Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine-Benzophenone Ligand in Alkyne Cyclotrimerization.

Adaptive ligands, which can adapt their coordination mode to the electronic structure of various catalytic intermediates, offer the potential to develop improved homogeneous catalysts in terms of activity and selectivity. 2,2'-Diphosphinobenzophenones have previously been shown to act as adaptive ligands, the central ketone moiety preferentially coordinating reduced metal centers. Herein, the utility of this scaffold in nickel-catalyzed alkyne cyclotrimerization is investigated. The complex [( p-tolL1)Ni(BPI)] ( p-tolL1 = 2,2'-bis(di(para-tolyl)phosphino)-benzophenone; BPI = benzophenone imine) is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes, selectively affording 1,2,4-substituted benzenes from terminal alkynes. In particular, this catalyst outperforms closely related bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction pathway involving a hemilabile interaction of the C=O unit with the nickel center. This is further borne out by a comparative study of the observed resting states and DFT calculations.

Introduction

The search for improved activity and selectivity in transition-<n class="Chemical">pan class="Chemical">span class="Chemical">metalpn>an> based homogeneous catalysts strongly relies on the development of steering ligands to tune the steric and electronic properties of the <spn class="Chemical">pan>an class="Chemical">metal center to the requirements of particular transformations. Recent years have seen the development of more sophisticated adaptive or cooperative ligands that can adjust their binding modes or engage in chemical transformations along a catalytic cycle.[1−4] The use of such ligands also plays an important role in the development of catalysts based on inexpensive and nontoxic first-row transition <spn>an class="Chemical">metals that can compete with, or even surpass, their counterparts based on precious metals, contributing to a more sustainable chemistry.[5] Perhaps one of the simplest implementations of the concept of adaptive ligands are hemilabile ligands, which are polydentate ligands featuring a weakly binding moiety that can reversibly (de)coordinate the metal center.[6] Since their introduction in synthetic chemistry by Jeffrey and Rauchfuss,[7] hemilabile ligands have played a significant role in the current transition from noble to base metals in homogeneous catalysis. The hemilabile interaction is important not only for opening up a masked coordination site but also to stabilize catalytic intermediates, by allowing the supporting ligand to adapt its binding mode throughout the reaction coordinate. Most commonly, weak donor groups, such as ether (OR2),[7,8] amine (NR3),[9] or imine derivatives (R2C=NR),[8b,10] are employed as the labile unit, often resulting in enhanced catalytic activity for different kinds of reactions. Examples include olefin oligomerizations and polymerizations,[8c,11] (cyclo)trimerization,[9c,11] carbonylation of methanol and methyl acetate,[12] (transfer) hydrogenation of ketones or dehydrogenation of alcohols,[9b,13,14] hydroacylation of alkenes and alkynes,[8d,15] and coupling reactions.[9c,11−19]

In addition to weak σ-<pan class="Chemical">spn>an class="Species">donorn>an> moieties, σ-acceptor groups[20] (e.g., <sppan>an class="Chemical">triarylborane)[21] or π-ligands[2] (e.g., C=C double bond)[22] can also act as hemilabile fragments. Our group recently reported the hemilabile character of the ligand 2,2′-bis(diphenylphospn>hino)<span class="Chemical">benzophenone[23] () bound to nickel[24] and other first-row transition metals.[25] was shown to function as a hemilabile π-acceptor ligand, with the central C=O bond coordinating to the <span class="Chemical">metal center in its more reduced forms (Figure ). Ru,[23,26] Rh,[27−29] Os,[30] and Ir[31] complexes of were also studied, more specifically in the catalytic hydrogenation of ketones,[23,26,28,30] in which the formation of a hydroxyalkyl species was accessible via the hydrogenation of the ketone moiety of the ligand.[28]

Figure 1

Previous work.[24] Coordination of a diphosphine benzophenone ligand, , to nickel(0), nickel(I), and nickel(II).

n class="Chemical">Pn>revious work.[24] Coordination of a <span class="Chemical">diphosphine <span class="Chemical">benzophenone ligand, , to nickel(0), nickel(I), and nickel(II).

In this pan class="Chemical">con>ntext, we set out to assess the potential utility of an η2-bound hemilabile π-accepn>tor moiety inpan class="Chemical">corporated in a pincer type architecture for catalysis, using the cyclotrimerization of alkynes as a benchmark reaction. Since its discovery by Reppe et al.[32] using a [Ni(CO)2(PCl3)2] catalyst, intermolecular [2 + 2 + 2] cyclotrimerization of <span class="Chemical">alkynes has been widely studied.[33] Metal catalyzed cyclotrimerization is an elegant method for the formation of cyclic frameworks, particularly convenient when the desired product is not accessible via traditional aromatic substitution reactions. Many transition metal systems have been developed (e.g., Ti,[34] Fe,[35] Co,[36] Ni,[37] Mo,[38] Ru,[39] Rh,[40] Pd,[41] Ir[42]). Literature precedents for the nickel-catalyzed conversion of alkynes to substituted benzene regio-isomers are known, both from an intramolecular[36c,37g−37l,37n,37p,37r,37s] and intermolecular approach.[9c,32,36a,37a−37f,37m,37o,37q,37t−37z] For the [2 + 2 + 2] intermolecular alkyne cyclotrimerization, it starts from the early example by Reppe et al.[32] to the combination of divalent nickel halide salts and reductants[36a,37f] or even activated Ni-particles.[37b] Phosphine,[9c,36a,37j,37m,37t,37v] NHC,[37v,37x] mixed phosphine/imino (P,N),[9c] or 1,4-diazadiene[37c,37e,37u,37w,37y,37z] ligands, have been introduced to Ni systems in order to improve selectivity and catalytic activity. Phosphine complexes generally favor the formation of benzene derivatives while 1,4-diazadiene complexes yield cyclooctatetraenes.

Besides their synthetipan class="Gene">c n>utility, <pan class="Chemical">span class="Chemical">alkynepan> condensations are also useful probes for ligand effects in catalysis.[43] The possible formation of different products and isomers, such as cyclotrimers, cyclotetramers, linear oligomers, <spn>n>an>an class="Chemical">polymers, and other compounds, provides useful information about the reactivity of a particular catalyst. For example, Uyeda[37w,44] and Ess[44] recently applied this approach to the evaluation of the effect of catalyst nuclearity, and Jones[9c] applied it to assessing differences in catalytic performance between bidentate PP and PN ligands (Figure ). In this study, we assess the effect of using hemilabile π-acceptor tridentate ligand (; Figure ) in alkyne cyclotrimerization through comparison with diverse nickel systems. <span class="Chemical">Ni(0) adopting a strong tridentate architecture (ligand = ; Figure ) and Ni complexes supported by a bidentate ligand (; Figure ) were selected for comparison with .

Figure 2

Examples of Ni(0) complexes used to evaluate the catalytic properties of a ligand–metal system by their reactivity with alkynes.[9c,37w] Blue bold font = property studied. R = Ph, CF3, tert-butyl.

Figure 3

Tridentate and bidentate ligands of comparison in this study.

Examples of <n class="Chemical">pan class="Chemical">span class="Chemical">Ni(0)pn>an> complexes used to evaluate the catalytic properties of a ligand–<spn class="Chemical">pan>an class="Chemical">metal system by their reactivity with <spn>an class="Chemical">alkynes.[9c,37w] Blue bold font = property studied. R = Ph, CF3, tert-butyl.

Tridentate and bidentate ligands of pan class="Chemical">con>mparison in this study.

Here we report the synthesis of <n class="Chemical">pan class="Chemical">span class="Chemical">nickelpn>an>(0) complexes of , , and featuring <spn class="Chemical">pan>an class="Chemical">benzophenone imine (<spn>an class="Gene">BPI) as a labile protecting ligand. Their stoichiometric reactivity with terminal alkynes is investigated, and the resulting Ni–alkyne complexes are thoroughly characterized. Then, a comparative study of these bi- and tridentate Ni(0)-catalysts in the cyclotrimerization of terminal alkynes is described, aiming to study the effect of a hemilabile π-acceptor system. For all tested alkyne substrates, outperforms and , as well as the structurally more different diphosphine rac-BINAP (), in terms of selectivity and activity. Reactivity studies combined with DFT calculations provide insight into the increased catalytic activity, suggesting that can adapt its binding mode throughout the reaction pathway, by the labile (de)coordination of its ketone unit.

Results and Discussion

Ni(0)-Benzophenone Imine Complexes

The <n class="Chemical">spn>an class="Chemical">diphn>an class="Chemical">osphine–<spn>n>an>an class="Chemical">benzophenone pincer-type ligand[23] 2,2′-bis(di(para-tolyl)phosphino)-benzophenone[25] (; Figure ) and the triphosphine <span class="Chemical">pincer-type ligand[45] bis(2-(di-(para-tolyl)phosphino)phenyl)phenylphosphine (; Figure ) were synthesized according to (adapted) literature procedures,[24,25,46] by lithiation of o-bromo(diarylphosphino)benzene[47] with n-BuLi followed by reaction with the appropriate electrophile (Scheme ). para-Tolyl substituents on the phosphine were introduced both to improve the solubility of the complexes and to provide a convenient 1H NMR handle for characterization. The 31P NMR spectrum of indicates the presence of an AB2 system (Δν ≈ JA,B)[48] with a coupling constant of 3JA,B = 155 Hz, as previously observed for its phenyl-substituted analogue.[46] The diphosphine ether ligand (49) was obtained from commercial sources and used as received.

Scheme 1

Synthesis of and from o-Bromo(di-(para-tolyl)phosphino)benzene

n class="Chemical">Con>ordination of the chelating <span class="Chemical">phosphine ligands to a <span class="Chemical">nickel(0) center was achieved by reaction with Ni(cod)2 (cod = cyclooctatetraene) in the presence of benzophenone imine (BPI), in a stoichiometric ratio, affording respectively complexes (i) [()Ni(BPI)] (; Scheme ), (ii) [()Ni(BPI)] (; Scheme ), and (iii) [()Ni(BPI)] (; Scheme ). BPI was chosen as coligand since it binds strongly enough to Ni(0) to prevent the formation of dimeric species[24] but weakly enough to be exchanged with different types of potential substrates such as alkenes, nitriles, and alkynes (see below; Scheme ).

Scheme 2

Ni0–Benzophenone Imine Complexes

(A) General synthetic route. (B) Isolated complexes. BPI = benzophenone imine.

Scheme 4

Ligand Exchange Reaction between the Benzophenone Imine from and Different Kinds of Substrates and Coligands

(A) General synthetipan class="Gene">c n>route. (B) Isolated pan class="Chemical">complexes. <pan class="Chemical">span class="Gene">BPI = <span class="Chemical">benzophenone imine.

NMR characterization of indicates that the <pan class="Chemical">spn>an class="Gene">Bpan class="Chemical">PIpan> ligand is bound in an η1 fashion. The <spn>n>an>an class="Chemical">13C NMR signal corresponding to the imine appears as a triplet (169.8 ppm,3JC,P = 6.1 Hz) close to that of the free imine at 177.3 ppm, consistent with retention of C=N double bond character. In the 1H NMR spectra, slight deshielding of the C=N–H proton by 0.24 ppm with respect to free <span class="Gene">BPI is also in accordance with η1(N) binding. Moreover, the ATR-IR spectrum displays a signal at 3152 cm–1 in the region corresponding the N–H stretch. In contrast, the backbone C=O moiety is bound side-on: the corresponding 13C NMR signal appears at 119.0 ppm (t, 3JC,P = 5.1 Hz), considerably shifted from the value of 197.4 ppm found in the free ligand.[25] This large shift is useful as a diagnostic tool to determine whether the CO moiety is coordinated to the Ni center. The 31P NMR spectra of consist of a single singlet signal at 15.5 ppm, indicating that the two phosphorus atoms are equivalent on the NMR time scale. However, from 1H and 13C NMR analysis, it appears that complex contains two chemically different methyl groups from the para-tolyl substituents (2.01 and 1.91 ppm in 1H NMR, 21.3 and 21.1 ppm in 13C NMR). Therefore, the para-tolyl substituents belonging to a single P-donor site are not equivalent on the NMR time scale, which can be used as a secondary indication for ketone coordination.

In a similar fashion, NMR data supn class="Chemical">port η1 binding of the <pan class="Chemical">span class="Gene">BPI ligand in the <spn>pan>an class="Chemical">triphosphine complex . The <span class="Chemical">13C NMR signal corresponding to the N=C bond is a doublet of triplets at 168.9 ppm (3JC,P = 8.1 Hz, 3JC,P = 7.4 Hz). The imine proton appears at 10.25 ppm in the 1H NMR spectrum (deshielding of 0.48 ppm compared to the free BPI ligand) and couples with the phosphorus nuclei (dt, 3JH,P = 3.2 Hz, 3JH,P = 2.7 Hz). The 31P NMR spectrum is consistent with an AK2 system (Δν ≈ 5JA,K)[48] with a coupling constant 2JA,K = 85 Hz. From 1H and 13C NMR analysis, it appears that the complex contains two chemically different para-tolyl substituents (2.14 and 1.95 ppm in 1H NMR, 21.4 and 21.1 ppm in 13C NMR), as also observed with complex .

In the n class="Chemical">1Hn> NMR of , the N–H signal from the coordinated <span class="Chemical">imine appears as a broad <span class="Chemical">singlet at δH = 9.71 in the expected 1:38 ratio compared to the aromatic region of the spectrum. The 31P NMR spectrum exhibits a single resonance at 32.4 ppm. No satisfactory 13C NMR data could be obtained due to the low solubility of the complex and its progressive decomposition, making the assignment of the binding mode of BPI in solution uncertain. The side-on binding of BPI observed in the solid state (see below) may be preserved in solution, in which case fast rotation of BPI ligand on the 1H NMR time scale would render the two phosphorus atoms equivalent. The N–H stretch from the bound imine appears as a broad weak band in the ATR-IR spectrum, located at 3200 cm–1.

More insights into the stpan class="Chemical">run>ctural and electronipan class="Gene">c properties of <pan class="Chemical">span class="Chemical">nickel complexes and were obtained by X-ray crystal structure determination (Figure ). Crystals suitable for X-ray diffraction were not accessible with but with the structurally related complex , in which the para-tolyl ligand is replaced by the phenyl analogue 2,2′-bis(di(phenyl)phosppan>hino)-<spn>n>an>an class="Chemical">benzophenone ().

Figure 4

Molecular structures of [()Ni(BPI)] () and [()Ni(BPI)] () in the crystal (50% probability level). Hexane solvent molecules () and C−H hydrogen atoms are omitted for clarity.

Molen class="Chemical">cun>lar structures of [()Ni(<span class="Gene">BPI)] () and [()Ni(<span class="Gene">BPI)] () in the crystal (50% probability level). Hexane solvent molecules () and C−H hydrogen atoms are omitted for clarity.

In acpan class="Chemical">con>rd with NMR data, the stpan class="Chemical">ructure of (Figure ; left) exhibits a side-on bound <pan class="Chemical">span class="Chemical">ketone moiety with Ni1–O11 and Ni1–C71 bond lengths of 2.0217(14) and 1.9760(19) Å, resppan>ectively. The C71–O11 bond length (1.320(2) Å) is found between those of unbound (1.213(3) Å)[23] and a C–O single bond (1.43 Å in <spn>n>an>an class="Chemical">ethanol[50]). The ketone is slightly less activated than in [()Ni(PPh3)],[24] which displays a C–O bond length of 1.310(2) Å, consistent with less pronounced π-back-donation arising from the weaker <span class="Species">donor character of BPI with respect to PPh3. The sum of bond angles (351.3(3)°) around the carbon atom lies between those expected for sp2 (328.5°) and sp3 (360°) hybridization at the carbon, in accordance with the Dewar–Chatt–Duncanson model. The N12–C12 bond length of 1.289(2) Å is close to the value of a C–N double bond (ca. 1.28 Å)[50] and comparable to the value found by Zhao et al. (1.294(3) Å)[51a] for η1-coordination mode in their synthesized NHC-based nickel(0) complex, [(IPr)Ni(η2-BPI)(η1-BPI)] (IPr = 1,3-bis(diisopropylphenyl)-imidazolium), showing that the π-back-donation to the BPI ligand is small. Accordingly, the valence angles around the imine carbon add up to 360.0(3)°. Finally, the torsion angle (∠Ni1–N12–C12–CPh) of 20.0(3)° shows that the nickel atom is slightly out of the plane of BPI, which is probably sterically driven.

In pan class="Chemical">con>ntrast to that of , the crystal stpan class="Chemical">ructure of (Figure , right) reveals a trigonal planar pan class="Chemical">complex in which the imine coligand is coordinated to the nickel in an η2 fashion (Ni1–C37 = 2.018(2) Å, Ni1–N1 = 1.8985(19) Å). The C–N bond axis is parallel to the metal coordination plane, which is thought to maximize π-back-donation from the high-lying in-plane d orbital with significant σ*(P–Ni) character. Accordingly, strong π-back-donation into the π*(N–C) orbital is evidenced by an elongated C37–N1 bond (1.373(3) Å vs 1.28 Å[50] in the free imine) and the pyramidalization of the C37 atom, which displays a sum of valence angles of 355.3(3)°, in line with known η2(C,N)-Ni(0) complexes.[51] As expected, the central oxygen atom from the ligand is not binding to the metal center (Ni1–O1 = 3.3668(16) Å), as also confirmed by a Wiberg bond index lower than 0.01 calculated by NBO analysis at a B3LYP/def2TZVP level of theory. The distance between the nickel and oxygen atoms is similar to the values reported from Ni(0) complexes bearing this ligand.[52] Finally, the measured P1–Ni1–P2 bite angle of 108.59(2)° and the Ni1–O1 distance of 3.3668(16) Å are in the same range as those found for [()Ni(Cl2)],[24] in which the C=O unit is not bound to the Ni ion (P1–Ni–P2 112.996(13)°; Ni–C = 3.4031(12) Å; Ni–O = 3.1012(10) Å), supporting the use of as a model complex for the unbound state of .

In summary, pan class="Chemical">con>mplexes and are geometrically similar and mainly difn>an class="Chemical">fer by the central coordinating unit the ligand (C=O vs pan class="Chemical">P). Complex differs in that the central oxygen atom is not coordinated and can be seen as a structural analogue of the unbound state of ligand (R = p-tolyl or Ph). Interestingly, moving from tridentate to bidentate mode is coupled to a change in binding mode of the BPI ligand from end-on to side-on, which can be understood from enhanced π-back-donation from a d10 L2Ni fragment (see Supporting Information for more insight by NBO analysis into the different coordination modes of the imine coligand; Table S8).

Ni(0)–Alkyne Complexes

With n class="Chemical">con>mplexes – in hand, their reactivity toward terminal <span class="Chemical">alkynes was investigated, leading to the generation of Ni–<spn>n>an>an class="Chemical">alkyne complexes [()Ni(HC≡CPh)] (), [()Ni(HC≡CCH2OMe)] (), [()Ni(HC≡CPh)] (), and [()Ni(HC≡CPh)] () (Scheme ). Ni-complexes and , bearing <span class="Chemical">phenylacetylene as coligand, are sufficiently stable to be isolated and are discussed first.

Scheme 3

Ni0-Terminal Alkyne Complexes

(A) General synthetic route. (B) Generated complexes. and were characterized in-situ in solution, while and were isolated. BPI = benzophenone imine.

(A) General synthetipan class="Gene">c n>route. (B) Generated pan class="Chemical">complexes. and were characterized in-situ in solution, while and were isolated. <pan class="Chemical">span class="Gene">BPI = benzophenone imine.

The <pan class="Chemical">spn>an class="Chemical">31pan class="Chemical">Ppan> NMR spn>n>an>ectrum of disn class="Chemical">ppan>lays two signals at 78.7 ppm (t, 2JP,P = 35.4 Hz) and at 28.1 ppm (d, 2JP,P = 35.4 Hz) in a ratio of 2:1, indicating that the ligand binds in a tridentate fashion. Two 1H NMR <span class="Chemical">singlets of the same intensity at 2.12 and 1.91 ppm correspond to the methyl group from diastereotopic para-tolyl substituents bound to the same <n class="Chemical">span class="Chemical">phosphorus atom. A single <spn>an class="Chemical">alkyne is bound to Ni as indicated by the terminal proton signal at 6.30 ppm, which appears as a doublet of triplets (3JH,P = 25.6 Hz, 3JH,P = 7.2 Hz) and couples with a carbon nucleus at 92.9 ppm in HMQC (1H–13C). Further confirmation of this assignment is provided by treatment of with d-phenylacetylene, resulting in the disappearance of the signal at 6.30 ppm in an otherwise identical 1H NMR spectrum (Supporting Information; Figure S68 and S76). In agreement with the spectroscopic characterization, the solid-state structure of (Figure , left) reveals a pseudo-tetrahedral geometry in which the alkyne moiety is coordinated in η2 fashion.

Figure 5

Molecular structure of [()Ni(HC≡CPh)] () and [()Ni(HC≡CPH)] () in the crystal (50% probability level). Toluene solvent molecules () and hydrogen atoms (except alkyne) have been omitted for clarity.

Molen class="Chemical">cun>lar structure of [()Ni(HC≡<span class="Gene">CPh)] () and [()Ni(HC≡<span class="Gene">CPH)] () in the crystal (50% probability level). Toluene solvent molecules () and hydrogen atoms (except alkyne) have been omitted for clarity.

pan class="Chemical">Con>mpound , supn>ported by the bidentate diphosphine ether ligand , displays two signals at 29.2 ppm and at 27.3 ppm coupling to each other (2JP,P = 14.0 Hz) and in a ratio of 1:1 in the <spn>n>an>an class="Chemical">31P NMR spectrum. This is consistent with a trigonal planar geometry in which rotation of the alkyne ligand is slow on the 1H NMR time scale; warming up the sample up to 100 °C does not result in coalescence of the two 31P NMR signals. This interpretation is supported by reaction of with 1 equiv of the symmetric alkyne diphenylacetylene, resulting in a symmetrical analogue of displaying a single 31P NMR <span class="Chemical">singlet at 28.1 ppm (Supporting Information; Figure S98). The proton from the alkyne gives a 1H NMR signal at 6.9 ppm, as revealed by HMQC (1H–13C) and comparison with deuterated analogue. The solid state structure (Figure , right) of is in accord with the NMR data and reveals a trigonal planar geometry with a P1–Ni–P2 bite angle of 104.94(2)°, where the oxygen atom is not bound to the nickel center (Ni1–O1 = 3.3317(15) Å). The alkyne is bound in an η2-coordination mode, the C37–C38 axis being in the Ni coordination plane.

While stable tripan class="Chemical">con>ordinate <pan class="Chemical">span class="Chemical">Ni(0) alkynepan> complexes with formally 16 valence electrons analogous to are well documented,[53,54] compound represents an unusual example of tetracoordinate, 18 VE <spn>n>an>an class="Chemical">Ni(0)–alkyne complex, to the best of our knowledge the first to be structurally characterized. Spectroscopic and structural data point toward a considerably weaker activation of the C≡C triple bond in this geometry (Table ). First, the <span class="Chemical">13C NMR chemical shifts of the two alkyne carbons (δC = 92.9 and 101.0) in are moderately deshielded in comparison with the free alkyne (δC = 77.5 and 83.3), indicating relatively weak rehybridization from sp to sp2. This contrasts with a stronger activation of the C≡C triple bond of phenylacetylene in evidenced by the 13C NMR signals from the coordinated phenylacetylene at 125.7 ppm (C≡C–H, dd, 2JC,P = 40.1 Hz, 2JC,P = 7.1 Hz) and 136.4 ppm (C≡C–Ph, dd, 2JC,P = 38.9 Hz, 2JC,P = 8.4 Hz), which were assigned with the assistance of APT 13C NMR and by comparison with the d-phenylacetylene complex. Second, the acetylic hydrogen signal of (δH = 6.30) is less shifted than that of (δH = 6.9), consistent with a stronger rehybridization toward sp2. Third, the C≡C stretch vibration from the alkyne in is found at 1823 cm–1 in the FT-IR spectrum versus 2126 cm–1 for free phenylacetylene and versus 1749 cm–1 for . Finally, the slightly elongated C47–C48 distance at 1.231(8) Å (vs 1.182–1.190 Å in free phenylacetylene)[55] and the rather large C47–C48–C49 angle at 152.0(6)° are also consistent with a relatively weak activation of the C–C triple bond, contrasting with the longer C37–C38 bond length (1.269(3) Å) and the more acute the C37–C38–C39 angle (144.07(19)°) found for . The C47–C48 bond length in is the shortest we are aware of for nickel(0)–alkyne complexes. Together, these data suggest that the π-back-donation from the nickel to phenylacetylene is more pronounced in , resulting in a stronger rehybridization toward sp2.

Table 1

Selected Spectroscopic and X-ray Crystal Structure Values of Phenylacetylene, and a

	phenylacetylene	p-tol5-Ph	Ph6-Ph
δC(C≡C–H) [ppm]	77.5	92.9	125.7
δC(C≡C–Ph) [ppm]	83.3	101.1	136.4
δH(C≡C–H) [ppm]	2.73	6.30	6.9
ν(C≡C) [cm–1]	2126	1823	1749
C≡C [Å]	1.182–1.190[55]	1.231(8)	1.269(3)
∠C≡C–CPh [deg]	177.39–179.49[55]	152.0(6)	144.07(19)

Vibrational frequencies are measured with ATR-IR (neat). NMR chemical shifts are given in C6D6.

The observations made for pan class="Chemical">con>mplex parallel those reported for mononuclear, 16 valence electron, tricoordinate Ni–<span class="Chemical">alkynepan>, in which the C–C bond is significantly more elongated than in , regardless of substituent groups on the <span class="Chemical">alkyne. For example, strong activation of the <span class="Chemical">alkyne is observed for bidentate diimine and mixed P,N supported species (up to 1.296(6) Å),[9c,53a,53b,53i,53l,53n,53r] while bidentate phosphorus ligands afford C–C bond lengths from 1.260(4) to 1.283(3) Å.[52b,53d−53f,53h,53j,53m,53o−53q] The substantially weaker rehybridization observed in can be assigned to its unique coordination geometry (i.e., tetracoordinate complex; 3 donor ligands; pseudo-T geometry) compared to and the reported Ni–alkyne complexes (i.e., tricoordinate complexes; 2 donor ligands; trigonal planar geometry). The weak activation of phenylacetylene in is presumably a consequence of the smaller splitting of the d-orbitals in its pseudo-tetrahedral geometry (3 donor ligands), resulting in less pronounced π-back-donation from Ni to the antibonding orbitals of the alkyne. Similar considerations were used by Lee and co-workers to explain the rather weak activation of CO2 in the tetracoordinate pseudo-tetrahedral nickel complex [(PPMeP)Ni(η2-CO2)] (PPMeP = PMe[2-PPr2–C6H4]2)[56] with respect to tricoordinate Ni-η2-CO2 adducts supported by bidentate phosphines such as [(dtbpe)Ni(η2-CO2)] (dtbpe = 1,2-bis(di-tert-butylphophino)-ethane).[57]

Finally, reaction of the <pan class="Chemical">spn>an class="Chemical">diphn>an class="Chemical">osphine-ketone complex with terminal <spn>n>an>an class="Chemical">alkynes generally led to a mixture containing the alkyne complex, [()Ni(HC≡CR1)] (; R1 = substituent on the alkyne), the starting material , <span class="Gene">BPI, the substrate, and cyclotrimerization products. This indicates that cyclotrimerization occurs concomitantly with ligand exchange, precluding the isolation of pure alkyne complexes. Nonetheless, in situ characterization of alkyne complexes was possible. Interestingly, 13C NMR of indicates that the ketone moiety is not bound to the metal center: addition of phenylacetylene to causes the disappearance of the triplet at 119.0 ppm, characteristic of the η2(C,O) coordination to nickel and the appearance of a triplet at 202.7 ppm (3JC,P = 4.9 Hz). In addition, all the para-tolyl groups in are equivalent at room temperature, with singlets appearing at 2.00 and at 21.2 ppm in 1H and 13C NMR, respectively. This is consistent with an unbound ketone allowing fast ring inversion of the chelate macrocycle. A single 31P NMR singlet at 33.0 ppm down to −80 °C indicates fast rotation of the alkyne, exchanging the two phosphorus atoms on the NMR time scale even at low temperature (Supporting Information; Figure S59).

The C≡C–<pan class="Chemical">spn>an class="Chemical">Hpan class="Chemical">1Hpan> NMR signal of the coordinating <spn>n>an>an class="Chemical">phenylacetylene could not be located, presumably hidden in the crowded aromatic region of the spectrum. In contrast, the assignment of the acetylic proton of the bound alkyne was possible in the 1H NMR spectrum of where it appears as a triplet of triplets at 6.34 ppm (3JH,P = 15.6, 4JH,H = 1.6 Hz) and integrates in a ratio of 1:12 compared to the four CH3 groups of the ligand, confirming that a single alkyne molecule is bound. The integrals, coupling constants, and multiplicity of CH2 (4.63 ppm, d, 4JH,H = 1.6 Hz) and OCH3 (3.32 ppm, s) of the bound <span class="Chemical">alkyne are in agreement with the proposed structure. Similar to , both 1H and 13C NMR of reveal that the ketone moiety is not bound to the metal center: the ketone resonance appears as a triplet at 203.1 ppm (3JC,P = 4.7 Hz) and the methyl groups from the para-tolyl are all equivalent, with a singlet at 2.03 and a doublet 22.2 ppm (4JC,P = 2.3 Hz) in 1H and 13C NMR, respectively. A singlet 31P NMR signal at 32.7 ppm at room temperature is consistent with fast rotation of the methyl propargyl ether similar to . However, gradually cooling the sample down to −85 °C first shows the appearance of a new unsymmetrical species at ca. −10 °C, as characterized by two doublets (2JP,P = 23.3 Hz) at 32.1 and 34.1 ppm (Supporting Information; Figure S67). The central signal corresponding to does split into two new doublets (2JP,P = 23.3 Hz) with a coalescence temperature between −50 and −60 °C. This indicates that the rotation of methyl propargyl ether can be frozen on the NMR time scale, but also that two distinct species are formed at low temperature. This is tentatively attributed to a secondary interaction of the oxygen atom from H–C≡C–CH2OMe with Ni. In addition, the second species observed at low temperature may be either a diastereomeric conformer or an isomeric structure in which the central ketone fragment binds to the nickel center.

The fast rotation of the <pan class="Chemical">spn>an class="Chemical">alkynepan> fragment in complex contrasts with the observed behavior of the tricoordinate . DFT calculations on the <sppan>an class="Chemical">acetylenen> complex reveal that the rotation of <span class="Chemical">acetylene is assisted by transient coordination of the <span class="Chemical">ketone moiety to the Ni center, resulting in a low overall rotation energy barrier of 5.5 kcal/mol (Figure ). The first transition state, , from the unbound ketone state to the bound state , is characterized by the coordination of the ketone moiety (ν = −113 cm–1; ΔGTS1 = 4.3 kcal/mol) to generate a four-coordinate intermediate . The second transition state, , corresponds to the rotation of acetylene (ν = −95 cm–1; ΔGTS2 = 4.1 kcal/mol) accompanied by partial decoordination of one of the phosphine arms. The low overall energy barrier of ΔGTS,overall = 5.5 kcal/mol is in qualitative agreement with the observed fast rotation of alkynes at room temperature. For comparison, a rotation energy barrier of 25.0 kcal/mol was calculated for the rotation of acetylene in the diphosphine ether complex (Supporting Information; Figure S112), in qualitative agreement with the observation of two 31P NMR signals for up to 100 °C.

Figure 6

(top) Energy diagram for the rotation of acetylene around the Ni-coordination plane (∠C1–C2–Ni–P1) of through two different transition states, = and , and one intermediate, = . Numbers in parentheses are G° values given in kcal/mol. ΔGTS1 = 4.3 kcal/mol. ΔGTS1′ = 2.9 kcal/mol. ΔGTS2 = 4.1 kcal/mol. ΔGTS,overall = 5.5 kcal/mol. (bottom) Optimized geometry of transition states and as well as intermediate at a B3LYP/6-31g(d,p) level of theory under vacuum. The imaginary frequency (−113 cm–1) of shows the coordination of the ketone (C3–O) to Ni, while the imaginary frequency (−95 cm–1) of is the rotation of acetylene around the C1–C2–Ni–P1 torsion angle. Selected bond lengths [Å] for : C3–O = 1.25; Ni–C3 = 2.35; Ni–O = 2.55; Ni–P1 = 2.17; Ni–P2 = 2.19; C1–C2 = 1.27. Selected bond lengths [Å] for : C3–O = 1.30; Ni–C3 = 2.01; Ni–O = 2.06; Ni–P1 = 2.18; Ni–P2 = 2.24; C1–C2 = 1.25. Selected bond lengths [Å] for : C3–O = 1.32; Ni–C3 = 1.96; Ni–O = 1.92; Ni–P1 = 2.14; Ni–P2 = 2.45; C1–C2 = 1.24. Hydrogen atoms have been omitted for clarity.

(top) Energy diagram for the rotation of <n class="Chemical">pan class="Chemical">span class="Chemical">acetylenepn>an> around the Ni-coordination plane (∠C1–C2–Ni–P1) of through two different transition states, = and , and one intermediate, = . Numbers in parentheses are G° values given in kcal/mol. ΔGTS1 = 4.3 kcal/mol. ΔGTS1′ = 2.9 kcal/mol. ΔGTS2 = 4.1 kcal/mol. ΔGTS,overall = 5.5 kcal/mol. (bottom) Optimized geometry of transition states and as well as intermediate at a B3LYP/6-31g(d,p) level of theory under vacuum. The imaginary frequency (−113 cm–1) of shows the coordination of the <spn class="Chemical">pan>an class="Chemical">ketone (C3–O) to Ni, while the imaginary frequency (−95 cm–1) of is the rotation of <spn>an class="Chemical">acetylene around the C1–C2–Ni–P1 torsion angle. Selected bond lengths [Å] for : C3–O = 1.25; Ni–C3 = 2.35; Ni–O = 2.55; Ni–P1 = 2.17; Ni–P2 = 2.19; C1–C2 = 1.27. Selected bond lengths [Å] for : C3–O = 1.30; Ni–C3 = 2.01; Ni–O = 2.06; Ni–P1 = 2.18; Ni–P2 = 2.24; C1–C2 = 1.25. Selected bond lengths [Å] for : C3–O = 1.32; Ni–C3 = 1.96; Ni–O = 1.92; Ni–P1 = 2.14; Ni–P2 = 2.45; C1–C2 = 1.24. Hydrogen atoms have been omitted for clarity.

The difn class="Chemical">fen>rences in electronic and structural properties of Ni–<span class="Chemical">alkyne complexes – were further investigated by DFT calculations (geometry optimization and NBO analysis), as summarized in Table . The optimized geometries of , , and (Table , entries 1–4) are consistent with the obtained crystal structures in which the C1–C2 bond length, C1–C2–C3 angle and Ni–C1 and Ni–C2 distances calculated for indicate weaker activation of the HC≡<span class="Gene">CPh substrate. Accordingly, Wiberg bond indexes (WBIs) calculated in the NBO basis at a B3LYP/def2TZVP level of theory show that the binding of phenylacetylene induces a decrease of the C1–C2 WBI (Table , entry 5) from 2.82 to 2.30 in versus 2.20 in and 2.17 in . The smaller Ni–C1 and Ni–C2 WBIs (Table , entries 6 and 7) of 0.31 in corroborate a weaker orbital interaction between the HC≡CPh ligand and the Ni-center in the triphosphine complex compared (WBIs of 0.43 and 0.39) and (WBIs of 0.42 and 0.39).

Table 2

Selected DFT Bond Distances and Angles and Wiberg Bond Indexes (WBI)a

entry	property	HC≡CPh	p-tol4-Ph	p-tol5-Ph	Ph6-Ph
DFT Bond Length and Angle [in Å and deg]
1	C1–C2	1.21	1.29	1.26	1.30
2	∠C1–C2–C3	180	141	150	141
3	Ni–C1		1.84	1.94	1.84
4	Ni–C2		1.89	1.97	1.89
Wiberg Bond Index (WBI)
5	C1–C2	2.82	2.20	2.30	2.17
6	Ni–C1		0.43	0.31	0.42
7	Ni–C2		0.39	0.31	0.39

Geometry optimizations of [()Ni(HC≡CCPh)] (), [()Ni(HC≡CPh)] (), and [()Ni(HC≡CPh)] () were performed at a B3LYP/6-31g(d,p) level of theory. WBIs were calculated by NBO analysis at a B3LYP/def2TZVP level of theory from the optimized geometries. Hydrogen atoms have been omitted for clarity.

Geometry optimizations of [()Ni(HC≡C<n class="Chemical">pan class="Chemical">span class="Gene">Cpn>an class="Chemical">Ph)] (), [()Ni(HC≡<spn>an>an class="Gene">CPh)] (), and [()Ni(HC≡CPh)] () were performed at a B3LYP/6-31g(d,p) level of theory. WBIs were calculated by NBO analysis at a B3LYP/def2TZVP level of theory from the optimized geometries. <span class="Chemical">Hydrogen atoms have been omitted for clarity.

In summary, we showed in this section that ligands – all form <pan class="Chemical">spn>an class="Chemical">Ni(0)pan>–<sppan>an class="Chemical">alkynen> complexes with <span class="Chemical">phenylacetylene. The <span class="Chemical">triphosphine binds in a tridentate mode in , resulting in unusually weak activation of the alkyne, while acts as a bidentate ligand in the tricoordinate , in which a strong Ni–alkyne interaction causes a high rotation barrier around the Ni–alkyne axis. The hemilabile ligand is able to sample both binding modes: it binds as a bidentate ligand in , resulting in a similar geometry as that of , but the observed low rotation barrier around the Ni–alkyne axis is indicative of the ability of to transiently adopt a tridentate mode.

Ligand Exchange Reactions on [()Ni(BPI)]

Having observed that the <pan class="Chemical">spn>an class="Chemical">ketonepan> moiety decoordinates upon binding of an <sppan>an class="Chemical">alkynen> to the ()Ni fragment, we sought to probe the generality of this hemilabile behavior. NMR-tube experiments showed that the <span class="Gene">BPI coligand can be reversibly exchanged with several types of ligands, such as <span class="Chemical">benzonitrile (Keq ≈ 2.7 × 10–3; ΔG298.15 ≈ 3.5 kcal/mol), styrene (Keq ≈ 1.5 × 10–2; ΔG298.15 ≈ 2.4 kcal/mol), and diphenylacetylene (Keq ≈ 6.1 × 10–3; ΔG298.15 ≈ 3.1 kcal/mol), as depicted in Scheme (see Supporting Information for more details). The proposed mode of coordination of the benzonitrile ligand in relies on DFT calculations, which predict the η2-(C,N) binding to be 9.4 kcal/mol energetically less stable than the η1-(N) coordination mode (Supporting Information; Table S11). With triphenylphosphine, the exchange is irreversible. In every case, with the exception of diphenylacetylene, 13C NMR demonstrates that the ketone moiety is still bound to nickel (no peaks between 190 and 210 ppm). The coordinated ketone appears at 120.4 ppm (dt, 2JC,P = 13.8 Hz, 2JC,P = 4.4 Hz) for the PPh3 complex and shows two chemically different methyl groups from the para-tolyl substituents at 1.97 ppm (s, 6H) and 1.96 ppm (s, 6H) in 1H NMR and at 21.2 and 21.1 ppm, respectively, in 13C NMR. Unfortunately for the PhCN complex and for the styrene complex , the carbon resonance of the bound ketone could not be assigned, as the weak signal is covered by those of the substrate and the starting complex . However, for all of these complexes, 1H NMR shows that the two methyl groups from the para-tolyl substituents on the phosphine are not equivalent (δH = 2.04 and 2.03 for , 2.05 and 2.01 for ), which is also indicative of the pincer-type binding mode of to nickel, with the central ketone moiety bound, as observed with and and in opposition to . With the diphenylacetylene complex , the absence of interaction of the ketone is demonstrated by the presence of a triplet at 198.3 of 3JC,P = 4.7 Hz and with all four C–H3 being chemically equivalent (δH = 1.95; δC = 21.2), similar to the closely related terminal alkyne complex . The difference in coordination between olefin and alkyne complexes is consistent with alkynes being stronger π-acceptors than olefins and illustrates the ability of the diphosphine ketone framework to adapt its binding mode to the electronic properties of a substrate.

Catalytic Comparison

Having shown that the hemilabile <pan class="Chemical">spn>an class="Chemical">diphn>an class="Chemical">osphine-ketone ligand is able to adapt its coordination mode to a substrate bound to <spn>n>an>an class="Chemical">Ni(0), we turn to its performance as a supporting ligand in the cyclotrimerization of alkynes. We compare hemilabile with the strong tridentate and the bidentate to delineate specific effects of the hemilabile behavior. In addition, the smaller bite-angle rac-BINAP () system was also tested.[58] Terminal alkyne substrate with diverse electronic properties were selected for comparison: first, <span class="Chemical">phenylacetylene (8), as a standard aryl-substituted substrate; second, methyl propiolate (9), as electron poor alkyne and since its additional oligomerization into cyclooctatetraene (COT) regio-isomers is commonly observed;[59] and finally, methyl propargyl ether (10) as an electron rich substrate. The catalytic outcome of the reaction of these alkynes with 0.5 mol % of isolated BPI-catalysts , , and and in situ generated catalyst at room temperature was analyzed. The organic products were separated from the reaction mixture as a mixture of 1,2,4- (a) and 1,3,5- (b) substituted arenes, in addition to cyclotetramerization products (when applicable), COTs (c), in which their relative ratio was determined by 1H NMR, as described in Table . To exclude a particular role of the BPI or cod coligands, the active catalysts were also generated in situ by mixing – with 1 equiv of Ni(cod)2 and 200 equiv of the corresponding alkyne. The reactions display similar activity as the values reported in Table (Supporting Information; Table S4) and form the same resting states (respectively, , , ) under these conditions.

Table 3

Catalytic Comparison for the Cyclotrimerization of Phenylacetylene (8), Methyl Propiolate (9), and Methyl Propargyl Ether (10), Using Catalysts to , and the Systema

entry	substrate (-R1)	catalyst	yield 1,2,4- (a) [%]	yield 1,3,5- (b) [%]	yield COTs (c) [%]	ratio a/b/c
1	phenyl acetylene, 8 (-Ph)	p-tol1	86.9	3.2	0	97:3:0
		p-tol2	3.1	1.9	0	62:38:0
		Ph3	2.8	0.2	0	94:6:0
		PhL4 + Ni(cod)2	4.6	1.9	0	70:30:0
2	methyl propiolate, 9 (−CO2Me)	p-tol1	90.2	6.3	2.5	91:7:2
		p-tol2	24.5	2.1	7.5	72:6:22
		Ph3	65.0	12.3	6.5	77:15:8
		PhL4 + Ni(cod)2	14.0	3.0	32	29:6:65
3	methyl propargyl ether, 10 (−CH2OMe)	p-tol1	71.6	6.9	0	90:10:0
		p-tol2	<1	<1	0	b
		Ph3	<1	<1	0	b
		PhL4 + Ni(cod)2	<1	<1	0	b

Similar results were obtained when the active catalysts were generated in situ using to , 1 equiv of Ni(cod)2, and the corresponding terminal alkyne (Supporting Information; Table S4). Yields and ratios were averaged over two runs (Supporting Information; Tables S2 and S3). Yields are isolated yields. Ratios were determined by 1H NMR.

For systems , , and , only trace amounts of products 10a and 10b were detected in which an accurate determination of the ratio between the two regio-isomers was not possible.

Similar results were obtained when the active catalysts were generated in situ using to , 1 equiv of <pan class="Chemical">spn>an class="Gene">Ni(pan class="Chemical">cod)2pan>, and the correspn>n>an>onding terminal <sppan>an class="Chemical">alkyne (Supporting Information; Table S4). Yields and ratios were averaged over two runs (Supporting Information; Tables S2 and S3). Yields are isolated yields. Ratios were determined by 1H NMR.

For systems , , and , only trace amounts of products 10a and 10b were detected in which an acn class="Chemical">pan class="Chemical">curate determination of the ratio between the two regio-isomers was not pn>pan class="Chemical">ossible.

Mixing <pan class="Chemical">spn>an class="Chemical">phenylacetylenen>an> (8) with 0.5 mol % of in <sppan>an class="Chemical">toluene at room temperature for 16 h yields 86.9% of <span class="Chemical">1,2,4-triphenylbenzene (8a) and 3.2% of 1,3,5-triphenylbenzene (8b), in an isomeric ratio of 97:3. Under the same conditions, the complexes and and the -Ni system afforded only low yields of 8a: 3.1%, 2.8%, and 4.6%, respectively. In addition, a lower regioselectivity toward the 1,2,4 isomers of 62:38 and 70:30 compared to the 1,3,5 isomer was observed with and + <span class="Gene">Ni(cod)2, respectively. With the more electron poor alkyne methyl propiolate (9), the reaction catalyzed by led to nearly exclusive cyclotrimerization with the formation of the 1,2,4-isomer 9a in a ratio of 91:7 compared to its 1,3,5 analogue 9b and with a yield of 90.2%. Only small amounts of the cyclotetramers, tetramethyl-cyclooctatetraene-tetracarboxylates (9c) are observed, in a ratio of 2:98 compared to the trimerization products. In contrast, the yields obtained for 9a with the other catalysts in the series range from 14.0% to 65.0%. More importantly, catalysts and and the system display lower selectivity. The tridentate triphosphine catalyst exhibits a regioselectivity of 92:8 (1,2,4-/1,3,5-isomer) and produces cyclotetramers in a 22:78 ratio compared to the cyclotrimerization products. The bidentate systems and are also less selective and lead to the formation of 9a in 84:16 and 82:18, respectively, compared to 1,3,5-benzene product 9b. The bidentate catalyst displays good chemoselectivity (92:8) toward cyclotrimerization, whereas the other diphosphine system appears to even favor cyclotetramerization products 9c (35:65). Finally, methyl propargyl ether (10), bearing an electron rich substituent was also tested and afforded lower yields than 8 and 9. With the ketone-based catalyst , 10 is converted into 1,2,4-tris(methoxymethyl)benzene (10a) in a yield of 71.6%, and the regioselectivity toward 10a remains high (90:10). Under the same conditions, the other complexes showed poor activity (less than 1% total yield). Overall, for the three examined substrates, catalyst , bearing the hemilabile diphosphine benzophenone ligand , was shown to be more active and more selective toward the formation of the 1,2,4-trisubstituted benzene product.

Additional Substrates

In addition to the substrates described in Table , catalyst was apn class="Chemical">plied to the cyclotrimerization of terminal <pan class="Chemical">span class="Chemical">alkynes 11an>an> to <sn class="Chemical">ppan>an class="Chemical">13a with high selectivity for 1,2,4-substituted <span class="Chemical">benzenes and in good isolated yields (>65%, Table ). Ethyl propiolate (11) (entry 1) affords triethylbenzene-1,2,4-tricarboxylate (11a) at room temperature. At 50 °C, 4-ethynylanisole (13) produces the desired 1,2,4-cyctrotrimerization product in a lower yield than the weakly electron-withdrawing analogue 1-ethynyl-4-fluorobenzene (12), supporting a preference for electron-withdrawing substrate (entries 2 and 3). The preference for electron poor alkynes is commonly observed in Ni-catalyzed cyclotrimerization of terminal alkynes.[37m,37v−37y] In comparison with the Ni-catalysts reported in literature,[37] complex is a competitive catalyst in terms of activity and regioselectivity (from 90:10 to 97:3). A catalyst loading of of 0.05 mol % mediated the cyclotrimerization of ethyl propiolate (11) into triethylbenzene-1,2,4-tricarboxylate (11a) in 87% isolated yield, reaching a TON of 1740 after 16 h.

Table 4

Additional Substrates for Alkyne Cyclotrimerization Catalyzed by c

Reaction temperature = room temperature.

Reaction temperature = 50 °C.

Reported yields are isolated yield for the 1,2,4-trisubstituted benzene isomer. Isomeric ratios between the 1,2,4 and 1,3,5 isomer and between the trimers and tetramers were determined by 1H NMR. Yields and ratios were averaged over two runs (Supporting Information; Table S5). Catalytic method A was applied (see experimental part).

Reaction temperature = room temn class="Chemical">perature.

Reported yields are isolated yield for the <n class="Chemical">pan class="Chemical">span class="Chemical">1,2,4-trisubstituted benzenepn>an> isomer. Isomeric ratios between the 1,2,4 and 1,3,5 isomer and between the trimers and tetramers were determined by 1H NMR. Yields and ratios were averaged over two runs (Supporting Information; Table S5). Catalytic method A was applied (see experimental part).

Mechanistic Considerations

The pan class="Chemical">con>mparison study between and the other Ni-systems (, , and ) strongly suggests that the hemilabile π-accepn>tor moiety pan class="Chemical">contributes to the high catalytic performance. In this section, the role of the <span class="Chemical">ketonepan> fragment in the improved catalytic performance of is studied and supported by computational modeling of some targeted compounds. To address the question, an overview of the general mechanism in <span class="Chemical">metal-catalyzed <span class="Chemical">[2 + 2 + 2] alkyne cyclotrimerization is discussed first (Scheme ), followed by differences in reactivity among the three catalysts tested in the comparison study.

Scheme 5

Proposed Intermediates for the Transition-Metal-Catalyzed [2 + 2 + 2] Cyclotrimerization of Acetylene, as Commonly Reported in Literature[33]

Scheme shows a pan class="Chemical">con>mmonly propn>osed mechanism for the cyclotrimerization of acetylene catalyzed by transition metals.[33] For base metals, most mechanistic studies have been carried out using cobalt complexes as catalysts.[60] From the <span class="Chemical">acetylene metal complex 14-C, the association of a second molecule of acetylene generates complex 14-(C, which requires an open coordination site in 14-C. Next, the oxidative coupling of the two coordinated alkyne fragments from 14-(C gives rise to the key metallacyclopentadiene intermediate. This step is generally thought to be rate determining. The metallacyclopentadiene intermediate can be best described as one of two resonance structures, 14-MCP or 14-MCP′, depending on the nature of metal complex used. For example Saá et al. calculated that CpRuCl (Cp = cyclopentadiene)-catalyzed alkyne cyclotrimerization proceeds via 14-MCP′, while the cobalt system CpCo proceeds via 14-MCP.[60d] Late transitions metals tend to favor the concerted-oxidative cyclization between two alkynes and the low-valent metal center.[60a] However, a stepwise zwitterionic diradical pathway, involving the formation of a σ(C–C) bond and one Ni–C bond cannot be excluded. When acetylene is substituted, the formation of the substituted metallacyclopentadiene is of importance as different 14-MCP regio-isomers can be generated. The substituted 14-MCP will thus dictate the overall selectivity toward the product formation of the 1,2,4- or 1,3,5-trisubstituted benzene regio-isomer (more precisely the transition state TS1 between the bisalkyne and metallacyclopentadiene intermediate).[61] In the last step (insertion of the third alkyne), different mechanisms have been proposed.[62] The insertion of a third alkyne could either proceed via a Diels–Alder type [4 + 2] cycloaddition, in which the metal does not participate in the bond formation (Scheme , pathway a),[60e] or with the assistance of the metal, prior to the new C–C bond formation (Scheme ; pathway b). 7-Metallanorbornadiene (14-[4 + 2])[63] is frequently presented as an unstable species and immediately collapses to arene, forming a metal–arene adduct. As regards pathway b, the coordination of the third alkyne is followed by either migratory insertion (metallacycloheptatriene 14-I) or [2 + 2] cycloaddition (cycloadduct 14-[2 + 2]). These complexes can also be formed in a concerted way, without involving the direct formation of an acetylene metallacyclopentadiene complex 14-AMCP, but still with participation of the metal center. It has also been suggested that an additional pathway could occur via an intermolecular [4 + 2] cycloaddition forming the η4-bound intermediate, 14-[4 + 2]′.[60d] Pathway a is likely to happen for strong donor ligands or solvents.[60d] The insertion of a fourth alkyne (especially if the intermediate formed is 14-I) leads to formation of COT side products.[61a] Eventually, the formation of the benzene product happens by either reductive elimination or ligand exchange of benzene with a molecule of acetylene (cycloadduct 14-[2 + 2] rearranges itself prior to the reductive elimination).

In this overall picture, several efn class="Chemical">pan class="Chemical">fects of the hemilabile π-accepn>tor <pan class="Chemical">span class="Chemical">ketonepan> moiety ion complex can be envisioned. First, the hemilabile interaction of the <spn>n>an>an class="Chemical">ketone unit can be thought to facilitate the alkyne uptake, especially in comparison with the tridentate phosphine system . In situ NMR spectroscopy experiments with the three complexes , , and showed that the corresponding <span class="Chemical">monoalkyne complexes – are the resting states of the catalyst in all cases. For the ketone catalyst and the bidentate catalysts , these resting states ( and ) are 16 VE species that can readily accept an incoming alkyne molecule, while the corresponding triphosphine complex is saturated (18 VE). Most probably, coordination of a second equivalent of alkyne to requires decoordination of one of the phosphorus atoms, which is likely to raise the overall reaction barrier and result in lower activity under the same conditions (Supporting Information; Scheme S4). The unique feature of the ketone unit is that the tricoordinate alkyne complex can be accessed directly from the tetracoordinate precursor . A similar process can be thought to happen at the end of the catalytic cycle: the ketone may accelerate product release by transient coordination to Ni(0), which would explain the lower propensity of to form cyclooctatetraenes.

Sepan class="Chemical">con>nd, the interaction of the <pan class="Chemical">span class="Chemical">ketonepan> with the <spn>n>an>an class="Chemical">nickel center could help to stabilize transient intermediates, such as the key metallacycle 14-MCP species. In order to assess structural differences between the <span class="Chemical">metallacyclopentadiene supported by the different ligands, , , and , we made use of geometry optimization by DFT calculation at a B3LYP/6-31g(d,p) level of theory using acetylene as a model substrate.

Unsurprisingly, the <n class="Chemical">pan class="Chemical">span class="Chemical">ketonepn>an> moiety is not bound in the <spn class="Chemical">pan>an class="Chemical">bisalkyne <spn>an class="Chemical">diphosphine benzophenone complex . However, geometry optimization of the metallacyclopentadiene intermediate formed by oxidative coupling reveals a pentacoordinate geometry in which the ketone is bound to the metal (, Scheme ), suggesting hemilabile behavior of in the key oxidative coupling step. Respective Ni–O and Ni–C distances of 1.93 and 1.99 Å, as well as an elongated C–O bond (1.35 Å), indicate a strong interaction with the metal center. In addition, a P1–Ni–P2 angle of 171° results in an approximate trigonal-bipyramidal geometry with apical P atoms. The fact that the C=O unit binds side-on to a formal Ni(II) center can be surprising at first sight in view of its low propensity to bind to divalent metal halides.[24,25] This can be understood by a synergistic interaction between a strongly σ-donating bidentate hydrocarbyl ligand and the strongly π-accepting ketone ligand in the equatorial plane of the trigonal bipyramid.

Scheme 6

C–C Coupling Reaction Step, as Modeled by DFT Calculations

R = para-tolyl.

For pan class="Chemical">con>mparison, the geometry of the metallacycles supported by the <spn>n>an>an class="Chemical">triphosphine and the diphosphine ether ligands were also optimized (Figure ). The triphosphine-supported [()Ni(C4H4)] () adopts a similar trigonal bipyramid geometry to . This binding mode may contribute to explain the differences in reactivity with the <span class="Chemical">rac-BINAP system, for which this mode is inaccessible. In contrast, [()Ni(C4H4)] () exhibits a distorted square planar geometry, in which the is bound in bidentate manner (N–O = 3.20 Å; WBI(Ni–O) < 0.01). Hence, the metallacycle in is not stabilized by its central ether donor group,[64] which may partly explain the lower regioselectivity of is toward the 1,2,4-trisubstituted arene cyclotrimerization product. Furthermore, no significant modifications of the P1–O–P2 bite angle from phenylacetylene analogue are visible.

Figure 7

C–C coupling reaction step. Comparison of metallacyclopentadiene molecular structures bearing the , , and ligands, optimized at a B3LYP/6-31g(d,p) level of theory. Hydrogen atoms have been omitted for clarity.

C–pan class="Gene">C n>pan class="Chemical">coupling reaction step. pan class="Chemical">Comparison of <span class="Chemical">metallacyclopentadiene molecular structures bearing the , , and ligands, optimized at a B3LYP/6-31g(d,p) level of theory. <span class="Chemical">Hydrogen atoms have been omitted for clarity.

At this point, we showed that the hemilabile character of the <n class="Chemical">pan class="Chemical">span class="Chemical">ketonepn>an> ligand contributes to higher activity and selectivity in the cyclotrimerization of terminal <spn class="Chemical">pan>an class="Chemical">alkynes. The combination of the two effects presented in this section, that is, decoordination and coordination of the C=O unit to assist the substrate uptake and stabilize the key <spn>an class="Chemical">metallacyclopentadiene intermediate, can explain its superior performance. Exchanging the C=O moiety in complex with a stronger donor atom, like a phosphine group (P–Ph) or with a bidentate ligand (O as central atom) decreases either the activity or the selectivity of the overall process.

Therefore, based on the pan class="Chemical">con>mputational and expn>erimental observations obtained from this study, a simpn>lified catalytipan class="Gene">c cycle is proposed for the cyclotrimerization of terminal alkynes catalyzed by the nickel diphosphine benzophenone system (Scheme ). The resting state, , can be generated by ligand exchange from or in situ from the ligand and <span class="Gene">Ni(cod)2 (1). The in situ system and operate at similar rates of product formation under the same tested conditions (similar percentage of isolated yields), showing that the dissociation of the coligand (cod vs BPI) and competitive binding with the metal center does not affect the final yields, as long as can be formed. At this step, the ketone is not bound, which facilitates coordination of the second alkyne molecule to generate Ni-bisalkyne (2). The next step (3) involves the C–C oxidative coupling, which is coupled to coordination of the ketone to nickel in η2-fashion. This interaction may favor the formation of the 2,5-disubstituted metallacyclopentadiene . The R1 substituents have been arbitrarily positioned in 2,5 positions as they promote the selective formation of the 1,2,4-trisubstituted benzene. From this intermediate, the insertion of the third alkyne to form the final benzene product could go via either [4 + 2] cycloaddition, migratory insertion, or [2 + 2] cycloaddition, followed by reductive elimination of the trisubstituted benzene and ligand exchange to regenerate for a new turnover (4). Throughout the reaction coordinate, the ligand can adapt its geometry by the labile interaction of C=O to Ni and stabilize intermediates.

Scheme 7

Proposed Simplified Catalytic Cycle for Cyclotrimerization of Terminal Alkynes Catalyzed by [()Ni(BPI)] or by in Situ Generation of the Active Intermediate with + Ni(cod)2

R = para-tolyl.

Conclusions

In pan class="Chemical">con>nclusion, we have reported here the synthesis and characterizations of Ni(0) complexes, incorporating a <spn>n>an>an class="Chemical">diphosphine ketone (), trisphosphine (), and diphosphine ether () ligand, in which the binding mode of the stabilizing <span class="Chemical">imine or alkyne coligand can change according to the structural and electronic characteristics of the supporting ligand. The characterization of [()Ni(HC≡CPh)] () provides a rare example of weak activation of alkynes by nickel complexes, attributed to its unique coordination geometry. We show in this study that [()Ni(BPI)] () is an effective catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes. In contrast, related Ni(0) complexes [()Ni(BPI)] () and [()Ni(BPI)] () are less active or less selective toward the 1,2,4-trisubstitued benzene cyclotrimerization product under similar conditions. We attribute the enhanced reactivity of to the hemilabile character of its diphosphine benzophenone ligand. In situ NMR spectroscopy and DFT calculations suggest that C=O hemilability may facilitate substrate uptake and assist the key oxidative coupling step. A more detailed mechanistic study of the reaction and applications of diphosphine-ketone ligands to other catalytic processes are actively investigated in our laboratories.

Experimental Section

Chemicals and Reagents

Unless otherwise noted, all reactions were carried out under an inert pan class="Chemical">N2n>(g) atmopan class="Chemical">sphere, using standard Schlenk line or glovebox techniques, and stirred magnetically. <pan class="Chemical">span class="Chemical">Silica gel <sppan>an class="Gene">P60 (SiliCycle) was used for column chromatography. Analytical thin layer chromatography was performed using SiliCycle 60 F254 silica gel (precoated sheets, 0.20 mm thick) from Merck KGaA (Darmstadt, Germany). Deuterated solvents were purchased from Cambridge Isotope Laboratory Incorporation (Cambridge, USA) and were degassed by standard freeze–thaw–pump procedure[65] and subsequently stored over molecular sieves. Common solvents were purified using a MBRAUN MB SPS-80 purification system or by standard distillation techniques or both.[65] They were degassed by bubbling N2(g) through the liquid for at least 30 min and then stored over molecular sieves. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran to confirm effective oxygen and moisture removal. Other solvents were checked for water content by the Karl Fischer titration or by 1H NMR. Liquid chemicals were first degassed by standard freeze–pump–thaw procedures or purged with N2(g) and then stored over molecular sieves prior to use. Phosphorus-containing compounds were checked for oxidation by 31P NMR before use. O-(Bromophenyl)-diphenylphosphine,[24,47]o-(bromophenyl)-di-p-tolylphosphine,[25] 2,2′-bis(diphenylphosphino)benzophenone (),[24] and 2,2′-bis(di(p-tolyl)phosphino)benzophenone ()[25] were synthesized according to reported procedures. All other reagents and starting materials were purchased from commercial sources and used without further purification, except when specified.

Physical Methods

The pan class="Chemical">1Hn>, <pan class="Chemical">span class="Chemical">13Cpan>, <spn>n>an>an class="Chemical">31P, and 19F NMR (400, 100, 161, and 400 MHz, respectively) spectra were recorded at 297 K on an Agilent MRF 400 spectrometer. All chemical shifts are reported in the standard δ notation of parts per million, referenced to residual peak of the solvent, as determined relative to Me4Si (δ = 0 ppm).[66] Variable-temperature (VT) NMR were recorded in d-toluene from −85 °C to room temperature. Infrared spectra were recorded using a PerkinElmer <span class="Chemical">Spectrum Two FT-IR spectrometer. For air-sensitive compounds, a N2 flow was used. Absorption spectra were recorded using a Lambda 35 UV–vis spectrometer. The UV–vis solutions were prepared in the glovebox, using degassed and dried solvent, and then stored in a cuvette sealed with a Teflon cap. The acquisition and analysis of the UV–vis data were performed with PerkinElmer UW WinLab and UV WinLab Data Processor and Viewer software. GC-MS measurements were conducted on a PerkinElmer Clarus 680 GC (column PE, Elite 5MS, 15 m × 0.25 mm ID × 0.25 μm) equipped with Clarus SQ8T MS and analyzed with TurboMass software. ESI-MS analysis was recorded with a Water LCT Premier XE spectrometer. Elemental analysis was provided by Mikroanalytisches Laboratorium Kolbe, Mülheim an der Ruhr, Germany, and Medac Ltd., Surrey, UK.

Computational Methods

DFT (density functional theory) results were obtained using the Gaussian 09 software package.[67] Restricted (R) geometry on class="Chemical">ptimizations use the B3LYpan class="Chemical">P (Becke, three-parameter, Lee–Yang–pn>an class="Chemical">Parr) functional and the 6-31g(d,p) basis set on all atoms. The structures were optimized without any symmetry restraints and are either minima or transition states. Frequency analyses were performed on all calculations. The transition states search was performed using the QST3 (synchronous transit-guided quasi Newton number 3) method. For NBO (natural bond orbital) calculation, the NBO6 program,[68] up to the NLMO (natural localized molecular orbital) basis set, was used at B3LYP/def2TZVP level of theory from the optimized geometry. Pictures derived from DFT calculations have been generated using the GaussView software. The B3LYP functional was chosen as it has been shown to be accurate for geometry optimization of related <span class="Chemical">metal compounds,[69] including closely related –<span class="Chemical">metal systems.[24,25]

Adapted from the n class="Chemical">procedure by Kpan class="Chemical">oshevoy et al.[46] To a suspension of (o-bromophenyl)di-p-tolylphosphine (5.01 g, 16.6 mmol) in dried and degassed THF (76 mL), a hexane solution of n-BuLi (1.6 M, 8.50 mL, 13.5 mmol) was added dropwise within 10 min at −78 °C under a N2 atmosphere. The reaction mixture was stirred at −78 °C for 1 h, and PPhCl2 (0.92 mL, 6.77 mmol) was added dropwise. The mixture was stirred at this temperature for one additional hour and then allowed to slowly warm up to room temperature. The solution was stirred at room temperature for three more hours and then quenched with MeOH (30 mL). The volatiles were removed in vacuo, and the yellow amorphous residue was washed with MeOH (5 × 15 mL) to afford L2 as a white solid, which was dried overnight under vacuum (3.52 g, 5.10 mmol, 76%). 1H NMR (400 MHz, C6D6, 25 °C): δH 7.40–7.33 (ArH, m, 6H), 7.33–7.25 (ArH, m, 6H), 7.19–7.16 (ArH, m, 2H), 7.01–6.94 (ArH, m, 5H), 6.93–6.87 (ArH, m, 6H), 6.84 (ArH, d, 3JH,H = 7.9 Hz, 4H), 2.03 (CH3, s, 6H), 2.01 (CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP – 14.8 (p-tolP, 2P), – 16.9 (PhP, 1P) [AB2 system, 2JA,B = 155 Hz]. 13C NMR (100 MHz, C6D6, 25 °C): δC 145.6–144.8 (m), 138.4–138.1 (m), 138.0 (d, JC,P = 12.7 Hz), 135.2–134.9 (m), 134.7 (t, JC,P = 4.4 Hz), 134.6–134.3 (m), 129.6, 129.4 (t, JC,P = 3.3 Hz), 129.1, 128.9, 128.2, 129.9, 21.2 (CH3), 21.2 (CH3). ATR-IR: ν [cm–1] = 3043, 1494, 1439, 1184, 1090, 806, 752, 504. HRMS (ESI, CH3CN, AgNO3): m/z calcd for [M + Ag]+ 793.1472; found 793.1605.

Synthetic Methods

Nickel Complexes

[()Ni(BPI)] ()

<pan class="Chemical">spn>an class="Gene">Ni(pan class="Chemical">cod)2pan> (249 mg, 0.91 mmol), (502 mg, 0.91 mmol), and <spn>n>an>an class="Chemical">benzophenone imine (181 mg, 0.92 mmol) were dissolved in dried degassed toluene (10 mL) under an inert atmosphere. The reaction mixture was stirred at room temperature for 20 min. Dried and degassed <span class="Chemical">hexane (5 mL) was added to the resulting black solution, causing the precipitation of a black solid. The precipitate was collected by filtration, washed with hexane (3 × 4 mL), and dried under vacuum to afford as a black powder (597 mg, 0.76 mmol, 83%). Single crystals suitable for X-ray diffraction and elemental analysis were obtained by slow exchange of hexane into a concentrated THF solution of . 1H NMR (400 MHz, C6D6, 25 °C): δH 9.88 (NH, br s, 1H), 7.90 (ArH, d, 3JH,H = 7.2 Hz, 2H), 7.83 (ArH, d, 3JH,H = 7.6 Hz, 2H), 7.75–7.70 (ArH, m, 4H), 7.23–7.19 (ArH, m, 2H) 7.09–6.79 (ArH, m, 26H), 6.73 (ArH, t, 3JH,H = 7.6 Hz, 2H). 31P NMR (161 MHz, C6D6, 25 °C): δP 16.4 (s, 2P). 13C NMR (100 MHz, d8-THF, 25 °C): δC 170.7 (C=N, t, 3JC,P = 5.4 Hz), 156.0 (t, JC,P = 18.0 Hz), 141.1, 140.2 (t, JC,P = 17.0 Hz), 138.7, 136.7 (t, JC,P = 11.2 Hz), 136.4 (t, JC,P = 13.7 Hz), 132.9 (t, JC,P = 7.7 Hz), 132.6, 131.8 (t, JC,P = 6.6 Hz), 129.62 (t. JC,P = 4.0 Hz), 128.9–126.7 (m) 125.8, 125.5 (t, JC,P = 8.1 Hz), 117.2 (C=O, t, 3JC,P = 7.6 Hz). ATR-IR: ν [cm–1]: 3163, 3050, 1583, 1404, 1432, 1478, 1432, 1303, 1249, 1091, 912, 778, 739, 692, 515. UV–vis (toluene): λmax [nm] 365, 574. Elemental analysis, Anal. Calcd for C50H39NNiOP2·1/2 hexane: C, 76.37; H, 5.56; N, 1.68. Found: C, 76.33; H, 5.58; N, 1.52. The crystal structure contains channels along the c-axis, which are filled with disordered hexane molecules (Supporting Information; Figure S110).

[()Ni(BPI)] ()

<pan class="Chemical">spn>an class="Gene">Ni(pan class="Chemical">cod)2pan> (108 mg, 0.39 mmol), (238 mg, 0.39 mmol), and <spn>n>an>an class="Chemical">benzophenone imine (71 mg, 0.39 mmol) were dissolved in dried degassed toluene (10 mL) under inert atmosphere. The reaction mixture was stirred at room temperature for 20 min. The solvent was evaporated, and the crude mixture was subsequently dissolved in dried and degassed <span class="Chemical">THF (5 mL). Dried and degassed hexane (5 mL) was added to the resulting black solution, causing the precipitation of a black solid. The precipitate was filtered, washed with hexane (3 × 4 mL), and dried under vacuum to afford as a black powder (221 mg, 0.31 mmol, 79%). Single crystals for elemental analysis were obtained by slow exchange of hexane into a concentrated toluene solution of . 1H NMR (400 MHz, C6D6, 25 °C): δH 10.01 (NH, s, 1H), 7.98 (ArH, d, 3JH,H = 7.4 Hz, 2H), 7.88 (ArH, dd, 3JH,H = 7.7, 4JH,H = 1.3, 2H), 7.66 (ArH, dt, 3JH,H = 7.9, 4JH,P = 4.6, 4H), 7.30–7.25 (ArH, m, 2H), 7.05 (ArH, dt, 3JH,H = 7.9 Hz, 4JH,P = 4.4 Hz), 6.97–6.88 (ArH, m, 6H), 6.86–6.73 (ArH, m, 14H), 2.11 (CH3, s, 6H), 2.09 (CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 15.5 (s, 2P). 13C NMR (100 MHz, C6D6, 25 °C): δC 169.8 (C=N, t, = 6.1 Hz), 156.7 (t, J = 18.1 Hz), 141.6 (t, J = 2.3 Hz), 141.2 (t, J = 16.9 Hz), 138.6 (Ar, t, J = 2.4 Hz,), 138.1, 137.2, 134.7 (t, J = 12.1 Hz), 133.9 (t, J = 14.7 Hz), 133.6, 133.5, 133.5, 133.4, 132.7 (t, J = 6.8 Hz), 130.5 (t, J = 4.3 Hz), 129.1 (t, J = 4.4 Hz), 129.0 (t, J = 4.0 Hz), 128.8, 127.5, 126.8, 126.5 (t, J = 8.1 Hz), 119.0 (C=O, t, J = 5.1 Hz), 21.3 (CH3), 21.1(CH3). ATR-IR: ν [cm–1]: 3152, 3053, 2917, 2860, 1598, 1496, 1448, 1394, 1250, 1185, 1092, 1018, 912, 803, 693, 627, 514. Elemental analysis, Anal. Calcd for C54H47NNiOP2: C, 76.61; H, 5.66. Found: C, 76.55; H, 5.93.

<pan class="Chemical">spn>an class="Gene">Ni(pan class="Chemical">cod)2pan> (163 mg, 0.59 mmol), (405 mg, 0.59 mmol), and <spn>n>an>an class="Chemical">benzophenone imine (108 mg, 0.59 mmol) were combined together and dissolved in dried degassed <span class="Chemical">toluene (7 mL) under inert atmosphere. The reaction mixture was stirred at room temperature for 1 h, and dried degassed hexane (7 mL) was added. The resulting solution was left in the freezer at −35 °C for 16 h, during which the precipitation of a black solid was observed. The solid was filtered, washed with cold hexane (5 × 4 mL), and dried under vacuum to afford as a black powder (315 mg, 0.34 mmol, 58%). 1H NMR (400 MHz, C6D6, 25 °C): δH 10.25 (NH, dt, 3JH,P = 3.2 Hz, 3JH,P = 2.7 Hz, 1H), 8.49 (ArH, d, 3JH,H = 7.6 Hz, 2H), 8.05 (ArH, dd, 3JH,H = 7.2 Hz, 3JH,P = 4.4 Hz, 2H), 7.66–7.60 (ArH, m, 5H), 7.55 (ArH, t, 3JH,H = 8.0 Hz, 2H), 7.12–6.86 (ArH, m, 18H), 6.83 (ArH, d, 3JH,H = 8.4 Hz, 6H), 6.72 (ArH, d, 3JH,H = 7.6 Hz, 4H), 2.14 (CH3, s, 6H), 1.95 (CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 37.1 (p-tolP, 2P) 28.7 (PhP, 1P) [AK2 system, 2JA,K = 85 Hz]. 13C NMR (100 MHz, C6D6, 25 °C): δC 168.5 (C=N, dt, 3JC,P = 8.1 Hz, 3JC,P = 7.4 Hz), 149.6–147.8 (m), 143.2, 141.3 (dt, JC,P = 11.1 Hz, JC,P = 2.7 Hz), 137.8 (dt, JC,P = 13.9 Hz, JC,P = 4.0 Hz), 137.2, 137.1–136.4 (m), 136.3, 132.8–132.6 (m), 132.4 (d, JC,P = 14.1 Hz), 131.8 (t, JC,P = 6.7 Hz), 131.3 (d, JC,P = 13.8 Hz), 130.6, 129.9–129.5 (m), 129.1 (t, JC,P = 4.2 Hz), 128.8–128.6 (m) 127.6, 127.1, 126.3, 21.4 (CH3), 21.1 (CH3). ATR-IR: ν [cm–1] 3048, 2972, 2917, 2864, 1664, 1598, 1496, 1445, 1187, 1114, 804, 695, 513. Elemental analysis, Anal. Calcd for C59H52NNiP3: C, 76.47; H, 5.66; N, 1.51. Found: C 75.97 H 5.49 N 1.56.

Under inert atmopan class="Chemical">spn>here, a dried and degassed <pan class="Chemical">span class="Chemical">toluenepan> solution (7 mL) of <spn>n>an>an class="Chemical">benzophenone imine (169 mg, 0.93 mmol) was added to a vial containing Ni(cod)2 (255 mg, 0.93 mmol), and (500 mg, 0.93 mmol). The reaction mixture was stirred at room temperature for 16 h in which a solid spontaneously precipitated. The resulting solid was filtered, washed with cold hexane (5 × 4 mL), and dried under vacuum to afford as an orange powder (582 mg, 0.75 mmol, 81%). Single crystals suitable for X-ray diffraction and elemental analysis were obtained by slow diffusion of <span class="Chemical">hexane into a C6D6 solution of . 1H NMR (400 MHz, C6D6, 25 °C): δH 9.71 (NH, s, 1H), 8.00–7.85 (ArH, br s, 2H), 7.70–7.64, (ArH, br s, 2H), 7.61–7.53 (ArH, br s, 6H), 7.13–6.90 (ArH, br s, 22H), 6.78–6.72 (ArH, br m, 4H), 6.56–6.52 (ArH, br m, 2H). 31P NMR (161 MHz, C6D6, 25 °C): δP 32.4 (s, 2P). ATR-IR: ν [cm–1] 3200, 3052, 1589, 1565, 1462, 1434, 1363, 1259, 1216, 1095, 880, 747, 693, 622, 505. Elemental analysis, Anal. Calcd for C49H29NNiOP2: C, 75.60; H, 5.05; N, 1.80. Found: C, 75.22; H, 5.69; N, 2.05. Due to poor solubility and progressive decomposition of in common solvents, no 13C NMR was recorded.

In Situ Generation of [()Ni(HC≡CR1)] ()

Under an inert atmon class="Chemical">spn>here, 1 equiv of [()Ni(<span class="Gene">BPI)] () was mixed with 1 equiv of a terminal <span class="Chemical">alkyne and dissolved in dried degassed C6D6 (0.6 mL). The solution was transferred into a Young-type NMR tube, and the mixture was analyzed. 31P and 1H NMR show the appearance of one new single species, [()Ni(HC≡CR1)] (), in addition to the partial release of BPI. High in situ yield was obtained with methyl propargyl ether (R1 = CH2OMe) as alkyne reactant, leading to the formation of [()Ni(HC≡CH2OMe)] (). The solution contains a mixture of 4-CH, , BPI, and methyl propargyl ether. 1H NMR (400 MHz, C6D6, 25 °C): δH 10.01 (, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H) 8.03 (, ArH, d, 3JH,H = 7.4 Hz, 2H), 7.92 (, ArH, dd, 3JH,H = 7.7, 4JH,H = 1.3, 2H), 7.70 (, ArH, dt, 3JH,H = 7.9, 4JH,P = 4.6, 4H), 7.54 (, p-tol-ArH, td, 3JH,P = 8.0 Hz, 3JH,P = 7.2 Hz, 8H), 7.35–7.29 (, ArH, m, 2H), 7.10 (, ArH, dt, 3JH,H = 7.9 Hz, 4JH,P = 4.4 Hz), 7.02–6.92 (, ArH, m, 6H), 6.91–6.76 (, ArH, m, 14H), 6.74 (, ArH, t, 3JH,H = 7.6 Hz, 2H), 6.66 (, ArH, td, 3JH,H = 7.6 Hz, 4JH,H = 1.6 Hz, 2H), 6.34 ppm (, ≡CH,3JH,P 15.6, 4JH,H = 1.6 Hz, 1H) 4.63 (, CH2, d, 4JH,H = 1.6 Hz, 2H), 3.70 (HC≡CCH2OMe, CH2,4JH,H = 1.6 Hz, 2H) 3.32 (, OCH3, s, 3H), 3.03 (HC≡CCH2OMe, OCH3, s, 1H); 2.10 (, CH3, s, 6H), 2.03 (, CH3, s, 12H), 1.96 (HC≡CCH2OMe, ≡CH,4JH,H = 1.6 Hz, 1H), 1.91 (, CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 33.3 (, s, 2P), 15.5 (, s, 2P).

<pan class="Chemical">spn>an class="Chemical">13Cpan> NMR was acquired in order to locate the resonances from the <sppan>an class="Chemical">carbonn>yl and methyl groups of the ligand of . <span class="Chemical">13C NMR (100 MHz, C6D6, 25 °C): δC 203.1 (C=O, t, 4JC,P = 4.7 Hz), 22.2 (H3C, d, 4JC,P = 2.3 Hz).

[()Ni(HC≡CPh)] ()

Under inert atmon class="Chemical">spn>here, [()Ni(<span class="Gene">BPI)] () (100 mg, 0.11 mmol), and <span class="Chemical">phenylacetylene (11 mg, 0.11 mmol) were mixed together and dissolved in dried degassed toluene (5 mL). The reaction mixture was subsequently stirred at room temperature for 1 h. The solvent was removed under vacuum, and the crude residue was dissolved in dried degassed THF (3 mL). Dried degassed hexane (3 mL) was added, and the mixture was put in the freezer at −35 °C for 16 h, after which a red precipitate was observed. The solid was isolated by filtration and washed with cold hexane (5 × 2 mL) to afford as a red powder (59 mg, 0.07 mmol, 66%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a concentrated THF solution of . 1H NMR (400 MHz, C6D6, 25 °C): δH 7.92 (ArH, dd, 3JH,H = 8.0 Hz, 3JH,P = 6.8 Hz, 2H), 7.63–7.53 (ArH, m, 9H), 7.45–7.39 (ArH, m, 2H), 7.10–6.95 (ArH, m, 1H), 6.88 (ArH, tt, 3JH,H = 7.2 Hz, 4JH,P 1.2 Hz, 1H), 6.83 (ArH, d, 3JH,H = 7.6 Hz, 4H), 6.72 (ArH, d, 3JH,H = 7.6 Hz, 4H), 6.29 (≡CH, dt, 3JH,P = 25.6 Hz, 3JH,P = 7.2 Hz, 1H), 2.12 (CH3, s, 6H), 1.91 (CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 78.7 (PhP, t, 2JP,P = 35.4 Hz, 1P), 28.1 (p-tolP, d, 2JP,P = 35.4 Hz, 2P). 13C NMR (100 MHz, C6D6, 25 °C): δC 150.0 (dt, JC,P = 47.1 Hz, JC,P = 16.4 Hz), 146.9 (d, JC,P = 36.0 Hz) 146.4 (dd, JC,P = 61.6 Hz, JC,P = 34.3 Hz),140.1 (dt, JC,P = 41.6 Hz, JC,P = 26.0 Hz), 137.9, 137.0–136.6 (m), 136.8, 133.7, 133.1 (t, JC,P = 28.0) Hz, 132.5 (t, JC,P = 28.0 Hz), 131.8–131.5 (m), 129.8, 129.7–128.9 (m) 101.1 (≡CPh, d, 2JC,P = 20.0 Hz), 92.9 (CH, br s), 21.3 (CH3, d, JC,P = 4.0 Hz), 21.1 (CH3, d, JC,P = 4.0 Hz). ATR-IR: ν [cm–1] 3042, 2918, 2861, 1823, 1589, 1479, 1440,1425, 1394, 1185, 1100, 1085, 1118, 805, 758, 690, 666, 625, 612, 539, 519. Due to the high sensitivity of , no elemental analysis data was obtained. The purity and identity of the compound was established by NMR and by its X-ray diffraction structure.

Alternative Synthesis of

Under inert atmon class="Chemical">spn>here, <span class="Gene">Ni(cod)2 (80 mg, 0.29 mmol), (200 mg, 0.29 mmol), and <span class="Chemical">phenylacetylene (32 mg, 0.31 mmol) were mixed together and dissolved in dried degassed toluene (5 mL). The reaction mixture was subsequently stirred at room temperature for 1 h. Precipitation of a red solid was observed after adding dried degassed hexane (10 mL) and leaving the solution to stand overnight in the freezer at −35 °C. The resulting solid was filtered, washed with cold hexane (5 × 5 mL), and dried under vacuum to afford as a red powder (169 mg, 0.20 mmol, 70%).

Synthesis of Deuterated Analogue of , [()Ni(DC≡CPh)]

The same procedure as for the synthesis of was an class="Chemical">pplied from <span class="Gene">Ni(n>an class="Chemical">cod)2 (30 mg, 0.10 mmol), (70 mg, 0.10 mmol), and <spn>pn>an>an class="Chemical">d-phenylacetylene (11 mg, 0.10 mmol). [()Ni(DC≡CPh)] was isolated as a red solid in a 74% yield (63 mg, 0.07 mmol). [()Ni(DC≡CPh)] can also be generated from reaction of with d-acetylene in a 1:1 stoichiometry. 1H NMR (400 MHz, C6D6, 25 °C): δH 7.92 (ArH, dd, 3JH,H = 8.0 Hz, 3JH,P = 6.8 Hz, 2H), 7.63–7.53 (ArH, m, 9H), 7.45–7.39 (ArH, m, 2H), 7.10–6.95 (ArH, m, 14H), 6.88 (ArH, tt, 3JH,H = 7.2 Hz, 4JH,P 1.2 Hz, 1H), 6.83 (ArH, d, 3JH,H = 7.6 Hz, 4H), 6.72 (ArH, d, 3JH,H = 7.6 Hz, 4H), 2.12 (CH3, s, 6H), 1.91 (CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 78.7 (PhP, t, 2JP,P = 35.4 Hz, 1P), 28.1 (p-tolP, d, 2JP,P = 35.4 Hz, 2P). 13C NMR (100 MHz, C6D6, 25 °C): δC 150.0 (dt, JC,P = 47.1 Hz, JC,P = 16.4 Hz), 146.9 (d, JC,P = 36.0 Hz) 146.4 (dd, JC,P = 61.6 Hz, JC,P = 34.3 Hz), 140.1 (dt, JC,P = 41.6 Hz, JC,P = 26.0 Hz), 137.9, 137.0–136.6 (m), 136.8, 133.7, 133.1 (t, JC,P = 28.0) Hz, 132.5 (t, JC,P = 28.0 Hz), 131.8–131.5 (m), 129.8, 129.7–128.9 (m) 101.1 (≡CPh, br m, 21.3 (≡CH3, d, JC,P = 4.0 Hz), 21.1 (CH3, d, JC,P = 4.0 Hz). ATR-IR: ν [cm–1] 3333, 3042, 2918, 2861, 1823, 1761, 1589, 1479, 1440,1425, 1394, 1185, 1100, 1085, 1118, 805, 758, 690, 666, 625, 612, 539, 519.

Under inert atmopan class="Chemical">spn>here and at room temperature, a suspension of [()Ni(<span class="Gene">Bpan class="Chemical">PI)] () (200 mg, 0.26 mmol) in dried degassed <sppan>an class="Chemical">toluene (3 mL) was combined with a toluene solution (3 mL) of phenylacetylene (26 mg, 0.26 mmol), resulting in a light yellow solution. The reaction mixture was subsequently stirred at room temperature for 10 min. Precipitation of a yellow solid was observed after adding dried degassed hexane (6 mL) and leaving the solution to stand overnight. The resulting solid was filtered, washed with hexane (5 × 5 mL), and dried under vacuum to afford as a yellow powder (112 mg, 0.16 mmol, 74%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a concentrated toluene solution of . 1H NMR (400 MHz, C6D6, 25 °C): δH 7.94–7.87 (ArH, m, 4H), 7.46–7.40 (ArH, m, 4H), 7.30 (ArH, dd, 3JH,H = 6.8 Hz, 4JH,H = 1.4 Hz, 2H), 7.05–6.80 (ArH and ≡CH, m, 18H), 6.75 (ArH, ddd, 3JH,H = 7.6 Hz, 4JH,P = 4.4 Hz, 4JH,H = 1.2 Hz, 1H), 6.66–6.56 (ArH, m, 3H), 6.39 (ArH, ddd, 3JH,H = 7.2 Hz, 4JH,P = 4.4 Hz, 4JH,H = 1.2 Hz, 1H), 6.32 (ArH, tt, 3JH,H = 7.6 Hz, 4JH,H = 1.0 Hz, 1H). 31P NMR (161 MHz, C6D6, 25 °C): δP 29.2 (d, 2JP,P = 22.5 Hz, 1P), 27.3 (d, 2JP,P = 22.5 Hz, 1P). 13C NMR (100 MHz, C6D6, 25 °C) δC 160.1 (d, 2JC,P = 9.1 Hz), 159.8 (d, 2JC,P = 11.4 Hz), 136.6 (≡CPh, dd, 2JC,P = 34.9 Hz, 2JC,P = 4.8 Hz), 136.1, 135.5 (dd, JC,P = 34.8 Hz, JC,P = 4.1 Hz), 134.9 (d, JC,P = 14.5 Hz), 133.8 (d, JC,P = 12.7 Hz), 132.9 (d, JC,P = 1.8 Hz), 130.7, 130.0, 129.8 (d, JC,P = 3.8 Hz), 129.6, 129.4 (d, JC,P = 1.7 Hz), 129.3, 129.0 (d, JC,P = 3.1 Hz), 128.8 (d, JC,P = 1.5 Hz), 125.3, 125.7(≡CH, dd, 2JC,P = 34.9 Hz, 2JC,P = 4.8 Hz) 124.9 (d, JC,P = 4.0 Hz), 124.1 (d, JC,P = 4.0 Hz), 122.9 (d, JC,P = 4.9 Hz), 118.3 (d, JC,P = 3.4 Hz). ATR-IR: ν [cm–1] 3286, 3171, 3052, 1749, 1588, 1564, 1480, 1461, 1434, 1259, 1213, 1095, 882, 834, 745, 692, 554, 503. Due to the high sensitivity of , no elemental analysis data was obtained. The purity and identity of the compound was established by NMR and by its X-ray diffraction structure.

Under inert atmon class="Chemical">spn>here, <span class="Gene">Ni(cod)2 (292 mg, 1.06 mmol), (567 mg, 1.05 mmol), and <span class="Chemical">phenylacetylene (108 mg, 1.06 mmol) were mixed together and dissolved in dried degassed toluene (7 mL). The reaction mixture was subsequently stirred at room temperature for 20 min. Precipitation of a yellow solid was observed after adding dried degassed hexane (10 mL) and leaving the solution to stand overnight. The resulting solid was filtered, washed with hexanes (5 × 5 mL), and dried under vacuum to afford as a yellow powder (387 mg, 0.55 mmol, 53%).

Synthesis of Deuterated Analogue of , [()Ni(DC≡CPh)]

The same experimental n class="Chemical">procedure as for the synthesis of was followed, using <pan class="Chemical">span class="Gene">Ni(pan class="Chemical">cod)2 (50 mg, 0.18 mmol), (99 mg, 0.18 mmol), and <spn>pan>an class="Chemical">d-phenylacetylene (19 mg, 0.18 mmol) as starting materials. [()Ni(DC≡<span class="Gene">CPh)] was isolated as a yellow powder (60 mg, 0.08 mmol, 48%). [()Ni(DC≡CPh)] can also be generated from reaction of with d-acetylene in a 1:1 stoichiometry. 1H NMR (400 MHz, C6D6, 25 °C): δH 7.94–7.87 (ArH, m, 4H), 7.46–7.40 (ArH, m, 4H), 7.30 (ArH, dd, 3JH,H = 6.8 Hz, 4JH,H = 1.4 Hz, 2H), 7.05–6.80 (ArH, m, 17H), 6.75 (ArH, ddd, 3JH,H = 7.6 Hz, 4JH,P = 4.4 Hz, 4JH,H = 1.2 Hz, 1H), 6.66–6.56 (ArH, m, 3H), 6.39 (ArH, ddd, 3JH,H = 7.2 Hz, JH,P = 4.4 Hz, 4JH,H = 1.2 Hz, 1H), 6.32 (ArH, tt, 3JH,H = 7.6 Hz, 4JH,H = 1.0 Hz, 1H). 31P NMR (161 MHz, C6D6, 25 °C): δP 29.2 (d, 2JP,P = 22.5 Hz, 1P), 27.3 (d, 2JP,P = 22.5 Hz, 1P). 13C NMR (100 MHz, C6D6, 25 °C): δC 160.1 (d, 2JC,P = 9.1 Hz), 159.8 (d, 2JC,P = 11.4 Hz), 136.4 (≡CPh, dd, JC,P = 34.9 Hz, JC,P = 4.8 Hz), 136.1, 135.5 (dd, JC,P = 34.8 Hz, JC,P = 4.1 Hz), 134.9 (d, JC,P = 14.5 Hz), 133.8 (d, JC,P = 12.7 Hz), 132.9 (d, JC,P = 1.8 Hz), 130.7, 130.0, 129.8 (d, JC,P = 3.8 Hz), 129.6, 129.4 (d, JC,P = 1.7 Hz), 129.3, 129.0 (d, JC,P = 3.1 Hz), 128.8 (d, JC,P = 1.5 Hz), 125.3, 124.9 (d, JC,P = 4.0 Hz), 124.1 (d, JC,P = 4.0 Hz), 122.9 (d, JC,P = 4.9 Hz), 118.3 (d, JC,P = 3.4 Hz). ATR-IR: ν [cm–1] 3052, 1588, 1564, 1461, 1434, 1259, 1213, 1095, 882, 745, 692, 503.

In Situ Generation of [()Ni(PhC≡CPh)] for Analytical Comparison with [()Ni(HC≡CPh)] ()

Under inert atmon class="Chemical">spn>here, <span class="Gene">Ni(cod)2 (7.6 mg, 0.028 mmol), (15.6 mg, 0.029 mmol), and <span class="Chemical">diphenylacetylene (5.0 mg, 0.028 mmol) were combined together and diluted in C6D6 (0.6 mL) at room temperature, turning the solution red. The mixture was transferred into a Young-type NMR tube and measured after 30 min of reaction. NMR analysis showed full conversion of the starting reagents and the release of cod, in addition to the single generation of a new species, [()Ni(PhC≡CPh)]. 1H NMR (400 MHz, C6D6, 25 °C): δH 7.71–7.61 (m, 8H), 7.18–7.13 (m, 4H), 6.97–7.88 (m, 12H), 6.86–6.82 (m, 6H), 6.73–6.66 (m, 4H), 6.55 (dd, 3JH,H = 8.0 Hz, 4JH,P = 2.8 Hz, 2H), 6.44 (t, 3JH,H = 7.6 Hz, 2H), 5.58 (cod, s, 4H), 2.21 (cod, s, 8H). 31P NMR (161 MHz, C6D6, 25 °C): δP 28.1 (s, 2P). 13C NMR (100 MHz, C6D6, 25 °C): δC 159.8 (t, 2JC,P = 5.0 Hz), 136.3 (≡CPh, t, 2JC,P = 6.8 Hz), 136.3, 136.0, 135.8 135.3, 135.2, 135.0 (d, JC,P = 3.2 Hz), 134.8, 134.7, 134.5 (t, JC,P = 7.0 Hz), 140.0, 130.4, 129.6, 129.5 (d, JC,P= 3 Hz), 129.3, 129.1, 128.8, 128.7, 124.4 (cod), 123.9, 121.1, 28.4 (cod).

Ligand Exchange Reactions (Scheme )

In Situ Generation of [()Ni(PPh3)] ()

In the glovebox, (5 mg, 5.9 μmol) and 1 equiv of <n class="Chemical">spn>an class="Gene">PPh3 (1.6 mg, 5.9 μmol) were dissolved in 0.6 mL of C6D6. The solution was transferred into a Young-type NMR tube, and an NMR spectrum was recorded approximately 15 min after the two reactants have been mixed tog<spn>n>an>an class="Chemical">ether, showing full conversion of . The analyzed solution contains a mixture of and BPI in a 1:1 ratio. [()Ni(PPh3)] (). [()Ni(<span class="Gene">PPh3)], with phenyl substituents on the phosphine ligand, has previously been reported.[24]1H NMR (400 MHz, C6D6, 25 °C): δH 7.86 (ArH, d, 3JH,H = 8.0 Hz, 2H), 7.62 (ArH, t, 3JH,P = 8.8 Hz, 6H), 7.44–7.39 (ArH, m, 2H), 7.38–7.33 (ArH, m, 4H), 7.25–7.19 (ArH, m, 4H), 6.93–7.80 (ArH, m, 9H), 6.67 (p-tolArH, d, 3JH,H = 7.6 Hz, 2H), 6.61 (p-tolArH, d, 3JH,H = 7.6 Hz, 2H), 1.97 (CH3, s, 6H), 1.96 (CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 38.5 (Ph3P, t, 2JP,P = 25.7 Hz 1P), 17.1 (p-tol2P, d, 2JP,P = 25.7 Hz, 2P). 13C NMR (100 MHz, C6D6, 25 °C): δC 155.1 (dd, JC,P = 19.1 Hz, JC,P = 16.0 Hz), 142.8 (dt, JC,P = 8.4 Hz, JC,P = 16.8 Hz), 138.4, 137.6 (dt, JC,P = 29.8 Hz, JC,P = 4.6 Hz), 136.9, 135.2 (t, JC,P = 11.4 Hz), 134.3–136.0 (m), 132.8 (t, JC,P = 7.0 Hz), 132.2, 130.6, 128.7–128.5 (m), 126.8, 120.4 (C=O, dt, 2JC,P = 13.8 Hz, 2JC,P = 4.4 Hz) 21.2 (CH3) 21.1 (CH3).

In Situ Generation of [()Ni(PhCN)] () and Keq Determination for the Reaction + PhCN ⇔ + BPI

In the glovebox, (4.9 mg, 5.8 μmol) was dissolved in 0.6 mL of C6D6, and 1 equiv of <pan class="Chemical">spn>an class="Chemical">pan class="Chemical">PhCNpan> was added via a microsyringe (0.6 μL, 5.8 μmol). The solution was transferred to a Young-type NMR tube, and an NMR spn>n>an>ectrum was recorded approximately 15 min after the two reactants have been mixed tog<sppan>an class="Chemical">ether. <span class="Chemical">31P NMR was recorded with a relaxation time of 21 s. For 1H NMR, the singlets at 10.01 ppm (1H), 2.10 ppm (6H), and 1.91 ppm (6H) for and the triplet at 6.63 (3H) for PhCN were selected to determine the relative concentration of the reactants. As regards the determination of the concentration of products, the two <span class="Chemical">singlets at 2.04 ppm (6H) and 2.03 ppm (6H) for and the singlet at 9.82 (1H) for BPI were selected. The same procedure was repeated at different stoichiometry of PhCN (i.e., 50 equiv and 200 equiv compared to ). 1H NMR (400 MHz, C6D6, 25 °C): δH 10.01 (, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H), 8.03 (, ArH, d, 3JH,H = 7.4 Hz, 2H), 7.92 (, ArH, dd, 3JH,H = 7.7, 4JH,H = 1.3, 2H), 7.70 (, ArH, dt, 3JH,H = 7.9, 4JH,P = 4.6, 4H), 7.35–7.29 (, ArH, m, 2H), 7.10 (, ArH, dt, 3JH,H = 7.9 Hz, 4JHP = 4.4 Hz), 7.02–6.92 (, ArH, m, 6H), 6.91–6.76 (, ArH, m, 14H), 6.63 (PhCN, ArH, t, 3JH,H = 7.6, 2H) 2.10 (, CH3, s, 6H), 2.04 (, CH3, s, 6H), 2.03 (, CH3, s, 6H), 1.91 (, CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 15.5 (, s, 2P), 12.7 (, s, 2P).

<pan class="Chemical">spn>an class="Chemical">13Cpan> NMR was acquired in order to locate the resonances from the <sppan>an class="Chemical">carbonn>yl and methyl groups of the ligand of . <span class="Chemical">13C NMR (100 MHz, C6D6, 25 °C): δc 22.8 (CH3). No <span class="Chemical">carbonyl peak around 200 ppm was detected.

In Situ Generation of [()Ni(C2H3Ph)] () and Keq Determination for the Reaction + C2H3Ph ⇔ + BPI

In the glovebox, (4.4 mg, 5.2 μmol) was dissolved in 0.6 mL of C6D6 and 1 equiv of <pan class="Chemical">spn>an class="Chemical">styrenepan> was added via a microsyringe (0.6 μL, 5.2 μmol). The solution was transferred to a Young-type NMR tube, and an NMR sppan>ectrum was recorded approximately 15 min after the two reactants have reacted. <n class="Chemical">sppan>an class="Chemical">31P NMR was recorded with a relaxation time of 21 s. For 1H NMR, the <span class="Chemical">singlet at 10.01 ppm (1H) for and the four sets of peaks at 7.23 ppm (2H), 6.58 ppm (1H), 5.60 ppm (1H), and 5,07 ppm (1H) for <span class="Chemical">C2H3Ph were selected to determine the relative concentration of the reactants. As regards the determination of the concentration of products, the four sets of peaks at 6.34 ppm (2H), 5.02–4.92 ppm (1H), 4.53 ppm (1H), 3.78–3.73 (1H) for and the singlet at 9.82 (1H) for BPI were selected. The same procedure was repeated at different stoichiometry of styrene (i.e., 50 equiv and 200 equiv compared to ). 1H NMR (400 MHz, C6D6, 25 °C): δH 10.01 (, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H), 8.03 (, ArH, d, 3JH,H = 7.4 Hz, 2H), 7.92 (, ArH, dd, 3JH,H = 7.7, 4JH,H = 1.3, 2H), 7.70 (, ArH, dt, 3JH,H = 7.9, 4JH,P = 4.6, 4H), 7.35–7.29 (, ArH, m, 2H), 7.23 (C2H3Ph, PhH, d, 3JH,H = 7.2 Hz, 2H); 7.10 (, ArH, dt, 3JH,H = 7.9 Hz, 4JHP = 4.4 Hz), 7.02–6.92 (, ArH, m, 6H), 6.91–6.76 (, ArH, m, 14H), 6.58 (C2H3Ph, =CH, dd, 3JH,H = 17.6 Hz, 3JH,H = 10.8 Hz, 1H), 6.34 (, PhH, d, 3JH,H = 7.2 Hz, 2H), 5.60 (C2H3Ph, =CH2, d, 3JH,H = 17.6 Hz, 1H), 5.07 (C2H3Ph, =CH2, d, 3JH,H = 10.8 Hz, 1H), 5.02–4.92 (, =CH, m, 1H), 4.53 (, =CH2, J = 9.6 Hz, d, 1H); 3.78–3.73 (, =CH2, m, 1H), 2.10 (, CH3, s, 6H); 2.05 (, CH3, s, 6H), 2.01 (, CH3, s, 6H), 1.91 (, CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 39.2 (, d, 2JP,P = 33.8 Hz, 1P), 15.9 (, d, 2JP,P = 33.8 Hz, 1P), 15.4 (, s, 2P).

<pan class="Chemical">spn>an class="Chemical">13Cpan> NMR was acquired in order to locate the resonances from the <sppan>an class="Chemical">carbonn>yl and methyl groups of the ligand of . <span class="Chemical">13C NMR (100 MHz, C6D6, 25 °C): δC 21.9 (CH3), 21.8 (CH3). No <span class="Chemical">carbonyl peak around 200 ppm was detected.

In Situ Generation of [()Ni(C2Ph2)] () and Keq Determination for the Reaction + C2Ph2 ⇔ + BPI

In the glovebox, (4.7 mg, 5.7 μmol) and 1 equiv of <pan class="Chemical">spn>an class="Chemical">diphenylacetylenen>an> (1.0 mg, 5.7 μmol) was dissolved in 0.6 mL of C6D6. The solution was transferred to a Young-type NMR tube, and an NMR sppan>ectrum was recorded approximately 15 min after the two reactants have reacted. <span class="Chemical">31P NMR was recorded with a relaxation time of 21 s. For 1H NMR, the <n class="Chemical">span class="Chemical">singlets at 10.01 ppm (1H), 2.10 ppm (6H), and 1.91 ppm (6H) for and the doublet of doublets at 7.52 (2H) for C2Ph2 were selected to determine the relative concentration of the reactants. As regards the determination of the concentration of products, the <spn>an class="Chemical">singlets at 1.95 ppm (12H) for and the singlet at 9.82 (1H) for BPI were selected. The same procedure was repeated at different stoichiometry of diphenylacetylene (i.e., 50 equiv and 200 equiv compared to ). 1H NMR (400 MHz, C6D6, 25 °C): δH 10.01 (, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H), 8.03 (, ArH, d, 3JH,H = 7.4 Hz, 2H), 7.92 (, ArH, dd, 3JH,H = 7.7, 4JH,H = 1.3, 2H), 7.70 (, ArH, dt, 3JH,H = 7.9, 4JH,P = 4.6, 4H), 7.52 (C2Ph2, PhH, dd, 3JH,H = 8.0 Hz, 4JH,H = 2.4 Hz, 2H), 7.35–7.29 (, ArH, m, 2H), 7.23 (C2H3Ph, PhH, d, 3JH,H = 7.2 Hz, 2H), 7.10 (, ArH, dt, 3JH,H = 7.9 Hz, 4JHP = 4.4 Hz), 7.02–6.92 (, ArH, m, 6H), 6.91–6.76 (, ArH, m, 14H), 6.70 (, ArH, d, 3JH,H = 8.0 Hz 8H), 2.10 (, CH3, s, 6H), 1.95 (, CH3, s, 12H), 1.91 (, CH3, s, 6H). 31P NMR (161 MHz, C6D6, 25 °C): δP 34.0 (, s, 2P), 15.4 (, s, 2P).

<pan class="Chemical">spn>an class="Chemical">13Cpan> NMR was acquired in order to locate the resonances from the <sppan>an class="Chemical">carbonn>yl and methyl groups of the ligand of . <span class="Chemical">13C NMR (100 MHz, C6D6, 25 °C): δC 198.3 (C=O, t, 4JC,P = 4.7 Hz); 21.2 (CH3).

Catalysis: General procedure for alkyne cyclotrimerization (Table and Table )

Method A

In the glovebox, a <pan class="Chemical">spn>an class="Chemical">toluenepan> solution (3 mL) of <sppan>an class="Chemical">alkynen> was added slowly, within 1 min, into a <span class="Chemical">toluene solution (3 mL) containing 0.5 mol % of the <span class="Chemical">nickel catalyst (Ni-cat. to Ni-cat. ). The solution was stirred at room temperature (substrates 8–11) or 50 °C (substrates 12 and 13) for 16 h. The solution was then opened to air, and the organic layer was extracted with HCl 1 M (1 × 10 mL) and water (2 × 10 mL). The aqueous layer was washed with Et2O (3 × 10 mL), and the organic portions were combined together, dried over MgSO4, filtered, and concentrated under vacuum. The product was extracted with Et2O and filtered through a silica plug. The final product was dried in vacuo and analyzed by GC-MS and NMR. The NMR characterization values of the organic catalytic products were compared with literature. The isomeric ratios were calculated and are reported according to 1H NMR. The reported values (yields and isomeric ratios) presented in Table and Table are the average values over two runs.

Method B

In the glovebox, a <pan class="Chemical">spn>an class="Chemical">toluenepan> solution (3 mL) of <sppan>an class="Chemical">alkynen> was added slowly, within 1 min, into a <span class="Chemical">toluene solution (3 mL) containing 0.5 mol % of the ligand ( to ) and 0.5 mol % of <span class="Gene">Ni(cod)2. The solution was stirred at room temperature for 16 h. The solution was then opened to air, and the organic layer was extracted with HCl 1 M (1 × 10 mL) and water (2 × 10 mL). The aqueous layer was washed with Et2O (3 × 10 mL), and the organic portions were combined together, dried over MgSO4, filtered, and concentrated under vacuum. The product was extracted with Et2O and filtered through a silica plug. The final product was dried under vacuum and analyzed by GC-MS and NMR. The NMR characterization values of the organic catalytic products were compared with literature. The ratios were calculated and are reported according to 1H NMR.

TON Experiment for the Cyclotrimerization of Ethyl Propiolate Catalyzed by

In the glovebox, a <pan class="Chemical">spn>an class="Chemical">toluenepan> solution (3 mL) of <sppan>an class="Chemical">ethyl propiolaten> (11, 232 mg, 2.37 mmol) was added slowly, within 1 min, into a <span class="Chemical">toluene solution (3 mL) containing 0.05 mol % of the <span class="Chemical">nickel catalyst (1 mg, 1.2 μmol). The solution was stirred at room temperature for 16 h. The solution was then opened to air, and the organic layer was extracted with HCl 1 M (1 × 10 mL) and water (2 × 10 mL). The aqueous layer was washed with Et2O (3 × 10 mL), and the organic portions were combined together, dried over MgSO4, filtered, and concentrated under vacuum. The product was extracted with Et2O and filtered through a silica plug. The ratios between 1,2,4- (11a) and 1,3,5- (11b) regioisomers in addition to a minor amount of tetraethyl-cyclooctatetraene-tetracarboxylates (11c) were determined by 1H NMR in a ratio of 91:7:2. Triethylbenzene-1,2,4-tricarboxylate (11a) was obtained in a yield of 87% (202 mg, 0.69 mmol).