Versatile Coordination and C-C Coupling of Diphosphine-Tethered Imine Ligands with Ni(II) and Ni(0).

Ligands that can adapt their coordination mode to the electronic properties of a metal center are of interest to support catalysis or small molecule activation processes. In this context, the ability of imine moieties to bind in either an η1(N)-fashion via σ-donation of the lone pair or, less commonly, in an η2(C,N)-fashion via π-coordination is potentially attractive for the design of new metal-ligand cooperative systems. Herein, the coordination chemistry of chelating ligands with a diphosphine imine framework (PCNP) to nickel is investigated. The imine moiety binds in an η1(N)-fashion in a Ni(II)Cl2 complex. The uncommon η2(C,N)-interaction is obtained in Ni(0) complexes in the presence of a PPh3 coligand. Increasing the bulk on the phosphine side-arms in the Ni(0) complexes, by substituting phenyl for o-tolyl groups, leads to a distinct binding mode in which only one of the phosphorus atoms is coordinated. In the absence of a coligand, a mixture of two different dimeric Ni(0) complexes is formed. In one of them, the imine adopts an uncommon η1(N)η2(C,N) bridging mode of the ligand to nickel, while the second one may involve reactivity on the ligand by the formation of a new C-C bond by oxidative coupling. The latter is supported by the isolation and structural characterization of a crystalline bis-CO derivative featuring a C-C bond formed by oxidative coupling of two imine moieties.

Introduction

In light of the ongoing interest in the substitution of precious metals by earth abundant metals in catalysis, the development of systems displaying metal–ligand cooperativity is flourishing.[1−4] Cooperative ligands can stabilize the metal center upon structural and electronic changes during catalytic processes by either adapting their binding mode or by reversibly accepting electrons, protons, or substrate fragments. For this purpose, ligands with versatile binding modes, facilitating hemilabile or adaptive behavior, or with the possibility of stabilizing multielectron redox processes, are of interest. In particular, π-ligands such as C=C double bonds have been shown to act as hemilabile ligands.[5] In this context, C=E (E = O, N) double bonds could constitute an attractive element of ligand design because of their ability to bind either through the E-centered lone pair or through the C=E π-system. Building on this idea, we recently reported that the diphosphine ketone ligand of 2,2′-bis(diphenylphosphino)benzophenone (Phdpbp) can act as a hemilabile acceptor ligand: the ketone moiety does not bind to the M(II)Cl2 fragment (M = Fe, Co, Ni) but binds in an η2-fashion to M(I)Cl.[6,7]

Imine functionalities are prevalent as ligands in organometallic chemistry, which is reflected in their numerous applications in homogeneous catalysis.[8,9] Examples are the NNN-pincer Fe and Co complexes independently reported by Gibson[10] and Brookhart[11] for alkene polymerization and investigated subsequently by Chirik for various reactions capitalizing on the redox noninnocent character of the imine moieties,[12] dinuclear Ni complexes as published by Uyeda that catalyze hydrosilylation reactions,[9j] or diphosphine imine complexes, reported by several groups, for olefin polymerization and oligomerization.[13−16] Generally, the imine nitrogen atom is reported to bind the metal center via its lone pair in an η1(N)-fashion, forming a σ dative bond. A second, and less common, possibility is formation of π-complexes via an η2(C,N)-coordination of the imine. This latter binding mode can be described by the Dewar-Chatt-Duncanson (DCD) model for coordination of π-ligands, i.e. the side-on bound and the metallo-aza-cycle extreme. Limited examples of an η2(C,N)-bound benzophenone-imine to Ni(0) are reported,[17] as well as a recently published bimetallic Ni complex with a chelating NNN-bis(imino)pyridine ligand.[18] Monometallic complexes with a chelating ligand with an η2(C,N)-coordinating imine functionality based on precious metals Pd, Rh, and Ir are described as well.[19−24]

Here, we investigate the versatile binding of an imine ligand flanked by two ortho-diphenylphosphino-substituted phenyl rings to nickel. The design of the ligand allows for a chelated binding structure via the phosphine arms to the metal center. Synthesis of the diphosphine-imine ligand PPhCNPPh (1, Scheme ) and its complexation to Co(II),[13,14] Ni(II),[13,15] and Pd(II)[14,15] with the imine backbone bound in an η1(N)-fashion has been reported previously, mainly for use in olefin oligomerization and polymerization reactions. In this contribution, the coordination chemistry of PCNP ligands to nickel is investigated, showing that the imine moiety is able to adopt a variety of coordination modes (end-on, side-on, bridging), changing upon varying the oxidation state from Ni(II) to Ni(0) and upon the addition of steric bulk on the phosphine tethers. Of particular interest is the observation of an oxidative C–C coupling reaction in dimeric complexes, in which a Ni(0)Ni(0) core is oxidized to Ni(II)Ni(0).

Scheme 1

Synthesis of PRCNPR Ligand 1

Results and Discussion

The diphosphine-imine ligand PPhCNPPh (1) was readily synthesized via an imine condensation of the diphenylphosphine substituted aldehyde and amine compounds (Scheme ).[15,25] A Ni(II) complex was synthesized from reaction of NiCl2(DME) with PPhCNPPh in CH2Cl2, resulting in Ni(PPhCNPPh)Cl2 (2) after isolation via precipitation from THF/hexanes and extraction of the product (Scheme ).[13] Analysis by 1H NMR spectroscopy at room temperature showed the formation of a paramagnetic species, with broad aromatic peaks in the region of δ 6 to 9, and one largely shifted broad peak at δ 35.1. Upon lowering the temperature to −80 °C, the peaks sharpen, and the peak at δ 35.1 shifts to δ 8.9, suggesting a spin transition. The latter possibly involves a high spin tetrahedral structure at room temperature–where one chloride anion is dissociated–converting to a diamagnetic low spin square planar structure at low temperature (Supporting Information, Figures S4, S5). X-ray crystal structure determination of crystals grown by slow vapor diffusion of hexanes into CH2Cl2 showed that 2 has a strongly distorted square pyramidal geometry around nickel (Figure ). The PPhCNPPh ligand is bound via both phosphorus atoms and the imine moiety in an η1(N)-fashion. The PPhCNPPh ligand is disordered by a 180° rotation around the imine bond in a ratio 0.550(6):0.450(6) (see the Supporting Information). Next to this, two chloride ligands are bound, of which the apical Ni–Cl bond is strongly elongated to 2.6545(6) Å, versus 2.1889(5) Å for the in-plane chloride ligand. Hence, the geometry can be seen as derived from a cationic square-planar Ni(II) complex weakly interacting with a Cl– counterion. This geometry is similar to that of the dibromide analogue of 2, as published by Sun and co-workers, with Ni–Br bond lengths of 2.3222(11) Å for the in-plane Br and 2.7754(11) Å for the apical Ni–Br bond.[13] Furthermore, Ni–P and Ni–N distances are similar for the Cl and Br analogues (2: Ni1–N1A: 1.963(3), Ni1–P1A: 2.1892(6), Ni1–P2A: 2.1837(6) Å (Table ). 2Br:[13] Ni–N: 1.956(6), Ni–P(N-side): 2.154(2), Ni–P(C-side): 2.1972(19) Å). The torsion angle for C–N=C–C is 175.4(6)°, indicating a slightly distorted flat configuration of the ligand backbone (close to 180°) and the largely sp2 character of the imine-carbon atom.

Scheme 2

Overview of the Ligands and Ni-Complexes

Figure 1

Molecular structures of 2, 3, 6, and 7 in the crystal. Displacement ellipsoids are drawn at the 50% probability level. Cocrystallized solvent molecules and hydrogen atoms are omitted for clarity: 2: 0.5 Et2O and 0.5 CH2Cl2; 7: THF. Selected distances (Å) and angles (deg): 2: only the major disorder component of the structure is shown here. Ni1–N1A: 1.963(3), Ni1–C7A: 2.913(4), N1A–C7A: 1.294(5), Ni1–P1A: 2.1892(6), Ni1–P2A: 2.1837(6), Ni1–Cl1: 2.1889(5), Ni1–Cl2: 2.6545(6), P1A–Ni1–P2A: 153.13(3), Cl1–Ni1–N1A: 169.69(10). 3: Ni1–N1: 1.943(3), Ni1–C7: 2.075(4), N1–C7: 1.364(5), Ni1–P1: 2.1888(11), Ni1–P2: 2.3234(11), Ni1–P3: 2.1761(11), P1–Ni1–P2: 121.89(4). 6: Ni1–N1: 1.969(2), Ni1–C1: 2.031(3), N1–C1: 1.358(4), Ni1–P1: 2.2996(8), Ni1–P2: 2.2189(8), Ni1–P3: 2.1904(9). 7: Ni1–N1: 1.864(3), Ni1–C1: 2.044(4), N1–C1: 1.368(4), Ni1–P1: 4.8493(12), Ni1–P2: 2.2048(11), Ni1–P3: 2.1475(12).

Table 1

Selected Bond Distances (Å) and Angles (deg) in the X-ray Crystal Structuresb

Constraints were used for the P atoms in the two disorder components.

2a is the major and 2b is the minor disorder component of complex 2. For clarity, the phenylene linkers in all structures are represented with gray lines.

An η2(C,N)-coordination mode of the imine moiety was accessed by synthesis of a Ni(0) complex. Reaction of ligand 1 with Ni(cod)2 (cod = 1,5-cyclooctadiene) in the presence of PPh3 as coligand in THF afforded the Ni(0) complex Ni(PPhCNPPh)(PPh3) (3) as a dark red solid after precipitation from THF/hexanes. Analysis by NMR spectroscopy displays a diamagnetic species with three signals in the 31P NMR spectrum: a sharp doublet at δ 29.5 and two broad signals at δ 6.3 and 44.1. The broadening is possibly caused by the presence of a small amount of coprecipitated free PPh3, causing the spectrum to broaden by exchange of the coligand. The 31P NMR signals sharpen both at −50 and 50 °C (Figure S7). The sharp low-temperature spectrum shows the splitting pattern and coupling constants for complex 3, from which it is evident that the three 31P nuclei couple with each other (δ 46.80 (d, JPP = 41 Hz), 30.00 (d, JPP = 28 Hz), 4.16 (dd, JPP = 28, 41 Hz)). Free PPh3 is not observed in this spectrum, which is probably due to its low concentration. The free imine is no longer present in 3, as the distinctive imine–CH peak in the 1H NMR spectrum at δ 9.32 in 1 is not observed, nor is the C=N band in IR analysis.[26]1H–13C HMQC 2D NMR analysis shows a cross peak for a signal at δ 84 in 13C NMR and δ 6 in 1H NMR spectrum, which is assigned to the imine–CH moiety (Figure S8). The considerable shift of the 13C NMR signal toward high field is indicative of a strong rehybridization toward sp3, i.e. a substantial metallacycle character of the M–C–N unit. It is, however, somewhat smaller than that observed in tricoordinate aldimine complexes of the (dippe)Ni fragment (dippe = bis(diisopropylphosphino)ethane),[17d] in which this signal is found at ca. δ 60. This difference likely arises from more efficient π-backdonation from the (Ni–P) σ-antibonding in-plane d-orbital in tricoordinate complexes. Crystallographic analysis of 3 showed a distorted tetrahedral geometry around the nickel center (Figure ).[27] The Ni(0) center is bound to 1 in a κ4(P,P,C,N)-fashion with an η2(C,N)-coordination of the imine backbone and to the PPh3 coligand. The C–N distance of the imine moiety is clearly larger in 3 (1.364(5) Å) than in the Ni(II) complex 2 (1.294(5) Å), as a result of π-backdonation to the antibonding π* orbital of the imine C=N bond.

Ligand Variation

The design of the ligand allows for facile incorporation of different substituents on phosphorus, including mixed ligands, as the building blocks of the imine condensation can be adjusted. The influence of additional bulk on the PRCNPR ligand was explored by the synthesis of the o-tolyl substituted ligands (Scheme ). The ligands PPhCNP (4) and PCNP (5) were synthesized accordingly, and subsequent complexation via a reaction of the ligand with Ni(cod)2 and PPh3 in toluene afforded Ni(0) complexes Ni(PPhCNP)(PPh3) (6) and Ni(PCNP)(PPh3) (7), respectively, after precipitation with hexanes and isolation of the solids.

Complex 6 gives rise to three broad signals in the 31P NMR spectrum at δ 36, 29, and 1, all in the region of nickel bound phosphorus compounds. The broad signals suggest fluxionality in the complex, which is assumed to be caused by the increased bulk. 1H NMR indicates a shift of the imine-CH moiety as the distinctive imine-proton (δ 9.33 in 4) is no longer present. The X-ray crystal structure on single crystals grown from vapor diffusion of hexane into THF showed a tetrahedral configuration around the nickel center, bound to PPh3, and 4 in a κ4(P,P,C,N)-fashion with an η2(C,N)-coordination of the imine backbone (Figure ). The imine C–N distance is 1.358(4) Å, which is similar to that in complex 3 (Table ). Likewise, the Ni–P distances closely resemble the analogues distances of complex 3.

The bulkier substituents on the phosphorus atoms in the tetra-ortho-tolyl PCNP ligand 5 and its nickel complex 7 result in decoordination of one of the phosphine arms: the 31P NMR spectrum of 7 shows again three signals, but in this case one peak appears as a broad singlet at δ −28, indicating a noncoordinated phosphorus atom. The remaining two signals–also broad singlets–are found at δ 11 and 40, consistent with coordination to Ni. Single crystal X-ray structure determination confirms this interpretation: nickel is bound to the ligand in a κ3(P,C,N)-fashion with an η2(C,N)-coordination of the imine moiety (Figure ). Next to this, PPh3 is bound, and the carbon-side phosphine of the PCNP ligand is dissociated. The imine C–N bond length is 1.368(4) Å for 7, which is comparable to the elongation of the imine backbone in 3 and 6, indicating a similar degree of π-backbonding despite the lower coordination number. The Ni–N bond distance of 1.864(3) Å is however shorter compared to 3 and 6, which is likely caused by the lesser amount of geometric strain due to the detachment of the second phosphine arm. The torsion angle C–N=C–C for 3, 6, and 7 is similar, with 151.4(3)° for 3, 147.0(3)° for 6, and 145.8(3)° for 7. These angles differ from an sp2 (180°) and an sp3 (120°), consistent with an intermediate hybridization, with a slightly higher degree of sp3 character for 7.

Dimeric Complexes

Reaction of 1 and Ni(cod)2 in toluene without the addition of a coligand resulted in a mixture of two species, major species 8a and minor species 8b (Scheme ). A workup was performed by precipitation of the products upon addition of hexanes to a THF solution, removing the majority of cod in the filtrate, followed by extraction of the solids in THF and evaporation of the solvent in vacuum and remaining cod by high vacuum.[28] The mixture mainly consists of 8a, allowing for its spectroscopic characterization, which shows the absence of a free imine moiety, as the imine-hydrogen peak is shifted in 1H NMR and the according band in the IR spectrum at 1621 cm–1 is not observed.[26] Four signals are observed in the 31P NMR spectrum: two doublets at δ 35.4 and −7.9 (JPP = 9 and JPP = 70 Hz) and two double–doublets at δ 35.5 and 22.9 (JPP = 2, 70 and JPP = 2, 9 Hz). This indicates that the four nonequivalent phosphorus atoms belong to a single product, which is likely to be caused by a dimeric nature of 8a. Heating a sample of 8a/8b up to 100 °C did not reveal exchange of the four inequivalent 31P signals of 8a on the NMR time scale (Figure S39). Next to 8a, the minor species 8b is present in approximately 10 to 20%, which shows two doublet signals in the 31P NMR spectrum at δ 1.1 and 38.3 (JPP = 43 Hz). No conditions could be identified that would allow for bulk isolation of either 8a or 8b, but the ratio between the species appears to be somewhat sensitive to the conditions of precipitation (see below), suggesting that 8a and 8b are kinetic products that have not reached equilibrium. To establish the overall composition of the mixture, one equivalent of PPh3 with respect to nickel was added. Quantitative conversion of 8a/8b into 3 was observed, confirming the overall composition of [Ni(PPhCNPPh)]2n (Scheme ) and demonstrating the synthetic use of the mixture as a source of the [Ni(PPhCNPPh)] fragment.

Scheme 3

Synthesis of Dimeric Complexes Derived from 1

P = PPh2, L is a solvent molecule.

Diffusion of hexanes into a THF solution afforded crystals suitable for X-ray diffraction, although only a minor fraction of the material crystallized. The crystalline material was covered by precipitation of a second compound, preventing bulk isolation. An NMR spectrum of the solid materials revealed a mixture of 8a and 8b (about 1:1), showing the increasing presence of the minor species after crystallization. A crystal could be harvested from the mixture and subjected to X-ray diffraction. The obtained crystal structure presents a dimeric species with two ligands and two nickel centers, identified as the minor component 8b (Figure ): even though no crystallographic symmetry is found, the overall structure of 8b possesses an approximate 2-fold rotation axis perpendicular to the Ni–Ni axis, which suggests that its 31P NMR spectrum should display only two signals. Each imine moiety acts as a bridge, binding side-on to one metal and end-on to the other, with the two P-donor sites of one ligand binding each to a different Ni center. The structure has a rather short Ni–Ni distance of 2.5595(3) Å which is likely sterically enforced by the geometrical arrangement of the ligands. An electronic Ni–Ni interaction is unlikely because both centers possess a d10 configuration. The C=N bond distances in 8b are 1.347(6) Å and 1.361(6) Å, both comparable to the monomeric Ni(0) complexes discussed above; thus, the additional σ(N)-interaction does not seem to contribute significantly to a more activated imine.

Figure 2

Molecular structures of 8b and 9b in the crystal. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): 8b: N1–C1: 1.347(6), N2–C2: 1.361(6), N1–Ni1: 2.033(4), N1–Ni2: 1.992(4), N2–Ni1: 2.004(4), N2–Ni2: 2.031(4), Ni1–Ni2: 2.5595(9), Ni1–C2: 2.063(5), Ni2–C1: 2.046(5). 9b: Formed C1–C2 bond is shown in red. C1–C2: 1.550(4), N1–C1: 1.448(4), N2–C2: 1.460(4), Ni2–P2: 2.1545(10), Ni2–P3: 2.1529(10), Ni2–N1: 1.884(3), Ni2–N2: 1.884(3), Ni1–P1: 2.2228(9), Ni1–P4: 2.2269(9), Ni1–C3: 1.765(4), Ni1–C4: 1.774(4).

The somewhat unusual μ–η1(N)η2(C,N) binding mode observed in 8b has been previously reported by de Bruin and co-workers in dinuclear rhodium(I) complexes formed by deprotonation of the α-CH2 group of a bridging dipicolylamine (dpa) ligand (Figure ).[19] Subsequent work by the same group has afforded a number of related (hetero)bimetallic compounds featuring the same binding mode, which could also be accessed directly from the imine analogue of the ligand (dpi).[21−23] Very recently, a similar CoI2(imine)2 core was observed by Rauchfuss and co-workers in the dimer [CoMe(Ph2PC2NHpy)]2, where Ph2PC2NHpy is a tridentate pyridine-imine-phosphine (pyCNP) ligand, differing from the PCNP framework by substitution of the second o-diphosphine-phenylene linker by a pyridine group (Figure ).[29] Notably, the Co–Co distance of ca. 2.9 Å is considerably longer than the Ni–Ni distance of 2.5595(9) Å in 8b, which is likely a result of the different local coordination geometry: 5-coordinate, trigonal bipyramidal (TBP) for CoI vs tetracoordinate, tetrahedral for Ni0.

Figure 3

Complexes by de Bruin and Rauchfuss. The bold line in the right drawing represents a pyridine ring of which only the bound C–N is shown.

The chemical nature of major dimeric species 8a could not be fully elucidated. Further analysis of the mixture shows the presence of two distinctive signals in the 1H NMR spectrum at δ 4.58 and 5.49 that belong to the major species 8a, besides numerous aromatic signals. Even though the low solubility of the compound leads to a low quality 13C NMR spectrum, the 1H–13C HMQC NMR spectrum shows clear cross peaks with 13C NMR signals at δ 78.7 and 86.90, respectively, which are both shown to be CH signals, according to APT 13C NMR analysis (Figures S35, S38). A possible explanation is the incorporation of a new C–C bond in the species. The absence of a large 3JHH between these two signals would then be due to a constrained H–C–C–H dihedral angle in the low-coupling Karplus region.[30] Upon arrangement of two imine bonds in close proximity to each other, possibly in the form of a dimer, the imine-carbon atoms can undergo a coupling reaction in which two electrons from a metal center are transferred to the ligand, forming a new carbon–carbon bond (Scheme ). A similar reaction was reported by Rauchfuss and co-workers,[31] where the coupling of two imine moieties from diphenylphosphino-2-benzimine ligands bound to iron(0) undergoes a coupling reaction upon addition of ferrocenium, analogous to a pinacol coupling observed on iron(0) in their earlier research.[32−35] However, it should be noted that a single dimeric molecule with two inequivalent η2(C,N)-coordinating imine moieties cannot be excluded on the basis of the obtained data, since a shift around δ 5 in the 1H NMR spectrum and δ 80 in the 13C NMR spectrum could also correspond to such a structure.[17d]

Evidence supporting the structure containing a new C–C bond arises from the reactivity of the 8a/8b mixture with CO. A solution of 8a/8b in C6D6 was exposed to CO (1 atm), after which the mixture was monitored in situ by NMR spectroscopy. 31P NMR data shows the formation of two species: major species 9a with two doublet signals at δ 33.0 and 16.2 (JPP = 11 Hz) and minor species 9b with two singlet signals at δ 39.8 and 22.1 (approximate ratio of 3.3:1) (Figure S42). Crystallization by slow vapor diffusion of hexanes into a benzene mixture resulted in single crystals of the minor product 9b (Figure , Scheme ). The X-ray crystal structure shows a dimeric, mixed valence Ni(0)Ni(II) complex with two added equivalents of CO both bound to the Ni(0) center. The Ni(II) center is surrounded by two neutral phosphine ligands and two anionic amido (R2N–) ligands, an uncommon coordination environment that was also observed in the Ni(II) complex of the P2N2 ligand N,N′-bis[2-(diphenylphosphino)phenyl]propane-1,3-diamine published by Duckworth, McPartlin, and co-workers.[34] Compared to Ni(II) complex 2, the Ni–N bonds are rather short, 1.963(3) Å in 2 (major disorder component) and 1.884(3) Å and 1.884(3) Å in 9b, originating from the strong π-donating nature of the nitrogen ligands. Noteworthy, the C–H hydrogen bound to C2 in the formed C–C bond has a rather short distance below 3 Å to the Ni(0) center. This is, however, attributed to the rigid geometry of the structure rather than a chemical interaction.

Crystals of 9b could be isolated from the 9a/9b mixture and were analyzed by NMR. 31P NMR analysis indeed shows the signals earlier attributed to 9b, without the presence of other species, and the 1H NMR spectrum shows a number of aromatic signals located in the range of δ 5.94 to 10.09. The large shift of the aromatic signals was confirmed by 1H–13C HMQC 2D NMR analysis, as coupling signals are observed for these peaks with the aromatic region of the 13C NMR spectrum. The exception is a broad singlet signal at δ 6.18, which has a coupling signal in the 13C NMR spectrum at δ 70.35 and is identified as the amido–CH functionality of the formed C–C bond. Furthermore, 2D 1H–31P HMBC NMR analysis shows a coupling with both phosphorus substituents of this proton (Figure S45). The IR spectrum of the crystals contains two signals for the CO bands at 1938 and 1999 cm–1. Complex 9b contains two Ni-centers, in two oxidation states, i.e. the Ni(0) and Ni(II) center. The formal oxidation state of the Ni(0) center is unchanged, starting from the Ni(0) precursor Ni(cod)2. The Ni(II) center, on the other hand, was formed via an intramolecular redox process, by transfer of its electrons to the formed C–C bond originating from the imine moieties (Scheme ).

The isolated compound 9b is however not the major species in the reaction mixture. The majority of the material (9a) shows two intense CO signals in the IR spectrum at 1929 and 1991 cm–1, slightly shifted from the CO signals in 9b, and NMR analysis shows the presence of aromatic signals in the 1H and 13C NMR spectra. In addition, the 1H–13C HMQC 2D NMR spectrum shows a coupling signal for a peak at δ 7.8 in 1H NMR and δ 155 in 13C NMR consistent with a noncoordinating imine-CH, which suggests the monomeric structure depicted in Scheme for compound 9a. More complex, possibly oligomeric structures can however not be fully excluded.

Combining the obtained data of compounds 8a, 8b and 9a, 9b suggests that the CO ligands act here as a trapping agent for the structure with the C–C bond, 8a, making its isolation possible. In the case of 8a, solvent molecules such as THF are likely coordinating to the Ni(0) center, which upon dissolution in benzene could be replaced by a benzene molecule.[36] Isolation is facilitated by replacing loosely bound solvents for stronger binding CO ligands, resulting in 9b. The higher apparent symmetry of 9b (two 31P NMR signals) with respect to 8a (four 31P NMR signals) may be due to higher fluxionality of the Ni(0) center in 9b. The formation of the interligand C–C bond by a two-electron transfer from the metal to the ligand resulting in 8a and 9b shows the possibility of the PCNP ligand to engage into ligand-centered redox processes, possibly opening new venues for cooperative processes besides the versatile binding as observed in complexes 2, 3, 6, and 7. Interestingly, the fact that the mixture 8a/8b can be quantitatively converted to the imine complex 3 by addition of PPh3 suggests that C–C bond formation may be reversible. These properties make Ni(PCNP)-complexes potentially interesting candidates for cooperative activation of substrates and subsequent catalysis. Further reactivity of these complexes is currently under investigation in our laboratories.

Conclusions

The coordination chemistry of chelating diphosphine-imine PRCNPR ligands to nickel was studied. The potential adaptive behavior of the phenyl-substituted ligand is exemplified in its coordination to Ni(II) and Ni(0): the imine moiety binds in an η1(N) fashion to Ni(II) and in an η2(C,N) fashion to the more electron-rich Ni(0). The addition of steric bulk to the PRCNPR framework in the form of o-tolyl substituents on the phosphorus atoms affords mixed-ligand complex 6, where both phosphine tethers of the PCNPPh ligand are bound to nickel, and tetra-o-tolyl ligand complex 7, where one phosphine arm is dissociated from the metal center. When no coligand is used in the synthesis of the Ni(0) complex with the PPhCNPPh ligand, a mixture of dimeric structures is obtained, with a) an η1(N)η2(C,N)-coordination of the two imine ligands to both Ni centers as shown by its X-ray crystal structure and b) a complex in which the imine functionalities seem to undergo a coupling reaction forming a new C–C bond. Addition of CO to the mixture leads to the isolation of a derivative of the C–C bound complex. The current study provides detailed insight into the coordination of η2(C,N)-bonding imine ligands to nickel, which are becoming prolific tools in the field of metal–ligand cooperativity. The observed adaptive behavior of the ligand upon different oxidation states and the redox-activity of the dimeric species make these complexes highly interesting for investigation toward their cooperative reactivity and catalytic activity. Further reactivity of these complexes is currently under investigation in our laboratories.

Experimental Section

General Considerations

All reagents were purchased from commercial sources and used as received unless stated otherwise. All reactions were performed under N2 using standard Schlenk and glovebox techniques unless stated otherwise. Deuterated benzene (C6D6) and deuterated dichloromethane (CD2Cl2) were degassed using the freeze–thaw–pump method (4×) and subsequently stored over molecular sieves. Dichloromethane (CH2Cl2) was distilled over calcium hydride, and tetrahydrofuran (THF) was distilled over sodium/benzophenone before use; both were degassed by bubbling N2 through it for 30 min and stored over molecular sieves. Dry diethyl ether (Et2O), hexanes, acetonitrile (MeCN), and toluene were acquired from a MBRAUN MB SPS-80 solvent purification system and further dried over molecular sieves before use. MeCN was filtered over alumina prior to use. 2-Diphenylphosphanyl-benzaldehyde[37−39] and 2-diphenylphosphinoaniline[40] were synthesized according to literature procedures.

1H, 13C, and 31P NMR spectra (respectively 400, 100, and 161 MHz) were recorded on an Agilent MRF400 or a Varian AS400 spectrometer at 25 °C. 1H and 13C NMR chemical shifts are reported in ppm relative to TMS using the residual solvent resonance as internal standard. 31P NMR chemical shifts are reported in ppm and externally referenced to 85% aqueous H3PO4. Infrared spectra were recorded using a PerkinElmer Spectrum One FT-IR spectrometer equipped with a general liquid cell accessory under a N2 flow. ESI-MS spectra were recorded on a Walters LCT Premier XE KE317 Micromass Technologies spectrometer. Compounds of which elemental analysis is reported were either recrystallized or precipitated and dried under high vacuum overnight before submission, and analysis was performed by the Mikroanalytisches Laboratorium Kolbe, Mülheim an der Ruhr, Germany.

2-Ph2PC6H4N=C(H)C6H4PPh2 (PPhCNPPh, 1)

This was adapted from a literature procedure.[15] 2-Diphenylphosphinobenzaldehyde (6.0 g, 0.021 mol), 2-diphenylphosphinoaniline (5.73 g, 0.021 mol), and p-toluene sulfonic acid (0.11 g, 0.58 mmol) were dissolved in toluene (500 mL) in a 3-necked round-bottom flask under an N2 atmosphere. A Dean–Stark trap was attached, and the collecting end was filled with molecular sieves (4 Å). The brown solution was heated to reflux for 17 h, after which all solvents were evaporated. The product was precipitated from a DCM/MeOH mixture, after which the solids were collected and washed with MeOH until the supernatant was colorless. Drying of the yellow solid resulted in the product with a yield of 60% (6.94 g, 0.013 mol). 1H NMR: δH 9.32 (d, J = 6 Hz, 1H, −N=CH−), 8.39 (dd, J = 4, 4 Hz, 1H, Ar–H), 7.45 (t, J = 7 Hz, 4H, Ar–H), 7.28 (m, 4H, Ar–H), 6.93–7.02 (m, 16H, Ar–H), 6.82–6.89 (m, 2H, Ar–H), 6.58 (dd, J = 3, 5 Hz, 1H, Ar–H) ppm. 13C NMR: δC 158.15 (dd, JCP = 2.0, 24.0 Hz), 155.04 (d, JCP = 18.1 Hz), 140.09 (d, JCP = 17.55 Hz), 138.67 (d, JCP = 19.3 Hz), 138.16 (d, JCP = 13.7 Hz), 136.89 (d, JCP = 11.1 Hz), 134.5 (dd, JCP = 2.6, 18.2 Hz), 133.63 (d, JCP = 13.8 Hz), 133.29 (d, JCP = 33.4 Hz), 131.15, 130.09, 129.29, 129.12, 129.01 (d, JCP = 7.0 Hz), 128.70, 128.63, 126.29, 117.50 ppm. 31P NMR: δP −13.5, −14.9 ppm. IR: 3054, 1621, 1561, 1465, 1432, 1358, 1309, 1263, 1189, 1157, 1090, 1069, 1026, 999, 751, 738, 695, 499, 409 cm–1. ESI-MS (MeCN/formic acid) m/z: [M + H]+ calcd: 550.1854, Found: 550.1780.

Ni(PCNP)Cl2 (2)

PPhCNPPh (1) (499 mg, 0.908 mmol) was dissolved in CH2Cl2 (2 mL). A solution of NiCl2(DME) (199.9 mg, 0.910 mmol) in CH2Cl2 (8 mL) was added, after which the color of the mixture turned from a bright yellow suspension to a yellow/brown solution. The mixture was stirred for 3 h 15 min, after which all solvents were removed in vacuum. The remaining solids were dissolved in a minimum amount of THF, and addition of hexanes caused precipitation. The solids were filtered off and collected via filtration with CH2Cl2. The product was obtained as a brown solid after evaporation of all solvents with a yield of 95% (0.584 g, 0.860 mmol). 1H NMR (CD2Cl2): δH 35.09, 8.63, 8.22, 7.94, 7.88, 7.76, 7.10, 7.01, 6.91, 6.75, 6.28 ppm. 31P and 13C NMR: no signal detected. ESI-MS (MeCN) [M – Cl]+m/z: calcd: 642.0817, found: 642.0851. IR: 3051, 1585, 1572, 1223, 1481, 1434, 1309, 1280, 1182, 1155, 1096, 1067, 998, 766, 747, 729, 691, 575, 521, 501 cm–1. Crystals for X-ray analysis were grown from CH2Cl2/hexanes. Elemental analysis, calcd: C 65.43, H 4.30, N 2.06, found: C 65.29, H 4.52, N 2.04.

Ni(PPhCNPPh)(PPh3) (3)

PPhCNPPh (1) (200 mg, 0.364 mmol), PPh3 (95.4 mg, 0.364 mmol), and Ni(COD)2 (100.1 mg, 0.364 mmol) were combined in a vial, and Et2O (9 mL) was added, resulting in a yellow-brown turbid mixture. The mixture was stirred for 5 h, in which the color changed to red-brown, after which the solvent was evaporated. THF (2 mL) was added to the solids, followed by addition of hexanes (6 mL) to precipitate the product as a red-brown solid, which was isolated via filtration and collected using THF. After evaporation of all solvents, 5 was obtained with a yield of 94% (299.3 mg, 0.344 mmol). 1H NMR (C6D6) due to extended overlap between the signals and with the solvent, integrals could not be assigned: δH 7.79 (m, Ar–H), 7.49 (t, J = 8 Hz, Ar–H), 7.39 (s, broad, Ar–H), 7.35–7.19 (m, Ar–H), 7.08–6.61 (m, Ar–H), 6.00 (t, J = 5 Hz, Ar–H) ppm. 13C NMR (C6D6): (note: precise assignment of signals hampered due to quality of the spectrum) δC 137.9, 137.5, 133.6, 133.5, 133.4, 133.2, 133.1, 132.9, 132.4, 132.2, 132.2, 132.0, 129.4, 128.2, 127.0, 125.1, 124.7, 120.4, 120.0, 99.6, 83.8 ppm. 31P NMR (d8-toluene, 25 °C): δP 44.1, 29.5, 6.1 ppm. 31P NMR (d8-toluene, −50 °C): δP 46.80 (d, JPP = 41 Hz), 30.00 (d, JPP = 28 Hz), 4.16 (d, JPP = 28, 41 Hz) ppm. IR: 3050, 1574, 1554, 1477, 1455, 1432, 1398, 1313, 1179, 1152, 1113, 1089, 1027, 815, 739, 693, 502, 436, 417 cm–1. Crystals for X-ray analysis were grown from THF/hexanes. The compound is too sensitive for transportation for elemental analysis.

2-Di-o-tolyl-phosphinoaniline

Triethylamine (0.25 mL, 1.79 mmol) was added to a solution of 2-iodoaniline (375 mg, 1.71 mmol) and di-o-tolyl-phosphine (365 mg, 1.70 mmol) in MeCN (9 mL) in a round-bottom flask under an N2 atmosphere, to which subsequently a suspension of Pd(PPh3)4 (20.4 mg, 0.018 mmol) in H2O/MeCN (12 mL, 1:2 ratio) was added. The pale orange mixture was refluxed for 64 h, after which all solvents were evaporated. Degassed CH2Cl2 (10 mL) was added, the organic layer was washed with degassed H2O (3 × 10 mL) and collected, and the solvents were evaporated. The pale brown solid was washed with cold, degassed MeOH (3 × 4 mL) and dried in vacuum, resulting in the product as an off-white to pink solid with a yield of 87% (452.1 mg, 1.48 mmol). 1H NMR (C6D6): δH 7.14 (m, 2H, Ar–H), 7.08–6.89 (m, 8H, Ar–H), 6.57, (t, J = 7 Hz, Ar–H), 6.32 (t, J = 7 Hz, Ar–H), 3.73 (s, 2H, −NH2), 2.39 (s, 6H, −CH3) ppm. 13C NMR (C6D6): δC 151.10 (d, JCP = 21 Hz), 142.80 (d, JCP = 27 Hz), 135.11 (d, JCP = 2 Hz), 134.19 (d, JCP = 8 Hz), 133.26, 130.74, 130.58 (d, JCP = 4 Hz), 129.13, 126.65, 117.10 (d, JCP = 6 Hz), 115.32 (d, JCP = 3 Hz), 21.32 (d, JCP = 20 Hz) ppm. 31P NMR (C6D6): δP −36.56 ppm. IR cm–1: 3457, 3344, 3055, 3005, 1613, 1599, 1475, 1439, 1304, 1245, 1161, 1081, 1033, 799, 746, 717, 556, 517, 456. Elemental analysis: calcd: C 78.67, H 6.60, N 4.59; found: C 78.86, H 6.77, N 4.57. ESI-MS (MeCN/formic acid) m/z: calcd: 306.1412, found: 306.1478.

2-Di-o-tolyl-phosphinobenzaldehyde

The compound is reported in the literature,[39] but an adapted synthesis method was used. 2-Bromobenzaldehyde (3.00 mL, 0.476 g, 25.7 mmol), di-o-tolyl-phosphine (5.51 g, 25.7 mmol), and Pd(PPh3)4 (16.2 mg, 0.014 mmol) were dissolved in toluene (90 mL) in a round-bottom flask under an N2 atmosphere, to which triethylamine (3.60 mL, 2.60 g, 25.8 mmol) was added. The orange suspension was refluxed for 8 h, after which the mixture was filtered, and washed with subsequently NH4Cl (3 × 50 mL) and brine (1 × 50 mL). The solvents were removed in vacuum, and the remaining solids were washed with cold degassed MeOH (3 × 40 mL) and dried in vacuum, resulting in the product as an off-white solid to brown with a yield of 93% (7.65 g, 24.0 mmol). 1H NMR (C6D6): δH 10.61 (d, J = 6 Hz, 1H, CH=O), 7.77 (ddd, J = 1, 3, 4 Hz, 1H, Ar–H), 7.06–6.84 (m, 11 H, Ar–H), 2.34 (s, 6H, −CH3) ppm. 13C NMR (C6D6): δC 190.62 (d, JCP = 22 Hz), 143.02 (d, JCP = 27 Hz), 140.02 (d, JCP = 26 Hz), 139.61 (d, JCP = 16 Hz), 135.03 (d, JCP = 11 Hz), 134.17, 133.77, 133.54, 130.67 (d, JCP = 5 Hz), 130.66, 129.42, 129.02, 126.75, 21.35 (d, JCP = 23 Hz) ppm. 31P NMR (C6D6): δP −28.54 ppm. IR: 3054, 2963, 2908, 2824, 1693, 1584, 1449, 1391, 1290, 1261, 1199, 1116, 1034, 843, 823, 800, 746, 716, 556, 528, 507, 481 cm–1.

PPhCNP (4)

The whole procedure was performed under inert conditions and with dry and degassed solvents. 2-Di-o-tolyl-phosphanyl-benzaldehyde (4.00 g, 0.013 mol), 2-diphenylphosphinoaniline (3.48 g, 0.013 mol), and p-toluene sulfonic acid (60 mg, 0.32 mmol) were dissolved in toluene (110 mL) in a 3-necked round-bottom flask under an N2 atmosphere. A Dean–Stark trap was attached, and the tap was filled with molecular sieves (3 Å). The clear yellow-brown solution was heated to reflux for 17 h, and the color changed to red/yellow-brown, after which all solvents were evaporated. CH2Cl2 was added (20 mL), subsequently MeOH was added (20 mL), and the mixture was cooled in an ice bath for 20 min to precipitate the crude product. The solids were washed with MeOH until the supernatant was colorless, and the solids were dried in vacuum resulting in 4 as a yellow powder with a yield of 47% (3.43 g, 5.94 mmol). 1H NMR (C6D6): δH 9.33 (d, J = 6 Hz, 1H, −N=CH−), 8.43 (ddd, J = 8, 4, 1 Hz, 1H, Ar–H), 7.46–7.42 (m, 4H, Ar–H), 7.06–6.93 (m, 16H, Ar–H), 6.87–6.80 (m, 4H, Ar–H), 6.59 (ddd, J = 8, 4, 1 Hz, 1H, Ar–H), 2.34 (s, 6H, −CH3) ppm. 13C NMR (C6D6): δC 158.08 (dd, JCP = 2, 28 Hz), 155.13 (d, JCP = 18 Hz), 142.76 (d, JCP = 25 Hz), 140.49 (d, JCP = 17 Hz), 138.14 (d, JCP = 12 Hz), 137.21 (d, JCP = 19 Hz), 134.79, 134.61 (d, JCP = 21 Hz), 133.95, 133.74, 133.59 (d, JCP = 13 Hz), 133.10, 131.34, 130.59 (d, JCP = 4 Hz), 130.09, 129.39, 129.33, 128.68, 128.60, 128.41 (d, JCP = 4 Hz), 126.84, 126.25, 117.47 (d, JCP = 2 Hz), 21.39 (d, JCP = 22 Hz) ppm. 31P NMR (C6D6): δP −13.49, −31.62 ppm. IR: 3048, 3004, 2965, 2941, 2912, 2875, 1698, 1627, 1618, 1561, 1466, 1431, 1357, 1265, 1191, 1118, 1093, 1068, 1024, 861, 766, 747, 740, 695, 555, 503, 457 cm–1. ESI-MS (MeCN/formic acid) m/z: calcd: 578.2167, found: 578.2456. Elemental analysis: calcd: C 81.09, H 5.76, N 2.42; found: C 80.68, H 5.89, N 2.41.

PCNP (5)

The same method was used as for 4 with adjusted amounts: 2-di-o-tolyl-phosphanyl-benzaldehyde (0.258 g, 0.810 mmol), 2-di-o-tolyl-phosphinoaniline (0.252 g, 0.825 mmol), p-toluene sulfonic acid (8 mg, 0.042 mmol), and toluene (20 mL). After the procedure 5 was obtained as an off-white/yellow powder with a yield of 82% (0.400 g, 0.66 mmol). 1H NMR (C6D6): δH 9.32 (d, J = 6 Hz, 1H, −N=CH−), 8.38 (m, 1H, Ar–H), 6.79–7.10 (m, 22H, Ar–H), 6.59 (dd, J = 3, 5 Hz, 1H, Ar–H), 2.51 (s, 6H, −CH3), 2.33 (s, 6H, −CH3) ppm. 13C NMR (C6D6): δC 158.12 (dd, JCP = 2, 27 Hz), 155.72 (d, JCP = 19 Hz), 142.92 (d, JCP = 26 Hz), 142.79 (d, JCP = 27 Hz), 140.59 (d, JCP = 18 Hz), 137.35 (d, JCP = 18 Hz), 135.96 (d, JCP = 13 Hz), 134.79 (d, JCP = 10 Hz), 134.15, 133.96, 133.52 (d, JCP = 46 Hz), 131.63 (d, JCP = 12 Hz), 131.28, 130.58 (d, JCP = 4.6 Hz), 130.29 (d, JCP = 4.4 Hz), 130.16, 129.30 (d, JCP = 6 Hz), 129.31, 128.81, 128.35, 126.82, 126.47, 126.31, 125.70, 21.61–21.28 (21.61, 21.49, 21.38, 21.28:4 signals, 2d, assignment unknown) ppm. 31P NMR (C6D6): δP −30.28, −31.56 ppm. IR: 3057, 2967, 2934, 2864, 1625, 1586, 1564, 1464, 1380, 1356, 1264, 1129, 1033, 800, 750, 733, 717, 555, 485, 453 cm–1. ESI-MS (MeCN/formic acid) m/z: [M]+ calcd: 606.2479, found: 606.2273. Elemental analysis: calcd: C 81.30, H 6.16, N 2.31; found: C 80.91, H 6.36, N 2.17.

Ni(PPhCNP)(PPh3) (6)

PPhCNP (4) (252.4 mg, 0.437 mmol), PPh3 (115.8 mg, 0.441 mmol), and Ni(cod)2 (120.5 mg, 0.438 mmol) were combined in a vial, and THF (5 mL) was added, resulting in a red/brown solution. The mixture was stirred overnight, after which the mixture was concentrated to ∼2 mL. Hexanes (6 mL) were added, and the vial was placed at −35 °C for 1 h. The solids were isolated by filtration and redissolved in toluene. After evaporation of all solvents, 6 was obtained with a yield of 90% (352.1 mg, 0.392 mmol). Single crystals for XRD analysis were grown by slow vapor diffusion of hexanes into THF. 1H NMR (C6D6): δH 7.77 (t, J = 9 Hz, 2H, Ar–H), 7.40 (s, broad, Ar–H), 7.30 (t, J = 6 Hz, Ar–H), 7.24 (m, Ar–H), 7.13–6.86 (m, Ar–H), 6.79–6.68 (m, Ar–H), 5.88 (t, J = 7 Hz, 1H, Ar–H), 2.54 (s, 3H, oTol–Me), 1.88 (s, 3H, oTol–Me) ppm. 13C NMR (C6D6): δC 166.83 (dd, JCP = 4, 28 Hz), 153.86 (d, JCP = 33 Hz), 142.06, 141.95, 138.41, 138.15, 137.87, 137.52, 137.28, 136.24, 136.09, 135.50, 133.97 (d, JCP = 13 Hz), 133.50 (d, JCP = 14 Hz), 132.95, 132.95 (d, JCP = 11 Hz), 131.83 (m), 131.49 (d, JCP = 4 Hz), 130.16, 129.34, 126.24, 126.03 (dd, JCP = 4, 11 Hz), 125.74 (m), 125.15 (d, JCP = 4 Hz), 120.27 (d, JCP = 5 Hz), 119.11 (broad), 22.98, 22.91 ppm. 31P NMR (C6D6): δP 36.02, 28.79, 1.09 ppm. IR: 3047, 1585, 1570, 1453, 1433, 1387, 1297, 1089, 1026, 818, 740, 693, 514, 484, 460, 407 cm–1. Crystals for X-ray analysis were grown from THF/hexanes. Elemental analysis: calcd: C 76.19, H 5.38, N 1.56; found: C 75.99, H 5.57, N 1.46.

Ni(PCNP)(PPh3) (7)

PCNP (5) (40.0 mg, 0.066 mmol), PPh3 (17.3 mg, 0.066 mmol), and Ni(cod)2 (18.2 mg, 0.066 mmol) were combined in a vial, and THF (5 mL) was added, resulting in a red/brown solution. The mixture was stirred overnight, after which the mixture was concentrated to ∼1.5 mL. Hexanes (8 mL) was added, and the vial was placed at −35 °C for 2 h. The solids were isolated by filtration, washed with hexanes (3 × 2 mL), and collected by dissolution in toluene. After evaporation of all solvents, 7 was obtained with a yield of 94% (57.5 mg, 0.062 mmol). Single crystals for XRD analysis were grown by slow vapor diffusion of hexanes into THF. 1H NMR (C6D6): δH 7.73 (broad singlet, 2H, Ar–H), 7.41 (broad singlet, 6H, Ar–H), 7.20 (broad singlet, 1H, Ar–H), 7.12–6.72 (m, 30H, Ar–H), 6.45 (t, J = 7 Hz, 1H, imine–CH (or Ar–H)), 6.17 (broad singlet, 1H, Ar–H (or imine–CH)), 5.81 (dd, J = 3, 8 Hz, 1H, Ar–H (or imine–CH)), 2.48 (broad singlet, 6H, oTol–Me) 2.43 (broad singlet, 3H, oTol–Me), 1.76 (broad singlet, 3H, oTol–Me) ppm. 13C NMR (C6D6): δC 165.23 (broad), 149.68 (broad), 143.10, 142,71, 141.82, 135.86, 135.49, 134.19 (d, JCP = 14 Hz), 133.87, 133.40, 132.98, 131.91, 131.28, 130.79, 130.34, 130.05, 129.11, 128.69, 126.68, 126.35, 126.04, 125.93, 123.47, 120.16, 117.71, 23.07, 21.70, 21.48, 21.12 ppm. 31P NMR (C6D6): δP 39.90, 10.80, – 27.72 ppm. IR: 3052, 2969, 2924, 2854, 1952, 1919, 1811, 1669, 1573, 1454, 1435, 1392, 1309, 1260, 1202, 1159, 1118, 1093, 1026, 803, 748, 716, 695, 572, 521, 487, 460 cm–1. Crystals for X-ray analysis were grown from THF/hexanes. The compound is too sensitive for transportation for elemental analysis.

[Ni(PPhCNPPh)]2 (8)

PPhCNPPh (1) (150.2 mg, 0.273 mmol) was suspended in toluene (2 mL). In a second vial, Ni(COD)2 (74.9 mg, 0.272 mmol) was suspended in toluene (6 mL) and transferred to the ligand suspension. The yellow suspension directly changed color to a turbid yellow-brown mixture and was left to stir for 5 h. All solvent was evaporated, and the remaining solids were dissolved in THF (1 mL). The product was precipitated by addition of 8 mL of hexanes, followed by cooling the mixture to −35 °C before the solids were collected by filtration. The solids were collected by dissolution in THF, after which the solvents were evaporated. The mixture was dried under vacuum for 2 nights, after which the product mixture was obtained with a yield of 96% (159.6 mg, 0.131 mol). 1H NMR (C6D6) major species 8a: δH 7.83–7.78 (m, Ar–H), 7.66–7.61 (m, Ar–H), 7.58–7.53 (m, Ar–H), 7.47 (ddd, J = 1, 3, 5 Hz, Ar–H), 7.35–7.30 (m, Ar–H), 7.25 (dt, J = 2, 8 Hz, Ar–H), 7.13–6.44 (m, Ar–H), 6.39–6.35 (m, 1H, Ar–H), 5.49 (d, J = 2 Hz, 1H, CH–CH or imine–CH), 4.58, (d, J = 6 Hz, 1H, CH–CH or imine–CH) ppm. Minor species 8b: δH 7.91 (t, J = 8 Hz, Ar–H), 7.21 (s, Ar–H), 7.19 (s, Ar–H), 6.29 (t, J = 8 Hz, Ar–H), 6.17 (t, J = 7 Hz, Ar–H), 5.31 (m), 3.88 (s) ppm. 13C NMR (C6D6) 8a + 8b: δC 166.28 (d, JCP = 38 Hz), 152.39 (d, JCP = 38 Hz), 151.91 (dd, JCP = 5, 15 Hz), 143.64 (d, JCP = 12 Hz), 143.16 (d, JCP = 11 Hz), 140.95, 140.66 (d, JCP = 10 Hz), 140.43, 140.16, 139.92, 139.12, 139.04, 138.78, 138.601 (d, JCP = 7 Hz), 140.65 (d, JCP = 10 Hz), 139.13, 139.04, 138.78, 137.61 (d, JCP = 7 Hz), 136.78 (d, JCP = 5 Hz), 136.42, 136.16, 135.72, 135.56, 135.08, 134.93, 133.70, 133.61, 133.52, 133.46, 133.39, 133.26, 133.11, 133.01, 132.88, 132.73, 132.63, 132.21, 132.09, 131.56, 130.72, 130.06, 129.39, 129.34, 129.27, 129.13, 128.91, 128.82, 128.69, 128.57, 128.50, 127.53, 127.33, 125.70, 123.54, 121.24, 120.39, 117.32, 87.24 (d, JCP = 26 Hz), 79.13 (d, JCP = 6 Hz). 31P NMR (C6D6) 8a: δP 35.5 (dd, JPP = 2, 70 Hz), 35.4 (d, JPP = 9 Hz), 22.9 (dd, JPP = 2, 9 Hz), – 7.9 (d, JPP = 70 Hz); 8b: 38.32 (d, JPP = 43 Hz), 1.05 (d, JPP = 43 Hz) ppm. IR: 3048, 2856, 1580, 1568, 1477, 1449, 1431, 1325, 1251, 1202, 1090, 1065, 1026, 919, 850, 735, 693, 497 cm–1.

[Ni(PPhCNPPh)]2(CO)2 (9a and 9b)

8 (8.4 mg, 6.9 μmol, the obtained mixture of 8a and 8b was used) was dissolved in C6D6 (0.6 mL) and transferred to a Young-type NMR tube. The mixture was degassed to remove the N2 atmosphere (2 freeze–pump–thaw cycles) and charged with CO (1 atm) upon thawing of the solution. The tube was regularly shaken, and the color of the mixture changed overnight from turbid red to clear orange. The progress of the reaction was monitored by NMR analysis (see Supporting Information, Figure S42). The NMR tube was placed back in the glovebox, after which the mixture was filtered and transferred to a small reaction tube. The reaction tube was placed in a vial, and hexanes (1 mL) was placed in the vial (crystallization setup) for crystallization to take place. After 2 days, crystals were formed, and the liquid phase was removed by decantation. The crystals were washed with hexanes (3 × 1.5 mL) and dried in vacuum for no longer than 5 min, resulting in 9b. The liquid phase was dried in vacuum as well, resulting in the bulk material 9a.

9b: 9b was isolated as red-brown crystals with a yield of 21% (1.8 mg, 1.4 μmol). 1H NMR (C6D6): δH 10.10 (dd, J = 4, 8 Hz, 2H, Ar–H), 7.91 (t, J = 2, 8 Hz, 4H, Ar–H), 7.75 (t, J = 8 Hz, 4H, Ar–H), 7.47 (m, 6H, Ar–H), 7.31 (m, 4H, Ar–H), 7.12–6.86 (m, 20H, Ar–H), 6.79–6.66 (m, 10H, Ar–H), 6.58 (t, J = 8 Hz, 2H, Ar–H), 6.50 (s, broad, 2H, Ar–H), 6.18 (t, J = 7 Hz, 2H, CH–CH), 5.95 (d, broad, J = 9 Hz, 2H, Ar–H) ppm. 13C NMR (C6D6): δC 198.24 (−CO), 150.97, 134.75, 134.69, 134.62, 133.65, 133.59, 133.21, 133.12, 133.07, 132.70, 132.14, 132.07, 131.61, 130.09, 129.96, 129.51, 128.69, 126.53, 113.81, 112.79, 70.35 ppm. 31P NMR (C6D6): δP 39.81, 22.12 ppm. IR: 3051, 2963, 2922, 2853, 1999, 1938, 1680 (broad), 1578, 1479, 1452, 1435, 1328, 1260, 1095, 1026, 799, 740, 692, 526, 511, 499, 456 cm–1.

9a (bulk): 9a was not isolated and only observed as a mixture together with an unknown byproduct. The mixture is a brown solid. Major signals in 1H NMR (C6D6): δH 8.03 (t, J = 3 Hz, Ar–H), 7.84 (d, J = 2 Hz, Ar–H or N=CH), 7.62–7.52 (m, Ar–H), 7.22 (t, J = 8 Hz, Ar–H), 7.12 (dt, J = 1, 7 Hz, Ar–H), 7.01 (d, J = 1 Hz, Ar–H), 7.00–6.98 (m, Ar–H), 6.94–6.89 (m, Ar–H), 6.87 (t, J = 1 Hz, Ar–H), 6.85 (t, J = 1 Hz, Ar–H), 6.76 (t, J = 8 Hz, Ar–H), 6.27 (ddd, J = 1, 3, 5 Hz, Ar–H) ppm. Major signals in 13C NMR (C6D6): δC 200.24 (dd, JCP = 5, 6 Hz, – CO), 156.55, 154.58, 154.43, 139.36 (d, JCP = 4 Hz), 139.04 (d, JCP = 4 Hz), 138.95 (d, JCP = 6 Hz), 138.78 (d, JCP = 1 Hz), 138.66 (d, JCP = 6 Hz), 138.08 (d, JCP = 7 Hz), 134.93 (d, JCP = 3 Hz), 133.96, 133.76, 133.62, 133.46, 133.33, 133.27 (d, JCP = 5 Hz), 130.98, 130.32 (d, JCP = 6 Hz), 129.33, 128.92 (d, JCP = 1 Hz), 128.68, 128.58 (d, JCP = 1 Hz), 128.50, 128.42, 128.36, 127.40, 127.06, 126.80, 15.85 (d, JCP = 4 Hz), 125.70, 117.76 (d, JCP = 5 Hz), 110.42 ppm. 31P NMR (C6D6): δP 33.04 (d, JPP = 11 Hz), 16.23 (d, JPP = 11 Hz) ppm. Unidentified impurity: δP 26.32 (d, J = 25 Hz), 23.08 (d, J = 25 Hz) ppm. Major signals in IR: 3054, 3002, 2957, 2925, 2855, 2068, 1991, 1929, 1629, 1578, 1460, 1434, 1266, 1184, 1090, 1028, 884, 766, 744, 695, 508 cm–1.