Flexible Coordination of N,P-Donor Ligands in Aluminum Dimethyl and Dihydride Complexes.

Aluminum hydrides, once a simple class of stoichiometric reductants, are now emerging as powerful catalysts for organic transformations such as the hydroboration or hydrogenation of unsaturated bonds. The coordination chemistry of aluminum hydrides supported by P donors is relatively underexplored. Here, we report aluminum dihydride and dimethyl complexes supported by amidophosphine ligands and study their coordination behavior in solution and in the solid state. All complexes exist as κ2-N,P complexes in the solid state. However, we find that for amidophosphine ligands bearing bulky aminophosphine donors, aluminum dihydride and dimethyl complexes undergo a "ligand-slip" rearrangement in solution to generate κ2-N,N complexes. Thus, importantly for catalytic activity, we find that the coordination behavior of the P donor can be modulated by controlling its steric bulk. We show that the reported aluminum hydrides catalyze the hydroboration of alkynes by HBPin and that the variable coordination mode exhibited by the amidophosphine ligand modulates the catalytic activity.

Introduction

class="Chemical">Aluminum hydrides such as class="Chemical">pan class="Chemical">LiAlH4, sodium bis(2-methoxyethoxy)aluminum hydride (RedAl), and AlH3 are ubiquitous in synthetic chemistry for their use as reducing agents.[1] Recently, the scope of the reactivity of these simple aluminum hydrides has been expanded into catalytic hydroboration of alkenes and alkynes, a development of significant environmental and economic importance because of the high abundance and relatively low toxicity of aluminum compared to platinum group metals.[2,3] Numerous other uncomplicated aluminum hydride compounds are also capable of hydroboration or even hydrogenation of unsaturated polar bonds such as aldehydes, ketones, or imines.[4,5] Aluminum hydride compounds with more complex ligands have also been investigated. For example, N-heterocyclic imine-coordinated aluminum hydrides catalyze carbonyl hydroboration[6] while the β-diketiminate-stabilized aluminum dihydride I (Figure ) also catalyzes the hydroboration of alkynes.[7] The dihydride I is also a precursor to β-diketiminate-stabilized aluminum(I) species (at least within the coordination sphere of a transition metal).[8]

Figure 1

Literature examples of aluminum dihydride and dimethyl complexes stabilized by N-based ligands (I–III) or mixed donor ligands (IV–VI).[9−11,15,20,21] (I and IV have Ar = 2,6-C6H3iPr2, and VI has R = R′ = Ph, iPr or R = Ph and R′ = iPr).

Literature examples of class="Chemical">aluminum dihydride and class="Chemical">pan class="Chemical">dimethyl complexes stabilized by N-based ligands (I–III) or mixed donor ligands (IV–VI).[9−11,15,20,21] (I and IV have Ar = 2,6-C6H3iPr2, and VI has R = R′ = Ph, iPr or R = Ph and R′ = iPr).

Reported class="Chemical">aluminum dihydride comclass="Chemical">plexes overwhelmingly use N-class="Chemical">pan class="Species">donor ligands (e.g., I–III; Figure ).[9−13] Typically, these ligands are also multidentate (to stabilize the intrinsically electron-poor Al center) and sterically hindered, in order to prevent dimerization or oligimerization by bridging interactions. In coordination chemistry, ligands greatly influence the chemistry at the metal center. Thus, the investigation and development of aluminum hydride chemistry using a diverse array of ligand classes is essential for the expansion of aluminum hydride chemistry and catalysis.

class="Chemical">Aluminum dihydrides or related sclass="Chemical">pecies with class="Chemical">pan class="Chemical">P-based ligands are much rarer. A few examples of dimethylaluminum complexes with mixed-donor ligands are known, in which bidentate ligands having one N donor also contain a “soft” donor, such as S or P (IV and V; Figure ).[14−18] The likely more labile Al–P interaction offers the possibility of hemilability, which can be useful in the stabilization of catalytic transition or resting states.[19] Indeed, Fryzuk et al. used NMR spectroscopy to demonstrate the fluxional coordination of P-donor atoms in V, resulting in an equilibrium between four- and five-coordinate Al centers.[20]

In compclass="Chemical">arison to mixed-class="Chemical">pan class="Species">donor methyl complexes, mixed-donor aluminum dihydride complexes are scarce, with only a single example.[21] Most P-coordinated aluminum hydrides are limited to simple adducts between phosphines and alane, with the exception of VI (Figure ), reported by Liang et al. in 2009, which was synthesized via the reduction of the corresponding aluminum dichloride using LiAlH4.[21,22] Hemilability of the P donors was not found in this example, likely because of the rigidity of the ligand backbone.

Herein, we describe novel class="Chemical">aluminum dimethyl and dihydride sclass="Chemical">pecies stabilized by mixed class="Chemical">pan class="Chemical">N,P-donor ligands that display flexible coordination modes based on a “ligand-slip” phenomenon.

Results and Discussion

The class="Chemical">amidophosphine ligands 1a–1c[23] (Figure ) have class="Chemical">previously been used to class="Chemical">preclass="Chemical">pclass="Chemical">pan class="Chemical">are nickel and palladium complexes, as well as to support reactive silicon(II) compounds.[24−27] The steric bulk around both the N and P centers of 1a–1c has not only enabled the isolation of reactive species such as silicon(II) hydrides but also modulates reversible SiII/SiIV oxidative additions/reductive eliminations. At the P donor in particular, both steric bulk and electron-donating ability are readily tunable. We were interested in whether this class of ligands could be employed to support Al centers and whether they could be used to modulate their structure and reactivity.

Figure 2

Mixed-donor ligands 1a–1c.

Mixed-pan class="Species">donor ligands 1a–1c.

Synthesis and Solid-State Structures of Aluminum Dimethyl Complexes

class="Chemical">Dimethylaluminum comclass="Chemical">plexes class="Chemical">pan class="Chemical">are a broad class of compounds that have been reported as catalysts or cocatalysts in alkene polymerization.[28−30] Complexes of dimethylaluminum stabilized by many N- or mixed-donor ligands have been reported, rendering this class of compounds ideal for benchmarking the coordination abilities of ligands 1a–1c. We decided to first investigate the coordination of ligand 1 to dimethylaluminum moieties.

The coordination of 1b and 1c to SiIV centers has been reported and was achieved by deprotonation before treatment with the appropriate class="Chemical">silicon halide.[27] Accordingly, ligands 1a–1c were declass="Chemical">protonated with nBuLi at −78 °C to afford yellow solutions of 2a–2c (Scheme ). A chclass="Chemical">pan class="Chemical">aracteristic resonance is observed in the 31P{1H} NMR spectra of these solutions in the form of a 1:1:1:1 quartet upfield compared to the free ligand. The 1:1:1:1 multiplicity indicates coordination to Li (e.g., 2a, 31P{1H} NMR δ 10.9, JPLi = 54 Hz). Similarly, in the 7Li NMR spectra, doublets are observed because of coupling with P (e.g., 2a, 7Li NMR δ 1.3, JLiP = 54 Hz).

Scheme 1

Lithiation of Ligand 1 To Form 2 Followed by Reaction with Dimethylaluminum Chloride To Form Dimethylaluminum Complexes 3a–3c

The class="Chemical">dimethylaluminum comclass="Chemical">plexes 3a–3c were obtained by reaction of the in situ generated lithiated ligand 2 with 1 equiv of class="Chemical">pan class="Chemical">dimethylaluminum chloride. Extraction of the products in pentane, followed by filtration and evaporation of the solvent afforded 3a–3c as yellow air-sensitive solids. Complexes 3a and 3c could be isolated as analytically pure solids by crystallization, while 3b was clearly identified but resisted purification attempts. All three complexes 3a–3c were extremely sensitive to air and moisture.

The solid-state structures of 3a and 3c were determined by X-ray crystclass="Chemical">allograclass="Chemical">phy (Figure ). Both comclass="Chemical">pounds have a tetrahedrclass="Chemical">pan class="Chemical">al Al center with coordinated N and P donors, forming a planar ring. The ring is heavily skewed with (as might be expected) a substantially shorter interaction between Al and the N donor than with the phosphine [e.g., 3a, Al1–N1 1.8985(14) Å vs Al1–P1 2.4800(6) Å]. Both the Al–N and Al–P distances are comparable to those previously reported, for example the N,P-coordinated dimethylaluminum complex IV [Al–N 1.894(6) Å; Al–P 2.477(3) Å].[15]

Figure 3

Molecular structures of 3a (left) and 3c (right) with thermal ellipsoids drawn at the 50% probability level. H and disordered ligand atoms are omitted for clarity. Selected bond distances (Å) and angles (deg) for 3a: N1–Al1 1.895(14), P1–Al1 2.4800(6), Al1–C1 1.9652(19), Al1–C2 1.970(2); N1–Al1–P1 86.67(4), N1–Al1–C1 116.12(8), N1–Al1–C2 115.25(8), P1–Al1–C1 114.53(7), P1–Al1–C2 114.00(7), C1–Al1–C2 109.00(9). Selected bond distances (Å) and angles (deg) for 3c: N1–Al1 1.917(3), P1–Al1 2.5304(9), Al1–C1 1.967(4), Al1–C2 1.964(4); N1–Al1–P1 85.59(8), N1–Al1–C1 116.14(19), N1–Al1–C2 116.19(19), P1–Al1–C1 115.98(13), P1–Al1–C2 115.99(14), C1–Al1–C2 106.4(2).

Moleculclass="Chemical">ar structures of 3a (left) and 3c (right) with thermclass="Chemical">pan class="Chemical">al ellipsoids drawn at the 50% probability level. H and disordered ligand atoms are omitted for clarity. Selected bond distances (Å) and angles (deg) for 3a: N1–Al1 1.895(14), P1–Al1 2.4800(6), Al1–C1 1.9652(19), Al1–C2 1.970(2); N1–Al1–P1 86.67(4), N1–Al1–C1 116.12(8), N1–Al1–C2 115.25(8), P1–Al1–C1 114.53(7), P1–Al1–C2 114.00(7), C1–Al1–C2 109.00(9). Selected bond distances (Å) and angles (deg) for 3c: N1–Al1 1.917(3), P1–Al1 2.5304(9), Al1–C1 1.967(4), Al1–C2 1.964(4); N1–Al1–P1 85.59(8), N1–Al1–C1 116.14(19), N1–Al1–C2 116.19(19), P1–Al1–C1 115.98(13), P1–Al1–C2 115.99(14), C1–Al1–C2 106.4(2).

The class="Chemical">Al–N bond distances of 3a and 3c class="Chemical">pan class="Chemical">are indistinguishable, but the Al–P bond length is slightly longer in the latter at 2.5304(8) Å, indicating that P is less strongly bound to the Al center. The aminophosphine donor of 3c is more electron-donating than the dialkylphosphine donor of 3a, which would be expected to give rise to the opposite trend.[31] The origin of the difference is likely due to steric effects: the greater steric bulk in 3c prevents the close approach of the phosphine to the Al center. Indeed, this can be observed in the C1–Al1–C2 angle, which is smaller in the case of 3c [106.4(2)°] than 3a [109.00(9)°] despite the similar bite angles of the two [3a, 86.67(4)°; 3c, 85.59(8)°].

Solution Behavior of 3a–3c

Despite their similclass="Chemical">ar solid-state structures, solution-class="Chemical">phase NMR sclass="Chemical">pectroscoclass="Chemical">py reveclass="Chemical">pan class="Chemical">aled differences in the coordination behavior among the dimethylaluminum complexes 3a–3c. No signals were observed for any of the compounds by 27Al NMR spectroscopy.

NMR spectroscopy of class="Chemical">dimethylaluminum comclass="Chemical">plexes 3a and 3b was consistent with the solid-state structure determined for 3a. class="Chemical">pan class="Chemical">31P{1H} NMR spectroscopy revealed a single resonance for each (3a, 1.6 ppm; 3b, 64.0 ppm) shifted upfield compared to the respective free ligand resonances [Δ∂(3a) = −54.4 ppm; Δ∂(3b) = −83.3 ppm]. The 31P{1H} NMR resonances for 3a and 3b were also significantly broadened in comparison to the free ligands 1a and 1b, presumably as a result of coordination of the P to the quadrupolar (I = 5/2) Al nucleus [3a, full width at half-maximum (Δν1/2) = 21.1 Hz; 1a, Δν1/2 = 2.7 Hz].

In the class="Chemical">1H NMR sclass="Chemical">pectra of 3a and 3b, resonances corresclass="Chemical">ponding to the class="Chemical">pan class="Chemical">aluminum methyl groups appear as doublets arising from coupling to P (3a, δ −0.33 and −0.19, 2JHP = 2.5 Hz). The 1H NMR spectrum also shows that each CH3 group in the 2,6-diisopropylphenyl (Dipp) substituent is inequivalent, indicating restricted rotation likely because of steric constraints.

Crystclass="Chemical">alline 3c was class="Chemical">pan class="Chemical">also characterized by solution-phase NMR spectroscopy. Surprisingly, the 31P{1H} NMR spectrum contained two resonances, at 99.9 and 49.7 ppm, in a ratio of 3:2 (the same ratio was observed by 1H NMR spectroscopy). The resonance at 49.7 ppm is broadened (Δν1/2 = 47.5 Hz) and downfield (Δ∂ = −40.9 ppm) from that of 1c and so is consistent with coordination of P to the Al center as in 3a and 3b. Conversely, the resonance at 99.9 ppm is sharp (Δν1/2 = 5.3 Hz) and close in chemical shift to that of the free ligand 1c (Δ∂ = +9.3 ppm), which indicates that P in this environment is not coordinated to the Al center.

On the basis of the class="Chemical">31P NMR sclass="Chemical">pectroscoclass="Chemical">pic data and by anclass="Chemical">pan class="Chemical">alogy with the behavior more fully studied in the hydride analogue 5c (see below), we propose that 3c exists in two forms in solution, in which the ligand exhibits a variable coordination mode, having either κ2-N,P or κ2-N,N coordination (Scheme ). In the solid state, κ2-coordination is exclusively observed. In solution, however, the two isomers are present as a result of the flexible coordination mode of the ligand.

Scheme 2

Proposed Structures of κ2-N,P- and κ2-N,N-3c

In the solid state, only κ2-N,P-3c is observed, while in solution, both the κ2-N,P- and κ2-N,N isomers are observed.

In the solid state, only κ2-class="Chemical">N,P-3c is observed, while in solution, both the κ2-class="Chemical">pan class="Chemical">N,P- and κ2-N,N isomers are observed.

The class="Chemical">1H NMR sclass="Chemical">pectrum of 3c is consistent with both the κ2-class="Chemical">pan class="Chemical">N,P and κ2-N,N isomers existing in solution, with two sets of resonances present in a ratio of 57:43 (consistent with the 3:2 ratio observed by 31P NMR). Multinuclear 2D NMR spectroscopic experiments verified that in both isomers the ligand backbone was intact and undisturbed. The possibility of a dimeric κ1-N isomer of 3c (with, e.g., bridging methyl ligands) was excluded based on analysis of the 1H DOSY NMR spectrum, which indicated that both of the observed isomers diffused at the same rate in solution. Similarly, high-resolution mass spectrometry (HRMS) also identified the product as 3c, with no evidence of a dimeric species observed.

Synthesis of Aluminum Dihydride Complexes

Following the prepclass="Chemical">aration of the class="Chemical">pan class="Chemical">dimethylaluminum complexes 3a–3c, we turned our attention to the preparation of aluminum dihydride complexes. Ligands 1a–1c do not react with Me2EtN·AlH3, in contrast to the observed reactivity of amidine ligands, which evolve H2 and form aminidinatoaluminum dihydrides.[32] Treatment with LiAlH4 also had no effect. Thus, we used the lithiated ligands 2a–2c as precursors instead.

Treatment of 2b with a single equivclass="Chemical">alent of class="Chemical">pan class="Gene">Me2EtN·AlH3 resulted in a yellow solution, the 31P NMR spectrum of which revealed a quartet (δ 110.8, 2JPH = 34 Hz), which collapsed to a singlet in the 31P{1H} NMR spectrum. This evidence, as well as further characterization by multinuclear NMR spectroscopy and mass spectrometry, confirmed formation of the aluminate complex 4b (Scheme ).

Scheme 3

Proposed Mechanism for the Reaction of 2 with Me2EtN·AlH3 (NR3 = NMe3 or NMe2Et) To Form the Aluminum Dihydride 5 via the Charged Intermediate 4

The addition of a second equivclass="Chemical">alent of class="Chemical">pan class="Gene">Me2EtN·AlH3 to solutions of 4b was monitored by 31P{1H} NMR spectroscopy, which revealed complete consumption of 4b and the formation of a new species represented by a broad singlet (61.3 ppm, Δν1/2 = 55.7 Hz), indicating P coordination to Al. Analysis of the 27Al NMR spectrum revealed the formation of LiAlH4. On the basis of this evidence, the reaction pathway shown in Scheme is proposed: the reaction of 2b with Me2EtN·AlH3 proceeds by forming 4b by displacement of the amine from Me2EtN·AlH3. The second 1 equivalent of Me2EtN·AlH3 abstracts a hydride from 4b, generating 5b and LiAlH4 and eliminating the amine.

When 2a was treated with 1 equiv of class="Gene">Me2EtN·class="Chemical">pan class="Chemical">AlH3, the resulting pale-yellow solution was revealed to contain a mixture of compounds by 31P{1H} NMR spectroscopy. In addition to residual lithiated ligand 2a, equal quantities of the aluminate intermediate 4a (8.0 ppm) and the neutral aluminum dihydride 5a (−10.1 ppm) were observed. LiAlH4 was also observed by 27Al NMR spectroscopy. The 2:1:1 ratio of the three species reveals that the lithiated ligand 2a and the intermediate aluminate 4a react at comparable rates with Me2EtN·AlH3 to generate a statistical mixture. This contrasts to the situation for 4b, where hydride abstraction by Me2EtN·AlH3 is much slower than its coordination to the lithiated ligand 2b. Upon the addition of a second equivalent of Me2EtN·AlH3 to 4a, the reaction mixture turned colorless and the 31P{1H} NMR spectrum showed complete conversion to 5a (7.5 ppm).

class="Chemical">Preclass="Chemical">pclass="Chemical">pan class="Chemical">aratively, the dihydride complexes 5a–5c were obtained in multigram quantities from treatment of the lithiated ligands 2a–2c with 2 equiv of Me3N·AlH3 or Me2EtN·AlH3. All three compounds could be isolated as colorless solids in excellent yields of 80–90%. Dihydrides 5b and 5c could be further purified by crystallization from hexane.

In the class="Chemical">1H NMR sclass="Chemical">pectra of 5a and 5b, class="Chemical">pan class="Chemical">Al–H resonances are visible as very broad singlets at 4.6 ppm (5a, Δν1/2 = 71.6 Hz; 5b, Δν1/2 = 125.3 Hz) because of the influence of the quadrupolar Al atom. Despite the lower steric influence of the hydride ligands compared to the methyl ligands of 3a and 3b, the methyl groups of the Dipp substituent remain inequivalent, indicating continued restricted rotation. Compound 5c has more complex solution behavior that will be discussed below.

IR spectroscopy of the solid-state samples of 5a–5c reveclass="Chemical">aled the exclass="Chemical">pected symmetric and antisymmetric class="Chemical">pan class="Chemical">Al–H stretches (5a, 1810 and 1786 cm–1; 5b, 1831 and 1816 cm–1; 5c, 1825 and 1801 cm–1) for a four-coordinate aluminum dihydride center.[33,34]

Solid-State Structures of 5b and 5c

The structures of 5b and 5c were verified by X-ray diffraction (Figure ). Broadly, the structures class="Chemical">are anclass="Chemical">pan class="Chemical">alogous to those of 3a and 3c. The amidophosphine ligand in each compound is κ2-N,P-coordinated, which together with the hydride ligands (located using a difference map and allowed to refine freely) results in a tetrahedral environment at the Al center. The two structures have statistically identical N–Al bond distances [5b, 1.8972(15) Å; 5c, 1.892(2) Å], which are essentially identical with those observed for the dimethyl analogues 3a and 3c. A more substantial difference is observed in the P–Al bond distances, which for the dihydride 5c is shorter than that in the corresponding dimethyl complex 3c [Al1–P1: 5c, 2.4791(10) Å; 3c, 2.5304(8) Å]. Contraction of this bond can be explained by the smaller size of the hydride substituents. Similarly, a comparison between the two dihydrides 5b and 5c reveals a shorter Al1–P1 distance for 5b as a result of reduced bulk at the P center in comparison to 5c [5b, 2.4442(7) Å; 5c, 2.4791(10) Å]. The larger bite angles for the dihydrides 5b and 5c [5b, 87.47(5)°; 5c, 86.60(6)°] compared to those of the dimethyl compounds are also due to the smaller hydride substituents compared to the methyl groups.

Figure 4

Molecular structures of 5b (left) and 5c (right). The aluminum hydride atoms were located using a difference map and allowed to refine freely. H and disordered ligand atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for 5b: N1–Al1 1.8972(15), P1–Al1 2.4442(7); N1–Al1–P1 87.47(5). Selected bond lengths (Å) and angles (deg) for 5c: N1–Al1 1.892(2), P1–Al1 2.4790(10); N1–Al1–P1 86.60(6).

Moleculclass="Chemical">ar structures of 5b (left) and 5c (right). The class="Chemical">pan class="Chemical">aluminum hydride atoms were located using a difference map and allowed to refine freely. H and disordered ligand atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for 5b: N1–Al1 1.8972(15), P1–Al1 2.4442(7); N1–Al1–P1 87.47(5). Selected bond lengths (Å) and angles (deg) for 5c: N1–Al1 1.892(2), P1–Al1 2.4790(10); N1–Al1–P1 86.60(6).

Solution-Phase NMR Characterization of 5c

Like its class="Chemical">dimethyl anclass="Chemical">pan class="Chemical">alogue 3c, the dihydride 5c exhibits variable coordination modes in solution. Upon dissolution of crystalline 5c, the 31P{1H} NMR spectrum revealed the presence of two broad singlets at 96.9 ppm (Δν1/2 = 137.9 Hz) and 47.8 ppm (Δν1/2 = 96.6 Hz) in a ratio of 1:2. By 1H NMR, two sets of resonances were also observed for all proton environments, including the dihydride ligands (signals at κ2-N,N-5c, 4.3 ppm, κ2-N,P-5c, 4.6 ppm; the ratio of the two species as measured by 1H NMR in a ratio of 35:65, consistent with that observed in the 31P NMR spectrum).

The two solution-phase isomers of 5c were determined to be κ2-N,class="Gene">P-5c, as observed in the solid state, and a κ2-N,N isomer in which the class="Chemical">pan class="Chemical">phosphine ligand has “slipped” and coordinates through one of the P-bound N atoms (Scheme ). Evidence for the κ2-N,N coordination mode is as follows:

Scheme 4

Proposed Structures of κ2-N,P- and κ2-N,N-5c

In the solid state, only κ2-N,P-5c is observed, while in solution, both the κ2-N,P and κ2-N,N isomers are observed.

In the solid state, only κ2-N,class="Gene">P-5c is observed, while in solution, both the κ2-class="Chemical">pan class="Chemical">N,P and κ2-N,N isomers are observed.

(1) The two isomers class="Chemical">are both monomeric sclass="Chemical">pecies, as reveclass="Chemical">pan class="Chemical">aled by 1H DOSY NMR measurements, which indicate similar diffusion coefficients. Thus, we were able to rule out the presence of a dimeric species with bridging hydrides (consistent with solution- and solid-phase IR spectroscopy, which did not reveal evidence of bridging hydride ligands).

(2) In the class="Chemical">31P{class="Chemical">pan class="Chemical">1H} NMR spectrum, the resonance at 96.9 ppm is assigned to the κ2-N,N isomer because of its similarity to that observed for the free ligand 1c (90.6 ppm), which indicates that the P center is not coordinated to Al. The resonance at 47.8 ppm is assigned to the κ2-N,P isomer observed in the solid state (confirmed by solid-state NMR measurements; see below).

(3) The class="Chemical">aluminum hydride stretching frequencies recorded for 5c in solution (1823 cm–1) and in the solid state (1825 and 1801 cm–1) class="Chemical">pan class="Chemical">are consistent with a four-coordinate aluminum dihydride species in both phases, ruling out a κ1-N isomer in which the phosphine is uncoordinated.

(4) Using density functionclass="Chemical">al theory (DFT), we class="Chemical">performed geometry oclass="Chemical">ptimization and frequency cclass="Chemical">pan class="Chemical">alculations on κ2-N,P isomers of 5a–5c at the M062X/Def2SVPP and M062X/6,31G+(d,p)/Lanl2DZ levels (Table S1). Following the lead of Crimmin et al., we found that calculations using the split basis set were essential to replicating experimentally observed Al–H stretching frequencies.[33] The calculations accurately reproduced the experimentally observed geometries and IR stretching frequencies for 5a–5c, enabling us to use this computational methodology to assign the identity of the solution-phase isomer of 5c.

(5) A relaxed potenticlass="Chemical">al energy surface (class="Chemical">pan class="Chemical">PES) scan of 5c in which the Al–P distance was increased systematically starting from the κ2-N,P geometry revealed two potential minima (Figure S1), which were reoptimized at the M062X/6,31G+(d,p)/Lanl2DZ level (Figure and Table S2). A κ1-N isomer was found to be 22.6 kcal mol–1 higher in energy than the κ2-N,P isomer (the calculated Al–H stretching frequencies for this three-coordinate aluminum dihydride of 1934 and 1922 cm–1 were also inconsistent with the experimental values). However, the κ2-N,N isomer located in the PES scan was found to be very close in energy to κ2-N,P-5c (−0.8 kcal mol–1 more stable; DFT does not replicate the experimentally observed order of stability, although it does correctly place the two species very close in energy). Calculated Al–H stretching frequencies for κ2-N,P- and κ2-N,N-5c (1863, 1845, and 1860, 1813 cm–1, respectively) are sufficiently close in order to explain the single peak observed in the experimental solution-phase spectrum (1829 cm–1).

Figure 5

Computed energies of κ2-N,P-, κ2-N,N-, and κ1-N-5c [M062X/6,31G+(d,p)/Lanl2DZ].

Computed energies of κ2-pan class="Chemical">N,P-, κ2-N,N-, and κ1-N-5c [class="Chemical">pan class="Mutation">M062X/6,31G+(d,p)/Lanl2DZ].

The ligand-slip reclass="Chemical">arrangement of 5c from κ2-class="Chemical">pan class="Chemical">N,P to κ2-N,N is likely driven by a preference for the “hard” N-donor functionality of the diaminophosphine donor over the “softer” P center. The increased proportion of the κ2-N,N isomer for the dimethyl complex 3c compared to the dihydride 5c suggests that the ring expansion that occurs as a consequence of isomerization from κ2-N,P to κ2-N,N may also be favorable as a route to relieve steric strain. The more restrained, sterically crowded, and less basic (due to the silyl substituent) tert-butylamino groups of 3b and 5b cannot favorably participate in the same isomerization as 3c and 5c.

Interconversion between κ2-class="Chemical">N,P- and κ2-N,N-3c or -5c in solution was not observable, and we were thus unable to determine the activation bclass="Chemical">pan class="Chemical">arriers for this process. Although resonances for the coordinated and free phosphine centers in both isomers of 5c are broad, using NMR spectroscopy, we could find no evidence for exchange between the two sites, even at elevated temperatures. The variable coordination mode of the ligand in both 3c and 5c appears to provide them with higher reactivity and renders them the most sensitive derivatives in these series. Indeed, 3c was found to be extremely challenging to handle because of its high sensitivity to air and moisture.

Solid-State NMR Spectroscopy

To further confirm our assignment of class="Chemical">31P resonances for the κ2-class="Chemical">pan class="Chemical">N,P and κ2-N,N isomers of 3c and 5c, we undertook solid-state NMR spectroscopy because from crystallographic studies κ2-N,P-coordination is exclusively observed. The 31P{1H} MAS NMR spectra of 3c and 5c are consistent with X-ray crystallography, revealing only a single-P environment for each compound (Figure ). In both cases, the solid-state chemical shift is almost identical with the solution-phase signal assigned to the κ2-N,P isomers (e.g., 3c, solid phase, 47.8 ppm, solution, 49.7 ppm; 5c, solid phase, 47.5 ppm, solution, 47.8 ppm). Furthermore, the line shapes observed in the 31P{1H} NMR spectra indicate quadrupolar coupling between Al and P, explaining the observed variation from the expected 1:1:1:1:1:1 sextet. No other resonances were observed in the 31P{1H} MAS NMR spectra, ruling out the presence of the κ2-N,N isomer in the solid state.

Figure 6

31P{1H} (9.4 T, 14 kHz, MAS) NMR spectra for 3c (top) and 5c (bottom).

class="Chemical">31P{class="Chemical">pan class="Chemical">1H} (9.4 T, 14 kHz, MAS) NMR spectra for 3c (top) and 5c (bottom).

For 3a, 3b, 5a, and 5b, which class="Chemical">all disclass="Chemical">play exclusive κ2-class="Chemical">pan class="Chemical">N,P coordination in solution, the observed 31P{1H} MAS NMR spectra each contain a single resonance extremely close in chemical shift to that observed in solution (e.g., 5a, solution phase, 8.0 ppm, solid phase, 8.9 ppm). Although we were unable to observe any resonances for any of the compounds reported here by solution-phase 27Al NMR spectroscopy, solid-state experiments were more successful. Details of the 27Al{1H} CPMG NMR spectra for 3a–3c and 5a–5c are provided in the Supporting Information.

Conclusions

In summclass="Chemical">ary, we have synthesized class="Chemical">pan class="Chemical">aluminum dimethyl and dihydride complexes with a series of amidophosphine ligands of varying steric bulk. The bulky bidentate ligands 1a–1c enable the isolation of reactive aluminum dihydrides, the synthesis of which was observed to proceed through five-coordinate aluminate intermediates (4a–4c). Evidence from X-ray crystallography and solid-state NMR spectroscopy indicates that, for all dimethyl and dihydride complexes, both N- and P-donor atoms are bound to the Al centers in the solid state. In solution, however, altering the steric bulk of the ligand enables control over the coordination mode at the Al center: for the bulkiest ligand employed, 1c, both the dimethyl and dihydride complexes 3c and 5c exist as a mixture of κ2-N,P and κ2-N,N isomers.

The vclass="Chemical">ariable coordination mode of the ligand is encouraging as a class="Chemical">potenticlass="Chemical">pan class="Chemical">al route to controlling the stoichiometric or catalytic reactivity of the aluminum dihydride centers. For example, preliminary results indicate that 5a–5c are active catalysts for the hydroboration of alkyl- and arylalkynes with HBPin (see the SI). The accessibility of the κ2-N,N coordination mode for 5c has a clear effect on the reactivity. While all three dihydrides catalyze the hydroboration of phenylacetylene with HBPin, 5a and 5b are significantly more efficient, with conversions of 79 and 83% after 2 h at 110 °C compared to 53% for 5c. We are now further exploring the coordination chemistry, reactivity, and catalytic applications of the dihydrides 5a–5c (Scheme ).

Scheme 5

Catalytic Hydroboration of Phenylacetylene and 2-Cyclooctyne Using 5a–5c

Experimental Section

General Procedures

class="Chemical">All maniclass="Chemical">pulations were cclass="Chemical">pan class="Chemical">arried out under an argon atmosphere using standard Schlenk or glovebox techniques. Reactions were carried out in glass Schlenk tubes, which were dried for 16 h at 110 °C before use. Solvents were obtained from an inert solvent purification system and stored over 4 Å molecular sieves. C6D6 and tetrahydrofuran (THF)-d8 were dried over potassium, then vacuum-distilled, and stored over 4 Å molecular sieves.

Ligands 1b and 1c,[23] their precursors [class="Chemical">imine[24] and class="Chemical">pan class="Chemical">chlorophosphines PCl(NtBu)2SiMe2[23] and PCl(NtBuCH2)2[35]], and [H3Al·NMe3][36] were synthesized according to literature procedures. SiMe2(NHtBu)2 was synthesized according to a modified literature procedure (see the SI). tert-Butylamine was dried over calcium hydride and vacuum-distilled prior to use. LiAlH4 was purified by extraction with diethyl ether and filtration to afford a white solid, which was stored under an inert atmosphere. Trimethylammonium chloride was dried under vacuum at 50 °C for 3 h prior to use. All other reagents were purchased from commercial suppliers and used without further purification.

General Synthesis of 2

To a solution of 1 in class="Chemical">THF cooled to −78 °C was added droclass="Chemical">pwise nBuLi (2.5 M in class="Chemical">pan class="Chemical">hexanes, 1 equiv). The cold bath was removed, and the resultant yellow solution was stirred at room temperature for 1 h. Monitoring by 31P{1H} NMR spectroscopy revealed the presence of the lithiated ligand 2, which was characterized in situ.

2a. class="Chemical">31P{class="Chemical">pan class="Chemical">1H} NMR (C4H8O, 202.5 MHz, 300 K): δ 10.9 (1:1:1:1 quartet, JP–Li = 54 Hz). 7Li NMR (C4H8O, 194.4 MHz, 300 K): δ 1.3 (d, JLi–P = 54 Hz).

2b. class="Chemical">31P{class="Chemical">pan class="Chemical">1H} NMR (C4H8O, 202.5 MHz, 300 K): δ 96.4 (1:1:1:1 quartet, JP–Li = 63 Hz). 7Li NMR (C4H8O, 194.4 MHz, 300 K): δ 1.1 (d, JLi–P = 63 Hz).

2c. class="Chemical">31P{class="Chemical">pan class="Chemical">1H} NMR (C4H8O, 202.5 MHz, 300 K): δ 68.6 (1:1:1:1 quartet, JP–Li = 54 Hz). 7Li NMR (C4H8O, 194.4 MHz, 300 K): δ 1.5 (d, JLi–P = 54 Hz).

General Synthesis of 3

To a solution of 1 in class="Chemical">THF cooled to −78 °C was added droclass="Chemical">pwise nBuLi (2.5 M in class="Chemical">pan class="Chemical">hexanes, 1 equiv). The cold bath was removed, and the resultant yellow solution was stirred at room temperature for 1 h. The reaction mixture was cooled to −78 °C, and Me2AlCl (1.0 M in hexanes) was added dropwise. The cold bath was removed, and the resultant solution was stirred at room temperature for 1 h. The solvent was removed in vacuo, and the product was extracted in hexane and dried to afford 3a–3c.

3a. 1a (0.40 g, 0.97 mmol), class="Chemical">THF (20 mL), nBuLi (0.39 mL, 0.97 mmol, 1.0 equiv), and class="Chemical">pan class="Chemical">Me2AlCl (0.97 mL, 0.97 mmol, 1.0 equiv) yielded 3a (0.34 g, 75%) as a pale-yellow solid. Colorless crystals suitable for X-ray crystallography were grown from a saturated diethyl ether solution at 4 °C. 1H (C6D6, 500 MHz, 300 K): δ −0.33 (d, 3JHP = 2.5 Hz, 3H, AlCH3), −0.19 (d, 3JHP = 2.5 Hz, 3H, AlCH3), 1.10 (m, 1H, 1/2CH2Norb), 1.19 (d, 3JHP = 8.6 Hz, 9H, CH3tBu), 1.22 (d, 3JHP = 8.6 Hz, 9H, CH3tBu), 1.24 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.26 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.37 (d, 3JHH = 6.8 Hz, 6H, CH3iPr), 1.43 (m, 2H, CH2CbridgeheadCP), 1.55 (m, 1H, 1/2CH2CbridgeheadCN), 1.61 (m, 1H, 1/2CH2Norb), 1.66 (m, 1H, 1/2CH2CbridgeheadCN), 2.50 (br s, 1H, PCCHbridgehead), 2.95 (br s, 1H, NCCHbridgehead), 3.44 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.61 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 7.17–7.19 (m, 3H, Haromatic). 13C NMR (C6D6, 126 MHz, 300 K): δ −5.7 (br s, AlCH3), −4.3 (br s, AlCH3), 25.2 (s, CH3iPr), 25.4 (s, CH3iPr), 25.7 (s, CH3iPr), 25.8 (s, CH3iPr), 25.9 (d, JCP = 2 Hz, CH2CbridgeheadCP), 27.6 (s, CHiPr), 27.7 (s, CHiPr), 30.0 (s, CH2CbridgeheadCN), 30.1 (d, JCP = 5 Hz, CH3tBu), 30.5 (d, JCP = 5 Hz, CH3tBu), 34.4 (d, JCP = 30 Hz, CtBu), 34.8 (d, JCP = 31 Hz, CtBu), 43.8 (d, JCP = 9 Hz, PCCH), 44.1 (d, JCP = 2 Hz, NCCH), 48.3 (d, JCP = 3 Hz, CH2Norb), 80.2 (d, JCP = 42 Hz, PCCH), 124.2 (s, Cmeta), 124.3 (s, Cmeta), 126.1 (s, Cpara), 141.7 (d, JCP = 3 Hz, NCAr), 147.1 (s, CCHiPr), 147.4 (s, CCHiPr), 185.1 (d, JCP = 21 Hz, NCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): δ 1.6 (s, Δν1/2 = 21.1 Hz). HRMS (APPI): m/z 469.341919 ([C29H49AlNP]+; theoretical m/z 469.341252). Elem anal. Found: C, 74.13; H, 10.38; N, 2.85. Calcd for C29H49AlNP: C, 74.16; H, 10.52; N, 2.98.

3b. 1b (0.3 g, 0.60 mmol), class="Chemical">THF (20 mL), nBuLi (0.24 mL, 0.60 mmol, 1.0 equiv), and class="Chemical">pan class="Chemical">Me2AlCl (0.60 mL, 0.60 mmol, 1.0 equiv) yielded 3b (0.17 g, 56%) as a yellow solid. Some impurities (less than 10%) were observed by NMR spectroscopy because of reaction with water but could not be separated because crystallization of 3b was not possible.

class="Chemical">1H NMR (C6D6, 500 MHz, 300 K): δ −0.26 (d, 3JHclass="Chemical">pan class="Chemical">P = 3.9 Hz, 3H, AlCH3), −0.12 (d, 3JHP = 3.9 Hz, 3H, AlCH3), 0.27 (s, 3H, SiCH3), 0.32 (s, 3H, SiCH3), 1.12 (d, 2JHH = 8.1 Hz, 1H, 1/2CH2Norb), 1.25 (s, 9H, CH3tBu), 1.26 (s, 9H, CH3tBu), 1.25 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.26 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.32 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.41 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.42 (m, 2H, CH2CbridgebeadCP), 1.60 (d, 2JHH = 8.1 Hz, 1H, 1/2CH2Norb), 1.69 (m, 2H, CH2CbridgeheadCN), 2.53 (br s, 1H, CHbridgeheadCP), 3.13 (br s, 1H, CHbridgeheadCN), 3.51 (sept, 3JHH = 6.8 Hz, CHiPr), 3.70 (sept, 3JHH = 6.8 Hz, CHiPr), 7.11–7.21 (m, 3H, Haromatic). 13C NMR (C6D6, 126 MHz, 300 K): δ −6.3 (br d, JCP = 24.8 Hz, AlCH3), −5.4 (br d, JCP = 19.9 Hz, AlCH3), 4.6 (d, JCP = 1.4 Hz, SiCH3), 6.7 (d, JCP = 3.7 Hz, SiCH3), 25.3 (s, CH3iPr), 25.4 (s, CH3iPr), 25.6 (s, CH3iPr), 26.0 (s, CH3iPr), 26.3 (s, CH2CbridgeheadCP), 27.6 (s, CHiPr), 27.8 (s, CHiPr), 29.4 (s, CH2CbridgeheadCN), 32.3 (d, JCP = 5.4 Hz, CH3tBu), 32.7 (d, JCP = 4.9 Hz, CH3tBu), 40.6 (d, JCP = 3.5, CHCN), 44.3 (d, JCP = 43.9, CHCP), 46.6 (d, JCP = 4.7 Hz, CH2Norb), 50.8 (d, JCP = 3.9 Hz, CtBu), 50.9 (d, JCP = 2.8 Hz, CtBu), 99.1 (d, JCP = 29.2 Hz, PCCH), 124.3 (s, Cmeta), 124.3 (s, Cmeta), 126.3 (s, Cpara), 141.2 (d, JCP = 3.1 Hz, NCAr), 146.6 (s, CCHiPr), 147.0 (s, CCHiPr), 186.8 (d, JCP = 33.9 Hz, NCCH). 31P{1H} (C6D6, 162 MHz, 300 K): δ 64.0 (s, Δν1/2 = 35.8 Hz). HRMS (APPI): m/z 555.371654 ([C31H55AlN3PSi]+; theoretical m/z 555.371277).

3c. 1c (0.43 g, 0.92 mmol), class="Chemical">THF (20 mL), nBuLi (0.37 mL, 0.92 mmol, 1.0 equiv), and class="Chemical">pan class="Chemical">Me2AlCl (0.92 mL, 0.60 mmol, 1.0 equiv) were mixed. To gain analytically pure material, 3c was further purified by recrystallization from hexanes at −20 °C to yield colorless crystals (0.23 g, 48%). Two isomers were identified in the NMR spectra with an approximate ratio of 4:3 of κ 2-N,N-3c to κ2-N,P-3c at 300 K (determined from the 1H NMR spectrum). Because of the high air sensitivity of this species, some impurities were observed in solution NMR spectra (31P{1H} NMR spectrum, 10% unidentified impurity at 75.1 ppm).

κ2-N,N-3c. class="Chemical">1H NMR (C6D6, 500 MHz, 300 K): δ −0.67 (s, 3H, class="Chemical">pan class="Chemical">AlMe), −0.13 (s, 3H, AlMe), 0.89 (m, 1H, 1/2CH2Norb), 1.18 (m, 1H, 1/2CH2Norb), 1.21 (s, 18H, CH3tBu), 1.24 (d, 3JHH = 6.8 Hz, 6H, CH3iPr), 1.27 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.45 (m, 2H, CH2CbridgeheadCP), 1.47 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.63 (m, 1H, 1/2CH2CbridgeheadCN), 1.72 (m, 1H, 1/2CH2CbridgeheadCN), 2.49 (br s, 1H, CHbridgeheadCP), 2.82 (m, 1H, 1/2NCH2), 2.83 (m, 1H, 1/2NCH2), 2.97 (m, CHbridgeheadCN), 3.15 (m, 1H, 1/2NCH2), 3.73 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.85 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.88 (m, 1H, 1/2NCH2), 7.15–7.20 (m, 3H, Haromatic). 13C NMR (C6D6, 126 MHz, 300 K): δ −5.5 (s, AlMe), −4.7 (s, AlMe), 24.3 (s, CH3iPr), 24.8 (s, CH3iPr), 25.75 (s, CH3iPr), 25.97 (s, CH2CHbridgeheadCP), 26.1 (s, CH3iPr), 27.0 (s, CHiPr), 28.2 (s, CHiPr), 29.3 (s, CH2CbridgeheadCN), 29.7 (d, JCP = 9.5 Hz, CH3tBu), 29.73 (d, JCP = 7.7 Hz, CH3tBu), 29.8 (d, JCP = 5.6 Hz, CH3tBu), 43.84 (s, CH2Norb), 43.88 (d, JCP = 12.6 Hz, NCH2), 45.3 (s, CHbridgeheadCP), 45.7 (d, JCP = 42.0 Hz, CHbridgeheadCN), 48.8 (d, JCP = 2.8 Hz, NCH2), 53.2 (d, JCP = 6.0 Hz, CtBu), 63.7 (d, JCP = 11.7 Hz, CtBu), 103.5 (d, JCP = 37.7 Hz, PCCH), 123.9 (s, Cmeta), 124.5 (s, Cmeta), 125.2 (s, Cpara), 144.1 (s, NCAr), 146.5 (s, CCHiPr), 147.0 (s, CCHiPr), 165.0 (d, JCP = 5.0 Hz, NCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): δ 99.9 (s, Δν1/2 = 5.3 Hz).

κ2-class="Chemical">N,P-3c. class="Chemical">pan class="Chemical">1H NMR (C6D6, 500 MHz, 300 K): δ −0.27 (d, 3JHP = 3.1 Hz, 3H, AlMe), −0.14 (d, 3JHP = 3.1 Hz, 3H, AlMe), 1.06 (dm, 2JHH = 8.1 Hz, 1H, 1/2CH2Norb), 1.25 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.26 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.30 (s, 9H, CH3tBu), 1.32 (s, 9H, CH3tBu), 1.33 (m, 1H, 1/2CH2CbridgeheadCP), 1.38 (m, 1H, 1/2CH2CbridgeheadCP), 1.40 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.42 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.56 (m, 2JHH = 8.1 Hz, 1H, 1/2CH2Norb), 1.63 (m, 2H, CH2CbridgeheadCN), 2.48 (br s, 1H, CHbridgeheadCP), 2.59 (m, 1H, 1/2NCH2), 2.66 (m, 1H, 1/2NCH2), 2.82 (m, 1H, 1/2NCH2), 2.89 (m, 1H, NCH2), 3.05 (br s, 1H, CHbridgeheadCN), 3.52 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.74 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 7.15–7.20 (m, 3H, Haromatic). 13C NMR (C6D6, 126 MHz, 300 K): δ −4.2 (s, AlMe), −3.5 (s, AlMe), 25.2 (s, CH3iPr), 25.4 (s, CH3iPr), 25.68 (s, CH3iPr), 25.84 (s, CH2CbridgeheadCP), 27.4 (s, CHiPr), 27.6 (s, CHiPr), 29.2 (s, CH3tBu), 29.4 (d, JCP = 2.2 Hz, CH2CbridgeheadCN), 29.7 (d, JCP = 7.6 Hz, CH3tBu), 42.0 (d, JCP = 4.2 Hz, CHbridgeheadCN), 43.6 (s, NCH2), 44.2 (d, JCP = 9.9 Hz, CHbridgeheadCP), 44.3 (s, NCH2), 46.6 (d, JCP = 5.3 Hz, CH2Norb), 52.5 (d, JCP = 11.7 Hz, CtBu), 52.8 (d, JCP = 8.8 Hz, CtBu), 95.5 (d, JCP = 33.5 Hz, PCCH), 124.29 (s, Cmeta), 124.32 (Cmeta), 126.2 (s, Cpara), 142.0 (d, JCP = 3.9 Hz, NCAr), 146.7 (s, CCH3iPr), 148.0 (s, CCH3iPr), 185.3 (d, JCP = 34.1 Hz, NCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): δ 49.7 (s, Δν1/2 = 47.5 Hz). HRMS (EI): m/z 525.37703 ([C31H53AlN3P]+; theoretical m/z 525.37871). Elem anal. Found: C, 70.71; H, 10.18; N, 8.03. Calcd for C31H53AlN3P: C, 70.82; H, 10.16; N, 7.99.

Synthesis of 4b

To a solution of 1b (0.10 g, 0.2 mmol) in class="Chemical">THF (20 mL) at −78 °C was added droclass="Chemical">pwise nBuLi (2.5 M in class="Chemical">pan class="Chemical">hexanes, 0.08 mL, 0.2 mmol, 1 equiv). The cold bath was removed, and the resultant yellow solution was stirred at room temperature for 1 h. The reaction mixture was cooled to −78 °C, and H3Al·NMe2Et (0.5 M in toluene, 0.4 mL, 1 equiv) was added dropwise. The cold bath was removed, the resultant yellow solution was stirred at room temperature for 20 min, and the solvent was removed in vacuo to afford the product as a yellow oil. No further purification was attempted.

class="Chemical">1H NMR (class="Chemical">pan class="Chemical">C4D8O, 500 MHz, 300 K): δ 0.29 (s, 3H, SiCH3), 0.33 (s, 3H, SiCH3), 0.84 (m, 1H, 1/2CH2Norb), 1.07 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.08 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.12 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.14 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.18 (m, 2H, CH2CbridgeheadCP), 1.19 (s, 9H, CH3tBu), 1.21 (s, 9H, CH3tBu), 1.25 (m, 1H, 1/2CH2Norb), 1.54 (m, 1H, 1/2CH2CbridgeheadCN), 1.62 (m, 1H, 1/2CH2CbridgheadCN), 2.19 (br s, 1H, CHbridgeheadCP), 3.35 (br s, 1H, CHbridgeheadCN), 3.54 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.63 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 6.83–6.90 (m, 3H, Haromatic). Note: It was not possible to locate the Al–H resonances even with the use of 1H{31P} NMR experiments, likely because of extremely high line width. 13C NMR (C4D8O, 126 MHz, 300 K): δ 6.5 (s, SiCH3), 8.3 (d, JCP = 5.7 Hz, SiCH3), 24.3 (d, JCP = 10.2 Hz, CH3iPr), 26.0 (d, JCP = 15.2 Hz, CH3iPr), 27.2 (s, CH2CbridgeheadCP), 28.11 (s, CHiPr), 28.13 (s, CHiPr), 30.1 (s, CH2CbridgeheadCN), 32.9 (d, JCP = 5.4 Hz, CH3tBu), 33.4 (d, JCP = 4.8 Hz, CH3tBu), 42.1 (d, JCP = 1.5 Hz, CHbridgeheadCN), 46.5 (s, CH2Norb), 47.2 (d, JCP = 5.5 Hz, CHbridgeheadCP), 51.4 (d, JCP = 11.5 Hz, CtBu), 51.6 (d, JCP = 14.6 Hz, CtBu), 111.2 (d, JCP = 43.1 Hz, PCCH), 123.1 (d, JCP = 10.0 Hz, Cmeta), 124.2 (s, Cpara), 146.7 (s, CCHiPr), 148.0 (s, CCHiPr), 150.7 (s, NCAr), 174.2 (d, JCP = 33.2 Hz, NCCH). 31P NMR (C4D8O, 162 MHz, 300 K): δ 110.8 (q, 2JPH = 34 Hz). 7Li NMR (C4D8O, 194.4 MHz, 300 K): δ −0.43 (s). HRMS (EI): m/z 528.34912 ([C29H52N3AlPSi]+; theoretical m/z 528.34891).

General Synthesis of 5

To a solution of ligand 1a–1c in class="Chemical">THF cooled to −78 °C was added droclass="Chemical">pwise nBuLi (2.5 M in class="Chemical">pan class="Chemical">hexanes). The cold bath was removed, and the resultant yellow solution was stirred at room temperature for 1 h. The reaction mixture was cooled to −78 °C, and a solution of H3Al·NMe3 in THF was added dropwise. The cold bath was removed, and the resultant colorless solution was stirred at room temperature for 1 h. The solvent was removed in vacuo, and the product was extracted in hexane and dried to afford a white solid. H3Al·NMe2Et (0.5 M in toluene) can be used in place of H3Al·NMe3. In this work, H3Al·NMe2Et was used for initial test reactions to synthesize up to 0.2 g of 5 using a procedure identical with that described above.

5a. 1a (1.63 g, 0.0039 mol), class="Chemical">THF (50 mL), nBuLi (1.6 mL, 0.0039 mol, 1.0 equiv), and H3class="Chemical">pan class="Chemical">Al·NMe3 (0.84 g, 0.0037 mol, 2.4 equiv) in THF (20 mL) yielded 5a (1.57 g, 91%).

class="Chemical">1H NMR (C6D6, 500 MHz, 300 K): δ 1.11 (dm, 2JHH = 8.1 Hz, class="Chemical">pan class="Chemical">1H, 1/2CH2Norb), 1.19 (d, 3JHP = 14.2 Hz, 9H, CH3tBu), 1.22 (d, 3JHP = 14.2 Hz, 9H, CH3tBu), 1.28 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.29 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.31 (m, 2H, CH2CbridgeheadCP), 1.42 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.44 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.51 (m, 1H, 1/2CH2CbridgeheadCN), 1.62 (dm, 2JHH = 8.1 Hz, 1H, 1/2CH2Norb), 1.67 (m, 1H, 1/2CH2CbirdgeheadCN), 2.53 (br s, 1H, PCCHbridgehead), 2.94 (br s, 1H, NCCHbridgehead), 3.42 (sept, 3JHH = 6.8 Hz, CHiPr), 3.63 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 4.6 (br s, 2H, AlH2), 7.16–7.22 (m, 3H, Haromatic). 13C NMR (C6D6, 126 MHz, 300 K): δ 24.0 (s, CH3iPr), 24.7 (s, CH3iPr), 25.2 (d, JCP = 1.8 Hz, CH2CbridgeheadCP), 25.8 (s, CH3iPr), 25.9 (s, CH3iPr), 28.0 (s, CHiPr), 28.1 (s, CHiPr), 29.6 (d, JCP = 4.5 Hz, CH3tBu), 30.1 (s, CH2CbridgeheadCN), 30.1 (d, JCP = 4.0 Hz, CH3tBu), 34.1 (d, JCP = 18.2 Hz, CtBu), 34.3 (d, JCP = 18.9 Hz, CtBu), 43.6 (d, JCP = 9.1 Hz, CHbridgeheadCP), 44.1 (d, JCP = 2.1 Hz, CHbridgeheadCN), 48.6 (d, JCP = 3.9 Hz, CH2Norb), 81.0 (d, JCP = 44.5 Hz, PCCH), 124.1 (s, Cmeta), 124.2 (s, Cmeta), 126.3 (s, Cpara), 141.4 (d, JCP = 3.0 Hz, NCAr), 146.8 (s, CCHiPr), 147.1 (s, CCHiPr), 185.1 (d, JCP = 20.1 Hz, NCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): δ 8.0 (s, Δν1/2 = 34.7 Hz). HRMS (EI): m/z 441.30855 ([C27H45AlNP]+; theoretical m/z 441.30996). Elem anal. Found: C, 73.11; H, 10.39; N, 3.13. Calcd for C27H45AlNP: C, 73.43; H, 10.27; N, 3.17. IR (solid, cm–1): 1810, 1786. IR (solution, cm–1): 1811.

5b. 1b (3.00 g, 0.0060 mol), class="Chemical">THF (80 mL), nBuLi (2.4 mL, 0.0060 mol, 1.0 equiv), and H3class="Chemical">pan class="Chemical">Al·NMe3 (1.28 g, 0.014 mol, 2.4 equiv) in THF (15 mL) yielded 5b (2.95 g, 93%). Colorless crystals suitable for X-ray crystallography were grown from a saturated hexane solution at −20 °C.

class="Chemical">1H NMR (C6D6, 500 MHz, 300 K): δ 0.24 (s, 3H, class="Chemical">pan class="CellLine">SiCH3), 0.29 (s, 3H, SiCH3), 1.13 (d, 2JHH = 8.1 Hz, 1H, 1/2CH2Norb), 1.28 (d, 4JHP = 0.8 Hz, 9H, CH3tBu), 1.29 (d, 4JHP = 0.8 Hz, 9H, CH3tBu), 1.30 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.31 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.38 (m, 2H, CH2CbridgeheadCP), 1.44 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.45 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.62 (d, 2JHH = 8.1 Hz, 1/2CH2Norb), 1.67 (m, 2H, CH2CbridgeheadCN), 2.57 (br s, 1H, CHbridgeheadCP), 3.12 (br s, 1H, CHbridgeheadCN), 3.53 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.74 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 4.6 (br s, 2H, Al–H), 7.17–7.23 (m, 3H, Haromatic). 13C NMR (C6D6, 126 MHz, 300 K): δ 4.4 (d, JCP = 1.6 Hz, SiCH3), 6.3 (d, JCP = 3.2 Hz, SiCH3), 24.1 (s, CH3iPr), 24.6 (s, CH3iPr), 25.6 (d, JCP = 1.0 Hz, CH2CbridgeheadCP), 25.8 (s, CH3iPr), 25.9 (s, CH3iPr), 28.0 (s, CHiPr), 28.3 (s, CHiPr), 29.3 (s, CH2CHbridgheadCN), 32.3 (d, JCP = 5.4 Hz, CH3tBu), 32.7 (d, JCP = 5.2 Hz, CH3tBu), 40.7 (d, JCP = 3.9 Hz, CHbridgheadCN), 43.9 (d, JCP = 9.7 Hz, CHbridgheadCP), 46.8 (d, JCP = 4.9 Hz, CH2Norb), 51.4 (d, JCP = 2.5 Hz, CtBu), 51.5 (d, JCP = 3.6 Hz, CtBu), 99.7 (d, JCP = 32.6 Hz, PCCH), 124.1 (s, Cmeta), 124.2 (s, Cmeta), 126.5 (s, Cpara), 140.8 (d, JCP = 3.5 Hz, NCAr), 146.4 (s, CArCHiPr), 146.5 (s, CArCHiPr), 187.6 (d, JCP = 33.9 Hz, NCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): δ 61.3 (s, Δν1/2 = 65.4 Hz). HRMS (EI): m/z 527.33886 ([C29H51AlN3PSi]+; theoretical m/z 527.33998). Elem anal. Found: C, 65.95; H, 9.66; N, 7.83. Calcd for C29H51AlN3PSi: C, 66.00; H, 9.74; N, 7.96. IR (solid, cm–1): 1831, 1816. IR (solution, cm–1): 1820.

5c. 1c (2.00 g, 0.0043 mol), class="Chemical">THF (100 mL), nBuLi (1.7 mL, 0.0043 mol, 1.0 equiv), and H3class="Chemical">pan class="Chemical">Al·NMe3 (0.91 g, 0.010 mol, 2.4 equiv) in THF (15 mL) were mixed. The final product was further purified by recrystallization from hexanes at −20 °C to yield 5c as colorless crystals (1.68 g, 79%). Two isomers were identifiable in the solution-phase NMR spectra in a ratio of 4:7 for κ2-N,N-5c to κ2-N,P-5c at 300 K (determined from the 1H NMR spectrum).

κ2-N,N-5c. class="Chemical">1H NMR (C6D6, 500 MHz, 300 K): δ 0.91 (m, class="Chemical">pan class="Chemical">1H, 1/2CH2Norb), 1.18 (d, 4JHP = 1.3 Hz, 9H, CH3tBu), 1.23 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.25 (m, 1H, 1/2CH2Norb), 1.31 (d, 3JHH = 6.8 Hz, 6H, CH3iPr), 1.32 (s, 9H, CH3tBu), 1.43 (m, 2H, CH2CbridgeheadCP), 1.60 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.65 (m, 1H, 1/2CH2CbridgeheadCN), 1.71 (m, 1H, 1/2CH2CbridgeheadCN), 2.56 (m, 1H, CHbridgeheadCP), 2.70 (m, 1H, 1/2NCH2), 2.78 (m, 1H, 1/2NCH2), 2.97 (m, 1H, CHbridgeheadCN), 3.33 (m, 1H, 1/2NCH2), 3.56 (m, 1H, 1/2NCH2), 3.85 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.98 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 4.3 (br s, 2H, AlH2) 7.18–7.21 (m, 3H, Haromatic). 13C NMR (CD6, 126 MHz, 300 K): δ 25.4 (s, CH2CbridgeheadCP), 25.6 (s, CH3iPr), 25.7 (s, CH3iPr), 25.92 (s, CH3iPr), 27.5 (s, CHiPr), 28.9 (s, CHiPr), 29.0 (d, JCP = 4.6 Hz, CH3tBu), 29.6 (d, JCP = 10.7 Hz, CH3tBu), 29.8 (d, JCP = 1.8 Hz, CH2CbridgeaheadCN), 44.2 (s, CH2Norb), 45.1 (s, CHbridgeheadCN), 45.7 (s, NCH2), 45.8 (d, JCP = 39.1 Hz, CHbridgeheadCP), 48.8 (d, JCP = 3.3 Hz, NCH2), 53.5 (d, JCP = 5.0 Hz, CtBu), 53.8 (d, JCP = 3.9 Hz, CtBu), 104.8 (d, JCP = 36.9 Hz, PCCH), 123.9 (s, Cmeta), 124.6 (s, Cmeta), 125.2 (s, Cpara), 143.22 (s, NCAr), 145.8 (CCHiPr), 146.53 (s, CCHiPr), 167.4 (d, JCP = 2.7 Hz, NCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): δ 96.9 (s, Δν1/2 = 137.9 Hz).

κ2-N,class="Gene">P-5c. class="Chemical">pan class="Chemical">1H NMR (C6D6, 500 MHz, 300 K): δ 1.07 (dm, 2JHH = 8.4 Hz, 1H, 1/2CH2Norb), 1.29 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.30 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.32 (s, 9H, CH3tBu), 1.35 (s, 9H, CH3tBu), 1.39 (m, 2H, CH2CbridgeheadCP), 1.42 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.45 (d, 3JHH = 6.8 Hz, 3H, CH3iPr), 1.56 (m, 1H, 1/2CH2Norb), 1.63 (m, 2H, CH2CbridgeheadCN), 2.52 (m, 1H, CHbridgeheadCP), 2.60 (m, 1H, 1/2NCH2), 2.72 (m, 1H, 1/2NCH2), 2.79 (m, 1H, 1/2NCH2), 2.88 (m, 1H, NCH2), 3.02 (m, 1H, CHbridgeheadCN), 3.54 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 3.76 (sept, 3JHH = 6.8 Hz, 1H, CHiPr), 4.6 (br s, 2H, AlH2), 7.18–7.21 (m, 3H, Haromatic). 13C NMR (C6D6, 126 MHz, 300 K): δ 24.0 (s, CH3iPr), 24.7 (s, CH3iPr), 25.3 (s, CH2CbridgeheadCP), 25.8 (s, CH3iPr), 25.86 (s, CH3iPr), 27.9 (s, CHiPr), 28.1 (s, CHiPr), 29.0 (d, JCP = 4.6 Hz, CH3tBu), 29.3 (s, CH2CbridgeheadCN), 29.9 (d, JCP = 4.6 Hz, CH3tBu), 42.7 (d, JCP = 4.4 Hz, CHbridgeheadCN), 43.6 (d, JCP = 9.9 Hz, CHbridgeheadCP), 43.9 (s, NCH2), 44.2 (s, NCH2), 46.6 (d, JCP = 4.9 Hz, CH2Norb), 53.4 (d, JCP = 9.8 Hz, CtBu), 53.6 (d, JCP = 5.4 Hz, CtBu), 97.0 (d, JCP = 35.3 Hz, PCCH), 124.1 (s, Cmeta), 124.3 (s, Cmeta), 126.4 (s, Cpara), 141.0 (d, JCP = 4.0 Hz, NCAr), 146.51 (s, CCHiPr), 146.8 (s, CCHiPr), 185.4 (d, JCP = 34.0 Hz, NCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): δ 47.8 (s, Δν1/2 = 96.6 Hz). HRMS (EI): m/z 497.35079 ([C29H49AlN3P]+; theoretical m/z 497.34741). Elem anal. Found: C, 69.80; H, 9.80; N, 8.33. Calcd for C29H51AlN3PSi: C, 69.99; H, 9.92; N, 8.44. IR (solid, cm–1): 1825, 1801. IR (solution, cm–1): 1823.