Ligand coordination modulates reductive elimination from aluminium(iii).

Oxidative addition of inert bonds at low-valent main-group centres is becoming a major class of reactivity for these species. The reverse reaction, reductive elimination, is possible in some cases but far rarer. Here, we present a mechanistic study of reductive elimination from Al(iii) centres and unravel ligand effects in this process. Experimentally determined activation and thermodynamic parameters for the reductive elimination of Cp*H from Cp*2AlH are reported, and this reaction is found to be inhibited by the addition of Lewis bases. We find that C-H oxidative addition at Al(i) centres proceeds by initial protonation at the low-valent centre.

Reductive elimination is a key reaction in organometallic chemistry, and is frequently both the product-forming and ratedetermining step in important stoichiometric and catalytic transformations. 1 The facility with which transition metal systems can undergo reversible oxidative addition and reductive elimination reactions is central to their widespread applications in catalysis. In this context, the analogy between the reactivity of transition metals and low-valent main-group compounds 2 has concentrated effort on expanding their capability towards oxidative addition and reductive elimination reactivity. The mechanisms of oxidative addition and reductive elimination at main group centres are diverse. Low valent group 14 carbene and alkyne analogues cleave dihydrogen through a concerted mechanism that involves simultaneous electron donation and acceptance to and from dihydrogen and the group 14 centre. [3][4][5][6][7][8][9] Stannylenes activate the N-H bond of ammonia in an apparently similar process, yet in this reaction a coordination/deprotonation mechanism involving two equivalents of NH 3 seems to be operative. 6,10 Activation of ammonia, as well as other protic compounds, by constrained geometry phosphorus(III) species probably follows a similar pathway. [11][12][13][14][15] Treatment of disilanes with Lewis bases can induce a formal reductive elimination, resulting in SiCl 4 and base-coordinated SiCl 2 fragments. 16,17 Meanwhile, reductive elimination of H 2 from arylstannanes, RSnH 3 , is also promoted by the addition of bases; in this case, the base does not coordinate the tin centre but instead initially deprotonates the tin hydride. 18 Although a stepwise reaction, this formally heterolytic (ionic) reductive elimination of dihydrogen is reminiscent of the concerted heterolytic dihydrogen activation achieved by frustrated Lewis pairs. 19 In transition metal chemistry, robust guiding principles exist that enable chemists to predict and select for oxidative addition/reductive elimination reactivity. In order to understand if the development of such principles for maingroup systems is possible, mechanistic studies of a range of main-group oxidative additions and reductive eliminations are required.
Aluminium(I) compounds have been shown to readily activate H-C, H-P, H-N, H-Si and H-B bonds through oxidative addition, 20 though the mechanism of these reactions is not well-  22 probably due to steric factors.
The coordination of the NHC ligands 3a or 3b to Cp* 2 AlH 1 was readily apparent in the 1 H NMR spectra of 4a and 4b. A dative Al-C interaction is confirmed by new signals observed for the now inequivalent methyl or isopropyl C-H groups of the NHC ligands (4a δ = 1.29 and 1.15 ppm; 4b δ = 6.08 and 3.76 ppm), which also display the expected downfield shifts observed for coordinated NHC ligands. 23 The typical upfield shift of NHC donor carbon resonances upon coordination could not be confirmed because these signals were not observable for 4a or 4b, likely because of line broadening due to quadrupolar 27 Al.
The chemical shift of the Cp* methyl groups is only slightly perturbed by coordination of the NHC ligands (4a δ = 1.98 ppm; 4b δ = 2.06 ppm; 1 δ = 1.91 ppm) and remains a lone singlet, indicating rapid sigmatropic shifts of the cyclopentadienyl substituents. 24,25 Coordination of the DMAP ligand in the adduct 4c is confirmed by the observation of two upfield-shifted signals (δ = 7. coordinated. 21 Clearly, the coordination of strong σ-donor to the aluminium centre of 1 is favoured over the weaker donation of electron density from the π-system of the Cp* ligands. Compound 4a is isostructural with its gallium analogue, 26 and the NHC bond distances in 4a and 4b are directly comparable to the very few reported NHC adducts of aluminium. 27,28 In contrast to the group 14 systems mentioned previously, the interaction of Lewis bases with the aluminium hydride 1 does not result in reductive elimination reactivity. Even after heating the NHC adducts 4a or 4b at 100 ˚C for several days, no elimination of Cp*H was observed. 29 However, heating solutions of the DMAP adduct 4c at 80 ˚C resulted in reductive elimination of Cp*H and formation of tetramer 2 as the only aluminium-containing product, along with uncoordinated DMAP. The rate of Cp*H elimination from 4c is significantly slower than that from Cp* 2 AlH 1 (for example, after 100 minutes at 353 K, 31.3 % of 4c was converted to the tetramer 2 whilst 90.7 % of 1 had been converted).
In order to explain our observations, we propose a mechanism involving the reversible dissociation of DMAP from the adduct 4c under the reaction conditions. Reductive elimination to form 2 can only take place from 1; the DMAP adduct 4c does not itself eliminate Cp*H (scheme 3). The formation of (Cp*Al) 4 is not observed when the NHC adducts 4a and 4b are heated because of the stronger coordination of these ligands to the aluminium centre.
The proposed reversible coordination of DMAP to 1 at higher temperatures is supported by the observation of time-  Why does base coordination to 1 inhibit reductive elimination, when in other main-group systems reductive elimination can be promoted by the coordination of donor ligands? We sought to understand this observation by undertaking a mechanistic study of reductive elimination from 1.
We initially confirmed Fischer's report 21 S11). By measuring the concentrations of (Cp*Al) 4 2, Cp* 2 AlH 1 and Cp*H we determined K eq for the equilibrium depicted in scheme 1 at a range of temperatures (Table S3). We were thus able to determine ΔG 0 300 as +13.83 ± 0.48 kJ mol -1 , indicating reductive elimination from 1 to 2 is an endothermic process, as might be expected for the reduction of Al III to Al I . 30 Having established experimental values for thermodynamic parameters of Cp*H reductive elimination, we studied the kinetics of this reaction. An important assumption we make is that the tetramerisation of Cp*Al to (Cp*Al) 4 , and the reverse process, proceeds with lower barriers than reductive elimination of oxidative addition of Cp*H. The tetramerisation energy for Cp*Al has been measured experimentally as 150 ± 20 kJ mol -1 , and tetramer and monomer are in rapid equilibrium under our reaction conditions. 31 Oxidative addition of Cp*H to Cp*Al is significantly faster than reductive elimination from 1; fitting our experimental data to the model in Scheme 1 we determined rate constants k 1 and k 2 at 333 K as 1.46 x 10 -3 ± 0.04 x 10 -3 s -1 and 35 x 10 -3 ± 4 x 10  figure S14) which was 92.80 ± 5.32 kJ mol -1 . Unexpectedly, the entropy of activation for reductive elimination is close to zero, and slightly negative, at -0.167 ± 2.64 J K -1 mol -1 , rather than the positive figure that could be expected for a reductive elimination reaction. Although coordination of an external Lewis base to 1 does not promote reductive elimination of Cp*H, we questioned if one of the Cp* ligands of 1 could play this role, particularly since X-ray crystallography reveals that the two Cp* ligands of 1 adopt η 2 and η 3 coordination modes. 21 A shift to higher hapticity of one Cp* ligand could explain the slightly negative entropy of activation for reductive elimination. An alternative explanation could be an ionic-type mechanism involving the dissociation of a Cp* ligand to form a transient [Cp*AlH] + species, with solvent ordering around the charged intermediates being responsible for the negative entropy of activation. 32 We examined the reductive elimination of Cp*H from Cp* 2 AlH using DFT (figure 2) in order to better understand the mechanism. Geometry optimisations were performed for compounds 1, 2, and Cp*H and the transition state that links them (geometries were optimised at the BP86/def2-SVP level of theory, and confirmed as minima by frequency calculations (ref to SI). The transition state for reductive elimination of Cp*H from 1, TS 1-2  Firstly, the presence of a strong electron donor substantially stabilises the high(er) oxidation state aluminium centre Secondly, coordination inhibits the aromatisation of the Cp* ligands required to enable reductive elimination. The combined effects of the π-donating Cp* ligands and the coordination of strong σ-donors in modulating the Al III /Al I process is similar to the recently reported effect of strong σ-donors in oxidative addition to germylenes. 34 Such ligands not only enable oxidative addition reactivity by narrowing the HOMO/LUMO gap in the low-valent species, but also favour the low oxidation state species by providing increased electron density. Continued study of reaction mechanisms of (reversible) oxidative addition and reductive elimination in low-valent main-group systems will be essential in developing effective principles for ligand design.