Proton-Transfer and H₂-Elimination Reactions of Trimethylamine Alane: Role of Dihydrogen Bonding and Lewis Acid−Base Interactions

Abstract

Proton-transfer and H(2)-elimination reactions of aluminum hydride AlH(3)(NMe(3)) (TMAA) with XH acids were studied by means of IR and NMR spectroscopy and DFT calculations. The dihydrogen-bonded (DHB) intermediates in the interaction of the TMAA with XH acids (CH(3)OH, (i)PrOH, CF(3)CH(2)OH, adamantyl acetylene, indole, 2,3,4,5,6-pentafluoroaniline, and 2,3,5,6-tetrachloroaniline) were examined experimentally at low temperatures, and the spectroscopic characteristics, dihydrogen bond strength and structures, and the electronic and energetic characteristics of these complexes were determined by combining experimental and theoretical approaches. The possibility of two different types of DHB complexes with polydentate proton donors (typical monodentate and bidentate coordination with the formation of a symmetrical chelate structure) was shown by DFT calculations and was experimentally proven in solution. The DHB complexes are intermediates of proton-transfer and H(2)-elimination reactions. The extent of this reaction is very dependent on the acid strength and temperature. With temperature increases the elimination of H(2) was observed for OH and NH acids, yielding the reaction products with Al-O and Al-N bonds. The reaction mechanism was computationally studied. Besides the DHB pathway for proton transfer, another pathway starting from a Lewis complex was discovered. Preference for one of the pathways is related to the acid strength and the nucleophilicity of the proton donor. As a consequence of the dual Lewis acid-base nature of neutral aluminum hydride, participation of a second ROH molecule acting as a bifunctional catalyst forming a six-member cycle connecting aluminum and hydride sites notably reduces the reaction barrier. This mechanism could operate for proton transfer from weak OH acids to TMAA in the presence of an excess of proton donor.

Dichotomous Si–H Bond Activation by Alkoxide and Alcohol in Base-Catalyzed Dehydrocoupling of Silanes

“…For some hydrides (e.g., (Me 3 N) 2 AlH 3 and (Ph 3 P) 2 Cu­(η 2 -BH 4 )), the reaction with alcohols was shown to proceed as concerted proton–hydride transfer through the cyclic 6-membered transition state involving two alcohol molecules. However, for phenylsilanes such transition state still has very high energy (Δ G ‡ Hex = 40.7–43.2 kcal/mol; TS B O(H)R , Table and Figure S35).…”

Comprehensive Insight into the Hydrogen Bonding of Silanes

“…This phenomenon was explored extensively for transition metal hydrides and particularly great success was achieved in the evaluation of DHB nature and structure . However, the studies of main group hydrides as proton acceptors are mostly limited to group 13 elements hydrides (boron, aluminum and gallium hydrides). Data on DHBs of group 14 elements hydrides are very scarce.…”

Boron, aluminium, gallium, indium and thallium

“…118 Proton-transfer and H 2 -elimination reactions of aluminium hydride AlH 3 (NMe 3 ) (TMAA) with XH acids have been studied by means of IR and NMR spectroscopy and DFT calculations. 119 AlH 3 (and GaH 3 ) adducts of the di-brominated NHC-carbenes have been shown to undergo an unusual thermaly promoted hydride-bromine exchange reaction, between hydrides on the metal centre and bromine atoms on the backbone of the NHC ligand, to produce the aluminium hydride-bromide complexes [MBr 2 HÁ(NHC)]. 120 The reactivities of the aluminium hydride complex [C 4 H 2 N(CH 2 NMe 2 ) 2 ]AlH 2 with an array of primary and secondary amines, phenols, ketones, and phenyl isothiocyanate in various ratios have been examined and the subsequent exchange and insertion products described.…”