An Elusive Hydridoaluminum(I) Complex for Facile C–H and C–O Bond Activation of Ethers and Access to Its Isolable Hydridogallium(I) Analogue: Syntheses, Structures, and Theoretical Studies

The reaction of AlBr3 with 1 molar equiv of the chelating bis(N-heterocyclic carbene) ligand bis(N-Dipp-imidazole-2-ylidene)methylene (bisNHC, 1) affords [(bisNHC)AlBr2](+)Br(-) (2) as an ion pair in high yield, representing the first example of a bisNHC-Al(III) complex. Debromination of the latter with 1 molar equiv of K2Fe(CO)4 in tetrahydrofuran (THF) furnishes smoothly, in a redox reaction, the (bisNHC)(Br)Al[Fe(CO)4] complex 3, in which the Al(I) center is stabilized by the Fe(CO)4 moiety through Al(I):→Fe(0) coordination. Strikingly, the Br/H ligand exchange reactions of 3 using potassium hydride as a hydride source in THF or tetrahydropyran (THP) do not yield the anticipated hydridoaluminum(I) complex (bisNHC)Al(H)[Fe(CO)4] (4a) but instead lead to (bisNHC)Al(2-cyclo-OC4H7)[Fe(CO)4] (4) and (bisNHC)Al(2-cyclo-OC5H9)[Fe(CO)4] (5), respectively. The latter are generated via C-H bond activation at the α-carbon positions of THF and THP, respectively, in good yields with concomitant elimination of dihydrogen. This is the first example whereby a low-valent main-group hydrido complex facilitates metalation of sp(3) C-H bonds. Interestingly, when K[BHR3] (R = Et, sBu) is employed as a hydride source to react with 3 in THF, the reaction affords (bisNHC)Al(OnBu)[Fe(CO)4] (6) as the sole product through C-O bond activation and ring opening of THF. The mechanisms for these novel C-H and C-O bond activations mediated by the elusive hydridoaluminum(I) complex 4a were elucidated by density functional theory (DFT) calculations. In contrast, the analogous hydridogallium(I) complex (bisNHC)Ga(H)[Fe(CO)4] (9) can be obtained directly in high yield by the reaction of the (bisNHC)Ga(Cl)[Fe(CO)4] precursor 8 with 1 molar equiv of K[BHR3] (R = Et, sBu) in THF at room temperature. The isolation of 9 and its inertness toward cyclic ethers might be attributed to the higher electronegativity of gallium versus aluminum. The stronger Ga(I)-H bond, in turn, hampers α-C-H metalation or C-O bond cleavage in cyclic ethers, the latter of which is supported by DFT calculations.