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-OC₄H ₇)[Fe(CO)₄] (4) and (bisNHC)Al(2-cyclo-OC ₅H₉)[Fe(CO)₄] (5), respectively. The latter are generated via C-H bond activation at the ∝-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 sp³ 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 ∝-C-H metalation or C-O bond cleavage in cyclic ethers, the latter of which is supported by DFT calculations.