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(<i>N</i>-Dipp-imidazole-2-ylidene)­methylene (bisNHC, <b>1</b>) affords [(bisNHC)­AlBr₂]⁺Br^– (<b>2</b>) 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 <b>3</b>, 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 <b>3</b> 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] (<b>4a</b>) but instead lead to (bisNHC)­Al­(2-<i>cyclo</i>-OC₄H₇)­[Fe­(CO)₄] (<b>4</b>) and (bisNHC)­Al­(2-<i>cyclo</i>-OC₅H₉)­[Fe­(CO)₄] (<b>5</b>), 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 sp³ C–H bonds. Interestingly, when K­[BHR₃] (R = Et, <i>s</i>Bu) is employed as a hydride source to react with <b>3</b> in THF, the reaction affords (bisNHC)­Al­(O<i>n</i>Bu)­[Fe­(CO)₄] (<b>6</b>) 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 <b>4a</b> were elucidated by density functional theory (DFT) calculations. In contrast, the analogous hydridogallium­(I) complex (bisNHC)­Ga­(H)­[Fe­(CO)₄] (<b>9</b>) can be obtained directly in high yield by the reaction of the (bisNHC)­Ga­(Cl)­[Fe­(CO)₄] precursor <b>8</b> with 1 molar equiv of K­[BHR₃] (R = Et, <i>s</i>Bu) in THF at room temperature. The isolation of <b>9</b> 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