| Sumario: | The reaction of AlBr with 1 molar equiv of the chelating bis(N-heterocyclic carbene) ligand bis(N-Dipp-imidazole-2- ylidene)methylene (bisNHC, l) affords [(bisNHC)AlBr]Br (2) as an ion pair in high yield, representing the first example of a bisNHC--A1(lll) complex. Debromination of the latter with 1 molar equiv of KFe(CO) in tetrahydrofuran (THF) furnishes smoothly, in a redox reaction, the (bisNHC)(Br)Al[Fe(CO)] complex 3, in which the Al(l) center is stabilized by the Fe(CO) 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 yie ld the anticipated hydridoaluminum(l) complex (bisNHC)Al(H)[Fe(CO)] (4a) but instead lead to (bisNHC)Al(2-cyc/o-OC4H7)[Fe(CO)] (4) and (bisNHC)Al(2-cydo-OCH)[Fe(CO)] (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 C -H bonds. Interestingly, when K[BHR] (R = Et, sBu) is employed as a hydride source to react with 3 in THF, the reaction affords (bisNHC)Al(OnBu)- [Fe(CO)] (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(l) complex 4a were elucidated by density functional theory (DFT) calculations. In contrast, the analogous hydridogallium(l) 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)] precursor 8 with 1 molar equiv of K[BHR] (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(l)— 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.
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