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 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 α-C–H metalation
or C–O bond cleavage in cyclic ethers, the latter of which
is supported by DFT calculations.