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

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• 作者：Gengwen Tan ; Tibor Szilv谩si ; Shigeyoshi Inoue ; Burgert Blom ; Matthias Driess
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：July 9, 2014
• 年：2014
• 卷：136
• 期：27
• 页码：9732-9742
• 全文大小：525K
• ISSN：1520-5126

文摘

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^{鈥?/sup> (2) as an ion pair in high yield, representing the first example of a bisNHC鈥揂l(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):鈫扚e(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鈥揌 bond activation at the 伪-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鈥揌 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鈥揙 bond activation and ring opening of THF. The mechanisms for these novel C鈥揌 and C鈥揙 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)鈥揌 bond, in turn, hampers 伪-C鈥揌 metalation or C鈥揙 bond cleavage in cyclic ethers, the latter of which is supported by DFT calculations.}