Low valent and hydride complexes of NHC coordinated gallium and indium

Abstract

The reactions of the N-heterocyclic carbene 1,3-dimesitylimidazol-2-ylidene (IMes) with Ga[GaCl₄], “GaI”, InCl₂ and GaBr₃ have been examined. All reactions using a low valent gallium or indium starting material led to species of the form [{MX₂(IMes)}₂], where M = Ga, X = Cl (1), I (2); M = In, X = Cl (3), with disproportionation and loss of gallium metal in the case of 2. Reaction of IMes with gallium tribromide yields the air and moisture stable complex [GaBr₃(IMes)] (4), which has been used as a precursor to the mixed bromohydrides [GaBrH₂(IMes)] (5) and [GaBr₂H(IMes)] (6) by (i) ligand redistribution with [GaH₃(IMes)], (ii) hydride–bromide exchange with triethylsilane, and (iii) alkylation with ⁿbutyllithium followed by β-hydride elimination (6 only). Attempts to prepare 1, or monovalent analogues such as [{GaCl(IMes)}_n], by thermally induced reductive elimination of dihydrogen from the chlorohydride congeners of 5 and 6 resulted in isolation of the known compounds [IMesCl][Cl] (IMesCl = 1,3-dimesityl-2-chloroimidazolium), and/or 1,3-dimesityl-2-dihydroimidazole, and gallium metal. Preliminary photochemical NMR spectroscopy and catalytic studies of 5 and 6 aimed at reductive dehydrogenation under milder conditions are reported. Compounds 1 and 4 have been characterised by single crystal X-ray structure determination.