Abstract
Protonolysis of [Cp*M] (M=Ga, In, Tl) with [(Me(4) TACD)H][BAr(4) (Me) ] (Me(4) TACD=N,N',N'',N'''-tetramethyl-1,4,7,10-tetraazacyclododecane; [BAr(4) (Me) ](-) =[B{C(6) H(3) -3,5-(CH(3) )(2) }(4) ](-) ) provided monovalent salts [(Me(4) TACD)M][BAr(4) (Me) ], whereas [Cp*Al](4) yielded trivalent [(Me(4) TACD)AlH][BAr(4) (Me) ](2) . Protonation of [(Me(4) TACD)Ga][BAr(4) (Me) ] with [Et(3) NH][BAr(4) (Me) ] gave an unusually acidic (pK(a) (CH(3) CN)=24.5) gallium(III) hydride dication [(Me(4) TACD)GaH][BAr(4) (Me) ](2) . Deprotonation with IMe(4) (1,3,4,5-tetramethyl-imidazol-ylidene) returned [(Me(4) TACD)Ga][BAr(4) (Me) ]. These reversible processes occur with formal two-electron oxidation and reduction of gallium. DFT calculations suggest that gallium(I) protonation is facilitated by strong coordination of the tetradentate ligand, which raises the HOMO energy. High nuclear charge of [(Me(4) TACD)GaH](2+) facilitates hydride-to-metal charge transfer during deprotonation. Attempts to prepare a gallium(III) dihydride cation resulted in spontaneous dehydrogenation to [(Me(4) TACD)Ga](+) .