A Brønsted Acidic Gallium Hydride: Facile Interconversion of NNNN-Macrocycle Supported [Ga I ] + and [Ga III H] 2 .

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] + .