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](+).