Abstract
Protonolysis of [Cp*M] (M=Ga, In, Tl) with [(Me4TACD)H][BAr4 Me] (Me4TACD=N,N′,N′′,N′′′‐tetramethyl‐1,4,7,10‐tetraazacyclododecane; [BAr4 Me]−=[B{C6H3‐3,5‐(CH3)2}4]−) provided monovalent salts [(Me4TACD)M][BAr4 Me], whereas [Cp*Al]4 yielded trivalent [(Me4TACD)AlH][BAr4 Me]2. Protonation of [(Me4TACD)Ga][BAr4 Me] with [Et3NH][BAr4 Me] gave an unusually acidic (pK a(CH3CN)=24.5) gallium(III) hydride dication [(Me4TACD)GaH][BAr4 Me]2. Deprotonation with IMe4 (1,3,4,5‐tetramethyl‐imidazol‐ylidene) returned [(Me4TACD)Ga][BAr4 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 [(Me4TACD)GaH]2+ facilitates hydride‐to‐metal charge transfer during deprotonation. Attempts to prepare a gallium(III) dihydride cation resulted in spontaneous dehydrogenation to [(Me4TACD)Ga]+.
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