Why Do Five Ga + Cations Form a Ligand‐Stabilized [Ga 5 ] 5+ Pentagon and How Does a 5:1 Salt Pack in the Solid State?

A Planar Five‐Membered Aromatic Ring Stabilized by Only Two π‐Electrons

Many chemicals known today are partially or fully aromatic, since a ring framework experiences additional stabilization through the delocalization of π‐electrons. While aromatic rings with equal numbers of π‐electrons and ring atoms such as benzene are particularly stable, those with the minimally required two π‐electrons are very rare and yet remain limited to three‐ and four‐membered rings if not stabilized in the coordination sphere of heavy metals. Here we report the facile synthesis of a dipotassium cyclopentagallene, a unique example of a five‐membered aromatic ring stabilized by only two π‐electrons. Single‐crystal X‐ray diffraction revealed a planar Ga₅ ring with almost equal gallium–gallium bond lengths, which together with computational and spectroscopic data confirm its aromatic character. Our results prove that aromatic stabilization goes far beyond what has previously been assumed as minimum π‐electron count in a five‐atom ring fragment.

A Brønsted Acidic Gallium Hydride: Facile Interconversion of NNNN-Macrocycle Supported [GaI]+ and [GaIIIH]2

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]⁺.