Beyond transition block metals: exploring the reactivity of phosphine PTA and its oxide [PTA(O)] towards gallium(iii)

The water-soluble cage-like phosphine PTA (1,3,5-triaza-7-phosphaadamantane) and its phosphine oxide derivative [PTA(O)] (1,3,5-triaza-7-phosphaadamantane-7-oxide) were used to explore their reactivity towards two gallium(iii)-halide precursors, namely GaCl₃ and GaI₃, for the first time. By using various reaction conditions, a series of N-mono-protonated phosphine salts with [GaCl₄]^- or [I]^- as counterions were obtained in all cases, while the formation of coordinated Ga-PTA and Ga-[PTA(O)] complexes was not observed. All compounds were characterized in solution using multinuclear NMR spectroscopy (¹H, ¹³C{¹H}, ³¹P{¹H} and ⁷¹Ga) and in the solid state using FT-IR spectroscopy and X-ray crystal diffraction. The new Ga-phosphine salts resulted stable and highly soluble in aqueous solution at room temperature. Density functional theory (DFT) calculations were also performed to further rationalize the coordination features of PTA with Ga³⁺ metal ion, highlighting that the phosphorus-gallium bond is about twice weaker than the phosphorus-metal bond commonly established by PTA with transition metals such as gold. Furthermore, the mono-protonation of PTA (or [PTA(O)]) makes the formation of ionic gallium-PTA coordination complexes thermodynamically unstable, as confirmed experimentally by the formation of Ga-phosphine salts reported herein.