The underappreciated influence of ancillary halide on metal–ligand proton tautomerism

Syntheses of Vaska-type complexes IrP₂X(CO)] (P = phosphine, X = halide) with all four common halides (fluoride, chloride, bromide, and iodide) was attempted using a protic and hemilabile imidazolyl di-tert-butyl phosphine ligand. In the solid-state, all four complexes were found to be ionic with the halides in the outer-sphere, and the fourth coordination site of the square plane occupied by the imidazole arm of the ligand. In solution, however, the chloride complex was found to be in equilibrium with an octahedral Ir^III–H species at room temperature. For the bromide and iodide analogs, the corresponding Ir^III–H species were also observed but only after heating the solutions. The neutral Ir^I Vaska''s analogs for X = Cl, Br, and I were obtained upon addition of excess halide salt, albeit heating was required for X = Br and I. The Ir^III–H species are proposed to originate from tautomerization of minor amounts of the electron rich neutral Vaska analog (halide inner-sphere and phosphines monodentate) that are in equilibrium with the ionic species. Heating is required for the larger anions of bromide and iodide to overcome a kinetic barrier associated with their movement to an inner-sphere position prior to tautormerization. For the fluoride analog, the Ir^III–H was not observed, attributable to strong hydrogen bonding interactions of the imidazolyl proton with the fluoride anion.

Ligand protonated Ir^I bisphosphine carbonyl complexes isolated as halide salts equilibrate with their neutral Ir^III–H congeners in solution. The equilibrium constant and energy barrier to interconversion are dependent on the identity of the halide.