Mobile Metal-Metal Bonds: Studies on Mixed Valence Ir3 and Ir4 Clusters

The dicationic clusters [(C₅Me₅)₄Ir₄S₄] ²⁺ ([1]²⁺) and [(C₅Me₅)₃Ir₃S₂] ²⁺ ([2]²⁺) were prepared by treatment of [(C₅Me₅)IrCl₂]₂ with (Me₃Si)₂S followed by ion exchange chromatography. Crystallographic characterization of the tetrairidium cluster [1]²⁺ reveals a cubane motif consisting of interpenetrated Ir₄ and S₄ tetrahedra. The Ir₄ core is distorted from idealized tetrahedral symmetry by virtue of a single Ir^IV-Ir^IV bonding contact of 2.764(1) Å. The four Ir^IV-Ir^III contacts are nearly equivalent at 3.5 Å, while the Ir^III⋯Ir^III distance is 3.683(1) Å. Variable-temperature ¹H NMR studies indicate that [I]²⁺ is fluxional with a coalescence temperature of 13 °C (400 MHz), corresponding to ΔG‡ = 57 kJ/mol. This dynamic process is attributed to migration of the metal-metal bond arising from internal Ir^IV/Ir^III self-exchange. Cyclic voltammetry studies of [1]²⁺ reveal a pair of reversible one-electron reductions at -218 and -487 mV vs Ag/AgCl. Chemical reduction of [1]²⁺ was effected with cobaltocene while the neutral cluster could be reoxidized with HC1/O₂. In the solid state [1]²⁺ features a trigonal-bipyramidal Ir₃S₂ core with average Ir-Ir contacts of 2.82 Å and Ir-S distances of 2.28 Å. Cyclic voltammetry studies indicate that this closo dication undergoes two single electron reductions at -712 and -993 mV. Cobaltocene reduction of [2]²⁺ afforded dark blue crystals of neutral [2]⁰. Variable-temperature ¹H NMR spectra of [2]⁰ reveal dynamic behavior, with coalescence at 60 °C (400 MHz), corresponding to ΔG‡ = 64 kJ/mol. These structural dynamics suggest migration of the nonbonding Ir⋯Ir interaction among the three edges of the Ir₃ triangle.