MOBILE METAL-METAL BONDS - STUDIES ON MIXED-VALENCE IR-3 AND IR-4 CLUSTERS

The dicationic clusters [(C(5)Me(5))(4)Ir4S4](2+) ([1](2+)) and [(C(5)Me(5))(3)Ir3S2](2+) ([2](2+)) were prepared by treatment of [(C(5)Me(5))IrCl2](2) with (Me(3)Si)(2)S followed by ion exchange chromatography. Crystalographic characterization of the tetrairidium cluster [1](2+) reveals a cubane motif consisting of interpenetrated Ir-4 and S-4 tetrahedra. The Ir-4 core is distorted from idealized tetrahedral symmetry by virtue of a single Ir-IV-Ir-IV bonding contact of 2.764(1) Angstrom. The four Ir-IV-Ir-III contacts are nearly equivalent at 3.5 Angstrom, while the Ir-III...Ir-III distance is 3.683(1) Angstrom. Variable-temperature H-1 NMR studies indicate that [1](2+) is fluxional with a coalescence temperature of 13 degrees C (400 MHz), corresponding to Delta G(double dagger) = 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 HCl/O-2. In the solid state [2](2+) features a trigonal-bipyramidal Ir3S2 core with average Ir-Ir contacts of 2.82 Angstrom and Ir-S distances of 2.28 Angstrom. Cyclic voltammetry studies indicate that this close 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 H-1 NMR spectra of [2](0) reveal dynamic behavior, with coalescence at 60 degrees C (400 MHz), corresponding to Delta G(double dagger) = 64 kJ/mol. These structural dynamics suggest migration of the nonbonding Ir...Ir interaction among the three edges of the Ir-3 triangle.