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

Abstract

The dicationic clusters ((CsMes)41r4S4)2+ ( (112+) and ((C5Me5)31r3S~)2+ were prepared by treatment of ( (C5Me~)IrCl2)2 with (Me3Si)zS followed by ion exchange chromatography. Crystallographic characterization of the tetrairidium cluster ( 112+ 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 IrIv-1rIv bonding contact of 2.764(1) A. The four IrIv-Irlll contacts are nearly equivalent at 3.5 A, while the Irlll...Irlll distance is 3.683( 1) A. Variable-temperature 1H NMR studies indicate that (112+ is fluxional with a coalescence temperature of 13 OC (400 MHz), corresponding to AG* = 57 kJ/mol. This dynamic process is attributed to migration of the metal-metal bond arising from internal Ir*v/IrIII self-exchange. Cyclic voltammetry studies of ( 112+ reveal a pair of reversible one-electron reductions at -21 8 and -487 mV vs Ag/AgCl. Chemical reduction of (1)2+ was effected with cobaltocene while the neutral cluster could be reoxidized with HC1/02. In the solid state (2)2+ features a trigonal-bipyramidal Ir3S2 core with average Ir-Ir contacts of 2.82 8, and Ira distances of 2.28 A. 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 (21°. Variable-temperature lH NMR spectra of (21° reveal dynamic behavior, with coalescence at 60 OC (400 MHz), corresponding to AG* = 64 kJ/mol. These structural dynamics suggest migration of the nonbonding Ir-Ir interaction among the three edges of the Ir3 triangle.