Metal−Metal Bonding in Pentanuclear Bow-Tie Metal Sulfido Clusters. Synthetic and Structural Studies on the Cationic Pentanuclear Clusters [(Cp*Ir)₂(μ₃-S)₂M(μ₃-S)₂(IrCp*)₂]<i>ⁿ</i>⁺ (M = Fe, Co, Ni; <i>n</i> = 1, 2)

Abstract

Reactions of [CpMCl(&mgr;(2)-SH)(2)MCpCl] (1, M = Ir; 2, M = Rh; Cp = eta(5)-C(5)Me(5)) with excess FeCl(2).4H(2)O in THF gave the paramagnetic trinuclear clusters [(CpM)(2)(&mgr;(3)-S)(2)FeCl(2)] (3, M = Ir; 4, M = Rh), which were further converted into the dicationic 78e(-) pentanuclear bow-tie cluster [(CpIr)(2)(&mgr;(3)-S)(2)Fe(&mgr;(3)-S)(2)(IrCp)(2)](2+) (5) by treatment with NaBPh(4). When complex 1 was allowed to react with CoCl(2) and NiCl(2).6H(2)O or Ni(cod)(2) (cod = cyclooctadiene), the related pentanuclear 79e(-) and 80e(-) bow-tie clusters [(CpIr)(2)(&mgr;(3)-S)(2)M(&mgr;(3)-S)(2)(IrCp)(2)](2+) (6, M = Co; 7, M = Ni) were obtained directly, respectively. Cyclic voltammograms of 5[BPh(4)](2), 6[BPh(4)](2), and 7[BPh(4)](2) showed two reversible reduction waves at -0.25 to -0.43 V and -1.04 to -1.34 V. In both redox couples, the redox potential was in the order Fe < Co < Ni. One-electron reduction of clusters 5[BPh(4)](2), 6[BPh(4)](2), and 7[BPh(4)](2) with Co(eta(5)-C(5)H(5))(2) gave the corresponding monocationic pentanuclear 79-81e(-) bow-tie clusters [(CpIr)(2)(&mgr;(3)-S)(2)M(&mgr;(3)-S)(2)(IrCp)(2)](+) (8, M = Fe; 9, M = Co; 10, M = Ni). The molecular structures of 3, 4, 5[BPh(4)](2).CH(2)Cl(2), 6[CoCl(3)(NCMe)](2), 7[NiCl(4)].CH(2)Cl(2), 8[BPh(4)], 9[BPh(4)], and 10[BPh(4)] were unambiguously determined by X-ray diffraction study. The structures of the pentanuclear bow-tie cluster cores remarkably changed stepwise as the core electrons increased from 78 to 81. Two of the M-Ir (M = Fe, Co) bonds in the 79e(-) clusters 6 and 8 show significant elongation in comparison with the Fe-Ir bonds in the 78e(-) cluster 5. Two different types of the bow-tie structures were observed for the 80e(-) clusters 7 and 9. Cluster 7 has a Z-shaped metal core with only two Ni-Ir bonds, while in cluster 9, the six metal-metal bonds in the bow-tie structure are retained with slight elongation of the Co-Ir bonds in comparison with the corresponding dication 6. The 81e(-) cluster 10 has two normal Ni-Ir bonds and one long Ni-Ir bonding interaction with the fourth nonbonding Ni-Ir contact. This structural variation is interpreted in terms of the total electron counts and molecular orbital calculations of the clusters.