Chemistry of Iridium Carbonyl Cluster Complexes. Synthesis and X-ray Crystal Structure of [PPh4]2[Ir12(µ-CO)5(CO)19], Containing an Unprecedented Metal Skeleton

The new dodecanuclear carbonyl cluster [Ir12(CO)24]2− can be prepared in good yields by thermal treatment of [Ir6(CO)15{Cu(NCMe)}]− in refluxing tetrahydrofuran: the anion has been characterized as the tetraphenylphos-phonium salt by a crystallographic study. [PPh4]2[Ir12(CO)24] crystallizes in the triclinic space group P1 with cell constants a = 14.614(5) Å, b = 25.435(6) Å, c = 11.977(5) Å, α = 90.30(3)°, β = 112.94(3)°, γ = 104.33(2)°, V = 3946(5) Å3, and Z = 2. Data were collected at room temperature to a maximum 2θ = 46°, giving 10 311 unique reflections. The structure was solved by direct methods. The final discrepancy indices were R = 0.050 and Rw = 0.057 for 5562 independent reflections with I >3σ(I). The metal framework of the anion is a v2 (14-atom) trigonal bipyramid, lacking one apical and one equatorial vertex. The Ir-Ir distances are scattered in the range 2.654-(1)–2.939(1) Å. Average bond distances for Ir-Cterminal, Ir-Cbridging, C-Oterminal, and C-Obridging are 1.82, 2.05, 1.19, and 1.19 Å, respectively. Electrochemistry shows that the dianion undergoes, in acetonitrile solution, a series of single-stepped two-electron transfers encompassing the sequence 2+/0/2−/4−/6−, mostly leading to transient congeners. Only the dianion/tetraanion redox change appears to be chemically reversible, but its electrochemical features suggest that [Ir12(CO)24]2− must experience severe stereochemical reorganization upon the addition of two electrons. © 1993, American Chemical Society. All rights reserved.