Benzylidyne-capped group 9 trinuclear clusters:: synthesis, structure and properties of trirhodium and cobalt-rhodium mixed-metal clusters [Co3-nRhnCp3(μ3-CPh)2] (n=1, 2, 3)

The benzylidyne-capped trirhodium and cobalt-rhodium mixed-metal clusters [M3Cp3(mu(3)-CPh)(2)] (M-3:Rh-3, 1; CoRh2, 2; Co2Rh, 3) were synthesized. Complex 1 was prepared from [CpRh(CO)(2)] and diphenylacetylene. Complexes 2 and 3 were synthesized by using a mixture of [CpRh(CO)(2)] and [CpCo(CO)(2)] and separated with silica-gel column chromatography. The molecular structures of 1, 2 and 3 were determined. The average Rh-Rh distance (2.617(8) Angstrom) in 2 is nearly equal to that in 1 (2.620(13) Angstrom), and the average Co-Rh distance (2.503(16) Angstrom) in 2 is similar to the average of the average Co-Co (2.382(8) Angstrom) length in [Co3Cp3(mu(3)-CPh)(2)] (4) and the average Rh-Rh distance in 1. Clusters 2, 3 and 4 showed a chemically reversible one-electron oxidation and a chemically reversible one-electron reduction response in MeCN. For 1, a chemically reversible one-electron reduction and irreversible oxidation waves were observed in MeCN, whereas in CH2Cl2, two chemically reversible oxidation waves were observed. The oxidation and the reduction potentials shifted to more positive and more negative potential, respectively, with an increase of the number of rhodium atoms. The anionic radical of 3 was generated by the reaction with potassium metal in 2-methyltetrahydrofuran and was examined with ESR. The isotropic hyperfine coupling constant of the Co-59 nuclei of 3(-) was slightly smaller than that of 4(-). Geometry optimization of model complexes [M3Cp3(mu(3)-CH)(2)] (M-3: Rh-3, 1'; CoRh2, 2'; Co2Rh, 3') was carried out by a density functional theory (DFT) calculation. Assignments of low-energy UV-Vis absorption bands for 1 and 4 are proposed based on transitions of 1' and [Co3Cp3(mu(3)-CH)(2)] (4) calculated by time-dependent DFT. (C) 2003 Elsevier Ltd. All rights reserved.