Synthesis, structure, and formation mechanism of the halogen-bridged tricobalt clusters, [Co3Cp3(μ3-CPh)2(μ-X)]+ (X=Cl, Br, and I)

Reactions of a benzylidyne-capped tricobalt cluster, [Co3Cp3(μ3-CPh)2] (1), with halogens (X2 = Cl2, Br2, and I2) in CH2Cl2 afforded halogen-adducts of 1. The structure of four isolated salts [Co3Cp3(μ3-CPh)2(μ-Cl)]PF6 · MeCN (2PF6 · MeCN), [Co3Cp3(μ3-CPh)2(μ-Br)]SbF6 (3SbF6), [Co3Cp3(μ3-CPh)2(μ-I)]SbF6 · CH2Cl2 (4SbF6 · CH2Cl2), and [Co3Cp3(μ3-CPh)2(μ-I)]I3 (4I3) determined by X-ray diffraction can be regarded formally as halide-adducts of 12+. The halogen atom in each structure lies in the Co3 plane. The halogen-bridged Co–Co edge was elongated (in 2PF6 · MeCN = 2.6072(4), in 3SbF6 = 2.6106(7), in 4SbF6 · CH2Cl2 = 2.622(2), and in 4I3=2.6718(9) Å), and the Co–Co distances that had no halogen-bridge remained unchanged from the Co–Co distance of 1 (2.382(8) Å), (in 2PF6=2.4037(8) and 2.3948(7), in 3SbF6=2.3888(6) and 2.4017(7), in 4SbF6 · CH2Cl2 = 2.393(2) and 2.388(1), and in 4I3 = 2.397(1) and 2.3868(9) Å). The UV–Vis absorption spectra of 2+, 3+, and 4+ had characteristic absorption peaks at 796, 819, and 844 nm, respectively. Cyclic voltammograms of 2PF6 in CH2Cl2 with 0.1 M nBu4NPF6 as the supporting electrolyte showed a chemically reversible oxidation (at a potential of 0.75 V versus Fc/Fc+), and an irreversible reduction wave at −0.57 V. The irreversible reduction resulted in the recovery of 1. The redox properties of 3+ and 4+ are very similar to that of 2+. Cyclic voltammetry of 1 in 0.1 M nBu4NCl/MeCN indicates that the formation of 2+ is a multi-step reaction. Initially, 1 is oxidized to 1+, and then, 1+ is coordinated by Cl− followed by immediate oxidation to 2+.