Ligand bridging and chelation of tetracobalt clusters with difunctional phosphines

Article abstract of DOI:10.1021/om00145a006

The interaction of the tetracobalt carbonyl cluster Co₄(CO)₁₀(μ₄-PPh)₂ (I) with various bidentate phosphines such as Ph₂P(CH₂)_nPPh₂ was examined under both thermal conditions at >80°C and electron-transfer chain (ETC) catalysis at 25°C. With diphosphines possessing short carbon backbones (n = 1 and 2), the disubstituted clusters Co₄(CO)₈(PPh)₂(P P)(III) were isolated in high yields under electron-transfer chain catalysis and shown to be the same as those derived in low yields from the thermal reaction. Structure elucidation by X-ray crystallography has established two modes of ligand binding to the tetracobalt cluster, in which the diphosphine either bridged a pair of adjacent cobalt atoms or chelated a single cobalt atom. The dynamic properties of such bridged and chelated clusters in solution were deduced from the temperature-dependent ³¹P and ¹³C NMR spectra of the diphosphine and carbonyl (terminal and bridging) ligands, respectively. The higher diphosphine homologues with n = 3 and 4 afforded mono-and disubstituted clusters, the relative amounts of which depended on whether the substitutions were carried out by ETC stimulation or under thermal conditions.