Thermolysis of the tetrahedrane cluster PhCCo3(CO)3(μ-CO)Cp2 with the unsaturated diphosphine ligands (Z)-Ph2PCH=CHPPh2 and 4,5-Bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd): Redox properties and molecular structure of PhCCo3(CO)(μ-CO)[(Z)-Ph2PCH=CHPPh2]Cp2

Abstract

Thermolysis of the tricobalt cluster PhCCo3(CO)3(μ-CO)Cp2 (1) with the diphosphine ligands (Z)-Ph2PCH=CHPPh2 and 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) has been examined and found to give the diphosphine-substituted clusters PhCCo3(CO)[(Z)-Ph2PCH=CHPPh2](μ-CO)Cp2 (2) and PhCCo3(CO)(bpcd)(μ-CO)Cp2 (3) in moderate yield. The new compounds 2 and 3 have been isolated and characterized in solution by IR and NMR (1H and 31P) spectroscopies. VT 31P NMR data reveal that the chelating diphosphine ligand is fluxional in solution and exhibits a rocking motion between the axial and equatorial sites that renders both phosphorus moieties identical at ambient temperature. The molecular structure of PhCCo3(CO)[(Z)-Ph2PCH=CHPPh2](μ-CO)Cp2 (2) has been determined by X-ray crystallography. PhCCo3(CO)[(Z)-Ph2PCH=CHPPh2](μ-CO)Cp2 crystallizes, as the CH2Cl2 solvate, in the monoclinic space P21/n, a = 16.822(2) A, b = 10.554(1) A, c = 23.135(3) A, β = 100.944(2)∘, V = 4032.4(8) A3, Z = 4, and dcalc = 1.537 Mg/m3; R = 0.0488, Rw = 0.0725 for 9431 reflections with I > 2σ(I). The solid-state structure of cluster 2 establishes the chelating nature of the ancillary (Z)-Ph2PCH=CHPPh2 ligand at the unique Co(CO)P2 center via coordination at an equatorial and an axial site. The redox behavior of clusters 2 and 3 has been explored by cyclic voltammetry and chronocoulometry. Each cluster reveals the presence of two one-electron oxidations of common origin due to the oxidation of a Co–Co bonding orbital. Whereas cluster 2 does not exhibit an accessible reduction process in CH2Cl2, a ligand-based one-electron reduction was found for cluster 3 given its low-lying π* LUMO associated with the bpcd ligand. The electrochemical data for clusters 2 and 3 are discussed with respect to the reported redox chemistry for this genre of tricobalt cluster and the bpcd ligand.