Mixed-Metal Cluster Chemistry. 13. Syntheses and Isomer Distribution Studies of Cp2W2Ir2(μ-CO)3(CO)7-n(PR3)n (n = 1, 2; R = Ph, Me): X-ray Crystal Structure of Cp2W2Ir2(μ-CO)3(CO)6(PPh3)
journal contributionposted on 21.06.1999, 00:00 by Susan M. Waterman, Mark G. Humphrey, Jeanne Lee, Graham E. Ball, David C. R. Hockless
Reactions of Cp2W2Ir2(CO)10 (1) with n equivalents of PPh3 and PMe3 (n = 1, 2) proceed in dichloromethane at room temperature to afford the clusters Cp2W2Ir2(μ-CO)3(CO)7-n(PPh3)n (n = 1 (2), 2 (3)) and Cp2W2Ir2(μ-CO)3(CO)7-n(PMe3)n (n = 1 (4), 2 (5)), respectively, in reasonable yields (38−63%). These products exhibit ligand fluxionality in solution, resolvable at low temperature into the constituent interconverting isomers. A structural study of one isomer of 2, namely 2a, reveals that the three edges of a WIr2 face of the tetrahedral core are spanned by bridging carbonyls and that the iridium-bound PPh3 ligates axially and the tungsten-bound cyclopentadienyl ligands coordinate apically and axially with respect to the plane of bridging carbonyls. Information from this crystal structure and 13C and 31P NMR spectral data have been employed to assign coordination geometries for the isomers of 2−5. NMR studies included development and use of 2D triple-resonance experiments with carbonyl cluster complexes; these 1H−31P−13C correlation experiments, designed to elucidate P−C coupling networks involving phosphorus and carbonyl ligands (a “HPCO experiment”), benefit from the sensitivity enhancement gained from using proton detection. All the isomers of clusters 2−5 contain a carbonyl-bridged WIr2 basal plane and an apical tungsten atom; ligands can be approximately coplanar (radial) to the basal plane or below the plane (axial). The configuration of 2a (axial phosphine, axial Cp, apical Cp), as assigned by NMR spectroscopy, is consistent with the structural determination. A second configuration (2b, 4b: axial phosphine, axial Cp, apical Cp) is the only one identified (within the temperature range 183−293 K) for the PMe3-substituted cluster 4. Two isomers, both with radial phosphine, axial phosphine, axial Cp, apical Cp configurations, have been identified for 5, namely 5a and 5b; it is likely that isomers of 3 adopt the same configurations. The WIr2-bridged form is seen exclusively across the isomers, indicating that the presence of two electropositive tungsten atoms may be polarizing the electron distribution toward the iridiums, although steric factors may also be involved.