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)

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 indium-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 indiums, although steric factors may also be involved.