Trimethylsilyl-substituted ligands as solubilizers of metal complexes in supercritical carbon dioxide

Abstract

The SiMe₃ group (TMS), introduced as a substituent at the cyclopentadienyl ligand, is found to magnify the solubility of the corresponding metal complexes in supercritical carbon dioxide (scCO₂). This is verified from comparative solubility measurements of the species (η⁵-Me₃SiC₅H₄)MoO₂Cl, 1a, (η⁵-Me₃SiC₅H₄)₂ZrCl₂, 2a, and (η⁵-Me₃SiC₅H₄)Co(CO)I₂·0.5(I₂), 3a (newly synthesised), and of their unsubstituted precursors 1b–3b, respectively. In spite of the increased solubility, the chemical, structural and reactivity properties of the TMS derivatives are scarcely affected. Confirmation comes from a detailed study of the cobalt complex 3a that includes X-ray structural determination. The geometry is most similar to that of the precursor 3b while an apparently different Co–CO interaction is observed in the carboxylated analogue [(η⁵–PhCH₂CO₂C₅H₄)Co(CO)I₂, 3c]. The problem is computationally tackled by using the DFT B3LYP method. The optimised geometries of the simplified models of 3a–3c are all very similar. In particular, the computed stretching frequency of the unique CO ligand is consistent with the insignificant influence of the TMS group while it suggests a reduced amount of metal back-donation in 3c. It is inferred that the TMS complexes 1a–3a, while having higher solubility in scCO₂, maintain almost unaltered the electronic and chemical features of their parent compounds. In particular, the role of 1a–3a as catalysts, that is well documented for homogeneous solutions, remains unaltered in the very different scCO₂ environment. The assumption is experimentally validated for 1a by performing with the latter two classic catalytic processes. The first process is the oxidation of PPh₃ that is achieved by using molecular oxygen as an oxidant. The second example concerns the epoxidation of cyclohexene achieved in presence of tert-butyl hydroperoxide (TBHP).