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Montilla, F., E. Clara, T. Aviles, T. Casimiro, A. A. Ricardo, and M. N. da Ponte. "Transition-metal-mediated activation of arylisocyanates in supercritical carbon dioxide." J Organomet Chem. 626 (2001): 227-232. AbstractWebsite

The reactivity of arylisocyanates in supercritical carbon dioxide (scCO(2)) was studied using the easily available complexes CpCo(CO)(2), CpCoPPh3Me2 and Ni(cod)(2) as catalysts. A study of the solubility of the catalysts in scCO(2) was undertaken in all cases. The complex CpCo(CO)(2) is very soluble, 1.7 x 10(-1) mol kg(-1), while CpCoPPh3Me2 has a lower solubility, 7.2 x 10(-3) mol kg(-1), and Ni(cod)(2) is insoluble in scCO(2). For comparison purposes, the reactions were performed in parallel in scCO(2), using toluene as a solvent and just with the neat liquid arylisocyanate. Reactions in scCO(2) either do not take place at all, when CpCo(CO), is used as catalyst, or occur with low yields affording the trimer of the corresponding arylisocyanate when CpCoPPh3Me2 or Ni(cod)(2) act as catalysts. No incorporation of CO2 into the organic substrate was observed. Better conversions to triarylisocyanate were obtained when the reactions were performed by direct mixture of the liquid arylisocyanate ArNCO (Ar = Ph, p-CH3C6H4, p-CH3OC6H4) and the catalyst. Using toluene as a solvent, the yields of the trimers were lower than those obtained in neat arylisocyanate, and in some cases they were not formed at all. For instance in the reaction of CpCo(CO), and tolylisocyanate either under stoichiometric or catalytic conditions the trimer is not obtained, instead the compound H2R3N3C2O2 (R = CH3C6H4), was isolated in low yield. In the reaction of Ni(cod)(2)/PPh3 with phenylisocyanate, the trimer was formed but in low yield. The lower yields of the trimers observed when the reactions were performed in scCO(2) or in toluene, compared to that observed in neat arylisocyanates, indicates that the decrease in reactivity is due to a decrease in concentration. (C) 2001 Elsevier Science B.V. All rights reserved.

Montilla, F., V. Rosa, C. Prevett, T. Aviles, M. N. da Ponte, D. Masi, and C. Mealli. "Trimethylsilyl-substituted ligands as solubilizers of metal complexes in supercritical carbon dioxide." Dalton T (2003): 2170-2176. AbstractWebsite

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 (scCO(2)). This is verified from comparative solubility measurements of the species (eta(5)-Me-3 SiC5H4)MoO2 Cl, 1a, (eta(5)-Me3SiC5H4)(2)ZrCl2, 2a, and (eta(5)-Me3SiC5H4)Co(CO)I-2.0.5(I-2), 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 [(eta(5)-PhCH2CO2C5H4)Co(CO) I-2, 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 scCO(2) 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).