Rhodium complexes modified by simple trialkylphosphines can be used to carry out homogeneous hydroformylation in supercritical carbon dioxide (scCO₂). The catalyst derived from PEt₃ is more active and slightly more selective for the linear products in scCO₂ than in toluene, and under the same reaction conditions [100°C, 40 bar of CO/H₂ (1∶1)] P(OPrⁱ)₃ is also an effective ligand giving good catalyst solubility and activity. Other ligands such as PPh₃, POct₃, PCy₃, and P(4-C₆H₄Bu^t)₃ are less effective because of the low solubility of their rhodium complexes in scCO₂. P(4-C₆H₄SiMe₃)_nPh_3−n (n = 3 or 1) and P(OPh)₃ impart activity despite their complexes only being poorly soluble in scCO₂. Under subcritical conditions, using PEt₃ as the ligand, C7-alcohols from hydrogenation of the first formed aldehydes are the main products whilst above a total pressure of 200 bar, where the solution remains supercritical (monophasic) throughout the reaction, aldehydes are obtained with 97% selectivity. High pressure IR studies in scCO₂ using PEt₃ as the ligand are reported.

[Mo(O)(O₂)₂(L)₂] compounds (L = pz, pyrazole; dmpz, 3,5-dimethylpyrazole) were reacted stoichiometrically, in the absence of an oxidant, with cis-cyclooctene in an ionic liquid medium where selective formation of the corresponding epoxide was observed. However, this oxo-transfer reaction was not observed for some other olefins, suggesting that alternative reaction pathways exist for these epoxidation processes. Subsequently, DFT studies investigating the oxodiperoxomolybdenum catalysed epoxidation model reaction for ethylene with hydrogen peroxide oxidant were performed. The well known Sharpless mechanism was first analysed for the [Mo(O)(O₂)₂(dmpz)₂] model catalyst and a low energy reaction pathway was found, which fits well with the observed experimental results for cis-cyclooctene. The structural parameters of the computed dioxoperoxo intermediate [Mo(O)₂(O₂)(dmpz)₂] in the Sharpless mechanism compare well with those found for the same moiety within the [Mo₄O₁₆(dmpz)₆] complex, for which the full X-ray report is presented here. A second mechanism for the model epoxidation reaction was theoretically investigated in order to clarify why some olefins, which do not react stoichiometrically in the absence of an oxidant, showed low level conversions in catalytic conditions. A Thiel-type mechanism, in which the oxidant activation occurs prior to the oxo-transfer step, was considered. The olefin attack of the hydroperoxide ligand formed upon activation of hydrogen peroxide with the [Mo(O)(O₂)₂(dmpz)₂] model catalyst was not possible to model. The presence of two dmpz ligands coordinated to the molybdenum centre prevented the olefin attack for steric reasons. However, a low energy reaction pathway was identified for the [Mo(O)(O₂)₂(dmpz)] catalyst, which can be formed from [Mo(O)₂(O₂)(dmpz)₂] by ligand dissociation. Both mechanisms, Sharpless- and Thiel-type, were found to display comparable energy barriers and both are accessible alternative pathways in the oxodiperoxomolybdenum catalysed olefin epoxidation. Additionally, the molecular structures of [Mo(O)(O₂)₂(H₂O)(pz)] and [Hdmpz]₄[Mo₈O₂₂(O₂)₄(dmpz)₂]·2H₂O and the full X-ray report of [Mo(O)(O₂)₂(pz)₂] are also presented.

The complex [Mo₄O₁₆(dmpz)₆] (1) has been isolated as part of a study of oxodiperoxomolybdenum catalysed epoxidation of olefin substrates with hydrogen peroxide in ionic liquids. Notably, 1 is the first dioxoperoxomolybdenum species to be structurally characterised.