The Catalytic Decarbonylation of Unstrained Ketones mediated by Platinum(II) Complexes

Cyclohexanone, an unstrained ketone, was found to undergo decarbonylation in the presence of soluble platinum(II) complexes of the general formula [Pt(tolpy)Cl(L)] (where tolpy is 2-(4-tolyl)pyridine and L is a neutral ligand) to afford the platinum carbonyl complex [Pt(tolpy)Cl(CO)] as well as carbon monoxide, methane and butane. As such, the activation of the cyclohexanone must proceed via the cleavage of three carbon–carbon bonds. However, the stoichiometric balance of the reaction required additional hydrogen, which implied a coupled transfer hydrogenation step. As part of a mechanistic investigation, a number of novel cycloplatinated complexes were prepared and characterised and their ability to catalyse the decarbonylation reactions was investigated. Many of them were identified as active catalyst precursors and, in particular, this was found to be true for [Pt(tolpy)Cl(CO)], suggesting that the reaction is catalytic. It was commonplace for reactions to be accompanied by decomposition to what was assumed to be colloidal platinum. In addition to cyclohexanone, a range of other carbonyl-containing substrates were investigated and examples of cyclic and acyclic ketones as well as aldehydes were found undergo decarbonylation under the conditions employed. A mechanistic investigation was undertaken involving in situ spectroscopic studies, dynamic light scattering, deuterium labelling and mercury poisoning experiments. A mechanism for the decarbonylation of cyclohexanone is proposed whereby fragmentation and transfer hydrogenation take place to afford acetaldehyde, which then undergoes decarbonylation to afford methane. For the family of complexes of formula [Pt(tolpy)Cl(L)] prepared to study the mechanism, a combination of spectroscopic and computational techniques were employed to study the structural and bonding properties of the complexes and the relative trans-influence of the ligands. An ordered series for the trans-influence of the ligands was identified using bond lengths obtained from analyses of single crystal X-ray data and this trend was also consistent with quantum chemical calculations. The trend was also analysed for possible correlations with chemical shifts and coupling constants obtained from 1H, 13C{1H}, 15N and 195Pt{1H} NMR spectroscopy.