Photo-isomerisation of alkenyl complexes of platinum(II): structural, spectroscopic, kinetic and computational...

In this work UVA and blue light have been used to study photo-isomerisation about the C=C double bond in complexes of the type [PtCl(-CH=CHAr)(tmeda)] [Ar = C6H5, (E)-2a; 4-CH3O-C6H4, (E)-2b; 3-NO2-C6H4, (E)-2c; and 3-CH3O-C6H4, (E)-2d]. The progress of the reaction has been monitored by NMR spectroscopy following irradiation of the NMR sample. The NMR data have been complemented with X-ray diffractometric analysis of compounds (E)-2a-c and (Z)-2a. The kinetic data clearly indicate that a monomolecular mechanism is operating with the energy of the irradiating light influencing the rate of isomerisation but not the equilibrium composition, which is only slightly in favour of the Z isomer. DFT and TD-DFT theoretical investigations have been carried out to elucidate the nature of the main electronic transitions in the UV-Vis region and the mechanism of the photo-isomerisation reaction appears to proceed through a C=C bond twist process similar to that involved in purely organic molecules such as stilbene. In the Z isomer, one ortho proton of the phenyl group can come close to platinum (Pt···H(ortho) distance of 2.632 Å in (Z)-2a). In the case of 2c, the difference in chemical shift between the two ortho protons varies from 3.30 ppm in the Z isomer, where interaction with Pt is possible, to 0.60 ppm in the E isomer, where such interaction cannot take place. The analysis of the DFT orbitals indicates that the most shifted H(ortho) is that with a greater positive charge, pointing to an H-bond type of interaction.