Structure-Activity Relationship To Screen Ni-Bisphosphine Complexes for the Oxidative Coupling of CO2 and Ethylene

Density functional theory calculations have been used to investigate competition between inner and outer-sphere reaction pathways in the oxidative coupling of CO2 and ethylene for a set of 12 Ni-bisphosphine complexes, in order to build a QSAR approach correlating catalyst structure to calculated energy barriers for CO2 activation. The ligands were selected to explore different substituents on the P atoms (cyclohexyl, phenyl, and tert-butyl) and different lengths of the tether connecting the P atoms, -(CH2)(n) - with n = 1-3. As expected, the conclusion is that the inner-sphere reaction pathway is favored with unhindered ligands, while the outer -sphere reaction pathway is favored with hindered ligands. To find a possible correlation with molecular descriptors, we started using the buried volume as a steric descriptor. A reasonable correlation could be found for the energy barrier along the inner -sphere pathway, while scarce correlation was found for the energy barrier along the outer-sphere pathway, indicating that the steric bulkiness of the ligand disfavors approach of CO2 to the metal center. Much stronger correlation between the ligand structure and the energy barrier along the inner-sphere pathway was achieved when the steric descriptor was augmented by an electronic descriptor, consisting of the partial charge on the Ni atom. The much better correlation suggests that bisphosphine ligands have a non-negligible electronic impact on the catalyst performance.