Theoretical Study on the Degree of CO2 Activation in CO2-Coordinated Ni(0) Complexes.

Abstract

The geometrical characteristic and the degree of CO2 activation of the CO2-coordinated Ni(0) complexes were investigated computationally by quantum chemical means for bidentate and tridentate ligands of PP, PPMeP, and PNP, and sometimes with co-complexing Fe(II) to differently coordinate CO2. We show that the coordination geometry of the central metal is determined by the ligand geometry. The charge and the energy decomposition analyses show that the charge transfer energy through orbital mixing has a strong correlation with CO2 net charge, while the binding energy cannot due to the lack of the coordination number and the deformation energy of the ligand. Among the examined ligands, PNP with negatively charged secondary amine makes Ni(0) an electron-rich atom, which results in an ∼20% higher CO2 activation than those of PP and PPMeP. In particular, Fe(II)-PNP in the CO2-bridged diatomic complex enhances CO2 activation by another ∼20%, partly through the inductive effect of Fe(II), which pulls electron density from Ni-PNP across the CO2-bridge and partly by the backward donation from Fe(II)-PNP. Therefore, the present study encourages us to design a strongly electron-donating ligand and a CO2-bridged diatomic complex to develop more efficient homogeneous catalyst.