Co-based molecular catalysts for efficient CO2 reduction via regulating spin states

A typical mode of CO2 activation is that d electrons at the d orbital of transition metals transfer to the unoccupied π* orbital of CO2. Thus the exploration of the relationship between d-electron behaviors and CO2 activation is of great importance. Herein, we demonstrate that high-spin state of 3d electrons in Co2+ facilitated the activation of CO2 over Co-salophen-X (X represents to Cl, Br, or I). Among these catalysts, Co-salophen-Br exhibited the highest Faradaic efficiency for CO. Notably, the Faradaic efficiency for CO over Co-salophen-Br reached 98.5 % at −0.70 V versus reversible hydrogen electrode, which was 1.5 and 1.2 times as high as those over Co-salophen-Cl (64.8 %) and Co-salophen-I (81.8 %), respectively. Density functional theory calculations revealed that high-spin state of Co sites decreased the reaction energy barrier for the formation of CO. Based on the analysis of electronic state, the ratio of high-spin state was 65.6 % for Co-salophen-Br, which was the highest among the three Co-based molecules. The Co sites with high-spin state promoted the electron transfer from high-energy 3d orbital (3dz2 and 3dx2-y2) of Co to the unoccupied π* orbital of CO2, improving catalytic performance.