An unexpected and robust nickel (II)-based ether-bonded homogeneous system: Effective for photocatalytic CO2-to-CO transformation in visible light

Three halogen-containing pyridine carboxylic acid ligands (6-fluoropicolinic acid, 6-chloropicolinic acid, and 6-bromopicolinic acid) were combined with nickel to form three types of complexes: Ni-O ((6,6′-oxydipicoline acid) Ni(H2O)2), Ni-(OH)2((6,6′-hydroxydipicoline acid) Ni(H2O)2) and Ni-Br2((6,6′-bromodipicoline acid) Ni(H2O)2). Interestingly, the nickel salt reacted with 6-fluoropicolinic acid to form the ether-type complex Ni-O, with 6-chloropicolinic acid to form the hydroxy-type complex Ni-(OH)2, and with 6-bromopicolinic acid to form a Ni-Br2 complex with unchanged ligands. The Ni-O and Ni-(OH)2 complexes are novel in structure and especially the Ni-O show an excellent catalytic performance on reduction of CO2 to CO. After 12 h of irradiation, the Ni-O complexes attained a CO turnover number (TON) of 5664.3 and 86.7 % selectivity. The configurations of the complex catalysts have been demonstrated to play a major role in photocatalysis, by integrating analysis of high resolution mass spectrometry (HRMS) as well as density-functional theory (DFT). A potential proton-coupled electron transfer (PCET) mechanism of two-stage photocatalytic process is proposed.