Rationalizing photo-triggered hydrogen evolution using polypyridine cobalt complexes: substituent effects on hexadentate chelating ligands.

We report four novel polypyridine cobalt(II) complexes based on a hexadentate ligand scaffold bearing either electron withdrawing (-CF 3 ) or electron donating (-OCH 3 ) groups in different positions of the ligand. Experiments and theoretical calculations have been combined to perform a systematic investigation of the effect of the ligand modification on the light-driven catalytic activity. The results indicate that the position, rather than the type of substituent, is the dominating factor in promoting efficient hydrogen evolution. The best performances are observed upon introduction of substituents on the pyridine moiety of the hexadentate ligand, which promotes the formation of the Co(II)H intermediate via intramolecular proton transfer reactions with low activation energy. Quantum yields of 11.3% and of 10.1%, maximum turnover frequencies of 86.1 min -1 and 76.6 min -1 , and maximum turnover numbers of 5520 and 4043 were obtained respectively with a -OCH 3 and a -CF 3 substituent.