Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene...

A molecular water oxidation catalyst based on the copper complex of general formula [(L_py)Cu^II]^2-, 2^2-, (L_py is 4-pyrenyl-1,2-phenylenebis(oxamidate) ligand) has been rationally designed and prepared to support a more extended π-conjugation through its structure in contrast with its homologue, the [(L)Cu^II]^2- water oxidation catalyst, 1^2- (L is o-phenylenebis(oxamidate)). The catalytic performance of both catalysts has been comparatively studied in homogeneous phase and in heterogeneous phase by π-stacking anchorage to graphene-based electrodes. In the homogeneous system, the electronic perturbation provided by the pyrene functionality translates into a 150 mV lower overpotential for 2^2- with respect to 1^2- and an impressive increase in the k_cat from 6 to 128 s^-1. Upon anchorage, π-stacking interactions with the graphene sheets provide further π-delocalization that improves the catalytic performance of both catalysts. In this sense, 2^2- turned out to be the most active catalyst due to the double influence of both the pyrene and the graphene, displaying an overpotential of 538 mV, a k_cat of 540 s^-1 and producing more than 5300 TONs.