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

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 pi-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 pi-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 kcat from 6 to 128 s(-1). Upon anchorage, pi-stacking interactions with the graphene sheets provide further pi-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.