Water oxidation has been considered as the key step in water splitting. It involves the transfer of four protons, four electrons and the formation of an O-O bond. The development of robust and efficient water oxidation catalysts is challenging but significant to achieve promising energy conversion and storage. The molecular water oxidation catalysts [Ru(bda)(pic)2] and [Ru(bda)(isq)2] (bdaH2 is 6,6’-dicarboxylic-2,2’-bipyridine; pic is 4-picoline; isq is isoquinoline), reported by Sun’s group, show high activities towards water oxidation, especially the latter due to π stacking, which can stabilize the O-O coupling transition state. Many modifications have been made to this kind of catalysts by introducing functional groups to both the bda2- backbone and aromatic axil ligands; however, these groups are always with different electronic and hydrophobic properties and no clear trend is established. The comparisons between the catalysts are oftentimes operating under different rate limiting steps, which lead to misleading interpretation of the data. This work systematically studied the influence of halogen atoms to the catalysis activity of [Ru(bda)(L)2]-type catalysts. We synthesized two series of [Ru(bda)(4-X-py)2] and [Ru(bda)(6-X-isq)2] (X is H, F, Cl, Br, I, OMe) complexes. Electrochemistry, Ce(IV)-driven water oxidation catalysis, stop-flow kinetics combined with DFT calculations were examined systematically of all the catalysts. We find the main contributor to the free energy of activation is entropy due to the highly organized transition states. We also find it is the electron-donating, not electron-withdrawing substituents, that decrease free energies of activation for bimolecular O-O coupling step. These substituents also affect the activation barrier through their contributions to the non-covalent interactions between the axial ligands. Based on our calculations, the catalysts with lower ΔG‡ are [Ru(bda)(4-I-py)2] and [Ru(bda)(6,7-(OMe)2-isq)2] for the pyridine and isoquinoline families, respectively. Our experimental results corroborated these predictions: the turnover frequency for [Ru(bda)(4-I-py)2] (330 s-1) is a ten-fold enhancement with respect to that of [Ru(bda)(py)2] and the turnover frequency for [Ru(bda)(6-OMe-isq)2] reaches values of 1300 s-1, faster than the oxygen-evolving complex.

O-O coupling: From detailed mechanistic understanding to enhanced water oxidation catalysis

254th National Meeting (2017)

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