The presence of the strong electron-donating carboxylate group and oxo group in the Mn4CaO5 complex of OEC plays an essential role in stabilizing the higher oxidation states of Mn and reducing the redox potential. The activity and structural arrangement of this complex motivated our synthesis of a new family of water oxidation catalysts (WOCs) by introducing a strong electron-donating motif to the ligand. Here, we report two complexes [Ru(tpy)(pic)2(OH2)](NO3)2 (1) and [Ru(EtO-tpy)(pic)2(OH2)](NO3)2 (2) (where tpy = 2,2':6',2''-terpyridine, EtO-tpy = 4'-(ethoxy)-2,2':6',2''-terpyridine, pic = 4-picoline), which have been synthesized and thoroughly characterized by NMR, UV Spectroscopy, EPR, and electrochemical analysis. Both complexes have been successfully used as catalysts for chemical water oxidation in presence of excess sacrificial oxidant ceriumIV ammonium nitrate (CAN) at pH = 1, 0.1M HNO3 solution. O2 evolution was measured by using Clark electrode in aerobic conditions. Electronic modification of the tpy ligand of 2 with the ethoxy group at the 4- position makes the catalyst ~5 times more active than the previously reported complex 1. O2 evolution rate was determined as 2nd order with respect to the catalyst and 1st order with respect to CAN concentration. The electrochemical and O2 evolution studies reveal that the electron-donating ethoxy group in 2 is more catalytically active than 1, as well as many highly studied mononuclear complexes. 1, 2

Molecular catalytic approach based on mononuclear Ru(II) complexes towards water oxidation

ACS Fall 2021

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