Five mononuclear RuII complexes supported by two pentadentate polypyridyl ligands, N,N‐bis(2‐pyridylmethyl)‐N‐(bis‐2‐pyridylmethyl)amine (N4Py) {[Ru(N4Py)(Cl)](PF6), 1Cl; [Ru(N4Py)(OH2)](PF6)2, 1Aq} and newly designed N‐benzyl‐N‐((6‐(6‐methylpyridin‐2‐yl)pyridin‐2‐yl)methyl)dipyridin‐2‐yl‐methanamine (N2Py‐MeBpy‐Bz) {[Ru(N2Py‐MeBpy‐Bz)(Cl)](PF6)·MeCN, 2Cl·MeCN; [Ru(N2Py‐MeBpy‐Bz)(OH2)](PF6)2·3H2O·MeOH, 2Aq·3H2O·MeOH and [Ru(N2Py‐MeBpy‐Bz)(MeCN)](PF6)2·0.5MeCN·H2O, 2ACN·0.5MeCN·H2O} were synthesized and characterized using different spectroscopic techniques such as UV/Vis, IR, 1D and 2D NMR spectroscopy, and mass spectrometry. The physicochemical properties of complexes 1Cl and 1Aq, and structural analysis of 1Aq were reported by Kojima and co‐workers (Chem. Sci. 2012, 3, 3421–3431). Structural characterizations of 1Cl, 2Cl·MeCN, and 2ACN·0.5MeCN·H2O were done by using single‐crystal X‐ray diffraction analyses. Catalytic water oxidation activities of aqua‐ligated RuII complexes, using CeIV as sacrificial electron acceptor at pH 1, were examined. Complex 2Aq shows higher activity as compared to 1Aq. Electrochemical study suggests that a formal [RuVI=O]4+ species is the active species which triggers the oxidation of water. Mechanistic investigation reveals that O–O bond formation takes place via water nucleophilic attack (WNA) pathway. The deactivation pathway of catalyst 2Aq has also been investigated. It was observed that complex 2Aq lost its water oxidation activity primarily due to ligand degradation via oxidative N‐debenzylation pathway.

The Cover Feature shows an aqua‐coordinated mononuclear ruthenium complex with a pentadentate ligand scaffold exhibiting oxidation of water, using cerium(IV) as sacrificial electron acceptor at pH 1. The picture also highlights an initial side reaction involving oxidative N‐debenzylation, which leads to deactivation of the catalyst. More information can be found in the https://doi.org/10.1002/ejic.201801099