z Catalysis Effect of Quinoline Substitution on Water Oxidation by [Ru (Ql–tpy)(bpy)(OH 2 )](PF 6 ) 2 Jully Patel, Karunamay Majee, Manaswini Raj, Aditi Vatsa, Surabhi Rai, and Sumanta Kumar Padhi* [a] The new ligand Ql-tpy (8-(2,6-di(pyridin-2-yl)pyridin-4-yl) quinoline) and the corresponding aqua complex [Ru(Ql- tpy)(bpy)(OH 2 )](PF 6 ) 2 (1) have been synthesized and character- ized by the different spectroscopic methods. This complex shows two pK a values, pK a 1 due to the deprotonation from the protonated N atom located on the quinoline moiety at 3.0 and another pK a 2 at 10.5 due to the deprotonation from aqua ligand. The catalytic activity of the complex 1 towards water oxidation was studied in CF 3 SO 3 H solution (pH  1) using Ce IV as an oxidant. The free N atom of the substituted quinoline moiety of the complex gets protonated at pH  1 and acts as an electron withdrawing group instead of electron donating group. Due to the electron withdrawing effect it shows a lower catalytic activity towards chemical water oxidation having turnover number (TON) of 18 (out of 25) and initial turnover frequency (TOF) 0.003 s 1 as compared to the previously reported [Ru II (QCl-tpy)(bpy)(H 2 O)] 2 + complex. Introduction Nowadays the world economy is getting affected due to the depletion of the limited non-renewable fossil fuels. Therefore the development of a clean and environmental friendly non- fossil based fuel source is the biggest challenge for the scientists. [1] The inexhaustable solar energy is the most attractive alternative for the fossil based fuel source. [2] The oxygen evolving complex (OEC) present in the photosystem II of the green plants utilizes the solar energy to split water to generate O 2 , proton and electron (equation 1). [3] 2H 2 O ! O 2 þ 4H þ þ 4e  ðþ1:23 V vs: NHE, at pH ¼ 0Þ ð1Þ This activity of OEC provides a bottle neck to the scientists for the development of a suitable water oxidation catalyst (WOC), which converts solar energy into chemical energy. By using the transition metals several types of WOCs have been developed for the photo-catalytic water oxidation. [4(a)4(h)] But amongst them both dinuclear and mononuclear ruthenium complexes show higher catalytic activity towards water oxidation. Berlinguette and Yagi groups have reported a series of mononuclear ruthenium complexes having [Ru II (tpy)(bpy)(H 2 O)] 2 + scaffold, wherein they revealed that electron-withdrawing group suppresses the catalytic activity (e. g., TOF decreases), however increases the catalyst stability (TON). [5–8] Similar effects were also observed in a family of [Ru II (tpy-R)(phen-X)Cl] + complexes, where tpy-R is the tridentate 4’-(4-methylmercapto- phenyl)-2,2’:6’,2’’-terpyridine ligand, “phen” is 1,10-phenanthro- line, X is 5-NO 2 or 5,6-dimethyl or 3,4,7,8-tetramethyl substitu- ents. [9] Recently Llobet and group have also reported a series of symmetrical and unsymmetrical dinuclear complexes having general formula {[Ru(R 2 -tpy)(H 2 O)][Ru(R 3 -tpy)(H 2 O)](m-R 1 - bpp)} 3 + ,wherein the bridged bpp  ligand and tpy ligands are substituted by the electron donating and withdrawing groups at the 4-position (where R 1 ,R 2 and R 3 are the electron donating and withdrawing groups i. e. H, CH 3 , OMe, NO 2 and NH 2 ). [10(a)] They also suggested that electron withdrawing group de- creases the catalytic activity, however the electron donating groups don’t affect significantly to the catalytic activity. Sun et al. have reported the mononuclear ruthenium complexes [Ru (bda)(isoq) 2 ] and [Ru(bda)(pic) 2 ] (where H 2 bda = 2,2’-bipyridine- 6,6’-dicarboxylic acid; isoq = isoquinoline and pic is 4-picoline), which shows high catalytic activity (TOF 300 s 1 ) for water oxidation in the presence of Ce IV as an oxidant under acidic conditions. [10(b)] Due to the strong s donating carboxylate groups of the ligand, it increases the stability of the reactive species in high oxidation states and their catalytic activity towards chemical water oxidation is comparable with the activity of the OEC. Llobet et al. have introduced a new family of Ru complexes, including compound with general formula [Ru n (tda)(py) 2 ] m+ tda 2  being [2,2’:6’,2’’-terpyridine]-6,6’’-dicar- boxylate (where n = x and m = x-2; x = 2 to 4), which shows an impressive catalytic activity towards water oxidation under both neutral and alkaline conditions having an impressive TOF = 8000 s 1 at pH 7.0. [10(c)] Concepcion et al. recently studied that the nature of the ligand was found to be important in facilitating electron transfer from the metal centre to the sacrificial oxidant. [11] [a] J. Patel, K. Majee, M. Raj, A. Vatsa, S. Rai, Dr. S. K. Padhi Artificial Photosynthesis Laboratory Department of Applied Chemistry Indian Institute of Technology (Indian School of Mines) Dhanbad, India, 826004 E-mail: padhi.sk.ac@ismdhanbad.ac.in Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201700074 Full Papers DOI: 10.1002/slct.201700074 3053 ChemistrySelect 2017, 2, 3053 – 3059  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim