Mononuclear Ruthenium-Water Oxidation Catalysts: Discerning between Electronic and Hydrogen-Bonding Effects Somnath Maji, † Isidoro Ló pez, † Fernando Bozoglian, † J. Benet-Buchholz, † and Antoni Llobet* ,†,‡ † Institute of Chemical Research of Catalonia, Avinguda Països Catalans 16, 43007 Tarragona, Spain ‡ Departament de Química, Universitat Autò noma de Barcelona, Cerdanyola del Valle ̀ s, 08193 Barcelona, Spain * S Supporting Information ABSTRACT: New mononuculear complexes of the general formula [Ru(trpy)(n,n′-F 2 -bpy)X] m+ [n = n′ = 5, X = Cl (3 + ) and H 2 O(5 2+ ); n = n′ = 6, X = Cl (4 + ) and H 2 O (6 2+ ); trpy is 2,2′:6′:2″-terpyridine] have been prepared and thoroughly characterized. The 5,5′- and 6,6′-F 2 -bpy ligands allow one to exert a remote electronic perturbation to the ruthenium metal center, which affects the combination of species involved in the catalytic cycle. Additionally, 6,6′-F 2 -bpy also allows through-space inter- action with the Ru-O moiety of the complex via hydrogen interaction, which also affects the stability of the different species involved in the catalytic cycle. The combination of both effects has a strong impact on the kinetics of the catalytic process, as observed through manometric monitoring. S ince the discovery by Zong and Thummel 1 that mononuclear ruthenium complexes were also active as water oxidation catalysts, there has been a large development in the field based on these types of complexes. In 2008, Meyer et al. 2 offered a mechanistic description of how the water oxidation occurred at a molecular level, where the O-O bond formation is proposed to occur based on the water nucleophilic attack pathway. This description has now been adopted to many mononuclear ruthenium complexes but also to those of iridium and other first-row transition metals, where water oxidation catalysis is claimed to proceed in a molecular manner. 3 Later on, Berlinguette and co-workers studied the strong influence that electronic perturbation of the metal center exerted through remote positions of the ligands over the whole water oxidation catalysis process. 4 Recent reports by Yagi and Fujita have shown how the presence of a nitrogen lone pair can influence the reactivity in isomeric 2-(2-pyridyl)-1,8-naphthyr- idine complexes. 5 In order to evaluate the electronic and hydrogen-bonding effects individually, we have designed complexes containing ligands that allow through-space interaction with the active RuOH 2 entourage in mononuclear complexes, in combination with others that only exert remote electronic perturbation. In the present paper, we report a new family of complexes of the general formula [Ru(trpy)(n,n′-F 2 -bpy)X] m+ [n = n′ = 5, X = Cl (3 + ) and H 2 O(5 2+ ); n = n′ = 6, X = Cl (4 + ) and H 2 O (6 2+ )] that allow us to discern and quantify the electronic and hydrogen-bonding effects. Additionally, we report their activity as water oxidation catalysts and compare them with the reference complex [Ru(tpry)(bpy)OH 2 ] 2+ (2 2+ ). 6 The synthetic strategy followed for the synthesis of complexes 3-6 uses [RuCl 3 (tpry)] (1) as the starting material and is similar to the one used for the preparation of 2 2+ . Synthetic details, together with a complete structural and spectroscopic characterization, are presented as Supporting Information (SI). An ORTEP plot of the X-ray structure of 6 2+ is given in Figure 1, whereas that of 5 2+ is presented as SI. In both cases, the Ru center presents an octahedrally distorted geometry around the metal center, and the bond distances and angles are unremarkable except for the hydrogen-interaction of the F atom with the aqua group in 6 2+ . This interaction is also responsible for rotation of one of the pyridyl groups of bpy, generating a dihedral angle of 11.6°, needed to be able to accommodate the F atom of the bpy ligand so close to the aqua group. In turn, this close and rigid interaction will ensure that all of the potential species that can be generated along the catalytic cycle will have an interaction with this group. The redox properties of complexes 5 2+ and 6 2+ were investigated with cyclic voltammetry and differential pulse voltammetry experiments in water at different pH values and are reported in Table 1, Figure 2, and the SI. The presence of the F substituent at the bpy ligand has a dramatic influence on the electronic structure of the metal center in the sense that for these Received: December 25, 2012 Published: March 21, 2013 Figure 1. ORTEP plot (50% probability) of the crystal structure of complex 6 2+ . Color code: Ru, cyan; N, navy blue; F, green; O, red; H, blue empty circles. Interesting metric parameters: d(H2W-F1B) = 2.32 Å; d(F1B-O1W) = 2.63 Å; ∠(O1W-H2W-F1B) = 100.7°; the dihedral angle between the pyridyl moieties of 6,6′-F 2 -bpy is 11.6°. Communication pubs.acs.org/IC © 2013 American Chemical Society 3591 dx.doi.org/10.1021/ic3028176 | Inorg. Chem. 2013, 52, 3591-3593