The synthesis of a library of molecular water oxidation catalysts based on the Co complex of tris(2-benzimidazolylmethyl)amine is described. Hydrophobicity was identified as the key variable in mediating the catalytic competence of the complexes. The change in this parameter correlates with both the conformational mobility of the ligand core and the structural changes in the local solvent environment around the metal site. The optimal Co complex identified is hydrophobic, because of three semifluorinated side chains. It catalyzes water electro-oxidation efficiently at neutral pH, with an overpotential of 390 mV and a turnover frequency (TOF) of 1.83 s −1 in the absence of soluble Co salts. The catalyst can be immobilized through physisorption, and it remains stable in prolonged electrolysis experiments. D evelopment of an easy means of photo/electrocatalytic water splitting is one of the main barriers to establishing a solar hydrogen economy. 1,2 Of the two half-reactions involved in splitting water into O 2 and H 2 , the water oxidation reaction (WOR) presents the greatest challenge, because of its mechanistic complexity. 3−5 A practical WOR catalyst must be highly active, yet inexpensive and indefinitely stable under harsh oxidative conditions. Several catalytic systems for water oxidation have been devised, incorporating either platinum group 6−13 or base metals. 11,14−22 Because of cost considerations, the latter are more suitable for practical applications. Heterogeneous catalysts based on metal-oxide phases are stable and readily accessible. 11,15,23−25 However, characterization, mechanistic studies, and rational tuning of properties remain a challenge for these inorganic phases. In contrast, molecular metalcomplex catalysts can be tailored using the full arsenal of organic chemistry. A mechanistic understanding, which is essential for the rational design of catalysts, is easier to achieve for these systems. 22,26−31 To date, application of molecular catalysts for WOR has been restricted by their synthetic complexity and limited stability under WOR conditions. 32−34 Decomposition of molecular species sometimes yields catalytically active oxide phases, 23,24,34−36 which may complicate the identification of the operational catalyst. 34,35 Recently, we reported on water electro-oxidation catalyzed by hydrophobic, insoluble perfluorinated cobalt phthalocyanine (CoFPc) physisorbed on fluorine-doped tin oxide (FTO). 37 This molecular catalyst is exceptionally active. Importantly, the stability of "fluorine-armored" CoFPc is vastly superior to that of previously described hydrophobic molecular catalysts, 38,39 and it is comparable to a particulate cobalt phosphate system (CoPi). 15 However, the inflexible and inert nature of the FPc ligand precluded an in-depth exploration of structure−activity relationships or improvement of the catalytic activity.Here, we describe a library approach to molecular catalysts for WOR based on the cobalt complex of tris(2-benzimidazolylmethyl)amine, (BimH) 3 . 40−42 This parent stru...