{[Ru(bda)]xLy}n cross-linked coordination polymers: toward efficient heterogeneous catalysis for water oxidation in an organic solvent-free system

Recently, the development of polymeric catalysts for water splitting has received an increasing amount of attention. In this study, we successfully developed a few novel cross-linked coordination polymers (CCPs) with the formula {[Ru(bda)](x)L-y}(n) as efficient heterogeneous catalysts for water oxidation in an organic solvent-free system, where Ru(bda) represents the catalytic center. Detailed water oxidation catalytic kinetics studies suggested that single-site water nucleophilic attack (WNA) is the general mechanism applied to these polymeric catalysts, which is different to the small-molecular reference, [Ru(bda)(pic)(2)] (pic = 4-picoline). The experimental evidence also indicated the importance of interfacial wettability and the existence of the Ru(bda)-macrocyclic fragments in the polymer network in determining the overall catalytic activity. More interestingly, end-capping modification via the pyridine/DMSO exchange reaction further removed the residual Ru(DMSO)(x) moieties on the surfaces of the polymer network, which leads to the improved performance with an impressive TOF of similar to 4.6 s(-1) and TON of similar to 750 in an organic solvent-free system, which are superior to [ Ru(bda)(pic)(2)]. The rate of catalysis is among the highest for a heterogeneous system reported to date. An electrochemical study showed the polymeric catalysts were also promising electrode materials for electrocatalytic water oxidation and an electrode based on CCP/Nafion/ITO maintained similar to 73% of its initial activity after 27 cycles under the optimal conditions.