Catalytic water oxidation by a single site [Ru(Fc-tpy)(bpy)OH2]2+ complex and it’s mechanistic study

Abstract

The mononuclear complex [Ru(Fc-tpy)(bpy)Cl]PF6 (Ru-Cl) and its corresponding aqua [Ru(Fc-tpy)(bpy)OH2](PF6)2 (Ru-OH2) complex (Fc-tpy = 4′-(2-ferrocenyl)-2,2′:6′2″-terpyridine) have been synthesized and characterized by 1H NMR, UV–Vis spectroscopy, Mass Spectrometry and the structure of the complex (Ru-Cl) was confirmed by Single Crystal X-Ray Crystallographic studies. Under strong acidic conditions the Fe(II) centre of ferrocene moiety (Fc) undergoes protonation to generate Fe(IV)hydrido species Fe(IV)H (FcH+). The Ru-OH2 complex shows one pKa value at 10.0 due to deprotonation of the aqua ligand. The resting potential of Ru-OH2 complex in pH 1.62 phosphate buffer indicates that the Fe center in ferrocene is in FeIII state, and the Ru center is in RuIII state. This complex acts as an efficient water oxidation catalyst at pH = 1.0 in triflic acid using ceric ammonium nitrate (CAN) as a sacrificial electron acceptor. The complex exhibits a turnover number (TON) of 12, whereas parent [Ru(tpy)(bpy)(OH2)]2+ complex exhibits a TON of 10 under the similar conditions. The presence of ferrocenyl group coupled with terpyridine has the potential to act as an electron donor, and can stabilize the higher oxidation state of ruthenium complex required towards chemical water oxidation. The rate of evolution of O2Ru-OH2 is much higher as compared to parent [Ru(tpy)(bpy)(OH2)]2+ complex with 230 mV of lower onset potential. The rate of O2 evolution is 1st order with respect to the catalyst as well as oxidant (CAN) concentration. The species distribution path involves [Fc+-RuII-OH2]3+, [Fc+-RuV=O]4+, [Fc+-RuVI=O]5+, [Fc+-RuIV-O-OH]4+, [Fc+-RuV-O-O]4+, [Fc+-RuIII-OH2]4+and [Fc+-RuIII-OH]3+. The rate of O⋯O bond formation (kO⋯O), interaction of [RuIV-O-OH]2+ with Ce4+ and O2 evolution (kO2) indicates that the interaction step is the rate-determining step, where k = 6.0 × 10−4 M−1 s−1.