A Tetranuclear Cobalt (II) phosphate Possessing a D4R Core: An Efficient Water Oxidation Catalyst

The reaction of Co(OAc)2·4H2O with the sterically hindered phosphate ester, LH2, afforded the tetranuclear complex, [CoII(L)(CH3CN)]4∙5CH3CN (1) [LH2 = 2,6‐(diphenylmethyl)‐4‐isopropyl‐phenyl phosphate]. The molecular structure of 1 reveals that it is a tetranuclear assembly where the Co(II) centers are present in the alternate corners of a cube. The four Co(II) centers are held together by four di-anionic [L]2- ligands. The fourth coordination site on Co(II) is taken by an acetonitrile ligand. Changing the Co(II) precursor from Co(OAc)2·4H2O to Co(NO3)2·6H2O afforded the mononuclear complex [CoII(LH)2(CH3CN)2(MeOH)2] · 2MeOH (2). In 2, the Co(II) is surrounded by two monoanionic [LH]‒ ligands, and a pair of methanol and acetonitrile solvents in a six-coordinate arrangement. 1 has been found to be an efficient catalyst for the electrochemical water oxidation under high basic conditions while the mononuclear analogue, 2, does not respond towards electrochemical water oxidation. The tetranuclear catalyst has excellent electrochemical stability and longevity, as established by the chronoamperometry and >1000 cycles durability test in high alkaline conditions. Excellent current densities of 1 and 10 mAcm‒2 were achieved with the overpotential of 354 and 452 mV respectively. The turnover frequency of this catalyst was calculated as 5.23 s−1 with excellent faradaic efficiency of 97%, indicating the selective oxygen evolution (OER) process happening with the aid of this catalyst. A mechanistic insight into the higher activity of complex 1 towards OER compared to complex 2 is also provided with the help of density functional theory-based calculations.