Theoretical Study of the Mechanisms of Two Copper Water Oxidation Electrocatalysts with Bipyridine Ligands

Hybrid density functional theory was employed to study the reaction mechanisms of two homogeneous copper water oxidation catalysts (WOCs), [(L2(2-))-Cu-II(OH2)(2)] (H2L2 = 6,6'-dihydroxy-2,2'-bipyridine) [turn-over frequency (TOF) = 0.4 s(-1) at an overpotential = 640 mV and pH 12.4] and [(L1)Cu-II(OH-)(2)] (L1 = 2,2'-bipyridine) (TOF = 100 s(-1) at an overpotential = 850 mV and pH 12.5). Interesting mechanistic insights were obtained by systematically exploring different oxidation and protonation states along reaction pathways. Two well-established protocols to compute the thermodynamics of proton and electron release processes were employed, and similar results were obtained. For the [(L2(2-))Cu-II(OH2)(2)] WOC, two distinct types of water oxidation mechanisms were found with similar rate-limiting barriers, cosubstrate and hydroxyl coupling, termed as such because of the characteristics of the O-O bond formation transition states. For the cosubstrate mechanism, after the starting species is oxidized twice, an O-O bond is formed between the copper-bound terminal oxyl radical ligand and its adjacent oxyanion substituent on the bipyridine radicaloid, showing an interesting mechanistic role for the electron-donating proton-responsive hydroxyl substituent. After the third oxidation of the complex, the organic peroxyl moiety is displaced into an inorganic superoxide by a free anionic hydroxide, which is the rate-limiting step in the cosubstrate mechanism. For the hydroxyl coupling mechanism, after two oxidations of the starting species, O-O bond formation occurs through coupling within the copper-bound (OHlig...OHfree)(center dot-) moiety, with hydrogen-bonding stabilization from the 6,6'-oxyanions and the ligating and free water molecules. For the parent [(L1)Cu-II(OH-)(2)] WOC, O-O bond formation was suggested to occur through coupling between the copper-bound oxyl radical and hydroxide ligands. The results could be useful for further improvement of Cu WOCs.