Electrocatalytic water oxidation by Cu^II complexes with branched peptides

Abstract

Two mononuclear Cu II complexes with tetrapeptides incorporating a L-2,3-diaminopropionic acid (dap) branching unit are reported to undergo PCET and catalyse water oxidation. C-terminal His extension of dap (L = 2GH) instead of Gly (L = 3G) lowers the pK a for Cu III H À2 L (9.36 vs. 9.98) and improves the TOF at pH 11 (53 vs. 24 s À1 ).Reactions requiring the synchronous transfer of multiple protons and electrons become energetically viable under mild conditions through the catalytic promotion of proton-coupled electron transfer (PCET) mechanisms that help circumventing high-energy intermediates. 1 Splitting water into its elements, which attracts growing attention as a prospective renewable tool to generate H 2 as an energy carrier, 2-4 ranks among reactions where PCET is of critical importance. Water oxidation catalysts (WOCs) can improve the efficiency of the oxidative half-reaction: 2H 2 O -O 2 + 4H + + 4e À , which has long been considered the bottleneck of the water splitting process. Bioinspired, homogeneous WOCs (Fe, 5 Co, 6 Ru, 7 or Ir 8 ), although inherently less robust than heterogeneous catalysts, 9 represent a meaningful source of mechanistic insight into the multiple proton and electron-transfer events associated with O 2 formation. A growing number of studies conclude that PCET helps in stabilising high-valent MQO or M-O intermediates by preventing charge accumulation upon oxidation and, as a consequence these intermediates can complete the O-O bond formation step. 10 Cu has rich oxygen chemistry, 11 yet, homogeneous WOCs involving this metal appeared only recently, when surprisingly robust Cu II complexes with 2,2 0 -bipyridine (bpy, TOF B 100 s À1 at pH 13) 12 and subsequently, with triglycylglycine (GGGG, or H-Gly-Gly-Gly-Gly-OH, TOF = 33 s À1

Water Oxidation Catalysts Constructed by Biorelevant First-row Metal Complexes

“…The group proposed a plausible catalytic cycle of 17, which is similar to the well-known catalytic cycle of mononuclear copper-based water oxidation catalysts 3 (Figure 8) , from which dioxygen is liberated to close the catalytic cycle. Similar copper-based catalysts housing a polypeptide ligand were also reported by Pap and Szyrwiel et al 37 Although several copper-based water oxidation catalysts have been developed, all catalysts mentioned above require alkaline conditions to drive the water oxidation reaction. Zhang et al first reported a copper-based catalyst that operates at a neutral pH.…”

Single Electron Transfer Steps in Water Oxidation Catalysis. Redefining the Mechanistic Scenario

“…1 top and an I2M mechanism is found when the favored species resemble those of the oxyl radical form depicted in the lower part of Figure 1 Recently several first row transition metal complexes have been reported as catalysts for the water oxidation reaction. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] While these catalysts are of interest because of their high abundance and low toxicity, their performance is much poorer than their Ru or Ir analogues, and in addition their mechanistic pathways are in most cases basically unknown. [28][29][30][41][42][43] We have very recently reported a new complex based on Cu, containing the amidate ligand OPBAN that can carry out the water oxidation reaction in a very efficient manner.…”

Branched peptide with three histidines for the promotion of CuII binding in a wide pH range – complementary potentiometric, spectroscopic and electrochemical studies

“…6) Instead of CV we applied square-wave voltammetry (SWV) to determine the E 0 values accurately, upon shiing the pH by small increments. 1 The E net (potential of I net ) vs. pH plots will directly give Pourbaix diagrams, when reversibility terms of the (Table S1 sums the data, Fig. S4 † illustrates reproducibility).…”

“…is the formal potential of the process in eqn (1). Fit of eqn (2) to E net data points yields the line in Fig.…”