Barrel-Shaped-Polyoxometalates Exhibiting Electrocatalytic Water Reduction at Neutral pH: A Synergy Effect

Two copper-based barrel-shaped polyoxometalates (POMs), namely, [{H₃O}₄{Na₆(H₂O)₂₂}][{Cu^I (H₂O)₃}₂{Cu^II (H₂O)}₃{B-α-Bi^IIIW^VI₉O₃₃}₂]·7H₂O (NaCu-POM) and Li₄[{NH₄}₂{H₃O}₃{Li(H₂O)₅}][{Cu^II(SH)}{(Cu^IICu^I_1.5)(B-α-Bi^IIIW^VI₉O₃₃)}₂]·9H₂O (LiCu-POM) have been synthesized and structurally characterized. The single-crystal X-ray diffraction analyses of NaCu-POM and LiCu-POM reveal the presence of penta- and hexa-nuclear copper wheels per formula units, respectively; these copper wheels are sandwiched between two lacunary Keggin anions {B-α-Bi^IIIW^VI₉O₃₃}^9- (BiW₉) to form the barrel-shaped title POM compounds. In both the compounds NaCu-POM and LiCu-POM, the mixed-valent copper centers are present in their respective penta- and hexa-nuclear copper wheels, established by X-ray photoelectron spectroscopy (XPS) as well as by bond valence sum (BVS) calculations. Compound LiCu-POM additionally shows the presence of a sulfhydryl ligand (SH^-), coordinated to one of the copper centers of its {Cu₆}-wheel, that is expected to be generated from the in situ reduction of sulfate anion present in the concerned reaction mixture (lithium-ion in ammonia solution may be the reducing agent). Interestingly, the title compounds, NaCu-POM and LiCu-POM exhibit an efficient electrocatalytic hydrogen evolution reaction (HER) by reducing water at neutral pH. Detailed electrochemical studies including controlled experiments indicate that the active sites for this electrocatalysis are the W(VI) centers of the title compounds, not the copper centers. However, a relevant tri-lacunary Keggin cluster anion {P^VW^VI₉O₃₃}^7- (devoid of copper ion) does not show comparable HER as shown by the title compounds. The intra-cluster cooperative interactions of the mixed-valent copper centers (Cu^II/Cu^I) with the tungsten centers (W⁶⁺) make the overall system electrocatalytically active toward water reduction to molecular hydrogen at neutral pH. High Faradaic efficiencies (89 and 92%) and turnover frequencies (1.598 s^-1 and 1.117 s^-1) make the title compounds NaCu-POM and LiCu-POM efficient catalysts toward electrochemical water reduction to molecular hydrogen.