Electrocatalytic water oxidation by molecular cobalt-terpyridine complexes in alkaline medium: experimental and theoretical study†

The current study reports the electrocatalytic water oxidation by mononuclear bis-terpyridine Co(II) complexes of general formula [CoL^1–4₂]X₂ (where L¹ = 4′-phenyl-2,2′:6′,2′′-terpyridine, L² = 4′-(3,4-dimethoxyphenyl)-2,2′:6′,2′′-terpyridine, L³ = 4′-(3,4,5-trimethoxyphenyl)-2,2′:6′,2′′-terpyridine, L⁴ = 4′-(4-fluorophenyl)-2,2′:6′,2′′-terpyridine, and X = ClO₄⁻/Cl⁻). All the complexes have been fully characterized by single-crystal X-ray diffraction and mass spectroscopy, which show the CoN₆ core structure in all the complexes. The complexes exhibit moderate to good electrocatalytic activity at pH 13.5. During the water oxidation study, an electrocatalytic wave in cyclic voltammetry appears near the Co(IV/III) couple in all four complexes (1–4). The electrocatalytic water oxidation occurs at an onset potential of 0.80 V, 0.81 V, 0.72 V, and 0.85 V vs. NHE for 1, 2, 3, and 4, respectively. The efficiency of the catalysts has been found to depend on the electron-withdrawing and -donating capacity of the substituents in the ligand scaffold. An electron-donating –OMe group in the terpyridine results in the maximum rate, while the electron-withdrawing –F group displays the lowest rate of the water oxidation reaction. Determination of the TOF values by both the peak current method (TOF_1,2,3,4 = 10 s⁻¹, 16 s⁻¹, 40 s⁻¹, 1 s⁻¹, respectively) and FOWA (TOF_1,2,3,4 = 22 s⁻¹, 34 s⁻¹, 194 s⁻¹, 2 s⁻¹) method follows the same trend. The faradaic efficiency of the complexes has been found to be 61%, 64%, 68%, and 35% for complexes 1, 2, 3, and 4, respectively. The TON calculated for complexes 1, 2, 3, and 4 have been found to be 11, 13, 18, and 5, respectively. The mechanism of water oxidation has been proposed based on mass spectral data of the electrochemically oxidised species and DFT calculations. All the molecules act as active OEC (oxygen-evolving complexes) photochemically with visible light in the presence of [Ru(bpy)₃]Cl₂ as photosensitizer and Na₂S₂O₈ as electron acceptor.