Insights into the directions to increase turnover frequency and turnover number during photochemical water oxidation with molecular Ru catalysts

Abstract

A series of four mononuclear water oxidation catalysts (WOCs), denoted as [A₂T_pz–Ru^II], in which A₂ represents two axial monodentate ligands, pyridine (Pyr), 4-picoline (4-Pic), isoquinoline (Iqn), and N-methyl imidazole (Imd), and T_pz represents a planar tetradentate ligand in the equatorial position (6,6′-di(1H-pyrazol-1-yl)-2,2′-bipyridine) were prepared. A series of four photosensitizers (PSs), [Ru(bpy)₃]²⁺, [Ru(bpy)₂{bpy-(CO₂Et)₂}]²⁺, [Ru(bpy){bpy-(CO₂Et)₂}₂]²⁺, and [Ru{bpy-(CO₂Et)₂}₃]²⁺, where bpy and bpy-(CO₂Et)₂ denote 2,2′-bipyridine and diethyl [2,2′-bipyridine]-4,4′-dicarboxylate, respectively, denoted as [PS-n]²⁺, where n = 0, 2, 4, and 6, were prepared. The three known WOCs with the related structures were also prepared. The catalytic performances of the above seven WOCs were evaluated in terms of turnover frequencies (TOFs) and turnover numbers (TONs), while varying PS, and their performances were compared. The measured TOFs were much smaller than those chemically measured. The low concentration of auxiliary oxidant, [PS-n]³⁺, arising from low photon flux density of the incident solar simulated light is proposed to be responsible for the large decrease in TOFs. The oxidation power of the oxidized form of [PS-n]²⁺ ([PS-n]³⁺), the oscillator strength (f) of [PS-n]²⁺ and the lifetime of the [PS-n]²⁺ in the excited state (τ_ps*) also sensitively affect the catalytic performance of a WOC. The results were also analyzed by the reaction schemes that were widely used for mononuclear Ru-based WOCs. For this, the corresponding energy-minimized structures, the energy levels of highest occupied molecular orbitals of the initially added WOCs and intermediates, and the free energy change (ΔG°) for each reaction step were also derived from DFT calculations. This work introduces a new promising series of WOCs, and gives insights into the directions to increase TONs and TOFs during photochemical evaluations of WOCs.