Substituent Effects on Core Structures and Heterogeneous Catalytic Activities of Mn^III(μ-O)₂Mn^IV Dimers with 2,2′:6′,2″-Terpyridine Derivative Ligands for Water Oxidation

Abstract

[(OH₂)­(R-terpy)­Mn­(μ-O)₂Mn­(R-terpy)­(OH₂) ]³⁺ (R-terpy = 4′-substituted 2,2′:6′,2″-terpyridine, R = butoxy (BuO), propoxy (PrO), ethoxy (EtO), methoxy (MeO), methyl (Me), methylthio (MeS), chloro (Cl)) have been synthesized as a functional oxygen-evolving complex (OEC) model and characterized by UV–vis and IR spectroscopic, X-ray crystallographic, magnetometric, and electrochemical techniques. The UV–vis spectra of derivatives in water were hardly influenced by the 4′-substituent variation. X-ray crystallographic data showed that Mn centers in the Mn^III(μ-O)₂Mn^IV cores for derivatives with R = H, MeS, Me, EtO, and BuO are crystallographically indistinguishable, whereas the derivatives with R = MeO and PrO gave the significantly distinguishable Mn centers in the cores. The indistinguishable Mn centers could be caused by rapid electron exchange between the Mn centers to result in the delocalized Mn­(μ-O)₂Mn core. The exchange integral values (<i>J</i> = −196 to −178 cm^–1) for delocalized cores were lower than that (<i>J</i> = −163 to −161 cm^–1) for localized cores, though the Mn···Mn distances are nearly the same (2.707–2.750 Å). The half wave potential (<i>E</i>_1/2) of a Mn^III–Mn^IV/Mn^IV–Mn^IV pair of the derivatives decreased with an increase of the electron-donating ability of the substituted groups for the delocalized core, but it deviated from the correlation for the localized cores. The catalytic activities of the derivatives on mica for heterogeneous water oxidation were remarkably changed by the substituted groups. The second order rate constant (<i>k</i>₂/mol^–1 s^–1) for O₂ evolution was indicated to be correlated to <i>E</i>_1/2 of a Mn^III–Mn^IV/Mn^IV–Mn^IV pair; <i>k</i>₂ increased by a factor of 29 as <i>E</i>_1/2 increased by 28 mV