Substituent Effects on
Core Structures and Heterogeneous
Catalytic Activities of Mn<sup>III</sup>(μ-O)<sub>2</sub>Mn<sup>IV</sup> Dimers with 2,2′:6′,2″-Terpyridine
Derivative Ligands for Water Oxidation
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Abstract
[(OH<sub>2</sub>)(R-terpy)Mn(μ-O)<sub>2</sub>Mn(R-terpy)(OH<sub>2</sub>) ]<sup>3+</sup> (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<sup>III</sup>(μ-O)<sub>2</sub>Mn<sup>IV</sup> 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)<sub>2</sub>Mn core. The exchange
integral values (<i>J</i> = −196 to −178 cm<sup>–1</sup>) for delocalized cores were lower than that (<i>J</i> = −163 to −161 cm<sup>–1</sup>) for
localized cores, though the Mn···Mn distances are nearly
the same (2.707–2.750 Å). The half wave potential (<i>E</i><sub>1/2</sub>) of a Mn<sup>III</sup>–Mn<sup>IV</sup>/Mn<sup>IV</sup>–Mn<sup>IV</sup> 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><sub>2</sub>/mol<sup>–1</sup> s<sup>–1</sup>) for O<sub>2</sub> evolution was indicated to be correlated to <i>E</i><sub>1/2</sub> of a Mn<sup>III</sup>–Mn<sup>IV</sup>/Mn<sup>IV</sup>–Mn<sup>IV</sup> pair; <i>k</i><sub>2</sub> increased by a factor of 29 as <i>E</i><sub>1/2</sub> increased by 28 mV