FTIR Spectra and Normal-Mode Analysis of a Tetranuclear Manganese Adamantane-like Complex in Two Electrochemically Prepared Oxidation States: Relevance to the Oxygen-Evolving Complex of Photosystem II

摘要

The IR spectra and normal-mode analysis of the adamantane-like compound [Mn4O6(bpea)4]ⁿ⁺ (bpea = N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, Mn^IV4 and Mn^IIIMn^IV3, that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn–O vibrational modes are identified with isotopic exchange, ¹⁶O→¹⁸O, of the mono-μ-oxo bridging atoms in the complex. IR spectra of the Mn^IIIMn^IV3 species are obtained by electrochemical reduction of the Mn^IV4 species using a spectroelectrochemical cell, based on attenuated total reflection [Visser, H.; et al. Anal. Chem. >2001, 73, 4374–4378]. A novel method of subtraction is used to reduce background contributions from solvent and ligand modes, and the difference and double-difference spectra are used in identifying Mn–O bridging modes that are sensitive to oxidation state change. Two strong IR bands are observed for the Mn^IV4 species at 745 and 707 cm⁻¹, and a weaker band is observed at 510 cm⁻¹. Upon reduction, the Mn^IIIMn^IV3 species exhibits two strong IR bands at 745 and 680 cm⁻¹, and several weaker bands are observed in the 510–425 cm⁻¹ range. A normal-mode analysis is performed to assign all the relevant bridging modes in the oxidized Mn^IV4 and reduced Mn^IIIMn^IV3 species. The calculated force constants for the Mn^IV4 species are fr^IV = 3.15 mdyn/Å, frOr = 0.55 mdyn/Å, and frMnr = 0.20 mdyn/Å. The force constants for the Mn^IIIMn^IV3 species are f_r^IV = 3.10 mdyn/Å, f_r^III = 2.45 mdyn/Å, f_rOr = 0.40 mdyn/Å, and f_rMnr = 0.15 mdyn/Å. This study provides insights for the identification of Mn–O modes in the IR spectra of the photosynthetic oxygen-evolving complex during its catalytic cycle.