Chloride ligation in inorganic manganese model compounds relevant to photosystem II studied using X-ray absorption spectroscopy

Abstract

Chloride ions are essential for proper function of the photosynthetic oxygen-evolving complex (OEC) of Photosystem II (PS II). Although proposed to be directly ligated to the Mn cluster of the OEC, the specific structural and mechanistic roles of chloride remain unresolved. This study utilizes X-ray absorption spectroscopy (XAS) to characterize the Mn-Cl interaction in inorganic compounds that contain structural motifs similar to those proposed for the OEC. Three sets of model compounds are examined; they possess core structures Mn(IV)(3)O(4)X (X=Cl, F, or OH) that contain a di-micro-oxo and two mono-micro-oxo bridges or Mn(IV)(2)O(2)X (X=Cl, F, OH, OAc) that contain a di-micro-oxo bridge. Each set of compounds is examined for changes in the XAS spectra that are attributable to the replacement of a terminal OH or F ligand, or bridging OAc ligand, by a terminal Cl ligand. The X-ray absorption near edge structure (XANES) shows changes in the spectra on replacement of OH, OAc, or F by Cl ligands that are indicative of the overall charge of the metal atom and are consistent with the electronegativity of the ligand atom. Fourier transforms (FTs) of the extended X-ray absorption fine structure (EXAFS) spectra reveal a feature that is present only in compounds where chloride is directly ligated to Mn. These FT features were simulated using various calculated Mn-X interactions (X=O, N, Cl, F), and the best fits were found when a Mn-Cl interaction at a 2.2-2.3 A bond distance was included. There are very few high-valent Mn halide complexes that have been synthesized, and it is important to make such a comparative study of the XANES and EXAFS spectra because they have the potential for providing information about the possible presence or absence of halide ligation to the Mn cluster in PS II.

Fig. 1

Schematic of core structures of trinuclear and binuclear Mn compounds examined: (A) [Mn₃O₄X(bpea)₃](ClO₄)₃ where X = Cl, F or OH; (B) [Mn₂O₂Cl₂(bpea)₂](ClO₄)₂ and [Mn₂O₂(O₂CCH₃) (bpea)₂](ClO₄)₃; and (C) proposed structure for [Mn₂O₂X₂ (tacn)₂](BPh₄)₂, where X = Cl or F. Compounds are labeled according to the abbreviations used in the text. Bridging ligands to Mn are shown in the core structure, while terminal ligands are shown separately in the inset boxes

Fig. 2

Normalized Mn K-edge XANES spectra

Fig. 3

Second derivatives of the XANES region shown in Fig. 2 calculated using a 5 eV differentiation interval. The IPE listed for each compound was determined from the zero-crossing position of the main absorption feature, as indicated by arrows

Fig. 4

Background-subtracted k-space Mn EXAFS weighed by k³

Fig. 5

Fourier transform spectra of k³-weighted Mn EXAFS shown in Fig. 4

Fig. 6

Fourier-filtered k-space EXAFS oscillation corresponding to FT peaks I and II of Mn₃Cl (shown in Fig. 5A), FT peaks I, II, and III of Mn₂Cl₂(b) (shown in Fig. 5B), and FT peaks I and II of Mn₂Cl₂(t) (shown in Fig. 5C). In each case, the isolated data are compared with the simulation that includes a Mn–Cl interaction shell. The atomic interaction shells used and the resulting parameters from each fit are detailed in Tables 1, 2, and 3 respectively