The Protonation States of Oxo-Bridged Mn^IV Dimers Resolved by Experimental and Computational Mn K Pre-Edge X-ray Absorption Spectroscopy

Abstract

In nature, the protonation of oxo bridges is a commonly encountered mechanism for fine-tuning chemical properties and reaction pathways. Often, however, the protonation states are difficult to establish experimentally. This is of particular importance in the oxygen evolving complex of Photosystem II, where identification of the bridging oxo protonation states is one of the essential requirements toward unraveling the mechanism. In order to establish a combined experimental and theoretical protocol for the determination of protonation states, we have systematically investigated a series of Mn model complexes by Mn K pre-edge X-ray absorption spectroscopy. An ideal test case for selective bis-μ-oxo-bridge protonation in a Mn-dimer is represented by the system [MnIV2(salpn)2(μ-OH(n))2](n+). Although the three species [MnIV2(salpn)2(μ-O)2], [MnIV2(salpn)2(μ-O)(μ-OH)]+ and [MnIV2(salpn)2(μ-OH)2]2+ differ only in the protonation of the oxo bridges, they exhibit distinct differences in the pre-edge region while maintaining the same edge energy. The experimental spectra are correlated in detail to theoretical ly calculated spectra. A time-dependent density functional theory approach for calculating the pre-edge spectra of molecules with multiple metal centers is presented, using both high-spin (HS) and broken-symmetry (BS) electronic structure solutions. The most intense pre-edge transitions correspond to an excitation of the Mn-1s core electrons into the unoccupied orbitals of local eg character (dz2 and dxy based in the chosen coordinate system). The lowest by energy experimental feature is dominated by excitations of 1s-α electrons and the second observed feature is primarily attributed to 1s-β electron excitations. The observed energetic separation is due to spin polarization effects in spin-unrestricted density functional theory and models final state multiplet effects. The effects of spin polarization on the calculated Mn K pre-edge spectra, in both the HS and BS solutions, are discussed in terms of the strength of the antiferromagnetic coupling and associated changes in the covalency of Mn-O bonds. The information presented in this paper is complemented with the X-ray emission spectra of the same compounds published in an accompanying paper. Taken together, the two studies provide the foundation for a better understanding of the X-ray spectroscopic data of the oxygen evolving complex (OEC) in Photosystem II.

High-Energy-Resolution Fluorescence-Detected X-ray Absorption of the Q Intermediate of Soluble Methane Monooxygenase

“…The obtained geometries were closer to experimental structures when the optimizations were done on the broken symmetry surface following published procedures on DFT single point broken symmetry calculations. 48 …”

Experimental and Computational X-ray Emission Spectroscopy as a Direct Probe of Protonation States in Oxo-Bridged MnIV Dimers Relevant to Redox-Active Metalloproteins

“…44 More details on the geometry optimization are given in Ref. 28 . The method for calculating XES spectra has been published and tested.…”

“…29 The assignment of protonation states by XAS is further strengthened by close correlation to theory, as explored in the preceding paper. 28 …”

X-ray Absorption and Emission Study of Dioxygen Activation by a Small-Molecule Manganese Complex

“…40 As with XES, analysis of the XAS using a DFT-based approach enhances the geometric and electronic structure information provided by this technique. 29,41 Unlike XES, however, these absorption processes generally require time-dependent DFT (TD-DFT) methods, and are at times less successful in their quantitative prediction of experimentally determined energies and intensities. 41,42 Transitions that are primarily metal-localized have been successfully reproduced using hybrid density functionals, but those that are charge transfer in nature are often poorly described.…”

“…29,41 Unlike XES, however, these absorption processes generally require time-dependent DFT (TD-DFT) methods, and are at times less successful in their quantitative prediction of experimentally determined energies and intensities. 41,42 Transitions that are primarily metal-localized have been successfully reproduced using hybrid density functionals, but those that are charge transfer in nature are often poorly described. 42 Nevertheless, it is certainly instructive to obtain qualitative insight into these transitions, particularly when used in conjunction with a similar analysis of XES data.…”