Tetranuclear [Mn^(III)Mn_3^(IV)O_4] Complexes as Spectroscopic Models of the S_2 State of the Oxygen Evolving Complex in Photosystem II

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the oxygen evolving complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S_2 state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure–electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [Mn^(III)Mn_3^(IV)O_4] cuboidal complexes as spectroscopic models of the S_2 state of the OEC. Resembling the oxidation state and EPR spectra of the S_2 state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed

Panoply of P: An Array of Rhenium–Phosphorus Complexes Generated from a Transition Metal Anion

We expand upon the synthetic utility of anionic rhenium complex Na[(BDI)ReCp] (1, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) to generate several rhenium-phosphorus complexes. Complex 1 reacts in a metathetical manner with chlorophosphines Ph2PCl, MeNHP-Cl, and OHP-Cl to generate XL-type phosphido complexes 2, 3, and 4, respectively (MeNHP-Cl = 2-chloro-1,3-dimethyl-1,3,2-diazaphospholidine; OHP-Cl = 2-chloro-1,3,2-dioxaphospholane). Crystallographic and computational investigations of phosphido triad 2, 3, and 4 reveal that increasing the electronegativity of the phosphorus substituent (C < N < O) results in a shortening and strengthening of the rhenium-phosphorus bond. Complex 1 reacts with iminophosphane Mes*NPCl (Mes* = 2,4,6-tritert-butylphenyl) to generate linear iminophosphanyl complex 5. In the presence of a suitable halide abstraction reagent, 1 reacts with the dichlorophosphine iPr2NPCl2 to afford cationic phosphinidene complex 6+. Complex 6+ may be reduced by one electron to form 6•, a rare example of a stable, paramagnetic phosphinidene complex. Spectroscopic and structural investigations, as well as computational analyses, are employed to elucidate the influence of the phosphorus substituent on the nature of the rhenium-phosphorus bond in 2 through 6. Furthermore, we examine several common analogies employed to understand metal phosphido, phosphinidene, and iminophosphanyl complexes

Tetranuclear [Mn^(III)Mn_3^(IV)O_4] Complexes as Spectroscopic Models of the S_2 State of the Oxygen Evolving Complex in Photosystem II

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the oxygen evolving complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S_2 state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure–electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [Mn^(III)Mn_3^(IV)O_4] cuboidal complexes as spectroscopic models of the S_2 state of the OEC. Resembling the oxidation state and EPR spectra of the S_2 state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed

CaMn3IVO4 Cubane Models of the Oxygen Evolving Complex: Spin Ground States S < 9/2 and the Effect of Oxo Protonation

We report single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn(3)(IV)O(4) and YMn(3)(IV)O(4) complexes as structural and spectroscopic models of the cuboidal subunit of the OEC. The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on spin state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn(3)(IV)O(4) complexes and variants with protonated oxo moieties need not be S = 9/2. Desymmetrization of the pseudo-C(3) symmetric Ca(Y)Mn(3)(IV)O(4) core leads to a lower S = 5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S = 3/2 spin ground state is observed in CaMn(3)(IV)O(3)(OH). Our studies complement the observation that the interconversion between the low spin and high spin forms of the S(2) state is pH dependent, suggesting that (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin state changes

CaMn3IVO4 Cubane Models of the Oxygen‐Evolving Complex: Spin Ground States S&lt;9/2 and the Effect of Oxo Protonation

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn3 IV O4 and YMn3 IV O4 complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn3 IV O4 complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo-C3 -symmetric Ca(Y)Mn3 IV O4 core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn3 IV O3 (OH). Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S2 state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes

Corpus Linguistics Research Trends from 1997 to 2016: A Co-Citation Analysis

Corpus linguistics is one of the fastest growing areas of linguistics because of its interface with neighboring academic disciplines and the data-processing capability of a large amount of empirical linguistic data. This study reviews research trends from the last two decades within the corpus linguistics fields. Specifically, the study applied systematic citation analysis procedures to summarize and identify the salient research themes and publications from citation-reference data of peer-reviewed research articles published and indexed in the Web of Science (WoS) between 1997 and 2016. The co-citation analysis of 5,600 research articles and their 172,352 references indicated that, over the four time spans of five years, the corpus linguistics research articles have cited works ranging from general linguistics journal titles to specialized journal titles and individual books. In terms of the research themes of corpus linguistics, the topics of the linguistics research have rapidly changed over the time span. More recently, the development of web-based large monitor corpora and corpus analysis software has contributed significantly to the dynamic and productive interaction of research in the discipline. This may indicate the evolving and juvenile nature of corpus linguistics and its possibility of growing into a multi-disciplinary field. Although there are exceptions to all of the research patterns found in the co-citation analysis, the current study also discusses the most up-to-date research trends and the future directions of corpus linguistics