Structural basis for the conformational protection of nitrogenase from O.

The low reduction potentials required for the reduction of dinitrogen (N) render metal-based nitrogen-fixation catalysts vulnerable to irreversible damage by dioxygen (O). Such O sensitivity represents a major conundrum for the enzyme nitrogenase, as a large fraction of nitrogen-fixing organisms are either obligate aerobes or closely associated with O-respiring organisms to support the high energy demand of catalytic N reduction. To counter O damage to nitrogenase, diazotrophs use O scavengers, exploit compartmentalization or maintain high respiration rates to minimize intracellular O concentrations.

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] (, BDI = -bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) to generate several rhenium-phosphorus complexes. Complex reacts in a metathetical manner with chlorophosphines PhPCl, NHP-Cl, and OHP-Cl to generate XL-type phosphido complexes , , and , respectively (NHP-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 , , and reveal that increasing the electronegativity of the phosphorus substituent (C < < O) results in a shortening and strengthening of the rhenium-phosphorus bond.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids.

Metalloenzymes with heteromultimetallic active sites perform chemical reactions that control several biogeochemical cycles. Transformations catalyzed by such enzymes include dioxygen generation and reduction, dinitrogen reduction, and carbon dioxide reduction-instrumental transformations for progress in the context of artificial photosynthesis and sustainable fertilizer production. While the roles of the respective metals are of interest in all these enzymatic transformations, they share a common factor in the transfer of one or multiple redox equivalents.

Coordination Number in High-Spin-Low-Spin Equilibrium in Cluster Models of the S State of the Oxygen Evolving Complex.

The S state of the Oxygen Evolving Complex (OEC) of Photosystem II (PSII) shows high-spin (HS) and low-spin (LS) EPR signals attributed to distinct structures based on computation. Five-coordinate Mn centers are proposed in these species but are absent in available spectroscopic model complexes. Herein, we report the synthesis, crystal structure, electrochemistry, SQUID magnetometry, and EPR spectroscopy of a MnMnO cuboidal complex featuring five-coordinate Mn.

MnO Model of the S Intermediate of the Oxygen-Evolving Complex: Effect of the Dianionic Disiloxide Ligand.

Synthetic complexes provide useful models to study the interplay between the structure and spectroscopy of the different S-state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the Mn core corresponding to the S state, the last observable intermediate prior to dioxygen formation, remain very rare. Toward the development of synthetic strategies to stabilize highly oxidized tetranuclear complexes, ligands with increased anion charge were pursued.

CaMn O Cubane Models of the Oxygen-Evolving Complex: Spin Ground States S<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 CaMn O and YMn O 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 CaMn O complexes and variants with protonated oxo moieties need not be S=9/2.

Dioxygen reacts with metal-carbon bonds in thorium dialkyls to produce bis(alkoxides).

Exposure of bis-amidinate and -guanidinate supported thorium dialkyl complexes to dioxygen results in facile oxygen atom insertion and formation of the corresponding thorium bis(alkoxide) species. Preliminary mechanistic studies suggest a radical propagation mechanism is operative. All new complexes were fully characterized by 1H and 13C NMR spectroscopy, IR, EA and X-ray crystallography.

Tetranuclear [MnMnO] Complexes as Spectroscopic Models of the S 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 state, which features two characteristic EPR signals at g = 2 and g = 4.1.

Two-electron oxidation of a homoleptic U(iii) guanidinate complex by diphenyldiazomethane.

Reaction of the first homoleptic U(iii) guanidinate complex with diphenyldiazomethane results in two-electron oxidation of U(iii) to U(v) and isolation of a U(v) hydrazido complex. Corresponding U(v) imido, U(v) oxo, and U(iv) azido complexes were also synthesized for structural comparison.