Syntheses and Structures of 5-Membered Heterocycles Featuring 1,2-Diphospha-1,3-Butadiene and Its Radical Anion.

LGa(P OC)cAAC 2 features a 1,2-diphospha-1,3-butadiene unit with a delocalized π-type HOMO and a π*-type LUMO according to DFT calculations. [LGa(P OC)cAAC][K(DB-18-c-6)] 3[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene radical anion 3⋅ was isolated from the reaction of 2 with KC and dibenzo-18-crown-6. 3 reacted with [Fc][B(C F ) ] (Fc=ferrocenium) to 2 and with TEMPO to [L Ga(P OC)cAAC][K(DB-18-c-6)] 4[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene anion 4 .

Selenium Valence-to-Core X-ray Emission Spectroscopy and Kβ HERFD X-ray Absorption Spectroscopy as Complementary Probes of Chemical and Electronic Structure.

Selenium X-ray absorption spectroscopy (XAS) has found widespread use in investigations of Se-containing materials, geochemical processes, and biologically active sites. In contrast to sulfur Kβ X-ray emission spectroscopy (XES), which has been found to contain electronic and structural information complementary to S XAS, Se Kβ XES remains comparatively underexplored. Herein, we present the first Se Valence-to-Core (VtC) XES studies of reduced Se-containing compounds and FeSe dimers.

Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes.

The electronic structure and ground spin states, S, observed for mixed-valent iron-sulfur dimers (Fe-Fe) are typically determined by the Heisenberg exchange interaction, J, that couples the magnetic interaction of the two metal centres either ferromagnetically (J > 0, S = 9/2) or antiferromagnetically (J < 0, S = 1/2). In the case of antiferromagnetically coupled iron centres, stabilization of the high-spin S = 9/2 ground state is also feasible through a Heisenberg double-exchange interaction, B, which lifts the degeneracy of the Heisenberg spin states. This theorem also predicts intermediate spin states for mixed-valent dimers, but those have so far remained elusive.

Probing Physical Oxidation State by Resonant X-ray Emission Spectroscopy: Applications to Iron Model Complexes and Nitrogenase.

The ability of resonant X-ray emission spectroscopy (XES) to recover physical oxidation state information, which may often be ambiguous in conventional X-ray spectroscopy, is demonstrated. By combining Kβ XES with resonant excitation in the XAS pre-edge region, resonant Kβ XES (or 1s3p RXES) data are obtained, which probe the 3d final-state configuration. Comparison of the non-resonant and resonant XES for a series of high-spin ferrous and ferric complexes shows that oxidation state assignments that were previously unclear are now easily made.

Kβ X-Ray Emission Spectroscopic Study of a Second-Row Transition Metal (Mo) and Its Application to Nitrogenase-Related Model Complexes.

In recent years, X-ray emission spectroscopy (XES) in the Kβ (3p-1s) and valence-to-core (valence-1s) regions has been increasingly used to study metal active sites in (bio)inorganic chemistry and catalysis, providing information about the metal spin state, oxidation state and the identity of coordinated ligands. However, to date this technique has been limited almost exclusively to first-row transition metals. In this work, we present an extension of Kβ XES (in both the 4p-1s and valence-to-1s [or VtC] regions) to the second transition row by performing a detailed experimental and theoretical analysis of the molybdenum emission lines.

The Spectroscopy of Nitrogenases.

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N to 2NH. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation.

Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K-Edge High Resolution X-ray Absorption Spectroscopy.

The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affords a unique probe wherein local Fe-Se interactions can be directly interrogated via high-energy resolution fluorescence detected X-ray absorption spectroscopic (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) studies. These studies reveal a significant asymmetry in the electronic distribution of the FeMoco, suggesting a more localized electronic structure picture than is typically assumed for iron-sulfur clusters.

X-ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin-Coupled Molybdenum(III) Center.

Nitrogenase enzymes catalyze the reduction of atmospheric dinitrogen to ammonia utilizing a Mo-7Fe-9S-C active site, the so-called FeMoco cluster. FeMoco and an analogous small-molecule (Et N)[(Tp)MoFe S Cl ] cubane have both been proposed to contain unusual spin-coupled Mo sites with an S(Mo)=1/2 non-Hund configuration at the Mo atom. Herein, we present Fe and Mo L -edge X-ray magnetic circular dichroism (XMCD) spectroscopy of the (Et N)[(Tp)MoFe S Cl ] cubane and Fe L -edge XMCD spectroscopy of the MoFe protein (containing both FeMoco and the 8Fe-7S P-cluster active sites).

Modulation of Proton-Coupled Electron Transfer through Molybdenum-Quinonoid Interactions.

An expanded series of π-bound molybdenum-quinonoid complexes supported by pendant phosphines has been synthesized. These compounds formally span three protonation-oxidation states of the quinonoid fragment (catechol, semiquinone, quinone) and two different oxidation states of the metal (Mo(0), Mo(II)), notably demonstrating a total of two protons and four electrons accessible in the system. Previously, the reduced Mo(0)-catechol complex 1 and its reaction with dioxygen to yield the two-proton/two-electron oxidized Mo(0)-quinone compound 4 was explored, while, herein, the expansion of the series to include the two-electron oxidized Mo(II)-catechol complex 2, the one-proton/two-electron oxidized Mo-semiquinone complex 3, and the two-proton/four-electron oxidized Mo(II)-quinone complexes 5 and 6 is reported.

Combination of redox-active ligand and lewis acid for dioxygen reduction with π-bound molybdenum-quinonoid complexes.

A series of π-bound Mo-quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal-arene interactions between Mo and the π system of the quinonoid fragment. The Mo-catechol complex (2a) was found to react within minutes with 0.

Dioxygen reactivity with a ferrocene-Lewis acid pairing: reduction to a boron peroxide in the presence of tris(pentafluorophenyl)borane.

Ferrocenes, which are typically air-stable outer-sphere single-electron transfer reagents, were found to react with dioxygen in the presence of B(C6 F5 )3 , a Lewis acid unreactive to O2 , to generate bis(borane) peroxide. Although several Group 13 peroxides have been reported, boron-supported peroxides are rare, with no structurally characterized examples of the BO2 B moiety. The synthesis of a bis(borane)-supported peroxide anion and its structural and electrochemical characterization are described.