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 MoII–quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons and electrons from the Mo^0 and Mo^(II) catechol complexes, 1 and 2, to H atom acceptor TEMPO suggests the presence of weak O–H bonds. Although thermochemical analyses are hindered by the irreversibility of the electrochemistry of the present compounds, the reactivity observed suggests weaker O–H bonds compared to the free catechol, indicating that proton-coupled electron transfer can be facilitated significantly by the π-bound metal center

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(C_6F_5)_3, a Lewis acid unreactive to O_2, 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 BO_2B moiety. The synthesis of a bis(borane)-supported peroxide anion and its structural and electrochemical characterization are described

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 0.5 equiv of O_2 to yield a Mo–quinone complex (3), H_2O, and CO. Si- and B-protected Mo–catecholate complexes also react with O_2 to yield 3 along with (R_2SiO)_n and (ArBO)_3 byproducts, respectively. Formally, the Mo–catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O_2 oxygens. Unreactive by itself, the Mo–dimethyl catecholate analogue reduces O_2 in the presence of added Lewis acid, B(C_6F_5)_3, to generate a MoI species and a bis(borane)-supported peroxide dianion, [[(F_5C_6)_3B]_2O_2^(2–)], demonstrating single-electron-transfer chemistry from Mo to the O_2 moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O_2 with pendant acids weaker than B(C_6F_5)_3. Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O_2 is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O_2 species

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. Supported by experimental small molecule model data in combination with time dependent density functional theory (TDDFT) calculations, the HERFD XAS data is consistent with an assignment of Fe2/Fe6 as an antiferromagnetically coupled diferric pair. HERFD XAS and EXAFS have also been applied to Se-substituted CO-inhibited MoFe protein, demonstrating the ability of these methods to reveal electronic and structural changes that occur upon substrate binding. These results emphasize the utility of Se HERFD XAS and EXAFS for selectively probing the local electronic and geometric structure of FeMoco

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 biological 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 under-explored. Herein, we present the first Se Valence-to-Core (VtC) XES studies of reduced Se-containing compounds and FeSe dimers. Se VtC XES is found to be sensitive to changes in covalent Se bonding interactions (Se–Se/Se–C/Se–H bonding) while relatively insensitive to changes in Fe oxidation states as selenide bridges in FeSe dimers ([Fe2Se2]2+ vs [Fe2Se2]+). Contrastingly, Se Kβ HERFD XAS is demonstrated to be quite sensitive to changes in Fe-oxidation state, with Se Kβ HERFD XAS demonstrating experimental resolution equivalent to K HERFD XAS. Additionally, computational studies reveal both Se VtC XES and XAS to be sensitive to selenium protonation in FeSe complexes

Synthesis, characterization, and reactivity of molybdenum-​quinoid complexes supported by a redox and acid​/base non-​innocent ligand

The propensity for earth-abundant transition metals to preferentially access single-electron chem. remains one of the more prominent obstacles in their utilization to facilitate efficient multi-proton, multi-electron activation of small mols. such as dioxygen or carbon dioxide. Incorporation of redox non-innocent ligands as well as pendant acid/base moieties has proven effective as a method for altering the reactivity of transition metal complexes. A series of low-valent molybdenum-quionoid complexes supported by a redox and acid/base non-innocent ligand have been synthesized and characterized demonstrating both hydrogen atom and proton and electron transfer chem. spanning a total of four electrons and two protons. Five different protonation/oxidn. state combinations were isolated from Mo(0)-catechol to Mo(II)-quinone. Substrate-based multi-proton, multi-electron reactivity was demonstrated through redn. of dioxygen to water, and mechanistic insight gained via substituting the protons for other Lewis-acidic moieties will also be presented

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 (<b>2a</b>) was found to react within minutes with 0.5 equiv of O₂ to yield a Mo–quinone complex (<b>3</b>), H₂O, and CO. Si- and B-protected Mo–catecholate complexes also react with O₂ to yield <b>3</b> along with (R₂SiO)_<i>n</i> and (ArBO)₃ byproducts, respectively. Formally, the Mo–catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O₂ oxygens. Unreactive by itself, the Mo–dimethyl catecholate analogue reduces O₂ in the presence of added Lewis acid, B­(C₆F₅)₃, to generate a Mo^I species and a bis­(borane)-supported peroxide dianion, [[(F₅C₆)₃B]₂O₂^2–], demonstrating single-electron-transfer chemistry from Mo to the O₂ moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O₂ with pendant acids weaker than B­(C₆F₅)₃. Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O₂ is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O₂ species