Seventeen Cu complexes with formal oxidation states ranging from Cu I to Cu III are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal−ligand bonding in high-valent, formally Cu III species is extremely covalent, resulting in Cu K-edge and L 2,3-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L 2,3edge data reveals that all formally Cu III species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs). DFT calculations provide further validation of the orbital composition analysis, and excellent agreement is found between the calculated and experimental results. The finding that Cu has limited capacity to be oxidized necessitates localization of electron hole character on the supporting ligands; consequently, the physical d 8 description for these formally Cu III species is inaccurate. This study provides an alternative explanation for the competence of formally Cu III species in transformations that are traditionally described as metal-centered, 2-electron Cu I /Cu III redox processes.

Terminal copper-nitrenoid complexes have inspired interest in their fundamental bonding structures as well as their putative intermediacy in catalytic nitrene-transfer reactions. Here, we report that aryl azides react with a copper(I) dinitrogen complex bearing a sterically encumbered dipyrrin ligand to produce terminal copper nitrene complexes with near-linear, short copper–nitrenoid bonds [1.745(2) to 1.759(2) angstroms]. X-ray absorption spectroscopy and quantum chemistry calculations reveal a predominantly triplet nitrene adduct bound to copper(I), as opposed to copper(II) or copper(III) assignments, indicating the absence of a copper−nitrogen multiple-bond character. Employing electron-deficient aryl azides renders the copper nitrene species competent for alkane amination and alkene aziridination, lending further credence to the intermediacy of this species in proposed nitrene-transfer mechanisms.

The metallobiochemistry underlying the formation of the inorganic N-N-bond-containing molecules nitrous oxide (N 2 O), dinitrogen (N 2 ), and hydrazine (N 2 H 4 ) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative, carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N-N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes, as well as synthetic model complexes.

The activation of dioxygen by FeII(Me3TACN)­(S2SiMe2) (1) is reported. Reaction of 1 with O2 at −135 °C in 2-MeTHF generates a thiolate-ligated (peroxo)­diiron complex FeIII 2(O2)­(Me3TACN)2(S2SiMe2)2 (2) that was characterized by UV–vis (λmax = 300, 390, 530, 723 nm), Mössbauer (δ = 0.53, |ΔE Q| = 0.76 mm s–1), resonance Raman (RR) (ν­(O–O) = 849 cm–1), and X-ray absorption (XAS) spectroscopies. Complex 2 is distinct from the outer-sphere oxidation product 1 ox (UV–vis (λmax = 435, 520, 600 nm), Mössbauer (δ = 0.45, |ΔE Q| = 3.6 mm s–1), and EPR (S = 5/2, g = [6.38, 5.53, 1.99])), obtained by one-electron oxidation of 1. Cleavage of the peroxo O–O bond can be initiated either photochemically or thermally to produce a new species assigned as an FeIV(O) complex, FeIV(O)­(Me3TACN)­(S2SiMe2) (3), which was identified by UV–vis (λmax = 385, 460, 890 nm), Mössbauer (δ = 0.21, |ΔE Q| = 1.57 mm s–1), RR (ν­(FeIVO) = 735 cm–1), and X-ray absorption spectroscopies, as well as reactivity patterns. Reaction of 3 at low temperature with H atom donors gives a new species, FeIII(OH)­(Me3TACN)­(S2SiMe2) (4). Complex 4 was independently synthesized from 1 by the stoichiometric addition of a one-electron oxidant and a hydroxide source. This work provides a rare example of dioxygen activation at a mononuclear nonheme iron­(II) complex that produces both FeIII–O–O–FeIII and FeIV(O) species in the same reaction with O2. It also demonstrates the feasibility of forming Fe/O2 intermediates with strongly donating sulfur ligands while avoiding immediate sulfur oxidation.

The detailed mechanism of CO2 reduction by a cobalt(i) complex is investigated using computations and kinetics.