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Reduction of nitrite anions (NO2−) to nitric oxide (NO), nitrous oxide (N2O) and ultimately dinitrogen (N2) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (−0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO22−), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N2O) and nitrite (NO2−). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO2−to NO.

Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L3edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)2]2+and [Fe(btbip)2]2+, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe–carbene bond length due to steric effects modify the metal–ligand bonding, including σ/π donation and π back-donation. We find that σ donation is significantly stronger in [Fe(bmip)2]2+, whereas the π back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)2]2+lead to approximately 1 eV destabilization of the quintet metal-centered 5T2gexcited state compared to [Fe(btbip)2]2+, providing an explanation for the absence of a photoinduced 5T2gpopulation and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)2]2+. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.

Successful deployment of carbon nanomaterials in many applications, such as sensing, energy storage, and catalysis, relies on the selection, synthesis, and tailoring of the surface properties. Predictive analysis of the behavior is difficult without detailed knowledge of the differences between various carbon nanomaterials and their surface functionalization, thus leaving the selection process to traditional trial-and-error work. The present characterization fills this knowledge gap for carbon nanomaterial surface properties with respect to chemical states and functionalization. We present an overview of the chemical trends that can be extracted from soft X-ray absorption spectroscopy (XAS) spectra on an extended set of nonideal carbon nanomaterials as a function of sp2bonded carbon and bond ordering. In particular, the surface chemical state, the presence of long-range order in the carbon matrix, and a qualitative estimation of the amount of oxygen and nitrogen and their respective functional group formation on the material surface, together with the detailed material fabrication parameters, are reported. The results expand our understanding of carbon nanomaterial functionalization, which can support material selection in practice, provided that the specifications of the application are known.

Nickel anions [(MeCN)Ni(CF3)3]−and [Ni(CF3)4]2–were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF3to [(dme)NiBr2] in the presence of the [PPh4]+counterion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands, surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF3)3]−and [Ni(CF3)4]2–, these complexes are better physically described as d9metal complexes. [(MeCN)Ni(CF3)3]−is electrophilic and reacts with other nucleophiles such as phenoxide to yield the unsupported [(PhO)Ni(CF3)3]2–salt, revealing the broader potential of [(MeCN)Ni(CF3)3]−in the development of “ligandless” trifluoromethylations at nickel. Proof-in-principle experiments show that the reaction of [(MeCN)Ni(CF3)3]−with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high-valent, unsupported, and formal organonickel(IV) intermediate. Evidence of the feasibility of such intermediates is provided with the structurally characterized [PPh4]2[Ni(CF3)4(SO4)], which was derived through the two-electron oxidation of [Ni(CF3)4]2–.

Seventeen Cu complexes with formal oxidation states ranging from CuIto CuIIIare 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 CuIIIspecies is extremely covalent, resulting in Cu K-edge and L2,3-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L2,3-edge data reveals that all formally CuIIIspecies 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 d8description for these formally CuIIIspecies is inaccurate. This study provides an alternative explanation for the competence of formally CuIIIspecies in transformations that are traditionally described as metal-centered, 2-electron CuI/CuIIIredox processes.

Li- and Mn-rich (LMR) layered cathode materials have demonstrated impressive capacity and specific energy density thanks to their intertwined redox centers including transition metal cations and oxygen anions. Although tremendous efforts have been devoted to the investigation of the electrochemically driven redox evolution in LMR cathode at ambient temperature, their behavior under a mildly elevated temperature (up to ∼100 °C), with or without electrochemical driving force, remains largely unexplored. Here we show a systematic study of the thermally driven surface-to-bulk redox coupling effect in charged Li1.2Ni0.15Co0.1Mn0.55O2. We for the first time observed a charge transfer between the bulk oxygen anions and the surface transition metal cations under ∼100 °C, which is attributed to the thermally driven redistribution of Li ions. This finding highlights the nonequilibrium state and dynamic nature of the LMR material at deeply delithiated state upon a mild temperature perturbation.

High-resolution nondestructive chemical analysis of both bulk and surface of application-ready carbon nanomaterials is needed to connect the material properties to the observed performance. This is needed to enable application-specific tailoring of carbon nanomaterials. However, detailed studies of effects of oxidizing treatments on the chemical composition and structural integrity of carbon and on the metal seed materials are rare. We show here a hybrid X-ray-based study retrieving this hard-to-access chemical and structural information of application-ready ferrocene-grown single-walled carbon nanotubes and their nitric acid- and oxygen plasma-treated versions. We have executed photoelectron, absorption, and X-ray emission spectroscopy (XES) measurements in the soft X-ray regime providing chemical and structural information with high energy resolution and throughput. We observed that the nitric acid treatment did not significantly alter the chemical state of the carbon matrix, whereas the oxygen plasma treatment was associated with strong effects and chemical conversions toward oxygen-functionalized groups such as carboxyl. Additionally, an Fe catalyst was present before and after the oxidizing treatments at considerable concentrations as detected by both X-ray absorption spectroscopy and XES, with a near complete conversion of Fe2+to Fe3+. Additional minor elements (metals <0.1 at. %) were detected only with XES. With this combined approach, we can study the effect of different treatments on application-ready carbon nanomaterials’ physicochemical and structural properties with unmatched precision, which helps understand the nature of these materials.