A tale of two topological isomers: Uptuning [FeIV(O)(Me4cyclam)]2+ for olefin epoxidation

TMC-anti and TMC-syn, the two topological isomers of [FeIV(O)(TMC)(CH3CN)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Me4cyclam), differ in the orientations of their FeIV=O units relative to the four methyl groups of the TMC ligand framework. The FeIV=O unit of TMC-anti points away from the four methyl groups, while that of TMC-syn is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC-syn reacts with HAT (hydrogen atom transfer) substrates at 1.3- to 3-fold faster rates than TMC-anti, but the reactivity difference increases dramatically in oxygen-atom transfer reactions. R2S substrates are oxidized into R2S=O products at rates 2-to-3 orders of magnitude faster by TMC-syn than TMC-anti. Even more remarkably, TMC-syn epoxidizes all the olefin substrates in this study, while TMC-anti reacts only with cis-cyclooctene but at a 100-fold slower rate. Comprehensive quantum chemical calculations have uncovered the key factors governing such reactivity differences found between these two topological isomers.

Theoretical study on hydrogen transfer in the dissociation of dimethyl disulfide radical cations

Hydrogen transfer (HT) is of crucial importance in biochemistry and atmospheric chemistry. Here, HT processes involved in the dissociation reaction of dimethyl disulfide radical cations (DMDS˙⁺, CH₃SSCH₃˙⁺) are investigated using quantum chemical calculations. Four HTs from the C to S atom and one HT from the S to S atom are observed and the most probable paths are proposed in the dissociation channel from DMDS˙⁺ to CH_nS⁺ (n = 2-4). The mechanisms of all these five HTs are described as hydrogen atom transfer (HAT) and four of them are accompanied by electron transfer (ET). Considering the catalytic effect of water molecules existing in organisms and the atmosphere, five HT processes in the dissociation of the [DMDS + H₂O]˙⁺ complex are further explored, which show lower free energy barriers. With the participation of water molecules acting as a base, two HTs from the C to the S atom, which have the largest decrease in energy barriers, are characterized as concerted proton-coupled electron transfer (cPCET). These results can be extended to understand the mechanism of the HT process during the dissociation of disulfide and help provide a strategy to design a rare cPCET mechanism for the activation of the C-H bond.

Terminal end-on coordination of dinitrogen versus isoelectronic CO: A comparison using the charge displacement analysis

The metal dinitrogen bonding in a wide series of terminal end-on dinitrogen complexes is investigated with the charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV). The effect of the σ donation and π backdonation on the NN bond are discussed and compared with the observations for a series of carbonyl complexes, published in 2016 by Tarantelli et al. The σ donation is relative invariant over the series of dinitrogen complexes and has no significant effect on the NN bond strength, whereas the π backdonation causes a considerable elongation of the NN bond. Some uncommon examples of weakly bound dinitrogen with blue-shifted stretching frequency compared to free N2 (ν = 2330 cm-1 ) are known. The dinitrogen bonding in these complexes is simulated with a point charge. Apparently, electrostatics account for the shortened N─N bond in these systems.

Understanding the Surprising Oxidation Chemistry of Au-OH Complexes

Au is known to be fairly redox inactive (in catalysis) and bind oxygen adducts only quite weakly. It is thus rather surprising that stable Au-OH complexes can be synthesized and used as oxidants for both one- and two-electron oxidations. A charged Au^III -OH complex has been shown to cleave C-H and O-H bonds homolytically, resulting in a one-electron reduction of the metal center. Contrasting this, a neutral Au^III -OH complex performs oxygen atom transfer to phosphines, resulting in a two-electron reduction of the hydroxide proton to form a Au^III -H rather than causing a change in oxidation state of the metal. We explore the details of these two examples and draw comparisons to the more conventional reactivity exhibited by Au^I -OH. Although the current scope of known Au-OH oxidation chemistry is still in its infancy, the current literature exemplifies the unique properties of Au chemistry and shows promise for future findings in the field.

Homolytic X‐H Bond Cleavage at a Gold(III) Hydroxide: Insights into One‐Electron Events at Gold

C(sp³)‐H and O−H bond breaking steps in the oxidation of 1,4‐cyclohexadiene and phenol by a Au(III)‐OH complex were studied computationally. The analysis reveals that for both types of bonds the initial X−H cleavage step proceeds via concerted proton coupled electron transfer (cPCET), reflecting electron transfer from the substrate directly to the Au(III) centre and proton transfer to the Au‐bound oxygen. This mechanistic picture is distinct from the analogous formal Cu(III)‐OH complexes studied by the Tolman group (J. Am. Chem. Soc. 2019, 141, 17236–17244), which proceed via hydrogen atom transfer (HAT) for C−H bonds and cPCET for O−H bonds. Hence, care should be taken when transferring concepts between Cu−OH and Au−OH species. Furthermore, the ability of Au−OH complexes to perform cPCET suggests further possibilities for one‐electron chemistry at the Au centre, for which only limited examples exist.

Modulation of a μ-1,2-Peroxo Dicopper(II) Intermediate by Strong Interaction with Alkali Metal Ions

The properties of metal/dioxygen species, which are key intermediates in oxidation catalysis, can be modulated by interaction with redox-inactive Lewis acids, but structural information about these adducts is scarce. Here we demonstrate that even mildly Lewis acidic alkali metal ions, which are typically viewed as innocent "spectators", bind strongly to a reactive cis-peroxo dicopper(II) intermediate. Unprecedented structural insight has now been obtained from X-ray crystallographic characterization of the "bare" Cu^II₂(μ-η¹:η¹-O₂) motif and its Li⁺, Na⁺, and K⁺ complexes. UV-vis, Raman, and electrochemical studies show that the binding persists in MeCN solution, growing stronger in proportion to the cation's Lewis acidity. The affinity for Li⁺ is surprisingly high (∼70 × 10⁴ M^-1), leading to Li⁺ extraction from its crown ether complex. Computational analysis indicates that the alkali ions influence the entire Cu-OO-Cu core, modulating the degree of charge transfer from copper to dioxygen. This induces significant changes in the electronic, magnetic, and electrochemical signatures of the Cu₂O₂ species. These findings have far-reaching implications for analyses of transient metal/dioxygen intermediates, which are often studied in situ, and they may be relevant to many (bio)chemical oxidation processes when considering the widespread presence of alkali cations in synthetic and natural environments.