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The reaction of three equivalents of LiCH(SiMe 3 ) 2 with TiCl 3 (NMe 3 ) 2 afforded the rare homoleptic Ti(III) alkyl Ti{CH(SiMe 3 ) 2 } 3 (1) which crystallized as blue needles in 32 % yield. Single crystal X-Ray data for 1 showed a trigonal pyramidal coordination geometry around titanium, which could be ascribed to weak interactions between the C-H bonds and the Ti(III) atom based on computational results. X-band EPR spectroscopy gives spectral parameters consistent with the proposed Ti(III) formulation. Solutions of 1 are unstable at room temperature owing to intramolecular C-H activation that gave a dimeric Ti(IV) complex [{(Me 3 Si) 2 HC}Ti{μ-CHSiMe 2 CHSiMe 3 }] 2 (2)., (© 2025 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbH.)

The low reduction potentials required for the reduction of dinitrogen (N 2 ) render metal-based nitrogen-fixation catalysts vulnerable to irreversible damage by dioxygen (O 2 ) 1-3 . Such O 2 sensitivity represents a major conundrum for the enzyme nitrogenase, as a large fraction of nitrogen-fixing organisms are either obligate aerobes or closely associated with O 2 -respiring organisms to support the high energy demand of catalytic N 2 reduction 4 . To counter O 2 damage to nitrogenase, diazotrophs use O 2 scavengers, exploit compartmentalization or maintain high respiration rates to minimize intracellular O 2 concentrations 4-8 . A last line of damage control is provided by the 'conformational protection' mechanism 9 , in which a [2Fe:2S] ferredoxin-family protein termed FeSII (ref.  10 ) is activated under O 2 stress to form an O 2 -resistant complex with the nitrogenase component proteins 11,12 . Despite previous insights, the molecular basis for the conformational O 2 protection of nitrogenase and the mechanism of FeSII activation are not understood. Here we report the structural characterization of the Azotobacter vinelandii FeSII-nitrogenase complex by cryo-electron microscopy. Our studies reveal a core complex consisting of two molybdenum-iron proteins (MoFePs), two iron proteins (FePs) and one FeSII homodimer, which polymerize into extended filaments. In this three-protein complex, FeSII mediates an extensive network of interactions with MoFeP and FeP to position their iron-sulphur clusters in catalytically inactive but O 2 -protected states. The architecture of the FeSII-nitrogenase complex is confirmed by solution studies, which further indicate that the activation of FeSII involves an oxidation-induced conformational change., Competing Interests: Competing interests: The authors declare no competing interests., (© 2025. The Author(s), under exclusive licence to Springer Nature Limited.)

We expand upon the synthetic utility of anionic rhenium complex Na[(BDI)ReCp] ( 1 , BDI = N,N' -bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) to generate several rhenium-phosphorus complexes. Complex 1 reacts in a metathetical manner with chlorophosphines Ph 2 PCl, Me NHP-Cl, and OHP-Cl to generate XL-type phosphido complexes 2 , 3 , and 4 , respectively ( Me NHP-Cl = 2-chloro-1,3-dimethyl-1,3,2-diazaphospholidine; OHP-Cl = 2-chloro-1,3,2-dioxaphospholane). Crystallographic and computational investigations of phosphido triad 2 , 3 , and 4 reveal that increasing the electronegativity of the phosphorus substituent (C < N < O) results in a shortening and strengthening of the rhenium-phosphorus bond. Complex 1 reacts with iminophosphane Mes*NPCl (Mes* = 2,4,6-tri tert -butylphenyl) to generate linear iminophosphanyl complex 5 . In the presence of a suitable halide abstraction reagent, 1 reacts with the dichlorophosphine i Pr 2 NPCl 2 to afford cationic phosphinidene complex 6 + . Complex 6 + may be reduced by one electron to form 6 • , a rare example of a stable, paramagnetic phosphinidene complex. Spectroscopic and structural investigations, as well as computational analyses, are employed to elucidate the influence of the phosphorus substituent on the nature of the rhenium-phosphorus bond in 2 through 6 . Furthermore, we examine several common analogies employed to understand metal phosphido, phosphinidene, and iminophosphanyl complexes.

Metalloenzymes with heteromultimetallic active sites perform chemical reactions that control several biogeochemical cycles. Transformations catalyzed by such enzymes include dioxygen generation and reduction, dinitrogen reduction, and carbon dioxide reduction-instrumental transformations for progress in the context of artificial photosynthesis and sustainable fertilizer production. While the roles of the respective metals are of interest in all these enzymatic transformations, they share a common factor in the transfer of one or multiple redox equivalents. In light of this feature, it is surprising to find that incorporation of redox- inactive metals into the active site of such an enzyme is critical to its function. To illustrate, the presence of a redox-inactive Ca 2+ center is crucial in the Oxygen Evolving Complex, and yet particularly intriguing given that the transformation catalyzed by this cluster is a redox process involving four electrons. Therefore, the effects of redox inactive metals on redox processes-electron transfer, oxygen- and hydrogen-atom transfer, and O-O bond cleavage and formation reactions-mediated by transition metals have been studied extensively. Significant effects of redox inactive metals have been observed on these redox transformations; linear free energy correlations between Lewis acidity and the redox properties of synthetic model complexes are observed for several reactions. In this Perspective, these effects and their relevance to multielectron processes will be discussed., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The S 2 state of the Oxygen Evolving Complex (OEC) of Photosystem II (PSII) shows high-spin (HS) and low-spin (LS) EPR signals attributed to distinct structures based on computation. Five-coordinate Mn III centers are proposed in these species but are absent in available spectroscopic model complexes. Herein, we report the synthesis, crystal structure, electrochemistry, SQUID magnetometry, and EPR spectroscopy of a Mn III Mn IV 3 O 4 cuboidal complex featuring five-coordinate Mn III . This cluster displays a spin ground state of S = 5/2, while conversion to a six-coordinate Mn upon treatment with water results in a spin state change to S = 1/2. These results demonstrate that coordination number, without dramatic changes within the Mn 4 O 4 core, has a substantial effect on spectroscopy.

Synthetic complexes provide useful models to study the interplay between the structure and spectroscopy of the different S n -state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the Mn IV 4 core corresponding to the S 3 state, the last observable intermediate prior to dioxygen formation, remain very rare. Toward the development of synthetic strategies to stabilize highly oxidized tetranuclear complexes, ligands with increased anion charge were pursued. Herein, we report the synthesis, electrochemistry, SQUID magnetometry, and electron paramagnetic resonance spectroscopy of a stable Mn IV 4 O 4 cuboidal complex supported by a disiloxide ligand. The substitution of an anionic acetate or amidate ligand with a dianionic disiloxide ligand shifts the reduction potential of the Mn III Mn IV 3 /Mn IV 4 redox couple by up to ∼760 mV, improving stability. The S = 3 spin ground state of the siloxide-ligated Mn IV 4 O 4 complex matches the acetate and amidate variants, in corroboration with the Mn IV 4 assignment of the S 3 state of the OEC.

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn 3 IV O 4 and YMn 3 IV O 4 complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn 3 IV O 4 complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo-C 3 -symmetric Ca(Y)Mn 3 IV O 4 core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn 3 IV O 3 (OH). Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S 2 state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes., (© 2021 Wiley-VCH GmbH.)

Exposure of bis-amidinate and -guanidinate supported thorium dialkyl complexes to dioxygen results in facile oxygen atom insertion and formation of the corresponding thorium bis(alkoxide) species. Preliminary mechanistic studies suggest a radical propagation mechanism is operative. All new complexes were fully characterized by 1H and 13C NMR spectroscopy, IR, EA and X-ray crystallography.

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the oxygen evolving complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S 2 state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure-electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [Mn III Mn 3 IV O 4 ] cuboidal complexes as spectroscopic models of the S 2 state of the OEC. Resembling the oxidation state and EPR spectra of the S 2 state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed.

Reaction of the first homoleptic U(iii) guanidinate complex with diphenyldiazomethane results in two-electron oxidation of U(iii) to U(v) and isolation of a U(v) hydrazido complex. Corresponding U(v) imido, U(v) oxo, and U(iv) azido complexes were also synthesized for structural comparison.