Search

A long-standing question in electron transfer research concerns the number and identity of collective nuclear motions that drive electron transfer or localisation. It is well established that these nuclear motions are commonly gathered into a so-called electron transfer coordinate. In this theoretical study, we demonstrate that both anti-symmetric and symmetric vibrational motions are intrinsic to bridged systems, and that both are required to explain the characteristic shape of their intervalence charge transfer bands. Using the properties of a two-state Marcus-Hush model, we identify and quantify these two coordinates as linear combinations of normal modes from ab initio calculations. This quantification gives access to the potential coupling, reorganization energy and curvature of the potential energy surfaces involved in electron transfer, independent of any prior assumptions about the system of interest. We showcase these claims with the Creutz-Taube ion, a prototypical Class III mixed valence complex. We find that the symmetric dimension is responsible for the asymmetric band shape, and trace this back to the offset of the ground and excited state potentials in this dimension. The significance of the symmetric dimension originates from geometry dependent coupling, which in turn is a natural consequence of the well-established superexchange mechanism. The conceptual connection between the symmetric and anti-symmetric motions and the superexchange mechanism appears as a general result for bridged systems., (© 2024 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Heteroleptic molybdenum complexes bearing 1,5-diaza-3,7-diphosphacyclooctane (P 2 N 2 ) and non-innocent dithiolene ligands were synthesized and electrochemically characterized. The reduction potentials of the complexes were found to be fine-tuned by a synergistic effect identified by DFT calculations as ligand-ligand cooperativity via non-covalent interactions. This finding is supported by electrochemical studies combined with UV/Vis spectroscopy and temperature-dependent NMR spectroscopy. The observed behavior is reminiscent of enzymatic redox modulation using second ligand sphere effects., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

For a future hydrogen economy, non-precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal-nitrogen-carbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN 4 ) show promising performance, but an optimization rooted in structure-property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN 4 centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [Fe II (P⋅)] - radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The application of an alkyne cyclotrimerization regime with an [Fe(salen)] 2 -μ-oxo (1) catalyst to triphenylmethylphosphaalkyne (2) yields gram-scale quantities of 2,4,6-tris(triphenylmethyl)-Dewar-1,3,5-triphosphabenzene (3). Bulky lithium salt LiHMDS facilitates a rearrangement of 3 to the 1,3,5-triphosphabenzene valence isomer (3'), which subsequently undergoes an intriguing phosphorus migration reaction to form the ring-contracted species (3''). Density functional theory calculations provide a plausible mechanism for this rearrangement. Given the stability of 3, a diverse array of unprecedented transformations was investigated. We report novel crystallographically characterized products of successful nucleophilic/electrophilic addition and protonation/oxidation reactions., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Nitride complexes are key species in homogeneous nitrogen fixation to NH 3 via stepwise proton-coupled electron transfer (PCET). In contrast, direct generation of nitrogenous organic products from N 2 -derived nitrides requires new strategies to enable efficient reductive nitride transfer in the presence of organic electrophiles. We here present a 2-step protocol for the conversion of dinitrogen to benzonitrile. Photoelectrochemical, reductive N 2 splitting produces a rhenium(V) nitride with unfavorable PCET thermochemistry towards ammonia generation. However, N-benzoylation stabilizes subsequent reduction as a basis for selective nitrogen transfer in the presence of the organic electrophile and Brønsted acid at mild reduction potentials. This work offers a new strategy for photoelectrosynthetic nitrogen fixation beyond ammonia-to yield nitrogenous organic products., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The properties and reactivities of transition metal complexes are often discussed in terms of Ligand Field Theory (LFT), and with ab initio LFT a direct connection to quantum chemical wavefunctions was recently established. The Angular Overlap Model (AOM) is a widely used, ligand-specific parameterization scheme of the ligand field splitting that has, however, been restricted by the availability and resolution of experimental data. Using ab initio LFT, we present here a generalised, symmetry-independent and automated fitting procedure for AOM parameters that is even applicable to formally underdetermined or experimentally inaccessible systems. This method allows quantitative evaluations of assumptions commonly made in AOM applications, for example, transferability or the relative magnitudes of AOM parameters, and the response of the ligand field to structural or electronic changes. A two-dimensional spectrochemical series of tetrahedral halido metalates ([M II X 4 ] 2- , M=Mn-Cu) served as a case study. A previously unknown linear relationship between the halide ligands' chemical hardness and their AOM parameters was found. The impartial and automated procedure for identifying AOM parameters introduced here can be used to systematically improve our understanding of ligand-metal interactions in coordination complexes., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N-N bond energy. Photochemical strategies that drive N 2 fixation are scarcely developed. Here, the synthesis of a dinuclear N 2 -bridged complex is presented upon reduction of a rhenium(III) pincer platform. Photochemical splitting into terminal nitride complexes is triggered by visible light. Clean nitrogen transfer with benzoyl chloride to free benzamide and benzonitrile is enabled by cooperative 2 H + /2 e - transfer of the pincer ligand. A three-step cycle is demonstrated for N 2 to nitrile fixation that relies on electrochemical reduction, photochemical N 2 -splitting and thermal nitrogen transfer., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The search for molecular catalysts that efficiently activate or cleave the dinitrogen molecule is an active field of research. While many thermal dinitrogen cleavage catalysts are known, the photochemical activation of N 2 has received considerably less attention. In this paper, the first computational study of the osmium dimer [Os (II,III) 2 (μ-N 2 )(NH 3 ) 10 ] 5+ , which was shown to be capable of dinitrogen photocleavage, is presented. Despite its deceptively simple geometry, it has a complex electronic structure with a valence-delocalized and electronically degenerate ground state. Using multiconfigurational methods, the electronic structure at the ground state geometry and along the dinitrogen cleavage coordinate was investigated. The results indicate that an unoccupied molecular orbital with σ-bonding character between osmium and μ-N atoms and σ-antibonding dinitrogen character is most affected by N-N distance elongation. This implies that a lower barrier for thermal or photochemical N 2 activation in linear M-N-N-M complexes can be achieved by lowering the energetic separation between this unoccupied orbital and the HOMO, representing a specific target for future catalyst design., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)