Pierre Kennepohl

Covalency is implicated in spin-phonon coupling in molec- ular qubits, yet direct, element-resolved measures of spin delocalization in working systems are scarce. Here, we combine multi-edge X-ray absorption spectroscopy (XAS)– including Cu L2,3-edge XAS to quantify metal character and S K-edge XAS to quantify ligand character–with pulsed and continuous-wave EPR and magnetic circular dichro- ism (MCD) spectroscopy. We correlate T1 with orbital reduction factors η calculated from both EPR and XAS measurements using ligand-field excitation energies from MCD. As symmetry-matched model complexes, we syn- thesized the S = 1 2 dithiolene complexes [Cu(dtsq)2]2− and [VO(dtsq)2]2− (dtsq2- = dithiosquarate) and com- pared them to the maleonitriledithiolate (mnt2-) bench- mark set. Resonance Raman spectroscopy and temperature- dependent pulsed EPR indicate that local vibrational modes do not dominate the observed differences in spin-lattice re- laxation (T1) in these systems, supporting a regime in which variations in SPC are dominated by electronic factors. This integrated analysis lays the groundwork for a covalency– relaxation relationship and shows that η tracks inversely with XAS-derived measures of spin delocalization.

Redox noninnocent ligands present opportunities to access reactive metal-radical complexes, but competing redox pathways often hinder their selective formation. Here, a pentadentate arylhydroxylamine-containing (ArNHOH) ligand directs copper salts to form a single, well-defined Cu^II-arylnitrosyl (ArNO^•-) radical complex via an unprecedented oxidative binding pathway. To the best of our knowledge, this is the first crystallographically characterized mononuclear κN-bonded Cu^II-(ArNO^•-) complex. Magnetometry, NMR, and density functional theory (DFT) reveal a pronounced difference in copper-radical antiferromagnetic coupling (AFC) between the solid state and solution (-1290 vs -428 cm^-1, respectively). This phase-dependent magnetism is associated with different geometries that impart different d_σ(Cu)-π*(ArNO) overlaps of the magnetic orbitals. Quantitative L-edge X-ray absorption spectra (XAS) together with DFT shows that the Cu^II-(ArNO^•-) interaction is strongly covalent, consistent with its robust antiferromagnetism. Mechanistic studies, including electrochemistry, establish conditions where oxidative binding is operative and show that product selectivity is governed by Cu oxidation state, base concentration, and the order and rate of addition of constituents. Taken together, this work introduces oxidative binding as a distinct route to access Cu-arylnitrosyl radical species and highlights how ligand design and geometric tuning dictate electronic structure and spin-state energetics in metal-radical complexes.

In chemistry, analyzing spectra through peak fitting is a crucial task that helps scientists extract useful quantitative information about a sample's chemical composition or electronic structure. To make this process more efficient, we have developed a new open-source software tool called SpectraFit. This tool allows users to perform quick data fitting using expressions of distribution and linear functions through the command line interface (CLI) or Jupyter Notebook, which can run on Linux, Windows, and MacOS, as well as in a Docker container. As part of our commitment to good scientific practice, we have introduced an output file-locking system to ensure the accuracy and consistency of information. This system collects input data, results data, and the initial fitting model in a single file, promoting transparency, reproducibility, collaboration, and innovation. To demonstrate SpectraFit's user-friendly interface and the advantages of its output file-locking system, we are focusing on a series of previously published iron-sulfur dimers and their XAS spectra. We will show how to analyze the XAS spectra via CLI and in a Jupyter Notebook by simultaneously fitting multiple data sets using SpectraFit. Additionally, we will demonstrate how SpectraFit can be used as a black box and white box solution, allowing users to apply their own algorithms to engineer the data further. This publication, along with its Supporting Information and the Jupyter Notebook, serves as a tutorial to guide users through each step of the process. SpectraFit will streamline the peak fitting process and provide a convenient, standardized platform for users to share fitting models, which we hope will improve transparency and reproducibility in the field of spectroscopy.

In chemistry, analyzing spectra through peak fitting is a crucial task that helps scientists extract useful quantitative information about a sample's chemical composition or electronic structure. To make this process more efficient, we have developed a new open-source software tool called SpectraFit. This tool allows users to perform quick data fitting using regular expressions of distribution and linear functions through the command line interface (CLI) or Jupyter Notebook, which can run on Linux, Windows, and MacOS, as well as in a Docker container. As part of our commitment to good scientific practice, we have introduced an output file-locking system to ensure the accuracy and consistency of information. This system collects input data, results data, and the initial fitting model in a single file, promoting transparency, reproducibility, collaboration, and innovation. To demonstrate SpectraFit's user-friendly interface and the advantages of its output file-locking system, we are focusing on a series of previously published iron-sulfur dimers and their XAS spectra. We will show how to analyze the XAS spectra via CLI and in a Jupyter Notebook by applying a global fitting routine. Additionally, we will demonstrate how SpectraFit can be used as a black box and white box solution, allowing users to apply their own algorithms to engineer the data further. The publication, along with its supplementary information, serves as a playbook to guide users through each step of the process. SpectraFit will streamline the peak fitting process and prove to be an invaluable resource for chemists and other professionals working in related fields.

A series of petroleum coke (petcoke)-derived solid acid catalysts were prepared via nitric acid treatment with or without ball milling pretreatment. The inherent sulfur in petcoke was converted to sulfonic groups, which were active sites for the esterification of octanoic acid and methanol at 60 °C, with ester yields of 14–43%. More specifically, samples without ball milling treated at 120 °C for 3 h had a total acidity of 4.67 mmol/g, which was 1.6 times that of the samples treated at 80 °C, despite their −SO₃H acidities being similar (~0.08 mmol/g). The samples treated for 24 h had higher −SO₃H (0.10 mmol/g) and total acidity (5.25 mmol/g) but not increased catalytic activity. Ball milling increased the defects and exposed aromatic hydrogen groups on petcoke, which facilitated further acid oxidation (0.12 mmol −SO₃H/g for both materials and total acidity of 5.18 mmol/g and 5.01 mmol/g for BP-N-3/120 and BP-N-8/90, respectively) and an increased ester yield. DFT calculations were used to analyze the pathways of sulfonic acid group formation, and the reaction pathway with NO₂• was the most thermodynamically and kinetically favourable. The activities of the prepared catalysts were related to the number of −SO₃H acid sites, the total acidity, and the oxygen content, with the latter two factors having a negative impact.

Ionic liquid (IL) based composite materials are promising antimicrobial agents, owing to their inherent germicidal properties, as well as their ability to stabilize metal nanoparticles (NPs), which may serve as a secondary antimicrobial reservoir.

Ionic liquid (IL) based composite materials have shown great promise as antimicrobial coatings, owing to their inherent germicidal properties, as well as their ability to stabilize metal nanoparticles (NPs), which may serve as a secondary antimicrobial reservoir. Here, we show that tetraalkylphosphonium ILs (TAPILs) on their own can annihilate pathogens by interfering with their cell membranes; however, the nature of the alkyl substituents on the central P atom and the nature of the anion play decisive roles in determining their antimicrobial activities. Con-comitantly, TAPILs can stabilize copper nanoparticles (Cu NPs) generated directly within the IL matrices without the addition of any second-ary stabilizers. The composites thus generated were thoroughly characterized and shown to be far more lethal to E. coli than just the TAPILs. The antibacterial effect demonstrated by the composite created from P[6,6,6,8]Cl (TAPIL-2) was orders of magnitude more lethal to microbes in comparison with P[6,6,6,8]Cl or copper nanoparticles alone. Neither the parent TAPIL-2 nor composite-2 were compromised by ambient storage conditions over a period of months with regards to their bactericidal effects. The composite with the best performance (composite-2) also proved to be effective against a panel of selected microbes. SEM studies were conducted to image E. coli after exposure to the TAPILs or composite-2; with the latter, only bacterial debris were noticed post-exposure, indicating total bacterial annihilation. The killing kinetic assay and regression analyses for time-dependent bactericidal activity of composite-2 against E. coli and S. aureus demonstrated increase in log re-duction values over time, indicating the effectiveness of composite-2 in reducing the viable cell counts for both bacterial strains. Finally, Cu K-edge XANES was used to investigate the fate of Cu NPs within the composites, revealing oxidative disintegration of the Cu NPs within the TAPIL matrices over time, releasing charged copper ions and/or small copper clusters which interfere with the integrity and the permeability of E. coli cell membranes, inducing cell death. This was confirmed by SEM of bacterial preparations before and after exposure to both the TAPILs themselves as well as to the composites. Exposing E. coli to composite-2 causes complete cellular destruction, leaving behind cellular debris as the only visible organic matter. Thus, these TAPIL-based composites containing ‘ion reservoir’ metal NPs are potent antimicrobial materials, deserving additional research.

The role of halogen bonding (XB) in chemical catalysis has largely involved using XB donors as Lewis acid activators to modulate the reactivity of partner Lewis bases. We explore a more uncommon scenario, where a Lewis base modulates reactivity via a spectator halogen bond interaction. Our computational studies reveal that spectator halogen bonds may play an important role in modulating the rate of S_N2 reactions. Most notably, π acceptors such as PF₃ significantly decrease the barrier to substitution by decreasing electron density in the very electron rich transition state. Such π-backbonding represents an example of a heretofore unexplored situation in halogen bonding: the combination of both σ-donation and π-backdonation in this “non-covalent” interaction. The broader implications of this observation are discussed.

Agostic interactions are examples of σ-type interactions, typically resulting from interactions between C–H σ-bonds with empty transition metal d orbitals. Such interactions often reflect the first step in transition metal-catalysed C–H activation processes and thus are of critical importance in understanding and controlling σ bond activation chemistries. Herein, we report on the unusual electronic structure of linear electron-rich d⁹ Ni(I) complexes with symmetric bis(C–H) agostic interactions. A combination of Ni K edge and L edge XAS with supporting TD-DFT/DFT calculations reveals an unconventional covalent agostic interaction with limited contributions from the valence Ni 3d orbitals. The agostic interaction is driven via the empty Ni 4p orbitals. The surprisingly strong Ni 4p-derived agostic interaction is dominated by σ contributions with minor π contributions. The resulting ligand–metal donation occurs directly along the C–Ni bond axis, reflecting a novel mode of bis-agostic bonding.