Christian Fischer

Among the many structural model complexes for the molybdenum and tungsten dependent oxidoreductases, none are specifically highlighted as being water stable and water soluble, and only very few may have such potential. Herein, complex salts with potassium cations and bisdithiolene oxido molybdate or tungstate anions are described, which were designed to be water‐tolerant and soluble in purely aqueous medium. Four different routes to water‐soluble complexes of the type K ₂ [MOx(dt) ₂ ] (M = Mo, W; dt = ene‐1,2‐dithiolate, x  = 1, 2) are presented, the applicability of which depends on the varied, distinct ligand precursors. The crystal structures of the salts are examined in detail, with particular attention to cation–anion interactions, alongside an analysis of the corresponding infrared spectra to support structural interpretations and interaction characteristics.

Fexinidazole, a drug active against trypanosomiasis and leishmaniasis, is a rare example of a nitroaromatic compound approved under the contemporary drug discovery framework. In an earlier study, we showed that the nitro group is absolutely required for antileishmanial activity. The current study employed X-ray crystallography to unveil the structural intricacies of fexinidazole and its principal metabolites, as well as electroanalytical analyses to characterize the reduction properties of the aromatic nitro group. Fexinidazole showcases a predominantly planar geometry with two distinct conformers. While most metrical parameters were conserved between fexinidazole and its metabolites, differences in the methyl ether bridge and S-methyl tail indicated distinctive preferences in molecular arrangement: conformer I of fexinidazole closely resembles the sulfone metabolite, while conformer II aligns with the sulfoxide metabolite. On the other hand, electroanalytical analysis of fexinidazole revealed a pH-dependent, two-step nitro group reduction mechanism, involving an initial concerted transfer of an electron and a proton, followed by the uptake of three electrons and three protons to likely form a hydroxylamine species. These findings characterize the molecular architecture and reduction mechanism of fexinidazole, providing valuable insights into its structural features and activation mechanism required for anti-infective activity.

The synthesis of [Cu(PteN^˄N)(P^˄P)][BF₄] complexes with pterin-fused phenanthroline (PteN^˄N) derivatives and bisphosphine (P^˄P) co-ligands was achieved through a mechanochemical approach. Due to the extremely poor solubility of PteN^˄N ligands, traditional solution methods are ineffective, whereas solid-state mechanochemistry reliably yielded the targeted heteroleptic—rather than homoleptic—complexes with considerable stability even in solution. The transformation from ligand to complex increased the solubility dramatically. The ligands and complexes were comprehensively characterised with a mixture of routine spectroscopic and spectrometric methods, the applicability of which depended to some extent on the compounds’ solubility, e.g., in the case of NMR spectroscopy. The photophysical properties of the complexes, which were not as exciting as anticipated, were assessed by absorption and emission spectroscopic methods, showing that further improvements are needed in complex design if these species are to be developed towards photocatalysis in the future.

Based on their general spacial flexibility, their Lewis and Brønsted basicity, and ability to mimic second sphere effects the 1,5‐diaza‐3,7‐diphosphacyclooctane ligand family and their complexes have regained substantial scientific interest. It was now possible to structurally analyze a recently reported member of this family with p‐tolyl and t‐butyl substituents on P and N, respectively, (P₂^p‐tolN₂^tBu). Notably, the ligand crystallizes with a ‘twisted’ backbone. This compound is the very first of its kind to have been unambiguously characterized with regard to its chemical and molecular structure as being in this conformation. A temperature‐dependent NMR study provides insight into the molecular dynamics of two isomers in solution, which are most likely also both twisted, as judged by the observed limited reactivity. Despite the in principle unfavorable conformation of the free ligand, it was successfully chelated to tungsten and molybdenum centers in mononuclear carbonyl complexes. The ligand, a derivative thereof and four new complexes were comprehensively characterized and analyzed in comparison. This includes single crystal XRD molecular structures of P₂^p‐tolN₂^tBu and all four complexes. P₂^p‐tolN₂^tBu, regardless of its twisted conformation, is able to coordinate to metal centers given that enough energy (heat) for a conformational change is provided.

The present work deals with the kinetic reasons for the inversion of the intermediates during catalytic selective reactions. By using a model system, typical ratios between diastereomer “catalyst–substrate” complexes are simulated by stable diolefin complexes [Rh(PP)(diolefin)]BF₄ of NBD (norborna-2,5-diene) and COD ((Z,Z)-cycloocta-1,5-diene). The detailed kinetic study demonstrates figuratively how the thermodynamically dominant species during the catalysis is not necessarily the major “catalyst–substrate” complex. By determination of all rate constants involved, the reasons for the unusual experimental behaviour of intermediate reversal under reaction conditions are depicted. The experimentally detected effect on a model reaction is of great importance for the explanation of the direction of, e.g., stereoselection and considerably extends the mechanistic understanding of selective reactions.

Heteroleptic molybdenum complexes bearing 1,5‐diaza‐3,7‐diphosphacyclooctane (P₂N₂) 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.

Heteroleptische Molybdänkomplexe mit 1,5‐Diaza‐3,7‐Diphosphacyclooctan (P₂N₂) und non‐innocent (nicht‐unschuldigen) Dithiolenliganden wurden synthetisiert und elektrochemisch charakterisiert. Dabei wurde festgestellt, dass die Reduktionspotentiale der Komplexe durch einen synergistischen Effekt moduliert werden, welcher durch DFT‐Berechnungen als Ligand‐Ligand‐Kooperativität über nicht‐kovalente Wechselwirkungen identifiziert werden konnte. Dieses Ergebnis konnte durch elektrochemische Studien in Kombination mit UV/Vis‐Spektroskopie und temperaturabhängiger NMR‐Spektroskopie bestätigt werden. Das beobachtete Verhalten erinnert an eine enzymatische Redoxmodulation durch Effekte basierend auf Interaktionen in der zweiten Ligandensphäre.

Triazolylidene ligands belong to a class of N-heterocyclic carbenes of growing chemical interest. Their precursors are readily available using Click chemistry and, therefore, highly modular for tuning their electronic characteristics. Due to their notable donor properties, these ligands are particularly suitable for modulating the electronic properties of the central ions of their complexes. Here, a bidentate bistriazolylidene which is a particularly strong donor ligand is combined with a low valent molybdenum(0) center and four carbon monoxide molecules as co-ligands. The novel complex exhibits characteristic electrochemical and IR-spectroscopic behavior. An X-ray structural analysis provides metrical details which are not entirely in agreement with spectroscopic data, likely going back to crystal packing effects. In comparison with precursor and ligand SCXRD data, notable geometrical changes induced by the coordination of the ligand to the metal can be observed. The analyses strongly support the bistriazolylidene ligand as being a particularly good donor of electron density towards the central metal. Potentially, these findings may support, in the future, the design of potent catalysts for the reductive activation of small molecules.

We present a method for the systematic investigation and quantification of the stability of solvate complexes of the type [Rh(diphosphine)(solvent)₂]⁺, using MeCN which replaces diolefins and forms stable Rh(i) MeCN complexes.

A new characteristic term, data point importance (DPI), is introduced in order to quantify the information ranking in multivariate data. DPI defines an easily calculable value corresponding to each row or column of data matrix to reflect its impact for keeping the pattern of the data structure. Usually, a lot of data points have DPIs equal or very close to zero so that they do not carry on useful information about keeping the data pattern. DPI values for some of the data points are significant, and they have been sorted regarding to their importance values. The applicability of the proposed procedure is tested for three typical examples of simulated and experimental data including hyperspectral image data. The possible incorporation of the DPI values for more informative score plots is suggested.