RESUMEN
A novel dinucleating bis(pyrazolyl)methane ligand was developed for tyrosinase model systems. After ligand synthesis, the corresponding Cu(I) complex was synthesized and upon oxygenation, formation of a µ-η2:η2 peroxido complex could be observed and monitored using UV/Vis-spectroscopy. Due to the high stability of this species even at room temperature, a molecular structure of the complex could be characterized via single-crystal XRD. Additional to its promising stability, the peroxido complex showed catalytic tyrosinase activity which was investigated via UV/Vis-spectroscopy. Products of the catalytic conversion could be isolated and characterized and the ligand could be successfully recycled after catalysis experiments. Furthermore, the peroxido complex was reduced by reductants with different reduction potentials. The characteristics of the electron transfer reactions were investigated with the help of the Marcus relation. The combination of the high stability and catalytic activity of the peroxido complex with the new dinucleating ligand, enables the shift of oxygenation reactions for selected substrates towards green chemistry, which is furthered by the efficient ligand recycling capability.
RESUMEN
In this study, a synthesis route of tri(quinolin-8-yl)amine (L), a recent member of the tetradentate tris(2-pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ4 mode, two cis-oriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinationsâbis(triflato), bis(acetonitrile), and mixed coligand speciesâcould be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mössbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time- and temperature-resolved UV/vis spectroscopy, Mössbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between high- and low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)]2+ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environmentâall of the {N6}-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.
RESUMEN
Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Mößbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N5O} to {N6} and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Mößbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch.
