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Bearing in mind the potential which tris(pyrazolyl) hydroborate ligands have demonstrated in various different fields of coordination chemistry, the behavior of a more rarely employed representative, namely bis(3-mesitylpyrazol-1-yl)(5-mesitylpyrazol-1-yl) hydroborate (TpMes,H*), towards a series of divalent metal ions has been investigated. Thus, the synthesis of two new heteroleptic metal chlorido [TpMes,H*MCl] complexes, 2, (Mn2+, 2-a, and Cu2+, 2-e) are described, which correspond to the typical precursor compounds that would be needed for subsequent research. Structural and spectroscopic properties of the two new and four published members of this series (M=Fe2+, 2-b, Co2+, 2-c, Ni2+, 2-d, Zn2+, 2-f) are discussed in detail for the solid and solution states. Furthermore, synthetic routes to five homoleptic [(TpMes,H*)2M] complexes, 3, bearing Mn2+, 3-a, Fe2+, 3-b, Co2+, 3-c, Ni2+, 3-d, and Cu2+, 3-e, ions as well as an alternative preparation method for [(κ2-TpMes,H*)2Zn], 3-f, are described. Their structures and spectroscopic parameters are compared to those of their heteroleptic counterparts. Additionally, the complex [(TpH,Mes*)2Cu], 4, is reported, representing the second example of a complex with the (3-mesitylpyrazol-1-yl)bis(5-mesitylpyrazol-1-yl) hydroborate ligand (TpH,Mes*). Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

The design of biomimetic model complexes for the cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) is reported, where the 3‐His coordination of the iron ion is simulated by three pyrazole donors of a trispyrazolyl borate ligand (Tp) and protected cysteine and cysteamine represent substrate ligands. It is found that the replacement of phenyl groups—attached at the 3‐positions of the pyrazole units in a previous model—by mesityl residues has massive consequences, as the latter arrange to a more spacious reaction pocket. Thus, the reaction with O2 proceeds much faster and afterwards the first structural characterization of an iron(II) η2‐O,O‐sulfinate product became possible. If one of the three Tp‐mesityl groups is placed in the 5‐position, an even larger reaction pocket results, which leads to yet faster rates and accumulation of a reaction intermediate at low temperatures, as shown by UV/Vis and Mössbauer spectroscopy. After comparison with the results of investigations on the cobalt analogues this intermediate is tentatively assigned to an iron(III) superoxide species.

Quercetin-Dioxygenasen (QueD) katalysieren die oxidative Spaltung von Quercetin, einem Pflanzenfarbstoff aus der Gruppe der Flavonole und bilden dabei das entsprechende Depsid und Kohlenstoffmonoxid. Interessanterweise werden in den QueDs natürlicher Quellen verschiedene zweiwertige Metallionen als Cofaktor im aktiven Zentrum des Enzyms gefunden. So stellt sich die Frage, welche Rolle dem Metallzentrum im Mechanismus der Katalyse zukommt. Um die Umgebung des Metallions im aktiven Zentrum mit einer biomimetischen, niedermolekularen Modellverbindung nachzuempfinden, wurde das Trispyrazolylborato-Ligandsystem (Tp) gewählt und als Substratanalogon diente 3-Hydroxyflavon (FlaH). So konnte ein strukturelles und funktionelles Modellsystem der NiQueD in Form von des Tp*NiFla-Komplexes synthetisiert und vollständig charakterisiert werden. Es erfolgte eine Variation dieses Systems, um verschiedene Einflüsse auf die Reaktivität mit Disauerstoff zu untersuchen. Der Austausch der Carbonylfunktion von FlaH durch C=S sowie C=Se Einheiten führte bei der Umsetzung mit O2 nicht zu der antizipierten Erhöhung der Reaktionsraten, stattdessen wurden zusätzlich Nebenreaktionen beobachtet. Die Veränderung der Substituenten am Tp-Ligandrückgrat zeigte, dass sterisch anspruchsvollere Gruppen zur Erhöhung der Reaktionsraten bei Umsetzungen mit O2 führen, vermutlich weil das Substrat für eine direkte Reaktion mit O2 leichter zugänglich wird. Durch systematische Variation der 3d-Übergangsmetallionen im Zentrum der Modellkomplexe wurde ein Einfluss auf die Redoxeigenschaften des metallgebundenen Flavonolats beobachtet. Die reversiblen Redoxpotentiale stehen in direktem Zusammenhang mit der Reaktionsrate. Ergebnisse mechanistischer Untersuchungen legen einen outer-sphere Elektronentransferprozess nahe, bei dem ein Elektron des Flavonolats direkt auf O2 übertragen wird. Durch Rekombination der entstandenen Radikale werden die nach biomimetischer Reaktion zu erwartenden Produkte gebildet., Quercetin dioxygenases (QueD) catalyze the oxidative cleavage of quercetin, a flavonol commonly found in fruits and leaves, forming the corresponding depside and carbon monoxide. Interestingly, quercetinases of various natural sources show a different selectivity towards the divalent metal ion incorporated as cofactor, raising the questions on the role of the metal center in the mechanism of catalysis. Synthetic can help to gain insight into the mechanistic pathway of the reaction and thus clearify such questions. In order to synthesize a biomimetic model compound, the trispyrazolylborato ligands (Tp) were used and 3 hydroxyflavone (FlaH) was chosen as substrate. The compound Tp*NiFla with a was synthesized and fully characterized as a structural and functional model for the NiQueD. Based on this, the system was varied in different ways in order to investigate the influence on the reactivity towards O2. It was shown that the substitution of the carbonyl function of FlaH by C=S and C=Se units did not lead to an increase in the reaction rates, but additionally to undesirable side reactions. By altering the residues on the Tp ligand backbone it turned out that sterically more demanding groups increase the rates of reaction with dioxygen, likely because the substrate is more accessible for direct reaction with O2. By systematic variation of the metal ions in the center of the model compounds, an influence on the redox properties of the metal-bound flavonolate was observed. For the first time, reversible redox reactions of flavonolate bound to 3d transition metals was demonstrated. Furthermore, a direct relation of the redox potentials to the reaction rates emerged. The results of mechanistic studies indicate that all model complexes react via an initial outer-sphere electron transfer process, in which an electron of the flavonolate is directly transferred to O2. By recombination of the formed radicals, the products expected for a biomimetic process can be obtained.