Mononitrosyl-iron compounds (MNICs) of the Enemark-Feltham {FeNO} 7 type can be divided into ad oublet (S = 1/2) and aq uartet (S = 3/2) spin variant. The latter relies on weak-field co-ligands such as amine carboxylates. Aquaonly co-ligation appears to exist in the long-known "brownring" [Fe(H 2 O) 5 (NO)] 2 + cation, whichw as preparedo riginally from ferrous salts and NO in sulfuric acid. Ac hloride variant of this species, the green [FeCl 3 (NO)] À ion, was first prepared analoguosly by using hydrochloric instead of sulfuric acid. As at etrahedral species, it is the simple prototypeo fs ulfurbonded {FeNO} 7 (S = 3/2) MNICs of biological significance. Although it has been investigated form ore than ac entury, neitherc lean preparative routes nor reliable structuralp arameters were available for the [FeCl 3 (NO)] À ion and related speciess uch as the [FeCl 2 (NO) 2 ] À ion, ap rototypical dinitrosylirons pecies( a" DNIC"). In this work, both issues have been resolved. In addition, we report on ac omputational study on the ground-and excited-state properties including an assignment of the chromophoric transitions. Photoinduced metastable isomersw ere characterisedi nacombined experimental and computational approacht hat resulted in the confirmation of as ingle photoinduced linkage isomer of the paramagnetic nitrosyl-metal coordination entity.
Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3-tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η(2) :η(2) )-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox , was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity.
Tyrosinase model systems pinpoint pathways to translating Nature's synthetic abilities for useful synthetic catalysts. Mostly, they use N-donor ligands which mimic the histidine residues coordinating the two copper centres. Copper complexes with bis(pyrazolyl)methanes with pyridinyl or imidazolyl moieties are already reported as excellent tyrosinase models. Substitution of the pyridinyl donor results in the new ligand HC(3-tBuPz) (4-CO MePy) which stabilises a room-temperature stable μ-η :η -peroxide dicopper(II) species upon oxygenation. It reveals highly efficient catalytic activity as it hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). Stoichiometric reactions with para-substituted sodium phenolates show saturation kinetics which are nearly linear for electron-rich substrates. The resulting Hammett correlation proves the electrophilic aromatic substitution mechanism. Furthermore, density functional theory (DFT) calculations elucidate the influence of the substituent at the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning.
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