Novel tripodal 3-iodopyridinium-based receptors were investigated through (i) UV-vis and NMR titrations with anions in solution, (ii) theoretical calculations, and (iii) X-ray diffraction studies. Their anion binding properties were compared to those of the monobranched model and/or non-halogenated model systems. Investigations in acetonitrile pointed out that the iodine atom in the meta position to pyridinium enhances anion affinity. According to computational studies, this effect seemed to depend on the electron-withdrawing nature of the iodine-substituents. Notably, 1 : 1 adducts were observed to form in solution with all the investigated anions. The strong de-shielding effect observed on the receptors' protons upon anion binding indicated their participation in hydrogen-bonds with the coordinated anion. This result was supported by theoretical calculations and, in the solid state, by X-ray diffraction studies on the complexes with nitrate and bromide. In the crystalline state, the pyridinium arms of the tripodal receptor assume a "2-up, 1-down" conformation. Both nitrate and bromide anions are included in the receptor's cavity, forming two hydrogen-bonding interactions with the protons of the "2-up" arms, and one halogen-bonding interaction with the C-I group of a second molecular cation. The combination of hydrogen and halogen bonds leads to supramolecular chains in the crystals

A dinuclear copper(ii) complex derived from the chiral N ligand (2S,2'S)-N,N'-(ethane-1,2-diyl)bis(2-((1-methyl-1H-imidazol-4-ylmethyl)-amino)-3-(1-trityl-1H-imidazol-4-yl)propanamide) (EHI) was synthesized and studied as a catalyst in stereoselective oxidation reactions. The ligand contains two sets of tridentate binding units, each of them giving rise to a coordination set consisting of a pair of 5- and 6-membered chelate rings, connected by an ethanediamide linker. Stereoselectivity effects were studied in the oxidations of a series of chiral l/d biogenic catechols and the pair of l/d-tyrosine methyl esters, in this case as their phenolate salts. The oxidation of β-naphthol has also been studied as a model monooxygenase reaction. The catechol oxidation was investigated in a range of substrate concentrations at slightly acidic pH and exhibited a marked dependence on the concentration of the [CuEHI] complex. This behavior has been interpreted in terms of an equilibrium between a monomeric and a dimeric form of the catalyst. Binding studies of l- and d-tyrosine were performed as a support for the interpretation of the stereoselectivity effects observed in the reactions. In general, [CuEHI] exhibits a binding preference for the l- rather than the d-enantiomer of the substrates, but it appears that in the catecholase reaction the oxidation of the d-isomer occurs at a faster rate than for the l counterpart. The same type of enantio-discriminating behavior is observed in the oxidation of l-/d-tyrosine methyl esters. In this case the reaction produces a complex mixture of products; the main product consisting of a trimeric compound, likely formed by radical coupling reactions, has been isolated and characterized. The oxidation of β-naphthol yields an additional product of the expected quinone but labeling experiments with 18-O show no oxygen incorporation into the product, confirming that the oxidation likely proceeds through a radical mechanism.

Recent advances in dinuclear copper complexes as mimics of the catalytic centers of tyrosinase and catechol oxidase allowed the reproduction of the structural and mechanistic aspects of the enzymes. However, a challenging objective is the development of chiral complexes for bioinspired enantioselective oxidation reactions. Here, we report the synthesis and characterization of a dinuclear copper(II) complex with a new chiral diamino‐m‐xylenetetra(benzimidazole) ligand (L55Bu4), which has chiral centers at the four 2‐methylbutyl substituents of the benzimidazole rings. The spectral characteristics, ligand binding properties, and reactivity of [CuII2L55Bu4]4+ in the catalytic oxidations of several biogenic catechols (L‐/D‐dopa, L‐/D‐DopaOMe, and L‐/D‐norepinephrine) and thioanisole are reported. The best discriminating properties are displayed towards the DopaOMe derivatives, for which the oxidation rate of the L enantiomer is approximately one order of magnitude larger than that of the opposite D isomer.

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The aim of mimicking enzyme activity represents an important motivation for the development of new catalysts. A challenging objective is the development of chiral complexes for bioinspired enantioselective oxidation reactions. Herein, we report a new chiral dinuclear copper­(II) complex based on a m-xylyl-bis­(histidine) ligand (mXHI) as a biomimetic catalyst for tyrosinase and catechol oxidase. The new ligand improves a previous system also containing two tridentate N3 units derived from l-histidine that were connected by a short, rigid ethanediamine bridge. In mXHI the bridge is provided by the more extended m-xylyl moiety. The dicopper­(II) complex [Cu2(mXHI)]4+ was studied as a catalyst for stereoselective oxidations of enantiomeric couples of chiral catechols of biological interest (L/D-dopa, L/D-dopa methyl ester, and (R/S)-norepinephrine), showing excellent discrimination capability, particularly for the methyl esters of dopa enantiomers. The catechol oxidation was studied in acetate buffer as slightly acidic medium, and a role of acetate as bridging ligand between the two coppers, preorganizing the dinuclear center in a more catalytic efficient structure, could be established. The oxidation of β-naphthol and 3,5-ditertbutylphenol was studied as a model monophenolase reaction. The oxidation proceeds stoichiometrically, and the partial incorporation of 18O into β-naphthol when the reaction was performed using 18O2 suggests the existence of two competitive reaction pathways, a genuine monooxygenase mechanism and a radical pathway. However, the more challenging reaction on derivatives of l-/d-tyrosine did not lead to the desired monooxygenase product but only to products of radical oxidation. Complex [Cu2(mXHI)]4+ was also used for the catalytic sulfoxidation of thioanisole in the presence of hydroxylamine as cosubstrate, in a preliminary attempt to model the reaction of external monooxygenases. The reaction proceeds with 25 turnovers, but the enantiomeric excess of sulfoxide was modest.