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RESEARCH PRODUCT

Copper(ii) and Zn(ii) coordination chemistry of tetraaza[n]cyclophanes

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The acid–base behaviour and Zn2+ and Cu2+ metal coordination chemistry of the novel orthocyclophane ligands 2,5,8,11-tetraaza[12]orthocyclophane (L2) and 2,5,9,12-tetraaza[13]orthocyclophane (L3) and metacyclophane 2,5,8,11-tetraaza[12]metacylophane (L1) are studied. Important differences in the chemistry of these compounds are found depending on the substitution of the aromatic ring. The ortho derivatives are much more basic in their first two protonation steps while the metacyclophane presents much larger constants in the third and fourth protonation stages. The crystal structure of the picrate salt of [H2L3]2+ shows an alternate disposition of the protons in the molecule with formation of N–H+⋯N hydrogen bonds between the protonated and non-protonated amino groups. While formation of mononuclear Cu2+ complexes is observed for L2 and L3, the metaderivative L1 forms also binuclear Cu2+ species which predominate largely for molar ratios M∶L 2∶1. The crystal structure of the complex [CuL3Cl](ClO4) shows a chain-like arrangement with the [CuL3]2+ units interconnected by chlorine atoms. The coordination geometry around the metal ion is a distorted octahedron with the nitrogen atoms of the macrocycle at the vertices of the equatorial plane and chloride anions asymmetrically disposed at the axial positions. The Zn2+ complexes of the ortho derivatives L2 and L3 are also much more stable than the meta counterparts. The crystal structure of [ZnL3Cl]Cl·H2O shows a square pyramidal geometry with the nitrogen atoms of the macrocycle at the vertices of the equatorial plane and one chloride anion occupying the axial position. In this case there are not chain arrangements. The involvement of all the nitrogen atoms in the coordination of Zn2+ by L3 in aqueous solution is proved by 1H and 13C NMR.