A Ferromagnetic [Cu_3(OH)_2]~(4+) Cluster Formed inside a Tritopic Nonaazapyridinophane: Crystal Structure and Solution Studies

摘要

Chemists working in coordination and/or supramolecular chemistry find a continuous source of inspiration in biomol-ecules and enzyme active sites. In this respect, trinuclear Cu~(II) centers have attracted a lot of interest due to their resemblance to multicenter blue copper oxidases like ascor-bate oxidase, ceruloplasmin, lacase oxidase, and particulate methane monooxygenase. These enzymes contain at least four copper centers, which are necessary for four-electron reduction of molecular oxygen to water. For instance, the high-resolution structure of ascorbate oxidase shows that the mononuclear electron-transfer copper site of type 1 (T1) is connected to the trinuclear site through a patch formed by a cysteine residue from which two histidine residues diverge tobind the T3 copper ions. In the oxidized form of the enzyme, the T3 Cu~(II) atoms, apart from the imidazole moiety of the histidine residues of the patch, are coordinated by another two imidazole units and by an OH group that bridges the two atoms, which are 3.71 A far apart from each other. The geometry around each metal ion can be best described as a trigonal bipyramid with a vacant equatorial position oriented towards the T2 copper atom, which completes the trinuclear center. In the T2 site the Cu~(II) center is coordinated in a very particular T-shaped geometry by two histidine residues and a monodentate hydroxo group or a water molecule.