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Intramolecular Spin Interactions in Bis(phenoxyl)metal Complexes of Zinc(II) and Copper(II)

1999; American Chemical Society; Volume: 38; Issue: 25 Linguagem: Inglês

10.1021/ic990396j

1520-510X

Eckhard Bill, Jochen Müller, Thomas Weyhermüller, Karl Wieghardt,

Porphyrin and Phthalocyanine Chemistry

The pendent arm macrocyclic ligand 1-ethyl-4,7-bis(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, H2L, forms stable complexes in methanol with zinc(II) and copper(II) ions: [ZnII(L)]·H2O (1); [CuII(L)]·0.5 CH2Cl2 (2); [CuII(LH)](ClO4) (3). The crystal structures of 1 and 2 have been determined by X-ray crystallography: 1 crystallizes in the orthorhombic space group Pbca with a = 21.100(4) Å, b = 10.267(2) Å, c = 28.896(6) Å, V = 6260(2) Å3, Z = 8; 2 crystallizes in the monoclinic space group C2/c with a = 14.447(2) Å, b = 25.522(4) Å, c = 17.296(3) Å, V = 6300(2) Å3, Z = 8. In CH2Cl2 solution complexes 1 and 2 can electrochemically be reversibly oxidized by two successive one-electron processes generating the stable phenoxyl mono- ([1]•+; [2]•+) and diradicals ([1]2•2+, [2]2•2+). In contrast, 3 containing a coordinated phenol and one phenolate can only be oxidized to the monoradical [3]•2+. The electronic structure of these mono- and diradicals have been established by UV/vis and EPR spectroscopy in fluid and/or frozen solution. All oxidations are ligand-centered generating coordinated phenoxyl radicals. In [1]2•2+ the two unpaired electrons interact with each other via exchange and weak dipolar couplings of the order of −3 and 10-2 cm-1, respectively. The monoradicals [2]•+ and [3]•2+ are nearly EPR-silent; an St = 1 excited state for both species is barely observable due to large zero-field splitting. In [1]•+ the phenoxyl radical electron is localized on one phenyl ring whereas for [2]•+ some degree of delocalization over both phenyl rings may be present. The diradical [1]2•2+ possesses a diamagnetic whereas [2]2•2+ has an St = 3/2 ground state.