Intramolecular spin interactions in bis(phenoxyl)metal complexes of zinc(II) and copper(II)

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: [Zn-II(L)].H2O (1); [Cu-II(L)].0.5 CH2Cl2 (2); [Cu-II(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) Angstrom, b = 10.267(2) Angstrom, c = 28.896(6) Angstrom, V = 6260(2) Angstrom(3), Z = 8; 2 crystallizes in the monoclinic space group C2/c with a = 14.447(2) Angstrom, b = 25.522(4) Angstrom, c = 17.296(3) Angstrom, V = 6300(2) Angstrom(3), 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 S-t = 1 ex:cited 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 S-t = 3/2 ground state.