Intramolecular electron transfer between CoII(SQ) and CoIII(Cat) species has been investigated for the series of complexes Co(py2X)(3,6-DBQ)2, where 3,6-DBQ are semiquinonate and catecholate forms of 3,6-di-tert-butyl-1,2-benzoquinone and py2X is bis(pyridine) ether and its heteroatomic analogs with X = S, Se, and Te. Transition temperature for Co(III)/Co(II) redox isomerism decreases in steps of approximately 30 K in toluene solution and in steps of 80 K in the solid state for the complexes with X = S, Se, Te. This appears to be primarily associated with an entropy increase that results from low-energy shifts in vibrational modes with increasing heteroatomic mass. Complexes containing py2O have been isolated at room temperature in two charge distributions, CoII(py2O)(3,6-DBSQ)2 and CoIII(py2O)(3,6-DBSQ)(3,6-DBCat). Crystallographic characterization on both forms of the complex [CoII(py2O)(3,6-DBSQ)2, monoclinic, P21/c, a = 11.0280(2) Å, b = 30.2750(9) Å, c = 12.1120(2) Å, β = 113.490(2)°, V = 3708.7(1) Å3, Z = 4, R = 0.056; CoIII(py2O)(3,6-DBSQ)(3,6-DBCat), monoclinic, P21/n, a = 9.882(3) Å, b = 20.915(5) Å, c = 17.579(4) Å, β = 91.57(2)°, V = 3632(2) Å3, Z = 4, R = 0.054] has shown that the py2O ligand adopts a planar structure for the Co(II) isomer that shifts to a folded, nonplanar structure with the smaller Co(III) ion. This structural change is responsible for hysteresis in the Co(III) → Co(II) and Co(II) → Co(III) electron transfer steps in the solid state. Optically induced shifts in charge distribution have been investigated using a low-energy polychromatic light source.

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1,1‘,5,5‘-tetramethyl-6,6‘-dioxobis(verdazyl) (BVD) reacts with copper(I) halides in acetonitrile and copper(II) halides in methanol to give copper(I) coordination polymers of composition [Cu2X2(BVD)] x . When X = Cl and X = Br, these polymers crystallize in orthorhombic unit cells with dimensions a = 6.684(1) Å, b = 12.524(3) Å, and c = 8.717(2) Å (X = Cl) and a = 12.680(2) Å, b = 6.744(1) Å, and c = 8.822(2) Å (X = Br). With X = I, powder diffraction indicates a monoclinic unit cell with dimensions a = 12.669 Å, b = 8.461 Å, c = 7.679 Å, and β = 91.88°, although poor crystal quality prevented a full structure determination. Magnetic susceptibility measurements taken on the three polymers indicate that the spins couple in one-dimensional chains with alternating exchange parameters:  J 1 = −190 cm-1, J 2 = −116 cm-1 (X = Cl); J 1 = −200 cm-1, J 2 = −110 cm-1 (X = Br); and J 1 = −271 cm-1, J 2 = −200 cm-1 (X = I). Variable temperature ESR measurements on the unstable monomeric complex resulting from reaction of BVD with (CH3O)3PCuI indicate an interradical exchange parameter J of −230 cm-1. ESR spectra of all three complexes show extremely broad, featureless lines as a result of very rapid spin−lattice relaxation. The reduction in exchange between the two halves of the bis(verdazyl) ligand upon coordination, and the unusual ESR properties, require a delocalized structure, with significant spin density on the copper atom. Variation of the auxiliary ligands on the copper atom allows tuning of the intramolecular exchange.