Bistability and molecular switching for semiquinone and catechol complexes of cobalt. Studies on redox isomerism for the bis(pyridine) ether series Co(py(2)X)(3,6-DBQ)(2), X=O, S, Se, and Te

Abstract

Intramolecular electron transfer between Co-II(SQ) and Co-III(Cat) species has been investigated for the series of complexes Co(py(2)X)(3,6-DBQ)(2), where 3,6-DBQ are semiquinonate and catecholate forms of 3,6-di-tert-butyl-1,2-benzoquinone and py(2)X 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 py(2)O have been isolated at room temperature in two charge distributions, Co-II-(py(2)O)(3,6-DBSQ)(2) and Co-III(py(2)O)(3,6-DBSQ)(3,6-DBCat). Crystallographic characterization on both forms of the complex [Co-II(py(2)O)(3,6-DBSQ)(2), monoclinic, P2(1)/c, a = 11.0280(2) Angstrom, b = 30.2750(9) Angstrom, c = 12.1120(2) Angstrom, beta = 113.490(2)degrees, V = 3708.7(1) Angstrom(3), Z = 4, R = 0.056; Co-III(py(2)O)(3,6-DBSQ)(3,6-DBCat), monoclinic, P2(1)/n, a = 9.882(3) Angstrom, b = 20.915(5) Angstrom, c = 17.579(4) Angstrom, beta = 91.57(2)degrees, V = 3632(2) Angstrom(3), Z = 4, R = 0.054] has shown the the py(2)O 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 electron transfer steps in the solid state. Optically induced shifts in charge distribution have been investigated using a low-energy polychromatic light source.