Bistability and Molecular Switching for Semiquinone and Catechol Complexes of Cobalt. Studies on Redox Isomerism for the Bis(pyridine) Ether Series Co(py₂X)(3,6-DBQ)₂, X = O, S, Se, and Te

Intramolecular electron transfer between Co^II(SQ) and Co^III(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, Co^II(py₂O)(3,6-DBSQ)₂ and Co^III(py₂O)(3,6-DBSQ)(3,6-DBCat). Crystallographic characterization on both forms of the complex [Co^II(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) ų, Z = 4, R = 0.056; Co^III(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) ų, 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.