Valence Tautomeric Transitions of Three One-Dimensional Cobalt Complexes

Three novel complexes, namely, [Co-III(3,5-DBCat)(3,5-DBSq)(bpe)] center dot 2CH(3)CN center dot 2H(2)O (1 center dot S), [Co((III))(3,5-DBCat)(3,5-DBSq)(azpy)] center dot 2CH3CN center dot 2H(2)O (2 center dot S), and [Co(II)(3,5-DBSq)(2)(bpb)][Co-III(3,5-DBCat)(3,5-DBSq)(bpb)](0.5) center dot 2CH(3)CN center dot 2H(2)O (3 center dot S), were synthesized and characterized by valence tautomeric (VT) X-ray diffraction and magnetic measurements [where 3,5-DBCatH2 = 3,5-di-tert-butyl-catechol, 3,5-DBSqH = 3,5-di-tert-butyl-semiquinone, bpe = trans-bis(4-pyridyl)ethylene, azpy = trans-4,4'-azopyridine, and bpb = 1,4-bis(4-pyridyl)benzene]. The three complexes have similar one-dimensional chain structure building from bidentate-bridging pyridine ligands and planar 3,5-DBCat/3,5-DBSq-fixed Co-II/(III)) entities. Complexes 1 center dot S and 2 center dot S could retain the crystallinity during desolvation, and the crystal structures of 1 and 2 were therefore able to be determined. Only when 1 center dot S and 2 center dot S desolvated above 310 K did the magnetic susceptibilities x temperatures values of the two complexes rise sharply, and then thermally induced complete, one-step VT transitions for 1 and 2 were available and repeatable. Complex 3 center dot S showed an incomplete, one-step VT transition independent of solvent molecules. Among these complexes, only 1 was sensitive to photoexcitation at low temperature, its photoinduced metastable state relaxed with temperature-independent behavior at low temperature range (5-10 K) and with thermally assisted behavior at high temperature range (above 20 K), respectively.