Valence tautomeric transitions of three one-dimensional cobalt complexes

Three novel complexes, namely, [Co^III(3,5-DBCat)(3,5-DBSq)(bpe)]·2CH₃CN·2H₂O (1·S), [Co^III(3,5-DBCat)(3,5-DBSq)(azpy)]·2CH₃CN·2H₂O (2·S), and [Co^II(3,5-DBSq)₂(bpb)][Co^III(3,5-DBCat)(3,5-DBSq)(bpb)]_0.5·2CH₃CN·2H₂O (3·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·S and 2·S could retain the crystallinity during desolvation, and the crystal structures of 1 and 2 were therefore able to be determined. Only when 1·S and 2·S desolvated above 310 K did the magnetic susceptibilities × 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·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.