Valence Tautomerism in One-Dimensional Coordination Polymers

The combination of the divergent bis-pyridyl linking ligands 1,2-bis(4-pyridyl)ethane (1,2-bpe), 4,4'-Trans-azopyridine (azpy), and 1,3-bis(4-pyridyl)propane (1,3-bpp) with cobalt and 3,5-di-tert-butyldioxolene (3,5-dbdiox) ligands has afforded the complexes [Co(3,5-dbdiox)(2)(1,2-bpe)]infinity (1), [Co(3,5-dbdiox)(2)(azpy)]infinity (2), [trans-Co(3,5-dbdiox)(2)(1,3-bpp)]infinity (3a), and [cis-Co(3,5-dbdiox)(2)(1,3-bpp)]infinity (3b). All species are 1D coordination polymers that crystallize as solvated forms; the geometric isomers 3a,b cocrystallize. Complexes 1, 2, and 3a exhibit around the Co centers a trans disposition of the N-donor atoms from the pyridyl linkers, while an unusual cis disposition is evident in 3b. Single-crystal X-ray structural analysis at 100 or 130 K of solvated forms of these complexes indicates that all complexes possess the {Co-III(3,5-dbcat)(3,5-dbsq)} (3,5-dbcat = 3,5-di-tert-butylcatecholate; 3,5-dbsq = 3,5-di-tert-butylsemiquinonate) charge distribution at the temperature of data collection. Variable-temperature magnetic susceptibility studies reveal that 1, 1 center dot 1.5MeCN center dot 2H2O, 2 center dot 2EtOH, and 3 center dot MeCN center dot H2O (3 = 3a center dot 3b) all exhibit thermally induced valence tautomeric (VT) transitions above 200 K. Multiple heating and cooling cycles indicate that in some cases the behavior is strongly dependent on desolvation processes. Most notably, further desolvation of 1 center dot 1.5MeCN center dot 2H2O above 340 K affords ?mT values that suggest unusual ferromagnetic coupling in the {hs-CoII(3,5-dbsq)2} valence tautomer. Compound 3 center dot MeCN center dot H2O exhibits a two-step VT transition that may be ascribed to the presence of the cis and trans geometric isomers. Compounds 1, 1 center dot 1.5MeCN center dot 2H(2)O, 2 center dot 2EtOH, and 3 center dot MeCN center dot H2O all also exhibit a single photoinduced VT transition, comparable to those generally observed for nonpolymeric cobaltdioxolene complexes.