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)]∞ (1), [Co(3,5-dbdiox)2(azpy)]∞ (2), [trans-Co(3,5-dbdiox)2(1,3-bpp)]∞ (3a), and [cis-Co(3,5-dbdiox)2(1,3-bpp)]∞ (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, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O (3 = 3a·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 1·1.5MeCN·2H2O above 340 K affords χmT values that suggest unusual ferromagnetic coupling in the {hs-Co(II)(3,5-dbsq)2} valence tautomer. Compound 3·MeCN·H2O exhibits a two-step VT transition that may be ascribed to the presence of the cis and trans geometric isomers. Compounds 1, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O all also exhibit a single photoinduced VT transition, comparable to those generally observed for nonpolymeric cobalt-dioxolene complexes.