Valence Tautomerism in One-Dimensional Coordination\nPolymers

Abstract

The combination of\nthe divergent bis-pyridyl linking ligands 1,2-bis­(4-pyridyl)­ethane\n(1,2-bpe), 4,4′-<i>trans</i>-azopyridine (azpy),\nand 1,3-bis­(4-pyridyl)­propane (1,3-bpp) with cobalt and 3,5-di-<i>tert-</i>butyldioxolene (3,5-dbdiox) ligands has afforded the\ncomplexes [Co­(3,5-dbdiox)₂(1,2-bpe)]_∞ (<b>1</b>), [Co­(3,5-dbdiox)₂(azpy)]_∞ (<b>2</b>), [<i>trans</i>-Co­(3,5-dbdiox)₂(1,3-bpp)]_∞ (<b>3a</b>), and [<i>cis</i>-Co­(3,5-dbdiox)₂(1,3-bpp)]_∞ (<b>3b</b>). All species are 1D coordination polymers that crystallize as solvated\nforms; the geometric isomers <b>3a</b>,<b>b</b> cocrystallize.\nComplexes <b>1</b>, <b>2</b>, and <b>3a</b> exhibit\naround the Co centers a trans disposition of the N-donor atoms from\nthe pyridyl linkers, while an unusual cis disposition is evident in <b>3b</b>. Single-crystal X-ray structural analysis at 100 or 130\nK of solvated forms of these complexes indicates that all complexes\npossess the {Co^III(3,5-dbcat)­(3,5-dbsq)} (3,5-dbcat = 3,5-di-<i>tert</i>-butylcatecholate; 3,5-dbsq = 3,5-di-<i>tert</i>-butylsemiquinonate) charge distribution at the temperature of data\ncollection. Variable-temperature magnetic susceptibility studies reveal\nthat <b>1</b>, <b>1</b>·1.5MeCN·2H₂O, <b>2</b>·2EtOH, and <b>3</b>·MeCN·H₂O (<b>3</b> = <b>3a</b>·<b>3b</b>)\nall exhibit thermally induced valence tautomeric (VT) transitions\nabove 200 K. Multiple heating and cooling cycles indicate that in\nsome cases the behavior is strongly dependent on desolvation processes.\nMost notably, further desolvation of <b>1</b>·1.5MeCN·2H₂O above 340 K affords χ_m<i>T</i> values that suggest unusual ferromagnetic coupling in the {<i>hs</i>-Co^II(3,5-dbsq)₂} valence tautomer.\nCompound <b>3</b>·MeCN·H₂O exhibits a two-step\nVT transition that may be ascribed to the presence of the cis and\ntrans geometric isomers. Compounds <b>1</b>, <b>1</b>·1.5MeCN·2H₂O, <b>2</b>·2EtOH, and <b>3</b>·MeCN·H₂O all also exhibit a single photoinduced VT transition, comparable\nto those generally observed for nonpolymeric cobalt–dioxolene\ncomplexes.