Variable-temperature structural studies on valence tautomerism in cobalt bis(dioxolene) molecular complexes

A variable-temperature single-crystal structural study of five valence tautomeric cobalt molecular complexes, Co-II(3,5-DBSQ)(2)(DBPy)(2) (1), Co-II(3,5-DBSQ)(2)(DBPy)(2)center dot 1.33C(7)H(8) (1S), Co-II(3,5-DBSQ)(2)(DCPy)(2)center dot C7H8 (2S), Co-II (3,5-DBSQ)(2)(TBPy)(2) (3) and Co-II(3,5-DBSQ)(2)(TCPy)(2) (4) (S = toluene, 3,5-DBSQ = 3,5-di-tert-butylsemiquinonate, DBPy = 3,5-dibromopyridine, DCPy = 3,5-dichloropyridine, TBPy = 3,4,5-tribromopyridine and TCPy = 3,4,5-trichloropyridine) is reported. The re-crystallization of (1S) in toluene at 277 K resulted in a concomitant formation of a solvent-free polymorph, Co-II(3,5-DBSQ)(2)(DBPy)(2) (1). Thermally induced valence tautomerism (VT) is observed only in (1S), (1) and (2S) [hs-Co-II(3,5-DBSQ)(2)L-2 <-> ls-Co-III(3,5-DBSQ)(3,5-DBCat)L-2 (hs = high spin, ls = low spin, 3,5-DBCat = 3,5-di-tertbutylcatecholate)], whereas (3) and (4) remain locked in the hs-Co-II(3,5-DBSQ)(2) state during cooling of the sample. Multi-temperature single-crystal studies demonstrate the change in cobalt coordination environment during the VT conversion. The non-solvated compound (1) shows a sharp VT transition (T-1/2 similar to 245 K with Delta T similar to 10 K) from hs-Co-II(3,5-DBSQ)(2)(DBPy)(2) to ls-Co-III (3,5-DBSQ)(3,5-DBCat)(DBPy)(2) oxidation state, whereas the other polymorph with lattice solvent (1S) results in a broad transition (T-1/2 similar to 150 K with Delta T similar to 100 K). This increase in the VT transition temperature for (1) relative to (1S) illustrates the effect of lattice solvent on the VT transition mechanism. Additionally, the influence of halogen substitutions on the pyridine ring is discussed with respect to observed VT behaviour in the studied compounds.