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, CoII(3,5-DBSQ)2(DBPy)2 (1), CoII(3,5-DBSQ)2(DBPy)2·1.33C7H8 (1S), CoII(3,5-DBSQ)2(DCPy)2·C7H8 (2S), CoII(3,5-DBSQ)2(TBPy)2 (3) and CoII(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, CoII(3,5-DBSQ)2(DBPy)2 (1). Thermally induced valence tautomerism (VT) is observed only in (1S), (1) and (2S) [hs-CoII(3,5-DBSQ)2L2 ↔ ls-CoIII(3,5-DBSQ)(3,5-DBCat)L2 (hs = high spin, ls = low spin, 3,5-DBCat = 3,5-di-tert-butylcatecholate)], whereas (3) and (4) remain locked in the hs-CoII(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 (T1/2 ∼ 245 K with ΔT ∼ 10 K) from hs-CoII(3,5-DBSQ)2(DBPy)2 to ls-CoIII(3,5-DBSQ)(3,5-DBCat)(DBPy)2 oxidation state, whereas the other polymorph with lattice solvent (1S) results in a broad transition (T1/2 ∼ 150 K with ΔT ∼ 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.