Structural and Magnetic Investigation of Cobalt Valence Tautomeric Complexes with Sulfur-Containing Ligands

We report a series of heteroleptic cobalt complexes of the general formula [Co(tBu2sq)2(L)2], where tBu2sq = 3,5-di(tert-butyl)-o-semiquinonato ligand and L represents pyridines functionalized with sulfur-containing substituents. The octahedral coordination of the Co metal center in these mononuclear complexes is formed by two chelating tBu2sq ligands in the equatorial plane and two axial pyridines in the axial positions. All complexes exhibit layered crystal packings, with S-containing substituents providing cohesion within the layers and the tBu substituents of dioxolenes protruding into the interlayer space, where disordered solvent molecules are located. The presence of thienyl substituents in the para position of the pyridine ring leads to the formation of molecular chains within the layers, due to efficient pi-pi interactions between the L ligands. Even more efficient packing with two-dimensional intermolecular pi-pi interactions is achieved when two thienyl substituents are present in the meta positions of the pyridine ring. Introduction of a terminal cyano or 1,3-dithiole-2-one substituent on the opposite end of the pyridyl-bound thienyl group leads to the disruption of the pi-pi interactions, thus decreasing the crystal packing efficiency. Such disruption, however, is not observed when the thienyl group is terminated with methyl-carboxylate. Magnetic measurements reveal an onset of valence-tautomeric spin-crossover (VT-SCO) from the [LS-CoIII(tBu2sq center dot)(tBu2cat)(L)2] state (low-spin, S= 1/2) to the [HS-CoII(tBu2sq center dot)2(L)2] state (high-spin, S= 5/2) above 300 K for the complexes with para-thienyl-substituted pyridines, while the complexes with a bithienyl substituent in the para position or two thienyl substituents in the meta positions exhibit only LS state up to 400 K. Adding a terminal group to the para-thienyl substituent lowers the VT-SCO temperature, leading to two-step spin-state conversion.