Four new cobalt(ii) coordination complexes: thermochromic switchable behavior in the process of dehydration and rehydration

Abstract

Four cobalt(II) coordination complexes, {[Co₁₁(L1)₆(HL1)₂(H₂O)₂₈]·12H₂O}_n (1), [Co₄(H₂L1)₄(HL1)₂(H₂O)₈]·2H₃L1·2H₂O (2), {[Co(H₂L3)(H₂O)₃]·2H₂O}_n (3) and [Co(HL1)(4,4′-bpy)(H₂O)₂]_n (4), (H₃L1 = 2,4,6-tri(3-carboxyphenylthio)-1,3,5-triazine, H₄L3 = 2,4-bis(3-carboxyphenylthio)-6-(4-carboxyphenylamino -1,3,5-triazine, 4,4′-bpy = 4,4′-bipyridine), was synthesized hydrothermally and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, IR, thermal analysis, UV-vis spectroscopy and magnetic measurements. Complex 1 shows a 2D layer structure composed of three types of secondary building units (SBUs), [M₁], [M₃] and [M₄] bridged by [HL1]²⁻ and [L1]³⁻ linkers. While complex 2 is a tetranuclear compound and further forms a 3D supramolecular network structure based on eight types of hydrogen-bonding interactions between the molecules. The 2D framework of complex 3 can be described as 1D infinite chains interconnected by [H₂L3]²⁻ ligands. Furthermore, the hydrogen-bonding interactions between the adjacent layers along the ac plane makes the 2D layer structure extend to a 3D network. The structure of complex 4 can be illustrated as a 1D zigzag infinite chain with 4,4′-bipyridine as the secondary ligand. Additionally, the π⋯π stacking interactions can be observed between the adjacent chains. An aquo-accessible thermochromic change in complexes 1–4 was induced by means of the reversible dehydration and rehydration of the crystals. Magnetic measurements showed antiferromagnetic coupling between the Co²⁺ ions in all of the four complexes.