Structural Transitions and Thermochromism of Linear Polynuclear Cobalt(II)-4-Octadecyl-1,2,4-triazole Complexes

Abstract

Combination of methanolic solutions of cobalt(II) chloride hexahydrate and 4-octadecyl-1,2,4-triazole (C18trz) leads to precipitation of a linear polynuclear complex (“coordination polymer”) established by Co2+ centers bridged by two C18trz molecules. The compound exists in a blue and a pink form in the solid, gel and dissolved state. The blue form represents tetrahedrally coordinated [Co(C18trz)2]2+ repeat units and the pink form octahedrally coordinated [Co(C18trz)2(H2O)2]2+ repeat units, therewith reflecting the affinity of Co2+ for both tetrahedral and octahedral coordination geometry. The blue and the pink form can be reversibly transformed into each other by humidification or, respectively, drying of the material. Thermochromism is observed in the dissolved state, where the pink form is reversibly transformed to the blue form upon increase in temperature. The results differ in some aspects from assumptions in previous reports on this subject.