Polymorphism in diaquatetrakis(6-chloro-2-hydroxypyridine)copper(II) perchlorate: Crystallographic, solution and theoretical studies and solid phase transformations

Abstract

Two polymorphs of [Cu(6C2HP)4(H2O)2](ClO4)2 and the analogous Cu(6C2HP)4(ClO4)2 complex have been prepared, 6C2HP = 6-chloro-2-hydroxypyridine. The three phases were characterized using single crystal X-ray diffraction, powder X-ray diffraction, TGA and DSC in the solid phase and UV-VIS spectroscopy in solution. The three compounds crystallize in the P21/c space group. The materials included the polymorph with β = 107.51°, henceforth polymorph I, and the polymorph with β = 121.16°, henceforth polymorph II, and a third complex, Cu(6C2HP)4(ClO4)2, henceforth the anhydrous complex. Heating both of the hydrated polymorphs (green color) at 110° for 5 min converts them to the anhydrous complex (yellow). The yellow anhydrous sample converts to green polymorph I upon exposure to a humid environment. These solid phase transformation processes are reversible. These transformations occur despite the absence of any significant porosity in the anhydrous crystal structure, which is rare. Experimental (DSC and TGA) and theoretical data indicate that polymorph II is the thermodynamically favorable product and polymorph I is the kinetically controlled product. The transformation processes were followed in solution using UV–Vis spectroscopy which agreed with the solid state study. The calculated UV–Vis spectra using TD-DFT theory (Time Dependent Density Functional Theory) agree with the measured ones.