Experimental and Computational Study on the Hydration-Dehydration Transitions in [Ni(cyclam)]₃M(CN)_n Coordination Polymers

Abstract

Hydration-dehydration transitions occurring in coordination polymers comprising complex Ni(cyclam = 1,4,7,11-tetraazacyclotetradecane) 2+ and polycyanometallate anions /Fe(CN) 6 3− , Cr(CN) 6 3− , or W(CN) 8 3− /, respectively, denoted NiFe, NiCr, or NiW, were studied by means of isothermal static adsorption and desorption, quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA), as well as Monte Carlo molecular simulations. Attribution of a single-step hydration-dehydration of NiW to the reversible single-crystal-tosingle-crystal phase transition was corroborated by the QE-TPDA measurements. Dehydration enthalpy NiW of 44 kJ/mol, determined based on the analysis of the QE-TPDA profiles, was very close to the calorimetric value. The content and location of water molecules in hydrated NiW were reproduced well in the results of molecular simulations. In contrast to NiW, multistep adsorption and desorption isotherms and the QE-TPDA profiles consisting of several desorption maxima and adsorption minima observed for NiCr and NiFe indicated complex hydration and dehydration mechanisms. A close correspondence of different hydration-dehydration patterns was confirmed by the similarity of the characteristic adsorption curves, calculated from isothermal or thermodesorption experimental data. The molecular simulations of hydrated NiCr and NiFe reproduced most of the locations of water molecules within their structures. Mechanisms of NiW, NiCr, and NiFe hydration-dehydration were discussed in terms of structure flexibility and stabilization energy of polycyanometallate anions.