Heat capacity and thermodynamic functions of crystalline forms of the metal-organic framework zinc 2-methylimidazolate, Zn(MeIm)2

Abstract

Zeolitic imidazolate frameworks (ZIFs) are composed of metal atoms connected with imidazole-like linkers, and these frameworks have potential for applications in molecular sieving, gas sequestration, and catalysis. In addition, these materials form true polymorphs with the same chemical composition but different topologies. In this paper, we present the results of low temperature heat capacity and inelastic neutron scattering studies of the sodalite (SOD) and diamondoid (dia) topologies of the popular zinc 2-methylimidazolate framework, Zn(MeIm)2. Molar heat capacities from 1.8 K to 300 K are presented, along with theoretical fits and the values of Cp,m°, Δ0TSm°, Δ0THm°, andΦm°calculated from those fits. The Gibbs energy of the transformation from SOD to dia is −(4.6 ± 2.2) kJ, and this transformation is primarily enthalpically driven. The results of this study are compared with previous measurements on the zinc 2-ethylimidazolate framework, Zn(EtIm)2. Inelastic neutron scattering measurements confirm the presence of low energy modes and suggest that the higher heat capacity of SOD at low temperatures is due to the dynamics of the methyl groups on the methylimidazolate linkers.