Ordering and Freezing-in Phenomena of Nanochannel Water in Crystalline Organic/Inorganic Self-Assembled Complex [Cr(H2bim)3](TMA)·23.5H2O

Abstract

The thermal properties of crystalline complex [Cr(H(2)bim)(3)](TMA) x 23.5 H(2)O were studied by adiabatic calorimetry to clarify the structural ordering and dynamic freezing-in behaviors of the nanochannel water within the pores possessing crystalline wall structure, where H(2)bim denotes 2,2'-biimidazole and TMA is 1,3,5-benzenetricarboxylic acid. Phase and glass transitions were found to occur at 233 K with the associated entropy of Delta(trs)S = 7.96 J K(-1) mol(-1) and at T(g) = 100 K, respectively, in the hydrated sample. The phase transition was interpreted as attributed to the crystallization-like formation of the hydrogen-bond network of the channel-water molecules. The glass transition was interpreted as a freezing-in phenomenon on the way of the development of the network, and its presence indicates that the network formation achieves no completion even at 100 K. The T(g) value is similar to those found previously in other channel-water systems of [Ni(cyclam)(H(2)O)(2)](3)(TMA)(2) x 24 H(2)O and porous silica. It is noted that the channel water within silica pores with their diameter below 1.8 nm undergoes no structural phase transition while the present one does. The origins of the phase and glass transitions and the implication of their presence are discussed based on the difference in the structures of pore wall interacting with the channel-water molecules.