Heat capacity and phase transition of the thermochromic complex bis(1,5-diazacyclo-octane)copper(II) nitrate

Abstract

The molar heat capacity of the thermochromic compound [Cu(daco) 2](NO 3) 2, where daco = 1,5-diazacyclooctane, has been measured with an adiabatic calorimeter in the range 14–400 K, and i.r. spectra have been recorded in the range 4000-30cm −1 at 108, 299 and 373 K. A large heat capacity anomaly associated with a thermochromic phenomenon was found at T C = 359 K. This anomaly exhibits both superheating and undercooling effects, indicating a first-order phase transition. The color of the crystal is brilliant orange in the low temperature phase, and turns violet above T C . The enthalpy and entropy of the phase transition were ΔH = 8400 ± 220 J mol −1 and ΔS = 23.37 ±0.83 J K −1 mol −1, respectively. The entropy gain is well accounted for in terms of conformational disordering of the daco ligand, Rln 16 (equal to 23.05 J K −1 mol −1), where R is the gas constant. The number of the microscopic states (16) is counted as follows. Each eight-membered daco ring statically coordinates to the central copper ion with a fixed chair-boat conformation in the low temperature phase. However, it may be assumed that dynamic interconversion between the chair and the boat form might take place in the high temperature phase and thus each daco ring will gain four conformations (i.e. chair-chair, chair-boat, boat-chair and boat-boat). Since two daco ligands are involved in the complex, the total configurations per [Cu(daco) 2] 2+ cation amount to 16 (4 × 4). The change in the ligand-field strength leading to thermochromism is most probably caused by a change in the thermal motion of the dago ligand, from being static to dynamically interconverting between different conformations. An improved method of sample preparation has been achieved, by which the yield is increased to ~90%.