Low-temperature heat capacity of Ir(C5H7O2)(C8H12)

• The sample of Ir(C5H7O2)(C8H12) was synthesized and purified.
• The temperature dependence of heat capacity of Ir(C5H7O2)(C8H12) has been measured.
• The thermodynamic functions at 298.15 K are presented.
• An anomaly of heat capacity has been discovered with a maximum at Tc = 287.20 K.

Iridium(I) (acetylacetonato)(1,5-cyclooctadiene) complex, Ir(C5H7O2)(C8H12), was synthesized and characterized by elemental CH-analysis, NMR, X-ray spectroscopy. The heat capacity of compound Ir(C5H7O2)(C8H12) has been measured by the adiabatic method within the temperature range (6–318) K. The thermodynamic functions: entropy, enthalpy and reduced Gibbs energy have been calculated. They have the following values at 298.15 K: C°p = (323.6 ± 0.6) J K−1 mol−1, Δ0298.15S°m = (347.0 ± 0.9) J K−1 mol−1, Δ0298.15H°m = (50.82 ± 0.09) kJ mol−1, Φ°m = (176.5 ± 0.7) J K−1 mol−1. The value of the absolute entropy were used to calculate the entropy of formation of Ir(C5H7O2)(C8H12) at T = 298.15 K. An anomaly of heat capacity has been discovered with a maximum at Tc = 287.20 K, which points to the phase transition of the complex. Anomalous contributions to entropy and enthalpy have been revealed.