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

Highlights: • 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 T_c = 287.20 K. - Abstract: 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, Δ₀^298.15S°_m = (347.0 ± 0.9) J K⁻¹ mol⁻¹, Δ₀^298.15H°_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 T_c = 287.20 K, which points to the phase transition of the complex. Anomalous contributions to entropy and enthalpy have been revealed.