Thermoanalytical Study of Linkage Isomerism in Coordination Compounds. Part 7. A DSC and DFT Investigation of Solid-state Linkage Isomerization in Bis(thiocyanato)-bipyridineplatinum(II) Complex

Abstract

The linkage isomerization of [Pt(SCN)2(bipy)] complex in solid-state was investigated by differential scanning calorimetry (DSC) at diverse heating rates. The conversion of the bis-thiocyanato isomer (–SCN)2 to the bis-isothiocyanato isomer (–NCS)2 was accompanied by the appearance of an exothermic peak. However, no DSC peak was obtained for isomerization of the (–NCS)2. The results imply that the (–SCN)2 isomer is metastable which can convert to the stable isomer (–NCS)2 at elevated temperatures. Assuming a two-stage irreversible isomerization, the enthalpy changes of the first stage ΔHSCN1 and also second stage ΔHSCN2 isomerization were obtained (–2.99 ± 0.44 and –2.81 ± 0.45 kJ·mol–1, respectively) using the mathematical resolution of the observed DSC peaks. The thermokinetic parameters of this conversion were determined using Kissinger method. The activation energy values for the first and second stages of isomerization are evaluated, 101.78 ± 7.58 and 106.26 ± 5.87 kJ·mol–1, respectively. The low values of the activation enthalpy, ΔH‡ and the high negative activation entropy ΔS‡, obtained by Eyring equation, supported an associative mechanism. A DFT study was employed to detect the electronic structures and the thermodynamic stabilities of the three linkage isomers and the transition states.