Alkyl Group as Entropy Reservoir in an MMX Chain Complex, Pt₂(n-PenCS₂)₄I

Abstract

Heat capacity of halogen-bridged one-dimensional binuclear metal complex (so-called MMX chain) having four n-pentyl groups, Pt2(n-PenCS2)4I, was measured by adiabatic calorimetry. A first-order phase transition was observed at 207.4 K when measurement was made after cooling from room temperature. The enthalpy and entropy of transition were determined to be 10.19 kJ mol(-1) and 49.1 J K(-1) mol(-1), respectively. A monotropic phase transition was observed at 324 K on heating, and the entropy of transition was essentially null. The sample once heated above 324 K never returned to the initial phase at room temperature and underwent a higher-order phase transition at 173 K and a first-order phase transition at 220.5 K. The enthalpy and entropy of the first-order phase transition were estimated to be 11.6 kJ mol(-1) and 52.4 J K(-1) mol(-1), respectively. The magnitude of the entropy gain at the phase transition from the initial room-temperature phase to the high-temperature phase at 324 K shows that in Pt2(n-PenCS2)4I a large amount of entropy reserved in alkyl chain is transferred to dithiocarboxylato groups upon the phase transition, as in the cases of Pt2(n-PrCS2)4I and Pt2(n-BuCS2)4I.

Conductive Nanostructures of MMX Chains

“…Direct current (DC) electrical conductivity measurements performed on eight different single crystals of [Pt 2 (n-pentylCS 2 ) 4 , similar to that previously reported for the same compound (s ¼ 0.84 S Á cm À1 ) [18] and slightly lower than those reported in the related [Pt 2 (RCS 2 ) 4 I] compounds (17-83, 5-30, and 13 S Á cm À1 for R ¼ n-But, Et, and Me, respectively). [9][10][11] Two different sequences were used for the conductivity measurements.…”

“…This sharp metal-insulator (M-I) transition is observed at a temperature slightly lower than that reported for the same compound from heat capacity measurements (207.4 AE 0.2 K). [18] The thermally activated regime follows the 1D variable range hopping model (1D-VRH; Fig. S3) used to explain conductivity in many conducting polymers.…”

“…7 V Á cm at 400 K. The transition at 330 K has also been observed in heat capacity measurements, although at a temperature of 324 K, and has been attributed to a moderate disorder of the alkyl chains. [18] When the temperature is decreased from 400 to 100 K, the resistivity smoothly increases to reach a maximum at ca. 390 K (Fig.…”

Calorimetric Investigation of Phase Transitions Occurring in Molecule-Based Magnets

“…138 Table 3 lists the transition temperatures and the entropy gains at the phase transitions calorimetrically determined for the [Pt 2 (n-C m H 2m+1 CS 2 ) 4 I] (1 e m e 5) complexes. The phase transitions with a large entropy gain always involve a orderdisorder mechanism of the conformational changes of the ligands.…”

“…Therefore, the interpretation of the phase transitions is not so easy as compared with other homologous complexes. Saito et al 138 proposed a more general interpretation of the RT-HT phase transitions occurring in [Pt 2 (n-C 3 H 7 CS 2 ) 4 I], [Pt 2 (n-C 4 H 9 CS 2 ) 4 I], [141][142][143] The calorimetric study 137 additionally detected a small phase transition at 114 K. e Data for as-grown sample. The phase transition was reported at 210 K. 143 f Data for the sample once heated above the transition at 324 K and then supercooled.…”