Structural Phase Transition and Valence Fluctuation Observed in a New MMX Chain Compound, Pt2(n-PrCS2)4I

Abstract

A new one-dimensional iodo-bridged mixed-valence diplatinum(II,III) complex (MMX chain compound) with dithiobutanoate (n-PrCS2-) as a ligand, Pt2(n-PrCS2)4I, was found to undergo a structural phase-transition at 358–359 K by the differential scanning calorimetry (DSC) measurement and X-ray crystal structure analyses. In X-ray diffraction photographs measured using synchrotron radiation of the SPring-8 facility, the weak but distinct Bragg spots corresponding to the periodicity of 2-fold repetition length of a –Pt–Pt–I– unit were observed at 300 K and changed to diffuse scattering at 350 K. The electric conductivity deviates from the typical semiconducting behavior in the temperature region above 330 K.

Calorimetric Study of a Halogen-Bridged MMX Chain Complex Having Alkyl Chains, Pt2(n-PrCS2)4I (n-Pr = Propyl Group)

“…18 This model was discussed in detail in ref 22. The experimental and model entropies of the LT-RT and RT-HT phase transitions are compared in Table 1.…”

“…Representatives of the MMX chain compounds having been studied are A 4 [Pt 2 (pop) 4 X]‚nH 2 O (A ) Li, K, Cs, NH 4 ; pop ) H 2 P 2 O 5 2-, diphosphonato; X ) Cl, Br, I), [4][5][6][7][8] and M 2 (RCS 2 ) 4 I (M ) Pt, Ni; R) alkyl chain). [9][10][11][12][13][14][15][16][17][18][19][20][21][22] M 2 (RCS 2 ) 4 I has four alkyl chains (R) in ligands. Elongation of alkyl chains in ligands is considered to enhance the onedimensionality of the electronic system on the MMX chains because of weak van der Waals S-S contacts between neighboring chains.…”

“…18,19 The structures of the room-temperature (RT) phase and the hightemperature (HT) phase have been determined by the singlecrystal X-ray analyses. 18 Their structures have been determined as shown in Figure 1. The HT phase (Figure 1b) appearing above ca.…”

“…The crystal structure of this phase was determined at 293 K, as shown in Figure 1a. 18 The lattice constant along the c-axis is tripled in the MMX unit. This periodicity describes only the structural disorder concerning the dithiocarboxylato groups and the propyl chains belonging to only the center complex in a unit cell length, similarly to Pt 2 (n-BuCS 2 ) 4 I.…”

“…20 The complex is reported as being diamagnetic below room temperature. 18,19 Here, it is noteworthy that the diamagnetism implies an evenly multiplied periodicity along the MMX chain because spins on the adjacent MMX units are to be strongly coupled in the diamagnetic state. In fact, weak superlattice reflections corresponding to the doubled periodicity are observed even at room temperature, 18 though they have not been used for structure analysis to obtain the average structure shown in Figure 1a.…”