Conjugated Crystals of Halogen-Bridged One-Dimensional Metal Complexes by Electrochemical Technique

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Halogen-bridged one-dimensional compounds having characteristic electron-phonon coupling seems to become high temperature superconductors, if electron rich or deficient columnar units could be coexists with the halogen-bridged linear chain in a crystal. In this project, we tried to prepare the new type of one-dimensional halogen-bridged Ni complexes by electrochemical technique. In addition, the phase transition of the iodo bridged Pt dimer complex with linear chain structure has been studied by the temperature dependent X-ray structure analysis.Two crystal forms (alpha and beta) of a novel halogen bridged one-dimensional Ni^<III>-Br-Ni^<III> compound, {[Ni(en)_2Br](ClO_4)_2}_*(en=C_2H_8N_2), have been prepared by electrochemical oxidation. X-ray crystal structure analysis of the alphaform revealed that a bridging Br ion is located on the midpoint between two Ni atoms and the Ni-Br distance of 2.640(1) * is significantly elongated by introducing a bulky ClO_4^- ion as a c anion. An ave … More rage Ni-Br distance of 2.693(2) * for the beta form can be estimated from the lattice parameter along the chain. Magnetic susceptibility data for bath forms show strong antiferromagnetic coupling (J=-1700K) between electronic spins (S=1/2) localized on the Ni^<III> on the linear chain.The halogen-bridged mixed-valence compound with MMX type linear chain structure, Pt_2(dta)_4I (dta=CH_3CS_2^-), exhibits metallic conduction between 300 and 340 K,which is the first observation in halogen-bridged one-dimensional metal complexes. The lattice parameters of Pt_2(dta)_4I have been measured by single crystal diffraction method in the temperature range of-160-190ﾟC,showing the anomalous hysteresis along with the first order (ca.90ﾟC)and the second order (ca.0ﾟC)phase transitions. Crystal structure analyzes have been made at several temperatures in the range of-89-130ﾟC by using an imaging plate detector installed to the Enraf- cad4 diffractometer. The crystal structure of high temperature phase with space group A2/m show the positional disorder of ligating S atoms of the dta ligands, which should be owing to the twisting motion of the Pt_2(dta)_4I unit, though the low-temperature phase of C2/c has no positional disorder. It appeared from the analysis of positional disorder in the intermediate phase that there is a molecular conformation which enhances the stretching amplitude of a Pt-Pt bond. The observed twisting thermal motion of the Pt_2(dta)_4 unit should be essential in a viewpoint of the electron-phonon coupling in the one-dimensional mixed-valence system which induce the electron transfer between platinum dimer units. Less