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

A Semiconductive Nature of Bis(dithiolato)nickelate Radical Salt Exhibiting Broadband Photoconduction

The fractional oxidation state [M(dmit)₂] (dmit^2- = 2-thioxo-1, 3-dithiole-4, 5-dithiolate) salts have long attracted attention in the molecular metal area owing to high conductivity and even superconductivity. In this study, we achieved a mixed-valence salt (1) of [Ni(dmit)₂]^0.5- with monovalent 1,3-N,N-dimethyl-imidazolium (DiMIm⁺) by a solvent evaporation approach under ambient conditions. The mixed valence of [Ni(dmit)₂]^0.5- has been characterized by an analysis of the IR spectrum and crystal structure. In the crystal structure of 1, two [Ni(dmit)₂]^0.5- anions overlap in an eclipsed mode to form a [Ni(dmit)₂]₂^1- dimer, featuring a radical bearing an S = 1/2 spin; the dimeric radicals stack into a column along the b axis, and the adjacent columns connect together via the lateral-to-lateral S···S contacts along the a axis, and through the head-to-head S···S contacts along the [101] direction. Salt 1 shows the magnetic behavior of an S = 1/2 Heisenberg antiferromagnetic uniform linear chain with J/k_B = -47.5(4) K and a semiconducting feature with σ = 2.52 × 10^-3 S cm^-1 at 293 K, 2.32 × 10^-2 S cm^-1 at 373 K, and E_a = 0.22 eV, as well as broadband photoconductivity under irradiation of green and white lights. This study suggests the possibility of designing new photoconductors based on the mixed-valence [Ni(dmit)₂]^0.5- salt.

One-dimensional electronic systems: metal-chain complexes and organic conductors

One-dimensional (1D) metal-chain complexes and organic conductors show many similarities as well as striking differences in structural and electronic properties, although constituent elements and orbitals that contribute to charge transfer in these systems are quite different. In this review, we highlighted the structural and electronic properties of neutral MMX-chain complexes (M = Pt2+/3+, X = I-) and tetramethyltetrathiafulvalene-based cation radical salts as typical examples of each group while comparing them with each other. This review primarily aims to construct a coherent body of knowledge of 1D electronic materials that might have been separately investigated. We have proposed future directions for the exploration of new and more advanced electronic materials not only having 1D character, but also residing in the dimensional crossover regime.

Reversiblechair↔skewconformational interconversion of 1,3,2-dioxaphosphorinane ring in the solid state

β-NH4[(MeO)2cDHAP] (where cDHAP is cyclic dihydroxyacetone phosphate) has been investigated by X-ray crystallography in the temperature range 350–100 K. Three reversible single-crystal-to-single-crystal phase transitions have been examined and four phases (high-, room-, medium- and low-temperature phase, HTP, RTP, MTP and LTP, respectively) have been structurally determined: HTP (at 350 K,P21/a,Z= 24), RTP (290 K, P \overline{1},Z= 12), MTP (205 K,P21/a,Z= 4) and LTP (100 K, P \overline{1},Z= 24). Various puckering modes of the 1,3,2-dioxaphosphorinane ring of the organic cyclic phosphate anion have been revealed in the crystal:chairandskew. Thechair↔skewring flipping, which has been shown to take place during the phase transitions, results in various populations ofskewandchairconformers in different phases. The flexibility in the coordination geometry of ammonium cations is considered to assist in the conformational flexibility of the dioxaphosphorinane ring.

On the nature of the multiple ground states of the MMX mixed-valence chain compound, [Pt(II/III)2(n-PenCS2)4I]∞

We present a comprehensive study of the temperature dependence of the crystal structure using single-crystal X-ray diffraction and diffuse scattering, and electrical transport and magnetic properties as well as some optical properties at room temperature to elucidate the origin and the form of multiple ground states demonstrated in a previous study of the heat-capacity of the MMX chain compound, [Pt(II/III)(2)(n-PenCS(2))(4)I](∞). The present results confirm the presence of the two phase transitions, one reversible of first order at 207 K and the other nonreversible monotropic at 324 K, separating the low temperature (LT), room temperature (RT), and high temperature (HT) phases. The unit cell displays a 3-fold periodicity of -Pt-Pt-I- in the RT and HT phases because of the structural disorder which is exhibited by the dithiocarboxylato groups and the n-pentyl groups belonging to the central diplatinum unit. In addition, for the HT-phase all the dimers show this disorder. This compound undergoes a metal-semiconductor transition at T(M-S) = 235 K. The presence of diffuse streaks corresponding to 2-fold -Pt-Pt-I- periodicity in the HT and RT phases indicates dynamic valence ordering of the type -Pt(2+)-Pt(2+)-I(-)-Pt(3+)-Pt(3+)-I(-)-or-Pt(2+)-Pt(3+)-I(-)-Pt(3+)-Pt(2+)-I(-)-. For the LT-phase the diffuse scattering is condensed into clear Bragg diffraction peaks while keeping the 3-fold periodicity. This fact suggests further localization through dimerization of charges and spins confirming the diamagnetic state in the magnetic susceptibility and the low electrical conduction below 207 K. The present results are further discussed in relation to those of previous studies on the homologues, [Pt(II/III)(2)(RCS(2))(4)I](∞), R = methyl, ethyl, n-propyl, and n-butyl.

(129)I Mössbauer spectroscopic study of a metallic MMX chain system

One-dimensional iodide-bridged mixed-valence binuclear platinum complexes (the so-called "MMX chains") and their Pt(III) dimer precursors were investigated with (129)I Mossbauer spectroscopy. Spectra consisting of two sets of octuplets were observed at low temperatures for a neutral MMX chain complex, Pt(2)(dtp)(4)I (dtp = C(2)H(5)CS(2)(-)), with various charge-ordering states at the Pt dimers, indicating that the charge-ordering state is in an alternate-charge-polarization phase (ACP: ...[Pt(2+)-Pt(3+)]-I(0.4-)-[Pt(3+)-Pt(2+)]...I(0.3-)...), which is consistent with a previous low-temperature X-ray diffraction study. The estimated valence states of the bridging iodines of [(C(2)H(5))(2)NH(2)](4)[Pt(2)(pop)(4)I] (pop = H(2)P(2)O(5)(2-)), with a charge-polarization phase (CP: ...[Pt(2+)-Pt(3+)]-I(0.4-)...[Pt(2+)-Pt(3+)]-I(0.4-)...), and [H(3)N(CH(2))(6)NH(3)](2)[Pt(2)(pop)(4)I], with a charge-density-wave phase (CDW: ...[Pt(2+)-Pt(2+)]...I(0.3-)-[Pt(3+)-Pt(3+)]-I(0.3-)...), suggest that the covalent bond interaction is dominant in the CDW phase, whereas the Coulomb interaction is dominant in the CP phase. The estimated absolute quadrupole coupling constant (QCC) values for negatively charged MMX chain complexes with pop ligands are larger than those for neutral MMX chain complexes with CH(3)CS(2)(-) (dta) ligands, implying that the Madelung potential formed by the more-negative pop ligands and countercations effectively contributes to the physical properties of the pop system. The three Pt(III) dimer complexes Pt(2)(dta)(4)I(2), Pt(2)(dtp)(4)I(2), and K(4)[Pt(2)(pop)(4)I(2)] showed almost the same isomer shifts, indicating that the valence state of the iodide ion (I(0.5-)) depends negligibly on the terminal ligand. The QCC value observed for K(4)[Pt(2)(pop)(4)I(2)] was larger than those for Pt(2)(dta)(4)I(2) and Pt(2)(dtp)(4)I(2), originating from the anisotropic arrangement of the iodide anions, which form layers lying on the ab plane in the crystal.

Cooperativity of dynamics of 18-crown-6 molecule forming one-dimensional chain in Cs2(18-crown-6)3[Ni(dmit)2]2

Heat capacity of Cs2(18-crown-6)3[Ni(dmit)2]2 was measured by adiabatic calorimetry. A broad thermal anomaly was observed around 225 K. The entropy gain (about 52 J K(-1) mol(-1)) is much larger than that expected for twofold disordering of 18-crown-6 assumed in the previous structure analysis. The shape of thermal anomaly was qualitatively explained by a linear Ising model developed for cooperative disordering in polymers. The 18-crown-6 molecules forming a one-dimensional chain in the crystal are orientationally disordered with moderate cooperativity.