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.

Roles of surface steps on Pt nanoparticles in electro-oxidation of carbon monoxide and methanol.

Design of highly active nanoscale catalysts for electro-oxidation of small organic molecules is of great importance to the development of efficient fuel cells. Increasing steps on single-crystal Pt surfaces is shown to enhance the activity of CO and methanol electro-oxidation up to several orders of magnitude. However, little is known about the surface atomic structure of nanoparticles with sizes of practical relevance, which limits the application of fundamental understanding in the reaction mechanisms established on single-crystal surfaces to the development of active, nanoscale catalysts. In this study, we reveal the surface atomic structure of Pt nanoparticles supported on multiwall carbon nanotubes, from which the amount of high-index surface facets on Pt nanoparticles is quantified. Correlating the surface steps on Pt nanoparticles with the electrochemical activity and stability clearly shows the significant role of surface steps in enhancing intrinsic activity for CO and methanol electro-oxidation. Here, we show that increasing surface steps on Pt nanoparticles of approximately 2 nm can lead to enhanced intrinsic activity up to approximately 200% (current normalized to Pt surface area) for electro-oxidation of methanol.

Hydrogen-bonded charge-transfer complexes of TTF containing a uracil moiety: crystal structures and electronic properties of the hydrogen cyananilate and TCNQ complexes.

[structure: see text] A novel TTF-based donor with a uracil moiety, TTF-(1-n-butyluracil-5-yl) (TnbU), was synthesized. Crystal structures of both TnbU and the charge-transfer complex of TnbU-hydrogen cyananilate possess complementary double hydrogen bonds through uracil moieties and pi-stacking dimer structures between TTF skeletons. Furthermore, the TnbU-TCNQ charge-transfer complex shows a high electrical conductivity underlying the partial charge-transfer accompanied by a hydrogen-bonding interaction, which was substantiated in terms of the measurements of the IR, electronic spectra, and conductivity.

Tuning of Spin Density Wave Strengths in Quasi-One-Dimensional Halogen-Bridged Ni(III) Complexes with Strong Electron Correlations, [Ni(III)(chxn)(2)X]Y(2).

A series of quasi-one-dimensional halogen-bridged Ni(III) complexes, [Ni(chxn)(2)X]Y(2) (chxn = 1R,2R-diaminocyclohexane; X = Cl, Br, and mixed halides; Y = Cl, Br, mixed halides, NO(3), BF(4), and ClO(4)) have been synthesized in order to investigate the effect of the bridging halogens and counteranions on their crystal, electronic structures, and moreover the spin density wave strengths. In the crystal structures, the [Ni(chxn)(2)] moieties are symmetrically bridged by halogen ions, forming linear-chain Ni(III)-X-Ni(III) structures. The hydrogen bonds between the aminohydrogens of chxn and the counteranions are constructed not only along the chains but also over the chains, forming the two-dimensional hydrogen-bond networks. While the Ni(III)-X-Ni(III) distances or b axes are almost constant in the compounds with the same bridging halogens, the c axes which correspond to the interchain distances in the directions of the interchain hydrogen bonds are remarkably lengthened with the increase of the ionic radius of the counterions; X < NO(3) < BF(4) < ClO(4). These compounds show the very strong antiferromagnetic interactions among spins on Ni 3d(z)2 orbitals through the superexchange mechanisms via the bridging halogen ions. Judging from the results of X-ray photoelectron spectra (XPS), Auger spectra, and single-crystal reflectance spectra, these Ni compounds are not Mott-insulators but charge-transfer-insulators. Their electronic structures or the spin density wave strengths are found to be tuned by the combinations of the counteranions and the bridging halogens.

Tuning of Charge Density Wave Strengths by Competition between Electron-Phonon Interaction of Pd(II)-Pd(IV) Mixed-Valence States and Electron Correlation of Ni(III) States in Quasi-One-Dimensional Bromo-Bridged Ni-Pd Mixed-Metal MX Chain Compounds Ni(1)(-)(x)()Pd(x)()(chxn)(2)Br(3).

A series of single crystals of quasi-one-dimensional bromo-bridged Ni-Pd mixed-metal MX chain compounds Ni(1)(-)(x)()Pd(x)()(chxn)(2)Br(3) (chxn = 1(R),2(R)-diaminocyclohexane) have been obtained by electrochemical oxidation methods of the mixed methanol solutions of parent Ni(II) complex [Ni(chxn)(2)]Br(2) and Pd(II) complex [Pd(chxn)(2)]Br(2) with various mixing ratios. To investigate the competition between the electron correlation of the Ni(III) states (or spin density wave states) and the electron-phonon interaction of the Pd(II)-Pd(IV) mixed-valence states (or charge density wave states) in the Ni-Pd mixed-metal compounds, IR, Raman, ESR, XP, and Auger spectra have been measured. The IR, resonance Raman, XP, and Auger spectra show that the Pd(II)-Pd(IV) mixed-valence states are influenced and gradually approach the Pd(III) states with the increase of the Ni(III) components. This means that in these compounds the electron-phonon interaction in the Pd(II)-Pd(IV) mixed-valence states is weakened with the strong electron correlation in the Ni(III) states.

A mixed-valence coordination polymer featuring two-dimensional ferroelectric order: {[Cu(I)4Cu(II)(Et2dtc)2Cl3][Cu(II)(Et2dtc)2]2(FeCl4)}n (Et2dtc- = diethyldithiocarbamate).

The first mixed-valence coordination polymer indicating ferroelectric properties, {[CuI4CuII(Et2dtc)2Cl3][CuII(Et2dtc)2]2(FeCl4)}n (Et2dtc- = diethyldithiocarbamate), has been synthesized and crystallographically characterized. This complex has a two-dimensional infinite square lattice structure, including both Cu(I) and Cu(II) ions, which is confirmed by XPS and SQUID measurements. In addition, the ferroelectric behaviors are revealed by the dielectric measurements with a LCR meter and a ferroelectric tester.

Framework control by a metalloligand having multicoordination ability: new synthetic approach for crystal structures and magnetic properties.

By utilizing the novel metalloligand l(Cu), [Cu(2,4-pydca)(2)](2)(-) (2,4-pydca(2)(-) = pyridine-2,4-dicarboxylate), which possesses two kinds of coordination groups, selective bond formation with the series of the first-period transition metal ions (Mn(ii), Fe(ii), Co(ii), Cu(ii), and Zn(ii)) has been accomplished. depending on the coordination mode of 4-carboxylate with Co(ii), Cu(ii), and Zn(ii) ions, L(Cu) forms a one-dimensional (1-d) assembly with a repeating motif of [-M-O(2)C-(py)N-Cu-N(py)-Co(2)-]: {[ZnL(Cu)(H(2)O)(3)(DMF)].DMF}(N)() (2), [ZnL(Cu)(H(2)O)(2)(MeOH)(2)](N)() (3), and {[ML(Cu)(H(2)O)(4)].2H(2)O}(N)() (M = Co (4), Cu (5), Zn (6)). the use of a terminal ligand of 2,2'-bipyridine (2,2'-bpy), in addition to the cu(ii) ion, gives a zigzag 1-d assembly with the similar repeating unit as 4-6: {[Cu(2,2'-bpy)L(Cu)].3H(2)O}(N)() (9). on the other hand, for Mn(ii) and Fe(ii) ions, L(Cu) shows a 2-carboxylate bridging mode to form an another 1-d assembly with a repeating motif of [-M-O-C-O-CU-O-C-O-]: [ML(Cu)(H(2)O)(4)](N)() (M = Mn (7), Fe (8)). this selectivity is related to the strength of lewis basicity and the electrostatic effect of L(Cu) and the irving-williams order on the present metal ions. according to their bridging modes, a variety of magnetic properties are obtained: 4, 5, and 9, which have the 4-carboxypyridinate bridge between magnetic centers, have weak antiferromagnetic interaction, whereas 7 and 8 with the carboxylate bridge between magnetic centers reveal 1-d ferromagnetic behavior (Cu(II)-M(II); M(II) = Mn(II), J/k(B) = 0.69 K for 7; M(II) = Fe(II), J/k(B) = 0.71 K for 8).

Spin-crossover cobalt(II) compound with banana-shaped structure.

A banana-shaped spin-crossover (SCO) cobalt(II) complex [Co(C16-terpy)2](BPh4)2 (1) with long alkyl chains, based on a terpyridine frame, was synthesized. Compound 1 exhibited very gradual SCO behavior and changes in the dielectric constant. This shows a way in which SCO materials can be used in electronic devices.

The NQR observation of spin-Peierls transition in an antiferromagnetic MX-chain complex [NiBr(chxn)2]Br2.

81Br Nuclear quadrupole resonance (NQR) measurement was performed in an S = 1/2 one-dimensional Heisenberg antiferromagnetic metal complex [NiBr(chxn)2]Br2 (chxn: 1R,2R-diaminocyclohexane), having a halogen-bridged MX chain structure -Br-Ni3+-Br-Ni3+-Br-. Two 81Br NQR signals were observed below 40 K, while a single signal was observed above 130 K, showing the presence of two nonequivalent bridging Br sites below 40 K. This NQR result together with previously reported magnetic susceptibility and X-ray results indicate the occurrence of a transition into a spin-Peierls state between 40 and 130 K. This communication reports the first spin-Peierls transition in metal complexes in which pure d electrons contribute to the magnetism. In addition, we demonstrated a new experimental method for studying a spin-Peierls system.