MMX Chain Research Articles

High Electrical Conductivity of Single Metal-Organic Chains.

Molecular wires are essential components for future nanoscale electronics. However, the preparation of individual long conductive molecules is still a challenge. MMX metal-organic polymers are quasi-1D sequences of single halide atoms (X) bridging subunits with two metal ions (MM) connected by organic ligands. They are excellent electrical conductors as bulk macroscopic crystals and as nanoribbons. However, according to theoretical calculations, the electrical conductance found in the experiments should be even higher. Here, a novel and simple drop-casting procedure to isolate bundles of few to single MMX chains is demonstrated. Furthermore, an exponential dependence of the electrical resistance of one or two MMX chains as a function of their length that does not agree with predictions based on their theoretical band structure is reported. This dependence is attributed to strong Anderson localization originated by structural defects. Theoretical modeling confirms that the current is limited by structural defects, mainly vacancies of iodine atoms, through which the current is constrained to flow. Nevertheless, measurable electrical transport along distances beyond 250 nm surpasses that of all other molecular wires reported so far. This work places in perspective the role of defects in 1D wires and their importance for molecular electronics.

Conducting Behavior and Valence Ordering of a One‐Dimensional MMX‐Type Coordination Polymer under High Pressure

One‐dimensional (1D) halogen‐bridged mixed‐valent binuclear metal compounds, the so‐called MMX chains, are well known as highly conductive coordination polymers because of the high electronic flexibility within their metal‐dimer unit. Herein, we report on the conducting behavior and electronic state of a highly conductive MMX‐chain compound, Pt2(dta)4I (dta: CH3CS2–) under high pressure. Electrical resistivity measurements showed that this compound exhibits semiconducting behavior in a broad pressure region, despite metallic conduction under ambient pressure. Raman spectra provided clear information about the semiconducting state of this compound under pressure, which was also supported by X‐ray diffraction (XRD) studies. Moreover, we successfully clarified the pressure–temperature phase diagram of this material. By comparing with an analogous material, we found that the difference in the alkyl chain length of the terminal ligand plays an important role for the conducting behavior and the electronic phase under pressure.

Electrochemically generated nanoparticles of halogen-bridged mixed-valence binuclear metal complex chains.

Spherical nanoparticles composed of MMX chains can be made by a polymerization strategy driven by electrochemical processes. In particular, the [Pt2(MeCS2)4I2] (MMI2) dimetal subunit is employed as a monomer for the formation of [Pt2(MeCS2)4I]n spherical nanostructures on surfaces. We have paid particular attention to elucidating the general mechanism of the deposition process on the basis of in situ electrochemical measurements. The reduction of MMI2 to give the electrodeposition of nanostructures agrees well with formation of the reduced [MMI2](-) species followed by a disproportionation mechanism mediated by iodide anions. The chemical composition of the particles was determined by energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) to reveal the MMI2 polymer.

ChemInform Abstract: Recent Progress in MMX‐Chain Complexes: Unique Electronic States and Characteristics Developed by Introducing Binary Countercations

AbstractReview: [57 refs.

Solid-State Electrochemistry of a Semiconducting MMX-Type Diplatinum Iodide Chain Complex

Electron-transfer-facilitated dissolution, ion insertion, and desorption associated with an MMX-type quasi-one-dimensional iodide-bridged dinuclear Pt complex (MMX chain) have been investigated for the first time. K2(NC3N)[Pt2(pop)4I]·4H2O (1) (NC3N(2+) = (H3NC3H6NH3)(2+); pop = P2H2O5(2-)) is a semiconductor with a three-dimensional coordination-bond and hydrogen-bond network included in the chain. The cyclic voltammetry of 1 was studied by using 1-modified electrodes in contact with acetonitrile solutions containing electrolyte. The chemical reversibility for oxidation of 1 depended on the electrolyte cation size, with large cations such as tetrabutylammonium (Bu4N(+)) being too large to penetrate the pores formed by the loss of K(+) and NC3N(2+) upon oxidation. The potential for reduction of 1 decreased as the cation size increased. The presence of the acid induced additional well-defined processes but with gradual solid dissolution, attributed to the breaking of the coordination-bond networks.

Recent Progress in MMX-Chain Complexes: Unique Electronic States and Characteristics Developed by Introducing Binary Countercations

Abstract MMX-Type quasi-one-dimensional iodide-bridged diplatinum complexes (MMX chains) provide various electronic states and attractive physical properties. In this review, we summarize the recent progress in MMX chains consisting of pyrophosphite (pop) ligands. The MMX chains with binary countercations, which have short Pt–I–Pt distance and robust frameworks, form a new electronic state. They show reversible dehydration–rehydration accompanied by the change of electronic states and electrical conductivity. Relatively high electrical conductivity in the MMX chains with binary countercations enables us to investigate the negative differential resistance (NDR) and electrochemical characteristics in the solid state for the first time in MMX chains.

Predominance of covalency in water-vapor-responsive MMX-type chain complexes revealed by (129)I Mössbauer spectroscopy.

(129)I Mössbauer spectroscopy was applied to water-vapor-responsive MMX-type quasi-one-dimensional iodide-bridged Pt complexes (MMX chains) in order to investigate their electronic state quantitatively. Two sets of octuplets observed in K2(H3NC3H6NH3)[Pt2(pop)4I]·4H2O (2·4H2O) and one octuplet observed in K2(cis-H3NCH2CH=CHCH2NH3)[Pt2(pop)4I]·4H2O (1·4H2O) and dehydrated complexes (1 and 2) indicate a unique alternating charge-polarization + charge-density-wave (ACP + CDW) electronic state and a charge-density-wave (CDW) electronic state, respectively. These spectra correspond to their crystal structure and the change of electronic states upon dehydration. Since these complexes consist of an alternating array of positively charged and negatively charged layers, the charge on the iodide ion (ρIS) was discussed on the basis of the isomer shift (IS). The ρIS of the water-vapor-responsive MMX chains was mainly -0.13 to -0.21, which are the smallest of all MMX chains reported so far. Hence, it indicates that the negative charge on the iodide ion is strongly donated to the Pt ion in these complexes. This covalent interaction predominates in the ACP + CDW state as well as in the CDW state. Therefore, the ACP + CDW state is in fact the CDW state with the ACP-type lattice distortion. Because the ρIS became smaller with the decreasing Pt-I-Pt distance, it can be concluded that the covalent interaction plays an important role in determining the electronic states of the MMX chains with pop (= P2H2O5(2-)) ligands.

Controlling the Electronic States and Physical Properties of MMX-Type Diplatinum-Iodide Chain Complexes via Binary Countercations

MMX-type quasi-one-dimensional iodide-bridged dinuclear Pt complexes (MMX chains) with binary countercations show a new alternating charge-polarization + charge-density-wave (ACP+CDW) electronic state and reversible switching of the electronic states and physical properties upon dehydration and rehydration process. By comparing several MMX chains with various binary countercations with previous chains, we found that the short backbone of the aliphatic diammonium ion was indispensable for realizing the ACP+CDW state because it induces a 2-fold periodicity along the chain axis via twisting of the ligands. Moreover, the reversibility of the changes in the structure and electrical conductivity upon dehydration and rehydration depend on the length of aliphatic diammonium ion. Short diammonium ions support a robust framework, which undergoes reversible structural changes. On the other hand, long and bent aliphatic diammonium ions weaken the framework, which causes partial degradation of the crystal and a decrease in the electrical conductivity when the structure changes. However, the decrease in the activation energy of the electrical conductivity after the dehydration process is independent of the robustness of the complex, indicating that the orbital overlap in MMX chains with binary countercations increases upon dehydration. Controllable electronic states and physical properties provide a platform for designing the multifunctional materials based on MMX chains.

Rationalization of the behavior of M2(CH3CS2)4I (M = Ni, Pt) chains at room temperature from periodic density functional theory and ab initio cluster calculations

The electrical conductivities and plausible charge-ordering states in the room temperature (r.t.) phase for MMX chains [Ni(2)(dta)(4)I](∞) and [Pt(2)(dta)(4)I](∞) (dta = CH(3)CS(2)(-)) have been analyzed with periodic density functional theory (DFT) and correlated ab initio calculations combined with the effective Hamiltonian theory. Periodic DFT calculations show a more delocalized nature of the ground state in [Pt(2)(dta)(4)I](∞) compared to [Ni(2)(dta)(4)I](∞), which features a rather large energy gap between the occupied and empty bands, and charge polarized dimer units. A larger electrical conductivity for the Pt chain can be expected, especially because the Fermi level lies within a band with contributions from Pt and I orbitals. Electronic structure parameters extracted from ab initio cluster calculations show that the large difference between the observed conductivities at 300 K for Ni and Pt compounds, of 3 orders of magnitude, cannot be explained from the parameters extracted from an embedded M(2)(dta)(4)I(2) dimer fragment alone. When tetramer fragments are considered, we observe that the interdimer transfer integral (t) between neighboring M(2) units connected by an iodine atom at correlated level is comparable in both chains. On the other hand, the energy to transfer an electron from a dimer to the neighboring one (Coulomb repulsion U) is three times larger in the Ni compound with respect to the Pt chain, in line with the poor conductivity of the former. The electronic structure of the M(4)(dta)(8)I(3) fragment points to an alternate charge-polarization state for Ni and an average valence state for Pt when the r.t. X-ray structure is considered.

Novel Countercation in MMX-Type Mixed-Valence Chain Compound: Coexistence of Neutral and Protonated Amino Substituents

The first MMX-type quasi-one-dimensional (Q1D) Pt chain complex (MMX chain) that contains a mono-protonated diamine as countercation, {o-(H3NC6H4NH2)}4[Pt2(pop)4I]·H2O (pop = P2H2O52–), was synthesized. According to the crystal structural analysis, –NH2 group was hydrogen-bonded to either lattice H2O molecule or –NH3+ group in addition to typical hydrogen bond between –NH3+ group and pop ligand. To control the partial deprotonation of the countercation will be an important method for achieving the high-conductive MMX-chain polymer by the hole doping.

On the Nature of the Multiple Ground States of the MMX Mixed-Valence Chain Compound, [PtII/III2(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.

Calorimetric Study of Correlated Ligand Dynamics in Mixed-Valence MMX Chain Complex [{Ni2(n-BuCS2)4}I

Abstract Heat capacity of a mixed-valence complex having a one-dimensional chain structure, [{Ni2(n-BuCS2)4}I] (n-Bu = butyl group), was measured between 10 and 325 K by adiabatic calorimetry. Anomalies due to phase transitions were detected around 135 and 260 K. The anomaly around 260 K was of multiple-peaks. The excess entropy acquired below the phase transition around 260 K is much smaller than the simple estimate based on the structural disorder found in the crystal structure, implying the presence of motional correlation in dynamics of n-BuCS2 ligands. Through the comparison with the corresponding Pt complex, [{Pt2(n-BuCS2)4}I], the phase transition around 135 K is also discussed in relation to the absence in the present complex of a spin-Peierls transition observed in other members of Ni complexes with similar chain structures.

Calorimetric Study of MMX Chain Complexes Having Alkyl Groups, Ni2(EtCS2)4I and Ni2(n-PrCS2)4I

Heat capacities of halogen-bridged one-dimensional binuclear metal complexes (so-called MMX chain) having alkyl groups, Ni 2 (RCS 2 ) 4 I (R = Et and n -Pr), were measured by adiabatic and relaxation calorimetry. For each complex, a small broad thermal anomaly due to a spin-Peierls transition was observed around 40 K. For Ni 2 ( n -PrCS 2 ) 4 I, a first-order phase transition was observed at 205.6 K. The enthalpy and entropy of transition were estimated to be about 4.05 kJ mol -1 and 19.7 J K -1 mol -1 , respectively. The magnitude of thermal anomaly shows that the effect of molecular dynamics of ligand on the electronic state in one-dimensional chain is small for Ni complexes having localized electrons in comparison with Pt complexes.

129I 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.

Syntheses, Structures and Solid-State Properties of MMX Mixed-Valence Chains, [NiII/III2(RCS2)4I]∞ (R = Et, n-Pr and n-Bu): Evidence of a Spin-Peierls Transition

The partial oxidation of [Ni(II/II)(2)(RCS(2))(4)] (R = Et (1), n-Pr (2), and n-Bu (3)) with iodine affords the MMX chain compounds [Ni(II/III)(2)(RCS(2))(4)I](infinity) (R = Et (4), n-Pr (5), and n-Bu (6)), respectively. The crystal structures of 4-6 consist of neutral one-dimensional (1-D) chains with a repeating -Ni-Ni-I- unit. The room-temperature (RT) structure of 4 indicates a charge-polarization (CP) state {-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)- (delta << 0.5)} close to an averaged valence state judged by the Ni-I distances. In contrast, 5 and 6 exhibit a 3-fold periodicity of a -Ni-Ni-I- unit due to the disorder of the dithiocarboxylato ligands. Compounds 4-6 show typical semiconducting behavior and exhibit an intense sharp absorption band centered at 5400 cm(-1), which is attributed to a Mott-Hubbard gap due to a relatively large on-site Coulomb repulsion energy U of the nickel atoms. The high-temperature magnetic susceptibilities of 4-6 can be described by a 1-D Heisenberg antiferromagnetic chain model with |J|/k(B) ranging from 898(2) to 939(3) K. Compounds 4 and 5 undergo a spin-Peierls (SP) transition at relatively high T(sp) = 47 and 36 K, respectively, which are accompanied by superlattice reflections corresponding to a 2-fold -Ni-Ni-I- period below T(sp). By determining the superstructure of 4 at 26 K, we conclude that the valence-ordered state changes from the CP in the RT phase to the alternate charge-polarization (ACP) state of -Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5+delta)+)-Ni((2.5-delta)+)-I(-)- in the SP phase. Such a spin-Peierls transition could not be observed for 6.

Metallicity in Individual MMX Chains

We present an ab initio study of the structural and electronic properties of the halogen-bridged MMX single polymer [Pt2(CH3CS2)4I]n and of various possible modifications of its sequence, in the framework of density functional theory. The computed band structure of the infinite regular polymer reveals a net metallic character; this evidence is compatible with the outcome of recent measurements done in the solid phase at room temperature. By taking the regular [Pt2(CH3CS2)4I]n polymer as our reference system, we analyzed the origin and the robustness of the metallic state along the chain with respect to a large set of geometrical and chemical perturbations of the subunits. In particular, we considered partial substitutions of the metal, halide, and dithiocarboxylate ligand subunits, as well as structural strain, defects, and magnetic effects. Our results demonstrate that the metallic character of single MMX chains is very resistant to a wide range of possible distortions that can occur in reality.

Pressure‐Induced Metal–Semiconductor–Metal Transitions in an MMX‐Chain Complex, Pt2(C2H5CS2)4I

AbstractThe electrical conductivity and X‐ray diffraction measurements were performed for a highlyconductive halogen‐bridged binuclear‐metal mixed‐valence complex (the so‐called MMX chain), Pt2(C2H5CS2)4I, under high pressure up to 2.5 GPa. The complex exhibited pressure‐induced metal–semiconductor–metal transitions at 0.5 and 2.1 GPa. The X‐ray diffuse scatterings were observed at k = n + 0.5 (n: integer) under ambient pressure, which are derived from the charge‐density wave (CDW: ···Pt2+–Pt2+···I–Pt3+–Pt3+–I···) fluctuation in the MMX chain. Above 0.5 GPa, where the pressure‐induced metal–semiconductor transition occurred, these scatterings disappeared. The electronic phases under high pressure (P) were found to be attributable to the metallic averaged‐valence state (AV: –Pt2.5+–Pt2.5+–I–Pt2.5+–Pt2.5+–I–) with CDW fluctuation of P &lt; 0.5 GPa, semiconducting charge‐polarization state (CP: ···Pt2+–Pt3+–I···Pt2+–Pt3+–I···) of 0.5 &lt; P &lt; 2.1 GPa, and metallic AV state of P &gt; 2.1 GPa. The electronic state of Pt2(C2H5CS2)4I is very sensitive to pressure, which implies that the phase competition among the CP, CDW, and AV phases is present in the MMX chain. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Direct Determination of Low-Dimensional Structures: Synchrotron X-ray Scattering on One-Dimensional Charge-Ordered MMX-Chain Complexes

A powerful method to determine the hidden structural parameters in functional molecules has been developed. Local valence arrangements that dominate the material properties are sometimes not three-dimensionally ordered. This method that comprises diffuse X-ray scattering and resonant X-ray scattering is suitable in such cases. Using this method, we present clear evidence of the low-dimensional valence arrangement in two halogen-bridged one-dimensional metal complexes, so-called MMX chains. This family allows us to control many physical and structural parameters by chemical substitution of bridging halogen, counterions, or metal ions, and one of our samples carries an unusual metallic phase. It is demonstrated with this complex that the present method makes it possible to have microscopic insight to low-dimensionally ordered systems.

Spin-Peierls transition in halogen-bridged mixed-valence MMX chain compounds

Spin-Peierls transition in halogen-bridged mixed-valence MMX chain compounds

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

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.