Single-component Molecular Metals Research Articles

The quest for single component molecular metals within neutral transition metal complexes

The first Single Component Molecular Metals (SCMM) were reported twenty years ago. This review will address their main design, synthetic routes and physical properties.

Correction: The quest for single component molecular metals within neutral transition metal complexes

Correction for ‘The quest for single component molecular metals within neutral transition metal complexes’ by Mariana F. G. Velho et al., J. Mater. Chem. C, 2021, DOI: 10.1039/d1tc01407b.

Syntheses, Structures, and Physical Properties of Neutral Gold Dithiolate Complex, [Au(etdt)2]·THF

In order to develop new types of single-component molecular conductors with novel electronic structures and physical properties, the neutral gold dithiolate complex with an etdt (= ethylenedithiotetrathiafulvalenedithiolate) ligand, [Au(etdt)2] was prepared. However, unlike the reported single-component molecular metals, the neutral gold complex [Au(etdt)2]·THF (2) contains a solvent molecule of tetrahydrofuran (THF). The crystals of 2 form a two-dimensional conducting layer structure, which are separated by the terminal ethylene groups and THF molecules. The fairly high room-temperature conductivity of 0.2 S/cm and semiconducting behavior with a low activation energy of 0.1 eV of 2, is consistent with the result of the density functional theory band structure calculations. The observed non-magnetic behavior of 2 is caused from the dimeric structure of [Au(etdt)2] molecules.

Antiferromagnetic Ordering in the Single-Component Molecular Conductor [Pd(tmdt)2

Crystals of [Pd(tmdt)2] (tmdt = trimethylenetetrathiafulvalenedithiolate) were prepared in order to investigate their physical properties. The electrical resistivity of [Pd(tmdt)2] was measured on single crystals using two-probe methods and showed that the room-temperature conductivity was 100 S·cm(-1). The resistivity behaviors implied that [Pd(tmdt)2] was a semimetal at approximately room temperature and became narrow-gap semiconducting as the temperature was decreased to the lowest temperature. X-ray structural studies on small single crystals of [Pd(tmdt)2] at temperatures of 20-300 K performed using synchrotron radiation at SPring-8 showed no distinct structural change over this temperature region. However, small anomalies were observed at approximately 100 K. Electron spin resonance (ESR) spectra were measured over the temperature range of 2.7-301 K. The ESR intensity increased as the temperature decreased to 100 K and then decreased linearly as the temperature was further decreased to 50 K, where an abrupt decrease in the intensity was observed. To investigate the magnetic state, (1)H nuclear magnetic resonance (NMR) measurements were performed in the temperature range of 2.5-271 K, revealing broadening below 100 K. The NMR relaxation rate gradually increased below 100 K and formed a broad peak at approximately 50 K, followed by a gradual decrease down to the lowest temperature. These results suggest that most of the sample undergoes the antiferromagnetic transition at approximately 50 K with the magnetic ordering temperatures distributed over a wide range up to 100 K. These electric and magnetic properties of [Pd(tmdt)2] are quite different from those of the single-component molecular (semi)metals [Ni(tmdt)2] and [Pt(tmdt)2], which retain their stable metallic states down to extremely low temperatures. The experimental results and the band structure calculations at the density functional theory level showed that [Pd(tmdt)2] may be an antiferromagnetic Mott insulator with a strong electron correlation.

Conducting films based on single-component molecular metals.

We demonstrate that single component molecular metals can be used as conductive inks for printed electronics. The resistance is 0.3 kΩ sq(-1), in a Ni complex, which is one order of magnitude better than that of commercial carbon based conductive inks.

Achievements and Challenges in Molecular Conductors

Molecular solids are generally highly insulating. The creation of conducting molecular solids proved to be a major scientific challenge. As in the case of Si technology, the challenge started as impurity doping in band insulators and then developed into highly doped polymers, which are not crystalline. More conducting materials in crystalline forms have been realized in charge transfer (CT) complexes with two different kinds of molecules, where electrons are transferred between them in solids. In such CT complexes, not only conducting, but also even superconducting systems were achieved in 1980 and today more than 100 different superconductors are known. The most remarkable achievement in this direction was the realization of a truly metallic state in molecular solids based on a single kind of molecule. These are called single component molecular metals (SCMM) and consist of a rich variety of electronic properties. In these conducting molecular solids, CT and SCMM, many interesting electronic properties resulting from mutual Coulomb interactions and electron-phonon interactions have been explored so far, and these will be reviewed briefly in this article from a theoretical viewpoint. Challenges to come, based on these achievements, are also discussed at the end of this review.

New copper thiophenedithiolenes for single component molecular metals

AbstractThe copper bisdithiolene complexes based on highly extended ligands incorporating fused TTF and thiophene moieties (α‐tdt = tiophenetetrathiafulvalenedithiolate and dtdt = dihydro‐thiophenetetrathiafulvalenedithiolate), were prepared and characterised by cyclic voltammetry and electrical transport measurements. The anionic complexes, (nBu4N)x[Cu(dtdt)2] (1) and (nBu4N)x[Cu(α‐tdt)2] (2), (1<x<2), obtained under anaerobic conditions, can be easily oxidised to the stable neutral state, [Cu(dtdt)2]0 (3) and [Cu(α‐tdt)2]0 (4), just by air or iodine exposure. Although no suitable single crystals of the neutral complexes could be obtained, room temperature conductivities measured in compressed powder pellets show values of σRT in the range of 0.10‐0.40 S/cm, a reasonable high electrical conductivity, for a powder compacted sample (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ChemInform Abstract: Studies on Molecular Conductors: From Organic Semiconductors to Molecular Metals and Superconductors

AbstractReview: 130 refs.

Studies on Molecular Conductors: From Organic Semiconductors to Molecular Metals and Superconductors

The field of molecular conductors was pioneered by works on the semiconducting properties of phthalocyanines and condensed aromatic hydrocarbons in the middle of the last century. Around three decades ago, the first organic superconductor was reported. Since then, various new molecular superconductors based on multichalcogen π molecules such as bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), dimethyl(ethylenedithio)diselenadithiafulvalene (DMET), BETS (or BEDT-TSF = bis(ethylenedithio)tetraselenafulvalene), and [M(dmit)(2) ] (M = Ni, Pd; dmit = 4,5-dimercapto-1,3-dithiole-2-thione) including unprecedented antiferromagnetic and field-induced organic superconductors and the first single-component molecular metals were developed by the members in the Department of Chemistry of the University of Tokyo along with outside collaborators. Studies on the physical properties--especially optical properties--of various types of molecular conductors are also extensively examined.

Single-Component Molecular Conductor [Cu(tmdt)2] Containing an Antiferromagnetic Heisenberg Chain

Traditional molecular conductors are composed of more than two chemical species and are characterized by low-dimensional electronic band structures. By contrast, the single-component molecular metals [M(tmdt)(2)] (M = Ni, Pt, Au; tmdt = trimethylenetetrathiafulvalenedithiolate) possess three-dimensional electronic structures that can be widely tuned by exchanging the central transition metal atom (M). In this study, the Cu atom was used to realize a new magnetic single-component molecular conductor exhibiting strong pi-d interactions. The crystal structure of [Cu(tmdt)(2)] was found to be essentially the same as those of the Ni, Pt, or Au-based systems with metallic states down to low temperature, but different from the structure of [Cu(dmdt)(2)] (dmdt = dimethyltetrathiafulvalenedithiolate) with its tetrahedrally coordinated dmdt ligands. A compressed pellet of microcrystals exhibited fairly high room-temperature conductivity (sigma(RT) approximately 7 S.cm(-1)), which increased almost linearly with pressure, reaching 110 S.cm(-1) at 15 kbar. This strongly suggests that the single crystal of [Cu(tmdt)(2)] is metallic at high pressure. Magnetic susceptibility measurements indicated one-dimensional Heisenberg behavior with |J| = 117 cm(-1) and an antiferromagnetic transition at 13 K. Density functional theory molecular orbital calculations revealed that the alpha-spin orbital of pdsigma(-) is distributed at the central part of the complex (CuS(4)), and alpha- and beta-sym-Lpi orbitals have almost the same energies and their spins are distributed mainly in the pdsigma(-) orbital. This is in contrast to the first single-component molecular metal [Ni(tmdt)(2)], which has stable metal bands formed from an almost degenerated sym-Lpi orbital (the highest occupied molecular orbital) and asym-Lpi(d) orbital (the lowest unoccupied molecular orbital). These results suggest that the alpha-pdsigma(-) state of [Cu(tmdt)(2)] exists just around the Fermi energy of the virtual metal band formed from the asym-Lpi(d) and sym-Lpi states. Thus, as expected, [Cu(tmdt)(2)] is a non-trivial single-component molecular conductor with pi-d multifrontier orbitals. In addition, ((n)Bu(4)N)(2)[Cu(tmdt)(2)] was synthesized, and its crystal structure was determined. Its Curie behavior (chi(rt) = 1.2 x 10(-3) emu mol(-1); C = 0.36 emu.K mol(-1)) indicates the existence of an isolated S = 1/2 spin on each dianionic molecule.

Structures and physical properties of single-component molecular metals

Structures and physical properties of single-component molecular metals

Electrical Resistivity of Tetramethyltetratelluronaphtalene Crystal at Very High Pressures Examination of the Condition of Metallization of π Molecular Crystal

Four-probe resistivity measurements were performed on the TMTTeN crystal by using a diamond anvil high-pressure cell up to 30 GPa. The crystal could not be metallized though the room-temperature resistivity decreased to a very small value (1.5 x 10-3 Omega cm). Although single-component molecular metals are composed of molecules, the pressure-induced metallization of a single-component pi molecular crystal while maintaining the initial molecular structure appears to be very difficult.

Electronic Structures of Single Component Molecular Metals Based on Ab initio Calculation

We perform a systematic study of electronic structures based on the ab initio calculations for single component molecular metals, Ni(tmdt) 2 , Au(tmdt) 2 , Au(tmstfdt) 2 , Cu(dmdt) 2 , and Zn(tmdt) 2 . First, we discuss the central metal dependence of the character of states around highest occupied molecular orbital (HOMO) [or singly occupied molecular orbital (SOMO)] and lowest unoccupied molecular orbital (LUMO) for isolated molecules and show that the stable structure of each isolated molecule is properly predicted. For solid state of Ni(tmdt) 2 , Au(tmdt) 2 , and Au(tmstfdt) 2 , the molecular shape is planar and the HOMO–LUMO separation in the Ni complex or the (HOMO - 1)–SOMO separation in the Au complexes is smaller than the inter-molecular interaction, which leads to the metallic state of Ni(tmdt) 2 and a unique antiferromagnetic state of Au(tmdt) 2 . For Cu(dmdt) 2 and Zn(tmdt) 2 , their two intra-molecular lingands are strongly twisted. Theoretically we obtain a stable antiferromagnetic solution for the solid state of the Cu complex. The theoretical electronic structures of solid phases of the Cu complex and Zn complex are compared with available experimental information.

Single-Component Molecular Metals as Multiband π–d Systems

Electronic states of single-component molecular metals M (tmdt) 2 ( M = Ni, Au) are studied theoretically. We construct an effective three-band Hubbard model for each material by numerical fitting to first-principles band calculations, while referring to molecular orbital calculations for the isolated molecules. The model consists of two kinds of base orbital for each molecule with hybridization between them, i.e., a π-character orbital for each of the two tmdt ligands, and, a p d π-orbital for M = Ni or a p d σ-orbital for M = Au centered on the metal site; this indicates that these materials can be considered as novel multiband π– d systems. We find that both orbitals contribute to realize the metallic character in Ni(tmdt) 2 . The origin of the antiferromagnetic transition observed in Au(tmdt) 2 is also discussed based on this model.

Conducting Dimerized Cobalt Complexes with Tetrathiafulvalene Dithiolate Ligands

To obtain novel single-component molecular metals, we attempted to synthesize several cobalt complexes coordinated by TTF (tetrathiafulvalene)-type dithiolate ligands. We succeeded in the syntheses and structure determinations of ((n)Bu(4)N)(2)[Co(chdt)(2)](2) (1), ((n)Bu(4)N)(2)[Co(dmdt)(2)](2) (2), [Co(dmdt)(2)](2) (3), and [Co(dt)(2)](2) (4) (chdt = cyclohexeno-TTF-dithiolate, dmdt = dimethyl-TTF-dithiolate, and dt = TTF-dithiolate). Structure analyses of complexes 1-4 revealed that two monomeric [Co(ligand)2]- or [Co(ligand)(2)](0) units are connected by two Co-S bonds resulting in dimeric [Co(ligand)(2)](2)(2-) or [Co(ligand)(2)](2) molecules. Complex 1 has a cation-anion-intermingled structure and exhibited Curie-Weiss magnetic behavior with a large Curie constant (C = 2.02 K x emu x mol(-1)) and weak antiferromagnetic interactions (theta = -8.3 K). Complex 2 also has a cation-anion-intermingled structure. However, the dimeric molecules are completely isolated by cations. Complexes 3 and 4 are single-component molecular crystals. The molecules of complex 3 form two-dimensional molecular stacking layers and exhibit a room-temperature conductivity of sigmart = 1.2 x 10(-2) S.cm(-1) and an activation energy of E(a) = 85 meV. The magnetic behavior is almost consistent with Curie-Weiss law, where the Curie constant and Weiss temperature are 8.7 x 10(-2) K x emu x mol(-1) and -0.85 K, respectively. Complex 4 has a rare chair form of the dimeric structure. The electrical conductivity was fairly large (sigmart = 19 S.cm(-1)), and its temperature dependence was very small (sigma(0.55K)/sigma(rt) = ca. 1:10), although the measurements were performed on the compressed pellet sample. Complex 4 showed an almost constant paramagnetic susceptibility (chi(300) (K) = 3.5 x 10(-4) emu x mol(-1)) from 300 to 50 K. The band structure calculation of complex 4 suggested the metallic nature of the system. Complex 4 is a novel single-component molecular conductor with a dimeric molecular structure and essentially metallic properties down to very low temperatures.

Transition Metal Bisdithiolene Complexes Based on Extended Ligands with Fused Tetrathiafulvalene and Thiophene Moieties: New Single‐Component Molecular Metals

The gold and nickel bisdithiolene complexes based on new highly extended ligands incorporating fused tetrathiafulvalene and thiophene moieties (alpha-tdt=thiophenetetrathiafulvalenedithiolate and dtdt=dihydro- thiophenetetrathiafulvalenedithiolate), were prepared and characterised by using cyclic voltammetry, single crystal X-ray diffraction, EPR, magnetic susceptibility and electrical transport measurements. These complexes, initially obtained under anaerobic conditions as diamagnetic gold monoanic [nBu(4)N][Au(alpha-tdt)(2)] (4), [nBu(4)N][Au(dtdt)(2)] (3) and nickel dianionic species [(nBu(4)N)(2)][Ni(alpha-tdt)(2)] (8), [(nBu(4)N)(2)][Ni(dtdt)(2)] (7), can be easily oxidised to the stable neutral state just by air or iodine exposure. The monoanionic complexes crystallise in at least two polymorphs, all of which have good cation and anion segregation in alternated layers, the anion layers making a dense 2D network of short SS contacts. All of the neutral complexes, obtained as microcrystalline or quasi amorphous fine powder, present relatively large magnetic susceptibilities that correspond to effective magnetic moments in the range 1-3 mu(B) indicative of high spin states and very high electrical conductivity that in case of the Ni compound can reach sigma(RT) approximately 250 S cm(-1) with a clear metallic behaviour. These compounds are new examples of the still rare single-component molecular metals.

Recent progress in development of single-component molecular metals

The novel nickel complex with the extended-TTF dithiolate ligand [Ni(tmdt) 2 ] (tmdt=trimethylene dithiotetrathiafulvalenedithiolate) is the first three-dimensional single-component molecular metal with metallic state down to very low temperature (0.6 K). Recently, the direct evidence for presence of electron and hole Fermi surfaces in [Ni(tmdt)2] was obtained by detecting the de Haars-van Alphen (dHvA) effect. We have synthesized analogous single-component molecular conductors with various ligands and central metal atoms and examined their structures and physical properties. The visible and ir spectra of single-component molecular conductors showed that these molecules have unprecedentedly small HOMO-LUMO gaps.

Development of single-component molecular metals based on extended-TTF dithiolate ligands

The novel nickel complex with the extended-TTF dithiolate ligand [Ni(tmdt) 2 ] (tmdt 2- =trimethylenedithiotetrathiafulvalenedithiolate) is the first three-dimensional single-component molecular metal. Structures and physical properties of analogous single-component molecular conductors [Ni(dmdt) 2 ] (dmdt 2- =dimethyltetrathiafulvalenedithiolate), [Ni(eodt) 2 ] (eodt 2- =ethylenedioxytetrathiafulvalenedithiolate) and [Pd(C3-tdt) 2 ] [(C3-tdt) 2- =dipropylthiotetrathiafulvalenedithiolate] were examined. The structure of [Au(tmdt) 2 ] was determined by synchrotron radiation powder diffraction experiments and the MEM/Rietveld analysis. The single-component paramagnetic molecule [Cu(dmdt) 2 ] was also studied to clarify the possibility of single-component magnetic molecular conductors.

Molecular design and development of single-component molecular metal

The requirements for designing single-component molecular metals were derived from the results of crystal structure analyses, resistivity measurements, band structure calculations, performed on molecular conductors with extended TTF-ligands [Ni(ptdt) 2] (ptdt: propylene dithiotetrathiafulvalenedithiolate). The development of π molecule with a small HOMO–LUMO gap and TTF-like skelton is a key step to obtain single-component molecular metal. We have prepared the crystals of [Ni(tmdt) 2] (tmdt: trimethylenetetrathiafulvalenedithiolate) and examined the crystal structure and resistivity. This single-component molecular crystal was found to be the first three-dimensional synthetic metal composed of planar molecules with metallic nature down to 0.6 K.

Molecular Design and Development of Single-component Molecular Metals with Extended TTF Ligands

The novel neutral nickel complex with the extended TTF dithiolato ligand, trimethylenedithiotetrathiafulvalenedithiolate[tmdt 2 m = (S 6 C 9 H 6 ) 2 m ], have been synthesized. The single crystal of [Ni(tmdt) 2 ] was metallic down to 0.6 K. The crystal structure and electronic band structure calculation showed [Ni(tmdt) 2 ] is the first single-component molecular metal with three-dimensional Fermi surfaces. Metallic behavior was also found in the crystal of an analogous molecule, [Ni(dmdt) 2 ][dmdt 2 m = dimethyltetrathiafulvalenedithiolate]. Similar highly conducting systems, [Au(dmdt) 2 ] and the molecular conductor with localized S = 1/2 spins, [Cu(dmdt) 2 ] were also prepared. extended TTF ligand single-component molecular metal three-dimensional molecular metal