Molecule-based Magnets Research Articles

Magnetically Ordered Molecule‐Based Assemblies

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A Three-Dimensional Ferromagnet, [Ni(dipn)]3[Cr(CN)6]2·3H2O (dipn = dipropylene triamine), Based on a Cubic Cr8Ni12 Unit

The combination of Ni2+, dipropylenetriamine (dipn), and [Cr(CN)6]3- affords the cyanide-bridged bimetallic assembly, [Ni(dipn)]3[Cr(CN)6]2.3H2O (1). This compound crystallizes in cubic space group Pa, with a = b = c = 20.9742(7) A and Z = 8. A three-dimensional network is constructed on the basis of a Cr8Ni12 cubane unit formed by an alternate array of [Cr(CN)6]3- and [Ni(dipn)]2+ units through Cr-CN-Ni-NC-Cr edges. Cryomagnetic studies reveal a ferromagnetic interaction between Cr(III) and Ni(II) ions and a long-range ferromagnetic ordering below 42 K with very small coercive field. To the best of our knowledge, this compound is the first "complete ferromagnet" providing three-dimensional ferromagnetic interaction through a three-dimensional bridging structure that is based on a cubic unit among general metal-oxide and molecule-based magnets. Magnetooptical studies demonstrate a strong correlation between magnetic and optical properties.

B(C6F5)3 Adducts of TCNE− and TCNQ−Vanadium Complexes as New Building Blocks for Molecule-Based Magnets

The TCNX ligands TCNE and TCNQ reacted in toluene with Cp2V in the presence of B(C6F5)3 to yield, depending on the stoichiometry, the divanadium(III) complexes [(Cp2V)2{(C6F5)3B·(μ4-TCNX)·B(C6F5)3}...

Understanding the gap in polyoxovanadate molecule-based magnets

We report a joint experimental and theoretical investigation of the transport gap, optical properties, and electronic structure of two chemically similar, inhomogeneously mixed-valent polyoxovanadate molecule-based magnets. We attribute the substantial gap in $[{\mathrm{NHEt}}_{3}{]}_{4}[{\mathrm{V}}_{8}^{\mathrm{IV}}{\mathrm{V}}_{4}^{\mathrm{V}}{\mathrm{As}}_{8}{\mathrm{O}}_{40}({\mathrm{H}}_{2}\mathrm{O})]∙{\mathrm{H}}_{2}\mathrm{O}$ to weak $p\text{\ensuremath{-}}d$ hybridization and a large on-site Coulomb repulsion $(U=6\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The reduced gap in $[{\mathrm{NHEt}}_{3}{]}_{3}[{\mathrm{V}}_{6}^{\mathrm{IV}}{\mathrm{V}}_{6}^{\mathrm{V}}{\mathrm{As}}_{8}{\mathrm{O}}_{40}({\mathrm{HCO}}_{2})]∙2{\mathrm{H}}_{2}\mathrm{O}$ is associated with a smaller value of $U$ $(4\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, at least from a molecular point of view, although the transport properties also reflect subtle organization of the molecular structure and differences between direct and indirect intermolecular charge transfer. A detailed analysis of the vibrational response supports the important role of local molecular distortions and hydrogen bonding in the intramolecular and intermolecular charge transport of $[{\mathrm{NHEt}}_{3}{]}_{4}[{\mathrm{V}}_{8}^{\mathrm{IV}}{\mathrm{V}}_{4}^{\mathrm{V}}{\mathrm{As}}_{8}{\mathrm{O}}_{40}({\mathrm{H}}_{2}\mathrm{O})]∙{\mathrm{H}}_{2}\mathrm{O}$.

Organic- and molecule-based magnets

The discovery of organic- and molecule-based magnets has led to design and synthesis of several families with magnetic ordering temperatures as high as ∼125°C. Examples of soft and hard magnets with coercivities as high as 27 kOe have also been reported. Examples from our laboratory of organic-based magnets using the tetracyanoethylene radical anion, [TCNE]•−, are discussed. In addition, several molecule-based magnets based on Prussian Blue structured materials as well as dicyanamide are discussed.

First-principle calculation on the electronic structure of the molecule-based ferromagnet Co[P(CN) 2] 2 and M[N(CN) 2] 2 (M = Co, Ni)

We have investigated the electronic and the magnetic properties of the molecule-based magnet M[P(CN) 2] 2 and M[N(CN) 2] 2 (M = Co, Ni) using the full potential linearized augmented plane wave (FP-LAPW) method. The relative stability of the ground state, the density of states and the electronic band structure were examined. The total energy calculations reveal that the ferromagnetic phase is a stable ground state for Co[P(CN) 2] 2 and M[N(CN) 2] 2 (M = Co, Ni). It is noteworthy that we predict the compounds Co[P(CN) 2] 2 and Co[N(CN) 2] 2 are ferromagnetic indirect semiconductor with a small band gap of 0.22 and 0.28 eV, respectively.

Synthesis of Cyano‐Bridged Magnetic Nanoparticles Using Room‐Temperature Ionic Liquids

A principally new exploit of ionic liquids as an alternative reaction medium in the synthesis of cyano-bridged coordination-polymer nanoparticles is reported. Stable colloid solutions containing nanoparticles of cyano-bridged molecule-based magnets, M)[Fe(CN)6]2/[RMIM][BF4] (M2+=Ni, Cu, Co) and Fe4[Fe(CN)6]3/[RMIM][BF4] (R=1-butyl (BMIM), 1-decyl (DMIM)), were prepared in the corresponding 1-R-3-methylimidazolium tetrafluoroborate [RMIM][BF4], which acts as both a stabilising agent and a solvent. By varying the length of the N-alkyl chain on the imidazolium cation of [RMIM]+ and the temperature, the growing process can be controlled to produce nanoparticles of different sizes. By studying the magnetic properties of frozen colloids it is shown that the relaxation of magnetisation is strongly influenced by interparticle interactions, which leads to the appearance of spin-glass-like dynamics in these systems.

Calorimetric Investigation of Phase Transitions Occurring in Molecule-Based Magnets

Calorimetric Investigation of Phase Transitions Occurring in Molecule-Based Magnets

Thin Films of Molecular Materials Grown on Silicon Substrates by Chemical Vapor Deposition and Electrodeposition

Chemical vapor deposition (CVD) and electrodeposition (ED) were applied to grow thin films of molecule-based magnets and conductors. Chemical vapor deposited films are illustrated by the M(TCNE)x (M=V, Cr, Nb, Mo) magnet series. V(TCNE)x and Cr(TCNE)x are room-temperature magnets. XANES/EXAFS studies on the air-stable Cr(TCNE)x have been performed to determine the chemical environment of the chromium within the polymeric network. Electrodeposition on a silicon working electrode was applied to process thin films of numerous molecule-based conductors, namely, non-integral oxidation state compounds, charge transfer complexes, and singlecomponent molecular conductors. Among the series of conductive thin films, TTF[Ni(dmit)2]2 and Ni(tmdt)2 exhibit a metal-like behavior.

Magnetically ordered molecule-based assemblies

The development of molecules and assemblies of molecules exhibiting technologically important bulk properties, such as magnetic ordering, is an important worldwide research focus. Organic- and molecule-based magnets have been discovered and several families have been reported with magnetic ordering temperatures exceeding room temperature and as high as approximately 125 degrees C. Examples of both hard and soft magnets have been reported with coercivities as high at 27 000 Oe (and exceeding commercially available magnets) have been reported. Several examples are based on the radical anion of tetracyanoethylene, S = 1/2 [TCNE].-. The include ionic zero-dimensional (0-D) [FeCp*2]*+[TCNE]*- (Cp* = pentamethylcyclopentadienide), 1-D [MnTPP]+[TCNE]*- [TPP = meso-tetraphenylporphinato] coordination polymers, and 3-D extended network structured M[TCNE](x).ySolvent (M = V, Mn, Fe, Co, Ni, Dy). This Perspective focuses on work in our laboratory that will be discussed at the Dalton Discussion 9 meeting entitled "Functional Molecular Assemblies." In addition to the overview of TCNE-based organic magnets, the targeted design, preparation, and chemical as well as magnetic characterization of a new family of magnets based on S = 3/2 mixed-valent [Ru(II/III)2(O2CR)4]+ (R = Me, Bu(t)) is described. In particular [Ru2(O2CMe)4]3[Cr(CN)6] prepared from aqueous media possess two interpenetrating cubic lattices and magnetically orders at 33 K. In contrast, [Ru2(O2CBu(t))4]3[Cr(CN)6] forms a 2-D layered lattice and orders at 37.5 K. Both exhibit hysteretic behavior, however, this is quite anomalous for the former cubic lattice. This as well as other anomalous magnetic behaviors is attributed to the presence of the second interpenetrating lattice.

Analysis of the magneto-structural correlations in the meso-tetraphenylporphyrinatomanganese(iii) tetracyanoethenide family of molecule-based magnets

The dependence of the nearest neighbor magnetic interaction, JAB, as a function of geometry for [Mn(porphyrin)][TCNE] materials has been evaluated by the results of CASSCF, CASPT2 and B3LYP calculations. The B3LYP results were validated for seven representative [Mn(porphyrin)][TCNE] molecule-based magnets and were found to be in good agreement with the experimental data for observed values of JAB/kB less than −150 K, and the CASPT2 method for JAB/kB greater than −150 K. Then, the JAB was systematically evaluated as a function of the intermolecular coordinates that define the position and orientation of [TCNE]˙− relative to [Mn(porphyrin)]+. The calculated JAB/kB (Ĥ = −2ΣJABŜAŜB) are antiferromagnetic in all regions, with values in the −112 to −457 K range. The most negative JAB values occur for the smallest plane angles between the [Mn(porphyrin)]+ and [TCNE]˙− planes. The B3LYP results were then compared to those obtained using the three spin exchange mechanisms (i.e. orthogonality, spin coupling, and CI mechanisms), and none of these mechanisms give trends in accord with the B3LYP results. Therefore, first principles methods, such as DFT or CASPT2, are needed for more detailed calculations.

Pyridine-substituted nitronyl nitroxide biradicals: a triplet (S = 1) ground state lasting out N-methylation

A 3,5-pyridine-substituted biradical of nitronyl nitroxide (1) has been designed, synthesized and the magnetic properties fully characterized. The ground-state spin multiplicity of 1 has been found to be triplet (S = 1) with a singlet–triplet energy gap of 2J/kB = 40 K from magnetic susceptibility measurements on a magnetically diluted system dispersed in organic polymer films. The 3,5-substituted pyridine 1 has a hydrogen-accepting site which is more accessible to hydrogen donors than previously known biradicals with sterically hindered 2,6-pyridine frameworks. N-Methylation of 1 has yielded a stable cationic species in a trifluoromethanesulfonate salt (2+·TfO−). The ground state of the cation 2+ has been found to be triplet as well with 2J/kB = 32 K from magnetic susceptibility measurements for magnetically diluted films. The magnetic susceptibility of neat crystalline solids of 1 and 2+·TfO− has been explained by Heisenberg exchange coupling models based on their X-ray crystal structures. It is well known that the energy preference of a high-spin ground state for m-phenylene, or m-xylylene, coupling units is disturbed in such cases as the π-conjugation is affected by heteroatomic substitution, an ionic charge, or molecular conformation. The present experimental results show that the high-spin preference in 1 and 2+ is little influenced by the heterocycle or the ionic charge. Intermolecular noncovalent bonds such as hydrogen bonding and electrostatic interactions are a driving force for crystallization of open-shell molecules in a controllable manner. The ground-state triplet biradicals serve as building blocks for molecule-based magnets of S > 1/2 based on intermolecular noncovalent bonding architecture.

Ab initio study of the electronic structure of the molecule-based ferromagnet Co[N(CN) 2] 2

We have investigated the electronic and magnetic properties of the molecule-based magnet Co[N(CN) 2] 2 using the full potential linearized augmented plane wave (FP-LAPW) method. The relative stability of the ground state, density of states and charge distributions were examined. Total energy calculations reveals that the ferromagnetic phase is a stable ground state for Co[N(CN) 2] 2 in agreement with the previous experimental findings. It is noteworthy that we predict the Co[N(CN) 2] 2 is a ferromagnetic semiconductor with a small band gap of 0.027 eV, and the semiconductor property can be connected to the strong crystal field splitting of Co 2+ 3d states for Co[N(CN) 2] 2. Such a molecule-based ferromagnetic semiconductor would offer a potential for semiconductor applications, therefore, an experimental confirmation of our theoretical predictions is encouraged.

Attractive magnetic Prussian blues

Four experimental demonstrations are proposed, which can be presented to large audiences: precipitation of Prussian blue on an overhead; attraction of a room temperature Prussian blue analogue; oscillating magnet, device transforming light in mechanical energy; thermal probe and magnetic switch. The experiments celebrate the 300th anniversary of the discovery of the first coordination compound by its synthesis. They illustrate simply the concept of Curie temperature, the transformation of light into mechanical energy and the use of molecule-based magnets as a thermal probe. Four experimental demonstrations are proposed, which can be presented to large audiences: precipitation of Prussian blue on an overhead; attraction of a room temperature Prussian blue analogue; oscillating magnet, device transforming light in mechanical energy; thermal probe and magnetic switch. The experiments celebrate the 300th anniversary of the discovery of the first coordination compound by its synthesis. They illustrate simply the concept of Curie temperature, the transformation of light into mechanical energy and the use of molecule-based magnets as a thermal probe.

Brief encounter at the molecular level: what muons tell us about molecule-based magnets

Spin-polarized muons can be implanted in various molecular magnetic materials in order to measure static and dynamic magnetic field distributions at a local level. The positively-charged muon is an unstable, radioactive particle which has spin–1/2, a lifetime of 2.2 μ S, about one-ninth of the proton mass and a magnetic moment of approximately 1/200 μ B. Both pulsed and continuous beams of muons can be produced with almost 100% spin polarization and significant intensity at various accelerator facilities. The subsequent decay of the muon into a positron allows the extraction of the muon-spin autocorrelation function which can be related to the magnetic field distribution inside a sample. This experimental technique has found particular application to the problem of hydrogen in semiconductors, as well as the study of the vortex lattice in both high-temperature and organic superconductors. Nevertheless, it has been most widely employed in the field of magnetism. We describe how our experiments using spin-polarized muons have been used to provide information about organic ferromagnets, molecular magnets, spin-chains and single molecule magnets.

Cyano-bridged One-dimensional SmIII–FeIII Molecule-based Magnet with an Ordering Temperature of 3.4 K

Abstract A new cynao-bridged SmIII–FeIII bimetallic coordination polymer has been prepared by the reaction of (n-Bu4N)3Fe(CN)6 and Sm(NO3)3·6H2O in a DMF–H2O media. The complex consists of alternate Sm(DMF)4(H2O)2 and Fe(CN)6 units bridged by two cis-cyano ligands to form a zigzag chain. It behaves as a molecule-based magnet with Tc = 3.4 K. At 2 K, hysteresis was observed with a coercive field of 200 Oe.

Molecule-Based Magnets in Heterospin Systems

Abstract Our approaches to the molecule-based magnets by using heterospin systems are reviewed. In our heterospin systems consisting of the organic 2p spins and the metal ion 3d spins, various pyridines carrying the organic spins were used as binding ligands for 3d metal ions. Carbenes generated by photolysis of diazo groups and persistent N-tert-butylaminoxyl radicals were employed as the organic spin sources. First, various model complexes were prepared and their magnetic couplings between the organic spin and the metal ion through the pyridine ligand were revealed. Based on the results of magnetic studies on model complexes, ferri- and ferromagnetic complexes having linear chain structures were designed and successfully prepared. When our heterospin systems were used under solution conditions, new fields were opened. Assemblies formed in frozen solution had magnetic behaviors exhibiting slow magnetic relaxations. Especially, when the anisotropic metal ion, high-spin cobalt(II) ion, was used as the metal ion in our heterospin system, the complex formed in frozen solution functioned as a single-molecule magnet (SMM). Such formation of a monometallic SMM was the first example reported.

Intermolecular Proton Transfer in Solid Phase: A Rare Example of Crystal-to-Crystal Transformation from Hydroxo- to Oxo-Bridged Iron(III) Molecule-Based Magnet

Intermolecular proton transfer in solid phase from the hydroxo bridge to a water molecule occurs in a new mu-hydroxo iron(III) compound of formula {EtNH3[Fe2(ox)2Cl2(mu-OH)].2H2O}n leading to a still crystalline compound in which the mu-oxo bridge replaces the mu-hydroxo one. Both three-dimensional compounds exhibit magnetic ordering at Tc ca. 70 K due to a spin canting.

Deflected spin transmission from radical substituent to Corannulene’s curved surface: Density functional theory calculations

Abstract Stable neutral radicals with spherical or curved-surface π-spin networks have drawn much attention in fullerene chemistry and molecule-based magnetism, because their intermolecular magnetic interactions are intrinsically three dimensional. Based on our previous experimental study on an oxoverdazyl radical directly conjugated with corannulene, we have carried out density functional theory calculations for a series of neutral organic π-radicals conjugated with this bowl-shaped hydrocarbon. The unpaired electron spin is delocalized from the radical substituent into the corannulene moiety, showing an uneven spin distribution with spin-rich and spin-poor regions on the curved surface of corannulene. The origin of the characteristic spin distributions was discussed on the basis of a comparison between the spin density distribution of corannulenoxyl having the curved surface structure with that of coronenoxyl bearing a flat surface structure. The magnitude of spin delocalization into the corannulene moiety significantly varies, depending on the radical employed.

Pressure effects on a dimetallic ferrimagnet [Mn(en)]3[Cr(CN)6]2·4H2O

Abstract We have investigated pressure effects on a dimetallic ferrimagnet [Mn(en)] 3 [Cr(CN) 6 ] 2 · 4H 2 O (en; ethylenediamine) through the magnetic measurements using a diamond anvil cell in the pressure region up to P = 4.7 GPa. This ferrimagnetic compound has an eminent high transition temperature ( T c) of 69 K at ambient pressure in the structurally characterized molecule-based magnet system. Under hydrostatic pressure, T c linearly increases against pressure, and exceeds 130 K at P = 4.7 GPa. The amount of the saturated moment hardly changes in the considered pressure region. This pressure experiment might become a prototype of artificial material control for the high- T c molecule-based magnet.