Molecule-based Magnets Research Articles

Structure–property correlations in a family of decamethylmetallocenium charge-transfer salt magnets using dialkyl dicyanofumarates as the one-electron acceptors: Ferromagnetism versus metamagnetism

The synthesis of molecule-based magnets derived from decamethylchromocene, CrCp2∗, and dialkyl dicyanofumarates, DRDCF, R = n-propyl (nPr) and isopropyl (iPr) yields two new members of a family of charge transfer (CT) salts, these being metamagnetic solids isomorphous with and reminiscent of the known diethyl (R = Et) homolog. The antiferromagnetic properties of these compounds contrast sharply with the previously reported ferromagnetic dimethyl (R = Me) compound. An examination of the four crystal structures provides an explanation for the observed behavior: although the pseudo-one-dimensional mixed stack structure is retained in all of the solids, a new relationship between the stacks is observed for the higher homologs that is different from that found for the DMeDCF salt. In the corresponding decamethylmanganocene, MnCp2∗, family, CT salts with DnPrDCF and DiPrDCF are both metamagnets that appear to enter a spin glass state well below their TN’s, exhibiting hysteresis with large very coercivity at 1.8 K, again similar to behavior previously seen for the corresponding diethyl compound and different from the dimethyl compound. These and other results reported herein indicate that meaningful structure–property correlations are possible in this class of compounds.

Building Blocks for 2D Molecule‐Based Magnets: The Diruthenium Tetrapivalate Monocation [RuII/III2(O2CtBu)4]+

Building Blocks for 2D Molecule‐Based Magnets: The Diruthenium Tetrapivalate Monocation [Ru^II/III₂(O₂C<i>t</i>Bu)₄]⁺

M[TCNQ]y-Based Magnets (M = Mn, Fe, Co, Ni; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane)

A family of molecule-based magnets of general formula M[TCNQ]y (M = Mn, Fe, Co, Ni; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) has been synthesized and characterized. The materials have been synthesized from both Ma(CO)b and [M(NCMe)6][SbF6]2 starting materials, complementing previous studies utilizing [M(NCMe)6][BF4]2. Magnetic ordering was observed for all materials with Tc values between 8 K [M = Ni from Ni(CO)4] and 60 K {M = Mn from [Mn(NCMe)6][SbF6]2}. The materials from [M(NCMe)6][SbF6]2 displayed the highest critical temperatures, followed by those from Ma(CO)b. With the exception of M = Ni, the lowest Tc's are obtained from the [M(NCMe)6][BF4]2 route, which was previously reported. The materials from [M(NCMe)6][SbF6]2 also exhibit the highest coercivity, followed by those from [M(NCMe)6][BF4]2. With the exception of M = Fe, the materials from Ma(CO)b exhibit the lowest coercive fields. In general, magnets made from [M(NCMe)6][SbF6]2 have a reduced χ‘ ‘(T) frequency dependence with respect to mat...

Organic-synthetic and supramolecular approaches to free radical-based magnets

Two approaches have been pursued to construct bulk magnets by ordering spins of unpaired 2p-electrons of organic free radicals. One is an intermolecular approach based on the crystal design of stable radicals. The other consists of intramolecular alignment of spins in polyradicals having many unpaired electron. The two approaches progressed complementarily to establish unprecedented molecule-based magnets. The second approach by our research group developed polyradicals, polycarbenes and their supramolecular assemblies with metal ions into permanent magnets, thereby deepening the understanding of chemical bonds and opening a new horizon of developing new magnets with other functionality. (Communicated by Hiroo INOKUCHI, M.J.A.)

Synthesis, Structure and Magnetic Properties of Chiral Molecule-Based Magnets

In this paper, we discuss the origin of magnetic chirality of chiral magnets made by chromium (III) and manganese (II) ions. Structurally characterized yellow needles: K 0.4 [Cr(CN) 6 ][Mn(S)-pn](S)-pnH 0.6 ((S)-pn = (S)-1,2-diaminopropane) and green needles: [Cr(CN) 6 ][Mn(S or R)-pnH(H 2 O)]H 2 O were obtained by the reaction among K 3 [Cr(CN) 6 ], Mn(ClO 4 ) 2 , (R or S)-pn.2HCl and KOH. The space groups of green needle and yellow needle are P2 1 2 1 2 1 and P6 1 , respectively. From the X-ray crystallographic analysis of them, the yellow needle has three-dimensional and green needle has two-dimensional magnetic connectivity. These ferrimagnets are good candidates for chiral spin order at low temperature. We present some theoretical supports, within classical phenomenologies, to understand characteristic features of the chiral magnetic materials and their magnetic properties.

Transfer matrix renormalization group studies on spin chains for molecule-based ferrimagnets

The transfer matrix renormalization group method has been applied to calculate the thermodynamic property of S=1 and S=1/2 alternating chains to show the possibility of a ferrimagnetic ordered state for organic molecule-based magnets. The S=1 site in the chain is composed of two S=1/2 spins coupled by a finite ferromagnetic interaction. From the calculation of the magnetic susceptibility temperature product chiT, we find that ferromagnetic behavior appears when the exchange interactions between the S=1 and S=1/2 moieties are much weaker than the biradical intramolecular interactions, which conclusion is consistent with experimental work. Through the study of the spatial symmetry lowering of intermolecular magnetic interactions, it is shown that the molecular ferrimagnetic alignment relies on the cooperative effects of intermolecular interactions. We also find that the maximum induced by an external field appears in chiT, following the minimum with decreasing temperature. In addition, we discuss the effect of dimerization along the spin chain on the stability of the ferrimagnetic state.

Electric‐Field‐Induced Conductance Switching in FeCo Prussian Blue Analogues.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stable Iminonitroxide Biradical in the Triplet Ground State

Abstract Magnetic properties and crystal structure of a pyridine-substituted iminonitroxide biradical, 2,6-bis(4,4,5,5,-tetramethyl-4,5-dihydro-1H-imidazol-2-yl-1-oxyl)pyridine (1), were examined. The molecular ground state of 1 was found to be triplet (S = 1) with a singlet–triplet energy gap of 7 K. The ground-state triplet biradical serves as a building block for molecule-based magnets of S &amp;gt; 1/2 based on intermolecular noncovalent bonding architecture.

Time-of-Flight Neutron Powder Diffraction Studies on a Chiral Two-dimensional Molecule-based Magnet

The magnetic structure of a chiral molecule-based magnet, [Cr(CN) 6 ][Mn( S )-pnH(H 2 O)](H 2 O), has been studied using neutron powder diffraction as a function of temperature in the range from 4 K to 300 K. The 020 and 022 magnetic reflections were clearly observed below T c = 38 K. On the basis of the intensity analysis of magnetic reflections, it was concluded that the magnetic structure is noncollinear ferrimagnetic with the magnetic (Shubnikov) space group P 2 1 2 1 '2 1 ', and magnetic moments of Cr and Mn atoms are mutually antiparallel along a direction near the a -axis. Our findings imply that the long-period magnetic structure, if exists, is not a helical structure but a conical one.

Cytosine-Substituted Nitronylnitroxide Radical: A Key Component for Bio-Inspired Molecule-Based Magnetics

A cytosine-substituted nitronylnitroxide radical (1) was synthesized. The nucleobase moiety plays a primary role in molecular packing in the crystal. It has been found from X-ray structure analyses and magnetic susceptibility measurements that the intermolecular magnetic interactions in the crystal are propagated by orbital overlaps between the phenylnitronylnitroxide moieties of the adjacent molecules, whereas the relative arrangement of the molecules are governed by the hydrogen bonding of the nucleobase moiety, giving a doubled chain structure. The hydrogen-bonded aggregation of the cytosine-substituted radical molecules can be a useful element for molecular assembly of bio-inspired molecule-based magnets in terms of crystal engineering for nucleobase-substituted radicals.

Muons as a probe of magnetism in molecule-based low dimensional magnets

We present the results of muon spin relaxation(μ+SR) studies on low dimensional molecular magnet systems.μ+SR measurements have been carried out on the Cu-based chain compoundsCuX2(pyz) (whereX = Br, Cl,NCS and pyz = pyrazine) as a function of temperature and applied longitudinal magnetic field.Oscillations in the time dependence of the muon polarization, characteristicof magnetic order at two distinct muon sites, are detected in bothCuBr2(pyz) (belowTN = 3.6(1) K)and CuCl2(pyz) (below TN = 3.2(2) K). No evidence of magnetic order is observed inCu(NCS)2(pyz) down to 0.35 K. The results are discussed in terms of the estimatedCu–X–Cu and Cu–(pyz)–Cu exchange constants. The theory ofμ+SR in high spin molecule (HSM) systems, which are effectivelyzero-dimensional magnets, is discussed and results are presented on[Ni12(chp)12(O2CMe)12(H2O)6(THF)6] (S = 12), [Mn9O7(OAc)11(thme)(py)3(H2O)2] (S = 17/2)and [Fe14(bta)6(O)6(OMe)18Cl6] (). Measurements made in applied longitudinal magnetic fields on HSM materials atdilution refrigerator temperatures strongly suggest that dynamic local magnetic fieldfluctuations are responsible for the relaxation of the muon spin ensemble. Trends intemperature and field dependent behaviour in these systems, as probed by the muon, arediscussed.

High Spin Chemistry Underlying Organic Molecular Magnetism: Topological Symmetry Rule as the First Principle of Spin Alignment in Organic Open-Shell Systems of π-Conjugation and Their Ions

This review paper deals with an overview of molecule-based magnetism as a rapidly developing interdisciplinary field, topological symmetry rule as the first principle of spin alignment in organic open-shell systems in the ground state, the proposal of organic through-bond 1D and 2D ferro- and super-paramagnets and the detection of the first organic high-spin molecule, m-phenylenebis(phenylmethylene) in the quintet ground state (S = 2), followed by extended organic high-spin systems with π-conjugation such as aromatic hydrocarbons having S = 3, 4, 5. The paper also describes a theoretical approach to the understanding of electronic spin structures of organic high-spin molecules by invoking both Heisenberg and Hubbard model Hamiltonians, weakly interacting intramolecular high-spin systems from both experimental and theoretical sides, the spin density distribution of the first organic high-spin molecule in terms of electron-nuclear multiple resonance spectroscopy and the detection and characterization of ionic high-spin hydrocarbons, emphasizing the establishment of high spin chemistry underlying organic molecular magnetism. (Communicated by Hiroo INOKUCHI, M.J.A., Feb. 12, 2004)

Electric-field-induced conductance switching in FeCo Prussian blue analogues.

FeCo Prussian blue analogues, which are known as typical molecule-based magnets, exhibited abrupt conductance switching by applying a high electric field as well as by varying the temperature. The current density versus electric field (J-E) curves of FeCo Prussian blue with Rb cations in interstitial sites shows so-called negative resistance effects at electric fields higher than the threshold voltage. This means that the FeCo Prussian blue analogues are multiproperty materials in the sense that their conducting, magnetic, and optical properties can be reversibly controlled by certain external stimuli.

A cyano-bridged molecule-based magnet containing manganese(III) Schiff base and octacyanotungstate(V) building blocks

A hetero-bimetallic complex K[Mn(acacen)] 2[W(CN) 8] · 2H 2O (acacen 2− = dianion of N, N′-ethylenebis(acetylacetonylideneaminato)) consists of two-dimensional gridlike {[Mn(acacen)] 2[W(CN) 8]} − layers that are linked by K + ions giving rise to a three-dimensional network. Ferromagnetic Mn(III)–W(V) coupling and magnetic ordering at 18.0 K ( T c) have been observed in the complex.

Vanadium 7,7,8,8-tetracyano-p-quinodimethane (V[TCNQ]2)-based magnets.

A new family of molecule-based magnets of general formula V[TCNQR(2)](2).zCH(2)Cl(2) has been synthesized and characterized (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane; R = H, Br, Me, Et, i-Pr, OMe, OEt, and OPh). In addition, solid solutions of V[TCNQ](x)()[TCNQ(OEt)(2)](2)(-)(x)().zCH(2)Cl(2) composition have been prepared. Except R = Br, magnetic ordering was observed for all materials, with T(c) values between 7.5 K (R = Me) and 106 K (R = OEt), with R = H at 52 K. The substitution of electron-donating OMe and OEt groups for H in TCNQ increased T(c), whereas the substitution of less electron-donating alkyl groups (with respect to alkoxy groups) decreased T(c). The results of MO calculations indicate that neither the spin nor charge densities of the disubstituted TCNQs are sufficiently different to explain the wide range of critical temperatures. Although the structures of the amorphous materials are not known, it is proposed that the oxygen atom of the [TCNQR(2)](*)(-) acceptor (R = OMe and OEt) and the V(II) interact to form a seven-membered ring. This interaction could stabilize the structure and enhance the magnetic coupling, leading to an increased T(c). The magnetic properties of V[TCNQ](x)()[TCNQ(OEt)(2)](2)(-)(x)().zCH(2)Cl(2) deviated from the expected linear relationship with respect to x, exhibiting magnetic behavior more characteristic of a step function in a plot of T(c) versus x.

Local structural order in the disordered vanadium tetracyanoethylene room-temperature molecule-based magnet

We determined the vanadium oxidation state and local coordination environment in disordered samples of magnetic $\mathrm{V}{[\mathrm{TCNE}]}_{x}(x\ensuremath{\approx}2)$ prepared by chemical vapor deposition (CVD). Systematic studies of the x-ray absorption near-edge structure (XANES) in this material and reference compounds show that V ions have a valence state near $2+$. Extended x-ray absorption fine structure (EXAFS) analysis shows that vanadium ions are coordinated by $6.04\ifmmode\pm\else\textpm\fi{}0.25$ nitrogen atoms at a room-temperature average distance of $2.084(5)\mathrm{\AA{}}$. The local environment is well defined with a distribution of V-N bond lengths comparable to that commonly found in ordered compounds. This distribution is mostly vibrational in origin, with static contributions being at least four fold smaller. The small disorder in V-N distances is a consequence of strong binding between $\mathrm{V}$ and TCNE, with an effective local force constant of $k=87\phantom{\rule{0.3em}{0ex}}\mathrm{N}∕\mathrm{m}$. This strong bonding leads to strong nearest neighbor coupling, which for the extended structure of $\mathrm{V}{[\mathrm{TCNE}]}_{x}$ with six $\mathrm{N}$ nearest neighbors results in magnetic ordering above room temperature. The strong V-N bonding explains in part the insoluble nature of this compared to other molecule-based magnets. The room-temperature XANES and EXAFS results for the CVD-prepared samples are compared to those for $\mathrm{V}{[\mathrm{TCNE}]}_{x}$ prepared as a powder from ${\mathrm{CH}}_{2}{\mathrm{Cl}}_{2}$ solvent, which has a similar magnetic ordering temperature but a magnetization that is more strongly temperature dependent. This comparison suggests that coordination of 6 nitrogens around each V(II) with little variation in the V-N distances is important for achieving the high magnetic ordering temperature of $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ associated with samples made by both the CVD and ${\mathrm{CH}}_{2}{\mathrm{Cl}}_{2}$ solution methods.

A molecule-based magnet structure above and below the critical phase transition temperature

A molecule-based magnet structure above and below the critical phase transition temperature

Diruthenium tetraacetate monocation, [RuII/III2(O2CMe)4]+, building blocks for 3-D molecule-based magnets.

Diruthenium tetracarboxylates monocations are utilized as building blocks for cubic 3-D network structured molecule-based magnets. [Ru(II/III)(2)(O(2)CMe)(4)](3)[M(III)(CN)(6)] [M = Cr (1a), Fe (2), Co (3)] were prepared in aqueous solution. Powder X-ray diffraction indicates that they have body-centered cubic structures (space group = Imm, a = 13.34, 13.30, and 13.10 A for 1a, 2, and 3, respectively), which was confirmed for 1a by Reitveld analysis of the synchrotron powder data [a = 13.3756(5) A]. [Ru(2)(O(2)CMe)(4)](3)[M(III)(CN)(6)].xMeCN [M = Cr, x = 1.8 (1b); M = Mn, x = 3.3 (4)] were prepared from acetonitrile. The magnetic ordering of 1a (33 K), 1b (34.5 K), 2 (2.1 K), and 4 (9.6 K) was determined from the temperature dependencies of the in-phase (chi') alternating current (AC) susceptibility. The field dependence of the magnetization, M(H), at 2 K for 1a showed an unusual constricted hysteresis loop with a coercive field, H(cr), of 470 Oe while the M(H) data for 1b, 2, and 4 showed a normal hysteresis loop with a coercive field of 1670, 10, and 990 Oe, respectively. The (57)Fe Mössbauer spectrum of 2 is consistent with the presence of low spin Fe(III) (delta = -0.05 mm/s; DeltaE = 0.33 mm/s) at room temperature, and the onset of 3-D magnetic ordering at lower temperature (<2 K). The effects of M(III) in [M(III)(CN)(6)](3-), and the large zero-field splitting (D) of diruthenium tetracarboxylates are discussed. The increasing critical temperatures T(c), with increasing S could not be accounted for by mean field models without significantly different J values for 1a, 4, and 2. By fitting the T(c) data with mean field models [H = -2JS(Ru).(S(M) - micro(B)(g(Ru)S(Ru) + g(M)S(M))H], J/k(B) are 4.46, 1.90, and 0.70 K for 1a, 4, and 2, respectively.

Remnant X-ray Magnetic Circular Dichroism Investigation of the Local Magnetic Contributions to the Magnetization of a Coercive Bimetallic Molecule-Based Magnet

X-ray magnetic circular dichroism (XMCD) measurements have been performed at the Cr and Fe K-edges for the two-dimensional oxalate-based magnet {[N(C4H9)4][FeIIFeIII0.77CrIII0,23(ox)3]}n (ox = C2O42-) in its long-range ordered magnetic state. First, the XMCD spectra recorded under a 2 T applied magnetic field are used to probe the local exchange interaction between the metal ions through the relative orientation of their spins in the magnetic field. The remnant local magnetic moments on the Cr and Fe ions have been clearly evidenced, analyzed, and compared to the total remnant magnetization measured by conventional magnetometry. The respective impact of single ion anisotropy, magneto-structural anisotropy, and exchange interaction on the remnant magnetic state is then discussed.

Structural and magnetic properties of two- and three-dimensional molecule-based magnets (cat)+6MIIMIII(C2O4)39$minus;

Structural and magnetic properties of two- and three-dimensional molecule-based magnets (cat)+6MIIMIII(C2O4)39$minus;