Molecule-based Magnetic Materials Research Articles

Recent advance on molecule-based magnetic refrigeration materials at low temperatures

Magnetic refrigeration technology is based on magnetic substances as working medium and achieve refrigeration by isothermal magnetization and adiabatic demagnetization. It displays a great potential for development of high-tech refrigeration technology. Compared to traditional gas-compression refrigeration, magnetic refrigeration is favored by more and more researchers due to its energy-efficient and environmentally friendly superiority. Recent studies indicate that molecular-based magnetic refrigerant materials exhibit great advantage in this field, compared with the alloy and nano magnetic refrigeration materials. In this review, the reported molecule-based magnetic refrigeration materials and our own work are summarized and discussed, and the development trends were evaluated.

Breathing Some New Life into an Old Topic: Chalcogen-Nitrogen π-Heterocycles as Electron Acceptors

Recent progress in the design, synthesis and characterization of chalcogen-nitrogen π-heterocycles, mostly 1,2,5-chalcogenadiazoles (chalcogen: S, Se and Te) and their fused derivatives, possessing positive electron affinity is discussed together with their use in preparation of charge-transfer complexes and radical-anion salts—candidate building blocks of molecule-based electrical and magnetic functional materials.

Synthesis, Crystal Structure, and Magnetic Properties of a Mn(III)-radical Complex

The design and synthesis of metal-radical complexes has attracted much attention in several filed of chemistry, optics, physics and material science for the purpose of exploiting new molecule-based magnetic materials. Among these metal-radical complexes, much attraction has been paid to the research of nitronyl nitroxide radicals since they are used as spin labels or paramagnetic building blocks in the construction of multidimensional magnetic materials. Moreover, they are usually stable in the open-shell organic molecule and exhibit coordination versatility in the structure in which they can act as bridging ligands or terminal ligands. A larger number of complexes based on the nitronyl nitroxide radical and transition metal ions, such as Mn(II), Co(II), Ni(II) and Cu(II), or rare earth metals assembly have been reported. 2-(4-Pyridyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3oxide (pPyNN) radical as one of the most attractive radicals has been widely used to assemble metal-radical complexes such as [M(II)(acac)2(pPyNN)] (M = Co, Ni, Mn). The Mn(III) ion has a large spin state (S = 2.0) and a strong axial magnetic anisotropy and has attracted much more interest in the design of molecular magnets especially single-molecule magnets (SMMs) and single-chain magnets (SCMs). However, to the best of our knowledge, magnetic complexes based on Mn(III) and nitronyl nitroxide radicals have not yet been reported. In the paper, we report the synthesis, crystal structure and magnetic properties of a new Mn(III)-radical complex [Mn(acacen)(pPyNN)(H2O)]ClO4 (1) (acacen = dianion of N,N'-ethylenebis(acetylacetonyl)-ideneaminato).

ChemInform Abstract: Framework‐Structured Weak Ferromagnets

AbstractReview: 273 refs.

Framework-structured weak ferromagnets

Framework-structured weak ferromagnets are new rising stars in molecule-based magnetic materials. The framework structures are powerful carriers for long-range ordering of spins. And weak ferromagnetism due to spin canting is an effective approach for magnets because of its frequent occurrence and desired spontaneous magnetization as long as the canting angle γ is large enough. In this critical review, we provide an overview of the various framework-structured weak ferromagnets based on different grades of ligands (from mono-atom to three-atom-like ligands). Particular emphasis is given to the relationships between structural features and the properties, rational employment of the ligands, and weak ferromagnetic strategies for molecule-based magnets with exciting properties and applications (273 references).

Molecule-based magnetic materials based on dinuclear ruthenium carboxylate and octacyanotungstate

Two structurally different polymeric complexes consisting of ruthenium(II,III) pivalate ([Ru2(piv)]4+) (Hpiv = pivalic acid) and octacyanotungstate(V) ([W(CN)8]3−) ions, [{Ru2(piv)4}3W(CN)8(H2O)] (1) and (PPh4)2[Ru2(piv)4W(CN)8]·0.5H2O (2), were synthesized. The infrared spectra as well as diffused reflectance spectra confirmed the existence of the ruthenium(II,III) pivalate dimer in 1 and 2. The X-ray crystal structure analysis of 2 showed an alternating arrangement of [Ru2(piv)]4+ and [W(CN)8]3− ions in a one-dimensional chain. The temperature-dependent magnetic susceptibility of 2 showed a ferrimagnetic-like behavior. However, the field dependent magnetization curve at 2 K indicates that this complex has an S = 0 ground state. The X-ray crystal structure analysis of 1 showed a two-dimensional layered structure made up of windmill-shaped 28 membered-rings of alternating arrangement of [Ru2(piv)]4+ and [W(CN)8]3−. The magnetic susceptibility and zero-field-cooled and field-cooled magnetization data of 1 showed that this complex is ferrimagnetic with Tc = 44 K. The field dependence of magnetization exhibited a hysteresis with a coercive field of 5600 Oe at 5 K.

Crystal structure of diaquabis[2-{2′-[(1′-methyl)benzimidazolyl]}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide]nickel(II)bisperchlorate dihydrate, [Ni(H2O)2(C15H19N4O2)2][ClO4]2 · 2H2O

Article Crystal structure of diaquabis[2-{2′-[(1′-methyl)benzimidazolyl]}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide]nickel(II)bisperchlorate dihydrate, [Ni(H2O)2(C15H19N4O2)2][ClO4]2 · 2H2O was published on December 1, 2010 in the journal Zeitschrift für Kristallographie - New Crystal Structures (volume 225, issue 4).

ChemInform Abstract: Structural Motifs and Topological Representation of Mn Coordination Clusters

AbstractReview: 212 refs.

Structural motifs and topological representation of Mn coordination clusters

Polynuclear coordination clusters have become of particular interest in recent times as a result of their relevance to bioinorganic chemistry and to the special area of molecule-based magnetic materials where cluster compounds behave as single-molecule magnets (SMMs). In this review we have focused on describing Mn coordination cluster complexes. Adopting our topological approach for the description of coordination clusters we present a means of classifying the structural motifs found in manganese clusters which range in nuclearity from 5 to 84, as well as some representative heterometallic Mn-M (M = K, Na, Ca, Sr, Ln) cluster complexes that have been reported. This sheds new light on the classification of the types of core structure accessible which, in turn, provides a useful means for developing the so-far missing magneto-structural correlation algorithm for these finite 0-D systems (212 references).

Design, structure, and properties of functional metal–ligand inorganic modules

From the standpoint of supramolecular chemistry major advances have been made concerning the functional properties of metal–ligand-based coordination complexes/extended solids/polymers. This paper reviews recent results from our laboratories on two contemporary areas of research: (i) spin-crossover Fe IIN 6 complexes which are representative examples of molecular bistability and (ii) synthesis of tailor-made transition-metal-based 1D coordination polymers and investigation of their magnetic exchange phenomena from the standpoint of molecule-based magnetic materials. Our primary focus is on the magnetic properties which seem to hold promise for future applications. It has been shown that covalent bond-driven discrete complexes/coordination polymers are further stabilized by association due to multiple weak, noncovalent interactions. In the design of Fe IIN 6 complexes our laboratories have so far relied on synthesis of discrete molecules. However, they are connected by noncovalent interactions, imparting cooperativity.

Ferromagnetic interaction between [Ni(bdt) 2] − anions in [Mn 2(Saloph) 2( μ-OH)][Ni(bdt) 2](CH 3CN) 2

A new molecule-based magnetic material [Mn 2(Saloph) 2( μ-OH)][Ni(bdt) 2](CH 3CN) 2 was prepared by the metathesis of [Mn(Saloph)(H 2O)(ClO 4)] ( S = 2) and TBA[Ni(bdt) 2] ( S = 1/2). In the crystal, [Ni(bdt) 2] − anions form square lattices which are separated from each other by the layers of antiferromagnetically coupled binuclear cations [Mn 2(Saloph) 2( μ-OH)] +. The magnetic susceptibility of the material coincides with the sum of the S = 2 van Vleck dimer model and S = 1/2 Heisenberg ferromagnetic square lattice model with 2 J = −92.4 and +4.5 K, respectively. The origin of the ferromagnetic interaction can be explained by the T-shaped intermolecular overlap mode of SOMOs which spreads to the ends of [Ni(bdt) 2] − molecules.

Nature of the Ferromagnetic Behavior and Possible Occurrence of the Ferrimagnetic Phase Transition in Genuinely Organic Molecule-Based Assemblages with an S = 1 and S = 1/ 2 Antiferromagnetic Alternating Spin Chain: A Green’s Function Approach

The temperature dependence of magnetic susceptibility and sublattice magnetizations were calculated for a Heisenberg Hamiltonian of an S = 1 and S = 1/2 antiferromagnetic alternating spin chain by means of the many-body Green's function theory to show the possible occurrence of a ferrimagnetic phase transition for genuinely 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 calculated results, it is found that the sublattice magnetization at low-spin S = 1/2 sites changes its sign from negative to positive with increasing temperature, giving rise to the spin alignments along the chain changing from antiferromagnetic to ferromagnetic ones, which indicates that there is a magnetic phase transition occurring. Because of the weak intermolecular antiferromagnetic interactions, the curves of the magnetic susceptibility multiplied by temperature (chiT) against temperature show a round peak at low temperatures, which is well consistent with recent experimental observations, and the ferrimagnetic phase transition could only be detected at an ultralow-temperature region and under very weak external magnetic fields in practical organic materials. From the analysis of the sublattice magnetizations, it is uncovered that the appearance of the low-temperature peak in the curves of the chiT originates from the ferromagnetic spin alignments for all the spins along the chain, and the intermolecular antiferromagnetic interactions play a pivotal role in ferrimagnetic spin alignments of the magnetic systems. It is also found that the higher spatial symmetry of the intermolecular antiferromagnetic interactions have contributions to stabilize the ferrimagnetic ordering state in the molecule-based magnetic materials.

Magnetic field-induced formation of molecule-based magnetic material [Co 1.5(N 3)(OH)(L)] n with antiferromagnetic coupling

Two identical hydrothermal self-assembly reactions were carried out with and without an applied magnetic field of 0.2 T. Two kinds of single crystals can be both obtained in the two reactions. It is shown that the crystal yield of the magnetic material is remarkably increased after the weak magnetic field is employed. We ascribe the magnetic field effect to the decrease of activation energy of antiferromagnetic complex during the course of the self-assembly reaction. And the decrease of activation energy is because the partial magnetic phase transits from paramagnetic phase to antiferromagnetic phase. Moreover, magnetic measurement data also reveal that the antiferromagnetic coupling interaction of the magnetic material has been enhanced.

Syntheses, crystal structures and magnetic properties of a new thienyl-substituted nitronyl nitroxide and its copper(II) complex

A new building block for molecule-based magnetic materials − thiophene-substituted nitronyl nitroxide, NIT2-thp, [1, NIT2-thp = 4,4,5,5-tetramethyl-2-(thiophenal-2-yl)imidazoline-1-oxyl-3-oxide] and its copper(II) complex [Cu(hfac)2]3(NIT2-thp)2 (2) (hfac = hexafluoroacetylacetonate) have been synthesized and characterized structurally and magnetically. For 1, dimers were formed and arranged through intermolecular interactions, the shortest contact between nitroxide groups is 4.115 Å among adjacent dimers. In 2 two types of copper interaction with 1 give three colinear Cu(II) ions linked by two μ − 1,4 bridging nitroxide ligands. The central metal ion is in a distorted octahedron, axially coordinated by two nitroxide oxygen atoms, while the two external metal ions are in distorted square-pyramidal environments with the nitroxide oxygen atom coordinated in the basal plane. Magnetic susceptibility data for 1 and 2 have been measured in the range 3–300 K. There are antiferromagnetic interactions (J = −3.89 cm−1) between the dimers of 1 and also ferromagnetic interactions in 2. The magnetic properties of 1 and 2 are discussed in connection with their crystal structures.

Ligand design for multidimensional magnetic materials: a metallosupramolecular perspective

The aim and scope of this review is to show the general validity of the 'complex-as-ligand' approach for the rational design of metallosupramolecular assemblies of increasing structural and magnetic complexity. This is illustrated herein on the basis of our recent studies on oxamato complexes with transition metal ions looking for the limits of the research avenue opened by Kahn's pioneering research twenty years ago. The use as building blocks of mono-, di- and trinuclear metal complexes with a novel family of aromatic polyoxamato ligands allowed us to move further in the coordination chemistry-based approach to high-nuclearity coordination compounds and high-dimensionality coordination polymers. In order to do so, we have taken advantage of the new developments of metallosupramolecular chemistry and in particular, of the molecular-programmed self-assembly methods that exploit the coordination preferences of metal ions and specifically tailored ligands. The judicious choice of the oxamato metal building block (substitution pattern and steric requirements of the bridging ligand, as well as the electronic configuration and magnetic anisotropy of the metal ion) allowed us to control the overall structure and magnetic properties of the final multidimensional nD products (n = 0-3). These species exhibit interesting magnetic properties which are brand-new targets in the field of molecular magnetism, such as single-molecule or single-chain magnets, and the well-known class of molecule-based magnets. This unique family of molecule-based magnetic materials expands on the reported examples of nD species with cyanide and related oxalato and dithiooxalato analogues. Moreover, the development of new oxamato metal building blocks with potential photo or redox activity at the aromatic ligand counterpart will provide us with addressable, multifunctional molecular materials for future applications in molecular electronics and nanotechnology.

Crystal structure of 4,4,5,5-tetramethylimidazoline-2-yl-(2’-chloro- 5′-nitrophenyl)-1-hydroxide-bis(hexafluoroacetyl acetonato)-zinc(II), Zn[C3HO2(CF3)2]2(C13H15ClN3O3)

C23H17ClF12N3O7Zn, triclinic, P1 (no. 2), a = 10.1626(9) A, b = 10.917(1) A, c = 16.157(2) A, 0 = 76.978(2)°, 1 = 77.780(1)°, " = 66.711(1)°, V = 1589.0 A, Z = 2, Rgt(F) = 0.071, wRref(F) = 0.237, T = 291 K. Source of material 4,4,5,5-tetramethylimidazoline-2-yl-(2+-chloro-5+-nitrophenyl)1-hydroxide) (IM-mNO2) was synthesized using a method in the literature [1]. The complex Zn(IM-mNO2)(hfac)2 (hfac = hexafluoroacetyl acetonate) was synthesized by adding Zn(hfac)2 (1 mmol) in boiling n-heptane (25 mL), and the solution was left to boil for 30 min and then cooled down to 65 °C, 1 mmol of the crystalline solid radicalwas slowly addedwhilst stirring followed by 5 ml of CH2Cl2. The solution was allowed to stand at room temperature for several days to give brown needle-like crystals. Experimental details The large R values are caused by the low quality of the crystals probably connected with the disorder (figure, bottom). Discussion New approaches in the design of novel molecular magneticmaterials have been the focus of active investigation during the past decades [2]. The popular family concerns the nitronyl nitroxide radicals since they are stable and easy to functionalize [3].M(hfac)2 is widely used in the synthesis of molecular magnets (M = transition metal) and the synthesis of molecule-based magnetic materials is one of the major subjects of materials science [4-7].The synthesis of new molecular magnetic materials that combine transition metal ions and pure organic radicals as ligating sites have attracted much more attention in the last few years [8-10]. In the title complex, the Zn(hfac)2with the IM-mNO2 ligand forms a distorted trigonal bipyramidwithN atom of the imidazoline unit, O4 andO6 in the basal plane,while the axial positions are occupied byO5 andO7 (figure, top).The Zn(II) atom lies 0.049A above the base plane, nearly coplanar with O4, O6 and N2, and a Zn1—N2 bond length of 2.046(5)A.TheN—O bond distance is 1.278(8)A, which is in the normal range for nitroxides being from 1.25 A to 1.32A [11,12]. The Zn—O (Zn1—O7, Zn1—O5) bond lengths in axial direction are 2.087(4) A and 2.063(4) A, respectively. Both are longer than those of the equatorial Zn—O (d(Zn1—O4) = 1.994(4) A, d(Zn1—O6) = 1.984(4) A). The bond length of Zn1—N2with 2.046(5)A is in a normal range [12].The angles of O6–Zn1–O4 (114.5(2)°), O6–Zn1–N2 (125.0(2)°), O4–Zn1–N2 (120.5(2)°) are approximately 120°, while theO5–Zn1–O7 angle is 179.5(2)°. The nitronyl nitroxide unit (imidazoline ring) is not coplanar with the phenyl ring to which the nitronyl nitroxide unit is directly linked, forming a dihedral angle of 73.16°. Z. Kristallogr. NCS 222 (2007) 181-183 / DOI 10.1524/ncrs.2007.0075 181 © by Oldenbourg Wissenschaftsverlag, Munchen

Trends and challenges in molecule-based magnetic materials

Trends and challenges in molecule-based magnetic materials

Synthesis, Structures, and Magnetic Properties of Heteronuclear Cu(II)−Ln(III) (Ln = La, Gd, or Tb) Complexes

Facile one-pot reactions led to the formations of dinuclear [CuLn(hmp)2(NO3)3(H2O)2] (Ln = Tb (1.Tb), Gd (1.Gd), or La (1.La)), and trinuclear [Cu2Ln(mmi)4(NO3)(H2O)2](ClO4)(NO3).2H2O (Ln = Tb (2.Tb) or Gd (2.Gd)) and [Cu2La(mmi)4(NO3)2(H2O)](ClO4).2H2O (2.La) with polydentate ligands 2-(hydroxymethyl)-pyridine and 2-hydroxymethyl-1-methyl-imidazole. In these complexes, each pair of Cu(II) and Ln(III) ions is linked by a double mu-alkoxo bridge. The temperature dependences of the magnetic susceptibilities of 1 and 2 were investigated in the range of 2-300 K. The dinuclear and trinuclear Cu-Gd complexes exhibit ferromagnetic interaction. The coupling constant J values in the heterodinuclear Cu-Gd complexes are correlated to values of the dihedral angles alpha between the two O-Cu-O and O-Gd-O fragments of the bridging CuO2Gd networks, with the largest J value associated with the smallest alpha value. The occurrence of a ferromagnetic interaction between Cu(II) and Gd(III) ions of the trinuclear entity is supported by the field dependence of the magnetization. The field dependence of the magnetization at 2 K of 1.Gd and 2.Gd confirms the nature of the ground state and of the Cu(II)-Gd(III) interaction, while alternating current susceptibility measurements demonstrates out-of-phase ac susceptibility signals of 1.Tb, which is the molecule-based magnetic material of the smallest nuclearity which exhibits frequency-dependent behavior within the 3d-4f mixed-metal systems.

Can DFT calculations help the molecular designer to construct molecule based magnetic materials?

The use of small molecule density functional theory calculations to enhance and complement experimental work in the area of molecule-based magnetic materials is highlighted through a review of the author's own work. Focus is on the spin density of radicals and the consequences this have on the magnetic coupling between interacting spins in solid-state compounds. Both examples of the McConnell I mechanism, based on an analysis of possible orthogonality and sign of the interacting spins, and of the McConnell II mechanism based on charge transfer are encountered. It is concluded that such relatively small and easy calculations on the molecular 'bricks' can often help in the analysis of spin interactions in the resulting material. They can give direct indications of the McConnell I type of exchange interaction or they can give hints about possible pathways for the McConnell II mechanism. However, care should be taken not to over-interpret the results and one should be aware of the limitations of the methods.

Calorimetric investigation of phase transitions in the layered antiferromagnetic molecule-based material {N( n-C 5H 11) 4[Mn IIFe III(ox) 3]} ∞(ox=oxalato)

Heat capacity studies of the layered molecule-based magnetic material {N( n-C 5H 11) 4[Mn IIFe III(ox) 3]} ∞ (ox=oxalato) have been made in the 2–280 K range. Two distinct heat capacity anomalies were detected at 27.1 K ( T N) and 226 K corresponding to magnetic and structural phase transitions, respectively, along with a hump around 23 K. The magnetic heat capacities above T N can be well represented by S=5/2 two-dimensional antiferromagnetic Heisenberg model of a honeycomb lattice with intralayer exchange interaction J/ k B=−3.3 K. Application of spin-wave theory indicated the existence of a very weak interlayer magnetic interaction below T N. The anomaly at 27.1 K is due to the antiferromagnetic transition, in conformity with the magnetic results reported earlier. The hump around 23 K might be associated with the existing uncompensated magnetic moments. The estimated magnetic entropy (33.22 J K −1 mol −1) is close to the expected magnetic entropy ( R ln (6×6)=29.80 J K −1 mol −1) for the spin multiplicity of high spin Mn II and Fe III ions. The heat capacity anomaly at 226 K may be assigned to a structural phase transition of order–disorder type due to increasing conformational change of the n-C 5H 11 chains in the organic cation.