Rotating Magnetocaloric Effect in an Anisotropic Two-Dimensional CuII[WV(CN)8]3– Molecular Magnet with Topological Phase Transition: Experiment and Theory

Switching of magnetic properties by topotactic reaction in a 1D CN-bridged Ni(II)-Nb(IV) system

Two 1D CN-bridged assemblies: the nearly straight Li2[Ni(cyclam)][Nb(CN)8]·7.5H2O (1) chains and the zigzag-shaped Li2[Ni(cyclam)][Nb(CN)8]·2H2O (2) chains, are obtained in the reaction between [Ni(cyclam)]2+ and [Nb(CN)8]4- in warm concentrated LiCl water solution. Both compounds are composed of alternating bimetallic Ni(II)-Nb(IV) chains and contain incorporated lithium cations, which compensate the negative charge of the coordination skeleton. The straight chain 1 (Ni-Nb-Ni angle = 153.2°) can be reversibly dehydrated under dry nitrogen flow at room temperature to an intermediate dihydrate phase 1d and further transformed to the zigzag-shaped chain 2 (Ni-Nb-Ni angle = 86.6°) by annealing at 150 °C. The process can be reversed by exposure to high humidity at room temperature, upon which 2 is converted back to 1. This water sorption-induced breathing effect is accompanied by changes in magnetic properties, most notably reflected in different values of saturation magnetization and critical field of metamagnetic transition, which indicate that both intra- and inter-chain interactions are affected by the structure reorganization.

Rotating magnetocaloric effect in highly anisotropic TbIII and DyIII single molecular magnets

The magnetocaloric effect (MCE) was investigated in highly anisotropic single crystals of two single molecule magnets (SMMs): [Ln^III(Zn^IIL)₂]CF₃SO₃, where Ln = Tb, Dy and L = tripodal hexadentate Schiff base ligand. The structure of these paramagnetic compounds consists of identically oriented linear trinuclear clusters in a trigonal system with an easy direction c∥Zn-Ln-Zn array and a hard plane ab⊥Zn-Ln-Zn array. The magnitude of MCE measured for c∥H was significantly greater than MCE for ab∥H at a wide temperature range regardless of the studied SMM. Therefore, the rotating magnetocaloric effect (RMCE) was evaluated. The maxima of the magnetic entropy change for RMCE were obtained at 2.0 K and moderate fields: 3.9 J K^-1 kg^-1 at µ₀H = 1.3 T for Ln = Tb and 3.3 J K^-1 kg^-1 at µ₀H = 1.1 T for Ln = Dy. The relative efficiency of RMCE compared to the MCE measured in c∥H was as high as 99% at low magnetic fields.

Magnetic cooling: a molecular perspective

The magnetocaloriceffect is considered as an energy-efficient and environmentally friendly technique which can take cooling technology to the next level. Apart from its commercial application at room temperature, magnetic refrigeration is an up-and-coming solution for the cryogenic regime, especially as an alternative to He³ systems. Molecular magnets reveal advantageous features for ultra-low cooling which are competitive with intermetallic and lanthanide alloys. Here, we present a guide to the current status of magnetocaloric effect research of molecular magnets with a theoretical background focused on the inverse magnetocaloric effect and an overview of recent results and developments, including the rotating magnetocaloric effect.

A rare isostructural series of 3d-4f cyanido-bridged heterometallic squares obtained by assembling [FeIII{HB(pz)3}(CN)3]- and LnIII ions: synthesis, X-ray structure and cryomagnetic study

A new series of cyanido-bridged {Fe^IIILn^III}₂ neutral molecular squares of general formula [Fe{HB(pz)₃}(CN)(μ-CN)₂Ln(NO₃)₂(pyim)(Ph₃PO)]₂·2CH₃CN [Ln = Ce (1), Pr (2), Nd (3), Gd (4), Tb (5), Dy (6) and Er (7); {HB(pz)₃}^- = hydrotris(pyrazolyl)borate, pyim = 2-(1H-imidazol-2-yl)pyridine and Ph₃PO = triphenylphosphine oxide] were obtained by reacting the low-spin [Fe{HB(pz)₃}(CN)₃]^- species with the preformed [Ln^III(pyim)(NO₃)₂(pyim)(Ph₃PO)]⁺ complex anions (generated in situ by mixing the nitrate salt of each Ln(III) ion with pyim and Ph₃PO molecules). Single-crystal X-ray diffraction studies show that 1-7 are isostructural compounds that crystallize in the triclinic P1̄ space group. Their crystal structures consist of centrosymmetric cyanido-bridged {Fe^IIILn^III}₂ molecular squares where two [Fe{HB(pz)₃}(CN)₃]^- units adopt bis-monodentate coordination modes towards two [Ln^III(pyim)(NO₃)₂(pyim)(Ph₃PO)]⁺ moieties. The cis-oriented convergent sites from both low-spin Fe^III and Ln^III fragments form a quasi square-shaped molecule in which the 3d and 4f ions alternatively occupy the corners of the square. Both Fe^III ions show a distorted octahedral surrounding (C_3v symmetry), whereas the Ln^III ions exhibit a distorted muffin-like geometry (C_s symmetry) in 1-7. The intramolecular Fe^III⋯Ln^III distances across the two cyanido-bridges range from ca. 5.48/5.46 up to ca. 5.58/5.61 Å. The molecular squares in 1-7 are interlinked through hydrogen bonds, weak π⋯π stacking and very weak C-H⋯π type interactions into three-dimensional supramolecular networks. The analysis of the solid-state direct-current (dc) magnetic susceptibility data of 1-7 in the temperature range 1.9-300 K reveals the occurrence of weak intra- and intermolecular antiferromagnetic interactions. The small intramolecular antiferromagnetic couplings in 4 compare well with those previously reported for parent systems. Although the coexistence of the spin-orbit coupling (SOC) of the low-spin iron(III) and lanthanide(III) ions in the remaining compounds together with the ligand field effects mask the visualization and make difficult the evaluation of the possible magnetic interactions in them, we were able to do it through a SOC model applied on exact or effective Hamiltonians. Frequency-dependent alternating current magnetic susceptibility signals in the temperature range 2.0-9.0 K under zero and non-zero static fields were observed for 5-7 which indicate slow magnetic relaxation (SMM) behavior. The usual absence of χ''_M maxima moved us to estimate their energy barriers through ln(χ''_M/ χ'_M) vs. 1/T plots, obtaining values from 25 to 40 cm^-1.

Engineering of the XY Magnetic Layered System with Adeninium Cations: Monocrystalline Angle-Resolved Studies of Nonlinear Magnetic Susceptibility

An original example of modular crystal engineering involving molecular magnetic {Cu^II[W^V(CN)₈]}^- bilayers and adeninium cations (AdeH⁺) toward the new layered molecular magnet (AdeH){Cu^II[W^V(CN)₈]}·2H₂O (1) is presented. 1 crystallizes within the monoclinic C2 space group (a = 41.3174(12), b = 7.0727(3), c = 7.3180(2) Å, β = 93.119(3)°, and V = 2135 ų). The bilayer topology is based on a stereochemical matching between the square pyramidal shape of Cu^II moiety and the bicapped trigonal prismatic shape of [W^V(μ-CN)₅(CN)₃], and the separation between bilayers is significantly increased (by ∼50%; from ca. 9.5 to ca. 14.5 Å) compared to several former analogues in this family. This was achieved via a unique combination of (i) a 1D ribbonlike hydrogen bond system {AdeH⁺···H₂O···AdeH⁺···}_∞ exploiting planar water-assisted Hoogsteen···Sugar synthons with (ii) parallel 1D π-π stacks {AdeH⁺···AdeH⁺}_∞. In-plane 2D XY magnetism is characterized by a T_c close to 33 K, H_c,in-plane = 60 Oe, and H_{c,out-of-plane} = 750 Oe, high values of in-plane γ critical exponents (γ_b = 2.34(6) for H||b and γ_c = 2.16(5) for H||c), and a Berezinskii-Kosterlitz-Thouless (BKT) topological phase transition, deduced from crystal-orientation-dependent scaling analysis. The obtained values of in-plane ν critical exponents, ν_b = 0.48(5) for H||b and ν_c = 0.49(3) for H||c, confirm the BKT transition (ν_BKT = 0.5). Full-range angle-resolved monocrystalline magnetic measurements supported by dedicated calculations indicated the occurrence of nonlinear susceptibility performance within the easy plane in a magnetically ordered state. We refer the occurrence of this phenomenon to spontaneous resolution in the C2 space group, a tandem not observed in studies on previous analogues and rarely reported in the field of molecular materials. The above magneto-supramolecular strategy may provide a novel means for the design of 2D molecular magnetic networks and help to uncover the inherent phenomena.

Low-Temperature Magnetocaloric Properties of V12 Polyoxovanadate Molecular Magnet: A Theoretical Study

The paper presents a computational study of the magnetocaloric properties of the V12 polyoxovanadate molecular magnet. The description is restricted to low-temperature range (below approximately 100 K), where the magnetic properties of the system in question can be sufficiently modelled by considering a tetramer that consists of four vanadium ions with spins S=1/2. The discussion is focused on the magnetocaloric effect in the cryogenic range. The exact and numerical diagonalization of the corresponding Hamiltonian is used in order to construct the thermodynamic description within a version of the canonical ensemble. The thermodynamic quantities of interest, such as magnetic entropy, specific heat, entropy change under isothermal magnetization/demagnetization, temperature change under adiabatic magnetization/demagnetization, refrigerant capacity, and magnetic Grüneisen ratio, are calculated and discussed extensively. The importance of two quantum level crossings for the described properties is emphasized. The significant ranges of direct and inverse magnetocaloric effect are predicted. In particular, the maximized inverse magnetocaloric response is found for cryogenic temperatures.

Octacyanidometallates for multifunctional molecule-based materials

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Multifunctional Molecular Magnets: Magnetocaloric Effect in Octacyanometallates

Octacyanometallate-based compounds displaying a rich pallet of interesting physical and chemical properties, are key materials in the field of molecular magnetism. The [M(CN)8]n− complexes, (M = WV, MoV, NbIV), are universal building blocks as they lead to various spatial structures, depending on the surrounding ligands and the choice of the metal ion. One of the functionalities of the octacyanometallate-based coordination polymers or clusters is the magnetocaloric effect (MCE), consisting in a change of the material temperature upon the application of a magnetic field. In this review, we focus on different approaches to MCE investigation. We present examples of magnetic entropy change ΔSm and adiabatic temperature change ΔTad, determined using calorimetric measurements supplemented with the algebraic extrapolation of the data down to 0 K. At the field change of 5T, the compound built of high spin clusters Ni9[W(CN)8]6 showed a maximum value of −ΔSm equal to 18.38 J·K−1 mol−1 at 4.3 K, while the corresponding maximum ΔTad = 4.6 K was attained at 2.2 K. These values revealed that this molecular material may be treated as a possible candidate for cryogenic magnetic cooling. Values obtained for ferrimagnetic polymers at temperatures close to their magnetic ordering temperatures, Tc, were lower, i.e., −ΔSm = 6.83 J·K−1 mol−1 (ΔTad = 1.42 K) and −ΔSm = 4.9 J·K−1 mol−1 (ΔTad = 2 K) for {[MnII(pyrazole)4]2[NbIV(CN)8]·4H2O}n and{[FeII(pyrazole)4]2[NbIV(CN)8]·4H2O}n, respectively. MCE results have been obtained also for other -[Nb(CN)8]-based manganese polymers, showing significant Tc dependence on pressure or the remarkable magnetic sponge behaviour. Using the data obtained for compounds with different Tc, due to dissimilar ligands or other phase of the material, the ΔSm ~ Tc−2/3 relation stemming from the molecular field theory was confirmed. The characteristic index n in the ΔSm ~ ΔHn dependence, and the critical exponents, related to n, were determined, pointing to the 3D Heisenberg model as the most adequate for the description of these particular compounds. At last, results of the rotating magnetocaloric effect (RMCE), which is a new technique efficient in the case of layered magnetic systems, are presented. Data have been obtained and discussed for single crystals of two 2D molecular magnets: ferrimagnetic {MnII(R-mpm)2]2[NbIV(CN)8]}∙4H2O (mpm = α-methyl-2-pyridinemethanol) and a strongly anisotropic (tetren)Cu4[W(CN)8]4 bilayered magnet showing the topological Berezinskii-Kosterlitz-Thouless transition.

Cd(II) and Zn(II) thiocyanate coordination compounds with 3-ethylpyridine: synthesis, crystal structures and properties

Reaction of Cd(NCS)2 and Zn(NCS)2 with 3-ethylpyridine leads to the formation of compounds of compositions M(NCS)2(3-ethylpyridine)4 (M=Cd, 1-Cd; Zn, 1-Zn) and M(NCS)2(3-ethylpyridine)2 (M=Cd, 2-Cd; Zn, 2-Zn). 1-Cd and 1-Zn are isotypic and form discrete complexes in which the metal cations are octahedrally coordinated by two trans-coordinating N-bonded thiocyanate anions and four 3-ethylpyridine co-ligands. In 2-Cd the cations are also octahedrally coordinated but linked into chains by pairs of μ-1,3-bridging anionic ligands. 2-Zn is built up of discrete complexes, in which the Zn cation is tetrahedrally coordinated by two N-bonded thiocyanate anions and two 3-ethylpyridine co-ligands. Compounds 1-Cd, 2-Cd and 2-Zn can be prepared in a pure state, whereas 1-Zn is unstable and transforms on storage into 2-Zn. If 1-Cd and 1-Zn are heated, a transformation into 2-Cd, respectively 2-Zn is observed. Luminescence measurements reveal that 1-Cd, 2-Cd and 2-Zn emit light in the blue spectral range with maxima at, respectively, 21724, 21654 and 22055 cm−1, assigned to ligand-based luminescence.

Dinuclear molecular magnets with unblocked magnetic connectivity: magnetocaloric effect

A detailed study of the magnetocaloric effect in two isostructural bimetallic compounds {[M^II(H₂O)₂]₂[Nb^IV(CN)₈]·4H₂O}_n (M = Mn, Fe) is presented. The substances show sharp phase transitions to the long-range magnetically ordered state with ferromagnetic coupling between M and Nb sublattices in the case of the Fe-based sample (FeNb, T_c = 43 K) and antiferromagnetic coupling for the Mn-based sample (MnNb, T_c = 50 K). The magnetic entropy change was found to reach 5.07 J mol⁻¹ K⁻¹ (9.09 J kg⁻¹ K⁻¹) for MnNb and 4.82 J mol⁻¹ K⁻¹ (8.65 J kg⁻¹ K⁻¹) for FeNb under the applied magnetic field change of 5 T. Isothermal entropy changes corresponding to different field changes are demonstrated to collapse on a single master curve, which confirms the magnetic transitions in FeNb and MnNb to be of the second order. The results obtained for FeNb and MnNb are discussed in the context of MCE tunability by un/blocking of magnetic connectivity through dis/reconnection of spatially extended ligands.

The study of magnetocaloric effect in two related bimetallic cyanide-bridged molecular magnets: {[M (H₂O)₂]₂[Nb (CN)₈]·4H₂O}_n (M = Mn, Fe) is presented.