Tadeusz Wasiutyński

Towards high Tc octacyanometalate-based networks

We present an overview of very recent advances in the engineering of magnetic networks based on octacyanometalates. The selected magnetic networks of CuIIWV, NiIIWV and MnIILNbIV (L – organic bridging linker) illustrate the possible strategies for tuning of the magnetic characteristics. The combination of magnetic ordering for 2D (two-dimensional) and 3D (three-dimensional) networks together with the solvent sensitivity of a cyano-bridged framework resulted in the development of a novel 3D {[MnII(imH)]2[NbIV(CN)8]} assembly with magnetic sponge character, characterized by Tc of 62 K, the highest ever observed for octacyanometalate-based networks.

A new family of magnetic 2D coordination polymers based on [MV(CN)8]3− (M=Mo, W) and pre-programmed Cu2+ centres

Abstract The self-assembly of [M(CN) 8 ] 3− (M=Mo, W) anion and polyamine complexes of Cu II [Cu(tetren)] 2+ and [Cu(dien)(H 2 O) 2 ] 2+ (tetren=tetraethylenepentamine, dien=diethylenetriamine) in acidic aqueous solution gives (tetrenH 5 ) 0.8 {Cu IIxa0 4 [W V (CN) 8 ] 4 }·7.2H 2 O 1 , (tetrenH 5 ) 0.8 {Cu IIxa0 4 [Mo V (CN) 8 ] 4 }·7.2H 2 O 2 , (dienH 3 ){Cu IIxa0 3 [W V (CN) 8 ] 3 }·4H 2 O 3 and (dienH 3 ){Cu IIxa0 3 [Mo V (CN) 8 ] 3 }·4H 2 O 4 2D coordination polymers. All compounds are structure-related: the crystal structures of isomorphous 1 – 2 and 3 – 4 , respectively, consist of double-layered cyano-bridged {Cu II [W V (CN) 8 ] − } n square grid backbones and non-coordinated fully protonated polyamine countercations as well as H 2 O molecules located between the sheets. The magnetic measurements reveal long range ferromagnetic ordering with sharp phase transitions at T C in range 28–37 K and coercivity in range 30–225 Oe at liquid helium temperature, T =4.3 K.

Exploring the formation of 3D ferromagnetic cyano-bridged CuII2+x{CuII4[WV(CN)8]4−2x[WIV(CN)8]2x}·yH2O networks

Two novel non-stoichiometric 3D cyano-bridged coordination networks of general formula CuII2+x{CuII4[WV(CN)8]4−2x[WIV(CN)8]2x}·yH2O were obtained according to two different synthetic strategies. The heterogeneous reaction between solid 2D cyano-bridged network (dienH3){CuII[WV(CN)8]}3·4H2O (1) (dienH33+ = protonated diethylenetriamine) and an aqueous solution of DyIII(NO3)3 results in the removal of dienH33+ cations and the formation of a 3D cyano-bridged CuII2.44{CuII4[WV(CN)8]3.12[WIV(CN)8]0.88}·5H2O (2) network. The direct combination of [CuII(H2O)6]2+ and [WV(CN)8]3− in aqueous media leads to the structurally related CuII2.97{CuII4[WV(CN)8]2.06[WIV(CN)8]1.94}·4H2O (3) assembly. The assemblies 2 and 3 were characterised by X-ray powder diffraction along with IR, X-ray absorption spectroscopy (XAS), proton induced X-ray emission (PIXE) and magnetic measurements. 2 and 3 crystallise in a tetragonal system, space group I4/mmm with cell parameters a = b = 7.2695(9) A; c = 28.268(5) A; Z = 2 (2) and a = b = 7.2858(9) A, c = 28.282(5) A, Z = 2 (3). Both networks are characterised by the increase of TC from 33 K to 40 K and coercivity from 0.2 to 2–2.5 kOe compared to 1. Despite their general structural and magnetic similarity, 2 and 3 reveal significant differences in magnetic dimensionality: compound 2 exhibits the features of a metamagnet with a threshold field of 1.8 kOe at 4.2 K, while compound 3 resembles a classical magnet with 3D ordering. This difference is discussed in terms of non-stoichiometry of the networks accompanied by the appearance of different numbers of non-magnetic “defects” due to the formation of diamagnetic W(IV) centres.

Double Magnetic Relaxation and Magnetocaloric Effect in the {Mn9[W(CN)8]6(4,4′-dpds)4} Cluster-Based Network

Cyanide-bridged {MnII9[WV(CN)8]6} clusters with the ground state spin SSG = 39/2 were connected by a 4,4-dipyridyl disulfide (4,4-dpds) linker into 2-D double-connected coordination layers of the I0O2 type, {MnII9(4,4-dpds)4(MeOH)16[WV(CN)8]6}·12MeOH (1). The intercluster contacts are controlled by the bridging MnII-(4,4-dpds)-MnII coordination modes and direct hydrogen bonds W-CN···HOMeOH-Mn in three crystallographic directions, with the vertex-to-vertex contact unprecedented in {Mn9W6}-based networks dominating over the typical edge-to-edge contacts. The resulting 3D supramolecular network of high-spin clusters was subjected to a thorough magnetic characterization in context of two critical issues. First, the intracluster WV-CN-MnII exchange coupling and intercluster interaction were successfully modeled through the combination of dc measurements, Quantum Monte Carlo simulations, and mean-field calculations, yielding a reasonable Jap = -8.0 cm-1, Jeq = -19.2 cm-1 (related to apical and equatorial CN bridges, depending on the angle they form with the S4 axis of dodecahedral [W(CN)8]3- units, respectively), and zJ = 0.014 cm-1 with the average gW = gMn = 2.0 parameter set. Continuing this approach, we simulated the magnetocaloric effect (MCE) and compared it to the experimental result of ΔSmax = 7.31 J kg-1 K-1 for fields >5.0 T. Second, two relaxation processes were induced by a relatively weak magnetic field, Hdc = 500 Oe, at an Hac field frequency range of up to 10 kHz, which are related to dipole-dipole interactions between high-spin (39/2) moieties. The observed relaxation times significantly differ from each other, the slow process with τslow at tenths of a second being temperature independent and the faster process being 3-5 orders of magnitude faster with the effective energy barrier Δeff = 17.6 K. These dynamic properties are surprising, since the compound is made up of isotropic high-spin molecules.

Self-Enhancement of Rotating Magnetocaloric Effect in Anisotropic Two-Dimensional (2D) Cyanido-Bridged MnII–NbIV Molecular Ferrimagnet

The rotating magnetocaloric effect (RMCE) is a new issue in the field of magnetic refrigeration. We have explored this subject on the two-dimensional (2D) enantiopure {[MnII(R-mpm)2]2[NbIV(CN)8]}·4H2O (where mpm = α-methyl-2-pyridinemethanol) coordination ferrimagnet. In this study, the magnetic and magnetocaloric properties of single crystals were investigated along the bc//H easy plane and the a*//H hard axis. The observed small easy plane anisotropy is due to the dipole-dipole interactions. For fields higher than 0.5 T, no significant difference in the magnetocaloric effect between both geometries was noticed. The maximal magnetic entropy change for conventional effect was observed at 32 K and the magnetic field change μ0ΔH = 5.0 T attaining the value of ∼5 J mol-1 K-1. The obtained maximal value of -ΔSm is comparable to previously reported results for polycrystalline octacyanidoniobate-based bimetallic coordination polymers. A substantial anisotropy of magnetocaloric effect between the easy plane and hard axis appears in low fields. This includes the presence of inverse magnetocaloric effect only for the a*//H direction. The difference between both geometries was used to study the rotating magnetocaloric effect. We show that the inverse part of magnetocaloric effect can be used to enhance the rotating magnetic entropy change up to 51%. This finding is of key importance for searching efficient materials for RMCE.

2-D soft ferromagnet based on [WV(CN)8]3− and CuII with a Tc of 34 K

Self-assembly of [Cu(tetren)]2+ (tetren = tetraethylenepentamine) and [W(CN)8]3- in acidic aqueous solution yields the double-layered square grid cyanide-bridged polymer of ((tetrenH5)0.8CuII4[Wv(CN)8](4).7.2H2O)n with Cu(II) centres of square pyramidal geometry coordinatively saturated solely by CN bridges supplied by five [W(CN)8]3- ions; it exhibits soft ferromagnetic behaviour with an ordering temperature Tc of 34 K.

A DSC study of K1−x(NH4)x SCN mixed crystals

Abstract The phase transitions in mixed crystals of the system K 1− x (NH 4 ) x SCN for a wide range of x values have been studied using differential scanning calorimetry (DSC). On the basis of the experimental data a phase diagram was constructed. The diagram contains two curves: (a) - the line of the order-disorder phase transition separating regions of the orthorhombic (phase II) phases, (b) - the line of coexistence of the monoclinic (phase III) and the orthorhombic one. The enthalpies of both transitions were determined. Careful investigations of the thermal hysteresis of the phase II-phase I transition showed no systematic dependence on the concentration. For most concentrations Δ hyst. T ≈ 0 and does not excite 0.8 K for any concentration. For pure ammonium thiocyanate the kinetics of the spontaneous transition of undercooled phase II into stable phase III has been investigated.

A Study of the Magnetic Ordering in KCo[Fe(CN)6]·xH2O

Abstract The results of the ac susceptibility and dc magnetization in the wide range of temperatures are presented for KCo[Fe(CN)6]·xH2O. Magnetic ordering was revealed at T = 13.2 K. Frequency dependence of the complex susceptibility and non-linear response suggest glassy like behavior.