Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation.

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(µ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

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.

Sorption and Magnetic Properties of Oxalato-Based Trimetallic Open Framework Stabilized by Charge-Assisted Hydrogen Bonds.

We report a new structure of {[Co(bpy)2(ox)][{Cu2(bpy)2(ox)}Fe(ox)3]}n·8.5nH2O NCU-1 presenting a rare ladder topology among oxalate-based coordination polymers with anionic chains composed of alternately arranged [Cu2(bpy)2(ox)]2+ and [Fe(ox)3]3- moieties. Along the a axis, they are separated by Co(III) units to give porous material with voids of 963.7 Å3 (16.9% of cell volume). The stability of this structure is assured by a network of stacking interactions and charge-assisted C-H…O hydrogen bonds formed between adjacent chains, adjacent cobalt(III) units, and alternately arranged cobalt(III) and chain motifs. The soaking experiment with acetonitrile and bromobenzene showed that water molecules (8.5 water molecules dispersed over 15 positions) are bonded tightly, despite partial occupancy. Water adsorption experiments are described by a D'arcy and Watt model being the sum of Langmuir and Dubinin-Serpinski isotherms. The amount of primary adsorption sites calculated from this model is equal 8.2 mol H2O/mol, being very close to the value obtained from the XRD experiments and indicates that water was adsorbed mainly on the primary sites. The antiferromagnetic properties could be only approximately described with the simple CuII-ox-CuII dimer using H = -J·S1·S2, thus, considering non-trivial topology of the whole Cu-Fe chain, we developed our own general approach, based on the semiclassical model (SC) and molecular field (MF) model, to describe precisely the magnetic superexchange interactions in NCU-1. We established that Cu(II)-Cu(II) coupling dominates over multiple Cu(II)-Fe(III) interactions, with JCuCu = -275(29) and JCuFe = -3.8(1.6) cm-1 and discussed the obtained values against the literature data.

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 {CuII[WV(CN)8]}- bilayers and adeninium cations (AdeH+) toward the new layered molecular magnet (AdeH){CuII[WV(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, ß = 93.119(3)°, and V = 2135 Å3). The bilayer topology is based on a stereochemical matching between the square pyramidal shape of CuII moiety and the bicapped trigonal prismatic shape of [WV(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 Tc close to 33 K, Hc,in-plane = 60 Oe, and Hc,out-of-plane = 750 Oe, high values of in-plane γ critical exponents (γb = 2.34(6) for H||b and γc = 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, νb = 0.48(5) for H||b and νc = 0.49(3) for H||c, confirm the BKT transition (νBKT = 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.

Chiral Photomagnets Based on Copper(II) complexes of 1,2-Diaminocyclohexane and Octacyanidomolybdate(IV) Ions.

Chiral photomagnets compose a class of multifunctional molecule-based materials with light-induced alteration of magnetization and chiral properties. The rational design and synthesis of such assemblies is a challenge, and only few such systems are known. Herein, the remarkable octacyanide-bridged enantiomeric pair of 1-D chains [Cu((R,R)-chxn)2]2[Mo(CN)8]·H2O (1R) and [Cu((S,S)-chxn)2]2[Mo(CN)8]·H2O (1S) exhibiting enantiopure structural helicity, which results in optical activity in the 350-800 nm range as confirmed by natural circular dichroism (NCD) spectra, is reported. The photomagnetic effects of 1R, 1S, and 1rac result from the blue light excitation (436 nm) of the photomagnetically active octacyanidomolybdate(IV) ions. In the excited state MoIVHS centers with S = 1 couple antiferromagnetically with the neighboring CuII centers with JCuMo values of -1.3, -1.0, and -1.1 cm-1 for 1R, 1S, and 1rac, respectively. The values of thermal relaxation energy barriers have been estimated as 142 and 356 K for 1R and 1S, being comparable with the energy range of the thermal bath. The value for 1rac reveals a significantly lower value of 75 K. On the basis of these results the value of gMoHS has been estimated to be in the range 4.8-5.8.

1D coordination polymer (OPD)2CoIISO4 showing SMM behaviour and multiple relaxation modes.

A novel one-dimensional CoII coordination polymer (OPD)2CoIISO4 was formed from alternating tetrahedral sulphate anions, Co centers and molecules of 1,2-phenylenediamine (OPD). The thermal stability of the structure was confirmed up to ∼230 °C using thermogravimetry and non-ambient powder X-ray diffraction in air. The distorted pseudo-octahedral coordination sphere of CoII ions promotes strong magnetic anisotropy. Therefore (OPD)2CoIISO4 was subjected to thorough characterization with ab initio and DFT calculations along with dc magnetic measurements both of which point to strong easy-axis anisotropy (D/kB≈-87 K, E/kB≈ 23 K). The analysis of ac susceptibility revealed field assisted magnetic relaxation in two field regimes: a low field one with one relaxation time and a high field one where three relaxation times were distinguished. The main role in relaxations of the fastest process (τ1) was attributed to the direct and Raman processes.

A mixture of cellulose production waste with municipal sewage as new material for an ecological management of wastes.

Chemical contaminants present in cellulose production waste and municipal sewage sludge condition the necessity to treat these wastes before they are introduced into the environment. Environmental use of the these wastes appears to be justified owing to the content of organic substances as well as the considerable fertilising value. The aim of the study was to assess chemical composition and ecotoxicity of cellulose production waste and municipal sewage sludge in terms of their eco-friendly application. The assessment was done based on the established soil environment according to regulations in Poland. The research on the chemical composition of the wastes was conducted under laboratory conditions, determining the content of macroelements, microelements and heavy metals with respect to using these wastes in the environment. Phytotoxkit and Microtox tests were performed in order to determine the ecotoxicity of the wastes in the aspect of their environmental use. It was established that, compared to the cellulose production waste, the municipal sewage sludge used in the incubation experiment had a higher content of macroelements and microelements. The content of heavy metals in the waste did not exceed the permissible content that conditions their use in agriculture and reclamation. Based on the results of bioassays it was established that the cellulose production waste was toxic, whereas the combination of cellulose production waste and sewage sludge was found to be of low toxicity to test organisms. The applied wastes and their mixtures were ranked according to phytotoxicity in the following order: cellulose production waste > cellulose production waste + soil > cellulose production waste + sewage sludge. The research results indicate that waste mixtures qualify for environmental use. With respect to the variable chemical composition of cellulose production waste and municipal sewage sludge as well as their ecotoxicity, it was established that each batch intended for environmental use must be subjected to chemical analysis and ecotoxicological tests.

The effect of cellulose production waste and municipal sewage sludge on biomass and heavy metal uptake by a plant mixture.

Environmental management of cellulose production waste and municipal sewage sludge appears to be substantiated due to various physicochemical properties of these wastes. The aim of the conducted research was to determine the effect of cellulose production waste and sewage sludge on yielding and heavy metal uptake by a plant mixture. The research was conducted under field experiment conditions, determining the fertilizer value of these wastes in the environmental aspect. The research was carried out in the years 2013-2016. Species composition of the plant mixture was adjusted to habitat conditions. It was established that, as compared with the cellulose production waste, the municipal sewage sludge used in the experiment had a higher content of macroelements. The content of heavy metals in the studied waste did not exceed the limits that condition their use in agriculture and reclamation. Applying only the cellulose production waste did not significantly decrease the yield of the plants. Municipal sewage sludge showed the highest yield-forming effect. Mixing the above-mentioned wastes and their application to soil had a significant effect on the increase in the plant mixture yield. The waste applied to soil also increased the content of Cr, Cd, Pb, Cu, and Zn in the plant mix. The level of heavy metal content in the plant mix did not exclude this biomass from being used for fodder or reclamation purposes. The cellulose production waste and municipal sewage sludge increased the heavy metal uptake by the plant mixture. The plant biomass extracted heavy metals from the sewage sludge more intensively than from the cellulose production waste. Among the analyzed heavy metals, the highest phytoremediation was recorded for Ni (30%), followed by Cd (20%), Cr (15%), Pb (10%), and the lowest for Cu (9%) and Zn (8%). Application of the cellulose production waste and sewage sludge to soil also increased the content of the studied heavy metals in soil. However, it did not cause deterioration of soil quality standards. Heterogeneity in the chemical composition of the wastes confirms that each batch intended to be used for environmental management should be subjected to chemical control.

Magnetic percolation in CN-bridged ferrimagnetic coordination polymers.

A series of diluted molecular magnets {[MnII(H2O)2]2[MIV(CN)8]·4H2O}n (M = Nb/Mo) exhibiting a ferrimagnetic phase have been synthesized and investigated to demonstrate the first example of site percolation in magnetic coordination polymers. It is the first case of a magnetic percolation study with selective site substitution, where magnetic ions of only one type are replaced by their diamagnetic analogues leaving the other magnetic sublattice untouched. By modifying the molecular field model the NbIV concentration dependence of the magnetic ordering temperature was reproduced and the percolation threshold was determined.

Irradiation Temperature Dependence of the Photomagnetic Mechanisms in a Cyanido-Bridged CuII2MoIV Trinuclear Molecule.

We report a new bimetallic cyanido-bridged trinuclear complex [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O (1) (enpnen = N,N'-bis(2-aminoethyl)-1,3-propanediamine) that shows reversible photomagnetic effect. The photo-induced increase of magnetization is characterized by the irradiation temperature-dependent shapes of the χM T( T) plots and different magnetization values at low temperature in high magnetic field, suggesting multiple photoexcited states. The photomagnetic effect in 1 is explained through two possible processes simultaneously: the light-induced metal-to-metal charge transfer (MMCT) in the CuII-NC-MoIV pair and the light-induced excited spin-state trapping (LIESST) effect in MoIV center. A numerical model assuming the simultaneous existence of three possible states after irradiation: the MMCT CuI-NC-MoV-CN-CuII state, the LIESST CuII-NC-MoIVHS-CN-CuII state, and the ground-state CuII-NC-MoIVLS-CN-CuII was applied to the data and resulted in Cu-Mo exchange coupling constants J1MMCT = 11 cm-1 and J2LIESST = 109 cm-1 for the MMCT and LIESST mechanisms induced states, respectively. Fractions of respective states after irradiations at different temperatures were also calculated, shedding light on the influence of irradiation temperature on the photomagnetic mechanism. The proposed model can provide the interpretative framework for testing and refinement of the mechanism of photomagnetic effect in other coordination networks with cyanido-bridged Cu-[Mo(CN)8]4- linkages.

Dinuclear molecular magnets with unblocked magnetic connectivity: magnetocaloric effect.

A detailed study of the magnetocaloric effect in two isostructural bimetallic compounds {[MII(H2O)2]2[NbIV(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.

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

Conventional (MCE) and rotating (RMCE) magnetocaloric effects have been explored in the two-dimensional (2D) coordination polymer {(tetren)H5)0.8CuII4[WV(CN)8]4·7.2H2O}n (WCu-t; tetren = tetraethylenepentamine). The unusual magnetostructural properties were exploited, including the bilayered Prussian Blue like coordination skeleton and the XY easy-plane magnetic anisotropy based on the in-plane correlation between WV and CuII spins of 1/2, underlying the Berezinskii-Kosterlitz-Thouless (BKT) topological phase transition to the long-range-ordered state at TC = 33 K. The magnetic properties were studied on single crystals along the H∥ac easy plane and H∥b hard axis. The maximal entropy change for MCE for easy-plane geometry at 38.0 K and the magnetic field change µ0ΔH = 7.0 T reached ∼4.01 J K-1 kg-1. The strong magnetic anisotropy was used to study the RMCE in which the maximal entropy change was observed at 35.5 K for 7.0 T, attaining 1.81 J K-1 kg-1. Moreover, easy-plane anisotropy introduces the inverse magnetocaloric effect for H∥b, which enhances the RMCE by up to 47%. This observation was confirmed by a theoretical investigation considering the XY model using a molecular field and cluster variational method in the pair approximation approach, dedicated to the bilayered systems with the adequate nearest neighbor number z = 5 and spin S = 1/2.

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.

Photoswitchable CuMoIV and CuMoIV cyanido-bridged molecules.

The self-assembly of copper(ii) complexes with two Schiff base ligands: L1O = N3 and L2 = N4 and octacyanidomolybdate(iv) ions yields two discrete molecules of odd nuclearity, namely pentametallic [Cu(L1O)(py)]4[MoIV(CN)8]·14H2O (1), Cu4Mo and trimetallic [Cu(L2)]2[MoIV(CN)8]·9H2O (2), Cu2Mo. Both molecular systems have been characterised structurally and magnetically, revealing a photomagnetic effect. In the case of (1) a metal-to-metal charge transfer (MMCT) mechanism is proposed. The analysis of magnetic interactions in the photogenerated state of (1) assumes the formation of the metastable cluster CuCuIMoV where metal centres in MoV-CN-CuII linkages are ferromagnetically coupled with J2 = 104(3) cm-1. In (2) the increase in the magnetisation is due to the singlet-triplet transition on the MoIV centre leading to the formation of the metastable CuMo. The presence of the paramagnetic Mo (S = 1) centre linking the CuII (S = 1/2) centres allows for effective ferromagnetic interaction of 3 paramagnetic centres with coupling constant J2 = 20.2(7) cm-1.

The first example of erbium triple-stranded helicates displaying SMM behaviour.

A series of isostructural C3-symmetrical triple stranded dinuclear lanthanide [Ln2L3](NO3)3 molecules have been synthesized using subcomponent self-assembly of Ln(NO3)3 with 2-(methylhydrazino)benzimidazole and 4-tert-butyl-2,6-diformylphenol, where Ln = Tb (1), Dy (2), Ho (3), Er (4), Tm (5), and Yb (6). The temperature dependent and field dependent magnetic properties of 1-6 were modeled using the van Vleck approximation including the crystal field term HCF, the super-exchange term HSE and the Zeeman term HZE. Ferromagnetic interactions were found in 1, 2, 4 and 6, while antiferromagnetic interactions were found in 3 and 5. The erbium analogue reveals field induced SMM behaviour.

Multifunctionality in bimetallic Ln(III)[W(V)(CN)8]3- (Ln = Gd, Nd) coordination helices: optical activity, luminescence, and magnetic coupling.

Two chiral luminescent derivatives of pyridine bis(oxazoline) (Pybox), (SS/RR)-iPr-Pybox (2,6-bis[4-isopropyl-2-oxazolin-2-yl]pyridine) and (SRSR/RSRS)-Ind-Pybox (2,6-bis[8H-indeno[1,2-d]oxazolin-2-yl]pyridine), have been combined with lanthanide ions (Gd(3+), Nd(3+)) and octacyanotungstate(V) metalloligand to afford a remarkable series of eight bimetallic CN(-)-bridged coordination chains: {[Ln(III)(SS/RR-iPr-Pybox)(dmf)4]3[W(V)(CN)8]3}n ⋅dmf⋅4 H2O (Ln = Gd, 1-SS and 1-RR; Ln = Nd, 2-SS and 2-RR) and {[Ln(III)(SRSR/RSRS-Ind-Pybox)(dmf)4][W(V)(CN)8]}n⋅5 MeCN⋅4 MeOH (Ln = Gd, 3-SRSR and 3-RSRS; Ln = Nd, 4-SRSR and 4-RSRS). These materials display enantiopure structural helicity, which results in strong optical activity in the range 200-450 nm, as confirmed by natural circular dichroism (NCD) spectra and the corresponding UV/Vis absorption spectra. Under irradiation with UV light, the Gd(III)-W(V) chains show dominant ligand-based red phosphorescence, with λmax ≈660 nm for 1-(SS/RR) and 680 nm for 3-(SRSR/RSRS). The Nd(III)-W(V) chains, 2-(SS/RR) and 4-(SRSR/RSRS), exhibit near-infrared luminescence with sharp lines at 986, 1066, and 1340 nm derived from intra-f (4)F3/2 → (4)I9/2,11/2,13/2 transitions of the Nd(III) centers. This emission is realized through efficient ligand-to-metal energy transfer from the Pybox derivative to the lanthanide ion. Due to the presence of paramagnetic lanthanide(III) and [W(V)(CN)8](3-) moieties connected by cyanide bridges, 1-(SS/RR) and 3-(SRSR/RSRS) are ferrimagnetic spin chains originating from antiferromagnetic coupling between Gd(III) (SGd = 7/2) and W(V) (SW = 1/2) centers with J1-(SS) = -0.96(1) cm(-1), J1-(RR) =-0.95(1) cm(-1), J3-(SRSR) = -0.91(1) cm(-1), and J3-(RSRS) =-0.94(1) cm(-1). 2-(SS/RR) and 4-(SRSR/RSRS) display ferromagnetic coupling within their Nd(III)-NC-W(V) linkages.

Magnetocaloric effect and critical behaviour in Mn2-pyridazine-[Nb(CN)8] molecular compound under press.

A comprehensive study of magnetocaloric effect (MCE) and critical behaviour in the ferrimagnetic Mn2­pyridazine­[Nb(CN)8] molecular magnet under hydrostatic pressure is reported. The pressure-induced structural changes provoke the strengthening of magnetic interaction between Mn and Nb centres. Consequently, an increase of critical temperature Tc is observed from 43 K for a sample at ambient pressure (A) to 52.5 K for a sample under a pressure of 1.19 GPa (AHP). The magnetocaloric effect was determined by the magnetization measurements. The application of a hydrostatic pressure of 1.19 GPa causes a decrease in the maximum value of magnetic entropy change ΔS, which for AHP is equal to 4.63 J mol−1 K−1 (7.73 J kg−1 K−1) at µ0ΔH = 5 T, while for A it is 5.36 J mol−1 K−1 (8.95 J kg−1 K−1) for the same magnetic field change. The temperature-dependent parameter n obtained for AHP, describing the field dependence of MCE, is consistent with other critical exponents determined from magnetization measurements. The critical exponents allow us to classify AHP to the 3D Heisenberg universality class, similar to the case of the non-pressurized sample.

Magnetocaloric effect in M-pyrazole-[Nb(CN)(8)] (M = Ni, Mn) molecular compounds.

We report a study of magnetocaloric effect (MCE) in cyanido-bridged {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)]·4H(2)O}(n) molecular compounds where M = Ni, Mn, pyrazole = C(3)H(4)N(2). The substances show a sharp phase transition to a long range magnetically ordered state, with ferromagnetic coupling between M and Nb sublattices in the case of the Ni-based sample 1 (T(c) = 13.4 K) and ferrimagnetic coupling for the Mn-based sample 2 (T(c) = 23.8 K). The magnetic entropy change ΔS due to applied field change ΔH as a function of temperature was determined by the magnetization and heat capacity measurements. The maximum value of ΔS at µ(0)ΔH = 5 T is 6.1 J mol(-1) K(-1) (5.9 J kg(-1) K(-1)) for 1 at T = 14 K and 6.7 J mol(-1) K(-1) (6.5 J kg(-1) K(-1)) for 2 at T = 25 K. MCE data at different applied fields have been presented as one universal curve, which confirms magnetic transitions in 1 and 2 to be of second order. The temperature dependences of the n exponent characterizing the dependence of ΔS on ΔH have been obtained. The n(T(c)) values, consistent with the shape of the magnetization curves, pointed to the 3D Heisenberg behaviour for 2 and some anisotropy, probably of the XY type, for 1. The (H/T(c))(2/3) dependence of the maximum entropy change has been tested in the ferrimagnetic Mn(2)-L-[Nb(CN)(8)] (L = C(3)H(4)N(2), C(4)H(4)N(2)) series.

W-knotted chain {[Cu(II)(dien)]4[W(V)(CN)8]}(5+)∞: synthesis, crystal structure, magnetism, and theory.

The self-assembly of [Cu(II)(dien)(H(2)O)(2)](2+) and [W(V)(CN)(8)](3-) in aqueous solution leads to the formation (H(3)O){[Cu(II)(dien)](4)[W(V)(CN)(8)]}[W(V)(CN)(8)](2)·6.5H(2)O (1). The crystal structure of 1 consists of an unprecedented {[Cu(II)(dien)](4)[W(V)(CN)(8)]}(5+)(∞) chain of (2,8) topology, nonbridging [W(CN)(8)](3-) anions, and crystallization water molecules. The analysis of magnetic behavior of 1 was performed by the density functional theory (DFT) method and magnetic susceptibility measurements. The DFT broken symmetry approach gave two J(CuW) coupling constants: J(ax) = +2.9 cm(-1) assigned to long and strongly bent W-CN-Cu linkage, and the J(eq) = +1.5 cm(-1) assigned to short and less bent W-CN-Cu linkage, located at the axial and the equatorial positions of square pyramidal Cu(II) centers, respectively, in the hexanuclear {W(2)Cu(4)} chain subunit. The dominance of weak-to-moderate ferromagnetic coupling within the chain was confirmed by magnetic calculations. Zero-field susceptibility of the full chain segment {WCu(4)}(n) was calculated by a semiclassical analytical approach assuming that only one W(V) out of five ½ spins of the chain unit WCu(4) is treated as a classical commuting variable. The calculation of the field dependence of the magnetization was performed separately by replacing the same spin with the Ising variable and applying the standard transfer matrix technique. The intermolecular coupling between the chain segments and off-chain [W(CN)(8)](3-) entities was resolved using the mean-field approximation set to be of antiferromagnetic character. The magnetic coupling parameters are compared with those of other low dimensional {Cu(II)-[M(V)(CN)(8)]} systems.