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.

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.

Ligand dependent topology and spontaneous resolution in high-spin cyano-bridged Ni3W2 clusters.

Two high-spin pentanuclear NiW clusters with diimine blocking ligands have been obtained: {[Ni(4,4'-MeObpy)2]3[W(CN)8]2}·12H2O (1) and {[Ni(phen)2(H2O)][Ni(phen)2]2[W(CN)8]2}·7H2O (2) (4,4'-MeObpy = 4,4'-dimethoxy-2,2'-bipyridine, phen = 1,10-phenanthroline). Despite the similarity of the building blocks and synthetic conditions the compounds show different topologies of the cluster core: 1 is a trigonal bipyramid while 2 is a decorated square. Both cluster structures are chiral with either ΔΔΔ or ΛΛΛ configuration around all three Ni centres. In 1 spontaneous resolution occurs and it crystallises in the P212121 space group forming a conglomerate containing both types of enantiomorphic crystals. 1Δ and 1Λ are the first pair of enantiomorphic structures of cyano-bridged clusters of trigonal bipyramidal topology obtained with achiral bidentate blocking ligands. 2 crystallises as a racemic compound in a centrosymmetric space group P1[combining macron] with both enantiomers present in the structure. 2 is an exceptional square-motif containing structure with an identical stereoconfiguration of all complex cations within one cluster. Ferromagnetic interactions are present in both clusters resulting in the ground spin state S = 4.

Crystal structures and magnetic properties of two low-dimensional materials constructed from [Mn(III)(salen)H2O]+ and [M(CN)8](3-/4-) (M = Mo or W) precursors.

The syntheses, X-ray structures, and magnetic behaviors of two new cyano-bridged assemblies, the molecular [Mn(III)(salen)H2O]3[W(V)(CN)8].H2O (1) and one-dimensional [Mn(salen)(H2O)2]2[[Mn(salen)(H2O)][Mn(salen)]2[Mo(CN)(8)]].0.5ClO4.0.5OH.4.5H2O (2), are presented. Compound 1 crystallizes in the monoclinic system, has space group P2(1)/c, and has unit cell constants a = 13.7210(2) A, b = 20.6840(4) A, c = 20.6370(2) A, and Z = 4. Compound 2 crystallizes in the triclinic system, has space group P, and has unit cell dimensions a = 18.428(4) A, b = 18.521(3) A, c = 18.567(4) A, and Z = 2. The structure of 1 consists of the asymmetric V-shaped Mn-NC-W-NC-Mn-O(phenolate)-Mn molecules, where W(V) coordinates with [Mn(salen)H2O] and singly phenolate-bridged [Mn(salen)H2O]2 moieties through the neighboring cyano bridges. The [W(V)(CN)8]3- ion displays distorted square-antiprism geometry. The structure of 2 consists of the cyano-bridged [Mn3(III)Mo(IV)]n- repeating units linked by double phenolate bridges into one-dimensional zigzag chains. The Mn(III) centers are bound to Mo(IV) of square-antiprism geometry through the neighboring cyano bridges. The magnetic studies of 1 reveal the antiferromagnetic intramolecular interactions through the CN and phenolate bridges and the relatively weak intermolecular interactions. Compound 1 becomes antiferromagnetically ordered below TN = 4.6 K. The presence of the magnetic anisotropy is documented with the MH measurements carried out for both polycrystalline and single-crystal samples. At T = 1.9 K, the spin-flop transition is observed in the field of 18 kOe applied parallel to the bc plane, which is the easy plane of magnetization. Field dependence of magnetization of 1 shows field-induced metamagnetic behavior from the antiferromagnetic ground state of ST = 3/2 to the state of ST = 5/2. The magnetic properties of 2 indicate a weak antiferromagnetic interaction between Mn(III) centers in double-phenolate-bridged [Mn(III)(salen)]2 dinuclear subunits and a very weak ferromagnetic interaction between them through the diamagnetic [Mo(IV)(CN)8]4- spacer.

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.