Tomasz Korzeniak

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.

Coordination polymers based on octacyanometalates(IV,V)(M = Mo, W) and aliphatic polyamine copper(II) tectons with [N3] donor atom sets

The cyano-bridged [CuII(tetrenH2)]2[WIV(CN)8]2·5H2O (tetren = tetraethylenepentaamine) (1), [CuII(tetrenH2)][CuII(tetrenH)][WV(CN)8][WIV(CN)8]·2.5H2O (2), [CuII(dien)]2[WIV(CN)8]·4H2O (dien = diethylenetriamine) (3) and its isomorphous molybdenum(IV) analogue (4) have been prepared and structurally characterised. 1 and 2 are built from the W2Cu2(μ-CN)4 squares extended into 1-D structure by cyano-bridges. 2-D 3 and 4 form a square grid pattern with tungsten atoms in the corners and –CN–Cu(dien)–NC– linkages on the edges of the squares. The magnetic behaviour of 1 and 3 indicates the presence of two isolated CuII spins S = 1/2 with a very weak antiferromagnetic coupling through the diamagnetic NC–WIV–CN bridges in the low temperatures. Assembly 2 exhibits a weak ferromagnetic interaction between CuII and WV isolated by diamagnetic [WIV(CN)8]4− spacer from another CuII centre within WV–CN–CuII–NC–WIV–CN–CuII unit and the antiferromagnetic interaction between the CuII2WVWIV units.

W-Knotted Chain {[CuII(dien)]4[WV(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.

Microwave-assisted construction of ferromagnetic coordination polymers of [W(V)(CN)8]3- with Cu(II)-pyrazole synthons.

The microwave-mediated self-assembly of [W(V)(CN)(8)](3-) with Cu(II) in the presence of pyrazole ligand resulted in the formation of three novel assemblies: Cu(II)(2)(Hpyr)(5)(H(2)O)[W(V)(CN)(8)](NO(3))·H(2)O (1), {Cu(II)(5)(Hpyr)(18)[W(V)(CN)(8)](4)}·[Cu(II)(Hpyr)(4)(H(2)O)(2)]·9H(2)O (2), and Cu(II)(4)(Hpyr)(10)(H(2)O)[W(V)(CN)(8)](2)(HCOO)(2)·4.5H(2)O (3) (Hpyr =1H-pyrazole). Single-crystal X-ray structure of 1 consists of cyanido-bridged 1-D chains of vertex-sharing squares topology. The structure of 2 reveals 2-D hybrid inorganic layer topology with large coordination spaces occupied by {Cu(Hpyr)(2)(H(2)O)(4)}(2+) ions. Compound 3 contains two types of cyanido-bridged 1-D chains of vertex-sharing squares linked together by formate ions in two directions forming hybrid inorganic-organic 3-D framework (I(1)O(2)). The magnetic measurements for 1-3 reveal a weak ferromagnetic coupling through Cu(II)-NC-W(V) bridges.

Photoswitchable CuII4MoIV and CuII2MoIV 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.

Reversible Single-Crystal-to-Single-Crystal Transformation in Photomagnetic Cyanido-Bridged Cd4M2 Octahedral Molecules

Two new hexanuclear octahedral cyanido-bridged clusters, {[CdII(bpy)2]4[WIV(CN)8]2}·10H2O (Cd4W2) and {[CdII(bpy)2]4[MoIV(CN)8]2}·10H2O (Cd4Mo2), have been obtained and characterized structurally and photomagnetically. Both compounds show a very rare and reversible single-crystal-to-single-crystal transformation upon dehydration accompanied by marked color changes in the case of Cd4W2. Moreover, irradiation of Cd4Mo2 using 436 nm light induces a reversible photomagnetic effect due to the LIESST-like singlet-triplet transition at the MoIV center. Analogous photomagnetic experiments for Cd4W2 did not lead to any significant change of its magnetic moment.

Studies of critical phenomena in molecular magnets by μSR spectroscopy

The rapidly developing field of molecular magnetism supplies a multitude of novel compounds of unprecedented properties and structure. Molecular magnets predominantly belong to the class of compounds involving well localized magnetic moments. This feature together with the fact that the nature and symmetry of magnetic interactions is encrypted in the critical behaviour makes them a perfect testing ground of the existing theoretical spin models. It is demonstrated that the experimental technique of the μSR spectroscopy is perfectly suited to study magnetic fluctuations and spin dynamics in the neighbourhood of a phase transition. This unique method can even dispense with the complementary measurements of the AC susceptibility or heat capacity to supply a complete set of the static and dynamic critical exponents. It can thus be used to pinpoint the universality class of the material of interest.

Magnetic and magneto-optical properties of nickel hexacyanoferrate/chromate thin films

One of the most important challenges of modern science and technology is the quest for novel and tuneable materials, the properties of which can be widely controlled by chemical modifications or external stimuli. Simultaneously, an interest in the development of magnetic thin films also gains significant attention. In the current paper we bring together both these challenges and present a study of a new type of low-dimensional nickel hexacyanoferrate/chromate system. Thin films were obtained by the “layer by layer” deposition technique, where the ratio of Fe/Cr was controlled by the dipping sequence. The scope of this work is a comprehensive analysis of structural, spectroscopic and magnetic properties of the compound and the investigation of the evolution of material properties induced by the change of the chemical composition.

X-ray absorption spectroscopy study of novel inorganic-organic hybrid ferromagnetic Cu-pyz-[M(CN)8]3- assemblies.

We present a unique interpretation of X-ray absorption spectroscopy (XAS) spectra at Cu:K, W:L(3), and Mo:K edges of structurally related magnetic Cu(II)-[M(V)(CN)(8)](3-) compounds. The approach results in description of the structure of novel three-dimensional (3-D) Cu(II)(3)(pyz)[M(V)(CN)(8)](2)·xH(2)O, M = W (1), Mo, (2) polymers. Assemblies 1 and 2 represent hybrid inorganic-organic compounds built of {Cu(II)[W(V)(CN)(8)](-)}(n) double-layers linked by cyanido-bridged {Cu(II)-(μ-pyz)(2+)}(n) chains. These Cu(II)-M(V) systems reveal long-range magnetic ordering with T(c) of 43 and 37 K for 1 and 2, respectively. The presence of the 3-D coordination networks and 8 cyanido-bridges at M(V) centers leads to the highest Curie temperatures and widest hysteresis loops among Cu(II)-[M(V)(CN)(8)](3-) systems.

An unprecedented copper(I,II)–octacyanotungstate(V) 2-D network: crystal structure and magnetism of [CuII(tren)]{CuI[WV(CN)8]}·1.5H2O

A novel two-dimensional cyanide-bridged polymer [CuII(tren)]{CuI[W(V)(CN)8]} . 1.5H2O (tren = tris(2-aminoethyl)amine) formed via the simultaneous in situ metal-ligand redox reaction of [Cu(tren)(OH2)]2+ and self-assembly with [W(V)(CN)8]3- consists of a {CuI[W(V)(CN)8]} square grid built of CuI centres of tetrahedral geometry coordinatively saturated by CN bridges and [W(V)(CN)8]3- capped by [CuII(tren)]2+ moieties; it exhibits ferromagnetic coupling J1 = +5.8(1) cm(-1) within the CuII-W(V) dinuclear subunits and weak antiferromagnetic coupling J2 = -0.03(1) cm(-1) between them through diamagnetic CuI spacers.