Beata Nowicka

Magnetic clusters based on octacyanidometallates

Octacyanidometallates make an important branch of cyanide-based molecular magnets that not only follow the trends in modern magnetochemistry and materials science but also have stimulated these fields from the very beginning and still blaze a trail by introducing new concepts such as heterotrimetallic systems, new functionalities and cross-effects like photo-switching of magneto-optical properties and setting new records in magnetic ordering temperatures or magnetic exchange interactions. The following paper focuses on a special class of molecular magnets which, to the best of our knowledge, have not been reviewed so far: octacyanide-based magnetic clusters. A complete list of all known molecules incorporating octacyanides with an extensive discussion of their structures, topologies and magnetic properties, with special attention paid to multifunctional systems, is provided. Several milestone-clusters are discussed thoroughly to emphasize their particular importance in the development of crystal engineering and molecular magnetism.

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.

Larger pores and higher Tc: {[Ni(cyclam)]3[W(CN)8]2·solv}n – a new member of the largest family of pseudo-polymorphic isomers among octacyanometallate-based assemblies

A new crystalline pseudo-polymorphic form of the 2D microporous honeycomb-like magnetic network {[Ni(cyclam)]3[W(CN)8]2·solv}n was obtained from the reaction between methanol-soluble {Ni[Ni(MeOH)3]8[W(CN)8]6} clusters and the [Ni(cyclam)]2+ complex in an acetonitrile-rich solvent mixture. In comparison to the three earlier characterised pseudo-polymorphs, which all crystallised in the triclinic space group P, the new form shows higher symmetry of C2/m, greater porosity of ca. 40% solvent accessible volume, and the highest magnetic ordering temperature of 12 K. It undergoes an irreversible single-crystal-to-single-crystal transformation to the hexadecahydrate form that is stable in air at ambient temperature. A new synthetic pathway leading directly to methanol solvate of the same network, which was previously obtained by sorption of MeOH into the anhydrous form, is also reported.

Syntheses and X-ray crystal structure of (dq) 2 [M (CN) 8 ] ·5H 2 O (M = Mo, W; dq = diquat)

Abstract Reaction between diquat (dq=6,7-dihydrodipyrido- [1,2-a:2′,1′-c] pyrazinediium) and [M (CN) 8] 4− (M=Mo or W) led to the isolation of (dq) 2 [M (CN) 8] ·5H2O salts. Both salts were found to be isomorphic. The [M (CN) 8] 4− anion has an approximately dodecahedral D2d configuration. The structure consists of a three dimensional hydrogen bonded network. The intensely coloured solids (hydrated and anhydrous) show ion-pair charge-transfer (IPCT) interaction which is discussed in terms of their structure.

Structure and properties of the ion pair charge-transfer complex of octacyanotungstate(IV) with the 2,2′-bipyridinium dication

The X-ray crystal structure of (bpyH2)2[W(CN)8]·4H2 O (bpyH2=2,2′-bipyridinium) is described. The [W(CN)8]4− anion has an approximately square antiprismatic (D4d) conformation, seemingly imposed by strong anion–water–cation hydrogen-bonding interactions. Bond distances in the anion are: WC 2.150(5), 2.163(5); CN 1.128(6), 1.145(6)Å and the angles WCN are 177.6(5), 178.3(5)°. The dihydrate and the anhydrous salt are both intensely black solids, exhibiting ion-pair charge-transfer interaction between cation and anion. E.s.r. spectra indicate that 30% of the tungsten is present as WV in the solid state, but that in solution only the WIV complex is present. The electron withdrawing effect of the cation is discussed and compared with that in a series of salts with different bipyridinium cations.

Geometrical isomerism in pentadecanuclear high-spin Ni9W6 clusters with symmetrical bidentate ligands detected

We have detected the existence of different geometrical isomers of high-spin Ni9W6 clusters with symmetrical bidentate blocking ligands, and have proposed the classifications of the isomers. A series of pentadecanuclear body centred full-capped octahedral clusters with symmetrical diimine blocking ligands of the general formula {Ni[Ni(L)(solv)]8[W(CN)8]6}, where L = 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine or 4,4′-ditertbutyl-2,2′-bipyridine and solv = H2O or EtOH, have been synthesised and structurally characterised. The presence of different crystalline forms and different geometrical isomers have been observed. Since geometrical isomerism in this type of cluster is discussed for the first time, the method of classification of the isomers is proposed. The characterised clusters represent four out of six possible geometrical isomers. Ferromagnetic interactions are present within all clusters resulting in the ground spin state S = 12.

Solvent effects on piezochromism of transition metal complexes

We report solvent effects on the piezochromic behaviour of wavelengths of maximum absorption for metal-to-ligand charge-transfer (m.l.c.t.) bands of several molybdenum–diimine–carbonyl and titanium or vanadium–cyclopentadienyl or ethylmaltolate–thiocyanate complexes. These and previously published data are collated in an attempt to establish a pattern for solvent effects on piezochromism of m.l.c.t. bands. Strategies for the design of compounds and complexes for use in establishing a clear pattern for such effects are outlined.

Dehydration of Octacyanido-Bridged NiII-WIV Framework toward Negative Thermal Expansion and Magneto-Colorimetric Switching

An inorganic three-dimensional [NiII(H2O)2]2[WIV(CN)8]·4H2O (1) framework undergoes a single-crystal-to-single-crystal transformation upon thermal dehydration, producing a fully anhydrous phase NiII2[WIV(CN)8] (1d). The dehydration process induces changes in optical, magnetic, and thermal expansion properties. While 1 reveals typical positive thermal expansion of the crystal lattice, greenish-yellow color, and paramagnetic behavior, 1d is the first ever reported octacyanido-based solid revealing negative thermal expansion, also exhibiting a deep red color and diamagnetism. Such drastic shift in the physical properties is explained by the removal of water molecules, leaving the exclusively cyanido-bridged bimetallic network, which is accompanied by the transformation of the octahedral paramagnetic [NiII(H2O)2(NC)4]2- to the square-planar diamagnetic [NiII(NC)4]2- moieties.

Exploration of a new building block for the construction of cyano-bridged solvatomagnetic assemblies: [Ni(cyclam)]3+

Two 1D CN-bridged coordination polymers based on the cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) complex of nickel in an unusual oxidation state III have been characterised in terms of structure and solvatomagnetic properties. The {[Ni(cyclam)][Cr(CN)6]·6H2O}n (1) and {[Ni(cyclam)][Fe(CN)6]·6H2O}n (2) chains are isostructural and crystallise in space group C2/m. They undergo reversible partial dehydration at 40 °C to {[Ni(cyclam)][Cr(CN)6]·3H2O}n (1d) and {[Ni(cyclam)][Fe(CN)6]·3H2O}n (2d), accompanied by marked changes in structure and magnetic properties. Strong ferromagnetic exchange interaction J along the chains, together with a much weaker inter-chain interaction, leads to magnetic ordering in the range from 2.4 to 5.8 K. Dehydration of 1 leads to a decrease in J from 32 to 18 K, while dehydration of 2 leads to an increase in J from 6.8 to 14.3 K (interaction Hamiltonian −2JSNiSM). The magnetic susceptibility for the CrIII–NiIII 3/2–1/2 spin alternating Heisenberg chain was analysed with the help of quantum Monte Carlo simulations, simultaneously including both intra- and inter-chain interactions for a bunch of chains. The compounds are the first bimetallic CN-bridged assemblies containing a magnetically coupled octahedral cationic building block of an LS d7 configuration.

Hybrid Organic–Inorganic Cyanide-Bridged Networks

Hybrid organic–inorganic CN-bridged networks are an important and versatile group of molecular magnets. Cyanide ligands mediate relatively strong magnetic interactions and at the same time allow easy design of polynuclear assemblies via building block approach. Introduction of organic ligands allows effective manipulation of topology and dimensionality, enabling formation of discrete polynuclear structures, chains and layers as well as intricate 3D architectures. Organic molecules in hybrid systems can act as blocking or bridging ligands as well as guest molecules. Most importantly, apart from directing the structure formation, organic ligands can be used to induce additional desired properties. In this chapter, we present numerous examples of hybrid CN-bridged assemblies to illustrate their diverse functionalities. They include single molecule (SMMs) and single chain magnets (SCMs), magnetic sponges, multi-switchable spin-crossover (SCO) and charge-transfer systems as well as materials combining magnetic ordering with optical activity or luminescence. Current efforts in the research of CN-bridged systems concentrate on several topics connected with their potential applications, like search for materials with high critical temperature of magnetic ordering, development of bistable systems responsive to multiple stimuli, or surface deposition and formation of heterostructures.