Magnetostructural Correlations in CuII−NC−WV Linkage: The Case of [CuII(diimine)]2+−[WV(CN)8]3− 0D Assemblies

Guest-Dependent Pressure-Induced Spin Crossover in FeII 4 [MIV (CN)8 ]2 (M=Mo, W) Cluster-Based Material Showing Persistent Solvent-Driven Structural Transformations

Discrete molecular species that can perform certain functions in response to multiple external stimuli constitute a special class of multifunctional molecular materials called smart molecules. Herein, cyanido-bridged coordination clusters {[Fe^II (2-pyrpy)₂ ]₄ [M^IV (CN)₈ ]₂ }⋅4 MeOH⋅6 H₂ O (M=Mo (1 solv), M=W (2 solv) and 2-pyrpy=2-(1-pyrazolyl)pyridine are presented, which show persistent solvent driven single-crystal-to-single-crystal transformations upon sorption/desorption of water and methanol molecules. Three full desolvation-resolvation cycles with the concomitant change of the host molecules do not damage the single crystals. More importantly, the Fe₄ M₂ molecules constitute a unique example where the presence of the guests directly affects the pressure-induced thermal spin crossover (SCO) phenomenon occurring at the Fe^II centres. The hydrated phases show a partial SCO with approximately two out-of-four Fe^II centres undergoing a gradual thermal SCO at 1 GPa, while in the anhydrous form the pressure-induced SCO effect is almost quenched with only 15 % of the Fe^II centres undergoing high-spin to low-spin transition at 1 GPa.

Octacyanidometallates for multifunctional molecule-based materials

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Molybdenum(III) Thiocyanate- and Selenocyanate-Based One-Dimensional Heteronuclear Polymers: Coordination Affinity-Controlled Assemblage of Mixed Spin and Mixed Valence Derivatives with Ni(II) and Co(II/III)

One-dimensional coordination polymers constructed with the 4d metallo-ligand [Mo(NCS)₆]^3- associated with Ni^II and Co^II complexes are reported. The first series consists of anionic NCS-bridged [Mo-M^II]^- coordination polymers associated with a discrete paramagnetic complex or a diamagnetic Co^III complex acting as cations. The latter takes advantage of the coordination preference of the S-ligand for the soft 3d ions and has led to hetero-trimetallic [Mo^IIINi^IICo^III] and mixed valence [Mo^IIICo^IICo^III] compounds. A second series concerns neutral chains in which trinuclear [Mo M₂^II] units are bridged by an additional NCS anion. The soft character of the S atom was also the key to a rare example of a compound involving both high-spin and low-spin Co^II centers associated with [Mo(NCS)₆]. A [Mo-Ni] derivative obtained with [Mo(NCSe)₆]^3- has been considered in order to evaluate the effect of Se versus S on the exchange interaction. The spin distribution for selenocyanate metallo-ligand has been assessed by density functional theory calculations. The crystal structures for all compounds have been characterized, and their magnetic behaviors have been investigated. These ferrimagnetic systems are characterized by antiferromagnetic Mo-M^II interactions in the range of -40 to -90 cm^-1 (based on H = -JS_aS_b formalism) operative both with Co^II and with Ni^II, thus demonstrating the potential of the Mo^III-NCS combination for molecular systems in which exchange interactions play an important role.

Auxiliary ligand-induced structural diversities of octacyanometalate-based heterobimetallic coordination polymers towards diverse magnetic properties

Three octacyanometalate-based bimetallic coordination polymers (CPs), {[μ₄-M^V(CN)₈][Co(DMF)₄]₂(ClO₄)}_n (M = W CP-1, Mo CP-2) and {[μ₄-W^IV(CN)₈]Co₂(azpy)₄}_n CP-3, were synthesized in the absence (for CPs 1 and 2) or presence (for CP-3) of auxiliary ligand 4,4'-azopyridine (azpy), respectively. CPs 1 and 2 exhibit the same three-dimensional (3D) polymeric cation frameworks with [ClO₄]^- as the counterions, while CP-3 displays a porous 3D framework in an unusual 2-nodal (4,6)-connected 4,6T155 topology with point symbol as {4²·6⁴}{4⁶·6⁴·7·8⁴}₂. Notably is that octacyanometalate [W^V(CN)₈]^3- has been reduced into [W^IV(CN)₈]^4- during the synthetic process of CP-3. The magnetic investigations indicate that CP-1 exhibits the typical ferromagnetic property due to the ferromagnetic coupling between W(v) and Co(ii) spin centers, while CP-3 displays the weak ferromagnetic interaction at low temperature, which is consistent with the valence change of W center in CP-3.

DFT Investigations of the Magnetic Properties of Actinide Complexes

Over the past 25 years, magnetic actinide complexes have been the object of considerable attention, not only at the experimental level, but also at the theoretical one. Such systems are of great interest, owing to the well-known larger spin–orbit coupling for actinide ions, and could exhibit slow relaxation of the magnetization, arising from a large anisotropy barrier, and magnetic hysteresis of purely molecular origin below a given blocking temperature. Furthermore, more diffuse 5f orbitals than lanthanide 4f ones (more covalency) could lead to stronger magnetic super-exchange. On the other hand, the extraordinary experimental challenges of actinide complexes chemistry, because of their rarity and toxicity, afford computational chemistry a particularly valuable role. However, for such a purpose, the use of a multiconfigurational post-Hartree-Fock approach is required, but such an approach is computationally demanding for polymetallic systems—notably for actinide ones—and usually simplified models are considered instead of the actual systems. Thus, Density Functional Theory (DFT) appears as an alternative tool to compute magnetic exchange coupling and to explore the electronic structure and magnetic properties of actinide-containing molecules, especially when the considered systems are very large. In this paper, relevant achievements regarding DFT investigations of the magnetic properties of actinide complexes are surveyed, with particular emphasis on some representative examples that illustrate the subject, including actinides in Single Molecular Magnets (SMMs) and systems featuring metal-metal super-exchange coupling interactions. Examples are drawn from studies that are either entirely computational or are combined experimental/computational investigations in which the latter play a significant role.

Heterotrimetallic complexes in molecular magnetism

The most representative examples of coordination compounds containing three different paramagnetic metal ions are reviewed, with a special emphasis on their magnetic properties.

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.

Electronic Structure and Magnetic Properties of Dioxo-Bridged Diuranium Complexes with Diamond-Core Structural Motifs: A Relativistic DFT Study

Electronic structures and magnetic properties of the binuclear bis(μ-oxo) U(IV)/U(IV) K2[{(((nP,Me)ArO)3tacn)U(IV)}2(μ-O)2] and U(V)/U(V) [{(((nP,Me)ArO)3tacn)U(V)}2(μ-O)2] (tacn = triazacyclononane, nP = neopentyl) complexes, exhibiting [U(μ-O)2U] diamond-core structural motifs, have been investigated computationally using scalar relativistic Density Functional Theory (DFT) combined with the Broken Symmetry (BS) approach for their magnetic properties. Using the B3LYP hybrid functional, the BS ground state of the pentavalent [U(V)(μ-O)2U(V)] 5f(1)-5f(1) complex has been found of lower energy than the high spin (HS) triplet state, thus confirming the antiferromagnetic character in agreement with experimental magnetic susceptibility measurements. The nonmagnetic character observed for the tetravalent K2[U(IV)(μ-O)2U(IV)] 5f(2)-5f(2) species is also predicted by our DFT calculations, which led practically to the same energy for the HS and BS states. As reported for related dioxo diuranium(V) systems, superexchange is likely to be responsible for the antiferromagnetic coupling through the π-network orbital pathway within the (μ-O)2 bridge, the dissymmetrical structure of the U2O2 core playing a determining role. In the case of the U(IV) species, our computations indicate that the K(+) counterions are likely to play a role for the observed magnetic property. Finally, the MO analysis, in conjunction with NPA and QTAIM analyses, clarify the electronic structures of the studied complexes. In particular, the fact that the experimentally attempted chemical oxidation of the U(V) species does not lead straightforwardly to binuclear complexes U(VI) is clarified by the MO analysis.

Ligand-directed assembly of trinuclear and one-dimensional heterotrimetallic CuIILnIIIFeIIIcomplexes: unusual antiferromagnetic CuIIFeIIIcoupling via cyano bridges

Rare phenoxo- and cyano-bridged CuLnFe trinuclear complexes were prepared. The 1D Cu₂DyFe complex displays both single-chain magnetic properties and the intrachain Cu(ii)–Fe(iii) antiferromagnetic coupling.

A new family of [CuIILnIIIMV] heterotrimetallic complexes (Ln = La, Gd, Tb; M = Mo, W): model systems to probe exchange interactions and single-molecule magnet properties

Four 3d–4f–4(5)d heterotrimetallic complexes, [L²CuLn(H₂O)₅(μ-NC)M(CN)₇], were obtained from the association of the binuclear 3d–4f complexes and {M^V(CN)₈}³⁻ metalloligands.

Magneto-structural variety of new 3d–4f–4(5)d heterotrimetallic complexes

Three families of heterotrimetallic chains with different topologies, have been obtained by reacting Schiff-base bicompartmental [Cu^II(L¹)] complexes and lanthanide(iii) salts, with (NHBu₃)₃[M(CN)₈] (M = Mo^V, W^V).

Magnetic clusters based on octacyanidometallates

A complete review of all known octacyanide-based clusters with an extensive discussion of their structures, magnetic properties and multifunctionality.

Chiral (LH)2L2Cu3 trinuclear paramagnetic nodes in octacyanidometalate-bridged helical chains

Trinuclear chiral (LH)2L2Cu3 (LH = 1,3-diamino-2-propanol, bdapH) assemblies linked by octacyanidometalate(IV) form isostructural one-dimensional (1D) chains consisting of right- and left-handed helixes arranged in an alternate manner: [(bdapH)2(bdap)2Cu(II)3][M(IV)(CN)8]·H2O (M = Mo 1, W 2). Each chain displays helicity with a long pitch around 17.2 Å. The direction of the helix rotation is strictly connected with the conformation of the (LH)2L2Cu3 unit. Right-handed helixes are based on Δ-S,S-(LH)2L2Cu3, whereas left-handed ones contain Λ-R,R-(LH)2L2Cu3 units. Magnetic studies reveal antiferromagnetic interactions through alkoxo-bridges inside trinuclear Cu(II) nodes leading to an ST = 1/2 ground state for both assemblies.

First magnets based on thiocyanato-bridges

2-D NCS-bridged {3d-4d} molecule-based magnets, formed by reaction of [Mo(III)(NCS)(6)](3-) with either Ni(II) or Co(II) complexes, have been found to exhibit magnetic ordering with T(C) = 39 K for {Co(3)Mo(2)} while {Ni(3)Mo(2)} has a non-magnetic ground state as a result of exact compensation by the individual sub-lattices.

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.

Microporous {[Ni(cyclam)]3[W(CN)8]2}n affording reversible structural and magnetic conversions

Methanol adsorption into the porous 2D bimetallic coordination network {[Ni(cyclam)](3)[W(CN)(8)](2)}(n) (cyclam = 1,4,8,11-tetraazacyclotetradecane) causes significant modification of its structure and magnetic properties. Fully reversible transformations between the three states of the network: anhydrous, hydrated and methanol-modified are observed with the transition to the magnetic ordered state at T(c) equal to 4.9 K, 8.3 K and 11.4 K respectively. All three phases have a metamagnetic character but the methanol-modified one most easily turns to ferromagnetic and shows a hysteresis loop with coercivity field of 250 Oe. The differences in magnetic behaviour of the anhydrous and guest-induced forms of the compound are discussed in terms of changes in the structure: CN-bridge geometry and distance between the layers.