Co-NC-W and Fe-NC-W Electron-Transfer Channels for Thermal Bistability in Trimetallic {Fe6Co3[W(CN)8]6} Cyanido-Bridged Cluster

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Electron transfer coupled spin transition in cyano-bridged mixed-valence {FeIII2FeII2} molecular squares

The research on electron transfer coupled spin transition regulating the valence state and spin state transition of metal ions is promising and challenging. Herein, we report a cyano-bridged {FeIII2FeII2} molecular square complex, {[Fe(Tp)(CN)3]2 [Fe(bnbpen)]2}(ClO4)2·8CH3OH (1·8CH3OH, bnbpen = N,N'-bis-(2-naphthylmethyl)-N,N'-bis(2-picolayl)-ethylenediamine), and its free of solvents form (1). Combined single-crystal X-ray diffraction, temperature-dependent infrared (IR) spectra, magnetic measurements, and Mössbauer spectra reveal that 1·8CH3OH and 1 exhibit reversible one-step and two-step electron transfer coupled spin transition (ETCST) with temperature change, between the low-temperature state {FeII,LS(μ-CN)FeIII,LS}2 (LS = low spin, HS = high spin) and the high-temperature state {FeIII,LS(μ-CN)FeII,HS}2, respectively.

Different delocalized ranges in mixed valence cyanido-metal-bridged Fe-Ru-Fe complexes controlled by terminal ligand substitution modification

In order to investigate the properties of metal to metal charge transfer (MMCT) influenced by the relative energy level between the bridging unit and the terminal unit, two groups of heterotrimetallic cyanido-metal-bridged complexes, trans-[Cp(dppe)Fe-CN-Ru(MeOpy)4-NC-Fe(dppe)Cp][X]n (1[X]n; n = 2, 3, or 4; X = PF6 or BF4) (Cp = cyclopentadiene, dppe = 1,2-bis(diphenylphosphino)ethane, MeOpy = 4-methoxypyridine) and [Cp*(dppe)Fe-CN-Ru(MeOpy)4-NC-Fe(dppe)Cp*] [X]n (2[X]n; Cp* = 1,2,3,4,5-pentamethylcyclopentadiene; n = 2, 3, or 4; X = PF6 or BF4) were synthesized and fully characterized. The crystallography data suggest different oxidation sites in the ground state for one-electron oxidation products 13+ and 23+, and the electrochemical and Mössbauer spectra suggest that in the one-electron oxidation compounds 13+, the charge is delocalized all along the trimetal backbone Fe-Ru-Fe, while in 23+, the charge is rather delocalized between the two metal parts Fe-Ru. Further oxidation of N3+ gives N4+ (N = 1 or 2), during which a spin transfer towards the terminal units is observed in both series.

Stimuli-responsive magnetic materials: impact of spin and electronic modulation

Addressing molecular bistability as a function of external stimuli, especially in spin-crossover (SCO) and metal-to-metal electron transfer (MMET) systems, has seen a surge of interest in the field of molecule-based magnetic materials due to their enormous potential in various technological applications such as molecular spintronics, memory and electronic devices, switches, sensors, and many more. The fine-tuning of molecular components allow the design and synthesis of materials with tailored properties for these vast applications. In this Feature Article, we discuss a part of our research work into this broad topic, pertaining to the recent discoveries in the field of switchable molecular magnetic materials based on SCO and MMET systems, along with some historical background of the area and related accomplishments made in recent years.

Site Selectivity for the Spin States and Spin Crossover in Undecanuclear Heterometallic Cyanido-Bridged Clusters

Polynuclear molecular clusters offer an opportunity to design new hierarchical switchable materials with collective properties, based on variation of the chemical composition, size, shapes, and overall building blocks organization. In this study, we rationally designed and constructed an unprecedented series of cyanido-bridged nanoclusters realizing new undecanuclear topology: Fe^II[Fe^II(bzbpen)]₆[W^V(CN)₈]₂[W^IV(CN)₈]₂·18MeOH (1), Na^I[Co^II(bzbpen)]₆[W^V(CN)₈]₃[W^IV(CN)₈]·28MeOH (2), Na^I[Ni^II(bzbpen)]₆[W^V(CN)₈]₃[W^IV(CN)₈]·27MeOH (3), and Co^II[Co^II(R/S-pabh)₂]₆[W^V(CN)₈]₂[W^IV(CN)₈]₂·26MeOH [4R and 4S; bzbpen = N¹,N²-dibenzyl-N¹,N²-bis(pyridin-2-ylmethyl)ethane-1,2-diamine; R/S-pabh = (R/S)-N-(1-naphthyl)-1-(pyridin-2-yl)methanimine], of size up to 11 nm³, ca. 2.0 × 2.2 × 2.5 nm (1-3) and ca. 1.4 × 2.5 × 2.5 nm (4). 1, 2, and 4 exhibit site selectivity for the spin states and spin transition related to the structural speciation based on subtle exogenous and endogenous effects imposed on similar but distinguishable 3d metal-ion-coordination moieties. 1 exhibits a mid-temperature-range spin-crossover (SCO) behavior that is more advanced than the previously reported SCO clusters based on octacyanidometallates and an onset of SCO behavior close to room temperature. The latter feature is also present in 2 and 4, which suggests the emergence of Co^II-centered SCO not observed in previous bimetallic cyanido-bridged Co^II-W^V/IV systems. In addition, reversible switching of the SCO behavior in 1 via a single-crystal-to-single-crystal transformation during desolvation was also documented.

Influence of the electronic effect of an ancillary ligand on MMCT and LMCT in localized cyanide-bridged complexes containing non-innocent ligands

For bimetallic MV complexes with non-innocent ligands, the MMCT energy in the localized system decreases significantly while the LMCT energy in the delocalized moiety increases slightly as the electronic effect of ancillary ligands is enhanced.

Evidence of protonation induced intra-molecular metal-to-metal charge transfer in a highly symmetric cyanido bridged {Fe2Ni2} molecular square

The possibility to induce intra-molecular metal-to-metal charge transfer in a cyanido bridged tetranuclear square shaped {Fe₂Ni₂} complex by employing protonation as an external stimulant is explored. Two cyanido bridged square shaped tetranuclear complexes, [{Ni(TPA)(μ₂-NC)₂Ni(CN)₂}₂]·4H₂O (2) and [{Ni(TPA)(μ₂-NC)₂Fe(bbp)(CN)}₂]·10H₂O (3) [TPA = tris(3,5-dimethylpyrazol-1-ylmethyl)amine and H₂bbp = bis(2-benzimidazolyl)pyridine], are synthesized and characterized. Low temperature magnetic measurements reveal that complex 3 has dominant ferromagnetic interactions between low-spin Fe^III (S = 1/2) and high-spin Ni^II (S = 1) ions. UV-visible spectrophotometric measurements and electrochemical studies establish that reversible intra-molecular metal-to-metal electron transfer can be triggered in 3 upon the addition of either an acid or a base.

Tuning of the phase transition between site selective SCO and intermetallic ET in trimetallic magnetic cyanido-bridged clusters

A series of crystalline phases composed of trimetallic 3d-5d-5d' {Fe₉[Re(CN)₈]_6-x[W(CN)₈]_x(MeOH)₂₄}·yMeOH (x = 1 (1), 2 (2), 3 (3), 4 (4) and 5 (5); y = 10-15) clusters were obtained by altering the octacyanidometalate composition. The temperature dependent studies involving SC XRD, SQUID magnetic measurements, IR spectroscopy and ⁵⁷Fe Mössbauer spectroscopy revealed reversible phase transition with the retention of single crystal character in each congener. The transition was assisted by reversible spin-crossover (SCO) ^HSFe^II↔^LSFe^II transition at the central Fe1(ii) site for Fe₉Re₅W₁ (1), Fe₉Re₄W₂ (2), Fe₉Re₃W₃ (3) and Fe₉Re₂W₄ (4). In contrast, the tungsten-rich congener Fe₉Re₁W₅ (5) exhibited nontrivial behavior with the SCO transition being stopped halfway through the cooling process, to be completed with single electron transfer (ET) from the external Fe2(ii) center towards one of the neighboring W(v) sites. The critical temperature T_c of SCO has been systematically increased from 193 K (1) to 247 K (4). All experimental data indicate the domination of the Fe(ii)-W(v) valence states in all crystals 1-5, however, with increasing quantity of [W(CN)₈]^3- (and decreasing quantity of [Re(CN)₈]^3-), the valence equilibrium Fe(ii)-W(v) ↔ Fe(iii)-W(iv) was systematically shifted to the right, starting from congener 3. The overall electronic configuration at low temperatures and variable amounts and location of spin carriers along the whole series suggest the remarkable competition between magnetic super-exchange Fe(ii)-CN-W(v) interactions and intermolecular interactions. The observed behavior is in line with the information collected previously for the bimetallic congeners Fe₉Re₆ and Fe₉W₆, to shed light on the role of the mixed tri-metallic composition in changing the properties observed for the relevant bimetallic cyanido-bridged skeletons.

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.

Investigating the effect of lanthanide radius and diamagnetic linkers on the framework of metallacrown complexes

By changing the stoichiometric ratios, the one-pot reaction of the glycinehydroxamic acid (H2glyha) ligand with copper(ii) and lanthanide(iii) salts in the presence of diamagnetic [Na2{Fe(CN)5(NO)}] led to two series of isostructural complexes, which can be designated as heterotrimetallic dimeric clusters [{LnCu5(glyha)5}{Fe(CN)5(NO)}(H2O)4]2·xNO3·yH2O (x = 2, y = 11 for La (1), x = 2, y = 11 for Pr (2), and x = 2, y = 11 for Nd (3)) and heterotetrametallic coordination polymers [Na{LnCu5(glyha)5}{Fe(CN)5(NO)}2(H2O)x·yH2O]n (x = 6, y = 4 for Sm (4), x = 6, y = 0 for Gd (5), x = 6, y = 4 for Tb (6), x = 5, y = 5 Dy (7), and x = 6, y = 4 for Ho (8)). Each molecular structure contains LnIII[15-metallacrown-5] nodes and diamagnetic [Fe(CN)5(NO)]2- linkers. The resulting products demonstrate diversified structural frameworks due to the radius effect of LnIII ions and different bridging fashions of diamagnetic [Fe(CN)5(NO)]2- linkers. An analysis of magnetic susceptibilities reveals that 7 exhibits ferromagnetic coupling between CuII and DyIII ions and field-induced SMM behavior.

Bulky ligands shape the separation between the large spin carriers to condition field-induced slow magnetic relaxation

Crystal engineering of magnetic relaxation in supramolecular networks based on almost isotropic cyanido-bridged {Mn9[W(CN)8]6L8(solv)8} clusters decorated by bulky 4,4'-di-tert-butyl-2,2'-bipyridine (But2bpy) and 4,7-diphenyl-1,10-phenanthroline (Ph2phen) ligands is presented. The three new compounds {MnII9[WV(CN)8]6(But2bpy)8(MeOH)8}·Pri2O·13MeOH (1), {MnII9[WV(CN)8]6(But2bpy)8(MeOH)6(H2O)2}·4Pri2O·2H2O (1a), and {MnII9[WV(CN)8]6(Ph2phen)8(MeOH)8}·29MeOH·6H2O (2) were characterized structurally and magnetically. Compound 1 exhibits unequivocal domination of repulsive intercluster contacts operating between the side But groups leading to intercluster distances exceeding 10 Å in all three dimensions. Compound 1a reveals a 1-dimensional (1D) supramolecular chain structure with very close intercluster distances of 6.7 Å realized through the direct W-CNHO-Mn hydrogen bonds along the chain, further isolated by the above repulsive ButBut synthons. Compound 2 shows significant separation in all three directions with the intercluster distances close to 10, 12 and 13.5 Å. However, in contrast to 1, these separations are accompanied by indirect hydrogen bond arrays and local π-π interactions of potential to assist in the transfer of weak magnetic interactions. The dc magnetic data show the signature of S = 39/2 in the ground state, which is typical in this group of compounds. The high-spin clusters are accompanied by different intercluster interactions, illustrated by the effective zJ' values of +0.010 cm-1 (1), +0.008 cm-1 (1a) and +0.001 cm-1 (2). The low temperature ac susceptibility measurements revealed a temperature- and field-dependent magnetic relaxation time for all 1, 1a and 2 compounds (τ1, τ1a-fast, and τ2-fast in the range 10-3-10-4 s). In contrast and only in the case of 1a and 2, an additional temperature independent slow process was detected (τ1a-slow and τ2-slow located between 0.1 s and 1 s). The magnetic relaxations were correlated with the obtained supramolecular networks, indicating the significant role of dipolar fields, weak non-covalent interactions, hydrogen bonds and π-π interactions.

A heterotrimetallic synthetic approach in versatile functionalization of nanosized {MxCu13–xW7}3+ and {M1Cu8W6} (M = Co, Ni, Mn, Fe) metal–cyanide magnetic clusters

The addition of 3d metal heteroatom into Cu^II(Me₃tacn)[W(CN)₈]ⁿ⁻ cluster system governs its structural and magnetic features.

Dynamic molecular crystals with switchable physical properties

The development of molecular materials whose physical properties can be controlled by external stimuli - such as light, electric field, temperature, and pressure - has recently attracted much attention owing to their potential applications in molecular devices. There are a number of ways to alter the physical properties of crystalline materials. These include the modulation of the spin and redox states of the crystal's components, or the incorporation within the crystalline lattice of tunable molecules that exhibit stimuli-induced changes in their molecular structure. A switching behaviour can also be induced by changing the molecular orientation of the crystal's components, even in cases where the overall molecular structure is not affected. Controlling intermolecular interactions within a molecular material is also an effective tool to modulate its physical properties. This Review discusses recent advances in the development of such stimuli-responsive, switchable crystalline compounds - referred to here as dynamic molecular crystals - and suggests how different approaches can serve to prepare functional materials.

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.

Modulation of the FeII spin crossover effect in the pentadecanuclear {Fe9[M(CN)8]6} (M = Re, W) clusters by facial coordination of tridentate polyamine ligands

N,N,N-tridentate ligands coordinated to {Fe₉[M(CN)₈]₆} (M = Re, W) induce the spin crossover on the external Fe sites of the cluster.

Tuning of Charge Transfer Assisted Phase Transition and Slow Magnetic Relaxation Functionalities in {Fe(9-x)Co(x)[W(CN)8]6} (x = 0-9) Molecular Solid Solution

Precisely controlled stoichiometric mixtures of Co(2+) and Fe(2+) metal ions were combined with the [W(V)(CN)8](3-) metalloligand in a methanolic solution to produce a series of trimetallic cyanido-bridged {Fe(9-x)Co(x)[W(CN)8]6(MeOH)24}·12MeOH (x = 0, 1, ..., 8, 9; compounds 0, 1, ..., 8, 9) clusters. All the compounds, 0-9, are isostructural, and consist of pentadecanuclear clusters of a six-capped body-centered cube topology, capped by methanol molecules which are coordinated to 3d metal centers. Thus, they can be considered as a unique type of a cluster-based molecular solid solution in which different Co/Fe metal ratios can be introduced while preserving the coordination skeleton and the overall molecular architecture. Depending on the Co/Fe ratio, 0-9 exhibit an unprecedented tuning of magnetic functionalities which relate to charge transfer assisted phase transition effects and slow magnetic relaxation effects. The iron rich 0-5 phases exhibit thermally induced reversible structural phase transitions in the 180-220 K range with the critical temperatures being linearly dependent on the value of x. The phase transition in 0 is accompanied by (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) charge transfer (CT) and the additional minor contribution of a Fe-based spin crossover (SCO) effect. The Co-containing 1-5 phases reveal two simultaneous electron transfer processes which explore (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) CT and the more complex (HS)Co(II) W(V) ↔ (LS)Co(III) W(IV) charge transfer induced spin transition (CTIST). Detailed structural, spectroscopic, and magnetic studies help explain the specific role of both types of CN(-)-bridged moieties: the Fe-NC-W linkages activate the molecular network toward a phase transition, while the subsequent Co-W CTIST enhances structural changes and enlarges thermal hysteresis of the magnetic susceptibility. On the second side of the 0-9 series, the vanishing phase transition in the cobalt rich 6-9 phases results in the high-spin ground state, and in the occurrence of a slow magnetic relaxation process at low temperatures. The energy barrier of the magnetic relaxation gradually increases with the increasing value of x, reaching up to ΔE/kB = 22.3(3) K for compound 9.

Structural anisotropy of cyanido-bridged {CoII9WV6} single-molecule magnets induced by bidentate ligands: towards the rational enhancement of an energy barrier

{CoII9[W^V(CN)₈]₆} clusters capped by odd and even number of bidentate ligands reveal the improved slow magnetic relaxation due to the significant structural anisotropy.

The solvent effect on the structural and magnetic features of bidentate ligand-capped {CoII9[WV(CN)8]6} single-molecule magnets

Cyanido-bridged {Co^II₉W^V₆} clusters combined with 2,2′-bipyridine N-oxide produce three crystalline phases with different solvents affecting their structural and magnetic properties.

A high-nuclearity metal-cyanide cluster [Mo6Cu14] with photomagnetic properties

A high-nuclearity metal-cyanide cluster [Mo₆Cu₁₄] has been prepared and its photomagnetic properties investigated.

Spin crossover and reversible single-crystal to single-crystal transformation behaviour in two cyanide-bridged mixed-valence {FeIII2FeII2} clusters

Two mixed-valence {FeIII2FeII2} clusters exhibit spin crossover behaviour, in which a distorted {Fe₄(μ-CN)₄} core and deviations of bent ∠Fe−NC angles play a key role in tuning the transition temperature.

12-Metal 36-membered ring based WV–CoIIlayers showing spin-glass behavior

Two cyano-bridged bimetallic W^V–Co^IIlayers were synthesized, which contain 12 metal 36 membered rings and present spin-glass behavior.

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.