Tristability in a Light-Actuated Single-Molecule Magnet

Light- and temperature-assisted spin state annealing: accessing the hidden multistability

Among responsive multistable materials, spin crossover (SCO) systems are of particular interest for stabilizing multiple spin states with various stimulus inputs and physical outputs. Here, in a 2D Hofmann-type coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂] (isoq = isoquinoline), a medium-temperature annealing process is introduced after light/temperature stimulation, which accesses the hidden multistability of the spin state. With the combined effort of magnetic, crystallographic and Mössbauer spectral investigation, these distinct spin states are identified and the light- and temperature-assisted transition pathways are clarified. Such excitation-relaxation and trapping-relaxation joint mechanisms, as ingenious interplays between the kinetic and thermodynamic effects, uncover hidden possibilities for the discovery of multistable materials and the development of multistate intelligent devices.

Two new two-stage manipulation protocols, namely light- and temperature-assisted spin state annealing (LASSA/TASSA), are applied to a spin crossover coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂], revealing the hidden multistability of spin states.

Hysteresis Photomodulation via Single-Crystal-to-Single-Crystal Isomerization of a Photochromic Chain of Dysprosium Single-Molecule Magnets

A one-dimensional coordination solid 1_c is synthesized by reaction of a bispyridyl dithienylethene (DTE) photochromic unit with the highly anisotropic dysprosium-based single-molecule magnet [Dy(Tp^py)F(pyridine)₂]PF₆. Slow magnetic relaxation characteristics are retained in the chain compound 1_c, and photoisomerization of the bridging DTE ligand induces a single-crystal-to-single-crystal transformation that can be monitored using photocrystallography. Notably, the resulting chain compound 1_o exhibits faster low-temperature relaxation than that of 1_c, which is apparent in magnetic hysteresis data collected for both compounds as high as 4 K. Ab initio calculations suggest that this photomodulation of the magnetic relaxation behavior is due to crystal packing changes rather than changes to the crystal field splitting upon ligand isomerization.

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.

Bistable molecular materials with dynamic structures

In this Feature Article, we introduce how to manipulate the motion of electrons or molecules by external stimuli, to achieve switchable properties in molecule-based single crystals.

External stimuli modulate the magnetic relaxation of lanthanide single-molecule magnets

The magnetic relaxation of lanthanide single-molecule magnets (Ln-SMMs) can be modulated reversibly by external stimuli including light irradiation, thermal treatment, protonation/deprotonation and oxidation/reduction etc.

Two ways of spin crossover in an iron(ii) coordination polymer associated with conformational changes of a bridging ligand

Structural phase transition in [Fe(bbtre)₃](ClO₄)₂·2CH₃CN (bbtre = 1,4-di(1-ethyl-1,2,3-triazol-5-yl)butane) plays the role of a switch, allowing spin crossover to be carried out in two ways.

Two Four-Coordinate and Seven-Coordinate CoII Complexes Based on the Bidentate Ligand 1, 8-Naphthyridine Showing Slow Magnetic Relaxation Behavior

For a long time, the cobalt(II) complex ([Co(napy)₄ ](ClO₄ )₂ ) (napy=1, 8-naphthyridine) has been considered as an eight-coordinate complex without any structural proof. After careful considerations, two complexes [Co(napy)₂ Cl₂ ] (1) and [Co(napy)₄ ](ClO₄ )₂ (2) based on the bidentate ligand napy were synthesized and structurally characterized. X-ray single-crystal structural determination showed that the cobalt(II) center in [Co(napy)₂ Cl₂ ] (1) is four-coordinate with a tetrahedral geometry (T_d ), while [Co(napy)₄ ](ClO₄ )₂ (2) is seven-coordinate rather than eight-coordinate with a capped trigonal prism geometry (C_2v ). Direct-current (dc) magnetic data revealed that complexes 1 and 2 possess positive zero-field splitting (ZFS) parameters of 11.08 and 25.30 cm^-1 , respectively, with easy-plane magnetic anisotropy. Alternating current(ac) susceptibility measurements revealed that both complexes showed slow magnetic relaxation behaviour. Theoretical calculations demonstrated that the presence of easy-plane magnetic anisotropy (D>0) for complexes 1 and 2 is in agreement with the experimental data. Furthermore, these results pave the way to obtain four-coordinate and seven-coordinate cobalt(II) single-ion magnets (SIMs) by using a bidentate ligand.

Dehydration-Hydration Switching of Single-Molecule Magnet Behavior and Visible Photoluminescence in a Cyanido-Bridged DyIIICoIII Framework

Microporous magnets compose a class of multifunctional molecule-based materials where desolvation-driven structural transformation leads to the switching of magnetic properties. Herein, we present a special type of microporous magnet where a dehydration-hydration process within a bimetal coordination framework results in the switching of emissive Dy^III single-molecule magnets (SMMs). We report a three-dimensional (3-D) cyanido-bridged coordination polymer, {[Dy^III(H₂O)₂][Co^III(CN)₆]}·2.2H₂O (1), and its dehydrated form of {Dy^III[Co^III(CN)₆]} (2), which was obtained through a reversible single-crystal-to-single-crystal transformation. Both phases are composed of paramagnetic Dy^III centers alternately arranged with diamagnetic hexacyanidocobaltates(III). The hydrated phase contains eight-coordinated [Dy^III(μ-NC)₆(H₂O)₂]^3- complexes of a square antiprism geometry, while the dehydrated form contains six-coordinated [Dy^III(μ-NC)₆]^3- moieties of a trigonal prism geometry. This change in coordination geometry results in the generation of Dy^III single-molecule magnets in 2, whereas slow magnetic relaxation effect is not observed for Dy^III sites in 1. The D_4d-to-D_3h symmetry change of Dy^III complexes produces also the shift of photoluminescent color from nearly white to deep yellow thanks to the modulation of emission bands of f-f electronic transitions. A combined approach utilizing dc magnetic data and low-temperature emission spectra confirmed an axial crystal field of trigonal prismatic Dy^III complexes in 2, which produces an Orbach type of slow magnetic relaxation. Therefore, we present a unique route to the efficient switching of SMM behavior and photoluminescence of Dy^III complexes embedded in a 3-D cyanido-bridged framework.

Relationship between the Molecular Structure and Switching Temperature in a Library of Spin-Crossover Molecular Materials

Structure-function relationships relating the spin-crossover (SCO) midpoint temperature (T_1/2) in the solid state are surveyed for 43 members of the iron(II) dipyrazolylpyridine family of SCO compounds. The difference between T_1/2 in the solid state and in solution [ΔT(latt)] is proposed as a measure of the lattice contribution to the transition temperature. Negative linear correlations between the SCO temperature and the magnitude of the rearrangement of the coordination sphere during SCO are evident among isostructural or near-isostructural subsets of compounds; that is, a larger change in the molecular structure during SCO stabilizes the high-spin state of a material. Improved correlations are often obtained when ΔT(latt), rather than the raw T_1/2 value, is considered as the measure of the SCO temperature. Different lattice types show different tendencies to stabilize the high-spin or low-spin state of the molecules they contain, which correlates with the structural changes that most influence ΔT(latt) in each case. These relationships are mostly unaffected by the SCO cooperativity in the compounds or by the involvement of any crystallographic phase changes. One or two materials within each subset are outliers in some or all of these correlations, however, which, in some cases, can be attributed to small differences in their ligand geometry or unusual phase behavior during SCO. A reinvestigation of the structural chemistry of [Fe(3-bpp)₂][NCS]₂·nH₂O [3-bpp = bis(1H-pyrazol-3-yl)pyridine; n = 0 or 2], undertaken as part of this study, is also presented.

Anionic guest-dependent slow magnetic relaxation in Co(ii) tripodal iminopyridine complexes

We report the syntheses and magnetic property characterizations of four mononuclear cobalt(ii) complex salts featuring a tripodal iminopyridine ligand with external anion receptor groups, [CoL5-ONHtBu]X2 (X = Cl (1), Br (2), I (3) and ClO4 (4)). While all four salts exhibit anion binding through pendant amide moieties, only in the case of 1 is field-induced slow relaxation of magnetisation observed, whereas in the other salts this phenomenon is absent at the limits of our instrumentation. The effect of chloride inducing a seventh Co-N interaction and concomitant structural distortion is hypothesized as the origin of the observed dynamic magnetic properties observed in 1. Ab initio computational studies carried out on a 7-coordinate Co(ii) model species survey the complex interplay of coordination number and trigonal twisting on the sign and magnitude of the axial anisotropy parameter (D), and identify structural features whose distortions can trigger large switches in the sign and magnitude of magnetic anisotropy.

Slow Magnetic Relaxation Triggered by a Structural Phase Transition in Long-Chain-Alkylated Cobalt(II) Single-Ion Magnets

The behavior of single-ion magnets (SIMs) that reflects large distortions of their coordination environments caused by the packing of long alkyl chains for two Co(II) complexes of the type [Co(C _n-terpy)₂](BF₄)₂ (C _n-terpy = 4'-alkoxy-2,2':6',2″-terpyridine; n = 10 (1), 16 (2)) is reported. 1·2MeOH, which features a highly distorted octahedral high-spin Co(II) center, exhibits field-induced slow magnetic relaxation under an applied dc field of 1000 Oe. Further detailed analysis of the relaxation process indicated the prevalence of the Raman process at low temperature. Surprisingly, 2 shows a reverse spin transition (rST) and also exhibits remarkable field-induced SIM behavior, revealing the presence of magnetic anisotropy for this high-spin Co(II) species that is triggered by a structural phase transition. We present here the first examples of the coexistence of field-induced slow magnetic relaxation and rST associated with structural phase transitions involving long-alkyl-chain conformational changes from gauche to anti. These results indicate the prospect of inducing SIM properties in other distorted high-spin Co(II) species bearing long alkyl chains.

Manipulating Spin Transition To Achieve Switchable Multifunctions

The spin transition of metal ions involves interconversion between electron configurations exhibiting considerably different functions and plays a substantial role in the chemical, physical, and biological fields. The photoinduced spin transition offers a promising approach to tune various physical properties with high spatial and temporal resolutions for producing smart multifunctional materials not only to explore their basic science but also to satisfy the demands of the next-generation photoswitchable-molecule-based devices. Therefore, it is attracting considerable interest to utilize photoinduced spin transition to simultaneously tune multifunctions. However, two issues are challenging in obtaining reversible and swift manipulation of functions: (1) the interconversion between different electron configurations of photoresponsive units should be reversibly switched via photoinduced spin transition; (2) effective coupling should be built between the photoresponsive and functional units to produce photoswitchable functions utilizing photoinduced spin transition. In this Account, we will review our recent advances in the usage of spin transition of metal ions as actuators for tuning the magnetic, dielectric, fluorescence, and mechanical properties, wherein the role of a photoswitchable spin transition is highlighted. We mainly focus on the study of two spin-transition categories, including spin-crossover (SCO) of one metal ion and metal-to-metal charge transfer (MMCT). Initially, we will describe a strategy for developing photoinduced reversible SCO and MMCT. The role of flexible intermolecular interactions, in particular, π···π interactions, is discussed with respect to a photoinduced reversible MMCT. Then, the SCO and MMCT units were assembled using metallocyanate building blocks to form a chain, wherein the spin states, anisotropy, and magnetic coupling interactions can be photoswitched to tune the single-chain magnet behavior. Besides magnetic properties, the photoinduced spin transition that is associated with the concomitant changing of charge distribution, bond lengths, and absorption spectra can be utilized to tune the multifunctions. Therefore, the transfer of an electron from a central cobalt site to one of the two iron sites in linear trinuclear Fe₂Co compounds resulted in the transformation of a centrosymmetric nonpolar molecule into an asymmetric polar molecule, and the molecular electric dipole and dielectric properties can be reversibly switched. Moreover, the spin transition usually involved significant expansion or contraction of the coordination sphere of metal ions because of the population/depopulation of the antibonding e_g orbitals. Therefore, colossal positive and negative thermal expansion behaviors were achieved in a layered compound by manipulating the spin-transition process and the rotation of the functional units, thereby providing a strategy for synthesizing phototunable nanomotors. Photoinduced spin transition can also be used to modulate the fluorescence properties by controlling the energy transfer between the fluorescent ligands and the metal sites showing SCO. Finally, we will provide a perspective and detail the remaining challenges that are associated with this research area. We believe that an increasing number of fascinating photoswitchable SCO and MMCT systems will emerge in the near future and that the materials exhibiting various properties and functions that can be manipulated using photoinduced spin transition will provide novel opportunities for the development of smart multifunctional materials and devices.

Slow magnetization dynamics in a six-coordinate Fe(ii)-radical complex

A new paramagnetic ligand, betaDTDA, and its coordination complex with Fe(hfac)2 are reported (betaDTDA = 4-(benzothiazol-2'-yl)-1,2,3,5-dithiadiazolyl; hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-). The neutral radical betaDTDA is the first dithiadiazolyl ligand designed to include an electropositive sulphur moiety outside the thiazyl heterocycle, increasing the capacity for supramolecular, structure-directing electrostatic contacts and enabling new pathways for magnetic exchange. The Fe(hfac)2(betaDTDA) complex is composed of a hs-Fe(ii) center with the three bidentate ligands arranged about the ion in a distorted octahedral 6-coordinate environment. The magnetic properties of crystalline Fe(hfac)2(betaDTDA) are consistent with strong antiferromagnetic (AF) coupling between the metal and ligand moments, giving rise to a well-defined Stotal = 3/2 ground state that is the only thermally populated state below 40 K. Below 4 K, this complex exhibits slow relaxation of the magnetization detected by ac susceptibility measurements consistent with a single-molecule magnet (SMM) behaviour.

Field Induced Single Ion Magnetic Behaviour in Square-Pyramidal Cobalt(II) Complexes with Easy-Plane Magnetic Anisotropy

Two penta-coordinate CoII complexes with formulae [Co(14-TMC)Cl](BF4) (1, 14-TMC = 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [Co(12-TBC)Cl](ClO4)·(MeCN) (2, 12-TBC = 1,4,7,10-Tetrabenzyl-1,4,7,10-tetraazacyclododecane) were synthesized and characterized. Structural analysis revealed that ligand coordinates to the CoII centre in a tetradentate fashion and the fifth position is occupied by chloride ion and the geometries around CoII centres are best described as distorted square pyramidal. Detailed magnetic measurements disclose the presence of significant easy-plane magnetic anisotropy and field induced slow magnetic relaxation behaviours of the studied complexes. More insight into the magnetic anisotropy has been given using ab initio theory calculations, which agree well with the experimental values and further confirmed the easy-plane magnetic anisotropy.

Photo-switched magnetic coupling in spin-crossover complexes

This feature article summarizes the recent progress in the magnetically coupled spin-crossover (SCO) complexes. The photo-switched molecular nanomagnet property, long range magnetic ordering, and the perspectives of SCO complexes are also presented.

A supramolecular chain of dimeric Dy single molecule magnets decorated with azobenzene ligands

Dy^III dimers decorated with photo-isomerizable azobenzene ligands behave as single-molecule magnets and self-organize into a supramolecular chain. Ab initio calculations, magnetic and optical properties are reported.

Electro-activity and magnetic switching in lanthanide-based single-molecule magnets

The present work reviews switching of single-molecule magnetic behaviour achieved through various stimuli such as temperature, light irradiation, redox processes, solvation/desolvation, and magnetic field.

Syntheses, structures, and magnetic properties of three two-dimensional cobalt(ii) single-ion magnets with a CoIIN4X2 octahedral geometry

Three two-dimensional Co^II SIMs with (4,4) layer structures have been synthesized and characterized structurally and magnetically.

Optorelaxers: Achieving real-time control of NMR relaxation

We present an approach to increase the detection sensitivity of NMR by shortening the spin-lattice relaxation time using transient paramagnetic species created by light irradiation of "optorelaxer" molecules. In the ultimate implementation of this concept, not yet realized here, these transient species are absent during the detection period, thereby avoiding the loss of spectral resolution caused by inhomogeneous broadening from paramagnetic species. Real-time control of NMR relaxation by visible light is demonstrated with Fe(II)(ptz)₆(BF₄)₂, (ptz = 1-propyltetrazole), abbreviated FePTZ. Illumination of FePTZ at 30 K results in a decrease of the ¹H NMR spin-lattice relaxation time T₁ due to formation of a high spin photoexcited state. The ¹H NMR of polystyrene containing a low concentration of FePTZ molecules shows a similar reduction in T₁, establishing that FePTZ can act as an optorelaxer for the protons of a matrix. Numerical modeling of the spin-diffusion processes from the protons in a FePTZ core to those in a shell of polystyrene accounts for the observed T₁ effects under both dark and light conditions. Additionally, ¹H MAS (magic-angle spinning) NMR results for pure FePTZ provide information on the isotropic and anisotropic portions of the electron-nuclear hyperfine interactions.

Impact of Ligand Substitutions on Multielectron Redox Properties of Fe Complexes Supported by Nitrogenous Chelates

Redox flow batteries (RFBs) have recently been recognized as a potentially viable technology for scalable energy storage. To take full advantage of RFBs, one possible approach for achieving high energy densities is to maximize a number of redox events by utilizing charge carriers capable of multiple one-electron transfers within the electrochemical window of solvent. However, past efforts to develop more efficient electrolytes for nonaqueous RFBs have mostly been empirical. In this manuscript, we shed light on design principles by theoretically investigating the effects of systematically substituting pyridyl moieties with imine ligands within a series of Fe complexes with some experimental validation. We found that such replacement is an effective strategy for reducing the molecular weight-to-charge ratios of these complexes. Simultaneously, calculations suggest that the reduction potentials and ligand-based redox activity of such substituted N-heterocyclic Fe compounds might be maintained within their +4 → -1 charge states. Additionally, by theoretically examining the role of coordination geometry, vis-à-vis reducing the number of redox noninnocent ligands within the first coordination sphere, we have demonstrated that Fe complexes with one such ligand were also capable of supporting multielectron reduction events and exhibited reduction potentials similar to their parent analogs supported by two or three of the same multidentate ligands. However, some differences in redox nature within the lower (+2 → -1) charge states were also noticed. Specifically, complexes containing two bidentate ligands, or one tridentate ligand, exhibited ligand-based reductions, whereas compounds with one bidentate ligand exhibited metal-centered reductions. The current results pave the way toward the design of the next-generation of Fe complexes with lower molecular weights and greater stored energy for redox flow batteries.

A mononuclear five-coordinate Co(ii) single molecule magnet with a spin crossover between the S = 1/2 and 3/2 states

Although a great number of single-ion magnets (SIMs) and spin-crossover (SCO) compounds have been discovered, multifunctional materials with the combination of SCO and SIM properties are extremely scarce. Here magnetic studies have been carried out for a mononuclear, five-coordinate cobalt(ii) complex [Co(3,4-lut)4Br]Br (1) with square pyramidal geometry. Direct-current magnetic measurement confirms the spin transition between the S = 1/2 and 3/2 states in the range of 150-290 K with a small hysteresis loop. Frequency- and temperature-dependent alternating-current magnetic susceptibility reveals slow magnetization relaxation under an applied dc field of 3000 Oe. The work here presents the first instance of the five-coordinate mononuclear cobalt(ii)-based SIM exhibiting the thermally induced complete SCO.

A Photomagnetic Sponge: High-Temperature Light-Induced Ferrimagnet Controlled by Water Sorption

"Converting" light energy to magnetization is the attribute of molecule-based compounds called photomagnets and is inaccessible for conventional magnetic solids. The design and synthesis of such compounds, however, is a formidable challenge, and only a few examples are known, all with rather low magnetic ordering temperatures well below the boiling point of liquid nitrogen. Herein, a cyanide-bridged coordination polymer, {[Mn^II(imidazole)]₂[W^IV(CN)₈]} _n, exhibiting the highest light-induced magnetic ordering temperature ever observed and a magnetic hysteresis loop up to 90 K is reported. The photomagnetic effect results from the blue light excitation (450 nm) of the constituent octacyanotungstate(IV) moiety, which then couples magnetically with manganese(II), resulting in light-induced ferrimagnetic ordering. The reported coordination framework shows also outstanding water sorption properties that are strongly correlated with the photomagnetic functionality. The photoswitching observed in the anhydrous state is completely quenched by the reversible capture of water, with the fully hydrated phase becoming practically non-photomagnetic.

Reversible on-off switching of both spin crossover and single-molecule magnet behaviours via a crystal-to-crystal transformation

The on-off switching of spin-crossover (SCO) and single-molecule magnetism (SMM) remains highly attractive, especially if it involves dynamic crystal-to-crystal transformation. Herein we report the first molecule, a mononuclear cobalt(ii) complex, that exhibits on-off switching between SCO and SMM reversibly during crystal-to-crystal transformation. Subtle structural transformation triggered by a simple dehydration-rehydration process induces significant geometrical changes of the CoII center and modification of the supramolecular interactions and switches its colour and magnetic properties (dark red/SCO-on/SMM-off ↔ orange/SCO-off/SMM-on). This work suggests that modification of the weak supramolecular interactions could be very effective in achieving switchable materials involving both SCO and SMM properties.

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.

The influence of pseudohalide ligands on the SIM behaviour of four-coordinate benzylimidazole-containing cobalt(ii) complexes

Three, mononuclear complexes of the formula [Co(bmim)2(SCN)2] (1), [Co(bmim)2(NCO)2] (2) and [Co(bmim)2(N3)2] (3) [bmim = 1-benzyl-2-methylimidazole] were prepared and structurally analyzed by single-crystal X-ray crystallography. The cobalt(ii) ions in 1-3 are tetrahedrally coordinated with two bmim molecules and two pseudohalide anions. The angular distortion parameter δ was calculated and the SHAPE program (based on the CShM concept) was used for 1-3 to estimate the angular distortion from an ideal tetrahedron. The molecules of 1-3 are effectively separated, and the values of the shortest distance of cobalt-cobalt are 8.442(6) and 6.774(8) Å for 1, 10.349(8) and 10.716(8) Å for 2 and 6.778(1) and 9.232(1) Å for 3. Direct current (dc) magnetic susceptibility measurements on the crushed crystals of 1-3 were carried out in the temperature range 1.9-295 K. The variable-temperature magnetic data of 1-3 mainly obey the zero-field splitting effect (D) of the 4A2 ground term of the tetrahedral cobalt(ii) complexes (2D being the energy gap between the |±1/2 and |±3/2 levels of the spin). The analysis of their magnetic data through the Hamiltonian H = D[S2z - S(S + 1)/3] + E(Sx2 - Sy2) + gβHS led to the following best-fit parameters: g = 2.29, D = -7.5 cm-1 and E/D = 0.106 (1), g = 2.28, D = + 6.3 cm-1 and E/D = 0.007 (2) and g = 2.36, D = + 6.7 cm-1 and E/D = 0.090 (3). The signs of D for 1-3 were confirmed by Q-band EPR spectra on powdered samples in the temperature range 4.0-20 K. Field-induced SIM behaviour was observed for 1-3 below 4.0 K, and the frequency-dependent maxima of χ''M were observed for 1 and only incipient signals of χ''M occurred for 2 and 3. The values of the exponential factor (τ0) and activation energy (Ea) for 1-3 which were obtained from the Arrhenius plot suggest a single relaxation process characteristic of an Orbach mechanism.

Slow Magnetic Relaxation in a Series of Mononuclear 8-Coordinate Fe(II) and Co(II) Complexes

A series of homoleptic mononuclear 8-coordinate Fe^II and Co^II compounds, [Fe^II(L²)₂](ClO₄)₂ (2), [Fe^II(L³)₂](ClO₄)₂ (3), [Fe^II(L⁴)₂](ClO₄)₂ (4), [Co^II(L¹)₂](ClO₄)₂ (5), [Co^II(L²)₂](ClO₄)₂ (6), [Co^II(L³)₂](ClO₄)₂ (7), and [Co^II(L⁴)₂](ClO₄)₂ (8) (L¹ and L² are 2,9-dialkylcarboxylate-1,10-phenanthroline ligands; L³ and L⁴ are 6,6'-dialkylcarboxylate-2,2'-bipyridine ligands), have been obtained, and their crystal structures have been determined by X-ray crystallography. The metal center in all of these compounds has an oversaturated coordination number of 8, which is completed by two neutral homoleptic tetradentate ligands and is unconventional in 3d-metal compounds. These compounds are further characterized by electronic spectroscopy, cyclic voltammetry (CV), and magnetic measurements. CV measurements of these complexes in MeCN solution exhibit rich redox properties. Magnetic measurements on these compounds demonstrate that the observed single-ion magnetic (SIM) behavior in the previously reported [Fe^II(L¹)₂](ClO₄)₂ (1) is not a contingent case, since all of the 8-coordinate compounds 2-8 exhibit interesting slow magnetic relaxation under applied direct current fields.

Single Ion Magnets from 3d to 5f: Developments and Strategies

Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc₂ Tb]^- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

Slow magnetic relaxation in five-coordinate spin-crossover cobalt(ii) complexes

We present the first examples of the coexistence of field induced slow magnetic relaxation and spin-crossover observed in five-coordinate cobalt(ii) complexes [Co(12-TMC)(CH₃CN)](X)₂ (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane, X = BF₄^-, 1; PF₆^-, 2). The direct-current (dc) magnetic data show a gradual and incomplete spin-crossover at high temperature. 1 and 2 display frequency- and temperature-dependent alternating-current (ac) magnetic susceptibility under an applied dc field of 2500 Oe, which originates from the S = 1/2 spin state of Co(ii) ions.

Switching single chain magnet behavior via photoinduced bidirectional metal-to-metal charge transfer

The preparation of single-chain magnets (SCMs) with photo-switchable bistable states is essential for the development of high-density photo-recording devices. However, the reversible switching of the SCM behavior upon light irradiation is a formidable challenge. Here we report a well-isolated double zigzag chain {[Fe(bpy)(CN)4]2[Co(phpy)2]}·2H2O (bpy = 2,2'-bipyridine, phpy = 4-phenylpyridine), which exhibits reversible redox reactions with interconversion between FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS (LS = low-spin, HS = high-spin) and FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS linkages under alternating irradiation with 808 and 532 nm lasers. The bidirectional photo-induced metal-to-metal charge transfer results in significant changes of anisotropy and intrachain magnetic interactions, reversibly switching the SCM behavior. The on-switching SCM behavior driven by light irradiation at 808 nm could be reversibly switched off by irradiation at 532 nm. The results provide an additional and independent way to control the bistable states of SCMs by switching in the 0 → 1 → 0 sequence, with potential applications in high density storage and molecular switches.

High-coordinate CoII and FeII compounds constructed from an asymmetric tetradentate ligand show slow magnetic relaxation behavior

A seven-coordinate Co^II compound and an eight-coordinate Fe^II compound based on an asymmetric tetradentate ligand have been reported, and both of them exhibited slow magnetic relaxation behaviour.

A linear trinuclear ferrous single molecule magnet

A linear trinuclear ferrous [FeII3] SMM exhibited clear magnetic hysteresis loops and a significant higher effective energy barrier in comparison with its cobalt(ii) analogue.

Heterometallic hexanuclear Ni4M2 (M = Dy, Y) complexes: structure and single-molecule magnet for the Dy(iii) derivative

We report here a 3d–4f heterometallic hexanuclear Ni₄Dy₂ single-molecule magnet with an S-shaped double-pocket skeleton.

A new series of tetrahedral Co(ii) complexes [CoLX2] (X = NCS, Cl, Br, I) manifesting single-ion magnet features

The influence of ligand field strength on the magnetic anisotropy of a series of isostructural tetrahedral Co^II complexes has been investigated by using a combined experimental and theoretical approach.

Effect of linear and non-linear pseudohalides on the structural and magnetic properties of Co(ii) hexacoordinate single-molecule magnets

Field-induced single-molecule magnets based on Co(ii) hexacoordinate complexes comprising pseudohalides are reported.