Asymmetric Coordination Toward a Photoinduced Single-Chain Magnet Showing High Coercivity Values

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.

3 nm-wide Cyanometallate Fe-Co Tape Exhibiting Single-Chain Magnet Behavior

Treatment of Co(OTf)2·6H2O, Li[(pzTp)FeIII(CN)3], and H3PMo12O40·nH2O in protic solvents afforded two structurally related Fe-Co cyanometallate complexes: [{(pzTp)Fe(CN)3}3Co3(MeOH)10][PMo12O40]·H2O·11MeOH (1, pzTp- = tetra(pyrazolyl)borate) and {[(pzTp)Fe(CN)3]4Co3(MeOH)5(H2O)3}n[HPMo12O40]n·3 nMeOH·6.5nH2O (2). Complex 1 consists of a cyanide-bridged hexanuclear [Fe3Co3] cage, characterized by the fused conjunction of two mutually perpendicular trigonal bipyramids (TBPs, [Fe2Co3] and [Co2Fe3]), while complex 2 showcases an intricate cyanide-bridged Fe-Co tape comprising a central chain backbone of vertex-sharing [Fe2Co3] TBPs alongside peripheral [Fe2Co2] squares. Complex 2 is among the widest one-dimensional coordination assemblies characterized by the single-crystal X-ray diffraction technique. Magnetic studies revealed that complex 2 behaved as a single chain magnet with an effective energy barrier (Ueff/kB) of 46.8 K. Our findings highlight the possibilities in the development of cyanometallate-POM hybrid materials with captivating magnetic properties.

Modulation of single-chain magnet behaviour in a heterometallic Fe2Co cyanide-bridged 2D sheet

A cyanide-bridged Fe2Co 2D sheet exhibiting electron transfer coupled spin transition (ETCST) with co-existence of magnetic ordering below 50 K is reported. The complex exhibits single-chain magnet behaviour where the uncoordinated water molecules act as an exchange-breaking impurity by allowing only a fraction of the molecule to undergo a spin state change. The paramagnetic centres prevail throughout the chain on desolvation, thereby increasing the number of correlated units in the chain.

Photoinduced large magnetic change at room temperature and radical-quenched spin glass in a cyanide-bridged MnII-FeIII compound

By the coordination assembly of a redox photoactive functional motif and a cyanide-bridged moiety, a cyanide-bridged MnII-FeIII compound with large photoinduced magnetic change at room-temperature due to photoinduced electron transfer was obtanied. This compound also shows unprecedented radical-quenched spin glass in molecule based magnets.

Ferromagnetically coupled single-chain magnets exhibiting a magnetic hysteresis of 0.42 Tesla in cyano-bridged FeIII2MII (M = Ni, Fe) coordination polymers

The synthesis, single-crystal structures and magnetic properties of two new double-zigzag-chain cyano-bridged heterobimetallic {[MII(Py-NOH)2][FeIII(Tp*)(CN)3]2}·H2O ([FeIII2MII]) (Py-NOH = 4-pyridinealdoxime, Tp* = tris(3,5-dimethylpyrazol-1-yl)borohydride, M = Ni (1), Fe (2)) compounds are reported. The crystal structures of both compounds were determined by single-crystal X-ray diffraction. Complexes 1 and 2 are isostructural, with the crystal structure comprising neutral double-zigzag (4,2-ribbon-like) bimetallic chains. The FeIII ion is coordinated by three cyanide carbon atoms and three nitrogen atoms of Tp* anions. However, the MII ion is surrounded by four cyanide nitrogen atoms and two nitrogen atoms from two Py-NOH ligands. The crystal structures and magnetic studies demonstrate that both complexes behave as single-chain magnetics (SCMs) with intrachain ferromagnetic coupling. Furthermore, [FeIII2NiII] exhibits an excellent coercive field of 0.42 T at 1.8 K, among cyano-bridged 3d transition-metal-based SCMs reported thus far. Preliminary theoretical calculations provide a deep understanding of the magnetic properties of [FeIII2NiII].

Magnetic Property, Heat Capacity and Crystal Structure of Mononuclear Compounds Based on Substitute Tetrazole Ligand

Three mononuclear compounds formulated as {M[(2-1H-tetrazol-5-yl)pyridine]2(H2O)2} (M = FeII (1), CoII (2), CuII (3)) were reported and synthesized. Their space group is monoclinic, P21/c, revealed by single-crystal X-ray diffraction. Antiferromagnetic interactions exist in Compounds 1, 2 and 3, as evidenced by magnetic and low-temperature heat capacity measurements. In addition, their thermodynamic functions were determined by a relaxation calorimeter, indicating that Compound 1 exhibits a Schottky anomaly in low-temperature heat capacity. This work can provide an important fundamental basis for the research of the thermophysical properties of molecular-based magnetic materials.

Confinement inside MOFs Enables Guest-Modulated Spin Crossover of Otherwise Low-Spin Coordination Cages

Confinement of discrete coordination cages within nanoporous lattices is an intriguing strategy to gain unusual properties and functions. We demonstrate here that the confinement of coordination cages within metal-organic frameworks (MOFs) allows the spin state of the cages to be regulated through multilevel host-guest interactions. In particular, the confined in situ self-assembly of an anionic FeII4L6 nanocage within the mesoporous cationic framework of MIL-101 leads to the ionic MOF with an unusual hierarchical host-guest structure. While the nanocage in solution and in the solid state has been known to be invariantly diamagnetic with low-spin FeII, FeII4L6@MIL-101 exhibits spin-crossover (SCO) behavior in response to temperature and release/uptake of water guest within the MOF. The distinct color change concomitant with water-induced SCO enables the use of the material for highly selective colorimetric sensing of humidity. Moreover, the spin state and the SCO behavior can be modulated also by inclusion of a guest into the hydrophobic cavity of the confined cage. This is an essential demonstration of the phenomenon that the confinement within porous solids enables an SCO-inactive cage to show modulable SCO behaviors, opening perspectives for developing functional supramolecular materials through hierarchical host-guest structures.

Syntheses and magnetic properties of a bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine and its metal complexes

A novel bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine, NIT-2-TrzPm (NIT-2-TrzPm = (2-(2'-triazolopyrimidine)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)) and six new transition metal complexes of this ligand, namely [M(hfac)₂(NIT-2-TrzPm)]·CH₂Cl₂ (M = Mn (1Mn) and Co (2Co)), [M(hfac)₂]₂(NIT-2-TrzPm) (M = Mn (3Mn) and Co (4Co)), [Mn(NIT-2-TrzPm)₂(MeOH)₂](ClO₄)₂·MeOH (5Mn), and [Co(NIT-2-TrzPm)₂(MeOH)₂]₂(ClO₄)₄·4MeOH (6Co) were prepared and characterized structurally and magnetically. These complexes can be selectively synthesized by controlling the reaction ratio of M(hfac)₂·2H₂O to the radical ligand (for 1Mn to 4Co) or using metal perchlorates as the starting materials (for 5Mn and 6Co). Single crystal X-ray crystallographic analyses confirmed that 1Mn and 2Co are isostructural 3d-2p M^II-radical complexes, in which the NIT-2-TrzPm radical acts as a terminal bidentate ligand chelating to one 3d ion, while 3Mn and 4Co are isostructural 3d-2p-3d M^II-radical-M^II complexes with the NIT-2-TrzPm radical acting as a bridging ligand between two 3d ions. For complexes 5Mn and 6Co, two NIT-2-TrzPm ligands from the equatorial positions coordinate with the metal center to form the 2p-3d-2p structures with the axial positions occupied by two methanol molecules. Magnetic analysis on the Mn^II complexes revealed the existence of a strong antiferromagnetic interaction between the Mn^II and the NIT radical spin, while weak ferromagnetic coupling for Mn⋯Mn and Rad⋯Rad in the Mn-NIT-Mn and Rad-Mn-Rad spins was confirmed. Interestingly, although the NIT-bridged complexes 3Mn and 4Co possess significantly different magnetic anisotropy, field-induced slow magnetic relaxation can be observed in both complexes, which was assigned to the phonon bottleneck effect for 3Mn and field-induced SMM behavior for 4Co. To the best of our knowledge, 3Mn is the first example of the NIT-bridged binuclear Mn^II complex undergoing slow magnetic relaxation.

Photo-Induced Magnetic Hysteresis in a Cyanide-bridged Two-dimensional [Mn2W] Coordination Polymer

2D magnetic materials have been opening a new horizon in materials science. It is challenging to switch the magnetic hysteresis of 2D magnetic materials via light irradiation, applicable for future...

Light actuated single-chain magnet with magnetic coercivity

A cyanide-bridged {Fe2Co}-based coordination polymer was synthesized. It showed photo-induced slow relaxation of magnetization and a coercive field of 400 Oe.

Polyoxometalate-Assisted Assembly of Pearl-Chain-Like Cyanide-Bridged Single-Chain Magnets

The introduction of Keggin-type POMs of [PMo₁₂O₄₀]^3- or [SiW₁₂O₄₀]^4- as counteranions into the Fe^III-M^II cyanometalate system afforded three chain complexes: [(Tp*)Fe(CN)₃Ni(DMF)₄]₂{[(Tp*)Fe(CN)₃Ni(DMF)₃(H₂O)]₂Ni(DMF)₄}[PMo₁₂O₄₀]₂·14DMF (1, Tp*= hydridotris(3,5-dimethylpyrazol-1-yl)borate) and {[(Tp*)Fe(CN)₃M(DMF)₃(H₂O)]₂M(DMF)₄}[SiW₁₂O₄₀]·3DMF (2, M = Ni^II; 3, M = Co^II). Complex 1 contains both discrete cationic [Fe₂Ni₂]²⁺ squares and less-studied {Fe₂Ni₃}_n pearl chains, namely 3,2-chains, while 2 and 3 consist of pure 3,2-chains due to the replacement of [PMo₁₂O₄₀]^3- with [SiW₁₂O₄₀]^4- bearing one more negative charge. Magnetic studies revealed that all of the complexes exhibit single-chain-magnet (SCM) behaviors with the effective thermal barriers of Δ_τ1/k_B = 61.6 K (infinite regime) and Δ_τ2/k_B = 36.5 K (finite regime) for 1, Δ_τ/k_B = 46.9 K for 2 (finite), and Δ_τ/k_B = 30.6 K for 3 (finite). The POM moieties may play a pivotal role for the realization of this promising archetype of favoring SCM property: (1) the highly negatively charged POMs may facilitate the formation of the uncommon highly positive "pearl chain"; (2) the nanosized POMs necessarily led to the good isolation of the chains in the title complexes, and (3) the employment of POMs with different charges may regulate the resultant complexes in both structure and magnetism.

Thermal and photoinduced spin-crossover of mononuclear FeII complexes based on bppCHO ligand

Two new iron(II) complexes [Fe(bppCHO)₂](ClO₄)₂ (1·ClO4) and [Fe(bppCHO)₂](BF₄)₂·H₂O (1·BF4) were synthesized by using 2,6-di(1H-pyrazol-1-yl)isonicotinaldehyde (bppCHO) as the ligand. The structures and spin states of the complexes were characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. Complex 1·ClO4 exhibits light-induced excited spin state trapping (LIESST) up to 53 K and two-step spin-crossover (SCO) over room temperature, in which the first step shows a thermal hysteresis of 26 K, whereas 1·BF4 exhibits a gradual SCO behavior. A magnetostructural study reveals that the difference in the SCO property is related to the supramolecular interactions and crystal packing effect.

A cyanide-bridged Fe-Co pearl-chain-like single-chain magnet containing 4-coordinate cobalt(II) ions

Treatment of CoCl₂·6H₂O and tris(pyrazolyl-1-yl)borate tricyanoiron(III) anions at an elevated temperature (55 °C) afforded two less-common pearl-chain-like compounds, {[(Tp^R)Fe(CN)₃CoCl₂]₂Co(DMF)₄}·nDMF (1, Tp^R = Tp^4-Me = hydridotris(4-methylpyrazol-1-yl)borate, n = 1 and 2, Tp^R = Tp*^Me = hydridotris(3,4,5-trimethylpyrazol-1-yl)borate, n = 4.5), in which the 4-coordinate Co(II) ions and [(Tp^R)Fe^III(CN)₃]^- units are alternately bridged by cyanide groups into squares, which are further linked with the 6-coordinate Co(II) ions into an infinite chain. Interestingly, the magnetic study revealed that 1 exhibits a typical single-chain magnet behaviour with an effective energy barrier of 28.0 K, while surprisingly no Glauber dynamics was observed for 2 despite their very similar structures. The variations of the local coordination environments of the cobalt ions and the cyanide linkages were evidenced, and they may account for the significant difference in their magnetic properties related to the global magnetic anisotropy and magnetic exchange of the chain.

Single-chain magnet behavior in a finite linear hexanuclear molecule

The careful monitoring of crystallization conditions of a mixture made of a TbIII building block and a substituted nitronyl-nitroxide that typically provides infinite coordination polymers (chains), affords a remarkably stable linear hexanuclear molecule made of six TbIII ions and five NIT radicals. The hexanuclear units are double-bridged by water molecules but ab initio calculations demonstrate that this bridge is inefficient in mediating any magnetic interaction other than a small dipolar antiferromagnetic coupling. Surprisingly the hexanuclears, despite being finite molecules, show a single-chain magnet (SCM) behavior. This results in a magnetic hysteresis at low temperature whose coercive field is almost doubled when compared to the chains. We thus demonstrate that finite linear molecules can display SCM magnetic relaxation, which is a strong asset for molecular data storage purposes because 1D magnetic relaxation is more robust than the relaxation mechanisms observed in single-molecule magnets (SMMs) where under-barrier magnetic relaxation can operate.