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Spin crossover (SCO) has long been a hot topic in the field of molecular magnetism owing to its unique bistability character. Rational control of thermal hysteresis and transition temperature ( T 1/2 ) is crucial for their practical applications, which rely on precise manipulation of the substituents of SCO coordinating ligands and molecular packing interactions. In this study, we designed two different bridging ligands (2-FDPB: 4,4'-(2-fluoro-1,4-phenylene)dipyridine; 2,3-FDPB: 4,4'-(2,3-difluoro-1,4-phenylene)dipyridine) featuring one and two fluoro substitution on the central benzene ring and applied a Schiff base-like equatorial tetradentate ligand {diethyl( E , E )-2,2'-[4,5-difluoro-1,2-phenyl-bis(iminomethylidyne)]bis(3-oxobutanoate)-(2-)- N , N ',O3,O3'} (H 2 L) to coordinate with the Fe II ion. Two Fe II -coordination chain polymers [Fe II (L)(2,3-FDPB)]·0.25CH 2 Cl 2 (1) and [Fe II (L)(2-FDPB)]·0.5CH 3 OH (2) were obtained. 1 crystallizes in the monoclinic P 2 1 / n space group with only one Fe II center, while 2 crystallizes in the triclinic P 1̄ space group with two independent Fe II centers. Unlike the identical 2D layer stacking in 1, 2 exhibited alternating stacking of the extending 2D layers and meshed chains. Magnetic measurements revealed the typical thermally induced spin crossover behavior (SCO): 1 exhibited a 41 K-wide thermal hysteresis with transition temperatures of T 1/2↑ = 245 K and T 1/2↓ = 204 K, while 2 showed a higher transition temperature ( T 1/2 = 330 K) with no thermal hysteresis. Magneto-structural correlation studies suggest that the electron-withdrawing effect present in the fluoro substituents does not have a significant impact on the SCO behaviors. Despite the fluoro substituents having a similar atomic radius of hydrogen atoms, variations in the number of these substituents can alter the crystallization behavior of these complexes, which in turn affects the solvents, molecular stacking patterns, and intermolecular interactions, ultimately influencing the SCO behaviors.

A dinuclear Fe(II) spin crossover (SCO) complex with the formula [Fe 2 L 5 (NCS) 4 ]·2DMF·2H 2 O (1) was synthesised from 1-naphthylimino-1,2,4-triazole (L). Complex 1 exhibits an incomplete thermally induced spin transition with a transition temperature T 1/2 of 95 K and a thermally trapped metastable high-spin state at low temperatures. Furthermore, it undergoes a reversible light-induced spin crossover by alternate irradiation with 532 and 808 nm lasers.

The design of magnetic molecular materials exhibiting multiple functions has garnered significant interest owing to their potential applications in molecular switches, sensors, and data storage devices. In this study, we synthesized a two-dimensional (2D) Fe II -based Hofmann-type coordination polymer, namely {Fe(DPPE) 2 [Ag(CN) 2 ] 2 }·2EtOH (1), using a luminescent ligand 1,1-diphenyl-2,2-di(4-pyridylbiphenyl)ethylene (DPPE). Single-crystal structural analyses and magnetic measurements revealed a thermally induced spin crossover (SCO) with the transition temperature T 1/2 = 231 K. Variable-temperature fluorescence emission spectra indicated the coexistence of spin crossover and fluorescence properties. Moreover, a pronounced dielectric change (Δ ε ' = 1.2 at 0.5 kHz) was observed during the SCO process, confirming the simultaneous magnetic and dielectric switching arising from the rearrangement of 3 d electrons and deformation of the Fe II -centered coordination sphere. This work provides an approach to explore the interplay between magnetic, dielectric, and fluorescence properties, and holds significance for developing multifunctional molecular materials.

Investigating spin crossover (SCO)-fluorescence bifunctional materials and establishing their structure-function relationships are attractive topics in chemistry and materials science. However, it remains challenging to preserve the fluorescence and SCO properties simultaneously in aggregated solid states. Herein, we design an ( E )-2,6-bis(1 H -pyrazol-1-yl)-4-(4-(1,2,2-triphenylvinyl)styryl)pyridine (tpe-bpp) ligand, which contains coordinated SCO and fluorescence units of an aggregation-induced emission luminogen (AIEgen). The coordination of the tpe-bpp ligand with different Fe II salts generated three mononuclear complexes: [Fe(tpe-bpp) 2 ](ClO 4 ) 2 ·5.75CH 2 Cl 2 (1), [Fe(tpe-bpp) 2 ](ClO 4 ) 2 ·CH 2 Cl 2 ·3CH 3 OH (2) and [Fe(tpe-bpp) 2 ](BF 4 ) 2 ·CH 2 Cl 2 ·3CH 3 OH (3). Single-crystal X-ray diffraction studies showed that they shared a similar [Fe(tpe-bpp) 2 ] 2+ complex cation. Their counterions and co-crystallized solvents were different. Magnetic measurements revealed that 1, 2, and 3 exhibited a complete SCO behavior with the transition temperatures T 1/2 of 375, 260, and 248 K, respectively. Fluorescence measurements confirmed the existence of the AIE property for both the tpe-bpp ligand and Fe(II) complexes. A monotonic decrease of the photoluminescence (PL) intensity upon increasing the temperature was behavior observed for all three complexes.

The self-assembly of multinuclear SCO complexes is appealing in which unique properties may be discovered due to enhanced intramolecular and intermolecular interactions. In this work, three dinuclear cobalt(II) complexes, named Co-1, Co-2, and Co-3, were prepared based on a complementary terpyridine ligand pair strategy. The complexes were accurately synthesized by the solvothermal method in which dinuclear complexes were directional assemblies from cobalt(II) ions, terpy bearing 2,6-dimethoxyphenyl substituents at the terpyridyl 6,6''-positions, and ditopic terpy built with different linkers (alkynyl for 1, diynyl for 2, and phenyl for 3). Single-crystal structure determinations reveal that all compounds possess a central symmetric molecular structure, so that two cobalt(II) units are identical in the solid state. Their spin crossover behaviours were investigated through variable-temperature magnetic susceptibility studies. Co-1 undergoes limited SCO with a large population of low spin state ( S = 1/2) in the measured temperatures. Co-2 and Co-3 exhibit solvent-modulated SCO behaviour. Impressively, the de-solvated samples show a repeatable thermal hysteresis loop around the room temperature region. This work demonstrates that complementary terpyridine ligand pairing is a practical approach to accurate and directional construction of multinuclear SCO-active compounds.

Two 3d transition metal mononuclear complexes, [(FeL 2 )(ClO 4 ) 2 ] 2 ·CH 3 CN (1) and (CoL 2 )(ClO 4 ) 2 ·2CH 3 CN (2), have been prepared from a rigid tetradentate bpybox (L = 6,6'-bis(2,5-dihydrooxazol-4-yl)-2,2'-bipyridine) ligand. Single crystal X-ray diffraction analyses together with the help of calculations show that both compounds are octa-coordinate. Direct current magnetic studies reveal their significant magnetic anisotropy. Impressively, field-induced relaxation of magnetism is observed in the two complexes and the apparent anisotropy barriers are 14.1 K for 1 and 21.6 K for 2, respectively. Theoretical calculations reveal that two Fe(II) centers in 1 have small negative D values of -4.897 and -4.825 cm -1 and relatively small E values of 0.646 and 0.830 cm -1 , indicating a uniaxial magnetic anisotropy. In contrast, the D and E values in the Co(II) center of 2 are 46.42 cm -1 and 11.51 cm -1 , featuring a rhombic anisotropy. This work demonstrates that field-induced slow magnetic relaxation in 3d transition metal complexes with high coordination numbers can be manipulated through rigid ligand design.

Four mononuclear 9-coordinate Dy-based complexes, [Dy(HL 1 ) 2 (NO 3 ) 3 (CH 3 OH)] (1Dy), [Dy(HL 2 ) 2 (NO 3 ) 3 (H 2 O)] (2Dy), [Dy(HL 3 ) 3 (NO 3 ) 3 ]·CH 3 CN (3Dy), and [Dy(HL 4 ) 3 (NO 3 ) 3 ] (4Dy), have been constructed by neutral Schiff-base ligands (1-[ N -(4-R)aminomethylidene-2( 1H )-naphthalenone, R = -Cl (HL 1 ), -NO 2 (HL 2 ), -OCH 3 (HL 3 ), -I (HL 4 )). By tuning the terminal substituent group of HL ligands, the number of HL ligands coordinated to the central Dy 3+ ion unexpectedly varies from 2 to 3, and the local symmetry around the Dy 3+ ion reduces from D 3h to C s . Magnetic measurements reveal that 2Dy can display single-ion magnet (SIM) behavior in zero dc field, while 1Dy, 3Dy and 4Dy show field-induced slow magnetic relaxation. Ab initio calculations were employed to elucidate magnetic anisotropy in the complexes, including g -tensors, averaged transition magnetic moments and magnetic easy axes. The difference in magnetic behaviors of the four complexes can be ascribed to the terminal substituent effect of neutral Schiff-base ligands.

Two new iron(II) complexes [Fe(bppCHO) 2 ](ClO 4 ) 2 (1·ClO4) and [Fe(bppCHO) 2 ](BF 4 ) 2 ·H 2 O (1·BF4) were synthesized by using 2,6-di(1 H -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.

The supramolecular self-assembly synthetic strategy provides a valid tool to obtain polynuclear Fe(II) complexes having effective communication between the metal centres and distinct spin crossover behaviour. Despite the great success in constructing various magnetic molecules, progress has not been made in SCO complexes based on azido bridges. In this article, the coordination-driven supramolecular assembly based on 3,6-substituted pyridazine and azide is presented to afford two Fe(II) grid-like complexes: [(L) 4 FeII4(N 3 ) 4 ][BPh 4 ] 4 ·sol (1, L = 3,6-bis(3,5-dimethyl-1 H -pyrazol-1-yl)pyridazine and 2, L = 3,6-di(pyridin-2-yl)pyridazine). The substitution of pyridinyl groups in 2 instead of pyrazolyl ones in 1 led to the only example exhibiting spin-crossover behaviour ( T 1/2 = 230 K) among the azido-bridged complexes. In addition, a temperature-dependent photoluminescence study of 2 demonstrates a visible synergetic effect between the SCO event and the luminescence.

The understanding of the correlation between the spin-state behaviour and the structural features in transition-metal complexes is of pronounced importance to the design of spin crossover compounds with high performance. However, the study of the influence of ligand symmetry on the spin crossover properties is still limited due to the shortage of suitable structural systems. Herein we report the magneto-structural correlations of three mononuclear Fe(ii) isomers with respect to their ligand symmetry. In this work, two phenyl-substituted meso and optically pure pybox ligands were employed to construct meso (1), optically pure (2), and racemic (3) ligand types of [Fe(pybox) 2 ] 2+ complexes. Their magnetic susceptibilities were measured via temperature-dependent paramagnetic 1 H NMR spectroscopy. We fitted the midpoint temperatures of the transition (T 1/2 ) of 260 K for 1(ClO 4 ), 247 K for 2(ClO 4 ), and 281 K for 3(ClO 4 ). The influence of structural symmetry on spin crossover was rationalized through density functional theory calculations. The optimized structures of [Fe(pybox) 2 ] 2+ complex cations show that the geometric distortion of the central FeN 6 coordination sphere is mainly caused by the steric congestions between adjacent phenyl substituents. In these compounds, there is a distinct correlation that more steric congestions produce larger coordination distortion and favor the electron configuration in the high-spin state, which reflects in the increase of T 1/2 . Additionally, the influence of the counter anion and lattice solvent on the meso series compounds was inspected. It is revealed that multiple factors dominate the spin-state behaviour in the solid state. This work provides deep insight into the effect of ligand symmetry on the spin transition behaviour in spin crossover compounds. It demonstrates that molecular symmetry should be considered in the design of spin crossover compounds.

A class of cyano-bridged 3d-4f zig-zag chain compounds, {RE[TM(CN) 6 ] (PNO) 2 (H 2 O) 4 }·(H 2 O) {RE = Y III , TM = [Fe III ] LS (1); RE = Dy III , TM = [Fe III ] LS (2), Co III (3)}, have been synthesized and characterized by single-crystal X-ray diffraction. The rare earth ions in these compounds are situated in a slightly distorted triangular dodecahedral (D 2d ) coordination environment. The magnetic properties of compounds 1-3 have been comparatively studied in detail. Under a zero dc field, the temperature dependence of ac susceptibility measurements for YFe (1) indicates the absence of magnetic relaxation stemming from the single anisotropic [Fe III ] LS ion. The dysprosium analogue DyFe (2) shows only magnetic relaxation behavior with a prominent QTM effect, while DyCo (3) exhibits SIM properties not completely covered by QTM, with an extracted energy barrier of 73 K under a zero dc field. The ab initio calculations indicate that both compounds 2 and 3 are SMMs with well-behaved magnetic relaxation properties primarily from the individual Dy III ion. Therefore, the different magnetic behaviors exhibited by compound 2 compared to 3 may be ascribed to the stronger QTM effect caused by the extra weak interaction of [Fe III ] LS ions in 2 as a fluctuating transverse field around the Dy III ion. The QTM effect for both 2 and 3 is suppressed under an applied dc field with an effective energy barrier of 134 and 150 K, respectively. Compared with compound 2, the higher extracted U eff /k B and χ''(T) peak temperature for 3 should be further attributed to its slightly higher single-ion axiality as calculated and the elimination of the transverse field from the [Fe III ] LS ion.

To conclude the design principle for high-performance erbium-based single-ion magnets (SIMs), two half-sandwich type erbium complexes 1Er [(LOMe)Er(COT)] (LOMe = [(η5-C5H5)Co{P([double bond, length as m-dash]O)(OMe)2}3], COT = cyclooctatetraenyl) and 2Er [(THF)2(OAr)Er(COT)] (Ar = 2,6-Dipp2C6H3, Dipp = 2,6-diisopropylphenyl) were synthesized, and structurally and magnetically characterized. Both of them exhibited SIM behavior. Their magnetic relaxation behaviours were further elucidated by ab initio calculations and near-IR emission spectroscopy. The results demonstrated that an axial ligand with a weaker electron-donating effect was preferential to construct high blocking barrier erbium-based SIMs.

Spin crossover and structural phase transition in three pairs of homochiral and heterochiral [Fe(pybox)2]2+ diastereomers were investigated through magnetic and crystallographic studies. Herein, we show that the spin transition properties of these compounds are strongly related to the homochiral or heterochiral assembly of the ligands and structural phase transition. Our work demonstrates that chirality is a feasible factor to modulate and influence the molecular geometry and thus the spin-crossover properties of complexes.

Herein, a series of light lanthanide-based complexes, Ln(fdh) 3 (bpy) (Ln = Ce III , Pr III , and Nd III and fdh = 1,1,1-fluoro-5,5-dimethyl-hexa-2,4-dione, bpy = 2,2'-bipyridine), were synthesized and characterized. The angle-resolved magnetometry studies reveal that the three complexes have Ising-type anisotropy, and the magnetic easy axes orient along the negative charge dense direction in the crystal field. The results were consistent with the ab initio calculations. This research demonstrates that the crystal field electron density distribution determines the anisotropy of light lanthanides.

Three penta-coordinate Co 2+ complexes with halide substitutes were synthesized. Their zero-field splitting (ZFS) parameters were determined by fitting dc magnetic data and far-infrared magneto-transmission spectra. The results gave an unusual case that complexes with heavier halide atoms bear smaller ZFS parameters.

The introduction of NaBPh(4) into a methanolic solution of CoCl(2)·(6)H(2)O and 2-[(pyridine-2-ylimine)-methyl]phenol (Hpymp) afforded {[Co(II)(3)(pymp)(4)(MeOH)(2)][BPh(4)](2)}·(2)MeOH (1) with a centro-symmetrically linear trinuclear structure. Magnetic analysis of 1 exhibited significant intracluster ferromagnetic exchange (2.4 cm(-1)) and slow relaxation of magnetization in both zero and non-zero static fields below 5 K, giving the first [Co(II)(3)] single molecule magnet with an effective energy barrier of 17.2(3) cm(-1) under a 500 Oe dc field.