Spin crossover iron complexes with spin transition near room temperature based on nitrogen ligands containing aromatic rings: from molecular design to functional devices

During last decades, significant advances have been made in iron-based spin crossover (SCO) complexes, with a particular emphasis on achieving reversible and reproducible thermal hysteresis at room temperature (RT). This pursuit represents a pivotal goal within the field of molecular magnetism, aiming to create molecular devices capable of operating in ambient conditions. Here, we summarize the recent progress of iron complexes with spin transition near RT based on nitrogen ligands containing aromatic rings from molecular design to functional devices. Specifically, we discuss the various factors, including supramolecular interactions, crystal packing, guest molecules and pressure effects, that could influence its cooperativity and the spin transition temperature. Furthermore, the most recent advances in their implementation as mechanical actuators, switching/memories, sensors, and other devices, have been introduced as well. Finally, we give a perspective on current challenges and future directions in SCO community.

Spin Crossover in [Fe(qsal-5-Brq)2]+ Complexes with a Quinoline-Substituted Qsal Ligand

Using a quinoline substituted Qsal ligand, Hqsal-5-Brq (Hqsal-5-Brq = N-(5-bromo-8-quinolyl)salicylaldimine), four FeIII complexes, [Fe(qsal-5-Brq)2]A·CH3OH (Y = NO3- (1NO3), BF4- (2BF4), PF6- (3PF6), OTf- (4OTf), were prepared and characterized. Structure analysis revealed that complex 2BF4 contained two species (2BF4(P1̅) and 2BF4(C2/c)). In these compounds except 3PF6, the [Fe(qsal-5-Brq)2]+ cations form 1D chains through π-π interactions and other weak interactions. Adjacent chains are connected to form the 2D "Chain Layer" structures and 3D structures through various supramolecular interactions. For 3PF6, a "Dimer Chain" structure is formed from the loosely connected dimers. Magnetic studies revealed that compounds 1NO3 and 2BF4(P1̅) displayed abrupt hysteretic SCO with the transition temperature T1/2↓ = 235 K, T1/2↑ = 240 K for 1NO3 and T1/2↓ = 230 K, T1/2↑ = 235 K for 2BF4(P1̅), while compounds 3PF6 and 4OTf are in the HS state. Desolvation of the complexes significantly modifies their SCO properties: the desolvated 1NO3 and 2BF4 show a gradual SCO, desolvated 3PF6 undergoes a two-step SCO, and desolvated 4OTf exhibits a hysteretic transition. Overall, this work reported the FeIII-SCO complexes of the quinoline-substituted Hqsal ligand and highlighted the potential of these ligands for the development of interesting FeIII-SCO materials.

Spin-Crossover Behaviors of Iron(II) Complexes Bearing Halogen Ligands in Solid State and Solution

Numerous Fe(II) spin-crossover (SCO) compounds have been developed in the past decades, while the reports on the SCO materials with halogen atoms acting as coordinating ligands remain rare. In this study, we synthesize three iron(II) halide complexes with a general formula of [Fe^II(Py5Me₂)X]⁺ (Py5Me₂ = 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine, X = Cl^- or Br^-) that undergo complete SCO transitions at near room temperature. The SCO properties of these compounds are investigated in detail by magnetic measurements, variable-temperature single-crystal X-ray diffractions, and Mössbauer spectra analyses. Because of the good stability of the coordination structures and suitable ligand-field strength, these compounds show robust spin transitions in both solid state and solution.

Structural and theoretical insights into solvent effects in an iron(III) SCO complex

Alcohol effects in a series of iron(III) spin crossover complexes [Fe(qsal-Cl)2]NO3·ROH (R = Me 1, Et 2, 1-Pr 3) are explored. Despite the solvents differing from each other by only...

New iron(ii) spin-crossover metallomesogen with long aliphatic chain terminated by a CC bond

A new Fe(ii) complex with long aliphatic chains and terminal CC bonds exhibits SCO behavior and liquid-crystalline properties.

Halogen-Substituted Spin-Crossover Fe(III) Compounds with Photoresponsive Properties

Halogen-substituted Fe(III) compounds, [Fe(Hphsalpm^X)₂]PTFB (Hphsalpm^X = 5-X-(R,S)-((phenyl(2-pyridyl)methylimino)methyl)phenol, PTFB = phenyltrifluoroborate; X = F for 1, Cl for 2, Br for 3) and [Fe(Hphsalpm^X)₂]PTFB·MeOH (X = I for 4·MeOH), were synthesized. Compounds 1, 4·MeOH, and its desolvated form 4 exhibited an invariant high-spin state in the whole temperature range, while 2 and 3 underwent gradual, nonhysteretic, and incomplete spin crossover (SCO) with transition temperatures (T_C) of 153 and 220 K, respectively. Interestingly, the SCO-active compounds 2 and 3 showed light-induced excited spin-state trapping (LIESST) effects at 10 K, and light-induced reversible ON/OFF switching behaviors were realized by alternately using 880 and 1064 nm light, while the thermally inert compound 4·MeOH unexpectedly showed a reverse-LIESST effect. These results may help to design and synthesize new photoresponsive SCO Fe(III) compounds for the development of switchable materials.

Interplay of halogen and hydrogen bonding in a series of heteroleptic iron(iii) complexes

The impact of the halogen substituent on supramolecular preferences that influence packing is explored in a series of heteroleptic iron(iii) complexes.

High temperature anionic Fe(III) spin crossover behavior in a mixed-valence Fe(II)/Fe(III) complex

Two ion-pair Fe(iii) complexes (PPh4)[FeIII(HATD)2]·2H2O (1, H3ATD = azotetrazolyl-2,7-dihydroxynaphthalene) and [FeII(phen)3][FeIII(HATD)2]2·3DMA·3.5H2O (2, phen = 1,10-phenanthroline, DMA = N,N-dimethylformamide) were synthesized by employing the tridentate ligand H3ATD. Crystal structure analyses reveal that complexes 1 and 2 consist of FeIII ions in an octahedral environment where a FeIII ion is coordinated by two HATD2- ligands forming the [FeIII(HATD)2]- core. The shortest cationanion distance between the phosphorus ion of the (PPh4)+ cation and the ferric ion of the [FeIII(HATD)2]- anion is 13.190 Å in complex 1, whereas that between the ferrous ion of the [FeII(Phen)3]2+ cation and the ferric ion of the [FeIII(HATD)2]- anion is 7.821 Å in complex 2. C-HC and C-HO hydrogen interactions between the [FeII(phen)3]2+ cation and the [FeIII(HATD)2]- anion are observed in 2. Face-to-face π-π stacking interactions between naphthalene rings with the separated interplanar center to center distances of 3.421-3.680 Å were observed, which result in a one-dimensional supramolecular chain in complexes 1 and 2. Magnetic measurements show that complex 1 is in the low-spin (LS) state below 500 K, whereas 2 undergoes a high temperature spin crossover (SCO) between 360 and 500 K. Magneto-structural relationship studies reveal that π-stacking, hydrogen interactions and Coulomb interactions between the [FeIII(HATD)2]- anion and the [FeII(phen)3]2+ cation play a crucial role in the high temperature Fe(iii) SCO behaviour of complex 2.

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.

Structure-function correlations in mononuclear manganese(iii) spin crossover systems with a big conjugated hexadentate Schiff-base ligand

This paper reports the syntheses, crystal structures and magnetic properties of spin crossover (SCO) salts of formulae [Mn(naphth-sal-N-1,5,8,12)]SbF6 (1), [Mn(naphth-sal-N-1,5,8,12)]AsF6 (2), [Mn(naphth-sal-N-1,5,8,12)]PF6·1/2CH3OH (3) and [Mn(naphth-sal-N-1,5,8,12)]ClO4 (4), where (naphth-sal-N-1,5,8,12)2- (2,2'-((1E,14E)-2,6,10,14-tetraazapentadeca-1,14-diene-1,15-diyl)diphenolate) is a big conjugated hexadentate Schiff-base ligand. Magnetic susceptibility measurements demonstrated that complexes 1 and 2 showed a gradual one-step SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis. The transition temperatures T1/2 of the SbF6 (1) and AsF6 (2) salts estimated from the magnetic susceptibility measurements are 164 and 171 K, respectively. The existence of the crystal solvent of complex 3 changes the supramolecular packing, leading to close ππ stacking interactions between the phenyl groups of the ligands. These close stacking interactions hinder the flexibility of the whole ligand, precluding the spin transformation of complex 3 and leading to its stabilization in the HS state in the temperature range of 2-300 K. For 4, crystal structure analysis indicates that the reduction in the anion size leads to close contacts between the naphthalene rings. These C-Hπ interactions provide a means for preventing the spin crossover occurring at low temperatures.

High temperature Fe(iii) spin crossover behaviours in three unprecedented FeIII–MII–FeIII (M = Fe, Cd) linear trinuclear complexes

An azo-based ligand of azotetrazolyl-2,7-dihydroxynaphthalene (H₃ATD) was used to synthesize three Fe^III–M^II–Fe^III (M = Fe, Cd) linear trinuclear complexes with different high temperature spin crossover (SCO) behaviours for the terminal Fe^III ions.

Influence of anion induced geometry change in Zn(ii) on the magnetization relaxation dynamics of Dy(iii) in Zn–Dy–Zn complexes

A series of [Zn₂Dy(L₁)₂(OAc)₄](X) where X = (NO₃)_0.92(Br)_0.08 (1), ClO₄ (2), Cl (3) and PF₆ (4) complexes were synthesized and their dc and ac magnetic data investigated. Experimental results are validated through MOLCAS calculations.