Insertion of a [FeII(pyimH)3]2+[pyimH = 2-(1H-Imidazol-2-yl)pyridine] Spin-Crossover Complex Inside a Ferromagnetic Lattice Based on a Chiral 3D Bimetallic Ox­alate Network

Homochiral Mn3+ Spin-Crossover Complexes: A Structural and Spectroscopic Study

Structural, magnetic, and spectroscopic data on a Mn³⁺ spin-crossover complex with Schiff base ligand 4-OMe-Sal₂323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal₂323)]X where X = ClO₄^– (1), BF₄^– (2), NO₃^– (3), Br^– (4), and I^– (5) crystallize isotypically in the chiral orthorhombic space group P2₁2₁2 with a range of spin state preferences for the [Mn(4-OMe-Sal₂323)]⁺ complex cation over the temperature range 5–300 K. Complexes 1 and 2 are high-spin, complex 4 undergoes a gradual and complete thermal spin crossover, while complexes 3 and 5 show stepped crossovers with different ratios of spin triplet and quintet forms in the intermediate temperature range. High-field electron paramagnetic resonance was used to measure the zero-field splitting parameters associated with the spin triplet and quintet states at temperatures below 10 K for complexes 4 and 2 with respective values: D_S=1 = +23.38(1) cm^–1, E_S=1 = +2.79(1) cm^–1, and D_S=2 = +6.9(3) cm^–1, with a distribution of E parameters for the S = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex 1 at room temperature reveal a 2:1 ratio of enantiomers in the chiral conglomerate, and solution CD measurements on the same sample in methanol show that it is stable toward racemization. Solid-state UV–vis absorption spectra on high-spin complex 1 and mixed S = 1/S = 2 sample 5 reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal₂323)]⁺ in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal₂323 ligand.

Zero-field splitting parameters of the spin triplet and quintet forms of a spin-crossover Mn³⁺ complex stabilized in lattices with different counterions are measured by high-field electron paramagnetic resonance at different frequencies. The homochiral crystallization of the enantiopure Δ or Λ forms of the chelate complex, despite the use of achiral anions, is attributed to the steric influence of the ligand substituent.

Sorption and Magnetic Properties of Oxalato-Based Trimetallic Open Framework Stabilized by Charge-Assisted Hydrogen Bonds

We report a new structure of {[Co(bpy)₂(ox)][{Cu₂(bpy)₂(ox)}Fe(ox)₃]}_n·8.5nH₂O NCU-1 presenting a rare ladder topology among oxalate-based coordination polymers with anionic chains composed of alternately arranged [Cu₂(bpy)₂(ox)]²⁺ and [Fe(ox)₃]³⁻ moieties. Along the a axis, they are separated by Co(III) units to give porous material with voids of 963.7 ų (16.9% of cell volume). The stability of this structure is assured by a network of stacking interactions and charge-assisted C-H…O hydrogen bonds formed between adjacent chains, adjacent cobalt(III) units, and alternately arranged cobalt(III) and chain motifs. The soaking experiment with acetonitrile and bromobenzene showed that water molecules (8.5 water molecules dispersed over 15 positions) are bonded tightly, despite partial occupancy. Water adsorption experiments are described by a D’arcy and Watt model being the sum of Langmuir and Dubinin–Serpinski isotherms. The amount of primary adsorption sites calculated from this model is equal 8.2 mol H₂O/mol, being very close to the value obtained from the XRD experiments and indicates that water was adsorbed mainly on the primary sites. The antiferromagnetic properties could be only approximately described with the simple Cu^II-ox-Cu^II dimer using H = −J·S₁·S₂, thus, considering non-trivial topology of the whole Cu-Fe chain, we developed our own general approach, based on the semiclassical model (SC) and molecular field (MF) model, to describe precisely the magnetic superexchange interactions in NCU-1. We established that Cu(II)-Cu(II) coupling dominates over multiple Cu(II)-Fe(III) interactions, with J_CuCu = −275(29) and J_CuFe = −3.8(1.6) cm⁻¹ and discussed the obtained values against the literature data.

Insertion of single-ion magnets based on mononuclear Co(II) complexes into ferromagnetic oxalate-based networks

The 1 : 2 and 1 : 1 Co(ii) complexes of the L ligand (L = 6-(3,5-diamino-2,4,6-triazinyl)2,2'-bipyridine) with formulas [CoII(L)2](ClO4)2·0.5MeCN·Et2O (1) and [CoII(L)(CH3CN)2(H2O)](ClO4)2·MeCN (2) have been prepared. The structural and magnetic characterization of the two compounds shows that they contain octahedral high-spin Co(ii) and present a field-induced slow relaxation of the magnetization. 1 has been inserted into a bimetallic oxalate-based network leading to a novel achiral 3D compound of formula [CoII(L)2][MnIICrIII(ox)3]2·(solvate) (3) exhibiting ferromagnetic ordering below 4.6 K. EPR measurements suggest a weak magnetic coupling between the two sublattices.

Metal-Organic Framework Magnets

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components. Despite these attributes, the realization of metal-organic magnets with high ordering temperatures represents a formidable challenge, owing largely to the typically weak magnetic exchange coupling mediated through organic linkers. Nevertheless, recent years have seen a number of exciting advances involving frameworks based on a wide range of metal ions and organic linkers. This review provides a survey of structurally characterized metal-organic frameworks that have been shown to exhibit magnetic order. Section 1 outlines the need for new magnets and the potential role of metal-organic frameworks toward that end, and it briefly introduces the classes of magnets and the experimental methods used to characterize them. Section 2 describes early milestones and key advances in metal-organic magnet research that laid the foundation for structurally characterized metal-organic framework magnets. Sections 3 and 4 then outline the literature of metal-organic framework magnets based on diamagnetic and radical organic linkers, respectively. Finally, Section 5 concludes with some potential strategies for increasing the ordering temperatures of metal-organic framework magnets while maintaining structural integrity and additional function.

A spin crossover porous hybrid architecture for potential sensing applications

Spin crossover cations have been successfully synthesized in the pores of a mesoporous robust Metal–Organic Framework (MOF) MIL-100(Al) through sequential introduction of Fe(iii) cations and a sal₂trien ligand.

Magnetic functionalities in MOFs: from the framework to the pore

In this review, we show the different approaches developed so far to prepare metal-organic frameworks (MOFs) presenting electronic functionalities, with particular attention to magnetic properties. We will cover the chemical design of frameworks necessary for the incorporation of different magnetic phenomena, as well as the encapsulation of functional species in their pores leading to hybrid multifunctional MOFs combining an extended lattice with a molecular lattice.

Design of 3d–4f molecular squares through the [Fe{(HB(pz)3)}(CN)3]− metalloligand

A new series of cyanido-bridged {FeIII2LnIII2} heterobimetallic molecular squares [Ln = La (1), Gd (2), Tb (3) and Dy (4)] has been prepared and magneto-structurally investigated.

Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2 ]2+ Derivatives

The following iron(II) complexes of 2,6-bis(oxazolinyl)pyridine (PyBox; L^H ) derivatives are reported: [Fe(L^H )₂ ][ClO₄ ]₂ (1); [Fe((R)-L^Me )₂ ][ClO₄ ]₂ ((R)-2; L^Me =2,6-bis{4-methyloxazolinyl}pyridine); [Fe((R)-L^Ph )₂ ][ClO₄ ]₂ ((R)-3) and [Fe((R)-L^Ph )((S)-L^Ph )][ClO₄ ]₂ ((RS)-3; L^Ph =2,6-bis{4-phenyloxazolinyl}pyridine); and [Fe((R)-L^iPr )₂ ][ClO₄ ]₂ ((R)-4) and [Fe((R)-L^iPr )((S)-L^iPr )][ClO₄ ]₂ ((RS)-4; L^iPr =2,6-bis{4-isopropyloxazolinyl}pyridine). Solid (R)-3⋅MeNO₂ exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T1/2 of 350 K. The discontinuity around 240 K lies well below T1/2 , and is unconnected to a crystallographic phase change occurring at 170 K. Rather, it can be correlated with a gradual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-crossover above room temperature (1 and (RS)-3), or remain high-spin between 5-300 K [(R)-2, (R)-4 and (RS)-4]. Homochiral (R)-3 and (R)-4 exhibit more twisted ligand conformations and coordination geometries than their heterochiral isomers, which can be attributed to steric clashes between ligand substituents [(R)-3]; or, between the isopropyl substituents of one ligand and the backbone of the other ((R)-4). In solution, (RS)-3 retains its structural integrity but (RS)-4 undergoes significant racemization through ligand redistribution by ¹ H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD₃ CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.

Photomagnetic properties of an Fe(ii) spin-crossover complex of 6-(3,5-diamino-2,4,6-triazinyl)-2,2′-bipyridine and its insertion into 2D and 3D bimetallic oxalate-based networks

The Fe(ii) complex of 6-diaminotriazyl-2,2′-bipyridine shows the LIESST effect and has been used as a countercation of two oxalate-based ferromagnets.