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.

Benchmarking magnetic and spectroscopic properties on highly stable 3d metal complexes with tuneable bis(benzoxazol-2-yl)methanide ligands

Two series a and b of 3d metal based complexes 1-4 [M^II{(4-R-NCOC₆H₄)₂CH}₂], (with M = Mn (1), Fe (2), Co (3), Ni (4) and R = H (a) or Me (b)) were synthesised and structurally characterized. The complexes were found to crystallize differently depending on the dication ionic radius and the ligand substitution. All complexes showed remarkable X-ray diffraction resolution that will allow further advanced diffraction experiments. Subsequently, their spectroscopic and magnetic properties were analysed. Complexes 3a and 3b notably show slow magnetic relaxation of their magnetization and represent simple model systems relaxing through a phonon-bottleneck process (3a) or as a field-induced single-molecule magnet (3b, U_eff = 45.0 cm^-1). Remarkably, the magnetic anisotropy in the manganese complex 1b results in induced slow magnetic relaxation. The influence of the dual 4-methylation of the ligands was investigated and found to generate important variations in the physical features of the corresponding complexes. Accessible via one-pot synthesis, these are highly robust against oxidation and moisture. Through smart ligand engineering, they represent stable and tuneable compounds for benchmarking purposes through standard and less-standard characterization methods.

Europium(III), Terbium(III), and Gadolinium(III) Oxamato-Based Coordination Polymers: Visible Luminescence and Slow Magnetic Relaxation

The reaction of aqueous solutions of Eu^III, Tb^III, and Gd^III ions with Na₂Hpcpa [H₃pcpa = N-(4-carboxyphenyl)oxamic acid] afforded three new isostructural oxamate-containing lanthanide(III) coordination polymers of general formula {Ln^III₂(Hpcpa)₃(H₂O)₅·H₂O}_n [Ln = Eu (1),Tb (2), and Gd(3)]. Their structure is made up of neutral zigzag chains running parallel to the [101] direction where double syn-syn carboxylate(oxamate)-bridged dilanthanide(III) pairs (Ln1 and Ln2) are linked by three Hpcpa^2- ligands, one of them with the μ-κ²O,O':κO″ coordination mode and the other two with the μ₃-κ²O,O':κO″:κO'''. Additionally, two of those chains are interlinked through hydrogen bonding and π-π type interactions, resulting in a porous structure with channels where water molecules are hosted. The emission properties of 1 and 2 are evaluated as a function of the temperature, exhibiting an emission in red and green, respectively. The external quantum yield for 2 is approximately 7 times that obtained for 1, indicating that the oxamate ligand is a better sensitizer for Tb^III ions. The temperature dependence of the dc magnetic properties of 1-3 reveals a different magnetic behavior depending on the nature of the Ln^III ion. A continuous decrease of χ_MT occurs for 1 upon cooling, and finally χ_MT tends to vanish, as expected for the thermal depopulation of the six magnetic ⁷F_J excited states (J = 1-6) of the Eu^III ion with a nonmagnetic ⁷F₀ ground state. χ_MT for 2 decreases sharply with decreasing the temperature due to the depopulation of the splitted m_J levels of the ⁷F₇ ground state of the magnetically anisotropic Tb^III ion. A very weak antiferromagnetic interaction between the magnetically isotropic Gd^III ions across the double carboxylate(oxamate) bridge is responsible for the small decrease of χ_MT at low temperatures for 3. The dynamic (ac) magnetic properties of 2 and 3 reveal a slow magnetic relaxation with very incipient frequency-dependent χ_M″ signals below 6.0 K (2) and frequency-dependent χ_M″ peaks below 10.0 K (3) under nonzero applied dc magnetic fields, being thus new examples of field-induced single molecule magnets (SMMs).

Spin crossover crystalline materials engineered via single-crystal-to-single-crystal transformations

This highlight illustrates the latest crystalline materials engineered via SCSC transformations, with emphasis on the onset and progress of spin crossover in a crystal control.

Heterometallic {DyIII2FeII2} grids with slow magnetic relaxation and spin crossover

The self-assembly of a Dy^III ion, an Fe^II ion and a multitopic H₂L ligand produces novel [2 × 2] {DyIII2FeII2} grids exhibiting slow magnetic relaxation and spin crossover.

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.

Unconventional Method for Fabricating Valence Tautomeric Materials: Integrating Redox Center within a Metal-Organic Framework

Due to the structural advantages displayed by Metal-Organic Frameworks (MOFs), integrating Valence Tautomerism (VT) systems within MOFs could be an effective strategy in order to break through the constraints of the traditional ones. Herein, we report the first successful integration of a VT system into a MOF termed VT-MOF-1. The structural characteristics of VT-MOF-1, such as dinuclear cobalt-catechol clusters and solvent-accessible pores, are both innovative and novel, potentially yielding new vitality within VT field. In addition, VT-MOF-1 exhibits specific behaviors responsive to temperature and different solvent molecules as n-butanol, tert-butanol, and isopropyl alcohol. The entropy values and configurations of the solvent molecules might be responsible for the tunable sensing behaviors.

Ion-pair complexes of Schiff base Fe(iii) cations and complex anions

We report on the crystal structure and magnetic properties of four new ion-pair complexes.

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.

Insights into the influence of ethylene group orientation on the iron(iii) spin state in the spin crossover complex [FeIII(Sal2-trien)]. Dalton Trans 2018;47:16040-16043. [PMID: 30387800 DOI: 10.1039/c8dt03619e]

The DFT calculations of the spin crossover complex [FeIII(Sal2-trien)]+ (1) with the following classification of conformers of 1 were performed. The study shows that rearrangements of ethylene group orientation in a coordinated ligand lead to the stabilization of the high-spin or low-spin iron(iii) state.

Lanthanide Discrimination with Hydroxyl-Decorated Flexible Metal-Organic Frameworks

We report two new highly crystalline metal-organic frameworks (MOFs), derived from the natural amino acids serine (1) and threonine (2), featuring hexagonal channels densely decorated with hydroxyl groups belonging to the amino acid residues. Both 1 and 2 are capable of discriminating, via solid-phase extraction, a mixture of selected chloride salts of lanthanides on the basis of their size, chemical affinity, and/or the flexibility of the network. In addition, this discrimination follows a completely different trend for 1 and 2 because of the different locations of the hydroxyl groups in each compound, which is evocative of steric complementarity between the substrate and receptor. Last but not least, the crystal structures of selected adsorbates could be resolved, offering unprecedented snapshots on the capture process and enabling structural correlations with the separation mechanism.

Efficient Capture of Organic Dyes and Crystallographic Snapshots by a Highly Crystalline Amino-Acid-Derived Metal-Organic Framework

The presence of residual organic dyes in water resources or wastewater treatment systems, derived mainly from effluents of different industries, is a major environmental problem with no easy solution. Herein, an ecofriendly, water-stable metal-organic framework was prepared from a derivative of the natural amino acid l-serine. Its functional channels are densely decorated with highly flexible l-serine residues bearing hydroxyl groups. The presence of such a flexible and functional environment within the confined environment of the MOF leads to efficient removal of different organic dyes from water: Pyronin Y, Auramine O, Methylene Blue and Brilliant Green, as unveiled by unprecedented snapshots offered by single-crystal X-ray diffraction. This MOF enables highly efficient water remediation by capturing more than 90 % of dye content, even at very low concentrations such as 10 ppm, which is similar to those usually found in industrial wastewaters. Remarkably, the removal efficiency is improved in simulated contaminated mineral water with multiple dyes.

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.