Solid-State Molecular Nanomagnet Inclusion into a Magnetic Metal-Organic Framework: Interplay of the Magnetic Properties

Nanostructures as the Substrate for Single-Molecule Magnet Deposition

Anchoringsingle-molecule magnets (SMMs) on the surface of nanostructures is gaining particular interest in the field of molecular magnetism. The accurate organization of SMMs on low-dimensional substrates enables controlled interactions and the possibility of individual molecules' manipulation, paving the route for a broad range of nanotechnological applications. In this comprehensive review article, the most studied types of SMMs are presented, and the quantum-mechanical origin of their magnetic behavior is described. The nanostructured matrices were grouped and characterized to outline to the reader their relevance for subsequent compounding with SMMs. Particular attention was paid to the fact that this process must be carried out in such a way as to preserve the initial functionality and properties of the molecules. Therefore, the work also includes a discussion of issues concerning both the methods of synthesis of the systems in question as well as advanced measurement techniques of the resulting complexes. A great deal of attention was also focused on the issue of surface-molecule interaction, which can affect the magnetic properties of SMMs, causing molecular crystal field distortion or magnetic anisotropy modification, which affects quantum tunneling or magnetic hysteresis, respectively. In our opinion, the analysis of the literature carried out in this way will greatly help the reader to design SMM-nanostructure systems.

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.

Magnetic Properties and Electronic Structure of the S = 2 Complex [MnIII{(OPPh2)2N}3] Showing Field-Induced Slow Magnetization Relaxation

The high-spin S = 2 Mn(III) complex [Mn{(OPPh₂)₂N}₃] (1_Mn) exhibits field-induced slow relaxation of magnetization (Inorg. Chem. 2013, 52, 12869). Magnetic susceptibility and dual-mode X-band electron paramagnetic resonance (EPR) studies revealed a negative value of the zero-field-splitting (zfs) parameter D. In order to explore the magnetic and electronic properties of 1_Mn in detail, a combination of experimental and computational studies is presented herein. Alternating-current magnetometry on magnetically diluted samples (1_Mn/1_Ga) of 1_Mn in the diamagnetic gallium analogue, [Ga{(OPPh₂)₂N}₃], indicates that the slow relaxation behavior of 1_Mn is due to the intrinsic properties of the individual molecules of 1_Mn. Investigation of the single-crystal magnetization of both 1_Mn and 1_Mn/1_Ga by a micro-SQUID device reveals hysteresis loops below 1 K. Closed hysteresis loops at a zero direct-current magnetic field are observed and attributed to fast quantum tunneling of magnetization. High-frequency and -field EPR (HFEPR) spectroscopic studies reveal that, apart from the second-order zfs terms (D and E), fourth-order terms (B₄^m) are required in order to appropriately describe the magnetic properties of 1_Mn. These studies provide accurate spin-Hamiltonian (sH) parameters of 1_Mn, i.e., zfs parameters |D| = 3.917(5) cm^-1, |E| = 0.018(4) cm^-1, B⁰₄ = B₄² = 0, and B₄⁴ = (3.6 ± 1.7) × 10^-3 cm^-1 and g = [1.994(5), 1.996(4), 1.985(4)], and confirm the negative sign of D. Parallel-mode X-band EPR studies on 1_Mn/1_Ga and CH₂Cl₂ solutions of 1_Mn probe the electronic-nuclear hyperfine interactions in the solid state and solution. The electronic structure of 1_Mn is investigated by quantum-chemical calculations by employing recently developed computational protocols that are grounded on ab initio wave function theory. From computational analysis, the contributions of spin-spin and spin-orbit coupling to the magnitude of D are obtained. The calculations provide also computed values of the fourth-order zfs terms B₄^m, as well as those of the g and hyperfine interaction tensor components. In all cases, a very good agreement between the computed and experimentally determined sH parameters is observed. The magnetization relaxation properties of 1_Mn are rationalized on the basis of the composition of the ground-state wave functions in the absence or presence of an external magnetic field.

Hydrolase-like catalysis and structural resolution of natural products by a metal-organic framework

The exact chemical structure of non-crystallising natural products is still one of the main challenges in Natural Sciences. Despite tremendous advances in total synthesis, the absolute structural determination of a myriad of natural products with very sensitive chemical functionalities remains undone. Here, we show that a metal-organic framework (MOF) with alcohol-containing arms and adsorbed water, enables selective hydrolysis of glycosyl bonds, supramolecular order with the so-formed chiral fragments and absolute determination of the organic structure by single-crystal X-ray crystallography in a single operation. This combined strategy based on a biomimetic, cheap, robust and multigram available solid catalyst opens the door to determine the absolute configuration of ketal compounds regardless degradation sensitiveness, and also to design extremely-mild metal-free solid-catalysed processes without formal acid protons.

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.

Macromolecular Probe Based on a NiII/TbIII Coordination Polymer for Sensitive Recognition of Human Serum Albumin (HSA) and MnO4. ACS OMEGA 2019;4:11949-11959. [PMID: 31460306 PMCID: PMC6682116 DOI: 10.1021/acsomega.8b03326]

Reported here are the design and fluorescence characters of a Ni^II/Tb^III polymer, [Tb₂Ni₃(HCAM)₆(H₂O)₁₂] _n (1) (H₃CAM = chelidamic acid). Under physiological conditions, the binding of biocompatible water soluble 1 to human serum albumin (HSA) was studied by spectroscopy techniques, which revealed that 1 could inherent the fluorescent light of HSA in a static quencher course and change the HSA second-level structure. The marked enhancement in 1 and its fluorescence intensity provide conclusive evidence that 1 can play the role of a "turn-on" sensor for recognition and detection of HSA in other biological interferents with a K _sv value of 7.68 × 10⁴ M^-1 and a detection limit of 0.14 μM. Luminescence experiments show that 1 has high selectivity and sensitivity to MnO₄ ^- in other anions. Its quenching efficiency (K _sv) is 5.54 × 10³ M^-1, and the detection limit is 0.29 μM.

Solvent-mediated structural transformations of copper(II) coordination polymers induced by different short-chain alcohols

A three-dimensional copper(II) coordination polymer (CP), {Cu(L-F)(N₃)}_n (1), was synthesized by reacting Cu(NO₃)₂ with 5-fluoronicotinic acid (HL-F) and NaN₃ in a water medium. Complex (1) shows a 3D network, in which the 1D [Cu₂(COO)N₃]_n chains are interconnected via L-F ligands. By immersing (1) into different short-chain alcohols (CH₃OH, C₂H₅OH and HOC₂H₄OH), three different CPs were isolated, including {Cu₃(L-F)₄(N₃)₂(CH₃OH)₂}_n (2), {Cu₃(L-F)₄(N₃)₂(C₂H₅OH)₂}_n (3) and {Cu_2.5(L-F)₃(N₃)₂(HOC₂H₄OH)_0.5}_n (4). CPs (2) and (3) display a similar structure, in which trinuclear subunit [Cu₃(COO)₂(N₃)₂(solvent)₂] is generated. Furthermore, such entities are interconnected via L-F ligands to give rise to a 3D network. As for (4), there are trinuclear [Cu₃(COO)₂(N₃)₂] and binuclear [Cu₂(COO)N₃] units, which are interconnected by L-F ligands to generate a 3D network. Notably, in (2) and (3), the coordination modes of CH₃OH and C₂H₅OH solvents are monodentate; whereas for (4), the HOC₂H₄OH solvent adopts a bridging mode to link two Cu atoms. Of further interest, these processes are solvent-mediated structural transformations, with obvious colour changes in the crystals. Structural changes and mechanisms of transformation are discussed in detail.

Metal Organic Frameworks Based Materials for Heterogeneous Photocatalysis

The increase in environmental pollution due to the excessive use of fossil fuels has prompted the development of alternative and sustainable energy sources. As an abundant and sustainable energy, solar energy represents the most attractive and promising clean energy source for replacing fossil fuels. Metal organic frameworks (MOFs) are easily constructed and can be tailored towards favorable photocatalytic properties in pollution degradation, organic transformations, CO₂ reduction and water splitting. In this review, we first summarize the different roles of MOF materials in the photoredox chemical systems. Then, the typical applications of MOF materials in heterogeneous photocatalysis are discussed in detail. Finally, the challenges and opportunities in this promising field are evaluated.

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.

Design and synthesis of a family of 1D-lanthanide-coordination polymers showing luminescence and slow relaxation of the magnetization

We have designed and synthesized eight isostructural 1D coordination polymers (CPs) with the general formula {[Ln(aapc)3(DMF)]}n [where Ln(iii) = Y (2), La (3), Nd (4), Eu (5), Gd (6), Tb (7), Dy (8), Er (9); and aapc = 3-((anthraquinone-1-yl)amino)propanoate]. These CPs consist of Ln-carboxylate infinite rods in which the bulky anthraquinone scaffolds arise from it in such a way that the resulting supramolecular packing exhibits isolated 1D chains. Solution structures have been corroborated through NMR methods including PGSE and EXSY NMR studies and, due to the presence of lanthanide ions, magnetic and luminescence properties have been studied. Alternating current magnetic measurements of compound 8 show slow relaxation of the magnetization, a characteristic of single molecule magnets (SMMs). The evaluation of solid-state photophysical properties reveals that the aapc scaffold sensitizes lanthanide(iii) based emission of compounds 4-9 both in the visible and near-infrared (NIR) regions at 10 K.

Electron Spin Resonance Study of Molecular Orientation and Dynamics of Phenyl Imino and Nitronyl Nitroxide Radicals in Organic 1D Nanochannels of Tris( o-phenylenedioxy)cyclotriphosphazene

Imino and nitronyl nitroxide (IN and NN, respectively) radicals such as phenyliminonitroxide (PhIN) and phenylnitronylnitroxide (PhNN), respectively, were dispersed in the organic 1D nanochannels of tris( o-phenylenedioxy)cyclotriphosphazene (TPP). Electron spin resonance (ESR) measurements were conducted on these inclusion compounds (ICs) in the temperature range 4.2-300 K. The modulated-septet ESR spectra of TPP ICs using PhIN observed in the range 165-258 K were reproduced with the EasySpin program package using a model in which some of the PhIN molecules underwent uniaxial rotational diffusion in the TPP nanochannels around the molecular long axis corresponding to the principal y-axis of the g tensor. However, for the TPP IC using PhNN, complicated ESR spectra were observed, which were not consistent with the modulated quintet observed in solution or in the fast-motion limit in solids. These spectra were reproduced by the superposition of a quintet originating from rotational diffusion of PhNN molecules and a septet based on rotational diffusion of PhIN molecules generated in the synthetic process. The rotational diffusion activation energies of PhIN and PhNN in the TPP nanochannels were estimated to be 19 and 45 kJ mol^-1 using an Arrhenius plot, respectively. These were consistent with that for NN radicals in the 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) (37-54 kJ mol^-1) with a larger pore diameter than TPP reported in our previous study, or with that for 4-substituted-2,2,6,6-tetramethyl-1-piperidinyloxyl (4-X-TEMPO) in TPP nanochannels (5-26 kJ mol^-1) with regard to molecular size or host-guest or guest-guest interactions. These results indicate that not only the NN group but also IN may be used for the clarification of chemical or biological structures of nanomaterials such as nanosized cavities.

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.

The effect of the disposition of coordinated oxygen atoms on the magnitude of the energy barrier for magnetization reversal in a family of linear trinuclear Zn-Dy-Zn complexes with a square-antiprism DyO8 coordination sphere

A series of trimetallic Zn-Dy-Zn complexes of the general formula [ZnX(μ-L)Dy(μ-L)XZn]Y·nS, where H₂L is the compartmental ligand N,N'-dimethyl-N,N'-bis(2-hydroxy-3-formyl-5-bromobenzyl)ethylenediamine, X is the coligand (X = Cl, Br, I and N₃), Y is the counteranion and S are the crystallization solvent molecules have been synthesized and magnetically characterized. In all these complexes, the Dy(iii) ions exhibit DyO₈ coordination environments with a slightly distorted square-antiprism D_4d symmetry. Due to the disposition of the oxygen atoms around the Dy(iii) ions, large easy-axis anisotropy is expected, which is responsible for the high thermal energy barriers for the reversal of the magnetization observed at zero field (in the 144-170 K range for all complexes). A preliminary correlation between the disposition of the oxygen atoms of the ligand (phenoxo and aldehyde) in the DyO₈ coordination sphere and the value of U_eff has been established.

2D and 3D mixed MII/CuIImetal–organic frameworks (M = Ca and Sr) withN,N′-2,6-pyridinebis(oxamate) and oxalate: preparation and magneto-structural study

Heterobimetallic coordination polymers with either antiferro- of ferromagnetic interactions are described herein.

Porous substrates as platforms for the nanostructuring of molecular magnets

This article highlights recent advances in the newly emerging field on the nanostructuration of molecular magnets using porous substrates.

Reversible solvatomagnetic switching in a single-ion magnet from an entatic state

A vast impact on molecular nanoscience can be achieved using simple transition metal complexes as dynamic chemical systems to perform specific and selective tasks under the control of an external stimulus that switches "ON" and "OFF" their electronic properties. While the interest in single-ion magnets (SIMs) lies in their potential applications in information storage and quantum computing, the switching of their slow magnetic relaxation associated with host-guest processes is insufficiently explored. Herein, we report a unique example of a mononuclear cobalt(ii) complex in which geometrical constraints are the cause of easy and reversible water coordination and its release. As a result, a reversible and selective colour and SIM behaviour switch occurs between a "slow-relaxing" deep red anhydrous material (compound 1) and its "fast-relaxing" orange hydrated form (compound 2). The combination of this optical and magnetic switching in this new class of vapochromic and thermochromic SIMs offers fascinating possibilities for designing multifunctional molecular materials.

Syntheses, structures, and magnetic properties of two unique Cu(ii)-based coordination polymers involving a crystal-to-crystal structural transformation from a 1D chain to a 3D network

Promoted by the DMF solvent, the 1D chain structure of 1 can be irreversibly transformed into the 3D sod network structure of 2 in a crystal-to-crystal fashion, which is accompanied by a drastic magnetic change.

Controlling the magnetic properties of dysprosium metallofullerene within metal–organic frameworks

The hysteresis and quantum tunneling of magnetism for a metallofullerene DySc₂N@C₈₀ single-molecule magnet were modulated through trapping in MOF-177.

Spin-crossover complex encapsulation within a magnetic metal–organic framework

We report the solid-state incorporation of a mononuclear iron(iii) spin-crossover (SCO) complex within the pores of a magnetic metal–organic framework (MOF).

Hydrogen bond-directed encapsulation of metalloporphyrin into the microcages of zeolite imidazolate frameworks for synergistic biomimetic catalysis

In efforts to replicate the 3D model and desirable function of haemoglobin, the zeolite imidazolate framework (ZIF-8) was delineated for an ideal host matrix to accommodate custom-designed porphyrin molecules via hydrogen bonding.