Modulating magnetic anisotropy in linear tetranuclear dysprosium(III) complexes via coordinated anions

We report the synthesis, structures, and magnetic properties of two novel linear tetranuclear complexes with the general formula [Dy4(Hheb)2(heb)4X2(MeOH)4] (X- = NO3-, OAc-; H2heb = (E)-N'-(1-(2-hydroxyphenyl)ethylidene)benzohydrazide, OAc- = acetate). The rigid ligands (Hheb-/heb2-) incorporate phenoxide groups and bridge the Dy3+ ions in an unusual tetranuclear linear assembly. Notably, we demonstrate through magnetic measurements and theoretical calculations how the anion (X) coordinated at the peripheral Dy3+ centers acts as a switch, significantly changing the magnetic anisotropy of the entire complex. This control over magnetic anisotropy through the selection of the coordinated anion offers a promising avenue for tailoring the functionality of single-molecule magnets.

The effect of co-ligands on the performance of single-molecule magnet behaviours in a family of linear trinuclear Zn-Dy-Zn complexes with a compartmental Schiff base

We present herein magneto-structural studies of three heterometallic Zn2Dy complexes: [Zn2Dy(L)2Cl2(H2O)](ClO4)·4H2O (1), [Zn2Dy(L)2Br2(H2O)](ClO4)·4H2O (2) and [Zn2Dy(L)2(OAc)I(H2O)]I3·4H2O (3), utilizing a new Schiff base ligand, N,N'-bis(3-methoxy-5-methylsalicylidene)-1,2-diaminocyclohexane (H2L). Complexes 1 and 2 exhibit remarkable magnetic relaxation behaviour with relatively high energy barriers in zero field (Ueff: 244 K for 1 and 211 K for 2) and notable hysteresis temperatures, despite the low local geometric symmetry around the central DyIII ions. The SMM performance of these complexes is further enhanced under an applied magnetic field, with Ueff increasing to 309 K for 1 and 269 K for 2, positioning them as elite members within the Zn-Dy SMM family. These findings emphasize the substantial influence of remote modulation on ZnII beyond the first coordination sphere of DyIII ions on their dynamic magnetic relaxation properties. Ab initio studies demonstrate that the relative orientation of the phenoxo-oxygen donor atoms around the DyIII ion is critical for determining the magnetic anisotropy and relaxation dynamics in these systems. Additionally, experimental and theoretical investigations reveal that the coordination of the bridging acetate towards the hard plane, combined with significant distortion from the ideal ZnO2Dy diamond core arrangement caused by the acetate ion, results in low magnetic anisotropy in complex 3, thereby leading to field-induced SMM behaviour. Overall, this study unveils the effects of co-ligands on the SMM performance in a series of linear trinuclear Zn-Dy-Zn complexes, which exhibit low local geometric symmetry around the DyIII centres.

Effect of diamagnetic Zn(II) ions on the SMM properties of a series of trinuclear ZnDy2 and tetranuclear Zn2Dy2 (LnIII = Dy, Tb, Gd) complexes: combined experimental and theoretical studies

To study the effect of diamagnetic ions on magnetic interactions, utilizing a compartmental ligand (Z)-2-(hydroxymethyl)-4-methyl-6-((quinolin-8-ylimino)methyl)phenol (LH2), two different series of ZnII-LnIII complexes, namely the trinuclear series of [DyZn2(L)2(μ2-OAc)2(CH3OH)2]·NO3·MeOH (1), [TbZn2(L)2(μ2-OAc)2(CH3OH)2]·NO3·5MeOH·H2O (2), and [GdZn2(L)2(μ2-OAc)2(CH3OH)2]·NO3·MeOH·CHCl3 (3) and the tetranuclear series of [Dy2Zn2(LH)4(NO3)4(μ2OAc)]·NO3·MeOH·H2O (4), [Tb2Zn2(LH)4(NO3)4(μ2-OAc)]·NO3·MeOH·2H2O (5), and [Gd2Zn2(LH)4(NO3)4(μ2-OAc)]·NO3·MeOH·2H2O (6), were synthesized. Trinuclear ZnII-LnIII complexes 1-3 consist of one LnIII ion sandwiched between two peripheral ZnII ions forming a bent type ZnII-DyIII-ZnII array with an angle of 110.64°. Tetranuclear ZnII-LnIII complexes 4-6 are basically a combination of two dinuclear moieties of [LnZn(LH)2(NO3)2]+ connected by one bidentate bridging acetate ion in μ2-OAc coordination mode. The detailed magnetic analysis reveals that complexes 1 and 4 are single molecule magnets having energy barriers of 34.98 K and 46.71 K with relaxation times (τ0) of 5.05 × 10-4 s and 5.24 × 10-4 s, respectively. Ab initio calculations were employed to analyze the magnetic anisotropy and magnetic exchange interaction between the ZnII and LnIII centers with the aim of gaining better insights into the magnetic dynamics of complexes 1-6.

Water-soluble chiral coordination polymers of Li+, Na+, K+, and Ba2+ with an anionic iron(III) complex of a L-threonine derivative and a significant red shift of visible spectra with Al3+ salt

Four salts of an anionic iron(III) bis-complex, [Fe(LL-thr)2]1-, were synthesized from water or methanol. H2LL-thr is a tridentate ligand derived from the L-threonine amino acid, and the cations used are Li+ (1), Na+ (2), K+ (3), and Ba2+ (4). Single-crystal X-ray diffraction showed that all the complexes are coordination polymers of different dimensionalities. The iron(III) complex binds to cations through its coordinated phenolate and non-coordinated carboxylate oxygen atoms. While Li+ forms a linear chain, all others have a pair of bridged cations intervening the iron(III) complexes. The 3D network of Ba2+ salt has a sizeable solvent-accessible space occupied by aquated chloride ions. The differences in circular dichroism (CD) spectra and significantly lower conductance values in water and methanol support partial retention of the polymeric nature in methanol. The visible spectra of 4 in methanol or water showed an ∼10 nm shift of the charge transfer bands from 3. However, the addition of Al3+ salt to 2 showed a significant colour shift. Further investigation confirmed that the colour shift is due to partial protonation of the complex with protons generated from salt hydrolysis. Most reports on visual aluminium detection consider aluminium's binding as the shift's source. The present results show that protonation due to hydrolysis of aluminium salt can skew the observation.

Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

Structural characterization of a new samarium-sodium heterometallic coordination polymer

Lanthanide-containing materials are of inter-est in the field of crystal engin-eering because of their unique properties and distinct structure types. In this context, a new samarium-sodium heterometallic coordination polymer, poly[tetra-kis-(μ2-2-formyl-6-meth-oxy-phenolato)samarium(III)sodium(I)], {[SmNa(C8H7O3)4]·solvent}n (Sm-1), was synthesized and crystallized via slow evaporation from a mixture of ethanol and aceto-nitrile. The compound features alternating SmIII and NaI ions, which are linked by ortho-vanillin (o-vanillin) ligands to form a mono-periodic chain-like coordination polymer. The chains propagate along the [001] direction. Residual electron density of disordered solvent mol-ecules in the void space could not be reasonably modeled, thus the SQUEEZE function was applied. The structural, vibrational, and optical properties are reported.

Insight into ferromagnetic interactions in CuII-LnIII dimers with a compartmental ligand

In the last two decades, efforts have been devoted to obtaining insight into the magnetic interactions between CuII and LnIII utilizing experimental and theoretical means. Experimentally, it has been observed that the exchange coupling (J) in CuII-LnIII systems is often found to be ferromagnetic for ≥4f7 metal ions. However, exchange interactions at sub-Kelvin temperatures between CuII and the anisotropic/isotropic LnIII ions are not often explored. In this report, we have synthesized a series of heterobimetallic [CuLn(HL)(μ-piv)(piv)2] complexes (LnIII = Gd (1), Tb (2), Dy (3) and Er (4)) from a new compartmental Schiff base ligand, N,N'-bis(3-methoxy-5-methylsalicylidene)-1,3-diamino-2-propanol (H3L). X-ray crystallographic analysis reveals that all four complexes are isostructural and isomorphous. Magnetic susceptibility measurements reveal a ferromagnetic coupling between the CuII ion and its respective LnIII ion for all the complexes, as often observed. Moreover, μ-SQUID studies, at sub-Kelvin temperatures, show S-shaped hysteresis loops indicating the presence of antiferromagnetic coupling in complexes 1-3. The antiferromagnetic interaction is explained by considering the shortest Cu⋯Cu distance in the crystal structure. The nearly closed loops for 1-3 highlight their fast relaxation characteristics, while the opened loops for 4 might arise from intermolecular ordering. CASSCF calculations allow the quantitative assessment of the interactions, which are further supported by BS-DFT calculations.

Part-II: an update of Schiff bases synthesis and applications in medicinal chemistry-a patent review (2016-2023)

INTRODUCTION

Schiff bases are compounds with characteristic features of azomethine linkage (-C=N-). Schiff bases are capable of coordinating with metal ions via azomethine nitrogen. Schiff base derivatives and their metal complexes are known for intriguing novel therapeutic properties. In organic synthesis, the Schiff base reaction is prime in creating the C-N bond. Synthetic accessibility and structural diversity are the salient features for facile synthesis of Schiff base hybrids via a condensation reaction between an aldehyde/ketone and primary amines.

AREA COVERED

This review aims to provide a comprehensive overview of the commendable medicinal applications of Schiff base derivatives and their metal complexes patented from 2016 to 2023.

EXPERT OPINION

Schiff base derivatives are exceptional molecules for their assorted applications in medicinal chemistry. Several Schiff base products are marketed as drugs, and plenty of room is available for the purposive synthesis of new compounds in a diverse pool of disciplines. Expansion in the derivatization of Schiff bases in innumerable directions with multitudinous applications makes them 'magical molecules.' These compounds have proved extraordinary, from medicinal chemistry to other fields outside medicine. This review covers the therapeutic importance of Schiff base derivatives and aims to cover the patents published in recent years (2016-2023).

Combined experimental and theoretical studies on a series of mononuclear LnIII single-molecule magnets: dramatic influence of remote substitution on the magnetic dynamics in Dy analogues

A series of Ln^III complexes of general formula [Ln(H₂L¹)₂(NO₃)₂(H₂O)](NO₃) (1-5) [Ln = Dy (1), Tb (2) Ho (3), Er (4), and Yb (5)] and an analogous Dy^III complex with ligand H₂L², [Dy(H₂L²)₂(NO₃)₃(H₂O)](NO₃) (6), where H₂L¹ and H₂L² stand for (E)-2-[(2-hydroxyphenyl)iminomethyl]-6-methoxy-4-methylphenol and (E)-2-[(2-hydroxy-5-methylphenyl)iminomethyl]-6-methoxy-4-methylphenol, respectively, have been synthesized and magneto-structurally characterized. All these complexes are isostructural and isomorphous, in which the zwitterionic form of the ligands predominantly coordinate the metal centers. The magnetic study revealed that complex 3 displays negligible SMM behaviour, while 1 and 6 are zero field SMMs, the performance of which can largely be improved in the presence of an applied dc field by lowering under barrier relaxation processes, and finally 2, 4, and 5 are field-induced SMMs. The most remarkable observation in the present study is the dramatically-enhanced SMM performance in 6 compared to 1, achieved by only a remote methyl substitution at the ligand framework to increase the intermolecular separation. Although SINGLE_ANISO ab initio calculations for 1 and 6 are very similar, the POLY_ANISO module revealed weak dipolar interactions in both the compounds but significant antiferromagnetic interaction in 1, thereby justifying the experimental fact. The present work discloses that even a small substitution such as a methyl group can adequately increase the intermolecular separation, leading to several-fold enhanced effective energy barrier.

Enantiomeric pairs of copper(II) complexes with tridentate Schiff bases derived from R- and S-methionine: the role of decorating organic groups of the ligand in crystal packing and biological activity

Three enantiomeric pairs consisting of copper(II) complexes with tridentate Schiff bases have been synthesized for employing in biological assessments: 1∞[Cu₂(R/S-salmet)₂(H₂O)] (1-R/S·H2O), 1∞[Cu(R/S-3-HOMe-5-Me-salmet)] (2-R/S), and 1∞[Cu(R/S-3-MeO-salmet)] (3-R/S) (where R/S-salmetH₂, R/S-3-HOMe-5-Me-salmetH₂, and R/S-3-MeO-salmetH₂ result from the condensation of R/S-methionine with salicylaldehyde, 2-hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde, and 3-methoxy-salicylaldehyde, respectively, in a 1 : 1 molar ratio). The crystal structures of 1-R·H2O and 2-R/S are reported. Moreover, the 1-R/S·H2O enantiomers have been subjected to a single-crystal-to-single-crystal (SC-SC) transformation by heating at 160 °C to afford their dehydrated forms, 1∞[Cu₂(R/S-salmet)₂] (1-R/S), whose structures have also been crystallographically determined. The coordination polyhedra of the metal centers, the binding modes of the ligands, and the 1-D double chain assemblies generated by the chiral mononuclear units are comparatively described. The diffuse reflectance UV-Vis and circular dichroism (CD) spectra of compounds 1-R/S·H2O, 1-R/S, and 2-R/S are analysed with respect to their structural peculiarities and compared to those of 3-R/S. The UV-Vis and CD spectra of solutions of 1-R/S, 2-R/S, and 3-R/S point to the collapse of the double chains via dissolution. Biological tests performed on the model eukaryote Saccharomyces cerevisiae indicated low toxicity for 1-R/S, 2-R/S, and 3-R, and moderate toxicity for 3-S. The S-type complexes were accumulated by cells in higher quantity compared to their R-type counterparts due to selective transport via the high-affinity S-methionine transporter, Mup1. A chemogenomic analysis of 3-S toxicity performed on a collection of yeast knockout mutants revealed that most of the deleted genes identified in the screen were involved in the cell response to oxidative stress, calcium-mediated response, or metal homeostasis. Altogether, it was concluded that 3-S accumulation may perturb the redox state of the cell, also interfering with the calcium-mediated response to oxidative stress or metal-related oxidative stress.

From Waste to Schiff Base: Upcycling of Aminolysed Poly(ethylene terephthalate) Product

Recycling plastic waste into valuable materials is one of the contemporary challenges. Every year around 50 million tons of polyethylene terephthalate (PET) bottles are used worldwide. The fact that only a part of this amount is being recycled is putting a burden on the environment. Therefore, a technology that can convert PET-based waste materials into useful ones is highly needed. In the present work, attempts have been made to convert PET-based waste materials into a precursor for others. We report an aminolysed product (3) obtained by aminolysis reaction of PET (1) with 1,2 diaminopropane (DAP, 2) under solvent and catalytic free conditions. The highest amount of monomeric product was obtained upon heating the mixture of diamine and PET at 130 °C. The resulting aminolysed product was then converted to a Schiff-base (5) in 25% yield. The chemical structure of the synthesized compounds was confirmed using multi-spectroscopic techniques. The results of this study will be a valuable addition to the growing body of work on plastic recycling.

Self-assembly of non-macrocyclic triangular Ni3Ln clusters

The synthesis and structural characterisation of four new heterometallic tetranuclear complexes is reported. Three L₃Ni₃Ln type complexes, where Ln = La (C1), Eu (C2), and Gd (C3), have been fully characterised including DC and AC magnetic measurements. A fourth complex featuring a diamagnetic Ba^II ion at its centre is also reported with structural characterisation. Structural elucidation showed that all four complexes successfully self-assembled from a stoichiometric mixture of the acyclic ligand, 1,4-diformylnaphthalene-2,3-diol, with nickel(II) nitrate and the appropriate heavy metal salt to produce the same near planar Ni₃MO₁₂ core. Ferromagnetic interactions were found to dominate the ground state of C3, exhibiting a maximal spin ground state of 13/2. The exchange coupling is quantitatively discussed along with the nickel(II) zero-field splitting effect. AC magnetic susceptibility experiments were carried out, but no frequency dependent signals were observed and thus no observable slow relaxation of magnetisation.

Field-induced mononuclear cobalt(II) single-molecule magnet (SMM) based on a benzothiadiazole-ortho-vanillin ligand

A unique π-conjugated benzothiadiazole-ortho-vanillin ligand (HL), characterized by single crystal X-ray diffraction and DFT calculations, has been prepared by condensation between 4-amino-benzothiadiazole (BTD) and ortho-vanillin. Its reaction with cobalt(II) acetate...

Three angular Zn2Dy complexes showing the effect of remote coordination at Zn and counter ions on slow magnetic relaxation at Dy centres

Three isostructural Zn2Dy complexes displaying the effect of remote coordination at Zn and counter ions on slow magnetic relaxation at Dy centres.

Influence of bridging and chelating co-ligands on the distinct single-molecule magnetic behaviours in ZnDy complexes

Four ZnDy complexes display an effect of bridging/chelating co-ligands on distinct single-molecule magnetic behaviours, relaxing through single to multi relaxation channels.

The first exploration of coordination chemistry using a methyl substituted o-vanillin based ligand: an example starting with Dy4/Zn2Dy2 systems displaying slow relaxation of magnetization

Two butterfly-shaped Dy4 and Zn2Dy2 complexes displaying slow relaxation of magnetization have been synthesized from a new methyl substituted o-vanillin based ligand, enlarging the scope for finding better SMMs.

Synthesis and crystal structure of a solvated CoIII complex with 2-hy-droxy-3-meth-oxy-benzaldehyde thio-semicarbazone ligands

The title CoIII complex, bis-[bis-(2-hy-droxy-3-meth-oxy-benzaldehyde thio-semi-carbazonato)cobalt(III)] di-thio-nate-dimethylformamide-methanol (1/4/3), [Co(C9H10N3O2S)2]2(S2O6)·4C3H7NO·3CH3OH, with monodeprotonated 2-hy-droxy-3-meth-oxy-benzaldehyde thio-semicarbazone as ligands crystallizes in the space group P . The asymmetric unit consists of two mononuclear [CoL 2]+ cations, one di-thio-nate anion (S2O6)2- as counter-anion and seven solvate mol-ecules (four di-methyl-methanamide and three methanol). Each CoIII ion has a moderately distorted octa-hedral S2N2O2 geometry. In the crystal, the components are linked by numerous N-H⋯O and O-H⋯O contacts.

A naphthyl thiourea-based effective chemosensor for fluorescence detection of Ag+ and Zn2

A naphthyl thiourea-based effective chemosensor HNC, (E)-2-(2-hydroxy-3-methoxybenzylidene)-N-(naphthalen-1-yl)hydrazine-1-carbothioamide, was synthesized. HNC showed quick responses toward Ag⁺ and Zn²⁺ through marked fluorescence turn-on in different solvent conditions, respectively. Binding proportions of HNC to Ag⁺ and Zn²⁺ were found to be 2:1 and 1:1, respectively. Detection limits of HNC for Ag⁺ and Zn²⁺ were calculated as 3.82 and 0.21 μM. Binding processes of HNC for Ag⁺ and Zn²⁺ were represented using Job's plot, DFT, ¹ H NMR titration, and ESI-MS.

Ortho-vanillin derived Al(iii) and Co(iii) catalyst systems for switchable catalysis using ε-decalactone, phthalic anhydride and cyclohexene oxide

Switchable catalysis is a useful one-pot method to prepare block polyesters utilising a single catalyst exposed to a mixture of monomers.

Enhancement of the coordinating flexibility in a Schiff–Mannich combo ligand: forced generation of a new NiII–Ophenoxo–LnIII–Oalkoxo–LnIII array (Ln = Gd, Tb, Dy and Ho)

A series of 3d–4f complexes, containing rare NiLn₂O₄ topology, were synthesized by designing a new type of combo Schiff base–Mannich ligand.

Synthesis, structures and magnetic properties of dysprosium(iii) complexes based on amino-bis(benzotriazole phenolate) and nitrophenolates: influence over the slow relaxation of the magnetization

Field-induced slow relaxation in dysprosium amino-bis(benzotriazole) phenolate complexes.

Schiff-base [4]helicene Zn(II) complexes as chiral emitters

The controlled preparation of chiral emissive transition metal complexes is fundamental in the field of circularly polarized luminescence (CPL) active molecular materials. For this purpose, enantiopure Zn(ii) complexes 1 and 2 based on a tetradentate salen ligand surrounded by [4]helicene moieties, together with their racemic counterpart 3, have been herein synthesized. Chirality is primarily brought about by chiral 1,2-cyclohexane-diamines. Alternatively, achiral complex 4 based on ortho-phenylene-diamine has been prepared as well. Single crystal X-ray diffraction analyses have been performed on helicenic intermediates 8 and 9 and complexes 1 and 4. Complexes 1 and 4 display the typical tetradentate O,N,N,O coordination around Zn(ii) characteristic of salen ligands, and bear two [4]helicene moieties. The zinc complexes are luminescent in the visible range around 560 nm at room temperature in aerated solutions with the QY reaching ca. 15% for a luminescence lifetime of 5.5 ns. The optical activities of these complexes have been assessed by CD and CPL, and compared to DFT calculations.

Lanthanide-Based Single-Molecule Magnets Derived from Schiff Base Ligands of Salicylaldehyde Derivatives

The breakthrough in Ln(III)-based SMMs with Schiff base ligands have been occurred for the last decade on account of their magnetic behavior, anisotropy and relaxation pathways. Herein, we review the synthetic strategy, from a structural point of view and magnetic properties of mono, di, tri and polynuclear Ln(III)-based single-molecule magnets mainly with Schiff bases of Salicylaldehyde origin. Special attention has been given to some important breakthroughs that are changing the perspective of this field with a special emphasis on slow magnetic relaxation. An overview of 50 Ln(III)-Schiff base complexes with SMM behavior, covering the period 2008–2020, which have been critical in understanding the magnetic interactions between the Ln(III)-centers, are presented and discussed in detail.