Slow Magnetic Relaxation in Silver(II) Macrocyclic Systems.

The spin-lattice relaxation time has been studied trough alternating-current susceptometry and ultralow-frequency Raman spectroscopy in a family of silver(II)-derived molecular systems with spin 1/2 and formulas [AgII(m-CTH)(NO3)2] (1) and [AgII(m-CTH)(ClO4)2] (2), where CTH = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. The combination of both techniques demonstrates the occurrence of slow spin magnetic relaxation induced by spin-phonon interaction. The magnetic behavior of these silver(II)-derived systems opens the door to a new cation in the scarce family of S = 1/2 systems with slow relaxation of magnetization.

Novel Co5 and Ni4 Metal Complexes and Ferromagnets by the Combination of 2-Pyridyl Oximes with Polycarboxylic Ligands.

The use of 2-pyridyl oximes in metal complexes chemistry has been extensively investigated in the last few decades as a fruitful source of species with interesting magnetic properties. In this work, the initial combination of pyridine-2-amidoxime (pyaoxH2) and 2-methyl pyridyl ketoxime (mpkoH) with isonicotinic acid (HINA) and 3,5-pyrazole dicarboxylic acid (H3pdc) has provided access to three new compounds, [Ni4(INA)2(pyaox)2(pyaoxH)2(DMF)2] (1), [Co5(mpko)6(mpkoH)2(OMe)2(H2O)](ClO4)6 (2), and [Co5(OH)(Hpdc)5(H2pdc)] (3). 1 displays a square-planar metal topology, being the first example that bears simultaneously HINA and pyaoxH2 in their neutral or ionic form. The neighbouring Ni4 units in 1 are held together through strong intermolecular hydrogen bonding interactions, forming a three-dimensional supramolecular framework. 2 and 3 are mixed-valent Co4IIICoII and Co2IIICoII3 compounds with a bowtie and trigonal bipyramidal metal topology, accordingly. Direct current and alternate current magnetic susceptibility studies revealed that the exchange interactions between the NiII ions in 1 are ferromagnetic (J = 1.79(4) cm-1), while 2 exhibits weak AC signals in the presence of a magnetic field. The syntheses, crystal structures, and magnetic properties of 1-3 are discussed in detail.

Family of Isomeric CuII-LnIII (Ln = Gd, Tb, and Dy) Complexes Presenting Field-Induced Slow Relaxation of Magnetization Only for the Members Containing GdIII.

The strategic design and synthesis of two isomeric CuII complexes, [CuLA] and [CuLB], of asymmetrically dicondensed N2O3-donor Schiff-base ligands (where H2LA and H2LB are N-salicylidene-N'-3-methoxysalicylidenepropane-1,2-diamine and N-3-methoxysalicylidene-N'-salicylidenepropane-1,2-diamine, respectively) have been accomplished via a convenient CuII template method. These two complexes have been used as metalloligands for the synthesis of three pairs of Cu-Ln isomeric complexes [CuL(µ-NO3)Ln(NO3)2(H2O)]·CH3CN (for complexes 1A-3A, L = LA, and for complexes 1B-3B, L = LB and Ln = Gd, Tb, and Dy, respectively), all of which have been characterized structurally. In all six isomorphous and isostructural complexes, the decacoordinated LnIII centers and pentacoordinated CuII centers possess sphenocorona and square-pyramidal geometries, respectively. The isomeric pair of Cu-Gd compounds shows field-induced slow relaxation of magnetization, although they present the typical isotropic behavior of GdIII complexes, indicating that slow relaxation is not due to the usual energy barrier originating from the magnetic anisotropy. The isostructural derivatives with the ion-anisotropic lanthanides TbIII and DyIII do not show slow magnetic relaxation with or without a direct-current bias field, demonstrating that the magnetic response of the isotropic system CuII-GdIII occurs through different mechanisms than the rest of the Ln cations.

Single-Ion Anisotropy and Intramolecular Interactions in CeIII and NdIII Dimers.

This article reports the syntheses, characterization, structural description, together with magnetic and spectroscopic properties of two isostructural molecular magnets based on the chiral ligand N,N'-bis((1,2-diphenyl-(pyridine-2-yl)methylene)-(R,R/S,S)-ethane-1,2-diamine), L1, of general formula [Ln2(RR-L1)2(Cl6)]·MeOH·1.5H2O, (Ln = Ce (1) or Nd (2)). Multifrequency electron paramagnetic resonance (EPR), cantilever torque magnetometry (CTM) measurements, and ab initio calculations allowed us to determine single-ion magnetic anisotropy and intramolecular magnetic interactions in both compounds, evidencing a more important role of the anisotropic exchange for the NdIII derivative. The comparison of experimental and theoretical data indicates that, in the case of largely rhombic lanthanide ions, ab initio calculations can fail in determining the orientation of the weakest components, while being reliable in determining their principal values. However, they remain of paramount importance to set the analysis of EPR and CTM on sound basis, thus obtaining a very precise picture of the magnetic interactions in these systems. Finally, the electronic structure of the two complexes, as obtained by this approach, is consistent with the absence of zero-field slow relaxation observed in ac susceptibility.

Exploring the Role of Intramolecular Interactions in the Suppression of Quantum Tunneling of the Magnetization in a 3d-4f Single-Molecule Magnet.

Hydroxide-bridged FeIII4LnIII2 clusters having the general formula [Fe4Ln2(µ3-OH)2(mdea)6(SCN)2(NO3)2(H2O)2]·4H2O·2MeCN {Ln = Y (1), Dy (2), mdea = N-methyldiethanolamine} were synthesized and magnetically characterized. The thermal relaxation of the magnetization for 2 and the diluted FeIII4DyIIIYIII complex 3 (with and without applied field) has been analyzed. The diluted sample shows a dominant QTM at low temperatures that can be removed with a 0.15 T dc field. Both 2 and 3 show moderately high Ueff barriers and exhibit hysteresis loops until 5 K.

Holmium(III) Single-Ion Magnet for Cryomagnetic Refrigeration Based on an MRI Contrast Agent Derivative.

The coexistence of field-induced blockage of the magnetization and significant magnetocaloric effects in the low-temperature region occurs in a mononuclear holmium(III) diethylenetriamine-N,N,N',N″,N″-pentaacetate complex, whose gadolinium(III) analogue is a commercial MRI contrast agent. Both properties make it a suitable candidate for cryogenic magnetic refrigeration, thus enlarging the variety of applications of this simple class of multifunctional molecular nanomagnets.

ZnII and CuII-Based Coordination Polymers and Metal Organic Frameworks by the of Use of 2-Pyridyl Oximes and 1,3,5-Benzenetricarboxylic Acid.

The simultaneous use of 2-pyridyl oximes (pyridine-2 amidoxime, H2pyaox; 2-methyl pyridyl ketoxime, Hmpko) and 1,3,5-benzenetricarboxylic acid (H3btc) provided access to five new compounds, namely [Zn(H2btc)2(H2pyaox)2]•2H2O (1•2H2O), [Zn(Hbtc)(H2pyaox)2]n (2), [Cu(Hbtc)(H2pyaox)]n (3), [Cu(Hbtc)(HmpKo)]n (4) and [Cu2(Hbtc)2(Hmpko)2(H2O)2]•4H2O (5•4H2O). Among them, 3 is the first example of a metal-organic framework (MOF) containing H2pyaox. Its framework can be described as a 3-c uninodal net of hcb topology with the layers being parallel to the (1,0,1) plane. Furthermore, 3 is the third reported MOF based on a 2-pyridyl oxime in general. 2 and 4 are new members of a small family of coordination polymers containing an oximic ligand. 1-5 form 3D networks through strong intermolecular interactions. Dc magnetic susceptibility studies were carried out in a crystalline sample of 3 and revealed the presence of weak exchange interactions between the metal centres; the experimental data were fitted to a theoretical model with the fitting parameters being J = -0.16(1) cm-1 and g = 2.085(1). The isotropic g value was also confirmed by electronic paramagnetic resonance (EPR) spectroscopy. Reactivity studies were performed for 3 in the presence of metal ions; the reaction progress was studied and discussed for Fe(NO3)3 by the use of several characterization techniques, including single crystal X-ray crystallography and IR spectroscopy.

From Bowls to Capsules: Assembly of Hexanuclear NiII Rings Tailored by Alkali Cations.

An anionic hexanuclear NiII metallamacrocycle with endo and exo linking sites has been employed as a building block to generate a series of capsules and bowls of nanometric size. The supramolecular arrangement of the {Ni6 } rings was tailored by the size of the alkali cations, showing the transition from {Ni6 -M2 -Ni6 } capsules (M=LiI and NaI ) to {Ni6 -M} bowls (M=KI and CsI ). The alkyl co-cations are determinant to stabilize the assemblies by means of CH⋅⋅⋅π interactions on the exo side of the metallamacrocycles. The effect on the topology of the supramolecular assemblies of the cation size, cation charge, Et3 NH+ or Me4 N+ counter cations has been analyzed. Magnetic measurements reveal the presence of ferromagnetic and antiferromagnetic interactions inside the rings that allow a S=0 ground state.

Chiral Versus Non-Chiral [MnIII 6 MnII NaI ], [MnIII 6 MnII 2 NaI 2 ] and [MnIII 3 MnII NaI ] Clusters Derived from Schiff Bases or the Fight for Symmetry.

The reaction in basic media of manganese chloride with Schiff bases derived from the condensation of o-vanillin with different chiral/racemic aminoalcohols yielded in a family of complexes in which the nuclearity, symmetry and magnetic behavior is controlled by changing the position of the chiral carbon. Chiral and racemic clusters with [MnIII 6 MnII NaI ], [MnIII 6 MnII 2 NaI 2 ] and [MnIII 3 MnII NaI ] metallic core have been structurally and magnetically characterized. The racemic clusters with an odd number of chiral ligands exhibit the anomalous mixing of ligands with different conformation. Related racemic compounds have been reviewed.

Multifunctionality in Two Families of Dinuclear Lanthanide(III) Complexes with a Tridentate Schiff-Base Ligand.

The employment of N-(2-carboxyphenyl)salicylideneimine in 4f metal chemistry has led to two families of dinuclear complexes depending on the lanthanide(III) used. Representative members exhibit interesting magnetic, optical, and catalytic properties.

Chiral [MnII MnIII 3 M'] (M'=NaI , CaII , MnII ) and [MnII MnIII 6 NaI 2 ] Clusters Built from an Enantiomerically Pure Schiff Base: Synthetic, Chiroptical, and Magnetic Properties.

From the reaction of manganese halides with the chiral Schiff bases obtained by condensation of o-vanillin and (R)- or (S)-phenylglycinol, 11 complexes based on pentanuclear cages with trigonal bipyramidal [MnII MnIII 3 M'] (M'=NaI , CaII , MnII ) or enneanuclear [MnII MnIII 6 NaI 2 ] cores were synthesized. Structural, supramolecular chirality, and optical properties were explored. The magnetic properties of related systems were reviewed, and the magnetic response of the new systems was rationalized to the bond parameters.

From Mesocates to Helicates: Structural, Magnetic and Chiro-Optical Studies on Nickel(II) Supramolecular Assemblies Derived from Tetradentate Schiff Bases.

The systematic reactions of a family of tetradentate pyridyl/imine and quinolyl/imine racemic or enantiopure Schiff bases with Ni(NO3 )2 or Ni(ClO4 )2 in the presence of sodium azide yielded, as a function of the starting racemic, chiral or achiral base, a set of chiral, meso or achiral complexes. In all cases, the compounds consist of two NiII cations linked by a double azido bridge in its end-on coordination mode. All the dimers exhibit a mesocate supramolecular structure and one of them, the unprecedented mix of helicate and mesocate in 2:1 ratio. The transition from mesocate to helicate conformation has been reached by tuning the flexibility of the central spacers of the Schiff bases and the size of the substituents. Electronic circular dichroism (ECD) studies have been performed for two pairs of enantiomers and interpreted by means of DFT calculations. Susceptibility measurements show a ferromagnetic coupling between the NiII cations mediated by the end-on azido bridges.

Triple Halide Bridges in Chiral MnII2MnIII6NaI2 Cages: Structural and Magnetic Characterization.

A family of decanuclear chiral clusters with a MnII2MnIII6NaI2 core have been synthesized from enantiomerically pure Schiff bases. The new systems consist of two MnIIMnIII3NaI units linked by rare triple chloro or bromo bridges between the divalent Mn cations. Susceptibility measurements point out the weak antiferromagnetic interaction mediated by these kinds of bridges and afford the first magnetic measurements for the (µ-Br)3 case.

Oximato-Based Ligands in 3 d/4 f-Metal Cluster Chemistry: A Family of {Cu3Ln} Complexes with a "Propeller"-like Topology and Single-Molecule Magnetic Behavior.

The organic chelating and bridging ligands 9,10-phenanthrenedione-9-oxime (phenoxH) and 9,10-phenanthrenedione-9,10-dioxime (phendoxH2) were synthesized and subsequently employed for the first time in heterometallic 3 d/4 f-metal cluster chemistry. The general reaction between CuCl2·2H2O, LnCl3·6H2O, phenoxH, and NEt3 in a 1:2:2:4 molar ratio, in a solvent mixture comprising MeCN and MeOH, afforded brown crystals of a new family of [Cu3LnCl3(phenox)6(MeOH)3] clusters (Ln = Gd (1), Tb (2), Dy (3)) that possess an unprecedented [Cu3Ln(µ-NO)6]3+ "propeller"-like core. Complexes 1-3 are the first {Cu3Ln} clusters in which the outer CuII and the central LnIII atoms are solely bridged by diatomic oximato bridges. The {Cu-N-O-Ln} bridging units are very distorted with torsion angles spanning the range 35.5-48.9° and 25.2-55.6° in 1 and 2, respectively. As a result, complexes 1-3 are antiferromagnetically coupled, in agreement with previously reported magnetostructural criteria for oximato-bridged Cu/Ln complexes. The magnetic susceptibility data for all complexes were nicely fit to an isotropic spin Hamiltonian (for 1) or a Hamiltonian that accounts for the spin of the CuII atoms, the spin component of the LnIII, the spin-orbit coupling (λ), an axial ligand-field component around the LnIII atoms (Δ), and the Zeeman effect (for the anisotropic 2 and 3). The resulting fit parameters were J = -1.34 cm-1 and g = 2.10 (1), J = -1.42 cm-1, gCu = 2.10, and Δ = -26.3 cm-1 (2), and J = -1.70 cm-1, gCu = 2.05, and Δ = -38.1 cm-1 (3). The reported fitting procedure, implemented in the PHI program, is here used for the first time. Even if this method is only valid in high-symmetry Ln environments, when it is properly used allows a very simple and efficient method to obtain the exchange parameters. In light of the negative anisotropy, compounds 2 and 3 were found to exhibit frequency-dependent tails of out-of-phase signals in the presence of a small external dc field, characteristic of the slow magnetization relaxation of a single-molecule magnet. By using the Kramers-Kronig equations, the effective energy barriers ( Ueff) were derived and reported as Ueff = 10.1 and 5.4 cm-1 for 2 and 3, respectively.

Transition Metal Single-Molecule Magnets: A {Mn31} Nanosized Cluster with a Large Energy Barrier of ∼60 K and Magnetic Hysteresis at ∼5 K.

The first {Mn31} cluster (1) has been prepared from carboxylate ions and the chelating/bridging ligand α-methyl-2-pyridine-methanol. Compound 1 possesses a unique nanosized structural topology with one of the largest energy barriers reported to-date for high-nuclearity 3d-metal clusters. Single-crystal magnetic hysteresis studies reveal the presence of hysteresis loops below 5 K, one of the highest temperatures below which molecular hysteresis has been observed for 3d-based SMMs.

Linked Nickel Metallacrowns from a Phosphonate/2-Pyridyloximate Blend of Ligands: Structure and Magnetic Properties.

In the present work, four new Ni(II) clusters with nuclearities ranging between Ni4 and Na2Ni8 were synthesized, employing the versatile ligand phenylphosphonate and 6-methylpyridylaldoximate as the coligand. Crystallographic data show that the tetranuclear complex [Ni4(6-MepaoH)4(PhPO3)2(OH)2(MeOH)4](OH)2 (1) consists of two dimers linked by phosphonate bridges, whereas [Cs2Ni6(6-Mepao)6(PhPO3)3(OH)2(H2O)8] (2), Cs[Ni8(6-MepaoH)6(6-Mepao)6(PhPO3)3](ClO4)5 (3), and [Ni8Na2(BzO)6(6-Mepao)6(PhPO3)3] (4) are built from phosphonato-linked {Ni3(6-Mepao)3} metallacycles. The [9-MC(Ni(II)(6-Mepao))-3] fragments in 2-4 show the unusual coordination of additional Cs(+), Na(+), and/or Ni(II) cations. Direct-current magnetic measurements were carried in the 300-2 K range. Analysis of the experimental data revealed a complex response with strong antiferromagnetic interactions mediated by the oximato bridges and weak interactions mediated by the phosphonate ones.

Cu(II)-Ln(III) (Ln = Gd, Tb and Dy) complexes of an unsymmetrical N2O3 donor ligand: field induced SMM behaviour of Cu(II)-Tb(III) complexes.

Three new hetero-metallic CuII-LnIII complexes [(CuL)Gd(NO3)3(CH3OH)]n (1), [(CuL)Tb(NO3)3(H2O)]·[CuL] (2) and [(CuL)Dy(NO3)3(H2O)]·[CuL] (3) have been synthesized using a mono-nuclear Cu(II) complex, [CuL], of an unsymmetrically di-condensed N2O3 donor Schiff base ligand, N-(3-methoxysalicylidene)-N-(salicylidene)-1,2-ethylenediamine (H2L). Single crystal X-ray crystallography revealed that complex 1 is a nitrate bridged 1D chain of dinuclear Cu(II)-Gd(III) units whereas in 2 and 3, the dinuclear Cu(II)-Ln(III) units are co-crystallized with a [CuL] unit. The Ln(III) centers are nine coordinated with the geometry of a spherical capped square antiprism for Gd and spherical tricapped trigonal prism for Tb and Dy. The geometry of the Cu(II) center is distorted octahedral for complex 1 and distorted square planar for complexes 2 and 3. Temperature-dependent molar magnetic susceptibility measurements in 1-3 revealed the presence of overall ferromagnetic coupling between the Cu(II) and Ln(III) centers. Notably, field induced single-molecule magnet behavior was witnessed in the Tb(III) derivative (2). The ab initio calculations indicated that upon application of an external magnetic field, the tunneling in the ground state of complex 2 gets reduced and thereby field-induced SMM behaviour is observed. Besides, in the case of complex 1, BS-DFT calculations were carried out to gain further insights into the magnetic exchange coupling interactions between the Cu(II) and Gd(III) centers.

Magnetocaloric efficiency tuning through solvent-triggered 3D to 2D interconversion in holmium(III)-based dynamic MOFs.

The neutral holmium(III) oxalate octadecahydrate {[Ho2(ox)3(H2O)6]·12H2O}n of mixed hexagonal/decagonal (63·103) 3D net topology shows important changes in the magnetocaloric efficiency upon dehydration/rehydration by heating and water vapor exposition to give the holmium(III) oxalate decahydrate {[Ho2(ox)3(H2O)6]·4H2O}n of hexagonal (63) 2D net topology through the intermediacy of the elusive amorphous anhydrous compound {Ho2(ox)3}n.

Magnetic and optical studies of a new family of multidimensional and multiproperty PO-lanthanide(III) derived systems.

A new family of lanthanide compounds has been synthesized using 1,2-bis(diphenylphosphino)ethane dioxide (dppeO2) as an O-donor ligand through the phosphoryl group to lanthanide(III) cations and structurally, magnetically and optically studied. Depending on the lanthanide, two different topologies appear: the two-dimensional structure [LnIII(dppeO2)1.5(NO3)3(H2O)0.5]n (Ln = Ce (1), Sm (2) and Dy (6)) and the one-dimensional structure [Ln(dppeO2)(NO3)3DMF]n (Ln = Eu (3), Gd (4) and Tb (5)). Some of the Ln-derived complexes have been used as structural probes, while others have been synthesized to use the specific characteristics of each cation to take advantage of their magnetic/luminescence properties. Complex 6 presents slow relaxation of the magnetization while 2, 3 and 5 present emitting properties in the visible range.

Interesting chemical and physical features of the products of the reactions between trivalent lanthanoids and a tetradentate Schiff base derived from cyclohexane-1,2-diamine.

The initial use of a tetradentate Schiff base (LH2) derived from the 2 : 1 condensation between 2-hydroxyacetophenone and cyclohexane-1,2-diamine in 4f-metal chemistry is described. The 1 : 2 reaction of Ln(NO3)3·xH2O (Ln = lanthanoid or yttrium) and LH2 in MeOH/CH2Cl2 has provided access to isostructural complexes [Ln(NO3)3(L'H2)(MeOH)] in moderate to good yields. Surprisingly, the products contain the corresponding Schiff base ligand L'H2 possessing six aliphatic -CH2- groups instead of the -CH-(CH2)4-CH- unit of the cyclohexane ring, i.e. an unusual ring-opening of the latter has occurred. A mechanism for this LnIII-assisted/promoted LH2 → L'H2 transformation has been proposed assuming transient LnII species and a second LH2 molecule as the H2 source for the reduction of the cyclohexane moiety. DFT calculations provide strong evidence for the great thermodynamic stability of the products in comparison with analogous complexes containing the original intact ligand. The structures of the PrIII, SmIII, GdIII, TbIII, and HoIII complexes have been determined by single-crystal X-ray crystallography. The 9-coordinate LnIII centre in the molecules is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms that belong to the bidentate chelating organic ligand, and one oxygen atom from the coordinated MeOH group. In the overall neutral bis(zwitterionic) L'H2 ligand, the acidic H atoms are clearly located on the imino nitrogen atoms and this results in the formation of an unusual 16-membered chelating ring. The coordination polyhedra defined by the nine donor atoms around the 4f-metal-ion centres can be best described as distorted, spherical capped square antiprisms. The EuIII, TbIII, and DyIII complexes exhibit LnIII-based luminescence in the visible region, with the coordinated L'H2 molecule acting as the antenna. Ac magnetometry experiments show that the DyIII member of the family behaves as an SIM at zero field and under external dc fields of 0.1 and 0.2 T without the enhancement of the peaks' maxima, suggesting that QTM is not the relaxation path. The GdIII complex behaves, rather unexpectedly, as a SIM with two different magnetic relaxation paths occurring at very close temperatures; this behaviour is tentatively attributed to a very small axial zero-field splitting (D ∼ 0.1 cm-1), which cannot be detected by magnetization or susceptibility experiments. The prospects of the present, first results in the lanthanoid(III)-LH2 chemistry are discussed.