Hydroxide-Free Cubane-Shaped Tetranuclear [Ln₄] Complexes

The reaction of the lanthanide­(III) chloride salts [Gd­(III), Tb­(III), and Dy­(III)] with a new chelating, flexible, and sterically unencumbered multisite coordinating compartmental Schiff-base ligand (<i>E</i>)-2-((6-(hydroxymethyl)­pyridin-2-yl)­methyleneamino)­phenol (LH₂) and pivalic acid (PivH) in the presence of triethylamine (Et₃N) affords a series of tetranuclear Ln­(III) coordination compounds, [Ln₄(L)₄(μ₂-η¹η¹Piv)₄]·<i>x</i>H₂O·<i>y</i>CH₃OH (<b>1</b>, Ln = Gd­(III), <i>x</i> = 3, <i>y</i> = 6; <b>2</b>, Ln = Tb­(III), <i>x</i> = 6, <i>y</i> = 2; <b>3</b>, Ln = Dy­(III), <i>x</i> = 4, <i>y</i> = 6). X-ray diffraction studies reveal that the molecular structure contains a distorted cubane-like [Ln₄(μ₃-OR)₄]⁺⁸ core, which is formed by the concerted coordination action of four dianionic L^2– Schiff-base ligands. Each lanthanide ion is eight-coordinated (2N, 6O) to form a distorted-triangular dodecahedral geometry. Alternating current susceptibility measurements of complex <b>3</b> reveal frequency- and temperature-dependent two-step out-of-phase signals under zero direct current (dc) field, which is characteristic of single-molecule magnet behavior. Analysis of the dynamic magnetic data under an applied dc field of 1000 Oe to fully or partly suppress the quantum tunneling of magnetization relaxation process affords the anisotropic barriers and pre-exponential factors: Δ/<i>k</i>_B = 73(2) K, τ₀ = 4.4 × 10^–8 s; Δ/<i>k</i>_B = 47.2(9) K, τ₀ = 5.0 × 10^–7 s for the slow and fast relaxations, respectively

<i>Rhombus</i>-Shaped Tetranuclear [Ln₄] Complexes [Ln = Dy(III) and Ho(III)]: Synthesis, Structure, and SMM Behavior

The reaction of a new hexadentate Schiff base hydrazide ligand (LH₃) with rare earth­(III) chloride salts in the presence of triethylamine as the base afforded two planar tetranuclear neutral complexes: [{(LH)₂Dy₄}­(μ₂-O)₄]­(H₂O)₈·2CH₃OH·8H₂O (<b>1</b>) and [{(LH)₂Ho₄}­(μ₂-O)₄]­(H₂O)₈·6CH₃OH·4H₂O (<b>2</b>). These neutral complexes possess a structure in which all of the lanthanide ions and the donor atoms of the ligand remain in a perfect plane. Each doubly deprotonated ligand holds two Ln­(III) ions in its two distinct chelating coordination pockets to form [LH­(Ln)₂]⁴⁺ units. Two such units are connected by four [μ₂-O]^2– ligands to form a planar tetranuclear assembly with an Ln­(III)₄ core that possesses a rhombus-shaped structure. Detailed static and dynamic magnetic analysis of <b>1</b> and <b>2</b> revealed single-molecule magnet (SMM) behavior for complex <b>1.</b> A peculiar feature of the χ_M″ versus temperature curve is that two peaks that are frequency-dependent are revealed, indicating the occurrence of two relaxation processes that lead to two energy barriers (16.8 and 54.2 K) and time constants (τ₀ = 1.4 × 10^–6 s, τ₀ = 7.2 × 10^–7 s). This was related to the presence of two distinct geometrical sites for Dy­(III) in complex <b>1</b>

<i>Rhombus</i>-Shaped Tetranuclear [Ln₄] Complexes [Ln = Dy(III) and Ho(III)]: Synthesis, Structure, and SMM Behavior

The reaction of a new hexadentate Schiff base hydrazide ligand (LH₃) with rare earth­(III) chloride salts in the presence of triethylamine as the base afforded two planar tetranuclear neutral complexes: [{(LH)₂Dy₄}­(μ₂-O)₄]­(H₂O)₈·2CH₃OH·8H₂O (<b>1</b>) and [{(LH)₂Ho₄}­(μ₂-O)₄]­(H₂O)₈·6CH₃OH·4H₂O (<b>2</b>). These neutral complexes possess a structure in which all of the lanthanide ions and the donor atoms of the ligand remain in a perfect plane. Each doubly deprotonated ligand holds two Ln­(III) ions in its two distinct chelating coordination pockets to form [LH­(Ln)₂]⁴⁺ units. Two such units are connected by four [μ₂-O]^2– ligands to form a planar tetranuclear assembly with an Ln­(III)₄ core that possesses a rhombus-shaped structure. Detailed static and dynamic magnetic analysis of <b>1</b> and <b>2</b> revealed single-molecule magnet (SMM) behavior for complex <b>1.</b> A peculiar feature of the χ_M″ versus temperature curve is that two peaks that are frequency-dependent are revealed, indicating the occurrence of two relaxation processes that lead to two energy barriers (16.8 and 54.2 K) and time constants (τ₀ = 1.4 × 10^–6 s, τ₀ = 7.2 × 10^–7 s). This was related to the presence of two distinct geometrical sites for Dy­(III) in complex <b>1</b>

Molecular Magnets Based on Homometallic Hexanuclear Lanthanide(III) Complexes

The reaction of lanthanide­(III) chloride salts (Gd­(III), Dy­(III), Tb­(III), and Ho­(III)) with the hetero donor chelating ligand <i>N</i>′-(2-hydroxy-3-methoxybenzylidene)-6-(hydroxymethyl)­picolinohydrazide (LH₃) in the presence of triethylamine afforded the hexanuclear Ln­(III) complexes [{Ln₆(L)₂(LH)₂}­(μ₃-OH)₄]­[MeOH]_<i>p</i>[H₂O]_<i>q</i>[Cl]₄·<i>x</i>H₂O·<i>y</i>CH₃OH (<b>1</b>, Ln = Gd­(III), <i>p</i> = 4, <i>q</i> = 4, <i>x</i> = 8, <i>y</i> = 2; <b>2</b>, Ln = Dy­(III), <i>p</i> = 2, <i>q</i> = 6, <i>x</i> = 8, <i>y</i> = 4; <b>3</b>, Ln = Tb­(III), <i>p</i> = 2, <i>q</i> = 6, <i>x</i> = 10, <i>y</i> = 4; <b>4</b>, Ln = Ho­(III), <i>p</i> = 2, <i>q</i> = 6, <i>x</i> = 10, <i>y</i> = 2). X-ray diffraction studies revealed that these compounds possess a hexanuclear [Ln₆(OH)₄]¹⁴⁺ core consisting of four fused [Ln₃(OH)]⁸⁺ subunits. Both static (dc) and dynamic (ac) magnetic properties of <b>1</b>–<b>4</b> have been studied. Single-molecule magnetic behavior has been observed in compound <b>2</b> with an effective energy barrier and relaxation time pre-exponential parameters of Δ/<i>k</i>_B = 46.2 K and τ₀ = 2.85 × 10^–7 s, respectively

Syntheses, Structures, and Magnetic Properties of a Family of Heterometallic Heptanuclear [Cu₅Ln₂] (Ln = Y(III), Lu(III), Dy(III), Ho(III), Er(III), and Yb(III)) Complexes: Observation of SMM behavior for the Dy(III) and Ho(III) Analogues

Sequential reaction of the multisite coordination ligand (LH₃) with Cu­(OAc)₂·H₂O, followed by the addition of a rare-earth­(III) nitrate salt in the presence of triethylamine, afforded a series of heterometallic heptanuclear complexes containing a [Cu₅Ln₂] core {Ln = Y­(<b>1</b>), Lu­(<b>2</b>), Dy­(<b>3</b>), Ho­(<b>4</b>), Er­(<b>5</b>), and Yb­(<b>6</b>)}. Single-crystal X-ray crystallography reveals that all the complexes are dicationic species that crystallize with two nitrate anions to compensate the charge. The heptanuclear aggregates in <b>1</b>–<b>6</b> are centrosymmetrical complexes, with a hexagonal-like arrangement of six peripheral metal ions (two rare-earth and four copper) around a central Cu­(II) situated on a crystallographic inversion center. An all-oxygen environment is found to be present around the rare-earth metal ions, which adopt a distorted square-antiprismatic geometry. Three different Cu­(II) sites are present in the heptanuclear complexes: two possess a distorted octahedral coordination sphere while the remaining one displays a distorted square-pyramidal geometry. Detailed static and dynamic magnetic properties of all the complexes have been studied and revealed the single-molecule magnet behavior of the Dy­(III) and Ho­(III) derivatives

Octanuclear Heterobimetallic {Ni₄Ln₄} Assemblies Possessing Ln₄ Square Grid [2 × 2] Motifs: Synthesis, Structure, and Magnetism

Octanuclear heterobimetallic complexes, [Ln₄Ni₄(H₃L)₄(μ₃-OH)₄(μ₂-OH)₄]­4Cl·<i>x</i>H₂O·<i>y</i>CHCl₃ (Dy³⁺, <i>x</i> = 30.6, <i>y</i> = 2 (<b>1</b>); Tb³⁺, <i>x</i> = 28, <i>y</i> = 0 (<b>2</b>) ; Gd³⁺, <i>x</i> = 25.3, <i>y</i> = 0 (<b>3</b>); Ho³⁺, <i>x</i> = 30.6, <i>y</i> = 3 (<b>4</b>)) (H₅L = <i>N</i>₁,<i>N</i>₃-bis­(6-formyl-2-(hydroxymethyl)-4-methylphenol)­diethylenetriamine) are reported. These are assembled by the cumulative coordination action of four doubly deprotonated compartmental ligands, [H₃L]^2–, along with eight exogenous −OH ligands. Within the core of these complexes, four Ln³⁺’s are distributed to the four corners of a perfect square grid while four Ni²⁺’s are projected away from the plane of the Ln₄ unit. Each of the four Ni²⁺’s possesses distorted octahedral geometry while all of the Ln³⁺’s are crystallographically equivalent and are present in an elongated square antiprism geometry. The magnetic properties of compound <b>3</b> are dominated by an easy-plane single-ion anisotropy of the Ni²⁺ ions [<i>D</i>_Ni = 6.7(7) K] and dipolar interactions between Gd³⁺ centers. Detailed ac magnetometry reveals the presence of distinct temperature-dependent out-of-phase signals for compounds <b>1</b> and <b>2</b>, indicative of slow magnetic relaxation. Magnetochemical analysis of complex <b>1</b> implies the 3d and the 4f metal ions are engaged in ferromagnetic interactions with SMM behavior, while dc magnetometry of compound <b>2</b> is suggestive of an antiferromagnetic Ni–Tb spin-exchange with slow magnetic relaxation due to a field-induced level crossing. Compound <b>4</b> exhibits an easy-plane single-ion anisotropy for the Ho³⁺ ions and weak interactions between spin centers

Cyclo- and Carbophosphazene-Supported Ligands for the Assembly of Heterometallic (Cu²⁺/Ca²⁺, Cu²⁺/Dy³⁺, Cu²⁺/Tb³⁺) Complexes: Synthesis, Structure, and Magnetism

The carbophosphazene and cyclophosphazene hydrazides, [{NC­(N­(CH₃)₂)}₂{NP­{N­(CH₃)­NH₂}₂}] (<b>1</b>) and [N₃P₃(O₂C₁₂H₈)₂{N­(CH₃)­NH₂}₂] were condensed with <i>o</i>-vanillin to afford the multisite coordination ligands [{NC­(N­(CH₃)₂)}₂{NP­{N­(CH₃)­NCH-C₆H₃-(<i>o</i>-OH)­(<i>m</i>-OCH₃)}₂}] (<b>2</b>) and [{N₂P₂(O₂C₁₂H₈)₂}­{NP­{N­(CH₃)­NCH-C ₆H₃-(<i>o</i>-OH)­(<i>m</i>-OCH₃)}₂}] (<b>3</b>), respectively. These ligands were used for the preparation of heterometallic complexes [{NC­(N­(CH₃)₂)}₂{NP­{N­(CH₃)­NCH-C₆H₃-(<i>o</i>-O)­(<i>m</i>-OCH₃)}₂}­{CuCa­(NO₃)₂}] (<b>4</b>), [{NC­(N­(CH₃)₂)}₂{NP­{N­(CH₃)­NCH-C₆H₃-(<i>o</i>-O)­(<i>m</i>-OCH₃)}₂}­{Cu₂Ca₂(NO₃)₄}]·4H₂O (<b>5</b>), [{NC­(N­(CH₃)₂)}₂{NP­{N­(CH₃)­NCH-C₆H₃-(<i>o</i>-O)­(<i>m</i>-OCH₃)}₂}­{CuDy­(NO₃)₄}]·CH₃COCH₃ (<b>6</b>), [{NP­(O₂C₁₂H₈)}₂{NP­{N­(CH₃)­NCH-C₆H₃-(<i>o</i>-O)­(<i>m</i>-OCH₃)}₂}­{CuDy­(NO₃)₃}] (<b>7</b>), and [{NP­(O₂C₁₂H₈)}₂{NP­{N­(CH₃)­NCH-C₆H₃-(<i>o</i>-O)­(<i>m</i>-OCH₃)}₂}­{CuTb­(NO₃)₃}] (<b>8</b>). The molecular structures of these compounds reveals that the ligands <b>2</b> and <b>3</b> possess dual coordination pockets which are used to specifically bind the transition metal ion and the alkaline earth/lanthanide metal ion; the Cu²⁺/Ca²⁺, Cu²⁺/Tb³⁺, and Cu²⁺/Dy³⁺ pairs in these compounds are brought together by phenoxide and methoxy oxygen atoms. While <b>4</b>, <b>6</b>, <b>7</b>, and <b>8</b> are dinuclear complexes, <b>5</b> is a tetranuclear complex. Detailed magnetic properties on <b>6</b>–<b>8</b> reveal that these compounds show weak couplings between the magnetic centers and magnetic anisotropy. However, the ac susceptibility experiments did not reveal any out of phase signal suggesting that in these compounds slow relaxation of magnetization is absent above 1.8 K

Mononuclear Lanthanide Complexes: Energy-Barrier Enhancement by Ligand Substitution in Field-Induced Dy^III SIMs

The sequential reaction of 2-((6-(hydroxymethyl)­pyridin-2-yl)-methyleneamino)­phenol (LH₂), LnCl₃·6H₂O, and 1,1,1-trifluoroacetylacetone (Htfa) in the presence of Et₃N afforded [Ln­(LH) (tfa)₂] [Ln = Dy³⁺ (<b>1</b>), Ln = Tb³⁺ (<b>2</b>), and Ln = Gd³⁺ (<b>3</b>)], while under the same reaction conditions, but in the absence of the coligand, another series of mononuclear complexes, namely, [Ln­(LH)₂]·Cl·2MeOH] [Ln = Dy³⁺ (<b>4</b>) and Tb³⁺ (<b>5</b>)] are obtained. Single-crystal X-ray diffraction analysis revealed that the former set contains a mono-deprotonated [LH]⁻ and two tfa ligands, while the latter set comprises of two mono-deprotonated [LH]⁻ ligands that are nearly perpendicular to each other at an angle of 86.9°. Among these complexes, <b>2</b> exhibited a ligand-sensitized lanthanide-characteristic emission. Analyses of the alternating current susceptibility measurements reveal the presence of single-molecule magnet behavior for <b>1</b> and <b>4</b>, in the presence of direct-current field, with effective energy barriers of 4.6 and 44.4 K, respectively. The enhancement of the effective energy barrier of the latter can be attributed to the presence of a large energy gap between the ground and first excited Kramers doublets, triggered by the change in coordination environments around the lanthanide centers

Novel Chemosensor for the Visual Detection of Copper(II) in Aqueous Solution at the ppm Level

A new water-soluble, multisite-coordinating ligand LH₇ was prepared by the condensation of tris­(hydroxymethyl)­aminomethane with 2,6-diformyl-<i>p</i>-cresol. LH₇ is a selective chemosensor for Cu²⁺, under physiological conditions, with visual detection limits of 20 ppm (ambient light conditions) and 4 ppm (UV light conditions). LH₇ can also be used in biological cell lines for the detection of Cu²⁺

Structure–Activity Relationships of Spectinamide Antituberculosis Agents: A Dissection of Ribosomal Inhibition and Native Efflux Avoidance Contributions

Spectinamides are a novel class of antitubercular agents with the potential to treat drug-resistant tuberculosis infections. Their antitubercular activity is derived from both ribosomal affinity and their ability to overcome intrinsic efflux mediated by the Mycobacterium tuberculosis Rv1258c efflux pump. This study explores the structure–activity relationships through analysis of 50 targeted spectinamides. Compounds are evaluated for ribosomal translational inhibition, MIC activity in Rv1258c efflux pump deficient and wild type tuberculosis strains, and efficacy in an acute model of tuberculosis infection. The results of this study show a narrow structure–activity relationship, consistent with a tight ribosome-binding pocket and strict structural requirements to overcome native efflux. Rationalization of ribosomal inhibition data using molecular dynamics simulations showed stable complex formation for halogenated spectinamides consistent with the long post antibiotic effects observed. The lead spectinamides identified in this study demonstrated potent MIC activity against MDR and XDR tuberculosis and had desirable antitubercular class specific features including low protein binding, low microsomal metabolism, no cytotoxicity, and significant reductions in bacterial burdens in the lungs of mice infected with M. tuberculosis. The structure–activity relationships detailed here emphasize the need to examine efflux-mediated resistance in the design of antituberculosis drugs and demonstrate that it is possible to overcome intrinsic efflux with synthetic modification. The ability to understand the structure requirements for this class has produced a variety of new substituted spectinamides, which may provide useful alternative candidates and promote the further development of this class