Single-Molecule Magnetism, Enhanced Magnetocaloric Effect, and Toroidal Magnetic Moments in a Family of Ln4 Squares

Three cationic [Ln4] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy4(\u3bc4-OH)(HL)(H2L)3(H2O)4]Cl2\ub7(CH3OH)4\ub7(H2O)8 (1), [Tb4(\u3bc4-OH)(HL)(H2L)3(MeOH)4]Cl2\ub7(CH3OH)4\ub7(H2O)4 (2) and [Gd4(\u3bc4-OH)(HL)(H2L)3(H2O)2(MeOH)2]Br2\ub7(CH3OH)4\ub7(H2O)3 (3). The structures are described as hydroxo-centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H2L2- ligand. Alternating current magnetic susceptibility measurements show frequency-dependent out-of-phase signals with two different thermally assisted relaxation processes for 1, whereas no maxima in \u3c7M" appears above 2.0 K for complex 2. For 1, the estimated effective energy barrier for these two relaxation processes is 29 and 100 K. Detailed ab initio studies reveal that complex 1 possesses a toroidal magnetic moment. The ab initio calculated anisotropies of the metal ions in complex 1 were employed to simulate the magnetic susceptibility by using the Lines model (POLY-ANISO) and this procedure yields J1=+0.01 and J2=-0.01 cm-1 for 1 as the two distinct exchange interactions between the DyIII ions. Similar parameters are also obtained for complex 1 (and 2) from specific heat measurements. A very weak antiferromagnetic super-exchange interaction (J1=-0.043 cm-1 and g=1.99) is observed between the metal centers in 3. The magnetocaloric effect (MCE) was estimated by using field-dependent magnetization and temperature-dependent heat-capacity measurements. An excellent agreement is found for the -\u394Sm values extracted from these two measurements for all three complexes. As expected, 3 shows the largest -\u394Sm variation (23 J Kg-1 K-1) among the three complexes. The negligible magnetic anisotropy of Gd indeed ensures near degeneracy in the (2S+1) ground state microstates, and the weak super-exchange interaction facilitates dense population of low-lying excited states, all of which are likely to contribute to the MCE, making complex 3 an attractive candidate for cryogenic refrigeration

Nickel(II)-lanthanide(III) magnetic exchange coupling influencing single-molecule magnetic features in {Ni2Ln2} complexes

Four isostructural [Ni(2)Ln(2)(CH3CO2)(3)(HL)(4)(H2O)(2)](3+)(Ln(3+)=Dy (1), Tb (2), Ho (3) or Lu (4)) complexes and a dinuclear [NiGd(HL)(2)(NO3)(3)] (5) complex are reported (where HL=2-methoxy-6-[(E)-2-hydroxymethyl-phenyliminomethyl]-phenolate). For compounds 1-3 and 5, the Ni2+ ions are ferromagnetically coupled to the respective lanthanide ions. The ferromagnetic coupling in 1 suppresses the quantum tunnelling of magnetisation (QTM), resulting in a rare zero dc field Ni-Dy single-molecule magnet, with an anisotropy barrier U-eff of 19K

Exploring the potential use of silver-exchanged zeolites for adsorption of radon traces in low background experiments

International audienceAbstract Radon is an important source of radioactive background in experiments searching for rare decays and in the field of low-energy particle physics. Here, we report the first temperature-dependent study of radon adsorption on silver-exchanged zeolites from several commercial producers. Among the three tested zeolites, Ag-ETS-10 showed the best result. Hence, it was chosen for the further study of internal radioactivity and radon emanation, which are important characteristics of materials used in low-activity experiments. The important role of silver in radon adsorption is demonstrated by comparison of the silver-exchanged zeolites with their unexchanged counterparts. Furthermore, the temperature-dependent measurements showed that the enhancement of the radon adsorption upon the introduction of silver in zeolite occurs due to the increase of the heat of adsorption. This opens a new perspective for the search for highly efficient radon adsorbents.</jats:p

Carboxylate free mu-oxo bridged ferric wheel with a record exchange coupling

Ahmed N, Upadhyay A, Rajeshkumar T, et al. Carboxylate free mu-oxo bridged ferric wheel with a record exchange coupling. Dalton Transactions. 2015;44(43):18743-18747.A novel oxo-bridged, carboxylate free 'ferric wheel' was isolated with the molecular formula [Fe-8(III)(mu-O)(4)(L2-)(8)] (1). Magnetic susceptibility measurements suggest that the Fe(III) ions are coupled anti-ferromagnetically and magnetic data modelling yields J(1) = -26.4 cm(-1), J(2) = -170 cm(-1) which are rationalized by DFT calculation. The exchange value for 1 of -170 cm(-1) (Fe-O(oxo)-Fe) is the largest exchange value known compared to any homometallic or heterometallic wheel reported to date

The Prospect of Salophen in Fluorescence Lifetime Sensing of Al³⁺

We have assessed the potential of salophen, a tetradentate Schiff base, in fluorescence sensing of Al³⁺ ions. While performing this investigation, we have noticed conflicting literature reports on the fluorescence spectral maximum and quantum yield of salophen. So, the compound has been purified by repeated crystallization. Fluorescence studies have been performed on samples in which the absorption and excitation spectra are completely superimposable. The purified compound exhibits a feeble fluorescence at 545 nm, associated with an ultrafast fluorescence decay. This is rationalized by excited state proton transfer and torsional motions within the molecule, which provide efficient nonradiative channels of deactivation of its excited state. The fluorescence quantum yield increases upon complexation of salophen with Zn²⁺ as well as Al³⁺. The increase is significantly more upon complexation with Al³⁺. However, fluorescence maxima are similar for the two complexes. This indicates that fluorescence intensity may not be a good parameter for Al³⁺ sensing by salophen, in the presence of a large excess of Zn²⁺. This problem can be circumvented if fluorescence lifetime is used as the sensing parameter, as the lifetime of the Al³⁺ complex is in the nanosecond time regime while that of the Zn²⁺ complex is in tens of picoseconds. The significant difference in the fluorescence quantum yield and lifetime between the two complexes is explained as follows: the Al³⁺ complex is monomeric, but the Zn²⁺ complex is dimeric. Quantum chemical calculations indicate a higher density of states near the locally excited state for the dimeric complex. This may lead to more efficient nonradiative pathways

Stabilizing Terminal Ni(III)–Hydroxide Complex Using NNN-Pincer Ligands: Synthesis and Characterization

The reaction of [Ni­(COD)2] (COD; cyclooctadiene) in THF with the NNN-pincer ligand bis­(imino)­pyridyl (L1) reveals a susceptibility to oxidation in an inert atmosphere ([O2] level <0.5 ppm), resulting in a transient Ni:dioxygen adduct. This reactive intermediate abstracts a hydrogen atom from THF and stabilizes an uncommon Ni­(III) complex. The complex is crystallographically characterized by a molecular formula of [NiIII(L1··)2–(OH)] (1). Various isotopically labeled experiments (16O/18O) assertively endorse the origin of terminal oxygen based ligand in 1 due to the activation of molecular dioxygen. The presence of proton bound to the terminal oxygen in 1 is well supported by NMR, IR spectroscopy, DFT calculations, and hydrogen atom transfer (HAT) reactions promoted by 1. The observation of shakeup satellite peaks for the primary photoelectron lines of Ni­(2p) in the X-ray photoelectron spectroscopy (XPS) unambiguously confirms the paramagnetic signature associated with the distorted square planar nickel ion, which is consistent with the trivalent oxidation state assigned for the nickel ion in 1. The variable temperature magnetic susceptibility data of 1 shows dominant antiferromagnetic interactions exist among the paramagnetic centers, resulting in an overall S = 1/2 ground state. Variable temperature X-band EPR studies performed on 1 show evidence for the S = 1/2 ground state, which is consistent with magnetic data. The unusual g-tensor extracted for the ground state S = 1/2 is analyzed under a strong exchange limit of spin-coupled centers. The electronic structure predicted for 1 is in good agreement with theoretical calculations

Flexible and luminescent fibers of a 1D Au( i )–thiophenolate coordination polymer and formation of gold nanoparticle-based composite materials for SERS

International audienceShaping of functional materials is of tremendous importance for applications. Among coordination polymer compounds, one of the challenges is to get fibers that can be self-standing and flexible, while maintaining their crystallinity and properties. Here, the synthesis and characterization of free-standing fibers of a Au(I)-based coordination polymer (CP) are reported. The flexible fibers of the 1D gold(I)–thiophenolate [Au(SPh)]n CP have an average diameter of 270 nm and length of a few micrometers. They are hydrophobic because of the presence of phenyl rings and exhibit high chemical stability in harshly acidic and basic conditions due to the strong Au(I)–S interactions. These fibers are red-emissive at room temperature because of the presence of aurophilic interactions. In addition, a composite material can be easily obtained through calcination, resulting in the formation of gold nanoparticles (AuNPs) on the CP fiber surface. Owing to the plasmonic resonance of AuNPs, this composite material exhibits good sensitivity towards the detection of molecules as observed through surface enhanced Raman scattering (SERS)

Exploratory study to evaluate tolerability, safety, and activity of Ashwagandha (Withania somnifera) in healthy volunteers

Ashwagandha (Withania somnifera) (WS), a "rasayana" drug, is recommended for balavardhan and mamsavardhan. The study was intended to evaluate dose-related tolerability, safety, and activity of WS formulation in normal individuals. The design was prospective, open-labeled, variable doses in volunteers. Eighteen apparently healthy volunteers (12M:6F, age:18-30 years, and BMI: 19-30) were enrolled. After baseline investigations, they received WS capsules (Rx) (aqueous extract, 8:1) daily in two divided doses with increase in daily dosage every 10 days for 30 days (750 mg/day x10 days, 1 000 mg/day x 10 days, 1 250 mg/day x 10 days). Volunteers were assessed for symptoms/signs, vital functions, hematological and biochemical organ function tests. Muscle activity was measured by hand grip strength, quadriceps strength, and back extensor force. Exercise tolerance was determined using cycle ergometry. Lean body weight and fat% were computed from skin fold thickness measurement. Adverse events were recorded, as volunteered by the subjects. Repeated measures ANOVA, McNemar′s test, and paired t test were employed. All but one volunteer tolerated WS without any adverse event. One volunteer showed increased appetite, libido, and hallucinogenic effects with vertigo at the lowest dose and was withdrawn from study. In six subjects, improvement in quality of sleep was found. Organ function tests were in normal range before and after the intervention. Reduction in total- and LDL- cholesterol and increase of strength in muscle activity was significant. Total body fat percentage showed a reduction trend. WS, in escalated dose, was tolerated well. The formulation appeared safe and strengthened muscle activity. In view of its traditional Rasayana use, further studies are planned to evaluate potential of this drug in patients of sarcopenia

Substituted versus Naked Thiourea Ligand Containing Pseudotetrahedral Cobalt(II) Complexes: A Comparative Study on Its Magnetization Relaxation Dynamics Phenomenon

A series of mononuclear tetrahedral cobalt­(II) complexes with the general molecular formula [Co­(L₁)₂X₂] [where L₁ = tetramethylthiourea ([(CH₃)₂N]₂CS) and X = Cl (<b>1</b>), Br (<b>2</b>), and I (<b>3</b>)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently reproduced by fitting both χ_M<i>T</i>(<i>T</i>) and <i>M</i>(<i>H</i>) simultaneously using the spin Hamiltonian (SH) parameters <i>D</i>_<b>1</b> = −18.1 cm^–1 and <i>g</i>_<b>1</b>,iso = 2.26, <i>D</i>_<b>2</b> = −16.4 cm^–1 and <i>g</i>_<b>2</b>,iso = 2.33, and <i>D</i>_<b>3</b> = −22 cm^–1 and <i>g</i>_<b>3</b>,iso = 2.4 for <b>1</b>–<b>3</b>, respectively, and the sign of <i>D</i> was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes <b>1</b>–<b>3</b> (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co­(L)₂X₂] [where L = thiourea [(NH₂)₂CS] and X = Cl (<b>1a</b>), Br (<b>2a</b>), and I (<b>3a</b>)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (<b>1</b>–<b>3</b>). Noticeably, the substitution of L by L₁ induces structural distortion in <b>1</b>–<b>3</b> on the level of the secondary coordination sphere compared to <b>1a</b>–<b>3a</b>. This distortion leads to an overall reduction in |<i>E</i>/<i>D</i>| of <b>1</b>–<b>3</b> compared to <b>1a</b>–<b>3a</b>. This may be one of the reasons for the origin of the slower relaxation times of <b>1</b>–<b>3</b> compared to <b>1a</b>–<b>3a</b>

Cyclic Solid‐State Multiple Phase Changes with Tuned Photoemission in a Gold Thiolate Coordination Polymer

International audienceA non-emissive amorphous AuI–thiolate coordination polymer undergoes two successive crystallisation phase changes on mild solid-state heating. The first high symmetric crystalline phase becomes slightly red-emissive and the second, which has a lower symmetry, exhibits intense luminescence due to the presence of aurophilic interactions. These transitions are reversible upon soft hand grinding, offering potential for non-volatile memories