Showing papers by "Sanjit Konar published in 2018"

Influence of the Coordination Environment on Easy-Plane Magnetic Anisotropy of Pentagonal Bipyramidal Cobalt(II) Complexes.

Abstract: A rational approach of modulating the easy-plane magnetic anisotropy of mononuclear pentagonal bipyramidal CoII single molecule magnets (SMMs) has been revealed in this paper. A class of three new pentagonal-bipyramidal complexes with formulas [Co(H2daps)(MeOH)2] (1), [Co(H4daps)(NCS)(MeOH)]·(ClO4)·(MeOH) (2), and [Co(H4daps)(NCS)2]·(MeOH)2 (3) (H4daps = 2,6-bis(1-salicyloylhydrazonoethyl) pyridine) were studied. In these complexes, the axial positions are successively replaced by different O and N donar ligands in a systematic way. Detailed magnetic measurements disclose the existence of large easy-plane magnetic anisotropy and field-induced slow magnetic relaxation behavior. Both experimental and ab initio theoretical calculations display that easy-plane magnetic anisotropy is maintained upon variation of coordination environments. Nevertheless, the magnitude of the D value was found to be increased in the case of weaker axially coordinated σ-donor ligands and a more symmetrical equatorial ligand. Addit...

3D isomorphous lanthanide coordination polymers displaying magnetic refrigeration, slow magnetic relaxation and tunable proton conduction

Abstract: Four new isostructural lanthanide-based three-dimensional (3D) coordination polymers (CPs), {[Ln4(OH)4(L)2(H2O)8]·4.6H2O·1.4CH3CN}n (Ln3+ = Gd3+ (1), Dy3+ (2), Ho3+ (3) and Er3+ (4)), have been constructed using a sulfonate-carboxylate-based ligand (Na2H2L = disodium-2,2'-disulfonate-4,4'-oxydibenzoic acid) and the corresponding lanthanide metal(iii) nitrates. All the CPs 1-4 contain [Ln4(μ3-OH)4]8+ cubane-like cores interconnected through L4- ligands to give rise to 3D coordination frameworks with 1D hydrophilic channels along the crystallographic c direction. From the topological perspective, the underlying 3D nets of the CPs can be classified as a 3,6,6-c net with an undocumented topology. Magnetic studies display that CP 1 exhibits a magnetocaloric effect with a significant magnetic entropy change (-ΔSm) = 34.6 J kg-1 K-1 for ΔH = 7 T at 3 K. CP 2 shows field-induced slow magnetic relaxation properties with energy barrier (Ueff/kB) = 30.40 K and relaxation time (τ0) = 2.47 × 10-7 s. Theoretical calculations have been performed to corroborate the magnetic exchange coupling constant value for CP 1 and to obtain a deeper understanding of the field-induced slow magnetic relaxation behavior of CP 2. Impedance analyses display high values of proton conductivity which reach 2.02 × 10-6, 2.96 × 10-6, 4.56 × 10-3 and 6.59 × 10-3 S cm-1 for CPs 1-4, respectively at high temperature (>75 °C) and 95% relative humidity (RH) in the order CP 1 Dy3+ > Ho3+ > Er3+). This demonstrates the effective employment of the lanthanide contraction strategy to tune proton conductivity while preserving proton-conducting pathways.

Field-Induced Slow Magnetic Relaxation and Anion/Solvent Dependent Proton Conduction in Cobalt(II) Coordination Polymers

Abstract: Three new coordination polymers (CPs), namely, 2D {[Co(L)2(H2O)2](ClO4)2·3DMA·0.4H2O}n (1), 3D {[Co(L)2(H2O)2]·(Cl)2}n (2), and 2D {[Co(L)2(NCS)2]}n (3) were obtained by the self-assembly of corresponding cobalt(II) salts and a linear semirigid linker bis(4-imidazol-1-yl-phenyl)diazene (L) designed with two terminal imidazole groups and an azo moiety in the middle. CPs 1 and 2 are composed of a cationic framework which leads to incorporation of anion in the framework along with solvent molecules (only in 1), whereas CP 3 possesses a neutral framework. The cationic core adopts a cis and trans configuration in 1 and 2, respectively. The structures consist of 2D net with sql topology in 1, eightfold interpenetrating 3-periodic network with dia topology in 2 and threefold interpenetrating 2-periodic network with sql (2,4L2) topology in 3. The structural diversity of CPs 1–3 is governed by several factors, including the different coordination ability of the anions, reaction conditions and intermolecular intera...

A new series of tetrahedral Co(ii) complexes [CoLX2] (X = NCS, Cl, Br, I) manifesting single-ion magnet features.

Abstract: A series of tetrahedral CoII complexes [CoLX2] (X = NCS (1), Cl (2), Br (3) and I (4); L = 9,9-dimethyl-4,5-bis(diphenylphosphino) xanthene) based on a P-donor ligand has been prepared to investigate the influence of terminal ligand field strength on the anisotropy of CoII single-ion magnets. It has been observed that heavier and softer terminal ligands are able to decrease the anisotropy of the tetrahedral CoII centers. Thorough analyses of experimental and theoretical studies show that all complexes have an easy-axis type magnetic anisotropy and slow relaxation behaviors of tetrahedral CoII centers. Detailed ab initio theory studies disclose that the changes in the ligand field strength imposed by the terminal ligands result in modifying the single ion anisotropy (D) of polyhedra 1-4. Furthermore, the isostructural ZnII analogue (5) has been prepared to examine the influence of dipolar interactions between adjacent CoII centres and magnetic dilution experiments were performed.

Achieving Amphibious Superprotonic Conductivity in a CuI Metal-Organic Framework by Strategic Pyrazinium Salt Impregnation.

Abstract: Treatment of a pyrazine (pz)-impregnated CuI metal-organic framework (MOF) ([1⊃pz]) with HCl vapor renders an interstitial pyrazinium chloride salt-hybridized MOF ([1⊃pz⋅6 HCl]) that exhibits proton conductivity over 10-2 S cm-1 both in anhydrous and under humid conditions. Framework [1⊃pz⋅6 HCl] features the highest anhydrous proton conductivity among the lesser-known examples of MOF-based materials exhibiting proton conductivity under both anhydrous and humid conditions. Moreover, [1⊃pz] and corresponding pyrazinium sulfate- and pyrazinium phosphate-hybridized MOFs also exhibit superprotonic conductivity over 10-2 S cm-1 under humid conditions. The impregnated pyrazinium ions play a crucial role in protonic conductivity, which occurs through a Grotthuss mechanism.

Field Induced Slow Magnetic Relaxation in a Non Kramers Tb(III) Based Single Chain Magnet

Abstract: Herein, we report a novel Tb(III) single chain magnet with the chemical formulae [Tb(μ-OH2)(phen)(μ-OH)(nb)2]n by using 4-nitrobenzoic acid (Hnb) and 1,10-phenanthroline (phen) as ligand system. The single-crystal X-ray diffraction reveals that 4-nitrobenzoic acid acts as a monodentate ligand, water and hydroxyl ions are the bridging ligand and the phen serves as a bidentate chelating ligand. The static magnetic susceptibility measurement (from 2 K to 300 K) shows ferromagnetic interaction at very low temperature (below 6 K). The alternating current (AC) susceptibility data of the complex show temperature and frequency dependence under an applied 2000 Oe DC (direct current) field. The phen moiety behaves as an antenna and enables the complex to show the green light fluorescence emission by absorption-energy transfer-emission mechanism. To calculate the exchange interaction, broken symmetry density functional theory (BS-DFT) calculations have been performed on a model compound which also reveals weak ferromagnetic interaction. Ab initio calculations reveals the anisotropic nature (gz = 15.8, gy, gy = 0) of the metal centre and the quasi doublet nature of ground state with small energy gap and that is well separated from the next excited energy state.