RESUMEN
Three lanthanide-based two-dimensional (2D) coordination polymers (CPs), [Ln(L)(H2 O)2 ]n , {H3 L=(HO)2 P(O)CH2 CO2 H; Ln=Dy3+ (CP 1), Er3+ (CP 2)} and [{Gd2 (L)2 (H2 O)3 }. H2 O]n , (CP 3) were hydrothermally synthesized using phosphonoacetic acid as a linker. Structural features revealed that the dinuclear Ln3+ nodes were present in the 2D sheet of CP 1 and CP 2 while in the case of CP 3, nodes were further connected to each other forming a chain-type arrangement throughout the network. The magnetic studies show field-induced slow magnetic relaxation property in CP 1 and CP 2 with Ueff values of 72â K (relaxation time, τ0 =3.05×10-7 â s) and 38.42â K (relaxation time, τ0 =4.60×10-8 â s) respectively. Ab-initio calculations suggest that the g tensor of Kramers doublet of the lanthanide ion (Dy3+ and Er3+ ) is strongly axial in nature which reflects in the slow magnetic relaxation behavior of both CPs. CP 3 exhibits a significant magnetocaloric effect with -ΔSm =49.29â J kg-1 K-1 , one of the highest value among the reported 2D CPs. Moreover, impedance analysis of all the CPs show high proton conductivity with values of 1.13×10-6 â S cm-1 , 2.73×10-3 â S cm-1 and 2, 6.27×10-6 â S cm-1 for CPs 1-3, respectively, at high temperature (>75 °C) and maximum 95 % relative humidity (RH).
RESUMEN
Two solvent-free mononuclear Fe(III) complexes [Fe(L)2]NO3 (1) and [Fe(L)2]ClO4 (2) have been synthesized by employing a new π-conjugated azo-phenyl substituted ligand, 2-(( E)-((2-(ethylamino)ethyl)imino)methyl)-4-(2-phenyldiazenyl)phenol (HL). The noncoordinated azo-phenyl part of the ligand adopts two different conformations which can exert a varied local distortion around the metal center affecting the spin crossover behavior. The magnetic data (2-450 K) reveal that complex 1 displays spin crossover above room temperature where the ligand is in linear form, while complex 2 shows an incomplete spin transition where the ligand adopts a skew form in the solid state. These complexes represent rare examples of high-temperature spin transition for mononuclear Fe(III) complexes with T1/2 > 350 K with very high thermal stability. Presence of strong intermolecular interactions and solvent-free nature of the complexes leads to exceptional thermal stability up to 485 K (for 1) and 496 K (for 2) as revealed by thermogravimetric analysis. The magnetic data for complex 1 have been analyzed by employing an Ising-like model with vibrations yielding the enthalpy change Δ H and entropy change Δ S of the spin transition along with the critical temperature T1/2 and the solid-state cooperativeness Γ. Spin crossover behavior of complex 1 has also been characterized by differential scanning calorimetry and electron paramagnetic resonance measurements. Ab initio calculations have been performed to analyze the difference in energies of the ground state and excited states of the complexes.
RESUMEN
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 < CP 2 < CP 3 < CP 4. Notably, the proton conductivities for CPs 3 and 4 are a few orders of magnitude higher than those of CPs 1 and 2 (10-3 S cm-1vs. 10-6 S cm-1), and the conductivity increases periodically following the decreasing order of ionic radius (Gd3+ > Dy3+ > Ho3+ > Er3+). This demonstrates the effective employment of the lanthanide contraction strategy to tune proton conductivity while preserving proton-conducting pathways.
RESUMEN
Two new lanthanide-based 3D metal-organic frameworks (MOFs), {[Ln(L)(Ox)(H2O)]n·xH2O} [Ln = Gd3+ and x = 3 (1) and Dy3+ and x = 1.5 (2); H2L = mucic acid; OxH2 = oxalic acid] showing interesting magnetic properties and channel-mediated proton conduction behavior, are presented here. Single-crystal X-ray structure analysis shows that, in complex 1, the overall structure originates from the mucate-bridged gadolinium-based rectangular metallocycles. The packing view reveals the presence the two types of hydrophilic 1D channels filled with lattice water molecules, which are strongly hydrogen-bonded with coordinated water along the a and b axes, whereas for complex 2, the 3D framework originates from a carboxylate-bridged dysprosium-based criss-cross-type secondary building block. Magnetic studies reveal that 1 exhibits a significant magnetic entropy change (-ΔSM) of 30.6 J kg-1 K-1 for ΔH= 7 T at 3 K. Our electronic structure calculations under the framework of density functional theory reveal that exchange interactions between Gd3+ ions are weak and of the antiferromagnetic type. Complex 2 shows field-induced single-molecule-magnetic behavior. Impedance analysis shows that the proton conductivity of both complexes reaches up to the maximum value of 4.7 × 10-4 S cm-1 for 1 and 9.06 × 10-5 S cm-1 for 2 at high temperature (>75 °C) and relative humidity (RH; 95%). The Monte Carlo simulations confirm the exact location of the adsorbed water molecules in the framework after humidification (RH = 95%) for 1. Further, the results from computational simulation also reveal that the presence of a more dense arrangement of adsorbed water molecules through hydrogen bonding in a particular type of channel (along the a axis) contributes more to the proton migration compared to the other channel (along the b axis) in the framework.