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Slow Magnetic Relaxation in {[CoCxAPy)] 2.15 H<sub>2</sub>O}<sub>n</sub> MOF Built from Ladder-Structured 2D Layers with Dimeric SMM Rungs
oleh: Ana Arauzo, Elena Bartolomé, Javier Luzón, Pablo J. Alonso, Angelica Vlad, Maria Cazacu, Mirela F. Zaltariov, Sergiu Shova, Juan Bartolomé, Constantin Turta
Format: | Article |
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Diterbitkan: | MDPI AG 2021-09-01 |
Deskripsi
We present the magnetic properties of the metal-organic framework {[CoCxAPy]·2.15 H<sub>2</sub>O}<sub>n</sub> (Cx = bis(carboxypropyl)tetramethyldisiloxane; APy = 4,4`-azopyridine) (<b>1</b>) that builds up from the stacking of 2D coordination polymers. The 2D-coordination polymer in the <i>bc </i>plane is formed by the adjacent bonding of [CoCxAPy] 1D two-leg ladders with Co dimer rungs, running parallel to the <i>c</i>-axis. The crystal packing of 2D layers shows the presence of infinite channels running along the <i>c</i> crystallographic axis, which accommodate the disordered solvate molecules. The Co(II) is six-coordinated in a distorted octahedral geometry, where the equatorial plane is occupied by four carboxylate oxygen atoms. Two nitrogen atoms from APy ligands are coordinated in apical positions. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization measurements on an isostructural compound {[Zn<sub>0.8</sub>Co<sub>0.2</sub>CxAPy]·1.5 CH<sub>3</sub>OH}<sub>n</sub> (<b>2</b>). The results show that the Co(II) ion has orthorhombic anisotropy with the hard-axis direction in the C<sub>2V</sub> main axis, lying the easy axis in the distorted octahedron equatorial plane, as predicted by the <i>ab initio</i> calculations of the <b><i>g</i></b>-tensor. Magnetic and heat capacity properties at very low temperatures are rationalized within a <i>S</i>* = 1/2 magnetic dimer model with anisotropic antiferromagnetic interaction. The magnetic dimer exhibits slow relaxation of the magnetization (SMM) below 6 K in applied field, with a t<sub>lf</sub> ≈ 2 s direct process at low frequencies, and an Orbach process at higher frequencies with <i>U</i>/k<sub>B</sub> = 6.7 ± 0.5 K. This compound represents a singular SMM MOF built-up of Co-dimers with an anisotropic exchange interaction.