Magnetic properties of the ladder-like MOF {[Co(Azo-Py)(Cx)]2.15H2O}n

Abstract

The majority of magnetic metal-organic-framework polymers (MOFs) are those containing metal centers and in particular, 3d- and 4f- transition metals, which may exist in different oxidation states, spin states. The developments in the metallopolymers also include silicone component, in which the metal is complexed through organic groups attached to the siloxane moiety. We present in this work the magnetic properties of the compound {[Co(Azo-Py)(Cx)]2.15H2O}n that builds up as a 2D coordination polymer, where Azo-Py is 4,4'-azopyridine and Cx is 1,3-bis(carboxypropyl)tetramethyldisiloxane. The 2D coordination network is built by [Co(Azo-Py)(Cx)] entities as a 1D ladder-like polymer, with Co(II) dimer rungs, with Co-Co separation at 4.041 ºA. The Co is coordinated by three carboxylate groups and two Azo-Py ligands building a six-coordinate distorted (O4N2) octahedral environment, the basal plane being occupled by four oxygen atoms and two nitrogen atoms in the apical positions. The Co dimer is supported by two carboxylate ligands which fulfil a bidentate-bridging (η1η1μ) function. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization ,easurements on an isostructural compound {[Co0.13Zn0.87(Azo-Py)(Cx)].2CH3OH.0.375H2O}n wher Co sits in the same C2v site. The results show that the Co(II) ion has uniaxial anisotropy with the easy-axis direction in the C2v main axis, as predicted by the ab-initio relativistic calculations of the g-tensor. From the fit of the high temperature magnetic susceptibility, the energy difference Δ/Kb=185 K between the ground state and the excited doublets is found. In the undiluted compounds, the Co-Co interaction within the dimer is found to be very small, of the order of a few tenths of K, as determined from low temperature heat capacity measurements. The coupling is antiferromagnetic, as is derived from the downwards deviation of the M(H) curve in the non-diluted sample with respect to the diluted one. Therefore it is shown taht the present compound is a ladder-type polyorganometalosiloxane with Co-Co antiferromagnetically coupled rungs.