EPR and magnetic properties of [Mn(μ-Cl)2(bpy)]n: An unusual ferromagnetic interaction in a Mn(II) chloro-bridged polymer

The study of the magnetic properties and EPR spectra of the one-dimensional bis(μ-chloro) Mn(II) polymer [Mn(μ-Cl)2(bpy)]n (1) (bpy = 2,2′-bipyridine) is reported. Magnetic susceptibility measurements reveal a weak ferromagnetic interaction between the Mn(II) ions (J = +0.19 cm−1). This is the first polymer of six-coordinated Mn(II) ions with a [Mn2(μ-Cl)2]2+ magnetic repeating unit, showing a ferromagnetic interaction. The coordination octahedra are elongated in the Cl⋯Cl direction and the elongation axes are parallel along the chain. Extended Hückel calculations show that the octahedra share an equatorial-apical edge, with the equatorial plane being parallel, and perpendicular to the Mn2(μ-Cl)2. Due to this disposition, the overlap through the chloride bridges is small, so the antiferromagnetic contribution must also be small and, therefore, a ferromagnetic behaviour is observed. The EPR spectra of a polycrystalline sample of 1 at different temperatures are also reported. A variation of the bandwidth with temperature is observed, which could be because this ferromagnetic interaction is weak: above 150 K the dipolar interaction is the predominant effect, while below 150 K the magnetic exchange interaction dominates.

The ferromagnetic behaviour observed for the one-dimensional compound [Mn(μ-Cl)2(bpy)]n could be due to the parallel disposition of the elongated octahedra; two coordination polyhedra share an equatorial-apical edge, with the equatorial plane being parallel. The “accordion effect” of the bandwidth EPR spectra for [Mn(μ-Cl)2(bpy)]n could be explained by the competition between the dipolar effect and the magnetic interaction.Figure optionsDownload as PowerPoint slide