Crystal structure and magnetic properties of a new ferrimagnetic chain containing manganese(II) and a nitronyl-nitroxide radical. Magnetic ordering in Mn(hfac)2NITR compounds

Abstract

A new nitronyl–nitroxide radical, 2-(p-dimethylaminophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hereafter denoted NIT(p-NMe₂)Ph, has been synthesized. It crystallizes in the P2₁/c, space group, with a= 15.742(2)Å, b= 10.875(4)Å, c= 19.011(5)Å and β= 113.26(2)°, the asymmetric unit containing two independent radical molecules. Its reaction with manganese hexafluoroacetylacetonate, Mn(hfac)₂, in the presence of chloroform leads to the solvent-containing compound Mn(hfac)₂[NIT(p-NMe₂)Ph]·0.25HCCl₃, which crystallizes in the C2/c space group, with a= 28.183(4)Å, b= 13.899(2)Å, c= 23.425(5)Å, β= 125.28(2)°. Its structure presents chains of 2₁ symmetry running along the b, axis, where the manganeses are bridged by the nitronyl-nitroxide radicals. The magnetic properties in the 5–300 K range are typically those of a ferrimagnetic chain where S= 1/2 and S= 5/2 spins alternate regularly. Below 5 K, a three-dimensional (3D) ferrimagnetic ordering occurs. Rationalization of the ferrimagnetic character of Mn(hfac)₂[NIT(p-Nme₂)Ph]·0.25HCCl₃ and of some other Mn(hfac)₂NITR chain compounds on the basis of magnetic dipolar energy calculations has been attempted. Results show that, as far as only collinear magnetic structures and localized moments are considered, the dipolar interaction alone cannot explain the type of magnetic order adopted by these compounds. This questions the possible role of other physical parameters, such as single-ion anisotropy, in the 3D ordering.