Crystal structure and magnetic properties of Cu(hfa)2NITPrn, a one-dimensional ferromagnet

Abstract

A linear chain compound formed by copper(II) and a nitronyl nitroxide radical of formula Cu(hfa)₂NITPrⁿ, where hfa = hexafluoroacetylacetonate and NITPrⁿ= 1-oxido-3-oxo-2-n-propyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidaz-ol-3-ium-1-yl has been synthesized. It crystallizes in the triclinic P1 space group with a= 10.071(3)Å, b= 10.460(1)Å, c= 16.003(2)Å, α= 95.54(1)°, β= 78.58(2)°, γ= 63.27(2)°, and Z = 2. The Cu(hfa)₂ moieties are bridged by the radicals which are coordinated to two metal ions through the two equivalent oxygen atoms on which the unpaired electron is equally delocalized. The product of magnetic susceptibility and temperature, χT, increases on lowering temperature and diverges as T tends to zero, as expected for an infinite chain of ferromagnetic coupled spins and a coupling constant between the metal and the radical J=–32.1(2) cm ^–1 has been estimated using the hamiltonian H=JS₁·S₂. The one-dimensional character has been confirmed also by single-crystal electron paramagnetic resonance spectra, but at low temperature the effects of weak inter-chain interactions are detected. The low-temperature magnetic behaviour of the title compound is compared with those of two other analogous derivatives comprising the methyl- and isopropyl-substituted radicals and the results confirm the hypothesis that the inter-chain dipolar interactions are mainly responsible for the deviation from one-dimensional behaviour.