Magnetic susceptibility measurements on a frozen solution of FeCl−4 anions in isopropanol in the 0.07–4.2 K temperature range

Abstract

Magnetic susceptibility measurements in the range 0.07–4.2 K have been performed on a 0.32 M solution of [FeCl4]− anions in isopropanol. At 0.1 K the experimental susceptibility is more than an order of magnitude smaller than that corresponding to a single [FeCl4]− monomer, a fact which clearly suggests the formation of dimers of [FeCl4]− with antiferromagnetic coupling. A careful analysis of the susceptibility curve has been performed allowing the existence of dimers as well as of monomers in the solution. Moreover, zero-field splitting terms of the form D[s2z−1/3 s(s+1)] have also been introduced in the theoretical description of both monomers and dimers. The experimental susceptibility can be satisfactorily explained along these lines and is consistent with the following conclusions: (1) Around 90% of the [FeCl4]− units present in the solution are in dimer form while the remainder are present as monomers; (2) The effective exchange interaction between the two ferric ions involved in a dimer is J=(0.15±0.02) cm−1. This value is consistent with a Fe–Cl–S–Cl–Fe superexchange path where S could be a solvent molecule; and (3) The [FeCl4]− units do not display a right Td symmetry but they are distorted by the interaction with the solvent giving rise to a value D=−(0.60±0.10) cm−1. The present results point out that magnetic susceptibility measurements in that range of temperature can provide us with useful and detailed information on the solute–solute and solute–solvent interactions in liquid solutions involving a paramagnetic complex.