Antisymmetric exchange in two tricopper(ii) complexes containing a [Cu3(μ3-OMe)]5+ core

Abstract

Reaction of CuX₂ (X⁻ = Cl⁻ or Br⁻) with 2 molar equivalents of 3{5}-(2,4,6-trimethylphenyl)pyrazole (Hpz^Mes) in MeOH in the presence of NaOH yields [Cu₃X(Hpz^Mes)₂(μ-pz^Mes)₃(μ₃-OMe)]X (X⁻ = Cl⁻ or Br⁻). Crystal structures of these compounds show almost identical triangles of Cu(II) ions, centred by a triply bridging methoxide ligand and with three edge-bridging pyrazolide groups. The mesityl substituents on the bridging pyrazolide ligands are arranged in HT, HH, TT fashion. χ_MT for both compounds decreases steadily with decreasing temperature, reaching 0.40 cm³ mol⁻¹ K at 70 K before decreasing further below 40 K. This low temperature behaviour could not be interpreted using conventional superexchange Hamiltonians, but was reproduced by an alternative model that incorporated an additional antisymmetric exchange term. This interpretation was confirmed by the Q-band EPR spectra of the two compounds. NMR experiments show that the structures of these compounds are not retained in solution, in contrast to other closely related tricopper compounds. These are the first examples of triangular Cu(II) compounds bearing a [Cu₃(μ₃-OR)]⁵⁺ (R ≠ H) core motif, and the first triangular compounds showing antisymmetric exchange to have been analysed by both susceptibility and EPR measurements.