Origin of Ferromagnetism and Magnetic Anisotropy in a Family of Copper(II) Triangles

Previously reported ferromagnetic triangles (NnBu₄)₂Cu₃(μ₃-Cl)₂(μ-4-NO₂-pz)₃Cl₃] ( 1 ), (PPN)₂Cu₃(μ₃-Cl)₂(μ-pz)₃Cl₃] ( 2 ), (bmim)₂Cu₃(μ₃-Cl)₂(μ-pz)₃Cl₃] ( 3 ) and newly reported (PPh₄)₂Cu₃(μ₃-Cl)₂(μ-4-Ph-pz)₃Cl₃] ( 4 ) were studied by magnetic susceptometry, electron paramagnetic resonance (EPR) spectroscopy and ab initio calculations to assess the origins of their ferromagnetism and of the magnetic anisotropy of their ground S=3/2 state (PPN⁺=bis(triphenylphosphine)iminium, bmim⁺=1-butyl-3-methylbenzimidazolium, pz^?=pyrazolate). Ab initio studies revealed the d character of the magnetic orbitals of the compressed trigonal bipyramidal copper(II) ions. Ferromagnetic interactions were attributed to weak orbital overlap via the pyrazolate bridges. From the wavefunctions expansions, the ratios of the magnetic couplings were determined, which were indeterminate by magnetic susceptometry. Single-crystal EPR studies of 1 were carried out to extend the spin Hamiltonian with terms which induce zero-field splitting (zfs), namely dipolar interactions, anisotropic exchange and Dzyaloshinskii–Moriya interactions (DMI). The data were treated through both a giant-spin model and through a multispin exchange-coupled model. The latter indicated that ≈62 % of the zfs is due to anisotropic and ≈38 % due to dipolar interactions. The powder EPR data of all complexes were fitted to a simplified form of the multispin model and the anisotropic and dipolar contributions to the ground state zfs were estimated.