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

• Published in: Chemistry : a European journal 2020-10, Vol.26 (56), p.12769-12784
• Main Authors: Mathivathanan, Logesh, Rogez, Guillaume, Ben Amor, Nadia, Robert, Vincent, Raptis, Raphael G., Boudalis, Athanassios K.
• Format: Article
• Language: English
• Subjects: ab initio calculations; Anisotropy; Chemical Sciences; Chemistry; Copper; Copper compounds; Couplings; Electron paramagnetic resonance; Electron spin resonance; EPR spectroscopy; exchange anisotropy; Exchanging; Ferromagnetism; Magnetic anisotropy; Nitrogen dioxide; or physical chemistry; Spectroscopy; spin triangles; Theoretical and; Wave functions
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202001028

Previously reported ferromagnetic triangles (NnBu4)2[Cu3(μ3‐Cl)2(μ‐4‐NO2‐pz)3Cl3] (1), (PPN)2[Cu3(μ3‐Cl)2(μ‐pz)3Cl3] (2), (bmim)2[Cu3(μ3‐Cl)2(μ‐pz)3Cl3] (3) and newly reported (PPh4)2[Cu3(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 dz2 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. Spin triangles: Single‐crystal EPR experiments combined with ab initio calculations are used to rationalize the ferromagnetism of copper(II) triangles and to quantify the anisotropy induced by anisotropic exchange and dipolar interactions.