Redox Switching of Magnetic Anisotropy in Copper(II) Verdazyl Systems

Reaction of copper(II)
triflate with 1-isopropyl-3,5-di(2′-pyridyl)-6-oxo-2H-tetrazine
(dipyvdH) and triethylamine gave the neutral species [Cu(dipyvd)2]. [Cu(dipyvd)2] can be oxidized in two one-electron
steps, leading to [Cu(dipyvd)2]+ and [Cu(dipyvd)2]2+ isolated as their PF6– salts. All three species are best formulated as copper(II) compounds
with localized anionic diamagnetic, or neutral radical ligands. Exchange
coupling between the neutral radical ligands and the copper(II) ion
is strongly ferromagnetic, leading to S = 1 and S = 3/2 ground states for [Cu(dipyvd)2]+ and [Cu(dipyvd)2]2+, respectively. High-field/high-frequency
EPR (HF-EPR) measurements reveal that, upon oxidation, the copper(II)
complexes become anisotropic, with zero-field splitting (ZFS) parameters
of D = 4.5 cm–1, E/D = 0.04 for [Cu(dipyvd)2]+ and D = 1.5 cm–1, E/D = 0.3 for [Cu(dipyvd)2]2+. DFT calculations suggest that the major component
of the ZFS tensor for [Cu(dipyvd)2]+ is aligned
with the Jahn–Teller elongation axis of the cations.