Slow Magnetic Relaxations in Cobalt(II) Tetranitrate Complexes. Studies of Magnetic Anisotropy by Inelastic Neutron Scattering and High-Frequency and High-Field EPR Spectroscopy

Three mononuclear cobalt­(II) tetranitrate complexes (A)<sub>2</sub>[Co­(NO<sub>3</sub>)<sub>4</sub>] with different countercations, Ph<sub>4</sub>P<sup>+</sup> (<b>1</b>), MePh<sub>3</sub>P<sup>+</sup> (<b>2</b>), and Ph<sub>4</sub>As<sup>+</sup> (<b>3</b>), have been synthesized and studied by X-ray single-crystal diffraction, magnetic measurements, inelastic neutron scattering (INS), high-frequency and high-field EPR (HF-EPR) spectroscopy, and theoretical calculations. The X-ray diffraction studies reveal that the structure of the tetranitrate cobalt anion varies with the countercation. <b>1</b> and <b>2</b> exhibit highly irregular seven-coordinate geometries, while the central Co­(II) ion of <b>3</b> is in a distorted-dodecahedral configuration. The sole magnetic transition observed in the INS spectroscopy of <b>1</b>–<b>3</b> corresponds to the zero-field splitting (2­(<i>D</i><sup>2</sup> + 3<i>E</i><sup>2</sup>)<sup>1/2</sup>) from 22.5(2) cm<sup>–1</sup> in <b>1</b> to 26.6(3) cm<sup>–1</sup> in <b>2</b> and 11.1(5) cm<sup>–1</sup> in <b>3</b>. The positive sign of the <i>D</i> value, and hence the easy-plane magnetic anisotropy, was demonstrated for <b>1</b> by INS studies under magnetic fields and HF-EPR spectroscopy. The combined analyses of INS and HF-EPR data yield the <i>D</i> values as +10.90(3), +12.74(3), and +4.50(3) cm<sup>–1</sup> for <b>1</b>–<b>3</b>, respectively. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal the slow magnetization relaxation in <b>1</b> and <b>2</b> at an applied dc field of 600 Oe, which is a characteristic of field-induced single-molecule magnets (SMMs). The electronic structures and the origin of magnetic anisotropy of <b>1</b>–<b>3</b> were revealed by calculations at the CASPT2/NEVPT2 level.