Conventional ESR Spectral Analyses and Quantum Chemical Calculations of High-Spin Cobalt(II) Complexes with Large ZFS Parameters Revisited: Exact g- and Hyperfine Tensors’ Relationships Between Fictitious Spin-1/2 and True spin-3/2 Hamiltonian Approaches
摘要
High-spin metallocomplexes having sizable zero-field splitting (ZFS) parameters feature in both exotic magnetic materials and many biological systems. The magnetic properties of such compounds have frequently been determined by X-band ESR spectroscopy with the help of fictitious spin-1/2 Hamiltonian approaches. The determined geff-values, however, never agree with those (gtrue-values) of the true g-tensors, which are obtained from precise quantum chemical calculations. In this work, we have revisited the X-band ESR spectra of four important penta-coordinated cobalt(II) complexes (complex 1 and 2, A. A. Fischer, et al., Dalton Trans. 46, 13, 229 (2017); complex 3 and 4, P. Kumar, et al., J. Am. Chem. Soc. 141, 10, 984 (2019), P. Kumar, et al., Inorg. Chem. 59, 16, 178 (2020)) in their quartet states. We use the exact relationships for the g- and hyperfine tensors derived using the fictitious spin-1/2 and true spin Hamiltonian (SH). Double perturbation treatments combined with the Zeeman and hyperfine interaction perturbations have been invoked to derive the relationships, giving physical insights into the complex exact relationships. The accuracy of the simplified relationships relevant to hyperfine tensors has been examined compared with the exact ones. The full sets of the principal values of the g-, hyperfine, and rank-2 ZFS tensors of the complexes have been evaluated from the canonical peaks of the |MS = ± 3/2 > -dominant transitions as well as the weak peaks attributed to the |MS = ± 1/2 > -dominant transitions. The absolute D-values amount to ~ 10 cm–1 (D < 0) for complexes 1–3. Quantum chemical calculations for the true magnetic tensors provide their salient electronic structures, which have not been considered in the previous works.