ESR analyses of picket fence MnII and 6th ligand coordinated FeIII porphyrins (S = 5/2) and a CoII(hfac) complex (S = 3/2) with sizable ZFS parameters revisited: a full spin Hamiltonian approach and quantum chemical calculations

Abstract

The fictitious spin-1/2 (effective spin-1/2) Hamiltonian approach has been the putative method to analyze the conventional fine-structure/hyperfine ESR spectra of high spin metallocomplexes with sizable zero-field splitting (ZFS) tensors since the early 1950s, and the approach gives salient principal g^eff-values far from g = 2 without explicitly affording their ZFS values in most cases. The experimental g^eff-values thus determined, however, never agree with those (g^true-values) of the true principal g-tensors, which are obtainable from reliable quantum chemical calculations. We have recently derived exact or extremely accurate analytical expressions for the g^eff/g^true relationships for the spin quantum number S's up to S = 7/2 (T. Yamane et al., Phys. Chem. Chem. Phys., 2017, 19, 24769–24791). In this work, we have removed the limitation of the collinearity between g- and ZFS tensors and derived the generalized g^eff/g^true relationships. To illustrate the usefulness of the present approach, we have revisited important typical high spin systems with large ZFS values such as picket fence metalloporphyrins with Mn^II (S = 5/2) (Q. Yu et al., Dalton Trans., 2015, 44, 9382–9390), a 6th ligand coordinated porphyrin with Fe^III (S = 5/2) (Y. Ide et al., Dalton Trans., 2017, 46, 242–249) and a pseudo-octahedral Co^II (S = 3/2)(hfac)₂ complex (D. V. Korchagin et al., Dalton Trans., 2017, 46, 7540–7548), completing the ESR spectral and magnetic susceptibility analyses and gaining significant physical insights into their electronic structures. The off-principal axis extra peaks overlooked in the documented spectra of the picket fence Mn^II porphyrins have fully been assigned, affording their accurate true g-, hyperfine and ZFS tensors, for the first time. For the Co^II complex, the occurrence of the non-collinearity between the g- and ZFS tensors has been discussed by using the generalized g^eff/g^true relationships. We have attempted to carry out reliable DFT-based and ab initio quantum chemical calculations of their magnetic tensors, in which spin–orbit couplings are incorporated, reproducing the experimental true tensors. We emphasize that the incorporation of multi-reference nature in the electron configuration is important to interpret the magnetic tensors for the Co^II complex.