Purely Spectroscopic Determination of the Spin Hamiltonian Parameters in High-Spin Six-Coordinated Cobalt(II) Complexes with Large Zero-Field Splitting

Abstract

Accurate determination of the spin Hamiltonian parameters in transition-metal complexes with large zero-field splitting (ZFS) is an actual challenge in studying magnetic and spectroscopic properties of high-spin transition metal complexes. Recent critical papers have convincingly shown that previous determinations of these parameters, based only on the magnetic data, have low accuracy and reliability. A combination of X-band electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry seems to be a more convincing and accurate approach. However, even in this case, the accuracy of the determination of the spin Hamiltonian parameters is strongly limited. In this work, we propose a purely spectroscopic approach, in which three complementary EPR spectroscopic techniques are used to unambiguously with high accuracy determine the spin Hamiltonian parameters for transition-metal complexes with S = 3/2. The applicability of this approach is demonstrated by analyzing the new quasi-octahedral high-spin Co(II) complex [Co(hfac) 2 (bpy)] (I). Along with the conventional X-band EPR spectroscopy, we also use such advanced techniques as multi-high-frequency EPR spectroscopy (MHF-EPR) and frequency-domain Fouriertransform THz-EPR (FD-FT THz-EPR). We demonstrate that the experimental data derived from the X-band and MHF-EPR EPR spectra allow determination of the g tensor (g x = 2.388, g y = 2.417, g z = 2.221) and the ZFS rhombicity parameter E/D = 0.158. The axial ZFS parameter D = 37.1 cm −1 is measured for I with the aid of FD-FT THZ-EPR spectroscopy, which is able to detect the high-energy EPR transition between the two Kramers doublets. CASSCF/NEVPT2 quantum-chemical calculations of magnetic parameters and magnetic direct current (dc) measurements are performed as well as testing options, and the results obtained in these ways are in good agreement with those derived using the proposed spectroscopic approach.

Insertion of single-ion magnets based on mononuclear Co(ii) complexes into ferromagnetic oxalate-based networks

“…More precise spectroscopic measurements should be necessary to avoid this correlation and to determine more precisely the E parameters. 28 These parameters are similar to those found for the related compounds. 21,22,28,29…”

The rationalized pathway from field-induced slow magnetic relaxation in CoII–WIV chains to single-chain magnetism in isotopological CoII–WV analogues

“…This Hamiltonian is often used for the interpretation of EPR spectroscopic data. 86 Its application is also a convenient route to discuss and compare magnetic anisotropy for Co II complexes, which is represented by the axial ZFS parameter, D , rhombic ZFS parameter, E , and the rhombicity parameter, E / D . 86,87 Based on the ab initio calculations, 86,88 we determined these ZFS parameters for all investigated Co II complexes (Table S11†).…”

Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets

“…The Hamiltonian acts on |S, L, MS, ML > functions with Usually, the magnetic anisotropy is treated with the spin Hamiltonian comprising the zero-field splitting and Zeeman terms; however, hexacoordinate Co II complexes with the shape of the chromophore close to octahedron or trigonal prism possess orbital angular momentum, hence the spin Hamiltonian is inappropriate. This was confirmed by ab initio calculations that predicted magnetic behavior based on E ground state for 1 and low lying (below 1000 cm −1 ) excited states for 2 and 3, both contradicting the use of spin Hamiltonian formalism [41] (vide infra).…”