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

Eugenii Ya Misochko; Alexander V Akimov; Denis V Korchagin; Joscha Nehrkorn; Mykhaylo Ozerov; Andrew V Palii; Juan Modesto Clemente-Juan; Sergey M Aldoshin

doi:10.1021/acs.inorgchem.9b02195

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

Inorg Chem. 2019 Dec 16;58(24):16434-16444. doi: 10.1021/acs.inorgchem.9b02195. Epub 2019 Nov 21.

Authors

Eugenii Ya Misochko¹, Alexander V Akimov¹, Denis V Korchagin¹, Joscha Nehrkorn^{2

3}, Mykhaylo Ozerov², Andrew V Palii^{1

4}, Juan Modesto Clemente-Juan⁵, Sergey M Aldoshin¹

Affiliations

¹ Institute of Problems of Chemical Physics of RAS , Chernogolovka , Russia.
² National High Magnetic Field Laboratory & Florida State University , Tallahassee , Florida , United States.
³ Max Planck Institute for Chemical Energy Conversion , Mülheim/Ruhr , Germany.
⁴ Institute of Applied Physics , Chisinau , Moldova.
⁵ Instituto de Ciencia Molecular , Universidad de Valencia , Paterna , Spain.

PMID: 31749359
DOI: 10.1021/acs.inorgchem.9b02195

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 Fourier-transform 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.