Cobalt(II) “Scorpionate” Complexes as Models for Cobalt-Substituted Zinc Enzymes: Electronic Structure Investigation by High-Frequency and -Field Electron Paramagnetic Resonance Spectroscopy

A series of complexes of formula Tp^R,R′CoL, where Tp^R,R′− = hydrotris(3-R,5-R′-pyrazol-1-yl)borate (“scorpionate”) anion (R = tert-butyl, R′ = H, Me, 2′-thienyl (Tn), L = Cl⁻, NCS⁻, NCO⁻, N₃⁻), has been characterized by electronic absorption spectroscopy in the visible and near-infrared (near-IR) region and by high-frequency and -field electron paramagnetic resonance (HFEPR). Reported here are also crystal structures of seven members of the series that have not been reported previously: R′ = H, L = NCO⁻, N₃⁻; R′ = Me, L = Cl⁻, NCS⁻, NCO⁻, N₃⁻; R′ = Tn, L = Cl⁻, NCS⁻. These include a structure for Tp^t-Bu,MeCoCl different from that previously reported. All of the investigated complexes contain a four-coordinate cobalt(II) ion (3d⁷) with approximate C_3v point group symmetry about the metal ion and exhibit an S = ³/₂ high-spin ground state. The use of HFEPR allows extraction of the full set of intrinsic S = ³/₂ spin Hamiltonian parameters (D, E, and g values). The axial zero-field splitting parameter, D, for all investigated Tp^R,R′CoL complexes is always positive, a fact not easily determined by other methods. However, the magnitude of this parameter varies widely: 2.4 cm⁻¹ ≤ D ≤ 12.7 cm⁻¹, indicating the extreme sensitivity of this parameter to environment. The spin Hamiltonian parameters are combined with estimates of 3d energy levels based on the visible−near-IR spectra to yield ligand-field parameters for these complexes following the angular overlap model (AOM). This description of electronic structure and bonding in pseudotetrahedral cobalt(II) complexes can enhance the understanding of similar sites in metalloproteins, specifically cobalt-substituted zinc enzymes.