Cobalt(II) "Scorpionate" complexes as electronic ground state models for cobalt-substituted zinc enzymes: Structure investigation by magnetic circular dichroism.

Zinc centers in pseudo-tetrahedral geometry are widely found in biology, often with three histidine ligands from protein. The trispyrazolylborate "scorpionate" ligand is used as a model for this tris(histidine) motif, and spectroscopically active Co is often used as a substitute for spectroscopically silent Zn. In this work, four pseudo-tetrahedral scorpionate complexes with the formula (Tp)CoL, where Tp = hydrotris(3-tert-butyl, 5-2'-thienyl-pyrazol-1-yl)borate anion and L = Cl, N, NCO, or NCS, were studied using variable-temperature, variable-field magnetic circular dichroism (VTVH MCD) spectroscopy. The major goal was to determine the axial and rhombic zero field splitting (ZFS) parameters (D and E, respectively) of these S = 3/2 systems and compare these ZFS parameters to those determined previously by high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy on the same (L = Cl and NCS) or closely related complexes. Additionally, HFEPR studies were undertaken here on the complexes with L = N, NCO. Crystal structures for these two complexes are also first reported here. The values of D determined by VTVH MCD were + 12.8 and + 3.6 cm for the L = Cl and NCS complexes, respectively. These values are in close agreement with those for the same complexes as previously determined by HFEPR. The values of D determined by VTVH MCD were + 3.0 and + 6.6 cm for the L = N and NCO complexes, respectively. These values were not as close to those determined by HFEPR in the present study, which are 4.2 cm ≤ |D| ≤ 5.6 cm in TpCoN, and 8.3 cm ≤ |D| ≤ 11.0 cm in TpCoNCO. The bands in MCD spectra of these complexes were assigned in C symmetry and a complete ligand-field analysis of the MCD data was made using the Angular Overlap Model (AOM), which is compared to previous results.