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

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^II is often used as a substitute for spectroscopically silent Zn^II. In this work, four pseudo-tetrahedral scorpionate complexes with the formula (Tp^t-Bu,Tn)CoL, where Tp^t-Bu,Tn = hydrotris(3-tert-butyl, 5-2'-thienyl-pyrazol-1-yl)borate anion and L = Cl^-, N₃^-, 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 = N₃^-, 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^-1 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^-1 for the L = N₃^- and NCO^- complexes, respectively. These values were not as close to those determined by HFEPR in the present study, which are 4.2 cm^-1 ≤ |D| ≤ 5.6 cm^-1 in Tp^t-Bu,TnCoN₃, and 8.3 cm^-1 ≤ |D| ≤ 11.0 cm^-1 in Tp^t-Bu,TnCoNCO. The bands in MCD spectra of these complexes were assigned in C_3v 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.