EPR Studies of Axial Ligation of a Low-Spin Cobalt(II) Macrocyclic Schiff Base Complex

EPR studies demonstrate that 1:1 adducts of cobalt(II) 5, 7, 12, 14-tetramethyldibenzo[b,i][l, 4, 8, ll]tetraazacyclotetra-decahexaene with ligands containing phosphorus, nitrogen, sulfur and oxygen donor atoms all have a (dxzdyzdx2_y2)6d221 ground-state configuration and similar spin-density distributions. The primary difference between the adducts is the energy separation between the ground state and the (dxzdx2-ydz2)6dyz1 state which decreases as the strength of the axial bond decreases, until in very weakly coordinating solvents such as neat toluene it is not possible to definitely say which is the ground state. All of the EPR spectra have rhombic symmetry which is due to a difference in the energies of the dyz1 and dxz1 states. This difference becomes larger as the strength of the axial bond increases. The EPR results are compared with results for analogous cobalt(II) porphyrin complexes. Solvent effects also occur and cause an orientation- and m1dependent broadening of the EPR Unes in a manner consistent with the ground-state assignment.