Kinetics and mechanisms of substitution of the axial and equatorial ligands in an iron(II) macrocyclic heme model complex

The complex Fe(imox) (Nmim)₂], where imox is a planar bis(iminooxime) macrocyclic ligand and Nmim=N-methyl imidazole, undergoes substitution reactions in the presence of 2,2′bipyridine (bipy), leading to a series of intermediate mixed complexes. The forward and reverse rate constants for the substitution of the Nmim ligand by water are k₁ = 0.23 s^?1 and k_?1=62 M^?1s^?1, respectively. The binding of a bipy to the Fe(imox) (Nmim) (H₂O)]⁺ complex proceeds according to k₂ = 4.7×10^?2 M^?1 s^?1 and k_?2 = 3.6 × 10^?4 s^?1, yielding Fe(imox) (bipy) (Nmim)]⁺, which eliminates Nmim according to k₃ = 1.6 × 10^?4 s^?1. Further substitution in the Fe(imox) (bipy)]⁺ complex with bipy takes place very slowly leading to the Fe(imox) (bipy)₂]⁺ and Fe(bipy)₃]²⁺ complexes. © 1993 John Wiley & Sons, Inc.