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 k 1 = 0.23 s −1 and k −1 =62 M −1 s −1 , respectively. The binding of a bipy to the [Fe(imox) (Nmim) (H 2 O)] + complex proceeds according to k 2 = 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 k 3 = 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.