KINETICS AND MECHANISM OF LIGAND SUBSTITUTION-REACTIONS OF PENTACYANOFERRATE(II) COMPLEXES WITH BRIDGING N-HETEROCYCLIC DICATIONS IN AQUEOUS-MEDIA

Three series of N-heterocyclic dicationic ligands, [R(CH2)nR]2+, [R(o- xyl)R]2+, and [R(p-xyl)R]2+, where R = pyrazine (pyz, n = 3-8), 4,4'-b ipyridine (bpy, n - 1-8), and trans- 1,2-bis(4-pyridyl)ethylene (bpe, n - 1-8), have been synthesized by the reaction of the N-heterocycle w ith the appropriate alpha,omega-dibromoalkane, alpha,alpha'-dibromo-o- xylene, or alpha,alpha'-dibromo-p-xylene. In addition, ligands with R = 3-methyl- and 3-aminopyrazine, in which the free N donor atom is ste rically hindered, have been prepared. These ligands react with the aqu apentacyanoferrate(II) ion, [Fe(CN)5OH2]3-, to form monomeric, [Fe(CN) sL]-, and bridged dimeric, [(NC)5Fe(mu-L)Fe(CN)5]4-, complexes in aque ous solution. The rate and activation parameters for the aqua substitu tion and ligand dissociation reactions of these complexes have been me asured and are found to be consistent with dissociative mechanisms of ligand substitution. The dependence of the rate constants for the form ation of [Fe(CN)5(R(CH2)nR)]- on the polymethylene chain length n decr eases in the order R = pyz > bpy > bpe, as the donor nitrogen atom is further removed from the site of the positive charges. Saturation kine tics are observed in the formation reactions with the dicationic ligan ds, consistent with an ion-pair dissociative mechanism, with [Fe(CN)5O H2 parallel-to L]- ion-pair constants in the range 40-100 M-1. The rat e constants for the rate-determining loss of the coordinated aqua liga nd decrease from approximately 300 to 90-130 s-1 upon ion-pairing with the dications. The presence of the amino or methyl substituents ortho to the nitrogen coordinated to the [Fe(CN)5]3- center results in liga nd dissociation rate constants which are 10(3) to 10(4) greater than f or ligands in which these substituents are in meta positions or are ab sent. Steric strain energies of 17 and 22 kJ mol-1 have been determine d for the amino- and methyl-substituted pyrazinium ligands, respective ly.