Kinetics and mechanism of ligand substitution reactions of pentacyanoferrate(II) complexes with bridging N-heterocyclic dications in aqueous media

Abstract

Three series of N-heterocyclic dicationic ligands, [R(CH2)"R]2+, [R(o-xyl)R]2+, and [R(p-xyl)R]2+, where R = pyrazine (pyz, = 3-8), 4,4'-bipyridine (bpy, = 1-8), and trans-1,2-bis(4-pyridyl)ethylene (bpe, -1-8), have been synthesized by the reaction of the N-heterocycle with the appropriate , -dibromoalkane, , '-dibromo-oxylene, or , '-dibromo-p-xylene. In addition, ligands with R = 3-methyl-and 3-aminopyrazine, in which the free N donor atom is sterically hindered, have been prepared. These ligands react with the aquapentacyanoferrate(II) ion, [Fe(CN)50H2]1 23", to form monomeric, [Fe(CN)5L]-, and bridged dimeric, [(NQsFeOt-LJFefCN^]4-, complexes in aqueous solution. The rate and activation parameters for the aqua substitution and ligand dissociation reactions of these complexes have been measured and are found to be consistent with dissociative mechanisms of ligand substitution. The dependence of the rate constants for the formation of [Fe(CN)5(R(CH2)"R)]-on the polymethylene chain length n decreases in the order R = pyz > bpy > bpe, as the donor nitrogen atom is further removed from the site of the positive charges. Saturation kinetics are observed in the formation reactions with the dicationic ligands, consistent with an ion-pair dissociative mechanism, with [Fe(CN)5OH2||L]~ion-pair constants in the range 40-100 M-1. The rate constants for the rate-determining loss of the coordinated aqua ligand 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)s]3-center results in ligand dissociation rate constants which are 103 to 104 56789greater than for ligands in which these substituents are in meta positions or are absent. Steric strain energies of 17 and 22 kJ mol-1 have been determined for the aminoand methyl-substituted pyrazinium ligands, respectively.

A study of mercury(II)-catalysed substitution of cyanide in hexacyanoferrate(II) by 2-methylpyrazine: effects of inhibitor on the catalysed reaction

“…He demonstrated that when [Fe (CN) 6 ] 4-is added to Hg(CN) 2 An I d mechanism is favoured over a D mechanism, as on decreasing water concentration, the reaction rate also decreases. This suggests the involvement of coordinated water in the activated complex.…”

“…Among transition metal cyano-complexes, pentacyanoferrate(II) has been intensively researched in aqueous as well as in micellar media [1][2][3][4][5]. Over the last few decades, hexacyanoferrate(II) complexes have attracted the attention of many researchers [6][7][8][9][10][11][12][13].…”

3-Amino-1-methylpyrazin-1-ium iodide

“…For the synthesis and characterization of the title compound, see: Foucher et al (1993). Additional preparative details of similar compounds are given by Goto et al (1968).…”

“…The title compound, (I), is prepared in moderate yield from the reaction of 2-aminopyrazine with methyl iodide in carbon tetrachloride (Foucher et al, 1993). The proximity of the amine group to one of the diazine nitrogen atoms makes it an ideal chelating ligand to metals and geometrically suggests the possibility for amine-imine tautomerism.…”

3-Amino-1-methylpyrazin-1-ium chloride

“…The title chloride compound, (I), was recovered from the ion exchange (Dowex 1-X8 ion exchange resin saturated with Clanions) of the iodide precursor of N-methyl-3-aminopyrazinium iodide (Foucher et al, 1993). The proximity of the amine group to one of the diazine N atoms makes it an ideal chelating ligand to metals and geometrically suggests the possibility for amine-imine tautomerism.…”