Substitution Kinetics of [Fe(PDT/PPDT)n(phen)m]2+(n≠m;n,m= 1,2) with 2,2′-Bipyridine, 1,10-Phenanthroline, and 2,2′,6,2″-Terpyridine

Abstract

The triazines 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PDT), 3-(4-phenyl-2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PPDT), and 1,10-phenanthroline (phen) were coordinated to the Fe2+ ion to form (1), (2), , (3) and (4). The complexes were synthesized and characterized by mass spectroscopy and elemental analysis. The rate of substitution of these complexes by 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), and 2,2′,6,2″-terpyridine (terpy) was studied in a sodium acetate–acetic acid buffers over the range 3.6–5.6 at 25, 35, and 45°C under pseudo–first-order conditions. The reactions are first order with respect to the concentration of the complexes. The reaction rates increase with increasing [bpy/phen/terpy] and pH, whereas ionic strength has no influence on the rate of reaction. Plots of kobs versus [bpy/phen/terpy] and 1/[H+] are linear with positive slopes and significant y-intercepts. This indicates that the reactions proceed by both dissociative as well as associative pathways for which the associative pathway predominates the substitution kinetics. Observed temperature-depended rate constants at the three temperatures at which substitution reactions were studied together with the protonation constants of the substituting ligands (phen, bpy, terpy) were used to evaluate the specific rate constants (k1 and k2) and thermodynamic parameters (Ea, ΔH#, ΔS#, and ΔG#). The reactivity order of the four complexes depends on the phenyl groups present on the triazine (PDT/PPDT) molecule. The π-electrons on phenyl rings stabilizes the charge on the metal center by inductive donation of electrons toward the metal center resulting in a decrease in reactivity of the complex, and the order is 1 < 2 < 3 < 4. The rate of substitution is also influenced by the basicity of the incoming ligand (bpy/phen/terpy), and it decreased in the order: phen > terpy > bpy. Higher rate constants, low Ea values, and more negative entropy of activation (−ΔS#) values were observed for the associative path, revealing that substitution reactions at the octahedral iron(II) complexes by bpy, phen, and terpy occur predominantly by the associative mechanism. Density functional theory calculations support the interpretations.