Acid and metal ion promoted hydrolysis of [N-o-carboxylphenyl]iminodiacetato(picolinato) chromate(III) complex: a mechanistic study

摘要

The sodium salt of the [N-(o-carboxyphenyl)iminodiacetato] picolinatochromate(III) complex has been synthesised and cahracterised. In acidic media, he complex has been shown to aquate to diaqua[N-(o-carboxyphenyl)imminodiacetato]chrominum(III) and picolinc acid as the ultimate products. The enahnced kinetic stability exhibited by the aromatic amino acid (cida) may be attributed to (i) rsonance interaction of the carboxyl and tertiary nitrogen group with the phenyl ring sresulting ina shortening of the bond distance and thus leading to a closer approach of the chelate donor groups, (ii) the presence of a larger number (two five-membered and on six-membered ring) of chelate rings in the ligated amino acid, in comparison to only one five- membered metal-picolinato ring. The variation of the observed pseudo-first-order rate constants with perchloric acid concentration suggests the operation of two concurrent uncatalysed and acid-catalysed pathways. The reaction exhibits a solvent deutrium isotope effect k_D/k_H = 1.5 at 45 deg C. The solvent deuterium isotope effect is consistent with a rapid pre-equilibrium protonation followed by rate-determining ring opening and excludes a mechanism involving concerted attack by [H_3O~+]. he metal ions, Cu~(2+), Ni~(2+), and Zn~(2+), promoted hydrolysis in weakly acidic medium has beens tudied as a function of metal ion concentration and temperature. The metal ion catalysed aquation is proposed to tak place through a rapid pre-equilibrium step to form a binuclear complex with the M~(2+) ion, followed by the cleavage of the Cr-O bond in a slow rate-determining step. The catalytic efficiency of the metal ion follows the Irving-Williams order of stability and also parallels the log k_(M-pic) values.