New chromium(III)–picolinamide complexes. Kinetics and mechanism of picolinamide liberation in HClO4 solutions

Abstract

Two new chromium(III) complexes with picolinamide (pica) and oxalates, [Cr(C2O4)2(N,N′-pica)]2− and [Cr(C2O4)2(N,O-pica)]−, were obtained and the kinetics of their aquation in HClO4 solutions were studied. The aquation leads to pica liberation and proceeds in two stages: (i) the chelate-ring opening at the Cr–amide bond and (ii) the Cr–N-pyridine bond breaking, which gives free pica and cis-[Cr(C2O4)2(H2O2)2]−. In the case of N,N′-bonded pica the kinetics of both stages was determined and in the case of the N,O-bonded pica only the second stage was investigated. The following rate laws were established: (kobs)1 = k0 + k1Q1[H+] and (kobs)2 = k2Q2[H+], where k0 and k1 are the rate constants of the chelate-ring opening in the unprotonated and protonated starting complex, and k2 is the rate constant of the pica liberation from the protonated intermediate. Kinetic parameters are calculated and the aquation mechanism is discussed.