New chromium(III)-picolinamide complexes.Kinetics and mechanism of picolinamide liberation in HClO_4 solutions

摘要

Two new chromium(III) complexes with picolinamide (pica) and oxalates,[Cr(C_2o-4)_2(N,N'-pica)]~(2-) and [Cr(C_2O_4)_2(N,O-pica)]~-,were obtained and the kinetics of their aquation in HClO_4 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(C_2O_4)_2(H_2O_2)_2]~-.In the case of N,N'-bonded pica the kinetics of both stages was determined and in the case of the N,0-bonded pica only the second stage was investigated.The following rate laws were established: (k_(obs))_1 = k_0 + k_1Q_1[H~+] and (k_(obs))_2 = k_2Q_2[H~+],where k_0 and k_1 are the rate constants of the chelate-ring opening in the unprotonated and protonated starting complex,and k_2 is the rate constant of the pica liberation from the protonated intermediate.Kinetic parameters are calculated and the aquation mechanism is discussed.