Synthesis ,characterization and kinetic studies of the formation of a new chromium( III) complex of mixed ligands L-cysteine and picolinic acid

Abstract

A new Cr(III) complex of mixed ligands of picolinic acid (Hpic) and Lcysteine
has been synthesized from acid catalyzed hydrolysis of the blue colored
solution of sodium salt of bis L-cysteinato(N,O,S) chromateIII complex with solution
of Hpic restrictive acidic media of pH=3-4 with gentle heating. The red-brown color
product complex of sodium bis L- cysteinato(N,O) monohydroxy
picolinato(N)chromateIII have been characterized by element analysis ,electronic and
i.r spectroscopy comparison to the properties of some well known related Cr(III)
complexes leads to the conclusion that Hpic binds to Cr(III) center via its nitrogen
donor atom.
The kinetics and mechanism of the formation of this complex, from the acid
catalyzed cleavage of Cr—S bond and subsequent Hpic substitution, have been
studied spectrophotometrically in a limited pH 3-4.5 range, which was adjusted by
HClO4 (μ =0.2M).The rate of the production shows two reaction paths ;one for
mono protic thiol,[ Cr(III) (L-cysH)(L-cysN,O,S) (H2O)] and other for diprotic both
thiol, [Cr (L-CysH)2 (H2O)2]+2,with Hpic ligand substitution on Cr(III) through N
atom.The speices of monoprotic reacts faster with an acid dependent than the diprotic
species reaction .The pseudo first order rate constant equation is of the form; k =
k1Ka2 [H+]-1 + k2 (where k1 represents the rate constant for first step, k2 for second
step reactions and Ka2 is acid dissociation constant for diprotic species) was obtained
with ΔH# and ΔS# for both paths are 67.195 kJ mol-1,- 25.41JK-1mol-1 and 68.96 kJ
mol-1,-93.64 JK-1 mol-1 respectively.