Interaction of Glycyl – Glycine with Hydroxopentaaquarhodium(Iii) Ion in Aqueous Medium: Kinetic and Mechanistic Studies

Abstract

The kinetics of interaction between glycyl-glycine (L-L'H) and[Formula: see text] [Rh(H2O)5OH] 2+ have been studied spectrophotometrically in aqueous medium as a function of [Rh(H2O)5OH] 2+], [glycyl - glycine], pH and temperature at constant ionic strength. At pH 4.3, the interaction with glycyl-glycine shows two parallel paths: both processes are ligand-dependent. The rate constant for the processes are: k1 ∼ 10 −3 s −1 and k2∼10−5 s−3. The activation parameters calculated from Eyring plots are: ΔH1≠ - 26.7 ± 0.3 kj mol−1, ΔS1≠ =-207+1 JK−1 mol−1, ΔH2≠ = 92.4 ± 1.3 kJ mol−1 and ΔS2≠ = −42±4 JK−1 mol−1. The ligand glycyl-glycine, with different donor atoms (N and O), attacks two rhodium(III) ions (Rh(III) being a borderline acid), with different reactivity. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for both interaction processes. From the temperature dependence of the outer-sphere association equilibrium constant, the thermodynamic parameters calculated are: ΔH1∘ = 21.8 ± 1.9 kJ mol−1, ΔS1∘ = 124± 6 JK−1 mol−1, ΔH2∘ = 12.7 ±0.5 kj mol−1 and ΔS2∘ 89±2 JK−1 mol″−1 which give a negative A G° value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer- sphere association complex for both the parallel paths. The product of the reaction has been characterized from IR and ESI-mass spectroscopic analysis.