Trans ligand influence in cobalt(III) Schiff base complexes: Electronic and steric effects on the kinetics of hydrolysis

Abstract

Rate constants for the hydrolysis (kh) of six different amines in trans-[Co((BA)2en)(amine)2]ClO4 complexes (amine = aniline 1a, para-toluidine 1b, benzylamine 1c (primary amines), pyrrolidine 2a, piperidine 2b, morpholine 2c (secondary amines), and (BA)2en = Bisbenzoylacetoneethylenediiminato) in mixed methanol/water (1:1) solvent have been determined between 30 and 55°C. The hydrolysis product of 2c, trans-[Co((BA)2en)(morpholine)(H2O)]ClO4, has been separately prepared and characterized by UV–vis and 1H NMR spectroscopy. Depending on the nature of the axial amine ligand the limiting first-order rate constants for the amine hydrolysis at 40°C range from (3.42 ± 0.10) × 10−5 to (5.32 ± 0.13) × 10−5 s−1. At the first glance, a reasonable trend cannot be established between kh and the basicity or the inductive trans effect of the amine ligands. However, when the complexes are classified into two groups, based on the type of the amine (primary and secondary), the values of kh correlate well with the basicity or inductive effect of the amine in each group. The observed trend in kh values for the complexes with primary amines is 1a (5.32 ± 0.13) × 10−5 s−1 > 1b (3.51 ± 0.14) × 10−5 > 1c (1.72 ± 0.03) × 10−5 (40°C), which is opposite to the amine basicity strength. In the case of the complexes with secondary amines, the observed trend in kh values is in accord with amine basicity (or inductive trans effect), i.e. 2a (5.02 ± 0.22) × 10−5 > 2b (4.18 ± 0.10) × 10−5 > 2c (3.42 ± 0.10) × 10−5 s−1 (40°C). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 387–393, 2002