Kinetics and mechanism of nucleophilic substitution of dichloro{1-methyl-2-(arylazo)imidazole}palladium(II) by pyridine bases |
| |
Authors: | Rauth Goutam Kumar Das Debasis Sinha Chittaranjan Bag Kanchan Mahapatra Ambikesh |
| |
Institution: | (1) Department of Chemistry, The University of Burdwan, Burdwan, 713104, India;(2) Department of Chemistry, Garbeta College, Midnapore, 721127, India;(3) Department of Chemistry, Kalyani University, Kalyani, 741235, India |
| |
Abstract: | The reaction of dichloro{1-methyl-2-(arylazo)imidazole}palladium(II), Pd(R aaiMe)Cl2 where R aaiMe = p-R –C6H4N=N–C3H2N2-1-Me; R = H(1), Me(2), Cl(3), with pyridine bases RPY: R = H (a), 4-Me (b), 4-Cl (c), 2-Me (d), 2,6-Me2
(e), 2,4,6-Me3
(f)] has been studied spectrophotometrically in MeCN at 451 nm. The products (4) have been isolated and characterised as trans-Pd(RPy)2Cl2. The kinetics of the nucleophilic substitution has been examined under pseudo-first-order conditions at 298 K. A single phase reaction step has been observed for bases such as Hpy (a), 4-MePy (b) and 4-ClPy (c) and follows the rate law: rate = (a + kRPy]2Pd(R aaiMe)Cl2]). The bases 2-MePy (d), 2,6-Me2Py (e) and 2,4,6-Me3Py (f) exhibits a bi-phasic reaction and follows the rate laws: rate–1 = (a + k RPy]Pd(R aaiMe)Cl2]) and rate–2 = (a + k RPy]Pd(R aaiMe)-Cl2]), where k is the third-order rate constant; k is the second-order first phase rate constant, k is the second-order second phase rate constant and a/a /a correspond to the solvent dependent constant of the respective reaction path. The rate data supports a nucleophilic association path. External addition of Cl– (LiCl) suppresses the rate, which follows the order: k/k /k (3) > k/k ,k (1) > k/k ,k (2). The k values are linearly related to the Hammett constants. The 2-substituted pyridines (d–f) remarkably reduce the rate and show a bi-phasic reaction behaviour as compared with 4-Rpy (a–c). This is attributed to the steric effect that destabilises the transition state. The rate decreases with increasing steric crowding at the ortho-position and follows the order: (d) > (f) > (e). The 4-substituted pyridines control the rate via an inductive effect and follow the order: (b) > (a) > (c). |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|