Abstract
The reaction of dichloro-{1-benzyl-2-(arylazo)imidazole}palladium(II), [Pd(R'aaiBz)Cl 2 ], where R'aaiBz= p -R'-C 6 H 4 -N=N-C 3 H 2 N 2 -1-CH 2 -C 6 H 5 ; 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-Me 2 (e), 2,4,6-Me 3 (f)] has been studied spectrophotometrically in MeCN at 468 nm. The products (4) have been isolated and characterised as trans -Pd(RPy) 2 Cl 2 . The kinetics of the nucleophilic substitution has been examined under pseudo-first-order conditions at 298 K. The single phase reaction step has been observed for the bases HPy (a), 4-MePy (b) and 4-ClPy (c) and follows the rate law, rate= k [RPy] 2 [Pd(R'aaiBz)Cl 2 ]. The bases, 2-MePy (d), 2,6-Me 2 Py (e) and 2,4,6-Me 3 Py (f) exhibit bi-phasic reaction: rate-1= k ' [RPy][Pd(R'aaiBz)Cl 2 ] and rate-2= k " [RPy][Pd(R'aaiBz)Cl 2 ]. The rate data support nucleophilic association path. External addition of Cl (LiCl) suppresses the rate, that follows the order; k / k '/ k " (3) > k / k '/ k " (1) > k / k '/ k " (2). The k 's are linearly related to Hammett constants. The 2-MePy, 2,6-Me 2 Py and 2,4,6-Me 3 Py remarkably reduce the rate compared with 4-substituted pyridine bases. This is attributed to 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 pyridine controls the rate via inductive effect and follows the order: (b)>(a)>(c).