KINETICS AND MECHANISM OF NUCLEOPHILIC-SUBSTITUTION OF DICHLOROARYLAZOPYRIDINEPALLADIUM(II) BY PYRIDINE BASES

The reaction of dichloroarylazopyridinepalladium(II) [Pd(aap)Cl-2, aap = 4-R'C6H4N=N-2-C5H4N; R' = H (1), Me (2), Cl (3)] with pyridine base s [RPy: R=H (a), 2-Me (b), 4-Me (c), 4-Cl (d), 2-Ph (e), 2-PhCH2(f)] h as been studied spectrophotometrically in MeCN at 400 nm. The products (4) have been isolated and characterized as trans-Pd(RPy)(2)Cl-2. The kinetics of the nucleophilic substitution have been examined under ps eudo-first-order conditions with respect to base at 298 K and follow t he rate law, Rate = k[RPy](2) [Pd(aap)Cl-2]. The rate data supports a nucleophilic association path. External addition of Cl- (LiCl) suppres ses the rate, which follows the order: k(3) > k(1) > k(2), where k val ues are linearly related to Hammet sigma constants. 2-Substitution in the pyridine base remarkably reduces the rate compared with 4-substitu tion and is attributed to a steric effect that destabilizes the transi tion state. The rate decreases with increasing steric crowding at the ol tho-position and follows the order: (e) > (f) > (b) The 4-substitut ed pyridines control the rate via the inductive effect and follow the order: (d) > (a) > (c).