Nucleophilic ligand displacement in dichloro[2-(arylazo)heterocycle]palladium(II) complexes by benzimidazole - a kinetic and mechanistic study

The reactions of benzimidazole (Bimz) with dichloro{2-(arylazo)pyridine}pal ladium(II), [Pd(aap)Cl-2 (3), aap = R'C6H4N= N-2-C5H4N; R' = H(a), o-Me(b), m-Me(c), p-Me(d), p-Cl(e)] and dichloro{2-(arylazo)pyrimidine}palladium(II ), [Pd(aapm)Cl-2 (4), aapm = R''C6H4N=N-2-C6H3N2; R'' = H(a), o-Me(b), m-Me (c), p-Me(d), p-Cl(e)] were followed separately. The kinetics were examined under pseudo-first order conditions with reference to Bimz in MeCN at 385 and 390 nm, respectively and at 298 K using a u.v.-vis. spectrophotometer. The product was isolated and characterised as trans-Pd(Bimz)(2)Cl-2. The re action between Pd(aap)Cl-2 and Bimz follows the rate law, rate = K[Bimz](2) [Pd(aap)Cl-2], a single step process, whereas biphasic behaviour is observe d for the reaction between Pd(aapm)Cl-2 and Bimz; each step is first order with respect to the concentration of complex and to Bimz. The rate data sup port a nucleophilic association path and the rate decreases upon addition o f Cl-(LiCl). The aryl ring substituent of the arylazoheterocycle influences the substitution rate as follows: k(e) > k(a) > k(d) > k(c) > k(b). The k- values are linearly correlated with Hammett sigma constants with usual devi ations for the m-Me and o-Me substituents because of their steric crowding. The rate follows the order: k(aap) > k(aapm). This is unusual with referen ce to the pi -acidity order of the heterocycles, pyridine < pyrimidine, as the increased pi -acidity will enhance the nucleophilic association. The ch arge density calculation by MNDO method shows that in the pyrimidine ring o f arylazopyrimidine, the peripheral m-N(uncoordinated) carries a high negat ive charge which may retard the nucleophilic association rate. This effect is absent in the pyridine ring of arylazopyridine and may be the reason for the faster single step nucleophilic substitution in Pd(aap)Cl-2.