Nucleophilic ligand displacement in dichloro[2-(arylazo)heterocycle]palladium(II) complexes by benzimidazole ± a kinetic and mechanistic study Ramkrishna Roy, Debasis Das, Manas Banerjee and Chittaranjan Sinha* Department of Chemistry, The University of Burdwan, Burdwan ± 713104, India Ambikesh Mahapatra Department of Chemistry, The University of Kalyani, Kalyani ± 741235, India Received 05 April 2000; accepted 06 June 2000 Abstract The reactions of benzimidazole (Bimz) with dichloro{2-(arylazo)pyridine}palladium(II), [Pd(aap)Cl 2 (3), aap = R¢C 6 H 4 N@N-2-C 5 H 4 N; R¢ = H(a), o-Me(b), m-Me(c), p-Me(d), p-Cl(e)] and dichloro{2-(arylazo)pyrimid- ine}palladium(II), [Pd(aapm)Cl 2 (4), aapm = R¢¢C 6 H 4 N@N-2-C 6 H 3 N 2 ;R¢¢ = H(a), o-Me(b), m-Me(c), p-Me(d), p-Cl(e)] were followed separately. The kinetics were examined under pseudo-®rst 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 reaction 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 observed for the reaction between Pd(aapm)Cl 2 and Bimz; each step is ®rst order with respect to the concentration of complex and to Bimz. The rate data support a nucleophilic association path and the rate decreases upon addition of Cl ) (LiCl). The aryl ring substituent of the arylazoheterocycle in¯uences the substitution rate as follows: k(e)> k(a)> k(d)> k(c)> k(b). The k-values are linearly correlated with Hammett r constants with usual deviations 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 reference to the p-acidity order of the heterocycles, pyridine < pyrimidine, as the increased p-acidity will enhance the nucleophilic association. The charge density calculation by MNDO method shows that in the pyrimidine ring of arylazopyrimidine, the peripheral m-N(uncoordinated) carries a high negative charge which may retard the nucleophilic association rate. This eect 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 . Introduction We are interested in exploring the reactions of arylazo- heterocycles with platinum metals [1±6]. 2-(Arylazo) het- erocycles with the azoimine function, AN@NAC@NA, where N refers to N(azo) and N¢ refers to the N(imine) group, is the unsymmetric N,N¢-chelator. Palladium(II) forms several complexes of the Pd(N,N¢)Cl 2 type with a cis-PdCl 2 con®guration [1±4]. The complexes exhibit various metal-centred [5] and ligand-centred reactivities [1, 6, 7]. The PdACl bond undergoes nucleophilic substitution with dierent anionic ligands, viz., catechol- ates [4] and salicylates [8], in the presence of bases. The reaction of pyridine bases (Rpy) with Pd(N,N¢)Cl 2 is unusual and is associated with chelate ring-opening followed by complete elimination of the ligand [9]. The reaction ®nishes with isomerisation and isolation of trans-Pd(Rpy) 2 Cl 2 . This result has prompted us to ascertain the reactivity pattern of other heterocycle bases towards palladium(II) complexes of dierent arylazoheterocycles. In this paper we have chosen Bimz as the nucleophile. Imidazole and its derivatives have been excluded because of the extremely fast reaction which could not be followed by the conventional u.v.±vis. spectrophotometer. Two classes of arylazohet- erocycles, viz., 2-(arylazo)pyridines (aap) and 2-(aryl- azo)pyrimidines (aapm) were used. They were selected in order to explore the role of p-acidity on the substitution process. The kinetics and mechanism of the reaction is described in this paper. Experimental 2-(Arylazo)pyridines (aap), 2-(arylazo)pyrimidines (aapm) and their palladium(II) complexes were prepared by reported procedures [1, 10]. Bimz and LiCl á H 2 O were purchased from the Sisco Research Laboratory. MeCN was puri®ed by a reported procedure [2]. I.r. spectra (KBr disk) were recorded on a JASCO FT-IR model 420 spectrophotometer and microanalyses were carried out on a Perkin-Elmer 2400 CHN elemental analyser. All kinetic and spectral measurements were * Author for correspondence Transition Metal Chemistry 26: 205±211, 2001. 205 Ó 2001 Kluwer Academic Publishers. Printed in the Netherlands.