Kinetics and mechanism of the reactions of Pd(ii) complexes with azoles and diazines. Crystal structure of [Pd(bpma)(H2O)](ClO4)2·2H2O

Abstract

Substitution reactions of the complexes [Pd(bpma)(H₂O)]²⁺, [Pd(bpma)Cl]⁺, [Pd(dien)(H₂O)]²⁺ and [Pd(dien)Cl]⁺, where bpma = bis(2-pyridylmethyl)amine and dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as triazole, pyrazole, pyrimidine, pyrazine and pyridazine, were studied in an aqueous 0.10 M NaClO₄ at pH 2.8 using variable-temperature and -pressure stopped-flow spectrophotometry. The second-order rate constants indicate that the Pd(II) complexes of bpma, viz. [Pd(bpma)(H₂O)]²⁺ and [Pd(bpma)Cl]⁺, are more reactive than the complexes of dien, viz. [Pd(dien)(H₂O)]²⁺ and [Pd(dien)Cl]⁺. Also, the aqua complexes, [Pd(bpma)(H₂O)]²⁺ and [Pd(dien)(H₂O)]²⁺, are much more reactive than the corresponding chloro complexes. The most reactive nucleophile of the five-membered rings is triazole and for the six-membered rings the most reactive one is pyridazine. Activation parameters were determined for all reactions and the negative entropies and volumes of activation (ΔS^‡, ΔV^‡) support an associative ligand substitution mechanism. The crystal structure of [Pd(bpma)(H₂O)](ClO₄)₂·2H₂O was determined by X-ray diffraction. Crystals are monoclinic with the space group P2₁/c. The coordination geometry of [Pd(bpma)(H₂O)]²⁺ is distorted square-planar. The Pd–N (central) bond distance, 1.958(5) Å, is shorter than the other two Pd–N distances, 2.007(5) and 2.009(5) Å. The Pd–O distance is 2.043(5) Å.