Kinetics and mechanism of the reaction of chelated Pd(ii) complexes with thiols in acidic aqueous solution. Synthesis and crystal structure of [Pd(bpma)Cl]Cl·H2O (bpma = bis(2-pyridylmethyl)amine)

Abstract

The kinetics of the complex-formation reactions between monofunctional palladium(II) complexes [Pd(N–N–N)H₂O]²⁺, where N–N–N is 2,2′:6′,2″-terpyridine (terpy), diethylenetriamine (dien) or bis(2-pyridylmethyl)amine (bpma), with L-cysteine, DL-penicillamine and glutathione, have been studied in an aqueous 0.10 M perchloric acid medium using variable-temperature and -pressure stopped-flow spectrophotometry. Second-order rate constants, k₁²⁹⁸, varied between 2.8 × 10² and 4.4 × 10⁴ M⁻¹ s⁻¹. The highest reactivity was observed for the [Pd(terpy)H₂O]²⁺ complex, whereas glutathione is the strongest nucleophile. Activation volumes for these reactions varied between −5.6 ± 0.3 and −10.7 ± 1.0 cm³ mol⁻¹. The negative entropies and volumes of activation support a strong contribution from bond making in the transition state of the substitution process. The crystal structure of [Pd(bpma)Cl]Cl·H₂O has been determined by X-ray diffraction at 190 K. Crystals are triclinic with space group P and consist of distorted square-planar [Pd(bpma)Cl]⁺ cations. The Pd–N distances are all equal to 2.005(7) Å. The Pd–Cl distance is 2.305(3) Å.