Kinetics and mechanism of the reactions of [Pt(terpy)H2O]2+ with thiols in acidic aqueous solution. Synthesis and crystal structure of [Pt(terpy)(tu)](ClO4)2 (tu = thiourea)

Abstract

The kinetics of the complex-formation reactions between [Pt(terpy)H₂O]²⁺, where terpy is 2,2′:6′,2″-terpyridine, with L-cysteine, DL-penicillamine, glutathione and thiourea (tu) were studied in an aqueous 0.10 M perchloric acid medium using variable-temperature and -pressure stopped-flow spectrophotometry. Thiourea is the best nucleophile with a second order rate constant, k₁²⁹⁸, of 1.72 × 10⁵ M⁻¹ s⁻¹, whereas glutathione is the strongest nucleophile of the studied thiols, for which k₁²⁹⁸ varied between 38 and 583 M⁻¹ s⁻¹. Activation volumes for the reactions with thiourea, L-cysteine, glutathione and DL-penicillamine, are −6.0 ± 0.3, −9.3 ± 0.4, −12.4 ± 0.6 and −20.6 ± 1.0 cm³ mol⁻¹, respectively. 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 [Pt(terpy)(tu)](ClO₄)₂ was determined by X-ray diffraction. Crystals are monoclinic with the space group P21/c and consist of the distorted square-planar [Pt(terpy)(tu)]²⁺ complex. The Pt–N (central) bond distance, 1.971(14), is shorter than the other two Pt–N distances, 2.078(15) and 2.045(15) Å. The Pt–S distance is 2.301(5) Å.