Kinetic and mechanistic study on the reactions of [Pt(bpma)(H2O)]2+ and [Pd(bpma)(H2O)]2+ with some nucleophiles. Crystal structure of [Pd(bpma)(py)](ClO4)2

Abstract

Substitution reactions of the complexes [Pd(bpma)(H₂O)]²⁺ and [Pt(bpma)(H₂O)]²⁺, where bpma = bis(2-pyridylmethyl)amine, with TU, DMTU and TMTU for both complexes and Cl⁻, Br⁻, I⁻ and SCN⁻ for the platinum complex, were studied in aqueous 0.10 M NaClO₄ at pH 2.5 using a variable-temperature stopped-flow spectrophotometer. The pK_a value for the coordinated water molecule in [Pd(bpma)(H₂O)]²⁺ (6.67) is a unit higher than that of [Pt(bpma)(H₂O)]²⁺. The observed pseudo-first-order rate constants k_obs (s⁻¹) obeyed the equation k_obs = k₂[Nu] (Nu = nucleophile). The second-order rate constants indicate that the Pd(II) complex is a factor of 10³ more reactive than Pt(II) complex. The nucleophile reactivity attributed to the steric hindrance in case of TMTU and the inductive effect for DMTU was found to be DMTU > TU > TMTU for [Pt(bpma)(H₂O)]²⁺ and DMTU ≈ TU > TMTU for [Pd(bpma)(H₂O)]²⁺. The trend for ionic nucleophile was I⁻ > SCN⁻ > Br⁻ > Cl⁻, an order linked to their polarizability and the softness or hardness of the metal. Activation parameters were determined for all reactions and the negative entropies of activation (ΔS^‡) support an associative ligand substitution mechanism. The X-ray crystal structure of [Pd(bpma)(py)](ClO₄)₂ was determined; it belongs to the triclinic space group P and has one formula unit in the unit cell. The unit cell dimensions are a = 8.522(2), b = 8.627(2), c = 16.730(4) Å; α = 89.20(2), β = 81.03(2), γ = 60.61(2)°; V = 1055.7(5) ų. The structure was solved using direct methods in WinGX's implementation of SHELXS-97 and refined to R = 0.054. The coordination geometry of [Pd(bpma)(py)]²⁺ is distorted square-planar. The Pd–N(central) bond distance, 1.996(3) Å, is shorter than the other two Pd–N distances, 2.017(3) and 2.019(3) Å. The Pd–N(pyridine) distance is 2.037(3) Å.