Acetonitrile and propionitrile exchange at palladium(II) and platinum(II)†

Abstract

Ligand exchange at square-planar [Pd(MeCN)₄]²⁺ and [Pd(EtCN)₄]²⁺ has been studied by ¹H NMR line broadening and at [Pt(MeCN)₄]²⁺ and [Pt(EtCN)₄]²⁺ by isotopic labelling using ¹H NMR spectroscopy in deuteriated nitromethane. Exchange takes place via two-term rate laws R_ex/4 = (k₁ + k₂[RCN])c_M with well defined k₁ paths. Rate constants per co-ordination site k₁²⁹⁸/s^–1, k₂²⁹⁸/kg mol^–1 s^–1 are 6.9 ± 1.6, 34 ± 3; 0.59 ± 0.12, 34 ± 3; 10.7 ± 1.8, 35 ± 4; (5.1 ± 2.3) × 10^–6, (2.8 ± 0.2) × 10^–5 and (5.5 ± 1.0) × 10^–6, (3.3 ± 0.2) × 10^–5 for [Pd(MeCN)₄][CF₃SO₃]₂, [Pd(MeCN)₄][BF₄]₂, [Pd(EtCN)₄][CF₃SO₃]₂, [Pt(MeCN)₄][CF₃SO₃]₂ and [Pt(EtCN)₄][CF₃SO₃]₂, respectively. For [Pd(MeCN)₄]²⁺ the k₁ path is much larger for triflate than for tetrafluoroborate as counter ion. Changing the tetrafluoroborate concentration has no effect on the exchange rate of acetonitrile at [Pd(MeCN)₄]²⁺. In this case the k₁ path is ascribed to an attack by solvent at the metal centre. For triflate saturation kinetics is observed. This can be rationalized in terms of ion-pair formation followed by reversible intramolecular exchange of nitrile for triflate within the ion pair, with an equilibrium constant K_ip³⁰⁰ = 8 ± 2 kg mol^–1 and a rate constant k³⁰⁰ = 12.5 ± 1.3 s^–1. All activation entropies are negative, indicating associative activation. A new, simple one-step synthesis of the substrate complexes as their triflate salts, using [M(acac)₂] (acac = acetylacetonate) as starting material, and of [Pd(MeCN)₄][BF₄]₂ using palladium(II) acetate, is described.