Reactivity studies of trans-[PtClMe(SMe 2) 2] towards anionic and neutral ligand substitution processes

Abstract

Reaction of trans-[PtClMe(SMe 2) 2] with the mono anionic ligands azide, bromide, cyanide, iodide and thiocyanate result in substitution of the chloro ligand as the first step. In contrast the neutral ligands pyridine, 4-Me-pyridine and thiourea substitute a SMe 2 ligand in the first step as confirmed by 1H NMR spectroscopy and the kinetic data. Detailed kinetic studies were performed in methanol as solvent by use of conventional stopped-flow spectrophotometry. All processes follow the usual two-term rate law for square-planar substitutions, k obs = k 1 + k 2[Y] (where k 1 = k MeOH[MeOH]), with k 1 = 0.088 ± 0.004 s −1 and k 2 = 1.18 ± 0.13, 3.8 ± 0.3, 17.8 ± 1.3, 34.9 ± 1.4, 75.3 ± 1.1 mol −1 dm 3 s −1 for Y − = N 3, Br, CN, I and SCN respectively at 298 K. The reactions with the neutral ligands proceed without an appreciable intercept with k 2 = 5.1 ± 0.3, 15.3 ± 1.8 and 195 ± 3 mol −1 dm 3 s −1 for Y = pyridine, 4-Me-pyridine and thiourea, respectively, at 298 K. Activation parameters for MeOH, N 3 - , Br −, CN −, I −, SCN −, and Tu are Δ H ≠ = 47.1 ± 1.6, 49.8 ± 0.6, 39 ± 3, 32 ± 8, 39 ± 5, 34 ± 4 and 31 ± 3 kJ mol −1 and Δ S ≠ = −107 ± 5, −77 ± 2, −104 ± 9,−113 ± 28, −85 ± 18, −94 ± 14 and −97 ± 10 J K −1 mol −1, respectively. Recalculation of k 1 to second-order units gives the following sequence of nucleophilicity: MeOH < N 3 - < Br - ∼ py < 4 - Me - py ∼ CN - < I - < SCN - < Tu (1:13:42:57:170:200:390:840:2170) at 298 K. Variation of the leaving group in the reaction between trans-[PtXMe(SMe 2) 2] and SCN − follows the same rate law as stated above with k 2 = 75.3 ± 1.1, 236 ± 4 and 442 ± 5 mol −1 dm 3 s −1 for X − = Cl, I and N 3, respectively, at 298 K. The corresponding activation parameters were determined as Δ H ≠ = 34 ± 4, 32 ± 2 and 39.3 ± 1.7 kJ mol −1 and Δ S ≠ = −94 ± 14, −86 ± 8 and −68 ± 6 J K −1 mol −1. All the kinetic measurements indicate the usual associate mode of activation for square planar substitution reactions as supported by large negative entropies of activation, a significant dependence of the reaction rate on different entering nucleophiles and a linear free energy relationship.