Low temperature kinetic study of very fast substitution reactions at platinum(ii) trans to olefins

Abstract

Ultra-fast substitution of chloride for bromide, iodide, azide and thiocyanate trans to ethene in Zeise's anion, [PtCl₃(C₂H₄)]⁻, 1, has been investigated in methanol solvent by use of cryo temperature diode array stopped-flow spectrophotometry. Reactions follow the usual two-term rate law for square-planar substitutions, k_obs = k₁ + k₂[Y] (where k₁ = k_MeOH[MeOH]), with k₁ = 118 ± 10 s⁻¹ and k₂ = (5.1 ± 0.2) × 10², (3.51 ± 0.07) × 10³, (11.8 ± 0.2) × 10³, and (56 ± 4) × 10³ mol⁻¹ dm³ s⁻¹ for Y = Br⁻, I⁻, N₃⁻ and SCN⁻, respectively, at 223 K. Activation parameters for MeOH, Br⁻, I⁻ and N₃⁻ are ΔH^≠ = 23 ± 2, 21 ± 2, 17 ± 1.0 and 11.9 ± 1.5 kJ mol⁻¹ and ΔS^≠ = −124 ± 10, −96 ± 9, −98 ± 4 and −111 ± 6 J K⁻¹ mol⁻¹, respectively. Recalculation of k₁ to second-order units gives the sequence of nucleophilicity MeOH < Br⁻ < I⁻ < N₃⁻ < SCN⁻ (1 ∶ 100 ∶ 700 ∶ 2500 ∶ 12000) at 223 K. This nucleophilic discrimination decreases with increasing temperature. Chloride for iodide substitution trans to allyl alcohol, vinyltrimethylsilane and cyclooctene at [PtCl₃(L)]⁻, (L = CH₂CHCH₂OH, 2; CH₂CHSiMe₃, 3; C₈H₁₄, 4) follow the same rate law with k₁ = 116 ± 5, 31.0 ± 0.3 and 23.6 ± 0.1 s⁻¹ and k₂ = (2.65 ± 0.06) × 10³, (0.273 ± 0.005) × 10³ and (0.119 ± 0.002) × 10³ mol⁻¹ dm³ s⁻¹ at 223 K. Activation parameters are ΔH^≠(k_MeOH) = 24.4 ± 1.3, 28.4 ± 0.6 and 29.9 ± 0.8 kJ mol⁻¹, ΔS^≠(k_MeOH) = −120 ± 5, −114 ± 2 and, −108 ± 3 J K⁻¹ mol⁻¹, ΔH^≠(k₂) = 19.9 ± 1.2, 24.6 ± 1.7 and 24 ± 3 kJ mol⁻¹ and ΔS^≠ (k₂) = −88 ± 5, −84 ± 7 and −93 ± 10 J K⁻¹ mol⁻¹, for 2, 3 and 4 respectively. The free energies of activation are dominated by the −TΔS^≠ terms. The crystal and molecular structures of Bu₄N[PtCl₃(CH₂CHSiMe₃)] and Bu₄N[PtCl₃(C₈H₁₄)] show slight Pt–Cl bond lengthening to 2.314(2) Å and 2.3238(16) Å trans to the olefins, similar to that found trans to ethene in Zeise's anion. All experiments support a model for the very fast substitution reactions trans to the olefins in which ground state labilisation is much less significant than transition state stabilisation. Extrapolation to ambient temperature together with literature data for related reactions in methanol solvent gives a quantitative measure of the trans effect of ethene as: SR₂ < Me₂SO < AsEt₃ < PR₃ < P(OR)₃ < C₂H₄ (1 ∶ 5 ∶ 400 ∶ 3500 ∶ 7000 ∶ 3 × 10⁶). The relative trans effect of the olefins studied is C₂H₄ ∼ CH₂CHCH₂OH > CH₂CHSiMe₃ ∼ C₈H₁₄, spanning a factor of between 5 and 30 depending on the nucleophile, and reflecting minor differences in steric and electronic properties of the olefins.