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, kobs = k1 + k2[Y] (where k1 = kMeOH[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≠(kMeOH) = 24.4 ± 1.3, 28.4 ± 0.6 and 29.9 ± 0.8 kJ mol−1, ΔS≠(kMeOH) = −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) A and 2.3238(16) A 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.