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

摘要

Ultra-substitution of chloride for bromide, iodide, azide and thiocyanate trans to ethane in Zeise's anion, [PtCl_3(C_2H_4)]~-, 1, has been invstigated in methanol solvent by the use of cryo temperature diode array stopped-flow spectrophotometry. Reactions 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 = 118 (+-) 10 s~(-1) and k_2 = (5.1 (+-) 0.2) X 10~2, (3.51 (+-) 0.070 X 10~3, (11.8 (+-) 0.2 ) X 10~3, and (56 (+-) 4) X 10~3 mol~(-1) dm~3 s~(-1) for Y = Br~-, I~-, N_3~- and SCN~-, respectively, at 223 K. Activation parameters for MeOH, Br~-, I~- and N_3~- are DELTAH~(dot =) 23 (+-) 2, 21 (+-) 2, 17 (+-) 1.0 and 11.9 (+-) 1.5 kJ mol~(-1) and DELTAS~(not =) - 124 (+-) 10, -96 (+-) 9, - 98 (+-) 4 and -111 (+-) 6 J K~(-1) mol~(-1), respectively. Recalculation of k_1 to second-order units gives the sequence of nucleophilicity MeOH < Br~- < I~- < N_3~- < 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 cyclooctane at [PtCl_3(L)]~-, (L = CH_2CHCH_2OH, 2; CH_2CHSiMe_3, 3; C_8H_14, 4) follow the same rate law with k_1 = 116 (=-) 5, 31.0 (+-) 0.3 and 23.6 (+-) 0.1 s~(-1) and k_2 = (2.65 (+-) 0.06) X 10~3, (0.273 (+-) 0.005) X 10~3 and (0.119 (+-) 0.002) X 10~3 mol~(-1) dm~3 s~(-1) at 223 K. Activation parameters are DELTAH~(dot = )(k_(MeOH) = 24.4 (+-) 1.3, 28.4 (+-) 0.6 and 29.9 (+-) 0.8 kJ mol~(-1), DELTAS~(not = )(k_MeOH) = - 120 (+-) 5, - 114 (+-) 2 and, - 108 (+-) 3 J K~(-1) mol~(-1), DELTAH~(dot =) (k_2) - 19.9 (+-) 1.2, 24.6 (+-) 1.7 and 24 (+-)3 kJ mol~(-1) and DELTAS~(not =)(k_2) = -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 -T DELTAS~(not =) terms. The crystal and molecular structures of Bu_4N[PtCl_3(CH_2CHSiMe_3)] and Bu_4N[PtCl_3(C_8H_14)] 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 reaction trans to the olefins in which ground state labilisation is much less significant than transition state stabilisation. Extrapolation to measure of the trans effect of ethane as: SR_2 < Me_2SO < AsEt_3 < PR_3 < P(OE)_3 < C_2H_4 (1:5:400:3500:700 3 X 10~6). The relative trans effect of the olefins studied is nucleophile, and reflecting minor differences in steric and electronic properties of the olefins.