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

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Summary, in English

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)=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+/-0.07) 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(not equal)=23+/-2, 21+/-2, 17+/-1.0 and 11.9+/-1.5 kJ mol(-1) and DeltaS(not equal)=-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 cyclooctene at [PtCl3(L)](-), (L=CH2CHCH2OH, 2; CH2CHSiMe3, 3; C8H14,4) follow the same rate law with k(1)=116&PLUSMN;5, 31.0&PLUSMN;0.3 and 23.6&PLUSMN;0.1 s(-1) and k(2)= (2.65&PLUSMN;0.06) x 10(3), (0.273&PLUSMN;0.005) x 10(3) and (0.119&PLUSMN;0.002) x 10(3) mol(-1) dm(3) s(-1) at 223 K. Activation parameters are &UDelta;H-&NOTEQUAL;(k(MeOH))=24.4&PLUSMN;1.3, 28.4&PLUSMN;0.6 and 29.9&PLUSMN;0.8 kJ mol(-1), &UDelta;S-&NOTEQUAL;(k(MeOH)) =-120&PLUSMN;5, -114&PLUSMN;2 and, -108&PLUSMN;3 J K-1 mol(-1), &UDelta;H-&NOTEQUAL;(k(2))=19.9&PLUSMN;1.2, 24.6&PLUSMN;1.7 and 24&PLUSMN;3 kJ mol(-1) and &UDelta;S-&NOTEQUAL;(k(2))=-8&PLUSMN;5, -84&PLUSMN;7 and -93&PLUSMN;10 J K-1 mol(-1), for 2, 3 and 4 respectively. The free energies of activation are dominated by the -T&UDelta;S&NOTEQUAL; terms. The crystal and molecular structures of Bu4N[PtCl3(CH2CHSiMe3)] and Bu4N[PtCl3(C8H14)] show slight Pt-Cl bond lengthening to 2.314(2) &ANGS; and 2.3238(16) &ANGS; 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 x 10(6)). The relative trans effect of the olefins studied is C2H4&SIM;CH2CHCH2OH>CH(2)CHSiMe(3)similar toC(8)H(14), spanning a factor of between 5 and 30 depending on the nucleophile, a

Publishing year

2002

Language

English

Pages

2354-2360

Publication/Series

Journal of the Chemical Society. Dalton Transactions

Issue

11

Document type

Journal article

Publisher

Royal Society of Chemistry

Topic

  • Inorganic Chemistry

Keywords

  • Fast kinetics
  • Olefin complex
  • Structure
  • Platinum(II)
  • Substitution reaction

Status

Published

ISBN/ISSN/Other

  • ISSN: 1472-7773