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

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.

[1]  S. Otto,et al.  Quantifying the trans influence of triphenylarsine. Crystal and molecular structures of cis-[PtCl2(SMe2)(AsPh3)] and cis-[PtCl2(AsPh3)2]·CHCl3 , 2002 .

[2]  M. Plutino,et al.  Reaction Mechanism for Olefin Exchange at Chloro Ethene Complexes of Platinum(II). , 1999, Inorganic chemistry.

[3]  V. Kukushkin,et al.  DEOXYGENATION OF COORDINATED SULFOXIDES AND OXIDATION OF THE METAL ION IN THE Pt(II) COMPLEXES USING HX (X = C1, Br): A CONVENIENT PROCEDURE FOR THE PREPARATION OF THIOETHER COMPOUNDS OF Pt(IV). X-RAY STRUCTURE OF POTASSIUM TRICHLORO(DIETHYL SULFOXIDE)PLATINATE(II) , 1992 .

[4]  W. Caseri,et al.  Hydrosilylation with platinum complexes. Preparation, low-temperature NMR spectra, and x-ray crystal structure of the novel bis-olefin catalyst cis-PtCl2(PhCH:CH2)2 , 1987 .

[5]  E. Stevens,et al.  Reexamination of the .pi. bonding in dichloro(cycloocta-1,5-diene)platinum , 1985 .

[6]  M. Tobe,et al.  Kinetics of the reversible displacement of chloride by amines under the trans effect of phosphines, phosphites, and arsines , 1983 .

[7]  L. I. Elding,et al.  Kinetics and Mechanism for Ligand Substitution Reactions of Square-Planar (Dimethyl sulfoxide)platinum(II) Complexes. Stability and Reactivity Correlations , 1978 .

[8]  T. Appleton,et al.  Preparation and properties of hydroxo(methyl)-1,2-bis(diphenylphosphino)ethaneplatinum(II). A trans-influence series including .sigma. carbon donor ligands based on platinum-phosphorus coupling constants , 1978 .

[9]  R. C. Elder,et al.  Tetraethylammonium trichloro(ethyl vinyl ether)platinum(II) , 1978 .

[10]  J. Hubert,et al.  Potassium trichloro(dimethyl sulphoxide)platinate(II) , 1976 .

[11]  P. Tucker,et al.  The crystal structure of tetrabutylammonium trichlorocarbonylplatinate(II) , 1976 .

[12]  L. C. Andrews,et al.  Neutron diffraction study of the structure of Zeise's salt, KPtCl3(C2H4).H2O , 1975 .

[13]  E. M. Haschke,et al.  Vinylmetallics as ligands III. Synthesis and characterization of potassium trichloro(trimethylvinylsilane)platinate(II) , 1973 .

[14]  R. H. Mais,et al.  The crystal structure of K2PtCl4 and K2PdCl4 with estimates of the factors affecting accuracy , 1972 .

[15]  Å. Oskarsson,et al.  Cis-trans influences in Pt complexes. Crystal structures of cis-[PtCl2(1,4-thioxane)2] and trans-[PtCl2(1,4-thioxane)2] , 1993 .

[16]  M. Tobe,et al.  Nucleophilic discrimination and the trans effect. The kinetics of displacement of X from trans-[Pt(L)(X)Cl2]n–(L = Me2SO, Et2S, PMe3, PEt3, PPh3, or AsEt3; X = Cl, n= 1; X = MeOH, n= 0) by neutral and anionic nucleophiles , 1988 .

[17]  L. Canovese,et al.  The kinetics of the displacement, by chloride, of heterocyclic nitrogen bases (am) from trans-[PtL(am)Cl2](L = C2H4, CO, or PMe3); the effect of steric hindrance in the leaving group on the trans effect of L , 1985 .

[18]  L. I. Elding,et al.  Solvent paths for square-planar substitutions. Part 2. Reactions between aqua chloroplatinates(II) and ethene , 1980 .

[19]  G. W. Bushnell,et al.  cis- and trans-Influences in platinum(II) complexes. X-Ray crystal structure analysis of tetraethylammonium trichloro(triethylphosphine)-platinate(II) , 1975 .

[20]  L. I. Elding The Stepwise Dissociation of the Tetrachloroplatinate(II) Ion in Aqueous Solution II. Kinetics of the First Step , 1966 .

[21]  L. A. Duncanson,et al.  586. Olefin co-ordination compounds. Part III. Infra-red spectra and structure: attempted preparation of acetylene complexes , 1953 .