Activation volumes for intramolecular oxidative C-X (X=H, F, Cl or Br) addition to platinum(II) imine complexes as a proof of the intimate mechanism

The kinetics of C-X (X = H, F, Cl or Br) bond activation of ring-substituted, PhCHNCH2Ph, type imines via intramolecular oxidative addition to platinum(II) complexes has been studied in acetone and toluene solution at different temperatures and pressures. Although the activation parameters determined are within the range expected, the latter is extremely large (Delta H-double dagger from 25 to 70 kJ mol(-1), Delta S-double dagger from -220 to -45 J K-1 mol(-1), Delta V-double dagger from -31.2 to -9.5 cm(3) mol(-1)). No differences were found for the reactions carried out in acetone or toluene, indicating that no polar transition state is formed during the reaction and that a common highly ordered three-centred C-Pt-X interaction is present for all the imines used. A good correlation was also obtained for the Delta S-double dagger and Delta V-double dagger values, independently of the solvent used, confirming the non-polarity of the transition state. A deviation from this pattern is observed only for fluorinated imines both in acetone and toluene solutions; this result is interpreted by considering an earlier transition state for the oxidative addition of C-F that has not yet produced an important volume contraction of the platinum centre despite the important spacial organization of the ligand, as shown by the very negative values of Delta S-double dagger.