Kinetic study and crystal structure of tetrakis(1,4-thioxane)palladium(II). A comparison between platinum(II) and palladium(II) reactivity

The compound [Pd(1,4-thioxane)(4)](BF4)(2) . 4CH(3)NO(2) crystallizes in the monoclinic space group P2(1)/c with a = 9.474(2), b = 9.529(5), c = 21.945(9) Angstrom, beta = 99.77(3)degrees, Z = 2; final R = 0.051 for 4325 observed reflections (Pd, C, S and O anisotropically refined). The thioxane ligands around Pd form a square-planar arrangement, with two nitromethane molecules at a distance of 3.115(2) Angstrom occupying sites perpendicular to the square-plane formed by the sulfur-bonding thioxanes giving an overall tetragonal bipyramidal coordination around Pd. Intermolecular exchange of 1,4-thioxane on [Pd(1,4-thioxane)(4)](BF4)(2) in deuterated nitromethane and intramolecular interconversion in the bound ligand have been studied as a function of temperature and pressure by H-1 NMR lineshape analysis. The first-order rate constant and activation parameters obtained for the intramolecular process, assigned to sulfur inversion, are as follows: k(i)(298) = 294 +/- 26 s(-1), Delta H-i(not equal) = 61.9 +/- 1.9 kJ mol(-1), Delta S-i(not equal) = + 10.2 +/- 6.5 J K-1 mol(-1), Delta Vi(not equal) = + 2.7 +/- 0.2 cm(3) mol(-1). The intermolecular process yielded the following second-order rate constant and activation parameters: k(2)(298) = 920 +/- 40 m(-1) s(-1), Delta H-2(not equal) = 39.5 +/- 0.9 kJ mol(-1), Delta S-2(not equal) = -55.6 +/- 2.8 J K-1 mol(-1), Delta V-2(not equal) = -9.5 +/- 0.3 cm(3) mol(-1). Second-order kinetics and negative values for the entropy and volume of activation indicate an associative Ia or A exchange mechanism. The reactivity of the analogous Pt2+ compound is estimated using the proposed empirical linear correlation function Delta G(not equal) (Pt) = -32.1 +/- 1.92 Delta G(not equal) (Pd). The calculated second-order rate constant and free energy of activation are 0.3 m(-1) s(-1) and 76 kJ mol(-1), respectively. This relationship indicates also that the well established reactivity order Pd2+ >> Pt2+ could be reversed for better pi-accepting ligands than MeNC and that, in this case, five-coordinated species on Pt(II) are more stable than the Pd(II) analogue.