Bridge-splitting kinetics, equilibria and structures of trans-biscyclooctene complexes of platinum(II)

Cyclooctene (C8H14, cot) complexes of platinum(II) have been synthesised and characterised by means of multinuclear NMR, UV-Vis spectroscopy and X-ray crystallography. The bridge-splitting equilibrium constants of the dinuclear, chloride-bridged trans-[PtCl2(cot)]2, 1, in dichloromethane solvent at 298 K with MeOH, MeCN and cot, yielding trans-[PtCl2(cot)(MeOH)], 2, trans-[PtCl2(cot)(MeCN)], 3, and trans-[PtCl2(cot)2], 4, were determined by UV-Vis measurements as K12= 0.0169 (plus-minus) 0.0015, K13= 9.7 (plus-minus) 0.9 and K14= 2.05 (plus-minus) 0.06 mol^-1 dm^3, respectively. Substitution of one cyclooctene from 4 by MeOH gives K42= 0.110 (plusminus) 0.009 and by MeCN K43= 2.25 (plus-minus) 0.03. The bis-cyclooctene complexes 1 and 4 react quantitatively with chloride to give [PtCl3(cot)]-, 5. The kinetics for bridge-splitting of 1 with MeOH, MeCN and cot was studied by conventional and cryo-stoppedflow spectroscopy. Second-order rate constants at 298 K are 0.128 (plus-minus) 0.003, 4.93 (plus-minus) 0.02 and 0.0637 (plus-minus) 0.0009 mol^-1 dm^3 s^-1, respectively. The corresponding activation parameters are (delta)H(not equal)= 43.7 (plus-minus) 1.8, 42.0 (plus-minus) 0.8 and 39.6 (plus-minus) 0.9 kJ mol^-1 and (delta)S(not equal)=-115 (plus-minus) 6, -91 (plus-minus) 3 and -135 (plusminus) 3 J K^-1 mol^-1, indicating associative activation with a relatively large -T(delta)S(not equal) contribution to (delta)G(not equal). Crystal and molecular structures of 1, 4, and the tetraphenylphosphonium salt of 5 have been determined, indicating a moderate ground-state trans influence of cyclooctene. The molecular structure of 4 with two C=C double bonds coordinated trans to each other and perpendicular to the platinum coordination plane, features a significant Pt-C bond lengthening compared to cyclooctene complexes with chloride or nitrogen trans to cyclooctene, leading to high reactivity and thermodynamic instability. Equilibrium data confirm that the thermodynamic stability of a cyclooctene coordinated trans to another cyclooctene is much lower than trans to a ligand with less (pi)back-bonding capacity.