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

Abstract

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 ± 0.0015, K₁₃ = 9.7 ± 0.9 and K₁₄ = 2.05 ± 0.06 mol⁻¹ dm³, respectively. Substitution of one cyclooctene from 4 by MeOH gives K₄₂ = 0.110 ± 0.009 and by MeCN K₄₃ = 2.25 ± 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-stopped-flow spectroscopy. Second-order rate constants at 298 K are 0.128 ± 0.003, 4.93 ± 0.02 and 0.0637 ± 0.0009 mol⁻¹ dm³ s⁻¹, respectively. The corresponding activation parameters are ΔH^≠ = 43.7 ± 1.8, 42.0 ± 0.8 and 39.6 ± 0.9 kJ mol⁻¹ and ΔS^≠ = −115 ± 6, −91 ± 3 and −135 ± 3 J K⁻¹ mol⁻¹, indicating associative activation with a relatively large -TΔS^≠ contribution to ΔG^≠. 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 CC 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 π-back-bonding capacity.