Dynamic equilibrium between cyclic oligomers. Thermodynamic and structural characterization of a square and a triangle

Abstract

A dynamic equilibrium has been found in CDCl₃ between a neutral molecular square, [cis-Mo₂(DAniF)₂]₄(O₂CC₆F₄CO₂)₄ (1) and triangle, [cis-Mo₂(DAniF)₂]₃(O₂CC₆F₄CO₂)₃ (2) (DAniF = the anion of N,N′-di-p-anisylformamidine). The two components have been crystallographically characterized and solution studies by ¹H and ¹⁹F NMR spectra of the concentration- and the temperature-dependence of the equilibrium have been performed. The conversion of three moles of molecular squares 1 to four moles of molecular triangles 2 has an equilibrium constant of 1.98(7) × 10⁻⁴ at 23.7 °C. At this temperature, the ΔG⁰ for the conversion of three moles of squares to four moles of triangles is 21.0 kJ mol⁻¹. The conversion is enthalpically disfavored (ΔH⁰ = 23.5 kJ mol⁻¹), but entropically favored (ΔS⁰ = 8.2 J K⁻¹ mol⁻¹).