An equilibrium between a loop and a triangle containing Mo24+ species and perfluorodicarboxylate linkers

A new family of supramolecular loops with flexible perfluorodicarboxylate linkers, [cis-Mo2(DAniF)2]2(O2C(CF2)2CO2)2 (1), [cis-Mo2(DAniF)2]2(O2C(CF2)3CO2)2 (2a), and [cis-Mo2(DAniF)2]2(O2C(CF2)4CO2)2 (3) have been prepared and characterized using X-ray structure analyses and NMR spectroscopy (DAniF = N,N′-di-p-anisylformamidinate). NMR spectra of 1 and 3 remain unchanged over long periods of time. However, the spectrum of 2a changes with time showing that there is a process that is consistent with a dynamic equilibrium between the molecular loop (2a) and molecular triangle [cis-Mo2(DAniF)2]3(O2C(CF2)3CO2)3 (2b) in the solution of 2a in CDCl3. A study of the concentration-dependent and temperature-dependent equilibrium using 1H and 19F NMR spectra of this hexafluoroglutarate-containing species suggested the existence of 2b and provided quantitative values for the thermodynamic equilibrium constant K, ΔH0, and ΔS0. Conversion of the molecular loops 2a to the molecular triangles 2b is enthalpically favored, but entropically disfavored with ΔH0 = −41.9 kJ mol−1, and ΔS0 = −82 J mol−1 K−1.

Three molecular loops containing quadruple bonded Mo24+ corner pieces and perfluorodicarboxylate linkers have been prepared and characterized crystallographically. In CD3Cl solution there is a dynamic equilibrium between loops and triangles for the hexafluoroglutarate species. A detailed analysis of the concentration-dependent and temperature-dependent NMR spectra provided the thermodynamic values K, ΔH0, and ΔS0, as well as the rate constant for the equilibrium at 0 °C.Figure optionsDownload as PowerPoint slide