Hydrogen-bonded complexes of pentaalkyl guanidines

Abstract

The hydrogen-bonded complexes of strongly basic pentaalkylguanidines (G) with 4-fluorophenol as a proton donor reference (FP) have been studied in CDCl₃ solution by means of ¹³C NMR spectroscopy. The equilibrium constants for formation and the chemical shifts of all the ¹³C NMR lines of the hydrogen-bonded complexes were determined. This rich spectroscopic characterization of the complexes allowed us to discuss their structure. The guanidines were found to be more basic than triethylamine, the bicyclic ones being the strongest bases, in accordance with expectations. The ion pair character of the hydrogen-bonded complexes was as high as 40% for the strongest bases. In addition to the expected GFP complex, a diassociation complex G(FP)₂ was observed at high fluorophenol concentrations. In this latter complex, one fluorophenol molecule is ionized by full proton transfer to the guanidine and the second fluorophenol molecule is bound by hydrogen bonding to the fluorophenate anion. A linear relationship between the ¹³C NMR chemical shift variations of 4-fluorophenol and log(K₁) was found, as in the original work of Gurka and Taft using ¹⁹F NMR.