Hydrogen-bonded complexes of pentaalkyl guanidines

The hydrogen-bonded complexes of strongly basic pentaalkylguanidines (G) wi th 4-fluorophenol as a proton donor reference (FP) have been studied in CDC l3 solution by means of C-13 NMR spectroscopy. The equilibrium constants fo r formation and the chemical shifts of all the C-13 NMR lines of the hydrog en-bonded complexes were determined. This rich spectroscopic characterizati on of the complexes allowed us to discuss their structure. The guanidines w ere 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) w as observed at high fluorophenol concentrations. In this latter complex, on e fluorophenol molecule is ionized by full proton transfer to the guanidine and the second fluorophenol molecule is bound by hydrogen bonding to the f luorophenate anion. A linear relationship between the C-13 NMR chemical shi ft variations of 4-fluorophenol and log(K-1) was found, as in the original work of Gurka and Taft using F-19 NMR.