Extraction of Pyridines into Fluorous Solvents Based on Hydrogen Bond Complex Formation with Carboxylic Acid Receptors

A molecular receptor embedded in a ‘poor-solvent' receiving phase, such as a fluorous phase, should offer the ideal
medium for selective extraction and sensing. The limited
solubility of most solutes in fluorous phases enhances
selectivity by reducing the extraction of unwanted matrix
components. Thus, receptor-doped fluorous phases may
be ideal extraction media. Unfortunately, sufficient data
do not exist to judge the capability of this approach. The
solubilities of very few nonfluorous solutes are known. As
far as we are aware, such important quantities as the
strength of a hydrogen bond in a fluorous environment
are not known. Thus, it is currently impossible to predict
whether a particular receptor/solute complex based on a
particular set of noncovalent interactions will provide
enough thermodynamic stabilization to extract the solute
into the fluorous phase. In this work, fluorous carboxylic
acids (a carboxylic acid-terminated perfluoropolypropylene oxide called Krytox and perfluorodecanoic acid
(PFDA)) were used as receptors and substituted pyridines
as solutes to show that the fluorous receptor dramatically
enhances the liquid−liquid extraction of the polar substrates from chloroform into perfluorohexanes. The method
of continuous variations was used to determine the
receptor−pyridine complex stoichiometry of 3:1. The free
energies of formation of the 3:1 complexes from one
pyridine and 3/2 H-bonded cyclic dimers of the fluorous
carboxylic acid are −30.4 (Krytox) and −37.3 kJ mol-1
(PFDA). The free energy required to dissociate the dimer
in perfluorohexanes is +16.5 kJ mol-1 (Krytox). The
crystal structure of the complex showed a 1:1 stoichiometry with a mixed strong−weak hydrogen-bonded motif.
Based on the stoichiometry, crystal structure, and UV and
IR spectroscopic shifts, we propose that the 3:1 complex
has four hydrogen bonds and the carboxylic acid transfers
a proton to pyridine. The resulting pyridinium carboxylate
N+HO- hydrogen bond is accompanied by a weak
pyridine ring CHO bond and is supported by two more
carboxylic acid H-bond donors. We estimate that the free
energy of formation of this complex from a free acid,
pyridine, and a carboxylic acid dimer to be ∼ −39 kJ
mol-1; this is the first reported hydrogen bond strength
in a fluorous environment.