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

Abstract

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 approximately -39 kJ mol-1; this is the first reported hydrogen bond strength in a fluorous environment.

Molecular and Ionic Hydrogen Bond Formation in Fluorous Solvents

“…We have previously reported a detailed study on this complex. 12 The electronic absorption spectrum of isoquinoline in FC-72 ( Figure S10) has a πfπ* band at 262 and an nfπ* band at 315 nm that is rich in vibronic structure. A constant-volume titration of isoquinoline with 1 in FC-72 (Figures S10-S12) reveals two new nfπ* bands at 320 and 327 nm with addition of 1.…”

“…These interactions become less important in competitive solvents; 10 thus, a matrix that is a poor solvent will provide a more selective environment for molecular recognition interactions. 11,12 Poorly solvating, highly nonpolar fluorous liquids are practically immiscible with both aqueous and organic phases 13 and, with the exception of some small gases, nonfluorous solutes are, in general, virtually insoluble in them. Thus, fluorous liquids have become useful for purification and reaction cleanup [13][14][15] by reducing the unintentional extraction of interfering species.…”

“…We have recently reported that a carboxylic acid-terminated poly hexafluoropropylene oxide, Krytox 157FSH (1), significantly enhances the extraction of pyridine (100-fold with excess 1 as compared to 1-free) and substituted pyridines from chloroform into a mixture of perfluorohexanes (FC-72). 12 (NsHO) and ionic complexes (NH + sO -) in which the proton is transferred are possible. There has been a great deal of interest in the degree of proton transfer in acid-pyridine complexes under various conditions; thus, relevant UV/vis, IR, and NMR spectroscopic signals of pyridines in the presence of acids are well-documented.…”

Stabilisers for water-in-fluorinated-oil dispersions: Key properties for microfluidic applications

“…However, it has been demonstrated that hydrogen bonds with the lone pair of nitrogen are more stable than the hydrogen bonds present in cyclic carboxylic acid dimers [85,86]. As a consequence, most nitrogen H-bond acceptors are more successful at competing for the carboxylic acid H-bonds than the carboxylic acids themselves [87]. This in turn results in the effective extraction of organic molecules comprising Lewis base characteristics as shown for pyridines [87,80].…”

A Fluorous Biphasic Solvent Extraction System for Lanthanides with a Fluorophilic β-Diketone Type Extractant

“…Weber and coworkers studied the extraction of pyridines and porphyrin from an organic solvent to fluorous solvents using a carboxylic acid receptor. 29 Kondo and coworkers examined the extraction of organic acids into a hydrofluoroether using tri-n-butylphosphate (TBP) and a mixture of TBP and tri-n-octylamine (TOA). 30 The Buhlmann group's research on fluorous chemistry applied to ion-selective electrodes (ISEs) revealed that ionophore-doped fluorous membranes showed drastically enhanced selectivity, because of their extremely low polarity.…”