Fluorous media for extraction and transport.

Selective partitioning can be useful for sample cleanup or the isolation and purification of desired compounds. Fluorocarbon solvents and polymers, and solvents or polymers with similar properties that are not composed solely of carbon and fluorine (so-called 'fluorous' solvents or polymers) have a low ability to dissolve or sorb solutes or penetrants. This lack of solvating ability can lead to selective extractions. Fluorous phases will solvate, and therefore extract or transport, fluorous solutes, or non-fluorous solutes that are stabilized in the fluorous phase by non-covalent interactions with a 'host' or 'receptor' molecule that is in the fluorous phase. In this review, there is a brief discussion of molecular recognition as applied to selective extraction. Fluorous solvents are introduced, and there is a description of some recent applications, chiefly in synthetic organic chemistry. In particular, it is important to understand solute partitioning behavior and methods to predict it when one of the solvents is fluorous. Fluorous polymers Teflon AF1600 and AF2400 have been used in separations. Their rather complex and still not completely understood properties in separations and transport are described. There is a discussion of molecular recognition in fluorous phases as well as a brief discussion of efficient methods of carrying out extractions for analytical or physicochemical purposes.

Extraction and metalation of porphyrins in fluorous liquids with carboxylic acids and metal salts.

Porphyrins have found application in a remarkable variety of areas such as sensors, ion selective electrodes, photodynamic therapy, and energy-transfer systems. Here, we demonstrate the extraction of 5,10,15,20-tetraphenylporphyrin (TPhP) and 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) into a mixture of perfluorohexanes (FC-72) through noncovalent interactions with Krytox (1), a carboxylic acid terminated perfluoropolyether. We found that 1 transfers two protons to the TPhP tetrapyrrole ring to create the porphyrin dication (H2TPhP2+) in FC-72 while up to six protons are transferred to the TPyP pyridyl and tetrapyrrole nitrogens to create a hexavalent cation macrocycle in the fluorous phase. The total charge on TPyP is controlled by adjusting the concentration of 1 in the fluorous phase. In addition, we observed extraction of ZnTPyP from CDCl3 with 1/FC-72, while ZnTPhP is not extracted by 1/FC-72. We prepared the Zn salt of 1 and found that it extracts (from CDCl3) and metalates TPyP but not TPhP. Competitive binding between the porphyrins and an ethanol cosolvent hinders the extraction of both TPhP and TPyP and inhibits the formation of the TPyP hexacation in FC-72. By controlling the concentration of porphyrin, 1, and ethanol, it is possible to reversibly solubilize TPyP in the fluorous phase through noncovalent interactions between the pyridyl moieties and 1 while leaving the tetrapyrrole ring available to interact with metals or other substrates. In addition, both porphyrins and ZnTPyP are easily recovered from the fluorous phase using commercially available fluorous solid-phase extraction cartridges. Understanding noncovalent interactions in fluorous matrices should lead to development of more robust devices for sensing and energy transfer.

Molecular and ionic hydrogen bond formation in fluorous solvents.

There are only a few studies of noncovalent association in fluorous solvents and even fewer that are quantitative. A full understanding, particularly of stoichiometry and binding strength of noncovalent interactions in fluorous solvents could be very useful in improved molecular-receptor-based extractions, advancements in sensor technologies, crystal engineering, and supramolecular chemistry. This work investigates hydrogen bonding between heterocyclic bases and a perfluoropolyether with a terminal carboxylic acid group (Krytox 157FSH (1)), chiefly in FC-72 (a mixture of perfluorohexanes). In particular, we were interested in whether or not proton transfer occurs, and if so, under what conditions in H-bonded complexes. Continuous variations experiments show that in FC-72 weaker bases (pyrazine, pyrimidine, and quinazoline) form 1:1 complexes with 1, whereas stronger bases (quinoline, pyridine, and isoquinoline) form 1:3 complexes. Ultraviolet and infrared spectral signatures reveal that the 1:1 complexes are molecular (B.HA) whereas the 1:3 complexes are ionic (BH+.A-HAHA). Infrared spectra of 1:3 ionic complexes are discussed in detail. Literature and experimental data on complexes between N-heterocyclic bases and carboxylic acids in a range of solvents are compiled to compare solvent effects on proton transfer. Polar solvents support ionic hydrogen bonds at a 1:1 mol ratio. In nonpolar organic solvents, ionic hydrogen bonds are only observed in complexes with 1:2 (base/acid) stoichiometries. In fluorous solvents, a larger excess of acid, 1:3, is necessary to facilitate proton transfer in hydrogen bonds between carboxylic acids and the bases studied.

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