O'Neal Kristi L, Geib Steven, Weber Stephen G
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260-3900, USA.
Anal Chem. 2007 Apr 15;79(8):3117-25. doi: 10.1021/ac062287+. Epub 2007 Mar 22.
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.
嵌入“不良溶剂”接收相（如氟相）中的分子受体，应能为选择性萃取和传感提供理想介质。大多数溶质在氟相中的有限溶解度通过减少不需要的基质成分的萃取来提高选择性。因此，掺杂受体的氟相可能是理想的萃取介质。不幸的是，目前尚无足够数据来判断这种方法的能力。已知的非氟溶质溶解度非常少。据我们所知，诸如氟环境中氢键强度等重要量值尚不清楚。因此，目前无法预测基于特定一组非共价相互作用的特定受体/溶质络合物是否能提供足够的热力学稳定性，以将溶质萃取到氟相中。在这项工作中，氟代羧酸（一种名为Krytox的羧酸封端的全氟聚环氧丙烷和全氟癸酸（PFDA））被用作受体，取代吡啶作为溶质，结果表明氟代受体显著增强了极性底物从氯仿到全氟己烷的液-液萃取。采用连续变化法确定受体-吡啶络合物的化学计量比为3:1。由一个吡啶和3/2个氟代羧酸的氢键连接的环状二聚体形成3:1络合物的自由能分别为-30.4（Krytox）和-37.3 kJ·mol⁻¹（PFDA）。在全氟己烷中使二聚体解离所需的自由能为+16.5 kJ·mol⁻¹（Krytox）。该络合物的晶体结构显示化学计量比为1:1，具有混合的强弱氢键基序。基于化学计量比、晶体结构以及紫外和红外光谱位移，我们提出3:1络合物有四个氢键，且羧酸将一个质子转移给吡啶。由此产生的吡啶羧酸N⁺H-O⁻氢键伴随着一个弱的吡啶环CH-O键，并由另外两个羧酸氢键供体支撑。我们估计由游离酸、吡啶和羧酸二聚体形成该络合物的自由能约为-39 kJ·mol⁻¹；这是首次报道的氟环境中的氢键强度。
