Detection and Differentiation of Neutral Organic Compounds by [superscript 19]F NMR with a Tungsten Calix[4]arene Imido Complex

Fluorinated tungsten calix[4]arene imido complexes were synthesized and used as receptors to detect and differentiate neutral organic compounds. It was found that the binding of specific neutral organic molecules to the tungsten centers induces an upfield shift of the fluorine atom appended on the arylimido group, the extent of which is highly dependent on electronic and steric properties. We demonstrate that the specific bonding and size-selectivity of calix[4]arene tungsten-imido complex combined with 19F NMR spectroscopy is a powerful new method for the analysis of complex mixtures. The selective detection and identification of organic compounds is of fundamental importance in environmental monitoring and biological studies.1,2 The desire for rapid and reliable analytical methods has led to extensive studies of chemical sensors that rely on changes in fluorescence3, resistance4 or other properties in response to a target analyte. However, the difficulty in direct conversion of a sensor response to precise structure and concentration information limits the use of these methods in the detection and differentiation of analytes in complex mixtures. Therefore, it is highly desirable to develop sensing platforms that provide outputs that are effectively analyte fingerprints. Herein, we report such a method by combining a calix[4]arene tungsten-imido receptor with 19F NMR for the selective detection and differentiation of organic molecules in complex mixtures.5 Calixarenes have found wide application in molecular recognition and sensing as a result of their utility as rigid scaffolds that present diverse functionality and encapsulate molecules.6 The interactions between unfunctionalized calixarenes and neutral organic molecules are weak, and their host-guest complexes are best characterized in the solid state.7 In contrast, metallated calixarene complexes tend to be higher affinity receptors as a result of the presence of the Lewis acidic metal centers.8 Among the various metallated calixarene complexes, tungsten-oxo complex stands out as being particularly stable and has a restrictive binding site capable of binding small Lewis basic organic molecules.9 Despite the highly specific recognition properties of these complexes, their application in sensing is largely unexplored. In 2002, we reported a conducting polymer incorporated with calixarene Corresponding Author: [email protected]. Supporting Information. Experimental procedures and characterization data for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public Access Author Manuscript J Am Chem Soc. Author manuscript; available in PMC 2014 December 18. Published in final edited form as: J Am Chem Soc. 2013 December 18; 135(50): 18770–18773. doi:10.1021/ja4106804. N IH -P A A uhor M anscript N IH -P A A uhor M anscript N IH -P A A uhor M anscript tungsten-oxo complex and observed a change of polymer’s conductivity in the presence of certain organic molecules.10 However, differentiation of various analytes, especially in complex mixtures, would still be challenging using that method. Put simply, more information from a sensor than a change in the intensity of an single observable signal is often needed to precisely identify an analyte in a confounding environment. To address this challenge we have designed a fluorinated tungsten-imido ca-lix[4]arene receptor/ chemosensor. Our hypothesis is that the binding of a Lewis basic organic molecule will induce an increase of the electron density on tungsten thereby changing the chemical shift of the fluorine that is connected by π-conjugation. Critical to the success of this method is the fact that the chemical shift range of 19F NMR spectroscopy is very large (>300 ppm), thereby allowing subtle differences in the electronic structure to produce observable changes (Scheme 1).11 Furthermore, the lack of organic fluorine compounds in nature combined with the highly specific recognition ability of calixarene tungsten-imido complex precludes interfering background signals and enables the analysis of complex mixtures. We began our investigations by preparing the t-Bu-calix[4]arene tungsten imido complex (1). Calix[4]arene tungsten-imido complexes were previously synthesized from the reaction of W(VI)(=NR)2Cl2 with calix[4]arene or the reaction of calix[4]arene tungsten(IV) olefin adduct with organic az-ides.12 These methods necessitated the preparation of air-sensitive intermediates, and as an alternative we attempted to incorporate the imido moiety by direct imination of the corresponding calix[4]arene tungsten-oxo which is stable and readily prepared.9 Although WOCl4 can be transformed to W(=NR)Cl4 by reaction with isocyanates,13 no conversion was observed under the same conditions with calix[4]arene tungsten-oxo as a result of the increased electron density on tungsten metal and rigid geometry imposed by the calix[4]arene. In analogy to Wittig reactions, which are driven to completion by the strong phosphorous oxygen bond,14 we envisioned the direct imination of the calix[4]arene tungsten-oxo could be achieved by an analogous iminophoshorane (Ph3P=NR) reagent.15 To our delight, the reaction proceeded smoothly in refluxed toluene, and 1 is obtained in good yield using a “one pot” method from tBu-calix[4]arene (Scheme 2). It should be mentioned that the presence of the two methyl groups on the arylimido ligand is crucial to the success of the reaction.16 The structure of 1 was further confirmed by X-ray crystallography. (Scheme 2, right) With 1 in hand, we explored its sensing potential with 19F NMR spectroscopy by adding various analytes to chloroform solutions of 1 at ambient temperature. Complex 1 only allows endo-coordination of analytes and the size selectivity of the bowlic calix[4]arene eliminates interference from larger analyte.8 Additionally, our approach is selective to strong binding of Lewis basic analytes that produce static structures on the NMR time scale and peaks at precise chemical shifts. This latter feature is important because we seek to differentiate between analytes wherein the dissociation is very fast and/or have weak interactions with the tungsten. In these cases the 19F NMR shift caused by the presence of the molecule could be considered as a solvent effect, which does not interfere with the sensing of the target molecule. As shown in Figure 1, only a single triplet is observed in 19F NMR experiments with ethyl acetate, acetone, and ethanol, which suggested that these analytes do not bond strongly to tungsten metal and/or the binding is too dynamic to induce Zhao and Swager Page 2 J Am Chem Soc. Author manuscript; available in PMC 2014 December 18. N IH -P A A uhor M anscript N IH -P A A uhor M anscript N IH -P A A uhor M anscript a shift in 19F NMR (Figure 1, a–c). In presence of a large excess of ethanol, an upfield shift was observed which can be considered as a solvent effect (Figure 1, e and f). In contrast, a new upfield peak was observed in the experiment with dimethyl sulfoxide (DMSO) indicating the association of DMSO and 1 in solution. The upfield shift is consistent with the assumption that the electron density on tungsten metal increases upon the coordination of analyte. Increasing the concentration of the DMSO led to the disappearance of the signal for 1 indicating full conversion to the inclusion complex. The chemical shift of the new peak remains at constant chemical shift, thereby indicating that the shift is not caused by a solvent effect. Furthermore, the binding with DMSO and N,N-dimethylformamide (DMF) produce much larger upfield shifts than acetonitrile (CH3CN) (Figure 1, g, i, and q), which indicates the response is highly dependent on the electron donating ability of the analyte. This hypothesis is further supported by experiments with various structurally diverse amides. The amide with more electron donating substituents on carbonyl group and nitrogen atom induced a more pronounced upfield shift (Figure 1, i–p). DMF and 2-pyrrolidinone, which possess similar electron density on oxygen and steric bulk resulted in similar shifts (Figure 1, i and k). Another observation from Figure 1 is that DMSO and amides show better coordinating ability than CH3CN which is consistent with a previous study.17 N,Ndibutylformamide failed to show a response with 1 because it is too bulky to bind in the cavity (Figure 1, j). The precise size selectivity of this method was further demonstrated by the different behavior between N-phenylformamide and N-cyclohexylformamide wherein only N-phenylformamide induced a change in the 19F NMR (Figure 1, m and n). Clearly the cavity effect of ca-lix[4]arene enables the size discrimination of analytes with the same function group. To test the application of the method in the analysis of complex mixture, 1 was mixed with DMF and an excess amount of acetone, ethanol, ethyl acetate and N,Ndibutylformamide in chloroform. As shown in Figure 1, only complex 1 and its adduct with DMF were observed in 19F NMR (Figure 1, r). Considering the diverse methods to elaborate calixarenes,6,18 tuning the selectivity of the method should be readily achieved by modifying the upper rim of calixarene. To test the feasibility of this idea, tungsten calixarene-imido 2 without t-Bu group was prepared using the same method as shown in Scheme 1. Indeed, 2 displays different selectivity and is capable of hosting larger molecules such as N-cyclohexylformimade and N,Ndibutylformamide, which failed to coordinate to 1 (Figure 2, b and c). We next explored potential applications of the method to detect biologically relevant amides. N-formylmethionine is known to play an important role in the protein synthesis of bacteria and is recognized by the human body to stimulate immune defense.19 The detection of N-formylmethionine is of interest as it is a characteristic structure motif of prokaryotic proteins.20 Both 1 and 2 were tested, and only 2 binds N-formylmethinine (Figure 2, e). The slight shift of the signal of unbound 2 is a result of the use of methanol as a co-solvent to dissolve N-formylmethionine. With an acquisition time of 24 min (800 scans), a detection limit of 200 μM could be achieved using this method (Figure 2, f). To further demonstrate applications in the analysis of complex mixtures, we have applied our method to the direct analysis of a crude reaction mixture. The complexity of reaction mixtures typically necessitates gas chromatography or liquid chromatography–mass Zhao and Swager Page 3 J Am Chem Soc. Author manuscript; available in PMC 2014 December 18. N IH -P A A uhor M anscript N IH -P A A uhor M anscript N IH -P A A uhor M anscript spectrometry to provide detailed information. However, these schemes require timeconsuming pre-purification steps and therefore, alternatives are desirable. As chemical reactions are accompanied by the bond cleavage and formation, the electronic properties and the size of the product is usually different from the starting material. To illustrate that subtle differences in a complex background can be observable by our tungsten-imido sensor we selected the Suzuki-Miyaura reaction as a model system. This complex mixture contains aryl halide, organoboronic acid, palladium metal, phosphine ligand, inorganic base and water.21 Moreover, side reactions, such as protonation or homo-coupling of organoboronic acid and the overlap of signals at aromatic range can make the crude proton NMR difficult to interpret. The Suzuki-Miyaura coupling with 4-cyanobenzenitrile and phenyl boronic acid was carried out using Pd(dppf)Cl2·CH2Cl2 as the catalyst with a mixture of DME and 2M Na2CO3 as the solvent. Small aliquots were taken and mixed with 1 in chloroform for 19F NMR analysis. As shown in Figure 3, only three species were observed in 19F NMR spectrum, which were identified as unbound 1, and adducts of 1 with the starting material 3 and product 4. As the reaction proceeded, the starting material decreased with a concurrent increase of product. The simplicity of the spectrum allowed for a clear monitoring of the reaction. In summary, we have demonstrated a new sensing scheme based on fluorinated calixarene imido complexes and their applications in the detection and differentiation of neutral organic molecules. The upfield shift of 19F NMR upon binding to the tungsten center are selfconsistent and support our model of coordination of Lewis bases to the tungsten centers transmitting greater electron density at the fluorine center. The power of this method is a clear and unambiguous detection of target molecules in complex mixtures and we have demonstrated the robust sensing in complex organic reactions. The combination of molecule recognition with 19F NMR technology not only allows the analysis of complex mixture but also provides valuable structure information. The application of this method can be easily widened by designing more fluorinated receptors. Supplementary Material Refer to Web version on PubMed Central for supplementary material.