Tetrakis(diisopropyl amide) substituted norbornadiene and quadricyclane derivatives were investigated
for their extraction and transport capabilities with alkaline earth metal cations. Both amides exhibited a
remarkably high preference of Ba2+ over any other alkali metal or alkaline earth cation. The binding geometries were determined by quantum chemical DFT calculations

Bayrhuber, Monika

Herges, Rainer

Sahlmann, Benjamin

Winkler, Torsten

Dalton Transactions

Torsten Winkler

Monika Bayrhuber

Benjamin Sahlmann

Rainer Herges

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Tetrakis(diisopropyl amide) substituted norbornadiene and quadricyclane are highly barium selective ligands

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DaltonTransactionsDynamic Article LinksCite this: Dalton Trans., 2012, 41, 7037www.rsc.org/dalton PAPERTetrakis(diisopropyl amide) substituted norbornadiene and quadricyclaneare highly barium selective ligands†Torsten Winkler, Monika Bayrhuber, Benjamin Sahlmann and Rainer Herges*Received 2nd February 2012, Accepted 4th April 2012DOI: 10.1039/c2dt30242jTetrakis(diisopropyl amide) substituted norbornadiene and quadricyclane derivatives were investigatedfor their extraction and transport capabilities with alkaline earth metal cations. Both amides exhibited aremarkably high preference of Ba2+ over any other alkali metal or alkaline earth cation. The bindinggeometries were determined by quantum chemical DFT calculations.IntroductionThe complexation and selective transport of alkaline earth metalcations in general and Ba2+ in particular have been thoroughlyinvestigated for a long time. Important applications for Ba2+extraction include the puriﬁcation of spent nuclear fuel1 as wellas sulfur analytics.2 Various binding domains have been success-fully established, e.g. calixarenes,3–5 cyclic oligopeptides6 orsubstituted crown ethers.7 Especially the use of the lipophilicdicarbollyl-cobaltate anion in combination with a wide variety ofdonor ligands was extensively studied by Makrlík et al.8,9Over the past two decades we have been investigating thewell-established norbornadiene-quadricyclane system regardingthe feasibility of active ion transport through artiﬁcial lipophilicmembranes.10–13 The light induced [2 + 2]-cycloaddition orcycloreversion results in a change of the size of the ligand’scavity, which subsequently affects the complexation propertiesfor metal ions. We synthesized a wide variety of symetricallyfour-fold-substituted norbornadiene and quadricyclane deriva-tives. Esters, polyethers as well as amides were considered assuitable binding domains mainly for alkali metal cations.14The tetrakis(diisopropyl amide) substituted isomers 1 and 2proved to be the most promising candidates in terms of activeion transport.15 The amides 1 and 2 showed remarkably differingassociation constants for Li+, Na+ and K+ and were successfullytested in passive transport experiments (Scheme 1).Alkaline earth ion complexationTo widen the scope of the amide substituted norbornadiene-quadricyclane carriers, the complexation and passive transportproperties of the amides 1 and 2 were studied using alkalineearth metals as central ions. To facilitate the understanding of thecomplexation mechanism, the system was further investigated byDFT calculations.Results and discussionThe substances 1 and 2 were synthesized according to our pre-viously published procedure.15 To determine the extraction con-stants (see eqn (1) and (2)), we used CRAM’s picrate extractionmethod using chloroform as the organic solvent.16,17MnþðaqÞ þ n AðaqÞ þ LðorgÞ  ! LMAnðorgÞ ð1ÞKex ¼ cðLMAnÞorg cðMnþÞaq1 cðAÞaqn cðLÞorg1 ð2ÞThe experimental results for both isomers are summarized inTable 1, assuming 1 : 1 stoichiometry and including the pre-viously determined extraction constants for alkali metal cations.Both carriers exhibit a remarkably high extraction constantwith Ba2+. The fact that the extraction constant for Ba2+ is morethan 20 times higher than for any other investigated alkalineearth and more than 300 times higher than for any alkali metalcation is particularly noteworthy. In comparison with numerousexamples of ligands with high extraction constants forBa2+,3,18–24 the performance of the amides 1 and 2 is well abovethe average. A future application as Ba2+ selective receptorslooks promising. In contrast, the selectivity between norborna-diene and quadricyclane reaches a maximum for Mg2+ and Cs+.A possible explanation for the outstanding extraction of Ba2+Scheme 1 The diisopropyl substituted norbornadiene 1 and the corre-sponding quadricyclane 2 are photoswitchable cation carriers.†Electronic supplementary information (ESI) available. See DOI:10.1039/c2dt30242jOtto Diels-Institute for Organic Chemistry, Otto-Hahn-Platz 3-4,24098 Kiel, Germany. E-mail: rherges@oc.uni-kiel.de;Fax: +49 431 1558; Tel: +49 431 1540This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 7037–7040 | 7037Published on 10 April 2012. Downloaded by Christian Albrechts Universitat zu Kiel on 08/07/2013 15:52:54. View Article Online / Journal Homepage / Table of Contents for this issuecould be the exclusive formation of a 2 : 1 complex due to thelarger ionic radius. To clarify the binding situation, we carriedout quantum chemical calculations.Transport ratesThe performance of the amides 1 and 2 in passive transportexperiments was investigated using a supported liquid membranesetup as ﬁrst described by Reinhoudt et al.25 2-Nitrophenyl octylether (NPOE) was used as the membrane phase with a micro-porous polypropylene foil as support.The ion ﬂow was continuously monitored by conductivitymeasurements in the acceptor phase over a period of 48 h. Thefollowing Tables 2 and 3 show the averaged ion ﬂow J48 as wellas the values averaged over the ﬁrst and second half of the dur-ation (J24A and J24B, respectively). Table 4 shows the ion ﬂowsobtained in blind experiments without any carrier in themembrane.The results of the transport experiments are consistent withthe extraction experiments. In all cases, the ion ﬂows of the nor-bornadiene amide 1 are signiﬁcantly higher than those of thecorresponding quadricyclane 2. As expected, the Ba2+ ion ﬂowexceeds any other observed ion ﬂow by a factor of about 10.Beyond that, no signiﬁcant decrease of the ion ﬂows for Ba2+and Sr2+ was observed during the experiments. Moreover, inaccordance to the extraction constants, the selectivity towardsBa2+ between the norbornadiene 1 and the quadricyclane 2 isnot very pronounced.In contrast to the extraction experiments, no signiﬁcant trans-port of Mg2+ and Ca2+ by the quadricyclane 2 could be detected.This fact gives rise to a comparatively high transport selectivitybetween isomers 1 and 2 for these two cations. The ion ﬂow ofMg2+ with norbornadiene 1 is about 25 times higher than withquadricyclane 2, which could prove useful in possible futurestudies towards active ion transport.Quantum chemical calculationsX-Ray crystal structures have shown that the norbornadieneamide 1 binds the central cation in a distorted quadratic pyrami-dal environment using the carbonyl oxygen atoms and the coun-terion as donors.15 To further elucidate the binding geometry, weconducted quantum chemical calculations of both 1 : 1 and 2 : 1complexes on a DFT level of theory. The example structures ofthe 1 : 1 complexes of the norbornadiene amide 1 with Mg2+ andBa2+ as well as the 2 : 1 complex with Ba2+ are shown in Fig. 1.The comparison of the 1 : 1 complexes with Mg2+ and Ba2+suggests the feasibility of a 2 : 1 complex with Ba2+, but notwith Mg2+.Geometry optimizations in vacuo were carried out to deter-mine the binding situation of various cations in the cavity of theion carriers 1 and 2. The calculation of complexation energiesproved difﬁcult as the solvation of both carrier and cation mustnot be neglected. Thus, we limited our focus on the differencesin heat of formation for the investigated cations. For the 1 : 1complexes, the distance of the central cation from the hypotheti-cal plane deﬁned by the four carbonyl oxygen atoms as well asthe bond distances between the cation and the oxygen atomswere of particular interest. The results are shown in Table 5.As expected, the distance of the central metal cation from theoxygen plane increases with the size of the cation. The centralcation’s distance from the oxygen plane in the quadricyclane 2 isalways considerably larger than in the corresponding norborna-diene 1 while the bond lengths are almost identical. The increasein bond length corresponds with the growing metal cation radii.These observations imply a rigid carrier backbone with onlylittle ability to adapt to the size of a complexated cation. Further-more, the distance from the oxygen plane is always higher forTable 1 Extraction constants between water and chloroform of thediisopropyl amide substituted carriers with alkaline earth as well asalkali metal cations,15 assuming 1 : 1 stoichiometryIona Kex (1)b Kex (2)bMg2+ 18.8 ± 2.1 6.3 ± 1.0Ca2+ 340.4 ± 28.2 149.8 ± 10.5Sr2+ 308.7 ± 26.9 190.6 ± 22.5Ba2+ 8566.5 ± 405.2 6142.1 ± 572.7Li+ 14.1 ± 0.4 13.9 ± 0.5Na+ 23.3 ± 1.1 8.6 ± 0.3K+ 6.2 ± 0.2 5.1 ± 0.3Cs+ 18.5 ± 0.5 6.9 ± 0.3a The corresponding picrates were used. b [L2 mol−2].Table 2 Transport rates of the diisopropyl amide substitutednorbornadiene 1 with alkaline earth ionsIon J48a J24Aa J24BaMg2+ 1.33×10−8 1.54×10−8 1.12×10−8Ca2+ 1.44×10−8 1.45×10−8 1.43×10−8Sr2+ 1.53×10−8 1.59×10−8 1.47×10−8Ba2+ 2.24×10−7 2.34×10−7 2.14×10−7aAll transport rates in mol m−2 s−1.Table 3 Transport rates of the diisopropyl amide substitutedquadricyclane 2 with alkaline earth ionsIon J48a J24Aa J24BaMg2+ 5.11 × 10−10 6.69 × 10−10 3.53 × 10−10Ca2+ 1.14 × 10−9 1.29 × 10−9 9.81 × 10−10Sr2+ 5.56 × 10−9 5.56 × 10−9 5.56 × 10−9Ba2+ 5.43 × 10−8 5.38 × 10−8 5.47 × 10−8aAll transport rates in mol m−2 s−1Table 4 Transport rates of alkaline earth ions without any carrierIon J48ha J24Aa J24BaMg2+ 1.33 × 10−9 1.45 × 10−9 1.21 × 10−9Ca2+ 9.33 × 10−10 7.85 × 10−10 1.08 × 10−9Sr2+ 4.33 × 10−10 4.72 × 10−10 3.93 × 10−10Ba2+ 5.93 × 10−10 8.08 × 10−10 3.77 × 10−10aAll transport rates in mol m−2 s−17038 | Dalton Trans., 2012, 41, 7037–7040 This journal is © The Royal Society of Chemistry 2012Published on 10 April 2012. Downloaded by Christian Albrechts Universitat zu Kiel on 08/07/2013 15:52:54. View Article Onlinethe quadricyclane 2, indicating a smaller cavity size whichcannot accommodate the cation as well as the larger cavity ofnorbornadiene 1.The calculation of the 2 : 1 complexes of the amide 1 resultedin stable minima only for Ca2+, Sr2+ and Ba2+. The binding ofan additional ligand to the 1 : 1 Sr2+ complex is 1.94 kcal mol−1less exothermic than for Ba2+. For Ca2+ the difference is evenlarger at 16.59 kcal mol−1. Considering hydration enthalpiesdecline with increasing cation radii, the difference in complexformation enthalpies will be even more pronounced in aqueoussolution.ConclusionsThe tetrakis(diisopropyl amide) substituted norbornadiene andquadricyclane derivatives 1 and 2 proved to be outstandinglyselective ligands towards Ba2+. The extraction constants betweenwater and chloroform are more than 20 times higher for Ba2+than for any other alkaline earth ion and more than 300 timeshigher than for any investigated alkali metal ion. The perform-ance of the amides 1 and 2 in passive transport experiments wasequally impressive, also showing high selectivity towards Ba2+.The results of the DFT calculations suggest that the formation ofa 2 : 1 complex may be responsible.ExperimentalPreparation of the solutions for the extraction experimentsThe picrate solutions needed for extraction were prepared bydissolution of dry picric acid in 0.01 M alkaline earth metalhydroxide solutions. The pH value after complete dissolution ofthe picric acid was determined to be pH 7.0 ± 0.1. The 0.01 Mligand solutions were prepared by dissolving the ligands inchloroform. Pure chloroform was used to perform the blanktests.General procedure for performing the extraction experimentsFor the extraction experiments 4 mL of each the ligand solutionand the picrate solution were ﬁlled in a 10 mL screw-cap glass.The cap was closed and the glass was shaken for 2 min at roomtemperature using a Heidolph Reax 2000 shaking machine.For the determination of the association constant the experimentwas repeated four times with identical samples and the meanvalue was formed. After separation of the phases the organiclayer was sucked off with a pipette and ﬁltered through glasswool. The ﬁltrate was investigated using UV-Vis spectroscopy.The thickness of the quartz cell was 1 cm. For the blank test anidentical sample was prepared that contained deionized waterinstead of the picrate solution. The concentration of the picrate inthe organic phase was calculated using Lambert–Beer’s law.General procedure for performing the transport experimentsThe transport experiments were carried out at room temperature(25 °C) in a quartz apparatus with an exchange area of8.5 × 10−4 m2. The aqueous donor phase consisted of 23 mL of0.15 M alkaline earth nitrate solution while 23 mL HPLC gradewater (Fluka) was used as the acceptor phase. Both phases werecontinuously stirred. A microporous Accurel® PP membrane(Akzo Nobel, thickness: 92.5 ± 17.5 μm) was used as supportfor the liquid membrane consisting of 15 mg of the carrier in285 mg of NPOE. The conductivity in the acceptor phase wasmonitored using a WTW LF-340A conductometer equippedwith a cell for pure water (type 325/01).Quantum chemical calculationsDensity functional theory calculations were carried out using theB3LYP/SDD method.27–29 All reported structures were veriﬁedas minima by frequency calculations. The Gaussian 09 softwarewas used for all calculations.30Fig. 1 Calculated geometries of the 1 : 1 complex of norbornadiene 1with Mg2+ (upper left side) and Ba2+ (upper right side) and the 2 : 1complex of 1 with Ba2+ (bottom center).Table 5 Distance between the plane deﬁned by the four carbonyloxygen atoms and the metal ion (dplane) and average bond lengthbetween oxygen and metal ion (dbond) for the calculated 1 : 1 complexesCation (radiusa,b)26Norbornadiene 1 Quadricyclane 2dplaneb dbondb dplaneb dbondbNa+ (1.02) 0.72 2.34 0.87 2.31Mg2+ (0.72) 0.42 2.03 0.53 2.03Ca2+ (1.00) 0.81 2.27 0.95 2.26Sr2+ (1.18) 1.05 2.43 1.19 2.41Ba2+ (1.35) 1.29 2.58 1.41 2.56a 6-Fold coordination. bAll radii and distances in Å.This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 7037–7040 | 7039Published on 10 April 2012. Downloaded by Christian Albrechts Universitat zu Kiel on 08/07/2013 15:52:54. View Article OnlineAcknowledgementsWe gratefully acknowledge ﬁnancial support for B.S. throughDFG SFB 677 “Function by Switching”.References1 S. C. Lee, J. D. Lamb, M. Cai and J. T. Davis, J. Inclusion Phenom.Macrocyclic Chem., 2001, 40, 51.2 Yu. M. Dedkov, N. V. Korsakova and V. A. Sychkova, J. Anal. Chem.,2006, 61, 1154.3 C. Tu, K. Surowiec and R. A. Bartsch, Tetrahedron Lett., 2006, 47, 3443.4 J. S. Kim, S. H. Lee, S. H. Yu, M. H. Cho, D. W. Kim, S. G. Kwon andE. H. Lee, Bull. Korean Chem. Soc., 2002, 23, 1085.5 E. Makrlík and P. Vaňura, J. Radioanal. Nucl. Chem., 2006, 267, 699.6 L. Mu, H. Huang, J. He, N. Zhang and J. Cheng, Chin. Sci. Bull., 2001,46, 219.7 M. J. Tapia, A. J. M. Valente, H. D. Burrows, V. Calderon, F. García andJ. M. García, Eur. Polym. J., 2007, 43, 3838.8 E. Makrlík, P. Vaňura and P. Selucky, J. Radioanal. Nucl. Chem., 2010,283, 615 and cited literature.9 E. Makrlík and P. Vaňura, Talanta, 1985, 32, 423.10 R. Herges and W. Reif, Chem. 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Tetrakis(diisopropyl amide) substituted norbornadiene and quadricyclane are highly barium selective ligands

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