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Jose´ C. Aponte1∗ James T. Dillon2 Yongsong Huang1 1 Department
of Geological Sciences, Brown University, Providence, RI, USA 2 Department of Chemistry, Brown University, Providence, RI, USA Received May 1, 2013 Revised May 21, 2013 Accepted May 21, 2013
Research Article
The unique liquid chromatographic properties of Group 11 transition metals for the separation of unsaturated organic compounds Silver(I) and copper(I) are known to form reversible complexes with bonds, which have been exploited in LC for separating unsaturated organic compounds. Prominent examples include the use of AgNO3 -impregnated silica gel in LC, and the use of copper(I) salts for selective extraction of alkenes from hydrocarbon mixtures. The Dewar–Chatt–Duncanson model is often invoked to explain the interaction between Ag(I) and Cu(I) and bonds. However, it is unclear if such a reversible interaction is directly related to their d10 outer electronic configurations. Particularly, Au(I) has not been reported to separate olefins with different numbers of double bonds in LC. Also, there has not been a systematic comparison of the liquid chromatographic properties of other d10 transition metal salts (e.g., Zn(II), Cd(II)), making it difficult to fully understand the observed reversible interactions of Ag(I) and Cu(I) with bonds. We demonstrate for the first time that silica gel impregnated with all three Group 11 transition metals with 1+ oxidation state strongly and similarly retain olefin compounds in LC, while transition metals from Groups 10 and 12 do not. We also tested a range of functionalized silica gels to improve the stability of Cu(I) and Au(I) ions on the surface of the silica. Keywords: Compexation chromatography / Olefin separation / pi bonds / Silver thiolate chromatographic material / Transition metal complexes DOI 10.1002/jssc.201300457
1 Introduction The ability of transition metals (TMs) to readily form metal– ligand complexes with organic functional groups such as double bonds (DBs) and exert major catalytic effects has been extensively explored in organic synthesis [1–3]. Among these interactions, Ag(I) and Cu(I) are known to form reversible charge-transfer complexes with bonds in organic molecules and have been widely applied for the separation of olefins from saturated compounds. For example, silica gel impregnated with silver nitrate (AgNO3 ), also called silverion chromatography, is used as the stationary phase to separate compounds by different degrees of unsaturation [4, 5]. Since it was first described in the early 1960s [6], silverion chromatography has been applied for the purification of natural products [7–10], in food chemistry [11–13], and has played an essential role in the elucidation of lipid struc-
Correspondence: Prof. Yongsong Huang, 324 Brook Street, Providence, RI 02912, USA E-mail:
[email protected] Fax: +1-401-863-2058
Abbreviations: AgTCM, silver thiolate chromatographic material; DB, double bond; DCM, dichloromethane; FID, flameionization detection; AgNO3 , silver nitrate; TM, transition metal C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
tures [14–16]. Similar to the use of silver ions, various cuprous salts have been used successfully for the selective liquidphase extraction/adsorption of alkenes from alkene/alkane mixtures [17–20]. To improve the stability and separation efficiency of TM-based LC, we recently successfully demonstrated the use of a silver thiolate chromatographic material [21–23] (AgTCM) for separating unsaturated compounds including hydrocarbons and fatty acids in flash chromatography, HPLC, and TLC [21–24] using normal phase solvents. The main advantages of normal phase separation are (1) higher loading capacity for preparative or industrial separations and (2) in the case of TM-impregnated silica gel for alkene separation, separation is based on only one mechanism (number of double bonds) leading to relatively simple chromatograms [22, 23]. The Dewar–Chatt–Duncanson model is commonly invoked to explain TM interactions with olefins [4, 5]. Ag(I) or Cu(I) form two types of interactions with a DB, a - and a -bonding interaction. The -bonding interaction is formed through the donation of the electrons from the occupied 2p bonding orbital of the olefin into the vacant 5s orbital of a silver ion. The -bonding interaction is formed through the back-donation of d electrons from the fully occupied 4d orbitals of Ag(I) to the unoccupied *-2p antibonding orbitals ∗ Current address: Jos´ e C. Aponte, NASA Postdoctoral Program at NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
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of the olefin. The stability of the metal–olefin complexes (socalled complexes) is thought to be largely determined by the degree of back-donation, but may vary greatly depending on the ligand and metal species [25, 26]. For applications in olefin/alkane separations, the right level of stability of the metal–olefin complexes is essential, since interactions must be readily reversible when solvents of different polarity are used to elute the compounds. A complex of too low stability will result in no retention, whereas too high stability, e.g., in the case of Pt(II) and Pd(II), will result in no elution (or even catalytic transformation) of the targeted compounds in LC [27]. Although both Cu(I) and Au(I) have also been reported to form stable metal–olefin complexes with ethylene as Cu(I) and Ag(I) do [28, 29], these salts have never been used to separate olefins in the context of LC. The bond dissociation energy of the Au(I)–ethylene complex in the gas phase is nearly twice as much as that of Ag(I)–ethylene [30], suggesting that Au(I) might exhibit strong chromatographic retention of organic compounds containing bonds. The absence of reports on relevant liquid chromatographic application of Cu(I) and Au(I) in LC may originate from the instability of Cu(I) and Au(I) salts. Cu(I) and Au(I) have no water-soluble nitrate salts as Ag(I) does. CuCl and AuCl have extremely low water solubility and are prone to disproportionation: they readily decompose in the presence of oxygen/water and selfdisproportionate into their corresponding metals and Cu(II) or Au(III) oxidation states, respectively [31, 32]. These properties and reactivity make impregnating Cu(I) and Au(I) salts onto silica gel extremely difficult using a similar approach employed in preparing silver nitrate silica gel. However, without impregnating Cu(I), Ag(I) and Au(I) and other TM ions onto the silica gel, it is impossible to fully compare their chromatographic properties, which have important implications for understanding the stability and reversibility of TM–olefin complexes. There are also various examples of TM interactions with DBs [33–36]. Ethylene and low-molecular-weight alkenes/alkynes have been found to readily form complexes with Group 9 to 11 TMs [37,38]. Polybutadiene can efficiently abduct Cu(I), Ag(I), and Au(I) during laser ionization in the ion source of a mass spectrometer [39]. A number of TMs such as Pd(II), Cu(II), Ni(II), Zn(II), Eu(II), and La(III) have been shown to enhance the gas chromatographic resolution of chiral, nucleophilic, and unsaturated compounds [40–44]. However, to the best of our knowledge, the only TMs studied for the liquid chromatographic separation of unsaturated compounds besides silver salts are rhodium(II) acetate [45] and thallium(I) nitrate [46], which were found to interact weakly with isomeric butenes. In this study, we selected five C14 hydrocarbons as our model compounds: one alkane, three alkenes (containing different numbers of DBs), and one polycyclic aromatic hydrocarbon, phenanthrene. These compounds were eluted under a similar solvent gradient using a flash column to obtain a direct comparison of the different chromatographic properties of TMs impregnated on silica gel. In the case of Cu(I) and C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. Structures of model C14 hydrocarbons and model structures for stationary phases 1–6 (all possible interactions are not shown).
Au(I), we also tested the feasibility of stabilizing these metal ions onto functionalized silica gel.
2 Materials and methods 2.1 Materials and standards All standards and TM salts (analytical grade) were obtained from Acros, Alfa Aesar, or Sigma Aldrich and were used as received. All solvents used for washing silica gel and column chromatography were HPLC grade. Plain silica gel (SiO2 ) was R ). All stationary phases obtained from Silicycle (SiliaFlash had 40–63 m particle size and 60 A˚ pore size (230–400 mesh, Silicycle). Mercaptopropyl-modified silica gel (≡Si–(CH2 )3 – SH) (1), was prepared using the procedure described below.
2.2 Flash column separation of model C14 hydrocarbons We prepared a mixture of C14 hydrocarbons consisting of ntetradecane (0-DB); 1-tetradecene (1-DB); 1,13-tetradecadiene (2-DB); 1,5,9,13-tetradecatetraene (4-DB), and phenanthrene (Fig. 1) in hexane solution with approximately equal concentrations (∼10 mg/mL). A total of 1 L of the hydrocarbon mixture was delivered using hexane to the top of the column (7.0 × 0.5 cm) packed with ∼1 g of the corresponding www.jss-journal.com
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stationary phase. Columns were further conditioned by eluting with 15 mL of dichloromethane (DCM) and hexane, respectively, before starting the chromatographic separation of the C14 hydrocarbon mixture, and then gently pressurized with nitrogen gas during elution. Separation was achieved by using a sequence of solvents with increasing polarities: hexane (6.4 mL), hexane/DCM 9:1 (6.4 mL), DCM (6.4 mL), and acetone (6.4 mL). Fractions of 400 L were collected in glass vials with inserts and then analyzed by GC with flameionization detection (FID).
2.3 GC The collected 400 L fractions containing hydrocarbons were directly analyzed on an Agilent 6890N GC system, equipped with an HP-1ms capillary column (30 m length, 0.32 mm diameter, 0.25 m film thickness) and an FID. The GC operating conditions were as follows: ultra-high-purity H2 gas as carrier gas (1.5 mL/min), constant flow mode, oven temperature program: initial isothermal at 60⬚C for 1 min, followed by 20⬚C/min to 315⬚C, and final isothermal at 315⬚C for 1 min, injector temperature 300⬚C, detector temperature 320⬚C.
2.4 Flash columns containing TMs other than Group 11 ˚ Flash columns were packed with silica gel (40–63 m, 60 A, SiliaFlash F60, SiliCycle) impregnated with 10% w/w TM salts. The typical procedure for the preparation of 10% TM impregnated silica starts by dissolving 150 mg of a TM salt (NiCl2 , PdCl2 , Cu(NO3 )2 , AuCl, AgNO3 , ZnCl2 , CdCl2 , and HgCl2 ) in 5 mL of methanol/water (1:1) and mixing it with 1.35 g of freshly activated (baked at 150⬚C overnight) silica gel. All silica gel impregnated with TM salts are white in color except NiCl2 (yellow), PdCl2 (dark red), Cu(NO3 )2 (green), and AuCl (light brown). The TM-loaded silica gel slurry was left drying overnight inside a fume hood at room temperature and then dried for 4 h at 150⬚C right before packing into the chromatographic column.
2.5 Flash columns containing Group 11 TMs For silica gel impregnated with CuCl, AgNO3 , and AuCl, we used a different procedure to minimize changes in the oxidation states of salts on the chromatographic columns. In total, 150 mg of these salts were dissolved in 30 mL of acetonitrile. The mixtures were shaken for 3 min, immediately the excess acetonitrile was decanted and the residue slurry was packed into a short glass pipette column. We further washed the column with an additional 15 mL of acetonitrile to remove nonadsorbed salts, this step was found necessary to avoid rapid self-disproportion reactions for CuCl and AuCl, followed by flushing with 15 mL of DCM and 15 mL of hexane to remove acetonitrile from the column for subsequent sepa C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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rations. The chromatographic separation was carried out immediately after salt impregnation, with no observable changes in color (Cu(II) and Au(III) are green and orange, respectively) that would have indicated changes in the oxidation states of the metal cations; silica gel remained white throughout the process.
2.6 Synthesis of mercaptopropyl-functionalized silica gel Mercaptopropyl-functionalized silica (≡Si–(CH2 )3 –SH) (1) was prepared by modifying the silica surface with (3mercaptopropyl)trimethoxysilane, as previously reported [47]. Briefly, silica gel (flash column grade, 40–63 m particle size, 60 A˚ pore size) was dried at 150⬚C for 16 h, then 20 g of the silica gel was suspended in 80 mL of o-xylene, 1 mL of n-butyl amine (20.24 mmol), and combined with 8 mL of (3-mercaptopropyl)trimethoxysilane (43.07 mmol). The mixtures were mechanically stirred (150 rpm) under solvent reflux and argon atmosphere for 24 h. The resulting mercaptopropyl silica gel 1, was filtered and successively washed with 200 mL of toluene, acetone, deionized water, and methanol, then dried at 60⬚C for 16 h. The model structure of 1 is presented in Fig. 1. The mercaptopropyl-functionalized silica gel prepared contained 4.18% sulfur (1.3 mmol/g) as found by elemental analysis (performed at Atlantic Microlab).
2.7 Synthesis of Cu(I) and Au(I) thiolate chromatographic materials CuCl (200 mg) was dissolved in 20 mL of degassed solutions of NaCl 3M, Na2 S2 O3 1M, and acetonitrile. AuCl (200 mg) was dissolved in acetonitrile and Na3 Au(S2 O3 )2 (500 mg) was dissolved in deionized water. Then 1.6 g of 1 and 50 L of pyridine were added and stirred for 4 h. The amount of Cu(I) and Au(I) ions trapped by the thiol functionality in 1 was less than one equivalent, however, the excesses of ionic copper(I) and gold(I) were verified for every mixture by taking an aliquot of the solution and precipitating the corresponding hydroxides. The prepared slurry was transferred to the flash column and rinsed with (a) 15 mL of water, methanol, DCM, and hexane for water solutions, or (b) 15 mL of acetonitrile, DCM, and hexane for acetonitrile solutions. The chromatographic material prepared in aqueous Na2 S2 O3 is brown in color, whereas other Cu(I) or Au(I) chromatographic materials were yellow.
2.8 Stabilization of Cu(I) on functionalized silica gel Functionalized silica gels: dimercaptotriazine (≡Si–DMT, 3.59% N), pentafluorophenyl (≡Si–PFP, 12.1% C), triaminetetraacetic acid (≡Si–TAAcOH, 1.92% N), and cysteine R (≡Si–Cys, 1.02% S) were obtained from Silicycle (SiliaBond www.jss-journal.com
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R and SiliaMetS , 40–63 m particle size and 60 A˚ pore size); while sulfonic acid functionalized silica gel (≡Si– SO3 H, 2.96% S) was prepared following a previously reported method [48] from 1. In total, 1.6 g of ≡Si–DMT, ≡Si–PFP, ≡Si–TAAcOH, ≡Si–Cys, or ≡Si–SO3 H was suspended in 20 mL of acetonitrile containing 200 mg of CuCl and stirred for 4 h. ≡Si–DMT and ≡Si–Cys became dark brown, ≡Si– TAAcOH turned blue, while ≡Si–Cys and ≡Si–SO3 H became slightly yellow when mixed with the CuCl acetonitrile solution (these two latter chromatographic materials quickly turn pale green when packing the column). The copper(I) excess was verified for every mixture by taking an aliquot of the solution and precipitating the corresponding hydroxide. The products: ≡Si–DMT–Cu (2), ≡Si–PFP + Cu+ (3), ≡Si– TAAcO− + 3Cu+ (4), ≡Si–Cys–Cu (5), and ≡Si–SO3 − + Cu+ (6) were filtered and washed thoroughly with acetonitrile and placed on the flash column right before chromatography. Model structures of 1–6 are presented in Fig. 1.
3 Results and discussion 3.1 Separation of unsaturated compounds by silica gel impregnated with TM salts The structures of the C14 model compounds are shown in Fig. 1. Alkanes (0-DB) show little to no retention in all systems, as expected. There is small retention for alkenes in blank silica gel containing no TMs, mainly due to the slight increase in polarity with increasing number of DBs (Fig. 2). Our results demonstrate a qualitative difference in olefin retention between the Group 11 TMs and other TM elements (Fig. 2). Importantly, we found metals belonging to Group 11 with an oxidation state of 1+ were the only TMs capable of separating the mixture of unsaturated hydrocarbons, while all the rest of the TM salts were ineffective or extremely weak in retention. The strongest retentions are found for silica gel impregnated with CuCl, AgNO3 , and AuCl (Fig. 2). Cu(NO3 )2 showed little to no effect, while only silica gel impregnated with NiCl2 and HgCl2 display a slightly enhanced retention for 4-DB alkene (Fig. 2). However, despite the presence of conjugated DBs across three aromatic rings, phenanthrene is not strongly retained by CuCl− , AgNO3 − , or AuCl-impregnated silica gel relative to other olefins.
3.2 The unique liquid chromatographic properties of group 11 TMs Certain levels of self-disproportionation for CuCl and AuCl appear to be unavoidable during our experimental workup: we noticed color changes when polar solvents (ethyl acetate and acetone) were added for elution. Since our operation is carried out in ambient air, self-disproportion reactions for Cu(I) and Au(I) are likely induced by the trace amounts of oxygen dissolved on the organic solvents used; however, we cannot C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. Flash-column LC of C14 hydrocarbons using plain silica and silica gel impregnated with TM salts. Separation was achieved by using a sequence of solvents with increasing polarities: hexane (6.4 mL), hexane/DCM 9:1 (6.4 mL), DCM (6.4 mL), and acetone (6.4 mL). Incremental fractions of 400 L were collected consecutively for analysis by GC–FID. The y axis is the GC– FID response of individual compounds normalized to the highest peak. a Data for blank silica gel and for AgNO3 were published previously [22] and are replotted here for comparison. b Silica gel impregnated with TM salt and flushed with 15 mL of acetonitrile prior to chromatography of hydrocarbons, see Sections 2.4 and 2.5.
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exclude light as a potential trigger as well. To avoid eluting the more soluble Cu(II) and Au(III) salts resulting from selfdisproportionation into our collection vials, we prerinsed the flash column with acetonitrile prior to chromatographic separation. To facilitate the comparison, we performed the same procedure for the silver nitrate column. Clearly, copper, silver, and gold have similar retentions (Fig. 2). Not surprisingly, if we do not rinse the 10% silver nitrate silica gel column with acetonitrile prior to separation, the retention is slightly greater (Fig. 2). Our findings shed important new light on the mechanisms of TM to -bond interactions. Because all tested metal salts except for Cu(I), Ag(I) and Au(I) are essentially ineffective in retaining DBs, forming a reversible while sufficiently strong complexation with DBs must be a unique and intrinsic property of the Group 11 elements in the periodic table. The electronic configurations of the Group 11 elements are ns1 (n–1)d10 (n = 3, 4 and 5). The single charged (1+) cations salts of these elements are s0 d10 , since Zn(II), Cd(II) and Hg(II) ions are also s0 d10 but with little retention, our results indicate electronic configurations of ions are not the determining factor in the complex formation with DBs in organic compounds. However, the absence of retention for Cu(NO3 )2 with s0 d9 suggests that electronic configuration does play a role in such interactions.
3.3 Analysis of the TM–olefin interactions Two theories have been proposed to explain the silver ion– olefin interaction of silver nitrate LC: (1) electrostatic interaction between silver cations and electrons in olefins [49, 50], (2) formation of a metallocyclopropane or a T-shaped structure [51, 52], through synergistic ligand → silver donation and ligand ← silver back-donation [25, 53]. Based on our results, the electrostatic interactions between TM cations and DBs are likely not important because the Cu(II) salt has no effect, whereas Cu(I) is strong (and many of the 2+ cations are ineffective as well (Fig. 2). A recent study shows that the Cu(I)–olefin linkage is mainly sustained by donation, lacking a substantial degree of back-donation [26]. In contrast, the olefin linkages with TMs such as Ni, Pd, and Pt are dominated by back-donation [54, 55]. Since the -donor interaction in Group 11 metal–alkene complexes is superior to the -acceptor interaction it would give the idea that having only donation occurring from the olefin 2p orbitals to the cation free ns and np orbitals is essential for strong chromatographic retention (where the cations include Cu(I), Ag(I) and Au(I)). Complexes based on a predominantly back-donation (such as in Group 10), arising from the full d10 orbitals in the TM to the olefin free antibonding 2p* orbitals, including those having full external s2 orbitals (Group 12), do not lead to sufficiently strong chromatographic retention. Further theoretical methods for the quantitative analysis of TM ion–olefin interactions [56, 57] are needed to confirm such hypotheses. C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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3.4 Stabilization of Cu(I) and Au(I) on 1 Since copper(I) and gold(I) salts are highly insoluble and much less stable than those of silver(I), their use in liquid chromatographic applications has been historically hampered. Recently, a stationary phase denominated AgTCM has been shown to significantly improve the liquid chromatographic properties associated with traditional silver-ion chromatography [21–24]. On AgTCM, silver(I) is covalently bonded to sulfur on the surface of the stationary phase without losing its ability to interact and retain unsaturated compounds. In our previous work, we reported the separation of C14 hydrocarbons using silica gel impregnated with Ag(I) nitrate, but not that of Cu(I) and especially Au(I) salts. Because now we show that Cu(I)- and Au(I)-impregnated silica gel exhibit similar bond retentions as Ag(I)-impregnated silica gel (Fig. 2), we tested the use of thiol-functionalized silica gel 1 to stabilize Cu(I) and Au(I) (Fig. 3). The use of CuCl dissolved in aqueous solutions of NaCl and Na2 SO3 in the presence of 1 is shown in Fig. 3. We found that the use of these solutions helped to improve the stabilization of copper(I), resulting in the separation of the unsaturated compounds, however, this retention was diminished after using the same column consecutively. In this case, a change in color, from pale yellow to pale green (indication of the formation of copper(II)), was observed after finishing the chromatographic separation. To reproduce the methodology used in the synthesis of AgTCM [21–24], we tried to stabilize CuCl and AuCl salts in acetonitrile in the presence of 1 (Section 2.7; Fig. 3). While gold(I) thiolate did not exhibit retention of any of the model hydrocarbons; copper(I) thiolate showed a selective retention of alkenes but not for the aromatic compound phenanthrene, however, this material turned green color and leached copper(II) right after finishing the column. Finally, we used water-soluble Na3 Au(S2 O3 )2 and combined it with 1; unfortunately the use of the thiosulfate salt showed no retention of the unsaturated hydrocarbons. Contrary to the results found for ionic group 11 metals Ag(I), Cu(I) and Au(I); which exhibit a high and similar retention of saturated hydrocarbons in LC, we found that only the silver and copper thiolates are able to reproduce such chromatographic properties. The reason for the no retention of gold(I) thiolate remains unclear at this point. However, the fact less than one equivalent of Au(I) was trapped to form the gold(I) thiolate complex may be important.
3.5 Stabilization of Cu(I) and Au(I) on functionalized silica gels Silica gels containing sulfur and other strong metal complexing ligands were used to stabilize CuCl (Figs. 1 and 4). We found that the stability of copper(I) is slightly improved using dimercaptotriazine silica gel 2, the resulting product was capable of retaining and separating the unsaturated hydrocarbons effectively. This enhancement in stabilization might www.jss-journal.com
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Figure 4. Liquid chromatographic separation of model compounds using stationary phases 2–6 containing Cu(I) ions. Proposed structures of stationary phases 2–6 are presented in Fig. 2. For explanation of the x and y axes see Fig. 2. Figure 3. Liquid chromatographic separation of model compounds using mercaptopropyl-functionalized silica gel (1) containing Cu(I) and Au(I) salts. Aqueous or organic solutions containing excess amounts of the TM ions were mixed with 1 and packed into the chromatographic column. The synthesis of Cu(I) and Au(I) thiolate chromatographic materials is described in Section 2.7. a Data for blank silica gel were published previously [22] and are replotted here for comparison. For explanation of the x and y axes see Fig. 2.
arise from the inductive effect produced by the thiol groups attached to a 1,3,5-triazine electron-withdrawing aromatic system. However, the effectiveness of 2 to interact with the unsaturated compounds was later reduced after consecutive chromatographic separations. We observed that when using acetone to regenerate the column for successive uses after a period of 24 h, a green color showed up in the solution presumably by the formation of CuCl2 dissolved in it. Proof of the formation of Cu(II) by the disproportionation of Cu(I) on 2 was provided by the reduced chromatographic resolution of the unsaturated compounds. The rest of the functionalized silica gels were much less effective but showed some interesting selectivity (Fig. 4). Material 3 exhibited a high selectivity for the retention of for 4-DB; while materials 4 and 5 exhibited selectivity for the aromatic compound phenanthrene. On C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
the opposite side, material 6 did not show strong retention or selectivity. These observed selectivities may partially originate from the specific type of functionalized silica gel, and not from their interaction with the copper salt, given that the disproportionation of copper(I) into copper(II) was observed for all these prepared materials.
4 Concluding remarks We have demonstrated for the first time that the chloride salts of Cu(I)- and Au(I)-impregnated silica gel strongly retain olefins in liquid chromatographic separation, whereas other TMs in Group 10 and 12 are ineffective. The retention of unsaturated compounds in Cu(I), Ag(I), and Au(I) are in fact very similar under the same chromatographic conditions. Therefore, Group 11 TMs are unique in their interactions with bonds for liquid chromatographic applications. We also investigated the stabilization of Cu(I) and Au(I) on mercaptopropyl-functionalized silica gel and various other functionalized silica gels. These results are important for a mechanistic understanding of the nature of TM–olefin www.jss-journal.com
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complexation and the development of novel chromatographic materials containing TMs. We are grateful to NASA-NNX09AM82G and NSF0902805 for the financial support of this research. We thank Rafael Tarozo for assistance during instrumental analyses. The authors have declared no conflict of interest.
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