- Email: [email protected]
A preparative study of some coronands and their complexation of silver(I), zinc(II), cadmium(II) and lead(II)
Inorganica Chimica Acta 357 (2004) 716–722 www.elsevier.com/locate/ica
A preparative study of some coronands and their complexation of silver(I), zinc(II), cadmium(II) and lead(II) Daniela Caiazza, Stephen F. Lincoln *, A. David Ward Department of Chemistry, The University of Adelaide, Adelaide, SA 5005, Australia Received 24 April 2003; accepted 25 June 2003
Abstract A study of the complexation of heavy metal ions by the coronands 3,12,20,29-tetraoxa-35,36-diazapentacyclo [29.3.1.1.14;18 .05;10 .022;27 ]-hexatriaconta-1(35),5(10),6,8,14,16,18(36),22(27),23,25,31,33-dodecaene (1); 2,3,11,12-bis (40 -methylbenzo)-1, 4,10,13-tetrathia-7,16-dioxacyclo-octadeca-2,11-diene (2); 7,16-diaza-1,4,10,13-tetraoxa-2,3,11,12-dibenzocyclooctadeca-2,11-diene (3); 2-[19-(2-hydroxy-2-phenylethyl)-7,8,9,10, 18,19,20,21-octahydro-6H,17H-dibenzo[b,k][1,4,10,13,7,16]tetraoxadiazacyclooctadecin-8-yl]-1-phenyl-1-ethanol (4); 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane (5); and 2-[16-(2-hydroxy-2-phenylethyl)-1,4,10, 13- tetraoxa-7,16-diazacyclo-octadecanyl]-1-phenyl-1-ethanol (6) is described. Coronands 1 and 3 were prepared by literature methods, improved methods were used to prepare 2, and 4 and 6 were prepared from 3 and 5 (obtained commercially), respectively. Potentiometric studies in N,N-dimethylformamide yielded (log K/dm3 mol1 ) ¼ 5.50, 6.49, 9.42 and 7.52 for [Ag 1]þ , [Ag 2]þ , [Ag 5]þ and [Ag 6]þ , respectively; <2, <2, 4.30 and <2 for [Zn 1]2þ , [Zn 2]2þ , [Zn 5]2þ and [Zn 6]2þ , respectively, <2, <2, 5.92 and >7.52 for [Cd 1]2þ , [Cd 2]2þ , [Cd 5]2þ , and [Cd 6]2þ , respectively, and 2.62, 2.38, 6.71 and >7.52 for [Pb 1]2þ , [Pb 2]2þ , [Pb 5]2þ , and [Pb 6]2þ , respectively. ESI-MS studies of the interactions of 1–6 with Agþ , Zn2þ , Cd2þ and Pb2þ are also reported. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Coronands; Silver(I) complexes; Heavy metal complexes; Stability constants
1. Introduction Industrialization has resulted in an increase in heavy metals in the environment and as a consequence of their detrimental effects on health there is a corresponding interest in their detection and sequestration [1]. We are particularly interested in two such ions, Cd2þ and Pb2þ , and their selective complexation with a view to the eventual development of detection and sequestration methods for environmental and biological samples. Because of its ubiquity in the environment it is important that Zn2þ should not compete with Cd2þ and Pb2þ in the complexation process. Accordingly, it is necessary to gain an understanding of the ligand coordination ÔcoreÕ that provides such selectivity. In seeking this core we chose the coronands 1–6 (Scheme 1) which provide a
range of denticities, donor atom types and structural rigidities as the basis of our studies. To obtain sufficient solubilities for complexation studies it was necessary to work in N,N-dimethylformamide (DMF) so that the metal ion selectivity of each coronand could be determined. (Obviously, DMF is unsuitable for most sequestration and detection applications, but it is envisaged that a coronand or similarly based system will either be water solubilized through the substitution of hydrophilic pendant groups onto the coronand, or they will be immobilized on a surface over which an aqueous analyte solution flows.)
2. Experimental 2.1. General
*
Corresponding author. Tel.: +61-8-308-5559; fax: +61-8-8303-4358. E-mail address: [email protected] (S.F. Lincoln).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.06.019
Melting points (uncorrected) were recorded on a Kofler hot-stage apparatus equipped with a Reichert
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
717
1,4,10,13-Tetraoxa-7,16-diazacyclo-octadecane (5) was obtained commercially (Aldrich). 2.3. 2-{2-[(2-{[2-(2-Hydroxyethoxy)ethyl]sulfanyl}-4methylphenyl)-sulfanyl]-ethoxy}-1-ethanol (9)
Scheme 1.
microscope. Microanalyses were performed by the Department of Chemistry, University of Otago, Dunedin, New Zealand. Electron impact mass spectra (EI-MS) and fast atom bombardment mass spectra (FAB-MS) were recorded on a Vacuum Generators ZAB 2HF mass spectrometer operating at 70 eV. High resolution mass spectrometry (HRMS) was performed by the Department of Chemistry, University of Tasmania, Hobart, Tasmania. High field 1 H NMR spectra were recorded using a 300 MHz Gemini Varian spectrometer. NMR spectra were recorded in CDCl3 , unless otherwise stated, with tetramethylsilane (TMS) as an internal standard. All chemical shifts are quoted as d in ppm, and coupling constants, J , are given in Hz. All multiplicities are abbreviated: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad. Solvents and reagents were purified and dried using standard laboratory procedures [2]. Pyridine was dried with sodium, followed by fractional distillation. All organic extracts were dried over anhydrous MgSO4 . 2.2. Syntheses 3,12,20,29-Tetraoxa-35,36-diazapentacyclo[29.3.1.1.14;18 0. 0.22;27 ]-hexatriaconta-1(35),5(10),6,8,14,16,18(36),22 (27),23,25,31,33-dodecaene (1) [3], 7,16-diaza-1,4,10,13tetraoxa-2,3,11,12-dibenzo-cyclo-octadeca-2,11-diene (3) [4,5] and 2-(20 -chloroethoxy)ethyl-200 -tetrahydropyranyl ether (7) [6] were synthesized by literature methods. 5;10
To a solution of 3,4-dimercaptotoluene (3.3 cm3 , 22 mmol) in dry n-butanol (10 cm3 ) was added solid sodium hydride (2.0 g, 49 mmol) and the solution was stirred for 20 min under a nitrogen atmosphere. 2-(20 Chloroethoxy)ethyl-200 -tetrahydropyranyl ether (7) (10.3 g, 49 mmol) in dry n-butanol (15 cm3 ) was added dropwise over 1 h, and the solution was refluxed for 20 h. The reaction mixture was quenched with water (25 cm3 ), and solvent was evaporated under reduced pressure. The remaining oily material was dissolved in dichloromethane (35 cm3 ), washed with water (2 20 cm3 ), and the organic layers were collected, dried and concentrated to afford the O-tetrahydropyranyl derivative 8 as a pale yellow oil. This crude product (8.5 g, 17 mmol) was dissolved in n-butanol (10 cm3 ). Hydrochloric acid (60 cm3 , 1 mol dm3 ) was added, and the solution was stirred at room temperature. After 14 h, t.l.c. analysis (20% acetone/dichloromethane) showed that the reaction had not reached completion. Accordingly, two more equivalents of hydrochloric acid (29 cm3 , 1 mol dm3 ) were added, and the solution was heated at 80 °C for a further 14 h. The reaction mixture was neutralized using sodium bicarbonate (aq., 10% (w/ v), 25 cm3 ), and the product was extracted with dichloromethane (3 25 cm3 ). The organic fractions were combined, washed with water (30 cm3 ), dried and evaporated at reduced pressure. After column chromatography of the residue (20% acetone/dichloromethane), the diol 9 was obtained as a pale yellow oil (1.26 g, 40%). HR-MS Calc. for C15 H24 S2 O4 : m=z 332.1150. Found: m/ z 332.1135. IR (film) mmax 3416 (O–H), 1292 (C–O) cm1 . 1 H NMR d 2.33 (s, 3H, CH3 ), 2.73 (br, 2H, 2 OH), 3.07–3.18 (m, 4H, 2 CH2 S), 3.54–3.75 (m, 12H, 6 CH2 O), 6.96–7.29 (m, 3H, ArH). EI-MS m=z 332 (M, 40%), 271 (M–C2 H5 O2 , 56), 243 (M–C4 H9 O2 , 68), 154 (M–C8 H18 O4 , 73). 2.4. 2-{2-[(5-Methyl-2-{[2-(2-{[(4-methylphenyl)sulfonyl]oxy}-ethoxy)-ethyl]-sulfanyl}-phenyl)sulfanyl]ethoxy}ethyl-4-methyl-1-benzenesulfonate (10) Under a nitrogen atmosphere, 9 (1.0 g, 5.1 mmol) was dissolved in dry pyridine (10 cm3 ) and cooled to )20 °C. A solution of p-toluenesulphonyl chloride (4.83 g, 25 mmol) in dry pyridine (10 cm3 ) was added dropwise to the cooled solution over 10 min, and the reaction mixture was maintained between )20 °C and )10 °C for 3 h. The mixture was then warmed to room temperature, and the solvent was removed under reduced pressure. The remaining solid was taken up into dichloromethane
718
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
(25 cm3 ), washed with water (2 20 cm3 ), and the organic fraction was collected, dried and concentrated to give a pale yellow oil. Purification by flash chromatography (30% ethyl acetate/hexane) afforded 10 as a viscous, colorless oil (1.17 g, 40%). (Found: C, 54.5; H, 5.8%. C29 H36 O8 S4 requires C, 54.4; H, 5.7%). 1 H NMR d 2.32 (s, 3H, CH3 ), 2.44 (s, 6H, 2 CH3 ), 2.94–3.06 (m, 4H, 2 CH2 S), 3.53–3.68 (m, 8H, 4 CH2 O), 4.12–4.18 (m, 4H, 2 CH2 OTs), 6.98–7.25 (m, 3H ArH), 7.32 (AA0 BB0 , J 8 Hz, 2H, ArH), 7.79 (AA0 BB0 , J 8 Hz, 2H, ArH). EI-MS m=z 640 (M, 6%), 469 (M-C7 H7 SO3 , 32). 2.5. 1,2-Di{[2-(2-chloroethoxy)ethyl]-sulfanyl}-4-methylbenzene (11) Neat 9 (1.37 g, 4.1 mmol) was stirred under a nitrogen atmosphere and cooled to 0 °C. Thionyl chloride (10 cm3 , 137 mmol) was added dropwise over 15 min, and the solution was warmed to room temperature and stirred for 14 h, followed by heating at 80 °C for 3 h. The excess thionyl chloride was removed under reduced pressure and the resulting brown oil was taken up in dichloromethane (40 cm3 ), washed with potassium carbonate {aq., 20% (w/v), 20 cm3 } and water (20 cm3 ). The organic layer was dried and concentrated at reduced pressure to afford a brown oily residue. Flash chromatography (1% acetone, dichloromethane) afforded the dichloride 11 as an orange oil, (0.253 g, 25%). (HR-MS: Found: m=z 368.0455. C15 H22 S2 O2 Cl2 requires m=z 368.0472). IR (film) mmax 746 (C–Cl), 666 (C–S) cm1 . 1 H NMR d 2.32 (s, 3H, CH3 ), 3.05–3.15 (m, 4H, 2 CH2 S), 3.58–3.75 (m, 12H, 4 CH2 O and 2 CH2 Cl), 6.95–7.28 (m, 3H ArH). EI-MS m=z 369 (M, 36%), 333.5 (M–Cl, 10), 298 (M–2Cl, 48). 2.6. 2,3,11,12-Bis (40 -methylbenzo)-1,4,10,13-tetrathia7,16-dioxacyclo-octa-deca-2,11-diene (2) 2.6.1. Method 1 Solid sodium hydride (0.14 g, 3.4 mmol) was added to a solution of 3,4-dimercaptotoluene (0.23 cm3 , 1.6 mmol) in dry n-butanol (5 cm3 ), and the solution was stirred at room temperature for 20 min under nitrogen. The ditosylate 10 (1.0 g, 1.56 mmol) in dry n-butanol and dry DMF {1:1(v/v), 20 cm3 } was added to the solution over a 30 min period. The resulting mixture was heated to reflux for 24 h. The solvents were removed under reduced pressure, and the solid residue was dissolved in dichloromethane (25 cm3 ) and washed with water (2 20 cm3 ). The organic layer was dried and evaporated at reduced pressure to afford a pale yellow solid. The crude material was purified by flash chromatography (2% ethyl acetate/dichloromethane) and recrystallized (1:1 ethyl acetate/hexane) to afford the title compound 2 as fine, white needles (0.2 g, 30%), m.p. 140–144 °C (lit. 147 °C [7]). The spectroscopic data for
the compound were consistent with that quoted in the literature. 2.6.2. Method 2 3,4-Dimercaptotoluene (0.09 cm3 , 0.6 mmol) was dissolved in dry n-butanol (5 cm3 ) under a nitrogen atmosphere. Solid sodium hydroxide (0.05 g, 1.3 mmol) was added to the solution, and it was warmed to 50 °C for 60 min. The dichloride 11 (0.22 g, 0.6 mmol) in dry n-butanol (10 cm3 ) was added dropwise to the pale yellow solution over a 15 min period. The solution was then heated to reflux for 24 h under nitrogen. The nbutanol was removed under reduced pressure, and the remaining orange solid was dissolved in dichloromethane (15 cm3 ), washed with sodium hydroxide (aq., 10% (w/v), 10 cm3 ) and water (2 20 cm3 ). The organic fraction was dried and concentrated under reduced pressure. Purification of the crude product by flash chromatography (30% ethyl acetate/hexane) and crystallization from 1:1 (v/v) ethyl acetate/hexane afforded 2 as a white, crystalline solid (0.05 g, 20%), m.p. 141–144 °C. 2.7. 2-[19-(2-Hydroxy-2-phenylethyl)-7,8,9,10,18,19,20,21octahydro-6H,17H-dibenzo[b,k][1,4,10,13,7,16]tetraoxadiazacyclooctadecin-8-yl]-1-phenyl-1-ethanol (4) The dibenzo-diaza-18-crown-6 derivative 3 (210 mg, 0.59 mmol) was dissolved in dry DMF (5 cm3 ) under nitrogen. ðRÞ-Styrene oxide (700 mg, 5.9 mmol) was added dropwise over 15 min and the solution was heated at 100 °C for 10 days. Removal of the solvent and excess ðRÞ-styrene oxide under reduced pressure gave the desired product as a crude orange oil. Purification of the oil by flash chromatography {30%dichloromethane/45% ethyl acetate/20% ethanol/5% ammonia (aq)}, followed by trituration using a 1:10 (v/v) DMF/water mixture afforded the diol 4 as a pale orange powder (55 mg, 20%). (HR-MS. Found: m=z 599.3103. C36 H42 N2 O6 : requires m=z 599.3043). IR (nujol) mmax 3356 (O–H), 1253 (C–O) cm1 . 1 H NMR d 2.59–2.63 (m, 2H, NCHHCHR2 ), 2.92–2.96 (m, 2H, NCHHCHR2 ), 3.30– 3.35 (m, 8H, 4 NCH2 ), 4.13–4.16 (m, 8H, 4 OCH2 ), 4.65–4.67 (m, 2H, CH2 CHR2 ), 6.93 (s, 8H, ArH), 7.26 (s, 10H, ArH). ESI-MS m/z 621 ((M+Na), 100%), 599 {(M + H), 56%}. 2.8. 2-[16-(2-Hydroxy-2-phenylethyl)-1,4,10,13-tetraoxa7,16-diazacyclo-octadecanyl]-1-phenyl-1-ethanol (6) ðRÞ-Styrene oxide (0.23 cm3 , 1.98 mmol) was added dropwise to a stirred solution of 5 (0.40 g, 0.76 mmol) in dry DMF (10 cm3 ) and the solution was heated to 100 °C for 24 h, under a nitrogen. Removal of solvent and excess (R)-styrene oxide under reduced pressure afforded an orange oil. Purification of the oil using ion exchange
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
chromatography {50% ethyl acetate/40% dichloromethane/9% methanol/1% ammonia (aq)} afforded 6 as a pale yellow oil (0.55 g, 65%). (HR-MS Found: m=z 503.3131. C28 H42 N2 O6 requires m=z 503.3043). IR (film) mmax 3355 (O–H), 1259 (C–O) cm1 . 1 H NMR d 2.58– 2.75 (m, 8H, 4 OCH2 CH2 N), 2.91–3.07 {m, 4H, 2 NCH2 CH(OH)Ph}, 3.57–3.76 {m, 16H, 8 OCH2 ), 4.67–4.73 (m, 1H, CH(OH)Ph}, 7.23–7.40 (m, 10H, 10 ArH). ESI-MS m=z 525 {(M + Na), 100%}, 503 {(M + H), 8%}. 2.9. Electrospray ionization mass spectrometry (ESIMS) ESI-MS were recorded using a Finnigan MAT ion trap LC-Q mass spectrometer, fitted with an electrospray ionization source in the positive ion mode. The electrospray needle was maintained at 4.25 kV. The tube lens offset was set at 30 V and the capillary voltage was maintained at 35 V. The capillary was maintained at 200 °C. Sample solutions were introduced into the ESI source at flow rates of 12–18 106 cm s1 and nitrogen was used as the nebulizing gas. Ions were detected by scanning the first quadrupole over the range 200–800 m=z. The sample solutions were prepared in HPLC grade methanol. Each solution was 106 mol dm3 in ligand and either 104 mol dm3 in a single metal salt or 5.0 105 in two metal salts in competitive experiments.
719
Research 720 digital analyzer was used to measure the change of the electrode potential as [Agþ ] changed during the titration. The reference vessel contained 20.0 cm3 of a 5.00 103 mol dm3 AgNO3 . For each titration to determine log(K/dm3 mol1 ) for [Ag L]þ , where K (dm3 mol1 ) is the stability constant, 20.0 cm3 of a 5.00 104 mol dm3 AgNO3 solution was titrated with a 5.00 103 mol dm3 L solution. The log(K/dm3 mol1 ) values for [M L]mþ complexes, where L ¼ 1, 2, 5 and 6 and Mmþ ¼ Zn2þ , Cd2þ and Pb2þ , were determined by competitive potentiometric titrations of 20.0 cm3 of 5.00 105 mol dm3 AgNO3 solution with a solution of 5.00 103 mol dm3 L and 2.50 103 mol dm3 M(ClO4 )2 , using a literature method [8]. All titrations were performed in triplicate.
3. Results and discussion 3.1. Ligand syntheses The coronands 1 [3,9] and 3 [4,5] were prepared by literature methods, 2 was synthesized by an improved
2.10. Potentiometric titrations DMF was dried over MgSO4 , distilled from MgSO4 and anhydrous CuSO4 (to remove any dimethylamine impurity) and stored under dry nitrogen. AgNO3 was dried under vacuum for 48 h over P2 O5 prior to use. Zn(NO3 )2 , Cd(ClO4 )2 and Pb(ClO4 )2 were dried over P2 O5 for 24 h. Tetraethylammonium perchlorate was prepared by the addition of HClO4 to a concentrated aqueous solution of NEt4 Br. The resulting white NEt4 ClO4 precipitate was repeatedly recrystallized from water until it was free of bromide and acid and dried under vacuum over P2 O5 for 45 h. [Caution: anhydrous nitrate and perchlorate salts are potentially powerful oxidants and should be handled with care.] All titration solutions were made up in dry DMF containing 0.05 mol dm3 NEt4 ClO4 under dry nitrogen. Titrations were carried out in a closed thermostatted (298.2 0.02 K) titration vessel connected to a thermostatted closed reference vessel by a salt-bridge. The salt bridge was filled with a solution of dry DMF containing 0.05 mol dm3 NEt4 ClO4 . The titration and reference electrodes were both silver wires. A stream of dry nitrogen gas was bubbled through the magnetically stirred titration solution for 20 min prior to and during the titration to remove any carbon dioxide and prevent the ingress of atmospheric gases and moisture. An Orion
Scheme 2.
720
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
method and 4 and 6 were prepared from 3 and 5 (obtained commercially), respectively. The improved synthesis of 2 (Scheme 2) commenced with the coupling of toluene-3,4-dithiol and two equivalents of 2-(20 -chloroethoxy)ethyl-200 -tetrahydropyranyl ether (7) in dry nbutanol in the presence of sodium hydride to give 8. The crude reaction mixture containing 8 was treated with hydrochloric acid to give 9 as a pale yellow oil after purification by flash chromatography which was then converted to either the ditosylate 10 or the dichloride 11. Cyclization was achieved through reaction of toluene3,4-dithiol with either ditosylate 10 or dichloride 11 in the presence of NaH to give 2 in an average yield of 25% as fine, white needles, which had characteristics in accord with those reported in the literature [7]. The synthesis of 4 (Scheme 3) was achieved through the heating (100 °C) of 3 and excess of ðRÞ-styrene oxide in dry DMF for 10 days, followed by flash chromatographic purification and recrystallization from aqueous DMF which gave 4 as a pale orange solid in 20% yield. The long reaction time required may be due to the bulk and structural stiffness of 3. (A similar type of dialkylation of 3 also required lengthy reaction times [10].) This appears to be borne out by the synthesis of the more flexible 6 by a similar method which required a reaction time of only 24 h to obtain 6 as a pale yellow oil in 65% yield after purification by ion exchange chromatography. 3.2. Electrospray ionization mass spectrometric (ESIMS) studies A positive ion ESI-MS method was used for a preliminary assessment of the Agþ , Zn2þ , Cd2þ and Pb2þ complexes in the gas phase generated from methanol solutions. Complexes were formed with 1–6 by Agþ as shown by peaks corresponding to each of the 107 Ag and 109 Ag isotopomers, but no complexes were detected for Zn2þ . Only 5 and 6 formed detectable complexes with Cd2þ for which the expected doubly charged ion of m=z of 187 was observed for [Cd 5]2þ , but instead of [Cd 6]2þ , the [Cd 6]2þ 3MeOH and [Cd 6]2þ 4MeOH ions were observed centered at m=z ¼ 356 and 373, respectively. Only 2 and 6 formed detectable complexes with Pb2þ at m=z ¼ 330 and 355, respectively.
Fig. 1. ESI-MS spectrum of a methanol solution 106 mol dm3 in 6 and 5.0 105 in each of Cd(ClO4 )2 and Pb(ClO4 )2 . The peaks at m=z 356 and 370 are assigned to the 114 Cd (28.86% natural abundance) isotopomers of [Cd 6]2þ 3MeOH and [Cd 6]2þ 4MeOH, respectively. The other multiplet components arise from the 110 Cd (12.39%), 111 Cd (12.75%), 112 Cd (24.07%), 113 Cd (12.26%) and 116 Cd (7.58%) isotopomers.
Competitive ESI-MS experiments were conducted on solutions of either 5 or 6 and excess Agþ and Cd2þ . The [Ag 5]þ /[Cd 5]2þ peak intensity ratio was 2:1 consistent with [Ag 5]þ being the more stable complex. No peak was observed for [Ag 6]þ while peaks for both [Cd 6]2þ 3MeOH and [Cd 62þ ] 4MeOH were observed consistent with these complexes being the most stable. Competitive ESI-MS experiments were also conducted on solutions that contained either 2 or 6 and excess Agþ and Pb2þ . The [Ag 2]þ /[Pb 2]2þ peak intensity ratio was 3:1 consistent with [Ag 2]þ being the more stable complex in the gas phase whereas the [Ag 6]þ /[Pb 6]2þ peak intensity ratio was 1:20 consistent with [Pb 6]2þ being the more stable complex. Finally, competitive experiments were carried out on a solution of 6 and excess Cd2þ and Pb2þ . Only peaks for [Cd 6]2þ 3MeOH and [Cd 6]2þ 4MeOH were observed (Fig. 1) consistent with them being more stable than [Pb 6]2þ . The lack of detection of any complexes of the divalent metal ions with 1, 3 and 4 is consistent with these complexes being unstable in the gas phase. On the basis of these data it was decided to carry out quantitative stability studies of the complexes formed by 1, 2, 5 and 6 with Agþ , Zn2þ , Cd2þ and Pb2þ in DMF which, in contrast to methanol, solubilized the ligands and their complexes sufficiently for potentiometric titration equilibrium studies. 3.3. Equilibrium studies The log(K/dm3 mol1 ) values for [M L]mþ , where M was either Agþ , Zn2þ , Cd2þ or Pb2þ and L is either 1, 2, 5 or 6, were determined by potentiometric titrations using silver measurement and reference electrodes in DMF solution. Thus, the variation of e.m.f. was monitored as titrant was added and the log(K/dm3 mol1 ) values were determined from these data as described in Section 2. The stabilities of [Ag L]þ were determined mþ
Scheme 3.
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
from the titration of 20.0 dm3 of 5.00 104 mol dm3 AgNO3 in DMF solutions with DMF solutions 5.00 103 mol dm3 in L. The stabilities of [M L]mþ , where Mmþ ¼ Zn2þ , Cd2þ and Pb2þ , were determined from the titration of 20.0 dm3 of 5.00 105 mol dm3 AgNO3 in DMF solutions with DMF solution 5.00 103 mol dm3 in L and 2.50 102 mol dm3 in either Zn(NO3 )2 , Cd(ClO4 )2 or Pb(ClO4 )2 . For all solutions I ¼ 0:050 mol dm3 adjusted with NEt4 ClO4 . A typical titration curve is shown in Fig. 2 and the derived log(K/dm3 mol1 ) values are shown in Table 1 [11,12]. For those [M L]mþ systems for which an accurate log(K/dm3 mol1 ) is reported the e.m.f. variation observed in the presence of Agþ and Mmþ differed from that observed in the presence of Agþ alone and log(K/ dm3 mol1 ) for [M L]2þ was determined from this difference. For those systems characterized by log(K/dm3 mol1 ) <2 the titration curve was indistinguishable from that carried out in the presence of Agþ alone and for those characterized by log(K/dm3 mol1 ) >7.52 no variation in e.m.f. occurred as the titrant solution containing L and Mmþ was added consistent with Agþ being unable to compete with Mmþ for L.
Fig. 2. Variation of e.m.f for 20.0 dm3 of a 5.00 105 mol dm3 AgNO3 solution in DMF titrated with a DMF solution 5.00 103 mol dm3 in 2 and 2.50 102 mol dm3 in Pb(NO3 )2 . For each solution I ¼ 0.050 mol dm3 adjusted with NEt4 ClO4 . The data points are shown as crosses and the best-fit of the algorithm for the competitive formation of [Ag 2]þ and [Pb 2]2þ is shown as a solid curve.
721
The log(K/dm3 mol1 ) values for the Agþ complexes are essential reference data required for the interpretation of the competitive titration of either Zn2þ , Cd2þ or Pb2þ against Agþ in the presence of a particular ligand. Stability increases in the order [Ag 5]þ > [Ag 6]þ > [Ag 2]þ > [Ag 1]þ . This is consistent with the flexibility of 5 most readily accommodating the linear N–Ag–N coordination favored by stable Agþ complexes [13]. Despite the potential for the phenylhydroxyethyl pendant arms of 6 to increase the statistical probability of coordination, the stability of [Ag 6]þ is about two orders of magnitude less than that of [Ag 5]þ . This is probably a consequence of steric crowding at the ternary nitrogens. Ligands 1 and 2 have structures stiffened by the incorporation of aromatic rings which probably lowers their ability to coordinate Agþ . While the donor atom set of 1 is identical to that of 5, the two oxygen– nitrogen–oxygen sets are more distant from each other in 1 than in 5 in which their separation appears close to optimal for coordinating Agþ , which has a six coordi [14]. Although the disposinate ionic radius of 1.15 A tion of the donor atoms of 2 about the macrocyclic ring is the same as in 5 and 6, the presence of four sulfur atoms in 2 in place of two oxygen and two nitrogen atoms in 5 and 6 combined with the stiffness of 2 are probably major causes of [Ag 2]þ being less stable than [Ag 5]þ and [Ag 6]þ . It is notable that [Ag 1]þ , [Ag 2]þ and [Ag 5]þ are much more stable than their Zn2þ , Cd2þ and Pb2þ analogues and that the stability of [Ag 6]þ is only exceeded by those of [Cd 6]2þ and [Pb 6]2þ (Table 1). This is attributable to the preference for Agþ to form strong coaxial bonds with two nitrogen donor atoms [13] and for their stabilizing influence only being exceeded in [Cd 6]2þ and [Pb 6]2þ where octadentate 6 evidently forms a coordination cavity of optimal size and stereochemistry for Cd2þ and Pb2þ . The complexation of Zn2þ and Cd2þ by 1 and 2 is weak probably because of the large cavity size of 1 and the lack of affinity of the soft Lewis base sulfur donor atoms of 2 for the borderline hard Lewis acids Zn2þ and Cd2þ . Both are coordinated more strongly by 5 consistent with 5 both incorporating an appropriate atom donor set for these border line hard acid metal ions and also possessing sufficientl flexibility to adapt its coordination cavity to the sizes of Zn2þ and Cd2þ (six-coor respectively). dinate ionic radii of 0.74 and 0.95 A,
Table 1 Variation of stability {log(K/dm3 mol1 )} of [M L]mþ in DMF at 298.2 K and I ¼ 0.050 mol dm3 (NEt4 ClO4 ) Mþ
L ¼ 1 log(K/dm3 mol1 )
L ¼ 2 log(K/dm3 mol1 )
L ¼ 5 log(K/dm3 mol1 )
L ¼ 6 log(K/dm3 mol1 )
Agþ Zn2þ Cd2þ Pb2þ
5.50 0.04 <2 <2 2.62
6.49 0.02 <2 <2 2.38
9.42 0.13a 4.30 0.07 5.92 0.02 6.71 0.12
7.52 0.01 <2 >7.52 >7.52
a
This compares with 8.08 in H2 O [11] and 9.91 in DMF {I ¼ 0:1 mol dm3 (NEt4 ClO4 )} [12].
722
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
However, [Zn 6]2þ is less stable than [Zn 5]2þ which is attributable to the steric crowding of the ternary nitrogens. This is offset in [Cd 6]2þ by the ability of Cd2þ to readily adopt a coordination number of eight (eight whereas this is less coordinate ionic radius ¼ 1.10 A) 2þ likely to occur for Zn . The variation of log(K/dm3 mol1 ) for [Pb L]2þ (for Pb2þ six- and eight-coordinate respectively) may be exionic radii ¼ 1.19 and 1.29 A, 2þ plained as for [Cd L] . Thus the overall effect is that 6 is selective for Cd2þ and Pb2þ over Zn2þ by >5 orders of magnitude as assessed in terms of log(K/dm3 mol1 ).
4. Conclusion Of the ligands studied, only 5 and 6 form stable Cd2þ and Pb2þ [M L]2þ complexes in DMF. While 5 shows substantial selectivity for Cd2þ and Pb2þ over Zn2þ , 6 shows >5 orders of magnitude selectivity for Cd2þ and Pb2þ over Zn2þ which is attributable to a combination of steric crowding at the ternary nitrogens of 6 and its denticity of eight which favors the coordination of eight-coordinate Cd2þ and Pb2þ over that of usually six-coordinate Zn2þ . This suggests that a strategy for designing such selectivity into coronand-based ligands is to exploit this maximum coordination number difference between Zn2þ , Cd2þ and Pb2þ in ligands which form a coordination cavity for M2þ of appropriate size. In principle, 6 may either be water solubilized to act as a sequestering agent or made to fluoresce on coordination of a heavy metal ion to act as a sensor by replacing the phenyl groups of the pendant arms by either hydrophilic groups or by a fluorophore sensitive to heavy metal ion coordination [15], respectively.
Acknowledgements The authors thank Dr. Karl Cornelius and Ms. Wendy Holstein (The University of Adelaide) for as-
sistance with the ESI-MS studies. Funding for this research by the Australian Research Council is gratefully acknowledged.
References [1] (a) L. Friberg, M. Piscator, G.F. Nordberg, T. Kjellstr€ om, Cadmium in the Environment, second ed., CRC Press, Cleveland, 1974; (b) J.M. Ratcliffe, Lead in Man and the Environment, Ellis Horwood, Chichester, UK, 1981; (c) J.-P. Vernet, Heavy Metals in the Environment, Elsevier, New York, 1991. [2] D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory Chemicals, third ed., Pergamon Press, Oxford, 1988. [3] G.R. Newkome, J.M. Robinson, J. Chem. Soc., Chem. Commun (1973) 831. [4] S.A. Hogberg, D.J. Cram, J. Org. Chem. 40 (1975) 151. [5] L.C. Hodgkinson, M.R. Johnson, S.J. Leigh, N. Spencer, I.O. Sutherland, R.F. Newton, J. Chem. Soc., Perkin Trans. 1 (1979) 2193. [6] E.P. Kyba, R.C. Helgeson, K. Madan, G.W. Gokel, T.L. Tarnowski, S.S. Moore, D.J. Cram, J. Am. Chem. Soc 99 (1977) 2564. [7] C.J. Pedersen, J. Org. Chem. 36 (1971) 254. [8] B.G. Cox, H. Schneider, J. Stroka, J. Am. Chem. Soc. 100 (1978) 4746. [9] D. Caiazza, S.F. Lincoln, E.R.T. Tiekink, A.D. Ward, Z. Kristallogr. NCS 216 (2001) 245. [10] B. Vaidya, J. Zak, G.J. Bastiaans, M.D. Porter, J.L. Hallman, N.A.R. Nabulsi, M.D. Utterback, B. Strzelbicka, R.A. Bartsch, Anal. Chem. 67 (1995) 4101. Malmsten, J. Inorg. Nucl. Chem. 43 (1981) 1299. [11] S. Kulstad, L.A. [12] B.G. Cox, P. Firman, H. Horst, H. Schneider, Polyhedron 2 (1983) 343. [13] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fifth ed., InterScience, New York, 1988. [14] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. [15] (a) A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515; (b) B. Valeur, I. Leray, Coord. Chem. Rev. 202 (2000) 3; (c) J.P. Geue, N.J. Head, A.D. Ward, S.F. Lincoln, J. Chem. Soc., Dalton Trans. (2003) 521.
A preparative study of some coronands and their complexation of silver(I), zinc(II), cadmium(II) and lead(II) Daniela Caiazza, Stephen F. Lincoln *, A. David Ward Department of Chemistry, The University of Adelaide, Adelaide, SA 5005, Australia Received 24 April 2003; accepted 25 June 2003
Abstract A study of the complexation of heavy metal ions by the coronands 3,12,20,29-tetraoxa-35,36-diazapentacyclo [29.3.1.1.14;18 .05;10 .022;27 ]-hexatriaconta-1(35),5(10),6,8,14,16,18(36),22(27),23,25,31,33-dodecaene (1); 2,3,11,12-bis (40 -methylbenzo)-1, 4,10,13-tetrathia-7,16-dioxacyclo-octadeca-2,11-diene (2); 7,16-diaza-1,4,10,13-tetraoxa-2,3,11,12-dibenzocyclooctadeca-2,11-diene (3); 2-[19-(2-hydroxy-2-phenylethyl)-7,8,9,10, 18,19,20,21-octahydro-6H,17H-dibenzo[b,k][1,4,10,13,7,16]tetraoxadiazacyclooctadecin-8-yl]-1-phenyl-1-ethanol (4); 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane (5); and 2-[16-(2-hydroxy-2-phenylethyl)-1,4,10, 13- tetraoxa-7,16-diazacyclo-octadecanyl]-1-phenyl-1-ethanol (6) is described. Coronands 1 and 3 were prepared by literature methods, improved methods were used to prepare 2, and 4 and 6 were prepared from 3 and 5 (obtained commercially), respectively. Potentiometric studies in N,N-dimethylformamide yielded (log K/dm3 mol1 ) ¼ 5.50, 6.49, 9.42 and 7.52 for [Ag 1]þ , [Ag 2]þ , [Ag 5]þ and [Ag 6]þ , respectively; <2, <2, 4.30 and <2 for [Zn 1]2þ , [Zn 2]2þ , [Zn 5]2þ and [Zn 6]2þ , respectively, <2, <2, 5.92 and >7.52 for [Cd 1]2þ , [Cd 2]2þ , [Cd 5]2þ , and [Cd 6]2þ , respectively, and 2.62, 2.38, 6.71 and >7.52 for [Pb 1]2þ , [Pb 2]2þ , [Pb 5]2þ , and [Pb 6]2þ , respectively. ESI-MS studies of the interactions of 1–6 with Agþ , Zn2þ , Cd2þ and Pb2þ are also reported. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Coronands; Silver(I) complexes; Heavy metal complexes; Stability constants
1. Introduction Industrialization has resulted in an increase in heavy metals in the environment and as a consequence of their detrimental effects on health there is a corresponding interest in their detection and sequestration [1]. We are particularly interested in two such ions, Cd2þ and Pb2þ , and their selective complexation with a view to the eventual development of detection and sequestration methods for environmental and biological samples. Because of its ubiquity in the environment it is important that Zn2þ should not compete with Cd2þ and Pb2þ in the complexation process. Accordingly, it is necessary to gain an understanding of the ligand coordination ÔcoreÕ that provides such selectivity. In seeking this core we chose the coronands 1–6 (Scheme 1) which provide a
range of denticities, donor atom types and structural rigidities as the basis of our studies. To obtain sufficient solubilities for complexation studies it was necessary to work in N,N-dimethylformamide (DMF) so that the metal ion selectivity of each coronand could be determined. (Obviously, DMF is unsuitable for most sequestration and detection applications, but it is envisaged that a coronand or similarly based system will either be water solubilized through the substitution of hydrophilic pendant groups onto the coronand, or they will be immobilized on a surface over which an aqueous analyte solution flows.)
2. Experimental 2.1. General
*
Corresponding author. Tel.: +61-8-308-5559; fax: +61-8-8303-4358. E-mail address: [email protected] (S.F. Lincoln).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.06.019
Melting points (uncorrected) were recorded on a Kofler hot-stage apparatus equipped with a Reichert
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
717
1,4,10,13-Tetraoxa-7,16-diazacyclo-octadecane (5) was obtained commercially (Aldrich). 2.3. 2-{2-[(2-{[2-(2-Hydroxyethoxy)ethyl]sulfanyl}-4methylphenyl)-sulfanyl]-ethoxy}-1-ethanol (9)
Scheme 1.
microscope. Microanalyses were performed by the Department of Chemistry, University of Otago, Dunedin, New Zealand. Electron impact mass spectra (EI-MS) and fast atom bombardment mass spectra (FAB-MS) were recorded on a Vacuum Generators ZAB 2HF mass spectrometer operating at 70 eV. High resolution mass spectrometry (HRMS) was performed by the Department of Chemistry, University of Tasmania, Hobart, Tasmania. High field 1 H NMR spectra were recorded using a 300 MHz Gemini Varian spectrometer. NMR spectra were recorded in CDCl3 , unless otherwise stated, with tetramethylsilane (TMS) as an internal standard. All chemical shifts are quoted as d in ppm, and coupling constants, J , are given in Hz. All multiplicities are abbreviated: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad. Solvents and reagents were purified and dried using standard laboratory procedures [2]. Pyridine was dried with sodium, followed by fractional distillation. All organic extracts were dried over anhydrous MgSO4 . 2.2. Syntheses 3,12,20,29-Tetraoxa-35,36-diazapentacyclo[29.3.1.1.14;18 0. 0.22;27 ]-hexatriaconta-1(35),5(10),6,8,14,16,18(36),22 (27),23,25,31,33-dodecaene (1) [3], 7,16-diaza-1,4,10,13tetraoxa-2,3,11,12-dibenzo-cyclo-octadeca-2,11-diene (3) [4,5] and 2-(20 -chloroethoxy)ethyl-200 -tetrahydropyranyl ether (7) [6] were synthesized by literature methods. 5;10
To a solution of 3,4-dimercaptotoluene (3.3 cm3 , 22 mmol) in dry n-butanol (10 cm3 ) was added solid sodium hydride (2.0 g, 49 mmol) and the solution was stirred for 20 min under a nitrogen atmosphere. 2-(20 Chloroethoxy)ethyl-200 -tetrahydropyranyl ether (7) (10.3 g, 49 mmol) in dry n-butanol (15 cm3 ) was added dropwise over 1 h, and the solution was refluxed for 20 h. The reaction mixture was quenched with water (25 cm3 ), and solvent was evaporated under reduced pressure. The remaining oily material was dissolved in dichloromethane (35 cm3 ), washed with water (2 20 cm3 ), and the organic layers were collected, dried and concentrated to afford the O-tetrahydropyranyl derivative 8 as a pale yellow oil. This crude product (8.5 g, 17 mmol) was dissolved in n-butanol (10 cm3 ). Hydrochloric acid (60 cm3 , 1 mol dm3 ) was added, and the solution was stirred at room temperature. After 14 h, t.l.c. analysis (20% acetone/dichloromethane) showed that the reaction had not reached completion. Accordingly, two more equivalents of hydrochloric acid (29 cm3 , 1 mol dm3 ) were added, and the solution was heated at 80 °C for a further 14 h. The reaction mixture was neutralized using sodium bicarbonate (aq., 10% (w/ v), 25 cm3 ), and the product was extracted with dichloromethane (3 25 cm3 ). The organic fractions were combined, washed with water (30 cm3 ), dried and evaporated at reduced pressure. After column chromatography of the residue (20% acetone/dichloromethane), the diol 9 was obtained as a pale yellow oil (1.26 g, 40%). HR-MS Calc. for C15 H24 S2 O4 : m=z 332.1150. Found: m/ z 332.1135. IR (film) mmax 3416 (O–H), 1292 (C–O) cm1 . 1 H NMR d 2.33 (s, 3H, CH3 ), 2.73 (br, 2H, 2 OH), 3.07–3.18 (m, 4H, 2 CH2 S), 3.54–3.75 (m, 12H, 6 CH2 O), 6.96–7.29 (m, 3H, ArH). EI-MS m=z 332 (M, 40%), 271 (M–C2 H5 O2 , 56), 243 (M–C4 H9 O2 , 68), 154 (M–C8 H18 O4 , 73). 2.4. 2-{2-[(5-Methyl-2-{[2-(2-{[(4-methylphenyl)sulfonyl]oxy}-ethoxy)-ethyl]-sulfanyl}-phenyl)sulfanyl]ethoxy}ethyl-4-methyl-1-benzenesulfonate (10) Under a nitrogen atmosphere, 9 (1.0 g, 5.1 mmol) was dissolved in dry pyridine (10 cm3 ) and cooled to )20 °C. A solution of p-toluenesulphonyl chloride (4.83 g, 25 mmol) in dry pyridine (10 cm3 ) was added dropwise to the cooled solution over 10 min, and the reaction mixture was maintained between )20 °C and )10 °C for 3 h. The mixture was then warmed to room temperature, and the solvent was removed under reduced pressure. The remaining solid was taken up into dichloromethane
718
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
(25 cm3 ), washed with water (2 20 cm3 ), and the organic fraction was collected, dried and concentrated to give a pale yellow oil. Purification by flash chromatography (30% ethyl acetate/hexane) afforded 10 as a viscous, colorless oil (1.17 g, 40%). (Found: C, 54.5; H, 5.8%. C29 H36 O8 S4 requires C, 54.4; H, 5.7%). 1 H NMR d 2.32 (s, 3H, CH3 ), 2.44 (s, 6H, 2 CH3 ), 2.94–3.06 (m, 4H, 2 CH2 S), 3.53–3.68 (m, 8H, 4 CH2 O), 4.12–4.18 (m, 4H, 2 CH2 OTs), 6.98–7.25 (m, 3H ArH), 7.32 (AA0 BB0 , J 8 Hz, 2H, ArH), 7.79 (AA0 BB0 , J 8 Hz, 2H, ArH). EI-MS m=z 640 (M, 6%), 469 (M-C7 H7 SO3 , 32). 2.5. 1,2-Di{[2-(2-chloroethoxy)ethyl]-sulfanyl}-4-methylbenzene (11) Neat 9 (1.37 g, 4.1 mmol) was stirred under a nitrogen atmosphere and cooled to 0 °C. Thionyl chloride (10 cm3 , 137 mmol) was added dropwise over 15 min, and the solution was warmed to room temperature and stirred for 14 h, followed by heating at 80 °C for 3 h. The excess thionyl chloride was removed under reduced pressure and the resulting brown oil was taken up in dichloromethane (40 cm3 ), washed with potassium carbonate {aq., 20% (w/v), 20 cm3 } and water (20 cm3 ). The organic layer was dried and concentrated at reduced pressure to afford a brown oily residue. Flash chromatography (1% acetone, dichloromethane) afforded the dichloride 11 as an orange oil, (0.253 g, 25%). (HR-MS: Found: m=z 368.0455. C15 H22 S2 O2 Cl2 requires m=z 368.0472). IR (film) mmax 746 (C–Cl), 666 (C–S) cm1 . 1 H NMR d 2.32 (s, 3H, CH3 ), 3.05–3.15 (m, 4H, 2 CH2 S), 3.58–3.75 (m, 12H, 4 CH2 O and 2 CH2 Cl), 6.95–7.28 (m, 3H ArH). EI-MS m=z 369 (M, 36%), 333.5 (M–Cl, 10), 298 (M–2Cl, 48). 2.6. 2,3,11,12-Bis (40 -methylbenzo)-1,4,10,13-tetrathia7,16-dioxacyclo-octa-deca-2,11-diene (2) 2.6.1. Method 1 Solid sodium hydride (0.14 g, 3.4 mmol) was added to a solution of 3,4-dimercaptotoluene (0.23 cm3 , 1.6 mmol) in dry n-butanol (5 cm3 ), and the solution was stirred at room temperature for 20 min under nitrogen. The ditosylate 10 (1.0 g, 1.56 mmol) in dry n-butanol and dry DMF {1:1(v/v), 20 cm3 } was added to the solution over a 30 min period. The resulting mixture was heated to reflux for 24 h. The solvents were removed under reduced pressure, and the solid residue was dissolved in dichloromethane (25 cm3 ) and washed with water (2 20 cm3 ). The organic layer was dried and evaporated at reduced pressure to afford a pale yellow solid. The crude material was purified by flash chromatography (2% ethyl acetate/dichloromethane) and recrystallized (1:1 ethyl acetate/hexane) to afford the title compound 2 as fine, white needles (0.2 g, 30%), m.p. 140–144 °C (lit. 147 °C [7]). The spectroscopic data for
the compound were consistent with that quoted in the literature. 2.6.2. Method 2 3,4-Dimercaptotoluene (0.09 cm3 , 0.6 mmol) was dissolved in dry n-butanol (5 cm3 ) under a nitrogen atmosphere. Solid sodium hydroxide (0.05 g, 1.3 mmol) was added to the solution, and it was warmed to 50 °C for 60 min. The dichloride 11 (0.22 g, 0.6 mmol) in dry n-butanol (10 cm3 ) was added dropwise to the pale yellow solution over a 15 min period. The solution was then heated to reflux for 24 h under nitrogen. The nbutanol was removed under reduced pressure, and the remaining orange solid was dissolved in dichloromethane (15 cm3 ), washed with sodium hydroxide (aq., 10% (w/v), 10 cm3 ) and water (2 20 cm3 ). The organic fraction was dried and concentrated under reduced pressure. Purification of the crude product by flash chromatography (30% ethyl acetate/hexane) and crystallization from 1:1 (v/v) ethyl acetate/hexane afforded 2 as a white, crystalline solid (0.05 g, 20%), m.p. 141–144 °C. 2.7. 2-[19-(2-Hydroxy-2-phenylethyl)-7,8,9,10,18,19,20,21octahydro-6H,17H-dibenzo[b,k][1,4,10,13,7,16]tetraoxadiazacyclooctadecin-8-yl]-1-phenyl-1-ethanol (4) The dibenzo-diaza-18-crown-6 derivative 3 (210 mg, 0.59 mmol) was dissolved in dry DMF (5 cm3 ) under nitrogen. ðRÞ-Styrene oxide (700 mg, 5.9 mmol) was added dropwise over 15 min and the solution was heated at 100 °C for 10 days. Removal of the solvent and excess ðRÞ-styrene oxide under reduced pressure gave the desired product as a crude orange oil. Purification of the oil by flash chromatography {30%dichloromethane/45% ethyl acetate/20% ethanol/5% ammonia (aq)}, followed by trituration using a 1:10 (v/v) DMF/water mixture afforded the diol 4 as a pale orange powder (55 mg, 20%). (HR-MS. Found: m=z 599.3103. C36 H42 N2 O6 : requires m=z 599.3043). IR (nujol) mmax 3356 (O–H), 1253 (C–O) cm1 . 1 H NMR d 2.59–2.63 (m, 2H, NCHHCHR2 ), 2.92–2.96 (m, 2H, NCHHCHR2 ), 3.30– 3.35 (m, 8H, 4 NCH2 ), 4.13–4.16 (m, 8H, 4 OCH2 ), 4.65–4.67 (m, 2H, CH2 CHR2 ), 6.93 (s, 8H, ArH), 7.26 (s, 10H, ArH). ESI-MS m/z 621 ((M+Na), 100%), 599 {(M + H), 56%}. 2.8. 2-[16-(2-Hydroxy-2-phenylethyl)-1,4,10,13-tetraoxa7,16-diazacyclo-octadecanyl]-1-phenyl-1-ethanol (6) ðRÞ-Styrene oxide (0.23 cm3 , 1.98 mmol) was added dropwise to a stirred solution of 5 (0.40 g, 0.76 mmol) in dry DMF (10 cm3 ) and the solution was heated to 100 °C for 24 h, under a nitrogen. Removal of solvent and excess (R)-styrene oxide under reduced pressure afforded an orange oil. Purification of the oil using ion exchange
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
chromatography {50% ethyl acetate/40% dichloromethane/9% methanol/1% ammonia (aq)} afforded 6 as a pale yellow oil (0.55 g, 65%). (HR-MS Found: m=z 503.3131. C28 H42 N2 O6 requires m=z 503.3043). IR (film) mmax 3355 (O–H), 1259 (C–O) cm1 . 1 H NMR d 2.58– 2.75 (m, 8H, 4 OCH2 CH2 N), 2.91–3.07 {m, 4H, 2 NCH2 CH(OH)Ph}, 3.57–3.76 {m, 16H, 8 OCH2 ), 4.67–4.73 (m, 1H, CH(OH)Ph}, 7.23–7.40 (m, 10H, 10 ArH). ESI-MS m=z 525 {(M + Na), 100%}, 503 {(M + H), 8%}. 2.9. Electrospray ionization mass spectrometry (ESIMS) ESI-MS were recorded using a Finnigan MAT ion trap LC-Q mass spectrometer, fitted with an electrospray ionization source in the positive ion mode. The electrospray needle was maintained at 4.25 kV. The tube lens offset was set at 30 V and the capillary voltage was maintained at 35 V. The capillary was maintained at 200 °C. Sample solutions were introduced into the ESI source at flow rates of 12–18 106 cm s1 and nitrogen was used as the nebulizing gas. Ions were detected by scanning the first quadrupole over the range 200–800 m=z. The sample solutions were prepared in HPLC grade methanol. Each solution was 106 mol dm3 in ligand and either 104 mol dm3 in a single metal salt or 5.0 105 in two metal salts in competitive experiments.
719
Research 720 digital analyzer was used to measure the change of the electrode potential as [Agþ ] changed during the titration. The reference vessel contained 20.0 cm3 of a 5.00 103 mol dm3 AgNO3 . For each titration to determine log(K/dm3 mol1 ) for [Ag L]þ , where K (dm3 mol1 ) is the stability constant, 20.0 cm3 of a 5.00 104 mol dm3 AgNO3 solution was titrated with a 5.00 103 mol dm3 L solution. The log(K/dm3 mol1 ) values for [M L]mþ complexes, where L ¼ 1, 2, 5 and 6 and Mmþ ¼ Zn2þ , Cd2þ and Pb2þ , were determined by competitive potentiometric titrations of 20.0 cm3 of 5.00 105 mol dm3 AgNO3 solution with a solution of 5.00 103 mol dm3 L and 2.50 103 mol dm3 M(ClO4 )2 , using a literature method [8]. All titrations were performed in triplicate.
3. Results and discussion 3.1. Ligand syntheses The coronands 1 [3,9] and 3 [4,5] were prepared by literature methods, 2 was synthesized by an improved
2.10. Potentiometric titrations DMF was dried over MgSO4 , distilled from MgSO4 and anhydrous CuSO4 (to remove any dimethylamine impurity) and stored under dry nitrogen. AgNO3 was dried under vacuum for 48 h over P2 O5 prior to use. Zn(NO3 )2 , Cd(ClO4 )2 and Pb(ClO4 )2 were dried over P2 O5 for 24 h. Tetraethylammonium perchlorate was prepared by the addition of HClO4 to a concentrated aqueous solution of NEt4 Br. The resulting white NEt4 ClO4 precipitate was repeatedly recrystallized from water until it was free of bromide and acid and dried under vacuum over P2 O5 for 45 h. [Caution: anhydrous nitrate and perchlorate salts are potentially powerful oxidants and should be handled with care.] All titration solutions were made up in dry DMF containing 0.05 mol dm3 NEt4 ClO4 under dry nitrogen. Titrations were carried out in a closed thermostatted (298.2 0.02 K) titration vessel connected to a thermostatted closed reference vessel by a salt-bridge. The salt bridge was filled with a solution of dry DMF containing 0.05 mol dm3 NEt4 ClO4 . The titration and reference electrodes were both silver wires. A stream of dry nitrogen gas was bubbled through the magnetically stirred titration solution for 20 min prior to and during the titration to remove any carbon dioxide and prevent the ingress of atmospheric gases and moisture. An Orion
Scheme 2.
720
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
method and 4 and 6 were prepared from 3 and 5 (obtained commercially), respectively. The improved synthesis of 2 (Scheme 2) commenced with the coupling of toluene-3,4-dithiol and two equivalents of 2-(20 -chloroethoxy)ethyl-200 -tetrahydropyranyl ether (7) in dry nbutanol in the presence of sodium hydride to give 8. The crude reaction mixture containing 8 was treated with hydrochloric acid to give 9 as a pale yellow oil after purification by flash chromatography which was then converted to either the ditosylate 10 or the dichloride 11. Cyclization was achieved through reaction of toluene3,4-dithiol with either ditosylate 10 or dichloride 11 in the presence of NaH to give 2 in an average yield of 25% as fine, white needles, which had characteristics in accord with those reported in the literature [7]. The synthesis of 4 (Scheme 3) was achieved through the heating (100 °C) of 3 and excess of ðRÞ-styrene oxide in dry DMF for 10 days, followed by flash chromatographic purification and recrystallization from aqueous DMF which gave 4 as a pale orange solid in 20% yield. The long reaction time required may be due to the bulk and structural stiffness of 3. (A similar type of dialkylation of 3 also required lengthy reaction times [10].) This appears to be borne out by the synthesis of the more flexible 6 by a similar method which required a reaction time of only 24 h to obtain 6 as a pale yellow oil in 65% yield after purification by ion exchange chromatography. 3.2. Electrospray ionization mass spectrometric (ESIMS) studies A positive ion ESI-MS method was used for a preliminary assessment of the Agþ , Zn2þ , Cd2þ and Pb2þ complexes in the gas phase generated from methanol solutions. Complexes were formed with 1–6 by Agþ as shown by peaks corresponding to each of the 107 Ag and 109 Ag isotopomers, but no complexes were detected for Zn2þ . Only 5 and 6 formed detectable complexes with Cd2þ for which the expected doubly charged ion of m=z of 187 was observed for [Cd 5]2þ , but instead of [Cd 6]2þ , the [Cd 6]2þ 3MeOH and [Cd 6]2þ 4MeOH ions were observed centered at m=z ¼ 356 and 373, respectively. Only 2 and 6 formed detectable complexes with Pb2þ at m=z ¼ 330 and 355, respectively.
Fig. 1. ESI-MS spectrum of a methanol solution 106 mol dm3 in 6 and 5.0 105 in each of Cd(ClO4 )2 and Pb(ClO4 )2 . The peaks at m=z 356 and 370 are assigned to the 114 Cd (28.86% natural abundance) isotopomers of [Cd 6]2þ 3MeOH and [Cd 6]2þ 4MeOH, respectively. The other multiplet components arise from the 110 Cd (12.39%), 111 Cd (12.75%), 112 Cd (24.07%), 113 Cd (12.26%) and 116 Cd (7.58%) isotopomers.
Competitive ESI-MS experiments were conducted on solutions of either 5 or 6 and excess Agþ and Cd2þ . The [Ag 5]þ /[Cd 5]2þ peak intensity ratio was 2:1 consistent with [Ag 5]þ being the more stable complex. No peak was observed for [Ag 6]þ while peaks for both [Cd 6]2þ 3MeOH and [Cd 62þ ] 4MeOH were observed consistent with these complexes being the most stable. Competitive ESI-MS experiments were also conducted on solutions that contained either 2 or 6 and excess Agþ and Pb2þ . The [Ag 2]þ /[Pb 2]2þ peak intensity ratio was 3:1 consistent with [Ag 2]þ being the more stable complex in the gas phase whereas the [Ag 6]þ /[Pb 6]2þ peak intensity ratio was 1:20 consistent with [Pb 6]2þ being the more stable complex. Finally, competitive experiments were carried out on a solution of 6 and excess Cd2þ and Pb2þ . Only peaks for [Cd 6]2þ 3MeOH and [Cd 6]2þ 4MeOH were observed (Fig. 1) consistent with them being more stable than [Pb 6]2þ . The lack of detection of any complexes of the divalent metal ions with 1, 3 and 4 is consistent with these complexes being unstable in the gas phase. On the basis of these data it was decided to carry out quantitative stability studies of the complexes formed by 1, 2, 5 and 6 with Agþ , Zn2þ , Cd2þ and Pb2þ in DMF which, in contrast to methanol, solubilized the ligands and their complexes sufficiently for potentiometric titration equilibrium studies. 3.3. Equilibrium studies The log(K/dm3 mol1 ) values for [M L]mþ , where M was either Agþ , Zn2þ , Cd2þ or Pb2þ and L is either 1, 2, 5 or 6, were determined by potentiometric titrations using silver measurement and reference electrodes in DMF solution. Thus, the variation of e.m.f. was monitored as titrant was added and the log(K/dm3 mol1 ) values were determined from these data as described in Section 2. The stabilities of [Ag L]þ were determined mþ
Scheme 3.
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
from the titration of 20.0 dm3 of 5.00 104 mol dm3 AgNO3 in DMF solutions with DMF solutions 5.00 103 mol dm3 in L. The stabilities of [M L]mþ , where Mmþ ¼ Zn2þ , Cd2þ and Pb2þ , were determined from the titration of 20.0 dm3 of 5.00 105 mol dm3 AgNO3 in DMF solutions with DMF solution 5.00 103 mol dm3 in L and 2.50 102 mol dm3 in either Zn(NO3 )2 , Cd(ClO4 )2 or Pb(ClO4 )2 . For all solutions I ¼ 0:050 mol dm3 adjusted with NEt4 ClO4 . A typical titration curve is shown in Fig. 2 and the derived log(K/dm3 mol1 ) values are shown in Table 1 [11,12]. For those [M L]mþ systems for which an accurate log(K/dm3 mol1 ) is reported the e.m.f. variation observed in the presence of Agþ and Mmþ differed from that observed in the presence of Agþ alone and log(K/ dm3 mol1 ) for [M L]2þ was determined from this difference. For those systems characterized by log(K/dm3 mol1 ) <2 the titration curve was indistinguishable from that carried out in the presence of Agþ alone and for those characterized by log(K/dm3 mol1 ) >7.52 no variation in e.m.f. occurred as the titrant solution containing L and Mmþ was added consistent with Agþ being unable to compete with Mmþ for L.
Fig. 2. Variation of e.m.f for 20.0 dm3 of a 5.00 105 mol dm3 AgNO3 solution in DMF titrated with a DMF solution 5.00 103 mol dm3 in 2 and 2.50 102 mol dm3 in Pb(NO3 )2 . For each solution I ¼ 0.050 mol dm3 adjusted with NEt4 ClO4 . The data points are shown as crosses and the best-fit of the algorithm for the competitive formation of [Ag 2]þ and [Pb 2]2þ is shown as a solid curve.
721
The log(K/dm3 mol1 ) values for the Agþ complexes are essential reference data required for the interpretation of the competitive titration of either Zn2þ , Cd2þ or Pb2þ against Agþ in the presence of a particular ligand. Stability increases in the order [Ag 5]þ > [Ag 6]þ > [Ag 2]þ > [Ag 1]þ . This is consistent with the flexibility of 5 most readily accommodating the linear N–Ag–N coordination favored by stable Agþ complexes [13]. Despite the potential for the phenylhydroxyethyl pendant arms of 6 to increase the statistical probability of coordination, the stability of [Ag 6]þ is about two orders of magnitude less than that of [Ag 5]þ . This is probably a consequence of steric crowding at the ternary nitrogens. Ligands 1 and 2 have structures stiffened by the incorporation of aromatic rings which probably lowers their ability to coordinate Agþ . While the donor atom set of 1 is identical to that of 5, the two oxygen– nitrogen–oxygen sets are more distant from each other in 1 than in 5 in which their separation appears close to optimal for coordinating Agþ , which has a six coordi [14]. Although the disposinate ionic radius of 1.15 A tion of the donor atoms of 2 about the macrocyclic ring is the same as in 5 and 6, the presence of four sulfur atoms in 2 in place of two oxygen and two nitrogen atoms in 5 and 6 combined with the stiffness of 2 are probably major causes of [Ag 2]þ being less stable than [Ag 5]þ and [Ag 6]þ . It is notable that [Ag 1]þ , [Ag 2]þ and [Ag 5]þ are much more stable than their Zn2þ , Cd2þ and Pb2þ analogues and that the stability of [Ag 6]þ is only exceeded by those of [Cd 6]2þ and [Pb 6]2þ (Table 1). This is attributable to the preference for Agþ to form strong coaxial bonds with two nitrogen donor atoms [13] and for their stabilizing influence only being exceeded in [Cd 6]2þ and [Pb 6]2þ where octadentate 6 evidently forms a coordination cavity of optimal size and stereochemistry for Cd2þ and Pb2þ . The complexation of Zn2þ and Cd2þ by 1 and 2 is weak probably because of the large cavity size of 1 and the lack of affinity of the soft Lewis base sulfur donor atoms of 2 for the borderline hard Lewis acids Zn2þ and Cd2þ . Both are coordinated more strongly by 5 consistent with 5 both incorporating an appropriate atom donor set for these border line hard acid metal ions and also possessing sufficientl flexibility to adapt its coordination cavity to the sizes of Zn2þ and Cd2þ (six-coor respectively). dinate ionic radii of 0.74 and 0.95 A,
Table 1 Variation of stability {log(K/dm3 mol1 )} of [M L]mþ in DMF at 298.2 K and I ¼ 0.050 mol dm3 (NEt4 ClO4 ) Mþ
L ¼ 1 log(K/dm3 mol1 )
L ¼ 2 log(K/dm3 mol1 )
L ¼ 5 log(K/dm3 mol1 )
L ¼ 6 log(K/dm3 mol1 )
Agþ Zn2þ Cd2þ Pb2þ
5.50 0.04 <2 <2 2.62
6.49 0.02 <2 <2 2.38
9.42 0.13a 4.30 0.07 5.92 0.02 6.71 0.12
7.52 0.01 <2 >7.52 >7.52
a
This compares with 8.08 in H2 O [11] and 9.91 in DMF {I ¼ 0:1 mol dm3 (NEt4 ClO4 )} [12].
722
D. Caiazza et al. / Inorganica Chimica Acta 357 (2004) 716–722
However, [Zn 6]2þ is less stable than [Zn 5]2þ which is attributable to the steric crowding of the ternary nitrogens. This is offset in [Cd 6]2þ by the ability of Cd2þ to readily adopt a coordination number of eight (eight whereas this is less coordinate ionic radius ¼ 1.10 A) 2þ likely to occur for Zn . The variation of log(K/dm3 mol1 ) for [Pb L]2þ (for Pb2þ six- and eight-coordinate respectively) may be exionic radii ¼ 1.19 and 1.29 A, 2þ plained as for [Cd L] . Thus the overall effect is that 6 is selective for Cd2þ and Pb2þ over Zn2þ by >5 orders of magnitude as assessed in terms of log(K/dm3 mol1 ).
4. Conclusion Of the ligands studied, only 5 and 6 form stable Cd2þ and Pb2þ [M L]2þ complexes in DMF. While 5 shows substantial selectivity for Cd2þ and Pb2þ over Zn2þ , 6 shows >5 orders of magnitude selectivity for Cd2þ and Pb2þ over Zn2þ which is attributable to a combination of steric crowding at the ternary nitrogens of 6 and its denticity of eight which favors the coordination of eight-coordinate Cd2þ and Pb2þ over that of usually six-coordinate Zn2þ . This suggests that a strategy for designing such selectivity into coronand-based ligands is to exploit this maximum coordination number difference between Zn2þ , Cd2þ and Pb2þ in ligands which form a coordination cavity for M2þ of appropriate size. In principle, 6 may either be water solubilized to act as a sequestering agent or made to fluoresce on coordination of a heavy metal ion to act as a sensor by replacing the phenyl groups of the pendant arms by either hydrophilic groups or by a fluorophore sensitive to heavy metal ion coordination [15], respectively.
Acknowledgements The authors thank Dr. Karl Cornelius and Ms. Wendy Holstein (The University of Adelaide) for as-
sistance with the ESI-MS studies. Funding for this research by the Australian Research Council is gratefully acknowledged.
References [1] (a) L. Friberg, M. Piscator, G.F. Nordberg, T. Kjellstr€ om, Cadmium in the Environment, second ed., CRC Press, Cleveland, 1974; (b) J.M. Ratcliffe, Lead in Man and the Environment, Ellis Horwood, Chichester, UK, 1981; (c) J.-P. Vernet, Heavy Metals in the Environment, Elsevier, New York, 1991. [2] D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory Chemicals, third ed., Pergamon Press, Oxford, 1988. [3] G.R. Newkome, J.M. Robinson, J. Chem. Soc., Chem. Commun (1973) 831. [4] S.A. Hogberg, D.J. Cram, J. Org. Chem. 40 (1975) 151. [5] L.C. Hodgkinson, M.R. Johnson, S.J. Leigh, N. Spencer, I.O. Sutherland, R.F. Newton, J. Chem. Soc., Perkin Trans. 1 (1979) 2193. [6] E.P. Kyba, R.C. Helgeson, K. Madan, G.W. Gokel, T.L. Tarnowski, S.S. Moore, D.J. Cram, J. Am. Chem. Soc 99 (1977) 2564. [7] C.J. Pedersen, J. Org. Chem. 36 (1971) 254. [8] B.G. Cox, H. Schneider, J. Stroka, J. Am. Chem. Soc. 100 (1978) 4746. [9] D. Caiazza, S.F. Lincoln, E.R.T. Tiekink, A.D. Ward, Z. Kristallogr. NCS 216 (2001) 245. [10] B. Vaidya, J. Zak, G.J. Bastiaans, M.D. Porter, J.L. Hallman, N.A.R. Nabulsi, M.D. Utterback, B. Strzelbicka, R.A. Bartsch, Anal. Chem. 67 (1995) 4101. Malmsten, J. Inorg. Nucl. Chem. 43 (1981) 1299. [11] S. Kulstad, L.A. [12] B.G. Cox, P. Firman, H. Horst, H. Schneider, Polyhedron 2 (1983) 343. [13] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fifth ed., InterScience, New York, 1988. [14] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. [15] (a) A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515; (b) B. Valeur, I. Leray, Coord. Chem. Rev. 202 (2000) 3; (c) J.P. Geue, N.J. Head, A.D. Ward, S.F. Lincoln, J. Chem. Soc., Dalton Trans. (2003) 521.