Langmuir 1998, 14, 2589-2592
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Hydrophobic Control of Diastereoselectivity in the Synthesis of Double-Chain Surfactant Co(III) Complexes David A. Jaeger,* Ven B. Reddy, Navamoney Arulsamy, and D. Scott Bohle Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071
David W. Grainger and B. Berggren Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received December 15, 1997. In Final Form: February 27, 1998 A series of double-chain surfactant octahedral Co(III) complexes 1-4 has been prepared in both water and 20:1 (v/v) ethanol-water by the reaction of an N-alkylethylenediamine ligand (5) with sodium hexanitrocobaltate(III), followed by the addition of sodium perchlorate or sodium nitrate: trans-dinitrocisoid-bis(N-alkylethylenediamine)cobalt(III) perchlorate (1) and nitrate (3), and trans-dinitro-transoidbis(N-alkylethylenediamine)cobalt(III) perchlorate (2) and nitrate (4) (alkyl ) hexyl, octyl, decyl, dodecyl, and hexadecyl). The cisoid 1(3)/transoid 2(4) diastereomer ratio increased on going from 20:1 ethanolwater to water, in particular by a factor of approximately 5 for the octyl and dodecyl perchlorate systems and approximately 8 for the hexadecyl perchlorate system. The increase in the relative amount of cisoid 1(3) is attributed to the hydrophobic effect associated with the aggregation of ligand 5 in water.
Surfactant transition metal complexes are anticipated to have a wide range of interesting stereochemical, redox, and physical properties. But there have been only a few reports1 of the synthesis, isolation, and characterization of surfactant transition metal complexes, in contrast to many reports of the formation and study of such surfactants in solution without isolation. In part, the number of examples of characterized isolated surfactants is small because of the highly variable lability of ligands in the coordination sphere of many first-row transition metals and the difficulty in selectively synthesizing and separating surfactant stereoisomers. As part of our studies of transition metal-based surfactants, we have synthesized and characterized double-chain surfactant octahedral Co(III) complexes 1-4. Herein we report control of diastereoselectivity in their syntheses by the hydrophobic effect. There have been only a few previous reports of hydrophobic control of diastereoselectivity in chemical reactions.2 * To whom correspondence should be addressed. Telephone: 307766-4335. Fax: 307-766-2807. E-mail:
[email protected]. (1) For examples, see: (a) Yashiro, M.; Matsumoto, K.; Yoshikawa, S. Chem. Lett. 1989, 985. (b) Yashiro, M.; Matsumoto, K.; Yoshikawa, S. Chem. Lett. 1992, 1429. (c) Yashiro, M.; Matsumoto, K.; Seki, N.; Yoshikawa, S. Bull. Chem. Soc. Jpn. 1993, 66, 1559. (d) Behm, C. A.; Creaser, I. I.; Korybut-Daszkiewicz, B.; Geue, R. J.; Sargeson, A. M.; Walker, G. W. J. Chem. Soc., Chem. Commun. 1993, 1844. (e) Behm, C. A.; Boreham, P. F. L.; Creaser, I. I.; Korybut-Daszkiewicz, B.; Maddalena, D. J.; Sargeson, A. M.; Snowdon, G. M. Aust. J. Chem. 1995, 48, 1009. (f) Arumugam, M. N.; Arunachalam, S. Ind. J. Chem. 1997, 36A, 84. (g) Bruce, D. W.; Denby, I. R.; Tiddy, G. J. T.; Watkins, J. M. J. Mater. Chem. 1993, 3, 911. (h) Mun˜oz, S.; Gokel, G. W. Inorg. Chim. Acta 1996, 250, 59. (i) Menger, F. M.; Lee, J.-J.; Hagen, K. S. J. Am. Chem. Soc. 1991, 113, 4017. (j) Sprintschnik, G.; Sprintschnik, H. W.; Kirsch, P. P.; Whitten, D. G. J. Am. Chem. Soc. 1997, 99, 4947. (k) Sakai, S.; Fuginami, T. Hyomen 1990, 28, 644 and references therein. (2) For examples, see: (a) Porter, N. A.; Ok, D.; Huff, J. B.; Adams, C. M.; McPhail, A. T.; Kim, K. J. Am. Chem. Soc. 1988, 110, 1896. (b) Porter, N. A.; Arnett, E. M.; Brittain, W. J.; Johnson, E. A.; Krebs, P. J. J. Am. Chem. Soc. 1986, 108, 1014. (c) Petter, R. C.; Mitchell, J. C.; Brittain, W. J.; McIntosh, T. J.; Porter, N. A. J. Am. Chem. Soc. 1983, 105, 5700. (d) Takagi, K.; Suddaby, B. R.; Vadas, S. L.; Backer, C. A.; Whitten, D. G. J. Am. Chem. Soc. 1986, 108, 7865. (e) Moss, R. A.; Chiang, Y.-C. P.; Hui, Y. J. Am. Chem. Soc. 1984, 106, 7506. (f) Jaeger, D. A.; Wang, J. J. Org. Chem. 1993, 58, 6745 and references therein.
The reaction of an N-n-alkylethylenediamine3 ligand (5) with sodium hexanitrocobaltate(III)4 in water or 20:1 (v/v) ethanol-water, followed by the addition of saturated aqueous sodium perchlorate, gave mixtures of diastereomeric surfactant Co(III) complexes 1 and 2 (ClO4counterion) (eq 1).5,6 The former was likely obtained as a mixture of (()-1A (only one enantiomer shown) and meso1B, and the latter was likely obtained as a mixture of (()-2A (only one enantiomer shown) and meso-2B. In each complex the NO2 groups are trans, and the alkyl groups R are cisoid in 1 and transoid in 2, as schematically (3) Linsker, F.; Evans, R. L. J. Am. Chem. Soc. 1945, 67, 1581. (4) Brauer, G. Handbook of Preparative Inorganic Chemistry; Academic Press: New York, 1965; Vol. II, p 1541. (5) Complexes with R ) H (NO3- counterion) (a, b) and R ) Me (ClO4-, Cl-, Br-, and NO2- counterions) (c, d) have been reported: (a) Holtzclaw, H. F., Jr.; Sheetz, D. P.; McCarty, B. D. Inorg. Synth. 1953, 4, 176. (b) Werner, A. Justus Liebigs Ann. Chem. 1912, 386, 1. (c) Buckingham, D. A.; Marzilli, L. G.; Sargeson, A. M. J. Am. Chem. Soc. 1967, 89, 3428. (d) Buckingham, D. A.; Marzilli, L. G.; Sargeson, A. M. Inorg. Chem. 1968, 7, 915.
S0743-7463(97)01375-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/22/1998
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illustrated above. Analogous complexes 3 and 4 (NO3counterion) were prepared by the substitution of sodium nitrate for sodium perchlorate in the procedure used for 1 and 2.6 Stereoisomers of 1-4 containing cis dinitro groups should be obtainable under different conditions.5a,b The presence of trans dinitro groups and the absence of cis dinitro groups were indicated by IR7 and UV-vis8 spectroscopy. In studies of cis- and trans-dinitrobis(ethylenediamine)cobalt(III) nitrate, it was found that the latter is the thermodynamic product.5a,b The structural assignments for 1-4 were based on their IR and 1H and 13C NMR spectra [270/67.9 MHz, CDCl3 and 5:1 (v/v) CDCl3-CD3OD], including 1H,1H COSY, 1H,13C COSY, and 1H,1H NOESY NMR spectra, and on a single-crystal X-ray diffraction study.9 A monolayer study of 1e and 2e was also performed. IR spectra indicated that in each case the NO2 group was attached to Co(III) by nitrogen (nitro ligand) as opposed to oxygen (nitrito ligand).7 Even though each of 1-4 was probably obtained as a mixture of stereoisomers A and B, each gave only one set of 1H and 13C NMR signals. Also, comparable 1H and 13C NMR spectra were obtained for all pairs of cisoid 1 and 3 and of transoid 2 and 4. The presumed mixture of stereoisomers A and B of 3a resulting from its synthesis was recrystallized (25 °C) from acetonitrile to give crystals for a single-crystal X-ray diffraction study. The molecular structure, represented as an ORTEP plot in Figure 1, corresponds to A.10,11 It is unclear whether A was formed to the exclusion of B or if A fractionally crystallized from a mixture of A and B. An inspection of CPK models indicates that, at their attachment to nitrogen, the alkyl chains of meso-B are sterically more congested than those of A. Monolayer experiments with 1e and 2e were performed on a subphase of water at 25 °C. The surface pressure (6) The general procedure for the synthesis of 1 and 2 in water is as follows. A mixture of 2.20 mmol of powdered/liquid 5 and 100 mL of water was sonicated (Branson 2200, 125 W) at 55 °C for 30 min to give a milky-white emulsion. Then a solution of 485 mg (1.20 mmol) of sodium hexanitrocobaltate(III) in 5.0 mL of water was added dropwise during 15 min, after which the reaction mixture was stirred for 1 h. The reaction mixture cooled from 55 to 40 °C during the sodium hexanitrocobaltate(III) addition and to 25 °C by the end of the reaction. The reaction mixture was extracted with 75 mL of dichloromethane, and the extract was washed with two 10-mL portions of saturated aqueous sodium perchlorate, followed by two 100-mL portions of water, and then it was rotary evaporated. A solution of the residue in 10 mL of dichloromethane was filtered through a 1.5-cm (i.d.) × 10-cm column of silica gel packed in dichloromethane with 1:3 (v/v) dichloromethane-methanol as eluant. The residue after rotary evaporation was chromatographed on a 1.5-cm (i.d.) × 24.5-cm column of silica gel packed in dichloromethane with dichloromethane-methanol as eluant to give 1 and 2. The general procedure for the synthesis of 1 and 2 in 20:1 water-ethanol paralleled that in water. Ligand 5, but not sodium hexanitrocobaltate(III), dissolved completely in the reaction mixture. Controls demonstrated that mixtures of cisoid 1d and transoid 2d do not fractionate, isomerize, or decompose during the column chromatography on silica gel used in their separation and isolation in the above synthetic procedures. (7) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley-Interscience: New York, 1986. (8) (a) Basolo, F. J. Am. Chem. Soc. 1950, 72, 4393. (b) Brasted, R. C.; Hirayama, C. J. Phys. Chem. 1959, 63, 780. (9) Each of compounds 1-4 gave acceptable carbon and hydrogen combustion analyses.
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Figure 1. Molecular structure of A of 3a; hydrogen atoms have been omitted for clarity.
Figure 2. Surface pressure-molecular area isotherms for cisoid 1e and transoid 2e on a water subphase at 25 °C with a compression rate of 3 Å2 molecule-1 min-1.
versus molecular area isotherms, shown in Figure 2, are dramatically different.12 In particular, the limiting molecular areas (just prior to monolayer film collapse) for 1e and 2e are approximately 66 and 112 Å2/molecule, respectively. The dissimilar values can be attributed to the different dispositions of the alkyl groups in the cisoid and transoid diastereomers, as schematically illustrated above. At the limiting molecular areas for 1e and 2e, the complexed cobalt head group is at the air-water interface and the two alkyl groups are extended into the air. The cisoid disposition of 1e, compared to the transoid disposition of 2e, allows for greater chain alignment and lateral association, with an overall greater packing density within the monolayer. It is interesting to note that the molecular cross section for A of 3a, determined in the X-ray diffraction study, is 65.4 Å2, the same as 1e’s limiting molecular area within its monolayer at the air-water interface. The cisoid 1/transoid 2 diastereomer ratios obtained in the syntheses of the Co(III) complexes displayed a dramatic dependence on the reaction solvent. Uniformly, the 1/2 ratios obtained in water were greater than those (10) Crystal data for (()-A of 3a: C16H40CoN7O7, MW ) 501.48, triclinic space group P1h , a ) 6.6001 (13) Å, b ) 9.891 (2) Å, c ) 19.266 (4) Å, R ) 84.83 (3)°, β ) 80.42 (3)°, γ ) 85.56 (3)°, V ) 1232.6 (4) Å3, Z ) 2, dcalc ) 1.351 mg m-3, λ ) 0.71073 Å, µ ) 0.744 mm-1, F(000) ) 536, T ) 213 K. Data were collected on a Siemens SMART CCD diffractometer for 2° < θ < 23.34°. The structure was solved by direct and Fourier methods and refined by least squares against F2 to R1 ) 0.0653 (wR2 ) 0.1500) and Sgoof ) 0.965 for 3412 unique intensity data points with I > 2σ(I). Alkyl chain disorder for C(7), C(8), and C(14) was revealed during later stages of refinement; the final model included a site occupancy factor for each of these carbons disordered over two sites. (11) Solid-state structures of (()- and meso-trans-dinitro-transoidbis(N-methylethylenediamine)cobalt(III) chloride, determined by singlecrystal X-ray diffraction, have been reported: Bernal, I.; Cai, J.; Myrczek, J. Acta Chim. Hung. 1993, 130, 555. (12) Each isotherm, obtained on a KSV 3000 Langmuir-Blodgett film balance, represents one of at least three identical curves for a given sample. Different compression rates had very little effect upon the isotherms.
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Table 1. Dependence of cisoid 1/transoid 2 Diastereomer Ratios on Reaction Solvent in the Synthesis of 1 and 2a system, R
reaction solvent
% yield 1 + 2b,c
1/2 ratioc
C8H17 (b) C12H25 (d) C16H33 (e) C8H17 (b) C12H25 (d) C16H33 (e)
20:1 (v/v) ethanol-water 20:1 (v/v) ethanol-water 20:1 (v/v) ethanol-water water water water
67 ( 3 64 ( 9 66 ( 7 67 ( 5 69 ( 4 74 ( 3
0.38 ( 0.01 0.36 ( 0.04 0.36 ( 0.06 2.0 ( 0.2 1.9 ( 0.3 2.8 ( 0.2
5b, 5d, 5e, and the tetradecyl and octadecyl homologues form bilayer membranes when dispersed into water by sonication. The ethylenediamine head groups should be partially protonated in water,17 resulting in positively charged aggregate-water interfaces that electrostatically attract hexanitrocobaltate(III) anions. The formation of the major cisoid diastereomer 1 in water likely involves the following pathway. A molecule of 5 within an aggregate displaces two nitrite ions from a hexanitrocobaltate(III) anion at the aggregate-water interface to give intermediate 6 (eq 2), a single-chain
a See the text for experimental details. b Yields for isolated products. c The limits of error are average deviations for at least three syntheses.
Figure 3. Ligand 5b at a water-aggregate interface; see the text for a description.
obtained in 20:1 ethanol-water. The results are summarized in Table 1 for the octyl (b), dodecyl (d), and hexadecyl (e) systems. On going from 20:1 ethanol-water to water, the 1/2 ratios increased by a factor of approximately 5 for the octyl and dodecyl systems and approximately 8 for the hexadecyl system. The 3/4 ratios (NO3- counterion) obtained in the syntheses of 3 and 4 in water and 20:1 ethanol-water were not measured but are qualitatively similar to the corresponding 1/2 ratios obtained in these solvents. At present it is unknown whether the 1(3)/2(4) ratios are kinetically or thermodynamically controlled, although the former is more likely. The source of the diastereoselectivity for cisoid 1 over transoid 2 in water is the hydrophobic effect associated with the aggregation of ligand 5 in water. In the syntheses,6 sonication of each of 5b, 5d, and 5e (0.022 M if completely dissolved) in water at 55 °C gave a milkywhite emulsion. Each emulsion should contain some molecularly dissolved 5, in addition to dispersed aggregates of 5. There could be a variety of aggregates, increasing in size from premicellar/prevesicular aggregates,13 to micelles/vesicles,14 to the macroscopic aggregates responsible for the overall milky-white appearance. The fraction of molecularly dissolved 5 should decrease on going from 5b to 5d to 5e, reflecting their increasing hydrocarbon contents. Except perhaps for premicellar/prevesicular aggregates, the majority of 5 within its aggregates will be oriented as illustrated for 5b in Figure 3, with the polar ethylenediamine head groups at the aggregate-water interface and the alkyl chains hydrophobically associated within the aggregate interior. For simplicity, (a) a flat interface is illustrated, whereas those of a micelle/vesicle and an emulsified droplet are curved, and (b) the molecules are shown in fully extended conformations, although the head group conformation is unknown and some of the alkyl chains are folded within a micelle.15 Liang and co-workers have reported16 that (13) Bunton, C. A.; Bacaloglu, R. J. Colloid Interface Sci. 1987, 115, 288. (14) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982.
surfactant that is aligned within an aggregate like 5 (Figure 3). Given the head-to-head disposition of 5 and 6, the reaction of a second molecule of 5 from within the aggregate with 6 gives a Co(III) complex with cisoid (1) rather than transoid (2) alkyl groups. As a positively charged cisoid Co(III) complex 1 is produced, it would be expected to form mixed aggregates with 5, since its shape should facilitate its accommodation at an aggregate-water interface.18 Mixed aggregates may in fact have greater ordering than aggregates of pure 5. In any event, the formation of cisoid 1 would also be expected within mixed aggregates. In water, formation of the minor amounts of transoid 2 could involve several pathways, including the following. Molecularly dissolved 5 reacts with the hexanitrocobaltate(III) anion within the bulk aqueous phase to give intermediate 6. A second molecularly dissolved molecule of 5 then reacts with 6 in transoid fashion to give 2. This stereochemistry derives from labilization of the nitro ligand trans to the alkyl-substituted nitrogen19 and the greater nucleophilicity of the secondary compared to the primary nitrogen of 5. In another pathway, analogous factors lead to the preferred formation of transoid 2 within premicellar/prevesicular aggregates.20 In both of these routes, the rate of the reaction of 5 with 6 must be greater than the incorporation of 6 into micelles/vesicles and macroscopic aggregates containing 5. Such incorporation would be followed by the reaction of 5 with 6 to give cisoid 1, by the route described above.21 The predominant formation of transoid 2 in 20:1 ethanol-water can be ascribed to less, or a complete lack of aggregation of 5, compared to its behavior in water. The reactions of molecularly dissolved 5, first with hexanitrocobaltate(III) anion and then with intermediate 6, which are in part responsible for the minor amounts of transoid 2 in water, lead to the major amounts of transoid (15) Menger, F. M.; Dulany, M. A.; Carnahan, D. W.; Lee, L. H. J. Am. Chem. Soc. 1987, 109, 6899 and references therein. (16) Lu, X.; Zhang, A.; Liang, Y. Langmuir 1996, 12, 5501. (17) For ethylenediamine in water at 25 °C and ionic strength ) 0.5, pK1 and pK2 ) 10.1 and 7.40, respectively (Nyman, C. J.; Murbach, E. W.; Millard, G. B. J. Am. Chem. Soc. 1955, 77, 4194). (18) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1992; Chapter 17. (19) (a) Pratt, J. M.; Thorp, R. G. Adv. Inorg. Chem. Radiochem. 1969, 12, 375. (b) Hartley, R. R. Chem. Soc. Rev. 1973, 2, 163. (20) For discussion of related features in a different system, see ref 2f. (21) Liang and co-workers (Lu, X.; Zhang, A.; Liang, Y. Langmuir 1997, 13, 553) have reported the in situ formation of 2:1 complexes of N-alkylethylenediamines and Cu2+, in which the proposed ligand geometry and stereochemistry vary with the length of the alkyl chain.
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2 in 20:1 ethanol-water. The fact that the 1/2 ratio obtained in 20:1 ethanol-water did not vary with the length of the alkyl group is consistent with a lack of aggregation of 5. It is interesting to note that Sargeson and co-workers5c,d obtained only transoid isomers in the syntheses in water of analogous complexes using Nmethylethylenediamine (R ) Me), which should not hydrophobically associate. Even though the procedures were different from that used for 1-4, the lack of formation of cisoid isomers with R ) Me is consistent with the above explanation for the selective formation of cisoid 1(3) over transoid 2(4) in water. But these considerations also suggest that the minor amounts of cisoid 1(3) in 20:1 ethanol-water result from at least limited hydrophobic association of 5. Of complexes 1-4, only 2a, 3a, 4a, and 4b have limited solubilities in water at 25 °C. The others do not disperse into water even with extended sonication at 60 °C. A study of the aggregate morphologies of the soluble
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complexes should prove interesting. The alkyl chains of 1(3) can readily associate hydrophobically on the same side of the cation, whereas those of 2(4) are disposed 180° apart, as in the gemini surfactants reported by Menger and co-workers.22 In summary, a series of new double-chain surfactant Co(III) complexes 1-4 has been prepared and characterized. The diastereoselectivity in the formation of cisoid 1(3) over transoid 2(4) in water can be attributed to the hydrophobic effect. Acknowledgment. D.A.J. and D.S.B. acknowledge the National Science Foundation (Grant CHE-9526188) and the Department of Energy (Grant DE-FC02-91ER), respectively, for the support of this research. D.W.G. and B.B. acknowledge support from a 3M Faculty Fellowship. LA971375+ (22) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451.
