Organometallics 2009, 28, 3567–3569
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Notes Synthesis and Total Spontaneous Resolution of an Octanuclear Organo(oxo)zinc Complex Anna Pettersen, Anders Lennartson, and Mikael Ha˚kansson* Department of Chemistry, UniVersity of Gothenburg, SE-412 96 Gothenburg, Sweden ReceiVed February 26, 2009 Summary: The reaction of diethylzinc, pyridine, and water followed by crystallization-induced asymmetric transformation yields homochiral batches of crystals containing the cornercapped octanuclear cubane [Zn8(Et)8O4(py)8]. The crystal structure exhibits channels containing pyridine molecules that are ordered along helices of the same handedness. While solutions of diethylzinc in nonpolar solvents are fairly unreactive toward carbonyl substrates, coordination of ligands to diethylzinc converts its linear structure into an approximate tetrahedral structure, which increases the nucleophilicity of the reagent. A chiral ligand not only activates the zinc reagent but can also control the stereochemistry of the organozinc addition. Addition of organozinc regents to prochiral aldehydes (using only catalytic amounts of chiral ligands) can thus yield chiral alcohols with high enantiomeric excess (ee). Soai and coworkers have taken the reaction one step further; by utilizatuion of the produced chiral alcohol as the activating ligand, a remarkable autocatalytic reaction is created.1,2 Chiral pyrimidyl or pyridyl alkanols not only act as asymmetric autocatalysts but also give significant amplification of the ee after each reaction. It seems that even stochastic fluctuations in a racemate can be amplified to give final ee’s over 99%, a result that may have implications for the origin of biomolecular homochirality.3-6 It is apparent that aggregation must be responsible for taking the minor enantiomer out of play.
Results and Discussion Diethylzinc forms complexes with pyridine (py) in hexane solution, which react cleanly with stoichiometric amounts of water. Colorless needles of high quality deposited (after several hours at ambient temperature) in good yield from such a solution, and a crystal structure could be determined. It revealed the octanuclear mixed oxo/alkyl aggregate [Zn8(Et)8O4(py)8] · 2py (1), which is displayed in Figure 1. To the cubane-shaped * To whom correspondence should be addressed. E-mail: hson@ chem.gu.se. (1) Soai, K.; Shibata, T.; Morioka, H.; Choji, K. Nature 1995, 378, 767. (2) Soai, K.; Shibata, T.; Sato, I. Acc. Chem. Res. 2000, 33, 382. (3) Mislow, K. Collect. Czech. Chem. Commun. 2003, 68, 849. (4) Singleton, D. A.; Vo, L. K. J. Am. Chem. Soc. 2002, 124, 10010. (5) Singleton, D. A.; Vo, L. K. Org. Lett. 2003, 5, 4337. (6) Soai, K.; Sato, I.; Shibata, T.; Komiya, S.; Hayashi, M.; Matsueda, Y.; Imamura, H.; Hayase, T.; Morioka, H.; Tabira, H.; Yamamoto, J.; Kowata, Y. Tetrahedron: Asymmetry 2003, 14, 185. (7) (a) Azad Malik, M.; O’Brien, P.; Motevalli, M.; Jones, A. C. Inorg. Chem. 1997, 36, 5076. (b) Marciniak, W.; Merz, K.; Moreno, M.; Driess, M. Organometallics 2006, 25, 4931.
(Zn-O)4 core, four corner-capping Zn centers are attached, thus completing a distorted-tetrahedral coordination geometry around each oxygen. The eight zinc centers coordinate an ethyl group each, and a tetrahedral coordination geometry arises around the four outer Zn atoms by coordination of pyridine ligands. Four of the eight pyridine ligand rings also participate (pairwise) in intramolecular offset π-π stacking (Figure 2). To our knowledge, alkyl(oxo)zinc aggregates have not been previously reported, although alkyl(amido)zinc complexes are known to react with water and form hexanuclear oxo aggregates.7a Alkyland hydrido(alkoxo)zinc clusters have also been reported.7b It is noteworthy that 1 crystallizes in the noncentrosymmetric space group P41212 (Table 1). This space group is one of the members of a pair of enantiomorphs (the other member is P43212), meaning that 1 forms a conglomerate. It is not immediately obvious how to characterize the chirality of the two enantiomorphic structures. With regard to the propellerlike orientation of the pyridine ligands (Figure 1) along the Zn4-O2 (or Zn3-O1) axis, the top half (Figure 2) of the aggregate resembles a P (clockwise) helix, while the bottom half exhibits L helicity. The whole aggregate can thus be described as being essentially a meso form. Figure 2 demonstrates that the top pair (and only the top pair) of stacked pyridine ligands participate in CH/N and/or CH/π interactions with a pyridine solvent molecule, which is cocrystallized and resident in channels throughout the structure (Figure 3). Although weak, similar CH/π interactions (involving the edge of an aromatic system) have been shown to be significant factors in the assembly of molecules to crystals.8-10 Since the cocrystallized pyridine solvent molecules are exclusively involved with the N4 pyridine ligands, they will be closer to the P helicity of the top molecule (as drawn in Figure 2). The chiral pockets in the channels confer perfect enantioselectivity. As shown in Figure 3, the pyridine molecules are arranged along homochiral 41 screw axes. The chirality of 1 is perhaps best understood in terms of the helical distribution of cocrystallized pyridine. The crystals turn into a highly air-sensitive powder on vacuum treatment, indicating that it is possible to remove the cocrystallized pyridine from the channels. Enantioselective pumping or loading of the channels could be monitored by solid(8) Nishio, M. CrystEngComm 2004, 6, 130. (9) Viswamitra, M. A.; Radhakrishnan, R.; Bandekar, J.; Desiraju, G. R. J. Am. Chem. Soc. 1993, 115, 4868. (10) Braga, D.; Grepioni, F.; Tedesco, E. Organometallics 1998, 17, 2669.
10.1021/om900153g CCC: $40.75 2009 American Chemical Society Publication on Web 05/06/2009
3568 Organometallics, Vol. 28, No. 12, 2009
Notes
Figure 1. Ortep drawing of 1, showing the crystallographic numbering (the cocrystallized pyridine ligands are not shown). Thermal ellipsoids enclose 50% probability. Selected bond distances (Å) and angles (deg): Zn1-O1 ) 2.056(3), Zn1-O2 ) 2.022(3), Zn1-O2* ) 2.017(3), Zn2-O1 ) 2.020(3), Zn2-O2 ) 2.055(3), Zn2-O1* ) 2.005(2), Zn3-O1 ) 1.877(2), Zn4-O2 ) 1.871(2), Zn1-C1 ) 1.996(5), Zn2-C3 ) 2.000(4), Zn3-C5 ) 1.998(5), Zn4-C7 ) 1.989(5), Zn3-N1 ) 2.187(4), Zn3-N2 ) 2.195(4), Zn4-N3 ) 2.183(4), Zn4-N4 ) 2.174(4); O1-Zn1-O2 ) 90.08(10), O1-Zn1-O2* ) 88.46(10), O2-Zn1-O2* ) 88.46(10), O1-Zn1-C1 ) 117.8(2), O1-Zn2-O2 ) 89.91(10), O1*-Zn2-O2 ) 89.11(10), O1-Zn2-O1* ) 87.50(10), O2-Zn2-C3 ) 115.2(2), Zn1-O1-Zn3 ) 120.37(13), Zn2-O1-Zn3 ) 119.94(13), Zn2*-O1-Zn3 ) 132.06(13), N1-Zn3-N2 ) 89.63(15), O1-Zn3-C5 ) 132.99(17), O1-Zn3-N1 ) 101.17(12), C5-Zn3-N1 ) 107.35(18), O1-Zn3-N2 ) 102.54(12), C5-Zn3-N2 ) 114.0(2). Symmetry code: (*) 1 - y, 1 - x, 1/2 - z. Table 1. Crystal and Refinement Data for 1 compd emp form fw cryst syst space group a, Å c, Å V, Å3 Z Dcalcd, g/cm3 final R (I > 2σ(I)) Flack param
[Zn8(Et)8O4(py)8] · 2py (1) C66H90N10O4Zn8 1610.44 tetragonal P41212 16.532(2) 26.081(3) 7128(1) 4 1.501 R1 ) 0.037, wR2 ) 0.082 0.02(2)
such aggregates in solution obviously must be very low, total spontaneous resolution is possible. Moreover, single-colony growth may result in that all the crystals in a batch display the same
Figure 2. Intramolecular π-π offset stacking between pairs of pyridine ligands (top and bottom) in 1. Only the top pair of ligands is involved in intermolecular CH/N or (CH/π vertex) interactions with the cocrystallized pyridine molecules.
state CD spectroscopy,11 and we are currently investigating the possibility of using highly air-sensitive compounds in solidstate CD spectroscopy. The intramolecular π-π offset stacking between the pyridine ligands at top of the molecule (as shown in Figure 2) is slightly different from that at the bottom, even without considering the cocrystallized pyridine, which could result in enantiomeric aggregates in solution. Since the enantiomerization barrier between (11) Kuroda, R.; Honma, T. Chirality 2000, 12, 269.
Figure 3. Channels along the c axis in 1 are filled with pyridine molecules. Since a 41 screw axis runs along each channel, the cocrystallized pyridine molecules are arranged in a helical manner, as indicated by the curved arrows.
Notes
Organometallics, Vol. 28, No. 12, 2009 3569
handedness.12-14 Since the solution stays racemic, such a process would be an example of absolute asymmetric synthesis.15-17 We have recently reported18 a method for determining the ee in a bulk sample of a stereochemically labile compound (using solid-state CD spectroscopy), but this method is not yet reliable for very air sensitive compounds. The only remaining option is to estimate the ee using anomalous dispersion data from individual crystals. We thus collected diffraction data using crystals, originating from a batch exhibiting single-colony growth. Indeed, as shown by the Flack parameters and their standard uncertainties,17,18 the crystals had the same absolute structure, indicating that absolute asymmetric synthesis was successful. Absolute asymmetric synthesis and autocatalysis (as demonstrated by Soai) are relevant in connection to the origin of biomolecular homochirality.12,21-23 The optical activity created during crystallization-induced asymmetric transformation of 1 is quickly lost in solution at ambient temperature, but alkylation of substrates in solid-state (intercrystal) reactions may transfer and trap the conformational chirality. Alternatively, the pyridine molecules could be pumped off and perhaps be replaced selectively by e.g. benzaldehyde molecules, whereafter (intracrystal) enantioselective ethyl addition might ensue on heating. Finally, the prospect that the reaction between alkylzinc reagents and water, to yield large alkyl(oxo)zinc aggregates, could be general needs to be explored. It is possible that this new class of compounds may prove useful, not primarily as alkylating agents but rather as precursors for well-defined materials.
Experimental Section All operations were carried out under nitrogen using Schlenk or low-temperature24 techniques. Commercial diethylzinc (Acros, 1.2 M in hexane) and pyridine (Aldrich) were used as purchased. (12) Vestergren, M.; Johansson, A.; Lennartson, A.; Hakansson, M. MendeleeV Commun. 2004, 6, 229. (13) Kondepudi, D. K.; Laudadio, J.; Asakura, K. J. Am. Chem. Soc. 1999, 121, 1448. (14) Kondepudi, D. K.; Kaufman, R. J.; Singh, N. Science 1990, 250, 975. (15) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38, 3419. (16) Vestergren, M.; Gustafsson, B.; Davidsson, O.; Hakansson, M. Angew. Chem., Int. Ed. 2000, 39, 3435. (17) Vestergren, M.; Eriksson, J.; Hakansson, M. Chem. Eur. J. 2003, 9, 4678. (18) Lennartson, A.; Vestergren, M.; Hakansson, M. Chem. Eur. J. 2005, 11, 1757. (19) Flack, H. D.; Bernardinelli, G. Acta Crystallogr. 1999, A55, 908. (20) Flack, H. D.; Bernardinelli, G. J. Appl. Crystallogr. 2000, 33, 1143. (21) Bonner, W. A. Chirality 2000, 12, 114. (22) Avalos, M.; Babiano, R.; Cintas, P.; Jimenez, J. L.; Palacios, J. C. Chem. Commun. 2000, 887. (23) Siegel, J. S. Chirality 1998, 10, 24. (24) Hakansson, M. Inorg. Synth. 1998, 32, 222.
Preparation of [Zn8(Et)8O4(py)8] · 2py (1). A Schlenk tube was charged with a solution of diethylzinc in n-hexane (2 mL, 1.2 M, 2.4 mmol), and pyridine (1 mL, 12 mmol) was added with stirring. Water (20 µL, 1 mmol) was added very slowly with a syringe. (Alternatively, a wet pyridine solution may be used; regardless, the synthesis is perfectly reproducible.) The solution was kept at ambient temperature (without stirring), and large colorless needles, frequently growing in a single colony, formed within 2 days. The crystals started to decompose within a few seconds when exposed to air. Upon gentle heating under vacuum they readily lost the cocrystallized pyridine, resulting in an extremely air-sensitive powder. Yield: 0.31 g, 65%. Total Spontaneous Resolution of 1. Single-colony growth (from crystallization as described above; stirring14 is not required in this system) resulted in homochiral crystal batches. This was proven by collecting single-crystal data from all crystals in a small batch. As shown by the Flack parameters and their standard uncertainties,17,18 all crystals had the same absolute structure. Crystals of the other enantiomorph dominated in other batches. Crystallographic Data. Several data sets were collected at 123 K, and for a typical crystal, intensities of 50 277 reflections were measured with a Rigaku R-AXIS IIc image plate system (using graphite-monochromated Mo KR radiation from a Rigaku RU-H3R rotating anode) and 6743 independent reflections were used during refinement. A multiscan absorption correction was applied by the REQAB program under CrystalClear. The structure was solved with SHELXS-9725 and refined using SHELXL-9725 operating in the WinGX program package.26 Anisotropic thermal displacement parameters were refined for all the non-hydrogen atoms. Carbon atoms C6 and C8 were refined with a disorder model consisting of double sites. Only one of the sites is shown in Figures 1 and 2. All hydrogen atoms were included in calculated positions and refined using a riding model. Structural illustrations have been drawn with ORTEP-3 for Windows.27
Acknowledgment. We thank Erica Wingstrand for preparative assistance and the Swedish Research Council (VR) for funding. Supporting Information Available: A CIF file giving crystallographic data for 1. This material is available free of charge via the Internet at http://pubs.acs.org. OM900153G
(25) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (26) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (27) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
