Imposition of chirality in a dinuclear triple-stranded helicate by ion pair formation.

Chirality is a fundamental property intrinsic to many supramolecular systems.2 Helical assemblies are inherently chiral, regardless of the stereotopic information contained within the individual building blocks.3-5 Metallo-supramolecular helicates, assembled from achiral ligands and metal centers, form racemic mixtures of clockwise and counterclockwise structures in which all the metal centers are homoconfigurational.6 How is chirality in helicates communicated, and can it be controlled through noncovalent forces in solution? Chirality in natural, biological supramolecular systems such as the helical iron complex of rhodotorulic acid, the dihydroxamate siderophore produced by the yeast Rhodotorula mucilaginosa, is determined by chiral centers within the individual ligand substituent.7,8 Likewise, chiral preference in synthetic supramolecular architecture is often introduced by covalently integrating a chiral moiety, such as a carbon stereocenter, into one or more subunits of the complete structure.9-16 We have previously demonstrated that the incorporation of stereocenters at the extremities of a bis-bidentate catecholate ligand leads to the exclusive formation of one dinuclear triple helicate enantiomer.17 There are very few systems in which resolution of enantiomers is achieved despite the absence of chiral components within the substructure. The best known abiological example is quartz.18 An unusual synthetic example reported by Lehn and co-workers19 is a helicate that spontaneously resolves into two enantiomers upon crystallization. Herein we report asymmetric induction with tunable chirality in a discrete dinuclear triple helicate, [Ga213] (H41 ) N,N′-bis(2,3-dihydroxybenzoyl)-1,4-phenylenediamine,20 an achiral ligand) by the use of a chiral counterion. The counterion, (S)-N-methylnicotinium21 (s-nic), is found to have stereospecific interactions with the helicate. Other examples of asymmetric induction to a supramolecular system were reported by Oda et al.22 and Lacour et al.23 There are two key distinctions between this [Ga213] helicate system and other reported cases of chiral resolution or determination in metallohelicates. First, the three bis-bidentate ligands that bridge the two octahedral metal centers are achiral. Therefore in the absence of an external stereogenic source, a racemic mixture of helicates is observed. Second, the labile gallium metal centers allow rapid racemization. Similar D3 symmetric homoconfigurational helicates based on gallium catecholate coordination have been reported to undergo very rapid intramolecular inversion at room temperature.17,24 Diffusion of THF into a methanolic solution of K6[Ga213] in the presence of excess s-nic selectively yields crystals of the ΛΛ isomer as K(s-nic)5[Ga213]. The crystal structure reveals five cocrystallized s-nic ions per helicate (Figure 1). One potassium ion, which coordinates strongly to the catecholate oxygens on the helical caps, remains. The chirality at the metal centers is determined by steric interaction from the flatly anchored pyridinium and the chiral pyrrolidine moiety. Each pyridinyl ring of s-nic, with the nitrogen 3.75 Å away from the centroid of the catecholate ring, is in parallel arrangement with the nearest catecholate ring. This conformation suggests strong cation-π interaction between the electron-poor pyridinium ring of s-nic (π-acceptor) and the electron-rich catecholate ring of the ligand (π-donor). Each pyrrolidine unit is angled away from the metal * Author to whom correspondence should be addressed. E-mail: Raymond@ socrates.berkeley.edu. Tel: +1-510-642-7219. Fax: +1-510-486-5283. (1) Coordination Number Incommensurate Cluster Formation. 16. Part 15: Ziegler, M.; Brumaghim, J. L.; Raymond, K. N. Angew. Chem., Int. Ed. 2000, 39, 4119-4121. (2) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (3) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63-68. (4) Suarez, M.; Branda, N.; Lehn, J.-M.; De Cian, A.; Fischer, J. HelV. Chim. Acta 1998, 81, 1-13. (5) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97, 2005-2062. (6) Some stable non-labile helicate mixtures have been resolved via column chromatography by the use of a chiral eluent. For an example, see: Hasenkopf, B.; Lehn, J.-M. HelV. Chim. Acta 1996, 79, 1643-1650. (7) Carrano, C. J.; Raymond, K. N. J. Am. Chem. Soc. 1978, 100, 53715374. (8) Carrano, C. J.; Cooper, S.; Raymond, K. N. J. Am. Chem. Soc. 1979, 101, 599-604. (9) Knof, U.; von Zelewsky, A. Angew. Chem., Int. Ed. 1999, 38, 303322. (10) Ghizdavu, L.; Kolp, B.; von Zelewsky, A.; Stoeckli-Evans, H. Eur. J. Inorg. Chem. 1999, 1271-1279. (11) Baum, G.; Constable, E. C.; Fenske, D.; Housecroft, C. E.; Kulke, T. Chem.sEur. J. 1999, 5, 1862-1873. (12) Masood, M. A.; Enemark, E. J.; Stack, T. D. P. Angew. Chem., Int. Ed. 1998, 37, 928-932. (13) Mamula, O.; von Zelewsky, A.; Bernardinelli, G. Angew. Chem., Int. Ed. 1998, 37, 290-293. (14) Woods, C. R.; Benaglia, M.; Cozzi, F.; Siegel, J. S. Angew. Chem., Int. Ed. Engl. 1996, 35, 1830-1833. (15) Enemark, E. J.; Stack, T. D. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 996-998. (16) Zarges, W.; Hall, J.; Lehn, J.-M. HelV. Chim. Acta 1991, 74, 18431852. (17) Meyer, M.; Kersting, B.; Powers, R. E.; Raymond, K. N. Inorg. Chem. 1997, 36, 5179-5191. (18) Natta, G.; Farina, M. Stereochemistry; Longman Group Limited: London, 1972. (19) Kramer, R.; Lehn, J.-M.; De Cian, A.; Fischer, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 703-706. (20) For the synthesis of N,N′-bis(2,3-dihydroxybenzoyl)-1,4-phenylenediamine, see: Caulder, D. L.; Raymond, K. N. Angew. Chem., Int. Ed. Engl. 1997, 36, 1440-1442. (21) For the synthesis of N-methylnicotinium iodide, see: Seeman, J. I.; Whidby, J. F. J. Org. Chem. 1976, 41, 3824-3826. (22) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; MacKintosh, F. C. Nature 1999, 399, 566-569. (23) Jodry, J. J.; Lacour, J. Chem.sEur. J. 2000, 6, 4297-4304. (24) Kersting, B.; Meyer, M.; Powers, R. E.; Raymond, K. N. J. Am. Chem. Soc. 1996, 118, 7221-7222. (25) Monoclinic, P21, orange, a ) 14.424(3) Å, b ) 24.772(5) Å, c ) 18.473(4) Å, â ) 107.88(3)°, -143 °C, Z ) 2, R1 ) 9.35%, GOF ) 1.519. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publication no. CCDC 139491. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: +44 1223336-033; e-mail: [email protected]). 2216 Inorg. Chem. 2001, 40, 2216-2217