Toward Nanodevices: Synthesis and Characterization of the Nanoporous Surfactant-Encapsulated Keplerate (DODA) 40 (NH 4 ) 2 [(H 2 O) n ⊂Mo 132 O 372 (CH 3 COO) 30 (H 2 O) 72 ] Dirk Volkmer,* ,† Alexander Du Chesne, ‡ Dirk G. Kurth,* ,§ Heimo Schnablegger, § Pit Lehmann, § Michael J. Koop, † and Achim Mu 1 ller † Contribution from the Department of Inorganic Chemistry 1, UniVersity of Bielefeld, P.O. Box 100 131, D-33501 Bielefeld, Germany, the Max-Planck-Institute for Polymer Research, P.O. Box 3148, D-55021 Mainz, Germany, and the Max-Planck-Institute of Colloids and Interfaces, D-14424 Potsdam, Germany ReceiVed July 7, 1999 Abstract: We describe the spontaneous self-assembly and the superstructure of a discrete surfactant-encapsulated cluster, (DODA) 40 (NH 4 ) 2 [(H 2 O) n ⊂Mo 132 O 372 (CH 3 COO) 30 (H 2 O) 72 ](2, n ≈ 50), which consists of a hollow giant isopolyoxomolybdate core covered by a hydrophobic shell of dimethyldioctadecylammonium (DODA) cations. The structural characterization of these nanoporous core-shell particles is based on small-angle X-ray scattering (SAXS) data on solutions of the encapsulated clusters, TEM investigations, FT-IR and UV-vis spectroscopy, as well as determination of the molecular area of 2 by Langmuir film investigations. Computer modeling of the solvent-accessible surface of the encapsulated cluster yields a central cavity with a volume of 1.5 nm 3 that is occupied by approximately 50 H 2 O molecules. The cluster bears (Mo-O) 9-ring openings with an average diameter of 0.43 nm. The covered surface area of 84 Å 2 /DODA indicates a rather tight packing of the amphiphile at the cluster surface. Due to the unique supramolecular architecture of 2 as well as its high solubility in common organic solvents, this compound shows promising perspectives for future applications in host-guest chemistry and homogeneous size-selective catalysis. Introduction The future design of functional nanodevices will most likely rely on the principles of molecular self-organization. 1 Current approaches to this ambitious task focus on supramolecular model systems which self-assemble according to a manageable set of combination principles from a limited number of building blocks. Discrete supramolecular structures have been assembled from suitably designed organic building blocks exploiting ligand-metal ion interactions, 2 π-π interactions, 3 or hydrogen- bonding mediated recognition processes. 4 With the recent discovery of self-assembling, discrete and nanosized polyoxo- molybdates (“giant wheels”, 5 “Keplerates” 6 ), novel inorganic components are now available, allowing access to the construc- tion of advanced nanodevices such as nanoreactors or sensors. A crucial point for POM applications that require further processing steps is the ability to control the surface properties and the grade of dispersion of the preferentially water-soluble clusters. This problem may be alleviated by encapsulating the inorganic clusters with a solubilizing protective shell of organic molecules, as we have recently shown in detail for the partially reduced heteropolyoxomolybdate [H 3 Mo 57 V 6 (NO) 6 O 183 - (H 2 O) 18 ] 21- . 7 Here we report on the synthesis and preliminary characteriza- tion of a novel type of partially reduced isopolyoxomolybdate, namely the surfactant-encapsulated Keplerate (DODA) 40 (NH 4 ) 2 - [(H 2 O) n ⊂Mo 132 O 372 (CH 3 COO) 30 (H 2 O) 72 ](2, n ≈ 50). Com- pound 2 was prepared from (NH 4 ) 42 [Mo 132 O 372 (CH 3 COO) 30 - (H 2 O) 72 ]‚ca. 300 H 2 O‚ ca. 10 CH 3 COONH 4 (1), the structure and synthesis of which have been described previously (ref 6). Results and Discussion For the surfactant-encapsulated cluster (SEC) 2, we propose the following structure (compare Figure 1a): a single anionic Keplerate cluster 1 resides in a hydrophobic shell of 40 DODA molecules, leading to a discrete, nearly spherical particle. The cationic headgroups of the surfactant molecules point toward the negatively charged surface of the cluster core 1, which itself possesses an average diameter of 3.0 nm, as derived from the crystal structure of 1 (ref 6). To determine the solvent-accessible surface (SAS) of 1, we assumed a surface probe radius of 0.28 † University of Bielefeld. ‡ Max-Planck-Institute for Polymer Research. § Max-Planck-Institute of Colloids and Interfaces. (1) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim 1995; pp 139-197. (2) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502-518. (3) Claessens, C. G.; Stoddart, J. F. J. Phys Org. Chem. 1997, 10, 254- 272. (4) (a) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D. N.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37- 44. (b) Rebek, J. Acc. Chem. Res. 1999, 32, 278-286. (5) (a) Mu ¨ller, A.; Krickemeyer, E.; Meyer, J.; Bo ¨gge, H.; Peters, F.; Plass, W.; Diemann, E.; Dillinger, S.; Nonnenbruch, F.; Randerath, M.; Menke, C. Angew. Chem., Int. Ed. Engl. 1995, 34, 2122-2124. (b) Mu ¨ller, A.; Krickemeyer, E.; Bo ¨ gge, H.; Schmidtmann, M.; Beugholt, C.; Ko ¨gerler, P.; Lu, C. Z. Angew. Chem., Int. Ed. Engl. 1998, 37, 1220-1223. (c) Mu ¨ller, A.; Das, S. K.; Fedin, V. P.; Krickemeyer, E.; Beugholt, C.; Bo ¨gge, H.; Schmidtmann, M.; Hauptfleisch, B. Z. Anorg. Allg. Chem. 1999, 625, 1187- 1192. (d) Mu ¨ller, A.; Krickemeyer, E.; Bo ¨gge, H.; Schmidtmann, M.; Beugholt, C.; Das, S. K.; Peters, F. Chem. Eur. J. 1999, 5, 1496-1502. (e) Mu ¨ ller, A.; Ko ¨ gerler, P.; Kuhlmann, C. Chem. Commun. 1999, 1347-1358. (6) Mu ¨ller, A.; Krickemeyer, E.; Bo ¨gge, H.; Schmidtmann, M.; Peters, F. Angew. Chem., Int. Ed. Engl. 1998, 37, 3360-3363. (7) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Co ¨lfen, H.; Koop, M. J.; Mu ¨ller, A.; Du Chesne, A. Chem. Eur. J. 2000, 6, 385-393. 1995 J. Am. Chem. Soc. 2000, 122, 1995-1998 10.1021/ja992350v CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000