Dendrimeric Tectons in Halogen Bonding-Based Crystal Engineering Pierangelo Metrangolo,* ,† Franck Meyer, † Tullio Pilati, ‡ Davide M. Proserpio, § and Giuseppe Resnati* ,† NFMLab c/o Department of Chemistry, Materials, and Chemical Engineering “G. Natta”, Politecnico di Milano Via L. Mancinelli 7, 20131 Milan, Italy, CNR-Institute of Molecular Science and Technology UniVersity of Milan, Via Golgi 19, 20133 Milan, Italy, and Department of Structural Chemistry and Inorganic Stereochemistry, UniVersity of Milan, Via Venezian 21, 20133 Milan, Italy ReceiVed September 11, 2007 ABSTRACT: The use of DAB dendrimers in halogen bonding-based crystal engineering is reported. The DAB-dendr-(NHC 6 F 4 I) 2 2 self-assembles with (E)-1,2-bis-(4-pyridyl)-ethylene in a 1:2 supramolecular adduct, whose crystal lattice is composed of 5-fold interpenetrated sql networks, which is the highest degree for a square topology reported so far. The synthesis and coordination properties of higher generation dendrimers are also described, both in the liquid state and in solution. Introduction Dendritic molecules have attracted massive interest because of their unique structures, properties, and potential for applica- tions in fields as diverse as materials chemistry, biomedicine, and nanotechnology. 1 When involved in molecular recognition and self-assembly processes, dendrimers tend to give a complex pattern of intra- and intermolecular interactions of comparable relative strength, whose balanced cooperation determines the structure of the resulting assembling system. As a probable consequence of the difficult control of such complex patterns of interactions, the use of dendrimers as tectons in crystal engineering, namely, as molecules whose interactions are dominated by particular associative forces that induce their self- assembly into organized networks with specific and pre- established architectural or functional features, 2 is rare. This article describes the synthesis of the poly(propylenimine) dendrimer DAB-dendr-(NH-C 6 F 4 I) 2 2 3a that is decorated at the periphery with four iodotetrafluorophenyl groups. These groups dominate the pattern of intra- and intermolecular interactions in this dendrimer. In particular, they promote a net of intramo- lecular hydrogen bonds that effectively preorganize the den- drimeric module to the point that the single crystal X-ray structure of the DAB-dendr-(NHC 6 F 4 I) 2 2 3a is obtained. Halo- gen bonding (XB) is the noncovalent interaction wherein halogens work as electrophilic sites. 3 The iodotetrafluorophenyl groups in 3a work as effective XB-donors, to the point that 3a functions as tetratopic tecton and self-assembles with (E)-1,2- bis-(4-pyridyl)-ethylene (6), a ditopic XB-acceptor (Scheme 1). The 1:2 supramolecular adduct 7 is formed, whose single crystal X-ray structure, showing the presence of very large two- dimensional (2D) square networks, is described. The overall lattice of 7 is composed of 5-fold interpenetrated square layers (sql) translationally related, 4 which is the highest interpenetration mode reported for a sql topology. 5 Self-assembly processes are usually a balance between enthalpic gain and entropic loss, so that the formation of heteromeric aggregates requires strong interactions and/or preorganized starting modules. In particular, introducing fluorine into the chemical formula of dendritic molecules has been shown to result in a total change in the self-assembly and self- organization patterns with respect to their nonfluorinated ana- logues; 6 interesting applications of fluorinated dendrimers are emerging in the fields of catalysis, new materials, and biology. 7 In our search for dendrimers that can work as tectons and drive the formation of well-ordered crystal lattices, we focused on DAB-dendr-(NH 2 ) 2 n+1 as they are commercially available and can be easily functionalized. To date, only two single crystal X-ray structures of DAB-dendr derivatives are described in the literature. 8 In these rare cases, the mobility of dendritic arms is greatly reduced by strong intramolecular hydrogen bonds, 8a or through diprotonation of core nitrogens. 8b On the assumption that the dendritic arms’ flexibility should be decreased to make the dendrimers work as tectons, the derivatization of terminal amine groups of DAB-dendr-(NH 2 ) 2 n+1 has to play a double role. First, it has to rigidify and preorganize the dendrimeric * To whom correspondence should be addressed. E-mail: pierangelo. metrangolo@polimi.it (P.M.); giuseppe.resnati@polimi.it (G.R.). Fax: +390223993180. † Politecnico di Milano. ‡ CNR. § University of Milan. Scheme 1. Synthesis of the Supramolecular Network 7 from 3a and 6 CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 654–659 10.1021/cg700870t CCC: $40.75  2008 American Chemical Society Published on Web 12/15/2007