pubs.acs.org/crystal Published on Web 01/05/2011 r 2011 American Chemical Society DOI: 10.1021/cg101447q 2011, Vol. 11 555–564 Organoamino Phosphonium Cations as Building Blocks for Hierarchical Supramolecular Assemblies Arvind K. Gupta, † Jennifer Nicholls, ‡ Suman Debnath, † Ian Rosbottom, ‡ Alexander Steiner,* ,‡ and Ramamoorthy Boomishankar* ,†,§ † Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam-781039, India, and ‡ Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, U.K. § Present Address: Department of Chemistry, Indian Institute of Science Education and Research (IISER) Pune, Sai Trinity Building, Pashan, Pune-411021, India Received October 31, 2010; Revised Manuscript Received December 6, 2010 ABSTRACT: Organoamino phosphonium cations of formula [P(NHR) 4 ] þ offer four hydrogen bonding sites that are arranged in tetrahedral fashion around the central phosphorus atom. This arrangement generates novel supramolecular building blocks that are distinct from the commonly planar NH donor systems, such as urea and guanidine. The supramolecular aggregation of these cations in the presence of chloride and several carboxylate anions is described. The carboxylate salts are obtained either by anion exchange reactions with phosphonium chlorides or by protonation reaction of the neutral phosphine imine P(NPh)- (NHPh) 3 . X-ray structure analyses of the title compounds show that they form hierarchical structures ranging from 1D chains to 2D sheets and 3D networks. Control over the dimensionality of the supramolecular structure can be achieved by modulating the steric bulk of the phosphonium cation. Introduction The design of new materials via crystal engineering has emerged as a major research topic over the past few decades. 1 While the main driving force for such approaches is to under- stand the supramolecular self-assembly process, the ultimate aim is to gain control over the supramolecular assembly pro- cess in order to produce designer crystals. 2 Several strategies have been developed to achieve this. For example, the network topology of metal-organic frameworks can be controlled to a great extent, which has enabled the synthesis of materials with specific properties. 3 As a result, metal-organic frameworks have shown enormous potential for applications ranging from host-guest chemistry and gas storage 4 to catalysis and sensors. 5 Thus the quest for new supramolecular building blocks that are able to generate structures of novel topologies and materials with certain properties, for example, metal-free organic frameworks, continues to grow at a steady rate. 6 Phosphonium cations containing alkyl and aryl substituents play an important role in chemical synthesis, for example, the Wittig reaction. 7 They have been widely applied as phase transfer catalysts, such as in the ring-opening polymerization of epoxides. 8 These compounds also play an important role in many stereoselective syntheses. Chiral tetraaminophospho- nium catalysts have been utilized in enantioselective transfor- mations, for example, the Mannich reaction, 9 the Henry reaction, and the hydrophosphonylation of aldehydes. 10 Ooi and co-workers used an enantiopure tetraamino phosphonium phenoxide salt containing a spirocyclic cation as a supramo- lecular ion-pair catalyst in which the chiral information is transferred to the substrate via hydrogen bonding. 11 Recently, we showed that the organoamino-substituted phosphonium cation [(PhNH) 4 P] þ can be deprotonated to form anions that are valence-isoelectronic to PO 4 3- , HPO 4 2- , and H 2 PO 4 - ions. 12 Such ligands can offer binding sites for both Lewis acids and Lewis bases. For example, the monoanionic ligand [(PhNH) 2 P(NPh) 2 ] - , which is the N-analogue of the dihydro- gen phosphate ion, contains a bidentate N-P-N site for the chelation of metal centers and a bidentate HN-P-NH site that can interact with anions via H-bonding. Tetra(organoamino)phosphonium cations of the general formula [P(NHR) 4 ] þ exhibit a tetrahedral arrangement of four RNH groups around the central phosphorus atom. As such they offer a unique coordination geometry for hydrogen bonding, which differs from that of the commonly planar NH donors such as urea and guanidinium (Chart 1). 13 The crystal structure of [P(NHPh) 4 ]Cl shows that the [P(NHPh) 4 ] þ ion binds two chloride ions via two HN-P-NH chelates, which are located at opposite sides of the cation and are twisted by 90° with respect to each other. 12a Spurred by the unique ligand geometry, we set out to explore the potential of these cations as supramolecular building blocks and the coordination modes of these cations in the presence of a range of anions. In particular, salts of anions containing carboxylate groups promise interesting structures, since these are potential double H-acceptors and thus compa- tible with the doubly H-donating HN-P-NH moiety of the cation. Carboxylic acids and carboxylic ions, especially those that contain several carboxylic groups, have been widely used as supramolecular synthons. 14 In addition, the steric and electronic properties of the [P(NHR) 4 ] þ ion can be tuned via Chart 1 *Corresponding authors. E-mail addresses: boomi@iiserpune.ac.in (R.B.); a.steiner@liv.ac.uk (A.S.).