N-H‚‚‚Cl 2 -M Synthon as a Structure-Directing Tool: Crystal Structures of Some Perchlorometallates D. Krishna Kumar, Amitava Das,* and Parthasarathi Dastidar* Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar - 364 002, Gujarat, India ReceiVed June 28, 2005; ReVised Manuscript ReceiVed September 15, 2005 ABSTRACT: A series of perchlorometallate salts, namely, [4,4′-H 2 diazastilbene][PdCl 4 ] 1; [H 2 -N-(4-pyridyl)isonicotinamide][MCl 4 ], M ) Pt(II) 2,M ) Pd(II) 2a; [H 2 -N,N′-bis(4-pyridyl)urea][MCl 4 ], M ) Pt(II) 3,M ) Pd(II) 3a; [H 2 -N-(3-pyridyl)isonicotinamide]- [MCl 4 ], M ) Pt(II) 4,M ) Pd(II) 4a; [H 2 -N-(4-pyridyl)nicotinamide][PtCl 4 ] 5; [H 2 -N,N′-bis(3-pyridyl)urea][PtCl 4 ] 6, have been synthesized and analyzed by single-crystal X-ray diffraction to study the frequency of occurrence, robustness, and reliability (as structure directing tools) of the bifurcated hydrogen bonding of the type N-H‚‚‚Cl 2 -M (synthon A). The results indicate that synthon A is indeed quite robust and reliable as structure-directing tool when the interacting cationic and anionic species are rigid. Structural parameters for synthon A indicate that the N-H‚‚‚Cl 2 M bifurcated hydrogen-bonding moiety is generally asymmetric and a “face approach” is more preferred in salts (4-6) derived from dications having angular cationic topology, whereas a nearly “edge approach” is preferred in salts (1-3a) derived from dications having linear cationic topology. Hydrogen-bonding interactions of the type N/C-H‚‚‚Pt are present in all the Pt salts except in 4, whereas no such interactions are observed in the cases of Pd salts. The metal center of the anionic moiety seems to have a profound effect on the molecular geometry of the cationic species as well as on the overall supramolecular architecture of the salts. Introduction Crystal engineering 1 and supramolecular syntheses 2 of solid- state materials are important interdisciplinary research fields in the area of chemistry and material science. The main impedi- ments in crystal engineering and supramolecular syntheses, whose ultimate goal is to build solid-state materials into order, are (i) the multiplicity of possible orientations of the molecules in crystals, (ii) the inaccuracies in estimating energies, and (iii) the entanglement of thermodynamic and kinetic contributions to crystal growth. 3 Thus, predicting the supramolecular assembly (crystal structure) even for a small molecule with modest structural complexity is a daunting task. Therefore, the primary goal of contemporary crystal engineering is to identify the molecular level building blocks (supramolecular synthons 2 ) that are capable of forming reliable substructural motifs as a result of various nonbonded interactions. Hydrogen-bonding 4 interactions, being reasonably strong and highly directional, are widely used as structure-directing tools in generating many molecular solids with novel properties. 5 Although the majority of these materials are based on nonionic hydrogen-bonding interactions, use of both directional hydrogen bonds and strong but less directional ionic interactions is also investigated. 6 On the other hand, in inorganic crystal engineer- ing, metal-ligand coordination remains the main strategy in generating various functional materials. 7 Deliberate syntheses of hydrogen-bond-based organic-inorganic hybrid materials have also gained widespread interest. 8 Recently, it has been proposed that “metal-bound chlorine often accepts hydrogen bonds”. 9 Since then, deliberate efforts have been made to construct intriguing supramolecular as- semblies using metal-bound halide-based hydrogen bonds. 10 The supramolecular synthon observed in these studies is the bifurcated hydrogen-bonded building block (A), which leads to the formation of a ribbon motif B with linear cations such as 4,4′-bipyridinium cation. However, the questions remain: How robust is the synthon A and how tolerant is it of other functional groups? What if the synthon occurs as a result of a far more intricate and incomprehensible poise between intermolecular forces? Is this synthon reliable to control the supramolecular network in such organic-inorganic hybrid composites? To address these points, we have recently studied some perchlorocuprates derived from a series of cations with various backbones and topology. 11 These results indicate that the N-H‚‚‚Cl-Cu hydrogen-bonding interaction is important in supramolecular syntheses of these solids. However, occurrence of bifurcated hydrogen bonding of the type N-H‚‚‚Cl 2 -Cu (synthon A) appears to be dependent on the topology of the cations, geometry of the anions, and other weak interactions such as C-H‚‚‚Cl-Cu. Since the coordination geometry of Cu(II), such as in [CuCl 4 ] 2- , is quite flexible and can adopt various geometries such as square planar, tetrahedral, distorted tetrahedral, pentagonal bipyramidal, etc., a great degree of structural complexity exists in these structures. Thus, it is not apparent how robust (or reliable as a structure-directing tool) synthon A (M ) Cu 2+ ) is in these inorganic-organic hybrid solids. To prove how important synthon A is or how reliable it is as a structure-directing tool, it is therefore important to reduce or completely remove the flexibility associated with the coordination geometry of the anionic metal center. Thus, we have decided to work on perchloroplatinate and/or perchloro- palladate wherein the metal centers, Pt(II) and Pd(II), are known to form rigid square planar geometry. In this paper, we report the syntheses and supramolecular structural description of nine * To whom correspondence should be addressed. Fax: +91-278- 2567562. E-mail: parthod123@rediffmail.com; amitava@csmcri.org; dastidar@csmcri.org. CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 216 - 223 10.1021/cg050296q CCC: $33.50 © 2006 American Chemical Society Published on Web 10/21/2005