M Synthon as a Structure-Directing Tool - American Chemical Society

N-H‚‚‚Cl2-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

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 216-223

ReceiVed June 28, 2005; ReVised Manuscript ReceiVed September 15, 2005

ABSTRACT: A series of perchlorometallate salts, namely, [4,4′-H2diazastilbene][PdCl4] 1; [H2-N-(4-pyridyl)isonicotinamide][MCl4], M ) Pt(II) 2, M ) Pd(II) 2a; [H2-N,N′-bis(4-pyridyl)urea][MCl4], M ) Pt(II) 3, M ) Pd(II) 3a; [H2-N-(3-pyridyl)isonicotinamide][MCl4], M ) Pt(II) 4, M ) Pd(II) 4a; [H2-N-(4-pyridyl)nicotinamide][PtCl4] 5; [H2-N,N′-bis(3-pyridyl)urea][PtCl4] 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‚‚‚Cl2-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‚‚‚Cl2M 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 engineering1

syntheses2

Crystal and supramolecular of solidstate materials are important interdisciplinary research fields in the area of chemistry and material science. The main impediments 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 synthons2) that are capable of forming reliable substructural motifs as a result of various nonbonded interactions. Hydrogen-bonding4 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 engineering, 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 assemblies 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 * To whom correspondence should be addressed. Fax: +91-2782567562. E-mail: [email protected]; [email protected]; [email protected].

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‚‚‚Cl2-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 [CuCl4]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 ) Cu2+) 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 perchloropalladate 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

10.1021/cg050296q CCC: $33.50 © 2006 American Chemical Society Published on Web 10/21/2005

N-H‚‚‚Cl2-M Synthon as a Structure-Directing Tool

Crystal Growth & Design, Vol. 6, No. 1, 2006 217

Table 1. Crystallograhpic Parameters for Salts 1-3a crystal data

1

2

2a

3

3a

CCDC Deposit No: empirical formula FW crystal size (mm) crystal system space group a/Å b/Å c /Å R/° β/° γ/° volume/Å3 Z Dcalc F(000) µ Mo KR (mm-1) temperature (K) observed reflections [I > 2σ(I)] parameters refined goodness of fit final R1 on observed data final wR2 on observe data

271600 C6H5Cl2N Pd0.50 215.21 0.54 × 0.28 × 0.18 triclinic P1h 7.023(4) 7.092(4) 8.304(5) 74.409(11) 68.551(10) 76.846(10) 366.9(4) 2 1.948 210 1.977 298(2) 863 88 1.168 0.0472 0.1243

271601 C11H10Cl4N3OPt 537.11 0.71 × 0.44 × 0.28 triclinic P1h 7.0209(16) 9.280(2) 11.633(3) 85.922(4) 83.343(4) 76.840(4) 732.3(3) 2 2.436 502 10.307 298(2) 1455 184 1.058 0.0387 0.1006

271602 C11H11Cl4N3OPd 449.43 0.43 × 0.28 × 0.16 triclinic P1h 7.091(3) 8.158(4) 13.722(7) 102.422(8) 95.689(9) 106.505(9) 732.3(6) 2 2.038 440 1.993 298(2) 1305 181 1.130 0.0627 0.1717

271603 C11H12Cl4N4OPt 553.14 0.54 × 0.32 × 0.18 monoclinic P21/c 14.8984(13) 7.0250(6) 15.9410(14) 90.00 110.8990(10) 90.00 1558.6(2) 4 2.357 1040 9.690 100(2) 1935 190 1.117 0.0155 0.0401

271604 C11H12Cl4N4OPd 464.45 0.72 × 0.45 × 0.32 monoclinic P21/c 17.171(2) 7.8632(10) 12.3623(16) 90.00 109.443(2) 90.00 1574.0(4) 4 1.960 912 1.859 100(2) 1993 190 1.199 0.0199 0.0557

Scheme 1

perchlorometallate salts (M ) either Pt(II) and/or Pd(II)) derived from a series of dications having various backbones and topologies to demonstrate the frequency of occurrence, robustness, and reliability (as a structure-directing tool) of synthon A. The dications chosen for this work are depicted in Scheme 1. While dications C-E have linear topology, F-H displays angular topology. Except C, all the cations chosen have potential hydrogen-bonding backbones. The cations are called “linear” or “angular” based on the relative position of the cationic centers with respect to one another. The supramolecular synthon A is expected to occur in these cationic centers, and the propagation of the assembly of the ion pair in the crystal lattice should follow in a manner dictated by the topology of the cations. It should be noted that perhalometallate salts have attracted attention as novel materials for various technological purposes, including the possibility of tunable magnetic, optical, and electronic properties.12 Results Efforts have been made to prepare X-ray quality single crystals of both Pt(II) and Pd(II) salts of all the dications shown in Scheme 1. However, single crystals of Pt(II) salt of C and

Pd(II) salts of G and H could not be prepared despite our best efforts. The perchlorometallate salts, namely, 1-6, have been prepared by treating the corresponding organic moiety with either K2PtCl4 or K2PdCl4 in either MeOH/HCl or water/HCl at room temperature. Single-crystal structures of all the salts (1-6) have been determined and analyzed to assess the frequency of occurrence, robustness, and reliability (as structuredirecting tools) of synthon A as well as the role of various backbones and hydrogen-bonding functionalities on the overall supramolecular architecture. The crystallographic parameters are listed in Tables 1 and 2. All the hydrogen atoms were geometricallyfixedat their correspondingnormalizeddistances10e,13 (N-H ) 1.01 Å and C-H ) 1.08 Å), and the hydrogenbonding parameters are given in Table S1 (Supporting Information). All the N-H‚‚‚Cl-M hydrogen-bond distances are found to be of normal dimensions.9 The π-π stacking interactions are based on the centroid-centroid distance and the twist angles to describe the extent of the twist of the molecular geometry of the dications are based on the angle between the two planes passing through the aromatic rings. X-ray powder diffraction (XRPD) patterns of all the salts were recorded. Reasonable agreement between the XRPD and simulated pattern obtained from single-crystal data is observed in all the cases (Supporting Information). Crystal Structures A. Single-Crystal Structures of Perchlorometallate Salts of Dications Having Linear Cationic Topology. [4,4′H2diazastilbene][PdCl4] (1). The crystal of 1 belongs to a centrosymmetric triclinic space group P1h, and both the organic cation and inorganic moieties are located on a center of symmetry. The molecular geometry of the cation is found to be absolutely planar (0°). Both the protonated ring nitrogen atoms of the organic moiety are involved in bifurcated N-H‚‚‚Cl2-Pd (synthon A) hydrogen bonding with the anionic moiety (H‚‚‚Cl ) 2.374, 2.390 Å; ∠N-H‚‚‚Cl ) 135.3, 138.6°). Such interactions lead to the propagation of a 1D hydrogenbonded network (ribbon motif B) of alternating cations and anions (Figure 1a). The ribbon is however slightly twisted displaying a twist angle of 17.2° between the molecular plane of the cation and anion. The ribbons are further packed in the crystal structure in a

218 Crystal Growth & Design, Vol. 6, No. 1, 2006

Kumar et al.

Table 2. Crystallographic Parameters for Salts 4-6 crystal data

4

4a

5

6

CCDC Deposit No: empirical formula FW crystal size (mm) crystal system space group a/Å b/Å c/Å R/° β/° γ/° volume/Å3 Z Dcalc F(000) µ Mo KR (mm-1) Temperature (K) observed reflections [I > 2σ(I)] parameters refined goodness of fit final R1 on observed data final wR2 on observed data

271605 C11H11Cl4N3OPt 538.12 0.65 × 0.44 × 0.18 monoclinic P21/n 10.6238(9) 11.1901(9) 12.9218(10) 90.00 97.5980(10) 90.00 1522.7(2) 4 2.347 1008 9.913 298(2) 1925 181 1.065 0.0259 0.0692

271606 C11H11Cl4N3OPd 449.43 0.51 × 0.36 × 0.22 monoclinic C2/c 24.1657(15) 7.9739(5) 15.2874(10) 90.00 92.6950(10) 90.00 2942.5(3) 8 2.029 1760 1.984 100(2) 3118 181 1.095 0.0339 0.0870

271607 C11H11Cl4N3OPt 538.12 0.54 × 0.33 0.14 orthorhombic Pbca 27.370(9) 6.884(2) 15.483(5) 90.00 90.00 90.00 2917.2(16) 8 2.450 2016 10.349 298(2) 1652 181 1.239 0.0417 0.1052

271608 C11H12Cl4N4O2Pt 569.14 0.68 × 0.48 × 0.26 triclinic P1h 6.7425(9) 8.1137(11) 16.254(2) 102.913(2) 90.129(2) 106.973(2) 826.80(19) 2 2.286 536 9.141 298(2) 1880 202 1.144 0.0241 0.0722

parallel fashion (Figure 1b). While each ribbon is in close contact with two other neighboring ribbons through edge-toedge interactions (C-H‚‚‚Cl ) 2.624-2.687 Å), its face-toface neighbor makes signification π-π stacking interactions (3.545 Å) through the aromatic rings of the cationic part. [H2-N-(4-pyridyl)isonicotinamide][MCl4], M ) Pt(II) (2), M ) Pd(II) (2a). Both the salts 2 and 2a crystallize in a centrosymmetric triclinic space group (P1h) with identical cell volume but not matching cell dimensions. Cationic moieties in both the salts are located on a general position, whereas the anionic moieties in 2 are on center of symmetry and that in 2a is sitting on a general position. The amide backbones of the cationic moieties in both the salts are found to be disordered. Similar disorder of the amide backbone of the parent compound has been reported in some crystal structures of coordination polymer.14 The angles between planes passing through the pyridyl moieties in the cations are found to be 28.2° and 11.4° in 2 and 2a, respectively, indicating significant nonplanarity of the cations. Synthon A is observed in both the salts (H‚‚‚Cl ) 2.2292.658 Å and 2.243-2.714 Å; ∠N-H‚‚‚Cl ) 127.2-148.6° and 122.4-151.9° in 2 and 2a, respectively). As a result, ribbon motif B is observed in both the salts. The ribbons are packed in parallel fashion in the crystal structures. In 2, each ribbon is in contact with three other neighboring ribbons through edge-

Figure 1. (a) Ribbon motif B and (b) packing of 1D chains in the crystal structure of 1.

to-edge interactions (C-H‚‚‚Cl ) 2.557-2.920 Å), whereas its face-to-face neighbor appears to have reasonable π-π stacking interactions (3.821 Å) through the aromatic rings of the cationic part. Because of severe disorder of the amide group in 2, the positions of the hydrogen atoms of the nitrogen are not defined, and, therefore, no hydrogen bonding is observed with the metalbound chlorine of the anionic moiety. On the other hand, in 2a, the ribbons self-assemble in a thick ribbon via hydrogen bonding of the type N-H‚‚‚Cl interactions (H‚‚‚Cl ) 2.635 Å; ∠N-H‚‚‚Cl ) 138.4°) involving the amide hydrogen atoms and π-π stacking interactions (3.569 Å) with its face-to-face neighbor. The ribbons are further packed in a parallel fashion in the crystal structure via various weak interactions such as C-H‚‚‚Cl (C-H‚‚‚Cl ) 2.541-2.784 Å) and π-π stacking interactions (3.835 Å). It may be mentioned here that C-H‚‚‚Pt (H‚‚‚Pt ) 2.899 Å) interaction involving aromatic C-H of one of the pyridyl moiety of the cation and one of the metal center in the asymmetric unit is observed in 2, whereas no C-H‚‚‚Pd interaction is observed in 2a. Illustrations of the part of the crystal structures of 2 and 2a described above are depicted in Figure 2. [H2-N,N′-bis(4-pyridyl)urea][MCl4], M ) Pt(II) 3, M ) Pd(II) 3a. Both 3 and 3a crystallize in a centrosymmetric

Figure 2. Illustration of crystal structures of 2 and 2a. (a) 1D network of cations and anions displaying ribbon motif B in 2 (amide backbone is found to be disordered); (b) Hydrogen-bonded ribbon motifs in 2a; parallel packing of the ribbons (c) in 2 and (d) 2a.



Figure 4. (a) Staircase-like 1D network of dimer in 4; (b) parallel packing of the 1D network in 4; (c) zigzag 1D network in 4a; (d) spacefilling model representation of planar sheets of zigzag network in 4a. (Two interacting sheets are shown in orange and purple.) Figure 3. Crystal structures of 3 and 3a. Panels (a) and (b) ribbon motif B in 3 and 3a, respectively; (c) parallel packing of the ribbon motifs in 3; (d) hydrogen-bonded rhombus grid in 3a; (e) interdigited packing of the rhombus grid in 3a shown in space-filling model (two interacting rhombus grids are shown in purple and orange).

monoclinic space group P21/c. The cations and anions in both the salts are located on a general position. While the cationic moiety in 3 is found to be significantly twisted (41.9°), it is considerably planar (9.0°) in 3a. The cationic centers in both the salts are involved in bifurcated hydrogen bonding (synthon A) with the anionic moieties (H‚‚‚Cl ) 2.198-3.140 Å and 2.200-3.110 Å; ∠N-H‚‚‚Cl ) 114.3-171.3° and 122.4151.9° in 3 and 3a, respectively) resulting into the linear ribbon motif B (Figure 3, panels a and b, respectively). In 3, the 1D ribbons of alternating hydrogen-bonded cations and anions are packed in parallel fashion further sustained by hydrogen bonding involving the urea nitrogen donors and halogen acceptors (H‚‚‚Cl ) 2.352 Å; ∠N-H‚‚‚Cl ) 161.8°) and also by various weak interactions such as C-H‚‚‚Cl (C-H‚‚‚Cl ) 2.555-2.947 Å) and π-π stacking interactions (3.888 Å) (Figure 3c). On the other hand, in 3a, the ribbons are not packed in parallel fashion; rather, through N-H‚‚‚Cl hydrogen-bonding interactions (H‚‚‚Cl ) 2.28-3.028 Å; ∠N-H‚‚‚Cl ) 107.9-164.6°) involving urea nitrogen atoms of one ribbon with the halogen atoms of the neighboring ribbon and weak C-H‚‚‚Cl (C-H‚‚‚Cl ) 2.761-2.882 Å) interactions among the neighboring ribbons result in a 2D hydrogen-bonded rhombus grid architecture. The angle between two such interacting ribbons is ca. 57.2°. The void dimension of the grid is ca. 5.4 × 4.0 Å (Figure 3d). However, such extended open frameworks of the rhombus grid are further packed on top of each other in an interdigited fashion presumably to fill the void space (Figure 3e). It is also noted that in Pt salt 3, a N-H‚‚‚Pt interaction (H‚‚‚Pt ) 2.440 Å) involving one of the urea N-H of the cation and metal center of the anion is observed, whereas no such interaction is observed in the Pd salt 3a. B. Single-Crystal Structures of Perchlorometallate Salts of Dications Having Angular Cationic Topology. [H2-N-(3pyridyl)isonicotinamide][MCl4], M ) Pt(II) 4, M ) Pd(II) 4a. Both the salts 4 and 4a crystallize in centrosymmetric monoclinic space group (P21/n and C2/c, respectively). Asymmetric units in both the salts are comprised of one cation and one anion sitting on general position. The cationic moiety is

found to be significantly twisted (49.5°) in salt 4, whereas it displays considerable planarity (7.2°) in salt 4a. In 4, both the cationic centers display synthon A through N-H‚‚‚Cl hydrogen bonding (H‚‚‚Cl ) 2.219-2.770 Å; ∠NH‚‚‚Cl ) 120.7-159.6°). Such interactions lead to the formation of a cyclic dimeric assembly of alternating cations and anions. The dimers are further held together by N-H‚‚‚Cl hydrogen bonding involving the amide nitrogen donors and halogen acceptors (H‚‚‚Cl ) 2.399 Å; ∠N-H‚‚‚Cl ) 138.6°) resulting in staircase-like 1D networks (Figure 4a), which are further packed in the crystal lattice in a parallel fashion (Figure 4b). Various C-H‚‚‚Cl interactions (Supporting Information) among the 1D networks are also found to be responsible for the stabilization of the crystal lattice. In 4a, the cationic center of the isonicotinic acid moiety of the dication is found to form only one N-H‚‚‚Cl hydrogenbonded contact (H‚‚‚Cl ) 2.155 Å; ∠N-H‚‚‚Cl ) 173.1°) with the anionic moiety, whereas the protonated pyridine nitrogen donor of the 3-aminopyridine moiety of the cation is involved in hydrogen bonding with the anionic moiety in the synthon A fashion (H‚‚‚Cl ) 2.163, 3.033 Å; ∠N-H‚‚‚Cl ) 118.3, 164.4°). Such interactions lead to the formation of 1D zigzag chain of alternating cations and anions. The chains are arranged in a planar sheet-like architecture sustained by various weak C-H‚‚‚Cl interactions (Supporting Information) (Figure 4c). The planar sheets are then packed on top of each other further stabilized by N-H‚‚‚Cl interactions involving the amide nitrogen (H‚‚‚Cl ) 2.599, 2.908 Å; ∠N-H‚‚‚Cl ) 116.7, 129.9°) (Figure 4d). [H2-N-(4-pyridyl)nicotinamide][PtCl4] 5. The space group assigned to the crystal structure of 5 is Pbca (orthorhombic). In the asymmetric unit, both the cation and the anion are located on a general position. The molecular geometry of the cationic species is found to be significantly twisted (34.9°). Involving synthon A on both the cationic centers (H‚‚‚Cl ) 2.318-2.695 Å; ∠N-H‚‚‚Cl ) 137.0-145.2°), the cations and anions are arranged in 1D zigzag chain. Through various C-H‚‚‚Cl interactions (Supporting Information), the chains are found to form sheet architectures, which are further packed on top of each other through N-H‚‚‚Cl hydrogen bonding (H‚‚‚Cl ) 3.133 Å; ∠N-H‚‚‚Cl ) 121.7 °) (Figure 5a, b). Significant N-H‚‚‚Pt interaction (H‚‚‚Pt ) 2.563 Å) involving amide N-H and metal center is also observed in this crystal.


Kumar et al. Scheme 2

Discussion

Figure 5. (a) Zigzag network of cations and anions in 5; (b) spacefilling model representation of planar sheets of zigzag network resting on each other through hydrogen-bonding interactions (two interacting sheets are shown in yellow and purple) in 5.

Figure 6. (a) 1D hydrogen-bonded network in 6; (b) parallel packing of the 1D chains in 6 (the chains are marked in purple and yellow in an alternating fashion).

[H2-N,N′-bis(3-pyridyl)urea][PtCl4] 6. Salt 6 crystallizes in a centrosymmetric triclinic space group P1h. One full cationic moiety, two anionic moieties sitting on inversion centers, and one full water molecule are located in the asymmetric unit. The molecular geometry of the cation is found to be significantly planar (3.69°). One of the cationic center of the dication forms synthon A (H‚‚‚Cl ) 2.349, 2.572 Å, ∠N-H‚‚‚Cl ) 129.6, 148.0°), whereas the other cationic center is involved in strong hydrogen-bonding interaction with the solvate water molecule (N‚‚‚O ) 2.664 Å, ∠N-H‚‚‚O ) 160.0°). The urea hydrogen atoms, on the other hand, form hydrogen bonding with one of the halogen of anionic moiety. Thus, involving N-H‚‚‚Cl interactions at one of the cationic center and urea nitrogen donor sites, 1D networks of alternating cations and anions are formed, which are further packed in parallel fashion in the crystal lattice sustained by hydrogen-bonding interactions between the solvate water molecule and urea oxygen atom (O‚‚‚O ) 2.835 Å) and various other weak interactions such as C-H‚‚‚Cl (Supporting Information) (Figure 6). In this crystal also, one of the metal centers in the asymmetric unit is involved in hydrogen-bonding interactions of the type N-H‚‚‚Pt (H‚‚‚Pt ) 2.702 Å) with one of the urea N-H protons.

Success of any crystal engineering design lies in reducing the number of possible ways of interactions among the interacting species thereby increasing the predictability of the resulting supramolecular architecture. Thus, choosing Pt(II) and/or Pd(II) proves to be beneficial because of their consistent occurrence of rigid square planar geometry unlike Cu(II), which has produced unexpected supramolecular structures in some perchlorocuprate salts earlier studied by our group,11 due to its flexible coordination geometry. In all the structures reported herein, the metal centers show almost an ideal square planar geometry. On the other hand, the backbones of the cations chosen in this study are significantly rigid allowing only C-C and/or C-N bond rotations. However, such a rotation, although it induces molecular nonplanarity in the cations, does not have any effect on the cationic topology. Thus, both the anions and the cations can be considered rigid enough for evaluating the robustness of synthon A. It is interesting to note that the molecular geometry of the cations are more planar in Pd(II) salts (twist ∠0, 11.4, 9.0 and 7.2° for 1, 2a, 3a, and 4a, respectively) compared to their Pt(II) counterparts (28.2, 41.9, and 49.5° for 2, 3, and 4, respectively). In Pt salt 5, the cationic moiety shows significant deviation from planarity (twist ∠34.9°); however, the cationic moiety in Pt salt 6 is found to be significantly planar (twist ∠3.69°). It may be noted here that except in the case of 4, all Pt salts 2-3 and 5-6 show C-H‚‚‚Pt or N-H‚‚‚Pt hydrogenbonding interactions, whereas in Pd salts, no such interactions are observed. Since weak interactions of the type C-H‚‚‚Cl are present in all the salts irrespective of the metal center, the secondary interactions of the type N/C-H‚‚‚Pt may be considered as one of the major reasons for the cationic species being significantly nonplanar compared to that in the Pd salts. A close look at the synthon A at the schematic level reveals that the directional approach of the cationic species toward the anionic moiety can have two possibilities: (1) edge and (2) face approaches, which are explained in Scheme 2. The structural description of the synthon A can be symmetric or asymmetric depending on Cl‚‚‚H distances and M-Cl‚‚‚H angles. Thus, it is important to define such parameters to describe synthon A in a more precise supramolecular sense. Thus, d1 and d2 are the distances between Cl and H; R1 and R2 are the angles between M-Cl‚‚‚H; and ω is the angle between the planes passing through MCl2 and Cl2‚‚‚H, which describes the deviation of the approaching proton from the anionic plane. Thus, if d1 ) d2 and R1 ) R2, then the synthon A can be considered as ideally symmetric, whereas deviation of ω from 0° will provide an idea of the extent of synthon A’s “face approach” (Scheme 2). The structures studied here fall into two categories based on the cationic topology of the dications used: (1) salts 1-3a having linear cationic topology, and (2) salts 4-6 having angular cationic topology.



Table 3. Synthon A Parameters in Salts 1-3a

1 2 2a 3 3a

d1/Å

d2/Å

R1/°

R2/°

ω/°

2.374 2.374 2.229 2.243 2.249 2.497 2.198 2.204 2.200

2.390 2.658 2.594 2.714 2.559 2.549 3.140 2.620 3.110

92.1 98.7 97.2 98.8 97.5 93.5 111.8 98.1 110.9

92.0 91.4 87.8 86.8 89.2 92.5 85.2 87.4 85.0

1.1 9.0 8.5 12.9 3.9 21.4 2.9 3.3 1.4

Table 4. Synthon A Parameters in Salts 4-6

4 4a 5 6

d1/Å

d2/Å

R1/°

R2/°

ω/°

2.351 2.219 2.163 2.362 2.318 2.349

2.509 2.770 3.033 2.695 2.555 2.572

92.2 87.6 105.4 95.9 96.1 87.1

88.4 75.7 82.0 87.7 89.9 92.6

26.0 56.5 23.6 29.2 14.4 30.6

In category (1), ribbon motif B involving both the cationic centers is observed in all the structures indicating highest possible occurrence of synthon A and direct translation of the cationic topology in the resulting supramolecular network. Thus, dications with linear cationic topology result in linear electrostatic polymers of alternating cations and anions. The synthon A parameters for these salts are listed in Table 3. It is clear from Table 3 that synthon A in salt 1 is nearly symmetric, whereas the rest of the salts display short d1 and long d2, and consequently unequal R1 and R2, indicating significant deviation from the symmetric structure. Except for salt 1 and one of the cationic centers of salt 3a, all other cationic centers in the rest of the salts show significant deviation of ω from 0°, indicating the extent of the face approach of the cationic centers in synthon A. It is also important to note that the overall packing of the ribbon motif B in these structures irrespective of their backbones are similar, i.e., parallel packing, except in 3a, wherein in the ribbon motifs B are packed in an angular fashion resulting in a rhombus grid-like architecture. It is remarkable that that by changing the metal center from Pt(II) in 3 to Pd(II) in 3a, such a difference in packing (parallel in 3 and angular in 3a) of the ribbon motif B is observed. On the other hand, in category (2), synthons A are observed in both the cationic centers of the dications except in salts 4a and 6. In 4a, while 3-pyridinium cationic center displays synthon A, one of the H‚‚‚Cl distances in the other cationic center is too long (3.216 Å) to consider a hydrogen-bonding distance.9 In 6, one cationic center displays synthon A and the other cationic center is involved in hydrogen bonding with one solvate water molecule. Synthon A parameters are listed in Table 4. Thus, it is clear from Table 4 that none of the synthons A are symmetric in nature in salts 4-6, and the approach of the cationic centers toward the anionic moieties is a “face” approach displaying high ω values. Propagation of the electrostatic network of organo cations and inorganic anions involving synthon A in a predictable fashion (i.e., a zigzag propagation from these dications of angular topology) exists in only two structures, namely, 4a and 5 in this category. In 4, however, a different approach of the cationic centers to the anionic moiety is observed. The cationic centers are approaching the anionic moiety in a cis fashion contrary to the trans fashion observed in ribbon motif B. These types of interactions lead to the formation of cation-anion hydrogen-bonded dimers. In salt 6, incorporation of one solvate water molecule interferes severely in the hydrogen-bonding

Figure 7. Space-filling model of synthon A observed in the crystal structures reported in this work.

network formation. It is remarkable to note that a change in the metal center from Pt(II) in 4 to Pd(II) in 4a results in a completely different supramolecular arrangement of the organicinorganic hybrids. It is also notable that the dications in 4 and 5 are isomeric having angular cationic topology. However, the resulting supramolecular architectures in both the structures are quite different from each other; while 4 displays a dimeric arrangement of cations and anions, a zigzag electrostatic polymeric network is observed in 5. Considering the fact that the shape fit and electrostatic interactions between the ions around the synthon A are important in providing access of the crystal structures observed in this work, it is worthwhile to examine the shape fit of synthon A observed in these structures by examining the space-filling model of synthon A. Figure 7 displays the synthon A observed in these structures in the space-filling model. It is clear that in most of the cases, synthon A displays a good shape fit between the pyridinium cation and the perchlorometallate anion except in the case of 3 and 3a, wherein the Cl‚‚‚H distances in synthon A belong to a “long” range of interactions.9 It is interesting to note that in none of these structures, the hydrogen-bonding-capable backbones display their typical hydrogen-bonding network. Thus, the amide backbone in 1-2a and 4-5 and the urea backbone in 3-3a and 6 do not show typical 1D hydrogen-bonding networks.15,16 Instead, the N-H protons of amide and urea moieties are involved in N-H‚‚‚Cl-M type hydrogen-bonding with the adjacent anion. This could be because the acidic condition of the crystallization might be preventing the hydrogen-bond acceptor atom oxygen (in both amide and urea moieties) to participate in its usual hydrogen-bonding interactions due to partial protonation during crystallization. It should be noted that the overall supramolecular architecture in a given crystalline solid arises from a balance of intermolecular forces. Although stronger forces such as electrostatic interactions, strong hydrogen bonding, etc. may dominate the overall supramolecular structure, the weak forces such as donor-acceptor interactions, dispersion forces, etc. are also important and may be a structure-determining factor if the number of such interactions is plenty.


Therefore, it is important to consider weak forces such as C-H‚‚‚Cl interactions17 in these solids. Data listed in Table S1 (Supporting Information) indicate that C-H‚‚‚Cl interactions must be contributing significantly to the overall supramolecular network observed in these solids. Thus, the crystal lattice is further stabilized by multiple C-H‚‚‚Cl interactions in all the solids. It should also be mentioned here that the hydrogenbonding acceptor capability of the amide/urea oxygen atom present in 2, 2a, 3a, 4a, and 5 is satisfied by weak C-H‚‚‚O interactions, whereas it is satisfied by strong hydrogen-bonding interactions with the solvate water molecule in 6. In the rest of the structures, the oxygen atom of the amide/urea moiety is found to be free from any notable interactions. It may be noted here that recently Valde´s-Martine´z et al. have studied perhalometallate (Cu and Zn) salts based on sterically hindered nonisomeric cations to demonstrate the importance of N-H‚‚‚Cl-M hydrogen-bonding interactions as structure-directing tools.18 Summary A systematic study of the supramolecular architectures in a series of perchlorometallate salts derived from various pyridyl dications having different backbone and cationic topologies clearly demonstrates the aptitude of crystal engineering concepts. The appearance of synthon A in all the structures indicate that the N-H‚‚‚Cl2-M bifurcated hydrogen-bonded interaction (synthon A) is indeed quite robust and reliable as structure directing tool when the cationic and anionic counterparts chosen are robust enough to reduce the number of possible ways of interactions. Therefore, choosing rigid dications and metal centers with fixed geometry (square planar) proves to be quite fruitful. Thus, propagation of the alternating cations and anions is governed by the cationic topology of the dication used in most of the structures [1-3a in category 1 and, 4a and 5 in category 2]. In the case of 4, an alternative propagation mode (cis mode) of the anionic moiety is observed. On the other hand, in 6, incorporation of solvate water molecule disrupts the formation of synthon A in one of the cationic centers, thereby resulting in a different supramolecular architecture. Synthon A parameters introduced in this work clearly indicate that in most of the cases, it is nonsymmetric and the face approach is more pronounced in the salts having angular cationic topology, i.e., in category 2. Both these parameters and shape fit representations indicate that the interactions between the cationic (N+-H) and anionic (MCl2) centers are mainly governed by stronger but less directional electrostatic interactions. It is also remarkable to note the influence of the metal center on the cationic conformation (palladium salts have more planar cations than that in platinum salts) and the resultant supramolecular architectures (the difference in 3, 3a, and 4, 4a, see Figures 3 and 4). The presence of C/N-H‚‚‚Pt hydrogenbonding interactions19 in all the Pt salts except in 4 appears to be the main cause for significant nonplanarity of the cationic species in these salts, whereas no such interactions are observed in Pd salts. Amide/urea nitrogen donors, instead of forming their usual 1D hydrogen-bonded network,15,16 invariably form N-H‚‚‚Cl-M hydrogen-bonding interactions with the anionic centers in all the structures reported herein, presumably because of the partial protonation of the urea oxygen atom during acidic crystallization condition. The presence of numerous weak interactions such as C-H‚‚‚Cl in all these solids suggests that these weak interactions play a significant role in stabilizing the network.

Kumar et al.

Experimental Section Syntheses. The organic moiety used in 1, K2[PtCl4] and K2[PdCl4] are commercially available (Aldrich) and were used without further purification. The syntheses and characterization of N-(4-pyridyl)isonicotinamide, N-(3-pyridyl)isonicotinamide and N-(4-pyridyl)nicotinamide the pyridyl amides used in this work were reported by our group.5f N,N′-Bis(4-pyridyl)urea and N,N′-bis(3-pyridyl)urea used in 3, 3a, and 6 were synthesized following a reported procedure.20 [4,4′-H2diazastilbene][PdCl4] (1). A solution of 4,4′-diazastilbene (111 mg, 0.61 mmol) in water (5 mL) and concentrated HCl (5 mL) was added dropwise, with stirring, to a solution of potassium tetrachloropalladate (200 mg, 0.61 mmol) in water (5 mL). The resulting orange suspension was stirred overnight, and then filtered. The solid obtained was washed with water (10 mL) and ethanol (10 mL), and then dried at the pump, (135 mg, 0.31 mmol, 50.81% yield). Single crystals were grown by dissolution of the product in hot aqueous HCl solution followed by slow cooling to room temperature over a period of 10 h. [H2-N-(4-pyridyl)isonicotinamide][PtCl4] (2). A solution of potassium tetrachloroplatinate (41.5 mg, 0.1 mmol) in concentrated HCl (5 mL) and water (10 mL) was taken in a beaker (25 mL). A solution of (4-pyridyl)isonicotinamide (19.9 mg, 0.1 mmol) in water (10 mL) and concentrated HCl (2 mL) was layered carefully over the above solution and allowed to evaporate at room temperature. After a period of one week, yellow crystals were collected by filtration, washed with water, followed by methanol, and dried (40 mg, 0.074 mmol, 74.3% yield). [H2-N-(4-pyridyl)isonicotinamide][PdCl4] (2a). A solution of potassium tetrachloropalladate (65.2 mg, 0.2 mmol) in concentrated HCl (5 mL) and water (10 mL) was taken in a beaker (25 mL). A solution of (4-pyridyl)isonicotinamide (39.8 mg, 0.2 mmol) in water (10 mL) and concentrated HCl (1 mL) was layered carefully over the above solution and allowed to evaporate at room temperature. After 2 days, along with some yellow powder, X-ray quality single crystals were obtained. [H2-N,N′-bis(4-pyridyl)urea][PtCl4] (3). A solution of N,N′-bis(4pyridyl)urea (21.4 mg, 0.1 mmol) in methanol (10 mL) and concentrated HCl (1 mL) was layered carefully over a solution of potassium tetrachloroplatinate (41.5 mg, 0.1 mmol) in concentrated HCl (2 mL) and water (10 mL) taken in a beaker (25 mL). After evaporation of the sample at room temperature, good-quality orange crystals were harvested (42 mg, 0.076 mmol, yield ) 75.94%). [H2-N,N′-bis(4-pyridyl)urea][PdCl4] (3a). A solution of N,N′-bis(4-pyridyl)urea (21.4 mg, 0.1 mmol) in methanol (10 mL) and concentrated HCl (1 mL) was layered carefully over a solution of potassium tetrachloropalladate (32.6 mg, 0.1 mmol) in concentrated HCl (2 mL) and water (10 mL) taken in a beaker (25 mL). After evaporation of the sample at room temperature, good-quality brown crystals were harvested (35 mg, 0.75 mmol, yield ) 75.43%). [H2-N-(3-pyridyl)isonicotinamide][PtCl4] (4). A solution of N-(3pyridyl)isonicotinamide (19.5 mg, 0.1 mmol) in water (5 mL) and concentrated HCl (1 mL) was carefully layered over a solution of potassium tetrachloroplatinate (41.5 mg, 0.1 mmol) in concentrated HCl (3 mL) and water (10 mL) taken in a beaker (25 mL) and evaporated at room temperature. Yellow-colored crystals thus obtained are filtered, washed with water, followed by methanol and dried (42 mg, 0.078 mmol, yield 78.06%). [H2-N-(3-pyridyl)isonicotinamide][PdCl4] (4a). A solution of N-(3pyridyl)isonicotinamide (39.8 mg, 0.2 mmol) in water (5 mL) and concentrated HCl (1 mL) was carefully layered over a solution of potassium tetrachloropalladate (65.2 mg, 0.2 mmol) in concentrated HCl (3 mL) and water (10 mL) taken in a beaker (25 mL) and evaporated at room temperature. Brown-colored crystals thus obtained were filtered, washed with water followed by methanol, and dried at the pump (70 mg, 0.156 mmol, yield 77.95%). [H2-N-(4-pyridyl)nicotinamide][PtCl4] (5). A solution of N-(4pyridyl)nicotinamide (19.9 mg, 0.1 mmol) in water (10 mL) and concentrated HCl (2 mL) was layered carefully over a solution of potassium tetrachloroplatinate (41.5 mg, 0.1 mmol) in concentrated HCl (3 mL) and water (10 mL) taken in a beaker (25 mL) and left for evaporation at room temperature. Yellow crystals thus formed were filtered, washed with water, followed by methanol, and dried at the pump (43 mg, 0.0799 mmol, yield 79.92%). [H2-N,N′-bis(3-pyridyl)urea][PtCl4] (6). A solution of N,N′-bis(4-pyridyl)urea (21.4 mg, 0.1 mmol) in methanol (10 mL) and

N-H‚‚‚Cl2-M Synthon as a Structure-Directing Tool concentrated HCl (2 mL) was layered carefully over a solution of potassium tetrachloroplatinate (41.5 mg, 0.1 mmol) in concentrated HCl (3 mL) and water (10 mL) taken in a beaker (25 mL). After evaporation of the sample at room temperature, good-quality yellow crystals were harvested (42 mg, 0.78 mmol, yield ) 78.06%). Single-Crystal X-ray Diffraction. X-ray single-crystal data were collected using Mo KR (λ ) 0.7107 Å) radiation on a SMART APEX diffractometer equipped with a CCD area detector. Data collection, data reduction, and structure solution/refinement were carried out using the software package of SMART APEX. Graphics were generated using PLATON21 and MERCURY 1.3.22 All structures were solved by direct methods and refined in a routine manner. In all cases, nonhydrogen atoms were treated anisotropically. Hydrogen atoms were fixed at their geometrically idealized position at a normalized distance (N-H ) 1.01 Å; C-H ) 1.08 Å, see also text) and refined with 1.5 times of the equivalent isotropic thermal parameters of the nonhydrogen atoms to which these hydrogen atoms are attached.

Acknowledgment. P.D. thanks the Department of Science & Technology, New Delhi, India, for financial support. D.K.K. thanks CSIR, New Delhi, India, for SRF fellowship. Supporting Information Available: Elemental analyses FT-IR data, hydrogen-bonding parameters, XRPD patterns, ORTEP diagrams for 1-6 (PDF), CIFs (in CIF format) for compounds 1-6. This material is available free of charge via the Internet at http://pubs.acs.org.

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M Synthon as a Structure-Directing Tool - American Chemical Society

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