Complexes on Packing - American Chemical Society

Article pubs.acs.org/crystal

Consequence of Presence and Absence of π‑Clouds at Strategic Locations of Designed Binuclear Pd(II) Complexes on Packing: SelfAssembly of Self-Assembly by Intermolecular Locking and Packing Mili C. Naranthatta, Deepika Das, Debakanta Tripathy, Himansu S. Sahoo, Venkatachalam Ramkumar, and Dillip K. Chand* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: Self-assembled binuclear coordination cages of general formula [Pd2(N−N)2(L)2](X)4, 1a/b−4a/b are prepared by the combination of N,N′-bis(m-pyridyl)urea, L, with a variety of cis-protected palladium(II) components, Pd(N− N)(X)2. The cis-protecting units “N−N” employed for the synthesis of 1−4 are ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen), respectively. The term “X” stands for nitrate and perchlorate for a and b, respectively. The assemblies are characterized by NMR and electrospray ionization mass spectrometry (ESI-MS) techniques, and in some cases (i.e., 1a, 2b, 3b, 4a, and 4b) the structures are confirmed by single crystal X-ray diffraction. The conformations of bound L in the crystal structures of all the Pd(II) complexes are found to be syn-syn. The influence of the presence and absence of π cloud at the cis-protecting units on the crystal packing has been studied in detail. In the packing of [Pd2(phen)2L2](NO3)4, 4a, one unit of [Pd2(phen)2L2]4+ is associated with two other units by π−π stacking interactions thus giving a one-dimensional growth as envisioned on the basis of a design principle. In the case of [Pd2(en)2(L)2](NO3)4, 1a, and [Pd2(tmeda)2(L)2](ClO4)4, 2b, such packing is not observed due to the absence of π-cloud at the strategic locations, instead notable H-bonding interactions are seen. However, [Pd2(bpy)2(L)2](ClO4)4, 3b, displays a π−π interactions using only two units of [Pd2(bpy)2(L)2]4+(ClO4)−.

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molecules where the value of “x” ranges from 1 to 4 and the general molecular formula is represented as [Pdx(N−N)x(L)x](monoanion)2x.1a,b,2 In this formula “N−N” stands for a chelating bidentate ligand, e.g., ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridine (bpy), and 1,10phenanthroline (phen), etc. The term L stands for a nonchelating bidentate ligand (i.e., a bis-monodentate ligand), frequently pyridine appended in nature. A single discrete structure is obtained in most of the designs, whereas in a few cases a dynamic equilibrium of more than one structures,3 for instance, M2L2 and M3L3,4 is observed. Design and synthesis of single discrete M2L2 complexes of the general formula [Pd2(N− N)2(L)2](monoanion)4 is well studied and reviewed.1a The relative positions of the two Pd(N)4 square planes confined in a M2L2 architecture is determined mostly by the spatial positions of the coordination vectors contributed by the nonchelating bidentate ligand component. In a majority of the structures, the two PdN4 square-planes are located almost in the same plane and are well separated from each other by two units of the bridging bidentate ligands as shown in Figure 1a.1a,5 However, only a few structures6 are reported where one of the square-

INTRODUCTION The construction of discrete and designed molecular architectures, using a selected metal component and an artistically chosen ligand system, through metal-driven selfassembly routes, has attracted intense interest in recent years.1 These self-assembled coordination cages have remarkable significance in crystal engineering because of their fascinating structural diversity. The creation of a tailor-made crystalline system where several numbers of the discrete structures are packed in a precise and targeted manner should be possible by cleverly coding the required information in the molecular framework. During the growth of crystals from a solution containing a discrete structure, the embedded information could be spontaneously decoded for the intended packing. Well-defined intermolecular interactions such as H-bonding and π-π stacking are the factors that govern the definite arrangement of the discrete molecules in the crystals. Such a predesigned packing could be termed as the self-assembly of self-assembly. Palladium(II) is one of the significant metal centers that is exploited for preparation of self-assembled coordination cage molecules by complexing suitable Pd(II) components with a variety of ligands.1a,b,2 Complexation of a cis-protected palladium(II) component with a nonchelating bidentate ligand is known to provide discrete MxLx type coordination cage © 2012 American Chemical Society

Received: July 30, 2012 Revised: October 16, 2012 Published: October 25, 2012 6012

dx.doi.org/10.1021/cg301085t | Cryst. Growth Des. 2012, 12, 6012−6022

Crystal Growth & Design

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Figure 3. Chemical structure of the ligand N,N′-bis(m-pyridyl)urea, L.

shown in Figure 3 is seemingly a suitable choice for the targeted packing indicated in our objective. This choice is made on the basis of a few factors including the reported crystal structure of the ligand. The ideal conformations of the free ligand L are synsyn, syn-anti, and anti-anti. Analysis of the reported crystal structures point out the presence of the syn-syn conformer in the absence of any solvent of crystallization and the anti-anti form in the presence of water of crystallization.7,8 No crystal structures are reported with the syn-anti form of the free ligand. There are only a few reports on the complexation study of the ligand L with transition metal ions.6d,7,8b,9 Analysis of all of these reported crystal structures made us consider that complexation of the ligand L with Pd(bpy)2+ should afford binuclear assembly belonging to M2L2 family where the complexed ligand moieties are anticipated to exist in syn-syn form. The nonbonded distance between the two Py(N) in the syn-syn conformer of the ligand L is 8.16 Å, and the coordination vectors are positioned in a slightly opened manner subtending an angle of approximately 23 degrees. Thus the resulting M2L2 framework would display the two πsurfaces due to the bpy moieties, one above the other. The dispositions of these two π-surfaces could facilitate intermolecular locking, in the solid state, as shown in Figures 2 and 5. We prepared eight discrete M2L2 cages (Figure 4) from the ligand L and a variety of cis-protected Pd(II) components having nitrate and perchlorate as counteranions (X). Complexation of the ligand L with Pd(bpy)(X)2 were carried out aiming at the intended packing in the solid state. However, the packing of the complexed cation present in [Pd2(bpy)2(L)2](ClO4)4 were not matched to the target; nevertheless the outcome was remarkable. Subsequently, another cis-protecting system with a somewhat larger π-surface than bpy was chosen. Thus, the M2L2 coordination cages constructed from complexation of L with Pd(phen)(X)2 were crystallized that resulted in success as the packing was found to be close to what was planned for (Figure 5). Four other analogous compounds were prepared by complexing L separately with non-π-surface containing metal components, i.e., Pd(en)(X)2 and Pd(tmeda)(X)2, for compar-

Figure 1. Diagram showing the relative positions of PdN4 square planes in the M2L2 self-assembly constructed from cis-protected Pd(II) and bidentate nonchelating ligands where the square planes are located (a) in same plane, and (b) in different planes but one above the other.

planes is located almost above the square-plane of the other as shown in Figure 1b, that resembles opened jaws. We considered new complexes of the type shown in Figure 1b to construct a special kind of packing in the solid-state. It was anticipated that the presence of a π-cloud in the cisprotecting unit of the opened jaws design should result in special intermolecular association through crystal packing as shown in Figure 2 where a given molecule is interlocked with two adjacent molecules by π-stacking. In this design the distance between the two π-surfaces in a M2L2 molecule is required to be around 10.2 Å, considering 3.4 Å as the ideal distance for π-stacking. Thus, judicial choice of the separation between the coordination sites and the relative direction of the coordination vectors in a ligand are the vital requirements for the packing of the M2L2 units in a manner envisioned in this work. Herein we report the complexation of N,N′-bis(m-pyridyl)urea (Figure 3) with a variety of cis-protected Pd(II) components (Figure 4) and successful demonstration of the targeted packing (Figures 2 and 5) through π-locking as realized in the crystal structure of [Pd2(phen)2(N,N′-bis(mpyridyl)urea)2](NO3)4. The structure/packing are analyzed and also compared with the structures obtained from few analogous crystals where the presence and absence of π-surfaces at the strategic locations were probed.

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RESULTS AND DISCUSSION Design of the Self-Assembly of Self-Assembly. The orientation of the pyridyl units and the distance between them in the urea spacered ligand7 N,N′-bis(m-pyridyl)urea, L, that is

Figure 2. A designed self-assembly of self-assembly where the M2L2 units are associated by intermolecular locking and packing by π-stacking in a linear sequence. 6013



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Figure 4. The self-assembled M2L2 complexes [Pd2(N−N)2(L)2](X)4, 1a/b−4a/b.

Figure 5. Intermolecular locking of [Pd2(phen)2(L)2]4+ units by π-stacking interactions. The hashed line between phen units represent π-stacking.

Figure 6. 500 MHz 1H NMR spectra in DMSO-d6 (TMS as external standard) for (i) ligand L; (ii) [Pd2(en)2(L)2](ClO4)4, 1b (iii) [Pd2(tmeda)2(L)2](ClO4)4, 2b; (iv) [Pd2(bpy)2(L)2](ClO4)4, 3b; and (v) [Pd2(phen)2(L)2](ClO4)4, 4b.

ison purposes. Since these resulting cages are handicapped with respect to the requisite π-surfaces at the strategic locations, the

crystal structures offered a dissimilar packing yet worth noting. Thus the influence and importance of the π-surface in the 6014



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Figure 7. Crystal structures of the complexed cations of 1a, 2b, 3b, and 4a. Views (a) across, and (b) along the Pd−Pd axis.

structures of the complexes 1a, 2b, 3b, 4a, and 4b also confirmed the binuclear architectures. More importantly, the packing in the solid state are found to be greatly influenced by the cis-protecting moieties crafted around the metal center. Crystal Structures. Single crystals were obtained for five out of the eight complexes prepared. The crystal structures of 1a, 2b, 3b and 4a are discussed in detail and that of 4b is provided in Supporting Information. The data obtained for 4b were not of good quality and hence complete refinement on 4b could not be done. However, the molecular structures of the complexed cations as well as the intermolecular locking comprehended in 4a and 4b are comparable. Thus, a preliminary comparison of the packing seen in 4a and 4b suggested that one can neglect the differences exhibited if any by the counteranions, i.e., nitrate and perchlorate respectively. Comparison of the crystal structures revealed a wide variety of intermolecular interactions upon varying the cis-protecting moiety crafted around the palladium(II) center. Two different views of the binuclear complexed cations of the crystal structures are shown in Figure 7. While the views across the Pd−Pd axis is depicted in Figure 7a, the views along the Pd−Pd axis is displayed in Figure 7b. The coordination geometry around the metal centers and the related bond lengths and bond angles in all the structures are well in the range of expectation. However, the relative positions of the two squareplanes in the binuclear complexes seem to depend on the nature of the cis-protecting group employed around palladium(II). The conformation of the ligand moieties in all the complexes is found to be syn-syn. The two Py-N atoms in a free ligand, in syn-syn form are separated by 8.136 Å. The Py(N) to Py(N) nonbonded distances in the two ligand strands in the complexes are 8.194/8.250 Å (for 1a), 8.063/8.078 Å (for 2b), 7.814/7.795 Å (for 3b), 8.157/8.222 Å (for 4a). Thus the order of the average distances between Py(N) to Py(N) is (8.222 Å) 1a > (8.189 Å) 4a > (8.136 Å) L > (8.070 Å) 2b ≫ (7.804 Å) 3b, which is also reflected in the bond angles around the urea moieties. The order of nonbonded distances between the metal centers is (9.349 Å) 1a > (9.164 Å) 4a > (8.817 Å) 2b > (8.174 Å) 3b, that is in line with above-mentioned order of Py(N) to Py(N) distances. Thus, upon complexation the overall geometries of the ligand moieties are slightly tempered but considerably so for 3b. In the case of 3b two units of the complexed cations are reasonably intercalated with each other by π−π interactions using the π-surfaces of the bpy moieties.

locking and packing was confirmed and the objective was accomplished. A further detail of the synthesis of the complexes and description of the crystal structures is described below. Synthesis and Characterization of the M2L2 SelfAssemblies. In a typical complexation reaction the ligand L was combined with an equimolar amount of a cis-protected palladium(II) component in acetonitrile−water(1:1) and stirred at room temperature. The clear solution so obtained afforded the corresponding binuclear complex upon evaporation at room temperature. Eight binuclear complexes (1a/b− 4a/b) were isolated by using four variants of the cis-protecting units (i.e., en, tmeda, bpy, and phen) and two different choices of counteranions (i.e., nitrate and perchlorate) as shown in Figure 4. Recently we have come across the synthesis of 1a but the crystal structure of the same is not reported.6d However, utilization of a phosphine based cis-protecting unit, i.e., 1,3bis(phenylphosphino) propane, in fact afforded a mixture of binuclear and trinuclear complexes in dynamic equilibrium where the binuclear compound was crystallized.6d The complexes were characterized by 1H NMR, 13C NMR, H−H COSY, HSQC, IR, electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray diffraction (XRD) methods. The 1H NMR spectra of the ligand and all the complexes have been recorded in DMSO-d6. A simple pattern in the 1H NMR spectra of each of the complexes indicated the formation of single, discrete compounds. All the signals with exception of Ha and He (Figure 6) could be assigned unambiguously. It was possible to distinguish the signal of Ha from He by using HSQC because one is connected to a carbon and the other to a nitrogen atom. The signals due to the protons Ha, Hb, Hc, and He of all the complexes (1a/b−4a/b) are downfield shifted with respect to that of the free ligand (Figure 6). A stacking diagram showing the 1H NMR spectra of the ligand L and the complexes 1b−4b is presented in Figure 6 for comparison. This downfield shift of the signals is attributed to the loss of electron density from the ligand L to the palladium(II) centers. In all the complexes Hd showed an upfield shift when compared with the signal of the ligand, probably due to some spatial location of the proton. The formation of the binuclear complexes of M 2 L 2 formulations was confirmed from ESI-MS data of the complexes. The mass spectra of 1a−4a show peaks at m/z = 442 for ([1a-2NO3]2+), 218 for ([2a-4NO3]4+), 1139 for ([3aNO3] +) and 250 for ([4a-4NO3]4+) respectively. The crystal 6015



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Table 1. Crystallographic Data and Structure Refinement Parameters (CCDC Nos. 888465−888467 and 890214) 1a empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) volume (Å)3 Z wavelength (Å) temperature (K) calculated density (g/cm3) absorption coefficient (mm−1) F(000) crystal dimensions (mm)3 θ min/max (deg) reflections collected/unique data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I) R indices (all data) largest diff peak and hole·Â

2b

3b

4a

C34H52N16O18Pd2 1185.72 triclinic P1̅ 11.9558(13) 13.9593(15) 4.9742(16) 75.258(4) 82.855(4) 88.532(4) 2398.0(4) 2 0.71073 298(2) 1.642 0.838

C40H71Cl3N14O15Pd2 1307.26 triclinic P1̅ 13.275(9) 15.216(16) 15.513(9) 110.66(3) 104.68(2) 101.53(3) 2688(4) 2 0.71073 293(2) 1.393 0.927

C42H36Cl4N12O19Pd2 1367.43 triclinic P1̅ 13.7721(11) 14.3283(11) 15.2061(11) 72.723(3) 78.255(3) 76.979(3) 2761.3(4) 2 0.71073 298(2) 1.645 0.926

C96H76N30O19Pd4 2379.47 triclinic P1̅ 12.1745(14) 15.9854(19) 16.9763(18) 66.886(3) 74.165(4) 69.919(3) 2817.2(6) 1 0.71073 173(2) 1.349 0.699

1208 0.25 × 0.20 × 0.18 1.51/28.36 29909/10335 [R(int) = 0.0479] 10335/7/601 1.025 R1 = 0.0493, wR2 = 0.1326 R1 = 0.0850, wR2 = 0.1557 0.992 and −0.859

1144 0.25 × 0.20 × 0.15 1.50/29.07 15377/8339 [R(int) = 0.0681]

1368 0.35 × 0.20 × 0.15 1.51/29.91 38587/13923 [R(int) = 0.0507] 13923/0/712 1.044 R1 = 0.0730, wR2 = 0.2204 R1 = 0.1292, wR2 = 0.2547 2.186 and −1.231

1146 0.25 × 0.20 × 0.15 1.32/27.24 19724/7255 [R(int) = 0.0807]

8339/8/670 0.860 R1 = 0.0781, wR2 = 0.2221 R1 = 0.1494, wR2 = 0.2869 1.377 and −1.270

7255/0/639 1.013 R1 = 0.1212, wR2 = 0.3178 R1 = 0.2046, wR2 = 0.3932 3.018 and −0.944

Figure 8. Crystal structure of 1a showing the views of (a) the H-bonding interactions around a complexed cation (b) the fit-in of two molecules where one unit uses the cavity of the other and (c) further growth showing the packing of fit-in units.

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Presumably this stacking along with binding of one anion in between the metal centers is responsible for the compression observed in the molecular structure along the Pd−Pd axis. The crystal structures of 2b and 3b, when viewed along the Pd−Pd axis, clearly display the oxygen atoms of the ligand tilted away from the plane of the concerned ligand. This tilting can be related to the H-bonding interactions of one molecule of the complex with a neighboring one, directly (as in 3b) or via water molecules (as in 2b). More detail on the intermolecular interactions observed in the crystal structures is discussed below and the crystallographic data are collected in Table 1. Crystal Structure of [Pd2(en)2(L)2](NO3)4, 1a. Single crystals suitable for X-ray diffraction studies were obtained by slow diffusion of dioxane into a solution of 1a in water. The complex crystallized in triclinic space group P1̅. The structure is composed of the complexed cation [Pd2(en)2L2]4+, four nitrate ions and two dioxane molecules. Several H-bonding interactions are present in the structure. As many as eight nitrates and one dioxane molecule are located and H-bonded around a complexed cation in the extended structure. The urea protons of the two ligand moieties are bonded to nitrate ions, one each. The amine protons present in the en moieties are also bonded to six nitrates and one dioxane molecule (Figure 8a). All the nitrates are further H-bonded utilizing the urea protons and en protons of neighboring units of the complex thus providing an extended network. Since two en units present in the complex are pointed away from each other, there is enough space in between them. Interestingly, two molecules are found to fit-in one another to fill the space in the cavity accessible between the cis-protected metal centers of a given molecule (Figure 8b,c). [Pd2(tmeda)2(L)2](ClO4)4, 2b. Single crystals suitable for Xray diffraction studies were obtained by slow evaporation of a solution of 2b in acetonitrile−water. The complex was crystallized in triclinic space group P1̅. The structure contains one [Pd2(tmeda)2L2]4+ cation, three perchlorate ions, two water molecules and one uncoordinated tmeda unit. One of the perchlorate ions could not be located in the structure. The origin of uncoordinated tmeda, located in the crystal lattice (see Supporting Information Figures S46 and S47), is attributed to the dissociation of the complex during the crystallization process. 1H NMR of a batch of sample, obtained by slow evaporation to grow crystals, shows some amount of free ligand L and free tmeda along with the parent complex. The urea protons of one of the two strands of the ligand are H-bonded to a perchlorate ion, and the urea oxygen is found to be free and not tilted away from the ligand. The perchlorate is not further H-bonded to any other groups. In the other strand of the ligands, the urea oxygen is remarkably tilted away from the complex, and the urea moiety thus exhibits a different fashion of binding. Here the urea protons are H-bonded to one water molecule which is further H-bonded to the tilted oxygen of the urea moiety of a neighboring unit of the complex. The neighboring unit also behaved similarly and returns the same favor, and hence a dimer of the complex is created. Each of these water molecules is further H-bonded to a water molecule followed by a perchlorate ion. Such a H-bonded dimer is shown in Figure 9. Interestingly, the two terminal perchlorates, bonded to water, are each encapsulated in the cavities of adjacent neighboring dimers. The zone where a perchlorate is encapsulated in the cavity is the one that exists between the two cis-protecting tmeda units. The packing thus formed in onedimension is shown in Figure 10. The encapsulated perchlorates, as shown in Figure 11, are held in the cavity

Figure 9. Crystal structure of 2b showing a H-bonded dimer composed of two units of the complexed cations, four water molecules and four perchlorates.

between the metal centers probably by short contact with several H-atoms delineated in the cavity, as well as by electrostatic interactions with Pd−O distances of 3.315 and 3.397 Å. The steric hindrance caused due to the methyl groups of tmeda apparently prevents the locking or fit-in between two units of the complex; rather a perchlorate ion is accommodated in the available space. Such encapsulation of perchlorate depends on several factors such as shape, size and binding modes of the anion inside the host molecule.10 [Pd2(bpy)2(L)2](ClO4)4, 3b. Single crystals suitable for X-ray diffraction studies were obtained by slow evaporation of a solution of 3b in acetonitrile−water. The complex crystallized in triclinic space group P1̅ . The structure contains [Pd2(bpy)2L2]4+ cation and four perchlorate ions. One of the perchlorates is present inside the cavity between the metal centers and other three are outside the cavity. The urea protons of one of the two strands of the ligand are H-bonded to a perchlorate ion and the urea oxygen is remarkably tilted away from the ligand toward the cavity pointing at the trenchlike zone between the ligands. The bonded perchlorate is not further H-bonded to any other groups. The tilted oxygen is involved in H-bonding to facilitate the dimerization of two identical units of the complex. The other strand of the ligand behaved somewhat differently in that the urea protons participate in H-bonding for dimerization, whereas the oxygen atom present in this urea moiety is not much tilted away from the ligand and not H-bonded with any groups. Two different views of the H-bonded dimer are shown in Figure 12. Interestingly, one H-bonded dimeric unit is found to be associated with two more dimeric units via π-stacking of the bpy moieties in an intercalating manner as shown in Figure 13. One of the two pyridine rings of a bpy moiety is involved in the stacking, though not completely, thus the π-surfaces are not fully utilized. As seen in previous reports, two bipyridine units usually interact in an offset or parallel displaced mode.6f,11 Nevertheless, one-dimensional growth is observed and attributed to π−π stacking interactions. The plane of a bpy rings is separated from the adjoining stacked bpy ring(s) by approximately 4 Å with the consequence of a severe 6017



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Figure 10. Interaction of a H-bonded dimer of 2b with two other units of the dimers in the packing.

compression within the structure. Since this is not the ideal distance for optimum interactions, the compression in the molecule is also assisted by the encapsulation of a perchlorate ion between the metal centers (Figures 13 and 14). A perchlorate ion is held in the cavity by electrostatic interactions with Pd−O distances of 3.092 and 3.007 Å. Thus, the overall packing is attributed to H-bonding between urea moieties, π−π interactions of bpy moieties and electrostatic interactions due to metal−perchlorate−metal contacts. [Pd2(phen)2(L)2](NO3)4] 4a. Single crystals suitable for Xray diffraction studies were obtained by slow diffusion of dioxane into a solution of 4a in acetonitrile−water. The complex crystallized in triclinic space group P1.̅ The structure contains [Pd2(phen)2L2]4+cation and two nitrate ions. Two other nitrates could not be located in the structure. Both the strands of the ligands are found to be almost identical. The urea protons of one ligand strand are H-bonded to one nitrate and the other strand to another nitrate ion (Figure 15). The oxygen of urea moieties possesses no H-bonding interactions. There is no further H-bonded network in the structure. However, the

Figure 11. Crystal structure showing the closest approach of two units of 2b and binding of one perchlorate ion per cavity.

Figure 12. Crystal structure of 3b showing different orientations of a H-bonded dimer composed from two units of the complexed cations and two perchlorates. 6018



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Figure 13. Interaction of H-bonded dimers of 3b in the packing by π−π stacking; also shown the encapsulated perchlorate ion one per cavity.

neighboring molecules by locking that continues to give a one-dimensional growth (Figures 16 and 17). Complexes are

Figure 14. Crystal structure showing the closest approach of two units of 3b and binding of a perchlorate ion per cavity.

Figure 16. Crystal structure showing the closest approach of the molecules in 4a by incorporating segment of two neighboring units per cavity.

packed in the crystal lattice in a perfectly stacking manner due to π−π stacking interactions. The π−π stacking becomes more favorable in the case of phenanthroline units because of its extended aromatic ring systems.11a Thus there is a near face to face π-stacked arrangement of phenanthroline units. The shortest distance between the two stacked phen units is found to be 3.79 Å. The Pd−Pd axes are not exactly collinear and shifted slightly toward one side and repeatedly as the molecules are associated (Figure 17b). Thus the allied units are not exactly one above the other in the crystal array. No anions are located in the cavity indicating strong preference of π−π stacking. [Pd2(phen)2(L)2](ClO4)4, 4b. Single crystals were obtained by slow evaporation of a solution of 4b in acetonitrile−water. The complex crystallized in monoclinic space group P2(1)/n. Two units of complexes are stacked due to π−π interactions in one unit cell itself. Six out of eight perchlorate anions and few other atoms were located all outside the cavity. Some of the urea protons of the ligand are hydrogen bonded to perchlorate anions directly or via water molecules. Since the quality of the crystals was not good we restrict detailed discussion on this structure. More importantly the focused intermolecular π−π

Figure 15. Crystal structure of 4a showing the H-bonding interactions around a molecule.

most important feature in this crystal structure is the intermolecular interactions through π-stacking to provide our intended packing. Each molecule is stacked with two 6019



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Figure 17. (a, b) Crystal structure showing two different views of the intermolecular locking and packing.

broad pattern at 1371 cm−1 along with a shoulder peak at 1426 cm−1 attributable to the N−O stretching frequencies. Presence of a shoulder peak indicates that all the three O of a given coordinated NO3− are possibly not in same environments. Further, the broad nature of the band does not allow precise interpretation of the number and variety of bonding modes around the four nitrate ions present in a molecule. In 2a, the N−O stretching frequency appears at 1379 cm−1 with a shoulder at 1420 cm−1. So the mode of binding of anions in the complex 2a may be comparable to 1a, at least with respect to bonding with urea protons. In 3a, N−O stretching band is very broad with signals at 1428, 1375, and 1340 cm−1. This suggests that the symmetry of nitrate anion is lost upon coordination and the anions are bonded in a somewhat different fashion compared to 1a and 2a. Possibility of nonbonded nitrates cannot be ruled out due to the very broad nature of the band. The nature of the band for 4a is comparable to 3a, and hence overall situations of the anions of 3a and 4a may be considered comparable. Occurrence of a very intense and a broad band around 1110 cm−1 is a characteristic feature of a free perchlorate ion. The Cl−O stretching frequency around this region is analyzed for the complexes 1b−4b. Broad Cl−O stretching appears around 1096, 1087, 1087, and 1078 cm−1 for 1b to 4b respectively. The broad bands indicate that the perchlorate ions are bonded in a variety of modes.

stacking present in 4b is quite similar to that of 4a. Notably perchlorate did not obstruct the stacking. Crystallographic data of 4b has been given in Supporting Information. Influence of cis-Protecting Moiety on the Packing in 1a, 2b, 3b and 4a. Most interesting observation has been the influence of the presence and absence of π-cloud on the intermolecular interactions in the solid state. In the case of 4a one unit of the complexed cation, i.e., [Pd2(phen)2 (L)2]4+, is found to be associated with two other units by π−π stacking interactions thus giving a one-dimensional growth like a rod. In the case of [Pd2(en)2(L)2]4+ of 1a no such interaction was seen in the packing due to the absence of π-cloud at the strategic locations. The Pd−Pd distances of 4a and 1a are comparable. However, to fill the available space in the cavity two units of the complexed cation of 1a are associated with each other in a fit-in manner, without tempering the overall framework. Such a fit-in is not observed in [Pd2(tmeda)2(L)2]4+ of 2b due to steric hindrance by the tmeda group. Rather a perchlorate anion is encapsulated in the cavity where the Pd−Pd distance is somewhat shortened indicating electrostatic interactions. Examination of the packing seen in [Pd2(bpy)2(L)2]4+ of 3b showed π−π stacking interactions between only two units of the complexed cation where one unit is intercalated with another unit causing a severe compression unlike the structure of 2b. This compression is also associated with the binding of a perchlorate per cavity, probably in a synergetic manner. IR Spectra. The N−O stretching frequency of a free nitrate anion appears around 1405 cm−1. Broad bands are observed around 1371, 1379, 1375, and 1375 cm−1 for the complexes 1a, 2a, 3a and 4a, respectively. The compound 1a displayed a

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CONCLUSION The nature of cis-protecting group around Pd(II) can have a significant influence on the growth of the crystal from a solution of self-assembled coordination cage molecules 6020



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ESI-MS: m/z = 442 corresponding to [1a−2NO3]2+. Single crystals were obtained by slow diffusion of dioxane into solution of 1a in water. FT-IR (cm−1): 3201, 3077, 1715, 1586, 1556, 1482, 1426, 1371, 1278, 1212, 1135, 1057, 904, 812, 699. 1b: Yield: (80%). M.P.: 220 °C. 1H NMR (500 MHz, DMSO-d6 external TMS/CDCl3) δ: 10.33 (s, 4H, a), 10.02 (s, 4H, e), 9.13(t, J = 3.5 Hz, 4H, b), 8.13 (d, J = 1 Hz, 8H, c and d), 6.05 (s, 8H, f), 3.22 (s, 8H, g).13C NMR (125 MHz, DMSO-d6) δ: 151.56, 145.80, 140.60, 137.92, 129.30, 126.26, 46.68. FT-IR (cm−1): 3242, 3073, 1715, 1593, 1549, 1483, 1433, 1382, 1281, 1213, 1096, 808, 697, 627. 2a: Yield: (93%). M.P.: 200 °C. 1H NMR (500 MHz, DMSO-d6 external TMS/CDCl3) δ:10.91 (s, 4H,e), 10.56 (s, 4H, a), 9.42 (d, J = 5.5 Hz, 4H, b), 8.14−8.12 (m, 4H, c), 8.03 (d, J = 8.5 Hz, 4H, d), 3.67−3.54 (m, 8H, h and i), 3.17−3.04 (m, 24H, j and k). 13C NMR (125 MHz, DMSO-d6) δ: 151.07, 144.86, 139.67, 138.70, 128.79, 126.89, 62.73, 62.02, 50.40, 50.11. ESI-MS: m/z = 218 corresponding to [2a−4NO3]4+. FT-IR (cm−1): 3234, 3060, 1709, 1546, 1480, 1379, 1275, 1207, 806, 702, 621,537. 2b: Yield: (72%). M.P.: 250 °C (decomposition). 1H NMR (500 MHz, DMSO-d6 external TMS/CDCl3) δ: 10.48 (d, J = 2.5 Hz, 4H, a), 9.92 (s, 4H, e), 9.42 (d, J = 5.6 Hz, 4H, b), 8.15−8.12 (m, 4H, d), 8.06 (d, J = 8.5 Hz, 4H, c), 3.67−3.52 (m, 8H, h and i), 3.16−3.04 (m, 24H, j and k). 13C NMR (125 MHz, DMSO-d6) δ: 150.90, 145.09, 139.40, 138.37, 129.31, 126.95, 62.09, 50.41, 50.22. Single crystals were obtained by slow evaporation of solution of 2b in acetonitrile− water. FT-IR (cm−1): 3349, 3084, 2926, 1715,1665, 1613, 1587, 1546, 1482, 1429, 1336, 1278, 1212, 1087, 1044, 1007, 954, 809, 705, 624. 3a: Yield: (85%). M.P.: 242 °C. 1H NMR (500 MHz, DMSO-d6, external TMS/CDCl3) δ: 10.74 (s, 4H, a), 10.42 (s, 4H, e), 9.44 (s, 4H, b), 9.28 (d, J = 8 Hz, 4H, o), 9.01 (t, J = 7.6 Hz, 4H, n), 8.31− 8.22 (m, 12H, d, c and m), 8.05 (d, J = 4.76 Hz, 4H, l). 13C NMR (125 MHz, DMSO-d6) δ: 155.77, 151.48, 150.01, 145.02, 142.66, 139.9, 138.84, 129.92, 128.61, 127.99, 124.52. ESI-MS: m/z = 1139 correspondingto [3a−NO3]+. FT-IR (cm−1): 3247, 3071, 1712, 1591, 1555, 1480, 1428, 1375, 1340, 1275, 1214, 1114, 1031, 777, 700. 3b: Yield: (90%). M.P: 223 °C. 1H NMR (500 MHz, DMSO-d6, external TMS/CDCl3) δ:10.72 (s, 4H, a), 10.18 (s, 4H, e), 9.4 (s, 4H, b), 9.22 (d, J = 7 Hz, 4H, o), 8.96 (bs, 4H, n), 8.25−8.19 (m, 12H, d, c and m), 8.01 (d, J = 5.5 Hz, 4H, l). 13C NMR (125 MHz, DMSO-d6) δ: 155.71, 151.43, 149.95, 144.96, 142.65, 139.90, 138.75, 130.04, 128.58, 127.94, 124.50. Single crystals were obtained by slow evaporation of solution of 3b in acetonitrile−water. FT-IR (cm−1): 3312, 3086, 1712, 1672, 1589, 1552, 1434, 1281, 1216, 1099, 807,772, 697, 625, 419. 4a: Yield: (88%). M.P.: 320 °C (decomposition). 1HNMR (500 MHz, DMSO-d6 external TMS/CDCl3) δ:10.8 (d, J = 1.5 Hz, 4H, a), 10.41(s, 4H, e), 9.58−9.56 (m, 4H, r), 9.51−9.50 (m, 4H, b), 8.88 (s, 4H, s), 8.53−8.50 (d, J = 5.1 Hz, 4H, q), 8.44−8.32 (m, 4H, p), 8.32− 8.30 (m, 8H, c and d). 13C NMR (125 MHz, DMSO-d6) δ: 151.57, 151.00, 145.96, 145.38, 141.44, 140.25, 138.91, 130.50, 130.10, 128.05, 126.82. ESI-MS: m/z = 250 corresponding to [4a−4NO3]4+. Single crystals were obtained by slow diffusion of dioxane into solution of 4a in acetonitrile−water. FT-IR (cm−1): 3271, 3234, 3058, 1710, 1598, 1544, 1481, 1415, 1375, 1283, 1207, 838, 711. 4b: Yield: (88%). M.P.: 280 °C (decomposition). 1HNMR (500 MHz, DMSO-d6 external TMS/CDCl3) δ: 10.79 (s, 4H, e), 10.28 (s, 4H, a), 9.61−9.59 (m, 4H, r), 9.49−9.48 (m, 4H, b), 8.91 (s, 4H, s), 8.55−8.52 (m, 4H, q), 8.46−8.45 (m, 4H, p), 8.34−8.33 (m, 8H, c and d). 13C NMR (125 MHz, DMSO-d6) δ: 151.30, 150.98, 145.81, 145.31, 141.47, 140.15, 138.83, 130.48, 130.16, 127.99, 126.85. Single crystals were obtained by slow evaporation of solution of 4b in acetonitrile−water. FT-IR (cm−1): 3274, 3062, 1710, 1663, 1600, 1585, 1542, 1519, 1480, 1426, 1328, 1278, 1205, 1139, 1108, 1078, 840, 806, 709, 623.

constructed from suitable ligand and cis-protected Pd(II) components. Particularly the packing can definitely influence the overall structure of the complexes in the solid state. The influence of solvent, counteranions, guest molecules, and variation of substituent on the ligand component as well as cis-protecting moieties may provide a rich chemistry and open a new avenue.

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EXPERIMENTAL SECTION

General. PdCl2 and AgClO4 were obtained from Aldrich, whereas AgNO3, nicotinic acid, 3-aminopyridine and all the common solvents were obtained from Spectrochem, India. The deuterated solvent (DMSO-d6) was obtained from Aldrich and Cambridge Isotope Laboratories. 1H and 13C NMR spectral data were obtained from a Bruker 500 MHz FT NMR spectrometer using external TMS in CDCl3 and residual solvent as reference, respectively. The mass spectra were obtained from a Micromass Q-TOF Mass Spectrometer. The crystal structures were determined using a Bruker X8 Kappa XRD instrument. The FT-IR spectra of the samples were recorded as KBr pellets using a JASCO FT/IR-4100 spectrometer. The samples were finely powdered with oven-dried spectroscopic-grade KBr and pressed into pellets. The ligand L was synthesized by slight modification of a reported procedure.7 The cis-protected Pd(II) components were obtained following well-known processes.12 Synthesis of the Ligand L. Nicotinic acid (0.732 g, 5.95 mmol) was added to 10 mL of thionyl chloride and the mixture was refluxed for 30 min. The resulting clear solution was reduced under a vacuum to yield white, shiny crystals of nicotinoyl chloride hydrochloride. To the suspension of nicotinoyl chloride hydrochloride in 10 mL of dry DCM, sodium azide (0.387 g, 5.95 mmol) was added and stirred for 8 h. The suspension so obtained was then washed with saturated sodium bicarbonate solution followed by water and extracted with DCM. The organic layer was dried over anhydrous Na2SO4. Evaporation of the DCM under a vacuum yielded the nicotinoyl azide intermediate as a white solid (0.498 g, 3.36 mmol), which was dissolved in 15 mL of toluene and refluxed for about 2.5 h to generate the isocyanate intermediate. To the yellow color solution so obtained, 3-aminopyridine (0.316 g, 3.36 mmol) was added and the mixture was refluxed for 2 h. The product was precipitated as a white solid which was collected by filtration, washed with DCM, and dried. Yield: (75%). M.P.: 225 °C. 1H NMR (500 MHz, DMSO-d6 external TMS/CDCl3) δ: 9.53 (s, 2H, e), 9.15 (d, J = 2.5 Hz, 2H, a), 8.75 (dd, J1 = 4.5 Hz, J2 = 1.5 Hz, 2H, b), 8.49 (m, 2H, d), 7.86 (m, 2H, c). 13C NMR (125 MHz, DMSO-d6) δ: 152.54, 143.01, 140.14, 136.04, 125.28, 123.49. Synthesis of the Complexes. A typical method for the synthesis of 1a and 1b is described below. The other complexes, 2a/2b, 3a/3b and 4a/4b, were prepared following the same procedure by taking appropriate cis-protected Pd(II) components. [Pd2(en)2(L)2](NO3)4,1a. To a solution of Pd(en)(NO3)2 (8.7 mg, 0.03 mmol) in 1:1 acetonitrile−water (3.0 mL), ligand L (6.4 mg, 0.03 mmol) was added and the mixture was stirred at room temperature for 48 h. The resulting solution was evaporated by standing at room temperature, washed with acetone, and dried under a vacuum to obtain the complex 1a as a yellow solid. [Pd2(en)2 (L)2](ClO4)4, 1b. To a solution of Pd(en)Cl2 (9.5 mg, 0.04 mmol) in 1:1 acetonitrile−water (4 mL), silver perchlorate was added (16.6 mg, 0 0.08 mmol) which led to immediate precipitation of AgCl. The resulting mixture was heated for 30 min and centrifuged. The yellow color solution of Pd(en)(ClO4)2 so formed was separated by filtration. To the clear yellow solution, ligand L (8.6 mg, 0.04 mmol) was added and the mixture was stirred at room temperature for about 48 h. The resulting solution was evaporated, washed with acetone, and dried under a vacuum to obtain the complex 1b as a white solid. Analytical Data of the Complexes. 1a: Yield: (90%). M.P.: 210 °C. 1H NMR (500 MHz, DMSO-d6 external TMS/CDCl3) δ: 10.35 (s, 4H, a), 10.15 (s, 4H, e), 9.15 (d, J = 4.5 Hz, 4H, b), 8.11 (d, J = 5.5 Hz, 8H, c and d), 6.13 (s, 8H, f), 3.23 (s, 8H, g). 13C NMR (125 MHz, DMSO-d6) δ: 151.48, 145.71, 140.53, 137.92, 129.09, 126.18, 46.63. 6021



Article

Single Crystal X-ray Diffraction. X-ray data collection was performed with a Bruker AXS Kappa Apex II CCD diffractometer equipped with graphite monochromated Mo(Kα) (λ = 0.7107 Ǻ ) radiation. Crystal fixed at the tip of the glass fiber was mounted on the Goniometer head and was optically centered. The automatic cell determination routine, with 32 frames at three different orientations of the detector was employed to collect reflections and the program APEX2-SAINT (Bruker, 2004) was used for finding the unit cell parameters. Four-fold redundancy per reflection was utilized for achieving good absorption correction using a multiscan procedure. Besides absorption, Lorentz polarization and decay correction were applied during data reduction. The program SADABS was used for absorption correction using the multiscan procedure. The structures were solved by direct methods using SHELXL-97 (Sheldrick, 1997)13 and refined by full-matrix least-squares techniques using SIR92 (WinGX) computer program. All hydrogen atoms were fixed at chemically meaningful positions and riding model refinement was applied. At the final stage of refinement, 4a and 4b were subjected to SQUEEZE calculations. Molecular graphics were generated using Mercury programs. The crystal data with refinement details are summarized in Table 1.

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ASSOCIATED CONTENT

S Supporting Information *

1 H NMR,13C NMR, H−H COSY, HSQC and IR spectra of 1a/ b−4a/b, ESI-MS spectra of 1a−4a, stacked 1H NMR spectra for1a/1b, 2a/2b, 3a/3b and 4a/4b, crystal packing diagrams of 4b, structural refinement table for 4b and X-ray crystallographic files for complexes 1a, 2b, 3b, 4a, and 4b in CIF format.This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91-4422574224, Fax: +91-4422574202, E-mail: dillip@ iitm.ac.in. Notes

The authors declare no competing financial interest. This article is dedicated to Prof. P. K. Bharadwaj on the occasion of his 60th birthday.

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ACKNOWLEDGMENTS The authors thank the Department of Science and Technology, Government of India (Project No. Sr/S1/IC-28/2009) for financial support. The NMR and ESI-MS facility at the Department of Chemistry, IIT Madras funded by DST, India is gratefully acknowledged. We profoundly acknowledge the single crystal X-ray Diffractometer facility funded by IIT Madras. We thank Prof. M. N. S. Rao for helping us in interpreting IR spectra.

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REFERENCES

(1) (a) Debata, N. B.; Tripathy, D.; Chand, D. K. Coord. Chem. Rev. 2012, 256, 1831−1945. (b) Northrop, B. H.; Zheng, Y.-R.; Chi, K.-W.; Stang, P. J. Acc. Chem. Res. 2009, 42, 1554−1563. (c) Teo, P.; Andy Hor, T. S. Coord. Chem. Rev. 2011, 255, 273−289. (d) De, S.; Mahata, K.; Schmittel, M. Chem. Soc. Rev. 2010, 39, 1555−1575. (e) Crassous, J.; Lescop, C.; Réau, R. In Phosphorus Compounds, Catalysis by Metal Complexes; Peruzzini, M.; Gonsalvi, L., Eds.; Springer: Dordrecht, 2011; Vol. 37, Chapter 11, pp 343−373. (f) Schroder, T.; Sahu, S. N.; Jochen, M. Top. Curr. Chem. 2012, 319, 99−124. (2) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853−908. (b) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022−2043. (c) Chakrabarty, R.; Mukherjee, P. S.; Stang, P. J. Chem. Rev. 2011, 111, 6810−6918. (d) Fujita, M. Chem. Soc. Rev. 6022


Complexes on Packing - American Chemical Society

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