CRYSTAL GROWTH & DESIGN
Crystal Engineering of “Porphyrin Sieves” Based on Coordination Polymers of Pd- and Pt-tetra(4-carboxyphenyl)porphyrin
2003 VOL. 3, NO. 5 855-863
Michaela Shmilovits, Yael Diskin-Posner, Mikki Vinodu, and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, 69978 Ramat Aviv, Tel Aviv, Israel Received May 4, 2003
ABSTRACT: Targeted solid-state synthesis of supramolecular porphyrin networks with palladium- and platinumtetra(4-carboxyphenyl)porphyrins as building blocks, in different reaction environments, afforded a series of new extended coordination polymers sustained by sodium, potassium, and copper metal ion cluster auxiliaries. Formulation of open channel-type architectures in these materials has been confirmed by X-ray crystallography at low temperature. Open, 0.50-0.65 nm wide, channels were found to propagate throughout the corresponding crystals. These channels are accessible to other guest/solvent molecules for inclusion in the crystalline lattice. The structural features of the analyzed materials resemble those of common molecular sieve solids, and represent a successful realization of the crystal engineering concepts of porphyrin-based supramolecular design evaluated in this laboratory. Introduction The area of porphyrin supramolecular chemistry has seen tremendous activity in recent years, leading to the design of a series of novel bulk as well as nanoscale systems with interesting properties and a wide range of applications in chemistry and biochemistry.1-5 In this context, we have introduced earlier the meso-tetra(4carboxyphenyl)porphyrin (TCPP) as a uniquely versatile building block for the construction of molecular sievetype architectures, which may bear structural as well as functional resemblance to the inorganic zeolites.6,7 The most attractive algorithms for the assembly of the TCPP units into rigid high-order arrays involve their tessellation to each other by cooperative intermolecular hydrogen bonding,6 and by coordination through external metal ion templates.7 The latter can readily interact with, and bridge firmly between, the carboxylic functions of adjacent building blocks. This is associated with deprotonation of the TCPP species during the reaction to account for charge balance, without the need to incorporate other counterions, thus allowing for the formation of open architectures. In most cases, the build-up of the crystalline lattice consists of flat layers of the interconnected porphyrin frameworks. Apart from the ion-pairing forces, the layered organization is sustained along the vertical dimension either by characteristic van der Waals stacking,6,8 or by additional ligands that can bridge axially between the porphyrin metal centers.7 Assembly of the layered multiporphyrin networks is facilitated by the square-planar shape and functionality of the individual units. Metalation of the porphyrin core generally serves to enhance the structural rigidity and to modify the axial functionality of the building blocks in the various formulations. The attractive zeolite properties (reversible absorption and desorption of water and organic solvents) of such TCPPbased coordination polymers, in which cobalt ions are inserted within and between the porphyrin species, has been well demonstrated in a most recent report.9 * E-mail:
[email protected].
Scheme 1a
a Porphyrin building block and the reaction reagents used in this study.
As part of our continuing effort to design porous porphyrin-based crystalline lattices, we report here a series of new open network coordination polymers involving Pd-TCPP and Pt-TCPP building blocks interlinked by external potassium and copper (as well as sodium) metal ion cluster auxiliaries (Scheme 1). The palladium and platinum ions inserted into the porphyrin core reveal high propensity for a square-planar coordination environment, and thus axial coordination to the metalloporphyrins should not occur (as opposed to the extensive axial coordination features of the most commonly investigated zinc and manganese metalloporphyrins). Correspondingly, a tight stacking of overlapping two-dimensional layers, associated with the formation of extended interporphyrin channels running perpendicular to them, has been anticipated. This is consistent with earlier observations that an offsetstacked layered arrangement of the tetraarylporphyrin species at a narrow distance range of 4-5 Å is a fundamental property of the porphyrin-porphyrin interaction required to optimize van der Waals stabilization in solids.8 Our previous investigations have shown that the interporphyrin voids in such metal-tessellated layers are 5-6 Å wide.6b Since the edge-length of the nearly square TCPP fragment is about 15 Å, interpenetration of the network arrays is unlikely in such cases, thus facilitating the formation of channel-type architectures.
10.1021/cg034071w CCC: $25.00 © 2003 American Chemical Society Published on Web 07/08/2003
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The compounds described below were obtained by solvothermal syntheses in diverse experimental conditions. Modification of the latter affected the nature of the evolving supramolecular architectures. To our knowledge, structural determinations of palladium and platinum metalloporphyrins have not been reported before. The polymeric assemblies referred to in this work include networks of Pd-TCPP intercoordinated by hydrated potassium (1) and copper-sodium ion clusters (3), and networks of Pt-TCPP intercoordinated by clusters of K+ (2), Cu2+(-Na+) (4), and Cu(NH3)62+ (5) species. We focus here on their structural features, as revealed by careful crystallographic investigations; the molecular-sieving properties will be evaluated in the future when adequate amounts (on more than a milligram scale) will become available.
solubilizing agents. The yields of the desired crystalline products were generally rather low ( 2σ no. refined parameters R1 (I > 2σ)a R1 (all data)a wR2 (all data)a |∆F|max e Å-3
C65.5H43.5Cu1.5N7.5Na1.5O9Pd 1315.77 triclinic P1 h 12.8740(2) 13.0790(2) 16.4710(4) 86.193(1) 85.954(1) 80.815(1) 2726.7(1) 2 0.99
1383.35 monoclinic C2/m 16.1940(8) 35.1320(13) 6.6930(3) 90.0 106.638(2) 90.0 3648.4(3) 2 2.55
110 1.473 55.7 6512
110 1.579 56.9 6631
110 1.603 55.9 12739
110 1.689 56.5 12901
110 1.259 55.8 4179
5510
5833
9058
9879
3209
331
331
778
786
186
0.036 (0.080) 0.045 (0.092) 0.094 (0.261) 0.75
0.033 (0.067) 0.040 (0.075) 0.082 (0.219) 1.55
0.053 0.091 0.119 0.92
0.052 0.080 0.116 1.80
0.058 0.084 0.158 1.73
a
The R-factor values in parentheses refer to convergence parameters before application of the “Squeeze” procedure.14
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compound 3 reactants mixture: Pd-TCPP, Cu(OAc)2‚xH2O (dissolved in MeOH), DMF, pyridine crystalline product: (Pd-TCPP)4-‚[Cu2+‚11/2Na+]‚ 1
/2[Cu+(C6H5N)3]‚2C6H5N‚H2O
compound 4 reactants mixture: Pt-TCPP, Cu(OAc)2‚xH2O (dissolved in MeOH), Et3N, pyridine crystalline product: (Pt-TCPP)4-‚[Cu2+‚11/2Na+]‚ 1
/2[Cu+(C6H5N) 3]‚2C6H5N‚H2O
compound 5 reactants mixture: Pt-TCPP, Cu(OAc)2‚xH2O, NH4OH(aq), pyridine crystalline product: (Pt-TCPP)4-‚2[Cu(NH3)62+]‚4H2O Crystallography. The diffraction measurements were carried out on a Nonius KappaCCD diffractometer, using graphite monochromated MoKR radiation (λ ) 0.7107 Å). The crystalline samples of the analyzed compounds were covered with a thin layer of light oil and freeze-cooled to ca. 110 K to minimize solvent escape, structural disorder, and thermal motion effects, and increase the precision of the results. The crystal and experimental data for all the compounds are summarized in Table 1. These structures were solved by direct (SIR-92/97)11 and Patterson methods (DIRDIF-96),12 and refined by fullmatrix least-squares on F2 (SHELXL-97).13 Intensity data of all compounds were routinely corrected for absorption effects. All non-hydrogen atoms were refined anisotropically. The hydrogens were either found in difference Fourier maps or located in idealized positions, and were refined using a riding model with fixed thermal parameters [Uij ) 1.2Uij (eq.) for the atom to which they are bonded]. Compounds 1-5 yielded small amounts of crystals, some requiring extended X-ray exposure times. The crystal lattices of 1 and 2 contained within the interporphyrin channels severely disordered solvent molecules (DMF or pyridine), which could not be reliably modeled. The residual electron-density maps after the conventional refinement contained several relatively high peaks (within 1.5-5 e/Å3 in 1 and 2-6 e/Å3 in 2 centered in these voids). These amounted to 118 and 136 electrons per unit-cell in 1 and 2, respectively (as calculated by PLATON),14 which is consistent with solvent content of 3-4 molecules. Additional refinement calculations were thus carried out using the “squeeze” method,14 in which the contribution of the disordered solvent to the diffraction pattern was subtracted from the observed data. The potential solvent accessible void space in 1 and 2 consists of 23.5-24.1% of the crystal volume (given in Table 1). In compounds 3 and 4, pyridine and the pyridine-solvated copper adduct act as guest species included within the channel voids. They were found disordered about inversion centers of the lattice. Similar disorder was observed when these structures were solved and refined in the noncentrosymmetric space group P1, confirming that the apparent disorder is genuine (due to the symmetry of the guest-occupied void space) and not imposed by choice of the P1 h space symmetry. An exceptionally low-density characterizes structure 5. The void volume in these crystals was assessed14 to be 29.5% of the crystal volume. Nevertheless, the residual electron count in the voids here amounted only to 26 electrons per unit-cell (with four residual peaks within 1.0-1.4 e/Å3), which may possibly represent the presence of 3-4 severely disordered molecules of water not accounted for in the refinement calculations. The low residual electron density found in the channels can be a genuine feature, or an artifact resulting from a severely disordered solvent.15 In view of these low values, the squeeze procedure was not applied in the refinement of 5. In all cases,
Figure 1. (a) Face-on view of the coordination complex in 1. The arrows indicate the position of the 2-fold symmetry axis the porphyrin molecule is located on. Note that only one of the carboxyphenyl rings is roughly perpendicular to the mean plane of the porphyrin core. O35, O36, and O37 represent water molecules, which complete the coordination sphere of the potassium ions and hydrogen bond to adjacent porphyrin molecules. (b) Edge-on view of the porphyrin fragment in 2, illustrating its four-saddle conformation. The atomic displacement parameters of the non-hydrogen atoms at ca. 110 K are represented by 50% probability thermal ellipsoids. the above guest/solvent disorder features seemed to have a negligible effect, allowing for a precise structural determination of the metalloporphyrin species and external metal ion clusters, as well as of the coordination and hydrogen-bonding motifs (see below).
Results and Discussion The reaction of Pd-TCPP and Pt-TCPP with copper biacetate in the presence of the strong KOH base led to isomorphous structures 1 and 2, respectively. In these basic conditions, a complete deprotonation of the carboxylic functions occurs. Somewhat surprisingly, however (at least in relation to an earlier report on a similar reaction of TCPP with cobalt biacetate),9 the potassium ions rather than the copper ones were found to coordinate to the four carboxylate sites. Ortep representation of the isostructural compounds, which are positioned on axes of 2-fold rotation, is depicted in Figure 1. The porphyrin core in 1 and 2 adopts a four-saddle conformation, with the four pyrrole rings being slightly twisted in an alternating manner either up or down with respect to the mean plane of the saddled porphyrin macrocycle. The four pyrrole N atoms are displaced by (0.06 Å (in 1) and (0.07 Å (in 2) from their mean N4 plane. The resulting dihedral angle between the planes of the N12 and N13 pyrrole rings twisted in the same direction with respect to the N4 plane is 21.3° (1) and
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Shmilovits et al. Table 2. Geometry of Coordination and Hydrogen-Bonding Interactions in Solids 1-5a compounds
1
2
Coordination Bonding Distance Range (Å) Pd/Pt-N(pyrrole) 2.014-2.021(2) 2.011-2.013(3) K+-COO-(carboxylate) 2.652-2.761(2) 2.647-2.745(2) K+ -O(water) 2.744-3.179(3) 2.766-3.024(3) Hydrogen Bonding Distance Range (Å) H2O‚‚‚O-(carboxylate) 2.799-2.903(3) 2.780-2.900(4) compounds
3
4
Coordination Bonding Distance Range (Å) Pd/Pt-N(pyrrole) 2.013-2.023(3) 2.007-2.019(4) Cu2+-O-(carboxylate) 1.932-1.993(2) 1.937-2.006(4) 3.008-3.109(2) 3.033-3.131(2) Cu2+ ‚‚‚Na+ Na+-O-(carboxylate) 2.289-2.474(3) 2.318-2.469(4) + Na -O(water) 2.374(3) 2.387(4) Hydrogen Bonding Distance Range (Å) H2O‚‚‚O-(carboxylate) 2.667(4) 2.649(6) compound
5
Coordination Bonding Distance Range (Å) Pd/Pt-N(pyrrole) 1.999-2.019(7) Cu2+-NH3 2.048-2.157(7) Hydrogen Bonding Distance Range (Å) NH3‚‚‚O-(carboxylate) 2.852-3.027(12) NH3‚‚‚O(water) 2.840-2.859(10) H2O‚‚‚O-(carboxylate) 2.691-2.740(7) a Detailed information on individual contacts is given in the supplementary CIF files (Supporting Information).
Figure 2. (a) Intercoordination through the large potassium ions of two inversion-related TCPP units in 1 and 2. K2 bridges between O27 and O34 sites; K3 links between porphyrin molecules of different layers. (b) A ball-and-stick representation of the open porphyrin layers, parallel to the ab plane of the crystal, in 1 and 2, held together by intercoordination through the hydrated potassium ion clusters. Color code: Pd/ Pt, pink; K, violet; O, red; N, light blue; C, gray; H, white.
20.1° (2). The dihedral angles between adjacent pyrrole rings around the macrocycle are 18.5, 12.2, 13.9, and 17.9(1)°. It is interesting to note that the entire tetraarylporphyrin fragment is considerably more flattened than it has been observed in related structures. Thus, the twist angles of the phenyl rings, with respect to the N4 plane (which may represent the average plane of the porphyrin core) are 46.2, 48.3, and 74.8° in 1 and 47.0, 47.0, and 74.4° in 2. In most of the other structures of tetraarylmetalloporphyrins, the aryl groups were found to be considerably more perpendicular to the central core of the molecule (with the twist angles of the aryl groups ranging from 65 to 90°, as it is also evident in 3-5 below).8 The observed flattening can be attributed to the electrostatic forces holding together the layered crystal structure (see below). The Pd-N and Pt-N bond distances are all within 2.011-2.021(2) Å. The C-O
bonds in the carboxyphenyl substituents range from 1.257(3) to 1.262(3) Å in 1 and from 1.249(3) to 1.268(4) Å in 2, characteristic to delocalized carboxylate groups. In the two structures, the porphyrin moieties are interconnected to each other through the hydrated cluster of potassium bridging auxiliaries, utilizing all four delocalized carboxylate functions as anchors to this end. Thus, O27, O28, and O34 coordinate directly to the two potassium ions at 2.65-2.76 Å, while the O19 sites serve as proton acceptors from the three water molecules bound to the potassium ions (at OH‚‚‚O distances of 2.80-2.90 Å). The distance between the two potassium nuclei is 3.840(1) Å in 1 and 3.825(1) Å in 2. The intermolecular organization in the crystal can be best described in terms of a flat-layered arrangement of the bridged porphyrin units roughly parallel to the ab plane of the unit-cell, which is sustained by multiple carboxylate-K+ coordination bonds (Figure 2). The interaction distances between the various species are summarized in Table 2. In the crystal, the flat networks described in Figure 2 are arranged tightly in an offset-stacked manner along the c axis of the unit-cell. The mean interlayer porphyrin-porphyrin distance is 3.70 Å. This rather short separation8 is associated primarily with the long-range electrostatic (ion-pairing) attractions between the layers (the potassium ions being located within, as well as between, the porphyrin layers), and is made feasible due to the considerable deformation of the porphyrin core from planarity. The overall crystal architecture is characterized by extended channels that propagate through the crystal, between the interlinked TCPP building blocks, parallel to the c axis. These channels are 5.0-5.5 Å wide (net van der Waals dimensions). They accommodate the disordered DMF (1) or pyridine
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Figure 3. (a) Side view of the stacked structure, approximately down b (a is horizontal). Note that the potassium ions (violet spheres) are distributed within, as well as between, the porphyrin layers (water molecules were omitted for clarity). (b) Spacefilling presentation of the channeled crystal structure of 1 and 2, viewed down the c axis. The van der Waals width of the nearly square-shaped channels, accessible to molecules of the solvent, is 5.0-5.5 Å. Color code: Pd/Pt, pink; K, violet; O, red; N, light blue; C, gray (H not shown).
(2) solvent, incorporated into the lattice during crystallization to fill the void space. The stacked arrangement of the porphyrin layers, and a space-filling model of the channeled structure are shown in Figure 3. In several respects, it reveals similar features to those observed in the structure of the K+-(CuTCPP)- salt reported earlier.7a Direct coordination of the copper ions to the porphyrin species was obtained, however, when the solvothermal reaction was performed in a less basic environment, applying triethylamine (instead of KOH), along with the pyridine or DMF reagents. In this case, the emerging material consisted indeed of supramolecular porphyrin networks sustained by the copper ion auxiliaries. Although somewhat different solubilizing mixtures (either pyridine or DMF) were used in the two preparative procedures, the corresponding copper-intercoordinated Pd-TCPP and Pt-TCPP compounds 3 and 4 (Figure 4) are characterized by isomorphous structures as well. The molecular features and the intermolecular interaction schemes herein appear nearly identical. The four carboxylic groups are deprotonated during the reaction, the bridging copper ion replacing the rejected proton.
Each carboxylate group of a given porphyrin unit is coordinated to a different copper ion, and every metal ion is linked through C-O-‚‚‚Cu coordination to four different TCPP molecules (Figure 4), thus leading to the formation of flat networks of square-planar symmetry (see below). The CO-‚‚‚Cu distances lie within 1.9321.993(2) Å in 3 and 1.937-2.006(4) Å in 4 (such squareplanar coordination scheme is characteristic to Cu2+ species). In addition, two sodium ions (present as impurity in the porphyrin starting material, but included in the formed crystals in stoichiometric amounts)10 are located on opposite sides of these copper ions at Cu‚‚‚Na distances of 3.01-3.13 Å. They lie also within coordinating distances of the carboxylate O atoms of the converging porphyrin building blocks, at Na‚‚‚O ) 2.292.47 Å. Na1 is located on center of inversion and further coordinated to a water molecule at 2.374(3) Å (3) and 2.387(4) Å (4); the other sodium ion is solvated by a pyridine species at Na2‚‚‚N(py) ) 2.413(4) Å (3) and 2.421(6) Å (4). The preferred linkage of the copper ions to only one of the carboxylate O atoms at each site allows for only partial delocalization within the carboxylate groups. Correspondingly, the C-O bond length
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Figure 4. (a) Face-on view of the coordination complex in the asymmetric unit of 3 (the guest/solvent components are omitted). Cu1, Na1, and Na2, are the metal ion auxiliaries that bridge between adjacent TCPPs in the crystal structure; O74 represents water molecule coordinated to one of the sodium ions. (b) Edge-on view of the porphyrin fragment in 4, illustrating its four-saddle conformation. The atomic displacement parameters of the non-hydrogen atoms in (a) and (b) at ca. 110 K are represented by 50% probability thermal ellipsoids.
values cluster within two distinct ranges: 1.2661.286(5) and 1.235-1.247(5) Å in 3 and 1.258-1.287(7) and 1.228-1.255(7) Å in 4 involving the copper-linked and the noncoordinated O atoms, respectively. The coordination geometries are summarized in Table 2. The porphyrin core in 3 and 4 is characterized by slightly saddled conformation, though it is less distorted than in 1 and 2. The dihedral angles between the mean planes of the trans-related pyrrole rings are within 9.610.5°, while the two pairs of the N-pyrrole atoms lie 0.06 Å below and above the median plane between them. In this case, the twist angles of the peripheral aryl rings with respect to the porphyrin macrocycle in the two structures lie also within the common range of 65-80°. The direct coordination of all four carboxylate groups to copper ions yields extended flat networks of the interconnected species, associated with the formation of open voids between adjacent molecular frameworks (Figure 5). The van der Waals width of these cavities, which are accessible to noncoordinated guest species, is about 5 Å. Each such layer is parallel to the (1,1,0) plane of the crystal. It is composed of molecules and ions displaced along the a-b and the c directions. The sodium ions are positioned above and below the layers. The layers are arranged parallel to one another along the normal a+b direction in a bilayered pattern (Figure 6). The mean interlayer distance within a given bilayer is 4.07 Å. One of the sodium ions (Na1) lies on inversion, and links between the copper centers located in neigh-
Shmilovits et al.
boring bilayers. The average spacing between the latter is 5.13 Å. The water molecule connects by coordination and hydrogen bonding between neighboring entities displaced along c. The porphyrin centers of adjacent parallel layers are shifted one with respect to the other (mostly) along a-b by about 4 Å within, and 12 Å between, the bilayers. This is related to significant and marginal overlap between the nearest porphyrin building blocks in the two layers, respectively. The copperintercoordinated porphyrin networks are inclined by about 47° to the b axis of the crystal, but are parallelstacked along it. This includes a columnar arrangement of the intralayer cavities, which combine into open channels of two types propagating through the structure along b. In 3 and 4, one channel accommodates molecules of the pyridine solvent, while the other channel includes a disordered pyridine-solvated complex of copper [Cu(py)3] and an additional uncoordinated molecule of pyridine.16 In both cases, the guest components are centered about the inversion sites within the interporphyrin channels. Another preparative experiment was carried out by reacting Pt-TCPP with copper biacetate in the presence of NH4OH(aq) as a mild base, instead of the KOH or Et3N reagent. Somewhat unexpectedly, this created a dramatic modification of the supramolecular architecture due to the strong affinity of the copper ions for NH3 ligands. While complete deprotonation of the TCPP was still achieved in the resulting compound [as evidenced by the observed carboxylate C-O bond lengths of 1.250(8) and 1.268(8) Å], the copper ion stripped of the acetates did not coordinate directly to the carboxylate groups. Instead, it formed the stable Cu(NH3)62+ cation, linking to the carboxylates only in the second coordination sphere. The molecular structure of the resulting complex 5, (Pt-TCPP)4-‚2[Cu(NH3)62+], which crystallized as a tetrahydrate is displayed in Figure 7. In the crystal of this compound, the metalloporphyrin and the Cu(NH3)6 moieties are located on the 2/m and 2 special positions, respectively. Thus, the porphyrin molecule exhibits a perfect C2h symmetry, and its macrocyclic core is planar. The observed Pt-N(pyrrole) bond lengths of 1.999 and 2.019(7) Å, and the Cu-NH3 coordination distances (which are affected to some extent by partial disorder of the cationic species) of 2.048, 2.153, and 2.157(7) Å, are in the expected range (Table 2). The crystal structure consists of columnar arrangements of the porphyrin anions, which stack in an offset manner along the short c axis (the planes of the porphyrin cores are inclined about 50° with respect to c). Along the columns, the parallel offset between the centers of neighboring molecules is about 4.7 Å (Figure 8b), and the intermolecular spacing is 4.54 Å. This represents the characteristic porphyrin-porphyrin dispersion interaction pattern observed in a large variety of porphyrin materials.8 The porphyrin columns are interspersed, and tessellated together, by positively charge columns of the copper complex. The electrostatic attraction is complemented by cooperative weak hydrogen bonding between the NH3 ligands and the neighboring carboxylate groups. The hydrogen bond distances and angles are N‚‚‚O 2.852-3.027(12), H‚‚‚O 1.98-2.14 Å, and N-H‚‚‚O 160-167° (Table 2, Figure 8). The additional water moiety hydrogen bonds simultaneously
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Figure 5. (a) The simultaneous coordination mode of Cu1 to four different porphyrins in 3 and 4; note that it is bound to only one O atom of each carboxylate function. (b) The copper-tesselated flat layers of Pd/Pt-TCPP in 3 and 4 (c is horizontal, a-b is vertical). The sodium ions (violet spheres) lie above and below the layer. Note the 5 Å wide open voids present between the interlinked porphyrin units. Color code: Pd/Pt, pink; Cu, brown; O, red; C, gray; N, light blue; H, white.
Figure 6. Intermolecular arrangement in 3 and 4. (a) View down b axis of the crystal through the stacked layers (c is horizontal), illustrating the channel type structure that forms (represented in ball-and-stick mode). The channels are occupied separately by the solvent and disordered guest moieties (described in the text and depicted in a line-mode), which are not coordinated to the main lattice. (b) Edge-on view of the stacked-porphyrin layers, approximately down the a-b axis (c is horizontal). Note the alternating spacing between the parallel layers. Color code: Pd/Pt, pink; Cu, yellow; Na, violet; O, red; N, light blue; C, green (H, omitted).
to the NH3 ligands (as proton acceptor) and the carboxylate functions of adjacent porphyrins (as proton donor). The resulting structure consists of alternating zones of lipophilic and hydrophilic moieties. Most interestingly, however, compound 5 reveals an attractive “porous” nature. Due to the large size of the porphyrin units and its square-planar geometry, optimization of the intermolecular ion-pairing as well as hydrogen-bonding interactions results in the creation of sizable channel voids between the columns. These channels propagate parallel to the c axis, between the porphyrin columns displaced by the a-translation of the lattice (Figure 9). Their diameter (after subtracting the van der Waals thickness of the hydrocarbon walls) is approximately 6.5 Å, wide enough to accommodate relatively large guest components. However, presence of such molecules in the channels was not evident from the X-ray diffraction data (see Experimental Procedures). Yet, the open crystalline lattice appears to be stable for an extended period of time (as monitored by repeated measurements of the unit-cell data).
Figure 7. The molecular structure of 5 (the water solvent, hydrogen bonded to N18, is omitted). The porphyrin and the Cu complex are located on 2/m and 2 special positions, respectively. The porphyrin core is perfectly planar, and the aryl rings are nearly perpendicular to it. The atomic displacement parameters of the non-hydrogen atoms at ca. 110 K are represented by 50% probability thermal ellipsoids.
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materials could be obtained consistently in diverse experimental conditions and with a variety of metal ion bridging auxiliaries. The relative stability of their open framework lattice is afforded mostly by effective cooperative coordination and ion-pairing interactions. Correspondingly, they provide a promising perspective for the formulation of organic zeolite analogues.7b,9 The successful syntheses of channel-type architectures in this study are generally consistent with the crystal engineering concepts of porphyrin-based networks developed by us earlier.5 Yet, they also indicate that the outcome does not always match ideally the expected result. Production of the crystalline (as opposed to amorphous) product is often associated with an unpredictable incorporation of impurities and hydration/ solvation reagents present in the reaction mixture, and diverse clustering of the metal ion auxiliaries.10 Pseudopolymorphism is sometimes also a problem. Thus, it is important to optimize, and achieve a better control of, the preparative procedures of such supramolecular syntheses (which still appear to be much less reliable then the methods of covalent synthesis) to be able to formulate porous architectures on a larger scale and in a more routine and consistent manner. Further studies are also required to evaluate the molecular sieving features of the compounds available thus far, and to extend the currently available database (which is still rather limited) by designing new forms of the TCPP polymers.
Figure 8. (a) The columnar arrangement of the crystal components in 5, viewed approximately down c* (b is horizontal). Small spheres represent the platinum and copper ions, as well as molecules of the water solvent. Note the channel voids created between the porphyrin columns. (b) Illustration of the parallel shift between adjacent moieties in the porphyrin stacks, which reflects optimization of dispersion forces between the overlapping porphyrin frameworks. Color code: Pt, pink; Cu, yellow; O, red; N, light blue; C, green (H, omitted).
Figure 9. Space-filling presentation of the channeled crystal structure of 5, viewed down the c axis of the crystal. The net van der Waals width of the channels is about 6.5 Å. Color code: Pt, pink; Cu, brown (covered by NH3 ligands); N, light blue; O, red; C, gray; H, white.
Conclusion The square-planar molecular-shape features of the TCPP building block and its high coordination and hydrogen bonding capacity have led readily to the construction of supramolecular lattices with channels voids, accessible to other components.6,7,9,10 Such porous
Acknowledgment. This research was supported in part by The Israel Science Foundation (Grant No. 68/01), as well as by the US-Israel Binational Science Foundation (BSF), Jerusalem, Israel (Grant No. 1999082). Supporting Information Available: Crystallographic data for compounds 1-5, in the crystallographic information file (CIF) format. This information is available free of charge via the Internet at http://pubs.acs.org.
References (1) For recent reviews on noncovalent assembly of porphyrin arrays and their potential applications see (a) Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. The Porphyrin Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R. Eds.; Academic Press: Orlando, FL, 2000; Vol. 6, Chapter 40, pp 1-42; (b) Chou, J.-H.; Nalwa, H. S.; Kosal, M. E.; Rakow, N. A.; Suslick, K. S. The Porphyrin Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R. Eds.; Academic Press: Orlando, FL, 2000; Vol. 6, Chapter 41, pp 43-132. (2) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.; Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S, J. The Porphyrin Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R. Eds.; Academic Press: London, 2000; vol. 3, pp 1-48. (3) (a) Drain, C. M.; Shi, X.; Milic, T.; Nifiatis, F. Chem. Commun. 2001, 287-288; (b) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D. Angew. Chem., Int. Ed. Engl. 1998, 37, 2344-2347. (c) Stang, P. J.; Fan, J.; Olenyuk, B. Chem. Commun. 1997, 1453-1454. (4) Suslick, K. S.; Rakow, N. A.; Kosal, M. E.; Chou, J.-H. J. Porphyrins Phthalocyanines 2000, 4, 407-413. (5) (a) Goldberg, I. Chem. Eur. J. 2000, 6, 3863-3870; (b) Goldberg, I. CrystEngComm 2002, 4, 109-116. (6) (a) Diskin-Posner, Y.; Goldberg, I. Chem. Commun. 1999, 1961-1962; (b) Dastidar, P.; Stein, Z.; Goldberg, I.; Strouse, C. E. Supramol. Chem. 1996, 7, 257-270. (7) Diskin-Posner, Y.; Dahal, S.; Goldberg, I. (a) Chem. Commun. 2000, 585-586; (b) Angew. Chem., Int. Ed. 2000, 39, 1288-1292.
Crystal Engineering of “Porphyrin Sieves” (8) (a) Krishna Kumar, K.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998, 37, 541-552; (b) Byrn, M. P.; Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.; Terzis, A.; Strouse, C. E. J. Am. Chem. Soc. 1993, 115, 9480-9497. (9) Kosal, M. E.; Chou, J.-H.; Wilson, S. R.; Suslick, K. S. Nat. Mater. 2002, 1, 118-121. (10) (a) Diskin-Posner, Y.; Goldberg, I. New J. Chem. 2001, 25, 899-904; (b) Diskin-Posner, Y.; Patra, G. K.; Goldberg, I. Eur. J. Inorg. Chem. 2001, 2515-2523. (11) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Polidori, G.; SIR-92, J. Appl. Crystallogr. 1994, 27, 435-436. (12) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garcia- Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C.; “The DIRDIF-96 Program System”. Technical Report of the Crystallography Laboratory, University of Nijmegen, The Netherlands, 1996. (13) Sheldrick, G. M. SHELXL-97. Program for the Refinement of Crystal Structures from Diffraction Data, University of Go¨ttingen, Germany, 1997. (14) Van der Sluis, P.; Spek, A. L. Acta Crystallogr. Sect. A 1990, 46, 194-201. The “Squeeze” method described therein is
Crystal Growth & Design, Vol. 3, No. 5, 2003 863 widely used in crystallographic analysis of compounds containing substantial amounts of disordered solvent that cannot be located precisely from diffraction data. (15) Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R. Nature 1994, 369, 727-729. (16) The Cu+(pyridine)3 entity (along with another molecule of the pyridine solvent) consistent with the diffraction data of 3 and 4 is located on, and disordered about, centers of inversion. The coordination sphere of the copper ions at each site consists of three molecules of pyridine and is characterized by an approximate trigonal symmetry which is typical to copper(I) ions (labeled as Cu2 in the CIF files of 3 and 4). The observed Cu2-N(py) coordination distances are within 2.014-2.144(5) Å in 3 and 2.037-2.142(8) Å in 4. The assigned oxidation states of the copper ions in this structure (which account for charge-balanced material) could not be determined independently by other means. It should be mentioned, however, that partial reduction of Cu2+ to Cu+ in basic amine environments is a well-known phenomenon (see e.g., in Cotton, F. A.; Wilkinson G., Advanced Inorganic Chemistry, Wiley & Sons: New York, 1980).
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