Porphyrin Supramolecular Solids Assembled with the Aid of Lanthanide Ions
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2651-2654
Sumod George, Sophia Lipstman, and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel AViV UniVersity, 69978 Ramat AViV, Tel AViV, Israel ReceiVed August 2, 2006; ReVised Manuscript ReceiVed October 31, 2006
ABSTRACT: Reactions of free-base tetra(4-carboxyphenyl)porphyrin with lanthanide ions as Pr3+, Dy3+, and Nd3+ led to open metalorganic framework solids sustained by polynuclear metal-carboxylate clusters. Their three-dimensional structures are characterized by channel voids accessible to other guest components (e.g., water, small organics). Crystalline solids obtained by the hydrothermal synthesis with Dy3+ and Nd3+ reagents reveal remarkable stability and represent the first example of hybrid porphyrin-lanthanide single framework architectures. The porphyrin macrocycle functionalized at its four meso positions by 4-carboxyphenyl groups [5,10,15,20-tetra(carboxyphenyl)porphyrin (TCPP)] is an extremely versatile building block for the self-assembly of framework solids. Its square-planar symmetry with diverging carboxylic functions is perfectly suited for the construction of open quadrangular supramolecular networks. In 1999 and 2000, we reported on the first successful constructions of TCPP-based molecular-sieve-type crystalline architectures involving either cooperative hydrogen bonding or/and coordination polymerization aided by external metal ions and organic ligands.1,2 Several examples reported since then attest to the high propensity of the TCPP scaffold to assemble into open, zeolite-like framework architectures that are suitable to incorporate guest components.3 When the TCPP networking is sustained by exocyclic metal bridges the porphyrin inter-coordination through them does not require introduction of foreign counter ions into the lattice, as the charge is readily balanced by deprotonation of the porphyrin tetra-acid.2,3 With common transition metals, however, use of this concept based on multiple metal-carboxylate interactions yielded three-dimensional (3D) single-framework crystalline polymers only on rare occasions. This includes a single outstanding example of a functional microporous solid obtained by chance when free-base TCPP was reacted with Co ions to yield bent Co-TCPP units that are interconnected by trinuclear cobalt ion bridges.4a In a related case, the intentional reaction of metallated-TCPP with Zn4O bridging moieties led to robust single-framework architectures,4b following the successful designs of Yaghi’s metal-organic frameworks with other molecular scaffolds.5 Yet, despite significant progress in the design and formulation of porphyrin-based solidstate receptors, the yield of sturdy crystalline solids is still rather low. The use of common transition metals to this end resulted in most cases in the formation of two-dimensional (2D) polymeric frameworks, the stacking of which along the third dimension is stabilized by dispersion, thus leading to soft materials of low stability.3,6 In this communication, we propose a systematic approach to the formulation of TCPP-based single-framework coordination polymers with open architectures. It is based on the observation that many of the acetate/oxalate salts of trivalent lanthanide ions consist in their stable form of polynuclear metal ion clusters bridged by several acetate/oxalate anions.7 This should be attributed to the large size and high coordination numbers (up to 9 for oxidation state +3) of the lanthanides,7,8 which makes them in the present context more advantageous to the common transition metals. The idea then is that during the reaction of the lanthanide’s salts with TCPP, the anionic bridges can be readily replaced by the porphyrin carboxylate groups, thus stimulating the multi-porphyrin self-assembly process around the metal clusters in three dimensions. We report here on * To whom correspondence should be addressed. E-mail: goldberg@ post.tau.ac.il.
the first examples of such framework architectures (after failing in this endeavor with the corresponding copper or molybdenum acetate reagents) obtained by the interaction of free-base TCPP with the Pr2(oxalate)3, Nd2(oxalate)3, and Dy2(oxalate)3 hydrated salts. The afforded compounds are framework coordination polymers of TCPP, sustained by the lanthanide ions, with 2:1 Pr/porphyrin (1) and 4:3 Dy/porphyrin (2) and Nd/porphyrin (3) constitutions.9 The connectivity motif in 1 is depicted in Figure 1. A pair of Pr-cations binds to the carboxylate arms of four different porphyrin units (residing on centers of crystallographic inversion), each of the carboxylates coordinating to both metal ions at Pr-O(carboxylate) distances within 2.413(3)-2.492(4) Å. These Pr2 clusters are further interlinked to each other on both sides by a bridging oxalate ligand at 2.497(3) and 2.520(3) Å, thus yielding a polymeric [-Pr2(porphyrin)-oxalate-]∞ coordination pattern along the a-axis of the crystal. Each of the Pr-ions was found to bind two additional molecules of water [at 2.515(4) and 2.525(4) Å] to satisfy its multiple coordination requirements. The Pr‚‚‚Pr distance within the dinuclear cluster is 4.1870(5) Å. As the same interaction modes (“synthons”) operate at the four corners of the TCPP unit, the assembly process yields a single framework architecture. It is composed of porphyrin bilayers joined by the Pr2 clusters, which are then interlinked in a roughly perpendicular direction by the oxalate ligands (Figure 2). This structure contains ∼0.6-nm-wide interporphyrin channel voids propagating parallel to the c-axis, which are accessible to other (solvent) molecules. Figure 3 shows additional 0.4-0.5 nm wide channels that run parallel to the a-axis of the crystal between the TCPP units. This channel-perforated framework makes up only 44% of the crystal volume, the remaining space being occupied (at least in part) by the severely disordered solvent (DMF, water, and acid species). The intermolecular networking in compounds 2 and 3 is similar (although not identical) to each other, but different than in 1 due to different preparative conditions (see below). Thus, only 2 will be discussed here. It crystallized in space group C2/c. The asymmetric unit contains two Dy ions, one porphyrin molecule in general position and another porphyrin unit located on an axis of 2-fold rotation. The fundamental tetranuclear interaction synthon in this structure is displayed in Figure 4. It consists of four bridged dysprosium ions connecting to 12 porphyrin molecules distributed between three different porphyrin layers. Each of the four porphyrins of the central layer coordinates through its carboxylate groups to the two “inner” Dy ions. Porphyrin species in the first and the third layer either link to the “outer” Dy ions or bridge between the outer and the inner Dy species. The inner Dy’s are characterized by coordination number 7, with an additional molecule of DMF attached to them. The outer Dy’s are engaged in six DyO(carboxylate) and two Dy-O(water) strong bonds. All the Dy-O coordinations are within 2.228(4)-2.509(4) Å. The two unique Dy‚‚‚Dy distances within this cluster are 3.9763(4) and 4.1791(4)
10.1021/cg060520r CCC: $33.50 © 2006 American Chemical Society Published on Web 11/14/2006
2652 Crystal Growth & Design, Vol. 6, No. 12, 2006
Communications
Figure 1. The continuous coordination pattern in 1, involving the Pr metal ions (denoted by small spheres), TCPP’s (“porph” - only half is shown at each site), and the oxalate anions (indicated by “ox”). The short sticks projecting from the Pr spheres represent the two coordinated water molecules. Figure 4. The tetranuclear coordination synthon in 2, showing the binding of 12 porphyrin units (organized in three different layers) to four Dy ions. Only the terminal carboxyphenyl groups are shown, “P” representing the remaining porphyrin framework. The metal ions are represented by small spheres. Note the additional DMF and water species coordinating to the inner and outer Dy’s, respectively.
Figure 2. Crystal packing of 1, viewed down the c-axis, showing the cross linking of the Pr2-porphyrin layers by the oxalate ligands. The ninecoordinate oxygen environment of the Pr’s composed of the carboxylate, oxalate, and water moieties is also indicated.
Figure 5. A single Dy-bridged porphyrin layer in 2. Note that within such a layer the nature of the bridging metal ion nodes alternates between a single Dy ion and double Dy2 linkages. For clarity, the other entities coordinating to the metal ions are represented by stick bonds only. Note the Cl atoms (small spheres) residing in the porphyrin cores.
Figure 3. Space-filling illustration of 1 projected down the a-axis. It shows 0.4-0.5 nm wide channel voids that propagate through the porphyrin layers parallel to a. The nodes between the porphyrins represent an end view of the [-Pr2-(oxalate)-] chains; only the water and carboxylate-O environment of the Pr ions (darker spheres) can be seen.
Å. Again, identical connectivity patterns operate at every carboxylate corner of the square-planar TCPP units, leading to the formation of interlinked porphyrin layers, with the tetranuclear Dy4 bridging clusters spanning between three adjacent layers. A single porphyrin layer is illustrated in Figure 5. The intralayer inter-porphyrin spacing is similar to that observed in TCPP-based networks bridged by main transition metals.6 Inspection of the crystal structure confirms that 2 represents a single framework coordination polymer. Neighboring Dy4 bridging clusters span over different three-layer sets of the porphyrins, as a given layer contains both an “outer” Dy ion and the “inner” Dy2 ion pair as linkers (Figure 5). The alternating vertical shift of the Dy4 clusters (acting as robust pillars) provides effective cross-linking between the porphyrin layers throughout the crystal. This framework solid is
more condensed, however, than 1. The porphyrin layers stack rather tightly along the [1,0,1] axis, as the oxalate anion is not part of the network (as opposed to the spacing nature of the oxalate anion in 1; Figure 2). Then, the DMF species, which are bound to the metal ions, protrude into the interporphyrin space. Stacking of the layered Dy-bridged porphyrin arrays along [1,0,1] creates solvent-accessible channel voids propagating parallel to this axis, as in 1, that make up about 30% of the crystal volume. Finally, noteworthy are the marked saddle-type distortions (from planarity) of the porphyrin cores in this structure. This can be attributed to the inclusion of Cl-atoms within the TCPP cores, and optimization of the numerous coordinative interactions with the metal ions. The overall structure of the neodymium compound 3 is nearly isomorphous with that of 2. The unit cell dimensions of the two crystals are isometric, and the intermolecular coordination schemes are essentially identical. However, 3 differs from 2 in two respects: it consists of free-base porphyrins (no Cl-atoms insert into the macrocycle in 3), and the diffraction data deviate significantly from monoclinic symmetry. It the latter context, the structure solution and crystallographic refinement were thus conducted in the triclinic space group P1h, with three porphyrin molecules and four Nd ions (along with the corresponding number of DMF and water ligating moieties) contained in the asymmetric unit. Otherwise, the two structures exhibit the same porphyrinmetal coordination schemes with Nd-O bonds within 2.3-2.5 Å,
Communications and similarly open architectures with periodically spaced channels propagating throughout the crystal perpendicular to the porphyrin layers. Detailed thermal analysis [thermogravimetric (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC)] was carried out on the structurally isomorphous and genuinely single framework solids 2 and 3. The TGA and DTA experiments indicate that in 2 (after evaporation of the solvent adsorbed on the crystal surface) the lattice-incorporated solvent, about 10% of the initial crystal mass, is lost around 186 ( 20 °C, and that the porphyrin carboxylate species disintegrate into black amorphous powder only near 391 ( 15 °C. In 3, similar phenomena occur at 180 ( 30 °C and 418 ( 15 °C, respectively. The corresponding DSC scans show a relatively sharp exothermic phase transition near 201 ( 8 °C in 2, possibly reflecting the collapse of the open metal organic frameworks into a more condensed phase. No phase transition was observed, however, in 3 below 400 °C. Similar experiments with our related TCPP materials interlinked by exocyclic mononuclear K, Cu, Cu(NH3)6, and Zn bridges6 resulted in a cascaded deterioration and transition between several different phases of the corresponding solid materials (their bulk structures lacking substantial stability in three dimensions) within the same temperature range. The above thermal behavior is consistent with the 3D coordination polymerization in 2 and 3 vs 2D framework formation in the latter materials. The actual composition of the lattice-included solvent phase in 1-3 could not be reliably characterized at this preliminary stage, but this has little relevance to the descriptions of the coordination polymerization and networking patterns in this study. Structure 1 is somewhat less stable than 2 or 3 due to the binuclear rather than tetranuclear nature of the metal ion bridges, the presence of the small oxalate ligand as part of the supramolecular framework, and the higher coordination number of the ions and thus weaker and longer Pr-O bonds. Formation of the two different polymeric architectures described above is due to the different conditions applied in their synthesis. 1 was obtained by slow evaporation at room temperature, and it exhibits an incomplete replacement of the oxalate anions in the coordination sphere of Pr by the porphyrin carboxylate entity and thus participation of the remnant organic ligand in the coordination polymerization scheme. Presence of the latter imparts some thermal instability to the formed framework, and therefore the conventional preparative route is less attractive and may not lead to materials of desired robustness. On the other hand, compounds 2 and 3 were obtained by hydrothermal synthesis, leading solely to acetate/oxalate-free lanthanide-TCPP frameworks.10 Indeed, nearly identical crystalline framework architectures of the second type have been obtained in due course at similar hydrothermal conditions with the Eu, Sm, Er, La, and Ce bridging ions, reflecting the high reproducibility of the hydrothermal synthetic approach. Full characteristics of these solids and their production on a larger scale, along with the evaluation of their functional porosity (via absorption isoterms) will be provided in a future full account of this work. It should be noted, however, that once formed, these framework coordination polymers are sparingly soluble in commonly used polar organic solvents as DMSO (another manifestation of their unique stability). This significantly hampers our continuing efforts to analyze the absorption/fluorescence properties of the lanthanide-TCPP polymers and to characterize their excited-state features. While this preliminary account involves preparative procedures only at the milligram scale to proof the structural concept, comprehensive characterizations of the lanthanide-TCPP materials and their potential utility will require largerscale formulations in the future. Metal-organic framework solids are of great fundamental structural interest and importance11 and may have appealing technological applications,12 providing a strong motivation for further research in this area. Their programmed formulations have been quite successful so far with small organic ligands but considerably less so with large macrocyclic units. We have
Crystal Growth & Design, Vol. 6, No. 12, 2006 2653 demonstrated hereby for the first time the effective use of the lanthanides in formulations of stable porphyrin-based open framework architectures based on the tetrafunctional TCPP derivative, taking advantage of their capacity to form robust polynuclear metal-carboxylate clusters.13 Acknowledgment. This research was supported in part by The Israel Science Foundation (Grant No. 254/04). Supporting Information Available: Crystallographic data for compounds 1-3, in the crystallographic information format (CIF) [CCDC612944 (1), CCDC-612945 (2), and CCDC-612946 (3)]. TGA, DTA, and DSC diagrams of 2 and 3. This information is available free of charge via the Internet at http://pubs.acs.org.
References (1) Diskin-Posner, Y.; Goldberg, I. Chem. Commun. 1999, 1961-1962. (2) (a) Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Angew. Chem., Int. Ed. 2000, 39, 1288-1292. (b) Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Chem. Commun. 2000, 585-586. (3) (a) Goldberg, I. Chem. Eur. J. 2000, 6, 3863-3870. (b) Goldberg, I. Chem. Commun. 2005, 1243-1254. (c) George, S.; Goldberg, I. Cryst. Growth Des. 2006, 6, 755-762. (4) (a) Kosal, M. E.; Chou, J.-H.; Wilson, S. R.; Suslick, K. S. Nat. Mater. 2002, 1, 118-121. (b) Suslick, K. S.; Bhyrappa, P.; Chou, J.-H.; Kosal, M. E.; Nakagaki, S.; Smitherny, W. D.; Wilson, S. R. Acc. Chem. Res. 2005, 38, 283-291. (5) O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-330. (6) (a) Shmilovits, M.; Diskin-Posner, Y.; Vinodu, M.; Goldberg, I. Cryst. Growth Des. 2003, 3, 855-863. (b) Shmilovits, M.; Diskin-Posner, Y.; Goldberg, I. Cryst. Growth Des. 2004, 4, 633-638. (7) Allen, F. H. The Cambridge Structural Database, Acta Crystallogr. 2002, B58, 380-388. (8) Cotton, F. A.; Wilkinson, G. In AdVanced Inorg. Chem., 4th Ed.; Wiley: New York, 1980. (9) The meso-tetra(4-carboxyphenyl)porphyrin (TCPP, Frontier Scientific), as well as the hydrates of Pr(acetate)3 contaminated with Pr2(oxalate)3, Nd2(oxalate)3, and Dy2(oxalate)3 (Aldrich), and the various solvents were obtained commercially and used without further purification. 1: TCPP (6 mg, 0.0075 mmol) dissolved in DMF was added to praseodymium salt (14.5 mg, excess) dissolved in water. The resulting solution was mixed well, excess glacial acetic acid was added to dissolve the remaining solids, and the solution was filtered. Diffraction quality crystals were obtained by evaporation after several days. 2 and 3: TCPP (4 mg, 0.005 mmol) dissolved in DMF was added to either dysprosium oxalate hydrate (3 mg, 0.005 mmol) or neodymium oxalate hydrate (2.8 mg, 0.005 mmol), dissolved in 1 mL of water and a few drops of concentrated HCl. Crystallizations in this case were carried out by a hydrothermal technique in a sealed reactor by gradually heating the samples to 150 °C for several hours and then cooling to room temperature. X-ray diffraction data were collected on a Nonius KappaCCD diffractometer at ca. 110 K. The crystal structures were solved and refined by standard methods, leading to reliable determinations of the supramolecular framework architectures. They all were found to contain severely disordered uncoordinated solvent (e.g., DMF, water, acids) within the intralattice voids, which could not be well modelled by discrete atoms. In the final calculations, the contribution of the disordered components was subtracted from the diffraction data by the Squeeze procedure,14 a common practice in similar situations.1,2,4 Crystal data (excluding the disordered solvent): 1: C50H34N4O16Pr2, M ) 1228.64, triclinic, space group P1h, a ) 9.4902(7), b ) 13.9123(13), c ) 16.7437(14) Å, R ) 83.044(3), β ) 75.802(5), γ ) 88.309(5)°, V ) 2127.4(3) Å3, Z ) 1, Fcalcd ) 0.959 g cm-3, µ(Mo-KR) ) 1.18 mm-1, 19 458 reflections measured (2θmax ) 56.6°), 9906 unique (Rint ) 0.064), final R ) 0.063 (wR ) 0.154) for 6690 reflections with I > 2σ(I) and R ) 0.087 (wR ) 0.162) for all data. Porphyrin and oxalate fragments lie on centers of inversion. The solvent-accessible volume is 1187 Å3/unit-cell, and the residual electron-density is 214 e. Conventional refinement with original data and the same solventexcluded model converged at R ) 0.105. 2: C150H94Cl3Dy4N14O30, M ) 3326.73, monoclinic, space group C2/c, a ) 21.6561(4), b ) 23.9803(5), c ) 30.9598(9) Å, β ) 106.264(2)°, V ) 15434.6(6) Å3, Z ) 4, Dc ) 1.433 g cm-3 and µ(Mo-KR) ) 2.046 mm-1, 66 159 reflections measured, 18 313 unique (Rint ) 0.075), final R ) 0.059 (wR ) 0.156) for 12256 reflections with I > 2σ(I) and R ) 0.091 (wR ) 0.168) for all data. The solvent-accessible volume is 4282
2654 Crystal Growth & Design, Vol. 6, No. 12, 2006 Å3/unit-cell, and the residual electron-density is 392 e. Conventional refinement with original data and the same solvent-excluded model converged at R ) 0.080. HCl/Cl- species (originating in the added hydrochloric acid) were found inserted into the porphyrin cores. 3: C150H100N14O30Nd4, M ) 3155.40, triclinic, space group P1h, a ) 16.1530(6), b ) 16.1512(6), c ) 31.2991(10) Å, R ) 99.842(2), β ) 99.877(2), γ ) 96.672(2)°, V ) 7836.7(5) Å3, Z ) 2, Fcalcd ) 1.337 g cm-3, µ(Mo-KR) ) 1.37 mm-1, 58 519 reflections measured (2θmax ) 50.7°), 27 434 unique (Rint ) 0.133), final R ) 0.092 (wR ) 0.183) for 12 929 reflections with I > 2σ(I) and R ) 0.187 (wR ) 0.212) for all data. The structure is geometrically pseudomonoclinic C (nearly isomorphous to 2), but the diffraction data deviate significantly from monoclinic symmetry. The solventaccessible volume is 2200 Å3/unit-cell, and the residual electrondensity is 213 e. Conventional refinement converged at R ) 0.15.
Communications (10) Hydrothermal synthesis was shown to provide optimal conditions for the formation of polynuclear metal-carboxylate clusters. See, for example, Burrows, A. D.; Cassar, K.; Friend, R. M. W.; Mahon, M. F.; Rigby, S. P.; Warren, J. E. CrystEngComm 2005, 7, 548550, and references cited therein. (11) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. 1998, 37, 14601494. (12) (a) Rowsell, J. L. C., Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670-4679. (b) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705-714. (13) Numerous examples of similarly linked discreet complexes of these lanthanide ions with small carboxylic acids are known.7 (14) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13.
CG060520R
