DOI: 10.1021/cg1011007
Supramolecular Crystal Chemistry of Tetra(3-pyridyl)porphyrin. 2. Two- and Three-Dimensional Coordination Networks with Cobalt and Cadmium Ions
2010, Vol. 10 5001–5006
Sophia Lipstman and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, Israel Received August 22, 2010; Revised Manuscript Received September 19, 2010
ABSTRACT: New hybrid coordination polymers have been self-assembled with the conformationally versatile tetra(3-pyridyl)porphyrin (T3PyP) scaffold. Hydrothermal reactions with cobalt ions resulted in metalation of the porphyrin core and selfcoordination of the CoT3PyP units through their endocyclic metal centers into two-dimensional (2D) polymeric arrays. These crystallized into interdigitated, either solvent-free or N,N-dimethylformamide-solvated, structures. Interaction of the T3PyP with cadmium ions yielded three-dimensional (3D) framework solids tessellated by exocyclic CdCl2 connectors to the peripheral pyridyl sites of neighboring units of either the freebase porphyrin or its CdT3PyP analogue. The complementary tetradenate nature of the porphyrin linkers and CdCl2 nodes resulted in pseudodiamondoid polymerization schemes.
Scheme 1
Introduction Formulations of porphyrin-based coordination networks with metal ion connectors are of particular interest due to the relative robustness of metal-organic frameworks and their potential utility in practical applications.1-4 In a recent article, we have explored the versatile molecular recognition features of the tetra(3-pyridyl)porphyrin (T3PyP) building block (Scheme 1) and its utility in the construction of porphyrinbased supramolecular assemblies via coordination as well as hydrogen bonding interaction synthons.5 In this short report, we highlight supplementary new results on coordination networking of the T3PyP scaffold with metal ions to emphasize its remarkable capacity to form hybrid coordination polymers (scarcely elucidated in the literature prior to our work6,7). The diversified supramolecular reactivity of T3PyP can be attributed to the conformational flexibility of this porphyrin framework, namely, the possibility of the four meso-substituted 3-pyridyl arms to align in different orientations with respect to the porphyrin plane.5 The two main conformers of the T3PyP may be characterized by (a) alternating up-down-up-down orientations of adjacent 3-pyridyl substituents along the macrocycle, the N-sites then pointing in tetrahedral directions, and (b) one pair of cisrelated pyridyls pointing “up” and the other pair directed “down”, giving rise to a chairlike structure. Formation of the recently observed coordination networks with manganese and copper ions is associated with these two conformers of the T3PyP.5 Other possible conformers with all four pyridyls pointing in the same direction, or with three pyridyl groups oriented up and the fourth one directed down, are less likely for thermodynamic considerations, as they have a lower probability to form continuous polymeric ensembles. Preferential occurrence of the chairlike conformation has been observed also in coordination polymers of the tetra(3-carboxyphenyl)porphyrin scaffold.8 *To whom correspondence should be addressed. Address: Prof. Israel Goldberg, School of Chemistry, Tel Aviv University, 69978 Ramat Aviv, Tel Aviv, Israel. Tel.: þ972-3-6409965. Fax: þ972-3-6409293. E-mail: goldberg@ post.tau.ac.il.
In spite of the tetradentate nature and coordination affinity to metal ions of the T3PyP ligand, only a small number of its coordination networks have been observed thus far. This includes two-dimensional (2D) polymeric arrays tessellated via mercury and zinc ions,6,7 and three-dimensional (3D) framework architectures (revealed in the first part of this work) tessellated by manganese and copper ion connectors.5 In order to expand the library of the T3PyP-based coordination networks assembled through either endocyclic or exocyclic metal ion linkers, and further demonstrate the important role this ligand can play in crystal engineering of framework solids, we report in this complementary study on the preparation and structural characterization of a series of new materials. The most recently synthesized and analyzed compounds include the two-dimensional self-coordinated polymer of CoIIT3PyP (1), its N,N0 -dimethylformamide (DMF) clathrate with the DMF solvent incorporated between the 2D layers (2), three-dimensional polymers of either freebase T3PyP or metalated Cd(DMF)T3PyP interlinked by CdCl2 exocyclic interporphyrin connectors (3 or 4, respectively), and a 3D polymer of CuIIT3PyP tessellated by tetranuclear (CuIBr)4 linkers (5). Survey of the Cambridge Structural Database indicates that the 3D connectivity schemes observed in compounds 3-5 have not been observed before in coordination networks with the closely
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Figure 1. Face-on view of the 2D coordination networks of the CoT3PyP ligands in (a) 1 and (b) 2. Wireframe illustration, with only the Co ions and N-sites denoted by small spheres. The Co-N(pyrrole) and Co-N(pyridyl) bond lengths are respectively 1.993(3)-2.003(3) A˚ and 2.296(3) A˚ in 1, and 1.994(2)-1.996(2) A˚ and 2.259(2) A˚ in 2. Note the different orientations of the noncoordinated pyridyl groups in the two structures with respect to the polymeric layer.
Figure 2. Interlayer organization of the CoT3PyP coordination networks (viewed edge-on) in (a) solvent-free 1 projected down the b-axis (a is vertical), and (b) its DMF clathrate 2, viewed down the b-axis (a is vertical). The Co ions, N and O (in b) sites are denoted by small spheres. Note the different content and appearance of the interface zone between the layers.
related and widely studied tetra(4-pyridyl)porphyrin analogue, due to the strictly equatorial (2D) disposition of the pyridyl N-sites in that ligand. Results and Discussion The focus of this paper is on the supramolecular reactivity of the previously little explored porphyrin ligand, which is prone to coordinate internally, via its pyrrole N-atoms, and externally, via the peripheral pyridyl N-sites, to transition metal ions, and engage in the formation of coordination networks. In compounds 1, 2, 4, and 5 the hydrothermal reactions of the freebase T3PyP with the corresponding metal reagents were associated with full metalation of the porphyrin core to the respective MT3PyP moieties (M = CoII, CdII, CuII). In 3 only residual metalation of the porphyrin ligand has been observed. The respective ionic radii of CuII and high-spin CoII ions are 0.73 and 0.74 A˚, and these ions fit perfectly into the porphyrin macrocycle (after extraction of the two inner H-atoms). On the other hand, the CdII ion is much bigger (with ionic radius of 0.95 A˚) to fit inside, and when coordinating to the N(pyrrole) sites it can be placed only in a perching position either above or below the macrocyclic ring. The endocyclic as well as the exocyclic metal ion auxiliaries (or in 5 also clusters of metal ions) can serve effectively as connecting nodes between the porphyrin scaffolds. Compounds 1 and 2 represent a 2D mode of direct interporphyrin association of the CoIIT3PyP units, through endocyclic (in the porphyrin center)
Co ions. It involves coordination of every such unit to four neighboring CoT3PyP moieties. The cobalt ion in the porphyrin core binds trans-axially to two pyridyl N-atoms of two adjacent CoT3PyP molecules, while the two trans-related pyridyl functions associate to the metal centers of two additional porphyrins (Figure 1). The coordination polymers in the two structures are similar to one another. In both structures, the CoT3PyP entities are located on centers of inversion, the metal ion revealing an octahedral coordination environment. The two axial Co-N(pyridyl) bonds are perpendicular to the plane of the porphyrin core. Neighboring molecules in the network are related by the glide-plane symmetry, their porphyrin planes being inclined by about 68 to each other. This herringbone arrangement of the molecular units within the network fills effectively the interporphyrin space. In the latter respect, it differs from similarly connected networks of the zinc-tetra(4-pyridyl)porphyrin derivative, which form polymeric layers of square-grid geometry with void space between the individual units.9,10 The upper and lower surfaces of the 2D nets in 1 and 2 are lined with the two uncoordinated pyridyl substituents oriented toward the interface between neighboring coordination layers (from above and below) in the crystal. These pyridyl substituents have a slightly different orientation with respect to the porphyrin framework in the two structures (Figure 1), as a result of the different modes of interlayer packing (Figure 2).
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Figure 3. (a) The diamondoid-type coordination scheme in 3, revealing tetrahedral binding geometry around the porphyrin component of the polymer. The CdCl2 connecting nodes and the N(pyridyl) sites are denoted by small spheres. (b) The doubly interpenetrated architecture of two polymeric nets, viewed down the b-axis of the crystal (a is horizontal). One of these nets is outlined by yellow shading.
In 1, the [CoT3PyP]¥ polymeric layers stack along the a-axis of the crystal with a perpendicular separation of 10.9 A˚ between them, allowing the outward pyridyls of one layer to interdigitate between those of an adjacent layer (Figure 2a). This interlayer arrangement increases to 14.0 A˚ upon intercalation of the DMF crystallization solvent between the polymeric arrays in 2 (Figure 2b). Structure 2 is isomorphous with the DMF intercalate of ZnT3PyP reported recently in considerable detail.7 It has been indicated that the [ZnT3PyP] 3 2DMF compound undergoes a phase transformation when the solvent molecules are removed by heating. Yet the structure of the desolvated phase could not be directly identified by single-crystal X-ray diffraction due to lack of suitable crystals. Structure determination of 1 in this work fills this gap, by precise characterization of the solvent-free crystalline polymer. Evidently, the interlayer spacing in the [CoT3PyP]¥ desolvated phase (10.9 A˚; in perfect agreement with the interlayer distance in the structure of solvent-free ZnT4PyP)9 is in fact much shorter than that derived earlier from X-ray powder diffraction data (accompanied by computational modeling) for the analogous [ZnT3PyP]¥ solvent-free structure (assessed to be 12.4 A˚).7 The proposed unit-cell data for the latter are also inconsistent with those of the observed structure of 1, indicating once again that crystal structure prediction even with knowledge of partial relevant information (essentially identical patterns of the 2D polymerization) is not an easy task for complex systems. It appears that the relative offset of neighboring layers and mutual orientations of their interdigitating 3-pyridyl arms required to optimize the intermolecular organization in the solvent-free structure could not be assessed reliably by modeling. It is also possible
that the calculated structure represents a less densely packed polymorph of the solvent free ZnT3PyP. Unfortunately, no single-crystal-to-single-crystal transformation between 1 and 2 could be observed to monitor precisely the structural changes that occur during the solvation/desolvation process. The coordination patterns in 1 and 2 represent nearly identical uninodal 4-connected 2D nets.11 Successful formulation of coordination networks has been observed also by reacting T3PyP with CdCl2. The stereochemistry of d10 CdII is similar to that of ZnII ions, both lacking ligand/crystal field stabilization effects and exhibiting diverse coordination geometries (most commonly tetrahedral, square-pyramidal, and octahedral). As the cadmium ion is too big to fit into the porphyrin core (see above), it can coordinate to the N(pyrrole) sites of the porphyrin core either from above or below, imparting to such an adduct a domed shape, and complement its coordination sphere by an axial ligand from the convex side. It may also serve as a perfect exocyclic connector to the peripheral pyridyl sites of several porphyrin units, as observed earlier for the tetra(4-pyridyl)porphyrin analogue.12 However, due to the conformational flexibility of the T3PyP scaffold new modes of coordination networking with metal ions are anticipated in the present case. Crystalline 3 and 4 compounds of different morphologies were obtained concomitantly from the same reaction pot, reflecting well the diverse possible orientations of the N-coordinating sites of the T3PyP indicated above. Compound 3 represents a 3D coordination network between the porphyrin and the CdCl2 constituents (Figure 3). Most of the porphyrin is present in these crystals in a free base form with only marginal metalation (12% by X-ray diffraction analysis)
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Figure 4. Compound 4. (a) Illustration of the chairlike conformation adopted by the Cd(DMF)T3PyP moiety, and its tetradentate coordination to exocyclic cadmium connectors. The endocyclic cadmium ions reveal 2-fold disorder, being positioned 0.65 A˚ either above or below the porphyrin macrocycle. The disordered DMF axial ligands to these ions are omitted. (b) Wireframe view (approximately down the c-axis, a is horizontal) of the single-framework polymeric architecture sustained by the CdCl2 connectors (denoted by small spheres). The intralattice voids are filled effectively by the disordered DMF axial ligands (not shown).
of the porphyrin core, which will be ignored in further discussion of this structure. The CdCl2 moieties were found to coordinate externally to the N-sites of the four 3-pyridyl groups at Cd-N distances of 2.387(5) and 2.395(4) A˚. The cadmium ions are characterized by an octahedral coordination environment with the chloride ions occupying trans positions, while the pyridyl functions of the four converging porphyrin units are arranged around the metal ions in a square-planar fashion. On the other hand, the T3PyP component assumes a tetrahedral conformation with two pairs of trans-related pyridyls oriented respectively above and below the slightly distorted (from planarity) porphyrin macrocycle. This connectivity scheme, where every porphyrin node is coordinated to four cadmium ions and each of the metal nodes is bound to four different porphyrin units describes an open coordination network of pseudodiamondoid topology which expands in 3D space. The entire crystal structure is composed of 2-fold interpenetrated polymeric nets, so that the framework of one net fills in an effective manner the empty space within the other net. Structure 4 represents another product from the same hydrothermal reaction between T3PyP and CdCl2 (Figure 4). It is associated with a different porphyrin building block, which is now metalated by cadmium ions (which in a given unit perch on the porphyrin core in a disordered manner from either above or below). The latter are five-coordinate to the four pyrrole N-sites [at Cd-N = 2.374(3) and 2.380(3) A˚] and axially from the other side to a DMF ligand [at Cd-O = 2.293(3) A˚]. The endocyclic cadmium ion deviates by 0.651(1) A˚ from the plane of the four pyrrole N-atoms toward the axial DMF. The porphyrin moiety also assumes in this case a chairlike conformation with two pairs of cisrelated pyridyls oriented in opposite directions (Figure 4a). Every porphyrin entity connects through the CdCl2 connectors to neighboring porphyrin scaffolds, the cadmium centers at each site linking in a square-planar geometry to four pyridyl functions of different porphyrin units (as in 3). The tetradentate nature of the Co(DMF)T3PyP and CdCl2 components leads to a single-framework polymeric architecture of the hybrid product (Figure 4b). Materials 3 and 4 represent binodal 4-connected coordination frameworks with 3D connectivity but are characterized by different topologies.11 Crystals of compound 4 deteriorate quickly upon heating, most probably due to desolvation of the
DMF component, as opposed to those of solvent-free 3, which appear to be stable for weeks. Reaction of the T3PyP with copper dibromide yielded a polymeric structure 5 (Table 1) with CuII ions inserted into the porphyrin core and (CuIBr)4 tetranuclear clusters as the exocyclic interporphyrin linkers. 5 is isometric and isomorphous to the structures obtained in similar reactions of the porphyrin with copper dichloride and copper iodide, as reported in a preceding paper.5 In all three cases, the resulting product represents a single-framework 3D coordination polymer with doubly interpenetrated diamondoid hybrid networks. The porphyrin entity adopts the tetrahedral-type conformation and the corresponding tetranuclear (CuX)4 (X = Cl, Br, I) cluster is characterized by a complementary tetrahedral coordination environment, an ideal situation for the construction of diamondoid nets. In 5, the endocyclic CuN(pyrrole) = 1.998(3) A˚ and the exocyclic Cu-N(pyridyl) = 2.025(3) A˚. Within the inorganic tetranuclear connector the Cu-Br and Cu-Cu bonds are 2.3675(6) and 2.7702(9) A˚, respectively, in perfect agreement with earlier observations. This structure type is discussed in full detail in a previous report.5 In summary, in spite of the initial difficulties experienced with the exploration of the supramolecular crystal chemistry of the T3PyP building block, we have been able to demonstrate the versatile molecular recognition features of this little explored scaffold in crystal engineering. We have shown here and in the previous report5 the utility of this ligand in the construction of 2D and 3D coordination networks with endocyclic as well as exocyclic manganese, cobalt, copper, and cadmium mononuclear as well as tetranuclear metal ion connectors. Several different modes of intermolecular association via hydrogen bonds into 3D supramolecular assemblies have been observed as well. The latter include hydrogen bonding of the porphyrin scaffold in its neutral as well as pyridyl-protonated forms to the coordination sphere of lanthanoid metal complexes, and self-hydrogen bonded networks of the protonated porphyrin moieties. The conformational versatility of T3PyP enriches the molecular recognition features of this scaffold, demonstrating new types of unique coordination architectures that can be realized with the tetrapyridylporphyrin building blocks, and providing a further challenge for the design of attractive porphyrin-based framework solids.
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Table 1. Crystal and Experimental Data for Structures 1-5 formula Fw crystal system space group a [A˚] b [A˚] c [A˚] R [] β [] γ [] V [A˚3] Z T [K} Fcalcd [Mg m-3] μ [mm-1] F(000) crystal size [mm3] θmax [] refln collected refln unique R(int) completeness refln with I > 2σ(I) refined parameters R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 [all data] wR2 [all data] (ΔFmax [e A˚-3] average C-C bond precision /A˚
1
2
3a
4b
5
C40H26CoN8 675.60 monoclinic P21/c 11.5703(5) 9.8865(4) 13.9700(9) 90.0 109.966(2) 90.0 1502.0(1) 2 110(2) 1.494 0.618 694 0.15 0.15 0.02 26.50 11167 3110 0.071 100% 2462 223 0.064 0.124 0.089 0.134 þ0.40, -0.67 0.005
C46H38CoN10O2 821.79 monoclinic C2/c 29.6174(10) 9.5015(3) 14.1841(5) 90.0 109.529(2) 90.0 3761.9(2) 4 110(2) 1.451 0.513 1708 0.20 0.10 0.05 27.90 13986 4466 0.119 99% 2376 299 0.060 0.105 0.146 0.129 0.45, -0.49 0.004
C20H12.9Cd0.56ClN4 407.07 orthorhombic Pnna 15.2583(3) 14.7895(3) 15.5834(5) 90.0 90.0 90.0 3516.6(2) 8 110(2) 1.538 0.880 1634.3 0.20 0.15 0.15 27.86 27680 4174 0.085 99% 2225 236 0.062 0.149 0.180 0.123 þ2.78, -1.04 0.009
C41H24Cd2Cl2N8O 940.38 orthorhombic Pnma 26.0670(2) 21.1688(6) 7.6354(7) 90.0 90.0 90.0 4213.3(4) 4 110(2) 1.482 1.176 1856 0.30 0.15 0.15 27.86 18214 5129 0.051 100% 3799 256 0.048 0.115 0.072 0.123 þ1.41, -0.98 0.005
C40H24Br4Cu5N8 1254.01 tetragonal I41/a 15.0113(3) 15.0113(3) 17.3605(4) 90.0 90.0 90.0 3912.0(1) 4 110(2) 2.129 6.803 2420 0.25 0.25 0.20 27.86 9214 2321 0.034 100% 1854 129 0.038 0.096 0.053 0.103 þ1.19,-0.76 0.005
a The porphyrin center is metalated with CdII ions at 12% occupancy. b The disordered -NMe2 fragments of the DMF axial ligand were excluded from the structural model.
Experimental Section Supramolecular Syntheses and Crystallization Procedures. The T3PyP freebase porphyrin compound, various transition metal reagents, and grade solvents were procured commercially and used without further purification. Compounds 1-5 were obtained by the following procedures: (1) T3PyP (0.010 mmol) and potassium hexacyanocobaltate(III) (0.009 mmol) were placed in sealed reactor with methanol/DMF (1:1, 2 mL) solution and heated to 120 C for 3 days in a dry bath. After gradual cooling to room temperature (at ∼0.2/min), two kinds of crystals were obtained; pale yellow rhomb crystals of unidentified cobalt salt and purple thin square crystals of CoIIT3PyP. Yield: 22% based on porphyrin. Reduction of the CoIII reagent to CoII has occurred due to the known tendency (and weak reductive nature) of the DMF to decompose at elevated temperatures to formic acid and dimethylamine. DMF is commonly used to dissolve the porphyrin component, sparingly soluble in other standard organic solvents. It is reasonable to assume that the involvement of DMF in the redox process in this case allowed the crystallization of the solvent-free material, which could not be obtained with the analogous ZnIIT3PyP compound.7 (2) T3PyP (0.010 mmol), dissolved in DMF, and cobalt(II) thiocyanate (0.020 mmol), dissolved in ethanol, were placed in a sealed reactor and heated to 100 C for 3 days in a dry bath. X-ray quality red crystals were obtained in the resulting solution after slow cooling (at ∼0.2/min) to room temperature. Yield: 74.2% based on porphyrin. (3 and 4) A 0.0088 M solution of T3PyP was prepared by dissolving 54.6 mg of the porphyrin in 10 mL of DMF followed by reflux for 2 h after filtration. When 1 mL of this solution was added to 1 mL of ethanol solution of cadmium(II) chloride (0.033 mmol), a precipitate was obtained immediately. The precipitate in DMF/ethanol was placed in a sealed reactor and heated to 100 C for 3 days in a dry bath. After slow cooling to room temperature (at ∼0.2/min), two kinds of crystals were obtained concomitantly: purple shining prism crystals of compound 3 (with the porphyrin component predominantly in a freebase form), and red rhomb crystals of compound 4 (with the porphyrin component
in the Cd(DMF) five-coordinate metalated form). The yield of the latter could not be determined as they deteriorated rapidly during the drying process. Upon extraction of the remaining crystalline product of 3, its yield was determined to be approximately 18% (based on porphyrin). Efforts to obtain a single crystalline phase of each compound have been unsuccessful thus far. (5) T3PyP (0.010 mmol) and copper(II) bromide (0.038 mmol) were placed in sealed reactor with methanol/DMF (1:1, 2 mL) solution and heated to 100 C for 3 days in a dry bath. After slow cooling, X-ray quality red very small cube crystals were obtained. Yield: 92% based on porphyrin. Further efforts to synthesize coordination polymers of T3PyP with zinc ions have led repeatedly to the already known DMF clathrate of self-coordinated ZnT3PyP compound (isomorphous with 2).7 All the crystalline products were kept in their respective crystallization solutions, until their mounting on the X-ray diffractometer at low temperature (110 K). The uniform identity of the formed crystal lattices (1-5) in a given reaction was confirmed in each case by repeated measurements of the unit-cell dimensions from different single crystallites of the same morphology. Crystallography. The X-ray measurements (Nonius KappaCCD diffractometer, MoKR radiation) were carried out at 110(2) K on crystals coated with a thin layer of amorphous oil to minimize crystal deterioration, possible structural disorder, and related thermal motion effects, and to optimize the precision of the structural results. These structures were solved by direct methods (SIR-97) and refined by full-matrix least-squares (SHELXL-97).13 The crystallographic refinements of structures 1-5 converged smoothly to relatively low R-values. In 3, the porphyrin units were found to be partly (0.12%) metalated by the CdII-ions. In compound 4, the DMF molecules located axially to the five-coordinate Cd-ions disordered about the porphyrin center were found rotationally disordered, and their -NMe2 fragments could not be modeled reliably by discrete atoms. The contribution of these fragments was subtracted from the diffraction pattern by the SQUEEZE procedure and PLATON software.14 CCDC reference numbers 795774-795778 (for structures 1-5, respectively). For crystallographic data in CIF format see Supporting Information.
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Acknowledgment. This research was supported by The Israel Science Foundation (Grant No. 502/08). Supporting Information Available: X-ray crystallographic files in CIF format for the five newly determined crystalline solids 1-5. This information is available free of charge via the Internet at http:// pubs.acs.org.
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