Porphyrin Framework Solids. Hybrid Supramolecular Assembly Modes

DOI: 10.1021/cg901505m

Porphyrin Framework Solids. Hybrid Supramolecular Assembly Modes of Tetrapyridylporphyrin and Aqua Nitrates of Lanthanoid Ions

2010, Vol. 10 1823–1832

Sophia Lipstman and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, 69978 Ramat-Aviv, Tel-Aviv, Israel Received December 2, 2009; Revised Manuscript Received January 17, 2010

ABSTRACT: New types of porphyrin-based framework solids were constructed by reacting meso-tetra(4-pyridyl)porphyrin (TPyP) with various aqua nitrate salts of lanthanoid metal ions [Ln(NO3)3(H2O)x]. They represent hybrid organic-inorganic crystalline compounds in which the tetradentate porphyrin units, having both coordination as well as hydrogen bonding functionalities, are interlinked through the inorganic connectors into self-assembled three-dimensional architectures. The lanthanoid complexes bear, in addition to the coordination capacity of the metal ions, multiple molecular recognition sites on their nitrate and water ligands, serving as effective linkers of the supramolecular arrays that form. Several different modes of the intermolecular association were revealed by single-crystal X-ray diffraction. They differ by the nature of the interaction synthons between the component species, the degree of protonation of the porphyrin entities in the given experimental conditions, and the topology of the resulting assemblies. An exceptional structure, in which a doubly protonated porphyrin unit and the lanthanoid species formed separate hydrogen bonded arrays (where one-dimensional chains of the latter are enclathrated within two-dimensional layers of the former) by reacting the meso-tetra(3-pyridyl)porphyrin (TPy(3)P) with a dehydrated La-salt, is described as well. All the compounds are characterized by open architectures, giving rise to latticeinclusion of solvent species. This study demonstrates that the TPyP building block may effectively engage in supramolecular constructs with the lanthanoid metal ions, either through direct coordination to the metal center and/or by hydrogen bonding to its coordination sphere ligands.

*To whom correspondence should be addressed. E-mail: goldberg@ post.tau.ac.il.

Fe,8 Hg,6b,9 Pb,6b and V.7b In addition, a number of structures with exocyclic coordination to the TPyP macroring of Ir, Rh, or Ru ions in discrete complexes have been reported as well.10 In contrast to the so intensive investigations of the TPyP/ MTPyP system in the coordination-driven self-assembly context, the hydrogen-bonding capacity of this scaffold in the formulation of supramolecular networks has not been explored systematically in the past. Construction of assemblies sustained by hydrogen bonds requires reaction of the TPyP Lewis base with, for example, complementary organic acid linkers. The porphyrin component has its pyridyl sites available for hydrogen bonding as proton acceptors and may interact best in a cooperative manner with tetradentate proton-donating partners of similarly square-planar geometry. Utilizing this concept, we have reported very recently on the structures of extended square-grid heteromolecular networks synthesized by reacting TPyP/ZnTPyP with benzene tri-, four-, and hexa-carboxylic acids.11 A small number of other structures, in which the TPyP entity is involved in supramolecular hydrogen-bonding, have been described in prior publications.12 During our targeted synthesis effort of porphyrin-based coordination polymers, we noticed that the use of lanthanoid ions as interporphyrin connectors was rather rare in earlier studies in spite of their attractive photophysical properties. This stimulated our further effort in this direction, with a successful construction of new hybrid organic-inorganic stable frameworks with open architectures composed of the TCPP moieties and various lanthanoid (Ln) ions.13 The coordination affinity of the oxophilic Ln metals for pyridyl sites seems to be considerably weaker than for the carboxylic sites in TCPP. Only a few coordination polymers and molecular complexes of Ln ions with simple bipyridyl ligands are

r 2010 American Chemical Society

Published on Web 02/09/2010

Introduction Crystalline engineering of porphyrin-based solid assemblies received remarkable attention in recent years due to their potential utility in a wide range of applications, including, for example, gas storage, molecular sensing, and heterogeneous catalysis. The tetrapyridylporphyrin (TPyP) and tetra(carboxyphenyl)porphyrin (TCPP) ligands in particular have played a key role in the construction of diverse polymeric architectures.1 They are characterized by rigid square-planar geometry, bear multiple diverging molecular recognition sites for metal coordination as well as hydrogen bonding, and consequently reveal an extraordinarily rich supramolecular chemistry. Of particular interest is the coordination polymerization of these scaffolds and their metal complexes (MTPyP and MTCPP) through exocyclic metal ion connectors,1-9 which often results in the formation of robust porous architectures with remarkable sorption and desorption features.2 The metal ions inserted into the porphyrin core may provide additional coordination capacity, allowing self-coordination of the porphyrin entities into framework architectures, without the use of external connectors.3 Noteworthy in the latter context is the synthesis of the robust MTPyP molecular sieve material of honeycomb architecture perforated by 0.5-0.6 nm wide channels and characterized by uniquely high thermal stability.3a,4 A survey of the Cambridge Crystallographic Database indicates that the TPyP/MTPyP moieties provide excellent building blocks for the formation of extended (twoor three-dimensional, 2D or 3D) hybrid coordination polymers of diverse topologies when reacted with various transition metal ion connectors. The latter include Ag,5 Cd,6 Cu,7

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Crystal Growth & Design, Vol. 10, No. 4, 2010

Lipstman and Goldberg

Scheme 1. Component Building Blocks for the Supramolecular Assembly in 1-11

known,14 but analogous compounds with the TPyP unit have not been explored thus far. On the other hand, a number of double-decker-type compounds in which Ln ions are sandwiched between, and coordinated by, the central pyrrole rings of two TPyPs,15 or between one TPyP and one phthalocyanine ring,16 are known. In this work, we explore the networking capacity of TPyP with several Ln ions, while taking advantage of the abovedescribed coordination as well as hydrogen bonding features of the porphyrin entity. This has been done by reacting the TPyP with neutral aqua nitrate salts of the trivalent metals [LnIII(NO3)3(H2O)x], as possible interporphyrin connectors (Scheme 1). The nitrate anions are tightly coordinated to the metal center, and occupy six of its coordination sites. Along with the metal-bound water molecules, they provide also attractive sites for hydrogen bonding with the surrounding moieties, both in the lateral and axial directions. In the supramolecular reaction, possible hydrogen bonding between the pyridyl functions of TPyP and the [LnIII(NO3)3(H2O)x] complex represents competing interaction to a direct linkage formation between the metal and the pyridyl sites of the porphyrin (e.g., through replacement of the H2O-Ln by Npy-Ln bond) in the coordination sphere of Ln. A preliminary report on the first complex of the TPyP moiety with lanthanum ion, where the coordination takes place on the porphyrin periphery between the metal ions and one of the pyridyl substituents of TPyP has been published.17 It demonstrates that the metal ion is coordinated to a single pyridyl site forming a 1:1 metal-porphyrin coordinated complex, while units of the latter are interlinked to one another in three dimensions by a network of hydrogen bonds between the metal-coordinated water ligands, nitrate ions, and pyridyl groups of adjacent species. Our preparative efforts have led in this work to the successful construction of an additional series of three-dimensionally interlinked (via coordination and/or hydrogen bonding) open architectures of hybrid organic-inorganic assemblies. The pores within the observed frameworks are occupied in all cases by additional molecules of the crystallization solvent, either o-dichlorobenzene (o-DCB) or benzene. The available crystalline framework solids that could be reliably analyzed in the present study include the following adducts: (a) [Ln(NO3)3(H2O)2] 3 TPyP 3 3(o-DCB); Ln=Nd (1), Sm (2), Gd (3), and Tb (4). (b) [Ln(NO3)3(H2O)3] 3 TPyP 3 2(C6H6); Ln = Dy (5) and Yb (6). (c) [Yb(NO3)3(H2O)3] 3 TPyP 3 2(o-DCB) (7) [Ln(NO3)3(H2O)2(EtOH)] 3 TPyP 3 3(o-DCB); Ln = Dy (8) and Yb (9). In the latter two structures, a molecule of the ethanol solvent replaced one molecule of water in the coordination sphere of the metal ions. The o-DCB turned out to be the preferred solubilizing reagent, due to its apparent tendency to

weakly interact (through the polarizable Cl-atoms) with the nitrate anions. The corresponding reactions were carried out in a large excess of the metal complex and an acidic aqueous environment, which led occasionally to additional products associated with protonation of the pyridyl groups on the porphyrin ligand and a correspondingly varying content of the metal-coordinated nitrate anions. Relevant examples include compounds [Sm(NO3)4(H2O)2]- 3 (TPyPH)þ 3 31/2(o-DCB) (10), and a product of the reaction between lanthanum trinitrate hexahydrate with the tetra(3-pyridyl)porphyrin (TPy(3)P) isomer [La(NO3)5(EtOH)]2- 3 (TPy(3)PH2)2þ 3 (o-DCB) (11) which is characterized by a unique intermolecular organization. Further attempts to construct other variants of the lanthanoid-TPy(3)P network materials (parallel to the lanthanoid-TPyP assemblies 1-10) have been unsuccessful so far, as their X-ray quality crystals could not be obtained. Results and Discussion Successful preparation of all the hybrid lanthanoidporphyrin compounds required the use of a large excess of the lanthanoid aqua nitrate salts, [LnIII(NO3)3(H2O)x=5,6], in the corresponding reactions. Crystal formation was in most cases by the diffusion-between-layers technique, and also by slow evaporation (see Experimental Section). In the solid structures, the metal ions are characterized by coordination numbers of 9-11.13 In compounds 1-9, the three doubly metal-bound nitrate anions account for six coordination sites. Two water molecules ligate to the metal in the coordination polymers 1-4, while three water/ethanol ligands bind to the metal center in the purely hydrogen bonded adducts 5-9. The Ln-coordination sphere in the former structures is complemented by two pyridyl ligands of adjacent porphyrin species. The facile protonation of the pyridyl N-atoms in the given experimental conditions (large excess of the aqueous nitrate salts) provides additional degrees of freedom and allows the inclusion of a varying number of the nitrate anions in the crystal lattice to balance the positive charge of the metal ions and the pyridinium groups (as in compounds 10 and 11). The afforded new crystalline products 1-11 were characterized by single-crystal X-ray diffraction. The crystallographic and experimental data are given in Table 1. Structures 1-10 exhibit three-dimensional connectivity schemes of the component species, in which the metal complexes act as connectors between neighboring porphyrin moieties by coordination and/or hydrogen-bonding cooperative interaction synthons. The corresponding bond-distance ranges are listed in Table 2. Structure 11 represents an outstanding example, where separate hydrogen-bonding arrays are formed, 2D assemblies

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Table 1. Crystal and Experimental Data for Structures 1-11 formula Fw crystal system space group a [A˚] b [A˚] c [A˚] R [°] β [°] γ [°] V [ A˚3] Z 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)]b wR2 [I > 2σ(I)] R1 [all data]b wR2 [all data] (ΔFmax [e A˚-3] average C-C bond precision/A˚ excluded solvent removed density [e/unit-cell]21

1a

2a

3a

4a

C40H30N11NdO11 984.99 monoclinic C2/m 13.0491(3) 30.6480(9) 30.4249(8) 90.0 102.120(1) 90.0 11896.6(5) 8 1.100 0.925 3960 0.25 0.15 0.05 25.00 24545 10650 0.095 99% 7005 574 0.068 (0.157) 0.156 0.104 (0.219) 0.166 (0.305) þ1.64, -0.64 0.009

C40H30N11O11Sm 991.10 monoclinic C2/m 12.8736(3) 30.7602(7) 29.9152(9) 90 101.086(1) 90.00 11625.2(5) 8 1.133 1.063 3976 0.45 0.10 0.10 27.88 41464 13830 0.098 98% 9824

C40H30GdN11O11 3 C6H4Cl2 1144.99 monoclinic C2/m 12.8400(3) 30.6950(9) 30.1003(8) 90.0 101.550(1) 90.0 11623.0(5) 8 1.309 1.293 4584 0.20 0.15 0.05 27.90 40159 14000 0.067 99% 10134 665 0.052 (0.095) 0.116 0.078 (0.127) 0.122 þ0.94, -0.78 0.006

C40H30N11O11Tb 3 C6H4Cl2 1146.67 monoclinic C2/m 12.8615(3) 30.5798(9) 30.3077(8) 90.0 102.060(1) 90.0 11657.0(5) 8 1.307 1.364 4592 0.50 0.20 0.02 27.96 60056 14172 0.101 99% 9848 665 0.060 (0.112) 0.117 0.095 (0.149) 0.125 þ1.19, -0.89 0.007

2C6H4Cl2 1423

2C6H4Cl2 1134

not refined

3C6H4Cl2 2161 5a

formula Fw crystal system space group a [A˚] b [A˚] c [A˚] R [°] β [°] γ [°] V [ A˚3] Z 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)]b wR2 [I > 2σ(I)] R1 [all data]b wR2 [all data] (ΔFmax [e A˚-3] average C-C bond precision/A˚ excluded solvent removed density [e/unit-cell]21

formula Fw crystal system space group a [A˚] b [A˚]

6a 1

1

7a

8 C42H36DyN11O12 3 3C6H4Cl2 1490.29 triclinic P1 14.5187(2) 15.0360(2) 17.8593(2) 65.5289(4) 89.1791(5) 62.7646(5) 3080.2(1) 2 1.607 1.545 1498 0.10 0.05 0.05 25.00 28357 10783 0.091 99 8786 812 0.064 0.150 0.085 0.161 þ2.30, -1.72 0.012

C40H32DyN11O12 3 1 /2C6H6 1138.43 triclinic P1 6.4267(1) 15.3468(4) 27.4025(8) 74.182(1) 88.248(1) 88.357(1) 2598.6(1) 2 1.455 1.508 1148 0.40 0.10 0.05 28.03 26466 12451 0.053 99 9176 658 0.049 (0.055) 0.107 0.078 (0086) 0.116 þ2.69, -1.91 0.006

C40H32N11O12Yb 3 1 /2C6H6 1148.97 triclinic P1 6.4060(2) 15.3026(4) 27.4509(7) 74.199(2) 88.230(2) 88.562(1) 2587.7(1) 2 1.475 1.878 1156 0.60 0.15 0.05 27.92 22791 12300 0.061 99 9291 658 0.062 (0.070) 0.157 0.086 (0.096) 0.166 þ1.88, -1.33 0.011

C40H32N11O12Yb 1031.81 monoclinic C2/c 28.6441(3) 31.7824(3) 12.7953(1) 90.0 101.8269(5) 90.0 11401.3(2) 8 1.202 1.697 4120 0.50 0.30 0.30 27.87 39263 13569 0.038 100 9980 577 0.039 (0.080) 0.096 0.055 (0.104) 0.101 þ2.53, -1.47 0.005

1/2 C6H6 37

1/2 C6H6 52

2 C6H4Cl2 988

9

10a

11

C42H36N11O12Yb 3 3C6H4Cl2 1500.83 triclinic P1 14.5139(2) 14.9753(2)

C40H31N12O14Sm 3 3C6H4Cl2 1495.09 orthorhombic Pbca 16.3656(2) 27.4265(3)

C42H34LaN13O16 3 C6H4C12 1262.72 monoclinic Cc 16.9127(2) 20.7331(3)

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Lipstman and Goldberg Table 1. Continued 10a

9 c [A˚] R [°] β [°] γ [°] V [ A˚3] Z 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)]b wR2 [I > 2σ(I)] R1 [all data]b wR2 [all data] (ΔFmax [e A˚-3] average C-C bond precision/A˚ excluded solvent removed density [e/unit-cell]21

17.8340(2) 65.5496(4) 89.3807(5) 63.1472(5) 3073.2(1) 2 1.622 1.854 1506 0.30 0.25 0.20 27.88 33660 14458 0.031 99% 13022 812 0.035 0.085 0.041 0.088 þ1.38, -1.77 0.005

11

30.3946(4) 90.0 90.0 90.0 13642.6(3) 8 1.456 1.164 6008 0.40 0.40 0.20 27.87 66781 16057 0.054 99% 9291 658 0.049 (0.067) 0.120 0.083 (0.106) 0.133 þ2.09, -1.13 0.006

14.8833(2) 90.0 102.1690(7) 90.0 5101.6(2) 4 1.644 1.028 2544 0.50 0.35 0.25 27.88 22516 10147 0.042 98% 9181 723 0.047 0.121 0.054 0.126 þ2.92, -0.89 0.007

1/2C6H4Cl2 504

a Excluding the solvent (bottom line) diffused within the interporphyrin channels (see Experimental Section). b Data in parentheses refer to refinements based on the uncorrected diffraction data.

Table 2. Coordination and Hydrogen Bonding Distance Ranges (A˚) in 1, 3-11 compound (type of metal)

M-Npy a

M-O (water/EtOH)

M-O (nitrate)

2.430-2.450(4) 2.358-2.393(2) 2.347-2.380(3)

2.522-2.639(4) 2.471-2.651(3) 2.451-2.649(3)

1 (Nd) 3 (Ga) 4 (Tb)

2.651-2.664(5) 2.589-2.603(3) 2.578-2.600(4)

M-O(water/EtOH)

M-O(nitrate)

5 (Dy) 6 (Yb) 7 (Yb) 8 (Dy) 9 (Yb)

2.307-2.339(3) 2.264-2.293(3) 2.250-2.286(2) 2.298-2.387(5)b 2.251-2.337(2)b

2.439-2.474(4) 2.397-2.438(3) 2.389-2.467(2) 2.423-2.515(5) 2.381-2.481(2)

10 (Sm) 11 (La)

NH 3 3 3 O(water)/NH 3 3 3 N 2.735(4) 2.617-2.644(6)

O(water) 3 3 3 Npy 2.678-2.722(7) 2.697-2.738(4) 2.686-2.717(5)

O(water) 3 3 3 Npy 2.660-2.741(4) 2.664-2.739(5) 2.636-2.724(3) 2.650-2.828(7) 2.659-2.809(3)

M-O(water/EtOH)

M-O(nitrate)

2.355-2.504(2) 2.503(4)b

2.490-2.575(3) 2.599-2.718(5)

O(water) 3 3 3 Npy 2.657-2.742(4)

O(water/EtOH) 3 3 3 O(nitrate) 2.756-2.860(6) 2.749-2.818(4) 2.752-2.844(4) O(water/EtOH) 3 3 3 O(nitrate) 2.867-2.869(4) 2.853-2.894(5) 2.808-2.874(3) 2.916(7)c 2.927(3)c O(water/EtOH) 3 3 O(nitrate) 2.907(3) 2.765(7)

a The indicated esd values for the distance-ranges represent an average esd for all bond distances within the given range. b The upper most value relates to the M-O bond of the ethanol ligand. c These values refer to the O(EtOH) 3 3 3 O(nitrate) hydrogen bond.

of the porphyrin units which enclathrate one-dimensional (1D) chains of the lanthanoid components. Compounds 1-4 are isomorphous and of identical composition, and therefore only the structural features of the representative and most precisely determined [Gd(NO3)3(H2O)2] 3 TPyP adduct (3) will be discussed. The assembly pattern in this compound can be best described in a modular way. The first motif is a zigzag-chain coordination polymer with alternating components, wherein every lanthanoid ion binds in a trans fashion to two pyridyl groups of two adjacent TPyP units, and every porphyrin molecule coordinates through two cis-related pyridyl functions to two neighboring lanthanoid connectors (Figure 1). The coordination sphere of the metal node includes two N-pyridyl ligands [Gd-N = 2.589(3) and 2.603(3) A˚], two molecules of water [Gd-O = 2.358(2) and 2.393(2) A˚], and three doubly associated nitrate ions [the six Gd-Onitrate bonds are within 2.471(3)-2.651(3) A˚] (Table 2), being characterized by a coordination number of 10 and a balanced charge. The surface of the polymeric chain thus

formed is lined with sites of hydrogen bonding capacity: the two uncoordinated pyridyl arms (which point outward) and the nitrate ions, as proton acceptors, and the two water ligands, as proton donors. Optimization of the hydrogen bonding interaction synthons leads to the formation of tightly packed layered 2D arrays of the polymeric chains linked to one another by HO-H 3 3 3 Npyridyl (O 3 3 3 N = 2.697(4) and 2.738(4) A˚) bonds (Figure 1, Table 2). These corrugated layers are aligned roughly parallel to the (-1, 0, 2) plane of the crystal. The second H-atom of each water ligand is involved in water 3 3 3 nitrate hydrogen bonding (O 3 3 3 O = 2.749(4) and 2.818(4) A˚) between such layers along the normal direction, as illustrated in Figure 2. The metal complex is involved in total of six hydrogen bonds. The molecular recognition scheme of this structure type represents therefore a continuous array of specific interaction synthons (a combination of coordination and hydrogen bonding) that propagates throughout the crystal lattice. The hydrogen bonded columns of the lanthanoidcomplex connectors serve as construction pillars of this

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Figure 1. Illustration of the layered supramolecular arrangement of the molecular components in 3 via coordination and hydrogen bonding interaction synthons. The polymeric zigzag coordination mode is indicated by a dashed line. Adjacent polymeric chains of intercoordinated species are denoted by different shading. The hydrogen bonds between the polymeric arrays are marked by dotted lines. The solvent species that accommodate the interporphyrin voids are excluded.

Figure 2. View of the crystal structure of 3 down the b-axis, showing the stacking of the corrugated supramolecular layers shown in Figure 1. The water 3 3 3 nitrate hydrogen bonds that operate between these layers are denoted by dotted lines, extending parallel to the a-direction.

structure taking part in the intralayer as well as interlayer binding of the component species. This leads to the formation of a hybrid 3D supramolecular array, which is perforated by channel voids that propagate through the layered arrays and parallel to the a-axis of the crystal. These canals are occupied by disordered molecules of the o-DCB solvent. The total noncoordinating solvent accessible void space in 1-4 amounts to nearly 35% of the crystal volume. In addition to the Nd, Sm, Gd, and Tb compounds, measurements of unitcell dimensions data suggest that isomorphous structures are formed in similar experimental conditions with the La and Ce ions as well. A different hybrid assembly pattern characterizes the two isomorphous compounds 5 and 6. The nine-coordinate lanthanoid-complex connector, Ln(NO3)3(H2O)3 (Ln = Dy, Yb), interacts with the four surrounding TPyP moieties only through H2O 3 3 3 Npyridyl hydrogen bonding. It also hydrogen bonds, through the two additional protons of the


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water ligands and the nitrate proton acceptors to two adjacent units of the complex (Figure 3a). Every porphyrin scaffold is then associated with four units of the inorganic complex in different directions, as shown in Figure 3b. The asymmetric unit of these structures contains two halves of nearly perpendicular porphyrin moieties, both revealing a similar tetradentate hydrogen bonding pattern, and the multiple porphyrin-lanthanoid interactions give rise to a three-dimensionally interconnected assembly. View of the crystal packing down the a-axis (Figure 4) exhibits segregation of the component species into three different zones: offset-stacked columns of TPyP and hydrogen bonded columns of the metal complexes (along a), and channels in between which propagate along the a-axis through the extensively hydrogen bonded lattice and accommodate the benzene solvent. The coordination and hydrogen bonding interaction distances for all the analyzed compounds are summarized in Table 2, revealing consistent features. Compounds 6 and 7 are composed of the same main building blocks, namely, Yb(NO3)3(H2O)3 3 TPyP, but contain a different crystallization solvent (benzene and o-DCB, respectively), and form different network architectures. In fact, 7 can be best described as a layered structure, wherein the metal complex and the porphyrin entity form flat squaregrid-type layers. Within the layers, each TPyP unit hydrogenbonds effectively in equatorial directions with its four pyridyl arms to four different metal complexes, and every complex links through its water ligands to four neighboring porphyrins (Figure 5a, Table 2). The inorganic component, containing additional hydrogen bonding sites, connects further in the axial directions to neighboring units of the complex located in adjacent layers and is involved in six hydrogen-bonding interactions, as in the preceding examples 5 and 6. In this case, instead of a herringbone-type arrangement of the porphyrin units around the Yb-complex linkers, an offset-stacked layered arrangement prevails with the square-grid networks being interconnected to each other through the hydrogen bonded pillars of the inorganic connectors (Figure 5b). The stacked layered arrangement gives rise to the creation of open channels that propagate perpendicular to these layers, incorporating into the crystal lattice the o-DCB solvent of crystallization. It should be emphasized, however, that the two different structure types (5-6 vs 7) exhibit similar connectivity features and denticity of the interacting units, with four and eight hydrogen bonds used by the TPyP and Ln(NO3)3(H2O)3 components, respectively. Another observed structural modification of the metalporphyrin adduct involves replacement of one of the water ligands in the coordination sphere of Dy or Yb by a molecule of ethanol present in the reaction mixture. It is represented by the two isomorphous compounds 8 and 9. Substitution of water by ethanol reduces the hydrogen bonding capacity of the metal complex Ln(NO3)3(H2O)2(EtOH) in relation to that containing three water ligands, as in compounds 5-7; only five H-atoms are available now for hydrogen bonding, as opposed to six in the previous examples. The consequent optimization of the intermolecular hydrogen bonding in 8 and 9 disrupts the layered organization of the hybrid grids and the columnar association of the metal complex observed earlier. Instead, the latter associate into a paired dimer via two OH(ethanol) 3 3 3 O(nitrate) hydrogen bonds, each unit hydrogen bonding also through the water ligands to the pyridyl groups of four different porphyrin moieties (Figure 6a). However, the porphyrin molecules are not in the same plane

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Figure 3. The hydrogen bonding pattern (dotted lines) around the (a) metal complex which is involved in eight bonds, and (b) TPyP involved in four bonds, in 5. The metal ion and the three water ligands are depicted as small spheres.

Figure 4. Space-filling illustration of the crystal packing in 5, viewed down the a-axis. The channel voids are occupied by the benzene solvent (not shown). Note that the metal coordinated water ligands are directed up and down in an alternating manner, providing connecting sites to neighboring species located above and below the wavy layer shown.

(as in 7), being characterized by a slightly bent conformation. They preserve, nevertheless, their tetradentate interaction to four different units of the metal complex. In order to facilitate the extended supramolecular association in this case the metal connectors are centered not in the equatorial plane of a given porphyrin but rather between the planes of neighboring porphyrin molecules they link to. The resulting architecture represents once again a three-dimensionally hydrogen-bonded assembly of the porphyrin and metal complex building blocks, with the o-DCB solvent filling the interstitial voids in the structure (Figure 6b). Structures 1-9 constitute of neutral building blocks, TPyP and a trinitrato complex of a trivalent lanthanoid metal ion, as well as molecules of the crystallization solvent (benzene or o-DCB). However, the large excess of the aqua nitrate salt in the reaction mixture creates acidic conditions which promote partial or full protonation of the TPyP Lewis base accompanied by the presence of additional nitrate anions. This has been observed in earlier studies of the tetranitrato lanthanide complexes with TPyP17 as well as with 4,40 -bipyridyl.14

Compound 10, [Sm(NO3)4(H2O)2]- 3 (TPyPH)þ 3 31/2(o-DCB), provides another example to this end, although it lacks direct coordination between the metal and the N-sites of the organic component observed in those examples. Instead, structure 10 reveals a 3D intermolecular connectivity pattern sustained by hydrogen bonding only. The Sm(NO3)4(H2O)2- and TPyPHþ ions assemble into a pleated-network structure with alternating disposition of the two components (Figure 7). The porphyrin entity associates by three HO-H 3 3 3 Npy and one NHpy 3 3 3 OH2 hydrogen bonds with four different Smcomplexes, while each one of the latter connects between four adjacent TPyPHþs within the pleated-sheet domain. In addition, it connects in the vertical direction to adjacent metal complexes located in neighboring sheets above and below (along the a-axis of the crystal) by two H2O 3 3 3 Onitrate interactions (Table 2). Side view of the stacked networks and the pillared arrangement of the metal connectors, which results in open 3D framework architecture is illustrated in Figure 8a. Occlusion of the o-DCB solvent between these layers as well as in the channel voids that propagate through the layered structure is depicted in Figure 8b. The final example of compound 11 is exceptional in the sense that it presents separately hydrogen-bonded assemblies of the organic and inorganic components (rather than supramolecular networks of hybrid constitution). It involves also the TPy(3)P porphyrin scaffold rarely used before in crystal engineering of polymeric ensembles.18 In the context of the present discussion, this compound is unique because it represents a doubly protonated bipyridyl-bipyridinium-type (rather than neutral) ligand, which required also the presence of five nitrate ligands to balance the total positive charge in this structure. The constituent species that were obtained in the given experimental conditions (an attempt was made to dehydrate the lanthanoid starting material prior to the supramolecular reaction in order to facilitate its coordination with the porphyrin, which however did not materialize) are then [La(NO3)5(EtOH)]2- and (TPy(3)PH2)2þ. The doubly protonated porphyrin ions are self-complementary for hydrogen bonding, bearing two proton donors and two proton acceptors. It has been established from difference-Fourier maps that the protonated pyridyls are trans to each other. The hydrogen bonding potential of the (TPy(3)PH2)2þ entities is fully satisfied by the formation of hydrogen-bonded porphyrin grids, sustained by NH 3 3 3 N hydrogen bonds and aligned

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Figure 5. Compound 7. (a) The hydrogen bonding pattern (dotted lines) and the layered porphyrin organization cross-linked by the inorganic connectors. (b) The crystal structure viewed down the b-axis. Note the alternating pillared zones of the organic and inorganic components, and the offset stacked organization of the square-grid layers viewed edge-on.

Figure 6. Compound 9, [Yb(NO3)3(H2O)2(EtOH)] 3 TPyP 3 3(o-DCB). (a) The hydrogen bonding (dotted lines) intermolecular interaction scheme cross-linked by the inorganic linkers. Different shading denotes porphyrin moieties located at different levels of the projected structure. (b) The crystal structure viewed approximately down the a-axis, showing the segregated zones of the organic (including the o-DCB solvent) and inorganic (as hydrogen-bonded dimers of the metal complex) moieties.

parallel to the (1, 0, -1) plane of the crystal (Figure 9, Table 2). These layers have a wavy surface due to the meta (rather than para in previous examples) position of the interacting N-atoms. The metal complex, in turn, has only one proton donor (the ethanol ligand), which directs a hydrogen-bonding assembly of these units into linear chains. Neither coordination nor hydrogen bonding specific interactions operate between the two domains. Instead, the hydrogen-bonded chains of the metal complexes pass through the interporphyrin voids of, and are enclathrated within, the porphyrin arrays (Figure 9). Conclusion The synthesis of framework hybrid assemblies composed of the meso-tetrapyridylporphyrin species and various lanthanoid complexes has been demonstrated here for the first time.

The successful construction of these materials is facilitated by the tetradentate functionality and square-planar geometry of the porphyrin building blocks, as well as the diverse binding capacity of the inorganic components which involves direct coordination to the metal ion and hydrogen bonding to the nitrate and water ligands in its coordination sphere. The spatial disposition of the latter, associated with the high coordination numbers that characterize the lanthanoid metal ions, induces continuous intermolecular interaction schemes. Compounds 1-10 are characterized by open architectures of 3D connectivity, yet they represent soft materials. Their structures collapse upon heating and removal/escape of the crystallization solvent. The hybrid 3D assemblies between the organic and inorganic components are sustained by a combination of 1D coordination polymerization and hydrogenbonding in the other directions in 1-4, and by the labile hydrogen bonding interactions in all three dimensions in

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5-10. There is little control over the topology of the different structures that form. Insertion of the pyridyl ligating group into the coordination sphere of the lanthanoid ion, usually associated with the replacement of the original water ligand, as opposed to the alternative “external” hydrogen-bonding of the porphyrin to ligands in the coordination sphere, represent competing processes in the supramolecular reaction between TPyP and Ln-complex. As a result, induction of extended polymeric arrays by direct TPyP-Ln coordination only is not straightforward. Our observations may suggest that coordination polymerization occurs more readily with the lighter lanthanoid ions (e.g., La through Tb), most probably due to the “lanthanide contraction” effect, as no coordination polymers with the heavier and smaller ions have been found so far. These results show that the supramolecular chemistry of the TPyP scaffold can be expressed not only in terms of coordinationdriven reactions4-9 but also as effective participation in

Figure 7. Pleated-sheet-type hydrogen-bonded (dotted lines) assembly of the Sm(NO3)4(H2O)2- and TPyPHþ components in 10, both revealing tetradentate connectivity within the layered ensemble.


hydrogen bonding assemblies with metal-complex connectors. They complement our earlier observations on the framework solids constructed with the TCPP and lanthanoid ion components.13 These crystal-engineering tools could be of potential significance for the design of related hybrid porphyrinlanthanoid assemblies in the bulk (crystal engineering)1,3-9 as well as in thin/mono layers (surface crystallization and metal-organic-frameworks).19 Experimental Section Supramolecular Syntheses. The porphyrin compounds, various lanthanoid salts, and grade solvents were procured commercially and used without further purification. The corresponding metal complexes were reacted with the porphyrin moieties as described below, using predominantly the diffusion between layers technique. (1) Nd(NO3)3(H2O)6 (1.000 mmol) and TPyP (0.0535 mmol) were completely dissolved in 12 mL of ethanol and 8 mL of oDCB, and this solution was refluxed for 3 days. Then, 6 additional mL of EtOH were added and the solution was filtered and stored in 20 mL glass tubes. The latter were covered by either 2-propanol or benzene. To control the crystallization process the glass tubes in this and the following samples were routinely covered by a pinholed parafilm. X-ray quality red crystals of 1 {[Nd(NO3)3(H2O)2] 3 TPyP 3 3(o-DCB)} were obtained after a week. It should be noted that replacement of the o-DCB by m-DCB solvent has led to an isomorphous product. (2) Sm(NO3)3(H2O)6 (0.4097 mmol) and TPyP (0.0181 mmol) were dissolved in 12 mL of ethanol and 12 mL of o-DCB. This solution was refluxed for 20 h, then filtered and stored in crystallization tubes. The first solution was covered with benzene and yielded after 2 weeks small red crystals of compound 2 {[Sm(NO3)3(H2O)2] 3 TPyP 3 3(o-DCB)}. Similar experimental procedures with La(NO3)3(H2O)6 and Ce(NO3)3(H2O)6 yielded isomorphous structures, as evidenced by nearly identical unit-cell dimensions for these compounds. However, the poor quality of crystals obtained with the Sm-, and even more so La- and Ce-complexes, did not allow their detailed structure determination. The second solution was covered with 2-propanol, yielding after one week big purple air-sensitive crystals of 10 {[Sm(NO3)4(H2O)2]- 3 (TPyPH)þ 3 31/2(o-DCB)}. (3) Gd(NO3)3(H2O)6 (1.13 mmol) and TPyP (0.028 mmol) were dissolved in 12 mL of ethanol (the porphyrin is only negligibly soluble in EtOH) and this suspension was refluxed

Figure 8. Crystal packing in 10. (a) View down the c-axis, showing edge-on the hydrogen-bonded pleated networks of the component species stacked along the a-axis. Note the intralayer (to the porphyrin units) and interlayer hydrogen-bonding (dotted lines) of the pillared metal connectors. (b) View down the a-axis, depicting the inclusion of the o-DCB solvent in the lattice. The asterisks denote the approximate location of additional disordered molecules of the solvent that could not be located reliably.

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Figure 9. Compound 11. (a) Illustration of the separate hydrogen bonding (dotted lines) patterns. The self-hydrogen-bonded TPy(3)PH2 porphyrin layers embed the hydrogen-bonded chains of the La(NO3)5(EtOH) metal complexes. (b) View of the crystal packing down the b-axis, showing edge-on the hydrogen-bonded porphyrin layers and the inorganic chains, as well as the occlusion of the o-DCB crystallization solvent in the interstitial voids.

(4)

(5)

(6)

(7)

for 4 h. Then, 12 mL of o-DCB were added until the solution was clear red. It was refluxed again for a short time and the solution was covered by a layer of benzene and left for crystallization by the solvent diffusion technique. Very thin air-sensitive crystals of 3 {[Gd(NO3)3(H2O)2] 3 TPyP 3 3(oDCB)} were obtained after 40 days. Tb(NO3)3(H2O)6 (0.248 mmol) and TPyP (0.0162 mmol) were completely dissolved in 2 mL of ethanol and 6 mL of o-DCB. After reflux for 3 h this solution was filtered, added 2 mL of EtOH and covered with benzene. X-ray quality small thin plate red crystals of 4 {[Tb(NO3)3(H2O)2] 3 TPyP 3 3(oDCB)} were obtained by slow evaporation on the solvent surface after 20 days. The same crystals were also obtained by reacting in a similar way 0.385 mmol of Tb(NO3)3(H2O)5 with 0.0175 mmol of TPyP. Dy(NO3)3(H2O)6 (1.24 mmol) and TPyP (0.0456 mmol) were completely dissolved in 6 mL of ethanol and 8 mL of o-DCB. This solution was refluxed overnight and then filtered. One portion of the solution was covered by a layer of benzene and yielded after 2 weeks thin red air-sensitive needle crystals of compound 5 {[Dy(NO3)3(H2O)3] 3 TPyP 3 2(benzene)}. Another portion of the same solution was kept for slow evaporation without adding any top layer. After 2 months, it yielded very small purple prisms of compound 8 {[Dy(NO3)3(H2O)2(EtOH)] 3 TPyP 3 3(o-DCB)}. Yb(NO3)3(H2O)5 (0.4110 mmol) and TPyP (0.0346 mmol) were partly dissolved in 12 mL of ethanol, and the resulting suspension was refluxed for 4 h. Then, 12 mL of o-DCB were added until the solution was clear red. This solution was refluxed again for a short time, filtered, and covered with a layer of benzene. After 20 days, two kinds of crystals were obtained: first, thin red needle crystals of compound 6 {[Yb(NO3)3(H2O)3] 3 TPyP 3 2(benzene)} and then, after benzene evaporation also big cube purple crystals of compound 7 {[Yb(NO3)3(H2O)3] 3 TPyP 3 2(o-DCB)}. Slight modification of the reaction conditions in another experiment led to the formation of compound 9. Thus, Yb(NO3)3(H2O)5 (0.838 mmol) was dissolved in 4 mL of EtOH and TPyP (0.041 mmol) was dissolved in 4 mL of EtOH and 4 mL of o-DCB. The two solutions were mixed and refluxed intensively for 3 h. The resulting mixture was still a little muddy. It was then filtered and kept undisturbed for slow evaporation, yielding after 27 days small red prism crystals of 9 {[Yb(NO3)3(H2O)2(EtOH)] 3 TPyP 3 3(o-DCB)}. La(NO3)3(H2O)6 (0.995 mmol), mp 65 °C, was placed in the oven at 120 °C for 20 min for dehydration, and then dissolved

in 12 mL of EtOH. TPy(3)P (0.0357 mmol) was dissolved completely in 12 mL of hot o-DCB. The two solutions were rapidly mixed and the mixture was refluxed for 24 h, filtered, placed in a tube and covered with a layer of 2-propanol. Big purple rhomb crystals of compound 11 {[La(NO3)5(EtOH)]2- 3 (TPy(3)PH2)2þ 3 (o-DCB)} were obtained at the bottom of the tube after 2 months. Reactions of the TPy(3)P ligand with other lanthanoid compounds did not produce crystalline products of adequate quality for crystal structure determination. The uniform identity of the formed crystal lattices (1-11) in a given reaction was confirmed in each case by repeated measurements of the unit-cell dimensions from different single crystallites. Crystallography. The X-ray measurements (Nonius KappaCCD diffractometer, MoKR radiation) were carried out at ca. 110 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)20a and refined by full-matrix least-squares (SHELXL-97).20b All nonhydrogen atoms were refined anisotropically. The hydrogen atoms were located in idealized/calculated positions and were refined using a riding model; most of those involved in hydrogen bonds were located in difference-Fourier maps but were not refined. The two inner pyrrole H-atoms were found (in the corresponding differenceFourier maps) disordered between the four pyrrole N-sites in all structures. Structures 1-10 were found to represent open hybrid lanthanoid-porphyrin 3D lattices with channel voids accommodating the noninteracting crystallization solvent. Compound 11 shows a layered structure of hydrogen-bonded networks of the selfcomplementary TPy(3)PH22þ porphyrin moieties, perforated by columns of hydrogen-bonded units of the La-complex. In structures 1, 3-7, and 10 the solvent species could be clearly recognized in difference electron-density maps, but (all, or parts, of it) could not be reliably modeled by discrete atoms due to their severe disorder. Correspondingly, the contribution of the disordered solvent moieties was subtracted from the diffraction pattern by the SQUEEZE procedure and PLATON software,21 allowing smooth convergence of the crystallographic refinements to acceptable low R-values, and precise description of the metal-porphyrin frameworks. In 3 and 4, the o-DCB solvent species included in the structure factor calculations were found located near, and 2-fold disordered about, planes of mirror symmetry. Six bond lengths restraints were applied to one of the refined dichlorobenzene species in 10 to avoid unreasonable distortion of its molecular geometry. In structures 8, 9, and 11,

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molecules of the crystallization solvent have been completely characterized. Structure 11 was refined as a racemic twin.

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 10, fully analyzed, crystalline solids 1 and 3-11. This information is available free of charge via the Internet at http:// pubs.acs.org.

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Porphyrin Framework Solids. Hybrid Supramolecular Assembly Modes

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