Self-Assembly of Supramolecular Porphyrin Arrays by Hydrogen

Self-Assembly of Supramolecular Porphyrin Arrays by Hydrogen Bonding: New Structures and Reflections Sumod George and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel AViV UniVersity, 69978 Ramat AViV, Tel AViV, Israel

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 3 755-762

ReceiVed NoVember 23, 2005; ReVised Manuscript ReceiVed December 20, 2005

ABSTRACT: This study relates to the self-assembly of the free-base 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP), sixcoordinate manganese-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin with molecules of water or methanol as axial ligands [MnIII(H2O)2-TCPP or MnIII(CH3OH)2-TCPP, respectively], and manganese chloride-5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin [MnIII(Cl)-TOHPP] into multiporphyrin hydrogen-bonding “polymers”. In the first case, the porphyrin units hydrogen bond directly to each other through their carboxylic acid functions. The two-dimensional (2D) square-grid polymeric arrays thus formed are sustained by characteristic intermolecular cyclic dimeric (COOH)2 hydrogen-bond synthons between a given porphyrin unit and four other neighboring species along the equatorial directions. They stack tightly one on top of the other in an offset manner along the normal direction, yielding channeled lattice architecture. In an aqueous basic environment insertion of MnIII into the porphyrin core involves deprotonation of one of the carboxylic groups, to balance the charge, and attraction of two water molecules as axial ligands. The MnIII(H2O)2-TCPP units hydrogen bond, however, directly to one another through their carboxylic/carboxylate functions in the equatorial plane in a catemeric (rather than cyclic dimeric) manner. The 2D networks that form in this case are interconnected in the normal direction by additional hydrogen bonds through the axial water ligands, yielding a three-dimensional (3D) hydrogenbonding architecture. In a methanolic solution of H3PO4, the methanol molecules replace water as axial ligands to the metal ion. The phosphate anions balance the extra positive charge of the trivalent metal, and they act also as effective bridges between adjacent MnIII(CH3OH)2-TCPP moieties by interacting as proton acceptors with the peripheral carboxylic functions of four different porphyrins. This affords open square-grid-type layers with alternating porphyrin and phosphate components. The 2D arrays stack in an offset manner, interconnecting to one another through hydrogen bonds involving the methanol axial ligands. The Mn(Cl)-TOHPP building blocks arrange in a unique tetragonal structure around axes of 4-fold rotation and self-assemble through multiple O-H‚‚‚Cl- attractions between neighboring units into a single-framework hydrogen-bonding polymer. The directional asymmetry introduced to them by the Cl-axial ligand, which is amplified by the preferred hydrogen-bonding scheme, induces the formation of a noncentrosymmetric crystal structure. The nanoporous nature of TCPP-based multiporphyrin assemblies and the chirality of the Mn(Cl)-TOHPP structure are highlighted. Introduction Supramolecular multiporphyrin assemblies with unique structural and functional properties provide attractive model systems in the study of various biological (e.g., charge and energy transfer) and chemical (e.g., homogeneous and heterogeneous catalysis) processes, and in the design of new materials and devices (e.g., new solid sensors and zeolite analogues).1,2 Their formulation can be driven by various molecular recognition elements readily incorporated into the porphyrin framework. In this context, we have introduced earlier the tetra(4-hydroxyphenyl)porphyrin (TOHPP) and tetra(4-carboxyphenyl)porphyrin (TCPP) moieties and their metalated derivatives as basic platforms for supramolecular networking,3a,4a and investigated their self-assembly reactions in a comprehensive effort to formulate ordered solids with open and/or chiral porphyrin architectures and to evaluate their potential applications. This study has led to the discovery, by us as well as by others, of a number of supramolecular porphyrin-based networks sustained by direct interporphyrin hydrogen bonding, or tessellated by external metal ion or organic ligand bridging auxiliaries.3-5 Noteworthy are the stable porous structures and zeolite analogues constructed from the Zn- and Co-TCPP building blocks,4b,4d,5a and interweaved networks based on the Zn-TOHPP units.3b,c To gain new insights to the subject of programmed construction of network solids, and to further extend the * To whom correspondence should be addressed. E-mail: goldberg@ post.tau.ac.il.

available database of the intermolecular modes of porphyrin selfassembly, we focus in this work on supramolecular arrays directed by hydrogen bonding using the free-base TCPP as well as Mn-metalated TCPP and TOHPP scaffolds. The results confirm our earlier observations on the ability to form open multiporphyrin arrays with interporphyrin voids readily accessible to other guest components, employing the highly versatile TCPP platform and different molecular recognition algorithms. They also demonstrate another useful strategy for the induction of noncentrosymmetric crystalline architectures by using suitably functionalized porphyrin building blocks of lower than D4h symmetry characterized by a polar axis. To this end, we describe here the crystal structures consisting of hydrogen-bonded multiporphyrin assemblies involving: (a) the free-base TCPP that crystallized from nitrobenzene (1), (b) diaqua-Mn-TCPP that cocrystallized with the tetrachloroethane solvate (2), (c) dimethanol-Mn-TCPP dihydrogenphosphate (3), and (d) aquaMn(Cl)-TOHPP (4) (Scheme 1). Experimental Section The porphyrin starting materials Mn(Cl)-TCPP and Mn(Cl)-TOHPP were procured commercially from Porphyrin Systems GbR. They usually contain minute amounts of the corresponding free-base porphyrins. Other grade chemicals were obtained from Aldrich and Merck and used without further purification. Several ground samples of the Mn(Cl)-TCPP were separately dissolved in a 3:1 mixture of 1,1,2,2tetrachloroethane and methanol. The resulting solutions were treated with a few drops of a strong base (either NaOH or KOH, dissolved in

10.1021/cg050624m CCC: $33.50 © 2006 American Chemical Society Published on Web 01/17/2006

756 Crystal Growth & Design, Vol. 6, No. 3, 2006

George and Goldberg Scheme 1 a

a

L ) H2O or CH3OH.

methanol) to facilitate deprotonation of the tetra-acid, and subsequently with a small amount (a few drops) of the nitrobenzene template to induce crystallization.4b These crystallization experiments (in open air at ambient temperature) yielded after several weeks single crystals of two different materials: a nitrobenzene solvate of TCPP (1) and a tetrachloroethane solvate of diaqua-Mn-TCPP (2). In another set of experiments, a 1:1 methanolic solution of ortho-phosphoric acid (instead of the hydroxy base) was reacted with the porphyrin material, yielding dark red crystals of a dihydrogenphosphate salt of the porphyrin compound (3). Dark crystals of aqua-Mn(Cl)-TOHPP (4) were obtained by first mixing thoroughly the corresponding porphyrin starting material with a 5-fold excess of sodium dihydrogenphosphate dihydrate. The resulting mixture was then dissolved in a 3:1 mixture of 1,1,2,2tetrachloroethane and methanol and left for crystallization in open air at ambient temperature. The yields of the desired crystalline products were generally rather low ( 2σ no. refined parameters R (I > 2σ) R (all data) Rw (I > 2σ) Rw (all data) |∆F|max e Å-3

1b 790.75d

2

3b 1004.75f

4c

triclinic P1h -163(2) 12.8446(8) 15.0021(11) 21.8715(16) 96.167(3) 97.670(4) 101.066(4) 4060.6(5) 2 0.05d 0.647d 1.05 25174

1214.38 triclinic P1h -163(2) 9.4265(1) 15.2987(3) 20.0481(4) 100.103(1) 97.252(1) 107.294(1) 2668.31(8) 2 0.71 1.511 1.50 17511

tetragonal P43212 -163(2) 18.6724(6) 18.6724(6) 16.8341(5) 90.0 90.0 90.0 5869.4(3) 4 0.31f 1.137f 1.25 11027

785.11 tetragonal I4 -163(2) 13.9892(5) 13.9892(5) 9.3840(3) 90.0 90.0 90.0 1836.4(1) 2 0.49 1.420 1.43 8417

14174

9383

5680

2047

0.079 6212

0.035 7684

0.043 3964

0.020 1866

543e

790

320e

141

0.073e 0.137e 0.160e 0.171e 0.26e

0.062 0.077 0.139 0.147 0.71

0.066e 0.091e 0.168e 0.180e 0.67e

0.043 0.048 0.113 0.118 0.45

a Empirical formulas: 1, C H N O ; 2, C H N O Mn(H O) ‚2C H Cl ; 48 30 4 8 48 27 4 8 2 2 2 2 4 3, C48H28N4O8Mn(CH3OH)2‚H2PO4; 4, C44H28N4O4Mn(Cl)(H2O). b This structure contains also disordered solvent, which could not be determined. c Structure was refined as a racemic twin. d Excluding the nitrobenzene solvent. e After subtracting the contribution of the solvent from the diffraction data (using the SQUEEZE procedure).7 f Excluding the methanol or methanol-water solvent.

Self-Assembly of Supramolecular Porphyrin Arrays

Crystal Growth & Design, Vol. 6, No. 3, 2006 757

Figure 1. ORTEP plots of 1-4 (showing 50% probability thermal displacement ellipsoids). The tetrachloroethane solvent in 2 and the phosphate anion in 3 are omitted. The N, O, Cl, and Mn atoms are represented by darkened ellipsoids. The observed Mn-N coordination distances are within 2.000-2.021(4) Å in 2, 2.012(3) Å in 3, and 2.006(2) Å in 4. The Mn-O ligation distances are 2.176(4) and 2.290(3) Å in 2, 2.237(3) Å in 3, and 2.351(5) Å in 4, wherein the manganese cation deviates slightly from the porphyrin plane toward the Cl anion at Mn-Cl of 2.624(2) Å. The porphyrin molecules are located on centers of inversion in 1, in general positions in 2, on axes of 2-fold rotation (propagating along the C10‚‚‚ Mn‚‚‚C24 axis) in 3, and on 4-fold rotation axes (propagating along the O1‚‚‚Mn‚‚‚Cl axis) in 4. being located on, and oriented perpendicular to, axes of 4-fold rotation. The water molecule weakly coordinated to the manganese metal center, trans to the axial Cl-ligand, is rotationally disordered to match this symmetry. The peripheral -OH substituents are involved in intermolecular hydrogen bonds with the Cl atoms of adjacent species. Because of the weak diffraction power of the disordered fragments, any potential deviation of the molecular structure from the 4-fold symmetry, even if it present or is concealed in the atomic thermal displacement parameters, could not be detected from the diffraction pattern. Crystals of 4 represent racemic twins, wherein the direction of the polar C4 axis of the structure appears to alternate randomly in opposite directions in different domains of the solid material. Measured densities of the crystalline materials (Table 1) are consistent with the calculated values for compounds 2-4, taking into account the Squeeze-assessed7 solvent content (12 molecules of methanol per unit-cell) in 3. On the other hand, the measured density of 1 reflects on a higher content of nitrobenzene than that assessed from diffraction data by summing up the residual and highly diffused, electron density in the inter-porphyrin channels of this solid (i.e., four molecules of nitrobenzene rather than two, per asymmetric unit). In all cases, the crystallographic evaluations provided reliable structural models of the porphyrin framework structures and of their supramolecular organization in the corresponding solids. Figure 1 illustrates the molecular structures of the porphyrin building blocks (Scheme 1) in the crystalline materials 1-4.

Results and Discussion TCPP is a uniquely versatile platform for the construction of multiporphyrin supramolecular arrays due to its square-planar symmetry, the self-complementarity of the terminal carboxylic acid functions in cooperative hydrogen bonding (i.e., their ability to act simultaneously both as proton donors and proton acceptors), and the capacity of these groups to ligate to metal centers in both their acidic and deprotonated forms.4,5 It can be metalated in the center to rigidify the molecular framework (to M-TCPP). When metal ions with five- or six-coordination preference are used to this end, they may provide additional

sites for axial coordination to other species. In fact, most of the earlier crystallographic characterizations of TCPP-based materials related to M-TCPP’s (M ) ZnII, CoII, CuII, PdII, PtII, MnIII), as the conformationally more robust building blocks seem to facilitate their organization in a periodic manner.8 Using earlier the Zn-TCPP units, we were able to self-assemble twodimensional (2D) polymeric arrays composed of multiple hydrogen-bonding species, wherein each porphyrin moiety connects to four different neighbors via four cyclic (COOH)2 hydrogen-bonding synthons.4a,b Because of the large size of the square-planar molecules, the networks that form are porous and contain about 1.6-nm-wide voids between the interacting carboxyphenyl arms of the surrounding porphyrins. In view of the thermodynamic lability of the hydrogen bonds interweaving of these networks is common, to minimize void space in the condensed phase,4a unless special precautions and/or suitable templates are applied. Indeed, under appropriate conditions (using nitrobenzene as one of the best templating solvents) stacked rather than interpenetrated network architectures can be obtained.4b In such cases, the structure is stabilized by typical porphyrin-porphyrin stacking interactions between neighboring nets,9 as well as by inclusion of the template/solvent within the interporphyrin voids. In the above context, we present here the first crystalline assembly of a porous supramolecular structure composed of nonmetalated TCPP units only, with nitrobenzene as template (1). The 2D networking scheme is illustrated in Figure 2a. Neighboring units in the open square-grid that forms are related to one another by a + b lattice translation and along the c-axis by crystallographic inversion at 1/2,1/2,0. Utilizing the hydrogenbonding potential of TCPP, the TCPP network is sustained by rather strong OH‚‚‚O hydrogen bonds within the O‚‚‚O distance range of 2.568-2.687 Å (Table 2).10 The van der Waals widths


George and Goldberg

Figure 2. (a) Open network arrays of free-base TCPP in 1. (b) Tight stacking of the 2D hydrogen-bonded polymers along the a-axis of the crystal, creating wide (0.9 × 1.5 nm2) channels as solvent/template accessible voids. The latter are partly occupied in 1 by severely disordered nitrobenzene solvent. The porphyrin molecules are shown in a space-filling mode. Table 2. Hydrogen-Bonding Parameters. O-H

a

A(O/Cl)

O-H (Å)

H‚‚‚A (Å)

O‚‚‚A (Å)

O-H‚‚‚A (°)

1.90 1.78 1.86 1.81

2.568(2) 2.633(3) 2.687(2) 2.619(3)

136 178 170 163

OH(20)a OH(29)a OH(20′)a OH(29′)a

O21′ (x - 1, y - 1, z)b O30′ (1 - x, 1 - y, 2 - z)b O21 (x + 1, y +1, z)b O30 (1 - x, 1 - y, 2 - z)b

Compound 1 0.84 0.85 0.84 0.84

OH(42)a OH(51)a OH(60a)a OH(60b)a OHa(61)c OHb(61)c OHa(62)c OHb(62)c

O32 (x - 1, y - 1, z)d O33 (x - 1, y - 1, z - 1)d O50 (x + 1, y + 1, z)b O50 (x + 1, y + 1, z)b O51 (-x, -y, -z)b O41 (-x, -y, 1 - z)b O32 (-x, 1 - y, 1 - z)d O50 (-x - 1, -y, -z)b

Compound 2 0.86 0.95 1.00 1.04 0.90 0.92 0.84 0.85

1.75 1.52 1.69 1.74 2.07 2.00 1.89 1.91

2.611(5) 2.462(4) 2.691(7) 2.784(9) 2.958(5) 2.865(5) 2.709(5) 2.735(5)

176 173 180 180 166 154 165 160

OH(19)e OH(25)e OH(34)a OH(35)g

O38 (-x, -y, z - 1/2)f O39 (1 - x, 1 - y, z - 1/2)f O38 (1 - y, -x, 1/2 - z)f O25 (1/2 - y, x - 1/2, z - 1/4)e

Compound 3 0.92 0.92 0.92 0.92

1.65 1.64 1.81 1.80

2.539(4) 2.511(4) 2.663(5) 2.715(4)

160 158 152 180

OH(2)h

Cl (x + 1/2, y + 1/2, z - 1/2)

Compound 4 0.88

2.43

3.287(5)

b

c

d

164

Carboxylic OH group. Carboxylic CdO group. Water. Deprotonated carboxylate. Delocalized carboxylic group. Phosphate anion. g Methanolic OH group. h Hydroxylic group.

of the distorted rectangular voids within the given multiporphyrin layer are approximately 1.5 and 2.0 nm. These layers are stacked in an offset manner one on top of the other along the a-axis of the crystal, being inclined by about 45° with respect to the stacking axis. Centers of the overlapping porphyrin units in successive layers (represented by the two crystallographically independent moieties of the asymmetric unit) are offset by about 5 Å along a + b, and their planar core macrocycles are slightly tilted with respect to one another by 4.6°. The mean interlayer distance is 3.75 Å, considerably shorter than commonly observed in layered tetraarylporphyrin structures,9 indicating a tight fit between neighboring nets. Thus formed intermolecular arrangement yields channeled three-dimensional (3D) porphyrin architecture, with the channel zones propagating (and combining the intralayer voids) through the stacked networks along the a-axis of the crystal (Figure 2b). These channels have a rectangular shape and are centered at y ) 0 and z ) 0, and their van der Waals cross-section is approximately 0.9 × 1.5 nm2. A few molecules of the severely diffused/disordered (and thus hardly diffraction-detectable) nitrobenzene template are trapped in the channels, preventing the crystal structure from collapsing despite the small percentage of the crystal volume (∼40%) being occupied by the porphyrin lattice alone. Similar observations

e

f

have been reported for other porphyrin framework materials, including our earlier similar example of a porphyrin-sieve-type structure based on the aqua-zinc TCPP units.4b,11 In a sense, the present material is a supramolecular isomer12 of the latter one, as the two crystal structures consist of similarly tessellated networks that are stacked in an overlapping manner to yield a similarly channeled architecture. Yet, the slight difference in the molecular building blocks [TCPP vs Zn(H2O)-TCPP] that afford in the present case layers with rectangular rather than square voids, in the offset of adjacent layers, and different tilts of the networks with respect to the stacking axis, result in very similar supramolecular organizations but with channel voids of different dimensions. The nearly perfect planarity of the freebase porphyrin core in 1 is consistent with the tight stacking of the porphyrin layers. Metalation of TCPP by a three-valent cation requires the presence of either an appropriate external counterion, or deprotonation of the tetraacid moiety, to balance the charge (metal insertion into the porphyrin core is associated with the expulsion of the two inner pyrrole protons). In a basic crystallization environment (e.g., with NaOH/KOH), the carboxyporphyrins are singly deprotonated to yield neutral MnTCPP building blocks. This also modifies the supramolecular


Figure 3. Space-filling illustration of the network arrays of Mn(H2O)2TCCP in 2, assembled in a catemeric mode of interporphyrin hydrogen bonding around the tetrachloroethane templates (the latter are shown at four sites).

organization of the porphyrin units in 2 (in relation to that in 1), giving rise to a catemeric hydrogen-bonding pattern. Every porphyrin with three carboxylic acid and one carboxylate functions bears now three proton donors but five main proton acceptor groups, and the intermolecular interaction synthons seem to be best optimized (involving now also some chargeassisted H-bonds) in a catemeric arrangement with closer proximity of the porphyrin molecules (Table 2). This includes relatively strong COOH‚‚‚COO- bonds at O‚‚‚O distances within 2.462-2.611 Å. Formation of such supramolecular crystalline networks is characterized by smaller interporphyrin voids than in 1. It is farther facilitated by the presence of a suitably sized template in the crystallization mixture around which the self-assembly occurs, a role played by the tetrachloroethane solvent in 2 (Figure 3). In the crystal, adjacent parallel stacked layers (at ca. 4.5 Å) are offset about half of the grid size along one of the porphyrin molecular axes, to position the axial water ligands of one layer in a close proximity to the hydrogen-bonding sites of neighboring arrays. This allows for the formation of multiple interlayer links from the axial water ligands to the carboxylic/carboxylate O-sites located above or below (Table 2, Figure 4). The charge-


assisted, as well as the neutral, hydrogen-bonding interactions presented in Table 2 reveal standard geometric features.10 The offset-stacked-layered assembly of the porphyrin units is still characterized by parallel channel voids (of approximate van der Waals width of 0.6 nm), which propagate throughout the crystal perpendicular to the layered arrays (Figure 4a). The latter are occupied effectively by the tetrachloroethane species. For the Mn(H2O)2-TCPP building blocks, a similarly assembled interporphyrin architecture was observed earlier with nitrobenze as the templating reagent.5e We have shown previously that it is possible to disrupt the intermolecular hydrogen bonding between the TCPP molecules, either by using exocyclic metal ion auxiliaries (which can coordinate conveniently to both the -COOH as well as the -COO- peripheral functions),5d,5f,8 or by introducing to the porphyrin solution strong nucleophilic reagents with better proton acceptor capacity than the carboxylic acid (as e.g., DMSO).4a The latter notion has led us to another potentially useful, but previously unexplored, approach to the hydrogenbonding-driven self-assembly of TCPP-based arrays. It involves the use of polyoxy acids that after deprotonation can act as excellent tessellating nodes between several neighboring TCPP units. Such anions can act as extremely effective proton acceptors in hydrogen bonding to the carboxylic groups of the TCPP moiety, strengthened by their negative charge. Structure 3 represents the first example of this strategy, afforded by reacting Mn(Cl)-TCPP with phosphoric acid. This reaction is accompanied by dissociation of the (stronger than carboxylic) inorganic acid, expulsion of HCl and creation of the Mn-TCPP cationic species without deprotonation of the carboxylic groups. In the methanolic solution, the manganese ion in the porphyrin core attracts two CH3OH molecules from above and below the porphyrin framework, due to its high affinity for octahedral coordination and for oxo-ligands. Self-assembly of the Mn(CH3OH)2-TCPP units into extended supramolecular arrays is then driven by the multidentate functionality and strong proton affinity of the phosphate anions. Although the latter have tetrahedral geometry, their small size leads to the formation of 2D hybrid hydrogen-bonding polymers, as illustrated in Figure 5. Equivalent P-O bonds, attesting to their delocalized nature, characterize the phosphate moieties.

Figure 4. (a) View of the crystal packing in 2, showing the offset stacking mode between two neighboring layered arrays and location of the CH2Cl4 molecules in channel voids that propagate through the crystal along the a-axis. Note that the axial water ligands of one layer protrude onto the hydrogen bonding sites of the next layer to facilitate interlayer hydrogen bonding. (b) Edge-on view of three neighboring porphyrin layers emphasizing the interconnection between them by hydrogen bonding through the axial water ligands (dotted lines).


Figure 5. Square-grid network arrays of Mn(CH3OH)2-TCCP in 3, tessellated by hydrogen bonding through the PO43- anions. The anionic bridges in this structure play a role similar to that of the exocyclic metal cations in metal-tessellated networks of TCPP.5d,5f,8

The layered hydrogen-bonded assemblies shown in Figure 5 arrange one on top of another along the c-axis of the crystal in an offset manner to allow interlayer hydrogen bonding between the axial methanol ligands of one layer to the carboxylic acid groups of adjacent layers from above and below (Figure 6a, Table 2). To facilitate the simultaneous interaction of the methanol entities with the Mn ions through coordination, and with the neighboring carboxylic groups by hydrogen bonding (as proton donors), the multiply hydrogen-bonded flat supramolecular assemblies are rotated with respect to each other (rather than parallel as in the previous examples). Thus optimized crystal packing of the porphyrin layers leaves small channel voids that propagate through the tetragonal crystal parallel to the 4-fold screw axes (Figure 6b). These solvent-accessible voids account for about 30% of the crystal volume and are occupied in 3 by disordered/diffused molecules of methanol/water. The present structure is one of the few examples of tetraarylporphyrins that crystallize in noncentrosymmetric chiral space groups (see also the next section), as their overall approximate D4h symmetry most often results in centrosymmetric organizations.13,14 This can be possibly attributed to the noncentrosymmetric relation of the two methanol axial ligands, resulting from the optimal enthalpic stabilization of the interlayer hydrogen bonds the methanol species are involved in. As part of our continuing research program on porphyrin network solids, we are involved also in deliberately designing porphyrin building blocks with reduced symmetry but still preserving their molecular recognition functions, aiming at the synthesis of multiporphyrin assemblies that lack mirror or

George and Goldberg

inversion symmetry elements. The 5-(3′-pyridyl)-10,15,20-tris(4′-carboxyphenyl)porphyrin and 5-(2′-quinolyl)-10,15,20-tris(4′-hydroxyphenyl)porphyrin building blocks serve as suitable examples, being characterized by a polar molecular axis.15 Evidently, the use of building blocks of polar nature (rather than those characterized, for example, by D4h symmetry) in the construction of supramolecular arrays, associated with specific directionality of the intermolecular interaction synthons, has higher the propensity of inducing a noncentrosymmetric architecture in three dimensions. This applies of course also to fivecoordinate metalloporphyrins of C4/C4V molecular symmetry, even when similarly substituted by equivalent functional groups at their four meso positions. In such compounds, the polarity is imparted to the porphyrin scaffold by the unidirectional metalaxial ligand bond, which is associated with deviation of the metal ion from the plane of the porphyrin core toward the axial ligand. In the above context, the five-coordinate Mn(Cl)-TOHPP compound provides a useful platform for polarized self-assembly (4). Most importantly at the outset is that in an inert crystallization environment the axial chloride ligand does not dissociate upon heating, considering the rather low acidity of the hydroxy substituents (which is not the case with the Mn-TCPP material). Then, this ligand gets readily involved (as proton acceptor) in hydrogen bonds with -OH functions, the O-H‚‚‚Cl- interaction representing a strong molecular recognition synthon abundant in molecular crystals,14 and has been already used by us in the construction of supramolecular multiporphyrin networks.16 The anticipated interaction geometry between neighboring porphyrins derives from the earlier examples (2 and 3), wherein an axial ligand (whether H2O or CH3OH) hydrogen bonds to the mesosubstituted functional groups of porphyrin moieties approaching from above. However, in structure 4 the role of the interacting sites exchange: the axial, partly charged, Cl-ligand acts as a proton acceptor (with stronger affinity for protons than that of the neutral hydroxylic function), while the peripheral OH groups of adjacent porphyrins act as proton donors. The mode of porphyrin networking in this case is shown in Figure 7. Compound 4 crystallizes in a rare tetragonal I4 space group. The porphyrin molecules organize in a body-centered type fashion, each unit interacting through hydrogen bonds with four porphyrin moieties located above and four other porphyrin entities located below, by utilizing in a cooperative manner both the equatorial and the axial recognition sites. It is thus involved

Figure 6. Crystal packing in 3. (a) Illustration of the stacking mode between neighboring porphyrin layers and of the hydrogen bonding (dotted lines) between them via the methanol axial ligands. Note the relative inclination of adjacent layers, which are related to one another by the 43 axis, to optimize these linkages. (b) View down the c-axis of the unit-cell, which depicts the channel voids at 1/2, 0, z and 0, 1/2, z. The latter propagate parallel to the 4-fold screw axes and accommodate the disordered methanol/water solvent.



Figure 7. The body-centered type intermolecular organization and the hydrogen-bonding pattern (dotted lines) in 4. (a) Section of the crystal structure showing hydrogen bonding of the four porphyrins in the upper layer to the axial chloride of the porphyrin unit below, and of the porphyrin in the middle layer to four different chloride ions in the lower level. The Mn ion deviates 0.13 Å from the porphyrin plane toward the Cl-ligand. (b) Top view of the unit-cell and hydrogen bonding down the tetragonal 4-fold axis.

in eight O-H‚‚‚Cl hydrogen bonds, with the axial chloride connecting to the hydroxylic groups of four molecules above, and the four hydroxylic groups donating their protons to four chloride ions of the molecules below. There is no hydrogenbonding interaction between the hydroxylic functions within each layer. Then, the water ligand residing on the concave face of the porphyrin (which is located on and disordered about the 4-fold rotation axis) is mainly filling space, interacting only weakly in a bifurcated fashion with the OH groups of neighboring porphyrin molecules. The corresponding nonbonding H(water)‚‚‚O(hydroxyl) distance is 2.8 Å. The asymmetry of the porphyrin building block incorporating a preferred axial direction, combined with optimization of the multiple hydrogen bonding that involves the four O-H groups of a given entity to point at the same direction and converge on a single chloride proton acceptor, thus impose the formation of a chiral architecture with a polar axis. The significance of this concept, combining features of molecular polarity and specific directional intermolecular interactions, is emphasized by earlier observations that five-coordinate metalloporphyrins lacking such molecular recognition functionality readily form centrosymmetric structures.13 The porphyrin lattice in 4 represents an efficiently packed structure, and is nonporous. Concluding Remarks This work demonstrates a number of useful guidelines in crystal engineering of porphyrin-based supramolecular architectures by hydrogen bonding that rely on multiple functionalities of the meso-tetraarylporphyrins. It confirms the general tendency of the TCPP building block to aggregate in the form of open multiporphyrin networks with very large voids (∼1.52.0 nm wide) not only in its metalated form but also as a freebase derivative. This aggregation involves utilization of the cyclic dimeric (COOH)2 interaction synthons between adjacent species in the network. Furthermore, it shows that by using preferred templates it is possible to prevent interpenetration of these networks and to obtain tightly stacked layered porphyrin architectures perforated by solvent-accessible channels (hence nicknamed “porphyrin sieves”).4b The intermolecular binding mode between the carboxylic functions within the porphyrin networks can be readily modified from cyclic dimeric to catemeric by replacing the TCPP units with, e.g., MnIII-(TCPP)moieties. As the manganese ion is preferentially six-coordinate, and attracts two additional axial ligands, the stacking mode of the 2D multiporphyrin layers can be also affected by possible additional interactions between the axial ligands of one layer

and the porphyrin networks above and below. Next, it has been shown that formulation of stable network arrays with Mn-TCPP can be obtained not only by direct interaction between the porphyrin building blocks, or by exocyclic metal bridges,8 but by means of bridging anionic auxiliaries as well. Polyoxy acids, which after deprotonation consist excellent proton acceptors in charge-assisted hydrogen bonding, may provide excellent auxiliaries to this end. Finally, we have illustrated the possibility to bias the intermolecular organization of inherently achiral species, as the Mn(Cl)-TOHPP, by directed self-assembly, and afford noncentrosymmetric porphyrin network structures. The above observations provide valuable insights into the crystal engineering strategies of porphyrin network solids. Acknowledgment. This research was supported in part by The Israel Science Foundation (grant No. 254/04). Supporting Information Available: Crystallographic data for compounds 1-4, in the crystallographic information (CIF) format. This information is available free of charge via the Internet at http:// pubs.acs.org.

References (1) (a) Drain, C. M.; Hupp, J. T.; Suslick, K. N.; Wasielewski, M. R.; Chen, X. J. Porphyrins Phthalocyanines 2002, 6, 243-258. (b) Milic, T. N.; Chi, N.; Yablon, D. G.; Flynn, G. W.; Batteas, J. D.; Drain, C. M. Angew. Chem., Int. Ed. 2002, 41, 2117-2119. (c) Suslick, K. N.; Rakow, N. A.; Kosal, M. E.; Chou, J.-H. J. Porphyrins Phthalocyanines 2000, 4, 407-413. (2) (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, 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, pp 43-132. (3) (a) Goldberg, I.; Krupitsky, H.; Stein, Z.; Hsiou, Y.; Strouse, C. E. Supramol. Chem. 1995, 4, 203-221. (b) Diskin-Posner, Y.; Patra, G. K.; Goldberg, I. Chem. Commun. 2002, 1420-1421. (c) DiskinPosner, Y.; Patra, G. K.; Goldberg, I. CrystEngComm 2002, 4, 296301. (d) Vinodu, M.; Goldberg, I. CrystEngComm 2003, 5, 490-494. (4) (a) Dastidar, P.; Stein, Z.; Goldberg, I.; Strouse, C. E. Supramol. Chem. 1996, 7, 257-270. (b) Diskin-Posner, Y.; Goldberg, I. Chem. Commun. 1999, 1961-1962. (c) Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Chem. Commun. 2000, 585-586. (d) Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Angew. Chem., Int. Ed. 2000, 39, 1288-1292. (5) (a) Kosal, M. E.; Chou, J.-H.; Wilson, S. R.; Suslick, K. S. Nat. Mater. 2002, 1, 118-121. (b) Diskin-Posner, Y.; Goldberg, I. New J. Chem. 2001, 25, 899-904. (c) Diskin-Posner, Y.; Patra, G. K.; Goldberg, I. Eur. J. Inorg. Chem. 2001, 2515-2523. (d) Shmilovits, M.; Diskin-Posner, Y.; Vinodu, M.; Goldberg, I. Cryst. Growth Des. 2003, 3, 855-863. (e) Shmilovits, M.; Vinodu, M.; Goldberg, I. New. J. Chem. 2004, 28, 223-227. (f) Shmilovits, M.; Vinodu, M.; Goldberg, I. Cryst. Growth Des. 2004, 4, 633-638.

762 Crystal Growth & Design, Vol. 6, No. 3, 2006 (6) (a) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Polidori, G. SIR-97. J. Appl. Crystallogr. 1994, 27, 435-436. (b) Sheldrick, G. M. SHELXL-97. Program for the Refinement of Crystal Structures from Diffraction Data, University of Go¨ttingen, Germany, 1997. (7) Spek, A. L. J. Appl. Crystallogr. 2003, 7-13. (8) Goldberg, I. Chem. Commun. 2005, 1243-1254. (9) Krishna Kumar, R.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998, 37, 541-552. (10) Jeffrey, G. A. The Introduction to Hydrogen Bonding; Oxford University Press: Oxford, 1977. (11) Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R. Nature 1994, 369, 727-729.

George and Goldberg (12) Champness, N. R.; Schro¨der, M. Encyclopaedia of Supramolecular Chemistry; Atwood, J. L., Steed, J., Eds.; Marcel Dekker: New York, 2004; p 1420. (13) 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 and references therein. (14) Allen, F. H. The Cambridge Structural Database. Acta Crystallogr. 2002, B58, 380-388. (15) Vinodu, M.; Goldberg, I. CrystEngComm. 2005, 7, 133-138. (16) Vinodu, M.; Goldberg, I. CrystEngComm. 2003, 5, 204-207.

CG050624M

Self-Assembly of Supramolecular Porphyrin Arrays by Hydrogen

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