Engineering Hydrogen-Bonded Molecular Crystals Built from 1,3,5

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Engineering Hydrogen-Bonded Molecular Crystals Built from 1,3,5-Substituted Derivatives of Benzene: 6,6′,6′′-(1,3,5-Phenylene)tris-1,3,5-triazine-2,4-diamines Fatima Helzy, Thierry Maris, and James D. Wuest*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1547–1553

Département de Chimie, UniVersité de Montréal, Montréal, Québec H3C 3J7 Canada ReceiVed August 23, 2007; ReVised Manuscript ReceiVed January 31, 2008

ABSTRACT: In 6,6′,6′′-(1,3,5-phenylene)tris-1,3,5-triazine-2,4-diamine, three trigonally directed diaminotriazinyl groups are attached to the 1,3,5-positions of a phenyl core. This introduces a significant capacity for intermolecular hydrogen bonding, because each diaminotriazinyl group can normally interact with two others to form a total of four hydrogen bonds. Derivatives 3 and 4, which have alkyl groups at the 2,4,6-positions, are designed to favor a conformation in which the diaminotriazinyl groups are held perpendicular to the phenyl core. This conformation is expected to direct the hydrogen bonding of each diaminotriazinyl group out of the plane of the phenyl core, leading to generation of a three-dimensional (3D) network in which each molecule is linked to six neighbors by a total of 12 hydrogen bonds. In fact, the observed networks all show a lower degree of connectivity, possibly because the cores of compounds 3 and 4 are too compact to accommodate six fully hydrogen-bonded neighbors. Nevertheless, compounds 3 and 4 have the following attractive features: (1) They have a well-defined molecular geometry that places multiple sites of hydrogen bonding in a predictable orientation, leading to the construction of 3D networks in which neighboring molecules are positioned logically by directional forces; and (2) their topologies make efficient packing difficult and favor open networks with significant volume available for the inclusion of guests. For these reasons, compounds with diaminotriazinyl groups attached to suitably substituted aryl cores are promising subunits for engineering crystals and other ordered molecular materials with novel structures and properties. Introduction Crystal engineering, which seeks to control the structures and properties of crystals, offers challenges and opportunities that make the field an exceptionally exciting area of contemporary science.1 In particular, crystal engineering is an increasingly powerful tool for reaching the visionary goal articulated by Feynman almost 50 years ago: ”What would the properties of materials be if we could really arrange the atoms the way we want them? They would be very interesting to investigate theoretically. I cannot see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do.”2 Even after decades of research, however, predictions of the detailed structures and properties of crystals by theoretical methods are not in general reliable.3–5 Nevertheless, important alternative strategies have emerged for producing crystals by design. As summarized by Dunitz in a recent review,3 “From a more qualitative and descriptive viewpoint has come the notion that certain groupings in organic molecules exercise attractive intermolecular interactions and so guide the molecules into distinctive patterns in their crystal structures. . .This has indeed become one of the tenets of crystal engineering.” An important degree of control over atomic arrangements can thereby be attained by building crystals from molecular subunits that have well-defined geometries and an ability to hold neighboring molecules in predetermined positions by engaging in strong directional interactions.6–8 Such subunits for planned molecular construction, which have been called tectons,9 can be made conveniently by choosing functional groups that engage in reliable patterns of molecular association, which have been called supramolecular synthons,10 and then by attaching them

Figure 1. Representation of the planar hexagonal network I built from 1,3,5-benzenetricarboxylic acid (1), with 1,3,5-trisubstituted phenyl groups represented by triangles and hydrogen bonds shown as broken lines.

* Author to whom correspondence may be addressed: james.d.wuest@ umontreal.ca.

An archetypal tecton is 1,3,5-benzenetricarboxylic acid (1),11,12 which incorporates three COOH groups oriented

to cores that orient the sticky sites properly and introduce other desired features.

10.1021/cg700798z CCC: $40.75  2008 American Chemical Society Published on Web 04/11/2008

1548 Crystal Growth & Design, Vol. 8, No. 5, 2008

Figure 2. Hydrogen-bonding motifs typically formed by diaminotriazinyl groups.

Helzy et al.

Figure 5. Side view of a single corrugated sheet in the structure of crystals of tecton 3 grown from DMSO/chlorobenzene, showing how diaminotriazinyl groups not involved in intertectonic hydrogen bonding are oriented above and below the sheets in alternation. Molecules of tecton 3 are drawn with all atoms in red or in pink, and hydrogen bonds are represented by broken lines.

Figure 3. View of the structure of crystals of tecton 3 grown from DMSO/chlorobenzene. A central molecule of tecton 3 is shown in red, and the four neighboring molecules that engage in hydrogen bonding according to motif IV (Figure 2) are drawn with carbon atoms in gray, hydrogen atoms in white, and nitrogen atoms in blue. Hydrogen bonds are represented by broken lines. In all these bonds, the N · · · H distances are less than 2.6 Å.

Figure 6. View of the structure of crystals of tecton 3 grown from DMSO/chlorobenzene, showing how diaminotriazinyl groups not involved in direct intertectonic hydrogen bonding interact with included molecules of DMSO and H2O. A central molecule of tecton 3 is shown in red, and hydrogen-bonded neighbors are drawn with carbon atoms in gray, hydrogen atoms in white, nitrogen atoms in blue, oxygen atoms in red, and sulfur atoms in yellow. Hydrogen bonds are represented by broken lines.

Figure 4. Representation of the corrugated four-connected hydrogenbonded sheets found in the structure of crystals of tecton 3 grown from DMSO/chlorobenzene. The centroid of each molecule of tecton 3 is shown as a red or blue sphere, and the lines connecting each sphere to four adjacent spheres correspond to the intertectonic hydrogen bonds shown in detail in Figure 3.

trigonally by a rigid core. Tecton 1 is programmed to generate planar hexagonal network I (Figure 1) by normal intermolecular association of the -COOH groups as cyclic hydrogen-bonded pairs.

Although the architecture of the sheets typically arises according to plan, the design has two notable shortcomings: (1) Each tecton participates in only six hydrogen bonds, so the resulting network is not highly robust; and (2) the relative orientation of adjacent sheets is hard to foresee because it is not controlled by strong directional forces. To eliminate the first shortcoming, we decided to replace the -COOH groups of 1,3,5-benzenetricarboxylic acid (1) by 2,4diamino-1,3,5-triazinyl groups, which are known to form multiple hydrogen bonds according to motifs II-IV (Figure 2).7 Motif II is the most frequently observed of these alternatives, presumably because it incorporates hydrogen bonds remote

6,6′,6′′-(1,3,5-Phenylene)tris-1,3,5-triazine-2,4-diamines

Figure 7. View of the structure of crystals of tecton 3 grown from DMSO/ chlorobenzene, showing three adjacent hydrogen-bonded sheets in red and blue. Included molecules of DMSO and H2O occupy spaces between the sheets. Atoms are represented by spheres of van der Waals radii.

from the sterically congested site where the diaminotriazinyl group is attached to the molecular core. Overcoming the second shortcoming cannot be achieved simply by attaching diaminotriazinyl groups in place of -COOH groups to create tris(diaminotriazine) 2. 2-Phenyl-1,3,5-triazines are known to favor conformations in which the phenyl and triazinyl rings are nearly coplanar,7 so we expected that the ability of tecton 2 to engage in multiple hydrogen bonds would be limited largely to a single plane, thereby leading again to the formation of parallel sheets with no dominant interconnections. We elected to solve this problem by making derivatives 3 and 4, in which substituents are introduced at the 2,4,6-positions to force the neighboring triazinyl groups out of the plane of the aromatic core, allowing them to use part of their capacity for hydrogen bonding to control cohesion of the primary sheets. In this way, we hoped to obtain three-dimensional (3D) hydrogen-bonded networks built by interconnecting hexagonal sheets. Results and Discussion Synthesis of Tectons 3 and 4. Methyl- and ethyl-substituted compounds 3 and 4 were synthesized in 84 and 86% yields,

Crystal Growth & Design, Vol. 8, No. 5, 2008 1549

respectively, by treating the known trinitriles 513 and 612 with dicyandiamide and KOH under standard conditions.14 Structure of Crystals of Tecton 3 Grown from DMSO/ Chlorobenzene. Exposing a solution of tecton 3 in DMSO to vapors of chlorobenzene induced the formation of crystals of composition 3 · 4DMSO · 2H2O.15 The crystals proved to belong to the orthorhombic space group P212121. Views of the structure appear in Figures 3–7, and crystallographic details are provided in Table 1. As planned, tecton 3 adopts a conformation in which the average planes of the 1,3,5-trisubstituted phenyl core and the three triazinyl rings are nearly orthogonal (84.2(2)°, 81.9(2)°, and 81.4(2)°). Unexpectedly, however, only two of the triazinyl groups engage in intertectonic hydrogen bonding, and they favor motif IV (Figure 2). In this way, each tecton forms a total of eight hydrogen bonds with four neighboring tectons (Figure 3), thereby creating corrugated sheets (Figure 4). The third triazinyl group of each tecton is directed perpendicular to the sheets, in a orientation opposite to that of the corresponding groups of the four hydrogen-bonded neighbors (Figure 5). Diaminotriazinyl groups not involved in the construction of sheets form hydrogen bonds with included DMSO and H2O (Figure 6), which separate adjacent sheets (Figure 7). Approximately 56% of the volume of the crystals is available for the inclusion of guests, as estimated by standard methods.16,17 The large fraction of accessible volume in crystals of compound 3 reflects the tendency of tectons to respect the normal geometric preferences of strong intermolecular interactions, even when they force packing to become inefficient. Removal of the crystals from the mother liquors led to rapid decomposition through loss of included guests. Structure of Crystals of Tecton 3 Grown from DMSO/ Toluene. By exposing a solution of tecton 3 in DMSO to vapors of toluene, we obtained crystals of approximate composition 3 · 1DMSO · 2toluene · g2H2O.15 The crystals were found to belong to the triclinic space group P1j. Views of the structure appear in Figures 8–11, and crystallographic details are provided in Table 1. Again, tecton 3 adopts the expected conformation, with nearly orthogonal average planes of the 1,3,5-trisubstituted phenyl core and the three triazinyl rings (68.2(1)°, 83.1(1)°, and 80.5(1)°). Crystals grown from DMSO/chlorobenzene and DMSO/toluene proved to have similar compositions and related structures, despite the difference in space groups. In crystals

Table 1. Crystallographic Data for Tectons 3 and 4

a

compound

3 · 4DMSO · 2H2O

3 · 2H2Oa

4 · 4DMSO · 3H2O

formula MW crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z T (K) Fcalc (g cm-3) λ (Cu KR Å) µ (Cu KR mm-1) R1, I > 2σ(I) (all) wR2, I > 2σ(I) (all) measured reflections independent reflections

C26H49N15O6S4 796.04 orthorhombic P212121 11.905(5) 12.714(5) 27.347(10) 90 90 90 4139(3) 4 226(2) 1.277 1.54178 2.577 0.0684 (0.0931) 0.1624 (0.1743) 47133 8134

C18H25N15O2 483.53 triclinic P1j 10.8239(5) 12.0117(7) 15.8500(9) 79.084(4) 86.515(4) 69.273(3) 1892.4(2) 2 223(2) 0.849b 1.54178 0.511 0.0766 (0.0920) 0.2099 (0.2256) 19111 7419

C29H57N15O7S4 856.14 triclinic P21 11.8963(2) 13.2249(3) 14.5323(2) 90 102.435(1) 90 2232.69(7) 2 295(2) 1.273 1.54178 2.441 0.0687 (0.0892) 0.1803 (0.2134) 26985 4647

Guests not identified unambiguously by crystallography are omitted from the composition; see Supporting Information. contribution from guests.

b

Calculated without

1550 Crystal Growth & Design, Vol. 8, No. 5, 2008

Helzy et al.

Figure 8. View of the structure of crystals of tecton 3 grown from DMSO/toluene. A central molecule of tecton 3 is shown in red, two neighboring molecules that engage in hydrogen bonding according to motif II (Figure 2) are drawn in green, and two other neighbors that form hydrogen bonds of type IV appear in yellow. Hydrogen bonds are represented by broken lines. In all these bonds, the N · · · H distances are less than 2.6 Å.

Figure 9. View of the structure of crystals of tecton 3 grown from DMSO/toluene. A central molecule of tecton 3 is shown in red, and two neighboring molecules in adjacent sheets that are connected to the central tecton by single hydrogen bonds are drawn in blue. Hydrogen bonds are represented by broken lines. In these bonds, the N · · · H distances are less than 2.6 Å, and all other N · · · H distances between the central tecton and its blue neighbors exceed 2.8 Å.

group not involved in the formation of sheets to donate an additional single hydrogen bond to a triazinyl group in an adjacent sheet (Figure 9).18 This raises the total of intertectonic hydrogen bonds per tecton to 10, and it links the individual sheets to form a 3D network (Figure 10). Approximately 52% of the volume of the crystals is available for the inclusion of guests,16 which are highly disordered and occupy parallel channels with cross sections that measure approximately 5 × 15 Å and run along the a-axis (Figure 11). Loss of these guests occurred readily when crystals were removed from their mother liquors. Figure 10. Representation of the six-connected 3D hydrogen-bonded network observed in crystals of tecton 3 grown from DMSO/toluene. The centroid of each molecule of tecton 3 is shown as a red or blue sphere. Red or blue lines corresponding to normal intertectonic hydrogen bonds of types II and IV (Figures 2 and 8) connect each sphere to four adjacent spheres, thereby defining corrugated sheets similar to those shown in Figure 4. Lines that are half-red and half-blue represent single intertectonic hydrogen bonds (Figure 9) and connect each sphere to two others in adjacent sheets, creating a 3D network.

grown from DMSO/toluene, each molecule of tecton 3 again uses two of its three diaminotriazinyl groups to form eight normal hydrogen bonds with four neighbors, thereby defining sheets similar to those observed in crystals grown from DMSO/ chlorobenzene (Figure 8). In crystals grown from DMSO/ toluene, however, each of the two diaminotriazinyl groups simultaneously engages in hydrogen bonding according to motifs II and IV, whereas crystals grown from DMSO/chlorobenzene show only motif IV. Moreover, in crystals grown from DMSO/ toluene, each molecule of tecton 3 uses the diaminotriazinyl

Structures of Crystals of Tecton 4 Grown from DMSO/ Toluene. Exposing a solution of ethyl-substituted tecton 4 in DMSO to vapors of toluene induced the formation of crystals of composition 4 · 4DMSO · 3H2O.15 The crystals proved to belong to the triclinic space group P21. Views of the structure appear in Figures 12 and 13, and crystallographic details are provided in Table 1. Like methyl-substituted analogue 3, ethylsubstituted compound 4 favors a conformation in which the average planes of the 1,3,5-trisubstituted phenyl core and the three triazinyl rings are approximately orthogonal (89.2(1)°, 78.9(1)°, and 86.7(1)°). Two of the three diaminotriazinyl groups of each molecule of tecton 4 form a total of eight hydrogen bonds with four neighbors according to motif IV (Figures 2 and 12). This yields parallel corrugated four-connected sheets essentially identical to those found in crystals of tecton 3 grown from DMSO/chlorobenzene (Figure 4). Diaminotriazinyl groups not involved in the construction of sheets form hydrogen bonds with included DMSO and H2O (Figure 13). Approximately 54%

6,6′,6′′-(1,3,5-Phenylene)tris-1,3,5-triazine-2,4-diamines

Figure 11. Representation of the structure of crystals of tecton 3 grown from DMSO/toluene, showing a 3 × 4 × 3 array of unit cells viewed approximately along the a-axis. Guests are omitted for clarity, and atoms are represented by spheres of van der Waals radii to show the cross sections of channels. Atoms of hydrogen appear in white, carbon in gray, and nitrogen in blue.

of the volume of the crystals is accessible to guests, as estimated by standard methods.16 Conclusions Tectons 3 and 4 are designed to hold trigonally directed diaminotriazinyl groups perpendicular to an aromatic core. In principle, this conformation should favor the assembly of 3D

Crystal Growth & Design, Vol. 8, No. 5, 2008 1551

hydrogen-bonded networks instead of 2D alternatives normally favored by planar trigonal tectons such as 1,3,5-benzenetricarboxylic acid (1). As confirmed by analysis of crystals grown under various conditions, tectons 3 and 4 reliably adopt the expected conformation. This conformation, when combined with the potential of each diaminotriazinyl group to participate in four hydrogen bonds according to established motifs II-IV (Figure 2), could give rise to 3D networks in which each tecton forms a total of 12 hydrogen bonds with six neighbors. In fact, the observed structures all show a lower degree of hydrogen bonding, possibly because the 1,3,5-trisubstituted phenyl cores of tectons 3 and 4 are too compact to accommodate six fully hydrogen-bonded neighbors. Inclusion of DMSO and H2O in all structures presumably helps compensate by allowing diaminotriazinyl groups to form additional hydrogen bonds with molecules smaller than tectons 3 and 4. Tectons 3 and 4 fail to fully exploit their potential for hydrogen bonding according to motifs II-IV. Nevertheless, their behavior reveals the following attractive features: (1) They have predictable molecular geometries; (2) they incorporate multiple sites of association that are reliably directed above, below, and within the plane of the aryl core, thereby allowing the construction of 3D networks in which neighboring molecules are positioned by directional forces; and (3) their complex nonplanar topologies make efficient packing difficult, leading to the formation of open networks with significant volume available for the inclusion of guests. For these reasons, tectons with diaminotriazinyl groups attached to suitably substituted aryl cores are promising subunits for engineering crystals and other ordered molecular materials with novel structures and properties. Experimental Section 6,6′,6′′-(2,4,6-Trimethyl-1,3,5-phenylene)tris-1,3,5-triazine-2,4-diamine (3). A mixture of 2,4,6-trimethyl-1,3,5-benzenetricarbonitrile (5; 0.580 g, 2.97 mmol),13 dicyandiamide (3.03 g, 36.0 mmol), and powdered KOH (0.396 g, 7.06 mmol) in 2-methoxyethanol (40 mL)

Figure 12. View of the structure of crystals of tecton 4 grown from DMSO/toluene. A central molecule of tecton 4 is shown in red, and the four neighboring molecules that engage in hydrogen bonding according to motif IV (Figure 2) are drawn with carbon atoms in gray, hydrogen atoms in white, and nitrogen atoms in blue. Hydrogen bonds are represented by broken lines. In all these bonds, the N · · · H distances are less than 2.6 Å.

1552 Crystal Growth & Design, Vol. 8, No. 5, 2008

Helzy et al. Supporting Information Available: Additional crystallographic details, including ORTEP drawings and tables of structural data in CIF format. This material is available free of charge via the Internet at http:// pubs.acs.org.

References

Figure 13. View of the structure of crystals of tecton 4 grown from DMSO/toluene, showing how diaminotriazinyl groups not involved in direct intertectonic hydrogen bonding interact with included molecules of DMSO and H2O. A central molecule of tecton 4 is shown in red, and hydrogen-bonded neighbors are drawn with carbon atoms in gray, hydrogen atoms in white, nitrogen atoms in blue, oxygen atoms in red, and sulfur atoms in yellow. Hydrogen bonds are represented by broken lines. was heated at reflux for 12 h. The resulting mixture was cooled, and a solid was separated by filtration. The solid was washed thoroughly with hot H2O, rinsed with CH3OH, and dried in vacuo to give a pure sample of 6,6′,6′′-(2,4,6-trimethyl-1,3,5-phenylene)tris-1,3,5-triazine-2,4-diamine (3; 1.11 g, 2.48 mmol, 84%) as a colorless solid: IR (KBr) 3323, 3184, 2980, 2880, 1637, 1541, 1445, 1365, 1253, 1122, 1080, 1034, 997, 829, 787 cm-1; 1H NMR (400 MHz, DMSO-d6, 293 K) δ 6.66 (s, 12H), 1.85 (s, 9H); 13C NMR (100.5 MHz, DMSO-d6, 293 K) δ 175.72, 168.08, 138.09, 130.35, 17.57; HRMS (FAB, 3-nitrobenzyl alcohol) calcd for C18H22N15 + H m/e 448.2183, found 448.2158. 6,6′,6′′-(2,4,6-Triethyl-1,3,5-phenylene)tris-1,3,5-triazine-2,4-diamine (4). An analogous reaction transformed 2,4,6-triethyl-1,3,5benzenetricarbonitrile (6; 0.710 g, 2.99 mmol)12 into 6,6′,6′′-(2,4,6triethyl-1,3,5-phenylene)tris-1,3,5-triazine-2,4-diamine (4; 1.26 g, 2.57 mmol, 86%), which was isolated as a colorless solid: IR (KBr) 3330, 3140, 1641, 1531, 1450, 1360, 1258, 1026, 991, 906, 829 cm-1; 1H NMR (400 MHz, DMSO-d6, 293 K) δ 6.67 (s, 12H), 2.28 (q, 3J ) 7.4 Hz, 6H), 0.95 (t, 3J ) 7.4 Hz, 9H); 13C NMR (100.5 MHz, DMSO-d6, 293 K) δ 175.80, 167.78, 137.55, 137.09, 24.72, 16.39; HRMS (FAB, 3-nitrobenzyl alcohol) calcd for C21H28N15 + H m/e 490.2652, found 490.2673. X-Ray Crystallographic Studies. Data were collected using a Bruker AXS SMART 2K/Platform diffractometer. Structures were solved by direct methods using SHELXS-97 and refined using SHELXL-97.19 All non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed in ideal positions and defined as riding atoms.

Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Ministère de l′Éducation du Québec, the Canada Foundation for Innovation, the Canada Research Chairs Program, and Université de Montréal for financial support.

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6,6′,6′′-(1,3,5-Phenylene)tris-1,3,5-triazine-2,4-diamines (16) The percentage of volume accessible to guests was estimated by the PLATON program.17 PLATON calculates the accessible volume by allowing a spherical probe of variable radius to roll over the van der Waals surface of the network. PLATON uses a default value of 1.20 Å for the radius of the probe, which is an appropriate model for small guests such as water. The van der Waals radii used to define surfaces for these calculations are C: 1.70 Å, H: 1.20 Å, and N: 1.55 Å. The percentage of accessible volume is given by 100Vg/V, where V is the volume of the unit cell and Vg is the guest-accessible volume as calculated by PLATON.

Crystal Growth & Design, Vol. 8, No. 5, 2008 1553 (17) (a) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University: Utrecht, The Netherlands, 2001. (b) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194–201. (18) All other intermolecular N · · · H distances exceed 2.8 Å and cannot be considered to involve hydrogen bonding. (19) Sheldrick, G. M. Program for the Solution of Crystal Structures and SHELXL-97, Program for the Refinement of Crystal Structures; Universität Göttingen: Germany, 1997.

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