Threefold interpenetration of hydrogen-bonded two-dimensional sheets with 4⁴ topology: supramolecular assembly of dimeric cyanuric acid nodes with four-fold connectivity

Abstract

Cyanuric acid forms a hydrogen-bonded framework with 2,7-diazapyrene that adopts a threefold interpenetrated two-dimensional sheet structure with an unexpected 4⁴ topology, generated via the supramolecular assembly of dimeric cyanuric acid nodes.

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Crystal engineering of extended frameworks can readily be achieved using a variety of supramolecular interactions.¹ Perhaps the most commonly studied supramolecular synthons are those based upon hydrogen bonds² and coordination bonds.³ Whereas coordination bonds can utilise the various preferred geometries of metal centres, in particular those of transition metals, leading to frameworks with a variety of connectivities, hydrogen-bonding interactions are limited by the geometries of organic molecular species, with some notable exceptions that utilise hydrogen bonding interactions involving transition metal species.⁴ Thus much crystal engineering that exploits the hydrogen-bonding interactions between organic species is limited to geometries based upon tetrahedral and trigonal planar arrangements. Fourfold connecting units are of course possible by appropriate design, such as tetra-substituted aromatic moieties,^5,6 but, with the exception of porphyrin-based species,⁷ rarely achieve the square-planar geometries afforded by metal cations.

One particular building block which has received considerable attention in supramolecular chemistry is cyanuric acid (CA, see Scheme 1). Interest in CA is not only focussed on crystal engineering,^8–10 but also in the solution phase¹¹ and, more recently, in surface¹² supramolecular chemistry. Indeed, the complementary and robust triple hydrogen bond between the imide moiety, observed along each edge of CA, and diaminopyridine group is one of the most widely studied synthons used in supramolecular chemistry.¹³

Scheme 1 (a) Cyanuric acid, CA; (b) the R²₂(8) hydrogen bonding arrangement of (CA)₂, observed in CA·diaz; (c) hydrogen bonded (CA)_∞ chains observed in both CA·4,4′bipyridine⁶ and CA·bis-(4-pyridyl)ethene.¹⁴

CA exhibits threefold molecular symmetry and provides three hydrogen bond donors and three hydrogen bond acceptors and is, therefore, capable of participating in multiple hydrogen bonds. The threefold symmetry of CA can be expressed in extended hydrogen bonded structures but in the absence of compatible hydrogen bond donors and/or acceptors, CA is forced to adopt less symmetric hydrogen bonding arrangements. We have been studying the role of hydrogen bond acceptor bridges in the construction of extended arrays and have reported the 2-D sheet structure adopted by the adduct between CA and bis-(4-pyridyl)ethene.¹⁴ This structure is very similar to that observed for the 4,4′-bipyrdine analogue,⁹ in that (CA)_∞ chains are formed with N–H⋯O interactions between CA units (Scheme 1) leaving two N–H groups to interact with pyridyl groups via N–H⋯N interactions. This arrangement leaves two of the carbonyl oxygen atoms of CA free to participate in weak C–H⋯O interactions.

We now report a surprising structure formed by the adduct between CA and 2,7-diazapyrene (diaz), a close analogue of 4,4′-bipyridine. Diaz has been studied in coordination polymer synthesis,¹⁵ as well as in coordination oligomer formation,¹⁶ and is found to behave in a highly similar manner to its more widely studied analogue 4,4′-bipyridine, but with a significantly increased tendency to form π–π interactions which often direct the extended solid-state structure.

A 1 ∶ 1 adduct of CA and diaz was prepared by addition of separate methanolic solutions of CA and diaz to hot MeOH followed by slow cooling to 45 °C. Single crystals of the adduct grew overnight. Although single crystal X-ray diffraction studies using synchrotron radiation† confirmed the formation of an adduct between CA and diaz, an unanticipated structure with an unusual topological arrangement was formed. Both intermolecular CA–CA and CA–diaz hydrogen-bonding interactions are observed in CA·diaz (Fig. 1, Table 1) as they are in other CA–pyridyl based adducts although the overall arrangement is quite different in the example reported herein.

Fig. 1 View of the N–H⋯N and N–H⋯O hydrogen bonding interactions observed in CA·diaz with numbering scheme adopted. Displacement ellipsoids drawn at the 50% probability level. Symmetry codes: i 1 − x, y + 1/2, −z + 3/2; ii x − 1, −y + 1/2, z − 1/2; iii x − 1, y − 1, z; iv −x, y − 1/2, −z + 3/2; v 1 − x, −y − 2, 1 − z.

Table 1 Hydrogen bond lengths and angles of N–H⋯O, N–H⋯N and C–H⋯O bond lengths observed in CA·diaz. Symmetry codes: i −x + 2, −y + 1, −z + 1; ii −x + 1, −y, −z + 1; iii −x + 2, y − 1/2, −z + 3/2 iv 2 − x, −1/2 + y, 3/2 − z; v x, 1 + y, z

	D–H/Å	H⋯A/Å	D⋯A/Å	D–H–A/°
N5- H5N⋯O3^i	0.90(2)	2.01(2)	2.8991(1)	169.2(15)
N4-H4N⋯N1^ii	0.95(2)	1.83(2)	2.7864(1)	174.2(16)
N3-H3N⋯N2^iii	0.93(2)	1.94(2)	2.8549(1)	168.7(16)
C4-H4A⋯O1^iv	0.95(2)	2.19	3.113(2)	165.0
C9-H9A⋯O2^v	0.95(2)	2.41	3.280(2)	152.0

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The CA molecules form dimeric units (Fig. 1) by adopting the highly familiar R²₂(8) hydrogen bonding arrangement observed in carboxylic acid and amide dimers.¹⁷ This arrangement leaves available four N–H groups per (CA)₂ dimer (two per CA molecule) to form N–H⋯N interactions with diaz molecules in a slightly distorted square-planar arrangement. Each diaz molecule acts as a linear linker between CA molecules, thereby linking the (CA)₂ dimers into a 2-D hydrogen bonded sheet of 4⁴ topology (Fig. 2). Such a square-planar nodal arrangement is unusual when using organic building blocks, particularly when generated by dimeric hydrogen-bonded building blocks, although it has previously been observed with a carboxylic acid species.¹⁸

Fig. 2 View of the hydrogen bonded CA·diaz sheets that exhibit 4⁴ topology via the formation of (CA)₂ dimeric units.

The 2-D 4⁴ sheets leave residual space which is filled by threefold parallel interpenetration of symmetry-related CA·diaz sheets (Fig. 3). Interpenetration in 2-D sheets can take two forms, parallel or inclined.¹⁹ Whereas the former, observed in the case of CA·diaz, leads to the formation of a 2-D superstructure, inclined interpenetration generates 3-D superstructures. Although interpenetrated 4⁴ sheets are common in coordination polymer chemistry, exhibiting both parallel and inclined modes,²⁰ such interpenetrated sheets are markedly less common amongst organic compounds.^21,22 Intriguingly these few examples include a hydrogen-bonded adduct containing diaz.⁶

Fig. 3 View of the threefold interpenetration observed in CA·diaz, the parallel nature of the interpenetration leads to a 2-D superstructure.

It is difficult to account for the differences observed between the structures of CA·diaz and CA·4,4′bipyridine⁹ or CA·bis-(4-pyridyl)ethene.¹⁴ In both CA·diaz and CA·bis-(4-pyridyl)ethene each NH group of the CA molecules is involved in either a N–H⋯N or a NH⋯O hydrogen bond and one of the carbonyl oxygen atoms is involved in a N–H⋯O hydrogen bond. Considering the strong hydrogen bonding interactions the only substantial difference between the two structural arrangements is the adoption of a R²₂(8) double hydrogen bond in CA·diaz in contrast to the (CA)_∞ chains observed in CA·bis-(4-pyridyl)ethene. It is likely that the R²₂(8) double hydrogen bond may be thermodynamically stronger than the chain structure observed in the CA·bis-(4-pyridyl)ethene structure. However, this observation does not explain in itself the structural differences.

Unlike many structures that contain diaz,^15,16 CA·diaz exhibits no face-to-face π–π interactions between diaz molecules in the structure but rather the closest diaz molecules adopt an interplanar angle of 31.13° with a shortest intermolecular distance of 2.94 Å between adjacent diaz molecules. Indeed the closest face-to-face π–π interplanar interaction is seen between diaz and CA molecules (interplanar angle = 4.3°; centroid–centroid distance = 3.97 Å; centroid–plane distance = 3.48 Å). Only the N1 terminus of the diaz molecules participates in this interaction and this may contribute to the observation that this end of the diaz molecules is significantly more coplanar with the CA molecule hydrogen-bonded to N1 (interplanar angle = 2.9°) in comparison to the CA molecule hydrogen-bonded to N2 (interplanar angle = 33.9°). The coplanar CA⋯bipyridine arrangement is similar to that observed in related CA·4,4′bipyridine⁹ or CA·bis-(4-pyridyl)ethene¹⁴ structures. In contrast to the N1 end of diaz, the N2 terminus of the diaz molecule does not participate in interplanar interactions with CA molecules from parallel interpenetrated sheets. It is also noticeable that the N–H⋯N interaction is shorter for N1⋯N4 than for N2⋯N3, reflecting a more favourable arrangement at the N1 terminus potentially induced by the CA⋯diaz π–π interaction and by the coplanar CA/diaz hydrogen bonded arrangement.

The contrast in behaviour between the two ends of the diaz molecule perhaps gives some indication as to why interpenetration is observed in this case and not in the previously reported examples.^9,14 However, the influence of CH⋯O interactions should not be overlooked. C–H⋯O interactions are observed between the central aromatic C–H groups of the diaz molecules and CA molecules on adjacent sheets (Table 1) and may provide further incentive for the structure to adopt an interpenetrated arrangement.

In summary, this study demonstrates that a highly unusual structural arrangement can be generated from simple building blocks by the serendipitous assembly of dimeric (CA)₂ units. The supramolecular organisation of the CA molecules ultimately controls the topology of the resultant product leading to the formation of the sheets in CA·diaz which exhibit threefold interpenetration of 2-D superstructures with 4⁴ topology. Our studies into the fascinating and diverse structures of cyanuric acid continue.

Acknowledgements

We are grateful to the UK Engineering and Physical Sciences Research Council (EPSRC) for financial support, under grant GR/S97521/01, and the University of Nottingham for supporting this work. We are also grateful to CCLRC for access to Station 9.8 on the Daresbury SRS and to Dr John E. Warren for experimental advice.

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Footnote

† Crystal data for CA·diaz: C₁₇H₁₁N₅O₃, M = 333.31, Monoclinic, space group P2₁/c, a = 8.2288(4) Å, b = 7.9971(4) Å, c = 21.7042(10) Å, β = 90.523(1)°, V = 1428.22(12) ų, Z = 4, D_c = 1.550 g m⁻³, μ(0.6749 Å) = 0.111 mm⁻¹, T = 150(2) K. 4289 unique reflections (R_int = 0.03). Final R₁ [3449 I > 2σ(I)] = 0.0444, wR₂ (all data) = 0.1330. Synthesis: CA (10 mg) and diaz (24 mg) were dissolved separately in MeOH (10 cm³ each). Both solutions were then added to hot MeOH (10 cm³) and then allowed to cool to room temperature over a day whereupon crystals slowly grew. Elemental analysis confirmed that although single crystals of CA·diaz could be separated manually the bulk sample was contaminated by crystals of another phase, most probably CA·2H₂O. IR (KBr/cm⁻¹): 3068 w, 2955 w, 2926 w, 1921 w, 1766 m, 1716 s, 1699 s, 1578 w, 1452 s, 1423 m, 1409 s, 1397 m, 1266 m, 1213 w, 1147 m, 1121 w, 1074 w, 1055 m, 972 w, 895 m, 827 m, 762 s, 753 m. CCDC reference number 288663. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b515668h

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