Control of Assembly of Dihydropyridyl and Pyridyl Molecules via Directed Hydrogen Bonding

The crystallization of two dihydropyridyl molecules, 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene ([C40H24N10]·2DMF, 1·2DMF; DMF = dimethylformamide) and 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)phenylbenzene ([C46H28N10]·2DMF, 3·2DMF), and their respective oxidized pyridyl analogues, 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)benzene ([C40H20N10], 2) and 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)phenylbenzene ([C46H24N10]·DMF, 4·DMF), has been achieved under solvothermal conditions. The dihydropyridyl molecules are converted to their pyridyl products via in situ oxidative dehydrogenation in solution. The structures of the four molecules have been fully characterized by single crystal and powder X-ray diffraction. The oxidized pyridyl products, 2 and 4, are more elongated due to aromatization of the dihydropyridyl rings at each end of their parent molecules 1 and 3, respectively. The solid-state supramolecular structures of the pyridyl molecules are distinct from the dihydropyridyl molecules in terms of their hierarchical assembly via hydrogen bonding due to the loss of primary N–H hydrogen bond donors in the two electron oxidized tectons. Overall, the geometrically shorter molecules 1 and 3 display close-packed structures, whereas the more extended 2 and 4 assemble into more open supramolecular systems.


■ INTRODUCTION
Supramolecular organic assemblies, including single-component and multiple-component complexes, are a family of crystalline materials composed of molecular species held together in the solid state by long-range, noncovalent interactions. 1 The crystal engineering of organic assemblies is developing apace due to the increasing knowledge of the underlying supramolecular chemistry that directs the assembly and packing of molecules in the sold state. This defines their crystal structures and underpins the design and synthesis of new organic supramolecular materials, 2,3 which have found applications in a wide range of areas including host−guest recognition, proton conductivity, and gas adsorption and separations. 4,5 Among them, the hydrogen bonded supramolecular organic frameworks (SOFs, also referred to as HOFs (hydrogen bonded organic frameworks)) have received attention due to their design potential 5 and their structural and conceptual similarities to metal−organic frameworks (MOFs), which can also function as porous substrates/absorbent. 6 However, most hydrogen bonds are primarily electrostatic in nature and vary in strength according to the different donor and acceptor properties of functional groups and their environment, and thus prediction of structures of supramolecular organic arrays driven by hydrogen bonding can be challenging. 7 Predicting and understanding the packing of molecular assemblies is, therefore, a practical target toward the goal of designing and engineering solid-based materials.

■ EXPERIMENTAL SECTION
Chemicals and General Methods. Commercially available reagents and organic solvents were used as received without further purification. Elemental analyses (C, H, and N) were carried out on an Elementar Vario EL III analyzer. Infrared (IR) spectra were recorded with a PerkinElmer Spectrum One as KBr pellets in the range 400− 4000 cm −1 , and 1 H NMR spectra on a Bruker DPX-400 spectrometer. Thermal gravimetric analyses (TGA) were performed under a flow of N 2 (20 mL·min −1 ) with a heating rate of 10°C·min −1 using a TA SDT-600 thermogravimetric analyzer, and X-ray powder diffraction (PXRD) measurements were carried out at room temperature on a PANalytical X'Pert PRO diffractometer using Cu Kα radiation (λ = 1.5418 Å) at 40 kV, 40 mA, at a scan speed of 0.02°/s and a step size of 0.005°in 2θ.
X-ray Crystallography. Single crystal X-ray data for compounds 1·2DMF, 2, and 3·2DMF were collected on Agilent SuperNova Atlas diffractometers, while the diffraction data for compound 4·DMF was acquired using an Oxford Diffraction Xcalibur Eos instrument. The structure was solved by direct methods and developed by difference Fourier techniques, both using the SHELXL software package. 11 Hydrogen atoms of the ligands were placed geometrically and refined using a riding model. The unit cell volume of compounds 3 and 4 includes disordered solvent molecules (DMF) which could not be modeled as discrete atomic sites. We therefore employed PLATON/ SQUEEZE 10 to calculate the contribution of the solvent region to the diffraction and thereby produced a set of solvent-free diffraction intensities. The solvent molecules of compounds 3 and 4 were not  (1), 980562 (2), 980572 (3), and 980586 (4) contain the supplementary crystallographic data for this paper. Data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. A summary of the crystallographic data for compounds 1−4 is given in Table 1.

■ RESULTS AND DISCUSSION
The 1,4-bis-(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl) derivatives (1 and 3, Scheme 1) were prepared from the reaction of 3-amino-3-(4-pyridinyl)propionitrile with 1,4-benzenedicarbaldehyde or 4,4′-biphenyldicarbaldehyde in glacial acetic acid at 120°C, respectively. Solvothermal reactions of appropriate amount of 1 or 3 in N,N-dimethylformamide (DMF) yielded colorless crystalline solid of 2 or 4 in a yield of ca. 12%, and these products are insoluble in water and common organic solvents at room temperature. The structural transformation of 1 to 2 and 3 to 4 via oxidative dehydrogenation under solvothermal conditions has been confirmed by NMR spectroscopy and single crystal X-ray diffraction. The supramolecular chemistry of all four compounds has been comprehensively studied, and it has been found that the oxidative products, 2 and 4, are somewhat elongated due to the aromatization of the dihydropyridyl rings located on each end of the parent molecules (1 and 3; Figure 1), Moreover, structures of 2 and 4 are distinct in that they are more linear than their parent zigzag dihdropyridyl parent molecules in terms of the hierarchical assemblies via hydrogen bonds.
Crystal Structures. 1·2DMF crystallizes in monoclinic space group P2 1 /c with the asymmetric unit containing one 1,4bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene molecule and two solvent DMF molecules. A feature of 1 is that the two dihydropyridyl rings are nearly perpendicular (dihedral angle of ca. 83.34°) to the central phenyl ring as a result of the sp 3 carbon at the 4-position of each dihydropyridyl ring. Thus, molecules of 1 display zigzag configurations with the N−H groups on the dihydropyridyl rings expected to act as dominant hydrogen bond donors, and the exo-pyridyl and lateral cyano groups behaving as potential hydrogen bond acceptors. Each molecule of 1 interacts with two solvent DMF molecules via hydrogen bonds from dihydropyridyl groups to amide groups (N−H···O interactions, 1.934(11) Å/1.971(15) Å; Table 2 and  Table S1 in the Supporting Information). Two of the four exopyridyl groups of 1 interact with two neighboring ones via hydrogen bonds from phenyl groups to pyridyl groups (C−H··· N interactions, 2.502(5) Å to 2.775(6) Å). Although DMF molecules block the connection of 1 to a neighboring molecule from the primary hydrogen bond donating sites, the pyridyl hydrogen bond accepting groups generate secondary C−H···N interactions and direct the assembly of 1 into a 2D hydrogen bonded square layer (Figure 2). A 3D supramolecular organic framework structure is thus formed through complex hydrogen bond and π−π stacking interactions. 1,4-Bis(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)benzene (2) crystallizes in the monoclinic space group C2/c. 2 possesses a linear central backbone with the two external pyridyl rings twisting around the central phenyl ring due to the rotation about C−C single bonds (torsion angles of 62.14°and 66.50°).

Crystal Growth & Design
Article 2 lacks the primary N−H hydrogen bond donating groups unlike its precursor 1 due to the oxidation of dihydropyridyl groups, and thus the exo-pyridyl and lateral cyano hydrogen bond accepting groups interact mainly with the C−H groups of pyridyl or phenyl moieties (Table 2 and Table S1 in the Supporting Information). A 2D supramolecular network can be identified through hydrogen bond interactions from the pyridyl groups on the central backbones of 2 to the pyridyl/phenyl C− H groups of neighboring ones (Figure 3). The cyano groups decorate the 2D layers and interact with the C−H groups in the adjacent layers through C−H···N hydrogen bonds (Table 2 and  Table S1 in the Supporting Information). Thus, a complex 3D supramolecular organic framework structure is formed through hydrogen bonding as well as π−π stacking interactions between adjacent aromatic rings (pyridyl and/or phenyl). The longer 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)phenylbenzene molecule (3) assembles into a 3D porous organic framework via hydrogen bonding. Single crystal X-ray diffraction confirms that compound 3 crystallizes in monoclinic space group P2 1 /c and adopts a zigzag configuration due to the sp 3 carbon at the 4-position on each dihydropyridyl ring. The two central phenyl rings are coplanar and the two dihydropyridyl rings are nearly perpendicular to the central phenyl rings (dihedral angle of ca. 80.22°) similar to that of 1. N−H groups on the dihydropyridyl rings of 3 participate in primary N−H···N hydrogen bond interactions (2.019(14) Å, Table 2 and Table S1 in the Supporting Information) with the pyridyl N atoms, which leads to the formation of a 2D hydrogen bonded square layer structure (Figure 4a). Secondary C−H···N interactions (Table 2 and  Table S1 in the Supporting Information) between the lateral cyano and C−H groups on the skeleton of 3 account for the formation and stabilization of a 3D supramolecular organic framework with free pyridyl functional groups pointing toward the 1D channels along the crystallographic b axis (Figure 4b). The opening to these channels is estimated to be ca. 2.5 Å × 5.0 Å and the total solvent accessible volume of compound 3 after the removal of guest DMF molecules was estimated to be ca. 30%, calculated using PLATON/VOID routine. 10 1,4-Bis(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)phenylbenzene (4) crystallizes in the orthorhombic space group Pbca as 4· DMF. Unlike 3, the two phenyl rings of 4 are twisted from each other with a torsion angle of ca. 46.38°and the neighboring pyridyl rings have torsion angles of ca. 45.09°and 47.36°with respect to each phenyl ring. One molecule of 4 interacts with four others, two at each end via C−H···N interactions between the N-donors and C−H groups on the pyridyl rings ( Figure 5, inset). In this way, a 3D supramolecular framework with 1D channels along the crystallographic a axis is formed ( Figure 5). The opening of the channels is estimated to be 7.0 Å × 9.2 Å. In addition, 4 incorporates two identical 3D supramolecular nets that interpenetrate through each other with interactions observed through C−H···N hydrogen bonds (Table 2 and  Table S1 in the Supporting Information). This framework interpenetration severely decreases the structural porosity to ca. 8% of solvent accessible volume in the solvent-free structure of 4 as confirmed by PLATON/VOID calculation. 10 It has been found that 2 and 4 are longer than the parent molecules 1 and 3 (Figure 1) due to the oxidative dehydrogenation of the dihydropyridyl moieties to form pyridyl rings. The supramolecular structures of the four molecules have been interpreted in terms of their hydrogen bonded structures    in the sold state. Among the multiple hierarchical hydrogen bonds which direct the self-assembly of the solvated compounds, N−H···A (A = N or O) appear to be the strongest amd most prevalent. Compounds 1 and 2 assemble into nonporous close-packed structures, whereas the biphenyl analogues, 3 and 4, build into potential porous phases. The supramolecular structure of 3·2DMF is reminiscent of the organic framework material SOF-1 in which a similar molecular tecton with an extra central anthracene moiety was used. 5g However, 3·2DMF shows fairly poor framework stability on solvent exchange and desolvation, probably due to the lack of strong π−π stacking interactions, which are observed in SOF-1, and we argue that it is these π−π interactions that are responsible for the exceptionally high thermal stability of SOF-1. 4·DMF possesses a promising porous 3D hydrogen bonded substructure; however, the observed framework interpenetration severely reduces the structural porosity. Thus, the balance between molecular size and topology of tectons and observed structural porosity of the resultant solid-state materials is crucial for the design and practical application of supramolecular organic assemblies.

Crystal Growth & Design
Article