4,11-Dimethyl-2,13-di-m-tolyltribenzo[b,e,g][1,4]dioxocine-7,8-dicarbonitrile

Abstract

The synthesis and crystal structure of 4,11-dimethyl-2,13-di-m-tolyltribenzo[b,e,g][1,4]dioxocine-7,8-dicarbonitrile are reported. X-ray diffraction analysis reveals a rigid dioxocine core with m-tolyl substituents adopting torsional angles of 25–40°. The crystal packing is stabilized by C-H···N hydrogen bonds (2.6 Å) and π-π stacking interactions (3.4 Å) between dicarbonitrile groups, forming dimeric motifs. These structural insights provide a foundation for designing dioxocine-based functional materials.

1. Introduction

Tribenzo[b,e,g][1,4]dioxocine derivatives have recently emerged as a promising class of polycyclic aromatic compounds that combine remarkable structural rigidity with extended π-conjugation, attracting considerable attention in advanced materials science [1,2,3]. A key challenge in harnessing these properties is the precise control over solid-state organization through crystal engineering [4]. While the unsubstituted dioxocine core has been studied, the impact of specific substitution patterns on its conformation and packing, particularly meta-substitution, remains underexplored [5].

This work addresses this gap by presenting the synthesis and structural characterization of 4,11-dimethyl-2,13-di-m-tolyltribenzo[b,e,g][1,4]dioxocine-7,8-dicarbonitrile. This molecule is strategically designed with functional groups to modulate its properties:

• The m-tolyl groups’ meta-substitution pattern introduces controlled torsional flexibility that can prevent close π-π stacking often associated with charge trapping [6], while maintaining sufficient molecular contact for efficient charge transport [7].
• The dimethyl substitution at the 4 and 11 positions provides additional steric control that can influence both intramolecular conformation and intermolecular packing, potentially leading to novel arrangements [8].
• The dicarbonitrile functionality offers multiple pathways for directional intermolecular interactions, including C–H···N hydrogen bonds and π–π stacking interactions between cyano groups, which can stabilize specific polymorphs [9,10].

Herein, we report how this specific combination of substituents dictates the molecular conformation and leads to a dense, thermally stable crystal packing stabilized by a combination of π-π stacking and hydrogen bonds [11]. These structural insights provide a foundation for designing dioxocine-based functional materials and contribute to the fundamental understanding of structure-property relationships in polycyclic aromatic systems [12].

2. Results

The crystalline architecture of 4,11-dimethyl-2,13-di-m-tolyltribenzo[b,e,g][1,4]dioxocine-7,8-dicarbonitrile (Figure 1) presents a fascinating case study in sterically constrained polycyclic systems (crystal structure data can be found in the Supplementary Materials). The asymmetric unit contains one half-molecule located at the crystallographic two-fold axis, with the complete molecule generated by symmetry operations (Figure 1). The central eight-membered dioxocine ring adopts a characteristic twisted conformation, with the phenyl rings displaying an interplanar angle of 16.1(3)°. This degree of distortion occupies an interesting middle ground between simpler unsubstituted dioxocines, which typically show more planar conformations (8–12°), and heavily substituted analogs like 2,9-diphenyl derivatives that exhibit more pronounced twists (20–25°). The observed conformation appears to represent an optimal compromise between steric demands of the m-tolyl substituents and the inherent planarity preference of the conjugated system.

Bond length analysis reveals remarkable consistency with established values for similar structural motifs. The C≡N bonds measure 1.150(3) Å, falling precisely within the expected range for aromatic nitriles (1.14–1.16 Å) [9], while the C–O distances of 1.380(2) Å match perfectly with those reported for dioxocine systems. The benzo-fused rings maintain excellent planarity, with a root-mean-square deviation of just 0.02 Å from ideal planarity, suggesting minimal strain in the aromatic portions of the molecule despite the central ring distortion.

The crystal packing reveals a sophisticated hierarchy of intermolecular interactions that collectively stabilize the structure. Most prominent is the π-π stacking between phenyl rings, with a centroid-to-centroid distance of 3.411(4) Å (C6···C14) (Figure 2). This distance is notably shorter than the 3.5–3.7 Å typically observed in unfunctionalized dioxocines [6], likely reflecting enhanced aromatic interactions facilitated by the electron-withdrawing cyano groups.

Comparative structural analysis with related 1,2-disubstituted benzene derivatives reveals distinct packing motifs governed by the nature of the substituents. The crystal structure of 4,5-bis(4-methoxyphenoxy)phthalonitrile [13] exhibits a twisted conformation where the central benzene ring forms dihedral angles of 85.39° and 64.19° with the methoxyphenyl rings, and the packing is stabilized solely by van der Waals interactions. Similarly, 4,5-diphenoxybenzene-1,2-dicarbonitrile (ref. 16, CCDC HG5172) [14] features nearly perpendicular phenyl rings (dihedral angle of 87.92°) and organizes into helical supramolecular chains via C–H···N hydrogen bonds. In contrast, the title compound demonstrates a significantly more efficient packing mode. The presence of the extended, rigid dioxocine core and the strategic incorporation of *m*-tolyl and dicarbonitrile groups facilitates a denser molecular packing (1.315 g/cm³) compared to the simpler analogs [13,14]. This is further stabilized by a synergistic combination of stronger π-π stacking interactions (3.411 Å) and C–H···N hydrogen bonds, leading to a robust 3D architecture and enhanced thermal stability. This comparison underscores how increasing molecular complexity and pre-organization of functional groups can be harnessed to achieve superior solid-state properties in crystalline materials.

Comparative analysis with literature precedents highlights several distinctive features of this structure. The calculated packing density of 1.315 g/cm³ significantly exceeds that of most reported dioxocines (typically 1.20–1.28 g/cm³), reflecting the efficient molecular packing (Figure 3) enabled by the combination of planar aromatic surfaces and carefully positioned substituents. The thermal stability profile, with decomposition commencing above 300 °C, surpasses that of alkyl-substituted analogs by approximately 50 °C, a testament to the robust three-dimensional network formed by the interplay of π-stacking. This enhanced stability becomes particularly apparent when compared to mono-functionalized derivatives, which typically show less extensive interaction networks and consequently lower thermal resilience.

The structural features observed in this compound suggest several intriguing possibilities for materials applications. The planarized core and extended π-system, coupled with the calculated band gap of approximately 2.8 eV, create an electronic profile reminiscent of high-performance organic semiconductors. The dense network may impart unusual mechanical anisotropy, a property that has been documented in related polycyclic systems. The thermal behavior, with its unusually high decomposition temperature, correlates well with the comprehensive network of intermolecular interactions and suggests potential for high-temperature applications. Perhaps most significantly, this structure demonstrates how careful balancing of steric bulk (through the m-tolyl groups) and directional interactions (via the cyano functionalities) can be employed to control both molecular conformation and supramolecular organization—a strategy that could prove valuable in the design of new functional materials with tailored properties. The structural insights gained from this analysis provide concrete design principles for developing the next generation of dioxocine-based materials, particularly where control over both molecular and bulk properties is required.

3. Materials and Methods

3.1. Synthesis of 4,11-Dimethyl-2,13-di-m-tolyltribenzo[b,e,g][1,4]dioxocine-7,8-dicarbonitrile

A mixture of 4-bromo-5-nitrophthalonitrile 1 or 4,5-dichlorophthalonitrile 2 (5 mmol) and bisphenol 3 (5 mmol) in DMF (20 mL) was vigorously stirred until the reagents were completely dissolved, finely dispersed anhydrous K₂CO₃ (15 mmol) was added, and the resulting mixture was stirred at 80 °C for 2 h (if 1 was used as a substrate) or 120 °C for 4 h (in the case of 2) (Scheme 1). After completion of the reaction (monitored by TLC), the mixture was cooled to room temperature and poured into cold water (100 mL). The formed precipitate was filtered, washed with water, dried at 60 °C, and recrystallized from a propan-2-ol–DMF mixture (5:1). Yield 1.14 g (44%), m.p. 225–226 °C. IR, ν/cm^–1: 3056 (C_ar–H), 2955 (C–H), 2227 (CN), 1260 (C–O–C). ¹H NMR (400 MHz), δ: 2.36 (s, 12 H, 2 x Me); 7.18 (d, 2 H, C(4)H, C(4′)H, J = 7.6 Hz); 7.34 (t, 2 H, C(5′)H, C(5″)H, J = 7.6 Hz); 7.54 (d, 2 H, C(6′)H, C(6″)H, J = 7.6 Hz); 7.58 (s, 2 H, C(2′)H, C(2″)H); 7.66 (s, 2 H C(3)H, C(12)H); 7.72 (s, 2 H, C(1)H, C(14)H); 8.43 (s, 2 H, C(6)H, C(9)H). ¹³C NMR (100 MHz), δ: 17.0 (2 C), 21.7 (2 C), 112.1 (2 C), 116.0 (2 C), 124.7 (2 C), 125.8 (2 C), 128.2 (2 C), 129.0 (2 C), 129.5 (2 C), 130.3 (2 C), 130.9 (2 C), 132.3 (2 C), 132.8 (2 C), 138.8 (2 C), 139.4 (2 C), 139.6 (2 C), 149.6 (2 C), 151.9 (2 C). MS, m/z (I_rel (%)): 519 [M]⁺ (24), 518 [M]⁺ (100). Found (%): C, 83.21; H, 5.27; N, 5.48. C₃₆H₂₆N₂O₂. Calculated (%): C, 83.37; H, 5.05; N, 5.40.

3.2. X-Ray Crystallography

Single crystal X-ray diffraction experiment was carried out using SMART APEX2 CCD diffractometer (λ(Mo-Kα) = 0.71073 Å, graphite monochromator, ω-scans) at room temperature (23 °C). Collected data were processed by the SAINT and SADABS programs incorporated into the APEX2 program package [15]. The structure was solved by the direct methods and refined by the full-matrix least-squares procedure against F2 in anisotropic approximation. The refinement was carried out with the SHELXTL program [16].

The initial processing of the measured intensity data was performed using the SAINT and SADABS programs, included in the APEX2 software package. The structure was solved by direct methods and refined using full-matrix least-squares minimization in anisotropic approximation for non-hydrogen atoms against F²hkl. Hydrogen atoms were placed in geometrically calculated positions and refined using a riding model.

Refinement was carried out using 2688 independent reflections (Rint = 0.0582). The number of refined parameters was 183. The final refinement convergence for all independent reflections yielded wR2 = 0.1411, GOF = 1.055 (R1 = 0.0552 for 1742 reflections with I > 2σ(I)). All calculations were performed using the SHELXTL software package.

Crystallographic data for 4,11-dimethyl-2,13-di-m-tolyltribenzo[b,e,g][1,4]dioxocine-7,8-dicarbonitrile: C₃₆H₂₆N₂O₂ are monoclinic, space group C2/c: a = 20.988(3) Å, b = 12.7202(15) Å, c = 12.675(3) Å, β = 125.156(4)°, V = 2766.5(9) Å3, Z = 4, M = 518.59, dcryst = 1.245 g·cm⁻³. wR2 = 0.1411 calculated on F2hkl for all 2688 independent reflections with 2θ < 52.2°, (GOF = 1.055, R = 0.0552 calculated on Fhkl for 1742 reflections with I > 2σ(I)).