5,10-Diiododibenzo[a,e]pentalene

Abstract

The crystal structure and the improved synthesis of the title compound (1) are presented. Treatment of 5,10-disilyldibenzo[a,e]pentalenes (2a and 2b) with iodine chloride (ICl) readily provided 1 in good yields. Recrystallization from a dichloromethane/hexane solution afforded single crystals suitable for X-ray diffraction studies. In the packing structure, iodine···iodine interactions classified as type II halogen bonding were found, forming the zigzag I…I chains along the ac-plane.

1. Introduction

Pentalene (C₈H₆) and its annulated derivatives such as dibenzo[a,e]pentalene (DBP) are an important class of compounds because of their 4nπ antiaromaticity and potential application in materials science [1,2,3,4,5]. For instance, Donor–π–acceptor-type compounds with a DBP as the central π-spacer were reported, and their photophysical properties were investigated [6,7]. Recently, carbon nanohoops incorporating DBP, in which benzene rings in cycloparaphenylenes are replaced by 5,10-dibenzopentalenediyl or 2,7-dibenzopentalenediyl, have been reported by the group of Esser, revealing the enhanced antiaromaticity of the curved DBP in the nanohoops [8,9,10,11,12]. DBP-based covalent organic frameworks (COFs) were also synthesized, and their applications, including solar-driven water splitting and Li-organic batteries, were demonstrated [13,14].

Considering this broad potential of DBP-based materials, the efficient and large-scale synthesis of halogenated DBP is highly demanded. However, previously reported syntheses of the title compound have some drawbacks: (1) sufficient quantities cannot be obtained [15], (2) a long reaction time is required, or (3) several steps are needed to synthesize the precursor [16,17]. Furthermore, no single-crystal X-ray diffraction studies have been reported for dihalogenated DBPs. Herein, we report the improved synthesis of the title compound, 5,10-diiododibenzo[a,e]pentalene 1, from the reaction of the 5,10-disilyl derivative and iodine chloride (ICl). The X-ray diffraction studies of 1, which are the first structural elucidation of 5,10-dihalogenated DBPs, were carried out to reveal the iodine···iodine interactions, which form the zigzag I…I chains along the ac-plane.

2. Results and Discussion

2.1. Improved Synthesis of 5,10-Diiododibenzo[a,e]pentalene 1

Considering that arylsilanes can be converted to synthetically useful iodoarenes by treating with ICl [18], the iodination of 5,10-disilyldibenzo[a,e]pentalenes 2a and 2b, where the silyl groups are triisopropylsilyl (2a) and tert-butyldimethylsilyl (2b), with ICl was investigated (Scheme 1). The reaction of 2a or 2b, which can be obtained by the reduction of the corresponding arylethynylsilanes with potassium followed by oxidation by I₂ [19], with 2.1 equiv of ICl at 0 °C provided 1 within 30 min with good yields (64% and 48% from 2a and 2b). Note that the previously reported synthesis of 1 using 2a and iodine gave only ca 50 mg and required a much longer reaction time as well as a refluxing condition [15]. It is necessary to pay attention to the amount of ICl, because increasing the ICl to 4.0 equiv gave a complex mixture. Since iodoarenes are promising candidates for organic semiconductors, including hole transport materials [20,21,22], this facile synthesis of 1 would accelerate investigations into pentalene-based materials.

2.2. X-Ray Diffraction Studies

Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of hexane into a dichloromethane solution of 1. Compound 1 crystallized in the orthorhombic space group Pbca. Figure 1 illustrates the molecular structure and the packing structures of 1. The pentalene core has a C−C bond alternation, as was found for other neutral DBPs (

Table 1. Selected bond lengths [Å].

Bonds 1	Lengths	Bonds 1	Lengths
C3−C4	1.484 (7)	C3−C6#	1.408 (7)
C4−C5	1.336 (7)	C5−C5#	1.479 (9)
C5−C6	1.482 (7)	C4−I1	2.704 (5)

¹ See Figure 1 for the atom labels.

) [19,23,24]. Each iodine atom has an intermolecular iodine…iodine contact (3.913(3) Å) (Figure 1b), which is shorter than the sum of the van der Waals Radii (3.96 Å) [25]. Halogen…halogen interactions, one of halogen bonding, are classified into two types depending on the geometries (Figure 1c): symmetrical interactions with the same R−X…X angles (θ₁ = θ₂) are named type I, and vertical interactions with one R−X…X angle of nearly 90° and another angle of nearly 180° are called type II [26,27]. Since the angles of C4−I1…I1^# and I1…I1^#−C4^# are 100.4(1)° and 168.4(1)°, the halogen bonding in 1 can be classified as type II. As a result of the type II halogen bonding, zigzag iodine…iodine chains along the ac-plane were formed, as illustrated in Figure 1d. Although halogen bonding is effective in realizing well-aligned packing structures in some organic semiconducting materials [28], no π…π interactions were detected, and only weak CH/π interaction, where the CH…C₆ ring center distance of 2.846 Å is in the upper region for those in the CH/π interaction [29], was found (Figure S1).

3. Materials and Methods

3.1. General Considerations

All manipulations were performed under an argon atmosphere by using standard Schlenk techniques. ICl was purchased from FUJIFILM Wako Pure Chemical Corporation and used as a stock solution of CH₂Cl₂ (2.0 M). The ¹H NMR spectrum was recorded on a JEOL ECZ-400 spectrometer at 20 °C. Chemical shifts are reported in δ and referenced to SiMe₄ (0 ppm). Multiplicities are abbreviated as doublet (d) and triplet (t). Diffraction data were collected on a Bruker APEX II with Mo Kα radiation (λ = 0.71075 Å) at −100 °C. The structure was solved by dual-space methods using SHELXT [30] and all non-hydrogen atoms were anisotropically refined on F_o² by full-matrix least-square techniques using SHELXL-2019/3 [31]. All hydrogen atoms were placed at the calculated positions with fixed isotropic parameters.

3.2. Synthesis of 5,10-Diiododibenzo[a,e]pentalene 1 from 2a

In a 100 mL two-necked flask, compound 2a (0.9058 g, 1.759 mmol) was dissolved in CH₂Cl₂ (25 mL), and the solution was cooled at 0 °C. ICl in CH₂Cl₂ (2.0 M. 1.85 mL, 3.7 mmol, 2.1 equiv) was added to the flask, and the mixture was stirred for 30 min at 0 °C. After quenching the reaction by adding a saturated Na₂S₂O₃ aqueous solution, the organic layer was extracted using CH₂Cl₂ (15 mL × 2). The collected organic layer was dried over MgSO₄ and filtrated. The removal of the volatile materials in vacuo provided a crude mixture, which was washed with hexane (5 mL × 2) to give analytically pure 1 (0.5119 g, 1.127 mmol, 64%) as an orange-brown powder. Single crystals that were suitable for X-ray diffraction analysis were obtained by slow diffusion of hexane into a CH₂Cl₂ solution. ¹H NMR (500 MHz, CDCl₃, Figure S2): δ = 7.43 (d, ³J_HH = 7 Hz, 2H); 7.08 (t, ³J_HH = 7 Hz, 2H); 7.02 (t, ³J_HH = 7 Hz, 2H); 6.84 (d, ³J_HH = 7 Hz, 2H) [15]. Crystal Data for C₁₆H₈I₂ (1) (M = 454.02 g/mol): orthorhombic; space group Pbca (no. 61); a = 5.580(5) Å; b = 14.838(13) Å; c = 15.956(15) Å; V = 1321(2) ų; Z = 4; T = 173(2) K; μ(MoKα) = 0.71075 mm⁻¹; D_calc = 2.283 g/cm³; 6841 reflections measured (5.1° ≤ 2θ ≤ 52.0°); 1285 unique reflections (R_int = 0.0255), which were used in all calculations. The final R₁ was 0.0297 (I > 2σ(I)), and wR₂ was 0.0655 (all data), with GOF = 1.020.

3.3. Synthesis of 1 from 2b

Compound 1 (0.0834 g, 0.184 mmol, 49%) was synthesized by the above-mentioned procedure using 2b (0.1630 g, 0.378 mmol).