Synthesis and Properties of Methylene-Bridged [6]Cyclo-2,6-naphthylene

Abstract

A novel structurally constrained nanobelt, methylene-bridged [6]cyclo-2,6-naphthylene ([6]MCN), has been synthesized by the nickel-mediated intramolecular aryl–aryl coupling of a triflated prism[6]arene. The structural and electronic properties of [6]MCN were revealed through NMR studies and photophysical measurements. In contrast to methylene-bridged [6]cycloparaphenylene, our previously synthesized benzene-based analogue, [6]MCN has a low strain energy (77.4 kcal·mol^–1) and displays bright fluorescence (Φ = 0.20).

Key words

cycloparaphenylenes - cyclonaphthylenes - molecular nanocarbons - carbon nanobelts - methylene-bridged nanobelts - prism[6]arenes

• References and Notes

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• 14 [6]MCN A 300-mL two-necked round-bottomed flask equipped with a magnetic stirrer bar and filled with N₂ gas was charged with 1 (2.00 g, 1.94 mmol), prepared by the reported procedure,^12b together with CH₂Cl₂ (300 mL), pyridine (2.81 mL, 34.9 mmol), and triflic anhydride (5.08 mL, 31.0 mmol) at 0 °C. The mixture was slowly warmed to rt and stirred for 24 h. H₂O (50 mL) was then slowly added at 0 °C, and the mixture was concentrated to remove CH₂Cl₂. The precipitate was collected by filtration, washed with copious amounts of H₂O, collected, and dried under a high vacuum to afford a purple solid. In an argon-filled glovebox, a second dry 300-mL two-necked round-bottomed flask equipped with a magnetic stirrer bar was charged with 2,2′-bipyridyl (843 mg, 5.40. mmol) and Ni₂(cod)₂ (1.49 g, 5.40 mmol), followed by NMP (150 mL. The resulting mixture was stirred for 40 min at rt until a dark-purple solution of the complex formed. About 3 min later, the product of the triflation reaction (1.18 g, 450 μmol) was added. The resulting mixture was stirred at 80 °C for 18 h then cooled to rt. Toluene (100 mL) and sat. aq NH₄Cl (200 mL) were added, and the organic layer was separated and washed with brine (3 × 300 mL). The organic layer was dried (Na₂SO₄), filtered, and concentrated, and the crude product was purified by precipitation through the gradual addition of an anhyd 1:2 toluene–hexane mixture to a solution of the crude product in anhyd toluene at about 20 °C to give a yellow solid; yield: 7.5 mg (1% over two steps from 1). ¹H NMR (600 MHz, CD₂Cl₂): δ = 7.70 (d, J = 8.9 Hz, 12 H), 7.64 (d, J = 8.9 Hz, 12 H), 4.51 (d, J = 20.6 Hz, 6 H), 3.76 (d, J = 20.6 Hz, 6 H). ¹³C NMR (150 MHz, CD₂Cl₂): δ = 145.73, 140.68,130.51, 124.80, 121.42, 37.72. HRMS (MALDI-TOF): m/z [M⁺] calcd for C₆₆H₃₆: 828.2812; found: 828.2835.