N-doped nonalternant aromatic belt via a six-fold annulative double N-arylation

The design and synthesis of nitrogen (N)-doped molecular nanocarbons are of importance since N-doped nanocarbons have received significant attention in materials science. Herein, we report the synthesis and X-ray crystal structure of a nitrogen-inserted nonalternant aromatic belt. The palladium-catalyzed six-fold annulative double N-arylation provided an aromatic belt bearing six nitrogen atoms in one step from cyclo[6]paraphenylene-Z-ethenylene, the precursor of the (6,6)carbon nanobelt. The C3i-symmetric structure of the aromatic belt in the solid state was revealed using X-ray crystallography. The multistep (electro)chemical oxidation behavior of the belt, which was facilitated by the six p-methoxyaniline moieties, was studied, and a stable dication species was successfully identified by X-ray crystallography. The present study not only shows the unique structure and properties of the N-doped nonalternant aromatic belt but also expands the scope of accessibility of synthetically difficult belt molecules by the conventional intramolecular contraction pathway.


Instrumentation and chemicals
Materials. Unless otherwise noted, all reactions were performed with dry solvents under an atmosphere of nitrogen in dried glassware with standard vacuum-line techniques. Materials were obtained from commercial suppliers and used without further purification. Silver salt Ag[B(C6F5)4] was prepared following the known procedure. 1 Macrocycle precursor 1 was prepared following our previous paper. 2 Dichloromethane (DCM) for reactions was purified by passing through a solvent purification system (Glass Contour). All work-up and purification procedures were carried out with reagentgrade solvents in air. Analytical thin-layer chromatography (TLC) was performed using E. Measurements. Melting points were measured on a MPA100 Optimelt automated melting point system. High-resolution mass spectra (HRMS) were determined on a JEOL JMS-S3000 SpiralTOF (MALDI-TOF MS) using polyethyleneglycol mixture (PEG) as internal standards and trans-2- [3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) as matrix with NaI as cationizing agent. Nuclear magnetic resonance (NMR) spectra were recorded on JEOL spectrometers (JNM-ECA-600: 1 H 600 MHz, 13 C 150 MHz;, JEOL ECA 500II: 1 H 500 MHz, 13 C 125 MHz). Chemical shifts for 1 H NMR are expressed in parts per million (ppm) relative to CHDCl2 (δ 5.32 ppm). 13 C NMR spectra were run using a protondecoupled pulse sequence. Chemical shifts for 13 C NMR are expressed in ppm relative to CD2Cl2 (δ 54.0 ppm). The following abbreviations were used to explain the multiplicities: s (singlet), d (doublet), t (triplet), sept (septuplet), m (multiplet). Coupling constants, J, are reported in Hz and with an accuracy of one unit of the last digit.
For photophysical measurements, dilute solutions in degassed spectral grade dichloromethane in a 1 cm square quartz cell were used. UV-vis absorption spectra were recorded on a Shimadzu UV-3510 spectrometer with a resolution of 0.2. UV-Vis-NIR absorption spectra were recorded on a JASCO UV V-570 spectrometer with a resolution of 2 nm between 250 nm to 2500 nm.
Fluorescence spectra were recorded with a Shimadzu RF6000 spectrofluorometer using a 0.1 S3 nm bandwidth in both excitation and emission. Absolute FL quantum yields were determined with the same instrument equipped with a calibrated integrating sphere. Electron paramagnetic resonance (EPR) spectra were recorded on JEOL ESR JES TE-200 instruments using quartz Schlenk tube filled with Ar.

X-ray crystallography
Details of the crystal data and a summary of the intensity data collection parameters are listed in Table S1. A suitable crystal was mounted with mineral oil on a MiTeGen MicroMounts and transferred to the goniometer of the kappa goniometer of a RIGAKU XtaLAB Synergy-S system with 1.2 kW MicroMax-007HF microfocus rotating anode (Graphite-monochromated Mo Ka radiation (l = 0.71073 Å)) and PILATUS200K hybrid photon-counting detector. Cell parameters were determined and refined, and raw frame data were integrated using CrysAlisPro (Agilent Technologies, 2010). The structures were solved by direct methods with SHELXT 3 and refined by full-matrix least-squares techniques against F 2 (SHELXL-2018/3) 4 by using Olex2 software package. 5 The intensities were corrected for Lorentz and polarization effects.

Titration of the N-belt (2) with oxidant
Dichloromethane solutions of cationic species of N-belt 2 prepared by chemical oxidation with AgSbF6 were used for measurement. The samples were prepared as following the procedure: To the 0.2 mM solution of 2 in DCM was added 2.0 mM AgSbF6 solution in DCM at -30 °C under argon atmosphere. Then, the reaction mixture was warmed gradually to the ambient temperature and stirred for 30 minutes at the same temperature. After stirring, the reaction mixture was diluted to 5.0 × 10 -5 M in DCM and transferred to quartz cell through membrane filter by syringe.

Electrochemical measurements of N-belt 2
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) of N-belt 2 were measured under the following conditions: 0.