Pyridinic Nanographenes by Novel Precursor Design

Abstract

In this work we present the solution-synthesis of pyridine analogues to hexa-peri-hexabenzocoronene (HBC)-which might be called superpyridines-via a novel precursor design. The key step in our strategy was the pre-formation of the C-C bonds between the 3/3' positions of the pyridine and the adjacent phenyl rings-bonds that are otherwise unreactive and difficult to close under Scholl-conditions. Apart from the synthesis of the parent compound we show that classical pyridine chemistry, namely oxidation, N-alkylation and metal-coordination is applicable to the π-extended analogue. Furthermore, we present basic physical chemical characterizations of the newly synthesized molecules. With this novel synthetic strategy, we hope to unlock the pyridine chemistry of nanographenes.

Keywords: Scholl oxidation; heterocycles; nanographene; polycyclic aromatic hydrocarbons; pyridine.

Figure 1

The old and the new. Previous work by Draper et al. for the synthesis of pyridinic HBCs (top) ^[5] compared to our method via a novel precursor design (bottom).

Figure 2

Synthesis of π‐extended pyridines. Green box: Synthesis of “lower‐half” precursors 4 a/b via functionalization of para‐nitroaniline: a) See ref. [13]; b) Isoamyl nitrite (2 equiv.), CHBr₃, 20 min, 80 °C, N₂, 47 %; c) 4 a: 1. nBuLi (2.5 m in hexanes, 1.2 equiv.), THF, 1.5 h, −72 °C, N₂, 2. B(OEt)₃ (1.5 equiv.) at −72 °C, 24 h, −72 °C to r.t., N₂, 50 %; 4 b: KOAc (3 equiv.), bis(pinacolato)diboron (1.1 equiv.), 4.5 mol % Pd(dppf)Cl₂xCH₂Cl₂, 1,4‐dioxane, 20 h, 80 °C, N₂, 70 %; Orange box: Synthesis of “top‐half” precursors 8 a/b with pre‐formation of the crucial 3/5 positions (highlighted in red): d) See literature [15]; e) 7 a: Phenylboronic acid (2.5 equiv.), Na₂CO₃ (8 equiv.), 8 mol % Pd(PPh₃)₃, toluene:EtOH:H₂O (10:2:3), 24 h, reflux, N₂, 98 %; 7 b: (3‐tert‐butylphenyl)boronic acid (2.5 equiv.), Na₂CO₃ (8 equiv.), 8 mol % Pd(PPh₃)₃, toluene:EtOH:H₂O (10:2:3), 16 h, reflux, N₂, 91 %; f) 8 a: CuBr₂ (1.2 equiv.), isoamyl nitrite (6 equiv.), CH₃CN, 24 h, 65 °C, N₂, 86 %; 8 b: CuBr₂ (3 equiv.), isoamyl nitrite (9 equiv.), CH₃CN, 5 h, 65 °C, 95 %; Blue box: Synthesis of the pseudo‐HAB precursors 9 a/b: g) 9 a: 8 a (1 equiv.)+4 a (1.1 equiv.), Cs₂CO₃ (2 equiv.), 10 mol % Pd(PPh₃)₄, THF:H₂O (4:1), 17 h, 80 °C, N₂, 80 %; 9 b: Method 1: 8 b (1 equiv.)+4 a (1.2 equiv.), Cs₂CO₃ (2 equiv.), 10 mol % Pd(PPh₃)₄, THF:H₂O (4:1), 24 h, 80 °C, N₂, 95 %; Method 2: 8 b (1 equiv.)+4 b (1.2 equiv.), Cs₂CO₃ (2 equiv.), 10 mol % Pd(PPh₃)₄, THF:H₂O (4:1), 24 h, 80 °C, N₂, 79 %; Red box: Synthesis of the final pyridine‐HBCs 10 a/b: h) 10 a: DDQ (7 equiv.), triflic acid (14 equiv.), CH₂Cl₂, 1 h, 0 °C, N₂, 81 %, 10 b: DDQ (7 equiv.), triflic acid (14 equiv.), CH₂Cl₂, 4 h, −50 °C to −20 °C, N₂, 83 %.

Figure 3

Post‐functionalization of N‐HBC 10 b. a) Coordination to the metal‐center of a zinc‐porphyrin to give the corresponding pyridine complex 11: tetrakis(4‐tert‐butylphenyl)‐zinc‐porphyrin (1 equiv.), C₆D₆, r.t. b) formation of the pyridinium salts achieved either by protonation or alkylation (12 as an example for methylation): 1. MeI (excess), CH₃CN, 2 h, r.t. N₂, 2. Ag(OTf) (2.1 equiv.), 15 min, r.t., N₂, 94 %. c) oxidation to the corresponding pyridine N‐oxide 13: mCPBA (1 equiv.), CHCl₃, 24 h, 0 °C to r.t., 93 %. Bottom right: aromatic signal region of the ¹H NMR spectra (400 MHz, C₆D₆, r.t.) of the zinc‐porphyrin (blue), 10 b (green) and zinc‐porphyrin‐pyridine complex 11 (dark red). The two most significant shifts and the corresponding hydrogen atoms are marked with red and blue dots. *CH₂Cl₂.

Figure 4

X‐ray structure analysis of 12. a) top view of a dimer of 12; b) side view of the dimer showing the π‐planes bending towards each other demonstrating the strong π–π interaction; c) larger cut‐out of the packing showing the dimers separated from each other with the space in between filled with solvent molecules (CHCl₃) and counterions (CF₃SO₃ ⁻). H atoms were omitted for clarity. For a) and b) solvent molecules and counterions were omitted as well. Deposition number 2021645 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures

Figure 5

a) Quantitative UV/Vis spectra of 10 b, 13, 14 (all in toluene) and 12 (in MeOH). The insert shows an enlargement of the α‐bands; b) fluorescence spectra of 10 b (λ_ex=360 nm), 13 (λ_ex=366 nm), 14 (λ_ex=360 nm) (all in toluene) and 12 (λ_ex=228 nm) (in MeOH); c) qualitative UV/Vis spectra of 10 b, 12 and 14 (all in toluene) with an excess of TFA; d) fluorescence spectra of 10 b (λ_ex=342 nm), 13 (λ_ex=380 nm) and 14 (λ_ex=360 nm) (all in toluene) with an excess of TFA; e) qualitative UV/Vis spectra of 12 in solvents of different polarity. The insert shows an enlargement of the most red‐shifted bands; f) fluorescence spectra of 12 in solvents of different polarity: toluene (λ_ex=343 nm), THF (λ_ex=340 nm), DMSO (λ_ex=341 nm). *Artifacts of 2nd order scattering at the double excitation wavelength.