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.

Abstract: In this work we present the solution-synthesis of pyridine analoguest oh exa-peri-hexabenzocoronene (HBC)-which might be called superpyridines-via an ovel precursor design.T he keys tep in our strategy was the pre-formationo ft he CÀCb onds between the 3/3' positions of the pyridine and the adjacent phenyl ringsbonds that are otherwise unreactivea nd difficult to close under Scholl-conditions. Apartf rom the synthesis of the parent compound we show that classical pyridine chemistry,n amely oxidation, N-alkylation and metal-coordination is applicable to the p-extended analogue. Furthermore, we present basic physicalc hemical characterizations of the newly synthesized molecules. With this novel synthetic strategy, we hope to unlockt he pyridine chemistryo f nanographenes.
Planar polycyclic aromatic hydrocarbons (PAHs) can be considered as small fragments of graphene. They play av ital role in the worldo fo rganic electronics and future materials. [1] Hence the tunability of these nanographenesis of utmost importance to match properties with desired applications. An effective way to achieve this tuning is the incorporation of heteroatoms (mainly:B ,N ,O ,P ,S )i nto the sp 2 carbon lattice. [2] Arguably the most prominenth eteroatom for this purpose is nitrogen as it fits very well into the benzene substructures of PAHs replacing one CÀHp osition. Up to date there are many smaller nitrogencontaining PAHs known in the literature, [2a] butl arger solutionsynthesized ones, which can be considered as real nanographenes(! 1nm 2 ) [3] are fairly rare. [2] One of the most famous and best studied nanographenes, HBC, also called superbenzeneand its alkylated, soluble derivatives are known for more than half ac entury. [4] Pyridinea nalogues of HBC however,p roved to be difficultt os ynthesise ( Figure 1). [5] Mostd efined, solution-synthesized nanographenes are made from polyphenylene precursors that are planarized via oxidative cyclodehydrogenationo rS choll-type reactions. [1a, 2b, 6] This procedure often fails for electron poor aromatics such as pyridines. [7] Pyrimidine containing HBCs can be made nonetheless [8] but for pyridinic ones this chemistry fails, giving only partially closed products.O nly one example was published by Draper et al. in which af ully closed terpyridine-HBC was synthesized. However this compound was only obtained in trace amounts with partially closeds peciesb eing the major products ( Figure 1). [6] The introduction of nitrogen at any desired positioni nto the framework of nanographenesa nd ultimately carbon allotropes is, however,a na ppealing target. Such compounds have av ariety of potentiala pplications, i.e., the replacement of expensive metalc atalysts [9] for oxygen reductioni nf uel cells, [10] as electrodes in solar cells, [11] or as active compounds for sensing. [12] In this publication,w ed escribe the bottom-up synthesis of pyridine-HBCs as nitrogen-containing nanographenes. This is achieved by an ovel precursor designb ased on the pre-formation of the bonds at the otherwise unreactive3 /3' positions of the pyridine. With the introduction of ap yridinic nitrogen, the properties of these nanographenes are altered. Additionally, the nitrogen gives access to protonation, oxidation,s ubstitution and coordinationc hemistry. N-Substitution and -coordination offer avast variety of possibilitiesa s2,6-unsubstituted pyridinesa llow the interaction and reactione ven with sterically demanding partners( e.g. in our case az inc-porphyrin). The synthetic protocol ( Figure 2) represents ac ombination of an adaptiono fo ur methodf or the synthesis of highly functionalized hexaarylbenzenes (HABs), [13] the pre-formationo ft he CÀC bond to non-activated3 /5-positions of the pyridine, the formation of a" pseudo-HAB" precursor and af inal Scholl oxidation. In af irst attempt, we synthesized pyridine-HBC 10 a with three tert-butyl groups in the backbone. However, the solubility of 10 a was rather low in common organic solvents. Therefore, we decided to attacht wo additional tert-butyl groups to provide highersolubility.This enables easier synthesis, purification, investigation and further modificationo ft he compound.
With the solution processable product 10 b at hand we were able to achieve post-functionalization at the nitrogen atom ( Figure 3). We focusedo nt hree different types of reactions: a) coordination;b )alkylation/cationization;c )oxidation. Methylation at the nitrogen atom yieldedapyridinium ion, which was isolated as its triflate salt. The oxidation of 10 b to its Noxide alters the properties compared to the pyridine itselfa nd the product could potentially serve as anovel p-extended pyridine N-oxide ligand.R egarding the coordinationc hemistry,w e demonstrated the interaction with tetrakis-(4-tert-butylphenyl)zinc-porphyrin. A 1 HNMR (Figure 3) of a1 :1 mixture of the zinc-porphyrin and 10 b in C 6 D 6 was measured showing the complex formation with an impressive shift of the 2/6 pyridine protons from 10.54 ppm to 4.60 ppm as they experience the shielding effect of the aromatic ring current in the porphyrin centre. Other protons of 10 b show this up-fields hifta sw ell, however not that pronounced. In contrast,t he signals of the porphyrin are down-field shiftedd ue to the deshielding effect . Post-functionalizationo fN-HBC 10 b.a)Coordination to the metal-center of azinc-porphyrin to give the corresponding pyridine complex 11:t etrakis(4-tert-butylphenyl)-zinc-porphyrin (1 equiv.), C 6 D 6 ,r .t. b) formationo ft he pyridiniums alts achieved eitherb yprotonation or alkylation (12 as an example for methylation): 1. MeI (excess),C H 3 CN, 2h,r .t. N 2 ,2.A g(OTf)(2.1 equiv.), 15 min, r.t.,N 2 ,94%.c )oxidation to the corresponding pyridine N-oxide 13: mCPBA (1 equiv.), CHCl 3 ,2 4h,08Ct or .t.,9 3%.B ottom right: aromatic signal region of the 1 HNMR spectra(400 MHz, C 6 D 6 ,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 correspondinghydrogena toms are marked with red and blue dots. *CH 2 Cl 2 . Chem. Eur.J.2021, 27,1 984 -1989 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH GmbH at the edge of 10 b (for more details see Supporting Information). This example shows that the extended pyridine derivative 10 b is able to act as ac hromophoric ligand and due to being 2/6 unsubstituted it can even interactw ith sterically quite shielded metal centres like the Zn-atome mbedded in the porphyrin core. Complex formation could not be observed in UV/Vis or fluorescences pectroscopy as there was no significant change of signals in the mixture compared to the individual components. We assume that the complex formation in high dilutions (10 À6 mol L À1 )i snotf avourable and therefore not observed in photophysical experiments. It was possible to follow this assumption by NMR dilutione xperiments,s howing that the complex is favoureda tc oncentrationsg reater % 5 10 À4 mol L À1 (Pyridine 2/6 protons visible at around4 .60 ppm). At lower concentrations( < % 5 10 À4 mol L À1 ), the signal for the respective protons broadens and finally disappears in the baseline. This indicates af ast exchange instead of an equilibrium favouring the complex (for details see Supporting Information). Still this novel complex is the first example of as upramolecular porphyrin-HBC conjugate. Covalently connected porphyrin-nanographene structures became very popular [18,19] as model compounds for porphyrin-graphene hybrids. [20] This novel example could serve as ab enchmark compound in further studies to investigate the non-covalent interactions in such systems.
Large crystalso f12 suitable for x-ray analysis grew from a saturated CHCl 3 solution overnight (Figure 4). The dominant motif in the packing of 12 is the formation of p-p aggregated dimers with the two pyridinium moietieso no pposite sides. To enable close packing,t he two molecules of the dimer are shifted slightly,l ikely increasingd ispersion interactions. Furthermore, the p-extended cores of the molecules bend inwardsr esulting in a p-p distance of 3.36 which is smaller than the interplanar distance for hexa-tert-butyl-HBC with 3.44 but very close to the one in graphite with3.35 . [21] Thisexample shows how polarization in the p-system increases the attractive forces and leads to ac loser aggregation. In al arger cutout of the packing (Figure 4c)t he dimers "on top of each other" are well separated with ad istance around 9 and solvent molecules as wella st he respective counterions in between. The dimers "next to each other" are in proximity witht heir tertbutyl substituents suggesting an aggregation via attractive van der Waals forces. Photophysical data of 10 b, 12, 13 and pentakis tert-butyl HBC 14 as ar eference substance were measured. Reference 14 is perfectly suited for our purpose as it has the same general structure ands ymmetry as 10 b,j ust replacing the pyridinic nitrogen by CÀH. The UV/Vis data (Figure 5a) shows that the spectra of 10 b and 14 are very similar.Am ajor difference is observed only when one takes ac loser look at the usually symmetry-forbidden a-bands [22] at around 420-500 nm. The peaks of oxidized derivative 13 are red shiftedc ompared to 10 b and overall broadened.F or the methylated,c ationic pyridinium salt 12 the UV/Vis absorptionc hanges drastically and the typical HBC fine structure of 10 b, 13 and 14 is lost. Instead an extreme broadening together with ad rop in the extinctionc oefficient in the area between3 40-390 nm is observed while on the other hand as ignificant absorption up to 520 nm is now present. In the fluorescencee mission ( Figure 5b)asimilar trend is observed. 10 b, 13 and 14 possess a fine structure with the individualp eaks at almosti dentical wavelengths but with varying relative intensities compared to each other. 12 shows again ad ifferent behaviour and exhibits just one very broad emission peak between 480 and 700 nm withoutadistinct fine structure.A se xpected, upon protonation with an excess of trifluoroacetic acid (TFA), 10 b behaves very similart o12 as observed in UV/Vis and fluorescence experiments (Figure 5c/d). Now 10 b shows almostt he same broadened and red shifteds pectra as 12.T his wasa lso observed when measuring 13 with an excess of TFA. Here we assume that the negatively charged oxygen of the N-O functionality is protonated leaving the nitrogen positively charged and therefore giving the molecule the pyridinium behaviour as observed before.A se xpected, reference compound 14 does not respondt oa ne xcesso fT FA and maintains its absorption and emissionfeatures.
Finally we observed as olvatochromic behaviour of 12 which is demonstrated here with 3d ifferent solvents (Figure 5e/f) namely toluene, THF and DMSO [solvent polarities:t oluene E T (30) = 33.9 kcal mol À1 ); THF (E T (30) = 37.4 kcal mol À1 ); DMSO (E T (30) = 45.1 kcal mol À1 ] [23] (forabroader range of solvents see the Supporting Information). The most bathochromic absorption is detected at 492.5 nm in toluene, 487.5 nm in THF and 485.5 nm in DMSO, respectively.F or the emission, the solvatochromic behaviour is more pronounced. Here the lowest energye mission is observed fort oluene at 506 nm, for THFa t 511.5 nm and for DMSO at 522 nm, respectively.A dditionally, in toluene the broad emission peak splits up with as houlder at 541 nm. For ac omplete summary of peaks and extinction coefficients see Supporting Information. This solvatochromic behaviour hints to an intramolecular charget ransfer character for 12.
The redox-features characteristics of 10 b, 12, 13 and 14 were determined by CV and DPV measurements. Ther esults are summarized in Ta ble 1. As expected form photophysical characterizations the band gaps for 10 b, 14 and 13 are very . Ha toms were omittedf or clarity.For a) and b) solvent molecules and counterions were omitteda sw ell. Deposition number 2021645 containst he supplementary crystallographic data for this paper.These data are providedf ree of charge by the joint Cambridge Crystallographic Data Centre and Fachinformations-zentrumK arlsruhe Access Structuress ervice www.ccdc.cam.ac.uk/structures similar ( % 3eV). However,e nergiesf or reduction and oxidation are shifted, indicating that 13 is least electron richa nd therefore exhibitst he highest electron affinity (E red1 : À1.66 eV) followed by 10 b (E red1 : À1.81 eV) and 14 (E red1 : À1.97 eV). 12 shows by far the smallest band gap (2.54 eV) and highest electron affinity (lowest E red1 with À1.07 eV) among all compounds. For details see Supporting Information.
To conclude, we achieved the wet chemicals ynthesis of the pyridine analogue of HBC. The pre-formationo ft he CÀCb ond at the 3/5 positiono ft he pyridine to the adjacent phenyl rings was the decisive step here. This precursor design allowed an efficient closure of the remaining bondsi nt he last step by a Scholl oxidation. The synthesis of the final products was easily carried out on ascale > 150 mg with room for further improvement. Classical pyridine chemistry such as protonation, N-alkylation, oxidation and coordination to az inc-porphyrin worked perfectly fine. In the future the chemistry andp hysicochemical properties of these novel compounds will be investigated in more detail.W ea re confident that our pre-formation strategy of CÀCb onds at positions that are unreactivef or oxidative cy-clodehydrogenation represents af urther step towards the ondemand introduction of nitrogen in PAHs and will ultimately lead to ap lethora of novel heteroatom containing nanographenes.  Chem. Eur.J.2021Eur.J. , 27,1984Eur.J. -1989 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH GmbH