π-extended [12]cycloparaphenylenes: from a hexaphenylbenzene cyclohexamer to its unexpected C 2-symmetric congener

Based on a π-extended [12]CPP, two different precursors for the bottom-up synthesis of CNTs were synthesized. The congested hexaphenylbenzene mode of connectivity of the two macrocycles reveals an improved oxidative cyclodehydrogenation over previous reported strategies.

The crude product was extracted with DCM (3x) and the combined fractions were dried over  8 (4 mg, 1.1 µmol, 1 eq.) were dissolved in THF (2 mL) and cooled to -78 °C. Sodium napthalide (0.06 mL, 55.0 µmol, 50 eq.) in THF (1 M) was slowly added. Upon addition, the reaction color turned blue. The reaction solution was stirred at -78 °C for 1 h and MeOH S12 (0.5 mL) was added. The colorless reaction solution was warmed to r.t.. After addition of water, the crude product was extracted with DCM (3x) and the combined fraction were dried over Na 2 SO 4 . The solvent was removed under vacuo and the product was purified by preparative GPC (THF). 3 (1.8 mg, 0.53 µmol, 48%) was obtained as a white solid.

X-Ray Crystallographic Analysis
Details of the crystal data and a summary of the intensity data collection parameters for 5 and 3 are listed in Tables S1 and S2. In each case, suitable crystals were measured with STOE IPDS 2T diffractometer. Graphite-monochromated Mo Kα radiation was used. The structures were solved by direct methods with SIR-97 and refined by the full-matrix least-squares techniques against F2 (SHELXL-97). The intensities were corrected for Lorentz and polarization effects. The nonhydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. The crystals structures were visualized using Mercury 3.3.
3.1 X-ray crystal structure of 5 Figure S1: X-ray crystal structure of 5.   Figure S5: NOESY-spectrum of 2 recorded at 298 K in C 2 D 2 Cl 4 . Intense through-space-coupling between the protons of the bridging phenylene with its neighboring 4-tBuPh and phenyl ring highlighted in black frames.

1 H-H 1 -COSY
The two colored rectangles belong to a split up A-B spin-system of compound 8 (see Figure S8): by integration and multiplicity (see Figure S8), these protons are attached to the phenylene ring, which bridges the cyclohexadiene and the tetrakis(4-tbutylphenyl)benzene moieties. However, instead of the expected two signals, the A-B spin-system is split up into a spin-system with four different signals. This splitting is due to the interlocking of the phenylene ring which renders all four protons magnetically different, as two of those point inwards, whereas the other point outwards of the macrocycle and thus experience magnetically different surroundings.
In this spectrum, only the 4 J-couplings are highlighted by two colored rectangles, whereas the intense 3 J couplings are not visualized. These weak signals denote couplings between the two lightly labeled protons and the couplings between the darkly labeled protons. 6

NOESY
The coupling of the backwards pointing methoxy groups with the light blue labeled protons in the aromatic region of compound 8 shows a higher coupling intensity than the more remotely lying light green labeled proton (see Figure S9). This can be explained by the distant-dependent signal intensity inherent for NOE measurement, as the nuclear Overhauser effect (NOE) scales with 1/r 6 , with r being the distance between the interacting nuclei. In addition, a slight axial twisting of the phenylene ring is also necessary to give rise to through-space coupling, since the nuclei of the proton is otherwise hidden behind it. The question arises for why one does not observe the other two protons in this NOESY-spectrum. This can rationalized by two arguments: first, the kinked 1,4-dimethoxycyclohexa-2,5-diene moiety has the phenylene point away from the methoxy group (see Scheme S1). Therefore, the lightly colored protons are on the "visual axis" in between the methoxy group and the dark colored protons. Second, the distance between the protons is larger, rendering the signal intensity smaller. Scheme S1: 3D-representation of macrocycle 8 based on the 2D-NMR spectra discussed above. The expected angle of around. 90 ° between the phenylenes the cyclohexadiene unit is labeled with red semicircles.

Computational Results
The geometry optimization of all structures reported in Scheme S2 has been performed using

Geometrical analysis
Here, the opening of the nanoring of the macrocycle is considered with the aim to understand whether the equilibrium structure of 10c is rather close to compound 3 or 9. By going from 3 to 10c, there is a nanoring opening of about 4 Å (see Figure S10). In contrast, the nanoring opening of the neutral structure 9 is 13 Å. Hence, we can conclude that the equilibrium structure of the tetraanionic intermediate 10c is shifted toward 3 in comparison to 9, due to its oval shape. To support this result, a dianionic structure 11 was analyzed and it was found that the doubly negatively charged macrocycle has a similar shape to 3, but with a smaller opening of the nanoring due to constrains of the two remaining hydrogen atoms bounded to the sp 3 -carbons. The bond length analysis of the structures 3 and 10c shows two peculiar situations (see Figure  S11): when the negatively charged structures 10c is considered, the bond length is close to the aromatic bond length, in between 1.4-1.45Å (see Figure S11, top view left, black double arrows); on the other hand, for the neutral structure 3, the bond length differs significantly. In fact, the bonds between the sp 3 -carbon and the neighboring atoms is 1.53 Å, while the distance between the two sp 2 -carbon atoms of the ring is 1.35 Å (see Figure S11, top view right, green double arrows). These results are in agreement with the bond angle analysis of the cyclohexadiene moieties. In fact, for the charged structures, the angles are close to the aromatic ring values, in S24 between 116-120 degrees, while for neutral structures the angles decrease to 110-113 degrees, close to a sp 3 hybridization of the carbon atoms (see Figure S11, top view, red dotted curves). The torsional angles analysis enforces this conclusion; in fact, the value of the out-of-plane bending of the phenyl substituents, bonded with the cyclohexadiene moiety, of ~38 degrees increases to ~65 degrees when passing to the neutral structure (see Figure S11, side view, black curves). Hence, we can conclude that the tetraaionic structure 10c is a resonant form in between 3 and 9, with a strong aromatic character of the anionic carbons that are sp 2 hybridized. Figure S11: geometrical analysis of structures 3 (sp 2 ) and 10c(sp 3 , tetraanion). Black double arrows refer to the double bond length, while green double arrows refer to the single bonds length. Red dotted curves refer to the bond angles of the cyclohexadiene moieties, while black lines refer to the out-of-plane torsion angles. S25

Frontier Orbital Analysis
The frontier orbitals of the neutral structures 8a and 3 are depicted in Figure S12. Figure S12: frontier orbital of 8a (above) and 3 (below)

S26
The HOMO of the neutral structures 8a and 3 is localized over the ring bridging the sp 3 -carbon atom, while the LUMO is localized over the sp 2 part of the macro ring. Due to the high symmetry of the structures, the last three occupied orbitals (namely HOMO-2, HOMO-1 and HOMO) are quasi-degenerate, as well as the first three virtual orbitals (LUMO, LUMO+1 and LUMO+2). Interestingly, for the neutral structure 9 (see Figure S13) both the HOMO and the LUMO are localized over the aromatized ring (previously sp 3 in 8a; see upper image Figure S12). When the tetraanionic species 10c is considered, the HOMO is localized over two aromatic rings, bridging the charge, while the LUMO is delocalized over the remaining part of the macro ring (see Figure S13). As for structures 3 and 8a, the strong symmetry of the system leads to a quasidegeneracy of the orbitals. When an intermediate case is considered, in which two negative charges are removed (structure 11), the HOMO is localized over the charged part of the molecule, while the LUMO is mainly localized over the cyclohexadiene moiety (see Figure S14). Figure S15: frontier orbital of dianionic macrocycle 3b.

Electrostatic Potential (ESP) Analysis
To gain insight into the formation of structure 3 from structure 8 when a mild reductive aromatization was performed, we need to analyze the distribution of charges of our molecules. The electrostatic potential (ESP) analysis is therefore performed to assess the charge migration proposed in the mechanism. To analyze the charge distribution, we consider the charge on different subunits of the structure; in particular, the charge over tetraphenyl-1,4-phenylene moieties as one group and the phenylene ring linking them as a second group (see Figure S16, top). Interestingly, we did observe a strong localization of the charge (~2e) over the cyclohexadiene moieties (see Figure S16, red circles) while the remaining charge (~2e) is delocalized over the substituted aromatic phenyl rings (see Figure S16, blue circles).

Energy analysis
The total energy of compound 8a and of the two isomers 9 and 3 are reported in the following table. Since 8a has two hydrogen atoms more than 9 and 3, we added the energy of H2 to the total energy of 9 and 3 to be comparable to 8a. The energy difference (in kcal/mol) is also reported.

Spectrum of 2 after oxidative cyclodehyrogenation
Oxidative cyclodehydrogenation of 2 for 1 d yielded a product mixture. Thus, the crude product was separated by preparative TLC to investigate the product distribution, in detail. In Figure S26, the mass spectrum of compound 2 after dehydrogenation is shown. The separation afforded various products ranging from 3389 m/z, as the most unpolar product, to 3408 m/z, as the most polar product. The lightest fraction was subjected to FeCl 3 again. No further decrease in mass could be observed. Room temperature and high temperature approaches for cyclodehydrogenation with other oxidizing agents, such as DDQ, Sc(OTf) 3 and/or TfOH, led to S92 decomposition of the starting material and gave product mixtures that did not yield masses in the expected product range.

C 2 -symmetric Congener
The C 2 -symmetric compound 3 was oxidatively cyclodehydrogenated with FeCl 3 . As can be seen from the crystal structure, the two pentaphenylenes that are bridged by two cyclohexa-2,5-dienes are only slightly bent. Thus, compound 3 is expected to undergo oxidative cyclodehydrogenation. In addition, cyclohexadienes are known to form phenylenes in the presence of oxidizing agents. [ Scheme S3: Oxidative cyclodehydrogenation of 108 with FeCl 3 The C 2 -symmetric compound 3 was subjected to FeCl 3 . The reaction progress was monitored by mass spectrometry. Spectra were recorded after 1 d and 3 d. In Figure S27, the mass spectrum of

S93
To understand the observed masses, first, a closer look is taken at the intermediate mass of 3355 m/z. The lower masses will be discussed in the following spectra (vide infra). Figure S28: section of the mass spectrum of compound 3 after cyclodehydrogenation for 1 d with FeCl 3 at r.t..
In Figure S28, a small section of the mass spectrum between 3350 m/z and 3360 m/z is shown. The sum formulas for two interemediates (red, left, 13; green, right, 11) are shown. These structures can be proposed based on the structure of the starting material, as the crystal structure hinted at a slightly bent pentaphenylene that should smoothly undergo cyclodehydrogenation to give a short nanoribbon segment. The sum formula of 11 the theoretical mass (green) deviates by 0.01 g/mol; such an error range has been observed for molecules with similar molecular weight, of which the structure had been confirmed by X-ray crystallography before MS measurements. Therefore, the experimental findings may very well be in agreement with the outlined sum formula. One can conclude that the above given sum formula matches with the experimental findings. After 3 d, the spectrum shows an increase in intensity for the signals at 3318 m/z and 3396 m/z, whereas the peaks around 3353 m/z, assigned to the cyclodehydrogenated dicyclohexadienebridged bisribbon 11, decreased in intensity and were additionally shifted by 2 m/z to lower masses; the peaks at 3370 m/z disappeared. In the range from 3340 m/z to 3260 m/z five different peaks can be observed. The mass difference between each of them equals 22 m/z. These findings are a first indicator that a side reaction has occurred, namely a chloro-de-t-butylation, as the mass difference between a chlorine atom and a t-butyl group equals 22.1 m/z. With this having said, the peaks around 3318 m/z were more closely investigated (see Figure S30), as this mass could have also hinted at a bottom-up synthesis of an ultrashort CNT. In Figure S30, sections of two mass spectra of 3 after cyclodehydrogenation for 3d at r.t. are shown. In the left spectrum, the hypothetical structure of an almost fully fused CNT is depicted. The calculated mass spectrum of this structure is shown in green. The difference between the observed and calculated mass is 1 m/z; the deviation is thus beyond the error limit. From these and the above described findings, one can conclude that during oxidative cyclodehyrogeation CNT 14 has not been formed. The mass spectrum for the simulated structure of 15 hints strongly at the formation of a bis(de-tbutylated) and concomittantly dichlorinated compound, as the mass and the corresponding isotopic pattern are in good agreement. One has to state, however, that only the sum formula can be concluded and a structural hypothesis, as shown here, is entirely based on chemical intuition.

UV-Vis spectra and Emission Spectrum
In this section, UV-Vis spectra and emission spectra are shown. For compound 2 and 3, the absorption spectra are depicted. An emission spectrum can only be shown for 2; for its C 2symmetric congener 3, no emission was observed. Figure S37: absorption spectra of the congested cyclohexamer (2, black) and its C 2 -symmetric congener 3 (red) are shown. The emission spectrum of 2 is depicted; no emission was observed for 3.
The absorption maxima at 258 nm and 263 nm for 2 and 3, respectively, are in the same range as the values reported for polyphenylene cylinders, consisting of a [9]CPP or a [15]CPP ring (see Angew. Chem. Int. Ed. 2014, 53, 1525-1528). The ring size and, correspondingly, the ring strain merely influence the absorption maxima. These results are in line with size-dependent measurements, which show very small differences for the absorption maxima. [5] For "naked" CPP rings, however, the absorption maxima are bathochromically shifted (ca. 350 nm), due to higher degrees of conjugation. The emission maximum of 2 is observed at 426 nm. This value is hypsochromically shifted in comparison to the [9]CPP based polyphenylene cylinder reported in Angew. Chem. Int. Ed. 2014, S104 53, 1525-1528; in comparison to its higher homologue -the [15]CPP based polyphenylene cylinder -its value is bathchromically shifted ([12]CPP has its emission maxium at 450 nm) This observation is in line with the general trend observed for CPPs and the explanation, that upon excitation the large ring strain and the distortion of these CPP scaffolds is released by partial planarization of neighboring phenyl rings, which results in large Stokes shifts.

HOMO-LUMO energies
CV measurements of both compounds were not successful, since the materials degraded during CV measurement. Due to the apparent molecular properties, we could not even record an irreversible oxidation/reduction potential. This holds also true for the previously reported In Figure S38, the HOMO/LUMO energies of compound 2 and 3 are depicted. The values of the HOMO energies have similar values as for [12]CPP (-5.26 eV). [6] However, the LUMO energies differ by 0.6 eV, with a higher lying LUMO of 2 and 3. This may be attributed the increased ring strain, arising from twisted neighboring phenylene units (due to the high sterical demand exerted by phenyl substituents). The calculated energy gaps between the HOMO and LUMO level energies of [n]CPPs are in the range of 2.5 eV (n = 4) to 3.7 eV (n = 20). For [12]CPP, the energy gap is 3.6 eV and thus significantly smaller than for 2 and 3 (4.6 eV and 4.5 eV, respectively). This can be rationalized by the computed structure of 2 and the X-ray crystal analysis of 3. Both show that neighboring phenylene units align in an almost perpendicular manner. Hence, the degree of conjugation within these macrocycles is reduced in contrast to its parent compound [12]CPP. In addition, the high sterical demand of the neighboring groups may also explain, why the emission maximum of 2 is hypsochromically shifted in comparison to [12]CPP.