Global Aromaticity and Antiaromaticity in Porphyrin Nanoring Anions†

Abstract Doping, through oxidation or reduction, is often used to modify the properties of π‐conjugated oligomers. In most cases, the resulting charge distribution is difficult to determine. If the oligomer is cyclic and doping establishes global aromaticity or antiaromaticity, then it is certain that the charge is fully delocalized over the entire perimeter of the ring. Herein we show that reduction of a six‐porphyrin nanoring using decamethylcobaltocene results in global aromaticity (in the 6− state; [90 π]) and antiaromaticity (in the 4− state; [88 π]), consistent with the Hückel rules. Aromaticity is assigned by NMR spectroscopy and density‐functional theory calculations.

Aromaticity is one of the oldest concepts in chemistry,and it is widely used to predict the properties of unsaturated cyclic molecules. [1] In essence,aromaticity describes the anomalous stability of certain molecules,a nd their peculiarly large anisotropic diamagnetic susceptibilities. [2] This unusual diamagnetism results from the tendency of aromatic molecules to sustain aring current when placed in amagnetic field. This effect is easily detected by NMR spectroscopy,b ut aromatic ring currents were first postulated by Pauling and Lonsdale before NMR spectroscopy had been invented. [3] While the earliest aromatic molecules were flat symmetric carbocyclic p-systems,t he concept has been extended to s-bonded systems,m etal clusters,e xcited states,t wisted molecules, and other systems. [4][5][6] Hückelsc alculations predict that an annulene (C n H n ) with ac onjugated circuit of [4n + 2] p-electrons will be aromatic. [7] Breslow extended this rule to state that annulenes with [4n] p-electrons are antiaromatic,with reduced stability and the opposite ring currents (paratropic rather than diatropic). [8] Although Hückelso riginal calculations were restricted to all-carbon, monocyclica nnulenes,t he rules also apply to aw ide range of heterocyclic and polycyclic molecules.H owever,t here are many macrocyclic p-conjugated rings with [4n + 2] or [4n] p-electrons that do not display global aromatic or antiaromatic ring currents.F or example, cycloparaphenylenes ([n]CPPs,F igure 1) have [4n] p-electrons in their circumferential conjugation path, yet are nonaromatic in their electronically neutral ground states. [9] Similarly,b utadiyne-linked porphyrin nanorings,s uch as c-P6·T6,d on ot exhibit global (anti)aromaticity in their neutral states. [10][11][12] These macrocycles consist of aromatic subunits,and the local aromaticity of the subunit dominates in the neutral ground states.( Anti)aromaticity can be induced by perturbing the electronic structure,f or example,b y oxidation or excitation. Ther emoval of two electrons from [8]CPP leads to global aromaticity (30 p-electrons). [9] Removal of 4or6electrons from the six-porphyrin nanoring c-P6·T6 leads to macrocyclic global antiaromaticity (80 pelectrons) or aromaticity (78 p-electrons), respectively. [13] Electronic excitation can lead to excited-state aromaticity, [4,14] and molecules such as c-P6 are aromatic in their lowest excited states. [15] Many other macrocycles with global aromaticity in the neutral, cationic,a nd excited states have been reported in the last decade. [16][17][18][19][20][21] Porphyrin oligomers have an exceptionally wide range of accessible oxidation states;f or example, c-P6·T6 can be reversibly oxidized up to the 12 + state and reduced to the 6À state. [13,22] Herein we demonstrate that global ring currents can also be established by reduction. We show that the tetraanion is antiaromatic (88 p-electrons), and the hexaanion is aromatic (90 p-electrons). Thec hemical reduction of annulenes was widely explored during the 1980s, [23] but there are few recent examples of macrocycles that become aromatic in anionic reduced states, [19] and it was not clear whether this strategy could be used in complex heterocyclic macrocycles,such as porphyrin nanorings.
We began this study by using computational techniques to predict the (anti)aromaticity in reduced states of c-P6·T6.The nucleus-independent chemical shift (NICS), [24] which evaluates the chemical shift at apoint in space,was calculated using density functional theory (DFT,B 3LYP/6-31 + G*) for the template-free c-P6.T he diatropic ring currents of aromatic molecules generate an induced field that opposes the external magnetic field inside the ring, thus leading to as hielding effect and lower chemical shifts.Outside the ring, the induced field reinforces the applied field, leading to NMR deshielding. Thep aratropic ring currents of antiaromatic molecules have the opposite NMR effects.Therefore,anegative NICS inside the nanoring indicates aromaticity,a nd ap ositive NICS indicates antiaromaticity.
Ther esults of NICS calculations ( Figure 2a nd Table 1) are consistent with predictions from the Hückel rule:the 4À state is antiaromatic (NICS(0) iso = 89 ppm), and the 6À state is aromatic (NICS(0) iso = À13 ppm). These NICS values are similar to those calculated previously for the 4 + and 6 + oxidation states. [13] Thep rediction of aromaticity in the 6À state is consistent across other functionals (M06-2X and wB97XD;s ee Table S1 in the Supporting Information). The 4À state undergoes symmetry breaking from D 6h to approximate C 2v symmetry,r eflecting the pseudo-Jahn-Teller distortion common to antiaromatic molecules.T he functionals M06-2X and wB97XD predict greater ellipticity than B3LYP (see Table S2), and corresponding lower NICS values (see Table S1). Calculations of the anisotropy of the induced current density (ACID) confirm the conclusions from the NICS calculations (see Figure S3). [25] Square-wave voltammetry of c-P6·T6 revealed six reversible reductions in the window À1.30 to À1.80 V( vs.F c/ Fc + ). [22] There are few reducing agents available in this potential range. [26] Alkali metals are strong reducing agents, but it is difficult to titrate them to avoid over-o ru nderreduction. Therien and co-workers used decamethylcobaltocene (CoCp* 2 )t op repare radical anions of linear porphyrin oligomers. [27] CoCp* 2 is ac onvenient reducing agent because it is sufficiently soluble in THF,a nd it has an oxidation potential of À1.84 Vvs. Fc/Fc + , [26] making it strong enough to access the 6À oxidation state of c-P6·T6.
Addition of excess CoCp* 2 to as olution of c-P6·T6 in [D 8 ]THF resulted in ar ed-brown solution with af airly well resolved 1 HNMR spectrum corresponding to c-P6·T6 6À . Addition of [D 5 ]pyridine (5 %byvolume of solvent) resulted in as lightly sharper spectrum (Figure 3c). Reduction was reversible and the addition of ferrocenium hexafluorophosphate cleanly regenerated c-P6·T6 (see Figure S7). Addition of 4equivalents of CoCp* 2 to neutral c-P6·T6 gave the 4À state,c haracterized by ab road chemical-shift dispersion (signals up to 18 ppm, Figure 3b). Before assigning the 1 HNMR spectra of c-P6·T6 4À and c-P6·T6 6À ,i ti sh elpful to recapitulate the assignment of neutral c-P6·T6 (Figure 3a). Theprotons of the solubilizing trihexylsilyl groups (THS i and THS o for those pointing inside and outside the ring, respectively) give rise to ab road overlapping set of peaks around 1ppm. Thep orphyrin b-pyrrole hydrogen atoms (labeled aa nd bi nF igure 3) resonate at d = 9.6 and 8.   Table S1 and Figures S1 and S2). [a] Data from ref. [13].

Angewandte Chemie
Communications spectrum of template-free c-P6 6À (see Figure S8). Ther esonances of the meso-aryl protons (o i , o o ,and p)are each related by a 4 J scalar coupling, and so exhibit acharacteristic coupling pattern in the 1 HT OCSY spectrum (see Figure S9). NOE correlations between the template protons and o i permit the assignment of the latter, and further NOE correlations to the THS manifold centered around À0.5 ppm confirm its assignment as THS i (see Figure S11). THS o and o o are related by NOE correlations,and p can then be assigned by elimination. Although the THS i resonance is shielded by the global aromatic ring current, the THS o resonance is at the same chemical shift as for the neutral ring because these protons sit at the border between the shielding and deshielding regions of the chemical-shift-anisotropy cone (see Figure S2). The chemical-shift difference between resonances on the inside and outside of the nanoring, Dd,g ives am easure of global (anti)aromaticity.F or c-P6·T6 6À , Dd o = À3.35 ppm, and Dd THS,CH3 = À1.18 ppm. These values are approximately 1.75 times higher than those for the aromatic hexacation, c-P6·T6 6+ , [13] thus suggesting that the hexaanion has astronger global diatropic ring current than the hexacation.
Thes pectrum of the 4À oxidation state is much broader than that of the 6À state,which made it impossible to record useful 2D 1 HTOCSY and 1 H-13 CHSQC spectra of c-P6·T6 4À , but NOESY spectra provided valuable correlations.T he THS o resonance is readily assigned, since it is unperturbed by global (anti)aromaticity.I tcan then be seen that the THS i resonance is deshielded (appearing at ca. 3-4 ppm), consistent with global antiaromaticity.NOEs between THS i and o i , and between THS o and o o ,p ermit the assignment of the signals for the ortho hydrogen atoms (see Figure S17). Both THS resonances also have equal-intensity NOEs to the p resonance.T he a-d template resonances at 13-17 ppm were identified by comparing the spectra of c-P6·T6 4À and template-free c-P6 4À (Figure 3b;s ee also Figure S16);t he extreme deshielding of these protons supports the assignment of antiaromaticity.
Theenergy barrier for rotating one porphyrin subunit by 1808 8 in the template-free c-P6 nanoring (swapping THS i and THS o , o i and o o )i sv ery sensitive to the oxidation state.T he transition state for this process must break the p-conjugation, so the barrier provides ap robe for the enhanced resonance energy in charged species.I nn eutral c-P6,r otation of porphyrin units is fast at room temperature.U pon cooling, the o i /o o resonances separate and enter the slow exchange regime,w ith ac oalescence temperature T c of 200-203 K, corresponding to ab arrier of (38.0 AE 0.5) kJ mol À1 (see Figure S22). EXSY experiments on c-P6 6À at 228 Kr evealed arotation barrier of (50.2 AE 0.5) kJ mol À1 (see Figure S25 and Table S4), which is similar to that previously measured for c-P6 6+ ((49.5 AE 0.4) kJ mol À1 at 213 K).
In conclusion, we have shown that [90 p]a romatic and [88 p]a ntiaromatic anions can be generated by chemical reduction of as ix-porphyrin nanoring of diameter 2.5 nm using decamethylcobaltocene.T his approach to establishing global (anti)aromaticity is probably applicable to many other redox-active p-conjugated macrocycles and provides auseful complement to chemical oxidation. Since larger aromaticring-current effects are observed by NMR spectroscopy for c-P6·T6 6À than for c-P6·T6 6+ ,reduction may prove more useful than oxidation for establishing global aromaticity in even larger macrocycles.Aromatic macrocycles with diameters on the order of 10-20 nm are expected to show mesoscale magnetic properties,s uch as an onlinear dependence of the induced ring current on the applied magnetic field, similar to that observed in mesoscopic metal rings. [28] Cartesian coordinates for computational structures are available in the Supporting Information and from https://doi. org/10.6084/m9.figshare.9405308.  Figure S6 of the SupportingI nformation. We did not observe any field dependence in the chemical shifts other than slight shifts associated with chemicale xchange processes.)