The Benzenium Ion â Aromatic as the Ï-Complex or Antiaromatic as

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

The Benzenium Ion - Aromatic as the #-Complex or Antiaromatic as the #-Complex Being Something Similar to the Cyclopentadienyl Cation? Erich Kleinpeter, and Andreas Koch J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b03121 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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The Journal of Physical Chemistry

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The Benzenium Ion − Aromatic as the π-Complex or Antiaromatic as the σ-Complex Being Something Similar to the Cyclopentadienyl Cation? Erich Kleinpeter* and Andreas Koch Universität Potsdam, Institut für Chemie, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam (Golm), Germany Supporting Information

ABSTRACT: The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of the benzenium cation C6H7+ 1 and of +/−I/M substituted relatives C6H6X+ 3−8 [X = −Me, −CF3, −NH2, −NO2, −NO, −SiH3] have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as isochemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to compare the spatial magnetic properties (TSNMRS) of benzene and the benzenium ion 1, and, when furthermore compared with the relatives 3−8, to answer the question if the electronic structure of 1 and 3−8 is still the one of aromatic species or something similar to the antiaromatic cyclopentadienyl cation 2, supported by structural data and δ(13C)/ppm values. ___________________________________________________________________________

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1. INTRODUCTION With the publication of the X-ray structure of [C6H7]+[Al2Br7]− in 20141 ended temporarily the long term discussion about the structure of the benzenium ion 1 (also called protonated benzene, σ- or π-complex, Wheland- or Meisenheimer intermediate in electrophilic aromatic substitutions) which proved to be the σ-complex. Before, with the same result, both 1H and

13C

NMR spectra in the solvent SbF5−FSO3H−SO2ClF−SO2F2 at low

temperature,2−5 IR spectra6,7 and a number of theoretical studies8−10 were examined. Actually, because of two hydrogen atoms at one carbon, the σ-complex of 1 could formally no longer be an aromatic species; on the other hand, it is planar, relatively stable obviously due to electron delocalization and could be isolated.1−7 Thus, the question arises: Is the electronic structure of 1 still aromatic (1a) or antiaromatic (1b) by something similar to the cyclopentadienyl cation 2 (cf. Scheme 1)? ______________________________________________________________________________ Scheme 1. Possible mesomeric contributors of benzenium cation 1 and structure of cyclopentadienyl cation 2

H H

1a

H H

H H

1b

2

_______________________________________________________________________________________________

We employed our through-space NMR shielding concept (TSNMRS)11−13 to qualify and quantify the spatial magnetic properties (actually, the widely used concept of anisotropy/ring current effect in 1H NMR spectroscopy) of the studied conjugated species; along this concept,11−13 the TSNMRS are calculated as NICS values14,15 for a grid of ghost atoms surrounding the molecules in order to locate diatropic and paratropic regions around the molecules. The TSNMRS values are visualized as iso-chemical-shielding surfaces (ICSS) of resulting NICS and were already successfully employed to visualize and quantify the anisotropic effects of functional groups and the ring current effect of aromatic species and, hereby, to indicate present (anti)aromaticity.11−13 While normally employed specifications from magnetic point of view to quantify (anti)aromaticity are non-measurable theoretical items16 (single NICS values or components in the latter, or traces of NICS or components of the latter starting from the centre of the (anti)aromatic compound up to 10 Å outwards), experimental Δδ/ppm in proton NMR spectra are the molecular response property of our ACS Paragon Plus Environment

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TSNMRS values.17 For this reason, the spatial magnetic properties (TSNMRS values) of studied molecules represent their electronic structures and should clear up also the position of present mesomerism (cf. Scheme 1). This is the main object of this paper. In order to compare the spatial magnetic properties of the benzenium cation 1 with both benzene and cyclopentadienyl cation 2, and to clear up presence of the mesomeric contributors 1a and/or 1b, some additional arenium cations 3−8 were studied as well (cf. Scheme 2); hereby, the influence of +I (−Me, −SiEt3), −I (−CF3), +M (−NH2) and −M (−N=O, −NO2) substituents on both structure and spatial magnetic properties of the benzenium cation 1 was studied. __________________________________________________________________________________________ Scheme 2. Structure of studied arenium cations 3−8 X H

X = -CH3 3

-CF3 4

-SiH3 5

-NH2 6

-NO2 7

-NO 8

______________________________________________________________________________

2. COMPUTATIONAL DETAILS The quantum chemical calculations were performed using the Gaussian 09 program package18 and carried out on LINUX clusters. The studied structures were fully optimized at the MP2/6-311G(d,p) level of theory without constraints. NICS values14,15 were computed on the basis of MP2/6-311G(d,p) geometries using the gauge-independent atomic orbital (GIAO) method19,20 at the B3LYP/6-311G(d,p)21−23 theory level.24 To calculate the spatial NICS, ghost atoms were placed on a lattice of 10 Å to +10 Å with a step size of 0.5 Å in the three directions of the Cartesian coordinate system. The zero points of the coordinate system were positioned at the centers of the studied structures. The resulting 68,921 NICS values, thus obtained, were analyzed and visualized by the SYBYL 7.3 molecular modeling software;25 different iso-chemical-shielding surfaces (ICSS) of −0.1 ppm (red) deshielding, and 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue) 0.5 ppm (green) and 0.1 ppm (yellow) shielding were used to visualize the TSNMRS of the studied structures in the various figures. ICSS are a quantitative indication of the diatropic ring current effect in 1H NMR spectroscopy;11−13 the closer the distance (in Å) of a certain



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shielding(deshielding) ICSS to the center of the molecules, the stronger the corresponding ring current effect measurable as δ(1H)/ppm in 1H NMR spectroscopy. TSNMRS were computed and appropriately examined herein as the central topic of this paper. Of significant note though, there have been some developments of the NICS index26,27 showing that it is not the average NICS, but rather only the NICS(0)πzz or NICS(1)πzz component as a single value that needs to be used to rigorously quantify aromaticity28 and single average NICS values have even been proven to be unsuitable generally for the quantitative evaluation of aromaticity.29−32

3. RESULTS AND DISCUSSION 3.1 Structures of studied compounds. More than 40 years ago, George Olah and coworkers2−5 gave more or less the answer on the present questions: At −135°C in solution of the special solvent SbF5−FSO3H−SO2ClF−SO2F2 benzenium 1 (and various other arenium salts) give both 1H and

13C

NMR spectra of the isolated σ-complex. The protonated carbon

[δ(13C) = 52.2 ppm] and the two attached protons [δ(1H) = 5.2 ppm] are truly aliphatic (i.e. this is a sp3 hybridized carbon atom). However, when increasing the temperature the respective signals in the two spectra coalesce and only one common signal has been observed [δ(1H) = 8.09 ppm; δ(13C) = 145.9 ppm]; the reason therefore is the 1,2-proton transfer process (cf. Scheme 3) with 1c as one of the transition states with an activation energy of about 10 kcal/mol proved by a number of theoretical calculations.33−35 This dynamic proton flip process is at −135°C slow, after coalescence at higher temperatures fast on the NMR time scale; here from and from transition state 1c of the proton flip dynamic process can be readily concluded that the aromatic structure of 1 is especially in solution not given up completely. ___________________________________________________________________________ Scheme 3. 1,2-Proton transfer process of benzenium ion 1 H H H

1a

H

H

1c

H

H

H

H

1a

___________________________________________________________________________ When frozen in solution2−5 and in the solid state1 (cf. Table 1), on the other hand, the exclusive presence of the σ-complex 1b (cf. Scheme 1) is proved: The benzenium ion 1 is ACS Paragon Plus Environment

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planar, the C−C−C bond angles are in between 118° and 123°, the C−C distances C2−C3/C3−C4/C4−C5/C5−C6 prove the delocalized electronic structure and the C1−C2/C6−C1 bond lengths are clearly elongated but still shorter than C−C single bonds; all together, the solid state structure of 1 proves to be similar to a delocalized electronic structure of an open chain pentadienyl cation.1

___________________________________________________________________________ Table 1. Comparison of the experimental (X-ray structure1 and 1H/13C NMR data)4 and the corresponding computed theoretical data of benzenium ion 1 ___________________________________________________________________________ 1H NMR data 13C NMR data [δ(1H/13C)/ppm]b Bond lengths (Ǻ)a C1,C2/C6,C1 C2,C3/C5,C6 C3,C4/C5,C4 H1 H2,6 H3,5 H4 C1 C2,6 C3,5 C4 ___________________________________________________________________________ 1.424/1.420 1.387/1.362 1.382/1.383 5.6 9.7 8.6 9.3 52.2 186.6 136.9 178.1 8.09c 145.9d 1.467 1.378 1.411 5.13 9.77 8.71 9.60 53.3 195.5 143.8 183.2 1.04e 1.46 e 1.27 e Data from the salt [C6H7]+[CB11H(CH3)5Br6]−. In SbF5−FSO3H−SO2ClF−SO2F2 solution at −135 °C. c In SbF −FSO H−SO ClF−SO F solution at −80 °C. 5 3 2 2 2 d In SbF −FSO H−SO ClF−SO F solution at −90 °C. 5 3 2 2 2 e WIBERG´s bond index.36 __________________________________________________________________________________________ a

b

Both solution NMR spectra and X-ray analyses of a number of substituted arenium ions have been also published5 but with about the same result. They exist as the corresponding σ-complexes: The chlorobenzenium cation,37 the pentamethyl chlorobenzenium cation,38 protonated mesitylene39,40 and the hexamethyl benzenium cations with various electrophiles (−Me, −Cl, −Br, −NO2 , SiEt3 and −NO);41 in the latter case, C6Me6-NO+, the only π-complex of arenium cations is reported41 (the nitroso cation located in central position of the absolutely planar arene ring). Further, the cationic silyl/arene complex42,43 was found to exhibit a relatively high degree of π-character.5,44 The bond length dSi−C1 = 2.18 Å, which is 0.24 Å larger than the sum of single bond covalent radii of C and Si. If QC calculations accompany the mentioned experimental solution NMR and/or solid state X-ray studies they confirm the experimental structural data, as our calculation at the MP2/6-311G(d,p) level of theory confirm the structure of the σ-complex of the benzenium cation 1 (cf. Table 1).



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3.2 Aromaticity of the benzenium cation 1. Aromaticity requires planar molecules with cyclic conjugation of 6π-electrons, in case of antiaromaticity of 4 π-electrons. This denotes for the benzenium cation 1 that it should be antiaromatic as the cyclopentadienyl cation 2 (cf. Scheme 4); on the other hand, the distance between C2...C6 in 1 is 2.51 Å, in cyclopentadienyl cation C3...C4 only 1.57 Å. And a useful distance is a fundamental precondition for the development of the paratropic ring current of homoantiaromaticity44 in case of 1 (see Supporting information).45 ___________________________________________________________________________ Scheme 4. Structures of benzenium cation 1, cyclopentadienyl cation 2 and pentadienyl cation 2a with artificially elongated C(3)−C(4) bond 6 5

4

1H H

5 3

2 3

1

2

2

2a

___________________________________________________________________________

In Figure 1 the TSNMRS of cyclopentadienyl cation 2 and the one with the artificially elongated C3−C4 bond length 2a [d3,4 (in 2a) = d2,6 (in 1) = 2.51 Å] are visualized by the various ICSS.11−13 The pure paratropic ring current in 2 (deshielding above/below plane, shielding in-plane) no longer exists in 2a (cf. Figure 1); only the combined anisotropy effect of the conjugated C3−C2−C1−C5−C4 moiety is visualized. Due to this artificial elongation of the d3,4 bond length in 2a (as d2,6 in 1) the complete ring conjugation which is the premise for the development of the paratropic ring current is prevented. For this reason, the benzenium cation 1 cannot be antiaromatic as suggested by the conjugation interrupting CH2 group in the σ-complex (cf. Scheme 4) − any different electronic structure of 1 must exist. In Figure 2, the spatial magnetic properties of benzene and the benzenium cation 1 are visualized by our TSNMRS approach.11−13 The ICSS are comparable, being in the benzenium cation somewhat smaller than in benzene (cf. also Table 2); a somewhat smaller ring current effect in 1 can be concluded. In addition, on the ICSS = +5 ppm (blue) shielding of the benzenium cation a dent above/below ring centre is observed; for this reason, the ICSS = +8 ppm (white) shielding is inserted. While this ICSS is closed in benzene, it is actually not in 1.


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Again, the smaller ring current effect in the benzenium cation compared with the one in benzene can be concluded. ___________________________________________________________________________ Table 2. TSNMRS values characterizing the ring current of benzenium cation 1 and benzene ___________________________________________________________________________ TSNMRS (Ring current effect)a) Benzenium cation Benzene ICSS = −0.1 ppm 6.7 Å 7.2 Å ICSS = +0.1 ppm 8.3 Å 8.8 Å ICSS = +0.5 ppm 4.6 Å 5.0 Å ICSS = +1.0 ppm 3.5 Å 3.9 Å ICSS = +2.0 ppm 2.6 Å 3.0 Å ICSS = +5.0 ppm 1.7 Å 2.0 Å ICSS = +8.0 ppm cavity 1.6 Å ICSS of various size and direction according to Figure 2: −0.1 ppm (red) deshielding (in-plane), and 8 ppm (white) 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green) and 0.1 ppm (yellow) shielding (above/below ring plane). __________________________________________________________________________________________ a)

This is of no influence on the assignment properties of the ring current effect (when calculated as TSNMRS values) in 1H NMR spectroscopy because TSNMRS values are employed to assign protons only from distances of protons larger than 3 Å from the next nearest atom of the ring current/anisotropy effect generating structure moiety.46−54 The corresponding δ(1H)/ppm can be measured and employed for the desired assignment purposes in 1H NMR spectroscopy and proves to be the quantitative measure of the present ring current/anisotropy effect on δ(1H)/ppm. However, the hole within the ICSS = +8 ppm shielding (white) in 1 will be of importance on decisions when concluding from the present ring current effect on the corresponding aromaticity of the studied compound because theoretical but non-measurable indexes of aromaticity measure NICS(0)πzz in the centre and/or NICS(1)πzz 1 Å above the molecular centre and should be considered with care [dependent on the index employed, the aromaticity of 1 was calculated to be from 37.5% up to 58.6% of the aromaticity of benzene].55−63



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___________________________________________________________________________

Figure 1. Visualization11−13 of the spatial magnetic properties (TSNMRS) of cyclopentadienyl cation 2 and the same molecule with the artificially elongated d3,4 = 2.51 Å bond length (2a) by different ICSS of −0.1 ppm (red) deshielding (above/below ring plane), and 8 ppm (white), 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green) and 0.1 ppm (yellow) shielding (above/below in-plane). __________________________________________________________________________________________


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___________________________________________________________________________

Figure 2. Visualization11−13 of the spatial magnetic properties (TSNMRS) of benzene (left) and the benzenium cation 1 by different ICSS of −0.1 ppm (red) deshielding (in-plane), and 8 ppm (white), 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green) and 0.1 ppm (yellow) shielding (above/below ring plane). __________________________________________________________________________________________

However, how the planar 6-membered ring in 1 with only five conjugated sp2 hybridized carbons but one the delocalization interrupting sp3 hybridized carbon atom (the σcomplex) can develop a diatropic ring current (not exactly but almost as large as the one in benzene) remains nebulous. This situation is only acceptable if the positive charge is more or less located at the C(1)−H proton or delocalized onto all hydrogen atoms being greatest at the site of protonation as suggested by the Mulliken population analysis of 1.5 Perhaps the



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substituted arenium cations 3−8 (cf. Scheme 2) can give an answer. 1H and

13C

NMR

chemical shifts of the hexamethyl analogues of 3 (X = −Me)5,41 and 7 (X = −NO2)41 prove the existence of σ-complexation at −80 °C in CD2Cl2 as 3 in the solid state41; the carbon at the site of substitution clearly reveals its sp3 hybridization. The solution

13C

NMR spectrum of

the hexamethyl analogue of 8 (X = −N=O)5,41, however, exhibit single aromatic and methyl signals; fast dynamic chemical exchange as along the ring proton flip in benzenium cation2−5 or π-complexation can be concluded; solid state structures prove the existence of πcomplexation.64−66 The corresponding spatial magnetic properties (TSNMRS) of 3−8 are given in Figures 3 and 4. All these arenium cations develop similar diatropic ring currents as the benzenium cation 1 itself, however, concerning the structure (σ-complex or π-complex) there are differences. 3.3 Influence of ±I substituents on the ring current effect of the benzenium cation. +I[−Me (3) and −SiH3 (5)] and −I-substituents [−CF3 (4)] have been studied. The TSNMRS values obtained are visualized in Figure 3 and the corresponding sizes of the ICSS are collected in Table 3. The spatial magnetic properties are comparable among themselves and with the one of benzene and benzenium cation 1 which were given in Figure 2. The existence of similar diatropic ring current effects in 1 and 3−5 can be concluded, however, there are few but notable differences. The structures of X = −Me and −CF3 (3 and 4) are identical, the remaining proton at C1 is positioned near to perpendicular to the benzenium ring plane. The ICSS values of X = −CF3 (4) are smaller compared with the one of 3 which is congruent with the stronger electronegativity of the −CF3 substituent, slightly destabilizing the diatropic ring current effect in 4. The ICSS = +8 ppm of both compounds 3 and 4 reproduce the centrecavity of the benzenium cation 1. ___________________________________________________________________________ Table 3. TSNMRS values characterizing the ring current of the benzenium cation 3−5 ___________________________________________________________________________ TSNMRS (Ring current effect)a) X = −Me X = −CF3 X = −SiH3 ICSS = −0.1 ppm 6.9 Å 6.7 Å 7.1 Å ICSS = +0.1 ppm 8.3 Å 7.8 Å 8.5 Å ICSS = +0.5 ppm 4.8 Å 4.3 Å 4.9 Å ICSS = +1.0 ppm 3.9 Å 3.3 Å 3.9 Å ICSS = +2.0 ppm 2.9 Å 2.4 Å 2.9 Å ICSS = +5.0 ppm 1.8 Å 1.6 Å 1.9 Å ICSS = +8.0 ppm cavity cavity 1.4 Å (no cavity)


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11 a) ICSS of various size and direction according to Figure 3: −0.1 ppm (red) deshielding (in-plane), and 8 ppm (white), 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green) and 0.1 ppm (yellow) shielding (above/below ring plane). __________________________________________________________________________________________


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_______________________________________________________________________________________________________________________

Figure 3. Effect of ±I substituents on the ring current effect of the benzenium cation − visualization11−13 of the spatial magnetic properties (TSNMRS) of −Me (3, left), −CF3 (4, middle) and −SiH3 (5) by different ICSS of −0.1 ppm (red) deshielding (in-plane), and 8 ppm (white), 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green) and 0.1 ppm (yellow) shielding (above/below ring plane). ______________________________________________________________________________________________________________________________________________


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_______________________________________________________________________________________________________________________

Figure 4. Effect of ±M substituents on the ring current effect of the benzenium cation − visualization11−13 of the spatial magnetic properties (TSNMRS) of −NH2 (6, left), −NO2 (7, middle) and −N=O (8) by different ICSS of −0.1 ppm (red) deshielding (in-plane), and 8 ppm (white), 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green) and 0.1 ppm (yellow) shielding (above/below ring plane). ______________________________________________________________________________________________________________________________________________



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Differently behaves the strongly electropositive substituent −SiH3 (5): ICSS = +8 ppm shows no cavity, the other ICSS are larger than the one of the benzenium cation 1 and more near to benzene than to 1, 3 and 4. Further, opposite to the structures of 3 and 4, the −SiH3 substituent in 5 is positioned near to perpendicular to the benzenium ring plane (as was the remaining C1 proton in 3 and 4). In addition, the bond length in 5 (dC−Si = 2.095 Å) proves to be 0.22 Å longer than the covalent C−Si bond (also this fact is different to 3 and 4; in these compounds dC−C(H3) and dC−C(F3) confirm the existence of covalent C−C bonds). Further, the TSNMRS values of the SiH3 analogue 5 join the structural differences: ICSS = −0.1 ppm and ICSS = +0.5 to 5.0 ppm are extended and there is no central cavity within ICSS = +8.0 ppm. These facts indicate that the −SiH3 substituent in 5 tend to the formation of the corresponding π-complex, but is still in relevant contact to C1 (vide infra). The two latter facts are in complete agreement with experimental structural data of already published Si-compounds 9 and 10 (cf. Scheme 5).5,41,67−69 A benzene-like structure with SiR3 substituent(s) attracting the positive charge of the structure on the way to the corresponding π-complex 11 is reported.5,41,67−69 Structure and spatial magnetic properties of 9

and 10 are identical to the one of 5 with the same structural peculiarities (see Supporting information). __________________________________________________________________________________________ Scheme 5. Structures of experimentally X-ray studied Si-substituted benzenium cations 9 and 10,5,41−43 and the structure of the potential π-complex 11 H

SiR3

9

R3Si

R3HSi

SiR3

10

SiH2R3

SiR3

11

___________________________________________________________________________

3.4 Influence of ±M-substituents on the ring current effect of the benzenium cation. +M substitution [X = −NH2 (6)] initially strengthens the amino-substituted benzene moiety due to extended conjugation; the −NH2 substituent is positioned more or less in-plane, the remaining hydrogen at C1 is tilted to the aromatic ring system. The angle aromatic system...H(C1) which is smaller than 90°, and the elongated bond length dH(1),C(1) = 1.22 Å (in benzenium 1.11 Å, see Supporting information), point to structure 6 with the C(1)−NH2 moiety in-plane and the C(1)−H proton facing π-complexation.


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Accordingly (cf. Figure 4 and Table 4) all ICSS in 6 are elongated compared with the benzenium cation 1 and approach the ICSS of benzene (cf. Table 2); elongated is the C(1)−H bond length [dC(1)−H = 1.221 Å; in 1 only 1.109 Å] as well. All this, the benzene-close TSNMRS values of 6 (cf. Figure 4 and Table 4) together with the more-or-less in-plane position of the −NH2 substituent and the more-or-less perpendicular (to benzene ring system) position of the C(1)−H proton and the elongated C(1)−H bond length in 6c reflect the existence of the σ-complex 6c approaching the π-analogue 6d in both structures locating the positive charge at the C(1)−H proton (cf. Scheme 6). ___________________________________________________________________________ Scheme 6. Potential structures of the σ- (6a−c) and the π-complex (6d) of the amino-benzenium cation 6, and of the anilinium cation 12

6a

H

H

H

NH2

NH2

NH2

6b

6c

H

NH3 NH2

6d

12

___________________________________________________________________________ It should be mentioned that the corresponding anilinium cation 12 was additionally found and proved to be 46.56 kcal/mol more stable than the amino-benzenium cation 6. TSNMRS data of 12, as expected, are comparable if not identical to the TSNMRS of benzene (see Supporting information).

___________________________________________________________________________ Table 4. TSNMRS values characterizing the ring current of the benzenium cation 6−8 ___________________________________________________________________________ TSNMRS (Ring current effect)a) X = −NH2 X = −NO2 X = −N=O ICSS = −0.1 ppm 7.1 Å 7.1 Å 8.4 (5.0) Å b ICSS = +0.1 ppm 8.4 Å 8.4 Å 7.6 Å b ICSS = +0.5 ppm 4.8 Å 4.8 Å 4.3 Å b ICSS = +1.0 ppm 3.7 Å 3.7 Å 3.3 Å b ICSS = +2.0 ppm 2.9 Å 2.4 Å 2.5 Å b ICSS = +5.0 ppm 1.9 Å 1.5 Å 1.5 Å b ICSS = +8.0 ppm 1.6 Å(no cavity) cavity cavity, resp. no cavity c ICSS of various size and direction according to Figure 3: −0.1 ppm (red) deshielding (in-plane), and 8 ppm (white), 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green) and 0.1 ppm (yellow) shielding (above/below ring plane). b) Measured from the NO averted direction. c) Measured from the NO facing direction. __________________________________________________________________________________________ a)



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When evaluating the outer shielding ICSS of the −M substituent −NO2 in the arenium cation 7 almost no differences to the −NH2 analogue 6 are observed (cf. Figure 4 and Table 4); anyway, the electron attracting character of the nitro group destabilizes the corresponding benzenium cation analogue 7 and reduces the ring current effect: ICSS = +2 ppm and +5 ppm are contracted and within the ICSS = +8 ppm shielding (white) the characterizing hole can be realized (cf. Figure 4 and Table 4). As in the cases of 3 (X = −Me), 4 (X = −CF3) and 6 (−NH2) also in 7 (X = −NO2) the C(1)−H proton is perpendicularly positioned to the benzene ring moiety and the corresponding C(1)−H bond length some elongated (dC(1)−H = 1.123 Å, normally 1.090 Å) suggesting a corresponding structure like 6d (cf. Scheme 6) and emphasize the fact that all four compounds have comparable structures of σ-complexes 13a approaching but not reaching yet the electronic structure of the corresponding π-complexes 13b (cf. Scheme 7). ___________________________________________________________________________ Scheme 7. σ- (left) and π-Complexes of 3 (X = −Me), 4 (X = −CF3), 6 (X = −NH2) and 7 (X = −NO2)

H

H X

13a

X 13b

___________________________________________________________________________

It remains the nitroso arenium cation 8. The solution

13C

NMR spectrum of the

hexamethyl analogue of 8 (X = −N=O)5,41 exhibit single aromatic and methyl signals; on fast dynamic chemical exchange as along the ring proton flip in benzenium cation2−5 or the alternative π-complexation can be concluded. The solid state structure of the hexamethyl analogue proves π-complexation.3,67−69 The same is true for 8 (cf. Figure 4 and Table 4): The nitroso substituent +N=O proved to be positioned above the benzene ring plane, the contact to C(1) is still existing [dC(1)...N(=O) = 2.8 Å] but the distance to the centre of the benzene moiety is already smaller [dN(=O)...centre = 2.6 Å], and the bond length of the C(1)−H proton in 8 proves normal − on the completed, intact aromatic 6π electron delocalization of the benzene moiety in 8 can be readily concluded. The TSNMRS values are confused due to the additional ACS Paragon Plus Environment

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anisotropy influences of the +NO substituent, but (cf. Table 4) the corresponding ICSS are comparable to 1 and 3−7. Here from, on comparable ring current effects of the benzene moiety, and, finally, on comparable π-electron distribution along the benzene moiety in 1 and 3−8 can be concluded. Thus, from the magnetic point of view the effect of protonation or the addition of an electrophile E+ to benzene is only of minor influence on the electron distribution, the delocalized aromatic 6π-electron system remains obviously intact. The positive charge is delocalized onto the hydrogen atoms being greatest at the site of protonation in the benzenium cation 1 or at the proton/electrophile in 3−8. However, it remains the matter of fact that in unusual, but structures stabilizing solvents at low temperature solely the σ-complex is unequivocally assigned; the same is true for the solid state structures. From the magnetic point of view, the most probable electronic structures of the compounds 1 and 3−8 studied are given in Scheme 8. __________________________________________________________________________ Scheme 8. Electronic structures of the compounds studied

H

H

X

H 1

3, 4, 6, 7

O N

SiH3 H 5

8

___________________________________________________________________________

4. CONCLUSIONS Both structures and spatial magnetic properties (TSNMRS) of the benzenium cation 1, a number of substituted analogues 3−8, (X = −Me, −CF3, −SiH3, −NH2, −NO2, −N=O), of the cyclopentadienyl cation 2 and an artificially deformed cyclopentadienyl cation 2a have been calculated at the MP2/6-311G(d,p) level of theory. Available experimental and computed structural data are coincident. Due to the artificial elongation of the d3,4 bond length in 2a (as d2,6 = 2.51 Å in 1), in cyclopentadienyl cation 2 1.57 Å only) 4π-electron conjugation which is the premise for the development of the paratropic ring current is prevented. For this reason, the benzenium cation 1 cannot be (homo)antiaromatic as suggested by the conjugation interrupting CH2 group in the σ-complex (1b) − due to comparable ring current effects (TSNMRS) of benzene and benzenium cation 1 the delocalized aromatic 6π-electron system



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remains intact, the positive charge is delocalized onto the hydrogen atoms being greatest at the site of protonation 1a. Identical results from the magnetic point of view can be reported for the structural analogues 3−8, but, dependent on the substituent X, notable structural differences were obtained: Generally, the diatropic ring current effect of the delocalized 6πelectron system remains intact in all analogues 3−8. While the −Me, −CF3, −NH2 and −NO2 substituents in 3, 4, 6 and 7, respectively, pursue the in-plane position with the benzene moiety, the corresponding C(1)−H protons are perpendicularly positioned to the benzene moiety and the corresponding C(1)−H bond length proved to be elongated (dC(1)−H = 1.120 − 1.128 Å, normally 1.090 Å); the contact is existing but the bond length moves into the direction of the corresponding π-complexation (cf. Scheme 8). This development continues in 5 and 8 (cf. Scheme 8): In the silicon compound 5 the C−Si bond length is elongated [dC−Si = 2.095 Å, 0.22 Å longer than the covalent C−Si bond), the C(1)−H bond length behaves normal]; obviously, 5 proves to be something in between the σ- and the π-complex. The nitroso analogue 8 finally, exists as π-complex [the contact to C(1) is still existing [dC(1)...N(=O) = 2.8 Å] but the distance to the centre of the benzene moiety is already smaller [dN(=O)...centre = 2.6 Å]; the bond length of the C(1)−H proton proves normal. In these two structures also, spatial magnetic properties TSNMRS (diatropic ring current effects) as in the benzenium cation 1 could be confirmed − on completed, intact aromatic 6π-electron systems as in 1, 3, 4, 6, and 7 can be concluded.

■ ASSOCIATED CONTENT Supporting information

The Supporting Information is available free of charge on the ACS Publications website at DOI: S1. Existence of the diatropic ring current effect in homoaromatic compounds. S2. Spatial magnetic properties (TSNMRS) of the anilinium cation 12. Table S1. Structure and NMR parameters of benzenium cation 1 and analogues 3−8 at the MP2/6-311G(d,p) level of theory. Table S2. Bond length reference values at the same MP2/6-311G(d,p) level of theory. Table S3. Coordinates and absolute energies of the studied compounds 1−8 at the MP2/6-311G(d,p) level of theory.


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■ AUTHOR INFORMATION Corresponding Author Erich Kleinpeter, Phone +49-331-977-5210; Fax: +49-331-977-5064; E-mail: [email protected] ORCID-ID: https://orcid.org/0000-0001-8993-0033 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS The calculation of the NICSπ,zz values with AROMA59 by Prof. Amnon Stanger (Technion, Haifa, Israel) is gratefully acknowledged.

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From NICS aromaticity index NICS(0)πzz = −18.0 ppm (−35.9 ppm for benzene).56



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Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. von Ragué, Which NICS aromaticity index for planar π ring is the best? Org. Lett. 2006, 8, 863−866.

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From the σ-only NICS(1)π,zz aromaticity index (−17.8±2.9, for benzene −34.8±3.5);58 from CMO-NICS(1)π,zz = −13.8 (benzene = −29.7);58 from σ-only ∫NICS(1)π,zz aromaticity index −41.5±7.2 (benzene = −101.9±12.3),58 from CMO-∫NICS(1)π,zz −33.0 (benzene = −87.4)58

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From the π current 0.046 au (benzene 0.079 au).10

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Choi, N.; Lickiss, P. D.; McPartlin, M.; Masagane, Ph. C.; Veneziani, G. L. Isolation and structure of a silicationic species with 1,3-aryl bridging between silicon atoms: A bis-silylated benzenium ion or a bridging Ph group? Chem. Commun. 2005, 48, 6023−6025.

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Table of Contents (TOC) Image ___________________________

H H

H H

H H

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The Benzenium Ion â Aromatic as the Ï-Complex or Antiaromatic as

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