Marcos
Mandado
* and
Nicolás
Ramos-Berdullas
Department of Physical Chemistry, University of Vigo, Lagoas-Marcosende s/n, 36310, Vigo, Spain. E-mail: mandado@uvigo.es
First published on 2nd June 2015
The unusual aromatic stability of cyclic bicalicene has been suggested to come from a tetraionic structure, where positive and negative charges are located on the cyclopropene and cyclopentadiene rings, respectively. Energetic, magnetic, geometric and electron delocalization analysis performed on a series of bicalicene derivatives, incorporating different electron donating and withdrawing groups, and electrically perturbed bicalicene structures provide additional proof of the role played by this tetraionic structure in the aromatic stability of bicalicene. In this work the aromatic stabilization is chemically and electrically tuned, enhancing or disrupting the electron delocalization and aromatic stability of the cyclopropene and cyclopentadiene rings by increasing or decreasing their corresponding charges. It is shown how the electron delocalization within these rings is similar to that of cyclopropene cation and cyclopentadiene anion for a perfect polarization of one electron.
The role played by the tetraionic form of Scheme 1 can be definitely revealed by analysing the changes exerted by different electron donating and electron withdrawing groups (EDGs and EWGs) as well as external electric perturbations on the global and local aromaticity of bicalicene. Thus, EDGs and EWGs will exert an opposite effect on the ring electron charges that must be reflected on the electron delocalization and aromatic stability. As long as the aromaticity of bicalicene has its origin in this tetraionic form, the changes experienced by the ring electron delocalization and aromatic stability must be ruled by the orientation of these groups. Additionally, the ring electron charge can be also ‘modulated’ by means of external electric perturbations. Thus, electric perturbations that enhance the polarization represented by Scheme 1 must reinforce also the electron delocalization within the cyclopropene and cyclopentadiene rings and the aromaticity of bicalicene. On the contrary, electric perturbations which tend to reverse this polarization must reflect just the opposite. In this work we show these arguments are indeed correct, demonstrating the tetraionic structure of Scheme 1 is the origin of the aromatic stabilization of bicalicene through local aromatic rings of cyclopropene and cyclopentadiene.
Multicenter indices and ring charges were calculated using the Mulliken atomic partitioning scheme through home-made Fortran codes. The density matrices required for the calculation of these magnitudes were obtained from Gaussian 09 program.13 The program GaussView 5.014 was employed for visualization of the electron deformation orbitals and associated electron densities, which were previously calculated with a home-made program. The reader is referred to ref. 12, 15–19 and 20 for a full description and theoretical backgrounds of multicenter delocalization indices and electron deformation orbitals, respectively. In this work, we have employed the renormalized version of multicenter delocalization indices defined in ref. 16. For the case of 3-center and 5-center delocalization indices (3-DI and 5-DI) using Mulliken atomic partition, they adopt the following forms;
(1) |
(2) |
In order to compare with aromaticity indices based on different criteria, we have also calculated the HOMA index (Harmonic Oscillator Model of Aromaticity),21 the NICS index (Nucleus Independent Chemical Shift)22 and the FLU index (Aromatic Fluctuation).23 In particular, we have employed here the zz component of the magnetic shielding tensor calculated 1 Å over the ring center, NICSzz(1), and also the π component of this index, NICSπzz(1), after classification of molecular orbitals (MOs) in σ and π symmetry. The orbital decomposition of the magnetic shielding tensor was performed with the help of the NBO program package.24 These indices were characterized as the best NICS indicators of aromaticity in planar polycyclic compounds with a hydrocarbonated frame.25 For the calculation of the HOMA indices we have employed the following expression,
(3) |
For the FLU index, we have opted for the π version (FLUπ), which does not require of a reference system and is the best choice for π aromatic compounds where the σ–π partitioning of molecular orbitals is possible. FLUπ is obtained from the following expression,
(4) |
Energies for homodesmotic reactions in Fig. 3 represent only the change in the electronic energy and do not include the zero point vibrational energy.
Fig. 2 Comparison of the B3LYP and HF energy differences (in kcal mol−1) calculated for the processes reported in Fig. 1. |
On the other hand, the effect of different external electric perturbations on the energetic stability of bicalicene is also shown in Fig. 1. The electric perturbations comprise positive and negative point charges placed outside and inside of the rings (structures 21–24) and external quadrupole electric fields (structures 25, 26). In all cases when the perturbation tends to concentrate positive and negative charges on the cyclopropene and cyclopentadiene rings, respectively, the stability increases significantly with respect to those that tend to polarize the charge in the opposite direction. Both HF and B3LYP energy differences reflect this trend with a small overestimation for the HF case (see Fig. 2). However, the total energy differences given in Fig. 1 depend on two terms (eqn (5));
ΔE = ΔEelec + ΔEdef | (5) |
ΔE | ΔEelec | ΔEdef | |
---|---|---|---|
21–22 | 38.3 | 38.1 | 0.2 |
23–24 | 171.3 | 144.5 | 26.8 |
25–26 | 112.5 | 108.7 | 3.8 |
3-Q | 5-Q | 3-Qπ | 5-Qπ | 3-DI | 5-DI | 3-DIπ | 5-DIπ | |
---|---|---|---|---|---|---|---|---|
0 | 0.199 | −0.568 | 0.465 | −0.442 | 0.658 | 0.882 | 1.136 | 0.836 |
1 | 0.282 | 0.145 | −0.052 | −0.056 | −0.536 | 0.076 | 0.113 | 0.034 |
2 | 0.164 | 0.407 | −0.129 | 0.028 | −0.212 | −0.203 | −0.229 | −0.193 |
3 | 0.032 | 0.159 | 0.052 | −0.004 | 0.082 | 0.011 | 0.070 | −0.027 |
4 | 0.175 | 0.108 | −0.106 | 0.111 | −0.235 | −0.142 | −0.201 | −0.138 |
5 | 0.256 | −0.016 | −0.091 | −0.059 | −0.571 | 0.075 | 0.067 | 0.072 |
6 | −0.003 | 0.292 | −0.016 | −0.084 | −0.001 | −0.049 | −0.022 | −0.043 |
7 | −0.263 | 0.191 | 0.147 | −0.202 | 0.645 | 0.148 | 0.180 | 0.170 |
8 | 0.327 | −0.344 | −0.156 | 0.126 | −0.702 | −0.092 | −0.057 | −0.122 |
9 | 0.012 | 0.353 | 0.009 | −0.084 | 0.033 | −0.007 | 0.015 | 0.009 |
10 | 0.328 | −0.017 | −0.100 | −0.004 | −0.602 | 0.002 | 0.020 | −0.001 |
11 | −0.274 | −0.156 | 0.140 | −0.123 | 0.618 | 0.176 | 0.169 | 0.173 |
12 | −0.006 | −0.317 | −0.063 | 0.136 | −0.089 | −0.096 | −0.087 | −0.126 |
13 | −0.030 | −0.005 | 0.040 | 0.046 | 0.060 | −0.044 | 0.053 | −0.076 |
14 | 0.012 | −0.067 | −0.058 | 0.092 | −0.369 | −0.113 | −0.178 | −0.109 |
15 | 0.059 | −0.004 | 0.003 | −0.036 | −0.054 | 0.043 | 0.046 | 0.043 |
16 | 0.002 | 0.018 | −0.010 | 0.000 | −0.007 | 0.018 | −0.014 | −0.014 |
17 | −0.052 | −0.355 | 0.066 | 0.001 | 0.104 | −0.047 | 0.088 | −0.048 |
18 | −0.323 | −0.093 | −0.131 | 0.167 | −0.992 | −0.207 | −0.275 | −0.200 |
19 | −0.264 | 0.022 | −0.085 | −0.068 | −0.243 | 0.086 | 0.071 | 0.083 |
20 | 0.011 | −0.315 | −0.022 | −0.073 | −0.024 | −0.081 | −0.035 | −0.082 |
21 | 0.014 | 0.048 | 0.102 | −0.101 | 0.043 | 0.132 | 0.150 | 0.121 |
22 | 0.023 | 0.021 | −0.122 | 0.122 | 0.097 | −0.156 | −0.182 | −0.148 |
23 | 0.067 | −0.072 | 0.051 | −0.050 | 0.065 | 0.044 | 0.049 | 0.054 |
24 | −0.068 | 0.074 | −0.055 | 0.054 | −0.065 | −0.052 | −0.052 | −0.057 |
25 | 0.187 | 0.126 | 0.286 | −0.281 | 0.348 | 0.311 | 0.342 | 0.267 |
26 | −0.113 | 0.315 | −0.503 | 0.505 | 0.332 | −0.526 | −0.625 | −0.506 |
Fig. 3 Molecular electrostatic potential (MEP) of bicalicene (0) and the difference in the MEP of electrically perturbed bicalicene structures (21–26) with respect to 0. |
Calculation of 3-center and 5-center delocalization indices, 3-DI and 5-DI, allows determining the effect of the charge polarization on the electron delocalization within the cyclopropene and cyclopentadiene rings (Table 2). Unsubstituted bicalicene displays a π 3-center delocalization index, 3-DIπ, of 1.136 and a π 5-center delocalization index, 5-DIπ, of 0.836, which are significant values but far from those displayed by the cyclopropene cation (1.758) and cyclopentadiene anion (1.491). However, changes in 3-Qπ and 5-Qπ, by effect of the different groups or the external electric perturbations correlate quite well with the values of 3-DIπ and 5-DIπ. Thus, a good linear correlation between both magnitudes have been found for all the derivatives and electrically perturbed bicalicenes (Fig. 4). Only slight deviations from the fitting have been found for the rings containing the groups –OH, –OCH3, and –F. This may be due to the fact that these groups exert both inductive and resonance effects with opposite result, whereas –CN, –CHO, –CH3 and –Li only exert significant resonance effects in the former two and inductive in the latter two. In the case of –NO2, which also displays resonance and inductive effects, they do not counteract each other since both are negative and the resonance is expected to be clearly dominant. It must be remarked that isolated rings of cyclopropene cation and cyclopentadiene anion perfectly fit in the correlations shown in Fig. 4. This definitely reflects that polarization approaches the electron delocalization of the cyclopropene and cyclopentadiene rings in bicalicene to that of isolated cyclopropene cation and cyclopentadiene anion rings enhancing their aromaticity. On the other hand, using total delocalization indices, 3-DI and 5-DI, the correlation with the charges disappear for the cyclopropene ring even at qualitative level due to the large σ 3-center electron delocalization within this ring (see differences between 3-DI and 3-DIπ data in Table 2). As will be discussed later, the large sigma electron delocalization within the cyclopropene ring is also reflected in the magnetic response.
A good correlation with the ring charges is also found for the FLUπ index (Fig. 4), with the exception of the points mentioned above. The FLUπ for these points, and for the isolated cyclopropene cation and cyclopentadiene anion, reflect a larger deviation with respect to the fitting line than the multicenter indices. In Table 3 the values of the FLUπ obtained for the cyclopropene and cyclopentadiene rings are collected together with the values of the HOMA index. The FLUπ index always reflects a larger aromaticity in the rings of the most stable conformation, i.e. with EDGs stabilizing the positively charged cyclopropene and EWGs the negatively charged cyclopentadiene rings with the only exception of the cyclopropene ring in the pair 9–10 with R = F (notice that smaller FLUπ reflects larger aromaticity). In the case of the HOMA index, the exceptions involve the pair 13–14 for the cyclopentadiene ring and the pairs 1–2, 5–6, 13–14 and 19–20 for the cyclopropene, although these exceptions reduces to only 5–6 and 13–14 when comparing the total HOMA for the molecule. This is not a surprising result, since the geometric criteria is expected to be more influenced by sigma bonding and steric factors associated to the nature of the substituent group and unconnected with the π aromaticity.
3-FLUπ | 5-FLUπ | Total-FLUπ | 3-HOMA | 5-HOMA | Total-HOMA | |
---|---|---|---|---|---|---|
0 | 0.083 | 0.068 | 0.302 | 0.934 | 0.944 | 3.757 |
1 | 0.015 | 0.057 | 0.143 | 0.920 | 0.949 | 3.738 |
2 | 0.142 | 0.124 | 0.532 | 0.930 | 0.926 | 3.712 |
3 | 0.067 | 0.066 | 0.266 | 0.933 | 0.941 | 3.748 |
4 | 0.111 | 0.092 | 0.407 | 0.933 | 0.930 | 3.726 |
5 | 0.028 | 0.055 | 0.165 | 0.912 | 0.954 | 3.733 |
6 | 0.092 | 0.072 | 0.329 | 0.934 | 0.951 | 3.769 |
7 | 0.022 | 0.029 | 0.101 | 0.980 | 0.967 | 3.895 |
8 | 0.086 | 0.074 | 0.319 | 0.814 | 0.918 | 3.463 |
9 | 0.076 | 0.056 | 0.265 | 0.935 | 0.954 | 3.777 |
10 | 0.063 | 0.068 | 0.260 | 0.833 | 0.947 | 3.561 |
11 | 0.026 | 0.039 | 0.129 | 0.983 | 0.957 | 3.881 |
12 | 0.104 | 0.074 | 0.356 | 0.935 | 0.913 | 3.695 |
13 | 0.073 | 0.081 | 0.309 | 0.933 | 0.928 | 3.723 |
14 | 0.107 | 0.086 | 0.387 | 0.935 | 0.932 | 3.735 |
15 | 0.066 | 0.061 | 0.253 | 0.944 | 0.948 | 3.784 |
16 | 0.088 | 0.070 | 0.315 | 0.934 | 0.941 | 3.751 |
17 | 0.063 | 0.074 | 0.276 | 0.933 | 0.938 | 3.741 |
18 | 0.140 | 0.104 | 0.489 | 0.835 | 0.923 | 3.516 |
19 | 0.023 | 0.053 | 0.153 | 0.923 | 0.955 | 3.756 |
20 | 0.112 | 0.101 | 0.426 | 0.930 | 0.940 | 3.739 |
21 | 0.045 | 0.047 | 0.184 | |||
22 | 0.126 | 0.095 | 0.443 | |||
23 | 0.074 | 0.059 | 0.265 | |||
24 | 0.093 | 0.078 | 0.341 | |||
25 | 0.000 | 0.018 | 0.036 | |||
26 | 0.253 | 0.172 | 0.851 |
Another proof of the changes exerted by the EDGs and EWGs in the aromatic stability of bicalicene is given by the energies of the homodesmotic reactions shown in Fig. 5. As one can see, when the groups –CN and –F are linked to cyclopentadiene rings and –OH and –Li groups to cyclopropene the reactions are more exothermic than that of bicalicene. The contrary happens when the same groups are interchanged between the rings. This reflects the aromatic stabilization energy of bicalicene, accounted for by the first homodesmotic reaction of Fig. 5, increases or decreases depending on the ring substitution and its effect on the ring electron charges.
Fig. 5 Homodesmotic reactions involving bicalicene and selected bicalicene derivatives (1, 2, 7, 8). |
Magnetic criteria of aromaticity also supports the aromaticity is larger in the bicalicene derivatives and electrically perturbed structures where the π electron charge is pushed from cyclopropene rings to cyclopentadiene. Thus, both total NICSzz(1) and NICSπzz(1) values collected in Table 4 are more negative in these structures with respect to their counterparts with the exception of the pair 5–6 (R = OH) and 19–20 (R = NO2) for NICSzz(1). Comparing ring by ring one finds more exceptions which also affects pairs 13–14 (R = CHO) for NICSzz(1) and 9–10 (R = F) for NICSπzz(1). Comparison with the unsubstituted bicalicene structure (0) reflects that the calculated NICS values are more negative for the structures 21 and 23 and less negative for 22 and 24.26 This is the expected trend, nevertheless, in the case of bicalicene derivatives the strong anisotropic effects associated to the resonance groups –CN and –OH do not only affect the strength of the ring currents and the NICS value can be significantly affected by other factors. Thus, comparison of the NICS values in these structures with those in structure 0 may not reflect properly the changes in the electron delocalization.
3-NICSzz(1) | 5-NICSzz(1) | Total-NICSzz(1) | 3-NICSπzz(1) | 5-NICSπzz(1) | Total-NICSπzz(1) | |
---|---|---|---|---|---|---|
0 | −16.02 | −21.77 | −75.58 | −1.41 | −18.02 | −38.86 |
1 | −13.12 | −20.02 | −66.27 | −2.31 | −16.56 | −37.74 |
2 | −12.34 | −18.94 | −62.58 | 1.13 | −16.22 | −30.18 |
3 | −17.37 | −21.42 | −77.58 | −2.59 | −17.98 | −41.14 |
4 | −14.28 | −20.85 | −70.26 | −0.69 | −17.07 | −35.52 |
5 | −11.66 | −19.80 | −62.91 | −0.94 | −16.05 | −33.98 |
6 | −13.97 | −19.49 | −66.93 | 0.52 | −16.86 | −32.68 |
7 | −19.13 | −21.69 | −81.63 | −1.32 | −19.26 | −41.16 |
8 | −9.60 | −19.60 | −58.42 | −0.52 | −15.23 | −31.50 |
9 | −14.97 | −20.35 | −70.65 | −0.31 | −17.69 | −36.00 |
10 | −10.35 | −19.77 | −60.25 | −1.13 | −15.87 | −34.00 |
11 | −19.93 | −23.30 | −86.47 | −2.20 | −19.74 | −43.88 |
12 | −15.23 | −21.78 | −74.02 | −0.79 | −17.56 | −36.70 |
13 | −17.27 | −20.46 | −75.47 | −2.63 | −17.56 | −40.38 |
14 | −14.24 | −21.21 | −70.91 | −0.85 | −17.36 | −36.42 |
15 | −15.28 | −21.58 | −73.72 | −1.17 | −18.00 | −38.34 |
16 | −15.22 | −20.40 | −71.24 | −0.80 | −17.42 | −36.44 |
17 | −17.78 | −20.02 | −75.60 | −3.02 | −17.36 | −40.76 |
18 | −11.21 | −19.88 | −62.18 | −0.68 | −16.27 | −33.90 |
19 | −11.30 | −19.67 | −61.93 | −0.79 | −16.04 | −33.66 |
20 | −13.68 | −19.29 | −65.94 | 0.75 | −16.72 | −31.94 |
21 | −17.41 | −22.36 | −79.54 | |||
22 | −14.43 | −21.36 | −71.58 | |||
23 | −16.65 | −22.81 | −78.91 | |||
24 | −15.34 | −20.70 | −72.07 |
An important difference between multicenter delocalization indices and the rest is the degree of aromaticity predicted for the cyclopropene and cyclopentadiene rings. According to the 3-DIπ and 5-DIπ values, cyclopropene ring is more aromatic than cyclopentadiene. On the contray, FLUπ and HOMA indicate the cyclopentadiene ring is slightly more aromatic. NICSπzz(1) values indicate a scarce ring current in the cyclopropene ring, in qualitative agreement with previous calculations of π ring currents plots in bicalicene.8 On the contrary, NICSzz(1), which includes σ orbitals, is largely negative for cyclopropene but less than cyclopentadiene, reflecting an important contribution of the σ orbitals to the total NICS value, which can be related to a significant σ ring current. This is in agreement with the large σ electron delocalization reflected by the 3-DI values. It must be clarified that the σ part of the 3-DI is a negative value, but the sign of the multicenter delocalization index derives from topological factors27,28 and is not related with possible antiaromatic character. Antiaromatic structures display almost null multicenter electron delocalization12 and the index is then not able to distinguish a priori between antiaromatic and nonaromatic systems. Origin of the differences between multicenter electron delocalization measurements and magnetic criteria of aromaticity has been extensively analysed and explained in the literature.29–33 The fact that multicenter indices reflect a larger aromaticity for the cyclopropene rings may be related to the nature of the renormalization employed and introduced in ref. 16. This renormalization does not warrant the comparison between multicenter indices of different order and just allows putting the indices in a more or less similar scale.
We have finally identified the main occupied and virtual π molecular orbitals involved in the π charge transfer between cyclopropene and cyclopentadiene rings. These molecular orbitals and the electron transfer associated to the combination of them were analysed with the help of the electron deformation orbitals (EDOs) induced by the electric perturbations. As can be seen in Fig. 6, the combinations HOMO → LUMO+2, HOMO−1 → LUMO+1, HOMO−2 → LUMO+5 and HOMO−3 → LUMO are the main responsible for the π electron transfer. The interesting picture reported by the deformation orbitals is that the electron density associated to the EDOs has almost identical shape in all the structures, despite depending on the orientation of the electric perturbation a net electron transfer occurs from the cyclopropene to the cyclopentadiene rings (structures 23 and 25) or in the opposite direction (structures 24 and 26). This reflects the orbital interactions upon the perturbation are identical even for opposite electric fields, so that the changes in the π electron delocalization are just originated in the electron transfer between rings and cannot be related to any fundamental difference in the molecular orbital nature or occupied–virtual orbital mixing.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp00990a |
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