Article pubs.acs.org/Organometallics
From Corannulene to Indacenopicene: Effect of Carbon Framework Topology on Aromaticity and Reduction Limits Sarah N. Spisak,† Jingbai Li,‡ Andrey Yu. Rogachev,*,‡ Zheng Wei,† Olena Papaianina,§ Konstantin Amsharov,§ Alexey V. Rybalchenko,∥ Alexey A. Goryunkov,∥ and Marina A. Petrukhina*,† †
Department of Chemistry, University at Albany, State University of New York, Albany, New York 12222, United States Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, United States § Institute of Organic Chemistry II, University Erlangen-Nuremberg, Henkestr 42, 91054 Erlangen, Germany ∥ Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory, 1-3, 119991, Moscow, Russia ‡
S Supporting Information *
ABSTRACT: The electronic structure, reduction limits, and coordination abilities of a bowl-shaped polycyclic aromatic hydrocarbon, indacenopicene (C26H12, 1), have been investigated for the first time using a combination of theoretical and experimental tools. A direct comparison with the prototypical corannulene bowl (C20H10, 2) revealed the effects of carbon framework topology and symmetry change on the electronic properties and aromaticity of indacenopicene. The accessibility of two reduction steps for 1 was predicted theoretically and then confirmed experimentally. Two reversible one-electron reduction processes with the formal reduction potentials at −1.92 and −2.29 V vs Fc+/0 were detected by cyclic voltammetry measurements, demonstrating the stability of the corresponding mono- and dianionic states of 1. The products of the doubly reduced indacenopicene have been isolated as rubidium and cesium salts and fully characterized. Their X-ray diffraction study revealed the formation of tetranuclear organometallic building blocks with the [M2(18-crown-6)]2+ (M = Rb (3) and Cs (4)) cations occupying the concave cavities of two C26H122− anions. The coordination of two outside exo-bound rubidium ions is terminated by crown ether molecules in 3 to form the discrete [Rb+4(18-crown-6)3(C26H122−)2] tetramer. In contrast, the larger cesium ions allow the 1D polymeric chain propagation in 4 to afford [Cs+2(18-crown-6)2(THF)(C26H122−)]∞.
■
INTRODUCTION Metalation of curved carbon surfaces is of great importance in organometallic and materials chemistry from both the fundamental and applied viewpoints. Significant outcomes range from superconducting alkali metal salts of fullerenes1 to conducting carbon nanotube materials with different metal junctions.2 In addition to extensive coordination chemistry of fullerenes3 these studies recently included bowl-shaped π-conjugated molecules (also referred to as open geodesic polyarenes,4 buckybowls,5 fullerene fragments, or π-bowls6). The latter represent a very interesting class of polycyclic aromatic ligands since their exo and endo π-surfaces are both open and available for metal binding. The number of such nonplanar polycyclic aromatic hydrocarbons (PAHs) has been rapidly expanding over the last decades.4−7 The known examples of π-bowls now range in size from the smallest curved fragments of C60-fullerene such as corannulene (C20H10)8 and sumanene (C21H12)9 to larger bowl-shaped polyarenes, C40H16,10 C50H10,11 and C56H38,12 providing unique carbon-rich containers for metal binding. However, the reported complexes of transition metals so far mainly include the two smallest π-bowls, corannulene13 and sumanene.14 These studies revealed a variety of coordination modes that are exhibited by bowl-shaped π-ligands, showing their unique versatility in metal-binding reactions. In contrast, only a very limited number © XXXX American Chemical Society
of metal complexes have been isolated and structurally characterized for larger π-bowls. Those include transition metal complexes of neutral dibenzocorannulene C28H14,15 monoindenocorannulene C26H12,16 and hemifullerene C30H12.17 At the same time, theoretical studies of large and deep π-bowls confirm their great potential for selective endohedral metal encapsulation.18 Furthermore, NMR investigations of alkali-metal-induced reduction of C26H12,19 C28H14,20 and C30H1221 predicted the formation of unique supramolecular assemblies in solution, showing that the ligating properties of neutral π-bowls can be enhanced by stepwise addition of electrons. However, none of the abovementioned products have been isolated in the solid state and crystallographically characterized. We have recently developed the preparation procedures to isolate crystalline products of the bowl-shaped corannulene upon stepwise electron addition. The doubly degenerate LUMO levels of C20H10 (C5v) allow it to accept up to four electrons,22 resulting in the set of consequently reduced carbanions with distinctly different binding abilities. On the basis of X-ray crystallographic studies, we have demonstrated that mono- and doubly reduced corannulene can form solvent-separated or contact ion pairs with Received: May 14, 2016
A
DOI: 10.1021/acs.organomet.6b00395 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics alkali metal ions in the solid state,23 while the very electron-rich triple- and tetrareduced corannulene anions tend to self-assemble into unique sandwich-type aggregates with a large number of metal ions encapsulated.24 The first product of sumanene (C3) reduction also revealed the tendency of sumanenyl trianions to self-assemble into a novel supramolecular sandwich with multiple K+ ions.25 These works24,25 showed that the bowl symmetry, along with other factors, affects the nuclearity of highly charged alkali metal belts sandwiched between the curved π-decks, prompting us to initiate investigation of bowl-shaped PAHs having larger surface areas and different carbon framework topology. The recently developed preparation of indacenopicene (C26H12, 1), using regiospecific aryl−aryl coupling in the cove region to form the product in high yield and pure form,26 opened up the possibility to evaluate its reduction and coordination abilities. Herein, we undertake the first investigation of the electronic structure and aromaticity of this new π-bowl followed by the studies of its properties using cyclic voltammetry and chemical reduction reactions with group 1 metals. Intrigued by the unique structure of this carbon-rich bowl having an extended π-surface and different symmetry (Cs) compared to the prototypical corannulene (C20H10, 2, Scheme 1), we set out to reveal its potential as a novel curved redox-active scaffold and new π-ligand for metal binding.
Figure 1. Optimized geometries (a) and NBO charge distribution (red) and Wiberg bond indexes (blue) calculated for neutral C26H12 and C20H10 bowls (b) (PBE0/cc-pVTZ level of theory; parameters are given only for symmetry-unique atoms and bonds).
Scheme 1
■
Figure 2. Molecular electrostatic potential maps of neutral C26H12 (a) and C20H10 (b).
RESULTS AND DISCUSSION Neutral Indacenopicene vs Corannulene. Single-crystal X-ray diffraction characterization of C26H12 (1) unambiguously revealed the geodesic shape of its extended carbon framework.26 It is expected that incorporation of any additional pentagons into the π-bowl surface imposes extra strain on the system, and, indeed, the bowl depth of 1, having two five-membered rings, is increased to 0.925(9) Å vs that of C20H10 (2) (0.875(2) Å).27 However, the formation of a more curved carbon surface in 1 unexpectedly resulted in the reduction of the bowl-to-bowl inversion energy barrier. Activation energy for this process was calculated at the PBE0/cc-pVTZ level of theory to be only +2.15 kcal/mol vs +9.98 kcal/mol for corannulene (which is in agreement with the experimental value reported for C20H1028). Direct structural comparison of C26H12 and C20H10 reveals their similarity (Figure 1). At the same time, the 3D charge distribution, illustrated by molecular electrostatic potential (MEP) maps (Figure 2), shows that the five-membered ring of 2 has a more positive character than the two symmetry-equivalent fivemembered rings of 1, which are clearly more negatively charged. Additional strain built into the carbon framework of 1 and changes in its electronic structure are expected to result in perturbation of the aromatic π-system. We therefore undertook a careful comparative evaluation of two bowl-shaped PAH molecules. The aromaticity of 1 and 2 was probed with the help of a set of descriptors, which include the structure-based harmonic oscillator model of aromaticity (HOMA), aromaticity electronic criteria based on Bader’s topological quantum theory para-delocalization
index (PDI), and aromatic fluctuation index (FLU), as well as nucleus-independent chemical shift (NICS). These results are collected in Table 1 (see SI for more details). Due to the high symmetry of neutral corannulene (C5v), it has only two independent rings, A and B (Table 1). At the same time, the lower symmetry of C26H12 (Cs) results in the presence of four different six-membered rings (A1, A2, A3, and A4) and one five-membered ring (B). This change is reflected in the perturbation of geometrical parameters of the indacenopicene carbon framework. In contrast to five identical hub carbon−carbon bonds of 1.409 Å in 2, analogous bonds in the five-membered ring of 1 cover the broad range of 1.397−1.504 Å. Notably, the open peripheral C2−C6 site exhibits the longest bond length distance (1.504 Å), indicating its single-bond character and the lack of conjugation. The HOMA indexes calculated for the five-membered ring clearly show low delocalization of electrons in 1 (0.196 vs 0.818 in C20H10). The same indexes computed for six-membered rings in neutral indacenopicene indicate their nonequivalency. The highest delocalization HOMA index is observed in the A4 ring (0.835), followed by the A1 and A3 rings (0.819 in both), with the A2 ring having the lowest delocalization index of 0.568. For comparison, the HOMA index calculated for the six-membered ring of corannulene is equal to 0.702. Next, these findings were supported by the aromaticity descriptors based on topological quantum theory (Table 1). The calculated indexes indicate the highest π-electron delocalization for the B
DOI: 10.1021/acs.organomet.6b00395 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Aromaticity Indexes Calculated for Neutral C26H12 and C20H10 Bowls (PBE0/cc-pVTZ)
AIMb
fuzzy atomic spacec
compound
ring
HOMAa
NICS
PDI
FLU
PDI
FLU
C26H12
A1 A2 A3 A4 B A B
0.819 0.568 0.819 0.835 0.196 0.702 0.818
−4.29 −0.98 −6.49 −4.41 10.68 −6.43 9.22
0.072 0.039 0.061 0.073
0.007 0.019 0.013 0.009 0.032 0.014 0.026
0.072 0.041 0.062 0.074
0.011 0.028 0.018 0.013 0.043 0.019 0.038
C20H10
0.059
0.060
Reference for C−C bond length in HOMA is 1.388 Å and α = 257.7 as obtained from the geometry of the benzene molecule optimized at the same level of theory. bAtomic overlap matrix is based on the basin integral in Bader’s AIM.29 cAtomic overlap matrix is based on the fuzzy atom space integral over the Becke atomic space.30 Reference delocalization index for the C−C bond for Bader’s FLU is equal to 1.3921e, as taken from benzene, whereas in Becke’s variant of FLU, it is equal to 1.4637e. a
A4 and A1 rings and the lowest delocalization for the A2 ring. The FLU index calculated for the five-membered ring (PDI descriptors cannot be computed for rings other than six-membered ones) also agrees with low π-delocalization in C26H12 compared with that in C20H10 (Table 1). The calculated nuclear-independent chemical shifts clearly show the strong aromatic character of the A1, A3, and A4 rings. At the same time, the A2 ring has neither aromatic nor antiaromatic character. In contrast, the behavior of five-membered ring B can be described as strongly antiaromatic in nature, similar to that in the parent corannulene.22c Mono- and Dianionic States of C26H12. The electronic structure of indacenopicene is different from that of corannulene, which has two low-lying degenerate LUMOs and thus is able to accept up to four electrons in stepwise reduction reactions.22 The orbital diagram of 1 shows no degeneracy for the lowestlying unoccupied molecular orbital (Figure 3). Thus, the C26H12
Figure 4. MEP maps for the monoanion (a) and dianion (b) of C26H12.
state, the A2 ring is notably more involved in negative charge localization than in the case of the monoanion. Thus, in the double-negatively charged C26H122− anion the preferable sites for attack by an electrophile (such as a metal cation, for example) are expected to be the rings A1 and A2. This finding correlates well with topology of the LUMO for the neutral C26H12 molecule (Figure 3). The energy of the LUMO in 1 is lower than that in 2 (−2.42 eV vs −1.77 eV), and that should facilitate the reduction processes. This was confirmed by calculations of the electron affinity for 1 and 2 to be 31.15 kcal/mol (1.35 eV) and 12.87 kcal/mol (0.56 eV), respectively. Together with the relatively high-lying doubly occupied HOMO (−6.07 vs −6.48 eV in C20H10), this should make the indacenopicene molecule significantly softer than corannulene (ΔEHOMO−LUMO = 3.65 eV vs 4.71 eV for 1 and 2, respectively). These theoretical predictions on reduction properties of 1 are supported by cyclic voltammetry (CV) measurements. The registered CV curves are shown in Figure 5, and peak potential values are summarized in Table 2. Two reversible one-electron reduction processes with the formal reduction potentials at −1.92 and −2.29 V vs Fc+/0 were detected with equivalent cathodic and
Figure 3. MO diagram for neutral C26H12 (PBE0/cc-pVTZ).
bowl should readily accept only up to two electrons with the subsequent formation of the singly and doubly reduced states of 1. The MEP maps show the 3D distribution of negative charge in both carbanions of 1 (Figure 4). Interestingly, in the dianionic C
DOI: 10.1021/acs.organomet.6b00395 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
of group 1 metals, namely, with rubidium and cesium countercations. The first product, [Rb+4(18-crown-6)3(C26H122−)2] (3), was prepared by the reduction of C26H12 in THF using an excess of rubidium metal over 24 h. The product was crystallized by layering of the THF solution with hexanes in the presence of 18-crown-6 ether at 10 °C. Dark purple blocks of 3·2THF were isolated in high yield in 5 days. An X-ray diffraction study revealed the tetrameric structure of 3 (Figure 6) with the [Rb2(18-crown-6)]2+ dication filling
Figure 5. CV curves for C26H12 (1 and 3 mM (inset), Pt, 0.15 M Bu4NBF4, o-DCB, 100 mV s−1) in the range from −2.7 to +1.6 V (inset: from −2.7 to +0.5 V). Peaks corresponding to the redox processes of 1 are marked with Roman numerals.
Figure 6. Molecular structure of the tetramer 3 (a); encapsulation of Rb+ ions in [Rb2(18-crown-6)(C26H122−)2] (b); and binding sites of rubidium ions (c).
Table 2. Cathodic and Anodic Peak (Epc and Epa) Potential Values vs Fc+/0, Epc − Epa (ΔE) Values, and Formal Reduction Potentials of 1a
a
peak
process
Epc/V
Epa/V
ΔE/V
(Epc + Epa)/2/V
I II III
0/1− 1−/2− 0/1+
−1.95 −2.33
−1.85 −2.25 +1.09
0.07 0.08
−1.92 −2.29
the concave cavities of two doubly reduced indacenopicene bowls. These internal rubidium ions bind asymmetrically to the endo surface of C26H122− with the average Rb···C distances to the centroids of the five- and six-membered rings being 2.962(4) and 3.025(4) Å, respectively. In addition, two rubidium ions are externally bound to the convex face of the C26H122− bowls in an η5-fashion with the corresponding Rb···C distances of 3.142(4)−3.382(4) Å. One 18-crown-6 ether molecule is shared by two endo-bound rubidium cations with Rb···O contacts of 2.945(3)−3.066(3) Å. The Rb+ ions bound to the exo surface of C26H122− complete their coordination by the terminal 18-crown-6 molecule having slightly shorter Rb···O distances of 2.881(3)−3.001(3) Å. The second product, [Cs+2(18-crown-6)2(THF)(C26H122−)] (4), was prepared by the reduction of 1 in THF with an excess of cesium metal over 5 h. The product was also crystallized from THF in the presence of 18-crown-6 ether upon layering with hexanes at 10 °C. Dark purple blocks of 4 were isolated in moderate yield in 4 days. An X-ray diffraction study revealed an extended polymeric chain structure in 4 with an otherwise close binding pattern of C26H122− to that in 3 (Figure 7). The cesium ions of the internal
1 mM C26H12, Pt, 0.15 M Bu4NBF4, o-DCB, 100 mV s−1.
anodic currents and direct/reverse peak separations close to 60 mV after iR compensation at a 100 mV s−1 scan rate. These results are indicative of the chemical and electrochemical reversibility of both reduction steps and stability of the corresponding mono- and dianionic states of 1 within the time scale of the CV experiment. As expected, both reduction steps of 1 proceed significantly easier compared to 2. Specifically, the first and second reduction potentials of 1 are positively shifted by ca. 0.5 and 0.7 V with respect to that of 2 (ca. −2.44 and −3.02 V vs Fc+/0).31 At the same time, the anodic oxidation of 1 irreversibly occurs at an Epa value of +1.09 V vs Fc+/0, likely due to dimerization processes or high reactivity of the resulting cation-radical toward the medium. Notably, the electrochemistry-based HOMO and LUMO energy level estimations (−5.8 and −3.0 eV) as well as their HOMO−LUMO gap (2.8 eV) are close to the theoretically predicted ones. The optical HOMO−LUMO gap, which is less sensitive to the dielectric properties of the medium used in CV experiments, was estimated from the absorption band onset observed for 1 at 510 nm (2.4 eV).26 X-ray Crystallographic Study. Consistently with the computational and electrochemical studies, we revealed that reduction of indacenopicene (C26H12, 1) with group 1 metals proceeds through two steps and stops at the formation of the dianion. The addition of an excess of alkali metals to 1 in THF allows the generation of an intense green color associated with C26H12−, which subsequently undergoes further reduction to form a deep purple solution of C26H122−. Even when an excess of Li metal was used for reduction of 1 over an extended period of time (these conditions allowed the facile formation of the tetrareduced corannulene24a), we observed the formation of C26H122− only (Figure S8). The isolation of the dianion in the solid state was accomplished with two heavier congeners
Figure 7. Fragment of the molecular structure of 4 (a); a space-filling model of the [Cs2(18-crown-6)]2+ cation encapsulated by two C26H122− bowls (b); and binding sites of cesium ions (c).
[Cs2(18-crown-6)]2+ dication occupy the concave cavities of two doubly reduced indacenopicene bowls (Figure 7b). One 18-crown-6 molecule is shared by these two endo-bound cesium ions with Cs···O contacts of 3.223(5)−3.525(7) Å. As in 3, the concave binding of the cesium ions is asymmetric, with the Cs···C distances to the centers of the five- and six-membered rings of C26H122− being 3.282(13) and 3.308(12) Å, respectively (Figure 7c). In contrast to 3, where exo η5-binding is observed, D
DOI: 10.1021/acs.organomet.6b00395 Organometallics XXXX, XXX, XXX−XXX
Organometallics
■
the external Cs+ ions are bound to the convex face of C26H122− in an η6-fashion (distance to the centroid is 3.344(14) Å). The exo-bound Cs+ ion is also coordinated by one 18-crown-6 ether molecule (Cs···Ocrown, 3.253(5)−3.627(5) Å) and one THF molecule (Cs···OTHF, 3.160(7) Å). Overall, the Cs···O distances are close to those previously reported for the salts of different carbanions crystallized with Cs+ ions solvated by O-donors.32 When Rb+ ions are replaced with larger Cs+ ions, the distance between the indacenopicene decks in the sandwich fragment is increased from 9.32 Å (3) to 9.60 Å (4) (Figures 6b, 7b). However, unlike 3, which exhibits a discrete tetrameric structure, larger Cs+ ions participate in chain propagation, with the [Cs2(18-crown-6)(THF)2]2+ cations serving as building blocks between the C26H122− anions to form a 1D organometallic polymer in 4 (Figure 8). The bowl depths of C26H122− in 3 and 4
Article
EXPERIMENTAL PART
Computational Methods. All calculations were performed at the PBE0/cc-pVTZ level of theory. In all cases, no symmetry restrictions were applied. All calculated structures correspond to local minima (no imaginary frequencies) on the corresponding potential energy surfaces, as determined by calculation of the full Hessian matrix, followed by estimation of frequencies in the harmonic approximation. All calculations were performed using the Firefly program (version 8.1.0).33 Electronic structures were then probed by the natural bond orbital method34 with the help of the NBO (version 6.0) program package.35 Using optimized geometries, a set of theoretical descriptors/indexes of aromaticity was calculated. This set includes (i) a structure-based harmonic oscillator model of aromaticity (as defined by Kruszewski and Krygowski36), (ii) nuclear independent chemical shift (introduced by von Rague Schleyer et al.37), and (iii) descriptors based on the topological quantum theory of atoms in molecule (QTAIM)38 approach such as para-delocalized index39 and aromatic fluctuation index.40 All QTAIM calculations were carried out by the Multiwfn 3.3.7 program.41 Calculations of NICS values were performed using the gauge-independent atomic orbitals (GIAO) approach with the help of the Gaussian 09 program42 at the PBE0/cc-pVTZ level of theory. Materials and Methods. All manipulations were carried out using break-and-seal43 and glovebox techniques under an atmosphere of argon. THF and hexanes were dried over Na/benzophenone and distilled prior to use. THF-d8 was dried over sodium−potassium alloy and distilled prior to use. Rubidium and cesium metals were purchased from Strem Chemicals. Crown ether, 18-crown-6 (99%), was purchased from SigmaAldrich and dried over P2O5 in vacuo for 24 h. Indacenopicene (C26H12) was prepared as described previously26 and sublimed at 135 °C under reduced pressure prior to use. The UV−vis spectra were recorded on a PerkinElmer Lambda 35 spectrometer. The 1H NMR spectra were measured on a Bruker AC-400 spectrometer at 400 MHz and were referenced to the resonances of the corresponding solvent used. Preparation of [Rb44+(18-crown-6)3(C26H122−)2] (3)·2THF. THF (1.5 mL) was added to a flask containing excess Rb (approximately 8 equiv), excess 18-crown-6, and indacenopicene (3 mg, 0.009 mmol). The initial color of the mixture was pale yellow-orange (neutral ligand). Within a few minutes, the color of the mixture turned green. The reaction mixture was stirred for an additional 2 h, resulting in a purple suspension. After a total of 24 h of stirring, the mixture was filtered and the purple filtrate was layered with hexanes (1.3 mL) and placed at 10 °C. Dark purple blocks of 3·2THF were present in high yield after 5 days. Yield: 11.9 mg, 70%. UV/vis (THF): λmax 492, 545, and 585 nm. 1H NMR (THF-d8, −80 °C): δ 2.71 (crown ether), 6.77 (br s, 1H), 7.16 (br m, 2H), 7.49 (br m, 2H), and 8.26 (br s, 1H). Preparation of [Cs22+(18-crown-6)(THF)(C26H122−)] (4). THF (1.5 mL) was added to a flask containing excess Cs (approximately 8 equiv) and indacenopicene (3 mg, 0.009 mmol). The initial color of the mixture was pale yellow-orange (neutral ligand). Within a few seconds, the color of the mixture turned green. The reaction mixture was stirred for an additional 3 h, resulting in a purple suspension. The mixture was filtered after being stirred at room temperature for a total of 5 h. The purple filtrate was layered with the solution of 18-crown-6 (2.5 equiv) in hexanes (0.75 mL) and placed at 10 °C. Dark block-shaped crystals of 4 were present in moderate yield after 4 days. Yield: 5.6 mg, 65%. UV/vis (THF): λmax 478, 549, and 589 nm. 1 H NMR (THF-d8, −80 °C): δ 2.34 (crown ether), 2.63 (crown ether), 6.66 (br s, 1H), 6.94 (br m, 2H), 7.34 (br m, 2H), and 8.07 (br s, 1H). Note: The 13C NMR spectra and EA could not be measured for 3 and 4, as crystals of the doubly reduced indacenopicene are extremely air and moisture sensitive and have low solubility in THF. Voltammetric Measurements. The measurements were carried out under an oxygen-free and moisture-free (
