Search Results for "circulene"

Tetrabenzo[7]circulene

I have discussed the circulenes in a few previous posts. Depending on their size, they can be bowls, flat disks, or saddles. A computational study of [7]circulene noted that C2 structure is slightly higher in energy than the Cs form,1 though the C2 form is found in the x-ray structure.2

Now, Miao and co-workers have synthesized the tetrabenzo[7]circulene 1 and also examined its structure using DFT.3

As with the parent compound, a C2 and Cs form were located at B3LYP/6-31G(d,p), and are shown in Figure 1. The C2 form is 7.6 kcal mol-1 lower in energy than the Cs structure, and the two are separated by a transition state (also shown in Figure 1) with a barrier of 12.2 kcal mol-1. The interconversion of these conformations takes place without going through a planar form. The x-ray structure contains only the C2 structure. It should be noted that the C2 structure is chiral, and racemization would take place by the path: 1-Cs1-Cs1-C2*, where 1-C2* is the enantiomer of 1-C2.

1-C2

1-TS

1-Cs

Figure 1. B3LYP/6-31G(d,p) optimized structures of 1.

References

1) Hatanaka, M., "Puckering Energetics and Optical Activities of [7]Circulene Conformers." J. Phys. Chem. A 2016, 120 (7), 1074-1083, DOI: 10.1021/acs.jpca.5b10543.

2) Yamamoto, K.; Harada, T.; Okamoto, Y.; Chikamatsu, H.; Nakazaki, M.; Kai, Y.; Nakao, T.; Tanaka, M.; Harada, S.; Kasai, N., "Synthesis and molecular structure of [7]circulene." J. Am. Chem. Soc. 1988, 110 (11), 3578-3584, DOI: 10.1021/ja00219a036.

3) Gu, X.; Li, H.; Shan, B.; Liu, Z.; Miao, Q., "Synthesis, Structure, and Properties of Tetrabenzo[7]circulene." Org. Letters 2017, DOI: 10.1021/acs.orglett.7b00714.

InChIs

1: InChI=1S/C44H22/c1-5-13-28-24(9-1)32-19-17-23-18-20-33-25-10-2-6-14-29(25)38-31-16-8-4-12-27(31)35-22-21-34-26-11-3-7-15-30(26)37(28)43-39(32)36(23)40(33)44(38)42(35)41(34)43/h1-22H
InChIKey=KVMXYGAVHDZMNP-UHFFFAOYSA-N

Aromaticity Steven Bachrach 01 May 2017 No Comments

[8]circulene

Circulenes are molecules where a central ring is composed of fused benzenoids. Corranulene can also be named [5]circulene and coronene is [6]circulene. In a previous post I discussed the topology of the circulenes. This earlier work suggested that [8]annulene 1 would have a saddle-shape. This hypothesis has now been confirmed with the synthesis of the substituted [8]circulene 2 by Wu and co-workers.1


1


2

The x-ray structure does show a saddle geometry for 2. The central 8-member ring is tub-shaped, even more puckered that cyclooctatetraene (COT) itself, though the bonds in 2 are nearly of equal length. The bond lengths involving the central carbon atoms appear consistent with an [8]radialene-type structure.

The ωB97X-D/6-31G** optimized geometries of the parent compound 1 and the synthesized compound 2 are shown in Figure 1. These computed structures are very similar to each other, along with being very similar to the x-ray structure of 2.

1

2

Figure 1. ωB97X-D/6-31G** optimized geometries of 1 and 2.
(Don’t forget that you can click on these structures – and any other structure on my blog – to interactively manipulate and visualize them, something worth doing here!)

The computed NICS(0) (at HF/6-31+G* – I would really rather have seen these computed with some density functional, preferably at ωB97X-D/6-31G**) values for the six-member rings of both 1 and 2 are negative, ranging from -8.9 ppm to -4.0 ppm, indicating aromatic character. The NICS(0) value at the center of the 8-member ring is +9.8 ppm in 1 and +12.2 ppm in 2. The authors argue that this value cannot discriminate the 8-member ring from that in COT (NICS(0) = 1.98 ppm, the expected value for a non-aromatic ring) and [8]radialene (NICS(0) = -1.2 ppm, also an expected value for a non-aromatic ring). However, they are silent on whether this might actually imply some antiaromatic character to the 8-member ring, which would be consistent with the equivalent bond lengths around the ring.

The authors note that there should be a second isomer of 2 resulting from a flip of the tub. Variable temperature NMR does not show any change in the spectrum, though with a different substituted [8]circulene they do see some coalescence, suggesting a large flipping barrier of at least 20 kcal mol-1. A computational search for this flipping/inversion might be interesting as the transition state is likely to not be planar.

References

(1) Feng, C.-N.; Kuo, M.-Y.; Wu, Y.-T. "Synthesis, Structural Analysis, and Properties of [8]Circulenes," Angew. Chem. Int. Ed. 2013, 52, 7791-7794, DOI: 10.1002/anie.201303875.

InChIs

1: InChI=1S/C32H16/c1-2-18-5-6-20-9-11-22-13-15-24-16-14-23-12-10-21-8-7-19-4-3-17(1)25-26(18)28(20)30(22)32(24)31(23)29(21)27(19)25/h1-16H
InChIKey=BASWMOIVIHXTRC-UHFFFAOYSA-N

2: InChI=1S/C96H80/c1-49-17-33-65(34-18-49)81-73-57(9)58(10)75-83(67-37-21-51(3)22-38-67)85(69-41-25-53(5)26-42-69)77-61(13)62(14)79-87(71-45-29-55(7)30-46-71)88(72-47-31-56(8)32-48-72)80-64(16)63(15)78-86(70-43-27-54(6)28-44-70)84(68-39-23-52(4)24-40-68)76-60(12)59(11)74(82(81)66-35-19-50(2)20-36-66)90-89(73)91(75)93(77)95(79)96(80)94(78)92(76)90/h17-48H,1-16H3
InChIKey=DEKWLSGHBADDAQ-UHFFFAOYSA-N

Aromaticity Steven Bachrach 03 Sep 2013 No Comments

Circulenes

What is the topology of a molecule made of fused benzene rings? Hopf and co-workers have examined the case where the benzene rings are fused in an ortho arrangement to complete a circle, the so-called [n]circulenes 1n.1 They computed the series of [3]- to [20]circulene at B3LYP/6-31G(d).


1n

The most common examples of this class are corannulene 15 and coronene 16. Hopf finds that the small circulenes, [3]- through [6]circulene, are bowls, consistent with many previous studies.


15, corannulene


16, coronene

The larger circulenes fall into two distinct topological categories. [7]circulene through [16]circulene are saddles, as shown in Figure 1a. When the compounds are even larger, namely [17]- through [20]circulene, they adopt a helical topology, as seen in Figure 1b. Unfortunately, Hopf does not supply the optimized geometries; there is no supporting material at all. So I have reoptimized [12]circulene at B3LYP/6-31G(d) and [18]circulene at AM1. It is a real shame that authors do not routinely deposit their structures, that referees do not call out the authors on this, and that editors of journals do not demand full geometrical descriptions of all reported computed structures.

a)

b)

112: [12]circulene

118: [18]circulene

Figure 1. Optimized structures of (a) [12]circulene (B3LYP/6-31G(d)) and (b) [20]circulene (AM1).
Note the hydrogens have been omitted for clarity.

Hopf does not provide a comparison of structures and their energies. For example, what is the energy difference between the bowl and saddle topologies of [7]circulene or the energy difference between the saddle and helical topologies of [17]circulene?

The change in topology of the circulenes is fascinating. One wonders if this change is strictly a function of a stringing fused hexagons in a circle and minimizing the surface. Or is their some π-π stacking that leads to the saddle and helical topologies? Further details would be interesting – as would be examining other types of ciculenes as hinted by the authors at the end of the paper regarding isomeric kekulenes 2.

Scheme 1 – examples of kekulenes 2

References

(1) Christoph, H.; Grunenberg, J.; Hopf, H.; Dix, I.; Jones, P. G.; Scholtissek, M.; Maier, G., "MP2 and DFT Calculations on Circulenes and an Attempt to Prepare the Second Lowest Benzolog, [4]Circulene," Chem. Eur. J. 2008, 14, 5604-5616, DOI: 10.1002/chem.200701837

InChIs

15: InChI=1/C20H10/c1-2-12-5-6-14-9-10-15-8-7-13-4-3-11(1)16-17(12)19(14)20(15)18(13)16/h1-10H
InChIKey: VXRUJZQPKRBJKH-UHFFFAOYAF

16: InChI=1/C24H12/c1-2-14-5-6-16-9-11-18-12-10-17-8-7-15-4-3-13(1)19-20(14)22(16)24(18)23(17)21(15)19/h1-12H
InChIKey: VPUGDVKSAQVFFS-UHFFFAOYAQ

112: InChIKey: FTLFLCQEVCSDMZ-UHFFFAOYAB

118: InChIKey: CRXJHICCESJVIG-UHFFFAOYAJ

Aromaticity Steven Bachrach 01 Jul 2008 3 Comments

Curved Aromatic molecules – 4 new examples

I have recently been interested in curved aromatic systems – see my own paper on double helicenes.1 In this post, I cover four recent papers that discuss non-planar aromatic molecules.

The first paper2 discusses the warped aromatic 1 built off of the scaffold of depleiadene 3. The crystal structure of 1 shows the molecule to be a saddle with near C2v symmetry. B3LYP/6-31G computations indicate that the saddle isomer is 10.5 kcal mol-1 more stable than the twisted isomer, and the barrier between them is 16.0 kcal mol-1, with a twisted saddle intermediate as well.

The PES is significantly simpler for the structure lacking the t-butyl groups, 2. The B3LYP/6-31G PES of 2 has the saddle as the transition state interconverting mirror images of the twisted saddle isomer, and this barrier is only 1.8 kcal mol-1. Figure 1 displays the twisted saddle and the saddle transition state. Clearly, the t-butyl groups significantly alter the flexibility of this C86 aromatic surface. One should be somewhat concerned about the small basis set employed here, especially lacking polarization functions, and a functional that lacks dispersion correction. However, the computed geometry of 1 is quite similar to that of the x-ray structure.


2 twisted saddle (ground state)


2 saddle (transition state)

Figure 1. B3LYP/6-31G optimized geometries of the isomer of 2.

The second paper presents 4, a non-planar aromatic based on [8]circulene 6.3 (See this post for a general study of circulenes.) [8]circulene has a tub-shape, but is flexible and can undergo tub-to-tub inversion. The expanded aromatic 4 is found to have a twisted shape in the x-ray crystal structure. A simplified model 5 was computed at B3LYP/6-31G(d) and the twisted isomer is 4.1 kcal mol-1 lower in energy than the saddle (tub) isomer (see Figure 2). The barrier for interconversion of the two isomers is only 6.2 kcal mol-1, indicating a quite labile structure.


5 twisted
0.0


5 TS
6.2


5 saddle
4.1

Figure 2. B3LYP/6-31G(d) optimized geometries and relative energies (kcal mol-1) of the isomers of 5.

The third paper presents a geodesic molecule based on 1,3,5-trisubstitued phenyl repeat units.4 The authors prepared 7, and its x-ray structure shows a saddle-shape. The NMR indicate a molecule that undergoes considerable conformational dynamics. To address this, they did some computations on the methyl analogue 8. The D7h structure is 309 kcal mol-1 above the local energy minimum structure, which is way too high to be accessed at room temperature. PM6 computations identified a TS only 0.6 kcal mol-1 above the saddle ground state. (I performed a PM6 optimization starting from the x-ray structure, which is highly disordered, and the structure obtained is shown in Figure 3. Unfortunately, the authors did not report the optimized coordinates of any structure!)

Figure 3. PM6 optimized structure of 8.

The fourth and last paper describes the aza-buckybowl 9.5 The x-ray crystal structure shows a curved bowl shape with Cs symmetry. NICS(0) values were computed for the parent molecule 10 B3LYP/6-31G(d). These values are shown in Scheme 1 and the geometry is shown in Figure 4. The 6-member rings that surround the azacylopentadienyl ring all have NICS(0) near zero, which suggests significant bond localization.

Scheme 1. NICS(0) values of 10

Figure 4. B3LYP/6-31G(d) optimized structure of 10.

Our understanding of what aromaticity really means is constantly being challenged!

References

1. Bachrach, S. M., "Double helicenes." Chem. Phys. Lett. 2016, 666, 13-18, DOI: 10.1016/j.cplett.2016.10.070.

2. Ho, P. S.; Kit, C. C.; Jiye, L.; Zhifeng, L.; Qian, M., "A Dipleiadiene-Embedded Aromatic Saddle Consisting
of 86 Carbon Atoms." Angew. Chem. Int. Ed. 2018, 57, 1581-1586, DOI: 10.1002/anie.201711437.

3. Yin, C. K.; Kit, C. C.; Zhifeng, L.; Qian, M., "A Twisted Nanographene Consisting of 96 Carbon Atoms." Angew. Chem. Int. Ed. 2017, 56, 9003-9007, DOI: 10.1002/anie.201703754.

4. Koki, I.; Jennie, L.; Ryo, K.; Sota, S.; Hiroyuki, I., "Fluctuating Carbonaceous Networks with a Persistent
Molecular Shape: A Saddle-Shaped Geodesic Framework of 1,3,5-Trisubstituted Benzene (Phenine)." Angew. Chem. Int. Ed. 2018, 57, 8555-8559, DOI: 10.1002/anie.201803984.

5. Yuki, T.; Shingo, I.; Kyoko, N., "A Hybrid of Corannulene and Azacorannulene: Synthesis of a Highly Curved Nitrogen-Containing Buckybowl." Angew. Chem. Int. Ed. 2018, 57, 9818-9822, DOI: 10.1002/anie.201805678.

InChIs

1: InChI=1S/C134H128/c1-123(2,3)57-37-65-66-38-58(124(4,5)6)42-70-74-46-62(128(16,17)18)50-82-94(74)110-106(90(66)70)105-89(65)69(41-57)73-45-61(127(13,14)15)49-81-93(73)109(105)119-113-97(81)85(131(25,26)27)53-77-78-54-87(133(31,32)33)99-83-51-63(129(19,20)21)47-75-71-43-59(125(7,8)9)39-67-68-40-60(126(10,11)12)44-72-76-48-64(130(22,23)24)52-84-96(76)112-108(92(68)72)107(91(67)71)111(95(75)83)121-115(99)103(78)118-104-80(56-88(134(34,35)36)100(84)116(104)122(112)121)79-55-86(132(28,29)30)98(82)114(120(110)119)102(79)117(118)101(77)113/h37-56H,1-36H3
InChIKey=GKUTUWMASUJSFD-UHFFFAOYSA-N

2: InChI=1S/C86H32/c1-9-33-34-10-2-14-38-42-18-6-22-46-50-26-30-55-56-32-28-52-48-24-8-20-44-40-16-4-12-36-35-11-3-15-39-43-19-7-23-47-51-27-31-54-53-29-25-49-45-21-5-17-41-37(13-1)57(33)73-74(58(34)38)78(62(42)46)84-70(50)66(55)81(65(53)69(49)83(84)77(73)61(41)45)82-67(54)71(51)85-79(63(43)47)75(59(35)39)76(60(36)40)80(64(44)48)86(85)72(52)68(56)82/h1-32H
InChIKey=MXCDWJZMTKLBDM-UHFFFAOYSA-N

3: InChI=1S/C18H12/c1-2-6-14-11-12-16-8-4-3-7-15-10-9-13(5-1)17(14)18(15)16/h1-12H
InChIKey=KVJJNMIHWIRGRP-UHFFFAOYSA-N

4: InChI=1S/C132H108O4/c1-125(2,3)53-29-65-66-30-54(126(4,5)6)34-70-74-38-58(130(16,17)18)42-78-86-46-82-63-51-91(135-27)92(136-28)52-64(63)84-48-88-80-44-60(132(22,23)24)40-76-72-36-56(128(10,11)12)32-68-67-31-55(127(7,8)9)35-71-75-39-59(131(19,20)21)43-79-87-47-83-62-50-90(134-26)89(133-25)49-61(62)81-45-85-77-41-57(129(13,14)15)37-73-69(33-53)93(65)109-110(94(66)70)114(98(74)78)122-106(86)118-103(82)104(84)120-108(88)124-116(100(76)80)112(96(68)72)111(95(67)71)115(99(75)79)123(124)107(87)119(120)102(83)101(81)117(118)105(85)121(122)113(109)97(73)77/h29-52H,1-28H3
InChIKey=ZLPRACZKLACDHX-UHFFFAOYSA-N

5: InChI=1S/C108H60O4/c1-37-13-49-50-14-38(2)18-54-58-22-42(6)26-62-70-30-66-47-35-75(111-11)76(112-12)36-48(47)68-32-72-64-28-44(8)24-60-56-20-40(4)16-52-51-15-39(3)19-55-59-23-43(7)27-63-71-31-67-46-34-74(110-10)73(109-9)33-45(46)65-29-69-61-25-41(5)21-57-53(17-37)77(49)93-94(78(50)54)98(82(58)62)106-90(70)102-87(66)88(68)104-92(72)108-100(84(60)64)96(80(52)56)95(79(51)55)99(83(59)63)107(108)91(71)103(104)86(67)85(65)101(102)89(69)105(106)97(93)81(57)61/h13-36H,1-12H3
InChIKey=ZSIVUKSPPZUSQL-UHFFFAOYSA-N

6: InChI=1S/C32H16/c1-2-18-5-6-20-9-11-22-13-15-24-16-14-23-12-10-21-8-7-19-4-3-17(1)25-26(18)28(20)30(22)32(24)31(23)29(21)27(19)25/h1-16H
InChIkey=BASWMOIVIHXTRC-UHFFFAOYSA-N

7: InChI=1S/C224H210/c1-211(2,3)197-99-169-85-183(113-197)184-86-170(100-198(114-184)212(4,5)6)157-66-149-67-158(79-157)172-88-187(117-200(102-172)214(10,11)12)188-90-174(104-202(118-188)216(16,17)18)161-70-151-71-162(81-161)176-92-191(121-204(106-176)218(22,23)24)193-95-179(109-207(123-193)221(31,32)33)165-74-153-75-166(83-165)180-96-195(125-208(110-180)222(34,35)36)196-98-182(112-210(126-196)224(40,41)42)168-77-154-76-167(84-168)181-97-194(124-209(111-181)223(37,38)39)192-94-178(108-206(122-192)220(28,29)30)164-73-152-72-163(82-164)177-93-190(120-205(107-177)219(25,26)27)189-91-175(105-203(119-189)217(19,20)21)160-69-150-68-159(80-160)173-89-186(116-201(103-173)215(13,14)15)185-87-171(101-199(115-185)213(7,8)9)156-65-148(64-155(169)78-156)141-50-127-43-128(51-141)130-45-132(55-143(150)53-130)134-47-136(59-145(152)57-134)138-49-140(63-147(154)61-138)139-48-137(60-146(153)62-139)135-46-133(56-144(151)58-135)131-44-129(127)52-142(149)54-131/h43-126H,1-42H3
InChIKey=ZDDKJXIESSWTIA-UHFFFAOYSA-N

8: InChI=1S/C182H126/c1-99-15-113-43-127(29-99)141-57-142-65-155(64-141)162-78-169-92-170(79-162)172-82-164-83-174(94-172)176-85-166-87-178(96-176)180-89-168-91-182(98-180)181-90-167-88-179(97-181)177-86-165-84-175(95-177)173-81-163(80-171(169)93-173)156-66-143(128-30-100(2)16-114(113)44-128)58-144(67-156)130-32-103(5)19-117(47-130)118-20-104(6)34-132(48-118)147-60-148(71-158(165)70-147)134-36-107(9)23-121(51-134)123-25-109(11)39-137(53-123)151-62-152(75-160(167)74-151)138-40-111(13)27-125(55-138)126-28-112(14)42-140(56-126)154-63-153(76-161(168)77-154)139-41-110(12)26-124(54-139)122-24-108(10)38-136(52-122)150-61-149(72-159(166)73-150)135-37-106(8)22-120(50-135)119-21-105(7)35-133(49-119)146-59-145(68-157(164)69-146)131-33-102(4)18-116(46-131)115-17-101(3)31-129(142)45-115/h15-98H,1-14H3
InChIKey=FJHGGHOTCCNJNI-UHFFFAOYSA-N

9: InChI=1S/C44H23N/c1-44(2,3)21-16-28-24-8-4-6-22-26-14-19-12-10-18-11-13-20-15-27-23-7-5-9-25-29(17-21)41(28)45-42(33(22)24)39-35(26)37-31(19)30(18)32(20)38(37)36(27)40(39)43(45)34(23)25/h4-17H,1-3H3
InChIKey=QHBWEZKXFSKCSM-UHFFFAOYSA-N

10: InChI=1S/C40H15N/c1-4-19-23-8-3-9-24-20-5-2-7-22-26-15-18-13-11-16-10-12-17-14-25-21(6-1)30(19)39-36-32(25)34-28(17)27(16)29(18)35(34)33(26)37(36)40(31(20)22)41(39)38(23)24/h1-15H
InChIKey=XWSUADIIRLXSBY-UHFFFAOYSA-N

Aromaticity Steven Bachrach 24 Sep 2018 No Comments

nano-Saturn

It never hurts to promote one’s science through clever names – think cubane, buckminsterfullerene, bullvalene, etc. Host-guest chemistry is not immune to this cliché too, and this post discusses the latest synthesis and computations of a nano-Saturn; nano-Saturns are a spherical guest molecule captured inside a ring host molecule. I discussed an example of this a number of years ago – the nano-Saturn comprised of C60 fullerene surrounded by [10]cycloparaphenylene.

Yamamoto, Tsurumaki, Wakamatsu, and Toyota have prepared a nano-Saturn complex with the goal of making a flatter ring component.1 The inner planet is modeled again by C60 and the ring is the [24]circulene analogue 1. The x-ray crystal structure of this nano-Saturn complex is shown in Figure 1.


1: R = 2,4,6-tri-iso-propylphenyl
2: R = H

Figure 1. X-ray crystal structure of the nano-Saturn complex of 1 with C60.

Variable temperature NMR experiments gave the binding values of ΔH = -18.1 ± 2.3 kJ mol-1 and TΔS = 0.8 ± 2.2 kJ mol-1 at 298 K. To gauge this binding energy, they computed the complex of C60 with the parent compound 2 at B3LYP/6-1G(d)//M05-2X/6-31G(d), unfortunately without publishing the coordinates in the supporting materials. The computed binding enthalpy is ΔH = -50.6 kJ mol-1, but this computation is for the gas phase. The computed structure shows close contacts of 0.29 nm between the fullerene and the C9-proton of the anthracenyl groups, in excellent agreement with the x-ray structure. These weak C-Hπ interactions undoubtedly help stabilize the complex, especially given that the fullerene carries a very tiny Mulliken charge of +0.08 e.

References

1) Yuta, Y.; Eiji, T.; Kan, W.; Shinji, T., "Nano-Saturn: Experimental Evidence of Complex Formation of an Anthracene Cyclic Ring with C60." Angew. Chem. Int. Ed. 2018, 57, 8199-8202, DOI: 10.1002/anie.201804430.

InChIs

1: InChI=1S/C174H180/c1-91(2)121-78-150(97(13)14)164(151(79-121)98(15)16)163-90-128-71-139-70-127-59-109(37-38-120(127)77-162(139)163)110-39-49-140-129(60-110)72-130-61-111(40-50-141(130)165(140)170-152(99(17)18)80-122(92(3)4)81-153(170)100(19)20)112-41-51-142-131(62-112)73-132-63-113(42-52-143(132)166(142)171-154(101(21)22)82-123(93(5)6)83-155(171)102(23)24)114-43-53-144-133(64-114)74-134-65-115(44-54-145(134)167(144)172-156(103(25)26)84-124(94(7)8)85-157(172)104(27)28)116-45-55-146-135(66-116)75-136-67-117(46-56-147(136)168(146)173-158(105(29)30)86-125(95(9)10)87-159(173)106(31)32)118-47-57-148-137(68-118)76-138-69-119(128)48-58-149(138)169(148)174-160(107(33)34)88-126(96(11)12)89-161(174)108(35)36/h37-108H,1-36H3
InChIKey=AMDNULXMAMDTMX-UHFFFAOYSA-N

2: InChI=1S/C84H48/c1-13-61-25-62-15-3-51-33-75(62)43-73(61)31-49(1)50-2-14-63-26-64-16-4-52(34-76(64)44-74(63)32-50)54-6-18-66-28-68-20-8-56(38-80(68)46-78(66)36-54)58-10-22-70-30-72-24-12-60(42-84(72)48-82(70)40-58)59-11-23-71-29-69-21-9-57(39-81(69)47-83(71)41-59)55-7-19-67-27-65-17-5-53(51)35-77(65)45-79(67)37-55/h1-48H
InChIKey=ZYXXLAYETADMDM-UHFFFAOYSA-N

Aromaticity &host-guest Steven Bachrach 28 Aug 2018 No Comments

Quadrannulene

The recent synthesis and characterization of the quadrannulene 1 once again stretches
our notions of aromaticity.1


1

The core of this system is a four-member ring with four fused-phenyl rings, forming the very small circulene (see this earlier post on circulenes). One might write other resonance structures for the molecule, which could include a central cyclobutadienyl fragment. However, the X-ray structure and computational analysis rejects any significant contribution of the cyclobutadienyl character. First, the four C-C bond of this central ring are 1.45 Å long, with an NBO bond order of 1.08, signifying single bonds. The bonds from the central 4-member ring are 1.36 Å long with bond order of 1.77 – these are double bonds. NICS computations attest to the central ring (+4.5 ppm) being more like [4]radialene (with a NICS value of -2.6 ppm) than like cyclobutadiene (with a NICS value of +16.5 ppm). The 6-member rings fused to the central ring have NICS values of -2.33 ppm, suggesting a non aromatic character, while the outer rings have NICS values of -10.7ppm, similar to that of benzene. The structure is clearly of radialene form. Nonetheless, the central ring possess extremely pyramidalized carbons, as seen in Figure 1, and their π-orbital axis vector, a measure of the pyramidalization, is 107°, which is similar to the idealized tetrahedral value of 109.47°. Despite this stain, the molecule is thermally stable to 170°C and reacts only slowly with air or base. This molecule will surely inspire further work in the small circulenes.

1

1a

Fig 1. B3LYP/6-311G** structures of 1 and its parent 1a (lacking the TMS groups).1

References

(1) Bharat, R. B.; Bally, T.; Valente, A.; Cyranski, M. K.; Dobrzycki, L.; Spain, S. M.; Rempala, P.; Chin, M. R.; King, B. T., "Quadrannulene: A Nonclassical Fullerene Fragment," Angew. Chem. Int. Ed. 2009, DOI: 10.1002/anie.200905633

InChIs

1: InChI=1/C44H48Si4/c1-45(2,3)33-21-29-30(22-34(33)46(4,5)6)38-27-19-15-16-20-28(27)40-32-24-36(48(10,11)12)35(47(7,8)9)23-31(32)39-26-18-14-13-17-25(26)37(29)41-42(38)44(40)43(39)41/h13-24H,1-12H3
InChIKey=CDVRNAINHDQBCN-UHFFFAOYAM

1a: InChI=1/C32H16/c1-2-10-18-17(9-1)25-19-11-3-4-12-20(19)27-23-15-7-8-16-24(23)28-22-14-6-5-13-21(22)26(18)30-29(25)31(27)32(28)30/h1-16H
InChIKey=QTVPEOVCCYEZNL-UHFFFAOYAK

Aromaticity Steven Bachrach 01 Feb 2010 No Comments