Expandability of Ultralong CC Bonds: Largely Different C1C2 bond lengths determined by low-temperature x-ray structural analyses on Pseudopolymorphs of 1,1-Bis(4-fluorophenyl)-2,2-bis(4-methoxyphenyl)pyracene

Article abstract of DOI:10.1246/cl.130872

Low-temperature X-ray analyses on several pseudopolymorphs (solvate crystals) revealed that the C1C2 bond length of the highly congested title molecule can adopt quite different values [1.700(6)1.739( 6)]. Such an unusual observation indicates that the ultralong covalent bond is endowed with expandability, thus the prestrained bond can be elongated or shortened very easily accompanied by only a minute change in energy, which can be compensated by intermolecular perturbation in the crystal.

Full text of DOI:10.1246/cl.130872

CL-130872
Received: September 21, 2013 | Accepted: September 30, 2013 | Web Released: October 9, 2013
Expandability of Ultralong C-C Bonds: Largely Different C1-C2 Bond Lengths Determined
by Low-temperature X-ray Structural Analyses on Pseudopolymorphs
of 1,1-Bis(4-fluorophenyl)-2,2-bis(4-methoxyphenyl)pyracene
Takanori Suzuki,*1 Yasuto Uchimura,1 Fumika Nagasawa,1 Takashi Takeda,1,³ Hidetoshi Kawai,1,³³ Ryo Katoono,1
Kenshu Fujiwara,1 Kei Murakoshi,1 Takanori Fukushima,2 Aiichiro Nagaki,3 and Jun-ichi Yoshida3
1Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503
3Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Kyoto 615-8510
(E-mail: tak@mail.sci.hokudai.ac.jp)
X
Y
Low-temperature X-ray analyses on several pseudopoly-
morphs (solvate crystals) revealed that the C1-C2 bond length of
the highly congested title molecule can adopt quite different
values [1.700(6)-1.739(6) ¡]. Such an unusual observation
indicates that the ultralong covalent bond is endowed with
“expandability,” thus the prestrained bond can be elongated or
shortened very easily accompanied by only a minute change in
energy, which can be compensated by intermolecular perturba-
tion in the crystal.
Me
Me
N
N
d
C1
C2
X
Y
a
: X = F; Y = OMe
b: X = F; Y = H
c
: X = F; Y = Cl
d: X = F; Y = Me
1
DSAP
Zn
C6H4Y-4
C6H4Y-4
4-XC6H4
4-XC6H4
4-XC6H4
C6H4Y-4
O
4-XC6H4
C6H4Y-4
Br Br
BuLi
diaryl ketones
TMSClO4
or TfOH
Not only from curiosity toward molecules with unusual
structural parameters1 but also from the viewpoint of the
development of potential functions,2,3 considerable recent
attention has been focused on ultralong C-C bonds with bond
lengths beyond 1.70 ¡ (standard: 1.54 ¡4). They are rare but
have actually been found in highly congested organic com-
pounds such as condensed hexaphenylethanes5 or caged-alkane
dimers.6 Based on rational design7 to stabilize the compounds
despite the presence of the C-C bond with a very small bond
dissociation energy (BDE), we succeeded in obtaining di(spiro-
acridan)pyracene (DSAP) with the longest C-C bond,8 whose
bond length [1.791(3) ¡ at 413 K; 1.771(3) ¡ at 93 K] is as large
as the distance of the shortest nonbonded C£C contact
[1.80(2) ¡].9
During the course of our further studies, we have found that
a series of 1,1,2,2-tetraarylpyracenes 1 having a common
hydrocarbon skeleton with DSAP can serve as an ideal platform
for scrutinizing the novel properties of extremely long C-C
bonds. The “front strain”10 among the four aryl groups in 1 is
large enough to expand the C1-C2 bond beyond 1.70 ¡ as
determined precisely (esd: 0.002-0.004 ¡) [e.g., 1F4: X =
Y = F, d = 1.761(4) ¡; 1H2-OMe2: X = H, Y = OMe, d =
1.714(2) ¡].2b,11 Although we did not get any indication that
suggests the electronic nature of different aryl groups affecting
the C1-C2 bond length (d), a wide variation of bond length (¦d:
0.047 ¡) was observed for a series of pyracenes 1, which cannot
be explained by considering intramolecular factors.
flow microreactor method
Scheme 1.
pseudopolymorphs,12 bis(4-fluorophenyl) derivatives 1a-1d
(X = F) were designed in this work. We found here that the
title compound 1a (X = F; Y = OMe) forms many pseudo-
polymorphs by incorporating a variety of solvent molecules in
the crystal lattice, each of which contains molecule(s) of 1a with
a different C1-C2 bond length (¦d: 0.039 ¡) as we expected.
The details will be shown herein.
By following a procedure we developed previously,2b,11
sterically hindered tetraarylpyracenes 1a-1d13 were prepared
from 5,6-dibromoacenaphthene and isolated as stable crystalline
solids (Schemes 1 and S114). The key step is the selective
introduction of two different diarylmethyl groups,15 which was
successfully realized by reaction integration using flow micro-
reactors.16 Upon recrystallization of 1a-1d from CHCl3-hexane
or CH2Cl2-hexane, single-crystalline samples suitable for X-ray
structural analyses were successfully obtained.14,17 Absence of
any electronic effects of aryl substituents on d was again verified
by the nonadditive substituent effects for 1b-1d18,19 [d =
1.740(3), 1.730(4), and 1.722(4) ¡, respectively]. Thus, the
d values are quite different from those expected from the
corresponding symmetric derivatives [1H4: X = Y = H, d =
1.754(2) ¡; 1Cl4: X = Y = Cl, d = 1.730(2) ¡; 1Me4: X = Y =
Me, d = 1.717(4) ¡]2b,11 (Figure S1).14
To confirm that the crystallographically determined d value
is largely affected by the intermolecular perturbation (e.g.,
crystal packing force), we planned to conduct X-ray analyses
of the polymorphs or pseudopolymorphs (solvated crystals) of a
certain derivative of 1, in which only the intermolecular factors
are responsible for the ¦d value, if observed. By considering
that fluorinated aromatic compounds often form polymorphs or
Different from 1b-1d or 1F4,2b 1a gave pseudopolymorphs
by including an equal number of solvent molecules in the unit
cell [1a¢CHCl3 and 1a¢CH2Cl2; isomorphous, P21/c, Z = 4].
Not only halogenated solvents but also ether and hexane were
found to be incorporated in the crystal lattice [(1a)2¢ether and
(1a)2¢hexane; isomorphous, Cc, Z = 4, two independent mole-
86 | Chem. Lett. 2014, 43, 86–88 | doi:10.1246/cl.130872
© 2014 The Chemical Society of Japan

Table 1. The d values in the pseudopolymorphs of 1a determined by X-ray analyses at 123 K along with the crystal dataa
Solvent
CHCl3
CH2Cl2
THF
1:1
CCl4-hexane
Ether
2:1
Hexane
2:1
Ratio
1:1
1:1
2:1 (CCl4)
4:1 (hexane)
1.735(7)
(1a:solvent)
C1-C2 bond
length (d/¡)
1.718(3)
1.700(6)
1.734(3)
1.732(7)
1.706(7)
1.739(6)
1.727(6)
1.718(7)
ꢀ
P1
ꢀ
P1
Space group
a/¡
b/¡
c/¡
¡/°
P21/c
P21/c
Cc
Cc
12.624(2)
22.527(4)
12.030(2)
90
102.014(2)
90
12.562(11)
22.28(2)
12.062(10)
90
102.39(2)
90
11.819(4)
12.132(3)
12.210(4)
78.759(12)
83.352(12)
75.349(10)
1657.3(8)
2
11.976(5)
12.469(4)
24.139(9)
71.90(3)
79.04(3)
78.22(3)
3323(3)
4
19.255(9)
15.420(7)
22.907(11)
90
109.521(7)
90
19.157(11)
15.524(8)
23.456(13)
90
108.802(6)
90
¢/°
£/°
V/¡3
Z
3346.1(10)
4
3297(5)
4
6410(5)
8
6603(6)
8
aThe latter three crystals contain two crystallographically independent molecules in the unit cell.
All the above crystallographic results demonstrate high
susceptibility of the d value to the intermolecular perturbation.
In general, the covalent bond length is the parameter with a high
uniformity since the slight change in length causes considerable
loss of BDE. In the case of a prestrained bond20 with much
smaller BDE, further expansion-contraction of the bond would
occur with only slight loss of BDE, which accounts for the
“expandability” in the ultralong covalent bond such as the C1-C2
bond of 1.
(a)
(b)
(c)
To obtain theoretical support for the “expandability,”
structural optimization (B3LYP/6-31G*) of tetraphenylpyracene
(1H4: X = Y = H) was conducted under the full-optimization
and constrained conditions. The d value in the fully optimized
1H4 was estimated to be 1.763 ¡, which is close to the
experimental value [1.754(2) ¡]. When the geometric optimiza-
tion processes were performed by constraining d at 13 different
values between 1.700-1.820 ¡, the scattering plot of the relative
energy vs. d suggests a parabolic relationship (Figure 2). The
characteristic feature is the very shallow potential curve with the
energy difference of 0.35 kcal mol¹1. Such a small amount of
energy-loss upon deviation of d from that of the optimized
structure can be easily compensated by the different crystal
packing force among pseudopolymorphs,21 which rationalizes
the observation of a variety of d values in the pseudopolymorphs
of 1a.
Figure 1. ORTEP drawings of tetraarylpyracene 1a in (a)
THF, (b) CHCl3, and (c) CH2Cl2 (1:1) solvate crystals
determined by X-ray analyses at 123 K. One chlorine atom in
(c) suffers from positional disorder.
cules of 1a]. The 1a¢THF solvate crystal adopts another space
ꢀ
group [P1, Z = 2], whose packing arrangement is similar to
ꢀ
(1a)2¢CCl4¢(hexane)0.5 [P1, Z = 2, two independent molecules
of 1a; hexane on the center of symmetry] but not to others.
Differences in the molecular ratio and the space group indicate
that the inclusion lattices formed by 1a in crystal have several
variations. Despite facile formation of pseudopolymorphs as
above, single crystals of unsolvated 1a were not obtained.
In accord with our assumption of “bond expandability” that
the prestrained bond can be elongated or shortened very easily,
structural refinement of the above six pseudopolymorphs of
1a14,19 revealed a considerable variation of C1-C2 bond length
[¦d: 0.039 ¡] although there are no direct and/or short contacts
between solvent molecules and the C1/C2 atom of 1a. The three
crystals including the solvent molecule in a 1:1 ratio (THF,
CHCl3, and CH2Cl2) have d values of 1.734(3), 1.718(3), and
1.700(6) ¡, respectively (Figure 1). It is interesting to note that
CHCl3 solvate and CH2Cl2 solvate are isomorphous, but the
packing force afforded different degrees of perturbation of 1a to
adopt different d values. This is also the case for another pair
of (1a)2¢ether [d: 1.732(7) and 1.706(7) ¡] and (1a)2¢hexane
[1.739(6) and 1.727(6) ¡] (Table 1). Moreover, the two crystal-
lographically independent molecules in (1a)2¢ether have the
quite different bond lengths (¦d: 0.026 ¡) despite being packed
in the same crystal.
Finally, for further validation of the d value determined
crystallographically on the ultralong C-C bonds, Raman spec-
troscopy was applied to the crystalline samples of difluoro (1b:
X = F, Y = H) and tetrafluoro (1F4) derivatives. The Raman
frequencies for the stretching vibration of the long C1-C2 bond
were compared with that of unsubstituted compound 1H4 studied
before.2b The values22 are 643 (1b), 640 (1H4), and 638
¹1
(1F4) cm (Figure S2),14 which are largely shifted compared
with that of ethane itself (995 cm¹1). Thus, the C1-C2 bond in 1
has a much smaller force constant than the ordinary C-C bonds.
Although the difference in the Raman frequency is not large
among 1b, 1H4, and 1F4, the observed values decrease in the
increasing order of the experimental d values [1.740(3),
1.754(2), and 1.761(4) ¡, respectively], suggesting that the
crystallographically determined bond lengths on the ultralong
Chem. Lett. 2014, 43, 86–88 | doi:10.1246/cl.130872
© 2014 The Chemical Society of Japan | 87

Sect. C: Cryst. Struct. Commun. 1996, 52, 177. b) F. Toda, K.
Tanaka, M. Watanabe, K. Tamura, I. Miyahara, T. Nakai, K.
Hirotsu, J. Org. Chem. 1999, 64, 3102. c) K. Tanaka, N.
Takamoto, Y. Tezuka, M. Kato, F. Toda, Tetrahedron 2001, 57,
3761. d) F. Toda, K. Tanaka, N. Takamoto, Tetrahedron Lett.
2001, 42, 7979. e) K. K. Baldridge, Y. Kasahara, K. Ogawa,
J. S. Siegel, K. Tanaka, F. Toda, J. Am. Chem. Soc. 1998, 120,
6167.
6
P. R. Schreiner, L. V. Chernish, P. A. Gunchenko, E. Y.
Tikhonchuk, H. Hausmann, M. Serafin, S. Schlecht, J. E. P.
Dahl, R. M. K. Carlson, A. A. Fokin, Nature 2011, 477, 308.
H. Kawai, T. Takeda, K. Fujiwara, M. Wakeshima, Y. Hinatsu,
T. Suzuki, Chem.®Eur. J. 2008, 14, 5780.
7
8
The bond of 1.781(15) ¡ in a silabicyclobutane derivative had
previously been the longest among the nonionic organic
molecules: G. Fritz, S. Wartanessian, E. Matern, W. Hönle,
H. G. V. Schnering, Z. Anorg. Allg. Chem. 1981, 475, 87.
J. L. Adcock, A. A. Gakh, J. L. Pollitte, C. Woods, J. Am.
Chem. Soc. 1992, 114, 3980.
Figure 2. Scattering plot of relative energy (E) of tetraphen-
ylpyracene 1H4 vs. d estimated by DFT calculations (B3LYP/
6-31G*).
9
10 B. Kahr, D. Van Engen, K. Mislow, J. Am. Chem. Soc. 1986,
108, 8305.
C-C bond are actually related to the properties of the C-C bond
in question (e.g., stretching vibration or force constant).
Studies are now in progress toward noncrystallographic
demonstration (e.g., 13C NMR) of the bond expandability as well
as proof for facile bond expansion-contraction in solution.
11 T. Suzuki, Y. Uchimura, Y. Ishigaki, T. Takeda, R. Katoono,
H. Kawai, K. Fujiwara, A. Nagaki, J.-i. Yoshida, Chem. Lett.
2012, 41, 541.
12 a) S. Terada, K. Katagiri, H. Masu, H. Danjo, Y. Sei, M.
Kawahata, M. Tominaga, K. Yamaguchi, I. Azumaya, Cryst.
Growth Des. 2012, 12, 2908. b) G. Kaur, P. Panini, D. Chopra,
A. R. Choudhury, Cryst. Growth Des. 2012, 12, 5096. c) A.
Schwarzer, E. Weber, Cryst. Growth Des. 2008, 8, 2862.
13 Experimental details and physical data for new compounds are
given in Supporting Information (ref 14).
14 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
15 a) A. Nagaki, N. Takabayashi, Y. Tomida, J.-i. Yoshida, Org.
Lett. 2008, 10, 3937. b) A. Nagaki, N. Takabayashi, Y.
Tomida, J.-i. Yoshida, Beilstein J. Org. Chem. 2009, 5, 16.
c) Y. Ishigaki, T. Suzuki, J.-i. Nishida, A. Nagaki, N.
Takabayashi, H. Kawai, K. Fujiwara, J.-i. Yoshida, Materials
2011, 4, 1906.
This work was performed by Grant-in-Aid for Scientific
Research on Innovative Areas: “Organic Synthesis Based on
Reaction Integration” (No. 2105) from MEXT, Japan. TS thanks
JSPS Grant-in-Aid for Challenging Exploratory Research on
“Maximum Function on Minimum Skeleton (MFMS)”
(No. 25620050). This work was also supported by the Cooper-
ative Research Program of “Network Joint Research Center for
Materials and Devices.”
In memory of Dr. Chizuko Kabuto.
References and Notes
16 a) H. Usutani, Y. Tomida, A. Nagaki, H. Okamoto, T. Nokami,
J.-i. Yoshida, J. Am. Chem. Soc. 2007, 129, 3046. b) A.
Nagaki, Y. Tomida, H. Usutani, H. Kim, N. Takabayashi, T.
Nokami, H. Okamoto, J.-i. Yoshida, Chem.®Asian J. 2007, 2,
1513. c) S. Suga, D. Yamada, J.-i. Yoshida, Chem. Lett. 2010,
39, 404. d) J.-i. Yoshida, K. Saito, T. Nokami, A. Nagaki,
Synlett 2011, 1189.
³
Present address: Institute for Multidisciplary Research for
Advanced Materials, Tohoku University, Sendai, Miyagi 980-
8577
³³ Present address: Department of Chemistry, Faculty of Science,
Tokyo University of Science, Shinjuku 162-8601
1
a) G. Kaupp, J. Boy, Angew. Chem., Int. Ed. Engl. 1997, 36,
48. b) J. S. Siegel, Nature 2006, 439, 801. c) T. Suzuki, T.
Takeda, H. Kawai, K. Fujiwara, in Strained Hydrocarbons:
Beyond the van’t Hoff and Le Bel Hypothesis, ed. by H.
Dodziuk, Wiley-VCH, 2009, Chap. 2.5. doi:10.1002/
9783527627134.ch2. d) T. Suzuki, T. Takeda, H. Kawai, K.
Fujiwara, Pure Appl. Chem. 2008, 80, 547. e) T. Takeda, Y.
Uchimura, H. Kawai, R. Katoono, K. Fujiwara, T. Suzuki,
Chem. Lett. 2013, 42, 954.
17 Crystal data and ORTEP drawings are given in Supporting
Information (ref 14).
18 X-ray analyses on unsolvated crystals of 1b-1d were con-
ducted at 123 K. They all have an extremely long C1-C2 bond,
and the ORTEP drawings show no anomalies in thermal
ellipsoids of the long C1-C2 bond or in electron density maps.
19 Details of crystallographic analyses on pseudopolymorphs of
1a (THF, CHCl3, CH2Cl2, hexane, CCl4-hexane, and ether)
and those of 1b-1d were deposited (CCDC 962022-962030).
20 a) K. Wada, T. Takeda, H. Kawai, R. Katoono, K. Fujiwara,
T. Suzuki, Chem. Lett. 2013, 42, 1194. b) E. Ōsawa, P. M.
Ivanov, C. Jaime, J. Org. Chem. 1983, 48, 3990.
21 J. D. Dunitz, J. Bernstein, Acc. Chem. Res. 1995, 28, 193.
22 The values were determined by using the same equipment to
reduce the machine-dependent errors. The absorption in 1H4
with the highest intensity (expt. 640 cm¹1) corresponds to the
normal mode with the largest amplitude along the trajectory
of the stretching vibration as verified by DFT calculation
(B3LYP/6-31G* (ref 2b)).
2
3
a) H. Kawai, T. Takeda, K. Fujiwara, T. Inabe, T. Suzuki,
Cryst. Growth Des. 2005, 5, 2256. b) T. Takeda, H. Kawai,
R. Herges, E. Mucke, Y. Sawai, K. Murakoshi, K. Fujiwara, T.
Suzuki, Tetrahedron Lett. 2009, 50, 3693.
Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H.
Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M.
Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, K. Nakasuji,
Nat. Mater. 2008, 7, 48.
4
5
F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
Orpen, R. Taylor, J. Chem. Soc., Perkin Trans. 2 1987, S1.
a) F. Toda, K. Tanaka, Z. Stein, I. Goldberg, Acta Crystallogr.,
88 | Chem. Lett. 2014, 43, 86–88 | doi:10.1246/cl.130872
© 2014 The Chemical Society of Japan