Unusual conformational preference of an aromatic secondary urea: Solvent-dependent open-closed conformational switching of N,N′- bis(porphyrinyl)urea

Article abstract of DOI:10.1039/c2cc37583d

N,N'-Bis(2,3,7,8,12,13,17,18-octaethylporphyrin-5-yl)urea (3) exhibits cis-trans conformational switching depending on the solvent properties, and can adopt the cofacial porphyrin dimer structure under certain conditions. The conformational preference can

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Unusual conformational preference of an aromatic
secondary urea: solvent-dependent open-closed
conformational switching of
Cite this: DOI: 10.1039/c2cc37583d
N,N0-bis(porphyrinyl)urea†
Received 17th October 2012,
Accepted 19th November 2012
Mio Matsumura,a Aya Tanatani,*a Isao Azumaya,b Hyuma Masu,c
Daisuke Hashizume,d Hiroyuki Kagechika,e Atsuya Muranaka*d and
Masanobu Uchiyama*df
DOI: 10.1039/c2cc37583d
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N,N0-Bis(2,3,7,8,12,13,17,18-octaethylporphyrin-5-yl)urea (3) exhibits on the other hand, changes the preferred conformation from
cis–trans conformational switching depending on the solvent trans to cis, and N,N0-diaryl-N,N0-dimethylureas (2) exhibit
properties, and can adopt the cofacial porphyrin dimer structure intrinsic preference for (cis, cis) conformation with the two
under certain conditions. The conformational preference can be aromatic groups located in a face-to-face arrangement.5 These
switched by electrochemical stimulus.
conformational preferences of aromatic ureas have been utilized
to construct various foldamers and macrocycles with well-defined
Urea derivatives have become important building blocks in geometries.6 Although a wide variety of aromatic ureas have been
organic chemistry, supramolecular chemistry, and materials reported to date, the origin of the conformational preferences of
sciences.1 The conformations of urea units are significantly N,N0-diarylureas remains unclear. As a part of our work to better
affected by substituents on the nitrogen atoms, and the pla- understand the nature of the conformational preference of aro-
narity of the urea linkage leads to well-defined organization of matic secondary ureas, we were interested in aromatic secondary
the nitrogen substituents. N,N0-Diarylureas (1) generally adopt a ureas containing ‘‘stackable’’ units. As shown in Fig. 1b, we
(trans, trans) conformation both in solution and in the solid speculated that aromatic ureas would adopt both (cis, cis) and
state, although they have three possible conformations (trans, trans) conformations if p–p interactions are sufficiently
(Fig. 1a). This is noteworthy, because the X-ray structures of strong. Clayden and coworkers reported that for N,N0-diphenyl-
more than 200 N,N0-diarylureas without N-alkyl groups reveal urea (1), (trans, trans) conformation is favored over (cis, cis)
that almost all of them adopt (trans, trans) conformation in the conformation by 9.0 kJ molꢀ1.7 Hunter and Sanders estimated
solid state.2–4 Introduction of N-methyl group(s) to N,N0-diarylureas, the energy of porphyrin–porphyrin interactions in solution to be
48 ꢁ 10 kJ molꢀ1.8 The energetic consideration motivated us to
study the synthesis and conformational preference of N,N0-
a Department of Chemistry, Faculty of Science, Ochanomizu University,
bis(2,3,7,8,12,13,17,18-octaethylporphyrin-5-yl)urea (3, Fig. 2). To
our surprise, compound 3 did not take (trans, trans) geometry
2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan.
E-mail: tanatani.aya@ocha.ac.jp; Fax: +81-3-5978-2716
b Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri
University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan
E-mail: i-az@kph.bunri-u.ac.jp
c Chemical Analysis Center, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan. E-mail: masu@faculty.chiba-u.jp
d Advanced Technology Support Division and Advanced Elements Chemistry
Research Team, RIKEN-ASI, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.
E-mail: atsuya-muranaka@riken.jp; Fax: +81-48-467-2869
e Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental
University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan.
E-mail: kage.chem@tmd.ac.jp
f Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: uchiyama@mol.f.u-tokyo.ac.jp;
Fax: +81-3-5841-0732
† Electronic supplementary information (ESI) available: Details of syntheses,
NMR spectra, details of X-ray crystallography, UV/vis. spectra, and cyclic
voltammetry. CCDC 897914, 897915 and 900116. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc37583d
Fig. 1 (a) Conformational preferences in N,N0-diphenylureas. (b) Cis–trans
isomerism of secondary aromatic urea with stackable units.
c
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Conformational analysis of 3 was conducted by means of
variable-temperature 1H NMR spectroscopy (Fig. 4). Compound
3 showed one set of signals in THF-d8 even at 223 K. The two
porphyrin rings are nonequivalent, and one proton on the urea
nitrogen atom is observed at higher field (5.80 ppm) than the
other proton (9.55 ppm), which indicates that compound 3
exists in (trans, cis) form, as observed in crystal A. The 1H NMR
spectrum of 3 in CDCl3 at room temperature showed broad
peaks, and contained two sets of signals with a 92 : 8 ratio at
223 K. The signals of the major conformer correspond well to
those observed in THF-d8, and therefore the conformation can
be assigned as (trans, cis). The two singlets observed between
8.0 and 8.3 ppm are attributable to the two inequivalent meso
protons in the 4 : 2 ratio of the minor conformer. Since these
proton signals were shifted to higher field compared to those of
the major conformer (9.1–9.8 ppm), the minor conformer can
be assigned to the (cis, cis) form with cofacial porphyrin rings.
Fig. 2 Structures of porphyrinylurea 3 and 4.
under our experimental conditions. Either (trans, cis) or (cis, cis)
structure was found to be favored for 3, depending on the solvent
properties.
Compound
3 was synthesized from 2,3,7,8,12,13,17,18-
octaethylporphyrin in 7 steps (Scheme S1, ESI†), and was crystal-
lized into two crystals, depending on the recrystallization
solvents, one from THF/n-hexane (crystal A) and another from
CHCl3/n-hexane (crystal B), both of which were suitable for single
crystal X-ray structure analysis.† The molecular structure of 3 in
crystal A shows (trans, cis)-urea structure, with the two porphyrin
rings located in an L-shaped orientation (skewed structure,
Fig. 3a). In the crystal, the urea bonds (cis-NH–CO–) of the
adjacent molecules formed intermolecular hydrogen bonds to
form the dimeric structure (Fig. S4, ESI†). The dihedral angle
between the two planes defined by the four nitrogen atoms of the
porphyrin rings is 1111. On the other hand, the molecular
structure of 3 in crystal B has a (cis, cis)-urea structure, with
the two porphyrin rings located in face-to-face positions (Fig. 3b).
The two porphyrins are almost parallel, and the dihedral angle
between the two planes of the four nitrogen atoms is 2.41. The
mean separation of the two planes is 3.47 Å. This value is close to
those reported for other cofacial porphyrin dimers, for example,
3.36 Å for a cis-ethene-bridged dimer,9 3.43 Å for a 1,2-phenylene-
bridged dimer,10 and 3.45 Å for a biphenylenediyl-bridged
dimer.11 These results suggest that similar p–p interactions
between porphyrin rings would exist in these dimers. It is
significant that compound 3, with a more flexible urea bond
than the linking group of other porphyrin dimers, showed a
cofacial porphyrin structure, in spite of the generally unfavorable
(cis, cis) conformation of the secondary urea bond.
Thus, the results suggested that compound
3 changes
its conformational equilibrium depending upon the solvent
properties. Since solvent-dependent (cis, cis) conformational
preference is not observed in simple N,N0-diarylureas such as
1, we examined the conformation of the reference compound 4
with one porphyrin ring. The crystal structure of 4 obtained
from THF showed (trans, trans) conformation, like that of 1
1
(Fig. S7, ESI†). The H NMR spectrum of 4 in THF-d8 at 223 K
showed an equilibrium between two conformers (3 : 1), while
that in CDCl3 showed only one set of proton signals (Fig. S2,
ESI†). In both cases, no aromatic proton signal is shifted to
higher field. Comparison of the chemical shifts of the aromatic
protons indicated that the major conformer in THF-d8 and
CDCl3 is (trans, cis), and the minor conformer in THF-d8 is
(trans, trans), corresponding to the crystal structure. Thus, the
solvent-dependent (cis, cis) conformational preference may be
caused by intramolecular interaction between the two por-
phyrin rings, as our energetic consideration mentioned above.
Cofacial porphyrin dimers are known to have distinctly
different UV spectra compared with those of their parent single
porphyrins.12 Thus, the UV-vis spectrum of compound 3 was
measured in order to investigate the solvent-induced conforma-
tional change in solution. Compound 3 exhibited a split Soret
band (400 and 414 nm) in THF (red curve in Fig. 5a). Such
splitting of the Soret band is associated with excitonic interactions
between two porphyrin chromophores, since compound 4, bearing
one porphyrin ring, has a sharp single Soret band at 400 nm
(Fig. 5b).13 The peak positions of 3 are similar to those of
Fig. 3 Crystal structures of 3 (a) in crystal A and (b) in crystal B. The solvent
molecules are omitted.
Fig. 4 1H NMR spectra (aromatic region) of 3 in (a) THF-d8 and (b) CDCl3 at 223 K.
c
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porphyrin dimers, which can be detected easily by UV spectro-
scopy, may find applications in solvent-controlled optical
molecular devices. We believe that the results of the present
study have important implications for the complete under-
standing of the mechanisms of conformational preferences in
N,N0-diarylureas.
Fig. 5 UV/vis spectra of (a) 3 (7.1 ꢂ 10ꢀ6 M) and (b) 4 (6.3 ꢂ 10ꢀ6 M) in THF/
n-hexane. The ratio of n-hexane is 0 (red), 50 (purple), 70 (blue) and 90% (green).
The insets show the enlarged view of the Soret band.
Notes and references
1 (a) N. Volz and J. Clayden, Angew. Chem., Int. Ed., 2011, 50, 12148;
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2 In this study, trans/cis nomenclature is used for the conformations
of N,N0-diarylurea bonds, based on the spatial relationship between
N-aryl group and carbonyl oxygen atom, as shown in Fig. 1a.
3 The results of our search on the CCDC database (WebCSD v.1.1.1,
Feb. 2012 version): F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci.,
2002, 58, 380.
trans-ethene-bridged bis(nickel octaethylporphyrin).14 Interest-
ingly, the intensity of the Soret band of compound 3 in THF
gradually decreased as the solvent ratio of n-hexane in THF was
increased, and a blue-shifted Soret band appeared at 388 nm when
the ratio of n-hexane : THF reached 90 : 10. Since no significant
spectral change was observed when n-hexane was added to a
solution of the porphyrin monomer (4) in THF, the spectral change
observed for compound 3 can be attributed to an intramolecular
conformational change from the extended form to the (cis, cis)
form with cofacial porphyrin rings. This assignment is supported
by the fact that the spectral pattern of 3 closely resembles that of
cis-ethene-bridged bis(nickel octaethylporphyrin), which has a
face-to-face arrangement.14 A similar solvent-induced spectral
change of 3 was observed using the CHCl3/n-hexane system as a
solvent (Fig. S8, ESI†). The observed spectral changes can be
explained as follows: the secondary urea-bridged bisporphyrin (3)
preferentially adopts the (trans, cis) form in THF or CHCl3 solution.
With increasing amounts of n-hexane, the (cis, cis) form becomes
favored, with the two porphyrin rings in the face-to-face arrange-
ment. This was confirmed by 1H NMR analysis of 3, in which the
ratio of (cis, cis) conformer in CDCl3-n-C6D14 (7 : 3) was larger than
that in CDCl3 (Fig. S9, ESI†). We believe this is the first spectral
evidence that the preferred conformation of an acyclic aromatic
secondary urea without an N-alkyl group can be switched from
(trans, cis) to (cis, cis).
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of the secondary urea bond, which is unfavorable for simple
N,N0-diarylureas without an N-alkyl group. We also found that
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