O...π and O-H...π interactions: The first disclosure of the nature of 1,3,4-oxadiazol...aromatic contacts

Article abstract of DOI:10.1039/c2ce26224j

A new type of O...π interaction has been found and is rationalized by Moller-Plesset second-order perturbation theory calculations. LMOEDA energy decomposition reveals that it is mainly dominated by electrostatic and dispersion components; the origin of the electrostatic component is illustrated by the molecular electrostatic potential.

Full text of DOI:10.1039/c2ce26224j

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COMMUNICATION
… …
O p and O–H p interactions: the first disclosure of the nature of
…
1,3,4-oxadiazol aromatic contacts{
Li Wang,{ae Zhenfeng Zhang,{*b Yibo Wang,c Yanbo Wud and Shengyong Zhang*a
Received 31st July 2012, Accepted 12th September 2012
DOI: 10.1039/c2ce26224j
…
A new type of O p interaction has been found and is
that compounds containing the 1,3,4-oxadiazole fragment have
been identified to exhibit diverse biological activities,5 and the
properties are often closely related with the noncovalent interac-
tions involving the 1,3,4-oxadiazole ring. In order to disclose the
noncovalent interacting motifs preferred by the oxadiazole ring, we
have synthesized a series of compounds of this kind, and examined
their X-ray crystal structures.
rationalized by Møller–Plesset second-order perturbation the-
ory calculations. LMOEDA energy decomposition reveals that
it is mainly dominated by electrostatic and dispersion compo-
nents; the origin of the electrostatic component is illustrated by
the molecular electrostatic potential.
…
Interestingly, we have discovered a new type of O p interaction
The ability of the p-cloud of benzene to act as a hydrogen bond
acceptor has been shown both theoretically and experimentally.1
In these cases, the phenyl p-cloud interacts with an electron-
deficient atom, a hydrogen attached to an electronegative atom. As
a general principle underlying hydrogen bond formation,2 a strong
hydrogen bond donor prefers to combine with a strong hydrogen
bond acceptor, and a weak to pair off with a weak. Therefore, it is
occurring between the oxadiazol oxygen and phenyl ring in
methyl-(2R,3S)-2-hydroxy-3-phenyl-3-{[(5-phenyl-1,3,4-oxadiazol-
2-yl)-carbonyl]amino}propanoate, 1 (Fig. 1).6 As is shown in
Fig. 1, the oxadiazol atom O1* lies above the C1–C6 ring; the
˚
distance of O1* to the centroid is only 3.344(2) A, which is
markedly shorter than the upper limit of the sum of van der Waals
7
…
˚
˚
radii, 3.42 A (the phenyl ring half-thickness is taken as 1.7–1.9 A).
The atom O1*, we think, acts as a p-electron acceptor and the
p-cloud of the C1–C6 ring acts as a p-electron donor. Despite a
strong electron-withdrawing inductive effect, O1* still exhibits
not surprising that a large number of C–H p hydrogen bonds
…
have been reported in recent times. By contrast, the O–H p (as
…
well as N–H p) hydrogen bonds involving benzene p-electrons
are relatively scarce in the literature.3
…
…
…
The C p, N p and O p interactions of the non-hydrogen
bond type are even more scarce,4 because unlike the H atom in the
…
H
p system, C, N or O atoms are usually not sufficiently
electron-deficient to serve as p-electron acceptors. However, the
presence of strong electron-donating group(s) in the aromatic ring
should lead to a substantial increase in p-electron density, thus the
reinforced phenyl p-cloud may obtain the ability to interact with a
…
4
non-hydrogen electron-deficient atom. In this situation, C p,
…
…
p and O p interactions, though rare, can actually occur.
N
Our interest in the 2-hydroxy-3-phenyl-3-{[(5-phenyl-1,3,4-
oxadiazol-2-yl)carbonyl]amino}propanoates stems from the fact
aDepartment of Chemistry, School of Pharmacy, Fourth Military
Medical University, Xian, P. R. China
bCollege of Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang 453007, P. R. China. E-mail: zzf5188@sohu.com;
Fax: +86-373-3326336
cDepartment of Chemistry, Guizhou University, Key Laboratory of
Guizhou High Performance Computational Chemistry, Guiyang 550025,
P. R. China
dInstitute of Molecular Science, Shanxi University, Taiyuan, 030006,
Shanxi, P. R. China
Fig. 1 The partial packing diagram of 1, showing the formation of a
eDepartment of Chemistry, School of Preclinical Medicine, Shanxi
Medical University, 56 Xinjian, South Road, 030001 Tai Yuan,
P. R. China
…
…
single O p interaction between O1* and the C1–C6 ring with an O Cg
˚
distance of 3.344 A (Cg is the centroid of the C1–C6 ring). For the sake of
clarity, the C11–C16 ring and H atoms not involved in the motif shown
have been omitted. Atoms marked with an asterisk (*) are at the symmetry
position: 1/2 + x, 1/2 2 y, 1 2 z.
{ CCDC 859574. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2ce26224j
{ Z. Zhang and L. Wang contributed equally to this work.
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relatively lower electron density due to its p-electron delocalization
over the whole oxadiazol ring; the higher density of p-electrons on
the oxadiazol ring is further delocalized over the bonded C1–C6
ring which possesses relatively low p-electron density, thus leading
to a decrease in p-electron density on oxadiazol oxygen and an
increase in electron density on the C1–C6 ring. The feature of
…
charge distribution facilitates formation of the O p electrostatic
interaction.
…
Recently, cation p interactions, as typical representatives of
electrostatic interactions, have been studied extensively by both
experimentalists8–10 and theorists.11–13 The results shows that
…
cation p interactions were energetically favourable and energy
minima were found when the cation is oriented along the principal
axis of the ring.
The atom O1* in compound 1, however, deviates by 16.7u from
being directly above the centre of the C1–C6 ring (Fig. 1). This
large deviation is mainly attributed to the steric hindrance
generated by the structural fragment bonded to C10, as well
Fig. 2 The dimeric structure of 2-phenyl-1,3,4-oxadiazole optimized at
WB97XD/6-311+G(d,p) level, showing the formation of a pair of
…
equivalent O p interactions.
…
as the C4*–H4* O4 interaction (H4* O4 = 2.44 A and
…
˚
…
polarization, and dispersion components. The LMOEDA
computations are performed in GAMESS at the MP2/maug-
cc-pVTZ level,15 and the results are summarized in Table 1. As the
C4*–H4* O4 = 159u). In addition, the steric hindrance and the
…
C4*–H O4 interaction, also prevents C1*–C6* ring from forming
…
an O p interaction with the O1 atom; the distance of O1 to
˚
C1*–C6* ring centroid is up to 3.675 A.
…
table shows, the O p interaction is mainly dominated by two
components: dispersion energy and electrostatic energy, which
contribute 62% and 31%, respectively, to the total dimerization
energy (211.87 kcal mol21).
If the above viewpoints are true, it should be probable to find a
…
pair O p interactions in 2-phenyl-1,3,4-oxadiazole, where the
…
steric factors and the C–H O interaction are removed.
We have also calculated and analyzed the electrostatic potential
(ESP) of the 2-phenyl-1,3,4-oxadiazole monomer to further probe
the origin of the electrostatic component. It is well known that
Keeping our motivation in mind, we replace C9 (Fig. 1) and the
bonded structural fragment with a H atom to build the monomer
and dimer of 2-phenyl-1,3,4-oxadiazole, the model system, for the
…
there is a negative ESP above the centroid of the C1–C6 ring. For
…
evaluation of O p interactions. To accurately investigate the
model system, a GGA-type density functional (WB97XD)14 with
long-range and semiempirical dispersion corrections was employed
in combination with 6-311+G(d,p) basis set used to geometry
optimization, and Møller–Plesset second-order perturbation the-
ory MP2 with a large maug-cc-pvTZ15 basis set including the basis
set superposition error (BSSE) correction used to dimerization
energy calculations. All the calculations were carried out by using
GAUSSIAN 0916a and GAMESS16b packages. At the selected
theoretical level, both the monomer and dimer of 2-phenyl-1,3,4-
oxadiazole are found to be the energy minima. The optimized
structures are shown in Fig. 2. As the figure shows, the core
O
p electrostatic bonding, the oxadiazole oxygen is expected to
show a positive ESP value for complementarity to the benzene
ring. Fig. 3 confirms this expectation showing positive ESP for the
oxadiazole oxygen. The electrostatic potential features are well
supported by the DFT-based Mulliken population analysis. The
representative result of charge distribution is illustrated in Fig. 4.
As can be seen from the figure, among all the oxadiazole ring
atoms, the atom O1 carries the largest Mulliken positive charge
(+0.157), and the benzene ring as a whole (not including
hydrogens) carries highest negative charge (21.152). Thus, the
features in electrostatic potential and charge distribution have well
…
…
structure of the O p system has changed significantly with the
clarified the origin of the electrostatic component of the O
p
interaction. Though the electrostatic interaction does not solely
dominate the total interaction energy, the result validates our
foregoing hypothesis that the oxadiazole oxygen in compound 1
exhibits relatively lower electron density due to its p-electron
delocalization.
substitution of bulky blocks with H atoms, and exhibits a pair of
…
equivalent O p interactions along the principal axis. The O1
atom lies within 7u of being directly below the centroid of C1*–C6*
˚
ring, and the distance of O1 to the centroid is 3.328 A. The dimer
geometry is in very good accordance with what we expected. The
total dimerization energy is 211.87 kcal mol21, showing that the
…
…
In order to further probe the generality of the O p interaction
in aromatic 1,3,4-oxadiazoles, we conducted a systematic search of
the CSD for structures that match the fragment shown in
Scheme 1. The two restrictions applied were that the distance (d)
O
p interaction is thermodynamically favorable. The results
validates our foregoing hypothesis that the steric hindrance does
…
prevent O1 from forming an efficient O p interaction with the
C1*–C6* ring in 1, and have also presented compelling evidence
…
that O p interaction(s) can actually occur between phenyl ring
Table 1 LMOEDA binding energy decomposition at MP2/maug-cc-
pVTZ level (kcal mol21 a
)
and 1,3,4-oxadiazol oxygen atom despite presence of steric
DEele
DEex+rep
DEdisp
DEpol
DEcp
hindrance in some cases.
…
To clarify the nature of the O p interaction, we employed the
28.34
a
15.23
216.86
21.91
211.87
localized molecular orbital energy decomposition analysis
…
(LMOEDA) approach of Su and Li17 to decompose the O
p
ele
dispersion, pol = polarization, and cp = total.
=
electrostatic, ex
=
exchange, rep
=
repulsion, disp
=
interaction energy into electrostatic, exchange, repulsion,
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Fig. 3 MP2/maug-pp-pVTZ electrostatic potential for the 2-phenyl-
1,3,4-oxadiazole monomer. The blue region represents the positive part of
the electrostatic potential, and the red region the negative part of the
electrostatic potential.
…
Scheme 1 Geometric parameters defining the O p interactions covered
in this work.
…
O
p interactions also exist in furan derivatives, but they are
weaker than those in phenyl-substituted 1,3,4-oxadiazoles.
…
between the centroid of phenyl ring and the oxadiazol oxygen
˚
atom be equal or less than 3.76 A and that the angle, a, defined by
This kind of O p interaction, despite its widespread existence,
…
is not reported so far in the literature. Though a number of O
p
the vector perpendicular to the phenyl ring and the vector passing
interactions have been reported recently,18 their nature is nothing
…
through the centroid to the oxygen atom, be equal or less than 20u.
˚
The 3.76A cut-off was chosen based on the van der Waals radius
but a lone-pair (lp) p interaction. In aryl 1,3,4-oxadiazoles, the
…
O
p interaction occurs between the electron-deficient oxadiazol
…
˚
of oxygen, taken as 1.52 A, and the phenyl ring half-thickness of
oxygen and the p-electron-rich phenyl ring, distinctly not a lp
p
7
1.7–1.9 A. The liberal value of 3.76 A was calculated as 1.52 + 1.9
˚
˚
interaction.
…
plus 10%, to capture all potential O p interactions. This search
We have also probed the vibrational characteristics of the
oxadizole ring in 1 in an attempt to observe this interaction in
solution. IR spectroscopic measurements of the stretching
frequency of the oxadizole in the crystalline material and in
solutions of acetone and acetonitrile did not show any dramatic
changes. The FTIR spectra of the crystalline sample in KBr
showed the characteristic vibrations at 1548 cm21. In acetone and
acetonitrile, there was no appreciable shift in stretching frequency
gave a total of 86 hits; these hits were scrutinized manually to
exclude those obviously inappropriate compounds including
disordered and polymorphic structures, and aggregates encapsu-
lated within obvious hydrogen bonding which presumably takes
…
precedence over a supposed O p interaction. Under these
constraints, 50 crystal structures remained, representing a total
…
of 109 unambiguous O p interactions (Table 2). Inspection of
these structures revealed: (i) no specific mention of the interactions
…
of the oxadizole ring. However, a moderate red shift (1544 cm21
)
was made in previous work and (ii) the majority of O p systems
…
exhibit the expected geometry for the O p interactions (Fig. 2),
was observed in CH2Cl2, maybe due to formation of a competitive
19
…
…
Cl p interaction. The result indicates that the O p interaction
in 1 has an effect on the stretching vibration of the oxadizole, but
this effect is considerably minimal.
whereas some others exhibit relatively large interacting geometry
…
due to steric hindrance. The results have confirmed that O
p
interactions do exist widespread in aromatic 1,3,4-oxadiazoles
Additionally, the hydroxyl O5–H5A in compound 1 could have
formed a conventional intramolecular H bond to the carbonyl O4
(Fig. 5), however, the hydroxyl atom H5A points not to the O4
atom but to the intermolecular C11*–C16* ring centroid of the
despite the presence of steric hindrance.
…
Given that O p interactions occur universally in phenyl-
…
substituted 1,3,4-oxadiazoles, it is possible to find similar O
p
…
interactions in 5-phenylfuran derivatives, and the O p interac-
tions there are expected to be weaker. In view of this, we also
conducted a systematic study of such compounds in the CSD, and
interestingly found that the furan oxygen actually makes a close
approach to the adjacent phenyl ring along the principal axis; the
…
neighboring translation-related molecule, the distances H5A Cg
…
˚
and O5 Cg being 2.69 and 3.460(2) A, respectively, and the
…
H-bond angle 161u. Hence, the O–H p interaction has all the
geometrical attributes of a hydrogen bond.20 In view of an offset of
˚
average distance of oxygen to the phenyl ring centroid is 3.46 A
and the average angle is 12.9u (Table 3). The result reveals that
…
Table 2 Result of database analysis of O p interactions in phenyl-
substituted 1,3,4-oxadiazoles
…
O
p interactions No. of entries Mean geometric parameters
b
d /A
ac/u
9.5
˚
50a
3.41
a
Refcodes: COVHOL, DAXBOU, EDOTAT, FEQNAR, LARNOI,
LEQPON, LEQPUT, NAXDIZ, NEVXIW, NEXHUU, QUJNUF,
QUJPER, QUJPOB, REYZOL, SUHWEY, WEPJUX, XOXLAX,
YETSIA, YETSOG, ADEKOJ, COZJOR, DAXBUA, GIKYOP,
GUPYUM,
HAXTAC,
HAXTEG,
JARBOU,
JOLDAP,
LARMUN, LODZUZ, MOMNAE, NEVXOC, NILQAB,
OBUJEB, OIXGUA, OJIFES, PUMGAG, QUVPON, RAXROY,
REYZUR,
WEDWUY, WEHQUW, WEKSAH, WONJOZ, XIWRIF,
SABPIV,
TUCYAS,
TUHXID02,
VAXYEZ,
b
XOZHAW. The distance from the oxadiazol oxygen to the phenyl
Fig. 4 Charge distribution of 2-phenyl-1,3,4-oxadiazole in the optimized
dimer (calculated at the WB97XD/6-311+G(d,p) level, and the no. of the
atom is the same as shown in Fig. 2).
c
ring centroid. The angle between the oxadiazol oxygen-phenyl ring
centroid vector and the vector perpendicular to the phenyl ring.
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Table 4 Crystallographic data for 1a
…
Table 3 Result of database analysis of O p contacts in 5-phenyl-
furan derivatives
Compound
1
…
O
p interactions No. of entries Mean geometric parameters
b
d /A
ac/u
12.9
Formula
Mr
Crystal system
Space group
C
19H15N3O5
367.36
Orthorhombic
˚
5a
3.46
a
Refcodes:
KOKMON. The distance from the furan oxygen to the phenyl ring
WULVEF,
NUKWUL,
TIBBIQ,
DUDWUV,
b
P212121
˚
c
centroid. The angle between the furan oxygen-phenyl ring centroid
vector and the vector perpendicular to the phenyl ring.
a/A
6.796(2)
11.993(3)
21.790(5)
1775.9(7)
4
1.374
˚
b/A
˚
c/A
3
˚
V/A
Z
˚
only 0.543 A of H5A with respect to the C11*–C16* centroid, the
…
O–H p bond is classed as ‘‘centroid type’’.
dc/g cm23
m/mm21
0.101
0.71073
0.18 6 0.29 6 0.39
˚
…
The O–H p H bonds of the ‘‘centroid type’’ are uncommon in
l/A
Crystal dimensions/mm23
Reflections measured
Independent reflections
(Rint y0.025)
the literature, especially in organic molecules.3a,b Some O–H
p
…
8686
3080
hydrogen bonds have been reported recently,3c,g however, they
belong to the ‘‘edge type’’, where the O–H group makes a close
approach to only two adjacent carbons of a particular phenyl
group.
Final R
wR2 (for all reflections)
a
Crystallization from ethyl acetate by slow evaporation in air yields
colorless block crystals. SADABS absorption correction. Standard
crystallographic computations (refinement on F2 of all reflections).
0.0246
0.0638
The crystal structure of 1 also exhibits two intramolecular
…
N–H N and C–H O hydrogen bonds (Fig. 5); the distances
…
Data/restraints/parameters
Bruker–Nonius, SMART CCD detector.
= 3080/0/253. Data collected on the
…
…
˚
N3–H N2 and C6–H O1 are 2.46 and 2.54 A, respectively,
and the angles 106 and 100u, respectively. The two H bonds
collaboratively maintain a planar conformation to the [(5-phenyl-
1,3,4-oxadiazol-2-yl)carbonyl]amino fragment, thus allowing
p-electrons to localize sufficiently between the oxadiazole and
great importance to understanding their biochemical and physical
behaviours.
…
C1–C6 ring. This sets the stage for the foregoing O p interaction.
In conclusion, in-depth structural studies have revealed a new
…
The crystallographic data are presented in Table 4.
type of O p interaction occurring in aromatic 1,3,4-oxadiazoles.
…
In addition to the O–H p and O p interactions, there are
…
Ab initio computations performed at the WB97XD/6-311+G(d,p)
level provided effective support to the interaction. LMOEDA
binding energy decomposition at the MP2/maug-cc-pVTZ level
has shown that although the dispersion component makes the
…
three intermolecular H bonds in 1, one of C–H N, and two of
…
C–H O type. The distances C12–H12 N1, C16–H16 O2 and
…
…
…
˚
C5–H5 O3 are 2.54, 2.50 and 2.68 A, respectively, and the
corresponding angles 177, 144 and 157u, respectively. These
noncovalent interactions synergistically constitute a 3D structure.
Considering no conventional hydrogen bonds involved in 1, we
…
largest contribution to the O p interaction, the electrostatic
component may be more important in controlling the direction-
ality and geometrical details of the interaction. Therefore, its origin
has been accounted for by molecular electrostatic potential and
Mulliken population analysis. The present work, we believe, may
…
think the O p interaction not only plays a crucial role in the
specific packing motif of aryl-1,3,4-oxadiazoles, but also may be of
…
be the first disclosure of the nature of 1,3,4-oxadiazol aromatic
contacts.
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