The annular tautomerism of 4(5)-phenylimidazole

Article abstract of DOI:10.1039/b109079h

A new polymorph of 4-phenylimidazole 1a has been characterised by X-ray crystallography. Both polymorphs present a secondary structure of chains, and the observed differences in the topology of their crystal packing are related to the conformational diffe

Full text of DOI:10.1039/b109079h

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The annular tautomerism of 4(5)-phenylimidazole
Rosa M. Claramunt,*a María Dolores Santa María,a Lourdes Infantes,b Félix H. Canob and
José Elgueroc
2
a Departamento de Química Orgánica y Biología, Facultad de Ciencias, UNED, Senda del Rey,
9, E-28040 Madrid, Spain. E-mail: rclaramunt@ccia.uned.es
b Departamento de Cristalografía, Instituto de Química-Física Rocasolano, CSIC, Serrano 119,
E-28006 Madrid, Spain
c Instituto de Química Médica, Centro de Química Orgánica ‘Manuel Lora-Tamayo’,
CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain
Received (in Cambridge, UK) 5th October 2001, Accepted 3rd December 2001
First published as an Advance Article on the web 17th January 2002
A new polymorph of 4-phenylimidazole 1a has been characterised by X-ray crystallography. Both polymorphs
present a secondary structure of chains, and the observed differences in the topology of their crystal packing are
related to the conformational differences in the primary and secondary structural levels. NMR experiments (13C and
15N) reveal that tautomer 1a is the only one observed in the solid state, being also the most abundant in solution
(80%) in agreement with high-level theoretical calculations which favour this tautomer over the 5-phenyl one.
The results obtained for this compound are relevant for the structure of the related 4(5)-porphyrinylimidazole.
calculated to be between 10 and 37,1 which corresponds to
Introduction
∆G298 between 5.7 and 8.9 kJ molϪ1. This result came from an
Although most problems related to tautomerism are well under-
old publication of Ridd and Smith based on the pKa of 1 and
stood,1–3 there remain some cases that still deserve attention
that of 1-methyl-4-phenylimidazole 3.9 The absence of data for
either due to their difficulty or to their relevance. An example of
1-methyl-5-phenylimidazole 4 made the determination of KT at
the latter is the annular tautomerism of 4(5)-phenylimidazole
best only an estimation. From the comparison of the electronic
1, i.e. the equilibrium between 4-phenyl 1a and 5-phenyl 1b
spectrum of 1 in ethanol with those calculated by using the π-
tautomers. For instance, the steric effect of 1 on the axial sub-
electron PPP method for 1a and 1b, it follows that calculations
stitution in an iron() phthalocyanine depends on its tauto-
for 1a tautomer match slightly better the experimentally
meric form.4 When 1 yields reversible silver() complexes, it has
observed spectra.3,10
been established that they are constituted by tautomer 1a.5
Finally, the biological properties of 2-substituted derivatives of
1 have been assigned to 4-phenyl tautomers.6
The X-ray molecular structure of 1 has been determined by
Sridhar, Prasad et al. and corresponds to tautomer 1a.11
However, this paper deserves some comments: i) it is not
reported in the Cambridge Structural Database (October
2000 release);12 ii) the title of the paper “Crystal Structures
of Two Triazoles” is misleading since only one of them is
a triazole; iii) even the triazole was named 3,5-diphenyltriazole
(a 1,2,4-triazole with both substituents on the carbon
atoms?) when the structure corresponds to 1,5-diphenyl-1,2,3-
triazole.
Recently, Kobuke et al. have discussed the related case of
4(5)-porphyrinylimidazole 2 [5,15-bis(imidazol-4-yl)-10,20-bis-
(4-dodecyloxyphenyl)porphyrin].7 According to these authors,
the supramolecular assembly formed by 2 is due to tautomer
2b. Moreover, they assigned the 13C NMR signals that appear
at 135 and 120 ppm to C-4 and C-2, respectively, which seems to
be in contradiction with our results on imidazole itself.8 For all
these reasons, we decided to reconsider the tautomerism of
4(5)-phenylimidazole.
Theoretical studies, which can be assimilated to the gas
phase, have been carried out on this compound. Ögretir et al.
reported AM1 and PM3 calculations on a series of imidazoles;
for compound 1 they found that 1a is more stable than 1b by
about 3 kJ molϪ1.13 Tomás et al. have calculated by mixed ab
initio–semiempirical methods (3-21G*/AM1) that tautomer 1a
is the most stable (6.4 kJ molϪ1).14 Maye and Venanzi have
calculated (ab initio 3-21G) the energies and rotational barriers
of both 1a and 1b.15,16 The difference in energy is 7.5 kJ molϪ1
in favour of 1a (planar, rotational barrier 23.4 kJ molϪ1)
compared to 1b (twisted 35.3Њ, rotational barriers 3.9 and
7.9 kJ molϪ1).
Let us summarise the information presently available on
compound 1. In water (basicity measurements) the tautomeric
equilibrium constant, as defined by KT = [1a]/[1b], has been
564
J. Chem. Soc., Perkin Trans. 2, 2002, 564–568
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b109079h

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Table 1 Selected geometrical parameters (distances, Å and angles, Њ)
N1–C2
C2–N3
N3–C4
1.343(6)
1.305(7)
1.384(5)
C4–C5
N1–C5
C4–C6
1.370(8)
1.368(6)
1.472(7)
C6–C7
C7–C8
C8–C9
1.383(8)
1.381(8)
1.392(9)
C9–C10
C10–C11
C6–C11
1.378(9)
1.386(8)
1.394(7)
C2–N1–C5
N1–C2–N3
C2–N3–C4
N3–C4–C5
N1–C5–C4
105.8(3)
N3–C4–C6
C5–C4–C6
C4–C6–C11
C4–C6–C7
C7–C6–C11
123.0(5)
C6–C7–C8
C7–C8–C9
C8–C9–C10
C9–C10–C11
C6–C11–C10
121.3(4)
113.6(3)
104.9(4)
108.9(4)
106.9(4)
128.0(4)
119.8(5)
121.3(4)
118.8(5)
119.8(6)
119.2(5)
121.0(5)
119.9(6)
C2–N3–C4–C6
Ϫ176.8(5)
N3–C4–C6–C7
38.7(8)
C4–C6–C7–C8
176.5(5)
Results and discussion
Ab initio calculations
At the HF/6-311G** level we found Ϫ1193212.57 kJ molϪ1 for
1a (dipole moment 3.91 D) and Ϫ1193204.77 kJ molϪ1 for 1b
(dipole moment 3.62 D), thus, the difference in stability is 7.80
kJ molϪ1. Therefore, at this high level, the result is very similar
to that obtained by Venanzi using the now abandoned 3-21G*
basis set,16 allowing us to conclude that in the gas phase the
4-phenyl tautomer should be greatly predominant (about 96%
at 25 ЊC). Since the dipole moments are similar, no important
effects of the condensed phases (solid state and solution) are
expected.
Crystal and molecular structure determination
We obtained suitable crystals of compound 1 by slow evapor-
ation of a saturated chloroform solution. The structure is dif-
ferent from that reported previously by Prasad et al. (solvent
of crystallisation not reported),11 thus constituting a new poly-
morph. The main molecular characteristics of our trigonal
polymorph of 4-phenylimidazole 1a are reported in Table 1
according to the numbering scheme depicted in Fig. 1. The
other polymorphic form of 1a, solved by Prasad et al., shows
monoclinic symmetry [Pa, lattice parameters: a = 7.0291(13),
b = 12.5142(8), c = 9.4476(12) Å, β = 100.639(12)Њ],11 but no
metric reduction of one form to another is possible.17 The bond
distances and angles of both polymorphs are not significantly
different, and the main difference between them lies in the
conformation around the bond joining the phenyl and the
imidazole ring [τ = 38.7(8)Њ for the trigonal polymorph, and
21.1(6)Њ and 21.2(7)Њ for the two independent molecules in the
monoclinic polymorph]. The geometry of these imidazole rings
follows a pattern of bond distances and angles similar to
the mean values retrieved from the Cambridge Structural
Database, CSD,12 for imidazole itself. Comparisons with the
optimised geometry of 1a and with the reported geometry of
1-methyl-4-phenylimidazole 3 [τ = 7.3(3)Њ]18 show that the ring
distances and angles are in all cases very similar. Therefore
3 is a good fixed model for tautomer 1a, while 1-methyl-5-
phenylimidazole 4, whose X-ray crystal structure has not been
determined, is probably not such a good model because both
rings should present an important torsion angle.
Fig. 1 View of a chain, through N–H ؒ ؒ ؒ N interactions, showing the
atomic numbering scheme. Ellipsoids are drawn at the 30% probability
level for non-H atoms.
has described the spectrum of 1 in DMSO-d6 but only a list of
chemical shifts without assignment is provided,22 Begtrup20 has
published the 13C NMR spectrum of compound 3, but his
assignment was wrong, and Kashima23 those of 3 and 4 with-
out assignment and moreover that of 4 incomplete. In Table 4
we have summarised the different results assigned through
standard, HMQC and HMBC experiments (we have assigned
those of Koskinen and Kashima by analogy and corrected that
of Begtrup).
Both polymorphs (trigonal and monoclinic) exhibit a
secondary structure of chains through N–H ؒ ؒ ؒ N inter-
molecular hydrogen bonds (HBs), Fig. 1, but the angles
between consecutive imidazole rings in the chains change, and
so the conformation of these chains is different ( ؒ ؒ ؒ NH, N,
NH, N ؒ ؒ ؒ ) (Table 2).
The chains in the title compound are interacting through
several C–H ؒ ؒ ؒ π contacts, giving the 3D structure represented
in Fig. 2 (Table 3). There are no voids in the structure and the
total packing coefficient is 0.64.19
We will discuss first the 13C NMR spectrum of a solid sample
of 1 where we know, from crystallography, that it is pure 1a.
Considering the deshielding effect of the N-methyl group on
C-2 and C-5,8,25,27 data of 3 allow the assignment of all the
signals of 1a in the solid state. Then, we recorded the spectrum
of 1 at 280 K in HMPA-d18, [(CD3)2N]3PO, a solvent known to
slow down the annular prototropic equilibria.1 The spectrum
corresponds to a mixture of 80% of 1a and 20% of 1b, the
most abundant tautomer being identified without ambiguity by
comparison with the CPMAS spectrum. Thus, for the first time,
all the signals of two tautomeric imidazoles were identified and
assigned. In DMSO-d6, only the 1H NH signals were split (but
NMR spectroscopy in solution and in the solid state
Several authors20–24 have reported the 13C NMR spectra of
imidazoles, amongst them, ourselves.8,25 For instance, Koskinen
J. Chem. Soc., Perkin Trans. 2, 2002, 564–568
565

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Table 2 Torsion angles (Њ) in the secondary structure to the polymorphs of 4-phenylimidazole
t1
t1Ј
t2
t2Ј
Monoclinic polymorph10
Trigonal polymorph (this work)
Ϫ65.8(7)
Ϫ34.1(11)
Ϫ119.5(6)
178.8(6)
Ϫ36.2(10)
34.1(11)
Ϫ128.2(6)
Ϫ178.8(6)
Table 3 Geometry of hydrogen bonds (distances, Å and angles, Њ). C0105 and C0611 represent the centroids of the imidazole and phenyl rings
respectively
D–H
H ؒ ؒ ؒ A
D ؒ ؒ ؒ A
D–H ؒ ؒ ؒ A
N1–H1 ؒ ؒ ؒ N3 (1 Ϫ y,1 Ϫ x,1/2 ϩ z)
C10–H10 ؒ ؒ ؒ C0105 (x Ϫ 1/3,x Ϫ y Ϫ2/3,z Ϫ 1/6)
C2–H2 ؒ ؒ ؒ C0105 (1 Ϫ y,x Ϫ y,z)
C8–H8 ؒ ؒ ؒ C0611 (4/3 Ϫ x ϩ y,2/3 Ϫ x,z Ϫ 1/3)
C5–H5 ؒ ؒ ؒ C0611 (1 Ϫ x ϩ y,y,1/2 ϩ z)
1.01(Ϫ)
1.08(Ϫ)
1.08(Ϫ)
1.08(Ϫ)
1.08(Ϫ)
1.88(Ϫ)
2.63(Ϫ)
2.58(Ϫ)
2.86(Ϫ)
3.15(Ϫ)
2.856(Ϫ)
3.581(Ϫ)
3.463(Ϫ)
3.805(Ϫ)
4.028(Ϫ)
165(Ϫ)
149(Ϫ)
135(Ϫ)
154(Ϫ)
141(Ϫ)
Table 4 1H, 13C and 15N chemical shifts of 4(5)-phenylimidazoles 1–3 and imidazole 5 (unless specified, all spectra at 300 K)
Compd. Solv.
H-1
H-2
H-4
H-5
C-2
C-4
C-5
Cipso
Cortho
Cmeta
Cpara
N-1
N-3
N.o.
1
CDCl3
N.o.a
7.70
CPh
CPh
——
CPh
7.39
CPh
CPh
CPh
——
——
6.85
——
7.35
7.59
——
7.75
CPh
——
——
——
CPh
——
7.05
——
135.5 138.7 115.2 133.0 124.9 128.7 127.0 N.o.
135.8 140.1 112.5 135.1 124.4 128.4 125.9 Ϫ211.4 Ϫ123.0
135.6 138.6 111.5 132.8 124.9 128.7 126.9 —— ——
135.6 140.9 113.0 136.4 124.8 128.3 125.6 Ϫ211.4 Ϫ123.9
136.3 125.8 131.1 131.6 124.3 129.0 126.4 Ϫ217.0 Ϫ117.3
136.6 142.0 113.1 134.9 127.9 127.9 127.9 Ϫ199.8 Ϫ130.2
137.4 141.7 115.5 133.6 124.3 128.1 126.2 ——
137.9 142.2 115.9 134.3 124.7 128.5 126.6 ——
138.1 128.0 133.0 130.3 128.2 128.8 128.7 ——
1
DMSO-d6 21 12.16 12.54 7.70
1
DMSO-d6
HMPA-d18
HMPA-d18
CPMAS
——
13.31
13.62
——
NMe
NMe
NMe
——
13.19
——
——
7.56
7.56
——
——
——
——
——
7.52
——
b
b
1a
1b
1a
3
20
CDCl3 23
——
——
——
3
CDCl3 23, c
CDCl3
4
5
CD3OD27, d
136.8 127.7 118.5 ——
135.2 128.0 115.7 ——
136.3 126.8 115.3 ——
——
——
——
——
——
——
——
——
——
Ϫ216.3 Ϫ143.4
Ϫ213.6 Ϫ119.9
Ϫ210
b
5
HMPA-d18
5
CPMAS8,26
Ϫ138
a Not observed. b 280 K. c The chemical shifts are those of the 1-butyl derivative. d 178 K.
should be reversed: the signal at 135 ppm should be C-2 and
that at 120 ppm C-4. Therefore, his assignment of protons
1
H-2 and H-4 (by means of a H 13C COSY spectrum) is also
wrong and this has important consequences for Kobuke’s
interpretation of the supramolecular assemblies through inter-
molecular N–H ؒ ؒ ؒ N hydrogen bonds.
Another consequence of the 13C chemical shifts of 1a and 1b
in HMPA-d18 is related to the dihedral angle between the phenyl
and imidazolyl rings.20 The difference, ∆δ, between ortho and
meta carbons is 3.5 ppm for 1a and 4.7 ppm for 1b and should
correspond, according to Venanzi’s calculations,15,16 to τ = 0Њ
for 1a and to τ = 35.3Њ for 1b. Our calculations yield 0Њ for both
tautomers which is more consistent with such small differences
in ∆δ. Note that in the 1-methyl derivatives ∆δ values are
3.8 ppm for 3,20,23 and 0.6 ppm for 4 (both N-methyl and N-
butyl derivatives).23 This is related to our comment that 4 is not
a good model for 1b.
Concerning 15N NMR, compound 1a presents in the solid
state two signals at Ϫ199.8 and Ϫ130.2 ppm, respectively,
attributable to N1 and N3.26,27 Tautomer 1a in HMPA-d18
shows the same signals at Ϫ211.4 (Ϫ11.6 ppm) and Ϫ123.9
(ϩ6.3 ppm). The differences are related to intermolecular
HBs, chains of N–H ؒ ؒ ؒ N in the solid state and N–H ؒ ؒ ؒ OP-
[(CD3)2N]3 in solution. Recently we have reported that the
annular tautomerism of imidazole itself, 5, is blocked in
CD3OD at 178 K (Table 4).26 The 13C NMR signals are close
to those reported for other compounds of Table 4 but the 15N
NMR signals in methanol, particularly those of N-3, are con-
siderably shifted by the solvent O–H ؒ ؒ ؒ N HBs in methanol.27
To check this result we have recorded the 15N NMR spectrum
of 5 in HMPA-d18 at 280 K and observed two signals at Ϫ213.6
Fig. 2 Crystal packing diagram as projected along the c axis.
note that imidazole protons at 7.59 and 7.70 ppm were broad),
the most abundant (12.16 ppm, 85%) belongs to 1a. The
remaining signals correspond to an average mixture very rich in
1a; the same happens in CDCl3 where the compound is rather
insoluble and a saturated solution was employed.
A first consequence of the 13C data reported in Table 4 is that
the assignment of Kobuke (135 and 120 ppm to C-2 and C-4)7
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J. Chem. Soc., Perkin Trans. 2, 2002, 564–568

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Table 5 Percentages, equilibrium constants and energy differences of 4(5)-phenylimidazole 1 annular tautomerism
N
Solvent
% 1a
% 1b
KT
∆G298/kJ molϪ1
ET
Gas phase
96
91
97
——
85
4
9
3
23
10
37
——
5.7
4.3
7.8
5.7
8.9
7.3
4.3
3.6
——
Water (lower limit)
Water (upper limit)
Water (average)
DMSO
1.000
1.000
1.000
0.444
0.315
——
15
19
HMPA
81
Table 6 Crystal data and structure determination details at room temperature
Chemical formula
Formula weight
Crystal habit
Crystal size/mm
Z
C9H8N2
144.2
Colourless needles
0.57 × 0.07 × 0.03
Crystal system
Space group
θ range for lattice parameters/Њ
No. of reflections for lattice parameters
Trigonal
R3c
2–35
48
18
Wavelength/Å
1.5418
5.80
705
663
5.3
a/Å
b/Å
20.627(2)
20.627(2)
9.766(1)
3598.6(6)
6.1
Absorption coefficient/cmϪ1
No. reflect. independent
No. reflect. observed [2(I ) criterion]
R(%)
c/Å
V/Å3
wR(%)
(N-1) and Ϫ119.9 ppm (N-3), thus verifying our assumption.
Note also that the signal of N-3 in the solid state (Ϫ138 ppm) is
also considerably affected by the HBs and that it cannot be used
as a reference for tautomeric studies.
ment. All the chemical shifts are expressed in ppm, 1H and 13
C
chemical shifts are reported with respect to external TMS and
15N chemical shifts to an external sample of pure nitromethane.
The NMR experiments (1H–13C) HMQC and (1H–13C) HMBC
with pulse field gradients38 were performed in order to assign
the signals obtained in the one-dimensional spectra. 15N
chemical shifts were obtained using (1H–15N) HMBC experi-
ments with pulse field gradients. Cross polarisation magic angle
spinning (CPMAS) NMR spectra have been obtained on a
Bruker AC-200 spectrometer at 298 K using a 7-mm BRUKER
DAB 7 probehead that achieves rotational frequencies of about
3.5–4.5 kHz. Samples (approximately 200 mg of material)
were carefully packed in ZrO2 rotors and the standard CPMAS
pulse sequence was applied.39 Recording of the 15N CPMAS
spectrum (natural abundance) required 25 h.
Conclusions
We have collected in Table 5 all the present information about
the tautomerism of 1. If one uses, for the tautomeric equi-
librium in water, an average value of ∆G298 = 7.3 kJ molϪ1, the
results in water, in DMSO and in HMPT are linearly correlated
N
with Reichardt’s ET solvent parameter:28 ∆G298 = (1.901
0.003) ϩ (5.400 0.005) ETN, n = 3, r2 = 1.000. This provided
conclusive evidence of the consistency of our results.
Experimental
Note added in proof
Calculations
Recently, the crystal structure of 4-phenylimidazole has been
published,40 and it corresponds to the structure described in
ref. 11, which is not quoted, probably because it is not reported
in the CSD.
The ab initio molecular orbital calculations were carried out
using the GAUSSIAN 98 program29 within the Spartan (release
5.0.2).30 Geometries for all structures were fully optimised. The
restricted Hartree–Fock calculations with the split-valence
6-311G** basis set were used in these calculations.31,32
Acknowledgements
X-Ray analysis†
Financial support was provided by the Spanish DGI/MCYT
(Project no. BQU2000-0252). One of us (L. I.) also thanks the
UNED for a grant (Project no. 2001V/PROYT/13 Ϫ I ϩ D).
A summary of the data collection and refinement process is
given in Table 6. All non-hydrogen atoms were found by direct
methods, SIR92,33 and the structure was refined with a full
matrix least squares procedure on Fobs using anisotropic dis-
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