The structure of two 1-(nitrophenyl)-Δ2-pyrazolines: A crystallographic and theoretical study

Article abstract of DOI:10.1071/CH04142

The crystal and molecular structures of two nitrophenylpyrazolines have been determined. The geometries have been used as starting geometries for density functional theory (DPT) calculations. The differences in conformation between both molecules and betw

Full text of DOI:10.1071/CH04142

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2004, 57, 1103–1108
The Structure of Two 1-(Nitrophenyl)-ꢀ2-pyrazolines:
a Crystallographic and Theoretical Study
Glenn P. A. Yap,A Ibon Alkorta,B Nadine Jagerovic,B and José ElgueroB,C
A Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA.
B Instituto de Química Médica, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain.
C Corresponding author. Email: iqmbe17@iqm.csic.es
The crystal and molecular structures of two nitrophenylpyrazolines have been determined. The geometries have
been used as starting geometries for density functional theory (DFT) calculations. The differences in conformation
betweenbothmoleculesandbetweenthesolidstateandgasphaseareexplainedintermsofstericeffects.Anattractive
intramolecular N· · ·N interaction between the nitro group and the pyrazoline N2 nitrogens has been found.Absolute
shieldings have been calculated (GIAO) and compared with experimental 1H and 13C chemical shifts.
Manuscript received: 29 May 2004.
Final version: 2 September 2004.
Introduction
compounds generally fails (for an exception see the 5-
hydroxy/5-trifluoromethyl intermediates).[21–24] Finally, the
2ꢀ,4ꢀ,6ꢀ-trinitrophenyl derivatives 3 have only been reported
in our work: compounds 1–3 are usually obtained by
N-arylationof1H-ꢀ2-pyrazolineswithp-fluoronitrobenzene,
1-fluoro-2,4-dinitrobenzene, and 1-chloro-2,4,6-trinitro-
benzene (picryl chloride).[25–39] Reports of crystal-structure
determinations of nitrophenylpyrazolines are limited to the
highly substituted derivative 4.[40]
There are five major sources of information on the chem-
istry of pyrazoles and their derivatives,[1–5] in all of them
the synthesis and reactivity of ꢀ2-pyrazolines (1H-4,5-
dihydropyrazoles)isextensivelydiscussed. Notsotheirstruc-
tural and spectroscopic aspects, which are only detailed in
the last two references.[4,5] However, these compounds have
important properties as organic light-emitting diodes[6] and
fluorescent probes,[7,8] and have other applications as intel-
ligent materials, a field particularly explored by Prasanna de
Silva et al.[9–15] 1-Nitrophenyl-ꢀ2-pyrazolines are promising
in these fields, being brightly coloured, but still unexplored.
A search for these compounds shows that the great major-
ity of such compounds belong to the p-nitrophenyl series 1
(Scheme 1).[16–18] The reason is that these compounds can
be obtained by cyclization of the p-nitrophenylhydrazones
of α,β-unsaturated carbonyl compounds and by 1,3-dipolar
cycloaddition of p-nitrophenylnitrilimines on olefins (includ-
ing fullerenes).[19,20] The 2ꢀ,4ꢀ-dinitrophenyl derivatives 2
are much less abundant because the cyclization of the
2,4-dinitrophenylhydrazones of α,β-unsaturated carbonyl
Results and Discussion
In this paper we will describe two compounds, one of type
2 and another of type 3, namely 1-(2ꢀ,4ꢀ-dinitrophenyl)-
2-pyrazoline 5 and trans-1-(2ꢀ,4ꢀ,6ꢀ-trinitrophenyl)-3,4,5-
trimethyl-ꢀ2-pyrazoline 6 (Scheme 2).
We have already published some information about
these two compounds. The synthesis,[27] reactivity,[29,34] 13C
NMR[35] and1HNMRspectra,[38] andpKa[39] ofcompound 5
have been reported. The 13C NMR[35] and 1H NMR[37] spec-
tra of compound 6 have also been reported, and the synthesis
of 6 is reported here.
CH3
H
H
R4
R4؅
R5
R5؅
H3C
R4
R4؅
R5
R5؅
H
R4
R4؅
R5
R5؅
R3
N
OH
CO2CH3
R3
N
R3
N
N
H
3
3
4
4
H
H
CH3
H
H
N
N
1
1
2
N
2
5
5
N
N
N
N
N
N
1؅
1؅
O2N
O2N
NO2
O2N
NO2
2؅
3؅
2؅
O2N
6؅
5؅
6؅
5؅
3؅
4؅
4؅
NO2
NO2
NO2
NO2
NO2
NO2
6
1
2
3
4
5
Scheme 1.
Scheme 2.
10.1071/CH04142
© CSIRO 2004
0004-9425/04/111103

1104
G. P. A. Yap et al.
Solid-State Structures
Density Functional Theory Calculations
Geometries
CrystaldataandrefinementdetailsaresummarizedinTable1.
The molecules are represented in Fig. 1 (top). Although there
is potentially free rotation between the phenyl and hetero-
cyclic rings during crystallization of 5, the molecule tends to
be flat in the solid with the angles between the mean planes
of the rings at 17.0(4) and 8.6(4)◦ for the two independent
molecules in the asymmetric unit. The molecules are packed
in a herringbone pattern on the 0 1 1 projection. The packing
diagram shows molecules having phenyl–phenyl distances of
3.5–3.8 Å, which suggests π-stacking. Note that the ortho-
nitro group is situated close to the pyrazoline N2 atom as
represented in 5. In the solid state, the methyl groups on the
heterocyclic rings appear in 6 to exert enough steric influ-
ence to twist the phenyl and heterocyclic rings from each
other, thereby increasing the angle between ring mean planes
to38.2◦.Theenhancedstericinfluenceaffordedbythemethyl
groups on the ring can also be observed from the increase in
puckering of the heterocyclic ring from a 0.064(5) Å aver-
age mean plane deviation in 5 to the considerably more
puckered 0.099(4) Å mean plane deviation in 6. The pyra-
zoline ring in this last compound adopts a Cs conformation.
In the case of 1-(2ꢀ,4ꢀ-dinitrophenyl)-ꢀ2-pyrazolin-5-ones,
the X-ray structure shows that the 2ꢀ-nitro group prefers the
proximity of the 5-C=O group than that of the N2 atom.[41]
We have optimized the geometries at the B3LYP/6–
311++G** level. Although we have not carried out a
complete exploration of the energy hypersurface, these
geometries are minima (no imaginary frequencies). To com-
pare the four situations depicted in Fig. 1, we have summa-
rized the most relevant geometrical parameters inTable 2, but
it is possible to visually notice the great similarity between
the experimental and calculated geometries.
In more detail, the calculated geometries confirm the fact
that in 1-(2ꢀ,4ꢀ-dinitrophenyl)pyrazolines 2, the ortho-nitro
group places itself close to the pyrazole N2, a particularly
novel result. We assign this fact (also observed, by 1H NMR
spectroscopy in solution, for the corresponding pyrazoles[42]
but not reported for pyrazolines) to an attractive interaction
between the positively charged N atom of the nitro group
and the lone pair of electrons of N2. The calculated N2· · ·
N(o-NO2) distance is a little longer than the crystallographic
one, perhaps because of crystal packing forces. The distance
is shorter for 6 than for 5. An examination of the calculated
geometries of these two compounds shows that the 6-nitro
group pushes the pyrazoline ring towards the 2-nitro group
(5: N2–N1–C1ꢀ 120.6◦, N1–C1ꢀ–C2ꢀ 123.3◦; 6: N2–N1–C1ꢀ
117.8◦, N1–C1ꢀ–C2ꢀ 121.3◦) shortening the N· · ·N distance.
Table 1. Summary of crystal data and refinement details for 5 and 6
Quantity minimized = R(wF2) = {ꢁ[w(Fo2 − Fc2)2]/ꢁ(wFo2)2}1/2: R(F) = ꢁꢀ/ꢁ(Fo), ꢀ = |(Fo − Fc)|:
w = [σ2(Fo2) + (aP)2 + bP]−1: P = [2Fc2 + max(Fo,0)]/3
Parameter
5
6
Empirical formula
Formula weight
C9H8N4O4
236.19
C
12H13N5O6
323.27
Colour
orange
block orange, plate
0.10 × 0.10 × 0.04
P21/c (no. 14)
4
Crystal dimensions [mm3]
0.10 × 0.10 × 0.10
Space group
Z
P1 (no. 2)
4
a [Å]
b [Å]
8.0211(14)
10.3927(19)
13.296(3)
76.261(8)
86.133(14)
75.246(9)
12.761(6)
8.704(2)
13.848(6)
90
112.97(2)
90
c [Å]
α [◦]
β [◦]
γ [◦]
Collection ranges
−10 ≤ h ≤ 10; −12 ≤ k ≤ 12;
−17 ≤ l ≤ 17
298(2)
1041.1(3)
1.507
−16 ≤ h ≤ 15; −11 ≤ k ≤ 10;
−18 ≤ l ≤ 17
120(2)
1416.2(10)
1.516
Temperature [K]
Volume [Å3]
Dcalc [mg m−3
]
Radiation
MoKα (λ 0.71073 Å)
0.122
SADABS
0.9879/0.9879
488
2.08–28.00
5210
MoKα(λ 0.71073 Å)
0.124
SADABS
0.9951/0.9877
672
2.83–28.08
7161
Absorption coeff. µ [mm−1
Absorption correction
Tmax/Tmin
]
F(000)
θ range for data collection [◦]
Observed reflections
Independent reflections
Data/restraints/parameters
Maximum shift/error
Goodness-of-fit on F2
4826 (Rint 0.0534)
4826/0/307
0.000
3335 (Rint 0.0365)
3335/0/208
0.000
1.042
1.056
Final R indices [I > 2σ(I)]
R1 0.0662, wR2 0.1719
0.288 and −0.244
R1 0.0529, wR2 0.1227
0.371 and −0.299
Largest diff. peak and hole [e Å−3
]

Structure of Two 1-(Nitrophenyl)-ꢀ2-pyrazolines
1105
The analysis of the electron density using the atoms in
molecules (AIM) methodology shows a bond critical point
connecting the N2 of the pyrazoline ring and the nitrogen
atom of the ortho-nitro group for the two systems studied
here. The presence of bond critical points is characteristic of
attractive interactions between the two atoms that generate it,
and in this case it can be explained based on the interaction
of the lone pair of N2 and the positive charge of the nitrogen
atom of the NO2 group.
The puckering of the five-membered rings, similar to
that of cyclopentene (envelope Cs conformation), principally
affects the C5 carbon opposite to the N2–C3 double bond. It
can be described using the N2–C3–C4–C5 torsion angle. Our
calculations reproduce the experimental data well for com-
pound 5 (−12◦ versus −11◦). In the case of compound 6 the
puckering determined by crystallography (15◦) appears to be
related to the crystal packing and not to the presence of the 3-,
4-, and 5-methyl groups on the pyrazoline ring. We reached
this conclusion because, in the calculated geometry that rep-
resents the conformation in the gas phase, the puckering is
only 6.8◦. The twist angle between both rings, as defined by
the N2–N1–C1–C2 angle, is similar in both compounds and
is similar in the solid state and in the gas phase.
The nitro groups are almost planar (the sum of the angles
about the N atom, ꢁθ, close to 360◦). The para-nitro group
(4ꢀ-NO2) is almost coplanar with the phenyl ring (167–178◦).
On the contrary, the ortho-nitro groups (2ꢀ-NO2 and 6ꢀ-NO2)
are near perpendicular.
The Possible Attractive Intramolecular N· · ·N
Interaction between the Nitro and
Pyrazoline Nitrogens
One of the most interesting observations of the present
paper is the existence of an interaction between the nitro
group nitrogen, which is acting as an acceptor to the pyra-
zoline N2 lone pair, an interaction that we will discuss
using the calculated geometries of 5 and 6, where dN···N is
2.78 and 2.69 Å, and the sum of the van der Waals radii is
3.16 Å (Kitaigorodsky) and 3.00 Å (Pauling).[43] Most weak
interactions involving nitro groups are hydrogen-bond inter-
actions between proton donors (including acidic CHs)[44] and
one of the negatively charged O atoms of the nitro group
[X–H· · ·O(NO)–R]. However, the electronic structure of the
nitro group presents a formal positive charge on the nitrogen
that can interact with a Lewis base, for instance, a lone pair.
This interaction should weaken the C2ꢀ–NO2 bond increas-
ing the C–N bond length.[45] In compound 5 the 2ꢀ-nitro
group has a dCN of 1.479 Å while the 4ꢀ-nitro group has a
dCN of 1.464 Å; the stronger interaction present in compound
6 has a corresponding dCN(2ꢀ) of 1.483 Å [dCN(4ꢀ) 1.467 Å
and dCN(6ꢀ) 1.470 Å]. Another possible criterion is that the
interaction should modify the planarity of the nitro group,
resulting in a slight pyramidalization towards the N atom. In
molecules 5 and 6, for all the nitro groups not involved in
N· · ·N interactions, the sum of the three angles, ꢁθ, amount
to 360.0◦, whereas ꢁθ of the 2ꢀ-nitro groups is 359.8◦. The
phenomenon described here bears some relationship with the
structure-correlation principle of Dunitz and Bürgi.[46] Note,
however, that the approach of an amine to the carbon atom
Fig. 1. X-Ray and optimized structures. Top-left, X-ray structure
of 5 (two independent molecules); top-right, X-ray structure of 6;
bottom-left, optimized structure of 5; bottom-right, optimized struc-
ture of 6.
Table 2. Characteristic geometrical parameters of 5 and 6
5a and 5b refer to the two independent molecules found in the unit cell
Geometry
5a (X-Ray)
5b (X-Ray)
5 (DFT)
6 (X-Ray)
6 (DFT)
Puckering (N2–C3–C4–C5) [◦]
−9.7
−11.9
−10.9
23.4
2.78
49.6/−134.5
−1.5/178.6
—
15.0
26.1
2.62
65.2/−117.8
−7.7/172.3
25.1/−150.7
6.8
29.1
2.69
49.1/−135.4
−1.8/178.2
40.7/−136.4
Inter-ring torsion (N2–N1–C1ꢀ–C2ꢀ) [◦]
Distance N2· · ·N(o-NO2) [Å]
23.5
14.9
2.70
2.74
Torsion of 2-NO2 (C1ꢀ–C2ꢀ–NO) [◦]
Torsion of 4-NO2 (C3ꢀ–C4ꢀ–NO) [◦]
Torsion of 6-NO2 (C5ꢀ–C6ꢀ–NO) [◦]
54.2/−129.5
−11.1/169.8
—
74.3/−106.8
12.4/−166.7
—

1106
G. P. A. Yap et al.
correlations with the experimental data[20,23] are excellent:
R
(CH3)2N
NO2
N
NO2
δH(exp) = (31.2 0.7) − (0.94 0.027)σ1H,
n = 12, r2 = 0.993
(1)
N
H
δC(exp) = (175.1 1.3) − (0.963 0.014)σ13C, (2)
n = 19, r2 = 0.997
DIWWEL
DAFVEL
dNN 2.73Å
d
NN 2.68 ꢀ 0.04Å
The intercepts are relatively close to the σ values for tetram-
ethylsilane (TMS) calculated at the same level: H (31.97
Ar
N
1
ppm) and 13C (184.75 ppm). When only sp2 carbons are cor-
related, like in Eqn (2), TMS is far away and the intercept
does not correspond to the calculated value. In two recent
publications, we have found intercepts of 175.8 0.9 and
176.7 1.7 ppm for 13C NMR correlations.[49,50]
N
NO2
N
NO2
N
O
VINMAG
NN 2.69Å
JAQTOK
dNN 2.72Å
d
Conclusions
Scheme 3.
The determination of the structures of compounds 5 and 6
fills a gap in the knowledge of 1-nitroarylpyrazolines, a series
of dyes that, conveniently modified, could have useful prop-
erties in non-linear optics.[51,52] The AIM analysis revealed
that the ortho-nitro substituent interacts with the lone pair
of the pyrazoline N2 atom thus explaining the preferred syn
conformation. This type of donor–acceptor interaction can
potentially have a large impact on the conformation of flexi-
ble molecules and may also play a role in controlling crystal
packing (for the intermolecular type of interaction).
N
NO2
H
H
H
H
H
Fig. 2. The calculated molecular structure of the C3H5N2O−2 anion.
Experimental
Synthesis of trans-1-(2ꢀ,4ꢀ,6ꢀ-Trinitrophenyl)-3,4,5-trimethyl-
2-pyrazoline 6
of a carbonyl group is the first step of a nucleophilic addi-
tion, while the approach to a nitrogen atom of a nitro group
cannot lead to any product (the only possible attack is on the
carbon with the NO2 as a leaving group).A search in the Crys-
tal Structure Database (version 5.25, update July 2004),[47]
shows several nitroaromatic derivatives with short N· · ·N dis-
tances (approx. 2.7 Å) the second nitrogen atom belonging to
an amino, imino, azo, or one heterocycle (see Scheme 3 for
some examples).
The most interesting case is DIWWEL, studied by
Egli et al.[48] The structure of 1-dimethylamino-8-nitro-
naphthalene has been observed in seven environments. The
shortest N· · ·N distance is 2.643 Å (DIWWEL01) with a sum
of the three angles ꢁθ of 359.9◦ (in other structures the sum
is 359.6◦), evidence, according to these authors, of ‘an attrac-
tion between the two groups’. The same conclusion is valid
for compounds 5 and 6. If one wants larger interactions, the
N atom should bear a negative charge as in the imaginary
molecule represented in Fig. 2.
From 3-methylpent-3-en-2-one[53] and hydrazine, 3,4,5-trimethyl-2-
pyrazoline was prepared.[54] Compound 6 was obtained by treating
3,4,5-trimethyl-2-pyrazoline[53] with picryl chloride in a 1 : 1 molar
ratio in boiling ethanol.[27] The compound was crystallized from
ethanol/benzene. Yield 87%, mp 118–120◦C (Found: C 44.3, H 4.1,
N 21.1. C12H13N5O6 requires C 44.6, H 4.1, N 21.3%).
Crystallographic Structural Studies of 5 and 6
Compounds 5 and 6 were crystallized by slow cooling of saturated
solutions in hot bromobenzene. Suitable crystals were selected, sec-
tioned as necessary, mounted on glass fibres with epoxy adhesive for 5,
and mounted with Paratone oil and flash-cooled to the data collection
temperature for 6. Unit cell parameters were obtained from 60 data
frames, 0.3◦, from three different sections of the Ewald sphere. No
symmetry higher than triclinic was observed for 5 and solution in the
centrosymmetric space group option yielded chemically reasonable and
computationally stable refinement. Systematic absences in the diffrac-
tion data and unit cell parameters for 6 were uniquely consistent with
the reported space group P21/c. SADABS absorption corrections were
applied based on redundant data.[55] Two crystallographically unique but
chemically equivalent molecules were located in the asymmetric unit of
5. All non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were treated as idealized contribu-
tions. Structure factors are contained in the SHELXTL 6.12-program
library.[50] The CIF is available from the Cambridge Crystallographic
Data Centre under the deposition numbers CCDC 226261 and 226262
for compounds 5 and 6, respectively.
For this molecule (B3LYP/6–311++G** calculations), a
dN···N of 2.567 Å and a ꢁθ of 356.3◦ were calculated.
Absolute Shieldings
Computational Details
We have calculated (GIAO on B3LYP/6–311++G**
1
geometries) the H and 13C absolute shieldings (σ, ppm,
The geometries of compounds 5 and 6 were initially optimized with
unreported) of compounds 5 and 6 [Eqns (1) and (2)]. The
the B3LYP method[56] using the 6-31G* basis set.[57] The minimum

Structure of Two 1-(Nitrophenyl)-ꢀ2-pyrazolines
1107
nature of all structures was confirmed by frequency calculations.[58] In
addition, B3LYP/6–311++G**[59] optimizations were carried out and
the structures thus obtained [also minima][58] were used to calculate the
absolute shieldings, σ, at the same computational level with the GIAO
approximation.[60] The complexes represented in Fig. 1 correspond to
the structure optimized at the B3LYP/6–311++G** level. All the cal-
culations were performed using Gaussian 98.[61] The electron density of
thecompoundshasbeenanalyzedbymeansoftheAIMmethodology.[62]
[21] M. D. Threadgill, A. K. Heer, B. G. Jones, J. Fluor. Chem.
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Acknowledgments
[26] J. Elguero, R. Jacquier, Bull. Soc. Chim. Fr. 1965, 769.
[27] J. Elguero, R. Jacquier, Bull. Soc. Chim. Fr. 1966, 610.
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Financial support was provided by the Spanish DGI/MCYT
(Projects no. BQU-2003-01251 and SAF-2003-08003-
C02-02).
[29] P. Bouchet, J. Elguero, R. Jacquier, Bull. Soc. Chim. Fr. 1967,
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[30] J.-L. Aubagnac, P. Bouchet, J. Elguero, R. Jacquier, C. Marzin,
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