X-ray structural and spectroscopic investigation of 1-piperidine-2,4-dinitrobenzene

The crystal structure of the title compound (C11H13N3O4) has been determined by single-crystal X-ray diffraction. The compound is monoclinic, space group P21/n, witha=9.968(2),b=9.156(2),c=13.249(2)Å, β=102.05(2)°, andDx=1.563 gcm−3 forZ=4. The aromatic ring shows a slight boat deformation. Theo- andp-NO2 groups are twisted out of the plane of the phenyl ring by 39.0(2)° and 4.4(1)°, respectively. The piperidine ring exhibits a slightly deformed chair conformation. Short C−H...O intermolecular contacts stabilize the three dimensional structure. UV and NMR data indicate that the molecule in solution presents a conformation similar to that of the the solid state.


Introduction
The investigation of the parameters that govern the molecular aggregation and the crystal packing of conjugated organic compounds presents interest due to the possible nonlinear optical behavior of these systems.~As part of a general study of the effect of substitution on the molecular geometry and organization of substituted 1-amine-2,4-nitrobenzenes 2-6 a single crystal X-ray study of the molecular structure of lpiperidine-2,4-dinitrobenzene (I) has been carried out and the UV, IH, and ~3C NMR spectra have been obtained.

Experimental
The title compound was synthesized as reported 7 and crystallized from slow evaporation of an ethanol tt~PROFiMO.Depto.de

Molecular geometry in the solid state
A view of the molecule with atom labeling is shown in Fig. 1.The aromatic ring presents a slight boat deformation, C(1) and C(4) atoms are out of the mean ring plane, 0.085(1),~ and 0.039(1).~,respectively.The Total Puckering Amplitude 12 is QT = 0.074 (2).
The o-nitro group is rotated out of the ring plane, the dihedral angle between the group and the mean plane of the aromatic ring is 39.0(2) ~ .The p-nitro group is also rotated out of the aromatic plane an angle The piperidine ring exhibits a slightly distorted chair conformation.The magnitude of the distortion can be described by the puckering parametersJ 2 QT = 0.544(2), qb = 48(1) ~ and 0 = 1.3(2) ~ The piperidine ring bond lengths are in agreement with the mean values found by Herbstein and Schwotzer 15 for 13 compounds containing the piperidine fragment.The piperidine ring bond angles present differences from the mean value obtained by the same authors.These discrepancies might be due to the partial double-bond character of the C(1)-N(I) bond.This double-bond character can be inferred from the short C(I)-N(1) bond length, 1.354(2)A, and the small displacement of the N(1) atom, 0.066(1 ),~, out of the plane defined by: C(I), C (7)

Influence of the substituents
The additivity of the effect of the substituents on the aromatic ring geometry Was analyzed using the angular parameters of Domenicano and Murray-Rust, 16 Table 4.It can be seen from the value of the nonadditivity parameters, NAP, t7 that additivity is not accomplished in this compound.As neither the piperidine nor the cyclohexane parameters were available we approximate the effect of the piperidine ring by the effect of the NMe2 group.For I the NAP parameter is larger than 2tr.This would indicate interactions between substituents that destroy the independence of the effect of each one.The interactions might be due not only to the conjugative effect described above but also to steric hindrance effects.Therefore, the deviation of the aromatic ring from the ideal geometry due to steric interaction between the o-nitro group and the piperidine ring was also analyzed.The repulsive deformation parameter, RDP [RDP = ~E i = i-2 { 0((exp) -0{(cal)}; where: 0{(cal) = 1/2{360 -0i(cal)}, 01 (exp) = ,(C(2)-C(I)-N(I) and 0~(exp) = ,( C(1)-C(2)-N(2)] 17 was used and the value obtained, 4.9(5) ~ showed that steric interactions are not negligible.

Crystal packing
The analysis of the intermolecular contacts shows that some C-H. 9 9 O distances are shorter than the sum of the van der Waals radii of the involved atoms (see Table 3).These contacts, that have been interpreted as attractive interactions by Taylor and Kennard 18 and Desiraju, 19 induce the formation of infinite polar chains along the [101] direction.The influence of weak interactions in the chain organization of nitroanilines and related compounds has been discussed previously by other authors.2~ They suggested that the chain organization might precede the crystallization process but they could not account for the relative arrangement of the chains.In the title compound the above described chains are disposed in an antiparallel fashion.Short intermolecular contacts are observed between o-nitro oxygens and piperidine hydrogens related by the inversion center in (1/2,0,1/2).The antiparallel array of the chains prevents the material from presenting a     The lack of coplanarity between the o-nitro substituent and the aromatic ring in a series of N-dialkylsubstituted dinitroanilines has been inferred from n3C NMR data.4 Chemical shifts of m were calculated using the additivity rule of substituent effects with data from the literature.2n The rotation of the 2-nitro group in I and II induces shielding in the ipso carbon (C(2)) and decreases the shielding in the ortho carbon (C(3)).Studies in the solid state have shown that when the size of the alkyl substituent of the amine increases, rotation of both groups (amino and o-nitro) contribute to reduce hindrance.4 The displacement of the n3C NMR signals of C(4) and C(6) in solution in I and II is in agreement with the twisting of both groups.It can be seen in Table 5 that the A~ for I is somewhere in between those of H and Ill indicating that the steric hindrance of the cyclic amine is smaller than that of diisopropyl group and bigger than that of the diethyl group.
There is a reduction of the shielding of C(I) in I (AS = +1,2), that can be compared with the value obtained for N,N-dicyclohexyl-2,4-dinitroaniline (IV) (A~ = 0.7), 5 which may indicate that the interaction of the lone pair of the nitrogen atom with the -rr system of the aromatic ring in I is small due to rotation of the group combined with the strain of the cyclic amine.On the other hand in IV the reduced resonance is due only to the extensive rotation of the amino and ortho nitro groups.
We can use the chemical shift of the methylene protons tx to N (see Table 6) to confirm the rotation of the amino group.The chemical shift of the hydrogens of the methylene groups in cyclohexane is 1.44 ppm.22 This signal appears 0.24 ppm at lower fields than that of methylene groups in alkanes (i.e., 1.2 ppm) 2z and the difference is attributed to ring strain.Assuming that the difference in chemical shifts between the methylene protons in Ill and the methyl protons in N,N-dimethyl-2,4-dinitroaniline (V) is the same as that of any methylene and methyl protons, we can calculate the chemical shift of the methylene protons in III as 3.36 ppm.Considering the ring strain, the value calculated for ct methylene protons in I is 3.60 ppm.The difference between this value and the observed one (i.e., 3.21-3.60= -0.39)shows a shielding of the protons in piperidine that could be attributed to the ring current produced by the electrons of benzene.23 Therefore, the piperidine moiety must be rotated with respect to the main plane of the aromatic ring.UV spectra.The rotation of the o-NO2 group out of the aromatic ring in solution can also be inferred from UV data.The UV spectrum of I shows only an absorption band at 374 nm (~ = 11500).It is known that 2,4-dinitroaniline (VI) 24 in methanol shows two charge transfer (CT) UV bands: one at 336nm (e = 14450) and the other at 390nm (e = 6460), these CT bands correspond to the electronic transitions from the amino group to the p-(band 1) and o-nitro (band 2) groups, respectively.As some of us have shown in a previous paper, N-monoalkylsubstitution of 2,4-dinitroaniline essentially does not modify the UV spectra.3 This was in accordance with previous tH NMR predic-tionsY On the other hand N,N-dialkylation induces a bathochromic displacement and steric enhancement of the resonance of band 1, while steric inhibition of band 2 is observed.3 These results indicate that the electron withdrawing ortho substituent is twisted out of the phenyl ring in solution.Besides, if the amino group was rotated out of the ring plane the intensity of band 1 would be reduced.We found that in I band 1 shows a bathochromic displacement compared with VI and band 2 disappears completely indicating a rotation of the o-nitro group out of the plane of the aromatic ring.The rotation of piperidine out of this plane in I is inferred from the comparison of the value of ~ of band I with that value in compound VI, as can be seen above.The examination of the values of hmax and of the CT band between the amino and p-nitro groups in II and Ill helps to analyze the I data.In fact, the bathochromic shift in the wavelength of maximum absorption (hrnax (I) : 374, hma x (I'I) : 375 and hma~ (HI) = 380 nm) is in accordance with the steric enhancement of the resonance.The decrease in the intensity of the band 1 in I and II relative to VI is consistent with a decrease in the probability of the transition due to the noncoplanarity of the amino group with the phenyl ring.

Conclusions
The present X-ray single crystal analysis, UV, and ~H and ~3C NMR results allow a comparison of the conformation of I in solid state and in solution.The comparison led us to conclude that the main features of the molecular conformation do not change due to the packing forces that induce the chain formation.

Fig. 1 .
Fig. 1.An ORTEP-II i~ view of the molecule showing the atom labelling and 50% probability ellipsoids.
. Crystal data and data collection and refinement are shown in Table 1.Atomic parameters are given in Table 2.The UV spectra were recorded on a Shimadzu UV 260 spectrophotometer.The tH and ~3C.NMR spectra were obtained on a Bruker ACE 200 spectrometer and chemical shifts are referred to TMS.

Table 1 .
Crystal data and structure refinement, esd's in parentheses."

Table 4 .
Experimental and calculated endocyclic angles, and nonadditivity parameter value, esd's in parentheses.

Table 5 .
Experimental a Experimental values, this work.b Experimental values taken from Punte et al. 4 c Calculated values.2~