Structural and theoretical studies of 4,4'-[1,4-phenylene-bis-(azanediyl)]dipent-3-en-2-one: evidence of a π -delocalized keto-enamine

In this work we present the synthesis of 4,4'-[1,4-phenylene-bis(azanediyl)]dipent-3-en-2-one. Evidence is proposed for the presence of various tautomers of this molecule which are complemented with theoretical density functional theory (DFT)-B3LYP/6-31G* calculations to characterize the potential energy surface of these species. The experimentally observed 3b isomer crystallizes as an orthorhombic Pbcn structure; a = 10.8412(10) Å, b = 8.9205(7)Å, c = 14.9949(13)Å, V = 1450.1(2) Å 3 , Z = 4 with a final R value is 0.038. From our X-Ray crystallographic analysis an intramolecular hydrogen N-H···O interaction is observed.


Introduction
Metallo-organic complexes containing conjugated nitrogen ligands represent an important class of nonlinear optical chromophores. 1Efficient systems involving organometallic and coordination compounds with donor-acceptor bipyridyl, 2 bipyridyl ligand bearing azo and imino-linked dyes, 3 enamines, 4 or Schiff-bases ligands have the focus of other recent works. 5The direct impact of dipolar effects in drug design of functionalised bis-armed quinoxaline has been suitability demonstrated 6a as well as that with 3-nitrozo-imidaz [1,2-a]pyridines.6b Additionally, the synthesis of square-pyramidal [Co 2 Cl 2 L 2 ]Cl 2 (HL = m-phenylenediiminobis(acetylacetone) (I), p-phenylenediiminobis(acetylacetone) (II), p-diphenyldiiminobis(acetylacetone) (III)) has been described by others 6c .The corresponding ESR measurements show intra-dimer zero splitting and tetradentate coordination through both O and N atoms as in the present investigation.

Results and Discussion
It is difficult to elucidate the correct tautomeric from the IR spectra of compound 3.The hydrogen-bonded N-H bands are extremely weak and broad, spanning from about 3700 to 2700 cm -1 with the center somewhere between 3400 and 2900 cm -1 , by which this region includes the C-H vibrations as well.There is only a difference of 1% transmittance between the peak center and the baseline which makes this a difficult task indeed.The C-C ring stretching mode is clearly shown as a strong band at about 1562 cm -1 however, the carbonyl stretch is nearly masked as a weak band at 1608 cm -1 .This latter mode has been assigned in the position isomer compound labeled as 4,4'- [1,3-phenylenebis (azanediyl)]dipent-3-en-2-one previously described.5d

Theoretical methods
Theoretical computations were performed to complement the experiments being described in the present experimental measurements.All calculations in this work where carried out with the AM1 level of theory as well as the Density Functional Theory (DFT)-B3LYP/6-31G* level of theory using the GAUSSIAN03 suite of programs. 7NMR shifts were computed using the GIAO method, and referenced relative to TMS for consistancy.Further information about these methods is available elsewhere. 8Previously reported, 9 carbon shifts (in TMS) at the B3LYP/6-311+G(2d,2p) level are 182.4572ppm, and the hydrogen shift (in TMS) are 31.6297ppm.
Figure 1 (based on the representative structures shown in Scheme 2) depict the optimized structures computed at the B3LYP/6-31G* level of theory.The theoretical vibrational spectrum are shown in Figure 2. In the structural representation, all bond lengths are in angstroms (Å) and bond angles are in degrees (°).For the theoretical vibrational spectra, the frequencies are in cm -1 , and the IR intensities in KM/mol (broadened by the Doppler method).Scheme 2. Different tautomeric forms of para-phenylenediamine derivative 3.
Generally speaking, the AM1 and B3LYP/6-31G* geometrical and thermodynamic results were similar with an RMSD value of 0.2724, 1.1504, 0.4678, 0.52554, 0.29085, 0.27761 for 3a-3f geometrical parameters, respectively.The largest difference in the structures was calculated for 3b but it is still minimal in comparison.This difference might be accounted for the complex nature of this species with respect to the other structures examined.The RMSD used is the relative differences in geometrical parameters for each structure computed at the AM1 and DFT level of theory.Figure 1 depicts various selected geometrical parameters for the structures however Table 4 displays both experimental and theoretical B3LYP/6-31G* data for the observed 3b isomer.As we can see the relative deviation is quite low and it serves as an example of the efficacy of the DFT methods in evaluating the physical properties of organic structures.Table 1 presents the relative energies for the structures in kcal/mol.The table shows that the lowest energy structure is 3f (which is given a relative energy value of 0.0).As we can see the AM1 and B3LYP results are substantially different.It is known, 9 that AM1 and B3LYP give similar geometries, but rather different energies, as is apparent from the table.This is relevant since both methods are derived from semi-empiracle models, however, it is logical that the DFT method has electron correlation which exceeds AM1 in energetic computations.Therefore, it can be expected that differences should exist which are observed, however the general trends remain consistantly similar which provides an an order of stability are 3f>3a>3e>3b>3d>3c.The differences in energy can be reasoned by an observation of the structures.The 3f molecule has internal hydrogen bonding that stabilizes the structure, as has been seen for the other structures which also posses internal hydrogen bond stabilization.The compounds without-NH groups are also stabilized (i.e.3a, 3e) in comparison to those that do (3b, 3d).The final structure 3c lacks stability due to the orientation of internal amino groups.We have attempted to locate forms of the structures 3c, 3d, 3e that form N-OH hydrogen bonds but these were only partially stable.The bond order between these was around 0.005 which is less than that of 0.10 for a typical hydrogen bond.While it is a viable mechanism for molecular stabilization it certainly is not the only factor but should be considered.The structures presented are the lowest energy isomers that we were able to calculate along the potential energy surface.
Along a similar note, in 3b there are two hydrogen bonds between NH and CO that form and in 3c between N and OH.Of course in 3d the N-OH and NH---CO hydrogen bonds also tend to stabilize the molecule but as the energies suggest the stability that arises from the NH---CO hydrogen bonds are ideal.The relative stabilities of 3b and 3d are higher than that observed for 3c due to the presence of NH---CO interactions.This is an important concept, since the NH---CO bond order is around 0.12 that is considered to be strong and lead to internal molecular stabilization.
Figure 2 displays the spectra for the tautomers.Therefore, it would appear that 3c, 3e share similar peaks due to the similar structure, and 3a, 3e as well as 3b, 3d also share similar peaks, again owing to their similar geometries.Experimentally we believe 3a and 3f will be the highest product of observed species.As we can see from the figures it is clear that peaks at around 1560 cm -1 which correspond to the C=O stretches can be observed in the structures as well as those for the C=N stretches at around 1510 cm -1 .For the 3b structure it appears that they are shifted towards the experimental value of 1562 cm -1 for the C=O shifts.In the 3f case we can observe that the value of this frequency is slightly shifted due to the fact that in 3b there is less contacts with neighboring ring structures.In most of the other structures this shift is relatively consistant and shows small displacements from the mean behavior.The peaks that we calculated for 3b are at 1516 cm -1 which are in excellent agreement to those experimentally verified.The others at 2990 cm -1 are found at around 3010 in the 3b species and slightly higher for the others.The peaks that were found at 2359 cm -1 experimentally are not found on the theoretical spectra and this may be accounted for due to impurities.For the most part it is safe to assume that the experimental and theoretical separations in the observed vibrational spectra are in fact minimal and do conincide on many observed fundamental bands.
Also, in 3a, 3c, 3e we observe peaks at 1550, 1585 and at 1534 cm -1 , respectively.Perhaps these may añsp be characterized as C=O stretches but the values vary based on the environment and torsional angles that separate the structural features of each.We can also see that a very high band at around 1750 cm -1 in all structures that may consistute non-fundamental modes.In the 1562 cm -1 there are also N-H modes, which are presented at 1560, 1565, 1573, 1578, 1580, 1601 cm -1 for 3a-3f respectively, that should also contribute.Perhaps these may also be a mixture of δ as (CH 3 )/(CH 2 ) scissoring modes, since they tend to occur in the same region 16 .
While we have only considered fundamental modes there is some knowledge available on the overtones and hot transitions for certain species 14 from a theoretical perspective.Experimentally, there is very little knowledge known about the hot transitions and overtones in molecules of this type.However, other investigations have shown that hot transitions and overtones can be adequately accounted for (in correlation to experiments) by using DFT methods and gaussian basis sets 15 .To the best of our knowledge limited information on these data points are available for the compounds investigated herein.The B3LYP/6-31G* basis set has been shown to adequately reproduce vibrational frequencies for large scale systems to a reasonably good level of approximation 9,[17][18][19] .We attempted a test calculation at the B3LYP/6-311++G** level of theory and as a result the differences in energies and vibrational frequencies for the 3b/3f separation are minimal.The results computed with the current basis yield decent geometries and structures which posses a high level of precision.Additionally, since these systems are large it is difficult to conduct such a calculation in a timely fashion.
As for the NMR frequencies limited information with regards to the experimental data can be made.The only useful conclusion that can be made is that the 13 C band correspond to a shift at 196.3, 136.1 is compared to the theoretical values of 197.2, 142.1 computed at the DFT level for 3b.For 3f these values are around 200.1 and 143.2 ppm which can be attributed to the addition of electron withdrawing NH groups that arise from the tautomerization process.The NH shift observed for the 1 H shift is experimentally measured at around 12.48 compared to 13.51 ppm computed for 3b at the DFT level of theory.For 3f we computed the same shift to be 11.45 which again has to do with steric hindrance and the electronic configuration surrounding the group in this configuration.For the other structures these shifts are quite different and probably will not be located experimentally.
Tables 1-2 shows the thermodynamic parameters for the product (3) where T (temperature in K), S (entropy in J mol -1 K -1 ), C p (heat capacity at constant pressure in kJ mol -1 K -1 ), and ∆H =H°-H°2 98.15 (enthalpy content, in kJ mol -1 ), T 1 = 100 K, T 2 =298.15K, and T 3 = 1000 K calculated using the B3LYP/6-31G* frequencies.These calculations are useful for future thermodynamic studies as well as for NIST database indexing.As we can see the structures (generally) with the highest values have the most stability.Therefore, 3a, 3f, are the highest, 3b, 3d are next, and 3e, 3c are last in agreement with the theoretical data in Table 2.The crystallographic details have been provided in Table 3 and the structure was solved by direct methods and refined by least-squares on F obs 2 by using the SIR-97 and SHELXL-97 programs, respectively. 10,11All H atoms were geometrically located to the ideal positions and treated as riding atoms, with N-H = 0.86, C-H = 0.93 -0.96 Å and U iso (H) = 1.5U eq (C) for methyl H atoms or U iso (H) = 1.2U eq (C) for the other H atoms.The molecule, 3b, is shown in Figure 3; the final atomic coordinates and equivalent isotropic thermal parameters for non-hydrogen atoms are listed in Table 3, with selected bond angles and hydrogen-bond parameters in Tables 4 and 5, respectively.In the crystal structure of 3b, the asymmetric unit contains only one half-molecule.A twofold rotation axis passes midline through ARKAT USA, Inc.
the phenyl ring atoms at the C7, C6 and C8 positions for which the bond lengths and angles are within normal ranges. 12The central ring A (C6 -C8 / C7a / C6a / C8a) is, of course, planar and the O1/N1/C1-C5 group is nearly planar, with a puckering amplitude of QT = 0.1026 (2) Å 13 with the dihedral angles between these planes being 46.6(2)°.The crystal packing is along the a axis which is depicted in Figure 5.   the asymmetrical synthesis of 3f should be feasible which is currently under active investigation in our collaborative teams.The computed geometrical parameters adequately demonstrate that the two structures share a physical resemblance to the experimental data.Even more interesting is the correlation between the AM1 and B3LYP levels of theory that suggests that low level calculations qualitatively are in agreement to the correlated DFT methods.Energetic analysis shows that the lowest isomer is the 3f species which is physically realistic due to the limitations on the steric effects.Further experimental elucidation is necessary before such claim can be adequately demonstrated.

Experimental Section
General Procedures.NMR spectra ( 1 H, 13 C) were recorded on a Bruker AM 300 (operating at 300.13 MHz for 1 H, at 75.47 MHz for 13 C) spectrometer.NMR data are listed in ppm and are reported relative to tetra-methylsilane ( 1 H, 13 C), residual solvent peaks being used as internal standard.Complete assignments of the 13 C spectra required non-decoupled 13 C NMR spectra with selective 1 H decoupling. Infrared spectra were recorded in KBr pellets using a Brucker IFS28 FTIR spectrometer.

Figure 1 .
B3LYP/6-31G* structures are shown for the six different tautomeric forms of paraphenylenediamine derivatives.All bond lengths are shown in angstroms (Å) and bond angles are shown in degrees (°)

Figure 3 .
Figure 3.The molecular structure of the title compound, showing the atom labeling scheme.Ellipsoids represent displacement parameters at the 50% probability level [Symmetry codes: (a) −x, y, 1/2 − z ].

Figure 4 .
Figure 4.The crystal packing of 3b, viewed along the a axis.

Table 1 .
Total energies (hartrees/particle) and relative energies (kcal/mol) relative to the lowest energy structure (3f).0 K is the sum of zero-point and electronic energies

Table 2 .
Physical properties of the computed at the B3LYP/6-31G* level of theory The crystal structure of 3b, C 16 H 20 N 2 O 2 , has been determined at room temperature.Molecules crystallize in the orthorhombic space group Pbcn and the asymmetric unit contains only one half-molecule.The molecule lies on a twofold rotation axis and hence posses C2 molecular symmetry.

Table 3 .
Crystal, X-ray data collection and refinement parameters for 3b