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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.1 Concepción mar. 2004

http://dx.doi.org/10.4067/S0717-97072004000100008 

IR AND 13C-NMR SPECTRAL PROPERTIES OF THE LAYERED
INCLUSION COMPOUND BIS(THIOUREA)HEXAMETHYLENETETRAMINE.

Paul Jara1, Guillermo González1,Víctor Manríquez1, Oscar Wittke2, and Nicolás Yutronic1

1, Faculty of Sciences, Universidad de Chile, Casilla 653, Santiago de Chile* nyutroni@uchile.cl.
2 Faculty of Physical and Mathematical Sciences, Universidad de Chile, Santiago de Chile

(Received: July 31, 2003 - Accepted: October 2, 2003)

ABSTRACT

The structure of the 1: 2 adduct of hexamethylenetetramine and thiourea was reanalyzed reinterpreting X-ray diffraction data and providing additional IR and NMR spectroscopic features. The product is thus described as supramolecular commensurate host-guest complex corresponding to the intercalation of hexamethylenetetramine in a layered thiourea matrix. FT-IR spectra evidence a simultaneous strengthening of both C-N and C-S bonds, that appearing as a pattern characteristic for described layered thiourea matrix. 13CP MAS-NMR spectra of the intercalated hexamethylenetetramine is essentially determined by nuclear dipolar magnetic interactions and host-guest chemical interactions arising from 13C-14N residual dipolar interaction not averaged to zero by MAS-NMR and from the magnetic non-equivalence of the guest carbon atoms caused by the hydrogen bonding interaction with the thiourea matrix .

Key Words: Thiourea, hexamethylenetetramine, inclusion compound.

INTRODUCTION

Host-guest supramolecular complexes are of special interest for understanding the chemistry in low dimensional spaces. The molecular recognition involved in the formation of such structures sometimes may be a relevant model for the kind of organized system usually found in living organisms. Matrix effects and anisotropic features which are habitual of the chemistry in restricted spaces also appear as useful for the development of new material of scientific and technological importance1).

Urea and thiourea clathrates constitute interesting systems in which the matrix being structured by hydrogen bond interactions has a relatively high liability to structural changes caused by the interaction with the host2). In the frame of studies directed to investigate the influence of the basicity of the guest on the structure of the clathrate in thiourea-amine supramolecular complexes3-7) we have focused our attention in the adduct hexamethylenetetramine-thiourea discovered by Mak et al8). This phase presents a structure which differs from that observed in the conventional thiourea clathrates, that possibly due to the relative strong interaction of the amine with the thiourea. We have therefore reanalyzed such structure providing further additional IR and NMR spectroscopic features in order to get a better understanding of the interactions which govern the formation of this kind of compounds.

EXPERIMENTAL

The product was prepared by mixing in a stoichiometric ratio 2:1 the thiourea (Aldrich) and the amine (Merck), both dissolved in methanol (Merck), at room temperature. Slow solvent evaporation leads to large plate-shaped crystals which were collected, washed with cold methanol, and dried under vacuum. The colorless crystalline product melts at 429 K has the composition C8N8H20S2 (Analytical data, found (calculated): C 33.03 (32.86); N 37.97 (38.32); H 7.18 (6.89).

X-Ray diffraction analysis was performed using a single crystal with dimensions 0.14, 0.20, 0.24 mm. The structure was solved by direct methods (Siemens R3m/V diffractometer with graphite monochromated Mo Kα radiation (λ= 0.71073 Ε)) at 298 K. Positions for the hydrogen atoms were calculated geometrically using the riding model with fixed isotropic temperature factors. Values of the obtained final indices were R = 4.04% and Rw = 4.78%. A final difference Fourier electron-density map showed a maximum peak of 0.22 eÅ-3. IR determinations were performed in a FT-IR Perkin Elmer 2000 spectrophotometer using spectroscopic potassium bromide disk (Merck) in the range 4000 a 450 cm-1.

RESULTS AND DISCUSSION

Structural Aspects

The ORTEP representation (Figure 1) and obtained structural data, some of which are reproduced in Table 1, are fully consistent with those mentioned in the paper of Mak et al8). In that work the macromolecular structure of the compound is described as formed by corrugated layers consisting each of infinite zigzag chains, constituted by 1:2 amine-thiourea molecular aggregates, cross-linked by relatively weaker hydrogen bonds. However, analyzing obtained structural data we have arrived to an alternative description of the same structure: A layered thiourea matrix with HMTU molecules located in the interlaminar spaces. This view, which emphasizes the host-guest complex nature of the product, permits the comparison of this entity with other thiourea supramolecular species, thus contributing to the understanding of the influence of the guest on the structural characteristic of the inclusion compound.


Fig. 1: ORTEP diagram of (2Thiourea)(HMTA). Thiourea and Hexamethylenetetramine are regularly associated. Each hexamethylenetetramine is tethered to two different thiourea molecule through H-bonds (Nhost(3a)-Nguest(1a)=3.73 Å)

Table 1. Structural parameters and other information relating to the crystal structure refinement of (2 Thiourea) (HMTA).

Space group C2/c
Lattice parameters a = 18.424(4) Å, b = 8.39(2) Å,
  C = 9.485(11) Å; a = b = 90°,
  g = 109.85 (3)°
No of unique reflexions with I > 4s (I) 548
R 0.0514


Atom

x

y

Z

U11/ Å2
U23/ Å2

U22/ Å2
U13/ Å2

U33/ Å2
U12/ Å2


S

0.345(1)

0.2528(1)

0.2028(1)

0.035(1)

0.043(1)

0.049(1)

0.007(1)

0.012(1)

- 0.002(1)

N(1)

0.4513(1)

0.1592(1)

0.6238(1)

0.023(1)
0.002(1)

0.045(1)
0.008(1)

0.040(1)
0.001(1)

N(2)

0.5517(1)

0.3662(1)

0.6954(1)

0.027(1)
0.000(1)

0.041(1)
0.022(1)

0.057(1)
- 0.002(1)

N(3)

0.3097(1)

0.0390(1)

0.3731(1)

0.054(1)
0.004(1)

0..051(1)
0.017(1)

0.044(1)
- 0.013(1)

N(4)

0.2169(1)

0.0838(1)

0.1453(1)

0.048(1)
- 0.001(1)

0.066(1)
0.014(1)

0.045(1)
- 0.020(1)

C(1)

0.5000

0.0601(1)

0.7500

0.043(1)
0.000(1)

0.034(1)
0.008(1)

0.043(1)
0.000(1)

C(2)

0.4030(1)

0.2623(1)

0.6811(1)

0.020(1)
0.011(1)

0.049(1)
0.011(1)

0.053(1)
0.004(1)

C(3)

0.5034(1)

0.2622(1)

0.5747(1)

0.041(1)
0.003(1)

0.052(1)
0.018(1)

0.043(1)
- 0.002(1)

C(4)

0.5000

0.4630(1)

0.7500

0.043(1)
0.000(1)

0.037(1)
0.035(1)

0.077(1)
0.000(1)

C(5)

0.2859(1)

0.1168(1)

0.2428(1)

0.044(1)
- 0.007(1)

0.036(1)
0.018(1)

0.039(1)
- 0.001(1)



Bond lengths (Å)

S¾ C(5)

1.708(1)

N(3)¾ C(5)

1.334(1)

N(4) ¾ C(5)

1.323(1)

N(1) ¾ C(2)

1.471(1)

N(1) ¾ C(3)

1.479(1)

N(1) ¾ C(1)

1.483(1)

N(29¾ C(3)

1.474(1)

N(2) ¾ C(4)

1.474(1)

N(2) ¾ C(2A)

1.471(1)

C(2) ¾ N(2a)

1.471(1)

C(4) ¾ N(2A)

1.474(1)

C(1) ¾ N(1A)

1.483(1)


Bond angles (°)

C(2)¾ N(1)¾ C(3)

108.1(1)

C(2)¾ N(1) ¾ C(1)

107.9(1)

C(3)¾ N(1)¾ C(1)

107.6(1)

C(3)¾ N(2)¾ C(4)

107.9(1)

C(3)¾ N(1)¾ C(1)

107.3(1)

C(4)¾ N(2)¾ C(2A)

107.8(1)

S¾ C(5)¾ C(3)

119.9(1)

S¾ C(5)¾ N(2A)

121.3(1)

N(3)¾ C(5)¾ N(4)

118.8(1)

N(1)¾ C(2)¾ C(2A)

113.1(1)

N(1)¾ C(3)¾ N(2)

112.8(1)

N(2)¾ C(4)¾ N(2A)

113.1(1)

N(1)¾ C(1)¾ N(1A)

111.8(1)

The basic unit of this kind of thiourea matrix is a dimer formed by two thiourea molecules hydrogen bonded each other via their N-H protons cis to sulfur as illustrated in scheme of Figure 2. Both the thiourea molecules and the dimers are practically planar species (mean deviation to planarity is about 0,002 Å). Each dimeric unit is in turn hydrogen bonded to other four ones oriented perpendicularly to the former via the same nitrogen and sulfur atoms involved in the formation of the dimer, a and b in the scheme above, thus bearing to the zigzag surfaces illustrated in Figure 3. One of the protons of the NH2 groups, labeled with c in Figure 2, does not participate in the formation of the thiourea network being directed towards the spaces between the thiourea layers where the guest species HMTA are located. Each amine molecule is indeed hydrogen bonded to two thiourea molecules in adjacent sheets, in the way described schematically in Figure 4, thus bearing to a pillared structured solid. Figure 5 shows two views of the supramolecular structure of (2 Thiourea)(HMTA) illustrating the layered arrangement of thiourea molecules are intercalated.


Fig. 2: Schematic representation of the dimeric basic unit in the matrix structure for the inclusionCompound (2 Thiourea)(HMTA). Hydrogen bond (N× × × S) distances: 3.45 Å.


Fig. 3: Two structural representation of the layers in the thiourea matrix for the compound (2 Thiourea)(HMTA). In (b) the dimeric basic units are represented by filled hexagons


Fig. 4: Scheme of the location and bonding of the hexamethylenetetramine molecules to two adjacentthiourea layers.


Fig. 5: The supramolecular structure of (2 Thiourea)(HMTA) illustrating the layered arrangement of thiourea molecule in which the hexamethylenetetramine molecules are intercalated. (a) View of the layers directional plane. (b) A view perpendicular to the layer, where hexamethylenetetramine are represented by filled polyhedra.

Infrared Spectra

The host structure described above is evidenced too by the IR spectrum of the product. As observed in Table 2 characteristic bands of the thiourea as well as some absorption arising from the guest may be identified. The effect of the inclusion on the thioamide bond can be there clearly observed. As shown in Table 2, shifts of both C-S and C-N stretching modes to higher frequencies respect to those in the orthorhombic thiourea are observed. Such a reinforcement of the C-N bond is also observed in the formation of typical clathrates (rhombohedric); however, the simultaneous strengthening of the C-S bond appears to be a feature characteristic for the layered thiourea matrix described here. The dimeric nature of the structural ground units (scheme of Figure 2) and the way by which these dimers are arranged in the thiourea network, in which both S-lone pairs are involved, lead to an amide bond description in which the typically sp2 sulfur hybrid atomic orbital display a relatively higher s character. The strengthening of the S-C bond caused by such an electronic configuration, in addition to the p delocalization proper for the thioamide bond, explains the observed spectroscopic features.

Table 2. Stretching frecuencies (cm-1) of thiourea pure and in some inclusion compounds.


Thiourea Compound

n (C-N)

n (N-H)

n (C-S)


Orthorhombic Thiourea

1465

3165

725

(6 Thiourea)(Dicyclohexylamine)

1490

3165

720

(3 Thiourea)(Quinuclidine)

1470

3165

720

(2 Thiourea)(Hexamethylenetetramine)

1480

3165

730


The strong bands associated to the N-H amide bonds, remains practically unaltered and no information about the host-guest interactions may be obtained by this method.

Table 3. Medium effects on 13C-NMR chemical shifts in hexamethylenetetramine.


Medium

13C-NMR Chemical Shifts (ppm)


Thiourea

73.96
75.78*

CDCl3a

74.89

CDCl3b

74.91

CCl4

75.78


a 1% p/v
b Saturated solution
*Doublet with intensity 2:1

13C-NMR Spectra

In table 3 are reported the 13C-NMR spectra of HMTA in the solvents CCl4 and CDCl3 as well as in the intercalation described above. As observed in Figure 6, the HMTA 13C-NMR spectrum in the intercalation product corresponds to two complexes, broad signals with maxima at 73.96 and 75.78 ppm, respectively. That contrast with the spectrum of free HMTA with six magnetically equivalent carbon atoms, for which only one 13C-NMR only one signal is expected. Thus in a CCl4 solution the observed chemical shift is indeed 75.78 ppm. The complexity of the spectrum in the solid phase essentially arises from two different factors, one due to nuclear dipolar magnetic interactions and the other to host-guest chemical interactions. A more detailed analysis of the spectrum can be obtained of the spectrum deconvolution considering both, the host-guest chemical interactions and dipolar nuclear magnetic interactions; results are illustrated by the segmented lines in Figure 6. The magnetic non-equivalence of the HMTA carbons is produced by the interaction with the TU-matrix. As shown in the structural description above, the amine is anchored to the matrix only by two of its four nitrogen atoms. As deduced from the interaction scheme illustrated in Figure 5, two of the six methylen groups should be more affected by the host-guest hydrogen-bond interactions, Thus, three signals with intensities 1:4:1 centered at 76.8, 74.0 and 73.0 ppm are expected. The chemical shifts indicate that the matrix effects on the methylene groups are similar to those occurring in the carbon tetrachloride and chloroform solutions, respectively. The relative magnitude of the effect on the two methylene groups are also in accordance with the acceptor capacities observed for these solvents which have acceptor numbers of 8.6 and 23.1 relative to n-hexane9). Moreover, each line is in turn splitted into two lines of the same intensity separated approximately 1.8 ppm, that agreeing with the observed signal intensities. That because for 13C nuclei directly bonded to nitrogen (14N, I=1), MAS of microcristalline samples frequently give patterns consisting in doublets caused by 13C-14N residual dipolar interaction, not averaged to zero by MAS-NMR10). The axis of quantization of the 14N nucleus is tilted from the direction of the static magnetic field as a consequence of the interaction between 14N nuclear moment and the electric field gradient at the 14N nucleus4, 10). That bearing he splitting into doublets of the two signals already commented. The residual dipolar interaction between 13C and 14N which results to be about 250 Hz, is about ten fold higher than in Ca of amine in typical clathrates3-7).


Fig. 6: HMTA 13C-NMR spectrum zone of (2 Thiourea)(HMTA) and its deconvolution, considering the host-guest and dipolar nuclear magnetic interactions.

Furthermore, the MAS 13C-NMR spectrum of the compound shows a resonance line at 182.0 ppm corresponds to the carbon of the thiourea molecule, different chemical values have been reported previously3-7) or amine-thiourea systems (about 180-182 ppm). 13C-NMR measurements of crystalline thiourea and of thiourea in CDCl3 solution show single resonance lines at 180.8 and 184.5 ppm, respectively.

CONCLUSIONS

The thiourea-hexamethylenetetramine supramolecular complex described here is a commensurate host-guest complex which can be described as an intercalation of hexamethylenetetramine in a layered thiourea matrix. The IR and 13C-NMR analysis of the compound indicate features that totally agree with the observed structure and show that both matrix and host are altered by the interaction. Indeed, the guest induces in this case a host structure different to classical thiourea clathrates and, simultaneously matrix effects are altering the electronic structure of the intercalated HMTA. The different nature of the hydrogen bonds and the structural versatility of the amide and thioamide matrices2) involved in such complexes make them suitable, at least in a first approximation, as alternative models for molecular biological systems. Dynamic NMR studies which could improve this perspective are in progress.

ACKNOWLEDGEMENTS

The research was financed by FONDECYT (1010891)

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