Study on self-assembling molecular complexes through hydrogen bonding

J o u r n a l of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 446 (1998) 103-108

Study on self-assembling molecular complexes through hydrogen bonding Ruifeng Zhang, Haipeng Zheng*, Jiacong Shen Key laboratoryfor Supramolecular Structure and Spectroscopy, Department of Chemistry, Jilin University, Changchun 130023, People's Republic of China

Received 8 July 1997; accepted 24 November 1997

Abstract A self-complementary hydrogen-bonding recognition unit--2,5-bis(alkylamino)-l,4-benzoquinone has been synthesized and studied. The formation of hydrogen bond in both solid and solution state has been further investigated by means of FFIR, IH NMR and UV-visible spectra, as well as X-ray diffraction. It can be concluded from the obtained results that the molecular complexes of this kind of compounds have a ribbon-like backbone which plays an important role in keeping the layer supramolecular structure. © 1998 Elsevier Science B.V. Keywords: Hydrogen bonding; Molecular complex; Aggregate

I. Introduction It has been learned from nature that organic molecules storing information of molecular recognition are capable, in principle, of self-assembling into supramoleular system presenting the desired structural and functional features. Hydrogen-bonding as well as m e t a l - l i g a n d and aromatic stacking interactions are well-known for being used as basic driving and directing force for molecular recognition and selforganization because of their unique steric requirement for interaction. Up to now, there has been a large amount of work focused on the design and synthesis of hetero- or self-complementary hydrogen-bonding recognition subunits which can form many kinds o f supramolecular architecture such as *Corresponding author. Fax: 00 86 431 892 3907; e-mail: [email protected]

cyclic arrays, infinite two-dimensional array and helical arrays as summarized by Lawrence et al. [1 ]. In this paper we are interested in those moieties creating stiff ribbon-like supramolecular structure through hydrogen bonding. Some original works have been carried out to develop many molecular complexes with ribbon-like structure. For example, 5,5-diethylbarbiturid acid has been employed to coordinate melamine derivatives [2] or 5-butylprimidine2,4,6-triamine [3], generating organized polymolecular tapes in both solution and solid state. In addition, 4,6-diamino-5-octylpyrimidine-2(1H)-one alone can also form a ribbon-like structure by crystallizing from D M F solution [4]. The c o m m o n feature of these molecular complexes is the formation of a triply hydrogen bonding network between subunits. Such a supramolecular structure may confer specific properties on the material at macroscopic level. The generation of an oriented series of functional side groups

0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0022-2860(97)00436-5

104

R. Zhang et al./Journal ~ff"Molecular Structure 446 (1998) 103-108

NHR_

O

O

along a strand will offer the opportunity of inducing such novel properties as non-linear optical [5-7], electron transfer or energy transfer. In this paper we studied a self-complementary hydrogen-bonding recognition unit--2,5-bis(alkylamino)- 1,4-benzoquinone. According to its chemical structure, on each side it has both a hydrogen bond donor and acceptor which tend to form intramolecular hydrogen bonding. At the same time the right side is a suitable partner of the left side for intermolecular hydrogen bonding. Although it has not been possible to grow a single crystal large enough for X-ray diffraction, we believe that this material has the same molecular structure (shown in Scheme 1) as those compounds in the previous works [8,9] proved by crystallographic analysis. The coexistence of inter- and intramolecular hydrogen bonding makes the compounds form the ribbon-like structure of main chain, which has a critical effect on the phase behavior of this series of compounds with different side groups.

as two typical compounds, were synthesized according to Scheme 1. DBBQ [2.66 g (0.01 mol)] was dissolved in 20 ml DMF, to which an excess of cyclohexylamine or dodecylamine was added. The reaction started rapidly accompanied with exothermicity and the appearance of a red precipitate, and continued 30 min with magnetic stirring. Then the mixture was heated to 100°C over the course of 10 min until all the precipitate disappeared to form a deep-color solution, after which the mixture could be cooled at room temperature until no more crystal was produced from the solution. The crystal was filtered and washed with ethanol, and the obtained product was recrystallized in hot ethanol and dried in a vacuum. The yield of BCBQ and BDBQ is 2.45 g (81%) and 4.08 g (86%), respectively. Calcd for BCBQ (C18H26N202): C, 71.52; H, 8.61; N, 9.27. Found: C, 71.49; H, 8.60; N, 9.29. Calcd for BDBQ (C30HsaN202): C, 75.95; H, 11.39; N, 5.91. Found: C, 75.91; H, 11.41; N, 5.92. BCBQ and BDBQ are less soluble in polar solvents such as DMF, ethanol and DMSO, but soluble in CHC13, THF and CH2C12 as a result of bearing alkyl groups on both sides of backbone. These compounds with short alkyl groups including BCBQ have no melt point, which can cause an exothermic reaction (the intra- and intermolecular hydrogen bonding network maybe causes a dehydrated reation) by DSC measurements (a Perkin-Elmer DSC 7 calorimeter, heating rate of 10°C/min) and form new products when heated to over 170°C, but BDBQ has melt point of 147°C. The structure of the obtained crystals were studied by FTIR (Nicolet-7199 model, KBr pellets) and X-ray diffraction (Rigaku D/MAX-IIA,o using copper K~ radiation of wavelength 1.542 A). The hydrogen bonding behavior of these two compounds in solution was studied by means of 1H NMR (Joel FX-400) and UV-visible spectra (UV-3100 spectrometer).

2. Experimental

3. Results and discussion

2,5-dibromo-l,4-benzoquinone(DBBQ) was first prepared according to Ref. [10] and recrystallized in THF or acetone-ethanol. Yield: 65%. m.p. = 190°C. Elem. Anal. Calcd for C6H2Br202: C, 27.07; H, 0.75; Br, 60.15. Found: C, 27.03; H, 0.76; Br, 60.21. 2,5-bis(cyclohexylamino)- 1,4-benzoquinone(BCBQ) and 2,5-bis(dodecylamino)- 1,4-benzoquinone(BDBQ),

First the two crystals BCBQ and BDBQ were studied by FTIR (see Fig. 1) and the assignment of the major peaks was summarized in Table 1, in which one important peak concerned with hydrogen bonding in solid state is the N - H stretching vibration at 3246cm -1 for BCBQ and 3252cm -j for BDBQ. Obviously both of them are shifted towards low

8r

RNH2 Ii,

-HBr

Br

RHN- "~

o

o

~

bonding

R

R

R

I

I

!

N ..2H -I

R

N E ; H ..:o I

R

I

R

Scheme 1. The synthesis of molecular complexes.

R. Zhang et al./Journal of Molecular Structure 446 (1998) 103-108

.=_ b.,

I

3000

I

2500

I

2000

1500

1000

Wavenumber(cm-I) Fig. 1. FTIR spectra of BCBQ (top) and BDBQ(bottom). wave number in comparison with the normal N - H vibration of around 3300 cm -j. A special peak at 1567cm < for BCBQ and 1582cm < for BDBQ ascribable to the deforming vibration of N - H bond is characteristic of hydrogen bonding in the form of N-H...O, and the C=O streching vibration occurs low-wavenumber shift compared with that of DBBQ at 1680 cm -j. These two kinds of peaks might suggest a strong conjugation of the molecular fragment O - C C - N , which is consistent with the resonance structures given by Kulpe [8]. The three most important contributors to the structure are 1, 2, and 3, giving the overall structure 4. o H

oN

N

O

_O

1

2

81-

+y-

"n"

I ~

-H

+N'-

~'++< I

"-#-- -H oil

3 4 The IH NMR spectra of BCBQ and BDBQ in CDC13 are shown in Fig. 2. All the peaks that appeared in the spectra are quite in accordance with the chemical structure given in Fig. 2. In fact, the protons involved in hydrogen bonding often exhibit

105

a downfield-shifted chemical shift compared with those free from hydrogen bonding. The peaks at 63.9 and 63.17 ppm on the top spectrum might be resonances of protons involved in both intra- and intermolecular hydrogen bonding and those only in the form of intramolecular hydrogen bonding located at the end of aggregating entity of BDBQ, respectively, showing that the coexistent hydrogen bonding is stronger. As for BCBQ, the two equivalent peaks appeared at 64.6 and 63.25 ppm. It should also be noted that hydrogen bonding is a kind of weak interaction in comparison with its covalent counterparts. Thus, aggregation of molecules in solution state based on hydrogen bonding is always in equilibrium. The discernment of these two types of protons made it possible to gauge the number of repeat units in aggregate from the ~H NMR spectra. According to the integration of these peaks, the average number of structural unit in the aggregate of BDBQ in solution is calculated to be 9.6 and that of BCBQ is 11.2. Further, it should be pointed out that the NMR peaks related to hydrogen bonds for both BCBQ and BDBQ were slightly split. This phenomenon can be explained in terms of the spatial configuration of hydrogen bonding backbone (see Fig. 3). In fact the ribbon-like backbone we studied is not a twodimensional structure and the benzoquinone rings in the structure are not placed on the same plane but have an angle between each other. Therefore the side alkyl groups have two different ways to be positioned as described in Fig. 3 which causes the splitting peaks of hydrogens involved in both inter- and intramolecular hydrogen bonding. We think that in solution state the side groups may transfer from one position to another quickly, so that the splitting of the peak is somewhat symmetrical. The UV-visible spectrum of BDBQ in CHC13 is almost the same as that of BCBQ, so only the spectrum of BCBQ is shown in Fig. 4. The main absorption at 361 nm (e = 22 300) can be attributed to the a ' - r * transition of benzoquinone derivative. The relatively small peak at 520 nm (~ = 95) corresponds to the n-a-* transition of unbonded electron on nitrogen atom. We can explain why the absorption of the n-Tr* transition is so weak compared with that of the 7r-Tr* transition--the rotation of alkylamino group around ~b-N bond has been hindered by stiff structure as a result of the formation of hydrogen bonding

106

R. Zhang et al./Journal


Structure

446 (I 998) 103- 108

b

a

Q a

b

a

b

c

8

7

6

4

5

3

2

I

0

G(ppm)

Fig. 2. ‘H NMR spectra of BCBQ and BDBQ in CDC13.

network. Therefore it is difficult for n orbit on nitrogen atom to overlap with a orbit on phenyl ring to make the n-?r* transition. It has also been found that the optical absorption of the n-R* transition increases dramatically when the temperature of solution rises from room temperature to 50°C. Fig. 5 displays X-ray diffractograms taken from BDBQ and BCBQ powder at room temperature. In the small angle region a sharp and strong reflection Table 1 Assignment R

Fig. 3. The possible configuration

of hydrogen

bonding backbone

of FTIR spectra N-H...0 str

Cyclohexyl

3246”

Dodecyl

3252

CH3 str

2953

“All the data are in wave number (cm-‘).

CHz str 2928 2853 2920 2851

asym sym asym sym

C=O...H str

N-H...0 def

6 str

CHz

16.50

156-l

1490

1448

1654

1582

1488

1452

def

CHj def

N-d str 1317

1431

1324

107

R. Zhang et al./Journal of Molecular Structure 446 (1998) 103 108

2.0

1.5

~= k0

0.5

0 300

400 500 Wavelength (nm)

600 10

Fig. 4. The UV-visible spectrum of BDBQ in CHCI3.

2(1 (degree)

20

30

Fig. 5. X-ray diffraction pattern of BCBQ (top) and BDBQ (bottom) powders.

together with its higher orders is observed. This reflection shifts to a lower angle when the length of the side group increases. According to our interpretation of the X-ray data, a layered structure can be proposed for both BDBQ and BCBQ as illustrated in Fig. 6. The layer spacing for BDBQ and BCBQ can be calculated to be 24.24 and 13.21 A, respectively, according to the Bragg equation. The side groups occupy the most part of the space between layers. Each layer is formed by a lateral packing of ribbon-like backbone, as can be elicited from the strong reflections at 20 -23.13 ° (3.84 A) for BDBQ and 20 = 24.62 ° (3.61 A) for BCBQ. These lateral space are close to the van der Waals' radius of the phenyl ring (3.45 A) suggesting that the rigid tapes in each layer are closely stacked. In fact, the relationship between molecular structure and phase behavior of this hydrogen bonding systems is very similar to that of those polymers composing stiff chains and flexible side chains [11-14]. Generally speaking, the stiff main chain in these polymers plays a key role in establishing and maintaining the ordered structure during phase transition and the flexible side chain acts as a bound solvent or an internal plasticizer, improving the solubility and fusibility of these polymers. This basic principle can also be seen in our hydrogen bonding systems, for example if the side group illustrated in Fig. 6 is methyl group, the obtained compound is less soluble in most organic solvents and infusible before decomposition just as the behavior of many conjugated polymers. As for BDBQ which contains long flexible side groups, it

rl

i~

R

~

',

R

I

R

~

~ ~'~,.,

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~,

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. ~.~'~, ',,

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R

.

~%

R

~,

~

,

~

t

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/

Fig. 6. The layered structure model for hydrogen bonding molecular complexes.

not only has a good solubility, but also displays thermotropic behavior like some liquid crystalline polymers when heating to a proper temperature region (between 105°C and 147°C) based on the fact that hydrogen bonding is strong enough to maintain its ribbon-like structure even in melt state.

Acknowledgements

The authors would like to thank the Polymer Physics Laboratory, Changchun Institute of Applied Chemistry, Chinese Academic of Sciences, for its financial support.

108

R. Zhang et al./Journal +~['Molecular Structure 446 (1998) 103-108

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[7] S.R. Marder, J.W. Perry, W.P. Schaefer, Science 245 (1989) 626. [8] S. Kulpe, J. Prakt. Chem. 312 (1970) 909. [9] S.J. Pettig, J. Trotter, Can. J. Chem. 53 (1975) 777. [10] J.F. Bagli, Ph. L'Ecuyer, Can. J. Chem. 39 (1961) 1037. [ 11 ] M. Ballauff, Makromol. Chem. Rapid Commun. 7 (1986) 407. [12] J.M. Rodrignez-Parada, R. Duran, G. Wegner, Macromolecules 22 (1989) 2507. [13] H.R. Kriecheldorf, A. Domschke, Macromolecules 27 (1994) 1509+ [14] H. Kim, S.-B. Park, J.C. Jung, W.C. Zin, Polymer 37 (1996) 2845.