Synthesis Nano Dendrimer Supramolecular with Melamine Core

Introduction

The first dendrimers, named “cascade” molecules, were produced by Vögtle et al. in 1978 [1]. Dendrimers are constructed in a stepwise manner through repeatable synthetic steps [2]. Each repetition cycle forms an additional layer of branches, called “generation” [3,4]. Two major synthetic approaches have appeared: the divergent approach in which growth starts from the inside (core) proceeding outwards, and the convergent approach proceeding “outside-in”,by first creating “dendrons” which are coupled to the core[5].Generally, synthetic chemists now have a considerable degree of control over the covalent synthesis of dendritic superstructures using a wide range of organic and inorganic methodologies[6]. Growing attention has turned to the production of dendritic structures using supramolecular synthetic methods in other words, synthesisingdendrimers held together by non-covalent (or supramolecular) interactions[7].Supramolecular chemistry lies beyond molecular chemistry based on the covalent bond and it is the chemistry of the entities generated via intermolecular noncovalent interactions[8–10]. These forces consist of hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi-pi interactions and electrostatic effects[11].The directionality and strength of the supramolecular bonding are significant features of systems that can be regarded as polymers and that behave based on well-established theories of polymer physics[12,13].Three main categories of supramolecular polymers can be distinguished: coordination polymers, polymers formed via π-π stacking of the monomeric units, and hydrogen bonded polymers. Hydrogen bonding interactions are very useful for the construction of supramolecular polymers due to their strength, directionality and reversibility[14].Both fully formed spherical dendrimers and individual dendritic branches (or dendrons) have been of the center of attention for their application in supramoleculardendrimer chemistry[15].Melamine and related triazine derivatives have the ability to form self-assembling, high molecular weight compound via organized intramolecular networks of hydrogen bonds and π-π aromatic ring stacking[16,17].The unique capabilities exhibited by melamine and related triazine derivatives present useful molecular scaffolding components employed by the field of supramolecular chemistry to produce a variety of sophisticated nano- or microscaled molecular compound[18,19]. In this article, a new melamine-based dendrimer is synthesized which its supramolecular and nano dimensions are studied.

Materials and Method

Materials

Melamine, methyl acrylate (Sigma- Alridch) were purified using distillation plant. Dimethyl sulfoxide (DMSO), methanol, sodium azide (Merck) were used for the synthesis.

Equipments

FT-IR analysis was performed using a SHIMADZU FTIR-8400S Spectrometer by KBr pellet technique.  ¹H and ¹³C NMR spectra were carried out on a Bruker AC 300 MHz nuclear magnetic resonance (NMR) spectrometer. The MS system employed in this study consisted of 5973 Network Mass Selective Detector of Agilent Technology (HP).

Method

Synthesis of dendrimers

Synthesis of the dendrimerG_0.5  with ester terminated group

A solution of freshly distilled melamine (5.0 g) and sodium azide (0.66 g) in DMSO (50 ml) was stirred for 1h at 25°C and there was drop wised added over period of 2h to a stirred solution of methyl acrylate (10.78 ml) in DMSO (50 ml) at room temperature. The final mixture was refluxed at 70°C temperature. The solvent was removed under reduced pressure at 70°C using a rotary evaporator and the resulting cream solid washed with methanol and dried under vacuum (10^-1 mm Hg, 80°C) to give the pure product (12 g, 80%) (Fig 1).

Spectral characterization of G_0.5 has been determined as follows:  FT-IR (υ_max/Cm^-1): 1651(C=O); ¹H-NMR(CDCl₃) δ_H: 6.222(3H,a), 3.60(9H,d), 3.475(6H,b), 2.515(6H,c);¹³C-NMR(CDCl₃) δ_C: 172.61(d), 167.66(a), 51.80(e), 40.78(b), 39.04(c); m/z:384, 341, 294, 281,252,212,126,110,86,68.

Figure 1: Synthesis of dendrimerG0.5  Click here to View figure

 

Preparation of amine terminated gruop G₁

A solution of precursor (G_0.5); (10 g) in DMSO (50 ml) was carefully added to a vigorously stirred solution of melamine (9.84 g) and sodium azide (1.32 g) in DMSO (50 ml) at 25°C. The mixture was stirred for 96h at 70°C. The solvent was removed under reduced pressure at 70°C using a rotary evaporator and the resulting cream solid washed with methanol and dried under vacuum (10^-1 mm Hg, 80°C) to give a pure product (17 g, 85%) (Fig 2).

Spectral characterization of PAMAM (G₁) is demonstrated as follows    

FT-IR (υ_max/Cm^-1): 1640(C=O), 3200(NH₂), 3400(NH); ¹H-NMR(CDCl₃) δ_H; 7.921(3H,d), 6.287(15H,a,e), 3.517(6H,b), 2.517(6H,c); ¹³C-NMR(CDCl₃) δ_C:180.15(f), 167.64(d),       167.55( a), 158.98(e), 40.73(b), 38.99(c); m/z: 667, 650, 638, 577 ,468, 431, 381, 369, 340, 327, 296, 212,126,85.

Figure 2: Synthesis of dendrimerG1. Click here to View figure

 

Results and Discussion

Experimental data are according to this fact that the various generation of denderimers have been synthesized.

¹H and ¹³C NMR spectra were also fully consistent with the proposed structures. It is interesting to note that the peaks corresponding to the aromatic ring were perturbed by the presence of the dendritic branching.

H-NMR data analysis  also confirmed the synthesis of dendrimers in which hydrogen of NH were appeared at 6.222 ppm and hydrogen of metoxy groups  at  3.60    ppm in  G_0.5  while hydrogen of amino groups have been  at  6.287   ppm and hydrogen of amido groups at 7.921 ppm for G₁ (Fig 3).

We expected to observe peaks related to NH and NH₂ groups in higher fields but they showed about 2 ppm shift toward low field which indicates hydrogen bond formation in dendrimer structure.

Figure 3: H-NMR spectrum of a: G0.5, b: G1  Click here to View figure

 

¹³C-NMR data analysis also confirmed the synthesis of dendrimers in which carbonyl functional groups (C = O) were appeared at 172.61 ppm andcarbone of metoxy groups at 51.80 ppmin G_0.5  and carbone of amido groups have been at 167.64 ppm and carbone of aromatic ring at 158.98 , 180.15 ppm for G₁ (Fig 4).

Figure 4: 13 C-NMR spectrum of a: G0.5, b: G1  Click here to View figure

 

FT-IR analysis for different generations indicates the presence of metoxy groups in G_0.5, which is replaced by amino groups in G₁ (Fig 5).

Considering frequency depletion of carbonyl group in IR spectrum it can be concluded that hydrogen formed.

Figure 5: FT-IR spectrum of a: G0.5, b: G1  Click here to View figure

 

Characterisation of the novel dendritic was achieved using all standard techniques. Mass spectrometry, which is particularly important for dendritic systems, was performedusing electrospray ionisation, and the ions were observed for G_0.5 and G₁ no significant fragment peaks or impurities were present. The isotope distributions observed for the mass spectral ions of the larger molecules were consistent with data calculated from isotopic abundances.

According to mass spectrum (Fig 6,a) of G_0.5, there are four distinct m/z peaks: at m/z 252 for losing three esteric groups from the dendrimer terminal, at m/z 212 for losing three methyl groups from the terminal of m/z 252, at m/z 126 for producing core melamine molecule and at m/z 58 for opening the melamine ring. M/z peak of G_0.5 at 382 has appeared very low which shows the instability of synthesized dendrimer.

Figure 6: Mass spectrum of a:G0.5 ,b: G1 Click here to View figure

 

At G₁ (Fig 6,b), m/z peak at 667 is similar to G_0.5  and is very low which shows its instability at reaction condition and m/z peak of melamine ring opening has appeared at 468. Other distinct peaks have appeared at m/z 369, 327, 296, 267, 212, 126 and 85 which are illustrated in (Fig 7).

Figure 7: m/z peak of G1 Click here to View figure

 

According to XRD spectrum and Scherrer’s formula, synthesized dendrimer of generation 1 has nano dimension (86 nm) (Fig 8).

Figure 8: XRD analysis Click here to View figure

 

Considering the widening of IR spectrum over 3000 cm^-1 and downfield shift of NMR peaks (4 ppm to over 6 ppm) for NH and NH₂ it can be concluded that the synthesized dendrimer is a supramolecule.

With these observations it can be speculated that synthesized dendrimer participates in hydrogen bonding through oxygen and nitrogen atoms.

Conclusion

According to spectral evidence it can be concluded that dendrimer generation 0.5 has been synthesized through reaction of melamine as core and methyl acrylate and then it reacted with added melamine monomer and dendrimers generation 1 have been obtained with amine terminal and also the formation of supramoleculedendrimer can be assured.

Acknowledgments

We are thankful to sophisticated test and instrumentation center BuAli Research Institute  and Khorasan Science and Technology Park (KSTP) of Mashhad.

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