Synthesis, spectral characterization, electron microscopic study and thermogravimetric analysis of a phosphorus containing dendrimer with diphenylsilanediol as core unit

Article abstract of DOI:10.3762/bjoc.6.85

A phosphorus containing dendrimer with a diphenylsilanediol core was synthesized using a divergent method. Several types of reactions were performed on dendrons of several sizes, either at the level of the core or the surface. The giant Schiff's base macro molecule possesses 12 imine bonds and 8 hydroxy groups on the terminal phenyl groups. The structures of the intermediate compounds were confirmed by IR, GCMS and ³¹P NMR. The final compound was characterized by ¹H, ¹³C, ³¹P NMR, MALDI-TOF MS and CHN analysis. Scanning electron microscopic and thermogravimetric analysis/differential scanning calorimetric studies were also performed on the final dendritic molecule.

Full text of DOI:10.3762/bjoc.6.85

Synthesis, spectral characterization, electron micro-
scopic study and thermogravimetric analysis of a
phosphorus containing dendrimer with
diphenylsilanediol as core unit
E. Dadapeer1, B. Hari Babu2, C. Suresh Reddy1
and Naga Raju Charmarthi*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 726–731.
1Department of Chemistry, Sri Venkateswara University, Tirupati,
Andhra Pradesh, India -517 502 and 2Department of Chemistry,
National Cheng Kung University, Tainan -701, Taiwan
doi:10.3762/bjoc.6.85
Received: 27 May 2010
Accepted: 29 July 2010
Published: 11 August 2010
Email:
Naga Raju Charmarthi* - rajuchamarthi10@gmail.com
Associate Editor: H. Ritter
* Corresponding author
© 2010 Dadapeer et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
diphenyl silanediol; divergent method; phosphorus containing
dendrimer; scanning electron microscopy; Schiff’s base;
thermogravimetric analysis
Abstract
A phosphorus containing dendrimer with a diphenylsilanediol core was synthesized using a divergent method. Several types of
reactions were performed on dendrons of several sizes, either at the level of the core or the surface. The giant Schiff’s base macro
molecule possesses 12 imine bonds and 8 hydroxy groups on the terminal phenyl groups. The structures of the intermediate com-
pounds were confirmed by IR, GCMS and 31P NMR. The final compound was characterized by 1H, 13C, 31P NMR, MALDI-TOF
MS and CHN analysis. Scanning electron microscopic and thermogravimetric analysis/differential scanning calorimetric studies
were also performed on the final dendritic molecule.
Introduction
Dendrimers constitute a new class of macromolecules that can The word was coined from a combination of dendron (Greek =
offer a potentially defect-free structure for a diverse range of tree) and polymer. Generally, a linear polymer typically consists
applications [1,2]. Indeed, these highly branched macromole- of an entangled mixture of individual molecular chains, each
cules possess precise constitutions and a large number of func- having only two terminal groups. But a dendrimer, by contrast,
tional groups which confer upon them unique properties [3]. has many branches, each with its own end groups, so that each
726

Beilstein J. Org. Chem. 2010, 6, 726–731.
molecule contains a very high number of terminal functional and rapidly with P-chlorides and this has the advantage that the
groups. This gives a molecule resembling a tree. Recently, only byproduct of the reaction is HCl which can be easily
dendrimers and dendrons have been defined in general terms for removed during the purification step. Thus, we envisaged that
a class of hyper branched and poly functional macromolecules –NH2, –CHO and –OH end groups would offer a large number
[4]. They are also called “cascade-type molecules”. Under ideal of possibilities to graft new functional groups. The type of
conditions, the molecular architecture is very uniform. A dendrimers prepared in the present work is based on the first
dendron is one of the major subdivisions of a dendrimer. Thus, method of synthesis and involves the condensation reaction
one of the major differences between linear polymers and between aldehydes and amines. Depending on the step consid-
dendrimers is their branched architecture. They can be ered, these dendrimers have either aldehyde or –NH2, or –OH
constructed by two fundamental approaches namely, the diver- as reactive end groups. From our previous research on tailored
gent and convergent methods. The divergent method, first dendrimers [15], we reasoned that the divergent method is a
reported by Vögtle is based on the successive attachment of more convenient approach to synthesize phosphorus containing
branching units to the core molecule [5]. As a result of this dendrimers [16], since this has the advantage of providing
growth, each subsequent reaction is characterized by the gener- maximum control on the growth of the dendrimer, and macro-
ation of an exponentially increasing number of functional molecules and a higher number of generations can be obtained.
groups on the periphery, so that a large spherical molecule is
formed. This is analogous to the burst of exploding firework. In Results and Discussion
contrast, the convergent approach involves the synthesis of Scheme 1 and Scheme 2 summarise the preparation of the
dendrimeric fragments followed by their subsequent addition to dendrimer G6 with diphenyl silanediol as the core unit. The first
the core. This method was initially described by Hawker and step of this synthesis is a condensation reaction between
Frechet [6]. The synthesis of dendrimers offers the opportunity diphenyl silanol and POCl3 in the presence of triethylamine at 0
to generate monodisperse, structure controlled macromolecular to −15 °C to afford the product G1. In the second step, G1 was
architectures [7-9]. Phosphorus containing dendrimers benefit treated with 3-hydroxybenzaldehyde at 30 °C in dry THF in
from the global interest in dendrimers, but they have also their presence of triethylamine to form G2 which was then reacted
own specificities and properties [10]. Recently, a synthesis has with 3-aminophenol in EtOH at reflux temperature to afford G3.
been developed and the properties of phosphorus containing Compound G3 was treated with POCl3 in dry THF in the pres-
dendrimers has been investigated [11] by several strategies of ence of triethylamine at 0 to −15 °C as before to yield G4. G4
functionalization of the surface [12] or the branches [13] of was then treated with 3-hydroxybenzaldehyde in dry THF at
dendritic compounds. Similarly, phosphorus-containing 30 °C to give G5. Finally, G5 was reacted with 4-aminophenol
dendrons [14] are also becoming of interest. Considerable effort in dry EtOH at reflux temperature to afford the dendrimer G6.
has been devoted to the functionalization of the periphery of As the size of the dendron increases the reaction is less effec-
dendrimers to explore new properties and applications. Indeed, tive and the yields are reduced. To improve the yield of G6, the
the properties of these compounds mainly depend on the type of reaction was carried out in excess ethanol at high temperature.
functional end groups they bear. The type of end groups most Scheme 1 and Scheme 2 summarize the preparation of the
generally used for further functionalization is limited and are Vögtle-type – dendrimer, the yield of the final compound (G6)
most often –NH2, –CHO and –OH groups. One of the common was moderate 65%. The formation of all the intermediates G1,
properties of all these end groups is that they are able to react G2, G3, G4 and G5 were characterized by IR, GC-MS and 31P
with large range of easily available functionalized compounds. NMR data. Their data is given in the Experimental Details,
The other property shared by all these end groups is that they Supporting Information File 1. The synthetic and analytical data
are directly obtained during the normal process of synthesis of of the dendrimer G6 is also given in the Experimental Details,
dendrimers at each generation. To expand the type of func- Supporting Information File 1. Compound G6 exhibited absorp-
tional groups on the periphery of dendrimers, it is desirable to tion bands for Ar–OH, P=O and CH=N in the regions 3360,
introduce new types of end groups possessing a high potential 1307, and 1616 cm−1, respectively. Two absorption bands for
of reactivity, even if they are not directly available from the P–O–C (aromatic) were found at 970 and 1197 cm−1 [17]. In
normal process. Ideally, the functionalization should be done in the 1H NMR spectra (500 MHz) of G6, the aromatic protons
a single easy step, without any modification of the general gave multiplets at δ = 6.01–6.88, the aromatic –OH protons
methods used for the synthesis and the growth of the resonated at δ 10.21 as singlet and the imine CH protons
dendrimers. Accordingly, we turned our attention toward the appeared as singlets at δ 8.69 and 9.04. The 13C NMR spec-
use of –NH2, –CHO and –OH end groups on the surface of trum for G6 (see Supporting Information File 2) showed the
dendrimers. Amino groups react easily with aldehydes to form aromatic carbons at δ = 115.1–156.9 and the methine carbon at
Schiff’s bases. Similarly, the phenolic –OH groups react easily δ = 170.8. Compounds G4, G5 and G6 showed two 31P reso-
727

Beilstein J. Org. Chem. 2010, 6, 726–731.
Scheme 1: Synthesis of G5.
728

Beilstein J. Org. Chem. 2010, 6, 726–731.
Scheme 2: Synthesis of dendritic macromolecule G6.
nance signals indicating two different types of phosphorus
atoms.
The mass spectral properties of the final dendrimer G6 was
studied by Matrix Assisted Laser Desorption/Ionisation
(MALDI) mass spectrometry. As expected, the mass obtained
from the MALDI measurements was in good agreement with
the calculated value (Table 1).
Table 1: Mass data of G6 obtained from MALDI.
Dendrimer Calculated mass
G6 3038.7
MALDI mass (MH+)
3039.8
Figure 1: SEM image of G6.
The surface topography of the molecule was studied by scan- TGA-DTA analysis
ning electron microscopy (SEM), whilst the molecular decom- Thermogravimetric analysis (thermal gravimetric analysis
position of the dendrimer G6 was investigated by both thermo- (TGA)) was carried out on G6 to determine weight changes
gravimetric analysis (TGA) and differential thermal analysis with respect to changes in temperature. Similarly, differential
(DTA).
thermal analysis (DTA), was performed on G6 to establish
whether the changes were either exothermic or endothermic.
Simultaneous TGA-DTA measures both heat flow and weight
Scanning electronic microscopic study
In order to get a deeper insight into the properties of the surface changes (TGA) in a material as a function of temperature in a
of our phenyl –OH terminated dendrimer, it was examined by controlled atmosphere. The TGA-DTA analysis of the com-
SEM. It revealed that highly resolved patterns with a line width pound was recorded up to 400 °C. The TGA graph shows that
of 1 µm. Figure 1 indicates the dispersion of layered polymeric the compound is stable up to 85 °C and minor decomposition
form of the molecule. The most widely used layered material starts at around 90 °C with a corresponding weight loss of
has negative charges or basic sites on the surface.
approximately 4% (loss of water) and continues up to 165 °C.
729

Beilstein J. Org. Chem. 2010, 6, 726–731.
The calculated water loss is 3.5%. At 165 °C decomposition is
more pronounced with a corresponding observed weight loss of
approximately 50% up to around 270 °C. This is attributed due
to loss of 3-((4-hydroxyphenylimino)methylene)phenoxy
groups. The calculated weight loss is 51%. The end-product is
stable as indicated by the plateau in the thermogram. An
overlay of TGA and DTG plots for compound G6 up to 400 °C
is shown in Figure 2. The TGA and DTA studies on G6 were
carried out in air. From the DTA curve, the heat of reaction was
calculated. The DTA profile (Figure 2) shows an endotherm at
90 °C corresponding to the loss of water and further endotherms
at 135, 150 and 165 °C, corresponding to the loss of branches
from the compound. The endotherm at 270 °C corresponds to
Supporting Information
Supporting Information File 1
Experimental details
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-85-S1.pdf]
Supporting Information File 2
Spectral details
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-85-S2.pdf]
the loss of 3-((4-hydroxyphenylimino)methylene)phenoxy Acknowledgements
group.
The authors express their grateful thanks to HRDG, CSIR, New
Delhi, India for sanctioning a major Research project
(01(2309)/09/EMR-II).
References
1. Astruc, D.; Chardac, F. Chem. Rev. 2001, 101, 2991–3024.
doi:10.1021/cr010323t
2. Cordova, A.; Janda, K. D. J. Am. Chem. Soc. 2001, 123, 8248–8259.
doi:10.1021/ja001819f
3. Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99,
1665–1688. doi:10.1021/cr970069y
4. Newkome, G. R.; Moorefield, C. N.; Vögtle, F. Dendritic Molecules;
Concepts, Synthesis, Perspectives; VCH: Weinheim, Germany, 1996.
5. Buhleier, E.; Wehner, W.; Vögtle, F. Synthesis 1978, 155–158.
6. Hawker, C.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112,
7638–7647. doi:10.1021/ja00177a027
7. Tomalia, D. A. Aldrichimica Acta 2004, 37, 39–57.
8. Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.;
Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466–2468.
doi:10.1021/ma00163a029
Figure 2: TGA and DTA curve of G6.
9. Tomalia, D. A.; Headstrand, D. M.; Ferritto, M. S. Macromolecules
1991, 24, 1435–1438. doi:10.1021/ma00006a039
10.Majoral, J. P.; Caminade, A. M.; Maraval, V. Chem. Commun. 2002,
2929–2942. doi:10.1039/b207194k
Conclusion
A simple synthesis of a dendritic macromolecule G6 is
presented. The final product G6 as well as intermediates G1,
G2, G3, G4 and G5 are stable in the range 50–60 °C. The deeper
surface topography of the dendritic molecule G6 was observed
by SEM. It reveals the dispersion of layered polymeric form of
the molecule due to presence of Si at the core. The thermal
stability and changes in weight in relation to change in tempera-
ture and the heat of the final dendritic molecule G6 were studied
by TGA-DTA analysis. It reveals that the compound G6 is
stable up to 85 °C, and then decomposition starts at 90 °C. The
loss of water (ca. 4% ) takes place up to 165 °C and between
165 °C to 270 °C there is almost a 50% weight loss. From
270 °C to 400 °C decomposition is very slow. The DTA curve
reveals the heat change is endothermic for the loss of water; for
the 50% decomposition of the compound the heat change is
endothermic at around 225 °C. The small endothermic peaks are
due to decomposition of branches.
11.Merino, S.; Brauge, L.; Caminade, A. M.; Majoral, J. P.; Taton, D.;
Gnanou, Y. Chem.–Eur. J. 2001, 7, 3095–3105.
doi:10.1002/1521-3765(20010716)7:14<3095::AID-CHEM3095>3.0.C
O;2-S
12.Le Derf, F.; Levillain, E.; Gorgues, A.; Sallé, M.; Sebastian, R. M.;
Caminade, A. M.; Majoral, J. P. Angew. Chem., Int. Ed. 2001, 40,
224–227.
doi:10.1002/1521-3773(20010105)40:1<224::AID-ANIE224>3.3.CO;2-
F
13.Majoral, J. P.; Larré, C.; Laurent, R.; Caminade, A. M.
Coord. Chem. Rev. 1999, 190–192, 3–18.
doi:10.1016/S0010-8545(99)00060-0
14.Maraval, V.; Laurent, R.; Caminade, A. M.; Majoral, J. P.
Organometallics 2000, 19, 4025–4029. doi:10.1021/om0005607
15.Larré, C.; Bressolles, D.; Turrin, C.; Donna dieu, B.; Caminade, A. M.;
Majoral, J. P. J. Am. Chem. Soc. 1998, 120, 13070–13082.
doi:10.1021/ja982778e
730

Beilstein J. Org. Chem. 2010, 6, 726–731.
16.Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s
Textbook of Practical Organic Chemistry, 5th ed.; Longman: U.K.,
1991.
17.Thomas, L. C. Interpretation of Infrared Spectra of Organophosphorus
compounds; Heydon and Sons: London, U.K., 1974.
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.6.85
731