Boron-substituted carboranе-carbosilane dendrimers: Synthesis and properties

https://doi.org/10.1016/j.reactfunctpolym.2020.104746Get rights and content

Highlights

  • A number of novel boron-substituted carborane-carbosilane dendrimers were synthesized in high yields.

  • The structures and purity of the compounds obtained were confirmed by using a set of physicochemical methods of analysis.

  • The properties of boron-substituted carborane-carbosilane dendrimers were studied by DSC, TGA, DLS, viscometry in solution.

Abstract

This paper presents a synthesis of new boron-substituted carborane-carbosilane dendrimers of the zero (G0-4 cages), first (G1-8 cages), third (G3-32 cages), and fifth (G5-128 cages) generations. The structures and purity of the compounds obtained were confirmed by using a set of physicochemical methods of analysis: 1H, 29Si, 13C{1H} and 11B{1H} NMR, IR spectroscopy, and gel permeation chromatography. Their properties were studied by DSC, TGA, DLS and viscometry in solution.

Introduction

Dendrimers are highly branched regular structures, characterized by nearly-ideal monodispersity and a wide variety of possible functional groups. They have been the subject of continuous studies in the past few decades [[1], [2], [3]]. Currently, studies of the packing problems and of the dependence of the dendrimer properties on the generation number become increasingly important [4]. For this reason, there is an increasing need to develop fast and well-controlled methods for the synthesis of such compounds with hybrid structure. From this point of view, carbosilane dendrimers seem to be ideal frameworks for creating new hybrid structures [[5], [6], [7], [8], [9], [10]].

The use of polyhedral carboranes as modifying agents of organosilicon compounds for creating different hybrid structures is due to a number of unique properties of boron clusters, such as high thermal stability, hydrophobic character, rigid molecular geometry, three-dimensional aromaticity, the presence of reactive -CH groups in their structure [[11], [12], [13], [14], [15], [16], [17], [18], [19]],which is confirmed by a significant number of publications [20].

Literature sources report a number of examples, where carboranyl derivatives of carbosilane dendrimers were synthesized by hydrosilylation [21] and nucleophilic substitution [22]. However, a search for more convenient methods for introducing carboranyl moieties into dendrimer structures continues. In this context, the use of azide-alkyne (CuACC) addition is very promising. Djeda and co-workers to incorporate 81 carboranyl moieties used this method. They noted that a further increase in the number of carborane-containing moieties involved certain difficulties [23]. Perhaps, this is due to the above-mentioned paper describes the preparation of carbon-substituted carboranyl derivatives. We can assume that certain spatial hindrance arises because the reaction involves rather bulky intermediate structures and rapid surface overflow. At the same time, few boron-substituted organosilicon compounds were reported [[24], [25], [26], [27]], and no carborane-carbosilane dendrimers were mentioned at all.

The hydrothiolation reaction has led to some progress in the synthesis of carbosilane dendrimers that comprise a sulfur heteroatom in their structures [[28], [29], [30], [31]]. It should be noted that incorporation of sulfur-containing moieties in the formation of the molecular framework of dendrimers, reduces the inertness of the chemical structure [32] that is one of the main advantages of carbosilane dendrimers [33]. However, this technique is expedient at the final stage, namely, in the formation of the outer layer. Therefore, the introduction of mercapto derivatives of polyhedral carboranes in synthetic practice [[34], [35], [36]] allows then to be considered as simple and convenient reagents that easily fit into the scheme of synthesizing carbosilane dendrimers at its final stage. This is very important in the preparation of non-functional derivatives convenient for the synthesis of model compounds.

The use of boron-substituted mercapto derivatives of polyhedral carboranes can allow one to not only modify the outer layer of a dendrimer without a catalyst, but in addition to studying stable derivatives as models for various research methods, to further functionalize reactive -CH groups of the carborane framework [37]. Thus, the use of mercapto derivatives of carboranes as modifying agents seems to be a more versatile approach compared to the use of lithium derivatives. Such modification of the synthetic scheme for producing carbosilane dendrimers would allow one to obtain new hybrid systems that combine the flexibility of the carbosilane structure with the rigidity of carborane polyhedra. The stability of these systems would allow them to be used in model studies, and the possibility to “activate” carborane moieties opens a way to the synthesis of new hybrid systems.

Thus, the purpose of this work was to synthesize new organoelement dendrimers based on polyallylcarbosilane dendrimers and a boron-substituted mercapto derivative of a polyhedral carborane as models of hybrid dendrimers of “flexible core – rigid shell” type.

Section snippets

Results and discussion

To achieve this goal, the well-studied radically initiated hydrothiolation corresponding to “click-reactions” was selected, as it is undemanding to the process conditions, gives high yields of target products, and involves few side processes [[38], [39], [40]]. This reaction is popular for obtaining dendrimers with various structures, including carbosilane ones [28,[41], [42], [43]].

Tetraallylsilane (as a zero-generation model dendrimer) and dendrimers of the first, third, and fifth generations

Experimental

All the solvents were purified before use as described earlier [51].

DMPA initiator (2,2-dimethoxy-2-phenylacetophenone) was purchased from Acros.

Carboranyl precursor (9-mercapto-m-carborane) was synthesized according to the published technique [[47]].

Carbosilane precursors were synthesized according to the published technique [44].

1H, 11B{1H}, 13C{1H}, and 29Si NMR spectra were recorded on a Bruker AvanceTM 500 spectrometer (Germany) (at 500.13, 160.46, 125.47, and 99.36 MHz for 1H, 11B{1H}, 13

Conclusion

Thus, a number of novel boron-substituted carborane-carbosilane dendrimers were synthesized in high yields. The hydrothiolation of allyl groups in carbosilane dendrimers with highly reactive 9-mercapto-m-carborane makes it possible to quickly and regioselectively incorporate a large number of polyhedral carboranes without any steric hindrance. This opens up the way to obtain a whole library of hybrid dendrimers and highly branched polymers with rigid shells. The hydrodynamic parameters of the

Author statement

E.O. Minyaylo-synthesis and purification of carboranecarbosilane dendrimers.

A.A. Anisimov -analysis of dendrimers by IR -spectroscopy.

A.V. Zaitsev -synthesis and purification of thio-carboranes.

S.A. Milenin- synthesis and purification of allyl functional dendrimers.

P.A. Tikhonov -analysis of the obtained dendrimers by GPC method.

O.V. Vyshivannaya -analysis of the obtained dendrimers by the DLS method.

V. A. Ol'shevskaya - DSC analysis of the obtained dendrimers.

G.G. Nikiforova- analysis of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (Grant of the President of the Russian Federation No. МК-1568.2019.3).

Synthesis of carbosilane and carboranyl precursors was funded by RFBR, project number 20-33-90249. Synthesis of novel boron-substituted carborane-carbosilane dendrimers was supported by the Ministry of Science and Higher Education of the Russian Federation (Grant of the President of the Russian Federation No. МК-1568.2019.3).

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