Synthesis and Hydrosilylation of Vinyl-Substituted Open-Cage Silsesquioxanes with Phenylsilanes: Regioselective Synthesis of Trifunctional Silsesquioxanes

Herein, we report an efficient synthesis and functionalization of trifunctional open-cage-type silsesquioxanes. The method proposed has been successfully applied for the synthesis of a library of incompletely condensed silsesquioxanes with vinyldiorganosilyl functional groups, which allows further modification. Detailed studies of hydrosilylation of sterically different open-cage vinylsilsesquioxanes with a wide spectrum of phenylsilanes catalyzed by platinum and rhodium compounds are also reported. The influence of the reaction parameters, types of reagents, and catalysts employed on the efficiency of the process was examined. Optimization of the process based on the above results permitted the design of a very attractive method for the synthesis of multifunctionalized silsesquioxanes with excellent yields and regioselectivity. Moreover, the results allowed the choice of the most efficient catalyst whose application led to the selective formation of substituted open-cage silsesquioxanes. These new compounds have been fully characterized and studied in terms of their thermal properties.


■ INTRODUCTION
Silsesquioxanes make up a group of organosilicon macromolecular cage compounds differing in the number and manner of combination of T units [the basic building blocks consist of one silicon atom covalently linked to three oxygen atoms (Figure 1)]. The research area focused on silsesquioxanes is one of the most dynamically developing in the chemistry of silicon compounds. Polyhedral oligomeric silsesquioxanes (POSSs) are characterized by structural diversity and the possibility of attachment of a variety of functional groups that significantly change their reactivity and solubility. 1−4 These properties can be easily modified by the introduction of various substituents by using a wide spectrum of catalytic and noncatalytic chemical reactions (e.g., Heck coupling, 5−9 metathesis, 10−16 silylative coupling, 13−18 and many others 1,2 ). Among all of the abovementioned methods, hydrosilylation of olefins (the addition of Si−H bonds to unsaturated systems, e.g., CC bonds, resulting in the formation of a C−Si bond) is one of the fundamental processes used in industry and laboratory. This transformation is most often catalyzed by platinum compounds, although rhodium, nickel, and other transition-metal complexes are also used. 19,20 In scientific literature, the most widely described group is that of completely condensed silsesquioxanes, but an interesting alternative to cubic POSS cages may be incompletely condensed open-cage silsesquioxanes (IC-POSSs; Figure 1), which, thanks to their unique structure, may exhibit different physicochemical properties and different reactivity. 21, 22 Most literature reports about incompletely condensed silsesquioxanes focus on the use of POSS trisilanols as precursors for the synthesis of completely condensed derivatives 1−3,23 and metallasilsesquioxanes complexes as molecular models for silica-grafted catalytic centers. 21,22, 24 Liu et al. investigated the hydrogen-bonded interaction in POSS silanols and the possibility of their usage as anion receptors to form host−guest complexes. 25 The scientific reports about trifunctional IC-POSSs are scarce. There are only a few examples of the synthesis and use of its vinylfunctional derivatives, which, because of the presence of several functional groups protruding from the cage, can be as multifunctional precursors and nanofillers of many materials used, e.g., in electronics and hybrid materials. Lorenz et al. have provided the first example of complete functionalization of the POSS trisilanol (with cyclohexyl groups in the corners) with chlorodimethylvinylsilane, leading to the formation of a product with three functional groups. 26 In 2010, Miyasaka et al. demonstrated that this compound can be functionalized by hydrosilylation, and it is used for the synthesis of branched polysiloxanes. 27 We have recently focused on open-cage POSSs, but in this work, the substrate scope was limited only to chlorodimethylvinylsilane and chlorodimethylvinylgermane with various kinds of commercially available POSS silanols. 28 Our recent work has been aimed at the synthesis of a library of IC-POSSs with three types of vinyldiorganosilyl groups and their further catalytic functionalization. Recently, Hreczycho's group has reported the Sc(OTf) 3 , 29 Bi(OTf) 3 , 30 and also Nafion 31catalyzed formation of Si−O−Si bonds, which has been successfully used in the functionalization of POSS silanols, and methods were expanded to the introduction of boron 32 and germanium 33 functionalities to silsesquioxanes. Laine's group has presented another approach to the functionalization of such structures by blocking hydroxyl groups in trisilanolphenyl POSS, followed by modification of inert phenyl groups, resulting in the synthesis of stilbene−silesquioxanes. 34 Other examples of using trivinyl-substituted open-cage silsesquioxanes are based on the substitution of one functional group with POSS or silane, followed by the formation of cyclic compounds and polymers. 35,36 There are also reports on the application of trifunctional silsesquioxanes with Si−H groups in film-forming materials, 37 amphiphilic molecules, 38 hyperbranched poly-mers, 39 carbazole-functionalized structures, 40 and composite films of poly(methyl methacrylate) matrixes with IC-POSS fillers. 41,42 It should be emphasized that the above-mentioned studies concerning hydrosilylation of trivinyl-substituted open-cage silsesquioxanes have not been devoted to optimization of the hydrosilylation process. Some of the above-mentioned literature reports only concerned the preparation of functional silsesquioxanes without their further modification. Therefore, this study was undertaken to comprehensively investigate hydrosilylation of trivinylsilyl-substituted open-cage silsesquioxanes with phenylsilanes (Scheme 1). The influence of the types of IC-POSSs and silane structures as well as the nature of the catalyst and reaction conditions on the effectiveness and selectivity of the hydrosilylation process was examined. The reactions were monitored with 1 H, 13 C, and 29 Si NMR and in situ Fourier transform infrared (FT-IR) spectroscopies. As follows from the perusal of relevant literature, it seems that no detailed comparison of the activity of various silanes and open-cage vinylsilsesquioxanes in the hydrosilylation reaction has been performed until now. To date, no one has ever compared a spectrum of catalysts in the hydrosilylation of trivinyl functional open-cage silsesquioxanes. Moreover, in our study for the first time, real-time FT-IR spectroscopy was applied to the monitoring of hydrosilylation of trifunctional IC-POSSs, which provided detailed data about the kinetics of hydrosilylation of these compounds and the influence of substrates structures on the course of the reaction. Additionally, the structure-dependent thermal properties of a series of IC-POSSs and their selected derivatives are presented.

■ RESULTS AND DISCUSSION
Synthesis of Incompletely Condensed Silsesquioxanes. The first and fundamental part of the study was the preparation of incompletely condensed silsesquioxanes with different steric hindrances protruding from the POSS core's were synthesized by the hydrolytic condensation of commercially available POSS trisilanols with chlorodimethylvinyl-, chloromethylphenylvinyl-, and chlorodiphenylvinylsilanes, according to Scheme 2.
In the process of reaction optimization, the effects of the amount of silane, triethylamine, and solvent used in condensation were determined. Because the efficiency and time of condensation reaction also strongly depend on the concentration of the reagents, we decided to use tetrahydrofuran (THF) as a reaction medium because silsesquioxane silanols are highly soluble in it. To avoid side-reactions, it was necessary to cool the reaction mixture in an ice bath during the addition of chlorosilane. A small excess of chlorosilane was used to fully convert POSS silanol (molar ratio [POSS trisilanol]:[chlorosilane] = 1:3.1). Then, the cooling bath was left to warm to room temperature. The progress of the condensation reactions was monitored by FT-IR spectroscopy as the disappearance of the signals assigned to the stretching vibrations of the −OH groups present in the silanol substrates and the simultaneous appearance of characteristic absorption bands at 1593 cm −1 attributed to CC bonds from vinyl groups in the formed product ( Figure 2).  Finally, the crude product was isolated from the reaction mixture and purified. After the reactions, the mixtures were filtered off by a glass filter type G4 with silica gel and Celite, and then the column was flushed by ethyl acetate to rewash all products. HCl formed during the reaction was bound to triethylamine, and the side product Et 3 NHCl (hardly soluble in the reaction mixture) was retained on the column. The excess of the solvent was evaporated, and the product was dried under vacuum. In the cases of 3, 6, and 9 (silsesquioxanes with C 8 H 17 inert groups in the framework), the solution after filtration was evaporated, the product was extracted with dichloromethane/ water, then the organic phase was dried over anhydrous magnesium sulfate for 24 h and filtered off, and the solvent was evaporated. All products 1−9 were obtained with good yields of 87−93%.
During the course of the reactions, it was noted that the time needed for complete conversion of POSS trisilanol increases with increasing steric hindrance of the chlorosilane used, in the order from chlorodimethylvinyl-to chlorophenylmethylvinyl-to chlorodiphenylvinylsilane. The same effect was also observed for the derivatives containing Ph and C 8 H 17 inert groups. The reaction times were 2.5−8 h for 1−6 and 6−12 h for 7−9 (data are compiled in Table 1). It should be added that the iBu and C 8 H 17 silanols react faster than the Ph silanol in the presented reaction. The reason for the longer times of condensation of POSS silanols with chlorodiphenylvinylsilane might be a greater  29 Si NMR spectra as changes in the resonance lines assigned to silicon atoms. For derivatives 3 and 9, containing in their structures inert 2,4,4-trimethylpentyl (C 8 H 17 ) groups, we observed the multiplicity of the signals in the Si NMR spectra probably caused by the interaction and rotation of these compounds. For all compounds, the chemical shifts of the three silicon atoms from the functional groups Si−CHCH 2 were changed because of the differences in the electronic effects of the substituents at silicon atoms. The resonance lines were shifted toward lower chemical shift values from alkyl to aryl substituents and with the increasing number of phenyl substituents. On the other hand, chemical shifts of the silicon atoms derived from the Si−O−Si core were just slightly changed. A similar effect has been reported for tetrakis(methylsilylated) double-decker octaphenylsilsesquioxane and tris(methylsilylated) heptaphenylsilsesquioxane. 43 Synthesis of Functionalized Open-Cage Silsesquioxanes. Having these sterically and electronically different vinyl IC-POSSs, we decided to functionalize them via the hydrosilylation reaction. Moreover, the employment of real-time FT-IR spectroscopy allowed us to examine the influence of the structural diversity of POSS compounds as well as applied hydrosilanes on the reactivity of the investigated derivatives in the trifunctional POSS derivatives functionalization. We chose the hydrosilylation reaction to selectively bond chromophores (in the form of phenylsilanes) to derivatives 1−9, which have three vinyl functional groups protruding from the silsesquioxane cage, thus enabling their further modification and making them molecular scaffolds for the bonding of chromophores.
Screening of Catalysts. At the beginning of this study, three IC-POSS derivatives with the same inert iBu group and three different types of open moieties (dimethylvinylsilyl, 1; phenylmethylvinylsilyl, 4; diphenylvinylsilyl, 7) and two silanes, namely, dimethylphenylsilane (a) and triphenylsilane (b), with significantly different steric properties were selected as our model systems for catalyst screening (Scheme 3). Optimization of the reaction conditions was carried out using different loadings of selected catalysts [PtO 2 , PtCl 2 , H 2 PtCl 6 , [Rh-(COD)Cl] 2 , and Karstedt's catalyst Pt 2 (dvs) 3 ] known for high catalytic activity in the hydrosilylation of olefins. 19,20 The conversion of all reagents and the formation of products were determined by 1 H, 13 C, and 29 Si NMR spectroscopy. The results of the experiments are presented in Tables 2 and 3.
In all experiments, the conversion of substrates was very high and amounted to over 96%. Moreover, hydrosilylation of 1, 4, and 7 with dimethylphenylsilane a resulted in the exclusive formation of β-isomeric products (1a, 4a, and 7a) with a selectivity of over 99% in the presence of platinum catalysts. Some test reactions with the bis(1,5-cyclooctadiene)dirhodium-(I) dichloride complex led to the formation of α and β (main) products and dehydrogenative coupling products (entry 14 in Table 2 and entry 9 in Table 3). However, hydrosilylation of selected silsesquioxanes with triphenylsilane b brought about In Situ FT-IR Experiments. On the basis of earlier results, we decided to perform several additional experiments using in situ FT-IR spectroscopy to more precisely explore the effect of the silsesquioxane structure on the reaction time. The applied methodology allowed precise monitoring of the experiments by measurement of the reaction trends and profiles in real time, providing highly specific information, e.g., about the kinetics, reaction initiation, and conversion of the reagents in the course of the reaction. For this purpose, we chose simple dimethylphenylsilane (a) and two groups of silsesquioxanes: the first of 1−3 with the same dimethylvinylsilyl opening moiety and different inert groups in the POSS cage and the second of 1, 4, and 7 with the same inert group and different types of vinyldiorganosilyl moieties. In these experiments, 1 equiv of silsesquioxane (1−4 or 7), 3 equiv of dimethylphenylsilane, and 1 mL of toluene were added to a glass reactor equipped with an in situ FT-IR probe and placed in an oil bath at 95°C, and then 10 −4 mol of platinum (per each Si−H group) of Karstedt's catalyst was added. The progress of hydrosilylation of selected silsesquioxanes with PhSiMe 2 H in the presence of Karstedt's complex was possible to control by monitoring of the high absorption bands assigned to Si−H units (v = 822 cm −1 ), which decreased over time. All results are shown in Figure 3.
The rate of hydrosilylation of the CC bond was found to strongly depend on the silsesquioxane structure. The time required for total hydrosilylation of CC bonds in the reactions between phenyldimethylsilane and POSS derivatives (1−3) reached 397, 1197, and 897 min, respectively. So, for the POSSs with alkyl inert groups, the reaction was faster than that for the aromatic (phenyl) group, although, at the beginning of the process, the reaction of 2 was faster than that of 3. However, eventually more time was needed to complete the addition of Si−H to CC bonds. Additionally, the smaller the alkyl substituent, the shorter the reaction (time iBu < time octyl ). The reason for such a reactivity could be the different electronic effects and steric hindrances of the alkyl and aryl groups. In the case of IC-POSSs with the same iBu inert cage groups and different moieties protruding from the cage dimethylvinylsilyl (1), phenylmethylvinylsilyl (4), and diphenylvinylsilyl (7), the reaction time increased with increasing steric hindrance of the functional groups. The reaction time was 397 min for 1, 867 min for 4, and 2522 min for 7, and differences in the reactivity and reaction times were quite significant. Figure 4 shows 2D and 3D illustrations of the disappearance of the characteristic Si−H signal in the course of an exemplary hydrosilylation reaction of 7 with dimethylphenylsilane, which leads to complete conversion of the substrates, selective functionalization of all functional groups, and formation of 7a.
Hydrosilylation of Open-Cage Silsesquioxanes with Phenylsilanes. The positive results achieved in the catalytic tests of hydrosilylation of selected IC-POSSs with silanes a and b prompted us to extend the range of POSSs and silanes. We decided to extend the described method to functionalization of all obtained IC-POSSs (1−9). Our choice of silanes was based on the tests with silanes with different steric hindrances (Tables  2 and 3). Therefore, we decided to perform the next experiments with silanes containing a dimethylsilylhydride moiety. In the earlier selected reaction conditions, new trisubstituted derivatives were obtained in the stoichiometric reaction of IC-POSSs  by selective functionalization of only one functional group in silane. We assume that, at the initial stage of reaction, monosubstitution of 1,4-bis(dimethylsilyl)benzene is observed. However, when conversion of the substrates is very high, the remaining vinyl groups may react with the dimethylsilyl units incorporated with the POSS molecules, which leads to the crosslinked structure. In order to avoid hydrosilylation of both dimethylsilyl groups in 1,4-bis(dimethylsilyl)benzene, the equilibrium of the reaction must be shifted toward the formation of monosubstituted products; therefore, for selective monofunctionalization, a slight excess of silane e was required. A similar effect was observed, e.g., for functionalization of 1,1,3,3,tetramethyldisiloxane with vinylsilanes; 44 however, for functionalization of POSS compounds with 1,4-bis(dimethylsilyl)benzene, a significantly lower excess of silane e had to be used compared with the disiloxanes, which could be explained by the steric properties of the POSS molecules. A definite advantage of 1d, 2d, 3d, 1e, and 2e is the presence of three bromine atoms and three Si−H groups, which may allow further functionalization of these compounds and their application, for example, in hybrid materials 45,46 and building blocks for organic optoelectronic materials. 47−51 Figure 5 shows an exemplary starting mixture and the postreaction mixture of 1e formation. Progress of the reaction was followed by attenuated-total-reflectance FT-IR. Because of overlap of the substrate signals at about 900 cm −1 , the progress of the reaction was monitored by observing decreasing signals assigned to Si−H groups at 2119 cm −1 , and additionally, as the reaction progresses, a band around 800 cm −1 started to appear. The reaction was completed after 24 h because changes in the conversion of Si−H were no longer observed. After the reactions, the mixtures were filtered off by a glass filter type G4 with silica gel and Celite. The solvent was evaporated, and the products were dried under vacuum.

■ THERMAL PROPERTIES
Compounds 1−9 exhibit the so-called open-cage structure. Besides various substituents at the protruding groups (which affect steric hindrance), these structures have three types of inert groups surrounding the POSS core (iBu, Ph, and C 8 H 17 ). All presented types of structural modifications affected not only the syntheses of the compounds but also their physical properties such as the physical state, solubility, and thermal properties. Thermogravimetric analysis (TGA) revealed that both types of substituents (inert groups in the POSS framework and opening moieties) affected the thermal properties of the open-cage-type silsesquioxanes and their derivatives. The influence of the structural diversity on the thermal properties of the samples studied was evaluated for IC-POSSs 1−9 and their selected derivatives on the basis of the 5% (T d 5% ) and 10% (T d 10% ) mass loss temperatures and the amount of residue at 1000°C (all results are shown in Table 4).
Considering the influence of the inert group, the highest thermal stability was found for the compounds with Ph groups (2, 5, and 8). Additionally, the presence of functional groups with greater steric hindrance increases the thermal stability of the derivatives (1 < 4 < 7), which is related to the generally greater stability of the phenyl compounds compared to the alkyl structures. 52,53 However, for compounds with the alkyl inert    (4−6) or diphenylvinylsilyl (7−9), the thermal stability increases in the order 6 < 4 < 5 and 9 < 7 < 8.
The observed impact of the type of functional group on the thermal stability of silesquioxanes may be caused by the changed distance between these groups and the wider opening of the silsesquioxane structure, making it more flexible. Furthermore, it should be strongly emphasized that the residue observed at 1000°C was the highest for the trisilanolphenyl POSS-derived products, which is consistent with the literature data. 53 Figures 6 and 7 show the TGA curves obtained for selected functionalized structures. We compared the influence of the modifier (substituted silane) and different inert groups in the compounds with the same modifier on their thermal properties. Analysis of the obtained results showed that functionalization of IC-POSSs with phenylsilanes strongly influenced the thermal properties of the resulting products (1a−1c); the T d 5% values significantly increased compared to those of the unmodified   Figure 6. TGA curves obtained for selected compounds: 1 and 1a−1c (influence of the modifier on the thermal properties).
Inorganic Chemistry pubs.acs.org/IC Article silsesquioxanes. A similar effect of the thermal behavior of IC-POSSs after the incorporation of aryl groups into the silsesquioxane core was observed by the Naka group. 42 However, for 4a and 7a, the T d 5% parameter was decreased for the modified structures probably because of differences in the steric hindrance of the silyl groups in the open corner of silsesquioxanes, which affected the arrangement of the aromatic units and interaction between them. For derivatives 1a, 4a, and 7a modified with the same dimethylphenylsilane but containing different inert groups, we observed increasing thermal stability in the order from iBu to C 8 H 17 and Ph. On the other hand, after the initial degradation, the decomposition of 7a progressed much faster compared with that of 4a, which clearly indicates higher resistance of the phenyl derivative than the alkyl one to thermooxidative decomposition.

■ CONCLUSION
In summary, we synthesized a library of trifunctional open-cage silsesquioxanes differing in the structure of the inert groups and opening moiety. Having these sterically and electronically different vinyl IC-POSSs, we modified them with a series of silanes of different structures, functionalities, and steric hindrances in hydrosilylation reactions. Different homogeneous and heterogeneous catalytic systems were examined to determine their impact on the effectiveness and selectivity of hydrosilylation evaluated by monitoring of the processes with 1 H, 13 C, and 29 Si NMR and in situ FT-IR spectroscopy.
The most interesting outcome of our study is the first example of a fully specified synthetic protocol allowing modification of trivinylsubstituted open-cage silsesquioxanes with silanes to be obtained via a hydrosilylation reaction. The applied methodology of synthesis does not require the use of complicated isolation of the final products. The hydrosilylation reactions catalyzed by Karstedt's catalyst proceeded in a highly regioselective manner to give exclusively anti-Markovnikov products.
Additionally, we studied the structure-dependent thermal properties of a series of IC-POSSs and their selected derivatives. Full characterization of the products enabled us to identify the relationship between the structures of the silsesquioxanes studied (e.g., the types of inert groups in the framework and silicon substituents in the functional group) and phenylsilanes (e.g., their arrangement, structure, etc.) and their physicochemical properties allowing the design of macromolecules with desirable properties.
We believe that the compounds based on open-cage silsesquioxanes can undoubtedly become very useful reagents for the synthesis of building blocks and scaffolds for hybrid materials, which, because of the unique structure and greater flexibility of the core, can compete with completely condensed silsesquioxanes.
General procedures for the synthesis of IC-POSSs (1−9) and for hydrosilylation of IC-POSSs with phenylsilanes, NMR and MALTI-TOF data and spectra of compounds synthesized in this paper, and TGA curves obtained for selected compounds 1−9 (PDF)  Inorganic Chemistry pubs.acs.org/IC Article