Facile Synthesis of Novel Functionalized Silsesquioxane

Article pubs.acs.org/Organometallics

Facile Synthesis of Novel Functionalized Silsesquioxane Nanostructures Containing an Encapsulated Fluoride Anion Youssef El Aziz,*,† Alan R. Bassindale,† Peter G. Taylor,*,† Peter N. Horton,‡ Richard A. Stephenson,‡ and Michael B. Hursthouse‡ †

Department of Chemistry, Open University, Walton Hall, Milton Keynes MK7 6AA, U.K. EPSRC National Crystallography Service, University of Southampton, Highfield, Southampton SO17 1BJ, U.K.

‡

S Supporting Information *

ABSTRACT: The presence of strongly electron withdrawing groups on alkoxysilanes, EWG-(CH2)n-Si(OEt)3 (where n = 1−3 and the electron-withdrawing group EWG contains an Si−C(sp3) bond), facilitates the formation and encapsulation of the fluoride anion in a silsesquioxane cage. Such species have been studied by 19F and 29Si NMR spectroscopy and X-ray crystallography together with MALDI-TOF and ESI mass spectrometry. The EWG must not be a good leaving group. Interestingly, this strategy led only to the T8 cage and excellent yields were obtained (81−95%) even without solvent. A wide range of functionalities were used. This new route offers an opportunity to build novel nanometer-sized 3-D molecular structures with a variety of functionalities which have not been accessible in the past.

1. INTRODUCTION Polyhedral oligomeric silsesquioxanes (POSS)1,2 are a family of inorganic−organic hybrid compounds that represent a new class of nanoscale molecule that has great scientific and technological potential with applications in the fields of catalysis,3 materials science, and coordination chemistry,4,5 together with the ability to take part in host−guest interactions.6−8 A POSS is a cagelike molecule where the repeating unit has the formula (RSiO3/2)n, abbreviated RnTn, where organic substituents (Rn) are attached to a silicon−oxygen cage. The most common octahedral POSS cages are the cubic silsesquioxanes, the T8 cages, which have the formula (RSiO3/2)8. They resemble the skeletal frameworks found in the crystalline forms of silica, particularly in zeolites.9 They are also of great interest because of their nanoscale inorganic core (Si8O12: 0.5 ± 0.7 nm) and highly symmetric octafunctionality, which makes these compounds ideal for use in the construction of inorganic/ organic hybrid nanomaterials. The most common class of functional POSS nanostructured molecules is that containing one or more covalently bonded reactive functionalities (e.g., Si−vinyl, Si−methacryloxypropyl, Si−aminopropyl) suitable for polymerization, grafting, surface bonding, and other transformations. The demand for advances in the performance and © XXXX American Chemical Society

properties of silsesquioxane materials has driven the search for new materials and, in particular, for high-yielding and selective synthetic routes to prepare them. Base-catalyzed polycondensation reactions have proven to be an excellent method to prepare large quantities of silsesquioxanes. Sprung and Guenther10 improved the yield for the synthesis of the octaphenyloctasilsesquioxane cage by using tetra-nbutylammonium hydroxide in a base-catalyzed process. Recently,we have developed a new synthetic route to simple silsesquioxane cages using tetra-n-butylammonium fluoride (TBAF in THF) in the presence of small amounts of water (5%), to catalyze the hydrolysis of organyltrialkoxysilanes11,12 (RSi(OR′)3, where R is a chemically stable constituent and R′ is alkyl). The yields are in the range 20−95%, which is a great improvement over other literature routes. We have also investigated the possibility of encapsulating a fluoride ion within a silsesquioxane cage. Indeed, we have developed an interesting synthetic strategy for this new class of silsesquioxane cages such that we now have examples of a fluoride ion encapsulated in cages with a range of functionalities6,7 when Received: April 5, 2012

A

dx.doi.org/10.1021/om300277g | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Scheme 1. Synthetic Route to T8[EWG-(CH2)n]8TBAF with Si−C(sp3)

the substituent on silicon is an sp2 carbon: T8[RC(sp2)]8TBAF. For example, we have made and fully characterized the vinyl, phenyl,6,7 styryl, p-chloromethylphenyl, p-tolyl, p-bromophenyl, p-chlorophenyl, and p-methoxyphenyl cages with a fluoride ion at the center.13 The X-ray crystal structures of these compounds have also been reported.13 In the past, we have been unable to synthesize an encapsulated fluoride ion within a silsesquioxane cage in which the pendant groups attached to the silicon atoms of the cage are connected via an sp3-hybridized carbon atom (Si−C(sp3)). Recently, we have developed a facile, single-step preparation of a fluoride ion encapsulated within a fluorinated polyhedral oligomeric silsesquioxane, 8−11 (F-POSS@F−) (Scheme 1), in which the pendant groups are connected to a silicon atom via an sp3-hybridized carbon atom.13 We have also demonstrated the capricious nature of RCsp3−Si substituents and the need for a balance between the electron-withdrawing group’s ability to stabilize the cage while at the same time not acting as a good leaving group. Recently, we have synthesized and isolated a wide variety of novel silsesquioxanes with an encapsulated fluoride ion in the middle of the cage, T8[RC(sp2)]8TBAF, using tetra-n-butylammonium fluoride and RC(sp2)−Si(OEt)3. However, this type of compound required precisely controlled conditions such as pressure and temperature during the workup which are often difficult to control and the yield is sometimes low, for example, with products such as T8(styryl)8TBAF. Herein, we report a facile, single-step strategy for the preparation of a novel class of polyhedral oligomeric silsesquioxane (POSS) compounds containing an encapsulated fluoride anion with electron-withdrawing groups attached, using tetrabutylammonium fluoride (TBAF) with scarce water, with or without a solvent. In this work we are interested in understanding the effects of other electron-withdrawing groups on encapsulating a fluoride anion in a silsesquioxane cage.

wide range of interesting functionalities of T8[EWG-(CH2)n]8TBAF (Scheme 1). Most of the products were isolated and characterized using NMR (1H, 13C, 19F, 31P, and 29Si) spectroscopy and MALDITOF and ESI mass spectrometry. A number of them have also been characterized using X-ray crystallography. This strategy for synthesizing cubic silsesquioxanes with an encapsulated fluoride ion, T8[EWG-(CH2)n]8TBAF, offers unique opportunities to build novel 3-D molecular structures based on a nanometer-sized octafunctional core with a variety of functionalities which were not accessible in the past. We have previously reacted TBAF with simple alkyltriethoxysilane groups; however, no encapsulated fluoride cage has been observed. Haddad and co-workers8 have used high-level calculations to show that when EWG-CH2CH2 arms (e.g., EWG = CF3) are attached to the cage, the HOMO is associated primarily with the atoms in the arms.8,14−16 This leads to a more positive charge in the center of the cage that can act as a host for the fluoride anion. Electron-donating alkyl groups lead to a HOMO associated with the cage atoms which interacts unfavorably with the fluoride ion inside the cage. The variety of EWG groups employed has demonstrated that the electronic effect exerted by an EWG modifies the electrostatic forces operating on a nearby cage center such that it has a positive charge. The results also show that the number of carbon atoms that separate the silicon atom from the EWG should not be more than three carbons. The EWG has been shown to have two roles: controlling the formation of the T8 cage and simultaneously favoring the hosting of F− ions in the middle of the cage. The incorporation of the fluoride anion into these oligomeric silsesquioxane cages, T8[EWG-(CH2)n]8TBAF, has been studied by 19F and 29Si NMR spectroscopy. 2.2. Methods of Preparation and Purification of T8(EWG-(CH2)n)8TBAF. The products were prepared from commercially available materials such as EWG-(CH2)n-Si(OEt)3, where n = 1−3 and EWG is cyano, alkanoate, O-methacrylate, O-acrylate, carboxylate, halo, pyridyl, P(O)(OEt)2, and 1H,1H,2H,2H-perfluoroalkyl in which the silicon atom is connected to an sp3-hybridized carbon atom (Si−C(sp3)). All syntheses were carried out in the same manner and by the reaction of EWG-(CH2)n-Si(OEt)3 with TBAF with 5% water (ratio of 1:2 with respect to the trialkoxysilane and TBAF) for 16 h at room temperature, under nitrogen (Scheme 1). The reaction can be carried out in a variety of solvents such as

2. RESULTS AND DISCUSSION 2.1. Synthesis of T8[EWG-(CH2)n]8TBAF. The products were prepared from commercially available materials such as EWG-(CH2)n-Si(OEt)3 (where n = 1−3 and EWG is an electron-withdrawing group). The electron-withdrawing group also needs to be a poor leaving group. Interestingly, this strategy led to only a T8 cage as the dominant product with excellent yields, even on a large scale (81−95%), and with a B


Organometallics

Article

Table 1. Yields of Fluoride Encapsulated Octasilsesquioxane Cages: 1−11 (T8[RC(sp3)]8TBAF) and 12−16 (T8[RC(sp2)]8TBAF) spacer n

group of EWG-(CH2)n and RCsp2 methacryloxymethyl acryloxymethyl acetoxymethyl 2-(carbomethoxy)ethyl 2-cyanoethyl 3-cyanopropyl 2-(diethylphosphato)ethyl CH2CH2CF3 CH2CH2CF2CF2CF2CF3 CH2CH2CF2CF2CF2CF2CF2CF3 CH2CH2CF2CF2CF2CF2CF2CF2CF2CF3 methacryloxypropyl chloroethyl 3-isocyanatopropyl hexyl octyl

T8[EWG-(CH2)n]8TBAF 1 1 1 2 2 3 2 2 2 2 2 3 2 3

compd

yield (%)

1 2 3 4 5 6 7 8 9 10 11

94 92 89 80 96 86 82 81 85 87 91 0 0 0 0 0

12 13 14 15 16

56 56 67 11 17

ref

13 13 13 13

T8[RC(sp2)]8TBAF p-chlorophenyl p-methoxyphenyl vinyl styryl p-bromophenyl

13 13 7 13 13

We therefore investigated the use of a cation-exchange resin for the purification of T8(EWG-(CH2)n)8TBAF . Parlow et al.17 have reported that the reaction of an excess of calcium sulfonate resin (Amberlyst A-15 (Ca)) with tetrabutylammonium fluoride (TBAF) in tetrahydrofuran resulted in complete sequestration of the TBAF, involving counterion exchange and precipitation of insoluble calcium fluoride. Filtration and evaporation left a residue-free vial. We have employed this methodology17,18 for the purification of our cage-encapsulated fluoride ions. The polymer-bound calcium sulfonate resin (C) was prepared by reaction of Amberlyst A-15 (hydrogen form) (A) with a saturated solution of aqueous calcium hydroxide (Scheme 2). A resin bed of Amberlyst A-15 (Ca) (C) was added to the crude T8(CH2CH2CH2CN)8TBAF (6) in THF. The resin reacted with the free TBAF by exchanging the calcium cation for a tetrabutylammonium cation; this simultaneously formed polymer-bound tetrabutylammonium sulfonate (D) and insoluble calcium fluoride (CaF2) as a white precipitate, which were easy to eliminate by filtration. Subsequent evaporation of the filtrate afforded the purified product. The 19F NMR spectrum of this purified product showed no signals corresponding to TBAF. The 29Si NMR revealed a major signal with a chemical shift of −72.3 ppm which corresponded to 6 together with a small signal at −79.5 ppm which corresponded to the nonencapsulated fluoride ion cage, T8(CH2CH2CH2CN)8. This result suggests that the sulfonate resin causes some of the fluoride ion to migrate outside of the cage without breaking the Si8O12 framework. We have also been able to develop a simple method of purification based on recrystallization by transferring the final reaction mixture (in toluene as solvent) immediately to the freezer (−20 °C). After 2 days colorless crystals were formed and the TBAF impurities remained in solution. After filtration, the crystals were washed with cold hexane, to give white crystals. A high degree of purity was achieved. Some of the products were recovered in 94% yield (in particular, T8(methacryloxymethyl)8TBAF

toluene, chloroform, dichloromethane, acetone, tetrahydrofuran, and acetonitrile. The cage-encapsulated fluoride anion was formed in solution and did not require any special workup. This was revealed by 19F NMR and 19Si NMR spectroscopic analysis of the reaction mixture. Interestingly, this reaction can also be carried out without solvent and still produces the desired products in good yield. This synthesis worked well for POSS cages, T8(EWG-(CH2)n)8, substituted with powerful electronwithdrawing groups when n = 1−3. It failed to produce any encapsulated fluoride anion for cages substituted with simple halogenated electron-withdrawing groups (EWG = Cl, Br, I where n = 2, 3) methacryloxy or isocyanate electron withdrawing groups where n = 3 simply gave no encapsulated fluoride ion (Table 1). In these cases, the majority of the organyltriethoxysilane reactions with TBAF led to a mixture of nonencapsulated cages or to insoluble gels. Different methods of purification have been investigated for T8(EWG-(CH2)n)8TBAF compounds. First of all, hexane was added to the reaction mixture, and the solution was transferred to a freezer. A white precipitate was obtained which contained free TBAF as an impurity. The precipitate was purified by flash column chromatography using silica gel, which led to the desired product. However, the yield was not as good as we were expecting, possibly due to the degradation of the product on the silica gel (Si−OH). We therefore examined alternative purification methods. We carried out recrystallization by the vapor diffusion method to get rid of the free TBAF. A small amount of a saturated solution in acetone was placed in a narrow tube, which was itself placed in a wider tube containing a portion of a precipitant (toluene or hexane). The precipitant was thus allowed to diffuse into the solution. After 4 days we observed the formation of small crystals at the bottom of the tube containing acetone. However, the crystal was still slightly contaminated with impurities of the excess of TBAF, as shown by 19F NMR. C


Organometallics Scheme 2.

Article

a

a The Amberlyst A-15 (H) was packed in a column and flushed with deionized water. A saturated solution of calcium hydroxide in deionized water was passed through the column until the eluent was basic (the resin displayed a distinct lightening in color as it exchanged with the calcium cation). Upon completion, the resin was rinsed with deionized water until the eluent was neutral. The resin was then removed from the column and rinsed three times with DCM, three times with THF, three times with ether, and then dried in vacuo.

29

and T8(acryloxymethyl)8TBAF). The high degree of purity was confirmed by 19F and 29Si NMR spectroscopy and MALDITOF and ESI mass spectrometry. The products 1−11 were formed in near-quantitative yield, even on a large scale, with or without solvent. Interestingly, this strategy led only to the T8 cage encapsulated fluoride anion as the dominant product. We have also observed that when we extended the perfluoroalkyl chain in the perfluorotriethoxysilane, the yield increased successively, possibly due to the increasing electronic effect of the electron-withdrawing group. A comparison of the yields of T8[RC(sp3)]8TBAF with those obtained for T8[RC(sp2)]8TBAF (Table 1) shows an improvement in the formation of silsesquioxane cage encapsulated fluoride ion with a good selectivity for T8 cages. 2.3. 19F and 29Si NMR Studies. The relevant 19F and 29Si NMR spectroscopic chemical shifts for the new T8[EWG(CH2)n]8TBAF compounds are given in Table 2. The 19F and

Si NMR chemical shifts for compounds 1−3, when the EWG is positioned at n = 1, vary between −24 and −25 ppm and between −77 and −78 ppm, respectively. For compounds 5, 7, and 8, when the EWG is positioned at n = 2, the 19F chemical shifts vary between −27 and −28 ppm and the 29Si NMR chemical shifts vary between −71 and −72 ppm. The 29Si NMR chemical shifts of the corresponding empty cages (Table 2) also vary with the increasing electronic effect of the EWG. For example, when n = 2 (compounds 16−18), the chemical shifts vary between −66 and −67 ppm. In the case of T8(3cyanopropyl)8TBAF (7) the electron-withdrawing cyano group is in position 3, such that it should have a very weak electronic effect through three carbon atoms to favor such encapsulation. This leads us to assume that there is an intramolecular electronic interaction in solution that favors the encapsulation of the fluoride ion. Interestingly, other small signals were seen in the 19F NMR spectrum within the range −24 to −30 ppm (up to 10% in some cases). These could arise from T8 cages that encapsulate the fluorine where one of the organic substituents has been substituted for fluorine, leading to another Si−F signal of equal intensity at about −132 ppm.19 2.4. Single-Crystal X-ray Diffraction Studies. Figures 1 and 2 show the molecular structures in the solid state of the fluoride encapsulated T8[EWG-(CH2)n]8TBAF cages. Selected bond distances and angles are shown in Table 3. A comparison of the crystal structure data of 1 and 4−6 with those of the nonencapsulated analogue T8(CH2CH2CO2CH3)820 (17) and T8[CH2CH2CH2N(COCH3)2]823 (20) (Figure 3), which are substituted cages with EWG-(CH2)n-Si-C(sp3) and T8[EWG-(CH2)n]8TBAF and have crystal structures reported, shows that fluoride ion entrapment inside the octasilsesquioxane cage causes the Si8O12 cage framework to contract slightly in the fluoride-containing cage (Table 3). This slight contraction of the framework is illustrated by the shrinkage of the mean Si−O−Si bond angle by 10° from those of the nonencapsulated fluoride ion cages (17 and 20), which may be due to repulsion of the surrounding oxygen atoms of the Si8O12 framework and fluoride ion. The average O−Si−O bond angle also increases, and there is a slight lengthening of the Si−Csp3 bond (1.65 Å) for nonencapsulated fluoride ion cages (17 and 20) and the encapsulated fluoride ion cages (1.86 Å), which is consistent with a weak silicon−fluoride ion interaction. We can also identify a decrease in the mean trans Si−Si bond distance. 2.5. Alternative Source of Fluoride Ion. Another source of fluoride ion which is suitable for our cage synthesis and is

Table 2. 19F and 29Si Chemical Shifts (ppm) for a Range of Fluoride Encapsulated Octasilsesquioxane Cages 1−8 (T8[RC(sp3)]8TBAF) and 12−15 (T8[RC(sp2)]8TBAF) and Nonencapsulated Octasilsesquioxane Cage Analogues 17−19 (T8[RC(sp3)]8) group

spacer n

methacryloxymethyl acryloxymethyl acetoxymethyl 2-(carbomethoxy) ethyl 2-cyanoethyl 3-cyanopropyl 2-(diethylphosphato) ethyl CH2CH2CF3 p-chlorophenyl p-methoxyphenyl vinyl styryl 2-(carbomethoxy) ethyl CH2CH2CF3 3-cyanopropyl a

compd

19 F NMR chem shifta

29 Si NMR chem shifta

ref

T8[EWG-(CH2)n]8TBAF 1 1 −25.1 1 2 −24.9 1 3 −25.1 2 4 −28.4

−77.6 −77.4 −78.2 −71.1

−28.9 −27.8 −27.5

−72.2 −71.3 −71.1

8 −28.4 T8[RC(sp2)]8TBAF 12 −27.2 13 −27.6 14 −25.4 15 −24.2 T8[EWG-(CH2)n]8 2 17

−70.9

13

−80.7 −80.2 −82.0 −81.3

13 13 7 13

−67.0

20

2 3

−66.7 −67.7

21 22

2 3 2

5 6 7

2

18 19

NMR solvent: CDCl3. D


Organometallics

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Figure 1. Molecular structures in the solid state (X-ray diffraction studies) of the fluoride encapsulated octakis(methacryloxymethyl)octasilsesquioxane fluoride (1) and octakis(2-(carbomethoxy)ethyl)octasilsesquioxane fluoride (4) (tetrabutylammonium salt) with thermal ellipsoids drawn at the 50% probability level. H atoms are omitted for clarity.

Figure 2. Molecular structures in the solid state (X-ray diffraction studies) of the fluoride encapsulated octakis(2-cyanoethyl)octasilsesquioxane fluoride (5) and octakis(3-cyanopropyl)octasilsesquioxane fluoride (6) (tetrabutylammonium salt) with thermal ellipsoids drawn at the 50% probability level. H atoms are omitted for clarity.

Table 3. Selected Bond Distances and Angles for Compounds 1, 4−6, 17, 20, and 21 cryst struct compd 1 4 5 6 T8(CH2CH2CO2CH3)8 (17)a T8[CH2CH2CH2N(COCH3)]8 (20)a 21 a

mean Si···F (Å)

mean Si−R(C(sp3)) (Å)

mean ∑O−Si−O (deg)

mean trans Si−Si (Å)

mean Si−O (Å)

mean Si−O−Si (deg)

2.64(1) 2.66(1) 2.65(1) 2.66(1)

1.87(1) 1.86(1) 1.86(1) 1.86(1) 1.87 1.85

339.4(1) 338.6(1) 338.7(2) 338.1(1) 328.9 326.7

5.29(1) 5.32(1) 5.30(1) 5.32(1) 5.40 5.40

1.62(1) 1.63(1) 1.62(1) 1.63(1) 1.62 1.62

140.4(2) 141.0(1) 141.0(1) 141.3(2) 148.1 148.1

2.65(1)

1.86(1)

338.9(1)

5.30(1)

1.62(1)

140.8(1)

The coordinates of the two previously known structures were taken to calculate their distances and angles.20,23

monomers, 2-cyanoethyltriethoxysilane and 1H,1H,2H,2Hnonafluorohexyltriethoxysilane,13 in toluene and in the presence of TEAF. The 29Si NMR of T8[2-cyanoethyl]8TEAF (21) exhibited a single sharp resonance with a chemical shift at −72.24 ppm. Additionally, the 19F NMR spectrum of 21 revealed a sharp signal with a chemical shift of −27.95 ppm which corresponds to a fluoride anion encapsulated within the T8[2-cyanoethyl]8 cage. We obtained a molecular structure in the solid state of T8(2-cyanoethyl)8TEAF (Figure 4), in which the tetraethylammonium cation was disordered. A comparison of the crystal structure data of the compounds T8(2cyanoethyl)8TEAF (21) and T8(2-cyanoethyl)8TBAF (5) shows that the selected bond angles and distances of the T8 cage encapsulated fluoride anion are essentially independent of the nature of the cation.

Figure 3. Molecular structure in the solid state (X-ray diffraction studies) of Si8O12[(CH2)3N(COMe)2]8 with thermal ellipsoids drawn at the 50% probability level. H atoms are omitted for clarity.23

3. CONCLUSION Previously, we have been unable to synthesize an encapsulated fluoride ion within a silsesquioxane cage in which the pendant

commercially available is tetraethylammonium fluoride (TEAF). We thus performed two separate experiments with different E


Organometallics

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spectrometer (J values are given in Hz). MALDI TOF mass spectra were carried out by the University of Southampton using 2,5dihydroxybenzoic acid as a matrix and dichloromethane as the solvent. Synthesis of Tetra-n-butylammonium Octakis(methacryloxymethyl)octasilsesquioxane Fluoride (1). Methacryloxymethytriethoxysilane (1.31 g, 5 mmol, 2 equiv) was dissolved in dry toluene (20 cm3), and then tetra-n-butylammonium fluoride (2.5 of 1 M solution in THF with 5% water, 1 equiv) was added. The mixture was stirred at room temperature under argon for 16 h. The reaction was stopped. The final mixture was transferred immediately to the freezer (−20 °C). After 2 days colorless crystals were formed. The crystals were washed with cold hexane to give white crystals. Recrystallization from toluene−acetone gave colorless crystals (0.87 g, 94%). Mp: 115−116 °C. 1H NMR (300 MHz, acetone-d6; ppm): δ 6.02−6.01 (m, 8H, CCH2), 5.52−5.50 (m, 8H, CCH2), 3.49− 3.43 (m, 8H, N-CH3), 1.88−1.88 (m, 16H, CH3), 1.88−1.78 (m, 16H, CH2), 1.50−1.38 (m, 8H, CH2), 0.98 (t, 3JHH = 7.32 Hz, 12H, CH3). 13 C NMR (75.5 MHz, acetone-d6; ppm): δ 168.0 (s, CO), 137.7 (CH2Cq), 124.9 (CH2), 59.4 (s, N-CH2-), 54.3 (Si-CH2-O), 24.4, 20.4 (CH2), 20.3 (-CH2-), 18.6 (−CH3), 13.8 (s, CH3). 29Si NMR (79.3 MHz, acetone-d6; ppm): δ −77.6. 19F NMR (376 MHz, acetoned6; ppm): δ −25.0. IR (Nujol; ν, cm−1): 2923 (νC−H), 2853 (νC−H), 1732 (νCO), 1633 (νCCH2 str), 1414, 1376, 1336, 1312, 1254, 1242, 1180, 1107 (νSi−O−Si as), 1016, 939, 812 (νSi−O−Si s), 746. Anal. Calcd for C48H76FNO28Si8: C, 45.72; H, 6.30. Found: C, 45.73; H, 6.29. MS (MALDI-TOF): m/z (%) 1471 [M−], 1227.1 [M − TBA]−, 1230.1 (isotope). This compound has been fully characterized, and X-ray data are given in the Supporting Information. Synthesis of Tetra-n-butylammonium Octakis(acryloxymethyl)octasilsesquioxane Fluoride (2). Acryloxymethytrimethoxysilane (10 g, 48.48 mmol, 2 equiv) was dissolved in dry toluene (60 cm3), and then tetra-n-butylammonium fluoride (24.24 cm3 of 1 M solution in THF with 5% water, 1 equiv) was added. The mixture was stirred at room temperature under argon for 24 h. The reaction was stopped and the mixture transferred to the freezer (−20 °C). After 3 days colorless crystals were formed. The crystals were washed with cold hexane. Colorless crystals were obtained (7.58 g, 92%). Mp: 82−84 °C. 1H NMR (300 MHz, acetone-d6; ppm): δ 6.32−6.25 (m, 8H, CCH2), 6.20−6.11 (m, 8H, CCH2), 5.85−5.73 (m, 8H, CH2CH), 3.71 (Si-CH2), 3.49−3.40 (m, 8H, N-CH2), 2.08−2.03 (m, 8H, N-CH3), 1.45−1.40 (m, 8H, CH2), 0.98 (t, 3JHH =7.32 Hz, 12H, CH3). 13C NMR (75.5 MHz, acetone-d6; ppm): δ 172.0 (s, CO), 167.0 (C O), 130.3 (CH2CH), 128.9 (CH2CH), 58.6 (N-CH2), 54.3 (SiCH2), 24.4 (CH2), 20.3 (CH2), 13.87 (CH3). 29Si NMR (79.3 MHz, acetone-d6; ppm): δ −77.2, −77.4 (major peak), −77.7. 19F NMR (376 MHz, acetone-d6; ppm): δ −24.8 (8% rel int), −24.9 (100% rel int), −26.3 (6% rel int). IR (Nujol; ν, cm−1): 2921, 2861, 1738, 1713 (νCO), 1632 (νCCH2), 1376, 1101 (νSi−O−Si as), 721. Anal. Calcd for C48H76FNO28Si8: C, 42.43; H, 5.46. Found: C, 42.45; H, 5.42. MS (MALDI-TOF): m/z (%) 1060.9 [T8(acryloxymethy)7(OMe) − TBAF]−, 1093.0 [T8(acryloxymethyl)7(OMe) + MeOH − TBA]−, 1114.9 [M − TBA]−, 1147.0 [M + MeOH − TBA]−, 1179.0 [M + 2MeOH − TBA]−, 1211.1 [M + 3MeOH − TBA]−. Synthesis of Tetra-n-butylammonium Octakis(acetoxymethyl)octasilsesquioxane Fluoride (3). Acetoxymethyltriethoxysilane (1.18 g, 5 mmol, 2 equiv) was dissolved in dry toluene (20 cm3), and then tetra-n-butylammonium fluoride (2.5 cm3 of 1 M solution in THF with 5% water, 1 equiv) was added. The mixture was stirred at room temperature under argon for 16 h. The reaction mixture was transferred to the freezer (−20 °C). After 3 days colorless crystals were formed. The crystals were washed with cold hexane to give white crystals (0.70 g, 89% yield). Mp: 153−154 °C. 1H NMR (300 MHz, acetone-d6; ppm): δ (m, 8H, N-CH3), (m, 16H), (m, 8H, CH2), (m, 8H, CH2), (t, 3JHH = Hz, 12H, CH3), (m, 16H). 13C NMR (400 MHz, CDCl3; ppm): δ 170.9 (s, CqO), 59.0 (s, N-CH2), 54.2 (s, Si-CH2), 23.9 (s, CH2), 21.0 (CH2), 19.8 (s, OC-CH3), 13.6 (s, -CH3). 29Si NMR (79.4 MHz, CDCl3; ppm): δ −78.2. 19F NMR (376 MHz, CDCl3; ppm): δ −24.7 (F−@T8), −25.9, −27.3. IR (Nujol; ν, cm−1): 2922 (νC−H), 2852 (νC−H), 1731 (νCO), 1375, 1298, 1249, 1106

Figure 4. Molecular structure in the solid state (X-ray diffraction studies) of the fluoride encapsulated octakis(2-cyanoethyl)octasilsesquioxane fluoride 21 (tetraethylammonium salt) with thermal ellipsoids drawn at the 50% probability level. H atoms are omitted for clarity.

groups attached to the cage silicon atoms are connected via an sp3-hybridized carbon atom. We have developed a facile, singlestep preparation of a new class of POSS compounds containing an encapsulated fluoride anion, using TBAF with scarce water, with or without solvent. The product T8[EWG-(CH2)n]8TBAF was obtained in near-quantitative yield, even on a large scale (81−95%). The products were formed in solution and at room temperature with only T8 cage encapsulated fluoride ion as the dominant product. We have also developed a simple method for their purification. Additionally, we also synthesized T8(methacryloxymethyl)8TBAF and T8(acryloxymethyl)8TBAF, which appear to be very stable materials that can be stored at room temperature and could lead to new polymerizable nanomaterials. We did achieve the synthesis and characterization of encapsulated fluoride ions within the T8(2-cyanoethyl)8 cages using tetraethylammonium fluoride, in good yield (90%). This work has provided evidence that it is preferable to have an electron-withdrawing group (EWG) that is not a good leaving group. Analysis by X-ray crystallography and solution 19F/29Si NMR spectroscopy of R8T8@F− reveal very similar environments for the encapsulated fluoride octasilsesquioxane cages. This new strategy offers access to a potentially new class of interesting functionalities that can be elaborated for further potential transformation. These T8 cage encapsulated fluoride anion complexes may serve as models for the development of systems for fluoride anion recognition and sensing.24−26 While these structures have not been investigated for these purposes as yet, we can envisage future work in this area.26−33 Further work will involve looking at the migration of a fluoride ion through the face of the T8 cage without breaking the Si8O12 structure in the presence of powerful bidentate Lewis acids or other strong fluoride anion receptors.13

4. EXPERIMENTAL SECTION Melting points were determined on an Electrothermal Digital melting point apparatus. Infrared spectra were obtained as Nujol mulls or thin films using sodium chloride plates or as KBr disks on a Nicolet 205 FT-IR spectrometer. NMR spectra were recorded as solutions in deuteriochloroform with tetramethylsilane as internal standard on a JEOL Lambda 300 NMR spectrometer or a JEOL EX 400 NMR F


Organometallics

Article

Table 4. Crystallographic Data for the Four Crystalline Systems Investigated by X-ray Diffraction Analysis chem formula formula mass, amu cryst syst a, Å b, Å c, Å α, deg β, deg γ, deg unit cell vol, Å3 temp, K space group no. of formula units per unit cell, Z abs coeff, μ, mm−1 no. of rflns measd no. of unique rflns Rint final R1 value (I > 2σ(I)) final wR2 value (I > 2σ(I)) final R1 value (all data) final wR2 value (all data) goodness of fit on F2

1

4

5

6

21

C56H92FNO28Si8 1471.03 triclinic 10.2479(3) 12.5527(4) 15.9489(5) 105.062(2) 106.841(2) 101.499(2) 1810.54(10) 120(2) P1̅ 1 0.230 37 088 8287 0.0552 0.0455 0.1089 0.0706 0.1206 1.046

C48H92FNO28Si8 1374.95 monoclinic 20.5416(2) 14.5201(2) 25.3577(3) 90 111.360(1) 90 7043.82(15) 160(2) C2/c 4 0.231 59 096 8034 0.0382 0.0433 0.1117 0.0487 0.1153 1.036

C40H68FN9O12Si8 1110.75 monoclinic 26.4378(11) 8.7798(4) 26.8273(10) 90 93.831(2) 90 6213.2(4) 120(2) C2/c 4 0.232 26 421 7115 0.0483 0.0541 0.1350 0.0673 0.1436 1.067

C48H84FN9O12Si8 1222.96 monoclinic 12.4124(4) 14.7103(5) 35.5306(13) 90 95.666(2) 90 6455.8(4) 120(2) P21/n 4 0.229 54 780 14 315 0.0907 0.0716 0.1801 0.1378 0.2157 1.083

C32H52FN9O12Si8 998.55 triclinic 8.6557(2) 11.7528(2) 12.4208(2) 106.041(1) 98.006(1) 96.123(1) 1188.45(4) 120(2) P1̅ 1 0.294 26 888 5461 0.0293 0.0342 0.1053 0.0388 0.1083 1.160

0.84 (t, 3JHH = 7.88 Hz, 12H, CH3). 13C NMR (75.5 MHz, acetone-d6; ppm): δ 121.8 (s, CN), 58.8 (N-CH2), 23.8 (CH2), 19.7 (CH2), 13.6 (CH3), 11.6 (CH2-CH2-CN), 10.04 (-CH2-CN).29Si NMR (79.3 MHz, CDCl3; ppm): δ −72.2. 19F NMR (376 MHz, CDCl3; ppm): δ −27.9, −28.0. IR (Nujol; ν, cm−1): 2621 (νC−H), 2862 (νC−H), 2246 (νCN), 1376, 1376, 1317, 1104 (νSi−O−Si as), 721, 665 (νSi−O−Si s). MS (MALDI-TOF): m/z (%) 1110.7 [M−], 867.0 (100%) [M − TBA]−, 869.0, 871.0 (isotopes). X-ray data are given in the Supporting Information. Synthesis of Tetra-n-butylammonium Octakis(3-cyanopropyl)octasilsesquioxane Fluoride (6). 3-Cyanopropyltriethoxysilane (1.15 g, 5 mmol, 2 equiv) was dissolved in dry toluene (20 cm3), and then tetra-n-butylammonium fluoride (2.5 cm3 of 1 M solution in THF with 5% water, 1 equiv) was added. The mixture was stirred at room temperature under argon for 16 h. The reaction was stopped, and hexane (150 cm3) was added to the mixture. The cloudy solution was transferred to the freezer (−20 °C). A white solid precipitated out and was filtered off and washed with cold hexane. Recrystallization from butanol−acetone afforded colorless crystals (0.66 g, 86%). 1H NMR (300 MHz, acetone-d6; ppm): δ 3.64−3.58 (m, 8H, CH2), 3.15−3.11 (m, 8H, N-CH3), 2.39 (t, 3JHH = 7.88 Hz, 16H, CH2), 1.65−1.57 (m, 8H, CH2), 1.47−1.39 (m, 8H, CH2), 1.03 (t, 3JHH = 7.52 Hz, 8H, CH2), 0.84 (t, 3JHH = 7.88 Hz, 12H, CH3). 13C NMR (75.5 MHz, acetone-d6; ppm): δ 121.8 (s, CN), 58.8 (N-CH2-), 23.8 (CH2), 19.7 (-CH2), 13.6 (-CH3), 11.6 (CH2-CH2-CN), 10.1 (CH2CN). 29Si NMR (79.3 MHz, CDCl3; ppm): δ −71.3. 19F NMR (376 MHz, CDCl3; ppm): δ −27.8. IR (Nujol; ν, cm−1): 2918 (νC−H), 2862 (νC−H), 2244 (νCN), 1650, 1392, 1376, 1318, 1284, 1196, 1100 (νSi−O−Si as), 997, 912, 786, 753, 722, 684 (νSi−O−Si s). Anal. Calcd for C48H84FN9O12Si8: C, 47.14; H, 6.92. Found: C, 47.15; H, 6.89. MS (MALDI-TOF): m/z (%) 1222.92 [M−], 979.1 (100%) [M − TBA]−, 981.1, 983.1 (isotopes). X-ray data are given in the Supporting Information. Synthesis of Tetra-n-butylammonium Octakis(diethylphosphatoethyl)octasilsesquioxane Fluoride (7). (Diethylphosphatoethyl)triethoxysilane (1 g, 3.04 mmol, 2 equiv) was dissolved in dry toluene (20 cm3), and then tetra-n-butylammonium fluoride (1.52 cm3 of 1 M solution in THF with 5% water, 1 equiv) was added. The mixture was stirred at room temperature under argon for 16 h. The reaction was stopped, and hexane (150 cm3) was added to the mixture. The cloudy solution was transferred to the freezer (−20 °C). A colorless solid gel precipitated out and was filtered off. A colorless solid gel was obtained (0.62 g, 82%). 1H NMR (400 MHz,

(νSi−O−Si as), 891, 763 (νSi−O−Si s). Anal. Calcd for C40H76FNO28Si8: C, 38.05, H, 6.07. Found: C, 38.33, H, 6.18. MS (MALDI-TOF): m/z (%) 1262.7 [M−], 1019.0 [M − TBA]−, 1021.0, 1023.0 (isotopes). Synthesis of Tetra-n-butylammonium Octakis(2-(carbomethoxy)ethyl)octasilsesquioxane Fluoride (4). 2-(Carbomethoxy)ethyltriethoxysilane (2 g, 9.61 mmol, 2 equiv) was dissolved in dry toluene (20 cm3), and then tetra-n-butylammonium fluoride (4.80 cm3 of 1 M solution in THF with 5% water, 1 equiv) was added. The mixture was stirred at room temperature under argon for 16 h. The reaction was stopped, and hexane (150 cm3) was added to the mixture. The cloudy solution was transferred to the freezer (−20 °C). A white solid precipitated out and was filtered off. Recrystallization from toluene−acetone offered colorless crystals (1.34 g, 80% yield). 1H NMR (300 MHz, CDCl3; ppm): δ 3.59 (s, 24H, OCH3), 3.40−3.50 (t, 8H, N-CH2), 2.33 (t, 16H, 3JHH = 7.72 Hz, CH2-CO), 2.00−1.70 (m, 8H, CH2), 0.98 (t, 3JHH = 6.96 Hz, SiCH2), 0.70 (t, 3JHH = 7.88 Hz, CH3). 13C NMR (75.5 MHz, acetone-d6; ppm): δ 175.6 (s, CO), 59.3 (N-CH2), 51.3 (OCH3), 23.9 (CH2), 18.8 (CH2), 16.4 (CH3), 13.89 (CH3). 29Si NMR (79.3 MHz, CDCl3; ppm): δ −71.1.19F NMR (376 MHz, CDCl3; ppm): δ −28.4 (93% rel int), −28.5 (6% rel int), −29.1 (1% rel int). IR (Nujol; ν, cm−1): 2961 (νC−H), 1731 (νCO), 1681, 1434, 1222, 1095 (νSi−O−Si as), 1009, 752. Anal. Calcd for C48H92FNO29Si8: C, 41.93; H, 6.74. Found: C, 41.91; H, 7.21. MS (ESI negative ion): m/z (%) 1063.06 [T8(2(carbomethoxy)ethyl)7F − TBA]−, 1075.08 [T8(2-(carbomethoxy)ethyl)7(OMe) − TBA]−, 1130.10 [M − TBA]−. X-ray data are given in the Supporting Information. Synthesis of Tetra-n-butylammonium Octakis(2-cyanoethyl)octasilsesquioxane Fluoride (5). 2-Cyanoethyltriethoxysilane (1.08 g, 5 mmol, 2 equiv) was dissolved in dry toluene (20 cm3), and then tetra-n-butylammonium fluoride (2.5 cm3 of 1 M solution in THF with 5% water, 1 equiv) was added. The mixture was stirred at room temperature under argon for 16 h. The reaction was stopped, and hexane (150 cm3) was added to the mixture. The cloudy solution was transferred to the freezer (−20 °C). A white solid precipitated out, and the supernatant was separated. A white solid precipitated out and was filtered off and washed with cold hexane. Recrystallization from toluene−acetone gave colorless crystals (0.67 g, 96%). Mp: 87−90 °C. 1 H NMR (300 MHz, acetone-d6; ppm): δ 3.15−3.11 (m, 8H, N−CH3), 2.39 (t, 3JHH = 7.88 Hz, 16H, CH2), 1.65−1.57 (m, 8H, CH2), 1.47−1.39 (m, 8H, CH2), 1.03(t, 3JHH = 7.52 Hz, 8H, CH2), G


Organometallics

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CDCl3; ppm): δ 4.06 (m, 16H, OCH3), 3.35−3.20 (m, 8H, N-CH2), 1.81−2.52 (m, 24H, -CH2PO and CH2), 1.45−1.35 (m, 8H, CH2), 1.31 (t, 3JHH = 6.21 Hz, CH3CH2PO), 0.98 (t, 3JHH = 7.71 Hz, CH3). 13C NMR (75.5 MHz, acetone-d6; ppm): δ 61.3 (CH2PO), 58.68 (N-CH2), 23.97 (CH2), 18.77 (CH2), 16.44 (CH3), 13.65 (CH3), 5.97 (POCH2), 5.89 (P(O)CH2CH2Si). 29Si NMR (79.3 MHz, CDCl3; ppm): δ −71.07. 19F NMR (376 MHz, CDCl3; ppm): δ −27.1, −27.2, −27.4 (F−@T8). 31P NMR (161.83 MHz, CDCl3; ppm): δ 35.9, 35.8, 35.7 (F−@T8). IR (Nujol; ν, cm−1): 3386, 2965, 2876, 2357, 16567, 1603, 1458, 1380, 1271, 1228, 1166 (νPO str), 1109 (νSi−O−Si as), 1054 (νP−O), 961, 881, 781, 731, 695 (νSi−O−Si s). Anal. Calcd for C64H148FNO36Si8P8: C, 38.45; H, 7.46. Found: C, 38.46; H, 7.42. MS (ESI negative ion): m/z (%) 1545.26 [T 8 [CH 2 CH 2 P(O)(OEt) 2 ] 7 CH 2 CH 2 F − TBA] − , 1663.28 [T 8 [CH 2 CH 2 P(O)(OEt) 2 ] 7 CH 2 CH 2 OCH 2 CH 3 − TBA] − , 1755.29 (100%) [M − TBA]−. Synthesis of Tetraethylammonium Octakis(2-cyanoethyl)octasilsesquioxane Fluoride (21). 2-Cyanoethyltriethoxysilane (1.08 g, 5 mmol, 2 equiv) was dissolved in dry toluene (20 cm3), and then tetraethylammonium fluoride hydrate (TEAF·xH2O) (373.12 g, 2.50 mmol, 1 equiv) was added. The mixture was stirred at room temperature under argon for 16 h. The reaction was stopped, and hexane (150 cm3) was added to the mixture. The cloudy solution was transferred to the freezer (−20 °C). A white solid precipitated out, and the supernatant was separated. Recrystallization from xylene− acetone afforded colorless crystals (0.59 g, 94%). 1H NMR (300 MHz, CDCl3; ppm): δ 3.33−3.29 (m, 8H, N-CH2), 2.41−2.37 (m, 16H), 1.35 (t, 12H, CH2), 0.85−0.82 (m, 16H, CH2). 13C NMR (75.5 MHz, acetone-d6; ppm): δ 121.9 (s, CN), 58.1 (N-CH2), 52.7, 18.5 (SiCH2), 10.1 (N-CH2CH3), 7.7 (-CH2CN). 29Si NMR (79.3 MHz, CDCl3; ppm): δ −72.2. 19F NMR (376 MHz, CDCl3; ppm): δ −27.9 (F−@T8). IR (Nujol; ν, cm−1): 2923 (νC−H), 2852 (νC−H), 2244 (νCN), 1392, 1376, 1318, 1284, 1195, 1099 (νSi−O−Si as), 997, 912, 786, 753, 724, 684 (νSi−O−Si s). MS (MALDI-TOF): m/z (%) 998.49 [M−], 867.0 [M − TBA]−, 869.0, 871.0 (isotopes). X-ray data are given in the Supporting Information. X-ray Crystallography. The crystallographic data collection of compounds 1, 4−6, and 21 was performed using a Nonius Kappa CCD diffractometer with Mo Kα radiation (λ = 0.710 73 Å) controlled by the Collect34 software package at either 120(2) K (1, 5, 6, 21) or 160(2) K (4). The data were processed using Denzo,35 and semiempirical absorption corrections were applied using SADABS.36 Crystallographic data are given in Table 4. The structures were solved by direct methods and refined by full-matrix least-squares procedures on F2 using SHELXS-97 and SHELXL-97,37 respectively. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were fixed using a riding model. Compound 5 also had some highly disordered solvent which could not be accurately modeled. As such, it was removed for the calculations using the SQUEEZE38 program within PLATON.39 Compounds 1 and 21 had disordered ammonium cations, for which distance restraints (SAME) were employed. CCDC 870856−870860 contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif.

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ACKNOWLEDGMENTS We wish to thank the EPSRC National Mass Spectrometry Service Centre (NMSSC) at Swansea and MEDAC Ltd. of Brunel University for elemental analysis.

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REFERENCES

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Figures giving NMR spectra and CIF files giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Facile Synthesis of Novel Functionalized Silsesquioxane

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