2.1 Silica surface

Among the large variety of accessible oxide supports, silica is probably the simplest and the best understood. Its surface is composed of relatively unreactive (at least for moderate temperatures of dehydroxylation) siloxane bridges ≡Si–O–Si≡, and surface hydroxyl groups ≡Si–OH. A huge amount of vibrational spectroscopy and solid-state NMR (¹H, ²⁹Si) studies have largely contributed to the determination, the distribution and the reactivity of these surface reactive sites. Some recent reviews are particularly interesting in this field [18–20]. The design of single-site supported organometallic complexes requires the perfect knowledge and the precise control of the surface reactive sites. Their nature and concentration are indeed strongly determining in the final structure of the surface organometallic fragments. A preliminary study of the surface morphology as a function of the pre-treatment temperature is then essential prior to chemical modification.

The support used for our studies was a pyrogenic (Degussa Aerosil^© 200). The specific area was 200 m² g^–1 for SiO_{2–(300,500)}, 180 m² g^–1 for SiO_{2–(700,800)} and 160 m² g^–1 for SiO_2–(1000). The influence of the dehydroxylation temperature on the nature and of the proportion of the reactive surface sites was studied by infrared and ¹H NMR spectroscopies.

The dehydroxylation of a compacted silica disk (10 mg) under vacuum was followed by infrared spectroscopy for temperatures ranging from 25 to 1000°C. The overall effect was an increase in the number of isolated silanol groups and an increase in the intensity of the 3747-cm^–1 IR band. The consumption of the H-bonded silanols was complete above 500 °C, and only isolated and terminal silanols at 3747 and 3720 cm^–1 were still observed at this temperature. For dehydroxylation temperatures above 500 °C, the low wavenumber asymmetry of the residual isolated silanol peak at 3747 cm^–1 disappeared and the band became narrower and more symmetrical. For the silica used in our studies, a qualitative approach was carried out by measuring the areas of each type of silanols (bridged and/or isolated) as a function of the dehydroxylation temperatures (Fig. 1). H-bonded silanols represented the majority of the surface hydroxyl groups initially present on silica. As the dehydroxylation temperatures increased, the sum of all silanols (▴) decreased in a same manner than that of bridged silanols (■). Due to the condensation of bridged silanols, the amount of isolated silanols (♦) increased and reached a maximum between 300–400 °C. For higher temperatures (≥ 500 °C), the condensation of isolated silanols occurred and their proportion decreased. At 600 °C, there was no more bridged silanols on the surface. However, since the extinction coefficients of each ν(O–H) vibration have not been calibrated, the relative quantification of the surface silanols by infrared was not possible.

Solid-state ¹H NMR spectra of silica samples dehydroxylated at temperatures below 500 °C displayed two peaks at 2.7 and 1.8 ppm. These peaks were respectively assigned to H-bonded, and isolated/geminal surface silanols. Isolated and geminal hydroxyl groups cannot be differentiated by solid-state ¹H NMR spectroscopy (the existence of geminal silanols was only evidenced by ²⁹Si NMR spectroscopy [21-23]. For dehydroxylation temperatures above 500 °C, the persistence of the sharp peak at 1.8 ppm demonstrated that only isolated silanols were present on the surface, as shown previously by infrared spectroscopy.

In NMR spectroscopy, the intensity of a signal is directly proportional to the total number of spins present in the sample. Then, relative intensities can be obtained from relative areas of resonance lines, and absolute intensities, from comparison with an external standard. For silica samples, the signals observed for isolated (1.8 ppm) or H-bonded (2.7 ppm) hydroxyls in ¹H NMR spectra are therefore proportional to the total amount of surface silanols, this latter being dependant on the mass of the samples and on the pre-treatment temperature. The peaks areas, divided by the mass of the corresponding samples, are representative of the total amount of surface silanols for a given dehydroxylation temperature. Solid-state ¹H NMR spectroscopy was then used to quantify the surface silanols for silica dehydroxylated at various temperatures, by reference to NH₄Cl as an external standard. The surface concentration α_OH of hydroxyl groups (OH/nm²) was then deduced from these data and the surface area as measured by nitrogen adsorption (Table 1).

It can be noted that the two analytical methods (¹H NMR and TGA) used for Aerosil silica were bulk methods measuring all OH groups, including internal and reactively inaccessible silanol groups. In conclusion, infrared spectroscopy brought qualitative aspects on the evolution and the distribution of surface reactive sites during the condensation process. Solid-state ¹H NMR allowed the quantification of all hydroxyl groups present on silica.

2.2 Reactivity of B(C₆F₅)₃ with silica surface

A large excess (5 equiv) of B(C₆F₅)₃ was reacted in toluene with SiO_{2–(500,700,1000)} for 2 h, at room temperature. The resulting pink solids were washed three times with toluene {these washings were collected for further analyses}and dried to give light pink (SiO_{2–(500,700)}) or white (SiO_2–(1000)) solids, 1.

The results of elemental analyses revealed a general loading of 0.1–0.2 w% of boron, which corresponds to a concentration of around 0.6 boron per surface silanol. After toluene washings, elemental analyses showed that 30 to 60% of boron has leached. This proved that only weak interactions were involved between B(C₆F₅)₃ and the silica surface (Table 2).

IR spectra of solids 1, the bands characteristic of the starting material at 1650, 1523, 1470, 1380 (C₆F₅ groups[16,24,25] and 1325 cm^–1are present (Fig. 2).

The most significant result was that in these spectra, (Fig. 2), the position and the intensity of the band due to the vibration of the free silanol, ν(≡Si–OH), at 3748 cm^–1 was identical before and after reaction with B(C₆F₅)₃. What is more, the ¹⁹F NMR spectra of the toluene filtrate (washings) of solids 1 showed only peaks at –129, –142 and –160 ppm, which were unambiguously attributed to ortho-, para- and meta-fluorine atoms of pure B(C₆F₅)₃. No C₆F₅H was detected by ¹⁹F NMR spectroscopy and gas chromatography in the filtrate solution. Likewise, the ¹H NMR spectrum of solids 1 revealed no significant difference in chemical shifts of surface isolated silanol groups Si–OH. (Δδ = 0.1 ppm) as expected if the surface Brønsted acidity has been created by interaction of B(C₆F₅)₃ with them [26]. These results indicated that the B–C₆F₅ bond was not cleaved (Eq. (2)), that there was no interaction of any type between B(C₆F₅)₃ and the oxygen atom of ≡Si–OH (Eq. (3)), and that B(C₆F₅)₃ was only physisorbed onto the silica surface (Eq. (4)).

In order to model the reaction of B(C₆F₅)₃ with silica surfaces, the reactivity of equimolar amounts B(C₆F₅)₃ and R–OH compounds (R = H, Ph₃Si, (c-C₅H₉)₇O₁₂Si₈) in benzene, at room temperature, was investigated by solution ¹H, ¹³C, ¹⁹F and ¹¹B NMR spectroscopy. ¹⁹F NMR spectra recorded for these solutions showed a general upfield shift of the para-fluorine after reaction with R–OH (Table 3). Such a shift was characteristic of a change in the boron geometry from trigonal to tetrahedral, induced by coordination with the hydroxy oxygen atom. The difference Δ(δ_{m p}) between the chemical shifts of para- and meta-fluorine atoms is strongly connected to the geometry at the boron centre and can be used to determine the strength of coordination of each R–OH compound [27]. The coordination strength at room temperature was shown to increase in the order Ph₃SiOH < (c-C₅H₉)₇O₁₂Si₈(OH) < H₂O (Table 3). It can be noted that no or few HC₆F₅, resulting from B-C₆F₅ bond cleavage, was detected in the reaction mixture in our conditions.

¹¹B NMR spectra recorded after addition of R–OH to B(C₆F₆)₃ at room temperature showed single signals shifted upfield from the initial peak of free B(C₆F₅)₃ (Table 3). ¹¹B chemical shifts are also strongly dependent on the boron geometry and go upfield from trigonal to tetrahedral (neutral then anionic) structures. This confirmed the coordination of B(C₆F₅)₃ on the hydroxylic groups. The upfield shifts observed in ¹¹B NMR spectra of B(C₆F₅)₃ after reaction with R–OH increased in the order Ph₃SiOH < (c-C₅H₉)₇O₁₂Si₈(OH) < H₂O (Table 3) and corroborated the coordination strengths evidenced by ¹⁹F NMR spectroscopy.

We can then conclude from these studies that at the opposite of silanol groups, the molecular compounds R–OH [ROH = Ph₃SiOH, (c-C₅H₉)₇O₁₂Si₈(OH)$\dot{}$H₂O] form a stable complex with B(C₆F₅)₃ (Eq. (5)).

2.3 Reactivity of B(C₆F₅)₃ with silica surface in presence of a Brønsted base

The above results demonstrated that there was no interaction between B(C₆F₅)₃ and ≡Si–OH. At the opposite, pyridine forms hydrogen bonds with ≡Si–OH [28-30] (Eq. (6)). This hydrogen bond induces a higher negative charge on oxygen atom of ≡Si–OH, which could favour the formation of ion pair [≡Si–O B(C₆F₅)₃]^–[HNC₅H₅]⁺. To verify this hypothesis, the reaction of toluene solution of C₅H₅N and B(C₆F₅)₃ (1:1) with SiO₂ was performed.

2.3.1 Reactivity of 1 with pyridine

Pink silica pellets of 1 prepared by impregnation of B(C₆F₅)₃ on SiO_{2–(500,700,1000)}, were contacted with a vapour pressure of pyridine under static vacuum at room temperature. The ν(≡Si–OH) band at 3747 cm^–1 due to free silanol disappeared and simultaneously a broad band resulting from hydrogen bonding between the surface silanols and pyridine appeared near 2900 cm^–1. Aromatic ν(C–H) and ν(C=C) vibrations were also observed at 3081, 3025, 3002 cm^–1 and in the [1650–1500 cm^–1] region respectively, and were characteristic of the pyridine. The bands initially present at 1648 and 1523 cm^–1 in 1 for the C₆F₅ groups were unaffected pyridine (respectively 1645 and 1519 cm^–1 after desorption), while the strong one at 1483–1473 cm^–1 was shifted by about 20 cm^–1 to lower wavenumbers (1465 cm^–1). Other vibrations observed in 1 at 1394, 1383 and 1323 cm^–1 disappear almost entirely after contact with pyridine. The same tendencies were noticed for the molecular compounds B(C₆F₅)₃ and (C₆F₅)₃B.NC₅H₅ adduct. The excess of pyridine was removed under vacuum (10^–5 mbar) at room temperature. In the infrared spectra, the position of the peaks at 1632, 1492 and 1447 cm^–1 indicates the presence of pyridine bonded to a Lewis acid site [28–30]. The shift of the peak of pyridine 8a mode [30] from 1583 to 1631 cm^–1 (Δν = 43 cm^–1) suggests the presence of strong strength Lewis acid. On the contrary, no bands at 1638, 1620 and 1540 cm^–1, characteristic of pyridine bonded to a Brønsted acid site on 1, are observed (Fig. 3).

Solid-state ¹H NMR spectrum of 1 after reaction with C₅H₅¹⁵N shows a broad peak at 2.3 ppm, attributed to hydrogen-bound silanol groups, and three peaks at 7.4, 7.8 and 8.6 ppm characteristic of meta, para and ortho protons of the heterocycle. The solid state ¹⁵N NMR spectra displayed a resonance at –150 ppm, characteristic of Lewis acidic site [26,30–34], unambiguously assigned to the Lewis acid-base adduct (C₆F₅)₃B–NC₅H₅, as demonstrated by comparison with the ¹⁵N NMR spectrum of (C₆F₅)₃B–(¹⁵N C₅H₅) in the solid state (δ¹⁵_N = –159.2 ppm). As by infrared studies, no Brønsted site acid was evidenced onto B(C₆F₅)₃-modified silica 1 by solid-state ¹H and ¹⁵N NMR spectroscopies. The strong Lewis acidity detected on 1 is only due to homogeneous (C₆F₅)₃B-(¹⁵N C₅H₅) physisorbed on the surface. The reaction of B(C₆F₅)₃ and C₅H₅N (1:1) in toluene afforded quantitatively the complex (C₆F₅)₃B.N C₅H₅, 2, which did not react with SiO₂, It was only physisorbed (Eq.(7)).

2.3.2 Reactivity of 1 with diethylaniline

The addition of one equivalent of NEt₂Ph to B(C₆F₅)₃ in aromatic solvents led instantaneously to a pink coloration of the solution, S. The stoichiometric reaction of B(C₆F₅)₃ and NEt₂Ph at room temperature, in an aromatic solvent, has been investigated by 1D and 2D NMR of ¹H,¹³C, ¹¹B, ¹⁹F and ¹⁵N [35]. No adduct Et₂PhN.B(C₆F₅)₃ was formed. An equilibrium has been evidenced between free (C₆F₅)₃B (40%) and Et₂NPh, imminium salt (HB(C₆F₅)₃)^–(HNEt₂Ph)⁺ (30% ), and zwitterionic stereoisomers E and Z-[PhEtN⁺=CH–CH₂B^–(C₆F₅)₃] in a 3:2 ratio (30%) [36]. In the presence of a protic compound, this equilibrium is totally displaced towards the ionic form [RO–B(C₆F₅)₃]^–[HNEt₂Ph]⁺ [36].

Silica pellets 2 were prepared by impregnation of S on SiO_{2–(300,500,800)}. The IR band ascribed to isolated silanols at 3747 cm^–1 disappeared totally and was replaced by broad bands at 3681, 3624 cm^–1 (ν(OH) of interacting silanols) and by a sharp band at 3232 cm^–1. The intensity of the bands at 3681, 3624 cm^–1 decreases inversely with the dehydroxylation temperature; simultaneously, that of the band at 3232 cm^–1 increases (Fig. 4).

The use of labelled ²H or ¹⁸O silica proved that the band at 3232 cm^–1 was due to a ν(N–H) vibration [36]. The structure of the supported species was confirmed by solid-state ¹H NMR [δC₆H₅ = 7.2, δCH₂ = 3.4, δCH₃ = 1 ppm], solid-state CP-MAS ¹³C NMR [δCH₃ = 39, δCH₂ = 51 ppm] and solid-state ¹¹B NMR [δ = –8 ppm], in agreement with the ¹¹B chemical shift (solid state) of molecular analogous (–6.9 and –7 ppm), or literature data [6]. It is characteristic of an anionic borate fragment, i.e. [≡SiO–B(C₆F₅)₃]^–, in which the boron atom is tetracoordinated.

Elemental analyses of 2 /SiO_{2–(300,500,700,800)} (Table 4) indicate a slight decrease in the weight percentage of boron, from 0.23 to 0.17%, when the dehydroxylation temperature of the silica is increasing from 300 to 800 °C. But no leaching of boron occurred after washing(see experimental part).

Table 4

Elemental analysis of 2 in function of the dehydroxylation temperature of SiO₂

T (°C)	OH (nm2)	%B a	%N a	B/N b	F/B b,c	C/(B+N) b	B/OHd	N/OH d
300	1.7	0.23	0.30	0.99	10.6	12.1	0.38	0.38
500	1.2	0.21	0.27	1.01	13.6	14.3	0.49	0.48
500	1.2	0.22	0.29	1	12	13	0.51	0.52
700	0.7	0.16	0.18	1.15	12.3	15.6	0.71	0.61
800	0.6	0.16	0.21	0.99	16.2	16.4	0.82	0.84
800	0.6	0.17	0.24	0.92	12.1	15.2	0.88	0.96
			Theory	1	15	14

^a Weight %.

^b Molar ratios.

^c Titration values obtained by conductimetry for fluorine in presence of boron were lowered by formation of BF₄^– anion.

^d Calculated from respective weight percentages of boron and nitrogen.

As demonstrated by infrared spectroscopy and elemental analysis, it must be emphasized that silanol groups could be only partially converted to ionic sites, depending on dehydroxylation temperatures. Indeed, weight percentages of boron corresponded to a proportion of modified silanols ranging from 37 to 96% when the pre-treatment temperatures of the silica increased from 300 to 800 °C.

In order to check if the experimental weight percentage of boron corresponded to the highest loading achievable on the surface, modelling of 1 was performed using Sybyl computer modelling program. This entity was then grafted on a model of a partially dehydroxylated silica particle SiO_2–(500) and minimized using the molecular mechanics Tripos force field [37–38]. A three-dimensional representation of the grafted model is depicted in (Fig. 5). The theoretical value obtained for the length of the B–O bond (1.567 Å) in the modelled complex was in the range of those reported for related silsesquioxane compounds (1.505–1.495 Å) [16,24–25]. The projected area of such a complex on the silica particle was estimated to 1.27 nm². According to the silica specific area (200 m² g^–1), this projection corresponds to a theoretical value of 0.24 w% of boron on a saturated surface. This result is in good agreement with the experimental percentages obtained on SiO_2–(500) (0.21–0.22 w%).

Al spectroscopic and analytic data, as well as the similarities with those of molecular compounds [HNEt₂Ph]⁺[(C₆F₅)₃BOR]^– (R = H, SiPh₃, Si₈O₁₂(c-C₅H₉)₇) [17,36,39] confirmed the already proposed [6,7,9] structure for 2 (Eq. (8)).

4.1 General procedures

Solid-state NMR spectra were recorded on a Bruker DSX-300 spectrometer equipped with a standard 4-mm double-bearing probe head and operating at 75.47, 96.31, 30.43, and 300.18 MHz for ¹³C, ¹¹B, ¹⁵N and ¹H, respectively. Chemical shifts were given with respect to external references (¹H, ¹³C, TMS at 0 ppm; ¹¹B, BF₃.OEt₂ at 0 ppm; ¹⁵N, CH₃NO₂ at 0 ppm). The samples were introduced in the zirconia rotor in a glovebox and tightly closed. Solution NMR spectra were recorded on Bruker AC 200 MHz (¹⁹F), AC 300 MHz (¹H, ¹³C), DRX 300 MHz (¹¹B) and DRX 500 MHz (¹H, ¹³C, ¹¹B, ¹⁵N) spectrometers. Chemical shifts were reported in ppm and referenced to residual solvent resonances (C₆D₆: 7.15 for ¹H, 128 for ¹³C; CD₂Cl₂: 5.32 for ¹H, 53.8 for ¹³C), or external standards (¹⁹F, CFCl₃ at 0; ¹¹B, BF₃.OEt₂ at 0; ¹⁵N, CH₃NO₂ at 0). Infrared spectra were recorded under vacuum on a Nicolet 550 FT spectrometer by using an infrared cell equipped with CaF₂ windows, allowing in situ studies. Typically, 16 scans were accumulated for each spectrum. KBR pellets were prepared for molecular compounds. Elemental analyses were performed by the Central Analysis Service of the CNRS at Solaize.

All operations were performed in the strict absence of oxygen and water under a purified argon atmosphere using glovebox (Jacomex or MBraun) or vacuum-line techniques. Toluene and pentane were distilled under argon from Na/K alloy, degassed and stored under argon over Na (toluene) or 3–Å molecular sieves (pentane). C₆D₆ (SDS – 99.6%) and CD₂Cl₂ (SDS – 99.6%) were degassed by three cycles of ‘freeze-pump-thaw’ and dried over freshly regenerated 3 Å molecular sieves. Pyridine (Aldrich, 99%) was dried on solid KOH and distilled under argon, degassed and stored over 3–Å molecular sieves. Labelled ¹⁵N pyridine (Aldrich, 99%) was degassed and store over 3Å molecular sieves. B(C₆F₅)₃ (Merck Chemicals, > 97%) was dried on Me₃SiCl [40], purified by vacuum sublimation and its purity was checked by ¹⁹F NMR before using. The 3,5,7,9,11,13,15-heptacyclopentyl-pentacyclooctasiloxan-1-ol and the triphenylsilanol were purchased from Aldrich Chemical and dried under vacuum before use. The synthesis of B(C₆F₅)₃.OH₂ [13] [16], B(C₆F₅)₃.N C₅H₅ [17], and [Z–O B(C₆F₅)₃]^–[HNEt₂Ph]⁺ with Z–OH, Z = H, Ph₃Si, (c-C₅H₉)₇O₁₂Si₈ [36], or silica were performed according to literature procedures. Silica support (Aerosil^® Degussa, 200 m² g^–1) was compacted to a disk (30 mg) for infrared studies or was hydrated, dried (80 °C) and crushed to prepare large quantities (1–2 g) for NMR studies and elemental analyses. Before reaction, to prepare large quantities, silica was calcinated at 400 °C in air for 4 h, and dehydroxylated at the desired temperature (500, 700, 800 or 1000 °C) under high vacuum (10^–5 Torr) for 12 h (referred respectively as SiO_2–(500), SiO_2–(700) and SiO_2–(1000)), except in the case of silica dehydroxylated at 300 °C (referred as SiO_2–(300)), which was calcinated at 400°C in air for 4 h, rehydrated with distilled water and then dehydroxylated at 300 °C for 12 h. The surface areas were measured by N₂ adsorption and treatment of data using BET method.

4.2 Preparation of the B(C₆F₅)₃-modified silica supports, 1, and reactivity with pyridine

4.3 Adsorption of pyridine on 1

4.4 Reactivity of B(C₆F₅)₃ with H₂O

Addition of 7 μl (0.39 mmol) of distilled water to a solution of 206 mg (0.4 mmol, 1 equiv) of B(C₆F₅)₃ in 12 ml of pentane led to precipitation of a white solid. The suspension was stirred overnight, and the crude product obtained after filtration was washed three times with pentane (4 ml) and dried 1 h under vacuum. Yield (175 mg, 82%) ¹H NMR (C₆D₆, δ): 4.6 (s, OH). ¹³C NMR (C₆D₆, δ): 148.0 (d, ¹J_C–F = 242 Hz, o-C₆F₅), 141.5 (d, ¹J_C–F = 254 Hz, p-C₆F₅), 137.6 (d, ¹J_C–F = 255 Hz, m-C₆F₅), 114.6 (br.s, i-C₆F₅). ¹⁹F NMR (C₆D₆, δ): –134.8 (d, ³J_F–F = 23 Hz, 6F, o-C₆F₅), –154.3 (t, ³J_F–F = 21 Hz, 3F, m-C₆F₅), –162.5 (t, ³J_F–F = 23 Hz, 6F, m-C₆F₅). ¹¹B NMR (C₆D₆, δ): 6.8 (s, (C₆F₅)₃B–OH₂). IR (KBr, cm^–1): 3565 (m), 3502 (m), 1652 (m), 1610 (w), 1523 (s), 1473 (s), 1382 (m), 1292 (m), 1270 (w), 1246 (w), 1122 (m), 1112 (m), 1093 (m), 1014 (w), 974 (s).

4.5 Reactivity of B(C₆F₅)₃ with Ph₃SiOH

In a NMR tube, 75 mg (0.15 mmol – 1 equiv) of B(C₆F₅)₃ was added at room temperature to a solution of 40 mg (0.14 mmol) of Ph₃SiOH in 0.5 ml of d₆-benzene. A clear colourless solution was obtained after stirring. ¹H NMR (C₆D₆, δ): 7.5 (d, ³J_H–H = 7.5 Hz, 6H, o-C₆H₅), 7.1 (m, 9H, p- and m-C₆H₅), 4.2 (br.s, 1H, –OH). ¹³C NMR (C₆D₆, δ): 148.2 (d, ¹J_C–F = 244 Hz, o-C₆F₅), 137.7 (d, ¹J_C–F = 255 Hz, m-C₆F₅), 135.6 (s, o-C₆H₅), 133.1 (s, i-C₆H₅), 131.0 (s, p-C₆H₅), 128.4 (s, m-C₆H₅), p- and i-C₆F₅ carbons non resolved. ¹⁹F NMR (C₆D₆, δ): –130.4 (d, ³J_F–F = 17 Hz, 6F, o-C₆F₅), –147.9 (br.s, 3F, p-C₆F₅), –161.0 (m, 6F, m-C₆F₅). ¹¹B NMR (C₆D₆, δ): 40 (br.s, Ph₃Si(OH)B(C₆F₅)₃).

4.6 Reactivity of B(C₆F₅)₃ with (c-C₅H₉)₇O₁₂Si₈(OH)

In a NMR tube, 75 mg (0.15 mmol, 1 equiv) of B(C₆F₅)₃ was added at room temperature to a solution of 135 mg (0.15 mmol) of (c-C₅H₉)₇O₁₂Si₈(OH) in 0.5 ml of d₆-benzene. A clear colourless solution was obtained after stirring. ¹H NMR (C₆D₆, δ): 5.2 (br.s, 1H, OH), 1.8 (m, 14H, CH₂ C₅H₉), 1.6 (m, 28H, CH₂ C₅H₉), 1.4 (m, 14H, CH₂ C₅H₉), 1.1 (m, 7H, CH C₅H₉). ¹³C {¹H} NMR (C₆D₆, δ): 148.4 (d, ¹J_C–F = 240 Hz, o-C₆F₅), 141.9 (d, ¹J_C–F = 254 Hz, p-C₆F₅), 137.6 (d, ¹J_C–F = 253 Hz, m-C₆F₅), 27.7, 27.6, 27.5, 27.4, 27.3 (s, CH₂ C₅H₉), 22.5, 22.3, 21.9 (s, CH C₅H₉). ¹⁹F NMR (C₆D₆, δ): –132.6 (br.s, 6H, o-C₆F₅), –152.0 (br.s, 3H, p-C₆F₅), –161.8 (m, 6H, m-C₆F₅). ¹¹B NMR (C₆D₆, δ): 27 (br.s, (c-C₅H₉)₇O₁₂Si₈(OH)B(C₆F₅)₃).

4.7 Preparation of (C₆F₅)₃B–NC₅H₅

To a solution of 0.248 g (0.48 mmol) of B(C₆F₅)₃ in 5 ml of toluene was added 0.042 ml (0.52 mmol) of pyridine to give a colourless solution. After stirring for 1 h, the toluene was evaporated under vacuum to leave a white solid which was washed twice with pentane (2 ml) and dried for 1 h at room temperature. Yield = 0.234 g (0.40 mmol – 82%). ¹H NMR (C₆D₆, δ): 7.94 (d, ³J_H–H = 5 Hz, 2H, o-C₅H₅N), 6.57 (t, ³J_H–H = 7 Hz, 1H, p-C₅H₅N), 6.24 (t, ³J_H–H = 7 Hz, 2H, m-C₅H₅N). ¹³C {¹H} NMR (C₆D₆, δ): 148.3 (d, ¹J_C–F = 243 Hz, o-C₆F₅), 146.4 (s, o-C₅H₅N), 141.7 (s, p-C₅H₅N), 140.8 (d, ¹J_C–F = 232 Hz, p-C₆F₅), 137.7 (d, ¹J_C–F = 242 Hz, m-C₆F₅), 124.9 (s, o-C₅H₅N), 118.6 (br.s, i-C₆F₅). ¹⁹F NMR (C₆D₆, δ): –131.4 (d, ³J_F–F = 21 Hz, 6F, o-C₆F₅), –155.4 (t, ³J_F–F = 21 Hz, 3F, p-C₆F₅), –162.6 (t, ³J_F–F = 19 Hz, 6F, m-C₆F₅). ¹¹B NMR (C₆D₆, δ): –3.7 (s, (C₆F₅)₃B–NC₅H₅). ¹⁵N NMR (C₆D₆, δ): –158.2 (s, (C₆F₅)₃B–NC₅H₅). IR (KBr, cm^–1): 1646 (m) C₆F₅, 1631 (m) L-bonded C₅H₅N, 1578 (w) C₅H₅N, 1520 (s) C₆F₅, 1495 (m) L-bonded C₅H₅N, 1462 (vs) C₆F₅, 1385 (m) C₆F₅, 1378 (m) C₆F₅, 1293 (m), 1283 (m), 1252 (m), 1242 (m), 1224 (m), 1162 (w), 1115 (m), 1098 (m), 1028 (w), 991 (s) C₆F₅, 981 (s) C₆F₅, 962 (m) C₆F₅.