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DOI: 10.1039/C1RA00186H (Paper) RSC Adv., 2011, 1, 128-134

Stable sulfate and nitrate borane-adduct anions†

Martin Becker a, Axel Schulz *ab, Alexander Villinger a and Karsten Voss a
aUniversität Rostock, Institut für Chemie, Albert-Einstein-Str.3a, 18059 Rostock, Germany
bLeibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany. E-mail: axel.schulz@uni-rostock.de

Received 20th May 2011 , Accepted 9th June 2011

First published on 18th July 2011

Abstract

The addition of potassium or silver salts of nitrate to a solution of B(C6F5)3 in diethyl ether affords salts containing very voluminous B(C6F5)3 mono adduct anions of the type [Ag(Et2O)3]+[NO3·B(C6F5)3]−, and [K(Et2O)2]+[NO3·B(C6F5)3]−. When the double negatively charged sulfate anion was used only for the potassium salt, adduct anion formation was observed upon addition of 18-crown-6. In this case K(18-crown-6)(CH2Cl2)]+[SO4·2 B(C6F5)3]− bearing the diadduct anion was isolated and fully characterized. These salts are thermally stable up to over 200 °C and dissolve in polar organic solvents.

Introduction

Almost ten years ago LaPointe and Bochmann et al. reported on weakly coordinating anions1 based on Lewis acid/Lewis base adducts2–4 and their application in catalysis.5 With the knowledge that a strong Lewis acid such as tris(pentafluorophenyl)borane, B(C6F5)3, easily forms stable donor–acceptor bonds with strong bases such as R–CN, the Bochmann group prepared weakly coordinating anions of the type [Z{B(C6F5)3}n]x− (Z = CN, Ni(CN)4, NH2; x = 1, 2; n = 2, 4). These anions are easily prepared in a simple acid/base reaction. For example, two equivalents of B(C6F5)3 are added to the strong base X− (X = CN,3,5c–d,C3N2H3 (imidazolyl) 4,6 or NH2,2 to give in almost quantitative yields adduct anions of the type [(F5C6)3B(μ-X)B(C6F5)3] with astonishingly stable borate units. This surprising stability can be observed for instance in reactions of Na+[(F5C6)3B(μ-NH2)B(C6F5)3]− with HCl in diethyl ether, resulting in the formation of [H(OEt2)2]+[(F5C6)3B(μ-NH2)B(C6F5)3]− and NaCl. For a labile anion decomposition products such as H3N·B(C6F5)3 and Et2O·B(C6F5)3 should be observed.5c,d Another example represents crystalline [CPh3]+[(F5C6)3B(μ-NH2)B(C6F5)3]−, which is stable over long periods in an oxygen and moisture atmosphere.5d

Both systems B(C6F5)/H2O and B(C6F5)/OH− have been intensively studied.7–11 For example, addition of water to the Lewis acid B(C6F5)3 gives H2O·B(C6F5)3·2H2O while the reaction between B(C6F6)3 and KOH–H2O in the presence of dibenzo-18-crown-6 gives [K(dibenzo-18-crown-6)]+[HOB(C6F5)3]2− which crystallizes together with the adductH2O·B(C6F5)3; the binuclear borate anion [(F5C6)3B(μ-OH)B(C6F5)3]− is formed as a salt with the cation [Ir(η5-C5H5)(C8H12)H]+ by addition of H2O to B(C6F5)3 in the presence of [Ir(η5-C5H5)(C8H12)].12

Furthermore, Siedle et al. described complexes of (C6F5)3B with water, alcohols, mercaptans, silanols and oximes, and their role in catalysis.13 Carboxylate adducts have been reported by Baird et al.14

The approach to generate weakly coordinating anions of the type [Z{B(C6F5)3}n]x can be generalized with respect to the Lewis base linker as well as to the bulky Lewis acid.15 A synthetically facile route to the bulky very weakly coordinating anions [N(CN·B(C6F5)3)2]−, [C(CN·B(C6F5)3)3]−, and [B(CN·B(C6F5)3)4]−, which were isolated as stable alkali, silver, EMIM and BMIM salts, was reported recently.15

Here we want to report the synthesis, structure and bonding of salts bearing the new nitrate [Ag(Et2O)3]+[NO3·B(C6F5)3]−, [K(Et2O)2]+[NO3·B(C6F5)3]− and sulfate K(18-crown-6)(CH2Cl2)]+[SO4·2 B(C6F5)3]2− adduct anions.

Results and discussion

Synthesis of [Ag(Et2O)3]+[NO3·B(C6F5)3]−, [K(Et2O)2]+[NO3·B(C6F5)3]− and [K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2−

Synthesis of the nitrates [Ag(Et2O)3]+[NO3·B(C6F5)3]−, [K(Et2O)2]+[NO3·B(C6F5)3]− and sulfate [K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2− is easily achieved by adding the corresponding MX salt (M = Ag, K; X = NO3, SO4) to a solution of B(C6F5)3 (slight excess of 1 equivalent) in diethyl ether. In case of the nitrate salts within a short period (ca. 30 min) the formation of a clear colorless solution can be observed, while for the doubly negatively charged sulfate a crown ether (18-crown-6) was additionally added to completely dissolve the adduct anion salt. It should be noted that it was impossible to carry out this reaction with Ag2SO4 due to its very low solubility in diethyl ether, only the K2SO4 diadduct formation for the sulfate anion was observed.

Removal of the solvent followed by a washing with n-hexane (removal of excess B(C6F5)3) gives the corresponding adduct anion salts in good yields. Recrystallization from diethyl ether or CH2Cl2 yields colorless crystals of the pure substances. Often solvent molecules are embedded in the crystals which, however, can be removed by thermal treatment (50–60 °C) of the crystalline material over a period of 6–24 h.

Properties of [Ag(Et2O)3]+[NO3·B(C6F5)3]−, [K(Et2O)2]+[NO3·B(C6F5)3]− and [K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2−

[Ag(Et2O)3]+[NO3·B(C6F5)3]−, [K(Et2O)2]+[NO3·B(C6F5)3]− and K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2− are neither air nor moisture sensitive. They dissolve in polar solvents such as CH2Cl2, diethyl ether, or THF, but slowly decompose in water under formation of the H2O·B(C6F5)3 adduct as shown by 19F and 11B NMR studies.15–18 The astonishing stability against water can be attributed mainly to kinetic hindrance by the B(C6F5)3 groups. For comparison, it is known that for instance in CH3CN·B(C6F5)3 the acetonitrile molecule can partially be removed by water, which has been studied by means of equilibrium titration.14 All mentioned salts are easily prepared in bulk and are indefinitely stable when stored in a sealed tube in the dark (silver salt). Salts bearing the anions [NO3·B(C6F5)3]− and [SO4·2 B(C6F5)3]− are thermally stable up to over 200 °C.
NMR studies. Selected 11B, and 19F data for compounds described in this work are listed in Table 1. 11B is particularly well suited to distinguish between three-coordinate borane and the four-coordinate boron found in the Lewis acid–base adducts for which the 11B resonance (between 0 and –5ppm) is shifted to lower frequency with respect to free B(C6F5)3 by more than 60 ppm (cf.B(C6F5)3 in CD2Cl2: 59.1 ppm).7 The data illustrate that when boron is directly attached to oxygen, the 11B{1H} resonance is sharp and is observed in the expected range (cf. −7 to −12 ppm when attached to a N atom of CN group).15 The 19F NMR spectra of all species are almost identical (besides the water adduct) and show resonances shifted to lower frequency compared to B(C6F5)3 but similarly broad.
Table 1 Selected NMR Data of B(C6F5)3, [Ag(Et2O)3]+[NO3·B(C6F5)3], [K(Et2O)2]+[NO3·B(C6F5)3]−, K(18-crown-6)]2+[SO4·2 B(C6F5)3]2−, K[HO·B(C6F5)3] and H2O·B(C6F5)3 (chemical shifts in ppm)
Species 11B 19F (o,m,p)a
a o,m,p = ortho, meta, para F atom in C6F5.
B(C6F5)3 59.1 −128.4, −161.3, −144.2
[Ag(Et2O)3]+[NO3·B(C6F5)3] 0.23 −134.7,−166.4, −160.6
[K(Et2O)2]+[NO3·B(C6F5)3]− −1.28 −135.0,−163.2, −156.2
K(18-crown-6)]2+[SO4·2 B(C6F5)3]2− −3.33 −132.7, −167.7, −163.4
K[HO·B(C6F5)3] −4.06 −135.9, −165.7, −161.7
H2O·B(C6F5)3 −0.67 −135.4, −163.2, −156.1

Structure and bonding. Adduct anions such as [NO3·B(C6F5)3]− and [SO4·2 B(C6F5)3]2− are typical charge transfer complexes and the bond between the B(C6F5)3 and the naked anions can be regarded as a donor–acceptor bond.15,19 According to NBO analysis (NBO = natural orbital analysis),20–23 the total charge transfer is −0.40 in [NO3·B(C6F5)3]− and −0.80 e in [SO4·2 B(C6F5)3]− which gives in the latter case −0.40 e per B(C6F5)3 group attached in [SO4·2 B(C6F5)3]2−. In the calculated monoadduct [SO4·B(C6F5)3]2− a charge transfer of −0.45 e was found which means that the charge transfer increases along NO3− < SO42− which can mainly be attributed to the larger negative charge in the sulfate.

According to this fairly large charge transfer, the anion charge is strongly delocalized over 38 ([NO3·B(C6F5)3]−), and 73 atoms ([SO4·2 B(C6F5)3]−). However, although very voluminous these anion adducts cannot be considered weakly coordinating anions (Fig. 1) as the NO3− and SO42− core anion is not completely protected by the B(C6F5)3 group allowing further complexation via the oxygen atoms (see below).


Molecular structure of [NO3·B(C6F5)3]− (left) and [SO4·2 B(C6F5)3]2− (right), as superposition of ball-and-stick and space-filling models. Color code: boron bronze, carbon dark grey, nitrogen blue, fluorine steel blue, sulphur yellow.
Fig. 1 Molecular structure of [NO3·B(C6F5)3]− (left) and [SO4·2 B(C6F5)3]2− (right), as superposition of ball-and-stick and space-filling models. Color code: boron bronze, carbon dark grey, nitrogen blue, fluorine steel blue, sulphur yellow.

The structures of [Ag(Et2O)3]+[NO3·B(C6F5)3]− (1, [K(Et2O)2]+[NO3·B(C6F5)3]− (2 and K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2− (3, (H2O)3·B(C6F5)3 (4, [H2O·B(C6F5)3]·2(THF) (5, Et2O·B(C6F5)3 (6 and THF·B(C6F5)3 co-crystallized with 18-crown-6 (7) have been determined. The latter three species were obtained during several different re-crystallization attempts of [K(18-crown-6)]+[HO·B(C6F5)3]−. The X-ray analysis of [K(18-crown-6)]+[HO·B(C6F5)3]− crystals was not good enough to be discussed. Table 2 presents the X-ray crystallographic data. ORTEP representations of 1–3 are shown in Fig. 2, 4 and 6 and for compounds 4, 5, 6 and 7 in the ESI.†


ORTEP drawing of the molecular structure of [Ag(Et2O)3]+[NO3·B(C6F5)3]− in the crystal. Thermal ellipsoids with 50% probability at 173 K. Selected bond lengths (Å) and angles (°): N–O3 1.208(4), N–O2 1.228(4), N–O1 1.332(3), O1–B 1.548(4), B–C1 1.645(5), B–C7 1.630(5), B–C13 1.635(5); O3–N–O2 125.1(3), O3–N–O1 115.8(3), O2–N–O1 119.1(3), N–O1–B 118.0(2), N–O2–Ag 107.7(2), O1–B–C7 106.0(3), O1–B–C13 112.7(3), C7–B–C13 116.2(3), O1–B–C1 102.7(2), C7–B–C1 114.1(3), C13–B–C1 104.6(3); O3–N–O1–B 179.1(3), O2–N–O1–B 1.6(4), O3–N–O2–Ag 8.3(4), O1–N–O2–Ag 171.1(2), N–O1–B–C7 63.5(3), N–O1–B–C13 64.5(3), N–O1–B–C1 176.5(2).
Fig. 2 ORTEP drawing of the molecular structure of [Ag(Et2O)3]+[NO3·B(C6F5)3]− in the crystal. Thermal ellipsoids with 50% probability at 173 K. Selected bond lengths (Å) and angles (°): N–O3 1.208(4), N–O2 1.228(4), N–O1 1.332(3), O1–B 1.548(4), B–C1 1.645(5), B–C7 1.630(5), B–C13 1.635(5); O3–N–O2 125.1(3), O3–N–O1 115.8(3), O2–N–O1 119.1(3), N–O1–B 118.0(2), N–O2–Ag 107.7(2), O1–B–C7 106.0(3), O1–B–C13 112.7(3), C7–B–C13 116.2(3), O1–B–C1 102.7(2), C7–B–C1 114.1(3), C13–B–C1 104.6(3); O3–N–O1–B 179.1(3), O2–N–O1–B 1.6(4), O3–N–O2–Ag 8.3(4), O1–N–O2–Ag 171.1(2), N–O1–B–C7 63.5(3), N–O1–B–C13 64.5(3), N–O1–B–C1 176.5(2).
Table 2 Crystallographic details of 1, 2, and 3
  1 2 3
Chem. Formula C30H30AgBF15NO6 C26H20BF15KNO5 C61.28H50.56B2Cl2.56F30K2O16S
Form. Wght. [g mol−1] 904.23 761.34 1835.45
Colour colourless colourless colourless
Cryst. system Monoclinic Monoclinic Triclinic
Space group P21/c P21/c P[1 with combining macron]
a/Å 9.3844(4) 13.0444(5) 14.309(5)
b/Å 16.4084(8) 14.1638(6) 22.551(9)
c/Å 22.847(1) 16.8260(7) 24.200(8)
α (°) 90.00 90.00 102.53(3)
β (°) 92.496(2) 90.394(2) 98.78(3)
γ (°) 90.00 90.00 100.40(3)
V/Å3 3514.8(3) 3108.7(2) 7347(4)
Z 4 2 4
ρ c/g cm−3 1.709 1.627 1.659
μ/mm−1 0.696 0.299 0.391
T/K 173(2) 173(2) 173(2)
Measured reflections 52170 77557 113434
Independent reflections 6517 10777 24030
Reflections with I > 2σ(I) 5358 7691 16214
Rint. 0.0395 0.0240 0.0414
F(000) 1808 1672 3695
R 1 (R [F2 > 2σ(F2)]) 0.0453 0.0416 0.0480
wR2 (F2) 0.1161 0.1267 0.1384
GooF 1.075 1.080 1.092
Parameters 493 504 2090

[Ag(Et2O)3]+[NO3·B(C6F5)3]− crystallizes in the monoclinic space groupP21/c with four formula units per cell (Fig. 2). The silver cation is coordinated by three oxygen atoms of the diethyl ether molecules at slightly different distances (Ag–O4 2.355(3), Ag–O5 2.314(3), and Ag–O6 2.405(3) Å; cf. 2.266(1), 2.274(1) and 2.341(1) Å in [Ag(Et2O)3][dca_2b]) and only one oxygen atom of the NO3− anion at significantly longer distance (Ag–O2 2.505(2) Å).15 Since O1 is attached to the boron atom, only O3 could also coordinate to the silver atom. The fairly long Ag–O3 distance of 3.033(3) Å is in the range of a weak van der Waals interaction (cf. dcov(Ag–O) = 1.91,24dvdW (Ag⋯O)) = 3.2 Å).25 The local coordination environment, therefore, is strongly distorted tetrahedral with O–Ag–O angles between 86–140° (e.g. O2–Ag–O6 122.2(1), O2–Ag–O4 86.6(1), O2–Ag–O5 117.9(1), O4–Ag–O5 138.3(1); Fig. 3).


Ag+ coordination in [Ag(Et2O)3]+[NO3·B(C6F5)3]−. Selected bond lengths (Å) and angles (°):O2–Ag 2.505(2), Ag–O4 2.355(3), Ag–O5 2.314(3), Ag–O6 2.405(3); O4–Ag–O5 138.3(1), O5–Ag–O6 88.8(1), O4–Ag–O6 106.8(1), O2–Ag–O5 117.9(1), O2–Ag–O4 86.6(1), O2–Ag–O6 122.2(1).
Fig. 3 Ag+ coordination in [Ag(Et2O)3]+[NO3·B(C6F5)3]−. Selected bond lengths (Å) and angles (°):O2–Ag 2.505(2), Ag–O4 2.355(3), Ag–O5 2.314(3), Ag–O6 2.405(3); O4–Ag–O5 138.3(1), O5–Ag–O6 88.8(1), O4–Ag–O6 106.8(1), O2–Ag–O5 117.9(1), O2–Ag–O4 86.6(1), O2–Ag–O6 122.2(1).

The nitrate ion distorts upon complexation from D3h to Cs symmetry but is still planar (O1–N–O2 119.1(3), O1–N–O3 115.9(3), O2–N–O3 125.1(3); Σ < N = 360.1°) with the Ag+ ion and the B atom also located in the NO3 plane. Three considerably different N–O bond lengths are found. The shortest NO bond is found for O3 (1.209(4) Å), followed by the weakly coordinated O2 atom (1.228(4) Å, weak van der Waals contact to Ag+) and the longest bond is observed for O1 (1.333(3) Å, cf. dcov(N–O) = 1.34 and dcov(N=O) = 1.17 Å),24 which is strongly coordinated to the boron center (B1–O1 1.550(4) Å). This B1–O1 bond can be compared to 1.56 Å (mean value) in [(C6F5)3B-OH-B(C6F5)3] salts,12,26 or to the value of 1.565(3) Å in (C6F5)3B(OH2)-dioxane-CH2Cl2,10 1.602(6) Å in the water adduct of Ph3B, or to the B–O distance of 1.660(4) Å in the THF adduct of Ph3B.27 Even shorter B–O bond lengths are observed in oxy- and hydroxytris(pentafluorophenyl)borates, e.g. 1.460(6) in (C5Me5)2ZrOB(C6F5)328 and 1.490(10) Å in [(C5Me5)2Zr(Me)OH]+[(C6F5)3BOH].29 Very short B–O bonds are found in the borinatoborate [(C6F5)3B-O-B(C6F5)2]−,11 with 1.306(2) Å which compares well with the values found in other borinic acid derivatives: 1.348 and 1.350 Å in [(C6H5)2BOH] and [(2,4,6-(CF3)3C6H2)2BOH], respectively.30

The central boron atom is tetracoordinated with an average B–C(aryl) bond length of 1.637 Å. The coordination geometry around boron in the tetrahedral BC3O core is slightly distorted with the smallest angle of 104.6(3), and the largest 116.2(3)°. The B–O–N angle amounts to 118.0(2)°.


ORTEP drawing of the asymmetric unit of [K(Et2O)2]+[NO3·B(C6F5)3]− in the crystal. Thermal ellipsoids with 50% probability at 173 K. Selected bond lengths (Å) and angles (°): N1–O1 1.340(1), N1–O2 1.206(2), N1–O3 1.229(2), O1–B 1.557(2), O1–K 3.068(1), O3–K 2.889(1), B–C1 1.631(2), B–C7 1.640(2), B–C13 1.632(2); O2–N1–O3 125.6(1), O2–N1–O1 121.1(1), O3–N1–O1 113.3(1), N1–O1–B 117.9(9), N1–O1–K 90.51(6), B–O1–K 139.35(7), O1–B–C1 105.68(9), O1–B–C13 111.6(1), C1–B–C13 116.2(1), O1–B–C7 102.49(9), C1–B–C7 115.0(1), C13–B–C7 105.07(9); O2–N1–O1–B 4.2(2), O3–N1–O1–B 176.8(1), N1–O1–B–C1 55.2(1),N1–O1–B–C7 176.0(1),N1–O1–B–C13 72.1(1).
Fig. 4 ORTEP drawing of the asymmetric unit of [K(Et2O)2]+[NO3·B(C6F5)3]− in the crystal. Thermal ellipsoids with 50% probability at 173 K. Selected bond lengths (Å) and angles (°): N1–O1 1.340(1), N1–O2 1.206(2), N1–O3 1.229(2), O1–B 1.557(2), O1–K 3.068(1), O3–K 2.889(1), B–C1 1.631(2), B–C7 1.640(2), B–C13 1.632(2); O2–N1–O3 125.6(1), O2–N1–O1 121.1(1), O3–N1–O1 113.3(1), N1–O1–B 117.9(9), N1–O1–K 90.51(6), B–O1–K 139.35(7), O1–B–C1 105.68(9), O1–B–C13 111.6(1), C1–B–C13 116.2(1), O1–B–C7 102.49(9), C1–B–C7 115.0(1), C13–B–C7 105.07(9); O2–N1–O1–B 4.2(2), O3–N1–O1–B 176.8(1), N1–O1–B–C1 55.2(1),N1–O1–B–C7 176.0(1),N1–O1–B–C13 72.1(1).

[K(Et2O)2]+[NO3·B(C6F5)3]− crystallizes in the monoclinic space groupP21/n with four formula units per cell. The asymmetric unit consists of one [K(Et2O)2]+[NO3·B(C6F5)3]− unit with significant cation⋯anion contacts (Fig. 4). A close inspection of the anion⋯cation interactions revealed three K⋯F(aryl) interactions (d(K⋯F) <4 Å) with three different C6F5 moieties (Fig. 5). The shortest K⋯F contact is found at 2.8154(9) Å (to F10), the other two amount to 3.104(1) (F12) and 3.376(1) Å (F15). Besides these weak van der Waals interactions strong symmetric intermolecular interactions between two nitrate anions and two K+ centers are observed (d(Bi–N5′) = 2.726(2) Å) leading to the formation of a centrosymmetric {[K(Et2O)2]+[NO3·B(C6F5)3]−}2 dimer (Fig. 5). While the O2 atom of the NO3− ion is almost uncoordinated (K⋯O2 3.904(5) Å), O1 bridges between B (1.557(2) Å, see discussion of B–O bonds above) and one K+ ion (O1–K 3.068(1) Å) and O3 bridges two K+ ions (K–O3 2.752(1), K–O3′ 2.889(1) Å) forming a planar four membered K2O2 ring. Both O1 and O3 adopt trigonal pyramidal arrangement, while the NO3− ion is still trigonal planar but as shown for the silver (see above) salt slightly distorted to Cs symmetry.


K+ coordination in [K(Et2O)3]+[NO3·B(C6F5)3]−. Disorder was omitted. Selected bond lengths (Å) and angles (°): O1–K1 3.0679(9), O3–K1 2.889(1), K1–O3′ 2.752(1), K1–O4a 2.604(2), K1–O5a 2.634(6), F10–K1 2.8154(9), F12‘‘‘‘–K13.104(1); O3′–K1–O3 57.26(4), O4a–K1–O5a 152.2(1), O4a–K1–O3′ 107.38(8), O5a–K1–O3′ 90.1(1), O4a–K1–F10 81.93(9), O5a–K1–F10 93.0(1), O3′–K1–F10 151.57(3), O4a–K1–O3 97.21(7), O5a–K1–O3 110.5(1), F10–K1–O3 95.46(3), O4a–K1–O1 85.60(9), O5a–K1–O1 113.4(1), O3′–K1–O1 99.35(3), F10–K1–O1 53.75(2), O3–K1–O1 42.09(3).
Fig. 5 K+ coordination in [K(Et2O)3]+[NO3·B(C6F5)3]−. Disorder was omitted. Selected bond lengths (Å) and angles (°): O1–K1 3.0679(9), O3–K1 2.889(1), K1–O3′ 2.752(1), K1–O4a 2.604(2), K1–O5a 2.634(6), F10–K1 2.8154(9), F12‘‘‘‘–K13.104(1); O3′–K1–O3 57.26(4), O4a–K1–O5a 152.2(1), O4a–K1–O3′ 107.38(8), O5a–K1–O3′ 90.1(1), O4a–K1–F10 81.93(9), O5a–K1–F10 93.0(1), O3′–K1–F10 151.57(3), O4a–K1–O3 97.21(7), O5a–K1–O3 110.5(1), F10–K1–O3 95.46(3), O4a–K1–O1 85.60(9), O5a–K1–O1 113.4(1), O3′–K1–O1 99.35(3), F10–K1–O1 53.75(2), O3–K1–O1 42.09(3).

ORTEP drawing of [K(Et2O)2]+[NO3·B(C6F5)3]− in the crystal. Thermal ellipsoids with 50% probability at 173 K. Only one component of the asymmetric unit is shown, for the second very similar structural data are observed. Selected bond lengths (Å) and angles (°): S–O1 1.521(2), S–O2 1.532(2), S–O3 1.428(2), S–O4 1.422(2), O1–B1 1.523(4), O2–B2 1.524(4), O3–K 2.619(2); O4–S–O3 117.1(1), O4–S–O1 111.4(1), O3–S–O1 106.2(1), O4–S–O2 107.9(1), O3–S–O2 109.5(1), O1–S–O2 104.0(1), S–O1–B1 134.6(2), B2–O2–S 128.8(2), S–O3–K 151.8(1), O4–S–O1–B1 31.9(3), O3–S–O1–B1 160.4(2), O2–S–O1–B1 84.1(3), O4–S–O2–B2 142.5(2), O3–S–O2–B2 14.0(2), O1–S–O2–B2 99.1(2), O4–S–O3–K 6.4(3), O1–S–O3–K 118.7(2), O2–S–O3–K 129.6(2).
Fig. 6 ORTEP drawing of [K(Et2O)2]+[NO3·B(C6F5)3]− in the crystal. Thermal ellipsoids with 50% probability at 173 K. Only one component of the asymmetric unit is shown, for the second very similar structural data are observed. Selected bond lengths (Å) and angles (°): S–O1 1.521(2), S–O2 1.532(2), S–O3 1.428(2), S–O4 1.422(2), O1–B1 1.523(4), O2–B2 1.524(4), O3–K 2.619(2); O4–S–O3 117.1(1), O4–S–O1 111.4(1), O3–S–O1 106.2(1), O4–S–O2 107.9(1), O3–S–O2 109.5(1), O1–S–O2 104.0(1), S–O1–B1 134.6(2), B2–O2–S 128.8(2), S–O3–K 151.8(1), O4–S–O1–B1 31.9(3), O3–S–O1–B1 160.4(2), O2–S–O1–B1 84.1(3), O4–S–O2–B2 142.5(2), O3–S–O2–B2 14.0(2), O1–S–O2–B2 99.1(2), O4–S–O3–K 6.4(3), O1–S–O3–K 118.7(2), O2–S–O3–K 129.6(2).

Besides strong interaction with the oxygen atoms of the nitrate anions, two further K–O interactions with two diethyl ether molecules are found (K–O4a 2.604(2), K–O5a 2.634(6) Å). If all five K–O and the one strong K⋯F interactions are considered, the overall coordination geometry at the K+ is strongly distorted octahedral, taking also the two weaker K⋯F interactions into account a 6+2 coordination can be discussed.

The structural parameters of the [NO3·B(C6F5)3]− anion in [K(Et2O)2]+[NO3·B(C6F5)3]− are very similar to those found in the silver salt (see Fig. 3, 4 and 5).

K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2− crystallizes in the triclinic space groupP[1 with combining macron] with four formula units (two independent molecules) per cell. The structure is composed of two K(18-crown-6) cations of which only one coordinates to one O atom of the sulfate dianion (K1–O3 2.619(2) Å, cf. 2.41–3.44 Å),31 while K2 unit is connected with the [SO4·2 B(C6F5)3]2− adduct dianion via three K⋯F van der Waals interactions (K⋯F26: 2.758(2), K⋯F10 3.096(2), K⋯F9 3.486(2) Å) (Fig. 6 and 7). Moreover, K1 weakly coordinates to one Cl atom of the CH2Cl2 molecule (K1–Cl1 3.339(2) Å). Both B(C6F5)3 molecules are attached to the SO42− anion via O1 and O2 (B1–O1 1.523(4), B2–O2 1.524(4) Å), O3 interacts with K1 (see above), while O4 remains uncoordinated (K1–O4 4.388(3), K2–O4 4.079(3) Å). As expected the SO42− anion adopts a distorted tetrahedral geometry with O–S–O angles between 104–112°. Two long and two short S–O bond lengths (S–O1 1.521(2), S–O2 1.532(2), S–O3 1.428(2), S–O4 1.422(2) Å; cf. 1.467(2)–1.478(2) Å in K2SO4) are observed depending on the coordination. Adduct formation results in a S–O bond lengths increase by ca. 0.1 Å.


K+ coordination in K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2−. Hydrogen atoms omitted. C6F5 groups reduced to the part which coordinates to K2 via F atoms. Selected bond lengths (Å) and angles (°):O3–K1 2.619(2), K1–Cl1 3.339(2), K1–O4 4.388(3), K2–O4 4.079(3), F9–K2 3.486(2), F10–K2 3.096(2), F26–K2 2.758(2), F9–F10 2.033(3), F9–F26 3.162(3), F10–F26 3.301(3); Cl1–K1–O3 156.80(6), F26–K2–F10 68.38(5), F26–K2–F9 59.49(5), F9–K2–F10 46.70(5).
Fig. 7 K+ coordination in K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2−. Hydrogen atoms omitted. C6F5 groups reduced to the part which coordinates to K2 via F atoms. Selected bond lengths (Å) and angles (°):O3–K1 2.619(2), K1–Cl1 3.339(2), K1–O4 4.388(3), K2–O4 4.079(3), F9–K2 3.486(2), F10–K2 3.096(2), F26–K2 2.758(2), F9–F10 2.033(3), F9–F26 3.162(3), F10–F26 3.301(3); Cl1–K1–O3 156.80(6), F26–K2–F10 68.38(5), F26–K2–F9 59.49(5), F9–K2–F10 46.70(5).

Conclusions

We have demonstrated the reactivity of the borane B(C6F5)3 with simple anions such NO3− and SO42− leading to the formation of adduct anions of the type [SO4·2 B(C6F5)3]2− and [NO3·B(C6F5)3]−. The reactions of AgNO3 and KNO3 with 1 equiv. of B(C6F5)3 in diethyl ether at room temperature yielded new salts [K(Et2O)2]+[NO3·B(C6F5)3]− and [Ag(Et2O)3]+[NO3·B(C6F5)3]−, while for the sulfate only the potassium salt was obtained from the reaction of K2SO4 with 2 equiv. B(C6F5)3 in the presence of crown ether (18-crown-6). This adduct anion salt formation can be regarded as Lewis acid/Lewis base reaction with a calculated charge transfer of ca. 0.4 e. The sterically crowded [NO3·B(C6F5)3]− and [SO4·2 B(C6F5)3]2− anions prevent the addition of further B(C6F5)3 groups. However, both anions cannot be considered weakly coordinating anions as they still can interact with their uncoordinated O atoms with smaller Lewis acids such as K+ or Ag+. Future work will focus on the utilization of a smaller Lewis acid in order to obtain the triple adduct anion [NO3·3LA]− and [SO4·4 LA]2− (LA = Lewis acid). Application of salts containing such adduct anion might arise from the fact that these anions are soluble in many organic solvents.

Experimental

General information

All manipulations were carried out under oxygen- and moisture-free conditions under Argon using standard Schlenk or drybox techniques.

Dichloromethane was purified according to a literature procedure,32 dried over CaH2 and freshly distilled prior to use. Diethylether was dried over Na/benzophenone and freshly distilled prior to use. n-Hexane was dried over Na/benzophenone/tetraglyme and freshly distilled prior to use. Tetrahydrofuran (THF) was dried over Na/benzophenone/tetraglyme and freshly distilled prior to use. B(C6F5)3 was prepared by a modified literature procedure originally developed by Massey et al.33AgNO3 (99%, VEB FeinchemieSebnitz), KNO3 (99%, Roth), KOH (>84%, Merck), K2SO4 (>99%, Merck), were dried in high vacuo for 3 h prior to use.

NMR

19F{1H}, 11B{1H}, 1H and 13C{1H} NMR spectra were recorded on Bruker spectrometers AVANCE 250, 300, or 500. The 1H and 13C NMR chemical shifts were referenced to the solvent signals (13C δCDCl3 = 77.0 ppm; 1H δCDCl3 = 7.25 ppm; 13C δCD2Cl2 = 54.0 ppm; 1H δCD2Cl2 = 5.31 ppm). The 19F and 11B chemical shifts are referred (δ = 0) to CFCl3 and B(OH)3 respectively.

IR

Nicolet 6700 FT-IR with Smart Endurance ATR device or Nicolet 380 FT-IR with Smart Orbit ATR module.

Raman

Bruker VERTEX 70 FT-IR with RAM II FT-Raman module, equipped with a Nd:YAG laser (1064 nm).

MS

Finnigan MAT 95-XP from Thermo Electron.

CHN analyses

Analysator Flash EA 1112 from Thermo Quest.

DSC

Thermoanalytical measurements were performed with a DSC 823e from Mettler-Toledo instrument. Two point calibrations with In (mp 156.6 °C) and Zn (mp 419.6 °C) were carried out. About 2–6 mg of the samples were weighed and contained in sealed aluminium crucibles. They were studied with a heating rate of 5 °C min−1; throughout this process the furnace was flushed with dry nitrogen. For the evaluation of the output the Stare software was employed.

X-Ray structure determination

X-Ray quality crystals of different samples were selected in Fomblin 1800 oil (Alfa Aesar) at ambient temperatures. All measurements were carried out at 173(2) K. The data were collected on a Bruker-Nonius Apex X8 CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods (SHELXS-97)34 and refined by full-matrix least squares procedures (SHELXL-97).35 Semiemperical absorption corrections were applied. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were included in the refinements at calculated positions using riding models.

Syntheses

Synthesis of [Ag(Et2O)3]+[NO3·B(C6F5)3]− (1). To a solution of B(C6F5)3 (2.04 g, 4 mmol, 4 equiv.) in 40 ml diethyl ether is added AgNO3 (170 mg, 1 mmol, 1 equiv.) at room temperature. After stirring for 30 min a clear colorless solution was observed. Removal of the solvent led to a white solid, which was washed with 60 mL of n-hexane (removal of the B(C6F5)3 excess). After discontinuation of the precipitate the excess n-hexane is removed with a syringe and discarded. The process is repeated one more time. Recrystallization from diethyl ether yields pure colorless crystals. Yield: 440 mg (65%). DSC: 76.3 °C (dec.).C18AgBF15NO3·Et2O (756.0): calcd C 34.95, H 1.33, N 1.85; found C 34.90, H 1.66, N 1.84. 11B-NMR (CD2Cl2, 96 MHz, 25 °C): δ = 0.23 (s). 13C NMR (CD2Cl2, 75 MHz, 25 °C): δ = 148.3 (d, o-C, 2C, 1JCF = 241.8 Hz); 139.8 (d, p-C, 1C, 1JCF = 249.4 Hz); 137.1 (d, m-C, 2C, 1JCF = 247.2 Hz); 118.4 (br, C-B, 1C). 19F NMR (CD2Cl2, 282 MHz, 25 °C): δ = –134.7 (d, o-F, 2F, 3JFF = 19.9 Hz); –160.6 (m, p-F, 1F); –166.4 (“t”, m-F, 2F, 3JFF = 19.9 Hz). IR (ATR, cm−1): ν = 2965 (w), 1645 (w), 1516 (s), 1465 (vs), 1381 (m), 1282 (s), 1261 (m), 1180 (w), 1088 (s), 1069 (s), 977 (vs), 854 (s), 871 (w), 782 (m), 748 (w), 717 (w), 984 (m), 662 (w), 607 (w), 575 (w). Raman (100 mW, 475 Scans, 25 °C, cm−1): ν = 2960 (7), 2912 (10), 2876 (7), 1648 (4), 1453 (3), 1290 (2), 1045 (4), 580 (6), 491 (4), 447 (4), 415 (4).
Synthesis of [K(Et2O)2]+[NO3·B(C6F5)3]− (2). To a solution of B(C6F5)3 (2.04 g, 4 mmol, 4 equiv.) in 40 ml diethyl ether is added KNO3 (101 mg, 1 mmol, 1 equiv.) at room temperature. After stirring for 30 min a clear colorless solution was observed. Removal of the solvent led to a white solid, which was washed with 60 mL of n-hexane (removal of the B(C6F5)3 excess). After discontinuation of the precipitate the excess n-hexane is removed with a syringe and discarded. The process is repeated one more time. Recrystallization from diethyl ether yields pure colorless crystals. Yield: 331 mg (54%). DSC: 134.8 °C (dec). C18BF15KNO3·2Et2O (761.33): calc C 41.02, H 2.65, N 1.84; found C 40.93, H 1.74, N 1.88. 11B-NMR (CDCl3, 96 MHz, 25 °C): δ = −1.28 (s). 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 147.8 (d, o-C, 2C, 1JCF = 243.5 Hz); 140.3 (d, p-C, 1C, 1JCF = 240.3 Hz); 137.0 (d, m-C, 2C, 1JCF = 246.7 Hz); 116.5 (br, C-B, 1C). 19F NMR (CDCl3, 282 MHz, 25 °C): δ = −135.0 (“d”, o-F, 2F, 3JFF = 24.2 Hz); −156.2 (t, p-F, 1F, 3JFF = 19.7 Hz); −163.2 (“t”, m-F, 2F, 3JFF = 22.4 Hz). IR (ATR, cm−1): ν = 3578 (w), 2986 (w), 1645 (m), 1517 (s), 1463 (vs), 1371 (s), 1283 (m), 1096 (s), 973 (s), 873 (w), 833 (w), 786 (m), 774 (m), 766 (m), 746 (w), 680 (m), 673 (m), 607 (w), 575 (w). Raman (100 mW, 1000 Scans, 25 °C, cm−1): ν = 2938 (1), 2880 (1), 2757 (1), 2487 (10), 2477 (10), 1647 (1), 1458 (1), 1308 (1), 582 (2), 492 (2), 414 (2), 392 (1).
Synthesis of K(18-crown-6)(CH2Cl2)]2+[SO4·2 B(C6F5)3]2− (3). To a solution of B(C6F5)3 (2.56 g, 5 mmol, 5 equiv.) in 60 ml diethyl ether is added K2SO4 (174 mg, 1 mmol, 1 equiv.) and18-Crown-6 (528 mg, 2 mmol, 2 equiv.) at room temperature. After stirring for 30 min a clear colorless solution was observed. Removal of the solvent led to a white solid, which was washed with 60 mL of n-hexane (removal of the B(C6F5)3 excess). After discontinuation of the precipitate the excess n-hexane is removed with a syringe and discarded. The process is repeated one more time. Recrystallization from dichloromethane yields pure colorless crystals. Yield: 950 mg (55%).DSC: 223.5 °C (dec).C60H48B2F30K2O16S (1726.8): calc C 41.73, H 2.80; found C 41.64, H 2.26. 1H NMR (CD2Cl2, 300 MHz, 25 °C): δ = 3.55 (s, CH2). 11B NMR (CD2Cl2, 96 MHz, 25 °C): δ = −3.33 (s). 13C NMR (CD2Cl2, 75 MHz, 25 °C): δ = 148.7 (d, o-C, 2C, 1JCF = 240.0 Hz); 139.3 (d, p-C, 1C, 1JCF = 242.9 Hz); 136.9 (d, m-C, 2C, 1JCF = 248.8 Hz); 122.2 (br, C-B, 1C); 70.5 (s, CH2, 12C). 19F NMR (CD2Cl2, 282 MHz, 25 °C): δ = −132.7 (d, o-F, 2F, 3JFF = 21.4 Hz); −163.4 (t, p-F, 1F, 3JFF = 21.2 Hz); −167.7 (“t”, m-F, 2F, 3JFF = 21.3 Hz). IR (ATR, cm−1): ν = 2891 (w), 1644 (w), 1514 (m), 1463 (s), 1353 (w), 1279 (m), 1175 (w), 1104 (s), 1085 (s), 839 (w), 805 (w), 763 (m), 677 (m), 621 (w), 576 (w), 550 (m). Raman: decomposition.

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Footnote

† Electronic supplementary information (ESI) available. CCDC reference numbers 817745–817751. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ra00186h
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