Synthesis and Characterization of Silylated Phosphonium [P(OSiMe3

Jul 9, 2018 - ... halogen, pseudohalogen, or even arenes)(7) to give cations (Scheme 1), which led to a long quest for salts bearing the solvent-free ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis and Characterization of Silylated Phosphonium [P(OSiMe3)4]+ and Phosphate [O2P(OSiMe3)2]− Salts Paul Felgenhauer,† Rene ́ Labbow,‡ Axel Schulz,*,‡,∥ and Alexander Villinger‡ †

Institut für Chemie, Carl von Ossietzky Universität Oldenburg, Carl-von-Ossietzky-Straße 9-11, 26129 Oldenburg, Germany Institut für Chemie, Universität Rostock, Albert-Einstein-Straße 3a, 18059 Rostock, Germany ∥ Leibniz-Institut für Katalyse an der Universität Rostock, Albert-Einstein-Straße 29a, 18059 Rostock, Germany ‡

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S Supporting Information *

ABSTRACT: Starting from an optimized synthesis of silylated phosphoric acid, OP(OSiMe3)3, a borate salt bearing the [P(OSiMe3)4]+ cation was generated in the reaction of OP(OSiMe3)3 with [Me3Si−H−SiMe3][B(C6F5)4], isolated, and fully characterized. Analogously to the protonated species, phosphoric acid (H3PO4) reaction of OP(OSiMe3)3 with a base led to the formation of the unknown [O2P(OSiMe3)2]− anion, which could be crystallized as potassium salt and structurally characterized, too. Both [P(OSiMe3)4]+ and [O2P(OSiMe3)2]− can be regarded as the formal autoprotolysis products of OP(OSiMe3)3.

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INTRODUCTION

and Lehmann et al. isolated [Me3Si][CHB11F11] featuring short cation−anion contacts.12 To get access to a truly tricoordinate free silylium ion without stabilization, bulkier substituents around the silicon center were needed as it is the case in [Mes3Si][CHB11Me5Br6]13 and [Pemp3Si]2[B12Cl12]214 (Mes = 2,4,6-trimethylphenyl, Pemp = pentamethylphenyl). As for the proton, also for [Me3Si]+ homoleptic (pseudo)halogen, (pseudo)chalcogen or pnictogen species are known as well as their silylated cations (Scheme 1), which feature small trimethylsilyl affinities (TMSA)2,3,15−17 similar to the small proton affinities (PA)18 known for the analogous protonated species. Hence, all these silylated cations represent highly labile, strong [Me3Si]+ transfer reagents, which are only stable in the presence of chemically robust and inert wca’s. Following our interest in [Me3Si]+ chemistry, we studied the similarities between ortho-phosphoric acid, OP(OH)3, and its silylated congener OP(OSiMe3)3 (1). Especially, we were intrigued by the idea to synthesize the formal autoprotolysis products of 2OP(OSiMe3)3 → [P(OSiMe3)4]+ + [O2P(OSiMe3)2]−. While the protonated species, [P(OH)4]+, has been known since 1999, when Minkwitz et al. used super acidic systems such as HF/MF5 (M = As, Sb) to protonate OP(OH)3, the existence of a tetrakis(trimethylsilyl)phosphonium ion, 19 [P(OSiMe3)4]+, was discussed in 1974 in the equilibrium of the reaction OP(OSiMe3)3 with Me3Si-X (X = halogen, NO3, ClO4, and SCN)20 but a full characterization (1H, 31P shifts,

At the first glance, the chemistry of the trimethylsilylium ion [Me3Si]+ seems to be quite different to that of a proton, although [Me3Si]+ is often referred to as a big proton.1−6 For example, the physical properties (e.g., melting (mp) or boiling point (bp)) of the halides or pseudohalides are quite different (Table S6). This term is more related to the fact that both cations are very strong electrophiles, reacting with almost any donor molecule (e.g., HX, X = halogen, pseudohalogen, or even arenes)7 to give cations (Scheme 1), which led to a long quest for salts bearing the solvent-free cations.8−10 By using weakly coordinating anions (wca),7 Reed et al. succeeded in preparing a solvent-free H[CHB11Cl11] salt,11 while Willner Scheme 1. Known Homoleptic Multiply Substituted Trimethylsilyl Substituted Cations2−5,12,15,21,22a

a

[(Me3Si)3O]+ and [(Me3Si)3S]+ were only observed by NMR spectroscopic experiments.23,24 © XXXX American Chemical Society

Received: May 15, 2018

A

DOI: 10.1021/acs.inorgchem.8b01323 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 2. Synthesis of Silylated Phosphorous Species 1, 2+, and 3−

−167.65 (m, m-CF, 1J(19F−13C) = 246 Hz), −163.84 (t, p-CF, J(19F−13C) = 245 Hz), −133.08 (s, o-CF, 1J(19F−13C) = 241 Hz). 29 Si INEPT NMR (25 °C, CD2Cl2, 59.63 MHz): δ = 35.61 (ddec, POSiCH3, 2J(29Si−1H) = 6.9 Hz, 2J(29Si−31P) = 1.6 Hz). 31P{1H} NMR (25 °C, CD2Cl2, 121.49 MHz): δ = −35.92 (s). IR (ATR, 8 scans, 25 °C, cm−1)*: 3668 (νH2O), 2969 (w), 2910 (w), 1643 (m, νC−Ctoluene), 1598 (w), 1556 (w), 1511 (m), 1459 (s), 1415 (w), 1382 (w), 1375 (w), 1367 (w), 1261 (m), 1116 (s), 1081 (s), 1029 (w), 975 (s), 908 (w), 852 (s), 827 (s), 765 (s), 756 (s), 725 (m), 707 (m), 682 (m), 659 (m), 609 (m), 603 (m), 572 (m). Raman (743 nm, 43 mW, 60 s, 10 acc., 25 °C, cm−1): 2973 (1), 2907 (1), 1643 (1, νC−Ctoluene), 1418 (1), 1377 (1), 1263 (1), 1100 (1), 859 (1), 820 (2), 767 (1), 700 (1), 642 (1), 583 (10), 575 (2), 492 (6), 475 (6), 448 (7), 423 (6), 390 (4), 357 (2), 345 (1), 277 (1), 243 (3), 159 (5). ESI + (M calc, (M found )): 387.14283 (387.144) [(Me3SiO4)4P]+. ESI− (Mcalc, (Mfound)): 678.97737 (678.9831) [B(C6F5)4]−. *Rapid hydrolysis during sample preparation. Prolonged exposition to ambient temperatures and moisture lead to decomposition of 2[B(C6F5)4]. Synthesis of [K-(18-crown-6)]3. To a clear, yellowish stirred solution of potassium tert-butoxide K[OC(CH3)3] (0.52 g, 4.63 mmol) and 1,4,7,10,13,16-hexaoxa-cyclo-octadecane (18-crown-6, 1.2 g, 4.63 mmol) in 20 mL DME, tris(trimethylsilyl) phosphate OP(OSiMe3)3 (1.46 g, 4.63 mmol) was added via syringe. This immediately led to a color change of the reaction solution to colorless. The reaction mixture was stirred for another 10 min and was afterward filtered through a G4 frit. Single crystals suitable for X-ray structure elucidation were grown overnight in the refrigerator (−20 °C). The crystals were washed with 2 × 5 mL of n-pentane. The remaining colorless crystals were dried in vacuo at 60 °C for 15 min, yielding 2.1 g (3.9 mmol, 85%) of [K-18crown-6][O2P(OSiMe3)2]. C18H42KO10PSi2 (544.76 g/mol). mp. 121 °C (dec). EA calc. (found), %: C, 39.69 (39.24); H, 7.77 (6.87). 1H NMR (25 °C, CD2Cl2, 300.13 MHz): δ = 0.16 (s, 18H, SiCH3, 1J(1H−13C) = 116.2 Hz, 2J(1H−29Si) = 6.8 Hz), 3.62 (s, 24H, OCH2, 1J(1H−13C) = 141.4 Hz). 13C{1H} NMR (25 °C, CD2Cl2, 75.47 MHz): δ = 1.40 (s, SiCH3, 1J(13C−29Si) = 60 Hz), 70.6 (s, OCH2). 17O NMR (25 °C, CD2Cl2, 67.83 MHz): δ = (no signal could be observed). 29Si INEPT NMR (25 °C, CD2Cl2, 59.62 MHz): δ = 10.28 (unresolved signal). 31 1 P{ H} NMR (25 °C, CD2Cl2, 121.51 MHz): δ = −13.01 (s, PO(Si)). IR (ATR, 16 scans, 25 °C, cm−1): 2954 (w), 2885 (m), 2829 (w), 1471 (w), 1454 (w), 1417 (w), 1349 (m), 1284 (w), 1243 (m), 1218 (m), 1103 (s), 1089 (s), 1010 (m), 962 (s), 939 (s), 865 (m), 835 (s), 756 (m), 686 (m), 599 (m), 532 (m). Raman (632 nm, 10 mW, 10 s, 20 acc., 25 °C, cm−1): 2961 (5), 2899 (10), 2846 (4), 2809 (2), 2732 (1), 2702 (1), 1477 (2), 1457 (1), 1412 (1), 1365 (1), 1289 (1), 1273 (2), 1246 (1), 1149 (2), 1141 (2), 1112 (1), 1094 (4), 1072 (1), 952 (1), 873 (5), 832 (2), 812 (1), 758 (1), 692 (1), 669 (1), 616 (9), 598 (3), 548 (1), 380 (1), 364 (1), 326 (1), 281 (3), 253 (1). 202 (1). MS (ESI−, m/z calc. (found)): 241.04867 (241.04905). (ESI+, m/z calc. (found)): 303.12045 (303.1197).

selected IR bands, conductivity in CH2Cl2, EA for P and I are known) including structural data remained to be carried out.

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1

EXPERIMENTAL SECTION

Synthesis of (Me3SiO)3PO 1. In a 250 mL three-neck flask with dropping funnel and pressure equalized reflux condenser, 9 g (66.1 mmol) of potassium dihydrogen phosphate KH2PO4 was suspended in 100 mL n-hexane. Trimethylsilyl chloride Me3SiCl (43 g, 396 mmol) was added within 30 min. The reaction mixture was refluxed for 9 h. After checking the reaction mixture by 31P NMR spectroscopy, the reaction mixture was filtered through a G4 frit, and the solvent was thermally distilled off. The residue was condensed into another flask in vacuo at 70 °C. This procedure yields in 19.9 g (315 mmol, 96%) of a colorless liquid (Me3SiO)3PO. C9H27O4PSi3 (314.54 g/mol). bp. 63 °C (0.9 mbar). 1H NMR (25 °C, CD2Cl2, 300.13 MHz): δ = 0.20 (s, 1J(1H−13C) = 119 Hz, 2 1 J( H−29Si) = 7.0 Hz). 1H NMR (25 °C, DMSO-[d6], 300.13 MHz): δ = 0.22 (s, 1J(1H−13C) = 119 Hz, 2J(1H−29Si) = 7.3 Hz). 13C{1H} (25 °C, CD2Cl2, 75.47 MHz): δ = 0.93 (d, 1J(13C−29Si) = 60.4 Hz, 2 13 J( C−31P) = 1.5 Hz). 13C{1H} (25 °C, DMSO-[d6], 75.47 MHz): δ = 0.24 (d, 1J(13C−29Si) = 60.2 Hz). 17O (25 °C, CD2Cl2, 67.80 MHz): δ = 83.1 (POSi, Δν1/2 = 315 Hz), 105.32 (d, PO, 1J(17O−31P) = 150 Hz, Δν1/2 = 100 Hz). 29Si INEPT (25 °C, CD2Cl2, 59.63 MHz): δ = 20.39 (m). 29Si INEPT (25 °C, DMSO-[d6], 59.63 MHz): δ = 20.40 (m). 31P{1H} NMR (25 °C, CD2Cl2, 121.51 MHz): δ = −25.81 (s). 31P{1H} NMR (25 °C, DMSO-[d6], 121.51 MHz): δ = −25.78 (s). IR (ATR, 8 scans, 25 °C, cm−1): 2962 (w), 2902 (w), 1457 (w), 1419 (w), 1276 (m), 1249 (s), 1004 (s), 835 (s), 757 (s), 696 (m), 607 (m). Raman (633 nm, 5 mW, 10 s, 20 acc., 25 °C, cm−1): 3115 (1), 2967 (3), 2904 (10), 2493 (1), 1416 (1), 1279 (1), 1255 (1), 1075 (1), 850 (1), 762 (1), 697 (1), 652 (2), 615 (4), 592 (2), 452 (1), 349 (1), 259 (1), 245 (1), 216 (1), 186 (2), 170 (2). Synthesis of 2[B(C 6F5)4]. The trimethylsilane adduct of trimethylsilylium tetrakis(pentafluorophenyl)borate [(Me3Si)2H][B(C6F5)4] (180 mg, 0.22 mmol) was suspended in 4 mL of toluene. The suspension was degasified three times by a freeze−pump−thaw procedure. Tristrimethylsilyl phosphate OP(OSiMe3)3 (69 mg, 0.22 mmol) was added to this suspension via microliter syringe. The suspension was set in an ultrasonic bath for 1 h. Single crystals suitable for X-ray structure elucidation were grown from this solution overnight at −20 °C. The supernatant was removed by syringe, and the colorless crystals were washed with a small amount of cold npentane (at −40 °C) and dried in vacuo at −20 °C, yielding in 185 mg (0.17 mmol, 83%) of tetrakis(trimethylsiloxy)phosphonium tetrakis(pentafluorophenyl)borate [(Me3SiO)4P][B(C6F5)4]. C36H36BF20O4PSi (1066.76 g/mol). mp. < 76 °C (dec.). EA calc. (found), %: C, 40.53 (40.47); H, 3.40 (3.58). 1H NMR (25 °C, CD2Cl2, 300.13 MHz): δ = 0.41 (s, SiCH3, 1J(1H−13C) = 120.5 Hz, 2 1 J( H−29Si) = 6.9 Hz). 11B NMR (25 °C, CD2Cl2, 96.29 MHz): δ = −16.56 (s). 13C{1H} NMR (25 °C, CD2Cl2, 75.47 MHz): δ = 0.77 (d, SiCH3, 1J(13C−29Si) = 60.8 Hz, 3J(13C−31P) = 1.7 Hz), (ipso-CF not observed), 136.81 (dm, m-CF, 1J(13C−19F) = 246 Hz), 138.59 (dm, p-CF, 1J(13C−19F) = 243 Hz), 148.74 (dm, o-CF, 1J(13C−19F) = 238 Hz). 17O NMR (25 °C, CD2Cl2, 67.80 MHz): δ = 78.4 (b, Δν1/2 = 160 Hz). 19F{1H} NMR (25 °C, CD2Cl2, 282.40 MHz): δ = B

DOI: 10.1021/acs.inorgchem.8b01323 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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RESULTS AND DISCUSSION Synthesis. OP(OSiMe3)3, generated from hexamethyldisiloxane and phosphoric anhydride, was first mentioned in 1944.25 Ever since different synthetic approaches have been published.26−30 An easy high-yielding synthetic process was published in 2014 by Wessjohann et al., who treated KH2[PO4] with an excess of Me3SiCl in formamide affording pure 1 in high yields.31 We have applied a slightly modified process by using n-hexane as solvent (yield 96%) as illustrated in Scheme 2 (eq 1). A similar H+/[Me3Si]+ exchange procedure was used to generate silylated pyrophosphoric acid (see Supporting Information) starting from Na4[P2O7].31 For the synthesis of a [P(OSiMe3)4]+ salt, we utilized a super Lewis acidic medium (OP(OSiMe3)3/[Me3Si][wca])2 to further silylate OP(OSiMe3)3 in analogy to the super acid medium used by Minkwitz.19 Hence, with freshly prepared OP(OSiMe3)3 in hand, we treated 1 with the [Me3Si]+ transfer reagent [Me3Si−H−SiMe3][(B(C6F5)4] affording the desired silylated phosphonium salt [P(OSiMe3)4][(B(C6F5)4] (2[B(C6F5)4], Figure 1, Scheme 2 eq 2, yield 83%). Highly

moisture and oxygen sensitive 2[B(C6F5)4] had to be stored at −40 °C because it decomposed slowly at ambient temperature even under an argon atmosphere but very fast at temperatures above 76 °C. Synthesis of salts containing bissilylated phosphate [O2P(OSiMe3)2]− (3−) was carried out again in analogy to the protonated species (Scheme 2, eq 3), by adding one equivalent of a base (KOCMe3) to “neutralize” exactly one [Me3Si]+ ion by ether formation yielding colorless crystals of [K-(18-crown6)][O2P(OSiMe3)2] ([K-(18-crown-6)]3) suitable for structure elucidation after adding a crown ether (18-crown-6) at −20 °C (yield 85%). Complexes, containing the anion [O2P(OSiMe3)2]−, have been known in the literature since the late 1990s.32−36 However, all these structures have in common that the anionic phosphate is coordinated by two metal centers (Al, Ti, Zn, and Ga). Herein, we report on a metal complex containing an almost naked bissilylated anion [O2P(OSiMe3)2]− weakly coordinated to only one potassium ion (see below). Crystals of [K-(18-crown-6)]3 were also moisture and oxygen sensitive but thermally considerably more stable than salts of 2+ (121 °C vs. 75 °C). Furthermore, the autoprotolysis equilibrium known for phosphoric acid (vide supra) could also be verified by NMR experiments: The reaction of exactly one equivalent 2+ with one equivalent of 3− led to the formation of the formal [Me3Si]+ exchange product 1 ([P(OSiMe3)4]+ + [O2P(OSiMe3)2]− → 2 OP(OSiMe3)3). Both isolated species 2+ and 3− were fully characterized by 1H, 13 C, 17O, 29Si, and 31P NMR techniques in CD2Cl2 and DMSO as well as IR/Raman spectroscopy (Table 1). We do want to stress that the back reaction represents only a formal autoprotolysis back reaction since by no means were the autoprotolysis products ([P(OSiMe3)4]+ + [O2P(OSiMe3)2]−) Table 1. Selected Experimental NMR (δ [ppm], Solvent = CD2Cl2), Computed TMSA/PA Values (kcal·mol−1), and NPA (Natural Population Analysis) Charges (e) 1

H C 29 b Si 31 P 17 O POSi PO q(P)d q(Si)d q(OSi)d q(Ofree)d q(Me3Si)d q(PO4) TMSAe PAf 13

Figure 1. Ball-and-stick representation of the molecular structure of 2+ (top) and 3− (bottom) in the crystal. Disorder not shown. Selected structural data (bond length [Å], angles [deg]): 2[B(C6F5)4] P1−O1 1.519(2), P1−O2 1.534(2), P1−O3 1.509(2), P1−O4 1.521(2), O1−Si1 1.713(2), O2−Si2 1.719(2), O3−Si3 1.707(2), O4−Si4 1.721(1); O3−P1−O1 110.36(8), O3−P1−O2 109.18(7), O3−P1− O4 111.28(8), O4−P1−O2 107.94(7), P1−O1−Si1 139.89(9), P1− O2−Si2 138.45(9), P1−O3−Si3 150.7(2), P1−O4−Si4 138.22(8); [K-(18-crown-6)]3 K1−O7 3.016(2), K1−O8 2.669(2), P1−O7 1.483(2), P1−O8 1.481(2), P1−O9 1.588(2), P1−O10 1.6(2), Si1− O9 1.64(2), Si2−O10 1.648(2); O7−P1−O9 111.30(8), O7−P1− O10 110.56(8), O9−P1−O10 99.61(8), O8−P1−O7 118.17(8), O8−P1−O9 107.85(9), O8−P1−O10 107.69(9), P1−O9−Si1 135.79(9), P1−O10−Si2 131.60(8).

3−

1

Ppaa

2+

0.16 1.4 10.3 −13.0 n.s.c n.s.c 2.580 1.975 −1.205 −1.177 0.592 −2.185 276.9 457.5

0.20 0.9 20.4 −25.8 83.1(315) 105.3(150) 2.649 1.960 −1.171 −1.133 0.665 −1.996 171.7 328.6

0.26 0.9 24.2 −31.5 82.4(325) 105.1 2.647 1.958 −1.165 −1.117 0.677 −1.913 163.5 308.0

0.41 0.8 35.6 −35.9 78.4(160) 2.755 1.945 −1.157 +0.718 −1.872 76.5 198.7

a

Ppa = pyrophosphoric acid = (Me3SiO)4P2O7. b29Si INEPT. cNo signal because 17O is a quadrupolar nucleus with I = 5/2 affording broad signals, often not to be observed. dFor more than one atom/ group the average value is given. eTrimethylsilyl affinity (TMSA) of A(g) is defined as the negative of the reaction enthalpy ΔH(g)°(298) in kcal/mol at 298.15 K for the reaction A(g) + [Me3Si]+(g) → [A(Me3Si)]+(g),2 [A(Me3Si)](g) = 3−, 1, Ppa, 2+; hence, the TMSA value given is for the conjugated acid−base pair A(g)/[A(Me3Si)]+(g). f Proton affinity (PA) is defined analogously like the TMSA; the corresponding [AH](g) species are [O2P(OH)2]−, OP(OH)3, H2[P2O7], and [P(OH)4]+. C

DOI: 10.1021/acs.inorgchem.8b01323 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

1.6(2) Å). This local symmetry decrease is also reflected by two different O−P−O angles (∠(O9−P1−O10) = 99.61(8), ∠(O8−P1−O7) = 118.17(8)°). Charge and Thermodynamic Consideration. To get some insight into the charge transfer upon silylation and desilylation, respectively, we computed the partial net charges of the elements and the [Me3Si] and [PO4] moieties within species 1, 2+, and 3− (Table 1) at the pbe0/aug-cc-pwCVDZ level of theory. Two interesting features can be derived from these data: (i) the atomic charges within this series do not change much upon increasing silylation degree. (ii) Hence, only a moderate charge change is found for the [Me3Si] and [PO4] moieties indicating that the charge difference of two between 2+ and 3− is compensated by delocalization over the entire species. However, in accord with our NMR data a small increase of the charge at the [PO4] and a small decrease at the [Me3Si] unit is found along the series 3− to 2+. Finally, the trimethysilyl affinities (TMSA, Table 1) of 1, 2+, and 3− were computed increasing along 76.5 (2+) < 171.7 (1) < 276.9 (3−) < 584.9 kcal/mol ([O3P(OSiMe3]2−) (cf. 32.8 [Me3Si−H− SiMe3]+, 94.7 [Me3Si][CHB11F11], and 109.8 kcal/mol [Me3Si][CHB11H11]).42 Hence, 2+ represents the best silylation species and most “naked” [Me3Si]+ species among the considered species here. Nevertheless, within [Me3Si−H− SiMe3]+, “[Me3Si]+” is less strongly bound and can be used to generate 2+ salts as experimentally demonstrated here and in accord with the computed TMSA value.

detected when OP(OSiMe3)3 was dissolved, e.g., in CH2Cl2; however, reaction 2+ with one equivalent of 3− affords the formation of 1 at once. Spectroscopic and Structural Details. The high-field 31P nuclear magnetic resonance shifts between −13 and −36 ppm observed for species 1, 2+, and 3− in CD2Cl2 at ambient temperatures are characteristic for phosphates (cf. δ[31P] = −31 (Me 3 SiO) 2 P(O)O−P(O)(OSiMe 3 ) 2 ; 31 34.4 [P(OSiMe3)4]I).20 Notably, the resonance of cation 2+ (δ[31P] = −35.9) was shifted to higher field by 10 ppm relative to compound 1 (−25.8), while the anion 3− was observed at considerably lower field (δ[31P] = −13.0). As expected, the 29 Si resonance of 2+ (δ[29Si] = 35.6) was shifted by 15 ppm to lower field compared to 1 (δ[29Si] = 20.4), while a high-field shift by 10 ppm was observed for 3− (δ[29Si] = 10.3, cf. 32 [Me-CN-SiMe3]+,3 computed 385.1 naked [Me3Si]+(g), see Supporting Information, Table S7) indicating strong interactions between the oxygen atom and the [Me3Si]+ moiety for all species. Interestingly, 17O NMR shifts were less sensitive with respect to the degree of silylation and charge. Only a small low-field shift of 5 ppm was detected for 2+ (78 vs. 83 for 1; cf. 113 [PO4]3− and 77 ppm of OP(OMe)3).37−39 Due to the large quadrupole moment (I = 5/2) of 17O, usually these signals were rather broad (Δν1/2 between 325−100 Hz), which was also the reason why a resonance was neither detected for 3− nor the 18-crown-6 compound. Despite these broad resonances, a rarely detected 1J(17O−31P) coupling of 150 Hz was found in the 17O NMR spectrum of 1 in CD2Cl2 solution (Figures S20/21, cf. 160 Hz in OP(OMe)3). The molecular structure of 2[B(C6F5)4 and [K-(18-crown6)]3 was unequivocally proven by single crystal X-ray diffraction (Figure 1). Both compounds crystallized in the triclinic space group P1̅. A toluene solvate 2[B(C6F5)4·2.5 toluene) was also crystallized (see Supporting Information). Besides, a rather poor data set for crystals of silylated pyrophosphoric acid (Ppa), establishing the connectivity, was also obtained (Figure S1). As depicted in Figure 1 (top), there are only three short contacts between 2+ and two fluorine atoms of one C6F5 ring of the [B(C6F5)4]− anion (d(H···F) 2.6−2.8 Å, cf. ∑rvdW(H···F) = 2.67 Å).40 Therefore, the molecular structure can be described as two weakly interacting almost sphere-shaped ions with the positive charge mainly localized at the phosphorus atom (q(P) = 2.75 e, Table 1) in 2+ and the negative charge delocalized over the 20 fluorine atoms of the borate anion. In 2+, which is isoelectronic to (Me3SiO)4Si and [(Me3SiO)4Al]−, the central phosphorus atom is almost tetrahedrally surrounded by four [OSiMe3] moieties with P(V)−O distances ranging between 1.509−1.534 Å in accord with structural data of H3PO4 (1.55 Å)41 and [P(OH)4]+ (1.53 Å).19 A slightly different situation is found in crystals of [K-(18-crown-6)]3, for which rather strong cation− anion interactions can be assumed (Figure 1 bottom). Since two oxygen atoms of the PO4 core in 3− remain unsilylated, they are utilized for coordinating to the K+ center, which is embedded in the 18-crown-6 ring system (d(K−O18‑crown‑6) = 2.82−2.97 Å). However, both K−OPO4 distances were found to be significantly different (d(K1−O7) = 3.016(2), d(K1− O8) = 2.669(2) Å). In contrast to 2+, 3− displays a strongly distorted tetrahedral central P atom with approximately local C2v point group symmetry of the PO4 core. Hence, two shorter P−O distances, displaying partial double bond character (d(P1−O7) = 1.483(2), d(P1−O8) = 1.481(2) Å), and two longer are found (d(P1−O9) = 1.588(2), d(P1−O10) =

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CONCLUSION In summary, highly labile salts bearing [P(OSiMe3)4]+ and [O2P(OSiMe3)2]− ions were generated utilizing super Lewis acidic media and bulky, chemical robust counterions similar to the chemistry known for analogous protonated species. Formally, [P(OSiMe3)4]+ and [O2P(OSiMe3)2]− ions might be regarded as the formal product of the autoprotolysis of OP(OSiMe3)3 and the big proton analogs of [P(OH)4]+ (protonated phosphoric acid) and [O2P(OH)2]− (dihydrogen phosphate).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01323. Experimental details; structure elucidation; starting and reference materials; synthesis of compounds; NMR spectra; IR and Raman spectra; computational details (PDF) Accession Codes

CCDC 1828618−1828620 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Axel Schulz: 0000-0001-9060-7065 D

DOI: 10.1021/acs.inorgchem.8b01323 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Deutsche Forschungsgemeinschaft (DFG SCHU 1170/12-1) is acknowledged for financial support. We thank L. A. Wessjohann (Leibniz Institute Halle) and M. Dessoy (University of Campinas) for helpful advice and Dr. D. Michalik (University of Rostock) for the measurement of 17O NMR spectra.

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DOI: 10.1021/acs.inorgchem.8b01323 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01323 Inorg. Chem. XXXX, XXX, XXX−XXX