Homoatomic cations: From [P₅]⁺ to [P₉]⁺

Abstract

Recent synthetic approaches to a series of [P₉]X salts (X = [F{Al(OR^F)₃}₂], [Al(OR^F)₄], and (R^F = C(CF₃)₃); Ga₂Cl₇) overcome limitations in classical synthesis methods that proved unsuitable for phosphorus cations. These salts contain the homopolyatomic cation [P₉]⁺ via (I) oxidation of P₄ with NO[F{Al(OR^F)₃}₂], (II) the arene-stabilized Co(I) sandwich complex [Co(arene)₂][Al(OR^F)₄] [arene = ortho-difluorobenzene (o-DFB) and fluorobenzene (FB)], or (III) the reduction of [P₅Cl₂][Ga₂Cl₇] with Ga[Ga₂Cl₇] as Ga(I) source in the presence of P₄. Quantum chemical CCSD(T) calculations suggest that [P₉]⁺ formation from [Co(arene)₂]⁺ occurs via the nido-type cluster [(o-DFB)CoP₄]⁺, which resembles the isoelectronic, elusive [P₅]⁺. Apparently, the nido-cation [P₅]⁺ forms intermediately in all reactions, particularly during the Ga(I)-induced reduction of [P₅Cl₂]⁺ and the subsequent pick up of P₄ to yield the final salt [P₉][Ga₂Cl₇]. The solid-state structure of [P₉][Ga₂Cl₇] reveals the anticipated D_2d-symmetric Zintl-type cage for the [P₉]⁺ cation. Our approaches show great potential to bring other [P_n]⁺ cations from the gas to the condensed phase.

The crystal structure determination of the phosphorus cation salt [P₉][Ga₂Cl₇] and the mechanism of its formation are described.

INTRODUCTION

Intensely colored oleum solutions of nonmetallic homopolyatomic cations of sulfur, selenium, and tellurium were likely the first examples for this elusive cation class and were observed as early as around 1800 (1). However, the first real evidence for representatives began with the synthesis and characterization of [O₂][PtF₆] in 1962 (2, 3), and many more examples of homoatomic cations of most nonmetallic elements have been isolated and characterized in the solid state in the past 60 years excluding examples of crystal structures of homoatomic phosphorus cations. For the latter element, experimental and theoretical work showed that it is possible to generate pure polyphosphorus cations in the gas phase (4). Moreover, Martin et al. (5) observed larger [P_n]⁺ cations (n = 2 to 24) in the gas phase by mass spectrometric studies as early as 1986, with cation [P₉₁]⁺ being the largest to date (6). More recent calculations suggest the smaller, odd-numbered cluster cations [P_n]⁺ with n up to 15 to be the most approachable in the condensed phase (7). The first evidence for the existence of [P₉]⁺ in the condensed phase was published only in 2012: The oxidant [NO][Al(OR^F)₄] (R^F = C(CF₃)₃) reacts with an excess of P₄ (8) to the C_2v-symmetric [P₄NO]⁺ cation, which acts toward P₄ as a “P⁺” donor. Very likely, the elusive [P₅]⁺ cation is formed and acts as an intermediate, which subsequently adds a second molecule P₄ to yield [P₉]⁺ (Fig. 1, I).

Fig. 1. Retrosynthesis concepts toward [P₉]⁺ and preparation of salts [P₉]X.

(I) X = [F{Al(OR^F)₃}₂]: P₄ (2.5 equivalents), NO[F{Al(OR^F)₃}₂] (1.0 equivalents), RT, exclusion of light, 4FB, (68%). (II) X = [Al(OR^F)₄]: P₄ (3.0 equivalents), [Co(o-DFB)₂][Al(OR^F)₄] (1.0 equivalents), RT, FB, (21%). (III) X = [Ga₂Cl₇]: P₄ (10.0 equivalents), [P₅Cl₂][Ga₂Cl₇] (1.0 equivalents), Ga[Ga₂Cl₇] (1.0 equivalents), RT, FB/CS₂, (77%). R^F, C(CF₃)₃.

RESULTS AND DISCUSSION

The aforementioned reaction has been studied in quite some detail, and despite the use of the larger and more robust weakly coordinating anion [F{Al(OR^F)₃}₂]⁻ in route I (Fig. 1) (9), it was not possible so far to crystallize any [P₉]⁺ cation salt [P₉]X (X = [F{Al(OR^F)₃}₂], [Al(OR^F)₄]; cf. table S5 and figs. S7 to S11). Nevertheless, the amorphous salt [P₉][F{Al(OR^F)₃}₂] obtained in 1,2,3,4-tetrafluorobenzene (4FB) as reaction medium in 69% yield reversibly dissolves in this solvent and shows a superior long-time stability of over 40 days, which is fundamentally different to the corresponding [Al(OR^F)₄] salt and allows stoichiometric reactions in the future.

From a retrosynthetic point of view and realizing that [P₅]⁺ plays a pivotal role in the synthesis of [P₉]⁺, we developed two new approaches. This includes the new reaction pathway II toward [P₉][Al(OR^F)₄] via oxidation of white phosphorus with the “naked” Co(I) sandwich cations [Co(arene)₂][Al(OR^F)₄] [arene = ortho-difluorobenzene (o-DFB) (10) and fluorobenzene (FB)], which was confirmed by nuclear magnetic resonance (NMR) spectroscopy (Fig. 1, II). Quantum chemical calculations suggest occurrence of a nido-cluster intermediate [(o-DFB)CoP₄]⁺ that resembles the isoelectronic [P₅]⁺ (cf. figs. S36 and S37). The crude nonmagnetic black by-product was analyzed by powder diffraction [powder x-ray diffraction (pXRD)] and energy-dispersive x-ray (EDX) measurements. The analysis agrees with its tentative assignment as the well-known cobalt phosphide CoP₃ (cf. fig. S22). (11–13)

Recent years gave access to cationic polyphosphorus compounds such as [P₅X₂]⁺ (X = Br, I) (14) and [P₅R₂]⁺ (R = alkyl, aryl) (15–17). Hence, for the most successful route III, particularly cation [P₅Cl₂]⁺ in combination with an adequate reducing agent that turned out to be the best [P₅]⁺ synthon (Fig. 1, III). However, the synthesis of a [Al(OR^F)₄]⁻ salt containing the cation [P₅Cl₂]⁺ following the reported procedure for the Br and I analogs only led to mixtures. By contrast, it succeeds as the salt [P₅Cl₂][GaCl₄], when P₄ is reacted in PCl₃ as the reaction medium with the required stoichiometric amount of GaCl₃ at room temperature. [P₅Cl₂][GaCl₄] can be isolated as colorless solid in excellent yields (86%) and starts to decompose in solution (CH₂Cl₂, FB, and o-DFB) and in the solid state at room temperature in the course of a few hours (solution) up to a few days (solid material) to P₄, GaCl₃, PCl₃, and an insoluble orange solid of unknown constitution, likely Schenck-type phosphorus (18). However, when stored in the dark and at −30°C, the salt is stable for several weeks. The molecular structure of [P₅Cl₂]⁺ was confirmed by single-crystal x-ray structure determination of the more stable [P₅Cl₂][Ga₂Cl₇] salt (cf. table S12 and fig. S23). The optimized synthesis of [P₉][Ga₂Cl₇] requires the slow treatment of [P₅Cl₂][GaCl₄] with GaCl₃ and Ga[Ga₂Cl₇] as reducing agent, in the presence of a huge excess of P₄ (10.0 equivalents), and a 5:1 solvent mixture of FB and CS₂ as reaction medium (Fig. 1, III).

Typically, the dissolved Ga(I) source Ga[Ga₂Cl₇] in FB is added dropwise to the FB/CS₂ solution of [P₅Cl₂][GaCl₄], GaCl₃, and P₄ over the course of 30 min, leading to the precipitation of [P₉][Ga₂Cl₇] as an orange microcrystalline material in 77% yield. Salt [P₉][Ga₂Cl₇] is stable when stored at low temperature (−35°C) for a long period of time (several month) but starts to decompose when heated above 60°C readily [simultaneous thermal analyzer (STA) measurement; see fig. S6]. The excess of P₄ can be conveniently and quantitatively recovered from the reaction medium. The purity of solid [P₉][Ga₂Cl₇] was confirmed by ³¹P MAS (magic angle spinning) NMR spectroscopy and Raman spectroscopy (Fig. 2, I and II), elemental analysis [inductively coupled plasma (ICP)], and pXRD (figs. S5 and S26). The salt is poorly soluble in common solvents such as CH₂Cl₂, FB, and o-DFB, but sufficiently concentrated solutions for liquid NMR studies and crystal preparations can be obtained by addition of small amounts of GaCl₃. Note that if the reduction is performed without CS₂ and only stoichiometric amounts of P₄ are present, then the formation of a much smaller amount of [P₉]⁺ is observed in the reaction mixture that is, however, sufficiently concentrated for NMR studies in solution. The solution of ³¹P NMR spectrum of dissolved crystalline [P₉][Ga₂Cl₇] reveals the expected A₂A′₂BC₂C′₂ spin system with signals at δ_A = 111.31 parts per million (ppm), δ_B = 60.91 ppm, and δ_C = −247.49 ppm. The chemical shifts and coupling constants retrieved from the iteration of the ³¹P{¹H} NMR spectrum compare well to all the aforementioned [P₉⁺] salts (Fig. 2, I; fig. S5; and tables S1 and S2).

Fig. 2. Characterization of [P₉][Ga₂Cl₇].

(I) ³¹P MAS NMR and liquid ³¹P{¹H} NMR spectra of the separated crystals in CD₂Cl₂; asterisks (*) denote spinning sidebands in the MAS NMR. (II) Raman spectrum and assignment. (III) Molecular structure of cation [P₉]⁺ including anion [Ga₂Cl₇]⁻.

Suitable crystals for XRD were obtained by overlaying either reaction solutions without CS₂ (polymorph 1, space group P2₁/n) or concentrated solutions of [P₉][Ga₂Cl₇] with additional GaCl₃ in o-DFB (polymorph 2, space group I2/a) with n-pentane at −30°C (Fig. 2, III, and cf. the Supplementary Materials). The second polymorph shows half of the salt composition in the asymmetric unit, and the P─P bond lengths of [P₉]⁺ compare well to those observed in related [P₅X₂]⁺ (X = Br, I) (14) and [P₅R₂]⁺ (R = alkyl, aryl) (15–17) cations. Similarly, the transannular P─P bond [P4–P5: 2.1871 (7) Å] is found to be the shortest, followed by the P─P bonds connected to the tetracoordinate P-unit [P1–P2: 2.1902 (6) Å and P1–P3: 2.1991 (5) Å]. The remaining P─P bonds show an average length of 2.239 Å (Fig. 2, III). The composition of the [P₉]⁺ cation shows the anticipated approximate D_2d-symmetric Zintl-type cage structure with slightly divergent bond and dihedral angles very likely caused by the stronger interaction of the cation with the [Ga₂Cl₇]⁻ anion. A Hirshfeld plot and fingerprint analysis confirm this stronger intramolecular interaction and also further explain the chemically nonequivalent phosphorus atoms in solid state and, thus, the rather complex ³¹P MAS spectrum (Fig. 2, I; table S15; and figs. S27 to S29). The pXRD diffractogram of the microcrystalline [P₉][Ga₂Cl₇] compares well with the simulated powder diffraction pattern of the second polymorph (cf. fig. S26) and indicates that the solid sample is phase pure.

To gain a deeper insight into the formation mechanism of [P₉][Ga₂Cl₇], quantum chemical calculations were performed at the (RI-)B3LYP(D3BJ)/def2-TZVPP level for structure optimizations and calculation of conductor-like polarizable continuum model (CPCM)-Gibbs solvation energies in 5:1 FB:CS₂ solution. The energies of all particles were refined at the rather definitive DLPNO-CCSD(T)/QZVPP level with ORCA 5. Because ⁷¹Ga-NMR spectroscopy proved the presence of [Ga^I(FB)₂]⁺ in Ga^I[(Ga^III)₂Cl₇] solutions in FB (fig. S30) (19–21), the starting point for the formation of [P₉]⁺ (Fig. 3) is the ion pairing reaction (Fig. 3, 1) of [Ga^I(FB)₂]⁺ with the [Ga^IIICl₄]⁻ counterion from [P₅Cl₂][Ga^IIICl₄], the strongest Lewis base in the system, to give salt [(FB)Ga^IGa^IIICl₄] (Δ_RG_(solv) = −59.4 kJ mol⁻¹) with Ga(I) and Ga(III) centers. Excess Ga^IIICl₃ leads to the formation of the observed counter anion [(Ga^III)₂Cl₇]⁻, which, however, is known to equilibrate readily in arene solution with [Ga^IIICl₄]⁻ and Ga^IIICl₃ (22). The ion pair [(FB)Ga^IGa^IIICl₄] then oxidatively adds via the Ga(I) ion into one of the two identical P─Cl bonds of [P₅Cl₂]⁺ (Fig. 3, 2), liberates the FB molecule, and forms [ClP₅(Ga^III)₂Cl₅]⁺ (Δ_RG_(solv) = −75.0 kJ mol⁻¹). This P-coordinated P₅Cl adduct to the very strong chloride acceptor [(Ga^III)₂Cl₅]⁺ reacts by a rearrangement reaction (Fig. 3, 3) via intermediate Cl coordination to give [P₅]⁺ under separation of neutral (Ga^III)₂Cl₆ [Δ_RG_(solv) = –19.7 kJ mol⁻¹]. The [P₅]⁺ intermediate generated picks up free P₄, which is present in a huge excess, to give the final product [P₉]⁺ [Δ_RG_(solv) = −43.8 kJ mol⁻¹] in an oxidative addition (Fig. 3, 4). With excess (Ga^III)₂Cl₆ in the solution, it crystallizes as salt [P₉][(Ga^III)₂Cl₇], facilitated by the rather low solubility of the formed salt. The suggested entire reaction path is highly exergonic by −197.9 kJ mol⁻¹, and although we evaluated alternative reaction paths as well, the one described here appears most likely to us (cf. the Supplementary Materials). However, all likely reaction pathways include the elusive [P₅]⁺ as intermediate (Fig. 1, retrosynthesis).

Fig. 3. Calculated mechanism toward the formation of [P₉]⁺ starting from [P₅Cl₂][Ga^IIICl₄], P₄, and in situ formed [Ga^I(FB)₂][(Ga^III)₂Cl₇].

Geometry optimization was performed at the (RI-)B3LYP(D3BJ)/def2-TZVPP level of theory, and single-point energies were calculated at the DLPNO-CCSD(T)/QZVPP level with ORCA. Solvation corrections were calculated in FB:CS₂ (v/v, 5:1), with the CPCM model (ε_r = 5.2) at the B3LYP(D3BJ)/def2-TZVPP level of theory.

With the crystal structure of [P₉][Ga₂Cl₇], the present work provides the first structural confirmation of a homopolyatomic phosphorus cation. The successful implementation of new synthetic strategies and a detailed elucidation of the possible reaction pathways showcase the elusive [P₅]⁺ as an intermediate, isoelectronic to the putative cyclopentadienyl cation (C₅H₅⁺) (23). The transition from the planar cyclic organic cation toward the Wade’ian nido type (24) was calculated to occur even by the substitution of only one CH group for the entire series [(HC)_xP_5-x]⁺, with x = 0 to 4 by Green and colleagues (25). However, only the synthesis of [3,5-tBu₂-1,2,4-C₂P₃][AlCl₄] was reported by Russell and colleagues (26, 27) so far. Now, the elusive [P₅]⁺ has also strong evidence to be present in condensed phases, and our synthetic strategies pave the way for the discovery of further novel molecules.

MATERIALS AND METHODS

General remarks for [P₉][Ga₂Cl₇]

Manipulations were performed in a Glovebox MB UNIlab or using Schlenk techniques under an atmosphere of purified nitrogen or argon, respectively. Dry, oxygen-free solvents (CH₂Cl₂, FB, o-DFB, and CS₂; distilled from CaH₂), and n-hexane and n-pentane (distilled from potassium) were used. Deuterated benzene (C₆D₆) was distilled from potassium. Deuterated dichloromethane (CD₂Cl₂) was dried over molecular sieve. All distilled and deuterated solvents were stored over molecular sieves (4 Å: CH₂Cl₂, CD₂Cl₂, FB, o-DFB, C₆D₆, n-hexane, n-pentane, and CS₂). All glassware were oven-dried at 160°C before use. Ga[Ga₂Cl₇] was purchased from Shanghai Richem International. All NMR measurements were performed with the Bruker AVANCE III HD Nanobay, 400-MHz UltraSield [¹H (400.13 MHz), ¹³C (100.61 MHz), ³¹P (161.98 MHz), and ⁷¹Ga (122.03 MHz)] equipped with a BBFO probe or on a Bruker AVANCE III HDX, 500-MHz Ascend [¹H (500.13 MHz), ¹³C (125.75 MHz), ³¹P (202.45 MHz), and ⁷¹Ga (152.52 MHz)] equipped with a BBO Prodigy CryoProbe. Chemical shifts were referenced to δ_TMS = 0.00 ppm (¹H and ¹³C, externally) and δ_H3PO4(85%) = 0.00 ppm (³¹P, externally). Chemical shifts (δ) are reported in parts per million. Coupling constants (J) are reported in hertz. ³¹P solid-state NMR spectra were recorded on a BRUKER Avance 300-MHz spectrometer using a commercial 2.5-mm MAS NMR probe and operating at a resonance frequency of 121.5 MHz. The MAS frequency was 15 and 23 kHz. NaH₂PO₄·H₂O was used as an external standard. Melting points were recorded on an electrothermal melting point apparatus (Büchi, Switzerland, Melting Point M-560) in sealed capillaries under argon atmosphere and are uncorrected. Infrared (IR) and Raman spectra were recorded at ambient temperature using a Bruker Vertex 70 instrument equipped with a RAM II module (Nd-YAG laser, 1064 nm). The Raman intensities are reported in percent relative to the most intense peak and are given in parenthesis. An attenuated total reflection (ATR) unit (diamond) was used for recording IR spectra. The intensities are reported relative to the most intense peak and are given in parenthesis using the following abbreviations: ≥0.8 = very strong (vs), ≥0.6 = strong (s), ≥0.4 = medium (m), ≥0.2 = weak (w), and ≤0.2 = very weak (vw). Elemental analysis and the Ga, P content were determined by ICP–optical emission spectrometry (OES) on an Optima 2000 DV (PerkinElmer), calibrated to two wave numbers for P (213.6 and 214.9 nm) and Ga (294.4 and 417.2 nm). Therefore, [P₉][Ga₂Cl₇] was digested in 5 ml of 100% HNO₃ with further heating at 120°C for 10 min. The STA measurement was performed on a STA 8000 (PerkinElmer) under argon atmosphere (25 ml min⁻¹) within a temperature range from 35° to 600°C (10 K min⁻¹).The ³¹P{¹H} spectra of compounds [P₉][Ga₂Cl₇] were fitted using the full line shape iteration of gNMR software (28) to determine the ³¹P─³¹P coupling constants.

Synthesis of [P₅Cl₂][GaCl₄]

A solution of GaCl₃ (710 mg, 4 mmol) in PCl₃ (7 ml) was added to a suspension of P₄ (500 mg, 4 mmol) in PCl₃ (5 ml). The reaction was stirred under exclusion of light for 1 hour at ambient temperature. After the addition of n-hexane (20 ml), a pale yellow precipitate was formed, which was filtered, washed with n-hexane (5 ml), and dried in vacuo to yield [P₅Cl₂][GaCl₄] as pale yellow, light sensitive, and pyrophoric solid in 86% yield (1.5 g). [P₅Cl₂][GaCl₄] decomposes in solution and slowly in the solid state when stored at room temperature to P₄, GaCl₃, PCl₃, and an insoluble orange solid material of unknown constitution. Figure S2 shows the ³¹P NMR spectrum of dissolved [P₅Cl₂][GaCl₄] in CD₂Cl₂ after 4 hours, indicating its decomposition. When stored at −30°C under the exclusion of light, solid [P₅Cl₂][GaCl₄] is stable for at least 3 months. ³¹P NMR (202.45 MHz, CD₂Cl₂, 300 K): δ = 148.7 ppm [2P, dt, ¹J(P_BP_A) = −341 Hz, ¹J(P_CP_A) = −142 Hz, P_A, [P₅Cl₂]⁺], 56.2 ppm [1P, tt, ¹J(P_BP_A) = −341 Hz, ²J(P_CP_B) = 27 Hz, P_B, [P₅Cl₂]⁺], and −269.2 ppm [2P, td, ¹J(P_CP_A) = −142 Hz, ²J(P_CP_B) = 27 Hz, P_C, [P₅Cl₂]⁺]. Raman (100 mW, 298 K): ν = 594 cm⁻¹ (10), 562 cm⁻¹ (13), 537 cm⁻¹ (100), 435 cm⁻¹ (33), 415 cm⁻¹ (69), 392 cm⁻¹ (26), 382 cm⁻¹ (20), 357 cm⁻¹ (38), 339 cm⁻¹ (59), 262 cm⁻¹ (23), 227 cm⁻¹ (18), 182 cm⁻¹ (51), 151 cm⁻¹ (24), 132 cm⁻¹ (18), and 118 cm⁻¹ (14). IR (ATR, 298 K): ν = 1212 cm⁻¹ (w), 630 cm⁻¹ (w), 585 cm⁻¹ (vs), 564 cm⁻¹ (s), 552 cm⁻¹ (vs), 536 cm⁻¹ (vs), and 412 cm⁻¹ (vs). Elemental analysis (solvent free): GaP₅Cl₆ calculated: N: 0.00, C: 0.00, H: 0.00; found: N: 0.03, C: 0.32; H: 0.01. Melting point: 62–64°C (decomposition).

Synthesis of [P₅Cl₂][Ga₂Cl₇]

GaCl₃ (528 mg, 3.0 mmol) in PCl₃ (3 ml) was added to P₄ (124 mg, 1.0 mmol). The mixture was stirred for 1 hour at ambient temperature under the exclusion of light. A colorless precipitate formed within 5 min. Converted to a colorless oil. The supernatant was decanted from the oil and washed with n-hexane (3 × 5 ml), and all volatiles were removed in vacuo leaving pure [P₅Cl₂][Ga₂Cl₇] as colorless microcrystaline material in 92% yield (599 mg). Dissolved and solid [P₅Cl₂][Ga₂Cl₇] also decomposes to P₄, GaCl₃, PCl₃ and an insoluble orange solid. It is also much longer stable in the dark at −30°C at least for several month. Crystals suitable for x-ray analysis can be grown from concentrated CH₂Cl₂ solution layered with n-pentane at −30°C. ³¹P NMR (202.45 MHz, CD₂Cl₂, 300 K): δ = 149.1 ppm [2P, dt, ¹J(P_BP_A) = −340 Hz, ¹J(P_CP_A) = −143 Hz, P_A, and [P₅Cl₂]⁺], 56.2 ppm [1P, tt, ¹J(P_BP_A) = −340 Hz, ²J(P_CP_B) = 27 Hz P_B, [P₅Cl₂]⁺], and −269.3 ppm [2P, td, ¹J(P_CP_A) = −143 Hz, ²J(P_CP_B) = 27 Hz, P_C, and [P₅Cl₂]⁺]. Raman (100 mW, 298 K): ν = 595 cm⁻¹ (15), 564 cm⁻¹ (14), 540 cm⁻¹ (100), 437 cm⁻¹ (37), 420 cm⁻¹ (59), 393 cm⁻¹ (42), 363 cm⁻¹ (80), 339 cm⁻¹ (30), 276 cm⁻¹ (12), 264 cm⁻¹ (29), 227 cm⁻¹ (22), 181 cm⁻¹ (54), 152 cm⁻¹ (20), 135 cm⁻¹ (43), and 102 cm⁻¹ (31). IR (ATR, 298 K): ν = 1189 cm⁻¹ (m), 637 cm⁻¹ (w), 590 cm⁻¹ (vs), 562 cm⁻¹ (vs), and 539 cm⁻¹ (vs). Elemental analysis (solvent free): Ga₂P₅Cl₉ calculated: N: 0.00, C: 0.00, and H: 0.00; found: N: 0.03, C: 0.40, and H: 0.144. Melting point: 52° to 55°C (decomposition).

Synthesis of [P₉][Ga₂Cl₇]

P₄ (140 mg, 1.2 mmol, 10 equivalents) dissolved in 1 ml of CS₂ was added to a solution of [P₅Cl₂][GaCl₄] (50 mg, 0.12 mmol, 1 equivalents) and GaCl₃ (21 mg, 0.12 mmol, 1 equivalents) in 2 ml of FB. Ga[Ga₂Cl₇] (54 mg, 0.12 mmol, 1 equivalents) dissolved in 1.5 ml of FB was added dropwise over a period of 30 min accompanied with the formation of a pale orange powder. The suspension was filtered, and the precipitate washed with 1 ml of CS₂ and 2 ml of n-pentane. After drying in vacuo, [P₉][Ga₂Cl₇] was obtained as orange microcrystalline powder in 77% yield (61 mg), which was characterized by ³¹P MAS spectroscopy, pXRD, IR, Raman, and ICP (vide infra). Note that, from the filtrate, the excess of P₄ can be quantitatively recovered by sublimation. The filtrate contains [P₉][Ga₂Cl₇], P₄, and Cl₃POGaCl₃ (fig. S3), of which the latter was identified by single-crystal x-ray analysis. We believe that the formation of Cl₃POGaCl₃ is a result of the presence of either very low amounts of moisture or the reaction of PCl₃ with the glass surface of the reaction vessels in the presence of the strong Lewis acid GaCl₃. This was independently confirmed by the diffusion of n-pentane in a solution of GaCl₃ in PCl₃ at −30°C, which leads to the deposition of colorless crystalline Cl₃POGaCl₃. The ³¹P NMR spectrum of the GaCl₃/PCl₃ solution is depicted in fig. S4 and the molecular structure is depicted in fig. S19. Note that [P₉][Ga₂Cl₇] is also obtained without additional GaCl₃, however, with a slightly decreased yield of 70%. ³¹P{¹H} NMR (202.45 MHz, CD₂Cl₂ + GaCl₃, 300 K): δ = 111.5 ppm (4P, m, P_A, [P₉]⁺), 61.0 ppm (1P, m, P_B, [P₉]⁺), and −247.4 ppm (4P, m, P_C, [P₉]⁺). IR (ATR, 298 K): ν = 554 cm⁻¹ (m), 523 cm⁻¹ (vs), and 501 cm⁻¹ (s). Raman (100 mW, 298 K): ν = 555 cm⁻¹ (11), 542 cm⁻¹ (100), 525 cm⁻¹ (32), 513 cm⁻¹ (12), 498 cm⁻¹ (62), 446 cm⁻¹ (30), 429 cm⁻¹ (5), 412 cm⁻¹ (74), 401 cm⁻¹ (12), 391 cm⁻¹ (32), 361 cm⁻¹ (48), 347 cm⁻¹ (10), 309 cm⁻¹ (6), 272 cm⁻¹ (5), 198 cm⁻¹ (96), 157 cm⁻¹ (80), 143 cm⁻¹ (16), 126 cm⁻¹ (9), 106 cm⁻¹ (17), and 95 cm⁻¹ (15) (assignment; see table S3). Melting point: Compound starts to decompose at temperature above 60°C. Elemental analysis with ICP-OES: calculated: P: 41.83 and Ga: 20.93; found: P: 41.92 and Ga: 20.41.

General remarks for [P₉][F{Al(OR^F)₃}₂] and [P₉][Al(OR^F)₄] (R^F = C(CF₃)₃)

All reactions were performed with standard Schlenk technique in special double Schlenk flasks with G4 frit and polytetrafluoroethylene valves (fig. S1) under argon atmosphere or in gloveboxes filled with argon (O₂ and H₂O < 1 ppm) or nitrogen (O₂ and H₂O < 0.1 ppm). o-DFB, 1,3-difluorobenzene (m-DFB), 4FB, FB, and CD₂Cl₂ were dried with CaH₂, distilled, and degassed. N-pentane and CH₂Cl₂ were dried using a conventional MBraun Grubbs apparatus. Co₂(CO)₈ (1 to 10% in hexane) was used as received. [Co(FB)₂][Al(OR^F)₄], [Co(o-DFB)₂][Al(OR^F)₄], [Ag][Al(OR^F)₄], and [NO][F{Al(OR^F)₃}₂] were synthesized according to literature (9, 10, 29). All NMR measurements were performed with a Bruker Avance II Widebore 400-MHz, a Bruker Avance III HD 300-MHz, or a Bruker Avance 200-MHz NMR spectrometer. All NMR samples were prepared using Schlenk technique or a glovebox. All spectra were processed with the software package TopSpin 3.6.2. ¹H NMR spectra were calibrated using solvent signals versus tetramethylsilane (TMS) [o-DFB = 6.965 ppm, m-DFB = 7.082 ppm, 4FB = 6.974 ppm, and CH₂Cl₂ (relative to CD₂Cl₂) = 5.33] (30). The NMR spectra of other nuclei (¹⁹F, ²⁷Al, and ³¹P) were calibrated with respect to TMS using the Ξ tables from IUPAC (31). Simulated NMR spectra were processed using the corresponding experimental chemical shifts and iterated literature known coupling constants (8). CV measurements were performed with a BioLogic Science Instruments potentiostat, and the data were processed with EC-Lab software V11.21 and OriginPro 2020. Element composition was determined using a Hitachi FEGHRSEM SU8220 with a Bruker XFlash FlatQUAD detector. The measurements were performed with an acceleration voltage of 15 kV and a working distance of 17 mm.

Experimental details for [P₉][F{Al(OR^F)₃}₂]

The following synthesis in 4FB typify for all other used solvents (CH₂Cl₂, o-DFB, and m-DFB). Inside, a glovebox P₄ (81 mg, 0.7 mmol, 2.5 equivalents) and [NO][F{Al(OR^F)₃}₂] (399 mg, 0.26 mmol, 1.0 eq.) were weighed in one side of a double Schlenk tube. Under reverse flow of argon, 4FB (4 ml) was added to the solids. Immediately, a dark red color of the solution was observed. The solution itself was stirred for at least 3 days in the dark. During this period, the color of the solution changed to yellow or orange, depending on the amount of solvent used for the reaction. The solution was filtered through the frit to the other side of the double Schlenk tube, and the solvent was slowly removed under reduced pressure. The desired product (320 mg, 0.18 mmol, 69%) was obtained as a yellowish powder and was characterized solely by NMR, because [P₉]⁺ and [F{Al(OR^F)₃}₂]⁻ are already known from literature (8, 9). ¹H NMR (400.17 MHz, 4FB, 298 K): only solvent signal at 6.97 ppm. ¹⁹F NMR (376.54 MHz, 4FB, 298 K): δ = −76.1 ppm (s, 54F, [F{Al(OC(CF₃)₃)₃}₂]⁻) and −184.8 ppm (br. s, 1F, [F{Al(OC(CF₃)₃)₃}₂]⁻). ²⁷Al NMR (104.27 MHz, 4FB, 298 K): δ = ~32 ppm (br. s, 1Al, [F{Al(OC(CF₃)₃)₃}₂]⁻). ³¹P NMR (161.99 MHz, 4FB, 298 K): δ = 123.2 ppm (m, 4P, P_A, [P₉]⁺), 69.2 ppm (m, 1P, P_B, [P₉]⁺), and −263.2 ppm (m, 4P, P_C, [P₉]⁺). Note the small impurities of unknown compounds at 50.5 and −236.2.

Experimental details for [P₉][Al(OR^F)₄] via [Co(arene)₂][Al(OR^F)₄]

The synthesis is equally feasible with arene = o-DFB or FB and is therefore only described for FB: In a special double Schlenk flask with G4 frit, P₄ (31 mg, 0.25 mmol, 3.0 equivalents) and [Co(FB)₂][Al(OR^F)₄] (102.0 mg, 0.0837 mmol) were dissolved in FB (2 ml) and stirred for 1 hour. The solution immediately turned into a black suspension with a black precipitate. This precipitate was further analyzed by EDX and pXRD (vide supra). After filtration, the solvent was removed, and the residue was extracted with n-pentane (3 × 5 ml). A brownish powder (42 mg, 0.034 mmol, 21%) was obtained and analyzed by NMR spectroscopy (fig. S12 to S15). ¹H NMR (300.18 MHz, o-DFB, 298 K): only solvent signal at 6.97 ppm (m, o-DFB). ¹⁹F NMR (282.45 MHz, o-DFB, 298 K): δ = −75.1 ppm (s, 36 F, [Al(OC(CF₃)₃)₄]⁻), −76.1 ppm (s, (F₃C)₃COH), and −139.4 ppm (m, o-DFB). ²⁷Al NMR (78.22 MHz, o-DFB, 298 K): δ = 35.1 ppm (s, 1 Al, [Al(OC(CF₃)₃)₄]⁻). ³¹P NMR (121.51 MHz, o-DFB, 298 K): δ = 116.8 ppm (m, 4 P, P_A, [P₉]⁺), 63.7 ppm (m, 1 P, P_B, [P₉]⁺), and −261.2 ppm (m, 4 P, P_C, [P₉]⁺).

Supplementary Materials

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Supplementary Materials and Methods

Supplementary Text

Figs. S1 to S37

Tables S1 to S19

References

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Data S1 to S6