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

Recent synthetic approaches to a series of [P9]X salts (X = [F{Al(ORF)3}2], [Al(ORF)4], and (RF = 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(ORF)3}2], (II) the arene-stabilized Co(I) sandwich complex [Co(arene)2][Al(ORF)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 D2d-symmetric Zintl-type cage for the [P9]+ cation. Our approaches show great potential to bring other [Pn]+ cations from the gas to the condensed phase.


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 [O 2 ][PtF 6 ] 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 [P 91 ] + 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 [P 9 ] + in the condensed phase was published only in 2012: The oxidant [NO][Al(OR F ) 4 ] (R F = C(CF 3 ) 3 ) reacts with an excess of P 4 (8) to the C 2v -symmetric [P 4 NO] + cation, which acts toward P 4 as a "P + " donor. Very likely, the elusive [P 5 ] + cation is formed and acts as an intermediate, which subsequently adds a second molecule P 4 to yield [P 9 ] + (Fig. 1, I).

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 [P 9 ] + cation salt [P 9 ]X (X = [F{Al(OR F ) 3  and energy-dispersive x-ray (EDX) measurements. The analysis agrees with its tentative assignment as the well-known cobalt phosphide CoP 3 (cf. fig. S22). (11)(12)(13) Recent years gave access to cationic polyphosphorus compounds such as [P 5 X 2 ] + (X = Br, I) (14) and [P 5 R 2 ] + (R = alkyl, aryl) (15)(16)(17). Hence, for the most successful route III, particularly cation [P 5 Cl 2 ] + in combination with an adequate reducing agent that turned out to be the best [P 5 ] + synthon ( Fig. 1 Cl 7 ] as reducing agent, in the presence of a huge excess of P 4 (10.0 equivalents), and a 5:1 solvent mixture of FB and CS 2 as reaction medium (Fig. 1, III).  (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 CH 2 Cl 2 , FB, and o-DFB, but sufficiently concentrated solutions for liquid NMR studies and crystal preparations can be obtained by addition of small amounts of GaCl 3 . Note that if the reduction is performed without CS 2 and only stoichiometric amounts of P 4 are present, then the formation of a much smaller amount of [P 9 ] + is observed in the reaction mixture that is, however, sufficiently concentrated for NMR studies in solution. The solution of 31 P NMR spectrum of dissolved   (19)(20)(21), the starting point for the formation of [P 9 ] + (Fig. 3) is the ion pairing reaction (Fig. 3, 1 with Ga(I) and Ga(III) centers. Excess Ga III Cl 3 leads to the formation of the observed counter anion [(Ga III ) 2 Cl 7 ] − , which, however, is known to equilibrate readily in arene solution with [Ga III Cl 4 ] − and Ga III Cl 3 (22). The ion pair [(FB)Ga I Ga III Cl 4 ] then oxidatively adds via the Ga(I) ion into one of the two identical P─Cl bonds of [P 5 Cl 2 ] + (Fig. 3, 2), liberates the FB molecule, and forms [ClP 5 (Ga III ) 2 Cl 5 ] + ( R G (solv) = −75.0 kJ mol −1 ). This P-coordinated P 5 Cl adduct to the very strong chloride acceptor [(Ga III ) 2 Cl 5 ] + reacts by a rearrangement reaction (Fig. 3, 3 −1 ] in an oxidative addition (Fig. 3, 4). With excess (Ga III ) 2 Cl 6 in the solution, it crystallizes as salt [P 9 ][(Ga III ) 2 Cl 7 ], 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 [P 5 ] + as intermediate (Fig. 1, retrosynthesis).
With  colleagues (25). However, only the synthesis of [3,5-tBu 2 -1,2,4-C 2 P 3 ][AlCl 4 ] was reported by Russell and colleagues (26,27) so far. Now, the elusive [P 5 ] + has also strong evidence to be present in condensed phases, and our synthetic strategies pave the way for the discovery of further novel molecules.

General remarks for [P 9 ][Ga 2 Cl 7 ]
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 (CH 2 Cl 2 , FB, o-DFB, and CS 2 ; distilled from CaH 2 ), and n-hexane and n-pentane (distilled from potassium) were used. Deuterated benzene (C 6 D 6 ) was distilled from potassium. Deuterated dichloromethane (CD 2 Cl 2 ) was dried over molecular sieve. All distilled and deuterated solvents were stored over molecular sieves (4 Å: CH 2 Cl 2 , CD 2 Cl 2 , FB, o-DFB, C 6 D 6 , n-hexane, n-pentane, and CS 2 ). All glassware were oven-dried at 160°C before use. Ga Chemical shifts were referenced to  TMS = 0.00 ppm ( 1 H and 13 C, externally) and  H3PO4(85%) = 0.00 ppm ( 31 P, externally). Chemical shifts () are reported in parts per million. Coupling constants (J) are reported in hertz. 31 P 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. NaH 2 PO 4 ·H 2 O 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, [P 9 ] [Ga 2 Cl 7 ] was digested in 5 ml of 100% HNO 3 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 31 P{ 1 H} spectra of compounds [P 9 ][Ga 2 Cl 7 ] were fitted using the full line shape iteration of gNMR software (28) to determine the 31 P─ 31 P coupling constants.
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 [P 5 Cl 2 ][GaCl 4 ] as pale yellow, light sensitive, and pyrophoric solid in 86% yield (1.5 g). [P 5 Cl 2 ][GaCl 4 ] decomposes in solution and slowly in the solid state when stored at room temperature to P 4 , GaCl 3 , PCl 3 , and an insoluble orange solid material of unknown constitution. Figure S2 shows the 31

Synthesis of [P 5 Cl 2 ][Ga 2 Cl 7 ]
GaCl 3 (528 mg, 3.0 mmol) in PCl 3 (3 ml) was added to P 4 (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 [P 5 Cl 2 ][Ga 2 Cl 7 ] as colorless microcrystaline material in 92% yield (599 mg). Dissolved and solid [P 5 Cl 2 ][Ga 2 Cl 7 ] also decomposes to P 4 , GaCl 3 , PCl 3 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 CH 2 Cl 2 solution layered with n-pentane at −30°C. 31

Synthesis of [P 9 ][Ga 2 Cl 7 ]
P 4 (140 mg, 1.2 mmol, 10 equivalents) dissolved in 1 ml of CS 2 was added to a solution of [P 5 Cl 2 ][GaCl 4 ] (50 mg, 0.12 mmol, 1 equivalents) and GaCl 3 (21 mg, 0.12 mmol, 1 equivalents) in 2 ml of FB. Ga[Ga 2 Cl 7 ] (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 CS 2 and 2 ml of n-pentane. After drying in vacuo, [P 9 ][Ga 2 Cl 7 ] was obtained as orange microcrystalline powder in 77% yield (61 mg), which was characterized by 31 P MAS spectroscopy, pXRD, IR, Raman, and ICP (vide infra). Note that, from the filtrate, the excess of P 4 can be quantitatively recovered by sublimation. The filtrate contains [P 9 ][Ga 2 Cl 7 ], P 4 , and Cl 3 POGaCl 3 ( fig. S3), of which the latter was identified by singlecrystal x-ray analysis. We believe that the formation of Cl 3 POGaCl 3 is a result of the presence of either very low amounts of moisture or the reaction of PCl 3 with the glass surface of the reaction vessels in the presence of the strong Lewis acid GaCl 3 . This was independently confirmed by the diffusion of n-pentane in a solution of GaCl 3 in PCl 3 at −30°C, which leads to the deposition of colorless crystalline Cl 3 POGaCl 3 . The 31 P NMR spectrum of the GaCl 3 /PCl 3 solution is depicted in fig. S4 and the molecular structure is depicted in fig. S19. Note that [P 9 ][Ga 2 Cl 7 ] is also obtained without additional GaCl 3 , however, with a slightly decreased yield of 70%. 31

General remarks for [P 9 ][F{Al(OR F ) 3 } 2 ] and [P 9 ][Al(OR F ) 4 ] (R F = C(CF 3 ) 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 (O 2 and H 2 O < 1 ppm) or nitrogen (O 2 and H 2 O < 0.1 ppm). o-DFB, 1,3-difluorobenzene (m-DFB), 4FB, FB, and CD 2 Cl 2 were dried with CaH 2 , distilled, and degassed. N-pentane and CH 2 Cl 2 were dried using a conventional MBraun Grubbs apparatus. Co 2 (CO) 8 (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. 1 H NMR spectra were calibrated using solvent signals versus tetramethyl silane (TMS) [o-DFB = 6.965 ppm, m-DFB = 7.082 ppm, 4FB = 6.974 ppm, and CH 2 Cl 2 (relative to CD 2 Cl 2 ) = 5.33] (30). The NMR spectra of other nuclei ( 19 F, 27 Al, and 31 P) 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 [P 9 ][F{Al(OR F ) 3 } 2 ]
The following synthesis in 4FB typify for all other used solvents (CH 2 Cl 2 , o-DFB, and m-DFB). Inside, a glovebox P 4 (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 [P 9 ] + and [F{Al(OR F ) 3 } 2 ] − are already known from literature (8,9). 1

Experimental details for [P 9 ][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, P 4 (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). 1