[P4H]+[Al(OTeF5)4]–: protonation of white phosphorus with the Brønsted superacid H[Al(OTeF5)4](solv)

The structure of protonated white phosphorus in solution has been revealed for the first time.


Introduction
White phosphorus (P 4 ), discovered by Henning Brand in 1669 while searching for the philosopher's stone, is the thermodynamically least stable and most reactive form of phosphorus at room temperature and consists of tetrahedral P 4 molecules. Despite its spontaneous ammability and severe toxicity, P 4 is the easiest form to produce on an industrial scale and is therefore the commercially most important allotrope. 1 Especially its conversion to PCl 3 is of high interest, as it is a base chemical for the production of many organophosphorus compounds.
From a historical point of view, two important chemical reactions of P 4 are described in every good textbook of inorganic chemistry: 2 (a) the slow oxidation of P 4 vapor to P 4 O 10 under emission of light. This chemoluminescence has coined the name phosphorus, which is derived from the greek mythology ("light-bearer"). (b) the activation and disproportionation of P 4 by aqueous solutions of alkali metal hydroxides. In this way, the industrially relevant phosphine gas (PH 3 ) is obtained in high purity next to the alkali metal salt of hypophosphorous acid (NaH 2 PO 2 ). More recent studies deal with the degradation of white phosphorus in the presence of other strong nucleophiles, such as organolithium and organomagnesium compounds, carbenes or silylenes, under the topic "P 4 -activation and functionalization". 3-6 From a mechanistic point of view, a charged nucleophile (Nu À ) interacts with one of the three energetically degenerate LUMOs of the P 4 molecule (Fig. 1a) under opening of the P 4 tetrahedron to yield a substituted buttery-like bicyclo [1,1,0]tetraphospha-butane anion (Fig. 1b, I). 7 Electrophiles, on the other hand, should react at an edge of the tetrahedron, as the two energetically degenerate highest occupied molecular orbitals (HOMO and HOMOÀ1) have large coefficients at two adjacent phosphorus atoms (Fig. 1a). The situation is, however, much more complicated and the formation of various products is usually observed in this seemingly simple reaction. In the case of Ph 2 P + and NO + , the insertion of these small molecules into one of the P-P bonds is indeed observed, as also theoretically predicted for NO + (Fig. 1b, II). [9][10][11] In the case of Ag + as an example of an electrophilic transition metal center, a weak coordination of Ag + to the edge of the P 4 tetrahedron occurs (Fig. 1b, III). 12 However, depending on the steric demand of the metal fragment, the coordination of P 4 via the apex can be enforced, even though the interaction of the energetically low-lying HOMOÀ5 with the electrophile is necessary to achieve this coordination mode (Fig. 1b, IV). 13 In contrast to the experimental observations made for the reaction of P 4 with nucleophiles as well as coordinatively and electronically unsaturated transition metal complexes, experimental proof for the structure of the elusive [P 4 H] + cation in solution is still missing in the literature. In fact, weak acids do not react with P 4 due to the rather poor nucleophilicity and weak basicity of white phosphorus. Common strong acids, such as sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ) cannot be used for the generation of [P 4 H] + as they directly oxidize P 4 to either phosphorous acid (H 3 PO 3 ) and sulfur dioxide (SO 2 ), or to phosphoric acid (H 3 PO 4 ), nitrogen oxide (NO 2 ) and water (H 2 O), respectively. Hydrogen chloride (HCl) can react with P 4 to form phosphine gas (PH 3 ) and phosphorus trichloride (PCl 3 ). Based on ab initio calculations, Fluck et al. 14 predicted in 1979 that the weakly bound proton in [P 4 H] + is located at the apex of the tetrahedron (Fig. 1b, V), while protonation at the edge was predicted to be energetically less favored (Fig. 1b, VI). The authors exclude protonation at the P 3 -face. More recent ab initio molecular orbital calculations at the MP2/6-31G(d,p) level of theory in 1996 by Abboud, Yáñez and co-workers reveal, however, that the thermodynamically most favourable process is the protonation at the edge under formation of a three-center two-electron (3c-2e) P-H-P bond (Fig. 1b, VI). 15 The same group determined the gas-phase basicity of P 4 by means of Fourier transform ion cyclotron resonance mass spectrometry. In 2000, Ponec and co-workers provided an additional theoretical support for the existence of a non-classical 3c-2e P-H-P bond in [P 4 H] + using the generalized population analysis. 16 More recently, Lobayan and Bochicchio used a topological analysis of the electron density to describe the 3c-2e P-H-P bond in [P 4 H] + . 17

Results
Taking the above-mentioned considerations into account, we anticipated that strong acids of conjugated weakly coordinating and non-reactive anions should be excellent reagents for the protonation of P 4 . Reed and Nixon, for instance, have shown that phosphabenzenes can be protonated by the in situ generated Brønsted superacid H(CHB 11 Me 5 Br 6 ). 18 Also these phosphorus heterocycles are known for their extremely weak basicity. As one of us 19 has recently reported a novel aluminumbased superacidic system containing the weakly coordinating anion [Al(OTeF 5 ) 4 ] À , we report here now the synthesis and the rst spectroscopic proof on the structure of [P 4 H] + in solution.
The reaction product of P 4 and the Brønsted superacid was obtained as a temperature-, moisture-and oxygen-sensitive salt. It shows a clean low-temperature proton-coupled 31 P NMR spectrum with two equally intense signals at d ¼ À481.7 and d ¼ À405.8 ppm with a weak roof effect (Fig. 2a). No other signals were observed in the 31 P NMR spectrum in the region between d ¼ 400 ppm and d ¼ À800 ppm.
This spectrum, which also reveals an additional splitting owing to the higher order of the system, is in accordance with an AX 2 Y 2 spin system ( 1 J( 31 P X , 31 P Y ) ¼ 233.95 Hz, 1 J( 1 H A , 31 P Y ) ¼ 36.70 Hz, 2 J( 31 P X , 1 H A ) ¼ 4.91 Hz). This can only result from the protonation of the P 4 molecule at the P-P-edge. Furthermore, a triplet of triplets at d ¼ À5.35 ppm appears in the 1 H NMR spectrum (Fig. 2c) showing the corresponding couplings of the proton to P X and P Y , respectively. Interestingly, both the chemical shis and coupling constants are in excellent agreement with the simulated spectra of [P 4 H] + (edge) , obtained by quantum-chemical calculations (Fig. 2b and c and S3, Table S1 ‡). For comparison reasons, Fig. 2d shows the simulated 31 P NMR spectrum of the species [P 4 H] + (apex) , which clearly differs from the experimental results. The NMR studies clearly prove the presence of [P 4 H][Al(OTeF 5 ) 4 ] and that P 4 is protonated at an edge of the tetrahedron as predicted by quantum-chemical calculations. [15][16][17] We further started to investigate the dynamics of the cation in solution. Interestingly, variable temperature NMR spectroscopy indicates a coalescence of the signals at T ¼ À10 C (Fig. 3).
The triplet of triplets observed at T ¼ À40 C in the 1 H NMR spectrum broadens with increasing temperature resulting in a broad singlet (approx. FWHM ¼ 75 Hz) at the coalescence temperature. The chemical shi slightly changes from d ¼ À5.35 ppm at T ¼ À40 C to d ¼ À5.19 ppm at T ¼ 0 C. In the 31 P NMR spectrum, a similar process is observed. The two signals are broadened at the coalescence temperature and shied to a higher eld by 0.5 ppm for P X and P Y at T ¼ 0 C. This process is reversible by re-cooling the sample to T ¼ À40 C again. No signal for P 4 is detected during this process. Based on these observations, we anticipate a dynamic intramolecular migration of the proton on the P 4 surface. From the experimental dynamic-NMR data, the corresponding barrier can be estimated to DG ‡ ¼ 54.2 kJ mol À1 .
We noticed, that if an excess of P 4 is present in the reaction mixture, the previously described [P 9 ] + cation 20 is formed next to [P 4 H] + . Upon warming a sample containing a mixture of [P 4 H] + and P 4 from T ¼ À40 C to T ¼ À10 C, a fast and full conversion to [P 9 ] + is observed, as detected by NMR spectroscopy. This observation indicates that activation of the P 4 molecule by protonation already occurs at low temperature, while broad band UV/Vis irradiation is necessary to form [P 9 ] + from a P 4 / [P 4 NO] + mixture, as reported in the literature before. 10 The [P 4 H] + cation was further analyzed by means of mass spectrometry. In the mass spectrum (positive mode), a signal allocated to [P 4 H] + appears at m/z ¼ 124. 9 4 ] by means of Raman spectroscopy both in an o-DFB solution at T ¼ À30 C and as a neat powder at T ¼ À78 C in the solid state (Fig. 4). In both spectra, two prominent bands can be observed. The band aroundñ ¼ 1615 cm À1 corresponds to the symmetrical P-H-P stretching mode and the band atñ ¼ 598 cm À1 occurs slightly   shied with respect to the breathing mode of neat P 4 and is assigned to the corresponding mode of [P 4 H] + . Both the experimental band positions agree well with the computed wavenumbers at the B3LYP/def2-TZVPP level of theory atñ ¼ 1569 cm À1 (A 1 ) andñ ¼ 584 cm À1 (A 1 ), respectively. Further Raman bands are predicted betweenñ ¼ 360 cm À1 and n ¼ 476 cm À1 but they are difficult to assign in the experimental spectrum due to their rather low intensities and their partial interference with the bands of [P 9 ] + impurities. It should be pointed out that special care must be taken by isolating [P 4 H][Al(OTeF 5 ) 4 ] as a solid, as one sample exploded during the Raman measurement at dry ice temperature aer approx. 300 scans at 75 mW, Fig. 4b. Attempts to record low-temperature IR spectra of the [P 4 H] + cation were unsuccessful, as the strongest IR band of [P 4 H] + is hidden by very prominent bands of the anion atñ ¼ 713 cm À1 andñ ¼ 695 cm À1 . Also, the strongest band of o-DFB occurs atñ ¼ 750 cm À1 . Nevertheless, low temperature (T ¼ À30 C) IR spectra of the liquid phase of [P 4 H][Al(OTeF 5 ) 4 ] have been recorded using a glass ber ATR head, which are provided in Fig. S10 and S11. ‡ Our quantum-chemical calculations at the coupled-cluster CCSD(T)/aug-cc-pVTZ level agree very well with the experimental results of the protonation of a P 4 edge. This position is also expected from the MO diagram, where the HOMO orbital is located along the P 4 edge (Fig. 1a), leading to a three-center twoelectron P-H-P bond. This gives rise to a C 2v symmetric structure with an elongation of the P Y /P Y bond of 20.4 pm compared to the bond length of 221.8 pm in the P 4 tetrahedral structure. The bond distances P Y -P X and P X -P X are less affected by 1.4 and 6.3 pm, respectively (Fig. 5). The computed minimum structure for protonation at the apex of the P 4 molecule is 61.4 kJ mol À1 higher in energy than the global minimum structure. An apex protonation would also lead to a computed P-H stretching mode atñ ¼ 2502 cm À1 , which is more thanñ ¼ 900 cm À1 above the experimentally observed band atñ ¼ 620 cm À1 . Surprisingly, the protonation and simultaneous opening of the tetrahedral structure lead to a P 4 -buttery type minimum structure (Fig. 5), while protonation of the triangle surface shows a higher order saddle point. Both structures will be higher in energy by 74.3 and 88.5 kJ mol À1 compared to the global minimum structure of [P 4 H] + , see Fig. 5. For details of the computed structural parameters see Tables S2 and S3. ‡

Conclusions
Based on these results, we could reveal for the rst time the structure of protonated white phosphorus in solution. Both experimental results and quantum-chemical calculations provide evidence for a protonation at the edge of the P 4 molecule. The opening of the P 4 -tetrahedron via the simplest electrophile (H + ) under formation of a three-center two-electron P-H-P bond is of fundamental interest for understanding the reactivity of this intriguing phosphorus allotrope. It is expected that this groundbreaking result is important for the development of chemical processes related to the activation and further functionalization of elemental phosphorus by electrophiles.

Conflicts of interest
There are no conicts to declare.