Synthesis and X-Ray Crystal Structure of the First Pure and Air-Stable Salt of Peroxymonosulphuric Acid: (Ph)4PHSO5

Universita di Padova, Dipartimento di Chimica Organica, Centro CNR di Studio sui Biopolimeri, ViaMarzolo 1, 35131 Padova, ItaliaFax: +39 49 8275239, E-mail: biop02@pdchor.chor.unipd.itReceived: 10 May 2000 / Accepted: 21 June 2000 / Published: 28 June 2000Abstract: In this paper we describe the synthesis of tetraphenylphosphonium peroxy-monosulphate, its crystal structure and packing mode. The asymmetric unit accomodates twoindependent molecules of the monopersulphate anion, which are held together by hydrogenbonds. In the packing mode, rows of such dimers are surrounded by four rows of tetraphenylcations. The consequence is that the highly water sensitive HSO


Introduction
Peroxymonosulphuric acid (1) (Caro's acid) [1] is a powerful oxidant (E o +1.82 V) [2] and it has been used in the preparation of a large variety of compounds.As an example, the syntheses of esters from ketones, glycols from olefins, iodoxybenzene from iodobenzene, and nitrosylcyclohexane from cyclohexylamine have been accomplished [3].Utilization of Caro's acid is hampered by its great instability.In fact, pure Caro's acid is a highly hygroscopic and explosive solid [4].Owing to these features, X-ray data for the pure Caro's acid have been obtained only quite recently at -130 o C [5].Furthermore, even pure and air stable salts of Caro's acid have never been reported up to now.While the mechanism of Caro's acid decomposition in aqueous solutions has been established [6], no data are available on the decomposition mechanism in the solid state.On the other hand, it may be envisaged that crystals hydration plays a role, thus triggering the decomposition within a highly concentrated solution of (1).The first stable derivative of Caro's acid prepared was the water soluble mixed salt (2), which is commercially available under the names of Oxone ® , Caroat ® or Curox ® .The X-ray structure of KHSO 5 in (2) [7] and then in (KHSO 5 )(H 2 O) [8]  (2), likewise (1), is a powerful oxidant with a wide range of applications.As an example, triple salt ( 2) is used for both the oxidation of water-soluble substrates [9] and for oxidations catalyzed by metal porphyrins in two-phase systems [10].Moreover, olefin epoxidations and alkane hydroxylations by (2) via the formation of a intermediate dioxirane have been accomplished [11].In order to extend the use of monopersulphate to organic solvents, the lipophilic salt (3) has been prepared by extracting aqueous solutions of (2) with organic solutions of tetrabutylammonium inorganic salts [12].However, compound ( 3) is never obtained with purity higher than 70-80% and its gum-like consistence prevents its further purification from sulphate and bisulphate salts, which always accompany organic extracts of (2).The presence of these contaminants greatly affects the monopersulphate anion reactivity.In fact, the high acidity of (Bu) 4 NHSO 4 inhibits the olefins epoxidation by (3) catalysed by metalloporphyrins [13].
In this paper we present the synthesis and the X-Ray structure of a stable lipophilic salt of Caro's acid, i.e. tetraphenylphosphonium monopersulphate (4), which can be obtained in pure form.Compound (4) is insoluble in water whereas it is fairly soluble in chlorinated organic solvents.Moreover, it is absolutely air stable.Owing to its stability and fair solubility in organic solvents, (4) is particularly suitable as oxygen donor for the mechanistic study of oxidative processes, for instance those involving metal complexes as catalysts.As an additional advantage, (4) can be obtained in crystalline form, allowing its purification simply by crystallization from a suitable solvent.The structure of compound (4) and its packing arrangement provide useful hints to rationalise its exceptional stability in the solid state.

Infrared Absorption
Spectra were recorded averaging 50 scans at 2 cm -1 nominal resolution on a Perkin-Elmer 1720X FTIR spectrometer, nitrogen flushed.For the solution spectra, CaF 2 cells with pathlength of 0.1 and 1.0 mm were used.In the solid state (KBr pellet), the O-H stretching falls at 3272 cm -1 , consistently with the H-bonding observed in the crystal structure.In 1,2-dichloroethane solution at 50 mM concentration, the O-H group give rise to two bands, at 3457 cm -1 (free O-H) and at 3224 cm -1 (H-bonded O-H).The intensity of the latter band sharply decreases upon dilution to 10 mM concentration, to eventually vanish at 1.0 mM concentration.

Conclusions
The segregation of the highly water sensitive monopersulphate anions inside the hydrophobic channels generated by tetraphenylphosphonium cation rows nicely accounts for the remarkable stability of (4).However, when (4) is dissolved in 1,2-dichloroethane, free monopersulphate anions are released in solution.In fact, infrared spectra of diluted solution of (4) (up to 1.0 mM) show only the absorption at 3457 cm -1 corresponding to free OH.Conversely, infrared spectra in the solid state show the OH stretching at 3272 cm -1 consistently with the H-bonding observed in the crystal structure.In conclusion, compound (4) represent a perfectly stable font of pure monopersulphate anions, which may be used for both synthetic and mechanistic purposes.

Synthesis and purification of Ph 4 PHSO 5
To a solution of Oxone ® (4.0 gr, 6.5 mmol), in deionized water (40 mL) a solution of tetraphenylphosphonium chloride (2.0 gr, 5.3 mmol), in distilled dichloromethane (40 mL) was added under vigorous stirring for 3 min.The organic phase was then separated and the crude product recovered after evaporation of the solvent.The crude material was washed with cold water (15 mL), dried under vacuum (0.05 mm Hg), and then dissolved in distilled dichloromethane (40 mL).The resulting turbid solution was filtered on paper and the solvent removed by rotavapor.The product was dissolved again in distilled dichloromethane (20 mL) and n-pentane was added dropwise until the solution becomes opaque.The solution was then frozen overnight in order to complete precipitation of the product.Crystals were filtered on a n. 3 Gooch filter and dried under vacuum (0.05 mm Hg).Ph 4 PHSO 5 (1.3 gr, 54% yield) was thus obtained with a purity better than 97% (determined by iodometric titration).Ph 4 PHSO 5 crystals are stable upon warming until 175 o C when they become opaque with crackling.With continuos warming, a melting point at 272-273 o C is observed.CAUTION: this peroxide should be considered as potentially explosive and despite a number of safe syntheses we never surpassed this preparation scale.

Figure 1 .
Figure 1.A view of the two independent molecules of (4) in the asymmetric unit.

Figure 2 .
Figure 2. Packing mode in the X-ray structure of (4) as viewed down the c axis.
confirmed the existence of a short and non symmetrical O-O bond (1.460A o compared to 1.453 in H 2 O 2 and 1.497 in S 2 O 8 2-) with a hydrogen atom on one side and SO 3 -group on the other side.The triple salt