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Molecules 2000, 5(6), 886-894;

Synthesis and X-Ray Crystal Structure of the First Pure and Air-Stable Salt of Peroxymonosulphuric Acid: (Ph)4PHSO5
Università di Padova, Dipartimento di Chimica Organica, Centro CNR di Studio sui Meccanismi di Reazioni Organiche, Via Marzolo 1, 35131 Padova, Italia
Università di Padova, Dipartimento di Chimica Organica, Centro CNR di Studio sui Biopolimeri, Via Marzolo 1, 35131 Padova, Italia
Author to whom correspondence should be addressed.
Received: 10 May 2000 / Accepted: 21 June 2000 / Published: 28 June 2000


In this paper we describe the synthesis of tetraphenylphosphonium peroxymonosulphate, its crystal structure and packing mode. The asymmetric unit accomodates two independent molecules of the monopersulphate anion, which are held together by hydrogen bonds. In the packing mode, rows of such dimers are surrounded by four rows of tetraphenyl cations. The consequence is that the highly water sensitive HSO5 anions are segregated inside hydrophobic channels composed by the lipophilic cations. This circumstance presumably accounts for the exceptional stability of the title compound.
Monopersulphate; X-Ray Crystallography; Oxidation


Peroxymonosulphuric acid (1) (Caro’s acid) [1] is a powerful oxidant (Eo +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].
H2SO5 2(KHSO5)(KHSO4)(K2SO4) (Bu)4NHSO5 Ph4PHSO5
1 2 3 4
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°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 KHSO5 in (2) [7] and then in (KHSO5)(H2O) [8] confirmed the existence of a short and non symmetrical O-O bond (1.460 Ao compared to 1.453 in H2O2 and 1.497 in S2O82-) with a hydrogen atom on one side and SO3 group on the other side. The triple salt (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)4NHSO4 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.

Results and Discussion

Crystal Data and Structure

Single crystals of (4) were grown from 1,2-dichloroethane by diethyl ether vapor diffusion, at room temperature. The crystals are stable without mother liquor, even when exposed to the air for several weeks. Crystals are triclinic, space group P-1, with a = 13.976(2), b = 14.672(3), c = 11.388(2) Å, α = 91.1(1), β = 109.6(1), γ = 81.5(1)°; V = 2174.5(7) Å3, M = 452.4 (HSO5 x PC24H20); Z = 4; Dcalc = 1.382 Mg/m3. A total of 6451 independent reflections were collected on a Philips PW 1100 diffractometer, using graphite-monochromated CuKα radiation (λ = 1.54184 Å) in the θ-2θ scan mode up to θ=60° with variable scan speed ranging from 1.5 to 0.375 deg/min. Index ranges: –15 ≤ h ≤ 14, from – 16 ≤ k ≤ 16, 0 ≤ l ≤ 12. An absorption correction based on psi-scan was applied to the data (max. and min. transmission factors 1.000 and 0.626, respectively, for a crystal of approximate dimension 0.20 x 0.20 x 0.10 mm and μ = 2.307 mm−1).
The structure was solved by direct methods of SHELXS 86 program [14] and refined by full-matrix least-squares on F2, using all data, with the SHELXL 93 program [15]. The non-H atoms were anisotropically refined. A planarity restraint was applied to all phenyl rings. H-atoms were calculated at idealized position, and during the refinement they were allowed to ride on their carrying atom. The analysis of possible intermolecular H-bonds in the structure indicated that, in principle, the H-atom of each monopersulphate molecule could be covalently bonded either to one of the terminal oxygen atoms or to the peroxo moiety. The latter assignment was assumed to be the correct one, on the basis of the relative acidities of the SOH and SOOH groups (pKa SOH <0; pKa SOOH =9.4) [16]. The refinement converged to R1 = 0.0677 [on F ≥ 4(σ)F] and wR2 = 0.2045 (on F2, all data). The data / restraints / parameters ratio was 6451 / 24 / 559. Fractional atomic coordinates of the non-H atoms, along with their equivalent isotropic displacement parameters, are listed in Table 1. Bond distances and bond angles of (4) are reported in Table 2 and Table 3, respectively.
The asymmetric unit is composed of two independent molecules of monopersulphate (A and B) and two of the tetraphenylphosphonium cation (see Fig. 1).
The three terminal S-O bond distances range from 1.396(6) to 1.458(6) Å at S1 (monopersulphate A), while from 1.421(5) to 1.445(5) Å at S2 (monopersulphate B). The S-O (peroxo) distance is 1.588(6) Å at S1 and 1.584(6) Å at S2. These values are in general agreement with those reported by Flanagan et al. [7] for HSO5 in (2). The O-O distances are 1.378(7) and 1.442(7) Å, in the two independent molecules A and B, respectively. In the crystal, each monopersulphate is intermolecularly Hbonded to a centrosymmetric counterpart. More specifically, the O5-H group of monopersulphate molecule A is H-bonded to O1 atom of its (1-x, 1-y, -z) symmetry equivalent, while in the monopersulphate molecule B the O10-H group is H-bonded to the O6 atom of its (-x, -y, 2-z) symmetry equivalent. The geometry of these rather strong H-bonds is defined by the following parameters: distance O5...O1 2.659(9) Å, distance HO5...O1 2.15 Å, angle O5-HO5...O1 120°; distance O10...O6 2.679(9) Å, distance HO10...O6 2.18 Å, angle O10-HO10...O6 119°. Because of the symmetry, such H-bonds give rise to dimers of each monopersulphate. In the packing mode, rows of monopersulphate dimers are formed along the z direction, each row being surrounded by four rows of tetraphenylphosphonium cations (see Fig. 2).
Additional stabilization of the structure may be attributed to some (phenyl)C-H...O(monopersulphate) interactions. By taking the H...O distance ≤ 2.8 Å as the cutoff criterion [17], seven C-H...O contacts have been detected for monopersulphate A and eight for monopersulphate B, with C...O distances ranging from 3.140(9) to 3.656(10) Å, and C-H--O angle ≥ 120°.

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, CaF2 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.


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 Ph4PHSO5

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). Ph4PHSO5 (1.3 gr, 54% yield) was thus obtained with a purity better than 97% (determined by iodometric titration). Ph4PHSO5 crystals are stable upon warming until 175°C when they become opaque with crackling. With continuos warming, a melting point at 272-273°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.


Financial support of the research project: "Efficient Processes for the Controlled Oxidations of Organic Compounds" by the Italian Ministry of the University and the Scientific Research (MURST) is gratefully acknowledged.

References and Notes

  1. Caro, H.Z. Angew. Chem. 1898, 11, 845.
  2. Balej, J. J. Electroanal. Chim. 1986, 214, 481.
  3. Kennedy, R.J.; Stock, A.M. J. Am. Chem. Soc. 1960, 25, 1901.
  4. Arnau, J.L.; Giguère, P.A. Can. J. Chem. 1970, 48, 3903.
  5. Frank, W.; Bertsch-Frank, B. Angew. Chem. Int. Ed. Engl. 1992, 31, 436.
  6. Ball, D.L.; Edwards, J.O. J. Am. Chem. Soc. 1956, 78, 1125.
  7. Flanagan, J.; Griffith, W.P.; Skapski, A.C. J. Chem. Soc. Chem. Commun. 1984, 1574.
  8. Schlemper, E.O.; Thompson, R.C.; Kay Fair, C.; Ross, F.K.; Appelman, E.H.; Basile, L.J. Acta Cryst. 1984, 1781.
  9. Block, R.; Abecassis, J.; Hassan, D. J. Org. Chem. 1985, 50, 1544.
  10. de Poorter, B.; Meunier, B. J. Chem. Soc. Perkin Trans. 2 1985, 1735.
  11. Mello, R.; Fiorentino, M.; Fusco, C.; Curci, R. J. Am. Chem. Soc. 1989, 111, 6749.
  12. Trost, B.M.; Braslau, R. J. Org. Chem. 1988, 53, 532.
  13. Campestrini, S.; Di Furia, F.; Labat, G.; Novello, F. J. Chem. Soc. Perkin Trans. 2 1994, 2175.
  14. Sheldrick, G.M. SHELXS 86. Program for the solution of crystal structures. University of Göttingen: Germany, 1985. [Google Scholar]
  15. Sheldrick, G.M. SHELXL 93. Program for refining crystal structures. University of Göttingen: Germany, 1993. [Google Scholar]
  16. Curci, R.; Edwards, J.O. Organic Peroxides; Swern, D., Ed.; Wiley-Interscience: New York, 1970; Vol. 1, pp. 199–264. [Google Scholar]
  17. Steiner, T.; Saenger, W. J. Am. Chem. Soc. 1993, 115, 4540.
  • Sample Availability: Available from the authors.
Figure 1. A view of the two independent molecules of (4) in the asymmetric unit.
Figure 1. A view of the two independent molecules of (4) in the asymmetric unit.
Molecules 05 00886 g001
Figure 2. Packing mode in the X-ray structure of (4) as viewed down the c axis.
Figure 2. Packing mode in the X-ray structure of (4) as viewed down the c axis.
Molecules 05 00886 g002
Table 1. Atomic coordinates ( × 104) and equivalent isotropic displacement parameters (Å2 × 103) for (4). Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
Table 1. Atomic coordinates ( × 104) and equivalent isotropic displacement parameters (Å2 × 103) for (4). Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
AtomxyzUeq AtomxyzUeq
S15778(2)5092(2)2327(2)97(1) S2-193(2)259(1)7759(2)81(1)
O16179(4)4688(4)1408(5)107(2) O6-827(3)531(3)8504(5)95(2)
O26181(5)5936(4)2808(6)147(3) O7-612(6)-351(4)6803(6)155(3)
O35728(6)4478(5)3219(7)185(3) O8183(4)1026(4)7369(6)131(2)
O44626(4)5543(4)1607(6)123(2) O9858(4)-262(4)8683(5)113(2)
O54046(4)4902(4)943(6)131(2) O10637(5)-1073(4)9178(6)134(2)
P15463(1)891(1)7658(2)55(1) P2487(1)5857(1)7797(2)56(1)
C15761(5)1437(4)9144(6)53(2) C25869(5)6441(4)9240(6)55(2)
C26675(5)1160(4)10077(6)69(2) C261884(5)6555(4)9732(6)63(2)
C36933(6)1644(5)11167(7)80(2) C272251(6)6982(4)10853(7)75(2)
C46277(6)2397(5)11309(7)79(2) C281582(7)7305(5)11458(7)83(2)
C55356(6)2674(5)10391(7)81(2) C29564(6)7212(4)10987(7)80(2)
C65078(5)2189(4)9288(7)72(2) C30183(5)6757(4)9851(7)72(2)
C76143(4)-258(4)7813(5)55(2) C31854(4)6462(4)6704(6)59(2)
C87212(5)-407(5)8097(6)71(2) C32567(5)7413(5)6582(6)68(2)
C97738(5)-1280(5)8194(6)79(2) C33782(5)7901(5)5703(7)78(2)
C107213(6)-2030(5)7989(7)85(2) C341279(6)7457(6)4961(7)85(2)
C116159(5)-1888(4)7699(7)83(2) C351599(6)6515(6)5106(7)88(2)
C125628(5)-1016(4)7617(6)69(2) C361395(5)6014(5)5991(7)68(2)
C134107(5)877(4)7025(6)53(2) C37-866(5)5835(4)7203(6)59(2)
C143594(5)593(4)7780(6)68(2) C38-1489(5)6287(4)6088(6)69(2)
C152558(5)530(4)7307(8)73(2) C39-2528(6)6212(5)5638(7)86(2)
C162038(5)744(4)6075(8)76(2) C40-2958(6)5714(5)6287(8)88(2)
C172516(5)1030(4)5307(7)77(2) C41-2344(6)5264(5)7399(8)83(2)
C183561(5)1099(4)5778(6)64(2) C42-1309(5)5327(4)7831(6)73(2)
C195800(4)1552(4)6602(6)56(2) C431095(4)4682(4)8005(6)58(2)
C206276(5)1153(5)5805(7)74(2) C441885(5)4368(4)9078(6)71(2)
C216461(6)1661(6)4926(7)89(2) C452342(5)3452(5)9226(7)86(2)
C226167(6)2610(6)4844(7)83(2) C461974(6)2844(5)8296(9)97(3)
C235705(5)3024(5)5644(7)76(2) C471187(6)3156(5)7241(8)96(3)
C245516(5)2509(4)6524(6)64(2) C48745(5)4050(4)7092(7)79(2)
Table 2. Bond lengths [Å] for (4).
Table 2. Bond lengths [Å] for (4).
Table 3. Bond angles [deg] for (4).
Table 3. Bond angles [deg] for (4).
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