Easy hydrolysis of white phosphorus coordinated to ruthenium

Abstract

The P₄ molecule bound to ruthenium as an η¹-ligand in [CpRu(PPh₃)₂(η¹-P₄)]Y (Y = PF₆, CF₃SO₃) undergoes an easy reaction with water in exceedingly mild conditions to yield PH₃, which remains coordinated to the [CpRu(PPh₃)₂] fragment, and oxygenated derivatives.

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In recent years the chemistry of white phosphorus in the presence of transition-metal fragments has been widely investigated and many compounds containing P_x units originating either from the degradation of the P₄ tetrahedron or reaggregation of fragments thereof have been synthesized and characterized.¹ The compounds, besides having unique geometric and electronic properties,² have also been found to be useful building blocks for networks of inorganic coordination compounds.³ Previous studies have shown that it is easier to cleave bond(s) of the cage than attaining coordination by the intact P₄ molecule.^2,4 Indeed, until some years ago, only a few compounds, thermally unstable and difficult to handle, containing the intact molecule had been described.^5,6 Accordingly, the reactivity of the coordinated molecule had not been investigated. Recently rhenium,⁷ iron⁸ and ruthenium⁸ complexes containing the intact P₄ molecule bound to the metal have been described. These tetrahedro-tetraphosphorus complexes are readily obtained and possess stability and solubility properties that make them suitable as starting materials for investigating the reactivity of the coordinated P₄ molecule and comparing the chemistry of the coordinated ligand with the free, highly reactive, molecule. Such a topic is of particular interest in view of the central role of the P₄ molecule in the synthesis of organophosphorus derivatives.⁹ The P₄ allotrope, which is activated only in harsh conditions,¹⁰ is in fact the starting material for the production of a large variety of organophosphorus compounds which rival the chemistry of carbon in terms of complexity and applications.⁹

Herein, we describe the synthesis and characterization of new tetraphosphorus complexes [CpRu(PPh₃)₂(η¹-P₄)]Y (Y = PF₆2a, CF₃SO₃2b) and report on the astonishing hydrolysis of the coordinated tetraphosphorus molecule, which takes place under exceedingly mild conditions to yield one equivalent of the phosphine complexes [CpRu(PPh₃)₂(PH₃)]Y (Y = PF₆3a, CF₃SO₃3b).

Reaction of [CpRu(PPh₃)₂Cl] (1) under argon in CH₂Cl₂–THF with one equivalent of P₄ in the presence of chloride scavengers (TlPF₆ or AgCF₃SO₃), leads to precipitation of TlCl or AgCl and coordination of the P₄ molecule (Scheme 1) to yield the orange [CpRu(PPh₃)₂(η¹-P₄)]Y complexes in excellent yield (>95%).† The compounds exhibit an exceptional stability in comparison to the few known η¹-P₄ derivatives^5,6 as well as to the free phosphorus molecule and, in this regard, they behave as the related Cp* analogous derivatives.⁸ The P₄ molecule remains firmly coordinated to the metal in solution yielding a temperature-invariant and anion independent first-order A₂FM₃ spin pattern in the ³¹P NMR spectra. The four phosphorus atoms of the cage yield the FM₃ part of the spectrum with the P_F atom coordinated to the metal featuring a quartet of triplets, due to the coupling to P_M and P_A, respectively, significantly shifted downfield with respect to the signal of the free P₄ (δ −526.9 ppm); the three naked P atoms yield a doublet at −487.0 ppm. The chemical shift of the naked P_M atoms is similar to that shown by [Cp*Ru(dppe)(η¹-P₄)]BPh₄, whereas the signal of the coordinated phosphorus (δ −348.2) is shifted at lower frequency with respect to that of the Cp* derivative (δ −308.46).⁸ The X-ray structure of the complex cation in 2a is shown in Fig. 1.‡ The P–P distances formed by the coordinating phosphorus atom are consistently shorter by an average of 0.043 Å than those among the distal P atoms, similarly to what has been found for the [Cp*Fe(dppe)(η¹-P₄)]⁺ cation⁸ and, to a lesser extent, for the [W(CO)₃(PCy₃)₂(η¹-P₄)] complex.⁶

Fig. 1 View of the [CpRu(PPh₃)₂(η¹-P₄)]⁺ cation in 2a. In this and the following ORTEP diagram thermal ellipsoids are at the 20% probability level and hydrogen atoms are not shown for clarity. Selected bond lengths (Å) and angles (°): Ru–P1 2.364(1), Ru–P2 2.360(1), Ru–P3 2.269(2), P3–P4 2.154(2), P3–P5 2.145(2), P3–P6 2.141(2), P4–P5 2.180(2), P4–P6 2.200(3), P5–P6 2.189(3); P1–Ru–P2 104.54(4), P1–Ru–P3 94.70(5), P2–Ru–P3 90.10(5).

Scheme 1

When a solution of 2a or 2b in acetone or THF is treated with excess water (1 ∶ 100), unexpected hydrolysis of the P₄ ligand occurs at room temperature, affording [CpRu(PPh₃)₂(PH₃)]Y (Y = PF₆3a, CF₃SO₃3b) in substantially quantitative ruthenium yield within 1 h.§ The reaction, intriguing in view of the well known stability of elemental phosphorus in water, is promoted by subtle changes in the properties of the metal fragment from those of [Cp*Ru(dppe)(η¹-P₄)]PF₆, whose solutions are indefinitely stable at room temperature in the presence of water.⁸ The single P-atom hydrogenation leading to 3a and 3b is accompanied (as revealed by NMR in solution) by formation of H₃PO₃ (ca. 100% with respect to 3a or 3b), while the rest of the P₄ atoms yield a red solid of difficult characterization, featuring a broad ³¹P NMR resonance at ca. 43 ppm in CD₂Cl₂. The nature of this solid, which contains hydrogen, phosphorus and oxygen in a ca. 1 ∶ 1 ∶ 2.5 ratio, according to XPS and elemental analysis, has not been clarified by mass spectrometry and variable-temperature 1D- and 2D-NMR measurements. The strict similarity in reactivity of 2a and 2b rules out any involvement of the anion in the hydrolysis. Complexes 3a and 3b represent rare examples of metal PH₃ derivatives.^11–17 A view of the complex cation [CpRu(PPh₃)₂(PH₃)]⁺, which was authenticated by X-ray diffraction analysis,‡ is reported in Fig. 2. The 2.283(1) Å Ru–P(PH₃) distance in 3a is at the low end of the 2.25–2.49 Å range of values found for distances formed by the PH₃ ligand with various metal atoms. The P–H distance in 3a of 1.27(2) Å is at the short end of the range of P–H distances recently reported (1.3–1.5 Å).^15–17

Fig. 2 Structure of the [CpRu(PPh₃)₂(PH₃)]⁺ cation in 3a. Selected bond lengths (Å) and angles (°): Ru–P1 2.283(1), Ru–P2 2.357(1), Ru–P3 2.335(1), P1–H 1.27(2); P1–Ru–P2 90.53(3), P1–Ru–P3 95.43(3), P2–Ru–P3 100.59(3), H–P1–H 94(2).

Complex 3a may be independently synthesized in almost quantitative yield by reaction of 1 in THF with gaseous PH₃ in the presence of TlPF₆.¶

Studies are in progress to highlight the reaction mechanism, the nature of the red solid accompanying the formation of 3a and 3b, and the scope of this surprising reaction, by extending the reactivity studies of 2a and 2b towards other nucleophiles. In this respect, preliminary tests confirm that methanol and other simple reagents may also easily react with coordinated P₄, thus paving the way to still unexplored paths of phosphorus chemistry.

Notes and references

1. K. H. Whitmire, Adv. Organomet. Chem., 1998, 42, 2; O. J. Scherer, Acc. Chem. Res., 1999, 32, 751; M. Peruzzini, I. de los Rios, A. Romerosa and F. Vizza, Eur. J. Inorg. Chem., 2001, 593; M. P. Ehses, A. Romerosa and M. Peruzzini, Top. Curr. Chem., 2001, 220, 107.
2. E. Urnèžius, W. W. Brennessel, C. J. Cramer, J. E. Ellis and P. von Ragué Schleyer, Science, 2002, 295, 832 , and references therein.
3. J. Bai, A. V. Virovets and M. Scheer, Science, 2003, 300, 781; J. Bai, A. V. Virovets and M. Scheer, Angew. Chem., Int. Ed., 2002, 41, 1737; J. Bai, E. Leiner and M. Scheer, Angew. Chem., Int. Ed., 2002, 41, 783.
4. A. P. Ginsberg and W. E. Lindsell, J. Am. Chem. Soc., 1971, 93, 2082; O. J. Scherer, M. Swarowsky and G. Wolmershäuser, Organometallics, 1989, 8, 841; F. Cecconi, C. A. Ghilardi, S. Midollini and A. Orlandini, Inorg. Chem., 1986, 25, 1766.
5. P. Dapporto, S. Midollini and L. Sacconi, Angew. Chem., Int. Ed. Engl., 1979, 18, 469; P. Dapporto, L. Sacconi and P. Stoppioni, Inorg. Chem., 1981, 20, 3834; M. Di Vaira, M. P. Ehses, M. Peruzzini and P. Stoppioni, Eur. J. Inorg. Chem., 2000, 2193.
6. T. Gröer, G. Baum and M. Scheer, Organometallics, 1998, 17, 5916.
7. M. Peruzzini, L. Marvelli, A. Romerosa, R. Rossi, F. Vizza and F. Zanobini, Eur. J. Inorg. Chem., 1999, 931.
8. I. de los Rios, J.-R. Hamon, P. Hamon, C. Lapinte, L. Toupet, A. Romerosa and M. Peruzzini, Angew. Chem., Int. Ed., 2001, 40, 3910.
9. D. E. C. Corbridge, Phosphorus, an Outline of its Chemistry Biochemistry and Technology, Elsevier, Amsterdam, 5th edn., 1995; K. B. Dillon, F. Mathey and J. F. Nixon, Phosphorus: the Carbon Copy, Wiley, Chichester, 1998.
10. L. Maier, Top. Curr. Chem., 1971, 19, 95 (Eur. Pat., WO9943612, 1999).
11. G. Huttner and S. Schelle, J. Cryst. Mol. Struct., 1971, 1, 69.
12. G. Huttner and S. Schelle, J. Organomet. Chem., 1973, 47, 383.
13. J. Bould, P. Brint, X. L. R. Fontaine, J. D. Kennedy and M. Thornton-Pett, J. Chem. Soc., Chem. Commun., 1989, 1763.
14. A. J. Deeming, S. Doherty, J. E. Marshall, J. L. Powell and A. M. Senior, J. Chem. Soc., Dalton Trans., 1993, 1093.
15. R. P. Hughes, J. M. Smith, C. D. Incarvito, K.-C. Lam, B. Rhatigan and A. L. Rheingold, Organometallics, 2002, 21, 2136.
16. G. Frenking, K. Wichmann, N. Frohlich, J. Grobe, W. Golla, D. Le Van, B. Krebs and M. Lage, Organometallics, 2002, 21, 2921.
17. U. Vogel and M. Scheer, Z. Anorg. Allg. Chem., 2001, 627, 1593.

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Footnotes

† A solution of [CpRu(PPh₃)₂Cl] (1) (2.00 g, 2.75 mmol) and TlPF₆ (960 mg, 2.75 mmol) in a mixture of CH₂Cl₂ (90 cm³) and THF (140 cm³) was added at room temperature to a solution of white phosphorus (380 mg, 3.07 mmol) in THF (30 cm³) under argon. The resulting slurry was stirred at room temperature for 4 h; the precipitated TlCl was filtered off and [CpRu(PPh₃)₂(η¹-P₄)]PF₆2a was obtained as orange microcrystals by evaporating the solvent under reduced pressure (2.50 g, 95%). The solid was recrystallized from CH₂Cl₂–hexane (Found: C 51.3; H, 3.8. C₄₁H₃₅F₆P₇Ru requires C 51.3; H 3.7%). 2b was obtained through the same procedure as 2a using AgCF₃SO₃ as chloride scavenger (Found: C 52.1; H, 3.9. C₄₂H₃₅F₃O₃P₆RuS requires C 52.3; H 3.7%). δ_P (121.5 MHz, (CD₃)₂CO, 25 °C, 85% H₃PO₄; complex cation A₂FM₃ pattern) 39.0 (2P, d, ²J(P_AP_F) 64 Hz, P_A), −348.2 (1P, tq, ¹J(P_FP_M) 235 Hz, P_F), −487.0 (3P, d, P_M), −143.10 (1P, sept, ¹J(PF) 714 Hz, PF₆, only for 2a). δ_H (300.0 MHz, (CD₃)₂CO, 25 °C, TMS) 7.60–7.10 (30H, m, Ph), 5.04 (5H, s, Cp).

‡ Crystal data: 2a: C₄₁H₃₅F₆P₇Ru, M = 959.55, monoclinic, space group P2₁/c, a = 11.611(5), b = 14.619(4), c = 24.489(5) Å, β = 94.95(3)°, U = 4141(2) ų, Z = 4, T = 293 K, µ(Mo-Kα) = 0.706 mm⁻¹. 28874 reflections collected, 11949 unique, 6783 (I > 2σ_I), R_int = 0.033. Final residual was R₁ = 0.067 for data with I > 2σ_I and wR₂ = 0.203 for all data. 3a: C₄₁H₃₈F₆P₄Ru, M = 869.66, monoclinic, space group P2₁/c, a = 10.107(1), b = 18.603(2), c = 20.228(3) Å, β = 95.95(3)°, U = 3782.8(8) ų, Z = 4, T = 293 K, µ(Mo-Kα) = 0.643 mm⁻¹. 31410 reflections collected, 12180 unique, 8354 (I > 2σ_I), R_int = 0.024. PH₃ H atom positions refined with one constraint. Final residual was R₁ = 0.055 for data with I > 2σ_I and wR₂ = 0.173 for all data. CCDC reference numbers 262402 (2a) and 262404 (3a). See http://www.rsc.org/suppdata/dt/b5/b504795a/ for crystallographic data in CIF or other electronic format.

§ Distilled water (1.08 g, 60 mmol) was added to a solution of [CpRu(PPh₃)₂(η¹-P₄)]PF₆2a (576 mg, 0.60 mmol) in THF or acetone (50 cm³) and the solution was stirred at room temperature for 2 h under argon. The solvent was removed under reduced pressure and the remaining red solid was extracted twice with acetone (2 × 5 cm³) leaving a reddish solid. [CpRu(PPh₃)₂(PH₃)]PF₆3a was obtained as a yellowish solid by concentrating the acetone solution. The crude 3a was recrystallized from CH₂Cl₂–hexane (420 mg, 80%) (Found: C 56.5; H, 4.5. C₄₁H₃₈F₆P₄Ru requires C 56.6; H 4.4%). 3b was obtained through the same workup as 3a (Found: C 57.5; H, 4.6. C₄₂H₃₈F₃O₃P₃RuS requires C 57.7; H 4.4%). δ_P (121.4 MHz, (CD₃)₂CO, 25 °C, H₃PO₄; complex cation A₂F pattern) 44.7 (2P, d, ²J(P_AP_F) 51 Hz, P_A), −113.2 (1P, t, P_F), −143.1 (1P, sept, ¹J(PF) 714 Hz, PF₆, only for 3a). δ_P (121.5 MHz, (CD₃)₂CO, 25 °C, 85% H₃PO₄; hydrogen coupled, complex cation A₂FX₃ pattern) 44.8 (2P, d, ²J(P_AP_F) 51 Hz, P_A), −113.1 (1P, qt, ¹J(P_FH) 358, P_F), −143.1 (1P, sept, ¹J(PF) 714 Hz, PF₆, only for 2a). δ_H (300.0 MHz, (CD₃)₂CO, 25 °C, TMS) 7.46–7.10 (30H, m, Ph), 4.91 (5H, s, Cp), 4.52 (3H, dt, ¹J(HP_F) 357 Hz, ³J(HP_A) 6 Hz, PH₃). The red solid (60 mg) yields a 1 ∶ 1 ∶ 2.5 ratio between hydrogen, phosphorus and oxygen (XPS and elemental analysis). δ_P (121.4 MHz, CD₂Cl₂, 25 °C, 85 % H₃PO₄) broad band centered at 43.0.

¶ PH₃ was gently bubbled for 5 min through a solution of [CpRu(PPh₃)₂Cl] (730 mg, 1.00 mmol) and TlPF₆ (350 mg, 1.00 mmol) in a mixture of CH₂Cl₂ (20 cm³) and THF (30 cm³). The resulting slurry was stirred at room temperature for 1 h; the precipitated TlCl was filtered off and yellow microcrystals of 3a were obtained by evaporating the solvent under reduced pressure. The solid was recrystallized from CH₂Cl₂–hexane (850 mg, 98%).

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