Successful oxidation of Ph₂P(CH₂)_nPPh₂ (n = 2, 4, 6) by tellurium leading to Ph₂P(Te)(CH₂)_nP(Te)Ph₂

Abstract

The first successful syntheses of Ph₂P(Te)(CH₂)_nP(Te)Ph₂ (n = 2, 4, 6) via oxidation of Ph₂P(CH₂)_nPPh₂ by tellurium powder are reported. The prepared compounds were characterized by elemental analysis, ³¹P and ¹²⁵Te NMR spectroscopy, and single-crystal X-ray analysis.

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The interest in tellurium compounds is based on their promising applications. Above all metal tellurides are widely used in solar cells¹ or as quantum dots.² One of the possible pathways to prepare metal tellurides is decomposition of single source precursors, which are often coordination compounds of selected main group and transition metals containing [R₂P(Te)NP(Te)R₂]⁻ ligands.³ The growing interest in the chemistry of tellurium compounds can be demonstrated also by an almost twofold increase in the number of publications per year during last decade.⁴

The oxidation of R₂PXPR₂ compounds (X = (CH₂)_n, NH, Fc, etc.; R = alkyl, aryl) by elemental chalcogen leading to R₂P(E)XP(E)R₂ (E = O, S and Se) has been well known for a long time.⁵ Such reactions take place quite easily in high yields, while similar reactions with elemental tellurium have not been described up to now.

To the best of our knowledge, only three reactions of R₂P(CH₂)_nPR₂ (n = 1–6, R = alkyl, aryl) with elemental tellurium were published to date. In two cases, the starting compound was a lithium salt Li[HC(PPh₂)₂]. The first reaction provided the lithium salt of the monotelluro ligand Li[HC(PPh₂Te)(PPh₂)] and subsequent metathesis led to a mercury(II) complex {Hg[HC(PPh₂Te)(PPh₂)]₂}.⁶ However, identification of the complex formation was based on ³¹P NMR spectra only. Afterwards,⁷ another monotelluro derivative, TMEDA·Li[HC(PPh₂Te)(PPh₂)] (TMEDA = N,N,N′,N′-tetramethylethylenediamine), was prepared and structurally characterized. The formation of the ditellurated compound TMEDA·Li[HC(PPh₂Te₂)₂] in the reaction mixture was confirmed spectroscopically (³¹P and ¹²⁵Te NMR).⁷ As mentioned earlier, the tellurated product was not obtained by the oxidation of Ph₂PCH₂PPh₂ by elemental tellurium, but via oxidation of its lithium salt. Recently,⁸ oxidation of iPr₂PCH₂PiPr₂ by tellurium led to monotelluro iPr₂P(Te)CH₂PiPr₂ species. So far, the only structurally characterized ditelluro R₂P(Te)(CH₂)_nP(Te)R₂ compound is Me₂P(Te)(CH₂)₂P(Te)Me₂, which was not prepared by direct oxidation by elemental tellurium, but by a tellurium transfer reaction between Et₃PTe and Me₂P(CH₂)₂PMe₂.⁹

In the case of structurally analogous R₂PNHPR₂ compounds (R = alkyl, aryl), a monotelluro P–H tautomer iPr₂P(Te)NP(H)iPr₂ was successfully prepared by reaction of iPr₂PNHPiPr₂ with elemental tellurium.¹⁰ The synthesis of ditelluro salts TMEDA·Na[R₂P(Te)NP(Te)R₂] was again successful when using the deprotonated [R₂PNPR₂]⁻ species (R = iPr,¹¹ tBu,¹² Ph¹³) as starting materials. However, further effort to synthesize R₂P(Te)NHP(Te)R₂ by direct oxidation of non-deprotonated ligands by elemental tellurium was not successful.^10,13

In this communication we present the first synthesis of two ditelluro Ph₂P(Te)(CH₂)_nP(Te)Ph₂ (n = 2, 6) and one monotelluro Ph₂P(Te)(CH₂)₄PPh₂ compounds by direct oxidation of the starting Ph₂P(CH₂)_nPPh₂ compounds by elemental tellurium.

In a typical synthesis, 100 mg of Ph₂P(CH₂)_nPPh₂ were dissolved in 10 cm³ of toluene. Then elemental tellurium was added to the solution (a molar ratio of Te : Ph₂P(CH₂)_nPPh₂ = 2 : 1) and the resulting suspension was stirred at 50 °C for a couple of hours. Unreacted tellurium was filtered off and the filtrate was cooled down to −27 °C. After the precipitation of a white crystalline compound, which was identified as a starting compound, 20 cm³ of petroleum ether (bp 40–60 °C) was added to the clear filtrate. The mixture was thoroughly mixed and cooled to −27 °C again. After a few days, yellow crystals of Ph₂P(Te)(CH₂)₂P(Te)Ph₂ (1), Ph₂P(CH₂)₄P(Te)Ph₂ (2) and Ph₂P(Te)(CH₂)₆P(Te)Ph₂ (3) were obtained in yields below 6%. These significantly lower values than those reported for alkyl analogues (51% in the case of ditelluro Me₂P(Te)(CH₂)₂P(Te)Me₂⁹ and 98% in the case of monotelluro iPr₂P(Te)CH₂PiPr₂ (ref. 8) species) were expected due to the lack of reactivity of the Ph₂P-moiety towards tellurium.¹³

Although the prepared compounds 1, 2 and 3 are air-sensitive (the decomposition can be prevented when stored at low temperature in inert atmosphere), they could be characterized by elemental analysis, ³¹P and ¹²⁵Te NMR spectroscopy, and single-crystal X-ray analysis.‡

The melting points determination‡ ended with decomposition of the compounds and amorphous Te was obtained as a final product. Obviously the P–Te bond can be easily broken by heat and potential coordination compounds could serve as precursors of metal tellurides.

The geometries of the molecules of 1 (Fig. 1), 2 (Fig. 2) and 3 (Fig. 3) are similar, the only difference being the number of CH₂ moieties connecting the phosphorus atoms in P1–(CH₂)_n–P2 motifs.

Fig. 1 ORTEP drawing of the molecular structure of Ph₂P(Te)(CH₂)₂P(Te)Ph₂ (1). Thermal ellipsoids are shown at the 50% probability level.

Fig. 2 ORTEP drawing of the molecular structure of Ph₂P(CH₂)₄P(Te)Ph₂ (2). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and positionally disordered atoms with minor occupancy have been omitted for clarity.

Fig. 3 ORTEP drawing of the molecular structure of Ph₂P(Te)(CH₂)₆P(Te)Ph₂ (3). Thermal ellipsoids are shown at the 50% probability level.

The P–Te bond lengths (the overview is shown in Table 1) are in good agreement with those of alkyl analogues Me₂P(Te)(CH₂)₂P(Te)Me₂ (ref. 9) and iPr₂P(Te)CH₂PiPr₂.⁸

Table 1 Bond lengths P–Te

	Bond length P–Te/Å
a The difference in P–Te bond lengths is commented in ESI.
Ph2P(Te)(CH2)2P(Te)Ph2 (1)	2.3629(8) (P1–Te1)
Ph2P(CH2)4P(Te)Ph2 (2)	2.3491(9) (P1–Te1A), 2.2416(14) (P2–Te1B)^a
Ph2P(Te)(CH2)6P(Te)Ph2 (3)	2.3654(13) (P1–Te1), 2.3768(13) (P2–Te2)
iPr2P(Te)CH2PiPr2 (ref. 8)	2.3603(7)
Me2P(Te)(CH2)2P(Te)Me2 (ref. 9)	2.357(2)

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When compared to literature data,¹⁴ significant intermolecular non-covalent contacts concerning P or Te atom are not observed in crystal structures (in 1 Te1⋯Te2 distance is 3.8058(6) Å; in 2 P1⋯Te1B 3.8951(18) Å and P2⋯Te1A 3.7796(12) Å).

The multinuclear (³¹P and ¹²⁵Te) NMR spectra measured at 303 K or 273 K show signals of products with chemical shift around 33 ppm in phosphorus spectra and a singlet in the range −636.5 and −653.0 ppm in tellurium spectra. Neither tellurium satellites in phosphorus spectra, nor splitting of signals to doublets in tellurium spectra was observed. Due to the presence of unoxidized Ph₂P(CH₂)_nPPh₂ in the sample, we propose to explain this by rapid exchange of tellurium atoms between PCP ligands, as described in literature.^8,15 It is curious, that measurements performed with compound 3 at 223 K (which should be sufficient temperature for slowing down the exchange process) did not lead to any significant changes neither in phosphorus, nor in tellurium NMR.

In conclusion, the first successful oxidation of Ph₂P(CH₂)_nPPh₂ by elemental tellurium, leading to Ph₂P(Te)(CH₂)_nP(Te)Ph₂, was performed. Three new compounds Ph₂P(Te)(CH₂)₂P(Te)Ph₂, Ph₂P(CH₂)₄P(Te)Ph₂ and Ph₂P(Te)(CH₂)₆P(Te)Ph₂ were isolated and characterized including single-crystal X-ray analysis.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: The detailed experimental procedure, materials and methods, discussion of NMR spectra, crystal data and structure refinement. CCDC 956544–46. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra00157e

‡ Elemental analysis (C, H). Calcd (found) for 1: C 47.78 (47.63), H 3.70 (3.76); for 2: C 60.70 (60.79), H 5.09 (5.14); for 3: C 50.77 (50.90), H 4.54 (4.61). Crystal data for 1: C₂₆H₂₄P₂Te₂, M_r = 653.59, monoclinic, space group C2/c, a = 17.383(4), b = 9.831(3), c = 14.5721(9) Å, β = 99.732(12), V = 2454.3(9) ų, Z = 4, D_c = 1.769 g cm⁻³, F(000) = 1256, μ = 2.519 mm⁻¹, 4737 reflections measured, 2416 unique, final R₁ = 0.0172, wR₂ = 0.0372, GOF = 0.955, R indices based on 2416 reflections with I > 2σ(I). CCDC 956546. Crystal data for 2: C₂₈H₂₈P₂Te, M_r = 554.04, monoclinic, space group P2(1), a = 9.0157(4), b = 14.2910(7), c = 9.7134(4) Å, β = 100.562(4), V = 1230.30(10) ų, Z = 2, D_c = 1.496 g cm⁻³, F(000) = 556, μ = 1.352 mm⁻¹, 8099 reflections measured, 3937 unique, final R₁ = 0.0240, wR₂ = 0.0562, GOF = 1.030, R indices based on 3937 reflections with I > 2σ(I). CCDC 956545. Crystal data for 3: C₃₀H₃₂P₂Te₂, M_r = 709.70, triclinic, space group P1̄, a = 9.513(2), b = 12.852(3), c = 13.247(3) Å, α = 100.80(2), β = 100.66(2), γ = 102.393(18), V = 1510.4(6) ų, Z = 2, D_c = 1.561 g cm⁻³, F(000) = 692, μ = 2.053 mm⁻¹, 11693 reflections measured, 5940 unique, final R₁ = 0.0361, wR₂ = 0.0911, GOF = 1.039, R indices based on 5940 reflections with I > 2σ(I). CCDC 956544. NMR spectroscopy. 1: ³¹P{¹H} NMR (CDCl₃, 303 K): −11.4 ppm (s), 32.5 ppm (d, ³J(³¹P, ³¹P) = 52.2 Hz), ¹²⁵Te NMR (CDCl₃, 303 K): −653.0 ppm (s). 2: ³¹P{¹H} NMR (CDCl₃, 303 K): −12.5 ppm (s), 33.0 ppm (s), ¹²⁵Te NMR (CDCl₃, 303 K): −636.5 ppm (s). 3: ³¹P{¹H} NMR (CDCl₃, 273 K): −12.1 ppm (s), 33.0 ppm (s), ¹²⁵Te NMR (CDCl₃, 273 K): −641.4 ppm (s). Further discussion of NMR results can be found in ESI. Melting point for 1: 90–92 °C (decomposition); 2: 99–101 °C (decomposition); 3: 116–118 °C (decomposition).

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