Heterometathesis of diphosphanes (R 2 P – PR 2 ) with dichalcogenides (R ’ E – ER ’ , E = O, S, Se, Te) †

The reactions of R 2 P – PR 2 with R ’ E – ER ’ , (where E = Se, S, O, Te) to give R 2 P – ER ’ have been explored experimentally and computationally. The reaction of Ph 2 P – PPh 2 with PhSe – SePh gives Ph 2 P – SePh ( 1 ) rapidly and quantitatively. The P – P/Se – Se reaction is inhibited by the addition of the radical scavenger TEMPO which is consistent with a radical mechanism for the heterometathesis reaction. Compound 1 has been fully characterised, including by X-ray crystallography. A range of other Ar 2 P – SeR (R = Ph, n Bu or CH 2 CH 2 CO 2 H) have also been prepared and characterised. The reaction of 1 with [Mo(CO) 4 (nbd)] (nbd = norbornadiene) gives two products which, from their characteristic 31 P NMR data, have been identi ﬁ ed as cis -[Mo(CO) 4 (Ph 2 PSePh – P ) 2 ] ( 8 ) and the mixed-donor complex cis -[Mo(CO) 4 (Ph 2 P – SePh – P )(Ph 2 P – SePh – Se )] ( 9 ). It is deduced that the P and Se atoms in ligand 1 have comparable capacity to coordinate to Mo(0). The reaction of Ph 2 P – PPh 2 with PhS – SPh gives Ph 2 P – SPh ( 2 ) quantitatively but no reaction was observed between Ph 2 P – PPh 2 and PhTe – TePh. Heterometathesis between Ph 2 P – PPh 2 and t BuO – O t Bu does not occur thermally but has been observed under UV irradiation to give Ph 2 P – O t Bu along with P( V ) oxidation by-products. DFT calculations have been carried out to illuminate why heterometatheses with dichalcogenides R ’ E – ER ’ occur readily when E = S and Se but not when E = O and Te. The calculations show that heterometathesis is predicted to be thermodynamically favourable for E = O, S and Se and unfavourable for E = Te. The fact that a metathesis reaction between Ph 2 P – PPh 2 with t BuO – O t Bu is not observed in the absence of UV radiation, is therefore due to kinetics.


Introduction
The chemistry of diphosphanes (R 2 P-PR 2 ) has attracted considerable attention, and the high reactivity of the P-P bond has been exploited for the diphosphination of a wide range of unsaturated substrates, including alkenes, [1][2][3][4] alkynes, 4-8 1,3dienes, 9 arynes, 10 CO 2 and CS 2 . 11We recently reported that tetra-aryldiphosphanes readily undergo homometathesis reactions (Scheme 1) under ambient conditions, most likely via homolysis of the P-P bond and the generation of Ar 2 P • radicals. 12Also shown in Scheme 1 are diphosphane reactions with diatomic X 2 that could be classed as examples of diphosphane heterometathesis reactions. 13,14ere, we report our experimental and computational investigations of the heterometatheses shown in Scheme 2 15 and show that this is an effective route for the synthesis of compounds of the type Ph 2 P-SeR.
Compounds containing P-S or P-Se bonds have been widely applied in organic synthesis as reagents for S or Se transfer (Lawesson's reagent 16 and Woollins' reagent 17 respectively).Furthermore, P/S and P/Se compounds are of interest in their own right [18][19][20] with potential applications ranging from semi-conductors to pesticides. 21However, to date, little attention has been given to compounds of the type R 2 P-SR or R 2 P-SeR which are amongst the simplest organophosphorus(III) compounds containing S or Se.Arbuzov reported the first examples of Ph 2 P-SR (R = alkyl) compounds over a century ago. 22In the 1960s, McLean reported the synthesis of Ph 2 P-SePh (1) and Ph 2 P-SPh (2) from Ph 2 PCl by the routes shown in Scheme 3, although no NMR spectroscopic data for these compounds were reported. 23More recently, Cui et al. 24 prepared thioaniline derivative 3 similarly (Scheme 3).

Diphosphane-diselenide metathesis
The reaction between equimolar quantities of tetraphenyldiphosphane and diphenyldiselenide in THF (Scheme 4) was monitored by 31 P{ 1 H} NMR spectroscopy.After 10 min, the signal for Ph 2 P-PPh 2 at δ(P) −15.0 ppm had been replaced by a singlet at δ(P) +29.4 ppm with 77 Se satellites ( 1 J PSe = 229 Hz); the 77 Se{ 1 H} NMR spectrum is a doublet at δ(Se) 307.5 ppm ( 1 J PSe = 229 Hz), consistent with the formation of the selanylphosphane, Ph 2 P-SePh (1).The product was isolated in 92% yield and further characterised by 1 H and 13 C{ 1 H} NMR, and mass spectrometry (see ESI † for the data). 25Crystals of 1 were grown from CH 2 Cl 2 /n-hexane via vapour diffusion and its X-ray crystal structure determined (see Fig. 1).
The P-Se bond length in 1 of 2.2620(5) Å is close to the mean average of 2.217 Å obtained from the 686 single crystal X-ray structures that feature a P-Se bond in the Cambridge Structural Database (CSD).There are no crystal structures in the CSD of compounds of the type Ar 2 P-SeAr′ (or Ar 2 P-SAr′) with which to compare 1, but the pyramidal geometry at P and bent geometry at Se are as expected.
The P 2 R 4 /Se 2 R 2 heterometathesis reaction has been extended to the preparation of the new selanylphosphanes 4-7 (Scheme 4) by combination of the appropriate diphosphane and diselenide.Each of these compounds was isolated in good yield (quantitative yields observed by in situ 31 P{ 1 H} NMR spectroscopy) as an oil or oily solid and then fully characterised by 31 P{ 1 H}, 77 Se{ 1 H}, 1 H, 13 C{ 1 H} and APCI mass spectrometry (see ESI † for the data).The modest isolated yields of 4 and 7

Dalton Transactions Paper
(shown in Scheme 4) are due to losses during work-up, since the NMR yields were quantitative.These reactions were carried out in THF, CH 2 Cl 2 or toluene and in no case was the solvent observed to affect the rate or outcome of the reaction, which in all cases had proceeded to completion within the time required (ca.30 s) to obtain a 31 P{ 1 H} NMR spectrum of the reaction mixture.Of particular note is the carboxylic acid 7, which shows that the P-Se bond is tolerant of reactive functional groups.The synthesis of 1 in CDCl 3 was repeated in an amberised NMR tube (to reduce the effect of photolysis) but again, complete conversion to 1 was observed within 30 s.
When the reaction of Ph 2 P-PPh 2 with PhSe-SePh was carried out in the presence of 4 equiv. of TEMPO in THF, the heterometathesis reaction was inhibited: in the absence of TEMPO, complete conversion was observed within 1 min, while in the presence of TEMPO, the reaction proceeded to 54% conversion after 90 min.Other products are detected associated with the formation of Ph 2 P-TEMPO species (see ESI †).This is consistent with radicals being involved in the reaction, as we previously demonstrated in the homometathesis of diphosphanes. 12As a result, the radical chain mechanism shown in Scheme 5 is proposed.7][28][29][30][31][32] It is therefore plausible that RSe-SeR cleavage to produce RSe • radicals initiates the radical chain process shown in Scheme 5. 33 McLean reported 23 that Ph 2 P-SePh ( 1) is unstable with respect to isomerisation to Ph 3 PvSe at 100 °C, over a period of 3-4 h.We have found that 1 is relatively stable under ambient conditions, which makes it easy to handle.Thus solid samples of 1 can be stored under argon at room temperature with <10% degradation observed (traces of Ph 2 PH detected) even after 4 months.When water was added to a solution of 1 in CDCl 3 and the emulsion shaken under nitrogen, only very slow hydrolysis to Ph 2 P(O)H was observed: after 2 days, the hydrolysis mixture contained unreacted 1 (ca.60%) along with a prominent signal (22%) at +32.5 ppm, consistent with the formation of Ph 2 P(O)OH; 34 crystals that grew from this solution were shown to match the structure of Ph 2 P(O)OH, 'DPPHIN' in the CSD.
The observed kinetic stability of selanylphosphane 1 prompted us to explore its coordination chemistry with Mo(0).To the best of our knowledge, selanylphosphanes have not previously been used as ligands for transition metals.6][37] Treatment of [Mo(CO) 4 (nbd)] (nbd = norbornadiene) with 2 equiv. of 1 in CD 2 Cl 2 , gave a mixture of two products in the ratio of ca. 4 : 1 according to 31 P{ 1 H} NMR spectroscopy (see Fig. 2), which have been assigned to the isomers of cis-[Mo(CO) 4 (1) 2 ] (8 and 9 in Scheme 6) on the basis of their characteristic 31 P NMR parameters.
The major product is a singlet at δ(P) +59.7 ppm with outer 77 Se satellites ( 1 J PSe = 333 Hz) that are themselves split into doublets ( 2 J PP = 25 Hz) and inner 77 Se satellites ( 3 J PSe = 40 Hz).This product is assigned the structure 8 where both ligands are P-bound cis on the Mo; this is supported by the large coordination chemical shift (Δδ P = +29.2) and the small 2 J PP .The inequivalence observed in the satellites is a result of the loss of symmetry in the 77 Se isotopologue (see Fig. 2).
The second product is assigned structure 9, containing one P-bound and one Se-bound ligand 1, on the basis of the 31 P NMR data.Two doublets are observed at δ(P 1 ) +62.4 ppm and δ(P 2 ) +17.8 ppm ( 3 J PP = 22 Hz).The Δδ P of +31.2 for P 1 in 9 is similar to the Δδ P of the P in 8 and is therefore consistent with P 1 being directly bound to Mo.The Δδ P of −12.7 ppm for P 2 is assigned to the Se-bound ligand in 9.After the mixture of products was heated to 60 °C in CDCl 3 for 3.5 h, complex 9 was the dominant species (ratio of complexes 9 : 8 was 5 : 1 according to 31 P NMR spectroscopy, see ESI †).In the IR spectrum of the mixtures of 8 and 9, several absorptions in the range 1892-2027 cm −1 were evident, as expected for the ν(CO) bands for these cis complexes present.Solutions of the molybdenum complexes 8 and 9 in a CH 2 Cl 2 /water emulsion did not undergo any changes after 17 h, indicating good water tolerance.
The analogous complexation reaction of selanylphosphane 7 with [Mo(CO) 4 (nbd)] in CH 2 Cl 2 produced a similar pattern of products according to 31 P NMR spectroscopy (see ESI †): two products in the ratio 4 : 1, with a singlet at +50.8 ppm (with 77 Se satellites) and two doublets at +54.0 and 17.7 ppm ( 3 J PP = 21 Hz) consistent with the formation of analogues of 8 and 9.
The notable conclusion from the Mo coordination chemistry is that, for Ph 2 P-SeR, the PPh 2 and SeR groups have comparable donor ability.The fine balance between the linkage isomers with P or Se coordination is likely a consequence of the donor atoms both being soft and the steric hindrance around the SeR being less than around the PPh 2 .

Diphosphane-disulfide metathesis
The reaction of tetraphenyldiphosphane with diphenyl disulfide or 4-aminophenyl disulfide gave the previously reported sulfanylphosphanes 2 and 3 respectively (see Scheme 7). 23,24,38Compared with the rapid diselenide reactions (Scheme 4), the disulfide analogues are sluggish, requir-ing up to 20 h to reach completion in an unagitated NMR tube.The reactions were significantly more rapid when they were stirred and were most rapid in chloroform (as with the diphosphane homometathesis). 12The conversions to 2 and 3 were apparently quantitative in the reactions monitored by 31 P{ 1 H} NMR spectroscopy but the isolated yields were modest (see Scheme 7); there were significant losses on the The diphosphane-disulfide metathesis was inhibited by the presence of TEMPO (4 equiv.),consistent with the reaction following the radical chain mechanism shown in Scheme 5, as for the diphosphane-diselenide process.The reaction of Ph 2 P-PPh 2 with PhS-SPh in CDCl 3 was followed by 31 P{ 1 H} NMR spectroscopy and 2 was formed in 34% yield after 1 h of reaction.When the same reaction was performed in the presence of 4 equivalents of TEMPO, 8% conversion to 2 was observed after 1 h (see ESI †).[41] Attempted diphosphane-ditelluride and diphosphaneperoxide metatheses When Ph 2 P-PPh 2 and PhTe-TePh were mixed in C 6 D 6 , no heterometathesis reaction (Scheme 2) was observed even after 12 days.The compound Ph 2 P-TePh has not been previously reported, although its P(V) isomer Ph 3 PvTe is readily prepared. 42imilarly, when Ph 2 P-PPh 2 and t BuO-O t Bu were mixed in toluene, no heterometathesis reaction (Scheme 8) to give the known 17 compound Ph 2 P-O t Bu was observed after 72 h.The only products detected by 31 P{ 1 H} NMR spectroscopy were the monoxide Ph 2 P(O)-PPh 2 (12%: 2 doublets at +32.7 and −24.7,J PP = 218 Hz) 43 and Ph 2 PH (12%, −40.2 ppm). 12Even when a large excess (∼15-fold) of t BuO-O t Bu was used and the reaction mixture was heated to 60 °C, only oxidation products were observed and no heterometathesis product.However, when this reaction was repeated in C 6 D 6 and irradiated with near-UV light, gradual formation of a new species was observed by 31 P{ 1 H} NMR spectroscopy characterised by a singlet at +87.1 ppm and assigned to the phosphinite Ph 2 P-O t Bu on the basis of the chemical shift being consistent with the reported value. 44After the irradiation was continued for a further 1 h, a significant quantity of the phosphinate Ph 2 P(O)-O t Bu (47% of the total 31 P integral, see ESI †) was identified by a singlet at +25.7 ppm, in good agreement with literature data. 45The formation of this P(V) species was further supported by the detection of the phosphinate [M + Na] + ion by mass spectrometry (297.1 m/z observed, 297.1 theoretical).In summary, there is evidence that under UV irradiation, the diphosphane/peroxide heterometathesis does indeed occur but that by-products associated with oxidation by t BuO-O t Bu contaminate the product (see Scheme 8).

DFT study of heterometathesis reactions of diphosphanes
The general reaction that has been calculated is shown in Scheme 9 and the results are given in Table 1.It was anticipated that the results for the diphosphane metatheses with diatomic molecules Z 2 would be the simplest to interpret.The DFT-calculated ΔE values for the addition of diatomic Z 2 to R 2 P-PR 2 , where R = Ph (A), or Me (B), are presented as entries 1-10 of Table 1.The values of ΔH estimated using the mean bond strengths for P-P (51 kcal mol −1 ), 46 Z-Z, and P-Z are also given in Table 1 and there is good agreement between ΔE and ΔH for Z = H, F, Cl, Br and I.The two consistent trends in the heterometathesis equilibria are: (i) ΔE becomes increasingly more favourable with increasing electronegativity of Z; (ii) Me 2 P-PMe 2 metatheses are more favourable (by 6-10 kcal mol −1 ) than the corresponding Ph 2 P-PPh 2 metatheses.The position of the equilibrium in Scheme 9 is determined by the relative Z-Z and P-Z bond energies, which in turn will be influenced by a combination of the following factors.(a) The very high H-H and very low F-F bond strengths dominate the explanation of why the metathesis equilibria with H 2 and F 2 are the least and most favourable respectively.(b) The electrostatic stabilising effect of the P δ+ -Z δ− dipole on the P-Z bond strength will increase with increasing electronegativity of Z. (c) The size of Z increases in the order F < Cl < Br < I and this will contribute to a lower P-Z bond strength (in that same order) due to increasingly poorer orbital overlap and increasing steric congestion.(d) The P-Z bond may be stronger in Me 2 P-Z than in Ph 2 P-Z, due to the lower steric hindrance of the PMe 2 group and the greater +I inductive effect of the Me substituents stabilising the δ+ charge on the PMe 2 . 47he calculated thermodynamics of the metathesis reactions of Ph 2 P-PPh 2 with PhE-EPh, where E = O, S, Se, or Te are given in Table 1, entries 12-15.These equilibria will depend on the relative PhE-EPh and Ph 2 P-EPh bond energies with potentially similar factors at play to those labelled (a)-(c) above, used for the diatomic Z 2 reactions.However, in the case of the dichalcogenides, the PhO-OPh equilibrium (entry 12) is exceptional in being extremely favourable (ΔE of −69.4 kcal mol −1 ) as a consequence of the low O-O bond energy and the high P-O bond energy (due in part to the large P δ+ -Z δ− dipole).By contrast, for the other dichalcogenides (entries 13-15), the ΔE values are relatively small, reflecting the fact that the P-P, E-E and P-E bond energies are similar due to the similarity of the electronegativities and sizes of P, S, Se and Te.The calculated ΔE for the diselenide and disulfide reactions are both ca.−5 kcal mol −1 and therefore favour the formation of Ph 2 P-EPh, in agreement with the experimental observations.The calculated ΔE of +7 kcal mol −1 for the ditelluride/ diphosphane metathesis disfavours the formation of Ph 2 P-TePh which is consistent with no reaction between Ph 2 P-PPh 2 and PhTe-TePh being observed experimentally.
Although the t BuO-O t Bu/Ph 2 P-PPh 2 metathesis (entry 11, Table 1) is very strongly favoured energetically for the formation of Ph 2 P-O t Bu, experimentally, the reaction (Scheme 8) only proceeded under UV irradiation and produced oxidation by-products; the lack of thermal reaction is therefore due to a kinetic barrier, possibly caused by the bulky t Bu substituents.

Conclusion
The heterometathesis reactions of Ph 2 P-PPh 2 with RE-ER to give Ph 2 P-ER proceed smoothly when E = S or Se but not when E = O or Te.We have shown that Ar 2 P-PAr 2 with RSe-SeR gives a range of Ar 2 P-SeR compounds (1 and 4-7) in quantitative spectroscopic yields and 100% atom-economy.The reaction of Ph 2 P-PPh 2 with ArS-SAr produces Ph 2 P-SAr, albeit more slowly than with the Se analogues.The inhibition by TEMPO of the heterometatheses of Ph 2 P-PPh 2 with RE-ER (E = Se or S) suggests radical processes are involved in the mechanism.DFT calculations have shown that the heterometatheses with RE-ER are thermodynamically favourable when E = S or Se but not when E = Te, which aligns with the experimental observations.However, the calculations also suggest that the heterometathesis of diphosphanes with peroxides (i.e.E = O) would be very strongly favoured thermodynamically.The lack of thermal reaction between Ph 2 P-PPh 2 with t BuO-O t Bu is therefore due to kinetics.There is evidence of formation of Ph 2 P-O t Bu when mixtures of Ph 2 P-PPh 2 and t BuO-O t Bu are photolysed but the reaction is complicated by the simultaneous formation of oxidation products.The selanylphosphane Ph 2 P-SePh (1) can be stored for weeks, largely unchanged at ambient temperatures and is only very slowly hydrolysed by water over several days.The ligand properties of 1 are of interest as it coordinates to Mo(0) to give products whose 31 P parameters are consistent with coordination via the P and Se being competitive.This is an unusual example of linkage isomerism that is worthy of further study.

Conflicts of interest
There are no conflicts to declare.

Fig. 2
Fig.231 P{ 1 H} NMR spectrum of the product mixture obtained from the reaction of Ph 2 P-SePh with [Mo(CO) 4 (nbd)].The large singlet at ca. 60 ppm is assigned to the symmetrical P,P-complex 8 and the expansion shows the signals associated with the 77 Se isotopologue of 8, which show satellite signals due to 1 J PSe ( ) and 3 J PSe ( ).The doublets at ca. +62 ( ) and +18 ( ) are assigned to 9, the P,Se-linkage isomer of 8 ( 77 Se satellites obscured by the noise).The signal at ca. 31 ppm is for unreacted ligand 1 with77  Se satellites (*).The minor signal labelled X at ca. +17 ppm is unassigned.

Table 1
Calculated ΔE for the heterometatheses shown in Scheme 9 a All energies are given in kcal mol −1 and are calculated at STP (298 K, 1 atm).D(Z-Z) and D(P-Z) are the average bond dissociation energies given in ref.46. a