Radical-initiated P,P-metathesis reactions of diphosphanes: evidence from experimental and computational studies †

By combining the diphosphanes Ar 2 P – PAr 2 , where Ar = C 6 H 5 , 4-C 6 H 4 Me, 4-C 6 H 4 OMe, 3,5-C 6 H 3 (CF 3 ) 2 , it has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations have shown that the stability of unsymmetrical diphosphanes Z 2 P – PZ ’ 2 is a function of the di ﬀ erence between the Z and Z ’ substituents in terms of size and electronegativity. Of the mechanisms that were calculated for the P,P-metathesis, the most likely was considered to be one involving Ar 2 P (cid:129) radicals. The observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent with a radical chain process. The P,P-metathesis reactions that involve ( o -Tol) 2 P – P( o -Tol) 2 are anoma-lously slow and, in the absence of photolysis, were only observed to take place in CHCl 3 and CH 2 Cl 2 . The role of the chlorinated solvent is ascribed to the formation of Ar 2 PCl which catalyses the P,P-metathesis. The slow kinetics observed with ( o -Tol) 2 P – P( o -Tol) 2 is tentatively attributed to the o -CH 3 groups quenching the ( o -Tol) 2 P (cid:129) radicals or inhibiting the metathesis reaction sterically.


Introduction
Dynamic and reversible chemical reactions have garnered increasing attention in medicine and biology, 1 supramolecular chemistry 2 and materials science, [3][4][5][6][7][8][9][10][11] and they are key to the rapidly growing field of dynamic covalent chemistry (DCC). 1,12,135][16] In the last decade, several other main group element-element bonds (O-O, 17,18 N-N 19 and Se-Se 9,20,21 ) have been investigated for their potential in dynamic chemistry with a view to their application in the field of polymeric materials.A notable omission from this group is diphosphanes (P 2 R 4 ) which contain a P-P bond whose strength (ca.52 kcal mol −1 ) 22 is intermediate between S-S (ca.62 kcal mol −1 ) 23 and Se-Se (ca.40 kcal mol −1 ) 22 and therefore diphosphanes should be candidates for DCC.
5][26][27][28][29][30][31] Several methods to prepare diphosphanes are available, including: Wurtz-type reductive coupling of chlorophosphines (with Li, Na, K, Mg and Hg); [32][33][34] salt metathesis between chlorophosphine and LiPR 2 ; 26,27 dehydrocoupling of HPR 2 ; [35][36][37][38][39] chlorosilane elimination; 40 P-N/P-P bond metathesis. 41The chemistry of diphosphanes can be divided into two categories: reactions where the P-P bond remains intact and reactions where the P-P bonds are cleaved. 42eactions involving P-P cleavage are pertinent to the chemistry described in this article and include additions of X 2 to the R 2 P-PR 2 to give monophos species R 2 PX (e.g.X = H, Cl) and additions of R 2 P-PR 2 to CvC or CuC bonds (diphosphination) to give diphos species (Scheme 1).It has also been shown that diphosphanes containing P-N bonds can add to CvS or CvO bonds. 43,44Mechanisms have been proposed for some of the P-P cleavage reactions including radical chain processes. 43,45iphosphane metathesis has not been systematically investigated, but an early 31 P NMR study by Harris et al. showed that rapid metathesis occurred between Me 2 P-PMe 2 and (F 3 C) 2 P-P(CF 3 ) 2 in CH 2 Cl 2 to form Me 2 P-P(CF 3 ) 2 . 46,47Gilheany et al. 48have shown that the calculated high energy barrier to P,P-metathesis via a concerted mechanism is not compatible with the observed rapid kinetics for the Me 2 P-P(CF 3 ) 2 system and suggested that pathways involving impurities in the diphosphanes may facilitate the P,P-metathesis process.Grubba et al. 26 have succeeded in the preparation of a variety of pure, unsymmetrical diphosphanes containing P-C(alkyl) and P-N bonds.By employing low temperatures, they avoided unwanted P,P-metatheses that would lead to symmetrical diphosphanes contaminating the products.Moreover, they reported that R 2 PCl or R 2 PLi can catalyse diphosphane metathesis.We have reported that diphosphane metathesis was a side reaction in the attempted synthesis of some unsymmetrical diphosphanes. 27n this study, we have investigated the P,P-metathesis reactions of tetra-aryldiphosphanes (Scheme 2) using 31 P{ 1 H} NMR spectroscopy and computational methods, with a view to exploring the dynamic nature of these P-P bonds.This has provided some new insights into the diphosphane metathesis reaction.

Thermodynamics of diphosphane metathesis
Experimental equilibrium studies.The tetra-aryldiphosphanes A 2 -E 2 (Chart 1) used in this study, were chosen in order to sample a range of steric and electronic effects.All have been previously reported and were prepared by modified literature procedures for the reductive coupling of the corresponding chlorophosphines by Mg in THF (see ESI † for details). 26,34In each of the diphosphane samples, residual chlorophosphine was not detected by 31 P NMR spectroscopy and was therefore estimated to be less than 0.05% (see ESI †).
When two of the diphosphanes A 2 -E 2 were mixed in CDCl 3 at ambient temperature, a reaction ensued to give an equilibrium mixture of homodiphosphanes X 2 and Y 2 , and the heterodiphosphane XY (Scheme 2).The data for the diphosphane metathesis reactions are given in Table 1.
The 31 P{ 1 H} NMR data for the XY species (Table 1) show that the chemical shifts for the two signals are close to the respective X 2 and Y 2 with 1 J XY ≃160 Hz.For example, Fig. 1 is the 31 P{ 1 H} NMR spectrum obtained after mixing diphosphanes B 2 and C 2 (entry 4, Table 1) and shows the singlets for the reactants along with an AB pattern for the product BC; the spectra for the other diphosphane combinations are given in the ESI.† The pure heterodiphosphane AE was prepared in order to establish the P,P-metathesis equilibrium with A 2 and E 2 (Scheme 2) from the opposite direction.Grubba et al. 26 have prepared a range of heterodiphosphanes (Z 2 P-PZ′ 2 ) by the addition of Z 2 PLi to Z′ 2 PCl at low temperatures.However, our attempts to extend this route to the preparation of AE were unsuccessful, even with the reaction mixture maintained at −78 °C throughout; instead, an equilibrium mixture of A 2 , E 2 and AE was obtained.This contrasting behaviour may be due to the lack of thermodynamic stability of heterodiphosphane AE compared with the Grubba heterodiphosphanes 26 (see below).The desired AE was prepared in >96% purity (after recrystallisation from methanol) via the protection/deprotection method (Scheme 3) that we previously developed for the synthesis of heterodiphosphanes. 27,49Solutions of pure AE in CDCl 3 do indeed equilibrate to a mixture of A 2 , E 2 and AE (see below for further details).
The diphosphane equilibrium constants for the P,P-metatheses (Scheme 2) fall in the range 3-30 (see Table 1), which corresponds to ΔG values between −0.5 and −2 kcal mol −1 , demonstrating that the diphosphanes in this study are very close in energy and insensitive to the steric and electronic effects of the aryl substituents.The equilibria that do not involve E 2 were established rapidly (see Table 1) despite the dilution (12.5 mM) which is a useful property for the potential application of diphosphanes in dynamic chemistry. 1The anomalously slow P,P-metatheses involving E 2 are discussed later.
Computational equilibrium studies.To understand better the factors that control the stability of heterodiphosphanes with respect to the corresponding homodiphosphanes, DFT calculations were carried out to determine the thermodynamics of the hypothetical equilibria shown in Scheme 4 in which the range of P-substituents Z and Z′ is greatly extended from the substituted aryls in Scheme 2. The values for ΔG are collected in Table 2, and for some of the combinations (Table 2, entries 1-3 and 7-10), the calculated |ΔG| is less than 3 kcal mol −1 and therefore, these particular P,P-metathesis equilibria are predicted to be finely balanced.The calculated   1, entry 7).Two gross trends emerge from the data in Table 2: (1) a bias in favour of the XY species is calculated when there is a large electronegativity difference between the X and Y fragments; thus Me 2 P-P(CF 3 ) 2 is predicted (and experimentally observed 50 ) to be highly favoured (Table 2, entry 6); (2) a strong bias in favour of the XY species is calculated when there is a large difference in steric bulk between the X and Y fragments; thus t Bu 2 P-PH 2 is predicted to be highly favoured (Table 2, entry 13).The largest calculated ΔG in favour of the XY species is for t Bu 2 P-PF 2 (Table 2, entry 20), at an apparent apotheosis of both trends.The observed trends in the calculated relative stabilities of the XY species in Scheme 4 can be rationalised as follows: (a) electronegative Z substituents (such as F) will lead to repulsive electrostatic interactions between the δ+ charges generated on the adjacent P atoms in diphosphane X 2 ; (b) bulky Z′ substituents (such as t Bu) will lead to steric congestion in diphosphane Y 2 ; (c) in XY, both of the P-P bond-destabilising effects identified in (a) and (b) will be reduced and, in some cases, a destabilising, repulsive, electrostatic (δ+/δ+) interaction is replaced by a stabilising, attractive, electrostatic (δ+/δ−) interaction.
The calculated ΔG values for some of the Grubba heterodiphosphanes 26 (Table 2, entries 16 and 18) suggest that these should have high thermodynamic stability with respect to the corresponding homodiphosphanes.By contrast, the heterodiphosphine AE, as reported above, is calculated to be of similar energy to the homodiphosphanes A 2 and E 2 (Table 2, entry 22).

Computational mechanistic studies of diphosphane metathesis
The mechanism of the diphosphane metathesis shown in Scheme 5 has been previously investigated computationally by Gilheany et al. 48The lowest energy route that they could find involved the ionic intermediate shown in Scheme 5 but the calculated barrier of 36.5 kcal mol −1 was too high to explain the rapidity of the P,P-metathesis that is observed experimentally.
We have investigated the mechanism of diphosphane metathesis computationally using the degenerate exchange reactions of Me 2 P-PMe 2 (Scheme 6).Tetramethyldiphosphane was chosen in order to limit the number of rotamers and thus the conformational noise in the calculations.It was hoped to distinguish between the three plausible pathways (i-iii) shown in Scheme 7: (i) a [2 + 2] concerted process involving a 4-mem-  The radical chain pathway (iii) in Scheme 7 requires homolysis of a P-P bond.Thermolysis is not viable under ambient conditions because the calculated P-P bond enthalpy is of the order of 52 kcal mol −1 .However, DFT calculations for Ph 2 P-PPh 2 (A 2 ) suggest that excitation from S 0 → S 1 or S 0 → S 2 would result in rapid homolysis of the P-P bond, yielding phosphanyl radicals (Ar 2 P • ) via intersystem crossing from S 1 or S 2 to the first excited triplet state (T 1 ) of A 2 ; the T 1 state is dissociative, and would lead to spontaneous radical formation.From the Ar 2 P • , the P,P-metathesis barrier generating XY species is only ∼5 kcal mol −1 , such that once radicals are generated an equilibrium would be expected to be established rapidly (see ESI † for details).
The DFT results suggested that Ar 2 P • radicals (generated photolytically) were viable intermediates in the P,P-metathesis reactions and so this hypothesis was pursued experimentally.

Qualitative experimental kinetic studies of diphosphane metathesis
The aryl diphosphane metathesis reactions with A 2 -D 2 occurred readily (typically <1 h) under ambient conditions in common organic solvents (C 6 D 6 , THF, PhMe, PhCl, CDCl 3 and CD 2 Cl 2 ).By contrast, when the diphosphane E 2 , containing o-tolyl substituents, was one of the reaction partners, P,P-metathesis was initially only observed to take place in CDCl 3 (typically over 6-12 h) and CD 2 Cl 2 (typically over more than 24 h); the anomalous behaviour observed for the P,P-metatheses involving E 2 will be discussed after consideration of the P,P-metatheses involving A 2 -D 2 .

Dalton Transactions Paper
When cold (−78 °C) THF solutions of B 2 and C 2 were mixed and the P,P-metathesis reaction monitored by low-temperature 31 P NMR spectroscopy, it was found that only traces of BC were detected at −80 °C after 1 h.By raising the temperature in increments of 20 °C, it was established that the P,P-metathesis at −20 °C progressed at a convenient rate to monitor the approach to P,P-metathesis equilibrium over a period of 2 h by 31 P NMR spectroscopy (see Fig. 3).Although a detailed kinetic study was not attempted, the data do not appear to fit a reaction order of 1, 2 or 0.5 (see ESI †).
Evidence for radicals in the general P,P-metathesis reactions In light of the computational results above suggesting that radicals may be involved in the mechanism of P,P-metathesis, experiments were carried out to determine if there was evidence of their presence.Two pieces of experimental evidence support the involvement of radicals in the P,P-metathesis process.
(1) When A 2 and C 2 were mixed in the presence of the radical scavengers 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) or tri(t-butyl)phenol (TTBP), the progress of the P,Pmetathesis reactions was greatly inhibited.Thus, a mixture of A 2 and C 2 proceeded smoothly to the metathesis equilibrium over a period of 2 h in CDCl 3 , at −20 °C (with 35% conversion at 1 h); by contrast, when the same A 2 /C 2 mixture was prepared in the presence of 5 equiv. of TEMPO, there was no formation of AC detected after 1 h.Moreover, when the reaction mixture containing TEMPO was allowed to warm to ambient temperature, <10% of AC was detected even after 16 h.Instead, in the presence of TEMPO, two singlet 31 P resonances at +30.5 and +31.5 ppm were observed (ca.10% of the total signal intensity), consistent with the formation of Ar 2 P(O)-TEMPO (Ar = Ph, p-anisyl) adducts (Scheme 8) which would be expected to be formed by the interception of the phosphanyl radicals (Ar 2 P • ) by TEMPO. 512) When A 2 and B 2 were mixed in a standard NMR tube in THF at ambient temperatures, equilibration was essentially complete within ca. 15 min.However, when the same A 2 /B 2 mixture in THF was prepared in an amberised NMR tube, to minimise the transmittance of UV and visible radiation, the equilibration to the same degree took ca.150 min.This is consistent with diphosphane photodissociation being a significant component of the mechanism of P,P-metathesis.
The UV-vis spectra of A 2 , C 2 and E 2 are shown in Fig. 4.Each spectrum has a major absorbance between 225 and 250 nm, and a shoulder at ca. 270 nm.TD-DFT calculations for A 2 were carried out using a range of functionals (CAM-B3LYP, PBE0 and M06-2X) and good agreement with experiment was found with CAM-B3LYP.These calculations suggest that the major peak in each spectrum corresponds to coincident aromatic π → π* and S 0 → S 2 (see Fig. 5(b)) transitions and the shoulder peak corresponds to S 0 → S 1 (see Fig. 5(a)).The transition electron density plots (Fig. 5) indicate that both transitions result in a significant delocalisation of electron density from the P lone pairs.The first triplet state, accessible from

Paper Dalton Transactions
either S 1 or S 2 via intersystem crossing, is dissociative, leading to spontaneous formation of phosphinyl radicals.There are small differences in the three spectra shown in Fig. 4. As expected, the p-OMe groups in C 2 produce a bathochromic shift in the π → π* band and there is a long 'tail' that extends towards the visible region, which explains why C 2 is pale yellow.
The evidence in support of radicals being present led to the radical chain mechanism proposal in Scheme 9 where homolysis of the Ar 2 P-PAr 2 is followed by attack of the Ar 2 P • on the second diphosphane Ar′ 2 P-PAr′ 2 .The termination steps are then the radical couplings with P-P bond formation.
The photodissociation requires UV radiation and it appears that there is sufficient background UVA-UVB radiation (280-400 nm) for the initiation step to proceed.
The two degenerate propagating steps, where Ar 2 P • and Ar′ 2 P • react with their respective dimers Ar 2 P-PAr 2 and Ar′ 2 P-PAr′ 2 , are omitted from Scheme 9.The reverse P,P-metathesis process would also be initiated by photodissociation of Ar 2 P-PAr′ 2 .The P,P-metathesis reactions generally proceed with a high degree of chemoselectivity but traces of Ar 2 PH were often evident after several days of reaction which would arise from termination via abstraction of an H radical, presumably from the solvent.
The radical mechanism for diphosphane metathesis in Scheme 9 is reminiscent of the radical mechanism for disulfide metathesis (Scheme 10) arrived at from calculations by Ruipérez et al. 52 and from experimental observations by Asua et al. 53 DFT and MD calculations were applied to two mechanisms of diaryldisulfide metathesis: a [2 + 2] concerted metathesis and a [2 + 1] radical chain reaction initiated by sulfanyl radicals (ArS • ). 47The transition state in the concerted process was calculated to be too high in energy to be viable, whereas the radical process was viable provided the ArS • radicals could be accessed.This reasoning mirrors ours for the mechanisms proposed for diphosphane metatheses shown in Scheme 7.
It has been reported that the diaryldisulfide metathesis between diphenyldisulfide and di(4-aminophenyl)disulfide proceeds spontaneously to equilibrium in ca. 12 h; this was accelerated by UV photolysis, and TEMPO completely inhibited the equilibration. 53These observations made on the disulfide metathesis reactions (and analogous diselenide studies) 54 are very similar to our observations on the diphosphane metatheses.

Anomalous kinetics with (o-Tol) 2 P-P(o-Tol) 2
The four P,P-metathesis reactions between tetra(o-tolyl)diphosphane (E 2 ) and A 2 -D 2 did not appear to proceed at all in THF, PhMe, PhCl, PhF or C 6 D 6 under ambient conditions after 72 h but do proceed in the chloroalkanes CH 2 Cl 2 and CHCl 3 , albeit slowly (see Table 1, entries 7-10).To probe this anomalous behaviour further, we have investigated the A 2 /E 2 system (Fig. 6) in more detail including approaching the equilibrium starting from an A 2 /E 2 mixture and starting from isolated AE.

Dalton Transactions Paper
The progress of the A 2 /E 2 metathesis in CDCl 3 was followed by 31 P NMR spectroscopy (see ESI † for details) with a spectrum recorded every 10 min for 12 h (see Fig. 7).The approach to equilibrium was measured from integration of the 31 P NMR signals to produce the plot shown in Fig. 7. Analysis of the curve revealed that it was not consistent with the integrated equations for an order of reaction of 0, 1 2 , 1 or 2; this indicates that the kinetics are complex, which would be expected if more than one mechanistic pathway was operating (see below).

Evidence for radicals in the anomalous (o-Tol) 2 P-P(o-Tol) 2 metathesis reactions
There are several pieces of evidence supporting a role for radicals in the P,P-metathesis equilibrium shown in Fig. 6: (1) The lack of any reaction between A 2 and E 2 in THF after 72 h was replaced by 30% AE formation when the solution was irradiated with a UV lamp for 2 h.Similarly, no equilibration was observed in a THF solution of pure AE over a period of 72 h but proceeded smoothly over 2 h upon irradiation of the same solution with a UV lamp.This suggests that the photodissociation mechanism shown in Scheme 6 for the general P,P-metathesis (Table 1, entries 1-6) is viable for the A 2 /E 2 metathesis but requires a more intense source of UV radiation than background UV for the initiation step.
(2) The slow A 2 /E 2 metathesis equilibration in CDCl 3 which takes place over a period of 12 h was extinguished in the presence of 4 equiv. of the radical scavenger TEMPO, as no AE was detected after 16 h.
(3) Pulse-sonication (20 kHz) at 0 °C of a CHCl 3 or CH 2 Cl 2 solution of A 2 and E 2 reduced the time to establish equilibrium from 12 h to <1 h.However, sonolysis had no accelerating effects on the reaction of A 2 with E 2 in THF or toluene.Fritze and von Delius reported the metathesis of disulfides (Scheme 10) in chloroform was accelerated by sonolysis and suggested that this proceeded via RS • radicals formed from the reaction of disulfides with Cl • generated by sonolytic chloroform degradation. 164) The onset of equilibration of solutions of AE in CDCl 3 was preceded by an induction period of up to 6 h; moreover, the time required to establish the P,P-metathesis equilibrium from mixtures of A 2 /E 2 in CDCl 3 varied from 6 to 12 h.These observations of rate-inconsistency are typical of radical processes. 55n addition to the evidence for a radical-promoted process, it is shown below that there is evidence of chlorophosphine involvement in the A 2 /E 2 metathesis.
Evidence for chlorophosphine intermediacy in the anomalous (o-Tol) 2 P-P(o-Tol) 2 metathesis reactions Under the conditions where a mixture of A 2 and E 2 in CDCl 3 would reach equilibrium with AE within 12 h, the presence of H 2 O (100 equiv.)suppressed the reaction to the extent that no  AE was detected even after 20 h (indicating that <1% AE had been formed).Instead, several minor signals (ca. 20% in total) were present and those at δ+ 23.5, +21.4 were tentatively assigned to Ar 2 P(O)H (Ar = Ph or o-Tol).These observations are consistent with the water reacting rapidly with the Ar 2 PCl compounds that are formed.It is notable that the addition of water to the P,P-metathesis reaction mixture of A 2 with C 2 in CDCl 3 had no discernible effect on the rapid formation of AC which indicates that Ar 2 PCl is not involved in the P,P-metathesis for this typical system.
When a solution of A 2 alone in CHCl 3 was irradiated with UV, some conversion to Ph 2 PCl was observed directly.The stoichiometric reaction of A 2 with (o-Tol) 2 PCl in CHCl 3 immediately produced a mixture of A 2 , AE, and Ar 2 PCl (Ar = Ph, o-Tol) (Scheme 11) along with several unidentified P-containing species.After 4 h, E 2 was detected along with small amounts of secondary phosphines Ar 2 PH (Ar = Ph, o-Tol).These observations confirm that Ar 2 PCl compounds are plausible promoters of P,P-metathesis.
To explore whether Ar 2 PCl could catalyse P,P-metathesis, a sub-stoichiometric amount (5 mol%) of Ph 2 PCl was added to a chloroform solution of pure AE.The equilibration (Scheme 11) was then monitored by 31 P{ 1 H} spectroscopy and compared with the same process carried out in the absence of Ph 2 PCl.In the absence of Ph 2 PCl, equilibrium was established in ca.70 h, whereas in the presence of 5 mol% of Ph 2 PCl, equilibrium was established in ca.7 h (see ESI †).
The key observations on the P,P-metathesis reaction between E 2 and A 2 are that: (1) the solvent (CHCl 3 or CH 2 Cl 2 ) is critical for the reaction to proceed under ambient conditions; (2) the reaction has the characteristics of being radical-initiated; (c) Ar 2 PCl catalyses the P,P-metathesis.
It is known that CHCl 3 is a source of Cl • radicals (especially under photolysis [56][57][58][59][60] or sonolysis [61][62][63][64] ) and therefore the process shown in Scheme 12 is proposed in which the role of The slow rate of the P,P-metathesis reaction between the o-tolyl diphosphane E 2 and A 2 contrasts with the rapid rate of the P,P-metathesis reaction between the isomeric p-tolyl diphosphane B 2 and A 2 .The explanation for the lower rates therefore lies in the proximity of the o-CH 3 group to the P-reaction centre.We have proposed that photolytic Ph 2 P • radical formation initiates the P,P-metathesis reactions of A 2 (see above) and, since the UV spectrum of E 2 resembles that of A 2 (see Fig.Alternatively, the o-CH 3 groups may provide a means of quenching the radical chain by H • intramolecular abstraction from the proximal CH 3 group to produce a benzylic radical by a process that is illustrated in Scheme 13.Calculations show that this would be highly unfavourable energetically (by 14.4 kcal mol −1 ) for the ground state (o-Tol) 2 P • radical but the initially formed P-centred diphosphane radical would have sufficient energy for H transfer to take place (see ESI † for details).The energetic C-centred radical could then abstract H • and the diphosphane reform thereby quenching the P,P-metathesis.
Both of these explanations are speculative and further work would be required to establish their veracity.

Conclusions
DFT calculations have shown that the thermodynamic stability of unsymmetrical diphosphanes (Z 2 P-PZ′ 2 ), with respect to their symmetrical analogues (Z 2 P-PZ 2 and Z′ 2 P-PZ′ 2 ), should increase with increasing difference in the electronegativity and/or steric bulk of the PZ 2 and PZ′ 2 constituents.In these terms, the observed stability of Me 2 P-P(CF 3 ) 2 and t Bu 2 P-PPh 2 can be understood.
The unsymmetrical diphosphanes (Ar 2 P-PAr′ 2 ) have been shown to be in finely balanced equilibria with their symmetrical analogues Ar 2 P-PAr 2 and Ar′ 2 P-PAr′ 2 which reflects their insensitivity to the aryl substituents employed (Me, MeO, CF 3 ).Significantly, the P,P-metathesis equilibria with the tetra-aryldiphosphanes were generally established rapidly (<10 min) at ambient temperatures, in a range of solvents, even in dilute solution; furthermore, the rate was increased by photolysis and inhibited by radical scavengers.The experimental and computational evidence points to the involvement of Ar 2 P • radicals generated photochemically.
Exceptional behaviour has been observed in the P,P-metathesis reactions involving (o-Tol) 2 P-P(o-Tol) 2 .These were only observed under UV-radiation or when the reactions were carried out in CHCl 3 and CH 2 Cl 2 .The evidence supporting the involvement of Ar 2 PCl in the metatheses with (o-Tol) 2 P-P(o-Tol) 2 in chlorinated solvents includes the effect of the presence of H 2 O: no P,P-metathesis is observed and the formation of hydrolysis products Ar 2 P(vO)H is detected.By contrast, the P,P-metatheses involving other Ar 2 P-PAr 2 proceed uninhibited by the presence of water and no Ar 2 P(vO)H species are detected.
The facility with which diphosphanes undergo P,P-metathesis is similar to the S,S-metathesis with disulfides and Se, Se-metathesis with diselenides.This augurs well for the application of diphosphanes in dynamic covalent chemistry.

Fig. 3
Fig. 3 (a) Plot of heterodiphosphane BC formation as a function of time over 2 h; (b) 31 P{ 1 H} NMR spectra of the reaction between B 2 and C 2 at −20 °C in THF, shown in 10 min increments over the course of 1 h (with inverse-gated decoupling).See Fig. 1 for the peak assignments.

Fig. 6
Fig. 6 31 P{ 1 H} NMR spectra for the mixture of A 2 and E 2 : (a) in toluene after 48 h, showing no detectable P,P-metathesis; (b) in CDCl 3 after 6 h, showing equilibrium with AE has been established.

Fig. 7
Fig.7(a) Plot of the data obtained from integration of the 31 P{ 1 H} NMR signals for the AE formed in the reaction between diphosphanes A 2 and E 2 ; (b)31 P{ 1 H} NMR spectra in CDCl 3 obtained by sampling over 12 h.See Fig.6for the peak assignments.

Chart 1 Table 1
Data for the P,P-metathesis equilibria shown in Scheme 2 Time by which equilibrium had been established according to 31 P NMR spectroscopy.b In THF.c In CDCl 3 .d In C 6 D 6 .
a e Estimated value (see details in ESI †).