Heterometathesis of diphosphanes (R2P–PR2) with dichalcogenides (R′E–ER′, E = O, S, Se, Te)

Abstract

The reactions of R₂P–PR₂ with R′E–ER′, (where E = Se, S, O, Te) to give R₂P–ER′ have been explored experimentally and computationally. The reaction of Ph₂P–PPh₂ with PhSe–SePh gives Ph₂P–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 Ar₂P–SeR (R = Ph, ⁿBu or CH₂CH₂CO₂H) have also been prepared and characterised. The reaction of 1 with [Mo(CO)₄(nbd)] (nbd = norbornadiene) gives two products which, from their characteristic ³¹P NMR data, have been identified as cis-[Mo(CO)₄(Ph₂PSePh–P)₂] (8) and the mixed-donor complex cis-[Mo(CO)₄(Ph₂P–SePh–P)(Ph₂P–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 Ph₂P–PPh₂ with PhS–SPh gives Ph₂P–SPh (2) quantitatively but no reaction was observed between Ph₂P–PPh₂ and PhTe–TePh. Heterometathesis between Ph₂P–PPh₂ and ^tBuO–O^tBu does not occur thermally but has been observed under UV irradiation to give Ph₂P–O^tBu 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 Ph₂P–PPh₂ with ^tBuO–O^tBu is not observed in the absence of UV radiation, is therefore due to kinetics.