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 identified 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.