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

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, Bu-n 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 P-31 NMR data, have been identified as cis-[Mo(CO)(4)(Ph2P-SePh-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 (BuO)-Bu-t-(OBu)-Bu-t does not occur thermally but has been observed under UV irradiation to give Ph2P-(OBu)-Bu-t 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 (BuO)-Bu-t-(OBu)-Bu-t is not observed in the absence of UV radiation, is therefore due to kinetics.