Synthesis of three-membered metallaphosphaheterocycles and their ring opening with Ph2PH on mixed-metal molybdenum-cobalt centres

Treatment of the alkyne-bridged molybdenum-cobalt complexes [(n'-CjHjHOQjMofu-R'CCR^CoCCCOj] (R1 = R2 = CO2Me la; R1 = H, R2 = Bu' Ib) with the phosphinoalkyne Ph2POCPh in refluxing toluene gave regiospecifically the phosphino-substituted butadiene-containing complexes [(r|5-C5H5)(OC)Mo{u-CR2=CR'CPh= C(PPh2)}Co(CO)2] (R1 = R2 = CO2Me 2a; R1 = H, R2 = Bu' 2b) in moderate yield. Single crystal X-ray diffraction studies of 2a and 2b reveal the phosphorus and molybdenum centres to be constrained into three-membered metallaphosphaheterocycles. Reaction of la with PhjPOCBu' has previously been shown to lead to P-C bond cleavage but, in contrast, reaction of Ib with this phosphinoalkyne under the same conditions affords two products, [(ri5-C5Hj)(OC)Mo{u-CBu'=CHCBu'=C(PPhj)}Co(CO)2] 2c, the analogue of 2a and 2b as the minor product, and [(n5-CjH5)(OC)Mo{u-CBu'=CHCBu'=C(PPh2O)}Co(CO)2] 3, as the major product. This major product is derived from 2c by oxidation of the phosphorus centre, with the P=O unit so formed then being incorporated into a four-membered Mo-O=P-C metallacycle. The reactivity of 2a and 2b towards diphenylphosphine has been examined and found to follow two different pathways. On reaction of 2a with Ph2PH in refluxing toluene the three-membered metallaphosphaheterocycle opens to give the vinyl-bridged complex [(ris-CJHJ)(OC)Mo{u-C(CO2Me)C(CPh=CHPPh2)C(OMe)O}(n-PPh2)Co(CO)] 4 while the vinylidene-bridged species Kn5-C5H5)(OC)Mo{u-C=CPhCH(CH2Bu')PPh2}(u-PPh2)2Co(CO)] 5 is isolated in the corresponding reaction of 2b. In addition to 2a and 2b, single crystal X-ray diffraction studies have been performed on 3,4 and 5. Possible reaction pathways for the formation of the new complexes are proposed and discussed.