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 [(eta(5)-C5H5)(OC)(2)Mo(mu-(RCCR2)-C-1)Co(CO)(3)] (R-1 = R-2 = CO2Me 1a; R-1 = H, R-2 = Bu-t 1b) with the phosphinoalkyne Ph2PC=CPh in refluxing toluene gave regiospecifically the phosphino-substituted butadiene-containing complexes [(eta(5)-C5H5)(OC)Mo{mu-CR2=(CRCPh)-C-1=C(PPh2)}Co(CO)(2)] (R-1 = R-2 = CO2Me 2a; R-1 = H, R-2 = Bu-t 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 1a with Ph2PC=CBut has previously been shown to lead to P-C bond cleavage but, in contrast, reaction of 1b with this phosphinoalkyne under the same conditions affords two products, [(eta(5)-C5H5)(OC)Mo{mu-CBut=CHCBut=C(PPh2)}Co(CO)(2)] 2c, the analogue of 2a and 2b as the minor product, and [(eta(5)-C5H5)(OC)Mo{mu-CBut=CHCBut=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 [(eta(5)-C5H5)(OC)Mo{mu-C(CO2Me)C(CPh=CHPPh2)C(OMe)O}(mu-PPh2)Co(CO)] 4 while the vinylidene-bridged species [(eta(5)-C5H5)(OC)Mo{mu-C=CPhCH(CH2But)PPh2}(mu-PPh2)(2)Co(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.