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 regiosp ecifically 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 co nstrained into three-membered metallaphosphaheterocycles. Reaction of 1a wi th Ph2PC=CBut has previously been shown to lead to P-C bond cleavage but, i n contrast, reaction of 1b with this phosphinoalkyne under the same conditi ons 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 ma jor 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 b een examined and found to follow two different pathways. On reaction of 2a with Ph2PH in refluxing toluene the three-membered metallaphosphaheterocycl e 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 isola ted in the corresponding reaction of 2b. In addition to 2a and 2b, single c rystal X-ray diffraction studies have been performed on 3, 4 and 5. Possibl e reaction pathways for the formation of the new complexes are proposed and discussed.