Treatment of the alkyne-bridged molybdenum–cobalt complexes [(η5-C5H5)(OC)2Mo(μ-R1CCR2)Co(CO)3] (R1 = R2 = CO2Me 1a; R1 = H, R2 = But1b) with the phosphinoalkyne Ph2PCCPh in refluxing toluene gave regiospecifically the phosphino-substituted butadiene-containing complexes [(η5-C5H5)(OC)Mo{μ-CR2CR1CPhC(PPh2)}Co(CO)2] (R1 = R2 = CO2Me 2a; R1 = H, R2 = But2b) 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 Ph2PCCBut 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, [(η5-C5H5)(OC)Mo{μ-CButCHCButC(PPh2)}Co(CO)2] 2c, the analogue of 2a and 2b as the minor product, and [(η5-C5H5)(OC)Mo{μ-CButCHCButC(PPh2O)}Co(CO)2] 3, as the major product. This major product is derived from 2c by oxidation of the phosphorus centre, with the PO unit so formed then being incorporated into a four-membered Mo–OP–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 [(η5-C5H5)(OC)Mo{μ-C(CO2Me)C(CPhCHPPh2)C(OMe)O}(μ-PPh2)Co(CO)] 4 while the vinylidene-bridged species [(η5-C5H5)(OC)Mo{μ-CCPhCH(CH2But)PPh2}(μ-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.
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