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

Abstract

Treatment of the alkyne-bridged molybdenum–cobalt complexes [(η⁵-C₅H₅)(OC)₂Mo(μ-R¹CCR²)Co(CO)₃] (R¹ = R² = CO₂Me 1a; R¹ = H, R² = Bu^t1b) 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 = Bu^t2b) 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 Ph₂PCCBu^t 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-CBu^tCHCBu^tC(PPh₂)}Co(CO)₂] 2c, the analogue of 2a and 2b as the minor product, and [(η⁵-C₅H₅)(OC)Mo{μ-CBu^tCHCBu^tC(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(CH₂Bu^t)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.