Alkyne–phosphinoalkyne coupling reactions on mixed-metal tungsten–cobalt centres; P–C(alkyne) bond cleavage versus P–C(alkyne) bond preservation

Abstract

Reactions of the alkyne-bridged tungsten–cobalt complexes [(η⁵-C₅H₅)(OC)₂W(μ-R¹CCR²)Co(CO)₃] (R¹ = R² = CO₂Me 1a; R¹ = H, R² = Bu^t1b) with Ph₂PCCPh in refluxing toluene result in two different reaction pathways. On reaction with 1a three products are isolated namely, [(η⁵-C₅H₅)(OC)₂W{μ-C₂(CO₂Me)₂}Co(CO)₃(PPh₂CCPh)] 2, the result of substitution of a cobalt carbonyl ligand by a phosphorus-bound molecule of Ph₂PCCPh, [(η⁵-C₅H₅)(OC)₂W{μ-PhCCC(CO₂Me)C(CO₂Me)PPh₂}Co(CO)₂] 3 and [(η⁵-C₅H₅)(OC)W{μ-C(CO₂Me)C(CCPh)C(OMe)O}(μ-PPh₂)Co(CO)₂] 4, in which phosphorus–carbon(alkyne) bond cleavage of the phosphinoalkyne has ocurred along with phosphorus–carbon bond formation (3) and/or carbon–carbon bond formation (3 and 4). In contrast, reaction of 1b with Ph₂PCCPh affords two products, [(η⁵-C₅H₅)(OC)W{μ-CBu^tCHCPhC(PPh₂)}Co(CO)₂] 5 and [(η⁵-C₅H₅)(OC)₂W{μ-CBu^tCHC(PPh₂)CPh}Co(CO)₂] 6, in which the bridging alkyne has coupled with an intact molecule of the phosphinoalkyne Ph₂PC_αC_βPh at either its β- or α-carbon atoms, respectively. However, on reaction of 1b with the tert-butyl-substituted phosphinoalkyne, Ph₂PCCBu^t, regiospecific coupling and oxidation of the phosphino moiety occur to give [(η⁵-C₅H₅)(OC)W{μ-CBu^tCHCBu^tC(PPh₂O)}Co(CO)₂] 8, as the sole product. The reactivity of 6 towards diiron nonacarbonyl has been explored and found to afford the trimetallic complex [(η⁵-C₅H₅)(OC)₂W{μ-CBu^tCHC(PPh₂Fe(CO)₄)CPh}Co(CO)₂] 7 in good yield. Single crystal X-ray diffraction studies have been performed on 4, 6, 7 and 8 and possible reaction pathways for the formation of the new complexes are proposed and discussed.