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 = But1b) 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-CButCHCPhC(PPh2)}Co(CO)2] 5 and [(η5-C5H5)(OC)2W{μ-CButCHC(PPh2)CPh}Co(CO)2] 6, in which the bridging alkyne has coupled with an intact molecule of the phosphinoalkyne Ph2PCαCβPh at either its β- or α-carbon atoms, respectively. However, on reaction of 1b with the tert-butyl-substituted phosphinoalkyne, Ph2PCCBut, regiospecific coupling and oxidation of the phosphino moiety occur to give [(η5-C5H5)(OC)W{μ-CButCHCButC(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-CButCHC(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.