Phosphorus-carbon bond formation in heterobimetallic molybdenum-tungsten alkyne complexes

The heterobimetallic complexes [MoW(μ-R¹CCR²)(CO)₄(η-C ₅H₅)₂] (R¹ = H, R² = Ph 1a, Me 1b or H 1c; R¹ = R² = CO₂Me 1d or Me 1e) have been isolated from the reaction of a mixture of [Mo₂(CO)₆(η-C₅H₅)₂] and [W₂(CO)₆(η-C₅H₅)₂] with the corresponding alkyne R¹C≡CR² in diglyme (2,5,8-trioxanonane). Thermolysis of complexes 1a-1d with PPh₂Cl proceeded via P-Cl bond cleavage and coupling of the diphenylphosphide unit with the alkyne in one of four different ways, depending on the nature of R¹ and R², to give the products [MoW(μ-Cl)(μ-Ph₂PCR¹=CR²)(CO ₂)(η-C₅H₅)₂] (R¹ = H, R² = Ph 2a; R¹ = R² = H 2c or CO₂Me 2d), [WMoCl₂(μ-Ph₂PC=CHR²) (μ-PPh₂)(η-C₅H₅)₂] (R² = Ph 3a, Me 3b or H 3c), [MoWCl₂(μ-Ph₂PC=CHR²)(μ-PPh ₂)(η-C₅H₅)₂] (R² = Ph 4a or Me 4b) and [WMo(μ-Cl){μ-Ph₂PC(CO₂Me)=CCO ₂Me}(CO)₂(η-C₅H₅)₂] 5d. The formation of complexes 2 and 3 can be rationalised as resulting from initial substitution of a molybdenum carbonyl ligand by the halogenophosphine and that of 4 and 5 by initial substitution of a tungsten carbonyl. The significantly higher yield of the former two complexes suggests that initial substitution of a molybdenum carbonyl is the preferred pathway. Attempts at the preparation of an organophosphine-substituted derivative of 1a via its reaction with PPh₃ gave [WMo(O)(μ-CH=CHPh)(μ-PPh₂)(CO)(η-C₅H ₅)₂] 6 as the only product. A single-crystal X-ray structure determination of complex 3a shows a Mo-W distance compatible with a double bond. The phosphorus atom and the α carbon of the four-membered metallacycle are σ bonded to tungsten and the carbon-carbon double bond is π-co-ordinated to molybdenum.