Photochemically Generated Transients from κ²- and κ³-Triphos Derivatives of Group 6 Metal Carbonyls and Their Reactivity with Olefins

The synthesis and characterization of (κ²-Triphos)­M­(CO)₄ derivatives, where M = Mo, W and Triphos = MeC­(CH₂PPh₂)₃, are reported. Photolyses of these metal carbonyls in dichloromethane or CO₂-saturated dichloromethane readily afford the (κ³-Triphos)­M­(CO)₃ complexes with no evidence of significant solvent or carbon dioxide interactions with the site vacated by CO. However, in the presence of 1-hexene a transient (κ²-Triphos)­M­(CO)₃(1-hexene) adduct was observed, which subsequently releases the olefin with formation of the stable κ³-tricarbonyl species. In the case of M = W the kinetic parameters for this process were assessed, with the rate of olefin replacement being inversely proportional to [1-hexene]. A dissociative rate constant of 25.6 ± 1.1 s^–1 at 298 K was determined for olefin loss, with the selectivity for 1-hexene vs free phosphine arm addition to the unsaturated intermediate being somewhat surprisingly large at 22. The activation parameters measured were ΔH^⧧ = 26.1 ± 0.4 kcal/mol and ΔS^⧧ = 36 ± 3 eu, which are consistent with a dissociative substitution reaction. The kinetic parameters for this transformation were unaffected in the presence of excess quantities of CO₂. Although no interaction of CO₂ with the transient species resulting from CO loss in the κ² complex was noted on the time scale of 50 ms, an intermediate described as an η²-HSiEt₃ complex was observed upon addition of triethylsilane. This latter transient species underwent dissociation with κ³-complex formation about 15 times as fast as its 1-hexene analogue. X-ray structures of the κ² complexes of Mo and W where the dangling phosphine arm has undergone oxidation are also reported.