Photochemically generated transients from κ ~2- and κ ~3-Triphos derivatives of group 6 metal carbonyls and their reactivity with olefins

摘要

The synthesis and characterization of (κ ~2-Triphos)M(CO) _4 derivatives, where M = Mo, W and Triphos = MeC(CH _2PPh _2) _3, are reported. Photolyses of these metal carbonyls in dichloromethane or CO _2-saturated dichloromethane readily afford the (κ ~3-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 (κ ~2-Triphos)M(CO) _3(1-hexene) adduct was observed, which subsequently releases the olefin with formation of the stable κ ~3-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 CO _2. Although no interaction of CO _2 with the transient species resulting from CO loss in the κ ~2 complex was noted on the time scale of 50 ms, an intermediate described as an η ~2- HSiEt _3 complex was observed upon addition of triethylsilane. This latter transient species underwent dissociation with κ ~3-complex formation about 15 times as fast as its 1-hexene analogue. X-ray structures of the κ ~2 complexes of Mo and W where the dangling phosphine arm has undergone oxidation are also reported.