Photochemically Generated Transients from κ 2 - and κ 3 -Triphos Derivatives of Group 6 Metal Carbonyls and Their Reactivity with Olefins
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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. © 2012 American Chemical Society.
author list (cited authors)
Kyran, S. J., Muhammad, S., Knestrick, M., Bengali, A. A., & Darensbourg, D. J.