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

• Published in: Organometallics 2012-04, Vol.31 (8), p.3163-3170
• Main Authors: Kyran, Samuel J, Muhammad, Sohail, Knestrick, Matthew, Bengali, Ashfaq A, Darensbourg, Donald J
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om300044k

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.