DFT and time-resolved IR investigation of electron transfer between photogenerated 17- and 19-electron organometallic radicals

The photochemical disproportionation mechanism of [CpW(CO)3]2 in the presence of Lewis bases PR3 was investigated on the nano- and microsecond time-scales with step-scan FTIR time-resolved infrared spectroscopy. Laser excitation (532 nm) was used to homolytically cleave the W–W bond, forming the 17-electron radicals CpW(CO)3 and initiating the reaction. With the Lewis base PPh3, disproportionation to form the ionic products CpW(CO)3PPh3+ and CpW(CO)3- was directly monitored on the microsecond time-scale. Detailed examination of the kinetics and concentration dependence of this reaction indicates that disproportionation proceeds by electron transfer from the 19-electron species CpW(CO)3PPh3 to the 17-electron species CpW(CO)3. This result is contrary to the currently accepted disproportionation mechanism which predicts electron transfer from the 19-electron species to the dimer [CpW(CO)3]2. With the Lewis base P(OMe)3 on the other hand, ligand substitution to form the product [CpW(CO)2P(OMe)3]2 is the primary reaction on the microsecond time-scale. Density functional theory (DFT) calculations support the experimental results and suggest that the differences in the reactivity between P(OMe)3 and PPh3 are due to steric effects. The results indicate that radical-to-radical electron transfer is a previously unknown but important process for the formation of ionic products with the organometallic dimer [CpW(CO)3]2 and may also be applicable to the entire class of organometallic dimers containing a single metal–metal bond.