Time-resolved infrared (TRIR) spectroscopy has been used to study the addition of trans-1,3-pentadiene (1,3-PD) and 1,4-pentadiene (1,4-PD) to Fe(CO)3 in the gas phase. The addition of either diene to Fe(CO)3 initially involves the formation of an excited-state triplet species, [(PD)Fe(CO)3]†. This excited-state intermediate can then either relax to form the ground-state singlet product, (η4-PD)Fe(CO)3, dissociate to regenerate the Fe(CO)3 and PD reactants, or rearrange to form the unsaturated intermediate, (η2-PD)Fe(CO)3. The rate constant for addition of either diene to Fe(CO)3 in the high-pressure limit is (1.7 ± 0.2) × 1014 cm3/(mol s), which approaches the gas-kinetic cross section for this process. The only significant difference between the reactive behavior of 1,3-PD and 1,4-PD with Fe(CO)3 is that addition of 1,4-PD leads to the formation of a small amount of an isomer of (η2-1,4-PD)Fe(CO)3. This species is generated from triplet [(PD)Fe(CO)3]† with an activation energy and preexponential factor estimated to be 12 kcal/mol and 4.5 × 1011 s-1, respectively. It is most likely that this isomer is (η2:CH-1,4-PD)Fe(CO)3, which contains an agostic M-H-C bond. The rates for addition of pentadiene to Fe(CO)4 have also been measured and are (7.0 ± 0.4) × 1011 and (3.5 ± 0.1) × 1011 cm3/(mol s) for 1,3-PD and 1,4-PD, respectively. The reaction mechanism for Fe(CO)3 + PD is compared to that for the Cr(CO)4 + PD system, which has been the subject of a prior study. Conclusions for the two systems are (1) neither the Cr(CO)4 + PD nor the Fe(CO)3 + PD mechanism is dominated by effects due to pentadiene conjugation, (2) the Cr(CO)4 mechanism is more influenced by geometric differences between 1,3-PD and 1,4-PD than is Fe(CO)3, and (3) the Cr(CO)4 mechanism occurs on one singlet potential surface whereas the Fe(CO)3 mechanism involves both triplet and singlet potential energy surfaces.
|Original language||English (US)|
|Number of pages||12|
|Journal||Journal of Physical Chemistry|
|State||Published - Dec 1 1993|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry