A Time-Resolved Infrared study of the gas-phase reactions of 1,3- and 1,4-pentadiene with Fe(CO)3 and Fe(CO)4

Steven J. Gravelle, Lambertus J. Van De Burgt, Eric Weitz*

*Corresponding author for this work

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10 Scopus citations

Abstract

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 languageEnglish (US)
Pages (from-to)5272-5283
Number of pages12
JournalJournal of Physical Chemistry
Volume97
Issue number20
StatePublished - Dec 1 1993

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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