1,4-Selective Hydrovinylation of Diene Catalyzed by an Iron Diimine Catalyst: A Computational Case Study on Two-State Reactivity

First-row
transition-metal catalysis has been attracting great attention
in recent years, partly due to its low toxicity and low cost, as well
as a wide variety in reactivities. However, the theoretical understanding
behind this important class of reactions is still quite limited, and
how the presence of low-lying high-spin states benefits their reactivities
is not well known. In this work, we have performed a detailed density
functional theory (DFT) study on a previously reported iron diimine-catalyzed
hydrovinylation, which we have found to exhibit an interesting “two-state
reactivity.” Specifically, we found that despite the fact that
the resting state of the reaction was experimentally determined to
be a triplet state previously, the rate-determining and product-determining
steps are found to preferably proceed at a singlet state. A triplet
state is better at stabilizing the intermediates and imposes fewer
geometric constraints for the substrate to adjust its conformations.
A singlet state allows an extra available metal d orbital for interaction
with ligand, which facilitates oxidative coupling of diene with an
incoming alkene, β-hydride transfer, and ligand substitution.
Through in-depth analysis of the electronic structures, we found that
the two-state reactivity phenomenon is due to the interplay between
orbital interactions, exchange interactions, and coordination geometry,
a conclusion that would also serve as an important step in the pursuit
of understanding in the first-row transition-metal catalysis and benefit
the future design of catalysis with earth-abundant metals.