Rebound or Cage Escape? The Role of the Rebound Barrier for the Reactivity of Non-Heme High-Valent FeIV=O species
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Date
2023
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Abstract
Owing to their high reactivity and selectivity, variations in the
spin ground state and a range of possible pathways, high-valent
FeIV=O species are popular models with potential bioinspired
applications. An interesting example of a structure–reactivity
pattern is the detailed study with five nonheme amine-pyridine
pentadentate ligand FeIV=O species, including N4py: [(L1
)FeIV=
O]2+ (1), bntpen: [(L2
)FeIV=O]2+ (2), py2tacn: [(L3
)FeIV=O]2+ (3),
and two isomeric bispidine derivatives: [(L4
)FeIV=O]2+ (4) and
[(L5
)FeIV=O]2+ (5). In this set, the order of increasing reactivity
in the hydroxylation of cyclohexane differs from that with
cyclohexadiene as substrate. A comprehensive DFT, ab initio CASSCF/NEVPT2 and DLPNO-CCSD(T) study is presented to
untangle the observed patterns. These are well reproduced
when both activation barriers for the C H abstraction and the
OH rebound are taken into account. An MO, NBO and
deformation energy analysis reveals the importance of π(pyr) !
π*xz(FeIII-OH) electron donation for weakening the FeIII-OH bond
and thus reducing the rebound barrier. This requires that
pyridine rings are oriented perpendicularly to the FeIII-OH bond
and this is a subtle but crucial point in ligand design for non heme iron alkane hydroxylation.