Rebound or Cage Escape? The Role of the Rebound Barrier for the Reactivity of Non-Heme High-Valent Fe^IV=O species

Owing to their high reactivity and selectivity, variations in the spin ground state and a range of possible pathways, high-valent Fe-IV=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 Fe-IV=O species, including N4py: [(L-1)Fe-IV=O](2+) (1), bntpen: [(L-2)Fe-IV=O](2+) (2), py(2)tacn: [(L-3)Fe-IV=O](2+) (3), and two isomeric bispidine derivatives: [(L-4)Fe-IV=O](2+) (4) and [(L-5)Fe-IV=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 pi(pyr) -> pi*(xz)(Fe-III-OH) electron donation for weakening the Fe-III-OH bond and thus reducing the rebound barrier. This requires that pyridine rings are oriented perpendicularly to the Fe-III-OH bond and this is a subtle but crucial point in ligand design for non-heme iron alkane hydroxylation.