Privileged Role of Thiolate as the Axial Ligand in Hydrogen Atom Transfer Reactions by Oxoiron(IV) Complexes in Shaping the Potential Energy Surface and Inducing Significant H‑Atom Tunneling

An H/D kinetic isotope effect (KIE) of 80 is found at −20 °C for the oxidation of 9,10-dihydro­anthracene by [FeIV(O)­(TMCS)]+, a complex supported by the tetramethylcyclam (TMC) macrocycle with a tethered thiolate. This KIE value approaches that previously predicted by DFT calculations. Other [FeIV(O)­(TMC)­(anion)] complexes exhibit values of 20, suggesting that the thiolate ligand of [FeIV(O)­(TMCS)]+ plays a unique role in facilitating tunneling. Calculations show that tunneling is most enhanced (a) when the bond asymmetry between C–H bond breaking and O–H bond formation in the transition state is minimized, and (b) when the electrostatic interactions in the O---H---C moiety are maximal. These two factorswhich peak for the best electron donor, the thiolate ligandafford a slim and narrow barrier through which the H-atom can tunnel most effectively.