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.
Bibliographical noteFunding Information:
This work was supported at the University of Minnesota by grants from the U.S. National Science Foundation (CHE-1361773 and 1665391 to L.Q.), a Feodor Lynen Research Fellowship from the Alexander von Humboldt Foundation to J.E.M.N.K., and a postdoctoral fellowship to W.-M.C. from the Ministry of Science and Technology, Taiwan. S.S. acknowledges support from the Israel Science Foundation (ISF grant 1183/13).
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