Indexed on: 29 Nov '17Published on: 29 Nov '17Published in: Journal of the American Chemical Society
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, which approaches the value previously predicted by DFT. 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 when: (a) 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.