The sluggish oxidants [FeIV(O)(TMC)(CH3CN)]2+ (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [FeIV(O)(TMCN-d12)(OTf)]+ (TMCN-d12=1,4,7,11-tetra(methyl-d3)-1,4,7,11-tetraazacyclotetradecane) are transformed into the highly reactive oxidant [FeIV(O)(TMCO)(OTf)]+ (1; TMCO=4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom. A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1. This rationalizes nature's preference for using O-rich ligand environments for the hydroxylation of strong C−H bonds in enzymatic reactions.
Bibliographical noteFunding Information:
Financial support from the DFG (Cluster of Excellence “Unifying Concepts in Catalysis”; EXC 314-2 to K.R.), MINECO (CTQ2014-59212-P and CTQ2015-70851-ERC to M.S.), GenCat (2014SGR1202 to M.S.), FEDER (UNGI10-4E-801 to M.S.), and COST action CM1305 “ECOSTBio” is gratefully acknowledged. K.R. also thanks the Heisenberg Program of the DFG for financial support. J.E. thanks the NTU for funding. W.N. and Y.-M.L. acknowledge financial support from the NRF of Korea through CRI (NRF-2012R1A3A2048842 to W.N.), GRL (NRF-2010-00353 to W.N.), and the Basic Science Research Program (2017R1D1A1B03029982 to Y.-M.L.). XAS experiments were conducted at the SSRL beamline 7-3 (SLAC National Accelerator Laboratory, USA), with support from the DOE Office of Science (DE-AC02-76SF00515) and the NIH (P30-EB-009998 and P41-GM-103393).
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- O ligands
- hydrogen atom transfer
- oxoiron(IV) complexes
- oxygen atom transfer
- spin state