TY - JOUR
T1 - Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction
AU - Sastri, Chivukula V.
AU - Lee, Jimin
AU - Oh, Kyungeun
AU - Yoon, Jin Lee
AU - Lee, Junghyun
AU - Jackson, Timothy A.
AU - Ray, Kallol
AU - Hirao, Hajime
AU - Shin, Woonsup
AU - Halfen, Jason A.
AU - Kim, Jinheung
AU - Que, Lawrence
AU - Shaik, Sason
AU - Nam, Wonwoo
PY - 2007/12/4
Y1 - 2007/12/4
N2 - The reactivities of mononuclear nonheme iron(IV)-oxo complexes bearing different axial ligands, [FeIV(O)(TMC)(X)]n+ [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH 3 (1-NCCH3), CF3COO- (1-OOCCF3), or N3- (1-N3)], and [FeIV(O)(TMCS)] + (1′-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4, 8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh3 and hydrogen atom abstraction from phenol O - H and alkylaromatic C - H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh3, the reactivity order of 1-NCCH3 > 1-OOCCF3 > 1-N3 > 1′-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)-oxo unit upon replacement of CH3CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C - H and phenol O - H bonds, i.e., 1′-SR > 1-N3 > 1-OOCCF3 > 1-NCCH3. Furthermore, a good correlation was observed between the reactivities of iron(IV)-oxo species in H atom abstraction reactions and their reduction potentials, Ep,c, with the most reactive 1′-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)-oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.
AB - The reactivities of mononuclear nonheme iron(IV)-oxo complexes bearing different axial ligands, [FeIV(O)(TMC)(X)]n+ [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH 3 (1-NCCH3), CF3COO- (1-OOCCF3), or N3- (1-N3)], and [FeIV(O)(TMCS)] + (1′-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4, 8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh3 and hydrogen atom abstraction from phenol O - H and alkylaromatic C - H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh3, the reactivity order of 1-NCCH3 > 1-OOCCF3 > 1-N3 > 1′-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)-oxo unit upon replacement of CH3CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C - H and phenol O - H bonds, i.e., 1′-SR > 1-N3 > 1-OOCCF3 > 1-NCCH3. Furthermore, a good correlation was observed between the reactivities of iron(IV)-oxo species in H atom abstraction reactions and their reduction potentials, Ep,c, with the most reactive 1′-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)-oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.
KW - Biomimetics
KW - High-valent iron-oxo intermediate
KW - Nonheme iron enzymes
KW - Oxygen activation
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U2 - 10.1073/pnas.0709471104
DO - 10.1073/pnas.0709471104
M3 - Article
C2 - 18048327
AN - SCOPUS:37649000653
SN - 0027-8424
VL - 104
SP - 19181
EP - 19186
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 49
ER -