TY - JOUR
T1 - Theoretical study of the mechanism of oxoiron(IV) formation from H 2O2 and a nonheme iron(II) complex
T2 - O-O cleavage involving proton-coupled electron transfer
AU - Hirao, Hajime
AU - Li, Feifei
AU - Que, Lawrence
AU - Morokuma, Keiji
PY - 2011/7/18
Y1 - 2011/7/18
N2 - It has recently been shown that the nonheme oxoiron(IV) species supported by the 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane ligand (TMC) can be generated in near-quantitative yield by reacting [FeII(TMC)(OTf) 2] with a stoichiometric amount of H2O2 in CH3CN in the presence of 2,6-lutidine (Li, F.; England, J.; Que, L., Jr.J. Am. Chem. Soc. 2010, 132, 2134 -2135). This finding has major implications for O-O bond cleavage events in both Fenton chemistry and nonheme iron enzymes. To understand the mechanism of this process, especially the intimate details of the O-O bond cleavage step, a series of density functional theory (DFT) calculations and analyses have been carried out. Two distinct reaction paths (A and B) were identified. Path A consists of two principal steps: (1) coordination of H2O2 to Fe(II) and (2) a combination of partial homolytic O-O bond cleavage and proton-coupled electron transfer (PCET). The latter combination renders the rate-limiting O-O cleavage effectively a heterolytic process. Path B proceeds via a simultaneous homolytic O-O bond cleavage of H2O2 and Fe-O bond formation. This is followed by H abstraction from the resultant Fe(III)-OH species by an •OH radical. Calculations suggest that path B is plausible in the absence of base. However, once 2,6-lutidine is added to the reacting system, the reaction barrier is lowered and more importantly the mechanistic path switches to path A, where 2,6-lutidine plays an essential role as an acid-base catalyst in a manner similar to how the distal histidine or glutamate residue assists in compound I formation in heme peroxidases. The reaction was found to proceed predominantly on the quintet spin state surface, and a transition to the triplet state, the experimentally known ground state for the TMC-oxoiron(IV) species, occurs in the last stage of the oxoiron(IV) formation process.
AB - It has recently been shown that the nonheme oxoiron(IV) species supported by the 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane ligand (TMC) can be generated in near-quantitative yield by reacting [FeII(TMC)(OTf) 2] with a stoichiometric amount of H2O2 in CH3CN in the presence of 2,6-lutidine (Li, F.; England, J.; Que, L., Jr.J. Am. Chem. Soc. 2010, 132, 2134 -2135). This finding has major implications for O-O bond cleavage events in both Fenton chemistry and nonheme iron enzymes. To understand the mechanism of this process, especially the intimate details of the O-O bond cleavage step, a series of density functional theory (DFT) calculations and analyses have been carried out. Two distinct reaction paths (A and B) were identified. Path A consists of two principal steps: (1) coordination of H2O2 to Fe(II) and (2) a combination of partial homolytic O-O bond cleavage and proton-coupled electron transfer (PCET). The latter combination renders the rate-limiting O-O cleavage effectively a heterolytic process. Path B proceeds via a simultaneous homolytic O-O bond cleavage of H2O2 and Fe-O bond formation. This is followed by H abstraction from the resultant Fe(III)-OH species by an •OH radical. Calculations suggest that path B is plausible in the absence of base. However, once 2,6-lutidine is added to the reacting system, the reaction barrier is lowered and more importantly the mechanistic path switches to path A, where 2,6-lutidine plays an essential role as an acid-base catalyst in a manner similar to how the distal histidine or glutamate residue assists in compound I formation in heme peroxidases. The reaction was found to proceed predominantly on the quintet spin state surface, and a transition to the triplet state, the experimentally known ground state for the TMC-oxoiron(IV) species, occurs in the last stage of the oxoiron(IV) formation process.
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U2 - 10.1021/ic200522r
DO - 10.1021/ic200522r
M3 - Article
C2 - 21678930
AN - SCOPUS:79960259713
SN - 0020-1669
VL - 50
SP - 6637
EP - 6648
JO - Inorganic chemistry
JF - Inorganic chemistry
IS - 14
ER -