title: Theoretical study of the mechanism of oxoiron(IV) formation from H2O2 and a nonheme iron(II) complex: O-O cleavage involving proton-coupled electron transfer.
abstract: 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 [Fe(II)(TMC)(OTf)(2)] with a stoichiometric amount of H(2)O(2) in CH(3)CN 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 H(2)O(2) 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 H(2)O(2) 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.
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371. hirao@ntu.edu.sg
authors.forename: Hajime
authors.initials: H
authors.lastname: Hirao
authors.forename: Feifei
authors.initials: F
authors.lastname: Li
authors.forename: Lawrence
authors.initials: L
authors.lastname: Que
authors.forename: Keiji
authors.initials: K
authors.lastname: Morokuma
mesh_terms: D004579:Electron Transport; D005296:Ferrous Compounds / Q000737:chemistry*; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D005296:Ferrous Compounds; D011522:Protons; C066317:ferryl iron; D006861:Hydrogen Peroxide; D007501:Iron; D010100:Oxygen