Mono- and Dinuclear Nonheme Iron Model Complexes: O-O Bond Activation, Structural Characterization and Reactivity Study

Abstract

The structures and reactivities of mono- and dinuclear nonheme iron model complexes were investigated. In Chapters 2 and 3, O-O bond activation of H2O2 by the dinuclear complexes [(FeIII2(μ-O)(μ-OH)L2]3+ (1A) and [(FeIII2(μ-OH)2L2]4+ (2A), L = tris(3,5-di-methyl-4-methoxypyridyl-2-methyl)amine, to form the high-valent [(FeIV2(μ-O)(OH)(O)L2]3+ (3A) and [(FeIV2(μ-O)2L2]4+ (4A) was studied. H2O2 and H2O competed for binding to the Fe centers of 1A and 2A, and [H2O2] was rate limiting under the concentrations studied. The presence of base increased the H2O2 activation rate for 2A, but not for 1A. The H2O2 activation rates by 1A and 2A were comparable to that of the mononuclear nonheme iron complex [FeII(TMC)]2+ (TMC = tetramethylcyclam) (J. Am. Chem. Soc. 2010, 2134-2135) after accounting for water inhibition. A crystal structure of [(FeIV2(μ-O)2L2]4+ (4A), or diamond core, was solved and described the Fe2O2 core in more detail than the original EXAFS structural assignment. In addition, structures of other complexes with Fe2O2L2 cores in different oxidation and protonation states were also studied and compared to the Fe2O2 cores of the high-valent enzymes intermediates RNR-X and sMMO-Q. In Chapter 4, iron complexes supported by the TMC ligand were studied by X-ray crystallography. A second isomer of the [FeIV(O)(TMC)]2+ complex was found, and the mechanism of conversion to the original isomer was explored. Additionally, the crystal structure of (TMC)FeIII(μ-O)Sc(NCCH3)(OTf)4 complex was obtained and used to reassign the Fe oxidation state of the originally reported (TMC)FeIV(μ-O)Sc(OH)(OTf)4 complex.(Nat. Chem. 2010, 756-759) In Chapter 5, the hydrogen atom transfer (HAT) rates of a series of S = 2 mononuclear nonheme iron complexes, [FeIV(O)TMG2dien(X)]2+,+ (X = CH3CN, Cl-, Br-, N3-, CH3CO2- and CF3CO2-; TMG2dien = 1,1-bis{2-[N2-(1,1,3,3- tetramethylguanidino)]ethyl}amine), were reported. Substitution of CH3CN with carboxylate and halide anions cis to the oxo ligand increased the HAT oxidation rate by as much as 15 times. A series of S = 1 nonheme iron complexes, [FeIV(O)TPA(Y)]2+,+ (Y = CH3CN, Cl-, CH3CO2- and CF3CO2-; TPA = tris(pyridyl-2-methyl)amine), was also investigated to explore what effect spin state has on reactivity. The HAT rates were similar for the [FeIV(O)TPA(Y)]2+,+ series, while OAT rates were much faster for the [FeIV(O)TPA(CH3CN)]2+ species.