Dioxygen Binding to Complexes with Fe^II₂(μ-OH)₂ Cores:  Steric Control of Activation Barriers and O₂-Adduct Formation

Abstract

A series of complexes with [Fe(II)(2)(mu-OH)(2)] cores has been synthesized with N3 and N4 ligands and structurally characterized to serve as models for nonheme diiron(II) sites in enzymes that bind and activate O(2). These complexes react with O(2) in solution via bimolecular rate-limiting steps that differ in rate by 10(3)-fold, depending on ligand denticity and steric hindrance near the diiron center. Low-temperature trapping of a (mu-oxo)(mu-1,2-peroxo)diiron(III) intermediate after O(2) binding requires sufficient steric hindrance around the diiron center and the loss of a proton (presumably that of a hydroxo bridge or a yet unobserved hydroperoxo intermediate). The relative stability of these and other (mu-1,2-peroxo)diiron(III) intermediates suggests that these species may not be on the direct pathway for dioxygen activation.

Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

“…Previously, we reported the oxygenation of the diiron(II) complex, [Fe 2 (-OH) 2 (21) and characterized to have a (-oxo)(-1,2-peroxo)diiron(III) core (22). This intermediate formed with kinetics that were first order in diiron(II) complex and first order in O 2 (23,24). Here, we report studies at Ϫ80°C that have led to the observation of intermediate species before the formation of 4, including spectroscopic evidence for a diiron-superoxo intermediate 2 as well as detailed mechanistic analysis for the conversion of 2 to 4.…”

Dioxygen Activation and Catalytic Aerobic Oxidation by a Mononuclear Nonheme Iron(II) Complex

“…The electrospray ionization mass spectrum (ESI MS) of 2 exhibited a prominent ion at a mass-to-charge ratio (m/z) of 477, whose mass and isotope distribution pattern correspond to [Fe(IV)(O)(TMC)(CF 3 SO 3 )] + (calculated m/z of 477) ( Figure 1). 4a The mass peak at m/z of 477 upshifted accordingly upon introduction of 18 O when 18 O-labeled dioxygen, 18 O 2 , was used instead of 16 The catalytic aerobic oxidation of organic substrates by 1 was then investigated in a solvent mixture of CH 3 CN and butyl ether (V/V ) 1:1) at 25°C (Supporting Information, Experimental Conditions). Upon addition of 100 equiv of PPh 3 to a reaction solution containing 2 that was generated from 1 in the presence of O 2 , 2 disappeared immediately.…”

“…15,16 However, other factors such as the change of spin states of iron ion in different solvents and steric effect of iron complexes may play an important role in the O 2 activation. 16 Thus, detailed investigations are needed to establish the solvent effect on the activation of O 2 by metal complexes. 13,16 Finally, a mechanism for the O 2 activation by 1 is proposed on the basis of the analogy of O 2 activation by iron(II) porphyrins (Scheme 1A), 9,15b in which two molecules of 1 react with O 2 to form a µ-peroxo-bridged diiron(III) species, [(TMC) 2 Fe III 2 (µ-O 2 )] 4+ (3).…”

“…16 Thus, detailed investigations are needed to establish the solvent effect on the activation of O 2 by metal complexes. 13,16 Finally, a mechanism for the O 2 activation by 1 is proposed on the basis of the analogy of O 2 activation by iron(II) porphyrins (Scheme 1A), 9,15b in which two molecules of 1 react with O 2 to form a µ-peroxo-bridged diiron(III) species, [(TMC) 2 Fe III 2 (µ-O 2 )] 4+ (3). 17 Subsequent O-O bond homolysis of 3 results in the generation of two molecules of 2 that are involved in oxygen atom transfer reactions.…”

Non-innocent ground state electronic structure of a polynuclear copper complex with picolinate bridges

“…In order to investigate directly these short lived intermediates tridentate (e.g. BnBQA -N-benzyl-N,N-di(quinolin-2-ylmethyl) amine) [5] tetradentate (e.g. TPA -tris-(2-pyridylmethyl)amine) [6], and pentadentate (e.g.…”