Selective Formation of an Fe^IVO or an Fe^IIIOOH Intermediate From Iron(II) and H₂O₂: Controlled Heterolytic versus Homolytic Oxygen–Oxygen Bond Cleavage by the Second Coordination Sphere

Abstract

We demonstrate that the devised incorporation of an alkylamine group into the second coordination sphere of an Fe II complex allows to switchits reactivity with H 2 O 2 from the usual formation of Fe III species towards the selective generation of an Fe IV -oxo intermediate.T he Fe IV -oxo species was characterizedb yU V/Vis absorption and Mçssbauer spectroscopy. Variable-temperature kinetic analyses point towards am echanism in which the heterolytic cleavage of the OÀOb ond is triggered by ap roton transfer from the proximalt ot he distal oxygen atom in the Fe II -H 2 O 2 complex with the assistance of the pendant amine.D FT studies reveal that this heterolytic cleavage is actually initiated by an homolytic OÀOc leavage immediately followed by ap roton-coupled electron transfer (PCET) that leads to the formation of the Fe IV -oxo and release of water through ac oncerted mechanism.

Catalytically Active Iron(IV)oxo Species Based on a Bis(pyridinyl)phenanthrolinylmethane

“…The minor quadrupole doublet with an isomer shift of 0.04 mm s À 1 close to zero and a corresponding quadrupole splitting of 0.54 mm s À 1 is well in line with the formation of a high-spin Fe(IV)oxo compound and comparable values were previously reported for tetradentate tpa Ph Fe(IV)O (δ = 0.09 mm s À 1 ; ΔE Q = 0.51 mm s À 1 ; S = 2; tpa Ph : tris(5-phenylpyrrole-2-ylmethyl) amine) [24] and other ferryl species. [25,26] In addition, a ferric species (δ = 0.48 mm s À 1 ; ΔE Q = 1.91 mm s À 1 ) is observed and a similar Mößbauer spectrum upon Fe(IV)oxo formation was previously reported by Nordlander and colleagues and assigned to an oxo-bridged iron dimer impurity. [27] We assume that the observed Fe(III) species results from the decomposition of the intermediate Fe(IV)oxo compound.…”

A designed second-sphere hydrogen-bond interaction that critically influences the O–O bond activation for heterolytic cleavage in ferric iron–porphyrin complexes

“…1,2 Heterolysis of metal peroxides generates high-valent metal oxo species which are very reactive and have been demonstrated to be able to catalyze oxidation of inert organic Compounds, a process that is highly relevant to the chemistry community. [3][4][5][6][7] The inspiration for such chemistry is derived from naturally occurring heme metalloenzymes (monofunctional peroxidases, 8 catalases, 9 bifunctional catalase-peroxidases (KatGs), 10 cyt P450 monooxygenases 11 and P450 peroxygenases 12 ) where detailed biochemical experiments heralded the presence of 1 st and 2 nd coordination sphere residues in the heme active site that are essential for the generation of high-valent catalytic intermediates. The resting state of peroxidases, peroxygenases and catalases is the ferric oxidation state of the heme iron and a catalytically-competent high-valent intermediate [Fe IV ]O Porc + ], Compound I, is formed by the reaction with hydrogen peroxide or alternative two-electron oxidants.…”

“…46 In a very recent work, incorporation of a pendent amine group into the 2nd coordination sphere of a non heme iron complex has been reported to form selectively the Fe IV ]O intermediate via heterolytic O-O bond cleavage with a lower activation energy barrier by the reaction of the non-heme Fe(II) complex with H 2 O 2 . 5 To date, the role of the histidine residue as an acid-base catalyst in the distal site of peroxidases remains to be successfully modeled in any synthetic heme-based model system. Peroxidases generally oxidize organic phenols and aromatic amines using an outer sphere electron transfer and oxidation of substrates like epoxidation and sulfoxidation, traditionally performed by monooxygenases, is reported.…”

“… 46 In a very recent work, incorporation of a pendent amine group into the 2nd coordination sphere of a non heme iron complex has been reported to form selectively the Fe IV O intermediate via heterolytic O–O bond cleavage with a lower activation energy barrier by the reaction of the non-heme Fe( ii ) complex with H 2 O 2 . 5 …”

Second-sphere effects on H2O2 activation by non-heme FeII complexes: role of a phenol group in the [H2O2]-dependent accumulation of FeIVO vs. FeIIIOOH

“…Such an activation of H 2 O 2 by Fe II complexes to form Fe IV O species has been reported in slightly acidic aqueous medium 43 or in the presence of external or preorganized internal bases, 33,44,45 which act as a proton shuttle. In these instances H 2 O 2 binds to Fe II and the base shuttles a proton from the proximal to the distal O atom, generating an H 2 O leaving group and favoring a heterolytic O-O bond cleavage to generate Fe IV O (Table 1).…”