Role of Fe(IV)-Oxo Intermediates in Stoichiometric and Catalytic Oxidations Mediated by Iron Pyridine-Azamacrocycles

Abstract

An iron(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand L1 (L1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene), 1, was prepared and characterized. Complex 1 contains low-spin iron(II) in a pseudo-octahedral geometry as determined by X-ray crystallography. Magnetic susceptibility measurements (298 K, Evans method) and Mössbauer spectroscopy (90 K, δ = 0.50(2) mm/s, ΔE(Q) = 0.78(2) mm/s) confirmed that the low-spin configuration of Fe(II) is retained in liquid and frozen acetonitrile solutions. Cyclic voltammetry revealed a reversible one-electron oxidation/reduction of the iron center in 1, with E(1/2)(Fe(III)/Fe(II)) = 0.49 V vs Fc(+)/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex 1 catalyzed the epoxidation of cyclooctene and other olefins with H(2)O(2). Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron(IV)-oxo intermediate in the reaction of 1 with H(2)O(2) and concomitant partial ligand oxidation. A soluble iodine(V) oxidant, isopropyl 2-iodoxybenzoate, was found to be an excellent oxygen atom donor for generating Fe(IV)-oxo intermediates for additional spectroscopic (UV-vis in CH(3)CN: λ(max) = 705 nm, ε ≈ 240 M(-1) cm(-1); Mössbauer: δ = 0.03(2) mm/s, ΔE(Q) = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (L1)Fe(IV)═O intermediate was established in rapid (k(2) = 26.5 M(-1) s(-1) for oxidation of PPh(3) at 0 °C), associative (ΔH(‡) = 53 kJ/mol, ΔS(‡) = -25 J/K mol) oxidation of substituted triarylphosphines (electron-donating substituents increased the reaction rate, with a negative value of Hammet's parameter ρ = -1.05). Similar double-mixing kinetic experiments demonstrated somewhat slower (k(2) = 0.17 M(-1) s(-1) at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (L1)Fe(IV)═O that could be generated from (L1)Fe(II) and H(2)O(2) or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe(IV)-oxo species, (L1)Fe(IV)═O, is a kinetically competent intermediate for cyclooctene epoxidation with H(2)O(2) at room temperature. Partial ligand oxidation of (L1)Fe(IV)═O occurs over time in oxidative media, reducing the oxidizing ability of the ferryl species; the macrocyclic nature of the ligand is retained, resulting in ferryl(IV) complexes with Schiff base PyMACs. NH-groups of the PyMAC ligand assist the oxygen atom transfer from ferryl(IV) intermediates to olefin substrates.

An Iron(II)[1,3‐bis(2′‐pyridylimino)isoindoline] Complex as a Catalyst for Substrate Oxidation with H2O2 – Evidence for a Transient Peroxidodiiron(III) Species

“…This metal-based oxidant appears to be mildly electrophilic, when considering that its Hammett ρ value for thioanisole oxidation of −0.40 is comparable to that of the closely related [Fe III 2 O(L) 4 (OH 2 ) 2 ] 4+ /H 2 O 2 system ( ρ = −0.55) [69] but 2.5- to 4-fold smaller than those obtained for oxygen-atom transfer reactions by [Fe III (salen)] + /H 2 O 2 ( ρ = −1.5), [70] [Fe IV (O)(TMP +• )] ( ρ = −1.2), [68] and [Fe IV (O)(PyMAC)] 2+ . [71] On the other hand, its Hammett ρ value of −0.85 for benzyl alcohol oxidation is twofold larger than those obtained with oxoiron(IV)(porphyrin radical cation) complexes (−0.4) and tenfold larger than those obtained with bona fide nonheme oxoiron(IV) complexes (−0.07). Furthermore the k H / k D value for benzyl alcohol oxidation by 1 /H 2 O 2 is smaller than those for corresponding oxidations by oxoiron(IV) complexes.…”

Asymmetric cyclopropanation of olefins catalysed by Cu(i) complexes of chiral pyridine-containing macrocyclic ligands (Pc-L*)

“…The absence of α-substituents on the styrene affected the diastereoselectivity of the reaction (Table 4, entries 1-7) yielding preferentially to the trans isomer (up to a 4 fold excess in the case of 4-Cl-styrene, entry 6). Steric hindrance at the α position does not hamper the reaction and good yields were obtained with 1,1-diphenyl ethylene (Table 4, entries [8][9][10].…”

“…8 These ligands combine the advantages of macrocyclic polyamines and those of aminopyridine ligands. 9 Despite the evident potential of this class of C 1 symmetric ligands, their application in asymmetric catalysis has not been fully exploited. 10 Indeed, beside porphyrins and related ligands, [11][12][13][14][15] tetraazamacrocycles have found a limited use in metal complexes catalytically active in enantioselective reactions.…”

Chemical characteristics of the products of the complexation reaction between copper(II) and a tetra-aza macrocycle in the presence of chloride ions

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Historically, transition metal tetra-aza complexes have been utilized extensively in biomimetic inorganic chemistry as functional models for superoxide dismutase, catalase, and copper oxidase active sites. [18][19][20][21][22][23][24][25] Metal complexes derived from tetra-aza macrocyclic ligands generally present large stability constants, on the order of log K ≥ 20.0 in the case of copper(II) complexes. 17,18,[26][27][28][29][30][31][32] In a 1969 paper by Margerum and Cabbiness, the copper(II) complex derived from a tetra-aza macrocycle was determined to be 10 4 times more stable than the non-cyclic tetraamine ligand.…”

“…16 Accordingly, tetra-aza macrocyclic ligands are well known for the tendency to accommodate uncommon oxidation states of first-row transition metal ions in solution. 23,35,[37][38][39][40][41][42][43][44][45][46][47][48][49][50] This is important because redox couples such as Cu(III)/(II) have rather positive reduction potentials (E 0 Cu(III)/(II) = 2.4 V) 51 and may be applied as redox agents in catalytic systems. [52][53][54] To date, several copper(II) tetra-aza complexes have been reported to undergo a reversible or quasi-reversible Cu(III/II) redox couple at solid electrodes, typically characterized by cyclic voltammetry (CV).…”