Factors Affecting the Carboxylate Shift Upon Formation of Nonheme Diiron‑O₂ Adducts

Several [Fe^II₂(N-EtHPTB)­(μ-O₂X)]²⁺ complexes (<b>1</b>·O₂X) have been synthesized, where N-EtHPTB is the anion of <i>N,N,N′N′</i>-tetrakis­(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O₂X is an oxyanion bridge. Crystal structures reveal five-coordinate (μ-alkoxo)­diiron­(II) cores. These diiron­(II) complexes react with O₂ at low temperatures in CH₂Cl₂ (−90 °C) to form blue-green O₂ adducts that are best described as triply bridged (μ–η¹:η¹-peroxo)­diiron­(III) species (<b>2</b>·O₂X). With one exception, all <b>2</b>·O₂X intermediates convert irreversibly to doubly bridged, blue (μ–η¹:η¹-peroxo)­diiron­(III) species (<b>3</b>·O₂X). Where possible, <b>2</b>·O₂X and <b>3</b>·O₂X intermediates were characterized using resonance Raman spectroscopy, showing respective ν_O–O values of ∼850 and ∼900 cm^–1. How the steric and electronic properties of O₂X affect conversion of <b>2</b>·O₂X to <b>3</b>·O₂X was examined. Stopped-flow analysis reveals that oxygenation kinetics of <b>1</b>·O₂X is unaffected by the nature of O₂X, and for the first time, the benzoate analog of <b>2</b>·O₂X (<b>2</b>·O₂CPh) is observed

Factors Affecting the Carboxylate Shift Upon Formation of Nonheme Diiron‑O₂ Adducts

Several [Fe^II₂(N-EtHPTB)­(μ-O₂X)]²⁺ complexes (<b>1</b>·O₂X) have been synthesized, where N-EtHPTB is the anion of <i>N,N,N′N′</i>-tetrakis­(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O₂X is an oxyanion bridge. Crystal structures reveal five-coordinate (μ-alkoxo)­diiron­(II) cores. These diiron­(II) complexes react with O₂ at low temperatures in CH₂Cl₂ (−90 °C) to form blue-green O₂ adducts that are best described as triply bridged (μ–η¹:η¹-peroxo)­diiron­(III) species (<b>2</b>·O₂X). With one exception, all <b>2</b>·O₂X intermediates convert irreversibly to doubly bridged, blue (μ–η¹:η¹-peroxo)­diiron­(III) species (<b>3</b>·O₂X). Where possible, <b>2</b>·O₂X and <b>3</b>·O₂X intermediates were characterized using resonance Raman spectroscopy, showing respective ν_O–O values of ∼850 and ∼900 cm^–1. How the steric and electronic properties of O₂X affect conversion of <b>2</b>·O₂X to <b>3</b>·O₂X was examined. Stopped-flow analysis reveals that oxygenation kinetics of <b>1</b>·O₂X is unaffected by the nature of O₂X, and for the first time, the benzoate analog of <b>2</b>·O₂X (<b>2</b>·O₂CPh) is observed

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

An iron­(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand <b>L1</b> (<b>L1</b> = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]­heptadeca-1(17),13,15-triene), <b>1</b>, was prepared and characterized. Complex <b>1</b> 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, Δ<i>E</i>_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 <b>1</b>, with <i>E</i>_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 <b>1</b> catalyzed the epoxidation of cyclooctene and other olefins with H₂O₂. Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron­(IV)-oxo intermediate in the reaction of <b>1</b> with H₂O₂ 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 CH₃CN: λ_max = 705 nm, ε ≈ 240 M^–1 cm^–1; Mössbauer: δ = 0.03(2) mm/s, Δ<i>E</i>_Q = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (<b>L1</b>)­Fe^IVO intermediate was established in rapid (<i>k</i>₂ = 26.5 M^–1 s^–1 for oxidation of PPh₃ at 0 °C), associative (Δ<i>H</i>^⧧ = 53 kJ/mol, Δ<i>S</i>^⧧ = −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 (<i>k</i>₂ = 0.17 M^–1 s^–1 at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (<b>L1</b>)­Fe^IVO that could be generated from (L1)­Fe^II and H₂O₂ or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl­(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe­(IV)-oxo species, (<b>L1</b>)­Fe^IVO, is a kinetically competent intermediate for cyclooctene epoxidation with H₂O₂ at room temperature. Partial ligand oxidation of (<b>L1</b>)­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

Rationalization of the Barrier Height for <i>p</i>‑Z-styrene Epoxidation by Iron(IV)-Oxo Porphyrin Cation Radicals with Variable Axial Ligands

A versatile class of heme monoxygenases involved in many vital functions for human health are the cytochromes P450, which react via a high-valent iron­(IV) oxo heme cation radical species called Compound I. One of the key reactions catalyzed by these enzymes is CC epoxidation of substrates. We report here a systematic study into the intrinsic chemical properties of substrate and oxidant that affect reactivity patterns. To this end, we investigated the effect of styrene and para-substituted styrene epoxidation by Compound I models with either an anionic (chloride) or neutral (acetonitrile) axial ligand. We show, for the first time, that the activation enthalpy of the reaction is determined by the ionization potential of the substrate, the electron affinity of the oxidant, and the strength of the newly formed C–O bond (approximated by the bond dissociation energy, BDE_OH). We have set up a new valence bond model that enables us to generalize substrate epoxidation reactions by iron­(IV)-oxo porphyrin cation-radical oxidants and make predictions of rate constants and reactivities. We show here that electron-withdrawing substituents lead to early transition states, whereas electron-donating groups on the olefin substrate give late transition states. This affects the barrier heights in such a way that electron-withdrawing substituents correlate the barrier height with BDE_OH, while the electron affinity of the oxidant is proportional to the barrier height for substrates with electron-donating substituents

Two-Step Binding of O₂ to a Vanadium(III) Trisanilide Complex To Form a Non-Vanadyl Vanadium(V) Peroxo Complex

Treatment of V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>1</b>) (Ar = 3,5-Me₂C₆H₃) with O₂ was shown by stopped-flow kinetic studies to result in the rapid formation of (η¹-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>2</b>) (Δ<i>H</i>^⧧ = 3.3 ± 0.2 kcal/mol and Δ<i>S</i>^⧧ = −22 ± 1 cal mol^–1 K^–1), which subsequently isomerizes to (η²-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>3</b>) (Δ<i>H</i>^⧧ = 10.3 ± 0.9 kcal/mol and Δ<i>S</i>^⧧ = −6 ± 4 cal mol^–1 K^–1). The enthalpy of binding of O₂ to form <b>3</b> is −75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of <b>3</b> and <b>1</b> to form 2 equiv of OV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>4</b>) occurs by initial isomerization of <b>3</b> to <b>2</b>. The results of computational studies of this rearrangement (Δ<i>H</i> = 4.2 kcal/mol; Δ<i>H</i>^⧧ = 16 kcal/mol) are in accord with experimental data (Δ<i>H</i> = 4 ± 3 kcal/mol; Δ<i>H</i>^⧧ = 14 ± 3 kcal/mol). With the aim of suppressing the formation of <b>4</b>, the reaction of O₂ with <b>1</b> in the presence of ^<i>t</i>BuCN was studied. At −45 °C, the principal products of this reaction are <b>3</b> and ^<i>t</i>BuC­(O)­NV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>5</b>), in which the bound nitrile has been oxidized. Crystal structures of <b>3</b> and <b>5</b> are reported

Role of Axial Base Coordination in Isonitrile Binding and Chalcogen Atom Transfer to Vanadium(III) Complexes

The enthalpy of oxygen atom transfer (OAT) to V­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>, forming OV­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>–O, and the enthalpies of sulfur atom transfer (SAT) to <b>1</b> and V­(N­[<i>t</i>-Bu]­Ar)₃, <b>2</b> (Ar = 3,5-C₆H₃Me₂), forming the corresponding sulfides SV­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>–S, and SV­(N­[<i>t</i>-Bu]­Ar)₃, <b>2</b>–S, have been measured by solution calorimetry in toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene) and Ph₃SbS as chalcogen atom transfer reagents. The V–O BDE in <b>1</b>–O is 6.3 ± 3.2 kcal·mol^–1 lower than the previously reported value for <b>2</b>–O and the V–S BDE in <b>1</b>–S is 3.3 ± 3.1 kcal·mol^–1 lower than that in <b>2</b>–S. These differences are attributed primarily to a weakening of the V–N_axial bond present in complexes of <b>1</b> upon oxidation. The rate of reaction of <b>1</b> with dbabhNO has been studied by low temperature stopped-flow kinetics. Rate constants for OAT are over 20 times greater than those reported for <b>2</b>. Adamantyl isonitrile (AdNC) binds rapidly and quantitatively to both <b>1</b> and <b>2</b> forming high spin adducts of V­(III). The enthalpies of ligand addition to <b>1</b> and <b>2</b> in toluene solution are −19.9 ± 0.6 and −17.1 ± 0.7 kcal·mol^–1, respectively. The more exothermic ligand addition to <b>1</b> as compared to <b>2</b> is opposite to what was observed for OAT and SAT. This is attributed to less weakening of the V–N_axial bond in ligand binding as opposed to chalcogen atom transfer and is in keeping with structural data and computations. The structures of <b>1</b>, <b>1</b>–O, <b>1</b>–S, <b>1</b>–CNAd, and <b>2</b>–CNAd have been determined by X-ray crystallography and are reported

Two-Step Binding of O₂ to a Vanadium(III) Trisanilide Complex To Form a Non-Vanadyl Vanadium(V) Peroxo Complex

Treatment of V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>1</b>) (Ar = 3,5-Me₂C₆H₃) with O₂ was shown by stopped-flow kinetic studies to result in the rapid formation of (η¹-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>2</b>) (Δ<i>H</i>^⧧ = 3.3 ± 0.2 kcal/mol and Δ<i>S</i>^⧧ = −22 ± 1 cal mol^–1 K^–1), which subsequently isomerizes to (η²-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>3</b>) (Δ<i>H</i>^⧧ = 10.3 ± 0.9 kcal/mol and Δ<i>S</i>^⧧ = −6 ± 4 cal mol^–1 K^–1). The enthalpy of binding of O₂ to form <b>3</b> is −75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of <b>3</b> and <b>1</b> to form 2 equiv of OV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>4</b>) occurs by initial isomerization of <b>3</b> to <b>2</b>. The results of computational studies of this rearrangement (Δ<i>H</i> = 4.2 kcal/mol; Δ<i>H</i>^⧧ = 16 kcal/mol) are in accord with experimental data (Δ<i>H</i> = 4 ± 3 kcal/mol; Δ<i>H</i>^⧧ = 14 ± 3 kcal/mol). With the aim of suppressing the formation of <b>4</b>, the reaction of O₂ with <b>1</b> in the presence of ^<i>t</i>BuCN was studied. At −45 °C, the principal products of this reaction are <b>3</b> and ^<i>t</i>BuC­(O)­NV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>5</b>), in which the bound nitrile has been oxidized. Crystal structures of <b>3</b> and <b>5</b> are reported

Electron-Transfer Studies of a Peroxide Dianion

A peroxide dianion (O₂^2–) can be isolated within the cavity of hexacarboxamide cryptand, [(O₂)⊂mBDCA-5t-H₆]^2–, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O₂)⊂mBDCA-5t-H₆]^2– with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler–Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O₂^2– occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O₂^2– within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal–air batteries and the use of [(O₂)⊂mBDCA-5t-H₆]^2– as a chemical reagent

Thermodynamic and Kinetic Study of Cleavage of the N–O Bond of N‑Oxides by a Vanadium(III) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V­(N­[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) from compounds containing N–O bonds with a range of BDEs spanning nearly 100 kcal mol^–1: PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N₂O (62) > MesCNO (53) > N₂O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis­(diisopropyl)­phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis­(diisopropyl)­phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N₂O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N₂O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-Bu]­Ar)₃ is strong (BDE = 154 ± 3 kcal mol^–1) compared with all the N–O bonds cleaved. In contrast, measurement of the N–O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol^–1) and that dissociation of dbabhNO to anthracene, N₂, and a ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts

Thermodynamic and Kinetic Study of Cleavage of the N–O Bond of N‑Oxides by a Vanadium(III) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V­(N­[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) from compounds containing N–O bonds with a range of BDEs spanning nearly 100 kcal mol^–1: PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N₂O (62) > MesCNO (53) > N₂O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis­(diisopropyl)­phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis­(diisopropyl)­phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N₂O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N₂O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-Bu]­Ar)₃ is strong (BDE = 154 ± 3 kcal mol^–1) compared with all the N–O bonds cleaved. In contrast, measurement of the N–O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol^–1) and that dissociation of dbabhNO to anthracene, N₂, and a ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts