Role of Fe(IV)-oxo intermediates in stoichiometric and catalytic oxidations mediated by iron pyridine-azamacrocycles

Article

pubs.acs.org/IC

Role of Fe(IV)-Oxo Intermediates in Stoichiometric and Catalytic

Oxidations Mediated by Iron Pyridine-Azamacrocycles

Wanhua Ye,^†Douglas M. Ho,^‡Simone Friedle,^§Taryn D. Palluccio,^†and Elena V. Rybak-Akimova*^,†

^†Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States

^‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States

^§Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

S

* Supporting Information

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

Mossbauer 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 H₂O₂. Low-temperature stopped-ﬂow kinetic studies

demonstrated the formation of an iron(IV)-oxo intermediate

in the reaction of 1 with H₂O₂and concomitant partial ligand

oxidation. A soluble iodine(V) oxidant, isopropyl 2-iodox-

ybenzoate, 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⁻¹cm⁻¹; Mossbauer: δ = 0.03(2) mm/s, ΔE = 2.00(2) mm/s) and kinetic

̈

Q

studies. The electrophilic character of the (L1)Fe^IVO intermediate was established in rapid (k₂= 26.5 M⁻¹s⁻¹for oxidation of

PPh₃at 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 (k₂= 0.17 M⁻¹s⁻¹at 0 °C), clean, second-order oxidation of cyclooctene into

epoxide with preformed (L1)Fe^IVO that could be generated from (L1)Fe^IIand H₂O₂or 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 H₂O₂at 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.

clear nonheme iron chemistry¹⁰⁻¹⁵demonstrate the impor-

INTRODUCTION

■

tance of Fe^IVO intermediates in the catalytic cycles of a

number of enzymes, including pterin-dependent phenylalanine

hydroxylase and α-ketoglutarate-dependent prolyl hydroxy-

lase.^4,5,16,17Several high-spin Fe(IV) intermediates in mono-

nuclear nonheme iron enzymes were spectroscopically

characterized and in certain cases were shown to be kinetically

competent oxidants of the enzyme’s biological targets.^5,16,18,19

Other examples of observable iron(IV) intermediates in

Increasing interest in synthetic applications of cheap, readily

available, nontoxic metals, such as iron, inspires the search for

biomimetic oxidation catalysts.¹Key mechanistic insight into

biological and biomimetic oxidations is gained by identifying

metal-based intermediates and characterizing their reactivity

with target substrates. High-valent iron-oxo intermediates, such

as (P^−.)Fe^IVO, are well-established active oxidants in heme

chemistry.^2,3Similarly, dinuclear nonheme iron enzymes, such

as methane monooxygenase, rely on the potent oxidizing ability

of high-valent diiron(IV) intermediates that are competent to

oxidize hydrocarbons.⁴⁻¹¹Recent developments in mononu-

Received: November 13, 2011

Published: April 25, 2012

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biological systems include soybean lipoxygenase-3,¹⁰naphtha-

lene dioxygenase,²⁰and α-keto acid-dependent dioxygenase

(TauD).^16,21

N4 systems that stabilize Fe(IV)-oxo intermediates include the

oxoiron(IV) generated from iron(II) complexes with TMC

(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane),²⁷

TAPM (1,4,8,12-tetramethyl-1,4,8,12-tetraazacyclopentade-

cane),²⁹and TAPH (1,4,8,12-tetraazacyclopentadecane).²⁹

Wieghardt et al. reported the generation of an oxoiron(IV)

intermediate based on the cyclamacetate (1,4,8,11-tetraazacy-

clotetradecane 1-acetate) macrocycle.³⁰Iron(IV)-oxo inter-

mediates were also characterized in systems containing

polydentate aminopyridine ligands such as TPA (tris(2-

pyridinemethyl)amine,²⁸N4Py (N,N-bis(2-pyridylmethyl)-N-

bis(2-pyridyl)methylamine),³¹and BPMCN (N,N-bis(2-pyr-

idylmethyl)-N,N-dimethyl-trans-1,2-diaminocyclohexane).³²

These oxoiron(IV) species can be generated by using single

oxygen atom donors such as iodosylbenzene (PhIO),^{29,31,33−36}

m-chloroperoxybenzoic acid (m-CPBA),^37,38peracetic acid

(CH₃COOOH),^28,34potassium monopersulfate (KHSO₅),³⁴

NaOX (X = Cl or Br)³⁹or O₃.^30,40

Iron complexes with polyamine or aminopyridine ligands are

frequently used as models of nitrogen-rich active sites of

nonheme oxidases and oxygenases.^10,15,22,23Similar to enzymes,

several types of metal−oxygen intermediates were identiﬁed in

reactions of these synthetic complexes with peroxides and other

oxygen atom donors. Recently discovered iron(IV)-oxo species

are of particular interest, with several of them crystallized at low

temperature and structurally characterized.²⁴⁻²⁶Notably, the

ﬁrst crystallographically characterized ferryl(IV) complex

contained a tetraazamacrocyclic (tetramethylcyclam) ligand,²⁷

and characterization of an Fe^IVO complex supported by an

acyclic aminopyridine ligand soon followed.²⁸A number of

related ferryl(IV) species were identiﬁed and characterized by

spectroscopic methods;²⁴⁻²⁶polyamine azamacrocycles or

acyclic aminoopyridine ligands were predominantly utilized in

this chemistry (Figure 1). Examples of macrocyclic tetradentate

Advances in our understanding of the mechanistic role of

Fe(IV)-oxo intermediates in nonheme iron enzymes and

synthetic accessibility of similar iron(IV) intermediates in

model complexes have prompted investigators to view high-

valent iron(IV)-oxo species as key intermediates responsible for

oxygen atom transfer to organic substrates in catalytic

oxidations.^{6−8,12,13,26,41−52}However, direct experiments that

characterize the reactivity of iron(IV) intermediates are limited

and include primarily stoichiometric reactions.^24,53Pregener-

ated ferryl(IV) species can participate in oxygen atom transfer

reactions with phosphines, sulﬁdes, and certain oleﬁns (e.g.,

cyclooctene),^49,54−57in hydrogen atom abstractions (e.g.,

formation of anthracene from dehydroanthracene),^46,50,58,59

and in electron transfer reactions.^60,61However, questions

about the intermediacy of ferryl(IV) in catalytic oxidations with

nonheme iron complexes remain.^{13,56,57,62,63}For example,

Figure 1. Selected tetra- and pentadentate ligands that support high-

valent iron-oxo intermediates.

Figure 2. Pyridine-containing macrocycles (PyMACs) used in this work.

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thickness, 0.25 μm). Helium at a ﬂow rate of 45 cm³/s was used as the

carrier gas. Injections were made in the splitless mode using an initial

column temperature of 50 °C. The temperature was raised at 15 °C/

min until 320 °C. Full scan was performed using ionization energy of

70 eV. Elemental analyses were performed by Schwarzkopf Micro-

analytical Laboratory, Inc. (Woodside, NY).

X-ray Crystallography. Single-crystal X-ray data for NiL1 (see

Supporting Information) and 1 were collected using a Bruker SMART

APEXII diﬀractometer equipped with an Oxford Cryosystems 700

Series Cryostream Cooler and Mo Kα (λ = 0.71073 Å) radiation at

173 K. All structures were solved by direct methods using SHELXTL⁶⁸

and were reﬁned against all reﬂections by full-matrix least-squares on

F²using SHELXTL. Speciﬁc crystal, reﬂection, and reﬁnement data for

eﬃcient oleﬁn epoxidation with hydrogen peroxide catalyzed by

Fe(TPA)²⁺or Fe(BPMEN)²⁺in the presence of acetic acid

proceeds under conditions that favor the formation of

(L)Fe^IVO. This intermediate was independently generated

and subjected to reactions with oleﬁns. Surprisingly, no epoxide

(or an alternative product, cis-diol) formed in these experi-

ments, clearly demonstrating that ferryl(IV) is not a competent

intermediate in these epoxidations or related dihydroxyla-

tions.⁵⁶Given that epoxidation is a synthetically useful and

mechanistically important process,⁶⁴further studies are clearly

needed to understand oxygen atom transfer in catalytic

reactions with synthetic nonheme iron complexes. Using

hydrogen peroxide, a nontoxic oxidant that generates water as

the oxidation byproduct, is particularly attractive.⁵⁴⁻⁵⁷

Table 1. Crystallographic Data, Experimental Conditions,

In the present work, we set out to explore the reactivity of

Fe(IV) intermediates derived from biomimetic iron complexes

that are catalytically active in oleﬁn epoxidation with H₂O₂. Our

group is interested in using pyridine-macrocycle ligands

(PyMACs), depicted in Figure 2, to explore the reactivity of

the iron complexes in catalytic oxidations. These ligands

combine the advantages of macrocyclic polyamines, which

prevent iron loss under catalytic conditions by forming

thermodynamically and kinetically stable iron complexes, and

those of aminopyridine ligands that are capable of stabilizing a

variety of iron-based intermediates. In our previous study,⁶⁵we

reported high activity of the iron(II) complex 3 in oleﬁn

epoxidation with H₂O₂in the presence of HOTf. To focus on

the role of iron(IV) intermediates in oxidations catalyzed by

Fe(PyMAC) complexes, we now prepared a simpler structural

analogue of 3, an iron(II) complex with a pyridine-containing

14-membered macrocyclic ligand L1 (L1 = 2,7,12-trimethyl-

3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-tri-

ene). This unfunctionalized complex, 1, generates an iron(IV)-

oxo intermediate (1b) using isopropyl 2-iodoxybenzoate as an

oxidant. Spectroscopic characterization of 1b and its reactivity

with substrates (oleﬁns or triarylphosphines) under stoichio-

metric and catalytic conditions are reported.

and Reﬁnement Details for 1

crystal data

1

chemical formula

Fw

C₂₂H₃₄F₆FeN₆O₆S₂

712.52

crystal system

space group

a (Å)

monoclinic

P2₁/c

16.4722(6)

14.1244(5)

13.3141(5)

100.968(3)

3050.21(19)

4

b (Å)

c (Å)

β (deg)

volume (Å³)

Z

D_c(g cm⁻³

crystal size (mm³)

abs coeﬀ (mm⁻¹

)

1.552

0.20 × 0.10 × 0.04

0.715

)

transmission range

no. reﬂns collected

no. indep collected

0.9754−0.8703

19365

5365

a

R₁[I > 2σ(I)]

0.0619

b

wR₂(all data)

0.1317

GOF (F²)

1.020

diﬀ peaks (e Å⁻³

)

0.537, −0.280

a

b

R₁= ∑||F_o| − |F_c||/∑|F_o|. wR₂= {∑[w(F_o− F_c²)²]}^1/2where w =

2

1/σ²(F_o) +(aP)²+ bP; R factors based on F²are statistically about

EXPERIMENTAL SECTION

■

twice as large as those based on F.

General Considerations. All reagents were obtained from

commercially available sources and used as received unless otherwise

noted. All manipulations of air-sensitive materials were carried out in

an MBraun glovebox under an atmosphere of ultra high purity argon.

Isopropyl 2-iodoxybenzoate was synthesized following a published

procedure.⁶⁶H₂¹⁸O₂(90% ¹⁸O-enriched, 2% solution in H₂¹⁶O) and

H₂¹⁸O (98% ¹⁸O-enriched) were purchased from Cambridge Isotope

Laboratories Inc. Fe(OTf)₂·2CH₃CN (OTf = CF₃SO₃¯) was prepared

from FeCl₂and CF₃SO₃SiMe₃in CH₃CN following an unpublished

procedure provided by Dr. Miquel Costas and Prof. Lawrence Que, Jr.

⁵⁷Fe(OTf)₂·2CH₃CN (ca. 40% ⁵⁷Fe with respect to total iron content)

was prepared by a literature procedure from elemental iron.⁶⁷The

oleﬁn substrates were puriﬁed by passing through a short column of

activated alumina and stored in a glovebox. All concentrations

intended for the stopped-ﬂow experiments are reported before mixing

unless otherwise noted.

1 are contained in Table 1. The macrocyclic cation in NiL1 was

located in the symmetry plane and suﬀered from whole molecule

disorder along with disorder of the counterions; complex 1 contained

only disordered triﬂate counterions (see Supporting Information for

details).

57

̈

Fe Mossbauer Spectroscopy. Mossbauer spectra were

̈

recorded on an MS1 spectrometer (WEB Research Co.) with a ⁵⁷Co

source in an Rh matrix maintained at room temperature. CH₃CN was

used as solvent for the preparation of all samples. A number of

separate samples of the intermediate and decomposition products

were prepared and measured to conﬁrm reproducibility. ⁵⁷Fe-enriched

samples of the reaction intermediate were prepared with ⁵⁷Fe-

(OTf)₂·2CH₃CN. In general, a solution of isopropyl 2-iodoxybenzoate

(20 mg, 0.5 mL) was added to a solution of 1 (4.5 mg, 0.35 mL) in

CH₃CN at −40 °C. After a short period of time (seconds), the

reaction mixture was transferred to a precooled nylon sample holder

(−40 °C) and quickly frozen in liquid nitrogen. Data were acquired at

Physical Methods. Room temperature UV−vis spectra were

recorded on a Jasco V-570 spectrophotometer. Electrospray

ionization-mass spectrometry (ESI-MS) was performed on a Thermo

Finnigan LCQ quadrupole ﬁeld mass spectrometer with electrospray

ionization in the positive ion detection mode. NMR spectra were

recorded in CDCl₃at 300 K on a Bruker DPX 300 MHz spectrometer;

chemical shifts in δ are reported versus tetramethylsilane. GC-MS

analyses were performed using a Shimadzu QP5050A mass

spectrometer connected to a Shimadzu GC-17A gas chromatograph

equipped with Rtx-XLB column (length, 30 m; i.d. 0.25 mm; ﬁlm

90 K without any applied magnetic ﬁelds. Mossbauer spectra were ﬁt

̈

to Lorentzian lines by using the WMOSS plot and ﬁt software.⁶⁹The

isomer shift (δ) values are quoted relative to natural iron foil used for

velocity calibration at 298 K.

Cyclic Voltammetry. Cyclic voltammetry was carried out using a

CH Instruments electrochemical analysis system (Model 830) using a

conventional three-electrode conﬁguration. A glassy carbon electrode

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Scheme 1. Synthesis of Iron(II) Complex 1

(3 mm diameter) was used as the working electrode and platinum

wires as counter and reference electrodes. Measurements were

performed with 2 mM analyte in CH₃CN at ambient temperature

under argon using 0.1 M tetra-n-butylammonium hexaﬂuorophosphate

as the supporting electrolyte. The range of potentials was scanned at a

rate of 100 mV s⁻¹. The ferrocene/ferrocenium (Fc/Fc⁺) reference

couple was used as an internal standard.

Measurements of Eﬀective Magnetic Moment in Solutions.

Measurements by Evans method were performed on a Bruker DPX-

300 spectrometer at 25 °C.⁷⁰⁻⁷²A solution of complex 1 (1 mM) in

0.5 mL of CD₃CN (with 1% tetramethylsilane, TMS) was prepared

under an argon atmosphere. The eﬀective magnetic moment (μ_eﬀ) was

calculated from μ_eﬀ= 8χ_gM_wT, where χ_g(cm₃g⁻¹) is the corrected

molar susceptibility derived from χ_g= 3δν/4πν_oCM_w+ χ_o.⁷³δν is the

shift in frequency (Hz) from the value found for the pure solvent, C is

the concentration of the complex (mol cm⁻³), M_wis the molecular

weight of the complex (g mol⁻¹), ν_ois the operating frequency of the

NMR spectrometer (Hz), and χ_ois the mass susceptibility of the pure

solvent (−0.682 × 10⁻⁶cm³g⁻¹for acetonitrile). The 4π/3 shape

factor is for a cylindrical sample in a superconducting magnet. The

number of unpaired electrons per molecule, n, can then be easily

derived from the magnetic moment using μ²= g²S(S + 1), where S is

Synthesis of [Ni^II(L1)](ClO₄)₂(NiL1). NiL1 and ligand L1 were

prepared following a literature procedure,^79,80except that the amine

1,7-diamino-4-methyl-4-azaheptane was used. Recrystallizing the

product from warm (65 °C) water allowed for separation of isomers.

The ﬁrst deep orange crystalline complex contains mostly meso ligand;

the latter orange complex contains mostly rac ligand. ESI(+)−MS

(CH₃CN) m/z: [Ni(L1)]²⁺calcd for 165.1; Found, 165.2; {[Ni-

(L1)]−H}⁺calcd for 329.2; Found, 329.4; {[Ni(L1)](ClO₄)}⁺calcd

for 429.1; Found, 429.2. Stereochemistry of Ni(L1)(ClO₄)₂was

conﬁrmed by X-ray crystallography (Supporting Information).

Synthesis of Ligand L1. A solution of NiL1 (0.05 g, 0.094 mmol)

in 5 mL of warm water, and sodium cyanide (0.028 g, 0.564 mmol)

was added. The solution changes color immediately, proceeding from

red-orange, through violet to yellow. After stirring for 10 min, the

solution was extracted with dichloromethane (3 × 15 mL); the

combined extracts were dried with anhydrous MgSO₄and the

dichloromethane was removed on a rotary evaporator to leave the

product as yellow oil. ESI(+)−MS (CH₃CN) m/z: (L1+H)⁺calcd for

276.2; Found, 276.5. ¹H NMR of a pure meso L1 (δ, ppm in CDCl₃):

1.41 (6H, d), 1.64−1.69 (4H, m), 1.97−2.56 (8H, m), 3.10 (4H, s),

3.63 (2H, quartet), 6.93 (2H, d), 7.48 (1H, t). ¹H NMR of a pure rac

L1 (δ, ppm in CDCl₃): 1.41 (6H, d), 1.64−1.69 (4H, m), 1.97−2.56

(8H, m), 3.10 (4H, s), 3.79 (2H, quartet), 7.05 (2H, d), 7.58 (1H, t).

Oxidative degradation of this material in boiling aqueous HNO₃/

Na₂S₂O₈, followed by neutralization and addition of excess

dimethylglyoxime did not aﬀord red Ni(dmg)₂product, indicating

essentially quantitative removal of nickel from NiL1.

the electronic spin and g the Lande

́

factor. This can be further

simpliﬁed into n(n + 2) for g = 2.⁷⁴

Catalysis Experiments. A solution of complex 1 was prepared in a

glovebox under an argon atmosphere (0.8 μmol, 5 mol % catalyst with

respect to oleﬁn substrate) in 1.0 mL of dry CH₃CN. cis-Cyclooctene

or 1-decene (25 μL, 0.016 mmol) was added to the catalyst solution

via syringe. To this mixture was added H₂O₂(30 μL, 0.024 mmol) all

at once. The reaction was run anaerobically at room temperature for 5

min followed by GC-MS. The molar ratio of catalyst/substrate/H₂O₂

is 1/20/30. In the ¹⁸O-labeling experiments, H₂¹⁸O₂or H₂¹⁸O (300

μL, 0.24 mmol, 0.8 M) was added to the catalyst solution following the

method described as above.

Stopped-Flow Kinetic Experiments. Stopped-ﬂow spectropho-

tometry was performed on a TgK Scientiﬁc (formerly, Hi-Tech

Scientiﬁc, U.K.) SF-61DX2 cryogenic double-mixing stopped-ﬂow

system⁷⁵equipped with a J&M TIDAS diode array spectrophotometer

(MCS UVNIR 500-3, 200−1024 nm).⁷⁶Low temperatures were

maintained through the use of a liquid-nitrogen-cooled ethanol bath

equipped with a temperature controller. All ﬂow lines of the

instrument were extensively washed with oxygen-free, argon-saturated

anhydrous CH₃CN before charging the driving syringes with reactant

solutions. CH₃CN solutions of the reagents were prepared in a

glovebox ﬁlled with argon and placed in Hamilton gastight syringes.

Kinetic Studio⁷⁷and J&M TIDAS software packages were used to

control the instrument and collect data. The raw kinetic data were

treated with Kinetic Studio software from TgK Scientiﬁc, and with

Spectﬁt/32 Global Analysis System software from Spectrum Software

Associates.⁷⁸

Synthesis of [Fe^II(L1)(OTf)₂](CH₃CN)₂(1). Fe(OTf)₂·2CH₃CN

(0.014 g, 0.033 mmol) dissolved in 3 mL of dry CH₃CN was added to

a solution of L1 (0.01 g, 0.036 mmol) in 3 mL of dry CH₃CN under

an argon atmosphere. Upon mixing, the solution turned red. Dark-red

crystals were obtained after 4−5 days by vapor diﬀusion of diethyl

ether into this solution. Yield: 0.0156 g (65%). ESI(+)−MS (CH₃CN)

m/z: {[Fe(L1)](OTf)}⁺calcd for 481.1; Found, 481.2. Anal. Calcd for

C₂₂H₃₄N₆FeO₆F₆S₂: C, 37.09; H, 4.81; N, 11.79; Fe, 7.84. Found: C,

37.15; H, 5.26; N, 11.76; Fe, 7.87. The preparation of [⁵⁷Fe(L1)-

(OTf)₂]²⁺⁵⁷1 was conducted in an analogous manner by using

⁵⁷Fe(OTf)₂·2CH₃CN.

RESULTS AND DISCUSSION

■

Iron complexes with pyridine-containing macrocycles (Py-

MACs) displayed notable catalytic activity in catalytic

oxidations with H₂O₂.^54,65To focus on the reactivity of the

unfunctionalized iron complex that is most closely related to

catalytically active complexes 2 and 3 (Figure 2), we present

here the synthesis and reactivity of 1. Methylation of one

nitrogen atom of the ring was expected to suppress oxidation

and hydrolysis of the ligand in the most vulnerable aliphatic

fragment. NH-groups adjacent to the pyridine ring were

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retained because their presence did not suppress the catalytic

activity of the functionalized PyMACs.⁶⁵The relatively simple

structure of the macrocycle allowed us to generate and

characterize high-valent iron intermediates in reactions of 1

with oxidants. The reactions of transient iron(IV) intermediates

with substrates (substituted triaryl phosphines or oleﬁns) were

studied by the stopped-ﬂow technique, providing insight into

the nature of reactive species in catalytic oxidations with H₂O₂.

Synthesis and Characterization of 1. Similarly to

previously reported syntheses of Fe(PyMAC)’s,^65,79the

macrocycle was assembled on a nickel(II) template, using

2,6-diacetylpyridine as the dicarbonyl component and 1,7-

diamino-4-methyl-4-azaheptane as the diamine component in

the Schiff base condensation (Scheme 1). Incorporation of

iron(II) required subsequent transmetalation. Transmetalation

of the Schiff base nickel(II) complex did not produce

satisfactory results because of the hydrolytic instability of the

free Schiff base ligand. Instead, CN double bonds in the

Ni(II) Schiff base complex were reduced with NaBH₄,

producing a mixture of meso- and rac-diastereomers that were

separated by recrystallization. The structure of the major

product, [Ni(meso-L1)](ClO₄)₂, was conﬁrmed by crystallog-

raphy (Supporting Information, Figure S1). Demetalation of

this complex aﬀorded pure aminopyridine macrocycle ligand,

meso-L1. The iron(II) complex was prepared from the

appropriate iron(II) salt and the free ligand under anaerobic

conditions, and was characterized by elemental analysis,

electrospray mass spectrometry, IR, UV−vis, and single-crystal

X-ray diﬀraction.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1

1

Fe(1)−N(1)

Fe(1)−N(2)

Fe(1)−N(3)

Fe(1)−N(4)

Fe(1)−N(5)

Fe(1)−N(6)

1.873(4)

1.996(4)

2.091(4)

2.2004(4)

1.937(4)

1.922(4)

N(1)−Fe(1)−N(2)

N(1)−Fe(1)−N(3)

N(1)−Fe(1)−N(4)

N(1)−Fe(1)−N(5)

N(1)−Fe(1)−N(6)

N(2)−Fe(1)−N(3)

N(2)−Fe(1)−N(4)

N(2)−Fe(1)−N(5)

N(2)−Fe(1)−N(6)

N(3)−Fe(1)−N(4)

N(3)−Fe(1)−N(5)

N(3)−Fe(1)−N(6)

N(4)−Fe(1)−N(5)

N(4)−Fe(1)−N(6)

N(5)−Fe(1)−N(6)

82.89(19)

177.61(15)

84.55(19)

88.78(16)

87.51(16)

96.48(18)

166.96(18)

88.50(17)

91.39(18)

96.21(17)

93.51(16)

90.20(16)

87.75(16)

91.55(16)

176.27(17)

Table S1). For 1, the values of the N1−Fe1−N3 angle

177.61(15)° and N2−Fe1−N4 angle 166.96(18)° are observed.

This geometry with a tetradentate macrocycle has also been

found in nickel and iron analogues.^65,81−84The Fe−N bond

lengths in 1 range from 1.873(4) to 2.2(4) Å with the shortest

bond length to the pyridine nitrogen 1.873(4) Å. The Fe−N

separation is similar to those of a reported low-spin iron(II)

complex (2.06−2.18 Å).^65,83

Magnetic susceptibility measurements using Evans method

were performed on 1 in CD₃CN. The peak of the standard,

TMS, did not shift in the presence of 1 (no splitting of the

TMS peak was observed for the sample containing a capillary

with pure TMS/CD₃CN inserted in the CD₃CN solution of 1),

indicating the eﬀective magnetic moment μ_eﬀ= 0 T J⁻¹.

Therefore, the low-spin conﬁguration of iron(II) in crystalline 1

is retained in acetonitrile solutions.⁸⁵

X-ray crystallographic analysis of complex 1 showed that the

iron(II) ion is in an octahedral environment, deﬁned by the

four nitrogen atoms from L1 and two coordinated acetonitrile

molecules in the axial positions (Figure 3). Selected bond

lengths and angles are listed in Table 2. The two methyl groups

on the ligand are on the same side of the tetradentate

macrocycle, as expected for the meso isomer identiﬁed in the

nickel(II) precursor, NiL1 (Supporting Information, Figure S2,

The Mossbauer spectrum of a frozen acetonitrile solution of

̈

1 (90 K, zero ﬁeld) conﬁrmed this conclusion. The spectrum

consisted of a sharp (Γ = 0.29 mm/s) quadrupole doublet with

an isomer shift of δ = 0.50(2) mm/s and a quadrupole splitting

parameter of ΔE_Q= 0.78(2) mm/s (Supporting Information,

Figure S3), indicating a low-spin iron(II) complex. Mossbauer

̈

parameters of 1 are comparable to the parameters found in

other low-spin ferrous complexes with macrocyclic ligands.⁸⁶⁻⁸⁸

A related iron(II) complex with a tetradentate aminopyridine

macrocycle, 2 (Figure 2), was also previously shown to

preferentially exist in a low-spin iron(II) form (μ_eﬀ= 0 T J⁻¹) in

acetonitrile solution, whereas 3, which contains a pentadentate

macrocycle, displayed an eﬀective magnetic moment of μ_eﬀ

=

5.2 T J⁻¹, characteristic of iron(II) in a high-spin conﬁg-

uration.⁶⁵It can be concluded that the denticity of PyMAC

ligands allows for a high degree of control over the spin state of

iron(II) in these complexes.

Close similarity in geometric and electronic structures

between complex 1 and other low-spin Fe(II)-PyMACs

suggested that their redox properties would likely be similar

as well. Electrochemical experiments support this assumption,

Figure 3. ORTEP diagram of the complex cation of 1 with thermal

ellipsoids at 40% probability. Hydrogen atoms are omitted for clarity.

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revealing a reversible Fe³⁺/Fe²⁺redox wave in the cyclic

voltammogram of 1, with E_1/2= 0.49 V (ΔE_p= 55 mV) vs Fc⁺/

Fc (Figure 4), a value very similar to previously measured half-

reaction, carried out under an argon atmosphere at room

temperature.

Complex 1 was shown to catalyze epoxidation of both 1-

decene and cyclooctene after 5 min of reaction, carried out

under argon atmosphere at room temperature. Although

epoxide yields and turnover numbers (18%, TON = 4 for 1-

decene, and 36%, TON = 8 for cyclooctene, Table 3) are

relatively low, the reaction is clearly catalytic. Longer reaction

times did not increase the product yields, suggesting that

epoxidation of oleﬁns with H₂O₂in the presence of 1 is rapid.

The reaction exhibited high selectivity (80%) with respect to

epoxides; other identiﬁed side products included cis-diol (ca.

0.5% with respect to 1-decene; ca. 0.7% with respect to

cyclooctene) and traces of allylic alcohol (ca. 0.4%).

Catalytic activity of 1 in oleﬁn epoxidations can be compared

to the reactivity of analogous, previously studied PyMAC

complexes 2 and 3 (Table 3). Complexes with tetradentate

macrocycles, 1 and 2, were catalytically active. Both complexes

contained low-spin Fe(II) in a distorted octahedral geometry

with two labile acetonitrile molecules occupying axial positions.

2 has shown a somewhat higher reactivity than 1 in both

epoxidations, suggesting a possible involvement of the NH

groups of the ligands in hydrogen peroxide activation. Complex

3, which contains high-spin Fe(II) in a square-pyramidal

geometry, turned out to be the least eﬃcient catalyst among

these three PyMACs. It should be noted, however, that

protonation of the aminopropyl arm in 3 with externally added

mineral acid dramatically improved its catalytic activity.⁶⁵

Although catalytic activity of 1 was relatively low, we did not

attempt to optimize it in the current work. By analogy with

previous studies of the other iron PyMACs, 2 and 3, addition of

acid can be suggested as a promising approach to increasing

oleﬁn conversion and epoxide yield in these systems and will

certainly be tested in future experiments. For the present

mechanistic study, however, we chose to avoid additives. The

observation of catalytic turnover in epoxidation of oleﬁns with

H₂O₂in the presence of 1 justiﬁed the subsequent search for

reactive intermediates in this system.

Figure 4. Cyclic voltammogram of complex 1 (2 mM in CH₃CN with

0.1 M [n-Bu₄N][PF₆] as the supporting electrolyte, under an argon

atmosphere; scan rate 100 mV s⁻¹) . * Represents the Fc⁺/Fc couple;

a minor peak at −0.3 V is due to the impurity in the solvent system.

wave potentials for 2 and (3−H⁺) (0.40 and 0.51 V,

respectively).⁶⁵Complex 1, featuring a simple macrocyclic

platform partially protected from oxidative decomposition by a

methyl substituent on the middle nitrogen atom, is therefore a

promising starting material for generating metal-based oxidizing

intermediates. With the long-term goal of identifying catalyti-

cally important intermediates in mind, we explored the

reactivity of new Fe(IV) intermediates generated from 1. As

a prerequisite for these studies, catalytic activity of 1 was

examined ﬁrst, followed by detailed investigations of the

individual steps related to catalysis.

Catalytic Oleﬁn Epoxidation. Catalytic activity of 1 was

tested in oleﬁn epoxidations with H₂O₂, a reaction catalyzed by

structurally related Fe-PyMAC’s 2 and 3. Epoxidation of

cyclooctene or 1-decene with H₂O₂was studied under mild

conditions identical to those used previously with 2 and 3

(Figure 2).⁶⁵cis-Cyclooctene is commonly used as a substrate

for catalyst screening in epoxidations with H₂O₂in the presence

of nonheme iron catalysts.⁷⁴Terminal oleﬁns, such as 1-decene,

are more challenging substrates that only undergo epoxidation

in the presence of highly reactive catalysts.⁵⁴Table 3 shows

epoxide yields and turnover numbers (TON) after 5 min of

Isotope labeling experiments provided initial insight into the

mechanism of catalytic epoxidation with H₂O₂/1. Nearly

quantitative incorporation of ¹⁸O into the epoxide product

was observed when the reaction was carried out in the presence

of H₂¹⁸O₂. ¹⁶O-epoxide was detected when the reaction was

carried out in the presence of H₂O₂and H₂¹⁸O. These

Table 3. Oleﬁn Epoxidation Reaction with H₂O₂Catalyzed by 1 and Related PyMAC Complexes

a

b

c

d

catalyst

oleﬁn

epoxide (%)

TON

convn (%)

selectivity (%)

ref

1a

1-decene

18

4

22

81

this work

this work⁶⁵

65

cyclooctene

cis-2-heptene

1-decene

36

8

42

85

f

33

7.2

8 (4)

64

52

e

2a

33 (20)

44(33)

71(43)

14

75 (61)

87 (74)

29

e

2a

cyclooctene

1-decene

62 (32)

12(6.4)

0.8

3a

4

6

cyclooctene

1.2

10

60

65

a

b

c

d

Yield of epoxide with the respect to oleﬁn. Moles of epoxide per mol of catalyst. Internal standard used: nitrobenzene. Selectivity = GC yield/

e

f

conversion. Catalyst (7.2 M):substrate (144 M):H₂O₂(216 M) = 1:20:30. Values shown in parentheses are taken from ref 65. Reaction was

performed in the presence of 1 equiv of triﬂic acid; no trans-epoxide was detected (side products included 2- and 3-heptanones).

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observations are consistent with a metal-based, H₂O₂-derived

oxidant that slowly exchanges with water. The cis-diol side

product, obtained from H₂¹⁸O₂experiments, contained about

one ¹⁸O atom per molecule. While low yield of cis-diol limited

the accuracy of isotope labeling studies, it appears that some

oxygen in this product was derived from H₂¹⁸O₂, and the

remaining oxygen was incorporated from water. Control

experiments demonstrated that the epoxide did not undergo

hydrolysis under our typical epoxidation conditions. Although

our isotope labeling results are limited, they agree with

previously published extensive studies of analogous systems,

which were interpreted as evidence for interconverting oxo-bis-

hydroxo-forms of a high-valent metal-based oxidant.^25,89,90

Subsequent experiments were designed to observe metal-based

intermediates in reactions of 1 with hydrogen peroxide or other

oxidants.

Reactions of 1 with Oxidants and Formation of

Iron(IV) Intermediates. (a). Reaction with Hydrogen

Peroxide. In an attempt to identify transient, metal-based

intermediates in the system containing 1 and hydrogen

peroxide, stopped-ﬂow experiments with spectrophotometric

registration were performed, revealing a rapid reaction that

occurs within seconds at 0 °C (Figure 5). During this time, a

plexes with Schiff base macrocycles⁶⁵that could result from

ligand oxidation. Mass spectra of this resulting solution (m/z at

479.2, [Fe^II(L1−2H)(OTf)]⁺and m/z at 477.2 [Fe^II(L1−

4H)(OTf)]⁺) corresponded to the iron(II) complex with an

oxidized ligand, suggesting that the ligand has been partially or

fully oxidized upon mixing of 1 with diﬀerent amount of H₂O₂.

Ligand oxidation was not observed in the absence of H₂O₂.

Rapid formation of the presumed Fe^IVO intermediate

from 1 and H₂O₂suggested that this species may be responsible

for oleﬁn epoxidation catalyzed by 1 and related Fe-PyMACs.

However, an interplay of several reactions, including rapid

ligand oxidation, in this seemingly simple system would

complicate the study of Fe^IVO and its role in catalytic

substrate oxidations with hydrogen peroxide. Using oxidants

other than H₂O₂was expected to provide additional insight into

ligand-based oxidation versus metal-based oxidation. Relatively

mild oxidants, such as O₂(air), were chosen to characterize

spectroscopic changes due to ligand oxidation. On the other

hand, strong oxygen atom donors, such as high-valent iodine-

oxo species, provided alternative chemical methods of

generating Fe^IVO.

(b). Reaction with O₂. When complex 1 was exposed to air

in acetonitrile solution, a slow reaction occurred. No near-IR

bands appeared in this reaction, but the spectral changes in the

500−600 nm region were qualitatively similar to the ones

observed in reaction of 1 with H₂O₂(λ_max(ε, L mol⁻¹cm⁻¹):

355 (sh, 2440), 375 (2640), 492 (720), 529 (800)) after

exposing 1 to air in CH₃CN overnight). The mass spectrum of

this red solution (m/z at 479.2, [Fe^II(L1−2H)(OTf)]⁺)

corresponded to the iron(II) complex with a partially oxidized

ligand. The formulation of this complex as 1c is supported by

its Mossbauer spectrum (Figure 6), with spectral parameters (δ

̈

Figure 5. Time-resolved spectral changes in the reaction of 1 (2 mM)

with 1 equiv of H₂O₂in CH₃CN at 0 °C, monitored by stopped-ﬂow

spectrophotometry. Concentrations are listed before mixing. Inset:

time course of the reaction monitored at 705 nm.

fairly broad, medium-intensity band was observed in the near-

IR region (λ_max= 690 nm, ε ≈ 100 M⁻¹cm⁻¹). This spectral

feature is characteristic of low-spin nonheme iron(IV) oxo

intermediates,^24,25suggesting that an Fe^IVO species has been

formed in the reaction of 1 with H₂O₂. The accumulation of

this species was a rapid process with a rate that depended

linearly on the concentration of H₂O₂(k₂= 405 M⁻¹s⁻¹)

(Supporting Information, Figure S4).

The lifetime of this intermediate at 0 °C was relatively short,

and it further decreased from 2−3 min to several seconds as the

concentration of H₂O₂increased from its stoichiometric

amount to a 10-fold excess with respect to iron complex. The

growth and decay of the absorption band at 690 nm was

accompanied by other spectral changes. Notably, another band,

with λ_max≈ 540 nm, grew in the spectrum of the reaction

mixture. Although the starting complex, 1, also had an

absorption band at 536 nm, albeit a weak one (ε = 148 M⁻¹

cm⁻¹), the consumption of 1 did not result in the decay in

absorbance intensity around 540 nm (Figure 5) because of the

overlap with reaction intermediate(s) and product(s) absorbing

in the same spectral range. Several iron complexes are known to

have relatively intense optical bands in the 500−600 nm region,

including Fe(III)-OOH intermediates⁹¹⁻⁹⁸and iron(II) com-

Figure 6. Zero-ﬁeld Mossbauer spectrum of 1c in frozen acetontrile

solution acquired at 90 K. The solid line displays the least-squares ﬁt

̈

to the spectrum, giving the parameters δ = 0.41(5) mm/s and ΔE_Q

=

1.14(5) mm/s (Γ = 0.50 mm/s). The sample was obtained by

oxidation of 1 in air overnight.

= 0.41(5) mm/s and ΔE_Q= 1.14(5) mm/s) consistent with a

low-spin iron(II) conﬁguration.⁸⁶⁻⁸⁸A small decrease in isomer

shift with concomitant increase in quadrupole splitting, as

compared with 1 (δ = 0.50(2) mm/s, ΔE_Q= 0.78(2) mm/s,

see discussion above), is typical for iron macrocycles with an

increasing degree of unsaturation, and the diﬀerence in ΔE_Q

(0.3 mm/s) may correspond to one additional CN bond as

reported for a series of tetraazamacrocyclic iron(II) com-

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plexes.⁹⁹Stepwise oxidation of similar nickel(II) pyridine

azamacrocycles with HNO₃has been reported,¹⁰⁰aﬀording

analogues of 1c and 1e (with nickel in place of iron). Analogous

Schiff base complexes were observed during and after the

reaction of 1 with hydrogen peroxide; mass spectrometry and

UV−vis spectra indicated the formation of 1c and 1e.

Importantly, electrochemistry of the iron(II) complexes with

partially and fully oxidized ligand, 1c and 1e, is qualitatively

similar to that of 1: both complexes exhibit reversible waves in

their cyclic voltammograms at progressively positive potentials

(Table 4, Supporting Information, Figure S5). It can be

iodoxybenzoate is shown in Scheme 2. The rate of formation

of 1b was determined in stopped-ﬂow experiments as described

below.

Scheme 2. Proposed Reaction Steps for the Interaction of

a

the Iron(II) Complex 1 with Isopropyl 2-Iodoxybenzoate

Table 4. Redox Potentials of Iron(II) Complexes Studied in

This Work

complex

E_1/2adjusted vs Fc⁺/Fc (V)

ΔE_p(V)

reversibility

1

0.49

0.51

0.62

0.055

0.11

R

1c

1e

0.034

concluded that ligand oxidation does not destroy the

macrocycle, and the resulting Schiff base macrocyclic complexes

of iron retain the ability to undergo reversible oxidations/

reductions.

(c). Reaction of 1 with Isopropyl 2-Iodoxybenzoate (iPr-

IBX ester): Generation of Fe(IV) Intermediates. In search for

clean methods of generating Fe^IVO intermediates from 1, an

oxygen atom transfer to the Fe(II) center in 1 was attractive.

Among oxygen atom transfer reagents, iodine-oxo compounds

have been known to act as clean and eﬃcient two-electron

oxidants.^66,101,102However, popular iodine(III)-oxo reagents,

such as iodosobenzene (PhIO), suﬀer from low solubility in

most organic solvents. Substituted iodosobenzenes with

somewhat higher solubilities, such as 2,6-dimethyliodosylben-

zene (Me2-PhIO),¹⁰³are still less-than-optimal for the

intended detailed kinetic studies of reactive intermediates,

where broad concentration and temperature ranges had to be

covered and low temperatures were often required. Unlike the

majority of iodine(III) compounds, selected iodine(V) oxidants

oﬀered a combination of high reactivity with high solubility in

common solvents (e.g., ca. 100 mM in CH₃CN at 25 °C).¹⁰²In

contrast to explosive Dess-Martin reagents (DMP) and related

hypervalent compounds, recently reported esters of 2-

iodylbenzoic acid (IBX esters) proved to be safe, thermally

stable reagents that are easy to synthesize and handle.⁶⁶

Furthermore, IBX esters were successfully utilized in iron-

catalyzed oxidations (e.g., iron-phthalocyanine catalyzed

oxidation of alcohols¹⁰¹and anthracenes¹⁰⁴). In the present

work, we took advantage of the high solubility of isopropyl 2-

iodoxybenzoate in acetonitrile (ca. 100 mM in CH₃CN at 25

°C)⁶⁶and tested this reagent for the oxygen atom transfer to an

iron(II) center in 1 at low-temperature. To the extent of our

knowledge, this is the ﬁrst report on applications of IBX esters

for the generation of high-valent iron-oxo intermediates in

nonheme iron systems.

a

The rate of formation of 1b was determined by stopped-ﬂow

spectrophotometry.

Treatment of 1 (0.5 mM) with 1−18 equiv of isopropyl 2-

iodoxybenzoate at low temperatures (from 0 to −40 °C) in

CH₃CN resulted in the decay of the absorption band of 1 and a

concomitant appearance of a new band with λ_max= 705 nm (ε

≈ 240 M⁻¹cm⁻¹) (Figure 7a). This chromophore indicated the

formation of the corresponding ferryl complex, which showed

the expected low-energy d−d transition band in the near-IR

region, characteristic of an S = 1 oxoiron(IV) center.^105,106ESI-

MS of this green species exhibits peaks corresponding to

[Fe^IV(L1)(O)]²⁺, 1b (Supporting Information, Figure S6). This

intermediate was stable for several hours at −40 °C, but

Iron(II) complex 1 rapidly reacted with isopropyl 2-

iodoxybenzoate; stopped-ﬂow spectrophotometry was neces-

sary to follow the initial steps of this process under most

practically important reaction conditions. Changes in time-

resolved UV−vis spectra with subsequent mass-spectrometric

analysis allowed us to also identify iron-containing oxidation

intermediates formed over longer time periods; a proposed

reaction scheme for 1 upon addition of isopropyl 2-

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iodoxybenzoate dissolved in CH₃CN at −40 °C. The raw

data were ﬁt to two quadrupole doublets (Figure 8).

Approximately 31% of the spectral area consists of a quadrupole

doublet (δ = 0.03(2) mm/s and ΔE_Q= 2.00(2) mm/s (Γ =

0.39 mm/s)) associated with the intermediate 1b. The isomer

shift of this doublet was in the range of the values reported for

low-spin S = 1 iron(IV)-oxo complexes (δ = −0.04 to 0.19

mm/s).^21,24,25Importantly, this isomer shift was signiﬁcantly

lower than the values typical of either Fe(II) or Fe(III)

complexes. Relatively large quadrupole splitting is uncommon

but not unprecedented for Fe(IV)=oxo complexes.^97,107−111

The remaining 69% of the spectral area exhibited Mossbauer

̈

parameters (δ = 0.40(2) mm/s and ΔE_Q= 0.50(2) mm/s)

associated with the decomposition product. These spectral

parameters corresponded best to low-spin iron(II) compounds,

although they diﬀered somewhat from the Schiff base products

of ligand oxidation, 1c or 1e. One possible structure of this

oxidation product(s) included coordination of the oxidant to

the iron(II) center in 1c or 1e (although such complexes would

be unstable, they may exist at low temperatures as transient

species).

Stopped-ﬂow studies of the rapid reaction between 1 and iPr-

IBX ester (^iPrIOO) revealed a rather complicated, multistep

process, which could be greatly simpliﬁed under certain

reaction conditions (Figure 9). The simplest kinetics was

Figure 7. Reaction of 1 (2 mM) with 2 equiv of isopropyl 2-

iodoxybenzoate in CH₃CN. (a) Stopped-ﬂow spectral changes at −20

°C. Inset: time course of the reaction monitored at 536 and 705 nm.

(b) UV−vis spectral changes at room temperature over 1 h.

decayed within 1 h at room temperature to a stable brick-red

species (λ_max= 492 nm, with an additional shoulder at 523 nm,

Figure 7b, consistent with the iron(II) complex with a partially

oxidized ligand; mass spectrometry results support this

formulation).

Mossbauer spectroscopy conﬁrmed the formulation of 1b as

̈

an [Fe^IV(L1)(O)]²⁺species. Figure 8 shows a zero-ﬁeld

Mossbauer spectrum of a frozen solution, ⁵⁷1b, generated

̈

Figure 9. Plot of k_obsagainst the concentration of ^iPrIOO for the

from the reaction between ⁵⁷1 (40% ⁵⁷Fe with respect to total

reaction of 1 (0.5 mM) with 1−18 equiv of ^iPrIOO in CH₃CN at −40

°C.

iron content) and about 10-fold excess of isopropyl 2-

observed under large excess of ^iPrIOO at low temperature,

where the decay of 1 (monitored at its λ_max= 536 nm) resulted

in clean, mixed second-order (ﬁrst order in each reactant)

formation of 1b (followed as a single-exponential growth of

absorbance at 705 nm, ε ≈ 240 M⁻¹cm⁻¹, Figure 7a). This

allowed us to plot k_obsvalues for the reaction against varying

excess ^iPrIOO concentrations (>8 equiv), outlined the rate law

in eq 1, and allowed for determination of the second-order rate

constant, k₂= 1.65 × 10³M⁻¹s⁻¹at −40 °C.

d[1]

dt

d[1b]

dt

−

=

= k_obs[1]

= k₂[1][isopropyl 2‐iodoxybenzoate]

(beyond 8 equiv)

(1)

Figure 8. Zero-ﬁeld Mossbauer spectrum of the intermediate ⁵⁷1b in a

frozen acetonitrile solution acquired at 90 K. The solid line displays

the least-squares ﬁt to the spectrum. The ﬁt for each subspectrum is

also displayed (thin black lines).

̈

Acquisition of kinetic data for the reaction of 1 (0.5 mM)

with an 8-fold excess of ^iPrIOO over a range of temperatures

(−40 to −25 °C) aﬀorded a linear Arrhenius plot (Supporting

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Information, Figure S10b) and yielded the eﬀective activation

energy of E_a^⧧= 35 kJ/mol.

oxidation does not represent a synthetic challenge, these

substrates are nevertheless useful as mechanistic probes in

studying electronic and steric eﬀects in oxygen atom transfer

reactions.

At longer reaction times, and especially at higher temper-

ature, the second, slower process becomes apparent, and mass

spectrometry clearly indicated partial ligand oxidation (Sup-

porting Information, Figure S7). Subsequent reaction of 1c

with excess ^iPrIOO resulted in the accumulation of the modiﬁed

high-valent iron-oxo intermediate, 1d (λ_max= 696 nm), which,

in turn, could further react with excess oxidant, yielding 1e and

1f (Scheme 2, Supporting Information, Figures S8, S9).

Importantly, partial ligand oxidation did not lead to complete

decomposition of the macrocycle; instead, the iron(II) Schiff

base macrocycles form, and continued to serve as precursors for

the (L)Fe^IVO species. The complexes with oxidized ligands

also retained reversible or quasi-reversible one-electron redox

couples (LFe^III/LFe^II) displayed by 1, at somewhat more

positive potentials because of relative stabilization of Fe(II) by

the unsaturated macrocycles (Supporting Information, Figure

S5).

Triarylphosphines were found to cleanly react with 1b

generated from complex 1 and ^iPrIOO (premixed in a 2:1 molar

ratio). To avoid direct interaction between PAr₃and ^iPrIOO,

stoichiometric or substoichiometric amounts of the oxidant

were used, and suﬃcient incubation times, determined from

independent single-mixing stopped-ﬂow experiments, ensured

quantitative incorporation of both oxygen atoms of ^iPrIOO into

the Fe^IVO intermediate 1b. Upon addition of PPh₃to the

preformed solution of 1b in the stopped-ﬂow instrument, a

relatively fast and smooth single-exponential decay of the

absorption band at λ_max= 705 nm occurred (Figure 10-a),

Ligand oxidation could be partially suppressed by using

smaller amounts of the oxygen atom donor, ^iPrIOO. However,

oxygen atom transfer with 0.5−6 equiv of ^iPrIOO with respect

to 1 was signiﬁcantly slower than the reactions with a larger

excess of ^iPrIOO (Figure 7a, Figure 9, Supporting Information,

Figure S10). This deviation from linearity in the plots of k_obsvs

the concentration of ^iPrIOO suggested that the initial

coordination of the oxidant to the iron center may facilitate

oxygen atom transfer from another molecule of ^iPrIOO.

Additionally, the second oxygen atom of the oxidant is

transferred to the iron center in a slower process. Despite

these complications, stoichiometric or substoichiometric

amounts of ^iPrIOO cleanly and quantitatively transferred an

oxygen atom to 1 within minutes at −40 °C (Supporting

Information, Figures S10, S11) and even faster at higher

temperatures. Spectrophotometric titrations showed that both

oxygen atoms of ^iPrIOO were eventually transferred to the iron

center. Therefore, 0.5 mol of isopropyl 2-iodoxybenzoate were

needed for essentially quantitative formation of 1b. Although

complete kinetic and mechanistic analyses of the reaction

between 1 and ^iPrIOO were precluded by the multiple

consecutive and parallel processes involved in this trans-

formation (Scheme 2), the limiting cases proved to be

revealing. With a large excess of ^iPrIOO, one oxygen atom of

the oxidant was rapidly transferred to the iron center, cleanly

generating (L)Fe^IVO (1b); initial coordination of another

molecule of ^iPrIOO likely accelerated this process. However, in

situations where excess oxidant had to be avoided (e.g., in

detailed studies of substrate oxidations with iron(IV)

intermediates), high-valent iron(IV)-oxo species could be

produced, in a somewhat slower multistep process, by

sequentially transferring both oxygen atoms from 0.5 mols of

^iPrIOO per mole of the iron(II) complex 1. Partially oxidized

macrocycles were capable of supporting iron(IV)-oxo inter-

mediates, such as 1d and 1e (Supporting Information, Figure

S12).

Figure 10. Reaction of 1b with triphenylphosphine. (a) Time-resolved

spectra upon addition, in the second mixing, of 5 equiv of PPh₃(40

mM) to 1b pregenerated by reaction of 1 (4 mM) with ^iPrIOO (2

mM) in CH₃CN with 6 s age time; CH₃CN, 0 °C, reaction time 50 s.

Inset: time course of the reaction monitored at 705 nm (red line) and

self-decay of 1b in the absence of PPh₃(blue line). (b) Determination

of the second-order rate constant by plotting k_obsagainst concentration

of P(C₆H₅)₃(5−25 equiv) at 0 °C; phosphine concentrations in the

cell during the reaction are shown on the x-axis of the plot.

quantitatively yielding Ph₃PO (as analyzed by GC-MS). The

observed pseudo-ﬁrst order rate constant increased linearly

with increasing concentrations of PPh₃(Figure 10b), indicating

ﬁrst order in this reagent and second order overall:

Reactivity of Oxoiron(IV) in Triarylphosphine Oxida-

tion. Identiﬁcation of the iron(IV)-oxo intermediates in

reactions of 1 with oxygen atom donors (such as IBX ester)

and optimizing the reaction conditions that produce Fe^IVO

in high yields set the stage for understanding the reactivity of

these intermediates in oxygen atom transfer to substrates.

Phosphines are frequently used as oxygen atom acceptors in

reactions with metal-oxo complexes. Although phosphine

Rate = k₂[1b][PPh₃]

The second order rate constant at 0 °C was found to be k₂=

26.5 M⁻¹s⁻¹(Table 5), and it decreased at lower temperatures

(down to −20 °C) (Supporting Information, Figure S14b),

yielding the activation parameters summarized in Table 5. A

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Table 5. Kinetic Parameters for Substrate Oxidation with 1b

k₂(M⁻¹s⁻¹

)

ΔH^⧧

ΔS^⧧

ΔG^⧧(kJ/mol)

273 K

(kJ/mol) (J/mol K)

298 K

PPh₃oxidation

26.5

53

−25

−169

60

73

a

cyclooctene

epoxidation

0.45 (0.17 )

23.0

a

This value was acquired from the reaction of substrate with 1b

generated by premixing of complex 1 with H₂O₂.

negative value of activation entropy suggests an associative rate-

limiting step in phosphine oxidation.¹¹¹

In the presence of excess phosphine, ferryl(IV) species were

reduced to the starting iron(II) complex 1, as followed from the

ﬁnal UV−vis (Figure 10a) and mass spectra (m/z = 481.2).

When 1 equiv of PPh₃was used, the reaction was relatively slow

and competing ligand oxidation became apparent (Supporting

Information, Figure S14), as evidenced by the growth of the

absorption bands at 492 and 523 nm and the appearance of the

peak at m/z = 479.2 (1c) in the mass spectrum.

The fast reduction of 1b into 1 upon addition of excess PPh₃

allowed us to perform repeated oxidation−reduction cycles

(monitored by UV−vis) (Supporting Information, Figure S15).

To a solution containing 1 and 50 equiv of PPh₃in CH₃CN, 1

equiv of ^iPrIOO was added, resulting in a rapid formation the

green species which immediately decayed, forming a red

solution with a UV−vis spectrum nearly identical to that of 1.

This sequence of reactions was repeated ﬁve times; stepwise

addition of the oxidant was necessary to prevent direct

oxidation of PPh₃with ^iPrIOO. In each addition, another

equivalent of ^iPrIOO again gave a ﬂash of green intermediate

that decayed into red product. The UV−vis monitoring showed

that the spectra of the resulting red solutions remained

essentially unchanged after repeated cycles. This suggested

that the iron(IV)-oxo could be converted back to the original

iron complex with multiple turnovers when the reduction step

was fast.

Iron(IV)-oxo species behaved as electrophilic oxidants in

several reported examples of substrate oxidations.^34,112−114To

determine whether the new iron-oxo intermediate 1b followed

this trend, substituted triarylphosphines were used as

substrates. In the electrophilic oxidation of triarylphosphines,

the rates were expected to increase with the electron-donating

properties of the para substituent.^115−117The kinetics of the

reactions of a series of derivatives of PPh₃with electron-

withdrawing (F, Cl) and -donating (OMe, Me) para phenyl

substituents were examined.

The reactivity of several investigated para-substituted

phosphines increased with the electron-donating properties of

the substituents. For this admittedly limited set of substrates,

this trend is graphically reﬂected in the correlation between the

reaction rates and the values of Hammett parameters, σ (ρ =

−1.05, R²= 0.789; the presence of three substituted aryl groups

in the molecules of substrates accounts for a factor of 3 in

plotting k_obsagainst 3 σ; Figure 11). The observed trend implies

the electrophilic character of the oxo group of the nonheme

oxoiron(IV) complex 1b and positive charge buildup on

phosphorus in the PPh₃oxygenation reactions. Thus, the

nonheme oxoiron(IV) complex 1b are capable of oxygenating

substrates via an electrophilic oxidation mechanism.¹¹⁸

Analogous electrophilic character of iron(IV)-oxo species was

observed in sulfoxidation^34,112and aromatic hydroxylation

reactions.^113,114

Figure 11. Hammett correlation for the observed rate constant of

triarylphosphine oxidations with 1b (generated by the reaction of 1 (4

mM) with ^iPrIOO (2 mM) in CH₃CN with 6 s age time); 1 equiv of

PAr₃(4 mM) was added, in the second mixing, at 0 °C. ρ = −1.05 (R²

= 0.789).

Reactivity of Oxoiron(IV) in Oleﬁn Epoxidation. To

understand the potential role of iron(IV) intermediates in

oleﬁn epoxidations with hydrogen peroxide catalyzed by

complex 1, the reactivity of independently generated iron-

(IV)-oxo species with oleﬁns was studied experimentally.

Similarly to phosphine oxidation experiments described

above, ferryl(IV) species were obtained in the reaction of 1

with ^iPrIOO. Identifying kinetically competent intermediates in

catalytic epoxidations required measurements of individual

reaction rates under various conditions. Ferryl(IV) intermedi-

ates were subjected to reactions with cyclooctene, and reaction

rates were determined by the addition of substrate (cyclo-

octene) to the preformed iron(IV)-oxo species in detailed

double-mixing stopped-ﬂow experiments. In the ﬁrst mixing

step, CH₃CN solutions of 1 (4 mM) and 0.5 equiv of isopropyl

2-iodoxybenzoate were combined at 0 °C and incubated for 6 s

to ensure maximum formation of iron(IV)-oxo while avoiding

excess oxidant. An acetonitrile solution of substrate (cyclo-

octene, 40 mM) was then mixed in, causing the decay of the

absorption band of the iron(IV) intermediate 1b (λ_max= 705

nm), with a clear isosbestic point at 555 nm (Figure 12a).

Under the same conditions, 1b remained stable upon mixing

with CH₃CN in a control reaction. These results demonstrated

that the intermediate oxoiron(IV) reacted with cyclooctene.

The kinetics of cyclooctene oxidation was monitored by the

disappearance of 1b. The rate of this process does not depend

on the concentrations of reagents that were used to generate

1b; experiments at substoichiometric ^iPrIOO (less than 0.3

equiv with respect to 1) and longer incubation times ensure the

dominance of 1b as the oxidant. The rate of epoxidation was

investigated over a range of substrate concentrations (excess

cyclooctene was always used); pseudo-ﬁrst order ﬁtting of the

kinetic data allowed us to determine k_obsvalues, which

increased linearly with cyclooctene concentration (20−100

equiv) (Figure 12b), indicating the second-order rate law (ﬁrst-

order in each reactant, eqs 2, 3).

d[1b]

dt

d[1c]

dt

−

=

= k_obs[1b] = k₂[1b][cyclooctene]

(2)

Electrophilic oxidant (L1)Fe^IVO transfers its oxygen atom to

the CC double bond of the oleﬁn substrate, most likely via a

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Table 6. Oleﬁn Epoxidation Reaction with Isopropyl 2-

a

Iodoxybenzoate

oxygen-donor (isopropyl 2-

reaction time

(min)

moles of

a

b

iodoxybenzoate)

epoxide

1 equiv

2 equiv

5 equiv

10 equiv

5

0.2

30

5

0.81

0.45

1.58

0.67

3.3

30

5

30

5

0.73

3

30

a

b

Moles of epoxide per mole of catalyst. Catalyst:cyclooctene:iso-

propyl 2-iodoxybenzoate =1:20:(1/2/5 or 10).

and quantiﬁed by GC-MS (Table 6). Control experiments

showed that ^iPrIOO did not oxidize cyclooctene under the

reaction conditions in the absence of 1.

Equimolar amount of ^iPrIOO with respect to 1 aﬀorded 0.2

mol of cyclooctene epoxide in 5 min (0 °C, 20-fold excess of

oleﬁn). However, epoxide yield increased over time, reaching

0.8 mol per mol of 1 after 30 min. Epoxidation appeared to be

slow under experimental conditions, but the epoxidation time

scale was consistent with the rate of cyclooctene-induced decay

of the iron(IV)-oxo intermediate determined from stopped-

ﬂow experiments. No diol was produced, providing indirect

evidence for water-assisted pathways for diol formation in

catalytic oleﬁn oxidations with H₂O₂.¹¹⁹Using 2 equiv of

^iPrIOO, the reaction aﬀorded up to 1.6 mols of epoxide in 30

min, and even larger excess of oxidant (5 or 10 equiv) increased

the epoxide yield to about 3 mols per mole of 1. These results

clearly showed the decay reaction upon addition of cyclooctene

to the preformed 1b yielded the corresponding epoxide in

about 30 min; less than quantitative yields of epoxide with

respect to oxidant (Table 6) could be attributed to partial

ligand oxidation (1c and 1e were detected by mass-

spectrometry).

To assess the eﬀects of ligand oxidation on the reactivity of

iron(IV)-oxo species with oleﬁns, cyclooctene epoxidation with

^iPrIOO was performed in the presence of 1c or 1e. The complex

with a single CN bond, 1c, yields 0.7 mols of epoxide using

excess oxidant. Moreover, this ligand oxidation reaction upon

the addition of cyclooctene to preformed 1d (generated by

premixing 1c and 1 equiv of ^iPrIOO) was also observed in

stopped-ﬂow experiments (Supporting Information, Figure

S16). These results demonstrated that 1d could transfer an

oxygen atom to cyclooctene to produce epoxide, although

additional ligand oxidation (leading to 1e) competed with

oxygen atom transfer to the substrate. Therefore, three types of

macrocyclic oxoiron(IV) species: 1b, 1d, and 1f consecutively

formed, and each could function as an eﬀective oxygen-atom

transfer reagent that can attack cyclooctene to form epoxide.

The ability of 1d or 1f to oxidize substrates does not

compromise the kinetic results described above for the

reactions of 1b with cyclooctene because the ferryl species

Figure 12. (a) Time-resolved spectral changes upon mixing of 1b

(generated by reaction of 1 (4 mM) with ^iPrIOO (2 mM) in CH₃CN

with 6 s age time) with 20 equiv of cyclooctene (40 mM) in CH₃CN

over 900 s at 0 °C. Inset: time course of the reaction monitored at 705

nm (red line) and a control experiment (self-decay of 1b solution, blue

line). (b) Determination of the second-order rate constant by plotting

k_obsagainst cyclooctene concentration (20−100 equiv); 0 °C. (c)

Eyring plot for the reaction of 1b with 60 equiv of cyclooctene (−20 to

0 °C) in CH₃CN.

concerted pathway, as follows from the retention of stereo-

chemistry in epoxidation of cis-2-heptene into cis-epoxide

(Table 3). The second-order rate constant for cyclooctene

oxidation, k₂= 0.45 M⁻¹s⁻¹at 0 °C, was signiﬁcantly lower

than the rate constant for the oxidation of PPh₃discussed above

(Table 5). The temperature dependence (T = −20 to 0 °C) of

the second-order rate constant ﬁts the linear form of the Eyring

equation, giving the activation parameters of ΔH^⧧= 23 kJ/mol

and ΔS^⧧= −169 J/(mol K), typical of associative processes

(Figure 12c).^106,114A similar approach has been applied to the

reaction of 1 and H₂O₂, showing k₂= 405 M⁻¹s⁻¹at 0 °C in

single mixing of 1 and H₂O₂(1−20 equiv) (Supporting

Information, Figure S13a) and k₂= 0.17 M⁻¹s⁻¹at 0 °C in

cyclooctene oxidation (Supporting Information, Figure S13b,

Table 4), which is comparable to the reaction of 1 and ^iPrIOO.

Kinetic experiments unambiguously proved bimolecular

decay of the iron(IV)-oxo intermediate upon reaction with

cyclooctene. In separate benchtop experiments, Fe(IV)O

species were generated from 1 and ^iPrIOO, treated with

cyclooctene, and the products of these reactions were analyzed

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1

2

supported by partially oxidized macrocycles do not form with

substoichiometric amounts of oxidant, especially on the time

scale of the stopped-ﬂow measurements. The reactivity of

ferryl(IV) intermediates with respect to cyclooctene appeared

to decrease as the number of CN bonds in the macrocycle

increased and the number of available secondary amino groups

decreased. Nevertheless, ligand oxidation of FePyMACs, such

as 1, did not lead to complete destruction of the catalyst.

Instead, partially oxidized Schiff base pyridine-containing

macrocycles were capable of supporting high-valent iron-oxo

intermediates that carry out relatively slow (minutes at room

temperature) but high-yielding (nearly quantitative in case of

1b) transfer of an oxygen atom from the oxidant to the oleﬁn

substrate.

(L)Fe^II+ ROOH → (L)Fe^III(OOR)

(L)Fe^III(OOR) → (L)Fe^IVO + ROO·

In our experiments, an iron(III)-hydroperoxo intermediate

was not directly observed, suggesting that this species, if

formed, rapidly undergoes O−O bond cleavage prior to the

rate-determining step. Detailed kinetic experiments revealed the

role of iron(IV) species in catalytic oleﬁn epoxidations by

determining the rates of individual reaction steps; a similar

approach was used by the van Eldik group in the studies of

catalytic oxidations with metalloporphyrins .^131,132

Accumulation of the presumed Fe(IV)-oxo intermediate in

the reaction of 1 with H₂O₂was rapid (k₂= 405 M⁻¹s⁻¹at 0

°C); the time scale of this process (Figure 5) was well within 5

min, the typical time used in oleﬁn epoxidation experiments,

suggesting that the Fe(IV)-oxo intermediate, 1b, may be

involved in catalytic epoxidations with H₂O₂. Unfortunately,

ligand oxidation occurs concurrently with the buildup of the

Fe(IV)-oxo intermediate upon interaction of 1 with hydrogen

peroxide, thus complicating direct studies of the reactivity of

Fe(IV) generated in this system.

DISCUSSION

■

Iron complexes with aminopyridine macrocycles are known to

be catalytically active in a number of “green” catalytic

oxidations.^49,54−57For example, complexes 2 and 3 (Figure

2) were found to be eﬃcient catalysts of oleﬁn epoxidation with

hydrogen peroxide, a nontoxic oxidant that produces water as

the only byproduct.⁶⁵A related macrocyclic complex with a 12-

membered ligand containing two pyridine rings (Figure 1)

catalyzes eﬃcient and selective oleﬁn dihydroxylation with

oxone in water.¹²⁰An iron(III) complex with a nonmethylated

analogue of L1, [Fe(CRH)]Cl₂(BF₄), is known to be one of the

best models of catalase,^121−125and hydrogen peroxide

decomposition is thought to involve an Fe(IV) intermediate.¹²⁶

In the present work, an iron(II) complex with a tetradentate

aminopyridine macrocycle L1(complex 1) was found to display

modest catalytic activity in oleﬁn epoxidation with hydrogen

peroxide. Complex 1 provided a convenient platform for

exploring the role of high-valent iron-oxo intermediates in

stoichiometric and catalytic substrate oxidations.

Oxygen atom transfer reagents are often used to cleanly

produce high-valent metal-oxo species. In our experiments, a

rapid and clean reaction of 1 with a potent oxidant, isopropyl 2-

iodoxybenzoate,⁶⁶aﬀorded an iron(IV) species that was

characterized by UV−vis, Mossbauer, and mass spectrometry.

̈

The transfer of the ﬁrst oxygen atom from the iodine(V)

reagent, ^iPrIOO, occurred rapidly at or below room temperature

(k₂= 1654 M⁻¹s⁻¹at −40 °C):

(L)Fe^II+ ^iPrI^VOO → (L)Fe^IVO + ^iPrI^III

O

It appears that the coordination of ^iPrIOO to the iron(II)

center in 1 accelerated the oxygen atom transfer process. It is

also likely that ^iPrIOO and/or ^iPrIO coordinated to the Fe(IV)-

oxo intermediate, similar to coordination of anions to high-

valent iron(IV) complexes supported by other polyamine- or

aminopyridine ligands.^{17,24,27,37,58,133}Under stoichiometric or

substiochiometric conditions, the reaction between 1 and

^iPrIOO was relatively slow (but still fast enough to reach the

maximum accumulation of Fe^IVin 20 s at −20 °C, Figures 7a),

and both oxygen atoms were being transferred from the oxidant

to the iron(II) center. Having access to the Fe(IV)-oxo

intermediate allowed us to investigate its reactivity. Quantitative

data on the reactivity of ferryl(IV) intermediates in nonheme

iron chemistry are still limited.^{29,31,58,111,134}

Complex 1 contained low-spin iron(II) in a pseudo-

octahedral geometry, as determined by X-ray crystallography.

Magnetic susceptibility measurements and Mossbauer spec-

̈

troscopy conﬁrmed that low-spin conﬁguration of Fe(II) was

retained in acetonitrile solutions. Electrochemistry revealed

reversible one-electron oxidation/reduction of the iron center

in 1, with E_1/2= 0.49 V vs Fc⁺/Fc, a value very similar to the

half-wave potentials of related macrocyclic complexes such as 2

and a protonated form of 3.⁶⁵Reversibility of repeated

electrochemical one-electron oxidations demonstrated the

stability of the ligand L1 on the cyclic voltammetry time

scale. However, chemical oxidations with a mild oxidant, air

oxygen, led to a slow (several hours) partial ligand oxidation,

aﬀording iron(II) complexes with Schiff base analogues of L1

(complexes 1c and 1e, Figure 7, Scheme 2). Fortunately, the

macrocycle remained intact, and amenable to reversible redox

processes at the metal center.

Similarly to other ferryl(IV) species,^{24,27,31,42,58}1b cleanly

reacted with triarylphosphines, forming the corresponding

phosphine oxides. Varying the electronic properties of

substituents on the aryl rings revealed the electrophilic nature

of the oxidant 1b, as followed from the Hammett correlation

with negative ρ (Figure 12). Analogous Hammett correlations

were previously found for oxidations of sulﬁdes,^34,112

phosphines,^115−117and other organic substrates,^113,114conﬁrm-

ing the electrophilic nature of nonheme ferryl intermediates.

The rate constant for the reaction of 1b with triphenylphos-

phine, 26.5 M⁻¹s⁻¹at 0 °C, was comparable to the analogous

values of the rate constants obtained with related nonheme iron

complexes. For example, the ﬁrst crystallographically charac-

terized nonheme iron(IV) complex with tetramethylcyclam

(TMC, Figure 1) reacted with PPh₃about ﬁve times slower

than 1b (k₂= 5.7 M⁻¹s⁻¹at 0 °C, for TMC¹³⁴). Fairly rapid

Reaction of the iron(II) complex 1 with hydrogen peroxide,

monitored by the stopped-ﬂow method, aﬀorded a mixture of

several colored intermediates, including a species with an

optical spectrum typical of low-spin iron(IV) complexes (λ_max

=

705 nm, ε = 240 M⁻¹cm⁻¹, Figure 5). Formation of iron(IV)

intermediates in reactions of iron(II) or iron(III) complexes

with hydroperoxides is well-documented.^{56,91,96,97,127,128}Gen-

erally accepted mechanisms for the Fe(IV) formation assume

the intermediacy of the Fe(III)-(hydro)peroxo intermedi-

ates:^129,130

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oxidation of PPh₃with 1b and full regeneration of 1 in this

process led to the possibility of performing multiple turnovers

in PPh₃oxidations with ^iPrIOO in the presence of

substoichiometric amounts of 1.

good reducing agents as substrates, ligand oxidation does not

occur (as was seen for triarylphosphine oxidations) or occurs

slowly, aﬀording both the substrate oxidation products and the

oxidized (yet still somewhat reactive) Schiff base macrocycles

(as was observed for cyclooctene). Fully oxidized macrocycle

1e, lacking NH−groups, is much less reactive in oleﬁn

epoxidation than the parent complex 1. These observations

parallel previously reported data on oleﬁn epoxidations; for

example, iron cyclam was catalytically active in oleﬁn

epoxidations with H₂O₂, but the analogous complex with

tetramethylcyclam (TMC) was inactive.¹³⁵For the PyMAC

complexes described in this work, the ferryl(IV) intermediate is

competent in oxidations of selected substrates (triarylphos-

phines and cyclooctene). However, ferryl(IV) does not have to

be the sole intermediate involved in substrate oxidations with

H₂O₂catalyzed by iron PyMACs; investigation into the

mechanisms of more challenging oxidations are currently in

progress.

Rather high activity of ferryl species supported by amino-

pyridine macrocycle L1 suggested that this intermediate might

play an important role in catalytic oleﬁn epoxidations. In a

series of kinetic experiments, we found that 1b reacted with

cyclooctene (k₂= 0.45 M⁻¹s⁻¹at 0 °C), forming epoxide. The

time scale of cyclooctene oxidation with the Fe(IV)-oxo

intermediate under catalytically relevant concentration con-

ditions, about 100 s at room temperature, was comparable to

the typical time of catalytic epoxidations with H₂O₂(5 min).

Interestingly, the rate constant of oleﬁn epoxidation with 1b

generated from 1 and ^iPrIOO was very close to the value of the

rate constant in the system where 1b was generated from 1 and

H₂O₂(Table 4). This suggested that cyclooctene epoxidation

was likely performed by ferryl(IV) species itself, rather than by

other oxidant-derived intermediates (e.g., macrocycle adducts

with the oxidant, or hydroxyl radicals). Independently

determined rates of ferryl(IV) formation and its subsequent

reaction with cyclooctene conﬁrmed that the Fe(IV)-oxo

species 1b was a kinetically competent intermediate for

cyclooctene epoxidation at room temperature; a similar

approach, measuring the rates of individual reaction steps to

identify a kinetically competent catalytic intermediate, was used

by van Eldik and co-workers in their analysis of heme-catalyzed

substrate oxidations.^131,132

ASSOCIATED CONTENT

* Supporting Information

■

S

Crystallographic data in CIF format. Further details are given in

Table S1 and Figures S1−S16. This material is available free of

charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: elena.rybak-akimova@tufts.edu.

■

A catalytic cycle for substrate oxidation with H₂O₂in the

presence of 1 is depicted in Scheme 3. It should be noted that

Notes

The authors declare no competing ﬁnancial interest.

Scheme 3. Catalytic Cycle for Substrate Oxidation with

H₂O₂/^iPrIOO in the Presence of 1

ACKNOWLEDGMENTS

■

This work was supported by the U.S. Department of Energy,

Oﬃce of Basic Energy Science, Grant DE-FG02-06ER15799.

The NMR facility, the ESI-MS spectrometer, and rapid kinetic

instrumentation at Tufts were supported by the NSF (Grants

CHE-9723772, MRI CHE 0821508, MRI CHE 0320783, and

CHE 0639138). We thank Prof. Stephen Lippard (MIT) for

generously allowing us to use Mossbauer spectrometer in his

̈

laboratories.

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Role of Fe(IV)-oxo intermediates in stoichiometric and catalytic oxidations mediated by iron pyridine-azamacrocycles

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