Dioxygen reactivity of biomimetic Fe(II) complexes with noninnocent catecholate, o-aminophenolate, and o-phenylenediamine ligands

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Open Access on 04/03/2015

Dioxygen Reactivity of Biomimetic Fe(II) Complexes with

Noninnocent Catecholate, o‑Aminophenolate, and

o‑Phenylenediamine Ligands

Michael M. Bittner,^†Sergey V. Lindeman,^†Codrina V. Popescu,*^,‡and Adam T. Fiedler*^,†

^†Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201, United States

^‡Department of Chemistry, Ursinus College, Collegeville, Pennsylvania 19426, United States

S

* Supporting Information

ABSTRACT: This study describes the O₂reactivity of a series of

high-spin mononuclear Fe(II) complexes each containing the facially

coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine

(

^Ph2TIP) ligand and one of the following bidentate, redox-active

ligands: 4-tert-butylcatecholate (^tBuCatH⁻), 4,6-di-tert-butyl-2-amino-

phenolate (^tBu2APH⁻), or 4-tert-butyl-1,2-phenylenediamine

(

^tBuPDA). The preparation and X-ray structural characterization of

[Fe²⁺(^Ph2TIP)(^tBuCatH)]OTf, [3]OTf and [Fe²⁺(^Ph2TIP)(^tBuPDA)]-

(OTf)₂, [4](OTf)₂are described here, whereas [Fe²⁺(^Ph2TIP)-

(

^tBu2APH)]OTf, [2]OTf was reported in our previous paper [Bittner

et al., Chem.Eur. J. 2013, 19, 9686−9698]. These complexes mimic the substrate-bound active sites of nonheme iron

dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is

oxidized in the presence of O₂, and the geometric and electronic structures of the resulting complexes were examined with

spectroscopic (absorption, EPR, Mossbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf

̈

reacts rapidly with O₂to yield the ferric-catecholate species [Fe³⁺(^Ph2TIP)(^tBuCat)]⁺(3^ox), which undergoes further oxidation to

generate an extradiol cleavage product. In contrast, complex [4]²⁺experiences a two-electron (2e⁻), ligand-based oxidation to

give [Fe²⁺(^Ph2TIP)(^tBuDIBQ)]²⁺(4^ox), where DIBQ is o-diiminobenzoquinone. The reaction of [2]⁺with O₂is also a 2e⁻

process, yet in this case both the Fe center and ^tBu2AP ligand are oxidized; the resulting complex (2^ox) is best described as

[Fe³⁺(^Ph2TIP)(^tBu2ISQ)]⁺, where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable

continuum of electronic structures ranging from [Fe³⁺(L²⁻)]⁺(3^ox) to [Fe³⁺(L^•−)]²⁺(2^ox) to [Fe²⁺(L⁰)]²⁺(4^ox). Notably, the O₂

reaction rates vary by a factor of 10⁵across the series, following the order [3]⁺> [2]⁺> [4]²⁺, even though the complexes have

similar structures and Fe^3+/2+redox potentials. To account for the kinetic data, we examined the relative abilities of the title

complexes to bind O₂and participate in H-atom transfer reactions. We conclude that the trend in O₂reactivity can be

rationalized by accounting for the role of proton transfer(s) in the overall reaction.

bidentate ligands provides the corresponding (di)(imino)-

semiquinonate radicals, and two-electron oxidation yields the

closed-shell (di)(imino)benzoquinones. Complexes that com-

bine the noninnocent ligands in Scheme 1 with redox-active

metal center(s) often possess ambiguous electronic structures,

since multiple assignments of ligand and metal oxidation states

are possible. Thus, careful examination with a variety of

experimental and computational methods is usually required to

obtain accurate electronic-structure descriptions.¹⁹⁻²³

1. INTRODUCTION

Catechols and their nitrogen-containing analogs (o-amino-

phenols and o-phenylenediamines; see Scheme 1) are well-

established members of the “o-phenylene family” of redox

noninnocent ligands.¹⁻¹⁸One-electron oxidation of these

Scheme 1. o-Phenylene Ligands

While the role of o-phenylene ligands in electron-transfer

series has been studied extensively, their ability to facilitate

proton-coupled electron transfers (PCETs) in transition-metal

complexes has received less attention. As shown in Scheme 1,

the free compounds are able to donate a total of two protons

(2H⁺) and two electrons (2e⁻) in various combinations.

Received: December 21, 2013

Published: April 3, 2014

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Coordination to a redox-active metal center is expected to

perturb the chemical and electronic properties of these ligands,

resulting in complexes with rich and unpredictable PCET

landscapes. Such complexes may ﬁnd applications in chemical

processes that require multiple proton and electron transfers,

including energy-related reactions like water oxidation, hydro-

gen production, and nitrogen ﬁxation.²⁴⁻²⁹For instance, an

iron complex with o-phenylenediamine ligands was recently

shown to undergo photochemical H₂-evolution via PCET

steps.³⁰Similarly, Heyduk et al. have found that zirconium(IV)

complexes with noninnocent bis(2-phenolato)amide ligands

react with O₂to yield [Zr⁴⁺₂(μ−OH)₂] speciesa process that

requires donation of 1H⁺and 2e⁻from each ligand.³¹

Our interest in noninnocent ligands stems from eﬀorts to

prepare synthetic mimics of mononuclear nonheme iron

dioxygenases. These enzymes carry out the oxidative ring-

cleavage of aromatic substrates (catechols, aminophenols, and

hydroquinones), and the catalytic cycles are thought to involve

formation of a ferrous-(substrate radical) intermediate.³²⁻³⁵

Recently, we reported the synthesis of two mononuclear Fe(II)

complexes (1 and [2]OTf) that model the substrate-bound

form of aminophenol dioxygenases.^36,37The high-spin ferrous

centers of 1 and [2]⁺are bound to the 2-amino-4,6-di-tert-

butylphenolate (^tBu2APH) “substrate,” and the enzymatic

coordination environment is replicated using a facially

coordinating N3 supporting ligand: hydrotris(3,5-diphenylpyr-

azol-1-yl)borate (^Ph2Tp) in the case of 1 and tris(4,5-diphenyl-

1-methylimidazol-2-yl)phosphine (^Ph2TIP) in [2]OTf (see

Scheme 2). These complexes were shown to engage in

Fe³⁺−^tBu2ISQ and Fe²⁺−^tBu2IBQ limits,³⁷where ^tBu2IBQ is

iminobenzoquinone with tert-butyl substituents at the 4- and 6-

positions.

In this Manuscript, we expanded upon our previous studies

by preparing monoiron(II) ^Ph2TIP-based complexes with

ligands derived from catechol and o-phenylenediamine. Like

the o-aminophenolate studies described above, we began with

the synthesis and X-ray structural characterization of mono-

nuclear, high-spin Fe(II) complexes, each containing a

bidentate ligand capable of both proton and electron transfer.

The catecholate complex [3]OTf was prepared using 4-tert-

butylcatechol (^tBuCatH₂) and the ^Ph2TIP supporting ligand

(Scheme 2). The o-phenylenediamine complex [4](OTf)₂has

the overall formulation of [Fe(^Ph2TIP)(^tBuPDA)](OTf)₂, where

^tBuPDA is 4-tert-butyl-1,2-phenylenediamine. Each of the three

Fe(II) complexes ([2]⁺, [3]⁺, and [4]²⁺) is air-sensitive, and the

products of the O₂reactions have been characterized with

spectroscopic (ultraviolet−visible (UV−vis) absorption, elec-

tron paramagnetic resonance (EPR), Mossbauer, resonance

̈

Raman) and computational (density functional theory (DFT))

methods. These studies revealed that the identity of the ligand

controls whether the O₂-driven oxidation is an Fe- or ligand-

based process (or a combination of both). In addition, O₂

reaction rates vary by greater than 5 orders of magnitude across

the series, despite the fact that the overall structures of the

Fe(II) complexes are quite similar. Thus, this unique series of

complexes has provided a valuable framework for exploring the

relationship between ligand-based PCET chemistry and the O₂

reactivity of Fe complexes. The implications of these results for

the O₂activation mechanisms of the ring-cleaving dioxygenases

are also discussed.

Scheme 2

2. EXPERIMENTAL SECTION

Materials. Reagents and solvents were purchased from commercial

sources and were used as received, unless otherwise noted. Air-

sensitive materials were synthesized and handled under inert

atmosphere using a Vacuum Atmospheres Omni-Lab glovebox. The

39

^Ph2TIP³⁸and ^tBuAPH₂ligands and the 2,4,6-tri-tert-butylphenoxyl

radical⁴⁰(TTBP^•) were prepared according to literature procedures.

Synthetic procedures for complexes [Fe(^Ph2TIP)(MeCN)₃](OTf)₂,³⁸

1,³⁶and [2]OTf³⁷were published previously by our group.

Physical Methods. Elemental analyses were performed at Midwest

Microlab, LLC in Indianapolis, IN. UV−vis absorption spectra were

measured with an Agilent 8453 diode array spectrometer equipped

with a cryostat from Unisuko Scientiﬁc Instruments (Osaka, Japan).

Fourier-transform infrared (FTIR) spectra of solid samples were

obtained with a Thermo Scientiﬁc Nicolet iS5 FTIR spectrometer

1

equipped with the iD3 attenuated total reﬂectance accessory. H and

¹⁹F NMR spectra were recorded at room temperature with a Varian

400 MHz spectrometer. ¹⁹F NMR spectra were referenced to the

benzotriﬂuoride peak at −63.7 ppm. Mass spectra were collected using

an Agilent 6850 gas chromatography−mass spectrometer (GC-MS)

with a HP-5 (5% phenylmethylpolysiloxane) column. Cyclic

voltammetric (CV) measurements were conducted in the glovebox

with an epsilon EC potentiostat (iBAS) at a scan rate of 100 mV/s

with 100 mM (NBu₄)PF₆as the supporting electrolyte. The three-

electrode cell contained a Ag/AgCl reference electrode, a platinum

auxiliary electrode, and a glassy carbon working electrode. Potentials

were referenced to the ferrocene/ferrocenium (Fc^+/0) couple, which

has E_1/2values of +0.52 V in CH₂Cl₂under these conditions.

EPR experiments were performed using a Bruker ELEXSYS E600

featuring an ER4415DM cavity that resonates at 9.63 GHz, an Oxford

Instruments ITC503 temperature controller, and an ESR-900 He ﬂow

cryostat. The program EasySpin⁴¹was used to simulate and ﬁt

experimental spectra. Resonance Raman (rR) spectra were measured

with excitation from either a Coherent I-305 Ar⁺laser (488.0 nm) or I-

ligand-based H-atom transfer (HAT) reactions to yield novel

species containing an Fe(II) center coordinated to an

iminobenzosemiquinonate (^tBu2ISQ) radical, thus providing

synthetic precedents for the putative Fe(II)/ISQ intermediate

of the enzyme. The Fe(II)/ISQ complexes can be further

oxidized by one electron, although it has proven diﬃcult to

determine whether this process is ligand- or iron-based.

Detailed crystallographic, spectroscopic, and computational

analyses suggest that the fully oxidized species have

intermediate electronic conﬁgurations between the

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Table 1. Summary of X-ray Crystallographic Data Collection and Structure Reﬁnement

a

b

[3]OTf·2DCE

[4](OTf)(BPh₄)·DCE·C₅H₁₂

[4(MeCN)](OTf)₂·MeCN·Et₂O

[5](OTf)·1.5CH₂Cl₂

empirical formula

formula weight

crystal system

space group

a, Å

C₆₃H₆₀Cl₄F₃FeN₆O₅PS

1298.85

C₉₀H₉₁BCl₂F₃FeN₈O₃PS

1590.34

C₆₈H₇₁F₆FeN₁₀O₇PS₂

1388.96

C_58.5H₅₁Cl₃F₃FeN₆O₅PS

1200.28

monoclinic

P2₁

monoclinic

P2₁/n

triclinic

P1

̅

16.0859(2)

21.4779(2)

17.8488(2)

90

19.6939(4)

18.3202(4)

22.2057(4)

90

15.3998(3)

15.6626(3)

17.9588(3)

88.191(2)

64.934(2)

61.081(2)

3351.4(1)

2

15.5548(2)

17.2278(3)

23.4768(4)

97.790(1)

91.828(1)

115.875(2)

5578.9(2)

4

b, Å

c, Å

α, deg

β, deg

90.1428(9)

90

92.009(2)

90

γ, deg

V, Å³

6166.6(1)

4

8006.8(3)

4

Z

D

_calc, g/cm³

1.399

1.293

1.376

1.429

λ, Å

μ, mm⁻¹

1.5418

0.7107

1.5418

4.642

0.350

0.388

4.654

θ range, deg

reﬂections collected

independent

reﬂections

6 to 148

60 871

6 to 58

6 to 147

77 363

74 982

71 466

23 500

19 234

16 362

21 895

[R_int= 0.0372]

23 500/7/1532

1.041

[R_int= 0.0418]

19 234/67/1058

1.051

[R_int= 0.0363]

16 362/7/898

1.050

[R_int= 0.0345]

21 895/129/1607

1.074

data/restraints/parameters

GOF (on F²)

b

R1/wR2 (I > 2σ(I))

0.0390/0.1022

0.0395/0.1028

0.0962/0.2182

0.1280/0.2375

0.0470/0.1170

0.0589/0.1253

0.0425/0.1169

0.0478/0.1215

b

R1/wR2 (all data)

a

b

The DCE solvate is only partially (80%) populated. The ethereal solvate is only partially (78%) populated.

302C Kr⁺laser (647.1 nm) with ∼50 mW of power at the sample. The

scattered light was collected using a 135° backscattering arrangement,

dispersed by an Acton Research triple monochromator equipped with

a 1200 grooves/mm grating and detected with a Princeton

Instruments Spec X 100BR CCD camera. Spectra were accumulated

at 77 K, and rR frequencies were referenced to the 983 cm⁻¹peak of

K₂SO₄.⁴²Low-ﬁeld (0.04 T) variable temperature (5−200 K)

dried under vacuum (yield = 83 mg, 20%). μ_eff= 4.7 μ_B(Evans

method). UV−vis [λ_max, nm (ε, M⁻¹cm⁻¹) in CH₂Cl₂]: 397 (940). IR

(neat, cm⁻¹): 3058 (w), 2955 (w), 1603 (w), 1505 (m), 1460 (m),

1

1443 (m), 1369 (m), 1241 (s), 1154 (s), 1028 (s). H NMR (400

MHz, CD₂Cl₂): δ = −27.52, −10.44, 2.22, 4.51, 5.98, 6.94, 9.53, 10.06,

11.47, 23.88, 29.58, 61.67, 65.98. ¹⁹F NMR (376 MHz, CD₂Cl₂): δ =

−80.3 (OTf) ppm. Elemental analysis calcd (%) for C₅₉H₅₂F₃-

FeN₆O₅PS·2DCE: C, 58.26; H, 4.66; N, 6.47. Found: C, 58.00; H,

4.98; N, 6.46.

Mossbauer spectra were recorded on a closed-cycle refrigerator

̈

spectrometer, model CCR4K, equipped with a 0.04 T permanent

magnet, maintaining temperatures between 5 and 300 K. Mossbauer

[Fe(^Ph2TIP)(^tBuPDA)](OTf)₂, [4](OTf)₂. Equimolar amounts of [Fe-

̈

spectra were analyzed using the software WMOSS (Thomas Kent,

SeeCo.us, Edina, Minnesota). The samples consisted of solid powders

(or crystalline material) suspended in nujol, placed in Delrin 1.00 mL

cups, and then frozen in liquid nitrogen. The [4](OTf)₂sample was

prepared from material crystallized from a mixture of 1,2-dichloro-

ethane (DCE) and Et₂O. The air-oxidized sample 4^oxwas prepared by

exposing a solution of [4](OTf)₂in CH₂Cl₂to air for 20 h, followed

by removal of solvent to give a dark green powder. (In this

Manuscript, “ox” designates species generated via reaction of the

Fe(II) starting complexes with O₂.) The isomer shifts are quoted at 5

K with respect to iron metal spectra recorded at 298 K.

(

^Ph2TIP)(MeCN)₃](OTf)₂(304 mg, 0.25 mmol) and ^tBuPDA (42 mg,

0.25 mmol) were dissolved in THF (10 mL), and the reaction was

stirred for 18 h. Removal of the solvent under vacuum yielded a white

solid that was dissolved in DCE (3 mL) and ﬁltered. Vapor diﬀusion of

Et₂O into this solution aﬀorded the product as a colorless solid (yield

= 123 mg, 39%) suitable for use in spectroscopic and kinetic studies.

The complex does not exhibit absorption features in the visible region.

μ_eff= 5.48 μ_B(Evans method). IR (neat, cm⁻¹): 3306 (w, ν(NH)),

3254 (w, ν(NH)), 3057 (w), 2960 (w), 1572 (w), 1443 (w), 1261 (s),

1147 (m), 1029 (w). ¹H NMR (400 MHz, CD₂Cl₂): δ = −28.8 (1H),

0.62 (9H), 4.98 (6H), 7.00 (3H), 7.32 (3H), 7.68 (6H), 8.53 (6H),

11.81 (1H), 12.89 (9H), 15.33 (6H), 19.01 (2H), 24.33 (2H), 31.36

(1H). ¹⁹F NMR (376 MHz, CD₂Cl₂): δ = −79.3 (OTf) ppm.

Elemental analysis revealed that a small amount of DCE solvent (0.5

equiv/Fe) remains after drying. Elemental analysis calcd (%) for

C₆₀H₅₅F₆FeN₈O₆PS₂·0.5DCE: C, 56.42; H, 4.42; N, 8.63; F, 8.78.

Found: C, 56.56; H, 4.51; N, 8.54; F, 8.32. X-ray quality crystals were

obtained by either slow diﬀusion of Et₂O into a concentrated MeCN

solution or pentane layering of a DCE solution containing 1 equiv of

NaBPh₄.

X-ray diﬀraction (XRD) data were collected with an Oxford

Diﬀraction SuperNova κ-diﬀractometer (Agilent Technologies)

equipped with dual microfocus Cu/Mo X-ray sources, X-ray mirror

optics, Atlas CCD detector, and low-temperature Cryojet device. The

data were processed with CrysAlis Pro program package (Agilent

Technologies, 2011), followed by an empirical multiscan correction

using SCALE3 ABSPACK routine. Structures were solved using

SHELXS program and reﬁned with SHELXL program⁴³within Olex2

crystallographic package.⁴⁴X-ray crystallographic parameters are

provided in Table 1, and experimental details are available in the CIFs.

[Fe(^Ph2TIP)(^tBuCatH)](OTf), [3]OTf. Equimolar amounts of [Fe-

[Fe(^Ph2TIP)(^Me2MP)]OTf, [5]OTf. Equimolar amounts of [Fe-

(

^Ph2TIP)(MeCN)₃](OTf)₂(143 mg, 0.12 mmol) and 2-methoxy-5-

(

^Ph2TIP)(MeCN)₃](OTf)₂(456 mg, 0.38 mmol) and ^tBuCatH₂(63

methylphenolate (^Me2MPH, 16.3 mg, 0.12 mmol) were dissolved in

THF (10 mL), followed by addition of NEt₃(20 μL, 0.15 mmol). The

mixture was stirred for 3 h, and the solvent was removed under

vacuum. The crude solid was taken up in CH₂Cl₂(5 mL) and layered

with hexanes. After several days, light green crystals suitable for X-ray

mg, 0.38 mmol) were dissolved in tetrahydrofuran (THF) (10 mL),

followed by addition of NEt₃(58 μL, 0.42 mmol). The dark yellow

solution was stirred for 30 min, and the solvent was removed under

vacuum. The crude material was taken up in DCE (5 mL) and ﬁltered.

Layering of this solution with hexane provided bright yellow crystals

suitable for X-ray diﬀraction. Crystals were washed with hexanes and

crystallography were collected (yield = 101 mg, 80%). UV−vis [λ_max

,

nm (ε, M⁻¹cm⁻¹) in CH₂Cl₂]: 390 (1400), 610 (500). IR (neat,

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cm⁻¹): 3048 (w), 1597 (w), 1499 (w), 1444 (m), 1396 (w), 1260 (s),

1220 (s), 1146 (s), 1073 (w), 1028 (s), 982 (m), 790 (s), 771 (s). The

crystals used for elemental analysis were prepared from a mixture of

DCE/hexanes. The results suggest that some DCE solvent (∼1 equiv/

Fe) remains after drying, consistent with the X-ray structure that found

1.5 equiv of uncoordinated CH₂Cl₂in the unit cell. Elemental analysis

calcd (%) for C₅₇H₄₈F₃FeN₆O₅PS·DCE: C, 60.47; H, 4.47; N, 7.17;

Found: C, 59.00; H, 4.63; N, 7.55.

zole rings attached at the 2-position to a central P atom, and the tert-

butyl substituents of the bidentate ligand were replaced with Me

groups. Geometry optimizations were performed for the Fe(II)

precursors, [Fe/O₂] adducts, and O₂under tight convergence criteria,

and the resulting models were used to obtain gas-phase vibrational and

thermodynamic data. Solvent eﬀects were calculated using the

conductor-like screening model (COSMO)⁶⁶with a dielectric constant

(ε) of 9.08 for CH₂Cl₂. The “spin-ﬂip” feature of ORCA was

employed to generate [Fe/O₂] wave functions for S_tot= 2 and 1 states.

Synthesis of ^tBuPDA with ¹⁵N at 2-Position. Using a published

procedure,⁴⁵acetic anhydride (2.64 g, 25.8 mmol) was added dropwise

to a solution of 4-tert-butylaniline (3.34 g, 22.4 mmol) in CH₂Cl₂(30

mL) at 0 °C. A white precipitate formed as the mixture was stirred for

30 min. After addition of 30 mL of hexanes, the solution was ﬁltered to

give 4-tert-butylacetanilide as a white solid. Without further

puriﬁcation, 680 mg (3.55 mmol) of the product was dissolved in

CHCl₃(15 mL). To this solution, H₂SO₄(0.3 mL) and ¹⁵N-labeled

HNO₃(1.0 g, 7.04 mmol, Aldrich, 98% ¹⁵N) were added dropwise.

The resulting dark orange solution was stirred for 1 h and was then

washed successively with H₂O, saturated NaHCO₃, and brine. After

drying the organic layer with MgSO₄the solvent was removed to yield

4-tert-butyl-2-nitroacetanilide. The protecting group was then removed

by reﬂuxing in EtOH with KOH (143 mg, 2.6 mmol). The mixture

was poured into ice−water, yielding a precipitate that was isolated by

ﬁltration, washed with cold H₂O, and dried in vacuo. The resulting

¹⁵N-labeled 4-tert-butyl-2-nitroaniline (300 mg, 1.54 mmol) was

dissolved in MeOH (20 mL), and 5% Pd/C catalyst (90 mg) was

added. The mixture was stirred under H₂(46 psi) for 5 h and ﬁltered

through Celite, and the solvent was removed under vacuum to yield a

dark purple solid (yield = 201 mg, 79%). ¹H and ¹³C NMR spectra of

the product were identical to those obtained with commercially

available ^tBuPDA.

3. RESULTS AND ANALYSIS

3.A. Synthesis, X-ray Structures, and Electrochemical

Properties. Reaction of [Fe(^Ph2TIP)(MeCN)₃](OTf)₂with

^tBuCatH₂or ^tBuPDA in THF generated the ^Ph2TIP-based

complexes [3]OTf and [4](OTf)₂, respectively; the synthesis

of [3]⁺also required 1 equiv of NEt₃.⁶⁷Yellow crystals of

[3]OTf were grown by layering a DCE solution with hexane.

Recrystallization of [4](OTf)₂by slow diﬀusion of Et₂O into a

DCE solution provided colorless and analytically pure material

that was used in subsequent reactivity and spectroscopic

studies.⁶⁸Crystals for X-ray diﬀraction (XRD) analysis were

obtained by either (i) slow diﬀusion of Et₂O into a

concentrated MeCN solution of [4](OTf)₂or (ii) pentane

layering of a DCE solution. In the latter case, the resulting

crystals diﬀracted poorly due to extensive disorder of the triﬂate

counteranions. However, by adding 1 equiv of NaBPh₄to the

DCE solution, we were able to grow well-diﬀracting crystals

with an overall composition of [4](OTf)(BPh₄).

As shown in Figure 1, the X-ray structure of [3]OTf features

a ﬁve-coordinate (5C) Fe(II) complex with bidentate

Dioxygen Reactivity Studies. Oxygenation studies were

performed by injecting anaerobic solutions of the Fe(II) complex

into O₂-saturated solutions of CH₂Cl₂at the desired temperature.

Formation of the oxidized species was monitored using UV−vis

spectroscopy. The concentration of O₂in CH₂Cl₂solutions at various

temperatures (T) was estimated using the formula: S = (LP_O2)/(TR),

where L is the Ostwald coeﬃcient (0.257 for CH₂Cl₂), P_O2is the

partial pressure of O₂, and R is the gas constant.^46,47The

determination of P_O2accounted for the vapor pressure of CH₂Cl₂

(P_solv) as a function of T: P_O2= 1 atm − P_solv. Following established

procedures,⁴⁸⁻⁵¹the decomposition products of the 3^ox+ O₂reaction

were isolated by removing the CH₂Cl₂solvent under vacuum, taking

the residue up in MeCN, and treating the solution with ∼3 mL of HCl

(2 M). After extraction of the aqueous layer with Et₂O, the solvent was

1

removed to give a residue that was analyzed with GC-MS and/or H

NMR spectroscopy. The ¹H NMR data was interpreted with the aid of

published spectra.⁵²

DFT Computations. DFT calculations were performed using the

ORCA 2.9 software package developed by Dr. F. Neese (MPI for

Chemical Energy Conversion).⁵³Calculations involving 4^oxemployed

Becke’s three-parameter hybrid functional for exchange along with the

Lee−Yang−Parr correlation functional (B3LYP).^54,55These calcu-

lations utilized Ahlrichs’ valence triple-ζ basis set (TZV) and TZV/J

auxiliary basis set, in conjunction with polarization functions on all

atoms.⁵⁶⁻⁵⁸In the geometry-optimized model, the ^Ph2TIP ligand was

modiﬁed by replacing the Ph-groups at the 5-position of the imidazolyl

rings with H-atoms. In addition, the tert-butyl substituent of the

^tBuPDA ligand was replaced with an Me group. To avoid spurious

transitions, time-dependent DFT (TD-DFT) calculations used a

truncated version of the optimized 4^oxmodel with Me groups (instead

of Ph groups) at the 4-positions of the imidazolyl rings. TD-DFT

calculations⁵⁹⁻⁶¹calculated absorption energies and intensities for 50

excited states with the Tamm−Dancoﬀ approximation.^62,63Isosurface

plots of molecular orbitals and electron-density diﬀerence maps

(EDDMs) were prepared with Laaksonen’s gOpenMol program.⁶⁴

Energetic parameters for the binding of O₂to the Fe(II) complexes

were computed using the Perdew−Burke−Ernzerhof (PBE) func-

tional⁶⁵with 10% Hartree−Fock exchange. These calculations

employed modiﬁed ^Ph2TIP ligands containing three N-methylimida-

Figure 1. Thermal ellipsoid plot (50% probability) derived from the

X-ray structure of [3]OTf·2DCE. Counteranions, noncoordinating

solvent molecules, and most hydrogen atoms have been omitted for

clarity.

monoanionic catecholate and facially coordinating ^Ph2TIP

ligands. The [3]OTf unit cell contains two symmetrically

independent complexes, and metric parameters for both are

provided in Table 2. The Fe(II) coordination geometry is

trigonal-bipyramidal for one cation (τ = 0.70)⁶⁹and distorted

square-pyramidal (τ = 0.24) for the other. The crystallographic

data provide solid evidence that the catecholate ligand is

monoanionic. First, there is a signiﬁcant diﬀerence in the

lengths of the O1−C49 and O2−C50 bonds (1.39 and 1.33 Å,

respectively). Second, the ^tBuCatH ligand binds asymmetrically

with Fe1−O1 bond distances that are ∼0.30 Å longer than the

corresponding Fe1−O2 bonds. The protonated donor is weakly

bound with an Fe1−O1 distance of 2.23 0.01 Å, while the

anionic donor exhibits a shorter Fe1−O2 distance of 1.92

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Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for [3]⁺, [4]²⁺, and [5]⁺Measured with X-ray Diﬀraction

a

[3]OTf·2DCE

b

(A)

(B)

[4](OTf)(BPh₄)·DCE·C₅H₁₂

[4(MeCN)](OTf)₂·MeCN·Et₂O

[5](OTf)·1.5CH₂Cl₂

Fe1−N1

Fe1−N3

Fe1−N5

Fe1−O1/N7

Fe1−O2/N8

Fe1−N9

2.118(3)

2.124(3)

2.192(3)

2.226(3)

1.922(3)

2.121(3)

2.155(3)

2.192(3)

2.241(3)

1.938(3)

2.089(4)

2.115(3)

2.181(3)

2.224(3)

2.131(4)

2.174(2)

2.162(2)

2.227(2)

2.237(2)

2.246(2)

2.214(2)

1.444(2)

1.454(2)

86.07(6)

91.74(6)

88.90(6)

167.47(6)

99.93(6)

89.59(6)

93.45(6)

172.48(6)

96.87(6)

75.38(6)

2.124(2)

2.120(2)

2.142(2)

2.229(2)

1.919(2)

O1/N7−C49

O2/N8−C50

N1−Fe1−N3

N1−Fe1−N5

N3−Fe1−N5

N1−Fe1−O1/N7

N3−Fe1−O1/N7

N5−Fe1−O1/N7

N1−Fe1−O2/N8

N3−Fe1−O2/N8

N5−Fe1−O2/N8

O1/N7−Fe1−O2/N8

1.390(5)

1.323(5)

93.5(1)

91.3(1)

85.4(1)

90.5(1)

90.7(1)

175.7(1)

130.7(1)

133.8(1)

103.7(1)

77.9(1)

0.70

1.395(5)

1.327(5)

94.2(1)

91.9(1)

85.9(1)

102.3(1)

89.6(1)

165.4(1)

114.5(1)

150.3(1)

100.0(1)

77.4(1)

0.24

1.445(5)

1.458(5)

95.3(1)

84.5(1)

91.1(1)

94.7(1)

96.9(1)

172.0(1)

145.3(1)

119.2(1)

92.2(1)

77.8(1)

0.45

1.390(3)

1.336(3)

94.03(7)

89.20(7)

85.81(7)

90.75(6)

90.21(6)

176.01(6)

124.35(7)

139.32(7)

105.70(7)

77.58(6)

c

τ-value

0.61

a

b

The [3]OTf·2DCE structure contains two symmetry-independent complexes per unit cell. Parameters are provided for both structures. The

[5]OTf·1.5CH₂Cl₂structure contains two symmetry-independent complexes per unit cell. Since the structures are nearly identical, parameters are

c

only provided for one complex. See reference 69 for deﬁnition of the τ value.

0.01 Å (Table 2). Such bond distances are generally similar to

those observed in the four previously reported Fe(II)

complexes with monoanionic catecholate ligands.^51,70−72The

triﬂate counteranion forms a hydrogen bond with the hydroxyl

group of the ^tBuCatH ligand, consistent with the O1···O3(Tf)

distance of ∼2.67 Å. The average Fe−N bond distance is 2.150

Å, similar to the corresponding distances measured for the

^tBu2APH-based complex ([2]⁺) and indicative of a high-spin,

pentacoordinate Fe(II) complex.³⁷Consistent with this fact, the

¹H NMR spectrum displays paramagnetically shifted peaks

ranging from 65 to −30 ppm (Supporting Information, Figure

S1).

The two X-ray structures of complex [4]²⁺shown in Figure 2

reﬂect the diﬀerent conditions under which the crystals were

generated (vide supra). Crystals grown in a DCE/pentane

mixture contain a 5C dicationic Fe complex associated with one

OTf and one BPh₄counteranion, in addition to DCE and

pentane solvent molecules (Table 1). The coordination

environment of the Fe(II) center is intermediate between

square-planar and trigonal-bipyramidal (τ = 0.45; Table 2). In

contrast, the structure arising from crystals grown in MeCN/

Et₂O features a six-coordinate (6C) Fe(II) center bound to a

solvent-derived MeCN ligand in addition to ^Ph2TIP and ^tBuPDA

(Figure 2). The increase in coordination number lengthens the

average Fe−N_TIPbond distance from 2.13 Å in [4]²⁺to 2.19 Å

in [4(MeCN)]²⁺. The ^tBuPDA ligand binds symmetrically in the

Figure 2. Thermal ellipsoid plots (50% probability) derived from

6C structure (Fe−N_PDAdistance of 2.24(1) Å), while the Fe1−

[4](OTf)(BPh₄)·DCE·C₅H₁₂(top) and [4(MeCN)](OTf)₂·MeCN·

N7/N8 distances diﬀer by 0.093 Å in the 5C structure. The

Et₂O (bottom). Counteranions, noncoordinating solvent molecules,

observed Fe−N bond lengths indicate that the Fe(II) centers

and most hydrogen atoms have been omitted for clarity. The phenyl

rings of the ^Ph2TIP ligands have also been removed to provide a clearer

are high-spin in both structures. This conclusion is supported

by the corresponding ¹H NMR spectrum (Supporting

Information, Figure S1) and the measured magnetic moment

of 5.48 μ_Bin CH₂Cl₂. The neutral charge of the ^tBuPDA ligands

is conﬁrmed by the presence of N−C bond lengths of 1.45(1)

Å, typical of aryl amines (anilide anions, in contrast, exhibit N−

C bond distances of ∼1.39 Å). In both structures, the ^tBuPDA

view of the ﬁrst coordination sphere.

ring tilts out of the plane formed by the N7−Fe1−N8 chelate

by ∼23°, and each triﬂate is hydrogen-bonded to an amino

group of ^tBuPDA.

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As described below, the noninnocent nature of the o-

phenylene ligands plays an important role in the reactions of

the corresponding Fe(II) complexes with O₂. To highlight this

phenomenum, we prepared a “control” ^Ph2TIP-based Fe(II)

complex with a completely innocent ligand (i.e., one incapable

of transferring either protons or electrons). For this purpose we

selected 2-methoxy-5-methylphenolate (^Me2MP; Scheme 2);

this ligand is structurally similar to ^tBuCatH, yet the second O-

donor is methylated instead of protonated. Complex

[Fe²⁺(^Ph2TIP)(^Me2MP)]OTf, [5]OTf, was prepared in a

manner similar to [3]OTf, and light green crystals were

obtained by layering a CH₂Cl₂solution with hexanes. The

resulting structure reveals a 5C, high-spin Fe(II) center bound

to ^Ph2TIP and ^Me2MP in a distorted trigonal-bipyramidal

geometry (Supporting Information, Figure S2). Importantly,

the Fe−O/N bond distances measured for [5]⁺are very similar

to those found for [3]⁺(Table 2); like ^tBuCat, the ^Me2MP ligand

binds in an asymmetric manner, with Fe−O distances of 1.92

and 2.23 Å. Thus, the overall structure of [5]⁺closely resembles

those in the o-phenylene series. However, methylation of the

−OH donor eliminates the possibility of ligand-based electron

transfer (ET) and proton transfer (PT), and this change causes

the O₂reactivities of [3]⁺and [5]⁺to diverge in dramatic

fashion (vide infra).

potential (−10 mV, ΔE = 145 mV), consistent with the

structural similarity between [3]⁺and [5]⁺noted above. In our

previous electrochemical studies of high-spin 5C Fe(II)

complexes with Tp or TIP ligands, the Fe²⁺/Fe³⁺couple

generally appears within 300 mV of the Fc^+/0reference.^37,73,74

Thus, it is reasonable to assign the ﬁrst oxidations of [3]⁺and

[5]²⁺to the Fe²⁺/Fe³⁺couple.

By comparison, the cyclic voltammogram of the phenyl-

enediamine complex [4](OTf)₂is less well-deﬁned, but two

events are clearly evident at +70 and 560 mV in the

corresponding square-wave voltammogram (dashed line in

Figure 3). We attribute the low-potential peak to the Fe²⁺/Fe³⁺

couple, which appears ∼100 mV higher than the corresponding

potentials for [3]⁺and [5]⁺. This anodic shift is likely due to

the neutral charge of ^tBuPDA compared to the monoanionic

^tBuCatH and ^Me2MP ligands. The high-potential redox events

for [3]⁺and [4]²⁺arise from oxidation of the catecholate or

phenylenediamine ligand, respectively. This assignment is

consistent with a previous study by Lever et al., which found

that the PDA ligand is oxidized at +500 mV when bound to a

Ru(II) center.⁷⁵Both redox events are irreversible for [4]²⁺;

indeed, the electrochemical behavior of [4]²⁺resembles that

reported previously for the o-aminophenolate complex 1, which

likewise exhibits an irreversible anodic wave near 0 mV, likely

due to an ET−PT process.³⁷For reasons that are not clear to

us, complex [2]OTf failed to exhibit well-deﬁned electro-

chemical features; however, on the basis of prior results,⁷³the

Fe²⁺/Fe³⁺potential of [2]OTf is likely ∼150 mV more positive

than the corresponding potential of 1.

Voltammetric studies of the Fe(II) complexes were

conducted in CH₂Cl₂at a scan rate of 100 mV/s with 0.1 M

(NBu₄)PF₆as the supporting electrolyte; redox potentials were

referenced to ferrocenium/ferrocene (Fc^+/0). The CV of the

catecholate complex [3]⁺displays an irreversible anodic wave at

3.B. Reaction with Dioxygen. Pale yellow solutions of

[3]OTf in CH₂Cl₂undergo rapid color change upon exposure

to air, yielding the blue-green chromophore 3^ox. The

corresponding electronic absorption spectrum, shown in Figure

4, consists of two broad bands at 700 and 905 nm with ε-values

of ∼1100 M⁻¹cm⁻¹. These spectral features are characteristic

of ferric-catecholate(2−) complexes and arise from ^tBuCat→

Fe(III) ligand-to-metal charge transfer (LMCT) transi-

tions.^51,71,72The EPR spectrum of 3^oxdisplays two S = 5/2

signals with E/D values of 0.14 and 0.25 (Supporting

Information, Figure S3), consistent with the presence of a

high-spin Fe(III) center. In addition, the Mossbauer (MB)

spectrum of 3^oxreveals two quadrupole doublets with isomer

shifts (δ) near 0.5 mm/s, typical of high-spin ferric ions (Table

3 and Supporting Information, Figure S4). The doublets have

diﬀerent splittings (ΔE_Q) of 0.82 and 1.24 mm/s. The

heterogeneity in E/D and ΔE_Qvalues likely arises from

diﬀerent orientations of the ^tBuCat ligand in the oxidized

complex, similar to the situation observed in the solid state for

[3]OTf. Collectively, the spectroscopic data indicate that 3^ox

has the formula of [Fe³⁺(^Ph2TIP)(^tBuCat)]OTf.

+740 mV (Figure 3) and a quasi-reversible couple with E_1/2

=

−30 mV (peak-to-peak separation, ΔE, of 120 mV). Complex

[5]⁺exhibits a quasi-reversible event at nearly the same

In the presence of air, the distinctive absorption bands of 3^ox

exhibit ﬁrst-order decay with a half-life (t_1/2) of 6300 s,

eventually yielding a nearly featureless spectrum. Previous

studies of related complexes indicate that this decomposition

corresponds to oxidation of the catecholate ligand via one (or

more) of the pathways shown in Scheme 3.^49,71,76To

determine reaction products, the ﬁnal mixture was analyzed

after treatment with acid and extraction into organic solvent

(MeCN/Et₂O). The extradiol cleavage products, 4-tert-butyl-2-

pyrone and 5-tert-butyl-2-pyrone, are generated in a 40:60 ratio

Figure 3. Cyclic voltammograms for [3]OTf (top, black), [5]OTf

(middle, red), and [4](OTf)₂(bottom, gray) collected in CH₂Cl₂with

0.1 M (NBu₄)PF₆as the supporting electrolyte and a scan rate of 100

mV/s. The corresponding square-wave voltammogram (dashed line) is

also shown for [4](OTf)₂. In all cases the voltammogram was initiated

by the anodic sweep.

1

with an overall yield of ∼30%, as determined by H NMR.

These compounds were also observed using GC-MS, although

the isomers are indistinguishable by this technique. When the

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reaction was performed with ¹⁸O₂, the ion signal arising from

the extradiol products shifted upward by two mass units,

providing conclusive evidence for incorporation of one O atom

from O₂(Supporting Information, Figure S5). The 3^ox

reactivity conforms to the previously established pattern that

iron(III)-catecholate complexes with facial, tridentate support-

ing ligands yield primarily extradiol products, while those with

tetradentate ligands provide intradiol products.^50,51,77,78

Interestingly, while [3]⁺converts to 3^oxin a matter of

seconds upon exposure to O₂, complex [5]⁺is relatively stable

in the presence of air. As shown in Supporting Information,

1

Figure S6, the H NMR spectrum of [5]⁺in CD₂Cl₂features

paramagnetically shifted peaks at 58 and −10 ppm that arise

from the ^Me2MP ligand. These peaks display only modest

decreases in intensity (relative to an internal standard) after

exposure to O₂for several days, indicating that the geometric

and electronic structures of [5]⁺remain essentially intact in

aerobic solution. UV−vis absorption spectra of [5]⁺in O₂-

saturated CH₂Cl₂were collected over a span of 24 h

(Supporting Information, Figure S7). These data revealed a

gradual increase of absorption intensity in the 500−900 nm

region, which corresponds to formation of the ferric complex,

[Fe³⁺(^Ph2TIP)(^Me2MP)]²⁺(5^ox). The absorption spectrum of

5^oxwas measured independently by treating [5]⁺with 1 equiv

of an acetylferrocenium salt (Supporting Information, Figure

S7). On the basis of these results, the conversion of [5]⁺→5^oxis

only 20% complete after 24 h. The stark contrast in O₂

reactivities between [3]⁺and [5]⁺is remarkable given that

the two complexes possess very similar geometric structures

and Fe^3+/2+redox potentials.

As shown in Figure 4, reaction of the o-aminophenolate

complex [2]OTf with O₂at room temperature generates a

green chromophore (2^ox) with absorption peaks at 790 and 420

nm. This spectrum is essentially identical to the one previously

obtained by treating [2]OTf with 2 equiv of a one-electron

oxidant (e.g., AgOTf).³⁷Thus, it is reasonable to assume that

2^oxcorresponds to [Fe(^Ph2TIP)(L_O,N)]²⁺, where the electronic

structure can be described as either Fe³⁺−^tBu2ISQ or

Fe²⁺−^tBu2IBQ (vide supra). These results indicate that O₂is

capable of extracting two electrons from [2]⁺, whereas initial

exposure of [3]⁺to O₂involves only one-electron oxidation of

the complex. In both reactions, the ET process is associated

with loss of a proton from the bidentate ligand. Similar to 3^ox,

complex 2^oxundergoes decomposition in the presence of O₂,

albeit at a much slower rate (t_1/2≈ 18 h).

Figure 4. Time-resolved absorption spectra for the reaction of [3]OTf

(top), [2]OTf (middle), and [4](OTf)₂(bottom) with O₂; spectra

were collected at intervals of 1, 20, and 14 400 s, respectively. Each

reaction was performed at room-temperature in O₂-saturated CH₂Cl₂

([O₂] = 5.8 mM). The path length of the cuvette was 1.0 cm.

Table 3. Experimental Mossbauer Parameters

̈

isomer shift (δ) mm/ quadrupole splitting (ΔE_Q)

complex

s

mm/s

2.52

reference

1

1.06

32

[2]OTf

1.06 (70%)/1.14

(30%)

2.08/2.93

2^ox

0.64

1.08

1.94

2.05

32

[3]OTf

this

work

3^ox

0.53/0.50

0.82/1.24

3.13/2.53

1.98/3.24

this

work

The o-phenylenediamine complex [4](OTf)₂is compara-

tively less reactive toward O₂, requiring days (instead of

minutes or hours) for complete oxidation. The resulting

[4](OTf)₂1.04 (75%)/1.05

this

work

(25%)

complex, 4^ox, displays an intense absorption band with λ_max

=

4^ox

1.03 (40%)/1.18

this

work

a

715 nm (ε = 4300 M⁻¹cm⁻¹) and a weaker feature at ∼500 nm

(Figure 4). This deep-green species is air-stable at room

temperature, allowing for crystallization from a MeCN/Et₂O

mixture. Unfortunately, extensive disorder within the crystal has

prevented the collection of a high-resolution structure. The

crude crystallographic data indicate that 4^oxcarries a +2 charge,

based on the number of counteranions present. The complex is

5C with the ^tBuPDA-derived ligand bound in a bidentate

manner, although sizable uncertainties in metric parameters

preclude reliable evaluation of Fe or ligand oxidation states.

Solutions of 4^oxare EPR silent with room-temperature

magnetic moments of 5.14 μ_B, indicative of a S = 2 paramagnet.

Since it was not possible to obtain a suitable X-ray crystal

structure of 4^ox, we employed spectroscopic and computational

(35%)

a

The remaining intensity (25%) arises from the starting material,

[4](OTf)₂.

Scheme 3

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techniques to gain insight into its geometric and electronic

structures, as described in the following section.

oxidized by two electrons upon exposure to O₂, although in the

PDA system the electrons are derived exclusively from the

ligand, and two protons are removed.

3.C. Spectroscopic and Computational Studies of 4^ox.

̈

3.C.i. Mossbauer Experiments. Low-temperature (5 K) MB

Table 3 summarizes the MB parameters reported here (and

previously) for complexes 1−[4]²⁺and their X^oxcounterparts.

The electronic structures of the Fe(II) precursors are quite

similar, with isomer shifts of δ = 1.09 0.05 mm/s and ΔE_Q

values between 2.05 and 3.13 mm/s. In contrast, there is

considerable variation in the X^oxparameters. Isomer shifts for

3^oxand 4^oxare characteristic of high-spin ferric and ferrous ions,

respectively, whereas the δ value of 2^ox(0.64 mm/s) precludes

an unambiguous assignment of oxidation state, as noted above.

Signiﬁcantly, the MB results reveal that the redox chemistry of

these complexes spans the entire gamut from iron-based to

ligand-based oxidations.

spectra of [4](OTf)₂collected before and after exposure to O₂

are shown in Figure 5. The parameters obtained from ﬁtting the

3.C.ii. DFT Calculations of 4^ox. Following the experimental

data, the geometry optimization of 4^oxassumed a 5C geometry,

S = 2 ground state, and overall charge of +2. Metric parameters

for the resulting model (4^ox-DFT) are provided in Figure 6.

Figure 5. Mossbauer spectra collected before and after exposure of

̈

[4](OTf)₂to O₂(top and bottom spectra, respectively). Both spectra

were recorded at 5 K in an applied ﬁeld of 0.04 T. The solid red lines

are least-squares ﬁts to the experimental data using the parameters in

Table 3. Both spectra were ﬁtted assuming nested doublets.

Approximately 25% of the area in the spectrum of the O₂-exposed

sample (bottom) was ascribed to [4](OTf)₂starting material.

data are provided in Table 3. The major component (75%) of

the [4](OTf)₂spectrum is a quadrupole doublet with an isomer

shift (δ) of 1.04 mm/s and large splitting (ΔE_Q) of 3.1 mm/s.

A minor feature (25%) is also evident with δ- and ΔE_Q-values

of 1.05 and 2.5 mm/s, respectively. Both signals are

characteristic of nonheme high-spin Fe(II) centers with N/O

coordination. Given the nearly identical isomer shifts, the two

doublets likely correspond to conformational isomers of [4]²⁺

that adopt diﬀerent geometries along the square-pyramidal to

trigonal-bipyramidal continuum; indeed, similar “τ-strain” was

observed in our previous MB studies of [2]OTf.^37,79Upon

exposure to O₂for 20 h, new features arising from 4^oxbecome

clearly evident (Figure 5), although starting material remains.

Adequate ﬁtting of the new signal required two equally intense

doublets with δ = 1.03 and 1.18 mm/s and ΔE_Q= 2.0 and 3.2

mm/s (Table 3). As with [4](OTf)₂, the observed hetero-

geneity is likely due to minor changes in coordination

geometries. Signiﬁcantly, the MB data provide conclusive

proof that the conversion of [4]²⁺to 4^oxby O₂does not involve

oxidation of the Fe center, as the isomer shifts remain above 1.0

mm/s in the ﬁnal complex.

Figure 6. (a) Bond distances (in Å) of the [Fe(DIBQ)]²⁺unit in the

4^ox-DFT model. (b) Isosurface plot of the spin-down (β) HOMO of

4^ox-DFT.

The short N−C bond distances of 1.29 Å and “four long/two

short” pattern of C−C bonds in the N,N-ligand are well-

established characteristics of DIBQ units.^3,30,80−83Mulliken

populations revealed that spin-density is found almost

exclusively on the Fe center (3.88 α spins), while the bidentate

ligand is largely devoid of unpaired spin. The most relevant

molecular orbital (MO) for evaluating the 4^ox-DFT electronic

conﬁguration is the highest-occupied (HO) spin-down (β)

MO, shown in Figure 6. The character of this MO is 74% Fe

and 14% N,N-ligand, with electron density mainly found in a 3d

orbital that bisects the N7−Fe1−N8 angle. Thus, there is only

modest transfer of electron density from the Fe(II) center to

the ^tBuDIBQ ligand, in agreement with the MB data presented

above.

With the MB data in hand, it is now possible to determine

the oxidation state of the ^tBuPDA-derived ligand in 4^ox. Since

the overall complex has a +2 charge, the ligand itself must be

neutral; thus, two possibilities exist: (diimino)-

benzosemiquinone radical or (diimino)benzoquinone

To aid in band assignments, the absorption spectrum of 4^ox-

DFT was calculated using TD-DFT. As shown in Supporting

Information, Figure S8, the computed spectrum exhibits two

bands at 635 and 460 nm (ε = 5.4 and 2.2 M⁻¹cm⁻¹,

respectively) that correspond to features in the experimental

spectrum. The higher-energy band arises primarily from a

^tBuDIBQ-based π−π* transition, as revealed in the electron

(

^tBuDIBQ). The former possibility is inconsistent with the

EPR-silent nature of 4^oxand its magnetic moment of 5.14 μ_B.

Therefore, 4^oxis best formulated as [Fe²⁺(^Ph2TIP)-

(

^tBuDIBQ)]²⁺a conclusion further supported by the DFT

and rR results described below. Similar to [2]⁺, complex [4]²⁺is

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density diﬀerence map (EDDM; Supporting Information,

Figure S8). In contrast, the intense near-IR (NIR) band

corresponds to an Fe(II)→^tBuDIBQ MLCT transition localized

on the N7−Fe1−N8 unit.

3.C.iii. Resonance Raman Experiments. The electronic

structure of the ^tBuPDA-derived ligand in 4^oxwas further

examined using resonance Raman (rR) spectroscopy. Since the

complex exhibits a pair of prominent absorption bands, data

were collected with two wavelengths of excitation (λ_ex): 647.1

nm light from a Kr⁺laser was used to probe the NIR feature

that arises from a MLCT transition, while 488.0 nm light from

an Ar⁺laser was selected to resonate with the ^tBuDIBQ-based

π−π* transition. The composite spectrum, shown in Figure 7,

Prior rR studies of metal-dioxolene complexes indicate that

these peaks correspond to modes that couple ν(N−C) and

ν(C−C) motions within the bidentate N,N-ligand.⁸⁴⁻⁸⁶On the

basis of its large ¹⁵N isotope shift (8 cm⁻¹), the experimental

peak at 1384 cm⁻¹matches the DFT-computed mode at 1372

cm⁻¹, which has primarily ν(N−C) character (calculated ¹⁵N

isotope shift of 7 cm⁻¹; Supporting Information, Figure S9).

The peaks at 1413 and 1453 cm⁻¹then correspond to ν(C−C)

motions of the ^tBuDIBQ ring with only minor amounts of ν(N−

C) character. Similarly, Lever and co-workers recently

published the crystal structure and rR spectrum of [Ru³⁺Cl₂-

(NH₃)₂(DIBQ)]⁺, where DIBQ is unsubstituted (diimino)-

benzoquinone.⁸⁶This complex displays three peaks between

1400 and 1500 cm⁻¹that the authors attribute to stretching

modes of the DIBQ ligand. The presence of resonance-

enhanced peaks at similar frequencies in the 4^oxspectrum

provides further evidence that this complex contains a ^tBuDIBQ

ligand.

3.D. Kinetic Analysis of O₂Reactivity. Kinetic studies

were generally conducted in O₂-saturated CH₂Cl₂solutions

([O₂] = 5.8 mM at 20 °C^41,42), and rates were measured by

monitoring the growth of absorption features associated with

the X^oxspecies. To ensure a large excess of O₂, concentrations

of the Fe(II) complexes never exceeded 1.0 mM. For the

reactions of [2]⁺and [3]⁺with O₂, initial rates increased

linearly with Fe and O₂concentrations, indicating that the

reactions are ﬁrst-order in both reactants (Supporting

Information, Figures S10 and S11). Interestingly, while the

[4]²⁺+ O₂reaction is also ﬁrst-order in Fe concentration, the

reaction rate displays only minor variations as [O₂] increases

from 0.2 to 5.4 mM (Supporting Information, Figure S11).

This zero-order [O₂] dependence indicates that O₂binding is

not the rate-limiting step in the conversion of [4]²⁺→4^ox.

As shown in Figure 8, the reaction of [3]OTf with O₂at

ambient temperature proceeds via pseudo-ﬁrst-order kinetics

with a rate constant (k₁) of 0.67(5) s⁻¹. The formation of 2^ox

and 4^oxunder the same conditions is more complex, however,

as indicated by the “S-shaped” kinetic traces (Figure 8). This

behavior suggests that these species are generated via multistep

mechanisms involving both ET and PTa common

occurrence for reactions that require net hydride (2^ox) or H₂

transfer (4^ox).^87,88Because of this mechanistic complexity, k₁

values for the reactions of O₂with [2]⁺and [4]²⁺were

measured using the initial rates approach. Interestingly, the

rates of formation span more than 5 orders of magnitude, with

k₁values of 0.67(5) (3^ox), 1.3(2) × 10⁻³(2^ox), and 4(2) × 10⁻⁶

s⁻¹(4^ox) in O₂-saturated CH₂Cl₂at room temperature. We also

measured an initial rate of 5 × 10⁻⁶s⁻¹for the one-electron

oxidation of [5]⁺to 5^ox(Supporting Information, Figure S7).

Thus, despite similar structures, the complexes examined here

diﬀer dramatically in their O₂reactivities.

Figure 7. Resonance Raman spectra of 4^oxin frozen CD₂Cl₂solutions

([Fe] = 7.8 mM) collected with 647.1 nm (left) and 488.0 nm (right)

laser excitation. The black (solid) spectra were obtained using natural

abundance (NA) complex, while the gray (dashed) spectra were

obtained using ¹⁵N-substituted complex (the ¹⁵N isotope was

incorporated at the 2-position of the PDA ligand). Frequencies (in

cm⁻¹) are provided for select peaks in the NA spectra, and the

corresponding ¹⁴N→¹⁵N shifts are shown in parentheses. Peaks

marked with an asterisk (*) arise from frozen solvent.

was obtained by irradiating frozen samples of 4^oxin CD₂Cl₂. In

some samples, the bidentate N,N-ligand was labeled with the

¹⁵N isotope at the 2-position to aid in peak assignments.

The low-frequency region of the 4^oxspectrum features two

intense peaks at 557 and 601 cm⁻¹with ¹⁵N isotope shifts of 2

and 8 cm⁻¹, respectively (Figure 7). The 601 cm⁻¹peak is

attributed to the breathing mode of the ﬁve-membered FeN₂C₂

chelate ring, based on its intensity and sizable isotope shift. This

assignment is supported by literature precedents⁸⁴⁻⁸⁶and DFT

frequency calculations performed with the 4^ox-DFT model

(computed frequencies and normal mode compositions are

provided in Supporting Information, Figure S9). The smaller

isotope shift of the 557 cm⁻¹peak suggests that the

corresponding normal mode involves substantial mixing of

Fe−N stretching motions with internal C−N/C−C vibrations

of the bidentate ligand. Both modes are strongly enhanced by

excitation into the NIR band at 715 nm, consistent with its

assignment as an Fe(II)→^tBuDIBQ MLCT transition.

Compared to the metallocycle-based features, peaks arising

from ligand-based modes (ν ≅ 1200−1600 cm⁻¹) are quite

weak in the 4^oxspectrum obtained with λ_ex= 647.1 nm.

However, the higher-energy peaks gain in relative intensity

when λ_exis changed to 488.0 nm, providing further

conﬁrmation that the absorption manifold near 500 nm arises

from ^tBuDIBQ-based transitions. Three isotopically sensitive

peaks are apparent at 1384, 1413, and 1453 cm⁻¹(Figure 7).

Activation parameters for the [2]OTf + O₂reaction were

determined by measuring rates at temperatures between 22 and

−30 °C. The linear Eyring plot (Supporting Information,

Figure S12) indicates an activation enthalpy (ΔH^‡) of 12(2)

kcal·mol⁻¹and a large negative activation entropy (ΔS^‡) of

−22(5) cal·mol⁻¹·K⁻¹. Such values are similar to parameters

obtained for similar Fe/O₂adducts^46,89and are consistent with

an associative reaction involving O₂binding to the Fe center as

the rate-determining step.

3.E. Reaction with H-atom Acceptors. In previous

sections, we demonstrated that formation of the oxidized

species 2^ox−4^oxunder aerobic conditions requires both electron

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spectral features that are similar to those of 4^ox, although not

identical (Supporting Information, Figure S14). This chromo-

phore is evident by UV−vis spectroscopy even when a single

equivalent of TTBP^•is added; this result suggests that the

species generated by removal of one H-atom from [4]²⁺

undergoes disproportionation to yield the starting complex

and a 4^ox-like species.

It is noteworthy that [3]⁺is the only complex in the series

capable of donating a H-atom to TEMPO^•, indicating that the

[3^ox−H] bond is very weak (BDFE < 66 kcal/mol). Bordwell

and Mayer have demonstrated that the BDFE the X−H bond

formed in a 1H⁺/1e⁻PCET reaction (i.e., X^•+ H⁺+ e⁻→ X−

H) is given by the following equation:

BDFE(X − H) = 1.37pK_a+ 23.06E^o+ C_G,solv

where C_G,solvis a solvent-dependent constant.^90,91In our case,

the relevant parameters are the E^ovalues of the starting Fe(II)

complexes and the pK_avalues of the one-electron oxidized

species. The complexes examined here exhibit greater variability

in ligand acidities than in redox potentials. As described above,

initial redox potentials diﬀer by ∼100 mV across the series,

accounting for a modest shift of ∼2.5 kcal/mol in BDFE. In

contrast, the pK_avalues of aromatic amines are generally ∼12

units higher than the corresponding phenolsa shift of 14

kcal/mol in BDFE.^90,92,93Thus, while the diﬀerences in redox

potential cannot be ignored, the weaker H-atom donating

ability of complexes [2]⁺and [4]²⁺compared to [3]⁺is largely

due to the greater acidity of the ^tBuCatH ligand relative to

^tBu2APH and ^tBuPDA.

These results have mechanistic implications for the O₂

reactivity of the complexes examined here. The strength of

the [3^ox−H] bond is comparable to that of perhydroxyl radical

•

•−

(HO₂), which is the product of HAT and O₂(BDFE of HO₂

≅ 60 kcal/mol).⁹⁰Therefore, complex [3]⁺may be able to react

directly with O₂via 1H⁺/1e⁻PCET without prior formation of a

ferric-superoxo intermediate. By contrast, complexes [2]⁺and

[4]²⁺cannot participate in HAT reactions with O₂due to the

greater strength of their N−H bonds. For these complexes, O₂

activation likely requires initial ET from Fe(II)→O₂, followed

by oxidation of the ligand via concerted (or stepwise) electron

and proton transfers. These mechanistic scenarios are

considered further in the Discussion section.

3.F. Computational Studies of O₂Reactivity. The

thermodynamics of O₂binding were examined with DFT

calculations. A previous study by Schenk et al. found that

hybrid functionals with a reduced amount (∼10%) of Hartree−

Fock (HF) exchange are most reliable for evaluating the

energetics of O₂(or NO) binding to nonheme Fe(II) centers.⁹⁴

We therefore employed the PBE functional⁶⁵with 10% HF

exchange to calculate geometries and thermodynamic param-

eters for O₂, the Fe(II) precursors, and the 6C [Fe/O₂]

adducts. Since exchange interactions between the unpaired

electrons of Fe and O₂give rise to three possible spin states

(S_tot= 1, 2, 3), it was necessary to optimize three models for

each [Fe/O₂] species. The resulting S = 2 [Fe/O₂] structures

are shown in Figure 9. (Supporting Information, Tables S1−S3

provide metric parameters for each model.) The [4/O₂]²⁺

adduct is dissociative on the S = 3 surface, with calculations

invariably converging to structures with very long Fe···O

distances (>3.75 Å). In all other cases, O₂coordinates in a bent

conformation with Fe−O−O angles between 120 and 140° and

Fe−O distances ranging from 1.95 to 2.18 Å. The O₂ligand can

adopt two possible orientations depending on whether the O−

Figure 8. Plots of absorption intensity as a function of time for the

reactions of [3]OTf (top), [2]OTf (middle), and [4](OTf)₂(bottom)

with O₂. All reactions were performed in O₂-saturated CH₂Cl₂at room

temperature ([Fe] ≈ 0.50 mM).

and proton transfers from the parent complexes, although the

rates of O₂reaction vary by a factor of 10⁵across the series.

Thus, it is plausible that the O₂activation mechanisms involve a

combination of electron and proton transfer from the

complexes to either dioxygen or superoxide. We therefore

examined the reactivity of the title complexes with two well-

established H-atom acceptors: TEMPO^•and 2,4,6-tri-tert-

butylphenoxyl radical (TTBP^•). Both reagents exhibit a strong

propensity to react via PCET mechanisms, but TTBP^•is a

much more eﬀective H-atom abstractor than TEMPO^•, as

indicated by the bond dissociation free energies (BDFE) of the

resulting H−O bonds (BDFE = 77.1 and 66.5 kcal/mol,

respectively, in MeCN).⁹⁰

Treatment of [3]⁺with either TEMPO^•or TTBP^•yields a

blue-green species with absorption features identical to those

observed for 3^ox(Supporting Information, Figure S13). Given

the formulation of 3^oxas [Fe³⁺(Cat)(^Ph2TIP)]⁺, this reaction is

classiﬁed as “separated PCET” because the electron and proton

originate from diﬀerent units of the [3]⁺complex, namely, the

Fe(II) center and CatH ligand. While complexes [2]⁺and [4]²⁺

are inert toward TEMPO^•, both react rapidly with TTBP^•. As

described in a previous manuscript,³⁷TTBP^•removes a

hydrogen atom from the ^tBu2APH ligand of [2]⁺to generate

the corresponding Fe(II)−^tBu2ISQ complex. Complex [4]²⁺

reacts with 2 equiv of TTBP^•to provide a species with

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On the basis of the DFT calculations, complex [2]⁺has the

greatest aﬃnity for O₂, followed in the series by [3]⁺and [4]²⁺

(Table 4). This trend correlates with the relative donor

strengths of the bidentate ligands (APH⁻> CatH⁻> PDA)

because formation of the Fe−O₂bond requires transfer of

electron density from an Fe d orbital to an empty O₂π* orbital.

However, our DFT results appear to contradict the kinetic

studies reported above, which found that [3]⁺is signiﬁcantly

more reactive than [2]⁺toward O₂. Possible explanations for

this discrepancy are provided in the following section.

4. DISCUSSION

In this Manuscript, we described the O₂reactivity of

monoiron(II) complexes bound to three types of o-phenylene

ligands (Schemes 1 and 2). The complexes resemble the

substrate-bound intermediates of nonheme Fe(II) dioxygenases

that catalyze the oxidative ring-cleavage of aromatic sub-

strates.³²⁻³⁴The ^Ph2TIP supporting ligand mimics the facial

triad of protein ligands in the active site and the substrate

ligands each coordinate in a bidentate manner, resulting in 5C

Fe(II) complexes capable of O₂binding. In an earlier study, we

demonstrated that one- and two-electron oxidation of the

^tBu2APH-based complex [2]OTf yields species containing

(imino)benzosemiquinone ligands.³⁷Like aminophenolates,

catecholates and phenylenediamines can serve as redox-active

ligands, although the ease of oxidation of the free ligands

increases across the series CatH₂< APH₂< PDA. We therefore

synthesized and structurally characterized the homologous

complexes [3]OTf and [4](OTf)₂to better understand the role

of redox-active ligands in modulating the O₂reactivity of Fe

complexes. The ligands are capable of donating protons as well

as electrons, but the acidities run counter to the redox

potentials. In other words, the most acidic ligand (^tBuCatH₂) is

the hardest to oxidize, while the most reducing ligand (^tBuPDA)

is the least acidic. This interplay between ET and PT

capabilities inﬂuences the rates of the O₂reactions as well as

the identities of the oxidized products.

Despite similar overall structures, the three Fe(II) complexes

in this study display remarkable diversity in their O₂reactivities,

as summarized in Scheme 4. The diﬀerences concern both the

total number of electrons transferred in the reaction (1e⁻or

2e⁻) and the source of these electrons (iron and/or ligand).

The resulting X^oxspecies have been characterized by various

spectroscopic (UV−vis, EPR, MB, rR) and computational

(DFT) methods. These results indicate that the [3]⁺→3^ox

conversion is an Fe-based 1e⁻process, while the [4]²⁺→4^ox

reaction involves 2e⁻oxidation of the ligand only. The [2]⁺→

2^oxreaction occupies an intermediate position, since substantial

Figure 9. DFT-calculated structures of the Fe/O₂adducts. Selected

bond distances (Å) and angles are provided (see Supporting

Information, Tables S1−S3 for additional metric parameters).

O vector is pointed toward (T) or away (A) from the bidentate

ligand. The two orientations are approximately isoenergetic

with diﬀerences less than the estimated error of the calculations

( 2 kcal/mol). In the remainder of this Paper, only the T

isomers of the [Fe/O₂] adducts are discussed, since these

models are more relevant from a mechanistic standpoint.

Table 4 summarizes the computed thermodynamic param-

eters for the eight Fe(II) + O₂→ [Fe/O₂] reactions considered

here. While the enthalpic contributions (ΔH^gas+ ΔSolv) are

slightly favorable in most instances, all of the reactions are

endergonic (ΔG = +7.0−13.0 kcal/mol) due to large and

unfavorable entropic eﬀects. A similar pattern has been

observed in DFT studies of O₂binding to nonheme Fe(II)

enzymes, where the calculated ΔG values range between +8

and 12 kcal/mol.⁹⁵⁻⁹⁸To understand why the O₂binding

reactions are decidedly “uphill,” it is instructive to examine the

computed properties of the O−O bonds in the [Fe/O₂]

intermediates. Superoxide ligands typically exhibit O−O bond

distances near 1.3 Å and ν(O−O) frequencies between 1050

and 1200 cm⁻¹.⁹⁹In contrast, our DFT-generated [Fe/O₂]

models have short O−O distances of 1.25 0.02 Å and ν(O−

O) frequencies greater than 1250 cm⁻¹(Table 4), indicating

that there is only partial charge transfer from Fe(II) to the O₂

ligand. The weakness of the Fe−O₂interactions is also reﬂected

in the low ν(Fe−O) frequencies, which range between 230 to

370 cm⁻¹.

Table 4. Energetics of O₂Binding to Complexes [2]⁺, [3]⁺, and [4]²⁺, and Comparison of O−O Bond Distances and Stretching

a

Frequencies in the Resulting Fe/O₂Adducts

b

c

reactants

spin (S_tot

)

ΔH^gas

TΔS

ΔSolv

ΔG

r(O−O) (Å)

ν(O−O) (cm⁻¹

)

[3]⁺+ O₂

S = 3

S = 2

S = 1

S = 3

S = 2

S = 1

S = 2

S = 1

−0.5

−0.2

−1.6

−2.0

−1.4

−2.9

+4.0

−1.8

−11.9

−12.2

−11.7

−10.0

−10.9

−11.4

−11.3

−12.2

−0.1

−1.1

+0.2

−1.1

−1.7

−0.2

−2.3

+0.4

+11.3

+10.9

+10.3

+6.9

1.25

1.27

1.25

1.26

1.27

1.25

1.23

1287

1260

1287

1254

1251

1303

1328

1406

[2]⁺+ O₂

[4]²⁺+ O₂

+7.8

+8.3

+13.0

+10.8

a

b

c

All energies in kcal/mol. Enthalpies of solvation were calculated using COSMO. ΔG = ΔH^gas+ ΔSolv − TΔS

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[Fe³⁺(L)(^tBu2ISQ)]⁺in the presence of O₂(where L is the

tris(2-pyridylthio)methanido anion).⁴⁸

Scheme 4

The enormous contrast between the O₂reactivities of [3]⁺

and [5]⁺provides the clearest evidence for the decisive role of

PT in determining reaction rates of the o-phenylene complexes.

These two complexes have nearly identical coordination

geometries and Fe^3+/2+redox potentials; however, replacing

the −OH group of ^tBuCatH with the −OCH₃donor of ^Me2MP

decreases the O₂reaction rate by 5 orders of magnitude.

Indeed, complex [5]⁺is quite stable in the presence of air, even

though slow oxidation to the ferric analog (5^ox) is observed

over the course of days. These results provide further

conﬁrmation that the one-electron oxidation of high-spin

Fe(II) centers by O₂is an unfavorable process, but the overall

reaction barrier can be lowered substantially if the ET is

coupled with PT (in either a stepwise or concerted manner).

The 3^oxintermediate reacts further with O₂to yield products

arising from extradiol ring cleavage. Previous studies have

proposed a mechanism that involves direct reaction of the

[Fe³⁺(Cat)]⁺unit with O₂to form a ferric-alkylperoxo species,

followed by rearrangement to the corresponding lactone

(Scheme 5a).^49,76In contrast, 2^oxand 4^oxare relatively air-

electron density is lost from both the Fe center and ^tBu2AP

ligand. This continuum in electronic structures is evident in the

isomer shifts (δ) of the X^oxspecies (Table 3), which range from

0.50 (3^ox, Fe³⁺) to 0.64 (2^ox, Fe^2.5+) to ∼1.1 (4^ox, Fe²⁺) mm/s.

In addition, each of the three possible o-phenylene oxidation

states (aromatic, semiquinone, benzoquinone; Scheme 1) is

represented in the X^oxseries. Thus, the [Fe²⁺(^Ph2TIP)(o-

phenylene)] framework supports a wide spectrum of redox and

O₂chemistry.

Scheme 5

Our kinetic analysis revealed that O₂reaction rates vary by a

factor of 10⁵across the series, following the order [3]⁺> [2]⁺>

[4]²⁺. It is somewhat counterintuitive that this order is inversely

related to the electron-donating abilities of the free ligands.

Moreover, these rates fail to correlate adequately with Fe^2+/3+

redox potentials, all of which fall within a 100 mV range near 0

mV (vs Fc^+/0). Finally, if one assumes that formation of the Fe/

O₂adduct is the rate-determining step, then our DFT

calculations of O₂-binding aﬃnities indicate that [2]⁺should

be more reactive than [3]⁺, even though the kinetic data

indicate that the reverse is true.

We believe these conﬂicting results can be reconciled by

considering the role of PT in the O₂reaction. The fact that [3]⁺

undergoes HAT with TEMPOa very weak H-atom accept-

orsuggests that ET and PT processes are tightly coupled in

the [3]⁺+ O₂reaction. A likely mechanism involves concerted

transfer of 1e⁻and 1H⁺to O₂as it approaches the Fe center. In

contrast, the intrinsically lower acidity of aromatic amines

(relative to phenols) prevents PT in the initial interaction of O₂

with [2]⁺and [4]²⁺. Deprotonation of these ligands in the

course of the O₂reaction likely requires complete oxidation of

the Fe center to lower the pK_aof the amino group(s). Thus,

these complexes cannot avoid the thermodynamically uphill ET

from Fe(II) to O₂; yet once the ferric complex is generated,

proton loss to superoxide or bulk solvent would be feasible, as

indicated by the CV data (vide supra). Importantly,

deprotonation destabilizes the redox-active MOs of the ligand,

making it possible for O₂to extract a second electron, thus

generating the ﬁnal 2^oxand 4^oxproducts. Thus, we propose that

the [2]⁺→2^oxconversion proceeds via a stepwise ET−PT−ET

mechanism, although PT may be coupled with the second ET

in a HAT reaction. This mechanism follows the one established

by Paine et al. for the oxidation of [Fe²⁺(L)(^tBu2APH)] to

stable, since they lack the two reducing equivalents necessary to

generate the critical alkylperoxo intermediate. Interestingly,

Paine and co-workers recently reported a 6C complex, [Fe²⁺(6-

Me₃-TPA)(^tBuAPH)]⁺, that (unlike [2]⁺) reacts with O₂to

yield the ring-cleaved product (6-Me₃-TPA = tris(6-methyl-2-

pyridylmethyl)amine).¹⁰⁰The proposed mechanism is analo-

gous to the one employed by ferric-catecholate complexes.

Paine’s system diﬀers from the one described here in that the

initial product of the O₂reaction is an [Fe³⁺(AP)]⁺species,

whereas our previous studies indicate that [2]⁺likely converts

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to a [Fe²⁺(ISQ)] intermediate. This diﬀerence in electronic

structure apparently controls subsequent reactivity, with the

latter species undergoing simple 1e⁻oxidation and the former

direct addition of O₂.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The results presented here highlight the mechanistic

sophistication of the ring-cleaving dioxygenases. Scheme 5b

provides a condensed version of the canonical enzymatic

mechanism. A recent DFT study by Christian et al. of extradiol

catechol dioxygenases has emphasized the role of the conserved

second-sphere histidine residue in the PT steps that occur after

O₂binding.⁹⁸This residue ﬁrst deprotonates the substrate

ligand, resulting in an imidazolium group that stabilizes the

superoxide ligand through H-bonding interactions. The proton

is eventually returned to the O₂unit after formation of the

bridging alkylperoxo intermediate (Scheme 5b). Thus, the

enzyme carefully “manages” the PT events to promote O₂

activation and discourage the autoxidation processes observed

in our models. Indeed, studies of homoprotocatechuate 2,3-

dioxygenase (HPCD) have demonstrated that if the His200

residue is mutated to Ala, the enzyme generates quinone and

H₂O₂instead of the ring-cleaved products.¹⁰¹Therefore, the

critical diﬀerence between the ring-cleaving dioxygenases and

the synthetic models reported here (and elsewhere) is the

ability to coordinate PT with O₂activation.

We thank Dr. Brian Bennett for allowing us to measure EPR

data at the National Biomedical EPR Center (supported by

NIH P41 Grant EB001980), and Thomas Brunold (Univ.

Wisconsin−Madison) for access to his resonance Raman

instrument. This research is generously supported by the

National Science Foundation (CAREER CHE-1056845 to

A.T.F, CHE-0956779 to C.V.P).

REFERENCES

■

(1) Kaim, W. Inorg. Chem. 2011, 50, 9752−9765.

(2) Kaim, W. Eur. J. Inorg. Chem. 2012, 2012, 343−348.

(3) Agarwala, H.; Ehret, F.; Chowdhury, A. D.; Maji, S.; Mobin, S.

M.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2013, 42, 3721−3734.

(4) Das, D.; Sarkar, B.; Kumbhakar, D.; Mondal, T. K.; Mobin, S. M.;

Fiedler, J.; Urbanos, F. A.; Jimenez-Aparicio, R.; Kaim, W.; Lahiri, G.

K. Chem.Eur. J. 2011, 17, 11030−11040.

(5) Das, D.; Das, A. K.; Sarkar, B.; Mondal, T. K.; Mobin, S. M.;

Fiedler, J.; Zalis, S.; Urbanos, F. A.; Jimenez-Aparicio, R.; Kaim, W.;

Lahiri, G. K. Inorg. Chem. 2009, 48, 11853−11864.

(6) Das, A. K.; Sarkar, B.; Duboc, C.; Strobel, S.; Fiedler, J.; Zalis, S.;

Lahiri, G. K.; Kaim, W. Angew. Chem., Int. Ed. 2009, 48, 4242−4245.

(7) Pierpont, C. G. Inorg. Chem. 2011, 50, 9766−9772.

(8) Pierpont, C. G. Coord. Chem. Rev. 2001, 219, 415−433.

(9) Brown, S. N. Inorg. Chem. 2012, 51, 1251−1260.

(10) Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A. Coord.

Chem. Rev. 2009, 253, 291−324.

It is noteworthy that none of the synthetic dioxygenase

models prepared to date follow the enzymatic mechanism in

proceeding through an Fe/O₂adduct. Even for those

complexes that carry out ligand cleavage, like [3]OTf, the

ﬁrst step always involves 1e⁻oxidation to the ferric complex

followed by direct reaction of the ligand with O₂. The enzyme

not only stabilizes the [FeO₂] adduct through H-bonding

interactions, it also prevents formation of the dead-end

intermediate that arises when the substrate proton is transferred

(11) Munha, R. F.; Zarkesh, R. A.; Heyduk, A. F. Dalton Trans. 2013,

42, 3751−3766.

(12) Khusniyarov, M. M.; Weyhermueller, T.; Bill, E.; Wieghardt, K.

J. Am. Chem. Soc. 2009, 131, 1208−1221.

(13) Khusniyarov, M. M.; Bill, E.; Weyhermueller, T.; Bothe, E.;

Harms, K.; Sundermeyer, J.; Wieghardt, K. Chem.Eur. J. 2008, 14,

7608−7622.

to O₂instead of H200.^101,102In this study, we have shown

•−

that PCET is an eﬀective strategy for bypassing the unfavorable

ET from Fe(II) to O₂; however, in the case of [3]OTf, the

PCET reaction does not lead to formation of the Fe(II)-

alkylperoxo intermediate (as in the enzyme) because the

resulting superoxide moiety has been deactivated through

protonation. By coupling O₂binding with PT to a second-

sphere His residue, the dioxygenases reap the energetic beneﬁts

of PCET while avoiding the pitfall that has plagued synthetic

models. Future modeling eﬀorts should therefore be directed

toward the generation of complexes capable of mimicking the

enzyme’s exquisite control of PT reactions.

(14) Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.;

Kokatam, S.; Ray, K.; Weyhermueller, T.; Neese, F.; Wieghardt, K.

Chem.Eur. J. 2005, 11, 204−224.

(15) Herebian, D.; Bothe, E.; Neese, F.; Weyhermueller, T.;

Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 9116−9128.

(16) Chlopek, K.; Bill, E.; Ueller, T. W.; Wieghardt, K. Inorg. Chem.

2005, 44, 7087−7098.

(17) Gorelsky, S. I.; Lever, A. B. P.; Ebadi, M. Coord. Chem. Rev.

2002, 230, 97−105.

(18) Rajput, A.; Sharma, A. K.; Barman, S. K.; Koley, D.; Steinert, M.;

Mukherjee, R. Inorg. Chem. 2014, 53, 36−48.

(19) Chirik, P. J. Inorg. Chem. 2011, 50, 9737−9740.

(20) Verma, P.; Weir, J.; Mirica, L.; Stack, T. D. P. Inorg. Chem. 2011,

50, 9816−9825.

ASSOCIATED CONTENT

■

S

* Supporting Information

(21) Ray, K.; Petrenko, T.; Wieghardt, K.; Neese, F. Dalton Trans.

2007, 1552−1566.

¹H NMR spectra of the Fe(II) complexes, crystallographic

structure of [5]OTf, EPR and Mossbauer data for [3]OTf and

̈

(22) Chlopek, K.; Muresan, N.; Neese, F.; Wieghardt, K. Chem.

3^ox, results from TD-DFT and frequency calculations of 4^ox,

plots of initial rates versus Fe(II) and O₂concentrations, Eyring

plot for the reaction of [2]OTf with O₂, absorption spectra of

the reactions of [3]OTf and [4](OTf)₂with organic radicals,

metric parameters for DFT-optimized models, and crystallo-

graphic data in CIF format. This material is available free of

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

Eur. J. 2007, 13, 8390−8403.

(23) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermuller,

T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213−2223.

(24) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.;

Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.;

Meyer, T. J. Chem. Rev. 2012, 112, 4016−4093.

(25) Huynh, M. H. V.; Meyer, T. J. Chem. Rev. 2007, 107, 5004−

5064.

(26) DuBois, D. L.; Bullock, R. M. Eur. J. Inorg. Chem. 2011, 1017−

AUTHOR INFORMATION

Corresponding Authors

*E-mail: adam.ﬁ edler@marquette.edu. (A.T.F)

*E-mail: cpopescu@ursinus.edu. (C.V.P) Fax: (+1) 414-288-

7066.

■

1027.

(27) Roubelakis, M. M.; Bediako, D. K.; Dogutan, D. K.; Nocera, D.

G. Energy Environ. Sci. 2012, 5, 7737−7740.

(28) Hoffman, B. M.; Lukoyanov, D.; Dean, D. R.; Seefeldt, L. C. Acc.

Chem. Res. 2013, 46, 587−595.

4059

dx.doi.org/10.1021/ic403126p | Inorg. Chem. 2014, 53, 4047−4061

Inorganic Chemistry

Article

(29) Hoffman, B. M.; Dean, D. R.; Seefeldt, L. C. Acc. Chem. Res.

2009, 42, 609−619.

(64) Laaksonen, L. J. Mol. Graphics 1992, 10, 33−&.

(65) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,

3865−3868.

(30) Matsumoto, T.; Chang, H.-C.; Wakizaka, M.; Ueno, S.;

Kobayashi, A.; Nakayama, A.; Taketsugu, T.; Kato, M. J. Am. Chem.

Soc. 2013, 135, 8646−8654.

(66) Klamt, A.; Schueuermann, G. J. Chem. Soc., Perkin Trans. 2 1993,

799−805.

(31) Lu, F.; Zarkesh, R. A.; Heyduk, A. F. Eur. J. Inorg. Chem. 2012,

2012, 467−470.

(67) Despite repeated eﬀorts, it was not possible to generate an

Fe(II) complex containing the monoanionic form of ^tBuPDA, reﬂecting

the weaker acidity of aryl amines relative to phenols and catechols.

(68) The yellow color of [3]OTf is due to an absorption band near

400 nm that arises from a catecholate-to-iron(II) charge transfer (CT)

transition. In the case of [4](OTf)₂, the ligand-based orbitals of neutral

^tBuPDA are stabilized relative to those of anionic ^tBuCatH. Thus, the

corresponding ^tBuPDA-to-Fe(II) CT transition is shifted to higher

energy and exists entirely in the UV region, resulting in a colorless

complex.

(32) Vaillancourt, F. H.; Bolin, J. T.; Eltis, L. D. Crit. Rev. Biochem.

Mol. Biol. 2006, 41, 241−267.

(33) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L. Chem. Rev.

2004, 104, 939−986.

(34) Lipscomb, J. D. Curr. Opin. Struct. Biol. 2008, 18, 644−649.

(35) Emerson, J. P.; Kovaleva, E. G.; Farquhar, E. R.; Lipscomb, J. D.;

Que, L. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 7347−7352.

(36) Bittner, M. M.; Lindeman, S. V.; Fiedler, A. T. J. Am. Chem. Soc.

2012, 134, 5460−5463.

(69) Addison, A. W.; Rao, T. N.; Reedijk, J.; Vanrijn, J.; Verschoor,

G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.

(70) Reynolds, M. F.; Costas, M.; Ito, M.; Jo, D.-H.; Tipton, A. A.;

Whiting, A. K.; Que, L., Jr. J. Biol. Inorg. Chem. 2003, 8, 263−272.

(71) Paria, S.; Halder, P.; Chakraborty, B.; Paine, T. K. Indian J.

Chem., Sect. A 2011, 50A, 420−426.

(37) Bittner, M. M.; Kraus, D.; Lindeman, S. V.; Popescu, C. V.;

Fiedler, A. T. Chem.Eur. J. 2013, 19, 9686−9698.

(38) Bittner, M. M.; Baus, J. S.; Lindeman, S. V.; Fiedler, A. T. Eur. J.

Inorg. Chem. 2012, 1848−1856.

(39) Khomenko, T. M.; Salomatina, O. V.; Kurbakova, S. Y.; Il’ina, I.

V.; Volcho, K. P.; Komarova, N. I.; Korchagina, D. V.; Salakhutdinov,

N. F.; Tolstikov, A. G. Russ. J. Org. Chem. 2006, 42, 1653−1661.

(40) Manner, V. W.; Markle, T. F.; Freudenthal, J. H.; Roth, J. P.;

Mayer, J. M. Chem. Commun. 2008, 256−258.

(41) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42−55.

(42) Michaud-Soret, I.; Andersson, K. K.; Que, L., Jr.; Haavik, J.

Biochemistry 1995, 34, 5504−10.

(72) Chiou, Y.-M.; Que, L., Jr. Inorg. Chem. 1995, 34, 3577−3578.

(73) Park, H.; Baus, J. S.; Lindeman, S. V.; Fiedler, A. T. Inorg. Chem.

2011, 50, 11978−11989.

(74) Baum, A. E.; Park, H.; Wang, D. N.; Lindeman, S. V.; Fiedler, A.

T. Dalton Trans. 2012, 41, 12244−12253.

(75) Masui, H.; Lever, A. B. P.; Auburn, P. R. Inorg. Chem. 1991, 30,

2402−10.

(43) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.

(44) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.;

Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339−341.

(45) Frantz, D. K.; Linden, A.; Baldridge, K. K.; Siegel, J. S. J. Am.

Chem. Soc. 2012, 134, 1528−1535.

(76) Comba, P.; Wadepohl, H.; Wunderlich, S. Eur. J. Inorg. Chem.

2011, 2011, 5242−5249.

(77) Ogihara, T.; Hikichi, S.; Akita, M.; Moro-oka, Y. Inorg. Chem.

1998, 37, 2614−2615.

(78) Jo, D.-H.; Que, L., Jr. Angew. Chem., Int. Ed. 2000, 39, 4284−

4287.

(46) Kryatov, S. V.; Rybak-Akimova, E. V.; Schindler, S. Chem. Rev.

2005, 105, 2175−2226.

(79) Since MeCN was not used in the preparation of the MB

samples, the minor species is not the six-coordinate structure obtained

by XRD.

(80) Peng, S. M.; Chen, C. T.; Liaw, D. S.; Chen, C. I.; Wang, Y.

Inorg. Chim. Acta 1985, 101, L31−L33.

(81) Bugarcic, T.; Habtemariam, A.; Deeth, R. J.; Fabbiani, F. P. A.;

Parsons, S.; Sadler, P. J. Inorg. Chem. 2009, 48, 9444−9453.

(82) Juestel, T.; Bendix, J.; Metzler-Nolte, N.; Weyhermueller, T.;

Nuber, B.; Wieghardt, K. Inorg. Chem. 1998, 37, 35−43.

(83) Venegas-Yazigi, D.; Mirza, H.; Lever, A. B. P.; Lough, A. J.;

Costamagna, J.; Latorre, R. Acta Crystallogr., Sect. C: Cryst. Struct.

Commun. 2000, C56, e281−e282.

(84) Holt, B. T. O.; Vance, M. A.; Mirica, L. M.; Heppner, D. E.;

Stack, T. D. P.; Solomon, E. I. J. Am. Chem. Soc. 2009, 131, 6421−

6438.

(85) Hartl, F.; Stufkens, D. J.; Vlcek, A. Inorg. Chem. 1992, 31, 1687−

1695.

(86) Kapovsky, M.; Dares, C.; Dodsworth, E. S.; Begum, R. A.; Raco,

V.; Lever, A. B. P. Inorg. Chem. 2013, 52, 169−181.

(87) Fukuzumi, S.; Kotani, H.; Prokop, K. A.; Goldberg, D. P. J. Am.

Chem. Soc. 2011, 133, 1859−1869.

(88) Fukuzumi, S.; Kotani, H.; Lee, Y.-M.; Nam, W. J. Am. Chem. Soc.

2008, 130, 15134−15142.

(89) Because of its fast nature, it was not possible to measure

accurate activation parameters for the reaction of [3]⁺with O₂using

conventional methods.

(47) Battino, R.; Cleve, H. L. Chem. Rev. 1966, 66, 395−463.

(48) Halder, P.; Paria, S.; Paine, T. K. Chem.Eur. J. 2012, 18,

11778−11787 , S11778/1-S11778/10.

(49) Bruijnincx, P. C. A.; Lutz, M.; Spek, A. L.; Hagen, W. R.;

Weckhuysen, B. M.; van Koten, G.; Gebbink, R. J. M. K. J. Am. Chem.

Soc. 2007, 129, 2275−2286.

(50) Lin, G.; Reid, G.; Bugg, T. D. H. J. Am. Chem. Soc. 2001, 123,

5030−5039.

(51) Jo, D.-H.; Chiou, Y.-M.; Que, L., Jr. Inorg. Chem. 2001, 40,

3181−3190.

(52) Shaikh, N.; Panja, A.; Banerjee, P.; Ali, M. Transition Met. Chem.

(Dordrecht, Neth.) 2003, 28, 871−880.

(53) Neese, F.ORCA - An Ab Initio, DFT and Semiempirical

Electronic Structure Package, version 2.9; Max Planck Institute for

Bioinorganic Chemistry: Muelheim, Germany, 2012.

(54) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.

(55) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B. 1988, 37, 785−

789.

(56) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100,

5829−5835.

(57) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97,

2571−2577.

(58) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7,

3297−3305.

(59) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.

1998, 109, 8218−8224.

(90) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110,

(60) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J.

Chem. Phys. 1998, 108, 4439−4449.

6961−7001.

(91) Bordwell, F. G.; Cheng, J. P.; Harrelson, J. A. J. Am. Chem. Soc.

1988, 110, 1229−1231.

(61) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256,

454−464.

(92) Bordwell, F. G.; Cheng, J. P. J. Am. Chem. Soc. 1991, 113, 1736−

1743.

(62) Hirata, S.; Head-Gordon, M. Chem. Phys. Lett. 1999, 314, 291−

299.

(93) Bordwell, F. G.; Zhang, X. M.; Cheng, J. P. J. Org. Chem. 1993,

58, 6410−16.

(63) Hirata, S.; Head-Gordon, M. Chem. Phys. Lett. 1999, 302, 375−

382.

4060

dx.doi.org/10.1021/ic403126p | Inorg. Chem. 2014, 53, 4047−4061

Inorganic Chemistry

Article

(94) Schenk, G.; Pau, M. Y. M.; Solomon, E. I. J. Am. Chem. Soc.

2004, 126, 505−515.

(95) Diebold, A. R.; Straganz, G. D.; Solomon, E. I. J. Am. Chem. Soc.

2011, 133, 15979−15991.

(96) Davis, M. I.; Wasinger, E. C.; Decker, A.; Pau, M. Y. M.;

Vaillancourt, F. H.; Bolin, J. T.; Eltis, L. D.; Hedman, B.; Hodgson, K.

O.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 11214−11227.

(97) Georgiev, V.; Borowski, T.; Blomberg, M. R. A.; Siegbahn, P. E.

M. J. Biol. Inorg. Chem. 2008, 13, 929−940.

(98) Christian, G. J.; Ye, S. F.; Neese, F. Chem. Sci. 2012, 3, 1600−

1611.

(99) Cramer, C. J.; Tolman, W. B.; Theopold, K. H.; Rheingold, A. L.

Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3635−3640.

(100) Chakraborty, B.; Paine, T. K. Angew. Chem., Int. Ed. 2013, 52,

920−924.

(101) Mbughuni, M. M.; Chakrabarti, M.; Hayden, J. A.; Bominaar,

E. L.; Hendrich, M. P.; Munck, E.; Lipscomb, J. D. Proc. Natl. Acad. Sci.

U.S.A. 2010, 107, 16788−16793.

(102) Mbughuni, M. M.; Chakrabarti, M.; Hayden, J. A.; Meier, K.

K.; Dalluge, J. J.; Hendrich, M. P.; Munck, E.; Lipscomb, J. D.

Biochemistry 2011, 50, 10262−10274.

4061

dx.doi.org/10.1021/ic403126p | Inorg. Chem. 2014, 53, 4047−4061

Article Doi

Dioxygen reactivity of biomimetic Fe(II) complexes with noninnocent catecholate, o-aminophenolate, and o-phenylenediamine ligands

DOI: 10.1021/ic403126p

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