Fe(II) complexes that mimic the active site structure of acetylacetone dioxygenase: O2 and NO reactivity

Article abstract of DOI:10.1021/ic3012712

Acetylacetone dioxygenase (Dke1) is a bacterial enzyme that catalyzes the dioxygen-dependent degradation of β-dicarbonyl compounds. The Dke1 active site contains a nonheme monoiron(II) center facially ligated by three histidine residues (the 3His triad); coordination of the substrate in a bidentate manner provides a five-coordinate site for O₂ binding. Recently, we published the synthesis and characterization of a series of ferrous β-diketonato complexes that faithfully mimic the enzyme-substrate intermediate of Dke1 (Park, H.; Baus, J.S.; Lindeman, S.V.; Fiedler, A.T. Inorg. Chem. 2011, 50, 11978-11989). The 3His triad was modeled with three different facially coordinating N3 supporting ligands, and substituted β-diketonates (acac^X) with varying steric and electronic properties were employed. Here, we describe the reactivity of our Dke1 models toward O₂ and its surrogate nitric oxide (NO), and report the synthesis of three new Fe(II) complexes featuring the anions of dialkyl malonates. Exposure of [Fe( ^Me2Tp) (acac^X)] complexes (where ^R2Tp = hydrotris(pyrazol-1-yl)borate with R-groups at the 3- and 5-positions of the pyrazole rings) to O₂ at -70 °C in toluene results in irreversible formation of green chromophores (λ_max ～750 nm) that decay at temperatures above -60 °C. Spectroscopic and computational analyses suggest that these intermediates contain a diiron(III) unit bridged by a trans μ-1,2-peroxo ligand. The green chromophore is not observed with analogous complexes featuring ^Ph2Tp and ^PhTIP ligands (where ^PhTIP = tris(2-phenylimidazoly-4-yl)phosphine), since the steric bulk of the phenyl substituents prevents formation of dinuclear species. While these complexes are largely inert toward O₂, ^Ph2Tp-based complexes with dialkyl malonate anions exhibit dioxygenase activity and thus serve as functional Dke1 models. The Fe/acac ^X complexes all react readily with NO to yield highspin (S = 3/2) {FeNO}⁷ adducts that were characterized with crystallographic, spectroscopic, and computational methods. Collectively, the results presented here enhance our understanding of the chemical factors involved in the oxidation of aliphatic substrates by nonheme iron dioxygenases.

Full text of DOI:10.1021/ic3012712

Article
pubs.acs.org/IC
Fe(II) Complexes That Mimic the Active Site Structure of
Acetylacetone Dioxygenase: O2 and NO Reactivity
Heaweon Park, Michael M. Bittner, Jacob S. Baus, Sergey V. Lindeman, and Adam T. Fiedler*
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881, United States
S
* Supporting Information
ABSTRACT: Acetylacetone dioxygenase (Dke1) is a bacterial enzyme that catalyzes
the dioxygen-dependent degradation of β-dicarbonyl compounds. The Dke1 active
site contains a nonheme monoiron(II) center facially ligated by three histidine
residues (the 3His triad); coordination of the substrate in a bidentate manner
provides a five-coordinate site for O2 binding. Recently, we published the synthesis
and characterization of a series of ferrous β-diketonato complexes that faithfully
mimic the enzyme−substrate intermediate of Dke1 (Park, H.; Baus, J.S.; Lindeman,
S.V.; Fiedler, A.T. Inorg. Chem. 2011, 50, 11978−11989). The 3His triad was
modeled with three different facially coordinating N3 supporting ligands, and
substituted β-diketonates (acacX) with varying steric and electronic properties were
employed. Here, we describe the reactivity of our Dke1 models toward O2 and its surrogate nitric oxide (NO), and report the
synthesis of three new Fe(II) complexes featuring the anions of dialkyl malonates. Exposure of [Fe(Me2Tp)(acacX)] complexes
(where R2Tp = hydrotris(pyrazol-1-yl)borate with R-groups at the 3- and 5-positions of the pyrazole rings) to O2 at −70 °C in
toluene results in irreversible formation of green chromophores (λmax ∼750 nm) that decay at temperatures above −60 °C.
Spectroscopic and computational analyses suggest that these intermediates contain a diiron(III) unit bridged by a trans μ-1,2-
peroxo ligand. The green chromophore is not observed with analogous complexes featuring Ph2Tp and PhTIP ligands (where
PhTIP = tris(2-phenylimidazoly-4-yl)phosphine), since the steric bulk of the phenyl substituents prevents formation of dinuclear
species. While these complexes are largely inert toward O2, Ph2Tp-based complexes with dialkyl malonate anions exhibit
dioxygenase activity and thus serve as functional Dke1 models. The Fe/acacX complexes all react readily with NO to yield high-
spin (S = 3/2) {FeNO}7 adducts that were characterized with crystallographic, spectroscopic, and computational methods.
Collectively, the results presented here enhance our understanding of the chemical factors involved in the oxidation of aliphatic
substrates by nonheme iron dioxygenases.
ment of the Fe center.8 However, in a subsequent computa-
tional study, Solomon and Straganz have set forth an alternative
1. INTRODUCTION
The oxidative cleavage of carbon−carbon bonds by mono-
mechanism that involves formation of an Fe/O2 adduct prior to
nuclear nonheme iron dioxygenases is a crucial step in the
substrate oxidation.9
microbial degradation of many organic pollutants.1 Well-
Dke1 has attracted the interest of synthetic inorganic
studied examples include the intradiol and extradiol catechol
dioxygenases,2 (homo)gentisate dioxygenases,3 and (chloro)-
chemists because of the presence of the unusual 3His triad in
hydroquinone dioxygenases.4 In 2003, Straganz and co-workers
demonstrated that a strain of Acinetobacter johnsonii is able to
the active site, as well as the enzyme’s ability to catalyze
aliphatic C−C bond cleavage. Interestingly, the first relevant
model system was reported a decade before the discovery of
use acetylacetonea toxic pollutantas its sole source of
carbon.5 The initial step of this process is performed by the
Dke1. In 1993, Kitajima and co-workers generated the complex
[Fe2+(iPr2Tp)(acac)(MeCN)] (where R2Tp = hydrotris-
enzyme acetylacetone dioxygenase (also known as β-diketone
(pyrazol-1-yl)borate with R-groups at the 3- and 5-positions
dioxygenase, Dke1), which uses O2 to convert acetylacetone to
of the pyrazole rings).10 Exposure to O2 in MeCN at room
temperature eventually produced crystals of the triiron(III)
complex [Fe3(μ-O)(μ-OH)(μ-OAc)4(iPr2Tp)2], where the
bridging acetate ligands derive from the acac group of the
ferrous precursor. Thus, this synthetic model exhibits Dke1-
type reactivity, although generation of the acetate ligands may
proceed via a different mechanism than the one employed by
the enzyme. In 2008, Limberg and Siewert demonstrated that
acetic acid and 2-oxopropanal.5 Biochemical and crystallo-
graphic studies revealed that the Dke1 active site contains a
monoiron(II) center facially ligated by three histidine (3His)
residues, a deviation from the 2-His-1-carboxylate facial triad
normally employed by nonheme iron dioxygenases.6 Dke1 is
capable of oxidizing β-diketones and β-ketoesters with a variety
of substituents at the 1-, 3-, and 5-positions; in each case, the
substrate coordinates to Fe as the deprotonated acac-type
anion.7 Initial mechanistic studies suggested that O2 reacts with
the bound acac ligand in a concerted two-electron process,
resulting in a peroxidate intermediate without direct involve-
Received: June 15, 2012
Published: September 13, 2012
© 2012 American Chemical Society
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Article
Scheme 1
the related complex, [Fe2+(Me2Tp)(acacPhmal)] (acacPhmal
=
equipped with a freezer set to −30 °C. Nitric oxide was purified by
passage through an ascarite II column, followed by a cold trap at −78
°C to remove higher nitrogen oxide impurities. The LN3 supporting
ligands K(Me2Tp),13 K(Ph2Tp),14 and PhTIP15 were prepared according
anion of diethyl phenylmalonate), reacts with O2 to give Dke1-
type products with incorporation of oxygen atoms from O2.11
The activated diethyl malonate anion was used because the
corresponding acac complex failed to exhibit oxidative cleavage.
Significantly, the [Fe2+(Me2Tp)(acacPhmal)] system is catalytic in
the presence of excess Li(acacPhmal) and O2 with a turnover
frequency of 55 h−1.
to literature procedures. With the exceptions of 1-acacOMe, 2-acacOMe
,
and 2-acacPhmal, the synthesis and characterization of Fe(II) β-
diketonato complexes in the 1-acacX, 2-acacX, and [3-acacX]OTf
series were reported in our earlier manuscript.12
Physical Methods. Elemental analyses were performed at Midwest
Microlab, LLC in Indianapolis, IN. UV−vis absorption spectra were
obtained with an Agilent 8453 diode array spectrometer equipped with
a cryostat from Unisoku Scientific Instruments (Osaka, Japan) for low-
temperature experiments. Infrared (IR) spectra were measured in
solution using a Nicolet Magna-IR 560 spectrometer. 1H and 19F
NMR spectra were collected at room temperature with a Varian 400
MHz spectrometer; 19F NMR spectra were referenced using the
benzotrifluoride peak at −63.7 ppm. Electron paramagnetic resonance
(EPR) experiments were performed using a Bruker ELEXSYS E600
equipped with an ER4415DM cavity resonating at 9.63 GHz, an
Oxford Instruments ITC503 temperature controller, and ESR-900 He
flow cryostat. Mass spectra were recorded using an Agilent 6850 gas
chromatography−mass spectrometer (GC-MS) with a HP-5 (5%
phenylmethylpolysiloxane) column.
Recently, our group published the synthesis and character-
ization of three series of Dke1 models featuring Fe(II)/acacX
units (acacX = substituted β-diketonates) bound to facially
coordinating N3 supporting ligands (LN3).12 These complexes
incorporated acacX ligands with a range of steric and electronic
properties. Following the labeling scheme employed in that
paper (Scheme 1), the 1-acacX and 2-acacX complexes contain
anionic Me2Tp and Ph2Tp donors, respectively. The [3-
acacX]OTf series utilizes the neutral tris(2-phenylimidazoly-4-
yl)phosphine (PhTIP) ligand, which was shown by spectro-
scopic and computational analysis to faithfully reproduce the
3His coordination environment of the Dke1 active site. The
present manuscript describes the reactivity of these complexes
with O2 and nitric oxide (NO) under a variety of conditions.
To compare our results with those previously published by
Siewert and Limberg, we have also generated three additional
complexes that incorporate β-diester anions (i.e., acacOMe and
acacPhmal in Scheme 1). Thus, we seek to expand upon previous
discoveries that have demonstrated that synthetic Fe/acacX
complexes are capable of performing Dke1-type chemistry. In
those earlier works, no attempt was made to observe reactive
intermediates, and each involved only one Fe complex. In this
study, using low-temperature techniques, we have identified
and characterized intermediates arising from treatment of our
Dke1 models with O2 and NO. We have also systematically
evaluated the role that solvent and ligand properties (both
steric and electronic) play in modifying the O2 reactivity of our
complexes. Thus, we have developed a more complete and
extensive picture of the nature of O2 activation at nonheme Fe
centers with relevance to Dke1.
Synthesis of (Me2Tp)Fe(acacOMe). Following the procedure
published by Siewert et al.,11 dimethyl malonate (1.0 mmol) was
deprotonated with lithium diisopropylamide (LDA; 1.1 mmol) in
CH3CN, followed by addition of FeCl2 (1.0 mmol) and K(Me2Tp) (1.0
mmol). The reaction mixture was stirred overnight, filtered, and the
volume reduced under vacuum. X-ray quality crystals were obtained by
storing this solution at −30 °C for several days. Anal. Calcd for
C20H29BFeN6O4: C, 49.62; H, 6.04; N, 17.36. Found: C, 49.72; H,
5.89; N, 17.82. UV−vis [λmax, nm (ε, M−1 cm−1) in toluene]: 315 (sh).
IR (KBr, cm−1): 2952, 2930, 2526 [ν(BH)], 1638 [ν(CO)], 1542,
1498, 1451.
Synthesis of (Ph2Tp)Fe(acacOMe). Li(acacOMe) was generated by
treating dimethyl malonate (0.92 mmol) with LDA (1.0 mmol) in
tetrahydrofuran (THF), followed by removal of the solvent to yield
the salt as a white powder. FeCl2 (0.92 mmol) and K(Ph2Tp) (0.92
mmol) were then combined with Li(acacOMe) in 15 mL of 1:3
CH2Cl2/CH3CN. The solution was stirred for 24 h, and the solvents
were removed under vacuum. The white product was taken up in 5 mL
of CH2Cl2 and filtered to remove insoluble salts. The resulting
solution was layered with CH3CN to yield colorless crystals suitable
for XRD studies. Anal. Calcd for C50H41BFeN6O4: C, 70.11; H, 4.82;
N, 9.81. Found: C, 70.02; H, 4.82; N, 9.85. UV−vis [λmax, nm (ε, M−1
cm−1) in toluene]: 330 (400). IR (KBr, cm−1): 3061, 2950, 2616
[ν(BH)], 1632 [ν(CO)], 1503, 1460, 1304.
2. EXPERIMENTAL SECTION
Materials. Unless otherwise noted, all reagents and solvents were
purchased from commercial sources and used as received. Acetonitrile,
dichloromethane, and tetrahydrofuran were purified and dried using a
Vacuum Atmospheres solvent purification system. The synthesis and
handling of air-sensitive materials were performed under inert
atmosphere using a Vacuum Atmospheres Omni-Lab glovebox
Synthesis of (Ph2Tp)Fe(acacPhmal). Diethyl phenylmalonate
(Phmal) (0.712 mmol) was stirred with LDA (0.790 mmol) for 30
min in 5 mL of THF. Removal of the solvent yielded a white salt that
was dissolved in a 3:2 mixture of MeCN/CH2Cl2. Addition of FeCl2
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Table 1. Summary of X-ray Crystallographic Data Collection and Structure Refinement
b
1-acacOMe·MeCN
2-acacOMe·1.5CH2Cl2
51.5H44BCl3FeN6O4
2-acacPhmal·MeCN
1-acacPhF3(NO)·MeCN·0.5CH2Cl2
empirical formula
formula weight
crystal system
space group
a, Å
C24H35BFeN8O4
566.26
C
C60H52BFeN7O4
1001.75
C27.5H32BClF3FeN8O3
681.71
983.94
orthorhombic
P212121
triclinic
monoclinic
P21/c
monoclinic
P21/n
P1
̅
12.62432(13)
16.70885(18)
13.70225(15)
90
12.2090(3)
13.0283(4)
17.0877(5)
95.921(3)
103.201(3)
117.095(3)
2287.5(1)
2
38.1369(8)
13.5597(3)
20.5303(4)
90
7.9307(2)
19.6395(3)
20.7317(5)
90
b, Å
c, Å
α, deg
β, deg
90
104.121(2)
90
91.126(2)
90
γ, deg
V, Å3
90
2890.32(5)
4
10295.9(4)
8
3228.4(1)
4
Z
D
calc, g/cm3
1.301
1.429
1.293
1.366
λ, Å
μ, mm−1
0.7107
0.7107
1.5418
1.5418
0.565
0.559
2.789
4.782
θ-range, deg
3 to 29
3 to 62
7 to 148
72082
9 to 148
28399
reflections collected
independent reflections
data/restraints/parameters
GOF (on F2)
89285
49672
7745 [Rint = 0.0286]
7745/0/354
1.057
13415 [Rint = 0.0278]
13415/24/609
1.031
20374 [Rint = 0.0351]
20374/0/1321
1.040
6454 [Rint = 0.0393]
6454/0/445
1.122
a
R1/wR2 (I > 2σ(I))
0.0226/0.0608
0.0364/0.0861
0.0463/0.0925
0.0525/0.1393
0.0570/0.1430
0.0478/0.1268
0.0546/0.1303
a
R1/wR2 (all data)
0.0251/0.0615
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
The CH2Cl2 solvate in 1-acacPhF3(NO)·MeCN·0.5CH2Cl2 is only partially
occupied.
(0.713 mmol) and K(Ph2Tp) (0.712 mmol) resulted in a cloudy gray
solution that was stirred overnight. The solvents were removed under
vacuum to yield a pale green solid that was taken up in 5 mL of
CH2Cl2 and layered with CH3CN. After one day, light green crystals
suitable for XRD were obtained. Anal. Calcd for C58H49BFeN6O4: C,
72.51; H, 5.14; N, 8.75. Found: C, 72.39; H, 5.11; N, 8.74. UV−vis
[λmax, nm (ε, M−1 cm−1) in toluene]: 350 (1806). IR (neat, cm−1):
3024, 2974, 2614 [ν(BH)], 1615 [v(CO)], 1594, 1544, 1478, 1463,
1448, 1433, 1405, 1375, 1337, 1303.
accounted for using the conductor-like screening model (COSMO).
The same basis set was used for time-dependent DFT (TD-DFT)
calculations,21 which computed absorption energies and intensities
within the Tamm−Dancoff approximation.22 In each case, at least 80
excited states were calculated. Finally, the gOpenMol program
developed by Laaksonen23 was used to generate isosurface plots of
molecular orbitals.
3. RESULTS AND ANALYSIS
Crystallographic Studies. Six novel complexes were characterized
using X-ray crystallography: 1-acacOMe, 2-acacOMe, 2-acacPhmal, 1-
acacPhF3(NO), Fe(Ph2Tp)2, and [Fe3(O)(OH)(OBz)4(Me2Tp)2] (5-
acacPhF3). Details concerning the data collection and analysis are
summarized in Table 1 and Supporting Information, Table S1. The
XRD data were collected at 100 K with an Oxford Diffraction
SuperNova kappa-diffractometer equipped with dual microfocus Cu/
Mo X-ray sources, X-ray mirror optics, Atlas CCD detector, and a low-
temperature Cryojet device. The data were processed with CrysAlisPro
program package (Oxford Diffraction Ltd., 2010) typically using a
numerical Gaussian absorption correction (based on the real shape of
the crystal) followed by an empirical multiscan correction using the
SCALE3 ABSPACK routine. The structures were solved using the
SHELXS program and refined with the SHELXL program16 within the
Olex2 crystallographic package.17 All computations were performed on
an Intel PC computer with Windows 7 OS. Some structures contain
disorder that was detected in difference Fourier syntheses of electron
density and accounted for using capabilities of the SHELX package. In
most cases, hydrogen atoms were localized in difference syntheses of
electron density but were refined using appropriate geometric
restrictions on the corresponding bond lengths and bond angles
within a riding/rotating model (torsion angles of methyl hydrogens
were optimized to better fit the residual electron density).
3.A. Synthesis, Solid State Structures, and Spectro-
scopic Features of Fe(II) Complexes. Following earlier
procedures,11,12 the iron(II) complexes 1-acacOMe, 2-acacOMe
,
and 2-acacPhmal were prepared by reacting equimolar amounts
of K(R2Tp), FeCl2, LDA, and dimethyl malonate or diethyl
phenylmalonate. The resulting X-ray crystal structures are
shown in Figure 1 and selected metric parameters are provided
in Table 2. Like other members of the 1-acacX series,12 1-
acacOMe features an octahedral Fe(II) center with a facially
coordinating Me2Tp ligand, bidentate acacOMe group, and
MeCN. The average Fe−NTp and Fe−Oacac distances of
2.170 and 2.087 Å, respectively, resemble the values found
for 1-acac and are characteristic of high-spin six-coordinate
(6C) Fe(II) complexes. The Ph2Tp complexes 2-acacOMe and 2-
acacPhmal exhibit distorted 5C square-pyramidal geometries (τ =
0.03 and 0.39, respectively24) with a pyrazole ligand in the axial
position. The lack of bound MeCN shortens the Fe−NTp and
Fe−Oacac distances in 2-acacOMe and 2-acacPhmal relative to 1-
acacOMe (Table 2). Because of steric interactions with the 3-Ph
group of the axial pyrazole, the acacOMe ligand is tilted out of
the N1−Fe1−N3 plane by ∼30°.
Density Functional Theory (DFT) Calculations. DFT calcu-
lations were performed using the ORCA 2.8 software package
developed by Dr. F. Neese.18 Geometry optimizations employed the
Becke−Perdew (BP86) functional19 and Ahlrichs’ valence triple-ζ basis
set (TZV) for all atoms, in conjunction with the TZV/J auxiliary basis
set.20 Extra polarization functions were used on non-hydrogen atoms.
The effect of toluene solvation on computational energies was
Most Fe(R2Tp)(acacX) complexes are brightly colored
because of the presence of Fe2+→acacX (CO*) metal-to-
ligand charge-transfer (MLCT) transitions in the visible
region.12 The acacOMe and acacPhmal complexes are colorless,
however, as the corresponding MLCT bands appear in the UV-
region, reflecting the electron-rich nature of the dialkyl
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Inorganic Chemistry
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the presence of a high-spin Fe(II) center. For both 1-acacOMe
and 2-acacOMe, resonances arising from the methyl groups of
the acacOMe moieties appear near 8 ppm in CD2Cl2. The
remaining R2Tp-derived peaks largely follow the patterns
previously described for complexes in the 1-acacX and 2-
acacX series,12,25 and detailed assignments and T1-relaxation
values are provided in Supporting Information, Table S2.
Attempts to generate the analogous Fe(II)/β-diester
complexes with the PhTIP ligand (i.e., [3-acacOMe +
] or [3-
acacPhmal]+) were not successful. This failure is perhaps due to
the elevated basicities of the dialkyl malonate anions, which are
incompatible with the unprotected H−N groups of the PhTIP
ligand.26
3.B. Oxygenation of Fe(II) Complexes. As shown in
Figure 2A, exposure of 1-acacOMe to O2 at −70 °C in toluene
Figure 1. Thermal ellipsoid plots (50% probability) derived from 1-
acacOMe·MeCN (A), 2-acacOMe·1.5CH2Cl2 (B), and 2-
acacPhmal·MeCN (C). Hydrogen atoms and noncoordinating solvent
molecules have been omitted for clarity. Selected metric parameters
are provided in Table 2.
Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) for 1-acacOMe·MeCN, 2-acacOMe·1.5CH2Cl2, and 2-
acacPhmal·MeCN
1-acacOMe
MeCN
·
2-acacOMe
·
2-acacPhmal
·
a
1.5CH2Cl2
MeCN
Fe−O1
Fe−O2
Fe−N1
Fe−N3
Fe−N5
Fe−N7
2.092(1)
2.083(1)
2.155(1)
2.162(1)
2.194(1)
2.252(1)
2.042(1)
2.046(1)
2.151(1)
2.154(1)
2.129(1)
1.982(2)
2.065(2)
2.214(2)
2.105(2)
2.076(2)
Figure 2. Time-dependent absorption spectra of the reaction of 1-
acacOMe (top) and 1-acacPhF3 (bottom) with O2 at −70 °C in toluene.
The molar absorptivities of the resulting 4-acacX intermediates are
based on the initial concentrations of the 1-acacX precursors.
Fe−Oacac (ave)
Fe−NTp (ave)
2.087
2.170
2.044
2.144
2.024
2.132
O1−Fe−O2
O1−Fe−N1
O1−Fe−N3
O1−Fe−N5
O2−Fe−N1
O2−Fe−N3
O2−Fe−N5
N1−Fe−N3
N1−Fe−N5
N3−Fe−N5
86.70(3)
90.18(3)
87.50(4)
92.16(4)
159.16(4)
108.31(4)
157.09(4)
88.54(4)
109.58(4)
83.68(4)
92.28(4)
92.29(4)
0.03
86.34(6)
97.02(7)
147.45(7)
120.57(7)
170.87(7)
90.05(7)
95.66(7)
82.54(7)
89.95(7)
91.98(7)
0.39
gives rise to a dark green intermediate (4-acacOMe) with an
intense absorption band at 756 nm (ε = 2400 M−1 cm−1 per
Fe). The oxygenation reaction is irreversible and the
intermediate quickly decays at temperatures above −60 °C.
Similar spectral changes are observed upon oxygenation of 1-
acac and 1-acacF3 (Supporting Information, Figure S1) in
toluene. Time-dependent absorption spectra of the reaction of
1-acacPhF3 with O2 (Figure 2B) display isosbestic points at 510
and 600 nm, indicative of clean conversion of 1-acacPhF3→4-
acacPhF3 without the build-up of intermediates. While each
green intermediate displays a prominent absorption band in the
near-IR region, the energy and intensity of this band varies
considerably within the series, suggesting that the acacX ligand
remains iron-bound and capable of modulating the electronic
structures of 4-acacX (vide infra). Interestingly, 1-acacF6 does
not give rise to a green intermediate upon treatment with O2,
suggesting that the electron-poor acacF6 ligand deactivates the
complex.
177.00(3)
94.25(3)
176.86(4)
96.17(3)
92.16(4)
86.95(4)
88.45(4)
84.78(4)
b
τ-value
a
The unit cell of 2-acacPhmal·MeCN contains two symmetrically
independent complexes with nearly identical geometries. Only
parameters of the first complex are shown. For a definition of the
τ-value, see reference 24. A five-coordinate complex with ideal square-
pyramidal geometry would have a τ-value of 0.0, while those with ideal
trigonal bipyramidal geometry would have a value of 1.0.
b
1
malonate anions. The H NMR spectra of these complexes
exhibit broad, paramagnetically shifted peaks consistent with
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Formation of the green intermediates is hindered by the
presence of coordinating solvents, such as MeCN and MeOH.
As shown in Figure 3, oxygenation of 1-acac in MeCN at −40
of [Fe(Me2Tp)(acacX)] complexes (where acacX is a β-
diketonate) with O2 in coordinating solvents results in cleavage
of the acacX ligand, in agreement with previous studies by
Kitajima10 and Limberg11 with related complexes.
Treatment of Fe(II) complexes in the 2-acacX and [3-
acacX]+ series with O2 failed to generate the corresponding
green intermediates or triiron complexes under any conditions.
These complexes react slowly, or not at all, with O2 in both
coordinating and noncoordinating solvents. For instance, the
UV−vis absorption and 19F NMR spectra of 2-acacF3, 2-
acacPhF3, and [3-acacF3]+ in O2-saturated toluene solutions
exhibit no significant changes over the course of several days at
room temperature. Only the β-diester complexes 2-acacOMe
and 2-acacPhmal exhibit perceptible reactivity toward O2.
Exposure of these complexes to O2 eventually yields colorless
crystals that were shown by XRD to consist of Fe(Ph2Tp)2
(Supporting Information, Figure S5), suggesting that the acac
ligands are degraded in the process.27 Monitoring the O2
Figure 3. Time-dependent absorption spectra of the reaction of 1-acac
(0.4 mM) with O2 at −40 °C in MeCN. Inset: Absorption intensity at
375 and 735 nm as a function of time.
1
reaction with H NMR spectroscopy in toluene-d8 revealed
that the paramagnetically shifted peaks of 2-acacOMe and 2-
acacPhmal slowly disappear, concomitant with the growth of
peaks arising from methyl 2-oxoacetate and ethyl benzoylfor-
mate, respectively. The formation of ethyl benzoylformate was
also verified by GC-MS. When the reaction with 2-acacPhmal
was performed with 18O-enriched dioxygen, GC-MS analysis
indicated that one O-atom of ethyl benzoylformate is derived
from O2, proving that the observed products are due to
dioxygenolytic cleavage of the ligand (the alkyl carbonate
products that are also generated quickly decompose to CO2
and the corresponding alkoxide). Thus, our Ph2Tp-based
complexes are capable of performing Dke1-type chemistry on
a reasonable time scale provided that the substrate ligand is
sufficiently activated. The rates of the O2 reactions in toluene
were measured using UV−vis absorption spectroscopy. 2-
°C initially generates a broad absorption feature at 735 nm that
likely corresponds to 4-acac. This feature never reaches full
intensity, however, because of a competing decay process that
eventually yields a species (5-acac) with two intense,
overlapping bands in the near-UV region. For other 1-acacX
complexes (X = OMe, F3, and PhF3), the transient absorption
band of the green intermediate is never observed during the
reaction with O2 in MeCN or MeOH; instead, these complexes
directly convert to intermediates with UV−vis features similar
to 5-acac (Supporting Information, Figure S2). The 1-acacX→
5-acacX conversion occurs rapidly at −40 °C in the case of 1-
acacOMe, whereas the process requires several days at room
temperature for compounds with fluorinated acacX ligands.
Significantly, the addition of MeOH to toluene solutions of
already-formed green intermediates results in immediate decay
to the corresponding 5-acacX species, even at −70 °C. This
behavior indicates that 4-acacX species are not stable in the
presence of coordinating solvents.
acacPhmal decays relatively quickly in the presence of O2 (kO2
=
3.4 × 10−4 s−1; Supporting Information, Figure S6), whereas 2-
acacOMe requires more than a day for complete reaction (kO2
4 × 10−5 s−1).
∼
The 5-acacX complexes are not simply the one-electron
oxidized forms of 1-acacX (i.e., [Fe3+(Me2Tp)(acacX)]+), since
treatment of 1-acacX complexes with ferrocenium salts yields
chromophores with very different absorption features (Support-
ing Information, Figure S2). Instead, the spectral features
resemble those reported by Kitajima et al. for the linear
trinuclear ferric complex [Fe3+3(O)(OH)(OAc)4(iPr2Tp)2] (6),
which is generated upon exposure of [Fe2+(iPr2Tp)(acac)] to O2
in MeCN (as noted in the Introduction).10 In support of this
assignment, the EPR spectra of 5-acac and 5-acacPhF3 display
an intense feature at g = 4.3 and a weaker peak at g = 8.83
(Supporting Information, Figure S3), identical to the spectrum
reported for 6. In addition, crystals of 5-acacPhF3 were obtained
by allowing an aerobic solution of 1-acacPhF3 in MeCN to stand
for several weeks. XRD analysis revealed a triiron complex with
Cs symmetry analogous to 6 (Supporting Information, Figure
S4). The Fe centers in 5-acacPhF3 are bridged by four benzoate
ligands derived from the acacPhF3 ligand, as well as two μ-O(H)
ligands. It is reasonable to assume that 5-acac and 5-acacF3
adopt similar structures. In the case of 5-acacOMe, however, the
resulting methyl carbonate anion would decompose to CO2
and methoxide, and therefore a trinuclear structure with
bridging carboxylate ligands is unlikely. Attempts to crystallize
5-acacOMe have not succeeded to date. Regardless, our
spectroscopic and crystallographic data prove that the reaction
The observed reactivity trends of the Dke1 models cannot be
rationalized on the basis of redox potentials. For instance, the
Fe3+/Fe2+ redox couple measured for the largely inert 2-acac
(−58 mV) is slightly more negative then the values for 1-acacF3
(−34 mV vs Fc+/0) and 1-acacPhF3 (0 mV)12 which react
rapidly with O2 to generate 4-acacX intermediates. In addition,
cyclic voltammetric studies indicate that 2-acacPhmal is actually
harder to oxidize than 2-acac (Supporting Information, Figure
S7); yet it is the former complex that carries out dioxygenolytic
cleavage. Within the 2-acacX series, it appears that O2 reactivity
is primarily governed by the relative energies of acacX π-
electrons, not the Fe redox potentials. This conclusion is
consistent with a prior study of Straganz that found a direct
correlation between kcat and the energy of the acacX highest-
occupied molecular orbital (HOMO).8
One might also ascribe the sluggish O2 reactivity of the 2-
acacX and [3-acacX]+ series to the increased steric bulk of the
phenyl substituents, which appear to block access to the vacant
coordination site in the X-ray crystal structures (Figure 1).
However, this assumption is contradicted by our experiments
with NO (vide infra). To understand the divergent O2
chemistry of these complexes, it is necessary to identify the
structure of the 4-acacX species formed upon reaction of some
1-acacX complexes with O2.
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3.C. Spectroscopic and Computational Character-
ization of 4-acacX Intermediates. Several pieces of evidence
suggest that the green intermediates (4-acacX) contain a
diiron(III)-peroxo unit. First, the observed spectral features and
the manner of preparation are very similar to those of the cis μ-
1,2-peroxo bridged complex [Fe2(O2)(μ-O2CR)2(iPr2Tp)2] (7;
R = Ph or CH2Ph) first prepared by Kitajima and co-workers.28
This complex is generated via the reaction of [Fe2+(O2CR)-
symmetry (BS) approach of Noodleman were not successful
because of SCF convergence difficulties. Regardless, previous
DFT studies of transition metal dimers have demonstrated that
HS and BS calculations generally provide similar geometries,
and thus the HS models are sufficient for our purposes.32
Significant metric parameters for the DFT-derived model of
4-acac (4DFT-acac) are provided in the caption of Figure 4. The
computed O−O bond distance of 1.39 Å is characteristic of
bridging peroxo ligands, and the Fe2O2 unit features Fe−O−O
angles of 123° and a Fe−O−O−Fe dihedral angle near 180°.
The trans μ-1,2 coordination of the peroxo moiety, combined
with the fewer number of bridging ligands, results in a much
larger Fe−Fe separation in 4DFT-acac (4.88 Å) than the crystal
structure of 7 (4.00 Å).28b As a result, the Fe−Operoxo bonds in
(
iPr2Tp)] with O2 in toluene at −20 °C and features a band at
682 nm (ε = 1730 M−1 cm−1 per Fe) that arises from a
peroxo→Fe(III) charge transfer transition.29 Even more
relevant to the current study is Kitajima’s report that
[Fe2+(acac)(iPr2Tp)] reacts with O2 in toluene to give a green
intermediate that is unstable even at −78 °C.10 Although no
spectroscopic characterization was performed, this species was
presumed to be a μ-peroxo species. Second, we examined 4-
acac with magnetic circular dichroism (MCD) spectroscopy,
which is sensitive to the ground-state spin of chromophores. As
shown in Supporting Information, Figure S8, MCD spectra of
4-acac fail to exhibit any temperature-dependent intensity in
the 600−900 nm region. This result provides strong evidence
in favor of an antiferromagnetically coupled diiron species (Stotal
= 0), while simultaneously ruling out a mononuclear Fe3+ or
high-spin Fe2+ assignment for 4-acac. Finally, EPR spectra of 4-
acacX are featureless apart from a weak peak at g = 4.3 from the
decomposition product (less than 10% of the Fe in the sample
based on double integration of the signal30). Collectively, these
results point to the formulation of 4-acacX as [Fe2(μ-
O2)(acacX)2(MeTp)2] intermediates. Unfortunately, further
confirmation with resonance Raman spectroscopy has not
been successful because of rapid photodecomposition of the
green intermediates even at low laser power.
4
DFT-acac are elongated by 0.09 Å relative to their counterparts
in 7.33
To validate the structure of 4DFT-acac on the basis of
experimental data, electronic transition energies were calculated
using time-dependent DFT (TD-DFT). The computed
spectrum (Supporting Information, Figure S9) features a
dominant band at 879 nm (11380 cm−1) with an ε-value of
6300 M−1 cm−1 per Fe, in reasonable agreement with the
experimentally observed feature at 740 nm (13510 cm−1) with
an ε-value of 3500 M−1 cm−1. As expected, TD-DFT attributes
this band to a peroxide(ππ*)→ Fe(3dxz) charge transfer
transition. The intensity of this feature is enhanced by the
planarity of the Fe2O2 unit in 4DFT-acac, which optimizes π-
overlap between the ππ* and 3dxz orbitals. For the sake of
comparison, the TD-DFT methodology was also applied to a
geometry-optimized model of complex 7. The resulting
calculation predicts a peroxide→ Fe CT transition at 788 nm
(12690 cm−1) with an intensity of 2200 M−1 cm−1 per Fe
(experimentally, this band appears at 682 nm with ε = 1730
M−1 cm−1 per Fe29). It therefore appears that TD-DFT
underestimates the transition energies and overestimates the
intensities of CT features in diferric-peroxo complexes.
Regardless, the computational results indicate that the principal
peroxide→Fe CT transition of the putative trans μ-1,2 peroxo
complex 4-acac should be lower in energy and greater in
intensity than the corresponding transition of the cis μ-1,2
peroxo complex 7, which is fully consistent with our
experimental observations.
Finally, computational methods were used to evaluate the
stabilities of the diiron(III)-peroxo complexes relative to their
5C Fe2+ precursors and O2. DFT-derived energies indicate that
formation of 7, via the reaction: 2Fe2+ + O2 → Fe2(μ-O2), is
more thermodynamically favorable than the formation of 4-
acac by nearly 14 kcal/mol.34 This computational result is
supported by numerous experimental observations that point to
the greater stability of 7 relative to 4-acac: (i) 7 is metastable at
−20 °C,28a whereas 4-acac decays rapidly at temperatures
above −60 °C, (ii) X-ray quality crystals of 7 were obtained at
low temperature,28b whereas crystallization of 4-acac has
proven futile to date, and (iii) 7 is more resistant to
photodecomposition than 4-acac. The enhanced stability of 7
relative to 4-acac is likely due to the presence of the two
bridging carboxylate ligands in the former.
Possible structures for the 4-acac intermediate were
evaluated using DFT calculations. In the X-ray structure of 7,
the peroxo ligand adopts a cis μ-1,2-peroxo mode with two
carboxylate ligands occupying μ-1,3 bridging positions between
the two Fe3+ centers.28b An analogous structure is not possible
for 4-acac, since the acac ligands are not capable of bridging in
a μ-1,5 fashion.31 Given that each Fe center is likely
coordinated to a single acac ligand, the most plausible structure
of 4-acac is a trans μ-1,2-peroxo diiron(III) complex with
idealized C2h symmetry, as shown in Figure 4. Geometry
optimizations based on this orientation were carried out for the
high-spin (HS), ferromagnetic state with Stot = 5. While it is
known from experiment that 4-acac exhibits antiferromagnetic
coupling, attempts to account for this fact with the broken-
On the basis of the spectroscopic and computational results
presented here, we are confident that the 4-acacX chromo-
phores correspond to diferric-peroxo complexes with trans μ-
1,2-peroxo orientations, as shown in Figure 4 for 4-acac.
3.D. Reaction with Nitric Oxide. Nitric oxide has been
frequently used to probe the ferrous states of nonheme Fe
Figure 4. Model of 4-acac derived from DFT geometry optimizations.
The computed structure exhibits almost perfect C2h symmetry.
Computed bond distances (Å) and angles (deg): Fe−O1(2) 1.994,
Fe−Neq (ave) 2.173, Fe−Nax 2.364, Fe−Oac (ave) 2.037, O1−O2
1.389, Fe···Fe 4.882, Fe−O1−O1 122.8.
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enzymes and related complexes. Each of our Dke1 models,
regardless of its supporting ligand, rapidly reacts with NO to
yield the corresponding {FeNO}7 species (according to the
Enemark-Feltham notation35). As shown in Figure 5 for the
Figure 6. Thermal ellipsoid plots (50% probability) derived from the
X-ray structure of 1-acacPhMe(NO)·MeCN·0.5CH2Cl2. Hydrogen
atoms and noncoordinating solvent molecules have been omitted for
clarity. The oxygen atom of the NO ligand (O3) is distorted over two
positions in a 3:1 ratio. Selected metric parameters are provided in
Table 3.
Table 3. Selected Bond Distances (Å) and Bond Angles
(deg) for 1-acacPhF3·MeCN and 1-
acacPhF3(NO)·MeCN·0.5CH2Cl2 Obtained via XRD
Figure 5. Absorption spectra of 2-acac in CH2Cl2 at room
temperature before and after addition of NO. Inset: X-band EPR
spectrum of the Fe-NO adduct of 2-acac in frozen CH2Cl2 ([Fe] = 1.5
mM). Instrumental parameters: frequency = 9.629 GHz; power = 2.0
mW; modulation = 12 G; temperature = 10 K.
a
Experiments
XRD
DFT
1-acacPhF3
MeCN
·
1-acacPhF3(NO)·
MeCN·0.5CH2Cl2
1-acacPhF3(NO)
Fe−O1
Fe−O2
Fe−N1
Fe−N3
Fe−N5
Fe−N7
Fe−Oacac (ave)
Fe−NTp (ave)
N7−O3
2.064(1)
2.073(1)
2.144(1)
2.164(1)
2.170(1)
2.255(1)
2.069
2.064(2)
2.069(2)
2.109(2)
2.112(2)
2.203(2)
1.813(3)
2.067
2.104
2.097
2.136
2.140
2.219
1.761
2.101
2.165
1.176
representative reaction of 2-acac with NO, formation of the
greenish-brown Fe-NO adduct, 2-acac(NO), is evident in the
appearance of two absorption bands near 440 and 620 nm that
are characteristic of 6C {FeNO}7 species36 (absorption spectra
of all Fe-NO adducts generated as part of this study are shown
in Supporting Information, Figures S10−S12). While iron-
nitrosyl complexes with R2Tp supporting ligands have long
lifetimes at room temperature (RT), the 3-acac(NO) species is
only moderately stable at −40 °C in MeCN. The nitrosyl
complexes are uniformly high-spin (S = 3/2), displaying nearly
axial EPR spectra with gx ≈ gy ≈ 4.0 and gz = 2.0 (Figure 5,
inset; see Supporting Information, Figures S12 and S13 for
additional EPR data). In spectra measured in frozen CH2Cl2,
poorly resolved superhyperfine splitting from the nitrogen
ligands is evident in the gx and gy features. FTIR spectra of the
NO adducts each revealed a prominent peak between 1720 and
1761 cm−1 arising from the stretching mode of the bound
nitrosyl ligand. These νNO frequencies are typical for high-spin,
nonheme {FeNO}7 complexes with 6C geometries.37
2.158
2.141
1.148(4)
Fe−N7−O3
O1−Fe−N3
O2−Fe−N1
147.7(3)
174.2(1)
170.0(1)
143.4
172.9
173.4
175.48(5)
177.83(5)
a
Metric parameters for the DFT-computed structure of 1-
acacPhF3(NO) are also included.
∼1.73 Å found in comparable nonheme {FeNO}7 complexes,
while the observed N−O distance of 1.148(4) Å is slightly
shorter than the average value of ∼1.16 Å.37
X-ray quality crystals of 1-acacPhF3(NO) were obtained by
slowly cooling a NO saturated solution of 1-acacPhF3 in 1:1
MeCN:CH2Cl2. The crystal structure confirms the coordina-
tion of a NO ligand to the Fe center, providing a distorted
octahedral geometry (Figure 6). Selected bond distances and
angles for 1-acacPhF3(NO) are provided in Table 3. The FeNO
unit is disordered over two positions in a 3:1 ratio with Fe1−
N7−O3(A) angles of 147.7(3)° (major) and 139.4(6)°
(minor).38 As shown in Figure 6, the NO ligand in the major
structural component is facing away from the acacPhF3 ligand,
such that the projection of the NO bond vector bisects the N1−
Fe−N3 angle (the N3−Fe1−N7−O3 dihedral angle is
41.5(5)°). In the minor structural component, the NO bond
vector points in the opposite direction, bisecting the O1−Fe1−
O2 angle of the acacPhF3 ligand; this orientation is similar to one
likely adopted by O2 in the Dke1 active site. Interestingly, the
Fe1−N7 distance of 1.813(3) Å measured for 1-acacPhF3(NO)
is significantly longer than the average Fe−NNO distance of
The geometric changes arising from NO binding are evident
in a comparison of the structure of 1-acacPhF3(NO) with its
precursor 1-acacPhF3 (Table 3). The trans influence of the
nitrosyl ligand results in a lengthening of the Fe−N5 bond
from 2.170(1) to 2.203(2) Å, and the Fe atom moves out of the
N2O2 plane by 0.13 Å. In contrast, the equatorial Fe−NTp
bonds shorten by an average of 0.044 Å upon NO coordination,
while the Fe−Oacac distances remain the same within
experimental error. The internal metric parameters of the
acacPhF3 ligand are unchanged to within experimental error.
Using the X-ray structure as a starting point, the electronic
structure of 1-acacPhF3(NO) was examined with spin-
unrestricted DFT calculations employing the nonhybrid BP86
functional.39 As shown in Table 3, the energy-minimized
structure of 1-acacPhF3(NO) is reasonably consistent with the
crystallographic data, exhibiting a rms deviation of 0.034 Å in
the first-sphere Fe−N/O bond distances. However, DFT fails
to reproduce the elongated Fe−N(O) bond observed for 1-
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Figure 7. Molecular orbital energy-level diagram obtained from DFT calculations of 1-acacPhF3(NO). MOs are labeled according to their principal
contributor. DFT-generated isosurface plots and compositions of selected orbitals are also provided.
Scheme 2
acacPhF3(NO), instead providing a more conventional value of
1.761 Å. DFT calculations employing hybrid functionals, such
as B3LYP, resulted in Fe−N(O) distances that were too long
(>1.84 Å), while also overestimating the Fe−Oacac/NTp
distances to a greater degree than the BP86 structure. Thus,
the BP model is likely the most accurate representation of the
gas-phase structure of 1-acacPhF3(NO).
The computed molecular orbital (MO) energy level diagram
for 1-acacPhF3(NO) in the S = 3/2 ground state is shown in
Figure 7. The frontier orbitals are primarily derived from the Fe
3d, NO(π*), and acacPhF3 orbitals. The highest-occupied acac-
based MO (acac HOMO) features a large lobe of electron
density on the central carbon atom, corresponding to the acac
“lone-pair”, while the lowest-unoccupied acac-based MO (acac
LUMO) has largely CO* character. The two lowest-
unoccupied spin-up (α) MOs are mainly composed of NO(π*)
character, indicating that the spin of the nitrosyl ligand is
aligned opposite to the Fe spin (i.e., antiferromagnetic
coupling). As illustrated by the contour plots shown in Figure
7, the Fe 3d(xz) and 3d(yz) orbitals interact strongly with the
two spin-down (β) NO(π*) orbitals. These MOs contain
roughly equal amounts of Fe 3d and NO(π*) character, and the
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same is true of their (unoccupied) antibonding counterparts.
The high covalency of the Fe−NO bond makes it challenging
to conclusively assign oxidation states for 1-acacPhF3(NO), a
difficulty encountered for nearly all iron nitrosyl complexes.
Nonheme NO complexes with S = 3/2 have typically been
described as high-spin Fe3+ centers (S = 5/2) coupled to NO−
(S = 1).36a Consistent with this formulation, the energies of the
Fe 3d(xy)- and 3d(x2 − y2)-based MOs, which are nonbonding
with respect to the axial ligands, decrease by approximately 0.9
eV upon conversion of 1-acacPhF3→1-acacPhF3(NO), indicating
a significant increase in the Fe effective nuclear charge. The
large amount of Fe 3d character (∼34%) in the spin-down
HOMOs, however, suggests that the electron is only partially
transferred from Fe to NO, resulting in an oxidation state
between Fe3+ and Fe2+.
Figure S4), as demonstrated previously by Kitajima and co-
workers.10 It is not certain if the formation of the triiron species
proceeds via breakdown of the corresponding diiron(III)-
peroxo intermediate or whether another mechanism is
operative. While 4-acac is indeed observed during the
oxygenation of 1-acac in MeCN (Figure 3), 4-acacX
intermediates are not evident in the 1-acacX→5-acacX
conversions of the remaining complexes, although this result
does not rule out transient formation of the green intermediates
in these cases. Limberg has suggested that cleavage of the acacX
ligand in Kitajima’s model (and in our systems, by extension)
may be due to trace amounts of H2O in MeCN, which could
hydrolyze the acacX ligand following oxidation of the Fe
center.11
Compared to the 1-acacX series, Fe(II)-acacX complexes with
Ph2Tp or PhTIP supporting ligands (2-acacX and [3-acacX]+,
respectively) exhibit dramatically diminished reactivity toward
O2. Complexes with β-diketonato ligands are stable in
oxygenated solutions at room temperature for several hours
or days, regardless of solvent. We initially assumed that this
inert behavior was due to the steric bulk of the phenyl
substituents, which might be expected to block O2 coordination
the Fe center. Yet this hypothesis is not consistent with the
facile formation of {FeNO}7 adducts upon exposure of the 2-
acacX and [3-acacX]+ complexes to NO, proving that formation
of an Fe/O2 species is sterically feasible for the Ph2Tp- and
PhTIP-based complexes. However, the size of the Ph groups
likely prevents the second step in the pathway to 4-acacX, that
is, formation of the diiron intermediate via reaction with a
second Fe(II) complex (Scheme 2). Our DFT model of 4-acac
possesses a high degree of steric crowding even with relatively
small methyl substituents on the pyrazole rings (Figure 4). The
presence of phenyl rings at these positions would undoubtedly
lead to severe steric clashes between opposing Ph2Tp (or PhTIP)
ligands, thereby prohibiting formation of the analogous 4-acacX
species.
The scenario illustrated in Scheme 2 requires the reaction
equilibrium of the first step (i.e., O2 binding) to lie far to the
side of Fe(II) + O2, since the putative iron(III)-superoxo
adduct is never observed. Indeed, a number of computational
studies have demonstrated that the reversible transfer of one
electron from a mononuclear nonheme Fe(II) center to O2 is
endergonic by approximately 5−10 kcal/mol.9,41 In our Me2Tp
systems, the uphill nature of this initial step is overcome by the
thermodynamic favorability of the second step, which “pulls”
the entire process forward to 4-acacX. However, when this
second step is hindered because of steric reasons, the ferrous
complexes are rendered much less reactive toward O2, as
observed for the 2-acacX and [3-acacX]+ series.
In many respects, 1-acacPhF3(NO) resembles an {FeNO}7
adduct recently synthesized and crystallographically charac-
terized by Berto and Lehnert, namely, [Fe(NO)(Cl)(BMPA-
Pr] (8; where BMPA-Pr is N-propanoate-N,N-bis(2-
pyridylmethyl)amine).37b Like 1-acacPhF3(NO), complex 8
exhibits an axial S = 3/2 EPR sigand, along with a relatively
long Fe−N(O) bond distance of 1.783 Å. DFT calculations of
8 revealed substantial mixing between the Fe 3d(xz/yz) and
NO(π*) orbitals in the β-manifold, similar to the bonding
scheme described above for 1-acacPhF3(NO). In these
situations, Berto and Lehnert argue that the NO− ligand
behaves as a strong π-donor, which lowers the effective nuclear
charge of the Fe(III) center.
Complex 1-acacPhF3(NO) serves as an approximate model of
the ferric−superoxo intermediate in the Dke1 mechanism. On
the basis of DFT calculations, Diebold et al. have proposed that
the superoxo group electrophilically attacks the acac HOMO to
yield a peroxo bridge between Fe and the central C atom of the
acac ligand.9 This crucial step in the catalytic cycle would be
facilitated by charge donation from the substrate HOMO to the
Fe3+ center in the initial Fe/O2 adduct, as likely occurs in the
mechanism of extradiol catechol dioxygenases. Yet our
calculations indicate that the acacPhF3 HOMO in 1-
acacPhF3(NO) is essentially nonbonding with respect to the
Fe-NO unit, and the amount of unpaired-spin density on the
carbons atoms of the acacPhF3 ligand is negligible for both 1-
acacPhF3(NO) and its ferrous precursor.
4. DISCUSSION
This manuscript has described the reactivity of the Fe(II) β-
diketonato complexes, [Fe2+(LN3)(acacX)] (LN3
=
Me2Tp,
Ph2Tp, PhTIP), with O2 and NO under various reaction
conditions. On the basis of our observations, we propose the
mechanistic framework shown in Scheme 2. With the exception
of 1-acacF6, complexes in the 1-acacX series readily react with
O2 in toluene to produce dark green intermediates (4-acacX)
that are only stable at temperatures less than −60 °C. A
combination of spectroscopic and computational results suggest
that the 4-acacX intermediates consist of two [Fe3+(Me2Tp)-
(acacX)]+ units linked by a trans μ-1,2-peroxide ligand (Figure
4). Given the generally accepted pattern of Fe/O2 chemistry,40
it is reasonable to assume that initial O2 binding generates an
iron(III)-superoxo intermediate (not observed) that quickly
reacts with a second 1-acacX equivalent to form the diferric-
peroxide intermediate, 4-acacX. In MeCN, reaction of some 1-
acacX complexes with O2 ultimately yields linear trinuclear
complexes (5-acac and 5-acacPhF3) featuring four bridging
carboxylates and two O(H) ligands (Supporting Information,
Obviously, the protein environment surrounding the Fe
center in Dke1 prevents formation of a diiron-peroxo
intermediate like 4-acacX. Instead, the enzymatic mechanism
likely involves formation of a peroxy bridge between Fe and
acac following initial formation of an iron(III)-superoxo
species. One would therefore expect the Ph2Tp and PhTIP
supporting ligands, which prevent dimerization, to be ideal
scaffolds for reproducing the catalytic activity of Dke1. Instead,
only 2-acacX complexes with activated dialkyl malonate anions
(2-acacOMe and 2-acacPhmal) react with O2 to give oxidized
products. Since these complexes are not capable of forming
diiron(III)-peroxo species like the Me2Tp-based complexes, the
dioxygenolytic cleavage mechanism likely replicates the
mechanism proposed by Diebold et al. for the enzyme.9
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However, even when dioxygenase activity is observed in our
synthetic models, the rate of reaction is orders of magnitude
slower than the rate of the enzyme with its natural substrate
(kcat = 6.5 s−1 for acetylacetone). Remarkably, Dke1 is capable
of oxidizing unactivated substrates like acacF3 and acacPhF3 at
measurable rates of 5.3 × 10−3 and 5.2 × 10−5 s−1;8 by contrast,
the corresponding synthetic complexes are stable in air for days.
The lackluster O2 reactivity exhibited by complexes in the 2-
acacX and [3-acacX]+ series highlights the obstacles Dke1 must
overcome to oxidize β-diketone substrates. As noted above,
formation of the initial Fe/O2 adduct has been shown to be
endergonic in most nonheme Fe enzymes studied to date;
therefore, enzymes rely on a favorable second-step to move the
catalytic cycle forward. In the extradiol catechol dioxygenases,
this process is facilitated by (partial or total) electron transfer
from the bound catecholate substrate to Fe, resulting in a
putative superoxo-Fe(II)-semiquinone intermediate. As dem-
onstrated by Siegbahn, the radical character on the aromatic
substrate lowers the barrier to peroxide formation.41b However,
this strategy is not feasible in the case of Dke1, which involves
oxidation of an aliphatic substrate. Our crystallographic and
computational studies of 1-acacX(NO) adducts, reasonable
models of the putative iron-superoxo intermediate in Dke1,
failed to detect any significant radical character on the carbon
backbone of the acacX ligands. Thus, there appears to be an
intrinsically large barrier to formation of the key alkylperoxo
intermediate. The protein environment in Dke1 probably plays
an important role in significantly lowering the barrier to O2
reduction. This conclusion is consistent with computational
and mutagenesis studies performed by Straganz and co-
workers, who have demonstrated that changes in second-
sphere residues (both hydrophilic and hydrophobic) signifi-
cantly alter the rate of acetylacetone oxidation.42 Nearby
residues may function as acid/base catalysts, as is the case for
ring-cleaving dioxygenases.43 In a recent computational study,
Christian et al. indicated that the barrier to alkylperoxo
formation is lowered by simultaneous transfer of a proton to
the proximal O-atom of the peroxo bridge.44 While this study
focused on the extradiol catechol dioxygenases, it is possible
that a similar mechanism is operative in Dke1. The
incorporation of these subtle second-sphere effects poses a
challenge to the development of synthetic complexes capable of
displaying Dke1-type reactivity.
ACKNOWLEDGMENTS
■
We thank Dr. Brian Bennett for generously allowing us to
perform EPR experiments at the National Biomedical EPR
Center (supported by NIH P41 Grant EB001980), and Dr.
Thomas C. Brunold at UW-Madison for access to his MCD
instrument. A.T.F. also thanks Marquette University and the
National Science Foundation (CAREER CHE-1056845) for
generous financial support.
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ASSOCIATED CONTENT
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