Factors affecting the carboxylate shift upon formation of nonheme diiron-O2 adducts

Article abstract of DOI:10.1021/ic302543n

Several [Fe^II₂(N-EtHPTB)(μ-O₂X)] ²⁺ complexes (1·O₂X) have been synthesized, where N-EtHPTB is the anion of N,N,N′N′-tetrakis(2-benzimidazolylmethyl)- 2-hydroxy-1,3-diaminopropane and O₂X i

Full text of DOI:10.1021/ic302543n

Article
pubs.acs.org/IC
Factors Affecting the Carboxylate Shift Upon Formation of Nonheme
Diiron‑O2 Adducts
Jonathan R. Frisch,† Ryan McDonnell,‡ Elena V. Rybak-Akimova,*,‡ and Lawrence Que, Jr.*,†
†Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant St. S.E., Minneapolis,
Minnesota 55455, United States
‡Department of Chemistry, Tufts University, 62 Talbot Ave., Medford, Massachusetts 02155, United States
S
* Supporting Information
ABSTRACT: Several [FeII (N-EtHPTB)(μ-O2X)]2+ complexes
2
(1·O2X) have been synthesized, where N-EtHPTB is the anion
of N,N,N′N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-di-
aminopropane and O2X is an oxyanion bridge. Crystal structures
reveal five-coordinate (μ-alkoxo)diiron(II) cores. These diiron-
(II) complexes react with O2 at low temperatures in CH2Cl2
(−90 °C) to form blue-green O2 adducts that are best described
as triply bridged (μ−η1:η1-peroxo)diiron(III) species (2·O2X).
With one exception, all 2·O2X intermediates convert irreversibly to doubly bridged, blue (μ−η1:η1-peroxo)diiron(III) species
(3·O2X). Where possible, 2·O2X and 3·O2X intermediates were characterized using resonance Raman spectroscopy, showing
respective νO−O values of ∼850 and ∼900 cm−1. How the steric and electronic properties of O2X affect conversion of 2·O2X to
3·O2X was examined. Stopped-flow analysis reveals that oxygenation kinetics of 1·O2X is unaffected by the nature of O2X, and for
the first time, the benzoate analog of 2·O2X (2·O2CPh) is observed.
yl)-2-hydroxy-1,3-diaminopropane] combined with 2 equiv of
iron(II) and 1 equiv of either benzoate, diphenylphosphinate,
or dimethylarsinate,29 we demonstrated that two different
(μ−η1:η1-peroxo)diiron(III) species can be formed upon
reduction of dioxygen. A doubly bridged (μ-alkoxo)(μ-1,2-
peroxo) intermediate was obtained from the benzoate-bridged
diiron(II) precursor at −40 °C, in contrast to the triply bridged
(μ-alkoxo)(μ-1,2-peroxo)(μ-1,3-dimethylarsinato) intermediate
produced by the dimethylarsinate-bridged diiron(II) precursor.
The diphenylphosphinate-bridged diiron(II) precursor was
unique in that it formed a triply bridged peroxo intermediate
as the initial kinetic product, which converted to a metastable
doubly bridged peroxo intermediate by a shift of the
phosphinate from a bridging mode to a terminal monodentate
binding mode before peroxo decomposition, analogous to the
“carboxylate shift” notion described by Lippard 20 years ago.30
We concluded that the nature of the oxyanion bridge (O2X)
strongly influenced the nature and stability of any (μ−η1:η1-
peroxo)diiron(III) species formed, but we felt further work was
required before the effects of bridge differences could be clearly
understood. This work is intended to address that issue.
To this end, we focused on three effects: (i) those produced
by O2X O···O bite distance differences, (ii) those produced by
O2X electronic differences, and (iii) those produced by O2X
steric differences. Examination required synthesis of several new
diiron(II) complexes (1·O2X) using N-EtHPTB and various
O2X ligands. We found that all of these new species reacted
INTRODUCTION
■
Nonheme diiron enzymes have attracted great interest because
they perform a wide variety of reactions despite having very
similar active sites.1−8 For example, soluble methane mono-
oxygenase inserts an oxygen atom into an alkane C−H bond;
toluene and o-xylene monooxygenases, as well as phenol
hydroxylase, insert oxygen atoms into aromatic C−H bonds;
the R2 subunit of class I ribonucleotide reductases extracts a
hydrogen atom, producing a stable radical; and, as its name
implies, stearoyl acyl carrier protein Δ9-desaturase dehydrogen-
ates a fatty acid hydrocarbon chain. In addition to having
similar active site structures, these enzymes all share one other
property. The putative catalytic cycles of these enzymes all
contain peroxide-bridged diiron(III) complexes formed upon
reduction of molecular oxygen. In some cases, these
intermediates are stable enough to be trapped and charac-
terized.9−17 While peroxo-diiron(III) species are endemic in
these enzymes, their precise role in catalysis is poorly
understood, even though they have been well-studied. Synthetic
attempts to study peroxo-diiron(III) intermediates have
resulted in a variety of biomimetic complexes.18−27 These
compounds often form peroxide-bridged diiron(III) intermedi-
ates, especially when stabilized by carboxylate bridges. They are
commonly reported as having a (μ−η1:η1-peroxo)diiron(III)
motif.
In a previous publication,28 we discussed how replacing the
carboxylate bridges commonly employed in synthesis of these
biomimetic complexes affected the behavior of the peroxo
intermediates formed. Using the dinucleating ligand N-
EtHPTB [anion of N,N,N′,N′-tetrakis(2-benzimidazolylmeth-
Received: November 20, 2012
Published: February 22, 2013
© 2013 American Chemical Society
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Article
with O2 in solution to produce blue-green (μ−η1:η1-peroxo)-
diiron(III) intermediates (2·O2X) at −90 °C. With one
exception, they then converted to deep-blue (μ−η1:η1-peroxo)-
diiron(III) species (3·O2X) before decaying to yellow products
(4·O2X) at higher temperature. In most cases, addition of
OPPh3 to solutions of 2·O2X led to conversion to purple-blue
species (3′·O2X) similar to 3·O2X. In this paper, we report
crystallographic details of three diiron(II) complexes, kinetic
data on their reactions with dioxygen, and spectroscopic
characterization of a variety of (μ−η1:η1-peroxo)diiron(III)
intermediates. Implications of O2X bite distances as well as
electronic and steric differences are discussed.
powder. The solid was filtered and dried in vacuo. Recrystallization
from MeCN and Et2O produced milky crystals. Yield: 130 mg (50%).
Anal. Calcd for [Fe2 (N-EtHPTB)(O2 CCMe3 )](OTf)2
(C50H58F6Fe2N10O9S2): C, 48.71; H, 4.74; N, 11.36. Found: C,
48.91; H, 4.77; N, 11.39.
1·O2CC6H2-3,4,5-(OMe)3. N-EtHPTB (72.9 mg, 0.101 mmol) was
dissolved in MeOH (∼10 mL) along with Et3N (0.077 mL, 0.56
mmol). 3,4,5-Trimethoxybenzoic acid (21.4 mg, 0.101 mmol) was
added and allowed to dissolve. Fe(OTf)2·2MeCN (94.2 mg, 0.216
mmol) was added, producing a yellow solution. After 5 min, NaBPh4
(87.2 mg, 0.255 mmol) was added, resulting in immediate
precipitation of a pale yellow powder. The solid was filtered and
dried in vacuo. Recrystallization from MeCN and Et2O produced
yellow crystals, some suitable for X-ray diffraction structural analysis.
Yield: 116 mg (74%). Anal. Calcd for [Fe2(N-EtHPTB)(O2CCH2-
3,4,5-(OMe)3)](BPh4)(OTf) (C78H80BF3Fe2N10O9S): C, 61.92; H,
5.33; N, 9.26. Found: C, 62.14; H, 5.55; N, 8.94.
1·O2CC6H3-3,4-(OMe)2. N-EtHPTB (100.0 mg, 0.138 mmol) was
dissolved in MeOH (∼10 mL) along with Et3N (0.097 mL, 0.69
mmol). 3,4-Dimethoxybenzoic acid (25.1 mg, 0.138 mmol) was added
and allowed to dissolve. Fe(OTf)2·2MeCN (120.3 mg, 0.276 mmol)
was added, producing a yellow solution. After 5 min, NaBPh4 (196.5
mg, 0.574 mmol) was added, resulting in immediate precipitation of a
pale yellow powder. The solid was filtered and dried in vacuo.
Recrystallization from MeCN and Et2O produced milky crystals. Yield:
162 mg (89%). Anal. Calcd for [Fe2(N-EtHPTB)(O2CC6H3-3,4-
(OMe)2)](OTf)2 (C54H58F6Fe2N10O11S2): C, 49.40; H, 4.45; N,
10.67. Found: C, 49.65; H, 4.44; N, 10.39.
1·O2CC6H3-3,5-(OMe)2. N-EtHPTB (146.1 mg, 0.202 mmol) was
dissolved in MeOH (∼10 mL) along with Et3N (0.142 mL, 1.02
mmol). 3,5-Dimethoxybenzoic acid (37.0 mg, 0.203 mmol) was added
and allowed to dissolve. Fe(OTf)2·2MeCN (187.8 mg, 0.431 mmol)
was added, producing a yellow solution. After 5 min, NaBPh4 (190.6
mg, 0.557 mmol) was added, resulting in immediate precipitation of a
pale green-yellow powder. The solid was filtered and dried in vacuo.
Recrystallization from MeCN and Et2O produced pale yellow crystals.
Yield: 238.5 mg (80%). Anal. Calcd for [Fe2(N-EtHPTB)(O2CC6H3-
3,5-(OMe)2)](BPh4)(OTf)2 (C77H78BF3Fe2N10O8S): C, 62.36; H,
5.30; N, 9.44. Found: C, 62.56; H, 5.11; N, 9.33.
1·O2CC6H4-4-OMe. N-EtHPTB (149.2 mg, 0.206 mmol) was
dissolved in MeOH (∼10 mL) along with Et3N (0.144 mL, 1.03
mmol). 4-Methoxybenzoic acid (31.3 mg, 0.206 mmol) was added and
allowed to dissolve. Fe(OTf)2·2MeCN (189.2 mg, 0.433 mmol) was
added, producing a yellow solution. After 5 min, NaBPh4 (205.4 mg,
0.600 mmol) was added, resulting in immediate precipitation of a pale
green-yellow powder. The solid was filtered and dried in vacuo.
Recrystallization from MeCN and Et2O produced milky crystals. Yield:
245.4 mg (93%). Anal. Calcd for [Fe2(N-EtHPTB)(O2CC6H4-4-
OMe)](OTf)2 (C53H56F6Fe2N10O10S2): C, 49.62; H, 4.40; N, 10.92.
Found: C, 49.41; H, 4.51; N, 11.08.
Physical Methods. UV−vis spectra were recorded on a Hewlett-
Packard 8453 diode array spectrophotometer (2-nm resolution)
equipped with an Unisoku Scientific Instruments cryostat (Osaka,
Japan). Resonance Raman spectra were collected on an ACTON AM-
506M3 monochromator with a Princeton LN/CCD data collection
system using a Spectra-Physics Model 2060 krypton laser. Low-
temperature spectra of the peroxo intermediates in CH2Cl2 and
MeCN were obtained at 77 K using a 135° backscattering geometry.
Samples were frozen onto a gold-plated copper coldfinger in thermal
contact with a Dewar flask containing liquid nitrogen. Raman
frequencies were referenced to the features of indene. Slits were set
for a band-pass of 4 cm−1 for all spectra.
EXPERIMENTAL SECTION
■
Materials and Syntheses. All reagents and solvents were
purchased from commercial sources and were used as received, unless
noted otherwise. The ligand N-EtHPTB was synthesized using a
published procedure.31 Solvents were dried according to published
procedures and distilled under Ar prior to use.32 The 18O2 (97%) used
in resonance Raman experiments was purchased from Cambridge
Isotope Laboratories, Inc., Andover, MA. Preparation and handling of
air sensitive materials were carried out under an inert atmosphere by
using either standard Schlenk and vacuum line techniques or a
glovebox. Elemental analyses were performed by Atlantic Microlab,
Inc., Norcross, GA.
1·O2CPh, 1·O2PPh2, and 1·O2AsMe2 were synthesized using
published procedures.28
1·O2PMe2. N-EtHPTB (157 mg, 0.217 mmol) was dissolved in
MeOH (∼10 mL) along with Et3N (0.19 mL, 1.4 mmol).
Dimethylphosphinic acid (21.6 mg, 0.230 mmol) was added and
allowed to dissolve. Fe(OTf)2·2MeCN33 (189 mg, 0.434 mmol) was
added, producing a yellow solution. After 5 min, NaBPh4 (149 mg,
0.435 mmol) was added, resulting in immediate precipitation of a
white powder. The solid was filtered and dried in vacuo.
Recrystallization from MeCN and Et2O produced colorless crystals,
some suitable for X-ray diffraction structural analysis. Yield: 153 mg
(80%). Anal. Calcd for [Fe2(N-EtHPTB)(O2PMe2)](BPh4)(OTf)
(C70H75BF3Fe2N10O6PS): C, 60.27; H, 5.42; N, 10.04. Found: C,
59.82; H, 5.49; N, 10.38.
1·O2P(OPh)2. N-EtHPTB (176 mg, 0.243 mmol) was dissolved in
MeOH (∼10 mL) along with Et3N (0.19 mL, 1.4 mmol).
Diphenylphosphoric acid (67.7 mg, 0.271 mmol) was added and
allowed to dissolve. Fe(OTf)2·2MeCN (222 mg, 0.509 mmol) was
added, producing a yellow solution. After 5 min, NaBPh4 (173 mg,
0.506 mmol) was added, resulting in immediate precipitation of a
white powder. The solid was filtered and dried in vacuo.
Recrystallization from MeCN and Et2O produced milky crystals,
some suitable for X-ray diffraction structural analysis. Yield: 331 mg
(79%). Anal. Calcd for [Fe2(N-EtHPTB)(O2P(OPh)2)](BPh4)2
(C103H99B2Fe2N10O5P): C, 71.87; H, 5.80; N, 8.14. Found: C,
71.77; H, 5.94; N, 8.41.
1·O2CCPh3. N-EtHPTB (208 mg, 0.288 mmol) was dissolved in
MeOH (∼10 mL) along with Et3N (0.19 mL, 1.4 mmol).
Triphenylacetic acid (83.3 mg, 0.289 mmol) was added and allowed
to dissolve. Fe(OTf)2·2MeCN (257 mg, 0.589 mmol) was added,
producing a yellow solution. After 5 min, NaBPh4 (205 mg, 0.598
mmol) was added, resulting in immediate precipitation of a white
powder. The solid was filtered and dried in vacuo. Recrystallization
from MeCN and Et2O produced colorless crystals. Yield: 370 mg
(73%). Anal. Calcd for [Fe2(N-EtHPTB)(O2CCPh3)](BPh4)2
(C111H104B2Fe2N10O3): C, 75.78; H, 5.96; N, 7.96. Found: C, 75.46;
H, 5.95; N, 7.96.
1·O2CCMe3. N-EtHPTB (153 mg, 0.212 mmol) was dissolved in
MeOH (∼10 mL) along with Et3N (0.19 mL, 1.4 mmol).
Trimethylacetic acid (21.7 mg, 0.212 mmol) was added and allowed
to dissolve. Fe(OTf)2·2MeCN (186 mg, 0.426 mmol) was added,
producing a yellow solution. After 5 min, NaBPh4 (152 mg, 0.443
mmol) was added, resulting in immediate precipitation of a white
Time-resolved spectra of rapid oxygenation reactions were acquired
with a Hi-Tech Scientific (Salisbury, Wiltshire, UK) SF-43 multimixing
anaerobic cryogenic stopped-flow instrument combined with either a
monochromator (low intensity light irradiation of the sample) or a
diode array rapid scanning unit (strong UV−vis irradiation of the
sample), or with a TgK Scientific (formerly HiTech Scientific,
Salisbury, Wiltshire, UK) SF-61DX2 cryogenic stopped-flow system
equipped with a J&M diode array (Spectralytics). All manipulations
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Table 1. Crystal Data and Structure Refinement for 1·O2PMe2(BPh4)(OTf)·MeCN, 1·O2P(OPh)2(BPh4)2·MeCN, and
(1·O2CC6H2-3,4,5-(OMe)3)2(BPh4)2(OTf)2·2MeCN
[1·O2PMe2](BPh4)(OTf)·MeCN
[1·O2P(OPh)2](BPh4)2·MeCN
[1·O2CC6H2-3,4,5-(OMe)3]2(BPh4)2(OTf)2·2MeCN
empirical formula
fw
C72H78BF3Fe2N11O6PS
1435.99
173(2)
C105H102B2Fe2N11O5P
1762.27
173(2)
C160H166B2F6Fe4N22O18S2
3108.29
T (K)
173(2)
Mo Kα λ, Å
space group
a (Å)
0.71073
P21/c
0.71073
P21/n
0.71073
P1
̅
16.0709(9)
15.6360(9)
28.4703(15)
90
23.063(4)
18.942(4)
23.888(4)
90
12.576(4)
19.153(7)
36.053(12)
96.743(5)
92.147(5)
103.100(5)
8381(5)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
103.5800(10)
90
99.205(3)
90
6954.1(7)
4
10301(3)
4
Z
ρ (calc), Mg/m3
1.372
1.136
1.232
abs coeff (mm−1
)
0.539
0.352
0.437
a
R1
0.0398
0.0446
0.0713
b
wR2
0.0984
0.1271
0.1955
a
b
2
2
R1 = ∑∥Fo| − |Fc∥/∑|Fo|. wR2 = [∑[w(Fo − Fc2)2]/∑[w(Fo )2]]1/2
.
Table 2. Selected Interatom Distances and Bond Angles for [Fe2(N-EtHPTB)(O2X)]2+
a
a
b
a
1·O2AsMe2
1·O2PPh2
1·O2PMe2
1·O2P(OPh)2
1·O2CC6H2-3,4,5-(OMe)3
1·O2CPh
τave
0.81
0.77
0.82
0.91
0.89
0.93
Interatom Distances (Å)
2.0184(18)
1.9995(18)
2.323(2)
Fe1−O1
Fe2−O1
Fe1−N1
Fe2−N2
Fe1−O2
Fe2−O3
Fe1−N3
Fe1−N5
Fe2−N7
Fe2−N9
As1/P1/C44−O2
As1/P1/C44−O3
Fe1···Fe2
O2···O3
2.0145(13)
2.0037(14)
2.3366(16)
2.3892(16)
1.9825(14)
1.9863(14)
2.1372(17)
2.1023(18)
2.1088(17)
2.0759(17)
1.6740(15)
1.6776(14)
3.5357(5)
2.004(2)
1.992(2)
2.294(3)
2.352(3)
2.008(2)
2.021(2)
2.086(3)
2.115(3)
2.075(3)
2.098(3)
1.514(2)
1.512(2)
2.0057(16)
2.0031(15)
2.3141(18)
2.309(2)
1.967(3)
1.970(3)
2.328(4)
2.314(4)
2.036(3)
2.025(3)
2.045(4)
2.046(4)
2.063(4)
2.044(4)
1.254(5)
1.273(5)
3.4879(13)
2.2253(46)
1.976(5)
1.964(5)
2.316(6)
2.280(7)
2.057(5)
2.019(6)
2.064(6)
2.069(6)
2.080(6)
2.064(6)
1.264(9)
1.253(9)
2.372(2)
2.0037(19)
2.0123(18)
2.105(2)
2.0562(16)
2.0506(16)
2.0668(19)
2.0732(19)
2.0551(19)
2.075(2)
2.127(2)
2.098(2)
2.080(2)
1.509(2)
1.4762(17)
1.4784(18)
3.6211(7)
2.5520(24)
1.517(2)
3.5405(10)
2.5600(32)
3.5364(6)
3.4749(31)
2.2251(74)
2.7991(21)
2.5545(28)
Bond Angles (deg)
123.32(9)
Fe1−O1−Fe2
O1−Fe1−O2
O1−Fe2−O3
Fe1−O2−As1/P1/C44
Fe2−O3−As1/P1/C44
123.27(6)
105.77(6)
107.92(6)
127.24(8)
128.15(8)
113.26(7)
124.76(11)
105.55(9)
99.44(9)
129.19(8)
98.99(6)
124.73(15)
98.27(13)
100.51(13)
138.1(3)
123.8(2)
98.6(2)
102.50(8)
103.98(8)
98.93(7)
101.5(2)
136.4(5)
133.9(5)
124.2(7)
132.01(14)
138.88(15)
115.59(13)
131.47(12)
132.54(12)
115.17(12)
133.92(11)
133.24(10)
119.47(10)
134.7(3)
O2−As1/P1/C44−O3
123.4(4)
a
b
Data from ref 28. The unit cell of this compound contains virtual twins of 1·O2CC6H2-3,4,5-(OMe)3 produced through a pseudoinversion center.
As there is no substantive difference between the two cations, values listed here are taken from a single cation.
with diiron(II) complexes and their solutions were done using an
argon atmosphere glovebox, airtight syringes, and the anaerobic
stopped-flow instrument to avoid contamination with air. Saturated
solutions of O2 in CH2Cl2 and CH3CN were prepared by bubbling the
dry O2 gas for 20 min in a septum-closed cylinder with the solvent at a
constant temperature (20 or 25 °C). The solubility of O2 was accepted
to be 5.8 mM in dichloromethane at 20 °C and 8.1 mM in acetonitrile
at 25 °C.34 Solutions of O2 with smaller concentrations were prepared
by diluting the O2-saturated solution with argon-saturated solvent
using graduated gastight syringes equipped with three-way valves. For
the kinetic experiments, dioxygen was always taken in large excess so
that its concentration did not change significantly during the reaction
with 1·O2X. The solutions of 1·O2X and O2 were cooled to a preset
temperature ( 0.1 °C) in the stopped-flow instrument before mixing.
Data analysis was performed with the IS-2 Rapid Kinetics Software
(Hi-Tech Scientific) for kinetic traces at a single wavelength.
X-ray Crystallography. X-ray diffraction data were collected on a
Bruker SMART platform CCD diffractometer at 173(2) K.35
Preliminary sets of cell constants were calculated from reflections
harvested from three sets of 20 frames. These initial sets of frames
were oriented such that orthogonal wedges of reciprocal space were
surveyed. The data collection was carried out using Mo Kα radiation
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Article
(graphite monochromator). Randomly oriented regions of reciprocal
space were surveyed to the extent of one sphere and to a resolution of
0.84 Å. The intensity data were corrected for absorption and decay
using SADABS.36 Final cell constants were calculated after integration
with SAINT.37 The structures were solved and refined using SHELXL-
97.38 The space groups P2 /c, P2 /n, and P1 were determined on the
̅
1
1
basis of systematic absences and intensity statistics. Direct-methods
solutions were calculated, which provided most non-hydrogen atoms
from the E-map. Full-matrix least-squares/difference Fourier cycles
were performed, which located the remaining non-hydrogen atoms. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were placed in ideal positions and
refined as riding atoms with relative isotropic displacement parameters.
The SQUEEZE function of the program PLATON39 was used to
remove 143 effective electrons in diffuse scattering from a volume of
1555.2 Å3 per cell of 1·O2P(OPh)2(BPh4)2·MeCN and 120 effective
electrons in diffuse scattering from a volume of 1097.4 Å3 per cell of
(1·O2CC6H2-3,4,5-(OMe)3)2(BPh4)2(OTf)2·2MeCN. Brief crystal
data and intensity collection parameters for the crystalline complexes
are shown in Table 1.
RESULTS
■
X-ray Crystallography. For this work, we synthesized
diiron(II) complexes using a variety of O2X ligands, namely
O2PMe2, O2P(OPh)2, and several aliphatic and aromatic
carboxylates, to augment a list that includes O2AsMe2,
O2PPh2, and benzoate. We were able to obtain crystal
structures of three of these 1·O2X compounds [O2X =
O2PMe2, O2P(OPh)2, and O2CC6H2-3,4,5-(OMe)3] (Table
2). Like those of previously published complexes,28 the new
complexes possess two distorted trigonal bipyramidal iron(II)
centers, with N-EtHPTB amine nitrogen atoms and oxygen
atoms from the bridging O2X moiety occupying the axial
positions. The equatorial sites are occupied by two
benzimidazole nitrogen atoms and the alkoxide oxygen atom
(Figure 1). With few exceptions, the respective atoms in the
first coordination sphere of each complex reflect approximately
equal interatomic distances and angles. Differences of note
include average τ values,40 O2X bite distances, O−X−O angles,
and inter-iron distances. The τ values can be grouped into two
sets with τave ∼ 0.8 (O2X = O2AsMe2, O2PPh2, and O2PMe2)
and τave ∼ 0.9 [O2X = O2P(OPh)2, O2CC6H2-3,4,5-(OMe)3,
and O2CPh]. Comparing bite distances of all complexes, we see
O···O distances of ∼2.55 Å (X = P) and ∼2.23 Å (X = C). The
lone X = As complex is in a class of its own with a bite distance
of ∼2.80 Å. It is also apparent that the O−X−O angle varies
with the identity of the X atom (X = As, 113.26°; X = P,
115.17°−119.47°; X = C, 123.4°−124.2°). The O−X−O angle
measurement correlates inversely with O2X bite distance; the
species with the longest O···O distance (1·O2AsMe2) has the
most acute O−X−O angle, while the X = C complexes have the
shortest bite distances and largest O−X−O angles, with the X =
P compounds falling in between these extremes. This inverse
Figure 1. Crystal structure of the cation 1·O2CC6H2-3,4,5-(OMe)3
(50% ellipsoids) with hydrogen atoms removed. Generic cartoon of
1·O2X: O2X = O2AsMe2 (1·O2AsMe2), O2PPh2 (1·O2PPh2), O2PMe2
(1·O2PMe2), O2P(OPh)2 (1·O2P(OPh)2), O2CC6H2-3,4,5-(OMe)3
(1·O2CC6H2-3,4,5-(OMe)3), and O2CPh (1·O2CPh).
of all the compounds, exceeding the distances found in
1·O2AsMe2 (3.5357 Å) and the two other X = P species
(3.5405 and 3.5364 Å) by ∼0.08 Å. This is greater than the
∼0.06 Å difference observed between the Fe···Fe distances of
the X = C complexes (3.4879 and 3.4749 Å) and those with
Fe···Fe distances of ∼3.54 Å. The unusually long Fe···Fe
distance in 1·O2P(OPh)2 is most likely not due to packing
effects, because a similar inter-iron distance of 3.649 Å was
reported when 1·O2P(OPh)2 crystallized in a different space
group (P1) with different anions (ClO ).41
̅
4
There are two crystallographically interesting observations
regarding the compounds we report here and those examined
in our previous work.28 First, we note that [Fe2(N-EtHPTB)-
(O2PMe2)](BPh4)(OTf)(MeCN) and [Fe2(N-EtHPTB)-
(O2AsMe2)](BPh4)(OTf)(MeCN) are crystallographically iso-
structural. Both fall into the space group P21/c with Z = 4 and
share virtually identical unit cells with respective values for a, b,
and c of 16.0709(9) vs 16.1151(12) Å, 15.6360(9) vs
15.7370(12) Å, and 28.4703(15) vs 28.473(2) Å. The α and
γ angles are exactly 90° and the β angles are respectively
103.5800(10)° and 103.4900(10)°. The respective cations,
anions, and solvent molecules in each unit cell lie in essentially
the same positions with the same orientations. The minor
variations in unit cell parameters and atom positions arise from
differences around the X atom of the O2X bridging moieties.
Small angle changes around X as well as X−C and X−O bond
length variations produce minor differences in the cations that
slightly alter the position of every other atom in the unit cell
due to packing effects. The second observation is that the unit
correlation reflects variations in X−O bond lengths (As−Oave
=
1.676 Å, P−Oave = 1.501 Å, C−Oave = 1.261 Å). Even though
the O−X−O angle decreases, concomitant lengthening of the
X−O bonds results in a greater O···O distance.
In four of the six diferrous complexes crystallographically
characterized, increased bite distance correlates to increased
Fe···Fe distance. With the other two complexes, we observed
that (i) 1·O2AsMe2 has an O···O distance ∼0.24 Å longer than
any other complex, although its inter-iron distance is within
0.01 Å of those found in both 1·O2PPh2 and 1·O2PMe2. (ii)
1·O2P(OPh)2 has an O···O distance on par with 1·O2PPh2 and
1·O2PMe2, yet its inter-iron distance (3.6211 Å) is the greatest
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Figure 2. ORTEP diagram (50% ellipsoids) of the unit cell containing two molecules of [Fe2(N-EtHPTB)(O2CC6H2-3,4,5-(OMe)3)]2+ and
accompanying anions and solvent molecules (hydrogen atoms removed for clarity). There is a pseudoinversion center (ic) between the cations.
Comparing the cations, we find that the “inversion” is almost true, whereas comparing the BPh4 ions reveals differences in thermal ellipsoid size and
minor differences in atom locations, indicating that the inversion center is not real.
cell containing the cation [Fe2(N-EtHPTB)(O2CC6H2-3,4,5-
(OMe)3)]2+ contains a pseudoinversion center lying between
cations (Figure 2). This effectively doubles the number of
unique elements per unit cell, thus doubling the size of the unit
cell. The cation “inversion” is reasonably true, but the anions
are farther away from the pseudoinversion center, resulting in
greater distortion during “inversion”. Because the differences
between the cations are not significant, we chose to focus on
one cation rather than redundantly discussing what essentially
amounts to duplicate cations.
Observation of Two Peroxo Intermediates. Upon
reaction with O2, the virtually colorless solutions of 1·O2X
produce blue-green O2 adducts (2·O2X), which in all cases
Figure 3. Selected UV−vis spectra obtained in the reaction of O2 and
except two convert to deep blue species (3·O2X) before
1·O2CCPh3 in CH2Cl2 at −90 °C showing initial formation of
decaying to yellow final products (4·O2X). Previously, we
2·O2CCPh3 (solid green line) and subsequent conversion to
reported the formation of 2·O2PPh2 and its conversion to
3·O2PPh2 at −30 °C.28 Corresponding studies with O2X =
benzoate afforded only evidence for the formation of 3·O2CPh
under these conditions.29,42,43 However, visible spectral
evidence was obtained for the formation of both 2·O2CR and
3·O2CR by going to −90 °C in CH2Cl2, as shown in Figure 3
for O2X = O2CCPh3 with respective λmax values at 708 and 630
nm. In general, the visible chromophores of 2·O2X and 3·O2X
fall in the ranges of 630−710 and 580−620 nm, respectively
(Table 3), which have been previously assigned to peroxo-to-
iron(III) charge transfer transitions for O2X = benzoate,
Ph2PO2, and Me2AsO2 by resonance Raman spectroscopy.28,42
Resonance Raman spectra for 2·O2CCPh3, 3·O2CCPh3,
2·O2P(OPh)2, and 3·O2P(OPh)2 shown in Figure 4 are
representative of the species observed in this study. 16O2 and
18O2 labeling experiments reveal O−O stretching frequencies
(Table 4) that by comparison to those previously published28,42
allow us to sort them into two clusters. The 2·O2X cluster
exhibits νO−O values ranging from 839 to 851 cm−1 that
3·O2CCPh3 (solid blue line). Dotted lines correspond to spectra
leading to the appearance of 2·O2CCPh3 within the first 40 s, while
dashed lines correspond to spectra associated with the conversion of
2·O2CCPh3 to 3·O2CCPh3 over the course of the following hour.
downshift to 791−807 cm−1 upon 18O substitution, while the
3·O2X cluster has νO−O values ranging from 897 to ∼910 cm−1
that downshift to 845−857 cm−1 with 18O incorporation. On
the other hand, no systematic difference is observed for the
νFe−O values of the 2·O2X and 3·O2X complexes, all of which
range from 457 to 479 cm−1 (Table 4). These features can be
assigned to the symmetric Fe−O stretch of the Fe−O2−Fe
moiety and often appear as Fermi doublets. The difference in
the νO−O values of 2·O2X and 3·O2X complexes has previously
been attributed to a change in the Fe−Fe distance in the O2
adducts on the basis of a systematic study of 1,2-peroxo-bridged
diiron(III) complexes by Fiedler et al.27 For the 2·O2X and
3·O2X complexes, the change in the Fe−Fe distance indicated
by the difference in the νO−O values is postulated to derive from
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Table 3. UV−Vis Properties of 2·O2X, 3·O2X and 3′·O2X in
Table 4. Fe−O and O−O Stretching Frequencies of 2·O2X
and 3·O2X Complexes Determined Using Resonance Raman
Spectroscopy
CH2Cl2
λmax (nm), [ε] (M‑1 cm‑1)
2·O2X
3·O2X
O2X
2·O2X
3·O2X
3′·O2X
νO−O
(cm‑1)
[18O2]
a
a
b
νO−O
(cm‑1)
[18O2]
O2AsMe2
O2PMe2
O2PPh2
632 [2100]
674 [1800]
678 [2100]
680 [2800]
708 [2500]
−
−
−
νFe−O (cm‑1)
νFe−O (cm‑1)
[18O]
616 [2400]
592 [2600]
594 [3600]
595 [3300]
577 [3800]
592 [2300]
592 [4300]
590 [5300]
592 [5600]
590 [4000]
O2X
O2AsMe2
[18O]
621 [1800]
576 [3100]
630 [2200]
580 [3100]
588 [1500]
590 [3200]
578 [6200]
588 [5400]
580 [3000]
464 [433]
845 [796]
839 [791]
−
−
−
−
O2P(OPh)2
O2PMe2
467, 479
O2CCPh3
[449, 460]
c
O2CCMe3
706 [−]
704 [−]
O2PPh2
465, 476
[455]
845, 853
[807]
477 [458]
479 [460]
897 [848]
897 [845]
c
d
O2CPh
O2P(OPh)2
O2CCPh3
O2CCMe3
457, 469
851 [806]
841 [792]
−
O2CC6H2-3,4,5-(OMe)3
O2CC6H3-3,4-(OMe)2
O2CC6H3-3,5-(OMe)2
O2CC6H4-4-OMe
707 [2600]
[436, 446]
c
704 [−]
706 [−]
705 [−]
466, 474
466, 479
903, 917
[857]
c
[447, 454]
[448, 456]
c
−
466, 475
897, 912
[846]
a
[447, 457]
Conversion to 3·O2X did not take place at any temperature.
b
a
Conversion to 3′·O2X did not take place at any temperature, even
O2CPh
−
−
−
−
476 [460]
900 [850]
c
with addition of 100 equiv of OPPh3. Conversion of 2·O2X to 3·O2X
began before complete formation of 2·O2X, so ε was not determined.
O2CC6H2-3,4,5-
(OMe)3
465, 476
900, 914
[853]
[449, 457]
d
Values from ref 42.
O2CC6H3-3,4-
(OMe)2
−
−
−
−
−
−
466, 474
898, 912
[847]
[449, 456]
O2CC6H3-3,5-
(OMe)2
465, 476
899, 913
[848]
[449, 458]
O2CC6H4-4-OMe
466, 474
899, 913
[847]
[448, 457]
a
Values from ref 42.
Scheme 1. Conversion of 2·O2X to 3·O2X or 3′·O2X
Figure 4. Resonance Raman spectra of 2·O2CCPh3 (A), 3·O2CCPh3
(B), 2·O2P(OPh)2 (C), and 3·O2P(OPh)2 (D). Solid red lines (16O2)
and dotted blue lines (18O2); T = 77 K, λex = 647 nm.
the conversion of the O2X moiety from a bidentate bridging
ligand in 2·O2X to a terminal monodentate ligand in 3·O2X
(Scheme 1).
Attempts to obtain resonance Raman spectra for either
2·O2CC6H2-3,4,5-(OMe)3 or 2·O2CC6H3-3,4-(OMe)2 were
unsuccessful, even though they exhibited a longer half-life
than 2·O2CCPh3. A color change was observed during the
freezing process. Their respective Raman spectra were
comparable to those of 3·O2CC6H2-3,4,5-(OMe)3 and
3·O2CC6H3-3,4-(OMe)2, indicating that conversion took
place during the phase change (Figure S1, Supporting
Information). On the other hand, the longer lifetimes of the
3·O2X intermediates allowed us to collect spectra of every one
(Figures 4 and S1, Supporting Information), with the exception
of 3·O2PMe2, which did not form in quantities sufficient for
characterization. The resonance Raman spectrum of 3′·O2PPh2,
formed by treatment of 2·O2PPh2 with an excess of OPPh3
(Figure 5), was also acquired. As it was nearly identical to the
previously reported spectrum of 3′·O2CPh,27 we did not carry
Figure 5. Resonance Raman spectra of 3′·O2PPh2 (16O2 = solid red
line, 18O2 = dotted blue line).
out the corresponding experiments with other 3′·O2X
intermediates.
Conversion of 2·O2X to 3·O2X. The spectral differences
between 2·O2X and 3·O2X make UV−vis absorption spectros-
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copy an excellent tool for following the transition from the first
peroxo intermediate to the second. On the basis of the data
shown in Figure 3, analysis of the spectral changes observed
upon oxygenation of 1·O2CCPh3 at −90 °C afforded a first-
order rate constant of 1.7 × 10−3 s−1 for the conversion of
2·O2CCPh3 to 3·O2CCPh3. This value is comparable to that
previously reported for the conversion of 2·O2PPh2 to
3·O2PPh2 at −40 °C, thus implicating a more facile conversion
for the carboxylate-bridged species. Figure 6 provides evidence
Table 5. pKa Values of HO2X and First-Order kobs Values for
Conversion of 2·O2X to 3·O2X and 2·O2X to 3′·O2X
kobs (s‑1)
pKa of
a
b
O2X
O2AsMe2
HO2X
2 → 3
2 → 3′
c
c
d
6.27
−
−
−
e
O2PMe2
3.08
1.8(4) × 10−4
(−40 °C)
e
f
O2PPh2
2.32
3.1 × 10−4
4.3(4) × 10−3
(−40 °C)
(−40 °C)
e
O2P(OPh)2
1.85
1.2(1) × 10−4
4.2(6) × 10−4
3.1(1) × 10−2
O2CC6H2-3,4,5-
(OMe)3
4.24
0.13(5)
O2CC6H3-3,4-
(OMe)2
4.36
6.6(9) × 10−4
0.12(1)
O2CCPh3
3.96
3.97
1.7(2) × 10−3
4.8(2) × 10−3
2.6(3) × 10−2
O2CC6H3-3,5-
(OMe)2
0.20(1)
O2CC6H4-4-OMe
O2CCMe3
4.50
5.03
4.19
4.7(4) × 10−3
1.8(1) × 10−2
0.12(1)
8.0(3) × 10−2
g
h
O2CPh
5.3(1) × 10−2
(−80 °C)
−
a
b
All rates measured at −90 °C, except where noted. All rates
Figure 6. Selected UV−vis spectra obtained in the reaction of O2 with
1·O2CPh at −90 °C showing evidence for the formation of two peroxo
intermediates. The solid green line represents the spectrum with the
largest fraction of 2·O2CPh formed, while the solid blue line
corresponds to the subsequently formed 3·O2CPh. Dotted lines
correspond to spectra leading to the maximum amount of 2·O2CPh
formed within the first 90 s after oxygenation, while dashed lines
correspond to spectra associated with the conversion of 2·O2CPh to
3·O2CPh over the course of the following 30 min.
measured after addition of 20 equiv of OPPh3 at −90 °C, except where
c
noted. Conversion to observable concentrations of 3·O2X did not
d
take place at any temperature. Conversion to observable concen-
trations of 3′·O2X did not take place at any temperature, even with
e
f
addition of 100 equiv of OPPh3. Measured in 7% EtOH (ref 53). At
−40 °C in CH2Cl2, 3·O2PPh2 starts to decay before complete
conversion from 2·O2PPh2 occurs. For this reason, kobs was calculated
from the y-intercept of the OPPh3 concentration dependence plot for
the conversion of 2·O2PPh2 to 3′·O2PPh2 (Figure S2, Supporting
g
Information). Rate measured at −80 °C using stopped-flow
h
techniques. By the time enough 2·O2CPh had formed to allow for
addition of OPPh3, significant conversion to 3·O2CPh had occurred,
preventing accurate rate determination for the conversion of 2·O2CPh
to 3′·O2CPh.
for the formation of two peroxo species at −90 °C even for the
well-studied complex 1·O2CPh,29,42,44 although they are not as
spectroscopically distinct as those of 2·O2CCPh3 and
3·O2CCPh3. Stopped-flow kinetic analysis at −80 °C revealed
the conversion from 2·O2CPh to 3·O2CPh with a kobs of 5.3 ×
10−2 s−1, much faster than that observed for 2·O2CCPh3 to
3·O2CCPh3 at −90 °C. This comparison provides a rationale
for why 2·O2CPh was not observed previously.29,42,44 The rate
of conversion of 2·O2X to 3·O2X decreased as electron-
donating substituents were introduced onto the benzoate ring
or as the steric bulk of the carboxylate bridge was increased
(Table 5). The rate of conversion was further decreased by
replacing the trigonal carboxylate moiety with tetrahedral
2·O2AsMe2 appears to be unaffected by addition of up to 100
equiv of OPPh3, even at temperatures as high as 20 °C.
After finding evidence indicating the formation of 2·O2CPh,
we opted to further examine the oxygenation kinetics of both
1·O2PPh2 and 1·O2CPh using stopped-flow techniques.
1·O2PPh2 offers an excellent opportunity to characterize the
initial oxygen coordination with the formation of 2·O2PPh2,
since the subsequent rearrangement of 2·O2PPh2 to 3·O2PPh2
is relatively slow. The reaction between 1·O2PPh2 and O2 in
either MeCN or CH2Cl2, which is accompanied by rapid
growth of the absorption band with λmax ≈ 680 nm, was
studied. At low temperatures (down to −80 °C in CH2Cl2), the
initially formed diiron-O2 intermediate 2·O2PPh2 is stable for at
least 1 h. However, noticeable decay of this intermediate was
observed under intense illumination with polychromatic, UV-
rich light of the arc lamp. In order to avoid complications from
undesirable photodecomposition, quantitative kinetic measure-
ments were performed in a single-wavelength mode, with low-
intensity monochromatic light and a sensitive photomultiplier
detector. Under these conditions, photobleaching was not
observed. In order to determine the rate law, kinetic traces were
acquired under a large excess of O2, showing single-exponential
growth of 2·O2PPh2 and yielding the values of observed
pseudo-first-order rate constants, kobs. The observed rate
constants did not depend on the initial concentration of
1·O2PPh2, in agreement with a rate law that is first-order in the
diiron precursor. The observed rate constants increased linearly
−
−
−
anions such as (PhO)2PO2 , Ph2PO2 , Me2PO2 , and
−
Me2AsO2 (Table 5).
In our previous publication,28 we postulated that the
conversion of 2·O2X to 3·O2X in acetonitrile involves a change
in O2X binding mode from bridging to terminal and
coordination of a solvent molecule in position L (Scheme 1).
For this work we added OPPh3 to solutions of 2·O2X in hopes
of substituting it into the L position. Upon OPPh3 addition,
purple-blue species (3′·O2X) indeed formed with λmax values at
wavelengths near each respective 3·O2X, although the
extinction coefficients of the new intermediates are higher
than those observed for each respective 3·O2X (Table 3), with
the exception of the O2CC6H3-3,4-(OMe)2-based species.
While facile conversion to 3′·O2X from most 2·O2X
intermediates was accomplished by adding 20 equiv of
OPPh3 at −90 °C, 2·O2PMe2 and 2·O2PPh2 had to be warmed
to −40 °C to attain reasonable rates of conversion. In contrast,
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with an increase in the concentration of O2 (Figure S3,
Supporting Information), indicating that the reaction was first-
order in dioxygen. It can be concluded that the formation of
2·O2PPh2 is a second-order process (first-order in diiron
complex and first-order in O2): v = k2[1·O2PPh2][O2]
As the temperature was raised, the oxygenation rate of
1·O2PPh2 increased modestly, and the conversion of 2·O2PPh2
to 3·O2PPh2 became pronounced. The Eyring plot (Figure 7)
kinetic trace becomes flat. At shorter wavelengths, a
biexponential absorbance increase was seen. Two-exponential
fit of the variable-wavelengths data gave the following value of
the observed rate constant: k1 = 0.22 s−1 at −80 °C (76 M−1 s−1
under the assumption of a second-order process). This rate
constant corresponds to the first reaction step, formation of
2·O2CPh. Interestingly, it is in excellent agreement with the
reported value of the rate constant for the reaction of 1·O2CPh
with O2 (135 M−1 s−1 at −75 °C in propionitrile, with the
activation parameters of ΔH⧧ = 15.4 kJ/mol, ΔS⧧ = −121 J K−1
mol−1).43 In that work, 3·O2CPh was characterized spectro-
scopically, and it was assumed that the kinetics of oxygenation
corresponded to the formation of 3·O2CPh. On the basis of our
studies, the kinetic parameters reported by Feig et al.43
correspond to the first oxygenation step, the formation of
2·O2CPh; the next step, conversion of 2·O2CPh to 3·O2CPh, is
very rapid at most temperatures.
DISCUSSION
■
One step in the catalytic activation of dioxygen by biological
diiron(II) systems often produces (μ−η1:η1-peroxo) diiron(III)
moieties. Some of these intermediates exhibit a level of stability
that allows for them to be trapped and characterized.9−16 The
same is true for many synthetic (μ−η1:η1-peroxo) diiron(III)
complexes,18,25,27,41,47 some of which are so stable they have
been crystallographically characterized.19−22 In previously
published work,28 we used the dinucleating ligand N-EtHPTB
and different oxyanions (O2X) to synthesize a set of three
dioxygen binding diiron(II) complexes (1·O2X). Our efforts
revealed that the (μ−η1:η1-peroxo) diiron(III) intermediates
produced upon oxygenation of the diiron(II) precursors come
in two forms, green-blue 2·O2X and deep-blue 3·O2X (Scheme
1). In the initial form, the oxyanion acts as a three-atom bridge
between the iron centers and the pendant benzimidazoles are
cis to each other on each iron. In many cases, this intermediate
converts to a second form, wherein the O2X ligand has moved
to a terminal position, allowing the pendant benzimidazoles of
the N-EtHPTB ligand to rearrange from a cis to a trans
disposition on each iron. We found that the stability of 2·O2X is
influenced by the identity of O2X (O2X = O2AsMe2, O2PPh2,
and O2CPh) and concluded that the dominant factor governing
2·O2X stability is the bite distance (O···O) of the O2X moiety
in 1·O2X as determined by X-ray crystallography. Anions with
greater bite distances are better able to accommodate the >3 Å
Fe···Fe distance in 2·O2X and gave rise to more stable 2·O2X
intermediates; for X = O2AsMe2, the 2·O2X intermediate was
so stable that no observable conversion to 3·O2X was observed
prior to decomposition.
Figure 7. Eyring plot for O2 binding to 1·O2PPh2 in CH2Cl2.
for the oxygen binding step was linear over a broad temperature
range (from −80 to −20 °C in CH2Cl2), yielding the activation
parameters that are summarized in Table 6. The kinetics of
oxygen binding to 1·O2PPh2 in MeCN was also examined, and
very similar activation parameters were extracted (Table 6).
Subsequent disappearance of 2·O2PPh2 was readily observed in
MeCN at temperatures above −40 °C (Figure S4, Supporting
Information). The spectral changes agreed well with the
conversion of 2·O2PPh2 into 3·O2PPh2 described in detail by
Frisch et al.28 Low activation enthalpies and large negative
activation entropies for the oxygenation of 1·O2PPh2 leading to
2·O2PPh2 are typical of associative O2 coordination at vacant or
labile iron sites,34,45 which are present in the starting diiron(II)
complex. The rates and activation parameters for the
oxygenation of 1·O2PPh2 in CH2Cl2 or in acetonitrile are
very similar to previously published kinetic parameters for the
oxygenation of 1·O2CPh in propionitrile.43 The kinetics of
oxygenation of five-coordinate diiron(II) complexes with a
related dinucleating ligand HPTP is also very similar (Table
6).46
A closer examination of dioxygen binding to 1·O2CPh
identified the individual reaction steps of the overall reaction.
Stopped-flow observation of the reaction of 1·O2CPh with
dioxygen (2.9 mM) in CH2Cl2 at −80 °C reveals a two-step
process (Figure S5, Supporting Information), in agreement
with sequential formation of intermediates 2·O2CPh and
3·O2CPh. At longer wavelengths (greater than ca. 650 nm),
the initial absorbance increase takes place within 15 s and is
followed by decay that is complete within 100 s. At 640 nm, the
initial absorbance increase was also seen, but after 15 s the
For this work, we synthesized several more 1·O2X complexes
to assess additional factors that may affect the stability of
2·O2X. Specifically, we were interested in examining effects
produced by electronic and steric changes in O2X and how
those results relate to effects produced by differences in O2X
Table 6. Kinetic Parameters for Reactions of Diiron(II) Complexes with Dioxygen
complex
1·O2PPh2
1·O2PPh2
1·O2CPh
[Fe2(HPTP)(O2CPh)]2+
[Fe2(HPTP)(O2CPh)]2+
solvent
T range (°C)
k (−40 °C) (M‑1 s‑1)
ΔH⧧ (kJ/mol)
ΔS⧧ (J/K·mol)
ref
CH2Cl2
MeCN
EtCN
−80 to −20
−40 to 0
−75 to −15
−40 to 0
4.7 × 102
2.55 × 103
1.03(12)x102
7.3 × 103
67
15.7(4)
13.7(4)
15.4(6)
15.8(4)
16.7(2)
−124(6)
−119(10)
−121(3)
−101(10)
−132(8)
this work
this work
43
MeCN
CH2Cl2
46
−80 to 0
46
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Inorganic Chemistry
Article
bite distances. We also investigated how OPPh3 could be used
to destabilize some 2·O2X intermediates. In addition, we used
low-temperature stopped-flow techniques to demonstrate that
the oxygenation of 1·O2CPh was in fact a two-step process like
those we have described for the other 1·O2X complexes.
Indeed, 2·O2CPh is short-lived, even at −90 °C, so the adduct
we originally observed at −40 °C and assigned to be 2·O2CPh
in 199029,42 is in actuality the more stable 3·O2CPh isomer.
Finally, we determined the activation parameters for the
oxygenation of 1·O2PPh2 in CH2Cl2 and in MeCN and found
them to be very similar to those reported earlier for 1·O2CPh in
EtCN (Table 6),43 strongly suggesting a common rate-
determining step for these reactions, corresponding to the
formation of 2·O2X.
meta and/or para positions of benzoate-based oxyanions is the
primary factor that stabilizes the resultant 2·O2X complexes.
The lifetimes of 2·O2CR complexes are shortened upon
addition of OPPh3. We previously found that OPPh3 had a
good binding affinity for the parent FeIII2(N-Et-HPTB)-peroxo
complex and exerted such a significant stabilizing effect that its
presence led to the crystallization and structural character-
ization of [FeIII2(μ-1,2-O2)(N-Et-HPTB)(OPPh3)2]3+.20 Addi-
tion of 20 equiv of OPPh3 to 2·O2CR at −90 °C in fact
accelerated the conversion to 3′·O2CR with rates of 0.03−0.2
s−1, representing a much smaller range of values than for the
conversions in the absence of OPPh3 (Table 5). These results
suggest that the binding of OPPh3 to the diiron center
facilitates the ligand rearrangement required to convert from
2·O2X to 3·O2X (Scheme 1).
The focus of the experiments reported in this paper has been
to gain further insight into the factors that affect the conversion
of 2·O2X to 3·O2X, in which irreversible conversion to 3·O2X is
preceded by movement of the O2X moiety from a bridging to a
terminal position (Scheme 1). We postulate that there is a rapid
equilibrium between bridging and terminal coordination modes
of the O2X ligand in 2·O2X and only the isomer with a terminal
O2X ligand can undergo conversion to 3·O2X during which the
benzimidazole arms of the N-EtHPTB ligand shift from a cis
relationship to each other to a trans configuration on both iron
centers. Thus, the initial preequilibrium should be affected by
the basicity of O2X ligands, with the more basic ligand favoring
the bridging mode and thereby decreasing the fraction of
monodentate O2X isomer available to undergo conversion to
3·O2X. On the other hand, as the transformation from 2·O2X to
3·O2X entails a significant rearrangement of the coordination
spheres about each iron center, the conversion should be
slowed down by an increase in the steric bulk of O2X.
Tetrahedral O2X bridges also increase the stability of 2·O2X.
In this series, we compared complexes with O2AsMe2, O2PMe2,
O2PPh2, and O2P(OPh)2 bridges that differ in steric properties
and basicity (pKa = 6.27, 3.08, 2.32, and 1.85, respec-
tively).51−53 The order of stability in this subset is O2AsMe2
> O2PMe2 > O2PPh2 > O2P(OPh)2 (Table 4), following the
trend of decreasing pKa values. Only 2·O2P(OPh)2 undergoes
conversion to the corresponding 3·O2X form at −90 °C, with
kobs = 1.2(1) × 10−4 s−1, a rate that is slower than observed for
five of the six 2·O2CR complexes at −90 °C (Table 5). While
2·O2PPh2 is indefinitely stable at −90 °C, it converts to
3·O2PPh2 when warmed to −40 °C, at a rate comparable to
that of 2·O2P(OPh)2 at −90 °C. Neither 2·O2PMe2 nor
2·O2AsMe2 generate an observable 3·O2X form; instead they
appear to decay directly to 4·O2X. However, 2·O2PMe2 can be
converted to 3′·O2PMe2 at −40 °C by addition of OPPh3,
whereas 2·O2AsMe2 does not convert to 3′·O2AsMe2, even
when reacted with 100 equiv of OPPh3 at room temperature.
The increasing stability of 2·O2X with the bite distance of the
oxyanion bridge (X = C, 2.23 Å; X = P, 2.55 Å; X = As, 2.80 Å)
reflects the ability of the O2X bridge to span the Fe···Fe
distance required by the dinucleating HPTB ligand framework
without imposing a strain on the six-member ring formed by
the three atoms of the oxyanion bridge, the two iron atoms, and
the alkoxide oxygen on the N-EtHPTB ligand. The O2AsMe2
complex, having the longest O···O bite distance, represents the
most stable of the 2·O2X intermediates.
The trend in the stability of the 2·O2X complexes is best
discerned by an examination of the kinetic data for the large
subset of complexes with carboxylate bridges. A perusal of
Table 5 suggests that, while it may be possible to observe an
effect of ligand basicity in comparisons of select pairs (e.g.,
O2CPh vs O2CC6H4-4-OMe, and O2CPh vs O2CCMe3), for
the most part steric considerations supersede ligand basicity
arguments. For example, the stability of 2·O2CR is enhanced
30-fold in the series, R = Ph < CMe3 < CPh3, commensurate
with the increase in steric bulk on the carbon atom adjacent to
the carboxylate function. Although the higher pKa (5.03) of
In this paper, we have investigated the various factors that
influence the conversion of 2·O2X O2 adducts to corresponding
3·O2X species. This conversion entails the shift of the O2X
ligand from a bidentate bridging mode to a terminal
monodentate mode, providing another example of a diiron
complex involved in a mechanistically important “carboxylate
shift” first recognized by Lippard and co-workers 20 years
ago.30 An analogous carboxylate shift has been suggested by Do
48
HO2CCMe3 relative to benzoic acid (pKa = 4.19)48 could be
used to rationalize the 3-fold longer lifetime of 2·O2CCMe3, the
30-fold greater stability of 2·O2CCPh3 than 2·O2CPh cannot be
explained by the slightly lower pKa of HO2CCPh3 (3.96)49 but
can easily be rationalized by the much greater bulk of the
triphenylmethyl group relative to phenyl.
et al.54 to occur upon protonation of O2 adducts of [FeII (N-
Complexes with methoxy-substituted benzoate bridges
support the above arguments. The conversion of 2·O2CC6H2-
3,4,5-(OMe)3 to its 3·O2X form is 100-fold slower that for
2·O2CPh, despite the fact that the two carboxylic acids have
essentially identical pKa values (4.24 and 4.19, respec-
tively).48,50 Therefore, the considerable difference in the
stabilities of 2·O2CC6H2-3,4,5-(OMe)3 and 2·O2CPh (Table
5) must arise from steric considerations. Other methoxy-
substituted benzoate complexes exhibit intermediate rates of
conversion; 2·O2CC6H3-3,4-(OMe)2 converts just slightly
faster than 2·O2CC6H2-3,4,5-(OMe)3, while 2·O2CC6H3-3,5-
(OMe)2 and 2·O2CC6H4-4-(OMe) are 10-fold faster. From
these results, it seems clear that additional steric bulk in the
2
EtHPTB)(μ-O2CR)]2+ complexes (R = Ph or C6F5) at −30 °C.
However, although unequivocal evidence was provided for the
protonation of the carboxylate ligand, it is clear from a
comparison with the spectroscopic data we have presented in
this paper that the O2 adducts studied by Do et al.54 must be
assigned structures we now associate with 3·O2CR complexes,
which have terminal monodentate carboxylates. On the other
hand, the corresponding 2·O2CR complexes (with bridging
carboxylates) are in fact formed upon oxygenation of the
diiron(II) precursors (as previously assumed29,42,54) but can be
observed only at −90 °C, as they readily undergo the
“carboxylate shift” at that temperature to isomerize to
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Inorganic Chemistry
Article
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corresponding 3·O2CR species. However, the key take-home
message of the study by Do et al.54 is that protonation of a
carboxylate ligand on a diiron complex can occur and give rise
to a complex with a carboxylic acid ligand. Indeed, a carboxylate
ligand may serve as a convenient conduit for delivering a
proton to a bound peroxide to facilitate O−O bond cleavage,
for example, in the conversion of intermediate P to Q in the
case of sMMO, for which a proton is clearly implicated.13,55
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Akimova, E. V. Inorg. Chem. 2005, 44, 8656.
(25) Kryatov, S. V.; Taktak, S.; Korendovych, I. V.; Rybak-Akimova,
E. V.; Kaizer, J.; Torelli, S.; Shan, X.; Mandal, S.; MacMurdo, V.;
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ASSOCIATED CONTENT
* Supporting Information
Figures S1−S5. This material is available free of charge via the
■
S
Internet at http://pubs.acs.org.
(28) Frisch, J. R.; Vu, V. V.; Martinho, M.; Munck, E.; Que, L., Jr.
Inorg. Chem. 2009, 48, 8325.
̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: elena.rybak-akimova@tufts.edu; larryque@umn.edu.
■
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́
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L., Jr. J. Am. Chem. Soc. 1990, 112, 6423.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(GM-38767 to L.Q.) and the Department of Energy (DE-
FG02-06ER15799 to E.R.-A.); stopped-flow instrumentation at
Tufts was supported by the NSF (CHE 0639138). We thank
Dr. Victor Young, Jr., of the X-ray Crystallographic Laboratory
at the University of Minnesota, for his invaluable assistance.
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