Characterization of two distinct adducts in the reaction of a nonheme diiron(II) complex with O2

Article abstract of DOI:10.1021/ic900961k

Two [Fe^II₂(N-EtHPTB)(μ-O₂X)] ²⁺ complexes, where N-EtHPTB is the anion of N, N,N N-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O ₂X is O₂PPh2 (1.O₂PPh₂

Full text of DOI:10.1021/ic900961k

Inorg. Chem. 2009, 48, 8325–8336 8325
DOI: 10.1021/ic900961k
Characterization of Two Distinct Adducts in the Reaction of a Nonheme Diiron(II)
Complex with O2
::
†
†
‡
,‡
,†
ꢀ
Jonathan R. Frisch, Van V. Vu, Marlene Martinho, Eckard Munck,* and Lawrence Que, Jr.*
†Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant St. S.
E., Minneapolis, Minnesota 55455, and ‡Department of Chemistry, Carnegie Mellon University, 4400 Fifth
Ave., Pittsburgh, Pennsylvania 15213
Received May 15, 2009
Two [FeII (N-EtHPTB)(μ-O2X)]2+ complexes, where N-EtHPTB is the anion of N,N,N0N0-tetrakis(2-ben-
zimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O2X is O2PPh2 (1 O2PPh2) or O2AsMe2 (1 O2AsMe2), have
2
3
3
˚
been synthesized. Their crystal structures both show interiron distances of 3.54 A that arise from a (μ-alkoxo)diiron(II)
core supported by an O2X bridge. These diiron(II) complexes react with O2 at low temperatures in MeCN (-40 °C)
and CH2Cl2 (-60 °C) to form long-lived O2 adducts that are best described as (μ-η1:η1-peroxo)diiron(III) species
(2 O2X) with νO-O ∼ 850 cm-1. Upon warming to -30 °C, 2 O2PPh2 converts irreversibly to a second (μ-η1:η1-
3
3
peroxo)diiron(III) intermediate (3 O2PPh2) with νO-O ∼ 900 cm-1, a value which matches that reported for [Fe2(N-
3
::
EtHPTB)(O2)(O2CPh)]2+ (3 O2CPh) (Dong et al. J. Am. Chem. Soc. 1993, 115, 1851-1859). Mossbauer spectra of
3
2 O2PPh2 and 3 O2PPh2 indicate that the iron centers within each species are antiferromagnetically coupled with
3
3
J ∼ 60 cm-1, while extended X-ray absorption fine structure analysis reveals interiron distances of 3.25 and 3.47 A for
˚
˚
2 O2PPh2 and 3 O2PPh2, respectively. A similarly short interiron distance (3.27 A) is found for 2 O2AsMe2. The
3
3
3
shorter interiron distance associated with 2 O2PPh2 and 2 O2AsMe2 is proposed to derive from a triply bridged
diiron(III) species with alkoxo (from N-EtHPTB), 1,2-peroxo, and 1,3-O2X bridges, while the longer distance
3
3
associated with 3 O2PPh2 results from the shift of the O2PPh2 bridge to a terminal position on one iron. The
differences in νO-O are also consistent with the different interiron distances. It is suggested that the O O bite
3
3 3 3
distance of the O2X moiety affects the thermal stability of 2 O2X, with the O2X having the largest bite distance
3
(O2AsMe2) favoring the 2 O2X adduct and the O2X having the smallest bite distance (O2CPh) favoring the 3 O2X
3
3
adduct. Interestingly, neither 3 O2AsMe2 nor the benzoate analog of 2 O2X (2 O2Bz) are observed.
3
3
3
Introduction
phenol hydroxylase, and stearoyl acyl carrier protein
Δ9-desaturase. The catalytic cycles of several of these
enzymes include peroxide-bridged diiron(III) intermediates
that are stable enough to be detected and characterized.9-16
The precise role of the peroxide moieties commonly observed
in biological reactions remains poorly understood, in spite of
the existence of a fairly large body of work describing them.
Over the past 20 years, a growing family of oxygen-
activating, nonheme diiron enzymes has been revealed and
cataloged.1-8 Most notably, this group includes soluble
methane monooxygenase, the R2 subunit of class I ribonu-
cleotide reductases, toluene and o-xylene monooxygenases,
*To whom correspondence should be addressed. E-mail: emunck@cmu.
edu (E.M.); larryque@umn.edu (L.Q.).
(1) Wallar, B. J.; Lipscomb, J. D. Chem. Rev. 1996, 96, 2625–2658.
(2) Que, L., Jr. In Bioinorganic Catalysis, 2nd ed.; Reedijk, J., Bouwman, E.,
Eds.; Marcel Dekker: New York, 1999; pp 269-321.
(3) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee,
S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J. Chem. Rev.
(9) Liu, K. E.; Valentine, A. M.; Qiu, D.; Edmondson, D. E.; Appelman,
E. H.; Spiro, T. G.; Lippard, S. J. J. Am. Chem. Soc. 1995, 117, 4997–4998.
(10) Bollinger, J. M., Jr.; Krebs, C.; Vicol, A.; Chen, S.; Ley, B. A.;
Edmondson, D. E.; Huynh, B. H. J. Am. Chem. Soc. 1998, 120, 1094–1095.
(11) Broadwater, J. A.; Ai, J.; Loehr, T. M.; Sanders-Loehr, J.; Fox, B. G.
Biochemistry 1998, 37, 14664–14671.
::
(12) Broadwater, J. A.; Achim, C.; Munck, E.; Fox, B. G. Biochemistry
2000, 100, 235–349.
::
(4) Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Muller, J.;
1999, 12197–12204.
(13) Lee, S.-K.; Lipscomb, J. D. Biochemistry 1999, 38, 4423–4432.
(14) Saleh, L.; Krebs, C.; Ley, B. A.; Naik, S.; Huynh, B. H.; Bollinger, J.
M. Biochemistry 2004, 43, 5953–5964.
(15) Murray, L. J.; Garcia-Serres, R.; Naik, S.; Huynh, B. H.; Lippard, S.
J. J. Am. Chem. Soc. 2006, 128, 7458–7459.
(16) Yun, D.; Garcia-Serres, R.; Chicalese, B. M.; An, Y. H.; Huynh, B.
H.; Bollinger, J. M., Jr. Biochemistry 2007, 46, 1925–1932.
Lippard, S. J. Angew. Chem., Int. Ed. 2001, 40, 2782–2807.
(5) Fox, B. G.; Lyle, K. S.; Rogge, C. E. Acc. Chem. Res. 2004, 37, 421–
429.
(6) Bollinger, J. M., Jr.; Diao, Y.; Matthews, M. L.; Xing, G.; Krebs, C.
Dalton Trans. 2009, 905–914.
(7) Sazinsky, M. H.; Lippard, S. J. Acc. Chem. Res. 2006, 39, 558–566.
(8) Murray, L. J.; Lippard, S. J. Acc. Chem. Res. 2007, 40, 466–474.
r
2009 American Chemical Society
Published on Web 07/17/2009
pubs.acs.org/IC

8326 Inorganic Chemistry, Vol. 48, No. 17, 2009
Frisch et al.
To complement these biological studies, a variety of biomi-
metic diiron complexes has been synthesized and character-
ized.17-26 Using synthetic compounds, it is also not unusual
to observe peroxide-bridged diiron(III) species, especially in
carboxylate-bridged complexes, wherein a common motif is a
(μ-η1:η1-peroxo)diiron(III) moiety.
an inert atmosphere by using either standard Schlenk and
vacuum-line techniques or a glovebox. Elemental analyses
were performed by Atlantic Microlab, Inc., Norcross, Georgia.
1 O2PPh2. N-EtHPTB (181 mg, 0.250 mmol) was dissolved
3
in MeOH (∼10 mL) along with Et3N (0.19 mL, 1.4 mmol).
Diphenylphosphinic acid (54.5 mg, 0.250 mmol) was added and
allowed to dissolve. Fe(OTf)2 2MeCN32 (238 mg, 0.546 mmol)
3
It is curious that nature evolved mechanisms that include
quasi-stable peroxide intermediates as steps along catalytic
pathways. The growing number of trapped and characterized
synthetic analogs to these peroxo moieties indicates that this
inherent stability is not limited to biological systems.17-21
Carboxylates are most often used as bridges supporting the
(μ-peroxo)diiron(III) species, with only two papers reporting
(μ-peroxo)diiron(III) complexes with noncarboxylate
bridges.27,28 To investigate how substitution of the carboxy-
late bridge would affect the properties of the (μ-peroxo)-
diiron(III) unit, we synthesized and crystallized two diiron-
(II) complexes using the ligand N-EtHPTB (anion of N,N,
N0N0-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diam-
inopropane), wherein the benzoate bridge of [Fe2(N-
was added, producing a yellow solution. After 5 min, NaBPh4
(180 mg, 0.526 mmol) was added, resulting in immediate pre-
cipitation of a white powder. The solid was filtered and dried in
vacuo. Recrystallization from MeCN and Et2O produced color-
less crystals, some suitable for X-ray diffraction structural
analysis. Yield: 341 mg (79%). Anal. for [Fe2(N-EtHPTB)-
(O2PPh2)](BPh4)2 and calcd for C105H102B2Fe2N11O3P: C,
72.89; H, 5.94; N, 8.90%. Found: C, 72.87; H, 5.97; N, 8.84%.
1 O2AsMe2. N-EtHPTB (146.3 mg, 0.202 mmol) was dissolved
3
in MeOH (∼10 mL) along with Et3N (0.141 mL, 1.01 mmol).
Dimethylarsinic acid (30.0 mg, 0.217 mmol) was added and
allowed to dissolve. Fe(OTf)2 2MeCN32 (182.7 mg, 0.419 mmol)
3
was added, producing a yellow solution. After 5 min, NaBPh4
(212.8 mg, 0.622 mmol) was added, resulting in immediate pre-
cipitation of a white powder. The solid was filtered and dried in
vacuo. Recrystallization from MeCN and Et2O produced pale
yellow crystals, some suitable for X-ray diffraction structural
analysis. Yield: 270 mg (93%). Anal. for [Fe2(N-EtHPTB)-
(O2AsMe2)](BPh4)(OTf) and calcd for C70H75AsBF3Fe2N10O6S:
C, 58.43; H, 5.25; N, 9.73%. Found: C, 58.62; H, 4.97; N, 9.74%.
EtHPTB)(O2CPh)]2+ (1 O2CPh), a complex first reported
3
29
ꢁ
in 1990 by Menage et al., is replaced with diphenylpho-
sphinate (1 O2PPh2) or dimethylarsinate (1 O2AsMe2). Re-
3
3
action of these new species with O2 produces (μ-η1:η1-
peroxo)diiron(III) moieties that exhibit surprising behaviors.
In this paper, we report the crystallographic details of the
precursor diiron(II) complexes and the spectroscopic char-
acterization of their dioxygen adducts.
1 O2CPh. N-EtHPTB (107.6 mg, 0.149 mmol) was dissolved
3
in MeOH (∼10 mL) along with Et3N (0.11 mL, 0.82 mmol).
Sodium benzoate (23.1 mg, 0.160 mmol) was added and allowed
to dissolve. Fe(OTf)2 2MeCN (144.4 mg, 0.331 mmol) was
3
added, producing a yellow solution. After 5 min, NaBPh4
(119 mg, 0.348 mmol) was added, resulting in immediate pre-
cipitation of a white powder. The solid was filtered and dried in
vacuo. Recrystallization from MeCN and Et2O produced pale
yellow-green crystals. Yield: 132 mg (71%). Anal. for [Fe2(N-
EtHPTB)(O2CPh)](OTf)2 and calcd for C52H54F6Fe2N10O9S2:
C, 49.85; H, 4.34; N, 11.18%. Found: C, 49.70; H, 4.38; N,
10.72%.
Experimental Section
Materials and Syntheses. All reagents and solvents were
purchased from commercial sources and were used as
received, unless noted otherwise. The 18O2 (97%) and the
16O2/16O18O/18O2 mixture (1:2:1) were purchased from Cam-
bridge Isotope Laboratories, Inc., Andover, Massachusetts.
The ligand N-EtHPTB was synthesized using a published
procedure.30 Solvents were dried according to published
procedures and distilled under Ar prior to use.31 Preparation
and handling of air-sensitive materials were carried out under
Physical Methods. UV-vis spectra were recorded on a Hewlett-
Packard 8453 diode array spectrophotometer. Resonance Raman
spectra were collected on an ACTON AM-506M3 monochroma-
tor 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
(17) Tshuva, E. Y.; Lippard, S. J. Chem. Rev. 2004, 104, 987–1012.
(18) Ookubo, T.; Sugimoto, H.; Nagayama, T.; Masuda, H.; Sato, T.;
Tanaka, K.; Maeda, Y.; Okawa, H.; Hayashi, Y.; Uehara, A.; Suzuki, M. J.
Am. Chem. Soc. 1996, 118, 701–702.
(19) Dong, Y.; Yan, S.; Young, V. G., Jr.; Que, L., Jr. Angew. Chem., Int.
Ed. Engl. 1996, 35, 618–620.
(20) Kim, K.; Lippard, S. J. J. Am. Chem. Soc. 1996, 118, 4914–4915.
(21) Zhang, X.; Furutachi, H.; Fujinami, S.; Nagatomo, S.; Maeda, Y.;
Watanabe, Y.; Kitagawa, T.; Suzuki, M. J. Am. Chem. Soc. 2005, 127, 826–
827.
(22) Kryatov, S. V.; Chavez, F. A.; Reynolds, A. M.; Rybak-Akimova, E.
V.; Que, L., Jr.; Tolman, W. B. Inorg. Chem. 2004, 43, 2141–2150.
(23) Korendovych, I. V.; Kryatov, S. V.; Reiff, W. M.; Rybak-Akimova,
E. V. Inorg. Chem. 2005, 44, 8656–8658.
frequencies were referenced to the features of indene. Slits were set
::
for a band-pass of 4 cm-1 for all spectra. Mossbauer spectra were
recorded with two spectrometers, using Janis Research Super-
Varitemp dewars that allowed studies in applied magnetic fields up
to 8.0 T in the temperature range from 1.5 to 200 K. Mossbauer
::
spectral simulations were performed using the WMOSS software
package (WEB Research, Edina, MN). Isomer shifts are quoted
relative to Fe metal at 298 K.
(24) Kryatov, S. V.; Taktak, S.; Korendovych, I. V.; Rybak-Akimova, E.
V.; Kaizer, J.; Torelli, S.; Shan, X.; Mandal, S.; MacMurdo, V.; Mairata i
Payeras, A.; Que, L., Jr. Inorg. Chem. 2005, 44, 85–99.
X-Ray Diffraction Crystallography. X-ray diffraction data
were collected on a Bruker SMART platform CCD diffract-
ometer at 173(2) K.33 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 KR radiation (graphite
monochromator). Randomly oriented regions of reciprocal
space were surveyed to the extent of one sphere and to a
(25) Yoon, S.; Lippard, S. J. Inorg. Chem. 2006, 45, 5438–5446.
(26) Fiedler, A. T.; Shan, X.; Mehn, M. P.; Kaizer, J.; Torelli, S.; Frisch, J.
R.; Kodera, M.; Que, L., Jr. J. Phys. Chem. A 2008, 112, 13037–13044.
(27) Than, R.; Schrodt, A.; Westerheide, L.; Eldik, R. v.; Krebs, B. Eur. J.
Inorg. Chem. 1999, 1537–1543.
(28) Yan, S.; Cheng, P.; Wang, Q.; Liao, D.; Jiang, Z.; Wang, G. Sci.
China, Ser. B: Chem. 2000, 43, 405–411.
::
ꢁ
(29) Menage, S.; Brennan, B. A.; Juarez-Garcia, C.; Munck, E.; Que, L.,
Jr. J. Am. Chem. Soc. 1990, 112, 6423–6425.
(30) McKee, V.; Zvagulis, M.; Dagdigian, J. V.; Patch, M. G.; Reed, C. A.
J. Am. Chem. Soc. 1984, 106, 4765–4772.
(31) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals; Butterworth-Heinemann: Oxford, U.K., 1997.
(32) Hagen, K. S. Inorg. Chem. 2000, 39, 5867–5869.
(33) Bruker SMART V5.054; Bruker Analytical X-Ray Systems: Madison,
WI, 2001.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8327
Table 1. Crystal Data and Structure Refinement for 1 O2PPh2(BPh4)2 MeCN and 1 O2AsMe2(BPh4)(OTf) MeCN
3
3
3
3
1 O2PPh2(BPh4)2 MeCN
3
1 O2AsMe2(BPh4)(OTf) MeCN
3
3
3
empirical formula
fw
T (K)
C
105H102B2Fe2N11O3P
C72H78BF3 Fe2N11O6S
1479.94
173(2)
0.71073
P21/c
16.1151(12)
15.6370(12)
28.473(2)
90
103.4900(10)
90
6977.0(9)
4
1.409
0.985
0.0338
0.0798
1730.27
173(2)
0.71073
P21/c
18.937(4)
23.855(5)
21.817(5)
90
114.903(4)
90
8940(3)
4
1.286
˚
Mo KR λ, A
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
F (calcd), Mg/m3
abs coeff (mm-1
R1a
)
0.402
0.0492
0.0957
wR2b
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2
.
˚
resolution of 0.84 A. The intensity data were corrected for
were used to reduce, average, and process the raw data using the
EXAFSPAK package,38 which was also used for X-ray absorp-
tion fine structure (EXAFS) fitting. Theoretical EXAFS ampli-
tude and phase functions were calculated using the FEFF
package (version 8.4). The input models for FEFF calculations
are shown in Figures S1 and S2 (Supporting Information). The
parameters r and σ2 were floated, while n was kept fixed for each
fit and systematically varied in integer steps between fits. The
scale factor was fixed at 0.9, and the threshold energy (E0) was
varied but maintained at a common value for all shells. The
goodness of fit was determined using eq 1:
absorption and decay using SADABS.34 Final cell constants
were calculated after integration with SAINT.35 The structures
were solved and refined using SHELXL-97.36 The space group
P21/c was determined on the 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 per-
formed which located the remaining non-hydrogen atoms. All
non-hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were placed in ideal
positions and refined as riding atoms with relative isotropic
displacement parameters. Brief crystal data and intensity collec-
tion parameters for the crystalline complexes are shown in
Table 1. Complete crystallographic details, atomic coordinates,
anisotropic thermal parameters, and fixed hydrogen atom
coordinates are given in the Supporting Information.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X
2
F ¼
k6ðχexp - χcalcdÞ =N
ð1Þ
where N is the number of data points.39
Results
X-Ray Absorption Spectroscopy. X-ray absorption spectros-
copy (XAS) data were collected on beamline 9-3 at Stanford
Synchrotron Radiation Lightsource (SSRL) of the SLAC Na-
tional Accelerator Laboratory and on beamline X3B at the
National Synchrotron Lightsource of Brookhaven National
Laboratory (NSLS). At SSRL, the synchrotron ring SPEAR
was operated at 3.0 eV and a 50-100 mA beam current. Energy
resolution of the focused incoming X-rays was achieved using
a Si(220) double-crystal monochromator, which was then
detuned to 50% of maximal flux to attenuate second harmonic
X-rays. At NSLS, the synchrotron ring was operated at 2.8 GeV
and a 100-300 mA beam current, and a Si(111) double-crystal
monochromator was used. Fluorescence data were collected
over the energy range of 6.8-8.0 keV using a 30-element Ge
detector (SSRL) or 13-element Ge detector (NSLS). The beam
spot size on samples was 5 mm (horizontal) ꢀ 1 mm (vertical).
Several diiron enzymes activate O2 via (μ-η1:η1-
peroxo)diiron(III) intermediates.9-14,16 In order to better
understand the behavior of these biological moieties, we
synthesized two diiron(II) complexes and characterized their
O2 adducts. Combining 2 equiv of Fe(OTf)2 2MeCN with
3
1 equiv of the ligand N-EtHPTB followed by the addition of
1 equiv of either diphenylphosphinic acid or dimethylarsinic
acid afforded complexes with the general formulation
[Fe2(N-EtHPTB)(O2X)](Y)2 (cation = 1 O2X where O2X =
3
O2PPh2 or O2AsMe2 and Y = counteranions). Recrystalliza-
tion of these compounds from MeCN and diethyl ether
produced crystals suitable for X-ray diffraction structural
analysis (Table 1). In solution, these complexes reacted with
dioxygen at low temperatures, producing intermediates that
A total of 13 scans were collected for 2 O2AsMe2 at NSLS
3
(55 min/scan), and 14 scans were collected for 2 O2PPh2 and
3
were characterized using UV-vis, resonance Raman,
::
3 O2PPh2 at SSRL (25 min/scan). All spectra were referenced
3
Mossbauer, and X-ray absorption spectroscopies.
against an iron foil.
Pre-edge quantification was carried out with the SSExafs
X-Ray Crystallography. The results from analysis of
the 1 O2PPh2 and 1 O2AsMe2 structures are compared
in Table 2 with information from the previously pub-
program using a standard procedure.37 Standard procedures
3
3
lished crystal structure of 1 O2CPh.29,40 Figure 1 shows
3
(34) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38.
(35) Bruker SAINTþ V6.45; Bruker Analytical X-Ray Systems: Madison,
WI, 2003.
an ORTEP view of 1 O2PPh2 with the hydrogen atoms
removed as well as a cartoon of the general structure
3
(36) Bruker SHELXTL V6.14; Bruker Analytical X-Ray Systems: Madison,
WI, 2000.
(37) Scarrow, R. C.; Trimitsis, M. G.; Buck, C. P.; Grove, G. N.; Cowling,
R. A.; Nelson, M. J. Biochemistry 1994, 33, 15023–15035.
(38) George, G. N.; Pickering, I. J. EXAFSPAK; Stanford Synchrotron
Radiation Laboratory, Stanford Linear Accelerator Center: Stanford, CA, 2000.
(39) Riggs-Gelasco, P. J.; Stemmler, T. L.; Penner-Hahn, J. E. Coord.
Chem. Rev. 1995, 144, 245–286.
ꢁ
(40) Dong, Y.; Menage, S.; Brennan, B. A.; Elgren, T. E.; Jang, H. G.;
Pearce, L. L.; Que, L., Jr. J. Am. Chem. Soc. 1993, 115, 1851–1859.

8328 Inorganic Chemistry, Vol. 48, No. 17, 2009
Frisch et al.
Table 2. Selected Interatom Distances and Bond Angles for [Fe2(N-EtHPTB)-
(O2X)]2+
1 O2PPh2
3
1 O2AsMe2
3
1 O2CPha
3
τave
0.77
0.81
˚
0.93
Interatom Distances (A)
Fe1-O1
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)
3.5405(10)
2.5600(32)
2.0145(13)
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)
3.4749(31)
2.2251(74)
Fe2-O1
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)
Fe1-N1
Fe2-N2
Fe1-O2
Fe2-O3
Fe1-N3
Fe1-N5
Fe2-N7
Fe2-N9
P1/C44/As1-O2
P1/C44/As1-O3
Fe1 Fe2
O2 O3
3 3 3
2.7991(21)
3 3 3
Bond Angles (deg)
Figure 1. Crystal structure ofthe cation1 O2PPh2 (50% ellipsoids) with
hydrogen atoms removed. Generic cartoon of 1 O2X: O2X = O2AsMe2
3
Fe1-O1-Fe2
O1-Fe1-O2
O1-Fe2-O3
Fe1-O2-P1/C44/As1 132.01(14)
Fe2-O3-P1/C44/As1 138.88(15)
O2-P1/C44/As1-O3
124.76(11)
105.55(9)
99.44(9)
123.27(6)
105.77(6)
107.92(6)
127.24(8)
128.15(8)
113.26(7)
123.8(2)
98.6(2)
101.5(2)
136.4(5)
133.9(5)
124.2(7)
3
(1 O2AsMe2), O2PPh2 (1 O2PPh2), O2CPh (1 O2CPh).
3
3
3
109.5°, the ligated oxygen atoms are forced apart to
accommodate the interiron distances of 3.5357(5)
115.59(13)
˚
(1 O2AsMe2) and 3.5405(10) (1 O2PPh2) A, producing
a Data for 1 O2CPh were re-refined using full-matrix least squares on
3
3
3
F2, and atoms were renamed to match the labeling scheme of 1 O2PPh2
and 1 O2AsMe2. The new solution was deposited directly with the
respective O-As-O and O-P-O angles of 113.26(7)
and 115.59(13)°.
3
3
Cambridge Crystallographic Data Centre (deposition number CCDC
731669). Values in the table represent recalculated distances and angles.
Interestingly, the interiron distances found in
1 O2AsMe2 and 1 O2PPh2 are greater than that found
in 1 O2CPh, in spite of the fact that the former complexes
produce sharper angles with their three-atom linkers.
Examination of the bond lengths between the oxygen
atoms and the central atom in the three-atom linker of
3
3
3
shared by all three cations. In each complex, both metal
atoms are five-coordinate iron(II) and form a six-mem-
bered ring along with three atoms of the bridging moiety
and the ligand alkoxide oxygen. Other similarities include
distorted trigonal-bipyramidal iron centers, the axial
positions of which are occupied by amine nitrogen atoms
and bridging moiety oxygen atoms. The equatorial sites
are occupied by benzimidazole nitrogen atoms and the
alkoxide oxygen atom. For the most part, the interatomic
distances and angles do not significantly fluctuate be-
tween species. However, variations of note include the
O-C-O, O-P-O, and O-As-O angles formed by the
three-atom linkers between the iron atoms, with values of
113.26(7) (1 O2AsMe2), 115.59(13) (1 O2PPh2), and
each complex reveals
1 O2AsMe2 being ∼0.16 A longer than those in
a trend with distances in
˚
3
˚
1 O2PPh2, which in turn contains distances ∼0.25 A
3
longer than those found in 1 O2CPh. The longer As-O
3
and P-O bond lengths found in 1 O2AsMe2 and
3
1 O2PPh2 result in significantly increased O O bite
˚
distances of 2.7991(21) (1 O2AsMe2) and 2.5600(32) A
˚
(1 O2PPh2) versus 2.2251(74) A (1 O2CPh). This shorter
distance in 1 O2CPh is reflected in a shorter interiron
distance, in spite of the fact that the three-atom linker has
3
3 3 3
3
3
3
3
3
3
a wider angle than the linkers in either 1 O2PPh2 or
1 O2AsMe2.
3
124.2(7)° (1 O2CPh), and the geometry about the iron
3
atoms, with average τ values41 of 0.81 (1 O2AsMe2), 0.77
(1 O2PPh2), and 0.93 (1 O2CPh).
3
3
UV-Vis Spectroscopy. The UV-vis spectrum of
1 O2PPh2 in CH2Cl2 has no notable features except for
a UV tail and an extremely weak absorbance maximum
around 1000 nm (Figure 2, inset). Bubbling dioxygen
through this solution at -80 °C elicits an intense green-
blue color (Figure 2, solid green line) with absorption
3
3
3
Clearly, the differences observed between the three-atom
linker angles in each complex result from the different
hybridization of the central atom. The central carbon atom
inthebenzoatebridgeissp2-hybridized, producing anangle
3° greater than the ideal 120° predicted for trigonal-planar
geometry. As the benzoate oxygen atoms bind to the iron
atoms, they must spread apart to accommodate the inter-
maxima at 368 and 678 nm (2 O2PPh2). Upon warming
3
to -30 °C, the green-blue solution becomes deep blue
over an 8 min period (Figure 2, dotted blue line). The
absorption maximum at 368 nm shifts to 344 nm while
gaining intensity. The maximum at 678 nm shifts to
621 nm with a concomitant loss of intensity, and a new
shoulder appears at 509 nm (Figure S3, Supporting
Information). The complex spectral changes and the lack
of true isosbestic points suggest the appearance of more
than one species during this time period. Further warming
˚
iron distance (3.4749(31) A), producing an O-C-O angle
of 124.2(7)°. The same effect is observed in the diphenyl-
phosphinate- and dimethylarsinate-bridged complexes,
where the central atom in each three-atom linker is
sp3-hybridized. However, instead of producing angles of
(41) Addison, A. W.; Rao, T. N.; Reedijk, J.; Rijn, J. v.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349–1356.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8329
Figure 2. UV-vis spectra of 1 O2PPh2 (inset), 2 O2PPh2 (solid green
3
3
line), and mostly 3 O2PPh2 (dotted blue line) in CH2Cl2.
3
Figure 3. UV-vis spectra recorded at -40 °C in MeCN for 2 O2PPh2
3
(solid green) and 3 O2PPh2 (dotted blue).
3
Figure 4. Resonance Raman spectra of frozen solutions of 2 O2PPh2
3
to room temperature produces the final product, a yellow
solution (4 O2PPh2) with no remarkable absorption fea-
in CH2Cl2 (A), 2 O2AsMe2 in CH2Cl2 (B), and 3 O2PPh2 in MeCN (C).
3
3
Solid red = 16O2. Dotted blue = 18O2. Dashed green = mixed O2.
3
tures in the visible spectrum.
Oxygenation of 1 O2PPh2 in MeCN at -40 °C pro-
3
3 O2PPh2 prior to complete decay to the yellow product
(4 O2AsMe2).
duces a green-blue solution (λmax = 686 nm) with spectro-
scopic characteristics (Figure 3, solid green line) similar to
3
3
Resonance Raman Spectroscopy. Resonance Raman
(rR) spectroscopy is a particularly effective technique
for examining vibrational transitions in complexes that
have strong chromophores. In light of this fact, rR spectra
of 2 O2AsMe2, 2 O2PPh2, and 3 O2PPh2 were collected,
those of 2 O2PPh2 in CH2Cl2 at -80 °C. However, in
3
contrast to complications observed when warming the
latter, warming the MeCN solution to -30 °C and
maintaining that temperature for 15 min results in clean
conversion to a deep blue solution (3 O2PPh2) with a
visible absorption maximum at 590 nm (Figure 3, dotted
blue line), corresponding to the 588 nm peak observed
3
3
3
3
analyzed, and compared with spectra presented in earlier
reports21,26,40,42-46 in an attempt to gain insight into their
individual molecular structures.
upon oxygenation of 1 O2CPh.29 Unlike the reaction in
CH2Cl2, conversion from 2 O2PPh2 to 3 O2PPh2 in
3
The rR spectrum of 2 O2PPh2 in frozen CH2Cl2 using
3
3
3
16O2 and 647.1 nm excitation shows two intense peaks at
845 and 853 cm-1 in addition to three peaks at 465, 475,
and 490 cm-1 (Figure 4A, solid red line). These features
suggest the presence of an iron(III)-peroxo chromophore.
With the use of 18O2 (Figure 4A, dotted blue line), the
peaks at 845 and 853 cm-1 shift to a single peak at
807 cm-1. This shift of 42 cm-1 is in agreement with the
change predicted for an O-O oscillator by application
of Hooke’s law, thereby assigning the peaks at 845 and
MeCN produces true isosbestic points at 363 and
618 nm (Figure S4, Supporting Information). Warming
to room temperature causes 3 O2PPh2 to decay to the
final product, a yellow solution (4 O2PPh2) with no
remarkable absorption features in the visible spectrum.
3
3
Oxygenation of 1 O2AsMe2 in MeCN at -40 °C
3
changes the almost colorless solution to a deep green-
blue color with UV-vis absorption maxima at 348 and
632 nm (Figure S5, Supporting Information) associated
with 2 O2AsMe2. It exhibits UV-vis absorption features
3
(42) Mirica, L. M.; Vance, M.; Rudd, D. J.; Hedman, B.; Hodgson, K. O.;
analogous to those of 2 O2PPh2, but the similarity ends
there. Instead of converting to a different species when
3
Solomon, E. I.; Stack, T. D. P. J. Am. Chem. Soc. 2002, 124, 9332–9333.
::
(43) Kim, E.; Shearer, J.; Lu, S.; Moenne-Loccoz, P.; Helton, M. E.;
::
Kaderli, S.; Zuberbuhler, A. D.; Karlin, K. D. J. Am. Chem. Soc. 2004, 126,
warmed to -30 °C, 2 O2AsMe2 is stable for more than an
3
12716–12717.
hour, at which point observation was aborted. Warming
this solution to 0 °C leads to slow decay with ∼25% loss of
intensity at 632 nm over a 4 h period. Gradual blue-shifting
of the absorbance maxima in conjunction with the pre-
sence of time traces that cannot be fit to first-order decay
ratesleads us to hypothesize thatthe systempasses through
(44) Chishiro, T.; Shimazaki, Y.; Tani, F.; Tachi, Y.; Naruta, Y.;
Karasawa, S.; Hayami, S.; Maeda, Y. Angew. Chem., Int. Ed. 2003, 42,
2788–2791.
(45) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. Rev.
2004, 104, 939–986.
(46) Yamashita, M.; Furutachi, H.; Tosha, T.; Fujinami, S.; Saito, W.;
Maeda, Y.; Takahashi, K.; Tanaka, K.; Kitagawa, T.; Suzuki, M. J. Am.
Chem. Soc. 2007, 129, 2–3.
an unobserved intermediate (3 O2AsMe2) comparable to
3

8330 Inorganic Chemistry, Vol. 48, No. 17, 2009
Frisch et al.
CH2Cl2 and MeCN solutions. Unfortunately, the chlor-
ine atoms of dichloromethane have a very high extinction
coefficient at 14.4 keV, and one can therefore not study
samples with a path length of more than 1 mm. However,
freezing a 1-mm-thick CH2Cl2 solution generates an
intolerably uneven sample thickness due to meniscus
formation. We found a simple solution to the problem
by dripping ca. 80 μL of dichloromethane solution onto a
Figure 5. Possible O2 coordination modes in Fe2O2-alkoxo core.
stack of five Fisher-brand paper filter discs stacked into
the 1-cm-diameter Mossbauer cup. After freezing, we
::
853 cm-1 as a Fermi doublet of νO-O. Additionally, with
the use of 18O2, the peaks at 465, 475, and 490 cm-1 are
replaced with peaks at 455 and 484 cm-1. Again, using
Hooke’s law, we can assign the peaks at 465 and 475 cm-1
to a Fermi doublet representing νFe-O, which collapses
to a single peak at 455 cm-1 upon 18O substitution. The
obtained homogeneous and firmly packed samples.
Figure 6 shows 4.2 K Mossbauer spectra of 2 O2PPh2
::
3
(A) and 3 O2PPh2 (B) in CH2Cl2 (black hash marks) and
MeCN (solid red lines). In both solvents, 2 O2PPh2
3
3
exhibits one doublet with quadrupole splitting ΔEQ
=
1.26 mm/s and isomer shift δ = 0.56 mm/s. The CH2Cl2
sample contains a high-spin iron(II) contaminant
(∼15%, arrow), most likely belonging to diiron(II) start-
ing material.
peak at 490 cm-1 clearly shifts to lower energy in the 18
O
isotopomer, but the change of only 6 cm-1 and the
relative weakness of the signal lead us to suspect that this
peak arises from a ligand vibration coupled to the Fe-O
stretch.
Figure 7 shows 8.0 T spectra of 2 O2PPh2 recorded at
3
4.2 K (A) and 80 K (B,C). The 4.2 K spectrum reveals that
the dinuclear complex, as expected, has a ground state
with cluster spin S = 0. The variable-temperature spec-
tra, taken at 50 K (not shown) and 80 K were analyzed
with a spin-Hamiltonian appropriate for an exchange-
coupled dinuclear complex comprising two high-spin
(S1 = S2 = 5/2) iron(III) ions (eq 2, where all symbols
have their conventional meanings). For the iron(III) sites
considered here, the zero-field splitting parameters D1,2
are generally on the order of 1 cm-1 and can be neglected.
Repeating this experiment using a mixture of 16O2,
18O2, and 16O18O (Figure 4A, dashed green line) allowed
us to gain insight into the peroxo binding mode. While the
ν
Fe-O region is not well-resolved, the νO-O region clearly
shows peaks at 806, 829, 845, and 853 cm-1, correspond-
ing to features arising from the 18O-18O, 16O-18O, and
16O-16O isotopomers, respectively. The peak at 829 cm-1
is assigned to ν16O-18O by comparison with the spectra
obtained from the 16O2 and 18O2 isotopomers. The ap-
pearance of only a single peak between 806 and 845 cm-1
with a line width comparable to those of the ν16O-16O and
ν18O-18O peaks indicates that the dioxygen moiety is
symmetrically ligated. Three possible symmetric coordi-
nation modes are shown in Figure 5.
^
^
ꢂ
H ¼ JS1 S2
3
X
^
^
^
^ ^
^
ꢂ
þ
f2βSi B þ AoSi Ii -gnβnB Ii þ HQðiÞg ð2Þ
3
3
3
i ¼1,2
The rR spectrum of 3 O2PPh2 generated by using 16O2
::
3
in MeCN shows peaks at 477 and 897 cm-1 (Figure 4C,
solid red line), corresponding to νFe-O and νO-O, respec-
tively. Using 18O2 to generate the sample respectively
shifts these peaks to 458 and 848 cm-1 (Figure 4C, dotted
blue line), confirming the assignments. Similar results are
The determination of J by Mossbauer spectroscopy has
been described in the literature.47,48 For an external field
B = 8.0 T, applied parallel to the observed γ-rays, one
compares the effective field at the iron nuclei, Beff(i) =
B þ Bint(i), at 4.2 K and some higher temperature, say
80 K. At 4.2 K, only the S = 0 ground state of the
diiron(III) cluster is populated, and thus Beff(i) = B. At
80 K, higher excited states of the spin ladder can become
populated (essentially the S = 1 manifold, S1 þ S2 = S),
and in the limit of fast electronic transitions among the
thermally populated spin levels, the two iron nuclei experi-
ence an internal magnetic field Bint(i) = -<Si>thAo(i) /
gnβn, where <Si>th is the thermally averaged expectation
value of the spin for site i. For nonheme octahedral sites
with N/O coordination, we can take Ao(i) = -21 T. The
expression for Bint is quite simple, because zero-field split-
tings can be ignored and because the magnetic hyper-
fine interactions of iron(III) sites are generally isotropic.
Figure 8 shows a plot of <S1z>th = <S2z>th versus J for
B = 8.0 T evaluated at T = 8.0 T.
obtained for 3 O2PPh2 generated in CH2Cl2 (Figure S6,
Supporting Information), but the samples were contami-
3
nated with residual 2 O2PPh2 due to incomplete conver-
sion, preventing precise assignment of νFe-O
3
.
Raman spectra of 2 O2AsMe2 (Figure 4B) generated in
3
CH2Cl2 using either 16O2 (solid red line) or 18O2 (dotted
blue line) reveal peaks assigned to νO-O at 845 and
796 cm-1, respectively. The peak at 464 cm-1 in the
16O2 spectrum shifts to 443 cm-1 upon 18O2 substitution
and is assigned to νFe-O
.
From an examination of the rR data summarized in
Table 3, we see that νO-O can be used to group the
oxygenated intermediates into two categories. Raman
shifts near 850 cm-1 are characteristic of the first cate-
gory, which includes the 2 O2X species. The second
category consists of complexes that exhibit νO-O near
3
The solid red line in the 4.2 K spectrum of Figure 7A is a
simulation assuming that only the S = 0 ground state is
900 cm-1 and includes the 3 O2X intermediates. In
3
addition to segregating the intermediates, the νO-O values
ranging from ∼850 to 900 cm-1 demonstrate that all of
the species in each group are peroxo complexes.
::
(47) Kauffmann, K. E.; Munck, E. In Spectroscopic Methods in Bioinor-
ganic Chemistry; Solomon, E. I., Hodgson, K. O., Eds.; American Chemical
Society: Washington, DC, 1998.
(48) Krebs, C.; Bollinger, J. M., Jr.; Theil, E. C.; Huynh, B. H. J. Biol.
Inorg. Chem. 2002, 7, 863–869.
::
::
Mossbauer Spectroscopy. Using Mossbauer spectros-
copy, we examined 2 O2PPh2 and 3 O2PPh2 in frozen
3
3

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8331
Table 3. Physical Properties of 2 O2X and 3 O2X Complexes
3
3
λ
max (nm) [ε]
ν
O-O (cm-1
18O2]
)
ν
Fe-O (cm-1
18O2]
)
complex
solvent
(M-1 cm-1
)
[
[
J (cm-1
)
δ (mm/s)
ΔEQ (mm/s)
ηf
˚
Fe Fe (A)
3 3 3
2 O2AsMe2
3
MeCN
CH2Cl2
MeCN
348, 632
[7300, 2100]
368, 678
[6000, 2100]
358, 686
[7400, 2200]
not observed
not observed
344, 509, 621
[7200, 1600, 1800]
338, 509, 590
[8300, 2000, 2300]
588
845a
[796]
845, 853
[829],b [807]
845, 853
[807]
464a
[443]
465, 475
455
465, 476
[455]
3.27
2 O2PPh2
3
57 ( 7
57 ( 7
0.56(1)
0.56(1)
-1.26(2)
-1.26(2)
0.4
0.4
2 O2PPh2
3
3.25
2 O2CPh
3
CH2Cl2
MeCN
CH2Cl2
3 O2AsMe2
3
c
3 O2PPh2
3
898
[845]
897
[848]
900
[850]
60 ( 10
60 ( 10
0.56(2)
0.54(2)
0.53(2)
0.86(5)
-0.52(3)
-1.03(4)
∼0
∼0
∼1
d
3 O2PPh2
3
MeCN
477
3.47
[458]
476
[460]
3 O2CPh e
CH2Cl2
3
[1500]
a Measured in CH2Cl2. b νO-O from 16O18O. c Resonance Raman spectroscopy indicates incomplete conversion; some 2 O2PPh2 remains, probably
red-shifting the peroxo-to-metal charge transfer to 621 nm. In addition, an unknown amount of 3 O2PPh2 has decayed to 4 O2PPh2, reducing the
3
3
3
::
measured extinction coefficients. d Mossbauer values represent 80% of the total iron. e Values from ref 40. f η = (Vxx - Vyy)/Vzz is the asymmetry
parameter of the electric field gradient tensor.
Figure 6. The 4.2 K zero field spectra of 2 O2PPh2 (panel A) in CH2Cl2
3
(black hash marks) and MeCN (solid red line) and 3 O2PPh2 (panel B) in
3
CH2Cl2 (black hash marks) and MeCN (solid red line). The high-energy
line of a diiron(II) contaminant is marked by an arrow.
Figure 8. Thermally averaged spin expectation values, <S1z>th
<S2z> , calculated at 80 K for an applied field B = 8.0 T, for an
=
th
antiferromagnetically coupled dimer comprising two high-spin (S1 = S2 =
5/2) iron(III) ions for the Hamiltonian H = JS1 S2 þ 2β(S1 þ S2) B. The
3
3
zero-field splittings of the iron(III) sites are small and can be ignored.
Simulating the 8.0 T spectrum with an “applied” field of
7.35 T yields the solid red line of Figure 7B. Taking Ao =
-21 T yields <Si,z>th = -0.035 and, thus from the
graph of Figure 8, a J value slightly smaller than 60 cm-1
.
Our best simulations using the full Hamiltonian of eq 2
(the 2Spin option of WMOSS) yielded J = 57 ( 7 cm-1
for 2 O2PPh2, taking also into account an 8.0 T spectrum
3
recorded at 50 K (not shown). The same J value was
obtained for 2 O2PPh2 in MeCN.
The zero-field spectra of 3 O2PPh2 depend on which
3
::
Figure 7. The 8.0T Mossbauerspectraof2 O2PPh2 in CH2Cl2 recorded
3
3
at 4.2 K (A) and 80 K (B). The solid red lines are theoretical curves using
the parameters listed in Table 3. The data in shown C are the same as in B;
the solid red line in C is a simulation obtained by assuming (wrongly) that
only the S = 0 ground state is occupied at 80 K (two-spin simulation for
J = 1000 cm-1). The solid red line in B was obtained for J = 57 cm-1
(alternatively, one can simulate the spectrumbyassuminga state withS=
0 and adjust the applied field to B = 7.35 T).
solvent is used (Figure 6B). The CH2Cl2 spectrum is best
represented by assuming two doublets of equal intensity
with δ(1) = 0.56 and δ(2) = 0.58 mm/s and ΔEQ(1) = 0.86
and ΔEQ(2) = -0.52 mm/s. In contrast, the spectrum of the
MeCN sample is best represented by one doublet with δ =
0.53 mm/s and ΔEQ = -1.03 mm/s (representing ∼80% of
populated. The solid red line drawn into the 80 K
spectrum of C is the same curve as shown in A. It can
be seen that the experimental splitting at 80 K is smaller
than expected for a strictly diamagnetic compound. At
80 K, the diiron center is magnetically isotropic, and
thus Bint is antiparallel (Bint < 0) to the applied field.
Fe); the remainder may belong to a doublet with ΔEQ
∼
-0.90 mm/s or, alternatively, belong to a distribution of
minority species. Analysis of the 8.0 T spectra of 3 O2PPh2
3
in MeCN, shown in Figure S7 (Supporting Information),
again suggests a J value of ∼ 60 cm-1, as do the 8.0T spectra
in CH2Cl2 (Figure S8, Supporting Information).

8332 Inorganic Chemistry, Vol. 48, No. 17, 2009
Frisch et al.
Figure 9. Fe K-edge X-ray absorption spectroscopy near-edge struc-
tures (XANES, fluorescence excitation) of 2 O2PPh2 (top), 2 O2AsMe2
(middle), and 3 O2PPh2 (bottom). Inset: Magnified pre-edge absorption
3
3
3
peaks.
Table 4. XANES Parameters for Complexes 2 O2PPh2, 2 O2AsMe2, and 3 O2PPh2
3
3
3
complex
E0 (eV) Epre-edge (eV) pre-edge area pre-edge width
2 O2PPh2 7126.2
2 O2AsMe2 7125.9
7114.6
7114.3
7114.7
15.8(3)
15.1(4)
16.1(2)
3.84(6)
3.66(8)
3.14(4)
3
3
Figure 10. Fourier transforms of the Fourier-filtered Fe K-edge EX-
AFS data k3χ(k) (inset) of 2 O2PPh2 (top), 2 O2AsMe2 (middle), and
3 O2PPh2 (bottom). Experimental data displayed with solid circles (b)
3
3
3 O2PPh2
7126.3
3
3
::
˚
and fits with solid lines (;). Back-transformation range ∼0.7-3 A
The Mossbauer parameters of each characterized com-
plex are summarized in Table 3. We found that, within
error, the J values are almost equivalent across the permu-
tations of species and solvents. The same is true of the
isomer shifts. However, the quadrupole splitting values
determined for this series of complexes range from -1.26
˚
(2 O2PPh2) and 0.7-3.5 A (2 O2AsMe2 and 3 O2PPh2). Fourier trans-
3
3
3
formed range, k = 2-15 A (2 O2PPh2), 2-13 A-1 (2 O2AsMe2), and
2-14.8 A (3 O2PPh2). Fit parameters are provided in Table 5 and
-1
˚
˚
3
3
-1
˚
3
Table S1 in bold italics.
The Fourier-filtered k3-weighted EXAFS data of
2 O2AsMe2, 2 O2PPh2, and 3 O2PPh2 and their corre-
sponding Fourier transforms are presented in Figure 10.
3
3
3
to 0.86 mm/s, and in the case of 3 O2PPh2 in CH2Cl2, the
3
presence of two distinct doublets revealed that the two iron
atoms in this complex exist in different environments, a
property unique to this species/solvent combination.
X-Ray Absorption Spectroscopy. To date, intermedi-
ates 2 O2PPh2, 3 O2PPh2, and 2 O2AsMe2 have not
Features of 2 O2PPh2 and 2 O2AsMe2 are similar in the
3
3
range of 2-8 A-1, where scattering from low Z atoms is
˚
dominant, but different from those of 3 O2PPh2. This
suggests that the first two complexes have similar ligand
geometries that differ from that of 3 O2PPh2. In the region
3
3
3
3
3
yielded crystals of sufficient quality for X-ray diffraction
characterization. To gain further insight into the struc-
tures of these complexes, we resorted to XAS, including
X-ray absorption near-edge structure (XANES) and
beyond 8 A-1, EXAFS data of 2 O2AsMe2 and 2 O2PPh2
˚
3
3
share similar oscillation phases, but 2 O2AsMe2 exhibits a
3
greater amplitude that most probably derives from a larger
contribution from the higher Z arsenic scatterer. Conse-
quently, the Fourier transforms of the 2 O2AsMe2 and
EXAFS analyses. Figure 9 shows XANES of 2 O2AsMe2,
2 O2PPh2, and 3 O2PPh2 with parameters shown in Ta-
3
3
3
3
2 O2PPh2 data are very similar with inner-shell peaks at
0
03
and r = 2.8 A, although the r = 2.8 A peak of the
ble 4. Edge energies (E0) assigned to the inflection points of
these spectra are ∼7126 eV, higher than the values typical
of diiron(III) clusters containing an oxo bridge (∼7123-
7124 eV), including diiron(III) peroxo complexes reported
by Fiedler et al.26 Westre et al. reported that (μ-oxo)diiron-
(III) complexes have edge energies ∼2 eV lower than those
found for (μ-hydroxo)diiron(III) complexes.49 Pre-edge
peaks observed at ∼7114-7115 eV in our complexes are
typical of diiron(III) clusters.26,49 Ithas been shown that an
oxo bridge distorts the geometry of a diiron cluster,
resulting in a more intense symmetry-forbidden 1s f 3d
transition.50 Ourmeasuredpre-edgeareas of ∼15-16units
are smaller than the ∼20 units (derived by the same
standard method developed by Scarrow et al.)37 found
for six-coordinate (μ-oxo)(μ-peroxo)diiron(III) clusters.26
It follows that the higher edge energies and smaller pre-
edge areas observed in the XANES parameters of
2 O2AsMe2, 2 O2PPh2, and 3 O2PPh2 are indicative of
0
˚
r = 1.6 and r = 2.0 A and oute0r-shell peaks at r = 2.3
2 O2AsMe2 spectrum is, as expected, much more intense
than the corresponding peak observed for 2 O2PPh2.
0
˚
˚
3
3
In the region beyond 8 A-1, data from 3 O2PPh2 have a
˚
3
different phase and a smaller amplitude than those from
2 O2AsMe2 and 2 O2PPh2. This difference suggests a
dissimilar geometry about the iron atoms in 3 O2PPh2 as
well as a smaller contribution from high Z atoms. The
3
3
3
Fourier transform of the 3 O2PPh2 data shares the same
general features as the other two complexes, but with a
3
0
˚
more complicated inner-shell peak near r = 1.6 A and an
0
0
˚
˚
outer-shell peak atr = 3.1A instead of an r = 2.8 A peak.
Table 5 shows some of the progressive fits for the three
complexes, with the best fits for each species shown in
bold-italics (see Table S1 for all of the fits including all
scatterers, Supporting Information). The inner-shell fea-
tures of the three complexes can best be fitted with a total
coordination number of six, with four to five ligands near
2.0-2.2 A and one ligand near 2.3 A, corresponding to
the Fe-Namine bond as observed in the crystal structures
of 1 O2PPh2, 1 O2AsMe2, and [Fe2(N-EtHPTB)(O2)-
3
3
3
six-coordinate diiron(III) complexes without oxo bridges.
˚
˚
(49) Westre, T. E.; Kennepohl, P.; DeWitt, J. G.; Hedman, B.; Hodgson,
K. O.; Solomon, E. I. J. Am. Chem. Soc. 1997, 119, 6297–6314.
(50) Roe, A. L.; Schneider, D. J.; Mayer, R. J.; Pyrz, J. W.; Widom, J.;
Que, L., Jr. J. Am. Chem. Soc. 1984, 106, 1676–1681.
3
3
(OPPh3)2]3þ 19
.
In 3 O2PPh2, a short Fe-O distance of
3

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8333
a
Table 5. Selected EXAFS Fitting Results for 2 O2PPh2, 2 O2AsMe2, and 3 O2PPh2
3
3
3
Fe-O/N
Fe-O/N
Fe-O/N
Fe
C
Fe
X
3 3 3
3 3 3
d
σ2
N
σ2
N
σ2
N
σ2
10.66 1P
N
σ2
F c
F 0
˚
R (A)
˚
R (A)
˚
R (A)
˚
R (A)
˚
R (A)
complex
fit
N
2 O2PPh2
3
A
B
C
D
4
4
4
4
1.99
2.00
2.00
2.00
6.14
6.54
6.69
6.57
1
1
1
1
2.29
2.31
2.31
2.30
3.31
3.64
2.71
1.50
5
5
5
5
2.94
2.99
2.99
2.96
3.12
2.21
3.28
3.25
3.14
3.25
3.19
3.28
3.27
3.21
3.13
3.21
3.27
3.45
3.47
3.61
3.50
3.44
-0.30 0.488 0.023
2.19
3.55
7.95
5C
2.26
3.18
4.06
3.00b
0.98
1.94
0.40
1.24
2.24
19.43
1.97
1.02
12.71
4.59
1.50
2.99
3.73
0.69
0.662 0.042
0.392 0.018
0.304 0.013
1
1
2.54
2.51
0.58
3.00b
1Fe
1Fe
1P
2 O2AsMe2
3
A
B
C
D
E
F
5
5
5
3
3
3
1.97
1.97
1.97
1.97
1.97
1.97
10.82
10.95
10.66
3.94
3.87
3.85
1
1
1
2
2
2
2.25
2.26
2.25
2.17
2.17
2.17
2.19
2.85
1.03
4.28
3.82
3.89
3
3
3
3
3
3
2.93
2.90
2.95
2.94
2.93
2.92
3.86
7.46
1.63
2.76
5.20
3.34
1Fe
1As
6C
0.611 0.040
0.595 0.038
0.809 0.079
0.381 0.018
0.369 0.019
0.344 0.022
1
1
1
2.35
2.36
2.35
5.35
3.80
4.24
1Fe
1As
1Fe
1As
1Fe
1As
1Fe
1P
G
3
1.97
3.84
2
2.17
3.90
1
2.36
4.71
3
2.94
2.22
0.344 0.022
3 O2PPh2
3
A
B
C
D
4
4
4
4
2.04
2.04
2.03
2.04
5.73
5.74
5.73
5.58
1
1
1
1
2.33
2.33
2.33
2.33
0.48
0.52
0.67
0.80
1
1
1
1
1.88
1.88
1.88
1.88
1.23
1.28
1.24
1.12
4
4
4
4
2.95
2.95
2.95
2.95
2.39
2.30
2.63
2.43
0.366 0.012
0.452 0.018
0.494 0.021
0.332 0.011
5C
1Fe
0.5P 3.39
a
2
-3
2
Resolution ∼ 0.12 A for 2 O2PPh2 and 3 O2PPh2 and ∼ 0.14 A for 2 O2AsMe2; σ = Debye-Waller factor in units of 10 A . b σ2 value held fixed
˚
˚
˚
√
3
3
3
P
during optimization. c F = goodness of fit calculated as F =
[
k6(χexp - χcal)2/N], where N = the number of data points.39 d F0 = F2/ν, where ν =
nidp - nvar, nidp is the number of independent points in each data set, and nvar is the number of variables used in each optimization step. F0 is used to
indicate the improvement of fit upon the introduction of a shell.39
˚
1.88 A can be resolved from other Fe-O/N distances and
not significantly improve the fit (fits F and G,
2 O2AsMe2). However, we favor the presence of both
is assigned to the peroxo ligand.19-21,26 Interestingly,
3
˚
introduction of a ligand at 2.5 A to the fit of 2 O2PPh2
˚
the 3.27 A Fe Fe and 3.21 A Fe As paths given the
˚
3
3 3 3
3 3 3
data improves the fit quality significantly (fit C,
2 O2PPh2). This same addition does not improve fit
following reasons: (i) 2 O2AsMe2 most likely contains
3
one arsenic and two iron atoms. (ii) Fe As distances of
3 3 3
3
0
˚
qualities for 2 O2AsMe2 and 3 O2PPh2. The r = 2.3 A
peaks of the three complexes correspond to three to five
˚
∼3.2-3.3 A have been found in related (μ-alkoxo)(μ-1,3-
3
3
dimethylarsinato)diiron(III) complexes by both X-ray
crystallography and EXAFS.27,52,53 (iii) the phase shift
and amplitude associated with iron and arsenic scatterers
as simulated by FEFF are very similar in the k range used
˚
Fe C paths near 2.95 A, which arise from carbon atoms
3 3 3
adjacent to the ligating nitrogen atoms of the benzimida-
zole rings of the N-EtHPTB ligand.
for 2 O2AsMe2; therefore, only one path with a small σ2
0
˚
The r = 2.8 A feature of 2 O2PPh2 can be well-
3
3
˚
simulated with an Fe Fe distance near 3.25 A. At-
tempts to simulate this distance with a single Fe P path
value is sufficient to obtain a good simulation of the data
(see Figures S9 and S10 in the Supporting Information for
a detailed description). The 3.21 A Fe As distance
found for 2 O2AsMe2 is slightly shorter than the average
˚
3.28 A Fe As distance found in the crystal structure of
3 3 3
3 3 3
or five Fe C paths respectively resulted in a negative σ2
value (fit A, 2 O2PPh2) or a lower fit quality (fit B,
˚
3 3 3
3 3 3
3
3
2 O2PPh2), indicating that these paths are not responsi-
0
3
3 3 3
˚
ble for the r = 2.8 A peak. However, introducing an
1 O2AsMe2. As is the case with the Fe P distances in
1 O2PPh2 and 2 O2PPh2, this difference is consistent
with differences in iron oxidation states. That the 3.21
3
3
3 3 3
˚
Fe P path at 3.14 A remarkably improved the fit
3 3 3
3
quality (fit D, 2 O2PPh2), indicating that a phosphorus
atom is present at this distance from the iron atoms in
3
˚
A Fe As distance is slightly longer than the 3.16 A
˚
3 3 3
˚
2 O2PPh2. Similarly, including an Fe P path at 3.23 A
Fe P distance found for 2 O2PPh2 comes as no sur-
3 3 3
3
3 3 3
3
˚
in addition to an Fe Fe path at 3.30 A remarkably
improved the fits to the EXAFS data for [Fe2(O)-
prise, given that arsenic has a greater atomic radius than
˚
phosphorus (respectively, 1.15 and 1.00 A).
3 3 3
54
(O2P(OPh)2)2(HB(pz)3)2].51 The 3.16 A Fe P distance
˚
The 3.1 A feature of 3 O2PPh2 is best fitted with an
3
˚
3 3 3
˚
Fe Fe path at 3.47 A (fit A, 3 O2PPh2). Replacing this
found for 2 O2PPh2 is slightly shorter than the average
˚
3.25 A Fe P distance seen in the crystal structure of
1 O2PPh2, an observation consistent with the different
iron oxidation states of these two complexes.
˚
The intense r = 2.8 A peak in 2 O2AsMe2 can be
3
3 3 3
3
Fe Fe path by either one Fe P path (fit B,
3 O2PPh2) or five Fe C paths (fit C, 3 O2PPh2) results
3 3 3
3 3 3 3 3 3
3
3
3 3 3
3
0
˚
in significantly lower fit quality. Thus, the r = 3.1 A
0
˚
feature is attributed to an Fe Fe path at 3.47 A. This
3
3 3 3
˚
simulated equally well with either a 3.27 A Fe Fe path
distance is longer than the Fe Fe distances found in
2 O2PPh2 and 2 O2AsMe2, consistent with the observed
difference in phase shift and the lower amplitude in the
3 3 3
3 3 3
or a 3.21 A Fe As path with relatively small σ2 values
˚
3 3 3
3
3
3
(fits D and E, 2 O2AsMe2). An attempt to replace the
Fe Fe/As paths with six Fe C paths yielded poor
k range of 8-13 A-1 for 3 O2PPh2 compared to the other
˚
3 3 3
3 3 3
3
results (compare fit C to fits A and B, 2 O2AsMe2).
Including both the Fe Fe and the Fe As paths does
3
(52) Eulering, B.; Ahlers, F.; Zippel, F.; Schmidt, M.; Nolting, H.-F.;
Krebs, B. J. Chem. Soc., Chem. Commun. 1995, 1305–1307.
(53) Eulering, B.; Schmidt, M.; Pinkernell, U.; Karst, U.; Krebs, B.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1973–1974.
3 3 3
3 3 3
(51) Hedman, B.; Co, M. S.; Armstrong, W. H.; Hodgson, K. O.;
Lippard, S. J. Inorg. Chem. 1986, 25, 3708–3711.
(54) Slater, J. C. J. Chem. Phys. 1964, 41, 3199–3204.

8334 Inorganic Chemistry, Vol. 48, No. 17, 2009
Frisch et al.
two complexes. Adding half an Fe P path to fit A
(3 O2PPh2) only increased the fit quality by ∼10% (fit D,
3 3 3
3
3 O2PPh2). Therefore, our EXAFS analysis cannot
unambiguously establish the presence of a phosphorus
atom at this distance from the iron atom.
3
All of the peroxo intermediates we have characterized
above decay upon warming to room temperature to
yellow products 4 O2X, which were only characterized
3
by UV-vis spectroscopy. For these complexes, we sug-
gest a generic formulation of [Fe4(N-EtHPTB)2(O2X)2-
(μ-O)2]4þ, by analogy to two tetranuclear iron(III) com-
plexes whose crystal structures were reported in 1988.55
These structures show two [FeIII2(HPTB)(O2CPh)]4þ
units connected by two oxo groups that bridge between
one iron(III) of one unit and the corresponding iron in the
other unit. This tetranuclear form was also proposed by
Feig et al. as the end product in their mechanistic studies
Figure 11. Peroxo O-O stretching ranges reported for various dicop-
per, copper-iron, monoiron, and diiron complexes.
a peroxo ligand-to-metal charge-transfer (LMCT) band, rR
spectroscopy is a good place to start, as it serves as an
excellent probe of O-O vibrations. The rR spectra of these
species exhibit features at ∼850-900 cm-1 that are assigned
to νO-O on the basis of isotopic substitution studies. Upon
18O substitution, the frequency downshifts observed for
of the reactions of O2 with 1 O2CPh and two sister
3
complexes.56 In the absence of crystal structures, we
cannot be sure that 4 O2PPh2 and 4 O2AsMe2 exist as
3
3
peaks around 850 cm-1 in the rR spectra of both 2 O2PPh2
tetra-iron species and leave open the possibility that either
complex may in fact remain dinuclear. We also note that
the reported tetranuclear structures may form as a result
of crystallization conditions and may not reflect the
3
and 2 O2AsMe2 unequivocally indicate the presence of a
3
ligated peroxide moiety. The rR spectrum produced by using
an isotopic mixture of O2 reveals that the peroxide in
::
nature of 4 O2X in solution.
3
2 O2PPh2 is symmetrically ligated. Because its Mossbauer
3
spectrum shows equivalent iron(III) atoms, the peroxide
must be ligated to the diiron(III) center in either a μ-η1:η1
or a μ-η2:η2 configuration. No (μ-η2:η2-peroxo)diiron(III)
complex has been reported to date, but there are examples of
dicopper and heme-copper species containing (μ-η2:η2-per-
oxo)dimetal cores. Figure 11 shows a trend wherein the O-O
stretch increases in frequency as the peroxide moiety moves
from μ-η2:η2 to μ-η1:η2 to η2 to μ-η1:η1 (Figure11).21,26,40,42-46
Comparing those frequencies to values observed for
2 O2PPh2, 2 O2AsMe2, and 3 O2PPh2 leads us to conclude
Discussion
A common step in dioxygen activation by biological
diiron(II) systems is the formation of (μ-η1:η1-per-
oxo)diiron(III) moieties,9-16 which are often stable en-
ough to be trapped and characterized. Synthetic (μ-η1:η1-
peroxo)diiron(III) complexes also exhibit enough stability
to allow characterization,17,24,26-28 and some in fact have
been crystallized.18-21 For the purpose of examining the
factors affecting the stability of this moiety, we synthesized
diiron(II) complexes using the dinucleating ligand N-
EtHPTB and diphenylphosphinate or dimethylarsinate in
lieu of more frequently employed carboxylate bridges.
Solutions of these complexes reacted with dioxygen, form-
ing (μ-η1:η1-peroxo)diiron(III) moieties, which were ex-
amined spectroscopically.
3
3
3
that the peroxide is ligated in a μ-η1:η1 mode in all three
complexes, as found in the crystal structure of [Fe2(N-
EtHPTB)(O2)(OPPh3)2]3þ 19
ity observed between the νO-O values of 2 O2X and 3 O2X
.
However, the ∼50 cm-1 dispar-
3
3
raises the interesting question of why they are different.
One possible explanation for the difference is a change in
Upon oxygenation, 1 O2AsMe2 forms 2 O2AsMe2, a
metastable green-blue intermediate, before decaying to the
the Fe Fe distance. Brunold et al. proposed a mechanical
3
3
3 3 3
coupling model for understanding (μ-η1:η1-peroxo)diiron-
(III) units wherein the stretching frequency of the peroxide
bond increases due to increased mechanical coupling to the
Fe-O stretch as the Fe-O-O angle opens from 90°.57 On
the basis of this model, it stands to reason that, as the iron
centers move apart, the Fe-O-O angle should increase, thus
yellow iron(III) end product. Complex 1 O2PPh2 also forms
3
a green-blue intermediate (2 O2PPh2), but this species con-
3
verts to a second intermediate (3 O2PPh2), a deep blue
species, before decaying to the yellow end product. This
3
second intermediate is reminiscent of 3 O2CPh, the (μ-
3
η1:η1-peroxo)diiron(III) intermediate observed upon oxy-
producing a higher-frequency νO-O.
58 This relationship be-
genation of 1 O2CPh.29 These results bring up three
tween νO-O values and Fe Fe distances is supported by a
3
3 3 3
questions: (i) What is the nature of 2 O2X and 3 O2X?
recent publication showing that an increase in the Fe Fe
3
3
3 3 3
(ii) How does 2 O2X convert to 3 O2X? (iii) Why are
2 O2CPh and 3 O2AsMe2 not observed?
We first address the identities of 2 O2PPh2, 3 O2PPh2, and
2 O2AsMe2, determination of which rests on evidence gath-
3
3
3
3
(57) Brunold, T. C.; Tamura, N.; Kitajima, M.; Moro-oka, Y.; Solomon,
E. I. J. Am. Chem. Soc. 1998, 120, 5674–5690.
3
3
3
(58) The Brunold and Solomon model also predicts that, as νO-O
increases, νFe-O will decrease. This predicted trend is consistent with our
ered from a variety of spectroscopic techniques. Because each
of these intermediates has a strong chromophore arising from
observations, but we found a decrease of only ∼7 cm-1 between 2 O2PPh2
3
and 3 O2PPh2, which is much smaller than the ∼110 cm-1 change predicted
3
by the model for an increase in νO-O from ∼850 to ∼900 cm-1. The disparity
may be due to differences in the Fe-O-O-Fe dihedral angle between our
intermediates (0° observed in the [Fe2(N-EtHPTB)(O2)(OPPh3)2]3þ crystal
structure19) and the complex used in their calculations (52.9°).20 Further
theoretical work extending the mechanical coupling model to other diiron-
O2 adducts would be desirable.
(55) Chen, Q.; Lynch, J. B.; Gomez-Romero, P.; Ben-Hussein, A.;
Jameson, G. B.; O’Connor, C. J.; Que, L., Jr. Inorg. Chem. 1988, 27,
2673–2681.
(56) Feig, A. L.; Becker, M.; Schindler, S.; van Eldik, R.; Lippard, S. J.
Inorg. Chem. 1996, 35, 2590–2601.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8335
Scheme 1. Conversion of 2 O2X to 3 O2X
3
3
distance can be directly correlated to the increase in the
stretching frequency of the peroxide bond in a series of Fe2-
(μ-η1:η1-O2)(μ-OR) complexes.26 Applying this correlation to
the values of νO-O observed for 2 O2PPh2, 2 O2AsMe2, and
(OPPh3)2]3þ, suggesting that, in 3 O2PPh2, the phosphinate
becomes a terminal ligand on one iron, thereby opening up a
coordination site on the other (Scheme 1). In acetonitrile, this
3
site is most likely occupied by solvent, making both iron
3
3
::
3 O2PPh2 generates respective Fe Fe distances of 3.16, 3.13,
centers six-coordinate and indistinguishable by Mossbauer
3
3 3 3
˚
and 3.40 A, values in reasonable agreement with distances of
spectroscopy. However, in CH2Cl2, the available coordina-
tion site cannot be filled by the noncoordinating solvent or
the BPh4 counterion. This produces a complex in which one
iron atom is six-coordinate and the other is five-coordinate, a
˚
3.25, 3.27, and 3.47 A determined from EXAFS analysis.
::
On the basis of this information, as well as Mossbauer
evidence for equivalent iron atoms in 2 O2PPh2 and the
3
bidentate phosphinate bridge observed in the crystal struc-
condition reflected by the presence of two quadrupole doub-
::
ture of 1 O2PPh2, we postulate that the diiron(III) center in
lets in the Mossbauer spectrum (Figure 6 and Table 3).
3
2 O2PPh2 is bridged by three groups: the alkoxo oxygen of
During conversion from 2 O2PPh2 to 3 O2PPh2 at
3
3
3
N-EtHPTB, the 1,2-peroxo moiety, and the phosphinate
-30 °C, the peroxo-to-iron(III) charge-transfer band blue-
shifts 96 nm in MeCN and 57 nm in CH2Cl2 (Table 3). It is
not clear why such large blue-shifts occur. An obvious
ligand. The structure of 1 O2PPh2 has one open coordination
3
site on each iron atom, both of which are properly positioned
to allow dioxygen to coordinate easily in the 1,2-peroxo
bridging mode found in 2 O2PPh2 (Scheme 1). The parallels
difference between the proposed structures of 2 O2X and
3
3 O2X (Scheme 1) is the binding site of the peroxide. In
3
3
::
observed in the UV-vis, rR, Mossbauer, and EXAFS
spectra of 2 O2PPh2 and 2 O2AsMe2 indicate that the latter
2 O2X, it is cis to the N-EtHPTB amine nitrogen atoms,
while in 3 O2X, it is trans to those atoms. There is also a
difference in the Fe-O-O angles due to different Fe Fe
distances in each intermediate. Because the degree of per-
oxide and iron orbital mixing affects the energy required for a
LMCT, any changes in the iron coordination sphere, espe-
cially changes involving the peroxide, can produce changes in
the visible absorbance spectrum. The smaller shift observed
in CH2Cl2 may be partially accounted for by incomplete
3
3
3
3
complex is also triply bridged, with the role of the arsinate
moiety analogous to that of the phosphinate moiety in
3 3 3
2 O2PPh2.
3
Complex 2 O2PPh2 is observed to convert to 3 O2PPh2 at
3
3
temperatures above -40 °C. In this conversion, the phosphi-
nate moiety is proposed to shift to a terminal position on one
iron, resulting in the larger interiron distance revealed by
EXAFS analysis and the increased νO-O seen in the rR
conversion to 3 O2PPh2, but we must also consider the
3
spectrum of 3 O2PPh2. On the basis of similar rR data and
presence of one five-coordinate iron center as another factor
3
similar electronic transitions in the visible range, we propose
that can affect the λmax (Scheme 1, 3 O2X).
3
that 3 O2PPh2 and 3 O2CPh share the same basic dibridged
The mechanism proposed in Scheme 1 would require the
N3 ligand set on either end of the 2-hydroxypropane ligand
backbone to rearrange from a facial to a meridional config-
uration during conversion from 2 O2PPh2 to 3 O2PPh2. Our
3
3
structure (Scheme 1). A Hammett study of [Fe2(N-EtHPTB)-
(O2)(O2X)]2þ (O2X = substituted benzoate) showed a cor-
relation between σ values of the benzoate substituent and the
3
3
lifetimes of the peroxo intermediates, with electron-with-
hypothetical structure of 2 O2PPh2 is based on the crystal
3
19
.
drawing substituents increasing t1/2
This effect indicates
structure of 1 O2PPh2 (Figure 1), which clearly shows an
3
that the benzoate remains coordinated to the diiron(III) unit
open coordination site on each iron atom cis to its respective
amine nitrogen. Dioxygen binding to these sites would be
expected to be facile, and the resulting peroxo complex would
have a geometry similar to that of the simplified cartoon
in a MeCN solution of 3 O2CPh at -10 °C. Since this 1996
3
report, we have noted the similarity of the resonance Raman
data from 3 O2CPh with those collected after adding
3
OPPh326,40 and deduce that the benzoate moiety in 3 O2CPh
representing 2 O2X shown in Scheme 1, with the N3 ligand
3
3
must occupy a terminal position. While this conclusion is at
odds with the bridging configuration proposed by the
authors who originally characterized this peroxo com-
plex,29,40 they had the benefits of neither the mechanical
coupling model of Brunold et al.57 nor a comparison of the
sets configured facially. The proposed structure of 3 O2PPh2
3
is based on the crystal structure of [Fe2(N-EtHPTB)(O2)-
(OPPh3)2]3þ, in which both N3 ligand sets of the dinucleating
N-EtHPTB ligand adopt meridional configurations and both
OPPh3 ligands coordinate trans to the alkoxo bridge.19 In
νO-O values of 3 O2CPh and its OPPh3 adduct.26 The
addition to N-ligand rearrangement, conversion of 2 O2X to
3
3
unchanged νO-O value recorded after OPPh3 ligation indi-
3 O2X also requires the bridging oxyanion in the initial
3
cates that the Fe Fe distance is comparable in the two
species to move to a terminal position in the second species.
Shifting O2X to a monodentate coordination mode forms the
unobserved species shown in the center of Scheme 1, which
can revert to 2 O2X or irreversibly rearrange to form 3 O2X.
3 3 3
˚
complexes. The interiron distance of 3.47 A measured in
˚
3 O2PPh2 closely matches the interiron distance of 3.462 A
3
reported19 in the crystal structure of [Fe2(N-EtHPTB)(O2)-
3
3

8336 Inorganic Chemistry, Vol. 48, No. 17, 2009
Frisch et al.
In both MeCN and CH2Cl2 at -30 °C, we observe
significant conversion of 2 O2PPh2 to 3 O2PPh2, indicating
3
3
that the latter is thermodynamically favored at this tempera-
ture. In contrast, no buildup of 3 O2AsMe2 is observed at
3
any temperature, so 2 O2AsMe2 must be the more favored
3
Figure 12. Generic representation of the bicyclic diiron core proposed
for 2 O2X.
form. In the case of O2X = benzoate, however, the equilib-
rium shifts in the opposite direction, and only 3 O2CPh has
been reported. Also, 2 O2CPh has not been observed despite
3
3
3
shifts alter the interiron distance in each active site and may
affect the ability of each diiron site to activate O2. When O2 is
activated, the iron-iron distance can change by as much as
the fact that the reaction of 1 O2CPh with O2 has been
3
investigated in various solvents and solvent combinations at
low temperatures,29,40 including one study by stopped-flow
methods in propionitrile at -75 °C.59
The distinct preferences of the different O2X moieties for
2 O2X versus 3 O2X suggest that the nature of the O2X
˚
1.5 A from the diiron(II) starting point to the diiron(IV) state
associated with methane oxidizing intermediate Q,3 so the
number and the nature of bridging ligands are of vital
importance for controlling oxygen activation. While this
current work does not directly address the question of how
to facilitate O-O bond cleavage at a diiron center, it does
shed light on the ability of O2X ligands to tune interiron
distances by changing coordination modes. We have demon-
strated that moving an O2X ligand from a μ-1,3 to a terminal
coordination mode changes the interiron distance of an
alkoxide-bridged (μ-η1:η1-peroxo)diiron(III) complex by
3
3
bridge in 2 O2X affects the equilibrium associated with the
conversion of 2 O2X to 3 O2X. Clearly, arsinate favors 2 and
3
3
3
benzoate favors 3, while phosphinate is intermediate between
the two. A comparison of the pKa values of thecorresponding
conjugate acids (HO2AsMe2, 6.27;60,61 HO2PPh2, 2.32;62
HO2CPh, 4.1963) does not reveal a trend that matches our
observations. However, an examination of the respective bite
distances of the bridging oxyanions in each of the diiron(II)
˚
∼0.2 A. This means relatively minor ligand rearrangements
precursors shows that the lifetime of 2 O2X increases with a
3
˚
˚
can produce substantial changes in interiron distances, which
in turn, affect the stability of the O-O bond.
larger O O distance (namely, 2.23 A for O2CPh, 2.56 A for
3 3 3
˚
O2PPh2, and 2.80 A for O2AsMe2, as deduced from the
In summary, we have synthesized two new diiron(II)
complexes and investigated their reaction with oxygen, pro-
ducing (μ-η1:η1-peroxo)diiron(III) species. By varying the
central atom of a three-atom chain bridging the iron atoms,
we were able to produce and stabilize a peroxo moiety
previously unobserved when using N-EtHPTB as a ligand
(Scheme 1). Under the right conditions, the diphenylpho-
sphinate-bridged form of this peroxo moiety converts to a
second peroxo-containing species, akin to other peroxo
complexes previously formed using this ligand. Although
(μ-η1:η1-peroxo)diiron(III) complexes of N-EtHPTB and
other ligands have been studied for years, this conversion
of one peroxo species to another is reported here for the first
time; we suspect that it takes place upon oxygenation of all
previously reported, three-atom bridged, diiron(II) N-
EtHPTB complexes, albeit often so rapidly that it has never
been observed. Further experiments are underway to confirm
this hypothesis.
structures of the three 1 O2X complexes). We speculate that
3
the bigger O O bite distance imposes less strain on the
3 3 3
bicyclic moiety (Figure 12) and results in the more stable,
triply bridged core of 2 O2AsMe2. On the other hand, the
3
smaller bite distance of the benzoate bridge makes it difficult
˚
to span an Fe Fe distance of ca. 3.2 A expected for
3 3 3
2 O2CPh and leads to facile conversion of the benzoate
bridge to a terminal ligand and formation of 3 O2CPh.
3
3
This movement of an O2X ligand from a bridging to a
terminal position corresponds to a phenomenon of some
importance in diiron enzymes, referred to as a carboxylate
shift.64 For soluble methane monooxygenase, toluene mono-
oxygenase, and ribonucleotide reductase, one of the con-
served glutamate residues (E243, E231, and E238,
respectively) of the common diiron active site alternates
between terminal and μ-1,1 or μ-1,3 coordination modes in
the diiron(III) and diiron(II) forms.65-69 These carboxylate
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Supporting Information Available: Crystallographic data for
1 O2PPh2 and 1 O2AsMe2. UV-vis, resonance Raman and
3
3
::
Mossbauer spectra. Complete EXAFS fitting results, including
FEFF models. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Acknowledgment. This work was supported by the
National Institutes of Health through Grants GM-
38767 (L.Q.) and EB-001475 (E.M.). We thank Dr. Victor
Young, Jr., Benjamin Kucera and Dr. William Brennessel
of the X-ray Crystallographic Laboratory at the Univer-
sity of Minnesota for their invaluable assistance.
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