Raman signature of the Fe2O2 'diamond' core

Article abstract of DOI:10.1021/ja973220u

We report the resonance Raman (RR) spectra of iron complexes containing the Fe₂(μ-O)₂ core. Frozen CH₃CN solutions of the Fe(III)Fe(IV) intermediate [Fe₂(μ-O)₂L₂](ClO₄)₃ (where L = TPA, 5-Me₃-TPA, 5-Me₂-TPA, 5-MeTPA, 5-Et₃-TPA, or 3-Me₃-TPA) show numerous resonance-enhanced vibrations, and among these, an oxygen-isotope-sensitive vibration around 667 cm^-1 that shifts ca. 30 cm^-1 when the samples are allowed to exchange with ¹⁸OH₂, and whose Raman shift does not vary with methyl substitution of the TPA ligand. Spectra of iron-isotope-substituted samples of [Fe₂(μ-O)₂(L)₂](ClO₄)₃ (⁵⁴Fe and ⁵⁷Fe for L = TPA, and ⁵⁴Fe and ⁵⁸Fe for L = 5-Me₃-TPA) show that this vibration is also iron-isotope-sensitive. These isotopic data taken together strongly suggest that this vibration involves motion of the Fe₂(μ-O)₂ core that is isolated from motions of the ligand. A frozen CH₃CN solution of the diiron(III) complex [Fe₂(μ-O)₂(6-Me₃-TPA)₂](ClO₄)₂ shows one intense resonance-enhanced vibration at 692 cm^-1 that shifts -30 cm^-1 with ¹⁸O labeling. Normal coordinate analysis of the Fe₂(μ-O)₂ core in [Fe₂(μ-O)₂(5-Me₃-TPA)₂](ClO₄)₃ supports the assignment of the Fermi doublet centered around 666.2 cm^-1 as an A₁ vibration of this core. Furthermore, we propose that this unique feature found in the region between 650 and 700 cm^-1 is indicative of a diamond core structure and is the Raman signature of an iron cluster containing this core.

Full text of DOI:10.1021/ja973220u

J. Am. Chem. Soc. 1998, 120, 955-962
955
Raman Signature of the Fe2O2 “Diamond” Core
Elizabeth C. Wilkinson,1a Yanhong Dong,1a Yan Zang,1a Hiroshi Fujii,1a
Robert Fraczkiewicz,1b Grazyna Fraczkiewicz,1b Roman S. Czernuszewicz,*,1b and
Lawrence Que, Jr.*,1a
Contribution from the Department of Chemistry and Center for Metals in Biocatalysis,
UniVersity of Minnesota, Minneapolis, Minnesota 55455, and Department of Chemistry,
UniVersity of Houston, Houston, Texas 77204
ReceiVed September 12, 1997
Abstract: We report the resonance Raman (RR) spectra of iron complexes containing the Fe2(µ-O)2 core.
Frozen CH3CN solutions of the FeIIIFeIV intermediate [Fe2(µ-O)2L2](ClO4)3 (where L ) TPA, 5-Me3-TPA,
5-Me2-TPA, 5-MeTPA, 5-Et3-TPA, or 3-Me3-TPA) show numerous resonance-enhanced vibrations, and among
these, an oxygen-isotope-sensitive vibration around 667 cm-1 that shifts ca. 30 cm-1 when the samples are
allowed to exchange with 18OH2, and whose Raman shift does not vary with methyl substitution of the TPA
ligand. Spectra of iron-isotope-substituted samples of [Fe2(µ-O)2(L)2](ClO4)3 (54Fe and 57Fe for L ) TPA,
and 54Fe and 58Fe for L ) 5-Me3-TPA) show that this vibration is also iron-isotope-sensitive. These isotopic
data taken together strongly suggest that this vibration involves motion of the Fe2(µ-O)2 core that is isolated
from motions of the ligand. A frozen CH3CN solution of the diiron(III) complex [Fe2(µ-O)2(6-Me3-TPA)2]-
(ClO4)2 shows one intense resonance-enhanced vibration at 692 cm-1 that shifts -30 cm-1 with 18O labeling.
Normal coordinate analysis of the Fe2(µ-O)2 core in [Fe2(µ-O)2(5-Me3-TPA)2](ClO4)3 supports the assignment
of the Fermi doublet centered around 666.2 cm-1 as an A1 vibration of this core. Furthermore, we propose
that this unique feature found in the region between 650 and 700 cm-1 is indicative of a diamond core structure
and is the Raman signature of an iron cluster containing this core.
Introduction
serve as the foundation for the hypothesis that such cores may
participate in the high-valent chemistry of dioxygen-activating
iron and copper enzymes such as methane monooxygenase,
ribonucleotide reductase, and tyrosinase.7,8 Indeed, evidence
suggesting such an Fe2O2 core has just been reported for the
high-valent intermediate Q in the methane monooxygenase
cycle.9 Finally, the recent demonstration that the M2(µ-O)2
diamond core can be in equilibrium with the isomeric M2(µ-
η2:η2-O2) core in the Cu(iPr3TACN) system8a,10 potentially unites
the oxygen activation chemistry of Fe and Cu enzymes and the
water oxidation chemistry of Mn in the photosynthetic apparatus
into a common mechanistic framework for oxygen chemistry
at nonheme dinuclear metal centers.
The M2(µ-O)2 diamond core has recently emerged as a
common feature in the high-valent chemistry of a number of
first-row transition metals. Complexes with such cores are now
characterized for Mn,2 Fe,3 and Cu,4 and serve as synthetic
precedents for the M2(µ-O)2 diamond core in corresponding
metalloenzymes. There is a large number of crystallographically
defined complexes with the Mn2(µ-O)2 core, representing the
MnIIIMnIII, MnIIIMnIV, and MnIVMnIV oxidation states.2 All
evidence points to the Mn2(µ-O)2 core as present in an inactive
superoxidized form of manganese catalase5 and, more impor-
tantly, as the basic structural motif in the various S states of
the oxygen-evolving complex in photosystem II.6 However,
only recently have iron3 and copper4 complexes with the M2-
(µ-O)2 diamond core been prepared and characterized. Their
remarkable structural, spectroscopic, and reactivity properties
As deduced from X-ray crystallography,2-4 the principal
features of the M2(µ-O)2 diamond core are its short metal-oxo
bonds (ca. 1.8 Å), its acute M-O-M angles (90-110°), and a
metal-metal separation of less than 3 Å. In the absence of
suitable crystals, evidence for such cores can also be obtained
(1) (a) University of Minnesota. (b) University of Houston.
(2) Manchanda, R.; Brudvig, G. W.; Crabtree, R. H. Coord. Chem. ReV.
1995, 144, 1-38.
(3) (a) Zang, Y.; Dong, Y.; Que, L., Jr. J. Am. Chem. Soc. 1995, 117,
1169-1170. (b) Dong, Y.; Fujii, H.; Hendrich, M. P.; Leising, R. A.; Pan,
G.; Randall, C. R.; Wilkinson, E. C.; Zang, Y.; Que, L., Jr.; Fox, B. G.;
Kauffmann, K.; Mu¨nck, E. J. Am. Chem. Soc. 1995, 117, 2778-2792. (c)
Dong, Y.; Que, L., Jr.; Kauffmann, K.; Mu¨nck, E. J. Am. Chem. Soc. 1995,
117, 11377-11378.
(4) (a) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang,
X.; Young, V. G., Jr.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am.
Chem. Soc. 1996, 118, 11555-11574. (b) Mahapatra, S.; Young, V. G.,
Jr.; Kaderli, S.; Zuberbu¨hler, A. D.; Tolman, W. B. Angew. Chem., Int. Ed.
Engl. 1997, 36, 111-113.
(6) (a) Kirby, J. A.; Robertson, A. S.; Smith, J. P.; Thompson, A. C.;
Cooper, S. R.; Klein, M. P. J. Am. Chem. Soc. 1981, 103, 5529-5537. (b)
Yachandra, V. K.; DeRose, V. J.; Latimer, M. J.; Mukerji, I.; Sauer, K.;
Klein, M. P. Science 1993, 260, 675-679. (c) Yachandra, V. K.; Sauer,
K.; Klein, M. P. Chem. ReV. 1996, 2927-2950.
(7) Que, L., Jr.; Dong, Y. Acc. Chem. Res. 1996, 29, 190-196.
(8) (a) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young,
V., Jr.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman, W. B. Science 1996, 271,
1397-1400. (b) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem.
ReV. 1996, 96, 2563-2605.
(9) Shu, L.; Nesheim, J. C.; Kauffmann, K.; Mu¨nck, E.; Lipscomb, J.
D.; Que, L., Jr. Science 1997, 275, 515-518.
(5) (a) Waldo, G. S.; Yu, S.; Penner-Hahn, J. E. J. Am. Chem. Soc. 1992,
114, 5869-5870. (b) Dismukes, G. C. In Bioinorganic Catalysis; Reedjik,
J., Ed.; Marcel Dekker: Amsterdam, 1992; pp 317-346. (c) Gamelin, D.
R.; Kirk, M. L.; Stemmler, T. L.; Pal, S.; Armstrong, W. H.; Penner-Hahn,
J. E.; Solomon, E. I. J. Am. Chem. Soc. 1994, 116, 2392-2399.
(10) Abbreviations used: N5, N-(hydroxyethyl)-N,N′,N′-tris(2-benzimi-
dazolylmethyl)-1,2-diaminoethane; R3TACN, 1,4,7-trialkyl-1,4,7-triazacy-
clononane; TPA, tris(2-pyridylmethyl)amine; modifiers preceding TPA
denote the position of the alkyl substituent on the pyridine ring and the
number of rings modified.
S0002-7863(97)03220-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/27/1998

956 J. Am. Chem. Soc., Vol. 120, No. 5, 1998
Wilkinson et al.
from X-ray absorption spectroscopy, EXAFS in particular. Due
to its rigidity, the M2(µ-O)2 diamond core exhibits a prominent
feature at ca. 2.7 Å in the Fourier-transformed spectrum due to
the scattering from the other metal center.3b,4a,6a,8a This short
metal-metal distance together with appropriately short metal-
oxygen bond lengths provides strong support for the M2(µ-O)2
diamond core. Indeed the EXAFS technique has proven useful
for establishing the presence of such cores in manganese
catalase,5 the photosynthetic oxygen-evolving complex,6 and
intermediate Q of methane monoxygenase.9
Resonance Raman spectroscopy is yet another spectroscopic
tool for probing the M2(µ-O)2 diamond core. Due to its novel
geometry, vibrations unique to this core may be observed and
serve as diagnostic features for the presence of such centers in
synthetic reaction mixtures and metalloenzyme intermediates.
Studies thus far show that there is an intense resonance-enhanced
vibration in the 600-700 cm-1 region in the spectra of synthetic
Mn,11 Fe,3b and Cu4 complexes. In this paper, we report a
resonance Raman analysis of complexes with the Fe2(µ-O)2
diamond core using Fe, O, and N isotope substitution and ligand
modification. With the available isotopic substitution data, we
have carried out a normal coordinate analysis for the Fe2(µ-O)2
core in [Fe2(µ-O)2(L)2]3+ (L ) TPA and its methyl-substituted
derivatives) and assign the nature of the unique vibration in
the 600-700 cm-1 region that is the Raman “signature” for
this diamond core.
Figure 1. (A) Crystal structure of [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2.
(B) Parameters of the Fe2(µ-O)2 core from the crystal structure of [Fe2-
(µ-O)2(6-Me3-TPA)2](ClO4)2. (C) Proposed structure of [Fe2(µ-O)2(5-
Me3-TPA)2](ClO4)3 with distances derived from EXAFS DFT calcu-
lations, and angles calculated using simple trigonometric relationships.
Results and Discussion
Geometric Features of the Fe2(µ-O)2 Diamond Core. The
complex [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2 (1) is the first and
only Fe2O2 complex to be crystallographically characterized to
date3a and as such provides important structural parameters for
the vibrational analysis of other Fe2O2 complexes whose crystal
structures are not yet available. The X-ray structure of 1 shows
an Fe2(µ-O)2 core with C2h symmetry, with geometric features
detailed in Figure 1. This novel structure exhibits an Fe-O-
Fe angle of 92.5°, significantly smaller than found for any other
(µ-oxo)diiron(III) structure as required by the diamond core;
indeed such small angles are present in related M2(µ-O)2
complexes (92-102°). Furthermore there is a difference of
nearly 0.08 Å between the two Fe-µ-O components of each
Fe-O-Fe unit. For comparison, the structure of the related
[Cu2(µ-O)2(d21-Bn3TACN)2]2+ complex shows a Cu2(µ-O)2 core
with nearly D2h symmetry, with Cu-O bond lengths differing
only by 0.005 Å, while for MnIIIMnIII and MnIVMnIV complexes
with Mn2(µ-O)2 cores, the Mn-O bond lengths are usually
comparable in length but can differ by as much as 0.05 Å.12
Notably the Fe2O2 core of [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2 has
the largest distortion toward C2h symmetry observed. While
the presence of different ligands trans to the oxo bridges can
contribute to the bond length differences, it is clear from these
structural comparisons that other factors yet unrecognized
exacerbate the asymmetry of the M-O-M unit in the diiron
complex.
spin Fe(III)-low-spin Fe(IV) complex; consequently the two
metal sites are essentially identical. The diamond core structure
is deduced from EXAFS analysis which requires an Fe-Fe
distance of 2.89 Å. As with [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2,
the Fe-O-Fe unit has unequal bond lengths; one Fe-µ-O
distance is 1.77 Å, while the other Fe-µ-O distance is included
in a three-scatterer shell averaging 1.94 Å. DFT calculations18
on [Fe2(µ-O)2(TPA)2](ClO4)3 predict Fe-µ-O bond lengths of
1.77 and 1.83 Å with an Fe-Fe distance of 2.83 Å, parameters
in good agreement with the EXAFS results.
[Fe2(µ-O)2] Core Vibrations. The resonance Raman spec-
trum of a frozen CH3CN solution of [FeIII2(µ-O)2(6-Me3-TPA)2]-
(ClO4)2 derived from excitation into its visible absorption band
at 500 nm shows an intense vibration at 692 cm-1 (Figure 2a).
This is the only feature observed in the spectrum of 1. The
energy of this band is intermediate between those observed for
heme FedO complexes (800-850 cm-1) and those typically
observed for (µ-oxo)diiron(III) complexes (450-550 cm-1). If
the sample is allowed to exchange with H218O at low temper-
ature, one oxo group becomes protonated, resulting in the
formation of [FeIII2(µ-O)(µ-OH)(6-Me3-TPA)2](ClO4)3, which
is then deprotonated to re-form [FeIII2(µ-O)2(6-Me3-TPA)2]-
(ClO4)2 (presumably with incorporation of 18O label), isolated
as a solid, and then redissolved in dry CH3CN. The resonance
Raman spectrum of a sample prepared in this manner (Figure
2b) shows a new feature at 660 cm-1, resulting from incomplete
exchange with H218O,13 in addition to the dominant feature at
692 cm-1. Attempts to obtain samples of [FeIII2(µ-O)2(6-Me3-
TPA)2](ClO4)2 containing a higher percentage of 18O label were
unsuccessful due to the sensitivity of the complex to water,
resulting in its decomposition.
The core structure of [Fe2(µ-O)2(5-Me3-TPA)2](ClO4)3 (2)
appears to exhibit a similar distortion toward C2h symmetry, as
shown in Figure 1c. While a crystal structure of this complex
has not been obtained, this geometry is deduced from a body
of spectroscopic data. On the basis of its EPR and Mo¨ssbauer
properties, 2 is best described as a valence-delocalized low-
(11) Czernuszewicz, R. S.; Dave, B.; Rankin, J. G. In Spectroscopy of
Biological Molecules; Hester, R. E., Girling, R. B., Eds.; Royal Society of
Chemistry: Cambridge, 1991; pp 285-288.
(12) Goodson, P. A.; Oki, A. R.; Glerup, J.; Hodgson, D. J. J. Am. Chem.
Soc. 1990, 112, 6248-6254.
(13) There is no evidence of a residual of [FeIII2(µ-O)(µ-OH)(6-Me3-
TPA)2](ClO4)3 in the sample. This complex has a λmax of 550 nm and a
distinct peak at 676 cm-1 in its Raman spectrum that shifts to 646 cm-1
upon exchange with H218O.

Raman Signature of the Fe2O2 “Diamond” Core
J. Am. Chem. Soc., Vol. 120, No. 5, 1998 957
Figure 2. Resonance Raman spectra obtained with 457.9 nm excitation
of a frozen CH3CN solution of (A) [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2
and (B) [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2 with incorporation of 18O.
Figure 3. Resonance Raman spectra obtained with 613 nm excitation
of frozen CH3CN solutions of (A) [Fe2(µ-O)2(5-Me3-TPA)2](ClO4)3,
(B) [Fe2(µ-18O)2(5-Me3-TPA)2](ClO4)3 (sample is allowed to exchange
with 100 equiv of of 98%-enriched H218O in solution at -40 °C), (C)
[Fe2(µ-O)2(TPA)2](ClO4)3, (D) [Fe2(µ-18O)2(TPA)2](ClO4)3), (E) [Fe2-
(µ-O)2(3-Me3-TPA)2](ClO4)3, and (F) [Fe2(µ-18O)2(3-Me3-TPA)2]-
(ClO4)3. These spectra were obtained at 1 cm-1 resolution.
The resonance Raman spectrum of the corresponding [FeIII-
FeIV(µ-O)2(TPA)2]3+ complex also exhibits an intense oxygen-
isotope-sensitive vibration at 666.1 cm-1 but has a number of
additional features to lower energy (Figure 3). To aid in
assigning these features, we have prepared a series of [Fe2(µ-
O)2(L)2]3+ complexes where L is TPA or one of its 3-methyl-
or 5-methyl-substituted derivatives. These complexes all show
an intense visible absorption band at 616 nm, and excitation
into this band with 613 nm laser light produces the resonance
Raman spectra shown in Figure 3. These spectra show a gross
pattern of features that are similar in Raman shift, intensity and
sensitivity, to oxygen isotope substitution. The prominent
vibrational features of this series (along with several other ligand
variations) and the shifts upon exchange with H218O are
summarized in Table 1. We have also prepared the following
isotopically labeled complexes: 54Fe/57Fe for L ) TPA and
54Fe/58Fe and 14N/15N at the tertiary amine position for L )
5-Me3-TPA; the data are listed in Table 1.
tions can combine to produce a band that has the correct energy
and symmetry to interact with the 666 cm-1 vibration (Figure
4).14 For L ) TPA, the combination band would be found at
616 cm-1stoo low in energy to interact with the fundamental
of either the 16O or 18O complex. For L ) 5-Me3-TPA, the
combination band would be found at 668 cm-1, which is close
in energy to the fundamental of the 16O complex, hence the
Fermi doublet at that frequency. For L ) 3-Me3-TPA, the
combination band would be found at 635 cm-1stoo low in
energy to interact with the fundamental of the 16O complex,
but of the correct energy to interact with the fundamental of
the 18O complex. The 18O complex where L ) TPA shows a
band centered at 637.9 cm-1 with weak splitting into nonre-
solved features at 633.9 and 641.9 cm-1. The band centered at
637.9 cm-1 is too high in energy to be in Fermi resonance with
the combination band that would be found at 616 cm-1, but
there is a weak band at 635 cm-1 (Figure 3c) that could be
causing the observed splitting.
Because the frequency of the ca. 666 cm-1 vibration appears
to be insensitive to the introduction of methyl groups on the
TPA ligand, it must involve only motion of the Fe2(µ-O)2 core
which is isolated from motion of the tripodal ligand. The
isotope shift for the ca. 666 cm-1 vibration is approximately
32 cm-1 for the five [Fe2(µ-O)2L2](ClO4)3 complexes with both
oxo bridges 18O-labeled (Table 1). These shifts correspond to
those expected for a diatomic Fe-O stretch; however, more
detailed 18O-labeling experiments demonstrate that this vibration
The most striking common feature in these spectra is the
prominent oxygen-isotope-sensitive band centered around 666
cm-1. The sample where L ) TPA shows a strongly resonance-
enhanced vibration at 666.1 cm-1 that shifts to 637.9 with 18
O
substitution. The 5-Me3-TPA analogue exhibits a symmetric
doublet centered at 666.2 cm-1, which collapses to a single peak
at 635 cm-1 upon exchange with H218O. The 3-Me3-TPA
analogue, on the other hand, has a resonance-enhanced vibration
at 667 cm-1, which becomes a doublet centered at 636 cm-1
upon exchange with H218O. The doublets observed in these
spectra derive from Fermi resonance with ligand vibrations in
the lower frequency region. Examination of this region identi-
fies two vibrations which are suitable candidates for a combina-
tion band that could be in Fermi resonance with the ca. 666
cm-1 feature: 285 and 331 cm-1 for L ) TPA, 312 and 356
cm-1 for L ) 5-Me3-TPA, and 306 and 329 cm-1 for L )
3-Me3-TPA (Figure 3). These two fundamental ligand vibra-
(14) Cotton, F. A. Chemical Applications of Group Theory; Wiley-
Interscience: New York, 1971; pp 330-333.

958 J. Am. Chem. Soc., Vol. 120, No. 5, 1998
Wilkinson et al.
Table 1. Resonance Raman Data for [Fe2(µ-O)L2](ClO4)3
oxygen-isotope-sensitive bandsa
other bandsa
522(w)c 470 447 374
L
isotope
5-Me3-TPA 16O, NAFe 676.7;655.7 (666.2)b
18O, NAFe 635
426.5 409.1
426
356
312(w)c 254
623
394
402
521(w)
470 448 374
356
313(w)
253
18O16O
not resolved
no change
677.5, 656.7
677.0, 652.3
676, 664 (670)b
15
N
amine
54Fe
58Fe
428.5 410.1
422.5 403.9
425
522(w)
521(w)
471 448 376
469 446 372
357
355
313(w)
309(w)
318(w)
254
254
5-Me2-TPA 16O
408
370(b)
d
18O
16O
18O
632
668
636
420
424
416
392(b)
410
392(b)
403
3709b)
318(w)
338(w) 322(w)
338(w) 322(w)
5-Me-TPA
TPA
442 obse
364(b)
473 446 345
16O, NAFe 666.1
655.3 416
514(w)
514(w)
331
285
272(w)
285
254
18O, NAFe 633.9, 641.9 (637.9)b 622.7 408
390
473 446 345
331
272(w)
18O16O
54Fe
644
667.0
663.3
667
654.4
651.2
54Fe
3-Me3-TPA 16O
18O
411
405
404
388
515(w)
514(w)
476 455 374
476 454 372
329
330
306
308
251(w)
228(w)
254(w)
228(w)
644, 628 (636)b
a Bands reported relative to the 922 cm-1 band of frozen CH3CN or the 836 cm-1 band of frozen CD3CN as internal standard. Bands with higher
precision were derived from spectra obtained with a scan increment of 0.1 cm-1 as described in the Experimental Section. b Number in parentheses
represents the center of the Fermi doublet. c w ) weak. d b ) broad. e obs ) obscured by solvent peak.
Figure 4. Graphical representation of transitions resulting (or not) in
Fermi doublets in the 666 cm-1 band of [Fe2(µ-O)2(L)2](ClO4)3, where
(A) L ) TPA, (B) L ) 5-Me3-TPA, and (C) L ) 3-Me3-TPA, based
on 1 cm-1 resolution data.
must involve both oxygen atoms of the Fe2(µ-O)2 diamond core.
The [Fe2(µ-O)2(TPA)2]3+ complex was chosen for this study
because the absence of Fermi resonance effects in the 600-
Figure 5. Resonance Raman spectra of 5 mM [Fe2(µ-O)2(TPA)2]-
700 cm-1 region would best allow a feature of intermediate
(ClO4)3 with (A) 300 equiv of H216O added, (B) 300 equiv of a 1:3
frequency to be observed. Thus, a sample of this complex
mixture of H218O/H216O added, and (C) 300 equiv of H218O added.
prepared in the presence of 75:25 H216O/H218O (Figure 5b) gives
rise to a new peak at 644 cm-1, intermediate between 666.1
and 637.9 cm-1. This peak has an intensity indicating that it
must arise from the complex with only one 18O incorporated.
Were the 666.1 cm-1 vibration associated with an Fe-O stretch
or an Fe-O-Fe deformation, the mixed labeling experiment
would produce only the peaks at 666.1 and 637.9 cm-1 and no
feature at intermediate frequency. So the appearance of this
feature at intermediate frequency demonstrates that the 666.1
cm-1 band arises from a vibration associated with the entire
Fe2(µ-O)2 diamond core. The 22 cm-1 isotope shift observed
may appear larger than expected at first glance, but will be
rationalized in a later section.
Another core vibration is observed in the lower frequency
region of the Raman spectra of these complexes. [Fe2(µ-O)2-
(5-Me3-TPA)2]3+ exhibits an iron and oxygen isotope vibration
at 409.1 cm-1. This vibration too must involve motion of both
oxygen atoms of the Fe2(µ-O)2 core, as introduction of H218O
results in the progressive appearance of new features at 402
and 394 cm-1, corresponding to a shift of 7.1 and 15.1 cm-1
respectively, for the singly- and doubly-labeled complex. The
,

Raman Signature of the Fe2O2 “Diamond” Core
J. Am. Chem. Soc., Vol. 120, No. 5, 1998 959
corresponding features in the spectra of both [Fe2(µ-O)2-
(TPA)2]3+ and [Fe2(µ-O)2(3-Me3-TPA)2]3+ exhibit similar iso-
tope shifts upon complete exchange with H218O but are
complicated by Fermi resonance with a metal-ligand mode.
This metal-ligand mode is found at 426.5 cm-1 for the 5-Me3-
TPA complex and is sufficiently distinct from the 409.1 cm-1
diamond core vibration not to interact. However, the corre-
sponding metal-ligand modes in the TPA and 3-Me3-TPA
complexes are closer in energy (408 and 405 cm-1, respectively,
as deduced from the 18O data) and thus are in Fermi resonance
with the ca. 409 cm-1 core vibration (Table 1).
Tripodal Ligand-Sensitive Vibrations. The 200-550 cm-1
region in the Raman spectra of the [Fe2(µ-O)2(TPA)2](ClO4)3
complexes is rich in features and can be divided into two groups
of vibrations. The three low-intensity vibrations at 446, 473,
and 514 cm-1 are somewhat sensitive to the iron isotope but
relatively insensitive to alkyl substitutions on the tripodal ligand.
Their modest resonance enhancement suggests that these
vibrations do not follow along the same coordinates as the
displacement in the electronic excited state; because of their
low intensity, they will not be discussed further.
Figure 6. Correlation of the energy of (A) νsym and (B) νasym for a
variety of complexes containing the Fe-O-Fe unit with the Fe-O-
Fe angle. Data for [Fe2(µ-O)2(TPA)2](ClO4)3 and [Fe2(µ-O)2(6-Me3-
TPA)2](ClO4)2 diamond cores are indicated by diamond symbols. Data
represented by solid circles are from ref 16a, open circles from ref
16c, and open triangles from ref 3b.
More interestingly, there are five vibrations between 200 and
430 cm-1 which are significantly affected by methyl substitution
of the TPA ligand and are likely to arise from metal-ligand
vibrations and/or tripodal ligand deformations. Since none of
these vibrations shift with introduction of 15N at the tertiary
amine in the 5-Me3-TPA ligand, it is unlikely that an Fe-Namine
vibration is observed in these spectra. The vibrations observed
are almost certainly metal-pyridine vibrations. Indeed features
assigned as metal-ligand vibrations can be found in this region
of the infrared spectra of transition metal (Viz., Co, Ni, Cu, and
Zn) complexes of 3- and 4-alkylpyridines. For the vibrations
of the [Fe2(µ-O)2(L)2](ClO4)3 complexes, there is no clear
correlation between the energy of the vibration and the mass
of the alkyl substituent. This is not surprising since it has been
noted that the interaction of metal-ligand stretching with the
pyridine-alkyl bending modes actually tends to oppose the
simple mass effect (of adding the alkyl group).15 Inspection of
the spectra of the 5-Me3-TPA complex show that the vibrations
at 374 and 312 cm-1 are sensitive to iron isotope substitution,
both exhibiting 4 cm-1 shifts between the 54Fe and 58Fe
isotopomers, while the vibration at 254 cm-1 is iron-isotope-
insensitive. While an exact assignment is not possible without
a normal coordinate analysis that includes all the atoms of the
pyridine, a number of symmetric and asymmetric combinations
involving equatorial and axial pyridine moieties are possible
on the basis of the geometry shown in Figure 1. The large
isotope shifts exhibited by the 374 and 312 cm-1 modes are
fully consistent with their assignment as Fe-Py stretching
vibrations where the iron atom undergoes a net displacement,
while the mode at 254 cm-1 is likely due to a symmetric stretch
involving a collinear Py-Fe-Py unit.
stretch (A1 symmetry) is seen as a strong polarized band in the
Raman spectrum in the region between 380 and 540 cm-1
,
which decreases in energy as the Fe-O-Fe angle increases
(Figure 6). The asymmetric Fe-O-Fe stretch (B1 symmetry)
is seen in the infrared spectrum (and sometimes in the Raman
spectrum) in the region between 720 and 870 cm-1, and the
energy of the vibration increases as the Fe-O-Fe angle
increases (Figure 6). The third vibration is an A1 deformation
that involves mainly a change in the Fe-O-Fe angle and is
found (if seen at all) in the Raman spectrum at very low energy
(ca. 100 cm-1). The addition of the second oxo bridge to the
Fe-O-Fe unit to form the Fe2(µ-O)2 unit results in an Fe-
O-Fe angle near 100°, much smaller than found for (µ-oxo)-
diiron(III) complexes. Nevertheless, the Fe-O vibrations at
ca. 660-700 cm-1 observed for [Fe2(µ-O)2(6-Me3-TPA)2]2+ and
the [Fe2(µ-O)2(3 or 5-R3-TPA)2]2+ complexes (R ) H or CH3)
fall in the region expected for (µ-oxo)diiron complexes when
the correlations are extrapolated to smaller angles (Figure 6).
In fact, the vibrations of the Fe2(µ-O)2 complexes appear to
correlate well with values associated with the asymmetric Fe-
O-Fe mode. Though the mixed oxygen isotope data demon-
strate that the 666 cm-1 feature is tetraatomic in nature,
vibrations of the triatomic and the tetraatomic cores are clearly
closely interrelated.
Symmetry analysis of this [Fe2(µ-O)2]3+ core in C2h symmetry
predicts six modes: two Ag (Raman-active) and two Bu (IR-
active) modes that involve only Fe-O stretching, an Au (IR-
active) torsion or “butterfly” mode, and an Ag (Raman-active)
mode that involves bending of the Fe-O-Fe and O-Fe-O
angles. A normal coordinate analysis (NCA) of the Fe2(µ-O)2
core was thus carried out using a geometry based on the EXAFS
analysis of [Fe2(µ-O)2(5-Me3-TPA)2](ClO4)3 as shown in Figure
1. The Fe-Fe distance of 2.89 Å and the short Fe-O distance
of 1.77 Å come directly from the EXAFS analysis.17 The longer
Fe-O bond in the C2h Fe2(µ-O)2 core, which is presumably cis
to the amine nitrogen as found in the X-ray structure of [Fe2-
(µ-O)2(6-Me3-TPA)2](ClO4)2,3a could not be resolved from a
[Fe2(µ-O)2] Core Vibration Assignments: Normal Coor-
dinate Analysis. Since only recently have examples of an Fe2-
(µ-O)2 diamond core become available in iron chemistry,3 the
data presented here represent all the available vibrational spectra
for this type of complex. There are numerous examples of
complexes containing the Fe-O-Fe unit, however. The
vibrational spectra of these triatomic cores have been studied
in great detail,16 and the results from these studies can provide
a starting point for understanding the vibrations of the Fe2(µ-
O)2 core. The C2V Fe-O-Fe unit gives rise to three vibrations.
As noted by Sanders-Loehr et al.,16a the symmetric Fe-O-Fe
(16) (a) Sanders-Loehr, J.; Wheeler, W. D.; Shiemke, A. K.; Averill, B.
A.; Loehr, T. M. J. Am. Chem. Soc. 1989, 111, 8084-8093. (b) Czernusze-
wicz, R. S.; Sheats, J. E.; Spiro, T. G. Inorg. Chem. 1987, 26, 2063-2067.
(c) Holz, R. C.; Elgren, T. E.; Pearce, L. L.; Zhang, J. H.; O’Connor, C. J.;
Que, L., Jr. Inorg. Chem. 1993, 32, 5844-5850.
(17) For details of the EXAFS analysis of [Fe2(µ-O)2(5-Me3TPA)2]-
(ClO4)3 the reader is referred to ref 3b.
(15) Burgess, J. Spectrochim. Acta 1968, 24A, 277-283.

960 J. Am. Chem. Soc., Vol. 120, No. 5, 1998
Wilkinson et al.
Table 2. Data Used in Normal Mode Analysis of the Fe2(µ-O)2
Core
Fe2(µ-O)2 core
geometry
internal coordinate
force constanta
bond stretch
K (stretching)
3.1509
3.1858
3.1858
3.1509
H (bending)
0.2115
0.2115
0.4104
0.4104
Fe1-O2 (r1)
1.84
1.77
1.77
1.84
Fe2-O2 (r2)
Fe1-O1 (r3)
Fe2-O1 (r4)
valence angle bending
Fe1-O1-Fe2 (R1)
Fe1-O2-Fe2 (R2)
O1-Fe1-O2 (R3)
O1-Fe2-O2 (R4)
106.35
106.35
73.64
73.64
τ (torsion)
0.3178
torsion angleb (τ1)
180.0
F (sretch-stretch)
0.2846
0.2846
0.0671
0.0671
F (stretch-bend)
0.0754
-0.0732
r1 + r2
r3 + r4
r1 + r3
r2 + r4
b
r1 + R3
r1 + R4
c
a K in mdyn/Å, H and τ in (mdyn‚Å)/rad2, F interaction constants
in mdyn/Å. b r1 + R3 ) r2 + R3 ) r4 + R1 ) r3 + R1. c r1 + R4 ) r2
+ R2 ) r3 + R4 ) r4 + R2.
Figure 7. Atomic displacement vectors for the normal modes calculated
for [Fe2(µ-O)2(5-Me3-TPA)2](ClO4)3.
shell of O/N scatterers with an average distance from the Fe of
1.94 Å. For the purpose of this analysis, we have assigned the
longer bond a length of 1.84 Å, 0.07 Å longer than the 1.77 Å
Fe-O bond. This bond length difference is commensurate with
the 0.08 Å difference observed for [Fe2(µ-O)2(6-Me3-TPA)2]-
cm-1 band of [Fe2(µ-O)2(5-Me3-TPA)2]3+ appears as a Fermi
doublet, its isotope shifts may be difficult to measure accurately,
so the analogous isotope shift study was carried out on [Fe2-
(µ-O)2(TPA)2]3+, which exhibits a single peak at 666.1 cm-1
.
This feature shows 54Fe/57Fe and 16O2/18O2 isotope shifts of -3.7
and -28.2 cm-1, respectively, values similar to those observed
for [Fe2(µ-O)2(5-Me3-TPA)2]3+. These shifts can be associated
with ν2 or ν3 but are more consistent with the ν2 assignment.
Where then is the other Ag stretching mode ν3? Closer
examination of the 600-700 cm-1 region of the spectrum of
[Fe2(µ-O)2(TPA)2]3+ reveals a shoulder on the low-frequency
side of the 666.1 cm-1 feature. This shoulder at 655.3 cm-1
exhibits 54Fe/57Fe and 16O2/18O2 isotope shifts of -3.2 and -32.6
cm-1. Given the similarities in their Fe and O isotope shifts,
the 666.1 and 655.3 cm-1 features cannot be unequivocally
assigned to either ν2 or ν3. One way to resolve this puzzle is
to consider the 16O2/16O18O isotope shift. The calculations show
that the mixed isotope shifts differ significantly for ν2 and ν3,
-21.3 vs -11.2 cm-1. The 16O18O derivative of [Fe2(µ-O)2-
(TPA)2]3+ shows a peak at 644 cm-1. This corresponds to an
isotope shift of -22.1 cm-1 relative to the 666.1 cm-1 feature
and -11.3 cm-1 from the 655.3 cm-1 shoulder, thus making it
possible that both peaks are superimposed at 644 cm-1 in the
16O18O spectrum. This result supports an assignment of the
666.1 cm-1 peak to ν2 and the 655.3 cm-1 feature to ν3.
An interesting question to discuss is why ν2 should be more
prominent than ν3. In triatomic Fe-O-Fe complexes, the more
intense feature usually arises from the symmetric stretch; this
vibration corresponds to ν3 of the Fe2(µ-O)2 core, the core
breathing mode where all Fe-O bonds stretch in phase. ν2 for
the Fe2(µ-O)2 core corresponds to the asymmetric stretch of the
Fe-O-Fe complexes which is usually quite weak in the Raman
spectrum. Exceptions arise when the Fe-O-Fe unit exhibits
some asymmetry. Indeed there are two examples where the
asymmetric stretch is the most prominent feature in the Raman
spectrum, [Cl3Fe-O-FeN5]+20 and [Fe2(µ-O)(µ-O2CtBu)2(Me3-
TACN)2]+.21 For these two complexes, there is a significant
difference in the lengths of the two Fe-µ-O bonds which leads
to enhancement of the asymmetric stretch and the corresponding
3a
(ClO4)2 and the 0.06 Å difference obtained from ab initio
LDF-DFT calculations on the TPA analogue [Fe2(µ-O)2-
(TPA)2]3+ 18
The calculated frequencies were not significantly
.
changed when longer Fe-O bond lengths were used in the
calculations.
Using the force field in Table 2, we calculated the six normal
modes for the C2h Fe2(µ-O)2 core in [Fe2(µ-O)2(TPA)2](ClO4)3,
and these modes, depicted with the displacement vectors
generated by the atomic displacement matrix, are shown in
Figure 7. The primary force constants used for the general
valence force field (Table 2) are typical for an Fe-µ-O-Fe
core,16 with the addition of k3 for the O-Fe-O bend and k4
for the Fe2O2 torsion, both internal coordinates that are
introduced by the addition of the second oxo bridge. The
magnitude and sign of the interaction force constants were
arrived at on the basis of previous calculations on the related
Mn2(µ-O)2 and the Cu2(µ-η2:η2-O2) cores.19 The calculations
using the force constants listed in Table 3 and the geometry
shown in Figure 1 result in the frequencies listed in Table 3 for
the six normal modes shown in Figure 7.
The prominent feature at around 666 cm-1 found in all the
[Fe2(µ-O)2]3+ complexes is sensitive only to the introduction
of Fe and O isotopes. It must arise from one of the two Ag
stretching modes of the Fe2(µ-O)2 core, ν2 or ν3, both of which
are calculated to have frequencies in the 600-700 cm-1 region,
with the 54Fe/58Fe, 54Fe/57Fe, and 16O2/18O2 isotope shifts listed
in Table 2. The 54Fe/58Fe and 16O2/18O2 isotopic shifts for the
666.2 cm-1 band of [Fe2(µ-O)2(5-Me3-TPA)2]3+ are -2.5 and
-31.2 cm-1, respectively, values which agree better with the
isotope shifts calculated for ν2 (Table 2). Because the 666.2
(18) Ghosh, A.; Almlo¨f, J.; Que, L., Jr. Angew. Chem., Int. Ed. Engl.
1996, 35, 770-772.
(19) Ling, J.; Nestor, L. P.; Czernuszewicz, R. S.; Spiro, T. G.;
Fraczkiewicz, R.; Sharma, K. D.; Loehr, T. M.; Sanders-Loehr, J. J. Am.
Chem. Soc. 1994, 116, 7682-7691.

Raman Signature of the Fe2O2 “Diamond” Core
J. Am. Chem. Soc., Vol. 120, No. 5, 1998 961
Table 3. Calculated Frequencies and Isotope Shifts (cm-1) for Vibrations of the Fe2(µ-O)2 Core in [Fe2(µ-O)2(5-R3-TPA)2](ClO4)3
∆(18O)2
∆(18O)(16O)a
∆54Fe/58Fea
∆54Fe/57Fea
PED (%)b
a
assignment
frequency
Calculated for the C2h [Fe2(µ-O)2] Core
ν1 (Bu)
ν2 (Ag)
ν3 (Ag)
ν4 (Bu)
ν5 (Au)
ν6 (Ag)
704.5
666.4
652.5
596.9
204.0
210.5
-31.1
-32.5
-27.1
-26.4
-9.0
-9.9
5.6
3.4
6.2
4.7
1.6
5.2
4.3
2.6
4.8
3.6
1.2
4.0
r1 (58), r2 (54), r1 + r2 (-10)
r1 (70), r2 (41), r1 + r2 (-10)
r1 (58), r2 (28), r1 + r2 (7)
r1 (46), r2 (42), r1 + r2 (8), R1 (6)
τ1 (100)
-21.3
-11.2
-16.2
-4.5
-3.4
-1.7
R3 (63), R1 (32)
Observed for [Fe2(µ-O)2(5-Me3-TPA)2](ClO4)3
ν2 or ν3
666.2
409.1
-31.2
-15.1
not resolved
2.5
6.2
a
2ν5
-7.1
Observed for [Fe2(µ-O)2(TPA)2](ClO4)
ν2 (or ν3)
ν3 (or ν2)
666.1
655.3
-28.2
-32.6
-22.1
3.7
3.2
not found
a ∆(18O)2 ) ν(18O)2 - ν(16O)2; ∆(18O)(16O) ) ν(18O)(16O) - ν(16O)2, ∆54Fe/58Fe ) ν(54Fe) - ν(58Fe), ∆54Fe/57Fe ) ν(54Fe) - ν(57Fe). b Potential
energy distributions calculated with respect to force constants.31 Symbols correspond to those in Table 2.
decrease in intensity of the symmetric stretch. The fact that
the Fe-µ-O bonds of each Fe-O-Fe unit of the Fe2(µ-O)2 core
have different lengths, i.e., C2h, rather than D2h, symmetry,
rationalizes the enhancement of the ν2 mode relative to the ν3
mode.
and ribonucleotide reductase,24 respectively. Resonance Raman
spectroscopy would thus be a useful tool to probe the structures
of such intermediates and ascertain whether such a diamond
core is involved in the catalytic cycle of nonheme diiron
enzymes.
The only other feature that is sensitive as well to Fe and O
isotopes is at 409.1 cm-1 in the spectrum of [Fe2(µ-O)2(5-Me3-
TPA)2]3+. This feature appears as a Fermi doublet in the spectra
of the TPA and 3-Me3-TPA analogs and is thus more difficult
to interpret in the latter complexes. The 54Fe/58Fe and 16O2/
18O2 isotope shifts observed for the 5-Me3-TPA complex are
6.2 and 15.1 cm-1, respectively. A perusal of Table 3 shows
no Ag mode near 400 cm-1. However, the first overtone of ν5,
the out-of-plane butterfly motion of the core (Au symmetry),
which is calculated to appear at 204.0 cm-1, would afford a
band at the right energy and with appropriate isotope shifts.
Significant resonance enhancement of the first overtone of the
IR-active out-of-phase breathing mode of the two linked FeS4
tetrahedra has also been observed in the Raman spectra of model
Fe2(µ-S)2 complexes.22 We thus assign the 409.1 cm-1 band
as 2ν5, the first overtone of ν5.
Experimental Section
General Procedures. All reagents and solvents were obtained from
commercial sources and used as received unless otherwise noted.
Oxygen isotope H218O (98% enriched) and iron isotopes 54Fe2O3 (99.9%
enriched), 57Fe2O3 (99.9% enriched), and 58Fe (89% enriched) were
obtained from commercial sources and used as received. Solvents were
dried according to published procedures.25 Caution! The perchlorate
salts in this study are all potentially explosiVe and should be handled
with care.
Tris(3-methyl-2-pyridylmethyl)amine (3-Me3-TPA). A mixture
of 4.85 g (24.8 mmol) of 2-(aminomethyl)-3-methylpyridine hydro-
chloride (prepared as described by Dong et al.3a) and 8.83 g (49.5 mmol)
of 2-(chloromethyl)-3-methylpyridine hydrochloride (prepared as de-
scribed by Hardegger26 and Baker et al.27) in 30 mL of water was
neutralized with 5.1 g (127.5 mmol) of NaOH in 10 mL of water at 0
°C. This solution was stirred at room temperature for 2 days, at the
end of which solid product (7.2 g) precipitated in 92% yield. The solid
was dissolved in CHCl3, and the solution was washed with water. The
purified product (5.0 g, 64% yield) was precipitated from the CHCl3
solution with ether. 1H NMR (CDCl3): δ (ppm) 1.65 (s, 3H, -CH3),
3.84 (s, 2H, CH2N), 7.09 (dd, 1H, â-pyr), 7.33, (d, 1H, R- or γ-pyr),
8.38 (d, 1H, R- or γ-pyr).
Conclusion
Resonance Raman spectra of complexes containing the high-
valent Fe2(µ-O)2 diamond core all show an oxygen- and iron-
isotope-sensitive vibration centered around 666 cm-1. This
vibration is unique to this type of complex, and through isotope
substitution, ligand substitution, and normal coordinate analysis,
we assign this vibration as ν2, the Ag mode of the C2h Fe2(µ-
O)2 core which involves stretching of the two shorter Fe-O
bonds alternating with stretching of the two longer Fe-O bonds
(Figure 7). A strongly resonance-enhanced, oxygen-isotope-
sensitive vibration between 600 and 700 cm-1 is also observed
for the diiron(III) complex [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2 as
well as for complexes with Mn2(µ-O)211 and Cu2(µ-O)2 cores.4
Taken together, these data indicate that a 600-700 cm-1 band
with significant oxygen isotope sensitivity is a characteristic
feature of the Raman spectra of complexes containing the M2-
(µ-O)2 diamond core. An Fe2(µ-O)2 diamond core has been
proposed7 as one of the structures for the high-valent intermedi-
ates Q and X in the redox cycles of methane monooxygenase9,23
N,N-Bis(5-methyl-2-pyridylmethyl)-N-(2-pyridylmethyl)ammo-
nium Perchlorate (5-Me2-TPA‚3HClO4). A mixture of 0.61 g (5.6
mmol) of 2-(aminomethyl)pyridine and 2.00 g (11.2 mmol) of 2-(chlo-
romethyl)-5-methylpyridine hydrochloride in 10 mL water was neutral-
ized with 0.90 g (22.5 mmol) of NaOH in 5 mL of water at 0 °C. This
solution was stirred at room temperature for 3 days and extracted with
CH2Cl2. The extract was washed with water, and the solvent was
(23) (a) Lee, S.-K.; Nesheim, J. C.; Lipscomb, J. D. J. Biol. Chem. 1993,
268, 21569-21577. (b) Lee, S.-K.; Fox, B. G.; Froland, W. A.; Lipscomb,
J. D.; Mu¨nck, E. J. Am. Chem. Soc. 1993, 115, 6450-6451. (c) Liu, K. E.;
Valentine, A. M.; Wang, D.; Huynh, B. H.; Edmondson, D. E.; Salifoglou,
A.; Lippard, S. J. J. Am. Chem. Soc. 1995, 117, 10174-10185.
(24) (a) Riggs-Gelasco, P. J.; Shu, L.; Chen, S.; Burdi, D.; Huynh, B.
H.; Que, L., Jr.; Stubbe, J. J. Am. Chem. Soc., in press. (b) Sturgeon, B. E.;
Burdi, D.; Chen, S.; Huynh, B.-H.; Edmondson, D. E.; Stubbe, J.; Hoffman,
B. M. J. Am. Chem. Soc. 1996, 118, 7551-7557. (c) Bollinger, J. M., Jr.;
Edmondson, D. E.; Huynh, B. H.; Filley, J.; Norton, J.; Stubbe, J. Science
(Washington, D.C.) 1991, 253, 292-298.
(20) Gomez-Romero, P.; Witten, E. H.; Reiff, W. M.; Backes, G.;
Sanders-Loehr, J.; Jameson, G. B. J. Am. Chem. Soc. 1989, 111, 9039-
9047.
(25) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Pergamon Press: New York, 1988.
(21) Cohen, J. D.; Payne, S.; Hagen, K. S.; Sanders-Loehr, J. J. Am.
Chem. Soc. 1997, 119, 2960-2961.
(22) Han, S.; Czernuszewicz, R. S.; Spiro, T. G. J. Am. Chem. Soc. 1989,
111, 3496-3504.
(26) Hardegger, E.; Nikles, E. HelV. Chim. Acta 1957, 40, 2428-
2433.
(27) Baker, W.; Buggle, K. M.; McOmie, J. F. W.; Watkins, D. A. M.
J. Chem. Soc. London 1958, 3594-3603.

962 J. Am. Chem. Soc., Vol. 120, No. 5, 1998
Wilkinson et al.
evaporated in Vacuo. The residue was treated with concentrated HClO4,
and the light yellow product (2.6 g, 74%) was precipitated from the
acetone solution with ether. 1H NMR (D2O): δ (ppm) 2.48 (s, 6H,
-CH3), 4.31 (s, 4H, CH2N of 5-Me-py), 4.35 (s, 2H, CH2N of py),
7.91-7.99 (m, 3H), 8.06-8.10 (d, 1H), 8.32-8.36 (d, 2H), 8.50-8.55
(m, 3H), 8.71-8.74 (d, 1H).
N-(5-Methyl-2-pyridylmethyl)-N,N-bis(2-pyridylmethyl)ammo-
nium Perchlorate (5-Me-TPA‚3HClO4). A mixture of 1.17 g (5.9
mmol) of bis(2-pyridylmethyl)amine and 1.05 g (5.9 mmol) of
2-(chloromethyl)-5-methylpyridine hydrochloride in 10 mL water was
neutralized with 0.47 g (11.8 mmol) of NaOH in 5 mL of water at 0
°C. This solution was stirred at room temperature for 3 days and
extracted with CH2Cl2. The extract was washed with water, and the
solvent was evaporated in Vacuo. The residue was treated with
concentrated HClO4, and the light yellow product (2.9 g, 81%) was
precipitated from the acetone solution with ether. 1H NMR (D2O): δ
(ppm) 2.45 (s, 3H, -CH3), 4.30 (s, 2H, CH2N of 5-Me-py), 4.34 (s,
4H, CH2N of py), 7.89-7.96 (m, 3H), 8.03-8.07 (d, 2H), 8.28-8.32
(d, 1H), 8.44-8.52 (m, 3H), 8.68-8.71 (d, 2H).
[Fe2O(3-Me3-TPA)2(OH)(H2O)](ClO4)3 was prepared by mixing
ligand (0.95 mmol) and Et3N (3.10 mmol) in 20 mL of methanol and
Fe(ClO4)3‚10H2O (0.500 g, 0.94 mmol) in 5 mL of methanol. The
solution was allowed to stand on the bench for 1 day, and dark green
crystals formed which were filtered. Anal. Calcd for [Fe2O(3-Me3-
TPA)2(OH)(H2O](ClO4)3‚2H2O (C42H55Cl3Fe2N8O17): C, 43.41; H, 4.77;
N, 9.64. Found: C, 43.62, H, 4.69, N, 9.36.
[Fe2O(5-Me2-TPA)2(OH)(H2O)](ClO4)3 and [Fe2O(5-Me-TPA)2-
(OH)(H2O)](ClO4)3 were prepared by mixing ligand HCL salt (0.94
mmol) and Et3N (4.5 mmol) in 20 mL of methanol and Fe(ClO4)3‚
10H2O (0.500 g, 0.94 mmol) in 5 mL of methanol. The solution was
allowed to stand on the bench for 1 day, and dark green crystals formed
which were filtered. Anal. Calcd for [Fe2O(5-Me2-TPA)2(OH)-
(H2O)](ClO4)3‚2H2O (C40H51Cl3Fe2N8O17): C, 42.36; H, 4.53; N, 9.88.
Found: C, 42.33, H, 4.79, N, 9.28. Anal. Calcd for [Fe2O(5-Me-
TPA)2(OH)(H2O)](ClO4)3‚2H2O (C38H47Cl3Fe2N8O17): C, 41.27; H,
4.28; N, 10.13. Found: C, 41.22, H, 4.17, N, 10.07.
(H2O)L2](ClO4)3 in a vial of CH3CN, placing the vial in a -40 °C
bath, and adding 1.5 equiv of H2O2 to generate the [Fe2(µ-O)2L2](ClO4)3
complex.
Raman Spectroscopy. Raman spectra were recorded at the
University of Minnesota on a Spex 1403 double spectrometer interfaced
with a Spex DM3000 data system using a Spectra Physics 2030-15
argon ion laser, and a 375B CW dye (Rhodamine 6G) laser. Spectra
were obtained at 77 K using a back-scattering geometry, and Raman
frequencies are referenced to appropriate solvent features that were in
turn calibrated with neon and mercury discharge lamps. Samples were
frozen onto a gold-plated copper cold finger in thermal contact with a
dewar containing liquid nitrogen.28 All spectra were recorded with slits
set for a band-pass of 2 cm-1. Most spectra were obtained with scan
increments of 1 cm-1, and peak positions are listed to 1 cm-1 precision.
A protocol reported by Nakamoto was used to obtain peak positions
reported to 0.1 cm-1 precision,29 which entailed averaging four highly
reproducible scans with scan increments of 0.1 cm-1
. To calculate
the peak positions, the region of interest was expanded, the peaks fit
with a Voigtian line shape, and the centroid taken as the peak frequency.
Normal Coordinate Analysis. Normal mode calculations were
carried out with the GF matrix method30 and a general valence force
field which included bond stretching, angle bending, and torsional force
constants. Also included were valence interaction force constants for
all coordinates sharing at least one atom. All calculations were carried
out on a Silicon Graphics INDY workstation using a UNIX version of
the RAMVIB software.31 The visualization of normal eigenvectors was
accomplished by transfer of the Cartesian atomic displacements into
the X-Mol molecular graphics program.32 Molecular parameters for
the Fe2(µ-O)2 unit of [Fe2(µ-O)2(5-Me3-TPA)2](ClO4)3 were obtained
from bond distances determined by EXAFS,3b from which the Cartesian
coordinates were determined by simple trigonometric relationships.
Acknowledgment. We thank the National Institutes of
Health for grants (GM33162 to L.Q. and GM48370 to R.S.C.)
and a predoctoral traineeship (GM07323 to E.C.W.) in support
of this work.
Samples. [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)2 was prepared according
to the published procedures.3a 54Fe and 58Fe samples of [Fe2(µ-O)2(5-
Me3-TPA)2](ClO4)3 and 54Fe and 57Fe samples of [Fe2(µ-O)2(TPA)2]-
(ClO4)3 were prepared according to the procedure for preparation of
57Fe samples.3b Samples of [Fe2(µ-O)2L2](ClO4)3 (L ) 3-Me3-TPA or
5-Me3-TPA) were prepared by dissolving the pure solid (prepared
according to the published procedure)3b in a vial of CH3CN that is
suspended in a -40 °C bath. All other samples of [Fe2(µ-O)2L2](ClO4)3
were prepared by dissolving the starting material [Fe2(µ-O)(OH)-
JA973220U
(28) Czernuszewicz, R. S.; Johnson, M. K. Appl. Spectrosc. 1983, 37,
297-298.
(29) Nakamoto, K. Angew. Chem., Int. Ed. Engl. 1972, 11, 666-674.
(30) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations;
McGraw-Hill: New York, 1955.
(31) Fraczkiewicz, R.; Czernuszewicz, R. S. J. Mol. Struct. 1997, 435,
109-121.
(32) X-Mol, version 1.3.1; Minnesota Supercomputer Center, Inc.:
Minneapolis, MN, 1993.