An iron(II)[1,3-bis(2'-pyridylimino)isoindoline] complex as a catalyst for substrate oxidation with H2O2 - Evidence for a transient peroxidodiiron(III) species

Article abstract of DOI:10.1002/ejic.201300162

The complex [Fe(indH)(solvent)₃](ClO₄)₂ (1) has been isolated from the reaction of equimolar amounts of 1,3-bis(2'-pyridylimino)isoindoline (indH) and Fe(ClO₄)₂ in acetonitrile and characterized by X-ray crystallography and several spectroscopic techniques. It is a suitable catalyst for the oxidation of thioanisoles and benzyl alcohols with H2O2 as the oxidant. Hammett correlations and kinetic isotope effect experiments support the involvement of an electrophilic metalbased oxidant. A metastable green species (2) is observed when 1 is reacted with H₂O₂ at -40 °C and has a Fe^III(μ-O)(μ-O₂)Fe^III core on the basis of UV/Vis, electron paramagnetic resonance, resonance Raman, and X-ray absorption spectroscopic data.

Full text of DOI:10.1002/ejic.201300162

FULL PAPER
DOI:10.1002/ejic.201300162
An Iron(II)[1,3-bis(2Ј-pyridylimino)isoindoline] Complex as
a Catalyst for Substrate Oxidation with H2O2 – Evidence
for a Transient Peroxidodiiron(III) Species
CLUSTER
ISSUE
József S. Pap,[a] Matthew A. Cranswick,[b] É. Balogh-Hergovich,[a]
Gábor Baráth,[a] Michel Giorgi,[c] Gregory T. Rohde,[b]
József Kaizer,*[a] Gábor Speier,[a] and Lawrence Que Jr.*[b]
Keywords: Iron / Tridentate ligands / N ligands / Peroxido ligands / Oxidation
The complex [Fe(indH)(solvent)3](ClO4)2 (1) has been iso-
lated from the reaction of equimolar amounts of 1,3-bis(2Ј-
pyridylimino)isoindoline (indH) and Fe(ClO4)2 in acetonitrile
and characterized by X-ray crystallography and several
spectroscopic techniques. It is a suitable catalyst for the oxi-
dation of thioanisoles and benzyl alcohols with H2O2 as the
oxidant. Hammett correlations and kinetic isotope effect ex-
periments support the involvement of an electrophilic metal-
based oxidant. A metastable green species (2) is observed
when 1 is reacted with H2O2 at –40 °C and has a FeIII(μ-O)(μ-
O2)FeIII core on the basis of UV/Vis, electron paramagnetic
resonance, resonance Raman, and X-ray absorption spectro-
scopic data.
brational features associated with a (μ-1,2-peroxido)di-
iron(III) unit.[17,23,24]
Introduction
Dioxygen activation by non-heme diiron enzymes occurs
in a number of metabolically important transformations,[1]
including the hydroxylation of methane by soluble methane
monooxygenase (sMMO),[2–4] the conversion of ribonucleo-
tides to deoxyribonucleotides by ribonucleotide reductase
(RNR),[5–7] the formation of unsaturated fatty acids by
fatty acid desaturases,[8] the biosynthesis of antibiotics
(CmlA, CmlI),[9] and the hydroxylation of eukaryotic initia-
tion factor 5a by human deoxyhypusine hydroxylase
(hDOHH) in the regulation of eukaryotic cell prolifera-
tion.[10] In general, O2 activation is thought to be initiated
by the binding of O2 to the diiron(II) center to form a per-
oxidodiiron(III) intermediate that in turn converts to the
oxidizing species. Peroxidodiiron(III) intermediates with
visible features between 600–750 nm have been identified
for sMMO,[11–13] RNR R2,[14–18] stearoyl-acyl carrier pro-
tein (Δ9D),[19,20] frog M ferritin,[21,22] and hDOHH.[10] Sev-
eral of these species have been characterized by resonance
Raman (rRaman) spectroscopy and shown to exhibit vi-
For sMMO and RNR R2, the peroxidodiiron(III) inter-
mediate undergoes O–O bond scission to generate a high-
valent oxidant capable of X–H bond cleavage. Kinetic stud-
ies of this conversion in sMMO show that a proton is re-
quired for O–O bond cleavage to occur,[25,26] which leads to
the formation of the high-valent intermediate, designated as
Q, which carries out methane hydroxylation. Intermediate
Q is generally accepted to possess a diiron(IV) center with
two bridging oxido ligands to form a [FeIV2(O)2] “diamond
core”, which cleaves the methane C–H bond.[27] On the
other hand, O–O bond cleavage of the peroxidodiiron(III)
intermediate of RNR R2 is initiated by proton-coupled
electron transfer, which results in the formation of an FeIII
–
O–FeIV intermediate called X;[28,29] this is the oxidant re-
sponsible for the formation of a nearby tyrosyl radical,
which in turn generates a cysteinyl radical by long distance
proton-coupled electron transfer to initiate the deoxygen-
ation of ribonucleotides.[7]
Understanding the factors that lead to O–O bond cleav-
age at non-heme diiron centers has been a major goal of
researchers in this field. Studies of synthetic complexes that
mimic O2 binding and activation can complement and aug-
ment the information available from enzyme studies. Com-
plexes for which intermediates along the O2-activation path-
way can be trapped are particularly useful. To date, poly-
dentate ligands with combinations of N-heterocycles and
amine donors have afforded peroxidodiiron(III) complexes
that can be spectroscopically characterized.[30–46] In this
study, we report our results with the rigid, meridional 1,3-
[a] Department of Chemistry, University of Pannonia,
8201 Veszprém, Wartha Vince u. 1., Hungary
Homepage: www.uni-pannon.hu
[b] Department of Chemistry and Center for Metals in
Biocatalysis, University of Minnesota,
207 Pleasant St. SE, Minneapolis, MN 55455, USA
Homepage: http://bioinorg.chem.umn.edu/quespace/
[c] Aix-Marseille Université, FR1739, Spectropole, Campus St.
Jérôme,
Avenue Escadrille Normandie-Niemen, 13397 Marseille cedex
20, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300162.
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bis(2Ј-pyridylimino)isoindoline (indH, Scheme 1), which
has been shown to function as
neutral[47–52] or
a
monoanionic ligand.[53–56] The isoindolines normally oc-
cupy three meridional sites at a metal ion and form either
1:1 or 2:1 (ligand/metal) complexes. We have previously re-
ported the preparation and crystal structures of a μ-oxido-
bridged diiron(III) complex, [(Fe(ind)Cl)2O] and a homo-
leptic iron(II) complex [Fe(ind)2].[57,58] Herein, we present
the synthesis and structure of a new iron(II) complex,
[Fe(indH)(CH3CN)3](ClO4)2 (1) and the spectroscopic
characterization of a transient green species (2) derived
from the reaction of 1 with H2O2. We also report an initial
investigation into the catalytic reactivity of 1 in combina-
tion with H2O2 that focuses on the oxidation of thioanisoles
and benzyl alcohols.
Figure 1. Molecular structure of 1 with crystallographic number-
ing. Hydrogen atoms (except for H4B), two perchlorate counter-
ions, and the water solvate are omitted for clarity. Thermal ellip-
soids are plotted at the 50% probability level. Selected bond dis-
tances [Å]: Fe–N1 2.0722(18), Fe–N3 2.1798(19), Fe–N5 2.219(2),
Fe–N6 2.184(2), Fe–N7 2.189(2), Fe–N8, 2.202(2), N(1)–C(1)
1.336(3), N(1)–C(8) 1.423(3), N(2)–C(8) 1.279(3), N(4)–C(1)
1.332(3).
Scheme 1. Structure of the indH ligand.
The geometry around the iron(II) center can be described
as distorted octahedral, and the equatorial plane is defined
by three N atoms of the tridentate ligand and the N atom
of one acetonitrile molecule with an average Fe–N distance
of 2.16 Å. The axial positions are occupied by two acetoni-
trile molecules with Fe–N distances of ca. 2.2 Å. The Fe–N
Results and Discussion
[58]
bond lengths are similar to those obtained for Fe(ind)2
and are consistent with the longer Fe–N distances associ-
ated with high-spin FeII centers.[59] The isoindoline mole-
cule is bound to the Fe center as a neutral ligand. The pro-
Synthesis and Structure of the Iron(II) Complex of indH (1)
Complex
1 was synthesized by mixing equimolar ton H4B, bound to N1 in the free ligand, resides on N4 in
amounts of Fe(ClO4)2·6H2O with free ligand (indH) in an- the complex; H4B was found on an electron difference map,
hydrous acetonitrile, which resulted in an immediate color and its position was confirmed by its participation in hydro-
change of the solution to brown. X-ray quality crystals of gen-bonding interactions with a perchlorate counterion
1 were obtained by vapor diffusion of Et2O into a concen- (Figure S3). An asymmetric bond-length pattern was ob-
trated MeCN solution of 1. Complex 1 is air stable and served for the nearly planar N2–C8–N1–C1–N4 fragment
affords a satisfactory elemental analysis for C, H, N, and of the bound indH ligand in 1, which shows typical bond
Cl.
lengths (see Figure 1 caption for data) for a metal-coordi-
Initial crystallographic refinement of 1 at room tempera- nated neutral isoindoline ligand.[52]
ture revealed a 1:1 iron–ligand complex with one acetoni-
Spectroscopic measurements corroborate the crystallo-
trile and two water ligands in a distorted octahedral geome- graphic conclusions. Complex 1 exhibits an electronic ab-
try. However, the refinement resulted in abnormal ellipsoids sorption spectrum with bands at 283 and 332 nm, which
for the MeCN ligand and an R1 value Ͼ 8% (see Figure S1 can be assigned to intraligand π–π* transitions, and weaker
and discussion in Supporting Information). Recrystalli- bands at 571 and 770 nm, which may arise from metal-to-
zation of 1 from anhydrous MeCN/Et2O afforded crystals ligand charge transfer (MLCT) transitions from the iron(II)
for which data collection at 173 K gave a solution with an center to the indH ligand. Similarly weak bands have been
R1 value of 4.1%. This structure (Figures 1 and S2) revealed observed in iron(II) complexes of bidentate α-keto acid li-
a 1:1 iron–ligand complex with three MeCN ligands. The gands.[60–62] The IR spectrum of 1 displays absorption
crystallographic refinement data of 1 are collected in Table bands at 1626 and 1613 cm–1, which indicates that the com-
S1, and selected bond lengths are listed in the caption of plex contains the neutral isoindoline ligand (indH), as com-
Figure 1 and in Table S2.
plexes with indH have multiple strong absorptions in the
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range 1600–1660 cm–1, whereas those with the monoanionic peaks at 560 and 520 cm–1 to 548 and 504 cm–1 (Figure 3).
ligand exhibit only weak bands above 1600 cm–1.[47,54]
Thus, the H218O2-sensitive features may be assigned to the
ν(O–O) and νsym(Fe–O2–Fe) vibrations of the Fe–O2–Fe
unit, respectively, and have frequencies comparable to those
of the complexes listed in Table 1. However, the 18O shift
for the ν(O–O) of –38 cm–1 is somewhat smaller than the
downshift of 50 cm–1 calculated for a simple O–O harmonic
oscillator, but it is similar to that observed for complex C
in Table 1,[39] which suggests that these features are coupled
to other vibrational modes of the respective complexes. On
the other hand, the presence of H218O-sensitive features
shows the presence of an additional Fe–O–Fe unit in 2. The
520 cm–1 peak is downshifted by 16 cm–1 upon H218O label-
ing and is assigned to νsym(Fe–O–Fe), based on the similar-
ity of these values to those reported for related [FeIII2(μ-
O)(μ-1,2-O2)] complexes A and B.[46,63] However, the assign-
ment of the 560 cm–1 band is uncertain, as its frequency
and H218O-derived isotope shift do not match with those
of any other feature in Table 1. Taken together, the Raman
data support the formulation of 2 as a peroxido-bridged
diferric complex with an additional single-atom bridge de-
rived from water.
Reaction of 1 with Hydrogen Peroxide
The addition of 4 equiv. H2O2 to an acetonitrile solution
of 1 generated an intermediate green species (2) at –40 °C
(Figure 2). Titration of 1 with H2O2 showed that the chro-
mophore associated with 1 disappeared upon introduction
of the first 0.5 equiv. of H2O2, but the chromophore associ-
ated with 2 was not observed until after additional H2O2
was introduced and achieved maximal formation with 4–
5 equiv. H2O2 (Figure S4). Complex 2 exhibited a signifi-
cant lifetime at –40 °C, which permitted further spectro-
scopic characterization (see below), but it decomposed
within 50 s at 25 °C.
Figure 2. UV/Vis spectra of 1 (1 mm, l = 1 cm, solid line) in CH3CN
at –40 °C and 2 (dashed line), which was formed after addition of
4 equiv. H2O2 to the 1 mm solution of 1 in CH3CN.
Complex 2 has been characterized by several spectro-
scopic techniques to be a peroxido-bridged diferric com-
plex. It exhibits a visible absorption spectrum with λmax at
690 nm (ε = 1500 m–1 cm–1 per Fe ion and assuming 100%
conversion). Such a feature likely arises from a peroxido-
to-iron(III) charge transfer transition, as found in related
peroxido-bridged diferric complexes.[17,30–46] Complex 2 is
essentially electron paramagnetic resonance (EPR) silent in
MeCN at 2.5 K, and only two weak signals are observed at
g = 4.3 and 2.2 (Figure S5), which represent only a very
small fraction (Ͻ5%) of the total 1 mm sample. The g = 4.3
Figure 3. Resonance Raman spectra of 2 generated by the addition
of 4 equiv. H216O2 to 1 (top), generation of 2 with 4 equiv. H216O2
in the presence of 100 equiv. H218O (middle), and 4 equiv. H218O2
(bottom) with 647.1 nm excitation of frozen CH3CN solutions; *
denotes laser plasma lines, and L is a Raman band from the bound
indH.
Figure 4 shows the excitation profiles for several Raman
signal is typical of a high-spin FeIII impurity, whereas that vibrations of 2. All four 18O-sensitive features discussed
at g = 2.2 has yet to be assigned. The EPR-silent nature of above are clearly in resonance with the peroxido-to-
2 suggests that it is either diamagnetic or, more likely, an iron(III) charge-transfer band at 690 nm. In particular, the
antiferromagnetically coupled diiron(III) complex.
ν(O–O) mode at 874 cm–1 is strongly enhanced. The fact
The resonance Raman spectrum of 2 obtained by exci- that vibrations associated with the Fe–O–O–Fe unit as well
tation at 647.1 nm reveals the presence of four vibrations at as those of the Fe–O–Fe units follow the same excitation
874, 560, 520, and 458 cm–1 (Figure 3), reminiscent of fea- profile demonstrates that these vibrations arise from one
tures previously reported for diferric–peroxido complexes molecule, rather than a mixture of complexes with either
with an additional oxido bridge (Table 1). Isotope-labeling an Fe–O–O–Fe or an Fe–O–Fe unit. Furthermore, oxido-
studies show that the preparation of 2 with H218O2 (2% bridged diiron(III) complexes without an associated perox-
solution in H216O) leads to shifts of the peaks at 874 and ido ligand typically have LMCT bands in the near-UV re-
458 cm–1 to 836 and 445 cm–1, whereas the generation of 2 gion,[64] so the Fe–O–Fe vibrations would be expected to
in the presence of 100 equiv. H218O results in shifts of the exhibit resonance enhancement at much shorter wave-
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Table 1. Resonance Raman features [cm–1] of 2 and related (μ-oxido or hydroxido)(μ-1,2-peroxido)diferric complexes.
Complex[a] λmax (εM)
[r(Fe···Fe)]
ν(O–O)
νsym(Fe–O2–Fe)
νasym(Fe–O2–Fe)
νsym(Fe–O–Fe)
νasym(Fe–O–Fe)
[ΔH218O, ΔH218O2] [ΔH218O, ΔH218O2] [ΔH218O, ΔH218O2] [ΔH218O, ΔH218O2] [ΔH218O, ΔH218O2]
2
690 (1500)
505 (1250), 650 (1300) [3.16 Å] 854 [0, –47]
928 [–2, –53]
494 (1100), 648 (1200) [3.14 Å] 847 [0, –44]
874 [0, –38]
458 [0, –13]
460 [0, –13]
468 [0, –6]
463 [–, –21]
465 [–, –19]
460 [–, –13]
471 [–, –16]
–
520 [–16, 0]
523 [–16, 0]
424 [–7, –11]
511 [–12, –]
–
560 [–12, 0][b]
714 [–42, –]
–
696 [–30, –6]
695 [–, –2]
–
–
A (μ-O)
511 [0, –19]
550 [–4, –17]
533 [–, –25]
–
548 [–, –18]
–
AЈ (μ-OH) 730 (2400) [3.46 Å]
B (μ-O)
C (μ-O)
CЈ (μ-OH) 644 (3000) [3.396 Å]
D (μ-OR) 588 (1500) [3.462 Å]
577 (1500) [3.171 Å]
847 [–, –33]
908 [–, –47]
900 [–, –50]
498 [–, –5]
–
[a] A = [FeIII2(μ-O)(μ-O2)(BnBQA)2(NCCH3)2]2+ [BnBQA = N-benzyl-bis(2-quinolylmethyl)amine]; AЈ = [FeIII2(μ-OH)(μ-O2)(BnBQA)2-
(NCCH3)2]3+, data from ref.[46]; B = [FeIII2(μ-O)(μ-O2)(6-Me3TPA)2]2+ [6-Me3TPA = tris(6-methyl-2-pyridylmethyl)amine], data from
refs.[43,63]; C = [FeIII2(μ-O)(μ-O2)(6-Me2BPP)2]2+ [6-Me2BPP = bis(6-methyl-2-pyridylmethyl)-3-aminopropionate]; CЈ = [FeIII2(μ-OH)(μ-
O2)(6-Me2-BPP)2]3+, data from ref.[39]; D = [FeIII2(μ-O2)(N-Et-HPTB)(OPPh3)2]3+ [N-Et-HPTB = tetrakis(2-benzimidazolylmethyl)-2-
hydroxy-1,3-diaminopropane], data from refs.[33,43] [b] Although it must arise from a water-derived ligand, the assignment of this vibration
is uncertain.
lengths, which is not observed for 2. Interestingly, the non- also been correlated with the Fe–O–Fe angle in (μ-oxido)-
18O-sensitive features at 637 and 658 cm–1 follow a different diiron(III) complexes,[65] and this correlation indicates an
profile with higher resonance enhancement as excitation Fe–O–Fe angle of 120° like those associated with (μ-ox-
shifts to shorter wavelengths, consistent with an indH li- ido)(μ-1,2-peroxido)diiron(III) complexes A and B in
gand vibrational mode that is in resonance with a higher Table 1. Thus, consideration of all Raman data leads us to
energy transition.
favor a (μ-oxido)(μ-1,2-peroxido)diiron(III) core for 2.
X-ray absorption spectroscopy studies were performed to
obtain further characterization of 2. The X-ray absorption
near-edge region of 2 exhibits a pre-edge feature at
7114.3 eV with an area of 13.3(5) and a K-edge energy of
7124.2 eV, values similar to those previously reported for
complexes with [FeIII2(μ-O)(μ-1,2-O2)] cores.[43] Notably,
these parameters are very similar to those of the recently
characterized A, a complex of the tridentate BnBQA ligand,
which has pre-edge and edge energies of 7114.4 and
7124.2 eV, respectively, and has a [FeIII2(μ-O)(μ-1,2-O2)]
core.[46] Protonation of the BnBQA complex to form a
[FeIII2(μ-OH)(μ-1,2-O2)] core results in a downshift of the
pre-edge and edge energies to 7112.4 and 7123.3 eV, respec-
tively
Table 2 details an analysis of 2 by extended X-ray ab-
sorption fine structure (EXAFS). The rЈ-space spectrum of
2 (Figure 5) shows a number of distinct features that can be
fitted to scatterers at 1.79, 1.96, 2.13, 2.99, 3.13, and 3.36 Å
(Table 2, Fit 13). For the first coordination sphere, the scat-
Figure 4. Excitation profiles for selected Raman vibrations of 2.
A comparison of the vibrational data for 2 with the re- terer at 1.79 Å represents the oxido bridge, the two at
lated diferric–peroxido complexes in Table 1 raises a ques- 1.96 Å correspond to the proximal O atom of the peroxido
tion regarding the nature of the water-derived bridging li- bridge and the central N donor of indH, and those at
gand, that is, whether 2 is an oxido-bridged complex like A, 2.13 Å arise from the remaining N donors on the indH li-
B, and C or a hydroxido-bridged one like BЈ and CЈ. No- gand and the likely solvent ligand. For the outer shell,
tably, the ν(O–O) band of 2 is 20–25 cm–1 higher in fre- features at 2.99 and 3.36 Å correspond to second-shell and
quency than those of A, B, and C but 26–54 cm–1 lower third-shell C atoms of the indH ligand, respectively, and
in frequency than those of BЈ, CЈ and D. The correlation that at 3.13 Å is an Fe scatterer. Together, the EXAFS
established by Fiedler et al.[43] between the observed ν(O– analysis supports
a 2
FeIII2(μ-O)(μ-1,2-O2) core for
O) frequencies and the Fe···Fe distances suggests that 2 (Scheme 2). Consistent with the titration of 1 with H2O2
should have an Fe···Fe distance of ca. 3.25 Å, which is described earlier (Figure S4), we propose that 2 is formed
about 0.1 Å longer than that for complexes with FeIII2(μ- by a two-step reaction involving the initial oxidation of
O)(μ-1,2-O2) cores but 0.2 Å shorter than that for com- 2 equiv. of iron(II) complex 1 by H2O2 to form a (μ-oxido)-
plexes with FeIII2(μ-OR)(μ-1,2-O2) cores (R = H or alkyl). diferric species and the subsequent reaction of the latter
In addition, the frequency of the νsym(Fe–O–Fe) mode has with a second H2O2 molecule to form 2 (Scheme 2).
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Table 2. Fit parameters for the EXAFS analysis of 2.[a]
Fit
Fe–N/O
Fe···Fe
r [Å]
Fe–C
E0
F
F factor
(ϫ103)
N
r [Å]
σ2
N
σ2
N
r [Å]
σ2
1
2
3
4
5
6
5
4
3
6
1
5
1
4
1
5
1
4
1
3
1
5
1
4
1
4
1
2
2.11
2.11
2.12
2.12
2.09
1.83
2.10
1.85
2.11
1.91
2.1
1.86
2.12
1.92
2.12
2.03
2.1
11
8.5
6.3
4.2
8.4
5.8
6.6
7.5
5.2
11.1
6.7
8
5.3
12.3
4.5
15.3
6.9
7.5
5.6
11.4
2
–
–
–
–
–
–
–
–
–
–
1
–
1
–
1
–
1
–
1
–
1
–
–
–
–
–
–
–
–
–
–
–
–
3.07
–
3.08
–
3.08
–
3.11
–
3.12
–
3.13
–
–
–
–
–
–
–
–
–
–
–
–
8
–
7.8
–
7.5
–
5.1
–
2.2
–
2.1
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
5
–
4
–
4
4
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.98
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
9
5.12
5.59
6.3
6.89
2.22
358
329
315
326
302
663
636
622
633
609
6
2.64
3.49
3.33
4.25
5.43
2.56
3.07
-0.4
297
302
250
253
259
270
253
206
605
609
554
557
564
576
558
503
7
8
9
10
11
12
13[b]
1.84
2.11
1.89
2.13
1.79
1.96
–
3.7
–
2.5
2.6
–
2.98
–
2.99
3.36
–
0.7
0.3
–
[a] k Range = 2–13 Å–1 and back-transform range ca. 0.6–3.3 Å. σ2 = mean-squared deviation in units of 10–3 Å2. Scale factor S0 = 0.9.
2
Goodness-of-fit calculated as F = ∑(χexp – χcalc)2. F factor = [∑k6(χexp – χcalc)2/∑k6χexp2 1/2. [b] The coordination number of seven for the
]
first coordination sphere in Fit 13 is acceptable, as it is well within the 25–30% uncertainty associated with coordination numbers obtained
from EXAFS fits.[66]
Scheme 2. Proposed steps in the formation of 2. In the proposed
structure for 2, N represents the N donors of the meridional indH
ligand.
Figure 5. Best fits (solid line) to the experimental (dashed line) un-
filtered EXAFS data (inset) and Fourier transform of 2 corre-
further product formation. Monitoring of the substrate
sponding to Fit 13 in Table 2. k range = 2–13 Å–1. Back-transfor-
concentration by GC during the course of the reaction
mation range ca. 0.6–3.3 Å.
showed an exponential decay of thioanisole with a pseudo-
first-order rate constant kobs of 5.6ϫ10–4 s–1.
krel(PhSOCH3/PhSCH3) value of 0.70 was also determined
A
Substrate Oxidation Studies
We have investigated the properties of 1 as a catalyst for by comparing an individual reaction with methylphenylsulf-
the oxidation of thioanisole and benzyl alcohol with H2O2 oxide as substrate with the PhSMe oxidation under iden-
to obtain an initial assessment of the oxidizing power of tical conditions. Oxidations of para-substituted thioanisoles
this catalyst/oxidant combination. Complex 1 is capable of were also performed, and kobs values were determined from
catalyzing both oxygen-atom transfer and hydrogen-atom ln[p-X-C6H4SCH3] vs. t plots for reactions executed individ-
abstraction. For example, thioanisole readily underwent ually for each p-X-C6H4SCH3 derivative. The ratios of the
oxidation to the corresponding sulfoxide and sulfone at various kobs values relative to that of PhSCH3 gave krel (kX/
25 °C. With 1 μmol 1, 0.25 mmol H2O2, and 0.1 mmol sub- kH) values that afforded a Hammett correlation with a ρ
strate in 10 mL of acetonitrile, 56 turnovers were observed value of –0.40 (Figure 6), which suggests that the oxidant
in the course of an hour, after which time there was no generated from 1/H2O2 is electrophilic.
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philic oxidant (Figure 6). Experiments to compare the rates
of oxidation of benzyl alcohol and benzyl [D7]alcohol re-
vealed a kinetic isotope effect (KIE) of 9, which excludes
the possibility that hydroxyl radicals are the oxidant and
implicates instead a metal-based oxidant.
Taken together, the oxidation results presented above
indicate the formation of an electrophilic metal-based oxi-
dant that is capable of both oxygen-atom and hydrogen-
atom transfer. Its oxidizing properties are compared with
those of related Fe complexes in Table 3. This metal-based
oxidant appears to be mildly electrophilic, as its Hammett
ρ value for thioanisole oxidation of –0.40 is comparable to
that of the closely related [FeIII2O(L)4(OH2)2]4+/H2O2 sys-
tem (ρ = –0.55)[69] but is 2.5–4 times smaller than those
obtained for oxygen-atom transfer reactions by [FeIII
(salen)]+/H2O2 (ρ –1.5),[70] [FeIV(O)(TMP+·)] (ρ
–1.2),[68] and [FeIV(O)(PyMAC)]2+ [71]
On the other hand,
-
Figure 6. Hammett plots for the oxidation of para-substituted
thioanisoles (᭿) and benzyl alcohols (ᮀ) with σ values taken from
ref.[67] See Table S3 for further details.
=
=
.
its Hammett ρ value of –0.85 for benzyl alcohol oxidation
is two times larger than those obtained with ox-
idoiron(IV)(porphyrin radical cation) complexes (ρ = –0.4)
and ten times larger than those obtained with bona fide
non-heme oxidoiron(IV) complexes (ρ = –0.07). Further-
more, the kH/kD value for benzyl alcohol oxidation by 1/
H2O2 is smaller than those for corresponding oxidations by
oxidoiron(IV) complexes. Thus, the comparisons in Table 3
suggest that the oxidant generated by 1/H2O2 has oxidizing
properties distinct from those of bona fide oxidoiron(IV)
species.
Another approach to analyze our thioanisole oxidation
data was presented in ref.[68] In this method, the log kobs
values were plotted against the E0 potentials of para-sub-
ox
stituted thioanisole derivatives and N,N-dimethylanilines
(DMAs, for which only electron transfer is possible) for
three different catalysts. Gradients of –8.5 and –10.5 were
associated with an electron transfer/oxygen rebound path-
way, based on the comparison with DMAs, but gradients
of much smaller magnitude (–2.2 and –2.3) were attributed
to direct oxygen-atom transfer to the sulfide as this pathway
should be less sensitive to variations in E0 of the sub-
ox
strates. When our data were analyzed in this manner, the Table 3. Comparison of ρ and KIE values of 1 with those of related
complexes.
resulting plot (Figure 7) has a gradient of –0.8, which is
Substrate
Oxidant[a]
ρ
kH/
Ref.
consistent with a direct oxygen-atom transfer mechanism
kD
for 1/H2O2.
p-X-C6H4SCH3
1/H2O2
–0.40
this
work
[69]
[Fe2OL4(H2O)2]4+/H2O2 –0.55
[70]
[68]
[71]
[FeIII(salen)]+/H2O2
[FeIV(O)(TMP+·)]+
[FeIV(O)(PyMAC)]2+
1/H2O2
–1.5
–1.2[b]
–1.05
–0.85
(p-X-C6H4)3P
p-X-C6H4CH2OH
9
this
work
[72]
[FeIV(O)(N4Py/TPA)]2+
[FeIV(O)(TMP+·/
Cl8TPP+·)]+
–0.07 Ն21
[72]
–0.4
Ն50
[73]
p-X-anthracene
[FeIV(O)(N4Py)]2+
–3.9
[a] L = (–)-4,5-pinenebipyridine; salen = ethylene-1,2-bis(salicylid-
eneamine); TMP = meso-tetramesitylporphyrin; Cl8TPP = meso-
tetrakis(2,6-dichlorophenyl)porphyrin; TPA
=
tris(pyridyl-2-
methyl)amine; N4Py = bis(pyridyl-2-methyl)bis(2-pyridyl)methyl-
amine; PyMAC
=
2,7,12-trimethyl-3,7,11,17-tetraazabicyclo-
[1.3.11]heptadeca-1(17),13,15-triene. [b] Data reported in ref.[68]
Figure 7. Plot of logkobs values for thioanisole oxidation versus the
oxidation potentials of the thioanisole substrates.
were used to calculate ρ.
The oxidation of benzyl alcohol by 1/H2O2 at 25 °C re-
An obvious candidate for the metal-based oxidant is 2,
sulted in the formation of benzaldehyde in 24% yield. A the peroxido species we observed at –40 °C and charac-
pseudo-first-order rate constant of 5.1ϫ10–5 s–1 could be terized in this paper. However (μ-oxido)(μ-1,2-peroxido)di-
extracted by monitoring substrate loss and product forma- iron(III) complexes characterized thus far such as A,[63]
tion. Reactions with para-substituted benzyl alcohols B,[46] and C[39] (Table 1) appear to be unreactive. To investi-
showed changes in oxidation rate as a function of the nature gate whether species 2 plays the role of metal-based oxi-
of the para substituent. A Hammett plot of krel values (kX/ dant, a solution of 2 was prepared by treatment of a 1 mM
kH) gave a ρ value of –0.85, which is indicative of an electro- solution of 1 in CH3CN with a 100-fold excess of H2O2 at
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CCDC-855411 (for T = 293 K) and -888012 (for T = 173 K) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
5 °C. Upon formation, 2 undergoes a monotonic decay over
the next hour at a rate of 1.5ϫ10–7 ms–1, which is unaffec-
ted by the addition of 0.2 or 0.4 m PhSCH3. Nevertheless,
the corresponding sulfoxide product forms, but at a rate
that is two orders of magnitude faster than the decay rate
of 2. Thus, it would appear that 2 is not directly involved
in substrate oxidation but is still somehow linked to the
observed oxidative reactivity. Related diiron(III)–peroxido
complexes that have previously been found to oxidize sub-
strates include the protonated derivative of the (μ-oxido)(μ-
1,2-peroxido)diiron(III) intermediate from the [Fe2OL4-
(H2O)2]4+/H2O2 system[69,74] (noted earlier in Table 3) and
the (μ-alkoxido)(μ-1,2-peroxido)diiron(III) intermediate ob-
tained at –40 °C from the reaction of O2 with the diiron(II)
complex of the dinucleating ligand HPTP [N,N,NЈ,NЈ-tet-
rakis(2-pyridylmethyl)-2-hydroxy-1,3-diaminopropane].[75]
Efforts are in progress to draw reactivity and mechanistic
parallels between 2 and these reactive intermediates to gain
insight into the factors that activate the FeIII2(μ-OX)(μ-O2)
core for oxidative reactivity. Answering this question would
enhance our understanding of how dioxygen activation can
take place at biological non-heme diiron active sites.[1–4,10]
X-band EPR spectra (9.64 GHz, 200 μW, 2.5 K) were obtained
with a Bruker Elexsys E-500 spectrometer equipped with an Oxford
ESR-910 cryostat and Oxford temperature controller in perpendic-
ular mode. Data collection was performed by using Xepr (Bruker).
Resonance Raman spectra were collected by using Spectra-Physics
Model 2060 Kr+ and 2030-15 Ar+ lasers and an ACTON AM-506
monochromator equipped with a Princeton LN/CCD data collec-
tion system. Low-temperature spectra in CH3CN or CD3CN were
obtained at 77 K by using a 135° backscattering geometry. Samples
were frozen onto a gold-plated copper cold finger in thermal con-
tact with a Dewar flask containing liquid nitrogen. Raman fre-
quencies were calibrated to indene prior to data collection. Ray-
leigh scattering was attenuated by using a holographic notch filter
(Kaiser Optical Systems) for each excitation wavelength. The
monochromator slit width was set for a band-pass of 4 cm–1 for all
spectra. The spectra of 2 were collected by using a laser excitation
power of 100 mW. The plotted spectra are an average of 32 scans
with collection times of 30 s. All spectra were intensity-corrected
to the 710 or 773 cm–1 solvent peak of CD3CN or CH3CN, respec-
tively.
X-ray Absorption Spectroscopy
Experimental Section
Data Collection: X-ray absorption data (XAS) were collected on
beam line 7-3 of the Stanford Synchrotron Radiation Lightsource
(SSRL) at the SLAC National Accelerator Laboratory. Fe K-edge
XAS data were collected for frozen samples prepared in SSRL
solution cells with [Fe]T = 5.9 mm. The SPEAR storage ring was
operated at 3.0 GeV and ca. 350 mA, and energy resolution of the
focused incoming X-rays was achieved by using a Si(220) double-
crystal monochromator, which was detuned to 50% of maximal
flux to attenuate second-harmonic X-rays. The sample temperature
(7–10 K) was controlled with an Oxford Instruments CF1208 con-
tinuous flow liquid helium cryostat. Harmonic rejection was
achieved by a 9 keV cutoff filter. Data was obtained as fluorescence
excitation spectra with a 25-element solid-state germanium detector
array (Canberra). In fluorescence mode, photon scattering “noise”
was reduced by using a 3 micron Mn filter and a Soller slit. An
Fe foil spectrum was recorded concomitantly for internal energy
calibration, and the first inflection point of the K-edge was as-
signed to 7112.0 eV. The edge energies were routinely monitored
during data collection for redshifts indicative of sample photore-
duction, but none were observed in the present study.
Materials and Methods
Materials: All manipulations were performed under a pure argon
atmosphere by using standard Schlenk-type techniques unless
otherwise stated. Solvents used for the reactions were purified by
standard methods and stored under argon. Iron(II) perchlorate was
purchased from commercial sources. The ligand 1,3-bis(2Ј-pyr-
idylimino)isoindoline (indH) was prepared according to published
procedures.[57,58]
Caution! Perchlorate salts are potentially explosive and should be
handled with care and in small quantities.
FeII(indH)(CH3CN)2(H2O)2(ClO4)2 (1):
A solution of indH
(0.269 g, 0.9 mmol) in acetonitrile (20 mL) was stirred at room tem-
perature for 2 h with Fe(ClO4)2·6H2O (0.362 g, 1 mmol). After ad-
dition of diethyl ether, dark brown crystals precipitated upon
standing for a day. The product was collected by filtration, washed
with diethyl ether, and dried in vacuo (0.35 g, 70%), m.p. 258–
260 °C. IR (KBr): ν = 3332 (w), 1654 (m), 1626 (s), 1613 (w), 1594
˜
(w), 1554 (m), 1524 (s), 1485 (s), 1467 (m), 1432 (m), 1375 (w),
1306 (w), 1262 (w), 1207 (m), 1145 (m), 1115 (m), 1082 (s), 864
(w), 780 (m), 712 (m), 625 (m) cm–1. UV/Vis (CH3CN): λmax (logε)
= 239 (4.73), 283 (4.50), 332 (4.52), 382 (4.47), 571 (2.39), 770
(1.86) nm. C22H23Cl2FeN7O10 (672.22): calcd. C 39.31, H 3.45, N
14.59, Cl 10.55; found C 39.67, H 3.30, N 14.61, Cl 10.03.
Data Analysis: Data reduction, averaging, and normalization were
performed by using the program EXAFSPAK.[76] The data for 2
are the average of 16 data sets. Following calibration and averaging
of the data, background absorption was removed by fitting a
Gaussian function to the pre-edge region and then subtracting this
function from the entire spectrum. A three-segment spline with
fourth-order components was then fitted to the EXAFS region of
the spectrum to extract χ(k).
Physical Measurements: Infrared spectra were recorded with an
Avatar 330 FTIR Thermo Nicolet instrument. UV/Vis spectra were
recorded with an Agilent 8453 diode-array spectrophotometer with
quartz cells. Microanalyses were done by the Microanalytical Ser-
vice of the University of Pannonia and Atlantic Microlab. The crys-
tal evaluation and intensity data collection for 1 were performed
with a Bruker-Nonius Kappa or Smart 1000 CCD single-crystal
Theoretical phase and amplitude parameters for a given absorber–
scatterer pair were calculated by using FEFF 8.40 and were utilized
by the “opt” program of the EXAFSPAK package during curve-
fitting. The FEFF parameters for 2 were calculated by using the
diffractometer with Mo-Kα radiation (λ = 0.71070 Å) at 293(2) or coordinates of other similar peroxidodiferric complexes. In all
173(2) K. Additional crystallographic data (Tables S1 and S2) and
details of the structure solution and refinement are given in the
Supporting Information.
analyses, the coordination number of a given shell was a fixed pa-
rameter and was varied iteratively, whereas bond lengths (r) and
mean square deviation (σ2) were allowed to freely float. The ampli-
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tude reduction factor S0 was fixed at 0.9, and the edge-shift param-
eter E0 was allowed to float as a single value for all shells [thus, in
any given fit, the number of floating parameters was typically equal
to (2 ϫ number of shells) + 1]. The goodness-of-fit F was defined
postdoctoral fellowship ES017390 to M. A. C.). X-ray absorption
spectroscopy was performed at the Stanford Synchrotron Radia-
tion Lightsource, a Directorate of SLAC National Accelerator Lab-
oratory and an Office of Science User Facility operated for the U.S.
simply as Σ(χexptl – χcalc)2. For fits to unfiltered data, a second Department of Energy Office of Science by Stanford University.
goodness-of-fit parameter
F
factor was defined as
The SSRL Structural Molecular Biology Program is supported by
the DOE Office of Biological and Environmental Research and by
the National Institutes of Health, National Center for Research
Resources, Biomedical Technology Program (P41RR001209).
[Σk6(χexptl – χcalc)2/Σk6χexptl2 1/2. To account for the effect that ad-
]
ditional shells have on improving fit quality, a third goodness-of-
fit metric FЈ was employed. FЈ = F2/(NIDP – NVAR), where NVAR is
the number of floated variables in the fit, and NIDP is the number
of independent data points and is defined as NIDP = 2ΔkΔr/π. In
the latter equation, Δk is the k range over which the data is fitted,
and Δr is the back-transformation range employed to fit the Fou-
rier-filtered data. Thus, FЈ is of principal utility in fitting Fourier-
filtered data, but can also be employed for unfiltered data by as-
suming a large value of Δr.
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mixing a solution of the thioanisole (0.1 mmol in 4 mL acetonitrile)
with solutions of the complex (1 μmol in 1 mL acetonitrile) and
H2O2 (0.25 mmol in 5 mL acetonitrile). The disappearance of sub-
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NO2) were determined (Table S3). The krel values for the Hammett
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For the oxidation of benzyl alcohols, the reactions were performed
under an argon atmosphere by mixing a solution of H2O2
(0.25 mmol in 5 mL acetonitrile) with solutions of 1 (1 μmol) in
acetonitrile (1 mL) and the benzyl alcohol (0.1 mmol) in acetoni-
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ferent substrates.
Supporting Information (see footnote on the first page of this arti-
cle): Additional crystallographic information, EPR spectrum of 2,
and kinetics data for the Hammett studies.
Acknowledgments
This work was supported by the János Bolyai research scholarship
of the Hungarian Academy of Sciences to J. S. P., by a grant from
the Hungarian Government and the European Union, with the co-
funding of the European Social Fund (grant number TÁMOP-
4.2.2.A-11/1/KONV-2012-0071) to J. K., and the U.S. National In-
stitutes of Health (NIH) (grant number GM-38767 to L. Q. and
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Received: February 1, 2013
Published Online: June 7, 2013
Eur. J. Inorg. Chem. 2013, 3858–3866
3866
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim