Thioether-ligated iron(ii) and iron(iii)-hydroperoxo/alkylperoxo complexes with an H-bond donor in the second coordination sphere

Article abstract of DOI:10.1039/c4dt00281d

The non-heme iron complexes, [Fe^II(N3PySR)(CH ₃CN)](BF₄)₂ (1) and [Fe^II(N3Py ^amideSR)](BF₄)₂ (2), afford rare examples of metastable Fe(iii)-OOH and Fe(iii)-OOtBu complexes containing equatorial thioether ligands and a single H-bond donor in the second coordination sphere. These peroxo complexes were characterized by a range of spectroscopic methods and density functional theory studies. The influence of a thioether ligand and of one H-bond donor on the stability and spectroscopic properties of these complexes was investigated. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00281d

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Thioether-ligated iron(II) and iron(III)-hydroperoxo/
alkylperoxo complexes with an H-bond donor
in the second coordination sphere†
Cite this: Dalton Trans., 2014, 43,
7522
Leland R. Widger,a Yunbo Jiang,a Alison C. McQuilken,a Tzuhsiung Yang,a
Maxime A. Siegler,a Hirotoshi Matsumura,b Pierre Moënne-Loccoz,*b
Devesh Kumar,c,d Sam P. de Visser*c and David P. Goldberg*a
The non-heme iron complexes, [FeII(N3PySR)(CH3CN)](BF4)2 (1) and [FeII(N3PyamideSR)](BF4)2 (2), afford rare
examples of metastable Fe(III)-OOH and Fe(III)-OOtBu complexes containing equatorial thioether ligands and
a single H-bond donor in the second coordination sphere. These peroxo complexes were characterized by a
range of spectroscopic methods and density functional theory studies. The influence of a thioether ligand
and of one H-bond donor on the stability and spectroscopic properties of these complexes was investigated.
Received 25th January 2014,
Accepted 26th March 2014
DOI: 10.1039/c4dt00281d
www.rsc.org/dalton
Introduction
Iron-(hydro)peroxo and (alkyl)peroxo species are postulated to
be critical intermediates in a number of biologically relevant
processes, from the utilization of dioxygen for substrate oxi-
dation, to the processing and detoxification of reactive oxygen-
derived species (ROS).1–5 Some specific examples of non-heme
Fe centers involved in this chemistry include isopenicillin
N-synthase (IPNS),6 superoxide reductase (SOR),7 aromatic
amino acid hydroxylase (AAH),8 Bleomycin,9 and cysteine dioxy-
genase (CDO) (Fig. 1).10 In one case, the alkylperoxo inter-
mediate of an extradiol ring-cleaving dioxygenase,
homoprotocatechuate 2,3-dioxygenase (2,3-HPCD), was struc-
turally characterized by X-ray crystallography.2 The physical
and spectroscopic properties of these species have been
studied in a number of inorganic model complexes,11–25 but
most of these models utilize only polyamino/polypyridyl
ligands. In contrast, some of the enzymes shown in Fig. 1 also
Fig. 1 Selected examples of iron-dioxygen complexes in biology.
include a sulfur donor in the first coordination sphere, but the
incorporation of sulfur or other heteroatoms in model com-
plexes is rare.23,25–41 In addition, little work has been done to
determine the effect of secondary sphere interactions, such as
hydrogen-bonding networks, on the structures and spectro-
scopic properties of iron-peroxo complexes,42,43 despite their
roles in many of the corresponding enzyme intermediates.
Recently our lab has focused on determining the first and
second coordination sphere effects on non-heme iron model
complexes, including the incorporation of S donors in the first
coordination sphere, and oxidizable substrates or H-bond
donors in the second coordination sphere.44–47
aDepartment of Chemistry, The Johns Hopkins University, 3400 North Charles Street,
Baltimore, Maryland 21218, USA. E-mail: dpg@jhu.edu; Fax: +1-410-516-8420;
Tel: +1-410-516-6658
bOregon Health & Science University, Institute of Environmental Health, Mail code:
HRC3, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA.
E-mail: moennelo@ohsu.edu; Fax: +1-503-346-3427; Tel: +1-503-346-3429
cManchester Institute of Biotechnology and School of Chemical Engineering and
Analytical Science, The University of Manchester, 131 Princess Street, Manchester,
M1 7DN, UK. E-mail: sam.devisser@manchester.ac.uk
dDepartment of Applied Physics, School of Physical Sciences, Babasaheb,
Bhimrao Ambedkar University, Vidya Vihar, Rae Bareilly Road, Lucknow 226-025,
India. E-mail: dkclcre@yahoo.com
†Electronic supplementary information (ESI) available: Computational details.
CCDC 976172–976174. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4dt00281d
The thiolate-ligated complex [FeII(N3PyS)(CH3CN)]BF4 was
among the first examples of a biomimetic iron(II) complex to
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undergo S-oxygenation with O2 to yield a doubly-oxygenated plexes were synthesized by metallation with Fe(BF4)2 of their
sulfinate product.44 This complex was also useful for studying respective purified ligand precursors. Both 1 and 2 possess a
the fundamental electronic structure and photolability of com- thioether ligand in the first coordination sphere, while
plexes while employing NO• as a surrogate for O2.45 We have complex 2 incorporates a single amide N–H group within
examined secondary coordination sphere effects by utilizing H-bonding distance of the preferred binding site for anionic
the aryl appended complexes, [FeII(N4Py2Ph)(CH3CN)](BF4)2 ligands. We demonstrate the generation of meta-stable FeIII-
and [FeII(N4Pyamide,2Ph)(CH3CN)](BF4)2. Without a stabilizing OO(H/R) complexes in the presence of the thioether donor
H-bond donor positioned near the open site on iron, the alkyl- ligands starting from 1 and 2. The influence of a first coordi-
peroxo complex [FeIII(N4Py2Ph)(OOtBu)]2+ likely undergoes nation sphere thioether donor and a second coordination sphere
rapid O–O bond cleavage to give a putative FeIV(O) intermedi- H-bond donor on the stability and spectroscopic properties of
ate. The FeIV(O) would then be situated for attack of the the (hydro/alkyl)peroxo iron(III) complexes is assessed.
appended aryl group, giving the arene-hydroxylated product.
However, in the presence of an amide H-bond donor, [FeIII(N4-
Pyamide,2Ph)(OOtBu)]2+ is stabilized and only the alkylperoxo-
iron(III) complex is observed.46 We also determined the influ-
ence of first and second coordination sphere modifications in
a system in which an equatorial thioether donor was included
in the first coordination sphere and an amide H-bond donor
was included in the second coordination sphere. The [FeII(N3-
PyamideSR)](BF4)2 starting material was shown to react with
O-atom donors to give [FeIV(O)(N3PyamideSR)]2+ in CH3CN at
−40 °C. This complex is a rare example of a metastable FeIV(O)
complex with a pendant thioether donor, and it does not
undergo intramolecular oxygen-atom transfer (OAT) to the
thioether group. However, this complex does exhibit rapid
intermolecular OAT to exogenous thioether substrates (PhSMe),
and it was proposed that amide H-bond donation to the termi-
nal oxo ligand was causing the significant rate accelerations
seen for intermolecular OAT.47
Results and discussion
Synthesis and characterization
Compound 1 is prepared by metallation of the ligand N3PySR
with Fe(BF4)2 in MeCN, followed by vapor diffusion of Et2O
into the resulting solution to give dark red blocks of 1 in 68%
yield (Scheme 1). The structure of the cation of 1 is shown in
Fig. 2 and structural data are summarized in Table 1. The
ferrous ion is bound in a pseudo-octahedral geometry by three
pyridyl and an aryl thioether donor in the equatorial plane,
with a tertiary nitrogen donor and solvent nitrile molecule
occupying the axial positions. The Fe–N bond distances
(1.9260(14)–1.9969(13) Å, Table 2) are consistent with a low
spin (ls) iron(II) center. The spin state was also confirmed by
observation of a diamagnetic 1H-NMR spectrum in CDCl3.
Metallation of the ligand N3PyamideSR with Fe(BF4)2, fol-
lowed by vapor diffusion of Et2O into the resulting dark orange
solution yielded dark red crystals of 2 in 88% yield. The struc-
ture of the cation of 2 (Fig. 3) was reported previously and is
repeated here for comparison with the other complexes.47 As in
1, the ferrous ion is low-spin and bound in a pseudo-octahedral
geometry with three pyridyl and one aryl thioether donor in the
equatorial plane, with the tertiary nitrogen donor occupying the
axial position. In 2 however, the amide ligand has displaced the
Herein we report the synthesis and characterization of
Fe(III)-OOH and -OOtBu complexes derived from two biomimetic
starting compounds, [FeII(N3PySR)(CH3CN)](BF4)2 (1) and
[FeII(N3PyamideSR)](BF4)2 (2), shown in Scheme 1. These com-
Fig. 2 Displacement ellipsoid plot of the cation of 1 shown at the 50%
Scheme 1 Convergent syntheses of 1 and 2.
probability level. H-atoms are removed for clarity.
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Table 1 Summary of the crystallographic data for 1, 3·CH3CN and 8
Complex
1
3·CH3CN
8
Formula
Formula weight
T (K)
Color, morphology
Class
Space group
a (Å)
b (Å)
C
29H28B2F8FeN6S
C34H37BF4.67FeN8OS
761.13
110
Yellow, lath
Orthorhombic
Pbcn
35.1087(4)
10.08753(9)
19.9277(2)
90
C44H42B2F8FeN6OS
932.37
110
Yellow, block
Triclinic
ˉ
P1
10.4419(3)
14.5834(3)
17.2950(4)
90.6304(19)
105.768(2)
106.791(2)
2415.01(10)
1.282
722.10
110
Dark red, irregular shape
Triclinic
ˉ
P1
10.6689(2)
12.0355(2)
13.0403(3)
80.5885(18)
88.0024(18)
68.543(2)
1536.85(5)
1.560
c (Å)
α (°)
β (°)
γ (°)
90
90
V (Å3)
7057.59(12)
1.433
4.546
ρ (g cm−3
)
μ (mm−1
)
0.640
0.65
0.425
0.62
(sin θ/λ)max (Å−1
)
0.62
No. reflections collected
No. unique reflections
Rint
35 673
7051
0.0247
425
44 026
6942
0.0273
511
29 736
9709
0.0305
648
No. variable parameters
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
0.0325
0.0877
0.0322
0.0791
0.0401
0.1117
R1 [all data]
R2 [all data]
0.0373
0.0905
1.061
−0.50, 0.63
0.0366
0.0821
1.026
−0.33, 0.63
0.0475
0.1155
1.102
−0.46, 0.58
Goodness-of-fit (GOF) on F2
Largest difference in hold and peak (e A−3
)
Table 2 Selected bond distances (Å) and angles (°) for 1
Bond lengths
Bond angles
Fe1 N6
Fe1 N4
Fe1 N3
Fe1 N5
Fe1 N2
Fe1 S1
1.9260(14)
1.9569(13)
1.9619(13)
1.9790(13)
1.9969(13)
2.2848(4)
N6 Fe1 N4
N6 Fe1 N3
N4 Fe1 N3
N6 Fe1 N5
N4 Fe1 N5
N3 Fe1 N5
N6 Fe1 N2
N4 Fe1 N2
N3 Fe1 N2
N5 Fe1 N2
N6 Fe1 S1
N4 Fe1 S1
N3 Fe1 S1
N5 Fe1 S1
N2 Fe1 S1
94.41(6)
97.35(6)
168.23(6)
94.24(5)
86.41(5)
92.09(5)
175.10(5)
83.35(5)
84.88(5)
81.29(5)
93.13(4)
87.26(4)
92.70(4)
170.63(4)
91.12(4)
Fig. 3 Displacement ellipsoid plot for the cation of 2 shown at the 50%
probability level. H-atoms, except the amide N–H, are removed for
clarity. The existence of the H atom attached to N4 was confirmed by
difference Fourier map.
bound solvent molecule and is coordinated through the amide
oxygen.
When the propionitrile protecting group of the ligand for 1
is removed prior to metal binding, the corresponding FeII-thio-
late complex is accessed, which has been shown to undergo
sulfur oxidation in the presence of O2.44 However, in com- bonding would be favored with the anionic ligands found in
pounds 1 and 2 the thioether donor appears to make these the axial position. Stirring of 2 with 1.0 equiv. NaN3 in MeCN,
complexes stable toward O2, with no visible decomposition followed by vapor diffusion of Et2O into the solution produced
over several days stored under aerobic conditions.
a small amount of yellow crystals that were manually separated
Prior to attempting to make iron-peroxo complexes from 1 from the bulk precipitate for X-ray structure determination.
and 2, we wanted to test the substitution chemistry of 2 toward This structure revealed the new compound, [FeII(N3PyamideSR)-
anionic donors to show that the amide group could be dis- (F/N3)]BF4 (3), which shows that the amide CvO donor has
−
placed and reoriented into a favorable hydrogen-bonding been displaced, and there is a mixture of F− (67%) and N3
position. We also wanted to confirm whether hydrogen (33%) anions that are occupying the desired axial position.
7524 | Dalton Trans., 2014, 43, 7522–7532
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shows that the pendant amide is positioned such that
H-bonding interactions to anionic ligands are favored (N6⋯N7
= 2.914(16) Å, N6⋯F1 = 2.784(5) Å, N6–H–N7 = 174(2)°, N6–H–
F1 = 170(2)°; amide N–H is located in the difference map).
This complex provides good evidence that Fe-OO(H/R) com-
plexes will likely exhibit the same H-bonding interaction.
Table 3 Selected bond lengths (Å) and angles (°) for 3
Bond lengths
Bond angles
Fe1 F1
Fe1 N7
Fe1 N2
Fe1 N1
Fe1 N3
Fe1 N5
Fe1 S1
1.850(4)
F1 Fe1 N2
N7 Fe1 N2
F1 Fe1 N1
N7 Fe1 N1
N2 Fe1 N1
F1 Fe1 N3
N7 Fe1 N3
N2 Fe1 N3
N1 Fe1 N3
F1 Fe1 N5
N7 Fe1 N5
N2 Fe1 N5
N1 Fe1 N5
N3 Fe1 N5
F1 Fe1 S1
N7 Fe1 S1
N2 Fe1 S1
N1 Fe1 S1
N3 Fe1 S1
N5 Fe1 S1
98.05(14)
105.6(5)
108.09(14)
102.8(4)
82.68(6)
173.68(12)
176.6(4)
77.62(6)
76.14(5)
98.62(14)
103.3(4)
95.76(5)
153.22(5)
77.41(5)
94.88(14)
87.7(5)
2.063(14)
2.1703(16)
2.1892(14)
2.2012(14)
2.2448(13)
2.6007(5)
FeIII-OOH complexes
Complex 1 reacts with excess H2O2 at 25 °C (Fig. 5) to generate
a new transient red species, 4 (t1/2 ≈ 1 min). Monitoring the
reaction by UV-vis shows isosbestic conversion of the peaks
corresponding to 1 (λmax = 360, 430 nm) into a new spectrum
for 4 (λmax = 542 nm, ε = 1000 M−1 cm−1). The peak at 542 nm
is characteristic of a hydroperoxo-to-iron(III) LMCT band.
Attempts to trap this species by generation at −40 °C were
unsuccessful, leading only to decomposition, with no observa-
ble UV-vis peaks corresponding to 4. Samples of 4 for analysis
by EPR and resonance Raman were therefore prepared at 25 °C
and flash-frozen immediately after mixing. From EPR and RR
(vide infra) complex 4 is assigned as the FeIII-OOH complex
[FeIII(OOH)(N3PySR)]2+.
Complex 2 reacts with excess H2O2 at −40 °C (Fig. 6), and
changes in color from orange to deep purple over the course of
30 min. The new purple species (5) persists for at least several
hours at −40 °C, but immediately decays upon warming to
25 °C. Monitoring by low-temperature UV-vis spectroscopy
shows isosbestic conversion from 2 (λmax = 350, 450 nm) to a new
peak (λmax = 567 nm, ε = 900 M−1 cm−1) that is characteristic of
a hydroperoxo-to-iron(III) LMCT band. The EPR and RR spectra
(vide infra), confirmed that the new species (5) is the FeIII-OOH
complex, [FeIII(OOH)(N3PyamideSR)]2+.
166.53(5)
96.89(4)
89.18(4)
78.52(4)
The Fe–S bond is rather long (2.6007(5) Å), but the sulfur is
oriented correctly for a weak bonding interaction with the Fe
center (Table 3). The FeII–N(py) distances (2.1703(16)–2.2448(13) Å)
for 3 are indicative of a high-spin (hs) ferrous center, and the
difference in Fe–F and Fe–N3 distances allows for reliable deter-
mination of the N3− ligand despite its low occupancy. The N3
-
−
containing component of 3 is shown in Fig. 4, as it more
closely resembles the structure of an anionic peroxo ligand
than does the F− component. The source of fluoride for the
F−-containing component of 3 can be assigned to BF4−. This
structure shows that the axial amide carbonyl is labile, and
can be displaced by anionic donors. The structure of 3 also
The X-band EPR spectra of 4 and 5 are shown in Fig. 5 and
6, respectively. Both complexes show axial spectra corres-
ponding to ls iron(III) centers with signals at g = 2.17, 2.11,
Fig. 5 Formation of [FeIII(OOH)(N3PySR)]2+ (4) (top). Changes in the
electronic absorption spectrum for the reaction of 1 + H2O2 in CH3CN
(25 °C), red = 1, blue = 30 s, green = 60 s (bottom left). X-band EPR
spectra of the reaction mixture of 1 + H2O2, flash frozen after mixing at
25 °C for 1 min (black line) and 3 min (red line) (bottom right). EPR para-
meters: T = 15 K, frequency = 9.46 GHz, power = 2.01 mW.
Fig. 4 Displacement ellipsoid plot of the N3−-containing disordered
component of the cation of 3 shown at the 50% probability level. H-
atoms, except the amide N–H, are removed for clarity. The existence of
the H atom attached to N6 was confirmed by difference Fourier map.
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Fig. 6 Formation of [FeIII(OOH)(N3PyamideSR)]2+ (5) (top). Changes in
the electronic absorption spectrum for the reaction of 2 + H2O2 in
CH3CN, over 30 min at −40 °C (bottom left). X-band EPR spectrum of
the reaction mixture of 2 + H2O2, flash frozen after mixing at −40 °C for
30 min. EPR parameters: T = 77 K, frequency = 9.46 GHz, power =
2.01 mW.
1.97, for 4 and g = 2.17, 2.16, 1.95, for 5. These values are
similar to other FeIII-OOH complexes reported in the litera-
ture.18 The mononuclear complex [FeIII(N4Py)(OOH)]2+ in
methanol has a signal at g = 2.17, 2.12 and 1.97,13 while the
TPA analogue [FeIII(TPA)(OOH)]2+ affords a spectrum with g =
2.19, 2.15 and 1.96 in acetone/acetonitrile.48 EPR spectra of
reaction mixtures of 4, taken at different timepoints (1–5 min)
clearly show the rapid decomposition of the ls-FeIII signal at
room temperature. Samples frozen after 1 min show a relatively
strong ls-FeIII signal along with an additional signal at g = 4.2.
After 3 minutes, the ls-FeIII signal appears to be mostly
decayed, and after 5 min no ls-FeIII signal is observed. The
Fig. 7 Resonance Raman spectra of complexes 1 and 4 (A), and 2 and
5 (B) in CD3CN.
peak at g = 4.2 does not increase with the decay of the FeIII- Fe–O–O–H unit with internal vibrations of the (N3PyS) ligand.
OOH feature, indicating it is not a decomposition product and Using Hooke’s law to calculate the ν(16O–16O) frequency based
is likely a byproduct of FeIII-OOH formation. The peak at g = on an isolated diatomic oscillator with a ν(18O–18O) at
4.2 is much more prominent in 4 relative to that seen in the 763 cm−1 leads to an 809 cm−1 value that matches the ν(O–O)
spectrum of 5, which was prepared at −40 °C and frozen after mode seen in D2O and the sharper component of the doublet
30 min, allowing for full formation of 5 with much less possi- observed with unlabeled complex 4. Accordingly, the 809 cm−1
bility for decay.
band is assigned to the ν(O–O) mode of 4. Using the same
The identity of the FeIII-OOH complex 4 was confirmed by approach to interpret the RR bands observed in the ν(Fe–O)
RR spectroscopy. Frozen CD3CN solutions were analyzed at region is less successful,49 presumably because of admixture
110 K with a 568 nm laser excitation to avoid overlap of RR between Fe–O stretch and Fe–O–O bend vibrations, as seen
bands with non-resonant vibrations from CH3CN. The RR previously with other Fe(III)-hydroperoxo complexes.13,50
spectra of 4 show a cluster of bands in the ν(Fe–O) region with
The pendant amide complex [FeIII(N3PyamideSR)(OOH)]2+ (5)
prominent bands at 615, 629, 647 and 664 cm−1, and two was also studied by RR spectroscopy under the same con-
bands at 787 and 809 cm−1 in the ν(O–O) region (Fig. 7A). ditions (Fig. 7b). In contrast with 4, the RR spectra of 5 shows
Labeled samples prepared with H218O2 exhibit greatly simpli- no evidence of vibrational coupling and assigning the ν(Fe–O)
fied spectra with unique ν(Fe–O) and ν(O–O) modes at 590 and and ν(O–O) bands is straightforward. The unlabeled complex
763 cm−1, respectively. RR spectra of samples prepared with shows a single ν(O–O) mode at 800 cm−1 that downshifts to
D2O2 in the presence of excess D2O also show a single set of 756 cm−1 with samples prepared with H218O2 and that is
ν(Fe–O) and ν(O–O) bands at 607 cm−1 and 809 cm−1. These unchanged in samples prepared with D2O2 in the presence of
data clearly indicate that the complexity of the RR spectra of excess D2O. A single band at 612 cm−1 in the ν(Fe–O) region
unlabeled complex 4 results from vibrational coupling of the shifts to 593 cm−1 with H218O2 and to 607 cm−1 with D2O2.
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Table 4 Comparison of RR data for ls non-heme FeIII-OOH complexes
Complex
ν(Fe–O), [ν(O–O)]
Δ
18O2
ΔD2O
[FeIII(N3PySR)(OOH]2+
615, [809]
−25, [−46] −8, [0]
−19, [−44] −5, [0]
−16, [−44] −5, [0]
[FeIII(N3PyamideSR)(OOH)]2+ 612, [800]
[FeIII(N4Py)(OOH)]2+ a
[FeIII(TPA)(OOH)]2+ b
632, [790]
624, [803]
−19, [44]
−3, [0]
a Ref. 13. b Ref. 51.
Comparing ν(Fe–O) and ν(O–O) frequencies for 4 and 5 with
previously characterized Fe(III)-OOH complexes (Table 4), con-
firms their identity and suggests that the amide H-bond donor
group exerts limited influence on the Fe–O–O unit.
Comparison of the RR data for complexes 4 and 5 with
[FeIII(N4Py)(OOH)]2+ reveals a 19 2 cm−1 downshift in ν(Fe–
O), and a 10 to 19 cm−1 (for 5 and 4, respectively) upshift of
the Δν(O–O) upon incorporation of the thioether donor. This
effect of the equatorial thioether donor on ν(O–O) differs from
Fig. 8 Formation of [FeIII(OOtBu)(N3PySR)]2+ (6) (top). Changes in the
electronic absorption spectrum for the reaction of 1 + tBuOOH in
CH3CN, over 10 min at −40 °C (bottom left). X-band EPR spectrum of
the reaction mixture of 1 + tBuOOH, flash frozen after mixing at −40 °C
a previous study, where the replacement of an axial triflate for 15 min. EPR parameters: T = 15 K, frequency = 9.46 GHz, power =
(OTf−) ligand with aryl-thiolate donors (ArS−) in hs
2.01 mW.
[FeIII(Me4[15]aneN4)(SAr)(OOR)]+ complexes was shown to have
little influence on ν(O–O) (Δν(O–O) = 1 cm−1).23 In contrast,
the effect of the equatorial thioether donor on ν(Fe–O) in 4
and 5 is similar to the effect previously seen for the aryl-thio-
late complexes, in which inclusion of sulfur induced a lower-
ing of this band.23
FeIII-OOtBu complexes
Complex 1 reacts with 10 equiv. of tBuOOH (Fig. 8) at −40 °C
in MeCN to give a new deep blue species (6) that persists at
low temperature for 15–20 min before slowly decaying. Moni-
toring the reaction by UV-vis spectroscopy shows the isosbestic
conversion of the peaks corresponding to 1, into a new spec-
trum (λmax = 600 nm, ε = 1670 M−1 cm−1) that is characteristic
of an alkylperoxo-to-iron(III) LMCT band and is in good agree-
ment with similar compounds reported in the literature.14,29
The UV-vis spectra reveal that 1 reacts with tBuOOH to form 6
within 10 min, but is only stable for a short period and begins
Fig. 9 Formation of [FeIII(OOtBu)(N3PyamideSR)]2+ (7) (top). Changes in
the electronic absorption spectrum for the reaction of 2 + tBuOOH in
CH3CN, over 10 min at −40 °C (bottom left). X-band EPR spectrum of
the reaction mixture of 2 + tBuOOH, flash frozen after mixing at −40 °C
for 15 min. EPR parameters: T = 77 K, frequency = 9.46 GHz, power =
2.01 mW.
to decay after 15 min. The new species (6) is assigned as the
alkylperoxo complex [FeIII(OOtBu)(N3PySR)]2+
.
Complex 2 also reacts with tBuOOH at −40 °C to give a
deep blue solution, similar to that seen for 1. This new
complex (7, Fig. 9) is formed via isosbestic conversion from 2,
to a new spectrum characteristic for FeIII-OOR complexes (λmax
= 620 nm, ε = 2000 M−1 cm−1) within 15 min. As seen for 6,
complex 7 is not stable at −40 °C, and slowly decays to a
broad, featureless spectrum over 1 h. This species (7) is
assigned as the ferric-alkylperoxo complex, [FeIII(OOtBu)-
(N3PyamideSR)]2+.
It has been shown that a lowering in energy of peroxo-to-
iron(III) charge transfer bands can be correlated with an
increasing number of pyridine ligands bound to the iron
center, as the π-accepting ability of py ligands lowers the
energy of LMCT bands.18,52 It is therefore interesting to
compare the energy of the LMCT band for compounds 6 and 7
to the parent [FeIII(N4Py)(OOtBu)]2+ compound (λmax = 560 nm,
ε = 2400 M−1 cm−1).15 Relative to N4Py, the thioether-ligated com-
plexes exhibit bands that are shifted to lower energy by 40 nm for
6, and 60 nm for 7. These shifts indicate that replacement of a
pyridine with a thioether donor causes a significant lowering
of the energy of the LMCT band. This change is consistent
with stabilization of the metal acceptor orbital caused by the
weaker ligand field presented by the thioether donor. In
addition, the LMCT bands for both the (hydro) and (alkyl)
peroxo-iron(III) complexes are further lowered in energy by the
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Dalton Transactions
addition of the H-bond donor, as evidenced by the ∼20 nm to changes in Fe–O–O–R angle and level of admixture between
red-shift in the absorption band for both 5 and 7, relative to Fe–O stretch and Fe–O–O bend rather than changes in Fe–O
the non H-bonded 4 and 6.
bond strength.
The X-band EPR spectra for 6 and 7 (Fig. 8 and 9) show that
the alkylperoxo species are also low-spin iron(III) with g values
of 2.14, 2.08, 1.96 (6), and 2.17, 2.11, 1.96 (7), that are in good
agreement with similar compounds previously reported in the
literature.14,28,29 As seen with the hydroperoxo complexes,
spectra of 6 and 7 do show a small peak at g = 4.3, consistent
with a small amount of high-spin decomposition product.
Resonance Raman spectra of 6 and 7 were collected on
frozen samples in CD3CN at 110 K with a 647 nm laser exci-
tation (Fig. 10). The spectrum of 6 shows two resonance-
enhanced vibrations at 700 and 796 cm−1 that are in the
expected range for ν(Fe–O) and ν(O–O), respectively. The RR
Density functional theory
Density functional computations at the UB3LYP-D level of
theory were utilized to model complexes 4–7 (see ESI† for
details). Our initial work explored the spin state ordering of
complex 5, for which we calculated the lowest lying doublet
(25), quartet (45), and sextet (65) spin states. The doublet spin
state was found to be the ground state by 1.5 and 8.5 kcal mol–1
over the sextet and quartet spin states, respectively. This spin
state ordering reproduces the experimental EPR characteri-
zation of 5, which indicates an S = 1/2 spin state. Optimized
geometries of 25 and 27 are shown in Fig. 11 and are consistent
with the proposed structures for complexes 5 and 7, where the
iron(III) center is bound in a pseudo-octahedral geometry by
the equatorial pyridine and thioether donors, and the tertiary
amine and (hydro/alkyl)peroxide ligands occupy the axial posi-
tions. The structures also show that the pendant amide H-
bond donor has reoriented in both cases, and reveals an
NH⋯O hydrogen bond, with calculated N⋯O distances of
2.774 Å (5) and 2.815 Å (7) and angles of 158.1° (5) and 153.3° (7).
spectrum of 7 shows similar vibrations at 691 and 796 cm−1
.
These ν(O–O) frequencies below 800 cm−1 are consistent
with the ls configuration of 6 and 7 and both ν(O–O) and ν(Fe–
O) closely match reported results for [FeIII(TPA)(OHn)-
(OOtBu)]n+ (see Table 5). The presence of the intramolecular
H-bond donor in 7 has no effect on the ν(O–O), but it results
in a −9 cm−1 downshift of the ν(Fe–O) relative to 6. RR spectra
of complex 7 after H/D exchange of the amide N–H group
showed that deuteration of the amide group has no effect on
the ν(Fe–O) and ν(O–O) signals, which suggests that the
H-bonding interaction might be weak. The 7 cm−1 downshift of
the ν(Fe–O) in 7, relative to [FeIII(TPA)(OOtBu)]2+, could be due
Fig. 10 Resonance Raman spectra of 6 and 7 in CD3CN.
Table 5 Comparison of RR data for ls non-heme FeIII-OOtBu
complexes
Complex
ν(Fe–O)
ν(O–O)
Reference
[FeIII(N3PySR)(OOtBu)]2+
[FeIII(N3PyamideSR)(OOtBu)]2+
[FeIII(TPA)(OOtBu)]2+
700
691
696
612
796
796
796
803
This work
This work
22
Fig. 11 DFT optimized structures of 25 (top) and 27 (bottom), calculated
at the UB3LYP-D level of theory with bond lengths given in angstroms.
[FeIII([15]aneN4)(SPh)(OOtBu)]+
23
7528 | Dalton Trans., 2014, 43, 7522–7532
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Steric influences in the secondary coordination sphere
Table 6 Selected bond distances (Å) and angles (°) for 8
In further efforts to stabilize the FeIII-OO(H/R) species, we syn-
thesized the new ligand shown in Scheme 2. The resulting
complex, [FeII(N3Pyamide,2PhSR)](BF4)2 (8), incorporates two
phenyl substituents in the second coordination sphere, provid-
ing additional steric protection around the putative peroxide
binding site (Fig. 12). The synthesis of the N3Pyamide,2PhSR
ligand is shown in Scheme 2. The key synthetic step, incorpor-
ation of the phenyl substituents onto the pyridine rings, was
accomplished in high yield using Suzuki–Miyaura cross coup-
ling reaction.46 Metallation of the free ligand with Fe(BF4)2 was
accomplished by stirring in MeCN, and single crystals of 8
were grown from vapor diffusion of Et2O into the solution.
The light yellow crystals reveal Fe–N bond distances
(2.1842–2.2267 Å, Table 6) consistent with a high-spin (hs) iron(II)
center. As in 2, the iron(II) ion is bound in a pseudo-octa-
hedral geometry with the 3 pyridine donors and aryl thioether
occupying the equatorial positions, while the tertiary amine
and the amide carbonyl are bound in the axial positions. The
incorporated phenyl substituents are projected orthogonal to
Bond lengths
Bond angles
Fe1 O1
Fe1 N4
Fe1 N3
Fe1 N1
Fe1 N2
Fe1 S1
2.0085(13)
2.1842(16)
2.1954(15)
2.1959(15)
2.2267(16)
2.6097(5)
O1 Fe1 N4
O1 Fe1 N3
N4 Fe1 N3
O1 Fe1 N1
N4 Fe1 N1
N3 Fe1 N1
O1 Fe1 N2
N4 Fe1 N2
N3 Fe1 N2
N1 Fe1 N2
O1 Fe1 S1
N4 Fe1 S1
N3 Fe1 S1
N1 Fe1 S1
N2 Fe1 S1
83.37(6)
159.27(6)
76.66(6)
111.24(6)
98.60(6)
77.62(6)
120.85(6)
154.51(6)
78.35(6)
80.75(6)
83.97(4)
84.45(4)
88.61(4)
164.70(4)
90.05(4)
the pseudo-equatorial plane, and are oriented to provide sig-
nificant steric protection around the axial binding site. In con-
trast to low-spin 1 and 2, as well as some other hs FeII
complexes,28,46 8 does not react with H2O2 or tBuOOH to
afford the corresponding ferric-peroxo complexes, and appears
to only undergo outer-sphere oxidation upon addition of a
large excess of oxidant (150 equiv.). The lack of reactivity may
be due to significant steric encumbrance around the metal
center with incorporation of the new phenyl substituents.
However, we should point out that the related diphenyl-substi-
tuted hs FeII complex, [FeII(N4Pyamide,2Ph)(CH3CN)]2+, does
react with tBuOOH to give an FeIII(OOtBu) complex at low
temperature,46 and therefore some additional factors may also
be involved in inhibiting the reactivity of 8.
Scheme 2 Synthesis of the ligand precursor (N3Pyamide,2PhSR) to 8.
Conclusions
We have synthesized a series of new biomimetic iron(II) model
complexes that incorporate a pendant thioether ligand in the
equatorial plane. Complexes 1 and 2 react to generate rare
examples of FeIII-hydroperoxo and -alkylperoxo complexes with
a pendant thioether donor (4–7). These peroxo species were
characterized by UV-vis, EPR and resonance Raman spec-
troscopy, and DFT calculations supported the proposed struc-
tures and spin states. All four peroxo complexes are low-spin
species with rhombic EPR resonances centered around g = 2,
and ν(O–O) at 800 10 cm−1. The hydroperoxo (4 and 5) and
alkylperoxo (6 and 7) complexes show ν(Fe–O) at 618 5 cm−1
and ν(Fe–O) at 695 5 cm−1, respectively, which are also in the
expected range for ls peroxo species. While the addition of an
equatorial thioether donor has minimal effects on the alkyl-
peroxo Fe–O–O vibrations, substantial ν(Fe–O) and ν(O–O)
upshifts are seen in the hydroperoxo complexes. The UV-vis
data show a significant decrease in the energy of the alkylper-
oxo-to-iron(III) charge transfer bands, indicating that incorpor-
ation of a thioether donor in the first coordination sphere
likely stabilizes the key metal acceptor orbital for the LMCT
bands in 6 and 7. Modification of the second coordination
Fig. 12 Displacement ellipsoid plot of the cation of 8 shown at the 50%
probability level. H-atoms, except the amide N–H, are removed for
clarity.
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sphere by addition of a single H-bond donor was also shown performed using previously calibrated and benchmarked
to affect the properties of these non-heme Fe complexes. Struc- methods53 and are explained in detail in the ESI.†
tural characterization of the azide/fluoride-ligated complex 3
Synthesis of reported compounds
[FeII(N3PySR)(CH3CN)](BF4)2 (1). The free ligand N3PySR
showed that the axial amide ligand was indeed labile toward
displacement by anionic donors, and that formation of a
single hydrogen bond between the amide N–H and anionic (399 mg, 0.89 mmol) was dissolved in CH3CN (10 mL). A slurry
ligands in the open site is favored. The inclusion of an amide of Fe(BF4)2·6H2O (298 mg, 0.88 mmol) in CH3CN (10 mL) was
H-bond donor in the second coordination sphere also appears added dropwise to the ligand, affording a red suspension. The
to shift the peroxo-to-iron LMCT band to lower energy by mixture was allowed to stir for 2 h, and filtered through celite.
∼20 nm in both the hydro- and alkylperoxo cases. In contrast, Diffusion of diethyl ether into a CH3CN solution afforded dark
the RR spectra of the peroxo complexes are minimally affected red blocks after two days. Yield: 409 mg (68%). 1H NMR
by the addition of the amide H-bond donor. The largest change (CDCl3): δ 9.48 (2H), 9.34 (1H), 8.23 (1H), 8.14 (1H), 7.99 (2H),
corresponds to a 9 cm−1 downshift of the ν(Fe–O) in the 7.76 (1H), 7.70 (1H), 7.56 (1H), 7.44 (3H), 7.21 (2H), 6.81 (1H),
FeIII-OOtBu complex 7. It appears that the addition of the 6.71 (1H), 4.40 (1H), 4.26 (1H), 3.88 (1H), 3.66 (1H), 3.10 (1H),
phenyl substituents in 8 inhibits formation of the FeIII-OO(H/R) 2.73 (1H), 2.60 (1H). Anal. Calc. for C29H28B2F8N6SFe: C, 48.24;
complexes, and further work is needed to tease out the factors H, 3.91; N, 11.64. Found: C, 48.22; H, 3.93; N, 11.70.
that contribute to this second coordination sphere effect. The
[FeII(N3PyamideSR)(F/N3)](BF4)·CH3CN (F− = 67%, N3
=
−
results presented here suggest that both first and second 33%) (3·CH3CN). A stock solution of NaN3 (5.8 mg in 0.5 mL
coordination sphere effects can be employed by non-heme MeOH) was prepared, and 50 μL of this solution was added to
iron complexes and proteins to subtly tune the reactivity and complex 2 (6.5 mg, 9 μmol) dissolved in 0.5 mL MeCN. Vapor
properties of FeII and FeIII-OO(H/R) species.
diffusion of Et2O into the mixture resulted in a small amount
of yellow crystals (3) suitable for X-ray structure determination,
which were separated manually from the bulk material. UV-vis
(CH3CN): λmax = 355, 430 nm, ε = 1620, 1680 M−1 cm−1
.
Experimental section
General considerations
[FeIII(N3PySR)(OOH)]2+ (4). A solution of 1 (0.8 mM) was
prepared in MeCN before H2O2 (30%, 5.0 equiv.) was added at
All reagents were purchased from commercial vendors and 25 °C. The yellow solution immediately turned red, then
used without further purification unless noted otherwise. The brown. Monitoring by UV-vis (Fig. 5) showed the decay of
N3PySR ligand44 and 247 were synthesized as previously peaks corresponding to 1 (λmax = 350, 450 nm), with isosbestic
reported. All reactions were carried out under an atmosphere conversion to a new spectrum (λmax = 542 nm) that corres-
of N2 inside a glovebox or under Ar by standard Schlenk and ponds to 4. Complex 4 was formed within 60 s and immedi-
vacuum line techniques. UV-visible spectra were recorded on a ately began to decay to a brown species with no prominent UV-
Varian Cary 50 spectrophotometer. Electron paramagnetic reso- vis features. UV-vis: λmax = 542 nm, ε = 1000 M−1 cm−1; EPR:
nance (EPR) spectra were obtained on a Bruker EMX EPR g = [2.17, 2.11, 1.97]; RR: ν(Fe–O) = 615, 629, 647 and
spectrometer controlled with a Bruker ER 041 XG microwave 664 cm−1, ν(O–O) = 809 cm−1
bridge at 15 or 77 K. The EPR spectrometer was equipped with
[FeIII(N3PyamideSR)(OOH)]2+ (5). An amount of 2 (1.25 mg,
.
a continuous-flow liquid He or N2 cryostat and an ITC503 1.7 μmol) was dissolved in 4.0 mL of MeCN and cooled to
temperature controller made by Oxford Instruments, Inc. NMR −40 °C. A solution of H2O2 (0.88 M, 25 equiv., 0.05 mL) in
was performed on a Bruker Avance 400 MHz FT-NMR spectro- MeCN was added and the color immediately began to change
meter at 25 °C. Elemental analysis was performed by Atlantic from orange to purple. Monitoring by UV-vis (Fig. 6) revealed
Microlab Inc., Norcross, GA. LDI-ToF mass spectra were full formation of 5 (λmax = 567 nm) within 30 min. UV-vis:
obtained using a Bruker Autoflex III Maldi ToF/ToF instrument λmax = 567 nm, ε = 900 M−1 cm−1; EPR: g = [2.17, 2.16, 1.95];
(Billerica, MA). Samples were dissolved in CH2Cl2 and de- RR: ν(Fe–O) = 612 cm−1, ν(O–O) = 800 cm−1
posited on the target plate in the absence of any added matrix.
[FeIII(N3PySR)(OOtBu)]2+ (6). A solution of 1 (0.8 mM) was
.
Samples were irradiated with a 355 nm UV laser and mass- prepared in MeCN and cooled to −40 °C. A solution of
analyzed by ToF mass spectrometry in the reflectron mode. tBuOOH (5.5 M, 10.0 equiv.) diluted in MeCN was added, and
Resonance Raman spectra were recorded at 110 K using a the yellow solution immediately turned green, then blue.
backscattering geometry with a McPherson 2061/207 spectro- Monitoring by UV-vis (Fig. 8) showed the peaks corresponding
graph and a liquid-N2-cooled CCD camera. The 568 and to 1 decay with isosbestic conversion to a new spectrum
647 nm excitations were obtained from a Kr and Ar ion laser, (λmax = 600 nm) that corresponds to 6. UV-vis: λmax = 600 nm,
respectively (Innova 300, Coherent). Longpass filters (Razor-
Edge, Semrock) were used to attenuate the Raleigh scattering. ν(Fe–O) = 700 cm−1, ν(O–O) = 796 cm−1
The samples were kept at 110 K inside a liquid-N2 coldfinger
ε
=
1670 M−1 cm−1
;
EPR:
g
=
[2.14, 2.08, 1.96]; RR:
.
[FeIII(N3PyamideSR)(OOtBu)]2+ (7). A solution of 2 (1.1 mM)
and spun continuously to prevent photodamage. Frequencies in MeCN was prepared and cooled to −40 °C. A solution of
were calibrated relative to an aspirin standard and are accurate tBuOOH (5.5 M, 10 equiv.) in MeCN was then added and
to 1 cm−1. Density functional theory calculations were allowed to stir for 20 min. The orange solution of 2 turns
7530 | Dalton Trans., 2014, 43, 7522–7532
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green and finally deep blue, as monitoring by UV-vis (Fig. 9) thanked for providing cpu time to SdV. H.M. acknowledges
shows the decay of the peaks corresponding to 2 with the support from the JSPS.
isosbestic growth of a new peak (λmax = 620 nm) corresponding
to 7. UV-vis: λmax = 620 nm, ε = 2000 M−1 cm−1; EPR: g =
[2.17, 2.11, 1.96]; RR: ν(Fe–O) = 691 cm−1, ν(O–O) = 796 cm−1
.
Secondary amine (iii). Primary amine (i)46 (825 mg,
2.5 mmol) and aldehyde (ii)47 (468 mg, 2.5 mmol) were dis-
solved in a 1 : 1 mixture of CHCl3–MeOH (see Scheme 2). Mole-
cular sieves (4 Å) were added and the mixture was stirred
under Ar for 48 h. Excess NaBH3CN (315 mg, 5.0 mmol) was
added and allowed to mix for 2 h before being quenched with
1 M HCl. The crude reaction mixture was concentrated, dis-
solved in CHCl3, washed with H2O, dried and re-concentrated,
before being purified on neutral alumina (EtOAc–hexanes) to
give 680 mg (53%) of secondary amine iii as a pale yellow
Notes and references
1 E. G. Kovaleva, M. B. Neibergall, S. Chakrabarty and
J. D. Lipscomb, Acc. Chem. Res., 2007, 40, 475–483.
2 E. G. Kovaleva and J. D. Lipscomb, Science, 2007, 316,
453–457.
3 E. I. Solomon, A. Decker and N. Lehnert, Proc. Natl. Acad.
Sci. U. S. A., 2003, 100, 3589–3594.
4 M. Costas, M. P. Mehn, M. P. Jensen and L. Que, Jr., Chem.
Rev., 2004, 104, 939–986.
1
solid. H-NMR (CDCl3): δ 8.09–8.07 (m, 4H), 7.72 (t, 2H), 7.63
(d, 2H), 7.54–7.49 (m, 6H), 7.46–7.42 (m, 6H), 5.30 (s, 1H), 4.06
(s, 2H), 3.06 (t, 2H), 2.47 (t, 2H).
5 E. I. Solomon, S. D. Wong, L. V. Liu, A. Decker
and M. S. Chow, Curr. Opin. Chem. Biol., 2009, 13, 99–113.
6 P. L. Roach, I. J. Clifton, C. M. H. Hensgens, N. Shibata,
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N3Pyamide,2PhSR. Secondary amine (iii) (200 mg, 0.4 mmol)
and Cs2CO3 (196 mg, 0.6 mmol) were combined in 100 mL
MeCN, followed by alkyl bromide (iv)46 (109 mg, 0.4 mmol) and
NaI (90 mg, 0.6 mmol) (see Scheme 2). After stirring for 72 h,
the crude mixture was filtered through celite and concentrated.
The crude solid was dissolved in CHCl3, washed with H2O, dried,
and purified by column chromatography on neutral alumina
(EtOAc–hexanes) to give 272 mg (97%) of the final ligand as a
9 L. Que, Jr., J. Biol. Inorg. Chem., 2004, 9, 684–690.
pale yellow solid. 1H-NMR (CDCl3): δ 8.07–8.03 (m, 5H), 10 S. Ye, X. Wu, L. Wei, D. M. Tang, P. Sun, M. Bartlam and
7.92–9.90 (m, 2H), 7.73 (t, J = 8.1 Hz, 2H), 7.67 (dd, J = 8.8, 1.0
Hz, 2H), 7.60 (t, J = 7.8 Hz, 2H), 7.50–7.39 (m, 9H), 7.33 (dd, J = 11 M. J. Vasbinder and A. Bakac, Inorg. Chem., 2007, 46, 2921–
7.6, 1.5 Hz, 1H), 7.25 (dd, J = 7.6, 1.3 Hz, 1H), 7.19 (td, 7.5, 1.8 2928.
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Hz, 2H), 2.37 (t, J = 7.1 Hz, 2H), 1.31 (s, 9H); 13C NMR (CDCl3):
δ 177.2, 160.1, 159.1, 156.5, 150.8, 141.8, 139.8, 138.7, 137.1,
133.0, 131.4, 130.4, 129.1, 128.9, 127.9, 127.8, 127.3, 123.0,
S. Genseberger, R. M. Hermant, R. Hage, S. K. Mandal,
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27.8, 18.2, 8.6. FAB-MS (+): m/z = 703.32127 [M + H]+ (calcd for
C44H43N6OS, 703.32191).
C. Zondervan, R. M. la Crois, E. P. Schudde, M. Lutz,
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[FeII(N3Pyamide,2PhSR)](BF4)2
(8). The
free
ligand
N3Pyamide,2PhSR (461 mg, 0.66 mmol) was dissolved in 5 mL 14 J. Kim, E. Larka, E. C. Wilkinson and L. Que, Jr., Angew.
MeCN before Fe(BF4)2·6H2O (221 mg, 0.66 mmol) was added.
Chem., Int. Ed., 1995, 34, 2048–2051.
After stirring for 2 h, the yellow solution was filtered through 15 J.-U. Rohde, S. Torelli, X. P. Shan, M. H. Lim, E. J. Klinker,
celite and vapor diffusion of Et2O gave 8 as yellow crystals suit-
J. Kaizer, K. Chen, W. Nam and L. Que, Jr., J. Am. Chem.
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able for X-ray structure determination (510 mg, 83% yield).
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32.37, 24.80, 20.33, 19.42, 13.40, 11.65, 9.19, 8.22, 7.38, 6.22,
2762–2767.
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m/z = 757.6 [M − H]+. UV-vis (CH3CN): λmax = 405 nm, ε =
2650 M−1 cm−1. Anal. Calc. for [8] (C44H42B2F8FeN6OS): predicted:
C, 56.68, H, 4.54; N, 9.01; Found: C, 56.53; H 4.72; N, 9.44.
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Acknowledgements
The NIH (D.P.G., GM62309 and GM101153, P.M.L. GM074785) 20 M. R. Bukowski, P. Comba, C. Limberg, M. Merz, L. Que, Jr.
is gratefully acknowledged for financial support. The National
Service of Computational Chemistry Software (NSCCS) is
and T. Wistuba, Angew. Chem., Int. Ed., 2004, 43, 1283–
1287.
This journal is © The Royal Society of Chemistry 2014
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7532 | Dalton Trans., 2014, 43, 7522–7532
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