Sulfide oxidation by O2: Synthesis, structure and reactivity of novel sulfide-incorporated Fe(II) bis(imino)pyridine complexes

Article abstract of DOI:10.1016/j.poly.2013.01.043

The unsymmetrical iron(II) bis(imino)pyridine complexes [Fe ^II(LN₃SMe)(H₂O)₃](OTf)₂ (1), and [Fe^II(LN_3-SMe)Cl₂] (2) were synthesized and their reactivity with O₂ was examined. Complexes 1 and 2 were characterized by single crystal X-ray crystallography, LDI-MS, ¹H NMR and elemental analysis. The LN₃SMe ligand was designed to incorporate a single sulfide donor and relies on the bis(imino)pyridine scaffold. This scaffold was selected for its ease of synthesis and its well-precedented ability to stabilize Fe(II) ions. Complexes 1 and 2 ware prepared via a metal-assisted template reaction from the unsymmetrical pyridyl ketone precursor 2-(O=CMe)-6-(2,6-(ⁱPr ₂-C₆H₃N=CMe)-C₅H₃N. Reaction of 1 with O₂ was shown to afford the S-oxygenated sulfoxide complex [Fe(LN₃S(O)Me)(OTf)]²⁺ (3), whereas compound 2, under the same reaction conditions, afforded the corresponding sulfone complex [Fe(LN₃S(O₂)Me)Cl]²⁺ (4).

Full text of DOI:10.1016/j.poly.2013.01.043

Polyhedron xxx (2013) xxx–xxx
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Sulfide oxidation by O2: Synthesis, structure and reactivity of novel
sulfide-incorporated Fe(II) bis(imino)pyridine complexes
⇑
Leland R. Widger, Maxime A. Siegler, David P. Goldberg
Department of Chemistry, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
a r t i c l e i n f o
a b s t r a c t
The unsymmetrical iron(II) bis(imino)pyridine complexes [FeII(LN3SMe)(H2O)3](OTf)2 (1), and [FeII(LN3-
SMe)Cl2] (2) were synthesized and their reactivity with O2 was examined. Complexes 1 and 2 were char-
acterized by single crystal X-ray crystallography, LDI-MS, 1H NMR and elemental analysis. The LN3SMe
ligand was designed to incorporate a single sulfide donor and relies on the bis(imino)pyridine scaffold.
This scaffold was selected for its ease of synthesis and its well-precedented ability to stabilize Fe(II) ions.
Complexes 1 and 2 ware prepared via a metal-assisted template reaction from the unsymmetrical pyridyl
ketone precursor 2-(O@CMe)-6-(2,6-(iPr2-C6H3N@CMe)-C5H3N. Reaction of 1 with O2 was shown to
afford the S-oxygenated sulfoxide complex [Fe(LN3S(O)Me)(OTf)]2+ (3), whereas compound 2, under
the same reaction conditions, afforded the corresponding sulfone complex [Fe(LN3S(O2)Me)Cl]2+ (4).
Ó 2013 Elsevier Ltd. All rights reserved.
Article history:
Available online xxxx
Keywords:
Bis(imino)pyridine
Sulfide
Iron(II)
Dioxygen
Non-heme Iron
Model Complexes
1. Introduction
harsh or toxic oxidizing agents (Fig. 2b) and the difficulty of stop-
ping oxidation at the sulfoxide [30,31] presents challenges that
There is broad interest in understanding the mechanisms by
which metalloenzymes activate O2 and control reactivity with sub-
strates [1]. Many non-heme iron oxygenases that carry out these
reactions typically employ a 2-His-1-carboxylate motif around
the catalytically active iron center. One exception is cysteine diox-
ygenase (CDO), which contains a high-spin ferrous center bound by
three His residues in a facial triad and one water molecule com-
pleting a pseudo-tetrahedral coordination sphere. This (His)3FeII
(H2O) center likely activates O2 upon binding of the cysteine sub-
strate, and is responsible for catalyzing the oxidation of cysteine
to cysteine sulfinic acid (Fig. 1). Mechanistic information for many
non-heme iron dioxygenases is available [2–7], but little is known
about the mechanism of CDO. There is debate in the literature
about the nature of the possible O2 derived intermediate(s) in
the catalytic cycle of CDO [8–18]. As the first step in mammalian
cysteine catabolism, CDO activity is critical for proper cellular
function. Loss of this activity has been implicated in many disease
states including cystinosis [19,20], Hallervoden–Spatz syndrome
[21], Alzheimer’s and Parkinson’s disease [22,23].
have attracted the interest of inorganic chemists. Furthermore, sul-
foxidation catalysts have recently attracted a great deal of atten-
tion for use in desulfurization of fossil fuels with O2 and other
applications to reduce toxicity (Fig. 2c) [28,32,33].
Some metal-based systems have been developed to oxidize sul-
fides under mild conditions. Some of these systems perform as cat-
alysts for asymmetric sulfide oxidation, and a few can utilize
dioxygen as the oxidant [34–60]. There are only a few reports of
iron-based sulfoxidation catalysts, but all of these systems utilize
FeIII centers in lieu of FeII [41,49,51,52]. Molecular O2 is used in
some of these cases as a co-oxidant with an FeIII source and an
additional oxidant generated in situ. For example, Wang and
coworkers have reported a catalytic Fe2O3 system where molecular
oxygen is proposed to work in concert with the FeIII catalyst to gen-
erate peracid oxidants in situ from various aldehydes [44,45].
These peracids, much like mCPBA, are ultimately responsible for
sulfide oxidation. Despite the efficiency of the Wang Fe2O3/O2/
aldehyde system, only modest yields are observed for sulfone
products. In addition, peracids are not selective for sulfides, and
are well known to be efficient reagents for olefin epoxidation, Bae-
yer–Villiger oxidation of ketones, and oxidation of secondary alco-
hols, even in the absence of a metal catalyst [43]. This limited
functional group tolerance severely restricts the scope under
which this methodology could be used in a synthetic scenario. In
other systems reported by Rossi et al. [49,51,61,62] aerobic oxida-
tion is carried out by catalytic amounts of NO2 (which is in equilib-
rium with N2O4), generated by an FeBr3 catalyst from HNO3. In
these reports, once NO2/N2O4 is generated there is an aerobic
In addition to the biological relevance of sulfur oxidation, sulf-
oxides are valued synthetic intermediates to organic chemists
[24,25] and are important in many pharmaceutically active com-
pounds (Fig. 2a) [26–28]. Sulfur oxidation is a well-known and
widely used synthetic strategy for a variety of applications in or-
ganic chemistry [29], but the use of stoichiometric amounts of
⇑
Corresponding author.
E-mail address: dpg@jhu.edu (D.P. Goldberg).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.043
Please cite this article in press as: L.R. Widger et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.01.043

2
L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
-O2C
H2N
ArSMe
FeBr3
O
OH2
FeII
HNO3
Cys
S
S
OH
O2
0.5 O2
NO (g) + NO2 (g)
FeII
-
NO+ / NO3
2 NO2 (g)
N2O4 (g)
-O2C
NH2
NHis
NHis
H2O
NHis
NHis
NHis
NHis
Cysteine Sulfinic Acid
(CSA)
ArSMe
[ArMeS+ NO] NO3
-
-
[ArMeSNO+] NO3
ArS(O)Me
Fig. 1. Oxidation of cysteine to cysteine sulfinic acid by CDO.
Fig. 3. Proposed catalytic mechanism of aerobic sulfoxidation by NOx species [61].
catalytic cycle of NOx species which gives NO and the S-oxygenated
product. NO then reacts with O2 to regenerate NO2 and turnover
the system (Fig. 3). Recent mechanistic studies have actually
shown that these types of systems, originally reported with FeBr3
to initiate the catalytic reaction, can in fact proceed in the absence
of a metal-based catalyst [61,63]. Catalytic systems based solely on
these types of gaseous equilibria are sensitive to a variety of envi-
ronmental conditions, and are likely to be deactivated in many
industrial circumstances [52].
Recent work in our lab has focused on the development of ir-
on(II) complexes with mixed N/S donor sets to serve as structural
and functional models for a variety of non-heme metalloenzymes,
including non-heme iron oxygenases which utilize O2 for sub-
strate oxidation reactions. Previous reports from our lab have in-
cluded examples where an FeII center was shown to mediate
sulfur oxygenation of thiolate ligands [64–66]. In these reports,
discrete thiolate-ligated FeII complexes reacted with O2 at room
temperature and atmospheric pressure to give the corresponding
FeII-sulfonato or sulfinato complexes. In addition to being among
the first reports of this reactivity [64–67], the assignment of an
oxidation state for the product as FeII was interesting due to
the similarity with the iron(II) resting state in CDO. We were
Fig. 4. Displacement ellipsoid plot (50% probability level) of the cation of 1. H-
atoms are removed for clarity.
2. Results and discussion
2.1. Synthesis of LN3SMe complexes
The bis(imino)pyridine (BIP) ligand scaffold has been widely
used due to its ease of synthesis, and facile steric and electronic
modification to give a range of ligand variants. In fact, from com-
mercially available 2,6-diacetylpyridine and 2 equiv. of a primary
amine, it is possible to synthesize a large array of symmetrical
BIP ligands in one pot in a matter of hours. The challenge lies in
the synthesis of unsymmetrical derivatives, of interest to us so as
to be able to incorporate sulfur-containing functional groups
around the metal center. For our purposes regarding iron-oxygen
chemistry, a key requirement for a ligand system is to be able to
include significant steric protection around the iron ion to help
prevent the formation of polymeric Fe complexes. We were there-
fore fortunate to come across the unsymmetrical precursor 2-
(O@CMe)-6-(2,6-(iPr2-C6H3N@CMe)-C5H3N, reported by Bianchini
et al. [68], which is easily prepared by mixing 2,6-diacetylpyridine
and 2,6-diisopropylaniline in MeOH with catalytic formic acid. The
interested in synthesizing
a
modified FeII(LN3SMe)X2 system
which incorporated sulfides in place of the thiolate donors to
determine the effect on S-oxygenation reactivity. Herein we re-
port the synthesis, characterization, and O2 reactivity of two no-
vel methylsulfide iron(II) complexes. The unsymmetrical
bis(imino)pyridine complexes [(FeII(LN3SMe)(H2O)3](OTf)2 (1)
(Fig. 4) and [FeII(LN3SMe)Cl2] (2) (Fig. 5), were synthesized by me-
tal-assisted template reactions. Complexes 1 and 2 were charac-
terized by single crystal X-ray crystallography, LDI-MS, 1H NMR,
and elemental analysis. Complex 1 reacts with O2 at modestly
elevated temperature to afford the sulfoxide complex [(Fe(LN3-
SOMe)(OTf)]2+ (3), while 2, under the same conditions, yields
the sulfone complex [Fe(LN3SO2Me)Cl]2+ (4).
O
O
O
N
O
O
N
AcOOH
S
S
pharmaceuticals
(a)
N
H
N
H
N
N
Omeprazole
O
S
S
CAN, O2
MeCN,
harsh conditions and
toxic reagents
(b)
(c)
O
S
0.5 equiv O2
S
detoxification
Cl
Cl
Cl
Cl
Mustard (HD)
(toxic)
Mustard (HDO)
(far less toxic)
Fig. 2. Examples of sulfoxides in synthetic chemistry and detoxification processes.
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L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
3
Cl
Cl
FeII
1) FeCl2
2)
S
, Et3N
N
O
NH2
N
N
N
N
S
EtOH, 60 - 80 °C
18 h
2
Scheme 2. Synthesis of complex 2.
Table 1
Summary of the crystallographic data.
1ꢀEt3NHOTf
[C37H55FeN4O12S4F9] [C29H37Cl2FeN3OS]
1102.94
110
red-purple
monoclinic
P21/n
13.73725(19)
16.1292(2)
22.9656(3)
95.0312(13)
5068.90(12)
4
1.445
0.553
25
36550
8932
0.0459
821
0.0450
2ꢀMeOH
Fig. 5. Displacement ellipsoid plot (50% probability level) of 2. H-atoms are
removed for clarity.
Formula
Formula weight
T (K)
Color
Class
Space group
a (Å)
b (Å)
602.43
110
product is insoluble in MeOH and precipitates out of solution be-
fore it can react further. This precursor includes pre-organized ste-
ric shielding via the 2,6-diisopropylphenyl substituent, and it
provides a convenient way for installing a single sulfur-containing
group through the available ketone (see Schemes 1 and 2).
We have previously reported the difficulty in direct condensa-
tion, or even reductive amination, of the second amine onto this
unsymmetrical precursor [69]. The difficulty we encountered while
attempting to synthesize these ligands resulted from the revers-
ibility of the Schiff-base reactions. Extended exposure of the
unsymmetrical precursor to Schiff base reaction conditions may
lead to scrambling of the imine groups, giving intractable mixtures
of labile imine products [69]. However, pre-binding a metal ion to
the 2-(O@CMe)-6-(2,6-(iPr2-C6H3N@CMe)-C5H3N precursor allevi-
ates this problem, and by using this metal template method we
have been successful at synthesizing several unsymmetrical, BIP-
derived compounds with an array of metal ions.
A template reaction was carried out to synthesize the new iro-
n(II)-sulfide complexes [FeII(LN3SMe)(H2O)3](OTf)2 (1), and [FeII
(LN3SMe)Cl2] (2). The appropriate FeII salt (Fe(OTf)2 or FeCl2) was
suspended in EtOH with 2-(O@CMe)-6-(2,6-(iPr2-C6H3N@CMe)-
C5H3N and heated to 60 °C for 1 h, or until all of the solids dis-
solved. As the solids dissolve, the solution becomes deep purple
or blue in color for the trifluoromethanesulfonate or chloride com-
plexes, respectively. This step is followed by addition of 2-(methyl-
thio)aniline added together with Et3N, and the reaction mixture is
allowed to stir at 80 °C for 24 h. The crude reaction mixture is then
concentrated to a residue and transferred to the glovebox for crys-
tallization. Single crystals suitable for X-ray structure determina-
tion were grown from vapor diffusion of diisopropyl ether into
solutions of 1 (in MeCN) and 2 (in MeOH). Crystallographic data
for complexes 1 and 2 are summarized in Table 1.
dark blue
monoclinic
P21/c
9.86440(19)
16.3351(2)
18.2562(2)
94.1396(15)
2934.07(7)
4
1.364
0.794
26.25
24149
5932
0.0274
358
0.0422
0.1078
0.0467
0.1103
1.084
c (Å)
b (°)
V (Å3)
Z
q
l
(g cm-3
)
(mmꢁ1
)
h (°)
No. reflections collected
No. unique reflections
Rint
No. variable parameters
R1 [I > 2
wR2 [I > 2
r
(I)]
(I)]
r
0.1222
0.0642
0.1292
1.041
R1 [all data]
wR2 [all data]
Goodness-of-fit (GOF) on F2
Largest difference in hole and
ꢁ0.58 and 0.65
ꢁ0.83 and 1.03
peak (e Aꢁ3
)
diisopropyl group is projected orthogonal to the N3 plane, which
contains the iron ion. The three water molecules lie in a plane com-
pleting the octahedral coordination environment; the locations of
all H atoms from the three water ligands were easily derived from
the difference Fourier maps. The sulfide moiety is not bound to the
iron center, being displaced by the water molecules from the coor-
dination sphere. The asymmetric unit contains three trifluoro-
methanesulfonate anions, one triethylammonium cation, and the
Fe complex, leading to the assignment of the metal center as
iron(II).
The structure of 2 is shown in Fig. 5. The three neutral
bis(imino)pyridine N donors and two chloride anions bind the fer-
2.2. X-ray crystallography
rous center in a distorted square pyramidal geometry (s = 0.18).
The diisopropyl groups are projected orthogonal to the N3
plane in which the iron sits, and as in 1 the sulfide moiety is not
bound to the iron center. One chloride counterion occupies the
axial position while the second occupies a pseudoequatorial posi-
tion completing the square pyramid. Additionally there is one
The ferrous center of 1 is bound in a distorted octahedral geom-
etry by the three neutral N donors from the BIP ligand and three
water molecules (Fig. 4). Although FeII-bis(imino)pyridine com-
plexes are usually found in five-coordinate geometries, there are
a few reported examples of octahedral complexes [65,70]. The aryl
H2O H2O
OH2
N
2+
1) Fe(OTf)2
-
(CF3SO3
)
2
2)
S
, Et3N
FeII
N
O
NH2
N
N
N
S
EtOH, 60 - 80 °C
18 h
1
Scheme 1. Synthesis of complex 1.
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4
L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
Table 2
Selected bond distances (Å) and bond angles (°) for iron(II) complexes.
a
1
[FeII(iPrBIP)(H2O)2(NCCH3)](OTf)2
2
[FeII(iPrBIP)Cl2]b
Fe1–N1
Fe1–N2
Fe1–N3
Fe1–O1
Fe1–O2
Fe1–O3
Fe–N4
Fe–Cl1
2.245(2)
2.102(2)
2.234(2)
2.086(2)
2.098(2)
2.116(2)
N/A
2.276(3)
2.086(3)
2.280(3)
2.075(2)
2.161(2)
N/A
2.105(5)
N/A
N/A
1.286(4)
1.284(4)
74.20(10)
147.36(9)
74.03(10)
102.98(10)
83.08(9)
N/A
168.56(10)
N/A
N/A
173.40(10)
N/A
87.59(10)
86.58(10)
2.200(2)
2.094(2)
2.186(2)
N/A
N/A
N/A
2.222(4)
2.091(4)
2.225(5)
N/A
N/A
N/A
N/A
N/A
N/A
N/A
2.3297(7)
2.2611(7)
1.287(4)
1.288(3)
73.14(8)
141.43(8)
73.11(7)
N/A
N/A
N/A
N/A
99.60(6)
152.16(6)
N/A
108.06(3)
N/A
2.3173(19)
2.2627(17)
1.301(7)
1.295(7)
73.67(16)
140.23(16)
72.59(16)
N/A
N/A
N/A
N/A
94.52(13)
147.90(13)
N/A
117.58(7)
N/A
Fe–Cl2
N1–C8
N3–C15
1.281(4)
1.287(4)
73.30(8)
146.95(8)
73.89(8)
176.45(9)
96.69(8)
89.82(9)
N/A
N/A
N/A
86.79(8)
N/A
86.64(9)
170.14(8)
N1–Fe1–N2
N1–Fe1–N3
N2–Fe1–N3
N2–Fe1–O1
N2–Fe1–O2
N2–Fe1–O3
N2–Fe1–N4
N2–Fe1–Cl1
N2–Fe1–Cl2
O1–Fe1–O2
Cl1–Fe1–Cl2
O1–Fe1–N4
O2–Fe1–N4
N/A
N/A
a
Ref. [65].
Ref. [72].
b
Fig. 6. Hydrogen bonding network in the crystal lattice of 1. Disorder and H-atoms are removed for clarity.
uncoordinated lattice solvent methanol molecule per Fe complex
in the structure of 2.
has two axial water ligands and an equatorial coordinated acetoni-
trile molecule, while 1 has three coordinated water molecules. Fe–
Nimine and Fe–Npyr distances of 2.276(3), 2.280(3) and 2.086(3) Å
are in good agreement with the observed Fe–N distances of
2.245(2), 2.234(2) and 2.102(2) Å in 1. Fe–OH2 distances in the
Bond distances for complexes 1 and 2 are consistent with high-
spin ferrous centers [71] and are in good agreement with similar
bis(imino)pyridine compounds reported in the literature
[65,70,72]. A comparison of bond lengths from cations 1 and 2 with
closely related compounds in the literature are summarized in Ta-
ble 2. The comparable symmetrical analogue of complex 1, [FeII
symmetrical
[FeII(iPrBIP)(H2O)2(NCCH3)](OTf)2
complex
are
2.075(2) and 2.161(2) Å, very close to the observed Fe–OH2 dis-
tances in 1 of 2.086(2), 2.098(2) and 2.116(2) Å, while Nimine–Cimine
distances are very much comparable. Similar results are derived
(
iPrBIP)(H2O)2(NCCH3)](OTf)2, reported previously by our lab [65]
has an additional diisopropyl phenyl group in lieu of the aryl sul-
from comparison of
2 with the closely related symmetrical
fide substitution. In addition, [FeII(iPrBIP)(H2O)2(NCCH3)](OTf)2
bis(diisopropylphenyl) compound [FeII(iPrBIP)Cl2], first reported
Please cite this article in press as: L.R. Widger et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.01.043

L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
5
Table 3
Selected bond distances (Å) and angles (°) for H-bonding interactions in the solid-
state structure of 1.
Donor
atom
(D)
Hydrogen
atom (H)
Acceptor
atom (A)
Distance
D–H
Distance
H–A
Distance
D–A
Bond
angle
DHA
O1
O1
O1
O2
O2
O2
O3
O3
N4
H1W1
H1W2
H1W2
H2W1
H2W1
H2W2
H3W1
H3W2
H4A
O6
0.82(2)
0.84(2)
0.84(2)
0.85(2)
0.85(2)
0.79(2)
0.82(2)
0.84(2)
0.93
1.95(2)
1.99(3)
1.94(2)
1.90(2)
1.81(3)
2.01(2)
2.07(3)
1.87(3)
1.90
2.759(3)
2.793(3)
2.77(2)
2.745(6)
2.636(19) 163(4)
2.789(3)
2.864(3)
2.702(4)
2.755(6)
174(4)
160(4)
174(4)
171(3)
O10
O100
O9
O90
O5
O4
O11
O7
171(4)
157(3)
170(4)
152
by Brookhart and coworkers [72] (see Table 2). The observed bond
angles in 1 and 2 are also in good agreement with their close ana-
logues from the literature (Table 2).
In the solid-state structure of 1, there does not appear to be any
p-stacking interactions between the individual cations of 1, as all
of the aryl groups are significantly offset from each other. In fact,
the individual cations are angled away from each other in order
to accommodate an extensive H-bonding network (Fig. 6). In this
network, two of the three trifluoromethanesulfonate anions (con-
taining S2 and S4) are found to be H-bond acceptors in two O–
Hꢀ ꢀ ꢀO hydrogen bonds, for which the coordinated water molecules
(O1, O2 and O3) are donors. One of the trifluoromethanesulfonate
ions bridges the cations of 1 between O2ꢀ ꢀ ꢀO5 and O4ꢀ ꢀ ꢀO3
through H-bonds, where O2 and O3 are the two axially coordinated
water molecules, and O4 and O5 are from the trifluoromethanesul-
fonate ion. The remaining trifluoromethanesulfonate anion (con-
taining S3) is found to be an H-bond acceptor in Owater–Hꢀ ꢀ ꢀO and
N–Hꢀ ꢀ ꢀO H-bonding interactions (the triethylammonium cation is
the donor in the latter). Distances and angles for H-bonds in the solid
state structure of 1 are given in Table 3. All values are consistent
with good H-bonding interactions and are in good agreement with
accepted values (D–A ꢂ 2.6–2.7 Å, DHA P 160°) although some var-
iation is expected based on the nature of the individual H-bond.
2.3. NMR spectroscopy
Compounds 1 and 2 were characterized by 1H NMR spectros-
copy. Both compounds exhibit paramagnetically shifted peaks,
consistent with the assignment of high-spin FeII from metal-ligand
bond distances. The 1H NMR spectrum for 1 exhibits peaks that are
paramagnetically shifted from 90 ppm to ꢁ42 ppm, while for 2 a
wider spread of resonances are observed, from 97 ppm to
ꢁ38 ppm (Figs. S1–S2). The spectra for compounds 1 and 2 are
significantly more complex relative to related, symmetrical BIP
complexes that we have prepared [65], likely due to the desym-
metrized nature of 1 and 2, as well as the possibility for ligand ex-
change at the metal center on the NMR timescale. Although
individual resonances cannot be assigned in the spectra of 1 and
2, the spectra are consistent and reproducible for analytically pure
samples.
Fig. 7. Reaction of 1 with excess O2 in CH2Cl2 at 60 °C as monitored by LDI-MS. The
peak at m/z 648.1 is assigned to [1-3H2O-OTf]+, and the peak at m/z 664.4 is
assigned to [Fe(LN3S(O)Me)(OTf)]+.
2.4. O2 reactivity
2.4.1. General remarks
All O2 reactions were carried out in a sealed reaction vessel,
equipped with a vacuum port that was connected above the Teflon
screw-cap. Solutions of compounds 1 and 2 in CH2Cl2 were trans-
ferred to the flask, and then the flask was charged with O2 by bub-
bling directly into the CH2Cl2 solution for 5 min. The vessel was
then sealed and heated to 60 °C for several days. Typical reaction
H2O H2O
OH2
N
O
S
2+
(CF3SO3
F3CSO3
FeIII
2+
-
)
2
FeII
excess O2
N
N
N
N
S
N
CH2Cl2, 60 °C, 96 h
1
3
Scheme 3. Reaction of 1 with O2 to form sulfoxide complex 3.
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6
L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
Cl
Cl
FeII
O
2+
O
Cl
FeIII
S
excess O2
N
CH2Cl2, 60 °C, 96 h
N
N
N
N
S
N
2
4
Scheme 4. Reaction of 2 with O2 to form the sulfone complex 4.
modestly elevated temperature in CH2Cl2, as seen in Scheme 3.
The reaction of 1–3 was followed by LDI-MS. As shown in Fig. 7,
a loss of the starting material at m/z 648.1 ([1-3H2O-OTf]+) is seen
over 4 days, along with the appearance of a singly-oxygenated sulf-
oxide species 3 at m/z 664.4 ([Fe(LN3SOMe)(OTf)]+). Further analy-
sis (vide infra) reveals that this complex is likely an FeIII species
which was reduced and observed as a monocation in the LDI-MS
experiment.
Complex 2 also exhibits reactivity toward O2. Solutions of com-
pound 2 react with excess O2 at 60 °C in a closed reaction vessel to
afford the sulfone 4 in 96 h (Scheme 4). Fig. 8 shows the loss of 2
(m/z 534.4, [2-Cl]+) with the concurrent appearance of a peak at
m/z 566.4, corresponding to the doubly-oxygenated species
[Fe(LN3S(O2)Me)Cl]+. As in the case of 3, this complex is assigned
as an FeIII product but is presumed to be reduced in the LDI-MS
experiment to the corresponding monocation. Spectra taken at
early time points (0–48 h) reveal the presence of a singly oxygen-
ated (possibly sulfoxide) intermediate (m/z 550.4, [Fe(LN3-
S(O)Me)Cl]+) along with sulfide starting material and sulfone
product. This intermediate species is not observed in significant
quantities, and after 96 h only the doubly-oxygenated sulfone
product 4 was observed (Fig. 10). Attempts to improve the yield
of the putative singly-oxygenated complex (m/z 550.4) by per-
forming the reaction at lower temperature and pressure gave only
small amounts of this species and the doubly-oxygenated product.
No significant improvement in yield could be obtained.
Control experiments were conducted, and exposure of 2-amino-
thiophenol to excess O2 under the same conditions for 3 d resulted
in no observed sulfoxide or sulfone products as measured by 1H
NMR. These control reactions indicated that the presence of the
ferrous complexes 1 and 2 were necessary for the observed
reactivity.
2.4.3. 18O-labeled water
The reaction of 1 was run in the presence of excess H218O
(100 equiv.). This experiment was performed to confirm that diox-
ygen, and not exogenous water, was the source of the oxygen atom
in the sulfoxide complex, and to determine if O-atom exchange
with water could occur during the S-oxygenation reaction. High-
valent ferryl (Fe@O) species are known to rapidly exchange with
H2O in non-heme iron complexes, resulting in the incorporation
of oxygen derived from water in oxidized substrates [73–75]. No
18O incorporation into 3 was observed by LDI-MS in the presence
of 18OH2. This result indicates that O2 is the sole source of oxygen
in the S-oxygenation reaction, and suggests that ferryl intermedi-
ates do not play a significant role. In addition, water (in small
amounts) does not appear to have any inhibitory effect on this
reaction.
Fig. 8. Reaction of 2 with excess O2 in CH2Cl2 at 60 °C as monitored by LDI-MS. The
peak at m/z 534.4 is assigned to [2-Cl]+, and the peak at m/z 566.4 is assigned to
doubly-oxygenated [Fe(LN3S(O2)Me)Cl]+. The small peak at m/z 550.4 is assigned to
singly-oxygenated [Fe(LN3S(O)Me)Cl]+.
times varied from 24 to 96 h, depending on concentration and
reaction scale.
2.5. Product characterization
2.4.2. S-Oxygenation reactions
2.5.1. Determination of the iron oxidation state
Although compound 1 is unreactive toward O2 at ambient tem-
perature and pressure, a new sulfoxide product is generated at
In our previous S-oxygenation studies, the final product(s) were
shown to be in the iron +2 oxidation state, making these complexes
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L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
7
Fig. 9. 1H NMR spectrum of 2-(methylsulfinyl)aniline recovered following demetalation of 3.
good functional models for CDO, and opening up the possibility for
catalytic activity. Product complexes 3 and 4 exhibit featureless
absorption spectra (Figs. S3 and S4), and also give paramagnetic
1H NMR spectra (Figs. S5 and S6) with no discernable resonances
that can be assigned to ligand-based peaks. 1H NMR spectra of 3
and 4 are significantly different from the sharp, paramagnetically
shifted spectra that is characteristic of high-spin iron(II), as seen
in the starting materials 1 and 2 (Figs. S1 and S2). These featureless
1H NMR spectra are consistent with high-spin (S = 5/2) iron(III)-
containing complexes. The X-band EPR spectrum of 3 (Fig. S7) re-
veals an intense signal near geff = 4.28 that is typical for a rhombic,
hs-FeIII ion. There is also a strong feature at geff = 2 that may arise
from hs-FeIII, ls-FeIII or intermolecular interactions [76–80]. The
EPR spectrum for 4 (Fig. S8) exhibits similar peaks near geff = 4.29
and 2.01, and while detailed simulations of these spectra have
not been attempted, they are clearly consistent with the S-oxygen-
ated products 3 and 4 being in the hs-FeIII oxidation state. In con-
trast, the EPR spectra of the starting complexes 1 and 2, which are
overlayed with the spectra from 3 and 4, show only very weak sig-
nals in the same regions, which can be attributed to a small
amount of oxidation by air during sample preparation.
A UV–Vis titration for iron(II) was employed to confirm the oxi-
dation state of the Fe product in these sulfide oxygenation reac-
tions. It is well known that 1,10-phenanthroline is a strong
chelator for iron(II) and iron(III), forming highly colored
[Fe(phen)3]2+ and [Fe(phen)3]3+ complexes, which exhibit distinct
spectra (Fig. S9). Titration of the iron from the crude reaction mix-
ture of 4 with 5.0 equiv. of 1,10-phenanthroline (Fig. S10) and com-
parison of the absorbance at k = 510 nm (the marker band for
[Fe(phen)3]2+) with a calibration curve (Fig. S11), revealed that
there is <6% FeII in this solution. These data confirm that 4 contains
a ferric (FeIII) center.
2.5.2. Identification of S-oxygenated products
Attempts to crystallize the sulfoxide and sulfone products in
Schemes 3 and 4 were unsuccessful. However, the S-oxygenated
products were easily identified when the ligand was separated
from the metal center and hydrolyzed with aqueous acid into
its component organic fragments (Scheme 5). The relevant or-
ganic compounds are distinguishable by the respective 1H NMR
chemical shifts of the RSCH3 moiety. The chemical shift for the
methyl sulfide of the starting material, 2-(methylthio)aniline, is
d(CDCl3) = 2.34 (s, 3H), while the corresponding sulfoxide (2-
(methylsulfinyl)aniline) and sulfone (2-(methylsulfonyl)aniline)
products have chemical shifts d(CDCl3) = 2.93 (s, 3H) and 3.03
(s, 3H) [81], respectively. Demetalation of the crude reaction
mixture of 3 was accomplished by stirring with 1 M HCl, neu-
tralization with NaHCO3, and separation of the organic layer.
Due to the polar nature of the sulfoxide moiety, 2-(methylsulfi-
nyl)aniline was easily separated from other organic material via
column chromatography on silica (EtOAc/hexanes) and its struc-
ture confirmed by 1H NMR (Fig. 9), d(CDCl3) = 2.93 (s, 3H). Final
recovery of 2-(methylsulfinyl)aniline was low (610%), contrast-
ing the apparent good conversion observed in the LDI-MS (vide
infra).
The demetalation of the reaction mixture of 4 was performed as
above, but in this case the S-oxygenated fragment, 2-(methylsulfo-
nyl)aniline, co-eluted with other organics and therefore its purifi-
cation by chromatography was not successful. However, in the
crude 1H NMR spectrum (Fig. 10) following demetalation, we can
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8
L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
Fig. 10. 1H NMR spectrum of the crude mixture following demetalation of 4. The 2-(methylsulfonyl)aniline is identified at 3.06 ppm, with minor peaks arising from the
intermediate sulfoxide (2.92 ppm) and starting sulfide (2.36 ppm).
O
S
F3CSO3
FeIII
2+
O
S
1) 1 M HCl (aq)
2) NaHCO3
O
O
N
N
N
NH2
CH2Cl2
NH2
ArS(O)Me
N
3
O
2+
O
Cl
O
O
1) 1 M HCl (aq)
S
O
O
S
2) NaHCO3
FeIII
N
N
N
N
NH2
CH2Cl2
NH2
ArS(O2)Me
4
Scheme 5. Hydrolysis of bis(imino)pyridine complexes.
clearly identify the characteristic resonance of the sulfone
(d(CDCl3) = 3.05), with only minor contributions from the starting
sulfide or sulfoxide methyl groups.
products (2-(methylsulfinyl)aniline or 2-(methylsulfonyl)aniline,
(d(CDCl3) = 2.92 or 3.05, respectively) gives only modest yields of
sulfoxide (15%) and sulfone (29%). However, low recovery of other
organic ligand fragments was also observed when compared with
the CH3NO2 standard, indicating that the polar, amino-functional-
ized fragments are likely lost in the aqueous workup. A comparison
of the methyl resonances from the sulfoxide and sulfone with the
recovered 2,6-diisopropylaniline resonance (d(CDCl3) = 1.27, d,
Given the above results, 1H NMR experiments were conducted
in an effort to quantitate the crude products immediately following
aqueous workup but prior to chromatographic separation
(Figs. S12 and S13). The addition of CH3NO2 as an external stan-
dard (d(CDCl3) = 4.32) and comparison with the S-oxygenated
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L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
9
12H) as an internal standard indicate a higher overall yield of the
sulfoxide (51%) and sulfone (45%) products.
onance (EPR) spectra were obtained on a Bruker Elexsys E580 EPR
spectrometer with a Bruker super high Q resonator (SHQE) at 15 K.
The EPR spectrometer was equipped with an Oxford Instruments
ESR900 liquid helium flow cryostat.
3. Conclusions
4.2. Synthesis
Herein we have reported the synthesis of two unsymmetrical
sulfide-incorporated, bis(imino)pyridine complexes which are
accessible via a metal-assisted template reaction. Complexes 1
and 2 were characterized by single crystal X-ray crystallography.
These complexes react with O2 at modestly elevated temperature
to yield S-oxygenated sulfoxide or sulfone complexes. The identi-
ties of the products were determined by mass spectrometry and
1H NMR studies. UV–Vis quantitation of the iron(II) ions with
1,10-phenanthroline in the reaction mixture of 4 indicated that 4
should contain mainly iron(III) ions. EPR spectroscopy of reaction
mixtures following oxygenation confirmed the presence of hs-FeIII
complexes in the case of both 3 and 4. Sulfide oxidation was con-
firmed by demetalation and hydrolysis of the bis(imino)pyridine li-
gands in 3 and 4. The 1H NMR of the ligand fragments allowed for
identification of the sulfoxide and sulfone products from com-
plexes 3 and 4, respectively. The S-oxygenation reaction for sulfide
ligands with dioxygen as the oxidant/O-atom source was demon-
strated for two non-heme iron(II) complexes. The nature of the
complexes (chloride versus triflate starting materials) determines
the extent of S-oxygenation, resulting in either sulfoxide or sulfone
products. Selective oxidation of organic substrates such as sulfides
is an important and desirable property of metal-mediated oxygen-
ations, and the reactivity reported here warrants further
investigation.
4.2.1. Synthesis of [FeII(LN3SMe)(H2O)3](OTf)2ꢀEt3NHOTf, 1ꢀEt3NHOTf
An amount of 2-(O@CMe)-6-(2,6-(iPr2-C6H3N@CMe)-C5H3N
(200 mg, 0.62 mmol) and Fe(OTf)2 (230 mg, 0.65 mmol) were sus-
pended in EtOH (10 mL) and heated at 60 °C for 1 h. The solids
slowly dissolved to give a deep purple solution, and a solution of
2-(methylthio)aniline (73
lL, 0.62 mmol) and triethylamine
(22 L, 0.62 mmol) in EtOH (1 mL) was added to the reaction mix-
l
ture. The reaction was stirred at 80 °C for 24 h to give a dark red
solution that was then cooled to room temperature. The crude
reaction mixture was evaporated to dryness and the resulting solid
residue was brought into a glovebox, where it was dissolved in a
minimum amount of CH2Cl2 and filtered through Celite. The filtrate
was layered with pentane to give 650 mg (95% yield) of 1 as a dark
red residue. Single crystals (red-purple blocks) suitable for X-ray
diffraction were grown from slow vapor diffusion of diisopropyl
ether into a solution of 1 in MeCN. LDI-MS (+): m/z 648.3 [1-
3H2O-OTf]+. ATR-IR,
m
(cmꢁ1): 3275, 2974, 1589, 1468, 1371,
1291, 1210, 1166, 1098, 1024, 803, 766, 636. Anal. Calc. for [FeII(L3-
NSMe)(H2O)3](OTf)2ꢀ0.5Et3NH+OTfꢁ
(C33.5H47F7.5FeN3.5O10.5S3.5):
Predicted: C, 41.17; H, 4.85; N, 5.02. Found: C, 41.29; H 5.02; N,
5.24%. Note: Although the X-ray structure of 1 contains one Et3NH+
OTfꢁ per Fe, samples for elemental analysis contain crystalline
material along with inseparable residue. Elemental data for the
bulk material fits best for 0.5 Et3NH+ OTfꢁ per Fe.
4. Experimental
4.2.2. Synthesis of [FeII(LN3SMe)Cl2]ꢀMeOH, 2ꢀMeOH
4.1. General remarks
An amount of 2-(O@CMe)-6-(2,6-(iPr2-C6H3N@CMe)-C5H3N
(200 mg, 0.62 mmol) and FeCl2 (82 mg, 0.65 mmol) were sus-
pended in EtOH (10 mL) and heated at 60 °C for 1 h. The solids
slowly dissolved to give a deep blue solution, and a solution of 2-
All reagents were purchased from commercial vendors and used
without further purification unless noted otherwise. All reactions
were carried out under an atmosphere of N2 inside a glovebox or
under Ar by standard Schlenk and vacuum line techniques unless
otherwise noted. Dioxygen gas (2.6 Grade) was purchased from
BOC Gases and dried by passage through a column of Drierite.
H218O (97%) was purchased from Cambridge Isotope Laboratories,
Inc. Iron(II) trifluoromethanesulfonate (98%) was purchased from
Strem. Iron(II) chloride (98%), 2-(methylthio)aniline (97%), and
2,6-diacetylpyridine (99%) were purchased from Sigma–Aldrich.
2,6-Diisopropylaniline (92%) was purchased from Acros. Dichloro-
methane was purified via a Pure-Solv Solvent Purification System
from Innovative Technology, Inc. Methanol, ethanol, and triethyl-
amine were distilled over CaH2. All solvents were degassed by re-
peated cycles of freeze-pump-thaw and stored in an N2-filled
glovebox. 2-(O@CMe)-6-(2,6-(iPr2-C6H3N@CMe)-C5H3N was syn-
thesized according to the published procedure [68]. LDI-TOF mass
spectra were obtained using a Bruker Autoflex III Maldi ToF/ToF
instrument (Billerica, MA). Samples were dissolved in CH2Cl2 and
deposited on the target plate. Samples were irradiated with a
355 nm UV laser and mass analyzed by ToF mass spectrometry in
the reflectron/linear mode. High resolution EI mass spectra were
obtained using a VG70S double-focusing magnetic sector mass
spectrometer (VG Analytical, Manchester, UK, now Micromass/
Waters) equipped with an MSS data acquisition system (MasCom,
Bremen, Germany). Attenuated total reflectance (ATR) infrared
spectra of neat crystalline material and crude reaction mixtures
were obtained with a Golden Gate Reflectance diamond cell in a
Nexus 670 Thermo-Nicolet FTIR spectrometer. Crude reaction mix-
tures were deposited on the reflectance window as a solution in
CH2Cl2 and allowed to dry to a residue. Electron paramagnetic res-
(methylthio)aniline (73 lL, 0.62 mmol) and triethylamine (22 lL,
0.62 mmol) in EtOH (1 mL) was added to the reaction mixture.
The reaction was stirred at 80 °C for 24 h and then cooled to room
temperature. The crude reaction mixture was evaporated to dry-
ness and the resulting solid residue was brought into a glovebox
where it was dissolved in a minimum amount of CH2Cl2 and fil-
tered through Celite. Layering the filtrate with pentane gave
250 mg (72% yield) of 2 as a dark blue residue. Single crystals of
2ꢀMeOH for X-ray diffraction were prepared by slow vapor diffu-
sion of diisopropyl ether into a MeOH solution of 2. LDI-MS (+):
m/z 534.3 [2-Cl]+. ATR-IR,
m
(cmꢁ1): 2964, 1585, 1463, 1370,
1318, 1269, 1204, 1105, 1059, 939, 803, 778, 738. Anal. Calc. for
[FeII(L3NSMe)Cl2]ꢀMeOH (C29H37Cl2FeN3OS): Predicted: C, 57.82;
H, 6.19; N, 6.97. Found: C, 57.74; H 6.30; N, 7.03.
4.3. O2 reactivity
4.3.1. Oxidation of 1 with O2
Compound 1ꢀ0.5 Et3NH+OTfꢁ (50.0 mg, 51
lmol) was dissolved
in 5 mL of CH2Cl2 and transferred to a pressure vessel equipped
with a vacuum port. Excess O2(g) was bubbled into the solution
for 5 min and the vessel was put under an atmosphere of O2 before
being sealed and heated to 60 °C. The reaction was monitored by
removal of aliquots of the reaction mixture for analysis by LDI-
MS. Complete disappearance of the starting material was observed
after 96 h. The crude reaction mixture was then stirred with 5 mL
of 1 M HCl for 1 h before being neutralized with NaHCO3. The or-
ganic layer was then collected, dried, and 2-(methylsulfinyl)aniline
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10
L.R. Widger et al. / Polyhedron xxx (2013) xxx–xxx
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was purified by column chromatography on silica with EtOAc/hex-
anes. 1H NMR (CDCl3) (@ 25 °C) d 7.23-7.22 (m, 2H), 6.75 (t, 1H),
6.69 (d, 1H), 2.93 (s, 3H). This spectrum matched that of an authen-
tic sample prepared as described in Section 4.3.4.
4.3.2. H218O study
Compound 1ꢀ0.5 Et3NH+OTfꢁ (34 mg, 35
lmol) was dissolved in
3.0 mL of CH2Cl2 and H218O (60
lL, 100 equiv.) was added. The
mixture was transferred to a reaction vessel and 16O2(g) was bub-
bled through the solution for 5 min. The vessel was sealed and
heated to 60 °C for 96 h. The LDI-MS of the crude reaction mixture
showed an isotopic cluster at m/z 684.1 for the unlabeled sulfoxide
3, and no significant peak corresponding to 18O incorporation at
m/z 686.1 was observed.
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Compound 2ꢀMeOH (14.0 mg, 23
lmol) was dissolved in 5.0 mL
of CH2Cl2 and transferred to a pressure vessel equipped with a
side-arm. Excess O2(g) was bubbled into the solution for 5 min
and the vessel was put under a slight positive pressure of O2 before
being sealed and heated to 60 °C. The reaction was monitored by
LDI-MS over the course of several days. After 96 h the reaction
was complete as evidenced by the absence of peaks for either the
sulfide starting material or sulfoxide intermediate observed in
the LDI-MS spectrum. The crude reaction mixture was then stirred
with 5 mL of 1 M HCl for 1 h before being neutralized with
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the
S-oxygenated
product,
2-(methylsulfonyl)aniline,
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d(CDCl3) = 3.06. This peak corresponds to the literature value [81].
4.3.4. Synthesis of 2-(methylsulfinyl)aniline
An amount of 2-(methylthio)aniline (44 mg, 0.316 mmol) was
dissolved in 7 mL of CH2Cl2 and mCPBA (55 mg, 0.316 mmol) was
added dropwise as a solution in 3 mL of CH2Cl2. After stirring for
several minutes, TLC analysis indicated the reaction was complete,
and the product was purified by column chromatography on silica
(EtOAc/hexanes) to give 10 mg (20% yield) of a pale solid. 1H NMR
(CDCl3) (@ 25 °C) d 7.27–7.22 (m, 2H), 6.75 (t, 1H), 6.69 (d, 1H),
2.93 (s, 3H). MS (EI+) m/z 155.1 [M+].
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