C=N Bond Activation and Hydration by an Iron(III) Complex with Asymmetric Sulfur Oxygenation

Article abstract of DOI:10.1002/ejic.201601565

The presence of asymmetric cysteine sulfur-oxygenation donors, consisting of sulfenate (SO), sulfinate (SO₂), and thiolate (S), around a trivalent iron is essential for the catalytic function of Fe- or Co-type nitrile hydratase (NHase). To gain

Full text of DOI:10.1002/ejic.201601565

A Journal of
Accepted Article
Title: C=N Bond Activation and Hydration by an Iron(III) Complex with
Asymmetric Sulfur-oxygenation
Authors: Yun-Ru Wu, Chia-Ming Chang, Chia-Chi Wang, Chang-Chih
Hsieh, and Yih-Chern Horng
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601565
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10.1002/ejic.201601565
European Journal of Inorganic Chemistry
COMMUNICATION
C=N Bond Activation and Hydration by an Iron(III) Complex with
Asymmetric Sulfur-oxygenation
Yun-Ru Wu, Chia-Ming Chang, Chia-Chi Wang, Chang-Chih Hsieh, and Yih-Chern Horng*
Dedicated to Professor Wen-Feng Liaw on the occasion of his 60th birthday
Abstract: The presence of asymmetric cysteine sulfur-oxygenation
donors, consisting of sulfenate (SO), sulfinate (SO2) and thiolate (S),
around a trivalent iron is essential for the catalytic function of Fe- or
Co-type nitrile hydratase (NHase). In order to gain insight into the
role of this asymmetric ligation, an octahedral bisthiolate low-spin
Fe(III) complex and two its S-oxygenation derivatives,
unprecedented mixed sulfinate/thiolate and O-bound bissufinate
complexes, were generated. The placement of anionic nitrogens
trans to thiolates in bisthiolate complex causes the axial imino C=N
bonds more polarized, resembling the activation of bound nitrile in
nitrile hydratase. The polarization is even more severe in
monosulfinate species, and the hydration of Fe-bound C=N bond by
OH- was observed. The generation of O-bound bissulfinate
octahedral complex implies the equatorial carboxamido and axial
thiolate ligation in Fe-NHase provides greater electron buffering
capacity in keeping constant electron density of Fe(III) ion even in
the presence of equatorial S-oxygenations.
molecules with sulfur oxidation exist,[9] and almost all S-
oxygenated low-spin Fe(III) complexes are derived from low-spin
Fe(III) precursor.[10] In the light of these structural mimics,
multiple anionic ligand system provides a greater chance for the
preparation of multiple S-oxygenated species. In addition, the
coordinatively saturated Fe(III) complex was suggested for
ensuring the isolation of S-bound sulfinato complex and avoiding
the unwanted O-bound sulfinato ligation.[9c]
Introduction
Scheme 1. The active site of NHase along with its catalytic reaction.
Nitrile hydratase (NHase) found in bacteria converts nitriles to
the corresponding less toxic amide products efficiently under
mild conditions.[1] The unique active site of NHase possesses a
monouclear low-spin non-heme iron(III) or non-corrin cobalt(III)
ion bound with two deprotonated carboxamido nitrogens from
the backbone and three cysteine sulfurs in the first coordination
sphere.[2] Two of the cysteine residues post-translationally S-
oxygenated to cysteinesulfenate (Cys-SO) and cysteinesulfinate
(Cys-SO2) in the deprotonated states,[3] are essential for its
catalytic activity.[4] The sixth ligand trans to the unmodified
cysteinate in the active form was proposed to be hydroxide (OH-)
or water molecule (Scheme 1).[5] Due to this unusual ligation
combination around metal center, along with the important
industrial application of nitrile hydrolysis,[6] the syntheses of
modeling compounds resembling both the structure and function
of NHase have been attempted.[7] So far, only Co(III) and bio-
inspired Ru(II) complexes have successfully implemented S-
oxygenated thiolate(s) as catalytically synthetic analogues of
NHase.[8] However, none of them clearly reveal the binding and
activation of nitrile on metal ion. No functional Fe(III) synthetic
mimics with or without S-oxygenation relevent to Fe-type NHase
are known. Only few examples of synthetic well-defined Fe(III)
Continued with our interests in nitrile activation[11] and
synthetic modeling of Fe-NHase,[12] we now utilized an aromatic
conjugated
NNS
tridentate
ligand
2-[(2-
mercaptophenyl)iminomethyl]pyrrole (HPyImSH) with thiolato,
imino, and pyrrolyl moieties for preparing the Fe(III) modeling
complex as well as its S-modification derivatives shown in
Figure 1. The pyrrole motif with an acidic N-H bond similar to
carboxamido group was installed. The placement of each
anionic nitrogen atom trans to thiolate in PyImS2- similar to the
active site of Fe-NHase have not been found in small molecule
active site mimics with NNS tridentate ligands as far. In addition,
the imino moiety is situated cis to coordinated nitrogen and
sulfur to examine the influence of the S-oxygenation on the
properties of C=N bond, resembling substrate nitrile bonding on
the active site.
Results and Discussion
The synthesizing strategy for [PPN][Fe(PyImS)2]
1
was
consulted with the early reports.[13] Treatment of two equiv of
(HPyImS)2[14] with [PPN][HFe(CO)4] in THF at room temperature
led to the generation of 1 in 61% yield via oxidative-addition
reaction with liberation of HPyImSH, HPyImS-, and all carbon
monoxide moieties. The wine-red complex 1 is stable in solid
state, but gradually transformed to oxygenated species (about
20%) in DMF within two weeks when exposing to air. The slow
conversion in air prompted us to employ other oxygenating
agents for the preparation of sulfur oxidation species. The S-
oxygenation derivatives [PPN][Fe(PyImS)(PyImSO2)] 2 (60%
Ms. Yun-Ru Wu, Mr. Chia-Ming Chang, Ms. Chia-Chi Wang, Dr.
Chang-Chih Hsieh, Prof. Yih-Chern Horng
Department of Chemistry, National Changhua University of
Education, Changhua 50058, (Taiwan)
E-mail: ychorng@cc.ncue.edu.tw
http://chem.ncue.edu.tw/en/members/Yih-Chern-Horng-51832368
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejic.201601565
European Journal of Inorganic Chemistry
COMMUNICATION
yield) and [PPN][Fe(PyImSO2)2] 3 (55% yield) were prepared by
using two and four equiv of an O-atom transfer reagent, tert-
butyl N-sulfonyloxaziridine (SOA), capable of oxygenating free
thiols[15] and Fe-bound thiolate.[9e] The reactions in DMF were
monitored and terminated (about 2 hrs) judged by silica gel thin
layer chromatography (TLC) (MeOH/Ethyl acetate 1:30): 1, Rf =
0.78; 2, Rf = 0.58; 3, Rf = 0.88. The purifications were
straightforward without the need of any column chromatography.
The isolation of yellowish green 3 also can be achieved through
the reaction of dark green 2 with 2 equiv SOA in DMF. Although
mono- and bissufinate derivatives of 1 were obtained without
knowledge 2 represents the first example of a structurally
characterized
mixed
S-bound
thiolate/sulfinate
Fe(III)
complex.[19] Notably, the two Fe-Nimino lengths in 1 are not
altered significantly (difference within 0.02 Å), but the unmodified
Fe-S bond distance in
1 becomes shorter upon doubly
oxygenation. The Fe-Npyrr bond length also shrinks due to S-
modification, reflecting the increased Lewis acidity of the
trivalent iron. Interestingly, the C=Nimino bonds in 1 is slightly
polarized as evidenced by the longer distances than typical C=N
double bonds (1.29 Å).[20] Although the elongation of C=Nimino
bond due to the constraints of bound ligand cannot be ruled out,
this polarization is even more severe in 2, especially in the
unmodified ligand frame, indicating the oxygenation of equatorial
thiolate also play a role on the activation of axial C-N bonds.
ambiguity,
efforts
to
obtain
pure
monosulfenate
[PPN][Fe(PyImS)(PyImSO)] and mixed sulfenate/sulfinate
[PPN][Fe(PyImSO)(PyImSO2)] for further characterization have
been unsuccessful.
Figure 1. S-oxygenation pathway of 1. The cations in complexes were omitted
for clarity.
Figure 2. ORTEP drawings of complexes 1, 2 and 3 and X-band EPR spectra
(77K, THF) of 1 and 2. The thermal ellipsoids were drawn at 35% probability
level. The [PPN] cations, solvent molecules and hydrogen atoms were omitted
for clarity. The g-values: complex 1, 2.14, 1.97; complex 2, 2.19, 2.13, 1.97.
Experiment parameters: microwave frequency = 9.46 GHz, microwave power
= 0.6 mW, modulation frequency = 30 kHz, modulation amplitude = 1.6 G.
The structures and key bond lengths of complexes 1, 2
and 3 were successfully determined by X-ray crystallography[16]
and are shown in Figure 2 and Table 1. The crystal structure of
complex 1 revealed that Fe(III) center, sitting in a N4S2
coordination environment, is ligated by two PyImS2- ligands. The
frameworks of two PyImS2- ligands, binding to Fe center in a mer
mode, are nearly planar due to extensive aromatic conjugation.
The pyrrolic NH is deprotonated and covalently bound to the
Fe(III) center at trans position of thiolate. Two thiolates, cis to
each other, and two pyrrolic nitrogen atoms occupy the
equatorial positions with two imino nitrogens in axial positions,
resembling the active-site structure of Fe-NHase. The averaged
Fe-S bond distance (2.26 Å) in 1 is slightly longer than those
observed in low-spin thiolato Fe(III) complexes in the range of
2.14~2.25 Å.[9b, 9c, 9e, 17]
Table 1. Selected bond lengths [Å ] for complexes 1, 2 and 3.
Complex[a]
Fe-S
Fe-Npyrr
Fe-Nimino
C=Nimino
1
2.2490(8),
2.2745(9)
1.949(2),
1.968(3)
1.936(3),
1.941(3)
1.315(4),
1.297(4)
2
3
2.2224(11),
2.2432(11)
1.928(3),
1.971(3)
1.932(3),
1.948(3)
1.321(4),
1.315(5)
The structure of doubly oxygenated product 2 displays that
the added two oxygen atoms are attached to the same sulfur
atom resulting a sulfinato species, instead of to two thiolates
forming a bissulfenato complex, similar to the product of Ru(II)
dithiolate complex added with two oxygen atoms.[18] Although
rare examples of Fe(III)-sulfinate complexes exist,[9b, 9d] to our
2.063(4),
2.062(4)
2.179(4),
2.185(4)
1.310(6),
1.321(7)
[a] The numbers in italic style indicate the bond distances derived from S-
oxygenated ligand.
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10.1002/ejic.201601565
European Journal of Inorganic Chemistry
COMMUNICATION
The crystal structure of 3 illustrated that the successful
process of four-oxygen-atom addition of 1 indeed obtained the
quadruple S-oxygenated species. To our surprise, this structure
displays an O-bound bissulfinate species. Notably, the pyrrolyl
and phenyl rings of chelating ligand in 1 and 2 are essentially
coplanar, however, they are non-coplanar in 3 with averaged
dihedral angle about 28. The averaged Fe-OSO bond distance
of 1.97 Å compares well with the ones of only three reported
9c, 9f]
Fe(III)-OSO complexes in the range of 1.91~2.03 Å.[9a,
Although this structure is irrelevant to Fe-NHase, we have not
found so far an O-bound sulfinate octahedral Fe(III) complex in
the literature. Thus, our results demonstrated the coordinatively
saturated Fe(III) complex cannot prevent the production of an O-
bound sulfinate species upon sulfur oxidation. The electron
deficiency of Fe(III) center and decreased electron donating
ability of sulfur atom of Fe-SO2 resulting in the switch to O-
bound bissulfinate and formation of a thermodynamically more
stable species even with severe torsion of imino C=N bond. The
isolation of S-bound bissufinate complex [Fe(PyPepSO2)2]-,[9b]
with amido-type NNS ligand reported by Mascharak, implies the
anionic carboxamido nitrogens provide more sufficient electron
buffering capacity than anionic pyrrolic nitrogens.
The X-band EPR signals of complexes 1 and 2 at RT and
77K revealed low-spin Fe(III) centers (Figure 2). Due to the
structural and electronic effects, the axial EPR signals of 1 were
altered to rhombic signals when the complex was oxygenated.
The g values of 2 are very similar to those from Fe-containing
NHase at pH = 9.0 (g= 2.20 2.12 and 1.99).[21] However,
complex 3 with Fe-O fragments exhibits an EPR signal with
features at g= 8.3 and 4.0 (77 K, THF), typical values for
octahedral high-spin Fe(III) systems with nitrogen and oxygen
ligation (Figure S1).[22] In CH3CN, 1 exhibits an absorption band
at max= 900 nm, generally assigned to a thiolate-to-metal
charge-transfer transition (LMCT). The doubly-oxygenation of 1
led to slightly blue shift of band at 900 nm, and appearing of a
new band at 616 nm (Figure S2). The generation of new band at
550~750 nm was also observed in reported octahedral Fe(III)
complexes upon S-oxygenation.[9b, 9d, 9e] Thus, the blue-shifted
band at 855 nm can be tentatively assigned as the LMCT band
of unmodified sulfur to metal transition.
Figure 3. The reaction mechanism proposed for the hydration of metal-bound
imino C=N moiety in 2 by hydroxide, and ESI-MS spectra before (a) and after
(b) the reaction of 2 with hydroxide in CH3CN.
Conclusions
In conclusion, a N4S2 six-coordinated low-spin Fe(III)
mimic of active form of Fe-NHase along with its S-oxygenation
derivatives, the monosulfinate and bissulfinate complexes, were
prepared and characterized. The crystal structures show that
unmodified thiolate and pyrrolic nitrogen donors can assist
maintaining the electron density of Fe(III) ion with single thiolate
oxidation. In addition, the presence of coordinatively saturated
Fe(III) center cannot avoid the isomerization of Fe-SO2 into Fe-
OSO fragment of bissulfinate complex for the preferential
interaction between Fe(III) center and anionic oxygen. Thus, the
presence of electron buffering system from equatorial
deprotonated amido and axial thiolate donors in Fe-NHase
maintains the Lewis acidity of Fe(III) ion and prevent the O-
bound bissufinate species upon multiple S-oxygenations. While
the effects of S-oxygenation on active nitrile hydration synthetic
Co(III) and Ru(II) catalysts have been reported,[8] we
demonstrated here for the first time that the activation and
further hydration by OH- of metal-bound C=N bond is influenced
by asymmetric S-oxidation (-SO2) by taking a closer look. The
isolation and purification of monosulfenate and mixed
sulfenate/sulfinate species, and also the investigations into the
nitrile hydration by these Fe(III) complexes are currently
underway.
The unusual elongation of imino C=N bonds in 1 and 2
prompted us to test the possibility of ligand hydration by
hydroxide. Although the extensively conjugated disulphide
(HPyImS)2, and 1 was not reacted with 4 equiv [Et4N][OH] in un-
dried CH3CN within 12 hrs as revealed by 1H NMR and
electrospray ionization mass spectrometry (ESI-MS) (Figure S3),
the reaction between 2 and 4 equiv hydroxide in 2 hrs led to the
disappearance of 2 (m/z 488.1) and formation of pyrrole-2-
carboxaldehyde (1H NMR, Figure S4), 2-aminobenzenesulfinate
-
(m/z 156.0), HPyImSO2 (m/z 233.1), 1 (m/z 456.1) and a few
uncharacterized dark brown precipitates (Figure 3). Despite the
fact that 2-aminobenzenethiolate cannot be detected by ESI-MS
-
due to the instrument limitation, the detection of HPyImSO2
indicates the hydration reaction occurred also at the metal- Acknowledgements
bound unmodified ligand frame. Thus, the equatorial sulfinate
group in 2 can aid the activation of axial C=N bond fragile to be
attacked by OH- through increasing Lewis acidity of Fe(III) center,
and exhibited no nucleophilic properties.
We gratefully acknowledge financial support from the Ministry of
Science and Technology (Taiwan) and the financial support of
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Metalloenzymes• Oxidation • S ligands
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European Journal of Inorganic Chemistry
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Sulfur Oxygenation*
Yun-Ru Wu, Chia-Ming Chang, Chia-Chi
Wang, Chang-Chih Hsieh, and Yih-
Chern Horng*
Page No. – Page No.
S-oxygenation of a low-spin bisthiolate Fe(III) complex with more Lewis acidic metal
center leads to the generation of mixed sulfinate/thiolate and O-bound bissulfinate
species. The polarization of Fe-bound C=N bonds is more severe in monosulfinate
species, and the hydration of C=N bond by OH- was observed.
C=N Bond Activation and Hydration
by an Iron(III) Complex with
Asymmetric Sulfur-oxygenation
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