Anionic mixed thiolate sulfide-bridged roussin's red esters [(NO) 2Fe(μ-SR)(μ-S)Fe(NO)2]- (R = Et, Me, Ph): A key intermediate for transformation of dinitrosyl iron complexes (DNICs) to [2Fe-2S] clusters

Article abstract of DOI:10.1021/ic9012679

Transformations of dinitrosyl iron complex (DNIC) [(NO)₂Fe(SEt) ₂]^- and the anionic Roussin's red ester (RRE) [(NO) ₂Fe(μ-SEt)₂Fe(NO)₂]^- into [2Fe-2S] clusters facilitated by HSC

Full text of DOI:10.1021/ic9012679

Inorg. Chem. 2009, 48, 9027–9035 9027
DOI: 10.1021/ic9012679
Anionic Mixed Thiolate-Sulfide-Bridged Roussin’s Red Esters
[(NO)2Fe(μ-SR)(μ-S)Fe(NO)2]- (R = Et, Me, Ph): A Key Intermediate for
Transformation of Dinitrosyl Iron Complexes (DNICs) to [2Fe-2S] Clusters
Tsai-Te Lu, Hsiao-Wen Huang, and Wen-Feng Liaw*
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
Received July 2, 2009
Transformations of dinitrosyl iron complex (DNIC) [(NO)2Fe(SEt)2]- and the anionic Roussin’s red ester (RRE)
[(NO)2Fe(μ-SEt)2Fe(NO)2]- into [2Fe-2S] clusters facilitated by HSCPh3/Me2S3 and [Fe(SEt)4]-, respectively, via
an intermediate anionic mixed thiolate-sulfide-bridged RRE [(NO)2Fe(μ-SEt)(μ-S)Fe(NO)2]- through the reas-
sembling process ([(NO)2Fe(SEt)2]- (2)/[(NO)2Fe(μ-SEt)2Fe(NO)2]- (4) f [(NO)2Fe(μ-SEt)(μ-S)Fe(NO)2]- (3-
Et) f [(NO)2Fe(μ-S)2Fe(NO)2]2- (5) f [(SEt)2Fe(μ-S)2Fe(SEt)2]2- (1)) were demonstrated. The anionic mixed
1
thiolate-sulfide-bridged RRE 3-Et was characterized by IR, UV-vis, electron paramagnetic resonance, H NMR,
cyclic voltammetry, and single-crystal X-ray diffraction. In contrast to the nucleophilicity displayed by complex 2, the
inertness of [(NO)2Fe(SPh)2]- toward HSCPh3 implicates how the reducing ability of the coordinated thiolates of
DNICs modulate the release of sulfide from HSCPh3 via reduction and the conversion of DNICs into the anionic mixed
sulfide-thiolate-bridged complex. The reversible interconversion between complex 3-Et and complex [(NO)2Fe(μ-
SPh)(μ-S)Fe(NO)2]- (3-Ph) via protonation and a bridged-thiolate exchange reaction, respectively, demonstrates
that the [{Fe(NO)2}9-{Fe(NO)2}9] motif displays a preference for the stronger electron-donating alkylthiolate-
bridged ligand over the phenylthiolate-bridged ligand. This study may signify that the anionic mixed thiolate-sulfide-
bridged RREs act as a key intermediate in the transformation of DNICs into [2Fe-2S] clusters. Also, the thiolate-
coordinate DNICs serve as not only the thiolate/electron carrier activating the incorporation of sulfide of HSCPh3
(Me2S3) to assemble the [Fe(μ-S)2Fe] core but also the Fe source in the biosynthesis of the [2Fe-2S] and [4Fe-4S]
iron-sulfur clusters.
Introduction
leading to dinitrosyl iron complexes (DNICs) with an elec-
tron paramagnetic resonance (EPR) signal at g = 2.03, for
example, modulating cellular iron homeostasis, altering the
metabolism of bacteria, regulating the expression of genes to
detoxify reactive nitrogen species (RNS) or repair/regenerate
essential [Fe-S] proteins,1-3 and lowering the activity of
human and mouse ferrochelatase to prevent the invading
pathogens from absorbing the heme/Fe source.4 In Escher-
ichia coli, degradation of the [2Fe-2S] cluster of SoxR by
nitric oxide yielding DNICs stimulatesthe expression of SoxS
to trigger the expression of proteins involved in detoxifying
RNS.3a Of importance, DNICs derived from the nitrosyla-
tion of Fe-storage protein (i.e., mammalian ferritin, cellular
labile/chelatable iron pool, or [Fe-S] cluster-containing
proteins) were actively delivered between cells via multidrug
resistance-associated protein 1 in the form of glutathione-
coordinated DNICs (DNIC-GSH).1c,d,5 Also, DNICs were
found to exert NO-related functions, such as S-nitrosation of
Nitric oxide modulates versatile physiological reactions by
nitrosylation of various nonheme Fe-containing proteins,
*To whom correspondence should be addressed. E-mail: wfliaw@mx.
nthu.edu.tw.
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2009 American Chemical Society
Published on Web 08/25/2009
pubs.acs.org/IC

9028 Inorganic Chemistry, Vol. 48, No. 18, 2009
Lu et al.
Scheme 1
bovine serum albumin or activation of soluble guanylate
cyclase.6 Interestingly, DNICs not only play a key role in the
storage and transport of NO but also regulate the Fe uptake
and cellular iron homeostasis by the efflux of Fe in the form
of DNIC-GSH.1
frataxin, which donates Fe to IscU facilitated by cysteine and
IscS.10 The donation of Fe to IscU facilitated by cysteine and
IscSimplicated that an [Fe-S-SR] complex was produced in
the interaction of IscA, IscS, and cysteine and delivered to
IscU for further assembling. A dual role of IscA was found in
which it acts as a scaffold protein by anchoring one [2Fe-2S]
cluster in the interface of two IscA monomers.10d
A requirement of the complex assembly machinery for the
maturation of [Fe-S] proteins was identified, but the assem-
bling mechanism remained for further investigation.7 It is
proposed that the biosynthesis of [Fe-S] clusters includes
three steps: (i) mobilizing iron and sulfur atoms from their
storage sources in nontoxic forms, (ii) assembling them into
an [Fe-S] core, and (iii) inserting the assembled [Fe-S] core
into apoprotein.7-10 Inorganic sulfur atoms (S0) are gener-
ated from cysteine catalyzed by cysteine desulfurase (carried
by cysteine desulfurase in the form of persulfide (RS-SH))
and inserted to form an [Fe-S] core via reductive cleavage of
the S-S bond by NADH mediated by Yah1 and Arh1.7,8 It is
known that persulfide and trisulfide (RS-S-SR0) were
observed on IscU upon incubation with IscS and cysteine,
that is, sulfur atom(s) anchored on IscU in the form of
persulfide or trisulfide is proposed to incorporate with Fe
to assemble [Fe-S] clusters.9 The Fe source for the biosynth-
esis of [Fe-S] clusters was proposed to derive from IscA and
In biological systems, NO-modified [2Fe-2S]/[4Fe-4S]
clusters in mitochondrial aconitase, HiPIP, and SoxR, that
is, DNICs and anionic Roussin’s red esters (RREs), were
automatically transformed into the original [2Fe-2S]/[4Fe-
4S] clusters in aerobically growing E. coli with no new protein
synthesis.11,12 Also, DNICs can be directly transformed back to
the ferredoxin [2Fe-2S] cluster by cysteine desulfurase (IscS)
and L-cysteine in vitro with no need of the addition of iron or
any other protein components in the repair of NO-modified
ferredoxin [2Fe-2S] clusters.13
It is known that iron sulfur clusters were synthesized via the
self-assembly pathway.14 Recently, the transformation of
[(NO)2FeS5]- into [S5Fe(μ-S)2FeS5]2- in the presence of the
NO-acceptor reagent [(C4H8O)Fe(S,S-C6H4)2]- by photoly-
sis was reported.15a Conversion of DNICs into [4Fe-4S]
clusters in the presence of [Fe(SR)4]- and S8 was also demon-
strated in a biomimetic model study.15b Also, degradation of
[2Fe-2S] clusters ligated by aromatic thiolates via nitrosyla-
tion was found to generate DNICs with elimination of the
elemental sulfur atoms.16 In this report, we attempt to adopt
::
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Article
Inorganic Chemistry, Vol. 48, No. 18, 2009 9029
Figure 1. Infrared νNO absorbance changes for the reaction of 2-PPN
with HSCPh3 in a 2:1 stoichiometry in CH3CN at ambient temperature.
The solid line curve (;) corresponds to 2-PPN, and the dashed line curve
(---) corresponds to the formation of complex 3-Et.
Figure 2. Conversion of 2-PPN to complex 3-Et monitored by UV-vis
spectrometry under the addition of half an equivalent of HSCPh3 to 2-
PPN in CH3CN at ambient temperature. Two absorption bands at 438
and 799 nm (;) disappeared followed by the simultaneous formation of
one absorption band at 376 nm (---).
the biomimetic model study to unravel the conversion of
the NO-modified [Fe-S] cluster into the [2Fe-2S] cluster.
Specifically, the addition of Me2S3 (or HSCPh3) to [(NO)2-
Fe(SEt)2]- results in the formation of anionic mixed thiola-
te-sulfide-bridged Roussin’s red ester [(NO)2Fe(μ-SEt)-
(μ-S)Fe(NO)2]-, which was characterized by IR, UV-vis
spectroscopy, 1H NMR, and single-crystal X-ray crystallography.
The anionic mixed thiolate-sulfide-bridged Roussin’s red
ester acts as a key intermediate in the transformation of
DNICs/anionic RREs into [2Fe-2S] clusters facilitated by
HSCPh3 (Me2S3) via the reassembling process [(NO)2-
Fe(SEt)2]-/[(NO)2Fe(μ-SEt)2Fe(NO)2]- f [(NO)2Fe(μ-SEt)-
(μ-S)Fe(NO)2]- f [(NO)2Fe(μ-S)2Fe(NO)2]2- f [(SEt)2Fe-
Scheme 2
(μ-S)2Fe(SEt)2]2-
.
Figure 2). The 1H NMR signal at 2.71 (q) and 1.40 (t) ppm
(CD3CN) of the ethyl group of [K-18-crown-6-ether]-
[(NO)2Fe(μ-SEt)(μ-S)Fe(NO)2] (3-Et-K) may rationalize
the absence of an EPR signal, presumably derived from
the antiferromagnetic coupling between the two
{Fe(NO)2}9 iron centers. In the previous study, 2-PPN
was demonstrated to act as a thiolate carrier and the
inherent reducing species.18 This study further supports
that the coordinated ethylthiolates of 2-PPN initiated the
release of sulfide from HSCPh3 via reduction to trigger
the transformation of 2-PPN into complex 3-Et. As
shown in Scheme 2, reaction of 2-PPN and HSCPh3
yielding complex 3-Et may be rationalized by the follow-
ing reaction sequences: consecutive reduction of HSCPh3
by the coordinated ethylthiolate of 2-PPN produces the
[PPN]2[(NO)2(SEt)Fe(μ-S)Fe(SEt)(NO)2] (A) intermedi-
Results
Nitrosylation of [(SEt)2Fe(μ-S)2Fe(SEt)2]2- (1) and
Transformation of [PPN][(NO)2Fe(SEt)2] (2-PPN)/[K-
18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (4) into [PPN]-
[(NO)2Fe(μ-SEt)(μ-S)Fe(NO)2] (3-Et). Similar to the nitro-
sylation of [(SPh)2Fe(μ-S)2Fe(SPh)2]2- leading to the
formation of complex [(NO)2Fe(SPh)2]-,16,17 reaction
of the PPNþ salt of complex [(SEt)2Fe(μ-S)2Fe(SEt)2]2-
(1-PPN) and 4 equiv of NO(g) at 0 ꢀC under the presence
of 2 equiv of PPh3 resulted in the formation of the
PPNþ salt of complex [(NO)2Fe(SEt)2]- (2-PPN) and
SdPPh3. Products 2-PPN and SdPPh3 were character-
ized by the IR νNO spectrum, displaying two strong
stretching bands (1722 and 1680 cm-1), and the 31P
NMR spectrum (31P NMR (CH3CN): δ 42.70 ppm),
respectively (Scheme 1a).
ate along with diethyl disulfide and HC(Ph)3 (Ea
=
-2.038 V (HSCPh3), Ea = -2.767 V ([EtS]-),18 Ec =
-0.194 V ({Fe(NO)2}9/{Fe(NO)2}8 (2-PPN)18 vs FeCp2/
FeCp2þ; Scheme 2). The subsequent elimination of the
ethylthiolate of intermediate A affords complex 3-Et. The
biomimetic trisulfide (sulfane sulfur carrier) employed as
a “S-donor species” to trigger the conversion of complex
[Fe(SPh)4]2- into complex [(SPh)2Fe(μ-S)2Fe(SPh)2]2-
was reported.19 In this study, the addition of dimethyl
trisulfide into 2-PPN leading to the formation of [PPN]-
[(NO)2Fe(μ-SMe)(μ-S)Fe(NO)2] (3-Me) characterized by
Treatment of 2-PPN with half an equivalent of HS-
CPh3 yielded [PPN][(NO)2Fe(μ-SEt)(μ-S)Fe(NO)2] (3-
Et) accompanied by byproducts [PPN][SEt], (EtS)2, and
1
HC(Ph)3 characterized by H NMR (Scheme 1b). The
shift of the νNO stretching frequencies from 1722 and 1680
cm-1 to 1718 and 1693 cm-1 (CH3CN) also corroborated
the formation of complex 3-Et (Figure 1). In addition, the
conversion of 2-PPN to 3-Et was also monitored by
UV-vis spectrometry; the intense bands at 438 and 799
nm disappeared, accompanied by the simultaneous for-
mation of one absorption band at 376 nm (complex 3-Et;
(18) Lu, T.-T.; Tsou, C.-C.; Huang, H.-W.; Hsu, I.-J.; Chen, J.-M.; Kuo,
T.-S.; Wang, Y.; Liaw, W.-F. Inorg. Chem. 2008, 47, 6040–6050.
(19) Coucouvanis, D.; Swenson, D.; Stremple, P.; Baenziger, N. C. J. Am.
Chem. Soc. 1979, 101, 3392–3394.
(17) Tsai, F.-T.; Chiou, S.-J.; Tsai, M.-C.; Tsai, M.-L.; Huang, H.-W.;
Chiang, M.-H.; Liaw, W.-F. Inorg. Chem. 2005, 44, 5872–5881.

9030 Inorganic Chemistry, Vol. 48, No. 18, 2009
Lu et al.
Scheme 3
Scheme 4
IR, UV-vis, 1H NMR, and single-crystal X-ray structure
was demonstrated.
Transformation of 2-PPN into the neutral RRE
salt (RRS) [K-18-crown-6-ether]2[(NO)2Fe(μ-S)2Fe(NO)2]
(5) (νNO: 1666 s, 1642 s cm-1 in CH3CN).20 The reduction
potential of -1.61 V (vs FeCp2/FeCp2þ) for 3-Et-K lies
between that of complex 5(-2.264 V vs FeCp2/FeCp2þ) and
that of complex [(NO)2Fe(μ-SEt)2Fe(NO)2] (-0.95 V vs
FeCp2/FeCp2þ).18 These results rationalize the trend of the
negative reduction potential and the shift of νNO stretching
frequencies to lower energy from complex [(NO)2Fe(μ-
SEt)2Fe(NO)2], complex 3, to complex 5. Compared to
complex 5 and [(NO)2Fe(μ-SEt)2Fe(NO)2], displaying two
absorption bands at 264 and 378 nm and at 310 and 360 nm,
respectively, in UV-vis spectrum, the anionic mixed thiola-
te-sulfide-bridged Roussin’s red ester 3-Et-K shows absorp-
tion bands at 259, 326, and 376 nm.
Transformation of 3-Et-K into 1-K. In contrast to the
inertness of HSCPh3 toward 3-Et-K, the addition of 1
equiv of ethylthiolate into a CH3CN solution of 3-Et-K
and HSCPh3 under a N2 atmosphere yielded the known
complex 5, characterized by IR (νNO stretching frequen-
cies shifting from 1718 and 1693 cm-1 to 1666 and 1642
cm-1) and UV-vis (Scheme 1e).20 In comparison, the
preliminary study showed that the spontaneous conver-
sion of the anionic mixed thiolate-sulfide-bridged RRE
3-Et into Roussin’s black salt (RBS) occurred upon
exposure of complex 3-Et to O2 or acid.
[(NO)2Fe(μ-SEt)2Fe(NO)2] driven by [(MeOH)n SEt]-
3 3 3
hydrogen-bonding interaction and the subsequent reduction
of RRE [(NO)2Fe(μ-SEt)2Fe(NO)2] by DNIC yielding the
anionic [PPN][(NO)2Fe(μ-SEt)2Fe(NO)2] were demon-
strated.18 Conversion of [K-18-crown-6-ether][(NO)2Fe(μ-
SEt)2Fe(NO)2] (4) into [K-18-crown-6-ether][(NO)2Fe(μ-
SEt)(μ-S)Fe(NO)2] (3-Et-K) under the presence of HSCPh3
was displayed and was monitored by IR νNO spectra. The
shift of NO stretching frequencies from 1673 s and 1655
s cm-1 to 1710 s and 1687 s cm-1 was consistent with the
formation of 3-Et-K accompanied by byproducts HC(Ph)3
and EtSSEt identified by 1H NMR when the THF solution
of complex 4 was reacted with HSCPh3 in a 1:1 molar ratio
(Scheme 1d). The elongation of the Fe-S bond of complex
4, compared to that of complex [(NO)2Fe(μ-SEt)2Fe(NO)2],
was observed upon the reduction of complex [(NO)2Fe(μ-
SEt)2Fe(NO)2].18 The electronic richness of the Fe center in
complex 4may enhance the dπ-pπrepulsion to promote the
thiolate reducing ability, since the theoretical computations
reported in the previous study have shown that the singly-
occupied molecular orbital of complex 4is composed of both
Fe atoms (∼50% dx2-y2) and its associated S ligands (25% Px
orbitals).18 Then, the transformation of complex 4 into 3-Et-
K under the presence of HSCPh3 may be accounted for by
the following reaction sequences. As shown in Scheme 3,
reduction of HSCPh3 by the bridged ethylthiolate of the
anionic RRE 4 led to the proposed intermediate B accom-
When a CH3CN solution of complex 5 was treated with
[Fe(SEt)4]- in a 1:2 molar ratio for 10 min at ambient
temperature, the appearance of two absorption bands at
456 and 427 nm and the shift of the νNO stretching
frequencies from 1666 s and 1642 s cm-1 to 1722 s and
1680 s cm-1 confirmed the formation of the known [K-18-
crown-6-ether]2[(SEt)2Fe(μ-S)2Fe(SEt)2] (1-K) and [K-18-
crown-6-ether][(NO)2Fe(SEt)2] (2-K). Consistent with the
conversion of [Fe4S4(NO)4]2- to [Fe4S4(SPh)4]2- facili-
tated by [Fe(SPh)4]-,15b the absence of formation of mono-
nitrosyl iron complex [(NO)Fe(SEt)3]- suggested that a
concerted dinitrosyl-dithiyl radical ([RS]•) exchange reac-
tion occurred around the Fe center in the transformation of
complex 5 into 1-K and 2-K triggered by [Fe(SEt)4]-.
Consequently, the conversion of complex 5 back to 1-K
triggered by 2 equiv of complex [Fe(SEt)4]- provided a
facile pathway for transformation of the NO-modified
[Fe-S] clusters into the original [Fe-S] clusters via a NO
radical-thiyl radical ([RS]•) exchange reaction.15b
panied by the liberation of byproducts HCPh3 and diethyl
0/1-
disulfide (E1/2
= -0.95 V (complex 4) vs FeCp2/Fe-
Cp2þ).18 The subsequent electrophilic attack of the terminal
iron-bound [•S]- ligand on the neighboring iron yields 3-Et-
K. In a similar fashion, transformation of complex 4 into 3-
Et-K was also observed under the presence of 1 equiv of
dimethyl trisulfide with the isolation of byproducts dimethyl
disulfide and diethyl disulfide, characterized by 1H NMR.
Compared to that of the neutral RRE [(NO)2Fe(μ-
SEt)2Fe(NO)2] (νNO: 1809 vw, 1774 s, 1749 s cm-1), the
IR spectrum (νNO: 1718 s, 1693 s cm-1) of 3-Et-K displays a
different pattern and position. Interestingly, the IR spectrum
of 3-Et-K exhibits a similar pattern to that of Roussin’s red
Interconversion between 3-Et-K and [(NO)2Fe(μ-SPh)-
(μ-S)Fe(NO)2]- (3-Ph). Transformation of 3-Et-K to [K-
18-crown-6-ether][(NO)2Fe(μ-SPh)(μ-S)Fe(NO)2] (3-Ph)
(20) (a) Thomas, J. T.; Robertson, J. H.; Cox, E. G. Acta Crystallogr.
1958, 11, 599–604. (b) Sanina, N. A.; Filipenko, O. S.; Aldoshin, S. M.;
Ovanesyana, N. S. Russ. Chem. Bull. 2000, 49, 1109–1112.

Article
Inorganic Chemistry, Vol. 48, No. 18, 2009 9031
core is ascribed to the capability of sulfide to bridge the two
{Fe(NO)2}9 fragments and the preference of {Fe(NO)2}9
motifs to be connected by the stronger electron-donating
thiolate. Obviously, the anionic mixed thiolate-sulfide-
bridged RREs 3-Et-K containing the bridged-sulfi-
de-ethylthiolate ligands and complex 3-Ph containing
the bridged-sulfide-phenylthiolate ligands are chemically
interconvertible via protonation and a bridged-thiolate
exchange reaction, respectively.
Structure. The single-crystal X-ray structure of the
complex 3-Et unit in the [PPN]þ salt is depicted in
Figure 3, and selected bond dimensions are presented in
the figure caption. As shown in Table 1, the geometry of
the [Fe(μ-S)(μ-SEt)Fe] core in complex 3-Et is best de-
scribed as a butterfly-like structure with a dihedral angle
of 171.7ꢀ (the intersection of the two Fe2S planes) in the
solid state. Two nitrosyl groups, bridging sulfide and
thiolate, define the distorted tetrahedral geometry of each
iron atom, leading to an acute angle Fe(1)-S(1)-Fe(2) of
72.69(4)ꢀ, Fe(1)-S(2)-Fe(2) of 73.39(4)ꢀ, S(1)-Fe-
(1)-S(2) of 106.71(5)ꢀ, and S(1)-Fe(2)-S(2) of
106.40(5)ꢀ. According to the crystallographic study, com-
plex 3-Me in [PPN]þ salt also exhibited a butterfly-like
structure with a dihedral angle of 169.2ꢀ, similar to
complex 3-Et (Supporting Information, Figure S1). Com-
Figure 3. ORTEP drawing and labeling scheme of the [(NO)2Fe(μ-
SEt)(μ-S)Fe(NO)2]- (3-Et) unit in [PPN]þ salt with thermal ellipsoids
˚
drawn at 50% probability. Selected bond distances (A) and angles (deg):
Fe(1) Fe(2), 2.6650(9); Fe(1)-N(1), 1.670(5); Fe(1)-N(2), 1.650(5);
3 3 3
Fe(2)-N(3), 1.667(5); Fe(2)-N(4), 1.674(5); Fe(1)-S(1), 2.2455(13); Fe-
(1)-S(2), 2.2280(13); Fe(2)-S(1), 2.2510(13); Fe(2)-S(2), 2.2317(14);
O(1)-N(1), 1.173(5); O(2)-N(2), 1.177(5); O(3)-N(3), 1.171(5); O-
(4)-N(4), 1.170(5); N(1)-Fe(1)-N(2), 118.9(2); N(3)-Fe(2)-N(4),
117.5(2); N(1)-Fe(1)-S(1), 103.86(14); N(2)-Fe(1)-S(1), 107.14(15);
N(1)-Fe(1)-S(2), 110.83(14); N(2)-Fe(1)-S(2), 108.65(17); N(3)-Fe-
(2)-S(1), 109.42(15); N(3)-Fe(2)-S(2), 107.92(14); N(4)-Fe(2)-S(1),
106.43(14); N(4)-Fe(2)-S(2), 108.64(16); S(1)-Fe(1)-S(2), 106.71(5);
S(1)-Fe(2)-S(2), 106.40(5); O(1)-N(1)-Fe(1), 168.2(4); O(2)-N-
(2)-Fe(1), 169.5(5); O(3)-N(3)-Fe(2), 169.4(4); O(4)-N(4)-Fe(2),
167.8(4).
˚
pared to the Fe-N bond distance of 1.6747(11) A found
in complex [(NO)2Fe(μ-SEt)2Fe(NO)2], the shorter
˚
Fe-N bond distance of 1.665(5) A observed in complex
3-Et and 1.653(7) A observed in complex 5 indicates the
˚
Table 1. Selected Metric Data for Complex 3-Et, Neutral Roussin’s Red Ester,
better metal π-back-donation to the NO antibonding
and Roussin’s Red Salt20
orbital.20 The mean N-O bond distance of 1.173(5) A
˚
in complex 3-Et is nearly at the upper end of the 1.178-
˚
[Fe(μ-SEt)-
(NO)2]2
[Pr4N]2[Fe(μ-S)-
(NO)2]2
9
(3)-1.160(6) A range for the anionic {Fe(NO)2} DNICs,
3-Et
and meanwhile, the mean Fe-N(O) distance of 1.665(5)
A in complex 3-Et approaches the lower end of the
˚
Fe Fe (A)
3 3 3
˚
2.7080(5)
2.2585(5)
1.6747(11)
1.1708(14)
3.615
73.671(14)
106.329(14)
116.10(5)
2.6650(9)
2.2391(14)
1.665(5)
1.173(5)
3.589
2.704(2)
2.225(2)
1.653(7)
1.178(8)
3.535
74.82(7)
105.2(1)
111. 5(3)
163.2(6)
180
a
˚
Fe-S (A)
9
˚
1.695(3)-1.661(4) A range for the anionic {Fe(NO)2}
a
˚
Fe-N (A)
DNICs.21 Presumably, the electronic structure of the
anionic mixed thiolate-sulfide-bridged Roussin’s red
esters [(NO)2Fe(μ-SR)(μ-S)Fe(NO)2]- is best described
as [{Fe(NO)2}9-{Fe(NO)2}9]. Furthermore, the strik-
a
˚
N-O (A)
˚
S (A)
S
3 3 3
Fe-S-Fe —(deg)
S-Fe-S —(deg)
N-Fe-N —(deg)
73.04(4)a
106.55(5)a
118.2(2)a
168.7(5)
171.7
˚
Fe-N-O —(deg)a 168.4(2)
3 3 3
ingly short Fe Fe distance of 2.6650(9) A observed in
˚
complex 3-Et, compared to 2.7080(5) A observed in
complex [(NO)2Fe(μ-SEt)2Fe(NO)2] and 2.704 (2) A ob-
served in complex 5, rationalizes the absence of para-
magnetism and the EPR signal.
dihedral —(deg)b
180
˚
a Average bond distance and angle. b Defined by the intersection of
the two Fe2S planes.
was displayed when 3-Et-K was reacted with 1 equiv of
thiophenol in THF (Scheme 1h). The higher-energy νNO
band of complex 3-Ph (νNO 1721, 1693 cm-1 (THF))
shifted by 6-11 cm-1 from that of 3-Et-K (νNO 1710,
1687 cm-1 (THF)) demonstrated the conversion via phe-
nylthiol protonation. The phenylthiol protonation of the
bridged-sulfide ligand of 3-Et-K and the concurrent nu-
cleophilic attack of phenylthiolate triggering the bridged-
ethylthiolate cleavage, and the subsequent elimination of
EtSH may rationalize the formation of complex 3-Ph
(Scheme 4). In contrast to thiolates [RS]- promoting
bridged-thiolate cleavage of RREs [(NO)2Fe(μ-SEt)2Fe-
(NO)2] to produce DNICs [(NO)2Fe(SR)2]- and the inert-
ness of complex 5 toward [RS]-,18 quantitative trans-
formation of complex 3-Ph to 3-Et-K was demonstrated
upon the addition of 1 equiv of [SEt]- into a THF solution
of complex 3-Ph (Scheme 1h). Presumably, the conversion
of the [Fe(μ-SPh)(μ-S)Fe] core into the [Fe(μ-SEt)(μ-S)Fe]
Discussion
The transformation of DNICs/anionic RREs into the anio-
nic mixed thiolate-sulfide-bridged RREs and the conversion
of the anionic mixed thiolate-sulfide-bridged RREs into RRS
triggered by the coordinated thiolate(s) of DNICs/anionic
RREs demonstrates that the generation of inorganic sulfide
occurs via the reduction of triphenylmethanthiol (HSCPh3;
Scheme 5). In contrast to the coordinated ethylthiolates of
[(NO)2Fe(SEt)2]- initiating the sulfide release via the reduction
of HSCPh3/Me2S3, the inertness of [(NO)2Fe(SPh)2]- toward
HSCPh3/Me2S3 may signify how the reducing ability of the
coordinated cysteinate ligands of DNICs was regulated by the
hydrophobic/hydrophilic environment to modulate the release
of sulfide from S-sulfanyl-cysteine in biological systems.
(21) Hung, M.-C.; Tsai, M.-C.; Lee, G.-H.; Liaw, W.-F. Inorg. Chem.
2006, 45, 6041–6047.

9032 Inorganic Chemistry, Vol. 48, No. 18, 2009
Lu et al.
Scheme 5
Scheme 6
Scheme 7
was delineated (Scheme 6c).24 It was reported that the partici-
pation of cytosolic Isu1 in the assembly of [Fe-S] clusters in
cytosol could not be excluded since the suppression of cytosolic
Isu1 impairs the activation of cytosolic [Fe-S] proteins.25
In this model study, the addition of HSCPh3 and [SEt]- into
the anionic mixed thiolate-sulfide-bridged RREs affords
Roussin’s red salt under anaerobic conditions, compared to
the spontaneous conversion of the anionic mixed thiola-
te-sulfide-bridged RREs into RBS upon exposure to O2 or
acid.15b These results show that the anionic mixed thiola-
te-sulfide-bridged RREs act as a key intermediate in the
transformation of DNICs into [2Fe-2S] and [4Fe-4S] clus-
ters, respectively. As shown in Scheme 7a, it is proposed that
the insertion of sulfide, derived from the reduction of the S-
sulfanyl cysteine of Nfs1 by the pendant cysteinate ligands of
DNICs, into DNICs results in the formation of the preas-
sembled protein-bound thiolate-sulfide-containing RREs.
The mixed thiolate-sulfide-bridged RREs then react with
the cysteinate ligand and cysteine desulfurase to afford RRS
under anaerobic conditions. The subsequent NO-radical/thiyl-
radical exchange reaction between RRS and rubredoxin-con-
taining proteins (the IscA-like protein) leads to the assembly of
a [2Fe-2S] cluster to mature [2Fe-2S] proteins. Instead, the
conversion of the anionic mixed thiolate-sulfide-bridged
The biomimetic model study of the reassembling process
([(NO)2Fe(SEt)2]-/[(NO)2Fe(μ-SEt)2Fe(NO)2]- f [(NO)2-
Fe(μ-SEt)(μ-S)Fe(NO)2]- f [(NO)2Fe(μ-S)2Fe(NO)2]2-
f
[(SEt)2Fe(μ-S)2Fe(SEt)2]2-) may signify the biosynthetic me-
chanism of the [2Fe-2S] cluster. Although the Fe source and
the assembling mechanism remain to be investigated, detailed
studies of the maturation of [Fe-S] proteins in eukaryotes were
reported by Lill and Muhlenhoff and by Rouault et al.,
respectively.7,22 As shown in Scheme 6, Fe from Yfh1 incor-
porating with sulfide, derived from the reduction of S-sulfanyl
cysteine of Nfs1 (cysteine desulfurase) by NADH mediated by
Yah1 and Arh1, leads to the assembly of an [Fe(μ-S)2Fe] core.
With the aid of Isu1/2 serving as a scaffold protein, the [Fe(μ-
S)2Fe] core finally matures [2Fe-2S] proteins (Scheme 6a).
Also, reductive coupling of two Isu1/2-bound [2Fe-2S] clus-
ters into an Isu1/2-bound [4Fe-4S] cluster activates [4Fe-4S]
iron-sulfur proteins (Scheme 6b).23 Concerning the matura-
tion of cytosolic [Fe-S] proteins, the [Fe-S] cluster precursor
glutathione-bound [X] exported by Atm1 to cytosol and
further assembled into the [Fe-S] cluster by CIA machinery
(22) (a) Land, T.; Rouault, T. A. Mol. Cell 1998, 2, 807–815. (b) Tong, W.
H.; Rouault, T. A. EMBO J. 2000, 19, 5692–5700.
(23) Fontecave, M.; Ollagnier-de-Choudens, S. Arch. Biochem. Biophys.
2008, 474, 226–237.
(24) (a) Isikawa, T.; Li, Z.-S.; Lu, Y.-P.; Rea, P. A. Biosci. Rep. 1997, 17,
189–207. (b) Roy, A.; Solodovnikova, N.; Nicholson, T.; Antholine, W.; Walden,
W. E. EMBO J. 2003, 22, 4826–4835.
(25) Tong, W. H.; Rouault, T. A. Cell Metab. 2006, 3, 199–210.

Article
Inorganic Chemistry, Vol. 48, No. 18, 2009 9033
Chart 1
RREs into RBS occurs spontaneously in an aerobic environ-
ment. The subsequent assembly of [4Fe-4S] clusters in the
presence of cysteine desulfurase and rubredoxin-containing
proteins activates [4Fe-4S] proteins, as proposed in the
previous study (the reassembling process ([(NO)2Fe(S-
Et)2]-/[(NO)2Fe(μ-SEt)2Fe(NO)2]- f [(NO)2Fe(μ-SEt)(μ-
S)Fe(NO)2]- f[Fe4S3(NO)7]-/[Fe4S3(NO)7]2-f [Fe4S4-
(NO)4]2- f [Fe4S4(SEt)4]2-; Scheme 7b).15b Additionally,
the anionic mixed thiolate-sulfide-bridged RREs produced
in mitochondria may be transformed into the anionic mixed
GS-sulfide-bridged RREs. The anionic mixed GS-sulfide-
bridged RREs are then exported to cytosol via Atm1 for
further assembling into [2Fe-2S]/[4Fe-4S] clusters by cyto-
soliccysteinedesulfurase and rubredoxin-containing proteins
(Scheme 7c).24 On the basis of this study, the protein-bound
RREs containing the bridging thiolate-sulfide ligands were
proposed to be a key intermediate in determining the bio-
synthesis of [2Fe-2S] and [4Fe-4S] iron-sulfur proteins.
as well as single-crystal X-ray diffraction. To the
best of our knowledge, the dinuclear dinitrosyl iron
complexes canthenbe classifiedinto (a) the neutral
RREs,20a (b) the anionic mixed thiolate-sulfide-
bridged RREs, (c) RRS,20b and (d) the anionic
RREs,18 as shown in Chart 1.
(2) Compared to the inertness of RRS [Fe(μ-S)-
(NO)2]22- toward[RS]- (R=Et, Ph), theaddition
of [EtS]- into RRE [(NO)2Fe(μ-SEt)2Fe(NO)2]
producing DNIC [(NO)2Fe(SEt)2]- via bridged-
thiolate cleavage was reported.18 In contrast, the
addition of the stronger electron-donating ethyl-
thiolate [EtS]- into the anionic mixed phenylthio-
late-sulfide-bridged RRE 3-Ph triggers a bridged-
thiolate exchange reaction to yield 3-Et. This result
clearly demonstrates that the dinuclear {Fe-
(NO)2}9-{Fe(NO)2}9 motif prefers the stronger
electron-donating sulfide and alkylthiolate over
phenylthiolate. Reversibly, protonation of the an-
ionic mixed ethylthiolate-sulfide-bridged RRE 3-
Et by PhSH induces the bridged-thiolate exchange
reaction to yield 3-Ph. These results show that the
anionic mixed thiolate-sulfide-bridged RREs
[(NO)2Fe(μ-SR)(μ-S)Fe(NO)2]- containing the
different bridged thiolates are chemically intercon-
vertable via thiol protonation and a thiolate-ex-
change reaction, respectively.
Conclusion and Comments
Studies on the transformation of DNICs to [2Fe-2S]
clusters via the anionic mixed thiolate-sulfide-bridged inter-
mediate [(NO)2Fe(μ-SR)(μ-S)Fe(NO)2]- and on the reactiv-
ity of the anionic mixed thiolate-sulfide-bridged RREs have
resulted in the following results:
(1) The anionic mixed thiolate-sulfide-bridged RREs
[(NO)2Fe(μ-SR)(μ-S)Fe(NO)2]- (R = Et, Me and
Ph) containing a {Fe(NO)2}9-{Fe(NO)2}9 core
were synthesized and characterized by IR,
UV-vis, EPR, 1H NMR, and cyclic voltammetry
(3) In contrast to the nucleophilicity (reducing abil-
ity) displayed by 2-PPN, the inertness of [PPN]-
[(NO)2Fe(SPh)2] toward HSCPh3 implicates how

9034 Inorganic Chemistry, Vol. 48, No. 18, 2009
Lu et al.
the reducing ability of the coordinated thiolates
of DNICs modulates the release of sulfide from
HSCPh3 via reduction accompanied by the con-
current conversion of DNICs into the anionic
mixed-sulfide-thiolate-bridged complex.
Scheme 8
(4) The transformation of DNICs into [2Fe-2S]
clusters facilitated by HSCPh3 or Me2S3 via the
reassembling process ([(NO)2Fe(SEt)2]-/[(NO)2-
Fe(μ-SEt)2Fe(NO)2]- f [(NO)2Fe(μ-SEt)(μ-S)-
Fe(NO)2]- f [(NO)2Fe(μ-S)2Fe(NO)2]2- f [(S-
Et)2Fe(μ-S)2Fe(SEt)2]2-) was consistent with the
repair of DNICs back to the ferredoxin [2Fe-2S]
cluster by cysteine desulfurase (IscS) and L-cysteine
in vitro with no need of the addition of iron or any
other protein components in E. coli.13
CH3CN using 0.2 M [n-Bu4N][PF6] as a supporting electro-
lyte. The potential was measured at 298 K versus a Ag/AgCl
reference electrode by using a glassy carbon working elec-
trode. 1H and 31P NMR spectra were obtained on a Varian
Unity-500 spectrometer. Analyses of carbon, hydrogen, and
nitrogen were obtained with a CHN analyzer (Heraeus).
Reaction of [PPN]2[(SEt)2Fe(μ-S)2Fe(SEt)2] (1-PPN) and
Nitric Oxide under the Presence of PPh3. Under a N2 atmo-
sphere, to a CH3CN solution (3 mL) of triphenylphosphine
(0.026 g, 0.1 mmol) and [PPN]2[(SEt)2Fe(μ-S)2Fe(SEt)2] (1-
PPN) (0.085 g, 0.05 mmol) was added NO(g) (48 mL (10%
NO þ 90% N2), 0.2 mmol) via a gastight syringe at 0 ꢀC. The
reaction solution was stirred and monitored by FTIR immedi-
ately. The IR νNO spectrum displaying two strong stretching
bands at 1722 and 1680 cm-1 was consistent with the formation
of [PPN][(NO)2Fe(SEt)2] (2-PPN). The addition of diethyl ether
to the THF solution led to the precipitation of red-brown solid
2-PPN (yield: 0.069 g, 89%), characterized by IR and UV-vis.
Evaporation of the THF-diethyl ether solution yielded
SdPPh3, identified by 31P NMR (δ 42.70 ppm in CH3CN).
Conversion of 2-PPN into [PPN][(NO)2Fe(μ-SEt)(μ-S)Fe-
(NO)2] (3-Et). HSCPh3 (0.055 g, 0.2 mmol) and 2-PPN (0.310
g, 0.4 mmol) were dissolved in CH3CN (5 mL) and stirred under
N2 at 0 ꢀC for 1 h. The reaction solution was then stirred
overnight at ambient temperature and monitored with FTIR.
An IR νNO shift from 1722 s and 1680 s cm-1 to 1718 s and 1693
s cm-1 was observed. The mixture solution was dried under a
vacuum, and the crude solid was redissolved in THF (5 mL).
Then, the solution was filtered through Celite to remove the
DNICs are known as one possible naturally occurring
form for the storage and transport of nitric oxide in biological
systems to exert NO-related functions, for example, S-nitro-
sylation of HSP70 transcriptional factor (HSF), triggering
trimerization and the activation of HSF to induce accumula-
tion of HSP70.6,26 Presumably, the thiolate-coordinate
DNICs serve as not only the thiolate/electron carrier activat-
ing the incorporation of sulfide to assemble an [Fe(μ-S)2Fe]
core but also the Fe carrier/source in the biosynthesis of
[2Fe-2S] and [4Fe-4S] iron-sulfur clusters (Scheme 8).
Since [Fe-S] proteins have been known to have a critical
influence on the uptake, intracellular distribution, and utili-
zation of iron, this biomimetic study supports the position
that DNICs may regulate Fe uptake and cellular iron home-
ostasis.1,7
Experimental Section
Manipulations, reactions, and transfers were conducted
under nitrogen according to Schlenk techniques or in a glove-
box (N2 gas). Solvents were distilled under nitrogen from
appropriate drying agents (diethyl ether from CaH2; acetoni-
trile from CaH2-P2O5; methylene chloride from CaH2;
methanol from Mg/I2; hexane and tetrahydrofuran (THF)
from sodium benzophenone) and stored in dried, N2-filled
˚
flasks over 4 A molecular sieves. Nitrogen was purged
through these solvents before use. Solvent was transferred
to the reaction vessel via stainless cannula under positive
pressure ofN2. Complexes[K-18-crown-6-ether][(NO)2Fe(μ-
SEt)2Fe(NO)2],18 [PPN][(NO)2Fe(SEt)2],18 [PPN][Fe(SEt)4],
and [PPN]2[(SEt)2Fe(μ-S)2Fe(SEt)2] were synthesized on the
basis of the reported literature.27 The reagents PPh3 (Wako),
methyltrisulfide (Acros), and HSCPh3 and HSPh (Aldrich)
were used as-received. The NO(g) (SanFu, 10% NO þ 90%
N2) was passed through an Ascarite II column to remove
higher nitrogen oxides before use.28 Infrared spectra of the
1
insoluble [PPN][SEt] solid characterized by H NMR (δ 7.79
(m), 7.46 (m), 2.37 (q), 1.13 (t) ppm in CD3CN). The addition of
hexane (20 mL) to the filtrate (THF solution) led to the
precipitation of brown solid [PPN][(NO)2Fe(μ-SEt)(μ-S)Fe-
(NO)2] (3-Et) (yield: 0.136 g, 79%). The upper solution
(THF-hexane solution) was dried under a vacuum to obtain
EtSSEt (δ 2.70 (q), 1.27 (t) ppm in CD3CN) and HC(Ph)3 (δ 7.29
(m), 7.21 (m), 7.13 (m), 5.59 (s) ppm in CD3CN), characterized
by 1H NMR. Dark-brown crystals suitable for X-ray diffraction
analysis were isolated from a THF solution of complex 3-Et
layered by hexane in a tightly sealed flask. IR νNO: 1709 s, 1686 s
cm-1 (THF); 1718 s, 1693 s cm-1 (CH3CN). Absorption spec-
trum (THF) [nm, λmax(M-1 cm-1, ε)]: 259 (8600), 326 (4100),
376 (5100). 1H NMR (CD3CN): δ 7.81 (m), 7.49 (m), 2.71 (q),
1.40 (t) ppm. Anal. Calcd for C38H35Fe2N5O4P2S2: C, 52.86; H,
4.09; N, 8.11. Found: C, 52.11; H, 4.20; N, 7.83.
ν
NO stretching frequencies were recorded on a PerkinElmer
model spectrumOneB spectrometerwithsealedsolution cells
(0.1 mm, CaF2 windows). UV-visspectrawererecordedona
Jasco V-570 spectrometer. Electrochemical measurements
were performed with CHI model 421 potentionstat (CH
Instrument) instrumentation. Cyclic voltammograms were
obtained from a 10 mM analyte concentration in O2-free
Transformation of 2-PPN into [PPN][(NO)2Fe(μ-SMe)(μ-
S)Fe(NO)2] (3-Me). Compounds methyltrisulfide (0.025 g, 0.2
mmol) and 2-PPN (0.310 g, 0.4 mmol) were dissolved in CH3CN
(5 mL) and stirred at 0 ꢀC under N2. The reaction was monitored
with FTIR, and an IR νNO shift from 1722 s and 1680 s cm-1 to
1719 s and 1697 s cm-1 was observed. After being stirred for 1 h,
diethyl ether (15 mL) was added, and the mixture solution was
filtered through Celite to remove the [PPN][SMe] (δ 7.82 (m),
(26) Chen, Y.-C.; Ku, W.-C.; Feng, L.-T.; Tsai, M.-L.; Hsieh, C.-H.; Hsu,
W.-H.; Liaw, W.-F.; Hung, C.-H.; Chen, Y.-J. J. Am. Chem. Soc. 2008, 130,
10929–10938.
(27) (a) Maelia, L. E.; Millar, M.; Koch, S. A. Inorg. Chem. 1992, 31,
4594–4600. (b) Han, S.; Czernuszewicz, R. S.; Spiro, T. G. Inorg. Chem. 1986,
25, 2276–2277.
(28) Works, C. F.; Jocher, C. J.; Bart, G. D.; Bu, X.; Ford, P. C. Inorg.
Chem. 2002, 41, 3728–3739.

Article
Inorganic Chemistry, Vol. 48, No. 18, 2009 9035
7.49 (m), 2.19 (s) ppm in CD3CN). Then, hexane was added into
the filtrate to precipitate [PPN][(NO)2Fe(μ-SMe)(μ-S)Fe(NO)2]
(3-Me) (yield: 0.136 g, 79%). Byproduct diethyl disulfide dis-
solved in the THF-diethyl ether-hexane solution was isolated
UV-vis,27 and the yellow-brown upper solution. The upper
solution (CH3CN-diethyl ether solution) was dried under a
vacuum and redissolved in THF. The yellow-brown solution
was filtered through Celite to remove the insoluble solid. The
addition of hexane into the filtrate led to the precipitation of red-
brown solid 2-K, characterized by FTIR and UV-vis.
Conversion of 3-Et-K into [K-18-crown-6-ether][(NO)2Fe(μ-
SPh)(μ-S)Fe(NO)2] (3-Ph). To the THF solution (3 mL) of 3-Et-
K (0.063 g, 0.1 mmol) was added HSPh (0.011 g, 0.1 mmol) at
0 ꢀC. After the mixture solution was stirred at 0 ꢀC for 1 h, the
reaction solution was further stirred overnight at ambient
temperature. The reaction was monitored with FTIR, and a
IR νNO shift from 1710 s and 1687 s cm-1 to 1721 s and 1693 s
cm-1 was observed. Hexane was then added to the THF solution
(3 mL), leading to the precipitation of complex 3-Ph (yield: 0.059
g, 88%). IR νNO: 1721 s, 1693 s cm-1 (THF). Absorption
spectrum (THF) [nm, λmax(M-1 cm-1, ε)]: 271 (7500), 379
(5000). 1H NMR(CD3CN): δ 7.3 (m), 3.52 (s) ppm.
Transformation of Complex 3-Ph into 3-Et-K. Complexes 3-
Ph (0.068 g, 0.1 mmol) and [K][SEt] (0.18 g, 0.3 mmol) were
dissolved in THF (5 mL) under a N2 atmosphere. The mixture
solution was stirred for 10 min at ambient temperature and
monitored by FTIR. The IR νNO spectrum displaying strong
stretching bands at 1710 s and 1687 s cm-1 (THF) shows the
formation of 3-Et-K. Diethyl ether (5 mL) was added to the
mixture solution, and then the mixture solution was filtered to
remove the insoluble [K][SPh]. Hexane was added to the filtrate
to precipitate 3-Et-K (yield: 0.057 g, 90%), characterized by IR
and UV-vis.
1
and characterized by H NMR. Recrystallization from THF
solution of complex 3-Me layered with diethyl ether at -20 ꢀC
for 4 days led to dark brown crystals suitable for X-ray crystal-
lography. IR νNO: 1709 s, 1688 s cm-1 (THF); 1719 s, 1697 s
cm-1 (CH3CN). Absorption spectrum (CH3CN) [nm, λmax(M-1
cm-1, ε)]: 253 (9000), 295 (4500), 377 (5500). 1H NMR
(CD3CN): δ 7.79 (m), 7.45 (m), 2.46 (s) ppm. Anal. Calcd for
C37H33Fe2N5O4P2S2: C, 52.32; H, 3.92; N, 8.24. Found: C,
52.51; H, 4.12; N, 8.63.
Transformation of [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe-
(NO)2] (4) into [K-18-crown-6-ether][(NO)2Fe(μ-SEt)(μ-S)Fe-
(NO)2] (3-Et-K). To a 30 mL Schlenk flask loaded with [K-18-
crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (4) (0.071 g, 0.1
mmol) and HSCPh3 (0.0276 g, 0.1 mmol; or Me2S3 (0.013 g,
0.1 mmol)) was added THF (3 mL). The reaction solution was
stirred under N2 at ambient temperature overnight and mon-
itored by FTIR. An IR νNO shift from 1673 s and 1655 s cm-1 to
1710 s and 1687 s cm-1 was observed. Hexane was then added to
lead to the precipitation of the brown solid [K-18-crown-6-
ether][(NO)2Fe(μ-SEt)(μ-S)Fe(NO)2] (3-Et-K; yield 0.054 g,
86%). Byproducts EtSSEt and HC(Ph)3 dissolved in a
1
THF-hexane solution were isolated and characterized by H
NMR. In a similar fashion, the reaction of complex 4 and Me2S3
led to the formation of 3-Et-K accompanied by the byproducts
EtSSEt and MeSSMe (δ 2.40 (s) ppm in CD3CN), characterized
by 1H NMR. IR νNO: 1710 s, 1687 s cm-1 (THF); 1718 s, 1693 s
cm-1 (CH3CN). Absorption spectrum (THF) [nm, λmax(M-1
cm-1, ε)]: 259 (8600), 326 (4300), 376 (5200). 1H NMR-
(CD3CN): δ 3.56 (s) (18-crown-6-ether), 2.71 (q), 1.40 (t) ppm
(SEt).
Crystallography. Crystallographic data and structure refine-
ment parameters of complexes 3-Et and 3-Me are summarized in
the Supporting Information. The crystal chosen for X-ray diffrac-
tion studies measured 0.45 ꢀ 0.4 ꢀ 0.35 mm for complex 3-Et and
0.42 ꢀ 0.18 ꢀ 0.02 mm for complex 3-Me. The crystal was mounted
on a glass fiber and quickly coated in epoxy resin. Unit cell
parameters were obtained by least-squares refinement. Diffraction
measurements for complexes 3-Et and 3-Me were carried out on a
Kappa CCD diffractometer with graphite-monochromated Mo
Transformation of 3-Et-K into [K-18-crown-6-ether]2[(NO)2-
Fe(μ-S)2Fe(NO)2] (5). A CH3CN solution (5 mL) of 3-Et-K
(0.126 g, 0.2 mmol) was transferred to a 30 mL Schlenk flask
loaded with HSCPh3 (0.055 g, 0.2 mmol), [K][SEt] (0.020 g, 0.2
mmol), and 18-crown-6-ether (0.053 g, 0.2 mmol) under a
positive N2 atmosphere. The reaction solution was stirred over-
night and monitored by FTIR. An IR νNO shift from 1718 s and
1693 s cm-1 to 1666 s and 1642 s cm-1 was observed. The
addition of diethyl ether into the CH3CN mixture solution led to
the precipitation of the known [K-18-crown-6-ether]2[(NO)2Fe-
(μ-S)2Fe(NO)2] (5; yield 0.148 g, 82%), characterized by IR and
UV-vis.20b Byproducts EtSSEt and HC(Ph)3 dissolved in the
CH3CN-diethyl ether solution were isolated and characterized
by 1H NMR.
˚
KR radiation (λ = 0.7107 A) between 2.25ꢀ and 25.37ꢀ for complex
3-Et and between 1.29ꢀ and 25.03ꢀ for complex 3-Me. Least-squares
refinements of the positional and anisotropic thermal parameters of
all non-hydrogen atoms and hydrogen atoms were fixed at calcu-
lated positions and refined as riding modes. A multiscan absorption
correction was made. The SHELXTL29 structure refinement pro-
gram was employed.
Acknowledgment. We gratefully acknowledge financial sup-
port from the National Science Council of Taiwan. The authors
thank Mr. Ting-Shen Kuo for single-crystal X-ray structural
determinations.
Reaction of Complex 5 and [K-18-crown-6-ether][Fe(SEt)4].
CH3CN solution (2 mL) of complex 5 (0.045 g, 0.1 mmol) was
added to the CH3CN solution of [K-18-crown-6-ether][Fe-
(SEt)4] (0.121 g, 0.2 mmol) in a dropwise manner at ambient
temperature. The reaction was monitored by FTIR. After the
reaction solution was stirred for 1 h, the IR νNO shifting from
1666 s, 1642 s cm-1 to 1722 s, 1680 s cm-1 implicated the
formation of [K-18-crown-6-ether][(NO)2Fe(SEt)2] (2-K). Di-
ethyl ether was added into the CH3CN solution to separate the
known red-brown precipitate [K-18-crown-6-ether]2[(SEt)2Fe-
(μ-S)2Fe(SEt)2] (1-K; yield 0.048 g, 47%), identified by
Supporting Information Available: X-ray crystallographic
files in CIF format for the structure determinations of
[PPN][(NO)2Fe(μ-SEt)(μ-S)Fe(NO)2] and [PPN][(NO)2Fe(μ-
SMe)(μ-S)Fe(NO)2]. This material is available free of charge
via the Internet at http://pubs.acs.org.
(29) Sheldrick, G. M. SHELXTL; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1994.