A new member of tetranuclear dinitrosyl iron complexes (DNICs) with 2-mercaptothiazoline ligand: Synthesis, structure and properties

Article abstract of DOI:10.1007/s11164-014-1599-4

A new tetranuclear dinitrosyliron complex [(μ-SC₃H ₄SN)Fe(NO)₂]₄ (2), each of a Fe center coordinated with two S or two N, was prepared by CO replacement from the reduced precursor (CO)₂Fe(NO)₂ with 1 equiv of HSC ₃H₄SN (2-mercaptothiazoline) in the presence of O _2(g). The structure of 2 is similar to [(Imid-iPr)Fe(NO) ₂]₄ (Imid-iPr = 2-isopropylimidazole) (Hess et al. J Am Chem Soc 133:20426-20434, 2011), and both complexes comprise a quadrilateral plane of irons with corresponding ligands, SC₃H₄SN ^- or Imid-iPr^-, bridging the edges and two nitrosyl ligands capping the irons at the corners. An additional equiv of SC ₃H₄SN^- was added to 2, which results in the mononuclear {Fe(NO)₂}⁹ (SC₃H ₄SN)₂Fe(NO) ₂ ^- (3), in the manner of N bound-[SC₃H₄SN]. Reaction of (TMEDA)IFe(NO) ₂ (TMEDA = tetramethylethylenediamine) and complex 3 leads to the formation of complex 2. Dinuclear complex [(μ-C₅H ₇N₂)Fe(NO)₂]₂ (4) can be synthesized by the ligand displacement of SC₃H₄SN^- to C₅H₇N₂ ^- (3,5-dimethylpyrazolate) of 2 (Chong et al. Can J Chem 57:3119-3125, 1979). Complexes 2-4 were characterized by IR and UV-Vis. The molecular structures of 2 and 3 were determined by X-ray single crystal diffraction.

Full text of DOI:10.1007/s11164-014-1599-4

Res Chem Intermed (2014) 40:2217–2227
DOI 10.1007/s11164-014-1599-4
A new member of tetranuclear dinitrosyl iron
complexes (DNICs) with 2-mercaptothiazoline ligand:
synthesis, structure and properties
•
Chien-Hong Chen Jheng-Hong Wang
•
•
Jing-Yi Huang Chung-Hung Hsieh
Received: 15 January 2014 / Accepted: 14 February 2014 / Published online: 30 March 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract A new tetranuclear dinitrosyliron complex [(l-SC3H4SN)Fe(NO)2]4 (2),
each of a Fe center coordinated with two S or two N, was prepared by CO
replacement from the reduced precursor (CO)2Fe(NO)2 with 1 equiv of HSC3H4SN
(2-mercaptothiazoline) in the presence of O2(g). The structure of 2 is similar to
[(Imid-iPr)Fe(NO)2]4 (Imid-iPr = 2-isopropylimidazole) (Hess et al. J Am Chem
Soc 133:20426–20434, 2011), and both complexes comprise a quadrilateral plane of
irons with corresponding ligands, SC3H4SN- or Imid-iPr-, bridging the edges and
two nitrosyl ligands capping the irons at the corners. An additional equiv of
SC3H4SN- was added to 2, which results in the mononuclear {Fe(NO)2}9
(SC3H4SN)2Fe(NO)-2 (3), in the manner of N bound-[SC3H4SN]. Reaction of
(TMEDA)IFe(NO)2 (TMEDA = tetramethylethylenediamine) and complex 3 leads
to the formation of complex 2. Dinuclear complex [(l-C5H7N2)Fe(NO)2]2 (4) can
be synthesized by the ligand displacement of SC3H4SN- to C5H7N2 (3,5-dim-
-
ethylpyrazolate) of 2 (Chong et al. Can J Chem 57:3119–3125, 1979). Complexes
2–4 were characterized by IR and UV–Vis. The molecular structures of 2 and 3
were determined by X-ray single crystal diffraction.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-014-1599-
4) contains supplementary material, which is available to authorized users.
C.-H. Chen
Department of Medical Education, Chung Shan Medical University Hospital, Taichung 40201,
Taiwan
C.-H. Chen (&) ꢀ J.-H. Wang
School of Applied Chemistry, Chung Shan Medical University, Taichung 40201, Taiwan
e-mail: cchwind@csmu.edu.tw
J.-Y. Huang ꢀ C.-H. Hsieh (&)
Department of Chemistry, Tamkang University, New Taipei City 25137, Taiwan
e-mail: chsieh@mail.tku.edu.tw
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Keywords Dinitrosyl iron complex ꢀ DNIC ꢀ Tetranuclear ꢀ Nitrosyl ꢀ
Nitric oxide ꢀ 2-Mercaptothiazoline
Introduction
One of the approaches of the current drug discovery research is to design a complex
with two or more medicinal functions [1]. For example, a dansyl derivative
[Ds-DAB = N-(2-Aminophenyl)-5-(dimethylamino)-1-naphthalenesulfonic Amide]
was prepared and used as a fluorescent chemosensor for peroxynitrite (ONOO-)
detection [2]. 2-Mercaptobenzimidazole has been explored as an inhibitor for the
synthesis of cellular DNA and RNA [3]. Interest in nitric oxide (NO) derives from
its physiological and biological functions in living organisms [4–7], the protein-
bound dinitrosyliron complexes (protein-bound DNICs) and low-molecular-weight
DNICs (LMW-DNICs), and has been suggested in vivo and in vitro, respectively
[8–16]. Both DNICs are possibly identified and characterized by their distinctive
electron paramagnetic resonance (EPR) signals at g = 2.03. Recently, the biological
protein-bound DNIC has been well characterized by an X-ray diffraction study via
the addition of a dinitrosyl diglutathionyliron complex into human glutathione
transferase P1-1 protein [17]. DNICs are also known to play an important role in
storage and transfer NO in vivo and in vitro. In fact, much evidence obtained in
animal models suggests that DNICs can indeed act as regulatory and cytotoxic
agents depending on the concentration of NO required by cells or tissues [18].
In order to develop a bifunctional medicine (an inorganic NO donor with a
targeting probe), heterocyclic analogs of 2-mrcaptobenzimidazole, such as 1,2,4-
triazole-3-thione, 1-methyl-imidazole-2-yl, 3-amino-5-mercapto-1,2,4-triazole,
3-mercapto-1,2,4-triazole, 5-mercapto-1-methyl-1H-tetrazole, and 2-mercapto-ben-
zothiazole were adopted to synthesize the DNIC complexes [19–23]. Several
coordination modes of thioamide with metals are shown in Chart 1b and two
possible tautomeric forms of the 2-mercaptothiazoline are shown in Chart 1a.
Basically, these type of ligands can bind to metals in a mono- or a bidentate
manners though S and N atoms to form a monomer or a cluster, but when it binds to
the Fe(NO)2 units, only mononuclear and dinuclear DNICs can be found in the
literature to date. In this work, a new tetrameric dinitrosyliron complex (DNICs)
[(l-SC3H4SN)Fe(NO)2]4 (2) and monomeric [cation][(SC3H4SN)2Fe(NO)2] (3),
SC3H4SN = 2-mercaptothiazoline; cation = Ph4P? and Et4N?, have been synthe-
sized. The formation and interconversion of 2 and 3 were studied by IR. Structural
identification of 2 and 3 was carried out by X-ray single crystal diffractometer.
Cyclic voltammetry (CV) data of 2 was also compared with earlier synthesized
tetranulcear DNIC [(Imid-iPr)Fe(NO)2]4 (Imid-iPr = 2-isopropylimidazole) [24].
Experimental
Infrared spectra of the nitrosyl m(NO) stretching frequencies were recorded on a
Thermo FT-IR Nicolet iS5/iD1 spectrophotometer/Bruker Alpha spectrophotometer
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A new member of DNICs with 2-mercaptothiazoline ligand
2219
(a)
(b)
Chart 1 a Tautomeric forms of 2-mercaptothiazoline. b Binding modes of thioamide ligand with
M = Fe(NO)2 fragments
with sealed solution cells (0.1 mm) and KBr windows. UV/Vis spectra were
recorded on a Thermo Genesys 10S spectrophotometer. X-band EPR spectrum was
recorded on a Bruker EMX spectrometer equipped with a Hewlett-Packard 5246 L
electronic counter. CV was carried out with a CH Instruments electrochemical
analyzer 611C. A three-electrode system consisted of a glassy carbon working
electrode, a platinum wire auxiliary electrode, and a 0.1 M Ag/Ag? reference
electrode. All CV data were recorded with the scan rate of 0.1 V/s in THF solution
with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. All
potential values are reported versus ferrocene/ferrocenium ion (Fc/Fc?). Analyses
of carbon, hydrogen, and nitrogen were obtained with a CHN analyzer (Heraeus).
Crystallography
Crystallographic data and structure refinements parameters of complexes [(l-
SC3H4SN)Fe(NO)2]4 (2) and [cation][(l-SC3H4SN)2Fe(NO)2]4 (cation = Et4N?
(3a) andPh4P? (3b)) are summarized in the Supplementary Information (Tables S1–
S6). Each 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 2 and 3 were carried out on a Nonius Kappa CCD
and Bruker SMART Apex CCD diffractometers using graphite-monochromated Mo
˚
Ka radiation (k = 0.7107 A). Least-squares refinement of the positional and
anisotropic thermal parameters for the contribution of all non-hydrogen atoms and
fixed hydrogen atoms was based on F2. A SADABS absorption correction was
made. The SHELXTL structure refinement program was employed [31–35].
Graphics and publication materials were plotted out by Mercury v.2.3 [36].
Preparation of [(l-SC3H4SN)Fe(NO)2]4 (2)
An amount of 40 mL of THF solution of freshly prepared 6.0 mmol (CO)2Fe(NO)2
was added to a Schlenk flask containing 2-mercaptothiazoline (0.730 g, 6.0 mmol).
The reaction solution was stirred for 2 h at room temperature and monitored with
FTIR. The IR spectrum showing mNO stretching frequencies at 1,997(s) cm-1 (mCO),
1,750(vs), 1,706(vs) cm-1 (mNO) (THF) was considered to the formation of
(SC3H5SN)(CO)Fe(NO)2 (1). Then, about 80 mL air was injected to the dark-brown
solution at room temperature. After stirring the reaction solution for 8 h at room
temperature, large amounts of dark-brown solid was yielded and dissolved with
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C.-H. Chen et al.
adequate THF. The THF solution was filtered through Celite and dried under
vacuum to afford a dark-brown solid. Then, the solid was washed with 10 mL EtOH
to afford complex 2 (Yield: 0.073 g, 52 %). Diffusion of diethyl ether into CH2Cl2
solution of complex 2 at -20 °C led to dark-brown crystals suitable for single-
crystal X-ray diffraction. IR mNO: 1,789(s), 1,735(vs) cm-1 (mNO) (THF); 1,789(vs),
1,734(s), 1,715(s) cm-1 (mNO) (KBr). Absorption spectrum (THF) [kmax, nm(e,
cm-1M-1)]: 241(35,653), 265(32,918), 342(9,837), 530(3,000). EPR (THF) at
298 K: g = 2.027; at 77 K. g = 2.023. Anal. Calcd for C12H16Fe4N12O8S8: C,
15.39; H, 1.72; N, 17.95. Found: C, 15.72; H, 1.88; N, 17.16.
Preparation of [cation][(SC3H4SN)2Fe(NO)2] (3) (cation = Et4N? (3a), Ph4P?
(3b))
The complex 2 (0.234 g, 0.25 mmol) and [Na][SC3H4SN] (0.141 g, 1.0 mmol)
were loaded into a Schlenk tube and dissolved in THF (10 mL). The reaction
mixture was stirred for 2 h at room temperature and added to a Schlenk flask
containing [Et4N][Br] (0.257 g, 1.2 mmol)/[Ph4P][Br] (0.402 g, 0.95 mmol)). After
being stirred for 5 h at room temperature, the mixture was filtered through Celite to
remove the insoluble solid [Na][Br] and concentrated to 2 mL under vacuum. Then,
diethyl ether was added to the dark-brown solution to precipitate complex 3a (Yield:
0.377 g, 78.0 %)/complex 3b (Yield: 0.619 g, 90 %). Diffusion of Hexane into THF
solution of complex 3a/3b at -20 °C led to dark-brown crystals suitable for single-
crystal X-ray diffraction. Complex 3a: IR mNO: 1,769(s), 1,704(vs) cm-1 (mNO
)
(THF); 1,751(s), 1,677(vs) cm-1 (mNO) (KBr). Absorption spectrum (THF) [kmax
,
nm(e, cm-1M-1)]: 238(16,843), 264(15,588), 345(3,745), 530(1,824). EPR (THF)
at 298 K: g = 2.030; at 77 K. g = 2.027. Anal. Calcd for C14H28FeN5O2S4: C,
34.85; H, 5.85; N, 14.51. Found: C, 35.08; H, 5.85; N, 14.34. Complex 3b: IR mNO
1768(s), 1705(vs) cm-1 (mNO) (THF); 1764(s), 1681(vs) cm-1 (mNO) (KBr).
:
Reaction of [Et4N][(SC3H4SN)2Fe(NO)2] (3a) with (TMEDA)IFe(NO)2
(TMEDA = tetramethylethylenediamine)
The complex 3a (0.097 g, 0.2 mmol) and (TMEDA)IFe(NO)2 (0.072 g, 0.2 mmol)
were loaded into a Schlenk tube and dissolved in THF (10 mL). The reaction
mixture was stirred for 3 h at room temperature and monitored with FTIR. The IR
spectrum showing mNO stretching frequencies at 1,789(s), 1,734(vs) cm-1 (mNO
)
(THF) was assigned to the formation of complex 2. After being filtered through
Celite to separate insoluble [Et4N][I], the filtrate was dried under vacuum to afford a
dark-brown solid. Then, the solid was washed with EtOH to afford complex 2
(Yield: 0.049 g, 58 %).
Reaction of [(l-SC3H4SN)Fe(NO)2]4 (2) with [Na][C5H7N2] (3,5-
dimethylpyrazolate sodium salt)
The complex 2 (0.094 g, 0.1 mmol) and [Na][C5H7N2] (0.047 g, 0.4 mmol) were
loaded into a Schlenk tube and dissolved in hexane (10 mL). The reaction solution
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A new member of DNICs with 2-mercaptothiazoline ligand
2221
was stirred for 48 h at room temperature and monitored with FTIR. The IR
spectrum showing mNO stretching frequencies at 1,806(s), 1,790(vs), 1,738(s),
1,724(vs) cm-1 (mNO) (hexane) was assigned to the formation of complex
[(C5H7N2)2Fe(NO)2]2 (4). Then, the resulting solution was filtered through Celite
and dried under vacuum to afford a dark-brown solid. The dark-brown solid was
washed with MeOH twice to afford complex 1 (Yield: 0.0760 g, 90.0 %).
Reaction of [(l-C5H7N2)Fe(NO)2]2 (4) with 2-mercaptothiazoline
The complex 4 (0.169 g, 0.4 mmol) and 2-mercaptothiazoline (0.195 g, 1.6 mmol)
were loaded into a Schlenk tube and dissolved in THF (5 mL). The reaction solution
was stirred for 48 h at room temperature and monitored with FTIR. The IR
spectrum showing mNO stretching frequencies at 1,787(s), 1,734(vs) cm-1 (mNO
)
(THF) was assigned to the formation of complex 2. Then, 5 mL hexane was added
to the resulting solution to precipitate a dark-brown solid. The solid was washed
with EtOH twice to afford complex 2 (Yield: 0.115 g, 61 %).
Result and discussion
Synthesis
For the preparation of [(l-SC3H4SN)Fe(NO)2]4 (2), addition of one equiv of
HSC3H4SN = 2-mercaptothiazoline to a freshly prepared (CO)2Fe(NO)2 in the
THF solution (Scheme 1) leads to a color change from dark orange to dark brown.
Infrared spectrum shows m(CO): 1,997(m); m(NO): 1,750(s), 1,706(vs) cm-1 in THF,
which was assigned to the formation of (HSC3H4SN)(CO)Fe(NO)2 (1), Fig. 1. Upon
injection of dry O2(g) into the THF solution of 1 under ambient temperature
(Scheme 1), the disappearance of the IR CO band and the blue shift of the NO
stretching frequencies (m(NO): 1,750(s), 1,706(vs) cm-1 of 1 shifts to 1,789(s),
1,734(vs) cm-1 of 2; Fig. 1) were attributed to the oxidation of 1 to 2. In the
meantime, water was detected as the by-product. Complex 2 is light and thermally
stable under N2 atmosphere.
Complex 2 was isolated and recrystallized from CH2Cl2/diethyl ether yielding dark
brown colored crystals suitable for the X-ray analysis. Addition of 4 equiv of
2-mercaptothiazoline sodium salt and [Et4N][Cl] or [Ph4P][Br] to a THF solution of
complex 2 led to red shifts of IR m(NO) stretching frequencies from 1,789(s),
1,734(vs) cm-1 to 1,769(s), 1,704(vs) cm-1, which indicated the formation of anionic
(SC3H4SN)2Fe(NO)-2 , (cation = Et4N? (3a), Ph4P? (3b)) (Scheme 1b). Isolation
and recrystallization from THF/hexanes yielded dark brown crystals of 3. Reaction of
complex 3 with (TMEDA)IFe(NO)2 (TMEDA = tetramethylethylenediamine) in 1:1
stoichiometry in THF at room temperature reproduced complex 2 in good yield.
Furthermore, complex [(l-C5H7N2)Fe(NO)2]2 (4) can be synthesized by reaction of
complex 2 and 4 equiv of [Na][C5H7N2] (3,5-dimethylpyrazolate sodium salt) in THF
solution (Scheme 1c). Interestingly, excess amounts of 2-mercaptothiazoline was
required (ca. molar ratio *1:4) to regenerate complex 2 from 4 (Scheme 1c0) [25].
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C.-H. Chen et al.
Scheme 1 Synthesis of Dinitrosyl Iron Complexes (DNICs) 1, 2, 3 and 4
Fig. 1 Overlaid IR spectra of (CO)2Fe(NO)2, 1 and 2 in THF solution. [filled circle, orange
(CO)2Fe(NO)2: m(CO): 2,092(m), 2,041(s), m(NO): 1,808(s), 1,761(vs) cm-1], [filled square, olive 1:
m(CO): 1,997(m), m(NO): 1,750(s), 1,706(vs) cm-1], [filled diamond, dark brown: 2: m(NO): 1,789(s),
1,734(vs) cm-1]. (Color figure online)
Molecular structures
A view of the ball and stick/capped sticks rendition of the molecular structure and
packing diagram of 2 is shown in Fig. 2a, b, respectively; full thermal ellipsoid plot
of 2 and metric data are provided in the Supplementary Information. The selected
bond distances and angles of complex 2 is similar to the published tetranuclear
DNIC [(Imid-iPr)Fe(NO)2]4 (Imid-iPr = 2-isopropylimidazole) [24] (Table 1). In
both structures: (1) the Fe–N–O units are slightly bent, avg. *165°, and oriented in
toward each other; (2) four irons construct a quadrilateral plane with a very small
diviation of Fe form Fe4 plane; (3) each iron contains two nitrosyl groups and the
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A new member of DNICs with 2-mercaptothiazoline ligand
2223
SC3H4SN- or Imid-iPr- ligands bridge the edge of the Fe4 quadrilateral; and (4) the
Fe4 in complex 2 is more closed to a diamond shape instead of an almost precise
square shape in [(Imid-iPr)Fe(NO)2]4. We also notice that two sets of dihedral
angles of opposite Fe(NO)2 planes are 51.11°, 6.72° of 2 and 12.52°, 11.85° of
[(Imid-iPr)Fe(NO)2]4, which is due to the binding mode of SC3H4SN- being more
flexible/asymmetrical than the Imid-iPr- ligand.
The X-ray diffraction analysis yielded the typical tetrahedral structures of
[cation][(SC3H4SN)2Fe(NO)2], cation = Et4N? (3a) and Ph4P? (3b), which are shown
in Fig. 3. Selected bond distances and angles of complex 3b are shown in Table 1. The
˚
average Fe–NO and N–O bond length of 3b, 1.688(4) and 1.175(5) A, falls in the range
˚
˚
of 1.661(4)–1.695(3) A and 1.160(6)–1.178(3) A, respectively, as observed in the
anionic {Fe(NO)2}9 (Enemark/Felthamelectronic notation [26])DNICs[27]. Itisworth
noting thattwo tautomericforms, thiolate andthioneforms, of2-mercaptothiazolinecan
befoundincomplexes2 and 3, respectively. Wepoint out thattheshorteninginthe a-C–
˚
N bond distance and the elongation of a-C–S bond length from 1.318(5) and 1.673(4) A
˚
for 3 to 1.294(4) and 1.722(3) A for 2 is consistent with the tautomeric thione/amide
form of complex 3 (Chart 1a, left) and the thiolate/imine form of complex 2 (Chart 1a,
right). Sanina et al. [23] have demonstrated the neutral {Fe(NO)2}9 mononuclear DNIC
(SC2H3N3)(SC2H2N3)Fe(NO)2 with two 1,2,4-triazol-3-thione ligands, one S atom is a
negative charge, bonding to the iron atom is covalent and the other S atom is formally a
neutral bond with the iron. Surprisingly, in the anionic complex 3, two ancillary
SC3H4SN- ligands are N bound to the iron in the thione/amide form. Even though both
complex 3 and (SC2H3N3)(SC2H2N3)Fe(NO)2 are in the {Fe(NO)2}9 form based on the
Enemark/Feltham electron count, we suggest that the influence of the non-innocent
properties of NO to the iron center in terms of the oxidation state of Fe in the neutral and
anionic compounds is different and may be the key factor to influence the binding mode
of the SC3H4SN- ligands. Further studies and computational investigations of these
possible linkage isomers will be explored and published in a separate article.
Electron paramagnetic resonance spectral data
The EPR spectra for complexes 2 and 3 (see SI) originate from paramagnetic
{Fe(NO)2}9 units (Enemark/Feltham electronic notation), which at 77 K display a
isotropic signal at g * 2.03 in THF solution, a characteristic of DNICs, and no
hyperfine coupling was observed. Only a few examples contains hyperfine coupling
from nitrogen in paramagnetic {Fe(NO)2}9. Tsai and Liaw et al.[28] published the
complex [(SC6H4-o-NHCOPh)(Imidazole)Fe(NO)2], whose EPR spectrum exhib-
ited a well-resolved nine-line signal which corresponds to N-hyperfine coupling of
two NOs and one imidazole of the {Fe(NO)2}9 unit. Until now, reasons for the
unresolved N-hyperfine couplings of oxidized DNICs are still unclear [29].
CV measurements
The cyclic voltammograms of complexes 2 and 3 were measured in 2 mM solution
with tetrabutylammonium hexafluorophosphate as a supporting electrolyte. All
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C.-H. Chen et al.
Fig. 2 a View of ball and stick (Fe) and capped sticks (N, O, S, C) of [(l-SC3H4SN)Fe(NO)2] (2). Red
O, blue N, orange Fe, yellow S, gray C; H atoms are removed for clarity. b Packing diagram of complex
2. (Color figure online)
˚
Table 1 Selected bond distances (A) and angle (°) for complex 2 [(Imid-iPr)Fe(NO)2]4 and 3b
{Fe(NO)2}9
2
[(Imid-iPr)Fe(NO)2]4 [24]
3b
Fe–NOavg.
N–Oavg.
1.686(3)
1.164(5)
2.000(2)
5.592, 5.617
5.671, 5.604
7.132, 8.689
2.303(1)
112.8(1)
165.0(3)
110.8(1)
–
1.712(1)
1.688(4)
1.175(5)
1.997(3)
–
1.194(1)
Fe–NThiaz./Imid.
FeꢀꢀꢀFe (adj)
2.036(1)
6.230, 6.253
FeꢀꢀꢀFe (cross)
Fe–S
8.898, 8.755
–
–
–
\NNO–Fe–NNO avg.
\Fe–N–O
110.4(1)
164.0(1)
–
108.5(2)
161.1(3)
–
avg.
\S–Fe–S
avg.
\NThiaz./Imid.–Fe–NThiaz./Imid. avg.
110.7(5)
117.7(1)
˚
Avg. deviation of Fe form Fe4 square plane (A)
0.033
0.015
–
Dihedral angles of opposite Fe(NO)2 planes (°)
51.11, 6.72
12.52, 11.85
–
The maximum deviations from the average distances and angles are shown. Full lists of metric parameters
are given in the Supplementary Information
potential values were recorded to the Ag/AgNO3 electrode using glassy carbon
working electrode and are referenced to ferrocene/ferrocenium ion (Fc/Fc?).
For the tetranuclear complex 2, three reductive events are observed at -1.25,
-1.56 and -1.67 V. The reductive events at -1.25 and -1.56 V are associate with
the oxidative events at -0.92 and -1.33 V, respectively. Similar to [(Imid-
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A new member of DNICs with 2-mercaptothiazoline ligand
2225
Fig. 3 Ball and capped sticks representation of the X-ray single crystal structure of complex
[cation][(SC3H4SN)2Fe(NO)2] (cation = Et4N? (3a) and Ph4P? (3b)). Red O, blue N, orange Fe,
yellow S, gray C, sienna P; H atoms are removed for clarity. (Color figure online)
iPr)Fe(NO)2]4 [24, 30], three reductive events are shown at -1.29, -1.67, and
-1.98 V. Again, the -1.29 and -1.67 V associated to oxidative events -1.09 and
-1.41 V, respectively, Fig. 4. The pattern of these multiple redox couples are
consistent with the previously reported tetrametallic CV data (Fig. 4a). The CV scan
of compound 3, measured on THF solution, displays an irreversible electrochemical
response within the THF solvent window with Epc at -1.66 V and Epa at -1.35 and
-0.96 V. These complicated events suggest somewhat that complex 3 is unstable
when the redox reaction occurred.
(a)
(b)
Fig. 4 Redox potential values for complex 2, [(Imid-iPr)Fe(NO)2]4 and 3. a In CH2Cl2 solution. b In
THF solution. c Redox potentials from Ref. [30]. d All the experiments were measured by using a glassy
carbon working electrode and Pt counter electrode. Redox potentials referenced to Fc/Fc? at 100 mV/s
scan rate. e This work
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C.-H. Chen et al.
Conclusions
A new tetrameric complex 2 forms a molecular diamond shape; each edge was
comprised of a 2-mercaptothiazoline ligand bridging to the {Fe(NO)2} units; to our
knowledge, only a few tetranuclear DNIC have been reported prior to this study.
However, many heterocyclic analogs of 2-mercaptothiazoline ligand have been
adopted to synthesize DNICs, but all the structures were characterized in the
dinuclear [19–23], a variation of Roussin’s red esters (RRE), or a mononuclear
S-bound DNIC [23]. In this paper, we report DNICs with 2-mercaptothiazoline
ligand in the tetranuclear and monoulear N-bound forms, but whether 2-mercapto-
thiazoline could serve as an inhibitor to the cancer cell or how NO releases from
DNICs may be the following challenge to be explored.
Acknowledgments We gratefully acknowledge financial support from the National Science Council of
Taiwan. The authors thank Dr. Gene-Hsiang Lee for the single-crystal X-ray structural determinations.
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