Synthesis of dinitrosyl iron complexes (DNICs) with intramolecular hydrogen bonding

Article abstract of DOI:10.1016/j.jorganchem.2008.08.034

HSCH₂CONHCH₃ and HSCH₂CON(CH₃)₂ containing a peptide bond are prepared for the synthesis of DNICs with/without intra-molecular hydrogen bonding, respectively. The IR ν(NO) bands of [(NO)₂Fe(SCH₂CONHCH₃)₂]^- (2) appears at 1751, 1700 cm^-1. In complex 2, the presence of intramolecular [NH?S] hydrogen bonding was verified by the observation of IR spectroscopy with N-H stretching frequency 3334 cm^-1 (CDCl₃) and subsequently confirmed by single-crystal X-ray diffraction showing N-S distance of 2.94 A?. Complex 2 displays the rhombic EPR spectrum with g₁ = 2.039, g₂ = 2.031 and g₃ = 2.013 at in frozen H₂O. Complexes 2 and 3 rapidly release NO when exposed to light. The time needed for photolysis reactions of 2 is two times faster than that of 3 in less polar solvent. Representative time courses for the photolability of 2 and 3 in THF display the NO-off ability: 2 > 3.

Full text of DOI:10.1016/j.jorganchem.2008.08.034

Journal of Organometallic Chemistry 693 (2008) 3582–3586
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of dinitrosyl iron complexes (DNICs) with intramolecular
hydrogen bonding
*
Show-Jen Chiou , Chien-Chu Wang, Chih-Ming Chang
Department of Applied Chemistry, National Chiayi University, No. 300 Syuefu Road, Chiayi 600, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 April 2008
Received in revised form 25 July 2008
Accepted 12 August 2008
Available online 3 September 2008
HSCH2CONHCH3 and HSCH2CON(CH3)2 containing a peptide bond are prepared for the synthesis of DNICs
with/without intra-molecular hydrogen bonding, respectively. The IR m(NO) bands of [(NO)2Fe(SCH2CON-
HCH3)2]ꢀ (2) appears at 1751, 1700 cmꢀ1. In complex 2, the presence of intramolecular [NHꢁ ꢁ ꢁS] hydro-
gen bonding was verified by the observation of IR spectroscopy with NꢀH stretching frequency
3334 cmꢀ1 (CDCl3) and subsequently confirmed by single-crystal X-ray diffraction showing NꢀS distance
of 2.94 Å. Complex 2 displays the rhombic EPR spectrum with g1 = 2.039, g2 = 2.031 and g3 = 2.013 at in
frozen H2O. Complexes 2 and 3 rapidly release NO when exposed to light. The time needed for photolysis
reactions of 2 is two times faster than that of 3 in less polar solvent. Representative time courses for the
photolability of 2 and 3 in THF display the NO-off ability: 2 > 3.
Keywords:
DNIC
Hydrogen bond
Photolysis reaction
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
thesis, structure and characterization of [PPN][(NO)2Fe(SCH2CON-
HCH3)2] (2) and [PPN][((NO)2Fe(SCH2CON(CH3)2] (3). Structural
Nitric oxide (NO) is one of the most important small molecules
in physiology [1–3]. It is produced in a variety of mammalian cells
by nitric oxide synthases (NOSs). As described as a ‘‘double-edged
sword”, it acts as critical endogenous regulator of blood flow,
aggregation of platelet and a neurotransmitter at physiological lev-
els (nano-molar); however, at higher levels (micro-molar), it is
cytotoxic. Dinitrosyliron complexes (DNICs) and S-nitrosothiols
(RSNOs) have been claimed as NO carriers that prolong its lifetime
and preserve its biological activities for physiological functions.
DNICs are classified into two categories, low-molecular-weight
and protein-bound DNICs. LMW-DNICs are further classified by
their electronic structures as EPR-active [L2Fe(NO)2]9, EPR-silent
[L02Fe(NO)]10 (L = thiolate, L0 = neutral ligands) and dimeric EPR si-
lent/-active DNICs [4–6]. Li et al. reported the X-ray structure of
DNIC with 1-methylimidazoles showing the diagnostic EPR signal
of DNICs, g = 2.03 [4–7]. Darensbourg et al. employed N2S2 ligands
mimicking the Cys–X–Cys motif to show that Fe(NO)2 acts as an
unit bound by different thiolate ligands [8]. Recently, Liaw and
coworkers synthesized a series of LMW DNICs, [S5Fe(NO)2]ꢀand
[(SR)2Fe(NO)2]ꢀ (SR = thiolates) and showed the NO-releasing abil-
ity of DNICs modulated by the coordinated thiolates [9–13]. In
addition to the NO-releasing ability regulation afforded by the
coordinated ligands, the effect of hydrogen bonding commonly
found in biology may further tune NO release. To probe the effect
of hydrogen bonding in this regard, in this paper we report the syn-
and spectroscopic data establish that the former complex contains
an intramolecular NꢀHꢁ ꢁ ꢁS bond. We have concluded that the NO-
release ability of DNICs is significantly regulated by the intramo-
lecular hydrogen bonding.
2. Experimental
All reagents were purchased from commercial sources and used
as received, unless otherwise noted. Solvents were distilled under
N2 and dried as indicated. THF and hexanes were freshly distilled
over Na/benzophenone, diethyl ether over CaH2, acetonitrile over
CaH2/P2O5, methylene chloride over CaH2 and methanol from
Mg/I2. N-methylmercaptoacetamide [14,15] and N,N-dimethyl
mercaptoacetamide [14,15] were prepared as described previ-
ously. GSH-DNICs (GSH = Glutathione) was prepared using the pre-
viously reported methods [16]. Manipulations involving
organometallic reagents were performed under a nitrogen atmo-
sphere on a Schlenk line. Infrared spectra of the mNO stretching fre-
quencies were recorded on a Perkin–Elmer model spectrum One B
spectrometer with sealed solution cells (0.1 mm, KBr windows).
UV–Vis were recorded on a Cintra 202 spectrometer. The collected
data were analyzed by Excel/Origin program. Analyses of carbon,
hydrogen, and nitrogen were obtained with a CHN analyzer
(Heraeus). Photolysis reactions were carried out in a 50 mL,
water-cooled quartz reactor equipped with 16 mercury arc 8-W
UV lamps (352 nm) outside the reactor. EPR spectra were recorded
at X-band (9.5 GHz) by using a Bruker X-band E500CW spectrom-
eter. The experiments at 77 K, the sample temperature was
* Corresponding author. Tel.: +886 963428025; fax: +886 52717901.
E-mail address: genechiou@mail.ncyu.edu.tw (S.-J. Chiou).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.08.034

S.-J. Chiou et al. / Journal of Organometallic Chemistry 693 (2008) 3582–3586
3583
maintained at –196 °C by immersion of the EPR sample tube into a
hexane. Yield: 131 mg (76%). IR (THF):
m
/cmꢀ1 = 1743(s, NO),
liquid nitrogen finger Dewar. Typical EPR measurement conditions
were as follows: microwave frequency, 9.5 GHz; microwave
power, 2 mW; modulation frequency, 100 kHz; modulation ampli-
tude, 0.5 mT; receiver gain, 6.32 ꢂ 103.
1693(vs, NO), 1666(s, CO). UV–Vis (THF) nm (e
, Mꢀ1 cmꢀ1): 267
(13809), 367 (3286), 429 (1393), 768 (209). Anal. Calc. for
C42H42FeN5O4P2S2: C, 58.40; H, 4.90; N, 8.07. Found: C, 57.82; H,
5.45; N, 7.72%.
Caution. N-methylmercaptoacetamide and N,N-dimethyl mer-
captoacetamide are flammable liquids that present a pungent
odour. The deprotonation reaction should be vented through an
aqueous solution of NaOCl and NaOH.
2.5. Preparation of [PPN][(NO)2Fe(SCH2CON(CH3)2)2] (3)
[(NO)2Fe(SCH2CON(CH3)2)2Fe(NO)2] (70 mg, 0.15 mmol) and
[PPN][SCH2CON(CH3)2] (202 mg, 0.31 mmol) were combined in
THF (25 mL). After stirring for 30 min, the solution was filtered
and the solvent was removed under reduced pressure. The result-
ing red brown solid was purified by recrystallization from THF/
Et2O/hexane. Yield: 160 mg (60.4%). FTIR (THF) 1730(s),
2.1. Preparation of [PPN][SCH2CONH(CH3)]
NaSCH2CONHCH3 (70 mg, 0.55 mmol) and PPNCl (287 mg,
0.5 mmol) were dissolved in 25 mL MeOH and refluxed for 4.5 h.
The solvent was removed under reduced pressure. The resulting
residue was dissolved in CH3CN. After stirring for 1 h, the solution
was filtered and the solvent removed under reduced pressure. The
resulting white solid was purified by recrystallization from CH3CN/
Et2O. Yield: 75%. 1H NMR (DMSO-d6) d 2.54 (CH3, d), 2.85 (CH2, s),
7.72 ꢃ 7.53 (PPN, m), 8.88 (NH, b).
1686(vs) cmꢀ1
M
(m (mCO). UV–Vis (THF) nm (e,
NO), 1642(s) cmꢀ1
ꢀ1 cmꢀ1): 267 (17191), 368 (3440), 429 (1067), 801 (223). Anal.
Calc. for C44H46FeN5O4 P2S2: C, 59.33; H, 5.20; N, 7.86. Found: C,
59.95; H, 5.16; N, 7.26%.
2.6. Crystallographic structural determinations
Crystal data collection and refinement parameters are given in
Table 2. Table 3 shows the atomic coordinates of complexes 1a
and 2b0. Typically, the crystals were removed from the vial with
a small amount of mother liquor and immediately coated with sil-
icon grease on a weighing paper. A suitable crystal was mounted
on a glass fiber with silicone grease and placed in the cold stream
2.2. Preparation of [PPN][SCH2CON(CH3)2]
NaSCH2CON(CH3)2 (78 mg, 0.55 mmol) and PPNCl (287 mg,
0.5 mmol) were dissolved in 25 mL MeOH and refluxed for 4.5 h.
The solvent was removed under reduced pressure. The resulting
mixtures were dissolved in CH3CN. After stirring for 1 h, the solu-
tion was filtered and the solvent removed under reduced pressure.
The resulting white solid was purified by recrystallization from
CH3CN/Et2O. Yield: 75%. 1H NMR (DMSO-d6) d 2.65 (CH3, s), 2.74
(CH2, s), 3.12(CH3, s), 7.72 ꢃ 7.53 (PPN, m).
of a Bruker APEX II with graphite-monochromated Mo Ka radiation
(k = 0.71073 Å) at 150(2) K.
3. Results and discussion
To mimic small peptide with cysteine residues and to establish
the desired intra/inter molecular hydrogen bonding within the iron
complexes, N-methylmercaptoacetamide was synthesized by reac-
tion of ethyl 2-mercaptoacetate and methylamine [14–15]. Reac-
tion of Fe(CO)2(NO)2 and N-methylmercaptoacetamide yields the
Roussin0s red ester (RRE) [(NO)2Fe(SCH2CONH(CH3))2Fe(NO)2] (1)
(Scheme 1a) characterized by IR and UV–Vis spectroscopies. The
2.3. Preparation of [(NO)2Fe(SCH2CONH(CH3))2Fe(NO)2] (1)
[PPN][Fe(CO)3(NO)] (355 mg, 0.5 mmol) and NOBF4 (140 mg,
1.2 mmol) were combined in 25 mL THF at 0 °C and stirred for
15 min. The filtrate was added into a 10 mL THF solution of the
thiol [PPN][SCH2CONH(CH3)] (643 mg, 1 mmol). After stirring for
1 h, the solution was filtered and the solvent was removed under
reduced pressure. The resulting black–brown solid was purified
by recrystallization from THF/Et2O. Yield: 268 mg (61%). IR (THF):
dinuclear complex 1 exhibits diagnostic IR bands at
(w), 1778 (vs), 1750 (vs) and
(CO) = 1686 (s) cm–1 with
(NO) = 28 cmꢀ1 (the separation of 1778 and 1750 cmꢀ1) in
THF. Consistent with RRE [(NO)2Fe(SC6H4NHCOPh)2Fe(NO)2] dis-
m(NO) = 1810
m
D
m
m
/cmꢀ1 = 1810(w, NO), 1778(vs, NO), 1750(vs, NO), 1686(s, CO).
UV–Vis (THF) nm
Mꢀ1 cmꢀ1): 218(58505), 312(3634),
(e,
playing (m(NO) = 1816 (w), 1787 (vs), 1759 (vs) and m(CO) = 1686
(s) cmꢀ1 (THF), complex 1 is described as a {Fe(NO)2}9–{Fe(NO)2}9
coupled dimer, As such it is diamagnetic, no EPR signal is observed
[11].
359(9596), 750(31). Anal. Calc. for C6H12Fe2N6O6S2: C, 16.38; H,
2.75; N, 19.10. Found: C, 16.25; H, 3.05; N, 18.85%.
2.4. Preparation of [PPN][(NO)2Fe(SCH2CONHCH3)2] (2)
Cleavage of the thiolate bridges in 1 by addition of 2 equiv. of
[PPN][SCH2CONHCH3] led to the formation of the dinitrosyliron
complex [PPN][(NO)2Fe(SCH2CONHCH3)2] (2) (Scheme 1b) [12].
Alternatively, oxidation of Fe(CO)2(NO)2 by NOBF4 followed by
addition of [PPN][SCH2CONHCH3] also yielded complex 2 (Scheme
1c). Presumably, [NO]+ oxidation of {Fe(NO)2}10 Fe(CO)2(NO)2
yields the {Fe(NO)2}9 [Fe(CO)2(NO)2]+ intermediate [7], with subse-
[(NO)2Fe(SCH2CONH(CH3))2Fe(NO)2] (91.2 mg, 0.2 mmol) and
[PPN][SCH2CONHCH3] (257 mg, 0.4 mmol) were combined in THF
(25 mL). After stirring for 30 min, the solution was filtered and
the solvent was removed under reduced pressure. The resulting
red brown solid was purified by recrystallization from THF/Et2O/
Scheme 1.

3584
S.-J. Chiou et al. / Journal of Organometallic Chemistry 693 (2008) 3582–3586
quent ligand-substitution between [PPN][SCH2CONHCH3] and
[Fe(CO)2(NO)2]+ producing complex 2. Brown red crystals of 2 ex-
hibit IR absorptions at 1740, 1695, (m(NO)) and 1664 (m
(CO)) cm–1
and UV–Vis spectrum shows three intense absorption bands at
367, 417, 747 nm in THF. EPR spectrum of complex 2 displays an
isotropic EPR signal at g = 2.028 at 298 K and the rhombic signal
with g1 = 2.039, g2 = 2.029 and g3 = 2.014 at 77 K in THF. Complex
2
displays a rhombic signal with g1 = 2.039, g2 = 2.031 and
g3 = 2.013 in frozen (and degassed) H2O (Fig. 3), consistent with
that of GSH-DNICs (GSH = Glutathione) [16]. Complex 2 is stable
in the degassed and deionized water. It is noted that the water-sol-
ubility of anionic {Fe(NO)2}9 DNICs is reportedly limited thus, re-
lated biological studies of complex
2 are ongoing in our
laboratory. The single-crystal X-ray structure of complex
2
(Fig. 1) shows the coordination sphere of Fe is pseudo tetrahedral
based on the NꢀFeꢀN and SꢀFeꢀS bond angles of 117.0° and
110.4°, respectively. The average FeꢀN(O) and NꢀO bond distances
of complex 2 are 1.665(3) Å and 1.187(3) Å, respectively. Average
FeꢀN(O) bond distance falls in the range of 1.695(3)ꢀ1.661(6) Å
and the average NꢀO bond distance is within the range of
1.178(3)ꢀ1.160(6) which is corresponding to the characteristics
of mononuclear {Fe(NO)2}9 DNICs [9,13].
The average FeꢀS bond lengths and IR stretching frequencies of
several thiolate ligated DNICs are listed in Table 1. The mean value
of the FeꢀS bond lengths in complexes 2, [PPN][(2-S-(C4H3S)2Fe
(NO)2], [PPN][(SC6H4-o-NHCOCH3)2Fe(NO)2] and [PPN][(SEt)2Fe
(NO)2] are 2.292(1), 2.296(1), 2.301(1) and 2.275(2), respectively.
The average FeꢀS bond length of complex 2, longer than that of
[PPN][(SEt)2Fe(NO)2], is presumably attributed to the intramolecu-
lar [NꢀH. . .S] interaction (N. . .S distance of 2.9721(26) Å in com-
plex 2) (Fig. 1) [17–19]. The Nꢁ ꢁ ꢁS distance as short as 2.97 Å has
been observed in bent NHꢁ ꢁ ꢁS hydrogen bonds such as those in
o-amino thiolate [20–22]. Zuppiroli et al. [23,24] have studied
HSCH2CONHCH3 with IR in CCl4 and the results show the ligand fa-
vored the cyclization as a five membered ring including the proton
on nitrogen. In the solid state, HSCH2CONHCH3 shows a tendency
to form the interligand NꢀHꢁ ꢁ ꢁO hydrogen bond, e.g. as found in
Hg(SR)2 (SR = SCH2CONHCH3). The X-ray structure of Mo[BH-
(Me2pz)3](NO)(SR)2 (SR = SCH2CONHCH3) synthesized by Walter
et al. shows the existence of both an intramolecular hydrogen bond
Fig. 2. ORTEP drawing and labeling scheme of {[Na][(NO)2Fe(SCH2CON
(CH3)2)2]} ꢁ 2{[Na][SC7H4SN]} (30ꢁ2{[Na][SC7H4SN]}). Selected bond distances (Å)
and angles (°): Fe(1)–N(1) 1.6870(15); Fe(1)–S(1) 2.2998(5); N(1)–O(1) 1.1715(19);
N(3)–Na(1) 2.4597(15); Na(1)–O(2) 2.7014(13); Na(1)–S(1) 2.9454(8); Na(1)–S(2)
3.0131(8); Na(1)–Na(2) 3.2646(10); Na(2)–O(3) 2.3590(14); Na(2)–O(2)
2.5043(13); Na(2)–S(2) 2.9410(5); N(1)–Fe(1)–S(1) 112.76(5); O(1)–N(1)–Fe(1)
164.84(15); N(3)–Na(1)–O(2) 88.84(4); N(3)–Na(1)–S(1) 93.46(4); O(2)–Na(1)–S(1)
71.59(3); N(3)–Na(1)–S(2) 58.27(4); O(2)–Na(1)–S(2) 90.72(3); S(1)–Na(1)–S(2)
147.54(3); O(3)–Na(2)–O(2) 167.46(5); O(3)–Na(2)–S(2) 78.54(3); O(3)–Na(2)–S(2)
78.54(3).
Fig. 3. EPR spectra of complexes 2, 3, and GSH-DNIC at frozen. (– : 2; : GSH-DNIC;
: 3)
NꢀHꢁ ꢁ ꢁS and an intermolecular hydrogen bond NꢀHꢁ ꢁ ꢁO; further-
more, the NꢀHꢁ ꢁ ꢁO linkage is likely disrupted in solution [14–15].
Herein, the structure of complex 2 reveals only an intramolecular
hydrogen bond NꢀHꢁ ꢁ ꢁS supported by the single IR
m(NH) at
Fig. 1. ORTEP drawing and labeling scheme of [((NO)2Fe(SCH2CONHCH3)2]– (2).
Selected bond distances (Å) and angles (°): Fe(1)–N(1) 1.661(3); Fe(1)–N(2)
1.670(3); Fe(1)–S(1) 2.2745(10); Fe(1)–S(2) 2.3093(10); N(1)–O(1) 1.182(3);
N(2)–O(2) 1.192(3); N(1)–Fe(1)–N(2) 116.99(13); N(1)–Fe(1)–S(1) 110.37(9);
S(1)–Fe(1)–S(2) 110.39(4); O(1)–N(1)–Fe(1) 168.7(3); O(2)–N(2)–Fe(1) 68.2(2).
1
3334 cmꢀ in CDCl3 corresponding to the hydrogen-bonded NꢀH
group [14–15]. The remarkable solubility of 2 in water may be
attributed to the hydrogen bond acceptor ability of the carbonyl
of amide group.

S.-J. Chiou et al. / Journal of Organometallic Chemistry 693 (2008) 3582–3586
3585
Table 1
Selected Fe–S bond lengths and IR stretching frequencies for the anionic thiolate
ligated {Fe(NO)2}9 DNICs
Anionic thiolate ligated
{Fe(NO)2}9 DNICs
mNO
D
mNO
Fe–S (Å) Reference
2.292(1) This work
(cmꢀ1
THF)
,
(cmꢀ1
)
2
1740,
1695
1730,
1686
1743,
1698
1752,
1705
1715,
1674
45
44
45
47
51
3
–
This work
[PPN][(2-S-(C4H3S)2Fe(NO)2]
2.296(1) [10]
2.301(1) [10]
2.275(2) [17]
[PPN][(SC6H4-o-
NHCOCH3)2Fe(NO)2]
[PPN][(SEt)2Fe(NO)2]
Fig. 4. Continuous monitoring the decay of absorbance values at 768, 801, and
775 nm of 2 (ꢀ), 3 (j) and 30 (N), respectively, in THF at 25 °C. The concentration of
complexes 2, 3 and 30 is 6.25 ꢂ 10ꢀ4 M.
Fig. 2 displays single-crystal X-ray structure of complex 30 co-crys-
tallized with two thiolates [Na][SC7H4SN] and two THF molecules.
The interactions of Na+ and the donor atoms, O, S, and N, in com-
plex 30 may serve to stabilize the structure. The cation exchange
reaction from [PPN]+ to [Na]+ was conducted to synthesized com-
plex 20.
Table 2
Selected IR stretching frequencies of complex 2, 20, 3 and 30 in THF
Complexes
cm–1
2
1740(s), 1695(vs), 1664(s), 1589(w)
1744(s), 1697(vs), 1663(b)
1730(s), 1686(vs), 1642(s), 1589(w)
1742(s), 1698(vs), 1637(b)
20
3
In the dark, both 2 and 3 are stable for weeks in the solid
state and solutions of CH2Cl2, THF and CH3CN. However, when
exposed to visible light, both complexes decompose gradually.
Besides the insoluble black powder, the IR spectrum show there
is no band of NO but the carbonyl group of thiolate in solution.
To compare the photolability of the bound NO, the photolysis
reactions of complexes 2 and 3 were conducted in CH2Cl2, THF
and CH3CN and DMSO. All reactions were monitored by FTIR
and UV–Vis spectrometry. The times needed for the reactions
to reach completion are collected in Table 3. Complex 2 releases
NO in the less polar solvents CH2Cl2 and THF about 2.5 times
faster than 3. Furthermore, the reaction times are similar in
more polar solvents, i.e. CH3CN and DMSO. These results suggest
a role for the intramolecular hydrogen bonding in 2 may en-
hance NO release in the less polar solvents and as the effect of
hydrogen bond is diminished in polar solvents, the rates of NO
release become similar. For complex 3, the NO release seems
to be dominated by solvent effects as well. In addition, based
on the structure of 30, the cation, Na+ is sustained by S and O
from ligands. The Na+ shares the electron density on S and
makes the time needed for NO release as faster as 2.
30
Table 3
Time required for photolysis reactions
Complex
CH2Cl2
THFa
CH3CNa
DMSOb
a
2
3
30
180( 5)
300( 5)
180( 6)
210( 4)
300( 5)
210( 7)
210( 6)
210( 5)
150( 7)
180( 4)
180( 4)
180( 3)
a
Determined by FTIR.
Determined by UV–Vis (unit: minute).
b
In contrast to the existence of intramolecular [NꢀHꢁ ꢁ ꢁS] hydro-
gen bonding observed in complex 2, the dinitrosyl complex
[PPN][(NO)2Fe(SCH2CON(CH3)2)2] (3) with no hydrogen-bonding
capacity was synthesized. As reported by Liaw and coworkers, li-
gand-substitution reaction occurred upon addition of the stronger
nucleophile ([SR0]ꢀ, R0 = SC6H4-o-NHCOCH3, Ph or 2-S-C4H3S) into
THF solution of [PPN][(NO)2Fe(SC7H4SN)2] leading to the formation
of complex [PPN][(NO)2Fe(SR0)2] [9–13]. Complex 3 was isolated
either from reaction of [PPN][(NO)2Fe(SC7H4SN)2] and
[PPN][SCH2CON(CH3)2] (N,N-dimethylmercapto-acetamide was
obtained from reaction of ethyl 2-mercaptoacetate and dimethyl-
amine) [14–15] in THF or the reaction as synthesis of complex 1
Another experiment to elucidate the effect of hydrogen bonding
and cation Na+ is the photodecomposition reactions monitored by
UV–vis spectrometry. Representative time courses for the photola-
bility of 2, 3 and 30 in THF are shown in Fig. 4. It displays the NO-off
ability: 2 ꢄ 30 > 3 (The slope of 2 ꢄ 30 is 20 ꢂ 10–5 and 7.0 ꢂ 10–5
for 3). The result is resembled to those reported by Liaw et al.
[10]; the less electron-donating, coordinated thiolate ligands, the
better NO-donor ability.
via
RRE,
[(NO)2Fe(SCH2CON(CH3)2)2Fe(NO)2]
(4)
and
[PPN][SCH2CON(CH3)2]. The ESI MS spectrum shows a M peak at
352 m/z which clearly indicates complex 3 with the stoichiometry
as [PPN][(NO)2Fe(SCH2CO N(CH3)2)2]. The present of NO group is
also supported by the appearance of the peak at 322 (M–NO).
The IR stretching frequencies of complex
(NO) = 1730 (vs), 1686 (vs) and
(CO) = 1642 (w) cm–1. The IR
stretching frequencies of complexes 2, 20, 3 and 30 are summarized
in Table 2. The IR
(NO) of complex 3 shifting by 10 cm–1, in com-
3
appear at
4. Conclusions
m
m
The water soluble complex [PPN][(NO)2Fe(SCH2CONHCH3)2] (2)
with intramolecular hydrogen bonding and [PPN][(NO)2Fe(SCH2-
CON(CH3)2] (3) were synthesized. The [NꢀHꢁ ꢁ ꢁS] intramolecular
interaction in complex 2 was characterized by means of IR spec-
troscopy and single-crystal X-ray diffraction. Complexes 2 and 3
exhibit rhombic EPR signal with gav = 2.03, the characteristic of
GSH-DNIC in biological system. The notable water solubility of 2
and 3 may be caused by the intermolecular hydrogen bonds
between these complexes and water. Both complexes are
photolabile and rapidly release NO upon illumination with visible
or UV light. The more efficient NO-releasing ability of complex 2,
compared to that of complex 3, is attributed to the existence of
[NꢀHꢁ ꢁ ꢁS] intramolecular hydrogen bonding. The cation Na+ also
m
parison with that of complex 2, indicates the intramolecular
hydrogen bonding in 2 decreases the electron back donation from
*
iron d
p
to NO
p
orbitals. Interestingly, the EPR spectrum of com-
plex 3 displays an isotropic EPR signal at g = 2.027 at 298 K and a
rhombic signal with g1 = 2.041, g2 = 2.032 and g3 = 2.012 at 77 K
in THF. The EPR spectrum of complex 3 displays a rhombic signal
with g1 = 2.039, g2 = 2.031 and g3 = 2.013 in frozen (degassed)
H2O. These results are similar to complex 2 and GSH-DNIC
(Fig. 3). In addition, reaction of [PPN][(NO)2Fe(SC7H4SN)2] and
[Na][SCH2CON(CH3)2] in THF solution at ambient temperature re-
sulted in the formation of [Na][(NO)2Fe(SCH2CON(CH3)2)2] (30).

3586
S.-J. Chiou et al. / Journal of Organometallic Chemistry 693 (2008) 3582–3586
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plays the similar role to affect NO release. The results here imply
that protein-bound DNICs preserve the sensitive {Fe(NO)2}9 core
in a hydrophobic environment and employ hydrogen bonding to
further tune the condition of NO release. On the other hand, Na+,
an essential ion in biological system, may have a similar influence
as hydrogen bonding while a hydrophilic environment dominates.
Acknowledgements
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We gratefully acknowledge financial support from the National
Science Council (Taiwan). We thank Professor Wen-Feng Liaw and
Charles G. Riordan for the discussion. Professor Steve Yu for mea-
surement of EPR and Professor Hon-Man Lee and Miss Chun-Yu
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Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2008.08.034.
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