Transformation and structural discrimination between the neutral {Fe(NO)2}10 dinitrosyliron complexes (DNICs) and the anionic/cationic {Fe(NO)2}9 DNICs

Article abstract of DOI:10.1021/ic0605120

Reaction of Fe(CO)₂(NO)₂ and sparteine/ tetramethylethylenediamine (TMEDA) in tetrahydrofuran afforded the electron paramagnetic resonance (EPR)-silent, neutral {Fe(NO)₂}¹⁰ dinitrosyliron complexes (DNICs) [(spa

Full text of DOI:10.1021/ic0605120

Inorg. Chem. 2006, 45, 6041−6047
Transformation and Structural Discrimination between the Neutral
10
{
{
Fe(NO)2
Fe(NO)2
}
}
Dinitrosyliron Complexes (DNICs) and the Anionic/Cationic
DNICs
9
Mu-Cheng Hung,† Ming-Che Tsai,† Gene-Hsiang Lee,‡ and Wen-Feng Liaw*,†
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan, and
Instrumentation Center, National Taiwan UniVersity, Taipei, Taiwan
Received March 27, 2006
Reaction of Fe(CO)2(NO)2 and sparteine/tetramethylethylenediamine (TMEDA) in tetrahydrofuran afforded the electron
10
paramagnetic resonance (EPR)-silent, neutral
{
Fe(NO)2
}
dinitrosyliron complexes (DNICs) [(sparteine)Fe(NO)2]
(1) and [(TMEDA)Fe(NO)2] (2), respectively. The stable and isolable anionic
{
Fe(NO)2
}
9 DNIC [(S(CH2)3S)Fe(NO)2]-
-
(4), with a bidentate alkylthiolate coordinated to a
{
Fe(NO)2
}
motif, was prepared by the reaction of [S(CH2)3S]2
9
and the cationic
{
Fe(NO)2
}
[(sparteine)Fe(NO)2]+ (3) obtained from the reaction of complex 1 and [NO][BF4] in
CH3CN. Transformation from the neutral complex 1 to the anionic complex 4 was verified via the cationic complex
3. Here complex 3 acts as an Fe(NO)2 -donor reagent in the presence of thiolates. The EPR spectra of complexes
2.032 and 2.031 at 298 K, respectively, the characteristic g value of
N(O) bond lengths of the single-crystal X-ray structures of the Fe-
core of DNICs can be unambiguously assigned.
{
}
3 and 4 exhibit an isotropic signal with g
)
9
{
Fe(NO)2
}
DNICs. On the basis of N
−O/Fe
−
{
9
10
(NO)2
}
/
{
Fe(NO)2
}
DNICs, the oxidation level of the
{
Fe(NO)2
}
The mean N
range of 1.650(7)
distances ranging from 1.178(3) to 1.160(6) Å and the mean Fe
1.661(4) Å are assigned as the anionic/neutral/cationic Fe(NO)2
−
O distances falling in the range of 1.214(6)
−
1.189(4) Å and the Fe
−
N(O) bond distances in the
10
−1.638(3) Å are assigned as the neutral
{
Fe(NO)2
}
DNICs. In contrast, the mean N−O bond
−
N(O) bond distances ranging from 1.695(3) to
9
{
}
DNICs. In addition, an EPR spectrum in
combination with the IR νNO (the relative position of the νNO stretching frequencies and their difference ∆νNO
)
spectrum may serve as an efficient tool for discrimination of the existence of the anionic/cationic/neutral Fe-
{
9
10
(NO)2
}
DNICs and the neutral
{
Fe(NO)2
}
DNICs.
Introduction
S-nitrosothiols or protein-bound DNICs.1,2 As observed in
cells or tissues, LMW-DNICs exerting cyclic GMP (gua-
nosine monophosphate)-independent effects were attributed
to nitrosylating modification of proteins via the transfer of
NO or a Fe(NO)2 unit, yielding protein/S-nitrosothiols or
protein-bound DNICs.3,4 In particular, the protein-bound
DNICs were characterized by X-ray crystallography through
nitrosylation of human glutathione transferase P1-1 with a
dinitrosyldiglutathionyliron complex in vitro/in vivo re-
cently.4
Dinitrosyliron complexes (DNICs) have been suggested
as intermediates of Fe-catalyzed degradation and formation
of S-nitrosothiols (RSNO) and as one of two possible forms
for storage and transport of NO in biological systems.1
Depending on the microenvironment, low-molecular-weight
DNICs (LMW-DNICs) can provide at least two types of
nitrosylating modifications of proteins, forming either protein/
* To whom correspondence should be addressed. E-mail:
wfliaw@mx.nthu.edu.tw.
As has been known, characterization of both protein-bound
and LMW-DNICs in vitro has been made possible via their
† National Tsing Hua University.
‡ National Taiwan University.
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Lorkovic, I. M. Chem. ReV. 2002, 102, 993-1017. (d) Butler, A. R.;
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10.1021/ic0605120 CCC: $33.50
Published on Web 06/22/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 15, 2006 6041

Hung et al.
distinctive electron paramagnetic resonance (EPR) signals
at g ) 2.03.1-5 To our knowledge, also known in inorganic
chemistry is the precedents for small-molecule DNICs in four
oxidation levels of the {Fe(NO)2} unit, including the EPR-
active, anionic {Fe(NO)2},9 neutral {Fe(NO)2},9 and cationic
{Fe(NO)2}9 DNICs as well as the EPR-silent, neutral {Fe-
(NO)2}10 DNICs coordinated by CO, PPh3, and N-containing
ligands.6-10 Here the electronic structure/state of the M(NO)2
unit of DNICs is generally designated as {M(NO)2}n (M )
transition metal).7 This formalism {M(NO)2}n invokes the
Enemark-Feltham notation, which stresses the well-known
covalency and delocalization in the electronically amorphous
M(NO)2 unit.7
Recently, we have shown that the detailed spectroscopic
analysis (EPR and IR νNO spectra) may provide a superior
level of insight on discrimination of the anionic {Fe(NO)2}9
DNICs, neutral {Fe(NO)2}9 DNIC [(SC6H4-o-NHCOPh)(1-
MeIm)Fe(NO)2] (1-MeIm ) 1-methylimidazole), and Rous-
sin’s red ester10a and that the reversible transformation of
complex [S5Fe(NO)2]- to the [S5Fe(µ-S)2FeS5]2- cluster by
photolysis in the presence of the NO-acceptor reagent
[(C4H8O)Fe(S,S-C6H4)2]- is consistent with reports of in vitro
repair of nitric oxide modified [2Fe-2S] ferredoxin by
cysteine desulfurase and L-cysteine.10b,11 We also demon-
strated that the NO-releasing ability of the anionic {Fe-
(NO)2}9 [(RS)2Fe(NO)2]- is finely tuned by the coordinated
thiolate ligands.10c Because of the lack of isolation and X-ray
structural data for the anionic {Fe(NO)2}9 DNICs [(RS)2-
Fe(NO)2]- containing alkylthiolate ligands coordinated to the
Fe reported,12 the property and reactivity of alkylthiolate-
containing DNICs have not been explored. Also, elucidation
of the structural features of the {Fe(NO)2}10/{Fe(NO)2}9
DNICs is important for the chemical understanding of these
species. The objective of this study was to delineate the
syntheses/reactivity of the neutral {Fe(NO)2}10 DNICs [(L)-
Fe(NO)2] [L ) sparteine (1), tetramethylethylenedi-
amine (TMEDA; 2)] and the anionic {Fe(NO)2}9 DNIC
[(S(CH2)3S)Fe(NO)2]- (4) containing a bidentate alkylthiolate
ligand coordinated to the {Fe(NO)2} unit and to investigate
the transformation of complex 1 into complex 4 via a cationic
{Fe(NO)2}9 DNIC [(sparteine)Fe(NO)2]+ (3). Of importance,
the structural discrimination between the {Fe(NO)2}10 and
{Fe(NO)2}9 DNICs was concluded.
Results and Discussion
Syntheses of the Neutral {Fe(NO)2}10 [(Sparteine)Fe-
(NO)2] (1) and [(TMEDA)Fe(NO)2] (2). Reaction of Fe-
(CO)2(NO)2 (0.2 mmol)13 and sparteine (0.2 mmol) in
tetrahydrofuran (THF) at ambient temperature yielded the
EPR-silent, neutral {Fe(NO)2}10 DNIC 1 isolated as a green
solid (35% yield) and characterized by IR, UV-vis, EPR,
and single-crystal X-ray diffraction. Complex 1 exhibits
diagnostic IR νNO stretching frequencies at 1622 (vs) and
1679 (vs) cm-1 (CH3CN) and 1633 (vs) and 1687 (vs) cm-1
(THF).8,9 In a similar fashion, synthesis of the green, neutral
{Fe(NO)2}10 DNIC 2 by reaction of Fe(CO)2(NO)2 and
TMEDA in 1:1 stoichiometry was investigated in THF under
a N2 atmosphere at ambient temperature. The shifts of IR
νNO to higher wavenumbers in complex 2 [νNO 1644 (vs)
and 1698 (vs) cm-1 (THF)] as compared to complex 1 show
the less electron-donating character of TMEDA as compared
to that of sparteine. The stretching frequencies νNO of
complexes 1 and 2 fall into the range from 1758 and 1807
cm-1 (THF) for the thermally unstable [(CO)2Fe(NO)2] to
1616 and 1673 cm-1 for the isolable [(1-MeIm)2Fe(NO)2].9,13
In contrast to the anionic {Fe(NO)2}9 DNICs [(RS)2Fe-
(NO)2]-,10c the {Fe(NO)2} motif of the neutral {Fe(NO)2}10
DNICs shows more affinity for the stronger electron-donating
ligands to yield the stable/isolable neutral [(L)2Fe(NO)2] (L
) N-containing ligands). Complexes 1 and 2 are soluble in
THF/CH3CN and exhibit air sensitivity in solution but are
stable to air for hours in the solid state.
Conversion of the Neutral 1 into the Anionic {Fe(NO)2}9
4 and [Fe2(µ-SPh)2(NO)4] (5). As presented in Scheme 1a,
upon the addition of [NO][BF4] into a CH3CN solution of
complex 1 in a 1:1 stoichiometry, a reaction ensued over
the course of 5 min to yield the cationic {Fe(NO)2}9 3
identified by EPR and IR spectra. The EPR spectrum of
complex 3 exhibits an isotropic signal with g ) 2.032 at
298 K (Figure 1), the characteristic g value of {Fe(NO)2}9
DNICs. The shift in νNO from 1622 (vs) and 1679 (vs) cm-1
(CH3CN) (1) to 1746 (vs) and 1814 (vs) cm-1 (CH3CN) is
in accordance with the formation of complex 3.14 Complex
3 is unstable and decomposes spontaneously to an insoluble
solid over the period of 5 h. We noticed that the lifetimes of
the cationic {Fe(NO)2}9 DNICs depend strongly on the
structure (monodentate/bidentate) and electron-donating abil-
ity of the coordinated ligands (sparteine vs TMEDA;
sparteine vs PPh3) because the cationic {Fe(NO)2}9 [(TM-
EDA)Fe(NO)2]+ cannot be observed spectrally (by EPR or
IR) at room temperature, whereas the cationic {Fe(NO)2}9
[(Ph3P)2Fe(NO)2]+ was isolated and characterized by single-
crystal X-ray diffraction.14
(5) (a) Foster, M. W.; Cowan, J. A. J. Am. Chem. Soc. 1999, 121, 4093-
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U.S.A. 1990, 87, 1223-1227.
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Karlin, K. D.; Rabinowitz, H. N.; Dewan, J. C.; Lippard, S. J. Inorg.
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(10) (a) Tsai, M.-L.; Liaw, W.-F., submitted for publication. (b) Tsai, M.-
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Lee, G.-H.; Wang, Y.; Liaw, W.-F. Inorg. Chem. 2004, 43, 5159-
5167. (c) 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. (d) Chen, T.-N.; Lo, F.-C.; Tsai, M.-L.; Shih, K.-N.; Chiang,
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Soc., Dalton Trans. 1996, 3491-3502.
6042 Inorganic Chemistry, Vol. 45, No. 15, 2006

Neutral {Fe(NO)2}10 Dinitrosyliron Complexes
Scheme 1
Upon the addition of 1 equiv of [S(CH2)3S]2- to the CH3-
CN solution of complex 3, a pronounced color change from
green to dark brown occurs under N2 at ambient temperature.
The IR, UV-vis, EPR, SQUID, and single-crystal X-ray
diffraction studies confirmed the formation of the anionic
{Fe(NO)2}9 DNIC 4 (yield 63%) with a bidentate alkylthi-
olate ligand [S(CH2)3S]2- coordinated to the {Fe(NO)2}9
motif (Scheme 1b). This result demonstrates that the facile
conversion of complex 1 to the anionic complex 4 was
carried out via the intermediate, the cationic complex 3. The
cationic complex 3 acting as an {Fe(NO)2}9-donor reagent
was also displayed by the reaction of complex 3 and
[(PhS)2Fe(NO)2]- (6); the shift in νNO from 1746 and 1814
cm-1 (3) to 1759 (s), 1786 (s), and 1817 (w) cm-1 (CH3-
CN) is in accordance with the formation of the EPR-silent,
Roussin’s red ester 5 (Scheme 1c).15
coordinated to the {Fe(NO)2}9 motif isolated and character-
ized by single-crystal X-ray diffraction.6,12 This complex
represents an important starting point to understand the
factors that are responsible for the stability and reactivity of
the anionic {Fe(NO)2}9 alkylthiolate-containing DNICs. The
IR spectrum of complex 4 in THF reveals two νNO stretching
bands at 1671 (s) and 1712 (s) cm-1. We noticed that the IR
spectra for the cationic complex 3 and the anionic complex
4 had the same pattern but differed in position [1746 and
1814 cm-1 for 3 versus 1676 and 1718 cm-1 for 4 (CH3-
CN)] and separation of NO stretching frequencies (∆νNO
)
68 cm-1 for 3 versus 42 cm-1 for 4). At 298 K, complex 4
exhibits an isotropic EPR signal at g ) 2.031 (Figure 2). As
observed in the previous study,10c the temperature-dependent
effective magnetic moment [the effective magnetic moment
(µeff) decreases from 2.25 µB at 300 K to 1.17 µB at 4 K] of
complex 4 may be contributed from both {Fe+(•NO)2}9 (ST
) 5/2) and {Fe-(+NO)2}9 (ST ) 1/2) electronic states. Complex
4 was alternatively obtained via a bridged-ligand breakage
reaction, a straightforward reaction of complex [Fe2(µ-
S(CH2)3S)(NO)4] with 1 equiv of [S(CH2)3S]2- in THF. The
quantitative conversion of complex [Fe2(µ-S(CH2)3S)(NO)4]
to 4 was monitored by Fourier transform infrared (FTIR);
the νNO stretching bands 1749 (s), 1775 (s), and 1806 (vw)
cm-1 (complex [Fe2(µ-S(CH2)3S)(NO)4]) disappeared ac-
companied by the simultaneous formation of the stretching
bands 1671 (s) and 1712 (s) cm-1.
The surprisingly stable complex 4 is the first example of
the anionic {Fe(NO)2}9 DNICs containing alkylthiolate
Interestingly, the formation of complex [Fe2(µ-SEt)2(NO)4]
[1750 (s), 1776 (s), 1807 (vw) cm-1 (CH3CN)] was observed
when complex 3 was treated with 2 equiv of [SEt]- in CH3-
CN at ambient temperature.6,15 This result shows that the
reaction of complex 3 with nucleophile [SEt]- occurs initially
(15) Rauchfuss, T. B.; Weatherill, T. D. Inorg. Chem. 1982, 21, 827-
Figure 1. EPR spectrum of complex 3 with g ) 2.032 at 298 K.
830.
Inorganic Chemistry, Vol. 45, No. 15, 2006 6043

Hung et al.
Figure 3. ORTEP drawing and labeling scheme of 1 with thermal
ellipsoids drawn at 50% probability. Selected bond distances (Å) and angles
(deg): Fe-N(1), 1.6501(19); Fe-N(2), 1.6430(19); Fe-N(3), 2.1324(17);
Fe-N(4), 2.1319(18); N(1)-O(1), 1.206(6); N(1)-O(1′), 1.220(6); N(2)-
O(2), 1.214(5); N(2)-O(2′), 1.217(5); N(2)-Fe-N(1), 112.08(10); N(2)-
Fe-N(4), 105.90(8); N(1)-Fe-N(4), 121.63(8); N(2)-Fe-N(3), 114.82(8);
N(1)-Fe-N(3), 115.59(8); N(4)-Fe-N(3), 84.03(7); O(1)-N(1)-O(1′),
14.0(4); O(1)-N(1)-Fe, 160.1(3); O(1′)-N(1)-Fe, 167.1(3); O(2)-N(2)-
O(2′), 18.9(3); O(2)-N(2)-Fe, 176.0(3); O(2′)-N(2)-Fe, 159.0(3).
electronic effect (electron-donating ability of thiolates) may
rationalize the stability/isolation of complex 4.
Reversibly, quantitative transformation of complex 5 to 6
was monitored by IR νNO spectroscopy; the shift of the
stretching frequencies from 1749 (s), 1775 (s), and 1806 (w)
cm-1 to the lower wavenumbers 1693 (s) and 1737 (s) cm-1
confirmed the formation of complex 6 (yield 86%) when a
THF solution of the dinuclear 5 was reacted with 2 equiv of
[SPh]- (Scheme 1e).10a Complex 4 does not react with [SPh]-
via thiolate ligand exchange to form complex 6.10c In contrast,
the coordinated [SPh]- ligands of complex 6 could be
replaced by the stronger electron-donating bidentate thiolate
[S(CH2)3S]2- to yield complex 4 when reacting complex 6
with [S(CH2)3S]2- in THF at ambient temperature, as shown
in Scheme 1f.10c The conversion of complex 4 to complex 6
was expected to be driven by protonation (PhSH) of complex
4; the reaction of complex 4 and 2 equiv of thiophenol in
THF does lead to isolation of the stable complex 6.
Presumably, protonation (electrophilic reaction of PhSH) of
complex 4 by thiophenol occurs only at the more accessible,
electron-rich sulfur site to form the well-known complex 6
(yield 82%) at ambient temperature (Scheme 1f′). Obviously,
complex 4 containing the coordinated alkylthiolate ligand
and complex 6 containing the coordinated phenylthiolate
ligands are chemically interconvertible.
Structures. Figures 3 and 4 display thermal ellipsoid plots
of the neutral complexes 1 and 2, respectively, and selected
bond distances and angles are given in the figure captions.
The strain effect of the chelating ligand in the coordination
sphere of complexes 1 and 2 explains that the geometry of
Fe is a distorted tetrahedral with N(3)-Fe-N(4) bond angles
of 84.03(7) and 82.57(12)°, respectively. The most striking
feature of the neutral {Fe(NO)2}10 DNICs and anionic {Fe-
(NO)2}9 DNICs is the difference of Fe-N(O)/N-O bond
lengths between the neutral {Fe(NO)2}10 and the anionic/
Figure 2. EPR spectrum of complex 4 (a) with an isotropic gav ) 2.031
signal at 298 K and (b) with a rhombic gx ) 2.048, gy ) 2.033, gz ) 2.015
signal at 77 K.
at the more accessible electrophile [Fe(NO)2]+ to yield the
charge-controlled, collision product [(EtS)2Fe(NO)2]-, which
subsequently transformed into the more stable form, complex
[Fe2(µ-SEt)2(NO)4]. Obviously, the two alkylthiolates [S(C-
H2)3S]2- and [SEt]-, rendering the {Fe(NO)2}9 unit in
different structural environments, induce differing stability
to {Fe(NO)2}9 fragment. In contrast, the coordinated sparteine
ligand of complex 3 could also be replaced by thiolate
[SPh]-. As shown in Scheme 1d, reaction of complex 3 and
2 equiv of [SPh]- in a CH3CN solution at room temperature
rapidly yielded the known 6 characterized by IR, UV-vis,
and EPR.10c,16 These results in combination with the previous
study elucidate that not only electronic but also structural
environments of the coordinated thiolate ligands of the
anionic {Fe(NO)2}9 DNICs play a key role in creating/
stabilizing DNICs. The νNO stretching frequencies [1671 and
1712 cm-1 (THF)] observed in complex 4 fall out of the
range from 1716 and 1766 cm-1 to 1693 and 1737 cm-1
observed in the known stable and isolable {Fe(NO)2}9
[(RS)2Fe(NO)2]-.10c The chelating effect of [S(CH2)3S]2-
coordinated to the {Fe(NO)2} motif overwhelming the
(16) Strasdeit, H.; Krebs, B.; Henkel, G. Z. Naturforsch. 1986, 41b, 1357-
1362.
6044 Inorganic Chemistry, Vol. 45, No. 15, 2006

Neutral {Fe(NO)2}10 Dinitrosyliron Complexes
Figure 5. ORTEP drawing and labeling scheme of 4 with thermal
ellipsoids drawn at 50% probability. Selected bond distances (Å) and angles
(deg): Fe-N(1), 1.674(2); Fe-N(2), 1.677(2); Fe-S(1), 2.2609(7); Fe-
S(2), 2.2544(7); N(1)-O(1), 1.181(3); N(2)-O(2), 1.174(3); N(2)-Fe-
N(1), 118.61(11); N(1)-Fe-S(2), 107.59(7); N(2)-Fe-S(2) 107.90(8);
N(1)-Fe-S(1), 108.54(7); N(2)-Fe-S(1), 111.72(8); S(2)-Fe-S(1),
100.92(2); O(1)-N(1)-Fe, 172.8(2); O(2)-N(2)-Fe, 167.4(3).
Figure 4. ORTEP drawing and labeling scheme of [(CH3)2N(CH2)2N-
(CH3)2)Fe(NO)2] with thermal ellipsoids drawn at 30% probability. Selected
bond distances (Å) and angles (deg): Fe-N(1), 1.639(3); Fe-N(2),
1.637(3); Fe-N(3), 2.117(3); Fe-N(4), 2.109(3); N(1)-O(1), 1.188(4);
N(2)-O(2), 1.197(4); N(2)-Fe-N(1), 115.36(16); N(2)-Fe-N(4),
115.26(15); N(1)-Fe-N(4), 112.54(15); N(2)-Fe-N(3), 114.81(14);
N(1)-Fe-N(3), 112.10(15); N(4)-Fe-N(3), 82.57(12); O(1)-N(1)-Fe,
169.9(3); O(2)-N(2)-Fe, 166.9(3).
S5]-.10b The constraint of the bidentate [S(CH2)3S]2- ligand
generates a 100.92(2)° S(2)-Fe-S(1) angle, enforcing a
significant distortion from a tetrahedral at the four-
coordinated Fe site in complex 4. The difference [∆Fe-N(O)
) 0.007 Å] of the Fe-N(O) bond distance between complex
4 [average 1.676(2) Å for complex 4] and the reported
anionic [(NO)2Fe(SR)2]- containing the coordinated phe-
nylthiolates {[(NO)2Fe(2-S-C7H4NS)2]- [average 1.684(7)
Å], [(NO)2Fe(SC6H4-o-NHC(O)CH3)2]- [average 1.682(2)
Å], and [(NO)2Fe(2-S-C4H3S)2]- [average 1.683(2) Å]}10c
may be attributed to the shorter Fe-S bond distance [average
2.2576(7) Å; the stronger electron-donating [S(CH2)3S]2-
ligand] of complex 4 compared to those of 2.294(2), 2.301-
(1), and 2.2962(6) Å of complexes [(NO)2Fe(2-S-C7H4-
NS)2]-, [(NO)2Fe(SC6H4-o-NHC(O)CH3)2]-, and [(NO)2Fe-
(2-S-C4H3S)2]-, respectively.10c The mean value of the N-O
bond lengths in complex 4 is 1.178(3) Å, slightly longer than
those in complexes [(NO)2Fe(2-S-C7H4NS)2]- [average
1.174(6) Å] and [(NO)2Fe(SC6H4-o-NHC(O)CH3)2]- [aver-
age 1.170(3) Å]. The Fe-N-O bond angle of 170.1(3)°
(average) in complex 4 is comparable to the Fe-N-O bond
angles of 169.7(5) (average) and 168.3(2)° (average) ob-
served in [(NO)2Fe(2-S-C7H4NS)2]- and [(NO)2Fe(2-S-
C4H3S)2]-,10c respectively.
Table 1. Selected Fe-N(O) and N-O Bond Lengths for the Neutral
{Fe(NO)2}10 and the Anionic/Neutral/Cationic {Fe(NO)2}9 DNICs
Fe-N(O)
N-O
(average) (average)
(Å)
(Å)
ref
Neutral {Fe(NO)2}10 DNICs
1.647(2)
1
2
1.214(6)
1.193(4)
1.189(4)
this work
this work
9
1.638(3)
1.649(3)
1.650(7)
[(1-MeIm)2Fe(NO)2]
[(PPh3)2Fe(NO)2]
1.190(10) 8a
Anionic/Neutral {Fe(NO)2}9 DNICs
4
1.676(2)
1.678(3)
1.683(2)
1.684(6)
1.178(3)
this work
10b
10c
10c
10c
18
10d
17
14
[S5Fe(NO)2]-
1.178(3)
1.178(2)
1.174(6)
1.170(3)
1.162(5)
1.170(5)
1.167(4)
1.160(6)
[(2-S-(C4H3S)2Fe(NO)2]-
[(2-S-C7H4NS)2Fe(NO)2]-
[(SC6H4-o-NHCOCH3)2Fe(NO)2]- 1.682(2)
[(SePh)2Fe(NO)2]-
[Se5Fe(NO)2]-
1.669(4)
1.675(4)
1.695(3)
1.661(4)
[(6-Me3-TPA)Fe(NO)2]+
[(PPh3)2Fe(NO)2]+
[(PPh3)(OPPh3)Fe(NO)2]+
1.668(13) 1.171(18) 14
[(SC6H4-o-NHCOPh)(Im)Fe(NO)2] 1.683(6)
1.167(6)
1.186(7)
1.161(4)
10a
6a
6b
[(H+bme-daco)Fe(NO)2]
[(C9H21N2S2)Fe(NO)2]
1.669(6)
1.680(3)
neutral {Fe(NO)2}9 DNICs (Table 1). Compared to complex
4, the mean N-O distances in complexes 1 and 2 have
increased by 0.037 and 0.015 Å and the mean Fe-N(O)
distances have decreased by 0.029 and 0.038 Å, respectively.
Specifically, the mean N-O distances fall in the range of
1.214(6)-1.189(4) Å and the mean Fe-N(O) distances are
within the range of 1.650(7)-1.638(3) Å for the neutral {Fe-
(NO)2}10 DNICs, compared to the mean N-O distances
ranging from 1.178(3) to 1.160(6) Å and the mean Fe-N(O)
distances ranging from 1.695(3) to 1.661(4) Å for the anionic/
neutral/cationic {Fe(NO)2}9 DNICs (Table 1).
Figure 5 displays the thermal ellipsoid plot of the anionic
complex 4, and selected bond distances and angles are given
in the figure caption. The X-ray structural determination of
complex 4 shows that the [FeS2C3] ring is in the chair
conformation, similar to that found in complex [(NO)2Fe-
Conclusion and Comments. Studies on the neutral {Fe-
(NO)2}10 1, 2, the anionic {Fe(NO)2}9 4, and the cationic
{Fe(NO)2}9 3 have resulted in the following conclusions.
(1) The facile transformation of the neutral complex 1 to
the anionic complex 4 may occur via the cationic complex
3. That is, the cationic {Fe(NO)2}9 complex 3 acts as an
[Fe(NO)2]-donor reagent in the presence of thiolates or
[(RS)2Fe(NO)2]-.
(2) This result in combination with the previous study
shows that the stability of the anionic [(RS)2Fe(NO)2]- was
finely modulated by the electronic and structural environ-
ments of the coordinated thiolate ligands.10c The bidentate
alkylthiolate-coordinated ligand of complex 4 promotes the
stability of the anionic {Fe(NO)2}9 DNICs compared to the
monodentate alkylthiolate-coordinated ligands. The chelating
Inorganic Chemistry, Vol. 45, No. 15, 2006 6045

Hung et al.
Table 2. Selected IR Data for the Neutral {Fe(NO)2}10 and the
Anionic/Neutral/Cationic {Fe(NO)2}9 DNICs
complex 4 to complex 6 may decipher the protein-bound
DNICs mobilized by the reaction with thiol such as glu-
tathione or free cysteine to form LMW-DNICs.5,6
νNO
∆νNO
complex
(cm-1
)
(cm-1
)
ref
Neutral {Fe(NO)2}10 DNICs
1622, 1679e
Experimental Section
1
2
57
54
57
46
this work
this work
9
8a
1644, 1698a
Manipulations, reactions, and transfers were conducted under N2
according to Schlenk techniques or in a glovebox (Ar gas). Solvents
were distilled under N2 from appropriate drying agents (diethyl ether
from CaH2; acetonitrile from CaH2/P2O5; methylene chloride from
CaH2; hexane and THF from sodium benzophenone) and stored in
dried, N2-filled flasks over 4-Å molecular sieves. N2 was purged
through these solvents before use. The solvent was transferred to
the reaction vessel via a stainless cannula under positive pressure
of N2. The reagents sparteine, TMEDA, thiophenol, iron pentac-
arbonyl, 1,3-propanedithiol (Aldrich), and bis(triphenylphospho-
ranylidene)ammonium chloride ([PPN][Cl]; Fluka) were used as
received. Compound [PPN][Fe(CO)3(NO)] was synthesized by
published procedures.13 IR spectra of the νNO stretching frequencies
were recorded on a PerkinElmer model spectrum one B spectrom-
[(1-MeIm)2Fe(NO)2]
[(PPh3)2Fe(NO)2]
1616, 1673b
1678, 1724c
Anionic {Fe(NO)2}9 DNICs
1671, 1712a
4
41
44
45
50
47
44
39
this work
10b
10c
10c
10c
[S5Fe(NO)2]-
1695, 1739a
[(2-S-(C4H3S)2Fe(NO)2]-
[(2-S-C7H4NS)2Fe(NO)2]-
[(SC6H4-o-NHCOCH3)2Fe(NO)2]-
[(SePh)2Fe(NO)2]-
1698, 1743a
1716, 1766a
1705, 1752a
1697, 1741d
1697, 1736a
18
10d
[Se5Fe(NO)2]-
Cationic {Fe(NO)2}9 DNICs
3
1746, 1814e
68
75
79
63
this work
[(6-Me3-TPA)Fe(NO)2]+
[(OPPh3)2Fe(NO)2]+
1726, 1801e
1734, 1813d
1746, 1809d
17
14
14
[(PPh3)(OPPh3)Fe(NO)2]+
1
Neutral {Fe(NO)2}9 DNIC
eter with sealed solution cells (0.1 mm, KBr windows). H NMR
[(SC6H4-o-NHCOPh)(Im)Fe(NO)2] 1722, 1786a
64
44
45
10a
6a
6b
spectra were obtained on a Varian Unity-500 spectrometer. UV-
vis spectra were recorded on a Jasco V-570 spectrometer. Analyses
of C, H, and N were obtained with a CHN analyzer (Heraeus).
[(H+bme-daco)Fe(NO)2]
[(C9H21N2S2)Fe(NO)2]
1740, 1696a
1740, 1695d
a THF. b Diethyl ether. c Tetrachloroethylene. d CH2Cl2. e CH3CN.
Preparation of [(Spartiene)Fe(NO)2] (Sparteine ) C15H26N2;
1). Sparteine (40 µL, d ) 1.02 g mL-1, 0.2 mmol) was added into
the THF solution of [Fe(CO)2(NO)2],13 freshly prepared from the
reaction of [PPN][Fe(CO)3(NO)] (0.141 g, 0.2 mmol) and [NO]-
[BF4] (0.024 g, 0.2 mmol), and stirred for 4 h under N2 at ambient
temperature. The reaction was monitored with FTIR. The IR
spectrum [νNO 1633 and 1687 cm-1 (THF)] was assigned to the
formation of 1. Diffusion of hexane into the THF solution of
complex 1 at -15 °C led to green crystals 1 (0.082 g, 35%) suitable
for single-crystal X-ray diffraction. IR: νNO 1633, 1687 cm-1
effect overwhelming the S-donating ability (electronic effect)
may rationalize the observed isolation/stability of complex
4.
(3) On the basis of N-O and Fe-N(O) bond lengths of
the single-crystal X-ray structures of the {Fe(NO)2}9/{Fe-
(NO)2}10 DNICs, the mean N-O distances fall in the range
of 1.214(6)-1.189(4) Å and the mean Fe-N(O) distances
are within the range of 1.650(7)-1.638(3) Å for the neutral
{Fe(NO)2}10 DNICs (Table 1).8,9 In contrast, the mean N-O
distances ranging from 1.178(3) to 1.160(6) Å and the mean
Fe-N(O) distances ranging from 1.695(3) to 1.661(4) Å are
assigned as the anionic/neutral/cationic {Fe(NO)2}9 DNICs
(Table 1).6-10,17,1817-18 Obviously, complex [(6-Me3-TPA)-
Fe(NO)2]+ is best described as a cationic {Fe(NO)2}9 DNIC
(Table 1).17
(4) EPR spectra in combination with IR νNO spectra (the
relative position of the νNO stretching frequencies and their
difference ∆νNO ∼ 45 cm-1 for the EPR-active, anionic {Fe-
(NO)2}9 DNICs, ∼65 cm-1 for the EPR-active, cationic {Fe-
(NO)2}9 DNICs, ∼55 cm-1 for the EPR-silent, neutral
{Fe(NO)2}10 DNICs, and ∼64 cm-1 for the EPR-active,
neutral {Fe(NO)2}9 DNICs) may serve as an efficient tool
for discrimination of the existence of the anionic {Fe(NO)2}9,
the cationic {Fe(NO)2}9, the neutral {Fe(NO)2}9, and the
neutral {Fe(NO)2}10 DNICs (Table 2).6-18
(THF); 1622, 1679 cm-1 (CH3CN). H NMR (CD3CN): δ 4.06
1
(d, C(15)HA, not first order), 3.68 (m, C(2)HA), 3.34 (m, C(17)-
HA), 3.02 (m, C(10)HA), 2.86 (m, C(17)HB), 2.62 (m, C(15)HB),
2.52 (m, C(10)HB), 2.21 (m, C(2)HB), 2.00 (m, C(8)HA, C(11)H),
1.87 (m, C(9)H), 1.81-1.61 (m) (C(6)H, C(4)HA, and C(13)HA),
1.60-1.25 (m, C(3)H2, C(14)H2, C(12)HA, C(7)H), 1.24-1.17 (m,
C(5)H2, C(12)HB, C(4)HB, and C(13)HB), 0.96 (m, C(8)HB).
Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 366 (816),
392 (781), 518 (143), 722 (113). Anal. Calcd for C15H26N4O2Fe:
C, 51.39; H, 7.42; N, 15.99. Found: C, 50.65; H, 8.05; N, 15.32.
Preparation of [(TMEDA)Fe(NO)2] (TMEDA ) N,N,N′,N′-
Tetramethylethenediamine; 2). TMEDA (30 µL, d ) 0.775 g
mL-1, 0.2 mmol) was added into the THF solution of [Fe(CO)2-
(NO)2], and the reaction solution was stirred for 4 h under N2 at
ambient temperature. The reaction was monitored with FTIR. IR
spectrum [νNO 1644 and 1698 cm-1 (THF)] was assigned to the
formation of 2. Diffusion of hexane into the THF solution of
complex 2 at -15 °C led to green crystals 2 suitable for single-
crystal X-ray diffraction (yield 0.056 g, 24%). IR: νNO 1644, 1698
1
cm-1 (THF). H NMR (C4D8O): δ 2.75 (s, CH2), 2.52 (s, CH3).
In this study, the isolation of complex 4, mimicking Cys-
(X)n-Cys binding of Fe(NO)2 to proteins or thiobiomol-
ecules, implicates that the chelating effect of thiobiomole-
cules or proteins may play a key role in creating/stabilizing
the protein-bound DNICs.5,6 The reversible conversion from
Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 369 (501),
388 (489), 508 (87), 726 (47). Anal. Calcd for C6H16N4O2Fe: C,
31.02; H, 6.94; N, 24.14. Found: C, 30.43; H, 6.29; N, 24.03.
Preparation of [(Sparteine)Fe(NO)2][BF4] (3). 1 (0.035 g, 0.1
mmol) and [NO][BF4] (0.012 g, 0.1 mmol) were dissolved in CH3-
CN and stirred for 5 min under N2 at ambient temperature. The
reaction was monitored with FTIR. IR [νNO 1746 and 1814 cm-1
(CH3CN); 1739 and 1808 cm-1 (THF)] and EPR (an isotropic g )
2.032 signal at 298 K, characteristic of LMW-DNICs) spectra
(17) Jo, D.-H.; Chiou, Y.-M.; Que, L., Jr. Inorg. Chem. 2001, 40, 3181-
3190.
(18) Liaw, W.-F.; Chiang, C.-Y.; Lee, G.-H.; Peng, S.-M.; Lai, C.-H.;
Darensbourg, M. Y. Inorg. Chem. 2000, 39, 480-484.
6046 Inorganic Chemistry, Vol. 45, No. 15, 2006

Neutral {Fe(NO)2}10 Dinitrosyliron Complexes
identified the formation of 3.14 Absorption spectrum (CH3CN) [λmax
nm (ꢀ, M-1 cm-1)]: 371 (1061), 525 (264), 1105 (58).
,
under N2 at ambient temperature. The reaction mixture was dried
under vacuum, and then diethyl ether was added to abstract the
red-brown product. Hexane was added to precipitate the product
[PPN]6 (yield 68%) characterized by IR and UV-vis.10c,16
Reaction of [PPN]6 and [PPN]2[S(CH2)3S]. A THF solution
(10 mL) of complex 6 (0.087 g, 0.1 mmol) was transferred to a
50-mL Schlenk flask loaded with [PPN]2[S(CH2)3S] (0.118 g, 0.1
mmol) by a cannula under positive N2 pressure at room temperature.
The reaction solution was stirred for 30 min at room temperature,
and the resulting mixture was filtered through Celite to separate
the insoluble solid. The solution was then concentrated, and diethyl
ether and hexane (5 and 15 mL, respectively) were added to
precipitate the dark-brown solid [PPN]4 (yield 0.0669 g, 88%)
characterized by IR and UV-vis.
Reaction of [PPN]4 and Thiophenol. Complex 4 (0.076 g, 0.1
mmol) and thiophenol (20 µL, 0.2 mmol) were dissolved in THF
(15 mL) under a N2 atmosphere. The reaction mixture was stirred
at room temperature for 1 h, and the resulting solution was filtered
through Celite. The solution was then concentrated under vacuum,
and diethyl ether and hexane (5 and 15 mL, respectively) were
added to precipitate dark-red solid [PPN]6 (yield 0.0716 g, 82%)
characterized by IR and UV-vis.
Preparation of [PPN][(S(CH2)3S)Fe(NO)2] (4). Method A. A
THF solution of Fe(CO)2(NO)2, freshly prepared from the reaction
of [PPN][Fe(CO)3(NO)] (0.071 g, 0.1 mmol) and [NO][BF4] (0.012
g, 0.1 mmol),13 was transferred to a 50-mL Schlenk flask loaded
with HS(CH2)3SH (10 µL, 0.1 mmol) by a cannula under positive
N2 pressure at room temperature. The reaction solution was stirred
at room temperature overnight. The IR spectrum (νNO 1749 s, 1775
s, and 1806 vw cm-1) shows the formation of [Fe2(µ-S(CH2)3S)-
(NO)4].10a,15 The THF solution (10 mL) of [Fe2(µ-S(CH2)3S)(NO)4]
(0.05 mmol) was transferred to [PPN]2[S(CH2)3S] (0.118 g, 0.1
mmol) by a cannula and stirred for 20 min. The resulting mixture
was filtered through Celite to separate the insoluble solid. The
resulting solution was then concentrated, and diethyl ether and
hexane (5 and 15 mL, respectively) were added to precipitate the
dark-brown solid [PPN]4 (yield 0.06 g, 78%). Diffusion of diethyl
ether into the THF solution of complex 4 at 0 °C for 1 week led to
dark-brown crystals suitable for single-crystal X-ray diffraction.
Method B. The acetonitrile solution of complex 3, freshly prepared
from the reaction of complex 1 (0.035 g, 0.1 mmol) and [NO]-
[BF4] (0.012 g, 0.1 mmol), was transferred to a 50-mL Schlenk
flask loaded with [PPN]2[S(CH2)3S] (0.236 g, 0.2 mmol) by a
cannula under positive N2 pressure at ambient temperature. The
reaction mixture was stirred for 10 min. The resulting mixture was
dried under vacuum and then redissolved in THF. The resulting
mixture was filtered through Celite to separate the insoluble solid.
The solution was then concentrated, and diethyl ether and hexane
(5 and 15 mL, respectively) were added to precipitate dark-brown
solid [PPN]4 (yield 0.0479 g, 63%). IR: νNO 1671, 1712 cm-1
(THF); 1676, 1718 cm-1 (CH3CN). Absorption spectrum (THF)
[λmax, nm (ꢀ, M-1 cm-1)]: 366 (5680), 430 (3837), 578 (1211),
807 (371). Anal. Calcd for C39H36N3O2S2P2Fe: C, 61.53; H, 4.73;
N, 5.52. Found: C, 60.97; H, 5.25; N, 4.93.
Addition of 3 to [PPN]6. An acetonitrile solution of complex
3, freshly prepared from the reaction of complex 1 (0.035 g, 0.1
mmol) and [NO][BF4] (0.012 g, 0.1 mmol) in CH3CN, was added
into [PPN]6 (0.218 g, 0.1 mmol) via a cannula under positive N2
pressure. The reaction solution was stirred for 5 min under N2 at
ambient temperature. The reaction was monitored with FTIR. The
IR spectrum [νNO 1759 s, 1786 s, and 1817 w cm-1 (CH3CN); 1749
s, 1775 s, and 1806 w cm-1 (THF)] was assigned to the formation
of [Fe2(µ-SPh)2(NO)4] (5).15 The solution was dried under vacuum,
and diethyl ether was added to redissolve the solid. The reacting
mixture was then filtered through Celite to separate the insoluble
solid. The final solution was dried under the vacuum to yield 5
(yield 0.044 g, 97%).
EPR Measurements. EPR measurements were performed at the
X band using a Bruker EMX spectrometer equipped with a Bruker
TE102 cavity. The microwave frequency was measured with a
Hewlett-Packard 5246L electronic counter. X-band EPR spectra
of complexes 3 and 4 in THF were obtained with a microwave
power of 20.020 mW, a frequency of 9.605 GHz, and a modulation
amplitude of 0.2 G at 100 kHz.
Magnetic Measurements. The magnetic data were recorded on
a SQUID magnetometer (MPMS5 Quantum Design Company)
under a 1-T external magnetic field in the temperature range of
4-300 K. The magnetic susceptibility data were corrected with
temperature-independent paramagnetism (2 × 10-4 cm3 mol-1) and
ligands’ diamagnetism by the tabulated Pascal’s constants.
Crystallography. Crystallographic data and structural refinement
parameters of complexes 1, 2, and 4 are summarized in the
Supporting Information (Tables S1-S3). Each crystal was mounted
on a glass fiber and quickly coated with an epoxy resin. Unit-cell
parameters were obtained by least-squares refinement. Diffraction
measurements for complexes 1, 2, and 4 were carried out on a
SMART Apex CCD diffractometer with graphite-monochromated
Mo KR radiation (λ ) 0.7107 Å). Least-squares refinement of the
positional and anisotropic thermal parameters of all non-H atoms
and fixed H atoms was based on F 2. A SADABS19 absorption
correction was made. The SHELXTL20 structural refinement program
was employed.
Reaction of 5 and [PPN][SPh]. A THF solution (10 mL) of
complex 5 (0.045 g, 0.1 mmol) was transferred to a 50-mL Schlenk
flask loaded with [PPN][SPh] (0.129 g, 0.2 mmol) by a cannula
under positive N2 pressure at room temperature. The reaction
mixture was stirred for 5 min at room temperature and then filtered
through Celite to separate the insoluble solid. The solution was
concentrated, and diethyl ether and hexane (5 and 15 mL,
respectively) were added to precipitate the dark-red solid [PPN]6
(yield 0.075 g, 86%) characterized by IR [νNO 1693 and 1737 cm-1
(THF)] UV-vis [absorption spectrum (THF) [λmax, nm (ꢀ, M-1
cm-1)]: 363 (4065), 484 (1991), 800 (473)].10c,16
Acknowledgment. We gratefully acknowledge financial
support from the National Science Council (Taiwan).
Supporting Information Available: Crystallographic data and
structural refinement parameters for complexes 1, 2, and 4 and
X-ray crystallographic files in CIF format for the structural
determinations of 1, [(CH3)2N(CH2)2N(CH3)2)Fe(NO)2], and [PPN]-
4. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0605120
Addition of 3 to [PPN][SPh]. An acetonitrile solution of
complex 3, freshly prepared from the reaction of complex 1 (0.035
g, 0.1 mmol) and [NO][BF4] (0.012 g, 0.1 mmol) in CH3CN, was
added into [PPN][SPh] (0.130 g, 0.2 mmol) via a cannula under
positive N2 pressure. The reaction solution was stirred for 5 min
(19) Sheldrick, G. M. SADABS, Siemens Area Detector Absorption Cor-
rection Program; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(20) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determi-
nation; Siemens Analytical X-ray Instruments Inc.: Madison, WI,
1994.
Inorganic Chemistry, Vol. 45, No. 15, 2006 6047