Nitrosylated iron-thiolate-sulfinate complexes with {Fe(NO)}6/7 electronic cores: Relevance to the transformation between the active and inactive NO-bound forms of iron-containing nitrile hydratases

Article abstract of DOI:10.1021/ic050108l

The five-coordinated iron-thiolate nitrosyl complexes [(NO)Fe(S,-C ₆H₃R)₂]^- (R = H (1), m-CH ₃ (2)), [(NO)Fe-(S,S-C₆H₂-3,6-Cl ₂)₂]^- (3), [(NO)Fe(

Full text of DOI:10.1021/ic050108l

Inorg. Chem. 2005, 44, 6670−6679
6/7
Nitrosylated Iron−Thiolate−Sulfinate Complexes with {Fe(NO)}
Electronic Cores: Relevance to the Transformation between the Active
and Inactive NO-Bound Forms of Iron-Containing Nitrile Hydratases
Chien-Ming Lee,† Chien-Hong Chen,† Hao-Wen Chen,†,‡ Jo-Lu Hsu,†,‡ Gene-Hsiang Lee,§ and
Wen-Feng Liaw*,†
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan, Department of
Chemistry, National Changhua UniVersity of Education, Changhua, Taiwan, and Instrumentation
Center, National Taiwan UniVersity, Taipei, Taiwan
Received January 24, 2005
The five-coordinated iron
−
thiolate nitrosyl complexes [(NO)Fe(S,S-C6H3R)2]- (R
)
H (1), m-CH3 (2)), [(NO)Fe-
-
-
(S,S-C6H2-3,6-Cl2)2]- (3), [(NO)Fe(S,S-C6H3R)2]2 (R
)
H (10), m-CH3 (11)), and [(NO)Fe(S,S-C6H2-3,6-Cl2)2]2
(12) have been isolated and structurally characterized. Sulfur oxygenation of iron
−
thiolate nitrosyl complexes 1
−
3
6
containing the
{
Fe(NO)
}
core was triggered by O2 to yield the S-bonded monosulfinate iron species [(NO)Fe-
2-
(S,SO2-C6H3R)(S,S-C6H3R)]- (R
)
H (4), m-CH3 (5)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2 (6),
7
respectively. In contrast, attack of O2 on the
by the minor products, [Fe(S,S-C6H4)2]2 and [NO3]- (yield 9%). Reduction of complexes 4
{Fe(NO)
}
complex 10 led to the formation of complex 1 accompanied
2-
−
6 by [EtS]- in CH3-
-
CN
−
THF yielded [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]2 (R
)
H (7), m-CH3 (8)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)-
9 with
7 core were oxidized by O2 to yield complexes
7 core caused by the coordinated sulfinate in complexes
9 toward O2. The iron sulfinate nitrosyl species with the
core exhibit the photolabilization of sulfur-bound [O] moiety. Complexes 1 10 (or 2 11 and
12) are interconvertible under sulfur oxygenation, redox processes, and photolysis, respectively.
(S,S-C6H2-3,6-Cl2)]2 (9) along with (EtS)2 identified by 1H NMR. Compared to complex 10, complexes 7
−
-
the less electron-donating sulfinate ligand coordinated to the {Fe(NO)}
4−6. Obviously, the electronic perturbation of the
{Fe(NO)}
7
{
−
9 may serve to regulate the reactivity of complexes 7
−
−
6/7
Fe(NO)
}
−4−7−
−5−8−
3
−6−9−
Introduction
consists of a paramagnetic, low-spin Fe(III) center coordi-
nated by two deprotonated carboxamido nitrogens from the
peptide backbone, three facial cysteine sulfurs, and a water
or hydroxide molecule.5 The X-ray crystallographic studies
on the inactive form of the Fe-NHase from Rhodococcus
sp. N-771 (and Rhodococcus sp. R312) revealed that the iron
center is coordinated by two deprotonated carboxamido
nitrogens, three Cys-S residues with two of them posttrans-
lationally modified to Cys-sulfinic (Cys-SO2) and Cys-
sulfenic (Cys-SO) groups, and a NO molecule.6 Such
Mononuclear non-heme iron centers are present in the
catalytic cycle of a number of proteins involved in oxygen
activation.1,2 Nitrile hydratase (NHase) is a metalloenzyme
which catalyzes the hydrolysis of nitriles to the corresponding
amides.3,4 Crystallographic studies have shown that the
active-site structure of the Fe-NHase (Fe-containing nitrile
hydratase) active form isolated from BreVibacterium R312
* To whom correspondence should be addressed. E-mail:
wfliaw@mx.nthu.edu.tw.
-
modified cysteines residues (sulfinate RSO2 and sulfenate
RSO- groups) are unprecedented.3-6 An inactive, NO-bound
† National Tsing Hua University.
‡ National Changhua University of Education.
§ National Taiwan University.
(1) Feig, A. L.; Lippard, S. J. Chem. ReV. 1994, 94, 759.
(2) Solomon, E. I.; Zhang, Y. Acc. Chem. Res. 1992, 25, 343.
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Kobayashi, M.; Nagasawa, T.; Yamada, H. Trends Biotechnol. 1992,
10, 402.
(4) (a) Brennan, B. A.; Alms, G.; Nelson, M. J.; Durney, L. T.; Scarrow,
R. C. J. Am. Chem. Soc. 1996, 118, 9194. (b) Sugiura, Y.; Kuwahara,
J.; Nagasawa, T.; Yamada, H. J. Am. Chem. Soc. 1987, 109, 5848.
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Y. Structure 1997, 5, 691.
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K.; Odaka, M.; Yohda, M.; Kamiya, N.; Endo, I. Nat. Struct. Biol.
1998, 5, 347. (b) Scarrow, R. C.; Stickler, B. S.; Ellison, J. J.; Shoner,
S. C.; Kovacs, J. A.; Cummings, J. C.; Nelson, M. J. J. Am. Chem.
Soc. 1998, 120, 9237.
6670 Inorganic Chemistry, Vol. 44, No. 19, 2005
10.1021/ic050108l CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/16/2005

Nitrosylated Iron-Thiolate-Sulfinate Complexes
Scheme 1
form is generated in cells grown in the dark, and the active
form of the Fe-NHase is regenerated with release of NO
molecule upon exposure to light.7,8 Interestingly, recent
studies demonstrated that atmospheric oxygen activates the
reconstituted nitrile hydratase through specific modifications
of Cys112 and Cys114 to Cys-SO2H and Cys-SOH, respec-
tively, and Cys112-SO2H is responsible for the catalytic
activity solely or in combination with Cys114-SOH.9 The
role/function of the NO ligand, NO binding to the iron active
site before or after Cys-S oxygenation, the oxidant(s)
responsible for the posttranslational modifications of the
bound Cys-S donors at the active site of the enzyme, the
function and biogenic mechanism of these modified cysteine-
sulfinic and -sulfenic groups, and the reason(s) for the
asymmetric oxygenation of the two Cys-S ligands are the
principal questions to be raised in the chemistry of Fe-
NHase.10-12
Some iron-thiolate nitrosyl model compounds have been
reported recently by Kovacs et al.,10 Mascharak et al.,11a,b
and Artaud et al.,11c respectively. In one interesting model
compound the diamagnetic Fe(III)-nitrosyl complex [Fe-
S2Me2N3(Pr,Pr)(NO)]+ (S2Me2N3(Pr,Pr) ) 3-methyl-3-mer-
capto-2-butanone) with the {Fe(NO)}6 core, obtained from
reaction of the paramagnetic [FeS2Me2N3(Pr,Pr)]+ and NO,
displayed antiferromagnetic coupling between d5 FeIII and
•NO.10 The S-bonded sulfinato iron(III) complexes [Fe(PyPep-
SO2)2]- was obtained from sulfur oxygenation of [Fe(PyPep-
S)2]- (PyPepSH2 ) N-2-mercaptophenyl-2′-pyridinecarnoxa-
mide) by H2O2.11a-b In a recent communication, we have
shown that the iron-nitrosyl complex containing S-bonded
monosulfinate [(NO)Fe(S,SO2-C6H4)(S,S-C6H4)]- (4) was
isolated from sulfur oxygenation of complex [(NO)Fe(S,S-
C6H4)2]- (1) (Scheme 1a).13 This result reveals that binding
of NO to the iron center promotes sulfur oxygenation by
dioxygen and stabilizes the S-bonded sulfinate iron species.
Reaction of complex 4 and excess PPh3 yielded complex 1
and Ph3PO (Scheme 1a′).13 Alternatively, upon photolysis
of CH2Cl2 solution of complex 4 under N2 purge at ambient
temperature, the UV-vis and IR spectra consistent with the
formation of complex 1 demonstrate that complexes 1 and
4 are photochemically interconvertible.13
In this paper, studies of the reactivity of the {Fe(NO)}n
(n ) 6, 7) iron-thiolate and iron-sulfinate nitrosyl com-
plexes toward O2, the influence of sulfinate ligand on Fe-
N-O bonding and on the electronic structure of the
{Fe(NO)}6/7 core, and the photochemical interconversion
between iron-thiolate and iron-sulfinate-thiolate nitrosyl
complexes may provide valuable insights into the intercon-
version pathways between the active and inactive NO-bound
forms of Fe-containing nitrile hydratase. Here, the formalism
{M(NO)x}y (where y ) the number of d-type electrons and
is the conventional number of d-electrons for the metal center
when the NO ligand is considered to be NO+) invokes the
Enemark-Feltham notation, which stresses the well-known
covalency and delocalization in the electronically amorphous
M(NO)x unit, i.e., without committing to a formal oxidation
state on M and NO.14 In addition, this study also details the
effect of S-atom electron density on the sulfur oxygenation
of the similar iron-thiolate-nitrosyl compounds, and the
reactivity of the iron-sulfinate nitrosyl complexes toward
O2 is compared with that of the iron-thiolate analogues.
(7) Noguchi, T.; Hoshino, M.; Tsujimura, M.; Odaka, M.; Inoue, Y.; Endo,
I. Biochemistry 1996, 35, 16777.
(8) Odaka, M.; Fujii, K.; Hoshino, M.; Noguchi, T.; Tsujimura, M.;
Nagashima, S.; Yohda, M.; Nagamune, T.; Inoue, Y.; Endo, I. J. Am.
Chem. Soc. 1997, 119, 3785.
(9) Murakami, T.; Nojiri, M.; Nakayama, H.; Odaka, M.; Yohda, M.;
Dohmae, N.; Takio, K.; Nagamune, T.; Endo, I. Protein Sci. 2000, 9,
1024.
(10) (a) Schweitzer, D.; Ellison, J. J.; Shoner, S. C.; Lovell, S.; Kovacs, J.
A. J. Am. Chem. Soc. 1998, 120, 10996. (b) Shearer, J.; Jackson, H.
L.; Schweitzer, D.; Rittenberg, D. K.; Leavy, T. M.; Kaminsky, W.;
Scarrow, R. C.; Kovacs, J. A. J. Am. Chem. Soc. 2002, 124, 11417.
(11) (a) Tyler, L. A.; Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K.
Inorg. Chem. 1999, 38, 616. (b) Noveron, J. C.; Olmstead, M. M.;
Mascharak, P. K. J. Am. Chem. Soc. 2001, 123, 3247. (c) Artaud, I.;
Chatel, S.; Chauvin, A. S.; Bonnet, D.; Kopf, M. A.; Leduc, P. Coord.
Chem. ReV. 1999, 190-192, 577. (d) Mascharak, P. K. Coord. Chem.
ReV. 2002, 225, 201.
Results and Discussion
Sulfur Oxygenation of the {Fe(NO)}6 Complexes. As
presented in Scheme 1a, the {Fe(NO)}6 [(NO)Fe(S,S-
C6H3R)2]- (R ) H (1), m-CH3 (2)) complexes reacted slowly
with molecular oxygen in CH2Cl2 solution at room temper-
ature to yield the corresponding S-bonded monosulfinate
(12) Noguchi, T.; Honda, J.; Nagamune, T.; Sasabe, H.; Inoue, Y.; Endo,
I. FEBS Lett. 1995, 358, 9.
(13) Lee, C.-M.; Hsieh, C.-H.; Dutta, A.; Lee, G.-H.; Liaw, W.-F. J. Am.
Chem. Soc. 2003, 125, 11492.
(14) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6671

Lee et al.
iron-nitrosyl complexes [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]-
(R ) H (4), m-CH3 (5)) in which the bound thiolato sulfur
is oxygenated to the sulfinato group.13,15,16 The dark purple
solids 4 and 5 were isolated in 21% and 20% yield after the
mixture solution was separated by silica gel chromatography
with THF and CH2Cl2 as eluant and recrystallized with THF
(or CH3CN) and diethyl ether, respectively.
Figure 1. ORTEP drawing and labeling scheme of complex 5. Selected
bonds (Å): Fe-N(1), 1.622(5); Fe-S(1), 2.2019(18); Fe-S(2), 2.1817-
(18); Fe-S(3), 2.282(2); Fe-S(4), 2.2345(18); N(1)-O(1), 1.162(6); S(4)-
O(2), 1.425(5); S(4)-O(3), 1.385(6). Selected angles (deg): Fe-N(1)-
O(1), 171.9(5); S(1)-Fe-S(3), 150.45(8); S(2)-Fe-S(4), 152.25(7); S(2)-
Fe-S(1), 88.94(6); S(3)-Fe-S(4), 85.51(7); N(1)-Fe-S(1), 109.39(18);
N(1)-Fe-S(2), 105.9(2); N(1)-Fe-S(3), 100.02(18); N(1)-Fe-S(4),
101.53(19).
Upon injection of O2 gas into a CH2Cl2 solution of
complex [(NO)Fe(S,S-C6H2-3,6-Cl2)2]- (3), a reaction ensued
over the course of 1 week to give the dimeric [(NO)Fe(S,-
2-
SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2 (6) identified by
X-ray crystal structure. In complex 6, the sixth site of each
iron is ligated to one of the bound thiolato sulfurs of the
other [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]- moi-
ety (Scheme 1a). The infrared spectrum of complex 6
showing strong bands at 1844, 1801 and 1217 m, 1083 m
cm-1 (KBr) corresponding to the ν(NO) and ν(SO) stretching
frequencies of the S-bonded sulfinate group, respectively.
Formation of complex 6 can be interpreted as coordinative
association of two monoanionic [(NO)Fe(S,SO2-C6H2-3,6-
Cl2)(S,S-C6H2-3,6-Cl2)]-. It may be attributed to the electron
deficiency at the iron center, derived from the existence of
one sulfinate group. In complexes 1-3, NO bound to an
iron center leading to an {Fe(NO)}6 system triggers sulfur
oxygenation to yield iron-sulfinate nitrosyl complexes 4-6.
To prove that the bound NO is responsible for sulfur
2-
oxygenation, we repeated the reaction of [Fe(S,S-C6H4)2]2
Figure 2. ORTEP drawing and labeling scheme of complex 6. Selected
bonds (Å): Fe(1)-N(1), 1.651(2); Fe(1)-S(1), 2.2986(8); Fe(1)-S(3),
2.2941(8); Fe(1)-S(2), 2.2829(8); Fe(1)-S(4), 2.2506(9); N(1)-O(1),
1.154(3). Selected angle (deg): Fe-N(1)-O(1), 177.1(2).
and [(C4H8O)Fe(S,S-C6H4)2]- with dioxygen in THF, re-
spectively. On the basis of IR ν(SO) spectra for both of the
solution sample and the insoluble solid sample, no iron-
sulfinate compounds were generated after the reaction
solution was stirred in THF/CH2Cl2 for 6 days; instead, an
insoluble yellow solid occurred. The insoluble solid (in CH3-
CN/CH3OH) has not been characterized at this time. How-
ever, we cannot unambiguously rule out the possibility of
formation of sulfinate/sulfenate species instead of iron-
sulfinate complexes.
UV-vis spectra of complexes 4-6 exhibit an intense
absorption around 980 nm with extinction coefficient ꢀ >
4000 L mol-1 cm-1 which is ascribed to the intervalence
transition M(L)(L•) T M(L•)(L) (L ) 1,2-benzenedithiolate
and L• ) 1,2-benzenedithiolate radical monoanion) in the
near-infrared and display frequencies around 1035 cm-1 in
the infrared (KBr) indicating the presence of one (L•) radical,
as elucidated by Wieghardt and co-workers.20 Presumably,
the less electron-donating sulfinate ligand coordinated to the
{Fe(NO)} core of complexes 4-6 triggers the intramolecular
electron transfer from the “redox-noninnocent” dithiophen-
olato(2-) to the {Fe(NO)} core to stabilize complexes 4-6.20
On the basis of UV-vis spectral, electron spin resonance,
X-ray structural, and magnetic data, the electronic structures
of the {Fe(NO)} cores of complexes 4-6 are best described
as a {FeI(NO+)}7 (a d7 FeI(NO+) core coordinated by one
[S,SO2-C6H3R]2- ligand as well as one dithio-o-semibenzo-
quinone(-) radical).20
O-atom transfer has been observed in the presence of PPh3
from the sulfenate group in the Ni complex.17 Conversion
of complex 6 to 3 was displayed when CH2Cl2 solution (3
mL) of complex 6 (0.1 mM) and PPh3 (1 mM) was
photolyzed under N2 purge at ambient temperature for 20
min (Scheme 1a′); the shift in electronic absorptions at 528
and 974 nm to 500 and 1248 nm is accompanied by a color
change from purple to dark reddish brown which is in accord
with the formation of complex 3.
1
The magnetic measurements and the H NMR spectra
confirm complexes 1-3 are diamagnetic species.13 Also,
diamagnetism of complexes 4-6 is confirmed by the EPR
X-ray crystal structures of complexes 5 and 6 are depicted
in Figures 1 and 2, respectively, and selected bond dimen-
sions for both complexes are presented in the figure captions.
1
silence, the H NMR spectra, and the magnetic measure-
ments. As can be inferred from the reported data,18-20 the
(15) Grapperhaus, C. A.; Darensbourg, M. Y. Acc. Chem. Res. 1998, 31,
451.
(16) Kumar, M.; Colpas, G. J.; Day, R. O.; Maroney, M. J. J. Am. Chem.
Soc. 1989, 111, 8323.
(17) Farmer, P. J.; Verpeaux, J.-N.; Amatore, C.; Darensbourg, M. Y.;
Musie, G. J. Am. Chem. Soc. 1994, 116, 9355.
(18) McCleverty, J. A. Chem. ReV. 2004, 104, 403.
(19) Eisenberg, R.; Meyer, C. D. Acc. Chem. Res. 1975, 8, 26.
(20) (a) Ray, K.; Bill, E.; Weyhermuller, T. Wieghardt, K. J. Am. Chem.
Soc. 2005, 127, 5641. (b) Herebian, D.; Bothe, E.; Bill, E.; Weyher-
muller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 10012. (c)
Sun, X.; Chun, H.; Hildenbrand, K.; Bothe, E.; Weyhermuller, T.;
Neese, F.; Wieghardt, K. Inorg. Chem. 2002, 41, 4295. (d) Hsieh,
C.-H.; Hsu, I.-J.; Lee, C.-M.; Ke, S.-C.; Wang, T.-Y.; Lee, G.-H.;
Wang, Y.; Chen, J.-M.; Lee, J.-F.; Liaw, W.-F. Inorg. Chem. 2003,
42, 3925.
6672 Inorganic Chemistry, Vol. 44, No. 19, 2005

Nitrosylated Iron-Thiolate-Sulfinate Complexes
Table 1. Selected Infrared Data for Complexes 1-12 and Fe-NHase12
compds
ν
NO (cm-1, KBr)
ν
SO (cm-1, KBr)
1
1776
2
1763
3
1796
4
5
6
7
8
9
1779
1761
1844 (m), 1801 (s)
1636
1648
1196, 1060
1202, 1057
1217, 1083
1160, 1020
1146, 1022
1156, 1035
Figure 3. ORTEP drawing and labeling scheme of complex 8. Selected
bonds (Å): Fe-N(1), 1.659(6); Fe-S(1), 2.241(2); Fe-S(2), 2.259(2); Fe-
S(3), 2.278(2); Fe-S(4), 2.249(2); N(1)-O(1), 1.158(7) Å. Selected angles
(deg): Fe-N(1)-O(1), 171.6(6); S(1)-Fe-S(3), 144.04(9); S(4)-Fe-S(2),
158.35(9); S(1)-Fe-S(2), 88.24(8); S(3)-Fe-S(4), 86.66(7); N(1)-Fe-
S(1), 108.1(2); N(1)-Fe-S(2), 102.9(2); N(1)-Fe-S(3), 107.9(2); N(1)-
Fe-S(4), 98.7(2).
1661
10
11
12
1605
1606
1615
Fe-NHase
1853a
a Measured in phosphate buffer (pH ) 7.5).12
compensation from the “redox-noninnocent” dithiophenolate-
(2-) ligand to the {Fe(NO)}7 core any more. These results
indicate that electronic structure of the {Fe(NO)}7 core may
exist as a low-spin d6 Fe(II) (S ) 0) coordinated by NO•
radical (S ) 1/2) to produce the total spin state of 1/2, i.e.,
the {Fe(NO)}7 7-9 having a low-spin d6 Fe(II)-NO• radical
electronic configuration in a distorted square pyramidal
ligand field18 even although that the intermediate-spin FeIII
(S ) 3/2) antiferromagnetically coupled to coordinated triplet
NO- (S ) 1) cannot be unambiguously ruled out.14,18,19
Figure 3 displays the thermal ellipsoid plot of the dianionic
complex 8, and selected bond distances and angles are given
in the figure captions. Analysis of the bond angles for
complexes 7-9 reveals that coordination geometry of the
iron center is best described as a distorted square pyramid
with a bent apical NO, and the iron atom is displaced from
the mean four sulfur atom plane (0.5382 vs 0.5462 Å for 4
vs 7, 0.5492 vs 0.5590 Å for 5 vs 8, and 0.4947 vs 0.5168
Å for 6 vs 9). In contrast to the linear Fe-N-O bonds of
complexes 4-6, the slightly bent Fe-N-O bonds with
angles of 156.8(9), 168.0(7), and 166.0(8)° for complexes
7-9, respectively, also support the presence of {FeII(•NO)}7
electronic configuration.18 Geometry around the Fe center
responding rapidly to the electronic change of the {Fe(NO)}
core may be rationalized according to the studies on the
qualitative MO energy level orderings for the five-coordi-
nated {Fe(NO)}6/7 metal-nitrosyl complexes.19 Computa-
tional studies have suggested that geometrical change from
square pyramidal with linear apical NO to square pyramidal
with bent apical NO or trigonal bipyramidal with linear
equatorial NO may occur for a five-coordinated metal-
nitrosyl complex upon the electronic changes going from
{Fe(NO)}6 to {Fe(NO)}7.19 Compared to complexes 4-6,
the electronic effect (from {Fe(NO)}6 to {Fe(NO)}7) may
explain the observed lengthening of the average Fe-N(O)
bond lengths (average 1.634 (4-6) vs average 1.672 Å (7-
9)) in complexes 7-9.18 As shown in Table 2, the N-O
bond lengths of 1.149(10) and 1.158(7) Å in complexes 8
and 9, respectively, are comparable to the N-O bond
distance of 1.154 Å in free •NO but far from that of 1.26 Å
in NO-.21
Geometry of the iron center in complex 4 is best described
as a distorted trigonal bipyramidal coordination environment
with NO and sulfinate groups occupying equatorial and axial
positions, respectively.13 In comparison, the geometry of iron
center in complex 5 is a distorted geometry midway between
tetragonal pyramid and trigonal bipyramid, whereas the
distorted octahedral structure is adopted in complex 6.
Presumably, the lengthening in average Fe-N(O) bond
lengths (1.599(2) vs 1.629(3) Å (1 vs 4), 1.606(3) vs 1.622(5)
Å (2 vs 5), and 1.596(3) vs 1.651(2) Å (3 vs 6)) are caused
by electronic perturbations from sulfur oxygenation and the
induced intramolecular electron transfer from the coordinated
dithiophenolato(2-) ligand to the {Fe(NO)} core. Consistent
with other published transition-metal sulfinate complexes,15
the S-O bond lengths average ca. 1.405(6) and 1.455(2) Å
in complexes 5 and 6, respectively.
Reduction of Complexes 4-6. When CH3CN-THF
solutions of complexes 4-6, respectively, were treated with
1 equiv of [RS]- (R ) Et, o-C6H4-NH2), a pronounced
change in color of the solution from dark purple to dark green
accompanied by the dark yellow green precipitate was
observed at ambient temperature. Instead of the direct
nucleophilic attack of [SEt]- on the coordinated NO+
2-
yielding EtSNO and [Fe(S,SO2-C6H3R)(S,S-C6H3R)]2
,
UV-vis, IR spectra, and X-ray diffraction studies confirmed
the formation of the dianionic, mononuclear {Fe(NO)}7
[(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]2- (R ) H (7), m-CH3
(8)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2-
1
(9) accompanied by a byproduct of (SEt)2 identified by H
NMR (Scheme 1b). The infrared spectra show bands at 1636
and 1160, 1020 cm-1 (KBr) (7) (1648 and 1146, 1022 cm-1
(KBr) (8); 1661 and 1156, 1035 cm-1 (KBr) (9)) (Table 1)
corresponding to the ν(NO) and ν(SO) stretching frequencies
of the S-bonded sulfinate group, respectively.15 The lower
energy ν(NO) bands of complexes 7-9 shifted by ∼140
cm-1 from those of complexes 4-6 also imply the reduction
process occurred in the {Fe(NO)} core. The proton chemical
shifts of 1,2-benzenedithiolate ligands of complexes 7-9
were not observed in NMR spectra at room temperature. The
effective magnetic moment was 1.76 and 1.85 µB for
complexes 7 and 9, respectively. No optical evidence (based
on the UV-vis spectrum) for the presence of [S,S-C6H3R]•-
was observed. It is believed that relief of the electron
deficiency due to reduction does not require electronic
(21) (a) Teillet-Billy, D.; Fiquet-Fayard, F. J. Phys. Chem. 1990, 94, 3450.
(b) Wong, M. W.; Nobes, R. H.; Bouma, W. J.; Radom, L. Chem.
Phys. 1989, 91, 2971.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6673

Lee et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes
1-12
compds
Fe-N
N-O
S-O
-Fe-N-O
1
2
3
4
5
1.599(2)
1.606(3)
1.596(3)
1.629(3)
1.622(5)
1.651(2)
1.779(9)
1.659(6)
1.684(8)
1.712(6)
1.692(3)
1.702(5)
1.66(3)
176.2(3)
176.0(3)
177.6(3)
171.0(3)
171.9(5)
177.1(2)
156.8(8)
168.0(17)
166.0(8)
160.5(6)
151.8(3)
153.4(4)
1.182(4)
1.163(4)
1.169(4)
1.162(6)
1.154(3)
1.002(9)
1.158(7)
1.149(10)
1.08(1)
1.469(2), 1.459(2)
1.425(5), 1.385(6)
1.443(2), 1.466(2)
1.429(8), 1.394(8)
1.460(5), 1.501(5)
1.462(9), 1.48(1)
6
7a
8
9
10a
11
12
1.193(4)
1.186(6)
a The atoms Fe(1), N(1), O(1), O(2), and O(3) in 7 and Fe(1), N(1), and
O(1) in 10 are structurally disordered at two equal occupancy locations.
Reduction of Complexes 1-3. To follow up on our earlier
communication with a more complete understanding of the
reactivity of the {Fe(NO)}n (n ) 6, 7) nitrosylated iron-
thiolates toward oxygen and their relevance as a model of
oxygen binding to sulfur in the Fe-containing nitrile hydratase
active site,13 the [(NO)Fe(S,S-C6H3R)2]2- (R ) H (10),
m-CH3 (11)) and [(NO)Fe(S,S-C6H2-3,6-Cl2)2]2- (12) com-
plexes with the {Fe(NO)}7 electronic core were synthesized
by addition of [SEt]- to complexes 1-3, respectively, in
THF at ambient temperature (Scheme 1c). The most note-
worthy characteristic of reduction of complexes 1-3 is the
disappearance of a low-energy absorption band at 1304 nm
(1) (1300 nm (2), 1248 nm (3)) with concomitant formation
of a 684 nm absorption band (10) (684 nm (11), 670 nm
(12)). The ν(NO) stretching frequencies of complexes 10-
12 (1605 (10), 1606 (11), and 1615 (12) cm-1 (KBr)) fall
into the range (1620-1675 cm-1) observed for five-
coordinated iron-nitrosyl complexes having the {Fe(NO)}7
configuration with S ) 1/2 ground state.18 The proton
chemical shift of 1,2-benzenedithiolate ligands of complexes
10-12 was not observed in the NMR spectra at ambient
temperature. The effective magnetic moment is 2.2 and 2.1
µB for complexes 10 and 12, respectively. Electrochemistry
of complex 12 (2.5 mM), measured in CH3CN with 0.1 M
[n-Bu4N][PF6] as supporting electrolyte (scan rate 100 mV/
s), reveals one reversible oxidation-reduction process at
-0.554 V (E1/2 vs Ag/AgNO3), as compared to -0.365 V
for complex 9 (2.5 mM) (Figure 4).
The X-ray crystal structure of complex 12 is displayed in
Figure 5, and selected bond distances and angles are
presented in the figure captions. The Fe-N-O bond angles
of 160.5(6), 151.8(3), and 153.4(4)° for complexes 10-12
are similar to those of their five-coordinated analogues,
averaging 160° in {Fe(NO)}7 species containing the [S4]
donor atom set.18 The changes in Fe-N-O bond angle from
the bent form (151.8(3) and 153.4(4)° for complexes 11 and
12, respectively) to the less bent form (168.0(7) and 166.0(8)°
for complexes 8 and 9, respectively) are, presumably, caused
by electronic perturbation from sulfur oxygenation (Table
2). In contrast to the lengthening of the average Fe-N(O)
bond length caused by sulfur oxygenation accompanied by
the induced electron-transfer from dithiophenolate(2-) to the
{Fe(NO)} core in complexes 4-6, the relief of the electron
Figure 4. Cyclic voltammograms (at 100 mV/s) of (a) complex 12 (one
reversible redox process at -0.554 V (E1/2 vs Ag/AgNO3)) and (b) complex
9 (one reversible redox process at -0.365 V (E1/2 vs Ag/AgNO3)) in CH3-
CN solution with 0.1 M [n-Bu4N][PF6] as supporting electrolyte at 25 °C.
Figure 5. ORTEP drawing and labeling scheme of complex 12. Selected
bonds (Å): Fe-N(1), 1.702(5); Fe-S(av), 2.2527(16); N(1)-O(1), 1.186(6).
Selected angles (deg): Fe-N(1)-O(1), 153.4(4); S(1)-Fe-S(4), 144.42(6);
S(2)-Fe-S(3), 158.08(6); S(2)-Fe-S(1), 88.09(6); S(3)-Fe-S(4), 87.58-
(5); N(1)-Fe-S(1), 106.48(16); N(1)-Fe-S(2), 96.47(16); N(1)-Fe-S(3),
105.45(16); N(1)-Fe-S(4), 108.99(16).
deficiency from reduction in combination with the less
electron-donating sulfinate in {Fe(NO)}7 complexes 8 and
9 may explain the observed shortening of the average Fe-
N(O) bond lengths from complexes 11 and 12 to complexes
8 and 9 (the average Fe-N(O) bond length of 1.697(5) Å in
complexes 11 and 12 vs the average Fe-N(O) bond length
of 1.672(8) Å in complexes 8 and 9). It is also noticed that
the N-O bond lengths of 1.193(4) and 1.186(6) Å in
complexes 11 and 12, longer than those of 1.158(7) and
1.149(10) Å in complexes 8 and 9, are nearly midway
•
between the N-O bond distance of 1.154 Å in free NO
and 1.26 Å in NO- (Table 2).21 Therefore, the electronic
structures of the {Fe(NO)}7 core of complexes 10-12 are
best described in terms of having {FeII(•NO)}7 (a low-spin
d6 FeII (S ) 0) coordinated by NO• (S ) 1/2) radical in a
distorted square pyramidal ligand field) or the having
presence of a resonance hybrid of {FeII(•NO)}7 and
{FeIII(NO-)}.718,19
Photochemical Studies of the {Fe(NO)}6/7 Complexes.
NO release upon photolysis has been observed in the {Fe-
6674 Inorganic Chemistry, Vol. 44, No. 19, 2005

Nitrosylated Iron-Thiolate-Sulfinate Complexes
Scheme 2
to the photochemically inert {Fe(NO)}7 electronic core.22
These results demonstrated that the [O] atoms of the
coordinated-sulfinate ligand are photolabile in both {Fe-
(NO)}6 and {Fe(NO)}7 iron-nitrosyl-sulfinate complexes
under irradiation.13
Figure 6. Conversion of complex 7 (0.1 mM in CH3CN solution) to 10
monitored by UV-vis spectrometry under irradiation (Hg lamp, Imax
)
Oxidation of the {Fe(NO)}7 Complexes. When a CH3-
CN solution of complexes 10/12 was stirred with 1 equiv of
dioxygen under controlled conditions at room temperature,
a pronounced color change from dark green to dark red
brown occurred. The UV-vis, IR, and X-ray diffraction
studies confirmed the formation of complexes 1/3 (major
product) accompanied by byproducts [NO3]- and the dimeric
[Fe(S,S-C6H3R)2]22- (minor product) (Scheme 2). Reactions
carried out by using 18O2 demonstrate that O2 is the source
of the [NO3]-. The infrared spectrum displays νNO vibrational
bands at 1363 and 1374 cm-1 (KBr), shifting from 1384 cm-1
(KBr), in the isotopic labeling experiments. It has been shown
that nitrosyl heme iron enzymes (nitrosylhemoglobin and
nitrosylmyoglobin) oxidized to metHb and deoxyMb by O2
generate nitrate in vivo.23a,b The influence of the electronic
structure of the {Fe(NO)}6/7 core on the chemical features
of complexes 1-3 vs 10-12 is clearly reflected in the
oxygen reaction: complexes 1-3 with {Fe(NO)}6 core
promote electrophilic attack of O2 on sulfur and O2 activation
to yield the S-bonded monosulfinate iron-nitrosyl species
(complexes 4-6), whereas an attack of O2 on the {Fe(NO)}7
complexes 10-12 lead to the formation of complexes 1-3
(yield 84% for 10, 72% for 12) and the minor products [Fe-
(S,S-C6H3R)2]22- and [NO3]- (yield 9% for 10, 11% for 12).
Thus, the electronic structure of the {Fe(NO)}n center appears
crucial for promoting oxygenation at either the sulfur or the
NO site.
366 nm). The absorption band at 684 nm (10) increases with decrease of
the band at 641 nm (7), and an isosbestic point was observed.
(NO)}6 complexes.22 The absorption band (497 nm for 1,
500 nm for 2 and 3) decays after irradiation (Hg lamp, Imax
) 366 nm) of CH2Cl2 (THF) solution of complexes 1-3
(0.1 mM) for 30 min and yields an insoluble, uncharacterized
yellow solid. In contrast to photolability of NO (decomposi-
tion) in complexes 1-3, irradiation of CH3CN solution of
complexes 10-12 did not cause dissociation of the bound
NO. Photolysis (Hg lamp, Imax ) 417 nm) of a CH2Cl2
solution of complex 4 yielded complex 1 with kobs ) 4.8 ×
10-4 s-1 (25 ( 2 °C), compared to photolysis of CH2Cl2
solution of complex 4 in the presence of triphenylphosphine
(1200-fold) with kobs ) 6.3 × 10-4 s-1 (25 ( 2 °C).
Obviously, the presence of triphenylphosphine in the pho-
tochemical removal of oxygen of complex 4 did not greatly
accelerate the rate of the deoxygenation reaction. The
dianionic S-bonded monosulfinate complexes 7-9 also
undergo oxygen transfer reaction in CH3CN solution with 2
equiv of PPh3 over the course of 4 days to yield complexes
10-12, respectively, and triphenylphosphine oxide identified
by 31P NMR spectroscopy. In addition, the transformation
of complexes 7-9 to complexes 10-12 was also displayed
when CH3CN solutions of complexes 7-9 were photolyzed
under N2 purge at ambient temperature (Scheme 1d). The
quantitative conversion of complex 7 to 10 was also
monitored by UV-vis spectrometry under irradiation (Hg
lamp, Imax) 366 nm); the band at 641 nm (complex 7) (630
nm, complex 9) disappeared accompanied by the simulta-
neous formation of one intense absorption band at 684 nm
(complex 10) (670 nm, complex 12) and the color changed
from dark green to brown yellow. The isosbestic point was
observed in successive optical spectra, as shown in Figure
6. During this transformation, no product with NO dissocia-
tion was detected spectrally. In contrast to complexes 4-6
possessing photolabile [O] atoms of the sulfinate ligand and
the photolabile NO, the direct transformation of complexes
7-9 to 10-12, respectively, under irradiation was ascribed
In contrast to the electrophilic attack of O2 on •NO of
complexes 10/12, the direct conversion of complexes 7-9
to complexes 4-6 was observed when CH3CN solutions of
complexes 7-9 were treated with O2 at ambient temperature.
Obviously, these iron-sulfinate nitrosyl complexes contain-
ing the {Fe(NO)}7 core undergo a reversible one-electron
redox process (Scheme 1b′). Consistent with complex
[(C4H8O)Fe(S,S-C6H4)2]-, complex [Fe(S,S-C6H4)2]2- does
not initiate O2 activation to yield iron-sulfinate/-sulfenate
(23) (a) Herold, S.; Ro¨ck, G. Biochemistry 2005, 44, ASAP. (b) Bruun-
Jensen, L.; Skibsted, L. H. Meat Sci. 1996, 44, 145. (c) Clarkson, S.
G.; Basolo, F. Inorg. Chem. 1973, 12, 1528. (d) Ford, P. C.; Lorkovic,
I. M. Chem. ReV. 2002, 102, 993. (e) Trogler, W. C.; Marzilli, L. G.
Inorg. Chem. 1974, 13, 1008. (f) Kubota, M.; Phillips, D. A. J. Am.
Chem. Soc. 1975, 97, 5637.
(22) (a) Patra, A. K.; Afshar, R.; Olmstead, M. M.; Mascharak, P. K. Angew.
Chem., Int. Ed. 2002, 41, 2512. (b) Afshar, R. K.; Patra, A. K.;
Olmstead, M. O.; Mascharak, P. K. Inorg. Chem. 2004, 43, 5736.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6675

Lee et al.
compounds identified by IR ν(SO); instead, a dimeric [Fe-
(S,S-C6H4)2]2 formed after the reaction solution of [Fe-
(S,S-C6H4)2]2- and O2 was stirred in CH2Cl2 for 1 min.
(3) In the {Fe(NO)}6 system, the lengthenings in average
Fe-N(O) bond lengths are caused by sulfur oxygenation
accompanied by the induced electron rearrangement from
dithiophenolato(2-) ligand to the {Fe(NO)} core (Table 2).
However, the Fe-N-O bond angle is not affected by the
sulfur oxygenation. In the {Fe(NO)}7 system, the shorter Fe-
N(O) bond distances are observed as a consequence of
substitution of thiolate ligand by the corresponding S-bonded
sulfinate ligand, as observed in the iron-thiolate {Fe(NO)}7
complexes 11 and 12 (Fe-N ) 1.692(3) Å (11), Fe-N )
1.702(5) Å (12)) and iron-sulfinate {Fe(NO)}7 complexes
8 and 9 (Fe-N ) 1.659(6) Å (8), Fe-N ) 1.684(8) Å (9))
(Table 2). The influence of sulfinate ligand on the electronic
environment of the {Fe(NO)}7 core is also reflected in the
change of Fe-N-O bond angle of complexes 11 and 12 vs
8 and 9 from a bent form (average 152.6(4)° for complexes
11 and 12) to a less bent form (average 167.0(17)° for
complexes 8 and 9).
2-
Conclusion and Comments
Studies on the mononuclear iron-sulfinate nitrosyl and
iron-thiolate nitrosyl complexes (1-12) with the {Fe(NO)}6/7
core have led to the following results:
(1) Monoanionic complexes 1-3 with the {Fe(NO)}6 core
initiate sulfur oxygenation of iron-thiolate by molecular
oxygen to yield S-bonded monosulfinate iron complexes 4-6
via the proposed intermediate A (below), whereas dianionic
complexes 10-12 with the {Fe(NO)}7 core triggers, to a
certain extent, the attack of O2 on the •NO radical of the
[FeII-NO‚] core yielding complexes 1-3 accompanied by
the minor products [NO3]- and [Fe(S,S-C6H3R)2]22- via the
proposed intermediate B (below).23c-f In the studies of
reaction of dioxygen and nitrosyl CoII-Schiff base com-
plexes, coordination of dioxygen to the N atom of the NO
group is the rate-determining step, leading to an intermediate
{CoIIN(O)(η1-O2)}.23c The proposed species, CoN(O)(η1-O2),
gains supports from the fact that the reaction rate increases
with increase electron density at the nitrosyl site. One
electron reduction of the {Fe(NO)}6 complexes 1-3 in-
creases electron density about the {Fe(NO)} core of the
complexes 10-12. Higher electron density facilitates oxida-
tion of the nitrosyl group in the {Fe(NO)}7 complexes 10-
12. It also explains why sulfur oxygenation occurs in the
{Fe(NO)}6 complexes 1-3 instead of the nitrate formation.
To our knowledge, two transition metal nitrato complexes,
nitrato cobaloxime23e and Ir(PPh3)2(CO)ClX(NO3),23f have
been reported.
(4) The presence of NO binding to the iron center forming
{Fe(NO)}6 iron-thiolate nitrosyl complexes appears crucial
in initiating oxygenation at sulfur and resulting in the
thermally stable S-bonded monosulfinate iron species.
(5) The iron-sulfinate nitrosyl species with the {Fe(NO)}6/7
electronic core exhibit the photolabilization of sulfur-bound
[O] moiety under irradiation, and the sulfinate ligand
coordinated to the {Fe(NO)}6/7 core does not promote the
photolability of the Fe-NO bond.
(6) X-ray structural data, magnetic measurements showing
diamagnetic properties and EPR silence, and IR and UV-
vis spectra suggest that the electronic structures of complexes
4-6 are best described as a [(NO+)FeI(dithio-o-semibenzo-
quinone(-) radical)(sulfinato-thiophenolato(2-))]- (C).18,20
(2) The less electron-donating sulfinate coordinated to the
{Fe(NO)}7 core stabilizing the dianionic S-bonded mono-
sulfinate iron species 7-9 was employed to explain that the
NO group of complexes 10-12 are much more O2-sensitive
than the corresponding complexes 7-9. The electronic
perturbation of the {Fe(NO)}7 core caused by the coordinated
sulfinate ligand in complexes 7-9 serves to regulate the
reactivity of complexes 7-9 toward O2. In contrast to the
attack of O2 on the •NO radical of the {Fe(NO)}7 core of
complex 10 yielding complex 1 along with the minor
products [NO3]- and [Fe(S,S-C6H4)2]22-, complexes 7-9
with less electron-donating sulfinate ligand coordinated to
iron were oxidized by O2 to yield complexes 4-6, respec-
tively. Presumably, factors contributing to the distinct
oxidation pathways are attributed to the less electron-donating
sulfinate ligand sufficient to place the {Fe(NO)}7 core in a
{FeII(•NO)}7 condition but insufficient to cause the formation
of a resonance hybrid of the {FeII(•NO)}7 and {FeIII(NO-)}7
states, as implicated in the N-O bond distances of complexes
11 and 12 vs 8 and 9.
It has been known that the inactive NO-bound form of
Fe-NHase with the iron center coordinated by two modified
-
cysteines residues (sulfinate RSO2 and sulfenate RSO-
groups) through sulfur atoms and one NO is generated in
cells grown in the dark and the active form is regenerated
with release of NO and [O] molecules upon exposure to
light.6-8 As shown in Scheme 3, the biomimetic reaction
cycle of complexes 1-4-7-10 (or complexes 2-5-8-11
and 3-6-9-12) may provide some clues to the transforma-
tion mechanism between the active form and inactive NO-
bound form of Fe-NHase.
On the basis of this study, we speculate that the intercon-
version between the active and inactive NO-bound forms of
Fe-NHase may involve intramolecular {FeNO}6 T {FeNO}7
electron-rearrangement processes.5-8
6676 Inorganic Chemistry, Vol. 44, No. 19, 2005

Nitrosylated Iron-Thiolate-Sulfinate Complexes
Scheme 3
was added to precipitate the dark red solid [PPN][(NO)Fe(S,S-C6H2-
3,6-Cl2)2] (3) (0.363 g, 87%).13 Diffusion of hexane into a THF
solution of complexes 3 at -15 °C for 4 weeks led to red crystals
Experimental Section
Manipulations, reactions, and transfers were conducted under
nitrogen according to Schlenk techniques or in a glovebox (argon
gas). Solvents were distilled under nitrogen from appropriate drying
agents (diethyl ether from CaH2; acetonitrile from CaH2-P2O5;
methylene chloride from P2O5; hexane and tetrahydrofuran (THF)
from sodium benzophenone) and stored in dried, N2-filled flasks
over 4 Å molecular sieves. Nitrogen was purged through these
solvents before use. Solvent was transferred to the reaction vessel
via stainless cannula under positive pressure of N2. The reagents
iron pentacarbonyl, 1,2-benzenedithiol, toluene-3,4-dithiol, 3,6-
dichloro-1,2-benzenedithiol, 2-aminobenzenethiol, and bis(triphen-
ylphosphoranylidene)ammonium chloride ([PPN][Cl]) were used
as received. [PPN][NO2] and [PPN][Fe(CO)3(NO)] were prepared
by published procedures.24 Infrared spectra of ν(NO) and ν(SO)
stretching frequencies were recorded on a Perkin-Elmer model
spectrum one B spectrophotometer with sealed solution cells (0.1
mm) and KBr windows. UV-vis spectra were recorded on GBC
Cintra 10e and Jasco V-570 spectrometers. 1H NMR spectra were
obtained on a Varian Unity-400 spectrometer. Cyclic voltammo-
grams were obtained from 2.5 mM analyte concentration in CH3-
CN using 0.1 M [n-Bu4N][PF6] as a supporting electrolyte.
Potentials were measured at 298 K vs a Ag/AgNO3 reference
electrode by using a Pt working electrode. Under the conditions
employed, the potential (V) of the ferrocenium/ferrocene couple
was 0.16 (CH3CN). Analyses of carbon, hydrogen, and nitrogen
were obtained with a CHN analyzer (Heraeus).
suitable for X-ray crystallography. IR(THF): 1802 vs (νNO) cm-1
.
1H NMR (CD2Cl2): δ 7.612-7.637 (m), 7.418-4.746 (m), 7.075
(s) ppm. Absorption spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]:
324 (26 841), 500 (3957), 1248 (5603). Anal. Calcd for C48H34-
OS4N2P2Cl4Fe: C, 55.29; H, 3.29; N, 2.69. Found: C, 55.44; H,
3.54; N, 2.80.
Preparation of [PPN][(NO)Fe(S,SO2-C6H3-m-CH3)(S,S-C6H3-
m-CH3)] (5) and [PPN]2[(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-
3,6-Cl2)]2 (6).13 To a THF solution (10 mL) of complex 2 (0.373
g, 0.4 mmol) (complex 3 (0.416 g, 0.4 mmol)) was bubbled dry
O2 was bubbled until the mixture solution became purple (1 week)
at room temperature. Hexane (20 mL) was added to precipitate the
purple solid, and then the purple solid was redissolved in CH2Cl2.
Purification using silica gel chromatography with CH2Cl2 as the
eluent first gave a trace of yellow-green band, and a reddish brown
band collected that was identified as the unreacted complex 2 (3).
The eluent was then changed to THF and the purple product eluted
from the column. The purple solution was reduced in volume under
vacuum, and hexane was added to precipitate the purple solid [PPN]-
[(NO)Fe(S,SO2-C6H3-m-CH3)(S,S-C6H3-m-CH3)] (5) (0.078 g, 20%)
([PPN]2[(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2 (6) (0.077
g, 18%)). Recrystallization by diffusion of diethyl ether into a
concentrated THF solution of complex 5 (CH3CN solution of
complex 6) at -15 °C for 4 weeks afforded the purple crystals
Preparation of [PPN][(NO)Fe(S,S-C6H2-3,6-Cl2)2] (3). A 10
mL volume of air was injected into the THF solution (10 mL)
containing [PPN][Fe(CO)3(NO)] (0.4 mmol, 0.283 g) and 3,6-
dichloro-1,2-benzenedithiol (0.8 mmol, 0.169 g) at ambient tem-
perature. After the mixture solution was stirred for 3 days, the
reaction solution was filtered through Celite and 20 mL hexane
suitable for X-ray diffraction analysis. Complex 5: IR 1786 (νNO
)
cm-1 (THF), 1761 (νNO), 1202, 1057 (νSO) (KBr) cm-1; H NMR
(CD2Cl2) δ 7.95 (d), 7.89 (s), 7.50 (d), 7.11 (d), 7.00 (d), 6.86 (d),
2.38 (s), 2.29 (d) (S,S-C6H3-m-CH3, S,S(O)2-C6H4-m-CH3) ppm;
absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)] 315 (25 868),
535 (3809), 995 (3615). Complex 6: IR 1803 (νNO) cm-1 (THF),
1844, 1801 (νNO), 1217, 1083 (νSO) (KBr) cm-1; 1H NMR (CDCl3)
δ 7.227 (d), 7.015 (d), 6.995 (d) (S,S-C6H2-Cl2, S,S(O)2-C6H4-Cl2)
ppm; absorption spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)] 316
1
(24) (a) Hedberg, L.; Hedberg, K.; Satija, S. K.; Swanson, B. I. Inorg.
Chem. 1985, 24, 2766. (b) Albano, V. G.; Araneo, A.; Bellon, P. L.;
Ciani, G.; Manassero, M. J. Organomet. Chem. 1974, 67, 413.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6677

Lee et al.
(43 991), 528 (8109), 974 (6756). Anal. Calcd for C96H68O6N4P4S8-
Cl8Fe2: C, 53.65; H, 3.19; N, 2.61. Found: C, 53.83; H, 3.61; N,
2.62.
[PPN][(NO)Fe(S,S-C6H4)2] (1) (0.1 mM) was sealed under positive
N2. The solution was then irradiated by a Hg lamp (8 W × 8, Imax
) 366 nm) at room temperature. The reaction solution was
monitored immediately by UV-vis. After 18 min of irradiation,
the resulting light yellow solution accompanied by the insoluble
yellow solid showed no absorption band in the UV-vis spectrum
(also, no νNO peak in the IR spectrum). The uncharacterized,
insoluble yellow solid does not show a νNO stretching band in the
IR (KBr) spectrum.
Photolysis of CH2Cl2 Solutions of [PPN][(NO)Fe(SO2,S-
C6H4)(S,S-C6H4)] (4) in the Presence and Absence of PPh3,
Respectively.13 For comparison of photochemical deoxygenation
of complex 4 in the presence/absence of triphenylphosphine, the
reaction time courses of [O] release (or trapped by 1200 equiv of
PPh3) under irradiation in CH2Cl2 at 25 ( 2 °C were studied by
monitoring the decay of complex 4 with an intense absorption at
970 nm. The concentration of complex 4 is 1.9 × 10-4 M. A 3 mL
(concentration 1.9 × 10-4 M) of complex 4 was loaded into a
septum-sealed UV cell wrapped with aluminum foil in the dark.
Then the 1200 equiv of PPh3 (180 mg) was added to the UV cell.
Preparation of [PPN]2[(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)] (R
) H (7), m-CH3 (8)) and [PPN]2[(NO)Fe(S,SO2-C6H2-3,6-Cl2)-
(S,S-C6H2-3,6-Cl2)] (9).13 To a Schlenk tube containing [PPN][S,-
NH2-C6H4] (0.133 g, 0.2 mmol) was added the THF solution (7
mL) of complex 4 (0.188 g, 0.2 mmol) (complex 5, 0.193 g, 0.2
mmol; complex 6, 0.215 g, 0.1 mmol) under N2 at ambient
temperature. After the reaction solution was stirred overnight, the
mixture solution was stood for 30 min to precipitate the dark green
solid. The upper yellow solution was transferred to another flask
and then dried under vacuum to obtain the (S,NH2-C6H4)2 solid
identified by 1H NMR. Then THF (15 mL) was added to wash the
dark green solid twice. The green solid was dried under N2 purge
to afford the product [PPN]2[(NO)Fe(S,SO2-C6H4)(S,S-C6H4)] (7)
(0.268 g, 91%) ([PPN]2[(NO)Fe(S,SO2-C6H3-m-CH3)(S,S-C6H3-m-
CH3)] (8) (0.335 g, 90%); [PPN]2[(NO)Fe(S,SO2-C6H2-3,6-Cl2)-
(S,S-C6H2-3,6-Cl2)] (9) (0.142 g, 44%)). Diffusion of diethyl ether
into a CH3CN solution of complexes 7 (8, 9) at -15 °C for 4 weeks
led to dark green crystals suitable for X-ray diffraction. Complex
7: IR 1636 (νNO), 1160, 1020 (νSO) cm-1 (KBr); absorption
spectrum (CH3CN) [λmax, nm (ꢀ, M-1 cm-1)] 412 (3200), 502
(1030), 641 (750), 877 (240). Anal. Calcd for C84H68N3O3P4S4Fe:
C, 68.38; H, 4.65; N, 2.85. Found: C, 68.38; H, 4.92; N, 2.64.
The mixture solution was irradiated by Hg lamp (8 W × 8, Imax
)
417 nm) at 25 ( 2 °C and then monitored immediately by UV-
vis. The collected data were analyzed by the Sigmaplot program.
Data of the reaction time courses of [O] release in the presence (or
absence) of PPh3 were collected in the interval (3 and 5 min,
respectively) of the first 75 (100) min. The kobs of the photochemical
reaction was conducted under pseudo-first-order conditions. Pho-
tolysis of CH2Cl2 solution of complex 4 yielded complex 1 in the
presence and absence of PPh3 with kobs ) 6.3 × 10-4 (R2 ) 0.9987)
and 4.8 × 10-4 (R2 ) 0.9964) s-1 (25 ( 2 °C), respectively. The
byproduct Ph3PO was identified by 31P NMR.
Complex 8: IR 1648 (νNO), 1146, 1022 (νSO) (KBr) cm-1
;
absorption spectrum (CH3CN) [λmax, nm (ꢀ, M-1 cm-1)] 485 (2108),
660 (1158). Complex 9: IR 1661 (νNO), 1156, 1035 (νSO) (KBr)
cm-1; absorption spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 332
(18 274), 426 (4122), 630 (1010), 932 (528). Anal. Calcd for
C84H64O3S4N3P4Cl4Fe: C, 62.54; H, 4.00; N, 2.60. Found: C, 61.24;
H, 4.07; N, 3.16. The elemental analysis did not show good
agreement with the calculated values because of the extreme air-
sensitivity of sample 9.
Photolysis of CH3CN Solutions of Complex 7 (9).13 Into a 4
mL quartz reactor was added CH3CN solution (3 mL) of complex
7 (0.1 mM) (or 9) by syringe, and the reactor was sealed under
positive N2. The solution was irradiated by Hg lamp (8 W × 12,
Imax ) 366 nm) at 20 °C. The reaction was monitored immediately
by UV-vis. After 20 min of irradiation, the resulting green-yellow
solution had the dominant absorption band at 684 nm, consistent
with the formation of complex 10 (670 nm for complex 12).
Photolysis of CH3CN Solutions of Complex 10.13 To a 4 mL
quartz reactor was added CH3CN solution (3 mL) of complex 10
(0.1 mM) by syringe, and the reactor was sealed under positive
N2. The solution was irradiated by Hg lamp (8 W × 8, Imax ) 366
nm) at 20 °C, and the reaction was monitored by UV-vis. After
27 min of irradiation, the green-yellow solution shows the electronic
absorption of 684 with the same intensity. This result demonstrated
the complex 10 is stable under irradiation at 20 °C.
Reaction of Complex 7 and O2. A CH3CN solution (15 mL) of
complex 7 (0.051 g, 0.03 mmol) was purged with pure O2 gas (3.0
mL, 14 psi, at 293 K) by syringe. The flask was tightly sealed, and
the reaction solution was stirred at ambient temperature for 72 h.
The color of solution gradually turned from yellow-green to brown-
purple accompanied by a trace of white solid precipitate. Solvent
was removed under vacuum to leave the brown-purple solid. THF
(10 mL × 2) was added to extract the purple product, and the
solution was transferred to another 50 mL flask. Hexane (15 mL)
was then added to precipitate the dark purple solid after the solution
was concentrated to 3 mL under vacuum. The dark purple solid
was identified as complex 4 (0.023 g, 80%) by UV-vis and IR
spectra.
Preparation of [PPN]2[(NO)Fe(S,S-C6H3R)2] (R ) H (10),
m-CH3 (11)) and [PPN]2[(NO)Fe(S,S-C6H2-3,6-Cl2)2] (12). The
degassed THF (10 mL) was added via cannula to the Schlenk tube
containing [PPN][S,NH2-C6H4] (0.133 g, 0.2 mmol) (or 0.018 g
[Me4N][BH4] or 0.029 g [Et4N][BH4]) and complex 1 (0.181 g,
0.2 mmol) (complex 2 (0.186 g, 0.2 mmol); complex 3 (0.208 g,
0.2 mmol)) under N2 at ambient temperature. The reaction solution
was stirred for 2 h, and then stood for 30 min to precipitate the
green-yellow solid. The upper yellow solution was transferred to
another flask under positive N2 pressure and then dried under
vacuum to obtain the (S, NH2-C6H4)2 solid identified by 1H NMR.
The green-yellow solid was washed twice by THF and then dried
under N2 purge to afford the product [PPN]2[(NO)Fe(S,S-C6H4)2]
(10) (0.246 g, 85%) ([PPN]2[(NO)Fe(S,S-C6H3-m-CH3)2] (11)
(0.335 g, 90%); [PPN]2[(NO)Fe(S,S-C6H2-3,6-Cl2)2] (12) (0.251 g,
79%)). Diffusion of diethyl ether into a CH3CN solution of complex
10 (11, 12) at -15 °C for 4 weeks led to green crystals suitable
for X-ray crystallography. Complex 10: IR 1605 vs (νNO) cm-1
(KBr); absorption spectrum (CH3CN) [λmax, nm (ꢀ, M-1 cm-1)] 405
(3380), 511 (930), 684 (710), 762 (460). Anal. Calcd for C84H68N3-
OP4S4Fe: C, 69.89; H, 4.70; N, 2.91. Found: C, 69.80; H, 4.70;
N, 2.94. Complex 11: IR 1606 vs (νNO) cm-1 (KBr); absorption
spectrum (CH3CN) [λmax, nm (ꢀ, M-1 cm-1)] 420 (4829), 685
(1084). Complex 12: IR 1615 vs (νNO) cm-1 (KBr); absorption
spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)] 340 (18 034), 420
(4688), 670 (764). Anal. Calcd for C84H64OS4N3P4Cl4Fe: C, 63.80;
H, 4.08; N, 2.66. Found: C, 63.47; H, 4.07; N, 2.97.
Reaction of Complex 10 and O2. A CH3CN solution (15 mL)
of complex 10 (0.148 g, 0.1 mmol) was purged with O2 (4.0 mL,
14 psi, at 293 K) gas via the syringe, and then the reaction mixture
Photolysis of CH2Cl2 Solutions of [PPN][(NO)Fe(S,S-C6H4)2]
(1). A 4 mL quartz reactor containing a CH2Cl2 solution (3 mL) of
6678 Inorganic Chemistry, Vol. 44, No. 19, 2005

Nitrosylated Iron-Thiolate-Sulfinate Complexes
was allowed to stir for 3 h at room temperature. Rapid change of
solution color from green-yellow to red occurred. The solution was
concentrated to 3 mL, and diethyl ether (20 mL) was then added
to precipitate the red purple solid. After 20 min of standing, solvent
was removed by cannula and the red purple solid was dried under
vacuum. A solvent mixture of THF-diethyl ether (20:5 mL) was
added to extract the dark red product. Recrystallization from THF-
hexane diffusion gave dark red crystals of [PPN][(NO)Fe(S,S-
C6H4)2] (1) (0.076 g, 84%) identified by IR and UV-vis spectra.
Then, the mixed solvent CH3CN-THF (5:5 mL) was added to
dissolve the remaining residues. Diffusion of diethyl ether into this
CH3CN-THF solution at room temperature for 1 week led to dark
purple crystals of [PPN]2[Fe(S,S-C6H4)2]2 (0.031 g, 9%) identified
by the UV-vis spectrum (λmax: 517, 560, 614 nm), white crystals
of [PPN][NO3] identified by the IR spectrum (VNO: 1384 cm-1
(KBr)), and an unknown product with the PPN+ cation.
Reaction of Complex 12 and O2. Complex 12 (0.158 g, 0.1
mmol) was loaded into a 50 mL Schlenk flask, and 5 mL of CH3-
CN was added to afford the green-yellow solution. After injection
of 30 mL of dry dioxygen gas, the solution was stirred at room
temperature for 6 h. The resulting mixture was dried under vacuum,
and 10 mL of THF was added. The resulting mixture solution was
filtered through a porous filter to separate the insoluble residue
and the filtrate. Hexane (20 mL) was added to the filtrate to
precipitate the red-brown solid. Diffusion of hexane into the THF
solution (3 mL) of the red-brown solid gave red crystals of complex
3 (0.076 g, 72%). The remaining residue was redissolved in CH2-
Cl2 (10 mL) and filtered through Celite. Hexane (30 mL) was then
added to precipitate the dark brown solid characterized by X-ray
diffraction as the known [PPN]2[Fe(S,S-C6H2-3,6-Cl2)2]2 (0.024 g,
11%) and the white [PPN][NO3] identified by the IR spectrum
(νNO: 1384 cm-1 (KBr)).
Crystallography. Crystallographic data and structure refinements
parameters of complexes 3 and 5-12 are summarized in the
Supporting Information. 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 3 and 5-12 were carried out on a SMART CCD (Nonius
Kappa 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-hydrogen atoms and
fixed hydrogen atoms was based on F2. A SADABS25 absorption
correction was made. The SHELXTL26 structure refinement pro-
gram was employed. In the case of complex 3, the Cl group is
found at disordered positions (Cl(1-4):Cl(1′-4′) ) 1/2:1/2), which
were refined by partial occupancies. The atoms Fe(1), N(1), O(1),
O(2), and O(3) in 7 and Fe(1), N(1), and O(1) in 10 having two
sites with equal occupancy factors of 50% were successfully refined,
but O(1) in 7 was refined by an isotropic temperature factor. The
site occupancy factor of the SO2 group in complex 12 is 2/3:1/3
(O(2) and O(3):O(2′) and O(3′)) and was refined by partial
occupancies.
Acknowledgment. We greatly acknowledge financial
support from the National Science Council (Taiwan).
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of [(NO)Fe(S,SO2-
C6H2Cl2)(S,S-C6H2-3,6-Cl2)]22-, [(NO)Fe(S,S-C6H2-3,6-Cl2)2]-,
[(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]2- (R ) H, m-CH3), [(NO)-
Fe(S,SO2-C6H2Cl2)(S,S-C6H2-3,6-Cl2)]2-, [(NO)Fe(S,S-C6H3R)2]2-
(R ) H, m-CH3), and [(NO)Fe(S,S-C6H2-3,6-Cl2)2]2-. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC050108L
Magnetic Measurements. The magnetic data were recorded on
a SQUID magnetometer (MPMS5 Quantum Design company)
under an 1 T external magnetic field in the temperature range 2-300
K. The magnetic susceptibility data were corrected for temperature-
independent paramagnetism (TIP, 1 × 10-4 cm3 mol-1) and ligands’
diamagnetism by the tabulated Pascal’s constants.
(25) Sheldrick, G. M. SADABS, Siemens Area Detector Absorption Cor-
rection Program; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(26) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determi-
nation; Siemens Analytical X-ray Instruments Inc.: Madison, WI,
1994.
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