Dinuclear iron(II)-cyanocarbonyl complexes linked by two/three bridging ethylthiolates: Relevance to the active site of [Fe] hydrogenases

Article abstract of DOI:10.1021/ic0261225

Dinuclear iron(II)-cyanocarbonyl complex [PPN]₂[Fe(CN)₂(CO)₂(μ-SEt)]₂ (1) was prepared by the reaction of [PPN]-[FeBr(CN)₂(CO)₃] and [Na][SEt] in THF at ambient temperature. Reaction of complex 1 with [PPN][SEt] produced the triply thiolate-bridged dinuclear Fe(II) complex [PPN][(CN)(CO)₂Fe(CO)₂(CN)] (2) with the torsion angle of two CN^- groups (C(5)N(2) and C(3)N(1)) being 126.9°. The extrusion of two σ-donor CN^- ligands from Fe(II)Fe(II) centers of complex 1 as a result of the reaction of complex 1 and [PPN][SEt] reflects the electron-rich character of the dinuclear iron(II) when ligated by the third bridging ethylthiolate. The Fe-S distances (2.338(2) and 2.320(3) A for complexes 1 and 2, respectively) do not change significantly, but the Fe^II-Fe^II distance contracts from 3.505 A in complex 1 to 3.073 A in complex 2. The considerably longer Fe^II-Fe^II distance of 3.073 A in complex 2, compared to the reported Fe-Fe distances of 2.6/2.62 A in DdHase and CpHase, was attributed to the presence of the third bridging ethylthiolate, instead of π-accepting CO-bridged ligand as observed in [Fe] hydrogenases. Additionally, in a compound of unusual composition {[Na·5/2H₂O][(CN)(CO)₂Fe(μ-SEt) ₃Fe(CO)₂-(CN)]}_n{1/2O(Et)₂} _n (3), the Na⁺ cations and H₂O molecules combining with dinuclear [(CN)(CO)₂Fe(μ-SEt)₃Fe-(CO)₂(CN)] ^- anions create a polymeric framework wherein two CN^- ligands are coordinated via CN^--Na⁺/CN^--(Na⁺)₂ linkages, respectively.

Full text of DOI:10.1021/ic0261225

Inorg. Chem. 2003, 42, 2783−2788
Dinuclear Iron(II)−Cyanocarbonyl Complexes Linked by Two/Three
Bridging Ethylthiolates: Relevance to the Active Site of [Fe]
Hydrogenases
,†
Wen-Feng Liaw,* Wen-Ting Tsai,‡ Hung-Bin Gau,‡ Chien-Ming Lee,† Shin-Yuan Chou,‡
Wen-Yuan Chen,‡ and Gene-Hsiang Lee§
Department of Chemistry, Naional Tsing Hua UniVersity, Hsinchu 30043, Taiwan, Department of
Chemistry, National Changhua UniVersity of Education, Changhua 50058, Taiwan, and
Instrumentation Center, National Taiwan UniVersity, Taipei 10764, Taiwan
Received October 22, 2002
Dinuclear iron(II)−cyanocarbonyl complex [PPN]2[Fe(CN)2(CO)2(µ-SEt)]2 (1) was prepared by the reaction of [PPN]-
[FeBr(CN)2(CO)3] and [Na][SEt] in THF at ambient temperature. Reaction of complex 1 with [PPN][SEt] produced
the triply thiolate-bridged dinuclear Fe(II) complex [PPN][(CN)(CO)2Fe(µ-SEt)3Fe(CO)2(CN)] (2) with the torsion
angle of two CN- groups (C(5)N(2) and C(3)N(1)) being 126.9°. The extrusion of two σ-donor CN- ligands from
Fe(II)Fe(II) centers of complex 1 as a result of the reaction of complex 1 and [PPN][SEt] reflects the electron-rich
character of the dinuclear iron(II) when ligated by the third bridging ethylthiolate. The Fe−S distances (2.338(2)
II
II
and 2.320(3) Å for complexes 1 and 2, respectively) do not change significantly, but the Fe −Fe distance contracts
II
II
from 3.505 Å in complex 1 to 3.073 Å in complex 2. The considerably longer Fe −Fe distance of 3.073 Å in
complex 2, compared to the reported Fe−Fe distances of 2.6/2.62 Å in DdHase and CpHase, was attributed to the
presence of the third bridging ethylthiolate, instead of π-accepting CO-bridged ligand as observed in [Fe]
hydrogenases. Additionally, in a compound of unusual composition {[Na‚5/2H O][(CN)(CO)2Fe(µ-SEt)3Fe(CO)2-
2
(CN)]}n{1/2O(Et)2}n (3), the Na+ cations and H O molecules combining with dinuclear [(CN)(CO)2Fe(µ-SEt)3Fe-
2
(CO)2(CN)]- anions create a polymeric framework wherein two CN- ligands are coordinated via CN-−Na+/CN-−
(Na+)2 linkages, respectively.
Introduction
crystal structures,3-6 in combination with FTIR data,6 suggest
that the dinuclear iron subcluster is ligated by terminal CO
and CN- diatomic molecules with the two iron atoms linked
by a di(thiomethyl)-amine ligand (DdHase)/two thiolate
ligands (CpHase) and a carbonyl group (DdHase). Upon
reduction, FTIR spectra and the crystal structure show that
the previously bridging CO shifts toward the distal iron that
most likely serves as the primary hydrogen binding site.6
Studies on the electronic structure of the active center of
[Fe]-only hydrogenases may provide insight into understand-
ing the catalytic mechanism of [Fe] hydrogenases. Mo¨ssbauer
study of the H cluster in [Fe] hydrogenases isolated from C.
pasteurianum (CpII) showed that the dinuclear Fe subcluster
Hydrogen metabolism (reversibly catalyzed H2 molecules)
in the biological energy cycle is mostly mediated by two
types of metal-containing enzymes, [Fe]-only hydrogenases
and [NiFe] hydrogenases.1,2 The recent report of high-quality
X-ray crystal structures of [Fe]-only hydrogenases isolated
from DesulfoVibrio desulfuricans (DdHase)/Clostridium pas-
teurianum (CpHase) revealed that the active sites are
composed of a conventional [4Fe-4S] subcluster linked by
a bridging cysteine to a dinuclear iron subcluster.3-6 The
* Author to whom correspondence should be addressed. E-mail:
wfliaw@mx.nthu.edu.tw.
† Naional Tsing Hua University.
‡ National Changhua University of Education.
(3) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853.
§ National Taiwan University.
(1) (a) Adams, M. W. W. Biochim. Biophys. Acta 1990, 1020, 115. (b)
Nicolet, Y.; Lemon, B. J.; Fontecilla-Camps, J. C.; Peters, J. W. Trends
Biochem. Sci. 2000, 25, 138. (c) Peters, J. W. Curr. Opin. Struct. Biol.
1999, 9, 670.
(2) (a) Albracht, S. P. J. Biochim. Biophys. Acta 1994, 1178, 167. (b)
Volbeda, A.; Charon, M. H.; Piras, C.; Hatchikian, E. C.; Frey, M.;
Fontecilla-Camps, J. C. Nature 1995, 373, 580.
(4) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps,
J. C. Structure 1999, 7, 13.
(5) (a) Lemon, B. J.; Peters, J. W. J. Am. Chem. Soc. 2000, 122, 3793.
(b) Lemon, B. J.; Peters, J. W. Biochemistry 1999, 38, 12970.
(6) Nicolet, Y.; de Lacey, A. L.; Verne´de, X.; Fernandez, V. M.;
Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001,
123, 1596.
10.1021/ic0261225 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/26/2003
Inorganic Chemistry, Vol. 42, No. 8, 2003 2783

Liaw et al.
contains two low-spin FeII-FeII sites in the reduced state
(Hred), while it is in the mixed-valence FeIII-FeII site in the
oxidized state (Hox and Hox-CO).7 In contrast, Nicolet et al.
argued that in the oxidized active state of the H cluster, the
Fe ions of the dinuclear subcluster could be either in ferrous
intermediate spin (S ) 1) or ferric low spin (S ) 1/2).6 Recent
studies implicate that FeI-FeI forms are also possible in the
dinuclear iron subcluster of the active-site of [Fe] hydrogen-
ases since the FeI-FeI distance of 2.517(12) Å in (µ-pdt)-
Scheme 1
2-
[Fe(CO)2(CN)]2 (pdt ) SCH2CH2CH2S)8 is closely com-
parable with the Fe-Fe distance of 2.6 and 2.62 Å observed
in [Fe] hydrogenases isolated from D. desulfuricans and C.
pasteurianum.3,4 The Fe-Fe distance of 2.6/2.62 Å observed
in DdHase and CpHase is significantly different from the
FeII-FeII distance of 3.473 Å found in [Fe(CO)2(CN)-
(-SC6H4S-)]22-,9 but comparable to that observed in (µ-
+
H)(µ-pdt)[Fe(CO)2(PMe3)]2 (2.578(1) Å), a functional
generally believed that the dinuclear Fe subcluster of DdHase
and CpHase may be assigned as [FeIIFeII], [FeIFeII], and [FeI-
FeI] in the fully oxidized (inactive), the oxidized (active),
and the reduced states, respectively.6,13,14
model of [Fe] hydrogenases in the catalytic isotopic scram-
bling of D2/H2O and H2/D2 mixtures.10,11 In particular, the
recent discovery by Rauchfuss and co-workers shows that
[HFe2(µ-SR)2(CO)4(CN)(PMe3)] serves as a catalyst for
proton reduction.12
Very recently, De Lacey et al. obtained high-quality FTIR
spectra for the Hox and the CO-inhibited Hox (Hox-CO) from
DdHase and indicated that the Hox and the Hox (fully oxi-
dized, inactive state) have similar structures.13 Inspection of
the νCO stretching frequencies (1965, 1940, 1802 cm-1) of
the Hox form of DdHase13 reveals that these bands are sig-
nificantly different from those of the Fe(II) bioorganometallic
complex [Fe(CO)2(CN)(-SC6H4S-)]22- (2011, 1961 cm-1).6,9
The distinct differences in νCO between (µ-pdt)[FeI(CO)2-
(CN)]22- (1964, 1924, 1885 cm-1)8,11 and [Fe] hydrogenases
isolated from D. desulfuricans (2016, 1972, 1963, 1811 cm-1,
CO-inhibited form)6,13 suggest the distinguishable electronic
structure of the iron atoms in the model compound (µ-pdt)-
Here we report two dinuclear iron-thiolate cyanocarbonyl
2-
compounds, [Fe(CN)2(CO)2(µ-SEt)]2 (1) and [Fe2(CN)2-
(CO)4(µ-SEt)3]- (2). The coordination chemistry of com-
plexes 1 and 2 suggests that the certain total number of
thiolate and cyanide ligands coordinated to Fe(II) center
provides significant stabilization to the dinuclear Fe(II)-
carbonyl complexes and reveals the significant FeII-FeII
distance contraction as the dinuclear iron(II)-cyanocarbonyl
complex is transformed from the doubly thiolate-bridged to
the triply thiolate-bridged forms. On the basis of IR νCO
stretching frequencies of complexes 1 and 2, these results
may provide insights into the electronic features of the
dinuclear iron subcluster of the [Fe] hydrogenase active site.
air
Results and Discussion
2-
[Fe(CO)2(CN)]2 and the CO-inhibited DdHase (CO is
When [PPN][Fe(Br)(CN)2(CO)3]] (0.5 mmol)15 was re-
acted directly with [Na][SEt] (0.5 mmol) in THF at room
temperature, dinuclear hexacoordinate d6 FeII complex
[PPN]2[Fe(CN)2(CO)2(µ-SEt)]2 (1) was isolated as light
yellow crystals after recrystallization from THF-hexane
(yield 18%) (Scheme 1a,b). A nucleophilic displacement and
the concomitant dimerization may account for the formation
of complex 1. Attempts to detect the extremely unstable
intermediate, the mononuclear [Fe(CN)2(CO)3(SEt)]-, by
FTIR were unsuccessful (Scheme 1a). The IR spectrum of
complex 1 in the aprotic solvent CH3CN reveals two weak
absorption bands for the CN- ligands at 2121 vw, 2108 w
cm-1 supporting a trans position of two CN- groups, while
the two strong absorption bands 2033s, 1984 s cm-1 assigned
to the carbonyl stretching frequencies support a cis position
known to be a reliable reporter ligand for electron density
changes). The inconsistency between the observed FeI-FeI
distance and the low-spin FeII-FeII assignment was noted
in the studies of the dinuclear iron-cyanocarbonyl-thiolate
model compounds and the Mo¨ssbauer spectra of C. pasteur-
ianum enzyme, respectively.4,6,7,9,10 Obviously, the oxidation
states of the diiron subcluster in different redox forms of
[Fe] hydrogenase are still controversial,13,14 although it is
(7) Popescu, C. V.; Μu¨nck, E. J. Am. Chem. Soc. 1999, 121, 7877.
(8) (a) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem.
Soc. 1999, 121, 9736. (b) Le Cloirec, A.; Best, S. P.; Borg, S.; Davies,
S. C.; Evans, D. J.; Hughes, D. L.; Pickett, C. J. Chem. Commun.
1999, 2285.
(9) Liaw, W.-F.; Lee, N.-H.; Chen, C.-H.; Lee, C.-M.; Lee, G.-H.; Peng,
S.-M. J. Am. Chem. Soc. 2000, 122, 488.
(10) (a) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710. (b)
Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. ReV. 2000,
206, 533.
(11) (a) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg,
M. Y. Angew. Chem., Int. Ed. 1999, 38, 3178. (b) Lyon, E. J.;
Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem.
Soc. 2001, 123, 3268.
(14) (a) Pierik, A. J.; Hulstein, M.; Hagen, W. R.; Albracht, S. P. Eur. J.
Biochem. 1998, 258, 572. (b) Patil, D. S.; Moura, J. J. G.; He, S. H.;
Teixeira, M.; Prickril, B. C.; Der Vartanian, D. V.; Peck, H. D., Jr.;
Legall, J.; Huyanh, B. H. J. Biol. Chem. 1988, 263, 18732. (c) Cao,
Z. X.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3734. (d) Liu, Z.-P.;
Hu, P. J. Am. Chem. Soc. 2002, 124, 5175.
(15) Liaw, W.-F.; Lee, J.-H.; Gau, H.-B.; Chen, C.-H.; Jung, S.-J.; Hung,
C.-H.; Chen, W.-Y.; Hu, C.-H.; Lee, G.-H. J. Am. Chem. Soc. 2002,
124, 1680.
(12) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476.
(13) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.;
Fernandez, V. M. J. Am. Chem. Soc. 2000, 122, 11232.
2784 Inorganic Chemistry, Vol. 42, No. 8, 2003

Dinuclear Iron(II)-Cyanocarbonyl Complexes
Figure 1. ORTEP drawing and labeling scheme of the [(CN)2(CO)2Fe-
2-
(µ-SEt)]2 anion.
Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes
(a) 1 and (b) 2
Complex 1
Fe(1)-C(1)
Fe(1)-C(3)
Fe(1)-S(1)
1.762(7)
1.924(6)
2.3462(15)
Fe(1)-C(2)
Fe(1)-C(4)
Fe(1)-S(1A)
1.777(6)
1.926(6)
2.3291(16)
C(1)-Fe(1)-C(2)
S(1)-Fe(1)-S(1A)
94.3(3)
82.86(5)
C(3)-Fe(1)-C(4)
Fe(1)-S(1)-Fe(1A)
176.4(2)
97.14(5)
Complex 2
Fe(1)-C(1)
Fe(1)-C(3)
Fe(1)-S(2)
Fe(2)-C(4)
Fe(2)-C(5)
Fe(2)-S(3)
1.786(9)
1.945(10)
2.318(3)
1.766(11)
1.954(9)
2.324(2)
Fe(1)-C(2)
Fe(1)-S(3)
Fe(1)-S(1)
Fe(2)-C(6)
Fe(2)-S(1)
Fe(2)-S(2)
1.745(9)
2.318(3)
2.324(2)
1.767(10)
2.317(3)
2.330(3)
Figure 2. (a) ORTEP drawing and labeling scheme of the [(CN)(CO)2Fe-
(µ-SEt)3Fe(CO)2(CN)]- anion. (b) A view along the Fe(2)-Fe(1) axis for
the [(CN)(CO)2Fe(µ-SEt)3Fe(CO)2(CN)]- anion. Hydrogen atoms are
omitted for clarity. Torsion angles (deg): C(4)-Fe(2)-Fe(1)-C(3), 10.5;
C(5)-Fe(2)-Fe(1)-C(3), 126.9; C(5)-Fe(2)-Fe(1)-C(2), 10.0; C(6)-
Fe(2)-Fe(1)-C(1), 0.5.
C(1)-Fe(1)-C(2)
C(1)-Fe(1)-C(3)
C(1)-Fe(1)-S(3)
S(2)-Fe(1)-S(1)
94.0(4)
87.4(4)
92.5(3)
81.47(9)
C(2)-Fe(1)-C(3)
C(2)-Fe(1)-S(3)
C(3)-Fe(1)-S(3)
Fe(1)-S(2)-Fe(2)
88.7(4)
173.2(3)
93.6(3)
82.78(9)
(1), C(3A)N(1A)) point into the antiparallel direction.
Formation of complex 1 can be interpreted as coordinative
association of two five-coordinate electron-deficient mono-
nuclear [Fe(CN)2(CO)2(SEt)]-, and attributed to the instabil-
ity of the monomeric [Fe(CN)2(CO)2(SEt)]- as well as the
avoidance of electron deficiency at the monomeric iron(II)
centers. The average Fe-CO and Fe-CN bond lengths are
1.769(7) and 1.925(6) Å, respectively.
Subsequent reactions of complex 1 with [PPN][SEt] in a
1:1 molar ratio in THF produced the thermally stable [PPN]-
[Fe2(CN)2(CO)4(µ-SEt)3] (2) (yield 23%), and the known
trans-[PPN]2[Fe(CN)4(CO)2] (3) (yield 8%)15,16 as identified
by IR and X-ray diffraction (Scheme 1c). Obviously, the
transformation of complex 1 into complex 2 is accompanied
by extrusion of two σ-donor CN- ligands from iron(II)
centers of complex 1, reflecting the electron-rich character
of the dinuclear iron(II) when ligated by the third bridging
ethylthiolate. That is, the nucleophilic reaction of [PPN][SEt]
occurs at the more electron-deficient iron(II) site to yield
complex 2. Structurally, the Fe-S distances (2.338(2) Å
(average) for complex 1 and 2.320(3) Å (average) for com-
plex 2) do not change significantly, but the FeII-FeII distance
decreases from 3.505 Å in complex 1 to 3.073 Å in complex
2, and this shortening of the FeII-FeII distance in complex
2 is accompanied by a decrease in the Fe-S-Fe angles
from 97.14(5)° to 82.88(9)° (Figure 2) (Table 1). The FeII-
of two CO ligands (vibrationally uncoupled CO groups and
CN- groups on adjacent iron(II) sites).9 The FeII-FeII
distance of 3.505 Å in complex 1 is considerably longer than
those reported for the dinuclear iron subcluster (2.6 and 2.62
Å) of the [Fe] hydrogenase active site (Figure 1).3,4 Here
the lack of the third bridging CO ligand and the nearly square
planar FeS2Fe core in complex 1, in contrast to butterfly
structure of the active-site FeS2Fe core observed in [Fe]
hydrogenases,3,4 are adopted to rationalize the significantly
longer FeII-FeII distance in complex 1. Presumably, the role
of increments of CN- ligands coordinated to the Fe(II)Fe(II)
center in complex 1, compared to dinuclear Fe(I)Fe(I)
2- 8,11
compound (µ-pdt)[FeI(CO)2(CN)]2
,
is to increase the
electron density in the FeII-FeII center as well as to stabilize
the FeII-FeII oxidation level.
Selected bond distances and angles are given in Table 1.
2-
The dinuclear [Fe(CN)2(CO)2(µ-SEt)]2 is electronically
dianionic, and therefore both irons should be divalent. Com-
plex 1 exhibits a diagnostic 1H NMR spectrum with the ethyl-
thiolate proton resonances at 2.698 (q), 1.270 (t) ppm, which
are consistent with the Fe(II) having a low-spin d6 electronic
configuration in an octahedral ligand field. Complex 1 pos-
sesses crystallographically imposed centrosymmetry. Each
iron atom is surrounded pseudooctahedrally by two bridging
ethylthiolates, two cyanides, and two carbonyl groups, and
two pairs of CN groups (C(4)N(2), C(4A)N(2A) and C(3)N-
(16) Jiang, J.; Koch, S. A. Angew. Chem., Int. Ed. 2001, 40, 2629.
Inorganic Chemistry, Vol. 42, No. 8, 2003 2785

Liaw et al.
FeII distance of 3.073 Å in complex 2 is considerably longer
than the FeI-FeI distance of 2.5090(6) Å in (µ-SCH2-
2- 18
NMeCH2S-)[Fe(CO)2(CN)]2
,
but comparable to the
FeII-FeII distance of 3.07 Å observed in [(PR3)(CO)2Fe(µ-
SR)3Fe(CO)2(PR3)]+ (R ) alkyl).19 Two Fe(II) atoms are
connected via three thiolate bridges, and the ethyl groups of
three bridging µ2-ethylthiolates in 2 form a regular propeller-
like arrangement around the S3 plane defined by the three
sulfurs (Figure 2). Two groups, [Fe(1)(CO)2(CN)] and [Fe(2)-
(CO)2(CN)], are in an eclipsed conformation with the torsion
angle of two CN- groups (C(5)N(2) and C(3)N(1)) being
126.9° (Figure 2b). The average Fe-CO and Fe-CN bond
lengths of 1.766(11) and 1.950(10) Å, respectively, closely
compare with those found in complex 1 (1.769(7) and
1.925(6) Å, respectively) (Table 1).
Figure 3. X-ray structural diagram of the three-dimensional lattice showing
the interactions of Na cations with the [(CN)(CO)2Fe(µ-SEt)3Fe(CO)2(CN)]-
anions and H2O molecules in complex 3.
The anionic complex 2 exhibits a one-band pattern in the
νCN region (2110 w cm-1 (THF)) and a three-band pattern
in the νCO region (2029 w, 2014 s, 1968 m cm-1 (THF)),
similar to the νCN and νCO spectra (νCN 2108; νCO 2048, 2024,
polymeric orange solid is insoluble in noncoordinating
solvents, such as hexane and CH2Cl2, but shows good
solubility in diethyl ether and THF, which allows one to
-
1987 cm-1) of (µ-H)(µ-pdt)[FeII(CO)2(CN)]2 reported re-
1
cently by Darensbourg and co-workers.10,11 Obviously, the
νCO stretching frequencies of complex 2 are much higher,
compared to those of the fully reduced state (Hred) (1965,
1940, 1916, 1894 cm-1) of the dinuclear iron subcluster of
DdHase.1,13,14 If complex 2 and the Hox-CO DdHase have
the same oxidation level for dinuclear iron, one would expect
that the dinuclear iron active site of the H cluster shows
higher νCO stretching frequencies than complex 2, since the
third bridging [SEt]- ligand of complex 2 is a better electron
donor than a bridging CO group observed in Hox-CO
DdHase, except that the [Fe4S4]-Scys unit serves as a stronger
electron donor. However, the νCO vibrational frequencies of
the triply ethylthiolate-bridged complex 2 are even higher
than those of the Hox-CO DdHase (2016, 1972, 1963, 1811
cm-1), of which the lowest band at 1811 cm-1 was assigned
to be the stretching mode of bridging CO in the H
cluster.6,13,14 This result may lend support for the Hox-CO
and Hox (active) states of DdHase, showing the νCO stretching
bands at 2016, 1972, 1963, 1811 cm-1 and 1965, 1940, 1802
cm-1, respectively, being the mixed-valent Fe(II)-Fe(I), a
prominent candidate responsible for H2 uptake, heterolytic
cleavage of H2.6,13,14 Alternatively, THF solution of a mixture
of [PPN][Fe(CO)4(CN)] (0.5 mmol) and I2 (0.5 mmol) was
added to [Na][SEt] (1 mmol) and stirred under N2 at 25 °C.
The reaction mixture finally led to the isolation of an orange
solid, which was recrystallized from diethyl ether-hexane
as orange crystals of {[Na‚5/2H2O][(CN)(CO)2Fe(µ-SEt)3Fe-
(CO)2(CN)]}n{1/2O(Et)2}n (3). The compound of unusual
composition, {[Na‚5/2H2O][(CN)(CO)2Fe(µ-SEt)3Fe(CO)2-
(CN)]}n{1/2O(Et)2}n, is a polymer containing dinuclear [(CN)-
(CO)2Fe(µ-SEt)3Fe(CO)2(CN)] units connected through
CN-‚‚‚Na+(H2O)2 interactions to form layers (chains). The
perform the solution NMR experiments. In the H NMR
spectrum, a singlet for the H2O molecules at δ 2.47 ppm is
observed. Three sharp quartet (δ 2.33, 2.50, 2.71 ppm) and
two triplet (δ 1.30, 1.48 ppm) H NMR resonances were
1
assigned to three nonequivalent -SCH2CH3 groups in C4D8O
1
solution. On the basis of solubility and H NMR spectro-
scopy, it is assumed that the structural feature (polymeric
structure) found for complex 3 in the solid state is retained
with tetrahydrofuran molecules displacing H2O molecules
and coordinating to Na+ cations in THF, although we cannot
unambiguously rule out the possibility of formation of the
monomeric dinuclear complex [Na‚(THF-H2O)n][(CN)-
(CO)2Fe(µ-SEt)3Fe(CO)2(CN)] in THF solution. Treatment
of complex 3 with [PPN][Cl] in THF for 1 h results in an
infrared νCN/νCO spectrum identical with that of complex 2
(νCN/νCO shift from 2120 w, 2113 w, 2032 m, 2019 s, 1976
m to 2110 w, 2029 m, 2014 s, 1968 m cm-1 in THF).
The X-ray structural diagram of the three-dimensional lat-
tice of complex 3 is shown in Figure 3. The Na cations and
H2O molecules combine with the homodinuclear [(CN)(CO)2-
Fe(µ-SEt)3Fe(CO)2(CN)]- anions to create a polymeric
framework wherein two CN ligands are coordinated via CN-
Na+/CN-(Na+)2 linkages (i.e., one cyanide group ligated
Fe and Na cation as Fe-CN-Na+ while the other cyanide
group ligated to one Fe and two Na cations by Fe-CN-
(Na+)2), respectively. The Na cations occur in pairs linked
by two bridging H2O molecules and one CN group, and the
inter-pairs of Na cations are linked by two H2O molecules.
Also, each Na+ center is additionally coordinated to one ter-
minal H2O ligand. The Fe-CO distances (1.775(5) Å, aver-
age) and Fe-CN distances (1.912(4) Å, average) are consis-
tent with those in complex 2 (Table 2). The FeII-FeII distance
of 3.087 Å in complex 3 is longer than that in complex 2.
Two fragments, Fe(3)(CO)2(CN) and Fe(4)(CO)2(CN), are
in an eclipsed conformation with the torsion angle of the
two CN groups (C(13)N(3) and C(16)N(4)) being 3.5°
(Figure 3).
(17) Rauchfuss, T. B.; Contakes, S. M.; Hsu, S. C. N.; Reynolds, M. A.;
Wilson, S. R. J. Am. Chem. Soc. 2001, 123, 6933.
(18) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Benard, M.; Rohmer, M.-
M. Angew. Chem., Int. Ed. 2001, 40, 1768.
(19) (a) Treichel, P. M.; Crane, R. A.; Matthews, R.; Bonnin, K. R.; Powell,
D. J. Organomet. Chem. 1991, 402, 233. (b) Schultz, A. J.; Eisenberg,
R. Inorg. Chem. 1973, 12, 518.
2786 Inorganic Chemistry, Vol. 42, No. 8, 2003

Dinuclear Iron(II)-Cyanocarbonyl Complexes
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complex 3
FTS-185 spectrophotometer with sealed solution cells (0.1 mm)
and KBr windows. H NMR spectra were obtained on a Bruker
1
Fe(1)-C(3)
Fe(1)-C(1)
Fe(2)-C(6)
Fe(1)-S(1)
Fe(1)-S(3)
Fe(2)-S(2)
Na(1)-N(3)
Na(1)-O(12)
Na(1)-Na(2)
1.765(5)
1.909(4)
1.778(5)
2.3182(12)
2.3264(12)
2.3345(12)
2.513(4)
2.422(3)
3.177(2)
Fe(1)-C(2)
Fe(2)-C(5)
Fe(2)-C(4)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(3)
Na(2)-N(3)
Na(1)-O(9)
Na(2)-O(9)
1.786(5)
1.771(5)
1.915(4)
2.3082(12)
2.3148(11)
2.3194(13)
2.554(3)
2.428(3)
2.343(3)
model AC 200 spectrometer. UV/vis spectra were recorded on a
Hewlett-Packard 71 spectrophotometer. Analyses of carbon, hy-
drogen, and nitrogen were obtained with a CHN analyzer (Heraeus).
Preparation of [PPN]2[(CN)2(CO)2Fe(µ-SEt)]2 (1). The com-
pounds [PPN][Fe(CO)3(CN)2(Br)] (0.5 mmol, 0.405 g)15 and [Na]-
[SEt] (0.5 mmol, 0.042 g) were dissolved in THF (10 mL) and
stirred overnight at ambient temperature. The reaction mixture was
then filtered to remove NaBr, and hexane (15 mL) was added to
precipitate the air-stable, orange yellow solid [PPN]2[(CN)2(CO)2Fe-
(µ-SEt)]2 (1). The yellow crystals [PPN]2[(CN)2(CO)2Fe(µ-SEt)]2
(1) suitable for X-ray diffraction were obtained by vapor diffusion
of hexane into a THF solution of complex 1 at -15 °C (yield 0.069
g, 18%). IR: 2124 vw, 2112 w (νCN), 2029 s, 1981 s (νCO) cm-1
(THF); 2121 vw, 2108 w (νCN), 2033 s, 1984 s (νco) cm-1 (CH3-
C(1)-Fe(1)-C(2)
S(1)-Fe(1)-S(2)
91.45(18) C(2)-Fe(1)-S(2)
80.53(4) Fe(2)-S(1)-Fe(1)
94.94(14)
83.19(4)
87.63(10)
82.18(11)
O(11)-Na(1)-O(10) 155.31(12) O(11)-Na(1)-O(9)
O(12)-Na(1)-N(3)
93.94(11) O(9)-Na(1)-N(3)
O(11)-Na(1)-Na(2) 131.83(9)
O(10)-Na(1)-Na(2) 46.37(7)
O(10)-Na(1)-N(3) 81.25(11)
N(3)-Na(1)-Na(2)
51.75(8)
Summary. Doubly and triply thiolate-bridged dinuclear
iron(II)-cyanocarbonyl complexes [PPN]2[Fe(CN)2(CO)2-
(µ-SEt)]2 (1) and [PPN][(CN)(CO)2Fe(µ-SEt)3Fe(CO)2(CN)]
(2) were prepared, respectively, by the reactions of [PPN]-
[FeBr(CN)2(CO)3] and [Na][SEt] in THF at ambient tem-
perature. The extrusion of two strong σ-donor CN- ligands
from iron(II) centers of complex 1 as a result of the reaction
of complex 1 and [PPN][SEt] reflects the electron-rich
character of the dinuclear iron(II) when ligated by the third
bridging ethylthiolate. Notably, the certain total number of
thiolate and cyanide ligands coordinated to the Fe center (1
vs 2 vs (µ-pdt)[Fe(CO)2(CN)]22-) stabilizes the binding of
carbon monoxide to iron metals in the intermediate oxidation
state.8-12 The FeII-FeII distance decreases from 3.505 Å in
complex 1 to 3.073 Å in complex 2. The considerably longer
FeII-FeII distance of 3.073 Å in complex 2, compared to
the reported Fe-Fe distances of 2.6/2.62 Å in [Fe] hydro-
genases isolated from Dd and Cp,3-5 can be attributed to
the presence of the third bridging ethylthiolate, instead of
π-accepting CO-bridged ligand as observed in [Fe] hydro-
genases. Presumably, the dinuclear iron linked by two
thiolates and one CO group observed in the active site of
[Fe] hydrogenases is constructed to provide short a Fe-Fe
distance (Fe-Fe interelectronic interaction) and to stabilize
the dinuclear Fe subunit.
1
CN). H NMR (CD3CN): δ 2.698 (q), 1.270 (t) (O-CH2-CH3)
ppm. Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 338
(4563). Anal. Calcd for C92H90O8N6P4S2Fe2: C, 64.72; H, 5.31;
N, 4.92. Found: C, 64.45; H, 5.93; N, 5.14.
Preparation of [PPN][(CN)(CO)2Fe(µ-SEt)3Fe(CO)2(CN)] (2).
The compounds [PPN][Fe(CO)3(CN)2(Br)] (0.6 mmol, 0.486 g),15
[Na][SEt] (0.9 mmol, 0.0756 g), and [PPN][Cl] (0.3 mmol, 0.1724
g) were loaded into a flask, and then 10 mL of THF was added
under positive N2 at room temperature. (Alternatively, a THF
solution (8 mL) of compound 1 (0.3 mmol, 0.459 g) was added to
[PPN][SEt] (0.3 mmol, 0.180 g) under positive N2 at ambient
temperature.) The reaction mixture was then stirred at 30 °C for
10 days. The upper solution was transferred to another flask via
cannula under a positive pressure of N2, and hexane (15 mL) was
added to precipitate the orange yellow solid [PPN][(CN)(CO)2Fe-
(µ-SEt)3Fe(CO)2(CN)] (2). To grow X-ray diffraction quality
crystals, crystallization vials containing a concentrated THF solution
of complex 2 were placed in a septum-sealed container surrounded
by hexane. Upon standing at 0 °C for 3 weeks, orange yellow thin-
layer crystals (0.069 g, 23%) of 2 formed. IR: 2110 w (νCN), 2029
w, 2014 s, 1968 s (νCO) cm-1 (THF); 2106 w (νCN), 2033 w, 2020
1
s, 1976 s (νCO) cm-1 (CH3CN). H NMR (CD3CN): δ 1.43 (t),
1.37 (t), 1.34 (t) ppm (-CH3); 2.70-2.31 (m) ppm (-CH2-).
Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 335 (2450).
Anal. Calcd for C48H45O4N3P2S3Fe2: C, 57.79; H, 4.54; N, 4.21.
Found: C, 57.63; H, 4.38; N, 3.83. The residue was dried under
vacuum and then redissolved in 8 mL of CH3CN. The yellow
solution was then filtered through Celite to remove NaBr and NaCl,
and diethyl ether (15 mL) was added to precipitate the light yellow
solid trans-[PPN]2[Fe(CN)4(CO)2] (0.032 g, 8%) identified by IR
and X-ray diffraction. IR: 2105 m (νCN), 2004 s (νCO) cm-1 (CH3-
CN).16,17
Preparation of {[Na‚5/2H2O][(CN)(CO)2Fe(µ-SEt)3Fe(CO)2-
(CN)]}n{1/2O(Et)2}n (3). A THF solution (6 mL) containing 0.368
g (0.5 mmol) of [PPN][Fe(CO)4(CN)] and 0.13 g (0.5 mmol) of I2
in THF (6 mL) was stirred at room temperature for 2 h. The reaction
was monitored immediately by IR. The spectrum (IR (THF): 2122
w (νCN), 2089 m, 2047 sh, 2093 s (νCO) cm-1) was assigned to the
formation of [PPN][Fe(CO)3(CN)(I)2]. In the same flask, 1 mmol
of [Na][SEt] (0.094 g) was added, followed by stirring at room
temperature for 2 days, and then diethyl ether (6 mL)-hexane (4
mL) was added to precipitate the byproducts (NaI, [PPN][I], and
presumably [PPN][SEt]). The reaction mixture was filtered, and
the orange-brown filtrate was dried under vacuum to afford the
product. Diethyl ether (10 mL) was then added to extract the pure
product {[Na‚5/2H2O][(CN)(CO)2Fe(µ-SEt)3Fe(CO)2(CN)]}n{1/2O-
(Et)2}n (3). Diffusion of hexane into a solution of complex 3 in
Experimental Section
Manipulations, reactions, and transfers of samples were con-
ducted under nitrogen according to standard 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 steel cannula under a positive
pressure of N2. The reagents iron pentacarbonyl, hexamethyl
disilazane sodium salt, cyanogen bromide, bis(triphenylphospho-
ranylidene)ammonium chloride ([PPN][Cl]), and sodium ethylthio-
late (Lancaster/Aldrich) were used as received. Compound [PPN]-
[Fe(CO)4(CN)] was synthesized and characterized by published
procedures.20 Infrared spectra of the carbonyl ν(CO) and cyanide
ν(CN) stretching frequencies were recorded on a Bio-Rad model
(20) (a) Ruff, J. K. Inorg. Chem. 1969, 8, 86. (b) Goldfield, S. A.; Raymond,
K. N. Inorg. Chem. 1974, 13, 770.
Inorganic Chemistry, Vol. 42, No. 8, 2003 2787

Liaw et al.
Table 3. Crystallographic Data of Complexes (a) 1, (b) 2, and (c) 3
for X-ray diffraction studies measured 0.10 × 0.10 × 0.06 mm,
0.45 × 0.25 × 0.16 mm, and 0.30 × 0.25 × 0.16 mm, respectively.
Each crystal was mounted on a glass fiber. Diffraction measure-
ments for complexes 1 and 3 were carried out at 150(1) K (293(2)
K for complex 2) on a Siemens SMART CCD diffractometer with
graphite-monochromated Mo KR radiation (λ 0.7107 Å) and θ
between 1.25° and 25.00° for complex 1, between 1.21° and 23.26°
for complex 2, and between 1.33° and 27.50° for complex 3. Least-
squares refinement of the positional and anisotropic thermal
parameters for the contribution of all non-hydrogen atoms and fixed
hydrogen atoms was based on F2. A SADABS21 absorption
correction was made. The SHELXTL22 structure refinement pro-
gram was employed.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre: CCDC-177274, & CCDC-
177275, and CCDC-186801. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, U.K. (Fax: (+44)1223-336-033; E-mail: deposit@ccdc.
cam.ac.uk.)
1‚2H2O‚2THF
2
3
chem formula
C92H90N6O8-
P4S2Fe2
1707.40
monoclinic
P21/n
0.7107
10.7955(8)
12.2741(8)
32.795(2)
90
95.903(2)
90
4322.5(5)
2
1.312
0.517
C48H45N3O4-
P2S3Fe2
997.69
monoclinic
P21/c
0.7107
10.9062(12)
13.1631(14)
34.002(4)
90
97.193(2)
90
4842.8(9)
4
1.368
0.840
C28H50S6Fe4O14-
Na2N4
1128.46
monoclinic
P21/c
fw
cryst syst
space group
λ, Å (Mo KR)
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
0.7107
9.5187(5)
18.2710(8)
28.4457(14)
90
99.0553 (9)
90
4885.5(4)
4
1.534
Z
d
calcd, g cm-3
µ, mm-1
T, K
1.496
150(1)
293(2)
150(1)
R
RWF
0.0599a
0.1142b
1.011
0.0449a
0.0839b
1.000
0.0453a
0.0717b
0.837
2
GOF
b
2
2
2
a R ) ∑|(Fo - Fc)|/∑Fo. RWF ) {∑w(Fo - Fc )2/∑[w(Fo )2]}1/2
.
2
Acknowledgment. We gratefully acknowledge financial
diethyl ether at -15 °C for 4 weeks led to orange crystals (yield
48%, 0.073 g) suitable for X-ray crystallography. IR: 2123 w (νCN),
2036 m, 2023 s, 1981 m (νCO) (diethyl ether); 2120 w, 2112 w
(νCN), 2033 m, 2018 s, 1975 m (νCO) (THF). 1H NMR (C4D8O): δ
1.30 (t), 1.48 (t), 2.33 (q), 2.50 (q), 2.71 (q) ppm (-SCH2CH3);
2.47 (s, br) ppm (H2O); 1.12 (t), 3.39 (q) ppm (-OCH2CH3).
Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 330 (4280),
422 (807) (sh).
support from the National Science Council (Taiwan).
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0261225
(21) Sheldrick, G. M. SADABS, Siemens Area Detector Absorption Cor-
rection Program; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(22) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determi-
nation; Siemens Analytical X-ray Instruments Inc.: Madison, WI,
1994.
Crystallography. Crystallographic data for complexes 1, 2, and
3 are summarized in Table 3 and in the Supporting Information.
The crystals of 1, 2, and 3 are chunky. The crystals of 1, 2, 3 chosen
2788 Inorganic Chemistry, Vol. 42, No. 8, 2003