Dinuclear and mononuclear iron(II)-thiolate complexes with mixed CO/CN- ligands: Synthetic advances for iron sites of [Fe]-only hydrogenases

Article abstract of DOI:10.1021/ja992300q

The pentacoordinate, 16-electron Fe(II) complex [PPN][Fe(CO)₂(CN)(S,NH- C₆H₄)] (1), stabilized by strong S, N π-donation of chelating [S,NH- C₆H₄]^2- ligand, was prepared by the reaction of 2-aminophenyl disulfide and [PPN][Fe(CO)₄(CN)]. Protonation of complex 1 by electrophiles (2- mercaptopyrimidine and 2-aminophenylthiol) yielded hexacoordinate iron(II)- thiolate cyanocarbonyl complexes [PPN][Fe(CO)(CN)-(S-C₄H₃N₂)₂] (5) and [PPN][Fe(CO)₂(CN)(S-C₆H₄NH₂)(S,NH₂-C₆H₄)] (3), respectively. The IR spectrum of complex 5 in the aprotic solvent CH₃CN displayed a weak υ(CN) band at 2090 cm^-1 and a strong υ(CO) band at 1945 cm^-1. Chemical oxidation of complex 5 in CH₃CN at - 20 °C with [Cp₂Fe] [PF₆] displayed absorption bands at 2096 and 1962 cm^-1 which were assigned to the υ(CN) and υ(CO) vibrational frequencies respectively of the thermally unstable neutral Fe(III)(CO)(CN)(S-C₄H₃N₂)₂. Complex 5 was reobtained upon addition of [PPN]-[BH₄] to Fe(III)(CO)(CN)(S-C₄H₃N₂)₂ in CH₃CN at -20 °C. The first dinuclear Fe(II)-thiolate cyanocarbonyl compound [PPN]₂[(CN)(CO)₂Fe(μ-S,S-C₆H₄)]₂ (4), the promising structural and functional model compound of the dinuclear iron active sites of [Fe]-only hydrogenases isolated from D. desulfuricans and C. pasteurianum, was prepared by reacting 1,2-benzenedithiol with complex 1 in THF at -10 °C. The X-ray structural analysis shows that complex 4 possesses crystallographically imposed centrosymmetry. Two six-coordinate Fe(II) centers are connected via two thiolate bridges, and both CN^- ligands point into the antiparallel direction. The IR spectrum of complex 4 in the aprotic solvent CH₂Cl₂ revealed a weak absorption band for the CN^- ligands at 2101 cm^-1, and two strong absorption bands for the CO groups at 2013 and 1960 cm^-1. When the CH₂Cl₂ solution of complex 4 was exposed to ¹³CO at 0 °C, absorbances at 1968 and 1915 cm^-1 appeared within 10 min. Reappearance of the 2013 and 1960 cm^-1 bands on the removal of the ¹³CO and replacement with ¹²CO atmosphere demonstrated reversibility of the CO ligand lability of complex 4. The vibrational spectroscopies of the Fe(CO)₂(CN)and Fe(CO)(CN) fragments (υ(CN) ranges from 2094 to 2105 cm^-1, υ(CO) ranges from 1928 to 2013 cm^{- 1}) found in complexes 1, 3, 4, and 5 may be regarded as spectroscopic references of [Fe] hydrogenases in the various enzymatic states.

Full text of DOI:10.1021/ja992300q

488
J. Am. Chem. Soc. 2000, 122, 488-494
Dinuclear and Mononuclear Iron(II)-Thiolate Complexes with Mixed
CO/CN- Ligands: Synthetic Advances for Iron Sites of [Fe]-Only
Hydrogenases
Wen-Feng Liaw,*,† Nan-Hung Lee,† Chien-Hong Chen,† Chien-Ming Lee,†
Gene-Hsiang Lee,‡ and Shie-Ming Peng§
Contribution form the Department of Chemistry, National Changhua UniVersity of Education, Changhua
50058, Taiwan, and Instrumentation Center and Department of Chemistry, National Taiwan UniVersity,
Taipei 10764, Taiwan
ReceiVed July 5, 1999
Abstract: The pentacoordinate, 16-electron FeII complex [PPN][Fe(CO)2(CN)(S,NH-C6H4)] (1), stabilized by
strong S, N π-donation of chelating [S,NH-C6H4]2- ligand, was prepared by the reaction of 2-aminophenyl
disulfide and [PPN][Fe(CO)4(CN)]. Protonation of complex 1 by electrophiles (2-mercaptopyrimidine and
2-aminophenylthiol) yielded hexacoordinate iron(II)-thiolate cyanocarbonyl complexes [PPN][Fe(CO)(CN)-
(S-C4H3N2)2] (5) and [PPN][Fe(CO)2(CN)(S-C6H4NH2)(S,NH2-C6H4)] (3), respectively. The IR spectrum of
complex 5 in the aprotic solvent CH3CN displayed a weak ν(CN) band at 2090 cm-1 and a strong ν(CO) band
at 1945 cm-1. Chemical oxidation of complex 5 in CH3CN at - 20 °C with [Cp2Fe][PF6] displayed absorption
bands at 2096 and 1962 cm-1 which were assigned to the ν(CN) and ν(CO) vibrational frequencies respectively
of the thermally unstable neutral FeIII(CO)(CN)(S-C4H3N2)2. Complex 5 was reobtained upon addition of [PPN]-
[BH4] to FeIII(CO)(CN)(S-C4H3N2)2 in CH3CN at -20 °C. The first dinuclear Fe(II)-thiolate cyanocarbonyl
compound [PPN]2[(CN)(CO)2Fe(µ-S,S-C6H4)]2 (4), the promising structural and functional model compound
of the dinuclear iron active sites of [Fe]-only hydrogenases isolated from D. desulfuricans and C. pasteurianum,
was prepared by reacting 1,2-benzenedithiol with complex 1 in THF at -10 °C. The X-ray structural analysis
shows that complex 4 possesses crystallographically imposed centrosymmetry. Two six-coordinate Fe(II) centers
are connected via two thiolate bridges, and both CN- ligands point into the antiparallel direction. The IR
spectrum of complex 4 in the aprotic solvent CH2Cl2 revealed a weak absorption band for the CN- ligands at
2101 cm-1, and two strong absorption bands for the CO groups at 2013 and 1960 cm-1. When the CH2Cl2
solution of complex 4 was exposed to 13CO at 0 °C, absorbances at 1968 and 1915 cm-1 appeared within 10
min. Reappearance of the 2013 and 1960 cm-1 bands on the removal of the 13CO and replacement with 12CO
atmosphere demonstrated reversibility of the CO ligand lability of complex 4. The vibrational spectroscopies
of the Fe(CO)2(CN) and Fe(CO)(CN) fragments (ν(CN) ranges from 2094 to 2105 cm-1, ν(CO) ranges from
1928 to 2013 cm-1) found in complexes 1, 3, 4, and 5 may be regarded as spectroscopic references of [Fe]
hydrogenases in the various enzymatic states.
Introduction
ligands bound to a [4Fe-4S] cluster via cysteinate bridge, and
also suggested that the unsaturated Fe center present in the
H-cluster of the enzyme can act as a binding site for the soft
The impressive synergism that has developed in the past few
years between biophysical and synthetic inorganic chemists with
regard to understanding the spectroscopic signals and functions
of the [Fe(CO)x(CN)y] active sites in [Fe] hydrogenases and
[NiFe] hydrogenases has highlighted the chemistry of iron-
thiolate cyanocarbonyl complexes.1-7 The recent report of high-
quality X-ray crystal structure of [Fe]-only hydrogenase from
DesulfoVibrio desulfuricans revealed that the active site contains
a dinuclear iron(II)-propanedithiolate with mixed CO and CN-
(2) (a) Gu, Z.; Dong, J.; Allan, C. B.; Choudhury, S. B.; Franco, R.;
Moura, J. J. G.; Moura, I.; LeGall, J.; Przybyla, A. E.; Roseboom, W.;
Albracht, S. P. J.; Axley, M. J.; Scott, R. A.; Maroney, M. J. J. Am. Chem.
Soc. 1996, 118, 11155. (b) Colpas, G. J.; Day, R. O.; Maroney, M. J. Inorg.
Chem. 1992, 31, 5053. (c) Roberts, L. M.; Lindahl, P. A. J. Am. Chem.
Soc. 1995, 117, 2565. (d) Marganian, C. A.; Vazir, H.; Baidya, N.; Olmstead,
M. M.; Mascharak, P. K. J. Am. Chem. Soc. 1995, 117, 1584. (e) De Lacey,
A. L. J. Am. Chem. Soc. 1997, 119, 7181.
(3) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-
Camps, J. C. Structure 1999, 7, 13.
(4) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853.
(5) (a) Pierik, A. J.; Hulstein, M.; Hagen, W. R.; Albracht, S. P. J. Eur.
J. Biochem. 1998, 258, 572. (b) Van Dam, P. J.; Reijerse, E. J.; Hagen, W.
R. Eur. J. Biochem. 1997, 248, 355. (c) Popescu, C. V.; Mu¨nck, E. J. Am.
Chem. Soc. 1999, 121, 7877.
(6) (a) Lai, C.-H.; Lee, W.-Z.; Miller, M. L.; Reibenspies, J. H.;
Darensbourg, D. J.; Darensbourg, M. Y. J. Am. Chem. Soc. 1998, 120,
10103. (b) Darensbourg, D. J.; Reibenspies, J. H.; Lai, C.-H.; Lee, W.-Z.;
Darensbourg, M. Y. J. Am. Chem. Soc. 1997, 119, 7903.
(7) Hsu, H.-F.; Koch, S. A.; Popescu, C. V.; Mu¨nck, E. J. Am. Chem.
Soc. 1997, 119, 8371.
† Department of Chemistry, National Changhua University of Education.
‡ Instrumentation Center, National Taiwan University.
§ Department of Chemistry, National Taiwan University.
(1) (a) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996,
96, 2239. (b) Volbeda, A.; Charon, M.-H.; Piras, C.; Hatchikian, E. C.;
Frey, M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580. (c) de Lacey, A.
L.; Hatchikian, E. C.; Volbeda, A.; Frey, M.; Fontecilla-Camps, J. C.;
Fernandez, V. M. J. Am. Chem. Soc. 1997, 119, 7181. (d) Happe, R. P.;
Roseboom, W.; Pierik, A. J.; Albracht, S. P. J.; Bagley, K. A. Nature 1997,
385, 126. (e) Pierik, A. J.; Roseboom, W.; Happe, R. P.; Bagley, K. A.;
Albracht, S. P. J. J. Biol. Chem. 1999, 274, 3331. (f) Happe, R. P.;
Roseboom, W.; Albracht, S. P. J. Eur. J. Biochem. 1999, 259, 602.
10.1021/ja992300q CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/08/2000

Synthetic AdVances for Fe Sites of [Fe]-Only Hydrogenases
J. Am. Chem. Soc., Vol. 122, No. 3, 2000 489
Scheme 1
Figure 1. Schematic drawing of the H cluster according to the crystal
structure D. desulfuricans [Fe]-hydrogenase.3,4
ligand H2.3 A more CO/CN--rich environment was found for
iron in the recently reported X-ray structure of [Fe] hydrogenases
from Clostridium pasteurianum (CpI), which indicated that the
active-site cluster consists of a [4Fe-4S] cubane subcluster
covalently bridged by a cysteinate to a dinuclear iron subcluster
with iron atoms coordinated by nonbiological CO and CN-
groups.4 A schematic drawing of the active centers of [Fe]
hydrogenases as deduced from the crystallographic studies is
shown in Figure 1. Moreover, the infrared spectra of [Fe]
hydrogenase from DesulfoVibrio Vulgaris concluded that the
weak absorption bands for the CN- ligand range from 2079 to
2106 cm-1, and the broad strong absorption bands (ranging from
2016 to 1894 cm-1) are assigned to the carbonyl stretching
frequencies under various manipulations of the protein.5a,b Very
recently, a Mo¨ssbauer study of the H cluster in [Fe] hydrogenase
isolated from C. pasteurianum (CpII) showed that the binuclear
Fe subcluster contains two low-spin FeII sites in the reduced
state (Hred), while the dinuclear Fe subcluster is in mixed-valent
FeIIIFeII site in the oxidized states (Hox and Hox-CO).5c Also,
the iron site in the [NiFe] hydrogenases active site from
DesulfoVibrio gigas has been established as a pyramidal [Fe-
(CN)2(CO)] unit with the opposite face coordinated to two
S-cysteines bridged to a nickel.1b-d,f
as the promising structural and functional model compounds
of the dinuclear iron active sites of [Fe]-only hydrogenases.3,4
Results and Discussion
When di(2-aminophenyl) disulfide (0.4 mmol) was reacted
directly with [PPN][Fe(CO)4(CN)] (0.4 mmol) in THF at room
temperature,9 substantial amounts of air-stable, pentacoordinate,
16-electron FeII complex [PPN][Fe(CO)2(CN)(S,NH-C6H4)] (1)
were isolated as a dark brown solid after recrystallization from
THF-hexane (Scheme 1). 2-Aminophenyl disulfide and H2O
were also obtained as byproducts identified by NMR. The IR
spectrum of complex 1 in the aprotic solvent CH3CN reveals a
weak absorption band for the CN- ligand at 2099 cm-1 while
the two strong absorption bands (1997 s, 1933 s cm-1) are
1
assigned to the carbonyl stretching frequencies. The H NMR
spectrum shows the expected signals for the chelate [S,NH-
C6H4]2- ligand in a diamagnetic d6 Fe(II) species. When a THF
solution of complex 1 is purged with 13CO, the IR νCO peaks at
1992 and 1929 cm-1 immediately shift to 1946 and 1885 cm-1
while the IR νCN stretching frequency remains unchanged. The
magnitude ∼45 cm-1 of the isotopic shift (∆νCO) is consistent
with the calculated position, based only on the difference in
masses between 12CO and 13CO.
Figure 2 presents the structure of complex 1 as an ORTEP;
significant bond distances and angles are given in Table 2. The
geometry around the iron(II) center is intermediate between
trigonal bipyramidal and square pyramidal, with the bite angle
of the chelating 2-thiolatophenylamido being 84.65(12)°. The
five-membered chelate ring [FeSNC2] is almost planar with a
deviation of 0.03 Å. The average Fe-CO and Fe-CN bond
lengths of 1.763(6) and 1.926(6) Å, respectively, closely
compare with those found in Fe(II) complexes [FeII(PS3)(CO)-
(CN)]2- (PS3 ) tris(2-phenylthiolate)phosphine) (1.710(8) and
1.950(8) Å)7 and [CpFe(CN)2(CO)]- (1.73(2) and 1.906(9) Å).6
The significantly shorter Fe-S bond of length 2.226(2) Å and
Fe-N bond of length 1.855(4) Å in complex 1, as comparing
with the reported Fe(II)-S bond of length 2.3452(8) Å in the
[Fe(dsdm)(bmes)Fe(CO)2] (H2dsdm ) N,N′-dimethyl-N,N′-bis-
(2-sulfanylethyl)ethylenediamine; H2bmes ) 2-bis(sulfanyleth-
yl)sulfide),10 were attributed to the strong π-donating ability of
Model compounds with donor ligands as close as possible to
the biological coordination are needed to serve as spectroscopic
references, explore biological pathways of reversible activation
of H2, and elucidate the electronic structure of active centers of
the binuclear iron subcluster. Recently two iron cyanocarbonyl
model complexes have been reported by Darensbourg et al.6
and Koch et al.,7 respectively. In one interesting model
compound iron is sandwiched between a cyclopentadienyl ring
and two cyanides, one carbon monoxide. The infrared and
structural properties of the potassium salt of this complex match
well to those of the Fe(CO)(CN)2 site in [NiFe] hydrogenases
isolated from D. gigas.6 The other model compound is a low-
spin six-coordinate iron(II)-thiolate cyanocarbonyl complex,
the first example of the mononuclear iron(II)-thiolate com-
plexes with mixed CO and CN- ligands.7 To our knowledge,
there is no example of the dinuclear iron(II)-thiolate complexes
with mixed CO and CN- ligands characterized by X-ray
crystallography.8 Reported herein are the syntheses and structural
characterizations of the mononuclear iron(II,III)-thiolate and
the first dinuclear iron(II)-thiolate compounds with mixed
diatomic ligands (CO and CN- ligands) bound to the Fe serving
(8) (a) Recently, the complexes [(µ-S-(CH2)3-S)FeI2(CO)6] (Fe(I)‚‚‚Fe-
(I) distance 2.510(1) Å), [(µ-S-(CH2)3-S)FeI2(CN)(CO)5]-, and [(µ-S-(CH2)3-
S)FeI2(CN)2(CO)4]2- were synthesized by M. Y. Darensbourg and co-
workers (personal communication). (b) Very recently, the crystal structure
of the dinuclear Fe(I)-thiolate cyanocarbonyl complex was reported by
Rauchfuss and co-workers. Schmidt, M.; Contakes, S. M.; Rauchfuss, T.
B. J. Am. Chem. Soc. 1999, 121, 9736.
(9) (a) Ruff, J. K. Inorg. Chem. 1969, 8, 86. (b) Goldfield, S. A.;
Raymond, K. N. Inorg. Chem. 1974, 13, 770.
(10) Kaasjager, V. E.; Henderson, R. K.; Bouwman, E.; Lutz, M.; Spek,
A. L.; Reedijk, J. Angew. Chem., Int. Ed. Engl. 1998, 37, 1668.

490 J. Am. Chem. Soc., Vol. 122, No. 3, 2000
Liaw et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for (a)
Complex 1 (b) Complex 4
Complex 1
1.743(6)
Fe-C(1)
Fe-C(2)
Fe-S
C(1)-O(1)
S-C(4)
Fe-C(3)
1.782(6)
1.855(8)
1.137(6)
1.147(6)
1.364(5)
1.926(6)
2.226(2)
1.141(6)
1.726(5)
Fe-N(1)
C(2)-N(2)
C(3)-O(3)
N(1)-C(9)
C(1)-Fe-C(3)
C(3)-Fe-N(1)
C(3)-Fe-C(2)
C(1)-Fe-S
94.6(2)
150.3(2)
91.8(2)
98.0(2)
84.65(12)
99.5(2)
C(1)-Fe-N(1)
C(1)-Fe-C(2)
N(1)-Fe-C(2)
C(3)-Fe-S
115.1(2)
94.2(2)
87.7(2)
90.0(2)
167.6(2)
124.4(3)
N(1)-Fe-S
C(2)-Fe-S
C(4)-S-Fe
N(2)-C(2)-Fe
C(9)-N(1)-Fe
178.4(5)
Complex 4
Fe(1)-C(1)
Fe(1)-C(3)
Fe(1)-S(2)
Fe(1A)-S(1)
S(1)-C(4)
1.784(6) Fe(1)-C(2)
1.941(6) Fe(1)-S(1)
2.288(2) Fe(1)-S(1A)
2.373(2) C(3)-N(1)
1.767(6) S(2)-C(9)
1.771(6)
Figure 2. ORTEP drawing and labeling scheme of the [Fe(CO)2(CN)-
(S,NH-C6H4)]- anion.
2.290(2)
2.373(2)
1.138(7)
1.747(6)
Table 1. Crystallographic Data of Complexes (a) 1, (b)
4‚C6H12‚(CH2Cl2)2, and (c) 5‚THF
C(1)-Fe(1)-C(2)
C(1)-Fe(1)-S(1)
C(1)-Fe(1)-S(2)
S(1)-Fe(1)-S(1A)
Fe(1)-S(1)-Fe(1A)
91.0(3)
94.2(2)
174.8(2)
83.80(6)
96.20(6)
C(1)-Fe(1)-C(3)
91.1(3)
1
4‚C6H12‚(CH2Cl2)2
5‚THF
C(1)-Fe(1)-S(1A) 94.2(2)
chem formula C45H34N3O2- C98H86N4O4-
C50H45N6O2-
P2S2Fe
943.83
monoclinic
C2/c
0.7107
24.4494(3)
12.2321(2)
31.6646(3)
90
96.737(1)
90
9404.5(2)
8
S(1)-Fe(1)-S(2) 88.27(6)
S(2)-Fe(1)-S(1A) 90.60(6)
C(3)-Fe(1)-S(1A) 86.8(2)
P2SFe P4S4Cl4Fe2
799.61 1889.33
triclinic
fw
cryst syst
space group
monoclinic
P21/n
0.7107
10.887(2)
17.574(2)
24.673(4)
90
92.059(12)
90
4717.8(11)
2
1.330
0.629
295(2)
0.0698a
0.2192b
1.096
P1h
λ, Å (Mo KR) 0.7107
1
brown to dark brown. The IR and H NMR spectra indicated
the formation of complex 1 accompanied by byproducts, H2O
and di(2-aminophenyl) disulfide (Scheme 1c). In this oxidative
reaction, the Fe(II) was not observed to undergo oxidation, and
consequently, the oxidation process was best assigned to the
terminal thiolate ligand [S-C6H4NH2]- to yield di(2-aminophe-
nyl) disulfide via radical ([S-C6H4NH2]•) recombination, and
the concomitant deprotonation of the amine proton of anion 3
(the coordination of N atom presumably increases the acidity
of amine protons to accelerate the deprotonation of NH2) led
to the formation of water.16
The ability of complex 1 to bind the second thiolate ligand
to form six-coordinate iron(II)-thiolate cyanocarbonyl complex
was also demonstrated by the facile reaction with 2-aminophe-
nylthiol. When a solution of complex 1 in THF was treated
with 2-aminophenyl thiol, an immediate color change from dark
brown to brown was observed. The absorption bands at 2101
w (νCN), 1992 s, and 1929 s (νCO) cm-1 disappeared, with
concomitant formation of a spectrum (2104 w (νCN), 2013 vs,
and 1958 s (νCO) cm-1 (THF)) assigned to formation of
hexacoordinate complex 3 (Scheme 1c′). Apparently, the anionic
pentacoordinate 1 and hexacoordinate 3 complexes are chemi-
cally interconvertible at ambient temperature. This result sup-
ports that the reactions of complex 1 with electrophiles
(2-aminophenylthiol, etc.) occur at the more electron-rich amide
site to yield charge-controlled, collision complexes.
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
9.8985(1)
14.1558(2)
14.4381(2)
90.094(1)
99.168(1)
92.304(1)
1995.56(4)
2
d
calcd, g cm-3
1.331
0.552
1.333
0.524
µ, mm-1
T, K
R
295(2)
295(2)
0.0859a
0.1237b
1.225
0.0578a
0.1323b
1.063
RWF
GOF
2
a R ) ∑|(Fo - Fc)|/∑Fo. RWF ) {∑w(Fo - Fc2)2/∑[w(Fo )2]}1/2
.
b
2
2
2
the bidentate [S,NH-C6H4]2- ligand which stabilized the un-
saturated Fe(II) complex 1.11-13
A reasonable reaction sequence accounting for the formation
of complex 1 is shown in Scheme 1a-c, based on the IR νCO
monitor of the reaction in THF at room temperature. An
oxidative addition/decarbonylation reaction yielded an unstable
intermediate fac-[FeII(CO)3(CN)(S-C6H4NH2)2]- (2) (IR: 2111w
(νCN), 2029 s, 1975 s (νCO) cm-1 (THF)) with two anionic
[S-C6H4NH2]- ligands bound to the Fe(II) metal in a mono-
dentate (S-bonded) manner (Scheme 1a).14 Chelation of one of
the [S-C6H4NH2]- ligands and concomitant dissociation of CO
resulted in the formation of unstable, oily [FeII(CO)2(CN)(S,-
NH2-C6H4)(S-C6H4NH2)]- (3) (IR: 2104 w (νCN), 2013 s, 195
s (νCO) cm-1 (THF)) (Scheme 1b).15 Upon contact with dry O2,
the color of complex 3 in THF solution immediately turned from
The first dinuclear Fe(II)-thiolate cyanocarbonyl compound
[PPN]2[(CN)(CO)2Fe(µ-S,S-C6H4)]2 (4) was prepared in a one-
step synthesis by treating 1,2-benzenedithiol with complex 1
in THF under N2 atmosphere at -10 °C (Scheme 2a). Complex
4 was isolated as an air-stable brown solid at -10 °C. The
thermally unstable complex 4 is soluble in CH2Cl2 and CH3-
(11) (a) Darensbourg, D. J.; Draper, J. D.; Reibenspies, J. H. Inorg. Chem.
1997, 36, 3648. (b) Darensbourg, D. J.; Klausmeyer, K. K.; Reibenspies, J.
H. Inorg. Chem. 1996, 35, 1535.
(12) (a) Sellmann, D.; Geck, M.; Knoch, F.; Ritter, G.; Dengler, J. J.
Am. Chem. Soc. 1991, 113, 3819. (b) Sellmann, D.; Weiss, R.; Knoch, F.;
Ritter, G.; Dengler, J. Inorg. Chem. 1990, 29, 4107.
(13) Liaw, W.-F.; Lee, C.-M.; Lee, G.-H.; Peng, S.-M. Inorg. Chem.
1998, 37, 6396.
(14) (a) Liaw, W.-F.; Chiang, M.-H.; Liu, C.-J.; Harn, P.-J.; Liu, L.-K.
Inorg. Chem. 1993, 32, 1536. (b) Liaw, W.-F.; Lai, C.-H.; Chiang, M.-H.;
Lee, C.-K.; Lee, G.-H.; Peng, S.-M. J. Chem. Soc., Dalton Trans. 1993,
2421.
(15) (a) Liaw, W.-F.; Chen, C.-H.; Lee, G.-H.; Peng, S.-M. Organome-
tallics 1998, 17, 2370. (b) Lee, C.-M.; Lin, G.-Y.; Hsieh, C.-H.; Hu, C.-
H.; Lee, G.-H.; Peng, S.-M.; Liaw, W.-F. J. Chem. Soc., Dalton Trans.
1999, 2393.
(16) (a) Okamura, T.; Takamizawa, S.; Ueyama, N.; Nakamura, A. Inorg.
Chem. 1998, 37, 18. (b) Sellmann, D.; Emig, S.; Heinemann, F. W.; Knoch,
F. Angew. Chem., Int. Ed. Engl. 1997, 36, 1201. (c) Crociani, L.; Tisato,
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1998, 120, 2973.

Synthetic AdVances for Fe Sites of [Fe]-Only Hydrogenases
J. Am. Chem. Soc., Vol. 122, No. 3, 2000 491
but shorter than that observed for cis-[Fe(CO)2(S-C5H4N)2]
(average Fe(II)-S ) 2.325(3) Å).15 The average Fe-CO and
Fe-CN bond lengths are 1.778(6) and 1.941(6) Å, respectively.
The Fe(II)‚‚‚Fe(II) distance of 3.472 Å in the model complex 4
is longer than those reported for the diiron subcluster active-
site (2.6 and 2.62 Å) of [Fe] hydrogenases,3,4 and excludes any
direct metal-metal interactions. Formation of complex 4 can
also be interpreted as coordinative association of two anionic
mononuclear [(CO)2(CN)Fe(S,S-C6H4)]-. It may be attributed
to the avoidance of electron deficiency at the iron(II) centers
as well as to the availability of the nonbonding electron pairs
of the sulfur atoms in [(CO)2(CN)Fe(S,S-C6H4)]-.18 Notably,
both the electronic and steric reasons are responsible for the
formation of complex 4.
Complex 4 exhibits a diagnostic 1H NMR spectrum with the
1,2-benzenedithiolate proton resonances at 6.60 (t), 6.79 (t), 7.22
(d), and 7.33 (d) ppm which are consistent with the Fe(II) having
a low-spin d6 electronic configuration in an octahedral ligand
field. The IR spectrum of complex 4 in the aprotic solvent CH2-
Cl2 reveals a weak absorption band for the CN- ligands at 2101
cm-1, and two strong absorption bands for the CO groups at
1960 and 2013 cm-1, as expected for the existence of cen-
trosymmetry in complex 4 (vibrationally uncoupled CO groups
and CN- groups on adjacent iron(II) sites).12b,19 When the CH2-
Cl2 solution of complex 4 was exposed to 13CO at 0 °C,
absorbances at 1968 and 1915 cm-1 appeared within 10 min.
Reappearance of the 2013 and 1960 cm-1 bands on removal of
the 13CO and replacement with 12CO atmosphere demonstrated
reversibility of CO ligand lability of complex 4.
Figure 3. ORTEP drawing and labeling scheme of the [(CN)(CO)2Fe-
(µ-S,S-C6H4)]2 anion with thermal ellipsoids drawn at the 50%
probability.
2-
Scheme 2
In contrast, protonation of complex 1 by 2 equiv of 2-mer-
captopyrimidine in THF under N2 at ambient temperature
yielded hexacoordinate iron(II)-thiolate cyanocarbonyl complex
[PPN][Fe(CO)(CN)(S-C4H3N2)2] (5) with two anionic
[S-C4H3N2]- ligands bound to the Fe(II) ion in a bidentate
manner (S,N-bounded) (Scheme 2b). The IR spectrum of
complex 5 in the aprotic solvent CH3CN displayed a weak ν-
(CN) band at 2090 cm-1 and a broad strong ν(CO) band at
1945 cm-1, well within the range observed for [Fe] hydrogenase
isolated from D. Vulgaris.5 The reversibility of CO ligand lability
of complex 5 was demonstrated by exposing the CH2Cl2 solution
of complex 5 to 13CO. The IR νCO peak at 1949 cm-1 shifted
to a single absorbance at 1904 cm-1. The formation of complex
5 is presumed to occur via protonation of amide (NH),
subsequent elimination of 2-aminophenylthiol, and concomitant
chelation of [S-C4H3N2]- ligands. The electrochemistry of
complex 5, measured in CH3CN with 0.1 M [n-Bu4N][PF6] as
supporting electrolyte (scan rate 1.5 V/s), reveals a reversible
oxidation-reduction process at +0.12 V (E1/2) (vs Ag/AgNO3)
(Figure 4). The infrared spectrum of solution obtained by the
chemical oxidation of complex 5 in CH3CN at - 20 °C with
[Cp2Fe][PF6] displays absorption bands at 2096 and 1962 cm-1
which are assigned to the ν(CN) and ν(CO) vibrational
frequencies of the thermally unstable neutral FeIII(CO)(CN)(S-
C4H3N2)2, respectively. Complex 5 was reobtained upon addition
of 1 equiv of [PPN][BH4] to FeIII(CO)(CN)(S-C4H3N2)2 in CH3-
CN at -20 °C.
CN and partially soluble in THF. The structure of complex 4 is
depicted in Figure 3. Selected bond distances and angles are
given in Table 2. Complex 4 consists of discrete PPN+ cations
and [(CN)(CO)2Fe(µ-S,S-C6H4)]22- anions. The dinuclear [(CN)-
(CO)2Fe(µ-S,S-C6H4)]22- is electronically dianionic and there-
fore both irons should be divalent. Complex 4 possesses
crystallographically imposed centrosymmetry. Two six-coordi-
nate Fe(II) atoms are connected via two thiolate bridges, and
both CN- ligands (C(3)N(1) and C(3A)N(1A) ligands) point
into the antiparallel direction. As can be seen from Figure 3,
two pairs of CO groups (C(2)O(2), C(2A)O(2A) and C(1)O(1),
C(1A)O(1A)) are also bonded to iron(II) atoms in a parallel
direction, respectively. Each iron atom is surrounded pseudooc-
tahedrally by two bridging thiolates, one terminal thiolate, two
terminal carbonyl groups, and one cyanide ligand, with the bite
angle of the chelating 1,2-benzenedithiolate being 88.27(6)° and
the obtuse angle (S(1)-Fe(1)-S(1A)) being 83.80(6)°.
The nearly identical Fe(1)-S(1) and Fe(1)-S(2) bond lengths
of 2.290(2) and 2.288(2) Å, respectively, well within the range
(2.3 Å) observed for [Fe] hydrogenases from D. desulfuricans,3
are considerably longer than the Fe(1A)-S(1) bond distance
of 2.373(2) Å in complex 4 (Figure 3). These differences
probably originate from the additional π-donating interactions
of the S(1) and S(2) with Fe(1) metal center. The Fe(1)-S(1)
and Fe(1)-S(2) bond distances of 2.289(2) Å (average) in
complex 4 are longer than those in [Fe(S,S-C6H4)(PMe3)3]
(2.205(3) Å)17 and in complex 1 (2.226(2) Å) stabilized by
incorporation of π-donating chelating [S,NH-C6H4]2- ligand,
Figure 5 displays an ORTEP plot of the anionic mononuclear
complex 5. The selected bond distances and angles are listed
(18) Liaw, W.-F.; Chiou, S.-J.; Lee, G.-H.; Peng, S.-M. Inorg. Chem.
1998, 37, 1131.
(19) (a) Liaw, W.-F.; Chen, C.-H.; Lee, C.-M.; Lin, G.-Y.; Ching, C.-
Y.; Lee, G.-H.; Peng, S.-M. J. Chem. Soc., Dalton Trans. 1998, 353. (b)
Liaw, W.-F.; Lee, C.-M.; Horng, L.; Lee, G.-H.; Peng, S.-M. Organome-
tallics 1999, 18, 782.
(17) Sellmann, D.; Kleine-Kleffmann, U.; Zapf, L.; Huttner, G.; Zsolnai,
L. J. Organomet. Chem. 1984, 263, 321.

492 J. Am. Chem. Soc., Vol. 122, No. 3, 2000
Liaw et al.
Table 4. Infrared Spectroscopic Data in ν(CO) and ν(CN) Regions
compd
ν(CN), cm-1
ν(CO), cm-1
references
this work
1
4
5
(CH3CN) 2099
(THF) 2101
(CH2Cl2) 2095
(CH3CN) 2102
(THF) 2106
(CH2Cl2) 2100
(CH3CN) 2090
(THF) 2094
(CH2Cl2) 2087
(air) 2106, 2087
1997, 1933
1992, 1928
2000, 1936
2011, 1961
2008, 1950
2013, 1960
1945
this work
this work
5a
1942
1948
D. Vulgaris
2007.5, 1983, 1847.5
[Fe] H2ase (H2/Ar) 2095, 2079 2016, 1972, 1965, 1940
(H2/CO) 2096, 2088 2016, 1971, 1964
(H2) 2079, 2041
2093, 2083
1965, 1941, 1916, 1894
1945
C. Vinosum
[NiFe]H2ase
1d
In comparison, protonation of complex 1 by HBF4 in CH2-
Cl2 under N2 at -10 °C yielded, presumably, the neutral
dinuclear iron(II)-thiolate cyanocarbonyl complex [(CN)-
(CO)2Fe(µ-S-C6H4-NH2)]2 (A), isostructural with complex 4.20
Figure 4. Cyclic voltammogram of a 2 mM solution of complex 5 in
0.1 M [n-Bu4N][PF6]/CH3CN with glassy carbon working electrode at
a scan rate of 1.5 V/s.
The IR spectrum of the extremely unstable complex A in the
aprotic solvent CH2Cl2 displayed a weak ν(CN) band at 2112
cm-1, and two broad strong ν(CO) bands at 2056 and 2008
cm-1. Attempts to recrystallize complex A have thus far been
unsuccessful.
Conclusion and Comments
Applications of anionic iron(0)-cyanocarbonyl for prepara-
tion of iron(II)-thiolate cyanocarbonyl proved a successful
approach in this direction. Isolation of the unsaturated complex
1 shows an access to the mononuclear iron(II)-thiolate cyano-
carbonyl compound containing a vacant coordination site, a
potential binding site for soft ligands.3,6a The bimetallic iron-
(II)-thiolate cyanocarbonyl complex 4 presented here is the
first example of the binuclear iron(II)-thiolate compounds with
mixed CO and CN- ligands serving as the promising structural
and functional model compounds of the dinuclear iron active
sites of [Fe] hydrogenases isolated from D. desulfuricans and
C. pasteurianum.3,4 We noticed that the thiolate-bridged bi-
nuclear Fe(II) complexes typically have an Fe‚‚‚Fe distance of
3.2(2) Å (3.472 Å in complex 4, 3.420(1) Å in [Fe(CO)(µ-
“S4”)]2 (“S4”2- ) 1, 2-bis(2-mercaptophenylthio)ethanato(2-
)),12b and 3.0747(7) Å in [Fe(dsdm)(bmes)Fe(CO)2]).10 However,
the Fe‚‚‚Fe distances of 2.6 and 2.62 Å found in [Fe]
hydrogenases respectively3,4 are well within the range of those
of dinuclear Fe(I) compounds.8,21 The vibrational spectroscopies
of the Fe(CO)2(CN) and Fe(CO)(CN) fragments (ν(CN) ranges
from 2094 to 2105 cm-1, ν(CO) ranges from 1929 to 2013
cm-1) found in complexes 1, 3, 4, and 5 may be regarded as
spectroscopic references of [Fe] hydrogenases in the various
enzymatic states (Table 4). In comparison with IR νCO and νCN
Figure 5. ORTEP drawing and labeling scheme of the [Fe(CO)(CN)-
(S-C4H3N2)2]- anion.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Complex 5
Fe-C(1)
Fe-N(4)
Fe-S(1)
C(1)-O(1)
S(1)-C(3)
1.823(3)
1.999(3)
2.3323(9)
1.130(4)
1.715(4)
Fe-C(2)
Fe-N(2)
Fe-S(2)
C(2)-N(1)
S(2)-C(7)
1.845(4)
2.001(3)
2.3550(9)
1.135(4)
1.735(4)
C(1)-Fe-C(2)
C(2)-Fe-N(4)
C(2)-Fe-N(2)
C(1)-Fe-S(1)
N(4)-Fe-S(1)
C(1)-Fe-S(2)
S(2)-Fe-S(1)
84.5(2)
C(1)-Fe-N(4)
C(1)-Fe-N(2)
N(4)-Fe-N(2)
C(2)-Fe-S(1)
N(2)-Fe-S(1)
N(4)-Fe-S(2)
173.17(12)
92.83(13)
90.80(10)
100.06(11)
70.88(8)
92.76(13)
170.53(12)
91.64(10)
95.02(8)
103.44(10)
160.81(4)
70.38(8)
in Table 3. The constraint of the 2-mercaptopyrimidine ligand
generates ca. 70.88(8)° S(1)-Fe-N(2) and 70.38(8)° S(2)-
Fe-N(4) angles, enforcing a severe distortion from octahedron
at the hexacoordinate iron(II) site. Because of disorder, the exact
Fe-CN and Fe-CO bond distances are poorly determined in
complex 5. The hexacoordinate sphere as well as the long
FeII-S and FeII-N bond distances (average 2.344(9) and 2.000-
(3) Å, respectively) of complex 5, comparing with the penta-
coordinate complex 1 (2.226(2) and 1.855(8) Å respectively),
reflect the fact that bidentate [S-C4H3N2]- may not serve as a
strong π-donating ligand.
(20) The analogue [(CO)3Mn(µ-S-C6H4-NH2)2Mn(CO)3] was synthesized
by reaction of [PPN][Mn(CO)3(S,NH-C6H4)] with HBF4 in THF, and was
characterized by IR and X-ray structural analysis. To be submitted for
publication.
(21) Dahl, L. F.; Wei, C. H. Inorg. Chem. 1963, 2, 328.

Synthetic AdVances for Fe Sites of [Fe]-Only Hydrogenases
J. Am. Chem. Soc., Vol. 122, No. 3, 2000 493
C6H4). Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 604
(3234), 525 (4318), 433 (6788), 327 (7270). Anal. Calcd for C45H35O2N3-
SP2Fe: N, 5.26; C, 67.59; H, 4.42. Found: N, 5.49; C, 67.14; H, 4.75.
Preparation of [PPN]2[(CN)(CO)2Fe(µ-S,S-C6H4)]2 (4). Compound
1 (0.5 mmol, 0.4 g) was loaded into a 25 mL Schlenk tube, and 2 mL
of dry THF was added by cannula under a positive pressure of N2.
1,2-Benzenedithiol (60 µL, 0.5 mmol) was added to the dark brown
solution of compound 1. After being stirred at -10 °C for 20 min, the
reaction solution was cooled to -15 °C and a brown precipitate was
deposited gradually, collected by filtration, and washed several times
with THF to give complex 4. The brown solid was dried under vacuum.
The yield of brown complex 4 was 0.154 g (37%). X-ray quality crystals
were obtained by diffusion of hexane-diethyl ether into a CH2Cl2
solution of complex 4 at -15 °C. IR: 2100 w (ν CN), 2013 s, 1960 s
(ν Co) cm-1 (CH2Cl2); 2102 w (ν CN), 2011 s, 1961 s (ν Co) cm-1 (CH3-
spectra of D. Vulgaris [Fe] hydrogenase, the IR νCO and νCN
spectra of the centrosymmetric model complex 4 (vibrationally
uncoupled CO groups and CN- groups on adjacent iron(II) sites;
two CO stretching bands and one CN stretching band) suggests
the lack of centrosymmetry in dinuclear iron subcluster active
sites of [Fe] hydrogenases.3-5 A ∼4 cm-1 shift in the ν(CN)
value and ∼23 cm-1 shift in the ν(CO) value were observed in
the case of changes in the coordination of the iron(II) center
from pentacoordinate Fe(II)-thiolate 1 to hexacoordinate Fe-
(II)-thiolate cyanocarbonyl 3. A ∼6 cm-1 shift in the ν(CN)
value and a ∼17 cm-1 shift in the ν(CO) value were observed
from Fe(II) complex 5 to the oxidized form FeIII(CO)(CN)(S-
C4H3N2)2. A ∼5 cm-1 shift in the ν(CN) value and a ∼20 cm-1
shift in the ν(CO) value were observed from anionic 1 to
dianionic 4. Notably, the response of ν(CN) to electron density
changes (i.e. coordination, ancillary ligand, and oxidation state
changes) around the Fe center shows less sensitivity as compared
with that of ν(CO) in these iron(II,III)-thiolate complexes with
mixed CO and CN- ligands.6,7
1
CN). H NMR (CD2Cl2): δ 6.60 (t), 6.79 (t), 7.22 (d), 7.33 (d) ppm
(S,S-C6H4). Absorption spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]:
426 (4807), 562 (3510), 650 (1548). Anal. Calcd for C90H68O4N4S4P4-
Fe2: N, 3.43; C, 66.10; H, 4.19. Found: N, 3.68; C, 65.89; H, 4.62.
Preparation of [PPN][Fe(CO)(CN)(S-C4H3N2)2] (5). To a stirred
solution of compound 1 (0.320 g, 0.4 mmol) in THF (5 mL) was added
2-mercaptopyrimidine (0.090 g, 0.8 mmol) under N2 at room temper-
ature. The reaction solution was stirred for 6 h, and then hexane was
added to precipitate the brown solid. The reaction mixture was filtered,
and the brown solid was dried under vacuum to afford the pure product
in essentially quantitative yield (0.141 g, 37.5%). Recrystallization from
saturated THF solution with hexane diffusion gave brown crystals of
complex 5 at room temperature. IR: 2090 w (ν CN), 1945 s (ν Co) cm-1
Experimental Section
Manipulations, reactions, and transfers of samples were conducted
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 purge was used on these solvents
before use and transfers to reaction vessels were via stainless steel
cannula under positive pressure of N2. The reagents iron pentacarbonyl,
2-aminophenyl disulfide, 1,2-benzenedithiol, hexamethyl disilazane
sodium salt, 2-aminophenylthiol, 2-mercaptopyrimidine, ferrocenium
hexafluorophosphate, bis(triphenylphosphoranylidene)ammonium chlo-
ride, and fluoroboric acid (Lancaster/Aldrich) were used as received.
Complex [PPN][Fe(CO)4(CN)] was synthesized by published proce-
dures.9 Infrared spectra of the carbonyl ν(CO) and cyanide ν(CN)
frequency region (2200-1700 cm-1) were recorded on a Bio-Rad
Model FTS-185 spectrophotometer with sealed solution cells (0.1 mm)
and KBr windows. 1H and 13C NMR spectra were obtained on a Bruker
Model AC 200 spectrometer. UV/vis spectra were recorded on a GBC
918 spectrophotometer. Cyclic voltammetric measurements were
performed on a BAS-100B electrochemical analyzer, using glassy
carbon as the working electrode. Cyclic voltammograms were obtained
from 2 mM analyte concentration in CH3CN using 0.1 M [n-Bu4N]-
[PF6] as supporting electrolyte. Analyses of carbon, hydrogen, and
nitrogen were obtained with a CHN analyzer (Heraeus).
Preparation of [PPN][Fe(CO)2(CN)(S,NH-C6H4)] (1). The com-
pounds [PPN][Fe(CO)4(CN)] (0.4 mmol, 0.293 g)9 and 2-aminophenyl
disulfide (0.4 mmol, 0.100 g) dissolved in 4 mL of THF were stirred
at ambient temperature. A vigorous reaction occurred with evolution
of CO gas. The reaction was monitored with FTIR. IR spectra [2111
w (ν CN), 2029 s, 1975 s (ν Co) cm-1 (THF), and 2104 w (ν CN), 2013
s, 1958 s (ν Co) cm-1 (THF)] were assigned to the formation of fac-
[PPN][Fe(CO)3(CN)(S-C6H4NH2)2] (2) and [PPN][Fe(CO)2(CN)(S,NH2-
C6H4)(S-C6H4NH2)] (3), respectively.13,14 The reaction mixture was
stirred overnight at room temperature, and a gentle stream of dry O2
was bubbled through the brown solution. The brown solution completely
converted into a dark brown solution accompanied by formation of
byproducts, di(2-aminophenyl) disulfide and H2O (identified by 1H
NMR in the separate NMR experiment). The dark brown solution was
then filtered through Celite and hexane (15 mL) was added to precipitate
the air-stable, dark brown solid [PPN][Fe(CO)2(CN)(S,NH-C6H4)] (1).
The dark brown solid was washed with hexane and recrystallized from
THF-hexane. The yield of dark brown product 1 was 0.48 g (75%).
Diffusion of hexane into a solution of complex 1 in THF at -15 °C
for 4 weeks led to dark brown crystals suitable for X-ray crystallogaphy.
IR: 2099 w (ν CN), 1997 s, 1933 s (ν Co) cm-1 (CH3CN); 2101 w,
1992 s, 1929 s cm-1 (THF); 2095 w, 2000 s, 1936 s cm-1 (CH2Cl2).
1H NMR (C4D8O): δ 10.34 (br) (N-H), 7.25 (m), 6.77 (m) ppm (S-
(CH3CN); 2094 w, 1942 s cm-1 (THF); 2087 w (ν CN), 1948 s (ν
)
Co
cm-1 (CH2Cl2). 1H NMR (CD2Cl2): δ 6.51 (t), 6.81 (t), 7.96 (m), 8.14
(m), 8.33 (m), 8.89 (m) ppm (SC4H3N2). Absorption spectrum (CH2-
Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 321 (9065), 385 (4320), 606 (104).
Anal. Calcd for C50H44O2N6S2P2Fe: N, 8.91; C, 63.69; H, 4.70.
Found: N, 8.81; C, 63.38; H, 4.75.
Reaction of [PPN][Fe(CO)(CN)(S-C4H3N2)2] with [Cp2Fe][PF6].
A solution containing 0.189 g (0.2 mmol) of complex 5 and 0.067 g
(0.2 mmol) of [Cp2Fe][PF6] in CH3CN (3 mL) was stirred at -20 °C.
The reaction was monitored immediately by IR. The IR spectrum (2096
w (ν CN), 1962 s (ν Co) cm-1) was assigned to the formation of neutral
FeIII(CO)(CN)(S-C4H3N2)2. In the same flask 0.2 mmol of [PPN][BH4]
(0.112 g) in CH3CN solution was added, followed by stirring at -20
°C for 5 min. The brown solid that was obtained upon removal of
solvent from the resulting solution was extracted with 5 mL of degassed
THF. Subsequent solvent removal produced complex 5 (identified by
IR) ,which was washed with hexane and dried under vacuum.
Crystallography. Crystallographic data of complexes 1, 4, and 5
are summarized in Table 1, and in the Supporting Information. The
crystals of 1, 4, and 5 are chunky. The crystals of 1, 4, and 5 chosen
for X-ray diffraction studies measured 0.40 × 0.23 × 0.03 mm, 0.55
× 0.50 × 0.45 mm, and 0.40 × 0.30 × 0.25 mm, respectively. Each
crystal was mounted on a glass fiber and quickly coated in epoxy resin.
Unit-cell parameters for complex 4 were obtained by the least-squares
refinement from 25 reflections with θ between 1.65 and 25.00°. Least-
squares refinement of the positional and anisotropic thermal parameters
for all non-hydrogen atoms and fixed hydrogen atoms contribution was
based on F2. Diffraction measurements were carried out on a Nonius
CAD 4 diffractometer with graphite-monochromated Mo KR radiation.22
A æ scan absorption correction was made. The NRCC-SDP-VAX
package of programs was employed,23 and atomic scattering factors
were from ref 24. Diffraction measurements for complexes 1 and 5
were carried out at 295(2) K on a Siemens SMART CCD diffractometer
with graphite-monochromated Mo KR radiation (λ 0.7107 Å) and θ
between 1.43 and 25.03° for complex 1 and between 1.30 and 27.49°
for complex 5. Least-squares refinement of the positional and aniso-
tropic thermal parameters for all non-hydrogen atoms and fixed
(22) North, A. C. T.; Philips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
(23) Gabe, E. J.; LePage, Y.; Chrarland, J. P.; Lee, F. L.; White, P. S.
J. Appl. Crystallogr. 1989, 22, 384.
(24) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, 1974; Vol. 4, Table 2.2B.

494 J. Am. Chem. Soc., Vol. 122, No. 3, 2000
Liaw et al.
hydrogen atoms contribution was based on F2. A SADABS25 absorption
correction was made. The SHELXTL26 structure refinement program
was employed.
Supporting Information Available: Tables of crystal data,
atomic coordinates, bond lengths and angles, anisotropic
displacement parameters, and hydrogen coordinates and isotropic
displacement parameters (PDF). An X-ray crystallographic file
for the structure determinations of [PPN][Fe(CO)2(CN)(S,NH-
C6H4)], [PPN]2[(CN)(CO)2Fe(µ-S,S-C6H4)]2, and [PPN][Fe-
(CO)(CN)(S-C4H3N2)2] (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We gratefully acknowledge financial
support from the National Science Council (Taiwan). We also
thank Prof. Marcetta Y. Darensbourg for helpful suggestions.
(25) Sheldrick, G. M. SADABS, Siemens Area Detector Absorption
Correction Program, University of Go¨ttingen, 1996.
(26) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Deter-
mination, Siemens Analytical X-ray Instruments Inc., Madison, WI, 1994.
JA992300Q