Six-coordinate and five-coordinate FeII(CN)2(CO)x thiolate complexes (x = 1, 2): Synthetic advances for iron sites of [NiFe] hydrogenases

Article abstract of DOI:10.1021/ja011504f

The dicyanodicarbonyliron(II) thiolate complexes trans, cis-[(CN)₂(CO)₂Fe(S,S-C-R)]^- (R = OEt (2), N(Et)₂ (3)) were prepared by the reaction of [Na][S-C(S)-R] and [Fe(CN)₂(CO)₃(Br)]^- (1). Complex 1 was obtained from oxidative addition of cyanogen bromide to [Fe(CN)(CO)₄]^-. In a similar fashion, reaction of complex 1 with [Na][S,O-C₅H₄N], and [Na][S,N-C₅H₄] produced the six-coordinate trans,cis-[(CN)₂-(CO)₂Fe (S,O-C₅H₄N)]^- (6) and trans,cis-[(CN)₂ (CO)₂Fe(S,N-C₅H₄)]^- (7) individually. Photolysis of tetrahydrofuran (THF) solution of complexes 2, 3, and 7 under CO led to formation of the coordinatively unsaturated iron(II) dicyanocarbonyl thiolate compounds [(CN)₂(CO)Fe(S,S-C-R)]^- (R = OEt (4), N(Et)₂ (5)) and [(CN)₂(CO)Fe(S,N-C₅H₄)]^- (8), respectively. The IR ν_CN stretching frequencies and patterns of complexes 4, 5, and 8 have unambiguously identified two CN^- ligands occupying cis positions. In addition, density functional theory calculations suggest that the architecture of five-coordinate complexes 4, 5, and 8 with a vacant site trans to the CO ligand and two CN^- ligands occupying cis positions serves as a conformational preference. Complexes 2, 3, and 7 were reobtained when the THF solution of complexes 4, 5, and 8 were exposed to CO atmosphere at 25 °C individually. Obviously, CO ligand can be reversibly bound to the Fe^II site in these model compounds. Isotopic shift experiments demonstrated the lability of carbonyl ligands of complexes 2, 3, 4, 5, 7, and 8. Complexes [(CN)₂(CO)Fe(S,S-C-R)]^- and NiA/NiC states [NiFe] hydrogenases from D. gigas exhibit a similar one-band pattern in the ν_CO region and two-band pattern in the ν_CN region individually, but in different positions, which may be accounted for by the distinct electronic effects betwen [S,S-C-R]^- and cysteine ligands. Also, the facile formations of five-coordinate complexes 4, 5, and 8 imply that the strong σ-donor, weak π-acceptor CN^- ligands play a key role in creating/stabilizing five-coordinate iron(II) [(CN)₂(CO)Fe(S,S-C-R)]^- complexes with a vacant coordination site trans to the CO ligand.

Full text of DOI:10.1021/ja011504f

Published on Web 02/20/2002
Six-Coordinate and Five-Coordinate FeII(CN)2(CO)x Thiolate
Complexes (x ) 1, 2): Synthetic Advances for Iron Sites of
[NiFe] Hydrogenases
Wen-Feng Liaw,*,† Jiun-Hung Lee,† Hung-Bin Gau,† Chien-Hong Chen,†
Shiou-Ju Jung,† Chen-Hsiung Hung,† Wen-Yuan Chen,† Ching-Han Hu,† and
Gene-Hsiang Lee‡
Contribution from the Department of Chemistry, National Changhua UniVersity of Education,
Changhua 50058, Taiwan, and Instrumentation Center, National Taiwan UniVersity,
Taipei 10764, Taiwan
Received June 20, 2001
Abstract: The dicyanodicarbonyliron(II) thiolate complexes trans,cis-[(CN)2(CO)2Fe(S,S-C-R)]- (R ) OEt
(2), N(Et)2 (3)) were prepared by the reaction of [Na][S-C(S)-R] and [Fe(CN)2(CO)3(Br)]- (1). Complex 1
was obtained from oxidative addition of cyanogen bromide to [Fe(CN)(CO)4]-. In a similar fashion, reaction
of complex 1 with [Na][S,O-C5H4N], and [Na][S,N-C5H4] produced the six-coordinate trans,cis-[(CN)2-
(CO)2Fe(S,O-C5H4N)]- (6) and trans,cis-[(CN)2(CO)2Fe(S,N-C5H4)]- (7) individually. Photolysis of tet-
rahydrofuran (THF) solution of complexes 2, 3, and 7 under CO led to formation of the coordinatively
unsaturated iron(II) dicyanocarbonyl thiolate compounds [(CN)2(CO)Fe(S,S-C-R)]- (R ) OEt (4), N(Et)2
(5)) and [(CN)2(CO)Fe(S,N-C5H4)]- (8), respectively. The IR νCN stretching frequencies and patterns of
complexes 4, 5, and 8 have unambiguously identified two CN- ligands occupying cis positions. In addition,
density functional theory calculations suggest that the architecture of five-coordinate complexes 4, 5, and
8 with a vacant site trans to the CO ligand and two CN- ligands occupying cis positions serves as a
conformational preference. Complexes 2, 3, and 7 were reobtained when the THF solution of complexes
4, 5, and 8 were exposed to CO atmosphere at 25 °C individually. Obviously, CO ligand can be reversibly
bound to the FeII site in these model compounds. Isotopic shift experiments demonstrated the lability of
carbonyl ligands of complexes 2, 3, 4, 5, 7, and 8. Complexes [(CN)2(CO)Fe(S,S-C-R)]- and NiA/NiC
states [NiFe] hydrogenases from D. gigas exhibit a similar one-band pattern in the νCO region and two-
band pattern in the νCN region individually, but in different positions, which may be accounted for by the
distinct electronic effects betwen [S,S-C-R]- and cysteine ligands. Also, the facile formations of five-
coordinate complexes 4, 5, and 8 imply that the strong σ-donor, weak π-acceptor CN- ligands play a key
role in creating/stabilizing five-coordinate iron(II) [(CN)2(CO)Fe(S,S-C-R)]- complexes with a vacant
coordination site trans to the CO ligand.
Introduction
(CO)] unit with the opposite face coordinated to two cysteines
bridged to a nickel (a pyramidal [Fe(CN)(CO)(SO)] unit was
An intriguing common feature in the active sites of [Fe]-
only and [NiFe] hydrogenase metalloproteins is the presence
of at least one Fe center coordinated by CO and CN- ligands
and a vacant coordination site at iron that may bind small
molecules (e.g., H2, H+, H-).1-8 The active site of [NiFe]
hydrogenase isolated from DesulfoVibrio gigas consists of a
(Scys)2Ni(µ-Scys)2(µ-X)Fe(CO)(CN)2 (X ) O, OH) bimetallic
complex that can exist in various redox states.1 The iron site in
the active form has been established as a pyramidal [Fe(CN)2-
(2) (a) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, E. C.; Fontecilla-Camps,
J. C. Structure 1999, 7, 13. (b) Nicolet, Y.; De Lacey, A. L.; Vernede, X.;
Fernandez, V. M.; Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem.
Soc. 2001, 123, 1596. (c) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.;
Seefeldt, L. C. Science 1998, 282, 1853. (d) De Lacey, A. L.; Stadler, C.;
Cavazza, C.; Hatchikian, E. C.; Fernandez, V. M. J. Am. Chem. Soc. 2000,
122, 11232. (e) Popescu, C. V.; Mu¨nck, E. J. Am. Chem. Soc. 1999, 121,
7877.
(3) (a) Higuchi, Y.; Ogata, H.; Miki, K.; Yasuoka, N.; Yagi, T. Structure 1999,
7, 549. (b) Higuchi, Y.; Yagi, T.; Yasuoka, N. Structure 1997, 5, 1671.
(4) (a) 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. (b)
Davidson, G.; Choudhury, S. B.; Gu, Z.; Bose, K.; Roseboom, W.; Albracht,
S. P. J.; Maroney, M. J. Biochemistry 2000, 39, 7468. (c) Hatchikian, E.
C.; Traore, A. S.; Fernandez, V. M.; Cammack, R. Eur. J. Biochem. 1990,
187, 635. (d) Happe, R. P.; Roseboom, W.; Albracht, S. P. J. Eur. J.
Biochem. 1999, 259, 602. (e) Van der Zwaan, J. W.; Coremans, J. M. C.
C.; Bouwens, E. C. M.; Albracht, S. P. J. Biochim. Biophys. Acta 1990,
1041, 101. (f) Liaw, W.-F.; Chen, C.-H.; Lee, C.-M.; Lee, G.-H.; Peng,
S.-M. J. Chem. Soc., Dalton Trans. 2001, 138.
† National Changhua University of Education.
‡ National Taiwan University.
(1) (a) Volbeda, A.; Charon, M.-H.; Piras, C.; Hatchikian, E. C.; Frey, M.;
Fontecilla-Camps, J. C. Nature 1995, 373, 580. (b) Volbeda, A.; Garcin,
E.; Piras, C.; de Lacey, A. L.; Fernandez, V. M.; Hatchikian, E. C.; Frey,
M.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 1996, 118, 12989. (c) Garcin,
E.; Vernede, X.; Hatchikian, E. C.; Volbeda, A.; Frey, M.; Fontecilla-
Camps, J. C. Structure 1999, 7, 557. (d) Happe, R. P.; Roseboom, W.;
Pierik, A. J.; Albracht, S. P.; Bagley, K. A. Nature 1997, 385, 126. (e)
Frey, M. Struct. Bonding 1998, 90, 98.
(5) (a) Lemon, B. J.; Peters, J. W. J. Am. Chem. Soc. 2000, 122, 3793. (b)
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1680 VOL. 124, NO. 8, 2002 J. AM. CHEM. SOC.
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II
6- and 5-Coordinate Fe (CN)2(CO)x Thiolate Complexes
A R T I C L E S
addition to serving as spectroscopic references, may provide
essential understanding about the active site construction and
function of [Fe] and [NiFe] hydrogenases, and the roles/
functions of CO and CN- ligands, and may elucidate the
electronic structure of active centers of the binuclear sub-
cluster.9-14 Two iron thiolate cyanocarbonyl model complexes
have been reported recently by Darensbourg et al.,9 Rauchfuss
et al.,11 Pickett et al.,12 and Koch et al.10 In one interesting model
compound each iron of the dinuclear Fe(I) thiolate cyano-
carbonyl complex is surrounded by one CN- and two CO
ligands,9,11,12 while the other model compound is a low-spin
six-coordinate, mononuclear iron(II)/iron(III) thiolate complex
with mixed CO and CN- ligands.10 Recent work in this
laboratory has shown that a dinuclear iron(II) thiolate cyano-
Figure 1. (a) Schematic drawing of the active site of [NiFe] hydrogenases
as deduced from crystallographic studies.1 (b) Schematic drawing of the
H-cluster of the CO-inhibited form of Clostridium pasteuranium [Fe]
hydrogenases as deduced from recent X-ray crystallographic studies.2,5
proposed in [NiFe] hydrogenases isolated from D. Vulgaris
Miyazaki F.2).1 Thus, a vacant coordination site around iron,
trans to the carbonyl ligand, appears to be a reactive site and
presumably plays an important role in biological hydrogen
activation.1,3,4,6 The nickel site has been proposed to be redox
active, while the iron site remains as Fe(II) in all spectroscopi-
cally defined redox states of the enzyme.4 Also, direct evidence
has shown that redox states C/SI can bind external CO in [NiFe]
hydrogenases.4 The exogenously added CO molecule is known
to inhibit activity in [NiFe] hydrogenases, and is bound to metal
in the active site in an end-on way, although it is not clear yet
to which metal, Ni or Fe (nickel was proposed as the binding
site of CO molecule recently).4 A schematic drawing of the
active site of [NiFe] hydrogenases as deduced from crystal-
lographic studies is shown in Figure 1a.1
The recent report of high-quality X-ray crystal structure of
[Fe]-only hydrogenases from DesulfoVibrio desulfuricans re-
vealed that the active site contains a dinuclear iron di-
(thiomethyl)amine with mixed CO and CN- ligands bound to
a [4Fe-4S] cluster via cysteine bridge, and also suggested that
the unsaturated Fe center present in the H-cluster of the enzyme
acts as a H2 binding site.2 The extrinsic CO binds to the distal
Fe atom of the H-cluster, and is vibrationally coupled to the
intrinsic CO ligand at this Fe atom.2d Upon reduction of the
active oxidized form, FTIR spectroscopic evidence and crystal
structure of the reduced active site of D. desulfuricans [Fe]
hydrogenase show that the previously bridging CO is now
terminally bound to the distal iron that most likely serves as
the primary hydrogen binding site.2b A schematic drawing of
the H-cluster of the CO-inhibited form of the Clostridium
pasteuranium [Fe] hydrogenases as deduced from recent X-ray
crystallographic studies is shown in Figure 1b.2d,5 Additionally,
recent investigations indicate that carbon monoxide causes
reversible inhibition of hydrogen oxidation, and that inhibition
can be reversed by illuminating with light in Clostridium
pasteuranium [Fe] hydrogenase.5
2-
carbonyl [Fe(CO)2(CN)(µ-S,S-C6H4)]2 complex was pro-
duced upon protonation of five-coordinate mononuclear iron(II)
thiolate cyanocarbonyl complex [Fe(CO)2(CN)(S,NH-C6H4)]-
by 1,2-benzenedithiol.13
Examples of cyanide (CN-) coordination to iron(II) and the
spectroscopic signals of dicyanide iron(II) thiolate carbonyl
complexes ([(CN)2(CO)FeII(SR)2]n-) are of much interest,
particularly in catalytically active site construction of the
(CysS)2Ni(µ-SCys)2Fe(CN)2(CO) active site of [NiFe] hydro-
genases. As far as we know, there is no report of the dicyanide
iron(II) thiolate carbonyl complexes characterized by X-ray
crystallography.15,16 By application of oxidative addition and
stepwise ligand exchange route, we have prepared trans,cis-
[(CN)2(CO)2Fe(S,S-C-R)]- (R ) OEt (2), R ) N(Et)2 (3)),
trans,cis-[(CN)2(CO)2Fe(S, O-C5H4N)]- (6), and trans,cis-
[(CN)2(CO)2Fe(S,N-C5H4)]- (7). These along with the precur-
sor complex [(CN)2(CO)3Fe(Br)]- (1) were isolated and char-
acterized by X-ray crystallography and infrared spectroscopy.
Results and Discussion
When cyanogen bromide (0.8 mmol) was reacted directly with
[PPN][Fe(CO)4(CN)] (0.5 mmol)17in tetrahydrofuran (THF) at
room temperature, air-stable hexacoordinate FeII complex [PPN]-
[(CN)2(CO)3Fe(Br)] (1) was isolated as a light yellow solid after
recrystallization from THF-hexane (yield 75%) (Scheme 1a).
An oxidative addition/decarbonylation reaction may account for
the formation of complex 1.18
Subsequent reactions of complex 1 with [Na][S-C(S)-R]
(R ) OEt, N(Et)2) in THF/CH2Cl2 produced the air and
(9) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M. Y.
Angew. Chem., Int. Ed. 1999, 38, 3178.
(10) Hsu, H.-F.; Koch, S. A.; Popescu, C. V.; Mu¨nck, E. J. Am. Chem. Soc.
1997, 119, 8371.
(11) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999,
121, 9736.
(12) Le cloirec, A.; Best, S. P.; Borg, S.; Davies, S. C.; Evans, D. J.; Hughes,
D. L.; Pickett, C. J. J. Chem. Soc., Chem. Commun. 1999, 2285.
(13) 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.
The roles of CN- and CO ligands, the function of dithiolate
bridge/protein-bound cysteine bridge, the active site construc-
tion, the photochemical properties of the CO-inhibited form,
and the requirement of two metals in the active sites of [Fe]
and [NiFe] hydrogenases (homodinuclear iron-iron in [Fe]
hydrogenases and heterodinuclear nickel-iron in the case of
[NiFe] hydrogenases) are the principal questions to be raised.1-8
Designing and synthesizing structural model complexes, in
(14) (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) Davies, S. C.; Evans, D. J.; Hughes, D. L.; Longhurst, S.; Sanders, J.
R. J. Chem. Soc., Chem. Commun. 1999, 1935. (f) Sellmann, D.; Geipel,
F.; Moll, M. Angew. Chem., Int. Ed. 2000, 39, 561.
(15) (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.
(16) Moreland, A. C.; Rauchfuss, T. B. Inorg. Chem. 2000, 39, 3029.
(17) (a) Ruff, J. K. Inorg. Chem. 1969, 8, 86. (b) Goldfield, S. A.; Raymond,
K. N. Inorg. Chem. 1974, 13, 770.
(6) (a) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. ReV. 2000,
206, 533. (b) Adams, M. WW; Stiefel, E. I. Curr. Opin. Chem. Biol. 2000,
4, 214.
(7) Bioinorganic Catalysis, 2nd ed.; Cammack, R., van Vliet, P., Reedijk J.,
Eds.; Marcel Dekker: New York, 1999; p 231.
(8) Adams, M. W. W.; Stiefel, E. I. Science 1998, 282, 1842.
9
J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1681

A R T I C L E S
Liaw et al.
Scheme 1
Table 1. Infrared Data for Complexes 1-8
compound
ν(CN), cm-1 (THF)
ν(CO), cm-1 (THF)
1
2
3
4
5
6
7
8
2139 vw, 2127 vw
2122 vw, 2112 w
2119 vw, 2112 w
2113 w, 2105 w
2109 w, 2102 w
2121 vw, 2109 w
2124 vw, 2113 w
2111 w, 2103 w
2099 m, 2056 s, 2035 m
2038 vs, 1984 vs
2027 vs, 1973 vs
1996 vs
1985 vs
2041 vs, 1982 vs
2036 vs, 1983 vs
1996 vs
thermally stable, yellow trans,cis-[PPN][(CN)2(CO)2Fe(S,S-
C-R)] complexes (R ) OEt (2), N(Et)2 (3)) individually
(Scheme 1b).4,6a The IR spectrum of complex 2 in the aprotic
solvent THF reveals two weak absorption bands for the CN-
ligands at 2122 vw and 2112 w cm-1 (2119 vw and 2112 w
cm-1 for 3) supporting a trans position of two CN- ligands,
while the two strong absorption bands 2038 s, 1984 s cm-1
(2027 s, 1973 s cm-1 for 3) assigned to the carbonyl stretching
frequencies support a cis position of two CO ligands (Table
1).19 When a THF solution of complex 3 is purged with 13CO,
the IR νCO peaks at 2027, 1973 cm-1 shift to absorbances at
1981 s, 1928 s cm-1 (trans,cis-[(CN)2(13CO)2Fe(S,S-C-
N(Et)2)]-). The isotopic shift of 45 cm-1 is consistent with the
calculated position, based only on the difference in masses
Figure 2. FTIR spectra (THF) of (a) complex 2, (b) complex 4, (c) complex
7, and (d) complex 8.
stretching frequencies and patterns 2038 s, 1984 s cm-1 (νCO
)
between 12CO and 13CO. The reappearance of the 2027, 1973
and 2122 vw, 2112 w cm-1 (νCN) (complex 3: 2027 s, 1973 s
cm-1 (νCO) and 2119 vw, 2112 w cm-1 (νCN)) shifting to
absorbances at 1996 s (νCO), 2113 w, 2105 w (νCN) cm-1 (1985
vs (νCO), 2109 w, 2102 w (νCN) cm-1) is consistent with the
formation of coordinatively unsaturated iron(II) dicyanide
carbonyl thiolate complex [PPN][(CN)2(CO)Fe(S,S-COEt)] (4)
([PPN][(CN)2(CO)Fe(S,S-CN(Et)2)] (5)) (Table 1) with two
cyanide ligands occupying cis positions (Figure 2a,b) (Scheme
1c).1,4a The 13C (CO) NMR spectra (δ (CD3CN) 210.79 (s) ppm
for 4 and 212.5 (s) ppm for 5) of complexes 4 and 5 also support
the presence of the low-spin five-coordinate d6 Fe(II) complexes
12
cm-1 bands on removal of the 13CO and replacement with
-
CO atmosphere demonstrated reversibility of CO ligand lability
of complexes 3.
In contrast, the isotopic shift of νCO was almost not observed
when complex 2 was treated with 1 atm of 13CO in THF at
room temperature overnight. We noticed that the distinct
electronic effects between diethyl dithiocarbamate ([S,S-CN-
(Et)2]-) and ethyl xanthate ([S,S-COEt]-) ligands has a
significant effect on CO ligand lability of trans,cis-[(CN)2-
(CO)2Fe(S,S-C-R)]- complexes.20
Upon photolysis of THF solution of complex 2 (or 3) under
CO atmosphere at room temperature, the IR νCO and νCN
(18) (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.; Horng, Y.-C.; Ou, D.-S.; Ching,
C.-Y.; Lee, G.-H.; Peng, S.-M. J. Am. Chem. Soc. 1997, 119, 9299. (c)
Liaw, W.-F.; Hsieh, C.-K.; Lin, G.-Y.; Lee, G.-H. Inorg. Chem. 2001, 40,
3468.
(19) (a) Liaw, W.-F.; Chen, C.-H.; Lee, G.-H.; Peng, S.-M. Organometallics
1998, 17, 2370. (b) 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.
(20) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley and Sons: New York, 1999.
9
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II
6- and 5-Coordinate Fe (CN)2(CO)x Thiolate Complexes
A R T I C L E S
Table 2. Predicted Reaction Energy and Enthalpy for the
Extrusion of CO from Complexes 2 and 3a,b
∆E
∆H(0 K)
∆H(298 K)
2
0.0
32.3
35.0
42.6
45.4
0.0
0.0
29.6
32.4
39.8
42.6
0.0
0.0
30.5
33.2
40.7
43.5
0.0
4 + CO
4b + CO
4c + CO
4d + CO
3
5 + CO
5b + CO
5c + CO
30.5
33.7
44.4
27.9
31.3
41.7
28.7
32.1
42.6
a Refer to Figure 3 for species notation. b Values are in kcal/mol.
with two CN- ligands occupying cis positions, respectively.
However, we cannot unambiguously rule out the possibility of
formation of the weak coordinative dimeric complex A in
photolysis of THF solution of complexes 2 and 3.
Density functional theory (DFT) was applied in this study.
We used Becke’s three-parameter hybrid functional,21 where
the exchange functional of Becke22 and correlation functional
of Lee, Yang, and Parr23 were chosen (B3LYP). The geometries
of all species studied were fully optimized using analytic
gradients at the B3LYP/6-31G level. Harmonic vibrational
frequency calculations were performed at the minima. The
vibrational frequencies were used in the evaluation of zero-
point vibrational energies and thermal corrections up to 298 K.
Enthalpies were obtained at the B3LYP/6-31G*//B3LYP/6-31G
level, including the thermal corrections obtained using B3LYP/
6-31G. The GAUSSIAN 98 suite of programs has been used in
this work.24
Relative energy (∆E) and enthalpy (∆H) computed of the
species involved in the extrusion of CO from compounds 2 and
3 are summarized in Table 2. The structures and some of the
important bond distances of these species are schematically
illustrated in Figure 3. We have located four minima which
correspond to the five-coordinate complex with the extrusion
of the CO ligand from 2 (4, 4b, 4c, and 4d; see Figure 3). Three
minima were located corresponding to the extrusion of CO from
3 (5, 5b, and 5c; see Figure 3). A common feature is observed
among these compounds: structures that are trigonal bypyra-
midal are more than 10 kcal/mol higher in energy than those
that are square pyramidal (4, 4b and 5, 5b). For the square
pyramidal complexes, it was found that those having vacant
sites trans to the CO ligand (4 and 5) are relatively stable. The
observation is consistent with the proposal made by several
research groups.25-31
Figure 3. Structures and bond distances for the species involved in the
extrusion of CO from 2 and 3; geometries were optimized at the B3LYP/
6-31G level.
The anionic complexes 4, 5 and Ni-A/Ni-C states in [NiFe]
hydrogenase from D. gigas exhibit a similar one-band pattern
in the νCO region and two-band pattern in the νCN region
individually,4 but at different positions, 1996 vs (νCO), 2113 w,
2105 w (νCN) cm-1 (THF) for complex 4, 1985 vs (νCO), 2109
w, 2102 w (νCN) for complex 5 (Table 1), and 1947 vs (νCO),
2093 w, 2083 w (νCN) cm-1 for Ni-A state [NiFe] hydrogenases
(21) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(22) Becke, A. D. J. Chem. Phys. ReV. A 1988, A38, 785.
(23) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785.
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, Jr. J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. d.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J.
V.; Stefamov, B. B.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M.
A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. GAUSSIAN 98, Revision A.3,
Gaussian, Inc.: Pittsburgh, PA, 1998.
(27) (a) Garcin, E.; Vernede, X.; Hatchikian, E. C.; Volbeda, A.; Frey, M.;
Fontecilla-Camps, J. C. Struct. Folding Des. 1999, 7, 557. (b) Higuchi,
Y.; Ogata, H.; Miki, K.; Yasuoka, N.; Yagi, T. Struct. Folding Des. 1999,
7, 549.
(28) Wang, H.; Ralston, C. Y.; Patil, D. S.; Jones, R. M.; Gu, W.; Verhagen,
M.; Adams, M.; Ge, P.; Riordan, C.; Maganian, C. A.; Maschrak, P.;
Kovacs, J.; Miller, C. G.; Collins, T. J.; Brooker, S.; Croucher, P. D.; Wang,
K.; Stiefel, E. I.; Cramer, S. P. J. Am. Chem. Soc. 2000, 122, 10544.
(29) Niu, S.; Thomson, L. M.; Hall, M. B. J. Am. Chem. Soc. 1999, 121, 4000.
(30) Dole, F.; Fournel, A.; Magro, V.; Hatchikian, E. C.; Bertrand, P.;
Guigliarelli, B. Biochemistry 1997, 36, 7847.
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P. J. J. Biol. Chem. 1999, 274, 3331. (b) Van Der Spek, T. M.; Arendsen,
A. F.; Happe, R. P.; Yun, S.; Bagley, K. A.; Stufkens, D. J.; Hagen, W.
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A R T I C L E S
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isolated from D. gigas,4 which may be accounted for by the
distinct electronic effects betwen [S,S-C-R]- (monoanionic
ligand with S,S-donor atoms) and cysteine ligands.15 This result
is consistent with the conclusion, reported by Darensbourg et
al., that the CO vibrational frequency to the electron density
changes around Fe(II) center is about 2.6 times more sensitive
than that for CN-.6a,15
Scheme 2
Exposure of complexes 4 and 5 to CO gives rise to changes
in the IR spectra that are highly dependent on the temperature.
Both 4 and 5 are unreactive toward CO at low temperature
(T < 0 °C) in THF. Complex 3 was reobtained on exposure of
the THF solution of complex 5 to CO atmosphere at 25 °C
overnight. (Scheme 1c′). Presumably, here the structural rear-
rangements occur concurrently as CO ligand coordinated to the
iron, and signify the low barriers of rearrangements of CO and
CN- ligands. The reversibility of CO ligand binding demon-
strates that the complexes 3 and 5 are interconvertible. In
contrast, complex 4 remains almost unchanged when treated
with 1 atm of CO in THF at 25 °C overnight. Obviously, the
two thiolates [S,S-COEt]- and [S,S-C-N(Et)2]-, rendering
the [(CN)2(CO)Fe] unit in different electronic environments,
induce differing stability to CO ligand. This observation may
explain that nucleophiles, such as Et3N and [BH4]-, do not react
with the unsaturated [(CN)2(CO)Fe(S,S-C-R)]- complexes
with a vacant site trans to CO ligand in THF. The more effective
electron-donating ligand [S,S-C-N(Et)2]-, compared with
[S,S-C-OEt]-, destabilizes the [(CN)2(CO)FeII] fragment (i.e.,
complex 5 is more apt to react with strong π-acceptor CO
ligand). This is in contrast to the observation that the stronger
π-donating bidentate [S,NH-C6H4]2- ligand stabilizes the
unsaturated complex [(CN)(CO)2Fe(S,NH-C6H4)]-.13 Coordi-
native association of the unsaturated complex [(CN)(CO)2Fe-
(S,S-C6H4)]- leading to dinuclear [(CN)(CO)2Fe(S,S-C6H4)]22-
explains the instability of [(CN)(CO)2Fe(S,S-C6H4)]-
(π-donating ability, [S,NH-C6H4]2- > [S,S-C6H4]2-).13,18c
These results imply that CN- ligands play a key role in creating/
stabilizing a five-coordinate Fe(II) [(CN)2(CO)Fe(S,S-C-R)]-
complex with a vacant coordination site trans to the CO ligand.25
(νCO 1958 s cm-1) (minor product) and 13C-enriched derivative
[(CN)2(13CO)Fe(S,S-C-N(Et)2)]- (νCO 1944 cm-1) (major
product) at 25 °C. Following extended periods of stirring at
room temperature for 5 days, complex 5 was completely
transformed into trans,cis-[(CN)2(13CO)2Fe(S,S-C-N(Et)2)]-.
Studies of these model compounds imply that electronic effects
transmitted to the [(CN)2(CO)Fe(S,S-C-R)]- unit should be
responsible for the optimized structure of the [(CN)2(CO)Fe-
(S,S-C-R)]- and signify ease of substrate binding and release.
In a similar fashion, the ligand-displacement reaction was
also displayed by complex 1 and [Na][S,O-C5H4N]. When a
THF solution of complex 1 was treated with 1 equiv of [Na]-
[S,O-C5H4N], an immediate change in color from light yellow
to dark brown was observed. The IR spectrum and X-ray crystal
structure identified formation of the d6 Fe(II) complex trans,cis-
[PPN][(CN)2(CO)2Fe(S,O-C5H4N)] (6) with a five-membered
ring (bidentate, S,O-bonded) (Scheme 2a). The four-membered
ring complex trans,cis-[PPN][(CN)2(CO)2Fe(S,N-C5H4)] (7)
(bidentate, S,N-bonded) was obtained via thermolytic conversion
of complex 6 on heating (40 °C) in THF (Scheme 2b). This
transformation slowly occurred at ambient temperature (25 °C),
during which period no intermediate was detected spectrally.
The loss of O atom from the Fe(II) center of complex 6 indicated
the lability of O atom coordinated to [FeII(CN)2(CO)2] unit and
suggested that the bidentate [S,N-C5H4]- enhances the stability
of [FeII(CN)2(CO)2] unit. Complex 6 is the first example of an
iron(II) cyanocarbonyl compound coordinated by mixed S and
O atoms serving as a promising structural and functional model
compound of the iron active site of [NiFe] hydrogenases, since
most of the oxygen-tolerant hydrogenases allow the binding of
the inhibitor (CO, sulfide, and O2) in such a way that it can
later be displaced.1-8
With the aid of isotopic 13CO labeling experiments, νCO and
νCN vibrational spectroscopic studies also have unambiguously
identified two CN- ligands occupying cis positions in complexes
4 and 5. Exposure of complex 4 to 13CO gives rise to a one-
band pattern (1953 cm-1) of IR νCO bands at 25 °C for 3 days
to afford 13C-enriched derivative [(CN)2(13CO)Fe(S,S-COEt)]-
(major) and a one-band pattern (1967 cm-1) of IR νCO bands
consistent with the observation that a single added CO molecule
binds to the trans position of CO ligand to yield, presumably,
the intermediate cis,trans-[(CN)2(13CO)2Fe(S,S-COEt)]- (mi-
nor). Following extended periods of stirring at 25 °C for 10
days, a THF solution of complex 4 completely converted into
trans,cis-[(CN)2(13CO)2Fe(S,S-COEt)]- identified by its IR νCO
bands at 1990 s, 1939 s cm-1 (THF). Here the “slow”
rearrangement of cis,trans-[(CN)2(13CO)2Fe(S,S-C-OEt)]- to
the more stable trans,cis-[(CN)2(13CO)2Fe(S,S-C-OEt)]- was
adopted to explain the formation of [(CN)2(13CO)Fe(S,S-C-
OEt)]- (major product) and cis,trans-[(CN)2(13CO)2Fe(S,S-C-
OEt)]- (minor product) in the initial reaction. Indeed, THF
solution of complex 5 on stirring overnight also undergoes a
ligand coordination/exchange process with 13C-labeled carbon
monoxide to afford cis,trans-[(CN)2(13CO)2Fe(S,S-C-N(Et)2)]-
The S,N-bonded complex 7 was alternatively obtained via
ligand-substitution reaction, a straightforward synthetic reaction
of complex 1 with 1 equiv of [Na][S,N-C5H4] (or HS-C5H4N)
in THF (Scheme 2c). When THF solution of complex 7 was
exposed to 13CO overnight, absorbances at 1990 s, 1939 s cm-1
(trans,cis-[(CN)2(13CO)2Fe(S,N-C5H4)]-) appeared. Reappear-
ance of the 2036 s, 1983 s cm-1 bands on removal of the 13CO
and replacement with 12CO atmosphere demonstrated revers-
ibility of CO ligand lability of complex 7.
Theoretical computations provide some insights in the extru-
sion of CO from 7 (forming 8); the structures are illustrated in
Figure 4. In contrast to that of complexes 4 and 5, only one
9
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II
6- and 5-Coordinate Fe (CN)2(CO)x Thiolate Complexes
A R T I C L E S
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Complexes 1 and 2
Complex 1
Fe-C(4)
C(4)-N(1)
Fe-C(1)
Fe-C(3)
C(2)-O(2)
Fe-Br
1.936(7)
Fe-C(5)
1.957(8)
1.013(8)
1.837(7)
1.029(9)
1.037(8)
1.017(8)
1.847(9)
1.852(7)
1.074(8)
2.459(1)
C(5)-N(2)
Fe-C(2)
C(1)-O(1)
C(3)-O(3)
C(2)-Fe-C(1)
C(2)-Fe-C(4)
C(2)-Fe-Br
90.0(3)
94.0(3)
91.4(2)
C(2)-Fe-C(3)
C(1)-Fe-C(4)
C(4)-Fe-C(5)
173.4(3)
87.9(3)
177.5(3)
Figure 4. Structures and bond distances for the species involved in the
extrusion of CO from 7; geometries were optimized at the B3LYP/6-31G
level.
Complex 2
Fe-C(2)
1.778(11)
1.919(10)
2.300(3)
1.051(9)
1.200(10)
1.361(13)
Fe-C(3)
1.810(10)
1.955(10)
2.326(3)
1.137(11)
1.149(10)
1.464(14)
Fe-C(1)
Fe-C(4)
Fe-S(1)
Fe-S(2)
C(1)-N(1)
C(3)-O(1)
C(5)-O(3)
C(4)-N(2)
C(2)-O(2)
C(6)-O(3)
S(1)-Fe-S(2)
C(3)-Fe-S(1)
C(4)-Fe-S(1)
N(2)-C(4)-Fe
75.22(11)
92.6(4)
86.7(3)
C(2)-Fe-S(1)
C(1)-Fe-S(1)
N(1)-C(1)-Fe
C(1)-Fe-C(4)
171.3(3)
91.2(3)
178.4(11)
177.4(4)
167.0(11)
Figure 5. ORTEP drawing and labeling scheme of the [(CN)2(CO)3Fe(Br)]-
anion with thermal ellipsoid drawn at 50% probability.
minimum was located. Complex 8 is square pyramidal with the
vacant site trans to the CO ligand. At the B3LYP/6-31G*//
B3LYP/6-31G level, the enthalpy of reaction is 26.0 kcal/mol.
To further add credibility to the CO ligand reversibly bound
to the FeII site in these model compounds, a straightforward
photolysis of THF solution of complex 7 under CO was also
conducted at room temperature. The bands at 2125 vw, 2112
w (νCN) and 2036 s, 1983 s cm-1 (νCO) disappeared, with
Figure 6. ORTEP drawing and labeling scheme of [(CN)2(CO)2Fe(S,S-
C-OEt)]- with thermal ellipsoid drawn at 50% probability.
Structures of Complexes 2 and 3. Crystals of complexes 2
and 3 were found to have monoclinic P21/c and P21/n space
group, respectively. Figure 6 displays a thermal ellipsoid plot
of the anionic complex 2. Selected bond distances and bond
angles are given in Table 3. The constraints of the ethylxanthate
ligand generates a S(1)-Fe-S(2) bond angle of 75.22(11)°
enforcing a severe distortion from an octahedron at the six-
coordinate iron site. The Fe-S(1) and Fe-S(2) bond lengths
of 2.300(3) and 2.326(3) Å (unsymmetrical chelate), respec-
tively, are within the range (2.2 Å) observed for [NiFe]
hydrogenases from D. gigas.1 The significantly shorter O(3)-
C(5) bond (1.361(13) Å), as compared to 1.464(14) Å for the
O(3)-C(6) bond, was attributed to the partial π-bond character
between O(3) and C(5) atoms.20
The structure of the trans,cis-[(CN)2(CO)2Fe(S,S-CN(Et)2)]-
unit as [PPN]+ salt is shown in Figure 7. The geometry about
iron is a distorted octahedral, with the bite angle of the chelating
[S,S-CN(Et)2]- ligand being 75.02(6)° (Table 4). The Fe-S
bonds of average length 2.316(2) Å (Fe(1)-S(1), 2.305(2) Å
and Fe(1)-S(2), 2.327(2) Å), Fe-C(O) bonds of average length
1.798(6) Å, and Fe-C(N) bonds of average length 1.906(6) Å
in complex 3 are within the range (2.2, 1.7, and 1.9 Å,
respectively) observed in [NiFe] hydrogenases from D. gigas.1
The interesting feature of complex 3 is the asymmetry in the
FeII-S bond lengths (2.327(2) and 2.305(2) Å), which shows a
difference of 0.022 Å. In complexes 2 and 3, the two carbonyl
groups are disposed in a cis arrangement and are trans to the
concomitant formation of a spectrum (2111 w, 2103 w (νCN
)
and 1996 cm-1 (νCO) (THF)) similar to that observed for
complexes 4 and 5 (Figure 2c,d; Table 1), i.e., formation of
five-coordinate [(CN)2(CO)Fe(S,N-C5H4)]- (8) complex with
two cyanide ligands occupying cis positions (Scheme 2d).
Additionally, complex 7 displayed two distinct doublet signals
13C,13
C
(δ 214.4 (d), 210.0 (d) ppm (CD3CN) (J
) 9 Hz)) in the
13C (CO) NMR spectra, consistent with the presence of two
nonequivalent CO groups in the 13C NMR spectra of complex
7 in acetonitrile-d3 solution. The occurrence of one sharp singlet
(δ 212.7 (s) ppm (CD3CN)) 13C (CO) NMR resonance was
assigned to the CO group of complex 8. The carbon monoxide
atmosphere does promote the formation of complex 7 when
complex 8 is exposed to 1 atm of CO in THF at room
temperature for 2 days (Scheme 2d′).
Structure of Complex 1. The molecular structure of the
complex 1 anion, Figure 5, is that of a distorted octahedron
with C(5)-Fe-C(4) bond angle of 177.5(3)°. The Fe-C(N)
average distance of 1.947(8) Å is significantly longer than those
of the five-coordinate [Fe(CO)2(CN)(S,NH-C6H4)]- (Fe-C(N)
) 1.926(6) Å) and octahedral [Fe(CO)(CN)(S,N-C4H3N)2]-
(Fe-C(N) ) 1.845(4) Å).13 Further appropriate comparison lies
in the C-N parameters: the C-N bond distance (average
1.015(8) Å) in complex 1 is significantly shorter than those of
[Fe(CO)2(CN)(S,NH-C6H4)]- (C-N ) 1.137(6) Å) and [Fe-
(CO)(CN)(S,N-C4H3N)2]- (C-N ) 1.135(4) Å) (Table 3).13
9
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A R T I C L E S
Liaw et al.
Figure 8. ORTEP drawing and labeling scheme of [(CN)2(CO)2Fe(S,O-
C5H4N)]- with thermal ellipsoid drawn at 50% probability.
Figure 7. ORTEP drawing and labeling scheme of [(CN)2(CO)2Fe(S,S-
C-N(Et)2)]- with thermal ellipsoid drawn at 50% probability.
Table 4. Selected Bond Distances (Å) and Angles (deg) for
Complexes 3, 6, and 7
Complex 3
Fe(1)-C(2)
Fe(1)-C(3)
Fe(1)-S(1)
N(1)-C(3)
N(3)-C(5)
N(3)-C(8)
S(1)-C(5)
1.785(6)
1.897(6)
2.305(2)
1.130(5)
1.316(5)
1.474(6)
1.719(5)
Fe(1)-C(1)
Fe(1)-C(4)
Fe(1)-S(2)
N(2)-C(4)
N(3)-C(6)
S(2)-C(5)
1.811(6)
1.915(6)
2.327(2)
1.096(6)
1.455(6)
1.709(5)
C(2)-Fe(1)-C(1)
C(1)-Fe(1)-C(3)
C(3)-Fe(1)-C(4)
C(2)-Fe(1)-S(2)
96.8(3)
87.3(3)
177.1(2)
166.7(2)
C(2)-Fe(1)-C(3)
C(2)-Fe(1)-C(4)
S(1)-Fe(1)-S(2)
C(1)-Fe(1)-S(2)
91.9(3)
90.9(3)
75.02(6)
96.4(2)
Figure 9. ORTEP drawing and labeling scheme of [(CN)2(CO)2Fe(S,N-
C5H4)]- with thermal ellipsoid drawn at 50% probability.
Complex 6
Fe(1)-C(1)
Fe(1)-C(3)
Fe(1)-S(1)
N(1)-C(3)
C(1)-O(1)
N(3)-O(3)
1.751(5)
1.899(5)
2.202(10)
1.153(5)
1.140(5)
1.47(2)
Fe(1)-C(2)
Fe(1)-O(3)
Fe(1)-C(4)
N(2)-C(4)
C(2)-O(2)
1.731(6)
1.97(2)
1.939(2)
1.130(4)
1.160(5)
7 (2.347(2) Å). The Fe(II)-O bond distance in complex 6 is
1.97(2) Å (Table 4).
The X-ray crystal structure of complex 7 is shown in Figure
9. The bond angles at the FeII center is considerably distorted
from the idealized octahedral limits due to the presence of the
four-membered N,S-chelate ring, a characteristic apparent in the
internal chelate angle, S(1)-Fe-N(3) of 70.71(8)° (Table 4).19a
The Fe-S(1) and Fe-N(3) bond lengths, 2.3546(9) and
1.971(3) Å in complex 7, are comparable to the values of
2.3323(9) and 2.000(3) Å in [(CN)(CO)Fe(S,N-C4H3N)2]-.13
C(1)-Fe(1)-C(2)
C(2)-Fe(1)-C(3)
C(2)-Fe(1)-O(3)
O(3)-Fe(1)-S(1)
92.8(3)
87.3(3)
93.9(6)
89.2(6)
C(1)-Fe(1)-C(3)
C(1)-Fe(1)-O(3)
C(3)-Fe(1)-C(4)
92.3(3)
172.7(6)
177.59(18)
Complex 7
Fe-C(3)
Fe-C(1)
Fe-N(3)
S(1)-C(5)
C(2)-N(2)
1.769(4)
1.892(4)
1.971(3)
1.723(4)
1.156(4)
Fe-C(4)
Fe-C(2)
Fe-S(1)
C(1)-N(1)
C(3)-O(1)
1.821(4)
1.928(3)
2.3546(9)
1.146(4)
1.145(4)
Conclusion and Comments
The following are the principal results of this investigation.
C(1)-Fe-C(2)
C(3)-Fe-C(4)
C(3)-Fe-S(1)
179.67(14)
94.62(16)
97.52(11)
N(3)-Fe-S(1)
C(3)-Fe-N(3)
C(4)-Fe-S(1)
70.71(8)
167.86(13)
167.64(13)
1. The iron(II) thiolate cyanocarbonyl complexes 2, 3, 6, and
7 are prepared, respectively, by the reaction of [Na][S-C(S)-
R]/[Na][ S,O-C5H4N]/[Na][S,N-C5H4] with [(CN)2(CO)3-
Fe(Br)]- which was obtained from oxidative addition of BrCN
to [Fe(CO)4(CN)]-.
sulfur atoms. The four-membered FeS2C rings for complexes 2
and 3 are almost planar with a dihedral angle between FeS2
and S2C planes of 5.7° and 0.2°, respectively.
2. Isotopic shift experiments demonstrate the lability of
Structures of Complexes 6 and 7. Definitive assignment of
the structure of complex 6 was obtained by X-ray crystal-
lography; the structure of trans,cis-[(CN)2(CO)2Fe(S, O-
C5H4N)]- unit as PPN+ salt is shown in Figure 8. The
coordination about Fe in [(CN)2(CO)2Fe(S,O-C5H4N)]- can
be considered as essentially octahedral, with the bite angle of
the chelating [S,O-C5H4N]- ligand being 89.2(6)° and the
positions trans to the [S,O-C5H4N]- bidentate ligand occupied
by two CO ligands. This feature, as observed in complexes 2
and 3, can be attributed to the electronic influence of the
bidentate [S,O-C5H4N]- ligand. Because of disorder of S(1)
and O(3), the exact Fe(1)-S(1) and Fe(1)-O(3) bond lengths
are poorly determined. The Fe(II)-S bond of length 2.202(10)
Å is significantly shorter than the Fe(II)-S distance in complex
carbonyl ligands of complexes 2, 3, 4, 5, 7, and 8.
3. Photolysis of THF solutions of complexes 2, 3, and 7 at
room temperature led to the formation of coordinatively
unsaturated iron(II) dicyanocarbonyl thiolate complexes 4, 5,
and 8, respectively, with two cyanides occupying cis positions
and a vacant site preference trans to the CO ligand.1-8,27-31
Additionally, density functional theory (DFT) calculations also
suggest the architecture of five-coordinate complexes 4, 5, and
8 with a vacant site trans to the CO ligand and two CN- ligands
occupying cis positions serves as a conformational preference.29-31
4. The strong σ-donor, weak π-acceptor CN- ligands play a
major role in creating and stabilizing a five-coordinate iron(II)
center with a vacant coordination site.13,25
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II
6- and 5-Coordinate Fe (CN)2(CO)x Thiolate Complexes
A R T I C L E S
diethyldithiocarbamic acid sodium salt, 0.045 g, 0.2 mmol) was added
to a THF solution containing 0.162 g (0.2 mmol) of complex 1 and
stirred at 50 °C for 3h. The reaction mixture was then filtered to separate
the precipitate NaBr, and then diethyl ether was added to precipitate
the yellow semisolid trans,cis-[PPN][(CN)2(CO)2Fe(S,S-C-OEt)] (2)
(0.075 g, 45%) (yellow semisolid trans,cis-[PPN][(CN)2(CO)2Fe(S,S-
C-N(Et)2)] (3),(0.090 g, 52%)). Recrystallization from concentrated
THF solution with diethyl ether diffusion gave yellow crystals used in
the X-ray diffraction study. Complex 2: IR: 2122 vw, 2112 w (νCN),
2038 vs, 1984 vs (νCO) cm-1 (THF); 2125 vw,br, 2112 w,br (νCN), 2053
vs, 2006 vs (νCO) cm-1 (MeOH). 1H NMR (CD3CN): δ 1.37 (t), 1.38
(t), 4.55 (q), 4.58 (q) (O-CH2-CH3) ppm.32 13C NMR (CD3CN): δ
5. The reversibility of CO ligand binding demonstrates that
complexes 2/4, 3/5, and 7/8 are photochemically interconvert-
ible. This result may imply the involvement of iron site in the
mechanism of hydrogen activation, and can be useful in
exploring the key step in H2 uptake mechanism in [NiFe]
hydrogenases.
6. Complexes 4, 5, and 8 on binding of the substrate (CO)
undergo considerable conformational changes, i.e., cis,trans-
[(CN)2(CO)2Fe(S,S-C-R)]- converted into trans,cis-[(CN)2-
(CO)2Fe(S,S-C-R)]-.
7. The vibrational spectra of the [FeII(CN)2(CO)] and [FeII-
(CN)2(CO)2] units (νCO and νCN) found for complexes 4, 5 and
2, 3 may be regarded as spectroscopic references for a variety
of [NiFe] hydrogenase enzymatic states,26 and may be the
exogenously added CO inhibited state, respectively.4
8. The two thiolate ligands [S,S-COEt]- and [S,S-C-
N(Et)2]-, rendering the [(CN)2(CO)Fe] unit in different elec-
tronic environments, induce different stability to CO ligand.1-8
9. All attempts to bind H- ligand terminally to [(CN)2(CO)-
Fe(S,S-C-R)]- unit were not observed spectroscopically.29
This investigation would perhaps allow a more extensive
discussion of the inhibition of the hydrogenases by exogenous
CO molecule and the photochemical properties of the CO
inhibitor complexes.
214.04 (s), 209.08 (s) ppm (CO). Absorption spectrum (CH2Cl2) [λmax
,
nm (ꢀ, M-1 cm-1)]: 332(2111), 342(1656) (sh). Anal. Calcd for
C43H35O3N3P2S2Fe: C, 62.70; H, 4.28; N, 5.10. Found: C, 62.47; H,
4.43; N, 5.63. Complex 3: IR: 2119 vw, 2112 w (νCN), 2027 vs, 1973
vs (νCO) cm-1 (THF); 2120 vw,br, 2108 w,br (νCN), 2043 vs, 1996 vs
(νCO) cm-1 (MeOH). 1H NMR (CD3CN): δ 1.17 (t), 1.20 (t), 3.64 (q),
3.67 (q) ppm (N-CH2-CH3).32 13C NMR (CD3CN): δ 214.1 (s), 209.6
(s) ppm (CO). Absorption spectrum (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]:
332(2111), 342(1656) (sh). Anal. Calcd for C45H40N4O2S2FeP2: C,
63.53; H, 4.74; N, 6.59. Found: C, 63.64; H, 4.88; N, 6.97.
Preparation of trans,cis-[PPN][(CN)2(CO)2Fe(S,O-C5H4N)] (6).
A solution containing 0.324 g (0.4 mmol) of complex 1 and 0.06 g
(0.4 mmol) of [Na][S,O-C5H4N] in THF (8 mL) was stirred at ambient
temperature overnight. The resulting mixture was filtered to remove
[Na][Br] and then diethyl ether (15 mL) was added to precipitate the
dark purple solid trans,cis-[PPN][(CN)2(CO)2Fe(S,O-C5H4N)] (0.063
g, 19%). Recrystallization from saturated THF solution with diethyl
ether diffusion at -15 °C gave dark purple crystals suitable for X-ray
crystallography. The THF solution of complex 6 was thermally unstable
which on stirring at 40 °C overnight was completely converted to
complex 7 as revealed from the IR spectrum. IR (THF): 2121 vw,
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 was purged through these solvents
before use. Solvent was transferred to the reaction vessel via stainless
steel cannula under positive pressure of N2. The reagents iron
pentacarbonyl, hexamethyldisilazane sodium salt, cyanogen bromide,
bis(triphenylphosphoranylidene)ammonium chloride, 2-mercaptopyri-
dine, 2-mercaptopyridine N-oxide sodium salt hydrate (Lancaster/
Aldrich), sodium xanthogenate (TCI), and diethyldithiocarbamic acid
sodium salt (Arcos) were used as received. Compound [PPN][Fe(CO)4-
(CN)] was synthesized and characterized by published procedures.17
Infrared spectra of the carbonyl ν(CO) and cyanide ν(CN) stretching
frequencies were recorded on a Bio-Rad Model FTS-185 spectropho-
tometer 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 Hewlett-Packard 71
spectrophotometer. Photolysis reactions were carried out in a 100 mL
water-cooled Pyrex reactor equipped with a mercury arc 450-W UV
lamp inside the reactor. Analyses of carbon, hydrogen, and nitrogen
were obtained with a CHN analyzer (Heraeus).
1
2109 w (νCN), 2041 vs, 1982 vs (νCO) cm-1. H NMR (CD3CN): δ
7.85 (d), 7.01 (t), 6.72 (d) (S,O-C5H4N) ppm. Absorption spectrum
(CH3CN) [λmax, nm (ꢀ, M-1 cm-1)]: 352(2035), 487(404) (sh). Anal.
Calcd for C45H34O3SN4FeP2: C, 65.23; H, 4.14; N, 6.76. Found: C,
64.83; H, 4.35; N, 6.54.
Preparation of trans,cis-[PPN][(CN)2(CO)2Fe(S,N-C5H4)] (7).
Complex 1 (0.4 mmol, 0.324 g) and [Na][S,N-C5H4] (0.4 mmol, 0.053
g) were dissolved in 8 mL of THF and stirred overnight under nitrogen
at ambient temperature. The resulting mixture was then filtered through
Celite to remove [Na][Br] and diethyl ether (15 mL) was added to
precipitate the light yellow solid trans,cis-[PPN][(CN)2(CO)2Fe(S,N-
C5H4)] (0.104 g, 32%). Diffusion of diethyl ether into THF solution of
complex 7 at -15 °C for 4 weeks led to yellow crystals suitable for
X-ray crystallography. IR (THF): 2124 vw, 2113 w (νCN), 2036 vs,
1
1983 vs (νCO) cm-1. H NMR (C4D8O): δ 8.11 (d), 7.25 (t), 6.63 (t),
6.52 (d) (S,N-C5H4) ppm. 13C NMR (CD3CN): δ 214.4 (d), 210.0 (d)
13 13
ppm (CO) (J
) 9 Hz). Absorption spectrum (THF) [λmax, nm
C,
C
(ꢀ, M-1 cm-1)]: 347(3498), 422(465) (sh). Anal. Calcd for C45H34O2-
SN4FeP2: C, 66.51; H, 4.22; N, 6.89. Found: C, 66.36; H, 4.23; N,
6.71.
Preparation of [PPN][(CN)2(CO)3Fe(Br)] (1). The compounds
[PPN][Fe(CO)4(CN)] (0.5 mmol, 0.365 g) and cyanogen bromide
(BrCN) (0.8 mmol, 0.084 g) were dissolved in 8 mL of THF and stirred
at ambient temperature for 2 h. The solution was then filtered through
Celite and hexane (15 mL) was added to precipitate the orange solid
[PPN][(CN)2(CO)3Fe(Br)] (0.277 g, 68%). Diffusion of hexane into a
THF solution of complex 1 at -15 °C for 4 weeks led to orange crystals
suitable for X-ray crystallography. IR (THF): 2139 vw, 2127 vw (νCN),
Photolysis of THF Solution of Complexes 2, 3, and 7. Complex
2 (0.2 mmol, 0.165 g) (3: 0.2 mmol, 0.171 g; 7: 0.2 mmol, 0.163 g)
was dissolved in 10 mL of THF and irradiated by UV lamp under CO
atmosphere at 20 °C for 15 min. The solution was then monitored with
FTIR. The IR spectra 2113 w, 2105 w (νCN), 1996 vs (νCO) (THF)
cm-1 indicated the formation of [PPN][(CN)2(CO)Fe(S,S-C-OEt)]
(4) (2109 w, 2102 w (νCN), 1985 vs (νCO) (THF) cm-1 and 2111 w,
2103 w (νCN), 1996 vs (νCO) (THF) cm-1 showed the formation of
[PPN][(CN)2(CO)Fe(S,S-C-N(Et)2)] (5) and [PPN][(CN)2(CO)Fe-
(S,N-C5H4)] (8) individually). The THF solutions of complexes 4, 5,
2099 m, 2056 s, 2035 m (νCO) cm-1. Absorption spectrum (THF) [λmax
,
nm (ꢀ, M-1 cm-1)]: 325(1990) (sh), 383(658). Anal. Calcd for C41H30-
BrFeN3O3P2: C, 60.77; H, 3.73; N, 5.19. Found: C, 60.88; H, 4.03;
N, 4.96.
Preparation of trans,cis-[PPN][(CN)2(CO)2Fe(S,S-C-R)] (R )
OEt (2), N(Et)2 (3)). Sodium xanthogenate (0.029 g, 0.2 mmol) (or
(32) Edgar, B. L.; Duffy, D. J.; Palazzotto, M. C.; Pignolet, L. H. J. Am. Chem.
Soc. 1973, 95, 1125.
9
J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1687

A R T I C L E S
Liaw et al.
Table 5. Crystallographic Data of Complexes 1‚THF and 2
Table 6. Crystallographic Data of Complexes 3, 6, and 7
1‚THF
2
3
6
7
chem formula
fw
cryst syst
space group
λ, Å (Mo KR)
a, Å
C45H38N3O4BrFeP2
882.48
monoclinic
P21/c
C43H35N3O3S2FeP2
823.65
monoclinic
P21/c
chem formula C45H40N4O2S2FeP2 C45H34O3SN4FeP2 C45H34O2SN4FeP2
fw
cryst syst
space group P21/n
λ, Å (Mo KR) 0.7107
a, Å
850.72
monoclinic
828.61
monoclinic
P21/n
0.7107
17.061(1)
15.871(1)
17.345(2)
90
117.404(2)
90
4169.7(6)
4
1.320
0.533
295(2)
0.0573a
0.0751b
0.898
812.61
monoclinic
P21/n
0.7107
17.6478(2)
13.1862(2)
17.8391(3)
90
104.9324(7)
90
4011.10(10)
4
0.7107
0.7107
12.2022(2)
13.2023(2)
25.5672(2)
90
90.834(1)
90
4118.37(10)
4
1.423
12.157(3)
13.341(3)
25.364(6)
90
92.462(5)
90
4109.8(17)
4
1.331
16.982(2)
b, Å
c, Å
b, Å
c, Å
15.9068(19)
17.576(2)
90
115.476(3)
90
R, deg
R, deg
â, deg
γ, deg
V, Å3
Z
â, deg
γ, deg
V, Å3
4286.1(9)
4
Z
d
calcd, g cm-3
d
calcd, g cm-3 1.318
1.346
0.551
µ, mm-1
T, K
1.461
0.589
µ, mm-1
T, K
0.566
150(1)
295(2)
293(2)
0.0473a
0.0702b
0.604
150(1)
R
RWF
0.0841a
0.2691b
1.119
0.0680a
0.1337b
1.047
R
RWF
0.0525a
0.0919b
1.036
2
2
GOF
GOF
b
2
2
2
2
b
2
2
2
2
a R ) ∑|(Fo - Fc)|/∑Fo. RWF ) {∑w(Fo - Fc )2/∑[w(Fo )2]}1/2
.
a R ) ∑|(Fo - Fc)|/∑Fo. RWF ) {∑w(Fo - Fc )2/∑[w(Fo )2]}1/2
.
and 8 are stable at low temperature (T < 0 °C). The THF solutions of
complexes 5 and 8 were stirred at ambient temperature (25 °C)
overnight; the IR spectra revealed the formation of complexes 3 and 7
along with the insoluble solids, presumably [PPN][S,S-C-N(Et)2] and
[PPN][S,N-C5H4], respectively. THF solutions of complexes 4, 5, and
8 were reduced to 3 mL and hexane (10 mL) was then added to
precipitate the orange semisolid complexes 4, 5, and 8. The thermally
unstable products 4, 5, and 8 were isolated, individually, by removing
the solvent (since decomposition occurred under vacuum, the yields
of the semisolid complexes were difficult to determine). Complex 4:
IR (THF): 2113 w, 2105 w (νCN), 1996 vs (νCO) cm-1. 1H NMR (CD3-
CN): δ 4.49 (q), 1.34 (t) (O-CH2CH3) ppm. 13C NMR (CD3CN): δ
210.79 (s) ppm (CO). Absorption spectrum (THF) [λmax, nm (ꢀ, M-1
cm-1)]: 336(1082), 360(908). Complex 5: IR (THF): 2109 w, 2102
w (νCN), 1985 vs (νCO) cm-1. 1H NMR (CD3CN): δ 1.16 (t), 1.20 (t),
3.61 (q), 3.64 (q) (N-CH2CH3) ppm. 13C NMR (CD3CN): δ 212.5 (s)
ppm (CO). Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]:
383(2395), 506(736), 593(575). Complex 8: IR (THF): 2111 w, 2103
and 27.50° for complex 1, between 1.61° and 25.00° for complex 2,
between 1.39° and 27.58° for complex 3, between 1.39° and 27.54°
for complex 6, and between 2.36° and 27.50° for complex 7. 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 SADABS33 absorption correction was made.
The SHELXTL34 structure refinement program was employed. In the
case of complex 2, the OCH2CH3 group (shown in Figure 6) is found
at disordered positions (O(3)C(6)C(7):O(3)′C(6)′C(7)′ ) 2/3:1/3) and
were refined by partial occupancies; in the case of 6 one sulfur and
one oxygen atom (S(1) and O(3) as shown in Figure 8) are found at
disordered positions and were refined by partial occupancies.
Acknowledgment. We thank the National Science Council
(Taiwan) for support of this work. We also thank Prof. Marcetta
Y. Darensbourg for helpful suggestions.
Supporting Information Available: X-ray crystallographic
file in CIF format for the structure determinations of [PPN]-
[(CN)2(CO)3Fe(Br)], [PPN][(CN)2(CO)2Fe(S,S-C-OEt)], [PPN]-
[(CN)2(CO)2Fe(S,S-C-N(Et)2)], [PPN][(CN)2(CO)2Fe(S, O-
C5H4N)], and [PPN][(CN)2(CO)2Fe(S,N-C5H4)]. Tables of
crystal data and structure refinement, atomic coordinates and
displacement parameters, and bond lengths and angles (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
w (νCN), 1996 vs (νCO) cm-1. H NMR (C4D8O): δ 7.77 (d), 7.21 (t),
6.60 (t), 6.44 (d) (S,N-C5H4) ppm. 13C NMR (CD3CN): δ 212.7 (s)
ppm (CO). Absorption spectrum (THF) [λmax, nm (ꢀ, M-1 cm-1)]:
334(5218), 408(3296).
Crystallography. Crystallographic data of complexes 1, 2, 3, 6, and
7 are summarized in Tables 5 and 6 and in the Supporting Information.
The crystals of 1, 2, 3, 6, and 7 are chunky. The crystals of 1, 2, 3, 6,
and 7 chosen for X-ray diffraction studies measured 0.33 × 0.12 ×
0.10 mm, 0.08 × 0.08 × 0.4 mm, 0.08 × 0.17 × 0.34 mm, 0.211 ×
0.211 × 0.42 mm, and 0.2 × 0.18 × 0.15 mm, respectively. Each crystal
was mounted on a glass fiber. Diffraction measurements for complexes
2, 3, 6, and 7 were carried out at 295(2) K (150(1) K for complexes 1
and 7) on a Siemens SMART CCD diffractometer with graphite-
monochromated Mo KR radiation (λ 0.7107 Å) and θ between 1.67°
JA011504F
(33) Sheldrick, G. M. SADABS, Siemens Area Detector Absorption Correction
Program; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(34) Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determination;
Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1994.
9
1688 J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002