Terminal pyridine-N ligation at [FeFe] hydrogenase active-site mimic

Article abstract of DOI:10.1016/j.jorganchem.2009.03.050

Diiron model complexes (μ-pdt)Fe₂(CO)₅L with L = pyridine ligands, e.g. py (A), etpy (B), btpy (C), were synthesized as active site analogues of [FeFe] hydrogenase, and characterized by X-ray crystallography and electrochemistry. Pyr

Full text of DOI:10.1016/j.jorganchem.2009.03.050

Journal of Organometallic Chemistry 694 (2009) 2576–2580
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Terminal pyridine-N ligation at [FeFe] hydrogenase active-site mimic
*
Yue Zhang, Ming-Qiang Hu, Hui-Min Wen, You-Tao Si, Cheng-Bing Ma, Chang-Neng Chen , Qiu-Tian Liu
Fujian Institute of Research on the Structure of Matter, No. 155 West Yangqiao Road, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Diiron model complexes (l-pdt)Fe2(CO)5L with L = pyridine ligands, e.g. py (A), etpy (B), btpy (C), were
Received 23 February 2009
Received in revised form 30 March 2009
Accepted 31 March 2009
Available online 14 April 2009
synthesized as active site analogues of [FeFe] hydrogenase, and characterized by X-ray crystallography
and electrochemistry. Pyridine-N ligation was found to be able to tune the redox properties of the diiron
centers of model complexes.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Bioinorganic chemistry
Electrochemistry
[FeFe] Hydrogenase
Organometallic complexes
Pyridine ligand
1. Introduction
for 2Fe2S model complexes. Herein, we report the preparation of
a series of active site analogues of [FeFe] hydrogenase with the
[FeFe] hydrogenase has sparked chemical mimic research inter-
est due to the capability of producing hydrogen efficiently. The
active site has been called ‘‘H-cluster”, which comprises a dithiol-
ate-bridged diiron subsite and a cuboidal Fe4S4(S-Cys)4 cluster,
linked up through a cysteine sulfur atom. Chemical model studies
have shown that classical Reihlen diiron dithiolate complexes
substitution of pyridine ligands, and their characterization and
electrochemistry.
2. Experimental
2.1. Reagents and measurements
(l-SR)2Fe2(CO)6 share several structural features with the diiron
subsite, e.g. short Fe–Fe distance (ca. 2.6 Å), bidentated dithiolate
bridge and diatomic CO ligands [1–3].
To obtain better structural and functional diiron model com-
plexes, good donor ligands, such as carbene [4–7], cyanide
[8–12], phosphine [13–18], thioether or sulfoxide [19,20], have
been introduced into diiron model complexes by the method of
All manipulations and reactions were carried out under dry
oxygen-free N2 using standard Schlenk techniques. All chemicals
were analytical reagents and used without further purification.
The compounds etpy, btpy (Supplementary) and (l-pdt)Fe2(CO)6
(1) [28,29] were prepared according to the literature. All solvents
were dried and distilled prior to use according to the standard
methods. Infrared spectra were recorded on a Perkin–Elmer Spec-
trum One FT-IR spectrophotometer using KBr pellets. 1H NMR
spectra were collected on a Varian Unity 500NMR spectrometer.
Mass spectra were recorded on an DECAX-3000 LCQ Deca XP
instrument. Elemental analysis was carried out on a Vario EL III
Elemental Analyser.
CO-displacement of (l-pdt)Fe2(CO)6 (1). While the structural
models of the active site of [FeFe] Hydrogenase have matured con-
siderably over recent years [21–25], little attention has been de-
voted to the influence of the N-ligation at diiron models, for it is
difficult to isolate the labile coordination of the amine to iron atom,
for instance in (l-pdt)Fe2(CO)5(NH2Pr) (2) which is unstable [26].
But the electron-donating capability of the nitrogen atom is ex-
pected in N-ligation of nitrogen-hetercycles, which are abundant in
nature and play important role in organometallic chemistry. We
thought it would be interesting to evaluate the role of N-ligation
in diiron model complexes with the introduction of pyridine or
other heterocyclic-N ligands [27], which was expected to extend
new promising models for structural or functional modification
2.2. Preparations
2.2.1. Preparation of (
l-pdt)Fe2(CO)5Py (A)
(l-pdt)Fe2(CO)6 (3 mmol, 1.158 g) (1) reacted with Me3-
NO Á 2H2O (3 mmol, 0.333 g) under the ice bath condition in
30 mL of CH3CN, and then pyridine (3 mmol, 0.237 g) was added.
The colour turned from red into black. After 24 h the solvent was fil-
tered and evaporated under vacuum. The remaining residue was
extracted with 30 mL of freshly distilled n-hexane and filtered. Dark
* Corresponding author. Tel./fax: +86 591 8379 2395.
E-mail address: ccn@fjirsm.ac.cn (C.-N. Chen).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.03.050

Y. Zhang et al. / Journal of Organometallic Chemistry 694 (2009) 2576–2580
2577
crystals were obtained after several days in the refrigerator at
monochromated Mo K
a
radiation (k = 0.71073 Å), while single
À20 °C. Yield: 70% (0.92 g). Anal. Calcd for Fe2S2C13O5H11N: C,
crystal of complex C was performed on a Rigaku Saturn70 diffrac-
tometer. Crystal data collection, refinement and reduction were
accomplished with the CRYSTALCLEAR processing program. The struc-
ture was solved by direct methods with SHELXS-97 [30] and refined
by using the SHELXL-97 crystallographic software package [31]. All
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were added in a riding model, while the pdt bridge in C
was refined with ‘‘envelope-flap” disorder. Crystallographic data
of A, B and C are summarized in Table 1.
35.73; H, 2.54; N, 3.21. Found: C, 35.54; H, 2.59; N, 3.14%. IR (cm–1
,
KBr pellet): (v
co) 2029 (s), 1980 (vs), 1939 (m), 1918 (m). 1H NMR
(CD3CN): d 8.36 (2H, Py), 7.33 (1H, Py), 7.20 (2H, Py), 2.15 (4H,
SCH2CH2CH2S), 1.85 (2H, SCH2CH2CH2S). m/z 437.7 [M+H]+.
2.2.2. Preparation of (l-pdt)Fe2(CO)5(etpy) (B)
The reaction steps were similar to those of A, but 4-(ethyl-
thio)pyridine (3 mmol, 0.418 g) was used to replace pyridine.
Yield: 60% (0.89 g). Anal. Calcd for Fe2S3C15O5H15N: C, 36.24; H,
3.04; N, 2.82. Found: C, 37.19; H, 3.11; N, 2.97%. IR (cmÀ1, KBr pel-
2.4. Electrochemistry
let): (v
co) 2033 (s), 1978 (vs), 1918 (m). 1H NMR (CD3CN): d 8.36
(2H, Py), 7.19 (2H, Py), 2.16 (4H, SCH2CH2CH2S), 2.13 (2H,
SCH2CH3), 1.85 (2H, SCH2CH2CH2S), 1.36 (3H, SCH2CH3). m/z:
498.7 [M+H]+, 414.9 [M+HÀ3CO]+.
Electrochemical measurements were made using a CH instru-
ment Model 630A Electrochemical Workstation. Cyclic voltammo-
grams were obtained in
a conventional and gastight three-
electrode cell under Ar and at room temperature. The working
electrode was a glassy carbon electrode (3 mm in diameter), the
reference electrode a Ag|AgCl electrode (ca. À0.43 V vs. Fc/Fc+),
and the counter electrode a platinum wire. The supporting electro-
lyte is 0.1 M Bu4NPF6 in CH3CN.
2.2.3. Preparation of [(l-pdt)Fe2(CO)5(btpy)] (C)
The reaction steps were similar to those of A, but 4-(n-butyl-
thio)pyridine (3 mmol, 0.501 g) was used to replace pyridine.
Yield: 64% (1.01 g). Anal. Calcd for Fe2S3C17H19O5N: C, 38.88; H,
3.65; N, 2.67. Found: C, 38.84; H, 3.78; N, 2.53%. IR (cmÀ1, KBr
pellet): (v
co) 2038 (s), 1969 (vs), 1953 (s), 1920 (m). 1H NMR
3. Results and discussion
(CD3CN): d 8.37 (2H, Py), 7.21 (2H, Py), 2.46 (2H, SCH2CH2CH2CH3),
2.15 (4H, SCH2CH2CH2S), 1.86 (2H, SCH2CH2CH2S), 1.69 (2H,
SCH2CH2CH2CH3), 1.35 (2H, CH2CH3), 0.95 (3H, CH2CH3). m/z:
525.9 [M+H]+.
3.1. Synthesis and characterization
(
l
-pdt)Fe2(CO)6 (1) was treated with Me3NO Á 2H2O under the
ice bath condition in CH3CN in the atmosphere of N2 [10], and then
pyridine ligands (py, etpy or btpy) were added. Considerable yields
of all the model complexes and their moderate stability in air as so-
lid were ensured to research their properties.
2.3. X-ray crystallography
Diffraction measurements of single crystals of complexes A and
B were made on a Rigaku Mercury diffractometer using graphite
The IR data of the CO bands for A, B and C show ca. 50 cmÀ1 shift
to lower frequency compared to that of the all-carbonyl complex 1
(Table 2), and similar average v(CO) and the frequency range of the
CO bands to those of different mono-subsituated complexes, e.g.
Table 1
Crystallographic data summary for complexes A, B and C.
(l-pdt)Fe2(CO)5(CN) (3) [10], (l-pdt)Fe2(CO)5(PPhMe2) (4) [15],
A
B
C
(
l
-pdt)Fe2(CO)5(SEt2) (5) [19], which prove the considerable elec-
Empirical formula
Formula weight
Space group
a (Å)
b (Å)
c (Å)
Fe2S2C13O5H11
437.07
N
Fe2S3C15O5H15
497.19
N
Fe2S3C17O5H19
N
tron-donating capabilities of pyridine ligands. The 1H NMR spectra
525.24
ꢀ
P2(1)/n
8.200(3)
12.661(5)
16.094(7)
90.00
90.717(5)
90.00
1670.6(12)
4
P2(1)/n
8.899(5)
22.291(10)
10.382(6)
90.00
106.380(8)
90.00
1975.9(17)
4
P1
of A, B and C each show typical signals for l-SCH2CH2CH2S at 2.15–
2.16 and 1.85–1.86 ppm in spite of the introduction of pyridine
groups. As shown in Table 3, it is interesting to observe that the
chemical shifts for
shifted as compared to those in non-coordinated pyridine com-
plexes, indicating -backdonation capability from diiron center
to the pyridine ring.
9.342(6)
11.528(7)
11.564(7)
66.769(17)
79.15(3)
77.62(2)
1110.3(12)
2
a- and b-H in pyridine groups are upfield-
a
(°)
b (°)
c
(°)
p
V (Å3)
Z
qcalc (g cmÀ3
)
1.738
2.007
1.671
1.810
1.571
1.615
3.2. X-ray crystallographic structures of complexes A, B and C
Absorption coefficient
(mm-1
)
Final R indices [I > 2
r(I)]
R1 = 0.0359
R1 = 0.0499
R1 = 0.0591
The structures of complexes A, B and C were determined by X-
ray diffraction. Their molecular diagrams are depicted in Fig. 1, and
the selected bond lengths and bond angles are given in Table 3.
Complexes A and B crystallize in monoclinic P21/n space group,
x
R2 = 0.0929
R1 = 0.0518
R2 = 0.1023
x
R2 = 0.0931
R1 = 0.0913
R2 = 0.1098
x
R2 = 0.1295
R1 = 0.1170
xR2 = 0.1734
R indices (all data)
x
x
Table 2
Listing of mCO infrared and electrochemical parameters for the model complexes 1–5, A, B and C.
Complex
Ligand
vCO (KBr)/cmÀ1
Average value of
v
CO/cmÀ1
Epc1/V vs. Ag|AgCl (FeIFeI/FeIFe0)
Reference
1
2
3
4
5
A
B
C
All-CO
NH2Pr
CNÀ
PPhMe2
SEt2
py
etpy
btpy
2071, 2026, 1989, 1972
1980, 1943, 1907, 1892a
2029, 1974, 1955, 1941, 1917
2040, 1980, 1921
2045, 1976, 1960, 1919
2029, 1980, 1939, 1918
2033, 1978, 1918
2015
1931a
1963
1980
1975
1967
1976
1970
À1.24
À1.36
À1.73
À1.88
À1.29
À1.21
À1.25
À1.23
[29]
[26]
[10]
[15]
[19]
This work
This work
This work
2038, 1969, 1953, 1920
a
vCO in THF.

2578
Y. Zhang et al. / Journal of Organometallic Chemistry 694 (2009) 2576–2580
Table 3
The chemical shifts for
Table 4
a
- and b-H in pyridine groups.a
-H
Selected bond lengths (Å) and angles (°) for complexes A, B and C.
a
b-H
Complex
A
B
C
py
A
etpy
B
btpy
C
8.59
8.36
8.44
8.36
8.68
8.37
7.35
7.20
7.26
7.19
7.36
7.21
Fe1–Fe2
Fe1–S1
Fe1–S2
Fe2–S1
Fe2–S2
Fe2–N1
2.5342(8)
2.2600(12)
2.2742(12)
2.2308(10)
2.2475(10)
2.029(2)
2.5450(14)
2.2654(14)
2.2755(14)
2.2229(15)
2.2519(15)
2.030(3)
2.506(2)
2.281(2)
2.2889(19)
2.285(2)
2.287(2)
2.030(5)
a
Fe1–Fe2–N1
S1–Fe2–N1
S2–Fe2–N1
C4–Fe2–N1
C5–Fe2–N1
103.26(8)
158.90(7)
86.08(7)
100.68(13)
92.87(12)
104.72(10)
160.80(10)
85.40(10)
99.66(18)
91.98(17)
146.24(14)
101.92(14)
99.39(14)
100.7(2)
The chemical shift of acetonitrile is 1.96.
ꢀ
each with Z = 4. But complex C belongs to triclinic P1 space group
with Z = 2. Although the core structures of these molecules are all
in the butterfly conformation, their sizes vary greatly (A,
1670.6(12) Å3; B, 1975.9(17) Å3; C, 1110.3(12) Å3).
100.5(2)
3.3. Cylic voltammograms and electrochemical proton reduction
As shown in Table 4, the displacement of one CO by pyridine li-
gands (py, etpy and btpy) has a slight influence on the Fe–Fe dis-
tance in A, B and C in comparison with that in 1 [29], as
expected for mono-substituted diiron complexes. The angles of
Fe(1)–Fe(2)–N(1) in complexes A and B are much smaller than that
in complexes C; accordingly, the basal coordination mode of py and
etpy on the Fe atoms in complexes A and B is different from the
apical configuration of btpy in complexes C (Fig. 1). The bond
lengths of the Fe(2)–N(1) in A, B and C lie in the narrow range
2.029(2)–2.030(5) Å, which are similar to that in 2 (2.033(5) Å)
[26] and indicate weak Fe–N coordination.
Complexes A, B and C were studied by cyclic voltammetry (CV)
to evaluate the effects of the pyridine ligand on the redox proper-
ties of the model complexes. All the CV measurements were car-
ried out in CH3CN (with 0.1 M n-Bu4NPF6 as the electrolyte) and
scanned in the cathodic direction (Fig. 2).
The electrochemistry of the complexes A, B and C all display
two irreversible reduction peaks: the first reduction peaks appear
at À1.21 to À1.25 V, and the second at À1.63 to À1.68 V. The first
reductive potentials of the complexes A, B and C are 100–500 mV
more anodic than those of 2–5 [10,15,19,26], and close to that of
A
B
C
Fig. 1. Molecular structures of A, B and C with thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for clarity.

Y. Zhang et al. / Journal of Organometallic Chemistry 694 (2009) 2576–2580
2579
Fig. 2. Cyclic voltammogram of complex B (1.0 mM) with HOAc [(a) 0–10 mM and (b) 10–50 mM]. Supporting electrode is 0.1 M n-Bu4NPF6 in MeCN; working electrode:
glassy carbon (0.0071 cm2); reference electrode: Ag|AgCl; scan rate = 0.1 V sÀ1
.
the all-CO complex 1 [29]. The electrochemical behavior conveys a
na (No. 20471061), the Science & Technology Innovation
message that, although the average electron density of the diiron
Foundation for Young Scholar of Fujian Province (No. 2007F3112)
for financial support of this work.
centers of A, B and C are comparable to those of 2–5, the
of the pyridine ring can render the uneven electron density of the
two iron atoms through an electron delocalisation. The strong
p-orbits
p
-
Appendix A. Supplementary data
accepting capability, also reported in substitutions of sulfoxides
[20], suggests pyridine rings to be potential candidates to tune
the electron properties of the diiron centers; this can be supported
by another evidence that the carbene–pyridine disubstituted com-
plex (l-pdt)Fe2(CO)4(NHCMePy) displays similar reductive behav-
iors to those of the mono-substituents [6].
CCDC 670552, 670553 and 670554 contain the supplementary
crystallographic data for A, B and C. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2009.03.050.
The electrochemical behavior of model B was analyzed in details
in presence of HOAc (0–10 mM) as a source of protons (Fig. 2). The
first reductive peak for B at ca. –1.24 V did not shift and the current
height showed a minor growing, indicating that H+ did not incorpo-
rate to the model complex B. It was of particular interest that the
second reduction peak for B shifted towards a more negative poten-
tial linearly with the addition of HOAc (0–10 mM), which electro-
chemical behavior features a proton relative process.
While HOAc was added continuously (10–50 mM), it is interest-
ing that the first reductive peak increased distinctly while the sec-
ond reductive peak reduced and disappeared at 30 mM of HOAc.
Another measurement was made, in which 30 mM HOAc was
added to the solution immediately. The first reductive peak at ca.
À1.20 V grew dramatically, while the second one at À1.67 V disap-
peared at once. Under the effect of the excessive acid, complex B
was proven to be reductive active at a rather low first reduction
potential, while many other reported diiron model complexes were
electrocatalytic inactive at the first reduction event in the presence
of HOAc [32]. Functional and structural improvement on pyridine-
N-ligands as good candidates to reduce the overpotentials of 2Fe2S
model complexes is in progress.
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Abbreviations
etpy
btpy
NH2Pr
NHCMePy
pdt
4-(ethylthio)pyridine
4-(n-butylthio)pyridine
propylamine
1-methyl-3-(2-pyridyl)imidazol-2-ylidene
propanedithiolate
py
pyridine
SEt2
Ethyl thioether
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