Selenium-bridged diiron hexacarbonyl complexes as biomimetic models for the active site of Fe-Fe hydrogenases

Article abstract of DOI:10.1039/b717497g

Three N-substituted selenium-bridged diiron complexes [{(μ-SeCH ₂)₂NC₆H₄R}Fe₂(CO) ₆] (R = 4-NO₂, 7; R = H, 8; R = 4-CH₃, 9) were firstly prepared as biomimetic models

Full text of DOI:10.1039/b717497g

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Selenium-bridged diiron hexacarbonyl complexes as biomimetic models for the
active site of Fe–Fe hydrogenases†
Shang Gao, Jiangli Fan, Shiguo Sun, Xiaojun Peng,* Xing Zhao and Jun Hou
Received 12th November 2007, Accepted 31st January 2008
First published as an Advance Article on the web 27th February 2008
DOI: 10.1039/b717497g
Three N-substituted selenium-bridged diiron complexes [{(l-SeCH2)2NC6H4R}Fe2(CO)6] (R = 4-NO2,
7; R = H, 8; R = 4-CH3, 9) were firstly prepared as biomimetic models for the Fe–Fe hydrogenases
active site. Models 7–9 could be generated by the convergent reaction of [(l-HSe)2Fe2(CO)6] (6) with
N,N-bis(hydroxymethyl)-4-nitroaniline (1), N,N-bis(hydroxymethyl)aniline (2), and
N,N-bis(hydroxymethyl)-4-methylaniline (3) in 46–52% yields. All the new complexes 7–9 were
characterized by IR, 1H and 13C NMR and HRMS spectra and their molecular structures were
determined by single-crystal X-ray analysis. The redox properties of 7–9 and their dithiolate analogues
[{(l-SCH2)2NC6H4R}Fe2(CO)6] (R = 4-NO2, 7s; R = H, 8s; R = 4-CH3, 9s) were evaluated by cyclic
voltammograms. The electrochemical proton reduction by 9 and 9s were investigated in the presence of
p-toluenesulfonic acid (HOTs) to evaluate the influence of changing the coordinating S atoms of the
bridging ligands to Se atoms on the electrocatalytic activity for proton reduction.
Introduction
Hydrogen evolution and uptake in nature is mostly catalyzed by
the metalloenzymes called hydrogenases.1 Among these, Fe–Fe
hydrogenases (Fe H2ases) have received more attention than other
types of hydrogenases because of their significant efficiency (6000–
9000 molecules H2 s−1 per site) in the production of hydrogen.2,3
Crystallographic and IR spectroscopy studies revealed the active
site of Fe H2ase contains a butterfly 2Fe2S subunit, in which
one of the iron atoms is linked to a cuboidal 4Fe4S cluster by a
cystenyl-S bridge (Fig. 1).4 The 4Fe4S unit is actually responsible
for the electron transfer while the 2Fe2S subunit is responsible
for hydrogen formation and activation. The two iron atoms in
the 2Fe2S subunit are decorated by the biologically unusual
carbon monoxide and cyanide ligands and are bridged by a three-
atoms-containing dithiolate ligand. Very recent crystallographic
studies with higher resolution,5 together with theoretical studies,6
suggest the bridging dithiolate ligand as azadithiolate (ADT)
SCH2NHCH2S or the N-substituted species, and the nitrogen
heteroatom plays an important role for the heterolytic cleavage
or formation of H2 in the natural system.
The structural elucidation of the active site of the Fe H2ases
led to a renaissance in the chemistry of the well-known transition
metal sulfides [(l-SR)2Fe2(CO)6], the structures of which resem-
bled the 2Fe2S subunit of the Fe H2ases active site, and raised their
profile as the potential bioinorganic hydrogen activation catalysts.
In this spirit, a great variety of diiron dithiolate complexes have
been prepared as the structural and functional models for the
Fe H2ases active site.7–10 Nevertheless, most of the efforts have
been performed on structure modifications by phosphane,7f –i,11
Fig. 1 Structure of the enzyme active site obtained from protein
crystallography.
cyanide,7a–c,10b,12 isocyanide13 and N-heterocyclic carbene14 lig-
ands exchange and X atom variation in the dithiolate bridge
–SCH2XCH2S–.15 To the best of our knowledge, there have been
few reports on the effects of changing the coordinating S atoms
of the bridging dithiolate ligands on the biomimetic models,16,17
and no biomimetic model bearing the –SeCH2NRCH2Se– bridge
with general formula [(l-SeCH2)2NR]Fe2(CO)6] (aza diselenide
diiron complexes) has been prepared. With this in mind, we
synthesized a series of aza diselenide diiron complexes [{(l-
SeCH2)2NC6H4R}Fe2(CO)6] (R = 4-NO2, 7; R = H, 8; R = 4-CH3,
9) (Scheme 1), in which the S atoms in [{(l-SCH2)2NR}Fe2(CO)6]
species were substituted with Se atoms. Se atoms were introduced
for the purposes: (i) the architectures of the diselenide diiron
complexes should be similar to those of their dithiolate analogues
without complicating the element group from sulfur to selenium;
(ii) the Se atoms should have influence on the reactivity and the
redox properties of the iron cores for their different electronic
properties as compared to S atoms. The attaching of a series
of aromatic rings with different inductive effects to the bridging
nitrogen atom would provide us an insight into the common
influence of Se atoms on the biomimetic models for the Fe H2ase
active site. Herein, we describe the preparation, the spectroscopic
and the structural characterization of the three aza diselenide
diiron complexes 7–9, as well as the redox properties of 7–9
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Zhongshan Road 158-40, Dalian, 116012, PR China. E-mail: pengxj@
dlut.edu.cn; Fax: +86 411/88993906; Tel: +86 411/88993899
† CCDC reference numbers 665465–665467. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b717497g
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paraformaldehyde in CH2Cl2,10a the yields were very low. To avoid
using the insalubrious reagent SOCl2, the halogenation of 1–3 was
not used but the hydroxymethylamines were used directly in the
next step without isolation.
Another precursor [(l-Se)2Fe2(CO)6] (4) could be obtained
according to literature methods.19,20 4 could be purified by column
chromatography using hexane as eluant and its structure was anal-
ogous to [(l-S)2Fe2(CO)6].21 It is noteworthy that 4 decomposes
rapidly in the solid state while in solution it can be stored at
0
◦C for several days. Treat the freshly prepared THF solution
of 4 with LiEt3BH, a reagent known to cleave Se–Se bond,22 at
−78 ◦C generated the lithium salt of hexacarbonyldiselenidediiron
(5).19,23 After the subsequent protonation of 5 with CF3COOH,
the resulting [(l-HSe)2Fe2(CO)6] (6) reacted with precursors 1–3
to give the corresponding aza diselenide diiron complexes 7–9 in
moderate yields. Unlike their precursor 4, complexes 7–9 are stable
to air in the solid state but moderately sensitive in solution.
Scheme 1 The prepared aza diselenide diiron complexes 7–9 and their
dithiolate analogues 7s–9s.
and their dithiolate analogues [{(l-SCH2)2NC6H4R}Fe2(CO)6]
(R = 4-NO2, 7s; R = H, 8s; R = 4-CH3, 9s) (Scheme 1).
The electrochemical response of 9 and 9s in the presence of p-
toluenesulfonic acid (HOTs) indicate that 9 has a slightly greater
electrocatalytic activity for proton reduction as compared to 9s.
1
Complexes 7–9 were characterized by H and 13C NMR, IR
and mass spectrometry. The results of the HR-MS analyses for
the three complexes are all in good agreement with the supposed
molecular weight. The absorption for the terminal carbonyls of
7–9 are listed in Table 1 together with their dithiolate analogues
7s–9s for comparison. All complexes show three strong bands
in the carbonyl region of 1983–2078 cm−1. The average v(CO)
values of 7–9 are shifted towards lower frequencies with respect
to those of their dithiolate analogues 7s–9s by 8, 9 and 11 cm−1,
Results and discussion
Synthesis and spectroscopic characterization of complexes 7–9
The synthetic methods for the target complexes 7–9 are
somewhat different from those of their dithiolate analogues
7s–9s9a,c,13c (Scheme 2). The N,N-bis(hydroxymethyl)amines
RC6H4N(CH2OH)2 (R = 4-NO2, 1; R = H, 2; R = 4-CH3, 3)
were readily accessible from the reaction of the primary amines
(4-nitroaniline, aniline, 4-methylaniline) with paraformaldehyde in
the methanol solution of KOH. This method was improved from
literature18 and the additional KOH made the paraformaldehyde
dissolved completely in methanol. The homogeneous reaction
took place immediately and completely after 4–5 hours. The
precursors 1–3 could be generated according ca. 80–90% yield.
Although 1–3 could be prepared by the heterogeneous proce-
dure in which the primary amines directly reacted with the
Table 1 A comparison of v(CO) bands of 7–9 and their dithiolate
analogues 7s–9sa
Complex
v(CO)/cm−1
Average value of v(CO)/cm−1
7
2070, 2033, 1999
2067, 2030, 1994
2064, 2023, 1983
2078, 2040, 2008
2076, 2038, 2002
2071, 2030, 2000
2034
2030
2023
2042
2039
2034
8
9
7s
8s
9s
a IR spectra were measured in CH2Cl2.
Scheme 2 The synthetic procedure of complexes 7–9.
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Fig. 2 ORTEP views of 7, 8 and 9 with 30% probability level ellipsoids.
respectively, which indicates a slight enhancement of the electron
density at the iron cores for 7–9. Such variation is attributed to
the better electron-donating capability of Se atoms which results
in the higher electron density of the iron cores. Consequently, the
enhanced feedback bonds from Fe atoms to CO cause the red-
shifts of the v(CO) frequencies of 7–9.
carbonyl ligands in a distorted square-pyramidal geometry. Com-
plexes 7–9 each contains two fused six-membered rings which
are formed by their two iron atoms and the N-substituted aza
diselenide bridge. The p-substituted phenyl moiety attached to
the bridged nitrogen atom lies in an equatorial position relative
to the metalloheterocycle for the all three complexes and slants
towards the Fe(2)(CO)3 unit. The long-range interaction between
the arene group and the closest Fe(CO)3 unit impacts the C–Fe–
Fe angle. Thus, compared with the C(1)–Fe(1)–Fe(2) angle, C(6)–
Fe(2)–Fe(1) angle was enlarged by 7.49◦ for 7, 7.64◦ for 8 and
6.89◦ for 9, respectively. The nitrogen long pair of 7–9 resides in
an axial position and the sum of the C–N–C angles around the
N atom is 357.6◦ for 7, 355.4◦ for 8 and 357.5◦ for 9. These sum
values agree well with those of 7s–9s (357.7◦ for 7s, 355.0◦ for 8s
and 357.4◦ for 9s)9a,c,13c and imply that the sp2-hybridized N atom
slightly deviates from the plane defined by the N(1), C(7), C(8)
and C(9) atoms (for 7–9) and the p–p conjugation between the
substituted phenyl ring and the p-orbital of the bridged N atom is
somewhat weakened.
Molecular structures of complexes 7–9
The molecular structures of 7–9 were confirmed by single-crystal
X-ray analysis. While ORTEP diagrams of 7–9 are shown in Fig. 2,
the selected bond lengths and angles are listed in Table 2. As
expected, the overall molecular geometries of 7–9 are analogous to
those of their dithiolate analogues 7s–9s9a,c,13c and other previously
reported aza dithiolate diiron derivatives.9,24
The 2Fe2Se cores of all three complexes adopt a butterfly
framework. Each iron atom is coordinated to three terminal
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complexes 7–9
7
8
9
Although the bond lengths in the structures 7–9 are remarkably
similar to those of 7s–9s, it is noteworthy that the Fe–Fe bond
Fe(1)–Fe(2)
Fe(1)–Se(1)
Fe(1)–Se(2)
Fe(2)–Se(1)
Fe(2)–Se(2)
Se(1)–C(7)
Se(2)–C(8)
N(1)–C(7)
N(1)–C(8)
N(1)–C(9)
Se(1) · · · Se(2)
2.5475(10)
2.3676(8)
2.3870(8)
2.3756(9)
2.3850(8)
1.990(5)
2.013(5)
1.430(6)
1.429(6)
1.403(6)
3.2704(7)
2.5572(5)
2.3903(5)
2.3791(4)
2.3703(4)
2.3840(4)
2.010(3)
2.021(3)
1.426(4)
1.407(4)
1.408(4)
3.2637(4)
2.5514(9)
2.3790(8)
2.3811(9)
2.3792(9)
2.3772(9)
2.018(4)
2.002(4)
1.427(5)
1.420(5)
1.407(5)
3.2718(8)
˚
˚
lengths are elongated to 2.5475(10) A for 7, 2.5572(5) A for 8,
˚
2.5514(9) A for 9, which are larger than those of 7s–9s (2.4998(10)
A for 7s, 2.5047(6) A for 8s and 2.5090(10) A for 9s)9a,c,13c and other
˚
˚
˚
reported hexa-carbonyl aza dithiolate diiron derivatives (2.49–
9,24
˚
2.51 A).
The Fe–Fe bond lengths of 7–9 are much close to
5
˚
the corresponding ones in the reduced form (2.55 A), although
a little shorter than those in the oxidized form (ca. 2.60 A) of
4a,b
˚
the metalloenzymes in the micro-biosystem. The recent research
revealed that the more electron rich is in the iron core, the longer
the Fe–Fe bond is.25 However, the average v(CO) values indicate
that the electron density at the iron cores of 7–9 is slight higher than
those of 7s–9s. Thus, the steric influence of the relatively bulky Se
atoms may primarily lengthen the Fe-Fe distances for 7–9.
There is a simple packing and no hydrogen bonds and no
p–p stacking interactions in the crystal cell packing graphs of
complexes 7–9. The van der Waals forces are primarily responsible
for the interactions between molecules, which is similar to those
of 7s–9s.9a,c,13c
C(1)–Fe(1)–Fe(2)
C(6)–Fe(2)–Fe(1)
Fe(1)–Se(1)–Fe(2)
Fe(1)–Se(2)–Fe(2)
Se(1)–Fe(1)–Se(2)
Se(1)–Fe(2)–Se(2)
C(7)–Se(1)–Fe(1)
C(8)–Se(2)–Fe(1)
C(7)–Se(1)–Fe(2)
C(8)–Se(2)–Fe(2)
C(1)–Fe(1)–Se(1)
C(1)–Fe(1)–Se(2)
C(6)–Fe(2)–Se(1)
C(6)–Fe(2)–Se(2)
C(7)–N(1)–C(8)
C(7)–N(1)–C(9)
C(8)–N(1)–C(9)
145.95(17)
153.44(17)
64.97(3)
64.53(3)
86.92(3)
147.72(10)
155.36(8)
64.979(13)
64.943(13)
86.359(15)
86.703(13)
110.08(11)
104.58(9)
110.27(7)
110.94(9)
103.03(9)
99.38(9)
104.56(8)
108.18(8)
113.7(3)
121.8(3)
119.9(2)
145.32(14)
152.21(15)
64.85(3)
64.85(2)
86.84(3)
86.78(3)
86.92(2)
107.89(17)
106.15(16)
111.68(16)
111.21(14)
101.61(19)
97.86(15)
102.74(19)
107.79(16)
114.5(5)
108.45(13)
105.82(13)
110.35(11)
111.12(12)
100.43(13)
98.36(15)
98.37(14)
112.13(16)
113.8(3)
Cyclic voltammograms of complexes 7–9
121.9(4)
121.2(4)
123.0(3)
120.7(3)
The iron core plays an important role in the process of H2
production. Thus, the redox properties of the iron cores of
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Table 3 Redox potentials of complexes 7–9 and their dithiolate analogous 7s–9sa
Complex
E
pa/V (FeIFeI/FeIFeII)
E
pa/V (FeIFeII/FeIIFeII)
Epc/V (FeIFeI/FeIFe0)
Epc/V (FeIFe0/Fe0Fe0)
7
0.66
0.58
0.57
0.76
0.64
0.64
−1.40
−1.49
−1.50
−1.41
−1.50
−1.50
−1.71
−2.02
−2.00
−1.76
−1.99
−1.99
8
0.84
0.89
9
7s
8s
9s
a 0.05 M n-Bu4NPF6 in CH3CN; scan rate: 100 mV s−1. All potentials were measured under the same conditions using a non-aqueous Ag/AgNO3 electrode
as reference.
complexes 7–9 were investigated by cyclic voltammograms (CVs).
To further understand these electrochemical characters, the CVs
of complexes 7s–9s were measured under the same conditions. All
the CV measurements were carried out in CH3CN solution. They
were initiated from the open circuit and proceeded in the cathodic
direction. While Table 3 lists the electrochemical data of 7–9 and
7s–9s, Fig. 3 shows their cyclic voltammograms.
Compared with 7, complexes 8 and 9 could be easier to oxidize
and more difficult to reduce. These observations indicated that
the strong inductive effect of the p-substituted NO2 group may
decrease the electron density in the p system of the phenyl ring
attaching to the bridged N atom, and slightly decrease the electron
accumulation on the iron core through the three atoms (N, C, Se),
resulting in the less negative reduction and more positive oxidation
potentials for complex 7 as compared to 8 and 9.
It has been demonstrated that 7 displayed two quasi-reversible
reduction and one irreversible oxidation events, while 8 and 9
manifested one quasi-reversible, one irreversible reduction and two
irreversible oxidation events. The defined waves are actually similar
to the typical ones of the ADT-bridged dithiolate series.9b,c,13c,24
The first and second reduction events of 7 (−1.40, −1.71 V), 8
(−1.49, −2.02 V) and 9 (−1.50, −2.00 V) (vs. Ag/Ag+, 0.01 M
AgNO3) could be assigned to the one-electron reduction processes
of FeIFeI to FeIFe0 and FeIFe0 to Fe0Fe0. Similarly, the first
oxidation events at 0.66 V for 7, 0.58 V for 8 and 0.57 V for
9 should be attributed to the one-electron oxidation processes
of FeIFeI to FeIFeII, while the second oxidation events for 8 at
0.84 V and 9 at 0.89 V were ascribed to the second electron-
transfer processes of FeIFeII to FeIIFeII. Noticeably, it could be
seen that the FeIFeI/FeIFeII oxidation events of 7–9 are apparently
shifted towards less positive potentials (DE = 70–100 mV) than
the corresponding oxidation events of 7s–9s (0.76 V for 7s, 0.64 V
for 8s and 9s). Meanwhile, no FeIFeII/FeIIFeII oxidation events
for 8s and 9s were observed within the electrochemical window
of solvent. These electrochemical behaviors demonstrate that
complexes 7–9 are somewhat easier to oxidize and convey the
same message, as shown by the v(CO) values of IR data, that the
average electron density of iron cores of 7–9 is slightly higher than
those of 7s–9s.
Interestingly, although the more negative FeIFeI/FeIFeII oxi-
dation potentials and the higher average v(CO) values of 7–9
compared to those of 7s–9s suggest that the average electron
density of iron cores in 7–9 is higher than that in 7s–9s as described
above, the first FeIFeI/FeIFe0 and the second FeIFe0/Fe0Fe0
reduction potentials of 7–9 (−1.40 and −1.71 V for 7, −1.49 and
−2.02 V for 8 and −1.50 and −2.00 V for 9) are roughly the same
relative to the corresponding ones of 7s–9s (−1.41 and −1.76 V
for 7s, −1.50 and −1.99 V for 8s and 9s). The enhancement of
the electron accumulations on the iron cores by changing the
coordinating S atoms of the bridging ligands to Se atoms dose
not result in the cathodic shifts for the reduction potentials of
the iron cores and it features the diselenide diiron complexes.
Whereas, further work is needed to elucidate this aspect of the
electrochemistry.
Electrocatalytic proton reduction by complex 9
To avoid the influence of the NO2 group on the cyclic voltam-
mograms, complexes 8 and 9 were chosen to investigate the
electrocatalytic proton reduction in the presence of HOTs
(0–9 mM). As complexes 8 and 9 display similar electrochemical
behaviors at variant acid concentrations, herein we reported the
Fig. 3 Cyclic voltammograms of complexes 7 and 7s (a), 8 and 8s (b), and 9 and 9s (c), 1.0 mM in 0.05 M n-Bu4NPF6/CH3CN solution at a potential
scan rate of 100 mV s−1
.
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Fig. 4 Cyclic voltammograms of complexes 9 (left) and 9s (right) with 1.0 mM complex and 0–9 mM HOTs in 0.05 M n-Bu4NPF6–CH3CN solution at
a potential scan rate of 100 mV s−1
.
cyclic voltammograms of 9 together with 9s under the same acid
conditions for a direct comparison of the electrocatalytic activity
(Fig. 4).
roughly the same as 9s (−1.37 V for 9 and −1.34 V for 9s). The
slightly higher electron density of iron core of 9 presumably makes
the reduced intermediates more stable and results in a relatively
high electrocatalytic activity of 9 as compared to 9s. Our results
indicate the affirmative effects of changing the coordinating S
atoms of the bridging ligands to Se atoms on the electrocatalytic
activity of model complexes and may provide us an insight into
designing new types of diselenide diiron model complexes.
Upon addition of 1 mM of HOTs, new reduction events were
observed at −1.37 V for 9 and −1.34 V for 9s. These events were
more positive than those of the initial FeIFeI/FeIFe0 reduction
processes and indicated the catalytic proton reduction.7d The
second reduction events still corresponded to the one-electron
FeIFeI/FeIFe0 reduction processes of 9 and 9s. The current
intensity of the first reduction events at −1.37 V for 9 and
−1.34 V for 9s show a significant electrocatalytic response and
gradually increase with increasing acid concentration, while their
potentials shift towards a relatively more negative potential. All
these electrochemical behaviors feature an electrocatalytic proton
reduction process.7d,h,i,9b,c,15b,26
Fig. 5 presents plots of current intensity of the electrocatalytic
events vs. acid concentration of HOTs in CH3CN for 9 and 9s.
The current intensity of the electrocatalytic events of 9 and 9s
both display a good linear increase with sequential increase of the
concentration of HOTs. The steeper slopes displayed by complex
9 are indicative of slightly greater sensitivity of the reduced species
to acid concentration, as compared to that exhibited by 9s under
the same measuring conditions. That is, complex 9 displayed a
relatively high electrocatalytic activity for reduction of protons
from HOTs, even though the electrocatalytic event potential is
Conclusions
Three selenium-bridged diiron hexa-carbonyl complexes [{(l-
SeCH2)2NC6H4R}Fe2(CO)6] (R = 4-NO2, 7; R = H, 8; R = 4-
CH3, 9) were firstly synthesized and well characterized as models
for the active site of Fe–Fe hydrogenases. As expected, the overall
molecular geometries of 7–9 are analogous to those of their
dithiolate analogues [{(l-SCH2)2NC6H4R}Fe2(CO)6] (R = 4-NO2,
7s; R = H, 8s; R = 4-CH3, 9s). The IR spectra indicate that
changing the coordinating S atoms of the bridging ligands to Se
atoms enhances the average electron density of the iron cores for
complexes 7–9. This affect is also consistent with the evidences: (i)
the shift of the FeIFeI/FeIFeII oxidation potentials of 7–9 to less
positive values; (ii) the appearance of the FeIFeII/FeIIFeII oxidation
events for 8 and 9. However, the FeIFeI/FeIFe0 and FeIFe0/Fe0Fe0
reduction potentials of 7–9 are roughly the same as those of 7s–
9s. As a typical object we chose, complex 9 has a slightly higher
electrocatalytic activity for reduction of protons from HOTs than
its dithiolate analogue 9s. These electrochemical observations are
particular and may indicate the better capability of catalyzing
electrochemical reduction of proton for the selenium-bridged
diiron complexes. To obtain more efficient proton reduction
electrocatalysts and further understand the electrochemical be-
haviors of such selenium-bridged system, structural modifications
of complexes 7–9 with good donor ligands are now in progress.
Experimental
Reagents and instruments
All reactions and operations related to organometallic complexes
were carried out under a dry, prepurified nitrogen atmosphere with
standard Schlenk techniques. All solvents were dried and distilled
Fig. 5 Dependence of current heights of electrocatalytic events on the
concentration of HOTs in CH3CN for complexes 9 and 9s.
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prior to use according to standard methods. Paraformalde-
hyde, 4-nitroaniline, aniline, 4-methylaniline and [Fe(CO)5] were
available commercially and used without further purifica-
tion. The reagents LiEt3BH and CF3COOH were purchased
from Aldrich. The starting complexes N,N-bis(hydroxymethyl)-
4-nitroaniline (1), N,N-bis(hydroxymethyl)aniline (2), N,N-
bis(hydroxymethyl)-4-methylaniline (3) and [(l-Se)2Fe2(CO)6]
(4) were prepared according to the literature methods.18–20
Three dithiolate analogues [{(l-SCH2)2N(4-NO2C6H4)}Fe2(CO)6]
(7s), [{(l-SCH2)2NC6H5}Fe2(CO)6] (8s) and [{(l-SCH2)2N(4-
CH3C6H4)}Fe2(CO)6] (9s) were generated following the published
procedures.9a,c,13c IR spectra were recorded on JASCO FT/IR 430
purified by column chromatography on silica gel using CH2Cl2–
hexane (1 : 2) as eluent to give complex 7 as a dark purple solid
(0.319 g, 46%). Recrystallization in the CH2Cl2/pentane solution
afforded crystals of 7 suitable for X-ray study. 1H NMR (CDCl3): d
8.234 (d, 2H), 6.825 (d, 2H), 4.594 (s, 4H) ppm; 13C NMR (CDCl3):
d 207.50, 147.96, 140.43, 126.71, 114.52, 42.12 ppm; IR (CH2Cl2):
mmax/cm−1 2070, 2033, 1999 (CO); HR-MS (EI): m/z calc. for [M+] :
603.7310; found: 603.7305.
Preparation of [{(l-SeCH2)2NC6H5}Fe2(CO)6] (8)
The similar procedure was followed as for 7, but 0.705 g (4.6 mmol)
of 2, prepared from aniline and paraformaldehyde, was employed
instead of 1. After purified by column chromatography on silica
gel using CH2Cl2–hexane (1 : 10) as eluent, 0.334 g (52%) of 8 was
obtained as a dark purple solid. Crystals suitable for X-ray study
1
spectrophotometer. H and 13C NMR spectra were collected on
a Varian Inova 400 NMR spectrometer with tetramethylsilane
(TMS) as internal standard. HR-MS spectra determinations were
made on a GCT-MS instrument (Micromass, England).
1
were grown from a mixed CH2Cl2–pentane solution. H NMR
(CDCl3): d 7.341 (t, 2H), 6.996 (t, 1H), 6.834 (d, 2H), 4.540 (s,
4H) ppm; 13C NMR (100 MHz, CDCl3): d 207.76, 142.91, 130.40,
120.66, 115.56, 43.86 ppm; IR (CH2Cl2): mmax/cm−1 2067, 2030,
1994 (CO); HR-MS (EI) : m/z calc. for [M+] : 558.7459; found:
558.7462.
Preparation of [{(l-SeCH2)2N(4-NO2C6H4)}Fe2(CO)6] (7)
LiEt3BH solution (1 M solution in THF, 2.3 mL, 2.3 mmol) was
added to a fresh prepared solution of [(l-Se)2Fe2(CO)6] (4) (0.5 g,
1.15 mmol) in THF (20 mL) by syringe at −78 ◦C over 20 min. The
mixture was stirred for 1 h at −78 ◦C and the color of the solution
turned from red–purple to dark red–brown. The resulting lithium
salt [(l-LiSe)2Fe2(CO)6] (5) solution was treated dropwise with
CF3COOH (0.17 mL, 2.3 mmol) to give [(l-HSe)2Fe2(CO)6] (6).
A solution of 1 (0.912 g, 4.6 mmol), prepared from 4-nitroaniline
and paraformaldehyde, in THF (10 mL) was added to the THF
solution of 6. After the mixture was warmed to room temperature
and continually stirred for 5 h, the solvent was removed on a rotary
evaporator to give a dark-brown solid. The crude product was
Preparation of [{(l-SeCH2)2N(4-CH3C6H4)}Fe2(CO)6] (9)
The similar procedure was followed as for 7, but 0.769 g (4.6 mmol)
of 2, prepared from 4-methylaniline and paraformaldehyde, was
employed in place of 1. After purified by column chromatography
on silica gel using CH2Cl2–hexane (1 : 10) as eluent, 0.323 g (49%)
of 9 was obtained as a dark purple solid. Crystals suitable for X-
ray study were grown from a mixed CH2Cl2–pentane solution. 1H
NMR (CDCl3): d 7.139 (d, 2H), 6.728 (d, 2H), 4.530 (s, 4H), 2.256
Table 4 Crystallographic data and structural refinement parameters for 7–9
7
8
9
Empirical formula
Mr
C14H8Fe2N2O8Se2
601.84
Monoclinic
C14H9Fe2NO6Se2
556.84
Monoclinic
C15H11Fe2NO6Se2
570.87
Triclinic
¯
P1
7.835(2)
9.930(3)
12.436(3)
100.133(3)
101.395(3)
90.643(3)
932.6(4)
2
Crystal system
Space group
P21
P21/n
˚
a/A
7.7173(3)
10.5330(4)
11.9968(4)
90.00
104.189(2)
90.00
945.43(6)
2
2.114
8.0764(6)
12.3471(8)
17.7225(12)
90.00
91.866(4)
90.00
1766.4(2)
4
2.094
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc./g cm−3
2.033
5.489
l/mm−1
5.429
5.793
F(000)
580
1072
—
0.33 × 0.26 × 0.15
2.01–28.00
22004
4270
0.0370
227
1.058
552
—
Flack parameter
0.018(9)
0.20 × 0.10 × 0.02
1.75–27.49
12039
4072
0.0472
253
0.960
0.0344, 0.0493
0.0557, 0.0551
0.385/–0.366
Crystal size/mm
0.43 × 0.05 × 0.05
2.09–29.17
5855
4317
0.0193
235
0.993
0.0368, 0.0784
0.0601, 0.0876
0.551/–0.472
◦
hmin/max
/
Reflections collected
Independent reflections
Rint
Parameters refined
GOF on F2
R1,a wR2 [I > 2r(I)]
0.0255, 0.0569
0.0356, 0.0596
0.919/–0.837
b
R1,awR2 (all data)
b
−3
˚
Dqmax/min/e A
ꢀ
ꢀ
ꢀ
2
a R1 = ꢀFoꢀ − |Fcꢀ. b wR2 = [ [w(Fo − Fc2)2]/ w(Fo2)2]1/2
.
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(s, 3H) ppm; 13C NMR (CDCl3): d 207.53, 140.55, 130.89, 130.18,
115.59, 44.29, 20.67 ppm; IR (CH2Cl2): mmax/cm−1 = 2064, 2023,
1983 (CO); HR-MS (EI) : m/z calc. for [M+]: 572.7616; found:
572.7614.
2 M. W. W. Adams, Biochim. Biophys. Acta, 1990, 1020, 115–145.
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X-Ray crystal structure determination of complexes 7–9
Diffraction measurements of 7–9 were made on a SMART APEX
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II diffractometer. Data were collected at 298 K using graphite-
˚
monochromated Mo-Ka radiation (k = 0.71073 A) in the x–2h
scan mode. Data processing was accomplished with the SAINT
processing program.27 Intensity data were corrected for absorption
by the SADABS program.28 The structures were solved by direct
2
methods and refined by full-matrix least-squares techniques on Fo
using the SHELXTL 97 crystallographic software package.29 All
non-hydrogen atoms were refined anisotropically. All hydrogen
atoms were located by using the geometric method, and their
positions and thermal parameters were fixed during the structure
refinement. Details of crystal data, data collections, and structure
refinements are summarized in Table 4.
CCDC reference numbers 665465 for 7, 665466 for 8 and 665467
for 9.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b717497g
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Electrochemistry studies of complexes 7–9
The acetonitrile (Aldrich, spectroscopy grade) used for electro-
chemical measurements was dried with molecular sieves and then
freshly distilled from CaH2 under N2. A solution of 0.05 M n-
Bu4NPF6 (Fluka, electrochemical grade) in CH3CN was used as
electrolyte in all cyclic voltammetric experiments. The supporting
electrolyte solution was degassed by bubbling with dry argon
for 10 min before measurement. Electrochemical measurements
were made with a BAS 100B electrochemical workstation at a
scan rate of 100 mV s−1. All voltammograms were obtained in
a conventional three-electrode cell under argon and at ambient
temperature. The working electrode was a glassy carbon disc
(diameter 3 mm) that was successively polished with 3-lm and
1-lm diamond pastes and sonicated in ion-free water for 15 min
prior to use. The experimental reference electrode was a non-
aqueous Ag/Ag+ electrode (0.01 M AgNO3–0.1 M n-Bu4NPF6 in
CH3CN) and the counter electrode was a platinum wire.
9 (a) H. Li and T. B. Rauchfuss, J. Am. Chem. Soc., 2002, 104, 726–727;
˚
(b) S. Ott, M. Kritikos, B. Akermark, L. Sun and R. Lomoth, Angew.
Chem., Int. Ed., 2004, 43, 1006–1009; (c) T. Liu, M. Wang, Z. Shi, H.
˚
Cui, W. Dong, J. Chen, B. Akermark and L. Sun, Chem.–Eur. J., 2004,
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Li, B. Akermark and L. Sun, Chem. Commun., 2005, 3221–3223; (e) L.
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M. M. Rohmer, Angew. Chem., Int. Ed., 2001, 40, 1768–1771; (c) S. Ott,
˚
M. Borgstro¨m, M. Kritikos, R. Lomoth, J. Bergquist, B. Akermark, L.
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˚
11 (a) P. Li, M. Wang, C. He, G. Li, X. Liu, C. Chen, B. Akermark and L.
Sun, Eur. J. Inorg. Chem., 2005, 2506–2513; (b) W. Dong, M. Wang, X.
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Acknowledgements
12 A. Le Cloirec, S. P. Best, S. Borg, S. C. Davies, D. J. Evana, D. L.
Hughes and C. J. Pickett, Chem. Commun., 1999, 2285–2286.
13 (a) J. L. Nehring and D. M. Heinekey, Inorg. Chem., 2003, 42, 4288–
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Chem., 2006, 4679–4686.
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Talarmin, Organometallics, 2005, 24, 2020–2022; (b) J. W. Tye, J. Lee,
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M. Y. Darensbourg, Inorg. Chem., 2005, 44, 5550–5552; (c) L. Duan,
M. Wang, P. Li, Y. Na, N. Wang and L. Sun, Dalton Trans., 2007,
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Organometallics, 2007, 26, 2042–2052.
We are grateful to the National Natural Science Foundation of
China (Projects 20725621 and 20706008) and the Cultivation Fund
of the Key Scientific and Technical Innovation Project, Ministry
of Education of China (707016) for financial support of this work.
Helpful discussions with Dr. Ping Li are gratefully acknowledged.
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