Synthetic and Structural Studies of [FeFe]-Hydrogenase Models Containing a Butterfly Fe/E (E = S, Se, or Te) Cluster Core. Electrocatalytic H2 Evolution Catalyzed by [(μ-SeCH2)(μ-CH2NCH2Ph)]Fe2(CO)<su

Article abstract of DOI:10.1021/acs.organomet.9b00022

As models of [FeFe]-H₂ ases, 13 butterfly Fe/E (E = S, Se, or Te) complexes (1-13) have been prepared by various synthetic methods. Treatment of [(μ-SCH₂)₂CH₂]Fe₂(CO)₆ (A) and [(μ-SCH₂

Full text of DOI:10.1021/acs.organomet.9b00022

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthetic and Structural Studies of [FeFe]-Hydrogenase Models
Containing a Butterfly Fe/E (E = S, Se, or Te) Cluster Core.
Electrocatalytic H2 Evolution Catalyzed by [(μ-SeCH2)(μ-
CH2NCH2Ph)]Fe2(CO)6
Li-Cheng Song,*,†,‡ Jin-Sen Chen,† Guo-Jun Jia,† Yong-Zhen Wang,† Zheng-Lei Tan,†
and Yong-Xiang Wang†
†Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin
300071, China
‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
S
* Supporting Information
ABSTRACT: As models of [FeFe]-H2 ases, 13 butterfly Fe/
E (E = S, Se, or Te) complexes (1−13) have been prepared
by various synthetic methods. Treatment of [(μ-
SCH2)2CH2]Fe2(CO)6 (A) and [(μ-SCH2)2CHO2CPh]-
Fe2(CO)6 (B) with the in situ-generated N-heterocyclic
carbenes (NHC) IMe/Mes, IVinyl/Mes, IMes, and IMe gave the
corresponding carbene-substituted butterfly [2Fe2S] com-
plexes [(μ-SCH2)2CH2]Fe2(CO)5(IMe/Mes) (1), [(μ-
S C H 2 ) 2 C H 2 ] F e 2 ( C O ) 5 ( I V i n y l / M e s
) ( 2 ) , [ ( μ -
SCH2)2CHO2CPh]Fe2(CO)5(IMes) (3), and [(μ-
SCH2)2CHO2CPh]Fe2(CO)4(IMe)2 (4), respectively. Although the N-p-methoxyphenyl-substituted 1,2,4-diselenazolidine C
reacted with Fe3(CO)12 to give the expected butterfly [2Fe2Se] complex [(μ-SeCH2)2NC6H4OMe-p]Fe2(CO)6 (5) and further
reaction of 5 with PPh3 afforded its PPh3-substituted derivative [(μ-SeCH2)2NC6H4OMe-p]Fe2(CO)5(PPh3) (6), the N-
arylmethyl-substituted 1,2,4-diselenazolidines D−F reacted with Fe3(CO)12 to give the first butterfly [2Fe1Se1N1C] complexes
[(μ-SeCH2)(μ-CH2NR)]Fe2(CO)6 (7, R = PhCH2; 8, R = NC5H4CH2-p; 9, R = CpFeC5H4CH2), unexpectedly. More
interestingly, on the basis of synthesizing the first ODTe-bridged butterfly [2Fe2Te] complex [(μ-TeCH2)2O]Fe2(CO)6 (10),
the NHC-substituted butterfly [2Fe2Te] complexes [(μ-TeCH2)2O]Fe2(CO)5(L) (11, L = IMe/Mes; 12, L = IMes; 13, L = IMe)
were prepared by reactions of complex 10 with the corresponding NHC ligands. In addition, while all of the new complexes 1−
13 were structurally characterized, complex 7 was found to be a H2-producing catalyst from benzoic acid and p-toluenesulfonic
acid under cyclic voltammetry conditions.
INTRODUCTION
■
The butterfly Fe/E (E = S, Se, or Te) cluster complexes have
received a growing amount of attention in recent years, mainly
due to their unique structures and novel properties and
particularly the close resemblance to [FeFe]-hydrogenases
([FeFe]-H2 ases).1−8 [FeFe]-H2 ases make up a class of natural
enzymes that catalyze the reversible proton reduction to
molecular H2 in a wide variety of microorganisms.9 X-ray
crystallographic,10−12 FTIR spectroscopic,13−15 and some
other experimental16,17 studies established that the active site
of [FeFe]-H2 ases (so-called H-cluster)18 comprises a
cubanelike [4Fe4S] cluster that is linked via a cysteine sulfur
atom to a butterfly [2Fe2S] cluster in which its two Fe centers
are coordinated by three CO ligands, two CN− ligands, and
one bridging azadithiolate (ADT) ligand (Figure 1). On the
basis of structural studies regarding the active site of [FeFe]-H2
ases, a wide variety of butterfly Fe/E (E = S, Se, or Te) cluster
complexes as [FeFe]-H2 ase models have been synthesized and
Figure 1. Basic H-cluster structure determined by FTIR and X-ray
crystallography.
characterized by chemists in bioinorganic and organometallic
fields.19−37 To further develop the synthetic methodology for
butterfly Fe/E cluster complexes and to develop the
biomimetic chemistry of [FeFe]-H2 ases, we have prepared
four types of new butterfly [2Fe2S], [2Fe2Se], [2Fe1S-
e1N1C], and [2Fe2Te] cluster core-containing complexes. In
Received: January 16, 2019
© XXXX American Chemical Society
A
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this work, we report their synthesis and structural character-
ization. In addition, the electrocatalytic H2 production
catalyzed by one of the prepared [2Fe1Se1N1C] cluster
core-containing models is also described.
(4) could be obtained by CO substitution of B with the in situ-
generated IMe from 1,3-bis(methyl)imidazolium salt IMe·HI
under the action of t-BuOK in THF at room temperature
(Scheme 2).
RESULTS AND DISCUSSION
■
Scheme 2. Synthesis of NHC-Substituted Butterfly [2Fe2S]
Complexes 3 and 4
Synthesis and Characterization of PDT-Bridged
Butterfly [2Fe2S] Complexes Containing One or Two
NHC Ligands. N-Heterocyclic carbene (NHC) ligands have
been extensively utilized as surrogates for phosphines to
prepare the H-cluster mimics of [FeFe]-H2 ases,23,24 because
NHC ligands have stronger σ-donation with negligible π-
accepting ability as well as greater electronic and steric
tunability in comparison with those of phosphine ligands.38−40
On the basis of our previous studies regarding the butterfly
[2Fe2S] cluster core-containing model complexes of [FeFe]-
H2 ases,28−30 we prepared a series of the new PDT (PDT =
propanedithiolate)-bridged butterfly [2Fe2S] core and NHC
ligand-containing [FeFe]-H2 ase models. Thus, as shown in
Scheme 1, the NHC ligand IMe/Mes (IMe/Mes = 1-methyl-3-
Scheme 1. Synthesis of NHC-Substituted Butterfly [2Fe2S]
Complexes 1 and 2
While complexes 1−3 are air-stable red solids, complex 4 is
an air-sensitive red solid. All complexes 1−4 were charac-
terized by elemental analysis and spectroscopy. For example,
the IR spectra of 1−4 showed three or four absorption bands
in the range of 2037−1871 cm−1 for their terminal carbonyls,
while 3 and 4 showed one additional band at 1709 or 1714
cm−1 for their ester carbonyls. The 1H NMR spectra of 1 and 2
displayed two singlets in the region of 2.10−2.36 ppm for the
three CH3 groups attached to benzene rings in their NHC
ligands. In addition, the 13C{1H} NMR spectra of 1−3
displayed one signal in the range of 189−194 ppm for their
carbene C atoms,31b,42 although the qualified 13C{1H} NMR
spectrum of 4 could not be obtained because of its instability
under the determined conditions.
The molecular structures of complexes 2 and 4 were further
confirmed by X-ray crystal diffraction analysis. While their
ORTEP views are shown in Figures 2 and 3, the selected bond
lengths and angles are listed in Table 1. As shown in Figure 2,
complex 2 contains one PDT ligand that is bridged between its
two iron atoms and one NHC ligand 1-vinyl-3-mesitylimida-
zol-2-ylidene located in the apical position of the square-
mesitylimidazol-2-ylidene)-containing butterfly [2Fe2S] com-
plex (μ-PDT)Fe2(CO)5(IMe/Mes) (1) could be prepared in
tetrahydrofuran (THF) at room temperature by CO
substitution of parent complex (μ-PDT)Fe2(CO)6 (A) with
the in situ-generated IMe/Mes by loss of one molecule of HI
from 1-methyl-3-mesitylimidazolium salt IMe/Mes·HI under the
action of n-BuLi.41 However, different from this, we further
found that the NHC ligand 1-vinyl-3-mesitylimidazol-2-ylidene
(IVinyl/Mes)-substituted complex (μ-PDT)Fe2(CO)5(IVinyl/Mes
)
(2) could be prepared by CO substitution of complex A with
the unexpectedly generated carbene IVinyl/Mes by thermal
decomposition (β-elimination) of the dicarbene
I*Mes(CH2)2I*Mes initially formed by loss of two molecules of
HBr from the ethylene-bridged imidazolium salt
[I*Mes(CH2)2I*Mes]·2HBr under the action of n-BuLi (Scheme
1).
Similarly, the benzoyloxy group-substituted derivative of
parent complex A, namely complex [(μ-SCH2)2CHO2CPh]-
Fe2(CO)6 (B), was found to react with the in situ-generated
NHC ligand IMes [IMes = 1,3-bis(mesityl)imidazol-2-ylidene]
from 1,3-bis(mesityl)imidazolium salt IMes·HCl under the
action of n-BuLi to give the IMes-monosubstituted complex
[(μ-SCH2)2CHO2CPh]Fe2(CO)5(IMes) (3), whereas the
NHC ligand IMe [IMe = 1,3-bis(methyl)imidazol-2-ylidene]-
disubstituted complex [μ-(SCH2)2CHO2CPh]Fe2(CO)4(IMe)2
Figure 2. ORTEP view of 2 with 50% probability level ellipsoids. All
hydrogen atoms have been omitted for the sake of clarity.
B
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C(17)C(16)C(15)S(2) with a chair conformation. The
benzoyloxy group is attached to the bridgehead C(16) atom
via the common equatorial C(16)−O(5) bond of the two
fused six-membered rings, to avoid the strong steric repulsions
of the equatorially attached benzoyloxy group with the apically
occupied carbonyl at Fe(1) and 1,3-dimethylimidazol-2-
ylidene ligand at Fe(2).4 In addition, the two NHC ligands
are unsymmetrically bound to a basal position and an apical
position of the square-pyramidal Fe(1) and Fe(2) atoms,
respectively.
Synthesis and Characterization of Butterfly [2Fe2Se]
and [2Fe1Se1N1C] Cluster Core-Containing Complexes.
On the basis of our previous study of the butterfly [2Fe2Se]
model complexes,31−33 we prepared the new azapropanedise-
lenolate (ADSe)-bridged butterfly [2Fe2Se] complexes 5 and
6. Thus, as shown in Scheme 3, when N-p-methoxyphenyl
Scheme 3. Synthesis of Butterfly [2Fe2Se] Complexes 5 and
6
Figure 3. ORTEP view of 4 with 50% probability level ellipsoids. All
hydrogen atoms have been omitted for the sake of clarity.
group-substituted 1,2,4-diselenazolidine C43 was treated with
Fe3(CO)12 in THF at 60 °C, the corresponding N-substituted
butterfly [2Fe2Se] core-containing complex [(μ-
SeCH2)2NC6H4OMe-p]Fe2(CO)6 (5) was obtained via the
oxidative addition of the Se−Se bond in diselenazolidine C
with diiron fragment Fe2(CO)8 [formed in situ by thermal
decomposition of Fe3(CO)12] and subsequent displacement of
its two CO ligands.35 In addition, upon further treatment of 5
with PPh3 in the presence of decarbonylating agent Me3NO·
2H2O in THF at room temperature, the PPh3-monosubsti-
tuted complex [(μ-SeCH2)2NC6H4OMe-p]Fe2(CO)5(PPh3)
(6) was produced (Scheme 3).
pyramidal Fe(2) atom. The C(12)C(13) bond length is
1.308 Å, whereas the Fe(1)−Fe(2) bond length is 2.5236 Å,
which is very close to that (2.55 Å) of the reduced state of
[FeFe]-H2 ases12 but much shorter than those (2.60 and 2.62
Å) of the oxidized state of [FeFe]-H2 ases.10,11 Figure 3 shows
that complex 4 contains one benzoyloxy group-substituted
PDT ligand and two NHC 1,3-dimethylimidazol-2-ylidene
ligands. The substituted PDT ligand is bridged between two
iron atoms to form two fused six-membered rings: one six-
membered ring Fe(1)S(1)C(17)C(16)C(15)S(2) with a boat
conformation and the other six-membered ring Fe(2)S(1)-
Table 1. Selected Bond Lengths (angstroms) and Angles (degrees) for 2 and 4
2
S(1)−C(6)
S(1)−Fe(1)
S(1)−Fe(2)
S(2)−Fe(1)
Fe(2)−S(1)−Fe(1)
Fe(1)−S(2)−Fe(2)
S(2)−Fe(1)−S(1)
S(2)−Fe(1)−Fe(2)
1.8261(18)
2.2596(6)
2.2687(8)
2.2597(6)
67.737(16)
67.581(15)
85.27(2)
Fe(2)−S(2)
Fe(2)−C(9)
Fe(1)−Fe(2)
C(9)−N(1)
S(1)−Fe(1)−Fe(2)
S(1)−Fe(2)−S(2)
S(1)−Fe(2)−Fe(1)
S(2)−Fe(2)−Fe(1)
2.2777(8)
1.9858(18)
2.5236(5)
1.377(2)
56.30(2)
84.641(18)
55.959(17)
55.868(17)
56.55(2)
4
Fe(1)−C(5)
S(1)−Fe(2)
S(2)−Fe(1)
S(2)−Fe(2)
S(2)−Fe(1)−S(1)
S(2)−Fe(1)−Fe(2)
S(1)−Fe(1)−Fe(2)
S(1)−Fe(2)−S(2)
2.099(6)
2.1392(16)
2.2256(16)
2.3120(16)
84.01(5)
57.84(4)
50.90(4)
89.42(6)
S(1)−Fe(1)
Fe(1)−Fe(2)
C(5)−N(1)
Fe(2)−C(10)
S(1)−Fe(2)−Fe(1)
S(2)−Fe(2)−Fe(1)
Fe(2)−S(1)−Fe(1)
Fe(2)−S(2)−Fe(1)
2.4508(16)
2.5245(11)
1.430(8)
2.066(6)
62.77(5)
54.58(4)
66.33(5)
67.58(5)
C
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Interestingly, although the N-methoxyphenyl-substituted
1,2,4-diselenazolidine C reacted with Fe3(CO)12 to give the
expected butterfly [2Fe2Se] complex 5, the N-benzyl-, N-
pyridylmethyl-, and N-ferrocenylmethyl-substituted 1,2,4-dis-
elenazolidines D−F, respectively [note that the three
diselenazolidines D−F and their precursors, 3,7-disubstituted
1,5,3,7-diselenadiazocanes X−Z, respectively, are new (see the
Supporting Information for their synthesis and character-
ization)], were found to react with Fe3(CO)12 under similar
conditions to give the first butterfly [2Fe1Se1N1C] core-
containing complexes [(μ-SeCH2)(μ-CH2NR)]Fe2(CO)6 (7,
R = PhCH2; 8, R = NC5H4CH2-p; 9, R = CpFeC5H4CH2),
unexpectedly (Scheme 4).
step is very similar to the previously reported intramolecular
nucleophilic attack at iron followed by loss of the Fe-bound
ligand that occurred in some butterfly Fe/S cluster
complexes.44,45 The third step involves extrusion of one Se
atom from the seven-membered metallacycle in M2 to give the
more stable six-membered metallacycle (2Fe1Se1N2C)-
containing butterfly [2Fe1Se1N1C] complexes 7−9. The
third step is actually similar to the previously reported
extrusion of Se from the p-MeC6H4SO2Se-bridged butterfly
Fe/E complexes (μ-RE)(μ-p-MeC6H4SO2Se)Fe2(CO)6 (E = S
or Se).46 It should be noted that although this proposed
pathway appears to be reasonable, some details for this
pathway need to be further studied.
While complex 9 is an air-stable red oil, complexes 5−8 are
air-stable red solids. The IR spectra of all-carbonyl complexes 5
and 7−9 showed three strong absorption bands in the range of
2060−1970 cm−1 for their terminal carbonyls; PPh3-mono-
substituted complex 6 displayed three bands in a much lower
region of 2036−1921 cm−1, obviously due to the increased
intensity of π-back bonding between iron atoms and the
attached carbonyls by CO substitution with the stronger σ-
donor PPh3.47 The 1H NMR spectra of 5 and 6 displayed one
singlet at 3.80 and 3.72 ppm for their CH3O groups,
respectively, whereas those of 7−9 showed the corresponding
multiplets for the H atoms in their benzene, pyridine, and
cyclopentadienyl rings. The 77Se{1H} NMR spectra of
complexes 5 and 6 exhibited one singlet at −103.6 ppm and
two singlets at −276.6 and −263.9 ppm for their two Se atoms,
respectively. The 77Se{1H} NMR spectra of complexes 7−9
displayed one singlet at 77.3, 71.4, and 84.6 ppm for their one
Se atom, respectively.
Scheme 4. Synthesis of Butterfly [2Fe1Se1N1C] Complexes
7−9
The molecular structures of complexes 6 and 7 were
unequivocally confirmed by X-ray crystallographic study.
Figures 4 and 5 display their ORTEP views, whereas Table 2
lists their selected bond lengths and angles. As one can see in
Figure 4, complex 6 contains an N-p-methoxyphenyl group-
substituted ADSe ligand that is bridged between two iron
atoms to form the two fused six-membered rings Fe(1)Se(1)-
C(6)N(1)C(7)Se(2) and Fe(2)Se(2)C(7)N(1)C(6)Se(1)
with a chair conformation and a boat conformation,
respectively. The p-methoxyphenyl group in this butterfly
[2Fe2Se] complex 6 is attached to the bridgehead N(1) atom
via the common axial N(1)−C(8) bond of the two fused six-
A possible pathway for the unexpected formation of
complexes 7−9 is proposed in Scheme 4, which includes the
following three major reaction steps. The first step, like the
formation of complex 5, involves oxidative addition of N-
substituted 1,2,4-diselenazolidines D−F with Fe3(CO)12 to
give butterfly [2Fe2Se] intermediates M1. The second step
involves the intramolecular nucleophilic attack of N atoms in
M1 (due to the fact that N-arylmethyl substituents in D−F are
stronger electron-donating groups than the N-p-methoxyphen-
yl substituent in C) at one of their two Fe atoms with cleavage
of one Se−Fe bond to give the seven-membered metallacycle
(2Fe2Se1N2C)-containing intermediates M2. Actually, this
Figure 4. ORTEP view of 6 with 50% probability level ellipsoids. All hydrogen atoms have been omitted for the sake of clarity.
D
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Synthesis and Characterization of the Parent ODTe-
Bridged Butterfly [2Fe2Te] Complex and Its NHC-
Substituted Derivatives. Although numerous butterfly
[2Fe2S] and [2Fe2Se] core-containing [FeFe]-H2 ase models
have been reported previously,20−35 only a few of the butterfly
[2Fe2Te] core-containing [FeFe]-H2 ase models have been
prepared so far,36,37 because they are less stable and more
difficult to prepare in comparison to the corresponding
butterfly Fe/S and Fe/Se complexes. We found that the first
oxapropaneditellurolate (ODTe)-bridged type of butterfly
[2Fe2Te] complex (μ-ODTe)Fe2(CO)6 (10) could be
prepared by reductive cleavage of the Te−Te bond in
ditellurium complex (μ-Te2)Fe2(CO)6 with 2 equiv of
Et3BHLi in THF at −78 °C, followed by treatment of the
resulting intermediate (μ-TeLi)2Fe2(CO)67 from −78 to 60 °C
with excess bis(chloromethyl) ether (Scheme 5).
Scheme 5. Synthesis of Parent Butterfly [2Fe2Te] Complex
10
Figure 5. ORTEP view of 7 with 50% probability level ellipsoids. All
hydrogen atoms have been omitted for the sake of clarity.
Having prepared the first ODTe-type model complex 10, we
found that the NHC-substituted and ODTe-bridged butterfly
[2Fe2Te] complexes (μ-ODTe)Fe2(CO)5(IMe/Mes) (11), (μ-
ODTe)Fe2(CO)5(IMes) (12), and (μ-ODTe)Fe2(CO)5(IMe)
(13) were prepared (under conditions similar to those for
preparation of the NHC-substituted and PDT-bridged
butterfly [2Fe2S] complexes 1−4) by CO substitution
membered rings, to avoid the strong steric repulsions between
the axially attached p-methoxyphenyl group and the bulky
PPh3 located in the apical position of the square-pyramidal
Fe(1) atom. The Fe(1)−Fe(2) bond length (2.5497 Å) is very
close to those of the previously reported ADSe-bridged diiron
complexes.33 As shown clearly in Figure 5, complex 7 indeed
includes a butterfly [2Fe1Se1N1C] cluster core with a
triangular Fe(1)Fe(2)Se(1) wing and a square-planar Fe(1)-
Fe(2)C(8)N(1) wing. In addition, the methylene C(7) atom is
bridged between N(1) and Se(1) atoms and the benzyl group
is bound to the N(1) atom via the C(9)−N(1) bond. The
Fe(1)−Fe(2) bond length is 2.5807 Å, which is longer than
those of the butterfly [2Fe2Se] complex 6 and the previously
reported ADSe-bridged diiron complexes.33
reactions of parent complex 10 with the NHC ligands IMe/Mes
,
I
Mes, and IMe generated in situ from the corresponding
imidazolium salts under the action of n-BuLi or t-BuOK
(Scheme 6).
While parent complex 10 is an air-stable red solid, its NHC-
monosubstituted derivatives 11−13 are air-stable black solids.
All of these new butterfly [2Fe2Te] complexes have been
characterized by elemental analysis and various spectroscopies.
For instance, the IR spectrum of parent complex 10 showed
Table 2. Selected Bond Lengths (angstroms) and Angles (degrees) for 6 and 7
6
Se(1)−C(6)
Se(1)−Fe(1)
Se(1)−Fe(2)
Se(2)−Fe(1)
Fe(2)−Se(1)−Fe(1)
Fe(1)−Se(2)−Fe(2)
P(1)−Fe(1)−Se(1)
Se(2)−Fe(1)−Se(1)
2.027(4)
2.4046(7)
2.3852(7)
2.4045(7)
64.322(19)
64.28(2)
Fe(2)−Se(2)
Fe(1)−P(1)
Fe(1)−Fe(2)
P(1)−C(15)
Se(2)−Fe(1)−Fe(2)
Se(1)−Fe(1)−Fe(2)
Se(1)−Fe(2)−Se(2)
Se(1)−Fe(2)−Fe(1)
2.3880(6)
2.2469(10)
2.5497(7)
1.829(3)
57.545(17)
57.47(2)
109.69(3)
84.81(2)
85.60(2)
58.208(19)
7
Se(1)−C(7)
Se(1)−Fe(2)
N(1)−Fe(1)
C(8)−Fe(2)
C(7)−Se(1)−Fe(2)
C(7)−Se(1)−Fe(1)
Se(1)−Fe(1)−Fe(2)
N(1)−Fe(1)−Se(1)
1.963(2)
2.3656(6)
2.0451(19)
2.069(2)
95.45(8)
76.37(7)
Se(1)−Fe(1)
Fe(1)−Fe(2)
C(7)−N(1)
C(8)−N(1)
Se(1)−Fe(2)−Fe(1)
C(8)−Fe(2)−Se(1)
Fe(2)−Se(1)−Fe(1)
N(1)−Fe(1)−Fe(2)
2.3748(9)
2.5807(7)
1.470(3)
1.479(3)
57.19(3)
85.38(7)
65.97(2)
74.39(6)
56.845(19)
72.76(6)
E
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Scheme 6. Synthesis of NHC-Substituted Butterfly
[2Fe2Te] Complexes 11−13
Figure 6. ORTEP view of 10 with 50% probability level ellipsoids. All
hydrogen atoms have been omitted for the sake of clarity.
one medium and four strong absorption bands in the range of
2054−1949 cm−1 for its terminal carbonyls, whereas those of
the NHC-monosubstituted derivatives 11−13 displayed three
strong absorption bands in the region of 2016−1897 cm−1.
The three absorption bands displayed by 11−13 lie at a
frequency range much lower than that displayed by their
parent complex 10, obviously due to the increased strength of
the π-back bonding between the Fe atoms and the attached
carbonyls by CO substitution with the stronger electron-
donating NHC ligands.38−40 The 1H NMR spectra of
complexes 10 and 11 displayed one singlet at 4.59 and 3.35
ppm for the two methylene groups in their ODTe ligands,
whereas 12 and 13 exhibited one multiplet in the range of
4.05−4.58 ppm for their methylene groups in their ODTe
ligands. The 13C{1H} NMR spectra of 11−13 showed one
signal in the range of 182−199 ppm for their carbene C
atoms.31b,42 In addition, the 125Te{1H} NMR spectrum of 10
and its substituted derivative 12 showed one signal at −376.5
and −85.0 ppm for their two Te atoms, respectively.
Figure 7. ORTEP view of 11 with 50% probability level ellipsoids. All
hydrogen atoms have been omitted for the sake of clarity.
To confirm the molecular structures of the ODTe-bridged
butterfly [2Fe2Te] complexes 10−12 and to obtain the
corresponding structural data, X-ray crystallographic studies of
10−12 were undertaken. The ORTEP views of 10−12 are
depicted in Figures 6−8, respectively, while Table 3 presents
their selected bond lengths and angles. Figures 6−8
demonstrate that the three complexes indeed have an ODTe
ligand that is bridged between their two iron atoms. While the
two iron atoms in parent complex 10 are all coordinated by
three terminal CO ligands, one of the two iron atoms in the
substituted complexes 11 and 12 is coordinated by three
terminal CO ligands but another iron atom is by two terminal
CO ligands and one NHC IMe/Mes or IMes ligand. In addition,
the NHC IMe/Mes ligand in 11 is located in a basal position of
the square-pyramidal Fe(2) atom, whereas the NHC IMes
ligand in 12 is located in the apical position of the square-
pyramidal Fe(2) atom. The Fe(1)−Fe(2) bond length of
parent complex 10 (2.6324 Å) is slightly shorter than those of
its substituted derivatives 11 (2.7084 Å) and 12 (2.6547 Å). In
addition, the Fe(1)−Fe(2) bond length of the parent ODTe-
type complex 10 (2.6324 Å) is longer than that of the parent
Figure 8. ORTEP view of 12 with 50% probability level ellipsoids. All
hydrogen atoms have been omitted for the sake of clarity.
ODSe-type complex (2.5599 Å),31b and in turn, the Fe(1)−
Fe(2) bond length of the parent ODSe-type complex is longer
than that of the parent ODT (ODT = oxapropanedithiolate)-
type complex (2.5113 Å),28 because the atomic radii of the
F
DOI: 10.1021/acs.organomet.9b00022
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 3. Selected Bond Lengths (angstroms) and Angles (degrees) for 10−12
10
Te(1)−C(7)
Te(1)−Fe(1)
Te(1)−Fe(2)
Te(2)−C(8)
2.190(5)
2.5428(8)
2.5343(9)
2.185(5)
Fe(2)−Te(2)
Te(2)−Fe(1)
Fe(1)−Fe(2)
O(7)−C(7)
2.5379(8)
2.5386(9)
2.6324(10)
1.392(6)
C(7)−Te(1)−Fe(2)
C(7)−Te(1)−Fe(1)
Fe(2)−Te(1)−Fe(1)
C(8)−Te(2)−Fe(2)
110.20(14)
105.86(13)
62.46 (3)
109.09(14)
C(8)−Te(2)−Fe(1)
Fe(2)−Te(2)−Fe(1)
Te(1)−Fe(1)−Fe(2)
Te(2)−Fe(2)−Te(1)
106.81(13)
62.47(3)
58.61(3)
86.93(3)
11
Te(1)−C(6)
Te(1)−Fe(1)
Te(1)−Fe(2)
Fe(2)−C(11)
2.186(3)
2.5319(7)
2.5252(8)
2.005(3)
Fe(2)−Te(2)
Te(2)−Fe(1)
Fe(1)−Fe(2)
C(11)−N(1)
2.5250(8)
2.5408(7)
2.7084(8)
1.361(4)
Fe(1)−Te(1)−Fe(2)
Fe(1)−Te(2)−Fe(2)
Te(1)−Fe(1)−Te(2)
Te(1)−Fe(1)−Fe(2)
64.762(16)
64.637(17)
86.32(2)
Te(2)−Fe(1)−Fe(2)
Te(1)−Fe(2)−Te(2)
Te(2)−Fe(2)−Fe(1)
Te(1)−Fe(2)−Fe(1)
57.40(2)
86.796(16)
57.963(18)
57.74(2)
57.50(2)
12
Te(1)−C(6)
Te(1)−Fe(1)
Te(1)−Fe(2)
Fe(2)−C(8)
Fe(1)−Te(1)−Fe(2)
Fe(1)−Te(2)−Fe(2)
Te(1)−Fe(1)−Te(2)
Te(1)−Fe(1)−Fe(2)
2.200(7)
2.5479(14)
2.5500(14)
1.997(6)
62.77(4)
62.42(4)
86.64(4)
58.66(3)
Fe(2)−Te(2)
Te(2)−Fe(1)
Fe(1)−Fe(2)
C(8)−N(1)
Te(2)−Fe(1)−Fe(2)
Te(1)−Fe(2)−Te(2)
Te(1)−Fe(2)−Fe(1)
Te(2)−Fe(2)−Fe(1)
2.5627(16)
2.5606(15)
2.6547(18)
1.380(8)
58.83(5)
86.55(4)
58.58(4)
58.75(3)
three chalcogen atoms decrease in the following order: Te > Se
> S.
Electrochemical and Electrocatalytic Studies of
Model Complex 7. Although the electrochemical and
electrocatalytic properties of some butterfly [2Fe2S],23,25,30
[2Fe2Se],33,34 and [2Fe2Te]36,37 models for [FeFe]-H2 ases
were previously studied by us and other groups, no
electrochemical or electrocatalytic study of the butterfly
[2Fe1Se1N1C] core-containing model complex has been
reported until now. To determine the electrochemical and
electrocatalytic properties of this type of model complex, we
chose complex 7 as a representative to determine its
electrochemical property and electrocatalytic ability. First, the
electrochemical property of complex 7 was determined in
MeCN with n-Bu4NPF6 as the electrolyte by cyclic
voltammetry. As shown in Figure 9, the cyclic voltammogram
of 7 consists of an irreversible reduction wave at −1.56 V, a
reversible reduction wave at −1.70 V (ΔEp = 80 mV; ipa/ipc
≈
Figure 9. Cyclic voltammograms of 7 (1.0 mM) in 0.1 M n-Bu4NPF6/
MeCN at a scan rate of 0.1 V s−1. Arrows indicate the starting
potential and scan direction.
1), and an irreversible oxidation wave at 0.40 V. Because the
current function (ip/υ1/2) values for the two reduction events
of 7 remain constant at all scan rates (see Figures S9 and S10),
each of the two reductions could be assigned to a one-electron
reduction process from FeΙFeΙ to FeΙFe0 and from FeΙFe0 to
Fe0Fe0 species.48,49 However, because the current height of the
oxidation peak of 7 is approximately twice that for each of its
two reduction peaks, the oxidation event of 7 could be
assigned to a two-electron oxidation process from FeΙFeΙ to
FeΙΙFeΙΙ species.28 It follows that such cyclic voltammetric
behavior for 7 is very similar to that displayed by the previously
reported butterfly [2Fe2Se] complexes [(μ-SeCH2)2NC(O)-
R]Fe2(CO)6 (R = Me and Ph).33 In addition, it is worth
noting that the two reduction processes of 7 are diffusion-
controlled because its two reduction peak currents versus the
square root of the scan rates (25−1250 mV s−1) are all linearly
correlated (see Figure S11).48
To assess the electrocatalytic H2-producing ability catalyzed
by complex 7, we recorded its cyclic voltammograms in the
presence of benzoic acid PhCO2H (pKaMeCN = 20.7)50 or p-
toluenesulfonic acid TsOH (pKaMeCN = 8.7).50 As shown in
Figure 10, during addition of the weak acid PhCO2H, the
intensity of the original first reduction peak of 7 increased
slightly, but the intensity of its original second reduction peak
increased remarkably with continuous addition of the acid.
However, in contrast to this, Figure 11 shows that during
G
DOI: 10.1021/acs.organomet.9b00022
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
SUMMARY AND CONCLUSIONS
■
Interestingly, four types of new model complexes (1−13) for
[FeFe]-H2 ases have been prepared by various synthetic
methods. The first type of model complex (1−4) contains a
PDT-bridged butterfly [2Fe2S] cluster core bearing five
carbonyls and one NHC ligand or four carbonyls and two
NHC ligands. While complexes 1, 3, and 4 are prepared by CO
substitution reactions of A and B with the in situ-generated
NHC ligands IMe/Mes, IMes, and IMe by loss of HX (X = Cl or I)
from the corresponding imidazolium salts under the action of
n-BuLi or t-BuOK, complex 2 is prepared by the CO
substitution reaction of A with the NHC ligand IVinyl/Mes
generated unexpectedly by in situ thermal decomposition of
the initially formed unstable dicarbene I*Mes(CH2)2I*Mes from
its HBr salt under the action of n-BuLi. The second type of
model complex (5 and 6) has an ADSe-bridged butterfly
[2Fe2Se] core bearing six carbonyls or five carbonyls and one
PPh3 ligand. While 5 is prepared by oxidative addition of N-
methoxyphenyl-substituted diselenazolidine C with Fe3(CO)12,
complex 6 is prepared by CO substitution of 5 with PPh3. The
third type of model complex (7−9) includes an azapropylse-
lenolate ligand-bridged butterfly [2Fe1Se1N1C] cluster core
bearing six carbonyl ligands. To our surprise, complexes 7−9
are obtained unexpectedly by reactions of the N-arylmethyl-
substituted 1,2,4-diselenazolidines D−F, respectively, with
Fe3(CO)12. Finally, the fourth type of model complex (10−
13) contains an ODTe-bridged butterfly [2Fe2Te] cluster core
carrying six carbonyls or five carbonyls and one NHC ligand.
While complex 10 is prepared by condensation of intermediate
(μ-TeLi)2Fe2(CO)6 with bis(chloromethyl) ether, complexes
11−13 are prepared by CO substitution of complex 10 with
the NHC ligands generated in situ from the corresponding
imidazolium salts. All of the new model complexes 1−13 and
the six new starting materials D−F and X−Z used for
preparation of complexes 7−9 are characterized by elemental
analysis and spectroscopy, and in particular, the molecular
structures of some of their representatives are confirmed by X-
ray crystal diffraction analysis. Finally, it should be indicated
that complex 7 as one representative of the first butterfly
[2Fe1Se1N1C] cluster core-containing complexes 7−9 has
been found to be an electrocatalyst for the production of H2
from PhCO2H and p-MeC6H4SO3H under cyclic voltammetry
conditions.
Figure 10. Cyclic voltammograms of 7 (1.0 mM) with varying
amounts of PhCO2H in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1
V s−1.
Figure 11. Cyclic voltammograms of 7 (1.0 mM) with varying
amounts of TsOH in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V
s−1.
addition of the strong acid TsOH, the intensity of the original
first reduction peak of 7 increased considerably with
continuous addition of the acid, while that of its original
second reduction peak was slightly increased. It is apparent
that such observations demonstrated that complex 7 can act as
the electrocatalyst for proton reduction to H2 from PhCO2H
and TsOH at its second and first reduction potentials,
respectively.28,30,35
The electrocatalytic H2-producing ability catalyzed by
complex 7 was further confirmed by controlled-potential
electrolysis (CPE) experiments of the MeCN solutions of 7
(0.5 mM) with excess PhCO2H (20 mM) at −2.15 V and
TsOH (20 mM) at −1.88 V. During a 1 h CPE of 7 with
PhCO2H, a total of 32.6 F/mol of 7 passed, which corresponds
to 16.3 turnovers (TONs); during a 1 h CPE of 7 with TsOH,
a total of 35.4 F/mol of 7 passed, which corresponds to 17.7
TONs. It follows that the H2-producing efficiencies catalyzed
by 7 from PhCO2H and TsOH are much higher than those
previously reported for the production of H2 from HOAc
catalyzed by the butterfly [2Fe2Se] model complexes [(μ-
SeCH2)2X]Fe2(CO)6 (X = O or S; TONs = 7.5 and 6.4,
respectively).31b
EXPERIMENTAL SECTION
■
General Comments. All reactions were carried out using standard
Schlenk and vacuum-line techniques under an atmosphere of highly
purified nitrogen. THF was purified by distillation under N2 from
sodium/benzophenone ketyl. Et3BHLi (1 M in THF), n-BuLi (2.5 M
in hexane), t-BuOK, Me3NO·2H2O, Ph3P, and (ClCH2)2O were
available commercially and used as received. Fe3(CO)12,51 1,3-
bis(mesityl)imidazolium chloride (IMes·HCl),52 1,3-dimethylimidazo-
lium iodide (IMe·HI),531-mesityl-3-methylimidazolium iodide
(IMe/Mes·HI),41 1,2-bis(mesitylimidazolium)ethane dibromide
[I*Mes(CH2)2I*Mes·2HBr],54 [(μ-SCH2)2CH2]Fe2(CO)6 (A),2 [(μ-
SCH2)2CHO2CPh]Fe2(CO)6 (B),55 (μ-Te2)Fe2(CO)6,56 and N-p-
MeOC6H4-substituted 1,2,4-diselenazolidine (C)43 were prepared
according to the published procedures. The three new starting
materials N-benzyl-1,2,4-diselenazolidine (D), N-pyridylmethyl-1,2,4-
diselenazolidine (E), and N-ferrocenylmethyl-1,2,4-diselenazolidine
(F) for preparation of complexes [(μ-SeCH2)(μ-CH2NR)]Fe2(CO)6
(7, R = PhCH2; 8, R = NC5H4CH2-p; 9, R = CpFeC5H4CH2) as well
as three new starting materials 3,7-dibenzyl-1,5,3,7-diselenadiazocane
(X), 3,7-dipyridylmethyl-1,5,3,7-diselenadiazocane (Y), and 3,7-
H
DOI: 10.1021/acs.organomet.9b00022
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 4. Crystal Data and Structural Refinement Details for
2 and 4
Table 5. Crystal Data and Structural Refinement Details for
6 and 7
2
4
6
7
molecular formula
C22H22Fe2N2O5S2
C24H26Fe2N4O6S2·
CH2Cl2
molecular formula C32H26Fe2NO6PSe2·
C15H11Fe2NO6Se
CH2Cl2
molecular weightt
crystal system
space group
a (Å)
b (Å)
c (Å)
570.24
triclinic
727.23
molecular weight 906.05
491.91
monoclinic
P2(1)/c
9.2923(19)
13.806(3)
13.961(3)
90
101.73(3)
90
1753.7(6)
4
monoclinic
P121/n1
8.4738(7)
12.0771(8)
29.2415(19)
90
94.698(4)
90
2982.5(4)
4
crystal system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P121/c1
9.3144(15)
10.3083(16)
36.214(6)
90
93.833(3)
90
3469.3(10)
4
P1
̅
8.086(2)
8.454(2)
17.831(5)
101.749(4)
90.301(4)
97.898(5)
1181.5(6)
2
α (deg)
β (deg)
γ (deg)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
V (Å3)
Z
Dc (g cm−3
)
1.603
1.620
Dc (g cm−3
)
1.735
1.863
absorption
1.441
1.338
absorption
3.180
3.760
coefficient
coefficient
(mm−1
)
(mm−1
)
F(000)
584
1488
F(000)
1800
968
index ranges
−10 ≤ h ≤ 10,
−11 ≤ k ≤ 10,
−23 ≤ l ≤ 23
−10 ≤ h ≤ 10,
−14 ≤ k ≤ 14,
−34 ≤ l ≤ 34
index ranges
−11 ≤ h ≤ 11,
−13 ≤ k ≤ 9,
−46 ≤ l ≤ 46
−9 ≤ h ≤ 12,
−18 ≤ k ≤ 17,
−16 ≤ l ≤ 18
no. of reflections
14972
20293
no. of reflections
30551
15470
no. of independent 5622
5234
no. of
7648
4171
reflections
independent
reflections
2θmax (deg)
2.33/27.91
1.83/25.00
0.0688
0.1767
1.087
1.315/−1.125
2θmax (deg)
R
Rw
2.05/27.20
0.0454
0.0821
2.10/27.88
0.0362
0.0708
1.098
0.544/−0.621
R
0.0272
Rw
0.0678
goodness of fit
1.022
goodness of fit
1.074
largest difference
0.350/−0.311
peak/hole (e Å−3
)
largest diff peak/ 0.712/−0.615
hole (e Å−3
)
C6H2); 13C{1H} NMR (75 MHz, CDCl3) δ 18.6 (s, o-CH3 of C6H2),
21.2 (s, p-CH3 of C6H2), 29.7, 39.6 (2s, CH2CH2CH2), 123.7, 124.2
(2s, NCHCHN), 129.2−139.0 (m, C6H2), 189.0 (s, NCN), 211.2,
215.6 (2s, CO). Anal. Calcd for C21H22Fe2N2O5S2: C, 45.18; H,
3.97; N, 5.02. Found: C, 45.38; H, 4.02; N, 4.95.
diferrocenylmethyl-1,5,3,7-diselenadiazocane (Z) for preparation of
diselenazolidines D−F, respectively, were prepared and characterized
(see the Supporting Information). Preparative thin layer chromatog-
raphy (TLC) was carried out on glass plates (26 cm × 20 cm × 0.25
cm) coated with silica gel H (10−40 μm). IR spectra were recorded
on a Bio-Rad FTS 135 or Bruker Tensor 27 FTIR spectrophotometer.
1H (13C, 31P, 77Se, 125Te) NMR spectra were obtained on a Bruker
Avance 300 NMR or 400 NMR spectrometer. Analysis and
Preparation of [(μ-SCH2)2CH2]Fe2(CO)5(IVinyl/Mes) (2). To a
stirred suspension of the imidazolium salt I*Mes(CH2)2I*Mes·2HBr
(0.756 g, 1.35 mmol) in THF (20 mL) was added n-BuLi (1.1 mL,
2.75 mmol) in a dropwise manner by a syringe to give an orange-red
solution. After the solution was stirred at room temperature for an
additional 45 min, it was filtered under anaerobic conditions through
a Celite-packed column and eluted with THF (15 mL) to give a
filtrate containing the air-sensitive NHC ligand IVi/Mes. To this filtrate
was added complex A (0.259 g, 0.67 mmol), and the new mixture was
stirred at room temperature for 6 h until A had been completely
consumed. The solvent was removed at reduced pressure, and then
the residue was subjected to TLC separation using CH2Cl2/
petroleum ether [1:5 (v/v)] as the eluent. From the main red
band, 2 (0.172 g, 45%) was obtained as a red solid: mp 197−199 °C;
1
assignments of the H NMR spectral data of complexes 7−9 were
supported by 13C−1H HMQC (heteronuclear multiple-quantum
correlation) and 13C−1H HMBC (heteronuclear multiple-bond
correlation) spectra (see the Supporting Information). Elemental
analyses were performed on an Elementar Vario EL analyzer. Melting
points were determined on a Yanaco Mp-500 or a SGW X-4
microscopic melting point apparatus and are uncorrected.
Preparation of [(μ-SCH2)2CH2]Fe2(CO)5(IMe/Mes) (1). n-BuLi
(0.8 mL, 2.0 mmol) was added in a dropwise manner by a syringe
to a stirred suspension of the imidazolium salt IMe/Mes·HI (0.500 g,
1.52 mmol) in THF (10 mL) to give a yellowish solution. After the
solution was stirred at room temperature for an additional 15 min, it
was filtered under anaerobic conditions through a Celite-packed
column and eluted with THF (15 mL) to give a filtrate containing the
air-sensitive NHC ligand IMe/Mes. To this filtrate was added [(μ-
SCH2)2CH2]Fe2(CO)6 (A) (0.386 g, 1.00 mmol), and the new
mixture was stirred at room temperature for 4 h until A had been
completely consumed as indicated by TLC. The solvent was removed
at reduced pressure, and then the residue was subjected to TLC
separation using CH2Cl2/petroleum ether [1:7 (v/v)] as the eluent.
From the main red band, 1 (0.452 g, 81%) was obtained as a red
solid: mp 171−173 °C; IR (KBr disk) νCO 2030 (vs), 1957 (vs),
1
IR (KBr disk) νCO 2035 (vs), 1963 (vs), 1910 (s) cm−1; H NMR
(300 MHz, CDCl3) δ 1.68−2.04 (m, 6H, CH2CH2CH2), 2.12 (s, 6H,
2-o-CH3 of C6H2), 2.36 (s, 3H, p-CH3 of C6H2), 5.07−5.30 (m, 3H,
NCHCH2), 6.95−7.50 (m, 4H, NCHCHN, C6H2); 13C{1H}
NMR (75 MHz, CDCl3) δ 18.4 (s, o-CH3 of C6H2), 21.2 (s, p-CH3 of
C6H2), 23.8, 29.6 (2s, CH2CH2CH2), 102.6, 119.2 (2s, NCHCH2),
124.8 (s, NCHCHN), 129.3−139.3 (m, C6H2), 193.7 (s, NCN),
210.7, 214.8 (2s, CO). Anal. Calcd for C22H22Fe2N2O5S2: C, 46.34;
H, 3.89; N, 4.91. Found: C, 46.39; H, 4.01; N, 4.71.
Preparation of [(μ-SCH2)2CHO2CPh]Fe2(CO)5(IMes) (3). To a
stirred suspension of the imidazolium salt IMes·HCl (0.300 g, 0.88
mmol) in THF (15 mL) was added n-BuLi (0.6 mL, 1.50 mmol) in a
dropwise manner by a syringe to give a yellow solution. After the
solution was stirred at room temperature for an additional 15 min, it
1
1911 (s) cm−1; H NMR (300 MHz, CDCl3) δ 1.64−1.89 (m, 6H,
CH2CH2CH2), 2.10 (s, 6H, 2-o-CH3 of C6H2), 2.36 (s, 3H, p-CH3 of
C6H2), 4.04 (s, 3H, NCH3), 6.87−7.12 (m, 4H, NCHCHN,
I
DOI: 10.1021/acs.organomet.9b00022
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Organometallics
Article
Table 6. Crystal Data and Structural Refinement Details for 10−12
10
11
12
molecular formula
molecular weight
crystal system
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
C8H4Fe2O7Te2
579.01
C20H20Fe2N2O6Te2
751.28
C28H28Fe2N2O6Te2
855.42
triclinic
monoclinic
P2(1)/c
10.062(2)
8.2258(16)
17.237(3)
90
95.18(3)
90
1420.9(5)
4
monoclinic
P2(1)/n
8.990(3)
20.439(6)
13.088(4)
90
93.303(5)
90
2400.8(12)
4
P1
̅
8.755(5)
10.782(6)
17.784(10)
102.576(6)
93.072(3)
111.666(10)
1506.2(14)
2
γ (deg)
V (Å3)
Z
Dc (g cm−3
)
2.070
2.079
1.886
absorption coefficient
6.087
3.628
2.905
(mm−1
)
F(000)
1056
1432
828
index ranges
−11 ≤ h ≤ 9, −9 ≤ k ≤ 9, −20 ≤ l ≤ 20 −11 ≤ h ≤ 11, −26 ≤ k ≤ 26, −17 ≤ l ≤ 15 −11 ≤ h ≤ 11, −14 ≤ k ≤ 9, −23 ≤ l ≤ 23
no. of reflections
9783
24920
14631
no. of independent
reflections
2487
5676
7091
2θmax (deg)
R
2.37/25.01
0.0301
1.85/27.84
0.0260
2.10/27.87
0.0642
Rw
0.0712
0.0548
0.1360
goodness of fit
largest diff peak/hole (e Å−3
1.014
0.763/−0.756
1.014
0.606/−1.118
1.071
1.108/−1.528
)
was filtered through an anaerobic Celite-packed column and then
eluted with THF (15 mL) to give a filtrate containing the air-sensitive
NHC ligand IMes. To this filtrate was added [(μ-SCH2)2CHO2CPh]-
Fe2(CO)6 (B) (0.120 g, 0.25 mmol), and the new mixture was stirred
at room temperature for 6 h until B had been completely consumed as
indicated by TLC. The solvent was removed at reduced pressure, and
then the residue was subjected to TLC separation using CH2Cl2/
petroleum ether [1:4 (v/v)] as the eluent. From the main red band,
complex 3 (0.086 g, 44%) was obtained as a red solid: mp 191−193
°C; IR (KBr disk) νCO 2037 (vs), 1972 (vs), 1948 (s), 1920 (s)
mL) was heated to 60 °C and stirred at this temperature for 2.5 h.
After removal of the solvent at reduced pressure, the residue was
subjected to TLC separation using CH2Cl2/petroleum ether [1:5 (v/
v)] as the eluent. From the orange-red band, 5 (0.230 g, 20%) was
obtained as a red solid: mp 146 °C dec; IR (KBr disk) νCO 2060 (s),
2016 (vs), 1987 (vs), 1970 (vs) cm−1; 1H NMR (400 MHz, CDCl3) δ
3.80 (s, 3H, OCH3), 4.52 (br s, 4H, CH2NCH2), 6.78, 6.91 (dd, J =
10.0 Hz, 4H, C6H4); 77Se{1H} (76 MHz, CDCl3, Me2Se) δ −103.6
(s). Anal. Calcd for C15H11Fe2NO7Se2: C, 30.70; H, 1.89; N, 2.39.
Found: C, 30.94; H, 1.98; N, 2.26.
1
cm−1; IR (KBr disk) νCO 1709 (s) cm−1; H NMR (300 MHz,
Preparation of [(μ-SeCH2)2NC6H4OMe-p]Fe2(CO)5(PPh3) (6).
A mixture of 5 (0.120 g, 0.20 mmol) and decarbonylating agent
Me3NO·2H2O (0.030 g, 0.27 mmol) in Me3CN (20 mL) was stirred
at room temperature for 10 min, and then PPh3 (0.054 g, 0.20 mmol)
was added. After the new mixture was stirred for an additional 2 h, the
solvent was removed at reduced pressure and then the residue was
subjected to TLC separation using CH2Cl2/petroleum ether [1:1 (v/
v)] as the eluent. From the orange-red band, 6 (0.090 g, 55%) was
obtained as a red solid: mp 155 °C dec; IR (KBr disk) νCO 2036
CDCl3) δ 1.53−2.50 (m, 22H, 2SCH2, 6CH3), 3.33−3.45 (m, 1H,
CH), 6.95−8.03 (m, 11H, NCHCHN, C6H5, 2C6H2); 13C{1H}
NMR (75 MHz, CDCl3) δ 18.6 (s, o-CH3 of C6H2), 21.2 (s, p-CH3 of
C6H2), 29.2 (s, SCH2), 75.1 (s, OCH), 125.1 (s, NCHCHN),
128.4−139.4 (m, C6H2), 164.5 (s, OCO), 190.3 (s, NCN), 214.4
(s, CO). Anal. Calcd for C36H34Fe2N2O7S2: C, 55.26; H, 4.38; N,
3.58. Found: C, 55.24; H, 4.39; N, 3.32.
Preparation of [(μ-SCH2)2CHO2CPh]Fe2(CO)4(IMe)2 (4). To a
1
stirred suspension of the imidazolium salt IMe·HI (1.120 g, 4.80
mmol) in THF (20 mL) was added t-BuOK (0.672 g, 6.00 mmol).
After the mixture was stirred at room temperature for 2 h, it was
filtered through an anaerobic Celite-packed column and then eluted
with THF (20 mL) to give a filtrate containing the air-sensitive free
carbene IMe. To this filtrate was added complex B (0.400 g, 0.84
mmol), and the new mixture was stirred at room temperature for 2 h
until B had been completely consumed. The solvent was removed at
reduced pressure, and then the residue was subjected to TLC
separation using CH2Cl2/petroleum ether [2:1 (v/v)] as the eluent.
From the main red band, 4 (0.218 g, 42%) was obtained as a red
solid: mp 256 °C dec; IR (KBr disk) νCO 1961 (s), 1924 (vs), 1901
(vs), 1978 (vs), 1921 (s) cm−1; H NMR (400 MHz, acetone-d6) δ
2.83, 4.40 (dd, J = 12.0 Hz, 4H, CH2NCH2), 3.72 (s, 3H, OCH3),
6.86 (s, 4H, C6H4), 7.57−7.86 (m, 15H, 3C6H5); 77Se{1H} (76 MHz,
CDCl3, Me2Se) δ −263.9 (s), −276.6 (s); 31P{1H} NMR (162 MHz,
acetone-d6, H3PO4) δ 68.2 (s). Anal. Calcd for C32H26Fe2NO6PSe2:
C, 46.81; H, 3.19; N, 1.71. Found: C, 46.63; H, 3.17; N, 1.64.
Preparation of [(μ-SeCH2)(μ-CH2NCH2Ph)]Fe2(CO)6 (7). A
mixture of the N-benzyl-substituted 1,2,4-diselenazolidine (D, 0.59
g, 2.0 mmol) and Fe3(CO)12 (1.00 g, 2.0 mmol) in THF (30 mL) was
heated to 60 °C and stirred at this temperature for 2 h. After removal
of the solvent at reduced pressure, the residue was subjected to TLC
separation using CH2Cl2/petroleum ether [1:6 (v/v)] as the eluent.
From the major orange-red band, 7 (0.200 g, 20%) was obtained as a
red solid: mp 107−108 °C; IR (KBr disk) νCO 2060 (s), 2012 (vs),
1971 (vs) cm−1; 1H NMR (400 MHz, acetone-d6) δ 2.57, 3.35 (dd, J
= 8.0 Hz, 2H, NCH2Fe), 2.98, 5.20 (dd, J = 8.0 Hz, 2H, NCH2Se),
3.54, 3.69 (dd, J = 14.0 Hz, 2H, C6H5CH2N), 7.38−7.40 (m, 5H,
C6H5); 77Se{1H} (76 MHz, CDCl3, Me2Se) δ 77.3 (s). Anal. Calcd
for C15H11Fe2NO6Se: C, 36.63; H, 2.25; N, 2.85. Found: C, 36.67; H,
2.40; N, 2.61.
1
(s), 1871 (s) cm−1; IR (KBr disk) νCO 1714 (s) cm−1; H NMR
(300 MHz, CDCl3) δ 1.54, 2.53 (2br s, 4H, 2SCH2), 4.08 (br s, 13H,
CH, 4NCH3, OCH), 6.94−7.03 (m, 4H, 2NCHCHN), 7.35−7.82
(m, 5H, C6H5). Anal. Calcd for C24H26Fe2N4O6S2: C, 44.88; H, 4.08;
N, 8.72. Found: C, 44.99; H, 4.05; N, 8.73.
Preparation of [(μ-SeCH2)2NC6H4OMe-p]Fe2(CO)6 (5). A
mixture of the N-p-methoxymethyl-substituted 1,2,4-diselenazolidine
(C, 0.62 g, 2.0 mmol) and Fe3(CO)12 (1.00 g, 2.0 mmol) in THF (30
J
DOI: 10.1021/acs.organomet.9b00022
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Organometallics
Article
Preparation of [(μ-SeCH2)(μ-CH2N-p-CH2C5H4N)]Fe2(CO)6 (8).
A mixture of N-pyridylmethyl-substituted 1,2,4-diselenazolidine (E,
0.58 g, 2.0 mmol) and Fe3(CO)12 (1.00 g, 2.0 mmol) in THF (30
mL) was heated to 60 °C and then stirred at this temperature for 2.5
h. After removal of the solvent at reduced pressure, the residue was
subjected to TLC separation using CH2Cl2/petroleum ether [1:6 (v/
v)] as the eluent. From the major orange-red band, 8 (0.180 g, 18%)
was obtained as a red solid: mp 120−121 °C; IR (KBr disk) νCO
2060 (s), 2013 (vs), 1973 (s) cm−1; 1H NMR (400 MHz, acetone-d6)
δ 2.61, 3.36 (2s, 2H, NCH2Fe), 3.02, 5.26 (2s, 2H, NCH2Se), 3.59,
3.77 (dd, J = 12.0 Hz, 2H, NC5H4CH2N), 7.40, 8.58 (2s, 4H,
NC5H4); 77Se{1H} (76 MHz, CDCl3, Me2Se) δ 71.4 (s). Anal. Calcd
for C14H10Fe2N2O6Se: C, 34.12; H, 2.04; N, 5.68. Found: C, 34.03;
H, 2.30; N, 5.35.
the new mixture was stirred at room temperature for 2 h until 10 had
been completely consumed. The solvent was removed at reduced
pressure, and then the residue was subjected to TLC separation using
CH2Cl2/petroleum ether [1:4 (v/v)] as the eluent. From the main
black band, 12 (0.079 g, 58%) was obtained as a black solid: mp 110−
112 °C; IR (KBr disk) νCO 2016 (vs), 1949 (vs), 1901 (s) cm−1; 1H
NMR (400 MHz, CDCl3) δ 2.19 (s, 12H, 4-o-CH3 of C6H2), 2.36 (s,
6H, 2-p-CH3 of C6H2), 4.05−4.09 (m, 4H, CH2OCH2), 7.03−7.20
(m, 6H, NCHCHN, 2C6H2); 13C{1H} NMR (100 MHz, CDCl3) δ
18.8 (s, o-CH3 of C6H2), 21.2 (s, p-CH3 of C6H2), 44.2 (s,
CH2OCH2), 124.7 (2s, NCHCHN), 129.6−139.3 (m, C6H2),
198.9 (s, NCN), 210.8, 212.2 (2s, CO); 125Te{1H} NMR (126
MHz, CDCl3, Me2Te)
δ −85.0. ppm. Anal. Calcd for
C28H28Fe2N2O6Te2: C, 39.31; H, 3.30; N, 3.27. Found: C, 38.98;
H, 3.27; N, 3.38.
Preparation of [(μ-SeCH2)(μ-CH2NCH2C5H4FeCp)]Fe2(CO)6
(9). A mixture of N-ferrocenylmethyl-substituted 1,2,4-diselenazoli-
dine (F, 0.63 g, 1.57 mmol) and Fe3(CO)12 (1.00 g, 2.0 mmol) in
THF (30 mL) was heated to 60 °C and stirred at this temperature for
2 h. After removal of the solvent in vacuo, the residue was subjected
to TLC separation using CH2Cl2/petroleum ether [1:4 (v/v)] as the
eluent. From the major orange-red band, 9 was obtained as a red oil
(0.200 g, 21%): IR (KBr disk) νCO 2058 (s), 2011 (vs), 1970 (vs)
cm−1; 1H NMR (400 MHz, CDCl3) δ 2.56, 3.02 (dd, J = 4.0 Hz, 2H,
NCH2Fe), 2.83, 4.53 (2s, 2H, NCH2Se), 3.17, 3.26 (dd, J = 14.0 Hz,
2H, C5H4CH2N), 4.10−4.22 (m, 9H, C5H4, C5H5); 77Se{1H} (76
MHz, CDCl3, Me2Se) δ 84.6 (s). Anal. Calcd for C19H15Fe3NO6Se:
C, 38.05; H, 2.52; N, 2.34. Found: C, 38.15; H, 2.64; N, 2.10.
Preparation of [(μ-TeCH2)2O]Fe2(CO)6 (10). A petroleum ether
solution (20 mL) containing (μ-Te2)Fe2(CO)6 (0.290 g, 0.50 mmol)
was mixed with THF (20 mL) and then cooled to −78 °C with a
liquid N2/acetone bath. LiEt3BH (1.0 mL, 1.0 mmol) was added in a
dropwise manner to this stirred solution by a syringe. At the midpoint
of the addition, the reaction solution turned from orange-red to dark
green, and for the rest of the addition, it gradually became brown-red.
After the resulting solution was warmed to room temperature,
(ClCH2)2O (90 μL) was added. The new mixture was heated to 60
°C and then stirred at this temperature for 12 h. The solvent was
removed at reduced pressure, and the residue was subjected to TLC
separation using CH2Cl2/petroleum ether [v/v (1:6)] as the eluent.
From the second main red band, 10 (0.060 g, 21%) was obtained as a
red solid: mp 135 °C dec; IR (KBr disk) νCO 2054 (m), 2010 (s),
Preparation of [(μ-TeCH2)2O]Fe2(CO)5(IMe) (13). To a stirred
suspension of the imidazolium salt IMe·HI (0.179 g, 0.77 mmol) in
THF (15 mL) was added n-BuLi (0.7 mL, 1.75 mmol) in a dropwise
manner by a syringe to give a yellow solution. After the solution was
stirred at room temperature for an additional 15 min, complex 10
(0.058 g, 0.10 mmol) was added. The new mixture was stirred at
room temperature for 1 h until 10 had been completely consumed.
The solvent was removed at reduced pressure, and then the residue
was subjected to TLC separation using CH2Cl2/petroleum ether [1:2
(v/v)] as the eluent. From the main black band, 13 (0.010 g, 16%)
was obtained as a black solid: mp 156−158 °C; IR (KBr disk) νCO
1
2011 (s), 1942 (vs), 1897 (s) cm−1; H NMR (400 MHz, CDCl3) δ
3.89 (s, 6H, 2NCH3), 4.46−4.58 (m, 4H, CH2OCH2), 6.93 (s, 2H,
NCHCHN); 13C{1H} NMR (100 MHz, CDCl3) δ 40.5 (s,
NCH3), 44.8 (s, CH2OCH2), 123.8 (s, NCHCHN), 182.6 (s,
NCN), 213.5, 219.2 (2s, CO). Anal. Calcd for C12H12Fe2N2O6Te2:
C, 22.27; H, 1.87; N, 4.33. Found: C, 22.31; H, 1.78; N, 4.12.
Electrochemical and Electrocatalytic Experiments. Acetoni-
trile (HPLC grade) was purchased from Amethyst Chemicals. For
electrochemical and electrocatalytic experiments in MeCN, a 0.1 M
solution of n-Bu4NPF6 was used as the supporting electrolyte. The n-
Bu4NPF6 electrolyte was dried in an oven at 110 °C for at least 24 h.
Argon was sparged through the solutions for at least 15 min before
measurements were taken. The measurements were taken using a BAS
Epsilon potentiostat. All voltammograms were obtained in a three-
electrode cell with a 3 mm diameter glassy carbon working electrode,
a platinum counter electrode, and a Ag/Ag+ (0.01 M AgNO3/0.1 M
n-Bu4NPF6 in MeCN) reference electrode under an atmosphere of Ar.
The working electrode was polished with 0.05 μm alumina paste and
sonicated in water for ∼10 min. A controlled-potential electrolysis
experiment was performed on a vitreous carbon rod (A = 2.9 cm2) in
a two-compartment, gastight, H-type electrolysis cell containing ∼25
mL of MeCN. All potentials are quoted against the Fc/Fc+ potential.
Gas chromatography was performed with a Shimadzu gas chromato-
graph (model GC-2014) under isothermal conditions with nitrogen as
a carrier gas and a thermal conductivity detector.
Determinations of the X-ray Crystal Structures of 2, 4, 6, 7,
and 10−12. While single crystals of 2, 4, 6, 7, and 10 were grown by
slow evaporation of their CH2Cl2/n-hexane solutions at −10 or −5
°C, those of 11 and 12 were grown by slow diffusion of n-hexane into
their CH2Cl2 solutions at room temperature. A single crystal of 2 or 4
was mounted on a Rigaku MM-007 (rotating anode) diffractometer
equipped with a Saturn accessory, and their data were collected using
a confocal monochromator with Mo Kα radiation (λ = 0.71070 Å) in
the ω scanning mode at 113 K. A single crystal of 6 was mounted on a
Rigaku MM-007 (rotating anode) diffractometer equipped with a
Saturn 724 CCD accessory, and its data were collected using a
multilayer monochromator with Mo Kα radiation (λ = 0.71075 Å) in
the ω scanning mode at 113 K. A single crystal of 7 or 10 was
mounted on a Rigaku MM-007 (rotating anode) diffractometer
equipped with a Rigaku Saturn CCD accessory, and their data were
collected using a confocal monochromator with Mo Kα radiation (λ =
0.71073 Å) in the ω-f scanning mode at 113 K. A single crystal of 11
or 12 was mounted on a Rigaku MM-007 (rotating anode)
diffractometer equipped with a Saturn 724 CCD accessory, and its
1
1987 (vs), 1962 (s), 1949 (s) cm−1; H NMR (300 MHz, CDCl3) δ
4.59 (s, 4H, CH2OCH2); 125Te{1H} NMR (126 MHz, CDCl3,
Me2Te) δ −376.5. Anal. Calcd for C8H4Fe2O7Te2: C, 16.60; H, 0.70.
Found: C, 16.58; H, 0.67.
Preparation of [(μ-TeCH2)2O]Fe2(CO)5(IMe/Mes) (11). n-BuLi
(0.7 mL, 1.75 mmol) was slowly added at room temperature to a
stirred suspension of the imidazolium salt IMe/Mes·HI (0.262 g, 0.80
mmol) in THF (15 mL) to give a yellow solution. After the solution
was stirred at this temperature for an additional 15 min, complex 10
(0.058 g, 0.10 mmol) was added. The new mixture was stirred at
room temperature for 1.5 h until 10 had been completely consumed
as indicated by TLC. The solvent was removed at reduced pressure,
and then the residue was subjected to TLC separation using CH2Cl2/
petroleum ether [1:3 (v/v)] as the eluent. From the main black band,
11 (0.029 g, 39%) was obtained as a black solid: mp 149−151 °C; IR
(KBr disk) νCO 2014 (s), 1954 (vs), 1901 (s) cm−1; 1H NMR (400
MHz, CDCl3) δ 2.17 (s, 6H, 2-o-CH3 of C6H2), 2.39 (s, 3H, p-CH3 of
C6H2), 3.82 (br s, 3H, NCH3), 4.35 (br s, 4H, CH2OCH2), 6.88−
7.10 (m, 4H, NCHCHN, C6H2); 13C{1H} NMR (100 MHz,
CDCl3) δ 19.3 (s, o-CH3 of C6H2), 21.2 (s, p-CH3 of C6H2), 40.7 (s,
NCH3), 44.5 (s, CH2OCH2), 124.2 (s, NCHCHN), 129.6−139.2
(m, C6H2), 189.1 (s, NCN), 213.5, 217.2 (2s, CO). Anal. Calcd for
C20H20Fe2N2O6Te2: C, 31.97; H, 2.68; N, 3.73. Found: C, 32.10; H,
2.71; N, 3.64.
Preparation of [(μ-TeCH2)2O]Fe2(CO)5(IMes) (12). To a stirred
suspension of the imidazolium salt IMes·HCl (0.269 g, 0.79 mmol) in
THF (20 mL) was added t-BuOK (0.155 g, 1.38 mmol). The mixture
was stirred at room temperature for 2 h to give a yellow solution. To
this solution was added complex 10 (0.094 g, 0.16 mmol), and then
K
DOI: 10.1021/acs.organomet.9b00022
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
data were collected using a multilayer monochromator with Mo Kα
radiation (λ = 0.71073 Å) in the ω-f scanning mode at 113 K. Data
collection, reduction, and absorption correction were performed using
CRYSTALCLEAR.57 The structures were determineed by direct
methods using the SHELXS program58 and refined by full-matrix
least-squares techniques (SHELXL)59 on F2. Hydrogen atoms were
located by using the geometric method. Details of crystal data, data
collections, and structural refinements are summarized in Tables 4−6.
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ASSOCIATED CONTENT
■
S
* Supporting Information
(7) Mathur, P.; Reddy, V. D. Reduction of (μ2-Te2)Fe2(CO)6 and
reaction of the dianion formed with metal dihalides, L2MCl2(L =
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Synthesis and characterization of X−Z and D−F,
ORTEP views of Z and F (Figures S1 and S2,
respectively), crystal data and structural refinement
details of Z and F (Table S1), 13C−1H HMQC and
HMBC spectra of complexes 7−9, and some electro-
chemical and electrocatalytic data and plots (Figures
S9−S11) (PDF)
Accession Codes
(11) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, E. C.; Fontecilla-
Camps, J. C. Desulfovibrio desulfuricans iron hydrogenase: the
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CCDC 1852760−1852768 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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(12) Nicolet, Y.; De Lacey, A. L.; Vernede, X.; Fernandez, V. M.;
Hatchikian, E. C.; Fontecilla-Camps, J. C. Crystallographic and FTIR
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Reduction of the Active Site of the Fe-Only Hydrogenase from
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lcsong@nankai.edu.cn.
ORCID
(14) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.;
Fernandez, V. M. FTIR Characterization of the Active Site of the Fe-
hydrogenase from Desulfovibrio desulfuricans. J. Am. Chem. Soc. 2000,
122, 11232−11233.
Li-Cheng Song: 0000-0003-0964-8869
Notes
The authors declare no competing financial interest.
(15) Chen, Z.; Lemon, B. J.; Huang, S.; Swartz, D. J.; Peters, J. W.;
Bagley, K. A. Infrared Studies of the CO-Inhibited Form of the Fe-
Only Hydrogenase from Clostridium pasteurianum I: Examination of
Its Light Sensitivity at Cryogenic Temperatures. Biochemistry 2002,
41, 2036−2043.
ACKNOWLEDGMENTS
■
The authors are grateful to the National Natural Science
Foundation of China (21772106) and Technology of China
(973 program 2014CB845604) for financial support.
(16) Silakov, A.; Wenk, B.; Reijerse, E.; Lubitz, W. 14N HYSCORE
investigation of the H-cluster of [FeFe] hydrogenase: evidence for a
nitrogen in the dithiol bridge. Phys. Chem. Chem. Phys. 2009, 11,
6592−6599.
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