Synthesis, structural characterization, and some properties of 2-acylmethyl-6-ester group-difunctionalized pyridine-containing iron complexes related to the active site of [Fe]-hydrogenase

Article abstract of DOI:10.1039/c4dt00335g

As biomimetic models for [Fe]-hydrogenase, the 2-acylmethyl-6-ester group-difunctionalized pyridine-containing iron(ii) complexes 1-4 have been successfully prepared via the following three separate steps. In the first step, the acylation or esterification of difunctionalized pyridine 2-(p-MeC ₆H₄SO₃CH₂)-6-HOCH₂C ₅H₃N with acetyl chloride or benzoic acid gives the corresponding pyridine derivatives 2-(p-MeC₆H₄SO ₃CH₂)-6-RCO₂CH₂C₅H ₃N (A, R = Me; B, R = Ph). The second step involves reaction of A or B with Na₂Fe(CO)₄ followed by treatment of the intermediate Fe(0) complexes [Na(2-CH₂-6-RCO₂CH ₂C₅H₃N)Fe(CO)₄] (M₁, R = Me; M₂, R = Ph) with iodine to afford 2-acylmethyl-6-acetoxymethyl or 6-benzoyloxymethyl-difunctionalized pyridine-containing Fe(ii) iodide complexes [2-C(O)CH₂-6-RCO₂CH₂C₅H ₃N]Fe(CO)₂I (1, R = Me; 3, R = Ph). Finally, when 1 or 3 is treated with sodium 2-mercaptopyridinate, the corresponding difunctionalized pyridine-containing Fe(ii) mercaptopyridinate complexes [2-C(O)CH ₂-6-RCO₂C₅H₃N]Fe(CO) ₂(2-SC₅H₄N) (2, R = Me; 4, R = Ph) are produced. While the structures of model complexes 1-4 are confirmed by X-ray crystallography, the electrochemical properties of 2 and 4 are compared with those of the two previously reported models. In addition, complexes 2 and 4 have been found to be catalysts for H₂ production in the presence of TFA under CV conditions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00335g

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PAPER
Synthesis, structural characterization, and some
properties of 2-acylmethyl-6-ester group-
difunctionalized pyridine-containing iron
complexes related to the active site of
[Fe]-hydrogenase†
Cite this: Dalton Trans., 2014, 43,
8062
Li-Cheng Song,* Fu-Qiang Hu, Miao-Miao Wang, Zhao-Jun Xie, Kai-Kai Xu and
Hai-Bin Song
As biomimetic models for [Fe]-hydrogenase, the 2-acylmethyl-6-ester group-difunctionalized pyridine-
containing iron(II) complexes 1–4 have been successfully prepared via the following three separate steps.
In the first step, the acylation or esterification of difunctionalized pyridine 2-(p-MeC6H4SO3CH2)-6-
HOCH2C5H3N with acetyl chloride or benzoic acid gives the corresponding pyridine derivatives 2-(p-
MeC6H4SO3CH2)-6-RCO2CH2C5H3N (A, R = Me; B, R = Ph). The second step involves reaction of A or B
with Na2Fe(CO)4 followed by treatment of the intermediate Fe(0) complexes [Na(2-CH2-6-
RCO2CH2C5H3N)Fe(CO)4] (M1, R = Me; M2, R = Ph) with iodine to afford 2-acylmethyl-6-acetoxymethyl
or 6-benzoyloxymethyl-difunctionalized pyridine-containing Fe(II) iodide complexes [2-C(O)CH2-6-
RCO2CH2C5H3N]Fe(CO)2I (1, R = Me; 3, R = Ph). Finally, when 1 or 3 is treated with sodium 2-mercapto-
pyridinate, the corresponding difunctionalized pyridine-containing Fe(II) mercaptopyridinate complexes
[2-C(O)CH2-6-RCO2C5H3N]Fe(CO)2(2-SC5H4N) (2, R = Me; 4, R = Ph) are produced. While the structures
of model complexes 1–4 are confirmed by X-ray crystallography, the electrochemical properties of 2 and
4 are compared with those of the two previously reported models. In addition, complexes 2 and 4 have
been found to be catalysts for H2 production in the presence of TFA under CV conditions.
Received 1st February 2014,
Accepted 13th March 2014
DOI: 10.1039/c4dt00335g
www.rsc.org/dalton
Introduction
Hydrogenases are natural enzymes, which catalyze the revers-
ible H2 metabolism at high rates in a wide range of
microorganisms.1–3 There are three major hydrogenases:
[FeFe]-hydrogenases, [NiFe]-hydrogenases, and [Fe]-hydrogen-
ase. While [FeFe]- and [NiFe]-hydrogenases catalyze the revers-
ible redox reaction of hydrogen with protons and electrons,4–6
[Fe]-hydrogenase (also named as Hmd, or iron–sulfur cluster-
free hydrogenase) catalyzes the reversible reduction of methe-
nyltetrahydromethanopterin (methenyl-H4MPT+) with H2 to
give methylenetetrahydromethanopterin (methylene-H4MPT)
and proton (Scheme 1).3,7,8 This stereospecific hydride transfer
reaction from H2 to C14a of methenyl-H4MPT+ is an intermedi-
Scheme 1 Reversible hydride transfer catalyzed by [Fe]-hydrogenase.
ate step in the reduction of CO2 to methane by methanogens
grown under Ni-deficient conditions.9
Spectroscopic10–12 and particularly X-ray crystallo-
graphic13–15 studies indicated that [Fe]-hydrogenase contains a
unique and intriguing active site, which consists of an iron
center ligated by a cysteine S atom, two cis-carbonyls, a 2-acyl-
methyl-6-pyridinol ligand, and an as yet unknown ligand,
which is probably a molecule of water (Scheme 2a).
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, and
Encouraged by such a well-elucidated structure, numerous
Hmd model complexes have been synthesized so far.16–28
However, among these reported models none contains the
2-acylmethyl-6-ester group-difunctionalized pyridine ligand,
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Nankai University, Tianjin 300071, China. E-mail: lcsong@nankai.edu.cn
†CCDC 983089–983092. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt00335g
8062 | Dalton Trans., 2014, 43, 8062–8071
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by treatment of the resulting Fe(0) intermediate [Na(2-CH2-6-
MeCO2CH2C5H3N)Fe(CO)4] (M1) with oxidant I2 (via in situ CO
migratory insertion followed by coordination of iodide and the
singly-bonded O atom of the ester group with its Fe(0) center)
to give the difunctionalized pyridine-containing Fe(II) iodide
complex 1 in 40% yield. Finally, in order to obtain a more
natural model, 1 was further treated with sodium 2-mercapto-
pyridinate in MeCN (via nucleophilic substitution of iodide by
2-mercaptopyridinate and subsequent displacement of the
singly-bonded O atom by the mercaptopyridine N atom) to
give the difunctionalized pyridine-containing Fe(II) mercapto-
pyridinate complex 2 in 59% yield. Actually, the suggested
pathway for production of complexes 1 and 2 is very similar to
that previously reported for the formation of acylmethyl/
hydroxymethyl- and acylmethyl/methoxybenzyl-difunctiona-
lized pyridine-containing model complexes.27,31
Scheme 2 (a) Proposed active site of [Fe]-hydrogenase. (b) Two repre-
sentative models reported in this article.
although some of them contain a similar difunctionalized
pyridine ligand, such as Hu’s acylmethyl/methoxy-
difunctionalized,23–25 Pickett’s carbamoyl/amino-difunctiona-
lized,29 Chen’s acylmethyl/hydroxy-difunctionalized,30 or our
acylmethyl/hydroxymethyl-,27 and acylmethyl/methoxybenzyl-31
difunctionalized pyridine ligands. Therefore, to further under-
stand the active site structure of [Fe]-hydrogenase and to see
the influence of the difunctionalized pyridine ligands towards
the structure and properties of such model complexes, we
recently launched a study aimed at synthesizing the hitherto
unknown 2-acylmethyl-6-ester group-difunctionalized pyridine
ligand-containing model complexes. Herein, we report the
studied results regarding the synthesis, structural characteriz-
ation, and some properties of such novel model complexes
(Scheme 2b). In addition, the synthesis and characterization of
two precursor compounds employed for preparing the model
complexes are also described.
In order to prove the suggested pathway starting from pre-
cursor A to model complex 1, we utilized in situ IR spec-
troscopy27,32 to monitor the existence of the highly reactive
intermediate M1 and its in situ conversion under the action of
iodide. As shown in Fig. 1, when A was added to a MeCN solu-
tion of Na2Fe(CO)4, the original νCO absorption bands of
Results and discussion
Synthesis and characterization of 2-acylmethyl-6-
acetoxymethyl-difunctionalized pyridine-containing model
complexes [2-C(O)CH2-6-MeCO2CH2C5H3N][Fe(CO)2I] (1)/
[2-C(O)CH2-6-MeCO2CH2C5H3N][Fe(CO)2(2-SC5H4N)] (2) and
their precursor 2-(p-MeC6H4SO3CH2)-6-MeCO2CH2C5H3N (A)
The synthetic route for preparing precursor A and model com-
plexes 1 and 2 includes three separate steps as shown in
Scheme 3. In the first step precursor A is prepared by acylation
of 2-(p-MeC6H4SO3CH2)-6-HOCH2C5H3N with acetyl chloride
in CH2Cl2 in the presence of ZnO in 57% yield. The second
step involves reaction of A with Na2Fe(CO)4 in MeCN followed
Fig. 1 In situ IR spectra for formation of 1 starting from Na2Fe(CO)4 via
intermediate M1 in MeCN at 0 °C.
Scheme 3
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Na2Fe(CO)4 at 1760 (vs), 1885 (m), and 1916 (w) cm−1 were
gradually diminished, and after ca. 10 min, they were comple-
tely replaced by a very strong band at 1887 cm−1 and a middle
band at 2001 cm−1. This implies that Na2Fe(CO)4 was totally
consumed and the intermediate M1 was formed. The in situ IR
spectroscopy further demonstrated that after addition of I2
(ca. 15 min), the aforementioned two νCO absorption bands
disappeared, and instead two middle bands at 2038 and
1978 cm−1 along with two weak bands at 1750 and 1681 cm−1
emerged. The four new bands can be attributed to the two
cis-arranged terminal carbonyls, one ester carbonyl, and one
acyl ligand for the newly formed complex 1, respectively.
Compounds A, 1, and 2 are air-stable yellow solids, which
have been characterized by elemental analysis and spectro-
scopy. The solid IR spectrum of A showed a very strong absorp-
tion band at 1745 cm−1 for its ester carbonyl, whereas 1 and
2
showed two strong absorption bands in the range
2030–1964 cm−1 for their two cis-arranged terminal carbonyls,
one band at 1769 or 1744 cm−1 for their ester carbonyl groups,
and one band at 1683 or 1656 cm−1 for their acyl groups,
Fig. 2 ORTEP plot of 1 with 30% probability level ellipsoids.
1
respectively. The H NMR spectrum of A displayed a singlet at
2.45 ppm for the CH3 group attached to its benzene ring and a
singlet at 5.13 ppm for the four H atoms in the two methylene
groups attached to its pyridine ring, whereas 1 and 2 displayed
two doublets in the regions 3.90–4.65 and 4.70–5.65 ppm for
their CH2CvO and CH2OCvO groups, respectively, since the
two H atoms in each of the two groups are diastereotopic. The
13C{1H} NMR spectrum of A showed one signal at 169.5 ppm
for its ester carbonyl, whereas 1 and 2 exhibited one signal at
252.2 and 263.2 ppm, respectively, characteristic of their acyl
C atoms attached to their Fe centers.23,27
The molecular structures of 1 and 2 were further confirmed
by X-ray crystal diffraction analysis. Table 1 lists their selected
bond lengths and angles, while their ORTEP views are
depicted in Fig. 2 and 3, respectively.
As shown in Fig. 2 and 3, model complexes 1 and 2 indeed
contain a difunctionalized 2-acylmethyl-6-acetoxymethyl pyr-
Fig. 3 ORTEP plot of 2 with 30% probability level ellipsoids.
Table 1 Selected bond lengths (Å) and angles (°) for 1 and 2
1
Fe(1)–C(3)
Fe(1)–N(1)
Fe(1)–O(4)
Fe(1)–I(1)
1.915(2)
O(3)–C(3)
O(5)–C(11)
N(1)–C(9)
N(1)–C(5)
1.202(2)
1.194(3)
1.344(3)
1.350(3)
1.9517(17)
2.2537(16)
2.6571(9)
idine ligand. While the difunctionalized pyridine ligand of 1
forms two five-membered (Fe(1)–C(3)–C(4)–C(5)–N(1) and
Fe(1)–N(1)–C(9)–C(10)–O(4)) ferrocycles with its iron center,
the difunctionalized ligand of 2 forms only one five-membered
(Fe(1)–C(8)–C(9)–C(10)–N(2)) ferrocycle since the acetoxymethyl
functionality is not coordinated with the Fe(1) center (in 2
there is still another four-membered Fe(1)–S(1)–C(7)–N(1)
ferrocycle formed by coordination of its 2-mercaptopyridinate
ligand with the Fe(1) center). In addition, just like the natural
[Fe]-hydrogenase14,15 and those previously reported [Fe]-hydro-
genase model complexes,7,8 both 1 and 2 contain two terminal
CO ligands that occupy the positions cis to their acylmethyl
ligands.
C(3)–Fe(1)–N(1)
C(3)–Fe(1)–O(4)
N(1)–Fe(1)–O(4)
N(1)–Fe(1)–I(1)
2
Fe(1)–C(8)
Fe(1)–N(2)
85.81(9)
161.41(7)
75.99(7)
89.71(6)
O(4)–Fe(1)–I(1)
O(5)–C(11)–C(12)
C(11)–O(4)–C(10)
C(4)–C(3)–Fe(1)
89.33(5)
126.5(2)
113.32(16)
110.91(15)
1.9272(18)
2.0538(15)
2.0822(16)
2.3699(10)
O(3)–C(8)
O(5)–C(16)
N(2)–C(10)
N(2)–C(14)
1.2137(19)
1.2009(19)
1.258(2)
Fe(1)–N(1)
Fe(1)–S(1)
1.3494(19)
C(8)–Fe(1)–N(2)
C(8)–Fe(1)–N(1)
N(2)–Fe(1)–N(1)
N(1)–Fe(1)–S(1)
83.42(6)
160.98(6)
96.53(5)
69.32(4)
C(8)–Fe(1)–S(1)
N(2)–Fe(1)–S(1)
C(7)–S(1)–Fe(1)
Fe(1)–C(8)–O(3)
91.69(5)
88.14(4)
79.82(6)
130.33(13)
8064 | Dalton Trans., 2014, 43, 8062–8071
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Synthesis and characterization of 2-acylmethyl-6-
for their CH2CvO and CH2OCvO groups since the two H
atoms in the two groups are diastereotopic. The 13C{1H} NMR
spectrum of B showed one singlet at 166.1 ppm for its ester
carbonyl, whereas the 13C{1H} NMR spectra of 3 and 4 each
displayed a signal at 252.3 and 262.8 ppm typical of their acyl
carbon atoms, respectively.
benzoyloxymethyl-difunctionalized pyridine-containing model
complexes [2-C(O)CH2-6-PhCO2CH2C5H3N][Fe(CO)2I] (3)/
[2-C(O)CH2-6-PhCO2CH2C5H3N][Fe(CO)2(2-SC5H4N)] (4) and
their precursor 2-(p-MeC6H4SO3CH2)-6-PhCO2CH2C5H3N (B)
In order to observe the influence of the 6-ester group upon the
structure and properties of 2-acylmethyl-6-ester group-difunc-
tionalized pyridine-containing model complexes, we decided
(after preparation of the aliphatic ester group-containing com-
plexes 1 and 2) to prepare the 6-aromatic ester group-contain-
ing model complexes 3 and 4. Actually, model complexes 3/4
and their precursor B can also be prepared via the synthetic
route similar to that for preparation of their analogues 1/2 and
A, as shown in Scheme 4. In the first step precursor B is pre-
pared by esterification reaction of 2-(p-MeC6H4SO3CH2)-
6-HOCH2C5H3N with benzoic acid under the action of DCC (as
an activator) and DMAP (as a catalyst) in 85% yield. The
second step includes reaction of B with Na2Fe(CO)4 in MeCN
followed by treatment of the resulting Fe(0) complex salt [Na(2-
CH2-6-PhCO2CH2C5H3N)Fe(CO)4] (M2) with iodine, via in situ
CO migratory insertion followed by coordination of iodide and
the doubly-bonded O atom (note that not the singly-bonded
O atom) of the ester group with its Fe(0) center, to afford the
difunctionalized pyridine-containing Fe(II) iodide complex 3 in
38% yield. Finally, the difunctionalized pyridine-containing
Fe(II) mercaptopyridinate complex 4 is produced in 62% yield
by reaction of 3 with sodium 2-mercaptopyridinate through
nucleophilic substitution of the doubly-bonded O atom by the
mercaptopyridine N atom.
The X-ray crystallographic study (Fig. 4 and 5, Table 2) indi-
cated that both 3 and 4 have a difunctionalized 2-acylmethyl-
6-benzoyloxymethyl pyridine ligand. While the difunctiona-
lized pyridine ligand of 3 constructs a five-membered (Fe(1)–
C(3)–C(4)–C(5)–N(1)) ferrocycle and a seven-membered (Fe(1)–
N(1)–C(9)–C(10)–O(4)–C(11)–O(5)) ferrocycle that is generated
by coordination of the doubly-bonded O(5) atom of the 2-acyl-
methyl-6-benzoyloxymethyl ligand, the difunctionalized pyri-
dine ligand of 4 constructs only one five-membered Fe(1)–
C(3)–C(4)–C(5)–N(1) ferrocycle with the Fe(1) center since the
benzoyloxymethyl functionality is like the acetoxymethyl func-
tionality in its analogue 2 not coordinated with the Fe center.
In addition, both 3 and 4 also have two terminal CO ligands
cis to their acylmethyl ligands, whereas complex 4 has an
additional four-membered Fe(1)–S(1)–C(18)–N(2) ferrocycle
constructed by coordination of its 2-mercaptopyridinate ligand
with the Fe(1) center. It follows that the structures of model
complexes 3 and 4 are very similar to those of their analogues
1 and 2 as well as the active site of [Fe]-hydrogenase.14,15
Electrochemical behavior of model complexes 2 and 4
Although the electrochemistry of many [FeFe]-hydrogenase
model complexes has been well-studied,32–36 very few [Fe]-
hydrogenase model complexes have been electrochemically
investigated to date.29,37 To make a comparison with the
electrochemical behavior of the previously reported Hmd
models29,37 and thus to see the influences of the ligands
around their iron centers, we determined the electrochemical
properties of model complexes 2 and 4 by cyclic voltammetric
techniques. The cyclic voltammograms of 2 and 4 are shown
in Fig. 6, whereas Table 3 lists their electrochemical data along
with those of the previously reported model complexes FeBr(2-
acylaminopyridine)(CO)2(PMe3) (C)29 and Fe(3,6-dichloro-1,2-
benzenedithiolate)(CO)2(PMe3)2 (D).37 As shown in Fig. 6 and
Table 3, both 2 and 4 display one irreversible reduction at Epc1
Compounds B, 3, and 4 are also air-stable solids. The solid
IR spectrum of B displayed a very strong absorption band at
1722 cm−1 for its ester carbonyl, whereas 3 and 4 exhibited two
very strong absorption bands in the range 2036–1964 cm−1 for
their two terminal carbonyls, one band at 1655 or 1657 cm−1
for their acyl groups, and one band at 1683 or 1723 cm−1 for
their ester carbonyl groups, respectively. The 1H NMR spec-
trum of B exhibited a singlet at 2.43 ppm assigned to its CH3
group and two singlets at 5.15 and 5.40 ppm assigned to its
1
CH2OSO2 and CH2OCvO groups, respectively, whereas the H
NMR spectra of 3 and 4 displayed two doublets in the range
3.90–4.65 ppm and two doublets in the range 4.90–5.85 ppm
Scheme 4
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Fig. 6 Cyclic voltammograms of 2 and 4 (1.0 mM) in 0.1 M n-Bu4NPF6–
Fig. 4 ORTEP plot of 3 with 30% probability level ellipsoids.
MeCN at a scan rate of 0.1 V s−1
.
Table 3 Electrochemical data of 2, 4, C, and Da
Epc1/V
Epc2/V
Epa/V
2
4
C
D
−1.86
−1.85
−1.87
−2.15
—
—
—
−2.34
+0.60
+0.68
+0.75
+0.10
a All potentials versus Fc/Fc+.
= −1.86/−1.85 V and one irreversible oxidation at Epa
=
+0.60/+0.68 V versus Fc/Fc+, respectively. In addition, as shown
in Table 3, the electrochemical behavior of complexes 2 and 4
is very similar to that of the previously reported complex C. For
example, they all display one irreversible reduction and one
irreversible oxidation at their respective potentials in close
proximity due to their structural similarity. However, in con-
trast to this, the electrochemical behavior of 2 and 4 is con-
siderably different from that of D. For example, complex D
exhibits two irreversible reductions and one reversible oxi-
dation, and particularly, the two peak potentials Epc1/Epa are
greatly shifted negatively to ca. 290–580 mV relative to the
corresponding two potentials of 2 and 4. Apparently, such
remarkably different electrochemical behavior between 2/4
and D is caused by their quite different structures in which the
two strong electron-donating PMe3 ligands present in D might
play a key role in the greatly negative shift of its two redox
potentials when compared to those of 2 and 4.
In order to observe if complexes 2 and 4 could be able to
catalyze proton reduction to H2, we further determined their
cyclic voltammograms in the presence of trifluoroacetic acid
(TFA). As shown in Fig. 7 and 8 (for comparison, their cyclic
voltammograms without TFA are also included), when TFA was
added, the current intensity of the original reduction peaks of
2 and 4 increased obviously and continued to increase con-
siderably with addition of the acid. Such a remarkable increase
of the acid-induced currents could be ascribed to the electro-
catalytic proton reduction processes.34,36
Fig. 5 ORTEP plot of 4 with 30% probability level ellipsoids.
Table 2 Selected bond lengths (Å) and angles (°) for 3 and 4
3
Fe(1)–I(1)
Fe(1)–C(3)
Fe(1)–N(1)
Fe(1)–O(5)
2.6749(8)
1.914(2)
2.0173(15)
2.1589(14)
O(3)–C(3)
O(4)–C(11)
O(5)–C(11)
N(1)–C(5)
1.211(2)
1.340(2)
1.216(2)
1.350(2)
C(3)–Fe(1)–N(1)
C(3)–Fe(1)–O(5)
N(1)–Fe(1)–O(5)
C(3)–Fe(1)–I(1)
4
N(1)–C(9)
Fe(1)–N(2)
85.18(7)
171.67(7)
93.58(6)
85.71(6)
N(1)–Fe(1)–I(1)
O(5)–Fe(1)–I(1)
C(11)–O(4)–C(10)
C(11)–O(5)–Fe(1)
92.29(4)
86.11(4)
115.89(15)
138.35(12)
1.365(5)
2.084(3)
2.079(3)
2.3823(11)
O(5)–C(11)
N(1)–C(5)
N(2)–C(18)
N(2)–C(22)
1.178(5)
1.351(4)
1.350(5)
1.350(5)
Fe(1)–N(1)
Fe(1)–S(1)
C(3)–Fe(1)–N(2)
C(3)–Fe(1)–N(1)
N(2)–Fe(1)–N(1)
N(1)–Fe(1)–S(1)
161.65(13)
82.88(14)
97.95(11)
89.34(9)
C(3)–Fe(1)–S(1)
N(2)–Fe(1)–S(1)
C(18)–S(1)–Fe(1)
Fe(1)–C(3)–O(3)
92.47(10)
69.24(8)
79.56(12)
130.7(3)
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that when the Ph3C+ cation as its Ph3C+BF4 salt was added to
the NMR tube containing an acetone-d6 solution of 2 or 4, the
original 1H NMR spectra of 2 and 4 were greatly changed. This
means that both 2 and 4 were seriously decomposed in the
presence of the strong Lewis acid Ph3C+ cation.38 So, we had to
use the weaker boron-based Lewis acid Ph3B39 to test whether
models 2 and 4 could activate H2 or not. For this purpose, the
sample solutions were prepared by addition of Ph3B to the
NMR tube containing an acetone-d6 solution of 2 or 4 followed
by purging the solutions with H2 and D2. As shown in Fig. 9
and 10, the 1H NMR spectra of the two sample solutions all
displayed one singlet at 4.54 ppm for H2,40 four doublets in
the region 3.93–5.80 ppm for the diastereotopic methylene
hydrogen atoms in CH2CvO and CH2OCvO groups of
complex 2 or 4 and the multiplets in the range 6.77–8.46 ppm
attributed to the hydrogen atoms of the pyridine and benzene
rings present in 2, 4, and Ph3B. In addition, the sample solu-
tion containing complex 2 showed a singlet at 2.07 ppm for its
CH3 group. Apparently, these observations demonstrate that
complexes 2 and 4 cannot heterolytically activate H2 and D2 in
−
Fig. 7 Cyclic voltammograms of 2 (1.0 mM) with TFA (0–20 mM) in
0.1 M n-Bu4NPF6–MeCN at a scan rate of 0.1 V s−1 (reverse scans were
partly truncated for clarity).
Fig. 8 Cyclic voltammograms of 4 (1.0 mM) with TFA (0–20 mM) in
0.1 M n-Bu4NPF6–MeCN at a scan rate of 0.1 V s−1 (reverse scans were
partly truncated for clarity).
Fig. 9 (a) Original 1H NMR spectrum of the sample solution consisting
of 2, Ph3B, H2, and D2 in acetone-d6. (b) The expanded 1H NMR spec-
trum in the range 3.75–5.50 ppm.
The electrocatalytic proton reduction catalyzed by 2 and 4
was further confirmed by a combination of controlled poten-
tial electrolysis and GC analysis. When a MeCN solution of 2
or 4 (0.5 mM) was electrolyzed with a large excess of TFA
(15 mM) at −2.0 V for 0.5 h, a total of 20.6 F mol−1 of 2 and
29.0 F mol−1 of 4 passed, which correspond to 10.3 and 14.5
turnovers (note that the turnover for the controlled potential
electrolysis of TFA catalyzed by complex D at −1.7 V for 0.5 h
was reported to be about 8).37 Gas chromatographic analysis
showed that the yields of H2 evolved in the cases catalyzed by 2
and 4 are all about 95%.
Attempted H2 activation by model complexes 2 and 4 in the
presence of Lewis acid Ph3B or Ph3C+ cations
As the H2 activation by [Fe]-hydrogenase occurs in the presence
of the hydride acceptor (methenyl-H4MPT+),3,7,8 we attempted
to activate H2 by our model complexes 2 and 4 in the presence
Fig. 10 (a) Original 1H NMR spectrum of the sample solution consisting
of 4, Ph3B, H2, and D2 in acetone-d6. (b) The expanded 1H NMR spec-
of Lewis acid Ph3B or carbocation Ph3C+. Initially, we found trum in the range 3.75–6.00 ppm.
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1
the presence of Lewis acid Ph3B, since no H NMR signal for spectra were recorded on a Bruker Avance 400 NMR spectro-
H–D was detected (note that the H–D signal is a triplet at meter. Elemental analyses were performed on an Elementar
4.57 ppm with J = 42.51 Hz).40,41
Vario EL analyzer. Melting points were determined on
SGW X-4 microscopic melting point apparatus and were
uncorrected.
Conclusions
Preparation of 2-(p-MeC6H4SO3CH2)-6-MeCO2CH2C5H3N (A)
We have synthesized the first [Fe]-hydrogenase model com- Acetyl chloride (1.40 mL, 20.00 mmol) was added dropwise to
plexes 1–4 each with a 2-acylmethyl-6-ester group-difunctiona- a stirred pink mixture consisting of 2-(p-MeC6H4SO3CH2)-
lized pyridine ligand. While 1 and 2 contain a 2-acylmethyl-6- 6-HOCH2C5H3N (2.800 g, 9.50 mmol) and ZnO (0.406 g,
acetoxymethyl-difunctionalized pyridine ligand,
3
and
4
5.00 mmol) in CH2Cl2 (100 mL). The new mixture was stirred
contain 2-acylmethyl-6-benzoyloxymethyl-difunctionalized at room temperature for 3 h to give a white suspension. The
a
pyridine ligand. The synthetic route to model complexes 1–4 suspension was washed with a saturated aqueous solution of
include three separate steps: (i) preparation of new precursor sodium bicarbonate (20 mL) and water (2 × 10 mL) succes-
compounds A and B by reaction of 2-(p-MeC6H4SO3CH2)-6- sively. After the separated organic phase was dried over anhy-
HOCH2C5H3N with MeCOCl in the presence of ZnO, or with drous magnesium sulfate, the solvent was removed at reduced
PhCO2H in the presence of DCC and DMAP; (ii) preparation of pressure to leave a residue, which was subjected to TLC separ-
complexes 1 and 3 via reaction of A or B with Na2Fe(CO)4 fol- ation using CH2Cl2–ethyl acetate (2 : 1, v/v) as an eluent. From
lowed by treatment of intermediate M1 or M2 with oxidant I2; the main band, A (1.820 g, 57%) was obtained as a pink solid,
and (iii) preparation of complexes 2 and 4 by reaction of 1 or 3 mp 54–55 °C. Anal. Calcd for C16H17NO5S: C, 57.30; H, 5.11; N,
with sodium 2-mercaptopyridinate. The X-ray crystallographic 4.18. Found: C, 57. 07; H, 5.24; N, 4.21. IR (KBr disk): νCvO
study reveals that model complexes 1–4 have some common 1745 (vs) cm−1 1H NMR (400 MHz, CDCl3): 2.15 (s, 3H,
.
structural features with the active site of [Fe]-hydrogenase, CH3CO2), 2.45 (s, 3H, CH3C6H4), 5.13 (s, 4H, 2CH2), 7.27–7.39
such as (i) they all contain a 2-acylmethyl-6-functionalized pyri- (m, 4H, C6H4), 7.72 (t, J = 7.8 Hz, 1H, 4-H of C5H3N), 7.83 (d,
dine ligand; (ii) all the 2,6-difunctionalized pyridine ligands in J = 7.6 Hz, 2H, 3,5-H of C5H3N) ppm. 13C{1H} NMR (100 MHz,
1–4 form a five-membered ferrocycle with their single Fe(II) CDCl3): 19.9 (CH3CO2), 20.6 (CH3C6H4), 65.4 (CH2OSO2), 70.5
centers; and (iii) they all have two terminal CO ligands that are (CH2OCvO), 119.8, 120.3, 127.1, 128.9, 131.6, 136.8, 144.1,
cis to their acyl ligands. Actually, model complexes 2 and 4 are 152.6, 154.4 (C6H4, C5H3N), 169.5 (CH2OCvO) ppm.
structurally more like the active site of [Fe]-hydrogenase than 1
Preparation of 2-(p-MeC6H4SO3CH2)-6-PhCO2CH2C5H3N (B)
and 3 since they have a sulfur atom located at the fifth coordi-
nation site of their Fe(II) centers. It is worth noting that the
A
pink mixture consisting of 2-(p-MeC6H4SO3CH2)-
reaction pathway from precursor A to model complex 1 via the 6-HOCH2C5H3N (1.732 g, 6.00 mmol), benzoic acid (1.465 g,
highly reactive intermediate M1 is monitored and proved by 12.00 mmol), DCC (1.800 g, 8.00 mmol), and DMAP (0.072 g,
in situ IR spectroscopy. In addition, the 1H NMR spectroscopic 0.60 mmol) in CH2Cl2 (10 mL) was stirred at room temperature
study indicates that complexes 2 and 4 cannot activate H2 and for 4 h to give a white suspension. After the same workup as
D2 in the presence of Lewis acid Ph3B, whereas complexes 2 that for A, from the main band B (2.040 g, 85%) was obtained
and 4 have been found to be catalysts for proton reduction to as a white solid, mp 64–65 °C. Anal. Calcd for C21H19NO5S: C,
hydrogen in the presence of TFA under CV conditions.
63.46; H, 4.82; N, 3.52. Found: C, 63.53; H, 4.64; N, 3.69. IR
(KBr disk): νCvO 1722 (vs) cm−1 1H NMR (400 MHz, CDCl3):
.
2.43 (s, 3H, CH3), 5.15 (s, 2H, CH2OSO2), 5.40 (s, 2H,
CH2OCvO), 7.33 (d, J = 7.6 Hz, 2H, 3,5-H of C6H4), 7.38 (d, J =
8.0 Hz, 2H, 2,6-H of C6H4), 7.46 (t, J = 7.6 Hz, 2H, 3,5-H of
C6H5), 7.59 (t, J = 7.4 Hz, 1H, 4-H of C6H5), 7.73 (t, J = 7.8 Hz,
Experimental section
General comments
All reactions were carried out using standard Schlenk and 1H, 4-H of C5H3N), 7.83 (d, J = 8.0 Hz, 2H, 3,5-H of C5H3N),
vacuum-line techniques under an atmosphere of highly puri- 8.09 (d, J = 7.6 Hz, 2H, 2,6-H of C6H5) ppm. 13C{1H} NMR
fied nitrogen. Acetonitrile and dichloromethane were purified (100 MHz, CDCl3): 21.7 (CH3), 66.8 (CH2OSO2), 71.6
by distillation over CaH2 prior to use. N,N′-Dicyclohexylcarbo- (CH2OCvO), 120.9, 121.2, 128.1, 128.5, 129.8, 130.0, 132.8,
diimide (DCC) and p-dimethylaminopyridine (DMAP), 2-mercapto- 133.3, 137.9, 145.1, 153.6, 155.8 (C6H5, C6H4, C5H3N), 166.1
pyridine, Ph3B, H2, D2, and some other materials were (CH2OCvO) ppm.
available commercially and used as received. Na2Fe(CO)4·(1,4-
42
Preparation of [2-C(O)CH2-6-MeCO2CH2C5H3N]Fe(CO)2I (1)
A suspension of Na2Fe(CO)4·(1,4-dioxane)1.5 (173 mg,
dioxane)1.5
,
2-(p-MeC6H4SO3CH2)-6-HOCH2C5H3N43 and
− 44
Ph3C+BF4
were prepared according to the published pro-
cedures. Solid IR spectra were recorded on a Bruker Tenser 27 0.5 mmol) in MeCN (15 mL) was cooled to 0 °C, and then
FT-IR spectrophotometer, whereas in situ IR spectra were 2-(p-MeC6H4SO3CH2)-6-MeCO2CH2C5H3N (A) (167 mg,
recorded on a ReactIR iC10 from Mettler-Toledo AutoChem 0.5 mmol) was added. After the mixture was stirred at this
fitted with a silicon-tipped (SiComp) probe. 1H (13C) NMR temperature for 1 h, I2 (127 mg, 0.5 mmol) was added, and
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then the new mixture was stirred at 0 °C for an additional 1 h Preparation of [2-C(O)CH2-6-PhCO2CH2C5H3N]Fe(CO)2-
to give a brown solution. After volatiles were removed at (2-C5H4NS) (4)
reduced pressure, the residue was subjected to column
It was prepared by the same procedure as that for complex 2,
chromatography (silica gel). Elution with CH2Cl2–acetone
but complex [2-C(O)CH2-6-PhCO2CH2C5H3N]Fe(CO)2I (3)
(40 : 1, v/v) developed a major yellow band, from which 1
(200 mg, 0.4 mmol) was used instead of 1. From the main
(86 mg, 40%) was obtained as a yellow solid, mp 121 °C (dec).
band complex 4 (118 mg, 62%) was obtained as a brown yellow
solid, mp 105–106 °C. Anal. Calcd for C22H16FeN2O5S: C,
55.48; H, 3.39; N, 5.88. Found: C, 55.25; H, 3.38; N, 6.03. IR
Anal. Calcd for C12H10FeINO5: C, 33.44; H, 2.34; N, 3.25.
Found: C, 33.61; H, 2.51; N, 3.13. IR (KBr disk): νCuO 2030 (vs),
1966 (vs); νCvO 1769 (s), 1683 (s) cm−1 1H NMR (400 MHz,
.
(KBr disk): νCuO 2028 (s), 1964 (s); νCvO 1723 (s), 1657 (s)
cm−1. 1H NMR (400 MHz, acetone-d6): 3.96, 4.61 (dd, J = 20 Hz,
2H, CH2CvO), 4.97, 5.80 (dd, J = 14 Hz, 2H, CH2OCvO), 6.77
(d, J = 8.4 Hz, 1H, 3-H of C5H4N), 6.96 (t, J = 6.0 Hz, 1H, 5-H of
C5H4N), 7.41 (t, J = 7.6 Hz, 1H, 4-H of C5H4N), 7.52–7.71
(m, 5H, C6H5), 8.02–8.06 (m, 3H, 3,4,5-H of C5H3N), 8.46 (d, J =
4.4 Hz, 1H, 6-H of C5H4N) ppm. 13C{1H} NMR (100 MHz,
acetone-d6): 63.7 (CH2CvO), 67.9 (CH2OCvO), 118.9, 122.6,
123.4, 125.9, 129.5, 130.5, 134.4, 137.0, 139.7, 152.6, 159.9,
163.9, 166.3, 169.4 (C5H3N, C5H4N), 177.8 (CH2OCvO), 209.7,
214.4 (CuO), 262.8 (CH2CvO) ppm.
acetone-d6): 2.39 (s, 3H, CH3), 4.49, 4.63 (dd, AB system, J =
20.4 Hz, 2H, CH2CvO), 5.55, 5.62 (dd, AB system, J = 14.4 Hz,
2H, CH2OCvO), 7.74 (br s, 2H, 3,5-H of C5H3N), 8.14 (br s,
1H, 4-H of C5H3N) ppm. 13C{1H} NMR (100 MHz, acetone-d6):
22.9 (CH3), 64.5 (CH2CvO), 69.8 (CH2OCvO), 122.5, 123.2,
141.2, 156.2, 161.2 (C5H3N), 171.1 (CH2OCvO), 210.4, 213.1
(CuO), 252.2 (CH2CvO) ppm.
Preparation of [2-C(O)CH2-6-MeCO2CH2C5H3N]Fe(CO)2-
(2-SC5H4N) (2)
To a solution of complex [2-C(O)CH2-6-MeCO2CH2C5H3N]Fe-
(CO)2I (1) (172 mg, 0.40 mmol) in MeCN (20 mL) was added
2-NaSC5H4N (53 mg, 0.40 mmol). The mixture was stirred at
room temperature for 2 h to give a brown solution. After vola-
tiles were removed at reduced pressure, and the residue was
subjected to column chromatography (silica gel). Elution with
CH2Cl2–acetone (20 : 1, v/v) developed a major yellow band,
from which 2 (97 mg, 59%) was obtained as a yellow solid, mp
130–132 °C. Anal. Calcd for C17H14FeN2O5S: C, 49.29; H, 3.41;
N, 6.76. Found: C, 49.25; H, 3.34; N, 6.56. IR (KBr disk): νCuO
Attempted H2 activation by model complex 2 in the presence
of Ph3B
A yellow solution of complex 2 (10 mg, 0.024 mmol) and Ph3B
(5.8 mg, 0.024 mmol) in acetone-d6 (0.50 mL) was added into
an NMR tube using a syringe and then a mixture of H2 and D2
in ca. 1 : 1 molar ratio was sparged through the solution for
3 min to give a sample solution, which was analyzed immedi-
ately by 1H NMR spectroscopy. 1H NMR (400 MHz, acetone-d6):
2.07 (s, 3H, CH3), 3.93, 4.58 (dd, J = 19.6 Hz, 2H, CH2CvO),
4.54 (s, H2), 4.73, 5.41 (dd, J = 13.6 Hz, 2H, CH2OCvO), 6.77
(d, J = 8.0 Hz, 1H, 3-H of C5H4N), 6.99 (t, J = 6.4 Hz, 1H, 5-H of
C5H4N), 7.35 (t, J = 7.2 Hz, 2H, (C6H5)3B), 7.41–7.49 (m, 8H,
(C6H5)3B), 7.55 (d, J = 7.6 Hz, 1H, 4-H of C5H4N), 7.62–7.67 (m,
2H, 1H of (C6H5)3B, 3-H of C5H3N), 7.79 (d, J = 6.8 Hz, 4H,
(C6H5)3B), 7.88 (d, J = 6.8 Hz, 1H, 5-H of C5H3N), 8.00 (t, J =
7.6 Hz, 1H, 4-H of C5H3N), 8.35 (d, J = 5.2 Hz, 1H, 6-H of
C5H4N) ppm.
2027 (s), 1964 (s); νCvO 1744 (m), 1656 (s) cm−1 1H NMR
.
(400 MHz, acetone-d6): 2.07 (s, 3H, CH3), 3.93, 4.58 (dd, J =
20.0 Hz, 2H, CH2CvO), 4.73, 5.41 (dd, J = 13.6 Hz, 2H,
CH2OCvO), 6.77 (d, J = 8.0 Hz, 1H, 3-H of C5H4N), 6.99 (t, J =
6.4 Hz, 1H, 5-H of C5H4N), 7.48 (t, J = 7.8 Hz, 1H, 4-H of
C5H4N), 7.63 (d, J = 7.6 Hz, 1H, 3-H of C5H3N), 7.55 (d, J =
8.0 Hz, 1H, 5-H of C5H3N), 8.00 (t, J = 7.6 Hz, 1H, 4-H of
C5H3N), 8.35 (d, J = 4.8 Hz, 1H, 6-H of C5H4N) ppm. 13C{1H}
NMR (100 MHz, acetone-d6): 20.7 (CH3), 63.8 (CH2CvO), 67.2
(CH2OCvO), 118.9, 122.6, 123.6, 125.9, 137.0, 139.6, 152.5,
159.8, 163.9, 170.6 (C5H3N, C5H4N), 177.7 (CH2OCvO), 209.8
(CuO), 214.42 (CuO), 263.2 (CH2CvO) ppm.
Attempted H2 activation by model complex 4 in the presence
of Ph3B
Preparation of [2-C(O)CH2-6-PhCO2CH2C5H3N]Fe(CO)2I (3)
The same procedure was followed as that described above,
It was prepared by the same procedure as complex 1, except except that complex 4 (10 mg, 0.021 mmol) and Ph3B (5.0 mg,
that was replaced by 2-(p-MeC6H4SO3CH2)-6- 0.021 mmol) were utilized instead of the corresponding
A
PhCO2CH2C5H3N (B) (198 mg, 0.50 mmol). 3 (94 mg, 38%) was materials. The 1H NMR data for this sample solution are as
obtained as a yellow solid, mp 136 °C (dec). Anal. Calcd for follows. 1H NMR (400 MHz, acetone-d6): 3.96, 4.62 (dd, J =
C17H12FeINO5: C, 41.25; H, 2.85; N, 2.83. Found: C, 41.44; H, 19.6 Hz, 2H, CH2CvO), 4.54 (s, H2), 4.97, 5.80 (dd, J = 14.0 Hz,
2.96; N, 3.01. IR (KBr disk): νCuO 2036 (vs), 1969 (vs); νCvO 2H, CH2OCvO), 6.77–6.82 (m, 2H, 3-H of C5H4N, 1H of
1
1683 (m), 1655 (s) cm−1. H NMR (400 MHz, acetone-d6): 4.35, (C6H5)3B), 6.96 (t, J = 6.0 Hz, 1H, 5-H of C5H4N), 7.14–7.20 (m,
4.63 (dd, AB system, J = 20.8 Hz, 2H, CH2CvO), 5.69, 5.85 (dd, 1H, 4-H of C5H4N), 7.35–7.56 (m, 10H, 2H of C6H5, 8H of
AB system, J = 13.6 Hz, 2H, CH2OCvO), 7.58–8.19 (m, 8H, (C6H5)3B), 7.65–7.71 (m, 4H, 2H of C6H5, 2H of (C6H5)3B), 7.80
C6H5, C5H3N) ppm. 13C{1H} NMR (100 MHz, acetone-d6): 62.5 (d, J = 6.8 Hz, 3H, 3H of (C6H5)3B), 7.88 (d, J = 7.2 Hz, 1H, 3-H
(CH2CvO), 67.3 (CH2OCvO), 122.3, 123.9, 128.8, 129.2, 130.3, of C5H3N), 8.02–8.09 (m, 3H, 1H of C6H5, 5-H of C5H3N, 1H of
134.4, 139.6, 155.3, 161.5 (C6H5, C5H3N), 170.7 (CH2OCvO), (C6H5)3B), 8.26 (t, J = 7.2 Hz, 1H, 4-H of C5H3N), 8.46 (d, J =
209.6, 211.2 (CuO), 252.3 (CH2CvO) ppm.
4.4 Hz, 1H, 6-H of C5H4N) ppm.
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Table 4 Crystal data and structure refinement details for 1–4
1
2
3
4
Formula
Mw
T (K)
C
12H10FeINO5
C17H14FeN2O5S
414.21
113(2)
C17H12FeINO5
493.03
113(2)
C22H16FeN2O5S
476.28
113(2)
430.96
113(2)
Cryst. syst.
Space group
a (Å)
b (Å)
c (Å)
α (°)
Monoclinic
C2/c
50.70(2)
7.723(4)
14.397(7)
90
Monoclinic
P2(1)/n
8.404(4)
20.146(9)
10.462(5)
90
Monoclinic
P2(1)/n
12.505(5)
8.365(3)
17.836(7)
90
Monoclinic
P2(1)/c
8.9860(18)
18.680(4)
12.028(2)
90
β (°)
92.112(7)
90
5633(5)
16
105.231(9)
90
1708.9(15)
4
109.982(6)
90
1753.4(11)
4
90.99(3)
90
2018.7(7)
4
γ (°)
V (Å3)
Z
Crystal size (mm)
0.20 × 0.18 × 0.12
2.033
3.280
0.20 × 0.12 × 0.10
1.610
1.036
0.20 × 0.18 × 0.12
1.868
2.647
0.20 × 0.18 × 0.12
1.567
0.889
Dc (g cm−3
)
μ (mm−1
F(000)
)
3328
848
960
976
Reflns collected
Reflns unique
θmin/max (°)
Final R
20 808
6684
2.41/27.90
0.0204
0.0462
0.992
17 703
4047
2.02/27.90
0.0270
0.0738
1.041
0.339/−0.449
16 626
4173
1.74/27.90
0.0189
0.0373
0.995
0.369/−0.375
16 568
3558
2.01/25.02
0.0519
0.1363
1.112
2.460/−0.645
Final RW
GOF on F2
Δρmax/min (e Å−3
)
0.678/−0.958
X-ray structure determination of 1–4
atography was performed with a Shimadzu gas chromatograph
GC-2014 under isothermal conditions with nitrogen as a
carrier gas and a thermal conductivity detector.
Single-crystals of 1–4 suitable for X-ray diffraction analyses
were grown by slow evaporation of their CH2Cl2–hexane solu-
tions at 0 °C. All the single-crystals were mounted on a Rigaku
MM-007 (rotating anode) diffractometer equipped with a
Saturn 724 CCD. Data were collected at 113 K using a confocal
monochromator with Mo-Kα radiation (λ = 0.71073 Å) in the
ω–ϕ scanning mode. Data collection, reduction, and absorp-
tion correction were performed with the CRYSTALCLEAR
program.45 The structures were solved by direct methods using
the SHELXS-97 program46 and refined by full-matrix least-
squares techniques (SHELXL-97)47 on F2. Hydrogen atoms
were located by using the geometric method. Details of crystal
data, data collections, and structure refinements are summar-
ized in Table 4.
Acknowledgements
We are grateful to the National Basic Research Program of
China (2014CB845604) and the National Natural Science Foun-
dation of China (21132001, 21272122) for financial support.
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