Synthesis and characterisation of three diiron tetracarbonyl complexes related to the diiron centre of [FeFe]-hydrogenase and their protonating, electrochemical investigations

Article abstract of DOI:10.1039/b903758f

The synthesis, characterisation of three diiron tetracarbonyl complexes, [Fe₂(SCH₂)₂C(Me)(CH₂OR)(PNP)(CO) ₄] (R = H and PNP = L¹: 2; R = H and PNP = L²: 3; R = Ts and PNP = L¹: 4) as models for the diiron sub-unit of [FeFe]-hydrogenase are described, where OTs, L¹ and L² are toluenesulfonate, (Ph₂P)₂NCH₂(2-C ₅H₄N) and (Ph₂P)₂NCH₂Ph, respectively. These complexes are fully characterised and the structure of complex 4 is crystallographically determined. Protonation of these complexes with HBF₄·Et₂O is probed by using infrared and NMR spectroscopies which reveals that no hydride can be formed upon addition of the acid. Instead addition of excess of the acid leads to protonating the N atom of the PNP skeleton, which is a weak base due to participating conjugating interactions with the Fe-Fe centre, as revealed by crystallographic analysis. Electrochemistry of these complexes and their electrocatalytic reduction of protons are also investigated. Our results suggest that the existence of the pendant pyridine group can lower the overpotential for proton reduction but does not seem to enhance electrocatalytic efficiency in our case.

Full text of DOI:10.1039/b903758f

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and characterisation of three diiron tetracarbonyl complexes
related to the diiron centre of [FeFe]-hydrogenase and their protonating,
electrochemical investigationsw
Yanwei Wang,a Zhimei Li,a Xianghua Zeng,a Xiufeng Wang,a Caixia Zhan,a
Yinqiu Liu,b Xirui Zeng,b Qiuyan Luob and Xiaoming Liu*a
Received (in Victoria, Australia) 24th February 2009, Accepted 28th May 2009
First published as an Advance Article on the web 10th July 2009
DOI: 10.1039/b903758f
The synthesis, characterisation of three diiron tetracarbonyl complexes,
[Fe2(SCH2)2C(Me)(CH2OR)(PNP)(CO)4] (R = H and PNP = L1: 2; R = H and
PNP = L2: 3; R = Ts and PNP = L1: 4) as models for the diiron sub-unit of
[FeFe]-hydrogenase are described, where OTs, L1 and L2 are toluenesulfonate,
(Ph2P)2NCH2(2-C5H4N) and (Ph2P)2NCH2Ph, respectively. These complexes are fully
characterised and the structure of complex 4 is crystallographically determined. Protonation of
these complexes with HBF4ꢀEt2O is probed by using infrared and NMR spectroscopies which
reveals that no hydride can be formed upon addition of the acid. Instead addition of excess of the
acid leads to protonating the N atom of the PNP skeleton, which is a weak base due to
participating conjugating interactions with the Fe–Fe centre, as revealed by crystallographic
analysis. Electrochemistry of these complexes and their electrocatalytic reduction of protons are
also investigated. Our results suggest that the existence of the pendant pyridine group can lower
the overpotential for proton reduction but does not seem to enhance electrocatalytic efficiency in
our case.
It is interesting to note that in modelling the H-cluster of the
Introduction
enzyme and its diiron sub-unit, the potential role of some amino
Since the detailed structure of [FeFe]-hydrogenase was revealed a
acid residues within the non-coordinating sphere, which may
have a similar role to play in proton transfer,24 have attracted
decade ago, this enzyme and its related chemistry have been under
intense investigations due to the fact that this metallo-
much less attention compared to modelling the central atom of
enzyme catalyses reversibly and rapidly hydrogen oxidation and
the bridgehead of the diiron sub-unit. A few systems bearing a
evolution with high preference to the latter reaction.1,2 In the
pendant basic group have been reported to date.25,26 Even rarer
diiron sub-unit of the H-cluster of the enzyme, the two iron atoms
are model complexes with such a base showing hemi-labile
coordination to the Fe–Fe centre.17 Recent reports by Wang,
of low oxidation state are bridged by a non-protein three-atom
dithiolate ligand and are coordinated with 3CO and 2CNꢁ ligands
Sun and Talarmin and their co-workers have shown that there
plus a water or OHꢁ occupying its vacant site in its rest state of the
were dynamic interactions between internal bases and metal
enzyme, Fig. 1(a).3,4 A very recent report also suggested that this
hydride of its host model complex.27,28
non-protein bridging linkage could be dithiomethyl ether
In modelling the diiron centre, both monodentate
(ꢁSCH2OCH2Sꢁ).5 To mimic the possible aza-containing nature,
phosphine ligands (PMe3, PPh3, P(OEt)3 PMe2Ph)29–34 or
bidentate ligands (dppm, dppe, dcpm, dppb)35–42 have widely
Fig. 1(a), Rauchfuss and co-workers published their ‘‘click’’
chemistry, synthesis of an aza-containing model complex.6 Since
been used as replacement of the cyanide. These phosphine-
then, complexes of this type have been intensely studied.7–10
based ligands not only more or less provide the needed
electron density as does CNꢁ in the enzyme, but also serve
These synthetic analogues as well as other artificial systems
enabled to replicate some key structural features of the metal
centre of the enzyme,2,11–23 which shed light on understanding
mechanistically the enzymatic catalysis.
as scaffolds to introduce internal bases. Among the bidentate
a Department of Chemistry/Institute for Advanced Study,
Nanchang University, Nanchang 330031, China.
E-mail: xiaoming.liu@ncu.edu.cn; Fax: +86 (0)791 3969254;
Tel: +86 (0)791 3969254
b School of Chemistry and Chemical Engineering, Jinggangshan
University, Ji’an 343009, China
w Electronic supplementary information (ESI) available: Spectral and
Fig. 1 (a) The H-cluster of [FeFe]-hydrogenase (X = NH, CH2 or O)
electrochemical details. CCDC reference number 705858 (complex 4).
and (b) attempt to synthesise a model complex possessing Fe(I)–OR
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b903758f
(R = organic moiety) bond for the diiron centre of the enzyme.
ꢂc
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phosphine ligands, PNP and PCP ligands, in which two
phosphorus atoms are linked by either a sp2 hybridised
N atom or a methylene (CH2) group, have a very small bite
angle. These ligands were employed for both mono-43,44 and
diiron complexes36,42 in the catalytic chemistry related to
hydrogen activation or evolution.
Without success in an attempt of synthesising a model complex
(1) possessing a Fe(I)–O(R) (R = organic moiety) bond, Fig. 1(b),
by using a hexacarbonyl diiron complex with an alcoholyl group
reported earlier, [Fe2{(SCH2)2CMe(CH2OH)}(CO)6], 1,17
we turned our attention to incorporate a PNP diphosphine ligand
with a pendant pyridinyl group, L1, (Ph2P)2NCH2(2-C5H4N), into
this complex to examine how the derived complex behaves upon
protonation and how the pendant internal bases affect its
electrocatalytic reduction of protons. For comparison, another
diphosphine ligand, L2, (Ph2P)2NCH2Ph), was also employed. In
this ligand, the pendant pyridinyl group was replaced by a phenyl
group. Substitution reactions of these ligands with complex 1, give
complexes 2, [Fe2{(SCH2)2CMe(CH2OH)}L1(CO)4] and 3,
[Fe2{(SCH2)2CMe(CH2OH)}L2(CO)4], respectively. To look into
how the organic moiety of the bridgehead of the diiron hexa-
carbonyl complex may affect the substitution reactivity by the
PNP ligand (L1) and hence the electrochemistry of the substituted
complex, we also synthesised another diiron hexacarbonyl complex
1Ts, [Fe2{(SCH2)2CMe(CH2OTs)}(CO)6] (Ts = toluenesulfonyl
group). From this complex, complex 4, [Fe2{(SCH2)2CMe-
(CH2OTs)}L1(CO)4], was derived, which is an analogue of
complexes 2 and 3. The synthesis and characterisation of the
three complexes, 2, 3 and 4 are described and their protonating
and electrochemical mechanisms probed by using infrared
spectroscopic, cyclic voltammetric and digital simulating
techniques. Electro-catalysis of the three complexes on proton
reduction was also investigated.
Scheme 2 Synthesis of complexes 2, 3 and 4.
time. The reaction sluggishness is mainly attributed to steric
hindrance from the OTs group. All three complexes show
four characteristic absorption bands of diiron tetracarbonyl
complexes in the region between 1910 and 2000 cmꢁ1 36–38,42
.
Due to the electron-withdrawing nature of both the pyridine
and the OTs groups, the infrared absorption bands for
complexes 2 and 4 shift towards higher frequencies by 2 and
5 cmꢁ1 on average, respectively, compared to that for complex
3. 31P NMR for the three complexes showed a nearly identical
single peak at about 120 ppm due to the rigidity in conformation
and electronic similarity in these complexes. This is in
agreement with other disubstituted diiron tetracarbonyl
complexes.38,42
X-Ray single-crystal diffraction analysis
Among complexes 2, 3 and 4, complex 4 was the most readily
crystallised to produce single crystals suitable for X-ray single
crystal diffraction analysis. In the refinement of the crystal
structure, 59 restraints were used. All hydrogen atoms except
those of water, which was refined with H–O distances at
0.85 A and with Uiso(H) = 1.5Ueq(O), were positioned
geometrically and treated as riding on their parent atoms with
C–H distances of 0.97 A (ethyl), 0.96 A (methyl) and 0.93 A
(aromatic rings) and with Uiso(H) = 1.2Ueq(C) (ethyl and
aromatic rings) and Uiso(H) = 1.5Ueq(C) (methyl). Atoms
C10, C11, C6, C7, C8 and C9 of the phenyl ring of the tosyl
group were refined to have approximately equal anisotropic
displacement parameters. The three adjacent atoms, N2, C22
and C18 of the pyridine ring were refined in the same manner.
The crystallographic data for complex 4 are summarised in
Table 1 and selected bonding parameters are tabulated
in Table 2. The structural view of this complex is shown in
Fig. 2. In the unit cell, some water molecules are also found,
which may be accidentally introduced during crystallisation.
As shown in Fig. 2, due to the bulkiness of the Ts group, it
unsurprisingly points away from the Fe–Fe centre and is
approximately opposite to the PNP moiety. The two phosphorus
atoms coordinate to the metal at a basal–basal position and
form a planar conformation together with the N1 atom due to
the rigidity of the ligand conformation. Three N1-centred
bond angles composed by P1, P2 and C17 are nearly perfectly
equal to 1201. Furthermore, the two P–N1 bond lengths are
about 1.7 A showing that they have partial double bond
character. All these bonding features suggest that the atom
N1 takes an ideal sp2 hybridisation and forms a conjugating
system within the plane composed by {Fe1–Fe2–P1–N1–P2}
Results and discussion
Synthesis of ligands L1, L2 and the diiron tetracarbonyl
complexes 2, 3 and 4
Ligand L1, (Ph2P)2NCH2(2-C5H4N), was prepared at ꢁ78 1C by
using methyl lithium to deprotonate pyridine-2-ylmethanamine.45
Our practice has shown that this ligand and its phenyl
analogue, L2, (Ph2P)2N(CH2Ph), could be prepared under
much milder reaction conditions in high yields (Scheme 1).
Heating a solution of the diiron hexacarbonyl complexes in
dry toluene with a slight excess of the ligand (L1 or L2)
afforded the desired disubstituted complexes, 2, 3 and 4 in
moderate to high yields, Scheme 2. IR monitoring showed that
the reaction was completed in 2 h for complexes 2 and 3.
However, the preparation of complex 4 took a much longer
Scheme 1 Synthesis of the ligands L1 and L2.
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Table 1 Crystallographic details for complex 4
Empirical formula
M
T/K
l/A
Crystal system
Space group
a/A
b/A
c/A
C46H46Fe2N2O9P2S3
1040.70
293(2)
0.71073
Monoclinic
Cc
13.5760(18)
20.7486(18)
17.305(3)
98.192(2)
4824.8(11)
4, 1.433
0.853
2152
b/1
V/A3
Z, Dc/Mg mꢁ3
m/mmꢁ1
F(000)
Crystal size/mm
y range for data collection
Limiting indices, hkl
Reflections collected/unique
Completeness (%) to y = 26.001
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I 4 2s(I)]
R indices (all data)
Absolute structure parameter
Largest diff. peak, hole/e Aꢁ3
0.26 ꢃ 0.16 ꢃ 0.08
1.81–26.00
ꢁ16 to 16, ꢁ25 to 25, ꢁ20 to 21
15 465/8600 (Rint = 0.0706)
98.4
None
Full-matrix least-squares on F2
8600/59/579
0.995
R1 = 0.0644, wR2 = 0.1471
R1 = 0.1194, wR2 = 0.1609
0.05(3)
0.749, ꢁ0.709
Table 2 Selected bond lengths (A) and angles (1) for complex 4
Fe(1)–P(2)
Fe(2)–P(1)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(1)–Fe(2)
2.2029(15)
2.2025(15)
2.2605(15)
2.2878(16)
2.4807(11)
Fe(2)–S(2)
Fe(2)–S(1)
P(1)–N(1)
P(2)–N(1)
2.2493(15)
2.2548(16)
1.744(4)
Fig. 2 Structural view of the model complex 4 (for clarity a stick
1.711(4)
model is adopted and hydrogen atoms are excluded).
about 5 cmꢁ1 (Fig. S2, ESIw) whereas for complex 3, its
infrared spectrum was unchanged (Fig. 3). This shows
evidently that the pyridinyl N atom in complexes 2 and 4 is
more basic than the other N atoms. However, upon addition
of the acid to over 15 equivalents, the infrared absorption
bands of all the three complexes shifted to higher frequencies
compared to parent complexes by about 40 cmꢁ1, Fig. 3 and
Fig. S2 (ESIw). These protonated species are not very stable
since neutralisation with triethylamine restored only partially
S(1)–Fe(1)–S(2)
P(1)–Fe(2)–Fe(1)
P(2)–Fe(1)–Fe(2)
S(1)–Fe(1)–Fe(2)
S(2)–Fe(1)–Fe(2)
S(2)–Fe(2)–S(1)
S(2)–Fe(2)–Fe(1)
S(1)–Fe(2)–Fe(1)
82.48(5)
94.64(5)
98.26(5)
56.56(4)
56.11(4)
83.47(6)
57.60(4)
56.78(4)
N(1)–P(1)–Fe(2)
N(1)–P(2)–Fe(1)
Fe(2)–S(1)–Fe(1)
Fe(2)–S(2)–Fe(1)
C(17)–N(1)–P(2)
C(17)–N(1)–P(1)
P(2)–N(1)–P(1)
114.30(15)
112.29(14)
66.65(5)
66.28(5)
120.2(3)
120.5(3)
119.4(2)
via dp–pp–dp interaction, which renders this N atom weak
basicity (vide infra). These results are in agreement with other
PNP analogues.42 As indicated in Table 2, typical bond
lengths and angles related to the Fe and S atoms are generally
similar to those of diiron tetracarbonyl analogues.36–38,42
However, it is noteworthy that the Fe–Fe bond length in this
PNP containing complex 4 is shorter by 0.04 A compared to
that of the PCP containing analogues,36,38 where C represents
a CH2 group, in spite of the smaller bond angle of 1151 for
+PCP compared to ca. 1201 for +PNP in complex 4. This
bond shortening is attributed to the stronger electronic inter-
action stemming from their conjugating interaction within the
five-membered ring, which does not exist in the PCP analogues.
Protonation of complexes 2, 3 and 4
The protonation reactions of complexes 2, 3 and 4 were
performed in acetonitrile by using HBF4ꢀEt2O. Upon the
addition of one equivalent of the acid, infrared absorption
bands of complexes 2 and 4 shifted towards higher energy by
Fig. 3 Protonation of complex 3 and its restoration upon the
addition of Et3N in acetonitrile.
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their parent complexes as judged by variations in infrared
band intensities before protonation and after neutralisation.
In one occasion, this recovery was over 80% for complex 3,
Fig. 3. It is noteworthy that these protonated species have a
rather similar spectral profile to that of their parent complexes.
Linear correlations are found between the absorption bands of
the protonated and those of their parent complexes, Fig. S3
(ESIw). This indicates that there is no significant change in
geometries upon the protonation when excess of the acid is
used.17
The question is where the proton is located in these instable
species. It has been known that direct protonation of the
Fe–Fe bond to form a hydride causes a ‘‘blue’’ shift for
carbonyl stretching frequencies by about 80 cmꢁ1 for either
diiron pentacarbonyl or tetracarbonyl complexes.17,29,36,37,46
This is about 2-fold higher than the 40 cmꢁ1 observed in this
work. Further, the spectral profile of these species derived
from the three complexes differ significantly from that of those
hydrides which usually comprise two major bands centred at
Fig. 4 Cyclic voltammograms of complexes 2, 3 and 4 at 3.0, 4.0 and
4.0 mM, respectively in 0.5 M [NBu4]BF4/THF. Inset: the oxidation
processes of the three complexes.
Electrochemistry of complexes 2, 3 and 4
about 2020 cmꢁ1 26,29,30,46
NMR spectroscopy did not show
.
The electrochemistry for complexes 2, 3 and 4 is presented in
Fig. 4. As shown in Fig. 4, complexes 2 and 3 exhibit some
similarity in electrochemical profiles and show multiple reductions
and oxidations. Due to the similarity of complexes 2 and 3 in
coordinating ligands, their reduction processes are nearly
superimposable. The presence of the tosyl group in complex
4 affects significantly the electrochemical behaviour. For the
convenience of discussion, the three reduction processes of
these complexes are designated as processes I, II and III as
illustrated in Fig. 4. All related redox peak potentials are
tabulated in Table 3.
any hydride signals even at ꢁ40 1C in the upfield region where
signals for iron hydrides appear usually.17,36,37 All these
experimental observations show that these unstable
protonated species are not metal hydrides. One of the
remaining sites for these protonated species is the N atom of
the PNP skeleton. In fact, the frequency shift (40 cmꢁ1) is
somewhat comparable with that (ca. 18 cmꢁ1) of protonated
aza-containing analogues.8,9,29,47,48 The slightly larger shift for
complexes 2, 3 and 4 is attributed to that the N atom of the
PNP skeleton is closer to the metal centre by one s-bond
distance compared to those of aza-containing analogues.
There was a report that the bridging thiolate in diiron
complexes could be also protonated and caused a huge ‘‘blue’’
Recently, Talarmin, Schollhammer and their co-workers
thoroughly investigated the electrochemistry of an analogous
complex and proposed that their diiron tetracarbonyl complex
followed an EE mechanism coupled with disproportionation
of the monoanion, and the reduction process at the most
negative potential was attributed to the reduction of a daughter
product associated with reductions at more positive
potentials.50 For the two successive reductions described in
this mechanism, the separation of their peak potentials
depends on the thermostability of the monoanion and the
second reduction potential could be more positive than the
first one.51–54 Since the complexes studied in this work possess
structural similarity with those reported,50 it is sensible to
assume that these complexes may follow similarly this electro-
chemical mechanism. To rationalise this assumption, electro-
chemical investigations and digital simulations were
carried out.
shift in CO stretching frequencies up to 150 cmꢁ1 49
;
this
however did not occur for complexes 2, 3 and 4.
For other diiron tetracarbonyl complexes with bidentate
diphosphine ligands of the skeletons P(CH2)nP (where
n = 1, 2 and 4) reported by Hogarth and co-workers, stable
hydrides were hardly observed.36,37 There was only one
exception that two substituents on each phosphorus atom of
the PCP ligand were cyclohexyl group, and this was attributed
to the electron-rich nature of the cyclohexyl group.36 The
general difficulty for hydride formation for the diiron
complexes with either PCP or PNP ligands was attributed to
the following two factors: electron density on the Fe–Fe centre
and the rigidity of the conformation of these ligands.37
However, we believe that a steric effect is the major governing
factor, based on following two arguments, (i) our recent report
showed that diiron pentacarbonyl complexes, whose infrared
absorption bands were at least 50 cmꢁ1 higher than those of
complexes 2, 3 and 4, formed reasonably stable bridging
hydrides,17 and (ii) other disubstituted analogues with two
monodentate phosphine ligands also readily formed bridging
hydrides.26,29,30,46 In the sense of a steric effect, both the
bulkiness of the substituents and the length of the linker
between the two phosphorus atoms determine the ease
of hydride formation. These observations may serve as
supportive evidence of the protonating pathway proposed
recently by both Talarmin and Hogarth.27,35
For process I, there is no doubt for its assignment to the
reduction of the parent complexes. For the reduction II, there
are two possibilities, that is, one is the further reduction of
the monoanion as suggested in the mechanism above,50 and
the other a daughter product generated from the decomposi-
tion of the monoanion. The observation that increasing
scanning rate enhanced process II, Fig. 5, implies that
process II is due to the further reduction of the monoanion.
By switching the working atmosphere from Ar to CO, Fig. 6,
the great suppression of process III shows that it was
associated with chemical reaction(s) of either the monoanion
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Table 3 Redox peak potentials (V) of the diiron complexes 2, 3 and 4 under Ar and CO atmospheres, respectively
Under Ar atmosphere
Reduction
Under CO atmosphere
Reduction
Oxidationa
Oxidation
Oxidationa
Oxidation
0.26, 0.36, 0.69
2
3
4
ꢁ2.70, ꢁ2.34, ꢁ2.25 ꢁ2.09, ꢁ1.83, ꢁ1.70, 1.33, ꢁ1.02
0.24, 0.35, 0.68 ꢁ2.66, ꢁ2.38, ꢁ2.27 ꢁ2.04, ꢁ1.11
ꢁ2.72, ꢁ2.36, ꢁ2.27 ꢁ2.05, ꢁ1.80, ꢁ1.69, ꢁ1.35, ꢁ0.98 0.27, 0.70, 0.98 ꢁ2.64, ꢁ2.31, ꢁ2.21 ꢁ2.04, ꢁ1.30, ꢁ1.16 0.23, 0.73, 1.01
ꢁ2.67, ꢁ2.41, ꢁ2.19 ꢁ2.00, ꢁ1.70, ꢁ1.47
Oxidation processes related to the reduction processes II and III.
0.42, 0.55, 0.83 ꢁ2.61, ꢁ2.29, ꢁ2.12 ꢁ1.98, ꢁ1.09
0.40, 0.53, 0.78
a
and steric causes. The former stems from the electron-
withdrawing nature of the OTs group in the bridgehead which
renders the diiron centre more tolerant towards reduction and
the latter from the bulkiness of the OTs group which may
disfavour a disproportionation reaction of the monoanion.
Although the above discussions suggest that processes I and
II generally follow the disproportionation mechanism,50 what
the origin of process III is and how CO is involved in the
related chemical processes are not clear. To shed some light on
these questions, digital simulations were performed by following
the disproportionation mechanism under both Ar and CO
atmospheres. The simulation was started with the first
reduction and thermodynamic parameters were finely tuned
so that under both Ar and CO atmospheres, there was a
globally satisfactory fit and minimal discrepancy between the
experimental and simulated cyclic voltammograms. Then the
simulation was further performed to include the second
reduction, process II. For complex 3, the comparison between
the experimental and simulated cyclic voltammograms are
shown in Fig. 7 and Fig. 8 under Ar and CO atmospheres,
respectively. For complex 2, the results are shown in Fig. S6
and Fig. S7 (ESIw). The mechanism followed in the
simulations is shown in Scheme 3 and related thermodynamic
parameters are tabulated in Table 4. As shown in Fig. 7 and
Fig. 8, and Fig. S6 and Fig. S7 (ESIw), the proposed
mechanism generally describes the chemical and electro-
chemistry at least for processes I and II. Simulated results at
various scanning rates are shown in Fig. S4 (ESIw).
Fig. 5 Cyclic voltammograms of complexes 2, 3 and 4 at 6.3, 6.9
and 6.8 mM, respectively under CO atmosphere. Inset: the cyclic
voltammogram of complex 2 (3.8 mM) at low temperature (253 K)
under Ar atmosphere.
However, attempts to incorporate process III into the
mechanism by assuming that the reduction was attributed to
either decomposed product P1 or P2 failed to generate
Fig. 6 Plot of the irped-I/ipred-II ratio of complexes 2, 3 and 4 vs. scanning
rate under Ar or CO atmosphere.
or the dianion, or perhaps both, involving CO-loss. Under the
same conditions, process II did not show noticeable change as
shown in Fig. 6 and Fig. S4 (ESIw) for complexes 2 and 3
whereas for complex 4, process II was significantly suppressed,
which suggests that this complex may follow a mechanism
different from that for the other two complexes. Indeed,
among the three complexes, complex 4 has the largest separation
in peak potential between the reductions I and II and thus
probably the most stable monoanion.52 The improvement in
stability of the monoanion can be ascribed to both electronic
Fig. 7 Cyclic voltammograms of complex 3 at various scanning
potential ranges under Ar atmosphere (inset: simulated ones).
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Fig. 4, close examination of process II reveals that it is broader
compared to the analogous peaks of the other two complexes.
Although following a similar mechanism to that for complexes
2 and 3 did not afford a good reproduction under Ar
atmosphere (Fig. S8, inset), the proposed mechanism of
Scheme S1 (ESIw) seems to nicely describe the electrochemistry
of the complex under CO atmosphere, Fig. S8. This indicates
that the proposed mechanism omitted some unknown
process(es) under Ar atmosphere, which probably contributed
to the broadness of process II and were suppressed under CO
atmosphere. Despite the unsatisfactory simulation under Ar
atmosphere, the generated thermodynamic parameters shown
in Scheme S1 (ESIw) indicate that the disproportionation of
the monoanion is slower by one to two orders of magnitude
compared to that for complexes 2 and 3, which is in agreement
with the earlier discussion that the monoanion of this complex
is thermodynamically more stable.
Fig. 8 Cyclic voltammograms of complex 3 at various scanning
potential ranges under CO atmosphere (inset: simulated ones).
In the above simulations, oxidations related to those
reduced species were not included but Fig. 9 and Fig. S5
(ESIw) do show that multiple oxidation processes were mainly
associated with two reductions, II and III, respectively.
Oxidations of the parent complexes are not considered either
in these mechanisms. These oxidation processes are shown in
Fig. 4 (inset) and apparently involve multielectron transfer
followed by chemical reactions as suggested by their significant
increase in current intensity and complexity.
Scheme 3 Proposed electrochemical mechanism for complexes 2 and
3; P1 and P2 are daughter products derived from related chemical
reactions.
Electrocatalysis of proton reduction by complexes 2, 3 and 4
The electrochemical responses of complexes 2, 3 and 4 upon
the addition of the acid HBF4ꢀEt2O are shown in Fig. 10. Since
they possess the same protonable sites, it is not surprising that
complexes 2 and 4 exhibit rather similar cyclic voltammetry in
this acidic medium. As shown in Fig. 10 (top), a new broad
reduction process appears at a potential more positive by over
300 mV compared to process I of their parent complexes. This
newly appeared reduction shows electrocatalytic activity with
a steady increase in peak current with acid addition. We
attribute this process to the protonated form of the two
complexes with a pendant pyridinium group. The effect of
the positive charge of this pyridinium group on the diiron
Table 4 Thermodynamic parameters for the electrochemistry and
related chemical processes of complexes 2 and 3 generated from digital
simulation
Entry EI1/2 ksa
EII
ksa
Keq kfdis Keq kf1 Keq kf2
dis
1
2
1/2
2
3
ꢁ2.14 0.0075 ꢁ2.25 0.0016 0.013 800 60 20 1500 40
ꢁ2.16 0.0035 ꢁ2.31 0.0023 0.003 1200 300 120 1300 100
Intrinsic electron transfer constant.
a
satisfactory results. The major problem encountered was that
the generated reduction peak was far weaker than the experi-
mentally observed peak. CO-suppression effect in the simula-
tions also did not match the experimental observations even
when CO-loss was introduced into the decomposition of the
dianion. This suggests that the species which gives reduction
III is derived from further reactions associated significantly
with CO-loss, probably involving P1 and/or P2. It is hard to
identify the species responsible for process III due to the
complexity. However, it is noteworthy that this species shows
better reversibility at low temperature, Fig. 6 (inset). There
were reports that reduction of diiron carbonyl complexes were
followed by chemical reactions which might give tetrairon
species.55,56 It was also known that polynuclear iron clusters
often exhibited reversible reductions.57,58 Thus, we speculate
that this species is probably a polynuclear product.
For complex 4, as described earlier, the tosyl group has
profound influence on its electrochemical behaviour. This is
particularly significant under Ar atmosphere. As shown in
Fig. 9 Cyclic voltammograms of complexes 2 and 4 (inset) at
different scanning potential ranges under Ar atmosphere, scanning
rate = 0.1 V sꢁ1
.
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Not surprisingly, complex 3 shows a different electro-
chemical response upon acid addition due to its lacking the
protonable pyridine base, Fig. 10 (bottom). The distinct
feature is that a sharp peak appeared at ca. ꢁ2.1 V, which
may be attributed to process B0 (right panel, Scheme 4) since a
species generated in the catalytic reduction of protons could be
reduced at a potential more positive than that of its parent
diiron hexacarbonyl complex.53,55 At higher acid concentra-
tion, a shoulder appearing prior to the main reduction may be
due to absorption rather than genuine reduction related to the
complex. Compared to the other two complexes, the major
difference for this complex is mechanistically that the
reduction of the complex is necessary prior to protonation
since we did not observe formation of either any hydride or
protonated species for the parent complex 3 at low acid
concentration (vide anti).
For the possible pathways, A, B and C for complexes 2 and
4 (left panel), and A0, B0 and C0 for complex 3 (right panel)
shown in Scheme 4, it is a non-trivial task to figure out which
one is dominant and/or whether they all operate or not under
the considered conditions. However, for all the complexes, the
electrocatalytic current against the acid concentration shows a
linear relationship with correlation coefficient 40.999, Fig. 11.
As indicated by the gradients of the three plots, complexes 2
and 3 have rather similar electrocatalytic efficiency, which is
about one third higher than that of complex 4. This suggests
that the pendant pyridine in both complexes 2 and 4 do not
seem to improve the electrocatalytic efficiency of proton
reduction except by lowering the proton reduction over-
potential, Fig. 10 (top).
Fig. 10 Cyclic voltammograms of complexes 4 (4.3 mM) and 2
(4.0 mM) (inset) (top) and 3 (4.2 mM) (bottom) in the presence of
HBF4ꢀEt2O in 0.5 M [NBu4][BF4]–THF at a scan rate of 0.1 V sꢁ1
.
centre drives its reduction to more positive potential.25,26 It is
clear that pre-protonation of the pendant pyridine group of
the complexes is essential for this process to occur.
Conclusions
As suggested by the complicated electrochemistry in acidic
medium, the electrochemical mechanism for complexes 2, 3
and 4 may be more complicated than that reported recently by
other researchers.59 Best and Talarmin and their co-workers
conducted detailed mechanistic studies for electrocatalysis
of proton reduction catalysed by diiron hexacarbonyl
complexes.53,55 In spite of the variation in the bridgehead of
the diiron complexes, the common features are that the
electrocatalysis follows a multipathway to evolve hydrogen.
As discussed in the electrochemistry of these complexes earlier,
the diiron complexes of the moiety {Fe2(CO)6ꢁnLn}
(n = 0, 1 and 2; L = non-carbonyl ligand, for instance,
phosphine) adopt essentially the same electrochemical mechanism
as described in Scheme 3, regardless of the variations in the
number of carbonyl ligands in the diiron core.50,51,53,54 Thus, it
is reasonable to assume that the complexes investigated in this
work may generally follow a similar mechanism as described
for those diiron hexacarbonyl complexes.53,55 As shown in
Fig. 10 (top), no substantial catalysis was observed, which
implies that the protonated species [HX]+ is not a strong
enough acid to protonate the successive reduced species. By
following these considerations, multipathway mechanisms for
the electrocatalysis of these complexes are presented in
Scheme 4 (left panel). The multipathway nature of the
mechanisms may explain the broadness of the cyclic voltammo-
grams shown in Fig. 10.
While the bulkiness of the organic moiety in the bridgehead of
complexes 1 and 1Ts significantly affects their substitution
reaction with both ligands L1 and L2, protonating the Fe–Fe
bond of their substituted complexes 2–4 is mainly hindered by
the rigidity of the PNP ligand and the steric effect originating
from the phenyl groups on the two P atoms. Of the three
complexes, complexes 2 and 3 exhibit rather similar electro-
chemistry. The first two reduction processes may follow the
mechanism recently proposed by Tarlamin and co-workers,50
an EE process coupled with disproportionation of the
monoanion to its parent complex and dianion. For
complex 4, however, reduction II may comprise additional
unknown process(es) compared to the other two complexes,
which was not further explored in this work. It is clear that
process III for all the three complexes is associated with
chemical reactions involving CO-loss. How the reactions are
related to the reductions and their decomposed products is
subject to further investigations.
The electrocatalytic behaviours of the three complexes have
shown that the pendant pyridine group after being protonated
can lower the overpotential for proton reduction but does not
seem to improve the electrocatalytic efficiency for proton
reduction. Recent reports by Wang, Sun, Talarmin and our
own work have shown that an internal base may play a role in
intramolecularly relaying protons.17,27,28 However, as to how
such a pendant base in a model complex may effectively
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Scheme 4 Proposed electrocatalytic mechanisms for the proton reduction catalysed by the three complexes 2, 3 and 4.
Protonation reactions were performed in acetonitrile under
Ar atmosphere. A typical procedure is as follows. To a solution
of complex 3 (0.006 g, 0.007 mmol) in CH3CN (2.5 mL) in a
Schlenk tube was added one equivalent of HBF4ꢀEt2O (1.0 mL,
0.0068 mmol) in a dropwise fashion (leading to no change in the
infrared spectrum). Further addition of an excess of HBF4ꢀEt2O
(15 mL, 0.10 mmol) shifted the IR absorption bands from 1911,
1925, 1964, 1994 cmꢁ1 to higher energy by about 43 cmꢁ1, to
1960, 1972, 2004, 2031 cmꢁ1. Neutralisation of the protonated
solution by adding triethylamine (14 mL, 0.10 mmol) restored
complex 3 by about 80%.
31P NMR of the protonated species, produced by using
excessive acid (HBF4ꢀEt2O), were recorded in situ in CD3CN
on a Bruker Advance III 600 (600 MHz) at ꢁ10 1C. For
complex 2, signals at 123.56 and 126.18 ppm were observed
for the first and the second protonated species, respectively.
For complex 4, the analogous corresponding chemical shifts
were at 123.37 and 125.99 ppm, respectively. For complex 3,
signals at 119.94 and 122.29 ppm were observed for the parent
complex and protonated species.
Fig. 11 Linear correlation of the electrocatalytic peak current (ip)
with the concentration of the added acid (the concentration of
complexes 2, 3 and 4 are the same as in Fig. 10).
enhance electrocatalytic efficiency of hydrogen evolution needs
still further exploration.
In the data collection for X-ray single crystal diffraction
analysis, standard procedures were used for mounting the
crystals on a Bruker Apex-II area-detector diffractometer at
293(2) K. The crystals were routinely coated with paraffin oil
before being mounted. Intensity data were collected using
Mo-Ka radiation (l = 0.71073 A) at 293 K using j- and
o-scan mode. The SAINT and SADABS programs in the
APPEX 2 software package were used for integration and
absorption correction.62 The structure of complex 4 was
solved by direct method using SHELXS-97 program63 and
refined on F2 with XSHELL6.3.1, all non-hydrogen atoms
being modelled anisotropically.
Experimental
All reactions were carried out under Ar atmosphere with
standard Schlenk techniques. Solvents were dried and distilled
prior to use by following standard procedures. Triiron
dodecacarbonyl and tetrafluoroboric acid/ether solution were
purchased from Alfa-aesar. Other chemicals were purchased
from local suppliers and further purified when necessary.
Complex 1, [Fe2{(m-SCH2)CMe(CH2OH)}(CO)6] and related
ligands
including
(4-methyl-1,2-dithiolan-4-yl)methyl-4-
methylbenzenesulfonate, which was used to synthesise the
complex [Fe2{(m-SCH2)CMe(CH2OTs)}(CO)6] (1Ts), were
prepared by following general methodologies.15,17 Electro-
chemistry was performed under an appropriate atmosphere
in 0.5 M [NBu4]BF4–THF at about 300 K unless otherwise
stated. Detailed procedures were described earlier elsewhere.60
Digital simulation was performed by using DigiElch 4.0 soft-
ware. In the simulation, general criteria as described earlier
were observed.61 Infrared spectra were recorded on a Varian
Synthesis of [Fe2{(l-SCH2)CMe(CH2OTs)}(CO)6], 1Ts
A solution of 4-methyl-4-(toluene-4-sulfonyloxy)methyl-1,2-
dithiolan (1.1 g, 3.6 mmol) and Fe3(CO)12 (1.8 g, 3.6 mmol)
in toluene (15 mL) was allowed to stir for 12 h at 90 1C. Then
the solvent was removed under reduced pressure and the crude
product was purified by chromatography on silica gel with
ethyl acetate–petroleum ether (1 : 3) as eluent. The complex
was obtained as a red solid (0.61 g, 29%). 1H NMR (CDCl3):
d 0.975 (s, 3H, CH3), 2.019 (d, J = 9.7 Hz, 1H, CCH2S), 2.199
(d, J = 14 Hz, 2H, CCH2S), 2.466 (s, 3H, PhCH3), 3.684
(s, 2H, CCH2O), 7.372 (d, J = 8.1 Hz, 2H, ArH), 7.766
1
Scimitar 2000 spectrophotometer. H and 31P NMR (CDCl3
solution) spectra were collected on a BRUKER DRX-400
NMR spectrometer, unless otherwise stated. The elemental
analysis service was provided by Nanjing University (China).
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(d, J = 8.2 Hz, 2H, ArH); IR (CH2Cl2, nCO): 2075 (s), 2036 (s),
2003 (s), 1994 (sh) cmꢁ1
CCH2S), 2.485–2.510 (m, 1H, CCH2S), 3.563 (d, J = 17 Hz,
.
2H, CCH2OH), 4.475 (t, J = 8.2 Hz, 2H, NCH2Ph), 5.925
(d, J = 7.6 Hz, 2H, ArH), 6.520 (t, J = 7.5 Hz, 2H, ArH),
6.657 (t, J = 7.1 Hz, 1H, ArH), 7.140 (d, J = 7.7 Hz, 6H,
ArH), 7.458 (s, 6H, ArH), 7.720 (s, 8H, ArH); 31P NMR
(CDCl3): d 122.9 (s); IR (CH2Cl2) n(CO): 1995s, 1962s, 1926s,
1913sh cmꢁ1. Microanalysis for 3 (C40H37Fe2NO5P2S2): calc.
(found) (%): C, 56.55 (56.31); H, 4.39 (4.37); N, 1.65 (2.45).
Synthesis of (Ph2P)2NCH2(2-C5H4N), L1 and
(Ph2P)2NCH2Ph, L2
To a solution of chlorodiphenylphosphine (1.8 ml, 10 mmol)
and triethylamine (1.4 ml, 10 mmol) in CH2Cl2 (30 ml) was
dropwise added pyridin-2-ylmethanamine (0.54 ml, 5 mmol) at
ice temperature under an atmosphere of argon. A precipitate
formed immediately and the reaction mixture turned deep
yellow. The reaction was further stirred for 18 h at room
temperature. Removal of the solvent, triethylamine and chloro-
diphenylphosphine under vacuum gave a yellow oily liquid,
which was re-dissolved in CH2Cl2 (40 ml) and washed with
degassed sodium hydroxide solution (saturated, 20 ml ꢃ 3).
The organic phase was dried with anhydrous MgSO4 and the
solvent was removed under reduced pressure after filtration,
which afforded an off-white solid. Recrystallisation of the
crude product in CH2Cl2–MeCN at low temperature produced
a white solid (L1, 1.78 g, 75%), which was washed with
acetonitrile (10 ml ꢃ 3) and dried in vacuum. Mp 168–170 1C
(uncorrected). 1H NMR (CDCl3): d 4.668 (t, J = 11, 2H,
CH2), 6.647 (d, J = 7.8, 1H, ArH), 6.972 (t, J = 6.0, 1H,
ArH), 7.266–7.312 (m, 13H, ArH), 7.415 (t, J = 6.7, 8H,
ArH), 8.385 (d, J = 3.9, 1H, ArH). 31P NMR (CDCl3): d 64.35
(s). Microanalysis for C30H26N2P2ꢀ0.4CH2Cl2: calc. (found)
(%): C, 71.53 (71.78); H, 5.29 (5.82); N, 5.49 (5.11).
Synthesis of [Fe2(SCH2)2CMe(CH2OTs)(L1)(CO)4], 4
Complex 4 was synthesised as a red solid in the same manner
as for complex 2 by using complex 1Ts as precursor but the
reaction was for much longer (18 h). The crude product was
first purified by using flash chromatography (eluent: ethyl
acetate–petroleum ether = 1 : 1) and then recrystallisation
from a mixed solvent of dichloromethane and hexane (1 : 1)
(0.35 g, 69%). The obtained crystals were suitable for X-ray
1
single crystal diffraction analysis. H NMR (CDCl3): d 1.185
(s, 3H, CCH3), 2.051–2.387 (m, 4H, 2 ꢃ CCH2S), 2.443 (s, 3H,
PhCH3), 3.692 (d, J = 9.3 Hz, 1H, CCH2O), 4.615 (d, J = 9.3
Hz, 1H, CCH2O), 4.891 (t, J = 8.3 Hz, 2H, NCH2Py), 5.593
(d, J = 7.6 Hz, 1H, ArH), 6.444–6.495 (m, 2H, ArH),
6.963-7.027 (m, 2H, ArH), 7.360 (d, J = 7.9 Hz, 2H, ArH),
7.466 (s, 6H, ArH), 7.838 (m, 11H, ArH); 31P NMR (CDCl3):
d 120.0 (s); IR (CH2Cl2) n(CO): 2000 (s), 1967 (s), 1931 (s),
1917 (sh) cmꢁ1. Microanalysis for 4 (C46H42Fe2N2O7P2S3):
calc. (found) (%): C, 54.99 (54.85); H, 4.21 (4.25); N, 2.80
(3.60).
The ligand L2 (white solid) was prepared in the same manner
as described for L1 (yield: 2.0 g, 83%). Mp 161–163 1C
1
(uncorrected). H NMR (CDCl3): d 4.410 (t, J = 10 Hz, 2H,
CH2), 6.690 (s, 2H, ArH), 7.012 (s, 3H, ArH), 7.211–7.278
Acknowledgements
We thank the Natural Science Foundation of China (Grant
No. 20571038, 20871064), Centre of Analysis and Testing at
Nanchang University (2005040, 2009001) and the State Key
Laboratory of Coordination Chemistry at Nanjing University
(China) for supporting this work.
(m, 20H, ArH).
Synthesis of [Fe2(l-SCH2)2CMe(CH2OH)(L1)(CO)4], 2
A solution of complex 1 (0.219 g, 0.5 mmol) and the ligand L1
(0.238 g, 0.5 mmol) in dry toluene (20 mL) was heated for 1.5 h
at 110 1C under stirring. After being cooled down to room
temperature, the solvent was removed under reduced pressure
to give a crude product. Purification by using flash chromato-
graphy with silica gel (eluent: ethyl acetate–petroleum ether =
1 : 1) produced a red solid (0.41 g, 96%). Storing a saturated
solution of the product in acetonitrile at 4 1C produced single
crystals. 1H NMR (CDCl3): d 1.212 (s, 3H, CH3), 2.135
(d, J = 11.3, 1H, CCH2S), 2.393 (d, J = 13.3 Hz, 2H,
CCH2S), 2.720 (d, J = 12.5 Hz, 1H, CCH2S), 3.591 (s, 1H,
CCH2OH), 3.694 (s, 1H, CCH2OH), 4.885 (t, J = 8.2 Hz, 2H,
NCH2Py), 5.617 (d, J = 7.6 Hz, 1H, ArH), 6.417–6.482
(m, 2H, ArH), 6.971 (s, 4H, ArH), 7.018 (s, 2H, ArH), 7.462
(s, 6H, ArH), 7.796 (s, 8H, ArH), 7.891 (d, J = 3.6 Hz, ArH);
31P NMR (CDCl3): d 121.9 (s); IR (CH2Cl2) n(CO): 1998 (s),
1964 (s), 1928 (s), 1914 (sh) cmꢁ1. Microanalysis for 2
(C39H36Fe2N2O5P2S2): calc. (found) (%): C, 55.08 (55.00);
H, 4.27 (4.23); N, 3.30 (3.24).
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