Controllable electrochemical synthesis of Mono- and Di-substituted diphosphine complexes related to [Fe-Fe]-hydrogenase

Article abstract of DOI:10.14233/ajchem.2013.14544

By the electrocatalytic reactions of [Fe₂(CO) ₆(μ-pdt)] (1) (pdt = SCH₂CH₂CH₂S) with bidentate phosphine complexes dppx (dppm = PPh₂CH ₂PPh₂, dppe = PPh₂(CH

Full text of DOI:10.14233/ajchem.2013.14544

Asian Journal of Chemistry; Vol. 25, No. 14 (2013), 7655-7659
http://dx.doi.org/10.14233/ajchem.2013.14544
Controllable Electrochemical Synthesis of Mono- and Di-Substituted
Diphosphine Complexes Related to [Fe-Fe]-Hydrogenase
1
2,*
2
3
WENWEN JIANG , ZHIMEI LI , XIANGHUA ZENG and GUOFEN WEI
1Department of Food Science and Engineering, Nanchang University, Nanchang 330031, P.R. China
2Department of Chemistry, Nanchang University, Nanchang 330031, P.R. China
3Department of Mathematics, Nanchang University, Nanchang 330031, P.R. China
*Corresponding author: Tel: +86 13576043206; E-mail: lizhimei@ncu.edu.cn
(Received: 10 September 2012;
Accepted: 8 July 2013)
AJC-13781
By the electrocatalytic reactions of [Fe2(CO)6(µ-pdt)] (1) (pdt = SCH2CH2CH2S) with bidentate phosphine complexes dppx (dppm =
PPh2CH2PPh2, dppe = PPh2(CH2)2PPh2, dppt = PPh2(CH2)4PPh2), three mono-substituted diphosphine complexes [Fe2(CO)5(µ-pdt)(k1-
dppm)] (2), ([Fe2(CO)5(µ-pdt)(k1-dppe)] (3) and [Fe2(CO)5(µ-pdt) (k1-dppt)] (4) were synthesized by keeping the controlling potential at
-1.75 V. Di-substituted phosphine complexes [Fe2(CO)4(µ-pdt)(k2-dppe)] (5), [Fe2(CO)4(µ-pdt)(µ-dppm)] (6), [Fe2(CO)4(µ-pdt)(µ-dppm)]
(7), [Fe2(CO)4(µ-pdt) (µ-dppt)] (8) were synthesized by holding the potential at -1.85 V. The mono-substituted diphosphine complexes 3
and 4 were characterized by elemental, NMR and IR analysis. We also discuss the catalytic mechanisms of the synthetic process.
Key Words: Mono-substituted diphosphine complexes, Controllable electrochemical synthesis, [Fe-Fe]-Hydrogenase.
decarbonylation reagents are low yielding and time consu-
ming11,12. It was also reported that treatment of the diiron hexa-
carbonyl complex [Fe2(CO)6(µ-pdt)](1)(pdt = SCH2CH2CH2S)
with PPh2(CH2)2PPh2(dppe) in refluxing toluene in the
presence of Me3NO afforded only the asymmetrical complex
[Fe2(CO)4(µ-pdt)(k2-dppe)] (5) and did not yield the mono-
substituted complex [Fe2(CO)5(µ-pdt)(k1-dppe)] (3). The need
for more effective methods for synthesizing these complexes
INTRODUCTION
As water is the only product in the process of oxidation
of H2, dihydrogen has the potential to act as a “clean’’alterna-
tive to fossil fuels. However, now-a-days, it is a fact that
efficient catalysts to produce H2 usually depend on noble
metals such as platinum. In nature, [Fe-Fe]-hydrogenase has
an active site composed of a diiron center that is coordinated
with two pairs of terminal CO and CN– ligands and by a carbonyl
in a bridging or terminal (or semi-bridging) position1-3. [Fe-Fe]-
hydrogenase exhibits high efficiency and a rapid reaction rate
in both the catalytic production of hydrogen or hydrogen oxid-
ation under mild conditions4-7.
prompted us to do more related research9,12
.
Here we report an effective method to synthesize some
mono-substituted complexes, [Fe2(CO)5(µ-pdt)(k1-dppm)] (2)
(dppm = PPh2CH2 PPh2), [Fe2(CO)5(µ-pdt)(k1-dppe)] (3),
[Fe2(CO)5(µ-pdt)(k1-dppt)] (4)(dppt = PPh2(CH2)4PPh2), the
asymmetrical di-substituted complexes, [Fe2(CO)4(µ-pdt)(k2-
dppe)] (5) and the symmetrical di-substituted complexes
[Fe2(CO)4(µ-pdt)(µ-dppm)] (6), [Fe2(CO)4(µ-pdt)(µ-dppm)] (7)
and [Fe2(CO)4(µ-pdt)(µ-dppt)] (8) by selecting the appropriate
potential. The key feature of these processes is their selectivity
We also discuss in this paper the possible mechanisms of
the processes from the mono-substituted complexes to the di-
substituted complexes.
The classical complexes, dithiolate-bridged diiron comp-
lexes, [Fe2(CO)n(µ-SXS) L6-n] (n = 6, 5, 4; X = R or
CH2N(R)CH2,L = PPh3, PMe3, CN– or other non-CO ligands),
are structurally and functionally similiar to the active metal
center of [Fe-Fe]-hydrogenase, have stimulated much active
interest in their chemistry8. In the catalytic conversion of protons
to hydrogen at the active centre, a key step is the protonation
of the active centre. Electron-donating groups, such as CN–,
can make the active center accept proton(s) easily. Since
phosphine ligands have an electron-donating ability similar
to that of the CN– group, a large number of phosphine-substi-
tuted derivatives have been prepared9,10. However, the produc-
tion of these substituted derivatives, based on the use of
EXPERIMENTAL
General procedures: All the experiments were carried
out under argon atmosphere. Solvents were dried and distilled
according to the standard methods. [Fe2 (CO)6(µ-pdt)], (1),

7656 Jiang et al.
Asian J. Chem.
was prepared as described in the literature13. The purification
of product was carried out with Schlenk technique.
Infrared spectra were recorded on a scimitar series Varian
2000 FT-IR. Elemental analyses were performed using an
Elementar vario MICRO instrument. 1H and31P NMR spectra
were recorded on aVarian INOVA 400 mHz NMR instrument.
Electrochemistry: Electrochemistry was carried out
under an argon atmosphere in freshly dried dichloromethane
using an Autolab Potentiostat 30. A conventional three-
electrode system was employed in which a Pt disk (φ = 1 mm)
was used as the working electrode, a vitreous carbon strip as
the counter electrode and anAg/AgCl (Metrohm) as the refer-
ence electrode. The inner reference solution is composed of
0.05 M [NBu4]Cl and 0.45 M [NBu4]BF4. The preparation and
purification of the supporting electrolyte [NBu4]BF4 was
described previously14. All the potentials were measured
against the ferrocene-ferrocenium couple, which was added
as an internal standard at the end of the experiments.
In dichloromethane, the electrolyte is 0.5 M [NBu4]BF4
to ensure optimum conductivity of the solution. When carrying
out bulk electrolyses, with a vitreous carbon strip (2 cm2) as
the working electrode, all coulometric experiments were
performed at constant potential under an argon atmosphere
and were followed by periodically measuring the cyclic
voltammogram in situ or extracting an aliquot of the catholyte
for IR analysis.
2H, µ-pdt) , 1.85(m, 4H, CH2), ppm. 31P NMR (CDCl3): δ(s)
57.6, -26.0 ppm.
Electrosynthesis of [Fe2(CO)4(µ-pdt)(k2-dppe)],
[Fe2(CO)4(µ-pdt) (µ-dppm)], [Fe2(CO)4(µ-pdt)(µ-dppe)] and
[Fe2(CO)4(µ-pdt)(µ-dppt)].
A similar procedure to that described above was carried
out to synthesize the complex di-substituted diphosphine
complexes at a constant potential of -1.85 V. Upon disappear-
ance of the IR band at 2044 cm-1 the reaction was continued
for an addtitional 5 min and then a sample of the solution
again checked by IR. After purification, three products were
obtained with yields of 65 % (121 mg) for product 6, 75 %
(142 mg) for product 7 and 71 % (139 mg) for product 8.
Product 6: IR (νCO) (CH2Cl2): 1987 m, 1956 vs, 1919 s, 1901
sh. Product 7: IR (νCO) (CH2Cl2): 1990 s, 1954 vs, 1920 s,
1901 w. Product 8: IR (νCO) (CH2Cl2): 1990 s, 1943 vs, 1925
s and 1886 m.
The electrolysis reaction was monitored by IR and then
stopped upon disappearance of the IR band at 2044 cm-1.
After the mixture was purified by column chromatography,
two products 5 and 7 were obtained. The yields of products 5
and 7 were 27 % (51 mg) and 61 % (117 mg), respectively.
Product 5: IR (cm-1) (νCO) (CH2Cl2): 2019 s, 1949 s and
1905 w.
RESULTS AND DISCUSSION
Electrosynthesis: Electrosynthesis of [Fe2(CO)5(µ-pdt)
(k1-dppm)], [Fe2(CO)5 (µ-pdt)(k1-dppe)] and [Fe2(CO)5(µ-pdt)
(k1-dppt)].
Electrosynthesis of mono- and di-substituted diphosphine
complexes: The diiron hexacarbonyl complex 1 was not
reactive towards diphosphines at room temperature before
electrolysis. However, the reactions of complex 1 with
diphosphines occurred rapidly if the electrodes [Ag/AgCl]
were simply inserted into the solutions and negative potentials
were applied. The value of the potential determined the selec-
tivity of the products. Table-1 showed that mono-substituted
diphosphine complexes 2, 3 and 4 were obtained at -1.75 V
(Scheme-I) and di-substituted diphosphine complexes 5, 6, 7
and 8 were obtained at -1.85 V in the solution of complex 1
with diphosphines (Scheme-II). Isolation of these products
was followed by IR analysis. Fig. 1 shows the IR spectra of
the solutions of complex 1 with dppe at -1.75 V and -1.85 V
before and after electrolysis, respectively. The carbonyl
region IR spectra of complexes synthesized at -1.75 V were
identical to the mono-substituted diphosphine complexes and
were also similar to results reported in the literature15. The
carbonyl region IR spectra of the complexes induced at -1.85
V are consistent with the di-substituted diphosphine complexes
reported in the literature9,12,16.The yields of these reactions are
listed in Table-1.
Three three-chambered electrolysis cells filled with argon
were charged with 0.100 g (0.26 mmol) [Fe2(µ-pdt)(CO)6], 1
and 0.100 g (0.26 mmol) dppm/0.103 g (0.26 mmol) dppe/
0.112 g (0.26 mmol) dppt in 10 mL of dichloromethane
containing 0.5 M TBA BF4. The cathodic current at constant
potential of -1.75 V was passed through the solution until the
cathodic current of the cells in the potentiostatic experiment
decayed to the background level, or the cathodic current was
observed to be relatively invariant at -0.03 mA for ca. 15 min
(passage of 0.24C/2.66C/3.49C). The cathodic electrolyte was
monitored by IR. The solvent was removed in vacuo. The red
solid residue was washed with hexane (3 mL × 3 mL) to
remove any remaining starting material. The solid was then
purified by column chromatography on a silica gel column by
elution with 10:1 v/v mixture of ethyl acetate and petroleum
ether. Removal of solvent in vacuo, yielded a red solid, 90 %
(0.173g)/92 % (0.181g)/87 % (0.180 g), respectively. Crysta-
llization twice from dichloromethane afforded a red solid for
IR analysis.
Product 2: IR (νCO) (CH2Cl2): 2043 m, 1982 vs, 1956
sh, 1924 w cm-1.
Product 3: IR (νCO) (CH2Cl2): 2044 m, 1982 vs, 1962
sh, 1927 w cm-1.1H NMR (CDCl3): δ 7.79-7.36 (m, 20H, C6H5),
3.19 (m, 4H, PCH2CH2P), 2.56(m, 4H, µ-pdt), 1.64 (m, 2H, µ-
pdt) ppm. 31P NMR (CDCl3): δ (s) 59.6, 32.4 ppm.Anal. calcd.
(%) for C34H30O5P2S2Fe2: C, 53.97; H, 3.97. Found (%): C,
54.36; H, 4.01.
PPh2(CH2)nPPh2
-1.75 V
Product 4: IR (νCO) (CH2Cl2): 2043 s, 1982 vs, 1956 sh,
1925 m cm-1. 1H NMR (CDCl3): δ 7.62(m, 8H, C6H5), 7.41(m,
12H, C6H5), 2.45 (m, 4H, PCH2), 1.85(m, 4H, µ-pdt), 1.52 (m,
Scheme-I: Process of mono-substituted diphosphine complexes 2, 3 and
4 were synthesised by the electrocatalytic reactions

Vol. 25, No. 14 (2013)
Controllable Electrochemical Synthesis of Mono- and Di-Substituted Diphosphine Complexes 7657
periodically sampling the catholyte and measuring the disap-
TABLE-1
ETC CATALYSIS OF DIPHOSPHINE
SUBSTITUTION OF COMPLEX 1 OF 0.26 mmol
pearance of the carbonyl band at 2074 cm-1 of complex 1 in
the infrared spectrum. More conveniently, the cathodic
currents in the potentiostatic experiment were observed to be
relatively invariant at -0.03 mA for approximately 5 min. The
latter observation signaled the completion of these substituted
reactions, as confirmed by IR analysis of the catholyte. Turn-
over numbers (TN) in excess of 43.5, 94.3 and 71.9 equiv of
the three mono-substituted complexes 2-4 per electron were
obtained (eqn. 1).
Ligand Potential (V)
-1.75
Product Yield (%)
Charge
0.24C
TN
3
92
-1.85
5 and 7
27 and 61
dppe
435
7
75
–
-1.75
-1.85
-1.75
-1.85
2
6
4
8
90
65
87
71
dppm
dppt
2.66C
3.49C
94.3
71.9
It should be noted that asymmetric di-substituted comp-
lexes were only observed in the dppe solution, an observation
confirmed by periodically sampling some catholyte for IR
analysis during the electrocatalytic reactions.At the beginning
of the electrocatalytic synthesis of complex 1 with dppe or
complex 3, the IR spectrum of the carbonyl region of the
product was similar to the asymmetric di-substituted9 complex,
5, which suggested that the reaction was followed by eqns. 2
or 3. During the electrocatalytic synthesis at -1.85 V in dppe
solution, the sharp peak at 2019 cm-1 disappeared completely
and a series of new peaks emerged, indicative of a symmetrical
di-substituted complex; the final product 716 exhibits symmetrical
di-substitution (eqn. 4). The complex 5 is an intermediate
between the mono-substituted diphosphine complex 3 and the
symmetrical diphosphine complex 7. Unlike the dppm and
dppt ligands, the mono-substituted diphosphine complexes 2
and 4 were transformed directly to the symmetrical diphos-
phine complexes 6 and 8, as shown in eqn. 5.
As mono-substituted complexes 3 and 4 were novel, they
were characterized by elemental, 1H and 31P NMR analysis.
Electrochemistry of electrosynthesis of mono- and di-
substituted diphosphine complexes and their possible
mechanisms
Electrochemistry of electrosynthesis of mono-substi-
tuted diphosphine complexes: Previous studies of the
substitution of complex 1 with monophosphines showed that
anionic species were involved in the ETC process17. In order
to assess whether the substitution of 1 with diphosphines is
also a possibility for the ETC process, it is necessary to discuss
the electrochemical reduction. As such, we examined the
cathodic behaviour of the parent complex 1 both in the absence
and in the presence of the diphosphines. Fig. 2 shows cyclic
voltammograms of complex 1 in the absence (dashed line)
and in the presence (solid line) of dppe and the mono-substi-
tuted complex 3 (dashed-dotted line). Compared to complex
1 (dashed line), strengthen of the current of the solution of
complex 1 with dppe at -1.78 V (dashed line) weakened
evidently. Two novel peaks occurred under the more negative
potentials of -1.97 and -2.36 V. Apparently, the peak at -1.78
V is associated with the complex 1. Its detailed mechanism
has been investigated by Stephen and his co-workers18. They
confirmed that the reduction process of complex 1 was a two-
electron process and the anionic 1 was a short-lived species.
As shown in Fig. 2 (dash-dotted line), complex 3 was reduced
at -1.97 and -2.36 V. Evidently, the new peaks in Fig. 2 are
associated with the mono-substituted complex 3. For com-
plex 1 with dppm or dppt, it had similar electrochemical
behaviour with dppe. The electrocatalytic mechanisms of CO
Scheme-II: Electrocatlytic synthesis of di-substituted diphosphine
complexes 5, 6, 7 and 8
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0
2100
2050
2000
1950
1900
1850
1800
Wavenumber (cm–1)
Fig. 1. IR spectrum of the complex 1 and the electrostimulated products
(3, 5, 7) of complex 1 with dppe at -1.75 and -1.85 V, respectively
To monitor for completion of the electrocatalytic reaction
during bulk electrolysis at constant potential, the reaction
course of the electrocatalytic displacement was followed by

7658 Jiang et al.
Asian J. Chem.
mically favoured. Likewise, diphosphines with rigid two-
carbon backbones such as dppe should favour asymmetrical
chelating coordination thus affording electronic and steric
asymmetry to the diiron centre15. Talarmin and his coworkers
have reported on the mechanism of the asymmetrical complex
5 to symmetrical complex 716. Herein, we focus on the mecha-
nism of the transformation of the mono-substituted complex
3 into the asymmetric di-substituted complex 5 as shown in
Fig. 4.
E (V)
Fig. 2. Cyclic voltammograms of complex 1 (0.03 mmol) in the absence
(dashed line) and in the presence of dppe (0.04 mmol) (solid line),
mono-substituted complex 3 (dashed-dotted line) and the solution
of the complex 1 with dppe after electrolysis at -1.85 V (dotted
line)
replaced by diphosphines at -1.78 V seem to correspond with
our previous studies (Fig. 3)19.
Fig. 4. Possible mechanism of the electrochemical transformation of the
mono-substituted complex 3 to the asymmetric di-substituted
complex 5
Conclusion
In this paper, we reported the successive synthesis of
mono- and di-substituted diphosphine complexes by control-
ling the potential. We obtained three mono-substituted
diphosphine complexes 2, 3 and 4; an asymmetric di-substi-
tuted diphosphine complex 5 and three symmetric di-substi-
tuted diphosphine complexes 6, 7, 8, in good yield and with
shorter reaction times. We also discussed the mechanisms of
these catalytic processes.
Fig. 3. Mechanism of the electrochemistry of complex 1 with diphosphines
Electrochemistry of mono-substituted complexes 2, 3,
4 and their possible mechanisms: In Fig. 2, two cathodic
peaks are seen at -1.97 and -2.36V in the solution of the mono-
substituted complex 3. The peak at -1.97 V is associated with
the reduced complex 3. Generally, the reduction of the mono-
substituted pentacarbonyl diiron complex is assigned to a
two-electron process20. As mentioned above, when the poten-
tial of -1.85 V was applied to the solution of complex 1 with
diphosphines or complex 3, di-substituted complexes were
obtained. This indicated that the reduction of the mono-
substituted complex 3 was followed by the loss of CO. In the
three mono-substituted complexes, asymmetric di-substituted
complexes were obtained only from the anionic product of
complex 3 and under bulk electrolysis, the asymmetric di-
substituted complex 5 transformed into another isomer,
symmetric di-substituted complex 7. The nature of the reaction
of mono-substituted complex leading to a range of products is
dependent upon the backbone flexibility of the diphosphine.
For diphosphines with a single methylene backbone (dppm),
symmetrically di-substituted complexes are thermodyna-
ACKNOWLEDGEMENTS
The authors thank the Natural Science Foundation of China
(Grant No. 20871064) and Centre of Analysis and Testing at
Nanchang University (2009001) for supporting this work.
Thanks are also due to Mr. Edmund Sorenson for editing the
paper.
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