Synthesis and characterisation of polymeric materials consisting of {Fe2(CO)5}-unit and their relevance to the diiron sub-unit of [FeFe]-hydrogenase

Article abstract of DOI:10.1039/c0dt00687d

By using click chemistry between a diazide and a diiron model complex armed with two alkynyl groups, two polymeric diiron complexes (Poly-Py and Poly-Ph) were prepared. The two polymeric complexes were investigated using infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), Moessbauer spectroscopy, and cyclic voltammetry (Poly-Py only, due to the insolubility of Poly-Ph). To probe the coordinating mode of the diiron units in the two polymeric complexes, two control complexes (3 and 4) were also synthesised using a monoazide. Complexes 3 and 4 were well characterised and the latter was further crystallographically analysed. It turns out that in both complexes (3 and 4) and the two polymeric diiron complexes, one of the two iron atoms in the diiron unit coordinates with one of the triazole N atoms. Our results revealed that both morphologies and properties of Poly-Py and Poly-Ph are significantly affected by the organic moiety of the diazide. Compared to the protonating behaviour of the complexes 3 and 4, Poly-Py exhibited proton resistance. In electrochemical reduction, potentials for the reduction of the diiron units in Poly-Py and hence its catalytic reduction of proton in acetic acid-DMF shifted by over 400 mV compared to those for complexes 3 and 4. It is likely that the polymeric nature of Poly-Py offers the diiron units a protective environment in an acidic medium and more positive reduction potential.

Full text of DOI:10.1039/c0dt00687d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and characterisation of polymeric materials consisting of
{Fe2(CO)5}-unit and their relevance to the diiron sub-unit of
[FeFe]-hydrogenase†
Caixia Zhan,a Xiufeng Wang,a Zhenhong Wei,a David J. Evans,b Xiang Ru,a Xianghua Zenga and
Xiaoming Liu*a
Received 20th June 2010, Accepted 14th September 2010
DOI: 10.1039/c0dt00687d
By using “click” chemistry between a diazide and a diiron model complex armed with two alkynyl
groups, two polymeric diiron complexes (Poly-Py and Poly-Ph) were prepared. The two polymeric
complexes were investigated using infrared spectroscopy, scanning electron microscopy (SEM),
transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), Mo¨ssbauer
spectroscopy, and cyclic voltammetry (Poly-Py only, due to the insolubility of Poly-Ph). To probe the
coordinating mode of the diiron units in the two polymeric complexes, two control complexes (3 and 4)
were also synthesised using a monoazide. Complexes 3 and 4 were well characterised and the latter was
further crystallographically analysed. It turns out that in both complexes (3 and 4) and the two
polymeric diiron complexes, one of the two iron atoms in the diiron unit coordinates with one of the
triazole N atoms. Our results revealed that both morphologies and properties of Poly-Py and Poly-Ph
are significantly affected by the organic moiety of the diazide. Compared to the protonating behaviour
of the complexes 3 and 4, Poly-Py exhibited proton resistance. In electrochemical reduction, potentials
for the reduction of the diiron units in Poly-Py and hence its catalytic reduction of proton in acetic
acid–DMF shifted by over 400 mV compared to those for complexes 3 and 4. It is likely that the
polymeric nature of Poly-Py offers the diiron units a “protective” environment in an acidic medium and
more positive reduction potential.
be an alternative green energy vector for our future has greatly
stimulated research interest in modelling this diiron sub-unit,
which is indicated by the rapid increase in the number of research
papers relating tothis enzyme. Reviews andpublications document
well the progress achieved in the past decade in synthesising diiron
model complexes which have some of the essential structural
features comparable to those found in the enzyme.3–14 These
achievements not only have greatly advanced our mechanistic
understanding of the catalysis of dihydrogen evolution catalysed
by systems consisting of low valence iron coordinated by both
sulfur and carbon monoxide and other ligands, as well as the
enzyme itself, but also the artificial system may bring us a step
closer towards assemblies which may be comparable to the natural
machinery in the operation of catalysis.
1
Introduction
In the course of evolution over a long period, nature “designed” a
delicate machinery, [FeFe]-hydrogenase, possessing unique struc-
tural and electronic features which render the natural system with
a high efficiency and a rapid rate in the catalytic production of
dihydrogen and vice versa. The key component of this system is
the diiron sub-unit as revealed nearly a decade ago,1,2 Fig. 1, where
the enzymatic catalysis occurs. The view that dihydrogen could
However, to move modelling chemistry further forward, two
critical issues need to be tackled, (i) a 3D structured environment
is desirable, in which synthetic models are harboured as the diiron
sub-unit is within the protein domains in the enzyme and (ii) an
appropriate scaffold is needed to combine with synthetic model
complexes. With the scaffold, synthetic models could be effectively
embedded into matrices in such a way that fabrication of an
electrode becomes possible. In such a system the amount of the
catalytic components can be delicately tuned and extra functional
groups may also be introduced to allow improvements in proper-
ties, for example, proton and electron transfer. To this end, there
are only a few reports. One approach was immobilisation of a
monolayer of diiron model complexes onto the surface of vitreous
carbon or gold electrodes.15,16 Pickett and co-workers reported
the immobilisation of a diiron complex into a polypyrrole film
Fig. 1 The diiron sub-unit of the H-cluster of [FeFe]-hydrogenase.1,2
aDepartment of Chemistry, Institute for Advanced Study, Nanchang Univer-
sity, Nanchang, 330031, China. E-mail: xiaoming.liu@ncu.edu.cn; Fax: +86
(0)791 3969254; Tel: +86 (0)791 3969254
bDepartment of Biological Chemistry, John Innes Centre, Norwich Research
Park, Colney, Norwich, UK NR4 7UH
† Electronic supplementary information (ESI) available: Supplementary
figures for related complexes in infrared spectra, schematic structure
(complex 3), cyclic voltammograms, Mo¨ssbauer spectra, TEM diagrams,
and bond lengths and angles. CCDC reference number 780635. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00687d
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via diffusion and its subsequent chemical reaction.17 Resin was
also employed by Darensbourg and co-workers as a support to
hold model complexes.18 Most recently, we reported that polyene
can be used to integrate diiron units together.19 To the best of
our knowledge, combining polymer materials with a synthetic
model had not been reported until this work. In the system
described, a polyene-based polymer functionalised with diiron
model complexes was achieved via homogeneous polymerisation,
the resultant polymer was successfully assembled into an electrode
which showed electrochemical activity.
Herein, we develop this approach further. Using “click” chem-
istry, a well known synthetic strategy,20 between an azide and
an alkynyl group under the catalysis of CuI, two polymeric
diiron complexes were prepared. The two polymeric materials
were characterised using a variety of techniques. One of the
polymers (Poly-Py) was also electrochemically investigated while
the same investigation was not possible for the other one (Poly-Ph)
due to its poor solubility. The organic moiety bearing the azide
groups dictates largely the physical properties and reactivity of the
polymeric diiron complexes. This polymeric effect may suggest an
approach to achieve novel artificial systems to mimic the diiron
sub-unit and its peripheral environment. Such a system may also
provide a scaffold for assembling a catalytic electrode.
inner reference solution is composed of 0.05 mol L-1 [NBu4]Cl
and 0.45 mol L-1 [NBu4]BF4. In 0.5 mol L-1 dichloromethane, the
potential of the ferrocenium/ferrocene couple is 0.55 V against
this reference electrode. All potentials were quoted against the
ferrocenium/ferrocene couple.
A typical procedure for electrochemistry is as follows. Under
an Ar atmosphere, Poly-Py (20.2 mg) was added into 0.1 mol L-1
[NBut4]BF4-DMF solution (4.0 mL). A scan rate of 0.1 V s-1
was used unless otherwise stated. In the investigation of catalytic
reduction, acetic acid was successively added in an appropriate
amount into the electrochemical cell and cyclic voltammetry was
accordingly measured.
2.2 Synthesis of the diazides, 2 and 2¢
2.2.1 Preparation of the dibromides, 1 and 1¢. A mixture of
AIBN (azobisisobutyronitrile, 100.4 mg, 0.61 mmol) and NBS
(N-bromosuccinimide, 10.70 g, 60.12 mmol) were dissolved in
CCl4 (100 mL), to which 2,6-dimethylpyridine (3.5 mL, 30 mmol)
was then added. The mixture was vigorously stirred to achieve a
light yellow mixture and then irradiated with 200 W tungsten-bulb
light under reflux for 12 h, the light yellow mixture turned finally
brown. After being cooled to room temperature, the reaction
mixture was filtered to remove any insoluble solids. Removal
of the solvent produced an oily residue which was purified
using flash chromatography on silica gel (ethyl acetate/petroleum
ether = 1: 6) to give a white crystalline solid (1.28 g, 17%), 2,6-
2. Experimental
2.1 Materials, instrumentation, and general procedures
◦
1
dibromomethylpyridine (1). M.p. 92 C (uncorrected), H NMR
(CDCl3, 295.5 K): 4.548 (s, CH2, 4H), 7.377 (d, Py, 2H, J =
7.76 Hz), 7.720 (q, Py, 1H).
Reactions and manipulations were performed using standard
Schlenk technique under argon when air-/moisture-sensitive
materials were handled. In extreme cases, handling was carried
out in an inert atmosphere box (Braun Lab-star). All solvents were
dried following standard methods, distilled prior to use and stored
under an argon atmosphere. General chemicals were purchased
from local commercial suppliers and further purifications were
performed when necessary. More specific chemicals were pur-
chased from Sigma-Aldrich and Alfa-Aesar. Diiron hexacarbonyl
complex, [Fe2(m-SCH2CCH)2(CO)6], and 11-azidoundecan-1-ol
were prepared as described in our recent work.19
Infrared spectra were recorded on Scimitar 2000 (Varian). NMR
spectra were recorded on Avance DRX 400 (Bruker) and Avance
600 (Bruker) in CDCl3. Scanning electron microscope (SEM)
images were obtained on Quanta 200F. Transmission electron
microscope (TEM) images were obtained on JEM2100. Thermal
gravimetric analysis (TGA) measurement was carried out on TA
SDT Q600 at a heating rate of 20 K min-1 under a nitrogen
purge. Mo¨ssbauer spectra were recorded in zero magnetic field
at 80 K on an ES-Technology MS-105 Mo¨ssbauer spectrometer
with a 220 MBq 57Co source in a rhodium matrix at ambient
temperature. Spectra were referenced against a 25 mm iron foil
at 298 K and spectral parameters were obtained by fitting with
Lorentzian curves. The X-ray single crystal diffraction data were
collected on Bruker Smart CCD diffractometer with graphite-
The other dibromide, 1,3-dibromobenzene (1¢), was analogously
prepared as a white crystalline solid (4.56 g, 29%). Mp 78 ◦C
1
(uncorrected). H NMR (CDCl3, 295.5 K): 4.49 (s, CH2, 4H),
7.33 (m, Ph, 3H), 7.42 (s, Ph, 1H).
2.2.2 Preparation of the diazides, 2 and 2¢. To a suspension
of NaN3 (6.50 g, 100.0 mmol) in DMF (50 mL) was added 2,6-
bis(bromomethyl)pyridine (1.75 g, 6.67 mmol). The mixture was
heated whilst stirring at 70 ◦C for 10 h. Water (100 mL) was
added to the reaction when being cooled to room temperature.
Extraction with ether (3 ¥ 50 mL) was performed and all the
extracts were combined and washed with brine (3 ¥ 20 mL).
After being dried with MgSO4, the solvent was evaporated under
reduced pressure to give a liquid residue. The residue was purified
using flash chromatography on silica gel (ethyl acetate/petroleum
ether = 1 : 3) to produce a colourless oil (1.22 g, 96%). IR (KBr):
(-N3) 2104 cm-1. 1H NMR (CDCl3, 295.5 K): 4.477 (s, CH2, 4H),
7.296 (d, Py, 2H, J = 7.60 Hz), 7.764 (q, Py, 1H).
1,3-bis(azidomethyl)benzene, 2¢, a colourless liquid (0.91 g,
84%) was prepared similar to compound 2. IR (KBr): (-N3)
2099 cm-1. 1H NMR (CDCl3, 295.5 K): 4.38(s, -CH2, 4H), 7.30(m,
Ph, 3H), 7.41(t, Ph, 1H).
2.3 Cyclisation between the diiron hexacarbonyl complex,
[Fe2(l-SCH2CCH)2(CO)6], and the diazide compounds via “click”
chemistry
˚
monochromated Mo-Ka radiation (l = 0.71073 A).
Electrochemical investigations were performed under argon in
a customised glass cell with a three-electrode system as described
elsewhere.21 In the setup, the working electrode was a glassy carbon
disc (f = 1 mm) and a glassy carbon strip electrode was used as a
counter electrode. Ag/AgCl was used as reference electrode whose
A red solution of [Fe2(m-SCH2C CH)2(CO)6] (1.20 g, 2.84 mmol)
in THF (40 mL) was added to a solution of Et3N (2.40 mL)
and CuI (17.07 mg, 0.09 mmol) in THF (10 mL) followed by
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˚
the addition of diazide 2 (0.54 g, 2.84 mmol). After stirring
overnight at 40 ◦C the reaction solution turned from red to a
black suspension. The suspension was poured into ethyl acetate
(1000 mL) with vigorous stirring for 10 min, after standing for
48 h, a black precipitate formed on the bottom of the reaction
vessel. The solid was collected by filtration, washed successively
with THF (250 mL), H2O (100 mL), Et2O/EtOH 1 : 1 (250 mL)
and finally Et2O (250 mL). The obtained black solid, Poly-Py
(0.66 g), was dried in vacuum.
Reaction of diazide 2¢ (0.37 g, 1.99 mmol) with the same diiron
hexacarbonyl complex (0.84 g, 1.96 mmol) produced a pale brown
solid, Poly-Ph (0.54 g). The preparation was identical to that for
Poly-Py except that methanol rather than ethyl acetate was used
as precipitating solvent.
ring; C–H = 0.93 A, with Uiso(H) = 1.2Ueq for alkyne group) and
constrained to ride on their parent atoms.
2.6 Reaction of Poly-Py with acid HBF4
Poly-Py (10.0 mg) was dissolved in DMF (6 mL). To the solution
was added acid HBF4 (6.4 mL) under CO. Infrared spectra were
recorded before and after the addition of the acid. The behaviour
of complex 3 upon the addition of the acid was examined in the
same manner.
3
Results and discussion
3.1 Synthesis
The synthesis of diazides is shown in Scheme 1. Bromination
of 2,6-bis(azidomethyl)pyridine and 1,3-bis(azidomethyl)benzene,
respectively, produced the dibromides (1 and 1¢), which were
further reacted with sodium azide in dimethylformide (DMF) to
give the desired diazides in yields over 90%.
2.4 Synthesis of Fe2(l-SCH2C CH)(l-SCH2R)(CO)5] (R =
(CH2)11OH, 3; R = CH2Ph, 4)
A
red solution of [Fe2(m-SCH2C CH)2(CO)6] (0.26 g,
0.61 mmoL) in THF (20 mL) was added to a solution of Et3N
(0.38 g, 0.5 mL) and CuI (7.3 mg, 0.04 mmol) in THF (10 mL). To
the mixture was added 11-azidoundecan-1-ol (0.13 g, 0.61 mmol).
After stirring at 40 ◦C for 3.5 h the red reaction solution turned to
a brown suspension. The solvent was evaporated under reduced
pressure and the product was purified using flash chromatography
on silica gel (ethyl acetate/petroleum ether = 1 : 2) to give a dark
red solid 3 (0.12 g, 48%). Layering its solution in dichloromethane
with hexanes produced dark-red plate-shaped crystals. IR (DCM):
(CO) 2041, 1996, 1966, 1937 cm-1, (–C CH): 3305 cm-1. 1H NMR
(CDCl3, 295.5 K): 1.5–1.19 (m, Hc, 18H), 1.83 (Ha, 1H), 2.26 (s,
Hh, 1H), 2.47–2.34 (m, Hg, 2H), 3.66–3.30 (m, Hf+b, 4H), 4.23
(tri, Hd, 2H), 7.24 (s, He, 1H) (please refer to Fig. S3† for the
labeling scheme of its non-CO carbon atoms). Microanalysis of
complex 3 (C22H29Fe2N3O6S2, MW = 607.30), calc. (found): C%,
43.51 (44.25); H%, 4.81 (4.43); N%, 6.92 (7.20).
Scheme 1 Synthesis of the diazides.
3.2 Formation of the two polymeric complexes, Poly-Py and
Poly-Ph
The “click” reaction of the diiron carbonyl complex, [Fe2(m-
SCH2C CH)2(CO)6], and diazides 2 and 2¢ under the catalysis of
CuI in THF generated polymeric diiron complexes, Poly-Py and
Poly-Ph, respectively, Scheme 2. Successful cyclisation between the
azides and the diiron complex was confirmed by the characteristic
absorptions of triazole rings ranging from 1400 to 1600 cm-1,
Fig. 2. The strong absorption bands between 2100 and 1850 cm-1
suggest the existence of carbonyl groups in the prepared polymeric
By analogy, reacting azidomethylbenzene (0.16 g, 1.2 mmol)
with [Fe2(m-SCH2C CH)2(CO)6] (0.51 g, 1.2 mmol) produced
a dark red solid (complex 4) (0.12 g, 19%). Its solution in
DCM layered with hexanes produced crystal blocks suitable
for X-ray single crystal diffraction analysis. IR (DCM): (CO)
1
2041, 1997, 1967, 1942 cm-1, (–C CH): 3303 cm-1. H NMR
(CDCl3, 295.5 K): 2.26 (s, –C CH, 1H), 2.47–2.34 (m, CH2,
2H), 3.61–3.26 (m, CH2, 2H), 5.42 (d, CH2, 2H, J = 11.7 Hz),
7.39–7.11 (m, Ph, 5H), 7.53 (s, triazol, 1H). Microanalysis for
complex 4 (C18H13Fe2N3O5S2, MW = 527.13), calc. (found): C%,
41.01 (40.12); H%, 2.49 (3.12); N%, 7.97 (7.68). Please note that
the relatively large discrepancy between the calculated and the
determined values may result from involvement of moisture during
the operation: 4·0.5H2O, calc. (found): C%, 40.32 (40.12); H%,
2.63 (3.12); N%, 7.84 (7.68).
2.5 Crystal structure determination
The crystal structure of complex 4 was solved using direct methods
and refined on F2 using full-matrix least-squares methods with the
SHELXL-97 program.22 All non-H atoms were anisotropically
refined. All H atoms were geometrically placed in idealised
Fig. 2 IR spectra (carbonyl region) of Poly-Py (solid line), Poly-Ph (dot
line) and complex 3 (dash line) (KBr). Inset: spectral region for the triazol
ring.
˚
positions (C–H = 0.99 A, with Uiso(H) = 1.2Ueq(C) for methylene
˚
groups; C–H = 0.95 A, with Uiso(H) = 1.2Ueq(C) for aromatic
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Scheme 2 Synthesis of Poly-Py, Poly-Ph, and complexes 3 and 4 via “click” chemistry.
materials. It is noteworthy that the rather weak absorption band
at around 2070 cm-1 in the infrared spectra of the two polymeric
complexes is likely attributed to terminal diiron hexacarbonyl units
of the polymeric chains. The broadness of those absorption bands
indicates the polymeric nature of the two materials.
Further examination of their spectral patterns reveals a high
similarity to those of diiron pentacarbonyl complexes reported
in the literature.23–27 As shown in Fig. 2, both materials possess
an identical spectral pattern, which rules out the possibility that
the pyridinyl N atom is involved in the formation of the diiron
pentacarbonyl unit in Poly-Py since both polymeric materials
exhibit nearly superimposable spectral profiles. In other words, one
of the triazolyl N atoms may coordinate to one of the Fe(I) atoms
in the diiron unit. To confirm this proposed coordinating mode,
two monomeric diiron complexes 3 and 4 were prepared from the
reaction of [Fe2(m-SCH2C CH)2(CO)6] with 11-azidoundecan-1-
ol and benzylazide, respectively, Scheme 2.
The identities of complexes 3 and 4 were well established using
infrared spectroscopy, NMR, and micro analysis. As shown in
Fig. 2 and Fig. S4† (infrared spectrum of complex 4), their spectral
profiles are highly similar to those of the two polymeric complexes.
Complex 4 was further crystallographically analysed. As shown in
Fig. 3, this complex shows largely the structural features found
for analogous diiron pentacarbonyl complexes reported in the
literature.4,9,14,24,26,28,29 But it is noteworthy that the Fe–N bond is
trans rather than cis to one of the two sulfur atoms as found in
those reported analogues. The triazole ring is co-planar with the
one defined by Fe2, S2, C9, C10, and N1 atoms. Crystallographic
details for this complex are tabulated in Table 1.
Fig. 3 Crystal structure of complex 4 (the ellipsoids were drawn
at a thermal probability of 30% and all hydrogen atoms are om◦it-
˚
ted for clarity) and selected bond lengths (A) and bond angles ( ):
The diiron centres in both polymeric materials were examined
using Mo¨ssbauer spectroscopy, Fig. S6.† Both Poly-Py and
Poly-Ph show slightly asymmetric, broad doublets, particularly
Poly-Ph. The measured parameters (mm s-1) are isomer shift (i.s.) =
0.15, quadrupole splitting (q.s.) = 1.01 (Poly-Py) and i.s. = 0.42,
q.s. = 0.84 (Poly-Ph), respectively. Except for the parameters for
Poly-Ph, the values are comparable to those for both diiron pen-
tacarbonyl and diiron hexacarbonyl complexes with an {FeIFeI}
core.19,25 However, it must be noted that the spectrum for a diiron
pentacarbonyl complex can be more complicated than that for
a diiron hexacarbonyl complex.25 On the other hand, higher
oxidation state irons may exist as impurities in the material,
which gives a high value of i.s. as observed in our recent work.19
Fe(1)–Fe(2) = 2.5411(4); Fe(2)–N(1) = 1.9655(13); Fe(1)–S(1) = 2.2630(5);
Fe(1)–S(2) = 2.2893(5); Fe(2)–S(1) = 2.2236(5); Fe(2)–S(2) = 2.2447(5);
∠Fe(2)–S(1)–Fe(1)
= 68.988(15); ∠Fe(2)–S(2)–Fe(1) = 68.166(14);
∠N(1)–Fe(2)–S(1) = 157.17(4); ∠N(1)–Fe(2)–Fe(1) = 101.01(4).
It is difficult, therefore, to precisely analyse the Mo¨ssbauer
data.
Thermal gravimetric analysis (TGA) of the two polymeric
complexes shows that three main stages of decomposition were
observed, roughly from 300 K to 470 K, 540 K to 720 K, and 720 K
to 1100 K, Fig. 4. This behaviour is, more or less, like that for the
polyene material functionalised with {Fe2(CO)6} units we reported
recently.19 It was found that the first stage of the decomposition
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Table 1 Crystal data and structure refinement for complex 4 (CCDC
780635)
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system, space group
Unit cell dimensions
C18H13Fe2N3O5S2
527.15
296(2) K
˚
0.71073 A
Triclinic, P1
¯
◦
˚
a = 7.8501(9) A, a = 81.0330(10)◦
˚
b = 8.1409(9) A, b = 84.0780(10)◦
˚
c = 18.077(2) A, g = 71.5380(10)
3
˚
Volume
1080.6(2) A
Z, Calculated density
Absorption coefficient
F(000)
2, 1.620 Mg m-3
1.570 mm-1
532.0
Crystal size
0.49 ¥ 0.45 ¥ 0.12 mm
Theta range for data collection 2.66 to 28.43◦
Limiting indices
-10 £ h £ 10, -10 £ k £ 10, -24 £ l £ 24
Reflections collected/unique
Completeness to theta = 28.43
Absorption correction
Refinement method
10273/5442 [R(int) = 0.0168]
96.8%
Semi-empirical from equivalents
Full-matrix least-squares on F2
5442/0/271
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
1.003
R1 = 0.0264, wR2 = 0.0705
Fig. 5 SEM diagrams of the two polymeric complexes: Poly-Py (A and
B) and Poly-Ph (C and D).
R1 = 0.0332, wR2 = 0.0752
-3
˚
Largest diff. peak and hole
0.460 and -0.186 e A
in morphology is undoubtedly ascribed to the nature of the
organic moiety which bears the two azide groups since the only
difference between Poly-Py and Poy-Ph is the aromatic ring
(Scheme 2). The variation in the organic skeleton not only changes
drastically their morphology, but also their solubility. Poly-Py
is highly soluble in DMF and DMSO, and slightly soluble in
polar solvents such as ethanol. But Poly-Ph is hardly soluble
in any common organic solvents. TEM images also indicate a
significant difference in internal structure between Poly-Py and
Poly-Ph (Fig. 6 and S7†). For the former, regular linear arrays and
even possibly orthogonal stacking structures were observed, Fig. 6.
These structural features were hardly found for Poly-Ph (Fig. S7†).
The controlling role played by the organic part in the diazide in
governing the properties of the polymeric complexes suggests that
varying the organic moiety would be an effective approach to tune
materials of this type to achieve desired properties, for example,
hydrophilicity and basicity. This work is currently underway in our
laboratory. Although estimation of its molecular weight turned out
Fig. 4 TGA diagrams of Poly-Py (dash line) and Poly-Ph (solid line)
measured under nitrogen flow at a heating rate of 20 K min-1.
is due to CO-loss. By analogy, the first stage decomposition
shown in Fig. 4 is attributed to the decomposition of the diiron-
carbonyl units. The major difference comes from the decompo-
sition pattern after losing CO. The polyene showed a simpler
pattern for its thermal decomposition. This difference originates
from the difference in the composition of these polymers. In the
previously reported polyene, its diiron units dangle at its backbone
whereas in both Poly-Py and Poly-Ph, the diiron unit is one of the
building blocks to constitute the polymeric backbone along with
the moiety of the diazide. Thus, at the stage of losing CO, it is
expected that the polymeric chain breaks into smaller fragments.
On further heating these fragments undergo more complicated
thermal decomposition compared to the polyene-based material.
The morphology of the two polymeric complexes, Poly-Py and
Poy-Ph, were explored using both scanning and transmission
electron microscopies. Fig. 5 shows SEM images of the two
polymeric complexes, which reveal rather different morphologies.
Poly-Py is essentially plate-like whereas Poy-Ph has a spherical
shape with a diameter of approximately 500 nm. This difference
Fig. 6 TEM diagram of Poly-Py showing both the linear arrays and
possibly orthogonal stacking structures.
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unsuccessful using a static light scattering technique, the length of
˚
the linear arrays (up to 300 A, Fig. S8†) may give a clue about
the number of diiron units in the linear chain of the polymer. By
considering the curling effect of the polymer chain, it is sensible to
˚
assume that each unit takes up about 10 A or less. Therefore, the
number of the diiron unit estimated could be over 30. Despite the
linear arrays observed in the TEM image of Poly-Py, the internal
structure (3D-structure) of both polymers is unknown.
3.3 Reaction of Poly-Py with the acid, HBF4
As discussed earlier, the diiron units in the two polymeric
complexes exist in the form of diiron pentacarbonyl. For diiron
model complexes of this coordinating mode, it has been well
established that bridging hydrides formed upon protonation and
under CO atmosphere, these hydrides could be converted to diiron
hexacarbonyl complexes.24,26 But addition of HBF4 acid into a
solution of the polymer in DMF did not produce either bridging
hydrides or protonated species. The acid has hardly any effect on
the diiron units, Fig. S1.†
To have a better understanding of the action upon the addition
of the acid, complex 3 which possesses the essential structural
feature of the diiron units in Poly-Py was reacted with the acid in
the same manner. Results showed that protonation is observed but
not as clean as those analogues reported previously.24,26 Although
some absorption bands around 2100 cm-1 may be an indication of
forming either a bridging hydride or perhaps a diiron hexacarbonyl
species, most of the complex decomposed upon the addition of the
acid, which is in strong contrast to the inertness of Poly-Py upon
addition of the acid. Neutralisation of the mixture after the acid
addition did recover a very small portion of the parent complex 3,
Fig. S2.† This further supports its partial protonation as described
above.
Fig. 7 Cyclic voltammograms of Poly-Py (5.1 mg mL-1, solid line) and
complex 3 (5.7 mmol L-1, dash line) in 0.1 mol L-1/DMF solution at a
scan rate of 0.1 V s-1 (298 K).
triazole ring. The electrochemical reduction of Poly-Py is shown
in Fig. 7, which exhibits a sharp difference to that of complex
3 in both reduction potential and electrochemical profile. The
broadness of the reduction process originates probably from the
polymeric nature of Poly-Py in which a range of macromolecules
possessing different numbers of diiron units exist. The striking
feature of the reduction of Poly-Py is the significant positive
shift in potential (ca. 400 mV based on the potential where the
reduction begins). As suggested by its infrared spectral absorption
bands and Mo¨ssbauer parameters, the diiron pentacarbonyl unit
essentially retains its structure in the polymeric material similar
to that of complex 4 (Fig. 3). Therefore, it is sensible that this
significant shift in reduction potential is correlated to the well
known electron-withdrawing nature of the diiron unit in the same
macromolecule.
The sharply contrasting behaviours in the reaction with the acid
indicate that in the polymeric form, the diiron units can not be
protonated, which ought to be attributed to its polymeric nature.
This may be a rationalisation of our initial idea that a polymeric
system into which a model complex is incorporated may offer the
metal centre a “protective” environment via the 3D structure of the
polymer system. Although it was not intended that the polymeric
complex Poly-Py was stable in the acidic medium, our results,
indeed, open up the possibility of a novel approach in modelling
the diiron sub-unit and the peripheral environment of the [FeFe]-
hydrogenase.
Despite the inertness of Poly-Py against protonation (vide ante),
this polymeric material shows catalytic reduction of protons in
a medium of acetic acid/DMF upon reduction, Fig. 8. It has
been well reported that the reduction of diiron complexes of the
core, {Fe2(CO)X } (X = 4–6) is complicated due to their reduction
processes coupling to chemical reactions.3,6,8,12,27,30–38 In catalysis of
proton reduction, the catalytic species is not simply the monoanion
3.4 Electrochemical investigations
We are particularly interested in the electrochemical behaviour
of the synthesised polymeric materials due to their relevance to
electrocatalytic reduction of protons. Although the electrochem-
ical investigation of Poly-Ph was entirely hampered since it is
insoluble in any solvent, a reasonable solubility of Poly-Py in
DMF allowed such an investigation. For comparison, both of the
control complexes 3 and 4 were also electrochemically examined.
As shown in Fig. 7 and Fig. S5,† complexes 3 and 4 exhibit nearly
identical electrochemical reductions to diiron pentacarbonyl ana-
logues reported in the literature23,25,27 except that the reduction
occurs at a slightly more negative potential, which is probably
ascribed to the decent electron-donating ability of the coordinated
Fig.
8
Cyclic voltammograms of Poly-Py (5.1 mg mL-1) in
0.1 mol L-1/DMF solution in the presence of acetic acid (0.0, 1.0, 4.0,
8.0, 10.0 eq.) at a scan rate of 0.1 V s-1 (298 K). Inset: Plot of the catalytic
peak current against the concentration of the acid.
11260 | Dalton Trans., 2010, 39, 11255–11262
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-1
The BBSRC (UK) supports DJE through a Core Strategic Grant
to the John Innes Centre.
{Fe2(CO)X } , instead its protonated form, for example, hydride
after being further reduced at a more positive potential compared
to that of the initial reduction is probably responsible for the
catalysis.30,33,35,39 This mechanistic complication may explain the
positive shift in the peak potential observed for complex 3 on
variation of the acid concentration, Fig. 9. Such a positive shift
was also observed in the catalysis of proton reduction with the
presence of Poly-Py as shown in Fig. 8, which suggests that this
polymeric complex may adopt a similar catalytic mechanism.
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4
Conclusions
Two polymeric diiron complexes as mimicking systems for the
[FeFe]-hydrogenase were prepared using “click” chemistry. Except
for the terminal diiron unit, the other diiron unit exists as a
pentacarbonyl core, {Fe2(CO)5}, in which one of the iron atoms
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We thank NSF (China) (Grant No: 20871064), Ministry of
Science and Technology (China) (973 program, Grant No:
2009CB220009), and the State Key Laboratory of Coordination
Chemistry at Nanjing University (China) for financial support.
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©