Design, synthesis and characterization of a modular bridging ligand platform for bio-inspired hydrogen production

Article abstract of DOI:10.1016/j.inoche.2012.04.034

Synthesis and characterization of a novel type of ambident bridging ligands joining together the functional prerequisites for visible-light absorption, photoinduced electron transfer and catalytic proton reduction is presented. This class of compounds consists of a chromophoric 1,2-diimine-based π-acceptor site and a rigid polyaromatic dithiolate chelator. Due to the presence of a common conjugated linker moiety with an intrinsic two-electron redox reactivity and a suitable orbital coupling of the subunits, a favourable situation for vectorial multielectron transfer from attached electron donors to a catalytic acceptor site is provided. As an example for the application of this kind of bifunctional ligand systems, a [FeFe]-hydrogenase enzyme model compound is prepared and structurally characterized. Electrocatalytic hydrogen formation with this complex is demonstrated.

Full text of DOI:10.1016/j.inoche.2012.04.034

Inorganic Chemistry Communications 21 (2012) 147–150
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Design, synthesis and characterization of a modular bridging ligand platform for
bio-inspired hydrogen production
Christoph Topf, Uwe Monkowius, Günther Knör ⁎
Institut für Anorganische Chemie, Johannes Kepler Universität Linz (JKU), 4040 Linz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis and characterization of a novel type of ambident bridging ligands joining together the functional
prerequisites for visible-light absorption, photoinduced electron transfer and catalytic proton reduction is
presented. This class of compounds consists of a chromophoric 1,2-diimine-based π-acceptor site and a rigid
polyaromatic dithiolate chelator. Due to the presence of a common conjugated linker moiety with an intrinsic
two-electron redox reactivity and a suitable orbital coupling of the subunits, a favourable situation for
vectorial multielectron transfer from attached electron donors to a catalytic acceptor site is provided. As an
example for the application of this kind of bifunctional ligand systems, a [FeFe]-hydrogenase enzyme model
compound is prepared and structurally characterized. Electrocatalytic hydrogen formation with this complex
is demonstrated.
Received 6 March 2012
Accepted 20 April 2012
Available online 28 April 2012
Keywords:
Multielectron transfer
Non-innocent ligands
Redox relays
Hydrogenase models
Iron catalysis
© 2012 Elsevier B.V. All rights reserved.
Due to its unexcelled energy density, hydrogen is an attractive
fuel medium for the chemical storage of renewable energy. The
search for more advanced H -releasing electro- and photocatalytic
2
systems based on earth-abundant materials therefore represents a
basic research area of paramount importance [1–4]. In living
organisms, [FeFe]-hydrogenase enzymes belong to the most
efficient class of catalysts for the reversible splitting of hydrogen
into protons and electrons. Within the last few years, there has
been a remarkable progress in understanding the structure and
function relationships of hydrogenase active sites and synthetic
analogues based on a functional Fe S -core [5–7]. Both native
2 2
enzymes and bio-inspired iron thiolate complexes have been
combined directly or indirectly with various photosensitizers in
2
order to achieve visible-light driven proton reduction and H -
release with such types of catalysts [8–13].
system consisting of a functional core unit (Scheme 1) that
electronically couples a reversible redox cofactor for multielectron
transfer chemistry with a rigid polyaromatic dithiolate site for the
attachment of robust iron-hydrogenase enzyme mimetics [17–20].
The central acenaphthene-based core 1 of the novel class of bridging
ligands presented here is obtained in three steps following published
procedures (Scheme 2) [21,22]. We have chosen this kind of linker
subunit as a starting point for the design of functionalized derivatives
of bis(arylimino)acenaphthene systems (BIAN-R) such as 2, which
can be readily modified in their electronic and solubility properties by
a variation of the aromatic substituents R (Scheme 1) [23,24]. BIAN-R
ligands are a non-innocent organic cofactors that can be exploited as
redox relays for the acceleration of biomimetic two-electron transfer
chemistry [4]. The corresponding orange- to red-colored compounds
(Fig. 1) are acting as strong π-acceptor chelates forming stable
coordination compounds with a variety of transition metals and main
group elements.
Especially, complex formation with non-precious-metal donor
fragments such as MLCT-chromophores based on copper or other
earth-abundant and environmentally benign elements is an attractive
option that can readily broaden the long-wavelength spectral
sensitization range of such systems for an optimized performance in
the context of solar energy utilization [4,25].
In addition to the functional 1,2-diimine subunit, the periphery of
the novel bridging ligand platform has been equipped with an
electronically coupled dithiolate binding site. This further modifica-
tion was carried out to enable efficient charge injection from a
chemically, electro- or photochemically reduced BIAN-R moiety in a
dyad architecture with attached electron acceptor systems such as
semiconductor surfaces, nanoparticles or molecular redox catalysts.
Since hydrogen formation in protic media is a net two-electron
process, a fundamental requirement for all such systems is the
efficient coupling of reducing-equivalents to the active site, thus
avoiding problems with destructive side reactions and back-electron
transfer. For this purpose, the native hydrogenases are wired to
additional iron-sulfur clusters acting as covalently linked redox relays
[14]. Thus, it is quite surprising that this important functional feature
has only recently been addressed in the design of synthetic
hydrogenase model compounds [15]. Following rational guidelines
for the development of artificial model enzymes [4,16], we are
2
presenting here a novel type of bio-inspired H -releasing catalyst
⁎
Corresponding author. Fax: +43 732 2468 9681.
E-mail address: guenther.knoer@jku.at (G. Knör).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.04.034

148
C. Topf et al. / Inorganic Chemistry Communications 21 (2012) 147–150
Scheme 1. a) Structural features of 1,2-diimine-fused naphthalene dithiolate ligands including possible sites for functionalization (R), coordination of metal-based donor fragments
M), and binding of catalytic subunits (cat) such as iron-hydrogenase model compounds to accept injected charges. b) π*-acceptor orbital (LUMO) of the bridging ligand
(
demonstrating the degree of electronic communication between the two binding sites.
We decided to demonstrate this potential by coupling the
precursor system 2 to a dinuclear iron carbonyl fragment in order to
obtain a functional model compound for an iron‐hydrogenase active
site. For this purpose, a toluene solution of 2 was metallated with
Fe (CO)12 and the crude product was purified by recrystallization
3
to yield the diiron cluster compound 3 (Scheme 2) as a deep red
crystalline material.
The intraligand ππ*-absorption bands of the substituted BIAN-R
derivative 2 in the UV and visible range are slightly blue-shifted upon
metallation (Fig. 1), while an additional spectral feature occurs at
spectrum of the iron-complex displays a set of three carbonyl bands
−
1
in the 1980–2080 cm
region (Fig. 2), which is consistent with the
properties of similar [FeFe]-hydrogenase model systems carrying
electron donating substituents [17]. The energetic range of the CO-
stretching vibrations, which are slightly shifted to lower frequencies
compared to compounds with an isolated carbonyl core [9,17],
indicates a significant degree of electronic communication between
the diiron moiety and the appended 1,2-diimine fragment, thus
confirming an orbital coupling of the linked subunits (Scheme 1).
The molecular structure of complex 3 is dominated by a regular
3
06 nm, which is attributed to the σσ*-absorption involving the
2 2
Fe S -butterfly core with an Fe-Fe distance of 2.509(1) Å attached to
metal-metal bond present in the diiron species 3. The infrared
the planar acenaphthene moiety of the BIAN-R ligand (Fig. 3). Further
details on selected bond lengths and angles of compounds 1 and 3 are
reported in the Supporting Information of this work.
Electrochemical properties of the iron carbonyl complex 3 were
studied by cyclic voltammetry at 298 K in DCM solution. On the
O
O
1
. NBS, DMF
2
. CrO , Ac O
cathodic scan, the voltammogram of 3 (200 μM, 0.1 M Bu
4
NPF
6
, 50
2
3
−
1
mVs
scan rate) displays a quasi-reversible primary reduction
+
3
. Na
S
2 2
, DMF
wave with a peak potential of −1.58 V versus Fc /Fc, which is in a
typical range for the reduction of a dithiolate bridged Fe
2
(CO)
6
S
S
i-Pr
i-Pr
i-Pr
1
a)
2
.0
.5
N
N
1
i-Pr
1.0
2
,6-Diisopropylaniline, HOAc, 150°C, 5h
0
0
.5
.0
2
S
S
300
400
500
600
i-Pr
i-Pr
i-Pr
Wavelentgh (nm)
b) 1
.0
.8
N
N
0
i-Pr
0.6
1
2
3
. Fe (CO)12, Toluene, 120°C, 5h
0
0
0
.4
.2
.0
. 150°C, 1h
S
S
3
00
400
500
600
OC Fe
Fe
CO
CO
3
Wavelentgh (nm)
OC
OC
CO
Fig. 1. Electronic absorption spectra of a) 7.0×10− M compound 2 and b) 2.0×10 −5
5
M
Scheme 2. Synthesis of compounds 1–3 from acenaphthene.
complex 3 in DCM-solution (298 K, 1-cm cell).

C. Topf et al. / Inorganic Chemistry Communications 21 (2012) 147–150
149
100
9
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
2
075
2
033
1 985
2000
2200
2100
1900
1800
-
1
Wavenumber (cm )
Fig. 2. ATR-FTIR spectrum of the iron carbonyl complex 3 in the solid state at 298 K.
Fig. 4. Cyclic voltammograms of acetic acid (0, 60 and 120 mM) dissolved in N
2
-purged
DCM in the absence and presence of 200 μM diiron complex 3 indicating electrocat-
4 6
alytic behaviour for hydrogen production (0.1 M Bu NPF
, 50 mVs− scan rate).
1
moiety not carrying strongly electron withdrawing groups [7,26]. A
second irreversible reduction process occurs at −1.90 V, which is
tentatively assigned to the aromatic 1,2-diimine core of the
complex. The corresponding free BIAN-R ligand shows a chemically
hundreds of mV clearly demonstrates that 3 can serve as a catalyst for
reversible reduction wave with
a
cathodic peak potential at
H
2
-generation in DCM solution.
Thus, the additional beneficial features such as an extended
+
−
2.14 V versus Fc /Fc [27]. In biomimetic hydrogenase model
systems favouring the net transfer of two electrons to the bimetallic
chromophoric π-electron system and the further metal complex
docking site attached to our novel catalyst system does not seem to
interfere negatively with its efficient function as an artificial
hydrogenase model compound.
iron site via sequential one-electron steps, the first reduction
7
8
process leads to a d –d precursor compound, which similar to
other metal-based mixed valent systems [3,28] represents a key-
intermediate for the catalytic release of hydrogen [26]. Therefore,
we also investigated the electrochemical response of DCM solutions
of the iron carbonyl complex 3 in the presence of acetic acid
In summary, we have introduced a novel versatile type of bridging
ligand that provides a favourable orbital communication between
covalently linked 1,2-diimine and rigid dithiolate chelator subunits.
The compound is deeply coloured and integrates a readily tunable
molecular architecture with an intrinsic two-electron redox reactiv-
ity. These advantageous properties can be exploited to combine the
complementary functions of visible-light absorption and multielec-
tron transfer catalysis in a synergistic manner. As a first example of
the latter reactivity, the promising electrocatalytic properties of a
novel biomimetic [FeFe]-hydrogenase model compound based on this
bifunctional ligand system have been investigated. Further studies are
currently underway to modify the features of such compounds by a
systematic variation of substituents R and by binding additional metal
complex fragments to the 1,2-diimine coordination site of the BIAN-R
subunit of the compounds (Scheme 1) to demonstrate the broad
potential of these systems as novel multielectron transfer photosen-
sitizers for artificial photosynthesis and bio-inspired photoredox
catalysis [4].
[29,30] as shown in Fig. 4.
While without compound 3 no reduction wave occurs in the
monitored potential window, the over-voltage for proton reduction
from AcOH is significantly lowered in the presence of the reduced
diiron complex. The observed dependence of the current on the acid
concentration together with the large anodic potential shift of several
Acknowledgement
Financial support of this work by the Austrian Science Fund FWF
project P21045: “Bio-inspired Multielectron Transfer Photosensi-
tizers” is gratefully acknowledged.
Appendix A. Supplementary material
Supplementary data to this article can be found online at doi:10.
1
016/j.inoche.2012.04.034.
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