Bachelor Project: Daniël Preschel (0592412)
Light-induced generation of hydrogen by supramolecular
assemblies based on Fe2-Hydrogenase models
Synthesis and characterization of new supramolecular linkers.
Using hydrogen as a primary energy source seems a promising solution towards our
dependence on classical fuels, and climate change as well. However this can only be considered
durable if hydrogen is produced in the same manner, for example by solar power, being the most
liable candidate supporting future large-scale applications (1). As quite often, nature is the main
source of inspiration in chemistry: photosynthesis is carried out by plants; absorbing the energy
of sunlight and storing it in a chemical form, as chlorophyll (a chromophore) captures photons
and provides excited electrons that are used for the reduction of substrates. Also, researchers
discovered the class of [Fe2] hydrogenase (H2-ase) enzymes, capable of catalyzing the reduction
of protons into molecular hydrogen. In theory, combining these valuable properties into a single
compound creates a photoactive catalyst, capable of generating hydrogen upon irradiation with
visible light.
Ever since the structure of the highly efficient enzyme has been determined in quite
some detail, researchers have tried to produce synthetic H2-ase mimics. Literature has already
reported photoactive artificial supramolecular complexes that are capable of generating
hydrogen indeed (2). The active site is composed of 2 Fe atoms, coordinated by carbonyl ligands.
In these experiments one of them is substituted by pyridyl-functionalized supramolecular
linkers, which automatically attach to the chromophore (ZnTPP), see fig. 1. The self-assembly of
the photo-sensitizer (PS) is a very convenient feature in this kind of research, as it facilitates the
synthesis of many closely related complexes in an easy way.
More recently, scientists have also coordinated an isocyanide ligand to the [Fe2] active
site, more faithfully resembling the highly efficient H2-ase enzymes found in nature (3). Indeed,
this system showed improved stability under actual photocatalysis conditions. This is a very
stimulating result, as deactivation of the catalyst remains a huge challenge thus far.
Inspired by the promising results found in literature, we present the synthesis and
characterization of a small series of new supramolecular linkers, based on pyridyl-functionalized
isocyanides. The new linkers are varied in length, and the position of the pyridine. This alters the
distance and orientation of the chromophore towards the [Fe2] active centre, determining the
spatial organization of the complete assembly. The goal was to relate the structural variations of
these supramolecular complexes to their performance and stability upon photoreduction by
means of reproducible photocatalysis experiments.
We have also synthesized a new supramolecular phosphoramidite ligand, which has
already proven to be very successful in producing H2. However, this type of ligand is very soluble
in organic solvents, preventing crystallization. Therefore, we modified the ligand with an
aromatic backbone, which is expected to improve the crystallization properties, enabling the
possibility to gather more information about the structure of this compound.
The synthesis of the new supramolecular linkers, and their [Fe2] complexes was
successful, but turned out to be challenging as especially isocyanides are very difficult to handle.
They were characterized by 1H- and 31P-NMR, and IR measurements, giving detailed information
about their structure. Luminescence titration experiments monitored by UV-vis showed the
quenching of fluorescent intensity of the chromophore, upon association with the Fe2-complex.
These measurements indicate the transport of excited electrons towards the [Fe2] catalytic
centre, but were not performed accurately enough to determine the association constant
between the [Fe2]-complex and ZnTPP. Also, the electrocatalytic reductions potentials of the new
[Fe2] complexes were established by means of cyclic voltammetry. In the presence of acid, the
current measured at the negative potential associated with the reduction of the catalytic centre
gradually increased. This significant electrocatalytic response proves the [Fe2] phophoramidite
complex is an active electrocatalyst indeed. However, no photocatalysis experiments have been
carried out yet, due to set-up problems of the GC equipment. So more research is still needed to
further investigate the photocatalytic properties of these supramolecular [Fe2] complexes.
fig.1. - a. Supramolecular photocatalytic assembly based on an [Fe2]-H2-ase mimic, using a
pyridine-substituted linker to connect ZnTPP (the PS) to the Fe2 core. Upon irradiation this
assembly forms H2. b. The [Fe2]-H2-ase enzyme found in nature: the Fe2 core is coordinated
by carbonyl and cyanide ligands, and electron transfer is facilitated by Ferredoxin.
References:
1. N. Amaroli, V. Balzani. The Hydrogen Issue. ChemSusChem, 2011, 4, p.21-36.
2. A.M. Kluwer, R. Kapre, F. Hartl, M Lutz, A.L. Spek, A.M. Brouwer, P.W.N.M. van Leeuwen,
J.H.N. Reek. Self-assembled biomimetic [2Fe2S]-hydrogenase-based photocatalyst for
molecular hydrogen evolution. PNAS, 2009, 106. No.26.
3. F. Wang, W.G. Wang, X. Wang, H. Wang, C. Tung, L Wu. A highly Efficient Photocatalytic
System for Hydrogen Production by a Robust Hydrogenase Mimic in an Aqueous
Solution Angewandte Chemie. 2011, 50, p.1-6.