A phosphoramidite-based [FeFe]H2ase functional mimic displaying fast electrocatalytic proton reduction

Article abstract of DOI:10.1039/c3dt53471e

A phosphoramidite modified [FeFe]H₂ase mimic is studied as a model for photodriven production of H₂. On cathodic activation, the pyridyl-phosphoramidite complex exhibits a strongly enhanced rate of proton reduction over the previously reported pyridylphosphine model at the same overpotential. Analysis of the cyclic voltammograms shows an apparent H ₂ evolution rate strongly influenced by the presence of both side-bound pyridyl and phosphorous-bound dimethylamino moieties at the phosphoramidite ligands. This difference is ascribed to the basic amines acting as proton relays. the Partner Organisations 2014.

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A phosphoramidite-based [FeFe]H2ase functional
mimic displaying fast electrocatalytic proton
reduction†
Cite this: Dalton Trans., 2014, 43,
8363
Sofia Derossi,‡a René Becker,‡a Ping Li,a František Hartl*a,b and Joost N. H. Reek*a
A phosphoramidite modified [FeFe]H2ase mimic is studied as a model for photodriven production of H2.
On cathodic activation, the pyridyl–phosphoramidite complex exhibits a strongly enhanced rate of proton
reduction over the previously reported pyridylphosphine model at the same overpotential. Analysis of the
cyclic voltammograms shows an apparent H2 evolution rate strongly influenced by the presence of both
side-bound pyridyl and phosphorous-bound dimethylamino moieties at the phosphoramidite ligands.
This difference is ascribed to the basic amines acting as proton relays.
Received 10th December 2013,
Accepted 3rd April 2014
DOI: 10.1039/c3dt53471e
www.rsc.org/dalton
Efficient photochemical generation of molecular hydrogen is
one of the key technologies our society needs in order to move
to a sustainable hydrogen-based economy.1 As a consequence,
a great deal of attention has been devoted to (photo-redox)-
catalysts performing proton reduction, and both homogeneous
and heterogeneous systems have been developed.2 [FeFe]
hydrogenases ([FeFe]H2ase) are natural-occurring enzymes that
catalyse reduction of protons in an extremely efficient way
(TOF ∼ 9000 s−1).3 Models of their bimetallic core based on
cheap and abundant materials are easily synthesised, which
has led to a plethora of diiron mimics.4 So far, the majority of
studies of such bioinspired systems have focused on electro-
catalysis,5 while the direct photodriven catalysis, using a light-
harvesting chromophore coupled to the [FeFe] core, has only
recently been addressed.6,7
The few initial attempts to assess light-driven H2 pro-
duction with homogeneous systems are roughly based on four
approaches: (i) simple mixing of the chromophore with the
catalyst in solution, (ii) electron-transfer mediation by addition
of an electron relay in donor–mediator–catalyst systems,
(iii) covalent attachment of the chromophore to the catalyst or
(iv) supramolecular coordination of the catalyst to the chromo-
phore.7 The first two strategies offer ease of screening but limit
the control over the spatial arrangement between the moieties.
Fig. 1 The active [FeFe]H2ase biomimetic supramolecular assembly
formed under photoreductive conditions from complex
1 in the
presence of two different porphyrins.
However, advantages of a well-defined system in the covalent
case come at the price of tedious syntheses and more rapid
charge recombination.
We envisaged a supramolecular approach combining the
chromophore with the precatalyst to be most advantageous.
With this philosophy in mind, we recently introduced
1·ZnTPP, a supramolecular dyad which (after disproportio-
nation into a disubstituted complex, Fig. 1) is capable of a photo-
catalytic conversion of protons into H2.7 In that case, ZnTPP
was chosen as the photosensitizer, while the catalytic mimic 1
was the [Fe2(μ-pdt)(CO)5L] precursor (Fig. 2). L represents the
template ligand8 pPyPPh2 (pPy = 4-pyridyl), which is co-
ordinated to the diiron centre via the phosphorus atom and to
ZnTPP via the pPy group (Fig. 1). Under photocatalytic con-
ditions, i.e. upon irradiation in the presence of a sacrificial
proton- and electron donor, the supramolecular assembly
1·ZnTPP exhibits H2 evolution, whereas the reference complex
(3; L = PPh3) is inactive.
aVan’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park
904, 1098 XH, Amsterdam, The Netherlands. E-mail: j.n.h.reek@uva.nl;
Fax: (+31)205255604; Tel: (+31)205255265
bDepartment of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD,
UK. E-mail: f.hartl@reading.ac.uk; Fax: +44 (0)1183786331; Tel: +44 (0)1183786795
†Electronic supplementary information (ESI) available: Syntheses and character-
Herein, we report development of our supramolecular dyad
approach, using mPyPA, a recently reported phosphoramidite
template ligand based on the binol motif and decorated with
two pyridyl groups (Fig. 2, complex 2).9 We have been intrigued
isation of complexes 2 and 4; detailed description of experiments, Fig. S1–S17
and Tables S1–S2. See DOI: 10.1039/c3dt53471e
‡Both authors contributed equally to this work.
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Table 2 Electrochemical data of compounds 1 to 4 in butyronitrile. For
an explanation on electrochemical reaction orders (n) and rate constants
(k’), see text. All potentials are vs. Fc/Fc+. Observed rate constant kobs at
7 mM acetic acid concentration
Complex
Epc1 [V]
Epc2 [V]
n
k′
kobs [s−1
]
1
2
3
4
−1.78
−1.88
−1.81
−1.90
−2.24
−2.28
−2.26
−2.38
1.79
3.58
2.31
2.85
17.8
6.00
1.44
0.93
580
6340
130
240
In line with this observation, cyclic voltammetry shows that
1–4 have their (irreversible) first cathodic waves placed at
roughly the same electrode potential (Table 2). The electrocata-
lytic activity of these complexes towards proton reduction was
studied at a static mercury drop electrode by examining the
growth of their cathodic waves upon addition of acetic acid
under identical experimental conditions (Fig. 3). Again, cata-
lytic waves for all four compounds were found at roughly the
same potential (−2.4 V to −2.5 V vs. Fc/Fc+).§ Adsorption of the
pyridine-functionalized complexes to the mercury surface was
not observed.¶
Although both cathodic and catalytic wave potentials for all
four studied complexes are similar, the current maxima and
shapes of the catalytic waves have indicated that there is a
remarkable difference in activity. To understand this differ-
ence, in-depth analysis was performed on the cyclic voltammo-
grams for each complex. The observed rate constant kobs has
been determined by the method of DuBois and co-workers
(eqn (1)),11 using the ratio between the second cathodic peak
current (ipc2) and the catalytic peak current (icat) under the
assumption that H2 formation is irreversible (ErCi mechan-
ism). Remarkably, catalytic efficiency is much higher for cata-
lyst precursor 2, as reflected in the much larger kobs (∼6000 s−1
at 7 mM acetic acid) compared to 1, 3 and 4 (Fig. 4). Since all
catalysts operate at similar potentials, the effect of differences
Fig. 2 The novel [FeFe]H2ase biomimetic catalyst 2 based on the phos-
phoramidite ligand appended with two 3-pyridyl groups (mPyPA) and
the reference complexes 1, 3 and 4.
by the properties such ligand could impart onto the diiron
core. Phosphoramidites have better π-acceptor properties com-
pared to the related phosphines, likely removing electron
density from the diiron core. This would lead to less negative
reduction potentials of the complex and, therefore, smaller
overpotential for H2 production. Furthermore, the two pyridyl
side groups on the ligand may facilitate the association of
macrocyclic chromophores to the metal centre and/or partici-
pate in proton transfer.
Complex
2
[Fe2(μ-pdt)(CO)5(mPyPA)] was synthesised
[Fe2(μ-pdt)-
together with the pyridyl-free complexes
3
(CO)5(PPh3)] and 4 [Fe2(μ-pdt)(CO)5(PhPA)] serving for control
experiments (Fig. 2). The syntheses of 2 and 4 involve substi-
tution of one carbonyl ligand in [Fe2(μ-pdt)(CO)6] for the phos-
phoramidite ligand: 31P NMR spectra as well as MS of the new
compounds revealed single CO displacement and phosphora-
midite coordination to the Fe centre via the phosphorus atom,
leaving, in each case, both pyridyl groups on the phosphor-
amidite free for coordination to the photosensitizer.
The IR spectra show a characteristic ν(CO) pattern of
[Fe2(μ-pdt)(CO)5L] (Table 1) and carbonyl stretching shifted
ca. 30 cm−1 to smaller wavenumbers with respect to [Fe2(μ-pdt)-
(CO)6]. Surprisingly, the ν(CO) values are very close to those for
the pyridylphosphine derivative 1, demonstrating that the
electronic difference between the pyridylphosphine and phos-
phoramidite ligands is not translated into Fe-to-CO π-back-
donation and hence into differences in electron density at the
iron core.
§Cyclic voltammograms of 1 in ref. 7 showed electrocatalytic proton reduction at
the potential coinciding with the first cathodic wave. However, these voltammo-
grams were recorded using a glassy carbon working electrode, which gives com-
peting direct reduction, thereby increasing the observed “catalytic” current.18
Repeating the same experiments using a static mercury drop electrode has
revealed that the true catalytic response is in fact not at the first cathodic wave,
but shifted more negatively by ca. 0.6 V.
¶It is known that pyridine (and pyridine-functionalized molecules) can adsorb
to mercury surfaces.19 If this were the case for complexes 1 and 2, their cyclic
voltammograms would diverge significantly from those recorded for complexes
3 and 4, respectively. Namely, the cathodic peaks would be broadened20 and
diffusion-related behaviour would be suppressed. However, we have observed
almost identical cyclic voltammograms for 1 and 3, and 2 and 4 (cf. ESI,
Fig. S11†) which are in their turn similar to cyclic voltammograms published in
literature (on Pt and glassy carbon).10 Furthermore, we have conducted cyclic
voltammetry of complex 2 at different scan rates (0.05 to 5.0 V s−1) using both
static mercury drop and platinum microdisc working electrodes and found a
linear relationship between the cathodic peak current and the square root of the
scan rate (cf. ESI, Fig. S13 and S14†). Since this behaviour corresponds with the
diffusion control of mass transport in the double-layer region of the working
electrode in line with the Randles–Sevcik equation, we assume no significant
surface adsorption for the major species in the electrolyte solution.
Table 1 The IR ν(CO) wavenumbers of complexes
1 to 4 in
dichloromethane
Complex
ν(CO) [cm−1
]
1 (ref. 7)
2
3 (ref. 10)
2048 (s), 1985 (s), 1966 (sh), 1937 (m)
2047 (s), 1993 (s), 1975 (s), 1962 (m)
2044 (s), 1984 (s), 1931 (m)
4
2045 (s), 1992 (s), 1973 (m), 1961 (sh)
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concentration was sought by equating kobs to ∂[H2]/∂t. The
obtained curve (kobs vs. [H+]) is then characterised by the rate
constant k′ and reaction order n in a pseudo rate equation pro-
portional to the kinetics of the reaction under study. Fitting
our data points to a power function (eqn (2)) has yielded
values for k′ and n for each complex under study (Table 2) with
R2 values between 0.993 and 0.999.
The obtained pseudo rate equation for (electro)catalysis
takes into account the step(s) just before the rate determining
step, including additional protonation equilibria (if any).14 On
this basis a correlation between rate order and constant, and
Brønsted basic sites has been found: Phosphine complexes
1 and 3 show a reaction order close to 2, whereas for phos-
phoramidite complexes 2 and 4, n has been found close to 3,
suggesting additional protonation equilibria. Since phosphor-
amidites can be protonated on the dimethylamine moiety,
proton preorganization might explain the differences in the
reaction order. It has been shown for multiple proton
reduction catalysts that a proton relay close to the active site
can indeed increase their activity, supporting the hypothesis of
proton preorganization in the case of the phosphoramidite
complexes.15
Fig. 3 Cyclic voltammetry of complex 2 (1.0 mM) in butyronitrile con-
taining 0.1 M (nBu)4NPF6 showing an effect of increasing acetic acid
concentration: 0 (dotted), 1, 2, 4, 5, 6, and 7 mM. Conditions: static
mercury drop (SMD) working electrode, scan rate 0.1 V s−1. The dashed
line represents direct (background) proton reduction for 7 mM acetic
acid in butyronitrile in the absence of catalyst.
Still, this does not explain why complex 2 is much more
active than its phenyl analogue 4. However, it has been found
that the pseudo rate constants for phenyl-functionalized 3 and
4 are considerably smaller than those for pyridyl-functiona-
lized 1 and 2. This pyridyl-induced rate increase might be
explained by an electronic communication between the ligand
and iron core or by stabilisation of the mono-reduced
intermediate.8a,9
From this analysis, it is believed that for the high electroca-
talytic rate observed for 2, both phosphoramidite and pyridyl
functionalities are mandatory. The dimethylamino moiety
might act as a proton relay, whereas the role of the pyridyl
functionality remains unclear. This distinct ligand effect also
implies that the active species contains at least one ligand
moiety. However, since it is known that Fe2(µ-pdt) complexes
undergo
a variety of transformations on one-electron
Fig. 4 Plotted dependence of the rate constant kobs on acetic acid con-
centration for complexes 1 to 4. Dashed lines are curve fits of the form
reduction, it is unclear whether the active species is in fact
one-electron reduced 2 or one of its secondary reduction
products.k Although its exact structure remains elusive, the
active species formed after reduction of precatalyst 2 shows a
much higher electrocatalytic activity than its phosphine
analogue.
y = axn. For fit parameters, R2 values and zoomed-in graph (kobs < 600 s−1
)
see the ESI.‡
in overpotential on the catalytic rate can be neglected,12 and
therefore the rate increase might well be caused by differences
in particular mechanisms of the H2 evolution.
ꢀ
ꢁ
kComplex 2 shows an irreversible one-electron reduction wave, consistent with
previously reported mono-substituted pentacarbonyl Fe2(µ-pdt) complexes.10
Furthermore, on one-electron reduction, complex 1 has been shown to dispro-
portionate into the parent hexacarbonyl and the disubstituted tetracarbonyl
complex.7 Spectroelectrochemistry (SEC) on 2 shows an identical behaviour
(cf. ESI, Fig. S15 to S17†). However, on the short time scale of cyclic voltammetry
(defined by ν = 100 mV s−1 as opposed to 2 mV s−1 for SEC), it is reasonable to
assume that during catalysis a rather complex chemical mixture is present, con-
sisting of precatalyst 2, one-electron reduced 2−, its disproportionation products
(the hexacarbonyl and the disubstituted complexes 5, see ESI†), their respective
decomposition products (a mixture of Fe2 and Fe4 clusters with or without
Fν icat
RT ip
kobs ¼ 0:0497
@½H2ꢀ
ð1Þ
n
¼ k′½Hþꢀ
ð2Þ
@t
In line with behaviour of the natural system and obser-
vations from recent model complexes,13 it seems eminent that
proton preorganization plays a role in accelerating the catalytic
reduction. To get more insight into the proton-reactive behav-
iour, a relation between the rate of the H2 formation and acid bridging CO and thiolate ligands) plus all possible protonated species.
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Having established that compound 2 is a precursor of a
good proton reduction catalyst, we analysed its photocatalytic
Acknowledgements
behaviour when combined with zinc tetraphenylporphyrin This work has been supported (S.D. and P.L.) by the Nether-
(ZnTPP). Comparison of the catalytic potential of 2 (roughly lands’ Organization for Scientific Research (NWO-CW, ECHO
−2.5 V vs. Fc/Fc+) with the second reduction potential of grant 700.57.042) and the BioSolar Cells program (R.B.). We
the porphyrin belonging to its singlet excited state (−1.75 V vs. acknowledge Dr A. M. Kluwer (InCatT BV) for fruitful discus-
Fc/Fc+) shows that the quenching of ZnTPP* by 2 is thermo- sions, and Dr R. Bellini (UvA) and Drs Bart van den Bosch
dynamically uphill, making photocatalysis for this system (UvA) for ligand synthesis.
unfeasible.**
Despite this, we still trust that the overall supramolecular
strategy is worth investigating, since using e.g. aromatic dithio-
lates instead of the propanedithiolate bridge in the studied
Notes and references
complexes (Fig. 2), the catalytic potential may well be shifted
towards a range well-accessible for ZnTPP.16 Therefore, we
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complex with ZnTPP. First, the binding of ZnTPP to the pyridyl
groups at the mPyPA ligand in complex 2 was determined by
means of UV-Vis titration (ESI†). Free mPyPA binds two ZnTPP
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8366 | Dalton Trans., 2014, 43, 8363–8367
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