Hydrogenase biomimetics: Fe2(CO)4(μ-dppf)(μ- pdt) (dppf = 1,1′-bis(diphenylphosphino)ferrocene) both a proton-reduction and hydrogen oxidation catalyst

Article abstract of DOI:10.1039/c3cc46456c

Fe₂(CO)₄(μ-dppf)(μ-pdt) catalyses the conversion of protons and electrons into hydrogen and also the reverse reaction thus mimicing both types of binuclear hydrogenase enzymes.

Full text of DOI:10.1039/c3cc46456c

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Hydrogenase biomimetics: Fe2(CO)4(l-dppf)(l-pdt)
(dppf = 1,10-bis(diphenylphosphino)ferrocene)
both a proton-reduction and hydrogen
oxidation catalyst†
Cite this: Chem. Commun., 2014,
50, 945
Received 23rd August 2013,
Accepted 27th November 2013
Shishir Ghosh,a Graeme Hogarth,*ab Nathan Hollingsworth,a Katherine B. Holt,*a
DOI: 10.1039/c3cc46456c
Shariff E. Kabirc and Ben E. Sancheza
www.rsc.org/chemcomm
Fe2(CO)4(l-dppf)(l-pdt) catalyses the conversion of protons also the oxidation of hydrogen to protons and electrons.9 We
and electrons into hydrogen and also the reverse reaction thus herein describe Fe2(CO)4(m-dppf)(m-pdt) (2) {pdt = S(CH2)3S, dppf =
mimicing both types of binuclear hydrogenase enzymes.
1,10-bis(diphenylphosphino)ferrocene} which we have shown is a
catalyst for both of these transformations.
Hydrogenases are enzymes capable of reversibly converting protons
Heating a toluene solution of equimolar amounts of Fe2(CO)6-
and electrons into hydrogen1 and over the past two decades their active (m-pdt) (1) with dppf initially leads to formation of the linked tetra-
sites have been discerned primarily from crystallographic studies,2–4 nuclear complex {Fe2(CO)5(m-pdt)}2(m,k1,k1-dppf)10 and unreacted
with three phylogenetically different enzyme types being identified. The dppf, which slowly rearranges to afford Fe2(CO)4(m-dppf)(m-pdt) (2)
two most widely studied of these are the so-called [FeFe]H2ase and in moderate yields‡ as an air-stable orange solid (Scheme 1) being
[NiFe]H2ase enzymes (Chart 1) the active sites of which contain two characterised by analytical and spectroscopic data together with a
transition metal atoms. While both enzyme types are able to catalyse single-crystal X-ray diffraction study, the results of which are sum-
both the reduction of protons and oxidation of hydrogen, [FeFe]H2ase marised in Fig. 1. The molecule is as expected and bond lengths and
enzymes are more efficient with respect to the former, while angles are generally within the ranges of those seen in related
[NiFe]H2ase enzymes are favoured for the oxidation of hydrogen.5
complexes.10,11 The iron–iron bond length of 2.6133(6) Å is, however,
Over the past 15 years, a range of structural and functional some 0.1 Å longer than is generally the case10–12 suggesting that the
biomimetics of the active sites of these enzymes have studied5,6 flexible nature of the dppf ligand allows this bond to relax. The non-
with key insights into the likely mechanism(s) being determined7 bonding iron–iron distances of 4.581 and 4.613 Å suggest that there is
and recently some impressive turnovers for the electrocatalytic no direct contact between the two redox centres in the molecule.
reduction of protons being reported.8 However, as far as we are
Cyclic voltammograms (CVs) of 2 were recorded in acetonitrile at
aware, no biomimetic has yet been shown to both be catalytic various scan rates as shown in Fig. 2. The complex undergoes an
for the reduction of protons and electrons to hydrogen and electrochemically reversible oxidation at E1/2 = 0.05 V (DE = 60 mV)
and a further reversible oxidation at E1/2 = 0.685 V (DE = 70 mV). The
former is associated with oxidation of the diiron centre and the latter
most likely with the ferrocene moiety (see later). The reversibility of
both oxidative processes is maintained at all scan rates. The complex
also shows two overlapping irreversible reduction peaks at Ep
=
ꢀ2.10 V and Ep = ꢀ2.19 V which become separated at higher scan
rates (Z0.25 V sꢀ1) (Fig. 2). Two small oxidation peaks are also
Chart 1 Structure of the active site of [FeFe]H2ase and [NiFe]H2ase.
a Department of Chemistry, University College London, 20 Gordon Street, London,
WC1H 0AJ, UK. E-mail: g.hogarth@ucl.ac.uk
b Department of Chemistry, King’s College London, Britannia House,
7 Trinity Street, London SE1 1DB, UK. E-mail: Graeme.hogarth@kcl.ac.uk
c Department of Chemistry, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh
† Electronic supplementary information (ESI) available: Experimental details,
crystallographic data. CCDC 956914. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3cc46456c
Scheme 1 Preparation of Fe2(CO)4(m-dppf)(m-pdt) (2).
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Fig. 1 Molecular structure of 2. Selected bond lengths [Å] and bond
angles [1]: Fe(1)–Fe(2) 2.6133(6), Fe(1)–P(1) 2.2256(6), Fe(2)–P(2)
2.2679(6), P(1)–Fe(1)–S(1) 174.34(2), P(2)–Fe(2)–S(1) 167.79(2).
Fig. 3 CVs of 2 in the absence and presence of 1–10 molar equivalents of
HBF4ꢁEt2O (1 mM solution in MeCN, supporting electrolyte [NBu4][PF6],
scan rate 0.1 V sꢀ1, glassy carbon electrode, potential vs. Fc+/Fc).
Complex 2 was first tested as a proton reduction catalyst in the
presence of HBF4ꢁEt2O in MeCN. Fig. 3 shows the CVs upon addition
of between 1–10 equivalents of acid. A new reduction wave appears
at Ep = ꢀ1.70 V upon addition of acid being associated with
reduction of 3, its height growing with increasing amounts of acid,
being characteristic of electrocatalytic proton reduction.6 At higher
amounts of acid (Z7 molar equivalents) this wave splits into two
distinct peaks possibly resulting from reduction of the putative
cation [HFe2(CO)4(m-H)(m-dppf)(m-pdt)]+ (see below). Another catalytic
wave is also observed at Ep = ꢀ2.10 V which competes with the direct
reduction of HBF4ꢁEt2O by the glassy carbon electrode as this
electrode becomes catalytically active beyond ꢀ2.00 V in presence
of strong acids.14 On the return scan a further catalytic wave is seen
Fig. 2 CVs of 2 in MeCN (1 mM solution, supporting electrolyte [NBu4][PF6],
glassy carbon electrode, potential vs. Fc+/Fc) at various scan rates.
observed at Ep = ꢀ1.80 V and Ep = ꢀ1.53 V on the return scan being at Ep = ꢀ1.55 V implying that the species responsible for the first
due to the product formed in the reductive processes, whilst the small catalytic wave is regenerated. Thus it appears that 2 enters into
reduction peak appeared at Ep = ꢀ0.35 V on the return scan is the catalytic cycle via a CE mechanism to generate the neutral
associated with the first oxidation product.
paramagnetic complex Fe2(CO)4(m-H)(m-dppf)(m-pdt)15 which either
Upon chemical oxidation, by addition of FcPF6 to a CH2Cl2 solution protonates or undergoes a further reduction before second protona-
of 2, new IR absorption bands appear at 2044 and 2013 cmꢀ1 (Fig. S1,
tion to liberate hydrogen. The peak heights of the oxidative processes
ESI†). The 60 cmꢀ1 shift of the first nCO band to higher energy is do not change during the experiment showing the robustness of 2
indicative of oxidation of the diiron centre allowing assignment of under the operating conditions.
the couple at 0.05 V vs. Fc+/Fc to this process. The second oxidation
Recent developments in hydrogenase biomimics suggest that H2
we believe is associated with the dppf ligand. Uncoordinated dppf activation can be favoured by the presence of a mild and chemically
undergoes an irreversible oxidation at 0.20 V which becomes inert oxidant in the diiron models.16 The concept was recently
reversible and shifts to more positive potentials upon coordination experimentally implemented by Camara and Rauchfuss17,18
to a metal centre.13 The relative position of the second oxidative who utilised (C5Me5)Fe(C5Me4)CH2PEt2 (FcP*) as the intramolecular
process vs. Fc+/Fc and its chemical reversibility is consistent with
oxidant, the FeII/III couple (E1/2 = ꢀ0.59 V) of which lies closer to the
the FeII/III couple of the ferrocene moiety.
H2/H+ couple vs. the Fc+/Fc couple.18 They showed that the dication
We next assessed the ability of 2 to bind a proton. Addition of of Fe2(CO)3(k2-Ph2PCHQCHPPh2)(k1-FcP*){m-SCH2N(Bz)CH2S} (A)
one molar equivalent of HBF4ꢁEt2O to a CH2Cl2 solution of 2 cleaves H2, being facilitated by an intramolecular electron-
(or two to an MeCN solution) resulted in the rapid and clean transfer in its doubly oxidised state, the electron transferring
formation of the cationic-hydride [Fe2(CO)4(m-H)(m-dppf)(m-pdt)][BF4] from the diiron unit to the pendent FcP* ligand i.e. switching from
(3).‡ Further, and unlike most related cationic complexes,11 Fe(III)Fe(II)Fe(I) to Fe(II)Fe(II)Fe(II).18 In contrast, an analogue of A in
addition of base leads to regeneration of the neutral complex. which FcP* is replaced by PMe3 is catalytically inactive towards H2
oxidation.18 That there is electronic communication between the
This suggests that while 2 is able to bind a proton, it is relatively
weakly held. Related Fe2(CO)4(m-diphosphine)(m-dithiolate) diiron core and the ferrocene in A despite the presence of a
complexes do not generally form stable cationic hydrides, the methylene linker unit prompted us to investigate the possibility
exceptions being Fe2(CO)4(m-Cy2PCH2PCy2)(m-pdt) and Fe2(CO)4- of electronic communication between the two redox-active metal
{m-Ph2P(CH2)4PPh2}(m-pdt) containing basic and flexible diphos- centres in 2. Indeed we found that 2 catalytically cleaves H2 in
phines respectively.11 Thus, the greater flexibility of dppf in 2 presence of a base (pyridine) in its 22+ state (Fig. 4). Thus, addition
appears to be the reason for its ability to bind a proton.
of equimolar amount of pyridine to an acetonitrile solution of 2
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R1 = 0. 0374, wR2 = 0.0929 (all data), for 511 parameters. CCDC number
956914. Synthesis of 3. To a CH2Cl2 (50 ml) solution of 2 (0.05 g,
0.06 mmol) was added a few drops of HBF4. The mixture was stirred at
room temperature for 20 min without any noticeable change. Volatiles
were removed under reduced pressure and the resulting deep red oily
solid washed with a small portion of Et2O to remove excess acid. The
remaining solid was dissolved in a minimum amount of CH2Cl2 which
was then layered with hexanes. Slow mixing of the solutions afforded
3 (0.04 g, 73%) as a dry red solid. IR n(CO)(CH2Cl2) 2058s, 2040s,
2002s cmꢀ1 1H NMR (CDCl3) d 8.11–7.33 (m, 20H, Ph), 4.74 (s, 2H,
.
CH), 4.68 (s, 2H, CH), 4.49 (s, 2H, CH), 4.32 (s, 2H, CH), 2.86
(br, 2H, CH2), 2.74 (m, 2H, CH2), 2.48 (br, 2H, CH2), ꢀ12.40 (t, J 17.6,
1H, m-H). 31P{1H} NMR (CD2Cl2) 44.8 (s) ppm.
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Ph2PCH2PPh2)(m-pdt)11 does not show catalytic waves under the
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In summary we have shown that a biomimetic of the diiron
hydrogenase can catalyse both the reduction of protons and H2
oxidation. We are currently developing a range of related bio-
mimetics containing different secondary redox-active centres20
and using density functional theory calculations in order to
more fully understand the electronic structure of 22+ and the
nature of the H2 oxidation process.
We are grateful to the Commonwealth Scholarship Commis-
sion for the award of a Commonwealth Scholarship to S.G. and
the EPSRC for a postdoctoral fellowship to N.H.
¨
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a colour change from orange to red-brown. After cooling to room tempera-
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residue. This was washed with hexanes (3 ꢂ 5 ml) and dried. Extraction into
a minimum volume of dichloromethane followed by addition of hexanes
and rotary evaporation gave 2 as a dry red solid (0.12 g, 52%). 2 can also be
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´
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