Electrocatalytic proton-reduction behaviour of telluride-capped triiron clusters: tuning of overpotentials and stabilization of redox states relative to lighter chalcogenide analogues

Article abstract of DOI:10.1039/d0dt00556h

Reaction of [Fe₃(CO)₉(μ₃-Te)₂] (1) with the corresponding phosphine has been used to prepare the phosphine-substituted tellurium-capped triiron clusters [Fe₃(CO)₉(μ₃-Te)₂(PPh₃)] (2), [Fe₃(CO)₈(μ₃-Te)₂(PPh₃)] (3) and [Fe₃(CO)₇(μ₃-Te)₂(μ-R₂PXPR₂)] (X = CH₂, R = Ph (4), Cy (5); X = NPrⁱ, R = Ph (6)). The directly related cluster [Fe₃(CO)₇(μ₃-CO)(μ₃-Te)(μ-dppm)] (7) was isolated from the reaction of [Fe₃(CO)₁₀(μ-Ph₂PCH₂PPh₂)] with elemental tellurium. The electrochemistry of these new clusters has been probed by cyclic voltammetry, and selected complexes have been tested as proton reduction catalysts. Each 50-electron dicapped cluster exhibits two reductive processes; the first has good chemical reversibility in all cases but the reversibility of the second is dependent upon the nature of the supporting ligands. For the parent cluster1and the diphosphine derivatives4-5this second reduction is reversible, but for the PPh₃complex3it is irreversible, possibly as a result of CO or phosphine loss. The nature of the reduced products of1has been probed by DFT calculations. Upon addition of one electron, an elongation of one of the Fe-Te bonding interactions is found, while the addition of the second electron affords an open-shell triplet which is more stable by 8.8 kcal mol^-1than the closed-shell singlet dianion and has two elongated Fe-Te bonds. The phosphine-substituted clusters also exhibit oxidation chemistry but with poor reversibility in all cases. Since the reduction potentials for the tellurium-capped clusters occur at more positive potentials than for the sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues, the tellurium-capped clusters1and3-5have been examined as proton reduction catalysts. In the presence ofp-toluenesulfonic acid (TsOH) or trifluoroacetic acid (TFA), these clusters reduce protons to H₂at both their first and second reduction potentials. Electron uptake at the second reduction potential is far greater than the first, suggesting that the open-shell triplet dianions are efficient catalysts. As expected, the catalytic overpotential increases upon successive phosphine substitution but so does the current response. A mechanistic scheme that takes the roles of the supporting ligands on the preferred route(s) to H₂production and release into account is presented.

Full text of DOI:10.1039/d0dt00556h

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Electrocatalytic proton-reduction behaviour of
telluride-capped triiron clusters: tuning of
overpotentials and stabilization of redox states
relative to lighter chalcogenide analogues†
Cite this: DOI: 10.1039/d0dt00556h
Ahibur Rahaman,a George C. Lisensky, *b Jess Browder-Long,b David A. Hrovat,c,d
e
Michael G. Richmond,d Ebbe Nordlander *a and Graeme Hogarth
*
Reaction of [Fe3(CO)9(μ3-Te)2] (1) with the corresponding phosphine has been used to prepare the phos-
phine-substituted tellurium-capped triiron clusters [Fe3(CO)9(μ3-Te)2(PPh3)] (2), [Fe3(CO)8(μ3-Te)2(PPh3)] (3)
and [Fe3(CO)7(μ3-Te)2(μ-R2PXPR2)] (X = CH2, R = Ph (4), Cy (5); X = NPri, R = Ph (6)). The directly related
cluster [Fe3(CO)7(µ3-CO)(µ3-Te)(µ-dppm)] (7) was isolated from the reaction of [Fe3(CO)10(µ-Ph2PCH2PPh2)]
with elemental tellurium. The electrochemistry of these new clusters has been probed by cyclic voltam-
metry, and selected complexes have been tested as proton reduction catalysts. Each 50-electron
dicapped cluster exhibits two reductive processes; the first has good chemical reversibility in all cases but
the reversibility of the second is dependent upon the nature of the supporting ligands. For the parent
cluster 1 and the diphosphine derivatives 4–5 this second reduction is reversible, but for the PPh3
complex 3 it is irreversible, possibly as a result of CO or phosphine loss. The nature of the reduced pro-
ducts of 1 has been probed by DFT calculations. Upon addition of one electron, an elongation of one of
the Fe–Te bonding interactions is found, while the addition of the second electron affords an open-shell
triplet which is more stable by 8.8 kcal mol−1 than the closed-shell singlet dianion and has two elongated
Fe–Te bonds. The phosphine-substituted clusters also exhibit oxidation chemistry but with poor reversi-
bility in all cases. Since the reduction potentials for the tellurium-capped clusters occur at more positive
potentials than for the sulfur and selenium analogues, and the redox processes also show better reversi-
bility than for the S/Se analogues, the tellurium-capped clusters 1 and 3–5 have been examined as proton
reduction catalysts. In the presence of p-toluenesulfonic acid (TsOH) or trifluoroacetic acid (TFA), these
clusters reduce protons to H2 at both their first and second reduction potentials. Electron uptake at the
second reduction potential is far greater than the first, suggesting that the open-shell triplet dianions are
efficient catalysts. As expected, the catalytic overpotential increases upon successive phosphine substi-
tution but so does the current response. A mechanistic scheme that takes the roles of the supporting
ligands on the preferred route(s) to H2 production and release into account is presented.
Received 14th February 2020,
Accepted 3rd April 2020
DOI: 10.1039/d0dt00556h
rsc.li/dalton
Introduction
Hydrogenases are enzymes that catalyse the production and
oxidation of molecular hydrogen using earth-abundant
metals1,2 There are three types of hydrogenases3,4 and all share
aChemical Physics, Department of Chemistry, Lund University, Box 124,
SE-221 00 Lund, Sweden. E-mail: Ebbe.Nordlander@chemphys.lu.se
bDepartment of Chemistry, Beloit College, 700 College Street, Beloit, WI 53511, USA
cCenter for Advanced Scientific Computing and Modeling, University of North Texas,
Denton, TX 76203, USA
common features; there is a proton/hydrogen binding site, a
redox centre, and π-acceptor (CO and/or CN−) and sulfur
ligands capable of stabilising metals in low oxidation states.
The most widely studied hydrogenases are the [FeFe]-hydroge-
nases that contain a diiron centre supported by carbonyl and
cyanide ligands and spanned by a dithiolate bridge.5–7 These
are extremely efficient catalysts for the reduction of protons to
give hydrogen, and a plethora of biomimetics of this active
site have been prepared and studied.8–17 Far less studied are
dDepartment of Chemistry, University of North Texas, 1155 Union Circle,
Box 305070, Denton, TX 76203, USA
eDepartment of Chemistry, King’s College London, Britannia House, 7 Trinity Street,
London SE1 1DB, UK. E-mail: graeme.hogarth@kcl.ac.uk
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0dt00556h
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biomimetic complexes of [FeNi]-hydrogenases.18–21 These
enzymes normally contain sulfur-bound cysteine ligands;
however, some variants that are more stable towards oxygen
also contain selenium (selenocysteine).18–21
Experimental
General procedures
Unless otherwise stated, purification of solvents, reactions,
In searching for simple, stable and efficient iron-based and manipulation of compounds were carried out under a
proton reduction catalysts, we22–24 and others25–31 have turned nitrogen atmosphere using standard Schlenk techniques.
our attention to chalcogenide-stabilised tri- and tetra-iron Reagent grade solvents were dried by standard procedures and
carbonyl clusters as potential candidates. These include the were freshly distilled prior to use. All chromatographic separ-
disulfide-capped cluster [Fe3(CO)9(μ3-S)2]27,28 together with ations and ensuing workup were carried out in air. Thin layer
various diphosphine derivatives.29 We and others32 have investi- chromatography was carried out on glass plates pre-coated
gated the proton-reduction chemistry of a series of sulfur and with Merck 60 0.5 mm silica gel. All phosphines were pur-
selenium-containing mono- and dichalcogenide-capped clusters chased from Acros Organics Chemicals Inc. and used as
[Fe3(CO)7(μ3-CO)(μ3-E)(μ-dppm)] and [Fe3(CO)7(μ3-E)2(μ-dppm)] received. The starting material [Fe3(CO)9(µ3-Te)2] (1) was pre-
(E = S, Se). While these clusters can act as proton-reduction cata- pared as previously reported.38 Infrared spectra were recorded
lysts, their activity is limited by the instability of the anions gen- on Nicolet 6700 FT-IR or Nicolet Avatar 360 FT-IR spec-
erated upon reduction, possibly due to carbonyl loss, and the trometers in solution cells fitted with calcium fluoride or
relatively high potentials required for efficient proton reduction. sodium chloride plates; subtraction of the solvent absorptions
While sulfur and selenium are widely found in nature, the was achieved by computation. Fast atom bombardment (FAB)
heavier congener tellurium has no apparent biological func- mass spectra were obtained on a JEOL SX-102 spectrometer
tion33 although tellurium-containing amino acids can be using 3-nitrobenzyl alcohol as matrix and CsI as calibrant.
found in some fungi34 and tellurium compounds have shown Proton and 31P{1H} NMR spectra were recorded on a Varian
anti-cancer activity.35 Tellurium is more metallic in nature Unity 500 MHz or a Bruker AMX400 spectrometer. Chemical
than sulfur and selenium, and, consequently, complexes con- shifts were referenced to residual solvent resonances or 85%
taining tellurium might be expected to be reduced at less nega- H3PO4.
tive potentials than analogous sulfur and selenium species,
while also generating relatively more stable reduced species.
Support for this hypothesis, and partial inspiration for our
Synthesis of [Fe3(CO)9(µ3-Te)2(PPh3)] (2) and
[Fe3(CO)8(µ3-Te)2(PPh3)] (3)55
current work on tellurium-containing iron carbonyl clusters, A benzene solution (20 mL) of 1 (60 mg, 0.09 mmol) and PPh3
comes from recent work by Song and co-workers36,37 who (23 mg, 0.09 mmol) was refluxed for 12 h. The solvent was
showed that for the series [Fe2(CO)6{μ-E(CH2)3E}] (E = S, Se, removed under reduced pressure and the residue was chro-
Te), the tellurium analogue is reduced at less negative poten- matographed by TLC on silica. Elution with n-hexane/CH2Cl2
tials than the sulfur or selenium analogues and that the anion (3 : 1 v/v) developed two bands. The faster moving band gave
generated was also more stable.
[Fe3(CO)8(µ3-Te)2(PPh3)] (3) (28 mg, 34%) as black crystals and
The chemistry of low-valent tellurium-containing iron clus- the second band afforded [Fe3(CO)9(µ3-Te)2(PPh3)] (2) (24 mg,
ters is well-developed and shows a rich and diverse array of 23%) as red crystals after recrystallization from hexane/CH2Cl2
cluster core geometries.38–55 A notable feature is their ability to at 4 °C. Characterization data for 2: IR (ν(CO), CH2Cl2): 2063w,
1
undergo addition reactions with a range of two-electron donor 2037s, 2012vs, 1972s cm−1. H NMR (CDCl3): δ 7.71–7.41 (m,
ligands to form adducts with one less iron–iron bond,42–52 15 H, Ph). 31P{1H} NMR (CDCl3): δ 46.11 (s). ESI-MS: m/z 936
which is in marked contrast with most related sulfur and sel- [M+, calc 937.11]. Characterization data for 3: IR (ν(CO),
1
enium-containing complexes, the related chemistry of which is CH2Cl2): 2053s, 2014vs, 1987s, 1937w cm−1. H NMR (CDCl3):
dominated by substitution resulting from CO loss.43,44 An δ 7.69–7.42 (m, 15 H, Ph). 31P{1H} NMR (CDCl3): δ 72.1 (s).
example of the addition chemistry discussed above are reac- ESI-MS: m/z 909 [M+, calc 909.10].
tions of the 50-electron cluster [Fe3(CO)9(μ3-Te)2] (1) with Lewis
Synthesis of [Fe3(CO)7(µ3-Te)2(µ-dppm)] (4)
bases (L) that result in high yield generation of 52-electron
clusters of the type [Fe3(CO)9(L)(μ3-Te)2], containing a single A benzene solution (30 mL) of 1 (100 mg, 0.15 mmol) and
iron–iron bond.43,56 This suggests that the one- and two-elec- dppm (Ph2PCH2PPh2, 57 mg, 0.15 mmol) was refluxed for
tron reduction products of [Fe3(CO)9(μ3-Te)2] (1) and related 30 min. The solvent was removed under reduced pressure,
phosphine derivatives may be particularly stable and provide and the residue was chromatographed by TLC on silica.
good access to proton reduction catalysis. Although the elec- Elution with n-hexane/CH2Cl2 (9 : 1 v/v) developed one band
tronic structure of 1 is well-known,42–46 the electrochemical which gave [Fe3(CO)7(µ3-Te)2(µ-dppm)] (4) (65 mg, 44%) as
and (electro)catalytic properties of 1 remain unexplored, to the black crystals after recrystallization from hexane/CH2Cl2 at
best of our knowledge. Herein we report the electrochemical 4 °C. Spectral data for 4: IR (ν(CO), cyclohexane): 2040s,
and electrocatalytic reduction of protons to hydrogen by a 1984vs, 1961w, 1942w, 1936w cm−1
.
1H NMR (CDCl3):
series of 50-electron telluride-capped clusters, and a compari- δ 7.80–7.09 (m, 20 H, Ph), 4.16 (m, 2H, CH2). 31P{1H} NMR
son of their behaviour with that of related sulfur- and sel- (CDCl3): δ 51.0 (d, J 62 Hz), 45.5 (d, J 62 Hz). ESI-MS: m/z 1003
enium-capped derivatives.27–32
[M+, calc 1003.19].
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Synthesis of [Fe3(CO)7(µ3-Te)2(µ-dcpm)] (5)
glass frit. Ferrocene was added as an internal standard57 and
potentials are referenced versus the ferrocenium/ferrocene
couple (Fc+/Fc). For the electrocatalytic studies, the catalyst
(cluster) concentrations were 1 mM. A Pine Wave Now poten-
tiostat was used for all electrochemical measurements.
Catalysis studies were carried out by adding known equivalents
of p-toluenesulfonic acid (TsOH) or trifluoroacetic acid (TFA)
(Sigma-Aldrich).
A benzene solution (30 mL) of 1 (92 mg, 0.14 mmol) and bis
(dicyclohexylphosphino)methane (dcpm) (55 mg, 0.14 mmol)
was refluxed for 25 min. The solvent was removed under
reduced pressure, and the residue was chromatographed by
TLC on silica. Elution with n-hexane/CH2Cl2 (4 : 1 v/v) devel-
oped two bands. The faster-moving band gave unreacted (1)
and the second band afforded [Fe3(CO)7(µ3-Te)2(µ-dcpm)] (5)
(15 mg, 11%) as black crystals after recrystallization from
hexane/CH2Cl2 at 4 °C. Spectral data for 5: IR (ν(CO): 2035s,
1968vs, 1940 m, 1915w cm−1. 1H NMR (CDCl3): δ 2.80 (br, 2 H,
CH2), 2.28–1.79 (m, 44 H, Cy). 31P{1H} NMR (CDCl3): δ 64.7 (d,
J 42 Hz), 51.7 (d, J 42 Hz). HRMS: m/z (ESI−): 1062.8556 [M +
Cl−, calc. 1062.8547].
Computational methodology
All calculations were performed with the hybrid DFT func-
tional B3LYP, as implemented by the Gaussian 09 program
package.58 This functional utilizes the Becke three-parameter
exchange functional (B3),59 combined with the correlation
functional of Lee, Yang and Parr (LYP).60 The iron atoms were
described by Stuttgart–Dresden effective core potentials (ecp)
and an SDD basis set, while the 6-31+G(d′) basis set was
employed for the remaining atoms. The reported geometries
were fully optimized, and the analytical second derivatives
were evaluated and found to possess only positive eigenvalues.
The geometry-optimized structures have been drawn with the
JIMP2 molecular visualization and manipulation program.61,62
Synthesis of [Fe3(CO)7(µ3-Te)2(µ-dppa)] (6)
A benzene solution (20 mL) of 1 (50 mg, 0.074 mmol) and bis
(diphenylphosphino)iso-propylamine
(dppa)
(55
mg,
0.074 mmol) was refluxed for 1.5 h. The solvent was removed
under reduced pressure and the residue was chromatographed
by TLC on silica. Elution with n-hexane/CH2Cl2 (4 : 1 v/v) devel-
oped two bands. The faster moving band gave unreacted 1 and
the second band afforded [Fe3(CO)7(µ3-Te)2(µ-dppa)] (6) (7 mg,
9%) as black crystals after recrystallization from hexane/
CH2Cl2 at 4 °C. Spectral data for 6: IR (ν(CO): 2036s, 1974vs,
Results and discussion
Synthesis and characterisation
1924w cm−1 1H NMR (CDCl3): δ 7.93–7.44 (m, 20H, Ph),
.
3.89–3.65 (m, 1H), 1.27 (m, 3H), 0.86 (m, 3H). 31P{1H} NMR
(CDCl3): δ 126.4 (d, J 78.1 Hz), 109.1 (d, J 78.1 Hz). HRMS: m/z
(ESI−): 1081.7101 [M + Cl−, calc. 1081.7091].
The parent cluster [Fe3(CO)9(µ3-Te)2] (1) was synthesized by a
literature procedure38 from Fe(CO)5 and K2TeO3, and the phos-
phine derivatives were prepared as shown in Scheme 1.
Treatment of 1 with PPh3 in benzene at 80 °C for 12 h afforded
a mixture of the addition product [Fe3(CO)9(µ3-Te)2(PPh3)] (2)
and the mono-substituted cluster [Fe3(CO)8(µ3-Te)2(PPh3)] (3).
The dppm complex, [Fe3(CO)7(µ3-Te)2(µ-dppm)] (4), was pre-
pared according to the method of Rauchfuss.43 This reaction
also proceeds via initial phosphine addition43 and CO loss,47
Synthesis of [Fe3(CO)7(µ3-Te)2(µ-dppm)] (4) and
[Fe3(CO)7(µ3-CO)(µ3-Te)(µ-dppm)] (7)
A CH2Cl2 solution (20 mL) of 1 (40 mg, 0.048 mmol) and
elemental tellurium (12.3 mg, 0.096 mmol) was refluxed for
24 h. The solvent was removed under reduced pressure, and
the residue was chromatographed by TLC on silica gel. Elution
with cyclohexane/CH2Cl2 (9 : 1 v/v) developed three bands. The
faster moving band gave the known cluster [Fe3(CO)7(µ3-Te)2(µ-
dppm)] (4) (4 mg, 7%),18 the second band gave unreacted 1
and the third band afforded [Fe3(CO)7(µ3-CO)(µ3-Te)(µ-dppm)]
(7) (5 mg, 11%) as red crystals after recrystallization from
hexane/CH2Cl2 at 4 °C. Spectral data for 7: IR (ν(CO), CH2Cl2):
2031s, 1983s, 1965vs, 1715w cm−1
.
1H NMR (CDCl3): δ
7.77–7.29 (m, 20H, Ph), 2.95–2.91 (m, H), 2.89–2.86 (m, H). 31
P
{1H} NMR (CDCl3): δ 55.1 (s). ESI-MS: m/z 1030 [M+, calc
1031.20].
Electrochemistry
Electrochemistry was carried out under solvent-saturated nitro-
gen in deoxygenated CH2Cl2 and MeCN solutions with 0.1 M
[NBu4][PF6] as the supporting electrolyte. The working elec-
trode was a 2 mm diameter glassy carbon electrode (GCE) that
was polished with a 0.3 μm alumina slurry as needed, the
counter electrode was a platinum wire, and the Ag/AgCl refer-
ence electrode was separated from the working electrode by a Scheme 1 Schematic depiction of synthetic routes to clusters 2–6.
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Scheme 2 Schematic depiction of the synthesis of 4 and 7 from [Fe3(CO)10(μ-dppm)].
but the intermediate nona- and octa-carbonyl complexes were
not isolated. The bis(dicyclohexylphosphino)methane (dcpm)
and bis(diphenylphosphino)iso-propylamine (dppa) deriva-
tives, [Fe3(CO)7(µ3-Te)2(µ-dcpm)] (5) and [Fe3(CO)7(µ3-Te)2(µ-
dppa)] (6), have not been reported previously. Both 5 and 6
were formed in low yields but gave sufficient amounts for full
characterisation. The 31P{1H} NMR spectra of 4–6 each dis-
played a pair of doublets at 51.0 and 45.5 ppm (d, J = 62 Hz)
for 4, 64.7 and 51.6 ppm (d, J = 42 Hz) for 5, 126.4 and
109.1 ppm (d, J = 78.1 Hz) for 6, showing that the diphosphine
bridges across a metal–metal bond leading to chemical and
magnetic inequivalence of the two phosphorus centres.43,47 It
may be noted that the sulfur and selenium analogues of 4 are
known.63–65 For both the S and Se analogues of 4, two isomers
are detected – one symmetric with the dppm ligand bridging
the two irons that are not directly bonded to each other, and a
second asymmetric isomer corresponding to the structure of 4
(Scheme 1). No symmetric isomer of 4 was detected in the
present work. This is in agreement with the observation that
the analogous reaction of [Fe3(CO)9(µ3-S)2] with dppm pro-
duces the asymmetric isomer of [Fe3(CO)7(µ3-S)2(µ-dppm)]
exclusively.63 While solutions of both 4 and 5 are stable in air,
this is not the case for 6, which oxidises rapidly to generate a
species tentatively assigned as [Fe3(CO)8(µ3-Te)2(µ-dppa)]+ on
the basis of related work we have recently carried out.56
Table 1 First and second reduction potentials of 1 and 3–5 together
with reduction potentials for related clusters (all in CH2Cl2). Peak widths
are reported for 100 mV s−1 scans
Potential (E1/2) of
Potential (E1/2) of
Complex
first reduction (V)
second reduction (V)
[Fe3(CO)9(μ3-Te)2] (1)
[Fe3(CO)8(μ3-Te)2(PPh3)] (3)
−0.97 (ΔE = 155 mV) −1.51 (ΔE = 145 mV)
−1.24 (ΔE = 190 mV) −1.91 (ΔE = 270 mV)
[Fe3(CO)7(μ3-Te)2(μ-dppm)] (4) −1.37 (ΔE = 110 mV) −1.77 (ΔE = 110 mV)
[Fe3(CO)7(μ3-Te)2(μ-dcpm)] (5) −1.51 (ΔE = 131 mV) −1.84 (ΔE = 145 mV)
[Fe3(CO)9(μ3-S)2]27
−1.03
−1.75
[Fe3(CO)9(μ3-Se)2]32
−1.03 (ΔE = 330 mV) −1.68 (ΔE = 350 mV)
−1.55 (ΔE = 215 mV)
[Fe3(CO)7(μ3-S)2(μ-dppm)]32
[Fe3(CO)7(μ3-Se)2(μ-dppm)]32 −1.45 (ΔE = 165 mV) −2.05 (ΔE = 235 mV)
Heating [Fe3(CO)10(μ-dppm)] with tellurium at 40 °C for
one day led to the isolation of small amounts of 4 and
the mono-capped cluster [Fe3(CO)7(µ3-CO)(µ3-Te)(µ-dppm)] (7)
(Scheme 2). Spectroscopic data for the latter are very similar to
that for the sulfur and selenium analogues;32 most notably the
triply bridging carbonyl ligand appears at 1715 cm−1 in the IR
spectrum while the 31P{1H} NMR spectrum contains only a
singlet at δ 55.1. Interestingly, while both [Fe3(CO)9(µ3-CO)(µ3-
S)]66 and [Fe3(CO)9(µ3-CO)(µ3-Se)]67 have been reported pre-
viously, the parent tellurium-capped cluster [Fe3(CO)9(µ3-CO)
(µ3-Te)] remains unknown. Due to the small amounts of 7 gen-
erated no further studies were carried out; current efforts are
focused on improving the yield of this cluster.
Fig. 1 Cyclic voltammograms of a 1 mM solution of [Fe3(CO)9(µ3-Te)2]
(1) in CH2Cl2 (at 10, 25, 50, 100, 250, 500, 1000, 2000, 4000, and
8000 mV s−1). No oxidation at more positive potentials (up to +1.3 V)
could be observed. The inset shows a plot of peak current vs. square
root of scan rate for the indicated peaks. The return wave at −1.51 V is
less electrochemically reversible at faster scan rates than the other
waves.
[Fe3(CO)9(µ3-Se)2] (−1.03 and −1.68 V)32 in CH2Cl2. Similar
behaviour has been noted in the [Fe2(CO)6{μ-E(CH2)3E}] (E = S,
Electrochemical studies
Cyclic voltammetry of 1 and 3–5 was investigated in CH2Cl2 Se, Te) series of diiron complexes,36,37,68,69 where the first
under a nitrogen atmosphere, and important features are sum- reduction potential of the tellurium derivative (at −1.58 V) was
marized in Table 1. The cluster [Fe3(CO)9(µ3-Te)2] (1) shows 0.03 and 0.09 V more positive than for the selenium and sulfur
two chemically reversible one-electron reduction processes at derivatives, respectively. A further difference between 1 and its
−0.97 V (ΔE = 155 mV) and −1.51 V (ΔE = 145 mV) (Fig. 1). sulfur and selenium analogues relates to the (chemical) rever-
These are at significantly less negative potentials than those sibility of the reduction processes. For 1, two sharp reduction
found for [Fe3(CO)9(µ3-S)2] (−1.03 and −1.75 V)27 and peaks are observed between scan rates of 0.01–8 V s−1 although
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Fig. 2 Cyclic voltammograms of a 1 mM solution of [Fe3(CO)8(µ3-
Te)2(PPh3)] (3) in CH2Cl2 (at 10, 25, 50, 100, 250, 500, 1000, and
2000 mV s−1). The inset shows a plot of peak current vs. square root of
scan rate for the indicated waves. The second wave is chemically
irreversible.
Fig. 3 Cyclic voltammograms of a 1 mM solution of [Fe3(CO)7(µ3-
Te)2(µ-dppm)] (4) in CH2Cl2 (supporting electrolyte [NBu4][PF6] at 10, 25,
50, 100, 250, 500, 1000, and 2000 mV s−1, glassy carbon electrode,
potential vs. Fc+/Fc). The inset shows a plot of peak current vs. square
root of scan rate for the reduction peaks. The return wave at −1.77 V is
less electrochemically reversible at faster scan rates than the other
reduction waves. The oxidation at +0.4 V is chemically irreversible.
at scan rates of 1 V s−1 and greater the peak height for the
reverse of the second reduction is less than expected. For both
the sulfide and selenide complexes, the first reduction poten-
tial is reversible but the second reduction displays far less
reversibility. The shift of the reductions to more positive poten-
tials and the greater chemical reversibility of the reductive pro-
cesses are encouraging signs for the use of 1 and related phos-
phine derivatives as proton reduction catalysts and suggest
that the larger tellurium with its more metallic character is
capable of stabilising these clusters in a wide range of redox
states.
We have also investigated the electrochemistry of
[Fe3(CO)8(µ3-Te)2(PPh3)] (3) and the three diphosphine deriva-
tives [Fe3(CO)7(μ3-Te)2(μ-R2PXPR2)] (4–6). Substitution of a car-
bonyl ligand in 1 for PPh3 predictably resulted in a shift of the
reduction potentials of [Fe3(CO)8(µ3-Te)2(PPh3)] (3) to more
negative potentials (Table 1) while the second reduction
becomes completely irreversible (Fig. 2). This suggests that
upon the second reduction either a carbonyl or the phosphine
is lost, resulting in chemical irreversibility. Carbonyl loss has
been proposed to occur at the second reduction potential of
[Fe3(CO)9(µ3-S)2].32
Fig. 4 Cyclic voltammograms of a 1 mM solution of [Fe3(CO)7(µ3-
Te)2(µ-dcpm)] (5) in CH2Cl2 (supporting electrolyte [NBu4][PF6] at 10, 25,
50, 100, 250, 500, 1000, and 2000 mV s−1, glassy carbon electrode,
potential vs. Fc+/Fc); the inset shows a plot of peak current vs. square
root of scan rate for the reduction waves. The return wave at −1.51 V is
less electrochemically reversible at faster scan rates than the other
reduction waves.
In contrast to the behaviour found for 3, CVs of all three
diphosphine-bridged complexes [Fe3(CO)7(μ3-Te)2(μ-R2PXPR2)]
(4–6) show two chemically reversible or quasi-reversible
reduction peaks, akin to those seen for 1. All are summarised shifted to −1.28 and −1.70 V in MeCN (Fig. S2†), while for the
in Table 1, and data for the dppm and dcpm derivatives 4 and more electron-releasing dcpm complex 5 these waves are seen
5 are shown in Fig. 3 and 4, respectively. The dppa derivative 6 at −1.51 V and −1.84 V. The reversibility of these waves pro-
shows quite different behaviour as seen in Fig. S1 (ESI†), with vides strong evidence that the Fe3Te2P2 core remains intact
both reductive processes being irreversible; this complex will upon addition of both one and two electrons.
not be discussed further. As expected, removal of a further car-
Both 4 and 5 also show oxidative chemistry that presumably
bonyl ligand and addition of an electron-releasing phosphine is outside of the solvent window for 1 and 3. The dppm
leads to a shift to more negative reduction potentials with complex 4 shows an irreversible oxidation at 0.4 V in CH2Cl2,
respect to those in 1. The dppm complex 4 (Fig. 3) shows two the height of which suggests that it may be a two-electron
reversible reduction waves at −1.37 V and −1.77 V, which are process, which is consistent with the two closely spaced oxi-
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dation waves seen in dcpm complex 5. Oxidation is irreversible
and in order to improve reversibility we also studied the
electrochemistry of 4 in the coordinating solvent MeCN
(Fig. S2†). Both the cathodic and anodic regions show similar
reductive and oxidative features to those observed in CH2Cl2,
the oxidation wave appearing at 0.3 V, the peak height of
which again suggests more than one electron may be involved.
Shifting of the oxidation wave by ca. 0.20 V to more negative
potential in MeCN as compared to CH2Cl2 was noted for the
selenium analogue32 and relates to the stabilisation of positive
charge by the coordinating solvent.
For the dcpm cluster 5 (Fig. 4), two oxidation waves are
seen, and both exhibited some reversibility. The first oxidation
occurs at 0.34 V (ΔE = 140 mV) and the second at 0.51 V (ΔE =
140 mV). When the scan rate was varied from 0.01 to 2 V s−1
for 4 and 5, no additional features were observed. Plots of
anodic and cathodic peak currents of the first reductive
process against the square root of the scan rate both give
straight lines (Fig. 4) indicating that they originate from
diffusion-controlled one-electron processes. Thus for 5, it was
possible to cycle through five different redox states with some
degree of reversibility with total electron counts ranging from
48–52 electrons.
In summary, while the electrochemical behaviour of the
series of clusters [Fe3(CO)9(µ3-E)2] (E = S, Se, Te) are quite
similar, the better reversibility of the reductive chemistry of
tellurium cluster 1, and the continued reversibility of the two
one-electron reductive processes upon addition of a dipho-
sphine, make the tellurium-containing clusters potential can-
didates for proton reduction catalysts.
Fig. 6 B3LYP/6-31+G(d’), SDD[Fe,Te] optimized Fe–Fe and Fe–Te inter-
nuclear distances in (a) 1, (b) 1−, (c) 312−, and (d) 112−. Internuclear dis-
tances in Angstroms.
The addition of a single electron to 1 results in significant
lengthening (0.923 Å) of one of the Fe–Fe bonds to give the
doublet radical anion, 21•−. Adding a second electron gives the
open-shell triplet dianion (312−) and results in a slight
lengthening of the remaining Fe–Fe bond and a slight shorten-
ing of the first Fe–Fe bond. This triplet dianion is calculated to
be 8.8 kcal mol−1 more stable than the closed-shell singlet
dianion (112−), which undergoes cleavage of one wingtip Fe–Te
bond, leading to polyhedral expansion of the cluster core. An
open-shell singlet dianion was also considered, but this
species was not stable and collapsed to the closed-shell
singlet. These results are summarized in Fig. 6.
Hence, it appears that upon addition of two electrons to 1,
the major structural change is the loss of one of the Fe–Fe
Density functional theory (DFT) calculations
3
bonding interactions with the formation of the diradical 12−
In order to gain more insight into the nature of the tellurium-
capped clusters upon one- and two-electron reduction we have
carried out a series of DFT calculations on [Fe3(CO)9(µ3-Te)2]
(1), 1− and 12−. Selected atomic charges and Wiberg bond
indices are given in Tables S1 and S2.† The HOMO and LUMO
of the neutral cluster are shown in Fig. 5. The HOMO (Fig. 5a)
consists primarily of Fe–Fe bonding character in the two
formal iron–iron bonds. The LUMO (Fig. 5b) is of most interest
with respect to the observed reduction chemistry and consists
primarily of Fe–Fe antibonding character in addition to some
Fe–Te antibonding character. This suggests that partial or com-
plete population of this orbital will lead to an expansion of
both the iron–iron and iron–tellurium bonds.
in which the triiron core is held together by two capping tellur-
ium atoms. This is most likely the reason that this process
shows such good reversibility on the electrochemical time
scale as neither ligand loss nor cluster fragmentation is facile.
3
The spin density in 11− and 12− is primarily distributed over
the three iron centers as depicted in Fig. 7. These metal sites
are electron rich and are expected to help direct the initial site
of protonation.
Spectroelectrochemical IR measurements on [Fe3(CO)9(µ3-
Te)2] 1 showed that the monoanion 1− is reasonably stable
Fig. 7 Computed B3LYP/6-31+G(d’), SDD[Fe,Te] net spin densities (ρα
ρβ) for (a) 1− and (b) 312−
−
Fig. 5 (a) HOMO and (b) LUMO of [Fe3(CO)9(µ3-Te)2] (1).
.
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during inert conditions (a total lifetime of ca. 30 min) while reduction (Fig. 9), which is characteristic of electrocatalytic
the corresponding dianion 12− was found to be relatively proton reduction. Fig. S3 (ESI†) shows the cyclic voltammo-
unstable and was rapidly deposited on the electrode surface. grams of the complex, of pure acid, and of acid with the
Neither 1− nor 12 were accessed by chemical means (cf. ESI†).
complex. The reduction potential is 0.5 V more positive in the
While we have not carried out DFT calculations for any of presence of the catalyst. An additional reduction peak is also
the related diphosphine complexes 4–6, we make the assump- seen at Ep = −2.20 V (Fig. S3(a)†), which increases sharply with
tion that their behaviour is similar, with the diphosphine most acid concentration, because the catalysis becomes competitive
likely spanning the relatively unaltered iron–iron bond after at this potential due to the direct reduction of acid by the elec-
two-electron reduction.32
trode. This reduction peak was not investigated further.
Similar results are obtained using tosylic acid (TsOH) with
small increases in catalytic current at the first reduction poten-
tial and larger increases at the second reduction potential, as
seen in Fig. S4 (ESI†). The catalytic activity of 1 is thus akin to
that which we have previously noted for [Fe4(CO)10(κ2-dppn)
(μ4-O)] {dppn = 1,8-bis(diphenylphosphino)naphthalene}.24
Next, the proton reduction ability of the phosphine deriva-
tives 3–5 was considered. The behaviour of the PPh3-substi-
tuted cluster 3 (Fig. S5, ESI†), [Fe3(CO)7(µ3-Te)2(µ-dppm)] (4)
(Fig. 10a) and [Fe3(CO)7(µ3-Te)2(µ-dcpm)] (5) (Fig. 10b) are
similar to that of 1 with some reduction activity at the first
reduction potential, but greater activity at the second
reduction. Both were examined. In each case, after addition of
one equivalent of acid a catalytic response was observed and
the chemical reversibility of the first and second reductions
disappeared completely. The dppm derivative 4 is particularly
well-behaved. At the first reduction potential, there is little
activity, but the dianion shows high activity and is able to
reduce protons very efficiently and is not saturated even upon
addition of 70 equivalents of acid. Cluster 5, containing the
more electron-donating dcpm ligand, behaves similarly until
the addition of ca. 20 equivalents of acid (light blue curve,
Fig. 10b), after which a second catalytic pathway becomes
active at slightly more negative potentials (ca. −2.2 V). This
leads to a significant broadening of the CV traces at higher
acid concentrations as the catalytic activities of these two sep-
arate species overlap. This also leads to large currents as com-
pared to the dppm-derivative 4. Thus, after addition of ca. 40
equivalents of acid, the total peak currents for 4 and 5 are ca.
380 μA and 470 μA, respectively (see insets, Fig. 10a and b, for
details). For both 4 and 5, the height of the oxidation peak
remains uniform upon addition of acid and is shifted to less
positive values, indicating that the catalysts are stable on the
time frame of the experiments. A further notable feature is
that there are no new reduction peaks at more positive poten-
tials than the first reduction of the neutral complex, support-
ing the IR protonation studies that suggest that neither
complex is rapidly protonated under the conditions of the cata-
lytic studies. This electrocatalytic proton reduction behaviour
is similar to that found for analogous sulfur and selenium
clusters,32 but hydrogen generation occurs at significantly less
negative potentials (ca. 0.25 and 0.14 V, respectively) than for
the lighter analogues, while peak currents are also signifi-
cantly higher for the telluride clusters under comparable
experimental conditions. Table 2 summarises the electro-
catalytic data, listing observed reduction potentials and associ-
ated derived rate constants (cf. ESI†) for proton reduction at
Electrocatalytic studies
Before electrocatalytic studies were undertaken, all complexes
were screened for their reactivity towards TsOH in CH2Cl2.
Under a nitrogen atmosphere, no significant changes in the IR
spectra were noted upon addition of acid, indicating that the
neutral complexes are not readily protonated under the uti-
lised conditions of catalysis (vide infra). In the presence of
oxygen, the addition of acid to the diphosphine-bridged com-
plexes 4–6 did lead to the slow formation of new absorptions
in the IR spectra, but these were associated with the respective
cations (vide supra), showing that oxidation rather than proto-
nation was the dominant feature.
In the ensuing electrocatalytic studies, all measurements
were conducted on 1 mM solutions of cluster. The electro-
catalytic activity of [Fe3(CO)9(µ3-Te)2] (1) towards proton
reduction was investigated in the presence of TFA. Fig. 8 shows
the cyclic voltammograms upon addition of a few equivalents
of acid to a CH2Cl2 solution of 1. It would appear that the
reduced complex is protonated and that the protonated
complex is more easily reduced than the parent compound but
the second reduction becomes lest reversible. Additional
increases in acid increase the peak current for the second
Fig. 8 Cyclic voltammograms of a 1 mM solution of 1 upon addition of
0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.8 and 2.0 equivalents of TFA (CH2Cl2,
supporting electrolyte [NBu4][PF6], scan rate 0.20 V s−1, glassy carbon
electrode, potential vs. Fc+/Fc). No dilution corrections have been made
for these small volume additions.
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Fig. 10 Cyclic voltammograms of
1
mM solutions of complex (a)
Fig. 9 Cyclic voltammograms of a 1 mM solution of 1 upon addition of
(a) 1–15 molar equivalents of TFA and (b) 5–140 molar equivalents of
TFA (CH2Cl2, scan rate 0.20 V s−1); the inset shows a plot of peak current
as TFA is added. Dilution corrections have been made so that the figures
represent the results at constant volume, as verified by ferrocene
serving as an internal concentration standard.
[Fe3(CO)7(µ3-Te)2(µ-dppm)] (4) and (b) [Fe3(CO)7(µ3-Te)2(µ-dcpm)] (5)
upon addition of TsOH (CH2Cl2, scan rate 0.25 V s−1). Dilution correc-
tions have been applied to represent the results at constant volume.
nature of the substituents at the triiron center. The neutral
50-electron clusters are not catalysts. One-electron reduction
these potentials. On the basis of the electrochemical and affords the 51-electron radicals [Fe3(CO)7(µ3-Te)2(L2)]•− and
electrocatalytic results presented above, we propose that a some catalytic activity is seen at this potential. This suggests
series of interlinked catalytic cycles are operating (Scheme 3). that these radical anions can be protonated, with higher cur-
The relative rates of these catalytic cyles will be dependent on rents being observed as the electron-releasing nature of the L2
the applied chemical potential, acid concentration and the moiety increases: dcpm (5) > dppm (4) > (CO)(PPh3) (3) > (CO)2
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Table 2 Electrocatalytic parameters for complexes 1, 3, 4–6 and related complexes. Fitted slopes of ic/ip vs. [H+] (cf. Fig. 9, 10 and S3 (ESI†); derived
rate constants for proton reduction at the specific reduction potential (cf. ESI† for derivation of rate constants)
Complex (250 mV s−1 except 3, 6 at 200 mV s−1
TsOH unless otherwise noted)
E1
Slope 1 ic/ip vs. [H+]
k1, M−2 s−1
E2
Slope 2 ic/ip vs. [H+]
k2, 103 M−2 s−1
[Fe3(CO)9(μ3-Te)2] (1)
−1.06
−1.09
−1.39
−1.44
−1.61
−1.14
−1.14
−1.37
−1.41
0.0165
0.0067
0.0094
0.0426
0.124
0.0499
0.0073
0.0135
0.0548
130
22
35
900
7600
980
26
−1.54
−1.51
−2.15
−1.90
−2.03
−1.64
−2.02
−1.85
Shifts
0.0694
0.157
0.245
0.531
0.644
0.503
0.324
0.272
0.252
2.4
12
24
140
200
100
52
[Fe3(CO)9(μ3-Te)2] (1) + TFA
[Fe3(CO)8(μ3-Te)2(PPh3)] (3)
[Fe3(CO)7(μ3-Te)2(μ-dppm)] (4)
[Fe3(CO)7(μ3-Te)2(μ-dcpm)] (5)
[Fe3(CO)8(μ3-Te)2(μ-dppa)] (6)
[Fe3(CO)9(μ3-Se)2]32
[Fe3(CO)7(μ3-S)2(μ-dppm)]32
[Fe3(CO)7(μ3-Se)2(μ-dppm)]32
90
1500
37
31
Scheme 3 Proposed electrocatalytic proton reduction pathways mediated by [Fe3(CO)7(µ3-Te)2(L2)] (L2 = diphosphine).
(1). This may relate to the increasing rate of protonation of the at more negative potentials (ca. −2.2 V) and this may be associ-
triiron centre upon successive carbonyl substitutions. In this ated with the less facile loss of hydrogen from [H2Fe3(CO)7(µ3-
respect, this behaviour differs from that noted for Te)2(μ-diphosphine)] and its subsequent reduction leading to
[Fe4(CO)10(κ2-dppn)(μ4-O)],24 where the one-electron reduced hydrogen elimination. For the less electron-rich clusters 1
cluster shows no catalytic activity, but it is similar to the (Fig. 8) and 3 (Fig. S3†), the major hydrogen generation also
behaviour of [Fe4(CO)12(μ4-N)]−,30 [Fe4(CO)12(μ4-C)]2− 31
and occurs at potentials greater than those required for generation
,
[Fe3(CO)7(µ3-E)2(μ-dppm)] (E = S, Se)26,32 where the activity is of the dianions. This suggests that there is also a pathway
also seen at the first reduction potential. The order of electron/ whereby these dianions (e.g. 12−) are not sufficiently basic to
proton addition after formation of the putative [HFe3(CO)7(µ3- undergo double protonation but rather react with acid to give
Te)2(μ-diphosphine)] intermediate is unknown but in view of [HFe3(CO)8(L)(µ3-Te)2]− (L = CO, PPh3), which are further
the slow rates of protonation of the neutral complexes a reduced to [HFe3(CO)8(L)(µ3-Te)2]2− before the second protona-
second reduction followed by protonation and hydrogen tion and hydrogen loss. These potential proton reduction path-
release seems most likely.
ways are summarized in Scheme 3.
While the ditelluride clusters are only relatively poor proton
reduction catalysts at their first reduction potentials, the doubly
reduced 52-electron clusters [Fe3(CO)7(µ3-Te)2(μ-diphosphine)]2−
especially clusters 4 and 5, show very large electron uptake at
,
Summary and conclusions
their second reduction potential. DFT calculations on 12− In this work we have investigated the utility of the non-biologi-
(vide supra) show that it exists in its triplet form and thus cal element tellurium as a replacement for the widely biologi-
we suggest that the triplet dianions [Fe3(CO)7(µ3-Te)2- cally-utilized lighter chalcogenides sulfur and selenium in the
(μ-diphosphine)]2− undergo facile addition of two protons to realm of proton-reduction catalysis. Our first target in this
generate the corresponding neutral 52-electron clusters respect was the 50-electron ditelluride cluster [Fe3(CO)9(µ3-
[H2Fe3(CO)7(µ3-Te)2(μ-diphosphine)]. Even upon addition of 70 Te)2] (1) and selected phosphine and diphosphine derivatives,
equivalents of TsOH to the dppm derivative 4 (Fig. 10a) no evi- that may be compared to related sulfur and selenium-clusters
dence of saturation associated with rate-determining loss of that are able to act as proton reduction catalysts.
hydrogen was noted, suggesting that hydrogen loss is facile
The synthesis of these clusters is relatively facile, and they
and leads to regeneration of the neutral cluster. For the more show good stability in both the solid state and solution.
electron-rich dcpm derivative 5, at high acid concentrations Electrochemical studies reveal that each cluster shows two
(>20 equivalents) a third hydrogen generation process appears reductive processes. The first of these has good reversibility in
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all cases, showing that the 51-electron radical anions are quite
Acknowledgements
stable. DFT calculations on 1− reveal that the only major struc-
tural change is an elongation of one of the Fe–Te bonding AR thanks the European Commission for the award of an
interactions. The chemical reversibility of the second reduction Erasmus Mundus pre-doctoral fellowship. GH and EN thank
process is highly dependent upon the nature of the supporting the Royal Society for an International Exchange Award. JBL
ligands. Thus, for the parent cluster 1 and the diphosphine thanks the Beloit Summer Science Scholars program. MGR
derivatives 4 and 5 this reduction is reversible, but for the thanks the Robert A. Welch Foundation (Grant B-1093-MGR)
PPh3 complex 3 it is completely irreversible. DFT calculations for financial support. Computational resources through the
on 12− suggest that addition of the second electron affords an High-Performance Computing Services and CASCaM at the
open-shell triplet with two elongated Fe–Te bonds that is more University of North Texas are acknowledged, and we thank
stable than the analogous closed-shell dianion. In all cases the Prof. Michael B. Hall (Texas A&M University) for providing us a
first reductive process occurs at more positive potentials than copy of his JIMP2 program, which was used to prepare the geo-
that of analogous sulfur and selenium-containing clusters32 metry-optimized structures reported here. We are indebted
and the availability (at relatively low potentials) and stability of to Dr Luca Zuppiroli (University of Bologna) for recording
the 52-electron dianions, [Fe3(CO)7(µ3-Te)2(μ-diphosphine)]2−
,
the high resolution mass spectra of complexes 5 and 6.
contrasts with the lighter chalcogenide analogues that are We thank Dr Georgia Orton (King’s College London) and
either not accessible (within the solvent window) or show little Prof. Frantisek Hartl (University of Reading) for spectroelectro-
stability.
Electrocatalytic proton reduction was assessed in dichloro-
chemical measurements on complex 1.
methane using trifluoroacetic acid (TFA) or p-toluenesulfonic
acid (TsOH) as the proton source. None of the iron telluride
clusters react with these acids on the time scale of the catalysis
experiments, but all are able to act as proton reduction cata-
lysts at both their first and second reduction potentials.
Electron uptake at the first reduction potential is relatively
small in all instances, while in contrast uptake at the second
potential is much larger, indicating that the open-shell triplet
dianions are efficient catalysts. The catalytic overpotential
increases upon successive phosphine substitution but so does
the electron uptake and we attribute this to an increasing
proton binding ability. For [Fe3(CO)7(µ3-Te)2(μ-dcpm)] (5), a
further electron uptake appears at high acid concentrations at
a slightly more negative potential than its second reduction,
suggesting that H2 loss from the putative intermediates
[H2Fe3(CO)7(µ3-Te)2(μ-dcpm)] or [H3Fe3(CO)7(µ3-Te)2(μ-dcpm)]+
may become rate-limiting. On the basis of the electrocatalytic
results a mechanistic scheme has been developed in order to
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