Electrocatalytic hydrogen generation by a trithiolato-bridged dimanganese hexacarbonyl anion with a turnover frequency exceeding 40000 s-1

Article abstract of DOI:10.1039/c4cc02016b

An unusual ionic manganese model complex [Mn(bpy)₃] ⁺[(CO)₃Mn(μ-SPh)₃Mn(CO)₃] ^-(bpy: 2,2′-bipyridine) has been synthesized, which bears some structural resemblance to the active site of [FeFe] hydrogenase. An overpotential of 0.61 V has been determined for the electrocatalytic proton reduction using this complex in CH₃CN with CF₃COOH as the proton source. A turnover frequency of 44600 s^-1 is achieved at high scan rates and in the presence of a large amount of acid. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02016b

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Electrocatalytic hydrogen generation by a
trithiolato-bridged dimanganese hexacarbonyl
anion with a turnover frequency exceeding
40 000 sꢀ1†
Cite this: Chem. Commun., 2014,
50, 6630
Received 18th March 2014,
Accepted 2nd May 2014
Kaipeng Hou, Hwa Tiong Poh and Wai Yip Fan*
DOI: 10.1039/c4cc02016b
www.rsc.org/chemcomm
An unusual ionic manganese model complex [Mn(bpy)3]+[(CO)3- proton reduction efficiencies with acetic acid or trifluoroacetic acid
Mn(l-SPh)3Mn(CO)3]ꢀ(bpy: 2,20-bipyridine) has been synthesized, which as proton sources.5–7 Recently, a Ni–Mn model complex has been
bears some structural resemblance to the active site of [FeFe] hydro- synthesized whereby an azadiphosphine (Ph2P)2NR-chelated nickel
genase. An overpotential of 0.61 V has been determined for the electro- center was linked to manganese by two thiolate ligands.8 The Artero
catalytic proton reduction using this complex in CH3CN with CF3COOH group introduced a dinuclear Ni–Mn complex as a bio-inspired
as the proton source. A turnover frequency of 44 600 sꢀ1 is achieved at mimic of the active site of [NiFe] hydrogenases catalyzing hydrogen
high scan rates and in the presence of a large amount of acid.
evolution from trifluoroacetic acid with an overpotential 0.86 V.9
Although many thiolate-bridged dimanganese complexes have been
Hydrogenase enzymes are divided into three types according to the prepared, no studies on their proton reduction catalytic efficiencies
different metals in the active sites: [NiFe], [FeFe] and [Fe] only.1 The have been reported. Furthermore the many different oxidation states
former two can catalyze reversible proton reduction to molecular exhibited by manganese should enable redox reactions to be carried
hydrogen at low overpotentials with remarkable efficiency.2 In the out in a facile manner.
[FeFe] hydrogenase, the two iron centres bearing CO and CN
In this work, we will show thatꢀa m3-thiolato dimanganese
ligands are bridged by a unique azadithiolate ligand for which hexacarbonyl anion, Mn2(SPh)3(CO)6 (complex 1 in Scheme 1),
the nitrogen atom serves as the protonation site during proton which structurally resembles the active site of [FeFe] hydrogenase,
reduction.3 The reduction is assumed to be a 2eꢀ process where the can catalyze electrochemical proton reduction with low overpotential
2Fe2S core provides one electron with the second electron supplied coupled with very high turnover rates. We will also present a
by the [Fe4S4] cluster (Scheme 1).4
mechanism based on the experimental results.
There have been many studies on synthetic [FeFe] hydro-
Complex 1 is synthesized via a one-pot UV photolysis of
genase models based on the m2-dithiolato-bridged diiron Mn2(CO)10, diphenyl disulfide and 2,20-bipyridine for 2 hours.
moiety (m2-RS)2Fe2 coordinated to a variety of ligands such as An alternative pathway to 1 is also carried out whereby
CO, amines and phosphines. Several reviews have resulted from Mn2(CO)10 and diphenyl disulfide are UV-irradiated to first form
extensive studies on their structures, infrared spectroscopy and the dithiolato-bridged complex [Mn(CO)4(m-SPh)]2. This complex
is further irradiated in the presence of 2,20-bipyridine to generate 1.
Suitable crystals of 1 are grown in acetone solvent and stored at low
temperature before being subjected to X-ray diffraction.
Complex 1 consists of a dinuclear manganese anion and a
mono-manganese cation (Fig. 1). The anionic part is [(CO)3Mn-
(m-SPh)3Mn(CO)3]ꢀ, which contains two manganese(I), each
coordinated by three terminal CO ligands and bridged by three
m-SPh ligands. The phenyl groups are not coplanar due to steric
Scheme 1 (a) Active site of [FeFe] hydrogenase and (b) model complex 1 repulsion but instead form a propeller-like arrangement. The
of this work.
distance between the two Mn atoms is 3.1646(10) Å which is too
long for a Mn–Mn single bond.10 The IR spectra of complex 1 show two
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
strong nCO absorptions at 1910 and 1992 cmꢀ1 whose patterns are
Singapore 117543, Singapore. E-mail: chmfanwy@nus.edu.sg
indicative of C3v symmetry around each of the Mn centre. The Mn(I)
† Electronic supplementary information (ESI) available. CCDC 991316. For ESI
center in the Mn(bpy)3+ cation is coordinated by three bidentate
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc02016b
bipyridine ligands, resulting in a highly-distorted octahedral geometry.
6630 | Chem. Commun., 2014, 50, 6630--6632
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Fig. 1 Two pathways to form complex 1 and the nCO IR spectra of
complex 1 in CH3CN.
Fig. 2 ORTEP view of the solid-state structure of 1. Hydrogen atoms are
omitted for clarity.
The Mn–N bond lengths range from 2.236(4) to 2.263(4) Å, which
are similar to those reported for Mn(II) octahedral tris(2,20-bipyridine)
complexes (Fig. 2).11
Fig. 3 (a) Cyclic voltammograms of 1 (1.0 mM) with TFA (0–20 mM) in
0.1 M Bu4NPF6 in CH3CN at a scan rate of 0.1 V sꢀ1. (b) Cyclic voltammo-
grams of 1 (1.0 mM) with TFA (0–300 mM) in 0.1 M Bu4NPF6 in CH3CN at a
scan rate of 1 V sꢀ1. The relationship of kobs with increasing acid concen-
tration (inset graph). 1 mm glassy-carbon working electrode, platinum
counter electrode at 295 K.
The route to the formation of 1 is complicated. Successive
nucleophilic substitution of the Mn2(CO)8(SPh)2 dimer by
2,20-bipyridine will eventually afford Mn(bpy)3 and PhSꢀ. Thꢀe
+
thiolate ion then reacts with the dimer to generate Mn2(CO)3(SPh)3
upon dissociation of two Mn–CO bonds.
The cyclic voltammetric diagram of 1 in the absence of acid reduction potential of the anion, which corresponds to an
shows a small reduction peak at ꢀ1.90 V (vs. Fc+/Fc in CH3CN), overpotential of 0.61 V. The electrocatalytic H2 production
which corresponds to the reduction of the Mn(bpy)3+ cation respec- was further confirmed by bulk electrolysis of 1 with excessive
tively (Fig. 3). The assignment is based on a previous study on the CF3COOH in CH3CN at ꢀ1.50 V vs. Fc+/Fc with consistently
reduction potentials of Mn(I) to Mn(0) bipyridine complexes.12 As the more than 75% Faraday yield.
addition of trifluoroacetic acid (TFA) causes the peak intensity to
The catalytic rate of proton reduction is investigated next. To
increase as well, it would appear that the cation itself is able to estimate the rate, an approximate model for pseudo-first-order
catalyze proton reduction albeit at a relatively high overpotential of catalytic systems previously used before is adopted here, as
1.01 V (TFA reduction in CH3CN occurs at ꢀ0.89 V vs. Fc+/Fc). shown by the following equation.13
Another smaller reduction peak is observed at ꢀ1.50 V, which we
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ic
n
RTkobs
have attributed to the reduction of the anion. Interestingly this peak
appears to have shifted to ꢀ1.20 V upon acid addition and remains
at constant height independent of the acid concentration. We believe
¼
ip 0:446
Fu
that protonation of the anion has taken place and hence reduces its ic is the catalytic current, ip is the peak current measured in the
reduction potential, as opposed to the higher charge repulsion absence of acid, n is the number of electrons involved in the
between the anion and the electron in the absence of acid. At the catalytic reaction, kobs is the observed first-order rate constant,
same time a large peak which grows with the addition of acid is R is the ideal gas constant, T is the temperature in Kelvin, F is
observed at ꢀ1.50 V to ꢀ1.60 V depending on the acid concen- Faraday’s constant, u is the scan rate. From our data (Fig. 3b), it
tration.8 The potential at ꢀ1.50 V is attributed to the catalytic proton can be observed that kobs increases linearly with acid
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manganese tetramer is shown to be sluggish which in turn enables
a high proton reduction catalytic rate to be achieved.
Although the overpotentials of many [FeFe] hydrogenase
model complexes such as the diiron dithiolato bridged complexes
fall within 0.2 V and 1.1 V, very few complexes actually exhibited
overpotentials smaller than 0.6 V.5 However complexes with low
overpotentials are also less efficient in generating large proton
reduction currents. The TOF for proton reduction catalysed by a
diiron complex16 can reach 58 000 sꢀ1 while the highest TOF reported
for a nickel complex in dry and wet CH3CN were 33 000 sꢀ1 and
106 000 sꢀ1, respectively, with an overpotential of 0.625 V.13,14
A comparison with these iron and nickel models shows that
complex 1 is indeed a fairly efficient catalyst for proton reduction
with an overpotential of 0.61 V. However if the homoconjugation
effect of TFA is considered, the overpotential would become
0.79 V while Helm’s method would give a value of 0.69 V.17,18
In summary, we have shown that a manganese complex, which
Fig. 4 Proposed mechanism for the proton reduction process based on
experimental results.
concentration, which suggests a first-order dependence of the is structurally similar to the active site of [FeFe] hydrogenase, can
catalytic rate on acid concentration. At the highest concentration catalyze proton reduction with high TOF. The unusual structure of
ic
complex 1 contains a dinuclear manganese anion bridged by three
m-SPh groups and a mono-manganese cation where Mn(I) is
coordinated by three 2,20-bipyridine ligands. A simple mechanism
showing how proton reduction takes place electrochemically is also
proposed. We have demonstrated that the manganese-only model
complex can also catalyze proton reduction at least as efficient as
some iron or nickel model complexes.
of CF3COOH (300 mM) studied, a
value of 152 was obtained,
ip
which corresponds to a turnover frequency of 44 600 sꢀ1 at 295 K.
From the CV measurements, the anion appears to play a
more important role in reducing the overpotential of proton
reduction, hence the anion-catalysed mechanism is studied in
more detail here. From the sequence of the process, the proton
reduction mechanism catalyzed by 1 is proposed to be a CECE
(chemical–electrochemical–chemical–electrochemical) process
similar to that in iron and nickel model complexes (Fig. 4).
The anion is first protonated to form intermediate 1H+.
Then 1H+ is reduced at ꢀ1.2 V vs. Fc+/Fc to generate a hydride
species 1H by an intrahydride transfer from sulfur to manganese.14
A second proton reacts with 1H to generate 1H2. Finally, dihydro-
gen is released and the anion is regenerated upon electron
reduction at ꢀ1.5 V vs. Fc+/Fc to complete the catalytic cycle.
To gain more insight into the mechanism, the reactivity and
stability of 1 in the presence of acids are first studied via
infrared spectroscopy. When one equivalent CF3COOH is
added to 1 in CH3CN, no change in the nCO peaks is observed
even after an hour. However, when the amount of acid is
increased to 50 equivalents, the initial peaks of 1 at 1910 cmꢀ1
and 1992 cmꢀ1 slowly decrease while two new peaks at 1932 cmꢀ1
and 2015 cmꢀ1 appear. Based on a literature search, we were able
to assign the 1932 and 2015 cmꢀ1 bands to the manganese
carbonyl tetramer Mn4(CO)12(SPh)4.15
The authors are grateful for a NUS research grant (143-000-
553-112).
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6632 | Chem. Commun., 2014, 50, 6630--6632
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