Reversible electrocatalytic production and oxidation of hydrogen at low overpotentials by a functional hydrogenase mimic

Article abstract of DOI:10.1002/anie.201108461

An efficient ligand combination: A new bis(diphosphine) nickel(II) complex (see picture) is described. A ΔG° value of 0.84 kcal mol^-1 for hydrogen addition for this complex was calculated from the experimentally determined equilibrium constant. This complex displayed reversible electrocatalytic activity for hydrogen production and oxidation at low overpotentials, which are characteristic for hydrogenase enzymes. Copyright

Full text of DOI:10.1002/anie.201108461

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DOI: 10.1002/anie.201108461
Hydrogen Catalysis
Reversible Electrocatalytic Production and Oxidation of Hydrogen at
Low Overpotentials by a Functional Hydrogenase Mimic**
Stuart E. Smith, Jenny Y. Yang,* Daniel L. DuBois, and R. Morris Bullock*
Interest in the use of hydrogen for energy storage, trans-
portation fuel, and as a reductant to generate other chemical
fuels has grown rapidly in recent years.[1,2] This has led to the
investigation of new catalysts composed of abundant and
inexpensive metals that can produce and use hydrogen
efficiently. In nature, hydrogen is also used as a fuel and is
an important source of energy in the microbial world.[3]
Hydrogen metabolism in nature is mediated by hydrogenase
enzymes, which operate at ambient conditions and pressures,
using the abundant metals nickel and iron in their active
sites.[4] One of the hallmarks of these enzymes is their ability
to reversibly catalyze the oxidation of hydrogen to protons
and electrons with high efficiency.[5] This capability indicates
that addition and release of hydrogen is nearly thermoneutral
and kinetically accessible.
Previous studies in our laboratory have focused on the
synthesis and catalytic performance of functional hydro-
genase mimics.[6] Our studies have focused on iron,[7] cobalt,[8]
and nickel[9] complexes with the diphosphine ligand PR2NR’
2
shown in Scheme 1, where R is the substituent on phospho-
rous, and R’ is the substituent on nitrogen. From our
experimental and theoretical[10] studies on [Ni(PR2NR’2)2]2+
derivatives, we have established how steric and electronic
properties of the R and R’ substituents affect the hydride
acceptor and proton acceptor abilities of these complexes.
These two quantities determine the free-energy of hydrogen
Scheme 1. Reaction of complex 1 with hydrogen (1 atm) in
[D3]acetonitrile at 238C.
addition to these complexes. The ability to vary these
properties by varying substituents on the ligand has led to
the design of catalysts that are biased towards hydrogen
production[9] or hydrogen oxidation[11] by adjusting the free-
energy for hydrogen addition to be either positive or negative,
respectively.
We present here the complex [Ni(PPh2NR’2)2](BF4)2, 1,
(R’ = CH2CH2OCH3) the hydrogen addition product of which
is nearly isoenergetic with the starting materials, 1 and free H2
(Scheme 1).
Additionally, this catalyst displays reversible electrocata-
lytic hydrogen oxidation and production activity with high
efficiency. Previously, only one synthetic molecular hydrogen
production catalyst has been shown to also oxidize hydrogen,
although the reaction was slow and this reaction was not
shown to be catalytic.[12] An electrode modified by phys-
isorption of a molecular catalyst was shown to perform
reversible hydrogen catalysis.[13] However, this is the first
example of electrocatalysis which comprises reversible hydro-
gen oxidation and production and is shown by a nonenzymatic
complex under homogeneous conditions.
[*] Dr. S. E. Smith, Dr. J. Y. Yang,[+] Dr. D. L. DuBois, Dr. R. M. Bullock
Center for Molecular Electrocatalysis
Pacific Northwest National Laboratory
P.O. Box 999, K2-57, Richland, WA 99352 (USA)
E-mail: morris.bullock@pnnl.gov
Homepage: http://efrc.pnnl.gov/
[+] Current address:
Joint Center for Artificial Photosynthesis
California Institute of Technology, Pasadena, CA 91125 (USA)
E-mail: jyy@caltech.edu
[**] This research was supported as part of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Office of Basic Energy
Sciences. Pacific Northwest National Laboratory is operated by
Battelle for the U. S. Department of Energy.
The PPh2NR’ ligand was synthesized by the condensation
2
of phenylphosphine, paraformaldehyde, and 2-methoxyethyl-
amine, in a procedure similar to those reported previously for
related cyclic ligands.[14] Reaction of the unpurified ligand
with 0.5 equivalents of [Ni(CH3CN)6](BF4)2 resulted in the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108461.
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formation of a crude sample of 1. Addition of hydrogen and
2.1 equivalents of the base N,N,N’,N’-tetramethylguanidine
led to the selective precipitation of the pure Ni0 complex
Ni(PPh2NR’2)2 (R’ = CH2CH2OCH3), 2, from the acetonitrile
solution. Complex 2 was oxidized with two equivalents of
voltammogram of 1 in acetonitrile with 5.9 mm 4-cyanoani-
linium tetrafluoroborate. Addition of acid results in an
observed increase in the cathodic current at À0.55 V attrib-
uted to catalytic proton reduction to produce hydrogen. The
rise and plateau shape of the current is indicative of catalytic
activity. Higher concentrations of acid did not result in any
further increase in current. Based on the modest current
Cp2Fe+BF4 to afford analytically pure samples of 1. These
À
complexes have been characterized by 1H, 13C{1H} and 31P{1H}
NMR spectroscopy and elemental analysis, as detailed in the
Supporting Information. All data are consistent with their
formulations.
enhancement, the turnover frequency is less than 0.5 sÀ1 [16]
.
The catalytic activity and H2 product was confirmed by
controlled-potential coulometry followed by gas chromato-
graphic analysis of the head space to measure the hydrogen
produced; the current efficiency was determined to be (101 Æ
5)%.
The NiII complex 1 reacts cleanly with H2 (1 atm) to form
an equilibrium mixture of Ni0 complexes with two protonated
amines, isomers 2A, 2B, and 2C, in a ratio of 0.8:1.0:0.1,
respectively (Scheme 1). The isomeric products differ in the
The onset potential for catalytic reduction in the acidic
solution is shifted about 360 mV positive of the NiII/I couple
observed in the absence of acid. Under catalytic conditions (in
the presence of the acid 4-cyanoanilinium tetrafluroroborate,
pKa = 7.0 in CH3CN)[17] the NiII complex 1 is doubly proton-
À
orientation of the N H bonds. Isomer 2A, in which the
protons are both in the endo position with respect to the metal
center, is the initial product of H2 addition, but intermolecular
proton exchange at room temperature leads to the formation
of additional isomers in which one or both protons are in the
exo position with respect to the metal and interact with both
nitrogen atoms of the ligand (isomers 2B and 2C). Character-
ization of the three isomers has previously been described in
detail for related systems.[15] Integration of the 1H NMR
resonances for complex 1 and the sum of the products 2A, 2B,
and 2C yields an equilibrium constant of 0.24(2) atmÀ1 at
238C and 1 atm H2, corresponding to DG8 = 0.84 kcalmolÀ1
for hydrogen addition. Integration of the 31P{1H} spectrum
yields a similar value, but because of the broadness of the
1
ated to form 3, as shown in Scheme 2. Both H and 31P{1H}
NMR studies on complex 1 in the presence of acid confirm
1
peaks, the value obtained from the H NMR integration is
Scheme 2. Complex 1 is protonated in the exo position on each ligand
upon addition of 4-cyanoanilinium tetrafluoroborate.
considered more accurate. In performing this study, we have
observed an inverse equilibrium isotope effect for this
reaction. Detailed experimental and theoretical studies on
this effect will be reported in a future publication.
The cyclic voltammogram of 1 in acetonitrile under N2,
shown as the black trace in Figure 1, displays two reversible
one-electron reduction waves at À0.94 V and À1.25 V versus
Cp2Fe+/0, and these are assigned to the NiII/I and NiI/0 couples
(DEp = 63 mV and 66 mV, respectively, at a scan rate of
50 mVsÀ1). The blue trace in Figure 1 shows the cyclic
that double protonation occurs in the “exo” positions, with
each proton shared between the pendant amines of their
respective ligands (see the Supporting Information). A
similar structure has been previously characterized for the
protonation of a related NiII complex.[18] Complex 3 has been
characterized by both 1H and 31P{1H} NMR spectroscopy (see
the Supporting Information). Addition of less than two
equivalents of acid leads to the doubly protonated complex 3
and unprotonated complex 1; the singly protonated complex
is not observed. The pKa of complex 3 in CH3CN was
1
determined relative to 4-cyanoanilinium. H NMR spectros-
copy was used to determine the ratio of 4-cyanoanilinium and
4-cyanoaniline, and integration of the 31P{1H} NMR spectra
was used to determine the ratio of 3 to 1. The two ratios were
used to determine an average pKa value of 7.3 in CH3CN for
the two sequential deprotonations of 3.
Formation of 3 under acidic conditions results in a positive
shift of 360 mV for the onset of the catalytic current for
proton reduction. Positive shifts of up to 440 mV have been
observed for related [Ni(PR2NR’2H)2]4+ complexes compared
to their unprotonated analogs.[18] The observed potential is
dependent on the pKa of the solution, indicating a proton-
coupled electron transfer. This large positive shift corre-
sponds to a lower overpotential for proton reduction. The
overpotential for this catalyst is 140[19] or 68 mV,[20] depending
on the literature value used for the standard potential for
Figure 1. Cyclic voltammograms of a 0.8 mm solution of complex 1
with no acid (black), and in 5.9 mm (blue), and 12 mm (red) 4-
cyanoanilinium tetrafluoroborate. Conditions: scan rate, 50 mVsÀ1
,
0.20m NEt4BF4 in acetonitrile solvent, glassy carbon working electrode.
Potentials are referenced to the Cp2Fe+/0 couple, shown at 0.0 V.
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proton reduction in the calculation for the thermodynamic
potential.
The low free-energy for hydrogen addition and the small
overpotential for catalytic hydrogen production indicate the
possibility for reversible electrocatalytic hydrogen production
and oxidation. The cyclic voltammograms shown in Figure 2
DG8 for H2 addition, and an equilibrium mixture with H2 is
readily observed by NMR spectroscopy. Furthermore, the
complex electrocatalytically produces and oxidizes hydrogen
close to the thermodynamic potential. This is the first
example of a synthetic homogeneous electrocatalyst capable
of performing this task, which is otherwise most commonly
observed in the hydrogenase enzymes.
Received: December 1, 2011
Published online: February 14, 2012
Keywords: catalysis · electrochemistry ·
.
homogeneous catalysis · hydrogen · hydrogenase enzymes
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by chemical oxidation because the slow rate of reaction
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to a solution of 1 in CH3CN, and D2 was sparged through the
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the amount of conjugate acid generated, giving eight turn-
overs after five minutes. A control reaction showed that the
À
D2 oxidation did not occur from the Cp2Fe+BF4
.
In summary, the new nickel complex reported here,
[Ni(PPh2NR’2)2]2+ (R’ = CH2CH2OCH3, 1) possesses a low
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