Hydrogen oxidation catalysis by a nickel diphosphine complex with pendant tert-butyl amines

Article abstract of DOI:10.1039/c0cc03246h

A bis-diphosphine nickel complex with tert-butyl functionalized pendant amines [Ni(P^Cy₂N^t-Bu₂) ₂]²⁺ has been synthesized. It is a highly active electrocatalyst for the oxidation of hydrogen

Full text of DOI:10.1039/c0cc03246h

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COMMUNICATION
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Hydrogen oxidation catalysis by a nickel diphosphine complex with
pendant tert-butyl amineswz
Jenny Y. Yang,*a Shentan Chen,a William G. Dougherty,b W. Scott Kassel,b
R. Morris Bullock,a Daniel L. DuBois,a Simone Raugei,a Roger Rousseau,a Michel Dupuisa
and M. Rakowski DuBoisa
Received 13th August 2010, Accepted 21st September 2010
DOI: 10.1039/c0cc03246h
A bis-diphosphine nickel complex with tert-butyl functionalized
pendant amines [Ni(PCy2Nt-Bu2)2]2+ has been synthesized. It is a
highly active electrocatalyst for the oxidation of hydrogen in the
presence of base. The turnover rate of 50 sꢀ1 under 1.0 atm H2
at a potential of ꢀ0.77 V vs. the ferrocene couple is 5 times
faster than the rate reported heretofore for any other synthetic
molecular H2 oxidation catalyst.
An important prerequisite for the expanded use of energy from
renewable sources is the ability to store and recover the energy
on demand. Chemical fuels are ideal in this context since
chemical bonds have high energy density. Hydrogen has
frequently been cited as a good candidate as a chemical fuel.
To minimize energy requirements, efficient catalysts are necessary
for both the production and utilization of hydrogen.
Currently, the best catalyst for hydrogen production and
oxidation is platinum, which is too costly for widespread use.
Such considerations have led to the development of molecular
catalysts employing more abundant metals, and several com-
plexes that contain nickel,1–5 cobalt,5–9 or iron10–14 have been
developed as electrocatalysts for the production of hydrogen.
In contrast, there are few molecular complexes that perform
hydrogen oxidation,15–20 and even fewer have been studied
for catalytic21–24 or electrocatalytic activity.1,25–27 We have
previously reported that [Ni(PCy2NBz2)2](BF4)2 functions as an
electrocatalyst for H2 oxidation with a turnover frequency of
Scheme 1
transfer events.28,29 In this communication we report a synthetic
variant of the nickel complex. Replacement of the benzyl
functionality on the pendant nitrogen base with a tert-butyl
group increases its proton acceptor ability which leads to
a greater driving force for hydrogen addition. The result is
the most active molecular catalyst for hydrogen oxidation
reported to date.
The new eight-membered cyclic ligand PCy2Nt-Bu was
2
synthesized by a procedure similar to that reported for other
ligands in this class.1,3,30,31 Two equivalents of the ligand were
added to [Ni(CH3CN)6](BF4)2 in acetonitrile to form the
complex [Ni(PCy2Nt-Bu2)2](BF4)2 (1), isolated as a crystalline
red solid. [Ni(PCy2Nt-Bu2)2](BF4)2 (1) in acetonitrile rapidly
reacts with H2 (1 atm, RT) to form a colorless product which is
identified by spectroscopic data as a Ni(0) complex with a
protonated amine in each ligand, as shown in Scheme 1. In the
1H NMR spectrum a resonance at 4.9 ppm is assigned to the
amine protons. No resonances are observed in the nickel
hydride region (ꢀ5 to ꢀ15 ppm). Both the 1H and 31P chemical
shifts (18.3 ppm) for the product are consistent with the isomer
in which the protonated chelate rings of the ligand retain their
10 sꢀ1 1 PR2NR0 denotes cyclic diphosphine ligands that
.
2
incorporate an amine base in each ligand chelate ring, where
R and R0 are substituents on the phosphorus and nitrogen
atoms, respectively (e.g., see Scheme 1).
The non-coordinating pendant bases in these complexes
have been found to contribute to high catalytic activity for
both H2 formation and oxidation by stabilizing the binding of
H2 to the metal center, assisting the heterolytic cleavage
or formation of the H–H bond, facilitating intra- and inter-
molecular proton transfer, and coupling proton and electron
boat conformations.2 The Ni0 complex Ni(PCy2Nt-Bu
) (2)
2 2
can be synthesized from the hydrogen addition product
by the addition of excess base (DBU, 1,8-diazabicyclo[5.4.0]-
undec-7-ene), also shown in Scheme 1. Both complex 1 and 2
have been characterized by 1H and 31P{1H} NMR spectro-
scopy, and elemental analysis, as detailed in the ESI.z All data
are consistent with their formulations.
a Center for Molecular Electrocatalysis, Chemical and Materials
Sciences Division, Pacific Northwest National Laboratory, Richland,
WA 99352, USA. E-mail: jenny.yang@pnl.gov;
Fax: +1 509-375-6660; Tel: +1 509-372-4639
b Department of Chemistry, Villanova University, Villanova,
Pennsylvania, USA
w This article is part of a ChemComm ‘Hydrogen’ web-based themed
issue.
z Electronic supplementary information (ESI) available: Synthetic and
electrochemical experimental details, crystallography tables, and
theoretical methods. CCDC 789420. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc03246h
X-Ray diffraction studies have been carried out on crystals
of 1 grown from a solution of acetonitrile and diethyl ether.
The dication of 1 (Fig. 1) is a distorted square planar complex
with a dihedral angle of 22.611, between the two planes defined
c
8618 Chem. Commun., 2010, 46, 8618–8620
This journal is The Royal Society of Chemistry 2010

Fig. 2 Cyclic voltammograms of a 0.57 mM Ni(PCy2Nt-Bu
) (2)
2 2
solution under nitrogen (black), under hydrogen (red), and in increasing
concentration of Et3N (colored). Conditions: scan rate = 50 mV sꢀ1
,
benzonitrile solvent, 0.2 M Bu4NPF6 as supporting electrolyte, glassy
carbon working electrode. Potentials are referenced to the ferrocenium/
ferrocene couple.
Fig.
1 The crystallographic structure of the cation of
[Ni(PCy2Nt-Bu2)](BF4)2 (1). Selected bond distances (A) and angles (1):
Ni(1)–P(1), 2.2205(8); Ni(1)–P(2), 2.2196(8); P(2)–Ni(1)–P(2), 80.83(2).
Thermal ellipsoids are shown at the 50% probability level. For clarity,
only the carbons attached to the phosphorous atoms are shown.
base, indicating a catalytic reaction with a turnover frequency
of 50 sꢀ1 under 1.0 atm H2.33
Previous kinetic studies of H2 oxidation catalysts by nickel
diphosphine complexes with pendant amines have shown that
the rate-limiting step is the addition/cleavage of H2 to the NiII
complex. We have investigated the thermodynamic factors
that govern this reaction, and these studies guided the design
of the new catalyst reported here. The thermodynamic driving
force for hydrogen addition can be tuned by modifying
the phosphorus and nitrogen functionalities. A large substi-
tuent on phosphorus, such as the cyclohexyl group, results
in a tetrahedral distortion of the complex. In a series of
[Ni(diphosphine)2]2+ derivatives this distortion was found to
lead to a decrease in the energy of the lowest unoccupied
molecular orbital, resulting from a decrease in the sigma
antibonding interaction between nickel and phosphorus. This
lower energy enhances the hydride acceptor abilities of the NiII
complexes34,35 and favors the addition of H2. The free
energy for hydrogen addition is also favored by increasing
the proton acceptor ability of the pendant amine. The use of
the tert-butyl substituent on the nitrogen increases the basicity
of the pendant amine compared to the previously studied
benzyl derivative, [Ni(PCy2NBz2)2]2+. As a result, the catalyst
[Ni(PCy2Nt-Bu2)2](BF4)2 has a greater driving force for hydrogen
addition. The catalytic rate of 50 sꢀ1 is the fastest reported for a
molecular hydrogen oxidation catalyst.
by the phosphorus atoms of each ligand and nickel. This
distortion from a planar geometry is likely the result of steric
interactions between the cyclohexyl substituents on phosphorus.
The small P–Ni–P bite angles for the cyclic diphosphine
ligands (80.641) are characteristic of first row metal complexes
with these PR2NR0 ligands, and are attributed to constraints
2
imposed by the second six-membered chelate ring in the
backbone of the cyclic ligand.1,8,9,26,32 From Fig. 1, it can be
seen that all four six-membered rings formed upon coordina-
tion of the diphosphine ligands to the metal are in boat
conformations, and the average non-bonding Niꢁ ꢁ ꢁN distance
is 3.25 A.
The cyclic voltammogram of [Ni(PCy2Nt-Bu2)2](BF4)2 (1)
recorded in benzonitrile displays two reversible one-electron
reduction waves at ꢀ0.81 V and ꢀ1.45 V vs. the ferrocenium/
ferrocene couple, which are assigned to the NiII/I and NiI/0
couples (DEp = 82 mV and 60 mV, respectively, at a scan rate
of 50 mV sꢀ1). The corresponding cyclic voltammogram of the
Ni0 complex, [Ni(PCy2Nt-Bu2)2] (2), recorded in benzonitrile
consists of two reversible one-electron oxidation waves
(DEp = 63 mV and 76 mV at a scan rate of 50 mV sꢀ1) at
ꢀ1.49 V and ꢀ0.87 V vs. the ferrocenium/ferrocene couple
(shown in black in Fig. 2).
Fig.
2 also displays the cyclic voltammogram of
[Ni(PCy2Nt-Bu2)2] (2) recorded in benzonitrile purged with
hydrogen (red trace), and upon addition of aliquots of triethyl-
amine (other colored traces). In the absence of base (red trace),
2 does not react with hydrogen until it is oxidized to the Ni2+
complex (1), upon which the oxidation wave at ꢀ0.87 V is
enhanced. Addition of triethylamine (up to 82 equivalents)
results in an increase in the anodic current observed for the
wave at ꢀ0.87 V (colored scans). Higher concentrations of
triethylamine did not result in any further increase in current.
The oxidative current for the NiII/I couple in the presence of
hydrogen plus triethylamine is 23 times larger than the current
observed for the NiII/I couple in the absence of hydrogen and
In conjunction with our experimental studies, we have
performed a computational electronic structure study (see ESIz
for relevant details) of the mechanism for H2 addition to
the nickel catalyst. The symmetric all boat conformer of
[Ni(PCy2NMe2)2]2+ is used in place of the NtBu derivative to
reduce the computational complexity. As shown in Fig. 3, the
H2 molecule forms an adduct with the Ni complex with a
relative free energy of +2 kcal molꢀ1 at T = 300 K. The
positive free energy arises from the loss of translational
entropy of H2 (B10 kcal molꢀ1) as it binds to the complex,
a loss partially offset by a 8 kcal molꢀ1 binding energy of H2 to
the Ni center. The H2 adduct then undergoes heterolytic H2
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8618–8620 8619

3 K. Fraze, A. D. Wilson, A. M. Appel, M. Rakowski DuBois and
D. L. DuBois, Organometallics, 2007, 26, 5003.
4 A. D. Wilson, K. Fraze, B. Twamley, S. Miller, D. L. DuBois and
M. Rakowski DuBois, J. Am. Chem. Soc., 2008, 130, 1061.
´
5 P.-A. Jacques, V. Artero, J. Pecaut and M. Fontecave, Proc. Natl.
Acad. Sci. U. S. A., 2009, 106, 20627.
6 L. A. Berben and J. C. Peters, Chem. Commun., 2010, 46,
398–400.
7 N. K. Szymczak, L. A. Berben and J. C. Peters, Chem. Commun.,
2009, 6729.
8 G. M. Jacobsen, J. Y. Yang, B. Twamley, A. D. Wilson,
R. M. Bullock, M. Rakowski DuBois and D. L. DuBois, Energy
Environ. Sci., 2008, 1, 167.
9 E. Wiedner, J. Y. Yang, W. G. Dougherty, W. S. Kassel,
R. M. Bullock, M. Rakowski DuBois and D. L. DuBois, Organo-
metallics, DOI: 10.1021/om100395r.
10 J. Chen, A. K. Vannucci, C. A. Mebi, N. Okumura,
S. C. Borowski, M. Swenson, L. T. Lockett, D. H. Evans,
R. S. Glass and D. L. Lichtenberger, Organometallics, DOI:
10.1021/om100396j.
Fig.
3
Calculated reaction profile for all boat confomer of
[Ni(PCy2NMe2)2]2+ + H2. Blue lines: energy; red lines: free energy.
11 C. Tard and C. J. Pickett, Chem. Rev., 2009, 109, 2245.
12 S. Ott, M. Kritikos, B. Akermark, L. Sun and R. Lomoth, Angew.
Chem., Int. Ed., 2004, 116, 1024.
13 D. Chong, I. P. Georgakaki, M. L. Miller, R. Mejia-Rodriguez and
M. Y. Darensbourg, Dalton Trans., 2003, 4158.
14 R. Meija-Rodriguez, D. Chong, J. H. Reibenspies, M. P.
Soriaga and M. Y. Darensbourg, J. Am. Chem. Soc., 2004, 126,
12004.
Dashed lines are only a guide to the eye.
cleavage resulting in a Ni2+ hydride species and a protonated
pendant amine. The transition state free energy for this step is
calculated to be +3.5 kcal molꢀ1 higher than the free energy
of the adduct. A homolytic cleavage of the H2 adduct to form
a dihydride intermediate (not shown) was also characterized
and found to have a free energy barrier about twice as large
(B7 kcal molꢀ1) as the heterolytic cleavage. The proton–
hydride complex converts to the experimentally observed
Ni(0) species with two protonated pendant amines with a free
energy barrier of only B1 kcal molꢀ1. The rate-determining
barrier to the overall mechanism of H2 oxidation is thus the
barrier associated with the heterolytic H2 bond cleavage
step in addition to the free energy (translational entropy) loss
due to the coordination of H2 to the complex. A detailed
account of the computational study will be given in a future
publication.36
´
15 L. I. Sima
Chem. Lett., 1975, 11, 773.
´
ndi, Z. Szeverenyi and E. Budo-Zahonyi, Inorg. Nucl.
´
´
´
16 J. P. Collman, L. M. Slaughter, T. A. Eberspacher, T. Strassner
and J. I. Brauman, Inorg. Chem., 2001, 40, 6272.
17 X. Hu, B. S. Brunschwig and J. C. Peters, J. Am. Chem. Soc., 2007,
129, 8988.
18 M. T. Olsen, B. E. Barton and T. B. Rauchfuss, Inorg. Chem.,
2009, 48, 7507.
19 Z. M. Heiden, G. Zampella, L. De Gioia and T. B. Rauchfuss,
Angew. Chem., Int. Ed., 2008, 47, 9756.
20 F. Gloaguen and T. B. Rauchfuss, Chem. Soc. Rev., 2009, 38, 100.
21 M. R. Ringenberg, S. L. Kokatam, Z. M. Heiden and
T. B. Rauchfuss, J. Am. Chem. Soc., 2008, 130, 788.
22 M. R. Ringenberg, M. J. Nilges, T. B. Rauchfuss and S. R. Wilson,
Organometallics, 2010, 29, 1956.
In summary,
a
new derivative in the class of
23 S. Ogo, Chem. Commun., 2009, 3317.
[Ni(PR2NR02)2](BF4)2 complexes has been synthesized. The
incorporation of the tert-butyl amine into the cyclohexyl
phosphine framework was designed to lower the free energy
of hydrogen addition/cleavage, the rate-determining step for
catalysis. This has led to an electrocatalyst that is 5 times faster
than the best molecular H2 oxidation catalyst previously
reported, the analogous nickel diphosphine derivative with
benzyl substituents on the nitrogens.1
24 S. Kuwata and T. Ikariya, Dalton Trans., 2010, 39, 2984.
25 C. J. Curtis, A. Miedaner, R. F. Ciancanelli, W. W. Ellis,
B. C. Noll, M. Rakowski DuBois and D. L. DuBois, Inorg. Chem.,
2003, 42, 216.
26 J. Y. Yang, R. M. Bullock, W. J. Shaw, B. Twamley, K. Fraze,
M. Rakowski DuBois and D. L. DuBois, J. Am. Chem. Soc., 2009,
131, 5935.
27 J. P. Collman, P. S. Wagenknecht, J. E. Hutchison, N. S. Lewis,
M. A. Lopez, R. Guilard, M. L’her, A. A. Bothner-By and
P. K. Mishra, J. Am. Chem. Soc., 1992, 114, 5654.
28 M. Rakowski DuBois and D. L. DuBois, C. R. Chim., 2008, 11,
805.
29 M. Rakowski DuBois and D. L. DuBois, Chem. Soc. Rev., 2009,
38, 62.
30 J. Y. Yang, R. M. Bullock, W. G. Dougherty, W. S. Kassel,
B. Twamley, D. L. DuBois and M. Rakowski DuBois, Dalton
Trans., 2010, 39, 3001.
This research was supported as part of the Center for
Molecular Electrocatalysis, an Energy Frontier Research
Center funded by the US Department of Energy, Office of
Science, Office of Basic Energy Sciences. Pacific Northwest
National Laboratory is operated by Battelle for the US Depart-
ment of Energy. Computational resources were provided by the
Environmental Molecular Science Laboratory (EMSL) and the
National Energy Research Scientific Computing Center
(NERSC) at Lawrence Berkeley National Laboratory.
31 V. G. Markl, G. Y. Jin and C. Schoerner, Tetrahedron Lett., 1980,
¨
21, 1409.
32 K. D. Welch, W. G. Dougherty, W. S. Kassel, D. L. DuBois and
R. M. Bullock, Organometallics, DOI: 10.1021/om100668e.
33 D. L. Pool and D. L. DuBois, J. Organomet. Chem., 2009, 694,
2858.
34 A. Miedaner, R. C. Haltiwanger and D. L. DuBois, Inorg. Chem.,
1991, 30, 417.
35 J. W. Raebiger, A. Miedaner, C. J. Curtis, S. M. Miller and
D. L. DuBois, J. Am. Chem. Soc., 2004, 126, 5502.
36 S. Chen, R. Rousseau, S. Raugei, M. Dupuis and R. M. Bullock, in
preparation.
Notes and references
1 A. D. Wilson, R. H. Newell, M. J. McNevin, J. T. Muckerman,
M. Rakowski DuBois and D. L. DuBois, J. Am. Chem. Soc., 2006,
128, 358.
2 A. D. Wilson, A. Miedaner, J. T. Muckerman, D. L. DuBois and
M. Rakowski DuBois, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 6951.
c
8620 Chem. Commun., 2010, 46, 8618–8620
This journal is The Royal Society of Chemistry 2010