Controlling proton movement: Electrocatalytic oxidation of hydrogen by a nickel(ii) complex containing proton relays in the second and outer coordination spheres

Article abstract of DOI:10.1039/c3dt53074d

A nickel bis(diphosphine) complex containing proton relays in the second and outer coordination spheres, Ni(P^Cy₂N ^(CH2)2OMe)₂, (P^Cy₂N ^(CH2)2OMe = 1,5-di(methoxyethyl)-3,7-dicyclohexyl-1,5-diaza-3,7- diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni(ii) complex results in rapid formation of three isomers of the doubly protonated Ni(0) complex, [Ni(P^Cy ₂N^(CH2)2OMe₂H)₂]²⁺. The three isomers show fast interconversion at 40 °C, unique to this complex in this class of catalysts. Under conditions of 1.0 atm H₂ using H ₂O as a base, catalytic oxidation proceeds at a turnover frequency of 5 s^-1 and an overpotential of 720 mV, as determined from the potential at half of the catalytic current. Compared to the previously reported Ni(P^Cy₂N^Bn)₂ complex, the new complex operates at a faster rate and at a lower overpotential.

Full text of DOI:10.1039/c3dt53074d

Volume 43 Number 7 21 February 2014 Pages 2697–3006
Dalton
Transactions
An international journal of inorganic chemistry
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COVER ARTICLE
Helm et al.
Controlling proton movement: electrocatalytic oxidation of hydrogen
by a nickel(II) complex containing proton relays in the second and outer
coordination spheres

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PAPER
Controlling proton movement: electrocatalytic
oxidation of hydrogen by a nickel(II) complex
containing proton relays in the second and outer
coordination spheres†
Cite this: Dalton Trans., 2014, 43,
2744
Parthapratim Das, Ming-Hsun Ho, Molly O’Hagan, Wendy J. Shaw, R. Morris Bullock,
Simone Raugei and Monte L. Helm*
A nickel bis(diphosphine) complex containing proton relays in the second and outer coordination spheres,
Ni(PCy2N(CH2)2OMe)2, (PCy2N(CH2)2OMe = 1,5-di(methoxyethyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphospha-
cyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni(II)
complex results in rapid formation of three isomers of the doubly protonated Ni(0) complex,
[Ni(PCy2N(CH2)2OMe2H)2]2+. The three isomers show fast interconversion at 40 °C, unique to this complex
in this class of catalysts. Under conditions of 1.0 atm H2 using H2O as a base, catalytic oxidation proceeds
at a turnover frequency of 5 s−1 and an overpotential of 720 mV, as determined from the potential at half
Received 30th October 2013,
Accepted 24th November 2013
of the catalytic current. Compared to the previously reported Ni(PCy2NBn
) complex, the new complex
2
DOI: 10.1039/c3dt53074d
www.rsc.org/dalton
operates at a faster rate and at a lower overpotential.
the reverse reaction, oxidation of hydrogen.6–13 We have found
Introduction
that [Ni(PCy2NR′2)2]2+ (R′ = alkyl) are electrocatalysts for oxi-
Obtaining energy from renewable sources such as wind, solar dation of H2.8–11 Experimental and computational data indi-
and biomass is necessary to reduce our dependence on fossil cate that removal of protons from the catalyst by an exogenous
fuels. Because of the intermittent nature of the generation of base is a potential rate-limiting step in the mechanism for
energy from solar and wind, there is a need to store energy electrocatalytic oxidation of H2.14 Recent studies have shown
during times of high production, so it can be used at times of that exogenous bases accelerate the isomerization between the
high demand. Platinum is currently used as a catalyst in fuel three isomers of the Ni(0) complex [Ni(PCy2NBn2H)2]2+ (e.g.
cells for hydrogen oxidation (energy production). Due to the Scheme 1) that results from addition of H2 to [Ni(PCy2NBn2)2]2+,
high cost and low abundance of platinum, catalysts based on indicating the rates of proton movement to and from the
abundant, inexpensive metals are attractive to facilitate more metal complex can be influenced.14 Recent reports of the
widespread use of renewable fuels.1 Transition metal mole- [Ni(PCy2N(3-pyridazyl)methyl2)2]2+ complex illustrated that the
cular electrocatalysts based on Ni and Co have been developed inclusion of an additional proton relay in the outer coordi-
for hydrogen production, and incorporation of pendant amines nation sphere of the complex facilitates rapid proton transfer
functioning as proton relays are beneficial for increasing between the doubly protonated isomers and lowers the over-
efficiency and turnover frequencies.2,3 There have also been potential for catalysis.9 This work illustrates that accelerated
recent reports of incorporating additional pendant groups in proton movement to and from the primary and secondary
the outer coordination sphere of nickel catalysts, resulting in coordination spheres of the metal complex can be controlled
significant rate enhancements for hydrogen production.4,5
by the ligand design and influence catalyst efficiency.
Compared to molecular electrocatalysts that produce hydro- Although rapid proton movement was observed in
gen, there are fewer examples of molecular electrocatalysts for [Ni(PCy2N(3-pyridazyl)methyl2)2]2+, rates of catalysis were signifi-
cantly inhibited, which was thought to be due to competitive
binding of the pyridazyl group with H2 to the Ni(II) center
during catalysis.9
Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest
National Laboratory, P.O. Box 999, K2-57, Richland 99352, Washington, USA.
In an effort to facilitate proton movement and removal from
E-mail: monte.helm@pnnl.gov
both the second and outer coordination spheres of the catalyst
†Electronic supplementary information (ESI) available. CCDC 969326. For ESI
without limiting the rate, we designed a new [Ni(PR2NR′2)2]2+
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
complex with additional relays in the outer coordination
c3dt53074d
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Scheme 1 The three doubly protonated Ni(0) isomers of [Ni(PCy2NR2H)2]2+
.
sphere that would not competitively bind with H2. We report a
new nickel complex bearing a methoxyethyl group appended
on the pendant amine that resides in the outer coordination
sphere of the compound. The three Ni(0) doubly protonated
isomers resulting from H2 addition (e.g. Scheme 1) undergo
rapid interconversion, providing mechanistic insight into
proton movement relevant to catalysis. Additionally, the inter-
action of the methoxyethyl groups with the pendant amine
results in a hydrogen oxidation catalyst that can operate at
both
a lower overpotential and a faster rate than the
[Ni(PCy2NBn2H)2]2+ complex using water as an exogenous base.
Importantly, this demonstrates our ability to control both rates
and overpotentials through a combination of modifying the
functionality of the second and outer coordination spheres.
Fig. 1 X-ray crystal structure of [Ni(PCy2N(CH2)2OMe)2](BF4)2·2CH3CN.
−
The BF4 counterions, CH3CN solvent molecules and H atoms have
Results
been omitted for clarity. Only the carbon attached to the phosphorus
atom of the cyclohexyl groups is shown for clarity. Thermal ellipsoids
are shown at the 50% probability level.
Synthesis and characterization of Ni(PCy2N(CH2)2OMe)2 and
[Ni(PCy2N(CH2)2OMe)2](BF4)2
The cyclic diphosphine ligand PCy2N(CH2)2OMe2 was synthesized
by the condensation of bis(hydroxymethyl)cyclohexyl-
phosphine,
CyP(CH2OH)2,
with
methoxyethylamine
ð2Þ
ð3Þ
(MeOCH2CH2NH2), as characterized by 31P{1H} NMR spec-
troscopy (eqn (1)). Attempts to isolate pure PCy2N(CH2)2OMe
2
ligand from the reaction mixture proved unsuccessful, so the
ligand was generated in situ and reacted with [Ni(CH3CN)6]-
(BF4)2 to form [Ni(PCy2N(CH2)2OMe2)2]2+. Isolation of the pure
Ni(II) complex from the reaction mixture was not accom-
plished, however, after reduction of the Ni(II) complex with H2
and deprotonation with tetramethylguanidine (TMG), the pure
Ni(0) complex Ni(PCy2N(CH2)2OMe2)2 precipitated from the reac-
tion mixture (eqn (2)). Oxidation of the Ni(0) species with two
equivalents of (Cp2*Fe)(BF4) (Cp* = η5-C5Me5) resulted in for-
mation and isolation of the pure Ni(II) complex (eqn (3)); both
the Ni(0) and Ni(II) species were fully characterized. An X-ray
diffraction study of [Ni(PCy2N(CH2)2OMe2)2]2+ indicates the
complex has a distorted square planar geometry around the
Ni(II) center (Fig. 1), as previously observed in other structu-
rally characterized [Ni(PR2NR′2)2]2+ complexes.8,10
X-ray quality crystals of [Ni(PCy2N(CH2)2OMe)2](BF4)2 were
grown by slow diffusion of Et2O into a saturated CH3CN solu-
tion of the complex. The [Ni(PCy2N(CH2)2OMe)2](BF4)2 complex
crystallizes with two non-coordinating CH3CN molecules per
−
unit cell, and no close contacts are observed between the BF4
anions and the [Ni(PCy2N(CH2)2OMe)2]2+ cation (Fig. 1). The
structure shows a distorted square planar geometry with all
four Ni–P bond distances nearly equal, ranging from
2.20–2.24 Å. The P–Ni–P bond angle for each of the dipho-
sphine ligands averages 82.2°, comparable to the 80.6° P–Ni–P
angle observed in the previously reported [Ni(PCy2NtBu2)2]2+
complex.7 The dihedral angle between the two planes defined
ð1Þ
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by the two phosphorus atoms of the bidentate P2N2 ligand and
Ni atom is 25.59(3)°, slightly larger than the 22.53° dihedral
angle reported for [Ni(PCy2NtBu2)2]2+.7 All four of the six-mem-
bered rings formed upon coordination of the PCy2NCH2CH2OMe
2
ligands to the Ni center are in a boat conformation, as also
seen in the four-coordinate structures of [Ni(PCy2NBn2)2]2+ and
[Ni(PCy2NtBu2)2]2+.7,10 Complete crystallographic information
along with bond distances and angles for [Ni(PCy2N(CH2)2OMe)2]-
(BF4)2 is contained in the ESI.†
Addition of H2 to [Ni(PCy2N(CH2)2OMe)2]2+
Room temperature addition of H2 (1 atm) to an acetonitrile
solution of [Ni(PCy2N(CH2)2OMe2)2]2+ leads to a color change
from red to yellow indicating formation of the doubly proto-
nated Ni(0) complex, [Ni(PCy2N(CH2)2OMe2H)2]2+, with protons
in an endo/endo position (eqn (4)), as previously observed for
other [Ni(PR2NR′2)2]2+ catalysts for oxidation of H2.8,10 The
doubly protonated [Ni(PCy2N(CH2)2OMe2H)2]2+ complex under-
goes isomerization at room temperature to form an equili-
brium distribution of 10% endo/endo, 40% endo/exo and 50%
exo/exo isomers (i.e. Scheme 1) in the time it takes to prepare
the sample (i.e. <15 min). The three isomers were identified by
their distinct 31P{1H} NMR spectrum (Fig. S1 in the ESI†), as
previously observed and reported for [Ni(PCy2NBn2H)2]2+.15
Phosphorus-31 EXSY experiments carried out at 40 °C (Fig. S2
in the ESI†) show strong exchange cross-peaks between both
the endo/endo and endo/exo isomers and the endo/exo and exo/
exo isomers at mixing times as short as 0.1 s, indicating that
all three isomers are in rapid exchange. At 25 °C, only the
exchange peaks between the endo/endo and endo/exo isomers
are observed, indicating slower exchange.
Fig. 2 Plot of the formation of the exo/exo isomer as a function of time
at 0 °C in CH3CN at different [Ni(PCy2N(CH2)2OMe2H)2]2+ concentrations.
This data shows that proton movement from endo to exo does not
depend upon catalyst concentration.
Fig. 3 Cyclic voltammogram of 1.0 mM [Ni(PCy2N(CH2)2OMe2)2](BF4) in
0.20 M [Bu4N][PF6]–CH3CN. Conditions: 1 mm glassy-carbon working
electrode; scan rate 0.05 V s−1 at 25 °C.
with observed E1/2 values (reported as the average of the
potentials of the maximum cathodic and anodic current) at
−0.81 V (NiII/I couple) and −1.37 V (NiI/0 couple) vs. the ferroce-
ð4Þ
The rate of isomerization was further investigated by moni-
toring the time to reach equilibrium by 31P{1H} NMR spec-
troscopy at 0 °C as a function of catalyst concentration (Fig. S3
in the ESI†). The resulting equilibrium concentrations as
measured by the integration of the 31P{1H} NMR spectra of
each isomer are identical to those obtained at room tempera-
ture. The time necessary to reach the equilibrium concen-
tration of the exo/exo isomer at three catalyst concentrations
showed no dependence on the catalyst concentration (Fig. 2),
indicating the rate limiting step for this isomerization process
is likely an intramolecular proton transfer process.
nium/ferrocene (Cp2Fe+/0
) couple. A plot of either the
anodic (ip) or cathodic peak current of the Ni(II/I) couple vs.
the square root of the scan rate shows a linear correlation,
implying diffusion-controlled electrochemical events. The
difference in the potential of cathodic and anodic peak poten-
tials (ΔEp) at a scan rate of 0.05 V s−1 for these processes is
measured to be 63–68 mV, indicating one-electron processes
in both couples. Cyclic voltammograms of the Ni(II)
complex [Ni(PCy2N(CH2)2OMe2)2]2+ and the Ni(0) complex
Ni(PCy2N(CH2)2OMe2)2 complexes give the same results.
Oxidation of the Ni(I) complex [Ni(PCy2N(CH2)2OMe2)2]+
complex in the presence of H2 results in its addition to the
Ni(II) complex and formation of the endo/endo isomer, allowing
for its electrochemical characterization (Fig. 4, blue). Electro-
chemical analysis of equilibrated mixtures containing all three
doubly protonated Ni(0) isomers of [Ni(PCy2N(CH2)2OMe2H)2]2+
allows for the electrochemical assignment of the exo/exo and
Electrochemical measurements
An equal stoichiometric mixture of the Ni(II) complex [Ni-
(PCy2N(CH2)2OMe)2]2+ and the Ni(0) complex Ni(PCy2N(CH2)2OMe
)
2
cleanly comproportionates to the Ni(I) [Ni(PCy2N(CH2)2OMe)2]+
complex. The cyclic voltammogram of [Ni(PCy2N(CH2)2OMe2)2]+
(Fig. 3) shows two one-electron reversible redox couples,
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Fig. 4 Cyclic voltammograms of (red) an equilibrated mixture of
0.5 mM [Ni(PCy2N(CH2)2OMe2H)2]2+ and (blue) 1.0 mM [Ni(PCy2N(CH2)2OMe2)2]+
in the presence of 1 atm H2. Conditions: 0.20 M [Bu4N][PF6]–CH3CN;
1 mm glassy-carbon working electrode; scan rate 0.05 V s−1 at 25 °C.
Table 1 Experimental (and computational) pKa values and electro-
chemical potentials (V) for Ni(PCy2N(CH2)2OMe
) and the doubly proto-
2 2
nated Ni(0) isomers of [Ni(PCy2N(CH2)2OMe2H)2]2+ in CH3CN. Redox
potentials are referenced to the Cp2Fe+/0 couple. The electrochemical
potentials for Ni(PCy2N(CH2)2OMe
)
2 2
are half-peak potentials for the
reversible waves. The electrochemical potentials for the doubly proto-
nated Ni(0) isomers of [Ni(PCy2N(CH2)2OMe2H)2]2+ are peak potentials of
the irreversible waves
Compound (isomer)
pKa
Ni(I/0)
Ni(II/I)
Fig. 5 Cyclic voltammograms of 1.0 mM [Ni(PCy2N(CH2)2OMe2H)2]2+ with
(A) subsequent additions of NEt3 and H2O and (B) subsequent additions
of H2O only. Conditions: 0.20 M [Bu4N][PF6]–CH3CN; 1 mm glassy-
carbon working electrode; scan rate (A) 1 V s−1 and (B) 0.05 V s−1 at
25 °C. Note the different operating potentials for the strong vs. the weak
base, which indicate a different mechanism of catalysis.
Ni(PCy2NCH2CH2OMe
)
2 2
—
−1.37
−0.81
(−0.81)
—
+
(−1.49)
−0.33
[Ni(PCy2NCH2CH2OMe2H)2]2+ (endo/endo)
[Ni(PCy2NCH2CH2OMe2H)2]2+ (endo/exo)
[Ni(PCy2NCH2CH2OMe2H)2]2+ (exo/exo)
15.1
(15.3)
15.2
(15.4)
15.4
(15.6)
(−0.25)
−0.43
—
—
(−0.33)
−0.79
(−0.61)
are independent of scan rate between 0.1–5 V s−1, indicating
steady state conditions; however, catalytic wave shapes and
plateaus were best defined at a scan rate of 1 V s−1, which was
used for calculating the turnover frequencies (kobs). The poten-
tial used to calculate the overpotential, Ecat/2, is defined as the
potential at half of the catalytic current used to calculate kobs
(Ecat/2, Fig. S4 in the ESI†). Turnover frequencies (kobs) were
determined from the ratio of icat/ip using eqn (5) where n is the
number of electrons (2 for H2 oxidation), R is the gas constant
endo/exo isomers (Fig. 4, red). Peak potentials for the three
doubly protonated Ni(0) isomers of [Ni(PCy2N(CH2)2OMe2H)2]2+
are reported in Table 1.
Electrocatalytic oxidation of H2
Standard conditions for catalytic electrochemical measure-
ments employed a 1 mM solution of the Ni complex dissolved
in a 0.2 M [Bu4N][PF6] CH3CN solution, 1 atm of H2 and sub-
sequent additions of an exogenous base to remove protons.
Catalytic H2 oxidation was identified by an increase in the
anodic peak current, icat (determined at the point where the
wave first plateaus (Fig. S4 in the ESI†). The use of NEt3 (pKa
[HNEt3]+ = 18.82 in CH3CN), tBuNH2 (pKa [tBuNH3]+ = 18.14 in
CH3CN), nBuNH2 (pKa [nBuNH3]+ = 18.26 in CH3CN) or H2O
(pKa [H3O]+ = 2.2 in CH3CN) as the exogenous base all result
in significant current enhancements and catalytic wave
shapes, establishing catalytic H2 oxidation (Fig. 5 and Fig. S5
(8.314 J K−1 mol−1), F is Faraday’s constant (9.65 × 104
C
mol−1), T is the temperature (298 K), and υ is the scan rate in
V s−1 (Table S4 in the ESI†).18–21 Based on previous studies
where water was shown to increase catalytic rates for
t
[Ni(PCy2NR′2)2](BF4)2 (R′ = Bu, Bn derivatives) complexes when
sterically bulky amines were used as the exogenous base,
aliquots of purified H2O were added subsequent to the com-
pletion of base additions, resulting in a further catalytic
current enhancement (Fig. 5, blue trace).8,11 In addition to
using amines as strong exogenous bases, H2O was also used
solely as a weak exogenous base. As expected based on
in the ESI†).16,17 The peak current for the catalytic waves, icat
,
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Table 2 Electrocatalytic data for hydrogen oxidation by 1 mM [Ni(PCy2N(CH2)2OMe2)2]2+ under 1 atm H2 in 0.2 M [Bu4N][PF6]–CH3CN
Exogenous base
Ecat/2 (V)
Overpotential (mV)
[Base]
kobs (s−1
)
[Base], [H2O]
kobs (s−1
)
NEt3
−0.82
−0.81
−0.81
−0.48
320a
290a
300a
720b
0.26 M
0.18 M
0.02 M
5.0 M
7
13
17
5
0.26 M, 0.05 M
0.18 M, 0.14 M
0.02 M, 0.01 M
—
10
17
18
—
tBuNH2
nBuNH2
H2O
a Determined by the method of Evans with the EH+/H2 = −0.028 V as determined by Roberts (Fig. S4 in the ESI).22,23 b Determined by open circuit
measurement as previously reported.9
previous reports, an alternative mechanism is observed in this to form the Ni(II) hydride, [HNi(PCy2N(CH2)2OMe2)2]+, as
case, that entails oxidation of the doubly protonated Ni(0) monitored by 31P{1H} NMR spectroscopy (eqn (8), see S8 and
complex, [Ni(PCy2N(CH2)2OMe2H)2]2+, before deprotonation.8 The S9 in the ESI† for 1H and 31P NMR spectra).24 Addition of
calculated turnover frequencies (kobs), overpotentials, con- 2,4,6-collidinium (pKa = 15.0 in acetonitrile) results in an equi-
ditions and half-peak potentials are reported in Table 2.
librium mixture of the doubly protonated Ni(0) isomers and
the Ni(II) hydride allowing the determination of the effective
pKa values for the endo/endo, endo/exo and exo/exo isomers of
[Ni(PCy2N(CH2)2OMe2H)2]2+. Using integration of 31P{1H} NMR
rffiffiffiffiffiffiffiffiffiffiffiffiffiffi
icat
ip
n
RTkobs
¼
ð5Þ
0:4463
Fυ
The rate law for the catalytic oxidation of H2 by [Ni- spectra to quantify the various nickel species and the ratio of
(PCy2N(CH2)2OMe2)2]2+ is shown in eqn (6). Using a constant base 2,4,6-collidinium to 2,4,6-collidine, the pKa of the doubly
concentration and measuring icat as a function of catalyst con- protonated isomers were determined to be 15.1 (endo/endo),
centration, the data show a first-order dependence on catalyst 15.2 (endo/exo) and 15.4 (exo/exo) using the thermodynamic
concentration (i.e. eqn (6), x = 1; Fig. S6 in the ESI†). Calcu- scheme in Fig. 6.
ð8Þ
lation of kobs was only done after subsequent base additions
The pKa and hydride donor ability (ΔG°Hꢁ ) of the Ni(II)
resulted in no additional catalytic current enhancement, indi- hydride complex, [HNi(PCy2N(CH2)2OMe2)2]+, can be calculated
cating that pseudo-zero-order conditions with respect to base from the Ni(II/I) redox couple of [Ni(PCy2N(CH2)2OMe2)]2+ (−0.81
concentration have been reached (i.e. eqn (7), y = 0; Fig. S7 in V) using previously reported linear correlations.25 The ΔGH° ꢁ of
the ESI†). The resulting calculated kobs is a function of k and 59.9 kcal mol−1 is calculated for [HNi(PCy2N(CH2)2OMe2)2]+ is
the H2 pressure (1 atm).
similar to those for other reported hydrogen oxidation cata-
lysts.8,10 The free energy of hydrogen addition (ΔGH° 2) to
[Ni(PCy2N(CH2)2OMe2)2]2+ can be calculated through the thermo-
chemical scheme shown in Fig. 7 and was found to be
x
Rate ¼ kobs½catꢀ
ð6Þ
ð7Þ
y
z
kobs ¼ k½baseꢀ PH
2
−4.7 kcal mol−1
.
Thermodynamic studies
Computational studies
Addition of one equivalent of a strong base such as TMG to Theoretical calculations have been carried out to investigate the
the doubly protonated Ni(0) isomers of [Ni(PCy2N(CH2)2OMe2H)2]2+ interaction of the methoxyethyl group and the protonated
results in deprotonation to form the mono protonated Ni(0) pendant amines in the doubly-protonated isomers of [Ni-
species that likely undergoes rapid intramolecular isomerization (PCy2N(CH2)2OMe2)2]2+. Calculated pKa values and electrochemical
Fig. 6 Thermochemical scheme used for calculating the average pKa value of [Ni(PCy2N(CH2)2OMe2H)2]2+
.
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Fig. 7 Thermochemical scheme used for calculating the ΔG°H2 of [Ni(PCy2N(CH2)2OMe2H)2]2+ 26
.
potentials are reported in Table 1. Additionally, the pro- is not favored. The calculated free energy profile for the CECE
posed mechanisms for electrocatalytic oxidation of H2 using mechanism using either NEt3 or H2O as the exogenous base
[Ni(PCy2N(CH2)2OMe2)2]2+ were also explored computationally.
and a potential of −0.75 V is shown in Fig. 10. Using NEt3 as
Hydrogen bonding. The interaction of the methoxyethyl the exogenous base results in deprotonation steps that decrease
group with the protonated pendant amine in the doubly-proto- in free energy and an overall ΔGrxn of −11.2 kcal mol−1 (Fig. 10,
nated Ni(0) endo/endo, endo/exo and exo/exo isomers was blue). On the other hand, using H2O as the exogenous base
explored computationally. The methoxyethyl groups were results in unfavorable deprotonation steps that result in an
found to form a weak hydrogen bond with both endo and exo overall ΔGrxn of +34.2 kcal mol−1 (Fig. 10, red). Clearly in the
protonated pendant amines (Fig. 8). The lengths of the case of H2O as the exogenous base, the CECE mechanism is not
O⋯HN, Ni⋯HN and N⋯HN hydrogen bonds are 2.6 Å, 2.6 Å favored. In the case of a strong exogenous base, the rate of H2
and 1.6 Å, respectively. The O⋯HN distance is similar for both oxidation catalyzed by [Ni(PCy2N(CH2)2OMe2)2]2+ is in between
endo and exo protons (Fig. 8). The Ni⋯HN hydrogen bond cal- that for the [Ni(PCy2NBn2)2]2+ and the [Ni(PCy2NtBu2)2]2+ com-
culated for the methoxyethyl derivative is slightly longer than plexes, as expected based on the intermediate ΔGH° 2
that of other Ni catalysts. For example, in [Ni(PCy2NMe2H)2]2+,
.
the calculated Ni⋯HN distance is 2.4 Å.27
Discussion
Mechanism of catalysis. The proposed CECE mechanism
of catalysis (where C represents a chemical deprotonation and
Previous studies have shown that [Ni(PR2NR′2)2]2+ complexes
are very active catalysts for H2 production and oxidation.2 Sys-
tematic variations of the R substituents on P and the R′ groups
on N have been used to control the hydride donor/acceptor
ability of the Ni center and the basicity or proton acceptor
ability of the pendant base, hence, controlling the free energy
of H2 addition (ΔG°H2) to these complexes. We have previously
reported that the complex [Ni(PCy2NtBu2)2]2+ is a catalyst for
hydrogen oxidation, with a turnover frequency as high as
58 s−1 under one atmosphere of H2 in the presence of nBuNH2
and H2O.7,8 In an effort to understand the role of water and
potentially increase intra- and intermolecular proton transfer
in this class of catalyst, we appended methoxyethyl groups in
the outer coordination sphere. To that end, we have syn-
thesized and studied [Ni(PCy2N(CH2)2OMe2)2]2+ that contains
pendant ether groups in the outer coordination sphere of the
metal complex. In addition to being an effective electrocatalyst
for H2 oxidation, this compound shows rapid proton transfer
between the three isomers of its H2 addition products. In this
paper, we discuss the differences in the H2 oxidation pro-
perties of [Ni(PCy2N(CH2)2OMe2)2]2+ compared to the previously
reported [Ni(PCy2NR′2)2]2+ catalysts.8–11
E represents an electrochemical oxidation) using strong and
weak exogenous bases was investigated through computational
analysis of the possible intermediates in the catalytic cycle.
Calculated pKa values for protonated species and oxidation
potentials are reported in Table 1 and Fig. 9 (red). The Ni(0)
endo/endo isomer was found to have a calculated pKa value of
15.3 and an oxidation potential of −0.25 V. In the case of
strong exogenous bases (NEt3, tBuNH2, nBuNH2) their pKa
values (18.1–18.8 are sufficient to allow for deprotonation to
form the Ni(0) endo isomer (Fig. 9, black CECE pathway). In
the case of H2O as the exogenous base, however, deprotonation
The reaction of [Ni(PCy2N(CH2)2OMe2)2]2+ with H2 results in
the formation of the doubly protonated Ni(0) product that
exists in the form of three isomers, endo/endo, endo/exo, and
exo/exo (Scheme 1) in the time it takes to prepare the sample
and collect the data (<15 min.). Similar isomers have been
reported previously in the reaction of other complexes of this
type with H2, and their structures were deduced by extensive
1H, 31P, and 15N NMR studies.7,8,11,15 The kinetic product of
Fig. 8 The structural characteristics of the hydrogen bond network
around the pendant amines in Ni(0) endo/endo and exo/exo isomers of
[Ni(PCy2N(CH2)2OMe2H)2]2+. Distances in Å are shown in red.
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Fig. 9 Proposed mechanism for the electrocatalytic oxidation of H2 with [Ni(PCy2N(CH2)2OMe2)2]2+ showing the CECE pathway (black arrows, coun-
terclockwise) and the ECEC pathway (orange arrows, clockwise). Experimental (black) and computational values (red) for the oxidation potentials
and pKa values are shown. The substituents on the phosphorus and nitrogen atoms were omitted for clarity.
Fig. 10 Calculated free energy profile at −0.75 V for the CECE mechanism for catalytic H2 oxidation with [Ni(PCy2N(CH2)2OMe2)2]2+ using NEt3 (blue)
or H2O (red) as the exogenous bases.
addition of H2 to [Ni(PCy2N(CH2)2OMe2)2]2+ is the endo/endo addition of water or exogenous base facilitates the intermole-
isomer, which has both protons endo with respect to the cular exchange, resulting in a more rapid approach to equili-
metal, as established in the previously studied [Ni(PCy2NR′2)2]2+ brium (<5 h).24 In the case of [Ni(PCy2N(3-pyridazyl)methyl2)2]2+ the
t
(R′ = Bn, Bn derivatives, Bu).7–10 The endo/endo isomer then isomerization process is extremely fast, occurring in less than
undergoes intermolecular deprotonation and reprotonation to 15 min. at room temperature, without the addition of water,
form the endo/exo and exo/exo isomers. In the case of attributed to the additional bases in the outer coordination
t
[Ni(PCy2NR′2)2]2+ (R′ = Bn or Bu) the rate of this isomerization sphere.9 The similar fast isomerization observed here in the
process to reach equilibrium is slow (>10 h). However, the absence of water indicates that the additional pendant groups
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in the case of [Ni(PCy2N(CH2)2OMe2)2]2+ are facilitating rapid [Ni(PCy2N(CH2)2OMe2H)2]2+; the Ni⋯HN hydrogen bond is desta-
proton movement similar to [Ni(PCy2N(3-pyridazyl)methyl2)2]2+. Iso- bilized through formation of an O⋯HN hydrogen bond
merization studies carried out at 0 °C with [Ni(PCy2N(CH2)2OMe2)2]2+ from the outer pendant ether group.9 The weakening of
and H2 at different catalyst concentrations suggest the rate- the hydrogen bonds results in a 150 mV lower overpotential
determining step of the isomerization is likely an intramolecular for [Ni(PCy2N(CH2)2OMe2)2]2+ than for [Ni(PCy2NBn2)2]2+ when a
process. This is in stark contrast to the intermolecular weak exogenous base is used for catalysis.
exchange process observed for the [Ni(PCy2NR2H)2]2+ (R′ = Bn
The mechanism for the electrocatalytic oxidation of H2 by
or tBu) derivatives, providing further evidence that the pendant [Ni(PR2NR′2)2]2+ compounds has been thoroughly studied both
ether group plays a role in the isomerization process for [Ni experimentally and computationally.10,27 Catalytic oxidation of
(PCy2N(CH2)2OMe2H)2]2+.14 While the rate of proton movement H2 by [Ni(PCy2N(CH2)2OMe2)2]2+ proceeds by two different cata-
does not influence catalysis for this complex, understanding lytic pathways consisting of chemical (C) and electrochemical
the movement of protons from the active site may be essential (E) steps depending on the steric size, strength and nature of
as the rates of catalysis become faster.
the exogenous base, as noted previously.8 In the case of a
An interaction of the methoxyethyl group with the endo strong exogenous base, deprotonation proceeds before oxi-
protons in the doubly protonated Ni(0) isomers of the [Ni- dation (Fig. 9, black CECE pathway) and catalysis occurs at
(PCy2N(CH2)2OMe2H)2]2+ complex is also suggested by the distri- more negative potentials.8 In contrast, when a weak exogenous
bution of isomers at equilibrium. A significantly higher popu- base is used, oxidation of the doubly protonated Ni(0) isomers
lation of exo/exo species is observed (≈50%) compared to the must occur before intermolecular deprotonation (Fig. 9,
t
[Ni(PCy2NR2H)2]2+ (R′ = Bn or Bu) derivatives (≈10%), indica- orange ECEC pathway). Although the first two steps (EC) of
tive a weakened endo Ni⋯HN hydrogen bond.7,14 The same this mechanism were previously reported, the following order
effect was also observed for the complex with an appended of deprotonation and oxidation steps was not established.8,9,11
pyridazine in the outer coordination sphere.9 The negatively Using computational analysis, we have identified that when a
shifted potentials for the doubly protonated isomers as com- weak exogenous base is used, oxidation occurs before deproto-
pared to [Ni(PCy2NBn2H)2]2+ are also suggestive of this nation in both steps (Fig. 9, orange ECEC pathway). After the
interpretation.9,28 This was characterized computationally and first oxidation and deprotonation, the calculated pKa of the Ni(I)
the Ni⋯HN hydrogen bond was found to be slightly longer, isomer was found to be 15.6, too low in acidity for deprotona-
lowering the barrier associated for the isomerization reaction, tion by water, requiring oxidation to the more acidic mono
as also seen in previous systems containing outer coordination protonated Ni(II) endo isomer first. Additionally, the calculated
sphere pendant groups.9,11 In contrast, the N⋯HN bond in exo oxidation potential of the mono protonated Ni(I) endo isomer
positions is of similar length and energy computationally is −0.51 V, slightly negative of the operating potential for cata-
observed for other systems, indicating that hydrogen bonding lysis (−0.47), indicating oxidation will proceed readily upon its
with the methoxyethyl moiety has little effect on the exo formation. The final step is a deprotonation, which is nearly
pinched hydrogen bond.27 The computational data corro- thermoneutral for water molecule based on the calculated pKa
borates that the fast rates for isomerization observed in for- of the Ni(II) endo isomer of 3.2, resulting in the starting Ni(II)
mation of the isomers of [Ni(PCy2N(CH2)2OMe2H)2]2+ is likely due complex. Thus the computational result supports the ECEC
to the appended basic groups that are not present in the mechanism.
[Ni(PCy2NBn2)2]2+ and [Ni(PCy2NtBu2)2]2+ compounds.
The cyclic voltammograms of the endo/endo, endo/exo and
exo/exo isomers of [Ni(PCy2N(CH2)2OMe2H)2]2+ (Scheme 1) show
different oxidation potentials, illustrated in Fig. 4 and com-
Conclusions
piled in Table 1, as previously reported for the [Ni(PCy2NR′2)2]2+
t
(R′ = Bn, Bn derivatives, Bu and (3-pyridazyl)methyl) and Ni- The [Ni(PCy2N(CH2)2OMe2)2]2+ complex has been shown to be an
(PtBu2NBn2)2 compounds.2,8,9,11,28 Each of the isomers has two electrocatalyst for the oxidation of H2. The pendant methoxy
hydrogen bonds consisting of either N⋯HN or Ni⋯HN hydro- pendant group on the ligand in the outer coordination sphere
gen bonds (Scheme 1). The differences in oxidation potentials of the metal complex facilitates isomerization of the H2
for the three isomers is likely due to the difference in the addition product, [Ni(PCy2N(CH2)2OMe2H)2]2+. The increase in
nature of the hydrogen bonding.28 The Ni⋯HN hydrogen the rate of isomerization is attributed to a weakening of the
bonds stabilize the HOMO of the Ni(0), which results in a Ni(0)⋯HN hydrogen bond by the methoxy group, consequently
higher oxidation potential for the metal in the endo/endo lowering the overpotential for H2 oxidation catalysis. Impor-
isomer. In the previously reported [Ni(PCy2N(3-pyridazyl) tantly, the rate of catalysis was faster than the [Ni(PCy2NBn2)2]2+
2H)2]2+ compound the additional pendant amine in the complex, demonstrating that the methoxyethyl provided both
methyl
outer coordination sphere is thought to destabilize the an additional driving force for H2 addition and an advan-
Ni⋯HN hydrogen bond through formation of an N⋯HN tageous interaction of the appended ether group to lower the
hydrogen bond from the nitrogen to the pendant amine.9 overpotential. Although the [Ni(PCy2N(CH2)2OMe2)2]2+ complex
As shown by the computational work, a similar effect is operates at a higher overpotential than the pyridazyl complex;
thought to be occurring in the endo/endo isomer of the rate is ∼10 times faster, indicating competitive binding of
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the outer coordination group to the Ni(II) center has been and the cyclic voltammograms were referenced to the
avoided. The mechanism with a weak base was investigated ferrocene/ferrocenium couple at 0.0 V.
computationally and shown to occur through an ECEC mech-
Computational details
anism, an important advancement in our understanding of
electrocatalytic H2 oxidation using water as an exogenous base. The structure of the species discussed in the paper was opti-
Work continues in our laboratories on understanding the roles mized without symmetry constrains within the density func-
pendant groups in the second and outer coordination spheres tional theory framework using the B3P86 exchange and
of molecular electrocatalysis play in proton movement.
correlation functional.29,30 The Stuttgart basis set with
effective core potential was used for Ni atom, whereas the
6-31G* basis set was used for all of the other atoms with one
additional p polarization function for the proton on pendant
amines. The optimized structures were confirmed by frequency
calculations. Harmonic vibrational frequencies were calculated
at the optimized geometries using the same level of theory to
estimate the zero-point energy (ZPE) and the thermal contri-
butions (298 K and 1 atm) to the gas-phase free energy. Free
energies of solvation in acetonitrile (which include the change
of thermodynamic conditions of P = 1 atm in the gas phase to
1 M solution) were then computed using a self-consistent reac-
tion field (SCRF) model at the same level of theory as for the
other steps. The conductor-like polarizable continuum model
(CPCM)31,32 was used with Bondi radii.33 The pKa values and
redox potential were calculated according to the isodesmic
scheme discussed by Chen et al.34 The [Ni(PCy2NBn2Hn)2]2+
complex was used as reference for the both pKa and for redox
potential calculations. All of the calculations were carried out
with the program Gaussian 09.35
Experimental section
General experimental procedures
All chemicals were purchased from Aldrich and used without
any further purification. Solvents were purified by passage
through neutral alumina using an Innovative Technology, Inc.,
PureSolv™ solvent purification system. Decamethylcobaltoce-
nium tetrafluoroborate ([CoCp*2](BF4)) was purchased from
Strem Chemicals Inc. (Newburyport, MA). Deuterated solvents
were purchased from Cambridge Isotope Laboratories Inc.
(Andover, MA). CD3CN and CD2Cl2 were dried from P2O5 and
CaH2, respectively. Ultra-high purity H2 (99.9%) was from
OXARC Inc. (Spokane, WA) and dried using an oxygen trap
(Oxy-PurgeTM N) from Alltech Associates (Deerfield, IL) and a
moisture trap (Model MT-4-S) from Agilent Technologies
(Santa Clara, CA). All manipulations and chemical reactions
were performed either under a nitrogen atmosphere using
standard Schlenk line or inert atmosphere glove box tech-
Ni(PCy2N(CH2)2OMe2)2. CyP(CH2OH)2 (4.11 g, 23 mmol) was
added to a 250 mL Schlenk flask and dissolved in 60 mL of
ethanol. The mixture was heated to 70 °C, then 2-methoxyethyl-
amine (1.75 g, 23 mmol) was added dropwise and the result-
ing mixture was stirred overnight at 70 °C. Solvents were
removed by vacuum to yield a white sticky oil, which was used
niques.
Bis(hydroxymethyl)cyclohexylphosphine
(CyP
(CH2OH)2) was synthesized according to the published pro-
cedure.7 Elemental analyses were performed at Atlantic Micro-
lab Inc. (Norcross, GA).
without further purification. Crude PCy2N(CH2)2OMe
,
31P{1H}
2
NMR (Ethanol, 202.2 MHz) δ −40.13 (br). Approximately 30 mL
of acetonitrile and 10 mL of tetrahydrofuran (THF) solution
Instrumentation
1H and 31P{1H} NMR measurements were carried out in Varian was used to dissolve the oil, following which 2.75 g (5.7 mmol)
Inova or VNMRS spectrometer (500 MHz for 1H NMR, of [Ni(CH3CN)6](BF4)2 dissolved in a minimum amount of
200 MHz for 31P NMR). Proton resonances were internally cali- acetonitrile was added and the combined mixture was stirred
1
brated to the residual H signals from the deuterated solvents, overnight. The resulting dark-red solution was sparged with H2
while 31P resonances were calibrated against 85% H3PO4 (exter- for 30 minutes. Following H2 sparging, 1,1,3,3-tetramethyl-
nal standard).
guanidine (TMG) (1.44 mL, 11.5 mmol) was added with
Electrochemical measurements were carried out using a CH continuous bubbling of H2, upon which yellowish-white
Instruments 660D potentiostat equipped with a standard solids were observed at the bottom of the flask. After
three-electrode cell. Prior to the collection of each cyclic vol- cooling to 0 °C overnight, a pale yellow solid of pure Ni-
tammogram (CV), the working electrode (1 mm PEEK-encased (PCy2N(CH2)2OMe
) was isolated after filtration and washing
2 2
glassy carbon, Cypress Systems EE040) was polished using the with cold CH3CN. Yield 1.17 g (22%). 31P{1H} NMR (C6D6,
1
diamond paste (Buehler). A 3-mm glassy carbon rod (Alfa) was 121.0 MHz): δ 9.47 (s). H NMR (C6D6, 299.9 MHz): δ 3.60 (t,
used as the counter-electrode, and a silver wire suspended in 8H, NCH2CH2OMe), 3.22 (s, 12H, NCH2CH2OCH3), 2.95 (t, 8H,
the electrolyte solution and separated from the analyte solu- NCH2CH2OMe), 2.86 (d, 8H, PCH2), 2.65 (d, 8H, PCH2), 1.86 (d,
tion by a Vycor frit (CH Instruments 112) was used as a refer- 8H, CyH), 1.76 (d, 12H, CyH), 1.64 (d, 8H, CyH), 1.10–1.50
ence electrode. Electrochemical measurements were carried (multiple peaks, 16H, CyH). Anal. calcd (found) for
out either under N2 or H2 in an anaerobic glove box and a Ni(PCy2N(CH2)2OMe
)
2 2
C44H88N4NiO4P4%C 57.46 (56.32), %H
0.2 M solution of tetrabutylammonium hexafluorophosphate 9.67 (9.64), %N 5.97 (6.09).
in acetonitrile was used as an electrolyte. [CoCp*2](BF4) was
[Ni(PCy2N(CH2)2OMe2)2](BF4)2. To 0.237 g of Ni(PCy2N(CH2)2OMe
)
2 2
used as an internal standard for hydrogen oxidation reactions (0.258 mmol) dissolved in
2
mL of CH3CN, 0.140
g
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(0.516 mmol) of decamethylferrocenium tetrafluoroborate (0.164 mg, 1.5 µmol) and the Ni(0) complex Ni(PCy2N(CH2)2OMe
)
2 2
(Cp*2Fe)BF4 dissolved in 2 mL of CH3CN was added and stirred (0.137 mg, 1.5 µmol) in 3 mL of acetonitrile. A portion (2 mL)
overnight. The reaction solution was concentrated until the of this solution was placed into the electrochemistry cell. A
precipitation of ferrocene began then stored overnight in a small amount of decamethylcobaltocenium tetrafluoroborate
−35 °C freezer. The mother liquor was then extracted with a (E° CoCp*2+/0 = −1.895 V vs. FeCp+/0) was added as an internal
pipette and the remaining solid was washed with 2 mL of cold standard. Hydrogen gas was sparged through the solution for
acetonitrile and dried under vacuum. The isolated solid was 5 minutes and the needle was placed above the solution to
dissolved in CH3CN and layered with diethyl ether resulting in maintain a constant pressure of hydrogen before measuring
precipitation of a red solid. Yield 0.266 g (95%). 31P{1H} NMR the cyclic voltammograms, which were collected at scan rates
1
(CD3CN, 202.2 MHz): δ 8.21 (s). H NMR (CD3CN, 499.9 MHz): of 50 mV s−1 to 10 V s−1. The working electrode was polished
δ 3.54 (br, 8H, NCH2CH2OMe), 3.30 (s, 12H, NCH2CH2OCH3), using the diamond paste (Buehler) and thoroughly cleaned
3.18 (d, 8H, NCH2CH2OMe), 3.10 (d, 4H, PCH2), 2.89 (br, 8H, and rinsed with acetonitrile. Base and water were added using
PCH2), 2.80 (br, 4H, PCH2), 1.86 (d, 8H, CyH), 1.76 (d, 12H, a micropipette or syringe.
CyH), 1.60–2.10 (multiple peaks, 24H, CyH), 1.10–1.50 (br,
20H, CyH). Anal. calcd (found) for [Ni(PCy2N(CH2)2OMe2)2]-
(BF4)2·0.5CH2Cl2 C44.5H89B2ClF8N4NiO4P4%C 47.05 (47.22),
%H 7.90 (7.83), %N 4.93 (5.21), %F 13.38 (13.56). The presence
Acknowledgements
of 0.5 equivalent of dichloromethane was confirmed by the
We thank Dr Jonathan Darmon for providing the cover artwork.
1H NMR spectra in CD3CN.
This research was supported as part of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded by
NMR experiments
the U.S. Department of Energy, Office of Science, Office of Basic
In a typical H2 addition experiment, 15 mg (0.014 mmol) of [Ni-
Energy Sciences. Pacific Northwest National Laboratory is oper-
(PCy2N(CH2)2OMe2)2](BF4)2 was dissolved in CD3CN and hydro-
gen gas was sparged into the solution for five minutes. The
resulting solution containing [Ni(PCy2N(CH2)2OMe2H)2]2+ was
analyzed by 31P{1H} NMR spectroscopy. The endo/endo and
ated by Battelle for the U.S. Department of Energy.
exo/exo isomers showed peaks at 18.6 and −10.1 ppm, respect-
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