A NiII-bis(diphosphine)-hydride complex containing proton relays - Structural characterization and electrocatalytic studies

Article abstract of DOI:10.1002/ejic.201402250

The synthesis of the 1,5-diphenyl-3,7-diisopropyl-1,5-diaza-3,7-diphosphacyclooctane ligand, P^iPr₂N^Ph₂, is reported. Two equivalents of the ligand react with [Ni(CH₃CN)₆](BF₄)₂ to form the bis(diphosphine)-Ni^II complex [Ni(P^iPr₂N^Ph₂)₂](BF₄)₂, which acts as a proton reduction electrocatalyst. In addition to [Ni(P^iPr₂N^Ph₂)₂]²⁺, we report the synthesis and structural characterization of the Ni⁰ complex Ni(P^iPr₂N^Ph₂)₂ and the Ni^II-hydride complex [HNi(P^iPr₂N^Ph₂)₂]BF₄. The [HNi(P^iPr₂N^Ph₂)₂]BF₄ complex represents the first Ni^II-hydride in the [Ni(P^R₂N^R₂)₂]²⁺ family of compounds to be structurally characterized. In addition to the experimental data, the mechanism of electrocatalysis facilitated by [Ni(P^iPr₂N^Ph₂)₂]²⁺ is analyzed by using linear free energy relationships recently established for the [Ni(P^R₂N^R₂)₂]²⁺ family.

Full text of DOI:10.1002/ejic.201402250

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DOI:10.1002/ejic.201402250
A NiII–Bis(diphosphine)–Hydride Complex Containing
Proton Relays – Structural Characterization and
Electrocatalytic Studies
Parthapratim Das,[a] Ryan M. Stolley,[a] Edwin F. van der Eide,[a] and
Monte L. Helm*[a]
Keywords: Homogeneous catalysis / Hydrogen / Proton transport / Nickel / Diphosphine
The synthesis of the 1,5-diphenyl-3,7-diisopropyl-1,5-diaza-
3,7-diphosphacyclooctane ligand, PiPr2NPh2, is reported. Two [HNi(PiPr2NPh2)2]BF4 complex represents the first NiII–
and the NiII–hydride complex [HNi(PiPr2NPh2)2]BF4. The
equivalents of the ligand react with [Ni(CH3CN)6](BF4)2 to
form the bis(diphosphine)–NiII complex [Ni(PiPr2NPh2)2]-
(BF4)2, which acts as a proton reduction electrocatalyst. In
addition to [Ni(PiPr2NPh2)2]2+, we report the synthesis and
hydride in the [Ni(PR2NRЈ2)2]2+ family of compounds to be
structurally characterized. In addition to the experimental
data, the mechanism of electrocatalysis facilitated by [Ni-
(PiPr2NPh2)2]2+ is analyzed by using linear free energy rela-
tionships recently established for the [Ni(PR2NRЈ2)2]2+ family.
structural characterization of the Ni0 complex Ni(PiPr2NPh
)
2 2
tentials and to control the movement of protons over longer
distances.[6–8] The rational design of coordination spheres
around metal complexes, from the primary coordination
sphere to the outer coordination sphere, illustrates one as-
pect of the progress made from Werner’s original work to
modern coordination chemistry 100 years later.
A major focus of our group has been the development
of NiII catalysts containing 1,5-diaza-3,7-diphosphacyclo-
octane (PR2NRЈ2) ligands.[5,9–12] These ligands are suitable
for substitution at both the phosphorus (R) and nitrogen
(RЈ) atoms to aid in tuning the electronic and steric proper-
ties of their metal complexes. Formation of NiII complexes
with PR2NRЈ2 ligands results in bis(diphosphine) complexes,
{[Ni(PR2NRЈ2)2]2+}, with N atoms in proximity, but not
bound to the Ni center. The positioned pendant amines are
suitable for relaying protons to and from the metal center
to exogenous acids and bases in solution.[13,14] The ability
Introduction
The production and oxidation of H2 can be used as a
reversible cycle to store electricity. Widespread use of elec-
trocatalytic cells for the storage of electrical energy from
renewable sources is currently limited by the scarcity and
high cost of platinum, the predominantly used catalyst. As
a result, there is much interest in the development of both
hetero- and homogeneous catalysts based on earth-abun-
dant metals to replace platinum as the primary catalyst in
fuel cells.[1–3] Efforts in our group have focused on the ratio-
nal design of earth-abundant transition metal complexes for
their use as molecular electrocatalysts for the production
and use of chemical fuels.[4] For the production and oxi-
dation of H2, we have used a modular, energy-based ap-
proach to design catalysts based on the functional compo-
nents of the [FeFe]-hydrogenase enzymes.[5] Namely, the
first coordination sphere around the metal center is primar-
ily used to tune the redox potential, hydride donor ability,
and pKa of the metal center. The second coordination
sphere, those atoms attached to the ligand structure that do
not directly interact with the metal center, is used to relay
protons between the surrounding solvent matrix and the
metal. Recent work pioneered by Shaw and co-workers has
extended the design of molecular catalysts to include the
use of the outer coordination sphere to influence redox po-
to tune the free energy of H2 addition (ΔG°H ) in the
2
[Ni(PR2NRЈ2)2]2+ family of compounds has resulted in pro-
ton reduction, H2 oxidation, and bidirectional catalysts.[5]
The mechanism of electrocatalysis for the production and
oxidation of H2 facilitated by the [Ni(PR2NRЈ2)2]2+ com-
plexes has been extensively studied and consists of a series
of electron transfer and inter- and intramolecular proton
transfer steps.[13,14] Recent computational work from our
group has established the linear free energy relationships
between the pKa of the pendant amine, Ni(II/I), and (I/0)
couples that predict the energy landscape of the catalytic cy-
cle.[15] These relationships enable a thorough mechanistic
analysis of catalysis by complexes of these types.
[a] Center for Molecular Electrocatalysis, Physical Sciences
Division, Pacific Northwest National Laboratory,
P. O. Box 999, K2-57, Richland, Washington 99352, USA
E-mail: monte.helm@pnnl.com
http://efrc.pnnl.gov
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402250.
In this work we report the synthesis of a new PR2NRЈ
2
ligand, 1,5-diphenyl-3,7-diisopropyl-1,5-diaza-3,7-diphos-
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phacyclooctane (PiPr2NPh2), and its NiII complex. We also
report the synthesis and structural characterization of the
NiII–hydride complex, [HNi(PiPr2NPh2)2]BF4. Additionally,
we report the electrochemical properties of the complex and
analyze the mechanism for H2 production by using compu-
tationally derived free energy landscapes.
Results
Synthesis and Structural Characterization
The PiPr2NPh ligand was synthesized through in situ
2
generation of iPrPH2 and bis(hydroxymethyl)isoprop-
ylphosphine, [iPrP(CH2OH)2], followed by condensation
with one equivalent of aniline (PhNH2) to form the cyclic
diazadiphosphine (Scheme 1). The ligand was isolated as a
white solid in more than 40% yield and fully characterized
by NMR spectroscopy and elemental analysis. Additional
characterization was performed by X-ray analysis of a sin-
gle crystal of Ni(PiPr2NPh2)Cl2, obtained from the reaction
of PiPr2NPh2 and NiCl2 as reported in the Supporting Infor-
mation (Figure S1).
Scheme 2. Synthesis of [Ni(PiPr2NPh2)2](BF4)2, Ni(PiPr2NPh
[HNi(PiPr2NPh2)2]BF4.
) and
2 2
Scheme 1. Synthesis of PiPr2NPh
.
2
Formation of the NiII complex, [Ni(PiPr2NPh2)2](BF4)2,
was achieved by reaction of two equivalents of PiPr2NPh
2
with [Ni(CH3CN)6](BF4)2 in CH3CN (Scheme 2). The NiII
complex was isolated as a crystalline red solid in 82% yield
and characterized by NMR spectroscopy and mass spec-
trometry. Formation of the Ni0 complex, Ni(PiPr2NPh2)2,
was accomplished through reduction of [Ni(PiPr2NPh2)2]-
(BF4)2 with KC8 in THF. The complex was isolated as a
yellow solid in 90% yield (Scheme 2). The Ni(PiPr2NPh
)
2 2
complex was characterized by 31P{1H} and H NMR spec-
troscopy and by single-crystal X-ray diffraction analysis
(Figure 1).
1
In addition to the NiII and Ni0 complexes described
above, the NiII–hydride, [HNi(PiPr2NPh2)2]BF4, was also
synthesized through the reaction of [Ni(PiPr2NPh2)2](BF4)2
with Na[HB(OMe)3] (Scheme 2). The hydride was isolated
in more than 85% yield and identified through its 31P{1H}
and characteristic 1H NMR spectra (Figure S2) that
showed the hydride resonance as a quintet (2JPH = 18 Hz)
at –11.2 ppm. In addition to the spectroscopic data, single-
crystal X-ray analysis of [HNi(PiPr2NPh2)2]BF4 was per-
formed (Figure 1). Formation of [HNi(PiPr2NPh2)2]+ was
Figure 1. Thermal ellipsoid plots rendered at 50% probability of
the structures of Ni(PiPr2NPh
)
(top) and [HNi(PiPr2NPh2)2]-
2 2
–
BF4 (bottom). Hydrogen atoms and the BF4 anion in [HNi-
also accomplished through protonation of the Ni0 complex (PiPr2NPh2)2]BF4 have been omitted for clarity.
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Ni(PiPr2NPh2)2 with one equivalent of [(DMF)H]+ or by re-
action of [Ni(PiPr2NPh2)2]2+ with H2 and a strong base such
as p-anisidine (see the Supporting Information for complete
details).
Compounds Ni(PiPr2NPh
)
2 2
and [HNi(PiPr2NPh2)2]BF4
were characterized by single-crystal X-ray crystallography,
and the structures are presented in Figure 1. Table 1 con-
tains selected average bond lengths and angles for these
compounds. The structures of the complexes show a tetra-
hedral coordination environment around the Ni center with
a dihedral angle between the planes formed from the P1–
Ni–P2 and P3–Ni–P4 atoms of 89.0° for Ni(PiPr2NPh2)2 and
88.0° for [HNi(PiPr2NPh2)2]+. In both structures, each phos-
phine ligand forms two six-membered chelate rings; the
metal center has a boat conformation in one ring and a
chair conformation in the other. In Ni(PiPr2NPh2)2, the
average Ni···Nboat nonbonding distance is observed to be
3.417 Å, and the Ni···Nchair nonbonding distance averages
3.766 Å. The average Ni···Nboat nonbonding distance in
Figure 2. Cyclic voltammogram of 1.0 mm [Ni(PiPr2NPh2)2]2+ in
0.10 m [nBu4N][PF6]/CH3CN. Conditions: 1 mm glassy carbon
[HNi(PiPr2NPh2)2]+ is observed to be 3.365 Å, and the working electrode; scan rate 0.10 Vs–1 at 25 °C.
Ni···Nchair nonbonding distance averages 3.822 Å. The
average Ni–P bond length increases by 0.078 Å from Ni-
The cyclic voltammogram of [HNi(PiPr2NPh2)2]+ shows
an irreversible oxidation peak at Ep = –0.28 V at a scan rate
of 0.1 Vs–1 (Figure 3). The wave remains irreversible at all
scan rates that were studied (0.5–10 Vs–1, Figure S3). The
peak potential for this oxidation is observed to shift in the
positive direction as a function of scan rate, which is indica-
tive of a kinetic potential shift caused by a rapidly following
chemical step. This data is consistent with the previously
reported irreversible oxidation of [HNi(PtBu2NR2)2]+ (R =
Bn, Ph), attributed to rapid intramolecular proton move-
ment of the hydride to the pendant amine, followed by
intermolecular deprotonation of the oxidized species.[16]
(PiPr2NPh
)
2 2
to [HNi(PiPr2NPh2)2]+. The average P–Ni–P
chelate bond angle for Ni(PiPr2NPh2)2 is 85.7°, and that for
[HNi(PiPr2NPh2)2]+ is 83.6°. In [HNi(PiPr2NPh2)2]+, the
Ni–H hydrogen atom was not located in the difference map;
however, after collection of the X-ray data, the single crystal
was dissolved in CD3CN and the characteristic peaks of
Ni–hydride were observed in the 31P{1H} and 1H NMR
spectra. Complete crystal data and bond lengths and angles
are presented in the Supporting Information (Tables S1–
S7).
Table 1. Selected average bond angles (°) and lengths (Å) for Ni-
(PiPr2NPh2)2 and [HNi(PiPr2NPh2)2]+.
Ni(PiPr2NPh
85.72
)
2 2
[HNi(PiPr2NPh2)2]+
83.61
P–Ni–P bite angle
Dihedral angle between
P–Ni–P ligand planes
Ni–P distance
Ni···Nboat distance
Ni···Nchair distance
89.02
2.135
3.417
3.766
88.90
2.213
3.365
3.822
Electrochemical Studies
The cyclic voltammogram of [Ni(PiPr2NPh2)2]2+ shows
two reversible redox couples with E1/2 values of –0.63 V
(NiII/I couple) and –1.06 V (NiI/0 couple) vs. the ferrocen-
ium/ferrocene (Cp2Fe+/0) couple (Figure 2). A plot of the
peak current (ip) vs. the square root of the scan rate shows
a linear correlation, which implies diffusion-controlled
electrochemical processes. The difference between the
potential of cathodic and anodic peaks (ΔEp) at a scan rate
of 0.1 Vs–1 is measured to be 65 mV, where the ΔEp of
Cp2Fe+/0 is 68 mV, which indicates a one-electron process
for the two couples.
Figure 3. Cyclic voltammogram of 1.0 mm [HNi(PiPr2NPh2)2]+ in
0.10 m [nBu4N][PF6]/CH3CN. Conditions: 1 mm glassy carbon
working electrode; scan rate 0.10 Vs–1 at 25 °C.
Electrocatalysis
When electrochemical measurements on [Ni(PiPr
-
2
NPh2)2]2+ were carried out in the presence of acid, an in-
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crease in the cathodic current was observed (Figure 4). troscopy (Figure S5). Rapid catalyst decomposition in the
Prior to the addition of acid, enough H2O was added to presence of acid prevented us from obtaining quantitative
result in a 1.1 m H2O concentration in CH3CN, which was rate data for the catalytic process.
previously shown to increase the rate of catalysis. In the
presence of 1.0 mm [Ni(PiPr2NPh2)2]2+ and 5.4 mm [(DMF)
H]OTf (pKa = 6.1 in MeCN), an increase in the cathodic
current was observed with a peak potential at –0.67 V (Fig-
Discussion
ure 4, top), indicative of proton reduction to form H2. Elec-
Synthesis and Structural Characterization
trochemical measurements carried out in the presence of
1.0 mm [Ni(PiPr2NPh2)2]2+ and 5.4 mm HOTf (pKa = 2.6 in
Due to the volatility of iPrPH2 required in the synthesis
MeCN) resulted in an increase in the cathodic current with
of the PiPr2NPh ligand, the primary phosphine was gener-
2
a peak potential at –0.60 V (Figure 4, bottom).[17] In the
ated in situ by reaction of K[P(TMS)2] with iPrCl to form
presence of either acid, a second wave in the cyclic voltam-
mogram near –1.2 V is observed. Over time the wave at
–1.2 V increases with a corresponding decrease in the cata-
iPrP(TMS)2 followed by protonation with EtOH, which
was used as the solvent in the next step.[18] Reaction of
iPrPH2 with paraformaldehyde results in formation of
lytic wave, which indicates decomposition of the complex
iPrP(CH2OH)2, which is then treated with PhNH2 to form
(Figure S4). The decomposition of [Ni(PiPr2NPh2)2]2+ in the
the PiPr2NPh ligand. The ligand was obtained in an overall
2
presence of acid was confirmed by 31P{1H} NMR spec-
yield of 42% as a white crystalline solid that was charac-
terized by heteronuclear NMR spectroscopy and elemental
analysis.
Formation of [Ni(PiPr2NPh2)2](BF4)2 was achieved
through the reaction of [Ni(CH3CN)6](BF4)2 with two
equivalents of PiPr2NPh2, as is common in the synthesis of
the [Ni(PR2NRЈ2)2]2+ family of compounds. In addition to
the NiII complex, the Ni0 and the NiII–hydride complex
were also obtained. The NiII–hydride, [HNi(PiPr2NPh2)2]+,
represents the first isolated and structurally characterized
hydride in the [Ni(PR2NRЈ2)2]2+ family of compounds.
Structural data for the Ni0 and NiII–hydride complexes
were obtained by single-crystal X-ray analysis. Both struc-
tures show similar distorted tetrahedral geometry around
the Ni centers, a notable difference being in the average Ni–
P bond lengths. The average Ni–P bonds in the [HNi-
(PiPr2NPh2)2]+ structure are 0.078 Å longer than those ob-
served in Ni(PiPr2NPh2)2. The increase in Ni–P bond length
is inconsistent with the expected trend based on a more
electron-poor NiII–hydride metal center than Ni0. Ad-
ditionally, the Ni···Nboat nonbonding distance of the pen-
dant amine in the boat confirmation of [HNi(PiPr2NPh2)2]+
is shorter than that in Ni(PiPr2NPh2)2, where the Ni···Nchair
nonbonding distance is longer. The increase in Ni–P bond
length and changes in the Ni···N nonbonding distances are
consistent with the interaction of the nitrogen atom of the
pendant amine in the boat confirmation with the hydride
ligand on the metal center (Figure 5). The interaction be-
tween the pendant amine and the hydride ligand has been
observed previously in spectroscopic and computational
Figure 4. Cyclic voltammogram of 1.0 mm [Ni(PiPr2NPh2)2]2+ in Figure 5. Illustration of the ligand distortion observed in the
0.10 m [nBu4N][PF6]/CH3CN with 1.1 m H2O before and after ad-
dition of [(DMF)H]+ (top) and HOTf (bottom). Conditions: 1 mm
glassy carbon working electrode; scan rate 0.10 Vs–1 at 25 °C.
crystal structure of [HNi(PiPr2NPh2)2]+ relative to that of Ni-
(PiPr2NPh2)2. Only a single PiPr2NPh ligand is shown, and the sub-
2
stituents on the P and N atoms are omitted for clarity.
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data of other [HNi(PR2NRЈ2)2]+ complexes.[13] The struc- most direct pathway (black arrows) proceeds through elec-
tural comparison of Ni(PiPr2NPh2)2 and [HNi(PiPr2NPh2)2]+ tron and protonation transfer steps to form endo-proton-
further supports this interaction.
ated isomers, leading to H2 elimination. A second pathway
(gray arrows) proceeds through proton-transfer steps that
result in exo-protonated isomers, requiring isomerization
steps before H2 elimination can occur.
Electrocatalytic Pathways
The free energy of H2 addition (ΔG°H ) for [Ni(PR
-
2
A recent publication from our laboratories highlights a
2
NRЈ2)2]2+ complexes can be calculated as the sum of the set of linear free energy relationships that can be used to
hydride donor ability of [HNi(PiPr2NPh2)2]+, the pKa of calculate free energy landscapes for the catalytic path-
[HNi(PiPr2NPh H)(PiPr2NPh2)]2+, and the heterolytic bond ways.[15] Analysis of the thermodynamic properties of [Ni-
2
cleavage energy of H2 in CH3CN (Figure 6). By using the (PiPr2NPh2)2]2+ complexes and the endo pathway (Figure 7,
reported free energy relationships, the hydride donor ability black arrows) for using either [(DMF)H]+ or HOTf are dis-
of [HNi(PiPr2NPh2)2]+ was found to be 63.8 kcalmol–1 and cussed below (see Figures S6 and S7 in the Supporting In-
the pKa of [HNi(PiPr2NPh2H)(PiPr2NPh2)]2+ was found to be formation for the exo pathway).
6.78.[15] These numbers are consistent with the experimen-
The free energy landscape for the ECEC endo pathway
tally determined values for other [Ni(PR2NPh2)2]2+ com- (Figure 7, black arrows) is illustrated in Figure 8. Catalysis
plexes (R = Ph, alkyl). Using these values, the ΔG°H for can proceed through different sequences of proton and elec-
2
[Ni(PiPr2NPh2)2]2+ was calculated to be 2.95 kcalmol–1 (Fig- tron transfer steps (i.e. EECC, CCEE, ECCE, etc.); how-
ure 6). Although the poor hydride donor ability of [HNi- ever, previous experimental and computational results indi-
(PiPr2NPh2)2]+ biases the catalyst towards H2 oxidation, the cate that an ECEC mechanism is the likely path-
low pKa of the N–Ph group provides compensation, which way.[12,14,20,21] Although quantitative rate data for electroca-
results in a positive ΔG°H and a catalyst biased for H2 pro- talysis in this system were not obtained, qualitative analysis
2
duction.[5]
of the cyclic voltammograms in Figure 4 indicates that ca-
As shown in Figure 4, [Ni(PiPr2NPh2)2]2+ can function as talysis with HOTf is significantly faster (greater than or
an electrocatalyst for the production of H2 as previously equal to five times current enhancement) than that ob-
observed for other [Ni(PR2NPh2)2]2+ (R = Ph, alkyl) served with [(DMF)H]+ under identical conditions. Analy-
compounds. The mechanism for H2 production by sis of the free energy landscapes illustrates the driving force
[Ni(PR2NRЈ2)2]2+ complexes has been well studied and con- for H2 production with HOTf (Figure 8, red) to be much
sists of alternating electron (E) and proton transfer (C) greater than that with [(DMF)H]+ (Figure 8, blue). As ex-
steps.[19–21] As shown in the mechanism outlined in Fig- pected, the major difference between the two free energy
ure 7, there are two possible pathways for catalysis; the landscapes occurs in the protonation steps. Specifically, the
Figure 6. Calculation of the free energy of H2 addition (ΔG°H ) to [Ni(PiPr2NPh2)2]2+ in CH3CN.
2
Figure 7. Proposed mechanism for catalysis facilitated by [Ni(PiPr2NPh2)2]2+
.
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Figure 8. Free energy landscape for electrocatalytic production of H2 by [Ni(PiPr2NPh2)2]2+. Free energy values are calculated at 1 atm H2,
an applied potential of 0.63 V, and [(DMF)H]+ (blue) and HOTf (red) as the protonating acid.
(Portland, OR), and was recrystallized three times from ethanol.
The ultra-high purity grade hydrogen gas used in this study was
from OXARC Inc. (Spokane, WA) and was passed through a hy-
drogen purification column from Vici Metronics (Poulsbo, WA).
All manipulations were carried out under an inert atmosphere by
either vacuum/Schlenk techniques or in an inert atmosphere
glovebox. K[P(SiMe3)2],[18] [(DMF)H]OTf,[22] and [Ni(CH3CN)
protonation steps with HOTf are significantly more favored
than those with [(DMF)H]+ because of the difference in the
pKa values of the two acids. In addition to the increase in
rate when using HOTf, a kinetic potential shift in the cata-
lytic wave of about 70 mV is observed, which indicates a
fast following chemical reaction after the electron transfer
event. When the free energy landscapes of the two acids are
compared, the first protonation step in the case of [(DMF)
H]+ is uphill by 1.63 kcalmol–1, whereas the same step with
HOTf is –3.42 kcalmol–1 downhill. This is one possible
source of the shift observed in the catalytic wave when using
HOTf as the protonating acid. Regardless of the precise
mechanism, protonation by the stronger acid, HOTf, of
catalytic intermediates will be more favored than that by a
weaker acid like [(DMF)H]+, which has been demonstrated
both experimentally and computationally.
[23]
6](BF4)2 were prepared according to the literature procedures.
Instrumentation: NMR spectra were recorded with a Varian Inova
1
spectrometer (500 MHz for H, 200 MHz for 31P) at 25 °C unless
noted otherwise. 1H NMR spectra were referenced against the pro-
teo impurity in the deuterated solvent. The 31P{1H} NMR spectra
were referenced to external phosphoric acid. Exact mass spectro-
metric analysis was performed by using an Exactive mass spectrom-
eter (Thermo Scientific, San Jose, CA) equipped with a custom
electrospray ionization (ESI) interface. Electrospray emitters were
custom-made by using 360 μm o.d. ϫ20 μm i.d. chemically etched
fused silica. The ion transfer tube temperature and spray voltage
were 250 °C and 2.2 kV, respectively. The mass spectrometer was
set to acquire data in the positive mode; data were collected from
300 to 1200 m/z units at an ultra-high resolution of 100k or be-
tween 200–600 m/z units at an ultra-high resolution of 100k, AGC
target set to ultimate mass accuracy, 5ϫ105. Samples diluted in
either methanol or acetonitrile to a concentration of 0.1 mm were
directly injected with a 250 μL Hamilton syringe at a flow rate of
1 μLmin–1.
Conclusions
Synthesis of [Ni(PiPr2NPh2)2]2+ has enabled the isolation
and first structural characterization of NiII–hydride in the
[Ni(PR2NRЈ2)2]2+ family of catalysts. Structural analysis of
[HNi(PiPr2NPh2)2]+ shows that the pendant amine in the
boat confirmation likely interacts with the hydride ligand,
which is consistent with previous spectroscopic and compu-
tational results. Although the catalyst decomposes in the
presence of acid, qualitative analysis of the experimental
data coupled with computationally derived free energy
landscapes yields insight into the mechanism for H2 pro-
Crystallography
Each crystal was coated in Paratone, affixed to a micromount, and
placed under streaming nitrogen (100 K) for analysis with a Bruker
KAPPA APEX II CCD diffractometer with 0.71073 Å Mo-Kα radi-
ation. Space groups were determined on the basis of systematic
absences and intensity statistics. The structures were solved by di-
rect methods and refined by full-matrix least-squares methods on
F2. Anisotropic displacement parameters were determined for all
non-hydrogen atoms. Hydrogen atoms were placed at idealized po-
sitions and refined with fixed isotropic displacement parameters,
by using a riding model.
The following programs were used: SAINT[24] for data reduction
and cell refinement; SADABS[25] for scaling and multi-scan absorp-
tion correction; SHELXS-2013 and SHELXL-2013[26] for structure
solution and refinement (respectively); OLEX2[27] was used as the
graphical user interface in which structure solution and refinement
duction facilitated by [Ni(PiPr2NPh2)2]2+
.
Experimental Section
Materials and Methods: All chemicals and materials were pur-
chased from Aldrich and were used without additional purification
unless otherwise noted. Tris(trimethylsilyl)phosphine was obtained
from Strem Chemicals, Inc. (Newburyport, MA), and used as re-
ceived. Tetrabutylammonium hexafluorophosphate, [nBu4N][PF6],
was obtained from Tokyo Chemical Industries (TCI), America
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were performed. Crystal data and information about data collec-
tion and refinement are listed in Table S1.
[HNi(PiPr2NPh2)2](BF4) (0.067 g, 85% yield). 31P{1H} NMR
(CD3CN, 202.4 MHz): δ = 22.9 (s) ppm. 1H NMR (CD3CN,
499.9 MHz): δ = 7.28 (t, J = 8 Hz, 8 H, ArH), 7.12 (d, J = 9 Hz,
8 H, ArH), 6.92 (t, J = 8 Hz, 4 H, ArH), 3.62 (d, J = 14 Hz, 8 H,
PCH2N), 3.57 (t, J = 14 Hz, 8 H, PCH2N), 2.19 (m, 4 H, iPrCH),
1.13 (m, 24 H, iPrCH3), –11.2 (quintet, J = 18 Hz, 1 H, Ni-H)
ppm.
CCDC-1000237 [for Ni(PiPr2NPh2)Cl2], -1000238 [for Ni(PiPr
-
2
NPh2)2], and -1000239 [for {HNi(PiPr2NPh2)2}BF4] contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details for alternative syntheses of [HNi-
(PiPr2NPh2)2](BF4) and additional crystallographic, spectroscopic,
cyclic voltammetry, and thermodynamic analyses.
PiPr2NPh2: To a stirring solution of 2-bromopropane (1.35 g,
0.011 mol) in THF (20 mL) was added dropwise a solution of
K[P(SiMe3)2] (1.943 g, 0.011 mol) in THF (20 mL). The resulting
solution was stirred at room temperature for 30 min, and a white
solid of potassium iodide was formed on the solution. The solid
was removed by filtration to yield a colorless solution. The solution Acknowledgments
was concentrated to dryness and dissolved in EtOH (20 mL). To
We thank Dr. Aaron Appel, Dr. Simone Raugei and Dr. Eric
Wiedner for helpful discussions. This research was supported as
part of the work at 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. Mass
spectrometry was provided at W. R. Wiley Environmental Molec-
ular Sciences Laboratory (EMSL), a national scientific user facility
sponsored by the Department of Energy’s office of Biological and
Environmental Research located at Pacific Northwest National
Laboratory. Pacific Northwest National Laboratory is operated by
Battelle for the U.S. Department of Energy.
this solution,
a suspension of paraformaldehyde (0.685 g,
0.022 mol) in ethanol was added. The resulting mixture was stirred
at 70 °C for 15 h to yield a colorless solution of isopropylbis-
(hydroxymethyl)phosphine. To this solution, aniline was added
dropwise over the course of 15 min, and the resulting solution was
stirred at 70 °C for 17 h. Cooling the reaction solution to room
temperature resulted in crystallization of the ligand as a white solid.
The ligand (0.922 g, 42% overall yield) was isolated as a white solid
through filtration and washed with cold EtOH. PiPr2NPh
,
2
C22H32N2P2 (386.45): calcd. C 68.37, H 8.35, N 7.25; found C
68.34, H 8.17, N 7.32. 31P{1H} NMR (CD2Cl2, 202.2 MHz): δ =
–37.1 (s) ppm. 1H NMR (CD2Cl2, 499.9 MHz): δ = 7.16 (t, J =
5 Hz, 4 H, ArH), 6.67 (d, J = 5 Hz, 4 H, ArH), 6.62 (t, J = 5 Hz,
2 H, ArH), 4.30 (t, J = 15 Hz, 4 H, PCH2N), 3.58 (d, J = 5 Hz, 2
H, PCH2N), 3.55 (d, J = 5 Hz, 2 H, PCH2N), 1.77 (m, 2 H, iPrCH),
1.29 (dd, J = 10 Hz, 12 H, iPrCH3) ppm.
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[Ni(PiPr2NPh2)2](BF4)2: To a stirring solution of the PiPr2NPh2 ligand
(0.225 g, 0.582 mmol) in CH3CN (20 mL), was added dropwise a
solution of [Ni(CH3CN)6](BF4)2 (0.139 g, 0.291 mmol) in CH3CN
(10 mL). The resulting mixture was stirred for 1 h, after which the
volume of the solution was reduced by 50% under vacuum. Ad-
dition of Et2O (10 mL) resulted in the formation of red crystals of
[Ni(PiPr2NPh2)2](BF4)2 (0.240 g, 82% yield), which were isolated by
filtration. 31P{1H} NMR (CD3CN, 121.4 MHz): δ = 16.0 (s) ppm.
1H NMR (CD3CN, 299.9 MHz): δ = 7.47 (t, J = 8 Hz, 8 H, ArH),
7.20 (d, J = 8 Hz, 8 H, ArH), 7.14 (t, J = 8 Hz, 4 H, ArH), 4.02
(d, J = 14 Hz, 8 H, PCH2N), 3.60 (t, J = 15 Hz, 8 H, PCH2N),
2.68 (m, 4 H, iPrCH), 1.29 (broad, 24 H, iPrCH3) ppm. HRMS
(ESI): calcd. for [Ni(PiPr2NPh2)2]2+ 415.1717; found 415.1719.
Ni(PiPr2NPh2)2: The [Ni(PiPr2NPh2)2](BF4)2 complex (0.096 g,
0.095 mmol) was dissolved in THF (10 mL), and a suspension of
KC8 (0.026 g, 0.095 mmol) in THF (10 mL) was added in a drop-
wise manner. The resulting solution was stirred overnight and fil-
tered to remove excess solids to yield a golden yellow solution.
Removal of solvents led to a pale yellow solid that was recrys-
tallized from benzene to yield Ni(PiPr2NPh2)2 (0.071 g, 90% yield).
31P{1H} NMR (C6D6, 202.2 MHz): δ = 18.9 (s) ppm. 1H NMR
(C6D6, 499.9 MHz): δ = 7.23 (t, J = 7 Hz, 8 H, ArH), 7.02 (d, J =
8 Hz, 8 H, ArH), 6.86 (t, J = 7 Hz, 4 H, ArH), 4.02 (d, J = 14 Hz,
8 H, PCH2N), 3.60 (t, J = 15 Hz, 8 H, PCH2N), 1.34 (m, 4 H,
iPrCH), 1.29 (dd, J = 14 Hz, 24 H, iPrCH3) ppm.
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[HNi(PiPr2NPh2)2](BF4): To a stirring solution of [Ni(PiPr2NPh2)2]-
(BF4)2 (0.086 g, 0.086 mmol) in CH3CN (5 mL) was added a slurry
of sodium trimethoxyborohydride (0.011 g, 0.086 mmol) in
CH3CN (5 mL). The resulting mixture was stirred for 30 min at
room temperature. The mixture was filtered to yield a yellow solu-
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solid was washed three times with pentane (5 mL) to recover pure
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
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Published Online: ᭿
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Nickel Hydride Electrocatalysts
P. Das, R. M. Stolley, E. F. van der Eide,
M. L. Helm* ..................................... 1–9
A new member of the [Ni(PR2NRЈ2)2]2+
family of electrocatalysts is reported. Com-
plex [Ni(PiPr2NPh2)2]2+ is an electrocatalyst
for proton reduction to form H2. Addition-
ally, we have isolated and structurally
characterized the NiII–hydride [HNi-
(PiPr2NPh2)2]+.
A
NiII–Bis(diphosphine)–Hydride Com-
plex Containing Proton Relays – Structural
Characterization and Electrocatalytic Stud-
ies
Keywords: Homogeneous catalysis / Hydro-
gen / Proton transport / Nickel / Diphos-
phine
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