Studies of a series of [Ni(PR2NPh 2)2(CH3CN)]2+ complexes as electrocatalysts for H2 production: Substituent variation at the phosphorus atom of the P2N<sub

Article abstract of DOI:10.1021/ic201461a

A series of [Ni(P^R₂N^Ph₂) ₂(CH₃CN)](BF₄)₂ complexes containing the cyclic diphosphine ligands [P^R₂N^Ph ₂ = 1,5-diaza-3,7-diph

Full text of DOI:10.1021/ic201461a

ARTICLE
pubs.acs.org/IC
Studies of a Series of [Ni(PR NPh ) (CH3CN)]2+ Complexes as
2
2 2
Electrocatalysts for H2 Production: Substituent Variation at the
Phosphorus Atom of the P2N2 Ligand
Uriah J. Kilgore,† Michael P. Stewart,† Monte L. Helm,†,‡,|| William G. Dougherty,§ W. Scott Kassel,§
Mary Rakowski DuBois,† Daniel L. DuBois,*,† and R. Morris Bullock*,†
†Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352,
United States
‡Department of Chemistry, Fort Lewis College, Durango, Colorado 81301
§Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085, United States
S Supporting Information
b
ABSTRACT: A series of [Ni(PR NPh2)2(CH3CN)](BF4)2 complexes contain-
2
ing the cyclic diphosphine ligands [PR NPh2 = 1,5-diaza-3,7-diphosphacyclooc-
2
tane; R = benzyl (Bn), n-butyl (n-Bu), 2-phenylethyl (PE), 2,4,4-trimethylpentyl
(TP), and cyclohexyl (Cy)] have been synthesized and characterized. X-ray
diffraction studies reveal that the cations of [Ni(PBn2NPh2)2(CH3CN)](BF4)2
and [Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2 have distorted trigonal bipyra-
midal geometries. The Ni(0) complex [Ni(PBn2NPh2)2] was also synthe-
sized and characterized by X-ray diffraction studies and shown to have
a distorted tetrahedral structure. These complexes, with the exception of
[Ni(PCy2NPh2)2(CH3CN)](BF4)2, all exhibit reversible electron transfer pro-
cesses for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the
production of H2 in acidic acetonitrile solutions. The heterolytic cleavage of H2
by [Ni(PR NPh2)2(CH3CN)](BF4)2 complexes in the presence of p-anisidine
2
or p-bromoaniline was used to determine the hydride donor abilities of the corresponding [HNi(PR NPh2)2](BF4) complexes.
2
However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H2,
suggesting that steric interactions between the alkyl substituents on phosphorus and the nitrogen atom of the pendant amines play an
important role in determining the overall catalytic rate.
’ INTRODUCTION
that the pendant amines dramatically accelerate one or more of the
elementary steps in0 the catalytic cycle. Additionally, the simple
synthesis of PR NR ligands permits easy modification of the
Renewable energy sources such as solar and wind have the
potential to significantly reduce fossil fuel consumption, but
the intermittent nature of electricity generated from these
sources makes efficient storage of this energy critical.1 An
attractive option is to store the excess energy available during
peak production in the form of fuels with a high energy
density, such as H2. In order to use H2 on a large scale,
inexpensive electrocatalysts that are capable of efficiently
converting electrical energy into chemical bonds are needed.
In nature, hydrogenase enzymes carry out oxidation of H2 and
reduction of H+ to produce H2.2 The active sit11es of these
enzymes contain nickel and iron, and these have inspired
attempts to develop synthetic complexes as structural models
capable of mimicking the functions of hydrogenases.3
2
2
substituents on P and N, and, thus, both the first and second
coordination spheres of these catalysts. Previous work by our group
has examined the effects of varying the basicity of the proton relay, a
second coordination sphere component. It was demonstrated that
varying the p-substituent of the aniline-based pendant amine
affected both the redox properties and the thermodynamic driving
force for H2 production, and ultimately the turnover frequency of
the catalyst.8 The work presented here examines the consequences
of varying the substituents at the phosphorus atoms of the 1,5-diaza-
3,7-diphosphacyclooctane (PR NPh2) ligands in nickel catalysts for
2
H2 production.
’ RESULTS
Previous0 studies from our laboratories have sho0wn that
[Ni(PR NR 2)2(CH3CN)]2+ complexes (where PR NR is a 1,
2
2
2
Synthesis and Characterization of Ligands and Nickel
Complexes. Most of the new and previously reported ligands
5-diaza-3,7-diphosphacyclooctane ligand) are highly active electro-
catalysts for oxidation of H2 or reduction of H+ to produce H2.4ꢀ9
0
Comparison of these [Ni(PR NR 2)2(CH3CN)]2+ complexes to
Received: July 10, 2011
2
other bis(diphosphine) complexes of nickel clearly demonstrates
Published: October 14, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Scheme 1. Synthesis of (PR NPh2) Ligands and [Ni(PR NPh2)2(CH3CN)]2+ Complexes
2
2
Figure 2. Structure of the [Ni(Pn‑Bu2NPh2)2(CH3CN)]2+ cation. BF4
anions and THF molecules are omitted, and only the ipso carbon atoms
of the phenyl groups on the N atom are shown for clarity. Thermal
ellipsoids are plotted at the 50% confidence level.
Figure 1. Structure of the [Ni(PBn2NPh2)2(CH3CN)]2+ cation. BF4
anions and THF molecules are omitted, and only the ipso carbon atoms
of the phenyl groups on the N atom are shown for clarity. Thermal
ellipsoids are plotted at the 50% confidence level.
1
and two multiplets in the H NMR spectra between 3.99 and
3.44 ppm are characteristic of the diastereotopic methylene pro-
tons. X-ray diffraction studies of two of these complexes have been
performed as described in the next paragraph, and additional data
supporting the formulations of these complexes is provided in the
Experimental Section.
used in this study were prepared using a single-flask two-step
reaction (reaction 1 in Scheme 1). In the first step, a primary
phosphine, RPH2, where R = benzyl (Bn), n-butyl (n-Bu),
2-phenylethyl (PE), 2,4,4-trimethylpentyl (TP), or cyclohex-
yl, was treated with 2 equiv of paraformaldehyde to form the
corresponding bis(hydroxymethyl)phosphine (reaction 1 of
Scheme 1). This product was dissolved in hot ethanol, and
then treated with an equivalent of aniline. The diazadipho-
sphacyclooctane (PR2NPh2) ligands were isolated as white
solids and have 31P{1H} NMR chemical shifts between ꢀ46
Dark red crystals of [Ni(PBn2NPh2)2(CH3CN)](BF4)2 and
[Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2 were grown by vapor
diffusion of hexane into a THF solution containing a few
drops of acetonitrile. For [Ni(PBn2NPh2)2(CH3CN)](BF4)2 the
unit cell consisted of a discrete dication, two tetrafluoroborate
anions, and disordered THF and acetonitrile solvent molecules.
For [Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2 the crystals consist of a
discrete dication, two tetrafluoroborate anions, three THF mol-
ecules, and one acetonitrile solvent molecule. The nickel cations are
depicted in Figures 1 and 2, and selected bond lengths and angles
are given in Tables 1 and 2 for [Ni(PBn2NPh2)2(CH3CN)](BF4)2
and [Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2, respectively. The struc-
tures are best described as trigonal bipyramids having two axial
and two equatorial phosphines, with the acetonitrile ligand
occupying the third equatorial position. The NiꢀP bond lengths
of 2.17ꢀ2.21 Å are typical for Ni(II) complexes with bidentate
phosphine ligands, and the PꢀNiꢀP bite angles of 80ꢀ82° for
the diphosphine ligands are also in the normal range observed for
this class of complexes.6,8 For both complexes the two six-
membered rings adjacent to the coordinated acetonitrile ligand
and ꢀ53 ppm, comparable to PPh2NC6H4X ligands reported
2
previously.8 1H NMR spectral data and mass spectrometric
data are also consistent with formation of 1,5-diaza-3,7-dipho-
sphacyclooctane (PR NPh2) ligands (see Experimental Section
2
for details).
The [Ni(PR2NPh2)2(CH3CN)](BF4)2 complexes were pre-
pared by addition of 2 equiv of the diazadiphosphacyclooctane
ligands to an acetonitrile solution of [Ni(CH3CN)6](BF4)2.
The NMR and mass spectra data are consistent with the structures
shown in reaction 2 in Scheme 1. The 31P{1H} NMR spectra of
these complexes all consist of a single resonance between 5 and 9
ppm, 47ꢀ62 ppm downfield of the free ligand resonances. Com-
plexation results in bicyclic diphosphine ligands that exhibit rapid
boatꢀchair interconversion of the six-membered chelate rings,
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Inorganic Chemistry
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Table 1. Selected Bond Distances and Angles for
[Ni(PBn2NPh2)2(CH3CN)](BF4)2
Table 3. Selected Bond Distances and Angles for
Ni(PBn2NPh
)
2 2
bond distance (Å)
bond angle (deg)
bond distance (Å)
bond angle (deg)
Ni(1)ꢀP(1)
2.2050(8)
2.1909(7)
2.2051(7)
2.2137(7)
1.975(2)
P(1)ꢀNi(1)ꢀP(3)
98.92(3)
80.28(3)
178.52(3)
122.80(3)
82.16(3)
99.32(3)
Ni(1)ꢀP(1)
2.1376(3)
2.1485(3)
2.1431(3)
2.1351(3)
P(1)ꢀNi(1)ꢀP(3)
P(1)ꢀNi(1)ꢀP(2)
P(3)ꢀNi(1)ꢀP(2)
P(1)ꢀNi(1)ꢀP(4)
P(3)ꢀNi(1)ꢀP(4)
P(2)ꢀNi(1)ꢀP(4)
116.510(11)
85.412(11)
128.726(11)
129.640(11)
85.297(11)
116.869(11)
Ni(1)ꢀP(2)
Ni(1)ꢀP(3)
Ni(1)ꢀP(4)
Ni(1)ꢀN(5)
P(1)ꢀNi(1)ꢀP(2)
P(3)ꢀNi(1)ꢀP(2)
P(1)ꢀNi(1)ꢀP(4)
P(3)ꢀNi(1)ꢀP(4)
P(2)ꢀNi(1)ꢀP(4)
Ni(1)ꢀP(2)
Ni(1)ꢀP(3)
Ni(1)ꢀP(4)
Table 2. Selected Bond Distances and Angles for
[Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2
bond distance (Å)
bond angle (deg)
Ni(1)ꢀP(1)
2.2124(5)
2.1775(5)
2.1898(5)
2.1672(5)
1.9859(16)
P(1)ꢀNi(1)ꢀP(3)
P(1)ꢀNi(1)ꢀP(2)
P(3)ꢀNi(1)ꢀP(2)
P(1)ꢀNi(1)ꢀP(4)
P(3)ꢀNi(1)ꢀP(4)
P(2)ꢀNi(1)ꢀP(4)
119.81(2)
82.68(2)
Ni(1)ꢀP(2)
Ni(1)ꢀP(3)
Ni(1)ꢀP(4)
Ni(1)ꢀN(5)
100.49(2)
98.48(2)
81.046(19)
177.33(2)
Figure 4. Cyclic voltammogram of 1 mM [Ni(Pn‑Bu2NPh2)2(CH3CN)]-
(BF4)2 in acetonitrile/0.2 M NEt4BF4 with a 1 mm glassy carbon
working electrode; scan rate = 0.1 V sꢀ1
.
angles of ∼85° are 2ꢀ4° larger than those observed for the
trigonal bipyramidal Ni(II) complexes. This is attributed to a
preference of Ni(0) complexes for a 109° PꢀNiꢀP bond
angle compared to a 90° PꢀNiꢀP angle in an ideal trigonal
bipyramid and a shorter NiꢀP bond distance of 2.14 Å. For this
complex, all of the six-membered rings adopt a boat conformation,
although a mixture of chair and boat conformations is likely
present in solution.10 The Ni N distances range from 3.34
3 3 3
to 3.53 Å.
Electrochemical Studies of [Ni(PR NPh2)2(CH3CN)](BF4)2
2
Complexes. With theexceptionof [Ni(PCy2NPh2)2(CH3CN)]2+,
cyclic voltammograms of each Ni(II) complex in this study exhibit
two distinct and reversible reduction waves assigned to the Ni(II/
I) and Ni(I/0) couples in acetonitrile. A typical voltammogram is
shown in Figure 4 (voltammograms for all complexes
are given in the Supporting Information). Plots of the peak current
(ip) versus υ1/2 are linear for both reduction waves, indicat-
ing that these reductions are diffusion-controlled.11 For [Ni
(PCy2NPh2)2(CH3CN)]2+, the Ni(I/0) reduction is irreversible
in acetonitrile (Figure S1) but reversible in benzonitrile (Figure
S3). The irreversible Ni(I/0) couple in acetonitrile is attributed to
the poor solubility of the Ni(0) complex in acetonitrile which
causes it to adsorb on the electrode. The cyclic voltammogram of
Ni(PBn2NPh2)2 in benzonitrile shows a quasireversible one-electron
redox wave for the Ni(II/I) couple and a reversible redox wave for
the Ni(I/0) couple. Identical electrochemical behavior is ob-
served for [Ni(PBn2NPh2)2(CH3CN)]2+ in benzonitrile. Redox
potentials (E1/2 reported vs Cp2Fe+/0 at 0.00 V), peak-to-peak
separations (ΔEp, υ = 100 mV sꢀ1), and the differences in the
Ni(II/I) and Ni(I/0) couples (ΔE1/2) are given in Table 4 for all
Figure 3. Structure of Ni(PBn2NPh2)2. Thermal ellipsoids plotted at
50% confidence level.
adopt chair conformations to minimize steric interactions with
acetonitrile, and the other two six-membered rings adopt boat
conformations. This results in Ni N(2) and Ni N(1)
3 3 3
3 3 3
distances of 3.423 and 3.830 Å, respectively, for [Ni(PBn2NPh2)2-
(CH3CN)](BF4)2, and Ni N(2) and Ni N(1) distances of
3 3 3
3 3 3
3.371 and 3.811 Å, respectively, for [Ni(Pn‑Bu2NPh2)2(CH3CN)]-
(BF4)2.
The Ni(0) complex Ni(PBn2NPh2)2 was synthesized by reducing
[Ni(PBn2NPh2)2(CH3CN)](BF4)2 under an atmosphere of H2 in
the presence of excess base, tetramethylguanidine. In the 31P NMR
spectrum of the product, a singlet at ꢀ1.03 ppm is assigned to the
equivalent phosphorus atoms, as expected for a tetrahedral structure.
Large yellow crystals of Ni(PBn2NPh2)2 suitable for X-ray dif-
fraction studies were grown by slow evaporation of an acetoni-
trile solution. The structure of the Ni(0) complex is depicted in
Figure 3, and selected bond lengths and angles are given in
Table 3. The coordination geometry is a distorted tetrahedron
with a dihedral angle between ligands of 78.5°. The PꢀNiꢀP bite
of the [Ni(PR NPh2)2(CH3CN)]2+ complexes.
2
Thermodynamic Properties of [Ni(PR NPh2)2(CH3CN)]2+
2
and [HNi(PR NPh2)2]+ Complexes. Hydride donor abilities were
2
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Inorganic Chemistry
ARTICLE
Table 4. Electrochemical Data for [Ni(PR NPh2)2(CH3CN)]2+ Complexes in Acetonitrile (0.2 M NEt4BF4) Referenced to
2
Cp2Fe+/0
Ni(II/I)
Ni(I/0)
R
E
1/2 (V)
ΔEp (mV)
E 1/2 (V)
ΔEp (mV)
ΔE 1/2 (mV)
n-butyl (n-Bu)
ꢀ0.93
ꢀ0.90
ꢀ0.89
ꢀ0.84
ꢀ0.83
ꢀ0.60
68
65
69
69
72
75
ꢀ1.23
ꢀ1.16
ꢀ1.17
ꢀ1.02
ꢀ1.12
ꢀ1.12
65
65
62
73
66
81
300
260
280
180
290
520
2-phenylethyl (PE)
2,4,4-trimethylpentyl (TP)
phenyl (Ph)
benzyl (Bn)
cyclohexyl (Cy)a
a Benzonitrile solvent.
Scheme 2. Thermodynamic Cycle for Determination of
ΔG°H‑
Scheme 3. Thermodynamic Cycle for Estimation of ΔG°H2
Table 5. Thermochemical Data for [HNi(PR NPh2)2]+
2
Complexes
R
ΔG°H- (kcal/mol)a
ΔG°H2 (kcal/mol)b
n-butyl
57.1
10.7
10.0
10.2
8.8
2-phenylethyl
2,4,4-trimethylpentyl
phenyl
57.8
57.6
59.013
benzyl
59.4
63.715
8.4
cyclohexyl
4.1
a Hydride donor ability (eq 6). b Free energy for H2 binding to
[Ni(PR2NPh2)2(CH3CN)]+ complexes; see Scheme 3 and Supporting
Information.
Figure 5. Plot of hydride donor abilities as a function of E1/2 of the
Ni(II/I) couple for all complexes listed. Line fit equation: ΔG°H‑ = 21.7
A plot of ΔG°H‑ for the [HNi(PR NPh2)2]+ complexes versus
2
E
1/2 Ni(II/I) + 77.2; R2 = 0.997.
the potential of the Ni(II/I) couple for the corresponding
[Ni(PR NPh2)2(CH3CN)]2+ complexes is linear, as shown in
2
Figure 5. The hydride donor ability decreases (ΔG°H‑ becomes
more positive) as the Ni(II/I) redox potential becomes more
positive. Similar correlations have been reported previously for
other [HNi(diphosphine)2]+ complexes.16,17 Protonation of
the Ni(II) hydride complex in acetonitrile leads to the formation
of [Ni0(PPh2NPhNPhH)2]2+ (Scheme 3), for which a pKa value
of 6.0 has been estimated.13,18 As shown in Scheme 3, the hydride
donor abilities and pKa data can be used to calculate free energies
calculated for [HNi(PR NPh2)2]+ complexes from the thermoche-
2
mical cycle shown in Scheme 2, by measuring equilibrium constants
for heterolysis of H2 to generate [HNi(PR NPh2)2]+ using p-anisidine
2
or p-bromoaniline as the base (B) in acetonitrile (Scheme 2, the
reverse of reaction 3). The Ni(II) complexes discussed here do not
add H2 (PH = 1 atm) in the absence of an external base. The
equilibrium c2onstants for reaction 3 in Scheme 2 were determined by
1
31P and H NMR spectroscopy as described in the Experimental
for H2 binding and cleavage by the corresponding [NiII(PR NPh2)-
2
(CH3CN)]2+ complexes (ΔG°H2; Table 5). Larger positive values
of ΔG°H2 indicate a more favorable driving force for HꢀH bond
formation and elimination of H2 from diprotonated intermedi-
ates during the catalytic cycle for H2 production.
Section. In eq 5 of Scheme 2, 76.0 kcal/mol is the free energy for
heterolytic cleavage of H2 in acetonitrile.12 These equilibrium con-
stants, the pKa values of the protonated base (BH+), and the free
energy for heterolytic H2 cleavage are used in eq 6 of Scheme 2 to
calculate the free energies (ΔG°H‑) for the heterolytic cleavage of the
NiꢀH bond in acetonitrile. This method for determining hydride
donor abilities is discussed in more detail elsewhere.13,14
Electrocatalytic Properties of [Ni(PR NPh2)2(CH3CN)](BF4)2
2
Complexes. Figure 6 shows a series of cyclic voltammograms for
[Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2 in the presence of increasing
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Figure 6. Cyclic voltammograms of 0.7 mM [Ni(Pn‑Bu2NPh)2(CH3CN)]-
(BF4)2 in acetontrile/0.2 M Et4NBF4 with a 1 mm glassy carbon
working electrode and internal ferrocene reference; scans taken in
absence of acid (black) and in the presence of various concentrations
Figure 7. Plot of icat/ip vs [acid] for [Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2
catalyst in acetonitrile/0.2 M Et4NBF4 with a 1 mm glassy carbon
working electrode in the presence of [(DMF)H]OTf.
of acid ([(DMF)H]OTf). Scan rate = 0.05 V sꢀ1
.
Table 6. Turnover Frequencies and Overpotentials for Pro-
ton Reduction in Acetonitrile.
concentrations of protonated dimethylformamide, [(DMF)-
H]OTf, as the acid (pKa
= 6.1).19 The black trace corre-
MeCN
turnover frequencies (s‑1) and overpotentials (mV)a
sponds to the complex in the absence of acid and consists of
reversible waves for the Ni(II/I) and Ni(I/0) couples. The wave
at 0.0 V is that of the ferrocenium/ferrocene couple resulting
from the addition of ferrocene as an internal standard. The
colored traces are for cyclic voltammograms recorded in the
presence of increasing acid concentrations as indicated in the
figure legend. The small decrease in catalyst concentrations
reflects dilution due to addition of aliquots of acid solution.
The value of ip (vide infra) is corrected for dilution when
determining the turnover frequency. It can be seen that there
is a significant increase in the current near the Ni(II/I) couple as
increasing amounts of acid are added. In addition, the waves
assume a plateau shape expected for catalysts operating in the
presence of excess acid substrate. At higher acid concentrations, the
waves are shifted to potentials that are positive of the Ni(II/I)
couple in the absence of acid. For the red trace (0.11 M acid)
the potential at the half-height of the catalytic wave is
shifted positive by 130 mV. This observation is consistent with
protonation of the Ni(I) species formed upon reduction of
Ni(II) at a rate that is significantly faster than the overall catalytic
rate. Under these conditions, the half-wave potential of the
Ni(II/I) couple is influenced by the rate of the following
protonation reaction, but the current is determined by the overall
catalytic rate. Slight changes in slope in the red trace suggest
that there may be as many as three different protonation
events that influence the catalytic potential. As discussed below,
these may represent different sites of protonation of the catalyst.
Hydrogen production for [Ni(PPE2NPh2)2(CH3CN)]2+ and [Ni-
(Pn‑Bu2NPh2)2(CH3CN)]2+ was confirmed by gas chromato-
graphic analysis of the headspace gas obtained in a bulk electrolysis
[(DMF)H]OTf
[cyanoanilinium]OTf
without
with
without
water
with
R
water
water
water
n-butyl
46
(450)
31
1820
(500)
1080
(450)
69
4
214
(440)
141
(330)
11
2-phenylethyl
2,4,4-trimethylpentyl
phenylb
(330)
<1
(290)
<1
(310)
19
(360)
590
(450)
720
(280)
28
(400)
72
(300)
7
(320)
130
(270)
3
(330)
28
benzyl
(410)
(460)
(230)
(320)
a Overpotentials in mV are indicated in parentheses. Determined using
the method of Evans et al.21 b Previously determined,8 included for
comparison.
Similar studies were carried out for the 2-phenylethyl and benzyl
derivatives using [(DMF)H]OTf as the acid, and their turnover
frequencies are listed in Table 6. The overpotentials calculated
according to the method of Evans et al. are also listed in Table 6.21
rffiffiffiffiffiffiffiffiffiffiffiffi
icat
ip
n
RTkobs
¼
ð12Þ
0:4463
Fυ
Evaluation of [Ni(PCy2NPh2)2(CH3CN)]2+ as a catalyst for
hydrogen production was precluded by its decomposition in
acidic solutions, and [Ni(PTP2NPh2)2(CH3CN)]2+ is a signifi-
cantly slower catalyst than the other derivatives in the presence of
>0.10 M [(DMF)H]+OTfꢀ, as can be seen by comparing the
cyclic voltammograms in Figure 8 with those in Figure 6. The
shift in the potential of the Ni(II/I) wave in the absence of
acid (black trace) to more positive potentials in the presence of
acid (red trace) indicates that, following reduction of Ni(II),
protonation of the Ni(I) species is rapid. However, catalysis
is slow, as indicated by the small increase in current in the
presence of a large excess of acid. We estimate that the catalytic
experiment. The current efficiency determined for the [Ni(PPE
-
2
NPh2)2(CH3CN)]2+ catalyst was found to be 95 ( 5%.
Figure 7 shows the ratio (icat/ip) of the catalytic current (icat)
to the peak current (ip) for the Ni(II/I) couple observed in the
absence of acid. This ratio increases as the acid concentration
increases up to 0.11 M acid, and at still higher concentrations the
ratio becomes constant. In general, this is the behavior expected
for a catalyst that saturates with substrate. Equation 12 can be
used to calculate a turnover frequency (TOF) of 46 sꢀ1 for
[Ni(Pn‑Bu2NPh)2(CH3CN)](BF4) in the acid independent region.7,20
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Inorganic Chemistry
ARTICLE
Scheme 4. Proposed Catalytic Cycle for Hydrogen Produc-
tion by [Ni(PR NPh2)2]2+ Catalystsa
2
Figure 8. Cyclic voltammograms of 0.7 mM [Ni(PTP2NPh)2(CH3CN)]-
(BF4)2 in acetonitrile/0.2 M Et4NBF4 with a 1 mm glassy carbon
working electrode; scans taken in the absence of (black) and in the
presence (red) of 120 mM acid ([(DMF)H]OTf). Scan rate = 0.1 V sꢀ1
.
a Phosphine substituents not shown and N-substituents simplified for
clarity.
0
type [Ni(PR NR 2)2(CH3CN)]2+, it has been proposed that the
2
intermediate shown by structure 1 is required for the formation of
the HꢀH bond during H2 production.
Figure 9. Cyclic voltammograms of 0.7 mM [Ni(PTP2NPh)2(CH3CN)]-
(BF4)2 in acetonitrile/0.2 M Et4NBF4 with a 1 mm glassy carbon
working electrode; scans taken in the absence of (black) and in the
presence of various concentrations of water and acid ([(DMF)H]+OTf).
Scan rate = 0.1 V sꢀ1
.
rate for this complex is <1 sꢀ1 under the same conditions where
[Ni(Pn‑Bu2NPh2)2(CH3CN)]2+ has a catalytic rate of 46 sꢀ1, at
least 1 order of magnitude faster.
For such an intermediate, the driving force and rate of H2 elim-
ination should increase as the acidity of the protonated pendant
amine increases and as the hydride donor ability of the metal
increases. The proposed catalytic cycle is shown in Scheme 4. This
cycle is consistent with the electrochemical observations discussed
The slow catalytic rate observed for [Ni(PTP2NPh2)2(CH3CN)]2+
provided an opportunity to assess the effect of water on the cata-
lytic rate. Addition of water (2.5 M) to an acetonitrile solution of
[Ni(PTP2NPh2)2(CH3CN)]2+ containing approximately 0.12 M
[(DMF)H]OTf results in relatively large increases in current, as
shown in Figure 9. The performance of this catalyst is enhanced
froma rateless than1 sꢀ1 to a rate of 69 sꢀ1. Similarenhancements
of the catalytic rates by 20ꢀ100 times are observed for all of the
above, as well as previously studied [Ni(PR NPh2)2]2+ electrocata-
2
lysts for H2 production.4ꢀ8 The mechanism in Scheme 4 shows
only endo protonation of the amines, leading to the formation of
H2. Scheme 5, discussed later, considers the results of exo proton-
ation, pathways that do not lead directly to the production of H2.
We recently reported a series of essentially isostructural [Ni-
(PPh2NC6H4X2)2](BF4)2 derivatives having pendant amines with
a wide range of Brønsted basicities, obtained by varying the
electron-donating character of X in the N-aryl para position. In
general, the catalysts having more acidic protonated pendant amines
experienced a larger driving force for H2 elimination from inter-
mediates with structure 1, and a good correlation was observed
between this driving force and the turnover frequencies for hydro-
gen formation.
[Ni(PR NPh2)2(CH3CN)]2+ catalysts with alkyl substituents on
2
phosphorus, as shown in Table 6.
Previous studies have demonstrated that the nature of the acid
plays an important role in determining the turnover frequencies
for H2 production using [Ni(PPh2NR ) (CH3CN)]2+ complexes
2 2
as electrocatalysts.8 Similar effects were observed for the present
series of complexes, and data shown in Table 6 indicate that the
rates of catalysis decrease by up to an order of magnitude when
bulkier anilinium acids are used in place of [(DMF)H]OTf.
Effect of Hydride Donor Ability on Catalytic Rates. The
present study was designed to examine how variation in the
’ DISCUSSION
On the basis of experimental6,8 and theoretical22 mechanistic
studies of both hydrogen production and oxidation catalysts of the
substituent on phosphorus within a series of [Ni(PR NPh2)2-
2
(CH3CN)]2+ complexes contributes to the observed catalytic rates
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ARTICLE
for H2 formation. Previous studies have shown that the hydride
donor abilities of [HNi(diphosphine)2]+ derivatives, as well as
the Ni(II/I) couple of the corresponding [Ni(diphosphine)2]2+
complexes, are strongly influenced both by the electron-donating
ability of substituents on the phosphorus atoms and by steric
interaction between the two diphosphine ligands.16,18,23 The
previously documented trend that sterically more demanding
substituents on phosphorus lead to more positive E1/2 values for
the Ni(II/I) redox couple and to weaker hydride donor abilities
(larger ΔG°H‑ values) has been attributed to a larger dihedral
angle between the two planes defined by the phosphorus atoms
of each diphosphine ligand and Ni in [Ni(diphsophine)2]2+
complexes. This tetrahedral distortion leads to lower LUMO
energies as a result of a decrease in the antibonding overlap
of the σ orbitals of phosphorus with the dxy orbital of nickel.23 In
the previously studied [Ni(PPh2NC6H4X2)2]2+ systems, in which
the acidities of theprotonatedamines werevaried, all P atoms were
substituted with phenyl groups and very little structural variation
Scheme 5. Protonation of Pendant Amines Endo or Exo with
Respect to Nickel
between catalysts was expected. For the series of [Ni(PR NPh2)2-
2
(CH3CN)]2+ derivatives studied in this work, all of the pendant
amines have a Ph substituent, so it is assumed that the basicity of
the protonrelay will not varysignificantly. Changes indrivingforce
for H2 production should be derived primarily from changes in
hydride donor ability associated with different substituents on the
phosphorus donors of the ligands.
on phosphorus is observed for the cyclohexyl substituted com-
plex, [Ni(PCy2NPh2)2(CH3CN)]2+ (ΔE1/2 = 0.52 V); the smallest
steric interaction is observed for the phenyl substituted complex,
[Ni(PPh2NPh2)2]2+ (ΔE1/2 = 0.18 V); and the complexes [Ni-
(Pn‑Bu2NPh2)2(CH3CN)]2+, [Ni(PBn2NPh2)2(CH3CN)]2+, [Ni-
(PTP2NPh2)2(CH3CN)]2+, and [Ni(PPE2NPh2)2(CH3CN)]2+ ex-
hibit ΔE1/2 values between 0.26 and 0.30 V. Despite the similar
degree of steric interaction between alkyl substituents of adjacent
phosphine ligands in the last group, large differences in turnover
frequencies are observed for this series, as described above.
Effect of Additional Steric Interactions. A consideration of
the mechanistic steps proposed4 for the production of H2 by
these catalysts suggests how additional steric interactions of the
phosphine substituents may be an important factor in determin-
ing the catalytic rate. The cyclic voltammograms shown in Figures 6
and 8 indicate that the reduction of Ni(II) to Ni(I) is followed by a
fast protonation reaction, and this proton addition is proposed to
occur at a pendant amine of the ligand, which serves as a proton
relay. This protonation may occur in either endo or exo positions
with respect to nickel as shown by structures 2-endo and 2-exo in
Scheme 5.
The data shown in Table 5 and Figure 5 show that the hydride
donor abilities determined for [HNi(PR NPh2)2]+ derivatives
2
and the resulting driving force for H2 elimination differ by
approximately 7 kcal/mol upon varying the alkyl substituent R
from cyclohexyl to n-butyl. In studies using [(DMF)H]OTf as
the acid, the complexes with ΔG°H2 values most favorable for H2
elimination, [Ni(Pn‑Bu2NPh2)2(CH3CN)]2+ and [Ni(PPE2NPh2)2-
(CH3CN)]2+, display the highest turnover frequencies for H2
production in this series. However, other data in Tables 5 and 6
indicate that in some cases the catalytic rate of H2 production does
not parallel the driving force for H2 elimination. For example, the
largest difference in turnover frequencies is observed for [Ni-
(Pn‑Bu2NPh2)2(CH3CN)] 2+ and [Ni(PTP2NPh2)2(CH3CN)]2+,
while the estimated driving force for H2 elimination differs by
only 0.5 kcal/mol, which is essentially the same within our
experimental uncertainty. In contrast, the turnover frequen-
cies for both [Ni(PBn2NPh2)2(CH3CN)]2+ and [Ni(PTP2NPh
)
2 2
Fast protonation in either position would result in the observed
positive shift in the reduction potential. However, in order for the
complex to enter the catalytic cycle, the site of protonation of the
Ni(I) intermediate must be endo with respect to the Ni atom. In
conjunction with the second electron transfer, the second proton-
ation must also occur in an endo position in order to form
intermediate 1.
(CH3CN)]2+ are similarly low for all conditions studied, despite
their comparatively large difference in ΔG°H2 (1.8 kcal/mol).
Unfortunately, the derivative with R = cyclohexyl is unstable in
the acidic solutions required for catalysis of H2 production, so it
could not be included in these comparisons. The lack of correla-
tion between the catalytic rates and the corresponding ΔG°H2
values for some of the complexes suggests that factors other than
the thermodynamics of HꢀH bond formation can play an
important role in determining the catalytic activities.
As discussed above, the first protonation of the catalyst occurs in
the Ni(I) oxidation state. Because both [NiI(diphosphine)2]+ and
Ni0(diphosphine)2 complexes exhibit distorted tetrahedral struc-
tures, the structure of Ni0(PBn2NPh2)2 (Figure 3) is of special
interest. The structure indicates that the region near the metal
center is shielded by considerable steric bulk and suggests that the
most likely site of protonation for this complex is the exo position.
Although the potentials of the Ni(II/I) couples are sensitive to
both electronic effects and interactions between the phosphine
substituents, the potentials of the Ni(I/0) couples for the [Ni-
(diphosphine)2]2+ complexes are determined almost exclusively
by the electronic character of the phosphorus substituent;16 [Pd-
(diphosphine)22+] complexes exhibit similar trends.24 Therefore,
a second way of evaluating the influence of steric effects between
diphosphines in this series of complexes is to consider the dif-
ference in the potentials of the Ni(II/I) and Ni(I/0) couples
(ΔE1/2). These ΔE1/2 values are listed in Table 4, and they fall into
three groups. The largest steric interaction between substituents
Previous NMR studies of the closely related Ni0(PPh2NBn
)
2 2
complex demonstrated that protonation by 2,6-dichloroanilinium
tetrafluoroborate in the exo postions is preferred.9 It is also
apparent from the structure of Ni0(PBn2NPh2)2 that the positions
of the phosphine substituents can give rise to unfavorable steric
interactions with the amine group of the opposing PBn2NPh
2
ligand, and have the potential to hinder endo protonation of the
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Inorganic Chemistry
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amines. This steric interaction corresponds with our observa-
tions that alkyl groups have a more pronounced effect on the
catalytic rates than phenyl substituents, and that larger alkyl
substituents on phosphorus result in slower catalytic rates. In
particular, it appears that alkyl substituents with groups attached
at a distance of two or more atoms from the phosphorus atom are
effective at retarding the rate of catalysis by blocking access of
acid to the endo positions of the N atoms. For example, the 2,4,
4-trimethylpentyl substituent does not manifest its steric bulk by
resulting in significant interactions between phosphines in
adjacent ligands in the Ni(II) oxidation state (as indicated by a
ΔE1/2 of 0.28 V for [Ni(PTP2NPh2)2(CH3CN)]2+ compared to
the ΔE1/2 of 0.30 V for [Ni(Pn‑Bu2NPh2)2(CH3CN)]2+). How-
ever, the very low catalytic rate for this complex can be under-
stood in terms of the distal bulk of the 2,4,4-trimethylpentyl
substituent preventing the formation of significant amounts of
the endo protonated intermediates necessary for catalysis.
Previous work with the [Ni(PPh2NC6H4X2)2]2+ catalysts has
demonstrated that water enhanced the rate of hydrogen pro-
duction.8 For example, addition of water (0.02ꢀ0.5 M) to reac-
tion mixtures containing 0.20ꢀ0.45 M [(DMF)H]OTf in-
creased the catalytic rates by 30ꢀ60%. In addition, the rates of
catalysis with this series of complexes were dependent on the
Figure 10. Plot of ΔG°H2 as a function of E1/2 of the Ni(II/I) couple,
superimposed on a bar graph of turnover frequencies.
phosphine ligands have been shown to influence the hydride
donor ability of [HNi(PR NPh2)2]+ derivatives. For some cases in
2
this series of catalysts, the turnover frequencies for H2 formation
were found to correlate with ΔG°H‑ and consequently ΔG°H2
,
nature of the acid substrate. Both cyanoanilinium (pKaMeCN = 7.0)
but for several of the complexes, catalytic rates do not follow the
trend predicted on the basis of these properties (Figure 10).
The catalyticreaction proceeds through protonation of pendant
amines in tetrahedral Ni(I) and Ni(0) intermediates, and the
turnover frequencies appear to be sensitive to the extent that the
steric bulk of the phosphine substituent hinders the endo proton-
ation of the amine of the adjacent ligand in these intermediates.
The addition of water to these systems leads to a significant
increase in catalytic rates, and this effect is attributed to the ability
of water to assist in the generation of catalytically active inter-
mediates that are protonated endo relative to nickel. These studies
suggest that structural modifications of the complexes that enable
the delivery of protons to endo sites should further increase the
turnover frequencies of these catalysts; studies to achieve this goal
are currently in progress.
MeCN
and 2,6 dichloroanilinium (pKa
= 5.0) were found to be
about an order of magnitude slower than those observed for
[(DMF)H]OTf (pKaMeCN = 6.1), although their pKa values
bracketed that of [(DMF)H]OTf.25
Similar effects were observed for the present series of com-
plexes, and these are alsoattributedtothesteric limitations to endo
protonation imposed by bulky phosphine substituents. In this
series of complexes the smaller acid [(DMF)H]OTf gave faster
catalytic rates than those observed for the larger cyanoanilinium
acid by as much as an order of magnitude. Similarly, water is likely
to assist in the delivery of protons to the endo position of these
catalysts, being smaller than either [(DMF)H]OTf or cyanoani-
linium, but capable of hydrogen bonding to serve as a relay to the
pendant base. The observation that water enhances the catalytic
rates for H2 production more for the [Ni(PR NPh2)2(CH3CN)]-
2
(BF4)2 catalysts reported in this study (by factors of 10 to >70,
Table 6) than observed previously for [Ni(PPh2NC6H4X2)2](BF4)2
catalysts (30ꢀ60%) is attributed to the greater steric bulk of the
alkyl substituents on phosphorus compared to the phenyl substituent
in the latter series. The more sterically demanding alkyl substit-
uents present a more restricted access for endo protonation, and
the preferential access by small water molecules acting as
intermolecular proton relays to the pendant amines should lead
to a larger enhancement of the catalytic rate.
’ EXPERIMENTAL SECTION
1
General Experimental Procedures. Instrumentation. H and
31P NMR spectra were recorded on Varian spectrometers (Inova or
1
1
VNMRS, 500 MHz for H) at 23 °C unless otherwise noted. All H
chemical shifts have been internally calibrated to the residual solvent
protons. 31P NMR spectra were externally referenced to H3PO4 at 0 ppm.
Electrospray ionization (ESI), chemical ionization (CI), and atmospheric
pressure chemical ionization (APCI) mass spectra were collected at the
Indiana University Mass Spectrometry Facility on a Waters/Micromass
LCT Classic (ESI/APCI) or Thermo MAT-95 XP (EI/CI) using anhydrous
solvents and inert atmosphere techniques.
’ SUMMARY AND CONCLUSIONS
In the catalytic production of H2 with[Ni(PR NPh2)2(CH3CN)]2+
Electrochemistry. Unless otherwise indicated in the text, voltam-
metric procedureswere conductedin0.2 M NEt4BF4/CH3CNatambient
temperature, 23 °C, under nitrogen using a Vacuum Atmospheres glove-
box. A standard three-electrode configuration was employed in conjunc-
tion with CH Instruments 660C or 1100C potentiostat interfaced to a
computer with CH Instruments 700 D software. All voltammetric scans
were recorded using glassy-carbon working electrode disks of 1 mm
diameter encased in PEEK (Cypress Systems EE040). The working elec-
trode was treated between scans by polishing with diamond paste
(Buehler) in sequence of decreasing sizes (3ꢀ0.25 μm) interspersed by
washings with purified H2O (vide infra). A glassy-carbon rod (Structure
Probe, Inc.) and platinum wire (Alfa-Aesar) were used as auxiliary electrodes
2
derivatives, the rate of H2 elimination from the proposed inter-
mediate of structure 1 should be favored by increased acidities of
the protonated amines and by stronger hydride donor abilities of
the metal center.
In order to determine the effect of hydride donor ability on the
rates of electrocatalytic proton reduction, a series of complexes of
the formula [Ni(PR NPh2)2(CH3CN)]2+ containing different
2
alkyl groups at the phosphorus atoms of the PR NPh ligands
2
2
were synthesized and characterized. Both electronic properties of
phosphine substituents and steric interactions between adjacent
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ARTICLE
and quasireference electrodes respectively. All glassware for electroche-
mical experiments was oven-dried overnight and allowed to cool under
vacuum. Ferrocene was used as an internal standard, and all potentials
reported within this work are referenced to the ferrocenium/ferrocene
couple at 0 V.
bath (70 °C) and stirred for two days. A singlet at ꢀ24 ppm in the 31
P
NMR spectrum of the crude reaction mixture was consistent with
formation of the bis(hydroxymethyl)-2-phenylethylphosphine. To the
reaction mixture was added degassed aniline (500 mg, 5.37 mmol)
portionwise to the hot solution over a period of 4 h. The resulting
suspension of white precipitate was heated for 8 h at 70 °C. The volume
of the reaction mixture was reduced under vacuum, and a white solid was
collected by filtration (yield 420 mg, 0.823 mmol, 30% based on aniline).
31P{1H} NMR (CDCl3, 202.2 MHz): δ ꢀ49.0 (s). 1H NMR (CDCl3,
499.7 MHz): δ 7.33 (t, J = 7.6 Hz, 4H, ArH); 7.26 (m, 4H, ArH); 7.23
(m, 2H, ArH); 7.18 (t, J = 7.7 Hz, 4H, ArH); 6.68 (td, J = 7.3 Hz, 7.3 Hz,
0.8 Hz, 2H, ArH); 6.59 (d, J = 8.7, 4H, ArH); 4.13 (t, J = 14.5 Hz, 4H,
CH2); 3.44 (dd, J = 15.0 Hz, 4.4 Hz, 4H, CH2); 2.95 (dd, J = 16.4 Hz, 8.3
Hz, 4H, CH2); 1.72 (m, 4H, CH2). CI-MS: Observed MH+ at 511.2450.
Calculated for MH+: 511.2432.
Synthesis and Materials. All reactions and manipulations were
performed under an N2 atmosphere using standard Schlenk techniques
or in a glovebox unless otherwise indicated. Solvents were dried using an
Innovative Technology, Inc. PureSolv solvent purification system.
Acetonitrile-d3 (Cambridge Isotope Laboratories, 99.5%D) was va-
cuum-distilled from P2O5. Chloroform-d (Cambridge Isotope Labora-
tories, 99.5%D) was degassed and stored over molecular sieves.
Tetraethylammonium tetrafluoroborate (Alfa-Aesar) was recrystallized
twice by vapor diffusion of diethyl ether into an acetonitrile solution; the
crystals obtained were dried under vacuum. Dimethylformamide-tri-
fluoromethanesulfonic acid, [(DMF)H]+OTfꢀ, was prepared by the
method of Faviera and Du~nach.19 2,4,4-Trimethylpentylphosphine
(Strem) was used as received. Water was dispensed from a Millipore
Milli-Q purifier and sparged with nitrogen. Ferrocene (Aldrich) was sub-
Synthesis of PTP2NPh2. A 100 mL Schlenk flask was charged with 2,4,4-
trimethylpentylphosphine (4.8 g, 33 mmol mmol), paraformaldehyde
(2.0 g, 67 mmol), and ∼20 mL of degassed absolute ethanol. The
resulting suspension was immersed in a hot oil bath (75 °C) and stirred
for 12 h, resulting in a clear solution. A 31P NMR spectrum recorded on
the reaction mixture at this point was consistent with formation of the
bis(hydroxymethyl)-2,4,4-trimethylpentylphosphine (ꢀ27 ppm, s; EtOH)
which was isolated as a clear, highly viscous oil by removal of all volatiles
in vacuo (6.2 g, 30 mmol, 94%). To a 100 mL Schlenk flask was added
1.0 g (4.8 mmol) of bis(hydroxymethyl)-2,4,4-trimethylpentylphosphine
and 30 mL of ethanol. To this solution was added a degassed toluene
solution (∼2 mL) of aniline (450 mg, 4.8 mmol) over a period of 2 h.
After 3 h of stirring at 65 °C a white suspension was observed. The
reaction mixture was heatedat50 °C overnight. Thesolid was collected by
filtration, washed with diethyl ether, and dried in vacuo (yield 747 mg,
1.4 mmol, 58%). 31P{1H} NMR (CDCl3, 202.2MHz):δ ꢀ53.0 (s). Anal.
Calcd for C32H52N2P2: C, 72.97; H, 9.95; N, 5.32. Found C, 73.18; H,
10.08; N, 5.29. APCI-MS: Observed MH+ at 527.3668. Calculated for
MH+: 527.3684.
limed under vacuum before use. [Ni(CH3CN)6](BF4)2] 1/2CH3CN,26
3
Ph 27
[Ni(P2CyN2Ph)2(CH3CN)](BF4)2,15P2CyN2
,
n-butylphosphine,28
benzylphosphine,28 and 2-phenylethylphosphine28 were prepared by the
literature methods.
Synthesis of PBn2NPh2. A 250 mL Schlenk flask was charged with
benzylphosphine (2.8 g, 23 mmol), paraformaldehyde (1.4 g, 47 mmol),
and ∼100 mL of degassed absolute ethanol. The resulting suspension
was immersed in a hot oil bath (70 °C) and stirred for 12 h, resulting in
a clear solution. The 31P{1H} NMR spectrum of the crude reaction
mixture exhibited a singlet at ꢀ14 ppm which is assigned to bis(hydroxy-
methyl)benzylphosphine. A degassed solution of aniline (2.1 g, 23 mmol)
in 10 mL of ethanol was added dropwise to the hot solution of bis-
(hydroxymethyl)benzylphosphine over a period of 2 h. The resulting
suspension of a white precipitate was heated for an additional 4 h at
70 °C and was further heated at 50 °C overnight. The solvent was removed
on a vacuum line, and the resulting product was suspended in Et2O and
stirred for 1 h. The white solid was collected by filtration and dried in vacuo
(yield 2.8 g, 5.8 mmol, 52%). 31P{1H} NMR (CDCl3, 202.3 MHz):
δ ꢀ46.0 (s). 1H NMR (CDCl3, 499.7 MHz): δ 7.33 (m, 10H, ArH); 6.99
(m, 4H, ArH); 6.59 (m, 2H, ArH); 6.16 (m, 4H, ArH); 4.21 (dd, J = 14.8
Hz, 3.8 Hz, 4H, PCH2N); 3.45 (dd, J = 14.8 Hz, 10.1 Hz, 4H, PCH2N);
2.83 (d, J = 2.8 Hz, 4H, PCH2Ar). CI-MS: Observed MH+ at 483.2118.
Calculated for MH+: 483.2113. Anal. Calcd for C30H32N2P2: C, 74.67; H,
6.68; N, 5.81. Found C, 74.67; H, 6.66; N, 5.86.
Synthesis of [Ni(PBn2NPh2)2(CH3CN)](BF4)2. To a stirred MeCN solu-
tion (15 mL) of [Ni(CH3CN)6](BF4)2 1/2CH3CN (101 mg, 0.202
3
mmol) was added 2 equiv of PBn2NPh2 (200 mg, 0.415 mmol). The blue
solution of [Ni(CH3CN)6](BF4)2 1/2CH3CN rapidly changed to a red
3
color. The resultant red solution was filtered through a plug of Celite
after stirring overnight. The remaining solvent was removed from the
filtrate under vacuum. The red powder was washed with diethyl ether
and dried in vacuo (yield 215 mg, 174 mmol, 86%). 31P{1H} NMR
1
(CD3CN, 202.2 MHz): δ 5.2 (s). H NMR (CD3CN, 499.7 MHz):
Synthesis of Pn‑Bu2NPh2. A 250 mL Schlenk flask was charged with
n-butylphosphine (2.2 g, 24 mmol), paraformaldehyde (5 g, 167 mmol),
and ∼100 mL of degassed absolute ethanol. The resulting suspension
was immersed in an oil bath (45 °C) and stirred for 12 h, resulting in a
clear solution. A singlet in the 31P NMR spectrum suggests formation
of the bis(hydroxymethyl)butylphosphine (ꢀ22 ppm; EtOH). Aniline
(1.6 g, 17 mmol) in toluene (∼2 mL) was added dropwise over 2 h to the
hot solution. The resulting white suspension was heated for 4 h at 70 °C
and was further heated at 50 °C overnight. The solvent was then
removed on a vacuum line, and the product was suspended and stirred in
diethyl ether. The suspension was filtered, and the white solid was dried
in vacuo (yield 1.6 g, 3.9 mmol, 45% based on aniline). 31P{1H} NMR
(CDCl3, 202.2 MHz): δ ꢀ50.0 (s). 1H NMR (CDCl3, 499.7 MHz): δ
7.20 (t, J = 6.5 Hz, 4H, ArH); 6.69 (d, J = 8.1 Hz, 6H, ArH); 4.20 (m, 4H,
PCH2N); 3.44 (m, 4H, PCH2N); 1.61 (m, 4H, CH2); 1.49 (m, 4H,
CH2); 1.41 (m, 4H, CH2); 0.98 (m, 6H, CH3). CI-MS: Observed MH+
at 415.2408. Calculated for MH+: 415.2426.
δ 7.40 (m, 12H, ArH); 7.33 (m, 8H, ArH); 7.16 (m, 8H, ArH); 7.04 (t,
J=7.3Hz, 4H, ArH);6.86(d, J=7.9Hz, 8H, ArH);3.99(d, J=14.1Hz, 8H,
PCH2N); 3.70 (s, 8H, PCH2Ar); 3.44 (d, J = 14.0 Hz, 8H, PCH2N) 1.96
(s, 3H, CH3CN). MS-ESI: Observed {[Ni(PBn2NPh2)2](BF4)}+ 1109.3481.
Calculated for {[Ni(PBn2NPh2)2](BF4)}+: 1109.3464.
Synthesis of [Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2. This complex was pre-
pared in a manner analogous to that described for [Ni(PBn2NPh2)2-
(CH3CN)](BF4)2. The product was isolated as a red powder in 91%
yield. The complex may be further purified by slow diffusion of hexanes
into a THF solution of the complex, resulting in red powder. 31P{1H}
1
NMR (CD3CN, 202.2 MHz): δ 7.8 (s). H NMR (CD3CN, 499.7
MHz): δ 7.39 (m, 8H, ArH); 7.08 (d, J = 7.3 Hz, 8H, ArH); 7.02 (t, 4H,
ArH); 6.85 (m, 4H, ArH); 3.83 (m, 8H, PCH2N); 3.68 (m, 8H,
PCH2N); 2.16 (m, 8H, CH2); 1.96 (s, 3H, CH3CN); 1.57 (m, 8H,
CH2); 1.46 (m, 8H, CH2); 0.92 (t, 12H, CH3). ESI-MS: Observed
{[Ni(P2N2)2](BF4)}+ 973.4071. Calculated for {[Ni(P2N2)2](BF4)}+:
973.4090.
Synthesis of PPE2NPh2. A 250 mL Schlenk flask was charged with
2-phenylethylphosphine (2.0 g, 14 mmol) dissolved in ∼80 mL of THF.
To this solution was added paraformaldehyde (1.6 g, 53 mmol) sus-
pended in degassed absolute ethanol. The flask was immersed in an oil
Synthesis of [(Ni(PTP2NPh2)2(CH3CN)](BF4)2. This complex was pre-
pared in a manner analogous to that described for [Ni(PBn2NPh2)2-
(CH3CN)](BF4)2. The product was isolated as a red powder in 76%
yield. 31P{1H} NMR (CD3CN, 202.2 MHz): δ 8.5 (s). 1H NMR (CD3CN,
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ARTICLE
499.7 MHz): δ 7.36 (m, 8H, ArH); 7.09 (m, 12H, ArH); 3.87 (m, 8H,
PCH2N); 3.77 (m, 8H, PCH2N); 2.17 (m, 8H, CH2); 2.02 (m, 4H,
CH); 1.96 (s, 3H CH3CN); 1.35 (m, 8H, CH2); 1.13 (m, 12H, CH3);
0.89 (s, 36H, CH3). ESI-MS: Observed {[Ni(P2N2)2](BF4)}+
1197.6565. Calculated for {[Ni(P2N2)2](BF4)}+: 1197.6594.
instance at the most positive potential beyond the Ni(II/I) E1/2 at which
current stopped increasing. Addition of acidic titrant was continued until
catalytic current enhancement ceased (ca. 0.12 M [(DMF)H]OTf).
Subsequent to the completion of acid additions, the described method
was repeated using aliquots of purified H2O as titrant. H2O was added by
Eppendorf automatic micropipeter in 3ꢀ10 μL increments until the
observed catalytic current enhancement ceased or catalytic current was
reduced.
Synthesis of [(Ni(PPE2NPh2)2(CH3CN)](BF4)2. This complex was pre-
pared in a manner analogous to that described for [Ni(PBn2NPh2)2-
(CH3CN)](BF4)2. The product was isolated as a red powder in 91%
yield. 31P{1H} NMR (CD3CN, 202.2 MHz): δ 6.8 (s). 1H NMR
(CD3CN, 499.7 MHz): δ 7.32 (m, 14H, ArH); 7.28 (m, 6H, ArH); 7.15
(m, 8H, ArH); 7.06 (t, J = 7.4 Hz, 4H, ArH); 6.98 (d, J = 7.9 Hz, 8H,
ArH); 3.84 (m, 8H, PCH2N); 3.71 (m, 8H, PCH2N); 2.98 (m, 8H,
CH2); 2.50 (m, 8H, CH2); 1.96 (s, 3H, CH3CN). Anal. Calcd for
C66H75B2F8N5NiP4: C, 61.23; H, 5.84; N, 5.41. Found C, 61.01; H,
6.07; N, 5.67. ESI-MS: Observed {[Ni(P2N2)2](BF4)}+ at 1165.4124.
Calculated for {[Ni(P2N2)2](BF4)}+: 1165.4090.
Catalytic Hydrogen Production. Controlled Potential Coulometry. A
three-necked flask having a total volume of 150 mL was used as the bulk
electrolysis vessel, and was assembled under an atmosphere of nitrogen
with each neck accepting an electrode fed through a pierced rubber
septum. The working electrode consisted of a copper wire attached to a
reticulated vitreous carbon cylinder; the reference and counter electro-
des were a Ag wire and graphite rod, respectively, placed into 5 mm glass
tubes terminating in Vycor fritted disks and filled via syringe with
electrolyte solution (0.3 M Et4NBF4 in acetonitrile). A solution of
[Ni(PPE2NPh2)2(CH3CN)](BF4)2 (13 mg, 0.010 mmol), ferrocene, and
NEt4BF4 (1.0 g, 4.6 mmol) in 10 mL of acetonitrile and a solution of
[(DMF)H]OTf (670 mg, 3.0 mmol) in 5 mL of acetonitrile were
prepared in the glovebox. The catalyst/ferrocene solution was then
transferred to the reaction vessel via syringe, and a cyclic voltammogram
was obtained. The acid solution was then added, and controlled-
potential coulometry was performed at ꢀ1.0 V versus the ferricinium/
ferrocene couple. After 17.5 C of charge was passed, a 200 μL sample of
the headspace gas was removed via gastight syringe and analyzed by gas
chromatography. Using the moles of H2 produced and the charge
passed, a current efficiency of 95 ( 5% was calculated for H2 production
(TON = 9, 25% decomposition). Gas analysis for H2 was performed
using an Agilent 6850 gas chromatograph equipped with a thermal
conductivity detector and fitted with a 10 ft long Supelco 1/800 Carbosieve
100/120 column, calibrated with two H2/N2/CO/CO2 mixtures of
known composition.
Synthesis of Ni(PBn2NPh2)2. To a stirred MeCN solution (7 mL) of
[Ni(CH3CN)6](BF4)2 1/2CH3CN (52 mg, 0.10 mmol) was added
3
2 equiv of PBn2NPh (100 mg, 0.207 mmol). The blue solution of
2
[Ni(CH3CN)6](BF4)2 1/2CH3CN rapidly changed to a red color. The
3
solution was stirred for 4 h at room temperature; then, tetramethylgua-
nidine (40 mg, 35 mmol) was added. A white precipitate presumed to be
P2N2 ligand was observed. The mixture was then bubbled with H2 gas for
10 min, followed by stirring for 2 days. The resulting mixture was filtered,
concentrated, and stored. After sitting for four days at room temperature
under N2, yellow crystals were observed and collected (yield 25 mg,
0.024 mmol, 24%). 31P{1H} NMR (C6D6, 202.2 MHz): δ ꢀ1.03 (s). 1H
NMR (C6D6, 499.7 MHz): δ δ 7.19 (m, 8H, ArH); 7.16 (m, 16H, ArH);
7.08 (m, 4H, ArH); 6.78 (m, 8H, ArH); 6.74 (d, 4H, ArH); 3.34 (m,
16H, PCH2N); 2.70(s, 8H, PCH2Ar). Anal. Calcd for C60H64N4NiP4:
C, 70.39; H, 6.30; N, 5.47. Found: C, 70.48; H, 6.37; N, 5.54.
Equilibration of [Ni(PR NPh2)2(CH3CN)](BF4)2 with H2 and Anisi-
2
dine/Anisidinium or Bromoaniline/Bromoanilinium in Acetonitrile-d3.
In a typical experiment, [Ni(PR NPh2)2(CH3CN)](BF4) and p-bromoa-
2
’ ASSOCIATED CONTENT
niline (pKa = 9.43) or p-anisidine (pKa = 11.83) were accurately weighed
into an NMR tube and dissolved in acetonitrile-d3 (0.5 mL). Hydrogen
gas was bubbled through the solution for 10 min. The solution was
allowed to sit for 15 min and was monitored by 1H NMR and 31P NMR
spectroscopy. The reaction came to equilibrium within the time of mixing
and was monitored every few hours for 24 h to ensure that the ratios
Supporting Information. CIF data for [Ni(PBn2NPh2)2-
S
b
(CH3CN)]2+, [Ni(Pn‑Bu2NPh2)2(CH3CN)]2+, and Ni(PBn2NPh
)
2 2.
Additional tabulated and graphical data for electrocatalytic hydro-
gen production. Cyclic voltammograms. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
had not changed. The H and 31P NMR resonances assigned to [Ni-
(PR NPh2)2(CH3CN)](BF4)2 and the corresponding hydrides, [HNi-
(PR2NPh2)2](BF4), were integrated, and the ratio was used to determine
’ AUTHOR INFORMATION
2
the equilibrium constant for the reaction. The ratio of [Hbase+]/[base]
was determined from the observed 1H NMR chemical shift. Three separate
experiments were run to determine the reproducibility. The equilibrium
constant (K = [NiH+][Hbase+]/PH2 [Ni2+][base]) was used to calculate
the hydride donor ability. Values of ΔG°H‑ are reported as an average of
three runs. The standard deviation for values reported is less than 0.5 kcal/
mol. Purging the samples with N2 gas for 15 min resulted in the observation
Corresponding Author
*E-mail: daniel.dubois@pnnl.gov (D.L.D.); morris.bullock@
pnnl.gov (R.M.B.).
Notes
Sabbatical Visitor at PNNL, 2010ꢀ2011.
of only [Ni(PR NPh2)2(CH3CN)](BF4)2.
2
Catalytic Hydrogen Production Using [Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2
as Catalyst and [(DMF)H]OTf as Acid. Typical Experimental Condi-
tions. [Ni(Pn‑Bu2NPh2)2(CH3CN)](BF4)2 (2.1 mg, 0.002 mmol) and
ferrocene (0.4 mg, 0.002 mmol) were weighed into an 8 mL glass vial
and dissolved in 2 mL of a supporting electrolyte solution (0.2 M [NEt4]-
[BF4]/acetonitrile). Purity of the electrolyte medium was confirmed
over the available electrochemical window through background scans
taken prior to addition of analyte. In a separate vial, 100 mg (0.45 mmol)
of [(DMF)H]OTf was dissolved in 0.6 mL of acetonitrile. Acidic titrant
was transferred to the electrochemical solution via volumetric syringe in
20 μL aliquots. Subsequent to each addition of titrant, the working
electrode was cleaned by polishing (vide supra), and a cyclic voltammo-
gram was recorded. The catalytic current (icat) was selected in each
’ ACKNOWLEDGMENT
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
under FWP 56073. Pacific Northwest National Laboratory is
operated by Battelle for the U.S. Department of Energy.
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