[Ni(PPh2NAr2)2(NCMe)][BF4]2 as an electrocatalyst for H2 production: PPh2NAr2 = 1,5-(di(4-(thiophene-3-yl)phenyl)-3,7-diphe

Article abstract of DOI:10.1016/j.jorganchem.2009.04.010

A new cyclic 1,5-diaza-3,7-diphosphacyclooctane ligand was prepared with phenyl substituents on phosphorus and (thiophene-3-yl)phenyl substituents on nitrogen. This ligand reacts with [Ni(CH₃CN)₆][BF₄]₂ to form

Full text of DOI:10.1016/j.jorganchem.2009.04.010

Journal of Organometallic Chemistry 694 (2009) 2858–2865
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
[Ni(PP2hN2Ar)2(NCMe)][BF4]2 as an electrocatalyst for H2 production:
PP2hNA2r = 1,5-(di(4-(thiophene-3-yl)phenyl)-3,7-diphenyl-1,5-diaza-3,7-
diphosphacyclooctane)
1
*
Douglas H. Pool , Daniel L. DuBois
Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States
Pacific Northwest National Laboratory, 908 Battelle Blvd., MS K2-57, Richland, WA 99354, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A new cyclic 1,5-diaza-3,7-diphosphacyclooctane ligand was prepared with phenyl substituents on phos-
phorus and (thiophene-3-yl)phenyl substituents on nitrogen. This ligand reacts with [Ni(CH3CN)6][BF4]2
to form the corresponding [Ni(PP2hN2Ar)2(NCMe)][BF4]2 complex, 3, which is an active electrocatalyst for H2
production. Kinetic studies indicate that the catalytic rate is first order in catalyst and second order in acid at
low concentrations of acid, but at higher acid concentrations the catalytic rate becomes independent of acid
concentration. The rate-determining step at high acid concentrations is attributed to the elimination of H2
from a reduced Ni species. The modest overpotential of 280 mV and a turnover frequency of 56 sꢀ1 confirm
that 3 is a relatively active catalyst for H2 production in acetonitrile solutions. Oxidation of the pendant thi-
ophene substituents of 3 results in the formation of films on glassy carbon electrode surfaces. However
these films are not electroactive, and electrocatalysis of proton reduction is not observed with these mod-
ified electrodes.
Received 27 February 2009
Received in revised form 3 April 2009
Accepted 7 April 2009
Available online 17 April 2009
Keywords:
Hydrogen production
Nickel
Catalyst
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
With the increasing use of alternative energy sources such as
nuclear, wind, and solar will come the need for an effective
means to store and transport the electrical energy produced
by these sources. One such energy carrier is hydrogen gas that
can be generated in a variety of ways including electrolysis [1].
Platinum metal is an excellent catalyst for the generation of
hydrogen as well as the oxidation of hydrogen in fuel cells,
but its cost and limited distribution provide strong incentives
for the development of catalysts based on more abundant and
less expensive metals. The hydrogenase enzymes found in nat-
ure catalyze hydrogen production and oxidation using the far
more abundant elements of nickel and iron in the active site
[2–7]. A drawing of a proposed structure of the [FeFe] hydrog-
enase active site based on X-ray diffraction studies [5] is shown
by structure 1.
* Corresponding author. Tel.: +1 509 375 2331.
E-mail addresses: douglas.pool@pnl.gov (D.H. Pool), daniel.dubois@pnl.gov (D.L.
DuBois).
A series of nickel complexes using cyclic 1,5-diaza-3,7-diphos-
0
phacyclooctane ligands of the general formula [Ni(PR2N2R )2]2+
,
1
Tel.: +1 509 372 4938.
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.04.010

D.H. Pool, D.L. DuBois / Journal of Organometallic Chemistry 694 (2009) 2858–2865
2859
structure 2, catalyze the electrochemical production or oxidation of
hydrogen [8–13]. The cyclic diphosphine ligands coordinate to the
metal through the two phosphorus atoms leaving the nitrogens as
single resonance at ꢀ37.3 ppm. The material is partially soluble
in DMSO-d6 and the 31P NMR spectrum of the resulting solution
has a main resonance at ꢀ49.3 ppm and a small resonance at
ꢀ35.2 ppm, tentatively assigned to isomers with cis and trans
phenyl groups with respect to the cyclooctane ring. This white
powder reacts with [Ni(NCMe)6][BF4]2 in acetonitrile to give a
red solution of [Ni(PP2hN2Ar)2(NCMe)][BF4]2 (3) (where P2PhNA2r is 1,5-
(di(4-(thiophene-3-yl)phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphospha-
cyclooctane) after four days of stirring (Scheme 1), consistent with
the proposed structure of the ligand [26]. Crystals of 3 form readily
with the diffusion of ethyl ether into acetonitrile, and the purity was
confirmed by elemental analysis. The proton NMR spectrum contains
a series of distinct resonances in the aromatic region and two dou-
blets at 4.29 and 3.98 ppm consistent with two sets of aliphatic
methylene groups expected for a bicyclic ligand. Decoupling experi-
ments and integration allow for the chemical shift assignments
listed in Section 5 and consistent with structure 3. The 31P NMR
spectrum of 3 recorded in acetonitrile-d3 consists of a single reso-
nance at 5.0 ppm similar to the value of 4.8 ppm reported for 2c,
which has been characterized by an X-ray diffraction study [9].
pendant bases similar to that proposed for the [FeFe] hydrogenase.
0
The substituents on the PR2N2R ligand tune the ability of these com-
plexes to bind or release H2 in a heterolytic manner. Variations at
the phosphine position affect the hydride acceptor ability of the Ni
center, while the groups at the nitrogen position affect the pKa of
the protonated base formed during the catalytic cycle [11,12–18].
The complex [Ni(PC2yN2Bz)2]2+ (2a) is a hydrogen oxidation catalyst
[9], [Ni(PP2hN2Bz)2]2+ (2b) is a better hydrogen production than oxida-
tion catalyst [11], and [Ni(PP2hNP2h)2]2+ (2c) is the best hydrogen pro-
duction catalyst of the series [10]. Particularly interesting features of
these catalysts are their relatively high catalytic rates with modest
overpotentials and their lack of inhibition by CO [9,19].
For practical reasons it is ultimately desirable for these molec-
ular catalysts to be attached to electrode surfaces [20]. There are
a number of methods that have been studied for electrode
modification [20–25], and in this paper we explore one of these,
the electrochemical generation of a polythiophene. Thiophene is
susceptible to oxidation, generating radicals that combine to form
polymers that precipitate at the electrode surface. Lennox and
coworkers demonstrated the ability to polymerize thiophenes with
tethered ferrocene groups that were redox active once confined to
the electrode surface [24]. In this work, we describe the synthesis
of a new 1,5-diaza-3,7-diphosphacyclooctane ligand with thio-
phene groups attached to the pendant nitrogen base and a corre-
sponding nickel complex. The resulting complex was investigated
for its ability to form coatings on electrode surfaces with redox
active Ni. Additionally the complex was used in solution to inves-
tigate its ability to reduce protons. The resulting data allows for a
2.2. Electrochemical studies
The cyclic voltammogram of 3 in benzonitrile has two revers-
ible one-electron waves with E1/2 = ꢀ0.80 V (with a peak to peak
separation (
D
Ep) of 56 mV) and E1/2 = ꢀ0.99 V (
DEp = 62 mV). Plots
of the peak currents of these waves vs. the square root of the scan
rate are linear indicating the electron transfers are diffusion con-
trolled. These two waves correspond to Ni(II/I) and Ni(I/0) couples.
When the cyclic voltammogram of 3 is recorded in acetonitrile, the
Ni(II/I) wave is at E1/2 = ꢀ0.82 V (
tions of 3, the Ni(I/0) wave is observable at E1/2 = ꢀ1.01 V
Ep = 58 mV) (Fig. 1). The Ni(I/0) couple becomes a stripping wave
DEp = 59 mV). At low concentra-
comparison to similar complexes and contributes to a broader
0
understanding of the [Ni(PR2N2R )2]2+ series.
(D
at concentrations greater than 0.7 mM in 3 in acetonitrile as a re-
sult of precipitation of the Ni(0) complex on the electrode surface.
The cyclic voltammogram of 3 also exhibits irreversible oxidation
waves at potentials more positive than +0.6 V.
2. Results
2.1. Synthesis
Complex 3 appears to polymerize onto the working electrode at
potentials greater than 0.6 V during cyclic voltammetry experi-
ments. Multiple sweep cyclic voltammograms in which the poten-
tial excursions remain negative of 0.6 V exhibit little or no decrease
in peak currents for the solution based Ni(II/I) and Ni(I/0) waves as
well as the oxidation current observed between 0.4 and 0.6 V.
Cycling to potentials greater than 0.6 V results in a decrease in
The thiophene substituted aniline, 4-(thiophen-3-yl)aniline, re-
acts with PhP(CH2OH)2 in acetonitrile to give an off-white solid
with limited solubility. Because of the low solubility, it was charac-
terized by solid state 31P NMR spectroscopy and found to have a
Fig. 1. Cyclic voltammogram of a 0.7 mM solution of 3 in acetonitrile containing
0.2 M NEt4BF4 using a scan rate of 50 mV/s with a 1 mm diameter glassy carbon
working electrode. The wave at 0.00 V corresponds to the ferrocenium/ferrocene
couple arising from added ferrocene, which was used as the internal standard.
Scheme 1.

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D.H. Pool, D.L. DuBois / Journal of Organometallic Chemistry 694 (2009) 2858–2865
Fig. 2. Cyclic voltammograms of a 1.1 mM 3 and 0.1 M NEt4BF4 acetonitrile solution
using a glassy carbon working electrode scanned at 0.1 V/s showing the Ni(II/I) and
Ni(I/0) reductions in solution: with a clean working electrode showing stripping
waves (a); after the potential was cycled from ꢀ0.4 V to 0.8 V and back to ꢀ0.4 V at
0.1 V/s (b); after the potential was cycled from ꢀ0.4 V to 1.1 V and back to ꢀ0.4 V
again at 0.1 V/s (c).
Fig. 4. Ten consecutive cyclic voltammograms scanning from ꢀ0.4 V to ꢀ1.4 V, then
positive to 1.6 V, and back to ꢀ0.4 V of an acetonitrile solution containing 0.6 mM 3,
20 mM thiophene, and 0.1 M NEt4BF4. The working electrode was glassy carbon,
and the scan rate was 0.1 V/s. The first scan shows the Ni(II/I) and Ni(I/0) couples of
3 in solution (a, inset) while the second scan no longer does (b, inset).
currents of greater than 1.0 [27–29]. While conducting the same
experiments with a mixture of 20:1 thiophene:3 shows increased
current between ꢀ0.4 and 1.6 V with consecutive scans associated
with the formation of polythiophene (Fig. 4), they failed to show
any electrochemically active nickel species confined to the electrode
surface (Fig. 4 inset).
the current observed for the Ni(II/I) and Ni(I/0) waves of 3 in solu-
tion on subsequent sweeps to negative potentials (Fig. 2). The oxi-
dation current observed in sequential scans to greater than 0.6 V
also decreases. Additionally when ferrocene is included in the solu-
tion as an internal reference, the return reduction wave for the
Fe(III/II) couple decreases after an initial sweep to positive poten-
tials, and the reduction wave is much broader with lower peak cur-
rents indicating the process is no longer diffusion controlled (Fig. 3,
trace a). This decrease in reduction current for the ferrocene couple
is not observed for cyclic voltammograms of the closely related
complex, [Ni(PP2hN2Ph)2NCMe][BF4]2 (2c), that lacks the thiophene
group, and which also has non-reversible oxidation peaks at poten-
tials more positive than 0.7 V (Fig. 3, trace b). When the working
electrode is coated by cycling to potentials greater than 0.7 V in a
solution of 3, removed from solution, placed in a separate electro-
chemical cell with only electrolyte, and cycled between 0.0 and
ꢀ1.4 V, no current is observed for the Ni(II/I) and Ni(I/0) couples.
The addition of cobaltocenium hexafluorophosphate to the solution
resulted in the observation of the Co(III/II) couple but no mediated
charge transfer for the reduction of nickel complexes confined to
the surface, as would be indicated by a ratio of cathodic to anodic
2.3. Electrocatalytic production of hydrogen
Although complex 3 does not remain electrochemically active
when attached to the electrode surface, 3 in acetonitrile solutions
does catalyze the reduction of protons from p-cyanoanilinium tet-
rafluoroborate at ꢀ0.83 V vs. the ferrocenium/ferrocene couple
(Fig. 5). This is indicated by the large increase of the current at
the potential of the Ni(II/I) couple of 3 in the presence of the acid
p-cyanoanilinium tetrafluoroborate. Two important parameters
for such a catalytic process are the overpotential and the catalytic
rate. The pKa of p-cyanoanilinium tetrafluoroborate in acetonitrile
is 7.0 [30], and using a potential of ꢀ0.14 V for the NHE vs. the fer-
rocenium/ferrocene couple in acetonitrile [31], the approximate
potential required for proton reduction with this acid is ꢀ0.55 V.
Fig. 3. CV experiments for 0.9 mM 3 (a) in benzonitrile with 0.2 M NBu4BF4 and
0.7 mM 2c (b) in acetonitrile with 0.2 M NEt4BF4. Both experiments have added
ferrocene as an internal reference and a scan rate of 0.1 V/s using a 1 mm diameter
glassy carbon working electrode.
Fig. 5. Cyclic voltammograms recorded on an acetonitrile solution containing
0.73 mM 3 and 0.2 M NEt4BF4 with added ferrocene as a function of acid (p-
cyanoanilinium tetrafluoroborate) concentration. The scan rate was 50 mV/s with a
glassy carbon working electrode.

D.H. Pool, D.L. DuBois / Journal of Organometallic Chemistry 694 (2009) 2858–2865
2861
Fig. 6. Plot of the ratio of the catalytic current (ic) to the peak current of the Ni(II/I)
couple in the absence of acid (ip) as a function of the concentration of p-cyano-
anilinium tetrafluoroborate at low acid concentrations. The solution was 0.66 mM 3
and 0.2 M NEt4BF4 in acetonitrile, the scan rate was 50 mV/s, and the working
electrode was glassy carbon.
Fig. 7. Plot of the ratio of the catalytic current (ic) to the peak current of the Ni(II/I)
couple in the absence of acid (ip) as a function of the concentration of p-cyano-
anilinium tetrafluoroborate. The solution was 0.73 mM 3 and 0.2 M NEt4BF4 in
acetonitrile, the scan rate was 50 mV/s, and the working electrode was glassy
carbon.
This gives an estimated overpotential of 0.28 V for the catalytic
reduction of the protons of p-cyanoanilinium tetrafluoroborate
using 3 as the catalyst. Information on the catalytic rate can be ob-
tained by plotting the ratio of the observed catalytic current (ic) to
the current of the Ni(II/I) reduction in the absence of acid (ip) vs.
the concentration of acid [9,11,32–34]. The straight line observed
at lower acid concentrations (Fig. 6) indicates the reaction is sec-
ond order in acid. A similar plot of ic verses the concentration of
3 gives a straight line indicating a first order dependence on cata-
lyst [32–34]. At acid concentration greater than approximately
0.2 M, the rate of reaction becomes independent of the acid con-
centration (Fig. 7).
A mechanism is shown in Scheme 2 that is consistent with the
observations described above and with previous studies of similar
complexes for both H2 production and oxidation [9,10,32]. In this
catalytic cycle, step 6 is the rate-determining step. Step 6 is the
reductive elimination of H2. There are two electron transfer steps
(1 and 3) and two protonation steps (2 and 4) that precede this
step. These two protonation steps will be governed by equilibria
that determine the fraction of the complex that is in the doubly
0
0
protonated Ni0 form, [Ni(P2RNR HNR )2]2+. As a result the rate of the
catalytic reaction is expected to obey Michaelis–Menton kinetics
with the rate expression shown in Eq. (1) where k4 and k may
ꢀ4
actually be a composite of forward and reverse rate constants for
step 2 as well. The equation for catalytic current, ic, for cyclic voltam-
metry is given by Eq. (2) where n is the number of electrons con-
sumed in the catalytic cycle, F is the Faraday constant, [Cat]T is the
total or initial concentration of the catalyst, D is the diffusion coeffi-
cient of the catalyst, and kobs is the observed rate constant for the
particular catalytic reaction under consideration [33–35]. The
derivation of this equation assumes that the concentration of the
substrate, BH+, is sufficiently large that it remains constant at the
initial value of the bulk solution. The value of n in Eq. (2) will be
Scheme 2.

2862
D.H. Pool, D.L. DuBois / Journal of Organometallic Chemistry 694 (2009) 2858–2865
2.0 because two electrons are consumed in the catalytic cycle (steps
1 and 3). Similarly, the expression for the peak current for a revers-
ible n0-electron reduction is given by Eq. (3). In this equation n0 will
be 1.0 because the wave at ꢀ0.8 V in the absence of acid corresponds
to a simple reversible one-electron process. Substitution of kobs from
Eq. (1) into Eq. (2) and dividing the result by Eq. (3) gives Eq. (4). At
low acid or protonated base (BH+) concentrations, Eq. (4) indicates a
linear dependence will be observed for ic/ip, as observed experimen-
tally. At these low concentrations the observed rate constant is a
composite of constants for several elementary steps. At high concen-
trations of BH+, Eq. (4) simplifies to Eq. (5) and the observed rate
constant corresponds to the simple unimolecular elimination of H2
for step 6. Using Eq. (5) a rate constant of 56 sꢀ1 can be obtained
for step 6 from the data in the acid independent region of the plot
shown in Fig. 7. This number represents the number of times a cat-
alyst molecule produces a molecule of H2 in a second, or the turn-
over frequency of this catalyst under these conditions. At
intermediate concentrations of BH+, the reaction order for this spe-
cies will change from second to zero order in acid (BH+) and the cat-
alytic current will change from first to zero order as observed in
Fig. 7.
trode surface thereby preventing electron transfer. However,
cobaltocenium reduction to cobaltocene is observed, and the cobal-
tocene produced in solution by electron transfer does not appear to
mediate nickel reduction as would be indicated by an enhanced
reduction current. Another possibility is that reduction of nickel is
not observed because the nickel complex is not able to undergo
structural rearrangements from square planar or trigonal bipyrami-
dal Ni(II) complexes [9] to tetrahedral Ni(I) and Ni(0) species that
typically occur upon reduction of Ni(II) complexes [36]. This may
be the result of multiple thiophene substituents per Ni(II) complex
being incorporated into the resulting polymer or simple physical
restriction of this motion by the backbone of the polymer. Finally,
because the oxidation of 3 is irreversible, the oxidation process itself
may also destroy the catalyst as it is polymerized. Whatever the rea-
son, the use of thiophene modified complexes does not result in elec-
trochemically active species confined to the electrode surface.
Although the thiophene groups of 3 did not work well as a
means to modify electrodes, it did demonstrate that adding sub-
stituents to the para position in the aniline portion of the ligand
system does not dramatically change the electrochemistry of the
nickel center in solution. When the cyclic voltammograms of 2c
and 3 in acetonitrile are compared, the values for the Ni(II/I) waves
are found to be very similar, with a small positive shift of 20 mV for
3 compared to 2c. The Ni(I/0) waves have an even smaller mea-
sured difference of +10 mV for 3 compared to 2c. As indicated in
Section 5, the pKa for the 4-(thiophen-3-yl)aniline was determined
by measuring the equilibrium constant for the reaction with
anilinium tetrafluoroborate. This equilibrium constant was used
to calculate a pKa value of 10.3 for 4-(thiophen-3-yl)anilinium tet-
rafluoroborate based on a pKa value of 10.62 for anilinium tetra-
fluoroborate [37]. This is fully consistent with the small positive
shifts in the Ni(II/I) and Ni(I/0) potentials upon replacing hydrogen
in 2c with a thiophene substituent in 3.
2
k6½BHþꢁ
rate ¼
½CatꢁT ¼ kobs½CatꢁT
ð1Þ
2
þ
k
ꢀ4þk6
þ ½BH ꢁ
k4
1=2
ic ¼ nFA½CatꢁT ðDkobs
Þ
ð2Þ
ð3Þ
1=2
1=2
ip ¼ 0:446n0FA½CatꢁT ðD
m
Þ
ðn0F=RTÞ
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
u
t
ꢀ
ꢁ
2
ic
2
RT
Fm
k6½BHþꢁ
¼
ð4Þ
ð5Þ
2
þ
k
ꢀ4þk6
ip 0:446
þ ½BH ꢁ
k4
Hydrogen production catalyzed by 3 using the weak acid
p-cyanoanilinium tetrafluoroborate was consistent with the
behavior reported for [Ni(PP2hNP2h)2NCMe]2+ (2c) [9,10]. The reaction
is first order in catalyst and second order in acid concentration at
lower concentrations. At high acid concentrations the rate of reac-
tion becomes acid independent, and the rate of this reaction corre-
sponds to the loss of H2 from the reduced form of the catalyst. The
rate of 56 sꢀ1 observed for 3 is somewhat less than the rates of
350 sꢀ1 observed for 2c [10] and 700 sꢀ1 reported for Ni–Fe hydrog-
rffiffiffiffiffiffiffiffiffiffi
ic
2
RTk6
¼
ip 0:4463
F
m
3. Discussion
Complex 3 was prepared for two reasons. First, only three deriv-
atives of complexes of type 2 have been prepared previously [9,11].
Although the complex [Ni(PP2hN2Ph)2]2+ (2c) is a good catalyst for H2
production with high catalytic rates (350 sꢀ1) and moderate overpo-
tentials (330 mV) [10,11], it is important to further determine how
changes in ligand substituents affect catalytic properties. Second,
the attachment of a thiophene substituent to the cyclic diphosphine
ligand could provide a convenient route for electrode modification
with these catalysts by electrochemically polymerizing the thio-
phene substituent of the catalyst with itself to form oligomers. Alter-
natively, a more extensive poly-thiophene chain might be formed by
copolymerization of 3 and thiophene. The inhibition of ferrocenium
reduction after complex 3 is oxidized at potentials positive of 0.6 V is
consistent with the formation of an insulating layer on the working
electrode. Because complex 2a also exhibits irreversible oxidations
at similar positive oxidation potentials, but does not impede charge
transfer between the working electrode and ferrocenium ions in
solution as observed for 3, it is reasonable to assume the thiophene
substituents are involved in the polymerization. While polymeriza-
tion or oligomerization appears to confine the coordinated nickel
species to the surface of the electrode, efforts to observe any redox
activity of the confined species at potentials close to those of the
Ni(II/I) or Ni(I/0) couples of the complex in solution failed, including
the use of thiophene copolymerization and mediation with
CoCp2BF4. One possible explanation for this behavior is that poly-
merization restricts the diffusion of the nickel catalyst to the elec-
enases [2]. The driving force for this reaction depends on the hydride
0
donor ability of the nickel hydride [HNi(PR2N2R )2]+ and the pKa of the
0
0
protonated N–Ar group in the H2 adduct [Ni0(P2RNR HNR )2]2+ [11].
Based on a comparison of the potentials of the Ni(II/I) couples of
2c and 3, the corresponding hydride of complex 3, [HNi(PP2hN2Ar)2]+,
should be a slightly weaker hydride donor than [HNi(PP2hN2Ph)2]+
(0.02 V ꢂ 19.2 kcal/V mol = 0.38 kcal/mol) [11]. Similarly, the
slightly lower pKa value of 4-(thiophen-3-yl)anilinium tetrafluorobo-
rate compared to anilinium tetrafluoroborate would suggest a
slightly more acidic NH proton for catalyst 3 compared to 2c
(2.303RT[10.62–10.34]) = 0.38 kcal/mol). As a result catalyst 3 would
be expected to be slightly less hydridic (by 0.38 kcal/mol) and
slightly more acidic than 2c (by 0.38 kcal/mol). Because these two
effects (hydride donor abilities and acidities) are in opposite direc-
tions, similar driving forces for H2 loss are expected for 2c and 3. It
should also be noted that 3 has a slightly lower overpotential of
280 mV compared to 330 mV required for 2c that should favor a
slightly higher rate for H2 production by 2c.
4. Summary
A new thiophene substituted catalyst of the type [Ni(PP2hN2Ar)2]2+
(where Ar = 4-(thiophen-3-yl)phenyl) has been prepared and

D.H. Pool, D.L. DuBois / Journal of Organometallic Chemistry 694 (2009) 2858–2865
2863
characterized. Although the thiophene substituted complex 3 has
not resulted in electrochemically active modified electrodes in our
hands, this complex is still a very active catalyst for H2 production
in acidic acetonitrile solutions with an overpotential of 280 mV
and a turnover frequency of 56 sꢀ1. This combination of high activity
and modest overpotentials confirms that the high activity observed
is a general feature of this class of complexes. High activities for elec-
trocatalytic H2 production or oxidation can be expected for similar
complexes with a variety of substituents on both phosphorus and
nitrogen.
spectra. 31P{1H} MAS NMR (96 MHz, 25 °C) d ꢀ37.3. 31P{1H} NMR
(DMSO-d6, 202.3 MHz, 25 °C) d ꢀ49.3 (main resonance), ꢀ35.2.
5.3. Preparation of [Ni(PP2hN2Ar)2NCMe][BF4]2 (3)
Solid PP2hN2Ar (0.232 g, 0.375 mmol) and [Ni(NCMe)6][BF4]2
(0.091 g, 0.18 mmol) were combined with acetonitrile (15 mL) and
stirred for 4 days to give a red solution with a small amount of sus-
pended solid material that was removed by filtration through celite.
The solvent was removed under vacuum and the residue was dis-
solved in 5 mL of acetonitrile. This solution was placed in a container
within a larger container with ethyl ether and sealed for slow diffu-
sion giving 0.203 g (0.134 mmol, 74%) of red crystals. A second crop
grown the same way from the remaining solution gave 0.018 g
(0.011 mmol, 6%) of red crystals. Anal. Calc. for C74H67B2F8N5NiP4S4:
C, 58.83; H, 4.47; N, 4.64. Found: C, 58.49; H, 4.31; N, 5.08%. 1H NMR
(CD3CN, 500 MHz, 25 °C) d 7.69 (d, J = 9 Hz, 8H, C-H aniline Ph), 7.58
(dd, J = 3.1 Hz, 4H, thiophene), 7.50 (dd, J = 5.3 Hz, 4H, thiophene),
7.46 (dd J = 5.1 Hz, 4H, thiophene), 7.43 (t, J = 8 Hz, 4H, C-H P-Ph),
7.36 (m, 8 H, C-H P-Ph), 7.27 (d, J = 9 Hz, 8H, C-H aniline Ph), 7.21
(t, J = 8 Hz, 8H, C-H P-Ph), 4.29 (d, J = 14 Hz, 8H, –CH2–), 3.98 (d,
J = 14 Hz, 8H, –CH2–). 31P{1H} NMR (CD3CN, 202.3 MHz, 25 °C) d 5.00.
5. Experimental
5.1. General methods
Acetonitrile from Alfa Aesar and ethyl ether from Honeywell
Burdick & Jackson were dried with a column of alumina and dis-
pensed under N2 from an Innovative Technology Pure Solv system.
Benzonitrile and DMSO-d6 from Aldrich were freeze pump thawed
and stored in a glove box. CD3CN from Cambridge Isotope Labs was
freeze pump thawed, dried with molecular sieves and stored in a
glove box. PhP(CH2OH)2 was prepared by reacting PhPH2 from
Strem Chemicals with 2 equiv. of para-H2CO from Aldrich. [Ni(NC-
Me)6][BF4]2 [38] and [Ni(PP2hN2Ph)2][BF4]2 [9] were prepared using lit-
erature methods. 4-(Thiophen-3-yl)aniline and thiophene were
obtained from Aldrich and used without further purification. Synthe-
sis and manipulation of compounds were performed using standard
Schlenk techniques or were done in a glove box.
Solution NMR spectra were recorded on a Varian Inova spec-
trometer (500 MHz for 1H). The 1H chemical shifts were internally
calibrated to the CD2HCN impurity of the deuterated solvent to the
value 1.93 ppm. Solid state 31P{1H} magic angle spinning (MAS)
NMR spectra were collected on a Varian VXR-300 operating at
121.4 MHz using a recycle delay of 200 s and a spin rate of
6150 Hz in a zirconia rotor. All 31P{1H} NMR spectra were exter-
nally referenced to phosphoric acid. Elemental analysis was done
by Columbia Analytical Services using V2O5 as a combustion cata-
lyst. The cyclic voltammetry experiments were conducted using a
computer aided CHI 1100A potentiostat. The working electrode
was a 1 mm glassy carbon PEEK coated electrode from Cypress Sys-
tems and was cleaned between scans unless otherwise noted using
Gamal from Fisher Scientific on a Buehler microcloth PSA and
5.4. Proton reduction
5.4.1. Order with respect to acid
A solution of 3 (0.66 mM) and NEt4BF4 (0.2 M) in acetonitrile
(10.0 mL) was prepared inside a glove box. The volumetric flask
was fit with a septum, and 2.0 mL of the solution was syringed into
an N2 purged cell with reference, counter, and working electrodes.
The cathodic current for the Ni(II/I) couple was measured to deter-
mine ip. Aliquots of a 0.4 M p-cyanoanilinium tetrafluoroborate
solution in acetonitrile were added and the current at ꢀ0.938 V
was measured to determine ic. The value for ip was adjusted to
compensate for dilution with the addition of the acid solution
assuming it is directly proportional to [3] in solution. The ratio of
ic/ip was plotted against the acid concentration present at each
measurement to get a linear relationship indicating the rate of
reaction is second order in acid (Fig. 6).
5.4.2. Acid Independent rate
The procedure described in the preceding paragraph was used
starting with a 0.73 mM solution of 3 in acetonitrile containing
0.2 M NEt4BF4 (1.0 mL) and adding increasingly larger amounts of
a 0.41 M solution of p-cyanoanilinium tetrafluoroborate in acetoni-
trile until a total added volume of 2.9 mL was reached. The value of
ic was measured at ꢀ0.946 V after each addition. A plot of ic/ip vs.
[acid] gives a curve that flattens at an ic/ip ratio of approximately
24 (see Fig. 7). Using equation 1 and a scan rate of 50 mV/s, a value
of 56 sꢀ1 was calculated for k.
rinsed with 18 MX water. A 3 mm diameter glassy carbon rod from
Alfa Aesar was used as an auxiliary electrode. A silver wire pseudo-
reference electrode was used with ferrocene as an internal stan-
dard. Electrochemistry was done under a nitrogen atmosphere in
acetonitrile with NEt4BF4 as the electrolyte except for the scan rate
current dependence check for 3 which was done in benzonitrile
using NBu4BF4 as the electrolyte. The nitrogen gas stream was bub-
bled through acetonitrile to saturate the gas stream for experi-
ments using acetonitrile solutions to minimize solvent loss.
5.4.3. Order with respect to [3]
A stock solution containing 22 mM p-cyanoanilinium tetrafluo-
roborate and 0.2 M in NEt4BF4 in acetonitrile was prepared. A sec-
ond solution was prepared from this stock solution to produce a
solution that was 2.0 mM in 3. Aliquots of the second solution were
added to 1.0 mL of the first stock solution maintaining constant
acid and supporting electrolyte concentrations while varying the
concentration of 3. The current ic was measured at ꢀ0.926 V. A plot
of ic vs. [3] gives a straight line indicating the rate of reaction is first
order in 3.
5.2. Preparation of PP2hNA2r
A solution of 4-(thiophen-3-yl)aniline (0.229 g, 1.31 mmol) in
acetonitrile (10 mL) was cannula transferred to a Schlenk flask con-
taining a solution of PhP(CH2OH)2 (0.222 g, 1.30 mmol) in acetoni-
trile (10 mL) and the mixture was heated to 73 °C for 3 h. A white
precipitate started forming after 1 h. The reaction mixture was stir-
red overnight before removing the solvent under vacuum. Inside a
glove box, the white residue was collected on a frit using 20 mL of
MeCN to rinse it from the flask and 20 mL of MeCN to wash the so-
lid. The solid was dried under vacuum (0.354 g, 0.572 mmol, 88%).
This material was not significantly soluble in any common solvent,
but it was sufficiently soluble in DMSO to obtain 31P{1H} NMR
5.5. Determination of pKa of 4-(thiophen-3-yl)aniline
A
mixture of 0.014 g (0.08 mmol) 4-(thiophen-3-yl)aniline
(ArNH2) and 0.011 g (0.061 mmol) anilinium tetrafluoroborate

2864
D.H. Pool, D.L. DuBois / Journal of Organometallic Chemistry 694 (2009) 2858–2865
was dissolved in 1 mL of CD3CN and transferred to a NMR tube. The
1H NMR spectrum of the sample has aromatic peaks at 7.54 and
ment was repeated at 1 V/s for the initial scan and 0.1 V/s for the
follow up cyclic voltammogram between ꢀ0.4 and ꢀ1.4 V. In this
case, waves corresponding to the Ni(II/I) and Ni(I/0) couples of 3
were observed, but they were broadened and the current was
diminished from the initial scan.
+
6.95 ppm assigned to ArNH2/ArNH3 and resonances at 7.32 and
+
7.04 ppm assigned to PhNH2/PhNH3
. A mixture of 0.003 g
(0.02 mmol) ArNH2 and 0.048 g (0.15 mmol) 2,6-dichloroanilinium
triflate, which has a pKa of 5.06 [36], was dissolved in 1 mL of
CD3CN, and the 1H NMR spectrum was acquired to determine
chemical shifts of 7.78 and 7.45 ppm for the aromatic resonances
of ArNH3+. Using these chemical shifts and the observed chemical
shifts of 7.39 and 6.66 ppm for ArNH2, the mole fraction of unprot-
onated ArNH2 in the equilibrium mixture was determined to be
0.61 and 0.63 (0.62 average) using each of the aromatic phenyl
chemical shifts. The same procedure was used for aromatic ortho
and meta chemical shift values of the PhNH2 located at 7.32 and
7.04 ppm for the equilibrium mixture, 7.53 and 7.39 ppm for
PhNH3+, and 7.08 and 6.63 ppm for PhNH2 to get a mole fraction
for PhNH2 of 0.47 and 0.46 (0.46 average). From this data, the equi-
librium constant Keq = [PhNH2][ArNH3+]/[PhNH3+][ArNH2] was
found to be 0.52. The experiment was repeated twice more using
0.003 g (0.02 mmol) of ArNH2 and 0.003 g (0.02 mmol) PhNH3BF4
as well as 0.010 g (0.057 mmol) of ArNH2 and 0.007 g (0.04 mmol)
of PhNH3BF4 to get Keq values of 0.52 and 0.53, respectively. The
5.8. Electrode transfer experiments
In this experiment two electrochemical cells were used. One
cell contained a solution of 3 as described in the preceding para-
graph. A second cell contained a 0.1 M solution of NEt4BF4 in ace-
tonitrile, but no 3. After various coating experiments described in
the preceding paragraph using solutions containing 3, the working
electrode was placed in the second cell containing a solution with
the electrolyte, but no dissolved 3, and scanned to ꢀ1.4 V. No ob-
servable current was observed for Ni(II/I) or Ni(I/0) couples above
the baseline current of the cyclic voltammogram.
5.9. Copolymerization with thiophene
A cyclic voltammogram was recorded on an acetonitrile solu-
tion (3.0 mL) containing 0.6 mM 3 and 0.1 M in NEt4BF4. The initial
potential was ꢀ0.41 V, and the potential was swept to ꢀ1.41 V,
+
+
pKa of ArNH3 is the sum of the pKa of PhNH3 and log Keq. Using
+
the pKa value of PhNH3 to be 10.62 [37] and the average Keq of
0.52, the pKa of ArNH3 is 10.34.
then to 1.59 V, and back to ꢀ0.41 V at 0.1 V/s. Thiophene (5
lL,
+
0.062 mmol, 20 mM) was added to the cell and mixed by bubbling
with N2 gas. Another cyclic voltammogram was recorded at 0.1 V/s
over the same potential range. Upon sweeping in the positive
direction, the onset of oxidative current was observed at 0.6 V sim-
ilar to the cyclic voltammograms recorded in the absence of thio-
phene. However, at 1.2 V the very large oxidation currents were
observed. On the return negative scan to ꢀ0.41 V, a peak at
0.54 V was observed. Scanning the same range again without
cleaning the working electrode showed no reduction current for
Ni(II/I) and Ni(I/0) couples, but increasing anodic current between
0.1 and 1.59 V and cathodic current between 1.23 and ꢀ0.41 V.
This process was repeated 9 more times sequentially with similar
effect (Fig. 4).
A similar experiment was performed using 3 mL acetonitrile
solutions containing 0.9 mM 3, 0.1 M NEt4BF4, and 40 mM thio-
phene. The coating of the electrode was done with a single scan
in the range outlined above at 0.1, 0.2, 0.5, and 1 V/s. The coated
working electrode was then placed in a CV cell containing a
0.1 M NEt4BF4 MeCN solution and a scan performed over the same
range as above. Polythiophene was observed at potentials positive
of 0.2 V but no Ni reduction peaks were observed.
5.6. Electrochemical oxidation of 3
The potential of a glassy carbon electrode in a benzonitrile solu-
tion containing 0.93 mM 3, 0.2 M NBu4BF4, and ꢃ1 mM ferrocene
was scanned from ꢀ0.5 V to 0.2 V at 0.1 V/s and a reversible Fc+/
Fc wave was observed. Without cleaning the electrode, a second
cyclic voltammogram was recorded in which the potential was
scanned from ꢀ0.5 V to 1.7 V and back to ꢀ0.5 V at 0.1 V/s. The
anodic wave for ferrocene oxidation was the same for both scans
and was used to reference the voltammogram.
For investigation of potential required to coat the working
electrode as a result of oxidation of 3, cyclic voltammograms
were recorded on an acetonitrile solution containing 1.0 mM 3
and 0.1 M NEt4BF4 in which the potentials were initially swept
in the negative direction to observe the Ni(II/I) and Ni(I/0) cou-
ples and then reversed and scanned to increasingly more positive
potentials using a scan rate of 0.1 V/s. A second scan was then re-
corded to observe any change in current from the initial cycle.
Sequential cyclic voltammograms scanned to a maximum of
0.6 V in the positive direction had similar peak currents for ferro-
cene and Ni(II/I) and Ni(I/0) waves with no change in potentials.
However, a similar cyclic voltammogram scanned initially to
0.7 V resulted in decreased and broadened waves for the Ni(II/I)
and Ni(I/0) couples for subsequent scans compared to those seen
before the cycling to 0.7 V. Cycling to 0.8 V resulted in significant
changes for the Ni reduction peaks observed in subsequent cyclic
voltammograms. Cycling to 1.1 V at 0.1 V/s resulted in no ob-
served current for the Ni(II/I) and Ni(I/0) couples in subsequent
cyclic voltammograms.
5.10. Attempted mediation with CoCp2PF6
A cell containing a solution of 0.7 mM 3 and 0.2 M NEt4BF4 in
MeCN was used for coating the working electrode with a single
scan to 1.8 V at 0.1 V/s before placing the working electrode in a
second cell containing 0.2 M NEt4BF4 and 7 mM CoCpPF6 in aceto-
nitrile. A cyclic voltammogram was recorded from ꢀ0.2 V to ꢀ1.2 V
and back to ꢀ0.2 V. Only the reversible couple for Co(III)/Co(II) was
observed with no enhanced cathodic current compared to the ano-
dic current.
5.7. Scan rate dependence
5.11. Cyclic voltammogram of 2c
An acetonitrile solution containing 0.5 mM 3 and 0.1 M NEt4BF4
was used in series of cyclic voltammograms with an initial poten-
tial of ꢀ0.4 V, sweeping to ꢀ1.4 V, then to 1.6 V, and finally back to
ꢀ0.4 V at 0.1 V/s. A subsequent cyclic voltammogram from ꢀ0.4 V
to ꢀ1.4 V and back to ꢀ0.4 V had no observable current for previ-
ously observed Ni(II/I) and Ni(I/0) couples. After cleaning the work-
ing electrode the experiment was repeated at 0.2 and 0.5 V/s with
the same result. After cleaning the working electrode the experi-
An acetonitrile solution containing 0.68 mM 2c, 0.2 M NEt4BF4,
and ferrocene was scanned from ꢀ0.22 V to ꢀ1.22 V, back to
1.78 V, and finally to ꢀ0.22 V at 0.1 V/s. The Ni(II/I) and Ni(I/0)
waves matched those previously reported. The ferrocenium/ferro-
cene couple was reversible and irreversible oxidations of 2c were
observed beginning at 0.6 V.

D.H. Pool, D.L. DuBois / Journal of Organometallic Chemistry 694 (2009) 2858–2865
2865
[15] C.J. Curtis, A. Miedaner, J.W. Raebiger, D.L. DuBois, Organometallics 23 (2004)
511–516.
Acknowledgements
[16] D.E. Berning, A. Miedaner, C.J. Curtis, B.C. Noll, M.C. Rakowski DuBois, D.L.
DuBois, Organometallics 20 (2001) 1832–1839.
[17] M.R. Nimlos, C.H. Chang, C.J. Curtis, A. Miedaner, H.M. Pilath, D.L. DuBois,
Organometallics 27 (2008) 2715–2722.
[18] G.M. Jacobsen, R.K. Shoemaker, M. Rakowski DuBois, D.L. DuBois,
Organometallics 26 (2007) 4964–4971.
This research was supported by the Laboratory Directed Re-
search and Development Program of the Pacific Northwest Na-
tional Laboratory. The Pacific Northwest National Laboratory is
operated by Batelle for the US Department of Energy.
[19] A.D. Wilson, K. Fraze, B. Twamley, S.M. Miller, D.L. DuBois, M. Rakowski
DuBois, J. Am. Chem. Soc. 130 (2008) 1061–1068.
[20] R.W. Murray (Ed.), Molecular Design of Electrode Surfaces; Techniques of
Chemistry, vol. XXII, John Wiley & Sons, New York, 1992.
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