Investigation of Immobilization Effects on Ni(P2N2)2Electrocatalysts

Article abstract of DOI:10.1021/acs.inorgchem.0c01669

A new synthetic route to complexes of the type Ni(P2N2)22+ with highly functionalized phosphine substituents and the investigation of immobilization effects on these catalysts is reported. Ni(P2N2)22+ complexes have been extensively studied as homogeneous and surface-attached molecular electrocatalysts for the hydrogen evolution reaction (HER). A synthesis based on postsynthetic modification of PArBr2NPh2 was developed and is described here. Phosphonate-modified ligands and their corresponding nickel complexes were isolated and characterized. Subsequent deprotection of the phosphonic ester derivatives provided the first Ni(P2N2)22+ catalyst that can be covalently attached via pendent phosphonate groups to an electrode without involvement of the important pendent amine groups. Mesoporous TiO2 electrodes were surface modified by attachment of the new phosphonate functionalized Ni(P2N2)22+ complexes, and these provided electrocatalytic materials that proved to be competent and stable for sustained HER in aqueous solution at mild pH and low overpotential. We directly compared the new ligand to a previously reported complex that utilized the amine moiety for surface attachment. Using HER as the benchmark reaction, the P-attached catalyst showed a marginally (9-14%) higher turnover number than its N-attached counterpart.

Full text of DOI:10.1021/acs.inorgchem.0c01669

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Investigation of Immobilization Effects on Ni(P2N2)2 Electrocatalysts
Felix M. Brunner, Michael L. Neville, and Clifford P. Kubiak*
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ABSTRACT: A new synthetic route to complexes of the type Ni(P2N2)2
with highly functionalized phosphine substituents and the investigation of
immobilization effects on these catalysts is reported. Ni(P2N2)22+ complexes
have been extensively studied as homogeneous and surface-attached
molecular electrocatalysts for the hydrogen evolution reaction (HER). A
synthesis based on postsynthetic modification of PArBr2NPh was developed
2
and is described here. Phosphonate-modified ligands and their correspond-
ing nickel complexes were isolated and characterized. Subsequent
deprotection of the phosphonic ester derivatives provided the first
2+
Ni(P2N2)2 catalyst that can be covalently attached via pendent
phosphonate groups to an electrode without involvement of the important
pendent amine groups. Mesoporous TiO2 electrodes were surface modified
2+
by attachment of the new phosphonate functionalized Ni(P2N2)2
complexes, and these provided electrocatalytic materials that proved to be
competent and stable for sustained HER in aqueous solution at mild pH and
low overpotential. We directly compared the new ligand to a previously
reported complex that utilized the amine moiety for surface attachment.
Using HER as the benchmark reaction, the P-attached catalyst showed a marginally (9−14%) higher turnover number than its N-
attached counterpart.
availability and high reactivity of primary phosphines, a key
component in the P2N2 ligand synthesis. Developing
alternative attachment strategies and directly comparing them
in a side-by-side study gives clearer insight into how surface
attachment affects these catalysts. Herein, we describe the first
synthesis of a P2N2 complex modified with a surface attachable
functional group on the phosphine moiety (6; Figure 1).
Mesoporous TiO2 electrodes modified with 6 are functional
electrode materials for HER in aqueous solution. Direct
comparison of the phosphine anchored catalyst 6 to the
previously reported complex 715 anchored through the
nitrogen moiety allows us to directly observe the differences
caused by changing the surface attachment substituent to the
phosphine.
INTRODUCTION
■
The P2N2 (1,5-diaza-3,7-diphosphacyclooctane) class of
ligands has been established as a very versatile platform and
used with various metals to catalyze a wide variety of different
reactions including the oxidations of amines, alcohols, and
formate as well as the hydrogen evolution reaction (HER) and
the reduction of CO2.1−4 In particular, Ni(P2N2)2 complexes
have been widely studied and proven to be exceptional
electrocatalysts for HER and H2 oxidation.5 To implement
these catalysts in practical electrochemical devices, efficient
ways to immobilize them on electrodes while preserving the
catalysts’ unique properties must be developed. In the past
decade, various strategies for the immobilization of Ni(P2N2)2
catalysts onto a variety of support materials ranging from
carbon nanotubes to silicon to metal oxide semiconductors
were developed.6−12 All of the surface-immobilized Ni(P2N2)2
catalysts examined to date have in common the attachment
through the amine moieties of the P2N2 ligand. This is not
optimal in view of mechanistic studies, which have
demonstrated the importance of free mobility of the amine
groups as “pendent base” proton shuttles in catalysis.
Presumably, pinning the pendent base to the surface could
adversely affect catalysis.13,14 An alternative attachment
utilizing the phosphine moiety would be advantageous from
this perspective. However, this route was previously not
pursued, likely due to synthetic challenges arising from the low
RESULTS AND DISCUSSION
■
Retrosynthetic Analysis of the Catalyst. Of the many
examples of surface-attached P2N2 ligands, the most impactful
utilize either amide coupling to modified carbon6 and gold9
Received: June 5, 2020
Published: November 16, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01669
© 2020 American Chemical Society
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were expected in the reaction to prepare the primary
phosphine.18 The greater versatility of aryl bromides in
subsequent modification steps make them the better choice
over chlorine. 4-Bromophenylphosphine (2) is a stable
molecule and can be converted into the corresponding P2N2
(3; Scheme 1). This makes 3 an ideal intermediate in the
synthesis of more functionalized P2N2 ligands by postsynthetic
functionalization (Scheme 1). This bromine substituted P2N2
3 is converted into the phosphonate ester (4; Scheme 1) or
other functional groups utilizing lithium chemistry. The
functionalized ligand can then be coordinated to the Ni center
and deprotected following known procedures.16
Synthesis and Characterization of [INi(PArPO3H22NPh2)2]
[I] • HI. Following the guidance of the retrosynthetic analysis
shown in Scheme 1, we developed a synthetic route to the
desired complex. Starting from readily available 1,4-dibromo-
benzene, the primary phosphine 4-bromophenylphosphine (2)
can be obtained in two steps: introduction of a phosphorus
group and subsequent reduction. For the first step, a previous
synthesis of diethyl (4-bromophenyl)phosphonate (1) was
modified to allow for easy scale-up.19 Selective halogen lithium
exchange of one bromine was followed by electrophilic
substitution with diethoxychlorophosphonate yielding 1. The
resulting phosphonate ester was then reduced with LiAlH4 to
the corresponding primary phosphine 2. Following an
established procedure20 for P2N2 synthesis, 2 was reacted
with formaldehyde and aniline to yield the P-bromine
Figure 1. Schematic representation of the two attachment methods of
Ni(P2N2)2 to a mesoporous TiO2 electrode. Attachment through a
modified amine moiety 7 (left) and through a modified phosphine
moiety 6 (right). All R-substituents are benzylphosphonic acid (7) or
phenylphosphonic acids (6) and are omitted for clarity.
electrodes or phosphonate groups for attachment to metal
oxide semiconductors.16 Our method for P-modified P2N2
ligands was designed to allow synthetic introduction of both
carboxylate and phosphonate substituents for surface attach-
ment. However, due to the harsh conditions necessary to
synthesize primary phosphines, it is not possible to directly
synthesize primary phosphines with either carboxylic or
phosphonic acid substituents. Therefore, an alternative strategy
was developed based on postsynthetic modification of the
ligands. Since the goal was to introduce substituents in the
para-position of a phenyl phosphine, we were limited to
reactions that can modify aromatics but are compatible with
phosphines. This excludes all oxidative reactions or any of
those that involve “phosphineophiles”. The most versatile
approach to directly modify an aromatic ring is through cross
coupling or organolithium reactions, which can be used to
introduce a variety of functional groups. A second requirement
for the modification is that the precursor group is inert to the
harsh reducing conditions necessary to make the primary
phosphine while also being unreactive toward that phosphine.
Halogens represent a class of functional groups that are
commonly used as precursors for further functionalization.
Literature precedent suggests that aryl halides are not reactive
toward phenyl phosphine.17 In the presence of LiAlH4, both
chlorine and fluorine aryl substituents are inert, while phenyl
iodide is readily dehalogenated. Aryl bromides were found to
react more slowly, and only minor degrees of dehalogenation
substituted ligand PArBr2NPh (3) (Scheme 2).
2
The bromine functionalized P2N2 presents a useful
intermediate for further functionalization of the phosphine
substituent through postsynthetic modification. Of the possible
reactions, lithium chemistry was identified as the cleanest
method to exchange the bromine for a phosphonate group
(Scheme 2). Attempts to cross couple the bromide derivative
in a Suzuki type reaction to further functionalize the ligand
were limited by its strong coordination affinity toward
transition metals commonly used as catalysts in such reactions.
Lithiation chemistry on the other hand is orthogonal to the
phosphine and nitrogen functionality of the ligand.
The halogen lithium exchange reaction is almost instanta-
neous and goes to completion. However, the resulting lithiated
intermediate is highly reactive, which can lead to deprotona-
tion of other parts of the ligand (see the Supporting
Information (SI) for details). These side reactions were
mitigated by carrying out the reaction at low temperatures
(−108 °C) and adding the electrophile diethyl chlorophos-
Scheme 1. Retrosynthetic Pathway to a Ni(P2N2)2 Complex Modified with a Surface Attachable Functional Group
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Scheme 2. Synthesis of [INi(PArPO3H22NPh2)2] [I] • HI
phate quickly after forming the lithium intermediate. Using this
method, we were able to isolate the phosphonate substituted
−0.73 V (Ni2+/+) and −0.89 V (Ni+/0) vs Fc+/0 and are fully
reversible. This represents one of the least negative reduction
potentials for the catalytically important second reduction
reported to date. For comparison, [Ni(PPh2NPh2)2]2+ requires a
potential over 100 mV more negative to reach the Ni0 state.
This positive shift in the CV of 5 is attributed to the strongly
electron withdrawing nature of the phosphonate ester groups.
Similar effects are observed for the complex with electron
withdrawing trifluoromethyl phenyl substituted ligands.23
To obtain a surface attachable catalyst, the phosphonate
groups on 5 must be deprotected. We followed a modified
procedure using TMS-I (trimethylsilyl iodide) to obtain the
surface attachable catalyst 6. Interestingly, this treatment also
P2N2 ligand PArPO(OEt2)2NPh (4) in 76% yield.
2
Ligand 4 was reacted with [Ni(MeCN)6] [BF4]2 in
acetonitrile yielding the corresponding Ni(P2N2)2 complexes
5. The complex was structurally and spectroscopically
characterized. The single crystal X-ray structure shows
approximately trigonal bipyramidal geometries with the four
phosphines and one MeCN solvent molecule coordinated to
the nickel center similar to previously published Ni(P2N2)2
complexes (Figure 2).5,21,22
The cyclic voltammogram (CV) of 5 in acetonitrile shows
two one-electron processes, characteristic for complexes of the
type [Ni(P2N2)2]2+ (Figure S6). The reductions occur at
−
leads to degradation of the BF4 counterion and liberation of
the iodide that coordinates to the nickel center to form a stable
five-coordinate complex (see SI for details). While not
explicitly discussed, the same process is evident in previous
reports with similar systems.10,16 After the removal of excess
TMS-I, hydrolysis in MeOH, and precipitation with diethyl
ether, 6 is isolated with a total of three iodides. Upon isolation,
the complex is insoluble in common organic solvents, which
limits the possibility of solution state studies, but the same low
solubility makes it ideal as a heterogenized catalyst. It is slightly
soluble in methanol acidified with excess triflic acid. In DMSO
and basic H2O, it will dissolve with slow degradation.
However, if deprotonated in basic MeOH solution, an orange
solution of 6′ is formed. This color is consistent with a four-
coordinate complex, indicating halogen dissociation due to the
strong donating nature of the ligand in this deprotonated state
(Scheme 2). These solutions are slightly air sensitive but are
stable under an inert atmosphere and can be stored in a
nitrogen filled glovebox for months without degradation.
Figure 2. X-ray crystal structure of the dication of 5. Only ipso
carbons of the nitrogen-bound phenyl rings are shown; counterions,
and solvent are omitted for clarity.
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Scheme 3. Schematic Representation of Surface Modification of Mesoporous TiO2 Electrodes with 6′ and Subsequent
a
Protonation of the Attached Catalyst
a
All R-substituents are phenylphosphonic acids and are omitted for clarity.
Figure 3. XPS spectra of mesoporous TiO2/6 electrode after protonation with HDMF • TfO, as used in all electrochemical experiments. XPS peak
positions (in eV): Ni (872.4, 854.9), F (688.5), N (399.8), S (165.9, 164.8), P (133.4, 132.6).
These solutions were used for NMR characterization of the
complex and for its further utilization for surface modification.
Surface Modification and Characterization. As a
support to heterogenize the catalyst, we chose mesoporous
TiO2. This material is well studied and commonly utilized to
surface immobilize electrocatalysts in electro- and photo-
catalytic systems.10,12,24 Furthermore, there is comprehensive
data12 available for the attachment of P2N2 catalysts to this
material, making it an excellent material to benchmark our new
attachment strategy versus previously reported attachment
through the amine moiety (7; Figure 1).12 The electrodes were
modified by soaking in methanolic solutions of the desired
catalyst. Electrodes modified with 6′ were found to be
susceptible to degradation in air. To convert the catalyst
back to its air stable fully protonated form, the electrodes were
protonated with dimethylformamidium trifluoromethanesulfo-
nate (HDMF • TfO). Both steps result in a color change of
the electrode, indicative of the attachment and subsequent
modification of the catalyst (Scheme 3). While the
deprotonation and protonation step were not necessary for
7, they were still included in the procedure for consistency. At
this point, the electrodes were taken out of the glovebox for
further study.
The catalyst loading was determined by desorption in 0.1 M
NaOH and recording the UV−vis spectra of the of solutions
obtained following a previously reported procedure (Figure
S7).12 The loading before catalysis was determined to be 63
1 nmol/cm2 (6) and 60 1 nmol/cm2 (7), respectively, which
is in agreement with previously reported data for similar
systems.12 This same procedure was followed to determine the
surface loading of the electrodes after the controlled potential
electrolysis (CPE) experiments. A catalyst retention of ∼70%
was observed for both systems supporting strong binding of
the catalyst to the electrode. While the nature of electrodes
modified with 7 is discussed in detail elsewhere,12 we further
confirmed the molecular integrity and surface binding of 6 with
XPS and FTIR spectroscopy. XPS measurements of the
electrodes were carried out using the air stable protonated
form that was used in all electrochemical experiments. The
survey scan (Figure 3) confirms the presence of Ni, F, O, Ti,
N, C, S, and P, which is in agreement with the composition of
the catalyst and support material. Two sharp peaks were
observed at 872.4 and 854.9 eV, respectively, characteristic for
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Figure 4. All experiments conducted in aqueous 0.5 M Na citrate buffer at pH 3. (a) CVs of TiO2/6 (blue), TiO2/7 (red), and TiO2 (black) at 25
mV/s. (b) Kinetic trace of TiO2/6 (blue), TiO2/7 (red), and TiO2 (black) electrodes’ UV−vis absorption at 800 nm. The blue area indicates the
duration of applied bias (E = −0.3 V vs RHE, 20 s). (c) SCP of TiO2/6 (blue) TiO2/7 (red), and TiO2 (black); 10 mV steps at 5 s hold time with
the end current of each step recorded. The data represents the average from three experiments for modified electrodes. The data for TiO2
represents a single experiment. (d) CPE (E = −0.3 V vs RHE) of TiO2/6 (blue), TiO2/7 (red), and TiO2 (black). Lines represent the average, and
light areas represent the standard deviation from three experiments for modified electrodes. The data for TiO2 represents a single experiment.
a NiII phosphine complex confirming that the complex is intact
on the electrode. Furthermore, a peak at 132.9 eV confirms the
presence of the phosphine and phosphonate groups of the
ligand. The presence of F and S signals are in agreement with
the presence of triflate ions from the treatment with HDMF •
TfO.
version, we used HER in aqueous solution as the benchmark
reaction. Screening of multiple electrolytes in aqueous solution
showed that the nature of the electrolyte, its concentration as
well as buffer capacity, has a major effect on catalysis. Figure
S10 shows CPE performed with TiO2/6 in different electro-
lytes. We found that a high electrolyte concentration with a
large buffer capacity at low pH produced the greatest current
density. One exception to this trend was 0.5 M HCl. With a
pH of only 0.3, a high activity would be expected. Instead, it is
comparable to pH 2 buffer systems. In agreement with
literature reports,3,23 we attribute the reduced activity to the
strong binding of chloride to the nickel center of the catalyst,
suppressing catalysis. To maximize the influence from the
pendent amine on catalysis, the reaction should be carried out
at a pH close to the pKa of the pendent amine. At higher pH,
the amine will not be protonated sufficiently and therefore not
contribute significantly to the catalytic activity. If the pH is
much lower, direct protonation of the Ni by solution species
will likely outcompete proton transfer through the amine,
bypassing the “proton shuttle”. The amines of complex 7 were
previously reported to have a pKa ≈ 3.27 We obtained the
same value for the amines of 6 by acid base titration of the
complex (see the SI for details). A system with excellent buffer
Further evidence for the retention of molecular structure
and covalent attachment of 6 to the TiO2 was gained from
attenuated total reflectance Fourier transform infrared (ATR-
FTIR) spectroscopy. Figure S8 shows a comparison of FTIR
spectra of powdered 5 and 6 and the surface-attached catalyst.
All three spectra show strong peaks around 1500 and 1600
cm−1 that are attributed to breathing modes of the aromatic
substituents and a strong peak around 1140 cm−1 that is
attributed to a P−C stretch confirming the presence of the
ligand on the surface. 5 exhibits a characteristic P=O stretch at
1244 cm−1; for both powdered 6 and TiO2/6, this feature is
extremely weak, likely due to strong intermolecular inter-
actions.25 A characteristic P−OH stretch is observed for 6 at
989 cm−1 that disappears after attachment. Instead, a new peak
at 1050 cm−1 emerges in agreement with the formation of a
Ti−O−P bond, indicating covalent binding to the surface.26
Effects of Attachment Moiety on Catalysis. To
compare the new P-attached catalyst to the N-attached
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capacity at this pH was found in 0.5 M citric acid/sodium
citrate buffer.
Hydrogen production was confirmed by CPE in aqueous 0.5
M Na citrate buffer at pH 3. CPE experiments for both
electrodes (TiO2/6 and TiO2/7) were conducted with three
independently prepared electrodes, each at a potential of E =
−0.3 V vs RHE for 2 h, and the headspace was analyzed by gas
chromatography. All electrodes tested showed continued HER
over the 2 h period, and hydrogen was produced in
quantitative Faradaic yield (Table S1). P-modified electrodes
produced an average of 14% more hydrogen during this period
compared to their N-attached counterparts. An unmodified
TiO2 electrode produced 2 orders of magnitude less H2,
suggesting that TiO2 in its native state does not significantly
contribute to HER in this system. The 14% difference between
the P- and N-modified is less than we expected given the
Cyclic voltammetry of 6- and 7-modified electrodes (TiO2/6
and TiO2/7) and unmodified TiO2 in aqueous 0.5 M Na
citrate buffer (pH 3) is shown in Figure 4 and exhibit catalytic
activity for the modified electrodes. The CVs of TiO2/6 and
TiO2/7 are both different in important details form the
noncatalytic blank TiO2 electrodes. The modified electrodes
exhibit a catalytic wave with an onset potential around 0 V vs
RHE. The shape of the electrochemical response is similar to
that previously published for CVs of TiO2/7.12 The
unmodified electrodes in contrast exhibit noncatalytic and
reversible semiconductor charging, typical for TiO2.12,28
The charging of the TiO2 is accompanied by a reversible
dark blue coloration of the electrode. The color change is due
to an absorption band around 800 nm that can be attributed to
a d−d transition of Ti3+, which is formed when the TiO2
conduction band is filled.12,28 This absorption can be leveraged
to observe charge transfer kinetics from the TiO2 to the
catalyst.12 Time resolved UV-spectroelectrochemistry (UV-
SEC) illustrated in Figure 4 shows, that upon applying a
potential (E = −0.3 V vs RHE, in aqueous citric acid buffer at
pH 3), an immediate increase in absorption at 800 nm is
observed for unmodified electrodes. The absorption persists
even after bias is removed. This is consistent with the
observation of a discharging current in the CV and suggests
that the electrons populating the TiO2 conduction band are
not consumed by any subsequent Faradaic reactions. The
electrodes modified with 6 or 7 also show a sharp increase in
absorption when biased (E = −0.3 V vs RHE) under the same
conditions. However, the initial absorption is lower than for
unmodified electrodes. This reduced absorption suggests that
there is efficient transfer of electrons from the TiO2 to the
catalyst, where they are consumed by the catalytic HER
reaction. This limits the electron population in the conduction
band. After bias was removed, we observe a rapid decrease in
absorption for both modified electrodes. The P-modified (t1/2
widely discussed importance of the pendent amine.13,14
A
commonly used parameter to benchmark molecular catalysts is
the comparison of turnover numbers (TONs). However, using
this metric is problematic, as the catalyst loading and activity
can change over the course of the experiment. For our
electrodes, we determined the loading before and after the
CPE experiment, which leads to three distinct ways to calculate
the TON. It can be calculated using the (i) initial loading, (ii)
the final loading, (iii) or the average. (Table S1). If calculated
assuming the initial catalyst loading, we obtained values of
1966 54 for TiO2/6 and 1802 26 for TiO2/7, respectively,
corresponding to a 9% difference in TON for the 2 h CPE
experiment. If calculated using postcatalysis loading, we
obtained values of 2773 380 for TiO2/6 and 2433 205
for TiO2/7, respectively, corresponding to a 14% difference in
TON. This difference between the two calculation methods
and the increased error can be explained by the variation in
postcatalysis loading. There is no positive correlation between
postcatalysis loading and hydrogen produced. One would
expect that electrodes that lose more catalyst also produce less
hydrogen. This would lead to a similar TON between different
experiments. For TiO2/6 electrodes, a reverse trend was
observed. The more current was passed, the lower the
postcatalysis loading.
To understand this phenomenon, we must consider the
underlying porous structure of the electrode and resulting
implications on mass transport. Considering the high activity
of the catalyst, a rapid consumption of the local proton supply
is expected, even in highly buffered solutions as previously
reported for other porous electrodes.29 Small differences in
electrolysis conditions can lead to slightly higher catalytic rates,
leading to an increased local pH that results in a loss of catalyst
(phosphonates are known to desorb under basic conditions).30
This would explain why electrodes that produce the most
hydrogen also lose the most catalyst.
UV-SEC allows for qualitative insight into the rate of
substrate depletion from the electrode. After the initial jump in
absorption (TiO2 conduction at band 800 nm) upon biasing
the electrode, we observed a gradual increase in absorption
that continued until the bias was removed. We attribute this
gradual increase to a slowing of the rate of catalysis caused by a
pH change in the local surface microenvironment, allowing the
TiO2 conduction band to be further populated by carriers.
Further evidence suggesting that we are indeed observing a
temporary increase in local pH induced by catalysis can be
found in the overproportionally large current drop compared
to the effective loss of catalyst. If this drop is due to a
temporary change, the measured current should recover after
the bias is removed and the system is given time to equilibrate.
= 3 s) electrode recovers twice as fast as the N-modified (t1/2
=
6 s). This suggests that the attachment through the phosphine
moiety enhances catalysis on a molecular level. This
observation is in stark contrast to the almost identical CVs
for TiO2 modified by 6 and 7. We hypothesize that the
underlying semiconductor properties (charging and discharg-
ing of the TiO2 conduction band) of the electrode material
obscure the differences in catalytic activity in the CV
experiments. To eliminate the background current of the
TiO2 and isolate the catalytic current in the system, we turned
to scanning sampled current polarography (SCP). SCP allows
the measurement of voltammograms of the catalytic response
while eliminating the noncatalytic currents (capacitive, semi-
conductor charging). As for the CV experiments, SCP was
conducted in aqueous 0.5 M Na citrate buffer at pH 3. We
used a step potential of 10 mV with a 5 s hold time, which was
sufficient to eliminate over 90% of the background current.
TiO2/6 electrodes showed a 27% higher current than TiO2/7
electrodes averaged over three independent experiments for
each material. Even taking the large standard deviation and a
5% difference in catalyst loading into account, the catalyst
attached through the amine is marginally slower than its P-
attached counterpart. This agrees with the observations from
UV-SEC and the expectation that “pinning” the amines to the
surface will lower the activity of the catalyst.
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To test this hypothesis, we conducted a series of 5 min CPE
experiments with sequential 5 min equilibrations between
electrolyses. A recovery of over 95% of the initial current was
observed (Figure S11). This time scale and previous SCP
experiments suggest that the current drop cannot be explained
by non-Faradaic processes (capacitive and semiconductor
charging). This leads us to the conclusion that the main cause
of loss of catalytic activity over time can be attributed to a
change in local surface environment caused by substrate
depletion and a steady state increase in local pH. However,
further studies are necessary to directly observe a pH increase
in our system, and other explanations for the current drop, like
a temporary catalyst deactivation, should also be considered.
Together, these results indicate that the catalyst attachment
through the phosphine moiety (TiO2/6) yields a slightly more
active electrocatalyst material, but the effect relative to amine
attachment (TiO2/7) is less than expected. While “pinning
down” the amine impedes catalysis on a molecular level, the
effect on a macroscopic level may be small. For example, the
overall limitations in diffusion through the porous support
material and fundamental differences in mass transport in
homogeneous and heterogeneous electrocatalyst systems may
be more important. Most of the early studies of Ni(P2N2)2
were carried out in aprotic organic solvents such as
acetonitrile.5,21 In contrast, the system described here operates
in an aqueous environment with an abundance of highly
mobile protons, where the beneficial effect of the pendent
amine proton relay may not be as important as it is in
nonaqueous solvents. Literature reports show that the addition
of water to organic solvents or ionic liquids can dramatically
enhance the turnover frequency (TOF) of Ni(P2N2)2.31,32 But
most importantly, the TOFs of the heterogenized catalysts are
nowhere near the extraordinary levels reported for homoge-
neous Ni(P2N2)2 HER catalysts.23,33 Thus, it is likely that
much simpler Ni-phosphine complexes without pendent
amines can achieve comparable performance in a surface-
immobilized system with a carefully designed microenviron-
ment at the electrode interface.
of results from the experiments reported here indicate that the
catalyst attachment though the phosphine moiety yields a
slightly more active electrocatalyst, but the effect is smaller
than expected. While “pinning down” the amine impedes
catalysis on a molecular level, the macroscopic effects of the
electrocatalyst material and environment that include mass
transport through the porous support and an abundance of
protons in the aqueous solution appear to determine
performance. Finally, we illustrated the importance of direct
comparative studies for understanding the nature of hetero-
genized molecular electrocatalysts and the many parameters
that can sometimes influence activities unpredictably.
EXPERIMENTAL SECTION
■
Materials and Methods. 13C NMR spectra were recorded on a
Varian 500 MHz spectrometer. 1H, 31P, and 19F spectra were recorded
on a Bruker 300 MHz spectrometer. The 13C and H chemical shifts
1
are referenced to deuterated solvent peaks (δTMS = 0), and 31P
spectra are referenced to 85% H3PO4. ATR-FTIR spectra were
recorded on a Bruker Alpha II. Microanalyses were performed by
NuMega Resonance Laboratories, San Diego, CA, for C, H, and N.
XPS was performed on a SSF-Kratos AXIS-SUPRA. Mass
spectrometry was performed on a Micromass Quattro Ultima. High
resolution mass spectrometry was performed on an Agilent 6230
Accurate-Mass TOFMS. UV−vis spectra were collected on a
Shimadzu UV-3600. Solvents were received from Fisher Scientific
and were dried on a custom solvent system (degassed with argon and
dried over alumina columns) and stored over 3 Å sieves. Deuterated
solvents were obtained from Cambridge Isotope Laboratories.
34
[Ni(CH3CN)6] [BF4]2 and 715 were synthesized following
previously reported procedures; aniline and triethylamine were
distilled form CaH2 and stored over 3 Å sieves in a N2 filled
glovebox. All other reagents were obtained from commercial sources
and used without further purification. Reactions were performed using
standard Schlenk line and glove box techniques under an atmosphere
of nitrogen. Flash column chromatography was performed on a
Teledyneisco CombiFlash Rf200 using SiO2 loaded columns.
Mesoporous TiO2 electrodes were obtained from Solaronix and cut
to fit the electrochemical cell. They were brought into a N2 filled
glovebox, where all surface modification was carried out.
X-ray Crystallography. Single crystal X-ray data was collected on
a Bruker Kappa APEX-II CCD diffractometer equipped with Mo Kα
radiation (λ = 0.71073 Å). The crystals were mounted on a CryoLoop
with paratone oil, and data were collected under a nitrogen gas stream
at 100 K using ω and φ scans. Data were integrated using the Bruker
SAINT software program and scaled using the software program. All
structures were solved via direct methods with SHELXS35 and refined
by full-matrix least-squares procedures using SHELXL21 within the
Olex2 small-molecule solution, refinement, and analysis software
package.36 All non-hydrogen atoms were refined anisotropically by
full-matrix least-squares (SHELXL-97). All hydrogen atoms were
placed using a riding model. Their positions were constrained relative
to their parent atom using the appropriate HFIX command.
Crystallographic data, structure refinement parameters, and additional
notes on structure refinement are summarized in the SI.
Electrochemistry. Electrochemical experiments with an immobi-
lized catalyst were carried out in a custom build five-port 90 mL glass
cell. The modified electrode was the working electrode, a graphite
rode separated from the solution by a porous glass frit was used as the
counter electrode, and an aqueous Ag/AgCl (3 M NaCl) electrode
separated from the solution by a Vycor tip was used as the reference.
The working, counter, and reference electrodes occupied one ground
glass joint each. The remaining two ports were sealed with rubber
septa, allowing for gas purging of the cell. The potential was
controlled by a BASi Epsilon potentiostat. The solution was stirred
during experiments. Homogenous electrochemical experiments were
carried out in acetonitrile solution in an oven-dried 20 mL vial with
[nBu4N][PF6] as the supporting electrolyte. A glassy carbon rod was
CONCLUSION
■
While there are many derivatives of Ni(P2N2)2 catalysts with
different functional groups on the pendent base amine groups,
there is only a very limited library of reported P2N2s with
variation in substituents on the phosphine. We presented a
versatile new synthetic strategy to overcome limitations of
previous approaches utilizing the 4-BrP2N2 ligand as a platform
that can be easily converted into other functional groups. This
approach enabled us to synthesize [Ni(PArPO(OEt)22NPh2)2]
[BF4]2 and [INi(PArPO3H22NPh2)2] [I] • HI. This represents
not only the first reported P2N2 ligand that has a reactive
functional group on the phosphine but also the first P2N2
complex that was successfully attached to an electrode surface
without utilizing the pendent amine. A mesoporous TiO2
electrode modified with the catalyst sustains HER in water at
mild pH and low overpotential. The catalytic performance for
HER was used as a benchmark for direct comparison of the
phosphine anchored catalyst 6 to the previously reported
complex 7 anchored through the amine moiety. Electrodes
prepared with 6 showed higher activity in UV-SEC and SCP
compared to 7. This is further confirmed by CPE, with 6
showing a 9−14% increase in activity for HER over 7.
However, these experiments also show the importance of the
microenvironment on the electrode surface. The combination
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used as the counter electrode, and a silver wire separated from the
solution by a Vycor tip was used as a pseudoreference electrode. All
potentials are referenced versus an internal ferrocene standard.
UV Spectroelectrochemistry. UV-SEC was performed in a
custom build UV-SEC cell. The cell was operated in transmission
mode with the electrode masked to expose a surface area of 0.36 mm2.
Catalytic UV-SEC experiments were carried out in aqueous Na citrate
buffer (pH 3.0, 0.5 M). The potential was controlled by a BASi
Epsilon potentiostat, and spectra were collected on a Shimadzu UV-
3600.
PArBr2NPh (3).
2
Surface Modification. Mesoporous TiO2 on fluorine doped tin
oxide (FTO) electrodes were purchased from Solaronix and cut to
size to fit the electrochemical cell. The TiO2 layer is made from 15 to
20 nm particles with a physical area of 6 by 6 mm and 12 μm
thickness (as determined by SEM; Figure S12). The electrodes were
modified by soaking in methanolic solutions (6′, 1 mM; 7, 0.1 mM)
of the desired catalyst and 10 equiv of triethylamine (NEt3) for 20 h.
The concentration of 7 compared to 6′ was adjusted to maintain the
same catalyst loading despite different adsorption kinetics. After being
rinsedwith MeOH, the electrodes were soaked in MeOH for 1 h to
remove any unattached catalyst. In this form, the electrodes modified
with 6′ are susceptible to degradation in air. To convert the catalyst
back to the air stable fully protonated form, the electrodes were
soaked in dimethylformamidium trifluoromethanesulfonate (HDMF
• TfO) (10 mM in THF) solution for 10 min, rinsed with THF, and
soaked in THF for 1 h.
In a 500 mL, two-neck, round-bottom flask with a reflux condenser,
(4-bromophenyl)phosphane (3.49 g, 16.6 mmol, 1.0 eq, 90%) and
para-formaldehyde (998 mg, 33 mmol, 2.0 equiv) were mixed in dry
degassed EtOH (150 mL) and heated to reflux for 17 h until a clear
solution was formed. Then, aniline (1.5 mL, 1.55 g, 17 mmol, 1.0
equiv) was added, and the solution was heated to reflux for 1 day,
during which a white precipitate was formed. After the solution was
cooled to room temperature, the solid was filtered off via a cannula
under N2 and washed with EtOH. After drying under vacuum, the
title compound was obtained as a white solid (3.0 g, 4.90 mmol,
59%).
1H NMR (300 MHz, CD2Cl2) δ 7.67 (dd, J = 21.9, 8.5 Hz, 4H,
H
ArP3), 7.54−7.46 (m, 4H, HArP2), 7.20 (t, J = 7.9 Hz, 4H, HArN3),
6.76−6.66 (m, 6H, HArN2, ArN4), 4.49−3.95 (m, 8H, HNCH2P).
31P{1H} NMR (121 MHz, CD2Cl2) δ/ppm 51.0.
IR (ATR) νmax [cm−1]: 3257 (wbr), 3088 (w), 3036 (w), 2959
(w), 2873 (w), 1902 (w), 1591 (s), 1498 (s), 1474 (m), 1450
(w),1425 (w), 1374 (m), 1343 (w), 1249 (m), 1230 (w), 1206 (m),
1161 (w), 1145 (w), 1108 (w), 1091 (w), 1065 (m), 1036 (w), 1004
(m), 991 (w), 955 (w), 934 (w), 908 (w), 877 (w), 859 (w), 806 (s),
738 (s), 720 (m), 680 (s), 601 (w), 511 (m), 476 (w).
Diethyl (4-Bromophenyl)phosphonate (1).
HR-ESI-MS: m/z calcd for [M + H]+ 611.0011 (found) 611.0007.
PArPO(OEt)22NPh (4).
In a 1 L Schlenk flask, 1,4-dibromobenzene (10.00 g, 42.4 mmol, 1.0
equiv) was dissolved in dry degassed Et2O (450 mL). At −78 °C, n-
butyl-lithium (29.2 mL, 46,6 mmol, 1.1 equiv of 1.6 M in hexane) was
added dropwise, and the solution was stirred at −78 °C for 1 h. The
solution was then added dropwise via a cannula to a stirred solution of
diethyl chlorophosphate (15.3 mL, 106.0 mmol, 2.5 equiv) in Et2O
(75 mL) at −78 °C. After the addition was completed, the solution
was stirred for 1 h at −78 °C and then warmed to room temperature
over 1.5 h. The reaction was quenched with water and extracted with
DCM. After the solvent was evaporated, the residue was purified by
flash column chromatography (hexane to ethyl acetate) to obtain the
title compound as a colorless oil (10.85 g, 37.02 mmol, 87%). NMR
spectroscopic data were consistent with literature values.19
2
A saturated solution of PArBr2NPh (500 mg, 816.61 μmol, 1.0 equiv)
2
in dry degassed THF (500 mL) was cooled to −108 °C in a glovebox.
n-Butyl-lithium (1.12 mL, 1.6 M in hexanes, 1.80 mmol, 2.2 equiv)
was added, and the solution was cooled back to −108 °C (4 min).
Diethyl chlorophosphite (590 μL, 4.01 mmol, 5.0 equiv) was added,
and the solution was warmed to room temperature over 3 h. The
solvent was removed under vacuum, and the crude was purified by
flash column chromatography (SiO2 treated with a 2% solution of
TEA in DCM and washed with DCM, DCM to DCM/5% MeOH).
The title compound was isolated as a white solid (448 mg, 617 μmol,
76%). This ligand is moderately air stable and can be handled out of
the glovebox for short periods (1 h) without significant degradation,
simplifying the purification by flash column chromatography.
1H NMR (300 MHz, CDCl3) δ/ppm 7.90 (dd, J = 13.1, 7.8 Hz,
4H, H4), 7.74−7.65 (m, 4H, H3), 7.22 (dd, J = 8.4, 7.7 Hz, 4H, H8),
6.78−6.67 (m, 6H, H7, H9), 4.50 (t, J = 14.0 Hz, 4H, H1), 4.28−4.10
(m, 8H, H10), 4.02 (dd, J = 15.2, 4.6 Hz, 4H, H1), 1.37 (t, J = 7.1 Hz,
12H, H11).
(4-Bromophenyl)phosphane (2).
A solution of diethyl (4-bromophenyl)phosphonate (2.00 g, 6.82
mmol, 1.0 equiv) in dry Et2O (10 mL) was degassed and added
dropwise to a suspension of LiAlH4 (647 mg, 17.06 mmol, 2.5 equiv)
in Et2O (20 mL) at 0 °C. Gas production was observed. After the
green suspension was stirred for 1.5 h at 0 °C, it was quenched by the
addition of degassed water (1.7 mL), resulting in a violent reaction,
with large quantities of gas produced. The off-white suspension was
warmed to room temperature, and the solvent was removed under
reduced pressure on a Schlenk line. The residue was extracted with
pentane (4 × 30 mL), and the solvent was evaporated under reduced
pressure at the Schlenk line. The title compound (extremely toxic with
a very strong unpleasant odor) was obtained as a white solid (605 mg,
3.02 mmol, 47%, 90% pure by NMR) and used without further
purification.
13C{1H} NMR (126 MHz, CDCl3) δ/ppm 145.16 (s, C6), (C2),
132.86−132.39 (m, C3), 132.27−131.83 (m, C4), 129.77 (d, J = 188.3
Hz, C5), 129.37 (s, C8), 117.46 (s, C9), 112.91 (s, C7), 62.46 c (d, J =
5.5 Hz, C10), 57.97 (d, J = 17.4 Hz, C1), 16.53 (d, J = 6.5 Hz, C11).
31P{1H} NMR (121 MHz, CDCl3) δ/ppm 17.78 (s, PO(OEt)2),
−49.44(s, NCH2P).
1H NMR (300 MHz, CDCl3) δ/ppm 7.50−7.20 (m, 4H, Ar), 3.93
(d, J = 202.3 Hz, 2H, PH2).
13C {1H} NMR (126 MHz, CDCl3) d/ppm 136.35 (d, J = 15.7 Hz,
C3), 131.65 (d, J = 6.0 Hz, C2), 127.68 (d, J = 8.7 Hz, C4), 122.87 (s,
C1).
IR (ATR) νmax [cm−1]: 3064 (w), 2982 (m), 2934 (w), 2902 (w),
1590 (s), 1500 (s), 1472 (w), 1380 (w), 1368 (s), 1343 (m), 1244
31P{1H} NMR (121 MHz, CDCl3) δ/ppm +123.1 (s).
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(s), 1214 (m), 1148 (w), 1135 (m), 1098 (w), 1014 (s), 943 (s), 893
(w), 874 (m), 795 (m), 773 (m), 741 (s), 684 (s), 648 (w), 585 (s),
537 (s), 447 (w).
IR (ATR) νmax [cm−1]: 2857 (br), 1594 (s), 1492 (s), 1383 (m),
1190 (s), 1140 (s), 989 (m), 921 (m), 882 (m), 814 (w), 747 (s),
692 (w), 569 (s), 531 (s).
Anal. Calcd for C56H60I2N4NiO12P8·HI·2H2O: C, 39.44; H, 3.84;
N, 3.29. Found: C, 39.36; H, 4.07; N, 3.20.
HR-ESI-MS: m/z calcd for [M + H]+ 727.2379 (found) 727.2372.
[Ni(PArPO(OEt)22NPh2)2] [BF4]2 (5).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01669.
■
sı
NMR quenching studies of Li-intermediate, degradation
of BF4 by TMS-I during deprotection reaction, solution
CV of [Ni(PPh2NPh2)2]2+ and 5, IR spectra, UV−vis
spectra of desorbed catalyst, pH titration of 6,
heterogenous electrochemistry data, SEM data, NMR
spectra, crystallographic data (PDF)
To a suspension of PArPO(OEt)22NPh2 (387 mg, 532.57 μmol, 2.0 equiv)
in MeCN (5 mL), a solution of [Ni(MeCN)6] [BF4]2 (127 mg,
266.28 μmol, 1.0 equiv) in MeCN (5 mL) was added. After the
mixture was stirred at room temperature for 1 h, the solvent was
removed under vacuum, and the residue was redissolved in minimal
MeCN. The addition of Et2O led to a precipitate that was filtered off
and dried under vacuum to yield the title compound as a dark red
solid (227 mg, 135 μmol, 98%). Single X-ray quality crystals were
grown by vapor diffusion of Et2O into a MeCN solution of the
complex.
Accession Codes
CCDC 1985396 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1H NMR (300 MHz, CD3CN) δ/ppm 7.56−7.38 (m, 24H,
H
ArP, N3), 7.29 (d, J = 8.1 Hz, 8H, HN2), 7.16 (t, J = 7.2 Hz, 4H, HN4),
4.30−3.88 (m, 32H, HNCH2P, PCH2Me), 1.38 (dd, J = 11.4, 6.9 Hz, 24H,
H
Me).
13C{1H} NMR (126 MHz, CD3CN) δ 152.1−152.0 (m, CP4, N1),
AUTHOR INFORMATION
Corresponding Author
■
133.8 (d, J = 186.4 Hz, CP1), 132.8 (d, J = 9.5 Hz, CP3), 132.1 (d, J =
14.4 Hz, CP2), 130.5 (s, CN3), 124.3 (s, CN4), 120.6 (s, CN2), 63.4 (t,
Jcc = 5.3 Hz, CCH2Me), 52.3 (d, J = 911.5 Hz, CNCH2P), 16.6 (d, J = 4.8
Hz, CMe).
Clifford P. Kubiak − Department of Chemistry and
Biochemistry, University of California, San Diego, La Jolla,
California 92093-0358, United States; orcid.org/0000-
0003-2186-488X; Email: ckubiak@ucsd.edu
31P{1H} NMR (121 MHz, CD3CN) δ/ppm 16.6 (s, PO(OEt)2), 6.3
(s, PNCH2P).
HR-ESI-MS: m/z calcd for [M + Cl− − 2[BF4]+] 1545.3655
(found) 1545.3666; [M + CN− − 2 [BF4]+] 1536.3997 (found)
1536.4027. (Isotope patterns match for both molecules. The presence
of Cl− and CN− is an artifact of the technique and not present in the
isolated compound).
Authors
Felix M. Brunner − Department of Chemistry and
Biochemistry, University of California, San Diego, La Jolla,
California 92093-0358, United States
Michael L. Neville − Department of Chemistry and
Biochemistry, University of California, San Diego, La Jolla,
California 92093-0358, United States
IR (ATR) νmax [cm−1]: 3060 (w), 2983 (m), 2935 (w), 2908 (w),
1595 (s), 1494 (s), 1442 (w), 1420 (w), 1387 (m), 1246 (s), 1191
(m), 1162 (w), 1136 (m), 1044 (w), 1011(s), 967 (s), 886 (w), 793
(w), 759 (w), 694 (w), 589 (s), 540 (m).
[INi(PArPO3H22NPh2)2] [I] • HI (6).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01669
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Air Force Office of Scientific
Research under Award No. FA9550-17-0198. We acknowledge
the UCSD Molecular Mass Spectrometry Facility for HRMS
sample analysis, Dr. Po Ling Cheung for recording SEM
images, and Dr. Ich Tran for the XPS measurements
performed at the UC Irvine Materials Research Institute
(IMRI) using instrumentation funded in part by the National
Science Foundation Major Research Instrumentation Program
under grant no. CHE-1338173.
To a solution of [Ni(PArPO(OEt)22NPh2)2] [BF4]2 (250 mg, 145 μmol
1.0 equiv) in dry degassed DCM (15 mL), TMS-I (270 μL, 1.88
mmol, 13.0 equiv) was added. The solution turns dark purple
immediately and was stirred at room temperature for 5 h before the
solvent was removed under vacuum. To the residue, dry degassed
MeOH (15 mL) was added, and the resulting suspension was stirred
overnight. The solvent was reduced to about one-third, and Et2O (25
mL) was added and stirred for 30 min. The resulting precipitate was
filtered, washed with Et2O, and dried under vacuum, yielding the title
compound as a dark purple solid (187 mg, 112 μmol, 77%) (NMRs
recorded of deprotonated species; 10 equiv of NEt3 added).
1H NMR (300 MHz, CD3OD) δ/ppm 8.18−6.30 (m, 36H, HAr),
4.77−3.42 (m, 16, HNCH2P).
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