Electrocatalytic oxidation of formate with nickel diphosphine dipeptide complexes: Effect of ligands modified with amino acids

Article abstract of DOI:10.1002/ejic.201300751

A series of nickel bis(diphosphine) complexes with dipeptides appended to the ligands were investigated for the catalytic oxidation of formate to carbon dioxide, a proton, and two electrons. Typical rates of approximately 7 s ^-1 were found, similar to that for the parent complex (ca. 8 s ^-1), with amino acid size and positioning contributing very little to the rate or operating potential. Hydroxy functionalities did result in lower rates, which were recovered by protecting the hydroxy group. The results suggest that the overall dielectric properties introduced by the dipeptides do not play an important role in catalysis, but free hydroxy groups do influence activity, implying contributions from intra- or intermolecular interactions. These observations are important in developing a fundamental understanding of the effect that an enzyme-like outer coordination sphere can have upon molecular catalysts. Electrocatalysts Ni(P^R₂N ^R′₂)₂ substituted with dipeptides were used for the oxidation of formate. Polar substituents resulted in a significant loss in activity, which was recovered upon protection of these groups. This work adds to our growing understanding of the role of the outer coordination sphere in molecular electrocatalysis. Copyright

Full text of DOI:10.1002/ejic.201300751

FULL PAPER
DOI:10.1002/ejic.201300751
Electrocatalytic Oxidation of Formate with Nickel
Diphosphine Dipeptide Complexes: Effect of Ligands
Modified with Amino Acids
Brandon R. Galan,[a] Matthew L. Reback,[a] Avijita Jain,[a]
Aaron M. Appel,*[a] and Wendy J. Shaw*[a]
Keywords: Formate / Oxidation / Nickel / Peptide catalysts / Electrocatalysts / Outer coordination sphere / Biocatalysts /
Bioinorganic chemistry
A series of nickel bis(diphosphine) complexes with dipept-
ides appended to the ligands were investigated for the cata-
lytic oxidation of formate to carbon dioxide, a proton, and
two electrons. Typical rates of approximately 7 s–1 were
found, similar to that for the parent complex (ca. 8 s–1), with
amino acid size and positioning contributing very little to the
rate or operating potential. Hydroxy functionalities did result
in lower rates, which were recovered by protecting the hy-
droxy group. The results suggest that the overall dielectric
properties introduced by the dipeptides do not play an im-
portant role in catalysis, but free hydroxy groups do influence
activity, implying contributions from intra- or intermolecular
interactions. These observations are important in developing
a fundamental understanding of the effect that an enzyme-
like outer coordination sphere can have upon molecular cata-
lysts.
straction by the pendant amine rather than a direct hydride
transfer, because the pendant amines are necessary for ca-
talysis and the rates of catalysis become faster with increas-
ing basicity of the pendant amine (Scheme 1). To date these
rates are the highest reported for formate oxidation by
homogeneous electrocatalysts and are the only catalysts
based on an earth-abundant metal such as nickel.
Introduction
Given the high global demand for petroleum fuels, re-
search into the development of catalysts for the utilization
of alternative carbon-based feedstocks has become an
active area of research. In particular, the reduction of car-
bon dioxide provides an attractive and potentially renew-
able route to simple carbon-based fuels such as formic acid,
carbon monoxide, formaldehyde, methanol, and meth-
ane.[1–7] Of these fuels, formic acid has received considerable
attention because of its applications in fuel cell technology
and energy storage.[8–10] As methods of formic acid pro-
duction for energy storage become more viable, the electro-
catalytic oxidation of formate as a fuel by using nonpre-
cious metals is critical.
We recently reported that nickel-based electrocatalysts,
[Ni(PR2NRЈ2)]2+ with R = Ph (phenyl) or Cy (cyclohexyl)
and RЈ = Ph, 4-MeOPh (methyloxyphenyl), Bn (benzyl), or
Me (methyl), were well suited for the oxidation of formate
with turnover frequencies of up to 15.8 s–1.[11] Mechanistic
studies of this system[11,12] suggest that after the binding of
formate, there is a rate-limiting proton transfer concomitant
Scheme 1. Proposed transition state for the electrocatalytic oxi-
dation of formate.[11,12]
The molybdenum- and iron-containing formate dehydro-
with a two-electron transfer process accompanied by the genase enzyme ([Se]FDHH), isolated from E. coli grown un-
loss of CO2. This process is thought to involve proton ab- der anaerobic conditions, is capable of interconverting for-
mate and CO2 at rates of up to 2800 s–1 for formate oxi-
dation.[13] The mechanism proposed for the catalytic system
[a] Physical Sciences Division, Pacific Northwest National
Laboratory,
above is analogous to many of the mechanisms proposed
Richland, WA 99352, USA
for formate dehydrogenase in which two electrons are trans-
ferred to the metal in the active site and the proton, H+, is
transferred to an adjacent heteroatom. The much faster
rates for the enzyme in comparison to molecular catalysts
E-mail: aaron.appel@pnnl.gov
wendy.shaw@pnnl.gov
http://www.pnnl.gov
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300751.
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are likely due in part to the protein scaffold, which can be sphere on the rates and potentials for the oxidation of for-
critical to the function by delivering and removing sub- mate.
strates and products, positioning functional groups to lower
the energy level of the transition state, or controlling the
polarity of the environment to optimize it for catalysis.[14–17]
We have recently reported that the incorporation of a di-
peptide into the outer coordination sphere of these Ni-
based electrocatalysts results in rates up to five times faster
Results
Synthesis and Characterization of Ligands and Complexes
Cyclic diphosphines containing the NNA residues 3-(4-
for the production of hydrogen than that of the parent Ni aminophenyl)propionic acid (para: p-NNA) or 3-(3-amino-
complex.[18–20] Amide or acidic/basic amino acid functional phenyl)propionic acid (meta: m-NNA) were prepared with
groups contribute to the increased rate of hydrogen pro- a carboxylic acid termination (PPh2Nm/pNNA2) by following
duction. This observation can be best explained by con- previously described protocols.[18,19] The dipeptide-func-
sidering that these functional groups help to concentrate tionalized ligands, PPh2Nm/pNNA-RЈ (Figure 1), were pre-
2
water and/or acid near the active site, and they may also pared by coupling the amino acid alanine or the amino acid
aid in proton delivery to the pendant amine;[18] these are esters AlaOEt, SerOMe, PheOMe, TyrOMe, or Tyr(tBu)-
features reminiscent of enzymatic function.
OMe to PPh2Nm/pNNA2 with use of standard TBTU/HOBT/
On the basis of the significant rate enhancements ob- DIPEA peptide coupling methods.[21,22] Nickel complexes
served for hydrogen production catalysis, as well as the po- were prepared by reacting two equivalents of a dipeptide-
tentially large impact that the outer coordination sphere functionalized ligand with one equivalent of [Ni-
can have on catalysis, the presented studies were undertaken (MeCN)6]2+ in acetonitrile, which resulted in a red solution
to assess whether similar catalytic enhancements could be from which a moderately air-sensitive metal complex was
observed for formate oxidation. In this work, we examine obtained in high yield (70–90%) and high purity. The li-
the influence of a series of known dipeptide catalysts gands and their [Ni(PPh2Nm/pNNA-RЈ2)2]2+ complexes were
1
[Ni(PPh2Nm/pNNA-RЈ2)]2+ [in which RЈ is alanine or various characterized by 31P NMR,[23] 1H NMR, and H-TOCSY
amino acid esters] on the electrocatalysis of formate oxi- NMR spectroscopy, mass spectrometry, and elemental
dation. The dipeptides consist of a non-natural amino acid, analysis; additionally, the metal complexes were charac-
3-(4-aminophenyl)propionic acid, coupled to the amino terized electrochemically.[13,20] All results of the characteri-
acid alanine or to esters of one of the following five amino zation are consistent with the proposed structures and sim-
acids: glycine, alanine, serine, phenylalanine, and tyrosine. ilar to those of other reported complexes of this
We also investigated the positioning of the amino acid func- type.[11,19,20,24,25] Hereafter, the nickel complexes are re-
tionality by substituting the para derivative with the corre- ferred to by the position and identity of the amino acid or
sponding meta derivative of the non-natural amino acid, 3- dipeptide substituent: for example m-SerOMe refers to the
(3-aminophenyl)propionic acid, as the first amino acid of nickel complexes that are substituted off the phenyl ring in
the dipeptide. The activities of these complexes are dis- the meta-position and contain the non-natural amino acid
cussed in terms of the influence of the outer coordination coupled to serine methyl ester.
Figure 1. Dipeptide esters investigated in this study (alanine: RЈЈ = H or ethyl, tyrosine: RЈЈ = H or tert-butyl).
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Electrochemical Studies
(Cp2Fe), because the ferrocene/ferrocenium couple overlaps
with the catalytic wave observed for formate oxidation.
Upon addition of formate (added as NBu4HCO2·
HCO2H),[11] an increase in current (icat) is observed at the
potential of the NiII/I couple (Figure 3), consistent with the
catalytic oxidation of formate. This current increases and
gradually shifts to more negative potentials as the concen-
tration of formate increases, previously interpreted as a fast
following reaction (formate binding) as the NiI species is
oxidized to form NiII.[11] This shift occurs at consistently
higher formate concentrations for the dipeptide complexes
than for the unsubstituted complex, suggesting that the
binding of formate is hindered for these bulkier catalysts.
A representative plot of the turnover frequency (TOF) of
the catalyst vs. formate concentration reveals similar behav-
ior to that observed for the parent catalyst (Figure 4).[11]
The catalytic reaction was shown to be first order in catalyst
(Figure S1), as previously observed for complexes without
the dipeptides.[11] These results suggest that the catalysts
that contain dipeptides operate by the same mechanism as
those without the dipeptides, for which the rate-determining
step at high formate concentration was proposed to be the
proton abstraction by the pendant amine concomitant with
electron transfer to the metal center.
As previously reported, each of the dipeptide ester com-
plexes shows two distinct, reversible reduction waves corre-
sponding to the NiII/I and the NiI/0 couples (Figure 2).[18]
Differences in redox potentials for all of the complexes are
small: E1/2 values are within 20 mV for both the NiII/I po-
tentials (–0.83 to –0.85 V) and the NiI/0 potentials (–1.03 V
to –1.05 V), which indicates that the substituents have mini-
mal impact on the metal center.
Figure 2. The cyclic voltammogram of p-SerOMe is representative
of the cyclic voltammograms for each of the complexes reported
–
here.[18] Conditions: 0.7 mm complex in 0.2 m Et4N+BF4 aceto-
nitrile solution, glassy carbon working electrode; scan rate 1 V/s. Po-
tentials are referenced to the ferrocenium/ferrocene couple (0.0 V).
Electrochemical Oxidation of Formate
Each of the dipeptide complexes shown in Figure 1 were
examined for their activity for the electrochemical oxidation
of formate. A representative cyclic voltammogram of p-Ser-
OMe as a function of formate concentration in benzonitrile
is shown in Figure 3, where benzonitrile was used because
of the higher solubility of the Ni0 complex. The reversible
wave at –1.33 V is due to the cobaltocenium/cobaltocene
couple, which was used as an internal standard for catalytic
runs. Cobaltocenium hexafluorophosphate {[CoCp2](PF6)}
was used as an internal reference instead of ferrocene
Figure 4. For dipeptide catalysts (shown for p-SerOMe), formate
oxidation is initially dependent on formate concentration, followed
by a region in which it is independent of formate concentration,
similar to the parent catalysts. The rates were obtained by using
the current in the formate concentration independent region (above
0.03 m).
The current enhancement (icat/ip) in the presence of form-
ate substrate can be converted to a catalytic rate (TOF)
using Equations (1) and (2). The terms icat and ip refer to
the current observed in the presence and absence of formate
substrate, respectively.
(1)
Figure 3. Representative cyclic voltammograms for the addition of
formate to any of the dipeptide complexes (shown for p-SerOMe),
which results in current enhancements consistent with the catalytic
oxidation of formate. Conditions: 1.0 mm catalyst in 0.2 m TBAPF6
benzonitrile solution, glassy carbon working electrode; scan rate
50 mV/s. Potentials are referenced to the ferrocenium/ferrocene
couple (0.0 V) using cobaltocenium/cobaltocene as an internal ref-
erence (–1.33 V).
(2)
Equation (1) can be rearranged to yield Equation (2) in
the formate-independent region. Equation (2) was used to
calculate the TOF for the catalytic oxidation of formate
using Faraday’s constant (F), the scan rate in V/s (υ), the
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number of electrons in the wave in the absence of formate
The lower catalytic activity for the TyrOMe derivatives
(np), the universal gas constant (R), the reaction tempera- may be the result of the free phenol residue present in the
ture in Kelvin (T), and the number of electrons involved complexes. To test this hypothesis, meta- and para-substi-
in the catalytic process (ncat).[26–28] The calculated rates of tuted catalyst derivatives were synthesized, with the phenol
formate oxidation are presented in Table 1 and range from protected as the tert-butyl ether to yield m/p-Tyr(tBu)OMe.
unobserved (p-AlaOH) up to 8.5 s–1 (m-GlyOMe), with an This protected complex performed similarly to the other
average of 6.1 s–1. Little difference was observed as a func- complexes that were studied (6–8 s–1) under identical reac-
tion of amino acid ester identity, and typically, the meta- tion conditions, suggesting that the unprotected phenol
substituted complexes were slightly faster than the para- group is indeed responsible for the reduction in rates. It is
substituted complexes. Notably, m/p-TyrOMe both had sig- not expected that the deprotonated substrate, formate,
nificantly lower TOFs than the other complexes, prompting would be able to appreciably deprotonate the phenol di-
the investigation of the m/p-Tyr(tBu)OMe. The alkylation peptides in TyrOMe, since the pKa of formic acid (which
of these complexes resulted in an increase in the catalytic has not been reported in MeCN to the best of our knowl-
TOF to the same range as the other dipeptide-containing edge) is expected to be similar to the pKa of acetic acid,
complexes, as shown in Table 1.
23.5,[29] and the pKa of phenol in acetonitrile, 29.1,[30] is
considerably higher than the expected value for formic acid.
A possible explanation for the role of the phenol group is
intra- or intermolecular interactions that result in a change
in ligand dynamics or an electronic effect. These may be
due to strong hydrogen-bonding interactions, possibly with
either formate or another phenol group. Regardless of the
precise cause, the presence of the protic group results in
lower rates than those for analogous complexes without this
functionality.
Table 1. TOF values for the electrocatalytic oxidation of formate
with dipeptide-substituted catalysts.
Entry[a] Catalyst
TOF
[s–1]
Entry[a] Catalyst
TOF
[s–1]
(RЈ)
(RЈ)
1[11]
2
PPh2NPh
7.4Ϯ0.7
8
9
10
11
12
12
14
p-PheOMe
6.0Ϯ0.9
6.8Ϯ0.9
3.3Ϯ0.4
1.8Ϯ0.4
2
p-GlyOEt 7.5Ϯ0.4
m-GlyOEt 8.5Ϯ0.3
p-AlaOEt 6.2Ϯ1
m-AlaOEt 7.3Ϯ1
p-SerOMe 4.6Ϯ1
m-SerOMe 6.1Ϯ0.5
m-PheOMe
p-TyrOMe
m-TyrOMe
3[b]
4
5
6
7
p-Tyr(tBu)OMe 6.2Ϯ0.1
m-Tyr(tBu)OMe 7.7Ϯ1.7
A similar mechanism may also be responsible for the de-
creased rate observed for p-SerOMe and the loss of catalytic
p-AlaOH
–
[a] Catalyst (1.0 mm) in a benzonitrile solution of TBAPF6 (0.2 m); activity observed for the acidic amino acid complex, p-
scan rate 50 mV/s. Average of three runs. [b] Average of two runs.
AlaOH. Previous studies have shown restricted motion in
the five-coordinate (solvent bound) p-Ala-OH complex,
best described by reduced structural interconversion of ax-
ial and equatorial phosphine groups due to intra- or inter-
Discussion
molecular interactions for this complex.[20] Consequently,
It has become widely acknowledged that the outer coor- the decrease in activity could also be related to intra- or
dination sphere is intimately involved with the catalytic intermolecular interactions that limit either formate associ-
function of metalloenzymes by providing catalytic control, ation or essential ligand flexibility such as chair/boat in-
speed, and efficiency unparalleled by small synthetic biomi- terconversions.
metics.[15,17] If these advantages are to be incorporated into
The use of polar and aromatic side chains such as tyro-
synthetic biomimetics, understanding and predicting how sine has been shown to assist in tuning active sites in en-
variations in the outer coordination sphere influence cataly- zymes.[15,17] Additionally, the global dielectric constant of
sis will be essential. The outer coordination sphere is cur- the outer coordination sphere will change as a result of the
rently not well understood in molecular catalysts, and the OH groups. However, the formate oxidation catalysts
dipeptide derivatives studied here provide a foundation for studied here had very little enhancement from the outer co-
developing predictive tools. The overall observation in com- ordination sphere. We previously demonstrated that this
paring the rates of formate oxidation with dipeptide-substi- series of catalysts had a significant impact on hydrogen pro-
tuted complexes is that there is very little change, regardless duction.[18–20] The catalyst activities ranged over almost an
of size or substitution position, when compared to the un- order of magnitude, the fastest catalysts operating five times
2+
modified complex, Ni(PPh2NPh
)
2 2
(TOF = 8 s–1).[11] The faster than the unmodified catalysts. In that case, the in-
exceptions to this pattern are the polar-substituted com- crease in activity was interpreted to be due to the ability to
plexes, m- and p-TyrOMe, which are four times slower than concentrate water and protons near the active site. In the
the unmodified catalyst, the p-SerOMe, which is two times case of formate oxidation, the rate-limiting step is proposed
slower, and the unprotected p-AlaOH which was not cata- to be an intramolecular proton-coupled electron trans-
lytic. It is tempting to conclude that the difference for these fer,[11,12] and therefore concentrating protons is not ex-
catalysts is related to size; however, it is clear that size is pected to enhance the rate. The expected effect would have
not the key factor, since Phe and Tyr are essentially the been due to the contributions of polar or aromatic groups
same size but m/p-TyrOMe is 2–4 times slower than m/p- altering the electronic environment near the active site or
PheOMe. Furthermore, p-PheOMe performs as well as the contributing to lower the energy barrier of the transition
smaller p-GlyOEt and p-AlaOEt derivatives, suggesting state. These effects were also not observed here, suggesting
little influence of the change in the sterics of the dipeptides. that the global dielectric properties around the active site
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1
are not important in the Ni(PR2NRЈ2)2 series of catalysts for
the oxidation of formate. Changes in local dielectric con-
stant close to the metal center and substrate may have
greater influence, and studies with positioned functional
groups are underway.
–49.83 (s) ppm. H NMR (CDCl3): δ = 7.60–6.65 (m, 26 H, Ar),
5.82 [d, 2 H, C(O)NH], 4.83 (m, 2 H, αCH), 4.45 (m, 4 H, PCH2N),
4.01 (m, 4 H, PCH2N), 3.61 (s, 6 H, COOCH3), 3.01 (d, 4 H,
βCH2), 2.80 [m, 4 H, CH2C(O)N], 2.42 (m, 4 H, N-PhCH2), 1.32
[s, 18H C(CH3)3] ppm.
PPh2NmNNA-Tyr(tBu)OMe [Tyr(tBu)OMe = O-tert-Butyl-
L-Tyrosine
2
Methyl Ester]: This complex was prepared in an analogous manner
to the synthesis described for PPh2NpNNA-Tyr(tBu)OMe with the fol-
2
Summary and Conclusions
lowing amounts: TBTU (2.0 equiv., 192.0 mg, 0.60 mmol) and
HOBT (2.0 equiv., 81.1 mg, 0.60 mmol) were added to a dichloro-
A series of dipeptide-substituted catalysts shows activity
for the oxidation of formate, and changing the amino acid
functional groups, size, or positioning had little influence
on the catalytic rates. Therefore, the global dielectric prop-
erties do not appear to affect the activity of these com-
plexes; however, the presence of protic groups did inhibit
catalysis, possibly due to intra- or intermolecular interac-
tions limiting ligand dynamics. This work provides ad-
methane solution containing PPh2NmNNA (1.0 equiv., 179.0 mg,
2
0.30 mmol) and DIPEA (2.2 equiv., 85.1 mg, 115 μL 0.66 mmol).
The mixture was stirred for 20 min and then O-tert-butyl-l-tyrosine
methyl ester hydrochloride (2.0 equiv., 172.0 mg, 0.60 mmol) was
added. (Yield: 80.3 mg, 0.075 mmol, 26%). 31P{1H} NMR
1
(CDCl3): δ = –47.8 (s) ppm. H NMR (CDCl3): δ = 7.60–6.48 (m,
26 H, Ar), 5.78 [d, 2 H, C(O)NH], 4.76 (m, 2 H, αCH), 4.44 (m, 4
H, PCH2N), 3.99 (dd, 4 H, PCH2N), 3.63 (s, 6 H, COOCH3), 2.87
(m, 4 H, βCH2), 2.80 [m, 4 H, CH2C(O)N], 2.40 (m, 4 H, N-
PhCH2), 1.30 [s, 18H C(CH3)3] ppm.
ditional insight into the role of a flexible outer coordination
2+
sphere on Ni(PR2NRЈ
)
2 2
molecular catalysts and provides
Ni(PPh2NpNNA-Tyr(tBu)OMe
L
) (BF4)2 [Tyr(tBu)OMe = O-tert-Butyl-
2 2
a starting point for introducing and evaluating a positioned
outer coordination sphere.
-Tyrosine Methyl Ester]: The purified PPh2NpNNA-Tyr(tBu)OMe li-
2
gand (29.6 mg, 0.03 mmol) was added to an acetonitrile solution
containing [Ni(CH3CN)6](BF4)2 (0.5 equiv., 7.19 mg, 0.015 mmol)
and stirred for 24 h. The resulting red solution was filtered through
Celite, and the solvent was removed under vacuum. The residual
red oil was then dissolved in a minimal amount of acetonitrile (ca.
2 mL) and added dropwise to 0 °C diethyl ether with stirring until
all of the solid had precipitated out of the acetonitrile/diethyl ether
solution. The resulting red solid was collected by filtration, washed
with diethyl ether, and dried in vacuo. (Yield: 20.1 mg, 8.8 μmol,
56.8%). 31P{1H} NMR (CD3CN): δ = 5.2 (s) ppm. 1H NMR
(CD3CN): δ = 7.40–6.83 (m, 52 H, Ar), 6.68 [d, 4 H, C(O)NH],
4.55 (m, 4 H, αCH), 4.15 (m, 8 H, PCH2N), 3.82 (m, 8 H, PCH2N),
3.53 (s, 12 H, COOCH3), 2.95 (m, 4 H, βCH2A); 2.83 (m, 4 H,
βCH2B), 2.80 [m, 8 H, CH2C(O)N], 2.42 (m, 8 H, N-PhCH2), 1.22
Experimental Section
Materials and Methods
All reactions were performed under an inert atmosphere of nitro-
gen by using standard Schlenk techniques or in a glovebox. Sol-
vents were de-oxygenated and purified with an Innovative Technol-
ogy, Inc. PureSolv™ solvent purification system. [D3]Acetonitrile
(Cambridge Isotope Laboratories, 99.5% D) was vacuum-distilled
from P2O5. [D]Chloroform (Cambridge Isotope Laboratories,
99.5% D) was degassed and stored over molecular sieves. Water
was dispensed from a Millipore MilliQ purifier at 18 MΩ and
sparged with nitrogen. 2-(1H-Benzotriazole-1-yl)-1,1,3,3-tetra-
methyluronium tetrafluoroborate (TBTU), 3-(4-aminophenyl)pro-
pionic acid, 3-(3-aminophenyl)propionic acid, ammonium chloride,
and trimethylsilyl chloride (Aldrich) were used as received. Diiso-
propylethylamine (DIPEA) (Aldrich) was degassed prior to use by
the freeze-pump-thaw method. The amino acid esters were pur-
chased from Nova Biochem or Sigma Aldrich and used as received.
[s, 36 H, C(CH3)3] ppm. MALDI MS: calcd. for [Ni(PPh
-
2
NpNNA-Tyr(tBu)OMe2)2]2+ 2186.93; found 2187.24.
Ni(PPh2NmNNA-Tyr(tBu)OMe
)
2 2
(BF4)2 [Tyr(tBu)OMe
=
O-tert-
Butyl-
L-Tyrosine Methyl Ester]: This complex was prepared in an
analogous manner to the synthesis described for [Ni(PPh
-
2
NpNNA-Tyr(tBu)OMe2)2](BF4)2
by
using
[Ni(MeCN)6](BF4)2
[Ni(MeCN)6](BF4)2 and PPh2Nm/p-NNA [p-NNA = 3-(4-amino- (0.5 equiv., 18.1 mg, 0.035 mmol) and PPh2NmNNA-Tyr(tBu)OMe
2
2
phenyl)propionic acid and m-NNA = 3-(3-aminophenyl)propionic
acid] were prepared by following literature methods.[18,31,32] All li-
gands and metal complexes were prepared as previously reported,
similar to the preparations for the Tyr(tBu)OMe ligands and com-
plexes reported below.[18]
(1.0 equiv., 78.7 mg, 0.07 mmol). (Yield: 48.2 mg, 24.7 μmol, 58%).
1
31P{1H} NMR (CD3CN): δ = 4.5 (s) ppm. H NMR (CD3CN): δ
= 7.41–6.83 (m, 52 H, Ar), 6.63 [d, 4 H, C(O)NH], 4.51 (m, 4 H,
αCH), 4.19 (m, 8 H, PCH2N), 3.88 (m, 8 H, PCH2N), 3.54 (s, 12
H, COOCH3), 2.98 (m, 4 H, βCH2A), 2.93 (m, 4 H, βCH2B), 2.83
[m,
8 H, CH2C(O)N], 2.43 (m, 8 H, N-PhCH2), 1.24 [s,
PPh2NpNNA-Tyr(tBu)OMe [Tyr(tBu)OMe = O-tert-Butyl-
L-Tyrosine
Methyl Ester]: TBTU (2.0 equiv., 107.0 mg, 0.33 mmol) and HOBT
2
36 H, C(CH3)3] ppm. MALDI MS: calcd. for [Ni(PPh
-
2
NmNNA-Tyr(tBu)OMe2)2]+2 2186.93; found 2187.20.
(2.0 equiv., 45.0 mg, 0.33 mmol) were added to a dichloromethane
solution containing PPh2NpNNA (1.0 equiv., 100.0 mg, 0.17 mmol)
Electrochemistry: All electrochemical experiments were carried out
in a glovebox under a nitrogen atmosphere by using a CH Instru-
ments 600 or 1100 series three-electrode potentiostat. The working
electrode was a glassy carbon disk (1 mm diameter), and the
counter electrode was a glassy carbon rod. A silver wire in electro-
lyte solution was separated from the working compartment by a
Vycor frit (4 mm, BAS) and was used as a pseudo-reference elec-
trode. All potentials were measured by using Cp2Fe (0 V) or
Cp2Co+ (–1.33 V) as internal references and all potentials are re-
ported vs. the Cp2Fe+/0 couple. All catalyst and
NBu4HCO2·HCO2H solutions[4] were freshly prepared in a tetra-
2
and DIPEA (2.2 equiv., 47.4 mg, 63.9 μL 0.38 mmol). The mixture
was stirred for 20 min and then O-tert-butyl-l-tyrosine methyl ester
hydrochloride (2.0 equiv., 96.0 mg, 0.33 mmol) was added and
stirred overnight. The solution was extracted with water to remove
residual chloride, and the dichloromethane solution was dried un-
der vacuum to yield a yellow viscous oil. This product was then
redissolved in a minimal amount of dichloromethane and flash pre-
cipitated from diethyl ether. This was repeated three more times.
The resulting off-white solid was collected and dried in vacuo.
(Yield: 29.6 mg, 0.03 mmol, 32.2%). 31P{1H} NMR (CDCl3): δ =
Eur. J. Inorg. Chem. 2013, 5366–5371
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
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in benzonitrile prior to analysis.
General Procedure for Electrocatalytic Formate Oxidation (Order
with Respect to Formate): In a glovebox, a stock solution of catalyst
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Acknowledgments
This work was funded by the US Department of Energy (DOE)
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences & Biosciences. Pacific Northwest National Laboratory
(PNNL) is a multiprogram national laboratory operated for DOE
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Received: June 15, 2013
Published Online: September 3, 2013
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim