Immobilization of an Amphiphilic Molecular Cobalt Catalyst on Carbon Black for Ligand-Assisted Water Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.7b03252

We have prepared the amphiphilic molecular catalyst [Co^III(L^OC₁₈)(pyrr)₂]ClO₄ (1), where L^OC₁₈ is the deprotonated form of N,N′-[4,5-bis(octadecyloxy)-1,2-phenylene]dipicolinamid

Full text of DOI:10.1021/acs.inorgchem.7b03252

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Immobilization of an Amphiphilic Molecular Cobalt Catalyst on
Carbon Black for Ligand-Assisted Water Oxidation
†
†
†
†
†
‡
Habib Baydoun, Jordyn Burdick, Bishnu Thapa, Lanka Wickramasinghe, Da Li, Jens Niklas,
Oleg G. Poluektov,‡ H. Bernhard Schlegel, and Clau
†
dio N. Verani*
,†
́
†
Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, Michigan 48202, United States
Chemical Sciences & Engineering Division, Argonne National Laboratory (ANL), Lemont, Illinois 60439, United States
‡
*
S Supporting Information
ABSTRACT: We have prepared the amphiphilic molecular
III OC18
OC18
catalyst [Co (L )(pyrr) ]ClO (1), where L
is the
2
4
deprotonated form of N,N′-[4,5-bis(octadecyloxy)-1,2-
phenylene]dipicolinamide. Species 1 can be anchored onto a
carbon black support to yield the assembly 1@CB, which can
catalyze water oxidation at an affordable onset overpotential of
2
0
.32 V, with a current density of 10 mA/cm at 0.37 V.
−1
Moreover, 1@CB displays TOF = 3850 h . A mechanism is
proposed based on the experimental and density-functional-
theory-calculated data.
INTRODUCTION
the oxidized ligand acts as a relevant electron reservoir but is
prone to degradation.
Recent efforts have focused on anchoring molecules onto
■
Surface functionalization is pivotal to enable the use of
homogeneous catalysts anchored onto conductive solid
supports. In this manner, future integrated water-splitting
devices can be developed in which water oxidation occurs at the
anode and proton reduction occurs at the cathode. Tradition-
ally, such electrodes are made with solid-state materials such as
metal oxides or phosphides. However, understanding the
surface chemistry of such extended and tridimensional materials
is challenging. On the other hand, understanding and
modifying the chemistry of surfaces via functionalization with
discrete and molecular coordination complexes is more
attainable because of the well-known and tunable electronic
signatures of molecular materials. As such, improving the
catalytic activity can be achieved using rational ligand design
along with electrochemical and spectroscopic methods.
The water-splitting reaction involves two half-reactions:
water oxidation and proton reduction. With a standard
potential of 1.23 VSHE, water oxidation is the more energetically
demanding of the two half-reactions. This high-energy
requirement is imparted by the need to perform four-electron
chemistry and form O−O bonds. The mechanisms of water
conductive solid supports in an attempt to favor the high-valent
n
n+
[
M L] product, enhancing the robustness of catalysts and
4
−6
favoring water splitting.
We have recently shown that
physisorbed Langmuir−Blodgett films of a procatalytic
phenolate-rich cobalt complex are effective on fluorine-doped
7
tin oxide. The resulting modified electrode is an excellent
water oxidation catalyst with an overpotential of 0.5 V.
However, structural rearrangement takes place after a potential
bias is applied. This transformation, coupled with the labor-
intensive layer-by-layer deposition process, precludes the large-
scale application of the approach.
Produced by the incomplete combustion of petrochemicals,
carbon black (CB) has been used as a solid support along with
8
−10
Nafion to drive catalytic water oxidation.
As such, we
hypothesized that CB can be functionalized with an amphiphilic
metal complex and that van der Waals interactions between the
substrate and complex will lead to more robust water oxidation
catalysts. Therefore, similar to other systems that we reported
7
,11−13
in the recent past,
we designed a new ligand with two
IV
octadecyloxy groups incorporated into the backbone. These
alkoxy groups play important functions: (i) they serve as
hydrophobic backbones to anchor the molecule onto CB, and
oxidation require the formation of high-valent oxo (M O or
V
III
M O) or hydroperoxo (M OOH) species. In order to lower
the activation barrier required to reach such highly oxidized
states needed for catalysis, redox-active ligands have been
1
,2
utilized. This is due to the formation of a radical-based
Special Issue: Applications of Metal Complexes with Ligand-
Centered Radicals
3
species instead of the expected high-valent tautomer in the
−
−
e
electron-transfer sequence [Mn−1L]0 ⎯⎯ →⎯ [Mn−1L ] ⇋ [M L] .
• +
n
+
Received: January 2, 2018
n−1 • +
When the last equilibrium step favors the [M L ] product,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b03252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
Scheme 1. Synthesis of 1, with R = C H
37
1
8
(ii) they lower the oxidation potential required for water
oxidation.
RESULTS AND DISCUSSION
■
OC18
The ligand H L
was prepared via the multistep synthetic
2
route outlined in Scheme 1, which involved the functionaliza-
tion of catechol with octadecyl groups to produce bis-
7
,12,14,15
(
octadecyloxybenzene).
This precursor was then
nitrated, yielding 1,2-dinitro-4,5-bis(octadecyloxy)benzene,
which was subsequently reduced to form the air-sensitive
precursor 4,5-bis(octadecyloxy)benzene-1,2-diamine. The dia-
mine was treated with picolinic acid to yield the ligand N,N′-
[
4,5-bis(octadecyloxy)-1,2-phenylene]dipicolineamide,
OC18 16
H2L
.
III OC18
OC18 2−
The complex [Co (L )(pyrr) ]ClO (1) contains a
2
4
doubly deprotonated (L
)
ligand and was prepared
Figure 1. (a) Cyclic voltammogram of 1 (1 mM) in CH
Cl , with
2 2
following a modification to the previously reported proce-
TBAPF as the supporting electrolyte, glassy carbon as the working
electrode, Ag/AgCl as the reference electrode, and platinum wire as
the auxiliary electrode.
6
1
7,18
OC
18
dure
in which stoichiometric amounts of H2L
were
treated with cobalt acetate in the presence of pyrrolidine. After
stirring under aerobic conditions, 1 was precipitated using
1
IV
5
V
NaClO . Catalyst 1 was thoroughly characterized using H
generation of the formal high-valent Co 3d and possibly Co
4
4
NMR, Fourier transform infrared (FTIR), mass spectrometry,
3d species but have been rather described as amido to amidyl
radical transformations in unsubstituted systems.
interesting to note that compared to our previously studied and
20,21
and elemental analysis. A strong peak in the FTIR spectrum at
It is
1
1
091 cm− confirmed the presence of a perchlorate counterion.
III
This observation, coupled with the occurrence of sharp peaks in
the NMR spectrum (Figure S1), suggests that we have formed
structurally related [Co (L)(pyrr) ]PF (2) system with a
2
6
similar nonalkoxy-functionalized ligand, the current oxidation
processes occur at potentials that are about 0.5 V less positive.
LS
6
22
a cobalt(III) complex in a pseudooctahedral 3d environment.
High-resolution mass spectrometry shows a peak cluster at m/z
This decrease in the oxidative potential is associated with the
electron-donating effect of the alkoxy chains.
6
911.5825 corresponding to the metal complex without the two
axial pyrrolidines. The elemental analysis results confirm these
structural assignments.
The cyclic voltammogram of 1 was taken in dichloromethane
The X-band electron paramagnetic resonance (EPR)
spectrum of the electrochemically generated singly oxidized
IV OC18
2+
cobalt complex, nominally [Co (L )(pyrr) ] , revealed a
2
(
CH Cl ) in the presence of tetrabutylammonium hexafluor-
narrow isotropic signal at g = 2.00, close to the free-electron g
value, with a line width of 3.5 mT (Figure 2). The typical EPR
signals expected for either high-spin or low-spin (LS)
2
2
ophosphate (TBAPF ) as the supporting electrolyte using
6
19
ferrocene as an internal standard. The cyclic voltammogram
Figure 1) shows four independent redox events consisting of
two oxidative and two reductive processes. The irreversible
reductive processes at −0.36 and −1.12 V (E ) are
23−25
(
cobalt(IV) complexes are absent.
spectrum is consistent with an S = / state associated with the
Therefore, the observed
1
2
III OC18
formation of a radical-bearing species like [Co (L •)-
SHE
pc
III
6
2+
tentatively assigned as the sequential reduction from Co 3d
to Co 3d and then Co to Co 3d . Two oxidation processes
(pyrr) ] . These conclusions are also supported by density
2
II
7
II
I
8
1
functional theory (DFT) calculations. For this S = / state, the
2
at 0.91 VSHE (ΔE = 92 mV; |I /I | = 0.94) and 1.41 V (ΔE
spin-density plot (Figure 3a) revealed a delocalized unpaired
electron on the diamine moiety of the ligand rather than on the
pa pc
SHE
=
94 mV; |I /I | = 1.45) are respectively associated with the
pa pc
B
DOI: 10.1021/acs.inorgchem.7b03252
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Inorganic Chemistry
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order to assess the catalytic properties of 1, we anchored the
molecule onto the surface of CB to yield an assembly denoted
as 1@CB. This was done by adding a solution of 1 to a
suspension of CB in CH Cl . Then, ethanol was added to
2
2
precipitate the resulting suspension. The precipitate was
isolated by centrifuging the sample. Following decantation of
the supernatant, the precipitate was dried overnight under
ambient conditions and resuspended using a 2:1:1:1 mixture of
ethanol/isopropyl alcohol/water/Nafion. The sample was
ultrasonicated to form the homogeneous ink that was used in
the catalysis. Inductively coupled plasma mass spectrometry
(
ICP-MS) analysis of the final ink revealed that 70% of the
cobalt present in the initial solution was anchored on CB. The
FTIR spectra of 1, CB, and 1@CB (Figure S2) show that the
characteristic peaks of 1 (where CB does not have significant
absorption) are retained in the ink, thus indicating that
molecular deposition took place without major structural
changes. Unfortunately, attempts to prepare a similar ink
Figure 2. cw X-band EPR spectrum of the electrochemically singly
IV OC18
2+
oxidized cobalt complex (nominally [Co (L )(pyrr) ] ) in
III
22
2
based on the unsubstituted [Co (L)(pyrr) ]ClO (2) were
2 4
CH Cl at T = 30 K.
2
2
unsuccessful.
The 1@CB ink was tested for catalysis using a rotating-disk
electrode operating at 1600 rpm in a 1 M potassium hydroxide
(
KOH) solution. Figure 4a shows the iR-corrected polarization
curves obtained for 1@CB (black trace) in comparison to the
blank CB (red trace). Significant current enhancement is
attained by 1@CB with an onset overpotential of 0.32 V. At an
overpotential of 0.37 V, the current density reaches 10 mA/
2
cm , an important figure of merit in the development of solar
31
cells operating at 10% efficiency. Remarkably, 1@CB has an
III OC18
2+
Figure 3. (a) Spin-density plot for [Co (L •)(pyrr)2] and (b)
III OC18
3+
the LUMO for [Co (L ••)(pyrr) ] .
2
metal center. This observation is in line with our previous
report on the redox activity of phenylenediamine bridging
2
6
moieties.
The doubly oxidized species is most stable as a diamagnetic
singlet (S = 0) with no observable spin density because of its
closed shell. However, the lowest unoccupied molecular orbital
(
LUMO; Figure 3b) was calculated as ligand-based. These data
suggest the assignment of the second oxidation as being ligand-
based like the first oxidation and best described as
III OC18••
3+
[
Co (L
)(pyrr) ] .
2
Although our observations agree with previous assign-
2
0,21
ments
that favor the formation of ligand radicals, Åkermark
2
7
et al. have shown that a related ruthenium complex can
efficiently catalyze water oxidation. Furthermore, Kim and co-
2
8−30
workers
have shown that iron, cobalt, and manganese ions
act as oxygen-atom-transfer agents in similar [N ] planar
4
environments and that high-valent oxo species were observed in
situ. These same oxo species stabilized during oxygen-atom
transfer are essential intermediates in the water oxidation
mechanism. These observations suggest that the species
III OC18
•
+
IV OC18
+
[
Co (L
)] and the putative [Co (L )] may be very
close in energy because the latter cannot be calculated without
converging into the former. As such, an oxo species may be
IV
OC18
+
better described as nominally [Co O(L )] . Therefore,
we hypothesized that 1 could perform water oxidation as a
molecular catalyst. The evaluation of these results allows us to
infer whether the metal or the bis(amidopyridine) ligand is
associated with the high oxidation state required for catalysis. In
Figure 4. (a) Polarization curves for 1@CB and the blank CB in a 1
mol/L KOH solution of pH 14 [RDE glassy carbon 1600 rpm (WE),
Ag/AgCl (RE), Pt wire (AE)]. (b) Plot of current versus time, under
an applied potential of 0.7 VAg/AgCl (0.89 VSHE) for a 2 h catalytic run.
C
DOI: 10.1021/acs.inorgchem.7b03252
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Inorganic Chemistry
Forum Article
III OCH
3
+
Figure 5. Proposed water oxidation mechanism of [Co (L
.04 au).
)(pyrr) ] . Spin-density plots are shown next to the paramagnetic species (isovalue =
2
0
2
39
overpotential at 10 mA/cm on par with recently reported
nanostructured materials.
catalytic cycle. Indeed, ICP-MS analysis of the postcatalytic
solution reveals the presence of 4 ppb of cobalt ion, against
trace amounts below the detection limit for the blank.
8
,9,31−34
To evaluate the catalytic activity of 1@CB, we performed a
bulk electrolysis experiment to determine the turnover number
We also performed X-ray photoelectron spectroscopy (XPS)
analysis on 1@CB to assess whether the surface-immobilized
(TON) using a custom-built H-type electrocatalytic cell. The
ink was deposited onto a carbon cloth working electrode, which
was then used during catalysis. Following 2 h of electrolysis at
an applied potential of 0.89 VSHE (overpotential = 0.49 V), 154
μmol of oxygen were produced that matched very well with the
theoretical amount of oxygen based on the net charge
consumption of 155 μmol (50.9 C). Considering that the
initial amount of cobalt present on the surface is estimated as
molecules of 1 rearrange into CoO -based nanoparticulates.
x
The results shown in Figure S3 reveal that the Co 2p3/2 and Co
2p1/2 energy peaks, respectively at 781 and 796 eV, remain
unchanged before and after catalysis. This observation was
further confirmed using electrochemical measurements (Figure
S4). We observed an electrochemical process at 0.08 V
SHE
III
II
before catalysis, which we ascribed to the Co /Co couple. As
expected for molecular catalysts, the process remained intact
after catalysis. These results point out that the molecular nature
of species 1 persists during catalysis.
0
.02 μmol, the assembly underwent 7700 turnovers, well above
35−38
other molecular catalysts in solution.
This TON value
corresponds to an observed averaged turnover frequency of
1
3
850 h− and a near-quantitative Faradaic efficiency of 99%.
In general, O−O bond formation can occur via two possible
pathways in the water oxidation mechanism. In the first
pathway, two CoO species combine to form a μ-peroxo
bridging intermediate (Co−O−O−Co). This mechanism is
unlikely to be present here because 1 is anchored onto CB with
limited surface mobility. We therefore favor a second pathway
Figure 4b shows the plot of current versus time. The slight
decrease in the current observed during catalysis suggests that
the catalyst is undergoing a slow deactivation process in which
the cobalt is possibly leaching out of the electrode. On the basis
of the recently published behavior of the analogous
22
−
unsubstituted 2 toward water reduction, this deactivation is
in which a nucleophilic attack by OH onto the CoO species
likely due to the formation of a ligand-based radical during the
takes place. This possibility was evaluated using DFT
D
DOI: 10.1021/acs.inorgchem.7b03252
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Inorganic Chemistry
Forum Article
calculations on a truncated analogue where −OCH replaces
Cyclic voltammograms in organic solvents were recorded using a BAS
3
5
0W electrochemical analyzer. TBAPF was used as the supporting
−
OC H , and our results suggest that catalysis is initiated by
6
18
37
electrolyte, Ag/AgCl was used as the reference electrode (RE), and a
platinum wire was used as the auxiliary electrode (AE). The cyclic
voltammograms were recorded under an argon atmosphere, and
the substitution of one pyrrolidine by a hydroxide ion, yielding
III OCH
3
0
[
Co (L
)(pyr)(OH)] (Figure 5). This transformation is
favored by 7.4 kcal/mol. The catalytic cycle ensues with
19
ferrocene was added as an internal standard at a scan rate of 0.1 V/s.
III OCH
3
0
[
Co (L
)(pyr)(OH)] undergoing a one-electron oxida-
XPS spectra were recorded using a Kratos Axis Ultra X-ray
photoelectron spectrometer, and all of the spectra were calibrated
using C 1s at 284.8 eV as a charge reference. XPS spectra of the
electrode itself did not show any discernible cobalt peaks; therefore,
functionalized CB was removed from the surface of the electrode by
sonicating it in ethanol. Then ethanol was removed, and the
concentrated sample was used for XPS analysis.
III OCH
3
+
tion to yield the ligand-oxidized [Co (L
a potential of 0.34 VSHE. Following this step, a proton-coupled
•)(pyr)(OH)] at
IV
electron transfer takes place to form the formally [Co O]-
III OCH
3
•
+
containing species [Co (L
•)(pyr)(oxo )] at 0.97 V . It
III •
SHE
is important to note that the unpaired electron on [Co O ]
is significantly delocalized over the metal center (Mulliken spin
densities of 0.11 on cobalt and 0.89 on oxo; Table S3).
Although the calculated potential for this step appears to be
higher than the experimentally determined onset potential, this
difference is well within the accepted values associated with
Synthesis of 1,2-Bis(octadecyloxy)benzene. A solution of 1-
bromooctadecane (23.4 g, 70.2 mmol) in dimethylformamide (DMF;
7
0 mL) was added dropwise under inert conditions to a solution of
catechol (3.50 g, 31.8 mmol) and anhydrous potassium carbonate
16.0 g, 116 mmol) in anhydrous DMF (53 mL). The reaction was
(
40
DFT-calculated electrochemical potentials. Then an iso-
allowed to reflux for 18 h, after which it was poured into ice-cold water
thermal transformation (ΔG = 1.5 kcal/mol) in which a
(150 mL) and subsequently extracted with CH Cl (100 mL). The
2
2
−
nucleophilic attack by OH onto the CoO oxyl species takes
organic layer was dried over anhydrous sodium sulfate (Na SO ). The
2 4
III OCH
3
place forms the diamagnetic hydroperoxo [Co (L
)(pyr)-
solvent was removed by rotary evaporation, and the remaining yellow
oil was dissolved in a 19:1 mixture of hexane and ethyl acetate. The
solution was filtered through silica under vacuum. The isolated yellow
solution was concentrated to half of its original volume and+
refrigerated to yield a white precipitate. Yield: 72%. ESI: m/z
0
III OCH
3
0
(
OOH)] . Species [Co (L
)(pyr)(OOH)] undergoes a
III OCH
3
one-electron oxidation at 0.32 V
to yield [Co (L
•)-
SHE
0
(
pyr)(OOH)] , which is deprotonated into a superoxide
III OCH
3
+
species described as [Co (L
)(pyr)(OO•)] (ΔG =
+
−1
6
2
37.5892 (100%) for [C H O + Na ]. IR (KBr, cm ): 2850−
4
2
78
2
−
19.8 kcal/mol). Following a further one-electron oxidation
1
919 (ν ), 1595 (ν
), 1508 (ν
), 1259 (ν
). H NMR
C−H
CC
CC
C−O−C
III
OCH
3
•
+
to form [Co (L
•)(pyr)(OO )] at 0.35 VSHE, a
Ph
OCH
2
(CDCl , 400 MHz): δ 6.866 (s, 4H ), 3.969 (t, 4H
), 1.792 (m,
3
−
CH
2
CH
2
CH
2
CH
3
nucleophilic attack by OH takes place to release O and
4H ), 1.444 (m, 4H ), 1.239 (s, 56H ), 0.863 (t, 6H ).
Synthesis of 1,2-Dinitro-4,5-bis(octadecyloxy)benzene. The
compound 1,2-bis(octadecyloxy)benzene (2.05 g; 3.33 mmol) was
dissolved in 48 mL of a 1:1 CH Cl /acetic acid mixture. The reaction
2
III OCH
3
0
regenerate [Co (L
)(pyr)(OH)] . It is interesting to note
that throughout our transformations the oxidation state of the
III
metal remains largely unchanged at [Co ], while the ligand
2
2
mixture was subsequently placed in an ice bath, and 3.50 mL of nitric
acid was added. Following the addition, the reaction was stirred at
room temperature for 30 min and then cooled back in an ice bath,
acts as the site were oxidizing equivalents are stored. This is in
contrast to most other reported systems in which pure metal
oxidation or highly limited ligand involvement are as-
when 8.50 mL of fuming HNO was added. Then the reaction mixture
2
,41−46
3
sumed.
was stirred at room temperature for 72 h before being poured into ice-
cold water (100 mL, 0 °C). The organic layer was washed with (3 ×
CONCLUSION
100 mL) saturated NaHCO and a brine solution. The organic layer
3
■
was subsequently dried over Na SO . The organic solvent was
2
4
In conclusion, we have reported on a novel alkoxy-substituted
cobalt complex that can be anchored onto CB. The resulting
@CB assembly, suspended in ink form, catalytically oxidizes
water under basic conditions with an affordable onset
overpotential of 0.32 V. The assembly reaches the required
removed by rotary evaporation and recrystallized from acetone to yield
+
yellow crystals. Yield: 86%. ESI: m/z 743.5341 (100%) for
1
+
−1
[
C H N O + K ]. IR (KBr, cm ): 2850−2918 (νC−H), 1587
42
76
2
6
(
ν
), 1539 (ν
), 1466 (ν
), 1335 (ν
), 1294 (ν
).
),
CC
CC
NO
NO
C−O−C
1
Ph
OCH
2
H NMR (CDCl , 400 MHz): δ 7.270 (s, 2H ), 4.073 (t, 4H
3
2 2 2
2
CH
CH
CH
current density of 10 mA/cm needed for solar cell develop-
1.846 (m, 4H ), 1.453 (m, 4H ), 1.234 (s, 56H ), 0.857 (t,
3
CH
ment at an overpotential of just 0.37 V, while retaining its
molecular nature. The affordable potentials are associated with
ligand oxidation, thus acting as an electron reservoir that
effectively lowers the activation barrier needed for catalysis.
This involvement, however, leads to slow catalyst deactivation
by demetalation due to an unfavorable equilibrium involving
6H ).
Synthesis of 4,5-Bis(octadecyloxy)benzene-1,2-diamine. The
compound 1,2-dinitro-4,5-bis(octadecyloxy)benzene (2.4 g, 3.4 mmol)
reacted with 6.3 mL of hydrazine monohydrate and 0.1 g of 10%
palladium/carbon in absolute ethanol (50 mL) under inert conditions.
The mixture was subsequently allowed to reflux for 18 h and then
filtered through Celite under inert conditions. The solvent was
reduced to dryness under vacuum to yield a white precipitate. Yield:
n−1
+
n
+
[
M
L•] ⇋ [M L] . As such, while observed TONs reached
1
Ph
notable values above 5000 for molecular catalysts, improve-
ments in design will be necessary to allow for the formation of
high-valence cobalt species expected to enhance the stability of
the catalyst.
7
4H
6%. H NMR (CDCl , 400 MHz): δ 6.350 (s, 2H ) 3.858 (t,
3
2 2 2 2
OCH
NH
CH
CH
), 3.121 (s, 4H ), 1.717 (m, 4H ), 1.410 (m, 4H ), 1.234
2 3
CH
CH
(s, 56H ), 0.856 (t, 6H ).
Synthesis of N,N′-[4,5-Bis(octadecyloxy)-1,2-phenylene]-
OC18H
37
dipicolineamide (H L
). A solution of 4,5-bis(octadecyloxy)-
2
benzene-1,2-diamine (1.05 g, 1.63 mmol) in anhydrous pyridine was
cannulated under inert conditions to a mixture of picolinic acid (0.401
g, 3.26 mmol) and triphenyl phosphite (1.01 g, 0.86 mL, 3.26 mmol)
in anhydrous pyridine (10 mL). The solution was heated in a water
bath for 5 h. Then the product crashed out by the addition of
methanol to obtain a pale-yellow precipitate. The precipitate was
subsequently filtered and washed with methanol. Yield: 56%. ESI: m/
EXPERIMENTAL SECTION
Materials and Characterization. Ultrapure water (18.1 MΩ.cm)
was used in all manipulations. All other solvents and reagents were
■
1
from commercial sources and were not further purified. H NMR
spectra were recorded on a Varian 400 or 600 MHz spectrometer.
FTIR spectra were recorded on a Bruker Tensor 27 spectrometer
using potassium bromide (KBr) pellets recorded between 4000 and
+
+
−1
z 855.6721 (100%) for [C H N O + H ]. IR (KBr, cm ): 3301
54
86
4
4
−1
6
00 cm . Elemental analysis was performed by Midwest Microlabs,
(νN−H), 2850−2920 (νC−H), 1660−1680 (ν
), 1609, 1591, 1570,
), 1236 (νC−O−C). H NMR
CO
1
Indianapolis, IN, and measured using an Exeter CE440 CHN analyzer.
1529 (ν
and ν
CC,aromatic
CN,pyridine
E
DOI: 10.1021/acs.inorgchem.7b03252
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Inorganic Chemistry
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NH
py
47
(
CDCl , 400 MHz): δ 10.096 (s, 2H ), 8.533 (dd, 2H ), 8.283 (dd,
E01). DFT calculations were carried out using the B3LYP*
3
py
py
py
Ph
48
2
4
0
H ), 7.875 (td, 2H ), 7.433 (td, 2H ), 7.407 (s, 2H ), 4.021 (t,
functional. Geometries were optimized using B3LYP* with the
SDD basis set with an effective core potential for the cobalt atom
and the 6-31G(d,p) basis set for all other atoms. The energetics of
structures with different spin states were compared, and the lowest
energy structures were used for the proposed catalytic cycle. The
structures with the lowest Gibbs free energy were reoptimized with the
SDD (for the cobalt atom) and 6-31+G(d,p) (for all other atoms)
OCH
2
CH
2
CH
2
CH
2
49
H
), 1.812 (m, 4H ), 1.488 (m, 4H ), 1.233 (s, 56H ),
CH
3
.855 (t, 6H ).
Synthesis of the Complex [Co (L
III OC18H
37
)(pyrr) ]ClO . The salt
2
4
−
of Co(CH COO ) ·4H O (0.060 g, 0.240 mmol) dissolved in 1 mL of
methanol was added dropwise to 0.200 g (0.232 mmol) of the ligand
3
2
2
OC18H
37
L
(
dissolved in 1 mL of CHCl . Following this addition, 0.5 mL
3
5
0,51
excess) of pyrrolidine was added. The solution was stirred overnight
basis sets to obtain more accurate energies.
The SMD implicit
at room temperature and then filtered to separate residual solids, and
oxygen was bubbled in for 10 min. Precipitation of the microcrystalline
product was induced by adding 0.020 g (0.269 mmol) of NaClO4.
Multiple attempts to yield crystals for X-ray studies were unfulfilled.
solvation model was used to incorporate solvation effects in water and
included in all of the geometry optimizations. Previous studies
5
2
53−56
have shown that a small number of explicit water molecules need to be
included to account for some of the short-range interactions while
+
−
Yield: 32 mg, 47%. ESI: m/z 911.58251 (100%) for
modeling anions such as OH in aqueous solution using the SMD
+
−1
[
(
(
C H N O Co ]. IR (KBr, cm ): 3430 (ν ), 2852−2921
implicit solvation model. Therefore, the lowest-energy structures
involved in the catalytic cycle were optimized, with two explicit water
5
4
84
4
4
N−H
), 1207 (ν
py
νC−H), 1625 (ν
), 1598 (ν
), 1091
CO
CN,pyridine
C−O−C
1
Ph
−
−
νClO4). H NMR (CDCl , 600 MHz): δ 9.76 (d, 2H ), 8.60 (s, 2H ),
molecules forming hydrogen bonds with OH/O /OOH/OO
3
py
py
OCH
2
), 3.05 (2HNH), 2.10
moieties in SMD implicit solvation. All of the geometries were
confirmed to be minima on the potential energy surfaces by
performing a vibrational frequency calculation and had no imaginary
frequency. Thermal and entropy contributions to the free energies
were calculated by standard statistical thermodynamical methods using
unscaled B3LYP* frequencies and a rigid rotor/harmonic oscillator
approximation at 298.15 K. The converged wave functions were tested
for self-consistent-field stability.
8
(
[
.26 (t, 2H ), 8.22 (d, 2H ), 4.06 (4H
pyr
CH
3
4H ), 1.25−1.66 (m, 72H), 0.86 (6H ). Anal. Calcd for
C H CoN O Cl]: C, 64.54; H, 8.91; N, 7.28. Found: C, 64.49;
6
2
102
6
8
H, 8.91; N, 7.02.
Ink Preparation. CB (6 mg) was suspended in CH Cl2 and
2
sonicated for 5 min. Following sonication, 0.125 mL of a stock
III OC18H
37
solution of [Co (L
)(pyrr) ]ClO (4 mg/mL in CH Cl ) was
2 4 2 2
added to CB. The mixture was sonicated for a further 15 min.
Following sonication, ∼40 mL of ethanol was added to the CB
suspension. The suspension was centrifuged, the solvent was decanted,
and CB was allowed to dry overnight under ambient conditions. The
dry carbon sample was suspended in 0.300 mL of water, 0.600 mL of
ethanol, and 0.300 mL of isopropyl alcohol. This suspension was
sonicated for 15 min, and then 0.300 mL of Nafion was added. The
suspension was further sonicated for 15 min to form the ink used for
catalysis. On the basis of the initial amount of 1 added, we expect the
final concentration of cobalt in the ink to be 15.6 μg/mL; however,
ICP-MS analysis revealed that the actual concentration of cobalt in the
ink is 10.9 μg/mL, which is a 70% adsorption efficiency.
n+
For a redox reaction, A(aq) → A (aq) + ne−(aq), the standard
n+
redox potential is given by eq 3, where ΔG * = G
e(g)
* − G
* −
(aq)
aq
A(aq)
A
G
* is the standard free energy change for a redox couple in
solution, Ge(g)* = 0.867 kcal/mol is the free energy of the electron at
5
3,57
2
98.15 K,
obtained from the literature, F is Faraday’s constant
(
23.06 kcal/mol·V), n is the number of electrons, and SHE is the
58−60
absolute potential of the standard hydrogen electrode (4.281 V).
−
ΔG *
nF
aq
Eaq° =
− SHE
(3)
The pH-dependent redox potential can be obtained by solving the
Nernst half-cell equation where E° is the standard redox potential (eq
Catalytic Measurements. Cyclic Voltammetry. All cyclic
voltammograms were recorded using an EC Epsilon potentiostat
equipped with an RDE2 rotating-disk electrode. The three-electrode
setup included an Ag/AgCl reference electrode, a platinum wire
4
). The redox potential at a particular pH can be obtained by including
the equilibrium concentration of each of the relevant species at that
pH using acid dissociation constants (K ’s). For low ionic strength, the
a
auxiliary electrode, and a glassy carbon working electrode (surface area
pH-dependent redox potential is given by eq 5, where AH is the
2
=
0.07 cm ). A total of 5 μL of the ink was deposited on the glassy
•+
reduced form, AH is the one-electron-oxidized form, and A• is the
carbon surface of the working electrode and dried under an IR heat
lamp for 5 min. The cyclic voltammograms were measured at a scan
rate of 10 mV/s at 1600 rpm in a 1 M KOH solution. After the
potentials against Ag/AgCl were measured, they were converted to the
standard hydrogen electrode (RHE) using eq 1. Additionally, using the
iR compensation function of the Epsilon software, the resistivity of the
solution was measured, and corrections were calculated using eq 2.
oxidized form after the proton-coupled electron-transfer process.
⎛
⎞
⎟
E1/2 = E° − Rn FT ln⎜
[Red]
⎝ [Ox] ⎠
(4)
(5)
−
pKa
−pH
RT ⎛10
⎞
+ 10
EpH = E
•
+
/AH° −
ln⎜
⎟
⎠
A ,H
−
(pK +pH)
a
nF
⎝
10
ESHE = EAg/AgCl + 0.197
(1)
The aqueous-phase free energy of the proton is calculated as in eq 6,
ESHE = EAg/AgCl + 0.197 − iR
+
(2)
where ΔG
* = −265.9 kcal/mol is the solvation energy of the
H (aq)
5
8−60
+
proton taken from the literature,
G
° = −6.29 kcal/mol is the
H (g)
Bulk Electrolysis and Faradaic Efficiency. Bulk electrolysis was
performed under an overpotential of 0.49 V in a custom H-type cell
that is split into two compartments by a fine frit. The auxiliary
electrode, a platinum coil, was placed in one compartment, and the
reference (Ag/AgCl) and working electrodes were placed in the other.
The working electrode was prepared by depositing 100 μL of the
functionalized ink (contains 0.02 μmol of 1) onto a 1 cm × 4 cm piece
of carbon cloth, allowing it to dry for 15 min, and threading it onto a
copper wire. A GOW-MAC series 400 gas chromatograph equipped
with a thermal conductivity detector and a 8 ft × / in., 5 Å molecular
sieve column held at 60 °C were used to quantify the amount of
oxygen produced. Helium was used as the carrier gas and flowed at a
rate of 30 mL/min. Atmospheric nitrogen was used as an internal
standard.
corr
gas-phase standard free energy of the proton, and ΔG = 1.89 kcal/
mol is the correction term for conversion of a standard state of 1 atm
in the gas phase to the standard state of a 1 M solution.
57,61
corr
G
+
* = G
+
° − ΔG
+
* − ΔG
H (aq)
H (g)
H (aq)
(6)
EPR Spectroscopy. Continuous-wave (cw) X-band (9−10 GHz)
EPR experiments were carried out with a Bruker ELEXSYS II E500
EPR spectrometer (Bruker Biospin, Rheinstetten, Germany), equipped
with a TE102 rectangular EPR resonator (Bruker ER 4102ST) and a
helium gas-flow cryostat (ICE Oxford, U.K.). An intelligent temper-
ature controller (ITC503) from Oxford Instruments, Oxford, U.K.,
was used. Measurements on the frozen solutions were done at a
cryogenic temperature (T = 30 K). Data processing was done using
Xepr (Bruker BioSpin, Rheinstetten, Germany) and Matlab 7.11.2
(The MathWorks, Inc., Natick, MA) environments.
1
8
Computational Methods. Electronic structure calculations were
performed using the Gaussian series of programs (Gaussian09, revision
F
DOI: 10.1021/acs.inorgchem.7b03252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
of Ni -xRuxP nanocrystals: enhancing activity by dilution of the noble
ASSOCIATED CONTENT
2
■
metal. J. Mater. Chem. A 2017, 5, 17609−17618.
*
S
Supporting Information
(11) Wickramasinghe, L. D.; Perera, M. M.; Li, L.; Mao, G. Z.; Zhou,
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03252.
Z. X.; Verani, C. N. Rectification in Nanoscale Devices Based on an
Asymmetric Five-Coordinate Iron(III) Phenolate Complex. Angew.
Chem., Int. Ed. 2013, 52, 13346−13350.
(12) Wickramasinghe, L. D.; Mazumder, S.; Gonawala, S.; Perera, M.
Experimental procedures and select figures (PDF)
M.; Baydoun, H.; Thapa, B.; Li, L.; Xie, L.; Mao, G.; Zhou, Z.;
Schlegel, H. B.; Verani, C. N. The Mechanisms of Rectification in Au|
Molecule|Au Devices Based on Langmuir−Blodgett Monolayers of
Iron(III) and Copper(II) Surfactants. Angew. Chem., Int. Ed. 2014, 53,
AUTHOR INFORMATION
■
*
Corresponding Author
E-mail: cnverani@chem.wayne.edu.
1
(
4462−14467.
13) Gonawala, S.; Leopoldino, V. R.; Kpogo, K.; Verani, C. N.
ORCID
Langmuir-Blodgett films of salophen-based metallosurfactants as
surface pretreatment coatings for corrosion mitigation. Chem.
Commun. 2016, 52, 11155−11158.
Jens Niklas: 0000-0002-6462-2680
Oleg G. Poluektov: 0000-0003-3067-9272
H. Bernhard Schlegel: 0000-0001-7114-2821
Claudio N. Verani: 0000-0001-6482-1738
́
(
14) Wickramasinghe, L. Redox-Active Trivalent Metallosurfactants
with Low Global Symmetry for Molecule-Based Electronics: Spectroscopic,
Electrochemical, and Amphiphilic Properties of New Molecular Materials
for Current−Voltage Measurements in M|lb-Monolayer|m Devices; Wayne
State University: Detroit, MI, 2014.
Notes
The authors declare no competing financial interest.
(15) Johnson, M. S.; Wickramasinghe, L.; Verani, C.; Metzger, R. M.
Confirmation of the Rectifying Behavior in a Pentacoordinate [N2O2]
Iron(III) Surfactant Using a “Eutectic GaIn | LB Monolayer | Au”
Assembly. J. Phys. Chem. C 2016, 120, 10578−10583.
ACKNOWLEDGMENTS
■
The authors thankfully acknowledge support from the National
Science Foundation (Grant NSF-CHE-1500201 to C.N.V. and
Grant NSF-CHE1464450 to H.B.S.) and from the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences (Grant DE-SC0001907 to C.N.V. and H.B.S.
and Contract DE-AC02-06CH11357 to J.N. and O.G.P. at
ANL). Jordyn Burdick is an undergraduate researcher.
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