Electroactive Co(iii) salen metal complexes and the electrophoretic deposition of their porous organic polymers onto glassy carbon

Article abstract of DOI:10.1039/c8ra04385j

This paper reports the CO₂ electroreduction properties of three bis-bromo Co(iii) salen metal complexes and their Porous Organic Polymers (POPs) as a platform for using the salen core as a multi-electron reducing agent. Although Co(iii) salen metal complexes have been studied extensively for their chemical catalysis with CO₂, their electrochemical behaviour, particularly their reduction, in the presence of CO₂ is much less explored. The discrete Co(iii) complexes enabled the reduction of CO₂ to CO in faradaic efficiencies of up to 20%. The reductive electrochemical processes of Co(iii) salen complexes are relatively unknown; therefore, the mechanism of reduction for the complexes was investigated using IR and UV-Vis-NIR spectroelectrochemical (SEC) techniques. The discrete bis-bromo salen complexes were incorporated into POPs with tris-(p-ethynyl)-triphenylamine as a co-ligand and were characterised using solid state NMR, IR, UV-Vis-NIR and Field Emission Scanning Electron Microscopy (FE-SEM). The POP materials were electrophoretically deposited onto glassy carbon under milder conditions than those previously reported in the literature. Direct attachment of the POP materials to glassy carbon enabled improved solid state electrochemical analysis of the samples. The POP materials were also analysed via SEC techniques, where a Co(ii/i) process could be observed, but further reductions associated with the imine reduction compromised the stability of the POPs.

Full text of DOI:10.1039/c8ra04385j

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PAPER
Electroactive Co(III) salen metal complexes and the
electrophoretic deposition of their porous organic
polymers onto glassy carbon†
Cite this: RSC Adv., 2018, 8, 24128
a
b
c
Marcello B. Solomon,
Clifford P. Kubiak,
Aditya Rawal,b James M. Hook,
Seth M. Cohen,
c
a
a
*
*
Katrina A. Jolliffe
and Deanna M. D'Alessandro
This paper reports the CO2 electroreduction properties of three bis-bromo Co(III) salen metal complexes
and their Porous Organic Polymers (POPs) as a platform for using the salen core as a multi-electron
reducing agent. Although Co(III) salen metal complexes have been studied extensively for their chemical
catalysis with CO2, their electrochemical behaviour, particularly their reduction, in the presence of CO2 is
much less explored. The discrete Co(III) complexes enabled the reduction of CO2 to CO in faradaic
efficiencies of up to 20%. The reductive electrochemical processes of Co(III) salen complexes are
relatively unknown; therefore, the mechanism of reduction for the complexes was investigated using IR
and UV-Vis-NIR spectroelectrochemical (SEC) techniques. The discrete bis-bromo salen complexes
were incorporated into POPs with tris-(p-ethynyl)-triphenylamine as a co-ligand and were characterised
using solid state NMR, IR, UV-Vis-NIR and Field Emission Scanning Electron Microscopy (FE-SEM). The
POP materials were electrophoretically deposited onto glassy carbon under milder conditions than those
previously reported in the literature. Direct attachment of the POP materials to glassy carbon enabled
improved solid state electrochemical analysis of the samples. The POP materials were also analysed via
SEC techniques, where a Co(II/I) process could be observed, but further reductions associated with the
imine reduction compromised the stability of the POPs.
Received 23rd May 2018
Accepted 20th June 2018
DOI: 10.1039/c8ra04385j
rsc.li/rsc-advances
nancial, logistical and environmental challenges. Other chief
concerns for CCS are the safety and environmental aspects
Introduction
that arise from the high pressures and large concentrations of
CO2. Processes that can combine the capture of CO2 with its
use as a feedstock may assist in the handling of emissions.
The benet of using CO2 as a feedstock lies in its abundance
and low cost; however, its use as a cheap feedstock is limited by
the presence of C in its most oxidised form. Investing energy to
convert CO2 into a more reactive intermediate could enable the
generation of commodity chemicals, which could eventually
provide a cleaner fuel source for the human population to use.
From a thermodynamic viewpoint, CO2 can accept an electron
to form a CO2cꢀ radical anion, but this is only observed directly
at high reduction potentials (Epc ¼ ꢀ1.90 V vs. NHE).12,13 The
proton-coupled reduction of CO2 provides a feasible alternative
to direct reduction and can lead to the generation of useful
products (carbon monoxide or formic acid) at lower reduction
potentials.13,14 It is important that there is a kinetic preference
for CO2 reduction over H2 due to the required presence of
a proton source to facilitate the process.
Since the industrial revolution, the consumption of fossil fuels
has been rapidly increasing to cater for the needs of an ever
growing world population. Industrialisation has been heavily
dependent on the use of coal, which today is responsible for
over 40% of the production of electricity worldwide.1 In excess
of 12 million tonnes of oil and 8 million cubic tonnes of
natural gas are consumed daily to provide energy, which has
resulted in excess carbon dioxide (CO2) emissions into the
atmosphere at a rate faster than the current carbon cycle can
mitigate.2–10 Carbon capture and sequestration (CCS) has long
been seen as a viable option as it allows for the retrotting of
existing power plants to separate CO2 from the ue stream
prior to its release into the atmosphere.11 A concern arising
from the storage of CO2 is its effective transport, which poses
aSchool of Chemistry, The University of Sydney, New South Wales 2006, Australia.
E-mail: deanna.dalessandro@sydney.edu.au; kate.jolliffe@sydney.edu.au; Fax: +61
2 9351 3329; Tel: +61 2 9351 3777
bNMR Facility, Mark Wainwright Analytical Centre, The University of New South
Porous Organic Polymers (POPs) are considered viable
candidates as materials that can both capture and convert CO2
into commodity chemicals. They are generally cheap to
produce, simple to synthesise on a larger scale, possess high
thermal and chemical stability,15 are lightweight, contain
Wales, 2052, Australia
cDepartment of Chemistry and Biochemistry, University of California, San Diego,
California 92093, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04385j
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hydrophobic pores from their extended aromaticity and are Not only did these polymers exhibit similar BET surface areas
chemically resistant. Hupp and co-workers have previously of 522–650 m2 gꢀ1, but they also converted CO2 to cyclic
shown that the porosity and stability of POPs can be exploited carbonates with yields of up to 94%.20 Xie and co-workers also
by generating a polymer based on cheap and abundant amine- used POPs to examine the capture of CO2 and its conversion to
and anhydride-bearing monomers.16 The activated polymer propylene carbonate under mild conditions.27 Surface areas up
demonstrated stability up to 500 ꢁC and retained porosity aer to 965 m2 gꢀ1 were achieved and the catalyst could be recycled
exposure to HCl. Patel and co-workers explored N2-phobicity up to 22 times without any signicant decrease in activity. To
within POPs by generating a class of nitrogen-rich azo POPs, the best of our knowledge, work has not yet appeared on the
which possessed high point selectivity for CO2/N2 (S ¼ 288 at electrochemical reduction of salen-based POPs.
323 K) and were chemically stable in boiling water.17 Zhou and
The electrochemical examination of POP materials requires
co-workers synthesised POPs containing tetratopic ada- their effective attachment to a conductive surface.28 A large
mantane cages which exhibited BET surface areas up to 5400 proportion of the literature addressing the surface attachment
m2 gꢀ1 and were suitable for CO2 and CH4 separation.18 It was of solid state materials (in particular Metal Organic Frame-
found through ideal adsorbed solution theory (IAST) that works (MOFs)) onto a conductive surface has reported Indium-
several high surface area POPs demonstrated selectivity for doped Tin Oxide (ITO) or Fluorine-doped Tin Oxide (FTO) on
CO2 at 295 K, suggesting their suitability for CO2 capture. glass as the substrate of choice.29–33 These two substrates are
Many early studies of POPs exploited porosity as their most both transparent, and therefore allow for the spectroscopic
important property; however, subsequent studies identify that study of the surface attached material prior to, during and
these materials can also have applications in catalysis.19 Bare aer reduction experiments using transmission measure-
metal sites can also be incorporated into POPs, which may ments;34 however, there are limitations to their use. For
promote their use in catalysis.20,21
example, ITO readily corrodes from glass substrates upon the
Creating a class of solid state materials that is capable of application of highly anodic and cathodic potentials, and
both the capture of CO2 and its catalytic conversion requires indium is a scarce and expensive element.35 Although FTO
the choice of multifunctional building blocks. The ubiquity of coated glass has a larger electrochemical window and uses
salen metal complexes over the past eighty years has shown more abundant uorine, it has a high resistance, which affects
that they are capable of diverse chemistry. Since the rst re- the reversibility of the ferrocene/ferrocenium ion (Fc0/Fc+)
ported preparation of the vibrantly coloured salen-based couple.36 Glassy carbon is more conductive than FTO and ITO
ligands in 1933 by Pfeiffer,22 this class of ligand has found coated glass, and is therefore is a strong candidate as the
extensive use in chemical catalysts,23 electrochemical agents,24 substrate for solid state electrochemistry. The major limita-
and charge transfer complexes,25 among other applications. tion for glassy carbon is that mechanical immobilisation of
The variability of substituents on the salen core enables the materials such as POPs is difficult. Electrophoretic deposition
preparation of diverse libraries of compounds that may be (EPD) has been successfully applied to synthesise homoge-
generated by systematic variation.26 There is scope to modify neous, continuous lms of UiO, MIL, NU,30 ZIF37 and
substituents along the backbone of the salen ligand to study porphyrin-based38 MOFs. The limitations of this particular
their steric and electronic inuence on catalysis (Fig. 1). The surface attachment method lie in the high voltages required
inherently redox-active nature of the salen metal complex for deposition to occur. It is likely that immobilising of a POP
allows for its properties to be modulated.
onto glassy carbon using EPD could be achieved at lower
The readily functionalisable core of the salen metal voltages.39
complex allows for its covalent immobilisation into a POP
Herein, we report the synthesis of three Co(III) salen metal
material. A class of salen-based POPs were reported by Chun complexes to explore their use as electrochemical reducing
and co-workers, who varied the bridging moiety in the salen to agents for CO2. The electrochemical properties of the discrete
incorporate the diaminocyclohexane functionality, and intro- complexes are analysed to examine the effect of modulating
duced Al(III), Co(III) and Cr(III) metals into the chelating site. the bridging diamine functionality of the complexes on their
electrochemical properties. We perform a detailed electro-
chemical study on the conversion of CO2 using these
complexes to address the gap in the literature regarding salen
complexes as reducing agents. We also report the inclusion of
the discrete complexes into novel POPs and examine the
impact of salen immobilisation on their redox properties. We
further report a method for the surface attachment of POPs
onto glassy carbon via EPD which uses milder potentials than
those previously reported for EPD onto an FTO substrate, thus
providing a benign pathway for attaching solid-state materials
to glassy carbon.
Fig. 1 The structure of the salen metal complexes used here. The
backbone can be systematically modified by varying Y to produce the
analogs Co1, Co2 and Co3.
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at room temperature. The 13C chemical shis were referenced
to the glycine CO peak at 176 ppm.
Experimental Methods
General
Cyclic voltammetry
All chemicals used were purchased from Aldrich, Alfa Aesar
and Merck, and were used without further purication unless
stated otherwise. Solvents were obtained from a PureSolv
system, or purchased and used without further purication.
Low resolution electrospray ionisation mass spectra (ESI-MS)
were acquired as a solution in MeCN, MeOH or DMF with
a 100 mL minꢀ1 ow rate on a Finnegan LCQ or amazon MS
detector. Spectra were collected over the mass range m/z 100
to 1000. An ESI spray voltage of 5 kV was applied with
a heated capillary temperature of 200 ꢁC and a nitrogen
sheath gas pressure of 60 psi. Melting points were measured
using a Gallenkamp melting point apparatus with the sample
placed in a glass capillary. The nal melting points are
uncorrected. Room temperature FT-IR spectra were obtained
using a PerkinElmer UATR 2 infrared spectrometer over the
range 400–4000 cmꢀ1 with a resolution of 4 cmꢀ1. Samples
were mechanically compressed on the surface of a diamond
crystal on which the background was collected. ICP-OES was
performed at the Mark Wainwright Analytical Centre at The
University of New South Wales.
Solution and solid state electrochemical measurements were
performed using a Bioanalytical Systems BASi Epsilon Electro-
chemical Analyser at 298 K. A single compartment cell was used,
consisting of either a glassy carbon working electrode (3.0 mm
diameter) for solution state or a glassy carbon plate (active area
of 2 cm2) with immobilised polymer attached through a steel
alligator clip to tinned copper wire for solid state, a platinum
wire auxiliary electrode and an electrolysed Ag/AgCl wire refer-
ence electrode separated from the solution by a CoralPor tip.
Cyclic and differential pulse voltammograms were performed
using a 10 mL solution of the analyte (1 mM complex in 0.1 M
[(n-C4H9)4N]PF6 in either MeCN or DMF). The solution was
purged with dried N2, Ar, or CO2 prior to each experiment.
Ferrocene (Fc) (1 mM) was added as an internal standard during
each experiment. All potentials are quoted in V vs. Fc0/Fc+.
Uncompensated resistance between the working and the refer-
ence electrodes was corrected by using iR compensation on the
potentiostat. Scan rate dependence studies were carried out for
each complex between 50–1600 mVsꢀ1 to ensure the homoge-
neity of the system.
Nuclear magnetic resonance
Infrared spectroelectrochemistry (IR SEC)
1
Solution state H and 13C{1H} NMR spectra were recorded on
Solution state IR SEC was performed on the discrete complexes
using the IR-SEC cell previously reported by Kubiak et al.41
a Bruker AVANCEIII 300, 400 or 500 spectrometer operating at
300, 400, 500 MHz for 1H and 75, 100, 125 MHz for 13C,
A
1
Pine Instrument Co. Model AFCBP1 potentiostat was employed
to control the cell potential, referenced to Ag/Ag+. Thin-layer
bulk electrolysis was measured by reectance IR off the elec-
trode as the potential was scanned. All experiments were con-
ducted in 0.1 M [(n-C4H9)4N]PF6/MeCN/DMF (9 : 1) with known
analyte loadings prepared under an inert atmosphere. FT-IR
spectra were recorded on a Thermo Scientic Nicolet 6700,
with resolution of 4 cmꢀ1. Unlike the CV or UV-Vis-NIR SEC
experiments, it was not possible to use pure DMF in the IR SEC
experiments, since the strong nC]O stretching vibration at
1740 cmꢀ1 overwhelmed the comparatively weaker nC]N stretch
respectively. H and 13C NMR chemical shis were referenced
internally to residual solvent resonances. Spectra were recorded
at 298 K and chemical shis (d), with uncertainties of ꢂ0.01 Hz
for 1H and ꢂ0.05 Hz for 13C, are quoted in ppm. Coupling
constants (J) are quoted in Hz and have uncertainties of
1
ꢂ0.05 Hz for H–1H. Deuterated solvents were obtained from
Cambridge Stable Isotopes and used as received. Chemical
shis for all ligands are reported relative to the deuterated
solvents used.40
The 13C cross polarisation magic angle spinning (CPMAS)
solid state NMR experiments on the diamagnetic discrete
complexes and their polymers were carried out at The Mark
Wainwright Analytical Centre, The University of New South
Wales, on a wide-bore Bruker Biospin AVANCEIII solids-300
MHz spectrometer operating at a frequency of 75 MHz for the
expected from the salen metal complex at ꢃ1600 cmꢀ1
.
Ultraviolet-visible-near infrared spectroelectrochemistry (UV-
Vis-NIR SEC)
13C nucleus. The sample (ꢃ80 mg) was placed into 4 mm Solution state UV-Vis-NIR SEC was measured over the range
zirconia rotors tted with Kel-f® caps and spun in a double 5000ꢀ35 000 cmꢀ1 using a CARY5000 spectrophotometer
resonance H-X probehead at 8 kHz magic angle spinning (MAS). interfaced to Varian WinUV soware. The absorption spectra
1
The 13C and H 90ꢁ radio frequency pulse lengths were opti- of the electrogenerated species were obtained in situ by the use
mised to 3.5 ms each. The 13C spectra were acquired with 1 ms of an Optically Semi-Transparent Thin-Layer Electrosynthetic
cross polarisation contact time with a total suppression of cell, path length 0.685 mm, mounted in the path of the spec-
spinning side bands (TOSS) scheme, followed by 1H decoupling trophotometer. Solutions for the spectroelectrochemical
at 75 kHz eld strength using spinal-64 decoupling. The 13C experiment contained 0.1 M [(n-C4H9)4N]PF6/MeCN or [(n-
non-quaternary suppression (NQS) spectra were recorded by C4H9)4N]PF6/DMF supporting electrolyte and ca. 0.4 mM of the
turning off the 1H decoupling for 40 ms during the TOSS period. compound for analysis. Appropriate potentials were applied by
For sufficient signal-to-noise, ca. 10 K transients were acquired using an eDAQ e-corder 410 potentiostat and the current was
for each sample with recycle delays of 3.0 s in between to ensure carefully monitored throughout the electrolysis. The electro-
sufficient relaxation of the 1H nuclei. The spectra were obtained generated species formed in situ, and their absorption spectra
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were recorded at regular intervals through the electrolysis. The Thermogravimetric Analyser. Dry N2 (0.1 L minꢀ1) owed over
attainment of a steady-state spectrum and the decay of the the sample during dꢀat1a collection. The sample temperature was
ꢁ
ꢁ
current to a constant minimum at a potential appropriately ramped at 1 C min from 25 to 600 C. Samples were loaded
beyond E1/2 (for the redox process) was indicative of the dry aer exposure to air.
complete conversion of the starting material.
Solid state diffuse reectance UV-Vis-NIR spectra of the
redox-active species were collected in situ using a CARY5000
UV-Vis-NIR spectrophotometer equipped with a Harrick Omni
Diff Probe attachment interfaced to Varian WinUV soware
over the range 5000–25 000 cmꢀ1 in a custom-made cell
previously reported by D'Alessandro et al.42 The cell consisted
of a Pt wire counter electrode and a Ag/Ag+ quasi-reference
electrode. The solid sample was immobilised by a thin strip
of Teon tape onto a 0.1 mm thick Indium-doped Tin Oxide
(ITO) coated quartz slide, which functioned as the working
electrode. The applied potential was controlled using an eDAQ
potentiostat. Continuous scans of the sample were obtained
and the potential increased gradually until a change in the
spectrum was observed.
Gas adsorption
Adsorption isotherms were measured using the Accelerated
Surface Area & Porosity (ASAP) 2020 or the 3-Flex, both supplied
by Micromeritics Instruments Inc. The sample (ꢃ50–100 mg)
was loaded into a glass analysis tube and outgassed for 24 h
ꢁ
under vacuum at 80 C, prior to analysis.
N2 adsorption and desorption isotherms were measured at
77 K and data were analysed using the Brunauer, Emmett and
Teller (BET) models to determine the surface area.44 Pore size
distributions were calculated using the Density Functional
Theory (DFT) cylindrical model in the Micromeritics Micro-
Active Soware Package Version 4.03.
Electrophoretic deposition (EPD) onto glassy carbon
Glassy carbon substrates were cut to 3 ꢄ 1 cm electrodes. The
POP (20 mg) was suspended in toluene (20 mL) and sonicated
for 30 s. Two identical glassy carbon substrates were dipped into
the deposition solution (1 cm separation distance) and
a constant DC voltage of 30 V was applied from an Extech
382270 High Precision Quad Output DC power supply. The
deposition occurred over a period of 6 h, with a stirred
suspension that was sonicated every 30 minutes to break up
larger aggregates of POP. Caution: electrical sparking due to the
accidental contact of electrodes and/or their leads can result in
the spontaneous ignition of toluene. Prior to the undertaking of
these experiments, ensure that the experiment is set up in an
empty fume hood, clear of ammables and with a blast shield.
Bulk electrolysis
Solution and solid state bulk electrolysis were performed in
a threaded 60 mL single compartment cell with a custom
airtight Teon top, as reported by Kubiak et al.43 The set-up
consisted of a carbon rod (surface area ¼ 7.4 cm2) for solution
state or glassy carbon plate with immobilised POP for solid state
as the working electrode (active area ¼ 2 cm2), a coiled Pt wire
counter electrode protected from the bulk solution by fritted
glass and an electrolysed Ag/AgCl pseudo reference, separated
from solution by a CoralPor tip. The analyte solution (ꢃ40 mL)
consisted of complex (1–2 mM) in 0.1 M [(n-C4H9)4N]PF6/MeCN/
DMF (8 : 2). The solution was purged with CO2 for 20 min prior
to each electrolysis experiment, with the optimal applied
potential determined by the CV experiments.
Field emission scanning electron microscopy
Gas phase analysis was performed by sampling 1 mL of the
headspace of the cell at 20 min intervals and injecting into
a Hewlett-Packard 7890A series gas chromatograph with two
molecular sieve columns (30 m ꢄ 0.53 mm i.d. ꢄ 25 mm lm).
The 1 mL injection was split between two columns, one with N2
carrier gas and one with He carrier gas to quantify both CO and
H2 respectively. Instrument specic calibration curves were
measured prior to analysis to determine the amount of each gas
produced.
Solution phase analysis was performed on the bulk solution
to quantify formic acid production by sampling the bulk elec-
trolysis solution (5 mL) aer electrolysis. D2O (1 mL) was added
to the solution and this was well mixed, prior to its dilution with
dichloromethane. The D2O layer was separated prior to the
addition of concentrated hydrochloric acid (1 drop). Samples
were analysed by 1H NMR and spectra were recorded on
a Bruker AVANCEIII 300 spectrometer operating at 300 MHz for
1H. Spectra were recorded at 298 K.
FE-SEM measurements were obtained at either the Nano3
facility at The University of California, San Diego or at the
Australian Centre for Microscopy & Microanalysis at The
University of Sydney. POP materials were deposited onto
a glassy carbon substrate, which was adhered to conductive
carbon tape on a sample holder disk. The disk was coated
using a Cr-sputter coating for 8 s. A Philips XL30 ESEM was
used for acquiring images using a 10 kV energy source under
vacuum at a working distance at 10 mm. ꢃ19 000ꢄ magnica-
tion images were collected.
Results and discussion
Synthesis and structural characterisation
The synthesis of salen ligands was achieved by the Schiff-base
condensation of 5-bromosalicylaldehyde with varying bridging
diamines to afford the free-base salen in good yield. Co(III)
metalation was achieved by the addition of Co(OAc)2$4H2O in
the presence of LiCl as an oxidising agent to afford all Co(III)
complexes in good yields (Scheme 1A). Salen metal complexes
Thermogravimetric analysis
TGA measurements were carried out on a TA Instruments Hi- were successfully incorporated into POPs via a Sonogashira–
Res 2950 Thermogravimetric Analyser or Discovery Hagihara palladium cross coupling reaction between tris(p-
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ethynyl)triphenylamine (TPA) and the bis-bromo salen metal polymeric systems that are more delocalised. The UV-Vis-NIR
complexes under an inert N2 atmosphere in the presence of spectra of the polymers all exhibited the n–p* charge transfer
[Pd(PPh3)4] and CuI to form the series of POPs. In all reaction band, consistent with TPA incorporation into the polymer
mixtures, the light yellow solution darkened and a brown (Fig. S3†). ICP-OES indicated that the Co(III) salen was incor-
precipitate formed (Scheme 1B).
porated into the POP; however, the lower than calculated values
Extensive characterisation was performed on the amorphous for the Co(III) content suggests that homocoupling between the
polymers to determine their composition. Despite their non- TPA may be occurring because of uncontrolled propagation, as
crystalline nature, TGA studies revealed that all POPs well as the possible leaching of the Co(III) species (Table S1†).
possessed thermal stability above 200 ꢁC, with a number of
Solid state 13C NMR experiments were performed to further
polymers showing thermal stability of up to 400 ꢁC, prior to characterise the salen-based POPs. Solid state 13C NMR
partial thermal degradation (Fig. S1†). IR experiments indicated spectra could be obtained for POPCo1, POPCo2 and POPCo3,
that the TPA co-ligand was covalently linked to the salen metal which all contain the Co(III) d6 species. This implies that the
complex. The IR spectrum of TPA exhibited a nC^H stretch at Co(III) salen metal complexes are low spin when incorporated
3264 cmꢀ1, which disappeared upon polymerisation. The weak into the POPs. The 13C CPMAS spectra of the polymers con-
nC^C stretch also shied from 2102 cmꢀ1 in the monomer to taining the diamagnetic salen metal complexes were collected
2191 cmꢀ1 in the polymer.45 The nC]N stretch belonging to the and compared to the discrete complexes to show incorpora-
imine in the salen metal complex was observed to shi upon tion of the salen into the POP (Fig. 2, S4–S6†). A shi in the
incorporation into the salen POP compared with the polymer position of the alkynyl peaks from those observed in discrete
(Fig. S2†). Further evidence of the covalent linkage between co- TPA previously reported,46 as well as their broadening,
ligand and salen was detected in the UV-Vis-NIR spectra of the suggests that a polymeric material has formed. Evidence for
monomers and polymers. The UV-Vis-NIR spectrum of TPA the incorporation of the salen metal complex came from the
exhibited a sharp n–p* charge transfer at 26 460 cmꢀ1 and p– appearance of peaks from the bridging diamine around 160–
p* charge transfer bands at 31 970 and 34 260 cmꢀ1. When 165 ppm, corresponding to the imine carbon and phenolic
compared to the discrete salen complexes, many of the char- carbon environments present in the salen. These varied
acteristic charge transfer bands in the salen metal complexes dependent on the bridging moiety. The comparatively small
were shied to lower energies in the polymers, consistent with signal intensity of the salen moiety suggests an incorporation
Scheme 1 (A) Synthesis of bromo-terminated Co(III) salen metal complexes (a) solvent: MeOH, 80 ꢁC, 2 h, (b) metal: Co(OAc)2$4H2O (1.1 eq.),
LiCl (4 eq.), solvent: EtOH, r.t., 48 h. (B) Synthesis of the Co(III) salen polymer (a) catalysts: [Pd(PPh3)4] (30 mol%), CuI (30 mol%), solvent: toluene/
EtOH (2 : 1), 85 ꢁC, 72 h.
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Fig. 2 13C CPTOSS of Co1 (above) and POPCo1 (below). The full 13C NMR spectrum is plotted in blue, while the spectrum in red is the non-
protonated methyl/carbon species detected after 40 ms of dipolar dephasing. The signals marked * are attributed to excess triethylamine, which
was unable to be completely removed from the pores.
of salen around 10%, which is consistent with the ICP-OES
data (Table S1†). Variation in the incorporated salen metal
complex could be deduced from the appearance of different
carbon environments at lower eld. For POPCo1, evidence for
the diaminocyclohexane bridging moiety came from the
appearance of peaks corresponding to the sp3 hybridised
secondary and quaternary carbons on the bridging diamine
between 20–30, and 70 ppm. The additional signals in the alkyl
region at 15 and 50 ppm have been tentatively assigned to
small quantities of residual triethylamine. For the aromatic
POPCo2 and POPCo3, a larger integration under the aromatic
signals from 110–160 ppm was detected, which implies addi-
tional carbon environments from the discrete complexes Co2
and Co3, respectively (Fig. S4 and S5†). Finally, the synthesis
was repeated in the absence of salen metal complex, forming
POPTPA. The 13C CPMAS spectrum for this deliberately
homocoupled polymer POPTPA demonstrated the absence of
peaks corresponding to the phenolic carbon and the imine of
the salen moiety at 162–165 ppm. Additionally, a single ethynyl
signal at 90 ppm was observed (Fig. S6†).
Gas adsorption in polymers
Methods for the activation of POPs were initially probed as
a step towards examining their gas sorption properties. The
activation of the polymers was achieved by initially immersing
the newly synthesised POP materials in DMF (10 mL) in an oven
at 100 ꢁC for 1 h, prior to ltration and repetition of the process
three times. The polymer was subjected to washing via a Soxhlet
ꢁ
washing process with methanol at 80 C for 48 h.
The porosities of the POPs were analysed using N2 gas
sorption experiments at 77 K to determine their BET surface
areas. All POPs demonstrated Type I BET isotherm behaviour,
which is indicative of a microporous material (Fig. 3A).47 There
is a signicant amount of hysteresis, which is commonly
observed within highly exible porous materials. The amor-
phous nature of the POPs is reected in the pore size distribu-
tion, where there is a wide variety of sizes, consistent with the
uncontrolled propagation of the POP (Table 1, Fig. S7†).
N2 and CO2 gas sorption experiments were performed at 298
K (Fig. 3B). The comparable N2 isotherms indicate that at 298 K,
there is a limited uptake of N2 and a preference for CO2 uptake
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Fig. 3 (A) N2 gas sorption isotherms at 77 K of POPCo1 (black), POPCo2 (red) and POPCo3 (blue). Closed squares represent N2 adsorption
isotherms while open squares represent N2 desorption isotherms. (B) CO2 adsorption isotherms (red) and N2 gas adsorption isotherms (black) at
298 K of POPCo1 (circles), POPCo2 (triangles) and POPCo3 (squares).
which could arise from differences in the salen backbone. Fig. S9†). The CV experiment for Co1 revealed one irreversible
Having to overcome the increased p-stacking from the aromatic redox process and two quasi-reversible processes. An additional
POPCo2 may hinder the adsorption of CO2, while an increased reduction process could be observed upon inclusion of an
bulk from the bridging diamine may disrupt these interactions, aromatic bridging diamine, suggesting that the increased
justifying why POPCo1 and POPCo2 have higher uptakes. The aromaticity of the salen improves its electronic properties. The
isosteric heats of adsorption indicate that there are phys- addition of two methyl groups on the bridging diamine of Co3
isorptive interactions between CO2 and the POP (Table 1, shied the reduction processes to more cathodic potentials
Fig. S8†). The slight variations in ꢀQst suggest that the back- relative to Co2, which was expected from a system containing
bone of the salen does not play a signicant role in the phys- increased electron density. The solution state CV of Co3 was
isorptive interactions between CO2 and the POP. Finally, the akin to that of Co2.
CO2/N2 selectivities (S) were taken with respect to the conditions
Co(III) salen POPs. Following the successful surface attach-
of a post-combustion ue stream at 298 K, consisting of N2 (P/P0 ment of the Co(III) salen POPs to glassy carbon, CV experi-
¼ 0.75) and CO2 (P/P0 ¼ 0.15) (Table 1).
ments were performed to examine their electronic behaviour.
For the reduction sweeps, up to four redox processes were
observed, corresponding to the reduction of the Co(III) salen.
There were fewer reduction processes observed for the
aromatic systems. The irreversible IIIR (in POPCo2 and
POPCo3) and IVR (in POPCo1) reduction process were assigned
to the reduction of the imine across the salen metal complex.
Incorporation of the salen into the POP appeared to bring the
reduction potentials to more cathodic potentials, which is
benecial for any electrochemical applications (Table 3,
Fig. S10†).
Surface attachment of Co(III) salen POPs to glassy carbon
POPCo1, POPCo2 and POPCo3 were successfully immobilised
onto glassy carbon via electrophoretic deposition by sus-
pending the POP in toluene, with sonication and stirring while
a constant DC voltage of 30 V was applied over a period of 6 h.
The reaction was paused every hour to allow for the further
sonication of the mixture to disrupt aggregation of the POP.
The experiment resulted in coverage of POPCo1, POPCo2 and
POPCo3, which could be imaged on the surface via FE-SEM
(Fig. 4). POPCo1 and POPCo2 demonstrated better coverage
of the glassy carbon plates than POPCo3.
Spectroelectrochemistry
Electrochemistry
Solution state SEC on Co(III) salen complexes. For all discrete
Discrete Co(III) salen complexes. The three discrete Co(III) complexes, UV-Vis-NIR SEC experiments were performed to
complexes were rst tested for their redox activity (Table 2, elucidate the mechanism for electron transfer (Scheme 2).
Table 1 BET surface areas, free pore volumes, pore size distributions, point selectivities and ꢀQst of the synthesised polymers
BET surface area
(m2 gꢀ1
Pore volume
(cm3 gꢀ1
)
Pore size distribution (A)
Point selectivity S
ꢀQst (kJ molꢀ1
)
˚
Polymer
)
POPCo1
POPCo2
POPCo3
657 ꢂ 1
669 ꢂ 1
688 ꢂ 1
0.36
0.37
0.36
4.9–7.7, 8.3–13
15.4
9.5
15.1
25–27
27–29
20–27
4.8–7.8, 8.4–13, 17ꢀ21
5.6–7.8, 8.8–12, 14ꢀ21
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Fig. 4 (A) Coverage of glassy carbon plates by Co(III) polymers. FE-SEM images of (B) POPCo1 (C) POPCo2 and (D) POPCo3.
The rst reduction processes in the complexes were consis- application of a potential at Epc ¼ ꢀ1.66 V vs. Fc0/Fc+, the
tent with the reduction of the Co(III) centre to Co(I). In the SEC observation of a weak IVCT band at 10 010 cmꢀ1 and a radical
for all complexes, an increase in the intensity of the d–d bands band at 12 860 cmꢀ1 are suggestive of the formation of the
(16 280 and 19 990 cmꢀ1 for Co1, 18 570 cmꢀ1 for Co2 and salen radical anion. The lower energies associated with the
15 040 cmꢀ1 for Co3) was consistent with a transition from d6 to radical anion band suggest that the Co(III) metal centre
d7 cobalt species (Fig. S11†) and decreases in LMCT bands promotes delocalisation. To the best of our knowledge, such
(24 470 in Co1, 20 980, 25 550 and 29 600 cmꢀ1 in Co2 and IVCT transitions have not been previously reported for Co(III)
21 720, 29 550 and 30 980 cmꢀ1 for Co3), as seen in other Co(III) salen metal complexes. The appearance of the IVCT band is
salen complexes.48,49 The maintenance of isosbestic points also accompanied by an increase in the p–p* transitions at
suggests that there is a clean conversion between the Co(III), 31 080 cmꢀ1, consistent with improved electron delocalisation
Co(II) and Co(I) species.
Interesting electronic properties appear aer the IIR reduc- evidence for a weaker IVCT transition. Changes appear in the
tion. Solution state differential pulse voltammetry (DPV) LMCT and n–p* transitions at 19 590 and 23 680 cmꢀ1
in the conjugated parts of the salen backbone. In Co2, there is
,
experiments on Co2 in 0.1 M [(n-C4H9)4N]PF6/DMF under N2 respectively. This is accompanied by the appearance of a weak
reveal a non-Gaussian peak, indicating that the redox couple IVCT band at 8340 cmꢀ1 and a weak radical band at
has a number of underlying processes (Fig. 5A). Despite 12 670 cmꢀ1. Similar processes in Co3 are observed. The
multiple processes observed, the integration of the area under dominant changes in the UV-Vis spectra are the decrease in
the combined processes is the same as the integration of the the transitions at 20 500, 25 120, 29 550 and 30 980 cmꢀ1, and
Fc0/Fc+ signal, indicating that only one electron is being trans- the appearance of new transitions at 22 490 and 23 870 cmꢀ1
.
ferred. UV-Vis-NIR SEC shows not only evidence for the Co(II/I) This is also supported by the fact that the IIIR process in Co3 is
couple (vide supra), but also an appearance of a radical band at more cathodic than its Co2 analogue, which is consistent with
12 690 cmꢀ1. IR SEC experiments were performed on Co2 (4 improved delocalisation and electron density increasing the
mM) in 0.1 M [(n-C4H9)4N]PF6/MeCN/DMF (9 : 1), which indi- voltage required for reduction of the Co3 backbone. The
cated that ligand-based processes are also occurring. The absence of an observable IVCT band suggests that the degree
appearance of a new IR stretch at n ¼ 1674 cmꢀ1, corresponding of aromaticity affects the nature of the IVCT transition (Fig. 6).
to a change in the nC–O stretch on the phenol, implies that there
may be evidence of charge transfer from the Co(I) metal centre SEC experiments were performed on POPCo1, POPCo2 and
to the ligand (Fig. 5B). POPCo3. No signicant changes were observed on the IR
Solid state SEC on Co(III) salen polymers. In situ solid state
Clear differences in the ligand based reductions are also reduction process of the polymer, corresponding to the loss of
apparent between the materials. In Co1, the third reduction the chloride anion. Upon the application of a cathodic potential
results in the generation of a number of important bands, corresponding to the IIR redox potential in all systems, there
consistent with intervalence charge transfer (IVCT). Upon was a darkening in the polymers that could be followed with
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Table 2 Redox features for Co(III) salen metal complexes. Ferrocene (1 mM) was used as an internal standard. (0.1 M C4H9NPF6/DMF) was the
supporting electrolyte. Where available, the peak–peak separation (DE) and the current ratio (ipa/ipc) of reversible and quasi-reversible redox
features have been described
Complex
IR
IIR
IIIR
IVR
Fc0/+
Co1
Epc ¼ ꢀ0.49 V
E1/2 ¼ ꢀ1.66 V
DE ¼ 64 mV
E1/2 ¼ ꢀ2.36 V
DE ¼ 60 mV
E1/2 ¼ 0 V
DE ¼ 60 mV
(ipa/ipc) ¼ 0.914
E1/2 ¼ 0 V
(ipa/ipc) ¼ 0.642
E1/2 ¼ ꢀ1.56 V
DE ¼ 35 mV
(ipa/ipc) ¼ 0.837
E1/2 ¼ ꢀ2.23 V
DE ¼ 63 mV
Co2
Co3
Epc ¼ ꢀ0.37 V
Epc ¼ ꢀ0.39 V
E1/2 ¼ ꢀ2.43 V
DE ¼ 84 mV
DE ¼ 61 mV
(ipa/ipc) ¼ 0.944
E1/2 ¼ 0 V
(ipa/ipc) ¼ 0.448
E1/2 ¼ ꢀ1.57 V
DE ¼ 33 mV
(ipa/ipc) ¼ 0.597
E1/2 ¼ ꢀ2.25 V
DE ¼ 58 mV
(ipa/ipc) ¼ 0.684
E1/2 ¼ ꢀ 2.46 V
DE ¼ 59 mV
DE ¼62 mV
(ipa/ipc) ¼ 0.918
(ipa/ipc) ¼ 0.540
(ipa/ipc) ¼ 0.705
(ipa/ipc) ¼ 0.859
Table 3 Redox features for Co(III) salen POPs. Ferrocene (1 mM) was used as an internal standard. 0.1 M LiBF4/CH3CN was the supporting
electrolyte
POP
IR
IIR
IIIR
IVR
Fc
POPCo1
POPCo2
POPCo3
Epc ¼ ꢀ0.12 V
Epc ¼ ꢀ1.42 V
Epc ¼ ꢀ0.86 V
E1/2 ¼ ꢀ0.80 V
Epc ¼ ꢀ2.35 V
Epc ¼ ꢀ1.86 V
Epc ¼ ꢀ1.10 V
ꢀ2.26 V
Epc ¼ ꢀ2.16 V
E1/2 ¼ 0 V
E1/2 ¼ 0 V
E1/2 ¼ 0 V
change in the bands at 17 410 and 19 940 cmꢀ1 (for POPCo1) For Co1, there was a slight increase in the magnitude of the
(Fig. S12†) and 18 170 cmꢀ1 and 20 510 cmꢀ1 (for POPCo3) current density of IIR for Co1, indicating that it was not t for
(Fig. 7). This change is consistent with a Co(II/I) process and is CO2 reduction (Fig. S13A†). The titration of TFE into the solu-
reected by corresponding changes in discrete complexes. tion did not enhance with the current; therefore, further elec-
There were small changes in the intensity of the peaks for trochemical measurements were not pursued. More
POPCo2. Upon reaching the IIIR potential, losses in intensity pronounced changes in the presence of CO2 were observed in
occurred in all of the bands in the spectra and the lack of stable Co2, where the IO redox process at Epa ¼ +0.37 V vs. Fc0/Fc+
isosbestic points indicated a degradation of the polymer. This is disappeared, with changes observed in the IR redox process at
also evident from the change in the electrolyte solution from Epc ¼ ꢀ0.37 V vs. Fc0/Fc+ (Fig. S13B†). A slight anodic shi was
clear and colourless to green.
observed for the IIR redox process at E1/2 ¼ ꢀ1.56 V vs. Fc0/Fc+,
corresponding to the Co(II/I) reduction. This may indicate weak
association between the reduced species and CO2. The IIIR
process at E1/2 ¼ ꢀ2.23 V vs. Fc0/Fc+ became signicant under
the CO2 atmosphere and continued to increase with the addi-
tion of aliquots of TFE, indicating that a proton source was
required to enhance activity. An absence of this increase under
N2 with TFE indicates that the process is not only related to the
proton reduction of H2, but is likely to arise from the reduction
of CO2. The reduction of CO2 by Co2 follows pseudo-rst order
kinetics. A plot of icat against the square root of [CO2] revealed
a linear relationship between these two variables, indicating
that the process is rst order in CO2 (Fig. S14A†). The plot of icat
vs. [H+] showed an initial second order dependence on the
proton source, reaching a maximum current density at 0.90 mA
cmꢀ2 aer 0.15 M TFE (Fig. S14B†). Plotting icat vs. [analyte]
revealed a linear relationship (Fig. S14C†). The concentration of
CO2 (0.23 M in DMF) was well in excess of the concentration of
the analyte (1 mM). The titration of TFE into the reaction
mixture under an atmosphere of CO2 resulted in the increase in
the current density of the IIIR redox process at Epc ¼ ꢀ2.23 V vs.
Electrochemical behaviour in the presence of CO2
Co(III) salen complexes. Co(III) salen complexes were evalu-
ated for their electrochemical behaviour in the presence of CO2.
Fc0/Fc+ to a maximum. The addition of TFE into the reaction
icat
mixture resulted in a peak ratio
of 3.34, corresponding to
ip
Scheme 2 Proposed mechanism of the Co(III) complexes observed
over the initial two reduction processes.
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Fig. 5 (A) Solution state DPV of Co2 (1 mM) (scan rate: 0.02 V sꢀ1, Fc (1 mM) was used as an internal standard) and (B) solution state IR SEC of Co2
(4 mM) upon changing the potential from ꢀ0.8 to ꢀ1.5 V vs. Ag/Ag+. (0.1 M [(n-C4H9)4N]PF6/MeCN/DMF (9 : 1) as the supporting electrolyte). (C)
The predicted mechanism of the IIR reduction process, showing possible charge transfer between the Co(I) metal and the salen backbone.
a TOF k[Q] of 2.16 sꢀ1 at 0.63 M TFE. Only a weak association of process at E1/2 ¼ ꢀ1.57 V vs. Fc0/Fc+, corresponding to the
CO2 with Co2 was observed (Fig. 8).
Co(II/I) process and indicating a weak association of CO2 to the
Solution state CV experiments performed on Co3 in the metal centre. The IIIR redox process at E1/2 ¼ ꢀ2.25 V vs. Fc0/Fc+
presence of CO2 resulted in a slight anodic shi in the IIR redox increased under CO2 saturation, and was further enhanced to
Fig. 6 Solution state UV-Vis-NIR SEC of (A) Co1 (0.61 mM) upon changing the potential from ꢀ1.5 V to ꢀ1.8 V vs. Ag/Ag+, (B) Co2 (0.51 mM) upon
changing the potential from ꢀ1.6 to ꢀ2.1 V vs. Ag/Ag+ and (C) Co3 (0.32 mM) upon changing the potential from ꢀ1.8 to ꢀ2.2 V vs. Ag/Ag+ (0.1 M
[(n-C4H9)4N]PF6/DMF as the supporting electrolyte).
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Fig. 7 Solid state Vis-NIR SEC of POPCo3 upon changing the potential from (A) ꢀ1.0 to ꢀ1.5 V vs. Ag/Ag+, (B) ꢀ1.7 to ꢀ2.0 V vs. Ag/Ag+ (0.1 M [(n-
C4H9)4N]PF6/MeCN as the supporting electrolyte).
a maximum upon the addition of successive aliquots of TFE, atmosphere of CO2 resulted in an increase in the current
indicating that a proton source is required to achieve optimal density of the IIIR process to a maximum at Epc ¼ ꢀ2.25 V vs.
activity. The absence of this enhancement under N2 with the Fc0/Fc+ aer only two aliquots. Addition of excess TFE aer this
presence of TFE indicates that the process is not solely related point resulted in no signicant change. The addition of TFE
icat
ip
to proton reduction, but is more likely to arise from the
reduction of CO2. Like Co2, Co3 also follows pseudo rst order
kinetics. A plot of icat against the square root of [CO2] showed
a linear relationship, suggesting rst order kinetics in [CO2]
(Fig. S15A†). The plot of icat vs. [H+] shows an initial second
order dependence on TFE, prior to reaching saturation
(Fig. S15B†). In Co3, saturation of the proton source occurs at
lower concentrations than its Co2 analogue, implying that
a lower concentration of a proton source facilitates the reduc-
tion of CO2. Plotting icat vs. [analyte] reveals a linear relation-
ship, indicating a rst order dependence of peak current on
[analyte] (Fig. S15C†). CO2 (0.23 M in DMF) is well in excess of
the analyte. Solution state CV experiments were performed on
Co3 (1 mM) with varying concentrations of TFE titrated into the
solution. The titration of TFE into the reaction mixture under an
into the reaction mixture yielded a peak ratio
of 2.98, cor-
responding to a TOF k[Q] of 1.72 sꢀ1 at 0.14 M TFE, which is
a smaller turnover than the Co2 analogue. A negligible associ-
ation with CO2 was observed.
CPE experiments were performed on solutions of both Co2
(3.60 mM) with TFE (0.63 M) and Co3 (1.11 mM) with TFE (0.14
M) in 0.1 M [(n-C4H9)4N]PF6/DMF/MeCN (8 : 2) under CO2. The
generation of CO was observed in the gas phase from both
complexes, as measured from the rst 3.73 turnovers over 3 h in
Co2 and measured from the rst 4.15 turnovers over the space
of 4 h in Co3 (Table 4). A linear relationship in the production of
gas was maintained throughout the duration of the CPE
experiments for both complexes, indicating that the true life-
time of the catalyst may be slightly longer, although there is
Fig. 8 Solution state CV showing the electrochemical response of (A) Co2 (1 mM) under saturation of N2 (black), CO2 (red) and CO2 with TFE (0.7
mmol-blue, 1.4 mmol-green, 2.1 mmol-light pink, 2.8 mmol-pink, 3.5 mmol-brown) and (B) Co3 (1 mM) under saturation of N2 (black), CO2 (red)
and CO2 with TFE (0.7 mmol-blue, 1.4 mmol-pink). Return sweeps are not shown for clarity (0.1 M [(n-C4H9)4N]PF6/DMF as the supporting
electrolyte, scan rate: 0.1 V sꢀ1, Fc (1 mM) was used as an internal standard).
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evidence for both H2 and CO production plateauing (Fig. S16†). upon the titration of successive aliquots of TFE. This was
The analyte sustained current densities of 2.57 mA cmꢀ2 for Co2 accompanied by the generation of a new irreversible process
and 0.4 mA cmꢀ2 for Co3, with CPE experiments revealing N1 at Epc ¼ ꢀ1.96 V vs. Fc0/Fc+. A CPE experiment was per-
a sustained current density over the period of the trans- formed in 0.1 M LiBF4/MeCN with TFE (0.5 M) at Epc ¼ ꢀ2.00 V
formation. This suggests that the electrochemical trans- vs. Fc0/Fc+ under CO2 saturation. Over the space of 3 h, the
formation of CO2 is more likely to be stoichiometric rather than initial current decreased, before reaching a maximum of 0.67
catalytic.
mA cmꢀ2 aer 1 h. Solution and gas phase analysis were
The solution of the electroanalyte was analysed via 1H NMR unable to detect products of CO2 reduction, which may be due
experiments for solution phase products post electrolysis. to the micromolar concentration of Co(III) metal sites in the
1
Aer aqueous work up into D2O, H NMR revealed a peak at polymer in the electrochemical cell and the detection limits of
d ¼ 8.00 ppm, corresponding to the generation of formic acid the GC and NMR. This may also suggest degradation of the
(Fig. S17†). An NMR time-course experiment was performed polymer under electrochemical stimulus.
concurrently with CPE experiments on Co2, which demon-
strated that the peak increased with the CPE. Broadening of
the peak may be indicative of paramagnetic metal ions
leaching into the solution. In the absence of CO2, the peak
Conclusions
corresponding to the formation of formic acid did not appear. This work sought to explore the use of Co(III) salen metal
No formic acid was observed during experiments where CO2 complexes and POPs as agents for the electrochemical reduc-
was present but Co2 was absent, indicating that the trans- tion of CO2. A series of discrete Co(III) bis-bromo terminated
formation of CO2 into formic acid is due to both the presence salen metal complexes were synthesised and characterised
of CO2 and Co2.
using NMR, UV-Vis-NIR, IR, CV and various SEC techniques to
The prime difference between the analytes of Co2 and Co3 understand their physical and electrochemical properties. The
lies in the generation of both gaseous phase and solution two salen metal complexes, Co2 and Co3, successfully reduced
phase products from the reduction of CO2. Although CO was CO2 to CO in 20% and 11% Faradaic efficiencies, respectively,
formed from both complexes, it was formed in lower effi- suggesting scope to use these complexes as multi-electron
ciencies from the more electron donating Co3. Following the reducing agents. Formic acid was also observed as a product
CPE experiments on Co3, the electroanalyte was analysed via of CO2 reduction in Co2 but not in Co3, implying that a more
1H NMR experiments aer work up into D2O. No evidence for electron poor salen metal complex improves the conversion of
formic acid was found, which may imply that selectivity for the CO2 into commodity chemicals. Further functionalisation of the
generation of CO over formic acid could be achieved by varying bridging diamine may allow for the selectivity of products to be
the electronics of the salen backbone. Future studies will tuned. This work has shown the potential for expanding the
require the analysis of more electron withdrawing species and studies of Co(III) salen complexes as reducing agents for two-
their effect on the reduction of CO2.
electron transfer reactions; however, additional work is
Co(III) salen POPs. Solid state CV measurements were per- required to improve the stability of the electrogenerated imine
formed on the polymer series in the presence of CO2. For radical that forms.
POPCo1 and POPCo2, there was a loss in intensity of all redox
The discrete salen complexes were successfully incorporated
processes, suggesting that CO2 switches off the redox activity into POP materials through the Sonogashira–Hagihara reaction
of the POP (Fig. S18†). Detailed CO2 reduction experiments with TPA to generate POPCo1, POPCo2 and POPCo3, and were
were therefore not pursued. Signicant changes were observed characterised by UV-Vis-NIR, IR, elemental analysis, FE-SEM,
in the solid state CV of POPCo4. Compared to the CV under the solid state NMR and gas sorption. The polymers were found
N2 atmosphere, a decrease in intensity of the IIR and IIIR to be porous, with BET surface areas up to 690 m2 gꢀ1
processes was observed, as well as an anodic shi of the IIR (comparable with those reported in the literature).21,27,49 The
process from Epc ¼ ꢀ1.86 V to ꢀ1.73 V vs. Fc0/Fc+, indicating successful reduction of CO2 into CO and formic acid by the
that CO2 could be irreversibly binding to the species formed discrete complexes led to the examination of the electro-
from the IIR process. The IIR process increased to a maximum chemical properties of the POPs through solid state CV, CPE
and SEC experiments. It was possible to use mild EPD condi-
tions to adhere the POP materials to the conductive glassy
carbon surface as a thin lm, which could be characterised
Table
4 Faradaic efficiencies for the generation of H2 and the
through FE-SEM. Solid state CV experiments revealed a number
of irreversible processes for these polymers, suggesting that the
instability of the POP material for electrochemical purposes
arises from the imine core. When exposed to CO2, the redox
behaviour was turned off, suggesting that there is an irrevers-
ible chemical transformation of CO2 in the polymer, which
could be conrmed from Vis-NIR SEC. There is further scope to
address this shortcoming by reinforcing the stability of the
imine.
conversion of CO2 into CO and HCOOH. All experiments were per-
formed in (0.1 M [(n-C4H9)4N]PF6/DMF/MeCN (8 : 2) with TFE as the
supporting electrolyte under CO2, Epc ¼ ꢀ1.85 V vs. Ag/Ag+)
Experiment
FCO (%)
FH (%)
FHCOOH (%)
2
Solvent
0
0
20
11
0
0
0
7
0
Solvent + TFE (0.7 M)
Co2 +TFE (0.63 M)
Co3 + TFE (0.14 M)
12
13
14
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The authors declare no conicts of interest.
Acknowledgements
This work was supported by the Australian Research Council
(ARC) and the Science and Industry Endowment Fund (SIEF) as
part of the ‘Solving the Energy Waste Roadblock’ venture. We
gratefully acknowledge Dr Danielle Kennedy at the Common-
wealth Scientic and Industrial Research Organisation (CSIRO)
for her support in the provision of an OCE PhD top-up schol-
arship for M. B. S. We are also grateful to Dr Charles Machan
from The University of Virginia and Dr Mark Reineke from The
University of California, San Diego for their assistance with IR
SEC and bulk electrolysis measurements, Dr Carol Hua for
guidance on UV-Vis-NIR SEC and Dr Dorothy Yu for the analysis
of the polymer samples via ICP-OES. S.M.C was supported by
a grant from the office of Basic Energy Sciences, Department of
Energy, under Award DE-FG02-08ER46519. C. P. K. gratefully
acknowledges NSF (CHE-1145893) for support.
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