Cobalt-based molecular electrocatalysis of nitrile reduction: Evolving sustainability beyond hydrogen

Article abstract of DOI:10.1039/c9dt00773c

Two new cobalt bis-iminopyridines, [Co(DDP)(H₂O)₂](NO₃)₂ (1, DDP = cis-[1,3-bis(2-pyridinylenamine)] cyclohexane) and [Co(cis-DDOP)(NO₃)](NO₃) (2, cis-DDOP = cis-3,5-bis[(2-Pyridinyleneamin]-trans-hydroxycyclohexane) electrocatalyse the 4-proton, 4-electron reduction of acetonitrile to ethylamine. For 1, this reduction occurs in preference to reduction of protons to H₂. A coordinating hydroxyl proton relay in 2 reduces the yield of ethylamine and biases the catalytic system back towards H₂.

Full text of DOI:10.1039/c9dt00773c

Dalton
Transactions
View Article Online
View Journal
COMMUNICATION
Cobalt-based molecular electrocatalysis of
nitrile reduction: evolving sustainability
Cite this: DOI: 10.1039/c9dt00773c
beyond hydrogen†
Received 20th February 2019,
Accepted 8th May 2019
Simon N. Child,a Radoslav Raychev,a Nathan Moss,a Benjamin Howchen,a
Peter N. Horton, b Christopher C. Prior,a Vasily S. Oganesyan a and
DOI: 10.1039/c9dt00773c
a
John Fielden
*
rsc.li/dalton
Two new cobalt bis-iminopyridines, [Co(DDP)(H2O)2](NO3)2 acetic acid plus solar electricity) that avoid the need for high
(1, DDP cis-[1,3-bis(2-pyridinylenamine)] cyclohexane) and H2 pressures, high temperatures and enable control through
[Co(cis-DDOP)(NO3)](NO3) (2, cis-DDOP cis-3,5-bis[(2- adjustment of electrochemical potential. Moreover, for multi-
=
=
Pyridinyleneamin]-trans-hydroxycyclohexane) electrocatalyse the electron processes, electrolysis driven by photovoltaics may
4-proton, 4-electron reduction of acetonitrile to ethylamine. For 1, prove more efficient than direct photoredox catalysis requiring
this reduction occurs in preference to reduction of protons to H2. multiple, reversible photoinduced electron transfers.7
A coordinating hydroxyl proton relay in 2 reduces the yield of
Herein, we present the electrocatalytic behaviour of two
new cyclohexane-supported cobalt bis-pyridyl imine com-
plexes: [Co(DDP)(H2O)2](NO3)2 (1) and [Co(cis-DDOP)(NO3)]
(NO3) (2) (Scheme 1). Cobalt bis-pyridyl imines are known as
low-cost, easily accessible molecular HECs,2b but we find that
in acetonitrile electrolyte with weak (acetic) acid, 1 is a better
catalyst for the 4 proton, 4 electron reduction of acetonitrile to
ethylamine than the kinetically more straightforward two-
electron reduction of protons to dihydrogen. Introduction of a
coordinating hydroxyl group, and resulting change of geometry
to trigonal prismatic in 2 biases the system back towards H2,
and increases current density, while still producing ethyl-
amine. To our knowledge this is the first confirmed demon-
stration of turnover for acetonitrile reduction with a molecular
electrocatalyst.
Compounds 1 and 2 are obtained by complexing the parent
ligands with Co(II) nitrate in methanol, vapour diffusion iso-
lates them as single crystalline materials suitable for X-ray
diffraction. Synthesis of the ligands is detailed in the ESI.†
The crystal structure of 1 (Fig. 1 and S1†) exhibits a very similar,
distorted octahedral geometry to the previously published trans-
hydroxy substituted complex,8 although the coordinated nitrate
ethylamine and biases the catalytic system back towards H2.
The last decade has seen an explosion of interest in molecular
electrocatalysts for production of dihydrogen (HECs).1–5
Tremendous progress has been made in increasing speed,1,3
lowering overpotential,1,4 and understanding the mechanism
of HECs.1,5 However, we are still some distance from molecule-
based systems that can produce H2 at industrial scale, and at a
cost low enough for hydrogen to play a major role in a nascent
mixed energy vector economy. Thus, it is worth considering
additional applications in sustainable chemistry for the knowl-
edge and materials developed in molecular HEC research.
Efficient, fast, electrochemically driven manipulation of
protons and hydrides is intrinsic to the performance of HECs.5
This implies that under appropriate conditions, many HECs or
derivatives thereof will be active in organic reductions, where
delivery of H atoms to the correct site is essential to achieving
the desired product.6 Thus, exploring HECs as catalysts for
organic chemistry could uncover new high value processes
where the advantages of molecular catalysts – exquisite struc-
tural control, providing for unrivalled tuneability and selecti-
vity – outweigh their drawbacks in cost and stability. Such pro-
cesses could be reductions using sustainable reagents (i.e.
aSchool of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK.
E-mail: John.Fielden@uea.ac.uk
bUK National Crystallography Service, School of Chemistry,
University of Southampton, Southampton, SO17 1BJ, UK
†Electronic supplementary information (ESI) available: Synthetic and other
experimental/computational details, CIF files. CCDC 1898421 and 1898426. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9dt00773c
Scheme 1 Catalysts 1 and 2.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Communication
Dalton Transactions
nation of the electron-donating hydroxyl and change in geome-
try in 2, and DFT calculations indicating that the HOMOs of
1− and 2− are predominantly metal based (Fig. 2, Fig. S5†).
The two subsequent reductions can be formally assigned to
the ligands for both complexes, however DFT-calculated fron-
tier orbitals suggest participation from both the imino-
pyridines and the metal centres. DFT also suggests that the
much larger ΔE for 2 on the last reduction results from a struc-
tural rearrangement involving de-coordination of both nitrate
counterion and hydroxyl group. Thus, the hydroxyl group may
act as a hemi-labile ligand that coordinates and de-coordinates
during catalytic turnover.
Fig. 1 Left: X-Ray crystal structures of the complex cations in 1 and 2.
Right: DFT calculated alternative structure of 2 (2-oct, +30 kJ mol−1 vs. 2)
with pseudo-octahedral Co. C is grey; O, red; N, blue; Co, purple; H, white
(CH omitted).
is replaced by a second water molecule – likely due to loss of the
hydrogen bonding ligand trans-OH group and resulting changes
to the crystal packing and hydrogen bonded network (Fig. S3†).
In 2 (Fig. 1 and S2†), coordination of the –OH group pulls Co
0.9743(8) Å out of the plane of the N4 pocket, leading to a trigo-
nal prismatic geometry where the two pyridyl imine chelators
are ca. 90° to one another and the shortest coordinate bond
(2.075(1) Å) is to the hydroxyl O. There is no coordinated water
molecule, and Co-pyridyl, imine and nitrato bond lengths are
all longer than in 1 (Table S2†). Large differences in the para-
magnetic 1H-NMRs of 1 and 2 indicate that these structural
differences are retained in solution (Fig. S4†). Even so, DFT
calculations estimate a small (ca. 30 kJ mol−1) ΔE between the
observed structure of 2, and an alternative 2-oct (Fig. 1) where
the pyridyl imines return to the equatorial plane (analogous
to 1) and the hydroxyl remains coordinated at a similar distance
(ca. 2.078 Å). This modest energy difference, comparable to a
strong hydrogen bond,9 suggests that 2 could change its con-
figuration in response to protonation of –OH and/or reduction
of Co, making the –OH group hemi-labile while retaining a
short O-to-Co distance.
Both complexes respond strongly to addition of acetic acid
(AcOH). For the first reduction of 1 (at 100 mV s−1) a shift of
ca. −200 mV in the return wave is observed if the subsequent
wave(s) are also scanned (Fig. 3, Fig. S7†). However, little
change occurs to this first wave when studied in isolation
(Fig. S8†), and at slow scan rates (Fig. S9†) it retains reversibil-
ity when scanning subsequent waves. For the second wave one
equivalent of AcOH roughly doubles the current associated
with the forward (reduction) process, but leaves the return
wave unchanged. Addition of more AcOH (up to 5 eq.) nearly
For both complexes, cyclic voltammetry (CV) reveals three
quasi-reversible reductions between −0.8 and −1.8 V vs. Fc/Fc+,
and an oxidation at ca. +0.5 V (Table 1, Fig. S6†). The large
peak separation (ΔE) of the oxidation indicates CoII/III, owing
to the associated spin-state change (Table 1). This ΔE for 2 is
especially large, because d6 CoIII strongly disfavours the trigo-
nal prismatic geometry10 and forces a structural change that
slows down electron transfer. The first reduction is assigned to
CoII/I for both 1 and 2, in accordance with prior work,2b and
consistent with the large (300 mV) negative shift due to coordi-
Fig. 2 DFT calculated structures and HOMOs of 2 in its one (1-), two
(2-) and three (3-) electron reduced states.
Table 1 Electrochemical data for 1 and 2 in acetonitrile
E1/2, V vs. Fc/Fc+ (ΔE, mV)a
1+/0b
0/1−
0/2−
0/3−
1
2
0.53 (130)
0.43 (393)
−0.85 (80)
−1.16 (91)
−1.37 (70)
−1.40 (81)
−1.77 (60)
−1.80 (200)
a Solutions ca. 10−3 M in analyte and 0.1 M in [NBu4][BF4] in aceto-
nitrile at a glassy carbon working electrode with a scan rate of 100 mV
−1. Potentials referenced to Ferrocene internal standard E1/2 = 0 V,
Fig. 3 CV showing effect of adding 1, 2 and 5 equivalents of AcOH to
the first two reduction waves of 1 in MeCN. Conditions as described
under Table 1.
s
ΔEp = 80 mV. b The “0” state is defined as the initially isolated Co2+
complex.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Communication
Table 2 Bulk electrolysis data for 1 and 2, for AcOH in MeCN
triples the current observed with no acid for the reduction
wave, which from 5 eq. onwards shifts to more positive poten-
tial by ca. 60 mV per −log[AcOH] (Fig. S7†), without further sig-
H2
EtNH2
nificant increase in current at 100 mV s−1. At 10 mV s−1
,
E/V vs. Fc/Fc+
Q/C
ηF
ηChem
ηF
ηChem
a
b
c
d
Catalyst
however, current continues increasing up to 30 eq. AcOH
(Fig. S9†). These observations are consistent with slow catalysis
on the second wave, which we assign as 2-proton, 2-electron
proton coupled electron transfer (PCET) followed by slow H2
evolution – as confirmed by detection of traces of H2 upon
Blank 1e
Blank 2e
1f
−1.87 V
−2.05 V
−1.87 V
−1.87 V
−2.05 V
6.7
7.3
12.1
12.8
17.4
56%
72%
15%
20%
38%
19%
27%
10%
13%
34%
0%
0%
22%
n.d.
11%
0%
0%
14%
n.d.
10%
1f/CD3CN
2f
bulk electrolysis at −1.5 V vs. Fc/Fc+. The slow nature of this Electrolysis performed with 0.2 mM catalyst, 40 mM AcOH, 0.1 M
[NBu4][BF4] in MeCN at a GC block electrode. a Faradaic efficiency
process (TOF ca. 0.3 s−1), occurring near the potential for
imine reduction, implies that H2 evolution is occurring from a
2-electron, 2-proton reduced imine ligand. Ligand-based H2 zene and 1H-NMR vs trimethoxybenzene standard. d Chemical yield
determined by gas chromatography. b Chemical yield based on AcOH.
c Faradaic efficiency determined by reaction with 2,4-dinitrofluoroben-
based on AcOH. e Electrolysis on GC block alone at tabulated potential
evolution is documented for more electron rich bis-iminopyri-
to match relevant catalyst. f Electrolysis conducted at catalytic half-
dines, at TOFs up to 19.4 s−1 11
Finally, a third, catalytic wave
.
wave potential, as tabulated. Data (H2 only) for the peak of the catalytic
develops at more negative potential (Fig. S7†). Its potential waves is presented in the ESI.
shifts positive and current increases in proportion to the con-
centration of acid, indicating PCET, and is likely initiated
through formation of a CoII hydride. Prior work on Co-bis-imi- turnover with any molecular electrocatalyst, and the first evi-
nopyridines found that in weakly acidic conditions metal pro- dence of any such reactivity with an earth abundant metal.13
tonation only occured upon transfer of a second electron to Thus for 1, it seems remarkable that ηF for EtNH2 exceeds that
Co, and postulated involvement of the imino-pyridines in the for the more kinetically straightforward reduction of H+ to H2.
reaction pathway.2b
Unlike existing nitrile hydrogenation catalysts,14 1 and 2
The behaviour of 2 is more complex (Fig. S10–12†). With produce no secondary or tertiary amines from MeCN.
AcOH present, three waves occur before the main catalytic However, the total ηF of H2 and EtNH2 never exceeds 50%,
process, corresponding to a similar series of electron transfers indicating presence of other processes. Current evidence
to those of 1: CoII to CoI, followed by two PCETs that reduce suggests: (i) Surface chemistry at GC may account for 25% or
the imino-pyridines and result in slow H2 evolution. The so of the charge – ηF for H2 in blank reactions is only 56–72%
reason for the split in the imino-pyridine PCETs is not clear, (>90% at higher potential), suggesting that some current
but may relate to lower symmetry inequivalencing the two reduces GC surface functionalities.15 This is supported by evi-
“arms” on the CV timescale, and/or a difference in the strength dence of quinones in GC-MS of the solutions (Fig. S27†). (ii)
of communication between them. Notably, as more acid is Reduction of NBu4+ to NBu3 is indicated by GC-MS (Fig. S27†),
added the PCETs begin to merge (Fig. S12†) and the catalytic and mass spectra of materials recovered after reaction with
half-wave potential for 2, initially ca. 230 mV more negative DNFB indicate trace quantities of butylamine and dibutyl-
than that of 1, shifts such that by 30 eq. AcOH the difference is amine adducts (Fig. S23–24†). (iii) A negative NH3 test (indo-
only 130 mV. This is consistent with acid encouraging de- phenol) implies the 2,4-dinitroaniline forms from a readily
coordination of –OH, producing behaviour more similar to 1, hydrolysed (enamine) 3-aminocrotonitrile-DNFB adduct.
and potentially enabling it to act as a proton relay that lowers 3-Aminocrotonitrile is a known product of electrochemical
the potentials associated with the metal centre.12
MeCN reduction,16 but both this and the triazine observed by
The observed response to a weak proton source encouraged GC-MS would be included in the ηF for H2 as they form via
investigation of the main catalytic wave by bulk electrolysis deprotonation of MeCN – BE of CD3CN electrolyte produced a
(BE) of solutions of acetic acid in acetonitrile (Table 2). Both higher H2 yield (Table 2), suggesting they are not major path-
catalysts have a lower faradaic efficiency (ηF) for H2 evolution ways. No other molecular products can be unambiguously
than the blank glassy carbon (GC) electrode, and for 1 the identified (Fig. S18–27†), but CV of the catholyte after reaction
chemical yield of H2 was also lower than for GC alone. This (Fig. S13–15†) reveals that the catalysts are degraded by the
indicates that H2 evolution cannot be a major pathway for 1, process, accounting for some of the current. Importantly, BE
and is not the only one for 2. A ninhydrin test indicated the performed with dirty electrodes from a prior run with 1
presence of amines in the catholyte, treatment with 2,4-dini- (Table S2†) produced no ethylamine. Although participation of
trofluorobenzene (DNFB) enabled quantification of ethylamine solution phase or metastable Co nanomaterials cannot be
(as N-ethyl-2,4-dinitroaniline) by 1H-NMR (Table 2, excluded by this test,17 these are usually associated with
Fig. S18–20†), and also revealed small quantities of 2,4-dini- efficient hydrogen evolution and our H2 yields are generally
troaniline – confirmed by MS (Fig. S23–25†). The faradaic yield lowered vs. blank electrodes. Thus it seems that EtNH2 exclu-
(ηF) of EtNH2 for 1 (22%) is double that of 2, and the TON is sively results from molecular electrocatalysis by 1 and 2.
higher (7 vs. 5). While the ηFs and TONs obtained are modest,
Bulk electrolysis underestimates turnover frequency (TOF)
the 4-electron, 4-proton process involved is challenging. To our as only a small portion of catalyst is active at any one time. So,
knowledge this is the first confirmation of MeCN → EtNH2 to estimate TOFs for EtNH2 we used CV, adding AcOH until
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Communication
Dalton Transactions
saturation was reached for 1, and until the concentration used the X-ray structure of 2, and Prof. Chris Pickett and Dr Saad
for BE was reached for 2 (Fig. S16†). Adjusting for ηF, this gives Ibrahim (UEA) for helpful discussions. This work was sup-
estimates of TOF(EtNH2) of 2.3 s−1 for 1 and 1.5 s−1 for 2, ported by: EPSRC (EP/L504944/1 DTA studentship to SNC),
under the BE conditions. While 2 appears capable of accessing RSC (Summer Research Bursary to RR) and the University of
faster rates, direct AcOH reduction current from the GC elec- East Anglia. As well as the ESI† and deposited cif files, data
trode becomes problematic as more acid is added. TOF(H2) is can be obtained by contacting the corresponding author.
not estimated for either catalyst as it cannot be delineated
from the electrode contribution. Use of H-terminated boron
doped diamond (BDD) electrodes (Fig. S17†),18 which have less
activity for AcOH reduction, indicates that 2 can indeed access
Notes and references
maximum current densities close to an order of magnitude
higher than those of 1, and thus much faster reaction rates
(for both H2 and EtNH2).
1 (a) J. R. McKone, S. C. Marinescu, B. S. Brunschwig,
J. R. Winkler and H. B. Gray, Chem. Sci., 2014, 5, 865;
(b) V. Artero, M. Chavarot-Kerlidou and M. Fontcave, Angew.
Chem., Int. Ed., 2011, 50, 7238; (c) N. Kaeffer, M. Chavarot-
Kerlidou and V. Artero, Acc. Chem. Res., 2015, 48, 1286;
(d) N. Queyriaux, R. T. Jane, J. Massin, V. Artero and
M. Chavarot-Kerlidou, Coord. Chem. Rev., 2015, 304–5, 3;
(e) A. Dutta, A. M. Appel and W. J. Shaw, Nat. Rev. Chem.,
2018, 2, 244.
2 (a) H. I. Karunadasa, C. J. Chang and J. R. Long, Nature,
2010, 464, 1329; (b) B. D. Stubbert, J. C. Peters and
H. B. Grey, J. Am. Chem. Soc., 2011, 133, 18070; (c) A. Dutta,
S. Lense, J. Hou, M. H. Engelhard, J. A. S. Roberts and
W. J. Shaw, J. Am. Chem. Soc., 2013, 135, 18490;
(d) M. Nippe, R. S. Khnayzer, J. A. Panetier, D. Z. Zee,
B. S. Olaiya, M. Head-Gordon, C. J. Chang, F. N. Castellano
and J. R. Long, Chem. Sci., 2013, 4, 3934; (e) J. Xie, Q. Zhou,
C. Li, W. Wang, Y. Hou, B. Zhang and X. Wang, Chem.
Commun., 2014, 50, 6520; (f) E. Deponti, A. Luisa,
M. Natali, E. Lengo and F. Scandola, Dalton Trans., 2014,
43, 16345.
Overall, current evidence supports a mechanism where 1
and 2 are first reduced from CoII to CoI, and then undergo a
ligand (imine) based 2-electron, 2-proton reduction that
results in slow H2 evolution. Additional PCET before H2 evol-
ution occurs results in reduction of acetonitrile solvent to
ethylamine, further reduction of the ligands, and for 2 likely
also initiates a faster hydrogen evolution process. This, and
the reaction by-products observed, indicates the likely involve-
ment of hydrides, implying that fast catalysis is initiated by
formation of [H2LCoII-H]. Tentatively, we suggest that 1 favours
EtNH2 production more than 2 because it requires simul-
taneous coordination of both MeCN and hydride – in 2, the
hydroxyl makes this more difficult to achieve. However, this
additional hydroxyl donor may help 2 access higher reaction
rates by stabilising against degradation as the acid concen-
tration increases. It may also act as a proton relay in the de-co-
ordinated geometries indicated by DFT (2−/23−, Fig. 1), notably
rate enhancements (up to 103×) have been seen for inclusion
of pendant amine groups in Fe and Ni based HECs.3a
Moreover, subsequent re-coordination of -OH would increase
the reactivity of resulting Co hydride species. However, current
data do not exclude direct PCET to coordinated MeCN, or
PCET from the reduced ligand to MeCN. Thus, work to elimin-
ate the electrode contribution and elucidate mechanism is
ongoing.
3 (a) M. L. Helm, M. Stewart, R. M. Bullock, M. Rakowski
DuBois and D. L. Dubois, Science, 2011, 333, 863;
(b) D. Khusnutdinova, B. L. Wadsworth, M. Flores,
A. M. Beiler, E. A. Reyes Cruz, Y. Zenkov and G. F. Moore,
ACS Catal., 2018, 8, 9888; (c) C. M. Clug, W. G. Dougherty,
W. S. Kassell and E. S. Wiedner, Organometallics, 2019, 38,
1269–1279.
In conclusion, we have shown that two complexes designed
as molecular HECs perform a challenging electrocatalytic
hydrogenation with high selectivity for the organic product.
Although this behaviour is previously unreported, it is likely
not unique and thus presents an interesting perspective for
investigation of HECs as electrocatalysts for more environmen-
tally benign organic reactions.
4 (a) X. Hu, B. S. Brunschwig and J. C. Peters, J. Am. Chem.
Soc., 2007, 129, 8988; (b) D. K. Bediako, B. H. Solis,
D. K. Dogutan, M. M. Roubelakis, A. G. Maher, C. H. Lee,
M. B. Chambers, S. Hammes-Schiffer and D. G. Nocera,
Proc. Natl. Acad. Sci. U. S. A., 2014, 111, 15001; (c) C. H. Lee,
D. K. Dogutan and D. G. Nocera, J. Am. Chem. Soc., 2011,
133, 8775; (d) M. Vennampalli, G. Liang, L. Katta,
C. E. Webster and X. Zhao, Inorg. Chem., 2014, 53, 10094.
5 (a) B. H. Solis, Y. Yu and S. Hammes-Schiffer, Inorg. Chem.,
2013, 52, 6994; (b) A. Lewandowska-Andralojc, T. Baine,
X. Zhao, J. T. Muckerman, E. Fujita and D. Polyansky, Inorg.
Chem., 2015, 54, 4310; (c) B. Mondal, K. Sengupta, A. Rana,
A. Mahammed, M. Botoshansky, S. G. Dey, Z. Gross and
A. Dey, Inorg. Chem., 2013, 52, 3381; (d) S. Raugei, S. Chen,
M.-S. Ho, B. Ginovska-Pangovska, R. J. Rousseau,
M. Dupuis, D. L. Dubois and R. M. Bullock, Chem. – Eur. J.,
2012, 18, 6493; (e) P. Wang, G. Liang, M. R. Reddy,
M. Long, K. Driskill, C. Lyons, B. Donnadieu,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank the UK EPSRC National Mass Spectrometry Facility
for MS, RC Treatt Ltd for GC-MS, Dr Hani El Moll (UEA) for
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Communication
J. C. Bollinger, C. E. Webster and X. Zhao, 12 J. Xie, Q. Zhou, C. Li, W. Wang, Y. Hou, B. Zhang and
J. Am. Chem. Soc., 2018, 140, 9219; (f) N. Elgrishi, X. Wang, Chem. Commun., 2014, 50, 6520.
B. D. McCarthy, E. S. Rountree and J. L. Dempsey, 13 Electro-reduction of MeCN to EtNH2 has been demon-
ACS Catal., 2016, 6, 3644.
strated for an osmium complex but no TON was reported,
see: M. H. V. Huynh, R. T. Baker, D. E. Morris, P. S. White
and T. J. Meyer, Angew. Chem., Int. Ed., 2002, 41, 3870.
14 (a) H. Li, Y. Wu, J. Zhang, W. Dai and M. Qiao, Appl. Catal.,
A, 2004, 275, 199; (b) D. J. Segobia, A. F. Trasarti and
C. R. Apesteguía, Appl. Catal., A, 2012, 445–6, 69.
6 For example, hydrogenation: H. U. Blaser, B. Pugin,
F. Spindler and M. Thommen, Acc. Chem. Res., 2007, 40,
1240.
7 Consider water splitting: an efficient (ca. 20%) commercial
solar cell connected to a PEM electrolyser (ca. 80%
efficient) will deliver a solar-to-H2 conversion efficiency of 15 K. M. Sundberg, W. H. Smyrl, Lj. Atanasoska and
ca. 15%. This is yet to be beaten by integrated solar water-
splitting devices or other photochemical systems.
8 J. Fielden, D.-L. Long, M. Speldrich, P. Kögerler and
L. Cronin, Dalton Trans., 2012, 41, 4927.
R. Atanasoski, J. Electrochem. Soc., 1989, 136, 434.
16 (a) B. Batanero, F. Barba and A. Martin, J. Org. Chem., 2002,
67, 2369; (b) J. K. Foley, C. Korzeniewski and S. Pons,
Can. J. Chem., 1988, 66, 201.
9 G. A. Jeffery, An Introduction to Hydrogen Bonding (Topics in 17 N. Kaeffer, A. Morozan, J. Fize, E. Martinez, L. Guetaz and
Physical Chemistry), Oxford University Press, New York, NY,
V. Artero, ACS Catal., 2016, 6, 3727.
1997.
18 (a) H. B. Suffredini, V. A. Pedrosa, L. Codognoto,
S. A. S. Machado, R. C. Rocha-Filho and L. A. Avaca,
Electrochim. Acta, 2004, 49, 4021; (b) G. R. Salazar-Banda,
A. E. de Carvalho, L. S. Andrade, R. C. Rocha-Filho and
L. A. Avaca, J. Appl. Electrochem., 2010, 40, 1817.
10 E. Cremades, J. Echeverría and S. Alvarez, Chem. – Eur. J.,
2010, 16, 10380.
11 E. J. Thompson and L. A. Berben, Angew. Chem., Int. Ed.,
2015, 54, 11642.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.