Photoinduced hydrogen evolution with new tetradentate cobalt(ii) complexes based on the TPMA ligand

Article abstract of DOI:10.1039/c6dt01705c

Hydrogen production from water splitting is nowadays recognized as a target, fundamental reaction for the production of clean fuels. Indeed, tremendous efforts have been devoted towards the research of suitable catalysts capable of performing this reaction. With respect to heterogeneous systems, molecular catalysts such as metal complexes are amenable to chemical functionalization in order to fine tune the catalytic properties. In this paper a new class of tris(2-pyridylmethyl)-amine (TPMA) cobalt(ii) complexes (CoL0-4) has been synthesized and employed as hydrogen evolving catalysts under photochemical conditions taking advantage of Ru(bpy)₃²⁺ (where bpy is 2,2′-bipyridine) as a light-harvesting sensitizer and ascorbic acid as a sacrificial electron donor. Tuning of the photocatalytic activity has been attempted through the introduction of different substituents at the catalyst periphery rather than through a direct chemical modification of the chelating TPMA ligand. The results show that CoL0-4 behave as competent hydrogen evolving catalysts (HECs), although the effects played by the different substituents on the catalysis are relatively modest. Possible reasons supporting the observed behavior as well as possible improvements of the aforementioned tuning approach are discussed.

Full text of DOI:10.1039/c6dt01705c

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: M. Natali, E.
Badetti, E. Deponti, M. Gamberoni, F. A. Scaramuzzo, A. Sartorel and C. Zonta, Dalton Trans., 2016, DOI:
10.1039/C6DT01705C.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/dalton

Please do not adjust margins
Dalton Transactions
Page 1 of 10
View Article Online
DOI: 10.1039/C6DT01705C
Journal Name
ARTICLE
Photoinduced hydrogen evolution with new tetradentate
cobalt(II) complexes based on the TPMA ligand.
Mirco Natali,*a Elena Badetti,b Elisa Deponti,a Marta Gamberoni,a Francesca A. Scaramuzzo,b
Andrea Sartorelb and Cristiano Zonta*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Hydrogen production from water splitting is nowadays recognized as a target, fundamental reaction for the production of
clean fuels. Indeed, tremendous efforts have been devoted towards the research of suitable catalysts capable of
performing this reaction. With respect to heterogeneous systems, molecular catalysts such as metal complexes are
amenable to chemical functionalization in order to fine tune the catalytic properties. In this paper a new class of tris(2-
pyridylmethyl)-amine (TPMA) cobalt(II) complexes (CoL0-4) has been synthesized and employed as hydrogen evolving
www.rsc.org/
2+
catalysts under photochemical conditions taking advantage of Ru(bpy)3 (where bpy is 2,2’-bipyridyne) as the light-
harvesting sensitizer and ascorbic acid as the sacrificial electron donor. Tuning of the photocatalytic activity has been
attempted through the introduction of different substituents at the catalyst periphery rather than through a direct
chemical modification of the chelating TPMA ligand. The results show that CoL0-4 behave as competent hydrogen evolving
catalysts (HECs), although the effects played by the different substituents on the catalysis are relatively modest. Possible
reasons supporting the observed behavior as well as possible improvements of the aforementioned tuning approach are
discussed.
macrocyclic cobalt complexes9 with cobaloximes playing the
major role.10 More recently, polypyridine cobalt complexes
have also emerged as active molecular catalysts for the HER in
Introduction
Hydrogen production from water splitting represents one of
the target processes for the production of clean fuels for a
sustainable development. Moreover, the use of a renewable
energy supply such as solar energy to drive this reaction would
in principle allow the mankind to solve both the energetic and
environmental problems of the current century.1 To this
purpose substantial efforts have been undertaken for the
development of efficient proton reduction catalysts, both at
the heterogeneous and homogeneous level, and to study their
possible coupling with light.2 Among the heterogeneous
catalysts, platinum metal,3 NiMoZn alloys,4 and W or Mo
sulfides5 have proven to be efficient and stable materials for
the hydrogen evolution reaction (HER) with the capability of
being also easily incorporated into artificial devices.6 As far as
homogeneous molecular catalysts are concerned, particular
attention has been mainly devoted to the study of noble-
metal-free species, including (i) diiron hydrogenase mimics,7
(ii) nickel compounds with diphosphine ligands,8 and (iii)
aqueous solutions, combining an efficient activity in light-
activated experiments with a good stability under turnover
conditions.11
Though being usually less stable than inorganic materials,
molecular catalysts are, on the other hand, amenable to
synthetic functionalization, e.g., by metal changing or ligand
substitution, with the possibility of tuning the catalytic activity
towards the optimization of the performances under both dark
(electrocatalytic)
and
light-activated
(photocatalytic)
conditions.11b,h,12 For instance, several works on cobalt-based
molecular catalysts have targeted the functionalization of
cobaloximes,13 polypyridine,14 or dithiolene cobalt complexes15
with different groups, either electron withdrawing or electron
donating ones, in order to finely adjust the reduction potential
of the cobalt center and thus its catalytic ability. The
functionalization of the
β-position of a cobalt corrole with
different halides was also recently exploited to optimize the
catalytic performance under electrochemical conditions.16 By
adopting a different approach to the catalyst tuning, a
carboxylic acid functional group was introduced at the
periphery of a cobalt porphyrin hydrogen evolving catalyst
resulting in a lower overpotential for the HER with respect to
the unfunctionalized porphyrin thanks to the possibility of
intramolecular proton-coupled electron-transfer.17
a.Department of Chemical and Pharmaceutical Sciences, University of Ferrara, Via
Fossato di Mortara 17-19, 44121 Ferrara, Italy, and Centro Interuniversitario per
la Conversione Chimica dell’Energia Solare (SolarChem), sez. di Ferrara, via L.
Borsari 46, 44121 Ferrara, Italy. E-mail: mirco.natali@unife.it
b.Department of Chemical Sciences, University of Padova, Via F. Marzolo 1, 35131
Padova, Italy. E-mail: cristiano.zonta@unipd.it
Electronic Supplementary Information (ESI) available: complete characterization of
L0-4 ligands and CoL0-4 complexes, electrochemistry, Stern-Volmer plots, transient
absorption spectroscopy.
Herein, we investigate the photocatalytic hydrogen evolution
ability of a new class of cobalt polypyridine complexes (CoL0-4
)
See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 2 of 10
View Article Online
DOI: 10.1039/C6DT01705C
ARTICLE
Journal Name
are dissolved in 50 mL of degassed H2O/toluene/CH3OH
(1:1:0.5) under N2 atmosphere. The mixture is stirred for 10 h
at 90°C. After an acid/base work-up of the reaction mixture
the final product is obtained in high yields. The syntheses have
also been optimized for ligands L2-4 on a gram scale without
requiring modification of the protocol. The obtained products
have been fully characterized by 1H-NMR, 13C-NMR, IR, ESI-MS,
and elemental analysis (see ESI). The general procedure for the
synthesis of Co(II) complexes CoL0-4 (Scheme 2) involves the
mixing of an equimolar amount of Co(ClO4)2·6H2O to the
corresponding ligands L0-4 in acetonitrile. The solutions are
left at room temperature for 30 minutes and the desired
compounds are obtained as powders by precipitation with
diethyl ether. These compounds have also been fully
characterized by IR, ESI-MS, and elemental analysis (see ESI).
Chart 1. Isostructural molecular cobalt(II) catalysts studied in this work. In all cases the
anion is perchlorate.
based on variations of the tetradentate tris(2-pyridylmethyl)-
amine (TPMA) ligand (Chart 1). In these systems, the
appropriate introduction of different functional groups in meta
position to a phenyl moiety in alpha to one pyridine ring is
performed in order to influence the second sphere
interactions of the catalytic center. The approach adopted
herein should provide an additional way, alternative to the use
of proton relays based on amine groups,8 in order to tune and
optimize the catalytic ability of molecular catalysts without
affecting the electronic properties of the metal center.
Electrochemical characterization
In order to get information on the effective ability of the
cobalt(II) complexes examined (CoL0-4) to act as hydrogen
evolving catalysts (HECs) under photochemical conditions,
electrochemical experiments were first performed. In
nitrogen-purged acetonitrile (0.1
processes can be detected at E1/2 = −1.23 V vs. SCE for CoL0, at
E1/2 = −1.22 V vs. SCE for both CoL2 and CoL3, and at E1/2
M
LiClO4) reversible
=
Results and discussions
−1.24 V vs. SCE for CoL4, which can be attributed to
Co(II)/Co(I) reductions (Figure 1a). In the case of CoL1 the
same process is slightly shifted towards more positive
potentials and becomes less reversible with a peak potential of
Epc = −1.19 V vs. SCE. The small differences (within ca 50 mV) in
terms of reduction potential within the class of complexes is
consistent with a weak electronic effect played by the different
substituents on the cobalt center. When the solvent is
changed from pure acetonitrile to a 50/50 acetonitrile/water
Synthesis
The general procedures for the synthesis of the TPMA ligands
and complexes CoL0-4 are summarized in Schemes 1 and 2,
respectively. We first synthesized the ligands L0-4, consisting
of the TPMA core bearing different substituents on one of the
three arms (Scheme 1). The method, already developed by us
for the synthesis of L0-1 18
compound via reductive amination of commercially available
,
consists in the formation of
1
mixture,‡ the Co(II)/Co(I) reduction process shifts by ca 100-
200 mV towards more negative potentials and becomes
6-bromo-2-pyridinecarboxaldehyde and di(2-picolyl)amine
followed by a Suzuki coupling with the boronic acid of interest.
In general, 4.9 mmol of ligand
1, 7.3 mmol of the desired
boronic acid, 12.2 mmol of Na2CO3 and 0.49 mmol of Pd(PPh3)4
Scheme 1. General synthetic procedure for the synthesis of the ligands L0-4; reaction yields are reported in brackets.
Scheme 2. General synthetic procedure for the synthesis of CoL0-4; reaction yields are reported in brackets.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 3 of 10
View Article Online
DOI: 10.1039/C6DT01705C
Journal Name
ARTICLE
one-electron reduction of Co(II) to Co(I) followed by a chemical
process such as protonation (eq 1), with formation of a key
Co(III)-hydride intermediate.19 Moreover, the onset of the
catalytic waves falls at an appreciably constant potential as
long as the TFA concentration is increased (i.e., the pH is
decreased). This can be taken as an indication19 that, under
these experimental conditions, an additional reduction of the
Co(III)-hydride (eq 2) is also required prior to hydrogen release.
The resulting Co(II)-H species should then be capable of
evolving hydrogen in either a heterolytic (protonation) or a
30
a)
CoL0
CoL1
CoL2
CoL3
CoL4
25
20
15
10
5
0
-5
homolytic (disproportionation) pathway (eq
respectively).20
3
or 4,
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
E vs SCE, V
Co(II) + H+ + e− → Co(III)-H
Co(III)-H + e− → Co(II)-H
Co(II)-H + H+ → Co(II) + H2
2 Co(II)-H → 2 Co(I) + H2
(1)
(2)
(3)
(4)
80
0 M
b)
1.0 mM
1.5 mM
2.0 mM
2.5 mM
60
40
20
0
It can be observed that for all CoL0-4 complexes the peak
currents of the catalytic waves display an appreciably linear
correlation with respect to the TFA concentration (Figure S3),
indicating that catalysis follows a second-order with respect to
the acid.21,22 The linear dependence of the catalytic current vs.
acid concentration together with the shape of the cyclic
voltammograms (Figure 1b and Figure S1) thus suggest that,
under these experimental conditions, catalysis is likely to fall
under the “total catalysis” regime defined by Savéant23 and is
thus limited by the diffusion of the substrate into the reaction
layer.§§ Under these kinetic conditions the extrapolation of
mechanistic information as well as the relevant kinetic
parameters from the catalytic peak current is not directly
applicable.24
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
E vs SCE, V
Figure 1. (a) Cyclic voltammetry (CV) of CoL0-4 in nitrogen-purged acetonitrile (0.1 M
LiClO4); (b) cyclic voltammetry (CV) of mM CoL0 in nitrogen-purged 50/50
1
acetonitrile/water (0.1 M LiClO4) upon addition of 0-2.5 mM TFA. Experimental
conditions: GC as working electrode, Pt as counter electrode, SCE as reference
electrode, room temperature, scan rate v = 100 mV/s.
generally broad and irreversible (see Figure 1b for CoL0 and
Figure S1 for the remaining complexes CoL1-4). Under these
conditions, water can indeed act as proton source on the
electrogenerated Co(I) species, thus explaining the
In order to attain a deeper mechanistic insight, taking CoL0 as
a reference for all complexes, it can be seen that, in the
presence of an excess TFA proton source, the catalytic current
is linearly dependent on the catalyst concentration (Figure S4),
thus suggesting a first-order process with respect to the
catalyst.21,22 This latter behavior is in principle compatible with
both a heterolytic mechanism (eq 1,2,3) and a homolytic
pathway (eq 1,2,4) with a rate-determining protonation step
(eq 1).25 However, the positive shift of the catalytic wave
relative to the Co(II)/Co(I) process suggests25,25 that the first
protonation is fast with respect to the following chemical
steps. This consideration thus supports25,25 the hypothesis
that, under these experimental conditions, the hydrogen
evolution catalysis by the CoL0-4 complexes most likely
proceeds via a heterolytic route (eq 3) rather than via a
homolytic one (eq 4).ǁ
irreversibility of the process.§ More interestingly, in all cases
the addition of trifluoroacetic acid (TFA) triggers the
appearance of new waves near the Co(II)/Co(I) process which
increase in intensity with increasing TFA concentration (Figure
1b for CoL0 and Figure S1 for the remaining complexes CoL1-4
)
and can be ascribed to catalytic proton reduction by the cobalt
complexes.11 This attribution is further confirmed by the
evolution of hydrogen upon bulk electrolysis at a constant
potential of E = −1.20 V vs. SCE on a 1 mM CoL3 solution in
50/50 acetonitrile/water (2.5 mM TFA, 0.1 M LiClO4 as
supporting electrolyte) using a carbon foil as the working
electrode (see Experimental Section for additional details and
Figure S2). Noteworthy, the ratio between the amount of
hydrogen formed and the theoretical amount expected from
the charge passed indicates a Faradaic Efficiency (FE) close to
90% (Figure S2c). The catalytic waves display an onset
potential of ca −1.00 V vs. SCE, irrespective of the substituent
present on the phenyl ring connected to the TPMA ligand.
Importantly, these waves precede the Co(II)/Co(I) reduction
steps, thus implying that catalytic hydrogen evolution by the
CoL0-4 complexes is triggered by an EC process, namely the
Photocatalytic hydrogen evolution
Photocatalytic hydrogen evolution experiments have been
carried out for the class of CoL0-4 catalysts upon continuous
visible irradiation (175 W Xe arc lamp, cut-off filter at 400 nm)
of argon-purged aqueous solutions (5 mL, 1 M acetate buffer
at pH 5) containing 75 ꢀM CoL0-4 catalyst,¶ 0.5 mM
Ru(bpy)3Cl2 as the photosensitizer, 0.1 M ascorbic acid as the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 4 of 10
View Article Online
DOI: 10.1039/C6DT01705C
ARTICLE
Journal Name
sacrificial electron donor and checking the gas-phase of the terms of TON. On the other hand, more efficient hydrogen
reactor by gas-chromatography. The choice of the pH was production is re-established when both the catalyst and the
mainly dictated by comparison with photocatalytic systems sensitizer are introduced after 1 hour photolysis with an
involving the same sensitizer and sacrificial donor and already overall recovery of up to 83% in terms of TON (Figure S5). The
reported in the literature.7a,11j,14 The kinetic traces (averages of inability to thoroughly restore the same initial hydrogen
three different experiments) are reported in Figure 2a, while evolving activity, in terms of both initial rate and maximum
related maximum turnover numbers (TONs) and frequencies TON, is very likely attributable to accumulation in solution of
(TOFs) are given in Figure 2b. In all cases hydrogen production decomposition products of the Ru(bpy)32+ photosensitizer,11c,14
levels off after approximately 2-3 hours of irradiation achieving competing in light absorption with the fresh sensitizer, and of
maximum TONs between 56 and 94 with maximum TOFs the ascorbic acid sacrificial electron donor, namely
between 2.1 and 3.3 min-1 depending on the catalyst. A dehydroascorbic acid, which is effective towards the short-
constant hydrogen production rate (where maximum TOFs circuiting quenching of transiently reduced species in solution
have been calculated) is established after a short (few (either of the sensitizer or of the catalyst).11j,27
minutes) induction period. As previously observed on similar In order to compare the photocatalytic activity of this class of
photochemical systems,9f,11c,d,j,26 this can be related to the cobalt complexes with related catalysts reported in the
required lag-time to accumulate an appreciable fraction of literature, it has to be considered that the hydrogen evolving
doubly reduced, catalytically active species at steady state performance is strictly dependent on the experimental
after that hydrogen evolution can take off.
conditions used, namely on the concentrations of the
reactants, on the solvent medium, on the pH, and on the
photon flux. Within these limitations, however, a qualitative
comparison of the photochemical hydrogen evolving activity of
the present systems with cobalt polypyridine complexes
reported in the literature working in purely aqueous solutions
40
a) 35
30
2+
in the presence of Ru(bpy)3 as the sensitizer and ascorbic
25
acid as the sacrificial electron donor can be made.11l Such a
comparison shows that CoL0-4 catalysts perform similarly to
the pentapyridine cobalt complexes reported by Long, Chang
and coworkers11a as well as to the pentadentate cobalt catalyst
described by Wang and coworkers11h and most of the
pentacoordinated cobalt complexes reported by Crowley,
Collomb, Blackman, and coworkers,11k but are less efficient
than other polypyridine cobalt analogs such as the
pentadentate complexes reported by Webster, Zhao, and
coworkers,14 the tetradentate catalysts by Castellano, Long,
Chang, and coworkers,11c the quaterpyridine catalyst reported
by Thummel and coworkers,11g the tetradentate complex
reported by Ott and coworkers,11f and a pentapyridine cobalt
complex recently reported by us.11j
20
15
CoL0
CoL1
CoL2
CoL3
CoL4
n
10
5
0
0
30
60
90
120
150
180
Time, min
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
TON
TOF
100
80
60
40
20
0
b)
Photochemical mechanism for hydrogen evolution
In homogeneous photocatalytic systems involving a hydrogen
evolving catalyst (HEC), a photosensitizer (P), and a sacrificial
electron donor (SD), the reduction of the catalyst to trigger
hydrogen evolution may in principle occur by two different
CoL0
CoL1
CoL2
CoL3
CoL4
Figure 2. Photocatalytic hydrogen evolution experiments in
1
M acetate buffer
solutions (5 mL, pH 5) containing 0.5 mM Ru(bpy)32+, 0.1 M ascorbic acid and 75 ꢀM
CoL0-4: (a) hydrogen evolution kinetics (averages of three different experiments), (b)
maximum turnover numbers and frequencies.
As far as the turnover limiting reactions are concerned, partial
decomposition of both the photosensitizer and the catalyst
can be deduced on the basis of the following considerations.
Addition of the same amount of either the catalyst or the
photosensitizer to a 1-hour photolyzed mixture (1 M acetate
Scheme 3. Possible photochemical mechanisms for catalyst reduction in
homogeneous system for hydrogen production involving catalyst (HEC),
a
a
a
buffer at pH 5, 0.1 M ascorbic acid, 0.5 mM Ru(bpy)32+, 75
ꢀM
photosensitizer (P), and a sacrificial electron donor (SD): (a) oxidative and (b) reductive
pathways.
CoL0-4) is such that the photocatalytic activity is only restored
to an extent of ca 20-30% and ca 40-60%, respectively, in
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 5 of 10
Journal Name
View Article Online
DOI: 10.1039/C6DT01705C
ARTICLE
mechanisms:9c,28 (i) an oxidative quenching pathway (Scheme monitored by laser flash photolysis technique and time-
3a), involving first photoinduced electron transfer from the resolved.10h,11c,d,f,j,26,33 Upon excitation at 355 nm of a solution
2+
photosensitizer to the catalyst followed by hole shift to the containing 0.1 mM Ru(bpy)3 and 0.1 M ascorbic acid in 1 M
2+
sacrificial donor, or (ii) a reductive quenching pathway acetate buffer at pH 5, the evolution of the Ru(bpy)3 triplet
+
(Scheme 3b) whereby the excited sensitizer reacts first with excited manifold to yield the reduced Ru(bpy)3 species via
the donor, yielding a photogenerated reductant which then reductive quenching by ascorbic acid is observed (Figure 3a).
undergoes a thermal electron transfer to the catalyst.
This process is accompanied by the decrease of the absorption
Basing on the first hypothesis (Scheme 3a), considering a redox at 380 nm and of the MLCT bleach centered at 450 nm (typical
2+
potential of −1.12 V vs. SCE for the Ru(bpy)33+/3*Ru(bpy)3
of the 3*Ru(bpy)32+) and the development of a transient
couple29 and the potentials for the Co(II)/Co(I) reduction absorption at 510 nm, which is characteristic of the reduced
(Figure 1a), oxidative quenching is thermodynamically Ru(bpy)3 species.11c,d,f,j,26,33,34 This signal then decays to the
+
unfavorable. On the other hand, reductive quenching of baseline in a time-scale of hundred
2+
ꢀs by bimolecular
excited Ru(bpy)3 by ascorbic acid is allowed,11,30 thus recombination with the oxidized ascorbate radical (Figure
demonstrating the feasibility of a reductive mechanism 3b).11d,33
(hypothesis ii, Scheme 3b). The bimolecular rate constant for
this quenching process can be estimated by a classical Stern-
Volmer analysis as k = 2.1 × 107 M-1s-1 (Figure S6).
wavelength, nm
400
500
600
700
800
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
0.01
0.20
0.50
ꢀ
ꢀ
ꢀ
s
s
s
Table 1. Relevant photochemical data of the three-component system.
a)
Entry
CoL0
CoL1
CoL2
CoL3
CoL4
kQ (109 M-1s-1) a
kET (109 M-1s-1) b
0.6
1.0
1.1
0.9
0.7
1.4
2.1
1.1
1.0
1.8
a.
2+
Bimolecular rate constants for quenching of the Ru(bpy)3 excited state
obtained from photoluminescence data and the related Stern-Volmer analysis
2+
b.
using a lifetime for the Ru(bpy)3 excited state decay of 400 ns; bimolecular
rate constants for the electron transfer from the photogenerated Ru(bpy)3+ to the
CoL0-4 catalysts obtained by laser flash photolysis (excitation at 355 nm, analysis
at 510 nm).
1.0
10
ꢀs
ꢀ
b)
s
100 ꢀs
2+
However, the luminescence of the Ru(bpy)3 photosensitizer
is quenched by CoL0-4 catalysts as well. From a Stern-Volmer
treatment (Figure S7) it can be estimated that complexes
2+
CoL0-4 quench the Ru(bpy)3 emission with bimolecular rate
constants between 0.6-1.1 × 109 M-1s-1 (Table 1). Being
oxidative quenching thermodynamically unfavorable, energy
transfer to low lying cobalt d-d states31 may very likely account
for the observed photoreaction.
400
500
600
700
800
wavelength, nm
Figure 3. Transient absorption spectra obtained by laser flash photolysis (excitation at
2+
Under the experimental conditions used in the photocatalytic
experiments, the ascorbic acid donor (0.1 M) is much more
355 nm) of a 1 M acetate buffer (pH 5) solution containing 100 ꢀM Ru(bpy)3 and 0.1
M ascorbic acid at (a) 0.01-0.50 ꢀs and (b) 1-100 ꢀs time delays.
concentrated than the CoL0-4 catalysts (75
ꢀM). This
translates into a higher pseudo-first order rate for the
reductive quenching by the sacrificial donor (2.1 × 106 s-1) than
for the catalyst quenching pathway (4.5-8.3 × 104 s-1). These
results thus establish that the reductive quenching of triplet
excited Ru(bpy)32+ is the primary photochemical process within
the donor/sensitizer/catalyst three-component systems
examined and thus hydrogen evolution is triggered by a
reductive photochemical pathway (Scheme 3b).
A similar sequence of photochemical events is also observed
when CoL0-4 catalysts are introduced in the same reaction
+
mixtures (formation of the reduced Ru(bpy)3 species via
bimolecular reductive quenching by the ascorbic acid donor),
differing essentially for a much faster disappearance of the
510-nm absorption. This faster decay can be reasonably
assigned to the electron transfer process occurring from the
+
photogenerated Ru(bpy)3 to the catalyst. Moreover, this
According to these considerations, one of the key processes
which clearly deserves much attention is the electron transfer
step from the photogenerated reducing sensitizer to the
transient absorption decays with an appreciable first-order
dependence on catalyst concentration and under pseudo-first
+
order kinetic conditions (i.e., [CoL0-4] >> [Ru(bpy)3 ]) a
+
catalyst. Indeed, highly reducing species such as Ru(bpy)3 are
bimolecular rate constant can be obtained for such an electron
transfer process (Figure 4 for CoL0 and Figure S8 for the
remaining CoL1-4 catalysts, data are reported in Table 1).
These estimates yield values in the range 1.0-2.1 × 109 M-1s-1,
prone to decomposition in solution32 and a fast electron
scavenging is thus pivotal to minimize such an unproductive
pathway. This electron transfer process can be conveniently
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 6 of 10
View Article Online
DOI: 10.1039/C6DT01705C
ARTICLE
Journal Name
close to the diffusion limit and within the same order of expected to display different molecular interactions with the
magnitude of the bimolecular electron transfer rates from solvent environment, i.e. with the surrounding water
+
Ru(bpy)3 to other polypyridine cobalt complexes,11c,d,f,j to a molecules. In other words, the substituents were expected to
tetracationic cobalt porphyrin,26 and to a cobaloxime.33
assist the proton coupled electron transfer (PCET) processes
These values are quite similar throughout the CoL0-4 series, as required to foster hydrogen evolution (i.e. the formation of
expected on the basis of the similar redox potentials and thus the cobalt(III)-hydride intermediate and the protonation of the
on the resulting driving forces for the electron transfer cobalt(II)-hydride species). A comparison of the photochemical
+
processes. Importantly, analysis of the decay of the Ru(bpy)3 hydrogen evolution performances among the CoL0-4
species at different wavelengths (taking CoL0 as
a
complexes shows that some differences do actually exist with
representative case for all catalysts, Figure S9) show no maximum TONs between 56 and 94 and maximum TOFs
appreciable formation and accumulation of additional between 2.1 and 3.3 min-1 (Figure 2).
transient signals, e.g. those expected for the generation of a On a TON basis, the highest values are obtained for complexes
Co(I) moiety, which is supposed to display featuring absorption CoL3 and CoL4 (90 and 94, respectively), while lower TONs are
signatures in the visible spectrum between 500-700 nm.11c,f
detected for the remaining catalysts in the order CoL0 > CoL2 >
CoL1 (67, 59, and 56, respectively). Therefore, the presence of
different substituents on the phenylpyridine moiety plays an
important role in the stabilization of the catalytic unit.
Although the degradation routes involving the cobalt catalyst
are actually unknown, it can be argued that the presence of
potentially reducible functional groups such as the aldehyde
and the amide might offer alternative reaction pathways to
the cobalt hydride intermediates. Accordingly, the
involvement of the ligand framework in the reaction can thus
result into an enhanced decomposition of the catalytic unit.
When the comparison is made in terms of maximum TOFs
(which can be intrinsically related to the quantum yield of
hydrogen evolution, being calculated in the linear part of the
kinetic traces), it can be observed that larger values are
measured in the case of complexes CoL3, and CoL4 (3.0, and
3.3 min-1, respectively), while lower values are measured in the
0 M
0.1 mM
0.2 mM
0.05
0.04
0.03
0.02
0.01
0.00
a)
0
5
10 15 20 25 30 35 40 45
time, ꢀs
30
25
20
15
10
5
b)
order for CoL0,
CoL2, and CoL1 (2.7, 2.4, and 2.1 min-1,
respectively). Since the primary photochemical step, namely
+
the photogeneration of the Ru(bpy)3 reducing agent, is
independent of the CoL0-4 catalyst used, possible reasons
accounting for the observed evidences must rely on the dark
catalytic process. Differences in terms of electron transfer
rates (eq 1,2) can be clearly ruled out since the rates of
catalyst activation (eq 1) by the photogenerated reduced
sensitizer are generally high and comparable along the CoL0-4
series (Table 1) and a similar trend can be also expected for
the second electron transfer reaction (eq 2). Accordingly,
differences must be found on the chemical protonation step.
On a tentative basis it can be envisaged that the hanging
groups can be involved in hydrogen bonding with water and
may consequently favor the formation of a network of solvent
molecules in proximity of the cobalt center, thus speeding up
the protonation step required in the HER mechanism (eq 3)
and therefore catalysis. The good hydrogen-bonding abilities
(both as donor and acceptor) of the hydroxo groups36 are
indeed consistent with this hypothesis. It must be remarked,
however, that the observed differences in terms of maximum
TOFs are not that large among the CoL0-4 series, pointing out
that the actual effect on the kinetics played by the dangling
substituents is indeed intrinsically small. This observation can
be rationalized considering the weak electronic effects exerted
by the dangling substituents on the Co(II)/Co(I) reduction
potential (see above), which strongly suggests that the phenyl
k
= 1.4 x 109 M-1s-1
0
0.00
0.05
0.10
0.15
0.20
[
CoL0], mM
Figure 4. (a) Kinetic traces at 510 nm obtained by laser flash photolysis (excitation at
355 nm) of a 1 M acetate buffer (pH 5) solution containing 100 ꢀM Ru(bpy)32+, 0.1 M
ascorbic acid, 0-0.2 mM CoL0; (b) plot of the observed rate (obtained from a single-
exponential fitting of the kinetic traces) vs. CoL0 concentration for the estimation of
the bimolecular rate constant.
This suggests that herein the electron transfer from the
+
photogenerated Ru(bpy)3 species most likely occurs together
with a protonation step as a concerted proton-coupled
electron-transfer process (see above eq 1), whose product,
namely the Co(III)-H species, is known to lack any appreciable
absorption above 500 nm.35
Effect of the substituent on hydrogen evolution
The approach adopted herein to tune the catalytic activity by
the tetradentate cobalt complex was motivated by the idea
that the different substituents at the phenyl ring, having
different hydrogen bonding donor or acceptor abilities,36 were
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 7 of 10
Journal Name
View Article Online
DOI: 10.1039/C6DT01705C
ARTICLE
ring is electronically decoupled from the pyridine, with
substantially wide dihedral angles. Under these assumptions,
the dangling functional groups should be in fact located far
apart with respect to the cobalt center, thus being only
marginally effective in promoting and assisting the formation
of the relevant catalytic intermediates.
Experimental Section
Materials. Chemicals were purchased from Sigma Aldrich and
TCI chemicals and used without further purification.
Acetonitrile for electrochemistry was of electrochemical grade.
Milli-Q ultrapure water and related buffers were used for the
electrochemical,
experiments.
spectroscopic,
and
photocatalytic
NMR. 1H and 13C{1H} NMR spectra were recorded at 301 K on
a Bruker AC-400, AC-300, and AC-200 MHz instruments. The 1H
NMR spectra were referenced to the solvent residual peak of
MeOD-d4 (3.31 ppm) or CD3CN-d3 (1.94 ppm); the 13C NMR
Conclusions
A series of new cobalt polypyridine catalysts (CoL0-4), based
on the tetradentate TPMA ligand bearing different functional
groups, has been synthesized and studied as hydrogen
evolving catalysts in light-assisted conditions in the presence
of the standard Ru(bpy)32+/ascorbic acid sensitizer/donor
couple. Upon continuous visible irradiation of the three-
component catalyst/sensitizer/donor system, hydrogen is
produced with TONs between 56 and 94, maximum TOFs
between 2.1 and 3.3 min-1. In all these cases the photocatalytic
activity is mainly limited by decomposition of both the
sensitizer and the catalyst. The mechanism towards hydrogen
evolution is triggered by bimolecular reductive quenching of
spectra (50 MHz) were referenced to MeOD (49.00
or CD3CN peaks (1.32 0.02 and 118.26 0.02 ppm).
±0.02 ppm)
±
±
ESI-MS. ESI-MS experiments were carried out in positive mode
with an Agilent Technologies LC/MSD Trap SL AGILENT
instrument (mobile phase methanol or acetonitrile). MS peaks
are reported as monoisotopic mass. Microanalyses were
performed with a Flash 2000 Thermo Scientific Analyser.
Electrochemistry. Cyclic Voltammetry (CV) measurements
were carried out with
a
PC-interfaced Eco Chemie
conventional three-
Autolab/Pgstat 30 Potentiostat.
A
2+
the Ru(bpy)3 photosensitizer by the ascorbic acid donor,
electrode cell assembly was adopted: a saturated calomel
electrode (SCE Amel) and a platinum electrode, with the
former separated from test solution by a glass frit, were used
as reference and counter electrodes, respectively; a glassy
carbon (GC) electrode (7 mm2 surface area) was used as the
working electrode. All experiments were performed in
nitrogen-purged solutions and the working electrode was
accurately polished with alumina slurry after every single scan.
Potential controlled electrolysis experiments were performed
in a gas-tight custom-made electrochemical cell using a carbon
foil (1 cm2 surface area, 0.5 mm thickness) as the working
electrode, a platinum wire as the counter electrode (separated
from the test solution by a frit) and a Ag/AgCl (3 M NaCl) as the
reference electrode (in the text, potentials are converted to
SCE for uniformity with CV experiments). The head-space of
the cell was connected to the gas-chromatography (GC)
apparatus (see below for details) for the determination and
quantification of hydrogen. The calibration was performed
through a galvanostatic electrolysis of a 0.1 M H2SO4 solution
at 2 mA current, using a platinum wire as the working
electrode, assuming 100% Faradaic yield towards hydrogen
formation.
followed by bimolecular electron transfer to the catalyst,
taking place at remarkable rates close to the diffusion limit.
The enhanced activity by CoL3 and CoL4 with respect to the
remaining complexes is attributable, on a TON basis, to the
enhanced stability under photocatalytic conditions arising
from the absence of potentially reactive functional groups as
in the case of CoL1 and CoL2, and, on a TOF basis, to faster
catalysis rates. As to the latter point, the presence of good
hydrogen-bonding substituents, such as the hydroxo group,
may indeed favor the formation of a water network near the
cobalt center thus accelerating the protonation step required
in the HER mechanism. Although this effect is present, the
substantially small differences observed within the CoL0-4
series demonstrate that the tuning approach adopted herein is
not that effective, which can be ascribed to the non-optimal
structural arrangement featuring large distances between the
dangling functional group and the cobalt center. Accordingly,
the modification of the ligand pool to increase the rigidity of
the phenylpyridine moiety can be a viable solution to enhance
the envisaged second-sphere effect on the HER catalysis.
Research towards this direction is currently planned in our
laboratories.
Optical spectroscopy. UV-Vis absorption spectra were
recorded on a Jasco V-570 UV/Vis/NIR spectrophotometer.
Emission spectra were taken on
a Horiba-Jobin Yvon
Acknowledgements
Fluoromax-2 spectrofluorimeter, equipped with a Hamamatsu
R3896 tube. Nanosecond transient measurements were
performed with a custom laser spectrometer comprised of a
Continuum Surelite II Nd:YAG laser (FWHM 6-8 ns) with
frequency doubled, (532 nm, 330 mJ) or tripled, (355 nm, 160
mJ) option, an Applied Photophysics xenon light source
including a mod. 720 150 W lamp housing, a mod. 620 power
controlled lamp supply and a mod. 03-102 arc lamp pulser.
Laser excitation was provided at 90° with respect to the white
light probe beam. Light transmitted by the sample was focused
The authors gratefully acknowledge Prof. Franco Scandola for
fruitful discussions. Financial support from the Italian MIUR
(FIRB RBAP11C58Y ‘‘NanoSolar’’, PRIN 2010 ‘‘Hi-Phuture‘‘,
PRIN-2010-11 2010CX2TLM_002), from the University of
Padova (PRAT-CPDA123307 E. B. fellowship, Progetto
Attrezzature Scientifiche finalizzate alla Ricerca 2014) and from
the COST action CM1202 ‘‘PERSPECT-H2O’’ is gratefully
acknowledged.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 8 of 10
View Article Online
DOI: 10.1039/C6DT01705C
ARTICLE
Journal Name
the reaction medium to evaluate the effect on electrocatalysis
by the different substituents since the interaction of protons
with the functional groups can be only mediated by water (i.e.,
via hydrogen bonding). To this aim we took advantage of a
50/50 acetonitrile/water mixture, chosen for comparison with
literature studies on related cobalt complexes.15a
onto the entrance slit of a 300 mm focal length Acton
SpectraPro 2300i triple grating, flat field, double exit
monochromator equipped with a photomultiplier detector
(Hamamatsu R3896) and a Princeton Instruments PIMAX II
gated intensified CCD camera, using a RB Gen II intensifier, a
ST133 controller and a PTG pulser. Signals from the
photomultiplier (kinetic traces) were processed by means of a
LeCroy 9360 (600 MHz, 5 Gs/s) digital oscilloscope.
§ A hint of catalytic wave can be indeed detected at potentials
compatible with the Co(II)/Co(I) process.
§§ This is also consistent with the fact that, with the exception of
CoL1 for which some decomposition is likely to occur particularly
at higher TFA concentration), comparable slopes (between 0.032
and 0.028 A·M-1, Figure S3) are observed in the linear correlation
between the catalytic peak current and [TFA].
ǁ It should be emphasized that the ECEC mechanism (eq 1,2,3)
for hydrogen evolution by the CoL0-4 complexes is only a
proposed catalytic pathway according to the available
experimental data. For a more conclusive account additional
experimental conditions should be indeed tested and further
analyses performed, including full determination of kinetic
constants. This is, however, out of the scope of this manuscript.
¶ Under the conditions explored, hydrogen evolving activity was
detectable in the presence of CoL0-5 catalysts at concentrations
Hydrogen evolution experiments. The hydrogen evolution
experiments were carried out upon continuous visible light
irradiation with a 175 W xenon arc-lamp (CERMAX PE175BFA)
of a reactor containing the solution (a 10 mm pathlength pyrex
glass cuvette with head space obtained from a round-bottom
flask). A cut-off filter at 400 nm and a hot mirror (IR filtering)
have been used to provide the useful wavelength range (400-
800 nm). The reactor is placed at a distance of 20 cm from the
irradiation source and the light beam is completely focused on
the reactor (all the solution is irradiated during the
experiment). The measuring cell is sealed during the
photoreaction: the head to which cell is attached has indeed
four ports, closed with Swagelok® connections, two of them
are part of a closed loop involving GC gas inlet and sample vent
in order to analyze head space content without an appreciable
gas consumption, and the other two are for the degassing
procedure (input and output). The gas phase of the reaction
vessel was analyzed on an Agilent Technologies 490 microGC
equipped with a 5 Å molecular sieve column (10 m), a thermal
conductivity detector, and using Ar as carrier gas. 5 mL from
the headspace of the reactor are sampled by the internal GC
pump and 200 nL are injected in the column maintained at
60°C for separation and detection of gases. The unused gas
sample is then reintroduced in the reactor in order to minimize
its consumption along the whole photolysis. The amount of
hydrogen was quantified through the external calibration
method. This procedure was performed, prior to analysis,
through a galvanostatic (typically 1 mA) electrolysis of a 0.1 M
H2SO4 solution in an analogous cell (same volume) equipped
with two Pt wires sealed in the glass at the bottom of the cell.
A 100% faradaic efficiency was assumed leading to a linear
correlation between the amount of H2 evolved at the cathode
and the electrolysis time. In a typical experiment, samples of 5
mL were prepared in 20 mL scintillation vials starting from a 1
M acetate buffer (pH 5) solution of Ru(bpy)3Cl2 (0.5 mM), and
further adding ascorbic acid (as solid) and the CoL0-4 catalyst
(small aliquots from a concentrated mother solution in
acetonitrile). The change in volume upon addition of the latter
≥10
photocatalytic performances within the class of CoL0-4
complexes a catalyst concentration of 75 M has been chosen
ꢀM. However, in order to evaluate the different
ꢀ
providing substantial hydrogen production for an accurate
comparison.
1
(a) J. H. Alstrum-Acevedo, M. K. Brennaman and T. J. Meyer,
Inorg. Chem. 2005, 44, 6802. (b) N. S. Lewis and D. G.
Nocera, Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729. (c) R.
Eisenberg and H. B. Gray, Inorg. Chem. 2008, 47, 1697. (d) D.
G. Nocera, Inorg. Chem. 2009, 48, 10001. (e) H. B. Gray, Nat.
Chem. 2009, 1, 7. (f) N. Armaroli, V. Balzani and N. Serpone,
Powering Planet Earth: Energy Solutions for the Future,
Wiley, New York, 2012. (g) D. Gust, T. A. Moore and A. L.
Moore, Faraday Disc. 2012, 155, 9. (h) J. R. McKone, N. S.
Lewis and H. B. Gray, Chem. Mater. 2014, 26, 407. (i) M.
Natali and F. Scandola. Supramolecular Artificial
Photosynthesis. G. Bergamini, S. Silvi (eds.), Applied
Photochemistry, Lecture Notes in Chemistry 92
10.1007/978-3-319-31671-0_1, Springer 2016.
, DOI:
2
3
J. R. McKone, S. C. Marinescu, B. S. Brunschwig, J. R. Winkler
and H. B. Gray, Chem. Sci. 2014, , 865.
(a) J. Kiwi and M. Grätzel, Nature 1979, 281, 657. (b) J.-M.
Lehn and J.-P. Sauvage, Nouv. J. Chim. 1977, , 449. (c) A. J.
5
1
Bard and M. A. Fox, Acc. Chem. Res. 1995, 28, 141.
4
5
S. Y. Reece, J. A. Amel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J.
H. Pijpers and D. G. Nocera, Science 2011, 334, 645.
(a) D. Merki, H. Vrubel, L. Rovelli, S. Fierro and X. Hu, Chem.
Sci. 2012, 3, 2515. (b) P. D. Tran, M. Nguyen, S. S. Pramana,
A. Bhattacharjee, S. Y. Chiam, J. Fize, M. J. Field, V. Artero, L.
H. Wong, J. Loo and J. Barber, Energy Environ. Sci. 2012, 5,
8912.
6
7
8
(a) D. G. Nocera, Acc. Chem. Res. 2012, 45, 767; (b) J. P.
Torella, C. J. Gagliardi, J. S. Chen, D. K. Bediako, B. Colon, J. C.
Way, P. A. Silver and D. G. Nocera, Proc. Natl. Acad. Sci.
U.S.A. 2015, 112, 2337.
(a) D. Streich, Y. Astuti, M. Orlandi, L. Schwartz, R. Lomoth, L.
Hammarström and S. Ott, Chem. Eur. J. 2010, 16, 60; (b) S.
Pullen, H. Fei, A. Orthaber, S. Cohen and S. Ott, J. Am. Chem.
Soc. 2013, 135, 16997.
(a) M. L. Helm, M. P. Stewart, R. M. Bullock, M. Rakowski
DuBois and D. L. DuBois, Science 2011, 333, 863. (b) M. P.
McLaughlin, T. M. McCormick, R. Eisenberg and P. L. Holland,
Chem. Commun. 2011, 47, 7989. (c) M. A. Gross, A. Reynal, J.
Durrant and E. Reisner, J. Am. Chem. Soc. 2014, 136, 356. (d)
A. S. Weingarten, R. V. Kazantsev, L. C. Palmer, M.
can be always considered negligible (dilution
≤ 1%). The
solution was then put in the reactor, degassed by bubbling Ar
for 30 min, and thermostated at 15°C. The cell was then
irradiated and the solution continually stirred during the
photolysis. The gas phase of the reaction was analyzed
through GC and the amount of hydrogen quantified.
Notes and references
‡ With respect to standard electrocatalytic studies usually
performed in neat acetonitrile, herein we introduced water into
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 9 of 10
View Article Online
DOI: 10.1039/C6DT01705C
Journal Name
ARTICLE
McClendon, A. R. Koltonow, A. P. S. Samuel, D. J. Kiebala, M.
R. Wasielewski and S. I. Stupp, Nat. Chem. 2014, 6, 964.
(a) S. Losse, J. G. Vos and S. Rau, Coord. Chem. Rev. 2010,
15368. (b) W. R. McNamara, Z. Han, C.-J. Yin, W. W.
Brennessel, P. L. Holland and R. Eisenberg, Proc. Natl. Acad.
Sci. U.S.A. 2012, 109, 15594.
9
254, 2492. (b) V. Artero, M. Chavarot-Kerlidou and M. 16 A. Mahammed, B. Mondal, A. Rana, A. Dey and Z. Gross,
Fontecave, Angew. Chem., Int. Ed. 2011, 50, 7238. (c) W. T. Chem. Commun. 2014, 50, 2725.
Eckenhoff, W. R. McNamara, P. Du and R. Eisenberg, 17 C. H. Lee, D. K. Dogutan and D. G. Nocera, J. Am. Chem. Soc.
Biochim. Biophys. Acta 2013, 1827, 958. (d) C. C. L. McCrory, 2011, 133, 8775.
C. Uyeda and J. C. Peters, J. Am. Chem. Soc. 2012, 134, 3164. 18 (a) F. A. Scaramuzzo, G. Licini and C. Zonta, Chem. Eur. J.
(e) S. Varma, C. E. Castillo, T. Stoll, J. Fortage, A. G. Blackman,
F. Molton, A. Deronzier and M.-N. Collomb, Phys. Chem.
Chem. Phys. 2013, 15, 17544. (f) C. Gimbert-Suriñach, J.
Albero, T. Stoll, J. Fortage, M.-N. Collomb, A. Deronzier, E.
2013, 19, 16809. (b) E. Badetti, K. Wurst, G. Licini and C.
Zonta, Chem. Eur. J. 2016, 22, 6515. (c) R. Berardozzi, E.
Badetti, N. A. Carmo dos Santos, K. Wurst, G. Licini, G.
Pescitelli and C. Zonta, Chem. Commun. 2016, 52, 8428.
Palomares and A. Llobet, J. Am. Chem. Soc. 2014, 136, 7655. 19 J. T. Muckerman and E. Fujita, Chem. Commun. 2011, 47
(g) R. Gueret, C. E. Castillo, M. Rebarz, F. Thomas, A.-A. 12456.
Hargrove, J. Pécaut, M. Sliwa, J. Fortage and M.-N. Collomb, 20 J. L. Dempsey, B. S. Brunschwig, J. R. Winkler and H. B. Gray,
,
J. Photochem. Photobiol. B 2015, 152, 82. (h) S. Roy, M.
Acc. Chem. Res. 2009, 42, 1995.
Bacchi, G. Berggren and V. Artero, ChemSusChem 2015,
3632.
10 (a) P. Du, K. Knowles and R. Eisenberg, J. Am. Chem. Soc.
8
,
21 A. J. Bard and L. R. Faulkner, Electrochemical Methods:
Fundamentals and Applications, 2nd ed.; John Wiley and
Sons: New York, 2001.
2008, 130, 12576. (b) A. Fihri, V. Artero, M. Razavet, C. 22 J. P. Bigi, T. E. Hanna, W. H. Harman, A. Chang and C. J.
Baffert, W. Leibl and M. Fontecave, Angew. Chem., Int. Ed. Chang, Chem. Commun. 2010, 46, 958.
2008, 47, 564. (c) T. M. McCormick, B. D. Calitree, A. 23 (a) J.-M. Savéant and K. B. Su, J. Electroanal. Chem. 1984,
Orchard, N. D. Kraut, F. V. Bright, M. R. Detty and R. 171, 341. (b) J.-M. Savéant, Chem. Rev. 2008, 108, 2348.
Eisenberg, J. Am. Chem. Soc. 2010, 132, 15480. (d) J. L. 24 E. S. Rountree, D. J. Martin, B. D. McCarthy and J. L.
Dempsey, J. R. Winkler and H. B. Gray, J. Am. Chem. Soc. Dempsey, ACS Catalysis 2016, , 3326.
2010, 132, 1060. (g) F. Lakadamyali, A. Reynal, M. Kato, J. R. 25 (a) C. Costentin and J.-M. Savéant, ChemElectroChem 2014,
6
Durrant and E. Reisner, Chem. Eur. J. 2012, 18, 15464. (h) M.
Natali, R. Argazzi, C. Chiorboli, E. Iengo and F. Scandola,
1, 1226. (b) C. Costentin, H. Dridi and J.-M. Savéant, J. Am.
Chem. Soc. 2014, 136, 13727.
Chem. Eur. J. 2013, 19, 9261. (i) M. Natali, M. Orlandi, C. 26 M. Natali, A. Luisa, E. Iengo and F. Scandola, Chem. Commun.
Chiorboli, E. Iengo, V. Bertolasi and F. Scandola, Photochem. 2014, 50, 1842.
Photobio. Sci. 2013, 12, 1749. (j) K. Peuntinger, T. Lazarides, 27 C. Bachmann, B. Probst, M. Guttentag and R. Alberto, Chem.
D. Dafnomili, G. Charalambidis, G. Landrou, A. Kahnt, R. P. Commun. 2014, 50, 6737.
Sabatini, D. W. McCamant, D. T. Gryko, A. G. Coutsolelos and 28 (a) E. S. Andreiadis, M. Chavarot-Kerlidou, M. Fontecave and
D. M. Guldi, J. Phys. Chem. C 2013, 117, 1647.
11 (a) Y. Sun, J. Sun, J. R. Long, P. Yang and C. J. Chang, Chem.
V. Artero, Photochem. Photobio. 2011, 87, 946; (b) K.
Ladomenou, M. Natali, E. Iengo, G. Charalampidis, F.
Scandola and A. G. Coutsolelos, Coord. Chem. Rev. 2015,
304-305, 38.
Sci. 2013,
D. Z. Zee, B. S. Olaiya, M. Head-Gordon, C. J. Chang, F. N.
Castellano and J. R. Long, Chem. Sci. 2013,
Khnayzer, V. A. Thoi, M. Nippe, A. E. King, J. W. Jurss, K. A. El
Roz, J. R. Long, C. J. Chang and F. N. Castellano, Energy 30 G. M. Brown, B. S. Brunschwig, C. Creutz, J. F. Endicott and N.
Environ. Sci. 2014, , 1477. (d) B. Shan, T. Baine, X. A. N. Ma, Sutin, J. Am. Chem. Soc. 1979, 101, 1298.
X. Zhao and R. H. Schmehl, Inorg. Chem. 2013, 52, 4853. (e) 31 B. Probst, M. Guttentag, A. Rodenberg, P. Hamm and R.
C. Bachmann, M. Guttentag, B. Spingler and R. Alberto, Alberto, Inorg. Chem. 2011, 50, 3403.
Inorg. Chem. 2013, 52 6055. (f) W. M. Singh, M. 32 C. V. Krishnan and N. Sutin, J. Am. Chem. Soc. 1981, 103
Mirmohades, R. T. Jane, T. A. White, L. Hammarström, A.
Thapper, R. Lomoth and S. Ott, Chem. Commun. 2013, 49
4, 118. (b) M. Nippe, R. S. Khnayzer, J. A. Panetier,
4
, 3934. (c) R. S. 29 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and
A. Von Zelewsky, Coord. Chem. Rev. 1988, 84, 85.
7
,
,
,
2141.
,
33 E. Deponti and M. Natali, Dalton Trans. 2016, 45, 9136.
8638. (g) L. Tong, R. Zong and R. P. Thummel, J. Am. Chem. 34 L. A. Kelly and M. A. J. Rodgers, J. Phys. Chem. 1994, 98
Soc. 2014, 136, 4881. (h) J. Xie, Q. Zhou, C. Li, W. Wang, Y. 6377.
Hou, B. Zhang and X. Wang, Chem. Commun. 2014, 50, 6520. 35 (a) C. Creutz, H. A. Schwarz, J. F. Wishart, E. Fujita and N.
(i) C.-F. Leung, S.-M. Ng, C.-C. Ko, W.-L. Man, J. Wu, L. Chen Sutin, J. Am. Chem. Soc. 1991, 113 3361. (b) A.
and T.-C. Lau, Energy Environ. Sci. 2012, , 7903. (j) E.
,
5
Lewandowska-Andralojc, T. Baine, X. Zhao, J. T.
Muckermann, E. Fujita and D. E. Polyansky, Inorg. Chem.
2015, 54, 4310.
Deponti, A. Luisa, M. Natali, E. Iengo and F. Scandola, Dalton
Trans. 2014, 43, 16345. (k) W. K. C. Lo, C. E. Castillo, R.
Gueret, J. Fortage, M. Rebarz, M. Sliwa, F. Thomas, C. J. 36 P. Gilli, L. Pretto, V. Bertolasi and G. Gilli, Acc. Chem. Res.
McAdam, G. B. Jameson, D. A. McMorran, J. D. Crowley, M.-
N. Collomb and A. G. Blackman, Inorg. Chem. 2016, 55, 4564.
(l) N. Queyriaux, R. T. Jane, J. Massin, V. Artero and M.
Chavarot-Kerlidou, Coord. Chem. Rev. 2015, 304-305, 3.
12 A. Call, Z. Codolá, F. Acuña-Pares and J. Lloret-Fillol, Chem.
Eur. J. 2014, 20, 6171.
2009, 42, 33.
13 (a) D. W. Wakerley and E. Reisner, Phys. Chem. Chem. Phys.
2014, 16, 5739. (b) A. Panagiotopoulos, K. Ladomenou, D.
Sun, V. Artero and A. G. Coutsolelos, Dalton Trans. 2016, 45
6732.
,
14 M. Vennampalli, G. Liang, L. Katta, C. E. Webster and X. Zhao,
Inorg. Chem. 2014, 53, 10094.
15 (a) W. R. McNamara, Z. Han, P. J. Alperin, W. W. Brennessel,
P. L. Holland and R. Eisenberg, J. Am. Chem. Soc. 2011, 133
,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Dalton Transactions
Page 10 of 10
View Article Online
DOI: 10.1039/C6DT01705C
TOC
2 H+
H2
N
Co
N
N
N
New cobalt(II) complexes based on the TPMA ligand have been synthesized and characterized as
molecular catalysts for photoinduced hydrogen evolution.