New Photosensitizers Based on Heteroleptic CuI Complexes and CO2 Photocatalytic Reduction with [NiII(cyclam)]Cl2

Article abstract of DOI:10.1002/chem.202001279

Earth-abundant metal complexes have been attracting increasing attention in the field of photo(redox)catalysis. In this work, the synthesis and full characterisation of four new heteroleptic Cu^I complexes are reported, which can work as photose

Full text of DOI:10.1002/chem.202001279

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Photocatalysis
New Photosensitizers Based on Heteroleptic CuI Complexes and
CO2 Photocatalytic Reduction with [NiII(cyclam)]Cl2
Lisa-Lou Gracia,[a] Luisa Luci,[a, b] Cecilia Bruschi,[a] Letizia Sambri,[b] Patrick Weis,[c] Olaf Fuhr,[d]
and Claudia Bizzarri*[a]
Dedicated to Prof. Luisa De Cola on the occasion of her 60th birthday
Abstract: Earth-abundant metal complexes have been at-
tracting increasing attention in the field of photo(redox)ca-
talysis. In this work, the synthesis and full characterisation of
four new heteroleptic CuI complexes are reported, which
can work as photosensitizers. The complexes bear a bulky
diphosphine (DPEPhos=bis[(2-diphenylphosphino)phenyl]
ether) and a diimine chelating ligand based on 1-benzyl-4-
(quinol-2’yl)-1,2,3-triazole. Their absorption has a relative
maximum in the visible-light region, up to 450 nm. Thus,
their use in photocatalytic systems for the reduction of CO2
with blue light in combination with the known catalyst
[NiII(cyclam)]Cl2 was tested. This system produced CO as the
main product through visible light (l=420 nm) with a TON
up to 8 after 4 hours. This value is in line with other photo-
catalytic systems using the same catalyst. Nevertheless, this
system is entirely noble-metal free.
Introduction
forts have been carried on for decades in order to achieve an
artificial photosynthetic system. In nature, water and carbon di-
oxide are the raw materials to start with, in order to produce
biomass and the solar photons give the energy that activates
these elaborated processes. Ideally, the exploitation of the
solar energy will lead to a circular economy,[4] where the fuels
are produced from the waste generated by their use, without
the assistance of other types of resources if not wholly renewa-
ble. Carbon dioxide is one of the major waste products of
combustion, and it is also a greenhouse gas. Striving for the
closure of the carbon cycle means then achieving the forma-
tion of fuels from CO2.[5] Although nature reduces carbon diox-
ide in the so-called dark reactions, for example, Calvin cycle, to
form carbohydrates and biomass, in artificial photosynthesis,
researchers aim at a direct photoreduction of CO2, employing
photocatalytic systems.[6] Usually, the major components of an
artificial photosynthetic system consist of a photosensitizer
(PS) and a catalyst (CAT), which sometimes can be the same
unit.[7] A sacrificial electron donor (e-D) is needed to close the
catalytic cycle and regenerate the ground-state of the photo-
sensitizer. To overcome the kinetic and thermodynamic stabili-
ty of CO2, heterogeneous as well as homogeneous systems
have been developed, from semiconductor materials[8] (e.g.,
TiO2, SiC) to molecular photocatalysts,[9] mainly based on ex-
pensive and rare metal complexes, such as Ru,[10] Re[11] or Ir.[12]
Many milestones have been reached so far. However, en-
deavours to achieve up-scalable systems are still needed. In
fact, from a practical point of view, the development of PS or
CAT based on cost-effective and earth-abundant material will
trigger viable industrial applications. More recently, molecular
systems entirely based on earth-abundant metals have been
designed,[13] and their results are inspiring in order to search
Solar light is essential for natural life. The exploitation of this
perennial primary resource for energetic human needs is a
must for sustainable development of processes.[1] The most
common use of solar energy is its conversion into thermal or
electrical energy.[2] Nevertheless, the best way to store energy
is in the form of fuels, namely in chemical bonds.[3] This is
what nature does with photosynthesis, and many research ef-
[a] L.-L. Gracia, L. Luci, C. Bruschi, Dr. C. Bizzarri
Institute of Organic Chemistry, Karlsruhe Institute of Technology
Fritz-Haber-Weg 6, 76137 Karlsruhe (Germany)
E-mail: claudia.bizzarri@kit.edu
[b] L. Luci, Prof. Dr. L. Sambri
Department of Industrial Chemistry “Toso Montanari”
University of Bologna
Viale Risorgimento 4, 40136 Bologna (Italy)
[c] Dr. P. Weis
Institute of Physical Chemistry, Karlsruhe Institute of Technology
Fritz-Haber-Weg 4, 76137 Karlsruhe (Germany)
[d] Dr. O. Fuhr
Institute of Nanotechnology and Karlsruhe Nano Micro Facility (KNMF)“
Karlsruhe Institute of Technology
Hermann von Helmholtz Platz 1, 76344
Eggenstein-Leopoldshafen (Germany)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202001279.
ꢀ 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of Creative Commons Attri-
bution NonCommercial License, which permits use, distribution and repro-
duction in any medium, provided the original work is properly cited and is
not used for commercial purposes.
Part of a Special Collection to commemorate young and emerging scien-
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accessible solutions intensively. Nevertheless, to meet global
demand, recycling is always required, and substantial invest-
ments in this direction are of utmost importance. Notably, CuI-
based coordination complexes have been considered potential
alternatives to RuII-polypyridine complexes, and only recently,
their potential use in photochemistry is rising their consider-
ation as photosensitizers or photoredox catalysts.[14,15] Copper
is a 3d transition metal, present in the earth’s crust at a con-
centration of about 67 ppm.[16] CuI complexes have a d10 elec-
tronic configuration; therefore metal central (MC) transitions
are forbidden, while they exhibit metal-to-ligand charge-trans-
fer (MLCT) transitions, often absorbing at relatively low energy,
in the visible region. This is an attractive property for their ex-
ploitation as photosensitizers. However, they may suffer from a
Jahn–Teller distortion in their excited state, decreasing the
charm of their photophysical properties. This effect is usually
reduced by the use of bulky substituents and the use of steri-
cally hindered ancillary ligands, such as chelating diphos-
phines. On the other hand, easy ligand replacements make
these compounds not very stable in the presence of a coordi-
nating solvent. Although this might be advantageous in pho-
toredox catalysis, so that the complex can coordinate reactant
and/or substrates,[17] stability of the photosensitizer is highly
desirable in photosensitised reactions. In this work, we present
four new photosensitizers based on CuI heteroleptic com-
plexes, a mononuclear and three dinuclear, of general formula
[Cu(NN)(PP)]BF4, where NN is the diimine ligand and PP is the
chelating phosphine. The bulky ancillary diphosphine is for all
compounds the bis[(2-diphenylphosphino)phenyl] ether (DPE-
Phos) and the chromophoric chelating diimines are 5-(quinol-
2’-yl)-1H-1,2,3-triazole derivatives. These complexes absorb the
blue light and are quite stable even in a coordinating solvent
such as acetonitrile. Their photo- and redox properties will be
hereafter illustrated, and their performance as photosensitizers
will be evaluated by employing these new complexes in com-
bination with a known catalyst,[18] [NiII(cyclam)]Cl2 (cyclam:
1,4,8,11-tetra-azacyclotetradecane) for the photoreduction of
CO2.
mosphere (Ar) in dry dichloromethane (DCM), (Scheme 1). The
complexes were obtained in very high yield (up to 95%). Re-
crystallisation from dichloromethane/cyclohexane solution
gave crystals 1, 2a and 2c of good quality for X-ray analysis.
The crystals of 2b were obtained by slow diffusion of diethyl
ether in a concentrated solution of dioxane. The structure of
the mononuclear CuI complex 1 is reported in Figure 1.
The structure of the dinuclear complexes 2a and 2b are
shown in Figure 2, while the structure of complex 2c is report-
ed in the Supporting Information (Figure S35). The crystal
system for compound 1 is monoclinic with P21/n space group,
while the dinuclear complexes 2a–c have a triclinic system and
¯
a P1 space group. The distances between Cu and triazoles
(CuꢀN2 in 1; Cu1ꢀN2 and Cu2ꢀN7 for 2a–c) are very similar in
each complex (1: 2.069 ꢁ; 2a: 2.092 ꢁ; 2b: 2.083 ꢁ; 2c:
2.046 ꢁ). This is true also for the distances between Cu and N
atom of the quinoline moiety, besides for 2b that has a slightly
longer distance (1: 2.093 ꢁ; 2a: 2.095 ꢁ; 2b: 2.110 ꢁ; 2c:
2.098 ꢁ). The distances between the atom of copper and the
phosphorus atoms are also similar for the four compounds (1:
2.249 ꢁ; 2a: 2.240 ꢁ; 2b: 2.226 ꢁ; 2c: 2.236 ꢁ, for Cu1ꢀP1 and
Figure 1. ORTEP drawing of crystals 1 shown at the 50% probability level.
Hydrogen atoms, counterion and solvent molecules were omitted for clarity.
Results and Discussion
The diimine ligands, responsible for the colour of the final CuI
complexes, are synthesised through a Cu alkyne-azide cycload-
dition (CuAAC)[19] from 2-ethynylquinoline and benzyl azide, in
case of the monochelating diimine (ligand 3 in Scheme 1), or
a,a’-diazido- (ortho, meta and para)-xylenes, (6a, 6b and 6c,
respectively) in case of the bis-chelating ligands (4a: o-substi-
tuted; 4b: m-substituted; 4c: p-substituted). In particular, the
diazido-xylenes were formed in situ from their bromo-deriva-
tives and NaN3 at room temperature. The starting alkyne was
synthesised through a Sonogashira cross-coupling reaction
from 2-bromoquinoline and trimethylsilylacetylene in good
yield (for details, please see experimental part). For the majori-
ty of the ligands, no further purification was needed, and the
diimines were used in the next step for the coordination of
CuI. Following an established procedure,[20] mono- and dinu-
clear heteroleptic complexes were synthesised under inert at-
Claudia Bizzarri received her B.Sc. and M.Sc.
in Chemistry from the University of Rome “Tor
Vergata” (Italy). In 2011 she obtained her
Ph.D. working on luminescent iridium(III) and
copper(I) complexes for optoelectronic appli-
cations under the supervision of Prof. Luisa
De Cola at the University of Mꢁnster (Germa-
ny). In 2007, she was a visiting student at the
University of Minnesota Duluth (USA) and in
2010 at the University College London (UK).
After a couple of years as a Process Engineer
at AIXTRON SE (Aachen, Germany), she was a
post-doc at the CAT Catalytic Center of the
University RWTH Aachen. Since the end of
2016, Claudia Bizzarri is an independent Junior Group Leader at the Karlsruhe
Institute of Technology. Her research interests focus on photoactive compounds
based on earth-abundant metals. Her coordination metal complexes find appli-
cations in the field of artificial photosynthesis and bioimaging.
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Scheme 1. Synthetic procedures for the synthesis of the monochelating, and bis-chelating ligands and CuI complexes thereof (1: mononuclear complex, 2: di-
nuclear complexes with xylenes as bridging unit: 2a (o-xylene); 2b (m-xylene); 2c (p-xylene)). (a) CuSO4; ascorbic acid; Na2CO3; (b) NaN3; CuSO4, ascorbic
acid; Na2CO3; (c) DPEPhos; Cu(CH3CN)4BF4; (d) double amount of equivalents as for (c).
Figure 2. ORTEP drawing of crystals 2a (left) and 2b (right) shown at the 50% probability level. Hydrogen atoms, counterions and solvent molecules were
omitted for clarity.
Cu1ꢀP2, and 2a: 2.225 ꢁ; 2b: 2.223 ꢁ; 2c: 2.267 ꢁ, for Cu2ꢀP3
and Cu2ꢀP4). The angles between the copper atom and the
chelating nitrogen atoms of the diimine ligand are similar (1:
79.88; 2a: 79.98; 2b: 79.68; 2c: 79.78). Interestingly, in the di-
nuclear complexes, the angles between the two planes formed
by Cu and the two N-atoms of the diimine ligand (N2-Cu1-N1
and N8-Cu2-N7) vary depending on the compound from being
parallel or almost in 2c and 2a, respectively, to a 608 angle for
2b (2a: 7.148; 2b: 63.38; 2c: 1808).
tions centred on the chelating diphosphine ligand. LC absorp-
tion bands centred on the diimine ligands are also occurring
up to 360 nm, where two sharp signals can be recognised at
1
320 and 340 nm ca. The characteristic MLCT band of CuI com-
plexes can be recognised above 370 nm with a relative maxi-
Photophysical characterisation of the CuI complexes
The new photosensitizers were fully characterised in spectro-
scopic dichloromethane and acetonitrile solutions. Absorption
spectra are very similar among the four complexes, as it is ex-
pected since they all bear the same core ligands (Figure 3).
The main difference is on the absorptivity coefficients e,
where the mononuclear species 1 presents almost the half
values in comparison to the e for the species 2a–c since it con-
tains only one chromophoric unit. In particular, they display in-
tense ligand centre (LC) absorption bands in the high-energy
UV region, below 320 nm, which are assigned to a p–p* transi-
Figure 3. Absorption spectra in acetonitrile. Inset: zoom-in in the range of
the MLCT.
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mum at 398 nm for 1, 388 nm for complexes 2a and 2b,
387 nm for 2c.
These transitions are assigned to charge transfer from the
metal core to the diimine ligand. Compared to the absorption
bands of previously reported CuI complexes based on pyrid-2-
yl-triazoles,[21] the MLCT band is bathochromic shifted, allowing
the complexes to absorb blue light (up to 450 nm). This is ra-
tionalised by the increased p-conjugation of the quinoline
compared with the pyridine ring.[22] In acetonitrile solutions, no
remarkable change could be observed in the absorption spec-
tra of all complexes.
The photophysical properties are reported in Table 1. The
emissions of the complexes in solution are broad and struc-
tureless, typical for MLCT emissions. Moreover, when the meas-
urements are done in air, the presence of the oxygen quench-
es the emission obviously, indicating that a triplet excited state
is involved. Therefore, all the emission and excitation spectra
were then recorded in an inert atmosphere (Ar). The excitation
spectra correspond nicely to the absorption signals.
Figure 4. Excitation (dashed plots) and emission (solid plots) spectra in Ar-
saturated acetonitrile solution. (*: solvent Raman peak). Emissions were re-
corded exciting at 400 nm.
Figure 4. Emission and excitation spectra in DCM can be found
in the Supporting Information (Figure S19). Despite their low
quantum yield and lifetime in acetonitrile, their use as photo-
sensitizers was tested. In fact, they present unexpected stabili-
ty in this coordinating solvent. While other heteroleptic com-
plexes are known to release ligands in solution, with simulta-
neous formation of the corresponding homoleptic complexes,
this phenomenon was not observed for our CuI complexes.
Table 1. Photophysical properties of the new CuI photosensitizers.[a,b]
Sample
labs [nm]
lem [nm]
PLQY[c]
t
[d] [ms]
1
395[a]
398[b]
386[a]
388[b]
388[a]
388[b]
388[a]
387[b]
633
0.014
<0.001[b]
0.013
2.25
640[b]
635
0.107[b]
1.26
2a
2b
2c
1
646[b]
630
<0.001[b]
0.017
0.109[b]
2.65
Their H NMR spectra in deuterated ACN presented the same
pattern even after a long time and also under irradiation, there
are no changes in their UV/Vis spectra (see stability tests in the
Supporting Information, Figures S10–S13 and S20–S23).
650[b]
636
<0.001[b]
0.023
<0.001[b]
0.125[b]
2.31
643[b]
0.128[b]
[a] In Ar-saturated DCM solution. [b] In Ar-saturated ACN. [c] Photolumi-
nescence quantum yields were measured with the relative method using
Ru(bpy)3Cl2 in aerated water solution as standard (PLQY=0.040).[23]
[d] Lifetimes were measured with TCSPC using Nanoled for excitation
(lexc =366 nm)
Electrochemical characterisation
Cyclic voltammetry was performed in acetonitrile solution with
0.1m tetrabutylammonium hexafluorophosphate (TBAPF6) and
a disk of Pt as the working electrode. The redox properties of
the four new heteroleptic CuI complexes are reported versus
Fc/Fc+ couple[25] in Table 2. All the species present two irrever-
sible oxidation processes. In particular, the first oxidation
(around 1 V) is usually attributed to the CuI/CuII process. The
second one is due to the oxidation of the chelating phosphine
The energy of the emission for the dinuclear complexes are
the same as the mononuclear complex 1 (maximum emission
lꢁ635 nm in DCM). In fact, they bear the same chromophoric
unit Cu(NN)(PP) and any particular cooperation effect in dinu-
clear compounds 2a, 2b and 2c cannot be observed, since
the coordinated metal parts are bridged with benzyl groups,
namely with no electronic communication.
Table 2. Redox properties of the new CuI photosensitizer in ACN (0.1m
TBAPF6).[a]
In acetonitrile solutions (ACN), the MLCT emission bands un-
dergo a bathochromic shift (lꢁ648 nm), caused by a stronger
stabilisation of the excited state in this polar solvent. In DCM
solutions, the complexes have small quantum yields (1% for
2a, 1% for 1 and 2b, 2% for 2c) and relatively long excited-
state lifetimes (t higher than 2 ms for compounds 1, 2b and
2c; tꢁ1 ms for 2a). Nevertheless, in acetonitrile, all complexes
turned out to present lifetimes of just a hundred nanoseconds,
as for the photoluminescence quantum yields (PLQY) that are
very low (less than 0.001). This can be rationalised by the pos-
sible quenching of the charge transfer excited state induced
by the polar solvent (ACN).[24] The emission and excitation
spectra in ACN for the four compounds are displayed in
Sample
Eox [V]
Ered [V]
Eox* [V][b]
Ered* [V][b]
1
0.95; 1.2
0.90; 1.3
1.00; 1.7
0.99; 1.9
ꢀ2.09
ꢀ1.75
ꢀ1.81
ꢀ1.31
ꢀ1.71
0.61
0.85
0.75
0.06
2a
2b
2c
ꢀ1.8; ꢀ2.5
ꢀ1.95
ꢀ2.6
[a] Estimated by cyclic voltammetry, at a scan rate of 100 mVsꢀ1, and re-
ported versus ferrocene/ferrocenium couple. [b] Redox potentials of the
excited states, calculated from the formulas Eox*=EoxꢀE00; Ered*=Ered+E00,
where E00 (ꢁ2.7 eV) is defined as the energy of the transition from the
lowest excited state in thermal equilibrium to the zero vibrational level of
the ground state. It was quantitatively estimated, according to equations
reported in ref. [26a] and [26b].
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(1.9 V and 1.7 V for 2c and 2b, respectively and 1.2 V and 1.3
for 1 and 2a). In the reduction part, one irreversible process is
visible in the electrochemical window for complexes 1 and 2c,
at ꢀ2.1 V and ꢀ2.6 V respectively, while for complexes 2a and
2b the irreversible processes can be recognised at ꢀ1.8 V and
ꢀ1.95 V respectively. The cyclic voltammetry plots recorded at
a scan rate of 100 mVsꢀ1 are displayed in the supporting infor-
mation (Figures S24–S27). The redox properties of the com-
plexes in their excited state in acetonitrile were calculated
using the spectroscopic and electrochemical data.[26] In their
excited state, all the complexes are better oxidants and better
reductants at the same time. However, if a reductive or an oxi-
dative quenching might occur, it is essential to see if the ther-
modynamics is feasible, that is the driving force of the electron
transfer (DG) must be negative. We have investigated one of
the most used sacrificial electron donors: 1,3-dimethyl-2-phe-
nylbenzimidazoline (BIH).[27] We prepared it according to re-
ported literature procedure and determined its oxidation po-
tential versus ferrocene/ferrocenium couple (ꢀ0.204 V; Fig-
ure S28). The driving force for the reductive quenching for the
new heteroleptic CuI complexes is exergonic, indicating that
the photoinduced electron transfer from BIH to the complexes
is energetically feasible (Table S1).
Table 3. Photocatalytic CO2 reactions and control experiments in ACN/
TEOA (5:1) after 4 h of irradiation at 420 nm.[a]
Entry
PS
[CAT]
[BIH]
CO [mmol]
TONCAT
1
2
3
4
5
6
7
8
9[b]
10
11[c]
12
13[d]
14
15[e]
16
17
18
1
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
0.1 mm
20 mm
20 mm
20 mm
20 mm
10 mm
10 mm
10 mm
10 mm
10 mm
10 mm
10 mm
0
10 mm
10 mm
10 mm
10 mm
20 mm
20 mm
1.73
1.94
3.24
1.83
1.02
1.40
2.00
0.70
1.13[b]
n.d.
4.3
4.9
8.1
4.6
2.6
3.5
5.0
1.8
2.8[b]
n.d.
2a
2b
2c
1
2a
2b
2c
1
2b
2b
2b
2b
none
1
Cu(ACN)4
1
2a
[c]
n.d.[c]
n.d.
n.d.
n.d.
n.d.[d]
n.d.
n.d.[d]
n.d.
n.d.
n.d.
n.d.
n.d.
2.9[f]
2.6[f]
7.3[f]
6.5[f]
[a] From headspace analysis, reactions were repeated twice. [b] ACN/
TEOA (4:1,v/v); [c] With trimethylamine instead of TEOA. [d] Without CO2
in Ar atmosphere; [e] in dark (n.d.=not detected). [f] After 10 h irradia-
tion.
Photoactivated CO2 reduction
sorption band at 350 nm, most probably due to the increased
BIH concentration, while the MLCT absorption band remains
constant. Notably, at high concentration of BIH (>5 mm), the
emission of the complexes 1 and 2a–c centred at ꢁ640 nm is
quenched and a new emission appears at 550 nm. The pres-
ence of oxygen does not influence the intensity of this emis-
sion. Thus, we assume it must be a singlet state. Further inves-
tigations to assign the nature of this new emission band are
currently ongoing and are out of the main aim of the present
work.
The application of these new CuI complexes as photosensitiz-
ers in the photochemical reduction of carbon dioxide was eval-
uated in combination with [Ni(cyclam)]2+, a NiII coordination
complex that can reduce CO2 both photochemically[18d,f–h,28]
and electrochemically.[18a,e,i,29] The photocatalytic experiments
were carried out dissolving all the components in a 4 mL
mixed solution of ACN and triethanolamine (TEOA) in 5:1
volume ratio. The concentration of the photosensitizers was
fixed at 1 mm for the mononuclear complex 1, and at 0.5 mm
for the dinuclear complexes 2a–c. At these concentrations, the
optical density of the photocatalytic solutions at the excitation
wavelength (420 nm) is 1.2 when the PS was the mononuclear
complex 1 and 0.9 when the dinuclear complexes 2a–c were
used (Figure S33). The sacrificial electron donor BIH was used
in 20 mm concentration unless otherwise specified. In order to
evaluate the photocatalytic system with the new PS, the ob-
served product is carbon monoxide, and the results are report-
ed in Table 3. All the new CuI complexes showed the ability to
photosensitise the reduction of CO2. Although [Ni(cyclam)]2+ is
known to produce also molecular hydrogen, this could not be
detected in our experiments. Upon absorption of light, the ex-
cited-state of the photosensitizers is populated and undergoes
reductive quenching by BIH. Stern–Volmer (SV) bimolecular
quenching experiments confirmed the favoured thermody-
namics already proven with the redox potentials of PS*, result-
ing in apparent quenching rate constants close to the diffusion
limit (k(1)=2.0ꢂ109 sꢀ1 mꢀ1; k(2a)=2.8ꢂ109 sꢀ1 mꢀ1; k(2b)=
4.3ꢂ109 sꢀ1 mꢀ1; k(2c)=1.91ꢂ109 sꢀ1 mꢀ1). The linear responses
of the SV experiments are displayed in the Supporting Informa-
tion, Figures S29–S32. The absorption spectra were also record-
ed. We observed, by adding BIH, an enhancement of the ab-
Photoinduced electron transfer from the BIH to PS* occurs,
and the reduced form of PS is oxidised back by the catalyst.
After 4 h of irradiation at 420 nm, analyses of the headspace
were performed by gas-chromatography, equipped with a
thermal conductivity detector (TCD). When the mononuclear
CuI complex 1 was used, 1.73 mmol of CO were produced. Very
similar results were obtained when PS was the binuclear com-
plex 2c. These values correspond to a turn-over number in re-
spect to the CAT (TONCAT) of 4.3 and 4.6 respectively. Interest-
ingly, when 2b photosensitised the photocatalytic reduction of
CO2, the amount of produced CO is almost double (3.24 mmol),
corresponding to a TONCAT of 8.1. This result is excellent com-
pared to homogeneous systems using [Ni(cyclam)]2+ as the
CAT (see Table S4).
Furthermore, to the best of our knowledge, our system is
the first one that exploits earth-abundant CuI photosensitizers
in combination with this catalyst. When a lower concentration
of BIH (10 mm) was used, the efficiency of the systems was
lower (entries 5–9 in Table 3). A weak improvement could be
achieved increasing the amount of TEOA (in ACN, 4:1 instead
of 5:1), where 1.13 mmol instead of 1.02 mmol were obtained
(entry 9 compared to entry 5 in Table 3). Control experiments
Chem. Eur. J. 2020, 26, 1 – 10
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under Ar atmosphere (entry 13, Table 3) did not give any pro-
duction of CO. Moreover, each component of our photocatalyt-
ic system herein presented was demonstrated to be necessary
for the photoinduced reduction of carbon dioxide, since con-
trol experiments carried out without BIH (entry 12), without PS
(entry 14), and without CAT (entry 10) produced no CO. The
same result was also obtained when the reaction was carried
out in the dark (entry 15). The new PSs are very stable in ACN
solution under irradiation, as it can be seen in Figures S20–S23,
where the absorption spectra present no modification even
after 5 h irradiation. However, a decrease of the MLCT absorp-
tion of the PSs could be observed, and the degradation of PS
started already after 30 min irradiation of the mixed photocata-
lytic reactions. It is not to exclude that TEOA may replace the
chromophoric ligand; in fact, a mass corresponding to DPE-
PhosCu(TEOA) (750 Da) was observed when HR-ESI mass analy-
ses were performed on the homogenous photocatalytic solu-
tions. Interestingly, the degradation of the PS happens only in
the presence of BIH and TEOA, when the atmosphere is
carbon dioxide. We assume that TEOA forms with CO2 a zwit-
terionic carbonate species[30] that induces complexation with
the reduced form of PS, developed after reductive quenching
by BIH. A detailed mechanism with the identification of the in-
termediate species is in process. Nevertheless, when the pho-
tocatalytic reaction was performed with Cu(ACN)4BF4 instead
of any of the new photosensitizers, CO could not be observed,
even after addition of one equivalent of DPEPhos (entry 16,
Table 3), so the new complexes are necessary for the successful
photocatalytic reduction of CO2. Longer reaction times increase
the amount of produced CO, so after 10 h of irradiation, the
produced CO was 2.9 mmol (TONCAT 7.3) when 1 was used and
2.6 mmol (TONCAT 6.5) when 2a was used (entries 17 and 18 in
Table 3). For those experiments, the results were plotted as
function of irradiation time in Figure S34 of the Supporting In-
formation. Increasing the concentration of the photosensitizer
1 and the catalysts to 3 mm and 0.3 mm, respectively, the
amount of CO was also higher (6.0 mmol). However, the TONCAT
was almost constant (5.1) after 4 h. The quantum efficiency of
the photocatalytic system was calculated, measuring the irradi-
ated light intensity by chemical actinometry (see the experi-
mental part for details).The calculated quantum yields for the
typical photocatalytic systems used (entries 1 to 4, Table 3) are
1.0% (1), 1.2% (2a); 2.1% (2b), 1.1% (2c). The results herein
reported have been promising and stimulated further research
in our working group, in order to develop new photosensitiz-
ers and photocatalysts based on earth-abundant metal com-
plexes.
particular, we presented the homogeneous photocatalytic re-
duction of CO2 with [NiII(cyclam)]Cl2 as the catalyst. After
4 hours of irradiation with LED lamps, the production of CO
could be observed in the presence of the reported complexes.
To the best of our knowledge, this is the first example reported
of CuI-based photosensitizers with [NiII(cyclam)]Cl2. Moreover,
the TONs obtained (up to 8) are in line with those results pro-
duced in previous works with noble-metal based photosensi-
tizers. These results motivate further development of new
earth-abundant metal-based photosensitizers and catalysts in
our research team.
Experimental Section
Experimental details and general remarks: All starting materials
were purchased from commercial suppliers and used as received
unless otherwise noted.
Synthesis of 2-ethynylquinoline (5): In a Schlenk-tube, under inert
atmosphere (Ar), 2-bromoquinoline (210 mg, 1.0 mmol, 1.0 equiv)
was dissolved in 20 mL diisopropylamine (DIPA), dried over CaH2.
Trimethylsilylacetylene (700 ml, 4,98 mmol, 5.0 equiv) was added,
followed by Pd(PPh3)2Cl2 (7.4 mg, 1 mol%) and CuI (3.6 mg,
2 mol%). The muddy brown reaction mixture was stirred at room
temperature for 4 hours. The 2-(trimethylsilylethynyl)quinoline is a
viscous brown oil that was purified by column chromatography in
silica, eluent mixture CH2Cl2/C6H12 (1:1), Rf =0.25. This intermediate
was then dissolved in methanol and K2CO3 (2 equiv) was added for
the deprotection of the triple bond. After completion of the reac-
tion (2 h), 5 mL of water was added, and the organic products
were extracted with dichloromethane. The organic phase was
washed with brine, dried over Na2SO4, filtrated and the solvent was
removed under reduced pressure. The desired product was puri-
fied by a silica gel chromatographic column using cyclohexene/
EtOAc (1:1) as eluent, Rf =0.75. (92 mg, 0.60 mmol) Yield: 60%.
1H NMR (300 MHz, CDCl3): d= 8.08 (d, J=8.4 Hz, 2H), 7.76 (d, J=
9 Hz, 1H), 7.72 (ddd, J1=8.6 Hz, J2=6.9 Hz, J3=1.5 Hz, 1H), 7.53
(dd, J 1=9 Hz, J2=, 6 Hz, 2H), 3.24 ppm (s, 1H).
Synthesis of the ligands: The mono-chelating ligand 3 was syn-
thesised following a modified procedure from literature,[19b] by dis-
solving 1 equivalent of 5 (120 mg, 0.80 mmol) with benzylazide
(107 mg, 0.80 mmol, 1.0 equiv) followed by sodium ascorbate
(63.4 mg, 0.32 mmol, 0.4 equiv), CuSO4(·5H2O) (25.5 mg, 0.16 mmol,
0.2 equiv) and sodium carbonate (57.2 mg, 0.54 mmol, 0.675 equiv)
into
a solution of ethanol and water (7:3), giving 200 mg
(0.696 mmol, yield 87%) of desired product. 1H NMR (500 MHz,
CDCl3): d=8.26 (d, J=8.6 Hz, 1H), 8.19 (s, 1H), 8.15 (d, J=8.6 Hz,
1H), 7.93 (d, J=8.5 Hz, 1H), 7.73 (dd, J1=8.1 Hz, J2= 1.4 Hz, 1H),
7.60 (ddd, J1=8.4 Hz, J2=6.9 Hz, J3=1.5 Hz, 1H), 7.42 (ddd, J1=
8.0 Hz, J2= 6.8 Hz, J3=1.0 Hz, 1H), 7.38–7.25 (m, 5H), 5.54 ppm (s,
2H).; 13C NMR (125 MHz, CDCl3): d=150.44, 149.08, 148.01, 136.89,
134.38, 129.74, 129.21, 128.97, 128.89, 128.32, 127.81, 127.78,
126.37, 122.71, 118.70,( 77.31, 77.26, 76.80, CDCl3) 54.48 ppm.
Conclusions
The bis-chelating ligands 4a–c were prepared following the gener-
al procedure showed hereafter. In a 100 mL flask, 1 equivalent
(120 mg, 0.80 mmol) of 2-ethynylquinoline 5 were dissolved in
10 mL of a solution of ethanol and water (7:3). The desired isomer
of dibromomethylbenzene (103 mg, 0.39 mmol, 0.5 equiv) was
added, followed by NaN3 (101 mg, 1.55 mmol, 2 equiv), sodium as-
corbate (61 mg, 0.4 equiv), CuSO4 (·5H2O) (0.04 g, 0.2 equiv) and
sodium carbonate (50 mg, 0.675 equiv). The reaction mixture be-
comes yellow-orange and stirred overnight at room temperature.
This work reports the synthesis, full characterisation and em-
ployment in CO2-reduction of four new coordination metal
complexes based on earth-abundant CuI. Their MLCT absorp-
tion falls in the visible region (up to 450 nm), and they are
very stable even in coordinating solvent such as acetonitrile.
Thus, we have studied their photophysical and electrochemical
properties in order to prove their use as photosensitizers. In
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The reaction was stopped by addition of a solution of NH4OH
(10%). The precipitate was filtered off, dissolved in dichlorome-
thane and washed with NH4OH (10% solution) three times. The or-
ganic phases were washed with brine, dried over MgSO4. The sol-
vent was removed under vacuum in a rotatory evaporator. The
product was obtained without further purification needed.
(162 MHz,
CDCl3):
d=ꢀ12.60 ppm.
HRMS
(ESI)
m/z
(C102H78Cu2P4N8O2 BF4): (z=1) 1785.38 (calc), 1785.40 (found) Ele-
mental analysis (C102H78Cu2N8O2P4B2F8): C=65.43, H=4.20, N=5.98
(calc.); C=65.10, H=4.321, N=5.91 (found).
2b: Under Ar atmosphere, 4b (227 mg, 0.46 mmol, 1.0 equiv) was
dissolved in dry dichloromethane with DPEPhos (495 mg,
0.92 mmol, 2.0 equiv) and Cu(CH3CN)4BF4 (288 mg, 0.92 mmol,
2.0 equiv) to give the desired product 2b (534 mg, 0.28 mmol).
4a: The desired product was obtained as a light yellow powder
1
(149 mg, 0.30 mmol) Yield: 38%. H NMR (300 MHz, CDCl3): d=8.78
1
(s, 2H), 8.42 (d, J=7.2, 2H), 8.18 (d, J=8.1 Hz, 2H). 7.98 (d, J=
7.8 Hz, 2H), 7.93 (d, J=8.7 Hz, 2H), 7.72 (dd, J1=8.1 Hz, J2=
7.2 Hz, 2H), 7.56 (dd, J1=7.8 Hz, J2=7.2 Hz, 2H), 7.41 (m, CH 2H),
7.38 (m, 2H), 6.00 ppm (s, 4H). 13C NMR (126 MHz, CDCl3): d=
150.17, 149.21, 148.05, 147.97, 136.83, 136.50, 133.22, 130.91,
130.14, 129.68, 129.25, 128.99, 128.22, 127.79, 127.73, 127.31,
126.70, 126.37, 126.15, 122.77, 118.61, (77.29, 77.03, 76.78, CDCl3),
53.44 ppm.
Yield: 62%. HNMR (500 MHz, CD3CN): d=8.50 (s, 2H), 8.46 (d, 2H),
7.94 (m, 4H), 7.80 (m, 2H), 7.46–6.60 (m, 64H), 5.58 ppm (s, 4H).
13CNMR (125 MHz, CD3CN): d=158.78, 149.87, 149.06, 147.54,
146.125, 135.89, 134.50, 132.52, 131.72, 130.43, 130.26, 129.00,
128.92, 128.76, 128.39, 127.86, 125.29, 124.75, 124.05, 123.93,
123.81, 120.77, 119.15, 55.76 ppm. 31PNMR (162 MHz, CD3CN): d=
ꢀ13.19 ppm. HRMS m/z (C102H78Cu2P4N8O2BF4): 1785.38 (calc.),
1785.39 (found). Elemental analysis (C102H78Cu2N8O2P4B2F8): C=
65.43, H=4.20, N=5.98 (calc.); C=65.29, H=4.242, N=5.93
(found).
4b: The desired product was obtained as a light yellow powder
(253 mg, 0.51 mmol) Yield: 64%.1HNMR (300 MHz, CDCl3): d=8.46
(s, 2H), 8.28(d, J=8.7 Hz, 2H), 8.22(d, J=8.7 Hz, 2H). 8.00 (d, J=
8.1 Hz, 2H), 7.8 (d, J=6 Hz, 2H), 7.67 (dd, J1=7.2 Hz, J2=7.8 Hz,
2H), 7.50 (dd, J1=6.3 Hz, J2=8.1 Hz, 2H), 7.37 (m, 4H) 5.64 ppm
(s, 4H). 13CNMR (126 MHz, CDCl3): d=150.29, 149.25, 147.99,
136.93, 135.67, 130.16, 129.77, 128.99, 128.66, 127.87, 127.84,
127.79, 126.42, 122.86, 118.68, (77.29, 77.03, 76.78, CDCl3), 54.04,
53.46 ppm.
2c: Under Ar atmosphere, 4c (163 mg, 0.33 mmol, 1.0 equiv) was
dissolved in dry dichloromethane with DPEPhos (355 mg,
0.66 mmol, 2.0 equiv) and Cu(CH3CN)4BF4 (207 mg, 0.66 mmol,
2.0 equiv) to give the desired product 2c (580 mg, 0.31 mmol).
Yield: 94% 1HNMR (500 MHz, CD3CN): d=8.48 (m, 4H), 7.92 (m,
4H), 7.78 (s, 2H), 7.43–6.58 (m, 64H), 5.66ppm (s, 4H). 13CNMR
(125 MHz, CD3CN): d=158.73, 147.49, 146.53, 146.3, 139.59, 135.63,
134.42, 132.47, 131.28, 130.41, 129.10, 128.83, 128.36, 127.88,
125.20, 124.80, 123,98, 123.87, 123.75, 120.69, 119.08, 100.26,
55.27ppm. 31PNMR (162 MHz, CD3CN): d=ꢀ13.30 ppm. HRMS m/z
(C102H78BCu2F4N8O2P4): 1785.38 (calc), 1785.39 (found). Elemental
analysis (C102H78Cu2N8O2P4B2F8): C=65.43, H=4.20, N=5.98 (calc.);
C=65.28, H=3.956, N=6.19 (found).
4c: The desired product was obtained as a light yellow powder
(174 mg, 0.35 mmol) Yield: 44%.1H NMR (300 MHz, CDCl3): d=8.36
(s, 2H), 8.33 (d, J not visible, under singlet, 2H), 8.27 (d, J=8.7 Hz,
2H). 8.05 (d, J=8.4 Hz, 2H), 7.83 (d, J=8.1 Hz, 2H), 7.69 (dd, J1=
9 Hz, J2=9 Hz, 2H), 7.52 (dd, J1=6.9 Hz, J2=8.1, 2H), 7.40 (s, 4H),
5.54 ppm (s, 4H). 13CNMR (126 MHz, CDCl3): d=150.31, 149.24,
148.01, 136.89, 136.50, 135.30, 134.55, 129.74, 129.04, 128.99,
128.93, 128.74, 127.79, 126.40, 122.77, 118.64, (77.32, 77.07, 76.82,
CDCl3), 53.93 ppm.
Synthesis of the CuI complexes: The general procedure was fol-
lowed as previously reported,[21a] by dissolving in 20 mL of dry di-
chloromethane 3 (183 mg, 0.64 mmol, 1.0 equiv) with DPEPhos
(344 mg, 0.64 mmol, 1.0 equiv) and Cu(CH3CN)4BF4 (200 mg,
0.64 mmol, 1.0 equiv) under Ar atmosphere. The desired product 1
(449 mg, 0.46 mmol) was obtained with a yield of 72%. 1HNMR
(500 MHz, CD3CN): d=7.92 (s, 1H), 7.86 (d, J=8.4 Hz, 1H), 7.71–
7.19 (3d, 3H), 6.76–6.42 (m, 25H), 6.34- 6.25 (m, 6H), 6.03–5.95 (m,
4H), 4.99 ppm (s, 2H). 13C-NMR (125 MHz, CDCl3): d=158.55,
147.76, 145.97, 146.60, 139.00, 134.51, 134.56, 131.89, 131.57,
130.93, 130.66, 130.26, 129.62, 129.47, 128.88, 128.68, 128.53,
128.29, 127.72, 127.11, 125.80, 124.88, 120.08, 119.86, 54.97 ppm.
31PNMR (162 MHz CDCl3) d=ꢀ12.73 ppm. HRMS (ESI) m/z
(C54H42CuN4OP2): 887.21 (calc.), 887.21 (found). Elemental analysis
(C54H42CuN4OP2BF4): C=65.51, H=4.43, N=5.74 (calc.); C=65.53,
H=4.357, N=5.54 (found).
Photocatalytic reactions: In a 12 mL vial, closed with an alumini-
um cap equipped with a rubber septum, 4 mL of solution, contain-
ing photosensitizers (1 mm for 1 and 0.5 mm for 2a, 2b and 2c),
catalyst (0.1 mm), and electron donor (20 mm), were introduced
and kept under dark. CO2 gas (99.995%, from Air Linde) was bub-
bled inside for 20 minutes. The vials were irradiated in the photo-
reactor LZC-IC2 from Luzchem with 4 fluorescent lamps (8 W each)
at 420 nm. The total photon flux of the system was measured by
K3Fe(C2O4)3 actinometry[31] and the amount is ~2.5ꢂ10ꢀ8 Esꢀ1. All
solutions were stirred continuously during irradiation at a tempera-
ture of 295 K. The produced CO was analysed by manual injection
of 200 mL of the headspace with a gas-tight syringe into our Gas-
chromatograph with a thermal conductivity detector (TCD) from
PerkinElmer Arnel (Clarus 590, HayeSep N and Molecular sieves col-
umns) using He as carrier gas. Calibration curves were performed
with 5 known standard injections of CO. Each reaction was tested
after 4 hours unless otherwise noted. The quantum yields for the
photocatalytic CO2 reduction reactions were determined by using
Equation (1):
2a: Under Ar atmosphere, 4a (138 mg, 0.28 mmol, 1.0 equiv) was
dissolved in dry dichloromethane, followed by DPEPhos (301 mg,
0.56 mmol, 2.0 equiv) and Cu(CH3CN)4BF4 (176 mg, 0.56 mmol,
2.0 equiv) to give the desired product 2a (477 mg, 0.25 mmol).
CO molecules ꢂ 2
F ð%Þ ¼
ꢂ 100
ð1Þ
incident photons ꢂ fap
1
in which the fap is the fraction of the absorbed photons of the pho-
tocatalytic system at the excitation wavelength (1–10-A). The
number of CO molecules was determined from the moles of CO
measured from the headspace of the reaction, using a calibrated
GC-TCD. The factor 2 is because CO is a 2 electron reduced prod-
uct of CO2. The number of incident photons was estimated using
K3Fe(C2O4)3 actinometry.
Yield: 91%. HNMR (500 MHz, CDCl3): d= 9.07 (s, 2H), 8.39 (d, J=
5.1 Hz, 2H), 8.11 (d, J=5.1 Hz, 2H), 7.94 (d, J=5.1 Hz, 2H), 7.81 (d,
J=5.1 Hz, 2H), 7.48 (m, 8H), 7.35 (dd, J1=4.5 Hz, J2=4.5 Hz, 2H)
7.28–7.24 (m, 12H), 7.22–7.19 (m, 8H), 7.10 (m, 4H), 7.04 (m, 4H),
6.93 (m, 6H) 6.84 (m, 8H), 6.72–6.69 (m, 4H), 6.60–6.57(m, 8H),
5.85 ppm (s, 4H). 13CNMR (125 MHz, CDCl3): d=158.62, 147.47,
146.16, 145.67, 139.04, 134.53, 134.09, 132.97, 131.97, 131.73,
131.03, 130.64, 130.35, 129.89, 129.49, 128.34, 128.32, 127.73,
127.35, 125.69, 124.69, 124.23, 120.13, 119.72, 51.52 ppm. 31PNMR
Photophysics: UV/Vis absorption spectra were recorded for all the
compounds in a solution of CH2Cl2 and CH3CN (concentrations
Chem. Eur. J. 2020, 26, 1 – 10
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ꢁ10ꢀ5 m). The instrument used was a Lambda 750 double-beam
UV/Vis-NIR spectrometer. Emission and excitation spectra were re-
corded with a Fluoromax 4 from Horiba Jobin. All the solutions
were air-free by letting Ar bubbling inside for 10 minutes at least.
Lifetime experiments were performed by time-correlated single-
photon counting method (TCSPC) with a DeltaTime kit for Delta-
Diode source on FluoroMax systems, including DeltaHub and Del-
taDiode controller. NanoLED 370 was used as the excitation source
(l=366 nm).
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Acknowledgements
The authors would like to thank generous funding from the
Deutsche Forschungsgemeinschaft (DFG), Collaborative Centre
SFB/TRR 88 “3MET” (Bizzarri, Bruschi and Gracia: Project B9;
Weis: Project C6). L.L. thanks the Erasmus plus program from
the University of Bologna. Prof. Stefan Brꢃse (KIT) is deeply ac-
knowledged for his scientific support. Prof. H.A. Wagenknecht
is greatly acknowledged for allowing us to measure with his
photophysical equipment.
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: [Ni(cyclam)]2+ · CuI complexes · earth-abundant
metal · photocatalytic CO2 reduction · photosensitizers
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Manuscript received: March 14, 2020
Revised manuscript received: June 18, 2020
Version of record online: && &&, 0000
Chem. Eur. J. 2020, 26, 1 – 10
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ꢀ 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Full Paper
doi.org/10.1002/chem.202001279
Chemistry—A European Journal
FULL PAPER
New photosensitizers based on earth-
abundant CuI absorb blue light and un-
dergo reductive quenching with an
electron donor (BIH). A second electron
transfer to a Nickel catalyst triggers the
catalytic reduction of CO2 to CO.
&
Photocatalysis
L.-L. Gracia, L. Luci, C. Bruschi, L. Sambri,
P. Weis, O. Fuhr, C. Bizzarri*
&& – &&
New Photosensitizers Based on
Heteroleptic CuI Complexes and CO2
Photocatalytic Reduction with
[NiII(cyclam)]Cl2
&
&
Chem. Eur. J. 2020, 26, 1 – 10
www.chemeurj.org
10
ꢀ 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!