Mononuclear and dinuclear heteroleptic Cu(I) complexes based on pyridyl-triazole and DPEPhos with long-lived excited-state lifetimes

Article abstract of DOI:10.1016/j.jorganchem.2018.07.013

A mononuclear and two dinuclear heteroleptic Cu(I) complexes have been successfully prepared, using the chelating bis [(2-diphenylphosphino)phenyl] ether (DPEPhos) and pyrid-2′-yl-1H-1,2,3-triazole as N^?N chelating ligands. They show good luminescence in solution at room temperature with long-lived excited states. Furthermore, bimolecular quenching experiments of these new complexes with the catalyst Ni(cyclam)Cl₂ encourage the use of such compounds as photosensitizers for the photoreduction of carbon dioxide.

Full text of DOI:10.1016/j.jorganchem.2018.07.013

Journal of Organometallic Chemistry 871 (2018) 140e149
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Mononuclear and dinuclear heteroleptic Cu(I) complexes based on
pyridyl-triazole and DPEPhos with long-lived excited-state lifetimes
a
,
*
b
c
c
d
Claudia Bizzarri , Andreas P. Arndt , Stephan Kohaut , Karin Fink , Martin Nieger
a
Institute of Organic Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 6, 76131, Karlsruhe, Germany
Light Technology Institute, Karlsruhe Institute of Technology, Karlsruhe, Germany
Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany
Department of Chemistry, University of Helsinki, Helsinki, Finland
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 March 2018
Received in revised form
A mononuclear and two dinuclear heteroleptic Cu(I) complexes have been successfully prepared, using
0
the chelating bis [(2-diphenylphosphino)phenyl] ether (DPEPhos) and pyrid-2 -yl-1H-1,2,3-triazole as
^
N N chelating ligands. They show good luminescence in solution at room temperature with long-lived
18 June 2018
excited states. Furthermore, bimolecular quenching experiments of these new complexes with the
Accepted 9 July 2018
catalyst Ni(cyclam)Cl
of carbon dioxide.
2
encourage the use of such compounds as photosensitizers for the photoreduction
©
2018 Elsevier B.V. All rights reserved.
Keywords:
Heteroleptic Copper(I) complexes
photosensitizers
carbon dioxide reduction
Ster-Volmer quenching
cooperativity
1
. Introduction
cannot occur and the excited state of a copper(I) complex is nor-
mally a metal-to-ligand charge-transfer state (MLCT). Nevertheless,
in the excited state they suffer from a JahneTeller flattening
distortion from the pseudo-tetrahedral geometry in the ground
state, which allows Lewis bases to form adducts and to deactivate
the excited state of the copper(I) complex [12,13]. Steric hindrance
imposed by the ligands in the coordination sphere avoids this
distortion to a certain extent and therefore increases the excited
state lifetime and the photoluminescence quantum yield. Lability of
the ligands might also complicate the development of such com-
Nowadays the reduced availability of materials and their recy-
clability are raising enormous interest in the research community.
Furthermore, the exploitation of environmentally sustainable e so
called ‘green’ e resources has become very important. Light can be
considered a ‘green’ reagent; definitely when it comes directly from
the sun, but also if it comes from artificial sources. Photons are
‘
green’ in view of the fact that they do not leave any waste products
1].
The development of molecules that can absorb electromagnetic
[
^
^
pounds. In fact it has been found that heteroleptic Cu(P P) (N N)
^
^
radiation to deliver a certain function is thus a central research
question. In particular, photosensitizers based on organometallic
complexes [2] have shown promising results in diverse catalytic
reactions [3e6] as well as in solar energy conversion [7e9]. Cop-
per(I) complexes represent an appealing alternative to expensive
photosensitizers based on ruthenium (II), iridium (III) or rhenium(I)
complexes, where P P is a chelating diphosphine and N N is a 1,10-
phenanthroline derivative, undergo an exchange of ligands in so-
^
^
lution, affording homoleptic Cu(N N)
2 2
and Cu(P P) complexes,
which are more favoured [14]. In the solid state, these drawbacks
are somehow limited and the utilisation of Cu(I) complexes in light-
emitting devices (OLEDs or LEECs) has attracted a strong interest
[15e18]. The pioneering work of Sauvage and co-workers with
[
10,11]. In fact, copper is more abundant in the earth's crust and is
therefore cheaper. Moreover, Cu(I) complexes are of easy prepara-
2
Cu(dap) Cl [19] as photoredox catalyst was published in 1987 [20],
tion. Being a d10 nucleus, metal centred electronic transitions
but apart from some contemporary publications [21], only recently
have Cu(I) complexes been applied as photoredox catalysts or
photosensitizers (PS) [22e32]. In artificial photosynthesis [33,34]
the photosensitizer absorbs the luminous radiation and transfers
*
Corresponding author.
E-mail address: claudia.bizzarri@kit.edu (C. Bizzarri).
https://doi.org/10.1016/j.jorganchem.2018.07.013
022-328X/© 2018 Elsevier B.V. All rights reserved.
0

C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
141
0
0
the energy to a catalyst, which then activates the small molecule
usually either water or carbon dioxide). The characteristics that a
(b) Synthesis 5-(6 methyl-pyrid-2 -yl)-4-phenyl-1H-1,2,3-triazole.
In 25 ml of dry N,N-dimethylformamide, 6-methyl-2-
phenylethynyl-pyridine (1.2 g, 6.2 mmol) was dissolved. Tri-
methylsilylazide was added (2.5 ml, 2.17 g, 19 mmol) was
(
PS should have are the following: good absorption in the visible
light region, long-lived excited state, and high stability. There are
still very few Cu(I)-based PS applied to artificial photosynthesis;
nevertheless, they are quite promising [35e40]. We are interested
in the development of new PS based on copper(I), which can be
ꢀ
added and the reaction temperature was set to 110 C. Tri-
methylsilylazide was added more than once, so that in total
other 20 mmol were added. The reaction stirred for 2 days.
The solvent was removed by reduced pressure. The residue
was purified by column chromatography in silica with
used in the homogeneous reduction of CO
heteroleptic Cu(I) coordination compounds based on pyrid-2 -yl-
2
. In this work three
0
1
H-1,2,3-triazole ligand and bis [(2-diphenylphosphino)phenyl]
CH
oil was obtained. Yield: 1.105 g, (75%). H NMR (300 MHz,
298 K,CDCl
: 7.68e7.64 (m, 2H); 7.54 (t, J¼7.7 Hz, 1H), 7.38
(m, 4H), 7.12 (d, J¼7.6 Hz, 1H); 2.56 (s, 3H).
2 2 3
Cl :CH OH (98:2) as eluent. A sticky very viscous orange
1
ether (DPEPhos) are presented. The dinuclear complexes have been
prepared using the pyrid-2 -yl-1H-1,2,3-triazole as chelating unit,
0
3
) d
as in mononuclear complex 1H, bridged by a phenyl ring,
substituted in meta (2H) or in para (3H) positions. A methyl group
alpha to the nitrogen atom of the pyridine increases the steric
hindrance in the excited state, allowing an increase in the quantum
yield in solution of such complexes. In the dinuclear complexes 2H
and 3H, this effect is more pronounced. The long lifetime of the
excited states and their good emission in solution make these
complexes very appealing for application as PSs. As a preliminary
test, a SterneVolmer bimolecular quenching experiment was con-
2
.2.2. Ligand L2
(
a) -Synthesis meta-bis((6‘-methyl-pyrid-2‘-yl)ethynyl)benzene.
Under Argon atmosphere, in 25 ml of dry diisopropylamine
the reagents were dissolved as following: 2-bromo-6-
methylpyridine
diethynylbenzene (0.24 ml, 0.228 g, 1.8 mmol). Then the
catalyst Pd(PPh )Cl (12 mg, 0.017 mmol) and the co-catalyst
(0.45 ml,
0.678 g,
4.0 mmol);
1,3-
ducted with the three complexes and Ni(cyclam)Cl
catalyst in the reduction of CO [42].
2
[41], a known
2
3
2
CuI (5.2 mg, 0.027 mmol) were added. The reaction was
ꢀ
2
. Experimental section
.1. General information and materials
The starting materials were purchased from commercial sup-
heated to 80 C until the starting materials are not present
anymore in the reaction mixture (almost after 6 h). Solvent
was removed under reduced pressure. The residue was dis-
2
solved in CH
were extracted with 3 ꢁ 10 ml DCM. The organic phases were
washed with brine and dried over Na SO . The residue sol-
vent was removed and the product was purified by column
chromatography in silica gel. Eluent was CH Cl :CH OH
(99:1). Yield: 0.480 g (88%). H NMR (300 MHz, 298 K,CDCl
: 7.819 (s, 1H), 7.59 (m, 4H), 7.36 (d, J¼7.8 Hz, 3H), 7.14 (d,
J¼7.8 Hz,2H), 2.60 (s, 6H).
(b) -Synthesis meta-bis(5’-(6’‘-methyl-pyrid-2’‘-yl)1H-1,2,3-
2 2
Cl and washed with water. The aqueous phases
pliers and used without further purification. Solvents for synthesis
were purified according standard procedure, if necessary.
Dichloromethane was dried over CaH
each reaction. All air- and water-sensitive experiments were carried
out in standard glassware under an inert Ar atmosphere, using
standard vacuum line techniques. Tetrakis (acetonitrile)copper(I)
tetrafluoroborate was synthesized according to literature
procedures.
2
4
2
and freshly distilled before
2
2
3
1
3
)
d
triazol-4’-yl)benzene. In 10 ml of N,N-dimethylformamide,
meta-bis((6‘-methyl-pyrid-2‘-yl)ethynyl)benzene (0.420 g,
1.36 mmol) was dissolved. Trimethylsilylazide (0.73 ml,
0.633 g, 5.5 mmol) was added and the reaction temperature
Warning! Azides and nitrogen-rich compounds can be explosive.
Handle with care!
1H NMR (300 MHz, 500 MHz) and 13C{H} NMR (150 MHz)
spectra were recorded with a Bruker Avance III spectrometers
ꢀ
was set to 110 C. The reaction stirred for 1 day. The solvent
operating at 298 K in the Fourier transform mode. Chemical shifts
was removed by reduced pressure. The residue was purified
1
are reported in
d
units (ppm) using residual CHCl
3
( H
d
7.26 ppm,
by column chromatography in silica with CH
2
2 3
Cl :CH OH
13
1
C
d 77.00) as reference.
(98:2) as eluent. Yield: 170 mg, (32%). H NMR (300 MHz,
2
2
98 K,CDCl
H), 7.10 (d, J¼7.6 Hz, 2H); 2.52 (s, 6H).
3
)
d
: 7.97 (s, 1H); 7.57 (m, 4H), 7.45 (d, J¼7.8 Hz,
2
2
.2. Synthesis
.2.1. Ligand 1
2.2.3. Ligand L3
(
a) Synthesis 6-methyl-2-phenylethynyl-pyridine. Under Argon
atmosphere, in 20 ml of dry diisopropylamine the reagents
were dissolved as following: 2-bromo-6-methylpyridine
(a) -Synthesis
para-bis((6‘-methyl-pyrid-2‘-yl)ethynyl)benzene.
Under Argon atmosphere, in 10 ml of dry diisopropylamine
the reagents were dissolved as following: 2-bromo-6-
(1.0 ml, 1.511 g, 8.7 mmol); ethynylbenzene (0.96 ml,
0
0
.897 g, 8.5 mmol). Then the catalyst Pd(PPh
.089 mmol) and the co-catalyst CuI (40.0 mg, 0. 21 mmol)
3
)Cl
2
(62.2 mg,
methylpyridine
diethynylbenzene (0.056 g, 0.44 mmol). Then the catalyst
Pd(PPh )Cl (2.3 mg, 0.003 mmol) and the co-catalyst CuI
(99
m
l,
0.15 g,
0.89 mmol);
1,4-
ꢀ
were added. The reaction was heated to 80 C until the
starting materials are not present anymore in the reaction
mixture (almost after 4 h). A white precipitate was filtered
out and the solvent was removed under reduced pressure.
The residue was purified by column chromatography in silica
3
2
(5.1 mg, 0.027 mmol) were added. The reaction was heated to
ꢀ
80 C until the starting materials are not present anymore in
the reaction mixture (almost after 6 h). Solvent was removed
under reduced pressure. The residue was dissolved in CH2Cl2
and the not soluble fraction was filtered out. The solution was
gel. Eluent was CH
oil. Yield: 1.2 g (71.3%). H NMR (300 MHz, 298 K,CDCl
.60e7.55 (m, 2H), 7.51 (d, 1H), 7.36e7.28 (m, 4H), 7.07 (d,
H), 2.56 (s, 3H).
2 2 6
Cl :C H12 (50:50). The product is yellow
1
3
)
d:
recrystrallized by slow evaporation of the solvent. Yield:
1
7
1
0.32 g (72.7%). H NMR (300 MHz, 298 K,CDCl
6H); 7.35 (d, 2H); 7.12 (d, 2H); 2.58 (s, 6H).
3
) d: 7.56 (m,

142
C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
(
b) -Synthesis
para-bis(5’-(6’‘-methyl-pyrid-2’‘-yl)1H-1,2,3-
2 2
minimum amount of CH Cl and recrystallized by slow diffusion of
triazol-4’-yl)benzene. In 10 ml of N,N-dimethylformamide,
para-bis((6‘-methyl-pyrid-2‘-yl)ethynyl)benzene (98.6 mg,
C6H12. The obtained yellowish-white crystals were filtrated and
dried under vacuum. Yield: 76.3 mg, (79.7%). H NMR (500 MHz,
1
0
2
.32 mmol) was dissolved. Sodium azide (184.3 mg,
298 K,CDCl3) d: 14.41 (broad s, 2H, NH), 7.66 (s, broad, 10H); 7.28 (q,
.83 mmol) was added and the reaction temperature was set
J ¼ 9.6, 7.5 Hz, 2H); 7.26 (CHCl3); 7.13 (s, broad, 10H), 7.01e6.93 (m,
ꢀ
13
to 110 C. The reaction stirred for 1 day. The solvent was
8H), 6.85 (m, 7H), 6.71 (m, 4H), 2.02 (s, 6H). C NMR (126 MHz,
removed by reduced pressure. The residue was purified by
298 K,CDCl3)
d: 158.47; 134.61; 132.19; 130.19; 128.78; 128.57;
31
column chromatography in silica with CH
2
Cl
as eluent. Yield: 100 mg, (79.2%). H NMR (300 MHz, 298 K,
CDCl : 7.76 (4H, broad singlet); 7.60 (t, 2H); 7.165 (d, 2H);
.61 (s, 6H).
2 3
:CH OH (95:5)
124.75; 120.23; 99.99; 78.24e77.94 (CDCl ); 26.94; 25.22. P NMR
(400 MHz, 298 K,CDCl )
3
1
3
d
: ꢂ13.99. HRMS (ESI): 1685.35 (z ¼ 1)
þ
þ
2þ
3
)
d
(C94H74Cu N O P BF ) ;1596.34 (z ¼ 2) (C
2
8
2
4
4
H
94 74
Cu N O P þH)
2 8 2 4
.
)
þ2
-
2
Calcd for C
H
Cu N O P
2(BF ) ꢃ CH Cl : C, 61.44; H, 4.12; N,
9
4
74
2
8
2
4
4
2
2
6.03. Found: C, 61.73; H, 4.09; N, 6.20.
2
.2.4. Complex 1H
In a Schlenk tube, under Ar atmosphere, DPEPhos (247.2 mg,
.46 mmol) and Cu(CH CN) BF (148.7 mg, 0.47 mmol) were dis-
Cl . The reagents
stirred for half an hour at room temperature, before the ligand L1
108.8 mg, 0.46 mmol) was added. The solution became immedi-
2.3. Photophysical data
0
3
4
4
UVevis absorption spectra were recorded with a Perkin Elmer
Lambda 750 double-beam UV/Vis-NIR spectrometer equipped with
solved in 15 ml of dry and freshly distilled CH
2
2
ꢀ
a 6 ꢁ 6 cell changer unit at 20 C. Luminescence at room temper-
(
ature was measured using a JobineYvon Fluoromax 4 fluorimeter
with a step width of 1 nm and an integration time of 0.8 s. All
measurements were performed in pressure resistant quartz cu-
vettes with septum from Hellma. CH Cl solvent for spectroscopic
ately bright yellow. After 4 h, the solvent was evaporated by
reduced pressure. The residue was dissolved in a minimum amount
of CH
2 2 6
Cl and recrystallized by slow diffusion of C H12. The ob-
2
2
tained white crystals were filtrated and dried under vacuum. Yield:
measurements was supplied by Merck (Uvasol). Photo-
luminescence quantum yields were determined utilizing as refer-
ence one of the following standards, depending on the excitation
and emission wavelengths: Ru (bpy) Cl (6 H O) in water; p-ter-
phenyl in cyclohexane. Lifetime measurements were performed by
time-correlated single-photon counting (TCSPC) with a DeltaTime
kit for DeltaDiode source on FluoroMax systems, includes DeltaHub
and DeltaDiode controller. The light-source was a NanoLED 366 nm.
For time-resolved photoluminescence spectroscopy at 77 K, an
actively Q-switched diode pumped solid-state laser (AOT-YVO-
20QSP, Advanced Optical Technology Ltd.) with pulse duration of
about 1 ns was used. All samples were excited at a wavelength of
1
3
02.5 mg, (71.1%). H NMR (300 MHz, 298 K,CDCl
3
) d: 14.41 (broad s,
1
7
2
1
H, NH), 7.75e7.58 (m, 10H), 7.53e7.38 (m, 12H), 7.27e7.25 (m, 5H),
.13 (m, 4H), 6.98e6.85 (m, 6H), 2.19 (s, 3H). 13C NMR (400 MHz,
98 K,CDCl : 159.56; 159.50; 159.44; 158.87; 153.04; 144.76;
39.62; 137.72; 136.24; 135.96; 135.52; 133.34; 132.59; 131.83;
30.56; 130.24; 129.797; 129.38; 129.09; 128.98; 127.76; 126.77;
3
2
2
3
)
d
1
1
26.65; 126.53; 125.34; 121.46; 121.12; 117.43; 78.29e77.65
(
(
CDCl
3
); 26.09. 31P NMR (400 MHz, 298 K,CDCl
3
)
d
: ꢂ14.85. HRMS
þ
þ
ESI): 837.20 (z ¼ 1) (C50
H40CuN
4
OP
2
). Calcd for C50
H
40CuN
4
OP
2
ꢂ
BF
4
: C, 64.91; H, 4.36; N, 6.06. Found: C, 64.67; H, 4.24; N: 6.15.
2
.2.5. Complex 2H
In a Schlenk tube, under Ar atmosphere, DPEPhos (193.4 mg,
.36 mmol) and Cu(CH CN) BF (119.1 mg, 0.38 mmol) were dis-
Cl . The reagents
stirred for half an hour at room temperature, before the ligand L2
70.4 mg, 0.18 mmol) was added. After 6 h, the solvent was evapo-
rated by reduced pressure. The residue was dissolved in a minimum
amount of CH Cl and recrystallized by slow diffusion of C 12. The
obtained yellowish-white crystals were filtrated and dried under
355 nm and with an excitation power of 60 mW. The photo-
luminescence was fibre-coupled into a spectrometer (Acton Spec-
traPro SP-2300, Princeton Instruments) and detected in a time-
dependent manner by a gated CCD-camera (PI-MAX4, Princeton
Instruments). Measurements at 77 K were performed by placing
cuvettes with the solutions into a liquid nitrogen bath.
0
3
4
4
solved in 20 ml of dry and freshly distilled CH
2
2
(
2
2
6
H
2.4. Electrochemical measurements
1
vacuum. Yield: 420.2 mg, (65.9%). H NMR (400 MHz, 298 K,CDCl
3
)
Cyclic voltammetry experiments were performed with a Met-
rohm Autolab PGSTAT101 inside a glovebox. The electrochemical
cell was equipped with a Pt-disc as working electrode, an Ag-wire
quasi-reference electrode and a Pt wire as counter electrode.
Ferrocene was used as internal standard. All the redox potentials
are reported versus the standard calomel electrode (SCE). The
electrochemical characterization, cyclic voltammetry (CV), was
performed in solutions of 0.1 M tetrabutylammonium hexa-
d
: 14.41 (broad s, 2H, NH), 8.07 (t, J¼7.9 Hz, 2H); 7.76 (t, J¼7.8 Hz,
1
H); 7.59e7.57 (d, J¼8.0 Hz, 2H); 7.53e7.45 (m, 11H), 7.31e7.27 (m,
1
3H), 7.26 (CHCl3); 7.24e7.18 (m, 6H); 7.13 (t, J¼7.8 Hz, 10H),
13
6
.98e6.92 (m, 8H), 6.78 (m, 8H), 6.68 (m, 4H), 2.01 (s, 6H). C NMR
: 159.496; 159.31; 147.44; 141.57; 141.10;
36.16; 135.55; 135.49; 135.23; 135.06; 133.01; 132.94; 132.78;
32.41; 131.80; 131.49; 131.76; 130.512; 129.68; 129.53; 129.49;
(
3
400 MHz, 298 K,CDCl ) d
1
1
1
27.36; 125.70; 125.47; 125.36; 121.10; 120.02; 78.24e77.94
6
fluorophosphate (TBAPF ). Electrochemical solvents used are dry
(
(
(
CDCl
3
); 26.11. 31P NMR (400 MHz, 298 K,CDCl
3
)
d
: ꢂ14.00. HRMS
acetonitrile (ACN) and N,N-dimethylformamide (DMF). The con-
centration of the samples was in the millimolar range: 2.8 mM
(1H); 1.6 mM (2H); 1.1 mM (3H); 17.9 mM (L1); 6.3 mM (L2);
þ
þ
ESI): 1685.35 (z ¼ 1) (C94
H
74Cu
2
N
8
O
H
2
P
4
BF
4
) ; 1596.34 (z ¼ 2)
þ
þ
)þ2
-
C
94
H74Cu
2
N
8
O
2
P
4
H ). Calcd for C94
74Cu
2
N
8
O
2
P
4
2(BF
4
) ꢃ
CH
2
Cl
2
: C, 61.44; H, 4.12; N, 6.03. Found: C, 61.93; H, 4.21; N, 6.03.
2 6
9.5 mM (L3); 14.1 mM ([Ni(cyclam)Cl ]). TBAPF (electrochemical
grade, 99%, Fluka) was used as the supporting electrolyte, which
ꢀ
2.2.6. Complex 3H
was recrystallized from an ethanol solution and dried at 60 C
In a Schlenk tube, under Ar atmosphere, DPEPhos (58.4 mg,
under vacuum.
0
.108 mmol) and Cu(CH
3
CN)
4
BF
4
(34.4 mg, 0.11 mmol) were dis-
Cl . The reagents
stirred for an hour at room temperature, before the ligand L3
21.3 mg, 0.054 mmol) was added (a small amount of CH CN was
solved in 10 ml of dry and freshly distilled CH
2
2
2.5. Computational details
(
3
All quantum chemical calculations were performed with the
Turbomole 7.2.1 package [43]. Models for the Ligands L1-L3 and the
complexes 1H-3H were fully relaxed at the DFT BP86-D3 level
added to increase the solubility of the ligand). After 4 h, the solvent
was evaporated by reduced pressure. The residue was dissolved in a

C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
143
(
neglecting the BF
4
anion in the calculations) with a mixed basis set
complexes were purified by recrystallization in cyclohexane. The
yield after purification of these complexes was close to 80%.
based on the def2-family [44,45]. The Cu ion and the coordinating P
and N atoms were described by a def2-TZVP basis, the rest of the
molecule by a def2-SVP basis set. Dispersion interactions in all
simulations were considered with the Grimme -D3 approach [46]
including a damping function as introduced by Becke and Johnson
3.2. X-ray structure of complex 1H
Crystallisation by slow diffusion of cyclohexane into a concen-
trated DCM solution of 1H provided colourless crystals, suitable for
X-ray analysis. The solved molecular structure is shown in Fig. 1,
and selected parameters of the molecular structure are given in
Table 1. The geometric parameters are comparable with the dis-
tances and angles reported for other mononuclear heteroleptic
Cu(I) complexes [49,50].
(
BJ) [47]. Solvent effects were modelled with an implicit solvent
model (COSMO) [48] utilizing a value of epsilon ¼ 8.93 for the
dielectric constant of dichloromethane. A fine grid (size 4) was used
in all numerical integrations using strict convergence criteria for
the energy (<10 H) and density (<10 ). Structural relaxations
were considered converged if the maximum norm of the Cartesian
ꢂ7
ꢂ7
ꢂ4
gradient had dropped below 10 H/Bohr. The ten lowest elec-
Quantum chemical calculations at the density functional theory
(DFT) level gave a structure in good agreement to the experimental
one (see Table S2 and Fig. S12 in ESI). However, compared to the
experimental obtained structure 1H the two N-Cu-P bond angles
are more symmetric. Inclusion of the solvent CH Cl by the
tronically excited states (singlets) were calculated for all models in
solution within the linear response formalism of the adiabatic
time-dependent density functional theory (TDDFT) employing the
hybrid BH-LYP functional (to take care of charge transfer excita-
tions) and the same basis as in the relaxations of the structural
parameters.
2
2
conductor like screening model (COSMO) did not change the
atomic positions significantly.
3
. Results and discussion
3.3. Spectroscopic properties
3.1. Synthesis of ligands and synthesis of complexes
3.3.1. Electronic absorption
The photophysical properties of the three heteroleptic Cu(I)
complexes were investigated at room temperature in dichloro-
methane as spectroscopic solvent. The absorption profiles of the
complexes were quite similar to each other and their absorptivity
coefficients are shown in Fig. 2. All three complexes show an
intense absorption around 270 nm, which can be attributed to
The ligands were prepared in two steps. First, 6-bromo-2-
methylpyridine reacted with the triple bond of an ethynyl-
derivative via a Sonogashira cross-coupling reaction. The second
step was a 1,3-Huisgen reaction between the so-formed alkyne and
a trimethylsilylazide at high temperature. The obtained 1H-1,2,3-
triazoles were substituted in the 4 and 5 positions, with a phenyl
and a pyrid-2ꢀ-yl ring, respectively (Scheme 1). In order to obtain
dinuclear complexes with metal centres that are electronically
coupled, we chose a phenyl ring to be the bridging unit.
p
e
p
* transitions, localised on the chelating phosphine and on the
N N ligand. In fact, in the high energy region, the electronic tran-
sitions are dominated by * and by ne * intra-ligand transi-
^
pe
p
p
tions. The absorptivity coefficients of the new ligands in
dichloromethane are shown in Fig. 2 (dashed lines) and in the
supporting information (Fig. S2). The difference in intensity be-
tween the dinuclear complexes and the mononuclear one, the
concentrations being equal, is due to the fact that the dimers
contain two DPEPhos units. The shoulders with highest absorption
peak close to 300 nm are related to the intra-ligand transitions
occurring on the ligands. The lower-energy absorptions are due to
the electronic transitions from the d-orbitals of the Cu(I) metal
The synthesis of the final copper complexes was conducted in
inert atmosphere, as it is well known that Cu(I) is easily oxidised to
Cu(II), especially in aerated solutions. The solvent used for the re-
action was dry and freshly distilled dichloromethane (DCM). The
coordination of the diphosphine ligand DPEPhos with the metal
centre occurred at first by reaction with the tetrakis (acetonitrile)
copper tetrafluoroborate. At a later time, the appropriate equiva-
^
lents of the chelating N N ligand were added. The final complexes
were salts. In fact, no deprotonation of the triazole ring occurred;
centre to the p* of the pyridyletriazole ligand, involving the pop-
therefore, the ligand stayed neutral, and upon coordination with
ulation of the singlet MLCT state. These results are supported by
TDDFT calculations for 1H (for details see ESI: Quantum chemical
calculations sections). In Fig. S12, the MLCT character of the lowest
excited state is shown.
ꢂ
the positively charged metal core, the BF
4
counteranion was pre-
sent in order to maintain electroneutrality. In Scheme 2 the syn-
thesis of the heteroleptic mononuclear Cu(I) complex 1H is shown,
together with the chemical structures of complexes 2H and 3H. The
The increase of the absorptivity coefficient of the dinuclear
0
Scheme 1. Synthesis of the monochelate 4-phenyl-5-(6’-methyl-pyrid-2 -yl)-1H-1,2,3-triazole L1 in two steps. The chemical structures of the bis-bidentate ligands L2 and L3 are
also shown.

144
C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
Scheme 2. Synthesis of the heteroleptic Cu(I) complex 1H in dry and freshly distilled DCM. In the same manner the complexes 2H and 3H (bottom) were prepared, after addition of
.5 equivalents of ligands L2 and L3, respectively.
0
Fig. 2. Absorption spectra in CH
2 2
Cl of the complexes investigated in this work: 1H
4
Fig. 1. Molecular structure of 1H. (BF anion and solvent are omitted for clarity;
displacement parameters are drawn at 50% probability level).
(red-squares line); 2H (blue-circles line) and 3H (black-triangles line). The absorptivity
coefficients of the ligands are reported in dashed lines (L1: red; L2: blue; L3: black). (In
the inset, an enlargement of the absorption between 330 and 450 nm is shown). (For
interpretation of the references to colour in this figure legend, the reader is referred to
the Web version of this article.)
compounds 2H and 3H in comparison to the mononuclear complex
1
H is due to the presence of two coordinated centres. Nevertheless,
the increase for the complex 3H is higher than 2H, suggesting that
an electronic communication between the two coordinated sites
should be present in 3H. For 2H and 3H, two transitions with the
same excitation energy but different transition dipole moments are
the origin of the peak at ca. 300 nm. In 2H, both transitions have an
appreciable intensity, whereas only one excitation contributes
significantly in 3H, where the transition dipole moments of the two
halves are almost parallel to each other. Taking both excitations into
account, 3H has a slightly higher intensity than 2H (see Fig.s S16,
S17 and S18).
A similar trend can be recognized in the absorptivity coefficients
of the ligands L1 and L2; in fact, their absorption spectra are almost
identical, but the intensity in L2 is almost twice the value of L1,
while ligand L3 shows a different profile.
3.3.2. Emission in solution at room temperature
Photophysical investigation of the free ligands and of the three
complexes was performed in CH Cl solutions. The photophysical
2 2
properties are reported in Table 2 and the emission and excitation
Table 1
Selected bond lengths (Å) and bond angles (deg) for 1H.
Bond lengths (Å)
Bond angles (deg)
Cu1-N11
Cu1-N1
Cu1-P2
Cu1-P1
2.038 (4)
2.095 (4)
2.2168 (13)
2.2742 (13)
N11-Cu1-N1
N11-Cu1-P2
N1-Cu1-P2
N11-Cu1-P1
N1-Cu1-P1
P2-Cu1-P1
79.93 (15)
123.20 (12)
125.78 (12)
102.51 (13)
104.44 (12)
114.59 (5)

C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
145
Table 2
Spectroscopic and photophysical properties at room temperature and at 77 K.
a
ema,b/nm
Fa,b,c
ta,b
4
ꢂ1
5
ꢂ1
emd/nm
td
/ms
(77 K)
Sample
l
(
abs /nm
l
/m
s
k
r
(x 10 )/s
k
nr (x 10 )/s
l
ꢂ1
ꢂ1
)
ε/M cm
(77 K)
3
0.038e
0.020e
0.221e
L1
L2
L3
250 (9.9 ꢁ 10 ),
352
352
369
0.007
0.007
0.003
563
1352
1463
2438
3
2
92 (10.5 ꢁ 10 )
4
250 (2.2 ꢁ 10 ),
298
3
2
92 (20.7 ꢁ 10 )
3
250 (9.9 ꢁ 10 ),
6875
3
2
95 (11.9 ꢁ 10 )
4
3
1H
2H
3H
281 (2.74 ꢁ 10 ), 350 (3.85 ꢁ 10 )
554
556
556
0.054
0.099
0.142
0.88
1.21
2.78
6.1
8.2
5.1
10.7
7.4
3.1
527
529
527
200
214
256
4
3
281 (5.59 ꢁ 10 ), 350 (9.04 ꢁ 10 )
5
4
281 (1.03 ꢁ 10 ), 350 (1.63 ꢁ 10 )
a
b
c
In DCM.
Ar-saturated solution.
Quantum yields were determined with the relative method using Ru (bpy)
In a glass matrix of EtOH/MeOH (4:1); emission decays were analysed with monoexponential fit.
in aerated DCM solution using p-terphenyl as reference (in aerated cyclohexane solution ф ¼ 0.92).
3
Cl
2
as reference (in aerated water solution ф ¼ 0.028).
d
e
spectra of the free ligands and of the Cu(I) complexes are shown in
Fig. 3 and in Fig. 4, respectively. Luminescence of the free ligands
can be described as fluorescence with maximum energy at 352 nm
for L1 and L2, while L3 emits at lower energy (maximum at
leads to a lower reorganisation of the ligands around the excited
Cu(I) core. This is evident comparing the photophysical properties
of 1H with the neutral complex C in Fig. 5. Complex C has an
unsubstituted pyrid-2 -yl-1H-1,2,3-triazole as N N chelating ligand.
The absence of any sterical hindrance in this ligand allows a great
flattening distortion in the excited state, diminishing the photo-
0
^
3
69 nm), clearly due to the extended conjugation of the system.
Photoluminescence quantum yield of ligand L3 is 22.1%, a much
higher value than L1 (3.8%) and L2 (2.0%). Lifetimes of their excited
state are in the range of nanoseconds. The three investigated
complexes are strongly luminescent and exhibit unstructured
broad emission with large Stokes shifts (Fig. 4). These features point
to an emissive excited state with a dominant metal-to-ligand
charge-transfer character. The emission maxima were 554 nm for
complex 1H, and 556 nm for complexes 2H and 3H. The excitation
spectra were recorded by monitoring the emission maximum of the
corresponding complex. Their profiles are very similar to each
other, and recall the observed absorption spectra. The emission
quantum yields measured in Ar-saturated atmosphere are reason-
ably high for Cu(I) complexes. The mononuclear Cu(I) complex 1H
luminescence quantum yield (
F
¼ 0.6% for C) [50]. Compounds A
and B have 1H-1,2,4-triazoles instead of 1H-1,2,3-triazole and,
^
although these N N ligands do not have any substituents in alpha to
the N atom of the pyridine, they have substituents in the triazole
ring. Neutral complex A has comparable photophysical properties
in respect to 1H, with almost identical emission energy (555 nm for
A) and quantum yield (5.0% for A) [49]. In the charged Cu(I) com-
plex B, the triazole coordinates the metal core with the N atom in
position 4, which is less common for this kind of triazole. The
presence of a t-butyl substituent in position 5 enhances the rigidity
in the excited state, affording higher quantum yield (14.0%) and
long-lived excited state lifetime (15.0 ms) [51]. Interestingly, the
has a photoluminescence quantum yield
F
of 5.4%, which is a good
dinuclear complexes showed much stronger emission than the
result with regard to other mononuclear heteroleptic complexes of
similar structure (Fig. 5) [49e51]. The increased emission might be
due to the steric hindrance caused by the presence of the methyl
group alpha to the N of the pyridine. The structural rigidity, in fact,
mononuclear complex 1H. The dinuclear complex 2H features two
coordinating units bridged in the meta position and has a
F of 9.9%,
whereas complex 3H, with a para-substituted bridging ligand,
Fig. 3. Excitation (dashed-dotted) and emission (solid) spectra of free ligands L1 (red
line), L2 (blue line) and L3 (black line) in air-equilibrated CH
¼ 295 nm). (For interpretation of the references to colour in this figure legend, the
reader is referred to the Web version of this article.)
2 2 ex-
Cl solution. (l
c
Fig. 4. Excitation (dashed-dotted) and emission (solid) spectra of complexes 1H, 2H
and 3H in Ar-saturated CH
2
Cl
2
solution. (
l
exc ¼ 365 nm).

146
C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
^
Fig. 5. Selected heteroleptic Cu(I) complexes, A [49], B [51], C [50], with N N chelating ligands that are analogue to L1, investigated in this work. The reported photophysical
2 2
properties were measured in CH Cl at room temperature.
ꢂ1
showed a
yield, the excited state lifetimes also increased. Mononuclear
complex 1H has an excited state lifetime of 0.88 s, and dinuclear
complexes 2H and 3H have lifetimes of 1.21 s and 2.78 s,
F
of 14.2%. Along with the increasing emission quantum
frozen matrix is dichloromethane (ca. 0.12 cm ). From the obser-
vations above, we can conclude that the more rigid environment of
the frozen matrix allows the emission from a MLCT state that is not
stabilised by solvent rearrangement, as it happens at room tem-
3
t
m
m
m
respectively. The phenomenon of significant increase of quantum
yields and lifetimes for dinuclear Cu(I) complexes compared to the
mononuclear one reflects the influence of the second metal centre
on the photophysical properties of these complexes, especially in
perature. The long decay lifetimes are in the range of 200
agreement with other Cu(I) complexes [49,52,53].
ms, in
This is supported by quantum chemical calculations. The Cu
coordination of the lowest triplet excited state is shown in Tables S2
and S3 and Fig. S12, where also the different density between
ground and excited state is shown, which supports the MLCT
character. The local structure at the Cu centre is only slightly dis-
torted. It is not as symmetric as in the ground state calculation. The
N-Cu-P angles are similar to the experimental values. The calcu-
lated Stokes shift amounts to 2.12 eV.
3
H. The fully p-conjugated system seems to mediate a strong
electronic communication between the metal cores, as already
indicated for the absorption spectra. Possible cooperative effects
between the metal centres are currently under investigation.
Emission in a glassy matrix at 77 K. Photoluminescence and
lifetime of the three complexes were also measured in frozen
matrices of ethanol/methanol (4:1). We measured the low-
temperature photoluminescence also in
a glassy matrix of
3
.4. Electrochemical properties
dichloromethane, (Fig. S6 and Table S1). Since the broad emission
structure persists also at low temperature, the MLCT state is the
emissive state even at 77 K, with no contribution from ligand-
centred transitions (Fig. 6). Moreover, for all three compounds
The three complexes were characterised also by cyclic voltam-
metry, in order to investigate their electrochemical behaviour. The
working and counter electrodes were a platinum disk and a plat-
inum wire, respectively. Since a silver wire was used as the pseu-
doreference electrode, ferrocene as internal standard was
ꢂ1
there is a hypsochromic shift (ca. 0.07 cm ), due to a rigidochromic
effect. This shift to higher energy is more pronounced when the
necessary [54]. The solvent was dry acetonitrile, with 0.1
centration of the supporting electrolyte, tetrabutylammonium
hexafluorophosphate (TBAPF ). The redox processes are not
M con-
6
reversible. The oxidation is expected to occur primarily on the
copper metal site (E1/2 ¼ 1.70 V, 1.75 V and 1.79 V versus standard
calomel electrode (SCE) for 1H, 2H and 3H respectively); a second
oxidation process is mainly localised on the chelating diphosphine
ligand. On the other hand, the reduction process should be
^
concentrated on the N N chelating ligands (E1/2 around ꢂ2.35 V
versus SCE for all three complexes 1H, 2H and 3H). Cyclic voltam-
metry of the free ligands was recorded in N,N-dimethylformamide
(
DMF), in order to have a wider solvent window in the reduction
process. The ligands present irreversible reduction (E1/2 ¼ ꢂ2.11 V
L1); ꢂ2.21 (L2); ꢂ2.05 (L3) vs SCE) (see Fig. S7 and S8). The redox
(
processes of 1H were investigated at different scan rates (see
Fig. S9). The irreversibility of the oxidation is retained, while the
reduction becomes quasi-reversible at a scan rate of 500 mV/s. As
expected from the photophysical studies, the difference in energy
between the first oxidation and the first reduction is very similar in
all three complexes. (See Table 3 and Fig. 7 for details). The frontier
orbitals HOMO and LUMO are indirectly related to the oxidation
and reduction potentials, thus we estimated their values using
empirical relations, utilizing the oxidation and reduction potentials
Fig. 6. Emission spectra of complexes 1H, 2H and 3H in a frozen matrix of EtOH/MeOH
(
4:1) at 77 K.

C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
147
Table 3
Table 4
þ
Electrochemical data of complexes in acetonitrile (0.1 M TBAPF
6
)
at room
Estimation of the excited state processes in ACN at room temperature versus Fc/Fc .
temperature.a
Sample
E
0-0/V
E
ox (PS*)/V
Ered (PS*)/V
Sample
E
ox/V
E
red/V
HOMOb
eV
LUMOc
/eV
DHOMO-LUMO/eV
1
2
H
H
3.23
3.20
3.22
ꢂ1.93
ꢂ1.85
ꢂ1.83
0.48
0.47
0.47
/
1H
2H
3H
1.30
1.35
1.39
ꢂ2.35
ꢂ2.33
ꢂ2.35
ꢂ6.10
ꢂ6.15
ꢂ6.19
ꢂ2.05
ꢂ2.07
ꢂ2.05
4.05
4.08
4.14
3H
d
d
d
L1
L2
L3
ꢂ2.04
ꢂ2.14
ꢂ1.98
ꢂ2.29
ꢂ2.19
ꢂ2.35
estimated the energy of the excited state E0-0 in acetonitrile for
each complex. Details are available in the supporting information.
The excited-state redox processes are reported versus Fc/Fc in
þ
a
Values obtained by cyclic voltammetry, scan rate 100 mV/s, reported versus the
standard calomel electrode (SCE).
Table 4.
b
These values are used to evaluate if a photoinduced electron
transfer process (reductive or oxidative quenching) is energetically
feasible (vide infra).
Estimated from oxidation potential versus ferrocene/ferrocenium potential.
Estimated from reduction potential versus ferrocene/ferrocenium potential.
c
d
6
In dry DMF (0.1 M TBAPF ).
3.5. SterneVolmer bimolecular quenching
Our target is to develop new photosensitizers (PS) that can be
used in the photoactivation of small molecules. The heteroleptic
complexes that we have presented here have a long-lived excited
state, which is an important characteristic for a PS. Hereafter we
report a preliminary experiment of SterneVolmer bimolecular
quenching of the three complexes with the catalyst Ni(cyclam)Cl
In fact, Ni(cyclam)Cl was used in both electrochemical and
photochemical reduction of CO to CO, showing high stability and
selectivity for CO over H O [42,56]. These experiments were
conducted in aerated acetonitrile solution
2
.
2
2
2
2
(
t
aer ¼ 54 ns (1H);
ꢂ3
5
7 ns (2H); 195 ns (3H)). A solution of Ni(cyclam)Cl
ACN was prepared and added in 50 mL steps to the PS solution in
2
(1 ꢁ 10 M) in
order to identify a possible quenching. Fig. 8 shows the ratio of the
emission intensity (I /I) as a function of the concentration of
Ni(cyclam)Cl ([CAT]). It reveals that the emission of the three
0
2
complexes suffers from the increasing presence of the Ni catalyst.
The SterneVolmer equation was used in order to fit the data in
ꢂ1
Fig. 8. The SterneVolmer constants are: 1.90 mM
(1H);
ꢂ1
ꢂ1
1
.16 mM (2H) and 3.27 mM (3H), from which we can estimate
1
0 ꢂ1 ꢂ1
the apparent quenching rate constant to be 3.5 ꢁ 10
s M
(1H);
10
ꢂ1 ꢂ1
10 ꢂ1 ꢂ1
2.0 ꢁ 10
s
M
(2H); 1.7 ꢁ 10
s
M
(3H). At this stage we
can observe that all three Cu(I) complexes are quenched by the
presence of the catalyst, which is a requirement in order to use the
6
Fig. 7. Cyclic voltammetry of complexes 1H, 2H and 3H in acetonitrile (0.1 M TBAPF )
at a scan rate of 100 mV/s.
of the complexes in respect to the oxidation potential of the
ferrocene [55].
In order to use 1H, 2H, and 3H as photosensitizers (PS) it is of
utmost importance to determine the excited state redox processes
Fig. 8. Plot of the ratio of the intensities (calculated at emission maximum) I
three complexes versus the concentration of the Ni(cyclam)Cl (CAT) in ACN. The
absorbance at the excitation wavelength was constant after each measurement.
0
/I of the
Eox (PS*) and Ered (PS*) (respectively the oxidation and the reduc-
2
tion potentials of the PS in its excited state). Therefore, we

148
C. Bizzarri et al. / Journal of Organometallic Chemistry 871 (2018) 140e149
system Cu(I) complex 3H and Ni(cyclam)Cl
2
as PS and CAT for the
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[
[
photoreduction of CO . This is also in accordance with the fact that
2
Cu(I) complexes undergo oxidative, rather than reductive,
quenching [23]. An estimation of the driving force for the oxidative
quenching can be calculated by the following equation:
€
D
G ¼ Eox
(
PS*) e Ered (CAT). In this case, CAT is Ni(cyclam)Cl
2
(Ered in
þ
ACN ¼ 1.73 V versus Fc/Fc ). Thus, the oxidative quenching of the
excited state by Ni(cyclam)Cl
investigated complexes (
G (1H) ¼ ꢂ0.20;
3H) ¼ ꢂ0.10).
2
results energetically feasible for the
D
DG (2H) ¼ ꢂ0.12;
DG
[
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from earth-abundant elements, Chem. Eur. J. 24 (2018) 2039e2058.
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[
diimineediphosphine Cu(I) complexes as an alternative towards noble-metal
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4
. Conclusion
[
[
12] O. Horv ꢀa th, Photochemistry of copper(I) complexes, Coord. Chem. Rev.
We have presented one mononuclear and two dinuclear heter-
135e136 (Supplement C) (1994) 303e324.
0
oleptic copper(I) complexes based on pyrid-2 -yl-1H-1,2,3-triazole
as N N chelating ligand and DPEPhos as chelating diphosphine.
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physics of coordination compounds: copper, in: V. Balzani, S. Campagna
^
(Eds.), Photochemistry and Photophysics of Coordination Compounds I,
The presence of the methyl group alpha to N of the pyridyl ring
increases the structural rigidity, leading to a lower structural
reorganisation in the excited state. Thus, these complexes possess
good luminescence properties in solution at room temperature. The
hypsochromic shift in frozen matrix at 77 K is due to a rigid-
ochromic effect. The excited-state lifetime of the dinuclear complex
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3
H is much longer than for the mononuclear compound, which we
postulate is due to electronic interaction between both metal
centres in the binuclear complexes. The excited-state redox pro-
cesses of the three complexes were evaluated and bimolecular
quenching experiments with Ni(cyclam)Cl
order to prove the photoinduced redox process, which is needed for
their application in the photoreduction of CO . This promising
[
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2
application is the subject of investigation in our laboratory.
[19] dap is 2,9-bis(para-anisyl)-1,10-phenanthroline.
Notes
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benzylic halides by a bis(di-imine) copper(I) complex, J. Chem. Soc., Chem.
Commun. 8 (1987) 546e548.
The authors declare no conflict of interest.
[
[
[
21] D. Li, C.-M. Che, H.-L. Kwong, V.W.-W. Yam, Photoinduced C-C bond formation
from alkyl halides catalysed by luminescent dinuclear gold(I) and copper(I)
complexes, J. Chem. Soc., Dalton Trans. 23 (1992) 3325e3329.
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Acknowledgement
We gratefully acknowledge funding support from the DFG-
funded Collaborative Research Centres (SFB) TRR 88/3MET “Coop-
erative Effects in Homo-and Heterometallic Complexes” (CB:
overhead of 3MET package (B2); KF, SK: project B1). CB thanks Prof.
S. Br a€ se (KIT) for his scientific support. We thank Prof. F. Breher
23] S. Paria, O. Reiser, Copper in photocatalysis, ChemCatChem 6 (9) (2014)
2477e2483.
[24] A.C. Hernandez-Perez, S.K. Collins, Heteroleptic Cu-Based sensitizers in pho-
toredox catalysis, Accounts Chem. Res. 49 (8) (2016) 1557e1565.
[
25] M.S. Lazorski, F.N. Castellano, Advances in the light conversion properties of
Cu(I)-based photosensitizers, Polyhedron 82 (Supplement C) (2014) 57e70.
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visible-light-driven photoredox catalyst in carbonecarbon bond-forming re-
actions, Chem. Eur J. 18 (24) (2012) 7336e7340.
(
KIT), Prof. H.-A. Wagenknecht (KIT) and Prof. U. Lemmer (KIT) for
[
giving access to electrochemical and photochemical equipment,
respectively. Dr. P. Weis is acknowledged for high-resolution mass-
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