Design and application of diimine-based copper(i) complexes in photoredox catalysis

Article abstract of DOI:10.1039/c9ob01331h

Structurally different bis(imino)copper(i) complexes were prepared in a highly modular manner and utilized as copper-based photocatalysts in the ATRA reactions of styrenes and alkyl halides. The new photocatalysts showed good catalytic activity and ensured efficient chemical transformations.

Full text of DOI:10.1039/c9ob01331h

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DOI: 10.1039/C9OB01331H
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Design and application of diimine-based copper(I) complexes in
photoredox catalysis†
Tamás Földesi,a Gellért Sipos,a, b Réka Adamik,a Bálint Nagy,a Balázs L. Tóth,a Attila Bényei,c
Received 00th January 20xx,
Accepted 00th January 20xx
Krisztina J. Szekeres,d Győző G. Láng,d Attila Demeter,e Timothy J. Peelen*f and Zoltán Novák,*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Structurally different bis(imino)copper(I) complexes were prepared to styrenes or alkenes, intramolecular cyclization and
in
a
highly modular manner and utilized as copper-based difluoromethylation of alkenes,11 intermolecular cyclization of
photocatalysts in ATRA reactions of styrenes and alkyl halides. The N-phenylmaleimide with N,N-dialkylaniline12 or azidation
new photocatalysts showed good catalytic activity and ensured reactions.13 It is worth mentioning that different heteroleptic
efficient chemical transformations.
copper complexes are also applicable as photocatalysts.14
Considering the structural features of these widely used
excellent copper photocatalysts we aimed to design new bis-
imine type N,N donor ligands and study the photocatalytic
activity of their copper complexes in atom transfer radical
addition (ATRA) reactions.15 Our target diimines 2 can be easily
prepared from ethylenediamine (4) and appropriate
benzaldehydes (5) in ethanol at room temperature in a simple,
efficient and highly modular manner. To build our ligand library,
we used 20 different benzaldehyde derivatives containing F, Cl,
Br, Ph, OMe, Me2N substituents in ortho, meta and para
positions. For the synthesis of copper complexes, two
equivalents of ligand 2 were used with Cu(MeCN)4PF6 and the
desired catalysts (6a‒t, representative examples are shown on
Scheme 1) were isolated in excellent yields (up to 86% yield for
two steps) by simple filtration after the addition of diethyl ether
to the reaction mixture.16 It is of note that, while some of these
diimine ligands and their copper complexes have been reported
previously17 to our best knowledge only few copper complex of
this type has been applied in a photocatalytic synthesis.18
The main bands of the absorption spectra can be
characterized with considerable large oscillator strengths (see
Figure SI1), while the luminescence is very weak: the
fluorescence yields are around 10-5 and similarly to the shape of
the spectra, slightly depend on the excitation wavelength. The
main component of the singlet decay is very short, so we believe
that most probably a triplet-like species induces the catalytic
reaction. Using laser flash excitation transient absorption (TA)
measurements a long-lived (30 ms) species can be detected at
460 nm, as well as the depletion of the ground state compound
at 400 nm. Both observations indicate that the absorption
spectrum of the long-lived triplet species is slightly shifted to
the red with excitation. Even more, at presence of CBr4 a further
kinetic increase of the TA signal may be observed at 460 nm
Utilization of visible light-driven photocatalytic transformations
in organic synthesis has become one of the most intensively
investigated fields of organic chemistry.1 In these photocatalytic
reactions, an excited state photosensitizing dye generates a
radical which can then take part in a catalytic cycle. One
drawback of this synthetic technique is the use of expensive
metal-based photocatalysts such as iridium and ruthenium
complexes. Despite their high prices, these noble metals are
used in a very broad area of synthetic chemistry.2 In order to
reduce the cost of these transformations, the change from
noble metals to cheaper organic dyes is one direction of
methodology developments in photoredox catalysis.3 Bearing in
mind the photochemical and redox properties there is
opportunity to use cheaper transition metal complexes as
sensitisers.4 In this respect, copper is a promising candidate, and
several successful applications of copper complexes were
already reported in different areas of photochemistry.5 Among
the most active copper-based catalysts are phenanthroline
complexes, such as homoleptic [Cu(dap)2]Cl (1,6 Scheme 1),
which shows impressive catalytic activity in photoredox
transformations7 such as addition of CBr4, fluoroalkyl iodide,8
aryl- and alkylsulfonyl,9 and trifluoromethylsulfonyl chlorides10
6
a. ELTE “Lendület” Catalysis and Organic Synthesis Research Group, Institute of
Chemistry, Eötvös Loránd University, Institute of Chemistry, Pázmány Péter s. 1/A,
H-1117 Budapest (Hungary) E-mail: novakz@elte.hu; web: zng.elte.hu
b. Current address: ComInnex Inc., Záhony u. 7, H-1031 Budapest (Hungary)
c. University of Debrecen, Department of Physical Chemistry, H-4032 Hungary,
Debrecen, Egyetem tér 1.
d. Eötvös Loránd University Laboratory of Electrochemistry & Electroanalytical
Chemistry, Eötvös Loránd University, Institute of Chemistry, Pázmány Péter stny.
1/A, H-1117 Budapest (Hungary)
e. Research Centre for Natural Sciences of the Hungarian Academy of Sciences. H-
1117 Hungary, Budapest, Magyar Tudósok körútja 2.
f. Department of Chemistry, Lebanon Valley College, Annville, PA (USA) 17003. E-
mail: peelen@lvc.edu
† Electronic Supplementary Information (ESI) available: [characterization data,
optimization studies. See DOI: 10.1039/x0xx00000x
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Table 1. Optimization of reaction conditionsa
View Article Online
Ar
Ar
Ar
Ar
DOI: 10.1039/C9OB01331H
Ar
Ar
N
N
N
N
N
N
Br Br
Cu(MeCN)4
N
N
N
N
catalyst
Br
Br
Cu+
-
Cu+
520-525 nm LED
PF6
PF6-
+
CBr4
DCM
Ar, 20 °C
3
-
1
7
2
PF6
Ar
Ar
Ar
Ar
6a-n
9a
8a
Cat.
mol%
1.0
2.5
1.0
2.0
3.0
1.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
yield (9a,
%)
23
73
0b
NH2
Entry
Catalyst
Ar (in 6)
O
+
2
diimine formation from
20 different aldehydes
H
Ar
NH2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
6a
6a
6a
2a
C6H5
C6H5
C6H5
C6H5
-
5
4
Cl
6
Synthesis of Cu-complexes
20 examples,
up to 86% yield (2 steps)
0c
Cu(MeCN)4PF6
0d
0e
8
Cl
Cl
Representative examples shown, Ar:
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
C6H5
6f
6j
82%
83%
6a
68%
4-Me2N-C6H4
4-MeO-C6H4
2-MeO-C6H4
2-Ph-C6H4
4-Cl-C6H4
3-Cl-C6H4
2-Cl-C6H4
2,4-Cl2-C6H3
2,6-Cl2-C6H3
2-F-C6H4
4-Br-C6H4
2-Br-C6H4
CV measurement
53
76
88
62
79
90
83
4
Br
F
OMe
6d
6l
6n
86%
83%
75%
X-ray structure
Br
6m
51%
88
88
93
Cu(MeCN)4PF6
+ 2.5 % 2m
19
-
2.5
41
Scheme 1 Design of new copper bis imine complexes.
Cyclic voltammogram of 6d (1) 100 mM TBABF4 in MeCN and (2) 1 mM
C36H40CuF6N4O4P (6d) / 100 mM TBABF4 in MeCN. v = 50 mV∙s–1; E = –1.73 – 0.77
V vs. SSCE. The working electrode was a GC (A = 0.031 cm2) plate.
. a reaction conditions: 1 equiv. CBr4, catalyst, abs. DCM, argon atmosphere, rt, 18
hours, green LED. b in the dark (under Al foil). c only ligand 2a. d only Cu(MeCN)4PF6.
e without argon atmosphere.
with a characteristic time parameter of 100-200 µs (see Figure
SI2). The 30 ms time range would be enough for a bimolecular
reaction, especially if one of the reactants makes a weak
complex with the catalysts even at ground state.
The shape of the cyclic voltammograms recorded on glassy
carbon and gold is significantly affected by the deposition-
dissolution kinetics of Cu. A quasi-reversible peak couple can be
observed at positive potentials, the E1/2 value varies between
0.49 V and 0.61 V vs. SCE.16
Complex 6m was also characterized by single-crystal X-ray
crystallography (Scheme 1).19 The cation has C2 symmetry and
the ligands take a slightly twisted conformation around the
central copper ion. The geometry around the Cu(I) centre is
pseudotetrahedral. The large angular distortion is due to the
small bite angle of the ligands (N5-Cu1-N8 = 84.30° and N1-Cu1-
N4 = 84.89°). Consequently, the interligand N-Cu-N angles are
larger than those of a tetrahedral complex (N1-Cu1-N5 =
114.94°, N1-Cu1-N8 = 132.79°, N4-Cu1-N5 = 133.76°, and N4-
Cu1-N8 = 113.29°, respectively). The angle of the CuN2 planes
is 72 degree and the average Cu-N distance is 2.035(5) Å while
the average imine C-N bond distance is 1.27(1) Å. The C=N
bonds show trans orientation. The average dihedral angle
between the two phenyl rings of the same ligand molecule is
55.6° while the phenyl rings at the same side of the two ligands
are close to parallel as the average dihedral angle is 5.0°. None
the less, intramolecular π-π stacking is unlikely as the two pairs
of rings are substantially displaced laterally (average centroid
distances are 5.2°).
With the series of complexes in hand, we first examined the
photocatalytic activity of the copper catalysts in the atom
transfer-radical addition (ATRA) of CBr4 (7) to styrene (8a) as the
chosen model reaction (Table 1).5k, 6 During the optimization
phase of the study, we examined the required catalyst loading
of 6a, and found that 2.5 mol% copper catalyst was necessary
to reach full conversion of styrene in dichloromethane (DCM) in
18 hours, and 9a was isolated in 73% yield (entry 2). When 1.0
mol% of copper photocatalyst was used, the yields of 9a were
significantly lower (entry 1). This finding shows slightly lower
stability of the catalyst than the known [Cu(dap)2] complexes
(catalyst loadings lower than < 1 mol% has been described). At
this stage, we checked the catalytic effect of the copper catalyst
and the importance of light irradiation. It was clearly
demonstrated that there was no reaction in the absence of light
or the copper complex (entries 3‒5). It was also found that inert
atmosphere is required for the successful addition (entry 6).
Next, an extended structure-activity relationship was
carried out using different diimino copper complexes. The
presence of electron donating (NMe2) group in para position of
the aryl ring had deleterious effect on the yield compared to 6a
having an unsubstituted phenyl ring (entry 7). However,
reaction with 4-methoxyphenyl derivative 6c gave an
acceptable 53% yield (entry 8).
2 | J. Name., 2012, 00, 1-3
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Ar
Ar
Method A
6d
520-525 nm LED
Method A
Method B
View Article Online
5 mol%
Br Br
6m
2.5 mol %
N
N
DOI: 10.1039/C9OB01331H
F
F
N
N
Br
Br
I
F
F
F
520-525 nm LED
Cu+
I
F
DCM, 3 h, Ar, 20 °C
O
O
+
CBr4
+
Method B
DCM, Ar
20 °C, 18 h
OEt
PF6
OEt
6d
1. 5 mol%
R
O
OEt
R
R
7
R
R
Ar
Ar
520-525 nm LED
DCM, 3 h, Ar, 20 °C
2. 3 equiv Et3N
9a-f
8a-f
12a-f
11a-f
8a-f
10
m
6
Ar: 2-Br-Ph
Br Br
DCM, rt, 18 h
Br Br
Br Br
Br
Br
Br
Br
Br
Br
Cl
Method A
I
F
I
F
F
I
F
F
O
F
O
Cl
O
OEt
9a
9d
9b
9e
9c
, 93%
, 87%
Br Br
, 89%
Br Br
OEt
OEt
Cl
F
Cl
11a,
11b,
11c,
75%
F
80%
83%
83%
Cl
Br Br
Br
Br
Br
Br
Br
Br
I
F
Cl
I
I
F
F
F
F
O
O
O
F
OEt
OEt
OEt
9f
, 91%
, 91%
, 54%
11e,
11f,
75%
F
11d,
80% (6 h)
Scheme 2 ATRA reactions of substituted styrene and carbon tetrabromide.
Method B
F
F
F
F
F
O
O
O
In order to enforce the pseudo-tetrahedral geometry
around the Cu(I) centre, we prepared a series of complexes with
ortho substituted aryl groups. Indeed, 6d featuring ortho
methoxy groups showed superior performance (76% yield,
entry 9). The proof for the importance of the ortho substituent
on the aryl rings led us to synthesize 2-biphenyl derivative 6e
having extended conjugation in the aromatic part. We found
increased catalytic activity in case of 6e and tetrabromo product
9a was obtained in 88% yield (entry 10). The influence of the
substituent position in the phenyl ring on the addition reaction
was clearly observed in case of the chloro derivatives. The same
trend was observed when the position of chlorine atoms was
altered in complexes 6f, 6g and 6h, and the reaction efficiency
increased in the order of para-meta-ortho substituted ligands,
and the adduct was isolated in 62%, 79% and 90% yield,
respectively (entries 11‒13). However, the occupation of both
ortho positions of the phenyl rings had deleterious effect (4%
yield, entry 15), presumably due to instability of complex 6j,
which was proved by electrochemical analysis.16 To probe the
importance of steric effects over electronic ones we performed
the reaction with 2,4-dichloro derivate 6k as catalyst, which
provided the desired product in good 83% yield (entry 14).
Besides Cl the presence of other halogen atoms such as F or Br
in the ligand of the copper catalyst also ensured efficient
transformations, and the application of the ortho-bromophenyl
derivative (6m) provided the highest yield (93%, entry 18) of the
adduct. Finally, the in situ formation of a [Cu(NN)(MeCN)n]+
species from Cu(MeCN)4PF6 + 2.5 mol% 2m resulted in only 41%
yield of 9a (entry 19). This result shows that the formation of a
monoligated active species by dissociation of one of the diimine
ligands during catalytic conditions is highly unlikely.
OEt
OEt
OEt
Cl
F
Cl
12a,
12b,
12e,
12c,
12f,
79%
F
64%
71%
73%
F
Cl
F
F
F
F
O
O
O
OEt
OEt
70% (6 h)
OEt
12d,
74%
Scheme 3 Synthesis of iodides 11 and alkenes 12.
iodine and CF2COOR group could be installed onto the C=C
double bond to access fluoroalkylated benzyl iodides as novel
compounds, ready for further functionalization. We found that
the addition reaction of ICF2COOEt (10) and styrene took place
in the presence of 6d catalyst under the previously used
reaction conditions, and 11a was obtained in 75% yield (Scheme
3, Method A). Next, a series of iodo compounds 11 were
prepared from substituted styrenes, and each target
compounds could be obtained in good yields (75‒83%) with the
use of the new photocatalyst in 3-6 hours.
Although, iodides 11 could be synthesized, purified by
chromatography, and characterized through the conventional
manner, these compounds showed tendency to slow
decomposition to alkenes 12 upon storage. This chemical
behaviour could be controlled and the process could be
accelerated by the addition of triethylamine as a base.
Therefore, another series of reactions were run in order to
prepare alkenes 12 following Method B (Scheme 3). After
irradiation for 3 hours, treatment of the reaction mixture with
3 equivalents of Et3N resulted in the formation of alkenes 12a‒f
in 64‒79% yield, depending on the type (Cl, F, Me) and position
(ortho, meta, para) of the substituent on the aromatic ring of
the styrene.
In conclusion, we designed and synthesized a series of
inexpensive and readily available bis-imino copper(I)-complexes
and studied their photocatalytic activity in different ATRA
reactions. We found that the complexes could serves as good
and economic alternatives to the existing copper based
photocatalyst for the addition reaction of CBr4 and ICF2COOEt
to styrenes. We demonstrated that in the presence of copper
complexes the photocatalytic transformations provide the
appropriate adducts in good yield. Current work in our
laboratory focuses on the application of this new photocatalyst
family in various organic photoredox transformations.
After finding the best catalyst system for the photocatalytic
ATRA reaction of styrene and CBr4 we performed the addition
reaction of various styrene derivatives in the presence of 6m
under the optimized reaction conditions (Scheme 2), and we
obtained the appropriate tetrabromopropyl benzenes 9a‒f in
moderate to excellent yields (54‒93%). It is of note that we also
tested different non-activated alkenes as substrate, but their
transformation resulted lower conversion.16
We also studied the ATRA-type reaction of styrenes in which
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Zambada and X. Hu, Organometallics, 2018, 37, 3928; (f) T. P.
The authors thank the support of the National Research,
Development and Innovation Office (Grant No. NN118172), and
the János Bolyai Research Scholarship of the Hungarian
Academy of Sciences (Z.N.). The research was also supported by
the EU and co-financed by the European Regional Development
Fund under the projects GINOP-2.3.2-15-2016-00008, GINOP-
2.3.3-15-2016-00004 and VEKOP-2.3.2-16-2017-00013 (K.Sz.
and G.G.L.). This work was supported by the ELTE Institutional
Excellence Program (1783-3/2018/FEKUTSRAT) and ÚNKP-17-1
(B. N.) New National Excellence Program of the Ministry of
Human Capacities. This material is based upon work supported
by the National Science Foundation under Grant No. 1358135
to T.J.P. There are no conflicts to declare.
Nicholls, J. C. Robertson, M. G. Gardiner and A. VCie.wBAisrtsicelemObnelinre,
DOI: 10.1039/C9OB01331H
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4 | J. Name., 2012, 00, 1-3
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