[Copper(phenanthroline)(bisisonitrile)]+-Complexes for the Visible-Light-Mediated Atom Transfer Radical Addition and Allylation Reactions

Article abstract of DOI:10.1021/acscatal.5b01071

A series of heteroleptic [Cu(phenantroline)(bisisonitrile)]⁺-complexes was synthesized, and their structural, spectroscopic, and electrochemical properties were investigated. The new copper(I) complexes were employed as photoredox-catalysts in the visible-light-mediated atom transfer radical addition (ATRA). Especially, [Cu(dpp)(binc)]BF₄ (6a-BF₄)(dpp = 2,9-diphenyl-1,10-phenanthroline; binc = bis(2-isocyanophenyl) phenylphosphonate) proved to be highly active owing to an enhanced excited-state lifetime compared to the commonly employed [Cu(dap)₂]Cl (1-Cl)(dap = 2,9-di(p-anisyl)-1,10-phenanthroline). Furthermore, the catalyst could be applied to allylation reactions with trimethylallylsilane under mild visible-light photoredox conditions.

Full text of DOI:10.1021/acscatal.5b01071

Page 1 of 10
ACS Catalysis
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[Copper(phenanthroline)(bisisonitrile)]+-Complexes for the
Visible Light-Mediated Atom Transfer Radical Addition and Al-
lylation Reactions.
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Matthias Knorna, Thomas Rawnera, Rafał Czerwieniecb, Oliver Reisera*
aInstitut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg (Germany)
bInstitut für Physikalische Chemie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg (Germany)
ABSTRACT: A series of heteroleptic [Cu(phenantroline)(bisisonitrile)]+-complexes was synthesized and their structural,
spectroscopic and electrochemical properties were investigated. The new copper(I) complexes were employed as photore-
dox-catalysts in the visible light-mediated atom transfer radical addition (ATRA). Especially [Cu(dpp)(binc)]BF4 (6a-BF4)
(dpp = 2,9-diphenyl-1,10-phenanthroline; binc = bis(2-isocyanophenyl) phenylphosphonate) proved to be highly active ow-
ing to an enhanced excited state lifetime compared to the commonly employed [Cu(dap)2]Cl (1-Cl) (dap = 2,9-di(p-anisyl)-
1,10-phenanthroline). Furthermore, the catalyst could be applied to allylation reactions with trimethylallylsilane under mild
visible-light photoredox conditions.
KEYWORDS: copper, isonitrile, visible light photocatalysis, atom transfer radical addition, allylation.
lecular atom transfer radical addition (ATRA) and allyla-
tion reactions under irradiation with green LEDs (λ = 530
nm) or even sunlight.7
INTRODUCTION
In the last years, visible-light mediated photoredox cataly-
sis developed rapidly. Besides the search for new applica-
tions, the continuous design of highly active catalysts pro-
motes this green technology. The most commonly em-
ployed visible-light photoredox catalysts are metal com-
plexes based on ruthenium or iridium,1 owing to their ex-
cellent stability and catalytic activity manifested in long
life times of their excited states combined with suitable re-
dox potentials to initiate electron transfer events with or-
ganic compounds. However, these metals are scarce in na-
ture, and hence the use of copper complexes is an attrac-
tive alternative, since it is less expensive, safer and environ-
mentally more benign. Nevertheless, there are only few re-
ports on copper based photoactive complexes used for vis-
ible-light driven organic synthesis.2
Figure 1. [Cu(dap)2]+ (1) (dap: 2,9-di(p-anisyl)-1,10-phenan-
throline).
In particular, the excited-state properties of copper(I)
diimine complexes attracted attention since long. As a ma-
jor drawback, such materials often suffer from short ex-
cited state lifetimes, caused by an excited-state reorganiza-
tion from a tetrahedral to a square-planar complex geom-
etry. The introduction of bulky substituents, e.g. in 2,9-po-
sition of the phenanthroline moiety, thus preventing such
a structural relaxation that facilitates the non-radiative re-
laxation to the ground state, has been attempted to over-
come this problem.3 Accordingly, [Cu(dpp)2]+ (dpp = 2,9-
diphenyl-1,10-phenanthroline) could be used for hydrogen
production from water4 as well as for the generation of aryl
radicals derived from diaryliodonium salts under visible-
light photoredox catalysis conditions.5 Recently, we suc-
ceeded in the application of [Cu(dap)2]+ (1) (dap = 2,9-di(p-
anisyl)-1,10-phenanthroline)6 as photoredox catalyst (Fig-
ure 1), utilizing its oxidative quenching cycle for intermo-
Very recently, fluoroalkylsulfonyl chlorides were intro-
duced as reagents for the [Cu(dap)2]Cl (1-Cl) catalyzed vis-
ible light ATRA reaction.8 Similar to ruthenium and irid-
ium based photocatalysts, net CF3Cl addition was observed
by the reaction of triflyl chloride with heteroatom contain-
ing alkenes. In contrast, an inner sphere mechanism which
suppresses SO2 extrusion was described for inactivated al-
kenes, leading to the trifluoromethylsulfonylated prod-
ucts, pointing to special opportunities copper based pho-
tocatalysts might offer beyond acting as electron transfer
reagents.8b
Nevertheless, with sub-microsecond (~ 300 ns) lumines-
cence decay times,5,6 these two copper complexes are dis-
advantageous with respect to their short excited-state life-
time compared to related ruthenium or iridium complexes,
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(e.g.: [Ru(bpy)3]2+ (bpy: 2,2’-bipyridyl): 1.1 μs9a, [Ir(dF(CF3)
other, thus coordination of 3a to a metal center must be
ppy)2(dtbbpy)]+ (dF(CF3)ppy: 2-(2,4-difluorophenyl)-5-tri-
fluoromethylpyridine; dtbbpy: 4,4’-di-tert-butyl-2,2’-di-
pyridyl): 2.3 μs9b) which limits their application in photo-
catalysis.
realized by rearrangement of the two diastereotopic isoni-
trile arms (Scheme 1). As counterparts, commercially avail-
able phenanthroline ligands 4a-c and readily synthesized
2,9-di(p-anisyl)-1,10-phenanthroline6,7a (4d) were em-
ployed in this study (Chart 1).
1
2
3
4
5
6
7
8
To increase the excited-state lifetime, specially designed
copper(I)bisphenanthroline complexes utilizing coopera-
tive steric hindrance were developed with reported excited
state lifetimes in the range of one microsecond.10 As an al-
ternative approach, mixed ligand copper(I) complexes
with a phenanthroline (NN) and a wide-bite-angle biden-
tate phosphine (PP) ligand have been proposed.11 With re-
ported room temperature lifetimes in the order of several
microseconds (e.g. [Cu(dmp)(DPEPhos)]+ (dmp: neocu-
proine, 2,9-dimethyl-1,10-phenanthroline; DPEPhos: bis(2-
(diphenylphosphanyl)phenyl)ether): 14.3 µs11c), these com-
plexes were extensively studied as sensitizers for the nobel-
metal free photocatalytic water reduction.12 Moreover, Col-
lins and co-workers reported the visible-light mediated
synthesis of helicenes and carbazoles,13 using different
[Cu(NN)(PP)]+-complexes. Very recently, a remarkable
study by Chen et al. appeared in which heteroleptic cop-
Besides the known [Cu(dap)2]Cl (1-Cl) the novel hetero-
leptic copper(I)-complexes 6a-e were investigated that
were obtained in quantitative yield by reacting an equimo-
lar solution of two different ligands in dichloromethane
with [Cu(MeCN)4]BF4 (Scheme 2).20 After precipitation in
diethyl ether, the light-brown to brown colored complexes
6 could be stored for extended periods without any sign of
decomposition.
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O
N
n
or
-BuLi LDA
O
N
OLi
N
R
R
Li
R
-78
°C
C
2
Ph
O
P
O
O
POPhCl2
R
R
per(I) complexes containing phenanthroline and
a
N
N
monoanionic nido-carborane-diphosphine ligand with ex-
cited-state lifetimes of 10-20 μs at ambient temperature
were applied for photoinduced cross-dehydrogenative
couplings.14
C
C
3
Following our interest15 in photocatalysis and the further
improvement and investigation of copper-complexes for
visible-light photoredox catalysis,7,8b we were intrigued by
a study of Mann et al.16 that reports emission lifetimes up
to milliseconds for specially designed heteroleptic cop-
per(I) complexes having phenanthroline and monodentate
isonitrile ligands. We envisioned that copper(I) phenan-
throline complexes bearing chelating bis(isonitrile) ligands
further impede structural reorganization upon irradiation
and thus extend the lifetimes of the excited state, which
might lead to improved photoredox catalysts. Here we re-
port the synthesis and characterization of new heteroleptic
[Cu(phenanthroline)(bisisonitrile)]+-complexes. The new
compounds have significantly improved photophysical
characteristics and can be used under mild visible-light
conditions for a variety of atom transfer radical additions
(ATRAs) and allylation reactions with trimethylallylsilane,
the latter being particularly challenging with previously
known catalysts.
Ph
O
O
P
O
N
N
C
C
3a
,
binc*
59%)
(
Ph
O
O
P
O
N
N
C
C
3b
binc
55%)
,
(
a
Synthesis of (bis((S)-2-isocyano-3,3-dimethylbutyl) phe-
nylphosphonate) (binc*, 3a) and (bis(2-isocyanophenyl)
phenylphosphonate) (binc, 3b) and their crystal structures.
Conditions: Oxazole 2 (1.0 equiv.), n-BuLi (1.6 M in hexane,
1.05 equiv.), THF (0.4 M), –78 °C, 1.5 h; POPhCl2 (0.53
equiv.), –78°C to rt, 2 h.
Scheme 1. Synthesis of bisisonitrile ligands 3.a
5
R2
4
R2
7
6
RESULTS AND DISCUSSION
a
=
=
=
=
=
a:
4
dpp
m
R1 Ph
R2
R2
H
H
1
R1
:
e
M
M
4b
d
p
Aiming at heteroleptic [Cu(phenanthroline)(bisisoni-
trile)]+ complexes, bisisonitrile ligands 3a17 and 3b18 were
synthesized, following the methodology introduced by us17
by treating oxazoles with n-BuLi or LDA and trapping the
resulting anion with phenylphosphonic dichloride
(Scheme 1). Bisisonitrile 3b was further characterized by X-
ray analysis.19 Its solid state structure revealed that both
isonitrile moieties are already pre-oriented for the chela-
tion of a metal. In contrast, in the X-ray structure17a of
bis(isonitrile) 3a both isonitriles point away from each
3
8
9
=
=
dpd
a
p
1R1
R2 Ph
c:
4
m
e
2
N
N
-
-
=
2
R1 p M O Ph R
H
1
:
e
4d
d p
R1
R1
:
x
4
d p
Chart 1. 2,9-diphenyl-1,10-phenanthroline (dpp, 4a);
2,9-dimethyl-1,10-phenanthroline (dmp, 4b); 2,9-di-
methyl-4,7-diphenyl-1,10-phenanthroline
(dpdmp,
4c); 2,9-di(p-anisyl)-1,10-phenanthroline (dap, 4d).
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ACS Catalysis
Single crystals suitable for X-ray analysis of 6b19 and 6c19
N1Cu1N2/
N1Cu1N2/
80.0
84.2
1
2
3
4
5
6
7
8
were obtained by vapor diffusion of diethyl ether into di-
chloromethane solution, which revealed a tetrahedral co-
ordination set up by the two different ligands around Cu(I)
with little distortion (Table 1). While the bond distances of
Cu to either the phenanthroline or the bisisonitrile ligands
are almost the same and in the expected range, the bite an-
gle of the bisisonitrile ligands in both copper complexes are
significantly larger (101° and 103°) than the bite angle of a
phenanthroline ligand (81-82° in agreement with other lit-
erature reports.11c,14,16) This should result in increased steric
interactions between both ligands upon planarization,
which corroborates our hypothesis that the lifetime of the
excited state in Cu(I) complexes can be increased by sub-
stituting one phenanthroline by one bisisonitrile ligand.
C15Cu1C20
C15Cu1C20
bond length [Å]
Cu1-N1
Cu1-N2
Cu1-C15
Cu1-C20
2.045
2.053
1.903
1.910
Cu1-N1
Cu1-N2
Cu1-C27
Cu1-C37
2.066
2.050
1.912
1.906
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The UV-Vis absorption spectrum of [Cu(dpp)(binc)]+ (6a)
(Figure 2) displays below 350 nm intense absorptions (e.g.
−
1
ε(299nm) = 3.9⋅104 mol dm3 cm ), which are assigned to π
→ π* transitions of the ligands. At longer wavelengths, a
−
1
much weaker tail (ε(450 nm) = 5⋅102 mol dm3 cm ) is ob-
served, which is attributed to metal-to-ligand charge-
transfer (MLCT) transitions involving an occupied 3d or-
bital of the copper ion and an empty π* orbital of the dpp
ligand (singlet-singlet d → π* transitions). This analysis is
in line with other Cu(I) complexes with phenathroline lig-
ands,11a,21 moreover, the MLCT character of the lowest sin-
glet excited state in [Cu(dpp)(binc)]+ (6a) is also supported
by the results of TD-DFT calculations (Figure 5).
D
M
C
+
+
ph
u
M
(
e
s son r e
bi
it il
)
enan ro ne
N
C
)
BF
]
i
(
th li
C
[
4
4
,
r
2 h
t
5
3
4
u
C
enan ro ne s son r e
th li
p bi i
h it il BF
]
)( )
[
(
4
6
Luminescence of [Cu(dpp)(binc)]+ (6a) was studied in
poly(methyl methacrylate) (PMMA). At ambient tempera-
ture the complex shows a broad unstructured emission
spectrum centered at λem = 560 nm, with a quantum yield
ΦPL of 3 %, accompanied by a decay time τ of 17 µs, which
is higher compared to [Cu(dap)2]+ (1) (τ = 560 ns in
PMMA)22 by a factor of about 30. Notably, 6a shows only
a very weak emission in dichloromethane, being most
likely a result of the greatly prolonged radiative lifetime
(570 µs in PMMA, vide infra) of 6a that make non-radiative
relaxation the dominating process in solution.23
R2
R1
Ph
N
N
N
O
P
O
P
N
C
C
N
N
C
C
O
O
u
C
u
C
Ph
Ph
N
R1
O
O
N
R2
Ph
+
a
=
+
*
nc
dpp b
i
]
)( )
a:
en
u
nc
e:
6
6
h
p
p
p
u
C
p
dpp
m
dpp b
i
C
C
[
[
u
]
)
)
)
(
)(
[
(
+
=
=
=
1
:
u
m
nc
p b
i
)(
e
n
h
6b
d
p
d
]
(
+
]
1
c:
6
m
n
i
)(
en
h
m
c
dpd p C dpd p b
[
(
[
+
1
:
u
C
n
c
e
n
h
a
a
6d
d p
d p bi
)(
]
)
(
Scheme 2. Preparation of the [Cu(phenanthroline)(bi-
sisonitrile)]+-complexes 6a-e.
Table 1. X-ray structures of [Cu(dmp)(binc)]+ (6b) and
[Cu(dpdmp)(binc)]+ (6c).
Figure 2. Electronic absorption and luminescence spectra of
[Cu(dpp)(binc)]BF4 (6a-BF4) at ambient temperature. Ab-
sorption spectrum was recorded in CH2Cl2 and emission was
measured in poly(methyl methacrylate) (PMMA) respectively.
The emission quantum yield ΦPL and lifetime τ (mean lifetime
resulting from a biexponential fit of the measured transient)
are 3 % and 17 µs, respectively.
bond angle [deg]
N1-Cu1-N2
81.9
N1-Cu1-N2
80.8
103.1
C15-Cu1-
C20
101.2
C27-Cu1-C37
Cu1-C15-N3
164.2
161.3
Cu1-C27-N3
Cu1-C37-N4
166.8
166.5
Cu1-C20-
N4
Electrochemical measurements reveal the reversible redox
behavior at 0.69 V (vs SCE) for the Cu(I)/Cu(II) couple in
dieder angle [deg]
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[Cu(dpp)(binc)]+ (6a). Considering this and the spectro-
[Cu(dpp)(binc)]BF4
(6a-BF4) (0.5)
4
5
455
–
7.5 / 20 88 / 89
scopic data,24 a reduction potential Cu(I)*/Cu(II) of –1.88 V
(vs SCE) could be appraised. Both values are higher com-
pared to [Cu(dap)2]+ (1) (0.62 and -1.43 V vs SCE),6 and es-
pecially the latter in combination with the long decay time
makes 6a promising for applications in photoredoxcataly-
sis.
1
2
3
4
5
6
7
8
[Cu(dpp)(binc)]BF4
(6a-BF4) (1.0)
20
n.r.
6
7
8
9
no catalyst
455
455
455
455
20
7.5
7.5
7.5
3
2
CuBF4 (1.0)
Having addressed the synthesis and characterization of the
complexes, as well as the spectroscopic and electrochemi-
cal properties of [Cu(dpp)(binc)]+ (6a), we started to inves-
tigate their catalytic activity for visible-light mediated pho-
toredoxcatalysis. As model system we choose the visible-
light induced atom transfer radical addition (ATRA) be-
tween N-boc-allylamine (7a) and diethyl-2-bromomalo-
nate (8a), for which we previously demonstrated that
[Cu(dap)2]+ (1) is a capable catalyst upon irradiation at 530
nm (Table 2, entry 1).7a Employing only half the catalyst
amount (0.5 mol%) of [Cu(dpp)(binc)]BF4 (6a-BF4) under
otherwise identical conditions gave 87% of the ATRA-
product 9 after 20 h, however, shortening the reaction time
resulted in a significant decrease of yield (entry 3). Further
optimization indicated that the reaction proceeds faster
upon irradiation at 455 nm (blue LED), which reduces the
reaction time to 7.5 h (entry 4), while also under these con-
ditions [Cu(dap)2]+ (1) gave inferior results (entry 2). It
should be noted that the absorption spectra of [Cu(dap)2]+
(1) and [Cu(dpp)(binc)]+ (6a) are very similar (see support-
ing information) with slightly larger extinction coefficients
found for 1 both at 455 and 530 nm. Thus, the higher activ-
ity of 6a with respect to 1 is not a reflection of a more effi-
cient absorption of light but rather of the greatly extended
life time of its excited state. No conversion in the presence
of 6a-BF4 was observed in the dark (entry 5), and further
control experiments omitting catalyst or ligands did not
lead to appreciable conversion (entries 6 and 7). As re-
ported for dap 4d7a, employing the dpp 4a alone promotes
dpp (4a) (0.5)
45
5
9
binc (3b) (0.5)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
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34
35
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
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[Cu(dmp)(binc)]BF4
(6b-BF4) (0.5)
10
11
455
455
455
7.5
7.5
7.5
13
12
[Cu(dpdmp)(binc)]BF
(6c-BF4) (0.5)
4
[Cu(dap)(binc)]BF4
(6d-BF4) (0.5)
12
78
88c
[Cu(dpp)(binc*)]BF4
(6e-BF4) (0.5)
13
455
7.5
a) Conditions: tert-butyl allylcarbamate (157 mg, 1.0 equiv., 1.0
mmol), diethyl 2-bromomalonate (0.34 ml, 2.0 equiv., 2.0
mmol), LiBr (174 mg, 2.0 equiv., 2.0 mmol), catalyst,
DMF:H2O (1:4) 1 ml, LED-Stick, freeze-pump-thaw (3x), rt;
n.r.= no reaction; b) see ref. 7a c) [α]ꢀ20 0 ° (c = 1 g/100 ml, CHCl3)
;
Subsequently, we applied the heteroleptic [Cu(phenan-
throline)(bisisonitrile)]BF4 complexes 6b-6e to this reac-
tion using the optimized conditions from entry 3. While
the yield was slightly reduced employing dap 4d as NN-
ligand (6d-BF4, entry 12), the crucial role of the aryl moiety
in 2,9-position of the phenanthroline can clearly be seen
by comparing the low yields of product that were obtained
with complexes 6b-BF4 and 6c-BF4 (entries 10 and 11). In
contrast, the exchange of binc 3b with its more flexible,
chiral version binc* 3a did not affect the catalytic perfor-
mance. No asymmetric induction was observed in this re-
action (entry 12), albeit 3a had been proven to be a capable
chiral promoter in iron complexes.17a
the reaction, but to
a
lesser extent than
[Cu(dpp)(binc)]BF4 (6a-BF4) (entry 8). The bisisonitrile
ligand 3b, however, shows no significant catalytic effect
(entry 9).
Table 2. Optimization and control experiment for the
visible-light mediated ATRA between boc-allylamine
(7a) and diethyl-bromomalonate (8a).a
,
ca a
s
r
t ly t LiB
LED
O
O
oc
B
oc
B
CO2Et
CO2Et
EtO
OEt
N
H
N
:
:
)
DMF H O 1 4
(
2
H
r
B
r
B
a
7
a
8
9
entry
catalyst (mol%)
λ
[nm]
time
[h]
yield
(%)
[Cu(dap)2]Cl
(1-Cl) (1.0)
a1b
2
530
24
75
[Cu(dap)2]Cl/BF4
(1-Cl or BF4) (0.5)
[Cu(dpp)(binc)]BF4
(6a-BF4) (0.5)
Figure 3. Kinetic study of the visible-light mediated ATRA be-
tween boc-allylamine (7a) and diethyl-bromomalonate (8a).
Conditions: tert-butyl allylcarbamate (157 mg, 1.0 equiv., 1.0
mmol), diethyl 2-bromomalonate (0.34 ml, 2.0 equiv., 2.0
mmol), LiBr (174 mg, 2.0 equiv., 2.0 mmol), catalyst (0.5
455
530
7.5
45 / 46
30 / 87
3
7.5 / 20
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ACS Catalysis
R1
X
+
*
mol%), DMF:H2O (1:4) 1 ml, LED-Stick (455 nm), freeze-
pump-thaw (3x), rt, 7.5 h.
u
nc
]
dpp b
i
C
[
( )(
)
1
2
3
4
5
6
7
8
- .
1 88 V
R1
X
u
The higher activity of the heteroleptic complex
[Cu(dpp)(binc)]BF4 (6a-BF4) compared to the homoleptic
copper [Cu(dap)2]Cl (1-Cl) was further confirmed by a ki-
netic study of the title reaction (Figure 3). For comparison,
we tested [Cu(dap)(DPEPhos)]BF4 (DPEPhos: 2,2'-ox-
ybis(2,1-phenylene)bis(diphenylphosphine) (10-BF4) for
this model reaction to quantify the effect of the bispho-
phine vs. the bisisonitrile substitution. Mixed ligand
[Cu(phenanthroline)(bisphosphine)]+-complexes have ex-
cited state lifetimes of several microseconds and promote
+
nc 2+
i
]
nm
C
[
dpp b
)(
LED 455
(
(
)
R
.
R
R1
0 69 V
+
u
nc
X
R
C
dpp b
i
( )(
[
]
)
R1
R1
R
X
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 3. Mechanistic concept of the Cu(I) catalyzed
ATRA under visible-light photocatalysis.
photocatalytic
reactions
effectively.12,13
Indeed,
[Cu(dap)(DPEPhos)]BF4 (10-BF4) is more active than
[Cu(dap)2]Cl (1-Cl), but did not reach the rate measured
for [Cu(dpp)(binc)]BF4 (6a-BF4).
The scope of ATRA reactions catalyzed by
[Cu(dpp)(binc)]BF4 (6a-BF4) was evaluated next. The ad-
dition of bromomalonate 8a to different olefins 7a-c pro-
ceeded smoothly with the exception of the sterically more
hindered diethyl-2-bromo-2-methylmalonate (8b) (Table
3, entries 1-4). Nitro substituted benzylhalides (8c-e)
cleanly reacted with styrenes 7d,e or silylenol ether 7f giv-
ing rise to the ATRA products 14-17 in repectable yields
(entries 5-8), although steric hindrance in the benzylhalide
has a detrimental effect (8d, entry 6). Attempts to utilize
4-cyano (8f) (entry 9) or 4-methylsulfonyl-benzyl bromide
(8g) (entry 10) resulted in no conversion of the starting ma-
terial. The higher reduction potential of the benzyl bro-
mides with less electron withdrawing groups (E1/2 = -0.95
V for 8c, -1.39 V for 8f and -1.43 V for 8g; vs SCE in MeCN)
apparently prevented the turnover with these substrates,
although based on the estimated reduction potential of 6a
(vide supra) we had assumed that this catalyst should be
able to activate these substrates.
Figure 4. [Cu(dap)(DPEPhos)]+ (10) in equilibrium with its
homoleptic representatives in solution.
The lower activity of 10-BF4 could be a consequence of the
known25 tendency of heteroleptic copper(I) complexes
combining biphosphine with sterically more demanding
phenanthroline ligands to form equilibria with their homo-
leptic representatives in solution that was also observed by
NMR for 10 (Figure 4). In contrast, NMR studies revealed a
low susceptibility of 6a to undergo ligand exchange. In
conclusion, [Cu(dpp)(binc)]BF4 (6a-BF4) was identified as
the most active catalyst, and especially outperforming the
up to now best copper catalyst for ATRA reactions
[Cu(dap)2]Cl (1-Cl) by a factor of at least two, which we
attribute to the increased lifetime as well as to the higher
reduction potential of its excited state.
While the utilization of allyltrimethylsilanes under visible-
light photoredox conditions was impressively shown in tri-
fluoromethylation reactions,27 its use in allylation reactions
of organohalides remains challenging. In our previous re-
port we could demonstrate the reaction of allyltributyltin
with organohalides under visible light photoredoxcata-
lyzed conditions, but our attempts using allyltrime-
thylsilane as ecologically more viable alternative, was met
with success only in one example.7a We questioned,
whether the new copper(I) catalysts could engage in this
barley investigated process.
The data obtained suggest a mechanistic picture for
[*Cu(dpp)(binc)]+ (6a) (Scheme 3) that is in agreement
with the previously reported mechanism for copper(I)-cat-
alyzed ATRA reactions under visible light irradiation.16,17
The excited catalyst species transfers an electron to the
ATRA reagent following the oxidative quenching cycle.
The generated radical adds to the alkene, forming an inter-
mediate which transfers its electron to the Cu(II) species
to regenerate the catalyst in a thermodynamically favored
process.26
Table 3. Visible-light induced ATRA reactions with
[Cu(dpp)(binc)]BF4 (6a-BF4) as photoredox catalyst
u
C
nc
,
dpp
b
i
BF
4
[
(
)(
)]
rea en
a
lk
ene
ro uc
p d
ATRA
t
g
t
r
LED455
t
.
-
7
8
7 5 24 h
OTMS
oc
B
s
T
HO
N
N
H
H
a
7
e
7
c
7
7
b
7d
7
f
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Page 6 of 10
ATRA rea-
gent
yield
(%)
entry
1
alkene
product
1
2
3
4
5
6
7
8
Table 4. Visible-light mediated allylation of organo-
halides with allyltrimethylsilan and
[Cu(dpp)(binc)]BF4 (6a-BF4) as photoredox catalyst.a
oc
B
CO2Et
CO2Et
N
H
r
B
7a
88
9
u
C
nc
dpp bi
)(
BF
[
(
)]
4
e
SiM
3
R
ro uc
p d
t
rea en
ATRA
g
t
O
O
,
r
s
T
LED455
t
CO2Et
CO2Et
N
=
=
e
:
:
a
M
CN
18
18b
R
R
H
8
EtO
OEt
H
r
B
e
M
2
3
7b
7c
68
80
r
B
11
a
8
9
ATRA
gent
rea-
Yield
[%]
HO
CO2E
t
Entry
Allylsilane
Product
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
r
B
CO2Et
12
1b
2c
3d
64
O
O
t
CO2E
O
O
oc
B
EtO
OEt
CO2Et
CO2Et
N
t
E O C 19
18a
n.r.
n.r.
2
r
EtO
OEt
B
H
r
B
4
7a
29
r
B
a
8
13
8b
r
B
O
O
r
B
CO2
Et
EtO
OEt
5
6
7
8
7d
7d
7e
7f
80
42
70
85
c
8
4
18a
18b
82
EtO C 20
O2N
14
r
2
B
NO2
8b
r
B
NO2
NO2
t
t
CO2E
r
B
e
M
E O2C
t
15
8d
O2N
NO2
NO2
NO2
21
CO2E
O
O
EtO
OEt
NO2
Cl
O
NO2
5
80
r
B
EtO2C
Cl
a
8
e
M
16
e
c
8
22
O2N
O2N
=
:
:
21 22 84 16
r
B
a)
Conditions: halide (0.5 mmol, 1.0 equiv.), allyltrimethyl-
8
17
silane (1.5 mmol, 3.0 equiv.), [Cu(dpp)(binc)]BF4 (6a-BF4) (1.0
mol%), MeCN (1.0 ml), blue LED (455 nm), freeze-pump-thaw
(3x), rt. b) 0.5 mol% [Cu(dpp)(binc)]BF4 (6a-BF4); c) dark reac-
tion; d) without catalyst.
r
B
r
B
9
n.r.
n.r.
R
7d
=
=
R
R
CN
SO2M
8f
8g
e
R
10
R1
X
+
]
Conditions: entries 1-4: alkene (1.0 mmol, 1.0 equiv.), ATRA re-
agent (2.0 mmol, 2.0 equiv.), LiBr (2.0 mmol, 2.0 equiv.),
[Cu(dpp)(binc)]BF4 (6a-BF4) (0.5 mol%), DMF/H2O mixture
(0.2 ml / 0.8 ml), blue LED (455 nm), freeze-pump-thaw (3x),
rt; entries 5-10: benzyl halide (1.0 mmol, 1.0 equiv.), alkene (5.0
mmol, 5.0 equiv.), Cu(dpp)(binc)BF4 (6a-BF4) (1.0 mol%),
MeCN (1.0 ml), blue LED (455 nm), freeze-pump-thaw (3x), rt.
*
u
nc
C
dpp bi
( )(
e
iM
S
[
)
3
- .
1 88 V
R1
X
+
u
nc 2+
i
]
nm
C
[
dpp b
( )(
LED 455
(
)
R1
e
iM
S
3
.
0 69 V
+
u
C
dpp b
( )(
nc
i
]
)
[
X
e
e
iM
S
3
X iM
S
3
Indeed, we found that 0.5 mol% [Cu(dpp)(binc)]BF4 (6a-
BF4), 3 equivalents of allyltrimethylsilane (18a) in MeCN
and irradiation with blue LED for 24 h furnishes the al-
lylated product 19 in 64% yield (Table 4, entry 1). The ne-
cessity of both, catalyst and irradiation with visible light
was shown by control experiments (entry 2-3). An even
higher yield could be obtained for the 2-methyl substituted
malonate 8b (entry 4). Furthermore, we successfully ap-
plied 2-butenyltrimethylsilane (18b) in this reaction (entry
5), which gave rise to a 84:16 mixture of the linear and the
branched isomer 21 and 22 in good yield.
Scheme 4. Proposed mechanism for the visible-light
mediated allylation of organohalides with
[Cu(dpp)(binc)]BF4 (6a-BF4) as photoredox catalyst.
To gain a deeper understanding of the radiative processes
of [Cu(dpp)(binc)]+ (6a), we determined the radiative rate
−
1
kr = ΦPL/τ ≈ 1.8⋅103 s from the measured values ΦPL = 3%
and τ = 17 µs (Figure 2), which corresponds to a radiative
lifetime of τr = 1/kr ≈ 570 µs. This large value points to a
forbidden character of the emission. The emission decay
profile recorded for 6a in PMMA (Supporting Information)
is distinctly non-monoexponential. In particular, signifi-
cant relaxation processes occurring in the timescale of a
few microseconds, e.g. with a time constant of 3.7 µs, dom-
Again, we assume an oxidative quenching cycle, in which
the copper catalyst acts as electron shuttle. After irradia-
tion, the excited Cu(I) catalyst 6a is transformed to its
Cu(II) species generating the reactive radical from the or-
ganohalide. Upon forming the product, a trimethylsilylrad-
ical is released which re-oxidizes the catalyst (Scheme 4).26
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ACS Catalysis
inate the decay behavior and lead to a substantial shorten-
ing of the mean excited state lifetime. The latter processes
1
2
3
4
5
6
7
8
can be assigned to bimolecular quenching processes such
as energy transfer and triplet-triplet annihilation, which
for extremely long-lived excited states can be effective even
at small concentrations. Therefore, the intrinsic radiative
rate kr for 6a is probably even smaller than the estimate of
CONCLUSION
In conclusion, we showed the synthesis and characteriza-
tion of a series of new, bench-stable heteroleptic copper-
(phenanthroline)(bisisonitrile)+-complexes. Referring its
applicability in visible light-mediated photoredox catalysis
we analyzed the spectroscopic and electrochemical prop-
erties of [Cu(dpp)(binc)]BF4 (6a-BF4). The correlation be-
tween its enhanced excited state lifetime compared to the
known [Cu(dap)2]Cl (1-Cl) and its catalytic activity could
be investigated by a kinetic study. The new complex
showed excellent activity in the visible light-mediated
atom transfer radical addition as well as in the so far
scarcely investigated allylation with trimethylallylsilane
under mild visible light conditions.
−
1
1.8⋅103 s (consequntly τr > 570 µs) based on the mean decay
time. Thus, the emitting state is assigned to the lowest tri-
plet state T1 and the emission of [Cu(dpp)(binc)]+ (6a) at
ambient temperature represents T1 → S0 phosphores-
cence. Such an assignment of the emitting state as a triplet
state T1 is, at first glance, surprising, since the ambient
temperature emission of numerous copper(I) complexes
with phenanthroline-type ligands was demonstrated to
represent thermally activated delayed fluorescence
(TADF).11a,21f,28 The latter process involves an emission from
the lowest excited singlet state S1, which is thermally pop-
ulated from the long-lived triplet state T1. Since the S1 →
S0 transition is spin allowed, the TADF decay times (radia-
tive) of such materials are usually in the order of few to
several microseconds, i.e. the TADF lifetimes are much
shorter than 570 µs determined for [Cu(dpp)(binc)]+ (6a).
This discrepancy can be rationalized by a relatively high
energy separation between the lowest triplet and singlet
excited states ∆E(S1-T1).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ASSOCIATED CONTENT
Experimental details. Compound characterization. Electro-
chemical measurements. X-ray data. NMR spectra. This mate-
rial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* E-Mail: oliver.reiser@chemie.uni-regensburg.de
Author Contributions
The manuscript was written through contributions of all au-
thors. All authors have given approval to the final version of
the manuscript.
Funding Sources
The authors thank the DFG (GRK 1626, Photocatalysis) and
the German Ministry of Education and Research (BMBF) for
financial support.
ACKNOWLEDGMENT
We thank Sabine Stempfhuber and Michael Bodensteiner,
Universität Regensburg, for carrying out the X-ray crystal
structure analysis of 4b, 6b and 6c.
Figure 5. Energy level diagram of the lowest energy excited
states of [Cu(dpp)(binc)]+ (6a) resulting from the TD-DFT cal-
culations and natural transition orbitals for the lowest excited
singlet (S1) and triplet (T1) states. Results obtained for the
ground-state molecular geometry.
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10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
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