Heteroleptic Copper(I)-Based Complexes for Photocatalysis: Combinatorial Assembly, Discovery, and Optimization

Article abstract of DOI:10.1002/anie.201800144

A library of 50 copper-based complexes derived from bisphosphines and diamines was prepared and evaluated in three mechanistically distinct photocatalytic reactions. In all cases, a copper-based catalyst was identified to afford high yields, where new heteroleptic complexes derived from the bisphosphine BINAP displayed high efficiency across all reaction types. Importantly, the evaluation of the library of copper complexes revealed that even when photophysical data is available, it is not always possible to predict which catalyst structure will be efficient or inefficient in a given process, emphasizing the advantages for catalyst structures with high modularity and structural variability.

Full text of DOI:10.1002/anie.201800144

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Accepted Article
Title: Heteroleptic Copper(I)-Based Complexes for Photocatalysis:
Combinatorial Assembly, Discovery, and Optimization
Authors: Shawn Collins, Clémentine Minozzi, Antoine Caron, Jeffrey
Santandrea, and Jean-Christophe Grenier-Petel
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800144
Angew. Chem. 10.1002/ange.201800144
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10.1002/anie.201800144
Angewandte Chemie International Edition
COMMUNICATION
Heteroleptic Copper(I)-Based Complexes for Photocatalysis:
Combinatorial Assembly, Discovery, and Optimization.
Clémentine Minozzi, Antoine Caron, Jean-Christophe Grenier-Petel, Jeffrey Santandrea and Shawn K.
Collins*[a]
Dedication ((optional))
Abstract: A library of 50 copper-based complexes derived from
bisphosphines and diamines was prepared and evaluated in three
mechanistically distinct photocatalytic reactions. In all cases, a
copper-based catalyst was identified to afford high yields, where new
heteroleptic complexes derived from the bisphosphine BINAP
displayed high efficiency across all reaction types. Importantly, the
evaluation of the library of copper-complexes revealed that even
when photophysical data is available, it is not always possible to
predict which catalyst structure will be efficient or inefficient in a
given process, emphasizing the advantages for catalyst structures
with high modularity and structural variability.
becomes highly advantageous, especially if a class of catalysts
are effective in three mechanistically distinct processes. Herein,
the rapid and facile combinatorial assembly, evaluation and
discovery of Cu(I)-complexes for photocatalysis in SET, ET and
PCET-based transformations is described.
+2
+1
+3
+1
N
Ru
N
N
Cr
N
C
Ir
N
N
N
N
N
N
N
N
N
N
N
N
N
C
N
N
Cu
Cu
C/N
This Work: Heteroleptic Copper-Based Photocatalysts
+1
Underexplored class of copper-based photocatalysts
Facile synthesis
Tunable redox properties and excited-state lifetimes
Applicable to photoredox, energy transfer and proton-
coupled electron transfer reactions
P
P
N
N
Molecular synthesis through the use of light has experienced a
renaissance in the past decade.[1] In concert, photocatalysis has
become increasingly important as a synthetic tool as it offers the
possibility to employ a wide variety of wavelengths and the
option to exploit different mechanistic pathways. Photocatalysts
can be used to promote single electron transfer (SET), to
engage in energy transfer (ET), or to participate in proton-
coupled electron transfer (PCET) reactions.[2] While the utility of
Ru- and Ir-based polypyridyl-type complexes as sensitizers has
been well demonstrated (Figure 1),[ 3 ] the development of
alternatives based upon more abundant metals has also
Combinatorial Assembly:
[Cu(MeCN)4]BF4
Diamine
Ligand
1. Bisphosphine
2. Diamine
Ligand
Ligand
THF or CH2Cl2
1 h, rt
THF or CH2Cl2 THF or CH2Cl2
1 h, rt 1 h, rt
+1
-BF4
+1
-BF4
P
N
N
N
N
N
Cu
Cu
N
P
Heteroleptic
Cu(I)(diamine)(bisphosphine)BF
Homoleptic
Cu(I)(diamine)2BF4
4
R1
R1
Tol
Diamine Ligands:
Bisphosphine Ligands:
N
N
R3
R3
4
]
emerged.[
Despite the rich history of copper-based
N
Ph2P
Fe
N
5
]
R2
R2
N
N
pytri
O
PPh2
photochemistry,[
synthetic applications of copper-based
Ph2P
R1 = OMe
R1 = t-Bu
Tol
N
N
PPh2
dmbp
dtbbp
R1
R1
N
N
dppf
photocatalysis have only begun to be revisited[ 6 , 7 ] While
homoleptic polypyridyl-type complexes of copper can be used as
photocatalysts for photoredox-type reactions,[ 8 ] only recently
have heteroleptic Cu(I) complexes been explored. [9,10] Despite
the potential of heteroleptic Cu(I)-based photocatalysts, they
remain underexplored in comparison to other organometallic
complexes who may appear more familiar to synthetic chemists.
However, familiarity does not always lead to successful
development of a photocatalytic reaction. Even when armed with
a suitable background of photophysical data, it is difficult to
predict a priori which photocatalyst will be optimal for a given
transformation. In addition to redox potentials and excited state
lifetimes, stability under the reaction conditions,[11] or desired
solubility profile can still be critical.[12 ] As such, the ability to
prepare catalysts in an efficient and straightforward manner
DPEPhos
N
N
R1 = R2 = R3 = H phen
quintri
R
1 = Me, R2 = R3 = H dmp
Ph2P
Ph2P
R1 = H, R2 = R3 = Me tmp
N
O
PPh2
N
Tol
dq
R
1 = R2 = H, R3 = Ph
batho
PPh2
N
XantPhos
N
N
N
BINAP
iquintri
Figure 1. Metal-based complexes for photocatalysis (top) and. combinatorial
assembly of Cu(I)-based photocatalysts (bottom).
An advantage of heteroleptic diamine/bisphosphine Cu(I)-based
13
]
photocatalysts is their facile synthesis and isolation.[
Sequential addition of bisphosphine and diamine to
Cu(MeCN)4BF4 is followed by isolation by precipitation (Figure 1).
To demonstrate the ability to rapidly generate a library of
photocatalysts for evaluation, different diamine and
bisphosphines were selected. The bisphosphines were chosen
to vary the nature of the chromophore and bite angle to
influence the photophysical properties and catalytic activity.[14]
The selection of diamines included bipyridines, phenanthrolines
as well as three triazole-based ligands known to stabilize the
LUMOs of photoactive complexes.[ 15 ] For comparison, the
corresponding homoleptic complexes were also prepared. In
summary, 50 different catalysts were prepared on gram scale as
crystalline solids. Several homoleptic complexes were found to
[a]
Clémentine Minozzi, Antoine Caron, Jean-Christophe Grenier-Petel,
Jeffrey Santandrea, and Prof. Dr. Shawn K. Collins
Department of Chemistry and Centre for Green Chemistry and
Catalysis
Université de Montréal
CP 6128 Station Downtown, Montréal, Québec, Canada H3C 3J7
E-mail: shawn.collins@umontreal.ca
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201800144
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COMMUNICATION
rapidly oxidize and/or have low solubility in common organic
solvents and were not included in the subsequent
characterization and evaluation. The UV-vis absorption
characteristics of the photocatalysts tend to vary very little with
respect to the bisphosphine.[ 16 ] In contrast, change of the
diamine ligand can produce significant changes in the UV-vis
spectra. None of the complexes explored with the four different
involved energy transfer from Ru(dtbbp)3(PF6)2 to promote
decomposition of azide 6 to the pyrrole 7 in excellent yield (99%)
in 3 h.
bisphosphine
ligands
exhibited
any disproportionation
behavior.[17] The library of heteroleptic copper complexes was
then applied in three mechanistically distinct photocatalytic
transformations. As photoredox processes have had a profound
impact on molecular synthesis, a representative photoredox
process was first selected. The visible-light decarboxylative
fragmentation of N-(acyloxy)phthalimides was targeted, as
several groups[ 18 ] have demonstrated the utility of the N-
(acyloxy)phthalimides in photochemical carbon-carbon bond
forming processes. Chen and co-workers had recently disclosed
a reductive decarboxylative Csp3–Csp bond coupling reaction to
construct substituted alkynes. The process was optimal when
employing N-(acyloxy)phthalimides with Ru(bpy)3(PF6)2 (1
mol %) as photocatalyst and a sulfonyl alkyne. They also
reported that simpler bromoalkyne coupling partners were not as
effective (1+2→3, 28% yield).[ 19 ] Consequently, the Csp3–Csp
bond coupling was investigated with the library of copper
heteroleptic complexes using N-(acyloxy)phthalimide
1 and
bromoalkyne 2 as coupling partners (Figure 2). Two controls
were performed in the absence of any catalyst at either 394 or
450 nm and no desired coupling product 3 was observed. From
the results, the heteroleptic complexes formed from the dq
ligand were effective for the Csp3–Csp coupling; the
Cu(dq)(BINAP)BF4 complex provided the highest yield of alkyne
3 (87 %, 0 % in absence of light).[20,21] The second photocatalytic
process investigated was a proton-coupled electron transfer
(PCET) reaction. PCETs are regarded as non-classical redox
processes in which a proton and electron are exchanged in a
concerted manner and have only recently been demonstrated as
a viable mode of activation in organic synthesis. As a model
transformation, the homolytic activation of ketones to generate
neutral ketyl radicals was selected (Figure 2). Knowles and co-
workers demonstrated that reductive coupling of ketone 4 and
subsequent intramolecular conjugate addition afforded the
bicycle 5 in 78 % using Ru(bpy)3(BArF)2 as catalyst.[22] When
ketone 4 was treated with Ru(bpy)3(PF6)2 under the blue LED
irradiation experimental set-up and conditions used for the
copper complexes, the desired bicycle 5 was obtained in 69%
yield. When the transformation was performed in the absence of
catalyst, a 40 % yield of bicycle 5 was observed under 394 nm
irradiation, however no conversion to the desired bicycle 9 was
observed at 450 nm. As such, all PCET reactions were
performed with blue LEDs. In examining the results from the
screening
of
the
Cu-based
photocatalysts,
the
Cu(dq)(BINAP)BF4 complex identified from the examination of
the photoredox process was again one of the most active
complexes (71
photocatalysts
%
of 5).
A
second grouping of active
ligands, with
Figure 2. Evaluation of the library of copper-based photocatalysts in
photoredox processes (top); in proton-coupled electron transfer process:
homolytic activation of ketones (middle) and. in an energy transfer process:
the visible-light sensitization of vinyl azides (bottom). Reactions irradiated with
394 nm light (charcoal) or 450 nm (light grey). Front entries without an
indicated phosphine ligand pertain to homoleptic Cu(diamine)2BF4 complexes.
bear triazole-based
Cu(quintri)(Xantphos)BF4 proving to be optimal (79% of 5).
Finally, the library of heteroleptic copper complexes was
evaluated in an energy transfer process.[ 23 ] A visible light
sensitization of vinyl azides recently described by Yoon[ 24 ]
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In investigating the synthesis of pyrroles (6→7) using the library
of copper-based photocatalysts, controls revealed that in the
absence of catalyst at 394 nm, complete conversion was
observed (99% of 7, Figure 2). Even at 450 nm, some pyrrole
was observed (19%), so all further reactions were performed at
450 nm. From the results, the heteroleptic complexes formed
from the bisphosphine Xantphos ligand tend to display the
highest yields regardless of amine ligand. Many of the catalysts
were highly effective, six catalysts provided the desired pyrrole 7
in >95% yield, while the complexes Cu(dq)(DPEPhos)BF4,
Cu(dmbp)(Xantphos)BF4 and Cu(dmp)(BINAP)BF4 all provided
the pyrrole 7 quantitatively.
having the longest excited state lifetime clearly afforded the
highest yield.[26]
Following the survey of the library of copper photocatalysts
in photocatalytic processes (Figure 2), absorption/emission data,
excited state lifetime and cyclic voltammetry experiments were
conducted for a series of photocatalysts: five complexes of the
type Cu(dq)(bisphosphine)BF4, and ten complexes of the type
Cu(diamine)(BINAP)BF4.[16] When examining the photoredox
processes (1+2→3), the yield obtained for each of the
photocatalysts versus their calculated excited state redox
potentials and excited state lifetimes was plotted in an effort to
discern any trends (Figure 3). When analyzing the influence of
the bisphosphine, the excited state redox potentials were shown
to vary between approximately -1.3 and -1.9 V (vs. SCE). The
Cu-based complexes with Xantphos and BINAP bisphosphines
possessed the highest redox potentials, however it was the dppf
and BINAP catalysts which afforded the highest yields for the
process (1→3). The reduction potentials for certain N-
hydroxyphthalimides are low enough (−1.28→−1.37
V vs.
SCE)[25] that most catalysts surveyed should be able to promote
the decarboxylation event. When examining the influence of the
diamine ligand, again no pattern could be readily identified
between the redox potential and the isolated yield in the
photoredox transformation. In addition, catalyst types having
longer excited state lifetimes did not necessarily afford greater
yields of product. In contrast to the photoredox process, the
PCET transformation surveyed (4→5, Figure 3) did demonstrate
trends between the yields and redox potential of the
photocatalyst, although again no trend with the excited state
lifetime was observed. The Cu(dq)(Xantphos)BF4 and
Cu(dq)(BINAP)BF4 complexes had the highest redox potentials
and best yields amongst the different bisphosphine complexes
surveyed. Knowles and co-workers had shown that a Ru-based
intermediate (RuI(bpy)3 (E1/2ox = −1.33 V vs. SCE))[22] possessed
a redox potential high enough to promote the PCET process.
Almost all of the catalysts surveyed possess a similar redox
potential capable of promoting the PCET process, but certain
classes of catalyst structures were clearly more efficient than
others, with the origins of the reactivity still not evident. When
examining the energy transfer reaction (6→7, Figure 3) the
triplet state energy of the corresponding Cu-based complex was
highest with the BINAP-based catalyst. The dienyl azide 6 could
be estimated to have a triplet energy of approximately 1.9 eV,[24]
which would imply that each catalyst should be able to promote
the transformation to pyrrole. However, it was the excited state
lifetimes which showed a trend with the observed yields: in the
Cu(diamine)(BINAP)BF4 series, the ET values remained
constant for all catalysts, however the dmp-containing complex
Figure 3. Comparison of yields obtained in using copper-based photocatalysts
versus relevant photophysical properties:
fragmentation of N-(acyloxy)phthalimides (top),
visible-light decarboxylative
homolytic activation of
a
ketones (middle), and visible-light sensitization of vinyl azides (bottom). Yields
are indicated as bars and are color-coded for reactions irradiated with 394 nm
(charcoal) and with 450 nm (grey). Redox potentials and triplet state energies
are indicated as discs (black).
Similarly, in the Cu(dq)(bisphosphine)BF4 series, the complex
having DPEPhos or XantPhos ligands had the highest excited
state lifetimes and highest overall yields of product.
In summary, Cu(diamine)(bisphosphine)BF4 complexes
possess characteristics that can significantly impact the growing
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10.1002/anie.201800144
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COMMUNICATION
area of photocatalysis. Their facile synthesis and modularity is
ideally suited to combinatorial synthesis and screening in
different photocatalytic processes. Preparation of a 50-complex
library was accomplished and evaluated in three mechanistically
distinct photocatalytic reactions. In each case, a copper-based
catalyst could be identified to provide ≥80 % isolated yield of the
desired product. The screening process identified new catalyst
structures based upon BINAP bisphosphines (for photoredox
and energy transfer processes) and triazole-based diamines (for
PCET processes). BINAP-based complexes were generally
efficient across all reaction types, and possessed higher redox
potentials and triplet energies (for photoredox and PCET:
Cu(dq)(BINAP)BF4, abs 472 nm, em 521 nm, Eo = -1.87 eV, ET
= 2.38 eV, =4 ns), although judicious choice of diamine was
necessary to extend excited state lifetimes for energy transfer
processes (for energy transfer Cu(dmp)(BINAP)BF4, abs 387
nm, em 445 nm, Eo = -2.04 eV, ET = 2.38 eV, =2188 ns). The
studies also demonstrate the first utility of copper-based
photocatalysts in synthetic PCET and energy transfer processes.
Efforts to rationalize catalyst efficiency through available
photophysical parameters could explain trends seen in a certain
transformation, but would not have been able to predict catalyst
behavior in another. Importantly, the above study represents a
rare evaluation of photocatalyst structure vs. activity in
photocatalytic processes, that is surprisingly absent from the
literature for other more well-established photocatalyst types.
The facile synthesis of heteroleptic Cu(NN)(PP)X, complexes
and the ability to vary both physical and photophysical
characteristics, should encourage further exploration and new
applications of the catalyst-class in photocatalysis.
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[16] See Supporting Information for full data on absorption/emission spectra,
tabular data for reaction yields and excited state lifetimes.
[17] 1H NMR analysis of various catalysts over time did not show any ligand
exchange processes.
Acknowledgements
The authors acknowledge the Natural Sciences and Engineering
Research Council of Canada (NSERC), the NSERC CREATE in
Continuous Flow Science, Université de Montréal, the Centre for
Green Chemistry and Catalysis (CGCC) for generous funding.
Prof. D. Rochefort and Dr. B. Gélinas are thanked for access
and helpful discussions regarding cyclic voltammetry. Profs. M.
Lafleur and W. Skene are thanked for helpful discussions.
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[20] Although the homoleptic copper-based sensitizers were ineffective,
Cu(dap)2Cl can promote the reaction: G. Kachkovskyi, C. Faderland, O.
Reiser, Adv. Synth. Catal. 2013, 355, 2240-2248.
Keywords: photocatalysis • copper complexes • photoredox •
proton-coupled electron transfer • energy transfer
[21] Inefficient catalysts do not afford higher yields at longer reaction times.
See Supporting Information for details.
[22] K. T. Tarantino, P. Liu, R. R. Knowles, J. Am. Chem. Soc. 2013, 135,
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[1]
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Rev. 2013, 113, 5322-5363. ET: D. M. Arias-Rotondo, J. K. McCusker,
Chem. Soc. Rev. 2016, 45, 5803–5820. PCET: E. C. Gentry, R. R.
Knowles, Acc. Chem. Res. 2016, 49, 1546-1556
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[24] E. P. Farney, T. P. Yoon, Angew. Chem., Int. Ed. 2014, 53, 793 –797;
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[26] Catalysts with limited absorption profiles did not afford higher yields
when light exposure was increased. See Supporting Information. For a
related example see: C. Le, M. K. Wismer, Z.-C. Shi, R. Zhang, D. V.
Conway, G. Li, P. Vachal, I. W. Davies, D. W. C. MacMillan,ACS Cent.
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Entry for the Table of Contents (Please choose one layout)
Bisphosphine
Photoredox
Ligand
Cu(MeCN)4BF4
PCET
Diamine
Ligand
Energy Transfer
Combinatorial
Assembly
Screening/
Evaluation
Catalyst Discovery
& Applications
COMMUNICATION
Clémentine Minozzi, Antoine Caron,
Jean-Christophe Grenier-Petel, Jeffrey
Santandrea and Shawn K. Collins*
Page No. – Page No.
Heteroleptic Copper(I)-Based
Complexes for Photocatalysis:
Combinatorial Assembly, Discovery,
and Optimization.
A library of 50 copper-based complexes derived from bisphosphines and diamines
was prepared and evaluated in three mechanistically distinct photocatalytic
reactions. In all cases, a copper-based catalyst was identified to afford high
yields/reactivities surpassing other transition-metal based photocatalysts,
emphasizing the advantages of using catalyst structures with high degrees of
modularity and structural variability.
This article is protected by copyright. All rights reserved.