Evaluating heteroleptic copper(I)-based complexes bearing π -extended diimines in different photocatalytic processes

Abstract

A series of 12 new copper-based photocatalysts of the type Cu(N^N)(P^P)BF 4 were synthesized bearing π-extended diimine ligands. The complexes have red shifted absorptions and larger extinction coefficients than complexes prepared with a parent diimine, dmp. The complexes were evaluated for their ability to promote three different photochemical transformations. Although the complexes were inactive in a reductive PCET process, the complexes afforded good yields in both SET and ET processes. Interestingly, homoleptic copper-complexes derived from the π-extended diimines were significantly more active in SET processes than analogous complexes with simpler diimines.

Full text

ARTICLE
Evaluating heteroleptic copper(I)-based complexes bearing
␲
-extended diimines in different photocatalytic processes
Johann Sosoe, Corentin Cruché, Émilie Morin, and Shawn K. Collins
Abstract: A series of 12 new copper-based photocatalysts of the type Cu(N^N)(P^P)BF
4
were synthesized bearing
␲
-extended
diimine ligands. The complexes have red shifted absorptions and larger extinction coefficients than complexes prepared with a
parent diimine, dmp. The complexes were evaluated for their ability to promote three different photochemical transformations.
Although the complexes were inactive in a reductive PCET process, the complexes afforded good yields in both SET and ET
processes. Interestingly, homoleptic copper-complexes derived from the
␲
-extended diimines were significantly more active in
SET processes than analogous complexes with simpler diimines.
Key words: copper, photocatalysis, photochemistry, Appel reaction, energy transfer.
Résumé : Nous avons synthétisé une série de 12 nouveaux photocatalyseurs à base de cuivre de type Cu(N^N)(P^P)BF
4
portant des
ligands diimine à système
␲
étendu. Ces complexes présentent un déplacement bathochrome dans leur spectre d’absorption et
des coefficients d’extinction plus élevés comparativement aux complexes préparés avec une diimine simple analogue, le ligand
dmp. Nous avons évalué la capacité de ces complexes à catalyser trois différentes transformations photochimiques. Si ces
complexes ont été inactifs lorsque soumis à une réduction par transfert couplé d’électron et de proton (PCET), ils ont tout de
même produit de bons rendements par les processus de transfert monoélectronique (SET) et de transfert d’énergie (ET). Il est
intéressant de noter que les complexes de cuivre homoleptiques dérivés de diimines à système
␲
étendu ont été nettement plus
actifs dans les processus SET que les complexes analogues dérivés de diimines plus simples. [Traduit par la Rédaction]
Mots-clés : cuivre, photocatalyse, photochimie, réaction d’Appel, transfert d’énergie.
Introduction
Photocatalysis using precious metal complexes has had a signifi-
cant impact in photochemical synthesis.
1–3
As issues of sustainability
become increasingly important,
4,5
research into photoactive base
metal complexes has widened, involving a diverse range of elements
demonstrating reactivity in a wide range of chemical transforma-
tions.
6
Copper-based complexes have a rich history in UV-based pho-
tochemistry,
7,8
with recent investigations involving their use in solar
energy sciences,
9
photocatalytic water-splitting,
10
and organic light
emitting diodes.
11
Synthetic photocatalysis has also made use of
copper-based complexes
12
when using irradiation in the UV
13–15
and
visible ranges.
16–18
The use of homoleptic polypyridyl-type com-
plexes of copper in particular have re-emerged
19
as photocatalysts of
interest for photoredox-type reactions.
20
Heteroleptic copper-based complexes of the type Cu(N^N)(P^P)X
were first reported by McMillin and co-workers
21
and highlighted
for their long excited state lifetimes when bearing the bisphos-
phine (P^P) DPEPhos. Our group first applied these catalysts to
synthetic photocatalysis for the synthesis of helical hydrocarbons
and heterocycle synthesis.
22
Although the catalysts have now
been explored by many more groups,
23,24
our group has focused
on trying to elucidate important structure–activity relationships
that optimize the catalysts for applications in single electron
transfer (SET),
25,26
energy transfer (ET),
27
and proton-coupled elec-
tron transfer (PCET) reactions.
28
Our group previously reported a
library of 50 copper complexes that were evaluated in three dif-
ferent bond-forming transformations. The library consisted of
four different bisphosphine ligands (P^P) and a series of diimines
(N^N), which were mainly phenanthroline-type ligands with dif-
ferent substituents at the 2-, 3-, and 4-positions (Fig. 1).
29
One
aspect of diimine structure that was not examined was the impact
of the
␲
-surface on photocatalysis. The study of extended aro-
matic ligands on photoactive ruthenium(II) polypyridyl com-
plexes has already been of interest. Representative complexes
have been studied for their cytotoxic activity involving modi-
fication of cell membrane function and DNA intercalation.
30
The effects of
␲
-extended diimines on heteroleptic copper(I)-
photosensitizers has also been studied with regards to proton
reduction experiments. Using Xantphos as the bisphosphine, the
investigation of photophysical properties of the four complexes
revealed large shifts in the cyclic voltamperograms and reduction
potential but relatively unchanged absorption spectra and weak
emission spectra.
31
However, a comparative assessment of the
impact of the
␲
-extended ligands on synthetic transformations
has not appeared. Our previous studies have shown that even with
suitable photophysical data, it is difficult to predict a priori which
photocatalyst will be optimal, or not, for a given transformation.
The efficiency of a synthetic process can be influenced by factors
such as photostability
32
and resisting attack from reactive radicals
in solution upon the catalyst framework.
33
Herein, we describe the
synthesis of 12 copper-based complexes of the type Cu(N^N)(P^P)BF
4
and evaluation in three photochemical processes. The photo-
Received 9 January 2020. Accepted 21 February 2020.
J. Sosoe, C. Cruché, É. Morin, and S.K. Collins. Département de Chimie, Centre for Green Chemistry and Catalysis, Université de Montréal, Montréal,
QC H3C 3J7, Canada.
Corresponding author: Shawn Collins (email: shawn.collins@umontreal.ca).
This paper is part of a special issue celebrating the 100th anniversary of the Department of Chemistry at the Université de Montréal.
Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.
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physical properties and yields are compared with a parent com-
plex, Cu(dmp)(P^P)BF
4
.
Results and discussion
Synthesis of a series of heteroleptic Cu(I)-based photocatalysts
using the known ligands
30,34
ddpq, ddppz, and dbdppz were at-
tempted with the following four bisphosphines: DPEPhos, Xantphos,
dppf, and BINAP. Commercially available Cu(MeCN)
4
BF
4
is placed
in solution and the corresponding bisphosphine ligand is added.
Following subsequent diimine addition into the same reaction
vessel, the desired heteroleptic complex is typically isolated by
precipitation (Table 1;Fig. 2). Although DPEPhos, Xantphos, and dppf
formed good yields of the desired complexes, the bisphosphine
BINAP was problematic. Analysis of the precipitates by
1
H NMR
and mass spectrometry showed that mixtures of the correspond-
ing heteroleptic and homoleptic complexes were formed. As such,
complexes derived from BINAP were excluded from the present
study. In addition, synthesis of the corresponding homoleptic
complexes was also performed. When examining the photophysi-
cal data, the UV–vis absorption characteristics of the photocata-
lysts tend to vary very little with respect to either the diimine or
bisphosphine, with absorption maxima all within the 380–
410 nm range, with the exception of the homoleptic complexes
that are farther red shifted. Extinction coefficients tend to grow
with increasing size of the diimine ligand, although differences
do occur with respect to the bisphosphine. Similar trends are
observed with the excited state lifetimes, although again the dif-
ferences vary only by a factor of 10 and the lifetimes remain in the
3–80 ns range. However, interestingly the
␲
-extended ligands sig-
nificantly decrease the excited state lifetime with regards to
the parent complex with the dmp ligand Cu(dmp)(DPEPhos)BF
4
␶
= 14 300 ns vs. Cu(ddpq)(DPEPhos)BF
4
␶
= 5 ns. It was similarly
difficult to discern general trends in excited state reduction po-
tentials. For example, when complexes bore the Xantphos or dppf
bisphosphine ligands, the potentials tended to decrease with in-
creasing
␲
-extension (Cu(N^N)(Xantphos)BF
4
: ddpq = –1.72 V;
ddppz = –0.82 V; dbdppz = –1.29 V); however, with the DPEPhos,
which bears a striking resemblance to Xantphos, the trend is
reversed and the excited state reduction potential increases with
␲
-extension of the diimine ligand (Cu(N^N)(DPEPhos)BF
4
: ddpq =
–1.26 V; ddppz = –1.12 V; dbdppz = –1.82 V).
With the library of copper-based complexes in hand, their eval-
uation in three mechanistically distinct photocatalytic transfor-
mations was pursued. Given the interesting changes in reduction
potential with regards to diimine and bisphosphines, evaluation
of the catalyst in a SET process was pursued first. The catalysts
were investigated in a visible-light mediated conversion of an
alcohol to a halide via a catalytic Appel-type reaction (Fig. 3;
Table 2).
35
Stephenson and co-workers had previously reported
the use of Ru(bpy)
3
Cl
2
to transform alcohols to halides, and our
group also investigated copper-based complexes for the transfor-
mation in continuous flow.
36
The 12 copper-based catalysts with
␲
-extended ligands were evaluated and compared with the parent
catalysts having the dmp diimine.
Control reactions performed in the absence of light or in the
absence of catalyst at 450 nm revealed no conversion to the alkyl
bromide 2, whereas most of the catalysts provide modest to good
yields of 2under irradiation. In general, the DPEPhos and Xantphos
ligands were superior to the dppf bisphosphine ligand. However, the
homoleptic complex Cu(dmp)
2
BF
4
was essentially inactive, increas-
ing the
␲
-extension provided homoleptic complexes that afforded
good yields of product.
37
The results would have been difficult to
rationalize when examining the excited state reduction potentials
and excited state lifetimes (Cu(dmp)
2
BF
4
,E= –1.12 V; Cu(ddpq)
2
BF
4
,
E= –1.36 V; Cu(ddppz)
2
BF
4
,E= –0.90 V; Cu(dbdppz)
2
BF
4
,E=
–1.34 V). All of the above catalysts possess excited state reduction
potentials that should promote SET to CBr
4
(E
½
= 0.30 V vs. SCE).
38
Apart from the homoleptic complex Cu(dbdppz)
2
BF
4
(78%, 2), the
best heteroleptic complexes were Cu(dmp)(Xantphos)BF
4
(78%, 2)
and Cu(dbdppz)(DPEPhos)BF
4
(72%, 2).
The catalysts were then evaluated in a reductive PCET reaction.
Such redox processes have surged in interest amongst synthetic
chemists for their ability to decrease the energy barrier to activat-
ing strong bonds. Our group has previously used the homolytic
activation of ketones to evaluate copper-based catalysts for PCET
processes (Fig. 4;Table 3).
39
Control reactions for the PCET process
(3¡4) showed no reaction in the absence of light and or catalyst.
In examining the results from the screening in the PCET process,
the Cu-based complexes bearing
␲
-extended ligands were very
poor catalysts, with only two catalysts (Cu(ddpq)(Xantphos)BF
4
and Cu(dbdppz)(DPEPhos)BF
4
) providing appreciable yields of bi-
cycle 4. Knowles and co-workers had previously shown that a
Ru-based intermediate (Ru
I
(bpy)
3
;E
1/2ox
= –1.35 V vs. SCE)
40
pos-
sessed a redox potential high enough to promote the PCET pro-
cess. Almost all of the catalysts bearing
␲
-extended ligands
surveyed possess an excited state redox potential capable of pro-
moting the PCET process but do not afford useful yields of the
bicyclic product 4. The inability of the
␲
-extended ligand com-
plexes to promote the PCET process could be due to the quenching
mechanism of the photochemical transformation. Recent work
with Cu-based photocatalysts has shown that a combination of
particular diimine and suitable electron donor (such as Hantzsch
esters) can allow Cu-based complexes to undergo reductive
quenching from their excited state.
41,42
If single electron reduc-
tion is unfavorable with the extended diimines, it would explain
why the PCET processes are challenging. Lastly, the new family of
Cu-based photocatalysts were evaluated in a visible light sensiti-
zation of vinyl azides as a representative ET process (Fig. 5;
Table 4).
43
Both Ru- and Cu-based catalysts have been shown to
promote the decomposition of the azide to corresponding azirine
and eventual pyrrole. In examining the conversion of 5¡6with
the
␲
-extended copper-based photocatalysts, controls showed a
slight background reaction in the absence of any catalyst at
450 nm (19% of 6).
From the results, all complexes provide good to excellent yields
of the desired pyrrole 6. The heteroleptic complexes formed from
the bisphosphine ligand Xantphos tend to display the highest
yields regardless of diimine ligand, with Cu(ddppz)(Xantphos)BF
4
providing quantitative yields of 6. The homoleptic complex
Cu(dmp)
2
BF
4
also provided a quantitative yield of the pyrrole 6.
Previous analyses of the ET process showed that if the catalysts
possess a suitable triplet state energy (E
T
), the length of the excited
state lifetime becomes the most important parameter in improv-
ing ET processes. The excited state lifetimes of copper(I) com-
plexes are lengthened through substitution on the ligands that
enforce a tetrahedral geometry about the Cu center.
20
Wide bite-
angle phosphines such as Xantphos complexes are known to ex-
tend excited state lifetimes,
20
so it is not surprising to see the
catalyst Cu(dbdppz)(Xantphos)BF
4
with the best yield. All Xant-
phos catalysts possess a triplet energy that is sufficient to promote
Fig. 1. Copper-based complexes for photocatalysis. [Colour online.]
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the reaction of dienyl azide 5(estimated to have a triplet energy
of approximately 1.9 eV).
44
However, the Xantphos-containing
copper complexes have widely different excited state lifetimes
(14 330, 3, 71, and 69 ns; Table 1). As such, predicting the “optimal”
catalyst strictly from the photophysical properties would have
been challenging.
In summary, a series of copper-based photocatalysts of the type
Cu(N^N)(P^P)BF
4
were synthesized bearing
␲
-extended diimine
ligands. Their behavior in several photocatalytic processes were
evaluated and revealed the following:
(1) Copper-based complexes with
␲
-extended diimine ligands
have absorptions that are red shifted and larger extinction coeffi-
cients than complexes prepared with smaller diimine ligands
such as dmp.
(2) The
␲
-extended diimine ligands afford photocatalysts that
can participate in oxidative quenching SET processes. However,
the heteroleptic complexes bearing the ddpq, ddppz, or dbdppz
ligands are not particularly more active than the complexes bear-
ing the parent dmp ligand. Interestingly, the
␲
-extended diimine
Table 1. Synthesis of Cu(I)-based photocatalysts and photophysical properties.
Entry N^N P^P Yield (%)
a
␭
max
(nm) ␧(L/mol cm)
␶
(ns)
␭
emm
(nm) E
T
(eV) E(*Cu
I
/Cu
II
)
1 dmp —97 463 17 200 90 750 1.65 –1.12
2 dmp DPEPhos 98 383 1 580 14 300 570 2.17 –1.68
3 dmp Xantphos 46 380 3 960 1 133 560 2.21 –1.76
4 dmp dppf 81 379 2 890 1 060 570 2.17 –1.56
5 ddpq —78 458 6 760 3 680 1.82 –1.36
6 ddpq DPEPhos 84 382 4 485 5 565 2.19 –1.26
7 ddpq Xantphos 85 386 3 444 3 560 2.21 –1.72
8 ddpq dppf 91 380 3 346 73 530 2.34 –1.15
9 ddppz —99 453 14 428 4 762 1.63 –0.90
10 ddppz DPEPhos 78 380 17 508 44 664 1.87 –1.12
11 ddppz Xantphos 91 380 12 489 71 634 1.95 –0.82
12 ddppz dppf 79 380 17 508 61 510 2.43 –1.59
13 dbdppz —82 412 25 891 78 567 2.19 –1.34
14 dbdppz DPEPhos 77 409 16 663 69 489 2.53 –1.82
15 dbdppz Xantphos 50 408 13 754 75 565 2.19 –1.29
16 dbdppz dppf 79
b
413 11 711 69 597 2.08 –0.80
a
Isolated yields following precipitation with Et
2
O.
b
In 1:1 CH
2
Cl
2
:MeCN [5 mmol/L].
Fig. 2. Synthesis of of Cu(I)-based photocatalysts.
Fig. 3. Evaluation of the
␲
-extended copper-based photocatalysts in
a photochemical Appel-type process. Column colour darkens with
increasing size of diimine ligand. Front entries without an indicated
bisphosphine ligand pertain to homoleptic complexes of the form
Cu(N^N)
2
BF
4
. No reaction is observed in the absence of light. [Colour
online.]
Table 2. Yield of 2.
2(%)
Ligand None DPEPhos Xantphos Dppf
dmp 1 0 78 13
ddpq 77 60 72 50
ddppz 70 55 73 53
dbdppz 78 72 61 53
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ligands do produce homoleptic complexes that were much more
active in the visible-light mediated Appel process than the
Cu(dmp)
2
BF
4
catalyst.
(3) The new complexes did not seem to participate in PCET
processes that likely pass through reductive quenching, suggest-
ing that any further development of photocatalysis via reductive
quenching should avoid using such diimine ligands.
(4) Almost all complexes have similar excited state lifetimes
and triplet state energies. Although the complexes possessed
characteristics suitable for the azide decomposition reaction de-
scribed herein, the use of the
␲
-extended ligands do not extend
excited lifetimes or afford advantages for tuning properties for ET
processes.
Supplementary data
Supplementary data are available with the article through the
journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/
cjc-2020-0014.
Acknowledgement
The authors acknowledge the Natural Sciences and Engineering
Research Council of Canada (NSERC, Discovery 1043344), Univer-
sité de Montréal and the Fonds de recherche Nature et technologie
via the Centre in Green Chemistry and Catalysis (FRQNT-2020-RS4-
265155-CCVC) for generous funding. Prof. D. Rochefort and Simon
Généreux are thanked for access and helpful discussions regard-
ing cyclic voltammetry. Dr. Daniel Chartrand, Prof. G. Hanan, Ms.
Clémentine Minozzi, and Dr. Antoine Caron are thanked for help-
ful discussions.
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ddppz 4 0 0 0
dbdppz 0 20 0 3
Fig. 5. Evaluation of the
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Ligand None DPEPhos Xantphos Dppf
dmp 99 75 75 27
ddpq 57 55 87 42
ddppz 65 75 99 30
dbdppz 50 65 85 27
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43
Given that 5used in the study possesses
additional conjugation, the triplet value may be slightly lower, but using the
45.4 kcal value provides a good standard for comparison (1.96 eV).
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