Cu Photoredox Catalysts Supported by a 4,6-Disubstituted 2,2′-Bipyridine Ligand: Application in Chlorotrifluoromethylation of Alkenes

Article abstract of DOI:10.1021/acs.organomet.8b00585

Interest in base metal catalysis motivates the development of Cu-based photoredox catalysts for organic synthesis. However, only a few Cu catalysts have been applied in photoredox reactions, the majority of which contain one or two 1,10-phenanthroline lig

Full text of DOI:10.1021/acs.organomet.8b00585

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Cu Photoredox Catalysts Supported by a 4,6-Disubstituted 2,2′-
Bipyridine Ligand: Application in Chlorotrifluoromethylation of
Alkenes
Murat Alkan-Zambada and Xile Hu*
́
́
́
Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de
Lausanne (EPFL), ISIC-LSCI, BCH 3305, Lausanne 1015, Switzerland
S
* Supporting Information
ABSTRACT: Interest in base metal catalysis motivates the develop-
ment of Cu-based photoredox catalysts for organic synthesis.
However, only a few Cu catalysts have been applied in photoredox
reactions, the majority of which contain one or two 1,10-phenanthro-
line ligands. Here we design a 4,6-disubstituted 2,2′-bipyridine ligand
for Cu. Two heteroleptic [Cu(N^N)(P^P)][PF6] complexes, where
N^N stands for the 2,2′-bipyridine ligand and P^P stands for a
bisphosphine ligand, have been synthesized and characterized. They
exhibit longer excited state lifetimes and higher Cu(I)/Cu(II)
potentials compared to the most widely used Cu catalyst, [Cu-
(dap)2]Cl. The complex with Xantphos as the P^P ligand is an
efficient catalyst for chlorotrifluoromethylation of terminal alkenes, especially styrenes, which had been challenging substrates
for previously reported photoredox reactions. This chlorotrifluoromethylation method enables the convenient introduction of a
trifluoromethyl group into organic molecules under mild conditions, which is important for medicinal chemistry.
ligands are reported, they have not been used in photoredox
catalysis.15−17
1. INTRODUCTION
Visible-light photoredox catalysis has become a powerful tool
in organic synthesis.1−3 Because of their long excited state
lifetime, tunable redox potentials, and high stability,
coordinatively saturated octahedral Ru and Ir complexes are
the most versatile photocatalysts (PC). Although Cu
complexes have often been employed as alternatives to Ru
and Ir complexes in many photochemical applications, they
have only started to be explored as visible-light photoredox
catalysts for organic synthesis.4−6 Homoleptic [CuI(N^N)2]+
complexes, especially [Cu(dap)2]Cl (1, dap = 2,9-bis(4-
anisyl)-1,10-phenanthroline; Figure 1), are the most com-
monly used Cu(I) PCs. However, they are limited by short
excited state lifetimes (5−10 times lower than Ru and Ir
counterparts).1,2,7,8 Pioneering work of McMillin showed that
bulky heteroleptic [CuI(N^N)(P^P)]+ complexes (2, Figure
1) could have excited state lifetimes similar to those of Ru and
Ir complexes.9 Accordingly, these complexes have been applied
to expand the scope of Cu-catalyzed photoredox cataly-
sis.5,10−13 So far, the majority of N^N ligands in homoleptic
and heteroleptic Cu PCs are 2,9-disubstituted 1,10-phenan-
throlines (3, Figure 1), which can be difficult to synthesize and
modify.14 Increasing the diversity of the N^N ligands, on the
other hand, is a promising approach to fine-tune the properties
and activity of the Cu-based PCs. Toward this goal, we decided
to develop substituted 2,2′-bipyridine (bpy) ligands for the
assembly of new [CuI(N^N)(P^P)]+ PCs. Although the
photophysical studies of some Cu complexes of bulky bpy
Ligand 4 (Figure 1) was designed for the following reasons:
(i) the steric bulk from the methyl group in the 6-position of
bpy should hinder excited-state structural relaxation and
solvent quenching, leading to a sufficiently long excited state
lifetime; (ii) the second pyridine moiety is unsubstituted so the
ligand is not too bulky to result in dynamic ligand exchange in
solution;18,19 (iii) the electron-rich aryl group at the 4-position
of bpy renders the ligand harder to reduce, thereby increasing
the reduction power of the excited state as the excited state is
formally [CuII(N^N)−(P^P)]+; (iv) the 2,4,5-substitution
pattern was chosen in order to twist the aryl-ring and prevent
extension of the aromaticity which might lower the reduction
power of the excited state. Here we report the synthetis,
characterization, and catalytic application of two [CuI(4)-
(P^P)][PF6] complexes (5 and 6, Figure 1), where P^P is
either bis{2-(diphenylphosphanyl)phenyl} ether (POP) or
4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xant-
phos). Complex [CuI(4)(Xantphos)][PF6] (5) is an efficient
catalyst for chlorotrifluoromethylation of alkenes, exhibiting
the broadest scope for a Cu-based PC. This reaction provides a
convenient method to introduce a trifluoromethyl group into
organic molecules, which is of high contemporary interest in
medicinal chemistry.
Received: August 15, 2018
© XXXX American Chemical Society
A
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Figure 1. Cu(I) complexes and their ligands in previous and current studies.
Figure 2. Synthesis of ligand 4 and complexes 5 and 6.
interaction between 4 and Xantphos or POP was observed
(Figure 3, Table 1).
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of Cu Complexes.
Extinction and emission spectra of 5 and 6 were recorded in
acetonitrile (MeCN) and show ligand centered (π → π*)
absorptions in the UV region as well as an MLCT band
(d(Cu) → π*(N^N), λ = 371 and 372 nm for 5 and 6,
respectively) in the blue region (Figure 4). The Xantphos-
containing complex 5 (ε = 2679 L mol−1 cm−1) has a
significantly higher MLCT extinction coefficient than the
POP-containing complex 6 (ε = 1240 L mol−1 cm−1). The
lower extinction coefficients of 6 were even visible by the eye,
as 6 gives a much paler solution compared to 5. Emission takes
place in the yellow region (550−600 nm). Both absorption and
emission in 5 and 6 are blue-shifted relative to 14 as well as to
[Cu(bpy)(P^P)]+ (383 and 388 nm for Xantphos and POP,
respectively),19 suggesting a destabilization of the ligand-
centered LUMO. In dichloromethane (DCM), the extinction
spectra did not scale with concentration at concentrations <0.1
mM, probably due to dynamic ligand dissociation.
̈
A Krohnke-type synthesis was developed for the bpy ligand 4.
Reaction of 2,4,5-trimethylbenzaldehyde with 2-acetylpyridine
gave the α,β-unsaturated carbonyl precursor 4-1 in 79% yield.
Condensation of 4-1 with α-pyridinium methyl ketone salt
furnished 4 in 17% yield. If the two steps were conducted in
one pot without isolation of the α,β-unsaturated carbonyl
precursor, the overall yield was improved to 42% (Figure 2).
Mixing of 4 with [Cu(MeCN)4][PF6] and Xantphos or POP
and recrystallization from DCM/Et2O yielded the Cu(I)
complexes 5 and 6 in 80−100% yields (Figure 2).
Single X-ray crystallography reveals a distorted tetrahedral
geometry for 5 and 6. The dihedral angle between the N−Cu−
N and P−Cu−P planes is 84.74° for 5 and 85.39° for 6, close
to the angle of 90° for a perfect tetrahedron. Notably, the two
pyridine rings are significantly twisted with a torsion angle of
18.97° for 5 and 18.65° for 6. The aryl substituent at the 4-
position is also twisted from the plane of the pyridine ring, with
a torsion angle of 50.39° in 5 and 44.15° in 6. The methyl
group in the 6-position of 4 lies over the (C6H4)2O unit of
POP and the xanthene “bowl” of Xantphos, as observed
previously for analogous Cu(I) compounds.18,20 No π-stacking
Excited state lifetimes were measured via time correlated
single photon counting (TCSPC) experiments (Supporting
Information, Table S5). The PMMA thin film samples showed
a biexponential decay that may arise due to thermally activated
B
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Figure 3. X-ray single-crystal structures of 5 (top left) and 6 (top right). The ellipsoids are plotted at the 50% probability level. Hydrogens,
counterions, and solvent molecules are omitted for clarity. Bottom: I = torsion angle between rings A and B, II = dihedral angle between the planes
containing N−Cu−N and P−Cu−P, III = torsion angle between rings A and C.
Table 1. Angles and Distances from X-ray Single Crystal Structures of 5 and 6
I (deg)
II (deg)
III (deg)
Cu−NA distance (Å)
Cu−NB distance (Å)
N−Cu−N angle (deg)
P−Cu−P angle (deg)
5
6
19.0(5)
18.6(4)
84.74
85.39
50.3(5)
44.1(4)
2.097(2)
2.104(2)
2.080(2)
2.070(3)
80.11(9)
80.05(9)
114.94(3)
113.38(3)
Figure 4. Extinction and emission spectra of 5 and 6 in MeCN (λex = 410 nm) and as PMMA thin films (λex = 400 nm).
delayed fluorescence processes or due to different confomers.
In DCM, the data was fitted to monoexponential decays
(Supporting Information, Figures S5−S14).
The excited state lifetimes are microseconds for PMMA thin
film samples (4.77 and 4.53 μs for 5 and 6, respectively) and
several hundred nanoseconds in DCM. Notably, in solution, 5
has a lifetime 300 ns longer than 6, 450 ns longer than 1, 600
ns longer than [Cu(bpy)(Xantphos)]+, and 10 times longer
than [Cu(bpy)(POP)]+.20
(Supporting Information, Figure S4 and Table S4). These
redox potentials are comparable to analogous [Cu(N^N)-
(P^P)][PF6] compounds (Supporting Information, Table
S4)18,20,21 and are about 0.6 V more positive than the
potential for complex 1 (0.6 V vs SCE). In MeCN, the Cu(I)/
Cu(II) oxidation is irreversible.
According to the photophysical and electrochemical data,
the excited state potentials of 5 and 6 were estimated via the
Rehm−Weller equation, Eox([Cu]*) = Eox(Cu(I)) − E0−0
3
Complexes 5 and 6 exhibit a quasi-reversible Cu(I)/Cu(II)
redox couple in DCM at 1.23 and 1.15 V vs SCE, respectively
(where E0−0 refers to MLCT energy, approximated from the
emission maximum), as −0.96 and −1.08 V vs SCE for 5 and
C
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6, respectively (Table 2). Thus, the photoexcited states of
complexes 5 and 6 exhibit similar reducing power as those of
redox catalyzed pathway to trifluoromethylation of styrenes is
fac-Ir(ppy)3 and gaseous CF3I.28 Alternatively, chlorotrifluor-
omethlyation can be performed using Ru-based catalysts at
high temperatures with limited functional group tolerance.29,30
Recently, Lin and co-workers also reported an electrocatalytic
method for chlorotrifluoromethlyation of olefins which
requires equimolar amounts of sacrificial oxidant and shows
limited compatibility with electron-rich styrenes.31
The reaction of the parent styrene (7) with CF3SO2Cl was
used as the test reaction for the optimization of conditions
(Table 3). First, complex 5 was used as the PC. The reaction
produced two major products: one from chlorotrifluorome-
thylation (8) and one from chlorotrifluorosulfonation (9).
Quantitative conversion was obtained at room temperature
(r.t.) after 3 h using 1 equiv of 7, 2 equiv of CF3SO2Cl, 2 equiv
of K3PO4 as base, 0.5 mol % of 5 as PC, DCM as solvent (3
mL for 1 mmol scale), LED450 illumination, and under N2.
Only 8 and 9 were observed as products by NMR, and the
ratio of 8 to 9 was 4:1 (entry 1, Table 3). When the LED
source is replaced by sunlight, an identical conversion and
product selectivity were obtained (entry 2, Table 3).
Table 2. Comparison of Photophysical and Electrochemical
Properties between 1, 5, and 6
a
b
c
λem
MLCT
(nm)
τave
Eox
[CuI]*/[Cu]II
(V vs SCE)
complex (nm)
(ns)
(V vs1S/2CE)
d
d
1
5
6
670
611
613
400−600
270
720
410
0.62
1.23
1.15
−1.43
−0.96
−1.08
ef
,
f
371
372
ef
,
f
a
b
Recorded in N2-purged DCM. Internally referenced vs Fc0/+ and
converted to SCE.23 Estimated via the Rehm−Weller equation.
c
d
e
f
From ref 6; excitation at 500 nm. Recorded in MeCN. Excitation at
410 nm.
[Ru(phen)3]2+, [Ru(bpy)3]2+, and [Ir(ppy)2(dtbpy)]+ (Sup-
porting Information, Table S6). They are all less reducing than
that of complex 1 (−1.43 V vs SCE) as well as other
heteroleptic Cu(I) complexes, which have excited state
potentials of −1.35 to −2.16 V vs SCE.5,10−12,22
Control experiments showed that light, catalyst 5, and N2
atmosphere are all necessary for the reaction (Supporting
Information, Figure S1). Without a base, no conversion was
observed (entry 3, Table 3). Other bases might be used, with
K2CO3 being the best for both yields and selectivity (entry 4,
Table 3 and Supporting Information, entries 1 and 2, Table
S1). Lowering the amount of K2CO3 to 1 equiv led to no
conversion (Supporting Information, entry 3, Table S1).
Replacing DCM by other solvents such as MeCN, THF, and
DMF significantly lowered the conversion (Supporting
Information, entries 4−6, Table S1). Further experiments
show that the concentration of reagents and temperature
influenced the product selectivity (Supporting Information,
2.2. Catalysis. The activity of 5 and 6 as photoredox
catalysts was tested in the chlorotrifluoromethylation reaction
of styrenes using CF3SO2Cl. While chlorotrifluomethylation of
α,β-unsaturated amides and esters as well as nonactivated alkyl
alkenes was achieved using complex 124,25 or [Ru(phen)3]-
26
Cl2 as a photoredox catalyst, styrenes have been unsuitable
substrates when CF3SO2Cl is used as the trifluoromethylating
reagent (except for one special substrate, p-nitrostyrene24).
Chlorotrifluomethylation of styrenes via photoredox catalysis
was previously only achieved using Ru(bpy)3(PF6)2 and
Umemoto’s reagent.27 Compared to Umemoto’s reagent,
CF3SO2Cl is less expensive and generates only gaseous
byproduct (SO2) which facilitates separation. Another photo-
Table 3. Summary of Optimization of Trifluoromethylation of Alkenes
a
entry
1
2
PC
base
n
x
temp (°C)
conversion (%)
8/9
5
5
5
5
5
5
5
5
5
6
1
K3PO4
K3PO4
no
2
2
2
2
2
2
2
2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
3
3
3
3
3
3
5
5
5
5
5
5
5
5
5
5
r.t.
r.t.
r.t.
r.t.
r.t.
40
r.t.
40
40
40
40
40
40
40
40
40
100
100
4:1
4:1
n.d.
5:1
b
c
c
3
4
5
n.r.
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
100
trace
100
100
100
100
50
d
n.d.
6:1
9:1
9:1
15:1
5:1
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6
7
8
9
10
11
12
13
14
15
16
20
Ru(bpy)3Cl2
n.r.
n.r.
n.r.
n.r.
n.r.
Ir(ppy)2(dtbbpy)PF6
[Cu(MeCN)4]PF6
Eosin Y
AIBN (10 mol %)
a
b
c
d
1
Determined by H NMR. Illumination by sunlight. n.r. = no reaction; n.d. = not determined. 1 equiv of base.
D
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Figure 5. Scope of chlorotrifluoromethylation with styrenes (A), terminal alkenes with adjacent electron withdrawing groups (B), and limitation of
chlorotrifluoromethylation with unsuccessful substrates (C).
Table S2). A higher 8/9 ratio was obtained when the
temperature was increased from r.t. to 40 °C (compare entries
1 and 6, Table 3) or when the solution volume was increased
from 3 to 5 mL (compare entries 1 and 7, Table 3). The
amount of CF3SO2Cl could be decreased to 1.2 equiv (entry 8,
Table 3). The best selectivity of 15:1 was achieved using 2
equiv of K2CO3 as base in 5 mL of DCM at 40 °C (entry 9,
Table 3). Replacement of 5 with 6 as PC reduced the
conversion to only 50% and the selectivity to 5:1 (entry 10,
Table 3). Other Ru, Ir, Cu, and organic PCs were tested, but
all gave lower to no conversions (Supporting Information,
Table S3). For example, using 1, the conversion was only 20%
(entry 11, Table 3); using Ir(ppy)3, the conversion was only
26% (Supporting Information, entry 4, Table S3). Ru-
(bpy)3Cl2, Ir(ppy)2(dtbbpy)PF6, [Cu(MeCN)4]PF6, and
Eosin Y were all ineffective (entries 12−15, Table 3). A
radical initiator, AIBN, also failed to initiate the reaction (entry
16, Table 3). During the attempt to isolate 8 and 9 from the
reaction conducted under the optimized conditions (entry 9,
Table 3), it was found that 9 is sensitive to water and rapidly
eliminates HCl during isolation to yield 10. The overall
isolated yield was 91%, and the 8/10 ratio was larger than 6:1.
The optimized conditions were successfully applied for
chlorotrifluoromethylation of a wide range of terminal styrenes
E
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Figure 6. Reactions with aliphatic terminal alkenes. See the Supporting Information and ref 24 for reaction conditions.
Figure 7. Proposed mechanism for trifluoromethylation and trifluorosulfonation of alkenes.
(Figure 5A). Though in some cases a minor amount of formal
trifluorosulfonation product (vide supra) was formed, the
majority of substrates underwent exclusive chlorotrifluomethy-
lation. Electron-neutral, -withdrawing, or -donating groups at
the para-, meta-, and ortho-positions are all tolerated (8a−8r,
10a−10e). Chlorotrifluoromethylation also worked for 2,4,6-
trimethylstyrene, suggesting that the reaction was insensitive to
steric hindrance at the aryl group (8s). Alkenes substituted by
other aromatic groups such as naphthyl (8t−8u, 10f−10g) and
pyridinyl (8v−8w) could be used as well. The α-position of
styrene could be further substituted by a methyl group (8x),
while analogous substitution at the β-position led to a
diminished yield (34%; 8y). On the other hand, 1,2-
dihydronaphthalene was chlorotrifluoromethylated in 70%
yield (8z). These results indicated that the reaction was
sensitive to the sterics at the β-position of the styrene. The
chlorotrifluoromethylation product of 1,1-diphenylethylene
was prone to HCl elimination (8aa). Thus, 21% of
chlorotrifluoromethylation and 56% of formal oxidative
trifluoromethylation products were obtained from this
reaction. The reactions tolerated various functional groups
such as ester (8d, 8f, 8o), nitrile (8e), nitro (8g), F (8h, 10d,
8k, 8p), Cl (8i, 10e, 8l, 8q), Br (8j, 8m, 8r), and pyridine (8v,
8w). The tolerance of this method was further demonstrated in
the successful chlorotrifluoromethylation of substrate derived
from estrone (8ab and 10h) and α-tocopherol (8ac). Besides
styrene derivatives, terminal alkenes with adjacent electron
withdrawing groups, such as ester (8ad−8af), phthalimide
(8ag), and amide (8ah), were also viable substrates (Figure
5b). The structure of 8ah was determined by single-crystal X-
ray crystallography. The limitations of the method were also
explored (Figure 5c). Significant polymerization was observed
for 4-vinylpyridine (8ai) and 1-methoxy-4-vinylbenzene (8aj)
under the reaction conditions. Internal alkenes such as stilbene
(8ak) and methylcinnamate (8al) were unreactive. Likewise,
alkenes with a N group at the 2-position (8am−8ao) did not
react.
Alkyl substituted terminal alkenes were then tested as
substrates. However, the reactions led to chlorosulfonation at
the 2-position and trifluomethylation at the 1-position (Figure
6). Complex 1 was previously shown to be an efficient catalyst
for such reactions.24 Indeed, for some substrates, e.g., 8aq and
8ar, the yields were higher using 1 instead of 5 as the PC.
Thus, the reactions catalyzed by 5 were not further explored.
On the basis of previous studies of analogous trifluomethy-
lation reactions of alkenes24,25,32 and the reaction outcomes
F
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observed in this study, a mechanistic hypothesis is proposed
(Figure 7).
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The photoexcited state of the Cu catalyst (5*) undergoes
oxidative quenching by CF3SO2Cl (Ered = −0.18 V vs SCE33)
AUTHOR INFORMATION
■
•
to give the CF3SO2 radical (11) and initiate the reaction.
Corresponding Author
*E-mail: xile.hu@epfl.ch.
ORCID
Xile Hu: 0000-0001-8335-1196
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
While some 11 is immediately trapped by the olefin, the
•
majority of it extrudes SO2 to give CF3 radical (12), which
then adds to the olefin as well. The addition product is an alkyl
radical (13) that can engage in various reaction pathways to
generate the observed products. When the alkene is substituted
by an aryl or electron withdrawing group, 13 might be oxidized
either by the oxidized Cu(II) PC or by CF3SO2Cl to give an
alkyl cation (14). In the former case, the PC is regenerated; in
the latter case, 11 is generated so the radical propagates. 14
then recombines with a Cl− anion to give the product.
Alternatively, 13 might abstract a Cl• radical from CF3SO2Cl
Notes
The authors declare no competing financial interest.
•
to give directly the product and a new CF3SO2 , propagating
ACKNOWLEDGMENTS
the radical chain reaction. These pathways might coexist. If the
alkene is substituted by a nonactivated alkyl group, then the
alkyl radical 13 prefers to attack the S instead of Cl of
CF3SO2Cl to give the chlorosulfonation product. We found no
evidence or reason to include the previously proposed inner-
sphere mechanism for Cu(I) photoredox catalyzed trifluor-
omethylchlorosulfonylation.24 A dedicated study is required to
elucidate the details of the mechanism.
■
We thank Laurent Liardet (EPFL) for help with electro-
chemical measurements and Dr. Anne-Sophie Chauvin
(EPFL) for conducting the TCSPC measurements. Dr.
Rosario Scopelliti (EPFL) and Dr. Farzaneh Fadaei Tirani
(EPFL) are acknowledged for the X-ray crystallography. This
work is funded by the Swiss National Science Foundation (no.
200020_162362).
The higher catalytic efficiency of 5 compared to other Cu
PCs for chlorotrifluoromethylation of styrenes warrants some
comments. 5 has higher extinction coefficients than 6, which
might be beneficial for absorption of photons leading to an
MLCT and thus photocatalysis. Furthermore, the longer
excited state lifetime of 5 compared to 6 may also contribute
to its higher catalytic efficiency. The Cu(I)/Cu(II) redox
couple of 5 is much more positive than that of 1. Thus, the
regeneration of 5 through the reaction of the alkyl radical 13
with the Cu(II) form of the catalyst is more facile than the
regeneration of 1 in an analogous reaction.
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Collins, S. K. Heteroleptic Copper(I)-Based Complexes for Photo-
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3. CONCLUSION
Two heteroleptic [CuI(N^N)(P^P)][PF6] complexes contain-
ing a new 4,6-disubstituted 2,2′-bipyridine ligand have been
synthesized and characterized. The complexes exhibit longer
excited state lifetimes and more positive Cu(I)/Cu(II) redox
couples than the classic Cu-based photoredox catalyst
[Cu(dap)2]Cl. Using complex [CuI(4)(Xantphos)][PF6] (5),
the first general photoredox catalyzed chlorotrifluoromethyla-
tion of styrenes has been achieved, with broad substrate scope
and high functional group tolerance. The work demonstrates
the potential of substituted 2,2′-bipyridine ligands for Cu-
based photoredox catalysis.
ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00585.
Experimental details and characterization data (PDF)
Accession Codes
CCDC 1850074−1850076 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
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DOI: 10.1021/acs.organomet.8b00585
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