Copper(I) complexes of 8-(diphenylphosphanyl-oxy)-quinoline: Photophysics, structures and reactivity

Article abstract of DOI:10.1016/j.inoche.2017.10.012

Copper(I) complexes are studied for various potential applications due to their luminescence properties. However, issues have been identified regarding the stability of heteroleptic compounds. As a novel strategy, we propose to modify existing copper(I) complexes by introduction of molecular bridges between the different ligands. We report the synthesis and chemical properties of the complexes of 8-(diphenylphosphanyl-oxy)quinoline (POQ), a combination of a phosphine and a N-heterocycle with CuX (X = Cl, Br, I and SCN). The photophysical properties of the materials were studied. However, all four compounds were found to be labile in solution upon contact with trace amounts of water. Two POQ complexes and the decomposition products were identified as tetraphenyldiphosphoxane complexes with single crystal X-ray diffraction. We propose a design rule to prevent this behavior in future development steps.

Full text of DOI:10.1016/j.inoche.2017.10.012

Accepted Manuscript
Copper(I) complexes of 8-(diphenylphosphanyl-oxy)-quinoline:
Photophysics, structures and reactivity
Manuela Wallesch, Martin Nieger, Daniel Volz, Stefan Bräse
PII:
DOI:
Reference:
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doi:10.1016/j.inoche.2017.10.012
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Inorganic Chemistry Communications
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1 May 2017
16 September 2017
11 October 2017
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Copper(I) complexes of 8-(diphenylphosphanyl-oxy)-quinoline: Photophysics, structures
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authors. Please check if appropriate. Inoche(2017), doi:10.1016/j.inoche.2017.10.012
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Key words: Copper(I) complex; degradation; multinuclear; POP; PPO; POQ
Abstract
Copper(I) complexes are studied for various potential applications due to their luminescence
properties. However, issues have been identified regarding the stability of heteroleptic
compounds. As a novel strategy, we propose to modify existing copper(I) complexes by
introduction of molecular bridges between the different ligands. We report the synthesis and
chemical properties of the complexes of 8-(diphenylphosphanyl-oxy)quinoline (POQ), a
combination of a phosphine and a N-heterocycle with CuX (X = Cl, Br, I and SCN). The
photophysical properties of the materials were studied. However, all four compounds were found
to be labile in solution upon contact with trace amounts of water. Two POQ complexes and the
decomposition products were identified as tetraphenyldiphosphoxane complexes with single
crystal X-ray diffraction. We propose a design rule to prevent this behavior in future
development steps.
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Copper(I) complexes of 8-(diphenylphosphanyl-
oxy)-quinoline: Photophysics, Structures and
Reactivity
Manuela Wallesch,a,b Martin Nieger,c Daniel Volz,d* and Stefan Bräse*a,b
Institute of Organic Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 6, 76131
Karlsruhe (Germany)
Luminescent copper(I) complexes are studied for various potential applications, e.g., as
sustainable emitters in organic light-emitting diodes (OLEDs)[1,2], light-emitting
electrochemical cells, and photo-catalysts[3,4,.5] Recently, the efficiency-gap between state-of-
the-art phosphorescent iridium emitters and copper-emitters, which often emit via thermally
activated delayed fluorescence (TADF) has been closed,[6] which puts copper emitters on par
with the most efficient molecules known for the transformation of UV-light or electrical energy
into visible light. However, the search for better materials is ongoing: Realizing deep-blue or red
emitters with copper is – with some exceptions[7–10] – still an issue and the general long-term
stability has to be enhanced to reach the durability that is needed for commercial products. This
requires a deep understanding of the chemical properties of copper emitters.[11,12] Recently,
Tsuge et al. reported that these materials are also interesting emitting materials and that their
color can be tuned by varying either the halides (X, Scheme 1) or the N-heterocycles.[13] From
quantum-chemical calculations, it is known that the emissive transition for similar compounds is
a result of a metal-halide-to-ligand charge transfer ((M+X)LCT) between the dinuclear copper-
halide subunit and the N-heterocycles.[9] The influence of the phosphine on the emission
properties is negligible.[9,14,15]
Scheme 1: Modification of heteroleptic copper(I) complexes by introduction of molecular
bridges. The color may be tuned by varying either the halides X or the N-heterocycles.[13]
Potential stability problems with such heteroleptic compounds,which are the result of a cleavage
of the N-Cu bond, were reported previously by our group.[16] The instability of these
compounds may hamper their durability in OLED devices. We propose a new strategy to
overcome these issues: The introduction of inert bridges between the phosphine and N-
heterocyclic ligands to stabilize those dinuclear structures with kinetically favored five- or six-
membered metallacycles.[6] A similar strategy was introduced by Yersin and Wesemann, where
five-membered rings were yielded by direct fusion of amines with phosphines.[7] Potential
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bridges are short alkyl-chains or ether- or thioether-bridges. In a first attempt, we chose a single
oxygen as a bridge between diphenylphosphine and a N-heterocycle. Because of the current lack
of efficient red copper emitters, we used the quinoline-moiety to test this hypothesis due to its
narrow HOMO-LUMO difference, high chemical stability and encouraging previous results with
similar structures.[10,13,17]
Fusing phosphine and N-donor moieties in one chelating ligand was achieved by combination of
diphenylphosphine as P-donor, quinoline as N-donor, and oxygen as the bridging unit, which
yields ligand 1 (POQ, (diphenylphosphanyl-oxy)-quinoline, see Figure 1). The synthesis of the
ligand is straight-forward and could easily be performed in large batches by reacting equimolar
amounts of chlorodiphenyl phosphine and 8-hydroxyquinoline in the presence of dry
triethylamine. Ligand 1 could be purified by multiple recrystallization to get a pure product. The
synthesis of the copper(I) complexes was performed in dry dichloromethane by mixing of
stoichiometric amounts of copper precursors with 1 and precipitation of the product. For the
synthesis of copper(I) complexes, we found that using non-crystallized ligand 1, which can be
obtained by filtering the reaction mixture of celite and removal of the solvent, is sufficient.
Figure 1. Synthesis of ligand 1 (POQ) and complexation with copper precursors yields dinuclear
complexes.
Reaction of POQ with precursor salts CuX (X =Cl, Br, I, SCN) was performed as previously
described in the literature.[18] The products were obtained by diffusion of cyclohexane into the
clear, yellow reaction mixtures or by precipitation from the reaction mixture with hexane (bulk
samples) to yield the complexes 1-Cl, 1-Br, 1-I and 1-SCN.
The photophysical properties of 1-Cl, 1-Br, 1-I and 1-SCN were studied in solid state under
3
nitrogen to prevent triplet quenching from dioxygen O2. We found that 1-Br is luminescent at
room temperature. Upon cooling to 77 K (liquid nitrogen bath), all samples showed weak or very
weak luminescence. However, only 1-Cl and 1-Br gave satisfactory PL spectra (Figure 2). At
room temperature, the photoluminescence quantum yield of 1-Cl was below 1%, while 1-Br
yielded a mediocre PLQY of 10 ± 1 %. As can be seen in Figure 2, all emission spectra are broad
and ill-structured, as it is expected for (M+X)LCT emission. The emission of 1-Br is centered in
the orange-to-red region around 590 nm at room temperature and around 610 nm at 77 K, while
1-Cl emits around 625 nm at 77 K. All spectra have a broadness of 0.4 ± 0.1 eV. Similar to
related compounds introduced by Tsuge and co-workers,[9,10,19] the large π-system of the
quinoline moiety leads to a stabilization of the LUMO and consequently to a profound red-shift,
which can be further enhanced by destabilizing the HOMO, by moving from bromide to
chloride. Also, from the fact that the color is profoundly red-shifted upon cooling (1-Br) and that
the PLQY seems to be increased at lower temperatures, one could speculate that the material is a
TADF-emitter, as it was found for other dinuclear Cu(I) complexes.[7] To confirm this
assumption, a deeper spectroscopic analysis is necessary.
Even though the ligand-fusion works in principle and yielded orange-to-red emitting compounds,
there remain open questions: First, the PLQY needs to be increased further. The fact that the
PLQY is low, but is increased upon cooling to low temperatures for 1-Cl, 1-Br, 1-I, and 1-SCN,
is a hint that there are strong, temperature-dependent quenching processes present. Second, it is
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noticeable that the bromide-derivative has the highest PLQY. In most cases, the PLQY either
increases or decreases when moving from Cl to Br to I.[9,17] The different behavior in this case
might be due to differences in the molecular structure of the samples. This will be further
elucidated in the section, where XRD results are discussed.
Figure 2. Excitation (λEm. = 600/625 nm) and emission (λEx. = 370/390 nm) spectra of powder
samples of complexes 1–Br at r.t. and 77 K/complex 1–Cl at 77 K.
The molecular structure of 1-Br and 1-I could be determined with single crystal X-ray
diffraction (Figures 3 and 4). As expected, both compounds consist of a dinuclear Cu2X2-unit
with two molecules of POQ chelating the copper atoms. However, there are fundamental
differences in the two structures: 1-Br is a centro-symmetric molecule with a planar Cu2Br2-unit,
which is similar to the related compound published by Tsuge et al.[13] On the other hand, 1-I
has mirror symmetry and features a bent, butterfly-shaped Cu2I2-unit, much like the NHetPHOS-
compounds developed by our group.[14,20,21] A similar behavior was found for other dinuclear
compounds by Yersin and Wesemann, where a bridging N,P ligand yielded a centro-symmetric
compound, similar to 1-Br with X = Cl, and a mirror-symmetric compound similar to 1-I with X
= Br and I.[7] This could be a reason for the abnormal trends found in the photophysical
experiments when moving from Cl to Br to I.
Figure 3: Molecular structure of complex 1-Br. Displacement parameter drawn at the 50%
probability level. The molecule possesses crystallographic Ci-symmetry.
Figure 4. Molecular structure of complex 1-I. Displacement parameter drawn at the 50%
probability level.
Table 1. Selected bond lengths (in Å) for 1-Br and 1-I.
1-Br
1-I
Cu−X
2.4348 (4)
2.4888 (6)
2.6097(5)
2.6600(6)
2.5699(5)
2.6462(6)
2.135(3)
2.169(3)
2.1789(9)
2.1854(9)
1.652(2)
1.650(2)
1.380(4)
1.387(4)
Cu−N
Cu−P
P−O
2.132 (2)
2.1532 (7)
1.6617 (18)
1.386 (3)
Cquin−O
Selected bond lengths for 1-Br and 1-I are given in Table 1. Comparison with reference
compounds (see Table 2, Table S2 in the supporting information) shows that the Cu-halide and
Cu-N bond lengths are similar to those in other copper(I) complexes: The Cu-halide bonds are in
the expected range, but will not be discussed further due to the rather larger variance (see Table
2). The Cu-N bonds are also in the expected range: While there are no known reference
compounds with Cu(I)-phosphine-quinoline-coordination, it can be seen that the Cu-N bond gets
longer when moving from Cu(I)-pyridine to Cu(I)-pyridine-phosphine and that the Cu-N bond
for Cu(I)-quinoline (2.13 Å) is in a similar range as for Cu(I)-pyridine-phosphine.
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However, the Cu-P and the P-O bond lengths deviate from the expected values: It is evident that
the Cu-P bonds for both compounds are roughly 0.04–0.14 Å shorter than expected. While in
most known cases, this bond is in the order of 2.19–2.29 Å, they are only 2.15/2.18 Å for 1-Br/1-
I, respectively. More anomalies can be found in the ligand backbone of the POQ-unit. So far,
only two other publications with POQ are available, studying the coordination chemistry with
Ni(II)[22] as well as Fe(II) and Ru(II).[23] Nevertheless, the P-O bond seems to be slightly
longer than expected (approx. 0.02–0.04 Å), while the Cquin-O bond is again close to the
expected value.
Table 2. Bond distances (in Å) in various representative copper(I) complexes from the literature.
bond,
source*
bond distances
compound
Cu-I
copper(I)iodide
[CuI(pyridine)]n
[CuI(PPh3)]4
[CuI(pyridine)(PPh3)2]
[CuI(quinoline)]2
[CuI(quinoline)]4
Cu-Br
[24]
[25]
[26]
[27]
[28]
[27]
2.6414
2.641(1)–2.689(1)
2.653(3)–2.732(3)
2.636(1)
2.657(3)–2.668(2)
2.680(6)–3.394(12)
copper(I)bromide
[CuBr(pyridine)]n
[CuBr(PPh3)]4
[CuBr(pyridine)(PPh3)2]
[CuBr(quinoline)]2
[CuBr(quinoline)]4
Cu-Cl
[24]
[25]
[29]
[27]
[30]
[31]
2.5201
2.450(5)–2.557(3)
2.491(2)–2.617(2)
2.459(2)
2.493(3)–2.680(3)
2.381(4)–2.851(2)
copper(I)chloride
[CuCl(pyridine)]n
[CuCl(PPh3)]4
[CuCl(pyridine)(PPh3)2]
[CuCl(quinoline)]2
[CuCl(quinoline)]4
Cu-N
[24]
[25]
[32]
[27]
[30]
[31]
2.3820
2.338(4)–2.530(2)
2.363(2)–2.505(2)
2.318(2)
2.357(7)–2.675(6)
2.311(5)–2.510(5)
[CuI(pyridine)]n
[CuBr(pyridine)]n
[CuCl(pyridine)]n
[CuSCN(pyridine)]n
[CuI(pyridine)(PPh3)2]
[CuBr(pyridine)(PPh3)2]
[CuCl(pyridine)(PPh3)2]
[CuSCN(pyridine)(PPh3)2]
[CuI(quinoline)]2
[CuBr(quinoline)]2
[CuCl(quinoline)]2
Cu-P
[25]
[25]
[25]
[33]
[27]
[27]
[27]
[34]
[28]
[30]
[30]
2.038(6)
2.00(1)
1.993(8)
2.054(5)–2.103(2)
2.131(5)
2.145(9)
2.130(2)
2.070(1)–2.091(2)
2.076(5)–2.104(5)
2.01(1)–2.07(2)
2.00(2)–2.08(2)
[CuI(PPh3)]4
[CuBr(PPh3)]4
[CuCl(PPh3)]4
[CuI(pyridine)(PPh3)2]
[CuBr(pyridine)(PPh3)2]
[CuCl(pyridine)(PPh3)2]
[CuSCN(pyridine)(PPh3)2]
[26]
[29]
[32]
[27]
[27]
[27]
[34]
2.251(6)–2.258(7)
2.206(3)–2.209(3)
2.192(2)–2.193(2)
2.283(2)–2.292(2)
2.277(3)–2.286(4)
2.256(1)–2.272(1)
2.1974(5)
*also refer to Table S1 in the supporting information
Figure 5. We hypothesize that the P-O bond is weakened upon coordination of POQ to
copper(I), whereas the Cu-P bond is stronger than in other complexes.
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If the difference in bond lengths is considered to be a hint for strengthened (shortening) or
weakening (elongation) of the respective bond, coordination of POQ to copper halides seems to
weaken the bond between the PPh2 unit and the quinolate unit, while the bond between PPh2 and
Cu(I) is stronger than usual. It is unlikely that the bond length anomalies are caused by steric
hindrance and the presence of a chelating ligand. Usually, such effects are more likely to cause
distorted bond angles, as it was found for other dinuclear copper(I) complexes.[14] The presence
of elongated or shortened bonds has two important consequences: i) upon photoexcitation, such
bonds could be cleaved or open up non-radiative relaxation pathways and ii) the elongated bond
is more likely to be cleaved.
Next, we will further discuss the degradation of POQ-Cu(I)-complexes. Various crystallization
experiments with a copper precursor (CuCl, CuBr, CuI, and CuSCN) and POQ by diffusion of
diethyl ether into acetonitrile solutions were performed. For compound POP-Br-2,
triphenylphosphine was added.
POP-Br-1 and POP-SCN were prepared by the recrystallization of 1-Br and 1-NCS,
respectively. POP-Br-2 was prepared from CuBr and POQ in the presence of PPh3. The
structures of the complexes are shown in Figure 6, 8, and 7, respectively.
We were able to isolate single crystal samples of three compounds (Figure 6, Figure 7, and
Figure 8). All three isolated compounds did not contain the initial ligand POQ. Instead, we
found a bridging bisphosphine ligand (PPh2)O(PPh2)/(POP). Tetraphenyl-diphosphoxane (POP)
is the tautomeric form of tetraphenyldiphosphine monoxide (PPO), which can be synthesized by
hydrolysis of chlorodiphenylphosphine. Without the presence of coordinating metal ions, PPO is
the dominating tautomer, while POP can be stabilized in the presence of transition metal ions.
Figure 6. Molecular structure of complex POP-Br-1. Displacement parameter drawn at the 50%
probability level. The molecule possesses crystallographic Ci-symmetry.
Figure 7. Molecular structure of complex POP-Br-2. Displacement parameter drawn at the 50%
probability level. Only the major isomer (ca. 96%) is depicted.
Figure 8. Molecular structure of complex POP-SCN. Displacement parameter drawn at the 50%
probability level. The molecule possesses crystallographic Ci-symmetry.
Early results for POP complexes were published in the 1970s with Cr, Mo, and W as metal
centers.[35–37] Depending on the metal fragments, POP may coordinate in a bridging or
chelating way, the latter yielding four-membered rings. Naktode and co-workers recently
reported the first copper(I) complexes of the POP ligand,[38] which was obtained by the
hydrolysis of N,N-bis-(diphenylphosphino)-aniline with copper halides in CH2Cl2:water
mixtures. Interestingly, this procedure yielded not only POP complexes, but also multinuclear,
heteroleptic complexes with POP as well as its isomer PPO, when additional triphenyl phosphine
was present.
The degradation only occurred when protic solvents such as acetonitrile or alcohols were present.
From analyses of aged solutions of complex 1-I in dichloromethane, non-dried acetonitrile as
well as acetonitrile-water (1:1) with gas chromatography, we were able to rule out a transfer of
oxygen from the oxyquinoline moiety: We could verify the formation of PPO and 8-
hydroxyquinoline, while neither quinoline, nor diquinolinylether or other coupling products were
found. Trace amounts of water, which may be present in acetonitrile, destabilize the complexes
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by protonation of the quinoline-moieties and serve as a source for the oxygen in the POP ligand.
Furthermore, the oxidation of the coordinated ligand by O2 does not seem to be of relevance for
our system, given that we were able to isolate to compounds 1-Cl, 1-Br, 1-I and 1-SCN as
powder samples at ambient conditions.
We performed no formal mechanistic studies regarding the degradation pathway from a Cu(I)-
POQ to Cu(I)-POP species. However, from our results and similar reactions from the literature,
we propose a mechanism for the reaction. While there is so far no concise analysis of a broad
range of POP complexes in one single publication, the synthetic pathways proposed so far can be
classified by either one of two groups (Scheme 2). Route I would be the thermal isomerization of
PPO in the presence of a metal precursors, as described by Wong and co-workers.[37] More
common is route II, where a phosphine precursor with a leaving group is employed. In our case,
LG- is the deprotonated form of 8-hydroxyquinoline, while Coetzee et al. used carboxylates as a
leaving group and Rh as a metal centre.[39] Zeiher and co-workers used sulfonates in
combination with Cr, Mo and W,[40] while Renz and co-workers used 3-hydroxypyridine and
Ag.[41] Older works describe the use of chloro-diphenylphosphine with chloride as a leaving
group for precursors.[42,43]
Scheme 2. Chemistry of the tetraphenyldiphosphoxane (POP) ligand.[35]
Scheme 3 contains our proposal for a degradation mechanism according to route II. We speculate
that the first, rate-determining step (i) is the cleavage of the Cu-N bond, yielding a semi-stable 16
valence electron Cu(I) species. From the molecular structures of 1-Br and 1-I, it is evident that
the bond between Cu and the quinoline-N is not weakened. In a second step, water may
coordinate (ii) and transfer a proton to the quinolate, which allows for the cleavage of the O-P
bond. This step (iii) yields a Cu(I)(OH)PPh2 species, which can react with an additional
equivalent of POQ to form POP.
Scheme 3. Proposed mechanism for the degradation of Cu(I)-POQ complexes.
In this study, we investigated a new approach to create chelated copper compounds by covalent
connection of two monodentate ligands, which yielded four copper(I) complexes. As expected,
we obtained orange-to-red emission. However, the materials seemed to be prone to degradation
in the presence of water. We formulated a hypothesis for the degradation mechanism based on
several distorted bonds identified by analysis of single crystal X-ray diffraction and chemical
analysis of the degraded solution.
From a material-developers point-of-view, the degradation reaction found in the POQ
compounds needs to be suppressed in a future development step. So far, only limited knowledge
and hypotheses exist regarding the degradation of Cu(I) OLED emitters. We believe that the
finding that ligands that contain potential leaving groups may lead to destruction of the emissive
compounds might be a general one. Also, the hypotheses that strained or compressed bonds need
to be avoided needs to be further investigated.
Corresponding Author
Dr. Daniel Volz, cynora GmbH; Prof. Dr. Stefan Bräse, Karlsruhe Institute of Technology.
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a Manuela Wallesch, Prof. Dr. Stefan Bräse, Institute of Organic Chemistry, Karlsruhe Institute
of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany. E-mail:braese@kit.edu
b Manuela Wallesch, Prof. Dr. Stefan Bräse, Institute of Toxicology and Genetics, Karlsruhe
Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany. E-mail:braese@kit.edu
cDr. Martin Nieger, Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55,
FIN-00014, Finland.
dDr. Daniel Volz, cynora GmbH, Werner-von-Siemensstraße 2-6, building 5110, 76646
Bruchsal, Germany. E-mail: volz@cynora.com
Acknowledgement. Financial support from the Deutsche Forschungsgemeinschaft (DFG) via
support for the transregional collaborative research center SFB/TRR 88 “3MET”, the
BioInterfaces program, and the Deutsche Telekom Stiftung are acknowledged.
Appendix: Supplementary material. Experimental procedures for the spectroscopic methods as
well as synthetic details and conditions used in the preparation of single crystal samples are
given in the supporting information. CCDC-1441665 (1-Br), CCDC-1441666 (1-I), CCDC-
1441671 (POP-Br-1), CCDC-1441677 (POP-Br-2) and CCDC-1441679 (POP-SCN) contain
the supplementary crystallographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Fig. 1
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Fig. 2
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Fig. 7
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Fig. 8
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Scheme 1
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Scheme 2
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Scheme 3
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Graphical abstract
Two POQ complexes and the decomposition products were identified as
tetraphenyldiphosphoxane complexes with single crystal X-ray diffraction.
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Highlights
 We modify existing Cu(I) complexes by introduction of bridges between ligands.
 We report the synthesis and properties of the POQ with various copper halides
 The photophysical properties and degradation of the materials were studied.

We propose a design rule to prevent this behavior in future development steps.
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