Copper(I) Complexes Bearing 1,2-Phenyl-Bridged P∧N, P∧N∧P, and N∧P∧N Chelate Ligands: Structures and Phosphorescence

Article abstract of DOI:10.1021/acs.inorgchem.6b02721

With the aim to obtain new phosphorescent Cu(I) compounds, several new 1,2-phenyl-bridged P^∧N, P^∧N^∧P, and N^∧P^∧N chelate ligands were designed and synthesized. These ligands were found to form complexes with Cu(I) ion readily via either solution reactions or solid-state grinding process. The new Cu(I) compounds based on this class of ligands display phosphorescence with emission color ranging from blue to red. The structure of the ligand and the nature of the N-heterocycle in the chelate ligands were found to have a significant impact on the phosphorescent properties of the Cu(I) compounds.

Full text of DOI:10.1021/acs.inorgchem.6b02721

Article
pubs.acs.org/IC
Copper(I) Complexes Bearing 1,2-Phenyl-Bridged P∧N, P∧N∧P, and
N∧P∧N Chelate Ligands: Structures and Phosphorescence
Chao Zeng,† Nan Wang,† Tai Peng,*,† and Suning Wang*,†,‡
†Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of
Technology, Beijing 100081, People’s Republic of China
‡Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: With the aim to obtain new phosphorescent Cu(I)
compounds, several new 1,2-phenyl-bridged P∧N, P∧N∧P, and
N∧P∧N chelate ligands were designed and synthesized. These ligands
were found to form complexes with Cu(I) ion readily via either
solution reactions or solid-state grinding process. The new Cu(I)
compounds based on this class of ligands display phosphorescence
with emission color ranging from blue to red. The structure of the
ligand and the nature of the N-heterocycle in the chelate ligands were
found to have a significant impact on the phosphorescent properties
of the Cu(I) compounds.
Chart 1. Examples of Commonly Used N-Heteroaryl Ligands
and Previously Reported P∧N Chelate Ligands for Cu(I)
Compounds and the Ligands Investigated in This Work
INTRODUCTION
■
Cu(I) complexes have recently attracted great attention as
promising luminescent materials, due to their low cost,
interesting phosphorescent properties, diverse structural
features, and potential applications in organic light-emitting
diodes (OLEDs),1−5 photosensitizers,6−8 sensing devices,9
biological imaging,10 and dye-sensitized solar cells
(DSCs).11−13 A large number of mono- and polynuclear
Cu(I) complexes have been developed and investigated for
various applications.14,15 In particular, Cu(I) complexes that
contain N-heteroaryl donors have been shown to be very
promising as luminescent materials.16,17 Many examples of
bright phosphorescent Cu(I)−halide complexes based on
mono- or bidentate N-heteroaryl ligands with tunable emission
colors have been demonstrated.18 Cu(I) complexes containing
a p-methylpyridine ligand, [CuX(PPh3)2(4-Mepy)] (X = Cl, Br,
I), have been shown to be a highly promising blue TADF
(thermally activated delayed fluorescence) emitter.19 However,
the poor stability of many of the previously reported Cu(I)
complexes in solution greatly hinders their practical applica-
tions.18,19 Combining the tunable electronic properties of N-
heteroaryl ligands with the strong stabilizing ability of
phosphine donors exerted on the Cu(I) center, the use of
P∧N chelate ligands has been shown to be an effective strategy
in achieving stable and bright luminescent Cu(I) compounds,
and many luminescent Cu(I) complexes based on P∧N,
N∧P∧N, and P∧N∧P ligands were investigated.20−26 However,
as shown in Chart 1, the previously investigated systems
focused mainly on the ligands with the phosphine unit being
bound directly to an N-heteroaryl ring or an aliphatic
group,24,27,28 with the former being usually bridging or
monochelate ligands due to the small chelate bite angle,
whereas the latter are often not effective in achieving bright
phosphorescent copper(I) compounds.23,24
With the aim to open the scope of P∧N chelate ligands and
to examine the impact of new P∧N chelate ligands on
phosphorescent properties of copper(I) compounds, we
designed and synthesized a series of 1,2-phenyl-bridged P∧N,
P∧N∧P, and N∧P∧N ligands shown in Chart 1 and investigated
the binding modes of the new ligands with the copper(I) ion
Received: November 11, 2016
© XXXX American Chemical Society
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Scheme 1. Synthetic Procedures for Ligands L2, L3, and L4
a
Scheme 2. Synthetic Procedures for the Cu(I) Complexes
a
The second yields for compounds 1, 2, and 4 are for the solid-state grinding procedure.
and the luminescent properties of the resulting copper(I)
complexes. The details are presented herein.
the aim to use them as tridentate chelate ligands to constrain
the geometry of the Cu(I) center.
To examine the impact of different structures and
compositions on phosphorescene, for ligand L1, three different
copper(I) complexes, a neutral monomer 1, a neutral dimer 2,
and a cationic monomer 3, were synthesized using the
procedure shown in Scheme 2 in 80−85% yield. For L2,
attempts were made to prepare both the neutral monomer and
the dimer, analogues of 1 and 2. However, only the monomer 4
was successfully isolated as a clean product in 90% yield. For
ligand L3 and L4, the reaction with CuI in CH2Cl2 or THF led
to the formation of the corresponding Cu(I) complexes 5 and 6
in 80 and 90% yield, respectively.
RESULTS AND DISCUSSION
■
Syntheses of Ligands and Copper Complexes. Ligand
L1 was prepared by a literature procedure.29 The synthetic
procedures for L2, L3, and L4 are shown in Scheme 1. The
precursor compounds 2-(2-fluorophenyl)pyridine30 and 2,6-
bis(2-fluorophenyl)pyridine31 for L2 and L3 were prepared
using literature procedures. Ligands L2 and L3 were obtained
by the reaction of the corresponding fluorine-substituted
precursor with KPPh2 in the presence of 18-crown-6 in 71
and 80% yield, respectively. In the case of ligand L4, the
attachment of a 2-pyridylphenyl group to the phosphine center
was accomplished by lithiation of 2-(2-bromophenyl)pyridine
with n-butyl lithium, followed by the addition of PPhCl2 in 57%
yield. Ligands L3 and L4 were designed and synthesized with
Inspired by the recent work of Kato32 and co-workers on
efficient mechanochemical synthesis of Cu(I) complexes, we
attempted the synthesis of the new Cu(I) complexes by the
solid-state grinding process. Indeed, compounds 1, 2, and 4 can
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be obtained readily in high yield (the second yield in Scheme 2)
by simply mixing and grinding the corresponding ligands with
CuI in a mortar, as shown by the photographs in Figure 1,
Figure 3. Crystal structures of 3 (left) and 4 (right).
Figure 1. Photographs showing the three-component reactant mixture
for compound 4 under ambient light (left) and UV light (right) before
(top) and after grinding (bottom). Ligand L2 emits a weak blue color
with λem = 460 nm in the solid state, which is distinctly different from
the blue−green emission color of 4 (λem = 504 nm) (see Figure S5a).
Figure 4. Crystal structures of 5 (left) and 6 (right).
tetrahedron. Ligand L1 chelates to the Cu atom, forming a
twisted six-membered chelate ring in compounds 1−3. The bite
angles of L1 in 1−3 are similar (88.54(7), 86.04(4), and
86.38(7)°, respectively), despite the different coordination
environment around the Cu center in these complexes. For L1-
based complexes, the Cu−I and the Cu−P bond lengths in
dimer 2 are somewhat shorter than those in monomer 1 due to
the reduced steric congestion in the dimer. The Cu(1)−Cu(1′)
separation distance in 2 is 2.7989(5) Å, similar to the sum of
the van der Waals radius of copper (2.80 Å), thus excluding any
significant metallophilic interactions. The cationic complex 3
also displays Cu−P and Cu−N bond lengths shorter than those
of 1, which may be attributed to the positive charge of the
Cu(I) atom that enhances ligand binding affinity. The crystal
data established that the triazolyl-based ligand L2 is an effective
bidentate chelate ligand for binding to the Cu(I) ion as its
pyridyl analogue L1 does. Ligand L2 forms a monomer
complex 4, which is an analogue of compound 1 with a similar
coordination environment. The chelate bite angle (89.8(1)°) of
L2 in 4 is similar to that of L1 in 1. The Cu−N bong length in
4 (2.076(3) Å) is, however, significantly shorter than that in 1
(2.155(2) Å). The pyridyl−phenyl unit in 1 is much more
twisted than the triazolyl−phenyl unit in 4, as evidenced by the
much larger dihedral angle in 1 (42.1°), relative to that in 4
(29.5°). As a consequence, the six-membered chelate ring in 4
is much less puckered than that in 1. In the crystal lattice,
compound 1 displays intermolecular π-stacking interactions
between the phenyl rings of L1 ligands (see Figure S2), while in
contrast, similar π-stacking interactions were not observed for 4
(see Figure S3).
which supports the generality of Kato’s procedure. Consistent
with the observation reported by Kato and co-workers, the
addition of a few drops of acetonitrile to aid the mixing of CuI
with the ligand is necessary in order for the grinding process to
work effectively.32 The new Cu(I) complexes were fully
characterized by NMR and elemental analyses. All new Cu(I)
compounds have a poor solubility in solvents such as CDCl3,
except the cationic compound 3 and compound 5, which have a
moderate solubility. Compounds 1 and 4 display two distinct
31P chemical shifts in 31P NMR spectra, consistent with the
presence of a PPh3 donor and a PPh2 donor from the chelate
ligand. Compound 5 displays a single peak at −6.79 ppm in the
31P NMR spectrum, indicating that both phosphorus atoms in
the L3 ligand are bound to the Cu(I) center in a symmetric
manner. NMR data indicated that compounds 1−6 are stable in
solution. To fully establish the structures of the new Cu(I)
compounds, single-crystal X-ray diffraction analyses were
performed for all compounds.
Crystal Structures of the Copper Complexes. The
crystal structures of the six new Cu(I) complexes are shown in
Figures 2−4, respectively. Important bond lengths and angles
are provided in Table 1. Solvent molecules such as CHCl3,
CH2Cl2, or THF were found in the crystal lattices of all
complexes, except that of compound 3. The geometry around
the Cu(I) center in all compounds, except in 5, is a distorted
Although both compounds 5 and 6 contain a tridentate
chelate ligand, their structures are very different. For 5, the
tridentate ligand L3 is bound to the Cu(I) center via the two
phosphorus atoms only, which along with the iodo ligand result
in a trigonal planar geometry around the Cu(I) center, as
indicated by the sum of the three bond angles around the Cu
center (360°). The pyridyl ring is approximately parallel to one
adjacent phenyl ring and perpendicular to the other phenyl
ring. The pyridyl N(1) atom in 5 is 2.626(1) Å away from the
Figure 2. Crystal structures of 1 (left) and 2 (right).
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Table 1. Important Bond lengths and Angles of the Cu(I) Complexes
bond lengths (Å)
bond angles (deg)
N(1)−Cu(1)−P(1)
1
2
3
4
Cu(1)−P(1)
Cu(1)−P(2)
Cu(1)−N(1)
Cu(1)−I(1)
2.2666(9)
2.2509(8)
2.155(2)
88.54(7)
P(1)−Cu(1)−P(2)
N(1)−Cu(1)−I(1)
N(1)−Cu(1)−P(2)
P(1)−Cu(1)−I(1)
P(2)−Cu(1)−P(1)
N(1)−Cu(1)−P(1)
P(1)−Cu(1)−I(1)
P(1)−Cu(1)−I(1′)
N(1)−Cu(1)−I(1)
N(1)−Cu(1)−I(1′)
I(1)−Cu(1)−I(1′)
N(1)−Cu(1)−N(2)
P(1)−Cu(1)−P(2)
P(1)−Cu(1)−N(1)
P(1)−Cu(1)−N(2)
P(2)−Cu(1)−N(1)
P(2)−Cu(1)−N(2)
P(1)−Cu(1)−N(2)
P(1)−Cu(1)−P(2)
P(1)−Cu(1)−I(1)
P(2)−Cu(1)−N(2)
P(2)−Cu(1)−I(1)
N(2)−Cu(1)−I(1)
P(1)−Cu(1)−P(2)
P(1)−Cu(1)−I(1)
P(2)−Cu(1)−I(1)
126.53(3)
104.90(7)
114.42(7)
112.12(2)
107.46(3)
86.04(4)
2.6564(4)
Cu(1)−P(1)
Cu(1)−N(1)
Cu(1)−I(1)
Cu(1)−I(1′)
Cu(1)···Cu(1′)
2.2337(5)
2.158(2)
120.82(2)
111.79(2)
110.18(4)
107.91(4)
115.48(1)
106.50(9)
135.54(3)
89.83(6)
2.6027(3)
2.6407(4)
2.7989(5)
Cu(1)−P(1)
Cu(1)−P(2)
Cu(1)−N(1)
Cu(1)−N(2)
2.2406(7)
2.2350(7)
2.074(2)
2.141(2)
113.40(6)
123.76(6)
86.26(6)
Cu(1)−P(1)
Cu(1)−P(2)
Cu(1)−N(2)
Cu(1)−I(1)
2.263(1)
2.240(1)
2.042(3)
2.6597(5)
89.8(1)
127.41(4)
106.54(3)
116.1(1)
108.02(3)
106.58(9)
123.90(2)
113.79(2)
122.31(2)
5
6
Cu(1)−P(1)
Cu(1)−P(2)
Cu(1)−I(1)
Cu(1)···N(1)
Cu(1)−P(1)
Cu(1)−N(1)
Cu(1)−N(2)
Cu(1)−I(1)
2.2510(6)
2.2150(6)
2.5259(3)
2.626(1)
2.183(1)
2.129(3)
2.138(3)
2.519(1)
P(1)−Cu(1)−N(1)
P(1)−Cu(1)−N(2)
N(1)−Cu(1)−N(2)
P(1)−Cu(1)−I(1)
N(1)−Cu(1)−I(1)
N(2)−Cu(1)−I(1)
90.27(9)
96.79(9)
94.6(1)
138.72(3)
115.35(8)
111.70(8)
Cu atom, with the pyridyl ring being approximately parallel to
the CuP2I plane, thus ruling out any significant σ binding
interactions with the Cu center. The pyridyl ring in 5 may have
only very weak π interaction with the Cu ion. Although three-
coordinate Cu(I) compounds are much less common than
four-coordinate ones, similar three-coordinate Cu(I) com-
pounds that display a weak π binding interaction between a
Cu(I) ion and an internal aryl ring have been reported
previously.24,33 For 6, the L4 ligand is bound to the Cu(I)
center through both nitrogen atoms and the phosphorus atom.
The two P∧N bite angles in 6 (90.27(9) and 96.79(9)°) are
greater than that in 1. The Cu(1)−P(1) bond length (2.183(1)
Å) in 6 is the shortest among the six Cu(I) compounds, which
may be attributed to the greater steric constraint in 6. Ligand
L4 in 6 is twisted to match the coordination geometry of Cu(I)
ion, with the dihedral angles between the pyridyl and the
adjacent phenyl rings being 42.0 and 49.3°, respectively. The
crystal structural data established that ligand L3 is not an
effective tridentate chelate ligand for Cu(I) due to the relatively
rigid and π-conjugated backbone of this ligand, whereas the
flexibility provided by ligand L4 makes it possible to bind to the
Cu(I) ion as a tridentate ligand. Crystal data also revealed the
presence of extensive π-stacking interactions between pyridyl
rings and between pyridyl and phenyl rings in the crystal lattice
of compound 6, similar to that observed for compound 1 (see
Figure S4).
Absorption Spectra of the Copper Complexes. All
copper(I) compounds reported here have a yellow or light
yellow color in solution and the solid state, except compounds
4 and 5, which are colorless. The absorption spectra of the
Cu(I) complexes 1−6 recorded in CH2Cl2 are shown in Figure
5, and the data are given in Table S1. Compared with the
absorption spectra of the free ligands L1, L2, L3, and L4 in
CH2Cl2 (see Figure S5), the intense absorption bands between
260 and 310 nm (ε > 20 000 M−1 cm−1) can be assigned to
ligand-centered (LC) π−π* transition. The shoulder peak at
∼310−350 nm can be assigned to the π−π* transition within
the N-heterocycle moiety. The broad and very weak low-energy
tails at ∼350−400 nm, which are not observed in the
corresponding free ligand spectrum, may be ascribed to the
charge transfer transition involving the copper ion or the iodide
ligand and are believed to be responsible for the observed pale
yellow or yellow color for some of the copper compounds.
To further understand the electronic properties of the
copper(I) compounds, time-dependent density functional
theory (TD-DFT) computational study was performed for all
complexes based on the optimized ground-state geometries.
The details of the TD-DFT data are given in Tables S2−S9. As
shown in Figure 6, for iodo-containing complexes 1, 2, and 4−
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pyridyl−phenyl unit of the chelate ligand with low oscillator
strengths (<0.01). For compound 3, the vertical excitations to
S1 and S2 involve mainly σ (Cu−P bonds) to the π* orbital of
the pyridyl−phenyl unit of the chelate ligand with a low
oscillator strength. The transitions to the higher excited state
do show appreciable oscillator strengths. For example, for
compound 1, the S0 → S3 transition has an oscillator strength
of 0.0141 (HOMO−2 → LUMO, 99%), whereas for 4, the S0
→ S4 transition has an oscillator strength of 0.0111 (HOMO−2
→ LUMO, 95%). Both of these transitions can be described as
σ (Cu−I bond) to π* (the P∧N chelate ligand) transitions. The
S0 → T1 transitions for all complexes involve mainly the
HOMO → LUMO transition. DFT data indicate that the
vertical excitation energy to S1 and T1 for 4 is about 0.2 eV
higher than that of the analogue 1. Interestingly, DFT data
revealed that the difference between the S1 and T1 energies for
complexes 1, 4, and 5 is very small (0.02, 0.03, and 0.01 eV,
respectively). Based on the DFT data, the low-energy
absorption tail of the copper complexes can be attributed to
MLCT/iodide-to-ligand charge transfer for the iodo-containing
complexes 1, 2, and 4−6 and MLCT/σ to π* transition for
complex 3.
Figure 5. UV−vis absorption spectra of compounds 1−6 in in CH2Cl2
at room temperature.
Phosphorescence of the Copper Complexes. Com-
pounds 1−6 are either non-emissive or weakly emissive in
solution at ambient temperature. However, these compounds
are all emissive in the solid state at ambient temperature with
distinct emission colors ranging from green−blue to red and
contrasting brightness, as shown by the photograph in Figure 7.
Figure 7. Photograph showing the colors of compounds 1−6 in the
crystalline state under ambient light (top) and UV light (bottom).
Therefore, the investigation on phosphorescent properties of
this class of compounds was focused on the samples in the solid
state, doped poly(methyl methacrylate) (PMMA) films or in a
frozen glass (2-Me-THF, 77 K). The emission spectra of
compounds 1−6 as powders and crystals at 298 K are shown in
Figure 8, and those of the doped PMMA films (5%) at 298 K
and the frozen solutions in 2-Me-THF at 77 K are shown in
Figure 9. All of the emission spectra for this class of compounds
are broad and unstructured, indicating that the emissions are
likely from charge transfer transitions,34,35 which appears to
agree with the TD-DFT data. The decay lifetimes of the copper
compounds are in the 5−177 μs range, consistent with
phosphorescence. The decay lifetimes at 77 K for all
compounds are much longer than those recorded at ambient
temperature due to the greatly reduced vibrational quenching/
structural relaxation at low temperature.
Figure 6. HOMO and LUMO diagrams for compounds 1−6.
6, the HOMO has a large contribution from the iodo ligand
and significant contribution from the Cu(I), whereas for the
cationic complex 3, the HOMO resides mainly on the Cu(I)
and Cu−P σ bonds. The LUMO is mainly located on the N-
heterocycle and the adjacent phenyl ring for all complexes. For
all compounds except the cationic molecule 3, the vertical
excitations to S1 and S2 states involve charge transfer transitions
from the iodide lone pair electrons to the π* orbital of the
As a neat solid, all six compounds display phosphorescence at
ambient temperature. For the three L1-based complexes 1, 2,
and 3, the emission wavelength varies significantly with λmax
=
601, 649, and 616 nm, respectively. The significant red shift of
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Figure 8. Normalized emission spectra of compounds 1−6 as powders (left) and as crystals (right) at 298 K.
Figure 9. Normalized emission spectra of compounds 1−6 doped PMMA film (5%) at 298 K (left) and in 2-Me-THF solution at 77 K (right).
Table 2. Phosphorescent Data of Copper Complexes 1−6
a
b
c
a
b
c
λmax (nm)
ΦPL
τav (μs)
kr (104 s−1
)
knr (104 s−1
)
λmax (nm)
ΦPL
τav (μs)
kr (104 s−1
)
knr (104 s−1
)
complex
1
3
5
2
4
6
d
powder
601
581
606
594
0.15
0.33
0.03
0.38
8.3
11.0
7.2
1.8
10.3
6.1
649
637
617
634
0.02
0.02
0.02
0.01
9.0
9.5
0.22
0.21
0.35
0.06
10.8
10.3
17.2
6.1
d
crystal
3.0
d
film
0.42
1.3
13.4
2.1
5.7
e
solution
29.0
16.1
complex
d
powder
616
606
589
571
0.01
0.01
0.02
0.01
9.0
7.8
0.11
0.13
0.25
0.02
11.0
12.9
12.3
1.56
504
504
518
500
0.54
0.27
0.16
0.97
29.9
36.1
19.1
177
1.80
0.75
0.84
0.55
1.5
d
crystal
2.0
d
film
8.0
4.4
e
solution
63.6
0.02
complex
d
powder
656
626
600
567
0.01
0.01
0.01
0.01
11.4
8.8
0.09
0.11
0.11
0.01
8.7
11.3
10.9
0.67
684
677
588
590
0.01
0.01
0.01
0.01
9.6
10.8
6.9
0.10
0.09
0.15
0.01
10.3
9.2
d
crystal
d
film
9.3
14.4
1.5
e
solution
148.2
67.0
a
Average lifetime (τav) is calculated by the equation τav = ΣAiτi2/ΣAiτi, where Ai is the pre-exponential for lifetime τi. The estimated standard
b
c
deviation for τav is 2−10%. Radiative rate constants (kr) were estimated from the equation kr = Φ/τav. Nonradiative rate constants (knr) were
d
estimated from the equation knr = kr(1 − Φ)/Φ. The data for neat powder, crystals, and PMMA films doped with 5 wt % of the Cu(I) compound
e
were recorded at room temperature. Glassy solution recorded in 2-Me-THF at 77 K.
the dimer complex 2 compared to the monomer 1 is consistent
with that in the previous report on related compounds.35 The
decay lifetimes of these three compounds are similar, 8.3−9.0
μs. Significantly, however, only compound 1 has a moderate
emission quantum efficiency (0.15), whereas 2 and 3 emit
weakly. These data illustrate that the ligands and the
environment about the Cu(I) center have a dramatic impact
on both emission energy and efficiency. As crystals, the
emission energy of compounds 1−3 follows a trend similar to
that of the powder sample, although the λmax of 1 and 2
experiences a 20 nm blue shift, which may be attributed to the
presence of CHCl3 solvent molecules in the crystal lattice that
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reduces intramolecular interactions. The emission quantum
yield of 1 is more than doubled (0.33) in the crystalline state
than that in the powder state. In 5% doped PMMA films, the
emission spectra of 2 and 3 experience a significant blue shift,
which may be attributed to reduced intermolecular interaction
due to dilution, compared to neat powders or crystals. The
emission energy of the PMMA film of compound 1 does not
change significantly from its powder state; however, its
emission efficiency decreases drastically from 0.15 to 0.02,
due to the great increase of nonradiative decay rate constant
(knr; see Table 2), caused perhaps by the reduced structural
rigidity in PMMA. In the frozen solution at 77 K, the emission
efficiency of 1 becomes impressive, reaching 0.38, which
supports that emission quenching via vibrational or structural
distortion is a main cause for its low emission efficiency at
ambient temperature. For 2 and 3, the emission efficiency of
the frozen solution at 77 K is still very low. Thus, the low
emission quantum efficiency of 2 and 3 is the intrinsic nature of
these two molecules. The dimer and the cationic structures are
therefore undesirable for achieving bright Cu(I) phosphor-
escent compounds based on the 1,2-phenyl-bridged P∧N
chelate ligands. For this reason, our investigation on L2-based
complexes focused on the monomer compound 4 only.
As an analogue of compound 1, compound 4 has an
extremely high emission quantum efficiency (0.97) in the
frozen solution at 77 K. Its emission efficiency as a neat powder
at ambient temperature is also very impressive (0.54). Unlike 1,
which becomes weakly emissive in PMMA, compound 4 retains
a moderate emission efficiency (0.16) in doped PMMA films.
Furthermore, consistent with the TD-DFT data, the emission
energy of 4 is much higher than that of 1, emitting a distinct
blue−green color with λmax = 500−518 nm, depending on the
physical state of the sample. Compound 4 also has the longest
lifetime among all the complexes. The reduced twist of the ph-
azolyl unit relative to the ph-py unit in 1 may be responsible for
the much smaller nonradiative decay rate constant, knr, and the
higher emission quantum efficiency of 4.
The trigonal planar compound 5 has a very low emission
quantum efficiency at ambient temperature and 77 K. Its
emission spectra undergo a significant and progressively blue
shift from powder, crystals, to PMMA film and the frozen
solution with a 90 nm of λmax difference between the powder
sample and the frozen solution. Intramolecular interactions and
the weak pyridyl−Cu π interactions in 5 may be responsible for
its low phosphorescent efficiency. The luminescent properties
of 6 resemble those of 5 with nearly a 90 nm blue shift of the
emission λmax from powder to the frozen solution and a
consistently low emission quantum efficiency. Again, the highly
twisted and distorted structure of 6 may be responsible for its
low emission efficiency. With the phosphorescent energy of the
copper complexes in the frozen solution at 77 K as the intrinsic
property of each molecule, the emission energy follows the
order of 4 > 3 ≈ 5 > 1 ≈ 6 > 2, which agrees with the TD-DFT
calculated trend of the vertical excitation energy to T1.
tetrahedral geometries for other complexes. The new Cu(I)
complexes are stable in the solutions, which can be attributed to
the chelation by the bidentate or tridentate ligands. All
complexes investigated in this work are phosphorescent in
the solid state or in doped PMMA films with emission colors
covering the entire visible region from blue to deep red.
Experimental and computational work demonstrated that the
phosphorescent properties of the Cu(I) compounds are highly
dependent on the N-heteroaryl ring, the geometry of the
ligands/the complexes, and the state of and the environment
around the complex.
EXPERIMENTAL SECTION
■
General Information. Air- and water-sensitive reactions were
carried out in oven-dried glassware under a nitrogen or argon
atmosphere. Solvents were dried using standard procedures prior to
use. All reagents are of analytical grade and were used without further
purification. Thin-layer chromatography was carried out on silica gel
plates (silica gel 60, F254, Merck) with detection by 254 or 365 nm
UV light. Purification was performed with preparative chromatography
using normal-phase silica gel (silica gel 60, 230−400 mesh) or basic
1
alumina. H NMR (400 MHz), 13C NMR (101 MHz), and 31P NMR
(162 MHz) spectra were recorded on a Bruker Avance-400
spectrometer. Chemical shifts are reported as δ values (ppm) relative
1
to SiMe4 for H NMR (internal standard) and 85% aqueous H3PO4
for 31P NMR (external standard). UV−vis spectra were recorded on an
Agilent Cary 300 scan UV−vis absorption spectrophotometer.
Excitation and emission spectra, time-resolved phosphorescence
spectra, and luminescent decay lifetimes were recorded on Edinburgh
Instruments FLS980 spectrophotometer, equipped with a xenon flash
lamp. All solutions for photophysical data measurements were
degassed under a nitrogen atmosphere. The absolute photo-
luminescence quantum yields were measured on a Hamamatsu QY
C11347-11 spectrometer. High-resolution mass spectra (HRMS) were
obtained from an Agilent Q-TOF 6520 LC-MS spectrometer.
Elemental analysis was performed on a EuroVector EA3000
instrument (EuroVector, SpA). Precursors 2-(2-fluorophenyl)-
pyridine,30 2,6-bis(2-fluorophenyl)pyridine,31 2-(2-bromophenyl)-
pyridine,36 and ligand 2-(2-(diphenylphosphanyl)phenyl)pyridine
(L1)29 were prepared according to previously reported procedures.
Synthesis of P∧N Ligands. 4-(2-Fluorophenyl)-1-methyl-1H-
1,2,3-triazole. A mixture of 1-ethynyl-2-fluorobenzene (0.732 g, 6.1
mmol), sodium azide (0.397 g, 6.1 mmol), iodomethane (0.866 g, 6.1
mmol), and CuI (0.012 g, 0.06 mmol) in 10 mL of water was heated
to 65 °C for 17 h. After being cooled to room temperature, CH2Cl2
(40 mL) was added. The organic layer was separated and washed with
water twice, dried over anhydrous Na2SO4, and evaporated under
vacuum. The crude product was further purified by column
chromatography on silica gel using petroleum ether/AcOEt (10:1)
as eluent to obtain the product as white powder. Yield: 0.73 g (68%).
1H NMR (400 MHz, CDCl3): δ 8.28 (td, J = 7.6, 1.9 Hz, 1H), 7.91 (d,
J = 3.7 Hz, 1H), 7.39−7.18 (m, 2H), 7.14−7.09 (m, 1H), 4.14 (s, 3H).
13C{H} NMR (100 MHz, CDCl3): δ 159.22 (d, J = 247.6 Hz), 141.39,
129.29 (d, J = 8.5 Hz), 127.80 (d, J = 3.6 Hz), 124.62 (d, J = 3.3 Hz),
123.73 (d, J = 12.6 Hz), 118.56 (d, J = 12.9 Hz), 115.65 (d, J = 21.7
Hz), 36.74.
4-(2-(Diphenylphosphanyl)phenyl)-1-methyl-1H-1,2,3-triazole
(L2). To solution of 4-(2-fluorophenyl)-1-methyl-1H-1,2,3-triazole
(0.481 g, 2.72 mmol) and 18-crown-6 (0.934 g, 3.53 mmol) in THF
(25 mL) was slowly added a 0.5 M solution of potassium
diphenylphosphide (6.6 mL, 3.50 mmol) in THF at 0 °C. The
mixture was stirred at room temperature for 24 h, after which water
(10 mL) was added, and it was concentrated in vacuum. Et2O (60 mL)
was added, and the organic phase was washed with water twice, dried
over anhydrous Na2SO4, and evaporated under vacuum. The crude
product was further purified by column chromatography on basic
alumina 90 column using petroleum ether/AcOEt (8:1) as eluent to
obtain the product as white solid. Yield: 0.66 g (71%). 1H NMR (400
CONCLUSIONS
■
A series of new phenyl-bridged P∧N, P∧N∧P, and N∧P∧P
ligands, as well as the corresponding copper(I) complexes, were
successfully synthesized and fully characterized. The simple and
solvent-free mechanochemical method was applied successfully
in the preparation of some of the new complexes in high yields.
X-ray crystallographic analysis revealed unusual distorted
trigonal planar coordination geometry for 5 and approximately
G
DOI: 10.1021/acs.inorgchem.6b02721
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
MHz, CDCl3): δ 8.03 (ddd, J = 7.8, 4.1, 1.1 Hz, 1H), 7.65 (d, J = 1.8
Hz, 1H), 7.49 (td, J = 7.6, 1.3 Hz, 1H), 7.39−7.33 (m, 6H), 7.31−7.24
(m, 5H), 7.01 (ddd, J = 7.7, 4.4, 1.1 Hz, 1H), 4.06 (s, 3H). 31P{H}
NMR (162 MHz, CDCl3): δ −11.10. HRMS (ESI) calcd for
C21H18N3P [M + H+] 344.1311; found 344.1331.
C46H36Cu2I2N2P2: C, 58.84; H, 4.18; N, 5.28. Found: C, 58.43; H,
3.82; N, 5.37.
[(2,6-Bis(2-(diphenylphosphanyl)phenyl)pyridine)2Cu·BF4] (3). In
the glovebox, Cu(CH3CN)4·BF4 (133 mg, 0.42 mmol) and L1 (287
mg, 0.82 mmol) were dissolved in THF (10 mL), and the mixture was
stirred for 10 h. After addition of diethyl ether (20 mL), the reaction
solution was stirred for an additional 0.5 h. The obtained yellow
precipitates were collected by filtration, washed with diethyl ether, and
dried under vacuum to obtain complex 3 as yellow powder. Yield: 283
2,6-Bis(2-(diphenylphosphanyl)phenyl)pyridine (L3). To solution
of 2,6-bis(2-fluorophenyl)pyridine (152 mg, 0.57 mmol) and 18-
crown-6 (391 mg, 1.48 mmol) in THF (15 mL) was slowly added a
0.5 M solution of potassium diphenylphosphide (2.7 mL, 1.37 mmol)
in THF at 0 °C. The mixture was stirred at room temperature for 48 h,
after which water (10 mL) was added, and it was concentrated in
vacuum. Et2O (50 mL) was added, and the organic phase was washed
with water twice, dried over anhydrous Na2SO4, and evaporated under
vacuum. The crude product was further purified by column
chromatography on basic alumina 90 column using petroleum ether
to petroleum ether/EtOAc (20:1) as eluent to obtain the product as
white solid. Yield: 0.27 g (80%). 1H NMR (700 MHz, CD2Cl2): δ 7.55
(t, J = 7.7 Hz, 1H), 7.43 (ddd, J = 7.6, 4.3, 1.2 Hz, 2H), 7.34−7.24 (m,
18H), 7.23−7.18 (m, 8H), 7.06 (ddd, J = 7.7, 3.9, 1.1 Hz, 2H). 31P{H}
NMR (162 MHz, CDCl3): δ −11.33. HRMS (ESI) calcd for
C41H31NP2 [M + H+] 600.2004; found 600.2045.
1
mg (81%). H NMR (400 MHz, CDCl3): δ 8.20 (d, J = 5.4 Hz, 2H),
7.78 (td, J = 7.8, 1.7 Hz, 2H), 7.71−7.62 (m, 4H), 7.45−7.34 (m, 8H),
7.30−7.12 (m, 18H), 6.91−6.84 (m, 2H). 31P{H} NMR (162 MHz,
CDCl3): δ −4.68. Anal. Calcd for C46H36BCuF4N2P2: C, 66.64; H,
4.38; N, 3.38. Found: C, 66.50; H, 4.54; N, 3.54.
[(4-(2-(Diphenylphosphanyl)phenyl)-1-methyl-1H-1,2,3-triazole)-
(PPh3)CuI] (4). To a dichloromethane solution (15 mL) of L4 (159
mg, 0.46 mmol) were added CuI (88 mg, 0.46 mmol) and PPh3 (121
mg, 0.46 mmol). The mixture was stirred at room temperature for 2 h
and then evaporated under reduced pressure. The white powder was
collected by filtration and then washed with diethyl ether and CH2Cl2
and dried under vacuum to obtain complex 3 as white powder. Yield:
1
340 mg (92%). H NMR (700 MHz, CD2Cl2): δ 7.75 (s, 1H), 7.66−
2,2′-((Phenylphosphanediyl)bis(2,1-phenylene))dipyridine (L4).
Under an argon atmosphere, n-BuLi (2.79 mL, 2.5 M in hexane)
was added dropwise to a solution of 2-(2-bromophenyl)pyridine
(1.635 g, 6.99 mmol) in THF (60 mL) at −78 °C. The resulting
mixture was stirred for 1 h at −78 °C. Then phenylphosphine
dichloride (0.619 g, 3.46 mmol) was added in one portion. After being
stirred for another 1 h, the reaction mixture was allowed to warm to
room temperature and stirred overnight. Subsequently, water (10 mL)
was added, and the reaction mixture was extracted with DCM (50 mL
× 3). Then the combined organic solution was dried over anhydrous
Na2SO4 and evaporated under vacuum, and the crude product was
further purified by column chromatography on silica gel using
petroleum ether/AcOEt (2:1) as eluent to obtain the product as
7.40 (m, 9H), 7.39−7.07 (m, 19H), 7.01 (s, 1H), 3.96 (s, 3H). 31P{H}
NMR (283 MHz, CD2Cl2): δ −2.59, −12.35. Anal. Calcd for
C39H33CuIN3P2: C, 52.14; H, 3.42; N, 2.64. Found: C, 51.98; H, 3.56;
N, 2.83.
[(2,6-Bis(2-(diphenylphosphanyl)phenyl)pyridine)CuI] (5). To a
dichloromethane solution (15 mL) of L3 (220 mg, 0.37 mmol) was
added CuI (70 mg, 0.37 mmol). The mixture was stirred at room
temperature for 6 h and then evaporated under reduced pressure. The
crude product was recrystallized from CH2Cl2 and petroleum ether to
1
obtain complex 5 as pale-yellow crystals. Yield: 232 mg (80%). H
NMR (400 MHz, CDCl3): 7.53−7.42 (m, 9H), 7.40−7.27 (m, 10H),
7.23−7.19 (m, 8H), 7.17−7.08 (m, 4H). 31P{H} NMR (162 MHz,
CDCl3): δ −6.79. Anal. Calcd for C41H31CuINP2: C, 62.33; H, 3.95;
N, 1.77 Found: C, 62.77; H, 4.20; N, 1.89.
1
yellow solid. Yield: 0.53 g (37%). H NMR (400 MHz, CDCl3): δ
8.45−8.43 (m, 2H), 7.55−7.52 (m, 2H), 7.45 (td, J = 7.6, 1.6 Hz, 2H),
7.36 (td, J = 7.5, 1.3 Hz, 2H), 7.25−7.22 (m, 9H), 7.17−7.14 (m, 2H),
7.04 (ddd, J = 7.5, 4.9, 1.0 Hz, 2H). 13C{H} NMR (100 MHz,
CDCl3): δ 158.65 (d, J = 3.3 Hz), 148.63, 145.37 (d, J = 25.3 Hz),
138.81 (d, J = 13.9 Hz), 136.84 (d, J = 17.0 Hz), 135.26, 134.80,
134.15 (d, J = 20.5 Hz), 129.52 (d, J = 4.7 Hz), 128.46, 128.27 (d, J =
6.6 Hz), 128.13, 128.12 (d, J = 3.4 Hz), 123.99 (d, J = 5.9 Hz), 121.56.
31P{H} NMR (162 MHz, CDCl3): δ −16.87. HRMS (ESI) calcd for
[(2,2′-((Phenylphosphanediyl)bis(2,1-phenylene))dipyridine)CuI]
(6). To a THF solution (15 mL) of ligand L4 (120 mg, 0.29 mmol)
was added CuI (55 mg, 0.29 mmol). The mixture was stirred at room
temperature for 10 h and the solvent removed under reduced pressure.
The red powder was collected by filtration and then washed with THF
1
and dried under vacuum. Yield: 160 mg (91%). H NMR (400 MHz,
CDCl3): δ 9.04 (d, J = 4.6 Hz, 2H), 7.62 (td, J = 7.8, 1.5 Hz, 2H),
7.56−7.48 (m, 2H), 7.45−7.30 (m, 7H), 7.24−7.11 (m, 6H), 7.07 (t, J
= 7.8 Hz, 2H). 31P{H} NMR (162 MHz, CDCl3): δ −19.56. Anal.
Calcd for C28H21CuIN2P: C, 55.41; H, 3.49; N, 4.62 Found: C, 55.54;
H, 3.90; N, 4.65.
Solid State Grinding Synthesis. Complexes 1: The mixture of
CuI (19 mg, 0.1 mmol), Ph3P (26 mg, 0.1 mmol), ligand L1 (34 mg,
0.1 mmol), and two drops of CH3CN was manually ground in a
mortar for 3 min. The obtained yellow powder was washed with
diethyl ether and CH2Cl2/petroleum ether (2:1) and dried under
vacuum. Complex 1 was obtained as yellow powder in a yield of 63 mg
(84%).
C28H21N2P [M + H+] 417.1515; found 417.1532.
Synthesis of Copper Complexes. [(2-(2-(Diphenylphosphanyl)-
phenyl)pyridine)(PPh3)CuI] (1). To a dichloromethane solution (15
mL) of L1 (170 mg, 0.5 mmol) were added CuI (95 mg, 0.5 mmol)
and PPh3 (131 mg, 0.5 mmol). The mixture was stirred at room
temperature for 2 h, and then solvent was removed under reduced
pressure. The yellow powder was collected by filtration and washed
with diethyl ether and dried under vacuum. The crude product was
recrystallized from CH2Cl2 to obtain complex 1 as yellow crystals.
1
Yield: 317 mg (80%). H NMR (400 MHz, CDCl3): δ 9.50 (s, 1H),
7.57 (td, J = 7.8, 1.7 Hz, 1H), 7.48 (td, J = 7.6, 1.2 Hz, 1H), 7.44−7.22
(m, 18H), 7.21−7.12 (m, 10H), 7.10−7.04 (m, 1H), 6.82 (t, J = 7.6
Hz, 1H). 31P{H} NMR (162 MHz, CDCl3): δ −1.39, −13.85. Anal.
Calcd for C41H33CuINP2: C, 62.17; H, 4.20; N, 1.77. Found: C, 62.02;
H, 4.16; N, 1.85.
Complexes 2: The mixture of CuI (19 mg, 0.1 mmol), ligand L1
(34 mg, 0.1 mmol), and two drops of CH3CN was manually ground in
a mortar for 3 min. The obtained yellow powder was washed with
diethyl ether and CH2Cl2 and dried under vacuum. Complex 2 was
obtained as yellow powder in a yield of 47 mg (89%).
[(2-(2-(Diphenylphosphanyl)phenyl)pyridine)2Cu2I2] (2). To a
dichloromethane solution (15 mL) of L1 (212 mg, 0.63 mmol) was
added CuI (119 mg, 0.63 mmol). The mixture was stirred at room
temperature for 2 h, and then solvent was removed under reduced
pressure. The yellow powder was collected by filtration and then
washed with diethyl ether. The crude product was recrystallized from
CH2Cl2 and petroleum ether to obtain complex 2 as yellow crystals.
Yield: 313 mg (85%). 1H NMR (400 MHz, CDCl3): δ 9.09 (d, J = 4.9
Hz, 2H), 7.58−7.48 (m, 9H), 7.48−7.39 (m, 4H), 7.32−7.22 (m, 7H),
7.21−7.16 (m, 10H), 6.98 (t, J = 6.6 Hz, 2H), 6.89 (t, J = 8.0 Hz, 2H).
31P{H} NMR (162 MHz, CDCl3): δ −13.94. Anal. Calcd for
Complexes 4: The mixture of CuI (19 mg, 0.1 mmol), Ph3P (26
mg, 0.1 mmol), ligand L4 (34 mg, 0.1 mmol), and two drops of
CH3CN was manually ground in a mortar for 3 min. The obtained
white powder was washed with CH2Cl2 and dried under vacuum.
Complex 4 was obtained as white powder in a yield of 71 mg (90%).
X-ray Crystallography. Single crystals were obtained by means of
a solvent diffusion method at room temperature. Thereinto, the single
crystals of compounds 1, 2, 3, and 5 were grown from chloroform,
crystal 4 was from dichloromethane, and crystal 6 from THF. Single-
crystal structure determinations were performed on a Bruker D8-
Venture diffractometer with Mo target (λ = 0.71073 Å). Data were
H
DOI: 10.1021/acs.inorgchem.6b02721
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
processed on a PC with the aid of the Bruker SHELXTL software
package and corrected for absorption effects. All non-hydrogen atoms
were refined anisotropically. The positions of hydrogen atoms were
calculated and refined isotropically. The details of crystal data,
collection parameters, and results of analyses are provided in the
Supporting Information. The crystal data of 1, 2, 3, 4, 5, and 6 have
been deposited to the Cambridge Crystallographic Data Center with
deposition numbers of CCDC 1492171, 1492178, 1492173, 1492172,
1492174, and 1494044, respectively.
Theoretical Calculations. The DFT calculations were performed
using Gaussian0937 software package. The ground-state geometries
were fully optimized at the Becke three-parameter hybrid exchange
and the Lee−Yang−Parr correlation functional (B3LYP)38−40 level
using the LanL2DZ41−44 basis set for Cu and I atoms and 6-
31G(d)45,46 basis set for all other atoms. Stationary points were further
characterized by frequency analyses. TD-DFT calculations were
performed to compute both singlet and triplet vertical excitation
energies and oscillator strengths in the gas phase.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02721.
■
S
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Crystallographic data for 1 (CIF)
Crystallographic data for 2 (CIF)
Crystallographic data for 3 (CIF)
Crystallographic data for 4 (CIF)
Crystallographic data for 5 (CIF)
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Crystallographic data for 6 (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: pengtai@bit.edu.cn.
*E-mail: wangs@chem.queensu.ca.
ORCID
(22) Musina, E. I.; Shamsieva, A. V.; Strelnik, I. D.; Gerasimova, T.
P.; Krivolapov, D. B.; Kolesnikov, I. E.; Grachova, E. V.; Tunik, S. P.;
Bannwarth, C.; Grimme, S.; Katsyuba, S. A.; Karasik, A. A.; Sinyashin,
O. G. Dalton Trans. 2016, 45, 2250−2260.
■
(23) Zink, D. M.; Grab, T.; Baumann, T.; Nieger, M.; Barnes, E. C.;
Klopper, W.; Brase, S. Organometallics 2011, 30, 3275−3283.
̈
Suning Wang: 0000-0003-0889-251X
(24) Wei, F.; Liu, X.; Liu, Z.; Bian, Z.; Zhao, Y.; Huang, C.
CrystEngComm 2014, 16, 5338−5344.
Notes
(25) Zink, D. M.; Bachle, M.; Baumann, T.; Nieger, M.; Kuhn, M.;
̈
̈
The authors declare no competing financial interest.
Wang, C.; Klopper, W.; Monkowius, U.; Hofbeck, T.; Yersin, H.;
Brase, S. Inorg. Chem. 2013, 52, 2292−2305.
̈
ACKNOWLEDGMENTS
This study was supported by the National Natural Science
Foundation of China (2157017, 21501011).
■
(26) Zink, D. M.; Baumann, T.; Friedrichs, J.; Nieger, M.; Brase, S.
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Inorg. Chem. 2013, 52, 13509−13520.
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