Photophysical Properties of Simple Palladium(0) Complexes Bearing Triphenylphosphine Derivatives

Article abstract of DOI:10.1021/acs.inorgchem.1c00631

Pd(0) complexes with monodentate phosphine ligands, [Pd(P)n] (n = 3, 4), are well-known catalysts. However, the nature of the Pd(0) complex, especially the basic photophysical properties of the Pd(0) complexes, has not been extensively explored. In this work, we measured the general photophysical properties and crystal structures of Pd(0)-bearing PPh3 derivatives in the solid state and in solution. In the solid state, four-coordinated Pd(0) complexes exhibited blue-yellow emission. On the other hand, three-coordinated Pd(0) complexes displayed yellow-orange emission. In solution, orange emission of three-coordinated complexes was observed, and prompt fluorescence was detected using time-resolved emission spectroscopy, which suggests a thermally activated delayed fluorescence mechanism. Density functional theory (DFT) and time-dependent DFT calculations show that the difference in the transition mechanism between the [Pd(PPh3)4] and [Pd(PPh3)3] complexes explains the different emission colors. The emitting states of both complexes have metal-To-ligand charge-Transfer character, but the metal-centered d → p transition is considerably incorporated for emission of the tris complex.

Full text of DOI:10.1021/acs.inorgchem.1c00631

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Article
Photophysical Properties of Simple Palladium(0) Complexes Bearing
Triphenylphosphine Derivatives
Daichi Kakizoe, Michihiro Nishikawa, Takuma Ohkubo, Masashi Sanga, Munetaka Iwamura,
Koichi Nozaki, and Taro Tsubomura*
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ABSTRACT: Pd(0) complexes with monodentate phosphine ligands, [Pd(P)n] (n = 3,
4), are well-known catalysts. However, the nature of the Pd(0) complex, especially the
basic photophysical properties of the Pd(0) complexes, has not been extensively explored.
In this work, we measured the general photophysical properties and crystal structures of
Pd(0)-bearing PPh3 derivatives in the solid state and in solution. In the solid state, four-
coordinated Pd(0) complexes exhibited blue-yellow emission. On the other hand, three-
coordinated Pd(0) complexes displayed yellow-orange emission. In solution, orange
emission of three-coordinated complexes was observed, and prompt fluorescence was
detected using time-resolved emission spectroscopy, which suggests a thermally activated
delayed fluorescence mechanism. Density functional theory (DFT) and time-dependent DFT calculations show that the difference
in the transition mechanism between the [Pd(PPh3)4] and [Pd(PPh3)3] complexes explains the different emission colors. The
emitting states of both complexes have metal-to-ligand charge-transfer character, but the metal-centered d → p transition is
considerably incorporated for emission of the tris complex.
PPh3 and its derivatives in the solid state and in solution. Some
interesting crystal structures of these molecules are also
described in this paper. These complexes show blue-to-orange
luminescence under UV light. This paper provides outstanding
photophysical properties of a series of palladium complexes
and discusses the mechanisms of emission.
INTRODUCTION
■
Pd(0) complexes with monodentate phosphine ligands have
been widely used as catalysts for various organic synthesis
reactions. Coordination numbers of the palladium(0) com-
plexes with monodentate phosphine ligands range from 2 to
4.1−4 In these complexes, a majority of the monodentate
phosphine complexes have three or four coordination
structures. In particular, tetrakis(triphenylphosphine)palladium
([Pd(PPh3)4]) is one of the most important catalysts for many
cross-coupling reactions.5−7 Despite the applications in
catalysis, the photophysical properties of the Pd(0) complexes
have not been extensively explored. More than 30 years ago,
Harvey, Gray, and Casper reported the emission of [Pd-
(PPh3)4] and its congener, [Pd(PPh3)3], in solution and in the
solid state.8−10 They reported that [Pd(PPh3)4] and [Pd-
(PPh3)3] showed green and orange emission, respectively, in
the solid state. However, [Pd(PPh3)4] is decomposed into
[Pd(PPh3)3] and a free ligand (triphenylphosphine, PPh3) in
solution; therefore, [Pd(PPh3)4] exhibits orange emission
originating from [Pd(PPh3)3] in solution. The photolumines-
cence of d10 metal complexes, especially for copper(I),
silver(I), or gold(I) species, has been extensively studied in
recent years.11−13 However, the number of reports focused on
the emission of Pd(0) species, which also have a d10 metal
configuration, is very limited.14−19 Furthermore, the crystal
structures of Pd(0) complexes bearing monodentate phos-
phine ligands have rarely been explored compared with the
vast number of studies of their catalytic activities.20,21 Here, we
describe the general photophysical properties of Pd(0)-bearing
EXPERIMENTAL SECTION
■
Materials and Methods. Triphenylphosphine (PPh3), tri-m-
tolylphosphine [P(m-tol)3], and tri-p-tolylphosphine [P(p-tol)3] were
purchased from Tokyo Chemical Industry Co., Ltd. Tris(3,5-
dimethylphenyl)phosphine [P(xylyl)3], tris(4-trifluoromethylphenyl)-
phosphine [P(p-CF3Ph)3], tris(3,5-ditrifluoromethylphenyl)-
phosphinepalladium(0) (63), dry solvent [methanol (MeOH) or
toluene], and ultrapure water were purchased from Wako Pure
Chemical Industry Co., Ltd. Tetrakis(triphenylphosphine)-
palladium(0) (14) and dry diethyl ether were purchased from
Kanto Chemical Co., Inc. Degassed solvent used in the synthesis was
prepared by at least Ar bubbling for 20 min. Schlenk tubes were dried
with a heat gun in vacuo. 14 was used as supplied, and 63 was
recrystallized by tetrahydrofuran (THF)/ether before use. 13 was
obtained as follows: 14 was dissolved in toluene at 60 °C and then
cooled to room temperature; thus, deposited yellow crystals were
Received: March 2, 2021
Published: June 9, 2021
https://doi.org/10.1021/acs.inorgchem.1c00631
© 2021 American Chemical Society
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a
Scheme 1. Syntheses and Abbreviations of the Complexes Used in This Study
a
The complexes are denoted by the phosphines and the coordination numbers: for example, 14 means [Pd(PPh3)4]. We use 13, 14, 23, 24, 33, 44, 53,
54, and 63 complexes in this study. Some complexes are commercially available, and such complexes were obtained from the vendors.
separated by filtration. The Pd complexes studied were unstable under
air; therefore, they were stored at −18 °C under an argon atmosphere,
and the synthetic yields were not measured. The 2-methylallylpalla-
dium chloride dimer [Pd2Cl2(2-methylallyl)2] was prepared by the
literature method and purified by recrystallization in CH2Cl2/n-
hexane.22
precipitates dissolved. The mixture was filtered and MeOH slowly
added. The mixture was stored at 0 °C to yield yellow crystals. Anal.
Found: C, 74.18; H, 6.00. Calcd for 23 (C63H63P3Pd): C, 74.22; H,
6.23.
Pd(P(p-tol)3)3 (33). 33 was synthesized according to a procedure
similar to that of 24 by employing (2-methylallyl)PdCl2 (25.6 mg,
0.065 mmol) and P(p-tol)3 (201 mg, 0.66 mmol) except that the
obtained crystals were washed with dry diethyl ether. The product
was obtained as orange crystals. Anal. Found: C, 74.12; H, 6.28. Calcd
for 33 (C63H63P3Pd): C, 74.22; H, 6.23.
Emission and absorption spectra in solution were obtained in a
solvent degassed by at least five freeze−pump−thaw cycles using a
quartz cell fitted with a Teflon vacuum stopcock. Absorption spectra
were measured on an Agilent 8453 UV−visible spectrometer.
Emission spectra, quantum yields of the emission, and emission
lifetimes were collected on a laboratory-made apparatus.23,24
Correction for the wavelength dependence of the sensitivity of the
system was done using a calibrated standard tungsten lamp
manufactured by USHIO Inc. Emission quantum yields in solution
were obtained using [Ru(bpy)3](PF6)2 as a standard (Φ = 0.018 in
air-saturated acetonitrile).25 The quantum yields in the solid state
were measured by the absolute method using an integrating sphere.
Picosecond time-resolved emission data were measured using a
laboratory-made time-correlated single-photon-counting (TCSPC)
system. The second harmonics (405 nm) of the fundamental of a
mode-locked Ti:sapphire laser (Tsunami, Spectra Physics, 810 nm, 1
W, 80 MHz repetition rate) was generated using β-barium borate
crystals (1 mm thickness) and separated with a CaF2 equilateral
dispersive prism. The 405 nm light was used for photoexcitation of
the sample. The emission from the sample was detected with a
monochromator (Jobin Yvon H10) equipped with a microchannel
plate photomultiplier (MCP-PM; Hamamatsu R3809U-51) that was
cooled at −30 °C. Signals from the MCP-PM (start pulse) and
photodiode (stop pulse) were amplified with preamplifiers (EG&G
Ortec VT120B and Hamamatsu C5594, respectively) and then
discriminated with picosecond-timing discriminators (EG&G Ortec
9307). Those output pulses were fed into a time-to-amplitude
converter (TAC; EG&G Ortec 567). The output of the TAC was
digitized using an analog-to-digital converter (10 MHz, 12 bit) and
accumulated on a computer. The full width at half-maximum of IRF
in our system is typically 35 ps. The TCSPC data were recorded at
every 20 nm in the range of 480−700 nm, and they were used to
reconstruct time-resolved emission spectra at each delay.
Pd(P(xylyl)3)4 (44). 44 was synthesized according to a procedure
similar to that of 24 by employing (2-methylallyl)PdCl2 (23.9 mg,
0.061 mmol) and P(xylyl)3 (209 mg, 0.60 mmol) with a modification:
the stirring time was changed from 90 to 60 min. The product was
obtained as yellow-green crystals. Anal. Found: C, 77.02; H, 7.12.
Calcd for 44 (C96H108P4Pd): C, 77.30; H, 7.30.
Pd(P(p-CF3Ph)3)3 (53). Under an argon atmosphere, (2-
methylallyl)PdCl2 (19.0 mg, 0.048 mmol) and P(p-CF3Ph)3 (180
mg, 0.39 mmol) were added to a mixture of 1 mL of MeOH (dry,
degassed) and 100 μL of ultrapure water (degassed) in a dry Schenk
tube. The reaction mixture was stirred for 120 min at room
temperature. The yellow precipitates were filtered off and recrystal-
lized by toluene (dry, degassed) at −18 °C. The obtained yellow
powder was washed with a small amount of MeOH. Anal. Found: C,
50.05; H, 2.38. Calcd for 53 (C61H36F27P3Pd): C, 50.27; H, 2.41.
Pd(P(p-CF3Ph)3)4 (54). Under an argon atmosphere, (2-
methylallyl)PdCl2 (15.9 mg, 0.040 mmol) and P(p-CF3Ph)3 (330
mg, 0.71 mmol) were added to 1 mL of MeOH (dry, degassed) in a
dry Schenk tube. The reaction mixture was stirred for 60 min at room
temperature and evaporated to dryness. The yellow precipitates were
recrystallized from toluene/MeOH at −18 °C. Anal. Found: C, 52.39;
H, 2.54. Calcd for 54 (C84H48F36P4Pd·•0.5toluene): C, 52.09; H,
2.60.
The phase purity of the above six complexes was investigated by
the measurements of powder X-ray diffraction (PXRD). Almost all
PXRD patterns agree with the simulated pattern calculated from the
single-crystal X-day diffraction data, as shown in Figure S7. Only the
pattern of 54 does not agree with the simulated one. Phase transition
by grinding or due to its air-sensitive nature may be possible
explanations.
X-ray Structure Determinations. X-ray crystallographic meas-
urements of 24, 23, 33, 44, 54, and 53 were performed on a Rigaku
Saturn 70 CCD area detector with graphite-monochromated Mo Kα
radiation. Images were collected, and the data were processed by
using CrystalClear26 or CrysAlisPro.27 The structures were solved by
direct methods SIR-9228 and SIR-201429 and refined by full-matrix
least-squares procedures with SHELXL.30 Calculations were per-
formed by using the CrystalStructure crystallographic software
package31 and Olex2.32 The disordered structure of 24 was analyzed
by the PART command of SHELXL. Crystallographic data have been
deposited to the Cambridge Crystallographic Data Centre as CCDC
2058648−2058653. Single crystals suitable for the analyses were
Synthesis. Pd(P(m-tol)3)4 (24). Under an argon atmosphere, (2-
methylallyl)PdCl2 (26.6 mg, 0.068 mmol) and P(m-tol)3 (205 mg,
0.67 mmol) were added to 2 mL of MeOH (dry, degassed) in a dry
Schenk tube. The reaction mixture was stirred for 90 min at room
temperature. The yellow-green precipitates deposited were filtered off,
washed with MeOH, and recrystallized from toluene/MeOH at −18
°C to yield yellow-green crystals. Anal. Found: C, 76.20; H, 6.44.
Calcd for 24 (C84H84P4Pd): C, 76.21; H, 6.40.
Pd(P(m-tol)3)3 (23). Under an argon atmosphere, (2-methylallyl)-
PdCl2 (28.1 mg, 0.071 mmol) and P(m-tol)3 (187 mg, 0.61 mmol)
were added to 3 mL of MeOH (dry, degassed) in a dry Schenk tube.
The reaction mixture was stirred for 50 min at room temperature.
Toluene was added to the reaction mixture until the yellow
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Figure 1. Crystal structures of complexes 2−5.
Although the synthesis of complex 24 appeared in a patent,47
the details cannot be retrieved. Complex 63, known as the
“superstable palladium complex”, is commercially available,
and its crystal structure has been published.48 In the present
study, complexes 23, 24, 33, 44, 53, and 54 were synthesized by
the reaction of a [PdCl(2-methylallyl)]2 dimer and mono-
dentate phosphine ligands [P(m-tol)3, P(p-tol)3, P(xylyl)3, and
P(p-CF3Ph)3] in MeOH (dry, degassed) at room temperature
(Scheme 1). The compounds were characterized using
elemental analysis and single-crystal X-ray diffraction studies.
The coordination number (bis, tris, and tetrakis) of the
monodentate phosphine complex [Pd(PR3)n] depends on the
bulkiness of the phosphines.49 We attempted to control the
coordination numbers, 3 and 4, of the complexes by changing
the ratio of the quantities of phosphine ligands to the [PdCl(2-
methylallyl)]2 dimer. However, the method was unsuccessful at
preparing pure products. Furthermore, the usual purification
method (recrystallization and/or washing) produced a mixture
of the tris and tetrakis complexes. Thus, we optimized
purification processes for each tris and tetrakis complex. For
the tris(m-tolyl)phosphine complexes, we successfully isolated
complexes 23 and 24 using the method described below.
Complex 23 was isolated when crystallization was performed at
low concentrations and ca. 0 °C. On the other hand, the
preferred conditions for the separation of the tetrakis complex
24 were a higher concentration of the complexes and a lower
temperature (ca. −18 °C.). However, it was difficult to
separate complexes 54 and 53 from each other using this
method. After performing a number of experiments, we found
that the content of water in the solution is important for
isolation. When using dry MeOH as the solvent, we obtained
complex 54. On the other hand, when using MeOH including
obtained by the following methods: the [Pd(P)4] type were obtained
by the slow diffusion of degassed dry MeOH in degassed dry toluene
at −18 °C; the [Pd(P)3] type were obtained by the slow diffusion of
degassed dry MeOH in degassed dry toluene at 0 °C. However, the
crystal of 53 was picked from a mixture of 54 and 53 crystals obtained
from the diffusion of degassed MeOH in degassed toluene with excess
ligands.
Computational Methods. Calculations of the electronic states
for 14 and 13 were performed by the Gaussian 09 software33 with the
ωB97XD34 functional using the initial atomic coordinates reported in
the literature by X-ray crystallography35,36 and the modified
coordinates of 44 (see the main text). Optimization in the triplet
states was performed using atomic coordinates determined by the
optimization in the ground state. Time-dependent density functional
theory (TD-DFT) was used to calculate the singlet and triplet excited-
state energies with the same, ωB97XD, functional. The transition
wavelengths in the singlet and triplet states were calculated by the
TD-DFT method using atomic coordinates optimized at the singlet
and triplet states, respectively. Basis sets were as follows: palladium,
CEP-121G;37−39 phosphorus, 6-31+G*; carbon, 6-31G*; hydrogen,
6-31G. Population and NTO analysis was carried out by the
AOMIX40 and Multifwn41 software, respectively. The pictures of the
orbitals have been depicted with the AVOGADRO, version 1.20,
software.42
RESULTS AND DISCUSSION
■
Synthesis. The triarylphosphines with various aryl groups
used in this work are shown in Scheme 1. In this paper, the
compounds are abbreviated as the combination of two
numerical characters, such as 13; the first number denotes
the ligand in the complexes, followed by the superscripted
number showing the coordination number. In addition to the
well-known compounds 13 and 14, the synthesis of complexes
23, 33, 53, and 54 was reported in previous papers.43−46
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a
Table 1. Bond Distance and Angles around the Pd atoms
complex
Pd−P/Å
P−Pd−P/deg
molecular symmetry point group
b
14
2.427
2.458
Pd−P1
Pd−P2
108.79
110.14
109.5
109.5
108.08(3)
110.83(3)
117.17
114.95
126.43
119.18(2)
119.81(2)
120.83(2)
113.21(2)
128.47(2)
118.28(2)
119.19(3)
120.08(3)
P1−Pd−P2
P2−Pd−P2*
C3
C3 axis through Pd−P1
T
T
C3
C3 axis through Pd−P1
C1
24
44
54
2.428(1)
2.438(1)
2.398(2)
2.425(2)
2.307
Pd−P1
Pd−P2
Pd−P1
Pd−P2
Pd−P3
Pd−P1
Pd−P2
Pd−P3
Pd−P1
Pd−P2
Pd−P3
Pd−P1
Pd−P2
P1−Pd−P2
P2−Pd−P2*
P1−Pd−P2
P2−Pd−P3
P3−Pd−P1
P1−Pd−P2
P2−Pd−P3
P3−Pd−P1
P1−Pd−P2
P2−Pd−P3
P3−Pd−P1
P1−Pd−P2
P2−Pd−P3
P3−Pd−P1
c
13
2.319
2.311
23
33
53
2.3079(6)
2.3026(6)
2.2888(6)
2.3155(6)
2.3086(5)
2.2968(5)
2.2827(9)
2.3054(9)
2.2912(8)
C1
C1
C1
Pd−P3
116.16(3)
c
a
b
The reported values for 14 and 13 are included. Reference 29. Reference 36.
Table 2. Photophysical Data of Complexes 1−6 in the Degassed Solvent and in the Solid State
solid
solution
ab
,
a c
,
ab
,
d
d
b
complex
λem,max /nm
τ
/μs
Φ
kr /104 s−1
knr /104 s−1
λabs,max /nm
λem,max /nm
τ/μs
e
e
e
14
24
44
54
13
23
33
53
63
533
542
506
584
600
601
613
f
3.1 (35%), 10 (65%)
1.9 (25%), 20 (75%)
1.4 (15%), 4.2 (85%)
1.5 (69%), 11 (31%)
3.7 (15%), 78 (85%)
6.0 (9%), 28 (91%)
8.5 (9%), 42 (91%)
f
0.33
0.53
0.24
0.04
0.46
0.29
0.27
f
4.4
3.4
6.3
0.9
0.7
1.1
0.7
8.9
3.1
20.0
21.8
0.8
322
320
653
7
e
e
e
635
3.1
e
e
e
322
637
7.3
e
e
e
325
322
320
320
667
0.22
e
e
e
653
6.0
3.9
7.7
e
e
e
e
2.7
1.8
635
e
e
651
e
e
e
325
667
0.22
0.12
g
g
g
g
627
4.4 (13%), 20 (87%)
0.07
0.4
5.2
315
652
a
b
c
In the solid state. Excited at 360 nm. The decay curve of these complexes was fitted to a double-exponential curve [I(t) = a exp(−t/τ1) + b
d
exp(−t/τ2)], and parentheses show the component ratio, aτ1/(aτ1 + bτ2) and bτ2/(aτ1 + bτ2), respectively. Radiative and nonradiative rate
constants are calculated from the values Φ and weighted-averaged values of τ. The values for the tetrakis complexes in solution are not actual values
of the tetrakis complexes. In degassed toluene. Not detected. In degassed THF.
e
f
g
10% water, we obtained complex 53. We confirmed the
reproducibility of the method. The reason why the method is
effective for separation is unclear, but the difference in the
hydrophobicity of the two complexes would explain the result.
Regarding the tris(p-tolyl)phosphine complex, only the tris
complex 33 was obtained, and [Pd(P(p-tol)3)4] was not
isolated as a pure compound after many attempts. On the
other hand, only the tetrakis complex 44 was obtained using
tris(xylyl)phosphine ligands. A lower ligand/Pd atom ratio was
applied to obtain the tris complex [Pd(P(xylyl)3)3], but only
the crystals of the tetrakis complex were deposited from the
reaction solution.
symmetry. Therefore, the P−Pd−P angles of complexes 24 and
44 are tetrahedral angles of 109.5°, and the four Pd−P lengths
are the same: 2.43 Å in complex 24 and 2.44 Å in complex 44.
On the other hand, the 54 molecule has a 3-fold axis on one of
the four Pd−P bonds, and thus the Pd atom and one of the P
atoms are located at special positions in the crystal. However,
no 2-fold axes are observed in the crystal. Therefore, the
symmetry of the complex is not strictly the same as that of
complexes 24 and 44, but the structure of complex 54 resembles
those of the other two complexes. Strictly, the 54 molecule
belongs to the C3 point group, but the bond angles around the
Pd atom are in the range 108.1−110.8° and are close to the
ideal tetrahedral angle. A characteristic of the structures of
complexes 24, 44, and 54 is each of the complex molecules
contains six pairs of aromatic groups, and each pair of aromatic
groups is aligned in a nearly parallel orientation in the crystals
(Figure 1). The shortest C−C distances between the aromatic
groups are 3.26, 3.30, and 3.27 Å in the crystals of complexes
24, 44, and 54, respectively. Notably, π-stacking interactions
should be observed in the crystals, although the phenyl groups
are not strictly parallel to each other. The crystal structure of
complex 14 was reported by Andrianov et al.35 (Figure S2).
Crystal Structures. The crystal structures and some
geometric parameters of complexes 24, 23, 33, 44, 54, and 53
are shown in Figure 1, and some crystal data are listed in Table
S1. The four-coordinated complexes 24, 44, and 54 show the
tetrahedral structures of typical d10 metals. A disordered
structure was observed for complex 24, and the details are
described in Figure S1. In particular, the crystal structures of
complexes 24 and 44 indicate very high symmetry. The space
groups of the crystals of complexes 24 and 44 are Fd3
the Pd atom is located at the special point with T(23)
̅
c and I23;
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Figure 2. (a) Corrected emission spectra of complexes 1−6 in the solid state at room temperature. Pictures of the solid-state samples under room
light (b) and UV light (c).
The overall conformation of complex 14 in the crystal is
definitely different from the conformations of complexes 24
and 44; the 14 molecule only has the C3 symmetry site and no
parallel aromatic groups. The Pd−P bond lengths and P−Pd−
P angles in complexes 24 and 44 are similar to those of complex
14 (2.43−2.46 Å and 109−110°),35 but the bond length in
complex 54 is somewhat shorter (2.40 and 2.42 Å) than that in
complex 14.
value is similar to the reported ε value of [Pd(PPh3)3].8,9 The
ligands do not show any absorption bands in the region >300
nm. The absorption band was mainly attributed to the metal-
to-ligand charge-transfer (MLCT) transition with some metal-
centered transition character, which resulted from the
theoretical calculations (see below). The absorption bands of
the tetrakis complexes in solution are identical with the band of
the corresponding tris complexes at wavelengths ranging from
320 to 450 nm, where PPh3 has no absorption bands (see
Table 2 and Figure S3 for a comparison of complexes 14 and
13, 24 and 23, and 54 and 53). The similarity of the spectra of
complexes 14 and 13 was reported in the 1980s. In general,
[Pd(PR3)4] is dissociated to [Pd(PR3)3] and free PR3 in
solution, as discussed in the section of the emission spectra in
solution.50 Therefore, we concluded that all tetrakis complexes
almost dissociate into the corresponding tris complexes, and
absorption bands based on the dissociated tris complexes are
observed.
Emission Spectra. The emission spectra and associated
data for the complexes in the solid state are presented in Figure
2 and Table 2, respectively. These complexes exhibit strong
emission in the solid state, except for complex 53, which emits
negligible luminescence under UV light. As shown in the
picture (Figure 2c), the complexes show a remarkable variation
in the emission color from blue to orange. The four-
coordinated complexes (14, 24, 44, and 54) show blue-to-
yellow luminescence (506−584 nm). On the other hand,
three-coordinated complexes (13, 33, 53, and 63) exhibit
yellow-orange luminescence (600−627 nm). Notably, com-
plexes 54 and 63 contain CF3 ligands, and both show red-
shifted emission compared with the complexes lacking CF3
congeners. The emission quantum yields of the complexes
range from 0.04 to 0.53. The emission decay curves of the
The crystal structures of the three-coordinated complexes
23, 33, and 53 are trigonal-planar structures, although they do
not exactly display any symmetry on the Pd atom. The bond
lengths (Å) and angles (deg) are 2.29−2.32 Å and 113−128°,
respectively (Table 1), and these values are similar to the
reported crystal structures of complexes 13 (2.31−2.32 Å and
115−126°)35 and 63 (2.29−2.31 Å and 113−127°).48 The
bond lengths are slightly shorter than those of the tetrakis
complexes by ca. 0.1 Å. The sums of the P−Pd−P angles of
complexes 23, 33, and 53 are 360°, 360°, and 355°, respectively.
Therefore, these complexes exhibit good planarity around the
Pd center. Complexes 23 and 33 contain a pair of phenyl
groups that are nearly parallel; the shortest interligand C−C
distances are 3.49 and 3.32 Å, respectively. In complex 53, the
three pairs of phenyl groups are oriented in parallel, and the
shortest interligand C−C distances between the phenyl groups
are 3.43, 3.44, and 3.79 Å in the pairs. These π−π interactions
are weaker than those observed in the tetrakis complexes.
Absorption Spectra. The shapes of the absorption spectra
(Figure S3) of complexes 23, 33, 53, and 63 are similar to the
reported spectrum of complex 13.8,9 They all show a maximum
band at 315−325 nm, as shown in Table 2. The exact molar
extinction coefficients (ε) of the bands are difficult to measure
because of the air-sensitive nature of the complexes, but these
values are estimated to be ca. 18000−20000 M−1 cm−1. The
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Figure 3. Excitation spectra of the complexes in the solid state at room temperature: (a) 33 (blue); 44 (orange); 54 (green); 63 (yellow). (b) 13
(brown); 14 (red); 23 (green); 24 (black). The intensities of the spectra are normalized at the maximum.
Figure 4. (a) Emission spectra of complexes 14 (light green) and 13 (orange) in degassed toluene. (b) Emission spectra of complexes 24 (green)
and 23 (red) in degassed toluene. (c) Emission spectra of complexes 54 (yellow) and 53 (black) in degassed toluene. (d) Emission spectra of
complexes 44 (blue) and 33 (magenta) in degassed toluene and complex 63 (purple) in degassed THF.
complexes follow a double-exponential function with a long-
lifetime component of 4−78 μs and a short-lifetime
component of 1.4−8.5 μs. The long lifetimes in the
microsecond range indicate that the emission involves the
triplet excited states. We discuss different emission colors
between the tetrakis and tris complexes.
24) to compare the tris and tetrakis complexes with the same
type of ligand. As shown in Figure 3a, the spectral bands of the
tris complexes 13 and 23 have higher intensities in the longer-
wavelength region than the tetrakis complexes 14 and 24. We
use the value of the wavelength at half-maximum intensity,
WHM, to discuss the excitation spectra. The WHM values for
complexes 13 and 14 are 449 and 430 nm, respectively, while
the values for complexes 23 and 24 are 452 and 422 nm,
First, we focused our attention on the excitation spectra of
the tris complexes (13 and 23) and tetrakis complexes (14 and
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Figure 5. (left panel) Emission spectra of complex 54 in degassed toluene at 296 K (λem,max = 667 nm), 266 K (λem,max = 703 nm), 233 K (λem,max
709 nm), and 196 K (λem,max = 720 nm). (right panel) Emission decay curves of complex 54 in the degassed toluene at 296, 266, 233, and 196 K.
=
respectively. This result clearly indicates that the energy of the
initially occupied excited state of the tris complexes after light
absorption is lower than that of the tetrakis complexes. The
WHMs of complexes 33 and 44 are 452 and 433 nm,
respectively, and they compare well to the WHMs of
complexes 13 (449 nm) and 14 (430 nm), respectively. The
WHM of complex 54, 450 nm, is similar to those of complexes
13 (448 nm) and 23 (452 nm), and the WHM of complex 63
shows the longest wavelength for the WHM, 499 nm, among
the complexes; the reason will be discussed in the last part of
the paper.
The emission spectra of complexes 1−6 in degassed toluene
or THF are presented in Figure 4. The emission spectra of
tetrakis and tris complexes in solution were observed at
approximately 600−700 nm. The emission maxima of the
tetrakis complexes are approximately the same as the
corresponding tris complexes, as shown in Figure 4a−c. In
solution, the decay curves of the emission of all complexes fit
well with single-exponential functions. The lifetimes of the tris
complexes (0.1−7 μs) also coincide with the corresponding
tetrakis complexes within 20% error. Tolman et al. reported
that PdL4 species [L = PPh3, P(p-C6H4CH3)3, and P(m-
C6H4CH3)3] completely dissociate in solution.43 For the
solution of 54, we successfully observed the 31P NMR signals of
the tris complex and free ligand separately at low temperature,
as shown in Figure S6. Therefore, we deduced that the
observed emission arises from the tris species that are formed
from the tetrakis complexes in solution. The emission maxima
of the tris complexes in solution are bathochromically shifted
from those in the solid state by 25−50 nm. This result may be
due to the more energetically relaxed excited structure in
solution than in the solid state.
temperatures. The emitting species in solution should be 53,
as described above. The emission spectra of complex 54 in
degassed toluene is shown in Figure 5. The emission maxima at
296, 266, 233, and 196 K were 677, 703, 709, and 720 nm,
respectively, which show a red shift of λem,max and an increase
in the emission intensity upon lowering of the temperature. If
we assume that the intensity of the absorption band at the
excitation wavelength is independent of the temperature, we
can calculate the quantum yields and radiative rates from the
relative area of the emission spectra after correction and the
decay rates. The quantum yields (lifetimes in parentheses) of
the emission are 0.13% (0.21 μs), 0.27% (1.0 μs), 0.33% (1.5
μs), and 0.54% (2.8 μs) at 296, 266, 233, and 196 K,
respectively. Therefore, the calculated radiative rate constants,
kr, are 6.2 × 103, 2.7 × 103, 2.2 × 103, and 1.9 × 103 s−1,
respectively. The data show a substantial decrease in the
radiative rate constants upon lowering of the temperature. In
addition, time-resolved emission spectra in the picosecond
time scale of complex 14 in degassed toluene at room
temperature are presented in Figure 6. The time-resolved
Figure 6. Time-resolved emission spectra of complex 14 in degassed
toluene.
Character of the Emitting States. We performed low-
temperature measurements and a picosecond time-resolved
study to elucidate the excited-state dynamics. We thought
studying the details of the photophysical data, e.g., temperature
dependence and time-resolved emission, in solution is suitable
in order to explore the nature of their emission because the
complexes in solution show single-exponential decay. PPh3
complexes are not suitable for measurements at low temper-
ature because of their low solubility, especially at low
temperature. We chose complex 54 for the low-temperature
study because of its good solubility in toluene at low
emission spectrum showed a blue-shifted emission band with a
short lifetime compared with the band observed in the longer
time range. The temporal profiles of the time-resolved
emission intensities from 0−1.5 ns are well reproduced by a
multiexponential function with time constants of ∼20 ps and a
very long function (7 μs), where the time resolution of the
system is ∼15 ps. Furthermore, the intensity of the ∼20 ps
component is typically more than 20 times larger than the long
lifetime component in the observed wavelength region (500−
700 nm), suggesting that rapid emission is prompt
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fluorescence. Our group previously reported that Pd(0)
complexes bearing diphosphine ligands [Pd(binap)2] and
[Pd(biphep)2], where binap and biphep denote 2,2′-bis-
(diphenylphosphino)-1,1′-binaphthyl and 2,2′-bis-
(diphenylphosphino)-1,1′-biphenyl, respectively, show ther-
mally activated delayed fluorescence (TADF). Typical TADF
shows a red shift of the emission with a decrease in the
intensity of the emission upon lowering of the temperature.
However, the intensity of the emission of [Pd(binap)2] and
[Pd (biphep)2] does not simply decrease with decreasing
temperature, but the radiative rate constants clearly show a
decrease with decreasing temperature; therefore, we concluded
that the emission of the binap and biphep complexes has a
TADF character. The tendency of the temperature dependence
of the monophosphine complex and the results of the
picosecond time-resolved emission study strongly suggest
that the emission of the complexes is TADF, at least in
solution. We propose that a similar mechanism may be
generally applied to mononuclear palladium phosphine
complexes. More detailed analyses are necessary for the exact
assignment of the emission mechanism, which is beyond the
scope of this study.
tris complexes. A detailed description of the method used for
the calculations is provided in the experimental section.
The starting coordinates used in the optimization calculation
were taken from the X-ray data of complexes 14 and 13.35,36 As
shown above, two different types of conformations are
observed for the tetrakis(triarylphosphine)palladium com-
plexes: one has a T point group, which has near-stacked
phenyl groups in the crystals of complexes 24 and 44, and the
other is found in the crystal structure of [Pd(PPh3)4] reported
by Andrianov et al.35
The structure of complex 14 was optimized from the three
different starting structures: (1) the structure generated by
removing methyl groups from the crystal structure of complex
44 with T symmetry, (2) the structure observed in the crystal
of [Pd(PPh3)4],35 (3) the T-symmetrized structure of the
previous structure. Three calculations successfully yielded
optimized structures: the symmetries of the optimized
structures are T, C1, and T, and the self-consistent-field
(SCF) energies are −4271.66963, −4271.69338, and
−4271.69333 au, respectively. Because the calculation of
structure (2) resulted in the lowest energy, we use the results
of structure (2) in the following discussion. Notably, TD-DFT
calculations using the coordinates based on these three
structures produce approximately the same results as those
for the transition wavelengths. Complex 13 has no symmetry in
the crystal,36 and the coordinates were used without
modifications of the initial coordinates for the optimization.
The important bond lengths and angles of the optimized
structures of complexes 14 and 13 in the ground state (S0) are
shown in Table S2. The calculated Pd−P distances are 2.42−
2.46 and 2.31−2.32 Å for complexes 14 and 13, respectively,
and the distances reproduce well the distances in the crystal
structures. Tetrakis and tris complexes with d10 configurations
tend to cause flattening and T-shaped distortion in the excited
states, respectively.51,52 Our calculation also shows that the
triplet-optimized structures of complexes 14 and 13 are
distorted toward square-planar and T-shaped triangular
geometries, respectively (Figure 7). The former distortion,
flattening, was frequently found for copper(I) complexes.
The distortion in the excited states may substantially alter
the emission energy. First, we postulated that the explanation
for why tris complexes generally emit low-energy light is
related to the extent of the distortion in the excited state. A
reasonable hypothesis is that the tris complexes show
apparently larger distortion than the tetrakis complexes in
the excited states because tetrakis complexes have a more
crowded structure. However, the calculations showed that both
complexes 13 and 14 exhibit large distortions in the excited
states. The dihedral angle between the two planes defined by
the three atoms, P1−Pd−P2 and P3−Pd−P4, for complex 14 is
distorted from 90° to 78.0° in the triplet excited state. For
complex 13, the maximum P−Pd−P angle changed from 126°
to 138°, and the sum of the three angles changed from 358° to
349°, which shows a T-shaped distortion and trigonal
pyramidization upon excitation. The maximum P−Pd−P
angle for complex 14 in the excited states is 123°. At the
ground states, the maximum P−Pd−P angles are 110.1° in the
crystal; therefore, the change in the P−Pd−P angle is 13° upon
excitation, which is similar to the change in complex 13 of 12°.
We compare the energy change induced by the distortion to
quantitatively evaluate the extent of distortion in complexes 14
and 13. The calculation showed that the stabilization energies
in the triplet excited states caused by the distortion for the tris
DFT Calculations. DFT calculations were performed to
discuss the photophysics of the four- and three-coordinated
complexes (Figures 7, 8, and S4 and Tables S2, S3, and S5).
Figure 7. Singlet- and triplet-optimized structures of complexes 14
and 13. DHA denotes the dihedral angles between the two planes,
which are defined by the three atoms: P1−Pd−P2 and P3−Pd−P4.
The functional ωB97XD, including long-range correction, was
employed in all calculations because this functional most
precisely reproduced the Pd−P distances in the functionals
B3LYP, PBE0, and BP97 for the complexes. In the present
study, the electronic states and optimized structures in the
ground and triplet (T1) states of complexes 14 and 13, which
are prototypes of the other complexes, were investigated. The
aim of the study is to clarify (1) the nature of the emitting
excited states of the complexes and (2) the reason why the
tetrakis complexes emit light at shorter wavelengths than the
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Figure 8. Molecular energy diagram calculated using the DFT method. These energies were obtained by calculating the singlet and triplet SCF
energies using the coordinates of the singlet- and triplet-optimized structures. The energies shown in black (atomic unit and wavenumber) are the
relative values to the ground states.
and tetrakis complexes are similar, at least when calculated for
the molecules in vacuum (Figure 8). DFT calculations showed
that the stabilization of the triplet state from the calculated
value for the singlet-optimized structure to the triplet-
optimized structure is estimated to be 6.6 and 6.8 K cm−1
for complexes 13 and 14, respectively. Therefore, we concluded
that both the singlet and triplet excited states of the tetrakis
complexes inherently have a higher energy than those of the
tris complexes, which may explain the shorter-wavelength
emission of the tetrakis complexes than the tris complexes.
The energies and components of the frontier Kohn−Sham
orbitals for the optimized structures of complexes 13 and 14 are
described in Table S3. The orbitals are illustrated in Figure S4.
In both complexes, the highest occupied molecular orbitals
(HOMOs) are composed of the 4d orbitals of the Pd atoms
and the 3p orbitals of the P atoms. Nodes are observed
between the Pd and P atoms, and thus the molecular orbital
can be regarded as an antibonding orbital of the Pd and P
atoms. The lowest unoccupied molecular orbitals (LUMOs)
mainly consist of the π* orbitals of the phenyl group of PPh3,
but nonnegligible contributions of the orbitals of the Pd and P
atoms are observed, especially for complex 13 (Table S3). The
HOMO−LUMO gaps for complexes 14 and 13 are similar,
7.42 and 7.35 eV, respectively.
at 636 nm than complex 14 at 500 nm; the difference equaled
4280 cm−1 (Table S6).
The calculation of complex 14 based on the ground-state
geometry indicates that the main components of the transition
from the ground state (S0) to the lowest-lying singlet excited
states (S0 → S1) are [HOMO and HOMO −1] to [LUMO+1,
LUMO+2, and LUMO+3] orbitals. Two sets of three orbitals,
[HOMO, HOMO−1, and HOMO−2] and [LUMO+1,
LUMO+2, and LUMO+3], are both nearly degenerate; the
former are composed of the orbitals of the Pd and P atoms,
and the latter are regarded as the π orbitals of the phenyl
groups (Table S3). Therefore, the transition is assigned as the
charge-transfer transition from the Pd and phosphine atoms to
the π* orbitals of the PPh3 moiety. On the other hand, the
lowest-energy transition for complex 13 is difficult to interpret.
The composition of both the HOMOs and LUMOs of
complex 13 contains considerable contributions of all atoms,
i.e., the metal atom, P atoms, and phenyl groups. We
performed natural transition orbital (NTO) analysis of the
complexes to obtain deeper insight into the transitions.
The NTO analysis of the results of TD-DFT calculations has
provided clearer pictures of the lowest-energy transitions of the
two complexes. Detailed results for the NTO calculations are
shown in Tables 3, S7, and S8. For complex 13, the analysis
The TD-DFT calculation for singlet-optimized structures
resulted in a large difference in the wavelengths of the lowest-
energy transition for the two complexes. The calculation of
complex 14 shows that three singlet−singlet transitions in the
lowest-energy region occur at 299.4−299.2 nm (ca. 33.4 K
cm−1), but the oscillator strengths of these transitions are small
(Table S4). Two calculated transitions with considerable
intensities at 289.4 and 289.6 nm are observed. For complex
13, the wavelength of the transition with considerable intensity
was calculated to be 298 nm, which is similar to complex 14.
However, the calculated lowest-energy transition is 367 nm
(27.02 K cm−1), which is lower than that of complex 14 by
6400 cm−1. Therefore, an important result is derived from
these data: the energy of the S1 state of the tris complex is
considerably lower than that of the tetrakis complex. The
difference observed in the excitation spectra of the tris and
tetrakis complexes should reflect the results. When the
calculations were performed using the geometry optimized in
the T1 state, complex 13 also showed a lower-energy transition
Table 3. Compositions of the NTOs of the Singlet Excited
States
a
14
13
atom
HOMO−1 HOMO LUMO LUMO+1 HOMO LUMO
Pd
P
C, H
50.5
28.9
20.6
49.1
30.9
19.9
5.0
28.5
66.4
5.7
29.2
65.1
55.3
30.7
14.0
40.9
34.8
24.2
b
a
b
They were calculated based on the ground-state geometries. The
contributions for all carbon and hydrogen atoms.
shows that the HOMO → LUMO transition in the NTOs
represents the lowest-energy singlet transition. On the other
hand, HOMO−1 and LUMO+1 are also involved in the
transition in complex 14 (Table S7). HOMO−1 and HOMO
in the NTOs of complex 14 are mainly attributed to metal ions
with some contribution of P atoms, but the orbitals localized
on the phenyl groups are the main components in LUMO and
LUMO+1. The NTOs are illustrated in Figure 9. The
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Figure 9. NTOs calculated for the S1 states. (a) HOMO, HOMO−1, LUMO, and LUMO+1 of complex 14. (b) HOMOs and LUMOs of complex
13 (isovalues 0.02).
transition of complex 14 is mainly characterized as a [Pd + P]-
to-[π* of the phenyl groups] charge transfer, which is a
variation of MLCT (Table S8). On the other hand, the result
of the NTO analysis of complex 13 produced considerably
different results (Table S9). If we see the NTO of the singlet
excited state, the composition of the NTO-HOMO of complex
13 is similar to complex 14; i.e., half of the HOMO is based on
the metal orbitals. However, the composition of the NTO-
LUMO is noticeably different from that of complex 14; the
composition of the phenyl groups in the LUMO is smaller than
that in complex 14, and the largest component is the metal
orbital. A deeper analysis of the NTO reveals that the major
orbital type of the Pd atom in the HOMO is the d-type orbital,
but the largest contribution in the LUMO is the pz orbital of
the Pd atom. Therefore, the transition in complex 13 exhibits a
metal-centered d → p transition character in addition to the
charge-transfer nature. These pictures are essentially similar for
the optimized geometry in the T1 state for both complexes
(Figure S5 and Tables S10 and S11). We concluded that the
lowest-energy transitions for complex 14, S0 → S1 and S0 → T1,
are characterized as [Pd + P]-to-π*(phenyl) charge transfer.
The transition for complex 13 should contain a metal-centered
d → p transition in addition to the charge-transfer character.
Previously, in the 1980s, the excited states of these complexes
were assigned as d → P transitions around the Pd center.9 The
assignment has been partially approved for complex 13 in the
present study. The difference in the transition nature between
the two complexes may explain why the excitation energy of
complex 14 is higher than that of complex 13.
As discussed above, the results of the TD-DFT analyses
clearly showed a difference in the transition energies between
complexes 14and 13, although the HOMO−LUMO energy
gaps of the two complexes are similar. However, variations in
the emission wavelengths are observed in the tetrakis and tris
complexes. Other factors that should be considered when we
discuss the emission color of the complexes studied here are
(1) electronic and (2) steric effects. The complexes bearing
tris(trifluoromethyphenyl)phosphine, 63 and 54, show red-
shifted luminescence compared with the PPh3 complexes, 13
and 14, respectively. The result should be reasonably explained
by the electron-withdrawing effect. The attachment of the
trifluoromethyl group should stabilize the LUMO and near-
LUMO orbitals, and thus the charge-transfer transition energy
decreases. Methyl groups on the phenyl groups may have little
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effect on the MLCT. Why does trixylylphosphine complex 44
emit light at a very short wavelength? Our preliminary
calculation showed that substantial distortion of complex 44
occurs, similar to complex 14, but the calculation was
performed for a single molecule in the gas phase. A reasonable
hypothesis is that the bulkiness of the ligand trixylylphosphine
may prevent the complex from distorting the crystal, which
may explain the blue emission of complex 44.
Japan; Present Address: M.N.: Research Institute for
Science and Technology, Tokyo University of Science,
2641 Yamazaki, Noda-shi, Chiba-ken 278-8510, Japan.
Takuma Ohkubo − Department of Materials and Life Science,
Seikei University, Musashino, Tokyo 180-8633, Japan
Masashi Sanga − Graduate School of Science and Engineering,
Toyama University, Gofuku, Toyama 930-8355, Japan
Munetaka Iwamura − Graduate School of Science and
Engineering, Toyama University, Gofuku, Toyama 930-8355,
Japan; orcid.org/0000-0002-4888-9086
CONCLUSIONS
■
Koichi Nozaki − Graduate School of Science and Engineering,
Toyama University, Gofuku, Toyama 930-8355, Japan
We synthesized four- or three-coordinated Pd(0) complexes
bearing PPh3 derivatives and reported their crystal structures,
basic photophysical properties, and the results of computa-
tional studies. In the solid state, the Pd(0) complexes indicated
colorful, blue to orange, emission at various wavelengths, high
quantum yields, and long lifetimes. Tetrakis complexes
generally emit at longer wavelengths than tris complexes. In
degassed solution, the main emitting species are three-
coordinated Pd(0) complexes, and they all show orange
emission. DFT and TD-DFT analyses indicate that both the S1
and T1 states of the tetrakis complexes have a higher energy
than those of the tris complexes. The NTO analysis revealed
that the nature of the transition differs between complexes 14
and 13. Both have [Pd + P]-to-π* charge-transfer character-
istics. However, complex 13 has a considerable metal-centered
(d → p) character. The difference in the transition nature
should be the main reason for the differences in the excited-
state energies between the tetrakis and tris complexes.
Additional steric and electronic effects may affect the emission
color. The photophysical nature of Pd(0) species has not
drawn the attention of many researchers compared with their
glorious success in catalysis. This study may provide an
opportunity for research in a new area of Pd(0) chemistry.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00631
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Grants-in-Aid from
MEXT of Japan (17K05816) and a grant from the Faculty of
Science and Technology, Seikei University. Prof. Shigeo
Satokawa is greatly acknowledged for the measurements of
PXRD.
REFERENCES
■
(1) Kendall, A. J.; Zakharov, L. N.; Tyler, D. R. Steric and Electronic
Influences of Buchwald-Type Alkyl-JohnPhos Ligands. Inorg. Chem.
2016, 55 (6), 3079−3090.
(2) Surry, D. S.; Buchwald, S. L. Dialkylbiaryl Phosphines in Pd-
Catalyzed Amination: A User’s Guide. Chem. Sci. 2011, 2 (1), 27−50.
(3) Martin, R.; Buchwald, S. L. Palladium-Catalyzed Suzuki−
Miyaura Cross-Coupling Reactions Employing Dialkylbiaryl Phos-
phine Ligands. Acc. Chem. Res. 2008, 41 (11), 1461−1473.
(4) Wolfe, J. P.; Singer, R. A.; Yang, B. H.; Buchwald, S. L. Highly
Active Palladium Catalysts for Suzuki Coupling Reactions. J. Am.
Chem. Soc. 1999, 121 (41), 9550−9561.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00631.
■
sı
(5) Christmann, U.; Vilar, R. Monoligated Palladium Species as
Catalysts in Cross-Coupling Reactions. Angew. Chem., Int. Ed. 2005,
44 (3), 366−374.
(6) Johansson Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.;
Snieckus, V. Palladium-Catalyzed Cross-Coupling: A Historical
Contextual Perspective to the 2010 Nobel Prize. Angew. Chem., Int.
Ed. 2012, 51 (21), 5062−5085.
(7) Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling
Reactions of Organoboron Compounds. Chem. Rev. 1995, 95 (7),
2457−2483.
(8) Harvey, P. D.; Gray, H. B. Low-Lying Singlet and Triplet
Electronic Excited States of Binuclear (d10-d10) Palladium(0) and
Platinum(0) Complexes. J. Am. Chem. Soc. 1988, 110 (7), 2145−
2147.
Figures showing crystal structures, absorption spectra,
and calculated orbitals and tables presenting crystal data,
optimized structural data, and the results of DFT and
TD-DFT calculations (PDF)
Accession Codes
CCDC 2058648−2058653 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(9) Caspar, J. V. Long-Lived Reactive Excited States of Zero-Valent
Phosphine, Phosphite, and Arsine Complexes of Nickel, Palladium
and Platinum. J. Am. Chem. Soc. 1985, 107 (23), 6718−6719.
(10) Harvey, P. D.; Schaefer, W. P.; Gray, H. B. Emission Properties
of Tetrahedral Bis[Bis(Diphenylphosphino)Propane]Palladium and
-Platinum, Complexes. Crystal and Molecular Structure of Pt(Dppp)-
2. Inorg. Chem. 1988, 27 (6), 1101−1104.
AUTHOR INFORMATION
Corresponding Author
Taro Tsubomura − Department of Materials and Life Science,
Seikei University, Musashino, Tokyo 180-8633, Japan;
orcid.org/0000-0002-2850-4636; Email: tsubomura@
st.seikei.ac.jp
■
(11) Barbieri, A.; Accorsi, G.; Armaroli, N. Luminescent Complexes
beyond the Platinum Group: The d10 Avenue. Chem. Commun. 2008,
No. 19, 2185−2193.
Authors
Daichi Kakizoe − Department of Materials and Life Science,
Seikei University, Musashino, Tokyo 180-8633, Japan
Michihiro Nishikawa − Department of Materials and Life
Science, Seikei University, Musashino, Tokyo 180-8633,
(12) Yam, V. W.-W.; Wong, K. M.-C. Luminescent Metal
Complexes of d6, d8 and d10 Transition Metal Centres. Chem.
Commun. 2011, 47 (42), 11579−11592.
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Inorganic Chemistry
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Article
(13) Yam, V. W.-W.; Au, V. K.-M.; Leung, S. Y.-L. Light-Emitting
Self-Assembled Materials Based on d8 and d10 Transition Metal
Complexes. Chem. Rev. 2015, 115 (15), 7589−7728.
(14) Henwood, A. F.; Lesieur, M.; Bansal, A. K.; Lemaur, V.;
Beljonne, D.; Thompson, D. G.; Graham, D.; Slawin, A. M. Z.;
Samuel, I. D. W.; Cazin, C. S. J.; Zysman-Colman, E. Palladium(0)
NHC Complexes: A New Avenue to Highly Efficient Phosphor-
escence. Chem. Sci. 2015, 6 (5), 3248−3261.
(15) Harvey, P. D.; Adar, F.; Gray, H. B. Spectroscopic Properties of
Binuclear Palladium(0) and Platinum(0) Dibenzylideneacetone
Complexes. J. Am. Chem. Soc. 1989, 111 (4), 1312−1315.
(16) Harvey, P. D.; Gan, L. Binuclear Compounds Containing
Pendant Electroactive Groups: Photophysical and Electrochemical
Properties of Tris(4-Ferrocenyldibenzylideneacetone)-
Dipalladium(0), Pd2[(C5H5)Fe(C5H4)(C6H4)CHCH(CO)-
CHCHC6H5]3. Inorg. Chem. 1991, 30 (17), 3239−3241.
(17) Tsubomura, T.; Ito, Y.; Inoue, S.; Tanaka, Y.; Matsumoto, K.;
Tsukuda, T. Strongly Luminescent Palladium(0) and Platinum(0)
Diphosphine Complexes. Inorg. Chem. 2008, 47 (2), 481−486.
(18) Ohkubo, T.; Takao, K.; Tsubomura, T. Blue to Orange
Emitters; Palladium(0) Monodentate Phosphine Complexes. Inorg.
Chem. Commun. 2012, 20, 27−29.
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.;
Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi,
R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar,
S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox,
J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A.
D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2009.
(34) Chai, J.-D.; Head-Gordon, M. Long-Range Corrected Hybrid
Density Functionals with Damped Atom−Atom Dispersion Correc-
tions. Phys. Chem. Chem. Phys. 2008, 10 (44), 6615−6620.
(35) Andrianov, V. G.; Akhrem, I. S.; Chistovalova, N. M.;
Struchkov, Y. T. Molecular structure of a palladium tetrakis-
(triphenylphosphine) complex Pd[P(C6H5)3]4·0.5C6H6. Zh. Strukt.
Khim. 1976, 17 (1), 135−141.
(36) Sergienko, V. S.; Porai-Koshits, M. A. Crystal Structure of
Tris(Triphenylphosphine) Palladium. J. Struct. Chem. 1988, 28 (4),
548−552.
(19) Tsubomura, T.; Murota, H.; Takao, K. Luminescent
Palladium(0) Complexes Bearing N-Heterocyclic Carbene and
Phosphine Ligands. Inorg. Chem. Commun. 2013, 35, 110−112.
(20) Wang, D.; Weinstein, A. B.; White, P. B.; Stahl, S. S. Ligand-
Promoted Palladium-Catalyzed Aerobic Oxidation Reactions. Chem.
Rev. 2018, 118 (5), 2636−2679.
(37) Stevens, W. J.; Basch, H.; Krauss, M. Compact Effective
Potentials and Efficient Shared-exponent Basis Sets for the First- and
Second-row Atoms. J. Chem. Phys. 1984, 81 (12), 6026−6033.
(38) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Relativistic
Compact Effective Potentials and Efficient, Shared-Exponent Basis
Sets for the Third-, Fourth-, and Fifth-Row Atoms. Can. J. Chem.
1992, 70 (2), 612−630.
(39) Cundari, T. R.; Stevens, W. J. Effective Core Potential Methods
for the Lanthanides. J. Chem. Phys. 1993, 98 (7), 5555−5565.
(40) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis,
V.6.81; http://www.sg-chem.net/; 2013.
(21) Hazari, N.; Melvin, P. R.; Beromi, M. M. Well-Defined Nickel
and Palladium Precatalysts for Cross-Coupling. Nat. Rev. Chem. 2017,
1 (3), UNSP 0025.
(22) Zhang, Y.; Yuan, Z.; Puddephatt, R. J. Allyl(β-Diketonato)-
Palladium(II) Complexes, Including Liquid Precursors, for Chemical
Vapor Deposition of Palladium. Chem. Mater. 1998, 10 (8), 2293−
2300.
(41) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction
Analyzer. J. Comput. Chem. 2012, 33 (5), 580−592.
(23) Nishikawa, M.; Wakita, Y.; Nishi, T.; Miura, T.; Tsubomura, T.
Long-Lived and Oxygen-Responsive Photoluminescence in the Solid
State of Copper(I) Complexes Bearing Fluorinated Diphosphine and
Bipyridine Ligands. Dalton Trans. 2015, 44, 9170.
(24) Kakizoe, D.; Nishikawa, M.; Fujii, Y.; Tsubomura, T.
Photophysical Properties of Three Coordinated Copper(I) Com-
plexes Bearing 1,10-Phenanthroline and a Monodentate Phosphine
Ligand. Dalton Trans. 2017, 46 (43), 14804−14811.
(25) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara,
T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Reevaluation of
Absolute Luminescence Quantum Yields of Standard Solutions Using
a Spectrometer with an Integrating Sphere and a Back-Thinned CCD
Detector. Phys. Chem. Chem. Phys. 2009, 11 (42), 9850−9860.
(26) CrystalClear 1.3; Rigaku Corp.: Tokyo, Japan, 2000.
(27) CrysAlis Pro; Rigaku Oxford Diffraction: Yarnton, England,
2015.
(28) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. SIR92 − a Program for
Automatic Solution of Crystal Structures by Direct Methods. J. Appl.
Crystallogr. 1994, 27 (3), 435−435.
(29) Burla, M. C.; Caliandro, R.; Carrozzini, B.; Cascarano, G. L.;
Cuocci, C.; Giacovazzo, C.; Mallamo, M.; Mazzone, A.; Polidori, G.
Crystal Structure Determination and Refinement via SIR2014. J. Appl.
Crystallogr. 2015, 48 (1), 306−309.
(30) Sheldrick, G. M. Crystal Structure Refinement with SHELXL.
Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71 (1), 3−8.
(31) CrystalStructure 4.24; Rigaku Corp.: Tokyo, Japan, 2017.
(32) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. a. K.;
Puschmann, H. OLEX2: A Complete Structure Solution, Refinement
and Analysis Program. J. Appl. Crystallogr. 2009, 42 (2), 339−341.
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
(42) Hanwell, M. D.; Curtis, D. E.; Lonie, D. C.; Vandermeersch, T.;
Zurek, E.; Hutchison, G. R. Avogadro: An Advanced Semantic
Chemical Editor, Visualization, and Analysis Platform. J. Cheminf.
2012, 4 (1), 17.
(43) Tolman, C. A.; Seidel, W. C.; Gerlach, D. H. Triarylphosphine
and Ethylene Complexes of Zerovalent Nickel, Palladium, and
Platinum. J. Am. Chem. Soc. 1972, 94 (8), 2669−2676.
(44) Stern, E. W. Homogeneous Autoxidation Catalysis by
Palladium(0) Complexes. J. Chem. Soc. D 1970, No. 12, 736−736.
(45) Tortosa Estorach, C.; Orejón, A.; Ruiz, N.; Masdeu-Bultó, A.
M.; Laurenczy, G. Hydrocarboxylation of Terminal Alkenes in
Supercritical Carbon Dioxide. Eur. J. Inorg. Chem. 2008, 2008 (22),
3524−3531.
(46) Nicholas, P. P. Amidation of Chloroalkenes Catalyzed by
Tertiary Phosphine Complexes of Palladium(0). J. Org. Chem. 1987,
52 (23), 5266−5272.
(47) Nozaki, K. Verfahren Zur Herstellung Von Carbonsaeuren
Und/Oder Carbonsaeurederivaten. Patent DE2410246(A1), Sept 12,
1974.
(48) Jakab, A.; Dalicsek, Z.; Holczbauer, T.; Hamza, A.; Pápai, I.;
Finta, Z.; Timári, G.; Soós, T. Superstable Palladium(0) Complex as
an Air- and Thermostable Catalyst for Suzuki Coupling Reactions.
Eur. J. Org. Chem. 2015, 2015 (1), 60−66.
(49) Kuran, W.; Musco, A. Synthesis and Characterization of
Tertiary Phosphine Pd(0) Complexes. Inorg. Chim. Acta 1975, 12 (1),
187−193.
(50) Amatore, C.; Pfluger, F. Mechanism of Oxidative Addition of
Palladium(0) with Aromatic Iodides in Toluene, Monitored at
Ultramicroelectrodes. Organometallics 1990, 9 (8), 2276−2282.
(51) McMillin, D. R.; McNett, K. M. Photoprocesses of Copper
Complexes That Bind to DNA. Chem. Rev. 1998, 98 (3), 1201−1220.
(52) Li, T.; Muthiah Ravinson, D. S.; Haiges, R.; Djurovich, P. I.;
Thompson, M. E. Enhancement of the Luminescent Efficiency in
Carbene-Au(I)-Aryl Complexes by the Restriction of Renner−Teller
9527
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Inorg. Chem. 2021, 60, 9516−9528

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Article
Distortion and Bond Rotation. J. Am. Chem. Soc. 2020, 142 (13),
6158−6172.
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Inorg. Chem. 2021, 60, 9516−9528