Strongly luminescent palladium(0) and platinum(0) diphosphine complexes

Article abstract of DOI:10.1021/ic700475z

The synthesis, structure, and photoluminescence of palladium(0) and platinum(0) complexes containing biarydiphosphines, biphep (biphep = 2,2′-bis(diphenylphosphino)-1,1′-biphenyl) and binap (binap = 2,2′-bis-(diphenylphosphino)-1,1′-binaphthyl) have been studied. X-ray structure analysis of [Pt(biphep)₂] revealed the distorted-tetrahedral geometry of the complex. The photophysical properties of the three complexes [Pd(biphep)₂], [Pt(biphep)₂], and [Pd(binap)₂] were investigated and compared with that of the previously reported [Pt(binap)₂] complex. The [Pd(biphep) ₂] complex shows the strongest luminescence with a high quantum yield (38%) and a long lifetime (3.2 μs) in a toluene solution at room temperature. The luminescence should be due to metal-to-ligand charge transfer excited states. At room temperature, radiative rate constants of the four complexes show similar values. The difference in the luminescent properties should reflect the different nonradiative rate constants of the complexes. The temperature-dependence of the luminescence spectra and lifetime of the complexes were also discussed.

Full text of DOI:10.1021/ic700475z

Inorg. Chem. 2008, 47, 481−486
Strongly Luminescent Palladium(0) and Platinum(0) Diphosphine
Complexes
Taro Tsubomura,* Yasuhiro Ito, Satoshi Inoue, Yu Tanaka, Kenji Matsumoto, and Toshiaki Tsukuda
Department of Materials and Life Science, Seikei UniVersity, Kichijoji-Kitamachi,
Musashino, Tokyo 180-8633, Japan
Received March 12, 2007
The synthesis, structure, and photoluminescence of palladium(0) and platinum(0) complexes containing biary-
diphosphines, biphep (biphep 2,2 -bis(diphenylphosphino)-1,1 -biphenyl) and binap (binap 2,2 -bis-
diphenylphosphino)-1,1 -binaphthyl) have been studied. X-ray structure analysis of [Pt(biphep) ] revealed the distorted-
tetrahedral geometry of the complex. The photophysical properties of the three complexes [Pd(biphep) ], [Pt(biphep) ],
and [Pd(binap) ] were investigated and compared with that of the previously reported [Pt(binap) ] complex. The
Pd(biphep) ] complex shows the strongest luminescence with a high quantum yield (38%) and a long lifetime (3.2
s) in a toluene solution at room temperature. The luminescence should be due to metal-to-ligand charge transfer
)
′
′
)
′
(
′
2
2
2
2
2
[
2
µ
excited states. At room temperature, radiative rate constants of the four complexes show similar values. The
difference in the luminescent properties should reflect the different nonradiative rate constants of the complexes.
The temperature-dependence of the luminescence spectra and lifetime of the complexes were also discussed.
1
0
5
Introduction
focused on the photophysics of d metal complexes such
6
7
as gold(I) and copper(I) complexes. A few platinum(0) and
palladium(0) complexes have been known as luminescent
in fluid solutions, but the photophysical study on the Pd(0)
species is limited. Our group reported that [Pt(binap)2]
(binap ) 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) shows
relatively strong luminescence (Φ ) 12%) and a long
lifetime (τ ) 1.3 µs) in solution. The unusual photophysical
properties of the complex were thoroughly studied, and the
luminescence was shown to be emitted mainly from a
thermally populated singlet metal-to-ligand charge transfer
Luminescent metal complexes have experienced a remark-
able development since the demonstration of the advantage
of triplet emitter over singlet emitter for organic light-
emitting diode devices by Thompson et al.1
8
,9
2
3
A large number of iridium(III) and platinum(II) com-
plexes that serve as highly luminescent materials have been
reported. On the other hand, the study of luminescent
10
4
palladium complexes is rare; the low energy level of d-d
excited states is believed to be the reason for the nonlumi-
nescent nature of the Pd(II) complexes compared with the
Pt(II) complexes. Recently, a significant research effort has
10
(
MLCT) state rather than the triplet MLCT state. Addition-
(5) (a) Kutal, C. Coord. Chem. ReV. 1990, 99, 213. (b) Che, C.-M.; Lai,
S.-W. Coord. Chem. ReV. 2005, 249, 1296.
6) (a) Fern a´ ndez, E. J.; Laguna, A.; L o´ pez-de-Luzuriaga, J. M. Coord.
Chem. ReV. 2005, 249, 1423. (b) Yam, V. W.-W.; Chan, C.-L.; Li,
C.-K.; Wong, K. M.-C. Coord. Chem. ReV. 2001, 216-217, 173. (c)
de la Riva, H.; Nieuwenhuyzen, M.; Medicute Fierro, C.; Raithby, P.
R.; Male, L.; Lagnus, M. C. Inorg. Chem. 2006, 45, 1418.
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Armaroli, N. Chem. Soc. ReV. 2001, 30, 113. (c) Harkins, S. B.; Peters,
J. C. J. Am. Chem. Soc. 2005, 127, 2030. (d) Tsubomura, T.; Enoto,
S.; Endo, S.; Tamane, T.; Matsumoto, K.; Tsukuda, T. Inorg. Chem.
2005, 44, 6373. (e) Tsuboyama, A.; Kuge, K.; Furugori, M.; Hoshino,
M.; Ueno, K. Inorg. Chem. 2007, 46, 1992.
(8) Caspar, J. V. J. Am. Chem. Soc. 1985, 107, 6718.
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(b) Harvey, P. D.; Shaefer, W. P.; Gray, H. B. Inorg. Chem. 1988,
27, 1101.
(
*
To whom correspondence should be addressed. E-mail:
tsubomura@st.seikei.ac.jp.
(
1) (a) Baldo, M. A.; O’ Brien, D. F.; You, Y.; Shoustikov, A.; Sibley,
S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Baldo,
M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S.
R. Appl. Phys. Lett. 1999, 75, 4.
(
2) (a) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. AdV. Mater.
2
005, 17, 1109. (b) Smith, L. H.; Wasey, J. A. E.; Samuel, I. D. W.;
Barnes, W. L. AdV. Funct. Mater. 2005, 15, 1839. (c) Tsuboyama,
A.; Iwasaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.;
Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno,
K. J. Am. Chem. Soc. 2004, 125, 12971.
(
3) Furuta, P. T.; Deng, L.; Garon, S.; Thompson, M. E.; Frechet, J. M.
J. J. Am. Chem. Soc. 2004, 126, 15388 and references therein.
4) Akaiwa, M.; Kanbara, T.; Fukumoto, H.; Yamamoto, T. J. Organomet.
Chem. 2005, 690, 4192.
(
(10) Siddique, Z. A.; Ohno, T.; Nozaki, K.; Tsubomura, T. Inorg. Chem.
2004, 43, 663.
1
0.1021/ic700475z CCC: $40.75
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 2, 2008 481
Published on Web 12/14/2007

Tsubomura et al.
Chart 1. Schematic Drawings of the Complexes and the Ligands
3
ally, we reported the energy transfer process from the MLCT
1
1
state to oxygen and the generation of singlet oxygen.
Recently, we found that some structurally related palladium-
0) complexes show very strong luminescence in solution at
room temperature. In this work, the photophysics of a series
of the palladium(0) and platinum(0) complexes (Chart 1) is
investigated in detail. The photophysics and its temperature-
dependence on the complexes are discussed.
(
Figure 1. ORTEP drawing of [Pt(biphep)2] viewed along the C2 axis.
The Pt1 atom is located on the C2 axis. Atoms with an asterisk are the
symmetrically located atoms. Important bond lengths (Å) and angles
(deg): Pt1-P1, 2.325(3); Pt1-P2, 2.327(3); P1-Pt1-P2, 94.1(1); P1-
Pt1-P1*, 120.8(1); P1-Pt1-P2*, 114.8(1); P2-Pt1-P2*, 120.4(2).
Results and Discussion
Synthesis and Crystal Structure of the Complexes. [Pd-
12
(binap)
2
]
3 2 3
was prepared from [Pd(P(o-tol) ) ] (P(o-tol) )
tris(o-tolyl)phosphine) by the method of Alcazar-Roman et
al. The solid samples of the complexes can be handled under
air, although the Pd(0) complexes show some degradation
of colors if stored in air. [Pd(biphep)
a similar method (biphep ) 2,2′-bis(diphenylphosphino)-1,1′-
biphenyl). [Pt(binap) ] and [Pt(biphep) ] were prepared from
the corresponding [PtCl (diphosphine)] complexes. Both [Pd-
biphep) ] and [Pt(biphep)
] give a single 31P NMR signal
in C at room temperature although the NMR spectrum
of the Pt complex contains Pt satellites. The Pt-P coupling
constant for [Pt(biphep) ] (3570 Hz) is comparable to that
for [Pt(binap) ] (3740 Hz).
The X-ray crystal structure analysis of the crystals of [Pt-
biphep) ] shows that the complex has a distorted-tetrahedral
structure as shown in Figure 1. The corresponding palladium
complex [Pd(biphep) ] has been revealed to have a similar
structure, although the crystallographic data is poor. We
have not succeeded in the crystallization of [Pd(binap) ], but
the crystal structure of [Pd(tol-binap) ] was already re-
ported, where tol-binap ) 2,2′-bis(di-p-tolylphosphino)-
,1′-binaphthyl. The two coordination structures of [Pt-
2
] was synthesized by
2
2
2
(
2
2
Figure 2. Absorption and the corrected luminescence spectra of the
complexes (1 × 10 M, toluene, room temperature).
6
D
6
-
5
195
2
complexes tend to have slight longer metal-phosphorous
bond lengths (2.38-2.40 Å). The geometrical data are shown
in Table S1 in the Supporting Information. One notable
difference between the biphep and binap complexes is in
the dihedral angles between the two aryl groups in the biaryl
2
(
2
2
moieties; the averaged angles are 66° for [Pt(biphep)
] and
], and 72° for [Pd(tol-
2
]. The result suggests that the binap ligand may favor
2
1
3
2 2
[Pd(biphep) ], 71° for [Pt(binap)
2
binap)
2
a large dihedral angle, which should be due to the steric
demand within the ligand. To confirm the hypothesis, we
examined the reported structures of the biphep and binap
complexes in the CCDC crystallographic database.15 We
found 25 structures of transition-metal complexes containing
one or more biphep ligands. Biphep derivatives (e.g., 3,3′-
dimethyl-biphep) were not included in the list because the
substituents may significantly affect the geometries of the
complexes. The average of the dihedral angles P-C-C-P,
which represent the twist angle of the biphenyl groups, in
the 25 structures is 67.4 (4.2)°. The standard deviation is
shown in parentheses. On the other hand, the average angle
1
2
1
1
4
2 2
(biphep) ] and [Pt(binap) ] resemble each other; for example,
the average Pt-P bond lengths are equal (2.33 Å) within
error limits. The structures of all complexes discussed here
are similar around the metal centers, although the palladium
(11) Tsubomura, T.; Abe, M.; Tarutani, M.; Yamada, H.; Tsukuda, T. Bull.
Chem. Soc. Jpn. 2003, 76, 2151.
(
12) Alcazar-Roman, L. M.; Hartwig, J. F.; Rheingold, A. L.; Liable-Sands,
L. M.; Guzei, I. A. J. Am. Chem. Soc. 2000, 122, 4618. We use
commercial (S)-binap as the binap ligand.
(13) The preliminary crystallographic data are shown in the Supporting
Information.
(
14) Tominaga, H.; Sakai, K.; Tsubomura, T. J. Chem. Soc., Chem.
Commun. 1995, 2273-2274.
(15) The Cambridge Structural Database: a quarter of a million crystal
structures and rising. Allen, F. H. Acta Crystallogr. 2002, B58, 380.
482 Inorganic Chemistry, Vol. 47, No. 2, 2008

Luminescent Pd(0) and Pt(0) complexes
Table 1. Absorption and Emission Maxima, Lifetimes (τ), Emission Quantum Yields (Φ), and Apparent Radiative and Nonradiative Rate Constants (kr
and knr) of the Pt(0) and Pd(0) Complexes in Degassed Toluene Solution at 25 °C
b
b
absorption νmax
emission νmax
τ
kr
knr
3
-1
3
-1
-6
5
-1
5
-1
complex
10 cm
10 cm
10
s
Φ
10 s
10 s
ref
[Pd(binap)2]
[Pt(binap)2]
[Pd(biphep)2]
[Pt(biphep)2]
19.2
18.9
22.9
22.7
13.4
13.1
15.0
14.5
2.6
1.25
3.2
0.32
0.12
0.38
0.27
1.2
0.99
1.2
2.6
6.8
1.9
3.0
this work
a
this work
this work
2.4
1.1
a
Ref 10. b The “apparent” rate constants are calculated as kr ) Φ/τ, knr ) (1 - Φ)/τ.
of the 169 structures of the binap complexes is 72.4 (4.6)°.
These values agree well with the angles found in our
complexes. Since steric bulkiness on the biaryl group should
favor the dihedral angle of 90°, the smaller dihedral angle
of the biphep ligand appears to be proof of the flexibility of
the ligand.
excited states toward planar structure and (2) the attack of
solvent molecule and formation of exciplexes may account
7
a
for the red shift in solution. One can see a significant
-
1
difference in the magnitude of the Stokes shift (5800 cm
-1
for the binap complexes and 7900-7200 cm for the biphep
complexes) which is independent of the metal species. The
biphep ligand is known to be more flexible than the binap
ligand, which is shown by the fact that binap can be optically
resolved but biphep cannot. The structural studies have also
shown the flexibility of the biphep ligand in comparison with
that of the binap ligand (vide supra). A larger distortion in
the biphep complexes in the excited states may be a reason
for the larger Stokes shift in the biphep complexes. However,
different participation of the solvent molecules on the binap
and biphep complexes is another possibility.
Absorption and Emission Spectra. These complexes
3
have absorption bands in the visible region (18-24 × 10
-
1
cm ). The photophysical data are listed in Table 1. We
already clarified that the absorption band in the visible region
1
10
2
of the [Pt(binap) ] complex is due to the MLCT transition.
The bands of the present complexes can be also assigned to
1
MLCT. The band of each biphep complex shows a blue
-1
shift by ca. 3700 cm relative to those of the corresponding
binap complexes, which should reflect the shrinkage of the
π-conjugated groups in the biaryl moieties. Furthermore, the
band maxima of the two palladium complexes are located
at 200-300 cm higher energy compared with those of the
corresponding platinum complexes.
As shown in Table 1, we can see that the apparent radiative
rate constants k
r
are similar in the four complexes at room
temperature, and the difference in the quantum yields of the
complexes is due to the variation of the nonradiative
processes. For both the binap and biphep complexes, the
platinum complexes have higher knr values than the corre-
-
1
Figure 2 shows the luminescence spectra of the complexes
measured in degassed toluene solution. The two palladium
complexes and [Pt(biphep) ] show strong luminescence in
2
degassed toluene solution. All the luminescence is effectively
quenched by oxygen. The highest quantum yield reaches
sponding palladium complexes. Moreover, [Pd(biphep)
2
] and
[
Pt(biphep) ] show smaller knr values than the corresponding
2
binap complexes, respectively. The higher quantum yield and
smaller knr values for the biphep complexes relative to those
of the binap complexes may be rationalized by the energy-
3
2
8%, and the lifetimes of the excited states are in the range
.6-3.2 µs for the palladium complexes. The luminescence
band of each Pt complex shows a slight red shift compared
with the corresponding Pd complexes. Furthermore, both the
biphep complexes have higher-energy luminescence bands
than the binap complexes. In recent years, some metal
complexes containing binap and related ligands were reported
17
gap law. The variation of knr values will again be discussed.
Solvent-dependence on the photophysical parameters of
the complexes has been observed (see Table S3, Supporting
Information). The solvent-dependence on the absorption
maxima is small. On the other hand, a red shift of the
emission maxima was observed in polar solvent. This result
and the large solvent dependence on knr suggests the attack
of the solvent to the excited states of the complexes. It should
be noted that no significant differences are observed for the
solvent effect between the platinum and palladium com-
plexes.
1
6
to luminesce. However, the luminescence of the Pd(0) and
Pt(0) biaryl complexes shown in this work is characterized
as very intense emission with a long lifetime in solution.
The long lifetime of the excited states should show the
participation of the triplet MLCT states in the emissive
excited states.
Emission was also observed for the solid samples. The
emission maxima for [Pd(binap) ], [Pt(biphep) ], and [Pd-
2 2
Temperature-Dependence of the Spectra. Recently, it
was shown that the major part of the luminescence intensity
3
3
3
-1
(
biphep)
2
] are 13.9 × 10 , 16.5 × 10 , and 15.6 × 10 cm ,
of the [Pt(binap)
2
] complex comes from the thermally
respectively. The values measured in toluene solution show
red shifts of 500-1500 cm when compared with the values
observed for the solid sample.
-1
populated singlet MLCT excited state, not from the triplet
1
0
state. Similar conclusion that room-temperature emission
is mainly “delayed fluorescence” was proposed by McMillin
As shown in Figure 2, these complexes show large Stokes
shifts, especially in solution. (1) A larger distortion in the
1
8
et al. for some Cu(I) systems and recently reexamined by
(
16) (a) Pawlowski, V.; Kunkely, H.; Vogler, A. Inorg. Chim. Acta 1994,
(17) Nonradiative rate constants of the T1 f S0 process often show an
exponential energy-gap dependence; knr ) a exp(-c∆E). See ref 21.
(18) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Paggio, A. D.;
Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380.
357, 1309. (b) Pawlowski, V.; Kunkely, H.; Lennartz, C.; B o¨ hn, K.;
Vogler, A. Eur. J. Inorg. Chem. 2004, 4242. (c) Kunkely, H.; Vogler,
A. Inorg. Chem. Commun. 1999, 2, 533.
Inorganic Chemistry, Vol. 47, No. 2, 2008 483

Tsubomura et al.
Chart 2 Energy Diagram of Two-Emitting-State Model
the luminescence decreases on lowering the temperature and
a slight red shift of the spectra is observed just above the
freezing point. However, different dependencies were ob-
2
served for the palladium complexes. For [Pd(binap) ], the
intensity of the luminescence initially increases on lowering
the temperature from 300 K. However, the intensity again
decrease as the temperature goes down below 250 K. A small
red shift of the spectra is observed at low temperature. For
[
Pd(biphep)
the change of the temperature, but the tendency of the
temperature-dependence is similar to that of [Pd(binap) ].
2
], the spectra show only a slight change with
2
The lifetimes of the excited states increase on lowering
the temperature for all of the complexes, as shown in Figure
S2 (Supporting Information). Each decay curve can be fitted
with a single exponential curve in all the temperature ranges
studied.
In the previous study of [Pt(binap)
2
],10 analysis of the
temperature change in the radiative rate constants afforded
the parameters for the two-states model including radiative
rate constants of singlet and triplet excited states and the
energy gap between the two states (Chart 2). In this model,
the ratio of the populations of the two states, K, is denoted
1
0
by the following equation:
1
3
∆E
RT
K ) exp -
(
)
In the present work, we tried to interpret the complex
Figure 3. Temperature-dependence of the corrected luminescence spectra
of the three complexes. These data were measured in degassed toluene
solution.
temperature-dependence by examining the temperature-
dependence of the apparent radiative and nonradiative rate
constants of the luminescence.
19
Nozaki et al. The characteristic was shown by the fact that
1) the intensity of the luminescence becomes lower as the
Temperature-Dependence in k . At room temperature,
r
(
apparent k values ()QE/τ) of all four complexes are similar,
r
temperature goes down and (2) the luminescence lifetime
increases with lowering the temperature. A slight red shift
of the emission bands has been observed in some complexes
as already stated above. Decreases of the k value at lower
temperature were observed for all complexes (see Figure S3,
Supporting Information). The energy gap ∆E between the
singlet and the triplet states and radiative rate constants of
the two states can be principally obtained by nonlinear least-
square analyses of the temperature-dependence of the kr
r
2
including [Pt(binap) ]. These spectral features can be un-
derstood if the emitters have thermally equilibrated two
emissive states and the radiative rate constant of the upper
level is much higher than that of the lower state. Thus, the
apparent rate constants calculated from the lifetime and
quantum yield values do not directly describe the emissive
properties of the excited-triplet states.
QE
τ
k1r K + k
3r
)
1 + K
data.10 The data are shown in the Supporting Information
Table S2). The result shows that ∆E values of the four
As shown in Figure 3, the complexes studied show
different temperature-dependence of the emission spectra.
The temperature-dependence of the luminescence of [Pt-
(
-
1
complexes are within the range of 900-1400 cm . This
means that K, the population of the upper singlet state, is
10
(
2 2
biphep) ] is similar to that of [Pt(binap) ]; the intensity of
0
.4-0.04%, even at 298 K. However, due to the high kr1/kr3
4
ratio (over 10 ), most of the emission intensity should come
(19) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem.
2003, 42, 6366.
from the thermally equilibrated upper singlet state. If we
484 Inorganic Chemistry, Vol. 47, No. 2, 2008

Luminescent Pd(0) and Pt(0) complexes
4
assume a population of 0.1% and a kr1/kr3 ratio of 10 , simple
calculation shows that 90% of the intensity of the emission
originates from the upper singlet state. Unfortunately, because
of the limitations of the temperature range and the errors in
the experimental data, noticeable errors are included in the
Conclusion. We found high-intensity luminescence of the
biaryldiphosphine complexes of Pd(0) and Pt(0) metals.
Many researchers may consider this result strange, because
many platinum complexes luminesce but palladium com-
plexes do not, especially at room temperature. This should
be true for divalent platinum and palladium complexes,
because low-lying d-d states effectively quench the CT
excitation energies of the palladium complexes. However,
the result for platinum(0) and palladium(0) complexes is
opposite that of the divalent complexes. It has been reported
that some platinum(0) complexes show luminescence with
a very short lifetime due to large knr values. For example,
k
r1 and kr3 values.20
Temperature-Dependence in knr. As stated before, the
difference in knr values almost determines the luminescence
quantum yield of the complexes at room temperature. The
temperature-dependence of knr values of [Pt(biphep)
2
] re-
sembles that of [Pt(binap) ]. As for the palladium complexes,
2
large temperature-dependence is observed compared with that
of the platinum complexes (see Figure S3, Supporting
Information). For the platinum complexes, the decrease in
[Pd(PPh
3 3
) ] has lifetime of 3.6 µs in solution at room
8
temperature, but [Pt(PPh ) ] has a lifetime of 0.7 µs. The
3 3
k
nr on lowering the temperature is gentler than that in k
r
;
variation of the photophysical properties of the complexes
described in the present work should be mainly ascribed to
the variation of nonradiative processes. Large spin-orbit
coupling in the platinum complexes may be the reason for
the large nonradiative deactivation. The palladium(0)-
phosphine complexes have long been known as precursors
of many homogeneous catalysts, but the interesting photo-
physical properties of the complexes have not been explored
so far. In this study, we shed light on additional promising
applications of the palladium complexes as luminescent
materials.
thus, the quantum yields of the luminescence essentially
10
decrease at low temperature. On the other hand, in the case
of [Pd(biphep) ], the k and knr curves closely resemble each
other, so the quantum yield does not change with tempera-
ture. The temperature-dependent knr data of [Pt(binap) ] were
2
fit with
2
r
1
- QE
τ
)
knr1 exp(-∆E′/RT) + knr2
an Arrhenius-type rate equation. In the treatment, knr1 denotes
the lifetime of the higher-lying state which gives rise to
deactivation, and knr2 is the intrinsic lifetime of the emitting
Experimental Section
3
Synthesis of the Complexes. Metal sources and phosphines were
obtained from Tanaka Kikinzoku Kogyo, Ltd. and Strem Chemicals,
respectively. The syntheses of the bis(diphosphine)complexes
should be done under argon atmosphere, although the obtained
crystals of the zerovalent complexes can be handled in air. [Pd-
MLCT. The temperature-dependence of the apparent knr data
(Figure S3, Supporting Information) clearly shows that knr2,
which is equal to the low-temperature limiting value of the
apparent knr value, is small for the palladium complexes as
compared with that of the platinum complexes. What is the
reason for the difference in the rate of the nonradiative
deactivation between the two metal species? We have shown
2
2
(
P(o-tol)
3
)
2
] was prepared by the reported procedure.
] was prepared according to the Alcazar-Roman
[
Pd(binap)
2
12
method.
Pd(biphep)
Benzene (3 mL) was added to a mixture of [Pd(P(o-tol) ) ] (0.14
that the geometry found in the two complexes, [Pt(biphep)
and [Pd(biphep) ], is similar, as clarified by X-ray analysis,
2
]
[
2
] was prepared by a similar method, as follows.
2
3
2
and furthermore, the solvent effect on the photophysical
parameters of the platinum and palladium complexes seem
to be similar. Harvey and Gray also observed a great
difference in the excited-state lifetime of the two binuclear
platinum(0) and palladium(0) complexes.9a The lifetime of
mmol) and biphep (0.28 mmol) and stirred until the complex
dissolved. The mixture was filtered under argon, and n-heptane was
added slowly and then kept in a refrigerator. Orange crystals (0.12
g) were afforded after several days. Yield, 72%. Anal. Calcd. for
C
72
H
56
P
4
Pd(C
P NMR (160 MHz, C
Pt(biphep) ] was synthesized from [PtCl
prepared from [PtCl (C CN) ] as follows. Portions of 0.25 mmol
of [PtCl (C CN) ] and 0.25 mmol of biphep were dissolved in
0 mL of benzene and stirred at 70 °C for 1 h. After cooling, the
6
H
6
)
3
: C, 78.00; H, 5.38. Found: C, 77.66; H, 5.39.
3
1
6 6
D ): δ ) 20.3.
the former is far smaller than that of the latter complex. They
concluded that the difference in the spin-orbit coupling may
be the reason for this result. It has been shown that spin-
orbit coupling must be included in the evaluation of
nonradiative rates for the relaxation between states of
different multiplicity.21 Since there is a large difference in
the spin-orbit coupling constants for the platinum atom
[
2
2
(biphep)], which was
2
H
6 5
2
2
6
H
5
2
5
solution was concentrated to 10 mL and was allowed to stand for
24 h at 5 °C. White powders of [PtCl (biphep)] deposited (77%
2
yield) were collected by filtration. To a THF solution (22.5 mL)
containing both the dichloride complex (0.13 mmol) and biphep
-
1
-1
(
4400 cm ) and palladium atom (1500 cm ), spin-orbit
(0.13 mmol) was added dropwise an aqueous solution (5 mL) of
coupling should be one of the essential reasons for the great
difference in the nonradiative rates of relaxation of the
platinum and palladium complexes.
NaBH (0.39 mmol) under argon atmosphere. The solution was
4
dried in vacuo. Toluene (20 mL) was added to the residue, and the
insoluble solid was separated by filtration. Heptane (20 mL) was
added to the toluene solution, and the solution was allowed to stand
(
20) In the least-squares calculation, results are seemingly accurate with
small standard deviations, but different weight function gave different
results, especially for k1r and k3r. Thus, the actual errors should be
considerably larger than the calculated standard deviations. Therefore,
we discuss ∆E values only in this section.
for a week at 5 °C. [Pt(biphep)
Yield, 48%. Anal. Calcd. for C72
NMR (160 MHz, C ): δ ) 13.2 (JPtP ) 3570 Hz).
2
] was obtained as orange crystals.
31
H
56
4
P Pt: C, 69.73; H, 4.55.
P
6
D
6
(
21) Adamson, A. W.; Fleischauer, P. D. Concepts of Inorganic Photo-
chemistry; John Wiley & Sons: London, 1975; p 69.
(22) Paul, F.; Patt, J.; Hartwig, J. F.; Organometallics 1995, 14, 3030.
Inorganic Chemistry, Vol. 47, No. 2, 2008 485

Tsubomura et al.
Crystallographic Study of [Pt(biphep)
2
]. The orange crystal
nitrobenzene with our recorded spectra.25 The quantum yields of
was glued on the tip of a glass fiber with epoxy resin and mounted
on a four-circle automated diffractometer (AFC-5S of Rigaku, Ltd.).
The detailed data collection parameters and the crystal data were
listed in a CIF file. The positions of the heavy atom were determined
by a direct method using the SIR92 program.23 The structure was
expanded and refined by the SHELX97 software.24 Two kinds of
toluene molecules of solvate were found in an asymmetric unit.
For one toluene molecule, the parameters of the carbon atoms of
the toluene molecules were refined with the multiplicities of each
atom set to unity. One more toluene molecule was found on a
crystallographic two-fold axis. The multiplicities of the carbon
atoms of the toluene molecule were set to 1 or 0.5 so that a
disordered pair of the molecule was located on the position.
Some C-C distances of the toluene molecules were restrained to
[Pd(biphep)
2
] and [Pt(biphep)
2
] complexes were calculated by the
following equation:26
2
-A*
n
S
I* 1 - 10
I
Φ ) Φ*
(
) ( )( )(
-A
)
n* S*
1 - 10
where n is refractive index of the solvent, S is the area of the
corrected emission spectra (quanta per wavenumber vs wavenum-
ber), I is the intensity of light at the excitation wavelength, and A
is the absorbance at the excitation wavelength. The asterisk refers
to the standard sample. At low temperature, a slight narrowing of
the absorption band widths and an increase in the absorption
maxima were observed, and then they were corrected for the
determination of the quantum yields. Rhodamine 101 was used for
the standard of the quantum yield (Φ ) 0.92).27 The wavelengths
1.36 Å.
of excitation were as follows: 440 nm for Pd(biphep)
2
and Pt-
2 6 5 3 3 80 4
Crystallographicparametersfor[Pt(biphep) ]‚(C H CH ) : C93H P -
(
biphep) and 525 nm for rhodamine 101. The intensity ratio of
2
Pt, M ) 1516.6, monoclinic, a ) 19.818(8) Å, b ) 14.219(3) Å,
the excitation, I*/I, was determined using a quantum counter of a
rhodamine 101 solution (5 g/L) dissolved in ethanol. Luminescence
decay curves were measured on a laboratory-made apparatus; the
sample was excited using a nitrogen laser (USHO AN-200), and
the emission light was focused into a Jovin-Ybon H-20 monochro-
mator equipped with a photomultiplier (Hamamatsu R-955). The
output of the photomultiplier was digitized by a Iwatsu-Lecroy DS-
3
c ) 26.677(2) Å, â ) 97.87(2)°, V ) 7446(3) Å , space group
-
1
C2/c, Z ) 4, T ) 293 K, µ ) 2.02 mm , reflections collected
2
9
(
146, unique 7561; final R
all).
Spectroscopy All spectroscopic measurements were carried out
using strictly degassed solutions by repeated freeze-pump-thaw
procedures. The spectroscopic properties of [Pd(binap) ] were
1
(F) ) 0.078 (I > 2σ(I)), R (F ) ) 0.162
2
2
4
262 digital oscilloscope and then downloaded to a PC. NMR
spectra were measured on a JEOL Λ-400 spectrometer using H
PO solution as an external reference.
studied by the methods described in the recent paper.10 In short,
the luminescence spectra of the complex up to 1000 nm were
measured with a polychromator equipped with a CCD detector.
3
-
4
The spectroscopic properties of [Pd(biphep)
2
] and [Pt(biphep)
2
]
Acknowledgment. We are grateful to Prof. Koichi
complexes were measured as follows. The UV-vis absorption
spectra and the luminescence spectra were recorded on a Shimadzu
UV-2100 spectrometer and a Shimadzu RF-5000 spectrofluorom-
eter, respectively. In the spectrofluorometer, a red-enhanced-type
photomultiplier tube (Hamamatsu R3896) was used. The concentra-
tions of the complexes were adjusted as the absorbance of the
solutions at the excitation wavelength was set to around 0.1. The
temperature was kept constant by means of circulating thermostatted
water for the measurements at around room temperature. For the
Nozaki of the University of Toyama for measuring the
luminescence spectra of [Pd(binap)
of Education, Culture, Sports, Science and Technology
MEXT) for their financial support of this research through
2
]. We thank the Ministry
(
the High-Tech Research Center Project and Grant-in-Aid for
Scientific Research.
Supporting Information Available: Crystallographic data of
2 2
[Pt(biphep) ] and [Pd(biphep) ]; geometrical parameters of the
2
low-temperature measurements, a custom-made LN cryostat
complexes; temperature-dependence of the photophysical data. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
manufactured by Jecc-Torisha Co., Ltd., and a temperature control-
ler (Scientific Instruments, Model 9700) were used. The correction
factors for the sensitivity of wavelength-dependence of the spec-
trometer were determined bycomparison of the reported spectral
curves of 4-dimethylamino-4′-nitrostilbene and 3-(dimethylamino)-
IC700475Z
(25) Lippert, E.; Nagele, W.; Seibold-Blankenstein, I.; Staiger, U.; Voss,
W. Fresenius’ Z. Anal. Chem. 1959, 170, 1.
(
26) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(
23) SIR92: Altomare, A.; Burla, M.C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994,
(27) Two conflicting values (1.0 and 0.92) were reported so far; we adopted
the newly reported value by Arden-Jacob et al.: Arden-Jacob, J.; Marx,
N. J.; Drexhage, K. H. J. Fluorescence 1997, 7(Suppl.), 91S. The value
of 1.0 was reported in the following paper: Karstens, T.; Kobs, K. J.
Phys. Chem. 1980, 84, 1871.
27, 435.
(24) SHELXL-97: Sheldrick, G.M. Program for the Refinement of Crystal
Structures; University of G o¨ ttingen: G o¨ ttingen, Germany, 1997.
486 Inorganic Chemistry, Vol. 47, No. 2, 2008