Structure and luminescence of a dinuclear copper complex bridged by a diphosphine ligand

Article abstract of DOI:10.1246/cl.2004.678

A new dinuclear copper(I) complex containing two 2,9-dimethylphenanthroline ligands and a bridging diphosphine ligand has been prepared. The structure of the complex was characterized by X-ray crystallography. A considerable blue shift was observed for the solid-state photoluminescence and absorption spectra of the complex as compared with the mononuclear mixed-ligand complex.

Full text of DOI:10.1246/cl.2004.678

678
Chemistry Letters Vol.33, No.6 (2004)
Structure and Luminescence of a Dinuclear Copper Complex
Bridged by a Diphosphine Ligand
Taro Tsubomura,ꢀ Naoki Takahashi, Ken Saito, and Toshiaki Tsukuda
Department of Applied Chemistry, Seikei University, Kichijoji-Kitamachi, Musashino, Tokyo 180-8633
(Received March 12, 2004; CL-040280)
A new dinuclear copper(I) complex containing two 2,9-di-
of 115.7–119.8ꢁ. The N (dmp)–Cu–N (acetonitrile) angles are
near the ideal tetrahedral angle. The bond lengths of Cu–P and
methylphenanthroline ligands and a bridging diphosphine ligand
has been prepared. The structure of the complex was character-
ized by X-ray crystallography. A considerable blue shift was ob-
served for the solid-state photoluminescence and absorption
spectra of the complex as compared with the mononuclear
mixed-ligand complex.
ꢀ
Cu–N (dmp) are 2.224(1) A and 2.10 A (the average of the two
ꢀ
Cu–N bonds), respectively. These values are typical for Cu(I)
4
ꢀ
complexes. The Cu–N (acetonitrile) bond length (2.015(3) A)
is notably shorter than that of the Cu–N (dmp) bonds.9 An inter-
molecular stacking interaction of dmp ligands is observed in the
crystal. The nearest interatomic CꢂꢂꢂC distance between two dmp
ꢀ
ligands is 3.410(7) A. Similar intermolecular stacking of dmp li-
gands has been reported.10 The interaction seems not to
influence the photophysical properties of the copper complex.
Luminescent copper(I) complexes containing diimine li-
gands are of considerable interest because of rich photophysical
and photochemical properties.1 Potential applications of these
complexes for photovoltaic cell or electroluminescent display
have been explored.2 Extensive studies reported by McMillin’s
group and other groups have made clear the properties and elec-
tronic structures of the luminescent metal-to-ligand charge trans-
fer (MLCT) states of the copper(I) diimine complexes.3
Recently, remarkable luminescent properties of the mixed-
ligand copper(I) complexes containing a diphosphine chelate
and a dmp ligand have been reported. For example, the or-
ange-colored [Cu(dppe)(dmp)]þ complex (dppe = bis(diphenyl-
phosphino)ethane, dmp = 2,9-dimethyl-1,10-phenanthroline)
shows photoluminescence in CH2Cl2 solution with a quantum
yield of 1%.4 As a part of our study of luminescent metal com-
plexes,5 we have been studying the structure and photophysical
properties of copper(I) complexes. We found that two types of
the mixed-ligand complexes containing dppe and dmp ligands
can be obtained when different solvents were used in the synthe-
sis of the complexes. In this work, the structure and properties of
the yellow-colored copper complex have been examined.
The mixed-ligand complexes were prepared as follows. To a
solution of [Cu(CH3CN)4]PF6 (0.5 mmol) in acetone (20 mL)
were added dmp (0.5 mmol) and dppe (0.5 mmol). Stirring for
1 h at room temperature and concentration of the solution afford-
ed [Cu(dppe)(dmp)]PF6 (1) as orange crystals.6 On the other
hand, if the similar reaction was performed in acetonitrile, a yel-
low powder was obtained. Recrystallization of the yellow com-
plex (2) from acetonitrile–diethyl ether gave yellow crystals.7
The structure of 2 is shown in Figure 1.8 The complex has a di-
nuclear structure in which a dppe ligand bridges the two cop-
per(I) atoms. Although there are a number of crystallographic
studies of copper(I) complexes containing dppe as a bridging li-
gand, this is the first example of the dinuclear copper complex
containing dppe and diimine ligands. The complex has a center
of inversion, which locates between the two methylene carbon
atoms of the dppe ligand. Each of the two copper(I) atoms is co-
ordinated by a bidentate dmp chelate, a bridged dppe ligand and
an acetonitrile ligand. The coordination geometry of the cop-
per(I) atom is a distorted tetrahedron. The N–Cu–N angle of
the dmp chelate is 79.6(2)ꢁ, and N–Cu–P angles are in the range
Figure 1. The structure of [Cu2(dmp)2(dppe)(CH3CN)2]2þ
.
The solid-state photoluminescence spectra of the dinuclear
complex 2 and the mononuclear complex 1 are shown in Fig-
ure 2. The dinuclear complex shows an emission maximum at
550 nm, whereas the monomer at 600 nm. The emission bands
are reasonably assigned to MLCT transitions.4 The diffusion
reflectance spectra also show the 50-nm blue shift for the dinu-
clear complex. Phosphine ligands are known to give strong crys-
tal field in metal complexes. The blue shift may be due to the
weakened crystal field in the dinuclear complex (N3P1 coordina-
tion) compared with that in the mononuclear (N2P2 coordina-
tion) complex. The coordination of the acetonitril molecule
causes a decrease in the energy of the higher-energy occupied
d orbitals.
The absorption spectra of the 1 and 2 in solution are shown
in Figure 3. Contrary to the above result, the absorption and
emission spectra of the two complexes in CH2Cl2 solution are
similar to each other. The bands centered at 400 nm are typical
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.6 (2004)
679
1. Researches on the luminescence of the complexes containing
a series of diphosphines as a bridge ligand are now in progress.
d
1
c
a
References and Notes
b
1
D. R. McMillin and K. M. McNett, Chem. Rev., 98, 1201
(1998); N. Armaroli, Chem. Soc. Rev., 30, 113 (2001).
Y.-G. Ma, W.-H. Chan, X.-M. Zhou, and C.-M. Che, New J.
Chem., 1999, 263; S. Sakaki, T. Kuroki, and T. Hamada, J.
Chem. Soc., Dalton Trans., 2002, 840.
2
3
4
5
M. K. Eggleston, D. R. McMillin, K. S. Koenig, and A. J.
Pallenberg, Inorg. Chem., 36, 172 (1997); Z. A. Siddique,
Y. Yamamoto, T. Ohno, and K. Nozaki, Inorg. Chem., 42,
6366 (2003).
D. G. Cuttell, S.-M. Kuang, P. E. Fanwick, D. R. McMillin,
and R. A. Walton, J. Am. Chem. Soc., 124, 6 (2002); S.-M.
Kuang, D. G. Cuttell, D. R. McMillin, P. E. Fanwick, and
R. A. Walton, Inorg. Chem., 41, 3313 (2002).
T. Tsubomura and K. Sakai, Coord. Chem. Rev., 171, 107
(1998); Z. A. Siddique, T. Ohno, K. Nozaki, and T.
Tsubomura, Inorg. Chem., 43, 663 (2004).
Preliminary X-ray study shows that the orange complex has a
mononuclear structure, [Cu(dppe)(dmp)]PF6.
0
400
500
600
700
λ
/ nm
Figure 2. The absorption spectra of 1 (a) and 2 (b), and emis-
sion spectra of 1 (c) and 2 (d). All spectra were recorded in
the solid state at room temperature. Absorption and emission in-
tensities (I) are in arbitrary units. The absorption spectra were
obtained by Kubelka–Munk transformation of the diffuse reflec-
tance spectra of the powdered samples.
for the MLCT band of the [Cu(dmp)(diphosphine)]þ com-
plexes.4 The shoulder found at 460 nm presumably shows the ex-
istence of [Cu(dmp)2]þ complex.11
6
7
8
Anal. Found: C, 52.92; H, 4.12; N, 6.31. Calcd for [Cu2-
(dmp)2(dppe)(CH3CN)2](PF6)2: C, 53.01; H, 4.14; N, 6.40.
A yellow crystal (C68H66N8P4F12Cu2) recrystallized from an
acetonitrile–benzene mixed solvent was investigated at
295 K on a Rigaku AFC-5S diffractometer (ꢀ ¼ 0:71069
The 31P NMR spectrum (160 MHz) of 2 measured in
CD2Cl2 consists of a broad singlet12 centered at ꢃ8:1 ppm and
ꢃ
a septet at ꢃ145 ppm which is attributable to the PF6 ion.
The 31P NMR of 1 shows a broad singlet at the same chemical
shift as observed for 2 associated with a very small and broad
signal at 6.2 ppm. The results indicate that the dominant phos-
phine-containing species in the solutions of 1 and 2 are identical.
As a conclusion, the dinuclear complex shows a significant
blue shift of the emission spectra compared with the mononu-
clear complex in the solid state. This work shows that the choice
of solvents is important to the structure, nuclearity and photo-
physical properties of the copper complexes. Although the struc-
ture of the species in solution needs further study, the spectral
data suggest that the emissive species in the CH2Cl2 solution
of 2 is identical with the mixed-ligand mononuclear complex,
ꢀ
ꢀ
ꢀ
ꢀ
A). a ¼ 12:721ð4Þ A, b ¼ 13:501ð4Þ A, c ¼ 10:587ð4Þ A,
ꢁ ¼ 92:44ð3Þꢁ, ꢂ ¼ 92:29ð3Þꢁ, ꢃ ¼ 108:06ð2Þꢁ, V ¼
1724:4ð9Þ A . Dcalcd ¼ 1:57 g/cm3, Z ¼ 1, space group =
ꢀ 3
ꢁ
P1. Of the 8333 reflections which were collected, 7906 were
unique (Rint ¼ 0:027). ꢄ ¼ 7:9 cmꢃ1. The structure was
solved by direct methods and expanded using Fourier tech-
niques. The nonhydrogen atoms were refined anisotropical-
ly. The positions of the hydrogen atoms were fixed except
those on methyl groups, which are refined using a riding
model. The final cycle of full-matrix least-squares refine-
2
ment was based on jFj of all unique reflections and 428 var-
iables. R1 ¼ 0:1437 (0.059 for Fo > 4ꢅðFoÞ), Rw2 ¼
0:1539, GOF = 1.01, Max.shift/esd = 0.06. Crystallograph-
ic data reported in this manuscript have been deposited with
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC 235471. Copies of the data can be ob-
tained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK;
fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
N. Vijayashree, A. G. Samuelson, and M. Nethaji, Curr. Sci.,
65, 57 (1993).
3000
2500
ε
a
2000
c
1500
d
b
1000
9
500
10 A. J. Blake, P. Hubberstey, W.-S. Li, D. J. Quinlan, C. E.
Russell, and C. L. Sampson, J. Chem. Soc., Dalton Trans.,
1999, 4261.
11 C. E. A. Palmer and D. R. McMillin, Inorg. Chem., 26, 3837
(1987).
0
350
450
550
λ
650
750
/ nm
12 The NMR line shape of the Cu(I) complexes has been dis-
cussed in terms of the quadrupolar relaxation of the Cu nu-
clei, see: B. Mohr, E. E. Brooks, N. Rath, and E. Deutsch,
Inorg. Chem., 30, 4541 (1991).
Figure 3. The absorption spectra of 1 (a) and 2 (b) in CH2Cl2.
The emission spectra of the CH2Cl2 solution of 1 (c) and 2 (d).
All spectra are measured at room temperature. Emission intensi-
ties (I) are in arbitrary units.
Published on the web (Advance View) May 10, 2004; DOI 10.1246/cl.2004.678