Luminescence ranging from red to blue: A series of copper(I)-halide complexes having rhombic {Cu2(μ-X)2} (X = Br and I) units with N-heteroaromatic ligands

Article abstract of DOI:10.1021/ic0510359

A series of Cu(I) complexes formulated as [CU₂(μ-X) ₂(PPh₃)(L)_n] were prepared with various mono- and bidentate N-heteroaromatic ligands (X = Br, I; L = 4,4′-bipyridine, pyrazine, pyrimidine, 1,5-naphthyridine, 1,6-naphthyridine, quinazoline, N,N-dimethyl-4-aminopyridine, 3-benzoylpyridine, 4-benzoylpyridine; n = 1, 2). Single-crystal structure analyses revealed that all the complexes have planar {Cu₂X₂} units. Whereas those with monodentate N-heteroaromatic ligands afforded discrete dinuclear complexes, bidentate ligands formed infinite chain complexes with the ligands bridging the dimeric units. The long Cu...Cu distances (2.872-3.303 A) observed in these complexes indicated no substantial interaction between the two Cu(I) ions. The complexes showed strong emission at room temperature as well as at 80 K in the solid state. The emission spectra and lifetimes in the microsecond range were measured at room temperature and at 80 K. The emissions of the complexes varied from red to blue by the systematic selection of the N-heteroaromatic ligands (λ^em_max: 450 nm (L = N,N-dimethyl-4-aminopyridine) to 707 nm (L = pyrazine)), and were assigned to metal-to-ligand charge-transfer (MLCT) excited states with some mixing of the halide-to-ligand (XL) CT characters. The emission energies were successfully correlated with the reduction potentials of the coordinated N-heteroaromatic ligands, which were estimated by applying a simple modification based on the calculated stabilization energies of the ligands by protonation.

Full text of DOI:10.1021/ic0510359

Inorg. Chem. 2005, 44, 9667−9675
Luminescence Ranging from Red to Blue: A Series of Copper(I)
−Halide
Complexes Having Rhombic
N-Heteroaromatic Ligands
{Cu2(µ-X)2} (X ) Br and I) Units with
Hiromi Araki, Kiyoshi Tsuge,* Yoichi Sasaki,* Shoji Ishizaka, and Noboru Kitamura
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity,
Sapporo 060-0810, Japan
Received June 23, 2005
A series of Cu(I) complexes formulated as [Cu2(
N-heteroaromatic ligands (X Br, I; L 4,4 -bipyridine, pyrazine, pyrimidine, 1,5-naphthyridine, 1,6-naphthyridine,
quinazoline, N,N-dimethyl-4-aminopyridine, 3-benzoylpyridine, 4-benzoylpyridine; n 1, 2). Single-crystal structure
analyses revealed that all the complexes have planar Cu2X2 units. Whereas those with monodentate
N-heteroaromatic ligands afforded discrete dinuclear complexes, bidentate ligands formed infinite chain complexes
with the ligands bridging the dimeric units. The long Cu‚‚‚Cu distances (2.872 3.303 Å) observed in these complexes
µ-X)2(PPh3)(L)n] were prepared with various mono- and bidentate
)
)
′
)
{
}
−
indicated no substantial interaction between the two Cu(I) ions. The complexes showed strong emission at room
temperature as well as at 80 K in the solid state. The emission spectra and lifetimes in the microsecond range
were measured at room temperature and at 80 K. The emissions of the complexes varied from red to blue by the
systematic selection of the N-heteroaromatic ligands (λemmax: 450 nm (L
nm (L pyrazine)), and were assigned to metal-to-ligand charge-transfer (MLCT) excited states with some mixing
) N,N-dimethyl-4-aminopyridine) to 707
)
of the halide-to-ligand (XL) CT characters. The emission energies were successfully correlated with the reduction
potentials of the coordinated N-heteroaromatic ligands, which were estimated by applying a simple modification
based on the calculated stabilization energies of the ligands by protonation.
Introduction
configuration are known to afford emissive complexes. Gold-
(I) complexes have been extensively studied because of their
strong emission and interesting photophysical properties
related to aurophilicity.6 Copper(I) complexes, which are
inexpensive, abundant, and as strongly emissive as d10
Au(I) complexes, have received increased attention.6e,7 Bis-
1,10-phenanthroline(phen)-Cu(I) analogues with substituted
Recently, there has been considerable interest in the
luminescent properties of transition-metal complexes, par-
ticularly in view of their potential applications as sensors
and light emitting diodes and in artificial photosynthesis.1-3
Ruthenium(II) and Pt(II) complexes with polypyridyl ligands
are representative, and have been investigated from various
viewpoints.4,5 The coinage metal ions with the d10 electron
(5) (a) Hissler, M.; McGarrah, J. E.; Connick, W. B.; Geiger, D. K.;
Cummings, S. D.; Eisenberg, R. Coord. Chem. ReV. 2000, 208, 115-
137. (b) van Slageren, J.; Klein, A.; Zalis, S. Coord. Chem. ReV. 2002,
230, 193-211. (c) Demas, J. N.; DeGraff, B. A. Coord. Chem. ReV.
2001, 211, 317-351.
* To whom correspondence should be addressed. E-mail: tsuge@
sci.hokudai.ac.jp (K.T.), ysasaki@sci.hokudai.ac.jp (Y.S.).
(1) Ward, M. D., Ed. ComprehensiVe Coordination Chemistry II: Ap-
plications of Coordination Chemistry; Elsevier: Oxford, U.K., 2004;
Vol. 9.
(2) Roundhill, D. M. Photochemistry and Photophysics of Metal Com-
plexes; Plenum Press: New York, 1994.
(6) Some selected references: (a) Vogler, A.; Kunkely, H. Coord. Chem.
ReV. 2001, 219-221, 489-507. (b) Zhang, H.-X.; Che, C.-M. Chem.s
Eur. J. 2001, 7, 4887-4893. (c) Fu, W.-F.; Chan, K.-C.; Cheung,
K.-K.; Che, C.-M. Chem.sEur. J. 2001, 7, 4656-4664. (d) White-
Morris, R. L.; Olmstead, M. M.; Balch, A. L. J. Am. Chem. Soc. 2003,
125, 1033-1040. (e) Yam, V. W.-W.; Lo, K. K.-W. Chem. Soc. ReV.
1999, 28, 323-334. (f) Tzeng, B.-C.; Chan, C.-K.; Cheung, K.-K.;
Che, C. M.; Peng, S.-M. Chem. Commun. 1997, 135. (g) Chao, H.-
Y.; Lu, W.; Li, Y.; Chan, M. C. W.; Che, C.-M.; Cheung, K.-K.; Zhu,
N. J. Am. Chem. Soc. 2002, 124, 14696-14706.
(3) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759-834.
(4) (a) Du¨rr, H.; Bossmann, S. Acc. Chem. Res. 2001, 34, 905-917. (b)
Serroni, S.; Campagna, S.; Puntoriero, F.; Loiseau, F.; Ricevuto, V.;
Passalacqua, R.; Galletta, M. C. R. Chim. 2003, 6, 883-893. (c)
Balzani, V.; Juris, A. Coord. Chem. ReV. 2001, 211, 97-115. (d)
Bignozzi, C. A.; Argazzi, R.; Kleverlaan, C. J. Chem. Soc. ReV. 2000,
29, 87-96.
(7) (a)McMillin, D. R.; McNett, K. M. Chem. ReV. 1998, 98, 1201-1220.
(b) Armaroli, N. Chem. Soc. ReV. 2001, 30, 113-124.
10.1021/ic0510359 CCC: $30.25
Published on Web 11/16/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 26, 2005 9667

Araki et al.
Chart 1. Ligands Used in This Study
phen ligands give a strong emission related to the structural
rigidity and the π* level of the ligands.7,8 In addition to
monomeric Cu(I) complexes, several polymeric Cu(I)-halide
compounds are known to be emissive.9 Of these, the
characteristic dual emissions observed for a series of tetra-
nuclear complexes {Cu4I4L4} (L ) py, substituted py) are
noteworthy. The emissions have been assigned to XLCT
(I-L charge-transfer) and CC (cluster-centered) excited states
on the basis of experimental and theoretical studies.9,10
The rational synthesis of the complexes with intense
emission at desired energies has important practical applica-
tions.11 Whereas colored transition-metal complexes may not
be suitable candidates for blue (or green) emissions, monov-
alent coinage metal ions can afford colorless complexes with
emissions over the entire visible region. In the oligomeric
Cu(I)-halide family, besides the strongly luminescent [Cu4-
(µ3-X)4L4] complexes, a number of complexes with the
rhombic {Cu2(µ-X)2} dimeric unit have been reported.
Although many {Cu2(µ-X)2} complexes with the unit have
been structurally characterized by X-ray diffraction,12 their
luminescent properties have been reported in only a few
cases. This is in contrast to the tetracopper complexes.
Reports on the photophysical properties of complexes with
this dinuclear unit are limited to [Cu2(µ-I)2(L)n] (L )
quinoline,10f pyridine (py),10d tetraethylethylenediamine
(Et4en)10d), [Cu2(µ-I)2(PPh3)2(L)n] (L ) py,9a 4,4′-bipyri-
(8) (a) Kuang, S.-M.; Cuttell, D. G.; McMillin, D. R.; Fanwick, P. E.;
Walton, R. A. Inorg. Chem. 2002, 41, 3313-3322. (b) Cuttell, D. G.;
Kuang, S.-M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A. J. Am.
Chem. Soc. 2002, 124, 6-7. (c) Cunningham, C. T.; Moore, J. J.;
Cunningham, K. L. H.; Fanwick, P. E.; McMillin, D. R. Inorg. Chem.
2000, 39, 3638-3644. (d) Crane, D. R.; Ford, P. C. Inorg. Chem.
1993, 32, 2391-2393. (e) Crane, D. R.; Ford, P. C. J. Am. Chem.
Soc. 1991, 113, 8510-8516. (f) Crane, D. R.; DiBenedetto, J.; Palmer,
C. E. A.; McMillin, D. R.; Ford, P. C. Inorg. Chem. 1988, 27, 3698-
3700. (g) Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J. P.;
Kirchoff, J. R.; McMillin, D. R. Chem. Commun. 1983, 513-515.
(h) McMillin, D. R.; Buckner, M. T.; Ahn, B. T. Inorg. Chem. 1977,
16, 943
(9) (a) Ford, P. C.; Cariati, E.; Bourassa, J. Chem. ReV. 1999, 99, 3625-
3647. (b) Kutal, C. Coord. Chem. ReV. 1990, 99, 213-252.
(10) (a)Vitale, M.; Ford, P. C. Coord. Chem. ReV. 2001, 219-221, 3-16.
(b) Vitale, M.; Ryu, C. K.; Palke, W. E.; Ford, P. C. Inorg. Chem.
1994, 33, 561-566. (c) Ford, P. C.; Vogler, A. Acc. Chem. Res. 1993,
26, 223. (d) Kyle, K. R.; Ryu, C. K.; Ford, P. C.; DiBenedetto, J. A.
J. Am. Chem. Soc. 1991, 113, 2954-2965. (e) Kyle, K. R.; Palke, W.
E.; Ford, P. C. Coord. Chem. ReV. 1990, 97, 35-46. (f) Rath, N. P.;
Holt, E. M.; Tanimura, K. J. Chem. Soc., Dalton Trans. 1986, 2303-
2310. (g) Ford, P. C. Coord. Chem. ReV. 1994, 132, 129-140. (h)
Dossing, A.; Ryu, C. K.; Kudo, S.; Ford, P. C. J. Am. Chem. Soc.
1993, 115, 5132-5137. (i) Ryu, C. K.; Vitale, M.; Ford, P. C. Inorg.
Chem. 1993, 32, 869-874. (j) Vitale, M.; Palke, W. E.; Ford, P. C. J.
Phys. Chem. 1992, 96, 8329-8336.
dine13), and [Cu2(µ-X)2(PPh3)2(L)n] (X ) Br, L ) py;9a
X
) Cl, L ) py,9a,14 pyrazine14). The emissive excited states
are metal-centered for [Cu2(µ-I)2(py)4] and [Cu2(µ-I)2(Et4-
en)2],10d and metal-to-ligand charge transfer (MLCT) or
halide-to-ligand CT (XLCT) for [Cu2(µ-Cl)2(PPh3)2(pyz)] and
[Cu2(µ-X)2(PPh3)2(py)2].9a,14 With the aim of understanding
the emissive properties of the {Cu2(µ-X)2} complexes and
designing complexes with emissions ranging over the visible
region, we have carried out the preparation of the complexes
with a {Cu2(µ-X)2} unit with a series of N-heteroaromatic
ligands. The selective preparation of complexes with
{Cu2(µ-X)2} units was difficult because of the coordination
lability of Cu(I) ions, particularly in polar organic solvents.
By using PPh3 as a co-ligand, however, we have successfully
prepared a series of mixed-ligand complexes [Cu2(µ-X)2-
(PPh3)2(L)n] (X ) I, Br; L ) 4,4′-bipyridine, pyrazine,
pyrimidine, 1,5-naphthyridine, 1,6-naphthyridine, quinazo-
line, N,N-dimethyl-4-aminopyridine, 3-benzoylpyridine, 4-ben-
zoylpyridine, piperazine; n ) 1, 2) as crystalline materials
using nine different N-heteroaromatic ligands (Chart 1). We
demonstrate here that the emissive properties of these
complexes can be controlled rationally by the proper choice
of N-heteroaromatic ligands.
(11) (a) Ma, B.; Li, J.; Djurovich, P. I.; Yousufuddin, M.; Bau, R.;
Thompson, M. E. J. Am. Chem. Soc. 2005, 127, 28-29. (b) Wang,
Q.-M.; Lee, Y.-A.; Crespo, O.; Deaton, J.; Tang, C.; Gysling, H. J.;
Gimeno, M. C.; Larraz, C.; Villacampa, M. D.; Laguna, A.; Eisenberg,
R. J. Am. Chem. Soc. 2004, 126, 9488-9489. (c) Lowry, M. S.;
Hudson, W. R.; Pascal, R. A., Jr.; Bernhard, S. J. Am. Chem. Soc.
2004, 126, 14129-14135.
(12) In the CCDC database, more than 200 structures have been reported.
For example: (a) Maeyer, J. T.; Johnson, T. J.; Smith, A. K.; Borne,
B. D.; Pike, R. D.; Pennington, W. T.; Krawiec, M.; Rheingold, A. L.
Polyhedron 2003, 22, 419-431. (b) Graham, P. M.; Pike, R. D.; Sabat,
M.; Bailey, R. D.; Pennington, W. T. Inorg. Chem. 2000, 39, 5121-
5132. (c) Bowmaker, G. A.; Hart, R. D.; Jones, B. E.; Skelton, B.
W.; White, A. H. J. Chem. Soc., Dalton Trans. 1995, 3063. (d)
Bowmaker, G. A.; Hanna, J. V.; Hart, R. D.; Healy, P. C.; White, A.
H. Aust. J. Chem. 1994, 47, 25. (e) Healy, P. C.; Pakawatchai, C.;
White, A. H. J. Chem. Soc., Dalton Trans. 1983, 1917. (f) Dyason, J.
C.; Engelhardt, L. M.; Healy, P. C.; Pakawatchai, C.; White, A. H.
Inorg. Chem. 1985, 24, 1950. (g) Hengefeld, A.; Kopf, J.; Nast, R.
Chem. Ber. 1977, 110, 3078. (h) Eller, P. G.; Kubas, G. J.; Ryan, R.
R. Inorg. Chem. 1977, 16, 2454.
Experimental Section
Reagents. Reagents were purchased from WAKO, Aldrich, and
TCI. All reagents were used as received.
Preparation of Complexes. [Cu2(µ-Br)2(PPh3)2(CH3CN)2]‚
2CH3CN. To an acetonitrile solution of CuBr (150 mg, 1.05 mmol,
(13) Li, R.-Z.; Li, D.; Huang, X.-C.; Qi, Z.-Y.; Chen, X.-M. Inorg. Chem.
Commun. 2003, 1017-1019.
(14) Henary, M.; Wootton, J. L.; Khan, S. I.; Zink, J. I. Inorg. Chem. 1997,
36, 796-801.
9668 Inorganic Chemistry, Vol. 44, No. 26, 2005

Copper(I)-Halide Complexes
60 mL) was added PPh3 (274 mg, 1.05 mmol) in 40 mL of CH3-
CN. Colorless crystals were obtained after a few days in 34% yield
(320 mg). Anal. Calcd for C44H42Br2Cu2N4P2: C, 54.16; H, 4.34;
N, 5.74. Found: C, 54.23; H, 4.44; N, 5.79.
[{Cu2(µ-I)2(PPh3)2}(µ-bpy)]∞ (1-I) (bpy ) 4,4′-bipyridine). To
a solution of CuI (36 mg, 0.19 mmol) and PPh3 (99 mg, 0.38 mmol)
in DMSO (5 mL) was added bpy (5.9 mg, 0.0038 mmol) to obtain
a clear yellow solution. After a few days, 1-I was obtained as small
yellow crystals in 98% yield (32 mg). Anal. Calcd for C46H38-
Cu2I2N2P2: C, 52.04; H, 3.61; N, 2.64. Found: C, 52.06; H, 3.72;
N, 2.40.
[{Cu2(µ-Br)2(PPh3)2}(µ-bpy)]∞ (1-Br). To a solution of CuBr
(2.9 mg, 0.02 mmol) and PPh3 (5.8 mg, 0.02 mmol) in DMSO (5
mL) was added bpy (0.65 mg, 0.004 mmol) to obtain a clear yellow
solution. After a few days, 1-Br was obtained as small yellow
crystals in 70% yield (2.6 mg). Anal. Calcd for C46H38Br2-
Cu2N2P2: C, 57.10; H, 3.96; N, 2.89. Found: C, 56.98; H, 4.07;
N, 2.79.
{[{Cu2(µ-I)2(PPh3)2}(µ-pyz)]‚2CH3CN}∞ (2-I‚2CH3CN) (pyz
) pyrazine). To a solution of CuI (38 mg, 0.20 mmol) and PPh3
(111 mg, 0.42 mmol) in CH3CN (30 mL) was added pyz (15.2
mg, 0.20 mmol) to obtain an orange-red solution. After 1 week,
2-I‚2CH3CN was obtained as small red crystals in 40% yield (40
mg). Anal. Calcd for C44H40Cu2I2N4P2: C, 49.14; H, 3.63; N, 4.09.
Found: C, 49.15; H, 3.77; N, 4.36. The crystal solvents are slowly
released in ambient conditions.
[{Cu2(µ-Br)2(PPh3)2}(µ-pyz)]∞ (2-Br). To a solution of CuBr
(12.2 mg, 0.085 mmol) and PPh3 (22.2 mg, 0.08 mmol) in CH3CN
(30 mL) was added pyz (3.4 mg, 0.04 mmol) to obtain an orange-
red solution. After 1 week, 2-Br was obtained as small red crystals
in solvated form in 55% yield (19.5 mg). The same compound was
also obtained using dichloromethane instead of CH3CN. The crystal
solvents are easily released in ambient conditions. Anal. Calcd for
C40H34Br2Cu2N2P2: C, 53.89; H, 3.84; N, 3.14. Found: C, 53.80;
H, 3.99; N, 3.06.
[{Cu2(µ-I)2(PPh3)2}(µ-pym)]∞ (3-I) (pym ) pyrimidine). To
the solution of CuI (37 mg, 0.20 mmol) and PPh3 (108 mg, 0.42
mmol) in CH3CN (30 mL) was added pym (15.4 µl, 0.20 mmol)
to obtain a yellow solution. After 10 days, 3-I was obtained as
small yellow crystals in 25% yield (24 mg). Anal. Calcd for C40H34-
Cu2I2N2P2: C, 48.75; H, 3.48; N, 2.84. Found: C, 48.71; H, 3.64;
N, 2.91.
[{Cu2(µ-Br)2(PPh3)2}(µ-pym)]∞ (3-Br). To a solution of CuBr
(8.2 mg, 0.06 mmol) and PPh3 (16.4 mg, 0.06 mmol) in CH3CN
(30 mL) was added pym (9.0 µl, 0.11 mmol) to obtain a yellow
solution. After 2 weeks, 3-Br was obtained as small yellow crystals
in 25% yield (11.4 mg). Anal. Calcd for C40H34Br2Cu2N2P2: C,
53.89; H, 3.84; N, 3.14. Found: C, 53.92; H, 3.84; N, 3.18.
[{Cu2(µ-Br)2(PPh3)2}(µ-1,5-nap)]∞ (4-Br) (1,5-nap ) 1,5-
naphthyridine). To a solution of CuBr (5.1 mg, 0.04 mmol) and
PPh3 (10.2 mg, 0.04 mmol) in DMF (15 mL) was added a DMF
solution (5 mL) containing 1,5-nap (1.0 mg, 0.008 mmol) to obtain
a yellow solution. After 3 days, 4-Br was obtained as small orange
crystals in 16% yield (3.0 mg). Anal. Calcd for C44H36Br2-
Cu2N2P2: C, 56.12; H, 3.85; N, 2.98. Found: C, 55.97; H, 3.90;
N, 2.66.
[{Cu2(µ-Br)2(PPh3)2}(µ-1,6-nap)]∞ (5-Br) (1,6-nap: 1,6-naph-
thyridine). To a solution of CuBr (78.9 mg, 0.55 mmol) and PPh3
(142.3 mg, 0.54 mmol) in a mixed solvent of CH2Cl2 (20 mL) and
CH3CN (10 mL) was added a CH3CN solution (3 mL) of 1,6-nap
(35.8 mg, 0.28 mmol). The orange precipitate that formed was
filtered, and then the yellow-orange filtrate was concentrated to 20
mL. After 1 day, the precipitate that deposited was filtered off.
The solution afforded orange crystals after two months. Yield 5.5%
(14.0 mg). Anal. Calcd for C44H36Br2Cu2N2P2: C, 56.12; H, 3.85;
N, 2.98. Found: C, 56.29; H, 4.03; N, 3.00.
[{Cu2(µ-Br)2(PPh3)2}(µ-quina)]∞ (6-Br) (quina ) quinazo-
line). To a solution of CuBr (17.1 mg, 0.12 mmol) and PPh3 (32.8
mg, 0.13 mmol) in CH3CN (15 mL) was added an aqueous solution
(10 mL) of quina (7.9 mg, 0.06 mmol/10 mL) to obtain an orange
solution. After 1 day, 6-Br was obtained as small orange crystals
in 35% yield (19.6 mg). Anal. Calcd for C44H36Br2Cu2N2P2: C,
56.12; H, 3.85; N, 2.98. Found: C, 56.13; H, 3.75; N, 3.02.
[Cu2(µ-Br)2(PPh3)2(dmap)2] (7-Br) (dmap ) N,N-dimethyl-
4-aminopyridine). To a dichloromethane solution (15 mL) of
[CuBr(PPh3)(CH3CN)]2 (232.4 mg, 0.26 mmol) was added dmap
(62.9 mg, 0.51 mmol) in 5 mL of CH2Cl2. After stirring for 2 h,
we added 16 mL of hexane to the resulting brown solution to afford
the brown precipitate, which was then filtered off. The filtrate gave
colorless 7-Br and blue crystals after a few days. Crystals were
isolated by filtration, and the blue crystals were removed by washing
with CH3CN. Yield of the colorless crystals, 9% (23.0 mg). Anal.
Calcd for C50H50Br2Cu2N4P2: C, 56.88; H, 4.77; N, 5.31. Found:
C, 56.96; H, 4.66; N, 5.28.
[Cu2(µ-Br)2(PPh3)2(3-bzpy)2] (8-Br) (3-bzpy ) 3-benzoyl-
pyridine). To a dichloromethane solution (10 mL) of [CuBr-
(PPh3)(CH3CN)]2 (33.8 mg, 0.04 mmol) was added 4-bzpy (13.8
mg, 0.08 mmol) in 0.5 mL of CH2Cl2. The resulting yellow solution
was concentrated to ca. 2 mL, and a small amount of hexane was
added. The complex 8-Br was obtained as yellow crystals in 78%
yield (34.7 mg). Anal. Calcd for C60H48Br2Cu2N2O2P2: C, 61.18;
H, 4.11; N, 2.38. Found: C, 61.04; H, 4.19; N, 2.20.
[Cu2(µ-Br)2(PPh3)2(4-bzpy)2] (9-Br) (4-bzpy ) 4-benzoyl-
pyridine). To a dichloromethane solution (20 mL) of [CuBr-
(PPh3)(CH3CN)]2 (32.9 mg, 0.04 mmol) was added 4-bzpy (13.6
mg, 0.07 mmol) in 0.5 mL of CH2Cl2. The resulting yellow solution
was concentrated to ca. 2 mL, and a small amount of hexane was
added. Red crystals of 9-Br were obtained in 55% yield (24.0 mg).
Anal. Calcd for C60H48Br2Cu2N2O2P2: C, 61.18; H, 4.11; N, 2.38.
Found: C, 60.97; H, 4.19; N, 2.23.
{[{Cu2(µ-I)2(PPh3)2}(pip)]‚2CH3CN}∞ (10-I‚2CH3CN) (pip )
piperazine). To an acetonitrile solution (15 mL) of CuI (10.1 mg,
0.05 mmol) and PPh3 (27.8 mg, 0.11 mmol) was added pip (4.1
mg, 0.05 mmol) in 5 mL of CH3CN. After 1 day, 10-I was obtained
as pale pink crystals in 18% yield (4.5 mg). Anal. Calcd for C40H40-
Cu2I2N2P2: C, 48.45; H, 4.07; N, 2.83. Found: C, 48.32; H, 4.09;
N, 2.74.
[{Cu2(µ-Br)2(PPh3)2}(pip)]∞ (10-Br). To an acetonitrile solution
(15 mL) of CuBr (7.5 mg, 0.05 mmol) and PPh3 (27.5 mg, 0.10
mmol) was added pip (4.1 mg, 0.05 mmol) in 5 mL of CH3CN.
After a few days, 10-Br was obtained as pale pink crystals in 26%
yield (5.8 mg). Anal. Calcd for C40H40Br2Cu2N2P2: C, 53.52; H,
4.49; N, 3.12. Found: C, 53.43; H, 4.64; N, 3.10.
Physical Measurements. Emission spectra were measured using
a photodiode array detector (Hamamatsu, PMA-11) and an
Nd:YAG laser (Continuum surelite, 355 nm, 7 ns pulse width) at
355 nm excitation. Emission lifetimes were measured using a streak
camera (Hamamatsu C3434) as a detector. Temperature was
controlled with a liquid N2 cryostat (Oxford instruments, model
01200).
X-ray Diffraction Studies. Suitable single crystals of 1-Br, 2-Br,
2-I‚2CH3CN, 3-I, 4-Br, 5-Br, 6-Br, 7-Br, 8-Br, 9-Br, and 10-I‚
2CH3CN were obtained as described in the preparation section.
The selected crystals were mounted onto a thin glass fiber.
Measurements were made on a Mercury CCD area detector coupled
with a Rigaku AFC-8S diffractometer and on a Mercury CCD area
Inorganic Chemistry, Vol. 44, No. 26, 2005 9669

Araki et al.
Table 1. Summary of X-ray Data Collection and Refinement
1-Br
2-Br‚2CH2Cl2
2-I‚2CH3CN
3-I
4-Br
formula
fw
cryst syst
space group
a (Å)
C46H38Br2Cu2N2P2
967.67
triclinic
P1h
9.078(3)
9.282(3)
13.677(3)
71.75(1)
66.70(1)
86.09(2)
1003.1(5)
1
C42H38Br2Cl4Cu2N2P2
1061.44
monoclinic
P21/n
8.854(2)
17.427(4)
13.507(3)
90
92.458(4)
90
2082.2(8)
2
C44H40Cu2I2N4P2
1067.68
monoclinic
P21/n
8.968(6)
18.15(5)
13.162(8)
90
93.22(2)
90
2139(6)
2
C40H34Cu2I2N2P2
985.57
triclinic
P1h
10.587(4)
12.396(4)
15.157(5)
92.921(5)
104.059(6)
103.157(3)
1867(1)
2
C44H36Br2Cu2N2P2
941.63
triclinic
P1h
8.289(4)
9.471(5)
13.98(1)
86.65(5)
72.77(4)
64.94(4)
947(1)
1
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (mm-3
Z
)
T (K)
153
1.602
3.174
6057
4066
0.019
104
1.693
3.313
12583
4682
0.027
153
1.657
2.548
11356
4560
293
1.753
2.91
15612
8128
0.028
153
1.652
3.360
7764
3672
0.036
F
calcd (g cm-3
)
µ (mm-1
)
no. of measured reflns
no. of unique reflns
Rint
0.033
no. of obsd reflns
(I > 2σ(I))
no. of params
R1a
3358
3872
2729
4027
2048
320
320
244
433
235
0.041
0.0858
1.085
0.0281
0.0745
0.972
0.0398
0.0729
1.064
0.0361
0.0496
0.825
0.0655
0.1387
1.266
wR2b
GOFc
5-Br
6-Br
7-Br
8-Br
9-Br
10-I‚2CH3CN
formula
C44H36Br2Cu2N2P2 C44H36Br2Cu2N2P2 C50H50Br2Cu2N4P2 C60H48Br2Cu2N2O2P2 C60H48Br2Cu2N2O2P2 C44H46Cu2I2N4P2
fw
941.63
triclinic
P1h
941.63
monoclinic
P21/a
15.05(1)
9.701(6)
27.03(3)
90
98.56(2)
90
3902(6)
4
1055.82
triclinic
P1h
1177.9
triclinic
P1h
1177.9
monoclinic
P21
9.269(3)
25.572(7)
11.738(4)
90
110.225(6)
90
2610(1)
2
1073.72
monoclinic
P21/n
9.134(3)
17.883(5)
13.321(4)
90
90.338(4)
90
2175(1)
2
cryst syst
space group
a (Å)
b (Å)
c (Å)
15.269(2)
15.255(2)
19.038(2)
74.781(7)
71.636(6)
69.557(6)
3885.1(8)
4
10.074(5)
14.565(6)
17.902(6)
65.15(2)
83.19(3)
87.93(3)
2366(1)
2
8.210(2)
12.632(3)
13.944(3)
65.776(9)
77.72(1)
83.80(1)
1288.1(5)
1
R (deg)
â (deg)
γ (deg)
V (mm-3
Z
)
T (K)
153
1.61
3.275
253
153
1.482
2.698
14278
9597
153
1.518
2.489
7834
5248
0.018
4281
153
1.498
2.456
15596
5712
153.1
1.639
2.506
12467
4733
F
calcd (g cm-3
)
1.603
3.261
24780
9344
0.031
7507
µ (mm-1
)
no. of measured reflns 23719
no. of unique reflns
Rint
15868
0.024
12931
0.047
6465
0.050
4270
0.020
4468
no. of obsd reflns
(I > 2σ(I))
no. of params
R1a
937
469
541
316
631
324
0.0382
0.0879
1.061
0.0653
0.1467
1.409
0.082
0.1889
1.776
0.0358
0.0811
1.036
0.0388
0.0768
0.811
0.0277
0.0916
1.417
wR2b
GOFc
2
2
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc )2]/∑[w(Fo )2]}1/2 with w ) {σ2(Fo ) + [x(max(Fo ,0) + 2Fc )/3]2}-1
.
c GOF ) ((∑(|Fo| - |Fc|)2)/
(No - Nv))1/2 with No ) no. of obsd reflns and Nv ) no. of params.
detector coupled with a Rigaku AFC-7R diffractometer with
graphite-monochromated Mo KR radiation (0.7107 Å). Final cell
parameters were obtained from a least-squares analysis of reflections
with I > 10σ(I). Space group determinations were made on the
basis of systematic absences, a statistical analysis of intensity
distribution, and the successful solution and refinement of the
structures. Data were collected and processed using Crystal Clear.15
An empirical absorption correction resulted in acceptable transmis-
sion factors. The data were corrected for Lorentz and polarization
factors.
Fourier techniques. Because of the problem of crystal size and
quality, the R and GOF values are relatively high for 7-Br, although
the basic structural features of the complex have been clearly
determined. Details of crystal parameters and structure refinements
are given in Table 1. Selected bond lengths and angles are shown
in Table S1. Interatomic distances around Cu atoms are listed in
Table S2.
Computational Methods. All of the molecular orbital calcula-
tions were performed with the Gaussian 03 program17 at the
B3LYP18 level using a 6-31G**19 basis set. The energy levels were
calculated for the optimized free and protonated N-heteroaromatic.
The single-point DFT and TD-DFT calculations were carried out
for model compound [Cu2Br2(PH3)2(py)2]. The model compound
was assumed to have an idealized C2h structure, with averaged
atomic distances and angles found in [Cu2Br2(PPh3)2(py)2].20 The
atomic parameters used for the calculation are shown in Table S3.
All the calculations were carried out on a Silicon Graphics O2
computer system using TEXSAN.16 The structures were solved by
direct methods, and were expanded using Fourier and difference
(15) Crystal Clear; Rigaku Corporation: Tokyo, 1999.
(16) teXsan version 1.11; Molecular Structure Corporation: The Wood-
lands, TX, 2000.
9670 Inorganic Chemistry, Vol. 44, No. 26, 2005

Copper(I)-Halide Complexes
Results and Discussion
Synthesis. Fourteen complexes with the general formula
[{Cu2(µ-X)2(PPh3)2}(L)n] were synthesized using nine N-
heteroaromatic ligands and one nonaromatic ligand, pipera-
zine. As described in the next section in detail, the X-ray
structural analyses revealed that all the complexes have {Cu2-
(µ-X)2} structural units. When the N-heterocyclic ligand (L)
is a bidentate bridging ligand, an extended chain structure
is formed. On the other hand, when L is a monodentate
ligand, the complex is of a discrete molecular type.
The reactions of Cu(I)-halide with N-heteroaromatic
ligands have been known to afford compounds with a variety
of structural types: monomer, oligomeric {CunXn} units, 1D
chain, stair-step polymers, and 2D sheet structures, depending
on the choice of solvent system and reaction ratios.12 This
is due to the geometric flexibility of Cu(I) and halide ions.
Hitherto, a rational synthetic approach to a specific oligo-
meric Cu(I)-halide core structure such as Cu2(µ2-X)2 and
Cu4(µ3-X)4 has not been established.
To restrict the structural types of the products, we chose
soft bulky PPh3 as an ancillary ligand, as it is expected to
coordinate to the soft Cu(I) ion more strongly than N-
heteroaromatic ligands, and its bulkiness may suppress the
higher degree of aggregation. This approach enabled us to
successfully prepare a series of complexes containing
{Cu2(µ-X)2} units with six bidentate N-heteroaromatic
ligands and nonaromatic piperazine, by the direct mixing of
the constituent chemical species.
As the compounds containing bidentate ligands are
insoluble in usual organic solvents because of their infinite
linear chain structure (vide infra), the purification of the
product was practically impossible once the products were
obtained. To obtain pure crystalline products, it was neces-
sary to carefully control the concentration of the reactants
in the solution at the optimum ratio. If an inappropriately
concentrated solution was used, the reaction mixture afforded
only powder material. Even if the elemental analyses
provided the expected values for these powder materials,
structural ambiguities remained because of possible structural
Figure 1. ORTEP drawing of discrete complex 8-Br with ellipsoids at
the 50% probability level. Hydrogen atoms are omitted for clarity. Symmetry
operation′: -x, -y, -z.
isomers.10f,12b Only pure crystalline products were used for
measurements.21
The molecular complexes with monodentate ligands 7-Br,
8-Br, and 9-Br were successfully synthesized by ligand
substitution of the acetonitrile complex [Cu2(µ-Br)2(PPh3)2-
(CH3CN)2] in dichloromethane. The yields are acceptable
for 8-Br and 9-Br, thereby indicating that the substitution
reaction is an effective way to prepare the dimeric complex
with the {Cu2(µ-Br)2(PPh3)2} moiety. The low yield of 7-Br
does not indicate the incomplete progress of the substitution
reaction; however, the low yield can possibly be attributed
to the relative instability of 7-Br to oxidation, because a blue
Cu(II) complex was obtained under the preparation condi-
tions. Unfortunately, our efforts to prepare the corresponding
iodide complex [Cu2(µ-I)2(PPh3)2(CH3CN)2] were unsuc-
cessful. Studies to prepare the molecular iodide analogues
are in progress at this laboratory.
The preparations of four complexes (1-I,13 3-I,22 3-Br,22
and 6-Br22) have been published during the course of our
study. However, the synthetic procedure for bulk samples
and single crystals differed in these reports. The preparation
of these compounds by our methods directly afforded single
crystals from the reaction mixture.
Structures of [{Cu2X2(PPh3)2}(L)n] (n ) 1, 2) Com-
plexes. (a) Discrete Complexes. The structures of discrete
complexes 7-Br, 8-Br, and 9-Br have the same features as
those reported previously for analogous dimeric complexes
[Cu2Br2(PPh3)2L2] (L ) pyridine,20 4-cyanopyridine,20 quino-
line,23 and piperidine24). Figure 1 shows the crystal structure
of 8-Br.
The complex has a {Cu2(µ-Br)2} rhombus as a structural
unit that has one PPh3 and one 3-bzpy ligand coordinating
to each Cu atom. The coordination geometry around the Cu
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; 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.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(18) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (b) Becke, A.
D. Phys. ReV. A 1988, 38, 3098-3100. (c) Lee, C.; Yang, W.; Parr,
R. G. Phys. ReV. B 1988, 37, 785-789.
(21) Though the crystal structures of 1-I and 3-Br have been reported, we
confirmed the structures of our samples by X-ray analysis. They
showed results identical to those previously reported.
(22) Maeyer, J. T.; Johnson, T. J.; Smith, A. K.; Borne, B. D.; Pike, R. D.;
Pennington, W. T.; Krawiec, M.; Rheingold, A. L. Polyhedron 2003,
22, 419-431.
(23) Jin, Q.-H.; Long, D.-L.; Wang, Y.-X.; Xin, X.-Q. Acta Crystallogr.,
Sect. C 1998, 54, 948.
(24) Bowmaker, G. A.; Hanna, J. V.; Hart, R. D.; Healy, P. C.; White, A.
H. J. Chem. Soc., Dalton Trans. 1994, 2621.
(19) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209.
(20) Engelhardt, L. M.; Healy, P. C.; Kildea, J. D.; White, A. H. Aust. J.
Chem. 1989, 42, 913.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9671

Araki et al.
Table 2. Emission Maxima (λemmax) and Lifetimes (τem)of the
Complexes
λmax (nm)
lifetime (µs)
rt
80 K
rt
80 K
X ) Br
1-Br (bpy)
595
707
579
616
614
780
599
642
655
668
468
599
730
3.1(1)
0.42(1)/0.06(2)
2.9(1)
5.6(1)
7.6(6)/0.69(2)
0.60(1)
7.5(3)/2.3(1)
0.75(1)
0.16(1)/0.039(2) 4.0(1)/0.096(3)
16(1)
3.2(3)/1.11(1)
29(2)
52(1)
23(1)/4.4(1)
10 (1)
2-Br (pyz)a
3-Br (pym)
4-Br (1,5-nap)
5-Br (1,6-nap)b 633
6-Br (quina)
7-Br (dmap)b
8-Br (3-bzpy)
9-Br (4-bzpy)b
644
450
579
689
430(10)/48(3)
9.0(1)
X ) I
1-I (bpy)c
2-I (pyz)a
3-I (pym)
542
648
562
542
671
576
4.0(1)
1.7(1)
2.7(1)
67(1)
31(1)/5.9(3)
37(1)
a Main components of decay curves were fit with a double-exponential
function for comparison with the other complexes. b Decay curves were fit
with a double-exponential function. c λem
for 1-I at room temperature
max
has been reported to be 535 nm. See ref 13.
Figure 2. ORTEP drawing of linear chain complexes with ellipsoids at
the 50% probability level. Hydrogen atoms are omitted for clarity. (a) 4-Br
symmetry operation′: -x, -y, -z. (b) 5-Br symmetry operation′: 1/2 + x,
1/2 - y, z.
atom is tetrahedral considering one N atom of 3-bzpy, one
P atom of PPh3, and two Br- ions. The {Cu2Br2} unit is
planar, with its crystallographic inversion center coinciding
with its midpoint. The structures of 7-Br and 9-Br are similar
to that of 8-Br; however, it is noteworthy that their molecular
structures do not have inversion centers, although their bond
distances and angles showed virtual Ci symmetry. The small
deviations bring two N-heteroaromatic ligands that are
inequivalent with respect to 7-Br and 9-Br, which is
interestingly reflected in dual emission lifetimes (vide infra).
Figure 3. Emission spectra of bromide complexes 1-Br-9-Br at room
temperature.
are used, the resulting linear chain structures are distorted
(Figure 2b). As the structure of 5-Br (Figure 2b) manifests
itself, the neighboring {Cu2Br2} units get oriented in different
directions along the chain, and are structurally nonequivalent.
Because of the skewed direction of the nitrogen lone pairs
in acentric ligands, neighboring {Cu2Br2} rhombuses are
compelled to assume an oblique arrangement. The planarity
of {Cu2Br2} units is intact for all complexes. The iodide
complexes are obtained as isostructural crystals correspond-
ing to the bromide complexes.
Emission Properties. (a) Emission Spectra and Life-
times. A series of complexes prepared in this study, except
for the piperazine complexes 10-I and 10-Br, show strong
emission in the solid state at room temperature.25 The
emission maxima (λemmax) and the lifetimes at room tem-
perature and at 80 K are shown in Table 2. Figure 3 shows
the emission spectra of the bromide complexes at room
temperature.
(b) Polymeric Complexes. The bidentate ligands em-
ployed here have two nitrogen donors located in a manner
so as to enable the interunit (-{Cu2X2}-) bridging coordina-
tion rather than a chelate or intraunit bridging coordination.
Such ligands afford linear chain complexes with the bidentate
ligands acting as a bridge between the two {Cu2X2} units.
The linear chain structure appears to be the common motif
for {Cu2X2} complexes with PR3 and the bidentate N-
heteroaromatic ligands.12a,b,13,14
Figure 2a shows the crystal structure of 4-Br, in which
an alternate arrangement of rhombic {Cu2Br2} units and
bridging 1,5-nap ligands form the 1-dimensional chain
structure. In the structure of 4-Br, two crystallographic
inversion centers are superimposed on the midpoint of the
{Cu2Br2} unit and on the center of the 1,5-nap ligand. The
complexes with other centric ligands (1-Br and 2-Br) show
similar chain structures. In these complexes, all the dimeric
units in the chain are structurally equivalent. When acentric
N-heteroaromatic ligands such as pym, 1,6-nap, and quina
The emission maxima of the complexes cover a wide
range, from 450 to 740 nm depending on the N-heteroaro-
(25) Using preliminary measurements with an integrating sphere at room
temperature, we determined the quantum yields of 1-Br and 1-I to be
0.18 and 0.23, respectively.
9672 Inorganic Chemistry, Vol. 44, No. 26, 2005

Copper(I)-Halide Complexes
[{Cu2(µ-I2)(PPh3)}2(pyz)], in that both spectra showed a
broad band centered around 500 nm (Figure S1).
(b) Structures and Emission Decay Patterns. As shown
in Figure 4, several complexes showed a deviation from
single-exponential decay. It is interesting to note that the
behavior is correlated with the crystal structures. Complexes
showing single-exponential decay are 1-Br, 1-I, 3-Br, 3-I,
4-Br, 6-Br, and 8-Br. The crystals for all these complexes
contain one crystallographically independent N-hetero-
aromatic ligand. The pym complex 3-Br and 3-I and the
quinazoline complex 6-Br showed single-exponential decay,
although the complexes contain two crystallographically
independent Cu(I) ions in the crystals. On the other hand,
complexes 5-Br, 7-Br, and 9-Br, which show a deviation
from single-exponential decay, have two crystallographically
independent N-heteroaromatic ligands. The results suggest
that each {Cu(I)-L} (L ) N-heteroaromatic ligands) unit
independently behaves as an emission center in the case of
discrete complexes with monodentate ligands. The results
also imply that the {Cu(I)-L-Cu(I)} units play a major role
in emission in the case of the polymeric complexes with
bidentate N-heteroaromatic ligands.28 The deviation from
single-exponential decay of 2-Br and 2-I that have one
crystallographically independent pyz may be explained by
the release of labile crystal solvents, resulting in structural
deviation from those determined by X-ray analysis.29
(c) Relationship between the Reduction Potential of the
Ligands and Emission Energies of the Complexes. The
emission energies of a series of the dicopper complexes are
strongly affected by the N-heteroaromatic ligands (L). The
emission was not observed for complexes with the non-π-
type ligand pip. Thus, the charge-transfer excited state to
π* orbitals of N-heteroaromatic ligands is assumed to be the
origin of the emission in the series of complexes.
In previous papers, several possible lowest excited states
have been suggested for the emissive Cu(I)-halide com-
plexes.9b These include metal-centered (d f s), MLCT,
XLCT, intraligand, and cluster-centered (CC) excited states.
The emission of the chloride complex [{Cu2(µ-Cl)2(PPh3)2}-
(pyz)] has been assigned to the Cu(I)-to-pyrazine MLCT
excited state on the basis of resonance Raman spectroscopy.14
Ford et al. reported the emission of a series of py complexes
[(Cu2(µ-X)2(PPh3)2)(py)2] (X ) I, Br, Cl), and pointed out
the possibility of the XLCT excited state.9a They also studied
the emission of the {Cu2(µ-I)2} complexes without PPh3,10d
and reported that the emission of [Cu2(µ-I)2(tetraeth-
ylethylenediamine)2] is attributed to a CC transition and that
the emission of [Cu2(µ-I)2(py)4] is mainly attributed to both
CC transition and XLCT.10b Both these transitions are
unlikely in our complexes, because no clear correlation
between Cu-Cu distances and emission maxima was
observed and also because the effects of bridging halides
were smaller than those of N-heteroaromatic ligands.
Figure 4. Emission decay of bromide complexes: (a) discrete complexes
7-Br-9-Br, (b) linear chain complexes 1-Br-6-Br.
matic ligands. The emissions of the iodide complexes are
higher in energy than those of corresponding bromide
complexes by 520-1900 cm-1 (Table 2). Figure 4 shows
the emission decay curves of the bromide complexes at room
temperature. The emission lifetimes are in the microsecond
range, indicating phosphorescence emissions from the triplet
excited state. At low temperatures, the lifetimes become
longer and the emission maxima shift to a higher-energy
region by 430-1300 cm-1.26 The conventional measurements
of absorption and excitation spectra of the present complexes
in solution were not possible because of the complexes’
insoluble natures and their possible decomposition in the
solution if dissolved.27 Even for those having absorption near
the UV region, the absorption and excitation spectra in the
solid state did not afford clear information because of the
scattered light. Nevertheless, the color of the complexes has
a good correlation with their emission bands (Table S4). The
only complex that provided meaningful information is
(26) 1-I did not show the bathochromic shift, but showed the slightly
structured spectrum at 80 K. The single-exponential decay of 1-I at
80 K ruled out the appearance of a new excited state.
(27) Molecular complexes are soluble to organic solvents such as CH2Cl2,
CHCl3, and acetonitrile. However, the extinction coefficients obtained
by the UV-vis absorption measurements did not obey the Lambert-
Beer law, showing the degradation of the complexes in the solvent.
NMR spectra show the signals corresponding to the free PPh3 and
N-heteroaromatic ligands, which also indicate the weak binding and
fluctuation of the ligands in solution.
(28) From this point of view, the structure can be described as the linear
chain composed of {Cu(I)-L-Cu(I)} and bridging halide ligands.
(29) The powder X-ray diffraction pattern of the 2-Br samples showed
the broad signals slightly deviated from those expected for the
structures determined by the single-crystal structure analysis.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9673

Araki et al.
Table 3. Reduction Potentials of N-Heteroaromatic Ligands (L) and
Table 4. Stabilization of LUMO Levels (∆E) on Protonation (eV)a
λem
of [Cu2Br2(PPh3)2(L)n]
max
first protonation
second protonation
total
L
n
redox potential (V)a
λemmax (nm)
bpy
pyz
pym
1,5-nap
1,6-nap
quina
dmap
py
5.19
6.52
6.48
5.41
5.48
5.47
5.60
6.40
4.84
4.83
3.85
6.37
6.01
5.03
4.68
5.04
9.01
12.89
12.49
10.44
10.16
10.51
5.60
6.40
4.84
4.83
bpy
pyz
pym
1,5-nap
1,6-nap
quina
4-bzpy
3-bzpy
py
2
2
2
2
2
2
1
1
1
-1.84
-2.10
-2.35
-1.82
-1.79
-1.74
-1.46
-1.60
-2.76b
595
707
579
616
633
644
686
579
487c
4-bzpy
3-bzpy
a In acetonitrile, vs Ag|AgCl. b Ref 31. c Ref 9a.
a Stabilization of LUMO levels in protonated forms is estimated as
∆E ) -(ELUMO(LH+) - ELUMO(L)).
As expected, the π* orbitals of the bidentate ligands are
more stabilized by protonation, as two protons are attached
to the ligands. Among the bidentate ligands, the total
stabilization energies of the π* orbitals are significantly
different by as much as 3.85 V, as exemplified by the
difference between bpy (9.04 V) and pyz (12.89 V). This
result is reasonable, because the π* orbital of pyz is confined
on one six-membered ring, making the π* orbital of pyz more
sensitive to protonation than that of bpy.33 We assume that
the stabilization energy of the π* orbital by the coordination
is linearly correlated with that by the protonation in terms
of these stabilization energies given by the formula E(π*)
) E(red) + aE(stabilization). When a is assumed to be 0.1,
the calculated energy and the emission energy showed good
linear correlation (Figure 5, filled symbols). Although the
strict estimation of emission energies needs more delicate
consideration for orbital energies in ground and excited states,
the linear correlation in Figure 5 shows that qualitative
prediction is possible, at least in our complexes, using the
redox potential and a simple energy calculation for free and
protonated ligands.
(d) Properties of HOMO and LUMO. From the afore-
mentioned discussion, the π* orbitals of N-heteroaromatic
ligands are clearly related to the emissive excited state in a
series of the dicopper complexes. In that context, the
available MO calculations of {Cu2(µ-X)2} complexes [Cu2I2-
(NH3)4],10b [Cu2I2(py)4],10b and [Cu2I2(PH3)4]34 are highly
relevant. For amine and phosphine model complexes, the
orbitals around frontier orbital levels have been reported10b,34
to mainly possess copper and iodine characters, showing that
the absorption and emission bands in the UV-vis region
can be ascribed to the transitions within cluster units. For
the pyridine model complex, py π characters appear in the
LUMO (lowest unoccupied molecular orbital) in addition to
copper and iodine characters. Thus the transitions have more
MLCT character in the pyridine complex.10b
Figure 5. Emission maxima and reduction potentials of N-heteroaromatic
ligands. Filled symbols show the values corrected for the protonation (see
text). The slope of the best fitting line is -1.6(3) with a correlation
coefficient of -0.87 and a reduced ø2 of 0.053.
We further discuss the emissive excited state in favor of
the MLCT transition. The linear correlation between the
MLCT transition energies and reduction potentials of the
ligands for homologous complexes has been assumed for
localized CT models.30 Table 3 lists the reduction potential
of the ligands and the λmax of the complexes.
The emission energy increases in the order of the reduction
potentials of the ligands in the case of discrete complexes
[(Cu2Br2(PPh3)2)(L)2] (L ) dmap, py, 3-bzpy, 4-bzpy).
However, the correlation becomes obscured when the chain-
type complexes are included (Figure 5).
We ascribed this inconsistency to the use of the reduction
potentials of free ligands themselves instead of the coordi-
nated ligands. The π* orbitals of N-heteroaromatic ligands
should be stabilized by the coordination to the Lewis acid,
Cu(I). Because of their insolubility, the direct measurement
of the reduction potentials in the complexes was not possible.
To estimate the extent of stabilization of the π* energy level
on coordination, we calculate the π* energy level for
protonated ligands as the simplest models of Lewis acid
complexes (Table 4).32
(32) Because Cu(I) is a d10 spherical ion, we assumed that the effects of
coordination to Cu(I) should be estimated by the protonated compounds
at the first approximation.
(33) The stabilization energies for the first protonation differ little (5.19
and 6.52 V). However, the stabilization energy of bpy for the second
protonation is much smaller than that of pyz (3.85 and 6.37 V,
respectively). The stabilization on first protonation is similar for bpy
and pyz because the LUMO orbital is mainly localized on the
protonated pyridine ring, as the (bpyH)+ is no longer symmetrical.
(34) Aslanidis, P.; Cox, P. J.; Divanidis, S.; Tsipis, A. C. Inorg. Chem.
2002, 41, 6875-6896.
(30) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277. (b) Juris,
A.; Campagna, S.; Balzani, V.; Gremaud, G. Inorg. Chem. 1988, 27,
3652-3655.
(31) Tabner, B. J.; Yandle, J. R. J. Chem. Soc. A 1968, 381-388.
9674 Inorganic Chemistry, Vol. 44, No. 26, 2005

Copper(I)-Halide Complexes
Table 5. Calculated Energies and Components of MO for
[Cu2Br2(PH3)2(py)2] around Frontier Orbital Levels
orbital of the N-heteroaromatic ligand. The mixing of the
Br orbitals to occupied MOs around the HOMO shows the
addition of XLCT properties to the excited state. The
observed satisfactory relationship between reduction poten-
tials of N-heteroaromatic ligands and emission energies is
explained by the almost pure py π* properties of the
unoccupied orbitals around the LUMO.
MO
energy (eV)
symmetry
Cu
Br
py
PH3
L + 4
L + 3
L + 2
L + 1
LUMO
HOMO
H - 1
H - 2
H - 3
H - 4
H - 5
0.68
-0.57
-0.58
-0.85
-0.85
-4.35
-4.62
-4.71
-4.74
-4.96
-5.05
au
bg
au
bu
ag
bu
au
bu
ag
ag
bg
0.29
0.01
0.00
0.02
0.02
0.70
0.74
0.68
0.86
0.83
0.76
0.00
0.00
0.00
0.00
0.01
0.23
0.23
0.19
0.02
0.04
0.23
0.01
0.99
0.99
0.97
0.97
0.02
0.01
0.06
0.08
0.05
0.01
0.70
0.00
0.00
0.00
0.00
0.05
0.02
0.06
0.04
0.08
0.00
Conclusion. A series of Cu(I)-halide complexes with the
formula [{Cu2X2(PPh3)2}(L)n] (L: monodentate, n ) 2; L:
bidentate, n ) 1) were prepared using nine different
N-heteroaromatic ligands (L). The emission energy strongly
depends on the N-heteroaromatic ligands, whereas the effects
due to the bridging halide ion and the structural distortions
in the {Cu2(µ-X)2} units are less significant. With changes
in N-heteroaromatic ligands, the emission energies vary
significantly over the visible region from red to blue. Easy
preparation and tunability of the emission energy of {Cu2-
(µ-X)2} complexes are useful for the design of emissive
materials. The structures of the complexes reveal that the
Cu‚‚‚Cu distances of {Cu2X2} units tend to be long enough
to neglect Cu‚‚‚Cu direct interactions. Due to the negligible
interaction between the two copper atoms, the emission of
the present complexes becomes simple; this is in contrast to
the temperature-dependent dual emission of related tetra-
nuclear {Cu4I4L4} complexes.9a,10 Although the redox po-
tentials of the present complexes are not measurable, the
emission energies were successfully explained by the reduc-
tion potentials of the N-heteroaromatic ligands that are
modified based on the MO calculation for protonated and
free ligands. The results provide useful information for the
construction of systems with desired emissive properties by
selecting the π* levels of monodentate and bidentate ligands.
We have carried out the MO calculation on the mixed-
ligand bromide model complex [Cu2Br2(PH3)2(py)2] by the
DTF method on Gaussian 03. The geometry of the model
compound was taken from the crystal structure of [Cu2Br2-
(PPh3)2(py)2],20 and virtual C2h symmetry was assumed. The
calculated energy levels around the frontier orbitals are
similar to that of the previous iodide-pyridine model
complex in that the orbitals are mainly composed of copper,
halide, and pyridine orbitals. Table 5 shows the energy and
components of each molecular orbital of [Cu2Br2(PH3)2(py)2]
around the frontier orbital levels.
The HOMO (highest occupied molecular orbital) was
mainly composed of Cu-3d and Br-3p orbitals. The other
occupied MOs around the HOMO are also mainly composed
of Cu-3d and Br-3p orbitals.35 The LUMO and L+1 to L+3
were essentially py π* orbitals. When the Cu-Cu distance
is short enough, the Cu-4s4p orbitals become the LUMO
because of the stabilization by Cu-Cu bonding interaction,
as has been previously reported for [Cu2I2(NH3)4].10b How-
ever, the long Cu-Cu distance in this system weakened the
Cu-Cu interaction leaving the Cu 4s and 4p orbitals at a
higher energy. Accordingly, the relatively stable py π* orbital
becomes LUMO in this model.
Acknowledgment. The authors are indebted to Prof.
Kiyoshi Tanaka (Hokkaido University) and Prof. Katsuaki
Nobusada (currently at the Institute for Molecular Science)
for the calculation. This work was partly supported by a
Grant-in-Aid for Young Scientists (B) (17750046) from the
Japan Society for the Promotion of Science (JSPS).
The lowest-energy transition calculated by the TD-DFT
method is mainly composed of a HOMO-LUMO transition
(HOMO f LUMO (85%) and HOMO - 3 f LUMO + 1
(8%)). The calculated transition energy was 2.72 eV (456
nm) with an oscillator strength of 0.0029. The calculation
on the model complex supports the fact that the emissions
of [Cu2X2(PPh3)2(L)n] are related to the excited state formed
by the charge transfer from the metal d orbital to the π*
Supporting Information Available: The absorption and excita-
tion spectra of 2-I, selected bond distances and angles of the
complexes, atomic parameters of the model complex, interatomic
distances around Cu atoms, color and emission maxima of the
complexes, and CIF files of the crystals. This material is available
free of charge via the Internet at http://pubs.acs.org.
(35) We carried out the MO calculation for the corresponding iodide
complex that shows results similar to those for the bromide complex.
The mixing of iodine and copper AOs around the HOMO was greater
than that found in bromide complexes.
IC0510359
Inorganic Chemistry, Vol. 44, No. 26, 2005 9675