Synthesis, structure, and emissive properties of copper(I) complexes [CuI2(μ-X)2(μ-1,8-naphthyridine)(PPh 3)2] (X = I, Br) with a butterfly-shaped dinuclear core having a short Cu-Cu distance

Article abstract of DOI:10.1021/ic7010925

New dinuclear copper(I) complexes [Cu₂(μ-X) ₂(μ-1,8-naphthyridine)-(PPh₃)₂] (X = I, Br) having the butterfly-shaped {CU₂(M-X)₂} unit show red phosphorescence at room temperature in the solid state. Molecular orbital calculations show that the emissions of the new complexes are not directly related to their short Cu...Cu separations [2.6123(5) and 2.6271(4) A] and are assignable to the triplet charge-transfer excited states from the {CU₂(μ-X)₂} core to 1,8-naphthyridine.

Full text of DOI:10.1021/ic7010925

Inorg. Chem. 2007, 46, 10032−10034
Synthesis, Structure, and Emissive Properties of Copper(I) Complexes
[CuI2(
-X)2( -1,8-naphthyridine)(PPh3)2] (X I, Br) with a
Butterfly-Shaped Dinuclear Core Having a Short Cu Cu Distance
µ
µ
)
−
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 4, 2007
New dinuclear copper(I) complexes [Cu2(
µ
-X)2(
µ
-1,8-naphthyridine)-
states from the {Cu2(µ-X)2} core to N-heteroaromatic ligands
by a comparison of the emission energies and reduction
potentials of the N-heteroaromatic ligands. The Cu-Cu
distances are in the range of 2.82-3.43 Å, which are long
enough to neglect Cu-Cu interaction in both the ground and
excited states. We are interested in the influence of short
Cu-Cu distances upon the emission behaviors of the
{Cu2(µ-X)2} complexes. In order to prepare the complexes
with short Cu-Cu distances, bridging ligands bearing short
bite distances would be useful. Here, we utilize 1,8-
naphthyridine (1,8-nap) as the intramolecular bridging N-
heteroaromatic ligand, which should bring two CuI ions into
closer contact with concomitant distortion of the {Cu2X2}
core. Hitherto, over 200 complexes with a {CuI2X2} unit have
been prepared with various ligands and most of them possess
a planar diamond core.4 The examples of a folded {Cu2X2}
unit by the small chelating ligand are limited to those with
N-heterocyclic carbene,5a bis(diphenylphosphino)ethane,5b,c
allylamine,5d dithiocarboxylato,5e and (phenylamino)bis-
(phosphonite).5f
The naphthyridine complexes [Cu2(µ-X)2(µ-1,8-nap)-
(PPh3)2] [X ) I (1), Br (2)] are prepared by the reaction of
CuX, PPh3, and 1,8-nap in organic solvents.6
The structures of the complexes 1 and 2 are determined
by the single-crystal X-ray analysis.7 Figure 1 shows the
structure of iodide complex 1. Two CuI ions are bridged by
two I ions and one 1,8-nap. Coordinated by PPh3 additionally,
each Cu atom is in a distorted tetrahedral geometry. The
bromide complex 2 adopts an almost identical structure;
(PPh3)2] (X I, Br) having the butterfly-shaped
)
{
Cu2( -X)2 unit
µ
}
show red phosphorescence at room temperature in the solid state.
Molecular orbital calculations show that the emissions of the new
complexes are not directly related to their short Cu‚‚‚Cu separa-
tions [2.6123(5) and 2.6271(4) Å] and are assignable to the triplet
charge-transfer excited states from the
naphthyridine.
{Cu2(µ-X)2} core to 1,8-
Monovalent group 11 metal ions, AuI, AgI, and CuI, are
known to afford emissive complexes.1 Several excited states
such as metal-centered transitions, intraligand transitions, and
charge-transfer (CT) transitions between metal and ligand
exist as emissive states, depending on the ligands and steric
factors.1a In addition, emissions originating from the metal-
metal interactions have also been known for multinuclear
complexes.1a,d,g Among them, recent developments of the
copper(I) halide complexes are particularly noteworthy
because of their intense emissions,2 tunable emission ener-
gies,3 and dual emissions due to juxtaposed excited states.1a
We have reported the solid-state emissions of a series of
copper(I) complexes having planar diamond {Cu2(µ-X)2}
units (X ) Br, I) with various mono- and bidentate
N-heteroaromatic ligands using PPh3 as the ancillary ligand.3
All of the complexes show emissions at room temperature,
which were unequivocally assigned to the triplet CT excited
* To whom correspondence should be addressed. E-mail: tsuge@
sci.hokudai.ac.jp (K.T.), ysasaki@sci.hokudai.ac.jp (Y.S.).
† Present address: Catalysis Research Center, Hokkaido University,
Sapporo 001-0021, Japan.
(1) (a) Ford, P. C.; Cariati, E.; Bourassa, J. Chem. ReV. 1999, 99, 3625.
(b) Vogler, A.; Kunkely, H. Coord. Chem. ReV. 2001, 219-221, 489.
(c) White-Morris, R. L.; Olmstead, M. M.; Balch, A. L. J. Am. Chem.
Soc. 2003, 125, 1033. (d) Yam, V. W.-W.; Lo, K. K.-W. Chem. Soc.
ReV. 1999, 28, 323. (e) McMillin, D. R.; McNett, K. M. Chem. ReV.
1998, 98, 1201. (f) Armaroli, N. Chem. Soc. ReV. 2001, 30, 113. (g)
Che, C.-M.; Mao, Z.; Miskowski, V. M.; Tse, M.-C.; Chan, C.-K.;
Cheung, K.-K.; Phillips, D. L.; Leung, K.-H. Angew. Chem., Int. Ed.
2000, 39, 4084.
(4) (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. (b) Graham, P. M.; Pike, R. D.; Sabat, M.; Bailey, R.
D.; Pennington, W. T. Inorg. Chem. 2000, 39, 5121. (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.
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B.-J.; Lobana, T. S.; Lin, Y.-W.; Wang, J.-C.; Liu, C. W. Inorg. Chem.
2005, 44, 9921. (c) Lobana, T. S.; Sharma, R.; Bermejo, E.; Castineiras,
A. Inorg. Chem. 2003, 42, 7728. (d) Vestergren, M.; Johansson, A.;
Lennartson, A.; Hakansson, M. MendeleeV Commun. 2004, 258. (e)
Okubo, T.; Kawajiri, R.; Mitani, T.; Shimoda, T. J. Am. Chem. Soc.
2005, 127, 17598. (f) Ganesamoorthy, C.; Balakrishna, M. S.; George,
P. P.; Mague, J. T. Inorg. Chem. 2007, 46, 848.
(2) Tsuboyama, A.; Kuge, K.; Furugori, M.; Okada, S.; Hoshino, M.;
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Chem. 2005, 44, 9667.
10032 Inorganic Chemistry, Vol. 46, No. 24, 2007
10.1021/ic7010925 CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/25/2007

COMMUNICATION
Figure 2. Emission spectra of complexes 1 (red) and 2 (blue) in the solid
state at room temperature excited at 337 nm.
Figure 1. ORTEP drawing of iodide complex 1 with a probability level
of 50%. H atoms are omitted for clarity.
indeed, 1 and 2 form the isomorphous crystals.7 The
Cu-Cu distances are 2.6123(5) and 2.6271(4) Å for 1 and
2, respectively, which are comparable with those found in
other copper(I) dimeric complexes bridged by 1,8-nap (2.51-
2.61 Å).8 The dihedral angles between two XCuX triangles
are 125.29(2)° and 127.87(2)° respectively for 1 and 2.
Except for the carbene-bridged {Cu2X2} unit,5a the {Cu2X2}
units in the present complexes are among the most distorted
cores because of the short Cu-Cu distances and parallel
direction of naphthyridine lone pairs.
Figure 2 shows the emission spectra of the bromide and
iodide complexes at 290 K in the solid state. The two
complexes show broad structureless bands with emission
maxima at 670 nm (1.49 × 104 cm-1) and 720 nm (1.39 ×
104 cm-1) for iodide and bromide complexes, respectively.
The emission lifetimes are 0.83 µs (X ) I) and 0.22 µs (X
) Br), indicating that the emissions are phosphorescence.9
The emission spectra at lower temperatures are almost
identical with those at room temperature for both 1 and 2
(Figure S1 in the Supporting Information). The decay curves
are well fitted as single exponential, and lifetimes become
longer at lower temperatures (Figure S2 and Table S1 in the
Supporting Information).
The apparent colors of the bromide and iodide complexes
are quite similar red-orange. In fact, both complexes show
the lowest energy band around 530 nm in absorption spectra
in the solid state. The excitation spectra of the complexes
are also similar between two complexes and correspond to
each absorption spectrum (Figure S1 in the Supporting Infor-
mation). Therefore, the red shift of the bromide complex is
difficult to explain from ground-state properties. Similar ob-
servations have been reported for the related copper(I) halide
systems, and plausible electron correlations in the excited
states have been suggested.10 Because the involved molecular
orbital (MO) corresponds to a weak Cu-X antibonding
orbital (vide infra), the bond strength in the excited state
may affect the differences of the emission energies.
In order to obtain information about the ground-state
electronic structure and the nature of the transitions, we
carried out MO calculations of the iodide and bromide
complexes using the density functional theory (DFT) method11
based on the crystal structures. The energy levels near frontier
orbitals of 2 are shown in Figure 3, and the components of
the MOs are listed in Table S2 in the Supporting Information.
The lowest unoccupied MO (LUMO), L + 1, and L + 2
were almost pure 1,8-nap π* orbitals. The highest occupied
MO (HOMO) and occupied orbitals near the HOMO are
mainly composed of copper and bromide orbitals, where the
(6) [Cu2(µ-I)2(µ-1,8-nap)(PPh3)2] (1). To the solution obtained by dis-
solving CuI (39.8 mg, 0.21 mmol) and PPh3 (131.8 mg, 0.50 mmol)
in CH3CN (16 mL) was added 1,8-nap (13.9 mg, 0.11 mmol) to obtain
a clear orange solution. After 1 week, red crystals were deposited in
40% yield (45.6 mg). Anal. Calcd for C44H36N2Cu2I2P2: C, 51.03; H,
3.50; N, 2.70. Found: C, 50.77; H, 3.59; N, 2.7. [Cu2(µ-Br)2(µ-nap)-
(PPh3)2] (2). To the solution obtained by dissolving CuBr (24.8 mg,
0.17 mmol) and PPh3 (45.7 mg, 0.17 mmol) in CH3CN (16 mL) was
added 1,8-nap (11.1 mg, 0.085 mmol) to obtain a clear orange solution.
After 1 week, red crystals were deposited in 55% yield (43.4 mg).
Anal. Calcd for C44H36N2Cu2I2P2: C, 56.12; H, 3.85; N, 2.98.
Found: C, 56.24; H, 4.03; N, 3.05. 1 and 2 were substantially insoluble
in common organic solvents such as acetonitrile, chloroform, dichlo-
romethane, N,N-dimethylformamide, dimethyl sulfoxide, tetrahydro-
furan, and alcohols, which prevented measurement of their properties
in solution.
(10) Vitale, M.; Ryu, C. K.; Palke, W. E.; Ford, P. C. Inorg. Chem. 1994,
33, 561.
(7) Crystal data for 1: C44H36Cu2I2N2P2, M ) 1035.63, monoclinic, space
group P21/a, a ) 18.068(2) Å, b ) 9.9984(7) Å, c ) 23.394(2) Å, â
) 107.103(4)°, V ) 4039.3(6) Å3, Z ) 4, T ) -120 °C, 17 545
measured reflections and 7815 independent reflections, and R1 )
0.0285 and wR2 ) 0.0879 for 7350 observed reflections [I > 2σ(I)]
and 469 parameters. Crystal data for 2: C44H36Cu2Br2N2P2, M )
941.63, monoclinic, space group P21/a, a ) 17.766(3) Å, b ) 9.701-
(2) Å, c ) 23.392(4) Å, â ) 107.009(3)°, V ) 3855(1) Å3, Z ) 4, T
) -170 °C, 23 521 measured reflections and 8470 independent
reflections, and R1 ) 0.0277 and wR2 ) 0.0697 for 6921 observed
reflections [I > 2σ(I)] and 469 parameters.
(8) (a) Maekawa, M.; Munakata, M.; Kitagawa, S.; Kuroda-Sowa, T.;
Suenaga, Y.; Yamamoto, M. Inorg. Chim. Acta 1998, 271, 129. (b)
Munakata, M.; Maekawa, M.; Kitagawa, S.; Adachi, M.; Masuda, H.
Inorg. Chim. Acta 1990, 167, 181.
(9) Preliminary measurements using an integrating sphere showed that
the quantum yield of the iodide complex 1 is ca. 0.2 whereas that of
the bromide complex 2 is lower than 0.05.
(11) 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.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10033

COMMUNICATION
oscillator strength.13 In the present complexes, the symmetry
of LUMO and the π* orbital of 1,8-nap do not match those
of the HOMO because LUMO and HOMO are antisymmetric
and symmetric to the virtual mirror plane on 1,8-nap, respec-
tively. For the π* orbital of 1,8-nap, the allowed transition
is from H - 5 because it contains the symmetrically suitable
d orbital by bending of the Cu2X2 core (Figure S4 in the
Supporting Information). The present results show that, for
the complexes having a {Cu(I)2X2} core with an N-hetero-
aromatic ligand, the absorption and emission energies depend
not only upon the energy of the π* orbital but also upon its
orientation and the shape of the {Cu2X2} core.
For the d10 dinuclear and multinuclear complexes that have
short metal-metal distances, the emissions related to metal-
metal interactions have been reported.1d,g For several copper-
(I) complexes where the π* orbitals are absent or have higher
energies, the emissions involving the orbitals perturbed by
the metal-metal interaction are suggested.1a,14 As shown in
Figure 3, in the present complexes, the vacant Cu orbitals
that are responsible for the Cu-Cu interaction are found
above the 1,8-nap π* and PPh3 orbitals. Therefore, although
the Cu-Cu distances are short in the present complexes, the
emissions originating from the Cu-Cu interaction were
unobservable because of the lower-lying CT state from
{Cu2X2} to 1,8-nap.
In summary, we show the characteristic difference of the
emissions between the butterfly-shaped and planar dinuclear
copper(I) halide complexes by using 1,8-nap as a bridging
ligand. Although the emissive excited states are related to
the triplet CT excited states from the {Cu2(µ-X)2} core to
1,8-nap, as in the case of complexes with {Cu2X2} diamond
cores with N-heteroaromatic ligands, the MO associated with
the emission is different because of the direction of the π*
orbital of the 1,8-nap ligand. It turns out that the short
Cu-Cu distance does not directly affect the emission
characteristics of the present butterfly-shaped complexes.
Figure 3. Energy levels of complex 2 near frontier orbitals. Each level
was colored according to the main components: blue, Cu 3d and Br 3p;
red, 1,8-nap; green, PPh3; black, Cu 4s and 4p.
Cu d orbitals mix well with the Br p orbitals, showing that
the Cu and Br ions afford mixed orbitals in the {Cu2Br2}
core. It is noted that, in spite of the short Cu-Cu distance,
the Cu-Cu interaction does not affect these occupied orbitals
significantly because the interaction between the Cu and
halide orbitals is dominant in the {Cu2X2} core. The
unoccupied orbital composed of the Cu 4s and 4p orbitals
that is somewhat perturbed by the Cu-Cu interaction was
observed above the vacant π* orbitals of 1,8-nap and PPh3.
The results for the iodide complex are very similar to those
of the bromide complex (Table S3 in the Supporting
Information).
The time-dependent (TD)-DFT calculation shows that the
HOMO-LUMO transitions are substantially forbidden for
1 and 2 and that the substantially allowed lowest singlet
excited states are the (H - 5)-LUMO transitions (Tables
S4 and S5 in the Supporting Information).12 As mentioned
above, the H - 5 is the Cu-X mixed orbitals in both
complexes (Figure S4 in the Supporting Information).
Therefore, the absorption and corresponding phosphorescence
can be ascribed to the singlet and triplet CT excited states
from the {Cu2(µ-X)2} core to 1,8-nap, respectively.
Both the diamond and butterfly-shaped {Cu2X2} com-
plexes have common natures of HOMO (Cu-X mixed
orbitals; Figure S5 in the Supporting Information) and
LUMO (ligand π* orbitals). The difference is that HOMO-
LUMO transitions are symmetry-allowed for the former and
symmetry-forbidden for the latter. This is due to the different
orientations of the aromatic ligands and the core shape. As
expected, in the case of the complexes with diamond
{Cu2X2} cores, the HOMO-LUMO transition has a large
Acknowledgment. This work was partly supported by a
Grant-in-Aid for Young Scientists (B) (Grant 17750046)
from the Japan Society for the Promotion of Science.
Supporting Information Available: Crystallographic data of
1 and 2 in CIF format, absorption and excitation spectra at room
temperature, temperature-dependent emission spectra and decay
curves, tables of emission lifetimes, tables of Mulliken population
near frontier orbitals, tables of calculated excited-state energies,
and figures of selected MOs. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC7010925
(12) TD-DFT calculations show that the energies and oscillator strengths
of S0 f S1 are 1.04 eV (1193 nm) and 0.0008 and 1.07 eV (1162
nm) and 0.0005 for iodide and bromide complexes, respectively
(Tables S4 and S5 in the Supporting Information). However, the
substantially lowest allowed transitions are S0 f S7; their calculated
energies and oscillator strengths are 2.22 eV (559 nm) and 0.0355
and 2.31 eV (536 nm) and 0.0404 for iodide and bromide complexes,
respectively. Besides the acceptable oscillator strengths, considerable
agreement of the calculated and observed energies also supports the
assignments of the lowest absorptions of 1 and 2. The S0 f S1 and S0
f S7 transitions mainly correspond to HOMO-LUMO and (H - 5)-
LUMO transitions for both iodide and bromide complexes (Tables
S4 and S5 in the Supporting Information).
(13) Using the single-crystal structure data, we calculate the transition
energies of the pyridine complexes, [{Cu2X2}(PPh3)2(pyridine)2],
which have diamond {Cu2X2} cores. The oscillator strengths of S0 f
S1, which correspond to the HOMO-LUMO transition, are 0.0183
and 0.0191 for X ) I and Br, respectively.
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V. K.; Eldabaja, M. G.; Elbjeirami, O.; Rawashdeh-Omary, M. A.;
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10034 Inorganic Chemistry, Vol. 46, No. 24, 2007