Structures and luminescence properties of two copper(I) halide complexes featuring 2-(2-benzimidazolyl)-6-methylpyridine

Article abstract of DOI:10.1016/j.cclet.2012.07.014

Two new copper(I) halide complexes bearing 2-(2-benzimidazolyl)-6- methylpyridine (Hbmp) and PPh₃ ligands, Cu(Hbmp)(PPh₃)X (X = Br (1); I (2)), have been synthesized and characterized. Single-crystal X-ray structure analysis reveals that the halide ligand has a significant impact on the CuN_pyridyl bond lengths. Complexes 1 and 2 show the solid-state emissions at ambient temperature, varying with the halide bound to the {Cu(Hbmp)(PPh₃)} motif, which are perhaps best attributed to the metal-to-ligand charge-transfer (Cu(I) → Hbmp) transition mixed with some ligand-to-ligand charge-transfer (X and PPh₃ → Hbmp) character.

Full text of DOI:10.1016/j.cclet.2012.07.014

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 1169–1172
www.elsevier.com/locate/cclet
Structures and luminescence properties of two
copper(I) halide complexes featuring
2-(2-benzimidazolyl)-6-methylpyridine
c
b
Li Hua He a, Jing Lin Chen b, , Jin Yun Wang , Xiu Xiu Chen ,
*
Xiu Zhen Tan b, Xing Fu Cao b
a School of Science, Jiangxi University of Science and Technology, Ganzhou 341000, China
b School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China
c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, China
Received 13 April 2012
Available online 26 September 2012
Abstract
Two new copper(I) halide complexes bearing 2-(2-benzimidazolyl)-6-methylpyridine (Hbmp) and PPh3 ligands,
Cu(Hbmp)(PPh3)X (X = Br (1); I (2)), have been synthesized and characterized. Single-crystal X-ray structure analysis reveals that
the halide ligand has a significant impact on the Cu–Npyridyl bond lengths. Complexes 1 and 2 show the solid-state emissions at ambient
temperature, varying with the halide bound to the {Cu(Hbmp)(PPh3)} motif, which are perhaps best attributed to the metal-to-ligand
charge-transfer (Cu(I) ! Hbmp) transition mixed with some ligand-to-ligand charge-transfer (X and PPh3 ! Hbmp) character.
# 2012 Jing Lin Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Copper(I) complex; 2-(2-Benzimidazolyl)-6-methylpyridine; Crystal structure; Photoluminescence
Copper(I) complexes have been widely studied in view of their intriguing photophysical properties and promising
applications in, for instance, organic light-emitting devices, dye-sensitized solar cells, and luminescence sensors [1].
The structural and electronic properties of the ligands have a great impact on the photophysical properties of the
resulting cuprous compounds [2]. For example, [Cu(1,10-phenanthroline)2]+ is non-emissive in solution, whereas its
derivatives with two bulky substituents at the 2,9-positions show luminescence behavior. Thus the appropriate choice
of the ligand is very important in modifying the emissive properties of the Cu(I) complexes.
Recently, we have initiated the study on photoactive copper(I) complexes of multidentate N-heterocyclic chelating
ligands [3]. It is shown that both N-heterocycle chelate and the ancillary ligand such as halide and phosphine have an
important influence on the photophysical properties of the Cu(I) systems. Similar to 1,10-phenanthroline, 2-(2-
benzimidazoly)-6-methylpyridine (Hbmp) can coordinate with the Cu(I) atom as a neutral chelate using its two sp2 N
atoms from the pyridyl and benzimidazole moieties. In addition, it is possible that the N–H group of the benzimidazole
may have some effects on the luminescence properties of the Cu(I) species. Herein, we report crystal structures and
luminescence properties of two new Cu(I) halide complexes with Hbmp, Cu(Hbmp)(PPh3)X (X = Br (1); I (2)). It is
* Corresponding author.
E-mail address: gzchenjinglin@126.com (J.L. Chen).
1001-8417/$ – see front matter # 2012 Jing Lin Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.07.014

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L.H. He et al. / Chinese Chemical Letters 23 (2012) 1169–1172
Table 1
Crystallographic data for 1 and 2.
Complex 1
Complex 2
Complex 1
Complex 2
Empirical formula
C31H26BrCuN3
PꢁEt2O
C31H26ICuN3Pꢁ1.5H2O
a [8]
b [8]
87.685(4)
90
Formula mass
689.09
688.98
69.758(3)
85.061(4)
1669.3(8)
10077/5426
115.80(3)
90
Temperature [K]
˚
Wavelength [A]
296(2)
293(2)
g [8]
3
V [A ]
˚
0.71073
Triclinic
0.71073
Monoclinic
5868(2)
12291/5928
Crystal system
Space group
Reflections
collected/unique
Data/restraints/
parameters
¯
P 1
C2/c
5426/5/379
5928/0/352
10.984(3)
11.315(3)
14.370(4)
34.999(7)
9.3376(19)
19.943(4)
Good-of-fit on F2
R1 [I > 2s(I)]
wR2
1.045
1.171
˚
a [A]
˚
b [A]
˚
c [A]
0.0435
0.1019
0.0823
0.1951
demonstrated that 1 and 2 are all emissive in the solid state, and the halide coordinated to the {Cu(Hbmp)(PPh3)} unit
has a notable effect on the emissive properties of the corresponding Cu(I) complexes.
1. Experimental
Hbmp was synthesized according to a literature method [4]. Elemental analyses were conducted on a Perkin-Elmer
model 240C elemental analyzer. Infrared spectra were recorded on a Nicolet 5700 FT-IR spectrometer. Solid-state
emission spectra were determined on a Perkin-Elmer LS55 luminescence spectrometer. Single-crystal X-ray
diffraction data of 1 (CCDC 873457) and 2 (CCDC 873458) were measured on a Bruker SMART CCD diffractometer,
and the crystallographic data of 1 and 2 were given in Table 1. All calculations (see Supporting Information) were
carried out by TD-DFT method at the PBE1PBE level with the help of Gaussian 03 program [5].
1.1. Synthesis of Cu(Hbmp)(PPh3)Br (1)
A mixture of PPh3 (52.5 mg, 0.200 mmol) and CuBr (14.3 mg, 0.100 mmol) in CH2Cl2 (20 mL) was stirred for 3 h
at room temperature. To the resulting colorless solution was added with stirring Hbmp (20.9 mg, 0.100 mmol). The
admixture was stirred for another 4 h, and then the solvent was removed at reduced pressure to give a yellow residue.
Yellow crystals were obtained by slow diffusion of Et2O into a 3:1 mixture of CH2Cl2 and CH3CN in several days.
Yield: 38.2 mg, 0.062 mmol, 62%. Anal. Calcd. for C31H26BrCuN3P: C, 60.5; H, 4.3; N, 6.8. Found: C, 60.8; H, 4.5;
N, 6.5%. Selected IR (KBr, cmꢀ1): 1435vs, 1096s, 743vs, 695vs, 524s, 507m, 490m (PPh3); 3437m (N–H), 1604m,
1574w, 1467s, 1398m, 1319m, 801w (Hbmp).
1.2. Synthesis of Cu(Hbmp)(PPh3)I (2)
A similar procedure to 1 was adopted except that CuBr was displaced with CuI. Orange-yellow crystals were
afforded by slow diffusion of Et2O into a 3:1 mixture of CH2Cl2 and CH3CN in a few days. Yield: 70%. Anal. Calcd.
for C31H26CuIN3P: C, 56.2; H, 4.0; N, 6.4. Found: C, 56.4; H, 4.3; N, 6.2%. Selected IR (KBr, cmꢀ1): 1434vs, 1096s,
743vs, 695vs, 524s, 505m, 493m (PPh3); 3432m (N–H), 1605m, 1574w, 1467s, 1397w, 1318m, 806w (Hbmp).
N
N
PPh3
X
X
X
Ph3P
Ph3P
CH2Cl2
Hbmp
CH2Cl2
Cu
Cu
Cu
PPh3
CuX + 2 PPh3
HN
(1)
(2)
X = Br
I
Scheme 1. Synthetic route of complexes 1 and 2.

L.H. He et al. / Chinese Chemical Letters 23 (2012) 1169–1172
1171
Fig. 1. ORTEP drawings of 1 (a) and 2 (b); pairwise stacking views of 1 (c) and 2 (d).
2. Results and discussion
Mono-copper(I) halide complexes of Hbmp, Cu(Hbmp)(PPh3)X (X = Br (1); I (2)), are obtained by treatment of
cuprous halide with two equivalents of PPh3 in CH2Cl2, followed by addition of one equivalent of Hbmp (Scheme 1).
In the above reaction, the non-isolated intermediate is actually a dicopper(I) cluster (PPh3)2Cu(m-X)2Cu(PPh3) [3b].
In 1 and 2, each four-coordinated Cu(I) ion is surrounded by two sp2 N atoms from one pyridyl and one imidazolyl,
one P atom of PPh3, and one halide to generate an N2PX distorted tetrahedral arrangement (Fig. 1a and b, Table S2).
˚
˚
The Cu–Nimidazolyl lengths (2.064 and 2.065 A) are shorter than the Cu–Npyridyl distances (2.162 and 2.147 A) for 1 and
2, implying that the imidazolyl-N donor has a stronger bonding with the Cu(I) ion, consistent with other Cu(I)
complexes bearing the benzimidazolyl [6]. It is interesting to note that only the Cu–Npyridyl length depends on the
halide, whereas the Cu–Nimidazolyl length is basically identical in 1 and 2, suggesting that the halide only has a
˚
significant impact on the pyridyl of Hbmp. Moreover, it is observed that the Cu–Npyridyl distance (2.162 A) in 1 is
˚
longer than that (2.147 A) of 2, consistent with the calculated higher Mayer bond order (0.566) of the Cu–Npyridyl bond
in 2 relative to that (0.538) of 1, which is most likely due to the different electron-donating nature of the halide
(Brꢀ < Iꢀ). Further support is given by the natural population analysis that Iꢀ (ꢀ0.7982e) which donates more
electrons holds less negative charge than Brꢀ (ꢀ0.8094e). The Cu–N, Cu–P, and Cu–X lengths in 1 and 2
are comparable to those observed in other tetrahedral Cu(I) halide complexes with one PPh3 [7]. It is observed that the
Hbmp ligands of two adjacent enantiomeric molecules in 1 and 2 are largely parallel and exhibit a pairwise p–p
˚
stacking (Fig. 1c and d). The interplanar separations are in the range of 3.2–3.5 A for 1 and 2, indicating the existence
of a weak intermolecular p–p interaction between two neighboring Hbmp ligands [8].
The free Hbmp emits a purple color with lmax = 373 nm in the solid state at room temperature, originating from
ligand-centered p ! p* transition. Complexes 1 and 2 exhibit a solid-state emission with the maximum at 569 and
531 nm at ambient temperature, respectively (Fig. 2). From the orbital plots and partial molecular orbital compositions
calculated via TD-DFT method (Fig. 2 and Table 2), it is seen that the highest occupied molecular orbitals (HOMO) of
1 and 2 are basically composed of Cu(I), halide, and PPh3, while the lowest unoccupied molecular orbitals (LUMO)
largely focus on Hbmp with ꢂ94% contribution. The conclusions on the orbital compositions of 1 and 2 are consistent
with the related work [6a,9]. Thus, the emissive excited states of 1 and 2 are probably best regarded as the metal-to-
ligand charge-transfer (Cu(I) ! Hbmp) transition with some mixing of the ligand-to-ligand charge-transfer (X and
PPh3 ! Hbmp) character. Moreover, the solid-state emission (lmax = 569 nm) of 1 has a red-shifting of around 38 nm
Fig. 2. HOMO and LUMO plots of 1 (left) and 2 (right) calculated by TD-DFT method at the PBE1PBE level; solid-state emission spectra of 1 and 2
at ambient temperature (middle).

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L.H. He et al. / Chinese Chemical Letters 23 (2012) 1169–1172
Table 2
Partial molecular orbital compositions (%) of 1 and 2 calculated by TD-DFT method at the PBE1PBE level.
Molecular orbital
Molecular orbital contribution (%)
Cu
1.68
X
PPh3
Hbmp
Complex 1
Complex 2
LUMO
HOMO
LUMO
HOMO
0.98
1.52
22.90
1.71
95.81
7.73
44.92
2.06
24.45
1.34
94.88
6.43
38.87
31.94
22.76
relative to that (lmax = 531 nm) of 2, consistent with the order of ligand field strength of the halide (Iꢀ < Brꢀ) [10].
Therefore, it is believed that the emissive properties of 1 and 2 are related to the electronic nature of the halide, further
supported via the variation of the Cu–Npyridyl bond length of 1 and 2.
3. Conclusion
In summary, two new luminescent Cu(I) halide complexes of 2-(2-benzimidazoly)-6-methylpyridine (Hbmp) have
been synthesized and characterized. It is demonstrated that the halide introduced to the {Cu(Hbmp)(PPh3)} motif
plays a major role in influencing the molecular structures and luminescence properties of Cu(I)–Hbmp complexes.
Further study on luminescent copper(I) halide complexes of N-heterocyclic chelating ligands is underway.
Acknowledgments
We thank financial support from the NNSFC (Nos. 21163009 and 21001057), the Key Project of Chinese Ministry
of Education (No. 211088), the Advanced Program for the Retuned Overseas Chinese Scholars of the MOHRSS (No.
474[2011]), the Training Program for Jiangxi Provincial Young Scientists, and the Foundation of State Key Laboratory
of Structural Chemistry (No. 20110015).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/
j.cclet.2012.07.014.
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