Phosphorescent copper(I) complexes bearing 2-(2-benzimidazolyl)-6- methylpyridine and phosphine mixed ligands

Article abstract of DOI:10.1016/j.inoche.2011.09.005

Two 2-(2-benzimidazolyl)-6-methylpyridine (Hbmp) copper(I) complexes bearing PPh₃ and 1,4-bis(diphenylphosphino)butane (dppb), namely, [Cu(Hbmp)(PPh₃)₂](ClO₄) (1) and [Cu(Hbmp)(dppb)](ClO₄) (2), have been synthesized. X-ray diffraction analysis reveals that the most significant influence of the phosphine ligands on the structures is on the P-Cu-P bond angle. Both two Cu(I) complexes exhibit a weak low-energy absorption at 360-450 nm, ascribed to the Cu(I) to Hbmp metal-to-ligand charge-transfer (MLCT) transition, perhaps mixed with some ILCT character inside Hbmp. The room-temperature luminescences are observed for 1 and 2, both in solution and in the solid state, which originate from the MLCT excited states and vary markedly with the phosphine ligands.

Full text of DOI:10.1016/j.inoche.2011.09.005

Inorganic Chemistry Communications 14 (2011) 1894–1897
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Phosphorescent copper(I) complexes bearing
2-(2-benzimidazolyl)-6-methylpyridine and phosphine mixed ligands
a
a
a
b
a
a
Jing-Lin Chen a,b, , Xing-Fu Cao , Wei Gu , He-Rui Wen , Lin-Xi Shi , Gan Rong , Pan Luo
⁎
a
School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2011
Accepted 7 September 2011
Available online 14 September 2011
Two 2-(2-benzimidazolyl)-6-methylpyridine (Hbmp) copper(I) complexes bearing PPh3 and 1,4-bis(diphe-
nylphosphino)butane (dppb), namely, [Cu(Hbmp)(PPh3)2](ClO4) (1) and [Cu(Hbmp)(dppb)](ClO4) (2),
have been synthesized. X-ray diffraction analysis reveals that the most significant influence of the phosphine
ligands on the structures is on the P–Cu–P bond angle. Both two Cu(I) complexes exhibit a weak low-energy
absorption at 360–450 nm, ascribed to the Cu(I) to Hbmp metal-to-ligand charge-transfer (MLCT) transition,
perhaps mixed with some ILCT character inside Hbmp. The room-temperature luminescences are observed
for 1 and 2, both in solution and in the solid state, which originate from the MLCT excited states and vary
markedly with the phosphine ligands.
Keywords:
Copper(I)
Mixed-ligand
Photoluminescence
2-(2-Benzimidazolyl)-6-methylpyridine
© 2011 Elsevier B.V. All rights reserved.
There has been considerable interest in luminescent transition-
metal complexes of multi-nitrogen heterocyclic ligands [1–3].
Copper(I) complexes are of interest in this context due to their attractive
photophysical properties with possible applications in solar energy
conversion, electroluminescent devices, luminescence-based sensors,
and biological labeling [4–7]. In Cu(I) systems, the selection of
N-heterocyclic chelating ligand is the key because it can modulate
the emissive properties from the metal-to-ligand charge-transfer
(MLCT) excited state [8–11]. In addition, the phosphine auxiliary
ligands play a positive role in stabilizing the Cu(I) center, albeit
not involved in the MLCT transition, and exert important effects
on the photophysical properties of Cu(I) complexes [12–14].
Recently, we have initiated research on photoactive copper(I)
complexes with N-heterocyclic chelate bridging ligands [11,15,16].
It is demonstrated that both N-heterocyclic chelate and the ancillary
ligand such as halide and phosphine have great effects on the formation
and photophysical properties of Cu(I) complexes. Additionally, to the
best of our knowledge, the structures of Cu(I) complexes with 1,4-bis
(diphenylphosphino)butane (dppb) as a bidentate chelating ligand
have not been reported hitherto. Herein, we describe the syntheses,
crystal structures, and photophysical properties of two new Cu(I)
complexes bearing 2-(2-benzimidazolyl)-6-methylpyridine (Hbmp)
and the two different phosphine ligands, i.e. [Cu(Hbmp)(PPh3)2]
(ClO4) (1) and [Cu(Hbmp)(dppb)](ClO4) (2). It is demonstrated that
complexes 1 and 2 are emissive at ambient temperature in both solution
and the solid state, and both the phosphine auxiliary ligand and the
P–Cu–P bond angle have an important impact on the photophysical
properties of cuprous complexes.
Mononuclear Cu(I) complexes possessing Hbmp and phosphine
mixed ligands could be prepared via two simple approaches
(Scheme 1) [17]. The PPh3 complex [Cu(Hbmp)(PPh3)2](ClO4) (1)
was firstly afforded by the treatment of Cu(ClO4)2·6H2O with 4
equivalents of PPh3, where PPh3 not only served as a monodentate
ligand but also as a reducing reagent, followed by slow addition of
1 equiv of Hbmp. However, in view of the bidentate nature of
dppb, a synthetic method different to 1 was executed to prepare
the dppb complex [Cu(Hbmp)(dppb)](ClO4) (2), in which copper
powder acts as the reducing agent of Cu(II). Complexes 1 and 2 display
a light-yellow color and are air-stable in both solution and the solid
state, and both were well-defined by NMR, IR, elemental, and X-ray
diffraction analyses. In the 1H NMR spectra of 1 and 2, the N–H signals
of the imidazole rings and the C–H signals from the methyl groups
were observed at around 12 and 2 ppm, respectively, besides the
aromatic proton signals (δ 6.95–8.44 ppm). In the 31P NMR spectra,
complex 1 displays one singlet 31P chemical shift at δ 0.91 ppm,
which is in agreement with those recorded in other related Cu(I)
PPh3 complexes [13,18,19], while one singlet peak is also observed
at δ −11.20 ppm in 2, comparable to those of previously reported
Cu(I) dppb complexes [20,21].
Single-crystal X-ray diffraction studies of 1 and 2 have been carried
out to reveal their structural features [22]. As depicted in Figs. 1 and 2,
each four-coordinated Cu(I) atom is bound to two nitrogen atoms of
Hbmp and two P donors to constitute a distorted tetrahedral geometry,
and the ClO4¯ anion is linked by hydrogen bond with the NH group of the
benzimidazolyl ring. The Cu–Npyridyl distances for the pyridyl rings (av.
⁎
Corresponding author at: School of Metallurgy and Chemical Engineering, Jiangxi
University of Science and Technology, Ganzhou 341000, PR China. Tel./fax: +86 797
8312204.
E-mail address: gzchenjinglin@126.com (J.-L. Chen).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.09.005

J.-L. Chen et al. / Inorganic Chemistry Communications 14 (2011) 1894–1897
1895
PPh3
PPh
HN
N
N
CH2Cl2
(1)
+
+
Cu(ClO4)2 4 PPh3 Hbmp
Cu
3 (ClO4)
Ph2
P
HN
N
N
CH2Cl2
(2)
+
+
+
Cu(ClO4)2 Cu 2 Hbmp 2 dppb
Cu
P
(ClO4)
Ph2
Scheme 1. Synthetic route of complexes 1 and 2.
2.152 Å) are obviously longer than the Cu–Nimidazolyl distances of the
imidazolyl rings (av. 2.079 Å), showing a stronger bonding with the
imidazolyl nitrogen, consistent with the related Cu(I) complexes
featuring the 2-(2′-quinolyl)benzimidazolyl and 2-(2′-pyridyl)ben-
zimidazolyl fragments [18,19]. The Cu–N and Cu–P distances in 1 and 2
are compatible to those observed in other related Cu(I) complexes
[13,18,19,23]. The N–Cu–N bite angles are similar with small variation
for 1 (79.44(13)°) and 2 (80.20(11)°). In contrast, however, the P–Cu–
P bond angles vary notably from 123.70(5)° of 1 to 111.49(4)° of 2,
and analogous structural phenomenon was also observed in other
Cu(I) phosphine complexes containing the 2-(2′-quinolyl)benzimida-
zolyl and 2-(2′-pyridyl)benzimidazolyl units [13,19]. The P–Cu–P bite
angle (123.70(5)°) of 1 resembles those found in Cu(I) PPh3 com-
plexes [Cu(Hqbm)(PPh3)](BF4) (124.24(3)°) [19] and [Cu(pbb)
(PPh3)](BF4) (124.12(6)°) [13], while the P–Cu–P bond angle
(111.49(4)°) of the dppb complex 2 approximates to those of Cu(I)
complexes bearing bis[(2-diphenylphosphino)phenyl]ether (DPE-
phos), [Cu(Hqbm)(DPEphos)](BF4) (112.40(4)°) [19] and [Cu
(pbb)(DPEphos)](BF4) (113.35(3)°) [13], which is obviously larg-
er than those documented in other reported metal complexes
with the chelating dppb [24,25]. In 1, it is noted that the phenyl
rings of the Hbmp ligands from two adjacent molecules are basically
parallel and show a favorable pairwise π–π stacking (Fig. 3). The
inter-phenyl separation is approximately 3.59 Å, implying the presence
of a weak π–π interaction between the intermolecular Hbmp ligands
Fig. 2. Molecular structure of 2 with atom-labeling scheme. Hydrogen atoms except
H3B are omitted for clarity. Selected bond distances (Å) and angles (°): Cu1–N1
2.148(3), Cu1–N2 2.080(3), Cu1–P1 2.2446(12), Cu1–P2 2.2851(13), H3B∙∙∙O3 2.03,
N1–Cu1–N2 80.20(11), N1–Cu1–P1 124.35(8), N1–Cu1–P2 102.26(8), N2–Cu1–P1
119.35(8), N2–Cu1–P2 115.31(8), P1–Cu1–P2 111.49(4), N3–H3B∙∙∙O3 160.0.
[26]. However, similar π–π stacking is not observed in the crystal lattice
of 2.
The UV–vis absorption spectra of Hbmp and its complexes 1 and 2
in CH2Cl2 at ambient temperature are depicted in Fig. 4. Hbmp shows
two absorptions at λmax ≈314 and 327 nm, attributable to the 1ππ*
transitions inside Hbmp. Complexes 1 and 2 exhibit multiple absorption
peaks (εN104 M−1 cm−1) in the 240–350 nm region, most likely
coming from Hbmp and PPh3 or dppb. Further support is provided
by a close match of the absorption spectra in high-energy region
(≤350 nm) of Hbmp and complexes 1 and 2. The red shift of the π–π*
absorptions of 1 and 2 is consistent with better conjugation of the coor-
dinated Hbmp relative to free Hbmp, resulting in a smaller π–π* energy
gap. In addition to the high-energy absorptions, complex 1 has a com-
paratively weak low-energy absorption tail (εb7.5×103 M−1 cm−1
)
at 360–450 nm, while a weak low-energy absorption with maximum
at 378 nm is also observed in 2. These weak low-energy absorptions
are identified as the metal-to-ligand charge-transfer (MLCT) transition
from the dπ orbital of Cu(I) center to the unoccupied π* orbital of
Hbmp, probably mixed with some intraligand charge-transfer (ILCT)
character inside Hbmp.
Fig. 1. Molecular structure of 1 with atom-labeling scheme. Hydrogen atoms except
H3B are omitted for clarity. Selected bond distances (Å) and angles (°): Cu1–N1
2.156(3), Cu1–N2 2.078(3), Cu1–P1 2.2614(12), Cu1–P2 2.2631(13), H3B∙∙∙O2 1.99,
N1–Cu1–N2 79.44(13), N1–Cu1–P1 109.16(9), N1–Cu1–P2 114.22(9), N2–Cu1–P1
113.38(10), N2–Cu1–P2 108.43(10), P1–Cu1–P2 123.70(5), N3–H3B∙∙∙O2 162.0.
Fig. 3. Side view depicting the pairwise stacking of Hbmp ligands between neighboring
cations of 1 in the crystal lattices.

1896
J.-L. Chen et al. / Inorganic Chemistry Communications 14 (2011) 1894–1897
order as the P–Cu–P bond angles (123.70(5)° for 1 and 111.49(4)°
Hbmp
for 2), consistent with the observations documented in related Cu(I)
phosphine complexes [13,19,27]. Hence, it is believed that the P–Cu–P
angle might also play a role in the photoluminescences of 1 and 2.
In sharp contrast to Cu(I) phosphine complexes of N-arylating 2-(2′-
pyridylbenzimidazolyl)benzene being non-emissive in the solid state
at ambient temperature [13], complexes 1 and 2 display a solid-state
emission at ambient temperature with maxima at 514 and
521 nm, respectively. It is possible that the arylation of the NH group
has some negative effects on the emissive excited state. The solid-
state emissions of 1 and 2 have a blue-shifting of 48–81 nm with respect
to their solution emissions, which are most likely from the rigidity
of solid-state medium impeding the distortion of the excited-
state geometries, and virtually such luminescence rigidochromism
is also observed in Ru(II) [28], Ir(III) [29], and Cu(I) systems [9,30].
It is notable that the blue-shift (48 nm) of the solid-state emission
of 1 versus its solution emission is much smaller than that (81 nm)
of 2, perhaps due to the inter-Hbmp π–π stacking of 1 (Fig. 3), not
found in 2, lowering the LUMO level, and thus decreasing the
HOMO–LUMO gap of 1 in the solid state.
0.4
0.3
0.2
0.1
0.0
1
2
250
300
350
400
450
500
Wavelength (nm)
Fig. 4. UV–vis absorption spectra of Hbmp (―■―), 1 (―●―), and 2 (―▲―) in diluted
dichloromethane solution at room temperature.
The photoluminescence properties of Hbmp and complexes 1 and
2 are investigated at ambient temperature (Fig. 5). The free ligand
Hbmp emits a purple color with λmax ≈361 and 373 nm in CH2Cl2
and in the solid state, respectively, attributed to the fluorescence
emission stemming from ligand-centered π→π* transition. The
solid-state emission of Hbmp has a small red-shift of 12 nm relative
to its solution emission, which comes from the inter-Hbmp π–π
stacking in the solid state, leading to a decrease of the π–π* energy
gap. Complexes 1 and 2 exhibit a broad emission profile with λmax at
562 and 602 nm in degassed CH2Cl2 at room temperature, respectively.
From previous work [4,13,18,19], the highest occupied molecular
orbitals (HOMO) of 1 and 2 are principally centered on Cu(I), perhaps
including some contributions from the phosphine and Hbmp ligands,
while their lowest unoccupied molecular orbitals (LUMO) are believed
to be primarily localized on Hbmp. Thus the lowest-lying emissive
excited states of 1 and 2 are best regarded as the 3MLCT character
from d(Cu)→π*(Hbmp). The emission of 2 (602 nm) is more red-
shifted than that of 1 (562 nm), and such a 40 nm red-shift can be related
to the replacement of PPh3 via a stronger electron-donating dppb, raising
the HOMO level, less influencing the LUMO energy, and hence leading to
a decrease of the HOMO–LUMO gap. It is reported that a wider P–Cu–P
bond angle can decrease the dσ* interactions and enhance the required
energy of the MLCT excitation [12]. The emission energies of 1 and 2
follow the sequence of 1 (562 nm)N2 (602 nm), which is the same
In summary, two new phosphorescent Cu(I)–Hbmp complexes
[Cu(Hbmp)(PPh3)2](ClO4) (1) and [Cu(Hbmp)(dppb)](ClO4) (2) have
been prepared. It is demonstrated that the phosphine auxiliary ligand
plays a role in regulating the P–Cu–P bond angle, and both have
significant effects on the photophysical properties of Cu(I) complexes.
It is suggested that the arylation of the imidazolyl-NH group
might has some important impacts on the luminescence properties
of cuprous complexes. Further investigation on Cu(I) complexes of
NH-functionalized Hbmp is underway.
Acknowledgments
The authors thank the National Natural Science Foundation of
China (Nos. 21163009, 21001057), the Key Project of Chinese Ministry
of Education (No. 211088), the Natural Science Foundations of Fujian
Province (No. 2008F3117) and Jiangxi Province (No. 2009GQH0038),
the Science Foundation of Education Bureau of Jiangxi Province
(No. GJJ10151), and the Science Foundation of State Key Laboratory
of Structural Chemistry (No. 20110015) for the financial support.
Appendix A. Supplementary data
CCDC 829377 and 806918 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data to this article can be found
online at doi:10.1016/j.inoche.2011.09.005.
1.0
Hbmp(l)
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[17] Synthesis. For 1, a mixture of PPh3 (123.0 mg, 0.469 mmol) and Cu(ClO4)2·6H2O
(43.5 mg, 0.117 mmol) was stirred in CH2Cl2 (25 mL) at RT for 8 h. To this resul-
tant colorless solution was added dropwise with stirring a solution of Hbmp
(24.5 mg, 0.117 mmol) in CH2Cl2 (5 mL). The mixture was stirred for another
3 h and the solvent was evaporated. Light-yellow crystals of 1 (Yield: 73%)
were afforded by slow diffusion of petroleum ether into a 1:3 mixture of CH2Cl2
and 1,2-dichloroethane. Anal. Calcd. for C49H41ClCuN3O4P2: C, 65.62; H, 4.61; N,
4.69. Found: C, 65.32; H, 4.23; N, 4.47%. Selected IR (KBr, cm−1): 1095 (s, ClO4).
1H NMR (400 MHz, CDCl3): δ 12.39 (s, 1H), 8.39 (d, 1H, J=7.6 Hz), 7.37–7.32 (m,
6H), 7.26–7.22 (m, 3H), 7.19–7.00 (m, 25H), 6.99 (t, 1H, J=7.6 Hz), 6.73 (d, 1H,
J=8.4 Hz), 2.28 (s, 3H). For 2, a mixture containing Cu(ClO4)2·6H2O (64.2 mg,
0.173 mmol), activated copper powder (55.8 mg, 0.878 mmol) and Hbmp
(72.4 mg, 0.346 mmol) was stirred for 30 min in dried acetonitrile (30 mL)
under N2 at RT, to which dppb (147.6 mg, 0.346 mmol) were added and stirred
for another 1 h. The resulting light-yellow solution was filtered to remove
unreacted Cu, the filtrate was evaporated. Light-yellow crystals of 2 (Yield:
62%) were obtained by slow diffusion of hexane into the CH2Cl2 solution. Anal.
Calcd. for C41H39ClCuN3O4P2: C, 61.65; H, 4.92; N, 5.26. Found: C, 61.32; H, 4.75;
N, 5.04%. Selected IR (KBr, cm−1): 1060 (s, ClO4). 1H NMR (400 MHz, CDCl3): δ
12.56 (s, 1H), 8.42 (d, 1H, J=7.6 Hz), 7.99 (d, 1H, J=8.0 Hz), 7.83 (t, 1H,
J=7.6 Hz), 7.69 (t, 1H, J=8.0 Hz), 7.51–7.44 (m, 2H), 7.37 (t, 1H, J=7.6 Hz),
7.26–7.22 (m, 13H), 7.20–7.11 (m, 6H), 6.98 (t, 1H, J=7.6 Hz), 2.58 (s, 4H),
2.14 (s, 4H), 1.91 (s, 3H).
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