Luminescent mononuclear copper(I) heteroleptic complexes with 6-cyano-2,2′-bipyridine

Article abstract of DOI:10.1016/j.inoche.2011.09.040

A new series of luminescent mononuclear Cu(I) heteroleptic complexes of 6-cyano-2,2′-bipyridine (cbpy), [Cu(cbpy)(PPh₃)X] (X = I (1); Br (2)) and [Cu(cbpy)(PPh₃)₂](ClO₄) (3), has been synthesized and characterized. It is revealed that phosphine in place of halide results in lengthening of the CuP bond from 2.21 A of 1 and 2.19 A of 2 to 2.27 A of 3. Complexes 1-3 display the weak low-energy absorption bands in the 350-600 nm region in dichloromethane, which are assigned to the Cu(I) to cbpy metal-to-ligand charge-transfer (MLCT) transitions, probably mixed with some halide-to-ligand charge-transfer (XLCT) character for 1 and 2. Complexes 1-3 show the solid-state emissions at ambient temperature, varying with the second auxiliary ligand coordinated to the {Cu(cbpy)(PPh ₃)} motif, which most likely originate from the MLCT excited states, perhaps involving some XLCT character for 1 and 2.

Full text of DOI:10.1016/j.inoche.2011.09.040

Inorganic Chemistry Communications 15 (2012) 65–68
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Luminescent mononuclear copper(I) heteroleptic complexes with
6-cyano-2,2′-bipyridine
a
a
a
a
a
a
Jing-Lin Chen a,b, , Xing-Fu Cao , Wei Gu , Bo-Tao Su , Feng Zhang , He-Rui Wen , Ruijin Hong
⁎
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 20 July 2011
Accepted 23 September 2011
Available online 4 October 2011
A new series of luminescent mononuclear Cu(I) heteroleptic complexes of 6-cyano-2,2′-bipyridine (cbpy),
[Cu(cbpy)(PPh3)X] (X=I (1); Br (2)) and [Cu(cbpy)(PPh3)2](ClO4) (3), has been synthesized and character-
ized. It is revealed that phosphine in place of halide results in lengthening of the Cu\P bond from 2.21 Å of 1
and 2.19 Å of 2 to 2.27 Å of 3. Complexes 1–3 display the weak low-energy absorption bands in the 350–
600 nm region in dichloromethane, which are assigned to the Cu(I) to cbpy metal-to-ligand charge-transfer
(MLCT) transitions, probably mixed with some halide-to-ligand charge-transfer (XLCT) character for 1 and 2.
Complexes 1–3 show the solid-state emissions at ambient temperature, varying with the second auxiliary li-
gand coordinated to the {Cu(cbpy)(PPh3)} motif, which most likely originate from the MLCT excited states,
perhaps involving some XLCT character for 1 and 2.
Keywords:
Copper(I)
Mixed-ligand
Photoluminescence
6-Cyano-2,2′-bipyridine
© 2011 Elsevier B.V. All rights reserved.
Copper(I) complexes are of considerable interest with respect to
their intriguing photophysical properties and possible utilization in
solar energy conversion, luminescence-based sensors, electrolumi-
nescence devices, and probes of biological systems [1–5]. The appro-
priate selection of N-heterocyclic chelating ligands is one key point to
tune and improve the luminescence properties of cuprous complexes,
which can be highly affected via the steric, electronic, and conforma-
tional effects of the coordinated chelates [6–8]. Additionally, the auxilia-
ry ligands such as halide and phosphine also have great effects on the
emissive properties of Cu(I) species [9–16].
Recently, we have initiated the investigation of photoactive cop-
per(I) complexes bearing multi-site N-heterocyclic chelate bridging
ligands [15–17]. It is shown that both N-heterocyclic chelate and
the auxiliary ligand such as halide and phosphine have a significant
impact on the construction and luminescence properties of the Cu
(I) systems. Moreover, to the best of our knowledge, metal complexes
of 6-cyano-2,2′-bipyridine (cbpy) have not been reported hitherto.
Herein, we report the syntheses, crystal structures, and photophysical
properties of a new series of mononuclear Cu(I) heteroleptic com-
plexes with cbpy, namely, [Cu(cbpy)(PPh3)X] (X=I (1); Br (2)) and
[Cu(cbpy)(PPh3)2](ClO4) (3). It is demonstrated that complexes 1–3
are all emissive in the solid state at ambient temperature, and the second
ancillary ligands introduced to the {Cu(cbpy)(PPh3)} motif exert signifi-
cant influences on the molecular structures and photophysical proper-
ties of the corresponding copper(I) complexes.
Mononuclear Cu(I) halide complexes of cbpy formulated as [Cu
(cbpy)(PPh3)X] (X=I (1); Br (2)) (Scheme 1) were facilely synthe-
sized via treatment of cuprous halide with two equivalents of PPh3
in dichloromethane, followed by addition of one equivalent of cbpy
[18]. In the above reactions, the non-isolated intermediates are actu-
ally a binuclear copper(I) compound [(PPh3)2Cu(μ-X)2Cu(PPh3)]
(X=I; Br) as confirmed by X-ray structural analysis [16], which are
considerably air-stable and highly soluble in dichloromethane. In ad-
dition, monomeric Cu(I) complex possessing PPh3 and cbpy mixed li-
gands, [Cu(cbpy)(PPh3)2](ClO4) (3) [18], which has no halide ligand,
was afforded by treatment of Cu(ClO4)2·6H2O with four equivalents of
PPh3, where PPh3 served as an unidentate ligand and as a reducing re-
agent of Cu(II), followed by addition of one equivalent of cbpy.
X-ray crystallographic studies [19] of the halide complexes 1 and 2
reveal that each four-coordinated Cu(I) atom is bound to two nitro-
gen atoms of cbpy, one PPh3, and one halide to generate a distorted
tetrahedral configuration because of the small bite angle of cbpy
(Fig. 1a and b). The dihedral angles between the N\Cu\N plane
and the P\Cu\X plane are around 89.6° and 86.9° for 1 and 2, re-
spectively. The Cu\N (av. 2.1015 Å) and Cu\P (2.2079 Å) distances
in
1 are slightly longer than the corresponding bond lengths
(2.0945 and 2.1913 Å) of 2, which are related to the ligand field
strengths of the halide ligands (I− bBr−) [20]. The Cu\N, Cu\P,
and Cu\X distances in 1 and 2 are comparable to those observed in
other tetrahedral Cu(I) complexes with halide and PPh3 mixed li-
gands [21–26]. The N\Cu\N and P\Cu\X bond angles of 1 (78.39
(14)° and 119.79(4)°) are largely identical with the corresponding
values of 2 (78.26(11)° and 118.87(4)°), while the N\Cu\P and
N\Cu\X angles of 1 and 2 have a small variation of 4–8°. It is notable
that the cbpy ligands of two adjacent enantiomeric molecules in 1
⁎
Corresponding author. 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.2011.09.040

66
J.-L. Chen et al. / Inorganic Chemistry Communications 15 (2012) 65–68
2.1065 Å) of 3 are slightly longer than those observed in 1 and 2,
Ph3P
Ph3P
X
X
CH2Cl2
whereas the Cu–P distances (av. 2.2711 Å) of 3 are much longer
than those of 1 (2.2079(12) Å) and 2 (2.1913(12) Å) due to introduc-
tion of the second π-acceptor ligand PPh3. The N\Cu\N bond angle
(79.10(19)°) in 3 is comparable to those of 1 and 2, whereas the
P\Cu\P angle (122.26(5)°) of 3 is slightly larger than the P\Cu\X
angles of 1 and 2, perhaps owing to the large steric hindrance be-
tween intramolecular two PPh3 ligands. Different to the stacking
manners of two neighboring cbpy ligands in 1 and 2, the π–π stacking
of 3 only occurs between the pyridyl rings of two adjacent cbpy li-
gands (Fig. 2b). The interplanar distance of 3 is approximately
3.48 Å, suggesting that a weak π–π interaction is likely operating be-
tween intermolecular two cbpy ligands.
CuX + 2 PPh3
Cu
Cu PPh3
PPh3
N
N
cbpy
Cu
N
CH2Cl2
X
(1)
(2)
X = I
Br
PPh3
PPh3
N
CH2Cl2
Cu
N
Cu(ClO4)2 + 4 PPh3 + cbpy
(3)
The UV–vis absorption spectra of cbpy and its complexes 1–3 in
dichloromethane at ambient temperature are shown in Fig. 3. The
free ligand cbpy shows two absorption bands at λmax ≈242 and
282 nm, attributable to the 1ππ* transitions inside cbpy. The Cu(I) ha-
N
(ClO4)
lide complexes
1 and 2 exhibit several strong absorptions
Scheme 1. Synthetic route of the Cu(I) cbpy complexes 1–3.
(εN104 M−1 cm−1) in the range of 230–350 nm, most likely coming
from cbpy and PPh3. Further support is given by a close matching of
the absorption spectra in high-energy region (≤350 nm) of cbpy
and its halide complexes 1 and 2. In addition to the high-energy ab-
sorptions, complexes 1 and 2 have a comparatively weak low-energy
absorption band (εb750 M−1 cm−1) with a maximum at around 422
and 428 nm, respectively, which is assigned to a metal-to-ligand
and 2 are essentially parallel and display a favorable pairwise π–π
stacking (Fig. 1c and d). The interplanar separations are approximate-
ly 3.48 and 3.62 Å for 1 and 2, respectively, indicating the existence of
a weak π–π interaction between two neighboring cbpy ligands [27].
To explore the effect of PPh3 in place of halide on the metric pa-
rameters of molecular structures, X-ray structural determination of
PPh3 substituted 3 was also carried out [19]. As depicted in Fig. 2a,
the Cu(I) atom is in a distorted tetrahedral geometry formed by two
nitrogen atoms of cbpy and two P donor atoms from PPh3 ligands.
The dihedral angle between the N\Cu\N plane and the P\Cu\P
plane is about 81.0° in 3, which is obviously smaller than those of
the halide complexes 1 and 2, probably owing to replacement of ha-
lide via highly encumbered PPh3. The Cu\N distances (av.
charge-transfer (MLCT) transition from the dπ orbital of the (3d10
)
Cu center to the unoccupied π* orbital of the cbpy ligand, probably
mixed with some halide-to-ligand charge-transfer (XLCT) character
[1,3,12–15,26]. Accordingly, for complex 3, besides the high-energy
absorption bands (≤350 nm) from the LC 1π–π* transitions, a weak
low-energy absorption (ε≈2440 M−1 cm−1) centered at 391 nm is
displayed and identified as the Cu(I) to cbpy MLCT transition [3,6–
11,15,17], which is notably blue-shifted relative to those of 1 and 2.
Fig. 1. ORTEP drawings (a and b) and pairwise stacking views (c and d) of 1 (top) and 2 (bottom). Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles
(°): for 1, Cu1\N1 2.090(3), Cu1\N2 2.113(3), Cu1\P1 2.2079(12), Cu1\I1 2.5828(6), N1\Cu1\N2 78.39(14), N1\Cu1\P1 117.06(10), N1\Cu1\I1 109.12(9), N2\Cu1\P1
116.87(10), N2\Cu1\I1 108.21(10), P1\Cu1\I1 119.79(4); for 2, Cu1\N1 2.082(3), Cu1\N2 2.107(3), Cu1\P1 2.1913(12), Cu1\Br1 2.4228(9), N1\Cu1\N2 78.26(11),
N1\Cu1\P1 123.12(8), N1\Cu1\Br1 105.69(8), N2\Cu1\P1 122.25(8), N2\Cu1\Br1 100.31(8), P1\Cu1\Br1 118.87(4).

J.-L. Chen et al. / Inorganic Chemistry Communications 15 (2012) 65–68
67
1.0
1
2
3
0.8
0.6
0.4
0.2
0.0
500
550
600
650
700
750
Wavelength (nm)
Fig. 4. Emission spectra of 1 ( ), 2 (●), and 3 ( ) in the solid state at ambient
temperature.
complexes 1 and 2 show a broad solid-state emission peaking at 594 and
613 nm at ambient temperature, respectively. With reference to previous
work [1,3,12–14,21,26], the highest occupied molecular orbitals
(HOMO) of the halide complexes 1 and 2 are believed to most likely
spread over the Cu(I) ion and the halide, probably including some con-
tributions from the phosphine, while their lowest unoccupied molecu-
lar orbitals (LUMO) are thought to be basically localized on the cbpy
ligand. Hence, the emissive excited states of the halide complexes 1
and 2 are perhaps best regarded as the metal-to-ligand charge-transfer
(MLCT) excited states with some mixing of the halide-to-ligand charge-
transfer (XLCT) character. The solid-state emission (λmax 594 nm) of 2
exhibits a small blue-shifting of approximately 19 nm compared with
1 (λmax 613 nm). Such a blue-shifting can be rationalized via the replace-
ment of the iodide by a weaker electron-donating bromide, lowering the
HOMO energy level, less influencing the LUMO energy, and thus result-
ing in an increase of the HOMO–LUMO energy gap. Complex 3, which
has no halide ligand, exhibits a broad solid-state emission maximum at
563 nm, displaying a blue-shift of ca. 50 nm with respect to that of 1,
which may be due to the substitution of the iodide via a good π-acceptor
ligand PPh3, decreasing the HOMO level, and hence leading to a lager
HOMO–LUMO gap. In addition, the mode of the π–π stacking between
intermolecular cbpy ligands (Figs. 1c and 2b) perhaps plays a positive
role in increasing the blue-shift of the emission of 3 relative to 1. For 3,
the LUMO is still believed to be mainly localized on cbpy, such that the
introduction of the second PPh3 would be anticipated to have little effect
on the LUMO energy. On the other hand, its HOMO is largely centered on
the Cu(I) center, probably involving some contributions from PPh3. Thus
the emissive excited state of 3 is reasonably assigned to an MLCT excited
state [3,6–11,17].
Fig. 2. ORTEP drawing and pairwise stacking view of the cation of 3. Hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and angles (°): Cu1\N1 2.111(4),
Cu1\N2 2.102(4), Cu1\P1 2.2683(13), Cu1\P2 2.2739(13), N1\Cu1\N2 79.10
(19), N1\Cu1\P1 114.33(12), N1\Cu1\P2 104.30(12), N2\Cu1\P1 108.23(12),
N2\Cu1\P2 120.29(11), P1\Cu1\P2 122.26(5).
The blue-shift can be rationalized in views of the lowering of the
HOMO energy upon replacement of the halide of 1 and 2 with a
good π-acceptor ligand PPh3, thus effectively increasing the HOMO–
LUMO energy gap.
The solid-state emissions of cbpy and its complexes 1–3 are investi-
gated at ambient temperature (Fig. 4). The free ligand cbpy emits a pur-
ple color with λmax=366 nm in the solid state, attributed to the
fluorescent emission from ligand-centered π→π* transition. The halide
0.4
0.025
1
2
0.020
3
cbpy
0.3
In summary, a new series of three luminescent mononuclear copper
(I) heteroleptic complexes with 6-cyano-2,2′-bipyridine (cbpy) has
been synthesized and characterized. It is demonstrated that the second
auxiliary ligand introduced, such as halide and phosphine, has a consid-
erable effect on both the molecular structures and the photophysical
properties of Cu(I) cbpy complexes. Further investigation on mononu-
clear copper(I) heteroleptic complexes with 6-monosubstituted 2,2′-
bipyridine is underway.
0.015
0.010
0.2
0.005
0.000
300
350
400
450
500
550
600
0.1
0.0
Acknowledgments
The authors thank the National Natural Science Foundation of
China (Nos. 21163009, 21001057), the Key Project of Chinese Minis-
try of Education (No. 211088), the Natural Science Foundations of
Jiangxi Province (No. 2009GQH0038) and Fujian Province (No.
2008F3117), the Science Foundation of Education Bureau of Jiangxi
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. UV–vis absorption spectra of cbpy (□), 1 ( ), 2 ( ), and 3 ( ) in diluted
dichloromethane solution at ambient temperature.

68
J.-L. Chen et al. / Inorganic Chemistry Communications 15 (2012) 65–68
[17] J.-L. Chen, W. Gu, X.-F. Cao, H.-R. Wen, R. Hong, Luminescent triply-bridged dicop-
per(I) complex possessing 3,5-bis{6-(2,2′-dipyridyl)}pyrazole as a bis-chelating
ligand, Journal of Coordination Chemistry 64 (2011) 1903–1913.
Province (No. GJJ10151), and the Science Foundation of State Key
Laboratory of Structural Chemistry (No. 20110015) for the financial
support.
[18] Synthesis. For 1, a mixture of PPh3 (129.0 mg, 0.492 mmol) and CuI (46.8 mg,
0.246 mmol) in CH2Cl2 (20 mL) was stirred for 3 h at room temperature. To this
resulting colorless solution was added dropwise with stirring a solution of cbpy
(44.6 mg, 0.246 mmol) in CH2Cl2 (5 mL). The mixture was stirred for another
2 h, and the solvent was evaporated at reduced pressure to give an orange resi-
due. Pure orange crystals were obtained by slow diffusion of petroleum ether
into a CH2Cl2 solution. Yield: 96.7 mg, 0.152 mmol, 62%. Anal. Calcd. for C29H22-
CuIN3P: C, 55.0; H, 3.5; N, 6.6. Found: C, 55.2; H, 3.8; N, 6.4%. For 2, an analogous
procedure to 1 was adopted except that CuI was displaced with CuBr. The orange-
yellow crystals were afforded by slow diffusion of petroleum ether into a CH2Cl2
solution. Yield: 58%. Anal. Calcd. for C29H22BrCuN3P: C, 59.4; H, 3.8; N, 7.2. Found:
C, 59.6; H, 4.1; N, 7.0%. For 3, a mixture of PPh3 (117.6 mg, 0.448 mmol) and Cu
(ClO4)2·6H2O (41.6 mg, 0.112 mmol) in CH2Cl2 (10 mL) was stirred for 5 h at
room temperature. To this resultant colorless solution was added dropwise
with stirring a solution of cbpy (20.3 mg, 0.112 mmol) in CH2Cl2 (5 mL). The mix-
ture was stirred for another 3 h, and the solvent was evaporated at reduced pres-
Appendix A. Supplementary material
CCDC 831454, 831453, and 831452 contain the supplementary
crystallographic data for 1–3, respectively. 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.2011.09.040.
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