4-[(E)-2-phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine and its dichlorocadmium(II) complex: Synthesis, luminescence, and supramolecular network

Article abstract of DOI:10.1002/zaac.201000105

A novel dipyrazinylpyridine ligand, 4-[(E)-2-phenylethenyl]-2,6-bis(2- pyrazinyl)pyridine (L) and its cadmium complex [CdLCl₂] were synthesized and characterized. Structural analysis revealed that there are extensive C-H???N and C-H???π unconventional hydrogen bonds and π-π interactions in the crystal structure of L, whereas in [CdLCl₂] rich C-H???Cl hydrogen bonding was the exclusive force in assembling the molecules and led to formation of a new 3,3,4,6-nodal topological network. Spectral analysis suggested that the absorption and luminescence behavior of [CdLCl₂] is ascribed to L based intraligand transitions. Coordination to Cd^II did not change the electronic structure of L but significantly enhanced the π* → n emission.

Full text of DOI:10.1002/zaac.201000105

ARTICLE
DOI: 10.1002/zaac.201000105
4-[(E)-2-Phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine and its
Dichlorocadmium(II) Complex: Synthesis, Luminescence, and
Supramolecular Network
Jing-Wei Dai,[a] Zhao-Yang Li,[a] Ying-Lin Chen,[a] Bin Cai,[a] Jian-Zhong Wu,*[a] and
Ying Yu[a]
Keywords: Hydrogen bonds; π interactions; Luminescence; Electronic absorption; Dipyrazinylpyridine
Abstract. A novel dipyrazinylpyridine ligand, 4-[(E)-2-phenyle- sive force in assembling the molecules and led to formation of a new
thenyl]-2,6-bis(2-pyrazinyl)pyridine (L) and its cadmium complex 3,3,4,6-nodal topological network. Spectral analysis suggested that the
[CdLCl2] were synthesized and characterized. Structural analysis re- absorption and luminescence behavior of [CdLCl2] is ascribed to L
vealed that there are extensive C–H···N and C–H···π unconventional based intraligand transitions. Coordination to CdII did not change the
hydrogen bonds and π–π interactions in the crystal structure of L, electronic structure of L but significantly enhanced the π* → n emis-
whereas in [CdLCl2] rich C–H···Cl hydrogen bonding was the exclu- sion.
which are isoelectronic to the terpyridines, are very scarcely
noticed, appeared in only five papers up to now [13–17]. Most
Introduction
Noncovalent interactions, such as hydrogen bonding and π–
π interactions are the structural basis for supramolecular self-
assembly and functionally importance of many chemical, bio-
logical, and material systems [1–7]. The molecules with delo-
calized π systems are possible to utilize the π–π interaction as
a factor for stabilization. Strong conventional hydrogen bonds
(X–H···Y) with both donor X and acceptor Y being highly elec-
tronegative atoms have been extensively studied and well
known even to beginner chemistry students. Unconventional
hydrogen bonds using weakly acidic C–H sites as proton donor,
such as C–H···O, C–H···N, C–H···Cl, and C–H···π interactions,
are relatively less focused. For a coordination compound, the
molecular structure and potential supramolecular network is
largely dependent on the ligand. Ligands with additional hydro-
gen donor or acceptor sites or extended π functionality may
direct the organization of the molecular packing. With this gen-
eral principle in mind, many interesting organic ligands could
be designed in order to obtain diverse architectures. Much more
work is required to extend our knowledge in this issue and
establish proper synthetic strategies that lead to the desired spe-
cies with predictable structures and properties.
recently we prepared two coordination compounds with one of
this family of ligands, 4-tolyl-2,6-bis(2-pyrazinyl)pyridine, and
found a novel N···π interaction involved with the two uncoordi-
nated pyrzinyl nitrogen atoms participating in the supramolec-
ular embrace interlocking motifs [18].
Herein we report a new dipyrazinylpyridine compound with
more extended π system, 4-[(E)-2-phenylethenyl]-2,6-bis(2-
pyrazinyl)pyridine (L) and its cadmium complex, CdLCl2
(Scheme 1) in order to explore the influence of the substituent
at the terminal position of the middle pyridine ring on the
structures. Moreover, the molecular electronic spectra of the
ligand and the complex are also investigated and compared.
Their electronic absorption and luminescence spectrum show
us the distinct enhancement of the π* → n emission in cad-
mium complex. This is different from other cadmium com-
plexes based on similar terpyridine ligands [19].
Terpyridine compounds are among the most extensively
studied tridentate ligands for their prominent photophysical,
photochemical, and electrochemical properties [8–12]. They
are also frequently used as building blocks in supramolecular
chemistry. However, the dipyrazinylpyridine compounds,
Scheme 1. Synthetic routes and the numbering scheme of L for NMR
assignment.
Results and Discussion
Synthesis
* Prof. Dr. J.-Z. Wu
E-Mail: wujzh@scnu.edu.cn
[a] School of Chemistry and Environment
South China Normal University
Guangzhou, Guangdong 510006, P. R. China
The new compound 4-[(E)-2-phenylethenyl]-2,6-bis(2-pyr-
azinyl)pyridine (L) was successfully obtained using 2-acetyl-
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J.-Z. Wu et al.
ARTICLE
pyrazine and cinnamaldehyde as starting materials in a one- 2.0(4)°, whereas that of C14–C15–C16–C21 is 17.5(4)°, which
pot reaction. This method is similar to the Kröhnke two-step implies that the conjugating system expands strongly to the
pathway frequently used for preparation of derivatives of C14–C15 double bond, but some weakly to the phenyl ring.
2,2':6',2''-terpyridine [8], but the reaction intermediate, which The phenyl plane twists from the pyridine plane by 21.64(12)°.
should be (E)-1,5-bis(pyrazin-2-yl)-3-styrylpentane-1,5-dione,
The N3 and N4 atoms in the two pyrazine moieties partici-
was not necessary to separate here. It is noteworthy that the pate in intermolecular C–H···N hydrogen bonds, as shown in
coupling constant of the protons of the vinyl double bond (CH1 Figure 2a and Table 1. The relevant H···N distances are 2.64
and CH2) is quite large (18.6 Hz), indicating these two protons and 2.67 Å, respectively, shorter than the sum of van der Waals
are in trans-geometric positions, which is further confirmed by radii of hydrogen and nitrogen atoms of 2.75 Å [25]. Such
its crystal structure vide infra. This is in agreement with the unconventional C–H···N hydrogen bonds were similarly ob-
finding that the coupling constant between trans protons served in several terpyridine compounds [26, 27]. Herein the
(range: 12–18 Hz) is always greater than that between cis pro- C–H···N hydrogen bonds lead two arrays of L molecules to
tons (< 12 Hz) in olefinic systems [19].
bind together and form an infinite planar tape-like structure
The complex [CdLCl2] was obtained by the hydrothermal with nano-sized width of about 2.5 nm. The tape overlap offset
reaction of L and CdCl2 with a high yield. The hydrothermal with adjacent parallel tapes so that the pyrazine and pyridine
method has recently been widely used to prepare coordination rings are able to interact through π–π interactions [28, 29].
polymers [21–24]. Although the two pyrazine rings in L are The N2/N3-contained pyrazine ring stacks with the N4/N5-
possible to act as metal ion bridges, and the chloride ion is a contained pyrazine ring or the pyridine ring in adjacent tapes
common bridging ligand, the complex we got is mononuclear. with the ring centroid distances being 3.73(1) and 3.62(1) Å,
In a hydrothermal reaction, water molecules are easily intro- respectively, as shown in Figure 2b and c. The π–π separation
duced into the crystal structure as a binding ligand or lattice parameters are very close to those reported for a phenyl-perflu-
component. However, there is no water in the crystal structure orophenyl stacking system [30]. The terminal phenyl ring does
of the product.
not participate in the π–π interaction. Instead it uses one of
the vinyl C–H groups to form a weak intermolecular C–H···π
unconventional hydrogen bond [31, 32] with the phenyl ring
in a neighboring but herringbone oriented “tape” (Table 1).
Molecular and Crystal Structure of L
Figure 1 shows the molecule structure of L. Steric hindrance
enforces that both pyrazine rings adopt a conformation trans
to the central pyridine moiety, i.e. either the pyrazine N3 or
N5 atom is trans to the pyridine N1 atom. The pyridine and
two pyrazine rings are strongly conjugated, since they are al-
most coplanar. The angles between the least-squares planes of
pyridine and N1- or N3-containing pyrazine rings are 1.14(11)°
and 6.16(12)°, respectively. The terminal phenyl and the cen-
tral pyridine rings are reasonably in E configuration positions
about the vinyl bond. The torsion angle of C9–C8–C14–C15 is
Figure 2. (a) Assembling of L molecules to a 2.5 nm wide tape via
Figure 1. ORTEP drawing of L with thermal parameters at a level of C–H···N interactions (dashed lines). (b) Packing of parallel tapes by
30 % probability.
π–π interactions. (c) π–π interaction details.
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Z. Anorg. Allg. Chem. 2010, 2475–2480

4-[(E)-2-Phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine Cadmium(II) Complex
Table 1. Hydrogen bonding geometric data /Å,° for L and [CdLCl2].
D–H···A
H···A
D···A
D–H···A
Symmetry code for A
L
C1–H1···N4
2.67
2.64
3.28
3.536(4)
3.447(3)
4.093(2)
154.9
146.0
147.0
0.5–x, 2.5–y, –z
–0.5+x, –0.5+y, z
1–x, y, 0.5–z
C12–H12···N3
C14–H14···Cg (C16–C21)
[CdLCl2]
C20–H20···Cl2
C13–H13···Cl2
C18–H18···Cl1
C15–H15···Cl1
C6–H6···Cl1
C3–H3···Cl1
C2–H2···Cl1
2.90
2.78
2.88
2.83
2.94
2.84
2.78
3.798(4)
3.546(3)
3.655(3)
3.725(3)
3.864(2)
3.734(3)
3.681(3)
161.5
140.8
141.2
162.5
174.8
161.5
163.2
x–1, –y+3/2, z–1/2
–x+1, –y+3, –z+2
–x, –y+1, –z+2
x, y–1, z
x, y–1, z
x, y–1, z
–x+1, y–1/2, –z+5/2
Molecular and Crystal Structure of [CdLCl2]
hydrogen bonding. The supramolecular embrace interlocking
motif attended by N···π interactions involving with the two
uncoordinated pyrzinyl nitrogen atoms in two complexes of
another dipyrazinyl ligand [4-tolyl-2,6-bis(2-pyrazinyl)pyri-
dine is also absent] [18]. Instead, the existence of chloride li-
gands leads to extensive intermolecular C–H···Cl hydrogen
bonding interactions between the mononuclear units. Cl1 and
Cl2 atoms act as acceptor of five and two intermolecular C–
H···Cl interactions, respectively, as shown in Figure 4a and Ta-
ble 1. The H···Cl distances fall slightly below the sum of van
der Waals radii (2.95 Å) and the C–H···Cl angles are fairly
linear, consistent with criteria for C–H···Cl interactions [34,
35]. Atoms H3, H6, and H15 from the same ligand L surround
atom Cl1 in a manner reminiscent of chelation. The Cd–Cl
coordination bond provides both charge assistance and direc-
tionality for strengthening the C–H···Cl interactions, similar to
other C–H···Cl cases [36, 37].
The molecular structure of monomeric [CdLCl2] is shown in
Figure 3. The CdII atom is pentacoordinate by a tridentate li-
gand L and two chloride ligands. The coordination arrange-
ment is between square pyramid and trigonal bipyramid. The
Cd–N bond lengths are 2.373(1) and 2.394(2) Å for the pyra-
zine nitrogen donors, and shorter as 2.305(1) Å for the pyri-
dine nitrogen atom. The Cd–Cl distances are 2.403(1) and
2.444(1) Å for the “bottom” chlorine atom (Cl2) and “apex”
chlorine atom (Cl1), respectively. These parameters are close
to those of previously reported complex dichloro(2,2':6',2''-ter-
pyridine)cadmium(II), in which the CdII atom has similar coor-
dination arrangement [33].
The C–H···Cl interactions connect the mononuclear units to
form a three-dimensional supramolecular network (Figure 4a).
The network topology is analyzed by TOPOS software [38].
The CdII atom is connected to one L molecule and two chlorine
atoms and thus could be regarded as a 3-connecting node. Sim-
ilarly, atoms Cl2 and Cl1 and molecule L could be treated as
3-, 4-, and 6-connecting nodes, respectively. Thus the network
could be simplified as shown in Figure 4b. This is a new
3,3,4,6-nodal net with Schäfli symbol {4·52·63·72·86·10}-
{4·52·63}{5·62}{62·8} [37].
Figure 3. ORTEP diagram showing the molecular structure of
[CdLCl2] with thermal ellipsoids at the 50 % probability level and the
atom-labeling scheme.
Electronic Absorption and Luminescence
The structure of coordinated L in [CdLCl2] is some different
For purpose of comparison, the electronic absorption and lu-
from that of uncoordinated L. Owing to the restrict of the minescence properties of both L and [CdLCl2] in solution
CdN3Cl2 coordination sphere, the dihedral angles between the (DMF/MeCN, 1/99 v/v) were investigated at room tempera-
central pyridine ring and the two side pyrazine rings are in- ture. As shown in Figure 5a, the absorption spectra of free L
creased to 4.64(14) and 11.65(14)°. But the styryl moiety is and [CdLCl2] are very similar, both having a broad UV band
more coplanar with the attached pyridine ring, as the torsion with the maximum at 289 nm and a shoulder at ca. 313 nm.
angles C6–C7–C14–C15 and C14–C15–C16–C21 decrease to The difference is that the absorbance of the latter is larger, as
1.1(5) and –3.1(5)°, respectively.
the molar absorption coefficients of L and [CdLCl2] at 289 nm
However, although the styryl moiety extends the π system, are 3.0 × 104 and 3.9 × 104 L·mol–1·cm–1, respectively. This
it does not induce π–π interaction in the crystal structure of implies that the absorption of [CdLCl2] is ligand centered. Co-
[CdLCl2]. There are also no remarkable C–H···N or C–H···π ordination to CdII does not alter the electronic structure of L,
Z. Anorg. Allg. Chem. 2010, 2475–2480
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J.-Z. Wu et al.
ARTICLE
Figure 4. (a) Supramolecular network of [CdLCl2] by intermolecular C–H···Cl interactions (dashed lines). (b) Topological representation of the
3,3,4,6-nodal net of [CdLCl2] with Cd, L and Cl as nodes.
but the photon absorption becomes more efficient than the free
ligand.
The difference in the photoluminescence is more obvious be-
tween L and [CdLCl2], as can be seen in Figure 5b. At first
glance it appears that the fluorescence peaks shift red on going
from L to [CdLCl2], like some similar CdII terpyridine com-
plexes [19], but careful inspection reveals that in fact the peaks
at 456 and 435 nm in the luminescence spectrum of [CdLCl2]
also emerge in that of L as shoulder peaks. The peaks at 408
and 423 nm for L also exist without positional shift for
[CdLCl2] as a peak and a shoulder peak, respectively. So the
excited state structure in [CdLCl2] is essentially the same as in
L. Their emission originates from relaxation of excited states
of L, π* → π and π* → n for shorter and longer wavelengths,
respectively. The remarkable emission enhancement in the low
energy range (approximately from 420 to 550 nm) for
[CdLCl2] compared to L is apparently not ascribable to the
absorption difference between these two compounds, implying
that π* → n relaxation is much stronger in [CdLCl2] than in
L. This conclusion could be explained by the fluorescence
quenching mechanism of aza-aromatic compounds previously
proposed, i.e. the quenching occurs via the photo-induced elec-
tron transfer (PET) from the lone electron pairs on aromatic
nitrogen atoms to solvent, and such PET process is inhibited
when the nitrogen atoms are coordinated to metal ions [39–
41]. So the nitrogen relevant π* → n based fluorescence is
much weaker in L than in [CdLCl2].
Conclusions
A new compound, 4-[(E)-2-phenylethenyl]-2,6-bis(2-pyrazi-
Figure 5. Electronic absorption (a) and luminescence (b) spectra of L
and [CdLCl2] in DMF/MeCN solution (1/99, 6.0 × 10–6 mol·L–1) at
nyl)pyridine, belonging to the rarely noticed dipyrazinylpyri-
dine family, and its complex dichloro[4-((E)-2-phenylethenyl)- room temperature.
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4-[(E)-2-Phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine Cadmium(II) Complex
by full-matrix least-squares on F2. All non-hydrogen atoms were re-
fined with anisotropic displacement parameters. All the hydrogen at-
oms were placed in geometrically idealized positions and refined using
a riding model.
2,6-bis(2-pyrazinyl)pyridine-κ3N,N',N'']cadmium(II) have been
successfully synthesized and characterized. Single-crystal anal-
ysis confirms that extensive C–H···N and C–H···π hydrogen
bonds and π–π interactions pack the molecules of 4-[(E)-2-
phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine into an interesting
supramolecular network. In the crystal structure of the com-
plex the C–H···Cl hydrogen bonds are the exclusive intermo-
lecular interactions and link the molecules into a new topologi-
cal net. Molecular absorption and emission studies suggest that
the coordination to CdII does not change the electronic struc-
ture of 4-[(E)-2-phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine
and significantly increases the probable π* → n fluorescence
by inhibiting the deactive relaxation of the excited states via
the nitrogen atoms.
Crystal Data for L: C21H15N5, M = 337.38, monoclinic, space group
C2/c, a = 21.816(3) Å, b = 5.4820(9) Å, c = 29.478(5) Å, α = 90°,
β = 109.121(2)°, γ = 90°, U = 3330.9(9) Å3, Z = 8, Dc = 1.346 Mg·m–3,
μ(Mo-Kα) = 0.084 mm–1, T = 298 K, 8364 reflections collected. Re-
finement of 3108 reflections (243 parameters) with I > 2σ(I) converged
at final R1 = 0.0515 (R1 for all data = 0.1189), wR2 = 0.1101 (wR2 for
all data = 0.1437), GOF = 0.943.
Crystal Data for [CdLCl2]: C40H38N12NiO10, M = 905.53, mono-
clinic, space group P21/c, a = 12.1417(7) Å, b = 8.6910(5) Å, c =
21.6171(10) Å, α = 90°, β = 116.880(2)°, γ = 90°, U = 2034.65(19) Å3,
Z = 4, Dc = 1.700 Mg·m–3, μ(Mo-Kα) = 1.354 mm–1, T = 298 K, 10731
reflections collected. Refinement of 3994 reflections (262 parameters)
with I > 2σ(I) converged at final R1 = 0.0301 (R1 for all data = 0.0352),
wR2 = 0.0685 (wR2 for all data = 0.0710), GOF = 1.022.
Experimental Section
Materials and Instrumentation: All the commercially available
chemicals were of analytically or chemically pure and used without
further purification. 1H NMR spectra were recorded with a Varian
NMR Systems 400 MHz spectrometer; chemical shifts are referenced
with internal TMS (δ = 0 ppm). Infrared (IR) samples were prepared
as KBr pellets, and spectra were recorded with a Perkin–Elmer Spec-
trum One FT-IR spectrophotometer in the range of 4000–400 cm–1.
Electrospray ionization mass spectra (ESI-MS) were obtained on a
Thermo LCQ DECA XP MAX mass spectrometer.
CCDC-757107 for 4-[(E)-2-phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine
and CCDC-757108 for dichloro[4-((E)-2-phenylethenyl)-2,6- bis(2-pyr-
azinyl)pyridine-κ3N,N',N'']cadmium(II) contain the crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2
1EZ, UK; Fax: +44-1223-336033; E-Mail: deposit@ccdc.cam.ac.uk).
Synthesis of 4-[(E)-2-Phenylethenyl]-2,6-bis(2-pyrazinyl)pyridine
(L): 2-Acetylpyrazine (1.0380 g, 8.5 mmol) and cinnamic aldehyde
(0.535 mL, 4.25 mmol) were dissolved in methanol (15 ml). The mix-
ture was stirred for 2 hours at room temperature, after which aqueous
sodium hydroxide (5 ml, 25 %) was added. Then excess aqua ammo-
nia was added slowly. The stirring continued for 12 hours. The result-
ing heterogeneous solution was filtered through a medium porosity
glass frit and the precipitate was washed with water, cold ethanol, and
diethyl ether. The collected solid was recrystalized from chloroform,
yielding highly pure yellow block crystals. Yield 67.2 %, m.p. 229.8–
Acknowledgement
Financial support from the Guangdong Natural Science Foundation
(grant No. 5005935) and the SRFROCS program, State Education
Ministry of China, are gratefully acknowledged.
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Received: February 26, 2010
Published Online: June 22, 2010
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