First coordination polymer of 1,4-dihydro-2,3-quinoxalinedione in ketoamine tautomeric form

Article abstract of DOI:10.1002/zaac.201000304

The first coordination compound of 1,4-dihydro-2,3-quinoxalinedione in ketoamine tautomeric form (denoted as H₂qdione) was reported. H ₂qdione was obtained by a solid-state reaction of o-phenylenediamine and oxalic acid. Reaction of this ligand with CdCl₂ solvothermally yielded a coordination polymer [Cd(H₂qdione)Cl₂] _n, which was structurally characterized by X-ray diffraction and IR spectroscopy. Continuous Cd₂Cl₂ diamonds form a double-sided comb with terminal H₂qdione-κ²O,O' as the comb teeth. Interaction of these combs through very extensive π-π stacking, C-HA...Cl, and N-HA...Cl hydrogen bonds leads to a novel 3D architecture and significant enhancement of solid-state luminescence of about 10 times compared to the free H₂qdione ligand.

Full text of DOI:10.1002/zaac.201000304

ARTICLE
DOI: 10.1002/zaac.201000304
First Coordination Polymer of 1,4-Dihydro-2,3-Quinoxalinedione in
Ketoamine Tautomeric Form
Ling Shi,[a] Bin Cai,[a] Guang Huang,[a] Jian-Zhong Wu,*[a] and Ying Yu[a]
Keywords: Coordination polymers; Cadmium; X-ray diffraction; 1,4-Dihydro-2,3-quinoxalinedione; Tautomers; Luminescence
Abstract. The first coordination compound of 1,4-dihydro-2,3-quinox- copy. Continuous Cd2Cl2 diamonds form a double-sided comb with
alinedione in ketoamine tautomeric form (denoted as H2qdione) was terminal H2qdione-κ2O,O' as the comb teeth. Interaction of these
reported. H2qdione was obtained by a solid-state reaction of o-phenyl- combs through very extensive π–π stacking, C–H···Cl, and N–H···Cl
enediamine and oxalic acid. Reaction of this ligand with CdCl2 solvo- hydrogen bonds leads to a novel 3D architecture and significant en-
thermally yielded a coordination polymer [Cd(H2qdione)Cl2]n, which hancement of solid-state luminescence of about 10 times compared to
was structurally characterized by X-ray diffraction and IR spectros- the free H2qdione ligand.
X-ray diffraction (XRD) analysis revealed that in each case
Introduction
As a special case of structural isomerism, tautomerism plays
an important role in stabilizing biological systems such as pro-
the ligand exists as doubly deprotonated 2,3-quinoxalinediolate
(qdiol2–), in κ2O,O' [11,12] or κ4O,O',N' binding mode.[13,14] The
ketoamine (dione) tautomeric form was only observed in non-
tein and nucleic acid structures.[1–3] The principle of tautomer-
coordinated compounds.[15–19] Herein, we present a new cad-
ism has been widely studied in molecular design.[4–7] Most of
mium coordination polymer with this ligand, which has an in-
the research on tautomerism is focused on pure organic com-
teresting supramolecular structure and with the ligand in neu-
pounds. Recently, utilization of suitable tautomeric organic li-
tral ketoamine tautomeric form.
gands in constructing coordination compounds has attracted
attention for potential application in molecular recognition,
photochromic, photomagnetic and molecular switch
materials.[8–10] As shown in Scheme 1, two common proto-
tropic tautomers are possible for the compound 1,4-dihydro-
2,3-quinoxalinedione (H2qdione). Research on this compound
has been undertaken for quite a long time, but investigation of
its coordination behavior is still very rare.
Results and Discussion
Synthesis and IR Spectra
The compound H2qdione could be obtained by heating a so-
lution of o-phenylenediamine and oxalic acid in hydrochloric
acid under reflux for 8 hours according to literature method.[15]
Another method, in which the ligand was prepared using o-
phenylenediamine and oxalic acid in presence of a relatively
small amount of CuCl2 under hydrothermal conditions, was
also reported.[17] Alternatively, we utilized a solid-state reac-
tion here and obtained the ligand satisfactorily. In the “dry”
preparation process, oxalic acid was used in slight excess and
the excessive oxalic acid could be easily removed at a tempera-
ture higher than its sublimation temperature (157 °C). The
heating time is shortened to one hour and after successive re-
crystallization the yield of the final microcrystals is higher as
91 %. The compound shows a very strong IR band at
1682 cm–1 (Figure 1), which implies that the compound exists
in the ketoamine tautomeric form, in agreement with the previ-
ous XRD results.[15–19]
Scheme 1. Two common prototropic tautomeric forms of 1,4-dihydro-
2,3-quinoxalinedione.
To the best of our knowledge, only three complexes of this
kind were reported before, i.e. two mononuclear complexes of
FeIII[11] and RuII,[12] and a coordination polymer of CuII.[13,14]
Heating a solution of CdCl2·6H2O and H2qdione (1:1) in a
mixture of water and ethanol (5:1 v/v) under solvothermal con-
dition gave crystals of [Cd(H2qdione)Cl2]n directly with some
high yield (71 % based on Cd). The most intense IR band
of this complex centers at 1653 cm–1, red-shifted by 29 cm–1
* 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
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1,4-Dihydro-2,3-Quinoxalinedione in Ketoamine Tautomeric Form
Table 1. Selected bond lengths /Å and angles /° for [Cd(H2qdione)Cl2]n.
Cd–O1
Cd–Cl1ii
Cd–Cl1
O1–C1
N1–C2
C2–C2i
C3–C4
2.3406(19)
2.5620(8)
2.6420(9)
1.231(3)
1.401(4)
1.380(6)
1.379(5)
Cd–O1i
Cd–Cl1iii
Cd–Cl1i
N1–C1
C1–C1i
C2–C3
2.3406(19)
2.5620(8)
2.6420(9)
1.338(3)
1.518(5)
1.401(4)
1.388(7)
C4–C4i
O1–Cd–O1i
O1i–Cd–Cl1ii
O1–Cd–Cl1iii
O1i–Cd–Cl1iii
Cl1ii–Cd–Cl1iii
O1–Cd–Cl1
O1i–Cd–Cl1
Cl1ii–Cd–Cl1
Cl1iii–Cd–Cl1
N1–C1–C1i
C2i–C2–N1
C2i–C2–C3
N1–C2–C3
71.18(9)
94.32(5)
94.32(5)
159.49(6)
103.06(4)
96.30(6)
83.47(6)
96.33(3)
83.85(2)
116.94(14)
118.82(15)
120.41(19)
120.8(3)
O1–Cd–Cl1ii
O1–Cd–Cl1i
O1i–Cd–Cl1i
159.49(6)
83.47(6)
96.30(6)
Cl1ii–Cd–Cl1i 83.85(2)
Cl1iii–Cd–Cl1i 96.33(3)
Cl1–Cd–Cl1i
Cdiii–Cl1–Cd
C1–O1–Cd
C1–N1–C2
O1–C1–N1
O1–C1–C1i
C4–C3–C2
C3–C4–C4i
179.71(3)
96.15(2)
115.06(16)
124.2(2)
123.7(2)
Figure 1. IR spectra of H2qdione (bottom) and [Cd(H2qdione)Cl2]n
(top).
119.31(15)
118.8(3)
120.76(19)
Symmetry transformations used to generate equivalent atoms: (i) 1–x,
y, 1.5–z; (ii) 1–x, 1–y, 2–z; (iii) x, 1–y, –0.5+z.
compared to the free H2qdione ligand, agreeing with the XRD
analysis as will be discussed vide infra.
Molecular and Crystal Structure of [Cd(H2qdione)Cl2]n
that there are two markedly different Cd–Cl bond lengths. The
chlorine atom trans to the oxygen atom of H2qdione is located
much closer to the central cadmium atom than the other chlo-
rine atoms cis to both oxygen atoms of H2qdione [2.562 versus
2.642 Å]. This observation can be ascribed to the much
stronger trans σ influence of electron-withdrawing carbonyl
groups than chlorine. A similar phenomenon was observed in
another chlorine-bridged coordination compound.[20]
Each chlorine atom bridges two cadmium atoms, which leads
to the formation of Cd2Cl2 diamonds. A one-dimensional (1D)
twofold helical chain is thus formed approximately along the
c direction, as illustrated in Figure 4a. The outstretching planar
H2qdione ligands make the 1D structure look like a double-
sided comb with the terminal H2qdione molecules as the comb
teeth. Two neighbored combs in the b direction intercalate,
which is stabilized by very extensive face-to-face π–π stacking
between the parallel diazo “pyrazine” or “piperazine” ring and
benzene ring (inter-centroid distance 3.558 Å and offset angle
21.43°), as well as the C(4)–H···Cl hydrogen bonds (Figure 4b
and Table 2). The six-membered aromatic rings and the chlo-
rine atoms are involved in these interactions, which in conse-
quence lead the 1D helices to form a two-dimensional (2D)
sheet in the bc plane.
High-quality crystals of the complex were obtained. The
asymmetric unit of the complex contains half a cadmium atom,
half an H2qdione ligand, and one chlorine atom. The coordina-
tion around CdII is best described as distorted octahedral, as
shown in Figure 2. The covalent bonding parameters are listed
in Table 1. To compare the complex with other relevant struc-
tures of organic or coordination compounds, some essential
bond lengths are collected in Figure 3. It appears that the two
tautomeric forms have distinct characteristic bond lengths and
the ligand used in this study resembles the ketoamine tauto-
meric form (i.e. H2qdione form) much closer. There are some
other crystallographic criteria to support this point; this ligand
should be neutral considering charge balance and the proton
H(1N) attached on N(1) could be clearly located from the dif-
ference Fourier map during the XRD structure determination.
In addition, the H(1N) atom plays an important structural role
as it forms a strong hydrogen bond (Table 2) and will be dis-
cussed later.
Each of the chlorine atoms is involved not only in an uncon-
ventional C–H···Cl hydrogen bond, but also in a traditional
N–H···Cl hydrogen bond (Table 2), which contributes to the
supramolecular assembly as well. The stronger N–H···Cl hy-
drogen bonds act like pillars to further connect the adjacent
2D sheets and finally to complete the construction of a three-
dimensional (3D) network (Figure 5).
Figure 2. ORTEP view (30 % thermal ellipsoids) of coordination
sphere in [Cd(H2qdione)Cl2]n. Symmetry codes: (i) 1–x, y, 1.5–z; (ii)
1–x, 1–y, 2–z; (iii) x, 1–y, –0.5+z.
In the previously prepared complexes using the H2qdione
ligand as
a
starting material, i.e. [FeIII(qdiol)3]3–,[11]
The Cd–O and Cd–Cl bond lengths are typical for CdII com- [RuII(NO)(Cp)(qdiol)],[12] and [CuII (qdiol)2Cl2(DMF)4]n,[13–14]
3
plexes with oxygen and chlorine donor atoms. It is noteworthy the ligand existed in the deprotonated form qdiol2– and thus no
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L. Shi, B. Cai, G. Huang, J.-Z. Wu, Y. Yu
ARTICLE
Figure 3. Bond lengths of similar structures (comparison). Refcodes and references: GARROG,[11] ZOKWOL,[12] VIXCUB01,[13] WIKSOZ,[21]
GURGAB,[22] HQOXDO,[19] HQOXDO01,[18] HQOXDO02,[17] QERCOF,[16] RAVFEA,[15] and LIVSEP.[23]
Table 2. Hydrogen bonding parameters /Å, ° for [Cd(H2qdione)Cl2]n.
excitation and emission spectra are shown in Figure 7. Both
compounds emit blue fluorescence around 420 nm, which sug-
gests that the broad emission band of [Cd(H2qdione)Cl2]n is
mainly due to the intra-ligand emission states as reported for
other CdII or d10 metal complexes with O donor ligands.[24–26]
It is remarkable that [Cd(H2qdione)Cl2]n emits about 10 times
stronger than the free H2qdione compound. This significant
fluorescence enhancement could be explained by the fact that
the incorporation of CdII effectively increases the conforma-
tional rigidity of H2qdione and reduces the loss of energy by
vibrational motions with a more stable 3D supramolecular net-
work as revealed from the XRD analysis vide supra. Consider-
ing the fact that [Cd(H2qdione)Cl2]n is insoluble in common
solvents such as alcohols, ketones, chloroform, benzene as
well as water, this complex could be a potential photoactive
solid material.
D–H···A
D–H
H···A
D···A
<DHA
N1–H1N···Cl1i
0.86
0.93
2.47
2.80
3.302(3)
3.683(3)
163
158
C4–H4···Cl1ii
Symmetry transformations used to generate equivalent atoms:
(i) –0.5+x, 0.5–y, –0.5+z; (ii) 1–x, –y, 2–z.
supramolecular interactions as observed in [Cd(H2qdione)Cl2]n
in this paper were present.
PXRD Analysis
The as-isolated bulky sample of [Cd(H2qdione)Cl2]n was
characterized by powder X-ray diffraction (PXRD) at room
temperature. As shown in Figure 6, the experimental PXRD
patterns are in good agreement with the simulated diffracto-
grams based on the single crystal data. It could be concluded
that the reaction conditions here afforded a pure phase of
[Cd(H2qdione)Cl2]n.
Conclusions
In this report we successfully prepared 1,4-dihydro-2,3-quin-
oxalinedione (H2qdione) using a solid state reaction not re-
ported before. We obtained a new coordination polymer
Photoluminescence Properties
The solid-state photoluminescent behavior of H2qdione and [Cd(H2qdione)Cl2]n by heating this ligand with CdCl2 under
[Cd(H2qdione)Cl2]n was investigated at room temperature. The solvothermal conditions. XRD analysis reveals that the organic
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1,4-Dihydro-2,3-Quinoxalinedione in Ketoamine Tautomeric Form
Figure 4. (a) Covalent-bonded 1D helical chain of Cd2Cl2 diamonds and the outstretching H2qdione planes form a double-sided comb. (b)
Intercalation of the comb teeth leads to a 2D sheet through face-to-face π–π stacking (dashed lines) and C–H···Cl (dashed lines) hydrogen bonds
in the bc plane.
ligand exists as the ketoamine tautomeric form and coordinates
Experimental Section
to the cadmium atom by the ketone O donors. Very extensive
Materials and Instrumentation: All commercially available chemi-
supramolecular interactions involving π–π stacking, C–H···Cl,
cals were of analytically or chemically pure and used without further
and N–H···Cl hydrogen bonds lead to a novel 3D architecture.
The coordination and supramolecular assembly of H2qdione
significantly enhances its blue fluorescence.
purification. The IR spectra were recorded with a Perkin–Elmer Spec-
trum One FT-IR spectrometer as KBr pellets. The C, H, N elemental
analysis was performed with a Perkin–Elmer 2400 element analyzer.
Z. Anorg. Allg. Chem. 2011, 306–311
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309

L. Shi, B. Cai, G. Huang, J.-Z. Wu, Y. Yu
ARTICLE
Figure 7. Luminescence spectra of H2qdione and [Cd(H2qdione)Cl2]n
in solid state at room temperature. λex = 360 nm, λem = 420 nm.
Preparation of 1,4-Dihydro-2,3-quinoxalinedione (H2qdione): A
mixture of o-phenylenediamine (3.0005 g, 0.03 mol) and oxalic acid
(8.5095 g, 0.051 mol) was ground into powder at room temperature.
Afterwards, the powder was placed in a muffle furnace for 1 h at
200 °C and cooled to room temperature to get a light yellow solid,
which was recrystallized from water, yielding colorless needle-like mi-
Figure 5. 3D network of [Cd(H2qdione)Cl2]n built up by N–H···Cl hy-
drogen bonds (dashed lines) between the 2D sheets in the bc plane.
Hydrogen atoms attached on carbon atoms are omitted for clarity.
crocrystals (91 %). IR (KBr) ν = 3781 (w), 2872 (m), 1682 (vs), 1403
˜
(m), 1392 (m), 836 (s), 753 (s), 524 (m) cm–1. ESI-MS: m/z 163 ([M
+ H]+), 347 ([2M + Na]+). Mp: 115–116 °C (Lit. 116.7 °C[17]).
Synthesis of [Cd(H2qdione)Cl2]n: A solution of H2qdione (0.6 mmol,
0.101 g), CdCl2·6H2O (0.4 mmol, 0.174 g) in water (10 mL) and etha-
nol (2 ml) in a 23 mL Teflon-lined stainless steel reactor was heated
at 140 °C for 72 h. The reaction system was cooled to 100 °C with a
rate of 3 °C·h–1 and to room temperature with a rate of 5 °C·h–1. Block
colorless crystals were collected and washed with distilled water (Yield
71 % based on Cd). C8H6CdCl2N2O2: calcd. C 27.81, H 1.75, N
8.11 %; found: C 27.37, H 1.68, N 7.91 %. IR (KBr) ν = 3386 (w),
˜
3222 (s), 1653 (vs), 1389 (m), 786 (s), 753 (m), 525 (m) cm–1.
Crystal Structure Determination:
A
single crystal of
[Cd(H2qdione)Cl2]n was carefully selected under a microscope and was
glued at the tip of a thin glass fiber with cyanoacrylate adhesive. X-ray
diffraction was performed with a Bruker Apex II CCD diffractometer
equipped with a normal focus at 298 K, sealed tube X-ray source (Mo-
Kα radiation, λ = 0.71073 Å) operating at 50 kV and 30 mA. The crys-
tal structure was solved by direct methods and refined on F2 by full-
matrix least-squares methods using the Apex2 program package.[27]
All non-hydrogen atoms were refined anisotropically. The hydrogen
atoms attached to carbon atoms were placed at the calculated positions,
whereas the hydrogen atoms attached to the nitrogen atoms were lo-
cated from difference maps.
Figure 6. PXRD patterns of simulated (bottom) and as-synthesized
(top) [Cd(H2qdione)Cl2]n at room temperature.
Crystal Data for [Cd(H2qdione)Cl2]n: C8H6CdCl2N2O2, Monoclinic,
space group C2/c, a = 13.079(2), b = 12.793(2), c = 7.0639(11) Å, β =
118.338(6)°, V = 1025.9(3) Å, Z = 4, Dc = 2.237 Mg·m–3, μ(Mo-Kα) =
2.626 mm–1, T = 298 K, Rint = 0.0372. Structure solution and refine-
ment based on 954 independent reflections with I > 2σ(I) and 70 pa-
rameters gave R1 (wR2) = 0.0240 (0.0646) and S = 1.10. Largest differ-
ence peak and hole: 0.574 and –0.657 e·Å–3.
The luminescent spectra in the solid state were recorded at room tem-
perature with a Hitachi F-2500 fluorometer. Electrospray ionization
mass spectra (ESI-MS) were obtained with a Thermo LCQ DECA XP
MAX mass spectrometer. Powder X-ray diffraction (PXRD) patterns
were recorded with a Rigaku D/M-2200T automated diffractometer for
Cu-Kα radiation (λ = 1.54056 Å), with a scan speed of 4°·min–1 and a CCDC-778834
contains
the
crystallographic
data
of
step size of 0.02° in the 2θ range of 5–50°.
[Cd(H2qdione)Cl2]n for this paper. These data can be obtained free
310 www.zaac.wiley-vch.de
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1,4-Dihydro-2,3-Quinoxalinedione in Ketoamine Tautomeric Form
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Received: August 3, 2010
Published Online: December 2, 2010
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