Crystal structures and luminescent properties of two zinc(II) complexes with 2-phenylquinoline-4-carboxylates

Article abstract of DOI:10.1080/15533174.2011.591306

Assembly of 2-phenylquinoline-4-carboxylic acid and Zn(OAC) ₂·2H₂O gives rise to two complexes [Zn ₂(C₁₆H₁₀NO₂)₄ (phen)2(H₂O)₂]·2H₂O (I) and [Zn

Full text of DOI:10.1080/15533174.2011.591306

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:798–804, 2011
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.591306
Crystal Structures and Luminescent Properties of Two
Zinc(II) Complexes with 2-Phenylquinoline-4-carboxylates
Yue Bing, Xing Li, Mei-Qin Zha, and Dong-Jie Wang
The State Key Laboratory Base of Novel Functional Materials and Preparation Science, Faculty of
Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang, P. R. China
carboxylate oxygen donor atoms are fine building motifs for
the construction of metal complexes. 2-Phenylquinoline-4-
carboxylic acid (Hpqc), aside from its significance in biolog-
ical systems,[7,8] also possesses fascinating coordination behav-
iors, such as asymmetric geometry and multiple coordination
sites, which have been widely used to design and synthesize
metal-organic coordination complexes because of the carboxy-
late group and/or pyridine nitrogen atom.[9–15] Zinc is a flexible
transition metal in adopting various coordination geometries
in its complexes. In this article, we report the synthesis and
characterization of compound I and compound II. Compound
I is binuclear structure, and further extends its units to form a
supramolecular framework via intermolecular hydrogen bond-
ing interactions (O–H···O) and π···π interactions. Compound
II has a one-dimensional (1D) chain formed by bridging of
4,4ꢁ-bipyridine.
Assembly of 2-phenylquinoline-4-carboxylic acid and
Zn(OAC)2·2H2O gives rise to two complexes [Zn2(C16H10NO2)4
(phen)2(H2O)2]·2H2O (I) and [Zn2(C16H10NO2)4(bpy)2]n (II)
(phen = 1,10-phenanthroline, bpy = 4,4ꢀ-bipyridine). Complexes I
and II have been characterized by x-ray single-crystal diffraction,
infrared (IR), powder x-ray diffraction (PXRD), thermogravi-
metric analysis (TGA), and fluorescence spectra. Structural
analysis reveals that I is binuclear in structure; two Zn(II) centers
are bridged by two carboxylate groups and the terminal water
molecule occupies a axial position of the octahedral geometry,
which further extends the binuclear unit into a supramolecular
framework via intermolecular hydrogen bonding and π···π
interactions. Complex II possesses a one-dimensional infinite
zigzag chain architecture by bridging of the 4,4ꢀ-bipyridne.
Keywords crystal structures, fluorescence, 2-phenylquinoline-4-
carboxylates, zinc complexes
EXPERIMENTAL
INTRODUCTION
Rapid development of metal-organic frameworks has been Materials and Measurements
made in recent years not only for their potential applica-
tions in materials science but also for fascinating architec-
tures and topologies.[1,2] Hydrogen bonding and π–π stack-
ing interactions are vital to design and construct supramolec-
ular compounds.[3–6] Meanwhile, assistant ligands such as
1,10-phenanthroline and 4,4ꢁ-bipyridine may contribute to the
various architectures of the supramolecular compounds. How
to control the molecular structures and topologies in the pro-
cess of self-assembly has attracted extensive attention. It has
been proved that the rigid ligands containing nitrogen and/or
All commercially available chemicals were of reagent grade
and used as received without further purification. The Fourier-
transform infrared (FT-IR) spectra were recorded from KBr
pellets in the range of 4000–400 cm−1 on a Shimadzu FTIR-
8900 spectrometer. All fluorescence measurements were carried
out on an LS 50B Luminescence Spectrometer (Perkin-Elmer,
Inc., USA). Thermogravimetric analyses (TGA) were carried
out in an air atmosphere with a heating rate of 10◦C/min on a
STA449C integration thermal analyzer. Powder x-ray diffrac-
tion (PXRD) data were collected on a Bruker D8 Focus x-ray
diffractometer using Cu Kα radiation. The calculated PXRD
patterns were produced using the SHELXTL-XPOW program.
Received 30 October 2010; accepted 8 March 2011.
This work was sponsored by the K. C. Wong Magna Fund at
Ningbo University and supported by the National Natural Science
Foundation of China (20971075), the “Qianjiang Talent” Projects of
Zhejiang Province (2009R10032), and the Program for Innovative Re-
search Team of Ningbo Novel Photoelectric Materials and Devices
(2009B21007).
Address correspondence to Xing Li, Faculty of Materials Sci-
ence and Chemical Engineering, Ningbo University, Ningbo, Zhejiang
315211, P. R. China. E-mail: lix905@126.com
Synthesis of the Complex
Preparation of [Zn2(C16H10NO2)4(phen)2(H2O)2]·2H2O (I)
2-Phenylquinoline-4-carboxylic acid (0.0123 g, 0.05 mmol),
Zn(OAc)2·2H2O (0.0210 g, 0.10 mmol), phen (0.0198 g, 0.10
mmol), and KOH (0.0028 g, 0.05 mmol) in H2O solution (10
mL) were placed in a 25-mL stainless-steel reactor fitted with
a Teflon liner and heated to 100◦C for 2 days, then cooled
to room temperature; colorless block-like crystals of (I) were
798

ZINC COMPLEXES WITH 2-PHENYLQUINOLINE-4-CARBOXYLATES
799
TABLE 1
Crystal data and structure refinement for I and II
Complex
(I)
(II)
Empirical formula
Formula weight
C88H64N8O12Zn2
1556.21
C84H56N8O8Zn2
1436.11
Temperature (K)
296(2)
296(2)
Wavelength (Å)
0.71073
0.71073
Crystal system
Space group
Monoclinic
P21/c
Triclinic
P-1
a (Å)
b (Å)
14.8487(17)
13.8206(16)
17.507(2)
90
97.5650(10)
90
12.9829(12)
14.2101(14)
19.9969(19)
91.3570(10)
106.5730(10)
101.9530(10)
3445.8(6)
2
c (Å)
α (◦)
β (◦)
γ (◦)
V (Å3)
3561.4(7)
2
Z
Crystal size (mm)
0.35 × 0.15 × 0.12
0.45 × 0.22 × 0.15
Density (calculated) (mg/m3)
Absorption coefficient (mm−1
F(000)
1.451
1.384
)
0.748
0.763
1608
1480
Theta range for data collection (◦)
Limiting indices
Reflections collected
1.88 to 27.54◦
1.47 to 27.47◦
–19 ≤ h ≤ 19, –17 < = k < = 16, –22 ≤ l ≤ 22
–16 ≤ h ≤ 16, –18 ≤ k ≤ 18, –25 ≤ l ≤ 21
30737
30364
Independent reflections
Completeness to theta
Refinement method
8209 [R(int) = 0.0625]
27.54/99.9%
Full-matrix least-squares on F2
8209 / 0 / 502
15453 [R(int) = 0.0383]
27.47/98.0%
Full-matrix least-squares on F2
15453 / 0 / 916
Data / restraints / parameters
Goodness of fit on Fˆ2
Final R indices [I > 2sigma(I)]
R indices (all data)
1.008
1.012
R1 = 0.0378, wR2 = 0.0974
R1 = 0.0573, wR2 = 0.1090
0.341 and –0.473
R1 = 0.0479, wR2 = 0.1270
R1 = 0.0904, wR2 = 0.1502
0.573 and –0.477
Largest diff. peak and hole (e.Å−3
)
obtained. The crystals were collected and dried in air. Yield: DATA COLLECTION, STRUCTURAL DETERMINATION,
74% (based on Zn). Anal.: Calcd. (%) for C88H64N8O12Zn2: AND REFINEMENT
C, 67.91; H, 4.15; N, 7.20. Found (%): C, 67.88; H, 4.23; N,
Crystal Structure Determination
7.35. IR (KBr, cm−1): 3428(vs), 3060(w), 1746(w), 1624(vs),
1574(vs), 1546(vs), 1519(m), 1425(m), 1389(vs), 1319(m),
841(w), 813(w), 770(vs), 724 (m), 698(m), 651(m).
A suitable crystal with dimensions of 0.35 mm × 0.15 mm ×
0.12 mm for I and 0.45 mm × 0.22 mm × 0.15 mm for II were
carefully selected under a polarizing microscope and glued at
the tip of a thin glass fiber with cyanoacrylate adhesive. Single-
Preparation of [Zn2(C16H10NO2)4(4,4ꢁ-bpy)2]n (II)
crystal determination was performed on a Bruker Apex II
CCD diffractometer equipped with graphite-monochromatized
Mo-Kα radiation (λ = 0.71073 Å) for data collection by using
an ω-scan mode at room temperature. The structures were
solved by a direct method[16] and refined with SHELXTL-97
by full-matrix least-squares technique on F2. All nonhydrogen
atoms were refined anisotropically, and the hydrogen atoms
were included in the final refinement on the calculated positions
bonded to their carrier atoms.[17] The crystallographic data and
other pertinent information of complexes I and II are summa-
rized in Table 1, and the selected bond lengths and bond angles
are given in Table 2. Hydrogen bond lengths and bond angles
are given in Table 3 for I. Crystallographic data for complex
I and II were deposited at the Cambridge Crystallographic
2-Phenylquinoline-4-carboxylic acid (0.0123 g, 0.05 mmol),
Zn(OAC)2·2H2O (0.0105 g, 0.05 mmol), 4,4ꢁ-bpy (0.0098 g,
0.05 mmol), and KOH (0.0028 g, 0.05 mmol) in H2O solution
(10 mL) were placed in a 25-mL stainless reactor fitted with
a Teflon liner and heated to 100◦C for 2 days, then cooled
to room temperature; colorless block-like crystals of (II) were
obtained. The crystals were collected and dried in air. Yield:
67% (based on Zn). Anal.: Calcd. (%) for C84H56N8O8Zn2: C,
70.25; H, 3.93; N, 7.80. Found (%):C, 70.14; H, 3.90; N, 3.83.
IR (KBr, cm−1): 3423(vs), 3059(w), 2925(w), 2362(w), 1620
(vs), 1569(m), 1542(m), 1419(m), 1389(vs), 1317(m), 1221(m),
1112(s), 1024(m), 813(m), 774(s), 699(m), 619(m), 566(w),
511(w).

800
Y. BING ET AL.
TABLE 2
Bond lengths (Å) and bond angles (◦) for I and II
Compound I
Bond lengths
Zn(1)–O(1)#1
Zn(1)–O(4)
Zn(1)–O(2)
2.0348(14)
2.1238(14)
2.1295(14)
Zn(1)–N(3)
Zn(1)–O(3)
Zn(1)–N(2)
2.1541(16)
2.1685(14)
2.1741(16)
Bond angles
O(1)#1–Zn(1)–O(4)
O(1)#1–Zn(1)–O(2)
O(4)–Zn(1)–O(2)
O(1)#1–Zn(1)–N(3)
O(4)–Zn(1)–N(3)
O(2)–Zn(1)–N(3)
O(1)#1–Zn(1)–O(3)
#1 –x + 2, –y + 1, –z
90.88(6)
91.73(5)
90.73(5)
101.51(7)
167.47(6)
90.73(6)
83.71(6)
O(2)–Zn(1)–O(3)
N(3)–Zn(1)–O(3)
O(1)#1–Zn(1)–N(2)
O(4)–Zn(1)–N(2)
O(2)–Zn(1)–N(2)
N(3)–Zn(1)–N(2)
O(3)–Zn(1)–N(2)
173.58(5)
85.78(6)
176.61(6)
90.46(6)
91.36(6)
77.06(6)
93.10(6)
Compound II
Bond lengths
Zn(1)–O(1)
Zn(1)–N(5)
Zn(1)–O(4)
O(6)–Zn(2)
O(8)–Zn(2)
N(8)–Zn(2)#1
1.932(3)
2.056(2)
2.090(3)
2.204(3)
2.271(2)
2.109(2)
Zn(1)–N(7)
Zn(1)–O(3)
O(5)–Zn(2)
O(7)–Zn(2)
N(6)–Zn(2)
Zn(2)–N(8)#2
2.093(2)
2.272(4)
2.160(3)
2.114(2)
2.090(2)
2.109(2)
Bond angles
O(1)–Zn(1)–N(5)
O(1)–Zn(1)–O(4)
N(5)–Zn(1)–O(4)
O(1)–Zn(1)–N(7)
N(5)–Zn(1)–N(7)
O(4)–Zn(1)–N(7)
O(1)–Zn(1)–O(3)
N(5)–Zn(1)–O(3)
O(4)–Zn(1)–O(3)
N(7)–Zn(1)–O(3)
N(6)–Zn(2)–N(8)#2
N(6)–Zn(2)–O(7)
#1 x + 1, y + 1, z
117.07(11)
104.07(13)
128.03(14)
116.10(12)
98.06(9)
90.41(11)
92.68(15)
90.05(12)
56.27(14)
141.38(14)
98.72(9)
N(6)–Zn(2)–O(5)
N(8)#2–Zn(2)–O(5)
O(7)–Zn(2)–O(5)
N(6)–Zn(2)–O(6)
N(8)#2–Zn(2)–O(6)
O(7)–Zn(2)–O(6)
O(5)–Zn(2)–O(6)
N(6)–Zn(2)–O(8)
N(8)#2–Zn(2)–O(8)
O(7)–Zn(2)–O(8)
O(5)–Zn(2)–O(8)
O(6)–Zn(2)–O(8)
92.11(10)
157.07(11)
103.02(10)
99.26(9)
98.36(10)
157.74(10)
59.77(10)
155.26(9)
87.00(9)
60.21(8)
91.57(9)
103.71(9)
95.15(9)
#2 x – 1, y – 1, z
Data Center (CCDC-795198 for I and CCDC-795199 for obtained. The reaction of 2-phenylquinoline-4-carboxylic acid,
II).
Zn(OAC)2·2H2O, and phen in a mole ratio of 1/2/2 gave rise to
compound I, and the reaction of 2-phenylquinoline-4-carboxylic
acid, Zn(OAC)2·2H2O, and 4,4ꢁ-bpy in a mole ratio of 1/1/1 gave
rise to compound II. When the mole ratio was changed, we could
not obtain high-quality crystals.
RESULTS AND DISCUSSION
Synthesis
Various kinds of structures of metal-organic complexes may
isolated by employing different synthetic techniques. The pH
value, temperature, and mole ratio are very important to ob-
Structure Description of Complex I
As shown in Figure 1, complex I is a binuclear structure, in
tain new complexes. Compound I and II were obtained in base which Zn1A is symmetrical component related by Zn1 (sym-
environment through hydrothermal reaction. When KOH was metry code: –x + 2, –y + 1, –z), and the two Zn(II) ions pos-
removed, or the temperature was raised to 140 ◦C, compounds sess the same coordination environment. The zinc ion adopts
I and II could not be obtained and only white deposition was a distorted octahedral coordinated geometry formed by two N

ZINC COMPLEXES WITH 2-PHENYLQUINOLINE-4-CARBOXYLATES
801
TABLE 3
Hydrogen bond lengths (Å) and bond angles (◦) for I
Donor–H
d(D–H)
d(H···A)
d(D···A)
∠DHA
Symmetry code
O(3)–H(1)
O(6)–H(2)
O(6)–H(2)
0.966
0.984
0.984
1.902
2.238
2.537
2.862
3.167
3.186
171.91
157.14
123.39
N(1)[–x + 2, y + 1/2, –z + 1/2]
O(4)[–x + 1, –y + 1, –z]
O(2)[–x + 1, –y + 1, –z]
atoms and four oxygen atoms, of which three are from differ- (Figure 2). The chain is further assembled into a supramolec-
ent 2-phenylquinoline-4-carboxylic acid ligands and one from ular framework by the other intermolecular hydrogen bonding
coordinated water, with Zn–O distances ranging from 2.035 to (O6-H2···O4 = 3.167 Å, symmetry code: –x + 1, –y + 1, –z;
2.168 Å and Zn–N distance being 2.154 Å and 2.174 Å.
In complex I, the centroid-to-centroid separation of the near-
est two benzene rings of pqc− ligands and 1,10-phen is 3.581 Å,
which implies an intermolecular π–π stacking interaction. The
O6–H2···O2 = 3.186 Å, symmetry code: –x + 1, –y + 1, –z).
Structure Description of Complex II
As shown in Figure 3, the asymmetry unit of compound
binuclear units are linked by intermolecular hydrogen bonding II consists of two Zn(II) ions together with two 4,4ꢁ-bpy and
(O3-H1···N1 = 2.862 Å, symmetry code: –x + 2, y + 1/2, –z + four 2-phenylquinoline-4-carboxylate ligands. The Zn1 is co-
1/2) and π–π interaction to generate a one-dimensional chain ordinated by three carboxyl oxygen atoms from two different
FIG. 1. Binuclear structure of compound I. Ellipsoids correspond to 30% probability. Hydrogen atoms and guest water are omitted for clarity. (Symmetry codes:
A: –x + 2,–y + 1, –z) (color figure available online.)

802
Y. BING ET AL.
FIG. 2. View of 1D chain with hydrogen bonding and π–π interaction of compound I (color figure available online.)
2-phenylquinoline-4-carboxylate ligands (O1, O3, O4) and two form the one dimensional structure in compound II and com-
nitrogen atoms from two different 4,4ꢁ-bpy ligands (N5, N7), pound I is the binuclear cluster by the carboxylate bridge and
forming a distorted pyramid. The Zn2 is coordinated by four phen chelate to Cd(II) ions, in which the different structures
carboxyl oxygen atoms from two different 2-phenylquinoline- are understandable in comparison with some previous reports
4-carboxylate ligands (O5, O6, O7, O8) and two nitrogen atoms in terms of coordination chemistry.[13,15]
from two different 4,4ꢁ-bpy ligands, forming a distorted oc-
Power X-Ray Diffraction Analysis
tahedral geometry. In the complex II, Zn–O distances range
from 1.932 (3) to 2.272 (4) Å and Zn–N distances from
In an attempt to confirm the homogeneity of the material
2.056(2) to 2.109(2) Å. The asymmetry units of compound synthesizedunder thehydrothermalcondition, wehaveanalyzed
II are bridged by 4,4ꢁ-bpy, giving rise to a zigzag zinc chain the PXRD pattern of I and II and correlated the results with the
(Figure 4).
simulated powder pattern obtained from the single crystal data.
As is well known, the bridging ligands are propitious to the As depicted in Figure 5, the basic features of the PXRD patterns
information of polymeric structures, and the terminal ligands resemble the simulated from single-crystal data for I and II,
are in favor of the construction of multinuclear clusters with the indicating that the bulk products obtained are homogeneous in
metal ions. Therefore, the 4,4ꢁ-bpy ligands link Cd(II) ions to nature.
FIG. 3. Structure unit of compound II. Ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity (color figure available online.)

ZINC COMPLEXES WITH 2-PHENYLQUINOLINE-4-CARBOXYLATES
803
FIG. 4. View of the 1D chain of compound II (color figure available online.)
IR Spectrum
The IR spectra of complexes I and II are very similar;
the characteristic bands of carboxylate groups are shown in
the range 1560–1620 cm−1 for an asymmetric stretching and
1370–1490 cm−1 for a symmetric stretching. The absorptions in
the range 1500–1420 cm−1 are attribute to characteristic peaks
of the C = N and C = C bonds of the aromatic ring. The broad
bands at ∼3300 cm−1 correspond to the vibration of the water
molecules in the complexes.
FIG. 6. TGA curve of the title complex I and II (color figure available online.)
[Zn2(C16H10NO2)4(4,4ꢁ-bpy)2]n from 260 to 500◦C, indicating
the stability of the framework of the complex up to 260◦C
(Figure 6).
Thermogravimetric Analyses
Thermogravimetric analysis (TGA) experiments were Fluorescent Emission
conducted to determine the thermal stability of the title
complexes, which was an important aspect for metal-organic
frameworks.[18,19] TGA was performed on crystalline samples
of I and II under an air atmosphere from 40 to 800◦C.
In complex I, the first weight loss of 2.5% (calcd. 2.3%)
corresponds to two guest water molecules per formula
unit [Zn2(C16H10NO2)4(phen)2(H2O)2]·2H2O from 100 to
170◦C, implying removal of the guest water. Then a sharp
drop of weight was observed after 283◦C, indicating the
decomposition of the framework of complex I. In compound
II, the first weight loss of 65.1% (calcd: 69.1%) corresponds
to the four pqc− (C16H10NO2) molecules per formula unit
Previous studies have shown that compounds containing
zinc ions could exhibit fine photoluminescent properties.[20,21]
Hence, we also investigated the photoluminescent properties of
complexes I and II (Figure 7). In the solid state, strong photo-
luminescence emission bands at 390 and 430 nm are observed
for complexes I and II, respectively, upon photoexcitation at
300 nm. The free 2-phenylquinoline-4-carboxylic acid exhibits
photoluminescence emission at 420 nm, which may originate
from the ligand-to-ligand πL–πL∗ charge transfer (LLCT) tran-
sition emission. The position blue-shift (comparing with pqc−)
and the red-shift (comparing with phen) of the strong emission
peak (390 nm) and the fluorescent enhancement of compound
FIG. 5. Experimental and simulated powder XRD patterns of I and II (color FIG. 7. Solid-state emission spectra of I and II at room temperature (color
figure available online.) figure available online.)

804
Y. BING ET AL.
I may be assigned to the intermolecular π–π stacking interac-
tion of phen and/or pqc− ligands or the effect of ligand-to-metal
or metal-to-ligand charge transfer (LMCT or MLCT) transition
emission similar to previous reports.[22] The emission bands at
430 nm of II could be attributed to the pqc− ligand donation
from the coordination interactions or crystal packing interac-
tions in the solid by radiationless decay of the intra-ligands.
7. Xi, P.X.; Xu, Z.H.; Chen, F.J.; Zeng, Z.Z.; Zhang, X.W. Study on synthesis,
structure, and DNA-binding of Ni, Zn complexes with 2-phenylquinoline-
4-carboylhydrazide. J. Inorg. Biochem., 2009, 103, 210–218.
8. Xu, Z.H.; Chen, F.J.; Xi, P.X.; Liu, X.H.; Zeng, Z.Z. Synthesis, char-
acterization, and DNA-binding properties of the cobalt(II) and nickel(II)
complexes with salicylaldehyde 2-phenylquinoline-4-carboylhydrazone. J.
Photochem. Photobiol. A 2008, 196, 77–83.
9. Qin, Z.Q.; Jenkins, H.A.; Coles, S.J.; Muir, K.W.; Puddephatt, R.J. Self-
assembly ofone-dimensionalpolymersby coordination and hydrogenbond-
ing in palladium(II) complexes. Can. J. Chem. 1999, 77, 155–157.
10. Qin, Z.Q.; Jennings, M.C.; Puddephatt, R.J.; Muir, K.W. Self-assembly of
polymer and sheet structures in palladium(II) complexes containing car-
boxylic acid substituents. Inorg. Chem. 2002, 41, 5174–5186.
11. Marsh, R.E. Space group P1: An update. Acta Crystallogr. 2005, B61,
359–361.
12. Blackburn, A.C.; Dobson, A.J.; Gerkin, R.E. 2-Phenylquinoline-4-
carboxylic acid. Acta Crystallogr. 1996, C52, 409–411.
13. Che, G.B.; Liu, C.B.; Cui, Y.C.; Li, C.B. Diaquatetrakis(µ-2-
phenylquinoline-4-carboxylato-κ2O:Oꢁ)dicopper(II)(Cu—Cu). Acta. Crys-
tallogr. 2005, E61, m2449–m2451.
CONCLUSIONS
Two new zinc complexes with 2-phenylquinoline-4-
carboxylates have been synthesized and characterized. TGA
analyses show high thermal stabilities of the frameworks for
I and II; the luminescence measurement suggests an intense
strong emission at solid state. This work may provide useful in-
formation for employing rigid carboxylate ligands to assemble
new functional materials.
14. Zhang, X.; Wei, P.; Li, B. Poly[bis µ -hemihydrogen 2-phenylquinoline-4
-carboxylato-κ2 N,O)silver(I)]. Acta Crystallogr. 2009, E65, m223–m232.
15. Shen, Y.C.; Li, Z.J.; Cheng, J.K.; Qin, Y.Y.; Yao, Y.G. Benzotriazole con-
trolled unusual building blocks in two zinc complexes and their fluorescence
properties. Inorg. Chem. Commun. 2007, 10, 888–890.
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