metal-organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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ISSN: 2056-9890

Bis(2,2′-bi­imidazole-κ2N,N′)bis­­(2-bromo­fumarato-κO)copper(II)

aState Key Laboratory, Base of Novel Functional Materials and Preparation Science, Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, People's Republic of China
*Correspondence e-mail: lianghongze@nbu.edu.cn

(Received 13 November 2007; accepted 12 December 2007; online 21 December 2007)

In the title compound, [Cu(C4H2BrO4)2(C6H6N4)2], the central CuII atom lies on an inversion center and is six-coordinated in an octahedral geometry by four N atoms from two chelating biimidazole mol­ecules in the equatorial plane and two O atoms from two 2-bromo­fumarate ligands in the axial positions. O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds lead to a three-dimensional network.

Related literature

For related literature, see: Atencio et al. (2005[Atencio, R., Ramírez, K., Reyes, J. A., González, T. & Silva, P. (2005). Inorg. Chim. Acta, 358, 520-526.]); Carraza et al. (2003[Carraza, J., Brennan, C., Sletten, J., Vangdal, B., Rillema, P., Lloret, F. & Julve, M. (2003). New J. Chem. 27, 1775-1783.]); Öhrström et al. (2001[Öhrström, L., Larsson, K., Borg, S. & Norberg, S. T. (2001). Chem. Eur. J. 7, 4805-4810.]); Sang & Xu (2006[Sang, R. L. & Xu, L. (2006). Eur. J. Inorg. Chem. pp. 1260-1267.]); Tadokoro et al. (1999[Tadokoro, M., Isobe, K., Uekusa, H., Ohashi, Y., Toyoda, J. & Nakasuji, K. (1999). Angew. Chem. Int. Ed. 38, 95-98.]). For the synthesis and crystal structure of 2-bromo­fumaric acid, see: Fischer (2006[Fischer, A. (2006). Acta Cryst. E62, o4190-o4191.]).

[Scheme 1]

Experimental

Crystal data
  • [Cu(C4H2BrO4)2(C6H6N4)2]

  • Mr = 719.77

  • Triclinic, [P \overline 1]

  • a = 7.1650 (14) Å

  • b = 8.6458 (17) Å

  • c = 9.841 (2) Å

  • α = 83.13 (1)°

  • β = 84.21 (3)°

  • γ = 87.56 (2)°

  • V = 601.9 (2) Å3

  • Z = 1

  • Mo Kα radiation

  • μ = 4.29 mm−1

  • T = 295 (2) K

  • 0.12 × 0.1 × 0.09 mm

Data collection
  • Rigaku R-AXIS RAPID diffractometer

  • Absorption correction: multi-scan (ABSCOR; Higashi, 1995[Higashi, T. (1995). ABSCOR. Rigaku Corporation, Tokyo, Japan.]) Tmin = 0.601, Tmax = 0.685

  • 5942 measured reflections

  • 2717 independent reflections

  • 1655 reflections with I > 2σ(I)

  • Rint = 0.072

Refinement
  • R[F2 > 2σ(F2)] = 0.068

  • wR(F2) = 0.196

  • S = 1.06

  • 2717 reflections

  • 172 parameters

  • 1 restraint

  • H-atom parameters constrained

  • Δρmax = 1.01 e Å−3

  • Δρmin = −0.74 e Å−3

Table 1
Selected geometric parameters (Å, °)

Cu—N4 2.001 (5)
Cu—N2 2.028 (5)
Cu—O4 2.627 (6)
N4—Cu—N2 81.9 (2)
N4i—Cu—N2 98.1 (2)
N4—Cu—O4 87.3 (2)
N4i—Cu—O4 92.7 (2)
N2—Cu—O4 88.9 (2)
N2—Cu—O4i 91.1 (2)
Symmetry code: (i) -x, -y, -z+2.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1⋯O3ii 0.86 1.90 2.756 (8) 174
N3—H4⋯O4ii 0.86 1.85 2.672 (8) 159
O2—H8⋯O1iii 0.85 1.90 2.743 (9) 172
C1—H2⋯O3iv 0.93 2.55 3.433 (10) 159
C5—H5⋯O1v 0.93 2.58 3.432 (10) 153
C6—H6⋯O2vi 0.93 2.56 3.329 (10) 141
Symmetry codes: (ii) -x+1, -y, -z+2; (iii) -x-1, -y+1, -z+1; (iv) x, y, z+1; (v) x+1, y-1, z; (vi) -x, -y, -z+1.

Data collection: RAPID-AUTO (Rigaku, 1998[Rigaku (1998). RAPID-AUTO. Rigaku Corporation, Tokyo, Japan.]); cell refinement: RAPID-AUTO; data reduction: CrystalStructure (Rigaku/MSC, 2002[Rigaku/MSC (2002). CrystalStructure. Rigaku/MSC Inc., The Woodlands, Texas, USA.]); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997[Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997[Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany.]); molecular graphics: ORTEPII (Johnson, 1976[Johnson, C. K. (1976). ORTEPII. Report ORNL-5138. Oak Ridge National Laboratory, Tennessee, USA.]); software used to prepare material for publication: SHELXL97.

Supporting information


Comment top

Because of its various deprotonation modes (H2biim, Hbiim-, biim2-), the 2,2'-biimidazole ligand exhibits rich coordination patterns with various metals such as AgI (Sang & Xu, 2006), NiII (Tadokoro et al., 1999), CuII(Atencio et al., 2005; Carraza et al., 2003) and CoIIIÖhrström et al., 2001). We report here the crystal structure of a CuII complex with neutral 2,2'-biimidazole molecule and 2-bromofumarate anion as ligands.

As illustrated in Fig. 1, the Cu atom shows a distorted octahedral coordination geometry, formed by four N atoms from two 2,2'-biimidazole molecules and two O atoms from carboxylate groups offered by two 2-bromofumarate ligands at the axial positions. The asymmetric unit contains an H2biim molecule and a 2-bromofumarate anion with a CuII atom lying on an inversion center. We can see that the lengths of Cu—N bonds [2.028 (5) and 2.001 (5) Å] are slightly asymmetric (Table 1). This behavior is similar to the reported Cu complex with H2biim [2.036 (2) and 2.010 (2) Å] (Atencio et al., 2005). Three types of strong hydrogen bonds are observed. The O—H···O hydrogen bonds are formed between two adjacent uncoordinated carboxylate groups. The N—H···O hydrogen bonds are formed between H2biim and the neighboring coordinated carboxylate group. Weak C—H···O hydrogen bonds also exist in the structure (Table 2). The complex molecules are assembled into two-dimensional layers via O—H···O and N—H···O hydrogen bonds. These layers are further assembled through C—H···O hydrogen bonds into a three-dimensional supramolecular structure.

Related literature top

For related literature, see: Atencio et al. (2005); Carraza et al. (2003); Öhrström et al. (2001); Sang & Xu (2006); Tadokoro et al. (1999). For the synthesis and crystal structure of 2-bromofumaric acid, see: Fischer (2006).

Experimental top

In a 50 ml two-neck bottle, the mixture of 2,2'-biimidazole (1.340 g, 10 mmol), 2-bromofumaric acid (0.195 g, 10 mmol) (Fischer, 2006), water (10 ml) and methanol (10 ml) was heated to 353 K, and then copper(II) chloride dihydrate (0.170 g, 10 mmol) was added. The suspension was stirred and kept at 353 K for 3 h. After cooling to room temperature, the solid was filtered off and the green solution was allowed to evaporate in air. After one day, block green crystals suitable for X-ray diffraction were formed.

Refinement top

H atoms on C and N atoms were positioned geometrically and refined as riding, with C—H = 0.93 Å, N—H = 0.86Å and Uiso(H) = 1.2Ueq(C,N). H atom attached to O atom was located in a difference Fourier map and fixed with Uiso(H) = 1.5Ueq(O).

Structure description top

Because of its various deprotonation modes (H2biim, Hbiim-, biim2-), the 2,2'-biimidazole ligand exhibits rich coordination patterns with various metals such as AgI (Sang & Xu, 2006), NiII (Tadokoro et al., 1999), CuII(Atencio et al., 2005; Carraza et al., 2003) and CoIIIÖhrström et al., 2001). We report here the crystal structure of a CuII complex with neutral 2,2'-biimidazole molecule and 2-bromofumarate anion as ligands.

As illustrated in Fig. 1, the Cu atom shows a distorted octahedral coordination geometry, formed by four N atoms from two 2,2'-biimidazole molecules and two O atoms from carboxylate groups offered by two 2-bromofumarate ligands at the axial positions. The asymmetric unit contains an H2biim molecule and a 2-bromofumarate anion with a CuII atom lying on an inversion center. We can see that the lengths of Cu—N bonds [2.028 (5) and 2.001 (5) Å] are slightly asymmetric (Table 1). This behavior is similar to the reported Cu complex with H2biim [2.036 (2) and 2.010 (2) Å] (Atencio et al., 2005). Three types of strong hydrogen bonds are observed. The O—H···O hydrogen bonds are formed between two adjacent uncoordinated carboxylate groups. The N—H···O hydrogen bonds are formed between H2biim and the neighboring coordinated carboxylate group. Weak C—H···O hydrogen bonds also exist in the structure (Table 2). The complex molecules are assembled into two-dimensional layers via O—H···O and N—H···O hydrogen bonds. These layers are further assembled through C—H···O hydrogen bonds into a three-dimensional supramolecular structure.

For related literature, see: Atencio et al. (2005); Carraza et al. (2003); Öhrström et al. (2001); Sang & Xu (2006); Tadokoro et al. (1999). For the synthesis and crystal structure of 2-bromofumaric acid, see: Fischer (2006).

Computing details top

Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO (Rigaku, 1998); data reduction: CrystalStructure (Rigaku/MSC, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson,1976); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 45% probability level. [Symmetry code: (i) -x, -y, 2 - z.]
Bis(2,2'-biimidazole-κ2N,N')bis(2-bromofumarato-κO)copper(II) top
Crystal data top
[Cu(C4H2BrO4)2(C6H6N4)2]Z = 1
Mr = 719.77F(000) = 355
Triclinic, P1Dx = 1.986 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 7.1650 (14) ÅCell parameters from 2750 reflections
b = 8.6458 (17) Åθ = 3.0–27.5°
c = 9.841 (2) ŵ = 4.29 mm1
α = 83.13 (1)°T = 295 K
β = 84.21 (3)°Platelet, green
γ = 87.56 (2)°0.12 × 0.1 × 0.09 mm
V = 601.9 (2) Å3
Data collection top
Rigaku R-AXIS RAPID
diffractometer
2717 independent reflections
Radiation source: fine-focus sealed tube1655 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.072
ω scansθmax = 27.5°, θmin = 3.4°
Absorption correction: multi-scan
(ABSCOR; Higashi, 1995)
h = 99
Tmin = 0.601, Tmax = 0.685k = 1111
5942 measured reflectionsl = 1012
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.068Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.196H-atom parameters constrained
S = 1.06 w = 1/[σ2(Fo2) + (0.0963P)2 + 0.1925P]
where P = (Fo2 + 2Fc2)/3
2717 reflections(Δ/σ)max < 0.001
172 parametersΔρmax = 1.01 e Å3
1 restraintΔρmin = 0.74 e Å3
Crystal data top
[Cu(C4H2BrO4)2(C6H6N4)2]γ = 87.56 (2)°
Mr = 719.77V = 601.9 (2) Å3
Triclinic, P1Z = 1
a = 7.1650 (14) ÅMo Kα radiation
b = 8.6458 (17) ŵ = 4.29 mm1
c = 9.841 (2) ÅT = 295 K
α = 83.13 (1)°0.12 × 0.1 × 0.09 mm
β = 84.21 (3)°
Data collection top
Rigaku R-AXIS RAPID
diffractometer
2717 independent reflections
Absorption correction: multi-scan
(ABSCOR; Higashi, 1995)
1655 reflections with I > 2σ(I)
Tmin = 0.601, Tmax = 0.685Rint = 0.072
5942 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0681 restraint
wR(F2) = 0.196H-atom parameters constrained
S = 1.06Δρmax = 1.01 e Å3
2717 reflectionsΔρmin = 0.74 e Å3
172 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Cu0.00000.00001.00000.0331 (3)
Br0.08553 (12)0.49244 (10)0.83673 (9)0.0604 (4)
N10.3926 (8)0.0593 (7)1.2738 (6)0.0444 (14)
H10.49760.10491.29150.053*
C10.2838 (12)0.0336 (10)1.3537 (8)0.053 (2)
H20.31080.06281.43690.064*
O10.3636 (9)0.4989 (7)0.6254 (6)0.0682 (18)
N20.1405 (7)0.0166 (6)1.1664 (6)0.0344 (12)
C20.1300 (11)0.0765 (10)1.2905 (8)0.0497 (19)
H30.03020.13821.32460.060*
O20.3039 (8)0.3652 (7)0.4506 (6)0.0654 (17)
H80.40870.41020.43430.098*
N30.4932 (8)0.2414 (7)1.0080 (7)0.0445 (15)
H40.59000.26261.05220.053*
C30.3013 (9)0.0657 (7)1.1606 (7)0.0344 (14)
O30.2811 (8)0.2094 (7)0.6488 (6)0.0643 (16)
N40.2205 (7)0.1400 (6)0.9522 (6)0.0334 (12)
C40.3456 (8)0.1495 (7)1.0427 (7)0.0336 (14)
O40.1940 (7)0.2317 (7)0.8670 (6)0.0568 (12)
C50.4601 (10)0.2935 (9)0.8885 (8)0.0493 (19)
H50.53730.36100.83990.059*
C60.2919 (10)0.2312 (9)0.8536 (8)0.0441 (18)
H60.23520.24670.77550.053*
C70.2662 (11)0.4104 (9)0.5622 (8)0.0477 (18)
C80.0839 (11)0.3384 (9)0.6025 (8)0.0514 (19)
H70.02300.27410.54160.062*
C90.0024 (10)0.3520 (8)0.7107 (8)0.0459 (17)
C100.1763 (10)0.2592 (10)0.7433 (10)0.0568 (12)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cu0.0202 (6)0.0442 (6)0.0367 (6)0.0114 (4)0.0126 (5)0.0077 (5)
Br0.0523 (6)0.0680 (6)0.0663 (6)0.0119 (4)0.0203 (5)0.0224 (5)
N10.032 (3)0.058 (4)0.046 (3)0.006 (3)0.017 (3)0.005 (3)
C10.042 (5)0.077 (5)0.044 (4)0.005 (4)0.021 (4)0.011 (4)
O10.058 (4)0.093 (4)0.060 (4)0.020 (3)0.040 (3)0.015 (3)
N20.021 (3)0.043 (3)0.040 (3)0.008 (2)0.006 (2)0.008 (3)
C20.045 (5)0.067 (5)0.042 (4)0.002 (4)0.004 (4)0.025 (4)
O20.050 (4)0.084 (4)0.066 (4)0.028 (3)0.018 (3)0.023 (3)
N30.023 (3)0.051 (3)0.059 (4)0.010 (3)0.010 (3)0.003 (3)
C30.017 (3)0.046 (3)0.040 (4)0.001 (3)0.008 (3)0.004 (3)
O30.044 (3)0.082 (4)0.068 (4)0.020 (3)0.020 (3)0.007 (3)
N40.017 (3)0.041 (3)0.042 (3)0.008 (2)0.007 (2)0.005 (2)
C40.017 (3)0.040 (3)0.045 (4)0.002 (2)0.006 (3)0.003 (3)
O40.032 (2)0.070 (3)0.068 (3)0.007 (2)0.022 (2)0.004 (3)
C50.032 (4)0.058 (4)0.060 (5)0.015 (3)0.006 (4)0.023 (4)
C60.030 (4)0.056 (4)0.048 (4)0.013 (3)0.009 (3)0.013 (4)
C70.033 (4)0.055 (4)0.052 (5)0.006 (3)0.009 (4)0.008 (4)
C80.043 (5)0.061 (5)0.051 (5)0.002 (4)0.011 (4)0.005 (4)
C90.036 (4)0.049 (4)0.053 (4)0.002 (3)0.010 (4)0.000 (4)
C100.032 (2)0.070 (3)0.068 (3)0.007 (2)0.022 (2)0.004 (3)
Geometric parameters (Å, º) top
Cu—N42.001 (5)O2—H80.8512
Cu—N4i2.001 (5)N3—C41.337 (8)
Cu—N22.028 (5)N3—C51.354 (9)
Cu—N2i2.028 (5)N3—H40.8600
Cu—O42.627 (6)C3—C41.441 (9)
Cu—O4i2.627 (6)O3—C101.242 (10)
Br—C91.883 (7)N4—C41.319 (8)
N1—C31.355 (8)N4—C61.369 (8)
N1—C11.358 (10)O4—C101.230 (10)
N1—H10.8600C5—C61.358 (10)
C1—C21.337 (11)C5—H50.9300
C1—H20.9300C6—H60.9300
O1—C71.200 (9)C7—C81.492 (10)
N2—C31.329 (7)C8—C91.304 (10)
N2—C21.377 (8)C8—H70.9300
C2—H30.9300C9—C101.494 (7)
O2—C71.266 (9)
N4—Cu—N4i180.000 (1)C5—N3—H4126.8
N4—Cu—N281.9 (2)N2—C3—N1111.6 (6)
N4i—Cu—N298.1 (2)N2—C3—C4117.0 (6)
N4—Cu—N2i98.1 (2)N1—C3—C4131.3 (6)
N4i—Cu—N2i81.9 (2)C4—N4—C6105.7 (5)
N2—Cu—N2i180.000 (1)C4—N4—Cu112.8 (4)
N4—Cu—O487.3 (2)C6—N4—Cu141.5 (5)
N4i—Cu—O492.7 (2)N4—C4—N3111.9 (6)
N2—Cu—O488.9 (2)N4—C4—C3116.9 (5)
N2i—Cu—O491.1 (2)N3—C4—C3131.2 (6)
N4—Cu—O4i92.7 (2)C10—O4—Cu115.9 (5)
N4i—Cu—O4i87.3 (2)N3—C5—C6107.7 (6)
N2—Cu—O4i91.1 (2)N3—C5—H5126.2
N2i—Cu—O4i88.9 (2)C6—C5—H5126.1
O4—Cu—O4i180.00 (17)C5—C6—N4108.3 (6)
C3—N1—C1106.0 (6)C5—C6—H6126.0
C3—N1—H1127.0N4—C6—H6125.7
C1—N1—H1127.0O1—C7—O2124.4 (7)
C2—C1—N1107.8 (6)O1—C7—C8125.4 (7)
C2—C1—H2125.7O2—C7—C8110.2 (7)
N1—C1—H2126.5C9—C8—C7129.5 (8)
C3—N2—C2104.6 (6)C9—C8—H7115.2
C3—N2—Cu111.4 (4)C7—C8—H7115.3
C2—N2—Cu143.8 (5)C8—C9—C10122.9 (7)
C1—C2—N2109.9 (6)C8—C9—Br121.5 (6)
C1—C2—H3125.5C10—C9—Br115.5 (6)
N2—C2—H3124.6O4—C10—O3126.1 (7)
C7—O2—H8105.0O4—C10—C9114.2 (8)
C4—N3—C5106.3 (6)O3—C10—C9119.5 (8)
C4—N3—H4126.9
Symmetry code: (i) x, y, z+2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O3ii0.861.902.756 (8)174
N3—H4···O4ii0.861.852.672 (8)159
O2—H8···O1iii0.851.902.743 (9)172
C1—H2···O3iv0.932.553.433 (10)159
C5—H5···O1v0.932.583.432 (10)153
C6—H6···O2vi0.932.563.329 (10)141
Symmetry codes: (ii) x+1, y, z+2; (iii) x1, y+1, z+1; (iv) x, y, z+1; (v) x+1, y1, z; (vi) x, y, z+1.

Experimental details

Crystal data
Chemical formula[Cu(C4H2BrO4)2(C6H6N4)2]
Mr719.77
Crystal system, space groupTriclinic, P1
Temperature (K)295
a, b, c (Å)7.1650 (14), 8.6458 (17), 9.841 (2)
α, β, γ (°)83.13 (1), 84.21 (3), 87.56 (2)
V3)601.9 (2)
Z1
Radiation typeMo Kα
µ (mm1)4.29
Crystal size (mm)0.12 × 0.1 × 0.09
Data collection
DiffractometerRigaku R-AXIS RAPID
Absorption correctionMulti-scan
(ABSCOR; Higashi, 1995)
Tmin, Tmax0.601, 0.685
No. of measured, independent and
observed [I > 2σ(I)] reflections
5942, 2717, 1655
Rint0.072
(sin θ/λ)max1)0.649
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.068, 0.196, 1.06
No. of reflections2717
No. of parameters172
No. of restraints1
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)1.01, 0.74

Computer programs: RAPID-AUTO (Rigaku, 1998), CrystalStructure (Rigaku/MSC, 2002), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), ORTEPII (Johnson,1976).

Selected geometric parameters (Å, º) top
Cu—N42.001 (5)Cu—O42.627 (6)
Cu—N22.028 (5)
N4—Cu—N281.9 (2)N4i—Cu—O492.7 (2)
N4i—Cu—N298.1 (2)N2—Cu—O488.9 (2)
N4—Cu—O487.3 (2)N2—Cu—O4i91.1 (2)
Symmetry code: (i) x, y, z+2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O3ii0.861.902.756 (8)174
N3—H4···O4ii0.861.852.672 (8)159
O2—H8···O1iii0.851.902.743 (9)172
C1—H2···O3iv0.932.553.433 (10)159
C5—H5···O1v0.932.583.432 (10)153
C6—H6···O2vi0.932.563.329 (10)141
Symmetry codes: (ii) x+1, y, z+2; (iii) x1, y+1, z+1; (iv) x, y, z+1; (v) x+1, y1, z; (vi) x, y, z+1.
 

Acknowledgements

This project was sponsored by the Scientific Research Foundation of the State Education Ministry for Returned Overseas Chinese Scholars (grant No. 2006331), the Educational Committee of Zhejiang Province (grant No. 20061696), the Starting Foundation of Zhejiang Province for Returned Overseas Chinese Scholars (grant No. 2005545), the Natural Science Foundation of Ningbo City (grant No. 2007A610021) and Ningbo University (grant No. 2005062). We thank Dr K.-W. Lei for structural discussions and Mrs W. Xu and D.-Y. Cheng for collecting the diffraction data.

References

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