Poly[bis­(N,N-dimethyl­formamide)tris­(μ4-trans-stilbene-4,4′-dicarboxyl­ato)­tricadmium(II)]: a two-dimensional network with an unusual 36 topology

Lee, D.-H.; Park, G.

doi:10.1107/S1600536808016267

metal-organic compounds

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ISSN: 2056-9890

Volume 64| Part 7| July 2008| Pages m861-m862

doi:10.1107/S1600536808016267

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Poly[bis(N,N-dimethylformamide)tris(μ₄-trans-stilbene-4,4′-dicarboxylato)tricadmium(II)]: a two-dimensional network with an unusual 3⁶ topology

Dong-Heon Lee ^a and Gyungse Park ^b ^*

^aDepartment of Chemistry, Chonbuk National University, Jeonju, Chonbuk 561-756, Republic of Korea, and ^bDepartment of Chemistry, Kunsan National University, Kusan, Chonbuk 573-701, Republic of Korea
^*Correspondence e-mail: parkg@kunsan.ac.kr

(Received 21 May 2008; accepted 29 May 2008; online 7 June 2008)

In the title compound, [Cd₃(C₁₆H₁₀O₄)₃(C₃H₇NO)₂]_n or [Cd₃(SDA)₃(DMF)₂]_n (H₂SDA is trans-stilbene-4,4′-dicarboxylic acid and DMF is dimethylformamide), the linear dicarboxylate ligand forms a two-dimensionally layered metal–organic network with the relatively uncommon 3⁶ topology. The structure reveals trinuclear secondary building units and has an octahedral geometry at a central metal ion (occupying a $[\overline{3}]$ symmetry site) and tetrahedral geometries at two surrounding symmetrically equivalent metal ions lying on a threefold axis. The six-connected planar trinuclear Cd^II centers, Cd₃(O₂CR)₆, play a role as potential nodes in generation of the relatively uncommon 3⁶ topology. The coordinated DMF unit is disordered around the threefold axis.

Related literature

For related literature, see: Chi et al. (2006 ); Dincâ & Long (2005 ); Dybtsev et al. (2004 ); Eddaoudi et al. (2002 ); Edgar et al. (2001 ); Hawxwell et al. (2006 ); Hill et al. (2005 ); Luan et al. (2006 ); Park et al. (2006 ); Rosi et al. (2003 ); Saalfrank et al. (2001 ); Seo et al. (2000 ); Wang et al. (2006 ); Williams et al. (2005 ).

Experimental

Crystal data

[Cd₃(C₁₆H₁₀O₄)₃(C₃H₇NO)₂]
M_r = 1282.11
Trigonal, $[R \overline 3]$
a = 16.4881 (5) Å
c = 16.7919 (10) Å
V = 3953.4 (3) Å³
Z = 3
Mo Kα radiation
μ = 1.27 mm⁻¹
T = 223 (2) K
0.30 × 0.30 × 0.30 mm

Data collection

Bruker SMART CCD diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.69, T_max = 0.69
6604 measured reflections
2105 independent reflections
1782 reflections with I > 2σ(I)
R_int = 0.104

Refinement

R[F² > 2σ(F²)] = 0.057
wR(F²) = 0.181
S = 1.18
2105 reflections
136 parameters
92 restraints
H-atom parameters constrained
Δρ_max = 1.70 e Å⁻³
Δρ_min = −1.53 e Å⁻³

Data collection: SMART (Bruker, 1997 ); cell refinement: SAINT (Bruker, 1997); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536808016267/bg2189sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808016267/bg2189Isup2.hkl

checkCIF report

Comment top

The study of one, two or three dimensional metal-organic frameworks (MOFs) has attracted much attention in the past decade due to their various intriguing framework topologies but also for their potential applications in gas storage (Rosi et al., 2003), separation (Dybtsev et al., 2004) and catalysis (Seo et al., 2000) etc. Many factors play important role in the synthesis of MOFs such as the coordination geometry of metal ions (Chi et al.,2006), the structure of organic ligands (Wang et al.,2006), the solvent system (Eddaoudi et al., 2002),the counteranion (Luan et al., 2006), and the ratio of ligands to metal ions (Saalfrank et al., 2001). The simplest 2D sheets are those which comprise just one kind of regular polygon based upon hexagons, squares and triangles. Since three hexagons, four squares and six triangles meet at a node in a 2D network with angles of 120°, 90° and 60°,respectively, the corresponding Schläfli topology symbols are 6³, 4⁴ and 3⁶, respectively (Hill et al., 2005). Although there were many examples of uninodal regularly tiled 2D metal–organic frameworks comprising linked squares or hexagons, however, a few examples comprising linked and tiled triangles have been reported only very recently (Edgar et al., 2001; Williams et al., 2005; Hawxwell et al., 2006; Dincâ & Long, 2005). Herein the formation of a two-dimensional metal-organic framework with an uncommon 3⁶ tessellated topology, [Cd₃(SDA)₃(DMF)₂], (I), constructed from tri-nuclear cadmium SBUs (secondary building units) linked by a novel 4,4'-stilbenedicarboxylate ligand (Park et al., 2006) is reported.

The two-dimensional 3⁶ tessellated network structure of 1 with the atomic numbering scheme is shown in Fig. 1 in which the coordinated DMF molecules are shown in only one of its three disordered components. The crystal structure of 1 is constructed from the tri-nuclear Cd₃(O₂CR)₆ SBUs cluster which contains two crystallographically equivalent four-coordinate terminal metal centers (Cd2) in which the O atom (O1S) of the DMF is axially coordinated and a six-coordinate central metal atom (Cd1). The coordination environment around the central Cd^II atom, Cd1, in the trinuclear center is an octahedron with all six positions occupied by one carboxylate oxygen, O1, from each half unit of six SDA ligands (Fig. 1) and that of the two symmetry equivalent neighbouring Cd^II atoms, Cd2, is a tetrahedron with three coordination sites occupied by the other carboxylate oxygen, O2, from a half unit of three SDA ligands and the vacant site occupied by an oxygen atom, O1S in the DMF molecule.

Related literature top

For related literature, see: Chi et al. (2006); Dincâ & Long (2005); Dybtsev et al. (2004); Eddaoudi et al. (2002); Edgar et al. (2001); Hawxwell et al. (2006); Hill et al. (2005); Luan et al. (2006); Park et al. (2006); Rosi et al. (2003); Saalfrank et al. (2001); Seo et al. (2000); Wang et al. (2006); Williams et al. (2005).

Experimental top

A mixture of Cd(NO₃)₂.6H₂O (0.122 g, 3.95 x 10 ^-4 mol) and H₂SDA (0.106 g,3.95 x 10 ^-4 mol) was suspended in DMF (1.3 ml), placed in a sealed-glasstube,and heated at 90°C for 3 days. Upon cooling to room temperature, the pale-yellow crystalline was formed, collected by filtration, washed with DMF,and driedunder a reduced pressure at room temperature for 5 h to give the product (0.178 g, 78%). Anal. Calcd. for [Cd₃(SDA)₃(DMF)₂]: C,50.59; H, 3.75; N, 2.18. Found: C, 50.69; H, 3.72; N, 2.12

Computing details top

Data collection: SMART (Bruker, 1997); cell refinement: SAINT (Bruker, 1997); data reduction: SAINT (Bruker, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

	Fig. 1. The trinuclear Cd₃(O₂CR)₆ SBU cluster for 1 showing the bridging SDA ligands and the coordinated DMF molecule. The remainder of the SDA is removed and only one of the threefold disordered DMF molecule is shown for clarity. Cd atoms are shown in green, O atoms in red, N atoms in blue and C atoms in grey.
	Fig. 2. [001] view of the structure showing the 3⁶ topology. (a) A single 2D layer . (b) Two overimposed close-packed layers, A and B. (c) Cubic close-packed layers, in ABC pattern.

Poly[bis(N,N-dimethylformamide)tris(µ₄-trans-stilbene-4,4'- dicarboxylato)tricadmium(II)] top

Crystal data top

[Cd₃(C₁₆H₁₀O₄)₃(C₃H₇NO)₂]	D_x = 1.616 Mg m⁻³
M_r = 1282.11	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3	Cell parameters from 6604 reflections
Hall symbol: -R 3	θ = 1.9–28.4°
a = 16.4881 (5) Å	µ = 1.27 mm⁻¹
c = 16.7919 (10) Å	T = 223 K
V = 3953.4 (3) Å³	Cubic, colourless
Z = 3	0.30 × 0.30 × 0.30 mm
F(000) = 1914

Data collection top

Siemens SMART CCD diffractometer	2105 independent reflections
Radiation source: fine-focus sealed tube	1782 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.104
ω scans	θ_max = 28.4°, θ_min = 1.9°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −21→21
T_min = 0.69, T_max = 0.69	k = −21→18
6604 measured reflections	l = −21→22

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.057	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.181	H-atom parameters constrained
S = 1.18	w = 1/[σ²(F_o²) + (0.0995P)² + 4.8382P] where P = (F_o² + 2F_c²)/3
2105 reflections	(Δ/σ)_max = 0.001
136 parameters	Δρ_max = 1.70 e Å⁻³
92 restraints	Δρ_min = −1.53 e Å⁻³

Crystal data top

[Cd₃(C₁₆H₁₀O₄)₃(C₃H₇NO)₂]	Z = 3
M_r = 1282.11	Mo Kα radiation
Trigonal, R3	µ = 1.27 mm⁻¹
a = 16.4881 (5) Å	T = 223 K
c = 16.7919 (10) Å	0.30 × 0.30 × 0.30 mm
V = 3953.4 (3) Å³

Data collection top

Siemens SMART CCD diffractometer	2105 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	1782 reflections with I > 2σ(I)
T_min = 0.69, T_max = 0.69	R_int = 0.104
6604 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.057	92 restraints
wR(F²) = 0.181	H-atom parameters constrained
S = 1.18	Δρ_max = 1.70 e Å⁻³
2105 reflections	Δρ_min = −1.53 e Å⁻³
136 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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Cd1	0.0000	1.0000	1.0000	0.0302 (2)
Cd2	0.0000	1.0000	0.79310 (3)	0.0424 (2)
O1	0.1245 (2)	1.0679 (2)	0.9162 (2)	0.0521 (7)
O2	0.1222 (2)	1.1401 (2)	0.8067 (2)	0.0599 (9)
C1	0.1562 (3)	1.1402 (3)	0.8740 (3)	0.0458 (9)
C2	0.2391 (3)	1.2283 (3)	0.9015 (3)	0.0579 (12)
C3	0.2624 (5)	1.3134 (4)	0.8665 (4)	0.0748 (17)
H3A	0.2259	1.3142	0.8238	0.090*
C4	0.3368 (6)	1.3965 (5)	0.8918 (5)	0.109 (3)
H4A	0.3496	1.4531	0.8677	0.131*
C5	0.3911 (6)	1.3967 (5)	0.9510 (5)	0.112 (3)
C6	0.4698 (8)	1.4929 (7)	0.9730 (7)	0.138 (4)
H6	0.4748	1.5449	0.9456	0.166*
C7	0.3714 (7)	1.3114 (7)	0.9864 (6)	0.133 (4)
H7A	0.4095	1.3120	1.0283	0.159*
C8	0.2968 (5)	1.2260 (5)	0.9610 (4)	0.099 (3)
H8A	0.2859	1.1690	0.9831	0.118*
O1S	0.0000	1.0000	0.6625 (11)	0.186 (4)
N1S	0.106 (2)	1.082 (2)	0.5571 (18)	0.188 (5)	0.33
C1S	0.067 (4)	1.086 (4)	0.625 (2)	0.190 (6)	0.33
H1S	0.0825	1.1436	0.6479	0.228*	0.33
C2S	0.136 (3)	1.014 (3)	0.547 (3)	0.188 (5)	0.33
H2S1	0.1426	0.9916	0.5985	0.282*	0.33
H2S2	0.1958	1.0429	0.5194	0.282*	0.33
H2S3	0.0899	0.9616	0.5156	0.282*	0.33
C3S	0.144 (3)	1.160 (3)	0.502 (2)	0.190 (5)	0.33
H3S1	0.0964	1.1771	0.4909	0.284*	0.33
H3S2	0.1615	1.1420	0.4531	0.284*	0.33
H3S3	0.1980	1.2129	0.5257	0.284*	0.33

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cd1	0.0307 (3)	0.0307 (3)	0.0291 (4)	0.01536 (14)	0.000	0.000
Cd2	0.0414 (3)	0.0414 (3)	0.0443 (4)	0.02070 (14)	0.000	0.000
O1	0.0407 (15)	0.0402 (15)	0.068 (2)	0.0147 (13)	0.0172 (14)	0.0081 (13)
O2	0.0493 (17)	0.061 (2)	0.0463 (17)	0.0106 (15)	0.0029 (13)	0.0043 (14)
C1	0.0361 (19)	0.044 (2)	0.047 (2)	0.0117 (16)	0.0112 (16)	0.0003 (16)
C2	0.052 (3)	0.048 (2)	0.048 (2)	0.006 (2)	0.0016 (19)	0.0048 (18)
C3	0.062 (3)	0.047 (3)	0.090 (4)	0.008 (3)	−0.004 (3)	0.013 (3)
C4	0.093 (5)	0.045 (3)	0.134 (7)	−0.006 (3)	−0.026 (5)	0.008 (4)
C5	0.108 (6)	0.067 (4)	0.091 (5)	−0.008 (4)	−0.016 (4)	−0.006 (4)
C6	0.137 (8)	0.095 (6)	0.130 (8)	0.018 (5)	−0.037 (6)	0.024 (5)
C7	0.113 (7)	0.112 (7)	0.094 (5)	−0.003 (5)	−0.057 (5)	0.013 (5)
C8	0.090 (4)	0.075 (4)	0.075 (4)	−0.001 (3)	−0.032 (3)	0.025 (3)
O1S	0.192 (4)	0.192 (4)	0.174 (6)	0.096 (2)	0.000	0.000
N1S	0.186 (6)	0.190 (6)	0.181 (6)	0.089 (4)	0.000 (4)	0.000 (4)
C1S	0.192 (8)	0.191 (7)	0.178 (7)	0.088 (6)	−0.002 (5)	−0.006 (5)
C2S	0.186 (6)	0.190 (6)	0.183 (6)	0.090 (4)	−0.001 (4)	0.001 (4)
C3S	0.189 (6)	0.190 (6)	0.184 (6)	0.090 (4)	0.000 (4)	0.001 (4)

Geometric parameters (Å, º) top

Cd1—O1ⁱ	2.269 (3)	C5—C7	1.408 (13)
Cd1—O1ⁱⁱ	2.269 (3)	C5—C6	1.509 (12)
Cd1—O1ⁱⁱⁱ	2.269 (3)	C6—C6^vi	1.279 (19)
Cd1—O1	2.269 (3)	C6—H6	0.9400
Cd1—O1^iv	2.269 (3)	C7—C8	1.395 (10)
Cd1—O1^v	2.269 (3)	C7—H7A	0.9400
Cd1—Cd2^v	3.4742 (5)	C8—H8A	0.9400
Cd1—Cd2	3.4742 (5)	O1S—C1Sⁱⁱⁱ	1.43 (4)
Cd2—O2	2.189 (3)	O1S—C1Sⁱ	1.43 (4)
Cd2—O2ⁱⁱⁱ	2.189 (3)	O1S—C1S	1.43 (4)
Cd2—O2ⁱ	2.189 (3)	N1S—C1S	1.323 (9)
Cd2—O1S	2.193 (19)	N1S—C3S	1.445 (9)
O1—C1	1.255 (5)	N1S—C2S	1.448 (9)
O2—C1	1.262 (6)	C1S—H1S	0.9400
C1—C2	1.484 (6)	C2S—H2S1	0.9700
C2—C3	1.386 (8)	C2S—H2S2	0.9700
C2—C8	1.394 (8)	C2S—H2S3	0.9700
C3—C4	1.373 (9)	C3S—H3S1	0.9700
C3—H3A	0.9400	C3S—H3S2	0.9700
C4—C5	1.336 (12)	C3S—H3S3	0.9700
C4—H4A	0.9400

O1ⁱ—Cd1—O1ⁱⁱ	180.00 (13)	C3—C2—C1	120.9 (5)
O1ⁱ—Cd1—O1ⁱⁱⁱ	85.62 (14)	C8—C2—C1	120.1 (5)
O1ⁱⁱ—Cd1—O1ⁱⁱⁱ	94.38 (14)	C4—C3—C2	122.4 (7)
O1ⁱ—Cd1—O1	85.62 (14)	C4—C3—H3A	118.8
O1ⁱⁱ—Cd1—O1	94.38 (14)	C2—C3—H3A	118.8
O1ⁱⁱⁱ—Cd1—O1	85.62 (14)	C5—C4—C3	119.8 (7)
O1ⁱ—Cd1—O1^iv	94.38 (14)	C5—C4—H4A	120.1
O1ⁱⁱ—Cd1—O1^iv	85.62 (14)	C3—C4—H4A	120.1
O1ⁱⁱⁱ—Cd1—O1^iv	180.000 (1)	C4—C5—C7	119.5 (6)
O1—Cd1—O1^iv	94.38 (14)	C4—C5—C6	114.1 (8)
O1ⁱ—Cd1—O1^v	94.38 (14)	C7—C5—C6	126.4 (8)
O1ⁱⁱ—Cd1—O1^v	85.62 (14)	C6^vi—C6—C5	123.3 (13)
O1ⁱⁱⁱ—Cd1—O1^v	94.38 (14)	C6^vi—C6—H6	118.4
O1—Cd1—O1^v	180.000 (1)	C5—C6—H6	118.4
O1^iv—Cd1—O1^v	85.62 (14)	C8—C7—C5	121.7 (7)
O1ⁱ—Cd1—Cd2^v	128.30 (9)	C8—C7—H7A	119.1
O1ⁱⁱ—Cd1—Cd2^v	51.70 (9)	C5—C7—H7A	119.1
O1ⁱⁱⁱ—Cd1—Cd2^v	128.30 (9)	C7—C8—C2	117.4 (7)
O1—Cd1—Cd2^v	128.30 (9)	C7—C8—H8A	121.3
O1^iv—Cd1—Cd2^v	51.70 (9)	C2—C8—H8A	121.3
O1^v—Cd1—Cd2^v	51.70 (9)	C1Sⁱⁱⁱ—O1S—C1Sⁱ	102 (3)
O1ⁱ—Cd1—Cd2	51.70 (9)	C1Sⁱⁱⁱ—O1S—C1S	102 (3)
O1ⁱⁱ—Cd1—Cd2	128.30 (9)	C1Sⁱ—O1S—C1S	102 (3)
O1ⁱⁱⁱ—Cd1—Cd2	51.70 (9)	C1Sⁱⁱⁱ—O1S—Cd2	116 (2)
O1—Cd1—Cd2	51.70 (9)	C1Sⁱ—O1S—Cd2	116 (2)
O1^iv—Cd1—Cd2	128.30 (9)	C1S—O1S—Cd2	116 (2)
O1^v—Cd1—Cd2	128.30 (9)	C1S—N1S—C3S	120.7 (11)
Cd2^v—Cd1—Cd2	180.0	C1S—N1S—C2S	120.1 (11)
O2—Cd2—O2ⁱⁱⁱ	118.93 (3)	C3S—N1S—C2S	116.8 (10)
O2—Cd2—O2ⁱ	118.93 (3)	N1S—C1S—O1S	119 (4)
O2ⁱⁱⁱ—Cd2—O2ⁱ	118.93 (3)	N1S—C1S—H1S	120.4
O2—Cd2—O1S	95.98 (9)	O1S—C1S—H1S	120.4
O2ⁱⁱⁱ—Cd2—O1S	95.98 (9)	N1S—C2S—H2S1	109.5
O2ⁱ—Cd2—O1S	95.98 (9)	N1S—C2S—H2S2	109.5
O2—Cd2—Cd1	84.02 (9)	H2S1—C2S—H2S2	109.5
O2ⁱⁱⁱ—Cd2—Cd1	84.02 (9)	N1S—C2S—H2S3	109.5
O2ⁱ—Cd2—Cd1	84.02 (9)	H2S1—C2S—H2S3	109.5
O1S—Cd2—Cd1	180.000 (4)	H2S2—C2S—H2S3	109.5
C1—O1—Cd1	131.5 (3)	N1S—C3S—H3S1	109.5
C1—O2—Cd2	105.6 (3)	N1S—C3S—H3S2	109.5
O1—C1—O2	122.1 (4)	H3S1—C3S—H3S2	109.5
O1—C1—C2	119.8 (4)	N1S—C3S—H3S3	109.5
O2—C1—C2	118.1 (4)	H3S1—C3S—H3S3	109.5
C3—C2—C8	119.0 (5)	H3S2—C3S—H3S3	109.5

Symmetry codes: (i) −x+y−1, −x+1, z; (ii) x−y+1, x+1, −z+2; (iii) −y+1, x−y+2, z; (iv) y−1, −x+y, −z+2; (v) −x, −y+2, −z+2; (vi) −x+1, −y+3, −z+2.

Experimental details

Crystal data
Chemical formula	[Cd₃(C₁₆H₁₀O₄)₃(C₃H₇NO)₂]
M_r	1282.11
Crystal system, space group	Trigonal, R3
Temperature (K)	223
a, c (Å)	16.4881 (5), 16.7919 (10)
V (Å³)	3953.4 (3)
Z	3
Radiation type	Mo Kα
µ (mm⁻¹)	1.27
Crystal size (mm)	0.30 × 0.30 × 0.30

Data collection
Diffractometer	Siemens SMART CCD diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.69, 0.69
No. of measured, independent and observed [I > 2σ(I)] reflections	6604, 2105, 1782
R_int	0.104
(sin θ/λ)_max (Å⁻¹)	0.668

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.057, 0.181, 1.18
No. of reflections	2105
No. of parameters	136
No. of restraints	92
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	1.70, −1.53

Computer programs: SMART (Bruker, 1997), SAINT (Bruker, 1997), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Acknowledgements

The authors acknowledge Professor Kimoon Kim and Mr Hyunuk Kim for the crystallographic work and helpful discussions.

References

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