A novel three-dimensional cadmium(II) coordination polymer incorporating 1,4-bis­(1,2,4-triazol-1-ylmethyl)­benzene and thio­cyanate

In the title complex, poly[cadmium(II)-[mu]₂-1,4-bis­(1,2,4-triazol-1-ylmeth­yl)benzene-di-[mu]₂-thio­cyanato], [Cd(NCS)₂(C₁₂H₁₂N₆)]_n, the Cd^II atom lies on an inversion centre in a distorted octa­hedral environment. Four N atoms from the thio­cyanate and 1,4-bis­(1,2,4-triazol-1-ylmeth­yl)benzene (bbtz) ligands occupy the equatorial positions, and two S atoms from symmetry-related thio­cyanate ligands occupy the axial positions. The benzene ring of the bbtz ligand lies about an inversion centre. Single thio­cyanate bridges link the Cd^II atoms into two-dimensional sheets containing novel 16-membered [Cd₄([mu]-NCS-N:S)₄] rings. The bbtz ligands further link these two-dimensional sheets into an unprecedented covalent three-dimensional network for the cadmium-thio­cyanate system.

S1. Comment

In the past decade, the design and synthesis of metal–organic frameworks based on the principles of crystal engineering have made rapid progress. The interest in these compounds is due to their intriguing structural topologies and potential application as microporous, magnetic, nonlinear and fluorescent materials (Kahn et al., 2000; Yaghi et al., 1998; Batten & Robson 1998; Moulton & Zaworotko, 2001). In the construction of one-, two- and three-dimensional frameworks, multidentate ligands are usually used to bridge metal centres to form polymeric structures. Cadmium thiocyanate adducts of organic ligands are an important class of compounds for the design and preparation of functional coordination frameworks (Zhang et al., 1999). A number of cadmium(II) thiocyanate complex adducts of monodentate organic ligands, such as methylsubstituted pyridines (Taniguchi et al., 1986, 1987), dibenzylamine (Taniguchi & Ouchi 1987), dimethylsulfoxide (DMSO; Chenskaya et al., 2000), 4-chloropyridine (Goher et al., 2003), 1H-1,2,4-triazole (Haasnoot et al., 1983) and imidazole (Chen et al., 1999), have been reported. All of these adducts exhibit chain structures, where each pair of adjacent Cd^II atoms are bridged by two inversely related µ-SCN⁻—N,S ligands and the remaining co-ordination sites are occupied by monodentate organic ligands. Few thiocyanate bridging cadmium(II) complexes consist of a two-dimensional network (Mondal et al., 2000; Yang et al., 2001).

Our synthetic approach started by focusing on the construction of novel topological frameworks using flexible ligands such as 1,2-bis(1,2,4-triazol-1-yl)ethane (Li et al., 2004), 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene (Li et al., 2005) and 1,2-bis(benzotriazol-1-yl)ethane (Zhou et al., 2004). As a part of our work towards the rational design and preparation of functional coordination frameworks, we recently reported two novel thiocyanate cadmium complexes, namely [Cd(NCS)₂(bte)]_n (bte is?; Li et al., 2004) and [Cd_1.5(NCS)₃(bbta)_1.5]_n (bbta is?; Zhou et al., 2004). In the present paper, we report the preparation and crystal structure of the title novel three-dimensional coordination polymer, [Cd(bbtz)(NCS)₂]_n, (I), in which 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene (bbtz) ligands bridge a two-dimensional network containing 16-membered [Cd₄(µ-SCN-N,S)₄] rings.

As shown in Fig. 1, the Cd^II atom of (I) lies on an inversion centre, in a distorted octahedral environment, coordinated by four N atoms from two symmetry-related thiocyanate ligands and bis-monodentate bbtz ligands in the equatorial positions, and two S atoms from two symmetry-related thiocyanate ligands in the axial positions. This coordination environment is similar to that in the chain structures of the cadmium(II) thiocyanate complex adducts of monodentate organic ligands, such as [Cd(NCS)₂(L)₂]_n [L = 2-, 3- or 4-methylpyridine (Taniguchi et al., 1986, 1987), dibenzylamine (Taniguchi & Ouchi 1987), 4-chloropyridine (Goher et al., 2003), 1H-1,2,4-triazole (Haasnoot et al., 1983) and imidazole (Chen et al., 1999), and that in the two-dimensional network cadmium thiocyanate complexes {[Cd(NCS)₂(nicotinamide)₂]·H₂O}_n and [Cd(NCS)₂(isonicotinamide)₂]_n (Yang et al., 2001). The NCS⁻ anion acts as a bridging ligand in the µ-NCS⁻(N,S) mode in (I), with single thiocyanate bridges linking two Cd^II atoms to form the two-dimensional network. The µ-NCS⁻(N,S) coordination mode for the Cd^II atom is one of the usual coordination fashions (Zhang et al., 1999), since the Cd^II atom is in the centre of the hard–soft range in the acid–base concept [Please check amended text].

The Cd—N_bbtz bond lengths (Table 1) are similar to those in other cadmium(II) bbtz complexes (Li et al., 2005), in [Cd(NCS)₂(bte)]_n (Li et al., 2004) and in [Cd(NCS)₂(trz)₂]_n (trz = 1H-1,2,4-triazole; Haasnoot et al., 1983). The Cd—N_NCS bond lengths (Table 1) are also similar to those in double thiocyanate-bridged cadmium(II) complexes, such as [Cd(NCS)₂(L)₂]_n (L are pyridine derivatives, such as 2-, 3- or 4-methylpyridine and 4-chloropyridine; Taniguchi et al., 1986, 1987; Goher et al., 2003), [Cd(dibenzylamine)₂(NCS)₂]_n (Taniguchi & Ouchi, 1987), [Cd(NCS)₂(bte)]_n (Li et al., 2004) and [Cd(NCS)₂(trz)₂] (Haasnoot et al., 1983), in the triple thiocyanate-bridged cadmium(II) complex [Cd_1.5(bbta)_1.5(NCS)₃]_n (Zhou et al., 2004), and in singly thiocyanate-bridged cadmium(II) complexes, such as [Cd(NCS)₂(dmen)]_n (dmen = N,N-dimethylethylenediamine; Mondal et al., 2000), {[Cd(NCS)₂(nicotinamide)₂]·H₂O}_n and [Cd(NCS)₂(isonicotinamide)₂]_n (Yang et al., 2001).

The Cd—S_NCS bond lengths, and Cd—N—C_NCS and Cd—S—C_SCN bond angles (Table 1), are similar to those in the above-cited thiocyanate-bridged cadmium(II) complexes. As illustrated in Fig. 2, each NCS⁻ anion in (I) coordinates to two Cd^II atoms in an µ-NCS⁻(N,S) mode and single thiocyanate bridges link the Cd^II centres into a two-dimensional sheet, resulting in an hourglass-shaped 16-membered [Cd₄(µ-NCS-N,S)₄] metallacycle. Such [Cd₄(µ-NCS-N,S)₄] metallacycle structures have rarely been reported. Two examples are [Cd(NCS)₂(nicotinamide)₂]·H₂O and [Cd(SCN)₂(isonicotinamide)₂] (Yang et al., 2001). The Cd···Cd separation through the NCS⁻ ligand is 6.2266 (7) Å, which is similar to the values of 6.231 (1) and 6.310 (1) Å in the singly thiocyanate-bridged cadmium(II) complexes {[Cd(NCS)₂(nicotinamide)2]·H₂O}_n and [Cd(NCS)₂(isonicotinamide)₂]_n (Yang et al., 2001).

Because the methyl C atoms of the bbtz ligand can freely rotate to adjust to the coordination environment, bbtz can exhibit trans–gauche and gauche–gauche conformations. The bbtz ligands exhibit the trans–gauche conformation in (I), similar to the situation in the free bbtz molecule (Peng et al., 2004), and in [Cd(bbtz)₂(H₂O)₂](BF₄)₂·3DMF and [Cd(bbtz)₂(H₂O)₂](ClO₄)₂·3DMF (DMF is dimethylformamide; Li et al., 2005). However, the bbtz ligands show the gauche–gauche conformation in [Cd(bbtz)₂(H₂O)₂](ClO₄)₂·2H₂O. The bbtz ligands present both trans–gauche and gauche–gauche conformations in [Cd₃(bbtz)₆(H₂O)₆](BF₄·H₂O (Li et al., 2005).

The three rings (two triazole rings and one benzene ring) of one bbtz ligand are not coplanar, either in (I) or in the free bbtz molecule (Peng et al., 2004). However, the dihedral angle between the two triazole planes is 0° by imposed crystallographic symmetry [both in (I) and in the free bbtz molecule], with the dihedral angles between the benzene and triazole planes being similar to those in the free bbtz molecule.

As illustrated in Fig. 3, each bbtz ligand in (I) coordinates to two Cd^II atoms through its two triazole N atoms, thus acting as a bridging bidentate ligand, further linking the [Cd(µ-NCS-N,S)₂]_n sheets into a novel covalent three-dimensional network. To the best of our knowledge, construction of such a covalent three-dimensional network by bridging ligands and the [Cd(µ-NCS-N,S)₂]_n two-dimensional network are unprecedented in cadmium–thiocyanate systems. A 34-membered ring is formed through four Cd^II atoms linked by two single µ-NCS⁻(N,S) bridges and two bbtz ligands. The Cd···Cd distance is 13.5938 (15) Å through the bridging bbtz ligand, similar to the corresponding range of 11.568 (1)–14.845 Å in the bbtz cadmium(II) complexes reported previously (Li et al., 2005). By way of comparison, N—H···O amide hydrogen bonds between two-dimensional [Cd₄(µ-NCS-N,S)₄]_n sheets extend the two-dimensional networks into three-dimensional structures in [Cd(NCS)₂(nicotinamide)₂]·H₂O and [Cd(NCS)₂(isonicotinamide)₂] (Yang et al., 2001). The bidentate bte ligands link the [Cd(µ-NCS-N,S)₂]_n chains into a two-dimensional rhombic network in [Cd(NCS)₂(bte)]_n (Li et al., 2004).

S2. Experimental

An H₂O–CH₃OH solution (1:1 v/v, 20 ml) of 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene (bbtz) (0.120 g, 0.50 mmol) and KNCS (0.194 g, 2.0 mmol) was added to one leg of an H-shaped tube and an H₂O–CH₃OH solution (1:1 v/v, 20 ml) of Cd(NO₃)₂·3H₂O (0.185 g, 0.6 mmol) was added to the other leg of the tube. Well shaped yellow crystals of (I) suitable for X-ray analysis were obtained after about one month. The product is stable in an ambient atmosphere and is insoluble in most common inorganic and organic solvents. Analysis, found: C 35.76, H 2.53, N 23.78%; calculated for C₁₄H₁₂CdN₈S₂: C 35.87, H 2.58, N 23.91%.

S3. Refinement

H atoms were placed in idealized positions and refined as riding, with C—H distances of 0.95 (triazole and benzene) and 0.99 Å (methane), and with U_iso(H) = 1.2 U_eq(C).

Computing details

Data collection: CrystalClear (Rigaku, 2000); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1998); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of the local coordination of the Cd^II atom in (I), with displacement ellipsoids drawn at the 50% probability level. Only the atoms of the asymmetric unit have been labelled.
	Fig. 2. A perspective view of the two-dimensional sheet in (I) (see Comment). Only the Cu and thiocyanate atoms are shown, for clarity.
	Fig. 3. The three-dimensional network in (I).

poly[cadmium(II)-µ₂-1,4-bis(1,2,4-triazol-1-ylmethyl)benzene-di-µ₂– thiocyanato]

Crystal data

[Cd(NCS)2(C12H12N6)]	F(000) = 928
Mr = 468.87	Dx = 1.823 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 3252 reflections
a = 22.749 (4) Å	θ = 3.4–25.4°
b = 9.5867 (15) Å	µ = 1.54 mm^−1
c = 7.9484 (14) Å	T = 193 K
β = 99.787 (5)°	Block, yellow
V = 1708.2 (5) Å^3	0.35 × 0.16 × 0.10 mm
Z = 4

Data collection

Rigaku Mercury CCD diffractometer	1553 independent reflections
Radiation source: fine-focus sealed tube	1457 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.023
ω scans	θmax = 25.4°, θmin = 3.4°
Absorption correction: multi-scan (North et al., 1968)	h = −26→27
Tmin = 0.723, Tmax = 0.861	k = −11→11
8122 measured reflections	l = −9→9

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.023	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.056	H-atom parameters constrained
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0313P)^2 + 1.9445P] where P = (Fo^2 + 2Fc^2)/3
1553 reflections	(Δ/σ)max = 0.001
116 parameters	Δρmax = 0.34 e Å^−3
0 restraints	Δρmin = −0.31 e Å^−3

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Cd1	0.7500	0.2500	0.0000	0.02509 (10)
S1	0.68453 (3)	−0.22428 (7)	−0.24249 (10)	0.04201 (19)
N1	0.91000 (8)	0.1834 (2)	0.3920 (2)	0.0289 (4)
N2	0.91536 (11)	0.0640 (3)	0.3051 (3)	0.0498 (6)
N3	0.83446 (8)	0.1815 (2)	0.1843 (2)	0.0280 (4)
N4	0.73165 (11)	0.0221 (2)	−0.0869 (3)	0.0407 (5)
C1	0.86861 (13)	0.0690 (3)	0.1821 (4)	0.0458 (7)
H1A	0.8600	−0.0021	0.0983	0.055*
C2	0.86169 (12)	0.2506 (2)	0.3189 (3)	0.0337 (6)
H2A	0.8484	0.3369	0.3577	0.040*
C3	0.95352 (12)	0.2226 (3)	0.5442 (3)	0.0362 (6)
H3A	0.9866	0.1543	0.5614	0.043*
H3B	0.9339	0.2203	0.6463	0.043*
C4	0.97804 (10)	0.3664 (3)	0.5243 (3)	0.0304 (5)
C5	0.94670 (10)	0.4841 (3)	0.5573 (3)	0.0357 (6)
H5A	0.9098	0.4738	0.5970	0.043*
C6	0.96796 (10)	0.6161 (3)	0.5336 (3)	0.0352 (6)
H6A	0.9457	0.6954	0.5568	0.042*
C7	0.71300 (10)	−0.0789 (2)	−0.1531 (3)	0.0283 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.02783 (16)	0.02189 (15)	0.02345 (15)	−0.00317 (9)	−0.00166 (10)	−0.00003 (9)
S1	0.0457 (4)	0.0410 (4)	0.0442 (4)	−0.0182 (3)	0.0214 (3)	−0.0185 (3)
N1	0.0258 (10)	0.0304 (11)	0.0286 (10)	−0.0039 (8)	−0.0009 (8)	0.0012 (9)
N2	0.0455 (14)	0.0438 (14)	0.0539 (15)	0.0129 (11)	−0.0096 (12)	−0.0133 (12)
N3	0.0279 (10)	0.0284 (10)	0.0266 (10)	−0.0029 (8)	0.0017 (8)	−0.0004 (9)
N4	0.0563 (14)	0.0248 (11)	0.0411 (13)	−0.0048 (10)	0.0085 (11)	−0.0053 (10)
C1	0.0491 (17)	0.0361 (15)	0.0456 (16)	0.0086 (13)	−0.0108 (13)	−0.0144 (13)
C2	0.0351 (14)	0.0342 (15)	0.0289 (13)	0.0057 (10)	−0.0027 (11)	−0.0046 (10)
C3	0.0304 (13)	0.0442 (15)	0.0299 (13)	−0.0069 (11)	−0.0062 (11)	0.0035 (11)
C4	0.0246 (12)	0.0434 (14)	0.0206 (11)	−0.0072 (10)	−0.0040 (9)	−0.0031 (10)
C5	0.0217 (12)	0.0498 (16)	0.0362 (14)	−0.0091 (11)	0.0072 (10)	−0.0079 (12)
C6	0.0235 (12)	0.0432 (15)	0.0381 (14)	−0.0012 (11)	0.0030 (11)	−0.0093 (12)
C7	0.0305 (12)	0.0280 (13)	0.0275 (12)	0.0040 (10)	0.0081 (10)	0.0028 (10)

Geometric parameters (Å, º)

Cd1—N3^i	2.3047 (19)	N4—C7	1.149 (3)
Cd1—N3	2.3047 (19)	C1—H1A	0.9500
Cd1—N4	2.308 (2)	C2—H2A	0.9500
Cd1—N4^i	2.308 (2)	C3—C4	1.505 (3)
Cd1—S1^ii	2.7394 (7)	C3—H3A	0.9900
Cd1—S1^iii	2.7394 (8)	C3—H3B	0.9900
S1—C7	1.646 (3)	C4—C5	1.383 (4)
N1—C2	1.320 (3)	C4—C6^iv	1.393 (3)
N1—N2	1.353 (3)	C5—C6	1.379 (4)
N1—C3	1.476 (3)	C5—H5A	0.9500
N2—C1	1.318 (4)	C6—C4^iv	1.393 (3)
N3—C2	1.320 (3)	C6—H6A	0.9500
N3—C1	1.330 (3)
N3^i—Cd1—N3	180.00 (15)	C7—N4—Cd1	165.8 (2)
N3^i—Cd1—N4	89.47 (8)	N2—C1—N3	114.8 (2)
N3—Cd1—N4	90.53 (8)	N2—C1—H1A	122.6
N3^i—Cd1—N4^i	90.53 (8)	N3—C1—H1A	122.6
N3—Cd1—N4^i	89.47 (8)	N3—C2—N1	110.6 (2)
N4—Cd1—N4^i	180.00 (12)	N3—C2—H2A	124.7
N3^i—Cd1—S1^ii	90.00 (5)	N1—C2—H2A	124.7
N3—Cd1—S1^ii	90.00 (5)	N1—C3—C4	110.8 (2)
N4—Cd1—S1^ii	92.57 (6)	N1—C3—H3A	109.5
N4^i—Cd1—S1^ii	87.43 (6)	C4—C3—H3A	109.5
N3^i—Cd1—S1^iii	90.00 (5)	N1—C3—H3B	109.5
N3—Cd1—S1^iii	90.00 (5)	C4—C3—H3B	109.5
N4—Cd1—S1^iii	87.43 (6)	H3A—C3—H3B	108.1
N4^i—Cd1—S1^iii	92.57 (6)	C5—C4—C6^iv	118.4 (2)
S1^ii—Cd1—S1^iii	180.00 (4)	C5—C4—C3	121.0 (2)
C7—S1—Cd1^v	100.06 (8)	C6^iv—C4—C3	120.6 (2)
C2—N1—N2	109.2 (2)	C6—C5—C4	121.2 (2)
C2—N1—C3	129.0 (2)	C6—C5—H5A	119.4
N2—N1—C3	121.9 (2)	C4—C5—H5A	119.4
C1—N2—N1	102.5 (2)	C5—C6—C4^iv	120.4 (2)
C2—N3—C1	102.9 (2)	C5—C6—H6A	119.8
C2—N3—Cd1	126.69 (16)	C4^iv—C6—H6A	119.8
C1—N3—Cd1	130.41 (17)	N4—C7—S1	177.8 (2)
C2—N1—N2—C1	0.3 (3)	N1—N2—C1—N3	0.1 (3)
C3—N1—N2—C1	179.9 (2)	C2—N3—C1—N2	−0.5 (3)
N4—Cd1—N3—C2	−159.0 (2)	Cd1—N3—C1—N2	177.81 (19)
N4^i—Cd1—N3—C2	21.0 (2)	C1—N3—C2—N1	0.7 (3)
S1^ii—Cd1—N3—C2	−66.4 (2)	Cd1—N3—C2—N1	−177.70 (15)
S1^iii—Cd1—N3—C2	113.6 (2)	N2—N1—C2—N3	−0.6 (3)
N4—Cd1—N3—C1	23.1 (2)	C3—N1—C2—N3	179.8 (2)
N4^i—Cd1—N3—C1	−156.9 (2)	C2—N1—C3—C4	−54.6 (3)
S1^ii—Cd1—N3—C1	115.7 (2)	N2—N1—C3—C4	126.0 (3)
S1^iii—Cd1—N3—C1	−64.3 (2)	N1—C3—C4—C5	81.7 (3)
N3^i—Cd1—N4—C7	4.2 (9)	N1—C3—C4—C6^iv	−96.1 (3)
N3—Cd1—N4—C7	−175.8 (9)	C6^iv—C4—C5—C6	0.1 (4)
S1^ii—Cd1—N4—C7	94.2 (9)	C3—C4—C5—C6	−177.7 (2)
S1^iii—Cd1—N4—C7	−85.8 (9)	C4—C5—C6—C4^iv	−0.1 (4)

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