Poly[μ₃-hydroxido-μ₆-sulfato-μ₃-(1,2,4-triazolato-κ³N¹:N²:N⁴)-dicadmium(II)]: a cadmium–triazolate coordination polymer with a pseudo-cubane-like tetra­nuclear cluster

The title complex, [Cd₂(C₂H₂N₃)(OH)(SO₄)]_n, is a three-dimensional metal–organic framework consisting of pseudo-cubane-like tetra­nuclear cadmium clusters, which are formed by four Cd^II atoms, two sulfate groups and two hydroxide groups. The tetra­nuclear cadmium clusters are connected into a layered substructure by Cd—O bonds and adjacent layers are linked by triazolate ligands into a three-dimensional network. A photoluminescent study revealed that the complex exhibits a strong emission in the visible region which probably originates from a π–π* transition.

S1. Comment

Metal–organic frameworks (MOFs) have gained significant attention over past decades because of their increasingly wide range of applications (Nishikiori et al., 2005; Qi et al., 2008). Polyazaheterocyclic compounds have attracted considerable attention as useful ligands to produce coordination polymers with useful functional properties such as gas storage (Zhang & Chen, 2008), magnetism (Ma et al., 2010; Wang et al., 2010) and catalysis (Wu et al., 2005). Among these polyazaheterocyclic compounds, 1,2,4-triazole and its derivatives have been extensively applied as organic spacers in constructing MOFs because of their potential for µ_1,2-, µ_2,4- and µ_1,2,4-bridging linkages (Lin et al., 2008; Zhai et al., 2007; Wei et al., 2010). Generally, the µ_1,2,4-bridging mode is preferred, if the ligand is deprotonated. MOFs are primarily constructed from mononuclear metal centres and organic ligands, and this has resulted in a tremendous number of intriguing network topologies and a variety of packing motifs (Kitagawa et al., 2004; Bradshaw et al., 2005). Recently, more attention has been paid to expanding the classical Aufbau principles to include polynuclear coordination clusters as building units, with the intention to utilize them as nodes in the design of coordination polymers (Marin et al., 2004; Perry et al., 2007). This represents an extension of the classical `node and spacer' approach (Robson, 2000), giving rise to a family of cluster-based polymers with an enhanced variety of coordination geometries compared with single metal ions. In this report, we describe a new polymer, [Cd₂(trz)(SO₄)(OH)], (I), in which the basic building block is a pseudo-cubane-like tetranuclear cluster. To the best of our knowledge, compound (I) is the first example of a tetranuclear cluster as a unique node among the Cd^II–trz-based coordination polymers.

The complex [Cd₂(trz)(SO₄)(OH)], (I), is a three-dimensional coordination polymer. There are two cadmium atoms, one triazolate group, one sulfate group and one hydroxy group in the asymmetric unit (Fig. 1). The Cd1 atom adopts a distorted octahedral geometry, which is composed of three sulfate oxygen atoms [Cd1—O 2.353 (4)–2.445 (5) Å], two µ₃-hydroxy groups [Cd1—O 2.260 (4)–2.238 (5) Å] and one nitrogen atom from the bridging µ₃-triazolate ligand [Cd1—N2 2.228 (6) Å]. Cd2 is also six coordinated, defined by three sulfate oxygen atoms [Cd1—O 2.319 (5)–2.360 (5) Å], two triazolate nitrogen donors [Cd1—N 2.301 (5)–2.338 (5) Å] and a µ₃-hydroxy ligand [Cd1—O 2.325 (5) Å]. The detailed bond distances and geometric parameters are given in Table 1. All the Cd—N or Cd—O bond lengths are comparable to those previously reported for Cd–triazolate complexes. The structure is based on {Cd₄(µ₃-OH)₂(µ₆-SO₄)₂} clusters; four cadmium sites are bridged by two µ₃-hydroxy and two sulfate oxygen atoms to produce the {Cd4O4} core. This core resembles the cubane motif but is highly distorted with the Cd—O atom distances ranging from 2.239 (3) Å to 3.157 (3) Å. The sulfate group connects to Cd1 and Cd2 sites of adjacent clusters forming a chain and connects to the Cd1 site of an adjacent chain to produce the layered substructure seen in Fig. 2. The sulfate group adopts an η⁴, µ₆-coordination mode. The layered substructures are aligned by triazolates. Each triazolate ligand adopts the N1,N2,N4-bridging mode, linking two Cd2 atoms of the layer and one Cd1 atom of the adjacent layer to connect the layered substructures into a network (Fig. 3).

Compared with the reported Zn analogue [Zn₂(trz)(SO₄)(OH)] (Ouellette et al., 2007), the differences in the radius and coordination number of the metal centres and the diverse coordination modes of the sulfate group lead to a different substructure. The building units of the title complex, (I), are tetranuclear clusters, and each sulfate adopts an η⁴, µ₆-coordination mode. By contrast, in the [Zn₂(trz)(SO₄)(OH)] complex, the building units are {Zn₃(µ₃-OH)} clusters, and each sulfate adopts an η³, µ₅-coordination mode. Compared with another Cd/trz/sulfate complex [Cd₈(trz)₄(OH)₂(SO₄)₅(H₂O)] (Ouellette et al., 2007), the difference in the ratio of SO₄^2–/OH^– influences the substructure construction. The title complex, (I), consists of a layered cadmium/sulfate/hydroxy substructure, while the cadmium/sulfate/hydroxy substructure of [Cd₈(trz)₄(OH)₂(SO₄)₅(H₂O)] is three-dimensional, which appears as cadmium sulfate layers stacking along the a axis and connected through sulfate linkages.

Luminescence properties have been explored at room temperature in the solid state (Fig. 4). The fluorescence spectrum shows that the title compound exhibits a broad and strong emission with a maximum wavelength of 447 nm upon excitation at 325 nm. The main chromosphere of this compound is the aromatic five-membered ring and its photoluminescence is assigned as originating from π–π* transitions.

Related literature

For related literature, see: Bradshaw et al. (2005); Kitagawa et al. (2004); Lin et al. (2008); Marin et al. (2004); Nishikiori et al. (2005); Ouellette et al. (2007); Perry et al. (2007); Qi et al. (2008); Robson (2000); Wei et al. (2010); Wu et al. (2005); Zhai et al. (2007); Zhang & Chen (2008).

S2. Experimental

The title compound was prepared under mild hydrothermal conditions. 3CdSO₄.8H₂O (0.167 mmol, 128 mg), 1,2,4-triazole (1.0 mmol, 69 mg) and water (10 ml) were placed in a Teflon-lined autoclave (23 ml), the pH of the mixture was carefully adjusted to 6.0 by slow addition of 1.0 mol l^-1 NaOH solution. The mixture was heated to 423 K for 5 d, and then cooled to room temperature at the rate of 5 K h^-1. Colourless crystals suitable for X-ray analysis were obtained.

S3. Refinement

H atoms bonded to C atoms were refined in idealized positions using the riding model approximation, with C—H = 0.93 and U_iso(H) = 1.2 U_eq(C). H atoms bonded to hydroxy O atoms were located in difference maps and treated as riding, with a DFIX restraint of O—H = 0.86.

Computing details

Data collection: CrystalClear (Rigaku, 2002); cell refinement: CrystalClear (Rigaku, 2002); data reduction: CrystalClear (Rigaku, 2002); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The structure of (I), showing the atom-numbering scheme and 30% probability displacement ellipsoids. H atoms have been omitted for clarity. [Symmetry codes: (i) -x, -y, -z + 1; (ii) -x, y, -z + 1/2; (iii) -x, -y + 1, -z + 1; (iv) x, -y + 1, z - 1/2.]
	Fig. 2. The layered substructure of (I), viewed in the bc plane. The tetranuclear cluster is marked by the ring and sulfate groups are represented by tetrahedra.
	Fig. 3. A view of the three-dimensional structure of (I) along the b axis.
	Fig. 4. The solid-state fluorescence spectrum of (I) at room temperature. Key: solid lines, emission spectra; dashed lines, excitation spectra.

Poly[µ₃-hydroxido-µ₆-sulfato-µ₃-(1,2,4-triazolato- κ³N¹:N²:N⁴)-dicadmium(II)]

Crystal data

[Cd2(C2H2N3)(OH)(SO4)]	F(000) = 1504
Mr = 405.93	Dx = 3.653 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 19.098 (4) Å	θ = 12–18°
b = 6.7985 (14) Å	µ = 6.04 mm^−1
c = 12.251 (3) Å	T = 298 K
β = 111.85 (3)°	Parallelepiped, colourless
V = 1476.4 (6) Å^3	0.22 × 0.08 × 0.07 mm
Z = 8

Data collection

Rigaku MODEL? CCD area-detector diffractometer	1678 independent reflections
Radiation source: fine-focus sealed tube	1650 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.036
scintillation counter scans	θmax = 27.6°, θmin = 3.5°
Absorption correction: multi-scan (RAPID-AUTO; Rigaku, 1998)	h = −24→24
Tmin = 0.566, Tmax = 0.655	k = −8→8
5862 measured reflections	l = −15→13

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.023	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.051	w = 1/[σ^2(Fo^2) + (0.0147P)^2 + 8.3173P] where P = (Fo^2 + 2Fc^2)/3
S = 1.16	(Δ/σ)max = 0.001
1678 reflections	Δρmax = 0.53 e Å^−3
123 parameters	Δρmin = −0.63 e Å^−3
1 restraint	Extinction correction: SHELXTL (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00148 (7)

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	0.039725 (14)	0.15289 (4)	0.40032 (2)	0.01470 (10)
Cd2	0.109946 (14)	0.51117 (4)	0.21902 (2)	0.01471 (10)
S1	−0.06795 (5)	0.32686 (13)	0.56168 (7)	0.01195 (18)
O1	−0.12596 (15)	0.3806 (4)	0.4492 (2)	0.0197 (6)
O2	−0.10217 (16)	0.2612 (4)	0.6442 (2)	0.0212 (6)
O3	−0.01887 (15)	0.1678 (4)	0.5473 (2)	0.0177 (5)
O4	−0.01889 (15)	0.5001 (4)	0.6126 (2)	0.0181 (6)
O5	0.07342 (15)	0.1948 (4)	0.2432 (2)	0.0163 (5)
N1	0.26806 (18)	0.0767 (5)	0.6701 (3)	0.0189 (7)
N2	0.15246 (18)	0.1526 (5)	0.5476 (3)	0.0178 (7)
N3	0.15957 (18)	0.2137 (5)	0.6580 (3)	0.0206 (7)
C1	0.2289 (2)	0.1646 (6)	0.7269 (3)	0.0230 (9)
H1	0.2486	0.1889	0.8074	0.028*
C2	0.2180 (2)	0.0717 (6)	0.5585 (3)	0.0191 (8)
H2	0.2281	0.0177	0.4961	0.023*
H3	0.100 (3)	0.108 (7)	0.232 (5)	0.051 (18)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.01438 (14)	0.01544 (16)	0.01228 (14)	0.00027 (10)	0.00265 (10)	0.00188 (10)
Cd2	0.01175 (14)	0.01656 (16)	0.01476 (15)	−0.00107 (10)	0.00371 (11)	−0.00039 (10)
S1	0.0123 (4)	0.0120 (4)	0.0113 (4)	−0.0002 (3)	0.0041 (3)	−0.0006 (3)
O1	0.0163 (13)	0.0264 (14)	0.0122 (12)	−0.0008 (11)	0.0005 (10)	0.0014 (11)
O2	0.0284 (15)	0.0208 (14)	0.0198 (13)	−0.0023 (12)	0.0151 (12)	−0.0013 (11)
O3	0.0199 (13)	0.0137 (13)	0.0230 (13)	0.0017 (10)	0.0119 (11)	0.0006 (10)
O4	0.0163 (13)	0.0126 (13)	0.0208 (14)	−0.0013 (10)	0.0016 (11)	−0.0031 (10)
O5	0.0159 (12)	0.0165 (13)	0.0163 (12)	0.0018 (11)	0.0058 (10)	0.0005 (11)
N1	0.0123 (14)	0.0205 (17)	0.0205 (16)	0.0040 (13)	0.0021 (13)	−0.0007 (13)
N2	0.0138 (14)	0.0226 (17)	0.0128 (14)	0.0002 (13)	0.0000 (12)	−0.0021 (12)
N3	0.0189 (16)	0.0258 (18)	0.0138 (15)	0.0090 (14)	0.0023 (13)	−0.0011 (13)
C1	0.0219 (19)	0.025 (2)	0.0170 (18)	0.0087 (16)	0.0012 (16)	−0.0006 (16)
C2	0.0156 (17)	0.023 (2)	0.0198 (18)	0.0007 (15)	0.0075 (15)	−0.0019 (16)

Geometric parameters (Å, º)

Cd1—N2	2.236 (3)	O1—Cd2^i	2.365 (3)
Cd1—O5^i	2.239 (3)	O2—Cd2^iii	2.326 (3)
Cd1—O5	2.263 (3)	O3—Cd1^ii	2.349 (3)
Cd1—O3^ii	2.349 (3)	O4—Cd2^vi	2.318 (3)
Cd1—O4^iii	2.388 (3)	O4—Cd1^iii	2.388 (3)
Cd1—O3	2.450 (3)	O5—Cd1^i	2.239 (3)
Cd1—Cd1^i	3.4217 (12)	O5—H3	0.82 (4)
Cd2—N1^iv	2.297 (3)	N1—C1	1.338 (5)
Cd2—O5	2.314 (3)	N1—C2	1.347 (5)
Cd2—O4^v	2.318 (3)	N1—Cd2^iv	2.297 (3)
Cd2—O2^iii	2.326 (3)	N2—C2	1.327 (5)
Cd2—N3^v	2.342 (3)	N2—N3	1.372 (4)
Cd2—O1^i	2.365 (3)	N3—C1	1.320 (5)
S1—O1	1.457 (3)	N3—Cd2^vi	2.342 (3)
S1—O2	1.463 (3)	C1—H1	0.9300
S1—O3	1.484 (3)	C2—H2	0.9300
S1—O4	1.490 (3)
N2—Cd1—O5^i	172.64 (11)	O1—S1—O2	110.64 (17)
N2—Cd1—O5	101.14 (11)	O1—S1—O3	111.18 (16)
O5^i—Cd1—O5	79.21 (11)	O2—S1—O3	108.92 (16)
N2—Cd1—O3^ii	89.36 (11)	O1—S1—O4	109.64 (16)
O5^i—Cd1—O3^ii	96.93 (10)	O2—S1—O4	108.96 (17)
O5—Cd1—O3^ii	118.71 (10)	O3—S1—O4	107.41 (16)
N2—Cd1—O4^iii	98.11 (11)	S1—O1—Cd2^i	128.23 (16)
O5^i—Cd1—O4^iii	74.57 (10)	S1—O2—Cd2^iii	117.34 (16)
O5—Cd1—O4^iii	84.96 (10)	S1—O3—Cd1^ii	117.71 (15)
O3^ii—Cd1—O4^iii	153.42 (10)	S1—O3—Cd1	126.82 (15)
N2—Cd1—O3	88.46 (11)	Cd1^ii—O3—Cd1	109.43 (10)
O5^i—Cd1—O3	90.04 (10)	S1—O4—Cd2^vi	125.45 (15)
O5—Cd1—O3	166.37 (9)	S1—O4—Cd1^iii	133.63 (16)
O3^ii—Cd1—O3	70.57 (10)	Cd2^vi—O4—Cd1^iii	100.71 (10)
O4^iii—Cd1—O3	84.11 (9)	Cd1^i—O5—Cd1	98.93 (11)
N2—Cd1—Cd1^i	140.95 (9)	Cd1^i—O5—Cd2	105.46 (11)
O5^i—Cd1—Cd1^i	40.79 (7)	Cd1—O5—Cd2	114.89 (11)
O5—Cd1—Cd1^i	40.28 (7)	Cd1^i—O5—H3	104 (4)
O3^ii—Cd1—Cd1^i	104.00 (7)	Cd1—O5—H3	116 (4)
O4^iii—Cd1—Cd1^i	86.13 (7)	Cd2—O5—H3	114 (4)
O3—Cd1—Cd1^i	130.55 (7)	C1—N1—C2	102.5 (3)
N1^iv—Cd2—O5	88.30 (11)	C1—N1—Cd2^iv	116.7 (3)
N1^iv—Cd2—O4^v	162.75 (11)	C2—N1—Cd2^iv	140.8 (3)
O5—Cd2—O4^v	74.56 (9)	C2—N2—N3	106.4 (3)
N1^iv—Cd2—O2^iii	94.00 (11)	C2—N2—Cd1	132.1 (3)
O5—Cd2—O2^iii	115.03 (9)	N3—N2—Cd1	120.5 (2)
O4^v—Cd2—O2^iii	95.06 (10)	C1—N3—N2	105.2 (3)
N1^iv—Cd2—N3^v	87.39 (12)	C1—N3—Cd2^vi	115.2 (3)
O5—Cd2—N3^v	163.85 (11)	N2—N3—Cd2^vi	130.8 (2)
O4^v—Cd2—N3^v	108.54 (10)	N3—C1—N1	113.7 (3)
O2^iii—Cd2—N3^v	80.81 (11)	N3—C1—H1	123.2
N1^iv—Cd2—O1^i	89.20 (11)	N1—C1—H1	123.2
O5—Cd2—O1^i	84.59 (10)	N2—C2—N1	112.2 (3)
O4^v—Cd2—O1^i	87.25 (10)	N2—C2—H2	123.9
O2^iii—Cd2—O1^i	160.17 (10)	N1—C2—H2	123.9
N3^v—Cd2—O1^i	79.79 (11)

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