Dy₂(SO₃)₂(SO₄)(H₂O)₂: the first lanthanide mixed sulfate–sulfite inorganic compound

The first lanthanide mixed sulfate–sulfite inorganic coordination polymer, poly[diaqua-μ₄-sulfato-di-μ₄-sulfito-didysprosium(III)], [Dy₂(SO₃)₂(SO₄)(H₂O)₂]_n, has been obtained, in which both sulfate and sulfite groups originate from the disproportionation of S₂O₃²⁻ under hydro­thermal and weakly acidic conditions. The crystal structure of the title compound exhibits a three-dimensional framework. The Dy^III ion is surrounded by eight O atoms from one water mol­ecule and two sulfate and five sulfite groups. These DyO₈ polyhedra have two shared edges and form an infinite zigzag Dy—O chain. In the bc plane, neighbouring chains are integrated through SO₃ trigonal pyramids, forming a two-dimensional sheet. Along the a-axial direction, the sulfate group, with the central S atom lying on a twofold axis, links adjacent two-dimensional sheets via two S—O—Dy connections, thus generating the three-dimensional framework.

S1. Comment

Over the past few decades, numerous solid inorganic materials with new topologies have been synthesized, and they have applications in ion-exchange, adsorption, catalysis and radioactive waste remediation. Compared with other transition metal cations, the lanthanides have flexible bond lengths and high coordination numbers ranging from 7 to 12, which offer the possibility of forming solid-state materials with novel topological structures (Allendorf et al., 2009; Feng et al., 2010).

Coordination polymers, open-framework materials and hybrid compounds built up from various anions have been much described in the literature. The structural diversities of the anions have constructed a variety of beautiful and topologically interesting structures, such as one-dimensional chains, two-dimensional grids, three-dimensional porous structures and interpenetrating networks. Among the different anions, sulfate and sulfite groups are two important kinds of sulfur-centred oxyanions. By using sulfate groups as bridges, Doran et al. (2002), the Louer group (Bataille & Louer, 2002) and Xing et al. (2003) have achieved impressive geometric constructions. Our laboratory has also obtained the crystal structure of oxonium neodymium bis(sulfate), (H₃O)Nd(SO₄)₂, which shows a two-dimensional layered framework assembled from SO₄ tetrahedra and NdO₉ tricapped trigonal prisms (Zhang & Zhang, 2010). For the sulfite group, the S atom is in the +4 intermediate oxidation state, which causes the sulfite anion to be readily oxidized to the sulfate ion and to appear unstable under hydrothermal and acidic conditions. For the purpose of synthesizing sulfite-containing materials, the main strategy nowadays is to introduce soft acids, for example, Cu^I, into the reactant system to stabilize the S^IV moieties through coordination from the soft base site (S atom) of the sulfite group to the soft acid (Li et al., 2007, 2009; Li & Mao, 2008, 2010; Abrahams et al., 2008). There are very few examples of metal sulfites synthesized without the assistance of soft acids (Rao & Rao, 2007). Due to this synthesis barrier, coordination polymers and open-framework structures containing both sulfate and sulfite anions are relatively rare. To the best of our knowledge, to date only one mixed sulfate–sulfite inorganic compound has been reported, the mineral orschallite, Ca₃(SO₃)₂(SO₄).12H₂O (Weidenthaler et al., 1993).

To synthesize sulfite-containing compounds without introducing soft acids, we therefore adopted the strategy of selecting and designating suitable sulfur sources to be used in reactions. Here, we report the synthesis and crystal structure of the first three-dimensional lanthanide mixed sulfate–sulfite inorganic coordination polymer, [Dy₂(SO₃)₂(SO₄)(H₂O)₂ ]_n. During the synthesis of the title compound, sodium hyposulfite, with better antioxidant ability, was chosen as the sulfur source instead of the normally used corresponding sulfite or hydrosulfite. The pH of the reactant solution was carefully controlled to be near 5.0. In this weakly acidic solution, S₂O₃^2- slowly disproportionated into H₂S, SO₃^2- and SO₄^2- under hydrothermal conditions and the title compound was subsequently obtained.

X-ray crystal structure analysis indicates that the asymmetric unit of Dy₂(SO₃)₂(SO₄)(H₂O)₂ contains one distinct Dy^III cation, one sulfite ion, half of the sulfate ions and one coordinated water molecule (Fig. 1). Atom Dy1 is coordinated by eight O atoms, viz. atom O6 of the water molecule, two sulfate O atoms (O4 and O5ⁱⁱⁱ) and five sulfite O atoms (O1, O2, O1^iv, O2ⁱⁱ and O3ⁱ) (symmetry codes as in Fig. 1 and Table 1). The Dy—O bond lengths are in the range 2.286 (4)–2.428 (4) Å (Table 1). The sulfite group makes five S—O—Dy linkages, where atom O3 coordinates to one Dy^III cation and atoms O1 and O2 each bridge two metal centres, while the sulfate group, with the central S2 atom lying on a twofold axis, makes four S—O—Dy connections where each of atoms O4 and O5 coordinates to one Dy^III cation.

The sulfite group bridges atoms Dy1ⁱ, Dy1 and Dy1ⁱⁱ using its two µ₂-O atoms (O1 and O2), forming a zigzag chain (symmetry codes as in Fig. 2). The remaining sulfite atom O3 coordinates to atom Dy1(x, -y, z + 1/2) of an adjacent chain, thus forming a cationic Dy—O—S sheet of composition [Dy(SO₃)]⁺ in the bc plane (Fig. 2). A similar structure of a sulfite-bridged lanthanide-centred sheet with composition [Ln(SO₃)]⁺ (Ln = lanthanide) has also been found in the previously reported two-dimensional open-framework compound (C₂H₁₀N₂)[Nd(SO₃)(SO₄)(H₂O)]₂ (Rao & Rao, 2007), which also contains both sulfate and sulfite ligands. In this structure, the unique eight-coordinate lanthanide centre is surrounded by eight O atoms of one water molecule, and five sulfite and two sulfate groups. The differences in the structures of these two compounds are mainly attributed to the coordination modes of the sulfate groups. In the title compound, the sulfate group simultaneously coordinates to two pairs of Dy^III cations from two neighbouring sheets and, as a consequence, a three-dimensional neutral framework is generated consisting of sulfate-bridged [Dy(SO₃)]⁺ sheets (Fig. 3). In (C₂H₁₀N₂)[Nd(SO₃)(SO₄)(H₂O)]₂, each sulfate group coordinates to only two lanthanide atoms of the same [Nd(SO₃)]⁺ sheet. Thus, the formation of a three-dimensional framework is terminated and a two-dimensional open framework is formed instead. The protonated organoamine cation C₂H₁₀N₂²⁺, which acts as a template in (C₂H₁₀N₂)[Nd(SO₃)(SO₄)(H₂O)]₂, induces the formation of anionic layers of composition [Ln(SO₃)(SO₄)(H₂O)]^- with an eight-coordinate Ln centre. The anionic layers and organoamine cations are stacked in the two-dimensional open framework through electrostatic forces and hydrogen bonds between the protonated amino groups and the sulfate O atoms. The occurrence of the organoamine cation in the two-dimensional open-framework stucture separates the [Ln(SO₃)(SO₄)(H₂O)]^- anionic layers, leading to lengthening of the distance between neighbouring [Ln(SO₃)]⁺ sheets. The distance along the a direction is 5.8680 (15) Å in the title compound and 9.0880 (3) Å in (C₂H₁₀N₂)[Nd(SO₃)(SO₄)(H₂O)]₂. Along the a direction, the [Nd(SO₃)]⁺ sheets of (C₂H₁₀N₂)[Nd(SO₃)(SO₄)(H₂O)]₂ are arranged in a parallel manner. In the title compound, all the [Dy(SO₃)]⁺ sheets are equivalent. However, facilitated by bridged tetragonal µ₄-sulfate groups, these sheets can be divided into two halves, which are parallel to each other within one half, whereas they show no stacking between the halves along the a direction. Sheets of different halves are packed alternately in the three-dimensional structure of the title compound. The coordinated water molecule in the title compound is fixed through O—H···O hydrogen bonds (cf. Table 2).

In summary, the first lanthanide mixed sulfate–sulfite inorganic compound with a three-dimensional framework structure was obtained by employing S₂O₃^2- as the sulfur source, which offers a new strategy to synthesize sulfite-containing solid inorganic materials. The structure of the title compound shows that an [Ln(SO₃)]⁺ inorganic sheet can not only occur isolated in two-dimensional open-framework compounds, as in (C₂H₁₀N₂)[Nd(SO₃)(SO₄)(H₂O)]₂, but can also be bridged to form porous three-dimensional neutral frameworks. Such porous frameworks would be important catalyst candidates. However, in the title compound, the sulfate group, which is the bridging ligand connecting neighbouring [Ln(SO₃)]⁺ sheets, is rather short, which leads to a tiny pore size in the framework. Further work will focus on introducing longer organic bridge ligands than the sulfate group in the title compound, in order to synthesize metal–organic frameworks (MOFs) with larger pores based on [Ln(SO₃)]⁺ inorganic sheets.

Related literature

For related literature, see: Abrahams et al. (2008); Allendorf et al. (2009); Bataille & Louer (2002); Doran et al. (2002); Feng et al. (2010); Li & Mao (2008, 2010); Li et al. (2007, 2009); Rao & Rao (2007); Weidenthaler et al. (1993); Xing et al. (2003); Zhang & Zhang (2010).

S2. Experimental

Crystals of Dy₂(SO₃)₂(SO₄)(H₂O)₂ were synthesized hydrothermally. A mixture of DyCl₃.6H₂O (0.30 mmol, 0.11 g) and Na₂S₂O₃.5H₂O (0.10 mmol, 0.025 g) was dissolved in distilled water (8 ml). The pH was carefully adjusted to near 5.0 using 1.0 M NaOH (aqueous) and the reaction mixture was placed in a Teflon-lined stainless steel autoclave (25 ml), which was heated at 433 K for 48 h. Block-shaped colourless crystals were collected (yield 54%, based on Na₂S₂O₃.5H₂O). The final solution gave off a pungent smell with little change in pH, which is undoubtedly ascribed to the formation of H₂S from the disproportionation of S₂O₃^2- during the hydrothermal reaction.

S3. Refinement

H atoms were located in a difference Fourier map and refined with distance restraints on the O—H bond lengths [0.85 (1) Å]. In the final stages of refinement, these atoms were refined as riding, with U_iso(H) = 1.2U_eq(O6).

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 (Bruker, 2004); data reduction: APEX2 (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXL97 (Sheldrick, 2008) and DIAMOND (Brandenburg & Putz, 2005); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The asymmetric unit of Dy2(SO3)2(SO4)(H2O)2, showing the atom-labelling scheme and the complete coordination environment around Dy1. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) x, -y, z - 1/2; (ii) -x + 1/2, y + 1/2, -z + 3/2; (iii) x - 1/2, y - 1/2, z; (iv) -x + 1/2, y - 1/2, -z + 3/2.]
	Fig. 2. The cationic sheet of [Dy(SO3)]^+. [Symmetry codes: (i) -x + 1/2, y + 1/2, -z + 3/2; (ii) -x + 1/2, y - 1/2, -z + 3/2; (iii) x, -y, z + 1/2.]
	Fig. 3. A view along the b axis, showing the three-dimensional framwork structure of Dy2(SO3)2(SO4)(H2O)2. Hydrogen bonds are represented by dashed lines.

Poly[diaqua-µ₄-sulfato-di-µ₄-sulfito-didysprosium(III)]

Crystal data

[Dy2(SO3)2(SO4)(H2O)2]	F(000) = 1120
Mr = 617.21	Dx = 4.115 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1242 reflections
a = 11.736 (3) Å	θ = 3.3–26.6°
b = 6.8010 (16) Å	µ = 15.58 mm^−1
c = 12.793 (3) Å	T = 296 K
β = 102.686 (2)°	Block, colourless
V = 996.1 (4) Å^3	0.15 × 0.15 × 0.10 mm
Z = 4

Data collection

Bruker SMART APEXII diffractometer	965 independent reflections
Radiation source: fine-focus sealed tube	854 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.028
ω scans	θmax = 26.0°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Bruker, 2000)	h = −14→14
Tmin = 0.13, Tmax = 0.22	k = −8→8
3145 measured reflections	l = −15→14

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)] = 0.022	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.049	H-atom parameters constrained
S = 1.09	w = 1/[σ^2(Fo^2) + (0.021P)^2] where P = (Fo^2 + 2Fc^2)/3
965 reflections	(Δ/σ)max < 0.001
78 parameters	Δρmax = 0.64 e Å^−3
0 restraints	Δρmin = −0.91 e Å^−3

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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
O1	0.2278 (3)	0.1888 (5)	0.8427 (3)	0.0108 (8)
O2	0.2594 (3)	−0.1496 (5)	0.8443 (3)	0.0143 (9)
O3	0.1896 (4)	0.0004 (5)	0.9927 (3)	0.0180 (10)
O4	0.4050 (4)	0.0426 (6)	0.6926 (4)	0.0198 (10)
O5	0.5431 (3)	0.2962 (6)	0.6719 (3)	0.0243 (11)
O6	0.0217 (3)	0.1796 (6)	0.6173 (4)	0.0306 (12)
H6A	−0.0397	0.1210	0.5848	0.037*
H6B	−0.0124	0.2676	0.6461	0.037*
S1	0.27979 (13)	0.02404 (19)	0.92397 (12)	0.0113 (3)
S2	0.5000	0.1727 (3)	0.7500	0.0140 (5)
Dy1	0.20158 (2)	0.01432 (3)	0.67341 (2)	0.00873 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.021 (2)	0.0048 (19)	0.007 (2)	−0.0006 (16)	0.0034 (17)	0.0023 (16)
O2	0.022 (2)	0.007 (2)	0.014 (2)	0.0034 (16)	0.0043 (18)	−0.0018 (16)
O3	0.024 (2)	0.019 (2)	0.012 (2)	0.0028 (17)	0.0058 (19)	0.0024 (18)
O4	0.012 (2)	0.018 (2)	0.027 (3)	−0.0047 (17)	0.0002 (19)	−0.0048 (19)
O5	0.016 (2)	0.024 (2)	0.032 (3)	−0.0060 (18)	0.004 (2)	0.009 (2)
O6	0.008 (2)	0.020 (2)	0.059 (4)	0.0014 (17)	−0.003 (2)	−0.005 (2)
S1	0.0139 (7)	0.0105 (7)	0.0090 (7)	0.0004 (5)	0.0017 (6)	0.0002 (6)
S2	0.0083 (10)	0.0090 (10)	0.0251 (13)	0.000	0.0047 (9)	0.000
Dy1	0.01101 (16)	0.00739 (16)	0.00794 (17)	−0.00039 (10)	0.00242 (11)	−0.00026 (11)

Geometric parameters (Å, º)

O1—S1	1.559 (4)	Dy1—O6	2.360 (4)
Dy1—O1	2.429 (4)	O6—H6A	0.8479
O2—S1	1.544 (4)	O6—H6B	0.8477
Dy1—O2	2.415 (4)	S1—Dy1	3.1352 (16)
O3—S1	1.526 (5)	Dy1—O3^i	2.287 (4)
O4—S2	1.486 (4)	Dy1—O2^ii	2.352 (4)
Dy1—O4	2.353 (4)	Dy1—O5^iii	2.376 (4)
O5—S2	1.477 (4)	Dy1—O1^iv	2.388 (3)
S1—O1—Dy1^ii	129.2 (2)	O3^i—Dy1—O2	146.87 (13)
S1—O1—Dy1	101.43 (17)	O2^ii—Dy1—O2	120.93 (9)
Dy1^ii—O1—Dy1	110.77 (14)	O4—Dy1—O2	82.32 (14)
S1—O2—Dy1^iv	143.3 (2)	O6—Dy1—O2	123.37 (15)
S1—O2—Dy1	102.50 (18)	O5^iii—Dy1—O2	77.47 (14)
Dy1^iv—O2—Dy1	112.54 (15)	O1^iv—Dy1—O2	67.43 (13)
S1—O3—Dy1^v	133.7 (3)	O3^i—Dy1—O1	152.90 (12)
S2—O4—Dy1	137.2 (3)	O2^ii—Dy1—O1	67.78 (13)
S2—O5—Dy1^vi	137.2 (2)	O4—Dy1—O1	86.56 (14)
Dy1—O6—H6A	122.1	O6—Dy1—O1	88.18 (14)
Dy1—O6—H6B	132.8	O5^iii—Dy1—O1	104.97 (14)
H6A—O6—H6B	96.5	O1^iv—Dy1—O1	123.44 (8)
O3—S1—O2	105.9 (2)	O2—Dy1—O1	57.52 (12)
O3—S1—O1	103.9 (2)	O3^i—Dy1—S1	166.78 (12)
O2—S1—O1	97.4 (2)	O2^ii—Dy1—S1	93.36 (10)
O3—S1—Dy1	120.17 (18)	O4—Dy1—S1	80.17 (11)
O2—S1—Dy1	48.77 (15)	O6—Dy1—S1	110.02 (12)
O1—S1—Dy1	49.40 (14)	O5^iii—Dy1—S1	94.28 (11)
O5—S2—O5^vii	110.7 (4)	O1^iv—Dy1—S1	94.49 (9)
O5—S2—O4	109.6 (2)	O2—Dy1—S1	28.73 (9)
O5^vii—S2—O4	110.0 (2)	O1—Dy1—S1	29.17 (8)
O5—S2—O4^vii	110.0 (2)	O3^i—Dy1—Dy1^ii	118.99 (9)
O5^vii—S2—O4^vii	109.6 (2)	O2^ii—Dy1—Dy1^ii	34.24 (9)
O4—S2—O4^vii	106.9 (3)	O4—Dy1—Dy1^ii	72.80 (10)
O3^i—Dy1—O2^ii	85.14 (13)	O6—Dy1—Dy1^ii	83.22 (10)
O3^i—Dy1—O4	86.83 (16)	O5^iii—Dy1—Dy1^ii	132.74 (11)
O2^ii—Dy1—O4	73.52 (13)	O1^iv—Dy1—Dy1^ii	140.39 (9)
O3^i—Dy1—O6	82.02 (16)	O2—Dy1—Dy1^ii	87.44 (9)
O2^ii—Dy1—O6	71.81 (14)	O1—Dy1—Dy1^ii	34.28 (8)
O4—Dy1—O6	144.28 (14)	S1—Dy1—Dy1^ii	59.25 (3)
O3^i—Dy1—O5^iii	95.16 (15)	O3^i—Dy1—Dy1^iv	114.16 (9)
O2^ii—Dy1—O5^iii	140.78 (14)	O2^ii—Dy1—Dy1^iv	147.20 (9)
O4—Dy1—O5^iii	145.70 (14)	O4—Dy1—Dy1^iv	81.05 (10)
O6—Dy1—O5^iii	69.43 (15)	O6—Dy1—Dy1^iv	134.35 (11)
O3^i—Dy1—O1^iv	79.50 (13)	O5^iii—Dy1—Dy1^iv	66.88 (10)
O2^ii—Dy1—O1^iv	144.49 (14)	O1^iv—Dy1—Dy1^iv	34.94 (9)
O4—Dy1—O1^iv	73.84 (13)	O2—Dy1—Dy1^iv	33.22 (9)
O6—Dy1—O1^iv	136.00 (14)	O1—Dy1—Dy1^iv	90.68 (8)
O5^iii—Dy1—O1^iv	72.91 (13)	S1—Dy1—Dy1^iv	61.63 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O6—H6A···O3^viii	0.85	2.00	2.843 (6)	176
O6—H6B···O4^ix	0.85	2.25	3.078 (6)	167

Symmetry codes: (viii) −x, y, −z+3/2; (ix) x−1/2, y+1/2, z.