A novel two-dimensional layered framework built of strontium(II) with 4-nitro­benzene-1,3-dicarboxylic acid

In poly[[aqua­strontium(II)]-μ₆-4-nitro­benzene-1,3-dicarb­oxyl­ato-κ⁷O¹:O²:O²:O³:O³,O⁴:O⁴], [Sr(C₈H₃NO₆)(H₂O)]_n, the Sr^II ion displays a distorted bicapped triangular prismatic configuration, defined by seven carboxyl O atoms from six symmetry-related ligands and one water mol­ecule. The ligand mol­ecules connect the Sr^II ions into a two-dimensional layered framework in the ac plane, with close O...O contacts between the nitro groups and with each nitro group providing one acceptor O atom for a weak inter­molecular C—H...O hydrogen bond.

S1. Comment

In recent years, great interest has been focused on the design and synthesis of coordination polymers due to their intriguing architectures and favourable properties (Biradha & Fujita, 2002; Lee & Lin, 2002; Lin et al., 2000; Moulton & Zaworotko, 2001; Sun et al., 2002; Tominaga et al., 2002). The selected ligand is an important factor that greatly influences the structure of the coordination polymer and the functionality of the complex formed. 4-Nitro-1,3-benzenedicarboxylic acid can act as a multifunctional organic ligand via various coordination modes to metal ions utilizing completely or partially deprotonated sites, and can provide suitable hydrogen-bond acceptors and donors. In addition, the noncoordinated –NO₂ functional group may assist in the creation of supramolecular assemblies through the simultaneous formation of dative and noncovalent bonds (Pedireddi & Varughese, 2004; Luo et al.,2003). The structures of alkali earth metal complexes have been explored less than the extensively investigated transition metal complexes. Here, we report a novel Sr^II coordination polymer, the title compound, [Sr(NO₂—BDC)(H₂O)]_n, (I) [NO₂—H₂BDC is 4-nitro-1,3-benzenedicarboxylic acid].

The asymmetric unit of (I) consists of one Sr^II ion, one NO₂—BDC ligand molecule and one coordinated water molecule (Fig. 1). Each Sr^II ion is eight-coordinate via seven O atoms of six symmetry-related NO₂—BDC ligands (see the caption to Fig. 1 for the designations of these ligands) and one O atom of a water molecule. The coordination environment of the Sr^II ion can be described as a distorted bicapped triangular prism. The dihedral angle between the two basal planes (O2D/O3/O4 and O1A/O4C/O7) is 18.38 (15)°, and the vertical distances between the two capping atoms, O2E and O3B, and their side planes O1A/O2D/O3/O4C and O1A/O2D/O4/O7 are 2.061 (3) and 1.847 (3) Å, respectively. The Sr—O bond distances fall in the range 2.458 (3)–2.783 (3) Å (mean 2.600 Å; Table 1) and are all within the range of those observed for other eight-coordinated Sr^II complexes with O-donor ligands (Davies et al., 2001; Deng et al., 2005; Marchetti et al., 2006; Stahl et al., 2006).

In (I), the O1/C1/O2 group of NO₂—BDC adopts a tridentate bridging coordination mode to bind three Sr^II ions, while the O3/C8/O4 group coordinates to one Sr^II ion in a bidentate chelate fashion and further bridges another two Sr^II ions through two three-coordinating O atoms (Fig. 1). The O1/C1/O2 group of NO₂—BDC is nearly coplanar with the phenyl ring, with an interplanar angle of 4.3 (5)°, while the O3/C8/O4 group shows a large distortion from planarity with the phenyl ring, with a dihedral angle of 62.5 (5)°. The identical C—O bond distances [1.249 (5) Å] in O3/C8/O4 are different from those in the O1/C1/O2 group (Table 1), as might be expected from the different coordination modes of the two carboxylate groups. However, the average carboxylate C—O bond distance of 1.250 (5) Å is identical to that in the complex [La₂(HL)₂(L)₂(H₂O)₆]·2H₂O [1.256 (4) Å; H₂L is 3-nitro-1,2-benzenedicarboxylic acid; Xiong & Qi, 2007] and that in [Mn(NO₂—HBDC)₂(H₂O)₄]·2H₂O [1.254 (3) Å; Xiong et al., 2007].

Interestingly, the Sr^II ions are bridged by three O atoms of carboxylic acid groups (O3, O2E and O4C) in a `three-bladed propeller' mode to form a one-dimensional chain with Sr—O—Sr connectivity parallel to the c axis of the unit cell with similar O—Sr—O angles (Table 1). This mode has been reported in the complex strontium bis-N,N-di(isopropyl)aminoethylamino-2-penten-4-onate (Sergej et al., 2005), but there the Sr—O—Sr connectivity is limited to trimer units.

The O1/C1/O2 groups of NO₂—BDC, acting as T-shaped units, link the Sr^II ions to form a one-dimensional chain with Sr—O—C—O—Sr connectivity parallel to the a axis of the unit cell. As a result, a two-dimensional layered architecture is obtained (Fig. 2) which has close contacts between the non-coordinating nitro O atoms [O5···O6ⁱ = 2.986 (5) Å; symmetry code: (i) x, 1/2 - y, -1/2 + z]. This kind of O···O contact has been reported in 1,2,3,4,5,7-hexanitrocubane (Gilardi et al., 2002).

In addition, there is an intramolecular O—H···O hydrogen bond between the bound water molecule and one nitro O atom (O5) and an intermolecular C—H···O hydrogen bond utilizing the other nitro O atom (O6) (Table 2). The latter provides a weak link between the two-dimensional layers and has a graph-set motif of R₂²(10) (Bernstein et al., 1995). A similar layered coordination polymer has been reported in polymeric diaquatetrakis(µ-1,2-benzenedicarboxylato)di-µ-nitrato-pentastrontium(II) (Stein & Ruschewitz, 2005), where the two-dimensional layers are only held together by weak van der Waals forces.

Related literature

For related literature, see: Bernstein et al. (1995); Biradha & Fujita (2002); Davies et al. (2001); Deng et al. (2005); Gilardi et al. (2002); Lee & Lin (2002); Lin et al. (2000); Luo et al. (2003); Marchetti et al. (2006); Moulton & Zaworotko (2001); Pedireddi & Varughese (2004); Sergej et al. (2005); Stahl et al. (2006); Stein & Ruschewitz (2005); Sun et al. (2002); Tominaga et al. (2002); Xiong & Qi (2007); Xiong, Qi, Yu & Liang (2007).

S2. Experimental

An aqueous solution (5 ml) of SrCl₂·6H₂O (0.2666 g, 1 mmol) was added dropwise to a hot aqueous solution (3 ml) of NO₂—H₂BDC (0.2210 g, 1 mmol). After mixing for 5 min, an aqueous solution (2 ml) of NaN₃ was slowly added. The resulting mixture was stirred for 10 min at room temperature and then filtered. Colourless block-shaped crystals of (I) were obtained from the filtrate at room temperature after four weeks. See the archived CIF for a warning and IR data.

S3. Refinement

H atoms attached to C atoms were placed in calculated positions, with C—H = 0.93 Å, and allowed to ride on their parent atoms, with U_iso(H) = 1.2U_eq(C). H atoms attached to O atoms were located in a difference map and refined as riding from their as-found positions, with O—H = 0.85–0.87 Å, with U_iso(H) = 1.2U_eq(O).

Computing details

Data collection: SMART (Bruker, 1997); cell refinement: SMART; data reduction: SAINT (Bruker, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); 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 coordination of the Sr^II atom in (I), showing the atomic numbering scheme. Displacement ellispoids are drawn at the 50% probability and H-atom radii are arbitrary. [Symmetry codes: (A) -1 + x, y, z; (B) 1/2 - x, y, 1/2 + z; (C) 1/2 - x, y, -1/2 + z; (D) -1/2 + x, 1 - y, 1/2 - z; (E) 1 - x,1 - y, -z. (F) -x, 1 - y, -z; (G) 1/2 + x, 1 - y, -1/2 - z; (H) 1/2 + x, 1 - y, 1/2 - z; (I) 1 + x, y, z.]

Fig. 2. The two-dimensional layered framework in (I), viewed down the b axis. H atoms have been omitted for clarity.

poly[[aquastrontium(II)]-µ₆-4-nitrobenzene-1,3-dicarboxylato- κ⁷O¹:O²:O²:O³:O³,O⁴:O⁴]

Crystal data

[Sr(C8H3NO6)(H2O)]	F(000) = 1232
Mr = 314.75	Dx = 2.100 Mg m^−3
Orthorhombic, Pccn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2ac	Cell parameters from 3159 reflections
a = 11.0883 (19) Å	θ = 3.0–26.4°
b = 26.405 (5) Å	µ = 5.44 mm^−1
c = 6.8010 (12) Å	T = 294 K
V = 1991.2 (6) Å^3	Block, colourless
Z = 8	0.28 × 0.18 × 0.16 mm

Data collection

Bruker SMART CCD area-detector diffractometer	1747 independent reflections
Radiation source: fine-focus sealed tube	1387 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.076
ϕ and ω scans	θmax = 25.0°, θmin = 1.5°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −12→13
Tmin = 0.311, Tmax = 0.416	k = −31→28
9126 measured reflections	l = −8→6

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)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.081	H-atom parameters constrained
S = 1.15	w = 1/[σ^2(Fo^2) + (0.0264P)^2 + 2.9895P] where P = (Fo^2 + 2Fc^2)/3
1747 reflections	(Δ/σ)max < 0.001
154 parameters	Δρmax = 0.74 e Å^−3
3 restraints	Δρmin = −0.77 e Å^−3

Special details

Experimental. Caution! Metal azide complexes are potentially explosive. Only a small amount of material should be prepared and should be handled with caution. IR (KBr pellet, ν, cm^-1): 3620 (w), 1650 (w), 1623 (w), 1591 (s), 1578 (s), 1516 (m), 1434 (m), 1371 (m), 1345 (m), 853 (m), 823 (m), 791 (m), 754 (m), 724 (m).

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
Sr1	0.16049 (3)	0.430282 (14)	0.17910 (6)	0.01555 (15)
O1	0.9499 (3)	0.45920 (12)	0.1695 (5)	0.0306 (8)
O2	0.7891 (3)	0.50677 (10)	0.1039 (4)	0.0200 (7)
O3	0.3764 (3)	0.41322 (12)	0.0371 (5)	0.0247 (7)
O4	0.3662 (2)	0.38796 (11)	0.3461 (4)	0.0207 (7)
O5	0.4329 (3)	0.29302 (13)	0.1683 (6)	0.0450 (10)
O6	0.5695 (4)	0.25904 (14)	0.3488 (7)	0.0533 (12)
O7	0.1166 (4)	0.33264 (14)	0.2162 (6)	0.0563 (12)
H7A	0.1215	0.3163	0.3275	0.068*
H7B	0.1372	0.3169	0.1121	0.068*
N1	0.5311 (4)	0.29380 (14)	0.2489 (6)	0.0288 (10)
C1	0.8400 (4)	0.46541 (15)	0.1469 (6)	0.0168 (9)
C2	0.7585 (4)	0.41996 (14)	0.1716 (6)	0.0164 (9)
C3	0.8065 (4)	0.37232 (16)	0.2071 (6)	0.0193 (10)
H3	0.8896	0.3682	0.2142	0.023*
C4	0.7321 (4)	0.33113 (17)	0.2320 (7)	0.0230 (11)
H4	0.7642	0.2991	0.2548	0.028*
C5	0.6082 (4)	0.33818 (16)	0.2225 (6)	0.0167 (10)
C6	0.5567 (4)	0.38525 (15)	0.1873 (6)	0.0153 (9)
C7	0.6339 (4)	0.42565 (15)	0.1598 (6)	0.0169 (9)
H7	0.6018	0.4574	0.1328	0.020*
C8	0.4226 (4)	0.39555 (15)	0.1898 (6)	0.0150 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr1	0.0106 (2)	0.0193 (2)	0.0167 (2)	−0.00074 (18)	−0.00010 (17)	0.00138 (19)
O1	0.0116 (16)	0.0316 (19)	0.049 (2)	0.0011 (14)	−0.0014 (16)	0.0068 (17)
O2	0.0182 (15)	0.0120 (15)	0.0298 (18)	−0.0005 (13)	0.0021 (14)	0.0016 (13)
O3	0.0162 (15)	0.0410 (19)	0.0167 (17)	0.0037 (14)	−0.0054 (14)	0.0044 (15)
O4	0.0187 (16)	0.0257 (17)	0.0178 (18)	0.0020 (13)	0.0056 (13)	0.0001 (14)
O5	0.0249 (19)	0.029 (2)	0.081 (3)	−0.0076 (16)	0.004 (2)	−0.008 (2)
O6	0.049 (2)	0.025 (2)	0.085 (3)	0.0001 (18)	0.008 (2)	0.025 (2)
O7	0.082 (3)	0.033 (2)	0.054 (3)	−0.010 (2)	0.000 (2)	0.005 (2)
N1	0.028 (2)	0.015 (2)	0.044 (3)	0.0013 (18)	0.011 (2)	−0.0013 (19)
C1	0.019 (2)	0.019 (2)	0.013 (2)	0.0019 (19)	−0.0003 (19)	0.0008 (17)
C2	0.019 (2)	0.016 (2)	0.014 (2)	0.0047 (17)	0.0000 (19)	0.0028 (19)
C3	0.015 (2)	0.021 (2)	0.022 (3)	0.0030 (18)	−0.0007 (18)	0.003 (2)
C4	0.020 (2)	0.016 (2)	0.032 (3)	0.0058 (19)	0.002 (2)	0.003 (2)
C5	0.018 (2)	0.015 (2)	0.018 (2)	0.0000 (18)	0.0029 (19)	0.0002 (18)
C6	0.013 (2)	0.019 (2)	0.014 (2)	0.0028 (17)	0.0000 (18)	−0.0010 (19)
C7	0.017 (2)	0.013 (2)	0.020 (2)	0.0025 (18)	0.0005 (18)	−0.0026 (19)
C8	0.015 (2)	0.009 (2)	0.021 (2)	−0.0017 (17)	0.0011 (19)	−0.0063 (19)

Geometric parameters (Å, º)

Sr1—O1^i	2.458 (3)	O7—H7A	0.8729
Sr1—O3^ii	2.510 (3)	O7—H7B	0.8528
Sr1—O4^iii	2.543 (3)	N1—C5	1.461 (5)
Sr1—O2^iv	2.604 (3)	C1—C2	1.511 (6)
Sr1—O3	2.621 (3)	C2—C3	1.387 (5)
Sr1—O7	2.636 (4)	C2—C7	1.393 (6)
Sr1—O2^v	2.641 (3)	C3—C4	1.375 (6)
Sr1—O4	2.783 (3)	C3—H3	0.9300
O1—C1	1.239 (5)	C4—C5	1.388 (6)
O2—C1	1.263 (5)	C4—H4	0.9300
O3—C8	1.249 (5)	C5—C6	1.389 (6)
O4—C8	1.249 (5)	C6—C7	1.380 (6)
O5—N1	1.220 (5)	C6—C8	1.511 (6)
O6—N1	1.219 (5)	C7—H7	0.9300
O1^i—Sr1—O3^ii	85.79 (10)	Sr1^iii—O3—Sr1	100.23 (10)
O1^i—Sr1—O4^iii	90.12 (11)	C8—O4—Sr1^ii	138.2 (3)
O3^ii—Sr1—O4^iii	140.01 (10)	C8—O4—Sr1	89.9 (2)
O1^i—Sr1—O2^iv	89.19 (10)	Sr1^ii—O4—Sr1	95.24 (9)
O3^ii—Sr1—O2^iv	150.08 (10)	Sr1—O7—H7A	123.7
O4^iii—Sr1—O2^iv	69.34 (9)	Sr1—O7—H7B	110.4
O1^i—Sr1—O3	155.74 (11)	H7A—O7—H7B	117.4
O3^ii—Sr1—O3	118.40 (9)	O6—N1—O5	123.4 (4)
O4^iii—Sr1—O3	72.70 (9)	O6—N1—C5	117.9 (4)
O2^iv—Sr1—O3	68.98 (9)	O1—C1—O2	125.6 (4)
O1^i—Sr1—O7	97.51 (12)	O1—C1—C2	118.0 (4)
O3^ii—Sr1—O7	72.63 (12)	O2—C1—C2	116.5 (4)
O4^iii—Sr1—O7	68.52 (11)	C3—C2—C7	119.2 (4)
O2^iv—Sr1—O7	137.29 (11)	C3—C2—C1	120.7 (4)
O3—Sr1—O7	92.06 (12)	C7—C2—C1	120.1 (4)
O1^i—Sr1—O2^v	109.42 (10)	C4—C3—C2	120.6 (4)
O3^ii—Sr1—O2^v	70.06 (10)	C4—C3—H3	119.7
O4^iii—Sr1—O2^v	146.85 (9)	C2—C3—H3	119.7
O2^iv—Sr1—O2^v	84.00 (7)	C3—C4—C5	118.8 (4)
O3—Sr1—O2^v	79.67 (9)	C3—C4—H4	120.6
O7—Sr1—O2^v	131.44 (11)	C5—C4—H4	120.6
O1^i—Sr1—O4	156.15 (10)	C4—C5—C6	122.4 (4)
O3^ii—Sr1—O4	70.47 (9)	C4—C5—N1	117.7 (4)
O4^iii—Sr1—O4	106.41 (9)	C6—C5—N1	119.9 (4)
O2^iv—Sr1—O4	112.46 (9)	C7—C6—C5	117.4 (4)
O3—Sr1—O4	48.11 (9)	C7—C6—C8	118.2 (4)
O7—Sr1—O4	73.72 (11)	C5—C6—C8	124.3 (4)
O2^v—Sr1—O4	65.29 (9)	C6—C7—C2	121.6 (4)
C1—O1—Sr1^vi	168.2 (3)	C6—C7—H7	119.2
C1—O2—Sr1^iv	128.9 (3)	C2—C7—H7	119.2
C1—O2—Sr1^vii	131.0 (3)	O3—C8—O4	124.2 (4)
Sr1^iv—O2—Sr1^vii	97.31 (9)	O3—C8—C6	117.5 (4)
C8—O3—Sr1^iii	160.2 (3)	O4—C8—C6	118.2 (4)
C8—O3—Sr1	97.6 (2)
O1^i—Sr1—O3—C8	178.7 (3)	O1—C1—C2—C3	−3.6 (6)
O3^ii—Sr1—O3—C8	3.5 (3)	O2—C1—C2—C3	176.1 (4)
O4^iii—Sr1—O3—C8	−134.5 (3)	O1—C1—C2—C7	176.0 (4)
O2^iv—Sr1—O3—C8	151.5 (3)	O2—C1—C2—C7	−4.3 (6)
O7—Sr1—O3—C8	−67.8 (3)	C7—C2—C3—C4	−0.5 (7)
O2^v—Sr1—O3—C8	64.0 (2)	C1—C2—C3—C4	179.2 (4)
O4—Sr1—O3—C8	−2.0 (2)	C2—C3—C4—C5	−0.5 (7)
O1^i—Sr1—O3—Sr1^iii	−9.8 (3)	C3—C4—C5—C6	0.5 (7)
O3^ii—Sr1—O3—Sr1^iii	174.97 (4)	C3—C4—C5—N1	179.9 (4)
O4^iii—Sr1—O3—Sr1^iii	36.90 (10)	O6—N1—C5—C4	27.9 (6)
O2^iv—Sr1—O3—Sr1^iii	−37.06 (10)	O5—N1—C5—C4	−151.0 (4)
O7—Sr1—O3—Sr1^iii	103.64 (12)	O6—N1—C5—C6	−152.7 (4)
O2^v—Sr1—O3—Sr1^iii	−124.55 (11)	O5—N1—C5—C6	28.4 (6)
O4—Sr1—O3—Sr1^iii	169.49 (16)	C4—C5—C6—C7	0.5 (6)
O1^i—Sr1—O4—C8	−178.7 (3)	N1—C5—C6—C7	−178.8 (4)
O3^ii—Sr1—O4—C8	−172.9 (2)	C4—C5—C6—C8	−175.5 (4)
O4^iii—Sr1—O4—C8	49.1 (3)	N1—C5—C6—C8	5.2 (7)
O2^iv—Sr1—O4—C8	−24.9 (2)	C5—C6—C7—C2	−1.6 (6)
O3—Sr1—O4—C8	1.9 (2)	C8—C6—C7—C2	174.7 (4)
O7—Sr1—O4—C8	110.1 (2)	C3—C2—C7—C6	1.5 (7)
O2^v—Sr1—O4—C8	−96.5 (2)	C1—C2—C7—C6	−178.1 (4)
O1^i—Sr1—O4—Sr1^ii	−40.3 (3)	Sr1^iii—O3—C8—O4	−150.5 (6)
O3^ii—Sr1—O4—Sr1^ii	−34.48 (9)	Sr1—O3—C8—O4	3.9 (4)
O4^iii—Sr1—O4—Sr1^ii	−172.47 (7)	Sr1^iii—O3—C8—C6	32.7 (10)
O2^iv—Sr1—O4—Sr1^ii	113.57 (9)	Sr1—O3—C8—C6	−172.9 (3)
O3—Sr1—O4—Sr1^ii	140.40 (15)	Sr1^ii—O4—C8—O3	−101.5 (5)
O7—Sr1—O4—Sr1^ii	−111.40 (13)	Sr1—O4—C8—O3	−3.7 (4)
O2^v—Sr1—O4—Sr1^ii	41.92 (9)	Sr1^ii—O4—C8—C6	75.3 (5)
Sr1^vi—O1—C1—O2	−140.8 (13)	Sr1—O4—C8—C6	173.1 (3)
Sr1^vi—O1—C1—C2	38.9 (17)	C7—C6—C8—O3	62.2 (5)
Sr1^iv—O2—C1—O1	50.9 (6)	C5—C6—C8—O3	−121.8 (5)
Sr1^vii—O2—C1—O1	−105.5 (5)	C7—C6—C8—O4	−114.8 (4)
Sr1^iv—O2—C1—C2	−128.9 (3)	C5—C6—C8—O4	61.2 (6)
Sr1^vii—O2—C1—C2	74.8 (4)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O7—H7A···O5^ii	0.87	2.47	3.294 (6)	157
C4—H4···O6^viii	0.93	2.48	3.335 (6)	153

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