catena-Poly[bis­{[mu]-dihydrogen [(5-carb­oxy­pentyl­imino)dimethyl­ene]diphosphon­ato-[kappa]²O:O'}­cadmium(II)]

The title compound, [Cd(C₈H₁₈NO₈P₂)₂]_n, synthesized by hydro­thermal methods, exhibits a layered structure in which the Cd^II ion, occupying a centre of symmetry, is coordinated by six O atoms from four phosphonate ligands. The crosslinkage of CdO₆ octa­hedra by bridging phosphonate ligands results in a cadmium(II) phosphonate layer. Within the layer, there exists a 16-membered ring incorporating four -Cd-O-P-O- linkages. The uncoordinated carboxyl group of the ligand is oriented so that it penetrates the adjacent layer, taking part in hydrogen bonding to two uncoordinated phosphonate O atoms to form a CO₂H/HO₂P motif.

S1. Comment

In recent years, the chemistry of metal phosphonates has been an active research area owing to their potential applications in the area of catalysis, ion exchange, proton conductivity, surface modification, intercalation chemistry, photochemistry and materials chemistry (Clearfield 1998). Metal phosphonates can exhibit structural types such as zero-dimensional (Ying, Chen et al., 2008), one-dimensional chain (Yang et al., 2005), two-dimensional layered (Zheng et al., 2002) or three-dimensional microporous (Burkholder et al., 2004). It has been proved that attaching an additional functional group, such as carboxyl, hydroxy, amine or crown ether groups, to the phosphonic acid is a very useful method for building open-framework structures (Cheetham et al., 1999; Stock, Frey et al., 2000; Serpaggi & Férey, 1999; Ying et al., 2006). Amongst these functional groups, the carboxyl group has been used widely because of its coordination ability. Phosphonic acids such as HOOC–R–PO₃H₂, HOOC–RN–(CH₂PO₃H₂)₂ and HOOC–RNHCH₂PO₃H₂, which contain only one carboxylate group, have been used extensively (Bauer et al., 2005; Tang et al., 2006; Stock, 2002; Stock, Stucky & Cheetham, 2000; Zhang et al., 2005). Amongst these three types of phosphonic acid ligand, several metal phosphonates of the form HOOC–RN–(CH₂PO₃H₂)₂ have been reported (Bauer et al., 2005; Tang et al., 2006), but to the best of our knowledge, metal phosphonates of the form HOOC–(CH₂)₅–N(CH₂PO₃H₂)₂ are rare. The only example, namely, Pb₂[O₂C(CH₂)₅N(CH₂PO₃)(CH₂PO₃H)] has been reported by our group recently (Ying, Li et al., 2008). This compound exhibits a three-dimensional structure and the metal ions are five-coordinate. Transition metal phosphonates formed by this ligand have not been reported before. Hydrothermal reactions of Cd^II acetate and HOOC–(CH₂)₅–N(CH₂PO₃H₂)₂ resulted in the title compound, (I). We report here its synthesis, characterization and crystal structure.

As shown in Fig. 1, in the title compound, the cadmium(II) ions, which reside on centres of symmetry are coordinated by six O atoms from four phosphonate ligands (O2, O4, O2A, O4A, O3C and O3B). The Cd—O distances range from 2.242 (2) to 2.287 (2) Å (Table 1). The cadmium(II) ions exhibit a distorted octahedral coordination geometry. The phosphonate anion chelates the Cd^II ion in a bidentate fashion, and also forms a bridge to second Cd^II ion. The amine group and carboxyl group of the ligand remain non-coordinated. The cross-linkage of CdO₆ octahedra by bridging phosphonate ligands results in a cadmium(II) phosphonate layer (Fig. 2). Within the layer, there is a 16-membered ring made up of four (Cd—O2—P1—O3–) sequences. If, for the purposes of classifying the net, we define the chelate ring as a single point of connection to Cd (making each ligand effectively a single linker), so that all Cd atoms can be defined as four-connected nodes, then the layer can be described as a (4,4) grid. Within the layer there is a single hydrogen bond (Table 2). The uncoordinated carboxylate group at the end of the hydrocarbon arm of the molecule penetrates the adjacent layer to form a double hydrogen-bonded CO₂H···HO₂P motif (Fig. 3 and Table 2).

Related literature

For related literature, see: Bauer et al. (2005); Burkholder et al. (2004); Cheetham et al. (1999); Clearfield (1998); Serpaggi & Férey (1999); Stock (2002); Stock et al. (2000a, 2000b); Tang et al. (2006); Yang et al. (2005); Ying et al. (2006, 2008a, 2008b); Zheng et al. (2002).

S2. Experimental

The phosphonic acid ligand was synthesized by a Mannich-type reaction according to previously reported procedures (Ying, Li et al., 2008). The title compound was synthesized as follows: a mixture of cadmium acetate (0.5 mmol, 0.130 g) and 6-carboxyl dimethylphosphonoaminohexane (0.5 mmol, 0.078 g) in distilled water (15 ml) was sealed in an autoclave equipped with a Teflon liner (20 ml) and then heated at 423 K 150°C for 4 d. Crystals of the title compound (colorless blocks) were obtained. Analysis found: C 25.62, H 4.87, N 3.70%; calculated for C₁₆H₃₆CdN₂O₁₆P₄ (Mr = 748.75): C 25.64, H 4.81, N 3.74%. IR data (KBr, cm^-1): 3436 (m), 3106 (s), 2960 (s), 1712 (m), 1677 (m), 1472 (m), 1437 (m), 1419 (m), 1375 (w), 1325 (s), 1290 (m), 1259 (s), 1235 (s), 1158 (s), 1083 (s), 1049 (s), 970 (m), 931 (s), 792 (m), 771 (m), 734 (m), 582 (s), 570 (s), 533 (m), 469 (s).

S3. Refinement

All the H atoms were positioned geometrically (C—H = 0.97 Å [please check change], O—H = 0.82 Å and N—H = 0.91 Å) and refined in the riding-model approximation [U_iso(H) = 1.2U_eq(C), 1.5U_eq(O) and 1.2U_eq(N), respectively].

Computing details

Data collection: SMART (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); 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

	Fig. 1. The molecular structure of the title compound with 30% probability displacement ellipsoids. [Symmetry codes: (A) -x + 1, -y + 1,-z + 1; (B) -x + 1, y + 1/2, -z + 1/2; (C) x, -y + 1/2, z + 1/2.]
	Fig. 2. The Cd^II phosphonate layer in the title compound. The C—PO3 tetrahedra are shaded. The OOC(CH2)5– group (which is approximately normal to this plane) and the H atoms have been omitted for clarity.
	Fig. 3. A view of the structure of the title compound, along the b axis. The C—PO3 tetrahedra are shaded and H atoms have been omitted for clarity. Hydrogen bonding is shown by dashed lines.

catena-Poly[bis{µ-dihydrogen [(5-carboxypentylimino)dimethylene]diphosphonato- κ²O:O'}cadmium(II)]

Crystal data

[Cd(C8H18NO8P2)2]	F(000) = 764
Mr = 748.75	Dx = 1.795 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 4271 reflections
a = 15.4321 (17) Å	θ = 2.6–28.4°
b = 9.4632 (11) Å	µ = 1.10 mm^−1
c = 9.9958 (11) Å	T = 273 K
β = 108.414 (2)°	Block, colourless
V = 1385.0 (3) Å^3	0.23 × 0.11 × 0.07 mm
Z = 2

Data collection

Bruker SMART CCD area-detector diffractometer	3219 independent reflections
Radiation source: sealed tube	2092 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.095
ω scans	θmax = 28.0°, θmin = 2.6°
Absorption correction: multi-scan (North et al., 1968)	h = −19→20
Tmin = 0.787, Tmax = 0.927	k = −12→12
10035 measured reflections	l = −13→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)] = 0.038	H-atom parameters constrained
wR(F^2) = 0.089	w = 1/[σ^2(Fo^2) + (0.0307P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.87	(Δ/σ)max = 0.025
3219 reflections	Δρmax = 1.27 e Å^−3
179 parameters	Δρmin = −1.20 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0019 (6)

Special details

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	0.50000	0.50000	0.50000	0.0196 (1)
P1	0.54772 (7)	0.22584 (10)	0.30183 (9)	0.0195 (3)
P2	0.28670 (7)	0.37672 (10)	0.28853 (9)	0.0225 (3)
O1	0.64054 (17)	0.1664 (3)	0.4018 (2)	0.0304 (9)
O2	0.54124 (18)	0.3807 (2)	0.3314 (2)	0.0237 (8)
O3	0.53347 (18)	0.1894 (2)	0.1498 (2)	0.0234 (8)
O4	0.35878 (17)	0.4086 (3)	0.4253 (2)	0.0254 (8)
O5	0.25164 (17)	0.4939 (3)	0.1826 (3)	0.0281 (8)
O6	0.20569 (18)	0.3046 (3)	0.3272 (3)	0.0292 (9)
O7	−0.0707 (2)	−0.2723 (4)	−0.0944 (3)	0.0566 (11)
O8	−0.0987 (2)	−0.4438 (3)	0.0386 (3)	0.0432 (10)
N1	0.3714 (2)	0.1136 (3)	0.2475 (3)	0.0208 (10)
C1	−0.0510 (3)	−0.3361 (5)	0.0160 (4)	0.0345 (14)
C2	0.0311 (3)	−0.2982 (4)	0.1398 (4)	0.0359 (16)
C3	0.0928 (3)	−0.1931 (5)	0.1017 (5)	0.0450 (16)
C4	0.1750 (3)	−0.1464 (4)	0.2229 (4)	0.0338 (14)
C5	0.2295 (3)	−0.0338 (4)	0.1772 (4)	0.0334 (14)
C6	0.3099 (3)	0.0162 (4)	0.2971 (4)	0.0264 (11)
C7	0.4626 (2)	0.1307 (4)	0.3586 (3)	0.0228 (11)
C8	0.3269 (3)	0.2506 (4)	0.1845 (3)	0.0235 (11)
H1A	0.66400	0.11530	0.35660	0.0460*
H2A	0.01060	−0.25900	0.21430	0.0430*
H2B	0.06560	−0.38340	0.17590	0.0430*
H3A	0.05700	−0.11010	0.06160	0.0540*
H3B	0.11430	−0.23430	0.02910	0.0540*
H4A	0.21390	−0.22740	0.25900	0.0400*
H4B	0.15450	−0.10970	0.29840	0.0400*
H5A	0.25100	−0.07150	0.10320	0.0400*
H5B	0.19000	0.04590	0.13860	0.0400*
H6A	0.34490	−0.06490	0.34400	0.0320*
H6B	0.28800	0.06570	0.36520	0.0320*
H6C	0.16230	0.29340	0.25530	0.0440*
H7A	0.48650	0.03770	0.39100	0.0270*
H7B	0.45390	0.18030	0.43830	0.0270*
H8A	0.27500	0.22640	0.10340	0.0280*
H8B	0.37010	0.30000	0.14880	0.0280*
H8C	−0.14410	−0.45530	−0.03040	0.0650*
H10A	0.38300	0.06690	0.17540	0.0250*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.0282 (2)	0.0200 (2)	0.0109 (2)	−0.0013 (2)	0.0068 (1)	−0.0003 (2)
P1	0.0267 (6)	0.0200 (5)	0.0118 (4)	−0.0004 (4)	0.0062 (4)	−0.0013 (4)
P2	0.0238 (6)	0.0244 (5)	0.0186 (5)	−0.0021 (4)	0.0057 (4)	−0.0006 (4)
O1	0.0291 (17)	0.0404 (16)	0.0194 (13)	0.0078 (13)	0.0043 (12)	−0.0015 (12)
O2	0.0397 (17)	0.0198 (13)	0.0146 (12)	−0.0027 (12)	0.0127 (11)	−0.0045 (10)
O3	0.0382 (17)	0.0225 (13)	0.0110 (11)	−0.0031 (12)	0.0099 (11)	−0.0029 (10)
O4	0.0253 (15)	0.0298 (14)	0.0200 (13)	−0.0043 (12)	0.0058 (11)	−0.0015 (11)
O5	0.0259 (15)	0.0256 (13)	0.0294 (13)	0.0001 (13)	0.0040 (11)	0.0043 (12)
O6	0.0237 (16)	0.0382 (16)	0.0250 (14)	−0.0051 (13)	0.0069 (12)	0.0003 (12)
O7	0.049 (2)	0.070 (2)	0.0376 (18)	−0.0267 (18)	−0.0053 (16)	0.0209 (17)
O8	0.0386 (19)	0.0468 (17)	0.0357 (16)	−0.0179 (15)	−0.0004 (14)	0.0048 (15)
N1	0.0268 (19)	0.0213 (16)	0.0153 (14)	−0.0028 (14)	0.0080 (13)	−0.0033 (13)
C1	0.028 (3)	0.039 (2)	0.037 (2)	−0.008 (2)	0.011 (2)	−0.006 (2)
C2	0.034 (3)	0.043 (3)	0.029 (2)	−0.009 (2)	0.0077 (19)	−0.005 (2)
C3	0.038 (3)	0.051 (3)	0.040 (2)	−0.016 (2)	0.004 (2)	0.000 (2)
C4	0.034 (3)	0.036 (2)	0.033 (2)	−0.004 (2)	0.013 (2)	−0.0059 (19)
C5	0.031 (3)	0.033 (2)	0.033 (2)	−0.0064 (18)	0.0057 (19)	−0.0019 (18)
C6	0.028 (2)	0.028 (2)	0.0255 (18)	−0.0044 (18)	0.0117 (16)	−0.0035 (17)
C7	0.032 (2)	0.0206 (19)	0.0136 (17)	0.0003 (17)	0.0039 (16)	0.0001 (15)
C8	0.025 (2)	0.029 (2)	0.0174 (18)	0.0005 (17)	0.0081 (16)	0.0016 (16)

Geometric parameters (Å, º)

Cd1—O2	2.280 (2)	N1—C8	1.509 (5)
Cd1—O3^i	2.2875 (19)	N1—H10A	0.9100
Cd1—O4	2.242 (3)	C1—C2	1.508 (6)
Cd1—O2^ii	2.280 (2)	C2—C3	1.507 (6)
Cd1—O4^ii	2.242 (3)	C3—C4	1.517 (6)
Cd1—O3^iii	2.2875 (19)	C4—C5	1.515 (6)
P1—O1	1.569 (3)	C5—C6	1.503 (6)
P1—O2	1.505 (2)	C2—H2A	0.9700
P1—O3	1.505 (2)	C2—H2B	0.9700
P1—C7	1.825 (4)	C3—H3A	0.9700
P2—O4	1.496 (2)	C3—H3B	0.9700
P2—O5	1.511 (3)	C4—H4A	0.9700
P2—O6	1.576 (3)	C4—H4B	0.9700
P2—C8	1.816 (4)	C5—H5A	0.9700
O7—C1	1.210 (5)	C5—H5B	0.9700
O8—C1	1.318 (6)	C6—H6A	0.9700
O1—H1A	0.8200	C6—H6B	0.9700
O6—H6C	0.8200	C7—H7A	0.9700
O8—H8C	0.8200	C7—H7B	0.9700
N1—C6	1.514 (5)	C8—H8A	0.9700
N1—C7	1.501 (4)	C8—H8B	0.9700
O2—Cd1—O4	91.82 (9)	C3—C4—C5	111.6 (3)
O2—Cd1—O3^i	87.83 (7)	C4—C5—C6	112.2 (3)
O2—Cd1—O4^ii	88.18 (9)	N1—C6—C5	112.0 (3)
O2—Cd1—O3^iii	92.17 (7)	P1—C7—N1	115.0 (2)
O3^i—Cd1—O4	95.14 (9)	P2—C8—N1	121.2 (2)
O2^ii—Cd1—O4	88.18 (9)	C1—C2—H2A	109.00
O3^iii—Cd1—O4	84.86 (9)	C1—C2—H2B	109.00
O2^ii—Cd1—O3^i	92.17 (7)	C3—C2—H2A	109.00
O3^i—Cd1—O4^ii	84.86 (9)	C3—C2—H2B	109.00
O2^ii—Cd1—O4^ii	91.82 (9)	H2A—C2—H2B	108.00
O2^ii—Cd1—O3^iii	87.83 (7)	C2—C3—H3A	108.00
O3^iii—Cd1—O4^ii	95.14 (9)	C2—C3—H3B	108.00
O1—P1—O2	109.15 (15)	C4—C3—H3A	108.00
O1—P1—O3	111.94 (14)	C4—C3—H3B	108.00
O1—P1—C7	103.13 (15)	H3A—C3—H3B	108.00
O2—P1—O3	115.08 (12)	C3—C4—H4A	109.00
O2—P1—C7	108.69 (16)	C3—C4—H4B	109.00
O3—P1—C7	108.12 (15)	C5—C4—H4A	109.00
O4—P2—O5	119.41 (16)	C5—C4—H4B	109.00
O4—P2—O6	106.36 (15)	H4A—C4—H4B	108.00
O4—P2—C8	111.47 (17)	C4—C5—H5A	109.00
O5—P2—O6	110.71 (16)	C4—C5—H5B	109.00
O5—P2—C8	101.10 (16)	C6—C5—H5A	109.00
O6—P2—C8	107.26 (18)	C6—C5—H5B	109.00
Cd1—O2—P1	132.76 (13)	H5A—C5—H5B	108.00
Cd1^iv—O3—P1	140.46 (12)	N1—C6—H6A	109.00
Cd1—O4—P2	138.27 (14)	N1—C6—H6B	109.00
P1—O1—H1A	109.00	C5—C6—H6A	109.00
P2—O6—H6C	109.00	C5—C6—H6B	109.00
C1—O8—H8C	109.00	H6A—C6—H6B	108.00
C7—N1—C8	114.4 (3)	P1—C7—H7A	108.00
C6—N1—C8	114.2 (3)	P1—C7—H7B	108.00
C6—N1—C7	111.2 (3)	N1—C7—H7A	109.00
C8—N1—H10A	105.00	N1—C7—H7B	109.00
C7—N1—H10A	105.00	H7A—C7—H7B	108.00
C6—N1—H10A	105.00	P2—C8—H8A	107.00
O8—C1—C2	114.4 (3)	P2—C8—H8B	107.00
O7—C1—C2	122.3 (4)	N1—C8—H8A	107.00
O7—C1—O8	123.3 (4)	N1—C8—H8B	107.00
C1—C2—C3	112.6 (3)	H8A—C8—H8B	107.00
C2—C3—C4	115.2 (4)
O1—P1—C7—N1	−153.6 (2)	C8—N1—C7—P1	−56.9 (3)
O2—P1—C7—N1	90.7 (3)	C6—N1—C8—P2	62.3 (4)
O3—P1—C7—N1	−34.9 (3)	C7—N1—C8—P2	−67.5 (4)
O4—P2—C8—N1	50.4 (4)	O7—C1—C2—C3	−10.3 (6)
O5—P2—C8—N1	178.4 (3)	O8—C1—C2—C3	169.6 (4)
O6—P2—C8—N1	−65.7 (4)	C1—C2—C3—C4	177.9 (4)
C7—N1—C6—C5	−164.5 (3)	C2—C3—C4—C5	−176.2 (4)
C8—N1—C6—C5	64.2 (4)	C3—C4—C5—C6	178.6 (4)
C6—N1—C7—P1	171.9 (2)	C4—C5—C6—N1	172.2 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O5^iv	0.82	1.87	2.652 (4)	159
O6—H6C···O7^v	0.82	1.79	2.605 (4)	178
O8—H8C···O5^v	0.82	1.90	2.718 (4)	176
N1—H10A···O2^iv	0.91	2.13	2.823 (4)	132
N1—H10A···O4^vi	0.91	2.42	3.172 (4)	140
C4—H4B···O8^vii	0.97	2.60	3.538 (5)	163
C6—H6B···O6	0.97	2.56	3.230 (5)	126
C7—H7A···O2^iv	0.97	2.59	3.023 (4)	107
C7—H7B···O4	0.97	2.59	3.256 (5)	126
C7—H7B···O3^iii	0.97	2.42	3.249 (4)	143

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