Poly[[diaqua­cadmium(II)]-μ₄-[N-(phosphonato­methyl)ammonio]acetato]

The title compound, [Cd(C₃H₆NO₅P)(H₂O)₂]_n, is a three-dimensional polymeric complex. The asymmetric unit contains one Cd atom, one N-(phosphono­methyl)glycine zwitterion [(O⁻)₂OPCH₂NH₂⁺CH₂COO⁻] and two water mol­ecules. The coordination geometry is a distorted CdO₆ octa­hedron. Each N-(phosphono­methyl)glycine ligand bridges four adjacent water-coordinated Cd cations through three phospho­nate O atoms and one carboxyl­ate O atom, like a regular PO₄³⁻ group in zeolite-type frameworks. One-dimensional zigzag (–O—P—C—N—C—C—O—Cd–)_n chains along the [101] direction are linked to one another via Cd—O—P bridges and form a three-dimensional network motif with three types of channel systems. The variety of O—H...O and N—H...O hydrogen bonds is likely to be responsible for stabilizing the three-dimensional network structure and preventing guest mol­ecules from entering into the channels.

S1. Comment

Metal complexes containing phosphonic acids attached to a variety of functional groups, such as aza-crown ethers (Sharma & Clearfield, 2000), amines (Kong et al., 2004; Jankovics et al., 2002) and carboxylic acid groups (Zhu et al., 2000; Mao et al., 2002), have shown many unusual structural and functional features. The N-(phosphonomethyl)glycine ligand has been reported as one of trifunctional aminocarboxylatephosphonates and features a diverse range of coordination modes in metal complexes (Stock, 2002; Ramstedt et al., 2004). Here, we report a new polymeric Cd^II complex using the trifunctional N-phosphonomethylglycine (PMG) ligand, (I), in which the coordination mode of PMG has not previously been reported in the literatures.

The structure of complex (I) is shown in Fig. 1. The asymmetric unit consists of one Cd^II ion, one PMG dianion and two water molecules. The Cd^II ion is six-coordinated and situated in a distorted CdO₆ octahedral environment, involving three O atoms from phosphonate groups of three neighbouring ligands, one carboxylate O atom from another ligand and two water O atoms. The Cd—O bond distances are in the range 2.208 (2)–2.387 (2) Å (Table 1), which are similar to those reported for another Cd^II aminocarboxylatephosphonate (Yang et al., 2003). The equatorial plane of the coordination environment is defined by atoms O1, O3ⁱⁱ, O4ⁱⁱⁱ and O6 [symmetry codes: (ii) ?; (iii) ? Please complete], with a mean standard deviation of 0.0454°. The trans-axial positions are occupied by atoms O7 and O2ⁱ [symmetry code: (i) ?; Please complete], with a O7—Cd1—O2ⁱ bond angle of 169.99 (9)°. The imino N atom is not coordinated. There are also O—H···O and N—H···O hydrogen-bonding interactions in the structure (Fig. 1 and Table 2).

The X-ray crystal structure reveals that compound (I) has a metal-to-ligand ratio of 1. It is also clear that both phosphonate and carboxylate groups of the ligand are deprotonated, whereas the Cd atom has an oxidation state of +2. Thus, the only way in which the neutrality of compound (I) can be achieved is by considering the possibility that the secondary amine may be 2H-protonated, including the –NH₂⁺– group. Overall, the ligand coordinates to the sphere of Cd^II carrying a double negative charge, as the zwitterion [^2-O₃PCH₂NH₂⁺CH₂COO^-] (Stock, 2002). Three phosphonate O atoms and one terminal carboxylate O atom in one zwitterion are utilized to connect the Cd²⁺ site, and therefore each [^2-O₃PCH₂NH₂⁺CH₂COO^-] zwitterion in effect behaves like a regular PO₄^3- group in zeolite-type frameworks (Zhu et al., 2000) and is four-connected to four Cd²⁺ sites. Such a coordination mode is different from those previously reported for PMG. To maintain a metal-to-ligand ratio of 1, each Cd²⁺ site is also four-connected to [O₃PCH₂NH₂COO]^2- sites (see scheme and Fig. 1).

A one-dimensional zigzag chain is formed in the structure of (I), containing a repeating (–O1—P1—C1—N1—C2—C3—O4—Cd1–) unit, with a distance of 9.586 Å (metal-to-metal) along the [101] direction. These chains are further linked to one another via two types of Cd1—O2—P1 and Cd1—O3—P1 bridges in a different direction, as the –PO^3-– groups link to Cd atoms in different chains (Fig. 2), with O—Cd—O bond angles in the range 78.87 (8)–163.42 (7)° (Table 1). The result of cross-linking chains in this manner is the formation of a three-dimensional network motif containing three channel systems that are nearly perpendicular to one another. These channels are created by 12-, 20- and 20-membered rings, respectively. Each 12-membered ring (A) is formed of three Cd atoms and three phosphonate groups, (–O—P—O—Cd–)₃. The two 20-membered rings (B and C) are similarly composed of two (–Cd—O—P—C—N—C—C—O—Cd–) units, sharing one Cd atom and one O—P—O bridge (Fig. 2).

Molecules of compound (I) show extensive hydrogen bonding between the coordinated water molecules and the phosphonate/carboxylate O atoms, between coordinated water molecules, and between the imino N atoms and the phosphonate O atoms. Such a complex hydrogen-bond network is likely to contribute to the overall stability of the crystal structure and prevents guest molecules entering into the above channels (Table 2 and Fig. 3).

Related literature

For related literature, see: Flack (1983); Jankovics et al. (2002); Kong et al. (2004); Mao et al. (2002); Ramstedt et al. (2004); Sharma & Clearfield (2000); Stock (2002); Yang et al. (2003); Zhu et al. (2000).

S2. Experimental

A mixture of cadmium(II) acetate dihydrate (0.134 g, 0.5 mmol), N-(phosphonomethyl)glycine (0.078 g, 0.5 mmol) and deionized water (16 ml) was heated in a Teflon-lined stainless steel autoclave (25 ml) for 144 h at 433 K, after which the autoclave was cooled to room temperature over a period of 12 h at a rate of 10 K h^-1. Colourless block single crystals of (I) were collected in about 18% yield.

S3. Refinement

H atoms on C and N atoms were positioned geometrically and were included in the refinement in the riding-model approximation, with C—H distances of 0.97 Å and N—H distances of 0.90 Å, and with U_iso(H) = 1.2U_eq(C,N). The water H atoms were located in a difference Fourier map and were refined subject to an O—H distance restraint of 0.82 (1) Å. The absolute structure could be determined by refining the Flack parameter (Flack, 1983), using 616 Friedel pairs.

Computing details

Data collection: CrystalClear (Rigaku, 1999); cell refinement: CrystalClear; data reduction: CrystalStructure (Rigaku/MSC, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 2000); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of the molecular structure of compound (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines and carbon-bound H atoms have been omitted for clarity. [Symmetry codes: (i) 1/2 + x, 1/2 + y, z; (ii) x, 1 + y, z; (iii) 1/2 + x, 1/2 - y, 1/2 + z.]
	Fig. 2. A view of the three types of 12-, 20- and 20-membered rings (A, B and C). The one-dimensioinal zigzag chains along the [101] direction are linked via two types of Cd1—O2—P1 and Cd1—O3—P1 bridges in different directions. Coordinated water molecules and H atoms have been omitted for clarity. [Symmetry codes: (i) 1/2 + x, 1/2 + y, z; (ii) x, 1 + y, z; (iii) 1/2 + x, 1/2 - y, 1/2 + z; (v) x, y - 1, z; (vi) -1/2 + x, 1/2 - y, - 1/2 + z; (vii) -1/2 + x, 3/2 - y, -1/2 + z; (viii) x, -1 - y, 1/2 + z; (ix) 1/2 + x, 1/2 + y, z - 1.]
	Fig. 3. The molecular packing in compound (I), showing the chains extending along different directions into a three-dimensional network motif via Cd—O—P bridges. Hydrogen bonds are shown as dashed lines and H atoms have been omitted for clarity.

Poly[[diaquacadmium(II)]-µ₄-N-(phosphonatomethyl)glycinato]

Crystal data

[Cd(C3H6NO5P)(H2O)2]	F(000) = 616
Mr = 315.49	Dx = 2.580 Mg m^−3
MonoclinicCc	Mo Kα radiation, λ = 0.71070 Å
Hall symbol: C -2yc	Cell parameters from 1695 reflections
a = 9.827 (2) Å	θ = 3.6–25.3°
b = 4.9326 (10) Å	µ = 2.90 mm^−1
c = 16.795 (4) Å	T = 153 K
β = 93.910 (4)°	Block, colourless
V = 812.2 (3) Å^3	0.30 × 0.25 × 0.07 mm
Z = 4

Data collection

Rigaku Mercury diffractometer	1359 independent reflections
Radiation source: fine-focus sealed tube	1348 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
Detector resolution: 7.31 pixels mm^-1	θmax = 25.4°, θmin = 4.2°
ω scans	h = −11→11
Absorption correction: multi-scan (Jacobson, 1998)	k = −5→5
Tmin = 0.430, Tmax = 0.817	l = −20→17
3642 measured reflections

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)] = 0.015	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.035	w = 1/[σ^2(Fo^2) + (0.0137P)^2 + 0.0465P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max < 0.001
1359 reflections	Δρmax = 0.60 e Å^−3
127 parameters	Δρmin = −0.47 e Å^−3
6 restraints	Absolute structure: Flack (1983), with 616 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.03 (2)

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	0.5379	0.42153 (3)	0.6762	0.00723 (7)
P1	0.34039 (9)	−0.09619 (15)	0.58567 (5)	0.00563 (18)
O1	0.3806 (2)	0.1080 (4)	0.65054 (14)	0.0105 (5)
O2	0.2009 (2)	−0.2227 (4)	0.59649 (13)	0.0087 (5)
O3	0.4490 (2)	−0.3069 (4)	0.57244 (14)	0.0095 (5)
O4	0.0635 (2)	0.3449 (5)	0.29362 (14)	0.0111 (5)
O5	0.0289 (3)	0.0494 (4)	0.39180 (14)	0.0125 (5)
O6	0.6839 (3)	0.6951 (5)	0.74746 (16)	0.0139 (5)
H6A	0.7570	0.6990	0.7267	0.021*
O7	0.3872 (3)	0.6469 (5)	0.76013 (15)	0.0117 (5)
H7A	0.4108	0.6157	0.8070	0.018*
N1	0.2003 (3)	0.2953 (5)	0.49885 (16)	0.0078 (6)
H1A	0.1267	0.2028	0.5127	0.009*
H1D	0.2213	0.4188	0.5372	0.009*
C1	0.3176 (4)	0.1030 (6)	0.4940 (2)	0.0094 (7)
H1B	0.4003	0.2041	0.4861	0.011*
H1C	0.3004	−0.0174	0.4487	0.011*
C2	0.1664 (4)	0.4381 (6)	0.4218 (2)	0.0082 (7)
H2B	0.2500	0.4865	0.3977	0.010*
H2C	0.1173	0.6039	0.4318	0.010*
C3	0.0799 (3)	0.2599 (6)	0.36479 (19)	0.0097 (7)
H7B	0.405 (4)	0.807 (3)	0.754 (3)	0.022 (12)*
H6B	0.652 (5)	0.841 (5)	0.760 (3)	0.027 (13)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.00718 (11)	0.00720 (11)	0.00730 (11)	−0.00118 (12)	0.00049 (7)	−0.00046 (11)
P1	0.0059 (4)	0.0054 (5)	0.0055 (4)	−0.0002 (3)	0.0000 (3)	0.0003 (3)
O1	0.0101 (12)	0.0102 (12)	0.0112 (13)	−0.0028 (9)	0.0001 (10)	−0.0018 (9)
O2	0.0085 (11)	0.0078 (11)	0.0100 (11)	−0.0001 (9)	0.0015 (8)	−0.0025 (9)
O3	0.0102 (13)	0.0087 (12)	0.0097 (13)	0.0028 (10)	0.0024 (10)	0.0024 (10)
O4	0.0152 (12)	0.0108 (11)	0.0068 (12)	−0.0007 (10)	−0.0025 (10)	−0.0006 (10)
O5	0.0180 (13)	0.0106 (13)	0.0085 (12)	−0.0052 (10)	−0.0010 (10)	−0.0001 (9)
O6	0.0095 (13)	0.0134 (15)	0.0193 (14)	−0.0033 (11)	0.0050 (10)	−0.0062 (11)
O7	0.0145 (13)	0.0109 (12)	0.0096 (13)	−0.0005 (11)	−0.0003 (10)	0.0011 (10)
N1	0.0078 (13)	0.0074 (14)	0.0080 (14)	−0.0010 (11)	−0.0020 (10)	−0.0006 (11)
C1	0.0085 (17)	0.0074 (17)	0.0125 (18)	0.0020 (13)	0.0012 (14)	0.0006 (13)
C2	0.0117 (17)	0.0070 (18)	0.0059 (18)	0.0014 (11)	0.0001 (14)	0.0038 (12)
C3	0.0083 (15)	0.0089 (16)	0.0117 (17)	0.0026 (14)	−0.0001 (13)	0.0017 (14)

Geometric parameters (Å, º)

Cd1—O1	2.208 (2)	O6—H6B	0.82 (3)
Cd1—O6	2.253 (2)	O7—H7A	0.8200
Cd1—O2^i	2.271 (2)	O7—H7B	0.820 (10)
Cd1—O3^ii	2.321 (2)	N1—C2	1.491 (4)
Cd1—O4^iii	2.369 (2)	N1—C1	1.500 (4)
Cd1—O7	2.387 (2)	N1—H1A	0.9000
P1—O1	1.516 (2)	N1—H1D	0.9000
P1—O3	1.517 (2)	C1—H1B	0.9700
P1—O2	1.528 (2)	C1—H1C	0.9700
P1—C1	1.828 (4)	C2—C3	1.517 (5)
O4—C3	1.267 (4)	C2—H2B	0.9700
O5—C3	1.251 (4)	C2—H2C	0.9700
O6—H6A	0.8200
O1—Cd1—O6	159.25 (9)	Cd1—O6—H6B	115 (4)
O1—Cd1—O2^i	100.24 (8)	H6A—O6—H6B	116.7
O6—Cd1—O2^i	92.94 (9)	Cd1—O7—H7A	109.5
O1—Cd1—O3^ii	92.01 (8)	Cd1—O7—H7B	103 (3)
O6—Cd1—O3^ii	104.27 (8)	H7A—O7—H7B	105.1
O2^i—Cd1—O3^ii	88.99 (8)	C2—N1—C1	112.3 (3)
O1—Cd1—O4^iii	78.89 (9)	C2—N1—H1A	109.2
O6—Cd1—O4^iii	82.10 (9)	C1—N1—H1A	109.2
O2^i—Cd1—O4^iii	106.12 (8)	C2—N1—H1D	109.2
O3^ii—Cd1—O4^iii	163.45 (8)	C1—N1—H1D	109.2
O1—Cd1—O7	89.29 (9)	H1A—N1—H1D	107.9
O6—Cd1—O7	78.81 (9)	N1—C1—P1	110.1 (2)
O2^i—Cd1—O7	169.98 (8)	N1—C1—H1B	109.6
O3^ii—Cd1—O7	87.58 (9)	P1—C1—H1B	109.6
O4^iii—Cd1—O7	78.59 (9)	N1—C1—H1C	109.6
O1—P1—O3	114.13 (14)	P1—C1—H1C	109.6
O1—P1—O2	112.20 (13)	H1B—C1—H1C	108.1
O3—P1—O2	112.54 (13)	N1—C2—C3	110.9 (2)
O1—P1—C1	105.04 (14)	N1—C2—H2B	109.5
O3—P1—C1	106.97 (15)	C3—C2—H2B	109.5
O2—P1—C1	105.10 (15)	N1—C2—H2C	109.5
P1—O1—Cd1	138.63 (15)	C3—C2—H2C	109.5
P1—O2—Cd1^iv	128.08 (12)	H2B—C2—H2C	108.0
P1—O3—Cd1^v	121.19 (14)	O5—C3—O4	126.1 (3)
C3—O4—Cd1^vi	127.0 (2)	O5—C3—C2	118.2 (3)
Cd1—O6—H6A	109.5	O4—C3—C2	115.7 (3)
O3—P1—O1—Cd1	61.5 (2)	O2—P1—O3—Cd1^v	−73.17 (17)
O2—P1—O1—Cd1	−168.99 (18)	C1—P1—O3—Cd1^v	171.90 (15)
C1—P1—O1—Cd1	−55.4 (2)	C2—N1—C1—P1	−172.9 (2)
O6—Cd1—O1—P1	−150.1 (2)	O1—P1—C1—N1	−64.9 (2)
O2^i—Cd1—O1—P1	−21.5 (2)	O3—P1—C1—N1	173.5 (2)
O3^ii—Cd1—O1—P1	67.8 (2)	O2—P1—C1—N1	53.7 (2)
O4^iii—Cd1—O1—P1	−126.1 (2)	C1—N1—C2—C3	80.9 (3)
O7—Cd1—O1—P1	155.4 (2)	Cd1^vi—O4—C3—O5	−34.1 (4)
O1—P1—O2—Cd1^iv	14.7 (2)	Cd1^vi—O4—C3—C2	147.8 (2)
O3—P1—O2—Cd1^iv	145.08 (14)	N1—C2—C3—O5	12.4 (4)
C1—P1—O2—Cd1^iv	−98.87 (17)	N1—C2—C3—O4	−169.3 (3)
O1—P1—O3—Cd1^v	56.19 (18)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O3^vii	0.90	2.07	2.881 (4)	149
N1—H1D···O2^ii	0.90	2.05	2.888 (4)	155
O6—H6A···O1^i	0.82	1.88	2.646 (3)	156
O6—H6A···O7^i	0.82	2.60	2.991 (3)	111
O6—H6B···O4^viii	0.82 (3)	1.88 (3)	2.697 (3)	174 (5)
O7—H7A···O5^iii	0.82	1.95	2.710 (3)	153
O7—H7B···O1^ii	0.82 (1)	2.28 (3)	2.924 (3)	136 (4)
O7—H7B···O4^viii	0.82 (1)	2.38 (3)	3.077 (3)	143 (4)

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