Polymeric aqua(nitrilotriacetato)erbium(III)

In the structure of the title compound, [Er(C₆H₆NO₆)(H₂O)]_n, the Er atoms are eight-coordinated by one N atom and six O atoms from three symmetry-related nitrilo­tri­acetate (NTA) ligands, and by one O atom of a water mol­ecule, adopting a distorted square-antiprismatic geometry. The Er atoms are linked by the NTA ligands into layers, which are interconnected via O—H...O hydrogen bonds between the water mol­ecules and the carboxyl­ate O atoms. The asymmetric unit contains one Er atom, one NTA ligand and one water mol­ecule, all of which are located in general positions.

S1. Comment

Owing to the enormous variety of intriguing structural topologies, much effort has been devoted to the research of novel coordination materials using multidentate organic ligands to coordinate to metal centres (Philp & Stoddard, 1996; Yaghi et al., 1998; Hagrman et al., 1999). The higher coordination numbers of lanthanide ions and the inherent flexibility of their coordination geometries might lead to some unprecedented topological architectures (Long et al., 2000, 2001; Pan et al., 2000). We are currently interested in pursuing synthetic strategies using lanthanide ions as nodes in the construction of polymeric frameworks. The nitrilotriacetate (NTA) ligand, which has six O atoms and one N atom as potential donors, is a rather versatile ligand for the synthesis of polymeric complexes containing lanthanide cations. To date, it is known that the NTA ligand may form at least three kinds of complexes with lanthanide elements, such as mononuclear cluster anions (Belyaeva et al., 1974; Starynowicz, 1987; Chen et al., 1989), polymeric complexes (Belyaeva et al., 1968; Martin & Jacobson, 1972a,b), and heterometallic polymers (Liu et al., 2000). As part of our investigations of polycarboxylic-acid-bridged new polymeric complexes, the title complex, Er[N(CH₂COO)₃](H₂O), (I), was obtained as red crystals and its structure is presented here. \sch

Compound (I) has a two-dimensional framework that is built up by connecting the crystallographically unique Er^III atom with its neighbours through bridging NTA ligands. For each NTA ligand, the N atom and one of the two O atoms in each carboxylate group coordinate to the same Er^III atom. The remaining three carboxylate O atoms in each ligand coordinate to three adjacent symmetry-related Er^III atoms. Six acetate O atoms, one N atom and one water ligand form a distorted square-antiprismatic coordination sphere about each metal centre (Fig. 1). The Er—N distance is about 0.24 Å longer than the average length of the Er—O(carboxylate) bond. The latter bonds show little variation, due to the similar coordination mode of the carboxylate groups (Table 1). The length of the Er—O bond involving the water ligand is similar to the Er—O distances involving the carboxylate groups.

Each NTA ligand acts as a bridging ligand, connecting four Er^III ions into a two-dimensional structure (Fig. 2). The two-dimensional polymer of (I) could be described as a trilayer. As shown in Fig. 3, all of the C, H and N atoms are in the middle `layer', while the two covering `sheets' are eight-coordinated Er^III centres, with all of the water ligands oriented to the outside of the trilayer. Furthermore, all of these trilayers are linked together through weak hydrogen bonding between the water ligands and the carboxylate O atoms of the NTA ligands, thus producing a three-dimensional framework in which the mean O···O hydrogen-bonding distance is 2.942 (9) Å.

Compared with the previously reported three-dimensional polymeric complexes, [LnNTA(H₂O)₂] (Ln is Nd, Pr or Dy; Belyaeva et al., 1968; Martin & Jacobson, 1972a,b), the replacement of one water ligand by one NTA carboxylate O atom causes the structure of (I) to become a two-dimensional lamellar structure. The difference results from the different coordination mode of the NTA ligand in (I), which prevents it from making the additional connections required to build a three-dimensional framework. This also suggests that the products obtained under hydrothermal reaction conditions are quite different from those obtained from syntheses conducted at room temperature.

S2. Experimental

The pH of a mixture of ErCl₃ [1.0 mmol, prepared from Er₂O₃ (0.191 g) dissolved in 35% HCl], nitrilotriacetic acid (0.19 g, 1.0 mmol) in H₂O (18 ml) was adjusted to 4.96 with 10% NH₃·H₂O under vigorous stirring. The reaction mixture was then heated at 443 K for 6 d under autogeneous pressure in a sealed 25 ml Teflon-lined stainless steel vessel. Red crystals of (I) were isolated after the reaction solution was cooled down gradually and washed with water and ethanol. Spectroscopic analysis: IR (solid KBr pellet, ν, cm^-1): 1599 (s), 1468 (m), 1429 (s), 1396 (m), 1338 (m), 1315 (s), 1300 (m), 1228 (m), 1132 (m), 1117 (m), 1024 (s), 991 (m), 970 (m).

S3. Refinement

The H atoms on C atoms were generated geometrically. The H atoms of the water ligand were clearly visible in difference maps. They were placed in the difference map positions and constrained to ride on their parent O atom. All H atoms were assigned fixed isotropic displacement parameters, with U_iso(H) = 1.2U_eq(parent atom). The highest residual peak (0.83 e Å^-3) is at the position (0.3360, 0.2275, 0.0774), which is 0.91 Å from the Er atom; the deepest hole (-1.04 e Å^-3) is at the position (0.6579, 0.1802, 0.6212), which is 0.83 Å from the Er atom.

Computing details

Data collection: SMART (Siemens, 1996); cell refinement: SMART and SAINT (Siemens, 1994); data reduction: SMART and SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Siemens, 1994); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A perspective view of the locally expanded unit for (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii [symmetry codes: (i) x - 1, y, z; (ii) 1/2 - x, y - 1/2, 1/2 - z; (iii) x, y - 1, z].
	Fig. 2. The crystal packing viewed down the c axis, showing the extended lamellar structure of (I). H atoms have been omitted for clarity.
	Fig. 3. The crystal packing in (I) viewed down the b axis. Hydrogen bonding is indicated by dashed lines. For clarity, H atoms on C atoms have been omitted.

Polymeric aqua(nitrilotriacetato)erbium(III)

Crystal data

[Er(C6H6NO6)(H2O)]	F(000) = 700
Mr = 373.39	Dx = 2.852 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 6.7262 (6) Å	Cell parameters from 920 reflections
b = 6.5427 (4) Å	θ = 2.1–25.1°
c = 19.800 (2) Å	µ = 9.67 mm^−1
β = 93.444 (4)°	T = 293 K
V = 869.8 (1) Å^3	Prism, red
Z = 4	0.16 × 0.08 × 0.06 mm

Data collection

Bruker SMART CCD area-detector diffractometer	1520 independent reflections
Radiation source: fine-focus sealed tube	1340 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
ϕ and ω scans	θmax = 25.1°, θmin = 2.1°
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996)	h = −5→7
Tmin = 0.328, Tmax = 0.560	k = −7→5
2640 measured reflections	l = −23→23

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)] = 0.032	H-atom parameters constrained
wR(F^2) = 0.072	w = 1/[σ^2(Fo^2) + (0.0119P)^2 + 12.9152P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max < 0.001
1520 reflections	Δρmax = 0.83 e Å^−3
137 parameters	Δρmin = −1.04 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0110 (5)

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
Er	0.23184 (5)	0.23483 (5)	0.104751 (17)	0.01708 (19)
O1	0.5554 (9)	0.1790 (10)	0.0721 (3)	0.0291 (14)
O2	0.8855 (8)	0.1769 (9)	0.0918 (3)	0.0271 (13)
O3	0.1142 (9)	0.4405 (10)	0.1927 (3)	0.0276 (14)
O4	0.1656 (9)	0.5791 (9)	0.2944 (3)	0.0273 (14)
O5	0.2049 (10)	0.8897 (9)	0.0817 (3)	0.0310 (15)
O6	0.1748 (9)	0.5596 (9)	0.0618 (3)	0.0261 (13)
O7	0.1781 (9)	0.2084 (9)	−0.0129 (3)	0.0282 (13)
H7A	0.1576	0.3224	−0.0291	0.034*
H7B	0.0875	0.1206	−0.0315	0.034*
N	0.4906 (10)	0.4853 (11)	0.1587 (4)	0.0230 (15)
C1	0.6957 (12)	0.3993 (14)	0.1572 (5)	0.028 (2)
H1A	0.7326	0.3374	0.2007	0.033*
H1B	0.7887	0.5094	0.1500	0.033*
C2	0.7122 (12)	0.2421 (12)	0.1025 (4)	0.0235 (18)
C3	0.4438 (13)	0.5275 (15)	0.2284 (4)	0.0266 (19)
H3A	0.4934	0.6623	0.2409	0.032*
H3B	0.5129	0.4292	0.2579	0.032*
C4	0.2283 (13)	0.5187 (12)	0.2395 (4)	0.0227 (19)
C5	0.4784 (13)	0.6706 (13)	0.1159 (5)	0.0285 (19)
H5A	0.5686	0.6558	0.0798	0.034*
H5B	0.5222	0.7875	0.1430	0.034*
C6	0.2723 (14)	0.7115 (13)	0.0854 (4)	0.0268 (19)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Er	0.0157 (2)	0.0137 (2)	0.0217 (2)	0.00028 (14)	0.00030 (13)	−0.00050 (14)
O1	0.018 (3)	0.033 (3)	0.036 (3)	0.002 (3)	0.000 (3)	−0.009 (3)
O2	0.017 (3)	0.026 (3)	0.040 (3)	0.002 (3)	0.005 (3)	−0.007 (3)
O3	0.020 (3)	0.034 (4)	0.029 (3)	0.001 (3)	0.000 (2)	−0.004 (3)
O4	0.028 (3)	0.029 (3)	0.025 (3)	−0.006 (3)	−0.002 (3)	−0.003 (3)
O5	0.039 (4)	0.020 (3)	0.033 (3)	0.002 (3)	−0.008 (3)	0.001 (3)
O6	0.035 (4)	0.016 (3)	0.026 (3)	0.000 (3)	−0.009 (3)	0.003 (2)
O7	0.030 (3)	0.027 (3)	0.028 (3)	−0.003 (3)	0.004 (3)	−0.003 (3)
N	0.016 (3)	0.023 (4)	0.030 (4)	0.000 (3)	0.003 (3)	−0.005 (3)
C1	0.016 (4)	0.025 (4)	0.042 (5)	0.005 (3)	−0.004 (4)	−0.006 (4)
C2	0.017 (4)	0.022 (4)	0.032 (5)	−0.001 (4)	0.008 (4)	0.001 (4)
C3	0.021 (4)	0.033 (5)	0.026 (4)	0.000 (4)	−0.002 (4)	−0.011 (4)
C4	0.036 (5)	0.016 (4)	0.016 (4)	0.000 (4)	−0.001 (4)	0.002 (3)
C5	0.028 (5)	0.023 (4)	0.034 (5)	−0.007 (4)	0.001 (4)	−0.003 (4)
C6	0.035 (5)	0.019 (5)	0.026 (4)	0.001 (4)	0.001 (4)	0.001 (3)

Geometric parameters (Å, º)

Er—O1	2.336 (6)	O7—H7A	0.8200
Er—O2^i	2.359 (6)	O7—H7B	0.9000
Er—O3	2.373 (6)	N—C3	1.459 (11)
Er—O4^ii	2.310 (6)	N—C5	1.479 (11)
Er—O5^iii	2.309 (6)	N—C1	1.492 (10)
Er—O6	2.312 (6)	C1—C2	1.502 (11)
Er—O7	2.343 (6)	C1—H1A	0.9700
Er—N	2.575 (7)	C1—H1B	0.9700
O1—C2	1.253 (10)	C3—C4	1.480 (12)
O2—C2	1.271 (10)	C3—H3A	0.9700
O3—C4	1.275 (10)	C3—H3B	0.9700
O4—C4	1.254 (10)	C5—C6	1.502 (13)
O5—C6	1.252 (10)	C5—H5A	0.9700
O6—C6	1.264 (10)	C5—H5B	0.9700
O4^ii—Er—O1	86.3 (2)	Er—O7—H7B	121.0
O5^iii—Er—O1	81.7 (2)	H7A—O7—H7B	109.1
O6—Er—O1	100.3 (2)	C3—N—C5	112.3 (7)
O1—Er—O2^i	151.1 (2)	C3—N—C1	110.0 (7)
O4^ii—Er—O2^i	105.4 (2)	C5—N—C1	108.5 (7)
O5^iii—Er—O2^i	75.9 (2)	C3—N—Er	109.5 (5)
O6—Er—O2^i	88.1 (2)	C5—N—Er	105.7 (5)
O7—Er—O2^i	77.7 (2)	C1—N—Er	110.7 (5)
O1—Er—O3	130.5 (2)	N—C1—C2	112.4 (7)
O2^i—Er—O3	78.5 (2)	N—C1—H1A	109.1
O4^ii—Er—O3	73.2 (2)	C2—C1—H1A	109.1
O5^iii—Er—O3	132.4 (2)	N—C1—H1B	109.1
O6—Er—O3	72.1 (2)	C2—C1—H1B	109.1
O7—Er—O3	137.1 (2)	H1A—C1—H1B	107.9
O5^iii—Er—O4^ii	75.8 (2)	O1—C2—O2	124.3 (8)
O4^ii—Er—O6	139.2 (2)	O1—C2—C1	118.4 (7)
O5^iii—Er—O6	144.8 (2)	O2—C2—C1	117.3 (7)
O1—Er—O7	78.7 (2)	N—C3—C4	113.7 (7)
O4^ii—Er—O7	147.9 (2)	N—C3—H3A	108.8
O5^iii—Er—O7	74.1 (2)	C4—C3—H3A	108.8
O6—Er—O7	71.9 (2)	N—C3—H3B	108.8
O1—Er—N	66.2 (2)	C4—C3—H3B	108.8
O2^i—Er—N	141.8 (2)	H3A—C3—H3B	107.7
O3—Er—N	65.3 (2)	O4—C4—O3	122.5 (8)
O4^ii—Er—N	76.3 (2)	O4—C4—C3	120.0 (7)
O5^iii—Er—N	138.4 (2)	O3—C4—C3	117.4 (7)
O6—Er—N	70.2 (2)	N—C5—C6	113.2 (7)
O7—Er—N	121.3 (2)	N—C5—H5A	108.9
C2—O1—Er	125.9 (5)	C6—C5—H5A	108.9
C2—O2—Er^iv	146.9 (6)	N—C5—H5B	108.9
C4—O3—Er	123.2 (5)	C6—C5—H5B	108.9
C4—O4—Er^v	142.2 (6)	H5A—C5—H5B	107.7
C6—O5—Er^vi	151.1 (6)	O5—C6—O6	122.2 (8)
C6—O6—Er	121.0 (5)	O5—C6—C5	120.8 (8)
Er—O7—H7A	109.5	O6—C6—C5	117.0 (8)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O7—H7B···O2^vii	0.90	2.30	2.983 (8)	133
O7—H7B···O5^viii	0.90	2.15	2.912 (9)	142
O7—H7A···O6^viii	0.82	2.42	2.932 (9)	122

Symmetry codes: (vii) −x+1, −y, −z; (viii) −x, −y+1, −z.