Polymeric hexa­aqua­hexakis(μ₃-2,2′-oxy­diacetato)­trizinc(II)­digadolinium(III) dodecahydrate

A polymeric heterometallic compound, {[Gd₂Zn₃(C₄H₄O₅)₆(H₂O)₆]·12H₂O}_n, comprising zinc(II) and gadolinium(III) cations bridged by carboxyl­ate groups from oxy­di­acetate ligands, is presented. The Gd^III cations lie at sites with crystallographic 32 symmetry and display a tricapped trigonal-prism arrangement, which is defined by six carboxyl and three ether O atoms. The Zn^II cations lie at sites with imposed 2/m symmetry and are octahedrally coordinated by four carboxyl O atoms and two apical water ligands, which form strong intramolecular hydrogen bonds. Comparison is made with the previously reported isostructural homologous copper–gadolinium complex.

S1. Comment

The design of new compounds with specific architectures is a rapidly developing field in modern inorganic chemistry. One successful strategy is based on the use of both the building bonding approach and the hydrogen-bonding capability of the ligands (Lehn, 1995). The oxydiacetate anion is a well known and versatile ligand, which is able to chelate and to bridge metal ions in a variety of ways, leading to the formation of polynuclear and low-dimensional systems (Grirrane et al., 2002). We have recently reported the study of a series of oxydiacetate-bridged copper(II)–lanthanide(III) three-dimensional polymers, formulated as [{Cu₃Ln₂(oda)₆(H₂O)₆}.12H₂O]_n (Ln is Y, Gd, Eu, Nd and Pr (Baggio et al.,2000); Ln is Dy, Ho, Er, Y (Rizzi et al., 2002); oda is oxydiacetate]. These crystallize in the hexagonal system, with space group P6/mcc (No. 192). Replacement of copper by zinc ions to obtain isostructural zinc lanthanide compounds appears possible, in principle, in spite of the lack of crystal-field stabilization of the Zn^II cation. Such compounds should be of interest in the study of magnetic exchange interactions between lanthanide ions, since Zn^II has no unpaired electrons. The first Zn–Gd compound bridged by carboxylate groups has been reported for the propionate derivative [Zn₂Gd₂(O₂CC₂H₅)₈(C₉H₇N)₂(NO₃)₂(H₂O)₂] (Cui, Zheng, Qian & Huang, 2001). The structure consists of two triply propionate-bridged dinuclear Zn–Gd subunits linked together by two tridentate propionates bridging the neighboring Gd^III ions. We report herein the isolation and X-ray structure of (I). The crystallographic results reveal that the Zn–Gd complex is isostructural to the other members of the copper lanthanide series, and, in particular the Cu–Gd member, (II).

The structure consists of two distinct types of building block, viz. GdO₉ and ZnO₆, as illustrated in Fig. 1. Two GdO₉ and three ZnO₆ cores form the unit cell that generates an extended, highly symmetric, three-dimensional structure. The Gd atom lies on the intersection of a three- and twofold axis and coordinates to the carboxylate O1 and O1' atoms (where ' denotes the twofold-related part of the ODA ligand) and to the ether O3 atom, of three symmetry-related oda ligands [Gd—O_carboxyl = 2.402 (1), Gd—O_ether = 2.475 (1) Å] leading to the usual tricapped trigonal prismatic geometry (TCTP), (Albertsson, 1968). This gadolinium coordination polyhedron is quite similar to the one found in (II), to the extent that a superposition of the two entities (XP in SHEXLXL) matched within an average deviation of less than 0.045 Å.

Each Zn atom lies on the intersection of a twofold axis and a mirror plane and coordinates to the outer carboxylate O2 and O2' atoms of four oda ligands [Zn—O_basal = 2.052 (1) Å], which define the equatorial plane. The apical sites are occupied by two water molecules at a Zn—O apical distance of 2.152 (1) Å, similar to the basal bonds. The geometries of the GdO₉ and ZnO₆ coordination polyhedra are unexceptional and their average bonding distances are similar to those obtained in a search for similarly coordinated cores in the Cambridge Structural Database [CSD; Allen, 2002; mean Gd—O₉ and Zn—O_{6 distances in the CSD (49 and 125 structures, respectively): 2.446 (83) and 2.098 (69) Å; measured for (I): 2.426 (36) and 2.085 (52) Å].}

The most noteworthy differences between (I) and (II) are found in the vicinity of the octahedral site, where the nearly isometric coordination of Zn in (I) contrasts with the strong Jahn–Teller distortion around Cu in (II) [Cu—O_basal = 1.956 (4) and Cu—O_apical = 2.519 (2) Å]. These differences in bond lengths propagate through the connecting chains, linking the three-dimensional structure in such a way as to modify the cell constants by decreasing a by ca 1.4% and increasing c by ca 4.5% when replacing copper with zinc. Therefore, the corresponding Zn···Zn and Gd···Gd separations have different values along different directions. Distances taken along the two-dimensional structures, nearly parallel to the apical O—Cu—O bonds ([100],[010],[110]), shrink, while distances taken normal to the two-dimensional structures, almost along the diagonal of the basal coordination plane([001]), expand when going fron Cu to Zn. The intramolecular Zn···Gd separation of 5.753 (1) Å, however, is very similar to the corresponding Cu···Gd value in (2), 5.695 (1) Å, due to the interatomic vector being nearly parallel to the almost invariant [111] direction.

All the outer carboxylate O atoms link to Zn atoms, so that each Gd atom is surrounded by six Zn atoms as nearest neighbors, while the Zn atoms have four Gd atoms in their vicinity, as expected from the Zn/Gd molar relationship. Through this connectivity pattern, the Gd polyhedra create a two-dimensional planar honeycomb structure, parallel to (001), at z heights of ca 0.25 and 0.75. Each pair of polyhedra at one side of the hexagonal motif is connected to its homologous pair one layer below (above) via a Zn atom bonded to four outer carboxylate O atoms (two from above and two from below). As a result of the translational symmetry along z, the two-dimensional structure develops into a three-dimensional honeycomb structure with columnar voids of nearly 6 Å diameter (Fig. 2). These, in turn, are occupied by a number of highly disordered water molecules. Their interaction with the main structure could not be studied, since the H atoms of the water molecules were not found. On the other hand, the only independent water H atom in the zinc coordination sphere strongly binds to atom O1 of the ligand and was clearly resolved in a difference Fourier map.

In spite of the differences in their octahedral sites, the whole array in (I) is very similar to that found in (II), suggesting that Zn^II can replace the Cu(II) ions in the octahedral CuO₆ sites in the Cu–Ln system, leading to an isomorphous Zn–Ln series. This fact could be useful in understanding the electronic and magnetic properties of these extended solids.

S2. Experimental

Chemicals for the synthesis of (I) were used as purchased (reagent grade), without further purification. Zinc acetate dihydrate (0.35 g, 1.5 mmol) and gadolinium acetate hydrate (0.35 g, 1 mmol) were dissolved in an aqueous solution (25 ml) of oxydiacetic acid (0.60 g, 4.5 mmol) and stirred at room temperature for 4 h. After a few weeks, colorless crystals of the compound suitable for X-ray diffraction were filtered off and dried in air.

S3. Refinement

The structure of the homologous copper–gadolinium complex was used as a starting model for refinement by least squares on F², using all data, with anisotropic displacement parameters for non-H atoms. H atoms belonging to the coordinated water molecules were found in the final Δ F synthesis, and fully refined with isotropic displacement factors. H atoms attached to C atoms were included at their expected positions and allowed to ride. The water molecules appeared to be very disordered in the columnar cavities of the structure, and it was impossible to determine their overall number by X-ray techniques alone. TGA measurements of freshly prepared samples showed a weight loss Δ m of 19.6% (expected for 18 water molecules: 19.8%), of which 6.6% correspond to the six water molecules and the rest (13.0%) to the remaining water of crystallization This? amounts to 11.8 (\sim 12) molecules per formula unit. Only a fraction of these molecules (ca 75%) were found in the difference Fourier map. They appeared to be scattered into five independent positions, with a total O-atom occupation of 8.9 (5). These molecules were refined with a common isotropic displacement factor without their H atoms.

Computing details

Data collection: SMART-NT (Bruker, 1999); cell refinement: SMART-NT; data reduction: SAINT-NT (Bruker, 1999); program(s) used to solve structure: XS in SHELXTL (Bruker, 1999); program(s) used to refine structure: XL in SHELXTL (Bruker, 1999); molecular graphics: XP in SHELXTL (Bruker, 1999); software used to prepare material for publication: XP in SHELXTL.

Figures

	Fig. 1. : Molecular diagram showing the way in which the three-dimensional network forms. Intramolecular hydrogen bonds are represented by dashed lines. Displacement ellipsoids are shown at the 40% probability level.
	Fig. 2. : Space-filling diagram of the structure of (I), viewed along the z axis. Note the columnar voids where the disordered water molecules reside. Projections of the hexagonal unit-cell nodes lie at the void centers.

hexaaquahexakis(µ₃-2,2'-oxydiacetato)trizinc(II)digadolinium(III) dodecahydrate

Crystal data

[Zn3Gd2(C4H4O5)6(H2O)6]·12H2O	Dx = 1.879 Mg m^−3
Mr = 1627.33	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6/mcc	Cell parameters from 15357 reflections
Hall symbol: -P 6 2c	θ = 1.6–28.0°
a = 14.5104 (11) Å	µ = 3.62 mm^−1
c = 15.7725 (16) Å	T = 293 K
V = 2876.0 (4) Å^3	Polyhedra, colorless
Z = 2	0.32 × 0.30 × 0.22 mm
F(000) = 1612

Data collection

Bruker SMART CCD 1K area detector diffractometer	1180 independent reflections
Radiation source: fine-focus sealed tube	962 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.091
Detector resolution: 8 pixels mm^-1	θmax = 28.0°, θmin = 1.6°
ω–scans	h = −18→15
Absorption correction: empirical (using intensity measurements) XPREP in SHELXTL (Bruker, 1999c)	k = −17→18
Tmin = 0.35, Tmax = 0.47	l = −20→19
15357 measured reflections

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)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.096	H atoms treated by a mixture of independent and constrained refinement
S = 1.07	w = 1/[σ^2(Fo^2) + (0.0556P)^2 + 0.947P] where P = (Fo^2 + 2Fc^2)/3
1180 reflections	(Δ/σ)max = 0.007
74 parameters	Δρmax = 0.78 e Å^−3
1 restraint	Δρmin = −0.72 e Å^−3

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Gd	0.6667	0.3333	0.2500	0.02765 (17)
Zn	1.0000	0.5000	0.5000	0.0302 (2)
O1	0.80431 (19)	0.41954 (17)	0.35441 (15)	0.0412 (5)
O2	0.9450 (2)	0.56231 (18)	0.40957 (17)	0.0623 (8)
O3	0.76513 (13)	0.5303 (3)	0.2500	0.0407 (7)
C1	0.8681 (3)	0.5181 (2)	0.3600 (2)	0.0432 (7)
C2	0.8491 (3)	0.5902 (3)	0.3049 (3)	0.0733 (14)
H2A	0.8324	0.6350	0.3398	0.088*
H2B	0.9128	0.6358	0.2726	0.088*
O1W	0.8555 (2)	0.3483 (2)	0.5000	0.0408 (7)*
H1W	0.829 (4)	0.367 (4)	0.543 (3)	0.111 (18)*
O2WA	0.803 (3)	1.033 (3)	0.5000	0.150 (7)*	0.29 (2)
O2WB	0.733 (3)	0.920 (4)	0.5000	0.150 (7)*	0.241 (19)
O2WC	0.769 (3)	0.977 (3)	0.451 (3)	0.150 (7)*	0.198 (15)
O2WD	0.845 (3)	0.984 (2)	0.311 (3)	0.150 (7)*	0.167 (11)
O2WE	0.923 (3)	1.073 (3)	0.379 (3)	0.150 (7)*	0.148 (14)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Gd	0.03199 (19)	0.03199 (19)	0.0190 (2)	0.01599 (10)	0.000	0.000
Zn	0.0271 (3)	0.0366 (4)	0.0237 (4)	0.0135 (3)	0.000	0.000
O1	0.0421 (12)	0.0409 (11)	0.0356 (12)	0.0169 (10)	−0.0127 (10)	0.0003 (9)
O2	0.0581 (16)	0.0468 (13)	0.0652 (16)	0.0136 (12)	−0.0350 (13)	0.0054 (12)
O3	0.0461 (14)	0.0360 (16)	0.0366 (17)	0.0180 (8)	−0.0157 (10)	0.000
C1	0.0402 (16)	0.0432 (17)	0.0387 (16)	0.0153 (14)	−0.0145 (13)	−0.0004 (14)
C2	0.071 (3)	0.0424 (19)	0.085 (3)	0.0120 (18)	−0.052 (2)	0.0013 (19)

Geometric parameters (Å, º)

Gd—O1^i	2.402 (2)	Zn—O2^viii	2.052 (2)
Gd—O1^ii	2.402 (2)	Zn—O1W^viii	2.152 (3)
Gd—O1	2.402 (2)	Zn—O1W	2.152 (3)
Gd—O1^iii	2.402 (2)	O1—C1	1.259 (4)
Gd—O1^iv	2.402 (2)	O2—C1	1.246 (3)
Gd—O1^v	2.402 (2)	O3—C2	1.390 (4)
Gd—O3^ii	2.475 (3)	O3—C2^iv	1.390 (4)
Gd—O3^v	2.475 (3)	C1—C2	1.488 (4)
Gd—O3	2.475 (3)	C2—H2A	0.9700
Zn—O2^vi	2.052 (2)	C2—H2B	0.9700
Zn—O2	2.052 (2)	O1W—H1W	0.88 (5)
Zn—O2^vii	2.052 (2)
O1^i—Gd—O1^ii	126.37 (10)	O1^iii—Gd—O3	136.07 (6)
O1^i—Gd—O1	87.85 (12)	O1^iv—Gd—O3	63.19 (5)
O1^ii—Gd—O1	78.15 (9)	O1^v—Gd—O3	74.39 (5)
O1^i—Gd—O1^iii	78.15 (9)	O3^ii—Gd—O3	120.0
O1^ii—Gd—O1^iii	87.85 (12)	O3^v—Gd—O3	120.0
O1—Gd—O1^iii	148.78 (11)	O2^vi—Zn—O2	88.08 (17)
O1^i—Gd—O1^iv	78.15 (9)	O2^vi—Zn—O2^vii	180.0
O1^ii—Gd—O1^iv	148.78 (11)	O2—Zn—O2^vii	91.92 (17)
O1—Gd—O1^iv	126.37 (10)	O2^vi—Zn—O2^viii	91.92 (17)
O1^iii—Gd—O1^iv	78.15 (9)	O2—Zn—O2^viii	180.0
O1^i—Gd—O1^v	148.78 (11)	O2^vii—Zn—O2^viii	88.08 (17)
O1^ii—Gd—O1^v	78.15 (9)	O2^vi—Zn—O1W^viii	86.79 (9)
O1—Gd—O1^v	78.15 (9)	O2—Zn—O1W^viii	86.79 (9)
O1^iii—Gd—O1^v	126.37 (10)	O2^vii—Zn—O1W^viii	93.21 (9)
O1^iv—Gd—O1^v	87.85 (12)	O2^viii—Zn—O1W^viii	93.21 (9)
O1^i—Gd—O3^ii	63.19 (5)	O2^vi—Zn—O1W	93.21 (9)
O1^ii—Gd—O3^ii	63.19 (5)	O2—Zn—O1W	93.21 (9)
O1—Gd—O3^ii	74.39 (5)	O2^vii—Zn—O1W	86.79 (9)
O1^iii—Gd—O3^ii	74.39 (5)	O2^viii—Zn—O1W	86.79 (9)
O1^iv—Gd—O3^ii	136.07 (6)	O1W^viii—Zn—O1W	180.0
O1^v—Gd—O3^ii	136.07 (6)	C2—O3—C2^iv	114.4 (4)
O1^i—Gd—O3^v	136.07 (6)	O2—C1—O1	126.1 (3)
O1^ii—Gd—O3^v	74.39 (5)	O2—C1—C2	115.9 (3)
O1—Gd—O3^v	136.07 (6)	O1—C1—C2	117.9 (3)
O1^iii—Gd—O3^v	63.19 (5)	O3—C2—C1	109.7 (3)
O1^iv—Gd—O3^v	74.39 (5)	O3—C2—H2A	109.7
O1^v—Gd—O3^v	63.19 (5)	C1—C2—H2A	109.7
O3^ii—Gd—O3^v	120.0	O3—C2—H2B	109.7
O1^i—Gd—O3	74.39 (5)	C1—C2—H2B	109.7
O1^ii—Gd—O3	136.07 (6)	H2A—C2—H2B	108.2
O1—Gd—O3	63.19 (5)	H1W—O1W—H1W^vi	100 (5)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1W···O1^vi	0.88 (5)	1.90 (5)	2.768 (3)	167 (4)

Symmetry code: (vi) x, y, −z+1.