Poly[tetraaqua-μ4-squarato-di-μ3-squarato-disamarium(III)]

The structure of the title compound, [Sm2(C4O4)3(H2O)4]n, consists of infinite-chain structural units, built from edge-sharing samarium SmO7(H2O)2 polyhedra and linked via bis-monodendate squarate (sq1) groups. The chains extend along [100] in a zigzag mode and are interconnected by bis-chelating squarate (sq2) ligands into layers parallel to (101). Interlayer hydrogen bonds strengthen the cohesion of the three-dimensional network. The samarium cation is coordinated by four O atoms from sq1 units and three O atoms from sq2 units, in addition to two water O atoms. The best representation of the samarium SmO7(H2O)2 polyhedron is distorted tricapped trigonal-prismatic. The sq1 ligand has one metal-free O atom and relates three Sm atoms in a bis-monodentate and chelation fashion, the second squarate, sq2, is strictly centrosymmetric and acts as a bis-chelating ligand.

The structure of the title compound, [Sm 2 (C 4 O 4 ) 3 (H 2 O) 4 ] n , consists of infinite-chain structural units, built from edgesharing samarium SmO 7 (H 2 O) 2 polyhedra and linked via bismonodendate squarate (sq1) groups. The chains extend along [100] in a zigzag mode and are interconnected by bis-chelating squarate (sq2) ligands into layers parallel to (101). Interlayer hydrogen bonds strengthen the cohesion of the threedimensional network. The samarium cation is coordinated by four O atoms from sq1 units and three O atoms from sq2 units, in addition to two water O atoms. The best representation of the samarium SmO 7 (H 2 O) 2 polyhedron is distorted tricapped trigonal-prismatic. The sq1 ligand has one metalfree O atom and relates three Sm atoms in a bis-monodentate and chelation fashion, the second squarate, sq2, is strictly centrosymmetric and acts as a bis-chelating ligand.

Data collection
Nonius KappaCCD diffractometer Absorption correction: none 3906 measured reflections 2340 independent reflections 2199 reflections with I > 2(I) R int = 0.029 Refinement R[F 2 > 2(F 2 )] = 0.023 wR(F 2 ) = 0.058 S = 1.08 2340 reflections 148 parameters 6 restraints H atoms treated by a mixture of independent and constrained refinement Á max = 1.54 e Å À3 Á min = À1.96 e Å À3 Table 1 Hydrogen-bond geometry (Å , ).  et al., 1988;Trombe et al., 1990) starting from heating hydrated lanthanide(III) squarates (Petit et al., 1990) in water inside a closed vessel. Unit-cell parameters of the whole compounds were determined from X-ray powder patterns. However, only crystal structure of cerium compound was determined using single-crystal X-ray techniques. Accordingly, much effort was given to experimental conditions in order to obtain single-crystals of the other lanthanides. Unfortunately, up to now no other single-crystal of lanthanide(III) squarate tetrahydrates could be obtained.
The compound (I) was synthesized during one of our attempts to create novel lanthanide(III) squarates. However, only [Sm(H 2 O) 2 ] 2 (C 4 O 4 ) 3 separated from solution and a view of the molecular structure is given in Fig. 1. Trombe et al.
reported the structure of cerium(III) squarate tetrahydrate (Trombe et al., 1988;Trombe et al., 1990). The samarium analog is isostructural, the compound crystallizing with similar unit-cell dimensions. Its molecular structure displays a layered structure based on infinite chains structural units, built from edge sharing distorted tricapped trigonal prism polyhedra SmO 7 (H 2 O) 2 (Fig. 2). These polyhedra are connected via regular squarate sq1 groups along [100] in zigzag mode and interconnected by bis-chelating squarate anions sq2 into layers parallel to (101) plan (Fig. 2). Hydrogen bonds strengthen the structure (table 1, Fig. 2). The two crystallographically independent squarate anions present a case of chelation. One (sq1) has no imposed symmetry. One of its oxygen atoms, namely O2, is not bound to any samarium atom. The oxygen atoms O3 and O4 chelate one samarium atom. The atom O4 binds also another samarium atom while the atom O3 binds only one samarium atom. Thus, the squarate sq1 ligand relates three metal centers. The second symmetric squarate sq2 ligand chelates on both side one samarium atom, and one more Sm atom is also bounded to the oxygen atom O5 of each bite , so that four metal centers are related.

S2. Experimental
For convenience, 3,4-dihydroxy-3-cyclobutene-1,2-dione (H 2 C 4 O 4 ) is named squaric acid hereafter. Pale yellow single crystals of the title compound were hydrothermally synthesized during an attempt to synthesize open frameworks of lanthanide squarates. A mixture of samarium chloride, SmCl 3 .6H 2 O, and squaric acid in molar ratio 2/3/704 were dissolved in 10 ml distilled water while stirring. The resulting mixture (pH = 2) was transferred into was transferred into a Teflon-lined acid digestion bomb (Parr) and heated at 150 °C for two days under autogenous pressure. Then the autoclave was cooled to room temperature by turning off the power. The pH after treatment remains unchanged. Products were filtered off, washed with distilled water and dried at room temperature. Crystals with cubic morphology were selected for single-crystal diffraction after checking under a polarizing microscope and identifying by X-ray powder diffraction.

S3. Refinement
All non-H atoms were refined with anisotropic atomic displacement parameters. All H atoms were localized on Fourier maps and refined isotropically with soft constraints on the distances to their relevant parent water oxygen atom. Within a molecule, the O-H and H-H distances were restrained to 0.96 Å and 1.55 Å, respectively, so that the H-O-H angle fitted the ideal value of the tetrahedral angle.  A diagram of the layered crystal packing in the unit cell of (I). Hydrogen bonds are shown as green dashed lines.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq