Hexaaquacadmium(II) bis{[N-(2-oxidobenzylidene)glycyl-l-leucinato]cuprate(II)} dihydrate

The title compound, [Cd(H2O)6][Cu(C15H17N2O4)]2·2H2O, has a chiral structure. Copper has a square-planar coordination with two N and two O atoms of the quadridentate chiral Schiff base ligand. The Cd2+ ion is coordinated by six aqua ligands with a slightly distorted octahedral configuration. Ions are linked by O—H⋯O hydrogen bonds, and the [Cd(H2O)6]2+ cations and [CuL]− anions (L = Schiff base derived from glycyl-l-leucine and salicylaldehyde) occupy a stacking structure within well separated columns along the a axis. The two crystallographically independent copper–Schiff base anions each have a chiral carbon centre with an S configuration. They are related by a non-crystallographic twofold rotation axis parallel to the [010] direction.


Related literature
Data collection: SMART (Bruker, 2002); cell refinement: SAINT-Plus (Bruker, 2003); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2000); software used to prepare material for publication: SHELXTL.  Table 1). The best-fit least-squares plane through the four basal and Cu atoms shows these atoms to be nearly coplanar. The Cd II is coordinated by six aqua ligands with a slightly distorted octahedral geometry. The six Cd-O bonds in the structure are in the range of 2.228 (2)-2.373 (2) Å.
The anions and cations linked by O-H···O hydrogen bonds (Table 2) form well separated columns along the a-axis in the stacking structure of (Fig. 3). The intermolecular and intramolecular hydrogen bonds in the title compound play an important role in the stabilization of the whole structure.

S2. Experimental
Glycyl-L-leucine (5 mmol), salicylaldehyde (5 mmol) and LiOH (10 mmol) were dissolved in MeOH/H 2 O (30 ml, v:v = 1:1) and refluxed for 30 min. Then Cu(ClO 4 ) 2 .6H 2 O (5 mmol) was added to the solution and the resulting solution was adjusted to the pH 9-11 by using 5 mol.L -1 NaOH solution. After stirring at room temperature (25 °C) for 1 hr, CdCl 2 .6H 2 O (2.5 mmol) was added. A violet precipitate was obtained immediately. After stirring for 30 min and then filtered, the precipitate was recrystallized in water. The violet crystals suitable for X-ray diffraction were obtained after 1 week.

S3. Refinement
The water H atoms were located in a difference Fourier map and refined in riding mode, with a distance restraint of O-H = 0.85 Å and U iso (H) = 1.5U eq (O). All other H atoms were positioned geometrically and constrained as riding atoms, with C-H distances of 0.93-0.98 Å and U iso (H) set to 1.2 or 1.5 eq (C) of the parent atom. The refinement of the structure supporting information sup-2 . E64, m94-m95 was performed using 361 least-squares restraints by applying SIMU and DFIX instructions of SHELXTL.

Figure 1
The molecular structure of the title compound, with atom labels and 50% probability displacement ellipsoids.  The packing of the title compound, viewed down the a axis, showing a separated columns stacking structure connected by O-H···O hydrogen bonds, indicated by dashed lines.

Hexaaquacadmium(II) bis{[N-(2-oxidobenzylidene)glycyl-L-leucinato]cuprate(II)} dihydrate
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.33 e Å −3 Δρ min = −0.28 e Å −3 Absolute structure: Flack (1983), 3005 Friedel pairs Absolute structure parameter: 0.008 (9) 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 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