Crystal structure of bis[(S)-2-(2-hydroxybenzylamino)-4-methylpentanoato-κ2 N,O 1](1,10-phenanthroline-κ2 N,N′)cadmium dihydrate

The crystal structure of [Cd(C13H18NO3)2(C12H8N2)] is non-centrosymmetric. Two complex molecules with similar bond lengths and angles are present in the asymmetric unit, each exhibiting a distorted octahedral N4O2 coordination environment around the CdII ion.


Chemical context
Schiff base metal complexes are an important research area with respect to inorganic and supramolecular chemistry (Burkhardt et al., 2008;Przybylski et al., 2009;Moroz et al., 2012). Such compounds have been found to exhibit a number of properties among which are antibacterial, antifungal, antitumor, herbicidal activities (Asadi et al., 2011), as well as having applications in pharmaceutical, agricultural and industrial chemistry (Anis et al., 2013). Unlike oximes, another azomethine ligand family (Sliva et al., 1997;Penkova et al., 2010;Pavlishchuk et al., 2010), Schiff base ligands containing additional polar or acidic groups are known for their enhanced reactivity and, as a consequence, instability upon coordination to metals (Casella & Gullotti, 1983). Thus, attempts to isolate Schiff bases derived from aminohydroxamic acids resulted in cyclization under the formation of 2-substituted 3-hydroxyimidazolidine-4-ones (Iskenderov et al., 2009). In attempts to achieve stable polydentate ligand systems retaining the initial donor sets, it was found that reduction of Schiff bases to amines allows the formation of stable complexes (Koh et al., 1996). Phenanthroline and phenanthroline-derived ligands also have important roles in many fields (Faizi & Sharkina, 2015;Faizi et al., 2017). Herein we report the synthesis and structure of a new hydrated cadmium complex, [Cd(C 13 H 18 NO 3 ) 2 (C 12 H 8 N 2 )]Á2H 2 O, with a phenanthroline ligand and two ligands derived from l-leucine.

Structural commentary
The asymmetric unit of the title complex contains two mononuclear molecules (Fig. 1). In each, the metal cation is located on a twofold rotation axis and is coordinated by three chelating ligands, leading to a distorted octahedral N 4 O 2 coordination sphere. The mixed-ligand complex is made up from one neutral phenanthroline ligand and two residues of the monodeprotonated l-leucine-derived ligand L. The L ligands of each complex are trans-N,N 0 disposed with respect to each other and comprise the chiral atoms C8 for the first and C27 for the second molecule. The Cd-O and Cd-N bond lengths in the first molecule are virtually the same in the Cd1O 4 N 2 octahedron with Cd1-O1, Cd1-N1 and Cd1-N2 = 2.346 (3), 2.341 (4) and 2.315 (4) Å , respectively. The second molecule also exhibits similar geometrical parameters [Cd2-O4, Cd2-N3 and Cd2-N4 = 2.322 (4), 2.351 (5) and 2.339 (4) Å , respectively]. All three sets of ligands form fivemembered chelate rings. Unlike the essentially planar chelate rings formed by the phenanthroline ligands, the ones involving the l-leucine-derived ligands exhibit a -conformation in both complex molecules. The deviations of the carbon atoms from the planes defined by the central atom and donor atoms are 0.258 (6) Å for C7, 0.599 (7) Å for C8, À0.417 (7) Å for C26 and 0.632 (5) Å for C27. In the second molecule, the highest deviations are found to be 0.160 and 0.232 Å for O4 and N4, respectively. The N-Cd-O and N-Cd-N bite angles are 73.01 (13) and 71.2 (2) , respectively, for the first molecule and 72.40 (14) and 70.8 (2) for the second. The phenolic O-H group remains protonated and is non-coordinating, albeit participating in an extensive intermolecular hydrogenbonding network. Intramolecular hydrogen bonds are also found to exist and take place between atoms H2A and O3 as well as between H4 and O6 of the l-leucine-derived ligands. To a minor extent, intramolecular C-HÁ Á ÁO interactions are also present between a methylene group and O4 (Table 1).

Supramolecular features
In the crystal structure, the complex molecules are linked via hydrogen-bonding interactions between phenolic O-H and C-O groups of l-leucine-derived ligands (Table 1, Fig. 2).
interactions take place between the central phenanthroline ring and the C14-C19 rings of two leucine-derived L ligands with distances between the centroids of the aromatic fragments being 3.813 (4) Å for the first molecule. The stacking interactions of the second molecule are between C33-C38 rings of two l-leucine-derived ligands L and the C23-C25/C23 0 -C25 0 (-x + 1, y, -z + 2) phenanthroline fragment with a centroid-to-centroid distance of 3.773 (4) Å . Structures of the two complex molecules in the title compound. Displacement ellipsoids are drawn at the 40% probability level. Atoms with primed labels are generated by the symmetry operations Àx + 1, y, Àz + 1 for complex Cd1 and Àx + 1, y, Àz + 2 for complex Cd2. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1 2 ; y À 1 2 ; Àz þ 3 2 ; (ii) Àx þ 1; y; Àz þ 2.  (Lou et al., 2005). This differs from the title compound in which the phenolic group is protonated and is non-coordinating. The second O atom of the -carboxylic group bridges the neighbouring Cd II units into a polymeric chain. In addition, there are four structures of complexes of homologous zinc and with 2-hydroxybenzyl derivatives of alanine (refcodes AZIROQ, AZIRUW, NOLYIW, NOLYOC). These compounds have a Zn 2 O 2 binuclear core, and the ligands also coordinate in an (O,N,O 0 )-tridentate manner, with an additional 2 -mode for the phenolic O atom (Lou et al., 2004;Ranford et al., 1998).

Synthesis and crystallization
Synthesis of (S)-2-(2-hydroxybenzylamino)-4-methylpentanoic acid (L) A mixture of l-leucine (1.00 g, 7.62 mmol) and LiOHÁH 2 O (0.323 g, 7.62 mmol) in methanol (25 ml) was stirred for 10 min to dissolve. A methanolic solution of o-salicylaldehyde (0.930 g, 7.62 mmol) was added dropwise to the above solution whereby the colour of the solution turned to yellow. Stirring was continued for 30 min before the solution was treated with NaBH 4 (0.580 g, 15.3 mmol), leading to a colourless solution. The solvent was evaporated under reduced pressure, and the resulting solid was dissolved in water. The clear solution was then acidified with diluted HCl (pH $5-7). The ligand precipitated as a white solid. The suspension was filtered, and the residue was washed thoroughly with water. The solid was dried in a vacuum desiccator (yield 1.65 g, 88%). Because of its poor solubility, the 1 H NMR spectrum for the ligand was recorded as the lithium salt of the ligand, prepared by adding 2 equiv. of LiOHÁ3H 2 O in CD 3 OD. 1  A methanolic solution of Cd(NO 3 ) 2 Á4H 2 O (0.130 g, 0.421 mmol) was added under stirring to 20 ml of a methanolic solution of L (0.200 g, 0.843mmol) and NaOH (0.034 g, 0.843 mmol), followed by addition of phenanthroline monohydrate (0.076 g, 0.421mmol) in 5 ml of methanol. A clear solution was formed within half an hour under constant stirring. After 2 h, the solvent was evaporated to dryness. The residue was subsequently washed with methanol and diethyl ether, and finally dried under vacuum. Empirical formula ), 6.6 (s, broad, 1H d ), 6.4 (s, broad, 1H c ), 6.6 (s, broad, 1H b ), 6.1 (s, broad,1H a ), 8.0 (s, broad, 2H n ), 8.1 (s, broad, 2H m ), 8.7 (s, broad, 2H l ), 9.1 (s, broad, 2H k ). ESI-Mass (-ve) at 829.18 (calculated 829.18). Suitable needle-shaped crystals for X-ray data collection were obtained by slow evaporation of a methanol: DMF (2:1 v:v) solution within a week.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Atoms O3, C3, C4 and C6 showed highly anisotropic displacement parameters and were modelled using the ISOR instruction in SHELXL (Sheldrick, 2015). The H atoms of the phenolic OH group were located from a difference-Fourier map and were constrained to ride on their parent atoms, with O-H = 0.82 Å and with U iso (H) = 1.5U eq (O). All C-bound H atoms were positioned geometrically and refined using a riding model with C-H = 0.93 Å and with U iso (H) = 1.2U eq (C).
After unsuccessful attempts to model disordered solvent molecules, their contributions to the diffraction data were removed by using the SQUEEZE routine in PLATON (Spek, 2015). PLATON calculated a solvent-accessible void volume in the unit cell of 629 Å 3 (15.4% of the total cell volume), corresponding to 151 electrons (residual electron density after the last refinement cycle) per unit cell, or 37.75 electrons per one complex molecule. This number agrees with two water molecules. Although not modelled in the refined structure, the two water molecules are included in the formula and other crystallographic data. The crystal packing of the title compound viewed along [010]. Hydrogen bonds are shown as dashed lines (see Table 1 for numerical details).   Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq