Structural characterization and DFT study of bis{(S)-2-[(2-hydroxybenzyl)amino]-3-(4-hydroxyphenyl)propanoato-κ2 N,O}(1,10-phenanthroline-κ2 N,N′)cadmium(II) tetrahydrate

In the crystal, the CdII atom, located on a twofold rotation axis, is coordinated by three chelating ligands, leading to a distorted octahedral CdN4O2 coordination sphere.


Chemical context
Schiff bases are widely known as an important class of organic compounds and ligands in coordination chemistry. In recent years they have found applications in the fields of analytical chemistry, medicine and biological processes, displaying antifungal, antibacterial and anticancer activities (Przybylski et al., 2009;Dhar & Taploo, 1982). Such systems are considered important ligands for coordination and supramolecular compounds (Moroz et al., 2012). Coordination complexes with Schiff bases have attracted the interest of researchers in the areas of pharmaceutical, agriculture and industrial chemistry (Anis et al., 2013). However, the use of Schiff base ligand systems having additional polar donor functions on contrary oximes (Sliva et al., 1997;Penkova et al., 2010;Pavlishchuk et al., 2010) is limited because of their enhanced reactivity or instability under complex formation (Casella & Gullotti, 1983). For example, Schiff bases derived from aminohydroxamic acids undergo spontaneous cyclization resulting in the formation of 2-substituted 3-hydroxyimidazolidine-4-ones (Iskenderov et al., 2009). One of the ways to overcome this drawback is the reduction of such compounds to amines. The formed ligands are more conformationally flexible at the coordination site, thus not necessarily forming planar chelate rings (Koh et al., 1996). In recent years it has also been found that phenanthroline, another ligand used in this study, has ISSN 2056-9890 extensive important roles in a variety of fields (Faizi & Sharkina, 2015;Faizi et al., 2017). In this paper we report the synthesis and structure of a new cadmium complex with an l-tyrosine-derived ligand synthesized by the reduction of a Schiff base precursor.

Structural commentary
The asymmetric unit of the title compound contains one half of the mononuclear complex of the mononuclear complex [Cd(L-H) 2 (phen)] and two water molecules of solvation ( Fig. 1). The central Cd II atom is located on a twofold rotation axis and coordinated by three chelating ligands, leading to a distorted octahedral CdN 4 O 2 coordination sphere. The mixed-ligand complex contains one neutral phenanthroline ligand, bisected by the twofold rotation axis, and two residues of monodeprotonated tyrosine-derived ligands. The latter are coordinated in a 2 N,O classical amino acid chelating mode and have a C9 chiral atom, exhibiting an (N,N 0 )-trans disposition. The Cd-O and Cd-N bond lengths are similar, being 2.325 (5), 2.335 (6) and 2.323 (6) Å for Cd1-O4, Cd1-N1 and Cd1-N2, respectively. All three ligands form fivemembered chelate rings. Unlike the chelate ring formed by the phenanthroline ligand which is virtually planar, the one created by the L residue exhibits an envelope conformation with the deviation of the Cd atom from the mean plane defined by the other four atoms being 1.0692 (3) Å . The N-Cd-O and N-Cd-N bite angles are 70.5 (2) and 72.0 (4) , respectively. The phenolic O-H group remains protonated and non-coordinating, albeit participating in an extensive intermolecular hydrogen-bonding network. An intramolecular N1-H1Á Á ÁO1 hydrogen bond (Table 1) occurs between the amino and phenolic groups of the same acido ligand.

Figure 2
The crystal packing of the title compound viewed along the a axis. Hydrogen bonds are shown as dashed lines (see Table 1 for details).

Figure 1
The molecular structure of the title compound, showing the atom labelling for the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level. The unabelled atoms are related to the labelled atoms by symmetry operation y, x, Àz donors and acceptors (Table 1, Fig. 2). Hydrogen bonds formed by the water molecules link the neighboring complex molecules, forming a three-dimensional structure.interactions take place between the central ring of phenanthroline and the C2-C7 aromatic rings of two tyrosine-derived ligands with centroid-to-centroid separations of 3.938 (6) Å .

DFT study
Density functional theory (DFT) calculations were performed to investigate the electronic structure and characteristic vibrations. The calculated frequencies were found within the range, shown in Table 2. Two factors could be responsible for the shift between the experimental and computed spectra (Fig. 3). The first is the environmental factor as the DFT calculations were performed for the gas phase while the experimental data were obtained for the solid state. The second reason for the shift is that the calculated values are only harmonic frequencies while the experimental values contain both harmonic and anharmonic vibrational frequencies, but the pattern of the spectra appear to be quite similar in both cases, which validates the experimental vibrational spectrum. Some animated images of the characteristic vibrations with displacement vector are given in supporting information.

Frontier molecular orbital analysis
The LUMO and HOMO orbital energy parameters are accountable to a significant extent for the charge transfer, chemical reactivity and kinetic/thermodynamic stability of a molecule. Metal complexes with a small energy gap (ÁE) between the HOMO and LUMO values are more polarizable, thereby acting as soft molecules with a higher chemical reactivity. However, complexes with large energy gap offer greater stability and lower chemical reactivity than those with a small HOMO-LUMO energy gap. The DFT study revealed that the HOMO, HOMO-1, HOMO-2 and HOMO-3 energies are localized on the N1, N4, O2, O3, O6, O7, C8, C9, C35, C36 and C37 atoms of the amino acid ligand, partially localized on the Cd centre, namely dx 2 À y 2 , as shown in Fig. 4

Figure 4
HOMO and LUMO frontier molecular orbitals with respective molecular orbital number. delocalized over phenanthroline moiety. It could be said that the HOMO and LUMO are mainly composed of and -type orbitals, respectively, and that intramolecular charge transfer occurred from the amino acid moiety to the phenanthroline moiety. The LUMO-HOMO gap of the complex was calculated to be 2.30 eV. The frontier molecular orbital energies are given in Table 3.

Hirshfeld surface analysis
The Hirshfeld surfaces of the title compound are illustrated in Fig. 5, depicting surfaces that have been mapped over a d norm range of À0.5 to 1.5 a.u., shape index (À1.0 to 1.0 a.u.) and curvedness (À4.0 to 0.4 a.u.). The d norm surface has a redwhite-blue colour scheme, whereas deep-red spots highlight shorter contacts i.e. hydrogen bonding. The white areas represent contacts around the van der Waals separation, such as HÁ Á ÁH contacts, and the blue regions are devoid of such close contacts. On the Hirshfeld surface mapped with the shape-index function, one can examine both red regions corresponding to C-HÁ Á Á interactions as well as 'bow-tie patterns', which indicate the presence of aromatic stacking (-) interactions. The curvedness surface indicates that the electron density of the surface curves around the molecular interactions. The fingerprint plots, presented in Fig. 6, can be decomposed to highlight particular atom-pair close contacts. This itemization allows visualization of the contributions from different interaction types, which overlap in the full finger-  Lou et al., 2005), a mononuclear complex with N-(2-hydroxybenzyl)-d,laspartic acid. In this complex, the doubly deprotonated (by the phenolic and -carboxylic groups) residue of the ligand is coordinated in an (O,N,O 0 )-tridentate mode including the phenolic oxygen, unlike the title compound in which the phenolic group is non-coordinating. The second oxygen atom of the -carboxylic group bridges the neighboring mononuclear Cd units into a one-dimensional chain. In addition, there are few structures of complexes with zinc or cadmium analogues (refcodes AZIROQ, AZIRUW, NOLYIW, NOLYOC) with 2-hydroxybenzyl derivatives of alanine. In all these complexes, the ligand is also coordinated in an (O,N,O 0 )tridentate manner, with an additional 2 -function of the phenolic oxygen, which results in the formation of a Zn 2 O 2 binuclear core in all cases (Lou et al., 2004;Ranford et al., 1998).

Synthesis and crystallization
Synthesis of (S)-2-[(2-hydroxybenzyl)amino]-3-(4-hydroxyphenyl)propanoic acid (L) A methanolic solution of o-salicylaldehyde (1.18 g, 5.51 mmol) was added dropwise to a stirring solution of l-tyrosine (1.00 g, 5.52 mmol) and LiOHÁH 2 O (0.23 g, 5.50 mmol) in methanol (25 mL). Stirring was continued for 2 h, followed by the addition of sodium borohydride (0.21 g, 5.55 mmol) with further stirring for 1 h. The solvent was evaporated and the resulting sticky mass was dissolved in water and acidified with dilute HCl. The pH of the solution was maintained between 5-7. The ligand precipitated as a brown solid. It was washed thoroughly with water and MeOH after filtration and dried in a vacuum desiccator. Yield 1.60 g (76% The two-dimensional fingerprint plots of interatomic interactions showing the percentage contributions to the Hirshfeld surface.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The O-H H atoms were located in a difference-Fourier map and 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). The crystal studied was refined as an inversion twin.

Computing details
Data collection: SMART (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). Absolute structure: Refined as an inversion twin Absolute structure parameter: 0.02 (7) 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.