Crystal structure and DFT study of bis{(S)-2-[(2-hydroxybenzyl)amino]-4-methylpentanoato-κ2 N,O}(1,10-phenanthroline-κ2 N,N′)nickel(II)

The title compound was prepared from an equimolar mixuture of nickel nitrate, phenanthroline and two equivalents of (S)-2-(2-hydroxybenzylamino)-4-methylpentanoic acid. The NiII complex shows a distorted octahedral geometry which is stabilized by intramolecular hydrogen bonds and a weak π–π interaction.

In the title compound, [Ni(C 13 H 18 NO 3 ) 2 (C 12 H 8 N 2 )], the Ni II cation shows a distorted octahedral coordination environment. It is formed by two N atoms from the phenanthroline ligand, as well as two N and two O atoms belonging to two 2-[(2-hydroxybenzyl)amino]-4-methylpentanoate ligands. Complex molecules are connected into layers propagating along the ab plane via hydrogen bonds formed by O atoms of carboxylate and phenoxide groups, which are further connected into a three-dimensional motif.

Chemical context
The design and synthesis of metal complexes have attracted considerable attention for their potential applications in catalysis, magnetism, materials science and pharmaceutical chemistry (Che & Siu, 2010). Mononuclear ethylenediaminediacetate complexes can be used to bind and cleave DNA under physiological conditions  and binuclear complexes containing bipyridyl or phenanthroline units in their structure show antiviral activity, as well as inhibition of proviral DNA synthesis (Rajendiran et al., 2007). On the other hand, using bifunctional ligands that are capable of simultaneously coordinating to a metal centre and providing hydrogen bonding gives important experimental data for a better understanding of the key tools in crystal engineering (Burrows, 2004). Metal complexes of 1,10-phenanthroline (phen) and its derivatives are of increasing interest because of their versatile roles in many fields, such as analytical chemistry (Chalk & Tyson, 1994), catalysis (Samnani et al., 1996), electrochemical polymerization (Bachas et al., 1997) and biochemistry (Sammes & Yahioglu, 1994). 1,10-Phenanthroline is a bidentate chelating ligand with notable coordination ability for transition metal cations. Over the last few decades, the complex formation of transition metal ions with amino acids has also been studied extensively (Auclair et al., 1984). Amino acid-metallic ion interactions are found to be responsible for enzymatic activity and the stability of protein structures (Dinelli et al., 2010). Nickel is also essential for the healthy life of animals since it is associated with several enzymes (Poellot et al., 1990) and plays a role in physiological processes as a cofactor in the absorption of iron from the intestine (Nielsen, 1980). Any change in its concentration leads to metabolic disorder (Kolodziej, 1994). With the discovery of the biological importance of nickel, it is essential to study its complex formation with amino acids in order to ISSN 2056-9890 gain a better understanding of the functions of their complexes (Faizi & Sharkina, 2015). Therefore, we report here the preparation and the crystal structure of a nickel(II) complex with the formula: [Ni(C 13 H 18 NO 3 ) 2 (C 12 H 8 N 2 )], (I).

Structural commentary
The complex molecule of I, represented in Fig. 1, contains one crystallographically independent Ni II cation, which is octahedrally coordinated by two molecules of deprotonated 2-[(2hydroxybenzyl)amino]-4-methylpentanoic acid via their N atoms and one of the carboxylate atoms each. The coordination environment is completed by one bidentate phenanthroline ligand. The C-O bond lengths in the deprotonated carboxylic acid groups differ significantly [1.239 (2) and 1.292 (2) Å ], which is typical for monodentate carboxylate groups (Wö rl et al., 2005a,b).
The values of the Ni-O bond lengths are similar to those reported in the literature for octahedral carboxylate nickel(II) complexes II-IV (see x5). However, the corresponding Ni-N separations of 2.101 (3)-2.149 (3) Å are somewhat shorter than found for III-IV and similar to that observed in II.

Figure 1
The molecular structure of compound I, showing the atom labelling. Displacement ellipsoids are drawn at the 40% probability level.

DFT study
The molecular structure used in the theoretical studies of the Ni complex was taken from the X-ray diffraction results, keeping all distances, angles and dihedral angles frozen. Single-point DFT calculations have been carried out using the scalar zeroth-order regular approximation Hamiltonian (Wü llen, 1998). Single-point ground-state calculations were carried out using the hybrid B3LYP functional as implemented in ORCA (Lee et al., 1988). The present calculation was performed using the additional approximation that the Coulomb integrals are approximated by sum of atom centred s, p, d functions, the auxiliary (or fitting) basis set (Yilmaz et al., 2006). This allows for efficient treatment of the Coulomb interactions and hence reduces calculation times. The Def2-TZVP main and Def2-TZVP/J auxiliary basis sets were used (Pantazis et al., 2008). The main basis set is of [5s3p2d] quality for Ni, (5s2p1d) for C, N and O, and (2s) for H (Weigend & Ahlrichs, 2005). The LUMO and HOMO orbital energy parameters are significantly accountable 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 are more polarizable, thereby acting as soft molecules with higher chemical reactivity. However, complexes with a large energy gap offer greater stability and low chemical reactivity compared to those with a small HOMO-LUMO energy gap. The DFT study of I revealed that the HOMO and HOMO-1 are localized on the N1, N2, O4, O5, O3, O6, C13 and C14 atoms of the amino acid ligand. In addition, the respective molecular orbitals are also partially localized on the Ni II cation, namely in the d x 2 Ày 2 orbital (Fig. 4). In contrast, LUMO and LUMO+1 are totally delocalized over the phenanthroline moiety. It could therefore be stated that research communications Table 2 Comparison of selected geometric data for I (Å , ) from calculated (DFT) and X-ray data.

Figure 3
A view along the b axis of the crystal packing of compound I. The C-HÁ Á Á interactions are illustrated by dashed lines. All H atoms have been omitted for clarity.

Figure 4
Electron distribution of the HOMO-1, HOMO, LUMO and LUMO+1 energy levels for I.
the HOMO and LUMO are mainly composed ofand -type orbitals, respectively, and that intramolecular charge transfer occurs from the amino acid moiety to the phenanthroline ligand. The HOMO-LUMO gap of I was calculated to 0.04212 a.u. and the frontier molecular orbital energies of I are also given in Fig. 4. A comparison of selected geometric data for I from calculated (DFT) and X-ray data is given in Table 2.

Synthesis and crystallization
For the preparation of 2-[(2-hydroxybenzyl)amino]-4-methylpentanoic acid (HAMA), l-leucine (1.00 g, 6.71 mmol) and LiOHÁH 2 O (0.284 g, 6.77 mmol) in anhydrous methanol (30 ml) were stirred for 30 min to dissolve. A methanolic solution of salicylaldehyde (1.44 g, 6.72 mmol) was added dropwise to the above solution. The solution was stirred for 1 h and then treated with sodium borohydride (0.248 g, 6.71 mmol) with constant stirring. The solvent was evaporated and the resulting sticky mass was dissolved in water. A cloudy solution was obtained, which was then acidified with dilute HCl. By maintaining the pH of the solution in the range 5-7 the ligand precipitated as a colourless solid. The solid was filtered off, washed thoroughly with water and finally dried inside a vacuum desiccator (yield 2.08 g, 85%). For the preparation of the title compound, HAMA (0.500 g, 1.43 mmol) was deprotonated with LiOHÁH 2 O (0.060 g, 1.44 mmol) in anhydrous methanol (25 ml), which resulted in a clear colourless solution after 30 min. A methanolic solution of Ni(NO 3 ) 2 Á6H 2 O (0.17 g, 0.71 mmol) was added dropwise to the ligand solution with stirring. The colour of the solution changed to green immediately. Phenanthroline (0.13 g, 0.71 mmol) was then added and the reaction mixture was stirred at room temperature for 16 h. The solution was evaporated to dryness with a rotary evaporator. Blue blockshaped crystals, suitable for single-crystal X-ray analysis, were obtained by slow diffusion of diethyl ether into a methanolic solution of the crude solid over a period of 2-3 d. The crystals were filtered off and washed with diethyl ether (yield 74%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The N-H hydrogens were located in a difference Fourier map and refined without constraints. The O-H hydrogens were also located in a difference Fourier map but were constrained to ride on their parent atoms, with U iso (H) = 1.5U eq (O). The C-bound H atoms were included in calculated positions and treated as riding atoms, with C-H = 0.95 Å and U iso (H) = 1.2-1.5U eq (C).  Refined as an inversion twin Absolute structure parameter À0.010 (18) graphics: Mercury (Macrae et al., 2008), SHELXTL (Sheldrick, 2008) and ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL2016 (Sheldrick, 2015b) and PLATON (Spek, 2009).

Bis{(S)-2-[(2-hydroxybenzyl)amino]-4-methylpentanoato-κ 2 N,O}(1,10-phenanthroline-κ 2 N,N′)nickel(II)
Crystal data 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.