Crystal structure and Hirshfeld surface analysis of (aqua-κO)(methanol-κO)[N-(2-oxidobenzylidene)threoninato-κ3 O,N,O′]copper(II)

The crystal structure of the amino acid Schiff base copper(II) complex with a tridentate ligand synthesized from salicylaldehyde, l-threonine and copper(II) acetate is reported.

In the title complex molecule, [Cu(C 11 H 11 NO 4 )(CH 4 O)(H 2 O)], the Cu atom is coordinated in a distorted square-pyramidal geometry by a tridentate ligand synthesized from l-threonine and salicylaldehyde, one methanol molecule and one water molecule. In the crystal, the molecules show intra-and intermolecular O-HÁ Á ÁO hydrogen bonds. The Hirshfeld surface analysis indicates that the most important contributions to the packing are HÁ Á ÁH (49.4%) and HÁ Á ÁO/ OÁ Á ÁH (31.3%) contacts.

Chemical context
Amino acid Schiff bases, which can be easily synthesized by condensation of primary amines with carbonyl components, are organic ligands having an azomethine (>C N-) group. They play an important and diverse role in coordination chemistry (Qiu et al., 2008;Li et al., 2010;Xue et al., 2009). On the other hand, copper has various oxidation states, of which the +2 oxidation state is the most stable. Copper ions readily form complexes and produce abundant coordination chemistry, while Schiff base-copper(II) complexes are known to increase the catalytic efficiency of redox reactions (Cozzi, 2004;Roy & Manassero, 2010).
One method of reducing highly toxic Cr VI compounds to less toxic Cr III compounds is the use of titanium(IV) oxide, a heterogeneous photocatalyst. Although useful for such redox reactions (Kitano et al., 2007;Sun et al., 2006;Tuprakay & Liengcharernsit, 2005), it is only active under UV illumination (Schneider et al., 2014). In our laboratory, a heterogeneous titanium (IV) oxide photocatalyst was combined with a Schiff base-Cu II complex and irradiated with visible light. The presence of a -conjugated ligand system increases the effi-ciency (Yoshida et al., 2017;Nakagame et al., 2019). It can be said that the Schiff base-copper complex has a photocatalytic effect. In the present study, the title Schiff base-copper complex was synthesized by microwave irradiation in order to shorten the synthesis time and to obtain high purity. The crystal structure is reported here.

Structural commentary
The molecular structure of the title compound consists of a tridentate ligand synthesized from l-threonine and salicylaldehyde, one methanol molecule, and one water molecule coordinating to copper (Fig. 1) in a distorted square-pyramidal coordination geometry. The C8 N1 double-bond distance is 1.286 (5) Å , close to a typical C N double-bond length for an imine. The Cu1-O2, Cu1-O3 and Cu1-O4 bond lengths are 1.968 (3), 1.937 (3) and 1.910 (3) Å , respectively, which are close to a typical Cu-O single bond length. The Cu1-N1 bond length of 1.922 (3) Å corresponds to the typical Cu-N single-bond length. These four atoms coordinated to Cu1 have similar bond-distance values, and the contribution degree of the electron cloud is almost the same. The Cu1-O6 bond [2.471 (3) Å ] has been lengthened by a pseudo Jahn-Teller effect. One intramolecular O-HÁ Á ÁO hydrogen bond (O5-H5Á Á ÁO6; Table 1) is observed between the methoxy function and the amino acid side chain (Fig. 2).

Synthesis and crystallization
l-Threonine (0.0234 g, 0.196 mmol) and salicylaldehyde (0.0295 g, 0.242 mmol) were dissolved in methanol (15 ml), which was treated for 5 min with microwave irradiation at 358 K to yield a transparent yellow ligand solution. To this solution, copper(II) acetate dihydrate (0.0421 g, 0.211 mmol) was added and treated for 5 min while being irradiated with microwaves at 358 K. The solution was placed in the air, and the solvent was removed. The title compound (0.0533 g, 0.169 mmol, yield 85.9%) was obtained as a green solid. IR (KBr, cm À1 ): 1633 (C N double bond). A part of the obtained solid was dissolved in a small amount of methanol Distribution of atomic charges (red: negative, blue: positive) on the Hirshfeld surface.   and left in air, and single crystals suitable for X-ray diffraction were obtained after several days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All C-bound H atoms were placed on geometrically calculated positions (C-H = 0.93-0.98 Å ) and were constrained using a riding model with U iso (H) = 1.2U eq (C) for R 2 CH and R 3 CH H atoms and 1.5U eq (C) for the methyl H atoms. The O-bound H atoms were located based on a difference-Fourier map. Atoms H4 and H5 of the terminal OH group were constrained using a riding model with O-H = 0.82 Å . H5 was assigned U iso (H) = 1.2U eq (O), while the U iso of H4 (attached to O6 was refined. Atoms H2 and H3 of the water molecule were refined freely.  (Sheldrick, 2008).

(Aqua-κO)(methanol-κO)[N-(2-oxidobenzylidene)threoninato-κ 3 O,N,O′]copper(II)
Crystal data [Cu(C 11 H 11 NO 4  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.