Crystal structure and Hirshfeld surface analysis of mono/bis(aqua-κO)[N-(2-oxidobenzylidene)valinato-κ3 O,N,O′]copper(II): dimeric Schiff base copper(II) complexes having different numbers of coordinated water molecules

The molecular structure of the title compound, [Cu(C12H13N2O3)(H2O)2]·[Cu(C12H13N2O3)(H2O)], consists of two different molecules in the asymmetric unit. Both of the structures consist of a tridentate ligand synthesized from l-valine and salicylaldehyde, and one water molecule or two water molecules coordinating to CuII.

The molecular structure of the title compound, [Cu(C 12 H 13 N 2 O 3 )(H 2 O) 2 ]Á-[Cu (C 12 H 13 N 2 O 3 )(H 2 O)], consists of two different molecules in the asymmetric unit. Both of the structures consist of a tridentate ligand synthesized from l-valine and salicylaldehyde, and one water molecule or two water molecules coordinating to Cu II . They have a square-planar (molecule 1) or a squarepyramidal (molecule 2) coordination geometry. In the crystal, the molecules form intra-and intermolecular O-HÁ Á ÁO hydrogen bonds involving the coordinated water molecules and other sites. A Hirshfeld surface analysis indicated that the most important contributions to the packing are from HÁ Á ÁH [52.9% (molecule 1) and 51.1% (molecule 2)] and HÁ Á ÁO/ OÁ Á ÁH [21.2% (molecule 1) and 25.8% (molecule 2)] contacts. In addition, an electrostatic potential map was also obtained from DFT calculations to support the discussion of the intermolecular interactions.

Chemical context
Amino acid Schiff bases, which can easily be synthesized by mixing primary amines and 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;Katsuumi et al., 2020). We recently published a review  of the synthesis of amino acid Schiff base-metal complexes. According to the literature, in general, Schiff bases and their metal complexes are multipurpose compounds and are extensively utilized in many research and industrial applications. These compounds can be utilized alone or for the preparation of various hybrid materials, such as, for example, supramolecular elastomers with imine-functionalized polysiloxanes (Hu et al., 2010), conducting metallopolymers for electrochemical sensing (Gonzá lez et al., 2021), while recently reported co-crystals of a Schiff base with lead iodide perovskite show photo-triggered ferroelectricity (Deng et al., 2022). Furthermore, Schiff base complexes are considered to be an important class of organic compounds with a wide range of biological properties, including free radical scavenging, antibacterial, antitumor activities (Mo et al., 2022). In our laboratory, novel mono-chlorinated Schiff base Cu II complexes have been synthesized and their antibacterial activities tested against Gram-positive and Gram-negative bacteria; the most active were then tested for their antioxidant activities, and as E. coli, in particular, was found to be sensitive to these compounds, their interaction with this bacterium was investigated (Otani et al., 2022). Microwave irradiation is suitable for the synthesis of amino acid Schiff bases Cu II complexes in order to shorten the synthesis time and to obtain high purity. In the present study, the title compound was therefore synthesized by using microwave irradiation (Otani et al., 2022). Differences in chemical properties as a result of differences in structure are remarkable and it is important to report different crystal structures to discuss these features. In this study, we report the structure of the title Schiff base Cu II complex ( Fig. 1) derived from l-valine and salicylaldehyde, which has a similar structure to that of one we reported previously (Katsuumi et al., 2020).
In contrast, in molecule 2 the Cu2 atom exhibits a squarepyramidal geometry, being coordinated by the same tridentate ligand in the equatorial plane and by two water molecules in the equatorial and axial positions. The C19 N2 double-bond distance is 1.278 (3) Å , which is again close to the typical C N double-bond length for imines (Katsuumi et al., 2020). The Cu2-O5, Cu2-O6, and Cu2-O7 bond lengths are 1.9432 (14), 1.9411 (15), and 1.9956 (14) Å , respectively, which are close to a typical Cu-O bond length (Katsuumi et al., 2020). The Cu2-N2 bond length of 1.9243 (17) Å corresponds to a typical Cu-N bond length (Katsuumi et al., 2020). Again, these four coordination bonds are almost the same length. The bond lengths involving the other interacting O atoms are Cu2Á Á ÁO8(1 + x, y, z) = 2.7937 (16) Å and Cu2-O9 = 2.3663 (16) Å ; the latter is longer than Cu2-O6 because of the pseudo-Jahn-Teller effect. The Cu2-O6 bond is slightly longer than Cu1-O2 as a result of the crowding that occurs as the number of coordinating water molecules increases.

Supramolecular features
Six intermolecular O-HÁ Á ÁO hydrogen bonds (Table 1 and Fig. 2) are observed in the unit cell; (O2-H2BÁ Á ÁO8, O2-H2AÁ Á ÁO9 i , O6-H6Á Á ÁO3 ii , O6-H10Á Á ÁO7 iii , O9-H11Á Á ÁO5 iv , and O9-H12Á Á ÁO4 ii ; symmetry codes as in Table 1). The angle O4(1 + x, y, z)-Cu1- N1 [102.50 (6) ] is tilted by much more than 90 as a result of the O9-H12Á Á ÁO4 ii hydrogen bond. Similarly, the angle O8(1 + x, y, z)-Cu2-N2 [98.58 (6) ] is tilted by more than 90 because of the effect of the O2-H2BÁ Á ÁO8 hydrogen bond. In the crystal, the molecules form an infinite chain as a result of the interaction of these six hydrogen bonds and the Cu II atom with the carbonyl groups of the ligands (Figs. 2 and 3). These six hydrogen bonds also form strong interactions between molecules 1 and 2 unit. The equatorial planes of ligands in the same type of molecule are parallel to each other, while those of molecules 1 and molecule 2 intersect at an angle of 52.29 (2) .  The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (iv) x À 1, y, z; Hirshfeld surface analysis (Spackman & Jayatilaka, 2009;McKinnon et al., 2007) was performed to better understand the intermolecular interactions and contacts. The intermolecular O-HÁ Á ÁO hydrogen bonds are indicated by brightred spots appearing near atoms O4, O5, O8, O9 and water H atoms on the Hirshfeld surfaces mapped over d norm and by two sharp spikes of almost the same length in the region 1.6 < (d e + d i ) < 2.0 Å in the 2D fingerprint plots (Fig. 4). In molecule 1, the contributions to the packing from HÁ Á ÁH, CÁ Á ÁC, CÁ Á ÁH/ HÁ Á ÁC and HÁ Á ÁO/OÁ Á ÁH contacts are 52.9, 0.3, 18.6 and 21.2%, respectively, and in molecule 2, the contributions of the HÁ Á ÁH, CÁ Á ÁC, CÁ Á ÁH/HÁ Á ÁC and HÁ Á ÁO/OÁ Á ÁH contacts are 51.1, 0.6, 17.3 and 25.8%, respectively (Fig. 4). A common feature of the two structures is the high values for the contributions of HÁ Á ÁH/HÁ Á ÁH and CÁ Á ÁH/HÁ Á ÁC contacts, with HÁ Á ÁH/HÁ Á ÁH representing the influence of van der Waals forces and CÁ Á ÁH/HÁ Á ÁC representing the influence of C-HÁ Á Á interactions as a result of the presence of aromatic rings in the structures. The reason for the low CÁ Á ÁC/CÁ Á ÁC ratio is thought to be that the aromatic rings do not overlap, as indicated by the packing structure, and thereby the contribution ofstacking is low. Compared to molecule 1, molecule 2 has a larger number of water molecules and a higher HÁ Á ÁO/ OÁ Á ÁH value, which seems to have resulted in a larger contribution from hydrogen bonding and corresponding decreases in the CÁ Á ÁH/HÁ Á ÁC and HÁ Á ÁH/HÁ Á ÁH values.

DFT calculations
Quantum chemical calculations were carried out to compare the structure in the gas phase with that of in the crystal. The optimized structure of the title compound in the gas phase was calculated by density functional theory (DFT) and the calculation was performed using the Gaussian 09W software package (Revision D.02; Frisch et al., 2009). The Lanl2DZ basis set was applied to the central metal atom (Cu), the 6-31G(d) basis set to the other atoms (C, O, N, H), and the effective core potential (ECP) to the central metal. Calculations were performed for square-pyramidal and square-planar geometries. The initial structure was obtained from X-ray refinement data. However, in the optimizing calculations for the square-pyramidal geometry, the molecule was unable to maintain the square-pyramidal structure, and the axial water molecules moved outside the first coordination sphere of the central metal atom. This indicates that the square-pyramidal structure of this molecule is not stable in the gas phase. The axial water molecules are stabilized by hydrogen bonds with research communications  The chains resulting from the coordination bonding of the carbonyl groups to the copper(II) atoms. Hydrogen atoms are omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ).  (2) 157 (3) Symmetry codes: (i) x À 1 2 ; Ày þ 3 2 ; Àz þ 1; (ii) x þ 3 2 ; Ày þ 3 2 ; Àz þ 1; (iii) x þ 1 2 ; Ày þ 3 2 ; Àz þ 1; (iv) x À 1; y; z.
neighboring molecules and the square-pyramidal structure is considered to be preserved. The bond lengths and bond angles of the square-planar sites are generally consistent between the crystal structure and the optimized structure (Table 2). In the DFT-optimized structure, the hydrogen atoms of the water molecule are oriented towards the carboxyl group and appear to be involved in an intermolecular hydrogen bond. The orientation of the water molecules in the crystal is largely influenced by the hydrogenbonding network. It can be seen that the carboxy group of the ligand is charged electron rich, while the hydrogen atom of water shows an electron deficiency. Therefore, the carboxy  Table 2 Comparison of selected (X-ray and DFT) bond lengths and angles (Å , ).  (7) N2-Cu2-O6 177.73 (7) 167.07

Figure 4
Hirshfeld surfaces mapped over d norm and two-dimensional fingerprint plots.
group is considered to be an electron donor and the water hydrogen atoms are electron acceptor, which is also consistent with the crystal structure (Fig. 5).  Korhonen & Hamalainen, 1979). In the crystal of YUYFUW, a chain along the a-axis direction is formed by one hydrogen bond while the other two hydrogen bonds form a hydrogen-bonded ring. The molecules are packed in a doublecolumn along the a-axis direction via these three hydrogen bonds. In the crystal of SLCDCU, two molecules form square planes by two intermolecular hydrogen bonds. The crystal of SAVACU has a very similar structure to that of the title complex. In SAVACU, there are two molecules in the unit cell. They have a coordination water per molecule in the unit cell, while the title compound has one or two coordination waters per molecule in the unit cell. 1450 cm À1 (s, C C double bond), 1455 cm À1 (m), 1535 cm À1 (m), 1600 cm À1 (s, C O double bond), 1639 cm À1 (s, C N double bond), 2960 cm À1 (w), 3300 cm À1 (br, O-H). UV-vis: 269 nm (" = 25000 L mol À1 cm À1 , n-* ); 367 nm (" = 9330 L mol À1 cm À1 , -* ); 664 nm (" =163 L mol À1 cm À1 , d-d).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All C-bound H atoms were placed in 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 R2CH and R3CH 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 and refined isotropically (H2B, H10, H11, and H12) or using riding model (H2A and H6) with O-H = 0.82 Å . Water H atoms were freely refined. Optimized structure and electrostatic potential map for the title compound.

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.