Synthesis, crystal structure and Hirshfeld surface analysis of [Cu(H2L)2(μ-Cl)CuCl3]·H2O [H2L = 2-hydroxy-N′-(propan-2-ylidene)benzohydrazide]

The synthesis and structural characterization of a dinuclear CuII complex, [trichloridocopper(II)]-μ-chlorido-{bis[2-hydroxy-N′-(propan-2-ylidene)benzohydrazide]copper(II)} monohydrate, [Cu2Cl4(C10H12N2O2)2]·H2O or [Cu(H2L)2(μ-Cl)CuCl3]·H2O is reported

The present study focuses on the synthesis and structural characterization of a novel dinuclear Cu II  The complex crystallizes in the monoclinic space group P2 1 /n with one molecule of water, which forms interactions with the ligands.The first copper ion is penta-coordinated to two benzohydrazine-derived ligands via two nitrogen and two oxygen atoms, and one bridging chloride, which is also coordinated by the second copper ion alongside three terminal chlorines in a distorted tetrahedral geometry.The arrangement around the first copper ion exhibits a distorted geometry intermediate between trigonal bipyramidal and square pyramidal.In the crystal, chains are formed via intermolecular interactions along the a-axis direction, with subsequent layers constructed through hydrogen-bonding interactions parallel to the ac plane, and through slipped �-� stacking interactions parallel to the ab plane, resulting in a three-dimensional network.The intermolecular interactions in the crystal structure were quantified and analysed using Hirshfeld surface analysis.Residual electron density from disordered methanol molecules in the void space could not be reasonably modelled, thus a solvent mask was applied.

Chemical context
Schiff bases are organic compounds that have important applications in many areas of chemistry, including organic synthesis and inorganic chemistry (Sinicropi et al., 2022).Over the years, Schiff bases have gained a lot of popularity as chelating ligands in coordination chemistry with transition metals, due to their versatility and ability to act as multiple linkers and their stability under various oxidizing and reducing conditions (DeepikaVerma et al., 2023).These ligands make excellent coordination molecules and can show variety in structures with metal complexes (Guo et al., 2011), thus leading to a variety of properties (DeepikaVerma et al., 2023).
Hydrazone ligands constitute a distinct category of Schiff bases, arising from the condensation reaction involving hydrazine and either an aldehyde or a ketone in the presence of an acid or a base.The literature has reported that coordination complexes formed between hydrazones and metals can be used in several areas, such as in catalysis for various reactions (Dile et al., 2016), as materials for gas adsorption (Roztocki et al., 2016), for the detection of heavy metals in the environment (Sharma et al., 2019), in electrochemistry (Toledano-Magan ˜a et al., 2015) and in molecular magnetism (Sadhukhan et al., 2018).In addition, these complexes are widely studied in pharmaceutical chemistry, (Haider & Khan, 2022) due to their potential as bioactive compounds, especially as anticancer (S ˇermuks ˇnyte ˙et al., 2022;Gaur et al., 2022), antituberculosis (Mathew et al., 2015;Teneva et al., 2023) and antifungal agents (Kajal et al., 2014) (Yankin et al., 2022), as well as for the design of drugs against Alzheimer's disease (Boulguemh et al., 2020) and Parkinson's disease (Kondeva-Burdina et al., 2022).
As a continuation of our research on the synthesis and the study of the biological and magnetic properties of new Schiff base-type ligands and their complexes (Ouilia et al., 2012;Boussadia et al., 2020;Boulguemh et al., 2020), we report here the synthesis, structural characterization and Hirshfeld surface analysis of a new dinuclear copper(II) complex [Cu(H 2 L) 2 (�-Cl)CuCl 3 ]•H 2 O with a hydrazine ligand (H 2 L = 2-hydroxy-N 0 -(propan-2-ylidene)benzohydrazide).

Structural commentary
The asymmetric unit of the title compound, which comprises a dinuclear Cu II complex and one water solvation molecule, is illustrated in Fig. 1.The first copper ion Cu1 is in pentacoordinated environment with trigonality index parameter � 5 = 0.516.The tau value for pentacoordinated complexes is calculated using the equation elaborated by Addison et al. (1984): � 5 = (� À �)/60, where � and � are the largest basal angles.� 5 equals 1 for an ideal trigonal bipyramid and 0 for a square-pyramidal coordination.The coordination geometry around the Cu1 ion lies between a distorted trigonal bipyramidal and square pyramidal.The copper ion Cu1 is coordinated to the two carbonyl oxygen atoms O1 and O3, and the two imine nitrogens N2 and N4 from two bidentate chelating H 2 L ligands.The fifth coordination site is occupied by a bridging chloride Cl1 with a Cu1-Cl1 distance of 2.5001 (14) A ˚, consistent with literature values (Comba et al., 1988).The Cu1-O bond lengths are Cu1-O1 = 1.971 (3) A ånd Cu1-O3 = 1.959 (3) A ˚, while the Cu1-N2 and Cu1-N4 bond lengths are 1.999 (4) and 2.009 (4) A ˚, respectively.The distorted tetrahedral site around the second copper ion, Cu2, is occupied by three terminal chloride ions, Cl2, Cl3, and Cl4 and a bridging chloride ion Cl1.The terminal Cu-Cl bond distances range from 2.2209 (16) to 2.2601 (12) A ˚, while the Cu-Cl bridging bond is slightly longer, with a Cu2-Cl1 distance of 2.2897 (15) A ˚(Table 1).These distances are comparable to those observed for other tetrachlorometallate (Vasilevesky et al., 1991;Ramos Silva et al., 2005;Comba et al., 1988).The geometry index for tetracoordinated copper ions, � 4 , is calculated as [360 � À (� + �)]/141 � (Yang et al., 2007), inspired by the � 5 index for five-coordinate complexes developed by Addison and Reedijk (Addison et al., 1984).The values of � 4 range from 1 for a perfect tetrahedral geometry to 0 for a perfect square-planar geometry.For the tetracoordinated coordination geometry around the Cu2 ion, � 4 = 0.61, indicating a very distorted tetrahedral geometry (Yang et al., 2007).This distortion has been noted in numerous salts containing [CuCl 4 ] 2À ions, with some displaying thermochromic properties attributed to the deformation of tetrachlorometallate ions in response to temperature changes (Willett et al., 1974).

Figure 1
The title compound showing the atom-labelling scheme with ellipsoids drawn at the 50% probability level and H atoms shown as small spheres of arbitrary radii.The Cu1-Cl1-Cu2 bridging angle of 135.00 (5) � is larger than those observed in the literature for yellow terminal tetrachlorometallate ligands (Ramos Silva et al., 2005).However, the intermetallic Cu� � �Cu distance observed in the title compound [4.426 (8) A ˚] is within the range observed for similar compounds (Comba et al., 1988;Ramos Silva et al., 2005).Some correlations between the magnetic and structural parameters for mono-�-chloro-copper chains have been observed, while the magnetic and structural data suggest a limited number of exchange pathways (van Albada et al., 2004;Alves et al., 2009).However, following these correlations, overall ferromagnetic behaviour can be expected for values of the quotient '/R (where ' is the Cu-Cl-Cu bridge angle and R is the Cu-Cl long bond length) lower than approximately 40 and higher than 57, whereas antiferromagnetic behaviour is observed when this quotient '/R is between these two values.However, the terminal halometallate counter-ion has no impact on the nature of the interaction.In the title compound, the Cu-Cl-Cu bond angle is 135.00 (5) � , with Cu-Cl distances of 2.5001 ( 14) and 2.2897 (15) A ˚, resulting in '/R ratios of 54 and 59, respectively.These values suggest antiferromagnetic behaviour.

Supramolecular features
In the crystal, the supramolecular network consists of an extensive set of intra-and intermolecular hydrogen-bonding interactions (numerical details are given in Table 2).Two intramolecular hydrogen bonds are formed between the imine N1 and N3 atoms and phenolic O2, O4 atoms of the ligand via the respective hydrogen atoms H1 and H3 (Fig. 1).While the carbon donor atoms C10 and C20 of the methyl groups are involved in hydrogen bonds with the acceptor atoms O3 and O1, respectively, of the carbonyl groups via the H10C and H20C atoms (Fig. 1).The solvent water molecule is linked to the complex molecule via the oxygen atom O2 of the phenolic group by a O2-H2� � �O1W hydrogen bond (Fig. 1).
The complex molecules are connected via two hydrogen bonds involving the water molecule, O1W-H1WB� � �Cl3 and O1W-H1WA� � �O3, leading to chains propagating along the a-axis direction (Fig. 2).The two-dimensional arrangement parallel to the ac plane is established by connecting two adjacent chains through two types of patterns.The first arrangement is formed by a succession of R 4 4 (20) and R 4 4 (24) rings, and the second one through a succession of R 4 6 ( 30) rings (Fig. 3).The first two ring structures are formed by two water and two complex molecules, except for the third ring, which is formed by two solvent water and four complex molecules.

Figure 2
Partial view of the crystal structure showing the intermolecular hydrogen bonds (indicated by green dashed lines) forming infinite chains propagating along the a-axis direction.The junction between the resulting two double chains via hydrogen bonds O4-H4� � �Cl2 and C10-H10A� � �Cl4 establishes two-dimensional layers parallel to the ac plane (Fig. 4).Slipped �-� stacking interactions are also observed in this structure, involving the aromatic rings of the ligands with an intercentroid distance Cg1� � �•Cg2( 3 2 À x, À 1 2 + y, 1 2 À z) of 3.683 (3) A ˚(where Cg1 and Cg2 are the centroids of the C2-C7 and C12-C17 rings, respectively), resulting in a threedimensional network by linking chains along the b axis (Fig. 5).

Database survey
A search of the Cambridge Structural Database (CSD version 5.45, updated in November 2023; Groom et al., 2016), revealed that crystal structures have been reported for complexes of several hydrazone derivatives with various metal ions, such as copper (Balsa et al., 2021), zinc (Dasgupta et al., 2020), cadmium (Govindaiah et al., 2021), cobalt (Han et al., 2020), magnesium (Khandar et al., 2019).Only one complex based on copper and benzoic acid, 2-(1-methylethylidene)hydrazide has been reported (Mohamad et al., 2019).No complexes containing two copper ions connected to each other by a chlorine atom and coordinated to two molecules of acetone hydrazone have been documented in the CSD.
To the best of our knowledge, there are only a few examples of asymmetric binuclear copper-based complexes reported in the CSD with some instances where a copper complex is bridged by any type of tetrametallate (Barz et al., 1998;Shi et al., 2014;Alves et al., 2014;Kaur et al., 2019;Singh et al., 2014;Comba et al., 1988;Ramos Silva et al., 2005).

Hirshfeld surface analysis
For further characterization of the intermolecular interactions in the title compound, we carried out a Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) using Crystal Explorer 21 (Spackman et al., 2021) and generated the associated twodimensional fingerprint plots (McKinnon et al., 2007).The HS of the title compound mapped over d norm in the range 0.4396 to +2.3676 a.u. is illustrated in Fig. 6 using colour to indicate contacts that are shorter (red areas), equal to (white areas), or longer than (blue areas) the sum of the van der Waals radii (Ashfaq et al., 2021).The red spots on the surface mapped over d norm (Fig. 6a) indicate the involvement of atoms in hydrogen-bonding interactions.The HS mapped over shapeindex (Fig. 6b) is used to check for the presence of interactions such as C-H� � �� and �-� stacking (Ashfaq et al., 2021).The existence of adjacent red and blue triangular regions around the aromatic rings conforms to the presence of �-� stacking interactions in the title compound (Fig. 6b), and the curvedness plots (Fig. 6c) show flat surface patches characteristic of planar stacking.The two-dimensional fingerprint plots provide unique information about the non-covalent interactions and the crystal packing in terms of the percentage contribution of the interatomic contacts (Spackman & McKinnon, 2002;Ashfaq et al., 2021).Fig. 7 shows the two-dimensional fingerprint plot for the overall interactions with their relative contributions to the Hirshfeld surface.The most important interatomic contact is H� � �Cl as it makes the highest contribution to the crystal packing (35.6%, Fig. 7b).The other major contributor is H� � �H interactions (32.3%, Fig. 7c).Other interactions contributing less to the crystal packing are C� � �H (9.9%, Fig. 7d), O� � �H (6.7%, Fig. 7e), C� � �C (4.7%, Fig. 7f), N� � �H (2.8%, Fig. 7g), C� � �O (1.7%, Fig. 7h), Cl� � �O (1.7%, Fig. 7i), N� � �C (1.5%, Fig. 7j) and O� � �O (0.8%, Fig. 7k).Other contacts make a contribution of 2.3% in total and are not discussed in this work.

Figure 3
Figure 3 Molecular view of the arrangement of chains (a) via R 4 4 (20) and R 4 4 (24) rings and (b) R 6 4 (30) rings along the ac plane.Hydrogen bonds are shown as dashed green lines.

Figure 4
Figure 4Crystal packing of the title compound shown in a projection of the twodimensional network connected through hydrogen bonds (shown as dashed green lines).

Figure 5
Figure 5Crystal packing of the title compound showing the 3D network and the chains parallel to the b axis formed by �-� stacking interactions between the aromatic rings of the ligands (shown in red).

Figure 6 A
Figure 6 A view of the Hirshfeld surface mapped over (a) d norm , (b) shape-index and (c) curvedness.

Table 3
Experimental details.
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