Bis[2-(4,5-diphenyl-1H-imidazol-2-yl)-4-nitrophenolato]copper(II) dihydrate: crystal structure and Hirshfeld surface analysis

A coordination geometry intermediate between square-planar and tetrahedral, defined by an N2O2 donor set, is found in the title CuII complex. Conventional O—H⋯O and N—H⋯O hydrogen bonding leads to a supramolecular layer in the crystal.

The crystal and molecular structures of the title Cu II complex, isolated as a dihydrate, [Cu(C 21 H 14 N 3 O 3 ) 2 ]Á2H 2 O, reveals a highly distorted coordination geometry intermediate between square-planar and tetrahedral defined by an N 2 O 2 donor set derived from two mono-anionic bidentate ligands. Furthermore, each six-membered chelate ring adopts an envelope conformation with the Cu atom being the flap. In the crystal, imidazolyl-amine-N-HÁ Á ÁO(water), water-O-HÁ Á ÁO(coordinated, nitro and water), phenyl-C-HÁ Á ÁO(nitro) and (imidazolyl)-(nitrobenzene) [inter-centroid distances = 3.7452 (14) and 3.6647 (13) Å ] contacts link the components into a supramolecular layer lying parallel to (101). The connections between layers forming a three-dimensional architecture are of the types nitrobenzene-C-HÁ Á ÁO(nitro) and phenyl-C-HÁ Á Á(phenyl). The distorted coordination geometry for the Cu II atom is highlighted in an analysis of the Hirshfeld surface calculated for the metal centre alone. The significance of the intermolecular contacts is also revealed in a study of the calculated Hirshfeld surfaces; the dominant contacts in the crystal are HÁ Á ÁH (41.0%), OÁ Á ÁH/HÁ Á ÁO (27.1%) and CÁ Á ÁH/HÁ Á ÁC (19.6%).

Chemical context
The title copper(II) complex, (I), was isolated during an ongoing research programme on the catalytic activity of copper borate (CuB 4 O 7 ) for C-N heterocyclic bond formation reactions. Complex (I) was formed during the attempted synthesis of a triarylimidazole derivative using benzil and the respective aromatic aldehyde with copper borate, using ammonium acetate as a nitrogen source. The single-crystal analysis of the synthesized product revealed that in the copper(II) complex, the triarylimidazole moiety acts as a bidentate ligand for the copper atom. During the successful synthesis of the triarylimidazole, the desired product formed in good yield at a temperature in the range 100-110 C. However, when the reaction was conducted at 130 C and above, the title copper(II) complex formed instead of the targeted triarylimidazole. The crystal and molecular structures of (I) are described herein, along with a detailed analysis of the molecular packing via an analysis of the calculated Hirshfeld surfaces.

Structural commentary
The crystallographic asymmetric unit of (I) comprises a complex molecule and two water molecules of crystallization. ISSN 2056-9890 The copper(II) centre in (I), Fig. 1, is bis-N,O-chelated by two 2-(4,5-diphenyl-1H-imidazol-2-yl)-4-nitrophenolate monoanions. The resulting N 2 O 2 donor set defines a highly distorted coordination geometry, as seen in the angles included in Table 1 and in the view of Fig. 2. The angles range from a narrow 89.36 (7) , for O1-Cu-O2, to a wide 147.34 (8) , for O1-Cu-N2. The distortion is highlighted in the dihedral angle between the best planes through the two chelate rings of 49.82 (7) . The value of 4 is a geometric measure of the distortion of a four-coordinate geometry (Yang et al., 2007). For (I), the value computes as 0.48 which is almost exactly intermediate between the values of 4 = 0 for an ideal tetrahedron and 4 = 1.0 for an ideal square-planar geometry. In fact, the six-membered chelate rings are not planar, each adopting an envelope conformation with the Cu atom being the flap atom. In this description, the r.m.s. deviation for the least-squares plane through the O1/N1/C1/C2 atoms is 0.036 Å with the Cu atom lying 0.410 (3) Å out of the plane. The comparable parameters for the O2-chelate ring are 0.033 and 0.354 (3) Å , respectively. The dihedral angle formed between the two planar regions of the chelate rings is 49.38 (8) . The dihedral angles between the best plane through the O1chelate ring and each of the fused six-and five-membered rings are 9.18 (12) and 5.54 (14) , respectively; the equivalent angles for the O2-chelate rings are 8.44 (8) and 2.71 (9) , respectively. The N1-imidazol-2-yl ring forms dihedral angles of 41.20 (11) and 37.46 (10) with the C10-and C16-phenyl substituents, respectively, and the dihedral angle between the phenyl rings is 59.92 (8) , i.e. all indicating splayed relationships. A similar situation pertains to the N2-imidazol-2-yl ring, where the comparable dihedral angles formed with the C31and C37-phenyl rings are 38.29 (10), 48.5 (9) and 50.84 (7) , respectively. Finally, the nitro groups are not strictly coplanar with the benzene rings to which they are connected, as seen in the dihedral angles of 14.2 (4) for C1-C6/N4/O3/O4 and 5.9 (3) for C22-C27/N6/O5/O6.

Supramolecular features
As each component of the asymmetric unit has hydrogenbonding functionality, conventional hydrogen bonds are found in the crystal of (I); the geometric parameters characterizing the identified intermolecular interactions operating in the crystal of (I) are collated in Table 2. Each of the imidazolylamine-N-H atoms forms a donor interaction to a water molecule to generate a three-molecule aggregate. The O1W water molecule forms donor interactions to the coordinated O2 atom and to a symmetry-related O2W water molecule. The O2W water molecule connects to the coordinated O1 atom as well as to a nitro-O3 atom. Hence, the O2W water molecule is involved in four hydrogen-bonding interactions. The fourth contact involving the O1W water molecule, a C-HÁ Á ÁO acceptor contact, is provided by the nitrobenzene ring. There is also a phenyl-C-HÁ Á ÁO(nitro) contact of note, Table 2. The aforementioned interactions combine to stabilize a supramolecular layer lying parallel to (101), as shown in Fig. 3(a). There are alsostacking and C-HÁ Á ÁO interactions in the crystal, Fig. 3  Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of the complex molecule in (I), showing the atom-labelling scheme and with displacement ellipsoids drawn at the 70% probability level.

Hirshfeld surface analysis
The Hirshfeld surface calculations for (I) were performed with CrystalExplorer17 (Turner et al., 2017) and published proto-cols (Tan et al., 2019), and serve to indicate the significant role of the two water molecules in the supramolecular association in the crystal. The involvement of both the water molecules in hydrogen bonds, Table 2, are evident as bright-red spots near the respective atoms on the Hirshfeld surfaces mapped over d norm for the O1W-, Fig. 4(a), and O2W-water, Fig. 4(b), molecules. In addition, the presence of faint-red spots near the O1W, O2W and H1W atoms in Figs. 4(a) and 4(b) are indicative of the other contacts of these atoms with those of the Cu II complex molecule ( Table 2). The donors and acceptors of the hydrogen bonds involving atoms of the complex molecule are also apparent as bright-red spots near the participating atoms in the views of the Hirshfeld surfaces calculated for the complex molecule shown in Figs. 4(c)-(e). The presence of a short interatomic CÁ Á ÁC contact between atoms C22 and C28 (Table 3) arises fromstacking between symmetry-related imidazole and nitrobenzene rings, and is observable as the faint-red spots near these atoms on the d norm -mapped Hirshfeld surface in Fig. 4(c). The pair of faint-red spots appearing near the phenyl-C36 and H36 atoms, and also near the nitro-O5 atom on the surface indicating short interatomic contacts that characterize the weak C-HÁ Á ÁO interaction, Table 3. The influence of the C-HÁ Á Á contact on the molecular packing is recognized from the three faint-red spots in the phenyl-(C16-C21) ring and another near atom H34 in Fig. 4(e). The donors and acceptors of this interaction are also evident as the blue bump and a brightorange spot enclosed within the black circle on the Hirshfeld surface mapped with the shape-index property in Fig. 5(a). The bright-orange region enclosed within a black circle in A view of the molecular structure of the complex molecule in (I), highlighting the distorted coordination geometry about the copper(II) atom.

Figure 3
The molecular packing in the crystal of (I): (a) a supramolecular layer parallel to (101) sustained by O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO interactions shown as orange, blue and green dashed lines, respectively, and (b) a view of the unit-cell contents in projection down the c axis, withand C-HÁ Á Á interactions shown as purple and pink dashed lines, respectively. intermolecular interactions through blue and red regions corresponding to positive and negative electrostatic potentials, respectively. Thestacking between symmetry-related imidazolyl and nitrobenzene rings are viewed as the flat regions enclosing them on the Hirshfeld surfaces mapped over curvedness in Fig. 7. On the Hirshfeld surfaces mapped over d norm illustrated in Figs. 4(c)-(e), faint-red spots also appear near other atoms indicating their involvement in other short interatomic contacts, as summarized in Table 3.
The Hirshfeld surfaces also provide an insight into the distortion in the coordination geometry formed by the N 2 O 4 donor set about the copper(II) centre in the complex molecule. This is performed by considering the Hirshfeld surface about the metal centre alone (Pinto et al., 2019). The distortion in the coordination geometry is observed on the Hirshfeld surface mapped with the shape-index property as the brightorange patches of irregular shape covering a major region for the Cu-O bonds in Fig. 8(a) and the small orange regions on the surface relatively far from the Cu-N bonds in Fig. 8 The different curvature of the Hirshfeld surfaces coordinated by the N 2 O 4 donor set in Figs. 8(c) and 8(d) also support this observation. The Cu-O and Cu-N bonds are rationalized in the two-dimensional fingerprint plot taking into account only the Hirshfeld surface for the copper atom shown in Fig. 9. The distribution of points in the fingerprint plot through the pair of aligned red points at different inclinations from d e + d i $ 2.0 Å for the Cu-N bonds (upper region) and the Cu-O bonds (lower region) are indicative of the distorted geometry (Pinto et al., 2019).
The overall two-dimensional fingerprint plot for (I), i.e. the entire asymmetric unit, Fig. 10(a), and those delineated into HÁ Á ÁH, OÁ Á ÁH/HÁ Á ÁO, CÁ Á ÁH/HÁ Á ÁC, CÁ Á ÁC and CÁ Á ÁO/OÁ Á ÁC contacts are illustrated in Figs. 10(b)-(f), respectively. The percentage contribution from different interatomic contacts to the Hirshfeld surfaces of the complex molecule and for overall (I) are summarized in Table 4. The presence of water molecules in the crystal of (I) increases the percentage contribution from OÁ Á ÁH/HÁ Á ÁO contacts (Table 4) to the Hirshfeld surface of the asymmetric unit compared with the complex molecule alone. This results in slight decreases in the percentage contributions from other interatomic contacts for (I) ( Table 4). A single conical tip at d e + d i $ 1.9 Å in the fingerprint plot delineated into HÁ Á ÁH contacts shown in Fig. 10(b) is the result of the involvement of the H12 atom in a short interatomic HÁ Á ÁH contact, Table 3. The points due to short interatomic contacts between amine hydrogen-H3N and water hydrogen atoms, H1W and H2W, Table 3 (Table 4) to the Hirshfeld surface of (I) reflect the combined influence of intermolecular C-HÁ Á Á interactions (Table 2) and the short interatomic CÁ Á ÁH/HÁ Á ÁC contacts, summarized in Table 3, and viewed as the distribution of points in the form of characteristic wings in Fig. 10(d). The presence of short interatomic CÁ Á ÁC contacts are evident as the points near a rocket shape tip at d e + d i $ 3.2 Å in the respective delineated fingerprint plot, Fig. 10(e), while the points correspondingstacking between the imidazole and nitrobenzene rings are distributed about d e = d i = 1.7 Å in the plot. The small, i.e. 2.7%, contribution from CÁ Á ÁN/NÁ Á ÁC contacts to the surface is also due to thesestacking interactions (delineated plot not shown). The contribution of 3.2% from CÁ Á ÁO/OÁ Á ÁC contacts is due to the presence of short interatomic contacts involving nitro-O atoms, Table 2, and are apparent as the pair of parabolic tips at d e + d i $ 3.2 Å in the delineated plot of Fig. 10 Two views of the Hirshfeld surface mapped with the shape-index property for the complex molecule in (I) from À1.0 to +1.0 arbitrary units highlighting (a) the donor and acceptor atoms of the C-HÁ Á Á interaction through a blue bump near the H34 atom and bright-orange curvature, enclosed within the black circle, and (b) the O2W-H4WÁ Á Á interaction by the bright-orange region enclosed within the black circle.  contribution from other interatomic contacts to the surface summarized in Table 4 have negligible influence on the molecular packing.   The two-dimensional fingerprint plot taking into account only the Hirshfeld surface calculated about the copper(II) atom.

Synthesis and crystallization
In a typical procedure, benzil (0.3 g, 1 mmol), ammonium acetate (0.19 g, 2.5 mmol), 2-hydroxy-5-nitrobenzalaldehyde (0.167 g, 1 mmol) and copper(II) borate (0.218 mg, 1 mmol) were ground in an agate mortar with a pestle. To this mixture, about 1.5 g of dried silica gel (column chromatography, 60-120 mesh) was added and the reaction mixture was ground again for 30 min. The whole reaction mixture was then transferred to a 100 ml round-bottomed flask and heated at 130 C with constant stirring for 4 h. The reaction mixture was then extracted with dry acetone and dried over MgSO 4 . After a few days, a dark-brown solid was obtained. The product was recrystallized from dry dimethylformamide and, after 5 d, light-blue crystals of (I) were obtained (yield 60%; m.p. > 300 C).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5    DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis[2-(4,5-diphenyl-1H-imidazol-2-yl)-4-nitrophenolato]copper(II) dihydrate
Crystal data [Cu(C 21  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.