Crystal structure of [2-({[2-(dimethylamino-κN)ethyl]imino-κN}methyl)phenolato-κO](1,10-phenanthroline-κ2 N,N′)copper(II) perchlorate

The asymmetric unit of the [Cu(L)(phen)](ClO4) complex {L= 2-[(2-dimethylaminoethylimino)methyl]phenol} and phen = 1,10-phenanthroline) contains two crystallographically independent molecules. It consists of alternating layers of he two types of molecules, which stack along the c axis.


Chemical context
The design and synthesis of mixed ligand copper(II) complexes have received much attention as they exhibit promising anticancer and nuclease activities compared to simple 1:1 complexes. Palaniandavar and co-workers (Sharma et al., 2020;Rajendiran et al., 2007;Selvakumar et al., 2006) and Chakravarty and co-workers (Goswami et al., 2012) have reported the X-ray crystal structures of several mixed ligand copper(II) complexes that have biological activity. Recently, our group has reported a series of mixed ligand copper(II) complexes and their biological applications (Karpagam et al., 2019(Karpagam et al., , 2022Radhakrishnan et al., 2021). Palaniandavar and coworkers (Jaividhya et al., 2012) prepared the title complex I and investigated its DNA binding, cleavage, and anticancer activity. It exhibits good cytotoxicity against MCF7 breast cancer cells with an IC 50 value of 1.20 AE 0.10 mM and against the ME180 human cervical epidermoid carcinoma cells with an IC 50 value of 24.6 AE 0.10 mM at 48 h incubation (Jaividhya et al., 2012). However, the crystal structure of complex I was not reported. In this work we report the crystal structure of this mixed ligand copper(II) complex.

Structural commentary
The title compound I is of the type {[Cu(L)(phen)](ClO 4 )} {where L is the deprotonated form of 2-[(2-dimethylaminoethylimino)methyl]phenol and phen is 1,10-phenanthroline} is a mononuclear mixed ligand copper(II) complex. The metal atom is coordinated to the tridentate Schiff base ligand (HL) through two N and one O atoms and to two N atoms of the 1,10-phenanthroline ligand, resulting in a five-coordinate complex.
Complex I (Fig. 1) crystallizes in the orthorhombic crystal system in the Pbca space group. The asymmetric unit contains two crystallographically independent complex cations (a and b) with a slightly different geometry around the Cu II ion. Selected geometrical parameters are listed in Table 1. The value of the trigonality index suggests that both cations, a and b, display a square-pyramidal distorted trigonal-bipyramidal (SPDTBP) geometry, with cation a being more distorted than cation b.
In cation a, the Cu1 atom is coordinated by the two nitrogen atoms (N1, N2) and the phenolate oxygen atom (O1) of the Schiff base primary ligand, and to two nitrogen (N3, N4) atoms of the phen co-ligand. The value of the trigonality index = 0.53 [ = ( À )/60, where = N1-Cu1-N3 = 175.79 (13) and = N2-Cu1-O1 = 143.82 (12) ; is 0 for a square-pyramidal geometry and 1 for trigonal-bipyramidal] reveals that the coordination environment around Cu1 is best described as having a square-pyramidal distorted trigonalbipyramidal (SPDTBP) geometry (Addison et al., 1984;Selvakumar et al., 2006). The amine nitrogen atoms (N1, N2) and the phenolate oxygen atom (O1) of the meridionally coordinated Schiff base ligand and one of the imine nitrogen atoms of phen (N3) occupy the corners of the (Cu1)N 3 O basal plane of this geometry. The other nitrogen (N4) of the phen ligand occupies the axial position at a distance of 2.251 (3) Å , longer than the equatorial distances [Cu1-O1 = 1.915 (3) Å , Cu1-N1 = 1.923 (3) Å , Cu1-N2 = 2.148 (3) Å , Cu1-N3 = 2.019 (3) Å ], which is due to the presence of two electrons in the d z 2 orbital of copper(II). The Cu1-N2 amine bond is longer than the Cu1-N1 imine bond formed by the Schiff base ligand, which is expected of sp 3 and sp 2 hybridizations of the amine (N2) and imine (N1) nitrogen atoms, respectively. The Cu1-N imine bond distance is shorter than that of trans Cu1-N phen ; this may be attributed to the fact that the azomethine nitrogen is a stronger base compared with the pyridyl nitrogen. The bond angles deviate from the ideal trigonal-bipyramidal angles of 90 and 120 , respectively, revealing the presence of significant distortion in the Cu1 coordination geometry.
In cation b, the Cu2 ion is coordinated by the two nitrogen atoms (N5, N6), the phenolate oxygen atom (O2) of the Schiff base primary ligand, and by the two nitrogen (N7, N8) atoms of the phen co-ligand. As for a, cation b also exhibits squarepyramidal distorted trigonal-bipyramidal (SPDTBP) geometry (Murphy, Nagle et al., 1997;Nagle et al., 1990;Rajarajeswari et al., 2014;Jaividhya et al., 2012;Radhakrishnan et al., 2021), but the value of the trigonality index is slightly smaller at 0.40 [ = ( À )/60, where = N5-Cu2-N7= 176.38 (14) and = O2-Cu2-N6 = 152.71 (12) ], indicating that it is less distorted than cation a. Similar to cation a, the amine nitrogen atoms (N5, N6) and the phenolate oxygen atom (O2) of the meridionally coordinated Schiff base ligand and one of the imine nitrogen atoms of phen occupy the corners of the (Cu2)N 3 O basal plane of this geometry. The other nitrogen (N8) of the phen ligand occupies the axial position at a distance of 2.238 (3) Å , again longer than the bonds to the equatorial donor atoms [Cu2-O2 = 1.913 (3) Å , Cu2-N5 = 1.919 (3) Å , Cu2-N6 = 2.121 (3) Å , Cu2-N7 = 2.030 (3) Å ) but shorter than the axial bond Cu1-N4 of cation a. As a result of a slight axial compression of the axial phen nitrogen in cation b, a slight increase of the equatorial phen nitrogen bond length (Cu2-N7) is observed. On the other hand, the other equatorial bonds in b are shorter than in cation a. Similar to cation a, the Cu2-N6 amine bond is longer than the Cu2-N5 imine bond formed by the Schiff base ligand, as expected for sp 3 and sp 2 hybridizations of the amine  Figure 1 Molecular structures of the crystallographically independent complex cations and the two perchlorate counter-ions with ellipsoids drawn at the 50% probability level; hydrogen atoms have been omitted for clarity.
(N6) and imine (N5) nitrogen atoms, respectively. The Cu2-N imine bond distance is shorter than the trans Cu2-N phen bond; this is also attributed to stronger basicity of the azomethine nitrogen compared to the pyridyl nitrogen. The deviations in the values of the bond angles with respect to the ideal square-pyramidal angles of 90 and 180 , respectively, again reveal a significant distortion in the Cu2 coordination geometry.
The layered packing arrangement onto the ab plane. Complex cations a (blue) and b (green) are shown on the left side of the figure. The two perchlorate ions are coloured in yellow and red. The relative arrangement of the two layers is shown on the right side of the image.  (Jaividhya et al., 2012) where bpy is 2,2 0 -bipyridine, dpq is dipyrido[3,2f:2 0 ,3 0 -h]quinoxaline and dmdppz = 11,12-dimethyldipyrido[3,2-a:2 0 ,3 0 -c]phenazine. Similar to the title compound, in these complexes the N,N,O-tridentate Schiff base ligand is coordinated meridionally to the Cu II ion and one of the diimine nitrogen atoms is coordinated in an axial position. The value of the trigonality index of the bpy complex ( = 0.13) is less than for the dpq ( = 0.37) and dmdppz ( = 0.39) complexes, as well as the title complex with phen (a, = 0.53; b, = 0.40), which exhibits the largest distortion. In addition to these diimine complexes, there are a few reports on fivecoordinate mixed ligand copper(II) complexes bearing L and an N,N-donor ligand such as benzimidazole and an O,O-donor ligand such as salicylaldehyde (Sathya & Murali, 2018). The N,N,O-tridentate Schiff base ligand is coordinated to the Cu II ion in a meridional fashion and the pyridine nitrogen of the benzimidazole ligand occupies the axial position, whereas in the salicylaldehyde complex, the carbonyl oxygen occupies the axial position. The former complex is distorted from a squarepyramidal geometry and shows a trigonality index of 0.25 but the latter complex exhibits only a slight distortion from an ideal square-pyramidal geometry. Similarly, Tadokaro et al. (1995) reported the molecular structure of a mixed ligand complex with L and bidentate mono-deprotonated 2,2 0biimidazolate (N,N-donor) ligands and discussed the existence of a capped-type dimeric hydrogen bond between the molecules. In another case, the authors attempted to synthesize an octahedral bis(N-b-dimethylaminoethylsalicyladiminato)copper(II) complex (Chieh & Palenik, 1972). They expected both the tridentate N,N,O-Schiff base ligands to coordinate to the Cu II ion and form an octahedral coordination geometry. However, the crystal structure revealed that the Cu II ion is pentacoordinate with one of the dimethylamino groups of the ligand not bonded to it. The resulting complex is highly distorted but appears closer to a trigonal-bipyramidal geometry rather than square pyramidal.

Synthesis and crystallization
The Schiff base-type ligand 2-[(2-dimethylaminoethylimino)methyl]phenol (HL) was prepared using the synthetic procedure reported by Jaividhya et al. (2012). Complex I was prepared by addition of a methanolic solution (10 mL) of 1,10phenanthroline (0.1802 g, 1 mmol) and HL (0.1949 g, 1 mmol) pretreated with triethylamine (139 mL, 1 mmol) to remove the phenolic hydrogen, to a solution of copper(II) perchlorate hexahydrate (0.37 g, 1 mmol) in methanol (15 mL) and then stirring at 313 K for 2 h. The green solid obtained was collected by suction filtration, washed with diethyl ether, and then dried under vacuum. A crystal suitable for X-ray diffraction analysis was obtained by dissolving the complex in methanol and allowing it to crystallize.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms were placed in idealized positions and constrained to ride on their parent atoms, with d(C-H) = 0.93 Å , U iso (H) = 1.2U eq (C) for aromatic, 0.97 Å , U iso (H) = 1.2U eq (C) for CH 2 and 0.96 Å , U iso (H) = 1.5U eq (C) for CH 3 atoms. The hydrogens bound to carbon were refined using standard riding models. The perchlorate ions are disordered. The first, Cl1/O3-O6, was successfully refined with two disorder components which refined to a ratio of 0.611 (15)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.94 e Å −3 Δρ min = −0.42 e Å −3 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. Refinement. Data collection: A crystal of complex I was mounted on a glass fiber. Data were collected on an Oxford Diffraction Xcalibur EOS Gemini Diffractometer at ambient temperature using graphite-monochromated Mo Kα radiation (λ = 0.7107 Å). The structure was solved with SHELXT (Sheldrick, 2015a) and refined with SHELXL (Sheldrick, 2015b). The graphic interface package PLATON (Spek, 2020), ORTEP (Burnett & Johnson, 1996) and Mercury (Macrae et al., 2020) were used for analysis and generation of images. Non-hydrogen atoms were refined anisotropically.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (