Crystal structure of a mononuclear copper(II) complex with 2-methoxy-N,N-bis(quinolin-2-ylmethyl)ethylamine (DQMEA)

The crystal structure of [Cu(DQMEA)(CH3CN)](ClO4)2 (DQMEA = N,N-bis(2-quinolylmethyl)methoxyethylamine) has been determined. The structure reveals a five-coordinate CuII center with a distorted square-pyramidal geometry.

ethylamine]copper(II) diperchlorate} by single-crystal X-ray diffraction reveals a complex cation with a tetradentate coordination of the DQMEA ligand along with monodentate coordination of a CH 3 CN ligand to a single Cu II center, with two perchlorate anions providing charge balance. The Cu II center has a distorted square-pyramidal geometry in which the nitrogen atoms of the DQMEA and CH 3 CN ligands occupy the equatorial positions, while the oxygen atom of the DQMEA ligand resides in the axial position with an elongated Cu-O bond. The quinoline ring systems are nearly co-planar in the structure, while the linear CH 3 CN ligand is tilted significantly below this plane, and the central nitrogen of DQMEA is above it. Within the complex, weak C-HÁ Á ÁN hydrogen bonding takes place between the nitrogen of CH 3 CN and a neighboring quinolyl group. The perchlorate ions are disordered within the structure, but undergo a number of weak intermolecular C-HÁ Á ÁO hydrogen-bonding interactions. Additional weak -stacking interactions between the quinolyl groups of neighboring complexes further stabilize the crystal packing.

Chemical context
Copper proteins are numerous in living systems, owing largely to their ability to bind and process dioxygen (Karlin & Tyeklá r, 1993;Karlin, 1993;Kopf & Karlin, 1999). Much of what is known about these proteins comes from modeling studies that involve the synthesis of low molecular weight copper complexes with organic-based ligands (Mirica et al., 2004;Lewis & Tolman, 2004;Hatcher & Karlin, 2004;Peterson et al., 2013). Many of these involve N-centered, tripodal, tetradentate ligands containing pyridine or quinoline moieties (Wei et al., 1994;Young et al., 1995;Kim et al., 2015). These ligands give stable complexes that provide access to both the Cu I and Cu II oxidation states, and leave open or solventbound coordination sites for the binding of dioxygen species (Wei et al., 1994).
More recently, copper(II) complexes have been targeted as potential anticancer agents (Santini et al., 2014). Indeed, copper(II) has been shown to promote tumor cell death through a variety of mechanisms while remaining less toxic systematically than platinum-based drugs (Angel et al., 2017). A number of the compounds that have been studied employ pyridyl, quinolyl, and other aromatic amine-containing ligands ISSN 2056-9890 because of their ability to form stable complexes with copper(II) ions that display promising anticancer activity (Angel et al., 2017;Santini et al., 2014). Given the rich variety of ligands of this type, copper(II) complexes with a range of coordination numbers, geometries, redox potentials, biological compatibility, and cytotoxicity are possible.
Based on their relevance to biology, we have begun to explore copper(II) complexes with novel N-tripodal ligands containing either pyridine or quinoline moieties. We report here the synthesis and structural characterization of [Cu(DQMEA)(CH 3 CN)](ClO 4 ) 2 ] [DQMEA = 2-methoxy-N,N-bis(quinolin-2-ylmethyl)ethylamine]. This compound is formed by the reaction of copper(II) perchlorate with DQMEA in acetonitrile, followed by the addition of diethyl ether (see reaction scheme) to afford dark-blue crystals suitable for X-ray diffraction studies.

Structural commentary
The title compound ( Fig. 1) crystallizes in the monoclinic P2 1 /n space group. The structure reveals a monomeric cation of [Cu(DQMEA)(CH 3 CN)] 2+ with two disordered perchlorate counter-anions. The copper(II) center is pentacoordinate with a distorted square-pyramidal geometry as indicated by the trigonality index, = 0.03 defined as = | À '|/60, where and ' are the two largest angles in the coordination sphere (Addison et al., 1984). These angles are 164.97 (8) and 163.04 (9) ( Table 1). According to this index, values of fivecoordinate complexes range from 0 for perfectly squareplanar to 1 for perfectly trigonal-bipyramidal geometries. The DQMEA ligand is tetradentate with its central (N1) nitrogen and two quinolyl nitrogen atoms (N2 and N3) lying in the equatorial plane, and the methoxy oxygen atom (O1) taking up the axial position. The fourth position in the equatorial plane is occupied by the nitrogen atom (N4) of a coordinated acetonitrile molecule. The two quinoline ring systems of DQMEA are nearly co-planar with each other [dihedral angle = 14.58 (7) ], which results in a steric interaction between hydrogen atoms H11 and H21 and the coordinated acetonitrile molecule. This causes the linear acetonitrile molecule to drop below the quinolyl plane, such that the bond angle that its nitrogen atom makes with the copper ion and the axial oxygen of DQMEA, O1-Cu1-N4, is 115.14 (8) . The bite angles imposed by the tetradentate chelation of the DQMEA ligand cause further constraints leading to some distortion of the structure. For example, the central nitrogen and methoxy oxygen atoms, spanning the equatorial and axial positions, form a five-membered metallocycle with an N1-Cu1-O1 bond angle of 81.40 (8) . This moves N1 slightly above the quinolyl plane, and causes the non-linearity of N2-Cu1-N3 [164.97 (8) ]. The equatorial bond angles N1-Cu1-N2 [84.06 (8) ] and N1-Cu1- N3 [80.92 (8) ] are also significantly reduced from 90 because of the constraints of the DQMEA coordination. The equatorial Cu-N bond lengths fall in the narrow range of 1.968 (2) to 2.0311 (19) Å , consistent with values reported previously (Wei et al., 1994), while the axial Cu-O bond is significantly longer at 2.3570 (19) Å . The latter is consistent with a weak axial interaction due to Jahn-Teller distortion as noted previously for square-pyramidal copper(II) complexes (Chavez et al., 1996;Warda, 1998;Rowland et al., 2002;Roy et al., 2011). Finally, a weak intramolecular C-HÁ Á ÁN hydrogen-bonding interaction takes place between a quinolyl hydrogen (H11) and the nitrogen atom of the acetonitrile ligand (N4), which may help stabilize the coordination of this monodentate ligand.

Supramolecular features
Within the crystal, a network of weak C-HÁ Á ÁO hydrogenbonding interactions (Table 2) takes place between the hydrogen atoms of the DQMEA ligand and the oxygen atoms of the perchlorate anions (Fig. 2). In addition, weakstacking interactions between nearby pyridine rings (Cg5Á Á ÁCg5) of a quinoline group and between the pyridine and phenyl rings (Cg5Á Á ÁCg7) of other nearby quinoline groups (where Cg5 and Cg7 are the centroids of the N3/C14-C17/C22 and C17-C22 rings, respectively) serve to further stabilize the crystal packing.  Table 1 Selected geometric parameters (Å , ).

Figure 1
Structure of the title compound, [Cu(DQMEA)(CH 3 CN)](ClO 4 ) 2 , with atom labels, shown with displacement ellipsoids drawn at the 30% probability level. Both perclorate anions are disordered, with oxygen occupancy ratios of 0.900 (10) (Table 2) are also present and contibute additionally to the crystal packing.

Database survey
To the best of our knowledge, a structure of the title compound has not been published previously. However, analogous structures of copper(II) complexes with tripodal ligands formed by tethering two quinolyl groups to either a chiral amino alcohol or amino acid have been reported (Holmes et al., 2005;Zahn et al., 2006). Within these chiral structures, the quinolyl groups are not coplanar, but are instead twisted relative to each other in a propeller-like fashion.

Synthesis and crystallization
All chemicals were obtained from commercial sources and used without further preparation. Deionized water was used throughout. The 1 H NMR spectrum was recorded with a JEOL ECX-300 NMR spectrometer and referenced against the 1 H peak of the chloroform solvent. IR spectra were recorded with a Perkin Elmer Spectrum 100 FT-IR.
[Cu(DQMEA)(CH 3 CN)](ClO 4 ) 2 ]. In a 50 mL roundbottom flask, 0.100 g (0.28 mmol) of copper(II) perchlorate hexahydrate and 0.104 g (0.28 mmol) of DQMEA were dissolved in 10 mL of acetonitrile. The reaction mixture was capped and allowed to stir for 30 minutes. Approximately 10 mL of anhydrous diethyl ether was added until crystals began to form on the side of the flask, and the mixture was capped and placed in a refrigerator. After seven days, 0.15 g (84%) of dark-blue crystals suitable for X-ray diffraction were Table 2 Hydrogen-bond geometry andstacking interactions (Å , ).

Funding information
Funding for this research was provided by: NSF-MRI (grant No. CHE-1039027 to Jerry P. Jasinski).

(Acetonitrile)[2-methoxy-N,N-bis(quinolin-2-ylmethyl)ethylamine]copper(II) bis(perchlorate)
Crystal data 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.