Crystal structure and characterization of a new copper(II) chloride dimer with methyl(pyridin-2-ylmethylidene)amine

The new copper(II) complex [CuLCl2]2, where L is a product of Schiff base condensation between methylamine and 2-pyridinecarbaldehyde, is built of discrete centrosymmetric dimers.

The new copper(II) complex, namely, di--chlorido-bis{chlorido[methyl-(pyridin-2-ylmethylidene)amine-2 N,N 0 ]copper(II)}, [Cu 2 Cl 4 (C 7 H 8 N 2 ) 2 ], (I), with the ligand 2-pyridylmethyl-N-methylimine (L, a product of Schiff base condensation between methylamine and 2-pyridinecarbaldehyde) is built of discrete centrosymmetric dimers. The coordination about the Cu II ion can be described as distorted square pyramidal. The base of the pyramid consists of two nitrogen atoms from the bidentate chelate L [Cu-N = 2.0241 (9), 2.0374 (8) Å ] and two chlorine atoms [Cu-Cl = 2.2500 (3), 2.2835 (3) Å ]. The apical position is occupied by another Cl atom with the apical bond being significantly elongated at 2.6112 (3) Å . The trans angles of the base are 155.16 (3) and 173.79 (2) . The CuÁ Á ÁCu separation in the dimer is 3.4346 (3) Å . In the crystal structure, the loosely packed dimers are arranged in stacks propagating along the a axis. The X-band polycrystalline 77 K EPR spectrum of (I) demonstrates a typical axial pattern characteristic of mononuclear Cu II complexes. Compound (I) is redox active and shows a cyclic voltammetric response with E 1/2 = À0.037 V versus silver-silver chloride electrode (SSCE) assignable to the reduction peak of Cu II /Cu I in methanol as solvent.

Chemical context
The crystal structure of the title compound was determined as part of our ongoing research focused on the design and synthesis of the organic-inorganic halometallates with substituted imidazo[1,5-a]pyridinium cations. The first cation in the series, 2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridinium, was obtained by the replacement of a conventional aqueous solution of methylamine with its solid hydrochloride salt in the reaction with 2-pyridinecarbaldehyde (2-PCA) in methanol (Buvaylo et al., 2015). The cation is a result of the acid-catalysed oxidative condensation-cyclization between two molecules of 2-PCA and one molecule of CH 3 NH 2 with the acid added as an adduct of the amine. The prepared in situ organic cation forms a halometallate salt in the subsequent interaction with divalent metal halides (M = Mn, Co, Zn, Cd) or can be isolated in salt form with Cl À /NO 3 À anions (Buvaylo et al., 2015;Vassilyeva et al., 2019aVassilyeva et al., ,b, 2020. The burgeoning research in the field of organic-inorganic halometallate-based hybrids in search of new applications (Wheaton et al., 2018;Yangui et al., 2019;Szklarz et al., 2020) prompted us to extend the developed reaction to the possible ISSN 2056-9890 preparation of a mixed-metal hybrid halometallate by combining two different metals with the organic precursors: Cu -NiCl 2 Á6H 2 O -CH 3 NH 2 ÁHCl -2-PCA -CH 3 OH in air Adhering to the direct synthesis approach (Kokozay et al., 2018), one of the metals was introduced in a zerovalent state. Our earlier studies showed that a metal powder was oxidized in solution to form a coordination compound in the presence of a proton-donating agent and dioxygen from the air, this being reduced to give H 2 O.
Such a complication of the reaction system had an adverse effect, precluding formation of the desired heterocycle with the imidazo[1,5-a]pyridinium skeleton but afforded the Schiff base 2-pyridylmethyl-N-methylimine (L) instead. The latter is a pale-yellow liquid usually accessible by a straightforward interaction of 2-PCA with a 40% aqueous solution of methylamine (Schulz et al., 2009). In the present work, the imine L was isolated as the copper(II) complex [CuLCl 2 ] 2 , (I), the dimeric structure of which has been established by X-ray crystallography. The title compound was characterized by elemental analysis, IR and EPR spectroscopy as well as cyclic voltammetry.

Structural commentary
The title complex crystallizes in the triclinic space group, P1; the dimeric molecule is situated on a crystallographic inversion centre. The coordination about the Cu atom can be described as distorted square pyramidal. The angular structural index parameter, = ( À )/60, evaluated from the two largest angles ( < ) in the five-coordinated geometry, which has ideal values of 1 for an equilateral bipyramid and 0 for a square pyramid, is equal to 0.31 (Table 1). The base of the pyramid consists of the two nitrogen atoms, N1, N22 from the bidentate chelate ligand L and the two chlorine atoms, Cl2 and the centrosymmetrically related Cl1 of the dimer (Fig. 1). Bond parameters are unexceptional ( Table 1). The apical position is occupied by the Cl1 atom with the apical bond being significantly elongated at 2.6112 (3) Å compared to the Cu1-Cl1 i bond length of 2.2835 (3) Å [symmetry code: (i) 1 À x, 1 À y, 1 À z]. The trans angles of the base are N22-Cu1-Cl2 = 155.16 (3) and N1-Cu1-Cl1 i = 173.79 (2) . The cis angles at the copper atom vary from 80.20 (3) to 108.803 (10) . The CuÁ Á ÁCu i separation in the dimer is 3.4346 (3) Å .

Figure 2
Fragment of crystal packing of [CuLCl 2 ] 2 (I) viewed along the c-axis direction.

Figure 1
Molecular structure and principal labelling of [CuLCl 2 ] 2 (I) with ellipsoids at the 50% probability level. coplanar, with the ring centroid distance being equal to the aaxis length [7.7054 (5) Å ], which is too great for -overlap.

IR and EPR spectroscopy measurements
A broad band centred at about 3440 cm À1 in the IR spectrum of (I) could be due to adsorbed water molecules (see supporting information). Several bands arising above and below 3000 cm À1 are assigned to aromatic CH and alkyl -CH stretching, respectively. The characteristic (C N) absorption of the Schiff base, which appears at 1652 cm À1 as a sharp and rather intense band in the IR spectrum of L (Schulz et al., 2009), is detected at 1648 cm À1 in the spectrum of (I). A number of sharp and intense absorptions are observed in the aromatic ring stretching (1600-1400 cm À1 ) and C-H out-ofplane bending regions (800-700 cm À1 ). The X-band polycrystalline EPR spectra of (I) (Fig. 3) show a typical axial pattern characteristic for the mononuclear Cu II complexes with no visible hyperfine structure. The spectra are almost temperature independent with a subtle change of their shapes seen between 295 and 77 K. The axial symmetry characteristics of (I), g || = 2.26 and g ? = 2.06, with a g || > g ? > 2.02 relation confirm a square-pyramidal coordination geometry for the metal centre suggested by the structural data. The additional low intensity lines at g eff = 2.18, 2.16 and 2.12 may indicate exchange interactions between copper(II) ions in the dimer that are probably very weak.

Cyclic voltammetry
Compound (I) is redox active and shows a cyclic voltammetric response in the potential range of À0.12 -0.047 V (E 1/2 = À0.037 V vs SSCE), which is assignable to the reduction peak of Cu II /Cu I (Fig. 4). The complex exhibits quasi-reversible behaviour as indicated by the non-equivalent current intensity of cathodic and anodic peaks (i c /i a = 0.422) and a large separation between them (167 mV) (Crutchley et al., 1990). Since Cu I prefers to be four-coordinate, the irreversibility of the Cu II /Cu I couple may be due to the dissociation of the dimers in solution. Reduction of copper(I) to copper(0) is associated with the irreversible peak II at À0.36 V vs SSCE. The latter process causes removal of the metal centre from the complex molecule. The resulting free ligand undergoes reduction at about À0.8 V, which is superimposed with the reduction peak of the solvent, as is evident from the comparison between the cyclic voltammograms of (I) and supporting electrolyte methanol solutions.

Synthesis and crystallization
2-PCA (0.19 ml, 2 mmol) was magnetically stirred with CH 3 NH 2 ÁHCl (0.27 g, 4 mmol) in 20 ml methanol in a 50 ml Erlenmeyer flask at room temperature (r.t.) for an hour. Dry NiCl 2 Á6H 2 O (0.23 g, 1 mmol) and Cu powder (0.06 g, 1.0 mmol) were added to the resulting yellow solution of the preformed Schiff base. The mixture immediately turned green and was magnetically stirred at 323 K in open air to achieve dissolution of the metallic copper (4 h). The resulting solution was filtered and left to evaporate at r.t. Green plate-like crystals of (I) suitable for X-ray analysis deposited over two days. They were filtered off, washed with diethyl ether and  Cyclic voltammogram of [CuLCl 2 ] 2 (I), 0.1 mM in methanol mixed with 0.1 M acetate buffer (pH 4) and NaClO 4 (70:28:2) as supporting electrolyte at a glassy carbon electrode and Ag/AgCl as reference electrode (scan rate: 100 mV s À1 ; T = 298 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Anisotropic displacement parameters were employed for the non-hydrogen atoms. All hydrogen atoms were added at calculated positions and refined by use of a riding model with isotropic displacement parameters based on those of the parent atom (C-H = 0.95 Å , U iso (H) = 1.2U eq C for CH, C-H = 0.98 Å , U iso (H) = 1.5U eq C for CH 3 ).

Funding information
Funding for this research was provided by: Ministry of Education and Science of .

Computing details
Data collection: CrysAlis PRO (Rigaku OD, 2016); cell refinement: CrysAlis PRO (Rigaku OD, 2016); data reduction: CrysAlis PRO (Rigaku OD, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 1999), Mercury (Macrae, 2020); software used to prepare material for publication: WinGX (Farrugia, 2012).  (2) 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. One low theta reflection was omitted from the refinement.