Synthesis of a new μ-chlorido-bridged tetra-nuclear copper(II) complex containing 2,3,5,6-tetra(2-pyridyl)pyrazine (tppz), chlorido and perchlorato ligands: non-planar central pyrazine rings

Abstract A μ-chlorido-bridged tetra-nuclear copper(II) complex containing 2,3,5,6-tetra(2-pyridyl)pyrazine (tppz), chlorido, and perchlorato ligands, starting from 2-aminomethyl pyridine (AMP) and Cu(OAc)2·H2O, was synthesized. The structure of the complex was determined by single-crystal X-ray diffraction (SCXRD), Fourier transform infrared spectroscopy (FT-IR), and electronic absorption spectroscopy (UV–Vis). The redox behavior of the complex was explored by cyclic voltammetry. The tetra-nuclear structure is formed by combining the di-nuclear copper complex of the two tppz ligands with a chloride bridge. The Cu(1) center adopts a distorted octahedral geometry, whereas the other three Cu centers adopt square pyramid geometries. Crystal data of the complex are monoclinic, P21/c, a = 13.602(4) Å, b = 13.438(4) Å, c = 29.412(8) Å, α = 90°, β = 95.258(8)°, γ = 90°, V = 5353(3) Å3, Z = 4. When the SCXRD structure was examined, it was determined that the two central pyrazine rings deviated from planarity, which is one of the four criteria for aromaticity. The aromaticity of the central pyrazine rings, whose planarity is disturbed, is decreased. Based on these bond lengths, the harmonic oscillator aromaticity model (HOMA) values of the non-planar central pyrazine rings were calculated as 0.964926 and 0.942886, respectively.

Ogura et al. developed an improved method to obtain the tppz ligand by the aerial oxidation of cobalt(II)-2-aminomethylpyridine complex [10].They reported that tppz synthesis was achieved with Co(II) ion in good yield, very low with Ni(II) ion, and no oxidation reaction with Cu(II) ion.They also found that the reaction yield increased in ethanol solution at higher temperatures and when the AMP/Co(II) ion molar ratio was 3:1.In my previous work [11], an octahedral diperchlorato copper(II) complex, [Cu(AMP) 2 (OClO 3 ) 2 ], was synthesized with 2-aminomethyl pyridine (AMP) ligand, and reported the synthesis, characterization and thermal properties.This reaction occurred at room temperature in aqueous medium, and a blue product was obtained.Because of our interests in Cu(II)-AMP complexes, it was desired to convert the Cu(II)-AMP complex to the Cu(II)-TPPZ complex by air oxidation under the conditions suggested by Okamoto et al., and the title compound was formed.
This study reports that the aromatic central pyrazine rings in the obtained l-chlorido-bridged tetra-nuclear copper(II) complex are highly non-planar, thus reducing their aromaticity.

Materials and instruments
All of the chemicals were purchased from commercial sources and used directly; AMP from Acros (Geel, Belgium); SP-Sephadex C-25 and sodium perchlorate from Sigma-Aldrich (St. Louis, MO); sodium chloride from Carlo Erba (Mannheim, Germany); potassium chloride from Riedel (Kufstein, Austria); copper(II) acetate monohydrate and methanol were purchased from Merck (Kenilworth, NJ).The complex was analyzed by FT-IR (Nicolet iS5, Thermo Scientific, Waltham, MA) coupled with the attenuated total reflection (ATR) technique (iD7, Thermo Scientific).The electronic spectrum of the complex was measured using a UV-Vis spectrophotometer (Genesys 10S UV-Vis, Thermo Scientific) from 200 to 800nm.Cyclic voltammetry was performed using a PalmSens4 potentiostat (PSTrace version 5.9 software, Houten, The Netherlands) with a screen-printed electrode (SPE) coated with a graphite working electrode, graphite counter electrode, and silver/silver chloride reference electrode, with range of potentials þ1 V and À1 V.

X-ray crystallographic data collection and structure refinement
Crystals suitable for single-crystal X-ray diffraction studies were obtained by slowly evaporating water at room temperature.The data were collected on a D8-QUEST diffractometer equipped with graphite-monochromatic Mo-Ka radiation at 296 K. Using Olex2 [12], the structure was solved with the Olex2.solve[13] structure solution program using Charge Flipping and refined with the Olex2.refine[13] refinement package using Gauss-Newton minimization and with SHELXL [14].Molecular diagrams were generated using OLEX2 [12] and MERCURY [15].Details of data collection and crystal structure determinations are given in Table 1.The crystallographic data of the structure reported in this document are deposited at the Cambridge Crystallographic Data Center with CCDC number 2217010.

General procedure for the synthesis of 1
[fCu 2 Cl 2 (OClO 3 ) 2 (N,N,N-j 3 -tppz)g(l-Cl)fCu 2 Cl 3 (N,N,N-j 3 -tppz)g] ( 1) was synthesized by modifying the literature method (see scheme 1) [10].A solution of AMP (1 g, 9.2 mmol)   and copper(II) acetate monohydrate (0.74g, 3.1 mmol) in 50 mL of methanol was heated (70 C) under aerial conditions with stirring for 24 h.The solution's color became blue, green, and reddish-brown during the reaction.After the solvent was evaporated entirely, it was dissolved in water, diluted to 500 mL, and loaded on a SP Sephadex C-25 cation exchanger resin column [11].It was tried to be eluted with 0.25 M NaClO 4 solution, but when the main band (brown) did not move, it was eluted with 1 M NaCl solution.The first blue and the following light green-brown bands, which were small in quantity, were not collected.The main brown band was collected, concentrated, and crystallized from water.The complex was obtained as dark brown crystals by slow evaporation at room temperature (yield 0.1 g, 27%

FT-IR characterization of 1
The complex was characterized by FT-IR(ATR) (see Figure S1).The FT-IR absorptions of 1 are listed in the Experimental section.The O-H vibrations of water molecules in the compound were at 3599, 3506, and 3401 cm À1 .The characteristic aromatic C-H stretch of 1 is at 3079 cm À1 .The C ¼ C, C ¼ N peaks of the tppz ligand are at 1595, 1493, 1471, and 1417 cm À1 .The peaks of monodentate perchlorate ligands were at 1081 and 1063 cm À1 .Cu-Cl, Cu-N, and Cu-O stretching frequencies are generally observed lower than 400 cm À1 , which is outside the detection limit of our device.

Electronic absorption study of 1
The UV-Vis spectrum of 1 was recorded in water at room temperature from 200 to 800 nm (see Figure S2).The high-intensity energy bands at 300 (e ¼ 24200 M À1 cm À1 ) nm and 362 (e ¼ 20,800 M À1 cm À1 ) nm observed of 1 are very close to the spectra of the previously reported "chloro-tppz-dicopper" complexes [5,6] and are due to the p!p Ã and n!p Ã transitions of the tppz ligand [16].

Cyclic voltammetry of 1
Cyclic voltammetry (CV) of 1 (10 À4 M) was performed at room temperature in an aqueous 0.1 M KCl solution (supporting electrolyte) at a scan rate of 1.0 Vs À1 (see Figure S3).The solution was deoxygenated by bubbling with N 2 for 15 min.The cyclic voltammogram for 1 shows only one reversible Cu(II)/Cu(III) redox couple at positive potentials and one reversible Cu(III)/Cu(II) and one irreversible Cu(II)/Cu(I) redox couple at negative potentials.The anodic peak at þ0.53 V (0.069 mA) is assigned to reversible one-electron oxidation of Cu(II).On the cathodic side, the first reduction peak occurred at À0.15 V (0.069 mA), indicating that the oxidized Cu(II) was reduced back to the original Cu(II) state.The second reduction peak was observed at À0.54 V (0.002 mA), indicating another reduction step.This additional reduction can be assigned to the reduction of Cu(II) to Cu(I) in a lower oxidation state.

Solid state molecular structure of 1
Details of the single-crystal X-ray data collection and refinement of 1 are summarized in Table 1.Selected bond lengths and angles for 1 are collected in Table 2.The bond lengths and selected torsion angles of the central pyrazine rings are collected in Table 3.The molecular structure of 1 with the labeling scheme is shown in Figure 1.

Conclusion
A new l-chlorido-bridged tetra-nuclear copper(II) complex based on tppz chlorido and perchlorato ligands was prepared and characterized.The tetra-nuclear complex was formed by coupling two tppz ligands with binuclear bis-tridentate coordination by a l-chlorido bridge.In the crystal structure, the Cu(1) center adopts distorted octahedral geometry, while the other three Cu centers have a square pyramidal geometry.In the SCXRD structure, the two central pyrazine rings deviated considerably from planarity, which is one of the four criteria for aromaticity.As a result of the significant distortion in the pyrazine rings, they did not wholly lose their aromaticity, but their aromaticity was reduced.HOMA aromaticity values were calculated as 0.964926 and 0.942886, respectively.

Figure 1 .
Figure 1.Molecular structure and partial labeling of 1.The thermal ellipsoids are shown at 50% probability.

Figure 2 .
Figure 2. The molecular packing view for 1.

Figure 3 .
Figure 3.The non-planar central pyrazine rings and partial labeling.

Table
. Single-crystal data and structure refinement.