Synthesis, crystal structure and photophysical properties of chlorido[(E)-3-hydroxy-2-methyl-6-(quinolin-8-yldiazenyl)phenolato]copper(II) monohydrate

A copper(II) complex with the (E)-2-methyl-4-(quinolin-8-yldiazenyl)benzene-1,3-diol ligand was prepared and structurally characterized. The UV–Vis absorption spectra of the ligand and the complex are reported.

The reaction between copper(II) chloride dihydrate and the (E)-2-methyl-4-(quinolin-8-yldiazenyl)benzene-1,3-diol ligand in acetonitrile leads to the formation of the title compound, [Cu(C 16 H 12 N 3 O 2 )Cl]ÁH 2 O. The ligand is deprotonated and coordinates with three donor atoms (tridentate) to the Cu II ion. Individual molecules of the Cu II complex are connected by chloride bridges, forming a one-dimensional coordination polymer. No photoisomerization to the cis isomer of the azo ligand was observed upon irradiation with UV light.

Chemical context
Azobenzene derivatives are well-known dyes with fascinating characteristics such as cis-trans photoisomerization and azohydrazone tautomerism. The combination of azo compounds with metal ions to form complexes is a promising approach for controlling their photophysical properties. In metal complexes with azo ligands, the metal centers and azo ligands can affect each other's properties. For example, cis-trans photoisomerization by irradiation with a single frequency of light has been achieved in azo-conjugated metal complexes by a combination of the photophysical and the redox properties of ligand and metal center (Nishihara, 2005). Azobenzene derivatives with hydroxy groups in the ortho or para position tend to form hydrazone tautomers (Jacques et al., 1979;Ball & Nicholls, 1982;Rauf et al., 2015). A hydrazone tautomer can be converted to an azo tautomer by complexation to the metal ion (Chen et al., 2012;Cai et al., 2016). In this study, we used the ortho and para isomer of the hydroxy-substituted azobenzene derivative, (E)-2-methyl-4-(quinolin-8-yldiazenyl)benzene-1,3-diol, to investigate azo-hydrazone tautomerism in its Cu II complex. The photophysical properties of the ligand and the Cu II complex were studied by UV-Vis spectroscopy to address the potential photoisomerization.

Structural commentary
The crystal structure of the Cu II complex is shown in Fig. 1. The asymmetric unit contains one Cu II complex and one solvent water molecule. The hydroxy group in the orthoposition of the azo ligand is deprotonated and is coordinated the Cu II center. In the asymmetric unit, the Cu II ion is 4-coordinated in a distorted square-planar geometry. The donor atoms comprise one nitrogen atom of the quinoline moiety, one nitrogen atom of the azo group, one deprotonated alcohol oxygen atom, and a chloride ion. The other hydroxy group of the azo ligand, in the para-position, remains protonated. The chlorido ligand is also weakly coordinated by an adjacent Cu II center occupying its apical position, resulting in an elongated square-pyramidal coordination polyhedron around the copper(II) ions. The Cu1-Cl1 i distance in the apical position is 2.7395 (10) Å , which is notably longer than the distances in the equatorial positions, Cu1-Cl1 = 2.2803 (8) Å , Cu1-O1 = 1.917 (2) Å , Cu1-N1 = 2.008 (3) Å , and Cu1-N2 = 1.945 (3) Å [symmetry code: (i) x + 1, y, z]. The N2-N3 bond distance of 1.293 (4) Å is typical for the N N double bond of an azo group. The structural features of the aromatic rings and the C11-O1 single-bond length of 1.300 (4) Å also indicate that the ligand adopts the azo structure, rather than the hydrazone structure, which is similar to the structures observed in analogous azo-metal complexes with other metals, including Ni, Cu, and Zn (Cai et al., 2016;Kochem et al., 2011Kochem et al., , 2014.
ConQuest ( While co-planarity of the aromatic moieties was observed in some of these structures, the formation of the column-type coordination polymeric structure of the title compound has no precedence in this group.

UV-Vis spectra for the azo ligand and Cu II complex
The UV-Vis spectra of the azo ligand and the Cu II complex in CH 3 CN are shown in Fig. 4. The maximum of the extinction (" max ) was observed at 406 nm for the ligand, while the Cu II complex showed decreased absorption and red-shifted maxima at 420 and 489 nm. To investigate the photoisomerization of the ligand and the Cu II complex, the samples were irradiated at maximum wavelength, but no photoisomerization to the cis isomer was observed for either compound.

Synthesis and crystallization
To synthesize the title ligand, an aqueous solution of 1.2 M NaNO 2 (3 mL) was slowly added to a cold solution of 8aminoquinoline (0.432 g, 3.00 mmol) in 0.5 M HCl (aq) (20 mL). The resulting solution was stirred at 277 K for 15 min, and an aqueous solution of (NH 2 ) 2 CO (0.180 g, 3.00 mmol) in 3 mL of water was then added to give a diazonium chloride solution. This solution was added to an aqueous 0.25 M NaOH solution of 2,6-dihydroxytoluene (0.372 g, 3.00 mmol) and stirred at 277 K for 30 min and then stirred at room temperature for 15 h. The reaction mixture was acidified with 1 M HCl(aq) (10 mL) and a red precipitate was formed.   UV-Vis spectra of the ligand and the title compound in CH 3 CN.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. The O-H hydrogen atoms of the solvent water molecules and the hydroxy group in the paraposition were found in the difference-Fourier map and were refined isotropically without restraints or constraints. Other hydrogen atoms were generated geometrically, and refined with a riding model with C-H = 0.98 Å , U iso (H) = 1.5 U eq (C) for methyl, and C-H = 0.95 Å , U iso (H) = 1.2 U eq (C) for aromatic hydrogen atoms. Two reflections were omitted as clear outliers.

Chlorido[(E)-3-hydroxy-2-methyl-6-(quinolin-8-yldiazenyl)phenolato]copper(II) monohydrate
Crystal data [Cu(C 16  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.