Synthesis and Crystal Structures of Schiff Base Copper(II) Complexes [CuBrL] and [CuL(NCS)]

CuBr₂ reacts with the Schiff base 4-bromo-2-[(pyridin-2-ylmethylimino)methyl]phenol (HL) and ammonium thiocyanate in absolute methanol to give the mononuclear copper(II) complexes [CuBrL] (1) and [CuL(NCS)] (2). Both complexes were characterized by elemental analysis, IR spectra, and single-crystal X-ray determination. The crystal of (1) crystallizes in the monoclinic space group P2₁/c, with a = 7.754(2), b = 10.542(3), c = 17.674(2) Å, β = 114.445(2)°, V = 1315.2(5) ų, Z = 4. The crystal of (2) crystallizes in the monoclinic space group P2₁/n, with a = 7.016(1), b = 15.559(2), c = 13.197(1) Å, β = 91.647(2)°, V = 1440.0(4) ų, Z = 4. The Cu atom in each of the complexes is in a square planar geometry.

Keywords: coordination compound; copper complex; crystal structure; Schiff base; synthesis

Transition metal complexes with Schiff bases are considered to be among the most important stereochemical models in coordination chemistry due to their preparative accessibility and structural variety.[[1]–[3]] The halide and pseudohalide anions are widely used coligands for the construction of complexes with Schiff bases.[[4]–[7]] In the present study, two new different mononuclear copper(II) complexes, [CuBrL] (1) and [CuL(NCS)] (2), derived from the reaction of the Schiff base ligand 4-bromo-2-[(pyridin-2-ylmethylimino)methyl]phenol (HL; Scheme 1) with copper bromide and ammonium thiocyanate have been obtained and characterized.

Graph: SCH. 1 HL.

5-Bromosalicylaldehyde and 2-aminomethylpyridine were purchased from Alfa Aesar (USA). All other chemicals and solvents were of analytical grade and used as obtained. The Schiff base ligand HL was prepared according to the literature method.[[8]] Microanalyses of the Schiff base ligand and the copper complexes were performed with a Vario EL III CHNOS elemental analyzer (Germany). The infrared spectra were recorded as KBr pellets with an FTS-40 spectrophotometer (BIO-RAD, USA).

Data were collected on a Bruker SMART 1000 CCD area diffractometer (Germany) using a graphite monochromator Mo Kα radiation (λ = 0.71073 Å) at 298(2) K. The data were corrected with SADABS programs (Bruker, Germany) and refined on F2 with Siemens SHELXL software (Germany).[[9],[10]] The structures were solved by direct methods and difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in calculated positions and included in the last cycles of refinement. Crystal data and details of the data collection and refinement are listed in Table 1. Selected coordinate bond lengths and angles are listed in Table 2.

Graph: FIG. 1 ORTEP plots (30% probability level) and numbering scheme for (1).

TABLE 1 Crystallographic data for complexes () and ()

A stirred solution of HL (0.291 g, 1 mmol) and ammonium thiocyanate (0.076 g, 1 mmol) in absolute methanol (20 mL) was mixed with CuBr2 (0.223 g, 1 mmol), and the resulting reaction mixture was refluxed on a water bath for 1 h, and then cooled to room temperature, yielding an amount of precipitation.

For (1), the filtrate was left to stand still at ambient temperature to slow evaporation of the solvent, yielding blue block crystals suitable for X-ray diffraction. Yield 0.083 g (19%). Anal. Calcd. (%) for C13H10Br2CuN2O: C, 36.0; H, 2.3; N, 6.5. Found (%): C, 35.9; H, 2.4; N, 6.5. Selected IR data (KBr, νmax/cm−1): 1605 (C=N).

For (2), the filter cake was redissolved by DMF, and deep brown needle single crystals suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether into the DMF solution of the precipitate. Yield 0.127 g (31%). Anal. Calcd. (%) for C14H10BrCuN3OS (%): C, 40.8; H, 2.4; N, 10.2. Found (%): C, 40.9; H, 2.5; N, 10.1. Selected IR data (KBr, νmax/cm−1): 1604 (C=N), 2085 (NCS).

The overview of the two complexes is shown in Scheme 2. Reaction between equimolar quantities of CuBr2 with HL and ammonium thiocyanate in absolute methanol afforded [CuBrL] and [CuL(NCS)]. The Schiff base ligand coordinates to the Cu atom through all the three donor atoms. Even though the Br and NCS groups readily bridge different metal atoms, forming versatile polymeric structures, in this work, they act as simple terminal ligands.

TABLE 2 Selected bond lengths (Å) and angles (°) for complexes () and ()

Graph: SCH. 2 [CuBrL] and [CuL(NCS)].

Graph: FIG. 2 ORTEP plots (30% probability level) and numbering scheme for (2).

Graph: FIG. 3 Molecular packing diagram for (1), viewed along the a axis.

Graph: FIG. 4 Molecular packing diagram for (2), viewed along the a axis.

TABLE 3 π···π interactions for () and ()

The crystal structures of (1) and (2) with atom numbering scheme are shown in Figures 1 and 2, respectively. Both compounds are mononuclear copper(II) complexes. The Cu atom in each complex is coordinated by the Schiff base ligand and one terminal secondary ligand, viz. Br for (1) and NCS for (2). The Schiff base ligand coordinates to the Cu atom through the phenolate oxygen, imine nitrogen, and pyridine nitrogen atoms, forming five- and six-membered chelate rings. The Cu atom is in a square planar coordination sphere. The displacement of the Cu atom from the least-squares plane defined by the four donor atoms is 0.0202 Å for (1), and 0.0001 Å for (2), indicating the well planarity of the square planar coordination. The bond lengths and angles in the two complexes are comparable to each other, and within normal values.[[11]–[13]] The cis coordinate bond angles are in the range 82.8(3)–95.9(2)° for (1) and 82.9(1)–93.4(2)° for (2), and the trans bond angles are in the range 173.4(3)–178.7(2)° for (1) and 176.2(1)–176.3(2)° for (2). In both complexes, the Schiff base ligands are approximately coplanar, with the dihedral angles between the benzene ring and the pyridine ring of 3.1(2)° for (1) and 6.6(2)° for (2).

In the crystal structure of the complexes, Figure 3 for (1) and Figure 4 for (2), the molecules are stacked together by weak π···π (Table 3) and C–Br···π (Table 4) interactions among the molecules.

TABLE 4 Y–X···Cg interactions for () and ()

The weak band centered at 3332 cm−1 of the free Schiff base ligand is assigned to the ν(O–H) vibration, which is absent in the spectra of the complexes. The strong bands at 1605 cm−1 in (1) and 1604 cm−1 in (2) are assigned to the azomethine groups, which are shifted to lower frequencies when compared with the free Schiff base ligand. The bands indicative of the Cu–O and Cu–N bonds are in the range 400–600 cm−1. The intense band indicative of the thiocyanate ligand in (2) is observed at 2085 cm−1.

Two new different mononuclear copper(II) complexes were prepared from the same starting materials. The Br coordinated complex (1) was obtained from the filtrate of the reaction mixture, and the NCS coordinated complex (2) was obtained from the filter cake of the reaction mixture. Both complexes were characterized by IR spectroscopy and single crystal X-ray diffraction. The Schiff base ligand coordinates to the Cu atom through the phenolate oxygen, the imine nitrogen, and the pyridine nitrogen atoms.

CCDC 860409 and 860410 contain the supplementary crystallographic data for this study. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data%5frequest/cif.

This work was financed by the National Technology Support Project (2010BAD02B03, 2010BAD02B04, 2010BAD02B06).