Synthesis and Characterization of a Copper ( II ) Complex with 6-Hydroxypicolinic Acid and 3-Picoline †

A copper(II) complex with 6-hydroxypicolinic acid (6-OHpicH) and 3-picoline (3-pic), [Cu(6-OHpic)2(3-pic)2] (1), was prepared by recrystallization of [Cu(6-OHpic)2(H2O)2] from 3-picoline and characterized by IR spectroscopy and thermal methods (TGA/DTA and DSC). The molecular and crystal structure of 1 was determined by single-crystal X-ray structural analysis. The copper(II) ion in 1 has a tetragonally compressed octahedral coordination environment, achieved by two N,O-bidentate 6-hydroxypicolinate ligands and by two 3-picoline molecules in trans positions. The crystal structure of 1 exhibits one strong intermolecular O–HO hydrogen bond, one weak intermolecular C–HO hydrogen bond and one C–Hπ interaction. Molecules of 1 are assembled into a 2D network in the (1 0 –1) plane, giving rise to C1(8) graph-set motif. These 2D networks are further assembled into 3D architecture only by weak Van der Waals interactions. A thermogravimetric study showed the initial loss of two coordinated 3-picoline molecules in 1, followed by a complete decomposition of the compound. (doi: 10.5562/cca2119)

As a part of our continuing interest in copper(II) complexes with molecules of biological importance, we have prepared a copper(II) complex with 6-hydroxypicolinic acid (6-OHpicH) and 3-picoline (3-pic), [Cu(6-OHpic) 2 (3-pic) 2 ] (1).The complex was characterized by IR spectroscopy and thermal methods (TGA/DTA, DSC) and its molecular and crystal structure was determined by single-crystal X-ray structural analysis.

EXPERIMENTAL Materials and Physical Measurements
All chemicals were of reagent grade, purchased from commercial sources and used as received, without further purification.
CHN analyses were performed by a Perkin-Elmer 2400 Series II CHNS analyzer in the Analytical Services Laboratories of the Ruđer Bošković Institute, Zagreb, Croatia.
The IR spectra were obtained from KBr pellets in the range 4000-450 cm -1 with a Perkin-Elmer Spectrum RXI FT-spectrometer.
TGA/DTA measurements were performed at heating rate of 5 ˚C min -1 in the temperature range of 25-500 ˚C, under nitrogen flow of 100 mL min -1 by a simultaneous TGA/DTA analyzer TA Instruments, Model SDT 2960.Approximately 10 mg of sample were placed in an open aluminum crucible.
DSC measurements were performed at heating rate of 10 ˚C min -1 in the temperature range of 25-500 ˚C, under nitrogen flow of 200 mL min -1 by a Mettler-Toledo DSC 823 e instrument.Approximately 5 mg of sample were placed in closed aluminum crucible (40 μL).

Single Crystal X-ray Diffraction Analysis and Structure Determination
A suitable single crystal of 1 was selected and mounted in air onto a thin glass fiber.The data collection for 1 was carried out by an Oxford Diffraction Xcalibur fourcircle kappa geometry diffractometer with Xcalibur Sapphire 3 CCD detector, using a graphite monochromated MoK  (λ = 0.71073 Å) radiation, and by applying the CrysAlis Software system, Version 171.32.29 at room temperature (296(2) K).Data reduction was done by the same program. 19he X-ray diffraction data were corrected for Lorentz-polarization factor and scaled for absorption effects by evaluation of multi-scans.The structure was solved by direct methods implemented in SHELXS-97.20 A refinement procedure by full-matrix leastsquares methods, based on F 2 values against all reflections, was performed by SHELXL-97, 20 including anisotropic displacement parameters for all non-H atoms.
The position of hydrogen atoms belonging to the carbon atoms Csp 2 and Csp 3 and to the hydroxyl O atom were geometrically optimized applying the riding model (Csp 2 -H, 0.93 Å, U iso (H) = 1.2 U eq (C); Csp 3 The calculations concerning the molecular geometry, the choice and verification of the space group, the analysis of hydrogen bonds and C-Hπ interactions Croat.Chem.Acta 85 (2012) 479.
were performed by PLATON. 21The molecular graphics were done with ORTEP-3 22 and MERCURY (Version 3.0). 23he crystal parameters, data collection and refinement results for 1 are summarized in Table 1.

Crystal Structure
An ORTEP-3 view of the molecular structure of 1 is depicted in Figure 1 and its crystal structure in Figure 2. The selected molecular geometry parameters are listed in Table 2 and the hydrogen bond geometry in Table 3.
The coordination environment of the copper(II) ion in 1 can be described as a distorted octahedron, revealing tetragonal compression due to the Jahn-Teller effect.The copper(II) ion in 1 is situated on a crystallo-graphic inversion center and it is coordinated by two 6-hydroxypicolinate ligands in the equatorial plane and by two 3-picoline molecules in the axial positions.The 6-hydroxypicolinate ligands are bound to a copper(II) ion in a bidentate N,O-chelated mode, forming fivemembered rings (Figure 1).The Cu1-N2 bond distance of 2.031(1) Å is significantly shorter than the Cu1-N1 bond distance of 2.328(1) Å, in accordance with the Jahn-Teller effect typical for d 9 electron configuration.The Cu-N and Cu-O bond distances (N and O atoms from 6-hydroxypicolinate) in 1 (Table 2) are longer than the corresponding ones reported in the literature for copper(II) complexes with picolinic, [24][25][26] 3-hydroxypicolinic, 11,14,27,28 6-hydroxypicolinic, 11,18,29 3-methylpicolinic, 15,16 6-methylpicolinic 17 and 6-bromopicolinic acid. 17he bond angles around the copper(II) ion, involving trans pairs of donor atoms, are exactly 180˚ due to the symmetry.The distortion of the octahedron is indicated by the angles with values from 76.37(4)˚ to 103.63(4)˚ for cis pairs of ligating atoms.One of the angles indicating the largest distortion from the ideal octahedral geometry is the bite angle O1-Cu1-N1 with the value of 76.37(4)˚ (Table 2).
The pyridine ring (defined by N1/C1/C2/C3/C4/ C5 atoms) is not perfectly coplanar with the chelated ring (defined by N1/C1/C6/O1/Cu1 atoms), as it is evident from the value of the dihedral angle between the mentioned rings (9.19(6)˚).There are one intermolecular O-HO and two weak intermolecular C-HO hydrogen bonds in the crystal packing of 1. Hydroxyl O3 and pyridine C4 and C9 atoms act as proton donors, while both coordinated (O1 atom) and uncoordinated (O2 atom) caboxylate oxygen atoms act as proton acceptors in hydrogen bonding (Table 3).Molecules of 1 are assembled into a 2D network in the (1 0 -1) plane by intermolecular O-HO hydrogen bonds, giving rise to C 1 1 (8) graph-set motif. 30f it is viewed in the ac plane, this network can be represented as a linear chain, extending along the [101] direction (Figure 2).These 2D networks are further assembled into 3D architecture only by weak Van der Waals interactions.There is also one C-Hπ interaction 31 between the C2 atom and the pyridine ring defined by atoms N2 i /C7 i /C8 i /C9 i /C10 i /C11 i (symmetry operator (i): x, 1+y, z; C2cg distance 3.592(2) Å; C2-H2cg angle 149˚; cg = centroid of the ring) in the crystal structure of 1.

IR Spectroscopy
Infrared spectroscopy data confirm the coordination of the 6-hydroxypicolinate ligand to the copper(II) ion via carboxylate group in 1.In the spectrum of 1, the band associated to the antisymmetric stretching vibrational mode,  asym (COO -), appears at 1668 cm -1 (1700 cm -1 in the uncoordinated ligand).The band associated to the symmetric stretching vibrational mode,  sym (COO -), appears at 1320 cm -1 (1295 cm -1 in the uncoordinated ligand).The value of Δ( asym (COO -) - sym (COO -)) amounts 348 cm -1 , indicating the presence of carboxylate group coordinated to copper(II) ion in monodentate mode 32,33 which is in agreement with the crystal structure of 1.The typical stretching vibrational mode of substituted pyridines,(CN), appears for the uncoordinated 6-hydroxypicolinic acid at 1606 cm -1 and it is shifted for 11 cm -1 to higher wavenumbers in 1.This shift is in agreement with the N,O-chelation, involving the pyridine N atom of 6-hydroxypicolinate ligand.

Thermal Analysis (TGA/DTA, DSC)
Complex 1 is thermally stable up to approximately 60 °C and decomposes in two endothermic steps.Two 3-picoline molecules are evolved in the first step (observed weight loss 31.0 %, calculated 35.4 %), with two endothermic peaks in the DTA curve (80.7 °C and 97.4 °C).The complex further decomposes in the second step, revealing the release of both 6-hydroxypicolinate ligands (observed weight loss 51.6 %, calculated 52.5 %), with two endothermic peaks (352.1 °C and 363.5 °C).The remained residue of 17.4 % (calculated 15.1 %) at 500°C is most probably CuO.
The DSC curve of 1 also shows two endothermic peaks for each of the two decomposition steps, similar as the DTA curve.Two peaks at 104.8 °C and 116.9 °C (80.7 °C and 97.4 °C in the DTA curve) in the first step indicate the release of two 3-picoline ligands, while the two peaks at 367.1 °C and 375.1 °C (352.1 °C and 363.5 °C in the DTA curve) in the second step correspond to the decomposition of both 6-hydroxypicolinate ligands.The peaks in the DSC curve are shifted by 10-25 °C to higher temperatures if compared to the peaks in the DTA curve.
The existence of two peaks in DTA and DSC curves for each step suggests that both the release of 3-picoline molecules and the decomposition of 6-hydroxypicolinate ligands may be composed of two different processes.However, this was not confirmed by TGA analysis.

Figure 1 .
Figure 1.An ORTEP-3 drawing of 1 with the atomic numbering scheme of the asymmetric unit.The thermal ellipsoids are drawn at the 50 % probability level at 296(2) K.

Figure 2 .
Figure 2. A view of the crystal structure of 1 in the ac plane.Molecules are linked by intermolecular hydrogen bonds of the O−HO type (shown by the dotted lines) into a 2D network, which is represented as a linear chain in this projection.These linear chains extend along the [101] direction and are assembled together only by weak Van der Waals interactions.

Table 1 .
The crystal data and details of the structure refinement for 1

Table 3 .
The hydrogen bond geometry for 1