Structural elucidation of novel mixed ligand complexes of 2-thiophene carboxylic acid [M(TCA)₂(H₂O)_x(im)₂] [x=2 M: Mn(II), Co(II) or Cd(II), x=0 Cu(II)]

Synthesis

To synthesize the Mn(II) and Cd(II) complexes, mixtures of metal salts and im were added to a solution of Na-TPC, which was prepared by addition of NaOH to HTPC solution. To prepare the Co(II) complex, a solution of the metal salt, HTPC and im, along with NH₃ were added. Suitable crystals were obtained by slow evaporation for X-ray single crystal analyses.

Interestingly, when we tried to obtain a copper compound by using our above-mentioned method or the method of Zheng et al., we continually isolated the known square pyramidal [Cu(H₂O)(Imd)₂(TCA)₂] (Zheng et al., 2007). We therefore prepared the copper complex by microwave-assisted hydrothermal synthesis.

Crystal structure

The structures of the complexes were confirmed by single crystal X-ray studies. All hydrogen atoms were placed in calculated positions. In Table 1, the experimental details for data collection and structure refinement of the complexes are summarized. Selected bond lengths and angles are provided in Table 2 and the intermolecular and intramolecular interactions are given in Table 3.

All complexes crystallized in the monoclinic space group P2₁/n and showed similar orientations. In all complexes, the TPC bonded to the metals in a monodentate fashion via one of its carboxylato-O atoms. The monodentate im ligand coordinated to the metal via its nitrogen atom (Figure 1).

Figure 1

The structures of the complexes. (M=Mn, Co or Cd).

While the geometry around metals for the Mn(II), Co(II) and Cd(II) complexes were distorted trans-octahedral, [MN₂O₄], the Cu(II) complex had a trans-square planar, [MN₂O₂], environment around the metal center.

The distorted octahedral environments around the Mn(II), Co(II) and Cd(II) metal centers, contain two TPC, two Im and two aqua ligands. The Cu(II) complex contains two TPC and two Im ligands to form the square planar environment around the metal center (Figure 1).

The M-O bond lengths in the complexes (except the Cu complex), reflected the van der Waals radius order of the metals and the Co-O distances were significantly shorter than the others. As expected, the Cu-O bond lengths were shorter than the other M-O distances in the square planar complex.

In the Mn(II), Co(II) and Cd(II) complexes, the aqua ligands, by using one of the hydrogen atoms, created intramolecular hydrogen bonds with the non-coordinated oxygen atoms (O2) of TPC ligands, and by using other hydrogen atoms formed intermolecular hydrogen bonds with the coordinated oxygen atoms (O1) of TPC ligands to connect the molecules along the a axis. Through this direction, the C–H‧‧‧π interaction was seen between the C3 atom of the thiophene ring and the im ring (Figures 2 and 3; Table 3).

Figure 2

The O-H···O, C-H···O, C-H···π and π ···π interactions in the Mn and Cd complexes.

Figure 3

The O-H···O, N-H···O, C-H···π and π ···π interactions in Co complex.

In addition to these interactions, in the Mn(II) and Cd(II) complexes, the C7 atom of the im ring formed intermolecular hydrogen bonds with the non-coordinated oxygen atom of a symmetry related TPC ligand (Table 3), and there were also detected π‧‧‧π interactions between the im and thiophene rings, so the molecules that located along the b and c axes, are linked to each other (Figure 2, Table 3).

In the Co(II) complex, N-H···O interactions were found between the N2 atom of the im ring and the O2 atom of the TPC ligand to extend molecules in the same manner (Figure 3) instead of C-H···O interactions which were detected for Mn(II) and Cd(II) complexes.

Therefore, overall, the Co(II), Mn(II) and Cd(II) complexes showed H bonded three-dimensional networks.

In the square-planar Cu(II) complex, the N-H of the im formed a hydrogen bond with the un-coordinated oxygen atom of TPC of the complex unit at x, 1+y, z, and additionally, there were found π‧‧‧π interactions between the im rings at 1-x, 1-y, -z. So, by contrast, the copper complex showed only a one-dimensional network (Figure 4, Table 3).

Figure 4

The N-H···O and π ···π interactions in the copper complex.

IR spectra

The IR spectrum of HTPC showed peaks at 2548 cm^-1, 1689 cm^-1 and 1353 cm^-1 attributed to OH, C=O+ COO^-_asym. and COO^-_sym. vibrations, respectively. The N-H and C=N peaks of im were seen at 3124 cm^-1 and 1669 cm^-1, respectively.

The bands of the free HTPC at 2548 cm^-1, disappeared in IR spectra of the complexes and indicated that the carboxylic acid proton is removed to form thiophen-2-carboxylato anion on complexation. In all complexes, due to the coordination of the metal environment, the C=N bands of im shifted to lower frequencies and were seen at 1613 cm^-1, 1600 cm^-1, 1602 cm^-1 and 1600 cm^-1 for Mn(II), Co(II), Cd(II) and Cu(II) complexes, respectively, and N-H peaks were shifted to the higher frequencies [for Mn(II) and Co(II)=3175 cm^-1, for Cd(II)=3167 cm^-1 and for Cu(II)=3139 cm^-1]. The band between 3370 cm^-1 and 3290 cm^-1 was attributed to the aqua ligands for Mn(II), Co(II) and Cd(II) complexes.

To determine the coordination modes of the carboxylato group, the difference between the asymmetric and symmetric carboxylato vibrations (Δ=υ¯asym-υ¯sym) was calculated. The differences between the frequencies were determined as 162, 154, 160 and 154 for Mn(II), Co(II), Cd(II) and Cu(II) complexes, respectively. These results for Δ values indicated that the carboxylato group participates in a monodentate manner for the complexes (Nakamoto, 1978; Zheng et al., 2007; Cagnin et al., 2010; Palanisami and Murugavel, 2011; Abbas et al., 2013).

UV-Vis spectra and magnetic susceptibility data

The room temperature magnetic moments which are in accord with the geometry around the metal centers, were found to be 4.87, 4.61, diamagnetic and 1.65 BM for the Mn(II), Co(II), Cd(II) and Cu(II) complexes, respectively.

The UV-Vis spectra of the complexes in DMF solutions showed one band [except the Cd(II) complex], at 494 nm (ε=9 lmol^-1 cm^-1), 566 nm (ε=23 lmol^-1 cm^-1) and 668 nm (ε=75 lmol^-1 cm^-1) for Mn(II), Co(II) and Cu(II) complexes, respectively.

Thermogravimetric analysis

In the thermal analyses works for [M(TCA)₂(H₂O)₂(im)₂] [M=Mn(II), Co(II) and Cd(II)] the complexes underwent complete decomposition in three stages and gave similar thermal analyses curves. The first stages are related to the removal of aqua ligands (Figures 5–7, Table 4). The Cu(II) complex decomposed in two stages (Figure 8, Table 4).

Figure 5

Thermal analyses curves of the Mn complex.

Figure 6

Thermal analyses curves of the Co complex.

Figure 7

Thermal analyses curves of the Cd complex.

Figure 8

Thermal analyses curves of the Cu complex.

It is not easy to explain reasonably, without mass spectra, the evolution of fragments during thermal decomposition for second and third decomposition stages for all complexes. However, it can be suggested, at the second and third stages, that the im and TCA ligands decomposed to form metals (Cu₂O for copper complex) and after 485°C, 565°C, 566°C and 539°C, increasing the temperature produced metal oxides for Mn, Co, Cd and Cu compounds, respectively.

In consideration of the DTA_max values of the anhydrous form of the complexes, the thermal stability of the complexes are suggested as Co (273°C)>Cu (229°C)>Cd (175°C)>Mn (167°C).

Materials and measurements

Only commercially available reagent grade chemicals were used. Fourier transform infrared spectroscopy (FT-IR) spectra (4000–450 cm^-1) were recorded on a Perkin-Elmer Spectrum 100 FT-IR spectrophotometer, with samples prepared as KBr pellets. Magnetic susceptibility was measured using a Sherwood Scientific MX1 Gouy Magnetic Balance (Cambridge, UK) at room temperature. UV-Vis spectra were recorded with a PG Instruments PG-T80+ UV-Vis spectrometer. An SII-O Extar 6000 thermal analyzer (Japan) was used to record TG, DTG, and DTA curves in static air atmosphere at a heating rate of 10 (°C)min^-1 from 30 to 700°C using platinum crucibles. Highly sintered α–Al₂O₃ was used as a reference. Elemental analysis for C, H, and N were performed using a Costech ECS 4010 CHNSO analyzer. A CEM Mars-Xpress microwave digestion system was used in the synthesis of the copper complex with a 50 mL Teflon tube.

Synthesis of the complexes

X-ray crystallography

By using Mo-Kα radiation, the crystal data were collected using ω-2θ scan techniques on a Bruker Smart Apex CCD diffractometer for the Mn, Co and Cd complexes and on an Agilent SuperNova diffractometer (with an Eos CCD detector) for the copper complex. Software programs on Bruker were used; Bruker APEX2 and Bruker SAINT for data collection for data reduction, respectively (Bruker 2007a,b). The CrysAlisPro software program was used for data collection, cell refinement and data reduction on an Agilent diffractometer (Agilent, 2011).

Using Olex2 (Dolomanov et al., 2009), the structures were solved by the SIR2011 (Burla et al., 2012) structure solution program by direct methods and refined with the ShelXL refinement package using least-squares minimization (Sheldrick, 2008). All H atoms were refined using a riding model. To prepare material for publication, Olex2 were used (Dolomanov et al., 2009).