Synthesis and Crystal Structures of Copper(II) Complexes with Schiff Base Ligands: [Cu₂(acpy-mdtc)₂(HBA)(ClO₄)]·H₂O and [Cu₂(acpy-phtsc)₂(HBA)]·ClO₄

Abstract

Two new Cu(II) complexes, $[Cu_2(acpy-mdtc)_2(HBA)(ClO_4)]{\cdot}H_2O$ (1) (acpy-mdtc- = 2-acetylpyridine S-methyldithiocarbamate and $HBA^-$ = benzilic acid anion) and $[Cu_2(acpy-phtsc)_2(HBA)]{\cdot}ClO_4$ (2) (acpy-$phtsc^-$ = 2-acetylpyridine 4-phenyl-3-thiosemicarbazate) have been synthesized and characterized by elemental analysis, infrared spectroscopy, thermogravimetric analysis, and single crystal X-ray diffraction. The X-ray analysis reveals that the structures of 1 and 2 are dinuclear copper(II) complexes bridged by two thiolate sulfur atoms of Schiff base ligand and bidentate bridging $HBA^-$ anion. For 1, each of the two copper atoms has different coordination environments. Cu1 adopts a five-coordinate square-pyramidal with a $N_2OS_2$ donor, while Cu2 exhibits a distorted octahedral geometry in a $N_2O_2S_2$ manner. For 2, two Cu(II) ions all have a five-coordinate square-pyramidal with a $N_2OS_2$ donor. In each complex, the Schiff base ligand is coordinated to copper ions as a tridentate thiol mode.

Introduction

Thiosemicarbazones1 and Schiff bases derived from S-alkyl/aryl esters of dithiocarbazic acid2 are among the most widely studied sulfur-nitrogen chelating agents. Interest in metal complexes of these ligands is stimulated by their interesting physico-chemical properties3 and significant biological activities.4

We have reported Mo(VI), V(IV)O, Mn(II), Co(II), and Zn(II) complexes of mono- or bis-Schiff base ligands result-ing from the condensation of salicylaldehyde or 2-acetyl-pyridine with dithiocarbamate or thiosemicarbazide.5 Recent-ly, dinuclear copper(II) complexes, [Cu2(L)2(CH3COO)](ClO4) (L = 2-benzoylpyridine S-methyldithiocarbazate)6 and [Cu2L2(SO4)] (L = di-2-pyridyl ketone N(4),N(4)-(butane-1,4-diyl)thiosemicarbazone)7 in which sulfur atom from Schiff bases ligand together with acetate or sulfate oxygen atoms bridges the two copper(II) Ions, respectively, were reported. Although many Cu(II)-Schiff base complexes have been reported, dinueclear Cu(II) complexes consist of Schiff base ligand and other the second ligand, especially, dinuclear complexes linked through the other second ligand except the solvent molecule or the counter anion of metal salt as starting material have been little published.8

In this work, the benzilic acid (H2BA) was taken as the second ligand. For metal ions, benzilic acid can provide a variety of chelating and/or bridging coordination modes dis-played by the carboxylic or hydroxy groups.9 Many frame-works constructed by BA2− or HBA− with transition metal ions or rare earth ions have been reported, mainly using hydrothermal synthetic method.10

As part of our long-standing interest in synthesizing and extending the dimensionality of coordination compounds with mixed N/S coordination spheres we report herein a dinuclear Cu(II) complexes of an acetylpyridine based dithiocarbamate or 4-phenyl-3-thiosemicarbazide. Thermal properties of the complexes are also discussed.

 

Experimental Section

Chemicals and Measurements. All chemicals are com-mercially available and were used as received without further purification. The ligands, acpy-mdtcH and acpy-phtscH, were prepared as described in the literature.1112 Elemental analyses (CHN) were performed on a Vario EL EA-Elementar Analyzer. Infrared spectra were recorded in the range from 4000 to 400 cm−1 on a Mattson Polaris FT-IR Spectrophoto-meter using KBr pellets. Thermogravimetric (TG) and differ-ential thermal analysis (DTA) were performed on a Shimadzu DTG-60 instrument with a heating rate of 10 °C·min−1.

Preparation of [Cu2(acpy-mdtc)2(HBA)(ClO4)]·H2O (1). To a methanol solution (20 mL) of acpy-mdtcH ligand (0.225 g, 1.00 mmol) was added Cu(ClO4)2·6H2O (0.371 g, 1.00 mmol). To the resulting solution was added a methanol solution (3 mL) of benzilic acid (0.228 g, 1.00 mmol) and triethylamine (0.101 g, 1.00 mmol). The solution turned to green and was refluxed for 3 h to yield green solid. The solid was isolated by filtration and air-dried. The green filtrate was kept at room temperature to give green block crystals in good quality for X-ray crystallography. Yield: 70％ (0.329 g) based on Cu. Elemental Anal. Calcd. for C32H33N6O8S4ClCu2: C, 41.76; H, 3.61; N, 9.13; S,13.93. Found: C, 41.83; H, 3.80; N, 9.02; S, 13.72％. Selected IR bands (KBr pellet, cm−1): 3467(w), 3063(w), 1590(s), 1562(m), 1486(w), 1431(s), 1418(s), 1402(s), 1371(m), 1152(m), 1099(s), 1039(s), 948(w), 767(w), 743(w), 702(w), 624(w).

Preparation of [Cu2(acpy-phtsc)2(HBA)]·ClO4 (2). A methanol solution (20 mL) of 2-acetylpyridine (0.121 g, 1.00 mmol), 4-phenyl-3-thiosemicarbazide (0.167 g, 1.00 mmol), and a drop of conc-HCl was heated to reflux for 2 h. To the cooled solution Cu(ClO4)2·6H2O (0.371 g, 1.00 mmol) was added with stirring. To the resulting solution was added a methanolic solution (3 mL) of benzilic acid (0.228 g, 1.00 mmol) and triethylamine (0.101 g, 1.00 mmol). The solution turned immediately to green and was stirred further for 3 h to yield a green solid. The solid was isolated by filtration and air dried. The filtrate was kept at room temperature for a week to isolate green single crystals for x-ray crystallography. Yield 58％ (0.273 g) based on Cu. Elemental Anal. Calcd. for C42H37N8O7S2ClCu2: C, 50.83; H, 3.76; N, 11.29; S, 6.46. Found: C, 51.09; H, 3.80; N, 11.31; S, 6.57％. Selected IR bands (KBr pellet, cm−1): 3315(m), 3056(w), 1598(m), 1575(m), 1563(m), 1541(m), 1498(s), 1457(s), 1433(vs), 1392(m), 1157(m), 1110(sh), 1063(s), 769(m), 748(s), 702(m), 690(m), 623(w).

X-ray Structure Determination. Single crystals of 1 and 2 were obtained by the method described in the above procedures. Structural measurement for the complexes were performed on a Bruker SMART APEX CCD diffractometer using graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at the Korea Basic Science Institute. The struc-tures were solved by direct method and refined on F2 by full-matrix least-squares procedures using the SHELXTL pro-grams.13 All non-hydrogen atoms were refined using aniso-tropic thermal parameters. The hydrogen atoms were includ-ed in the structure factor calculation at idealized positions by using a riding model, but not refined. Images were created with the DIAMOND program.14 The crystallographic data for complexes 1 and 2 are listed in Table 1.

Table 1.Crystal Data and Structure Refinement for Complexes 1 and 2

 

Results and Discussion

The complexes of 1 and 2 were prepared from the meth-anolic solution of Cu(ClO4)2·6H2O, Schiff base ligand, and benzilic acid. The molecular structures of ligands used are shown in Scheme 1. Our first aim in this work was to obtain the coordination polymers which metal centers are bridged by the benzilic acid as spacer. Unfortunately, attempts to obtain the high dimension network of the complexes by varying stoichiometry, metal salts, and other reaction para-meters proved to be generally unsuccessful.

Scheme 1.Ligands used.

Table 2.Selected Bond Lengths (Å) and Angles ( ° ) for Complexes 1 and 2

Figure 1.(a) Molecular structure of complex 1 with atomic labeling. (b) The 2D layer framework of complex 1 formed by H-bond. All H-atoms in (b) have been omitted for clarity.

Description of the Structures. The molecular structures of 1 and 2 were determined using single crystal X-ray diffraction techniques. Selected bond parameters are listed in Table 2. The molecular structure of complex 1 contains dinuclear [Cu2(acpy-mdtc)2(HBA)(ClO4)] in which two thiolate sulfur atoms and bidentate bridging HBA− anion bridge the two copper(II) and two hemihydrate molecule centers (Figure 1(a)). Each of the two copper atoms in [Cu2(acpy-mdtc)2(HBA)(ClO4)] has different coordination environments. Cu1 adopts a five-coordinate square-pyrami-dal (τ = 0.145, the geometric parameter τ = |β − α|/60, where β and α are the two largest angles around the central atom; τ = 0 and 1 for the perfect square pyramidal and trigonal bipyramidal geometries, respectively.15) with a N2OS2 donor. The pyridine nitrogen (N4), the azomethine nitrogen atom (N5) and the thiolate sulfur atom (S3) together with the carboxyl oxygen atom (O1) from HBA− ligand comprise the basal plane of the square-pyramid whereas the thiolate sulfur atom (S1) of another ligand occupies the apical position. The maximum displacement of them from the coordination plane is −0.059(5) Å (N4). Cu1 atom displaces 0.157(1) Å out of the plane. The behavior of acpy-mdtc− results in the formation of two five-membered rings around Cu1 atom. Two planes [Cu1–N4–C14–C15–N5 and Cu1–N5–N6–C17–S3] are nearly planar with mean deviation of 0.045(6) and 0.058(4) Å, respectively, the dihedral angle between them being 8.043(1)°.

The environment around Cu2 atom can be best described as a distorted octahedral geometry in a N2O2S2 manner. One thiolate sulfur atom (S1), one azomethine nitrogen atom (N2) and one pyridine nitrogen atom (N1) from one acpy-mdtc− ligand and one oxygen atom (O2) from HBA− ligand occupy the basal positions, the two remaining positions in the octahedral geometry are the axial ones which are occupied by one thiolate sulfur atom (S3) from the another ligand and one perchlorate oxygen atom (O5). The bridging thiolate sulfur atoms (S1 or S3) connect the two copper atom centers. The large difference between the two Cu–S distances (Cu1–S3 2.273(2)/Cu2-S1 2.257(2) Å) in the basal plane and Cu1–S1 2.797(2)/Cu2-S3 3.036(2) Å in the apical position) can be ascribed to a Jahn-Teller distortion.16 Similar thiolate sulfur-bridging has also been observed in the complexes [Cu(apsme)(NCS)]2 (apsme = 2-acetylpyrazine S-methyldithiocarbazate)17 and in [Cu(fpytsc)X]2 (fpytsc = anionic form of 2-pyridinecarbaldehydethiosemicarbazone; X = Cl, Br).18

The C–S bond length in acpy-mdtc− increases from the typical of thione linkage 1.671(4) Å7 to C8−S1 1.750(6) and C17−S3 1.757(6) Å, respectively. Similarly, C8–N3/C17-N6 suffer a significant decrease from the normal single bond of 1.52 Å19 to 1.298(8)/1.311(8) Å, respectively. A comparison of the N2–N3/N5–N6 distance [1.389(7)/1.374(7) Å] with that in S-methyldithiocarbazate20 shows that the bond is shorter than a single N–N bond (1.44 Å) indicating that a significant π-charge delocalization occurs along the C–N–N–C moiety. These changes indicate the ligand in the present complex is coordinated in its deprotonated thiolate form as observed in most complexes derived from carbamate/semi-carbazone.21 Also, the two acpy-mdtc− ligands have slightly different Cu–N(pyridine) bond distances and they are longer than the Cu–N(azomethine) distances, this may be attributed to the fact that the azomethine nitrogen is a stronger base compared with the pyridine nitrogen.622

The HBA− ligand together with each thiolate sulfur atom bridges two Cu(II) centers. The Cu···Cu distance is 3.265(1) Å which is smaller than 3.453 Å23 of dimeric Cu(II) complex with only the μ-thiolate bridge of the thiosemicarbazone ligand. The Cu2–O5 bond length (2.743(8) Å) is significant-ly longer than the distance of Cu2−O2 1.943(4) Å in the HBA−, denoting the strength of the perchlorate oxygen coordination.

Two planes [C21–C26 and C27–C32] in HBA− are nearly planar with mean deviation of 0.003(7) and 0.005(8) Å, respectively, the dihedral angle between them being 87.3(2). The Cu1-S1-Cu2/Cu1-S3-Cu2 bridging angle is 79.71(5)/74.34(5)°, respectively, which is much shorter than the value of 87.22° reported for the complex having the μ-phenolato and μ-acetate bridging groups as the smallest value found in this series of complexes.24

The dinuclear units are further stabilized and consolidated into a three-dimensional network by the intra- and inter-molecular hydrogen bonds (Table 3). The perchlorate ion coordinated to the Cu(II) ion links the adjacent dinuclear unit by H-bonding (C29–H29···O6) through HBA− ligand to give 1D chain network along a-axis. The chains constructs 2D framework by the H-bonds (C12-H12···O4) between perchlorate oxygen atoms and adjacent pyridine rings (Figure 1(b), and finally, 3D network is accomplished by H-bonds between pyridine ring and water molecule (C11-H11···O8), between perchlorate and water (O6···O9 3.43(3) Å), and between water molecules (O8···O9 2.69(3) Å).

Table 3.Symmetry codes: i) 1−x, 1−y, 2−z; ii) 1−x, 1−y, 1−z; iii) 0.5−x, 0.5−y, 1− z; iv) 2−x, 1−y, 1−z; v) 1+x, 1−y, −0.5+z.

In contrast to the complex 1, in complex 2 two Cu(II) ions all have a five-coordinate square-pyramidal (τ = 0.23 for Cu1 and 0.16 for Cu2) with a N2OS2 donor (Figure 2(a)). The perchlorate ion is not coordinated to the Cu(II) ion. The shortest distance between Cu(II) ion and perchlorate oxygen atoms is 4.750(5) Å, compared to complex 1 (Cu2-O5 2.743(8) Å). There is no other significant structural differences between complexes1 and 2, except for the acpyphtsc − ligand coordinated to Cu(II) ion. As the complex 1, acpy-phtsc− ligand acts as thiolate form. Complex 2 is also further stabilized through intra- and inter-molecular H-bonds (Table 3) and it constructs 1D chains along c-axis by Hbonds between perchlorate oxygen atoms and pyridine ring (C4-H4⋯O4, C15-H15⋯O7, C16-H16⋯O5) or hydroxyl oxygen atom from HBA− (O3-H3⋯O7) (Figure 2(b)). The 1D chains form double chain by H-bond between perchlorate oxygen and phenyl amine nitrogen atom (N8-H8⋯O6). The double chains are also interlinked through H-bonds (N4- H4A⋯O5 and C4-H4⋯O4) between perchlorate oxygen atom and phenyl amine nitrogen or pyridine rings to form 2D network along a-axis (Figure 2(c)).

Figure 2.(a) Molecular structure of complex 2 with atomic labeling. (b) 1D-chain networks of cmplex 2 along with c-axis. (c) 2D layer in complex 2. All H atoms in (b) and (c) have been omitted for clarity.

The IR spectra of the free ligands have prominent bands appearing at ca. 3232, 1622, and 1059 cm−1 due to ν(N-H), ν(C=N), and ν(C=S) for Schiff base ligands,25 and the bands at ca. 1700, 1347, 1246, 1173, and 1051 cm−1 due to ν(C=O), δ(O-H)COH, δ(O-H)COOH, ν(C-O)COH, ν(C-O)COOH stretching modes for H2BA, respectively.26 On complexation these all bands for the Schiff base, except the ν(N-H) band at 3315 cm−1 for phenyl amine of complex 2 disappeared and the ν(C=N) bands shifted to 1590 and 1598 cm−1, respective-ly. These results indicate NNS coordination mode of the ligand in the thiol form. In addition, for all complexes the C=O stretching and O-H bending of the carboxylic acid group for the H2BA disappeared and the O-H bendings of COH (1371 and 1392 cm−1 for 1 and 2, respectively) and the C–O stretching bands (1152, 1039 for 1, and 1157, 1110 cm−1 for 2) of the COH and COO− are observed, respectively, supporting that HBA− is coordinated to Cu(II) through carboxyl group. The IR spectra also show the bands at 3461 cm−1 (for 1) corresponding to the vibration absorption of lattice water.27 The intense bands at 1099 (for 1) and 1063 cm−1 (for 2) can be assigned to the ClO4 group.6,28

The TG-DTA analysis was performed to examine the stability and decomposition pattern of compounds. Their thermal decomposition behaviors are similar to each other (Figure 3). For 1, the weight loss of 2.05 ％ from 37 to 70 °C displays the release of 1 mole of lattice water molecule per formula unit (calc. 1.96 ％). The release of lattice water was accompanied by endothermic effect on the DTA curve observed at 65 °C. The solvent-free species, in general, decompose in two steps. The first weight loss (25.74 ％) takes place in the narrow temperature range of 190-220 °C with an exothermic peak at 214 °C, followed by the weight loss of 54.66 ％ in a broader temperature range (220-800 °C). The total weight loss of 80.4 ％ corresponds to the loss of all organic compo-nents and perchlorate ion (calc. 84.24 ％). However, the observed weight loss is less than the calculated value. This indicates the decomposition ends above 800 °C. Complex 2 is stable up to 220 °C and then it undergoes a rapid weight loss of 30.95 ％in the temperature range of 220-252 °C with an exothermic peak at 246 °C. The second gradual weight loss of 54.74 ％take place up to 782 °C. This weight loss was accompanied by exothermic effect on the DTA curve with maximun at 456 and 566 °C, respectively. The total weight loss of 85.69 ％ corresponds to the loss of all organic compo-nents and perchlorate anion (calc. 87.2％). The remaining residue of 14.7 ％presumably corresponds to the formation of CuO (calc. 16.03％).

Figure 3.Thermogravimetric curves of 1 (a) and 2 (b).

 

Conclusion

Two new dinuclear copperl(II) complexes, [Cu2(acpy-mdtc)2(HBA)(ClO4)](H2O) (1) and [Cu2(acpy-phTsc)2(HBA)(ClO4)] (2) have been synthesized and characterized by elemental analysis, infrared spectroscopy, thermogravimetric analysis, and single crystal X-ray diffraction. In each com-plex, two copper(II) ions are bridged by HBA− anion together with thiolate sulfur atom from Schiff base ligand. Also, the structures are further extended into supramolecular frame-work by hydrogen bonds. It is noteworthy that two copper(II) centers are connected through benzilic acid. To our know-ledge, these structures are not common.