Characterization of two new zinc(II) complexes with saccharinate and imidazole or benzimidazole as ligands

The crystal structure of the complexes [Zn(sac) 2 (im) 2 ] ( 1 ) and [Zn(sac) 2 (bzim) 2 ] 2 · 2Et-OH · H 2 O ( 2 ) (sac = saccharinate anion; im = imidazole; bzim = benzimidazole; EtOH = ethanol) was determined by single crystal X-ray diffractometry. Complex 1 crystallizes in the monoclinic P 2 1 = n space group with a = 9 : 1585(5), b = 16 : 4409(6), c = 15 : 0249(5) ˚A, ﬂ = 94 : 079(1) ◦ , and Z = 4, whereas complex 2 belongs to the triclinic space group P 1 with a = 10 : 8500(2), b = 12 : 4860(2), c = 13 : 5640(3) ˚A, ﬁ = 115,696(1), ﬂ = 100 : 086(1), (cid:176) = 102 : 169(1) ◦ , and Z = 1. In both complexes, the Zn(II) cations are in a slightly distorted tetrahedral ZnN 4 environment, coordinated to two saccharinate anions and to two imidazole ( 1 ) and to two benzimidazole ( 2 ) molecules. The complexes were also characterized by means of infrared spectroscopy and their thermal behavior investigated by means of thermogravimetric

As a continuation of our own research on this type of coordination compounds [10][11][12][13][14] we have now prepared and thoroughly characterized two new tetrahedral zinc(II) complexes containing the saccharinate anion together with imidazole or benzimidazole as the second ligand.

Synthesis of the complexes
The starting material for the synthesis was tetraquabis(saccharinato)-zinc(II) dihydrate, obtained according to literature procedures. 15,16or the preparation of [Zn(sac) 2 (im) 2 ] (1), 0.537 g (1 mmol) of this precursor was dissolved in 10 mL of distilled water and this solution was added dropwise to 10 mL of an aqueous solution containing 0.136 g (2 mmol) of imidazole.The reaction mixture was stirred for 1 h and, after filtration, evaporated slowly at room temperature.After a few days, colorless crystals were collected by filtration, washed with small portions of cold distilled water, and left to dry at room temperature.The other complex, [Zn(sac) 2 (bzim) 2 ] 2 •2EtOH•H 2 O (2), was prepared by same procedure, but working in ethanolic instead of in aqueous solutions, and employing 0.236 g (2 mmol) of benzimidazole.The obtained crystals were only stable in the mother liquor, becoming unstable in air because of the loss of the ethanol molecules.

Instrumentation
The IR spectra were recorded on a Perkin Elmer 580 B spectrophotometer using the KBr pellet technique.
Thermogravimetric (TG) and differential thermal analysis (DTA) were performed on a Shimadzu thermoanalytical system (models TG 50 and DTA 50, respectively), between room temperature and 800 • C, working in Pt-crucibles, at a heating rate of 10 • C•min −1 and under a constant oxygen flow of 50 mL min −1 .Alumina was used as DTA standard.

X-ray structure determinations
Relevant crystallographic data and details of the refinement are presented in Table 1.All hydrogen atoms of [Zn(sac) 2 (im) 2 ] (1) were found in a difference Fourier map.However, they were positioned stereochemically and refined with the riding model.Most H-atoms of [Zn(sac) , including the ones of the water molecule, were also found in a difference Fourier map.The saccharinato and benzimidazole H-atoms were positioned and refined as described for (1).The water hydrogen atoms were refined with O---H distances restrained to a target value of 0.82(2) Å.The ethanol hydroxyl hydrogen was positioned at a tetrahedral location and the corresponding torsion angle re-fined to maximize the observed electron density at the calculated position.The methyl hydrogen atoms were treated in the refinement as rigid bodies and allowed to rotate along the corresponding C---C bond such as to maximize the sum of the observed electron density at the three calculated H-positions.The structures were resolved by direct and Fourier methods and the final molecular models obtained by anisotropic full-matrix leastsquares refinement of the non-hydrogen atoms.Programs used were DENZO and SCALEPACK 17 for data reduction and corrections and SHELXS-97 18 and SHELXL-97 19 for structure solution and refinement, respectively.Intensity date were corrected for Lorentz, polarization, and experimental absorption effects. 20Unit cell dimensions were obtained by least-square refinement of the angular settings for 17287 (1) and 20714 (2) reflections in the 1.84 < θ < 25 • (1) and 1.75 < θ < 26.00 • (2) ranges.

Description of the structures
Figures 1 and 2 show the ORTEP 21 plots of both structures including the atom labeling of the non-hydrogen atoms and their vibrational ellipsoids at 30% probability.Intramolecular bond distances and angles around the Zn(II) centers are presented in Tables 2 and 3.
In [Zn(sac) 2 (bzim) 2 ] 2 •2EtOH•H 2 O (2) the two independent complexes can be related to each other through a noncrystallographic improper rotation.The homologous atom of all but one of the saccharinate ligands can be brought into a close superposition.The corresponding rms deviation from the best least-squares fit of the partial As expected, the molecular skeleton of the saccharinate ligands in both complexes are nearly planar (rms deviation of atoms from the mean plane less than 0.027 Å).The imidazole (in complex 1) and the benzimidazole (in complex 2) lig-ands are also planar within experimental accuracy.The metal cation departs from the ligand planes in less than 0.434 Å in complex 1 and 0.293 Å in complex 2.

Some of the most representative and characteristic vibrational modes of the two new
-The high-frequency region is relatively complicated because of the superimposition of ν(CH) and ν(NH) vibrations and bands originating from nonfundamental modes (cf.also 28 ).In the case of complex 2, the ν(OH) band of the water molecule could be identified.Bands related to the EtOH molecules could not be found.These molecules are probably lost during the preparation of the KBr pellets for measurement.-In both complexes, the saccharinate C ------O stretching mode is observed at higher frequencies than in the free saccharinate anion, in agreement with the fact that these bonds are slightly stronger in the complexes.-The characteristic SO 2 and ν s (C---N---S) vibrational modes remain practically unchanged after coordination.-The behavior of the 936 cm −1 ring mode of free imidazole (corresponding probably to the medium-intensity 933 cm −1 band in benzimidazole) which, as known, 29,30 is strongly dependent on the coordination geometry of the complexes, is very interesting.In the present cases, this band is displaced to higher energies after coordination and its intensity is strongly enhanced.Most of the other skeletal vibrations are scarcely affected by the coordination.-The ν(Zn---N) bands are located in the 350-280 cm −1 region, as expected by comparison with other similar systems. 13,25,31,32These stretchings lie at comparable energies in agreement with the not very different Zn---N bond distances in both complexes (cf.Tables 2 and 3).

Thermal behavior
Both complexes show a comparable thermal behavior as derived from the analysis of its TG and DTA data.[Zn(sac) 2 (im) 2 ] (1) presents a fourstep degradation pattern with steps between 20 and 320 • C, 320 and 460 • C, 460 and 550 • C, and 550 and 700 • C. The first three weight losses are accompanied by some weak DTA signals located at 287 • C (endo), 400 • C (exo), and 536 • C (exo), respectively.The final pyrolysis process is related to two very strong exothermic signals at 607 and 619 • C. Weight constancy is attained at around 700 • C and the final solid pyrolysis residue was ZnO, as confirmed by IR spectroscopy.The total experimental weight loss of 85.70% is in excellent agreement with the calculated one (85.60%).
In the case of [Zn(sac) 2 (bzim) 2 ] 2 •2Et-OH•H 2 O (2) it was observed that the weakly bonded ethanol molecules are lost during the crushing and weighing of the crystals.The TGA trace of this complex shows that in the temperature range between 20 and 180 • C only the water molecule is given off (experimentally found mass loss 1.50%; calculated value = 1.33%) and this process is accompanied by a weak endothermic DTA signal at 58 • C. The degradation of the anhydrous complex also occurs in four steps, as in the case of complex 1: between 180 and 380 Weight constancy is attained at around 650 • C and the final solid pyrolysis residue was also in this case, ZnO.The total experimental weight loss of 88.00% practically coincides with the calculated value (87.95%).

Fig. 1 .
Fig. 1.Plot of the structure of [Zn(sac) 2 (im) 2 ] displaying the non-H labeling and their displacement ellipsoids at 30% probability.Full lines indicate zinc-ligand bonds.The crystals are further stabilized by a net of intermolecular N---H• • •O bonds involving the imidazole (in 1) and benzimidazole (in 2) ligands of a complex and the saccharinate carbonyl and sulfonyl oxygen atoms of neighboring complexes.In complex 2, the EtOH and H 2 O molecules act as acceptors in N---H• • •O bonds and the water molecule is a donor in Ow---H• • •O bonds with a carbonyl and a sulfonyl oxygen atoms.

Table 4 .
Assignment of Some Characteristic IR Bands (in cm −1 ) of Sodium Saccharinate, Imidazole, Benzimidazole, and the Two Investigated Complexes