Novel Organotin ( IV ) Complexes , Derived from Chiral Benzimidazoles : Synthesis , Molecular Structure and Spectral Properties

The synthesis of homoleptic five-coordinate heteroaryl tin(IV) compounds using three chiral N-heterocyclic substituents, 2-(1-aminoethyl)benzimidazole (1), 2-(pyrrolidin-2-yl) benzimidazole (2) and 2-(1-amino-2-mercaptopropylyl)benzimidazole (3) ligands is described. It has been found that ligands 1 and 2 behave in the bidentate N,N donor fashion, however, ligand 3 behave in the bidentate N, S donor. Overall ligands furnished, with moderate yields, their corresponding complexes [(CH3)2Sn(C9H10N3)Cl] (4), [(CH3)2Sn(C9H12N3)Cl] (5) and [(CH3)2Sn(C9H10N3S)Cl] (6). The characterization of all compounds by various spectroscopic methods UV-Vis, FT-IR, NMR, RX and elemental analyses provided insights into the diversity of complex formation.

Structural studies have always been prominent in organotin chemistry, and particularly the structural changes which occur between the solution and solid states.The tin atom in the complexes can be penta-coordinated [12][13][14] or hexa-coordinated [15][16][17][18] and the complexes often show fluxional stereochemical behavior 16,19 .
On the other hand, benzimidazoles are known to exhibit a wide range of pharmacological properties including antiviral and antibacterial activities [20][21][22][23] .Another feature of the benzimidazole ligands is functionalization at the non-coordinated nitrogen of the benzimidazole moiety, which provides a mode for modification of their properties.
In addition, chiral organometallic complexes exhibit various pharmacological properties.The binding of chiral drugs to DNA usually shows stereoselectivity, which affect their pharmacokinetic profiles and biochemical activities [24][25][26] .
Organotin complexes (Me 2 SnABzCl) 4-6 were prepared by reaction of 2-Alkylaminobenzimidazole derivatives (ABzH) 1-3 with Me 2 SnCl 2 in aqueous alkaline medium at room temperature (Scheme 2).The composition of isolated complexes was confirmed by the findings obtained from elemental analyses.It has been found that in all AbzH 1-3, one chloride was substituted by monoanionic ligands and yielded their corresponding mononuclear complexes.Thus, ligands (1) and ( 2) behave in the Bidentate N, N donor fashion, however, ligand 3 behave in the bidentate N, S donor.This result may be explained by the considerable simultaneous acidic and nucleophile behave of sulfur in mercaptan group 29 (Table 1).Characterization of synthesized complexes and their nature of bonding were examined by spectroscopic investigations.

FT-IR
All ligands (ABzH) 1-3 display broad medium stretching vibrational band (n(N-H)) at 3450 cm -1 , which does not change on complexation 28 .However, as well used as hydrochloride benzimidazoles, the broad bands at 3150-2400 cm -1 corresponding to n(N+-H) stretching vibration disappear after reaction to create new bands at 500-600 cm -1 related to Sn-C stretching vibration.Thus, a medium or relatively weak band at 1625 cm -1 with a small shoulder at 1522 cm -1 due to stretching vibration of C=N , were considerably become larger and sharper (1633 and 1507 cm -1 ) confirming the complexation with tin atom in all complexes 4-6 28 .Moreover, the medium intensity band at 2525 cm −1 , assigned to the ν(S+−H) vibration in the free ligand 3, has not been observed in the spectra of the corresponding complex (6), while, a short band has been appeared in the 421 cm -1 range assigned to the ν(Sn+−S) (Fig. 1).The above discussed data are in agreement with the deprotonation of the thiolic group after coordination on the tin center 29 .

UV-Vis studies
The solutions of complexes 4-6 were prepared with DMSO at room temperature, and their concentration was 1×10 -5 mol/L.The spectrum showed the general appearance of absorption maxima for 2-alkylaminobenzimidazole moiety 30 (Figure .2).

NMR spectroscopy 1 H NMR spectroscopy
The  32 , and the noncoordination of -NH-group of tryptophan moiety on the tin atom 28 .In addition, the ligands ( 1) and ( 3) revealed NH 2 proton signal at 1.61-1.76ppm 33 which has disappeared in the complexes ( 4) and ( 5) respectively, indicating coordination of the -NH 2 group on the tin atom.Moreover, the ligand (2) showed NH (pyrrolidine group) proton signal at 2.16 34 which has disappeared in the complex (5)  suggesting probably coordination of -NH-moiety to the atom center.In complex ( 5), the 1 H NMR signals at 1.80-2.40,3.09-3.30,and 4.64 ppm indicates the presence of pyrrolidine group 34 .Furthermore in complex (6), cysteine moiety was confirmed by the presence of signals at 2.57 and 4.08 ppm 33 .Thus, the appearance of signals at 1.63-1.78indicates the presence of -NH 2 group which confirms coordination of the deprotonated thiolate group on the tin center instead of the latter amino group 29 .
119 Sn NMR spectroscopy To investigate the ligand-binding modes to the tin atom in solution 35 , 119 Sn NMR spectra was measured.It is generally recognized that 119 Sn chemical shifts are very sensitive either to the coordination modes and ligand types 36 .Thus, a range of d values from +200 to -60 have been reported for four-coordinated, -90 to -190 for five-coordinated and -210 to -400 for six-coordinated diorganotin(IV) complexes in solution 37 .The chemical shift values for the synthesized compounds 4-6 are -186, -204, and -207 ppm, respectively confirming the penta-coordination mode.

XRD analysis
The XRD powder diffraction spectra of synthesized complexes were recorded with a scan rate about 0.02 o min. - and range 2θ = 5-55 degree (Fig. 3).The registered spectra revealed peak with maxima at 2θ = 12.48-12.42,d = 7.23-7.05´, and FWHM = 0.2503-0.2435(Fig. 4).All data are summarized in Table 2.The obtained results demonstrate closer parameters regarding to the first (maxima) peaks of XRD spectra for all complexes.

Calorimetric measurements
The differential scanning calorimetry measurements of synthesized complexes were recorded on heating between 200 and 800 K with a scanning rate 5 K min. - (Fig. 6).The first thermogram showed the combustion of organic part of complex (4) which located at 592 K.The values of enthalpy and entropy were resulted from the peak area are as follow: ΔH = -774.8kJ/mol and ΔS = -1.5 kJ mol -1 K -1 .
The second thermogram revealed the combustion of complex (5), which was achieved in three steps; at 473, 568, and 638 K, correspondently.The values of enthalpy and entropy were determinate from the peak area are as follow: ΔH = -668 kJ/mol and ΔS = -1.2kJ mol -1 K -1 .
The third thermogram displayed the combustion of complex (6), which was completed in three steps located at 555 K, 588 K, and 652 K, respectively.The values of enthalpy and entropy were obtained from the peak area are as follow: ΔH = -664 kJ/mol and ΔS = -1.3kJ mol -1 K -1 .

Apparatus and materials
The melting point of complexes was recorded by a differential scanning calorimetric instrument (DSC 131 evo-Setaram, France).The IR spectra were measured using a Nicolet iS5 Infrared Spectrometer in KBr (Thermo Scientific, USA).). 1 H-NMR, 13 C-NMR and 119 Sn spectra were recorded by Bruker Avance 300 spectrometer (Bruker, France) at 300 and 75 MHZ, respectively.Chemical shifts were reported in ppm from internal TMS for 1 H, 13 C and 119 Sn.Elemental analysis was obtained by means an Exeter Analytical CE-440 Elemental instrument (Exeter Analytical, UK).X-ray spectra were recorded by an Oxford X-calibur Gemini diffractometer (Santa Clara, California, USA) equipped with EOS CCD detector using Typical Procedure for the synthesis of complexes 4-6 ABzH 1-3 (5 mmol) were taken in 10 mL aqueous solution of NaOH (5 mmol) and stirred at room temperature for 20 minutes.After that, an aqueous solution of dimethyltin dichloride (5.5 mmol) was added dropwise with continuous stirring.After couple of minutes stirring, a pale yellow powder was precipitated, which was filtered out, washed simultaneously with water and ethanol and recrystallized from a mixture ethanol/DMSO (95/5).

CONCLUSION
Chiral penta-coordinate organotin(IV) complexes were successfully prepared and characterized using different analytical techniques.The obtained spectroscopic data were accordant with the proposed structures of products.The complexes (4) and ( 5) were resulted via N,N coordination to tin atom by their analogous ligands (1) and (2), respectively, whereas, the complex (6) was achieved by N,S chelation of tin center.The XRD spectra of compounds revealed closer parameters about the first (maxima) peak.DSC studies showed that the decomposition of the complexes proceeded via exothermic combustion.

Table 2 : First peak (maxima) parameters of XRD spectra
(*) d-spacing [Å] values were obtained by X'pert High Score software.