1. Introduction

Our research is based on the reactions of organic and inorganic Tin, Lead, and Mercury compounds with multi-dentate ligands. The removal of Tin, Lead, and Mercury compounds from the environment is considered a top priority for local, state, and federal governments as these metals are very toxic to biological life [1-5]. In this study, we investigate whether inorganic Tin and Lead Chlorides will bind with certain mono- and bidentate ligands. These reactions will serve as a starting point to investigate the possible use of these mono- and bidentate ligands as environmental remediation agents. Our research focuses on the preparation and use of air stable compounds. Most organotin and organolead compounds prepared using similar reagents tend to be air-sensitive [6]. The ligands used in this study are meta and para aminophenol (3-aminophenol and 4-aminophenol respectively), and p-toluidine (4-aminotoluene). The effect of differing mole ratios of ligand to metal was also investigated. It is proposed that the ligands used in this study bind to the metal via an elimination reaction rather than by adduct formation (Figures 1 and 2).

Figure 1. Schematic showing proposed reaction schemes between 3-aminophenol and Tin (IV) Chloride pentahydrate in a 1:1 mole ratio

Figure 2. Schematic showing proposed reaction schemes between 3-aminophenol and Tin (IV) Chloride pentahydrate in a 2:1 mole ratio

2. Procedure

For all experiments the same basic procedure was used. A specific example is shown below. For each mono- and bidentate ligand, experiments were carried out with a 1:1 mole ratio, and a 2:1 mole ratio between ligand and metal respectively. Each mole ratio reaction was carried out at room temperature.

Reaction of meta-aminophenol with Tin (IV) Chloride pentahydrate (2:1 mole ratio).

Meta-aminophenol (0.939 g, 0.0086 mol) was stirred into a mixture of isopropanol (8.0 mL) and ethanol (7.0 mL). To this, triethylamine (1.20 mL) was added. The resulting solution was stirred. In a separate beaker a solution of Tin (IV) Chloride pentahydrate (1.5 g, 0.0043mol) was dissolved in isopropyl alcohol (20.0 mL) and ethanol (10.0 mL). The solution containing the phenol was added to the solution containing the tin chloride. A white solution was formed. The resulting solution was stirred at room temperature for 24 hours. The white solution turned yellow. The solvent was removed to leave behind a yellow solid. A melting point determination, elemental analysis, and a FT-IR were carried out on the solid. The molar mass of the product was determined by using the Rast method [7]. Solubility tests were also performed on the yellow solid.

3. Results and Discussion

The IR spectra of the starting ligands and the products of their reactions with Tin (IV) chloride pentahydrate and Lead (II) chloride were recorded. Some important assignments are show in Table 3. A significant feature in the spectra of the products is the absence of broad v(OH) bands for the phenolic OH groups expected in the region 3000 - 3700 cm^-1, and the absence of the v(NH₂) bands for the aromatic amino groups expected in the region 3300-3400 cm^-1. This indicates oxygen and/or nitrogen bonding to the Tin moiety after replacement of the phenolic and/or amino hydrogen. In adduct formation; the phenolic OH group and/or the aromatic NH₂ group would still be present in the final product. Further evidence for bonding between the metals and the ligands is seen in the IR frequencies for the new Sn-O and Pb-O bonds. The Sn-O frequencies are observed in the range of 550-560 cm^-1 for compounds 1-4. The IR spectra also show that the metals also form a bond to the other binding site of the aminophenols, with the formation of Sn-N and Pb-N bonds. The Sn-N frequencies are observed in the range of 501-504 cm^-1 for compounds 1-6. The v(Sn-O) modes are reported to be at 613–515 cm^-1 for different organotin(IV) complexes of Schiff bases and at 500 ± 10 cm^-1 for some methyltin(IV) complexes [8-11]. Frequencies for v(Sn-N) have been reported at 450-500 cm^-1 for organotin salicylideneimine ligands [8-13].

The Pb-O frequencies are observed in the range of 480-483 cm^-1 for compounds 7-10. As with the tin complexes, the lead compounds also bind to the nitrogen of the ligands. The Pb-N frequencies are observed in the range of 395-403 cm^-1 for compounds 7-12. The v(Pb-O) modes have been reported for organolead salicylideneimine ligands at 450-531cm^-1, and v(Pb-N) at 305-350cm^-1 [14].

The IR spectra thus suggest that the metal coordinates to the ligand in the fashion shown in Figures 1 and 2.

Melting point determinations confirmed that the products of the reactions were different from the starting materials, demonstrating that a chemical reaction had taken place between the metal halides and ligands. After the solid reaction products had been purified, melting points were carried out on the stored solids several days apart. The melting points were similar over a period of 1-2 weeks (data not shown), indicating the products were air stable.

Molecular weight studies were carried out on the 1:1 and 2:1 molar ratio products to determine if the products were the monomer or dimer units. The Rast method for determining molecular weights was used, using camphor as the solvent. Table 2 shows that 1:1 mole ratio reactions produced monomer units, while 1:2 mole ratio (ligand:metal) produced dimer units (that is two ligands are bonded to the metal).

The values obtained from the elemental analysis of the products are in good agreement with the proposed formulae, and further serve to support the idea of complexation of the metal to the ligand (as opposed to adduct formation), plus the idea of dimer formation when a 2:1 mole ratio of ligand to metal is used.

Table 1. Melting/Freezing points of metal chlorides and ligands (from Aldrich Handbook of Fine Chemicals)

Table 2. Elemental Analysis, melting points (°C), and molar mass (g/mol) of products formed from the reaction of Tin (IV) chloride (Compounds 1 – 6) and Lead (II) chloride (Compounds 7 – 12) with ligands used in this study

Table 3. Infrared Spectroscopy frequencies [in cm-1] of Tin (IV) Chloride (1-6) and Lead (II) Chloride (7-12) compounds

4. Conclusions

Our studies show that inorganic Tin (IV) and Lead (II) Chlorides bind to the mono- and bi-dentate ligands used in this study by an elimination reaction pathway rather than by adduct formation. The presence of Sn-O and Sn-N bands in the IR, and the absence of OH and NH bands support this hypothesis. It has also been shown that varying the ligand to metal ratio has an effect on the structure of the product formed.

ACKNOWLEDGEMENTS

To the Department of Natural Sciences and LaGuardia Community College for providing research facilities.