Abstract
Monomeric five coordinated adducts of bis(O-ethyldithiocarbonato)copper(II) of general formula [Cu(C2H5OCS2)2(L)], [L = 2-, 3-, 4-methylpyridines and 2-, 3-, 4-ethylpyridines] have been synthesized and characterized by elemental analysis, i.r. and electronic spectroscopy, magnetic and conductivity measurements. Analytical results show that the adducts have 1 : 1 stoichiometry. The adducts were found to be paramagnetic and their magnetic moments at room temperature lie within the 1.81–1.94 B.M. range and this indicates the presence of one unpaired electron. All the adducts have distorted square pyramidal geometry.
1. Introduction
O-Alkyldithiocarbonates, popularly known as xanthates derivatives, have found a wide pra ctical application as addition to lubricating oils, antioxidants for polyolefins, and so forth. However, alkylxanthates are mainly used in flotation concentration of nonferrous metal sulfide ores. These are extensively used as pharmaceuticals, fungicides, pesticides, and quite recently in therapy for HIV infections [1–3]. Divalent transition metals xanthates are partly unsaturated and can therefore form 1 : 1 adducts with electron donors such as neutral nitrogen, oxygen, phosphorus, or sulfur donors in which the coordination geometry ranges from square pyramidal to trigonal bipyramidal [4]. Such adducts of many transition metals such as Ni, Zn, Mn, V, Pt, and Pd are well studied, but little attention has been paid to the xanthates of copper owing to their instability. Here we report synthesis and investigation of the 1 : 1 adducts of some adducts of copper(II)xanthates with substituted pyridines such as 2-, 3-, 4-methylpyridines and 2-, 3-, 4-ethylpyridines.
2. Experimental
2.1. Preparation of Potassium Salt of O-ethyldithiocarbonate
Potassium salt of ethyl xanthate was prepared by the method reported in literature [5]. Into a 500 mL round bottomed flask, fitted with a reflux condenser, was placed 0.75 mole of potassium hydroxide pellets and 120 g of ethanol. The reaction mixture was heated under reflux for 1 hour. The mixture was then cooled and liquid from the residual solid was decanted off into another dry 500 mL flask. To this flask was added 45 mL (0.75 mole) of carbon disulfide slowly with constant shaking. The residual solid yellow mass was filtered (after cooling in ice) on a sintered glass funnel at the pump. It was washed with three 25 mL portions of ether. The resulting potassium salt of ethyl xanthate was dried in a vacuum desiccator over anhydrous calcium chloride. The yield was about 75 g. It was then recrystallized from ether.
2.2. Preparation of Adducts of Bis(O-ethyldithiocarbonato)copper(II) with Substituted Pyridines
The yellow coloured parent compound copper(II)xanthate could not be isolated because it changed rapidly into brown coloured copper(I)xanthate. The addition complexes were prepared by the direct reaction of solution of metal salt, solution of potassium salt of ethylxanthate, and substituted pyridine. About 20 mL solution of cupric chloride dihydrate (0.0025 mole) in acetone was prepared. To this solution, a solution of potassium salt of ethyl xanthate (0.005 mole) and substituted pyridine (0.0025 mole) [2-methylpyridine = 0.235 g, 3-methylpyridine = 0.235 g, 4-methylpyridine = 0.235 g, 2-ethylpyridine = 0.27 g, and 3-ethylpyridine = 0.27 g, 4-ethylpyridine = 0.27 g] prepared in 50 mL of acetone was added slowly with constant stirring. Dark green coloured precipitates so obtained were filtered, washed with acetone, and dried over anhydrous calcium chloride in a vacuum desiccator at room temperature. A scheme of the proposed structure is shown in Scheme 1.
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3. Results and Discussion
The addition complexes of bis(O-ethyldithiocarbonato)copper(II) are microcrystalline solids which are light green in colour. These are insoluble in common organic solvents such as ethanol, acetone, and benzene. However, these are soluble in dimethylformamide and dimethylsulfoxide. On the basis of elemental analysis (Table 1) they have been assigned the general formula Cu(S2COC2H5)2L [L = 2-, 3-, 4-methyl and 2-, 3-, 4-ethylpyridines]. Conductance values of these complexes in DMF fall in the range of 3.50–5.00 ohm−1 mole−1 cm2 (Table 1). These values are lower than the values expected for any uni-univalent electrolytes in this solvent suggesting that these complexes are neutral and nonionic in character [6, 7]. The magnetic moments of 1 : 1 complexes of bis(O-ethyldithiocarbonato)copper(II) with substituted pyridines fall in the range 1.81–1.94 B.M (Table 1) which is in agreement with magnetic moment values observed for distorted square pyramidal complexes of copper(II) [8–10].
Free dithiocarbonate shows vibrations due to C–O–C stretching in the range of 1270–1280 cm−1 and the C–S stretching vibration appears as two bands in the range of 1057–1070 cm−1. In the present work, a single sharp band of high intensity has been observed in all the complexes in the range of 1011–1035 cm−1 (Table 2). The appearance of the only C–S band suggests symmetrical bidentate binding of the dithiocarbonate moiety. There is a positive shift of 10–30 cm−1 in comparison to the free ligands. This indicates that the dithiocarbonate ligand coordinates with the metal through sulfur atoms. Moreover, the complexes show a band in the region 1190–1220 cm−1 which is attributed to (C–O) of dithiocarbonate moiety. In these complexes C–H out of plane bands occur at lower energy positions on complexation, indicating red shifts, which confirm that these ligands interact with the metal ion through their respective ring nitrogen atom. A new band of medium to strong intensity, observed in the range of 320–352 cm−1, may be attributed to Cu–S stretching mode [11]. The electronic spectra of 1 : 1 addition complexes of bis(O-ethyldithiocarbonato)copper(II) have been recorded in DMF. The copper(II) five coordinate complexes are expected to show a strong broad band appearing in the region 12000–17500 cm−1 (Table 2). The band maximum appears around 16000 cm−1 and is assigned to transition. A weak shoulder is associated with it due to transition. Third possible transition is orbitally forbidden and merges with the broad band. The relative energy order of these transitions depends upon the extent of axial ligand-metal interaction.
The complexes under observation show an intense band in the range of 15500–17500 cm−1 which is attributed to d-d transitions as observed in most of the copper complexes (). This main absorption band around 16000 cm−1 can be assigned to (Table 2). The appearance of one band with the intensity pattern suggests square pyramidal geometry around copper(II) ion in these 1 : 1 adducts [12] Figure 1.
(a)
(b)
4. Conclusion
The results obtained from various physicochemical and spectral techniques suggest that the adducts of bis(O-ethyldithiocarbonato)copper(II) with substituted pyridines have 1 : 1 stoichiometry with square pyramidal geometry around copper(II) ion.