KOOPMHHAUHOHHAa XHMHH, 2007, moM 33, № 4, c. 282-288
y^K 541.49
DIPYRIDINIUM TETRAISOTHIOCYANATODIOXOTUNGSTATE(VI)
AND RELATED COMPOUNDS
© 2007 M. S. Pathania, H. N. Sheikh, and B. L. Kalsotra
Departament of Chemistry, University of Jammu, 180006 India Received March 4, 2006
Eight tetraisothiocyanatodioxotungstate(VI) salts with general formula, (BH)2[WO2(SCN)4] (where B = pyridine, piperidine, quinoline, isoquinoline, 2-, 3-, and 4-picolines) are reported. These salts have been prepared by the reaction of sodium tungstate dihydrate with ammonium thiocyanate in the presence of HCl and isolated as pyridin-ium or related salts. The compounds have been characterized by elemental analysis, IR and electronic absorption spectra, conductance and magnetic susceptibility measurements, TGA/DTA, and molecular modeling studies.
INTRODUCTION
The chemistry of transition metal-oxocompounds is an area of particular interest with potential relevance to oxygen atom transfer processes in a wide range of catalytic reactions. Molybdenum-oxocompounds hold an important place in this field in systems going from met-alloenzymes to solid-state metal oxide surfaces [1-3]. Another important transition metal-oxo of group VI is tungsten. Several tungsten enzymes are known [4, 5], and many models have been synthesized and studied [6, 7]. Furthermore, tungsten-oxo- or peroxocomplexes are also known to be active in oxidation reactions especially in aqueous media [8].
Isothiocyanato complexes of dioxotungsten(VI) with morpholinomethyl urea and related ligands have been reported recently [9]. Arzoumanian et al. have synthesized and characterized the thiocyanato and chlorodioxotung-sten(VI) complexes and have also compared their oxygen atom transfer capability with molybdenum analogs [10]. The dioxotungstate anion [WO2(SCN)4]2- has been obtained by the procedure reported for a molybdenum analog and isolated as its tetraphenyl phosphonium salt, [PhtP]2[WO2(NCS)4]2-, by a metathetic cation exchange under phase-transfer conditions [11].
Anionic complexes containing pyridinium and related N-base cations are well known for transition metals and some rare earths. Pyridinium chlorochromate, (PyH)CrO3Cl, is long known as an efficient reagent for the oxidation of primary and secondary alcohols to car-bonyl compounds [12]. Salts of [Yb(oxalate)2]-, [HOCl5]2-, and [CeCl5]3- of pyridinium and related cations have been prepared and characterized [13, 14]. The reaction of uranyl chloride with hydrochlorides and hydrooxalates of N-containing heterocycles produces (PyH)2[UO2Cl4] and related compounds [15].
In the present paper, the tetraisothiocyanatodioxo-tungstate(VI) complexes with pyridinium and related cations are reported. The [WO2(NCS)4]2- anion was obtained by the interaction of Na2wO4 ■ 2H2O and
NH4NCS in an aqueous medium in the presence of HCl in situ and was isolated as its pyridinium or related salts by treating with corresponding bases with the general formula (BH)2[WO2(NCS)4], where B = pyridine (Py) (I), piperidine (Pipy) (II), quinoline (Qn) (III), isoquinoline (Iqn) (IV), morpholine (Morp) (V), 2-pi-coline (2-Pic) (VI), 3-picoline (3-Pic) (VII), and 4-pi-coline (4-Pic) (VIII).
EXPERIMENTAL
Morpholine (Fluka), piperidine (SDS), pyridine (SDS), quinoline (SDS), isoquinoline (SDS), and 2-, 3-, and 4-picolines (Merck) were purified by distillation after keeping these bases over potassium hydroxide overnight. Sodium tungstate dihydrate (Sisco-Chem-Indus-tries, Bombay) was used as supplied. The analysis of tungsten was carried out gravimetrically by a reported method [16]. Carbon, hydrogen, nitrogen, and sulfur analyses were performed by microanalytical methods. Molar conductivity in DMF (10-3 M) at room temperature was measured by an Elico conductivity bridge type CM82T having a conductivity cell with a cell constant of 0.74. IR spectra of the complexes over the region 4000-400 cm-1 were recorded on a FT-IR spectrophotometer Vector-22 using KBr disc. Electronic absorption spectra of the complexes were run in DMSO/DMF on a Double Beam UV-vis spectrophotometer Elico SL-164 in a range of 190-800 nm. The thermogravi-metric analysis (TGA/DTA) of the complexes were recorded on a DtG 60 thermoanalyser at a heating rate of 10 K/min. Molecular modeling studies were carried out using HyperChem Release 7.52 [17].
Synthesis of (BH)2[WO2(SCN)4], where B = pyridine, piperidine, quinoline, and isoquinoline. Sodium tungstate dihydrate (1.2 g, 0.00363 mol) and ammonium thiocyanate (2.9 g, 0.038 mol) were dissolved in water (30 ml) at room temperature, and 7.5 ml of 11 M HCl was added to it. The resulting yellow solution was cooled in an ice bath. To this solution base (0.00726 mmol): pyridine
(0.585 ml), piperidine (0.713 ml), quinoline (0.938 ml), and isoquinoline (0.938 ml), respectively, dropwise with constant stirring was added. A yellow solution was immediately formed in each case, which changed into an oily liquid with red color. The reaction mixture was heated in a water bath for about 1 h with constant stirring when the oily liquid separated as an orange red solid. The complex obtained in each case was washed with distilled water containing few drops of HCl, filtered, and dried in vacuo (yield: 70-80%).
Synthesis of (morpholinium)2[WO2(SCN)4]. Sodium tungstate dihydrate (1.2 g, 0.00363 mol) and ammonium thiocyanate (2.9 g, 0.038 mol) were dissolved in water (30 ml) at room temperature, and 7.5 ml of 11 M HCl was added to it. The resulting yellow solution was cooled in an ice bath. To this solution morpholine (0.00726 mol, 0.632 ml) dropwise with constant stirring was added. Initially no reaction occurred in cold but just on slight heating on a water bath, a yellow precipitate was obtained, which settled down. It was filtered, washed with distilled water containing few drops of HCl, and dried in vacuo (yield: 78%).
Synthesis of (BH)2[WO2(SCN)4], where B = 2-, 3-,
and 4-picolines. Sodium tungstate dihydrate (1.2 g, 0.00363 mol) and ammonium thiocyanate (2.9 g, 0.038 mol) were dissolved in water (30 ml) at room temperature, and 7.5 ml of 11 M HCl was added to it. The resulting yellow solution was cooled in an ice bath. To this solution base (0.00726 mol): 2-picoline (0.676 ml), 3-picoline (0.676 ml), and 4-picoline (0.676 ml), respectively, dropwise with constant stirring was added. In the case of 2-picoline, an orange red precipitate was formed immediately, which settled down. However, in the case of 3- and 4-picolines, an oily product was obtained, which has decanted and then extracted with eth-anol, when greenish yellow precipitates were formed. The precipitates formed in each case were filtered, washed with distilled water containing few drops of HCl, and dried in vacuo (yield: 75-80%). The analytical data of the tetraisothiocyanatodioxotungstate(VI) complexes are given in Table 1. The complexes were characterized by IR and UV-vis spectra, magnetic susceptibility measurements, conductivity measurements, TGA/DTA, and molecular modeling.
RESULTS AND DISCUSSION
All the complexes are mononuclear with general formulae, (BH)2[WO2(SCN)4], where B = Py, Pipy, Morp, Qn, Iqn, 2-Pic, 3-Pic, and 4-Pic. All the complexes are colored, stable in air, insoluble in common organic solvents but fairly soluble in DMF and DMSO.
The characteristics IR spectral bands of the complexes are presented in Table 2. The IR spectra of all the complexes exhibit bands in the region 860—880 and 935-958 cm1 due to the vas(O=W=O) and vs(O=W=O) modes, respectively. This observation indicates the presence of the cis-WO2 unit in these complexes [18,
19]. Bands for v(CN), v(CS), and 5(NCS) were observed in the spectra of the complexes at 2065-2070, 763-810, and 480-500 cm-1, respectively, suggesting that thiocyanate is N-bonded [20].
The presence of pyridinium, piperidinium, quinolin-ium, isoquinolinium, morpholinium, and picolinium cations in their respective compounds is indicated by various peaks observed in their spectra. The IR spectra of these cations result mainly from the same vibrational modes as observed for bases [21].
The IR spectrum of (PyH)2[WO2(NCS)4] shows C-H stretching frequencies at 3091 and 3066 cm-1, and the ring stretching vibrations are observed at 1606, 1535, 1485, and 1400 cm-1. The C-H in-plane bending at 1250 and 1060 cm-1, ring breathing modes at 1038 cm-1 and C-H out-of-plane deformation at 740 cm-1 are observed. The band at 3460-3100 and 1545 cm-1 can be assigned to v(N-H) stretching and bending vibrations, respectively.
The IR spectrum of (PipyH)2[WO2(NCS)4] shows bands at 3450 and 3340 cm-1 assigned to v(N-H) stretching vibrations of the (PipyH)+ cation. The v(C-H) stretching bands are observed from 3101 to 2985 cm-1. The v(C-N-C) stretching vibrations of the piperidine ring at 1150 and 1140 cm-1 have a negative shift of 15-20 cm-1 in the spectrum complex indicating the formation of the (PipyH)+ cation in the complex.
The IR spectra of quinolinium and isoquinolinium tet-raisothiocyanatodioxotungstate(VI) show C-H stretching frequency at 3101-2823 cm-1, the ring stretching vibrations are observed at 1643, 1600, 1560, and 1488 cm-1, the C-H in-plane deformations are found at 1303, 1280, and 1130 cm-1, and the presence of the N-H group is characterized by bands in the regions 3250-3170 and 12451240 cm-1, which can be assigned to v(N-H) symmetric stretching and bending vibrations.
In the IR spectrum of (MorpH)2[WO2(NCS)4], the bands at 3056 and 2855 cm-1 can be assigned to v(C-H) stretching vibrations, and bands at 1635, 1620, 1510, and 1440 cm-1 can be assigned to v(N-H) bending and v(C-N) stretching vibrations, indicating negative shifts of 10-20 cm-1 as compared to morpholine. The v(C-N-C) stretching mode at 1186 and 1160 cm-1 and v(C-O-C) vibrations at 1110 cm-1 observed for morpholine are also shifted to lower frequencies in the complex, which indicates the protonation and formation of the (MorpH)+ cation in the complex.
The complex
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