Synthesis and characterization of oxovanadium (IV) dithiocarbamates with pyridine

ARTIGO

SYNTHESIS AND CHARACTERIZATION OF OXOVANADIUM (IV) DITHIOCARBAMATES WITH PYRIDINE

Antonio L. Doadrio ^* , José Sotelo and Ana Fernández-Ruano

Departamento de Química Inorgánica y Bioinorgánica, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain

*e-mail: [e-mail]

Recebido em 19/02/01; aceito em 12/12/01

SYNTHESIS AND CHARACTERIZATION OF OXOVANADIUM (IV) DITHIOCARBAMATES WITH PYRIDINE We report the synthesis and study of a new series of oxovanadium (IV) dithiocarbamate adducts and derivatives with pyridine and cyclohexyl, di-iso-butyl, di-n-propyl, anilin, morpholin, piperidin and di-iso-propyl amines. The complexes have been characterized by analytical, magnetochemical, IR, visible-UV spectral and thermal studies, and are assigned the formulas [VO(L)₂].py, where L=cyclohexyl, di-iso-butyl, di-n-propyl, anilin dithiocarbamate and [VO(OH)(L)(py)₂]OH.H₂O (L=morpholin, piperidin and di-iso-propyl dithiocarbamate).

The effect of the adduct formation on the p_V=0 bound is discussed in terms of the IR (V=O, V-S and V-N stretching frequencies) and electronic spectra (d-d transitions).

Keywords: vanadium(IV); dithiocarbamates; pyridine.

INTRODUCTION

In former investigations, we have synthesized and characterized five-coordinated oxovanadium(IV) complexes. Oxovanadium (IV) complexes with dithiocarbamates show a square pyramidal structure¹, which can react with Lewis bases to form mainly stable adducts, in which the base occupies the sixth coordination position in an octahedral complex², as in the oxovanadium (IV) xanthates³ and dithiocarboxylates^4,5. So, the adduct formula is [VO(L)₂].B (L=bidentate ligand, B=base).

In this paper, we report the synthesis and characterization of a new series of oxovanadium(IV) dithiocarbamates complexes with pyridine, which also form complexes of the general formula [VO(L)₂].py, noted as adducts, and alternating complexes of the general formula [VO(OH)(L)(py)₂]OH.H₂O, that are soluble in water and noted as derivatives.

We have studied the variations in the IR and electronic spectra caused by the addition of the organic base.

The study of these complexes by IR and electronic spectroscopy, magnetic susceptibilities, thermal methods and analytical data, shows that the adducts stoichiometry is 1:1 (base:complex) and in the derivatives 2:1 (base: metal) with lost of a ligand. From IR and electronic spectral data, we assign a six-coordinated distorted octahedral structure for the adducts [VO(L)₂].py, and possible six-coordinated structure for the derivatives [VO(OH)(L)(py)₂]OH.H₂O from the metallic center. We have also observed a relationship between the V=O, V-S stretching modes and the ²B₂®²B₁ , ²B₂®²E(I) transition frequencies.

EXPERIMENTAL PART

MATERIALS AND METHODS

Starting materials

VOSO₄.5H₂O and Pyridine were Merck commercial products and used as supplied. Cyclohexyl, di-iso-butyl, di-n-propyl, anilin, morpholin, piperidin and di-iso-propyl amines were Merck or Aldrich commercial products. The solvents used were Merck (analytical grade). Solutions for absorption spectra were prepared using pyridine. All these reagents were supplied by Micron Analítica S.A. (Madrid, Spain).

Analytical procedures

C, H and S elemental analyses were made on a Perkin-Elmer model 240B (Boston, Massachusetts, USA) analyzer and N elemental analysis using a Leco model SC32 analyzer (Leco Corporation, St Joseph, USA). Vanadium was determined by atomic absorption spectrophotometry after decomposition of the adduct by heating in a 1:1 concentrated HNO₃:H₂SO₄ mixture or dissolving the derivative in water. The melting points were determined on a melting point apparatus by using open capillary tubes and the conductivity measurements were made on a Metrohm Herisau E365B conductometer at room temperature.

Methods

Magnetic susceptibilities were measured by the Gouy method at room temperature on a Mettler H-51A.R. balance (Mettler Toledo, Greifensee Switzerland) using a Newport electromagnet, made from Oxford Instrument (Oxfordshire, UK). Molar susceptibilities were corrected for the diamagnetism of the constituent molecules.^{6, 7}

The IR spectra were recorded as KBr pellets on a Perkin Elmer recording spectrophotometer model 283.

The visible/near U.V. spectra of the complexes were determined in the range of 300¾900 nm on a Beckman DK 2A (Beckman Coulter, Inc, Fullerton, USA) recording spectrophotometer using solution of the complexes in pyridine (derivatives in water).

Thermograms were recorded on a Mettler HE20 thermobalance with Mettler DSC20 module and DSC were determined on a Mettler TA3000 system in static air with a heating rate of 10 ºC per minute.

Preparations

Oxovanadium(IV) dithiocarbamates complexes VO(RNCS₂)₂ were prepared by the same general method described in another paper.¹

{[C₆H₁₁HNCS₂ ]₂VO}.py: 1.5 g (3.03.10^-3 mol) of the complex previously obtained (oxovanadium(IV) cyclohexyl dithiocarbamate) was dissolved in pyridine (50mL) by continuous stirring for 3 hours at room temperature. After cooling for 24 hours, the adduct was separated by filtration in vacuo, washed repeatedly with cool water and dried over P₄O₁₀ in a nitrogen atmosphere. Yield: 50%.

Similar procedure was used for the other complexes with di-iso-butyl, di-n-propyl, anilin, morpholin, piperidin and di-iso-propyl dithiocarbamate as ligands. The yields were 50-70 %. (see Table 1).

RESULTS AND DISCUSSION

The adducts [VO(L)₂].py and the [VO(OH)(L)(py)₂]OH.H₂O derivatives were prepared by the direct reaction between the VO(RNCS₂)₂ complex and pyridine at room temperature. The source of the water and consequently the OH^- could be the solvent pyridine. Cyclohexyl, di-iso-butyl, di-n-propyl and anilin dithiocarbamate ligands are adducts, and morpholin, piperidin and di-iso-propyl dithiocarbamates are derivatives. The adducts are soluble in pyridine and DMSO, but less soluble in dichloromethane, benzene or hexane. The derivatives are soluble in water, methanol or ethanol, and insoluble in organic solvents.

The complexes obtained in this study gave analytical results which are concordant with the formulas assigned, as summarised in Table 1.

The molar conductivity values calculated from the conductivities measured on millimolar solutions of the derivatives complexes [VO(OH)(L)(py)₂]OH.H₂O, in water, support the electrolytic nature of the complexes.

All these complexes are paramagnetic, with values of the magnetic moments between 1.6 to 1.75 BM (see Table 1). These results show the existence of monomeric species of oxovanadium (IV).

The most relevant bands in the infrared spectra of the complexes are presented in Table 2. The IR spectra of the adducts [VO(L)₂].py, exhibit a very strong band at 990¾975 cm^-1 and the derivatives, [VO(OH)(L)(py)₂]OH.H₂O at 975- 970 cm^-1, which is attributed to the stretching vibration of the terminal V=O bond. If we compare the VO(RNCS₂)₂ complex with the adduct or derivative, we observed that these show the V=O band displaced to lower frequencies (990¾970 cm^-1) than the complex (1000-980 cm^-1). Similar results are obtained in other complexes of oxovanadium(IV) with dithiocarbamates⁸, dithiocarboxylates⁹, 8quinolinate¹⁰, benzoylacetonate¹¹ or dibenzoylmethanate¹² as ligands. This displacement can be attributed to the electronic donation of the base to the vanadium (N®V), which increases the electron density on the metal d-orbitals, and consequently the p_p®d_p donation from the oxygen atom to vanadium is expected to be reduced.

The IR spectra of the adducts and derivatives exhibit a medium band at 340¾325 cm^-1 which are assigned to the stretching vibration of the V-N_(base) bond.^{2, 10-12}This band is not present in the complex VO(RNCS₂)₂.

The IR spectra of all the complexes studied in this work show two bands at 440¾370 cm^-1 and 410¾340 cm^-1, which are assigned to the antisymmetrical and symmetrical vibrations respectively, of the stretching V-S_ligand. Both bands are displaced to greater frequencies than in the complex VO(RNCS₂)₂, according to the displacement to lower frequencies observed in the stretching vibration V=O bond.

One C-S stretching frequency (n(CSS), 1180¾1100 cm^¾1) is observed in the IR spectra of all the complexes. The presence of the an only C-S band can be due to the greatest contribution of the resonant form (RN⁺CS₂^-) in the adducts and derivatives.

The IR spectra of the derivatives [VO(OH)(L)(py)₂]OH.H₂O exhibit a band at 2480-2500 cm^-1, which is assigned to the stretching vibration of the NH⁺ bond, and a band at 750-740 cm^-1, which can be assign to the vibration of the VOH bond. These bands are not present in the adducts [VO(RNCS₂)₂].py nor in the VO(RNCS₂)₂ complexes.

Finally, one stretching frequency at 1520¾1420 cm^-1 is observed in the IR spectra of all the complexes, and can be assign to the stretching vibration of the C-N bond.

The electronic spectra of the adducts and derivatives, exhibit three bands (Table 3). The first band at 14000 cm^-1, can be attributed to a d-d transition ²B₂®²E(I) and it is displaced to lower frequencies than in the complex VO(RNCS₂)₂, according to the displacement to lower frequencies of the nV=O in the IR spectra, which is indicative that this d-d transition is very sensitive to the electronic O®V donation. The second band at 20230¾20400 cm^-1 can be assigned to a ²B₂®²B₁ transition. This band is displaced to higher frequencies in the adducts and derivatives, due to the introduction of the base into the sixth-coordination position, and is according to the displacement of the n(V-S) in the IR spectra. This d-d transition is sensitive to the V¾S bonding force variation. The third band about 24000 cm^-1 is attributed to a ²B₂®²A₁ transition.

However, the coordination of a sixth ligand as pyridine to the complex VO(RNCS₂)₂ apparently displaces the sulphur atoms towards the oxygen atom with concomitant reduction of the O-V-S angles, with destabilization of the b₁^* level relative to the e_p^* and renders the Balhausen-Gray ¹³ scheme applicable to the adducts and derivatives. In the VO(RNCS₂)₂ complexes, the three d-d transitions are assigned to ²B₂®²B₁ , ²B₂®²E(I) and ²B₂®²A₁ transitions respectively, according to the inverse energetic levels scheme proposed by Selbin¹⁴ for a square pyramidal structure.

The data on thermal decomposition of the adducts and derivatives complexes are given in Table 4. The thermograms of the derivatives complexes showed a first decomposition (endothermic) corresponding to loss of water molecule, a second decomposition (endothermic) corresponding to loss of 2 OH^- species, a thrid and fourth decomposition (endothermic) corresponding to loss of pyridine molecules and a formation of corresponding metal oxide (V₂O₅) at 430-500 ºC. On heating, the adducts showed a decomposition (exothermic) with metal oxide formation.

REFERENCES

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Publication Dates

• Publication in this collection
  26 Aug 2002
• Date of issue
  July 2002

History

• Received
  19 Feb 2001
• Accepted
  12 Dec 2001