Synthesis and characterization of gallium (III) derivatives of sterically hindered heterocyclic

β-diketone

 

Jyoti Bhomia, Jyoti Sharma, Yashpal Singh^*

 

Department of Chemistry, University of Rajasthan, Jaipur 302004 INDIA.

Email: [email protected]

 

Abstract          Some new organic derivatives of heterocyclic β-diketone [4-chlorobenzoyl-​3-​methyl-​1-​phenyl-​2-​pyrazolin-​5-​one (CMPPOH)] of the type Ga(OPrⁱ)_3-n (CMPPO)_n (where n=1,2and 3) have been synthesized by the reactions of Ga(OPrⁱ)​₃ and heterocyclic β-diketone in different molar ratios. These derivatives were characterized by elemental analyses, mol. wt. measurements and their probable structures have been proposed on the basis of IR and ¹H NMR spectral studies.

Keywords- Elemental analyses, spectral studies, sterically hindered heterocyclic β-diketones

 

INTRODUCTION

4-Acyl-​3-​methyl-​1-​phenyl-​2-​pyrazolin-​5-​ones constitute an interesting class of ligands because of their pharmacological¹ and potential biological activities, such as, anti-inflammatory², antitumor³, anticonvulsant⁴, antimicrobial⁵, antiviral⁶, antihistaminic⁷ fungicidal activities⁸ and anti-HIV-1 activity.^9-11 Acyl pyrazolones are good chelating ligands.^12-13 In this paper we have report the synthesis of Ga (OPrⁱ)_3-n (OPPMC)_n ( n=1,2and 3) compounds. These compounds have been characterized by elemental analysis molecular weight measurements and their probable structures have been proposed on the basis of IR, and ¹H NMR spectral data.

 

MATERIALS AND METHODS

All the reactions were carried out under anhydrous conditions and the solvents were dried by standard procedure.¹⁴ Heterocyclic β-diketone ligands have been synthesized by literature method.¹⁵ Gallium isopropoxide has been prepared by literature method.¹⁶ Gallium was estimated as oxinate.¹⁷ Isopropyl alcohol in the azeotrope was estimated by oxidimetric method.^{18 1}HNMR spectra have been recorded on a JEOL 400 MHz spectrophotometer. ¹H NMR has been recorded in CDCl₃ using TMS as internal reference. IR spectra have been recorded on 8400 SHIMADZU FT-IR spectrophotometer as nujol mull on KBr cell in the range 4000-400 cm^-1.Carbon and hydrogen were analysed by a coleman-33 carbon hydrogen analyser. Molecular weights were determined ebulioscopically using Beckmann thermometer. Since similar methods have been used to synthesize these compounds, the preparative detail of one representative compound is given in detail. Synthetic and analytical data of the other analogous compounds are summarized in Table-1.

Synthesis of Ga(OPrⁱ)₂ (OPPMC)

CMPPOH (0.75 g; 8.27 m mol) was added to toluene solution (~50ml) of Ga(OPrⁱ )₃ (0.59m; 8.46 mol) and the solution was refluxed on a fractionating column for about 3 hours. The isopropanol in the reaction was fractionated out azeotropically with toluene. The product was soluble in toluene. After stripping off the solvent under reduced pressure, a light brown coloured solid was obtained in good yield which was purified by recrystallization from a mixture of toluene and hexane

RESULT AND DISCUSSION

The reactions of Ga(OPrⁱ )₃ with ( 4-chlorobenzoyl-​3-​methyl-​1-​phenyl-​2-​pyrazolin-​5-​one(CMPPOH) ligand was carried out in 1:1, 1:2 and 1:3 molar ratios in refluxing toluene.

 

Ga(OPrⁱ )₃ + n CMPPOH             Ga(OPrⁱ)_3-n (OPPMC)_n + n Prⁱ OH

 (Where n=1,2and 3)

The isopropanol liberated during the reaction was fractionated out azeotropically with toluene.. After the completion of the reaction the removal of solvent under reduced pressure yield coloured solid compounds. These derivatives are soluble in common organic solvents like chloroform, methanol, THF etc. and are purified by toluene and n-hexane mixture.

All these derivatives are light brown (1) or yellow (2-3) colored solids. The molecular weight determinations indicate that the compounds Ga(OPrⁱ) (OPPMC)₂ and(OPrⁱ)Ga(CMPPO)₂ are of dimeric nature whereas the compound Ga(CMPPO)₃ is monomeric.

I. R. Spectra

The appearance of a new band at 440, 610 and 700 cm^-1 (Table-2) due to ν Ga-O indicates, the deprotonation of OH group and the formation of gallium- oxygen bond The stretching bands due to C–O of the isopropoxy groups appears at 1180-1130 and 930 cm^-1. Another band due to ν C=O in the region 1490-1510 cm^-1 is shifted during complexation. The medium intense absorption bands observed in the spectra of the free ligand in the region 1570 and 1590 cm^-1 due to C=C and phenyl group remain unchanged in the spectra of gallium complex. This indicates that nitrogen atom of the heterocyclic ring does not take part in the complexation.

¹H NMR Spectra

The signal due to enolic proton of the free CMPPOH ligand at 12.3 ppm is absent in all these complexes indicating the deprotonation of this group and formation of Ga-O bond (Table-2). The signals due to ClC₆H₄ group protons attached with C = O group in ligand shows an upfield shift in the specra of complexes due to the involvement of ClC₆H₄ -C=O group in the bonding. A doublet appears at 1.20 ppm and a broad signal at 3.4-4.0 ppm in the spectrum of the derivative Ga(OPrⁱ) (OPPMC)₂ have been assigned to gem dimethyl and –CH protons respectively. Presence of one set of signals in this range in the compound Ga(OPrⁱ)(OPPMC)₂ for isopropoxy group indicates the presence of only bridging isopropoxy groups. Two sets of signals for the methyl (doublet) and methine (septet) signals for isopropoxy groups in the spectra of these derivatives Ga(OPrⁱ)₂ (OPPMC) appear in the range of 0.9 - 1.30 ppm and 3.6-4.3ppm respectively. Presence of two sets of signals in the spectra of this compound indicates the presence of terminal and bridging isopropoxy groups. Other proton signals are observed at their expected positions.

 

CONCLUSION

In view of the monomeric nature of tris derivative; Ga (CMPPO)₃ and monofunctional bidentate nature of ligand, the following structure (Fig-1) has been proposed-

 

Table 1: Synthetic and analytical data of Ga(OPrⁱ)_3-n (OPPMC)_n complexes

 

Table 2: I. R and ¹HNMR data of complexes Ga(OPrⁱ)_3-n (OPPMC)_n

 

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