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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.1 Concepción mar. 2003

http://dx.doi.org/10.4067/S0717-97072003000100001 

MIXED LIGAND COMPLEXES OF ALKALINE EARTH METALS. PART-X. Mg(II),
Ca(II), Sr(II) and Ba(II) COMPLEXES WITH
5-BROMOSALICYLALDEHYDE AND b-DIKETONES

R.N. PRASAD*, MITHLESH AGRAWAL, MADHULIKA SHARMA

Department of Chemistry, University of Rajasthan, Jaipur-302004, India
e-mail : prasadraghunandan@yahoo.com
(Received: August 27, 2001 - Accepted: July 23, 2002)

ABSTRACT

Magnesium(II), calcium(II), strontium(II) and barium(II) complexes of the type [MLL¢(H2O)2] (where HL = 5-bromosalicylaldehyde and HL¢ = pentane-2,4-dione, 1-phenylbutane-1,3-dione or 1,3-diphenylpropane-1,3-dione) have been synthesized by 1:1:1 molar reactions of metal chlorides with 5-bromosalicylaldehyde and b-diketones. The resulting complexes have been characterized by elemental analyses, TLC, IR and 1H NMR spectra.

INTRODUCTION

b-Diketonate complexes of alkaline earth metals of the type ML2 have been widely studied. Acetylacetonates, benzoylacetonates, dibenzoylmethanates and dipivaloylmethanates have been synthesized1-3 and electronic4 and NMR5,6 spectra of acetylacetonates and benzoylacetonates have been reported. The crystal and molecular structures of Mg(acac)2(H2O)27, Mg(dbzm)2(DMF)28, Ca(dbzm)2.1/2 EtOH8, Sr(dbzm)2.1/2 MeCO8, [Ca(acac)2(H2O)2] H2O9 and [Ca(CH3COO)(acac)(H2O)2]10 have been determined. Fenton et al.11,12 prepared complexes of the type ML2L¢ by the reactions of acetylacetonates and fluorinated acetylacetonates with various ligands. Crystal and molecular structures of M(dpm)2(phen)2 type complexes of Ca(II), Sr(II) and Ba(II) have been reported recently by Sobotera et al.13 Reports on mixed ligand complexes of alkaline earth metals of the type MLL¢(H2O)2 are very scanty except few publications from our laboratories.14-21 Mass spectral studies of such complexes have also been reported earlier from these laboratories.22-23 However, such mixed ligand complexes with substituted salicylaldehydes have not been studied so far. In the present paper mixed ligand complexes, MLL¢(H2O)2 [where M = Mg(II), Ca(II), Sr(II) and Ba(II); HL = 5-bromosalicylaldehyde and HL¢ = pentane-2,4-dione, 1-phenylbutane-1,3-dione or 1,3-diphenylpropane-1,3-dione] are reported.

EXPERIMENTAL

Materials
Pentane-2,4-dione (IDPL, India) was purified by distillation and 1-phenylbutane-1,3-dione (Fluka), 1,3-diphenylpropane-1,3-dione (S. D’s, India) and 5-bromosalicylaldehyde (Aldrich) were purified by recrystallization from ethanol. MgCl2.6H2O (Glaxo), CaCl2.2H2O (E. Merck), SrCl2.6H2O (Glaxo) and BaCl2.2H2O (Glaxo) used were of AR grade.

Synthesis of Mixed Ligand Complexes of Mg(II), Ca(II), Sr(II) and Ba(II)
4 m mol of metal chloride were dissolved in 15 cm3 of ethanol and a mixture of two carbonyl compounds (4 m mol each) in ethanol (15 cm3) was added to it. A clear solution was obtained and the temperature was maintained at 50-60°C. The pH of the reaction mixture was raised to ~6 to 6.5 by adding 5% sodium hydroxide solution dropwise with constant stirring. About 1 ml of 5% sodium hydroxide solution was required to attain the desired pH. The pH was measured with the help of a pH paper and stirring was continued for one and a half hour. The solid separated was filtered and dried in vacuo.

Analytical Methods
Magnesium, calcium and strontium were determined volumetrically by EDTA. Barium was determined gravimetrically. Carbon and hydrogen were analysed on a Coleman C, H Analyser Model-33. Specific conductances were measured at room temperature in methanol by a Systronics Direct Reading Conductivity Meter-304, using a glass cell having cell constant 1.0. Infrared spectra were recorded in the region 4000-200 cm-1 on a Nicolet Magna-550 FT IR Spectrophotometer. Spectra were recorded mostly as KBr pellets. 1H NMR spectra were recorded in DMSO-d6 on Jeol FX 90 Q FT NMR Spectrometer at 90 MHz using TMS as a reference.

RESULTS AND DISCUSSION

The reactions of the hydrated metal ions with 5-bromosalicylaldehyde and b-diketones such as pentane-2,4-dione, 1-phenylbutane-1,3-dione or 1,3-diphenylpropane-1,3-dione have been carried out in 1:1:1 molar ratios and can be represented as given in Scheme-I.

The resulting mixed ligand complexes have been isolated as yellow solids. These are quite stable and are not sensitive to exposure to atmosphere. However, on heating they decompose. The complexes are soluble in methanol and DMSO but insoluble in water, carbon tetrachloride and chloroform. Their characteristics, yields, analyses and temperatures of decomposition are recorded in Table-1. The conductances of the complexes are very low, indicating their non-electrolytic nature. Thus, the metal atom appears to be hexa-coordinated and the probable geometry may be octahedral. Octahedral geometry has been confirmed by X-ray crystal structure studies of bis(b-diketonato) complexes of alkaline earth metals viz. Mg(acac)2(H2O)27, [Ca(acac)2(H2O)2].H2O9, Mg(dbzm)2(DMF)28, Ca(dbzm)2(EtOH)1/28 and Sr(dbzm)2(Me2CO)1/28.


Thin Layer Chromatography
TLC of all the mixed-ligand complexes was performed on silica gel G using benzene-pet ether (1:1) as the solvent and retention times were compared with those of the corresponding bis-complexes. All the mixed ligand complexes show single spots with Rf values being the intermediate of the two corresponding symmetrical bis-complexes. This indicates that these are mixed complexes rather than a mixture of the two corresponding bis-complexes.

Infrared Spectra
IR spectra of all the mixed ligand complexes have been recorded and spectrum of one representative complex Mg(5-Brsal)(acac)(H2O)2 is given in Fig.1. In the spectra of mixed ligand complexs weak to medium intensity absorption bands in the region 400-500 cm-1, which are absent in free ligands, may be attributed to nM-O vibrations. Similar bands in this region have been assigned to nM-O vibrations by Nakamoto et al.24 in metal b-diketonates such as Al(acac)3, Al(bzac)3(490 cm-1), Al(dbzm)3(450 cm-1) and Cu(acac)2 (455 cm-1). The coordination of water molecules to the metal atom is confirmed by the appearance of weak to medium intensity absorption bands in the region 200-380 cm-1 which may be assigned to metal coordinated OH stretching.


Fig1. IR spectrum of Mag (5-brsal)(acac)(H2O)2

The bands in the region 1600-1680 cm-1 may be assigned to coordinated C=O groups and those in the region 1500-1580 cm-1 to C=C stretching modes. The C=O frequencies of the complexes are in the lower region as compared to those of the free ligands. The IR spectra of b-diketones and their metal complexes have been reported by Bellamy and Beecher25. For 2,4-pentanedione bands at 1724 cm-1 and 1608 cm-1 have been assigned to nC=O and nC-O respectively. In 1-phenyl-1,3-butanedione bands due to C=O and C-O have been reported at 1724 cm-1 and 1600 cm-1 respectively and in case of 1,3-diphenylpropane-1,3-dione C-O band appears at 1600 cm-1. In case of Cu(II) complexes, bands at 1580 cm-1, 1550 cm-1 and 1524 cm-1 have been assigned to nC=O frequencies and those at 1389 cm-1, 1389 cm-1 and 1391 cm-1 have been assigned to C-O frequencies of pentane-2,4-dione, 1-phenylbutane-1,3-dione and 1,3-diphenylpropane-1,3-dione ligand moieties respectively. However, Bellamy and Branch26 observed C=O absorption bands at 1524 cm-1 and 1552 cm-1 in Cu(II) complexes of 1-phenylbutane-1,3-dione and 1,3-diphenylpropane-1,3-dione respectively. In case of Mg(acac)2 Bellamy and Branch26 have observed nC=O absorption band at 1575 cm-1 whereas Holtzclaw and Collman27 have assigned the band at 1615 cm-1 to these vibrations. Fenton and Newman12 reported the nC=O frequency at 1660 cm-1 in Ca(hfac)2(tmed). Thus on coordination the nC=O frequency of b-diketone ligands is shifted to lower side, as has been observed in the mixed ligand complexes of Mg(II) synthesized during the present investigations.

The substitution of methyl group by a phenyl group affects the perturbed C=O bands. Holtzclaw and Collman27 suggested that a phenyl group attached to a carbonyl group in the ligand migtht set up an interfering conjugation through a quinoid like structure decreasing the double bond character of the adjacent carbonyl group. Thus the frequency of the perturbed C=O bond is lowered and the strength of metal-oxygen bond in Cu(II) complexes decreases. This was illustrated by the spectrum of Cu(bzac)2, which exhibited perturbed carbonyl band at 1561 cm-1 as compared to 1580 cm-1 for Cu(acac)2 which is known to possess a stronger Cu-O bond. Bellamy et al.26 reported similar absorption band at 1560 cm-1 in Be(II) complexes of acetylacetone.

Perturbation calculations for Cu(II) and Ni(II) complexes of dibenzoyl-methane have been carried out by Nakamoto et al.28 It was observed that the phenyl substitution slightly increased the nC=C and nM-O force constants and slightly decreased the nC=O force constant. Due to mesomeric interaction of the phenyl group with the quasiaromatic metal chelate ring, the resonance shifts of p-electrons to the chelate ring increase the negative charge on oxygen atom strengthening the s as well as to some extent the p character of the M-O bond. In case of magnesium which does not form p-bonds, a smaller shift of the M-O band to the higher region has been reported. In the IR spectra of the mixed ligand complexes Mg(II) synthesized during the present investigations similar shifts of nC=O frequency to lower side and of nM-O to higher side have been observed.

Nuclear Magnetic Resonance Spectra
1H NMR spectra of a few complexes were recorded in DMSO-d6 and spectrum of one representative complex Mg(5-Brsal)(bzac)(H2O)2 is given in Fig. 2. In free 5-bromosalicylaldehyde the aldehydic CH proton gives a signal at d 10.34 ppm which is shifted in the mixed ligand complexes Mg(OC6H3BrCHO)(CH3COCHCOCH3)(H2O)2 and Mg(OC6H3BrCHO)(CH3COCHCOC6H5)(H2O)2 and appears at d 9.64 and 9.51 ppm respectively. This confirms the coordination of the C=O group of the aldehyde to metal atom.


In free acetylacetone the resonance due to CH3 and CH protons have been reported at d 1.99 and 5.54 ppm respectively29. In the mixed ligand complex Mg(OC6H3BrCHO)(CH3COCHCOCH3)(H2O)2, singlets due to CH3 and CH protons are observed at d 1.78 and 5.15 ppm respectively. Upfield shifts of these protons confirm the coordination of the acetylacetone ligand to magnesium atom. In the NMR spectrum of Mg(acac)2 these peaks have been reported to shit upfield (CH3 1.70, CH 5.07 ppm) in comparison of the ligand5. Similar upfield shifts have been reported in the acetylacetonates of other metals also.

Appearance of a single methyl resonance of acetylacetone moiety in the mixed ligand complex confirms its octahedral structure in which two trans positions are occupied by two water molecules which might possibly be replaced by DMSO molecules when the complex is dissolved in this solvent. In cis conformation two methyl resonances would have been observed as reported by Smith and Wilkins in case of tin acetylacetonate complexes of the type Sn(acac)2L230.

Substitution of methyl group of acetylacetone by phenyl group causes downfield shift of CH proton. In free benzolyacetone this has been reported at d 6.18 ppm29. In the mixed ligand complex of benzolyacetone Mg(OC6H3BrCHO)(CH3COCHCOC6H5)(H2O)2 the singlet due to CH proton is observed at d 6.04 ppm. Thus the upfield shift of this is an evidence for the coordination of the metal atom to this ligand. Recca et al.6 have reported this CH peak at d 5.88 ppm in Mg(bzac)2 and in the similar region in other alkaline earth metal benzoylacetonates.

Appearance of the CH signal of the b-diketones in a region very close to that of the aromatic protons supports the pseudoaromatic character of the ring involving the metal atom and the b-diketone ligand.

In free benzoylacetone, CH3 protons have been reported to appear at d 2.19 ppm.29 In the mixed ligand complex Mg(OC6H3BrCHO)(CH3COCHCOC6H5)(H2O)2 these protons exhibit a singlet at d 1.91 ppm. This peak has been reported in similar region in Mg(bzac)2 (d 1.92 ppm) and in benzoylacetonates of other alkaline earth metals.6 Upfield shift of these CH3 protons also supports the coordination of magnesium atom to the ligand.

Aromatic protons of benzoylacetone moiety are observed in the lower field (d 7.9 ppm) as compared to those of 5-bromosalicylaldehyde moiety. The aromatic protons of 5-bromosalicylaldehyde moiety are more resolved and the splitting pattern can be easily interpreted. In the complexes Mg(OC6H3BrCHO)(CH3COCHCOCH3)(H2O)2 and Mg(OC6H3BrCHO)(CH3COCHCOC6H5)(H2O)2 CHa proton gives a doublet at d 7.35 ppm, CHb proton gives a doublet at d 6.55 ppm and CHc proton exhibits a singlet at d 7.53 ppm.

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