INTRODUCTION:

In an attempt to explore the mechanism of selective absorption of alkaline earth metal ions by plants, our attention has been drawn to the naturally occurring ligands i.e. 3-hydroxy-2-methyl-1, 4-Napthaquinone (thiocol), 5-hydroxy-1, 4-napthaquinone (juglone), because of its presence of many plant system (Rodd, 1956).¹ Complexes of transition metals with Lawson and Juglone have been studied and reported earlier (Bottie and Mc Eachern, 1970).² The present communication describes the study of the complexes having the general formula ML₂ or ML_2.nH₂O where M=Mg, Ca, Sr or Ba; L= thiocol, L’=juglone and n=0 or 2.

EXPERIMENTAL:

3-hydroxy-2-methyl-1, 4-napthaquinone (Burton and Praill,1952)³ and 5-hydroxy-1, 4-napthaquinone (Wheeler and Willstatter,1914)⁴ were prepared by reported method.

Preparation of the Complexes:

Magnesium thiocolate and juglonate–To a warm solution of ligands in ethanol added aqueous solution of magnesium acetate in 2:1 mole ratio. The content was refluxed two hours with continuous stirring. On cooling and concentrating the solution, magnesium thiocolate got separate but in the case of magnesium juglonate, adduct got separated on addition of ammonium hydroxide.

Calcium, Strontium and Barium thiocolate and juglonate–To a suspension of metal hydroxide in 95% ethanol added slightly excess solution of ligand in 95% ethanol in 1:2 mole ratio. The content was refluxed for two to three hours with constant stirring. On cooling adduct got separated.

All the adduct were filtered, washed with ethanol and finally dried in an electric oven at 100^oC.

RESULTS AND DISCUSSION:

Some physical properties of the ligand and newly formed chelates are listed in Table 1.

Table 1

Compound	Colour	M.P./ decomp. /transition temp.(^oC)	Condictivity (Ohm^-1cm²mol^-1)	(%) found		M	(%) (Calculated)		M
Compound	Colour	M.P./ decomp. /transition temp.(^oC)	Condictivity (Ohm^-1cm²mol^-1)	C	H	M	C	H	M
Thiocol (HL)	Yellow	173m		70.10	4.4		70.00	4.3
MgL₂	Reddish Brown	300	6.8	64.86	3.60	5.85	66.33	3.51	6.03
CaL₂	Deep Red	272 d	8.5	61.98	3.45	9.50	63.76	3.38	9.66
SrL₂	Dull Red	300 d	8.9	56.50	3.10	18.50	57.14	3.03	19.04
BaL₂	Red	308 d	10.1	50.28	2.80	24.90	51.66	2.73	26.81
Juglone (HL’)	Yellow	185 m		68.80	3.47		68.96	3.44
MgL₂.2H₂O	Purple	300	9.0	59.00	2.48	5.90	59.11	2.46	5.91
CaL₂	Brick red	295 d	10.1	62.00	2.62	10.35	62.17	2.49	10.36
SrL₂.2H₂O	Brownish pink	290 d	8.6	50.85	2.15	18.60	51.06	2.12	18.72
BaL₂	Rosy red	300 d	9.1	48.80	2.15	28.00	49.68	2.07	28.36

All the complexes are coloured and stable under dry condition. They show no changes in stoichiometry or in physical properties even after a long time. In general the complexes are slightly soluble in methyl alcohol, ethyl alcohol, benzene etc. But insoluble in water. All these complexes undergo a transformation at temperatures which are considerably higher than the melting point of the ligand. This indicates then greater thermal stability.

Infrared Spectra:

Infrared measurements for the title and their hitherto unknown mixed ligand complexes with alkali metals were made between 4000-650 cm^-1in Nujol mulls. Pertinent IR data for these compounds are recorded in Table 2.

Table-2 Pertinent IR Spectral Bands in cm^-1

Compounds	-OH	Free y C=0	Y C=0/C=C	Y C-0
Thiocol (HL)	3350	1670 s	1630m	1290 m
MgL₂	-	1668 m	1605 sh, 1590m	1280 m
Cal₂	-	1670 m	1610 sh, 1590m	1270 s
Srl₂	-	1670 m	1600 sh,1585 m	1275 s
BaL₂	-	1668 m	1610 sh,1590 m	1270 s
Juglone (HL)	-	1663 m	1635 m	1300 m
MgL₂.2H₂O	3500 s	1660 m	1620 s	1280 m
CaL₂	-	1665 m	1625 m	1290 w
SrL₂	3350 s	1660 m	1615 sh	1295 m
BaL₂	-	1658 m	1620 m	1290 m

The moderately strong band at 3350 cm^-1 in thiocol is attributed to the stretching –OH frequency. The absence of – OH absorption band in the region 3500 – 1800 cm^-1in juglone suggests that there is strong hydrogen bonding (Flett, 1948).⁵

The –OH stretching frequency is absent in the alkaline earth metal complexes of thiocol and juglone (except in magnesium and strontium complexes of juglone) because hydrogen atom of – OH group has been replaced by alkaline earth metal. In the complexes of Mg and Sr with juglone, the two water molecules are coordinated and lose at a temperature 140⁰showing strong association. Sharp –OH peaks have been observed at 3500 cm^-1 for Mg/Sr complexes respectively. The rocking H₂O has also been spotted at 840cm^-1for strontium complexes.

Lower shifting of the v(C-O) strong frequency at 1290 cm^-1thiocol and 1300 cm^-1(juglone) indicate the coordination through oxygen atom of the phenolic group.

Strong absorption band of carbonyl group at 1630cm^-1 (thiocol) and 1635cm^-1((juglone) are shifted down by 20-30 cm^-1 due to chelation.

Conductivities:

Molar conductivities of all the complexes were measured in DMF at 23⁰C at a concentration of 10^-3 M.A very lower values (∆m=6.8 to 10.1 Ohm^-1cm²mol^-1) of molar conductivities of the complexes suggest their non electrolyte nature.

Structure and Bonding:

On the basis of the elemental analysis and IR spectral data of the following probable structures have been assigned to the complexes.

ACKNOWLEDGEMENT:

Once of the authors (Dr. Avinash Kumar) is thankful to Principal MJK College, Bettiah for providing Lab facilities.

REFERENCES:

1. Rodd, E.W. (1956). Chemistry of Carbon Compounds, Vol. IIIB; p. 1320.

2. Bottei, R.S and Mc Eachern, C.P. (1970). J. Inorg.Nuci. Chem., 32; p. 2653.

3. Burton W. And Praill F.G.P. (1952) J. Chem. Soc.; p. 758.

4. Wheeler and Willstatter. (1914).47; p.2796.

5. Flett, M. Stc. (1948). J. Chem.Soc.; p.1441.

Received on 31.03.2020 Modified on 17.04.2020

Asian J. Research Chem. 2020; 13(4):253-254.

DOI: 10.5958/0974-4150.2020.00049.8