Chapter VIII STUDIES ON SOME MIXED LIGAND COMPLEXES

Size: px

Start display at page:

Download "Chapter VIII STUDIES ON SOME MIXED LIGAND COMPLEXES"

Paul Brooks
6 years ago
Views:

1 Chapter V STUDES ON SOME MXED LGAND COMPLEXES During the past two decades, studies on mixed ligand complexes have evoked considerable interest among coordination chemists, mainly because of the involvement of such type of complexes in biological processes. Mixed ligand complexes should be able to cater to the special needs of metal ions better. While there may be 'mismatch' between optimum electron requirement of a metal ion and that provided by one kind of ligand alone, the electron density provided by a set of different ligands may approach the optimum closely, thus forming complexes of enhanced stability. 299 Apart from these aspects a comparative study of isostructural mixed ligand complexes can bring out the influencing factors of stability and can thus provide valuable pointers to the nature of metal-ligand bonds in these complexes. 300 Allured by these observations we thought it is interesting to prepare a series of base adducts of [La(SAT)Ch] and a comparative thermal decomposition study of these complexes was undertaken to evaluate the factors affecting thermal stability.

2 175 EXPERMENTAL Preparation of the base adducts All the mixed ligand complexes containing heterocyclic base (pyridine, a-, - or y-picoline) or amine (ethylamine, diethylamine or triethylamine) were prepared using a general procedure. Lanthanum() chloride was first prepared from La by the method described earlier. Lanthanum() chloride solution in ethanol was refluxed with an ethanolic solution of the primary ligand (SAT) in 1: 1 molar ratio for 10 h. The refluxing was continued further with about 10 ml of amine or heterocyclic base for 8 h. Solution thus obtained was concentrated and kept overnight. The solid mass obtained was washed successively with alcohol and ether and dried under vacuum. RESULTS AND DSCUSSON The mixed ligand complexes obtained analytically pure are listed in Table V. 1. Formulation of these complexes has been made on the basis of the elemental analytical data (Table V. ), molar conductance values, molecular masses etc. Molar conductance values adequately confirmed the non-electrolytic nature of these complexes.

3 Table VD.l Analytical data and other details ofmixed ligand complexes Complex [La(SAT)(EtNH 2 )Chl nitial decompo- sition temperature C Analytical data (%) Molar conductance in Metal C H N S DMSO ohm- l cm 2 morl (21.56) (39.10) (3.57) (6.52) (4.96) [La(SAT)(Et 2 NH)ChJ (20.67) (41.04) (4.31) (624) (4.76) [La(SAT)(Et3N)CbJ (19.84) (42.82) (4.71) (5.99) (4.57) l 0 [La(SAT)(py)ChJ [La(SAT)(a.-pico)Chl [La(SAT)(~-pico)Chl (20.48) (42.44) (3.39) (6.19) (4.72) (20.07) (43.32) (4.19) (6.06) (4.62) (20.07) (43.32) (4.19) (6.06) (4.62) [La(SAT)(y-pico)Chl (20.07) (43.32) (4.19) (6.06) (4.62) Calculated values are given in brackets

4 177 The mixed ligand complexes gave characteristic spectral bands for coordinated SAT in the relevant regions (chapter V). A strong band cm- l which is assignable to D(NH) of the indole ring indicating that NH of isatin moiety is not involved in coordinaton. A strong band observed cm- l in the complexes showing coordination of ester carbonyl with lanthanide ion. A strong band appeared cm- l in the complexes indicating the coordination of carbonyl oxygen of isatin moiety with lanthanide ion. A medium intensity band appeared cm- l in the complexes showing the coordination of azomethine group with lanthanide ion. Apart from these vibrations characteristic of the monodentate bases have been observed in the relevant regions. Thus for example, the bands of pyridine nucleus at 1478, 1217 and 911 cm- l have been raised in frequency by 20 ± 5 cm- l coordination. 30,302 in the spectra of the pyridine adduct as a result of The possibility of coordination by ring sulphur atom of thiophene moiety has been ruled out, because vibrations occurring at 1525, 1410 and 1370 cm- l characteristic of substituted thiophene ring did not show any appreciable change in the spectra of metal complexes. This is further confirmed by the absence of D(M-S) bands in far infrared spectra of the complexes.

5 178 Thermal stability The TO and DTO profiles of the complexes are given in Fig V. 1 to Fig V. 7. As far as the metal complexes are concerned, the initial decomposition temperature is frequently used to determine the relative thermal stabilities. 303 t is interesting to note that in all the cases the thermal stability increased when compared to [La(SAT)C3J. A close examination of the results reveal that all the complexes show almost same thermal behaviour except their initial decomposition mainly because of their structural similarities. All the three complexes exhibited a single stage decomposition pattern. As far as [La(SAT)(Et NH2)C3J is concerned, it is stable up to 200 C and then shows a wide range of decomposition from C, which is denoted by a DTG peak at 410 dc. There is no further change, even after heating the complex to about 800 C and the residue left has been analysed to be La203. This agrees fairly well with the mass loss obtained in independent pyrolysis experiments and the final product obtained has been La203. However, [La(SAT)(Et2NH)ChJ has been found to be more stable. This complex has been stable up to 230 C and undergoes a similar single stage decomposition in a wide range, C, which is indicated by the DTG peak at 420 C. The mass loss

6 179 r '".s. '" '"< ::; TEMPERATURECC)-- Fig V. 1 [La(SAT)(EtNH2)ChJ

7 BOO TEMPERATURECC} - Fig V. 2 [La(SAT)(Et2NH)CbJ

8 ;' f f f soo TEMPERATURECC) _ Fig V. 3 [La(SAT)(Et 3 N)CbJ

9 s r i s 1 '" 1 J < 3 J :: TEMPERATURE ("C) - Fig V. 4 [La(SAT)(y-pico)Ch]

10 183 i " f, " TEMPERATURE eel-- Fig V. 5 [La(SAT)(P-pico)CbJ

11 J (...' SOO TEMPERATURECC) - Fig V. 6 [La(SAT)(Py)ChJ

12 s J ( J loo TEMPERATURECC)-- Fig V. 7 [La(SAT)(a-pica)CbJ

13 186 obtained corresponds to the formation of La20 3 which is stable beyond 800 C. This also agreed fairly well with the mass loss obtained in independent pyrolysis experiments and the final decomposition product has been analysed as La But, [La(SAT)(Et 3 N)Cl 3 ] decomposes at a lower temperature with the same thermogravimetric profile. The decomposition occurred in a wide range, C with a DTG peak at 409 C. Here also the final residue obtained has been La which is stable beyond 800 C. The above observations show that the metal-amine bond strength varies in the order_ tertiary amine < primary amine < secondary amine. On the basis of the electron releasing character of alkyl group, order of basicity expected for the amine series is : (C 2 Hs) 3 N > (C2 Hsh NH > (C2 Hs) NH 2. However, bond angles of the amine get compressed in the amine complexes. Since the resulting strain increases with increasing substitution of the amine, tendency to release strain by dissociation from the complex should be in the order C2 Hs NH 2 < (C2 Hsh NH < (C2 Hs) 3 N. Hence the observed order of thermal stability of the amine complex is a compromise between the two important but opposing factors, basic strength and strain.

14 187 Pyridine and picoline adducts The above factors have been further examined by studies on the thermal decomposition of complexes containing the heterocyclic bases, such as pyridine, u-picoline, B-picoline and y-picoline. All the complexes showed two stage decomposition pattern. The pyridine complex is stable up to 280 C and undergo decomposition in two stages as indicated by the DTG peak obtained at 325 C and 560 dc. The decomposition of the complexes corresponds to the mass loss of 31.74% and 47.62%. The first stage corresponds to the decomposition of pyridine and isatin moiety and the second stage is due to the loss of anions and formation of La203. The mass of the residue left agrees with the mass of the residue obtained in independent pyrolysis. Similar is the case with u-picoline, B-picoline and y-picoline complexes but they differ in the stabilities. The initial decomposition temperatures for the u-picoline, B-picoline and y-picoline complexes have been found to be 240 C, 300 C and 350 C respectively. n all cases the final product obtained has been identified to be La203 which is stable beyond 800 C. Variation in stability is due to the position of methyl group. Since u-picoline is as basic as y-picoline, the observed lower thermal stability of the u-picoline complex is fully attributable to the steric

15 188 factor. Greater stability of the y-picoline complex is due to the position of methyl group in the para position. The stability order is found to be [La(SAT)(y-pico)Ch] > [La(SAT)(P-pico)Cl3] > [La(SAT)(py)Cl3] > [La(SA T)( a-pico)ch]. 304 n this case it appears that the thermal stability of the complexes depends on steric factors. n this context, it may be noted that attempts to prepare analogous quinoline complex were not successful. Apart from the above instructive and interesting results, thermogravimetric studies also provided strong support for the formulation of the complexes, since weight loss expected and found for the removal of the unidentate bases, agreed in all cases.

Studies on Coordination Behaviour of a Polydentate Schiff Base in Presence of Dimethyl Sulphoxide on Rare Earth Metal Ions

Studies on Coordination Behaviour of a Polydentate Schiff Base in Presence of Dimethyl Sulphoxide on Rare Earth Metal Ions Jisha. M. J 1, G. Rajendran 2 1 Department of Chemistry, Christian College Kattakada,