| dc.description.abstract | Certain observations and conclusions regarding the investigations presented in Chapters 5 to 9 are described here. The analysis is in relation to the ‘SCOPE OF THE WORK’ indicated in Chapter 1.
The present work is mainly concerned with the reactions of nickel(II), copper(II) and palladium(II) complexes of bivalent tetradentate and monovalent bidentate Schiff bases and structural characterization of their products, with particular emphasis on the linkage isomerism of the oximino group (=C=NOH). It also deals with the X?ray single?crystal diffraction studies on bis(4?ethylimino?2,3?dionepentane?5?oximato)palladium(II). In some cases, complexes have also been prepared by alternative routes to examine the possibility of obtaining structurally different isomers.
The results of the investigation have led to the following conclusions:
The nitrosation reaction of nickel(II) and copper(II) complexes of Schiff bases, derived from ??diketones and 1,2?diamines, suggests the nucleophilic character of the imine chelate ring. It is of significance to note that unlike the conventional substitution reactions of coordinated ??diketones and ??ketoimines, the nitrosation reaction is accompanied by change in the ligational environment around the metal ion. The oximino group introduced at the ??CH position gets itself coordinated to the metal ion by dislodging the already coordinated C=O group. This further produces chelate linkage isomers depending upon the metal ion.
The quasiaromatic character of ??ketoimine chelate ring (even in the asymmetric environment), in the intermolecular chelate linkage isomeric nickel(II) complexes, (3?oximino?4,9?dimethyl?5,8?diazadodeca?4,8?diene?2,11?dionato)nickel(II), Ni(OIMD) and (3?oximino?4,7,9?trimethyl?5,8?diazadodeca?4,8?diene?2,11?dionato)nickel(II), Ni(OTMD), is demonstrated by the bromination of these complexes.
Amine?exchange reactions of complexes of bis(oximino???ketoimino) nickel(II), copper(II) and palladium(II) complexes show the electrophilic character of the chelate ring.
Three types of chelate linkage isomerism involving the coordination of the oximino group have been established:
i. Intermolecular chelate linkage isomerism – this type refers to the bifunctional coordination of the oximino group when it is involved in two different complex molecules.
ii. Intra?ligand chelate linkage isomerism – here the oximino groups of the same ligand exhibit dual mode of coordination to the same metal ion.
iii. Intramolecular chelate linkage isomerism – it pertains to the ambidentate coordination of the oximino group when it occurs in two different/same ligands coordinated to the same metal ion.
All the three types of chelate linkage isomeric complexes are formed by nickel(II), but copper(II) shows intramolecular chelate linkage isomerism as in bis(4?imino?2,3?dionepentane?3?oximato)copper(II), Cu(IDPO)(IDPO’) and (4?methylimino?2,3?dionepentane?5?oximato),(4?imino?2,3?dionepentane?5?oximato)copper(II) Cu(Me?IDPO)(IDPO’). In addition, copper(II) forms symmetric complexes involving either oximino?nitrogen bonding, Cu?N?, or oximino?oxygen bonding, Cu?N?O? in bis(4?methylimino?2,3?pentanedione?3?oximato)copper(II), Cu(Me?IDPO)? and 1,2?diamine?bridged mono? and di?nitrosated copper(II) complexes, respectively. It is in this respect that copper(II) differs from nickel(II) in its bonding mode with the oximino group.
For the first time, intra?ligand chelate linkage isomerization has been observed in some intermolecular chelate linkage isomers of nickel(II). Each of the isomers of (3?oximino?4,7,9?trimethyl?5,8?diazadodeca?4,8?diene?2,11?dionato)nickel(II), Ni(OTMD), and isomer A (oximino?oxygen bonded) of (2?oximino?1,10?diphenyl?3,6,8?trimethyl?4,7?diazadodeca?3,7?diene?1,10?dionato)nickel(II), Ni(ODPTMD), isomerize in CDCl? solution to an equilibrium mixture of oximino?oxygen and oximino?nitrogen (isomer B) bonded isomers.
Similarly, the brominated complexes, Ni(ODMD?Br) and Ni(OTMD?Br), undergo structural transformation, which depends on the 1,2?diamine residue.
Based on the observations made in 4 and 5, it is found that the ambidentate coordination of the oximino group derived from vinylic functionality is modified by the following factors:
i. nature of the metal ion,
ii. electronic properties of the terminal substituents of the oximino???ketoimines,
iii. azomethine nitrogen substituent, =C=NR,
iv. solvent, and
v. temperature. | |
