English

The principles of coordination chemistry e.g. ligand field theory constitute a common ground for molecular magnetism, biomimetics and bioinspired chemistry. The subject of molecular magnetism is the center of this thesis. Summarily, this thesis mainly describes exchange-coupled homo and heteropolynuclear complexes, containing different paramagnetic metal ions, with particular emphasis on the interactions of spin carriers based on different topological approach with irregular spin state structures towards building high-spin molecules. These polynuclear complexes described here are characterized structurally and spectroscopically so that magnetostructural correlations can be made. This work is divided into eight chapters. The first chapter gives an introduction relevant to this work, considering the background of "Molecular Magnetism" and the importance of the exchange coupled polynuclear complexes in “molecular magnetism” and magnetic molecular materials. The importance of oxime ligands as backbones for polynuclear complexes due to their versatility at bonding modes is discussed. A few examples of well characterized high spin molecules, relevant to this thesis, are reviewed. The second chapter is concerned with the synthesis, characterization and magnetostructural study of exchange coupled trinuclear oximate complexes. It is to be mentioned here that new exchange pathways can be expected for heteropolynuclear complexes where unusual sets of magnetic orbitals can be made to overlap with each other and hence investigations of a series of heteropolynuclear complexes might be more informative in comparison to those of homometal complexes. Three trinuclear complexes, NiIIMnIIINiII 1, NiIICrIIINiII 2 and NiII3 3 based on (pyridine-2-aldoximato)nickel(II) units are described. Two of them, 1 and 2, contain metal-centers in linear arrangement, as is revealed by X-ray diffraction. Complex 3 is a homonuclear complex in which the three nickel(II) centers are disposed in a triangular fashion. The compounds were characterized by various physical methods including cyclic voltammetric and variable-temperature (2–290 K) susceptibility measurements. Complexes 1 and 3 display antiferromagnetic exchange coupling of the neighboring metal centers, while weak ferromagnetic spin exchange between the adjacent NiII and CrIII ions in 2 is observed. The experimental magnetic data were simulated by using appropriate models. The third chapter presents linear tetranuclear “homo and heteropolymetallates” constructed using a dinucleating oxime ligand. One dinuclear and four tetranuclear complexes, MnIIMnII 4, MnIIIMnIIMnIIMnIII 5, MnIVMnIIMnIIMnIV 6, FeIIIMnIIMnIIFeIII 7 and CrIIIMnIIMnIICrIII 8 based on (2,6-diformyl-4 methyl phenoldioximato)manganese(II) units are described. All of them contain metal-centers in linear arrangement, as is revealed by X-ray diffraction. The compounds were characterized by various physical methods including cyclic voltammetric and variable-temperature (2–290 K) susceptibility measurements. Complexes display overall antiferromagnetic exchange coupling with extremely low-lying states. The fourth and fifth chapters discuss the building up of high spin polynuclear complexes based on different molecular topology such as, butterfly, star-shaped etc, and a “parallel spin coupled” system using “accidental ferromagnetism” and “planned ferromagnetism” both governed by the common principle of orthogonal orbital overlap. It also discusses irregular spin state structures due to spin frustration or competing exchange interaction. Two tetranuclear complexes, FeIII2CuII2 9, CuII2CrIII2 10 based on (2,6-diacetyl pyridinealdoximato)copper(II) units and Me3TacnMX3 (where M = Fe(III), Cr(III) and X = Cl or Br) are described. Both of them, 9 and 10, contain metal-centers disposed in "butterfly" fashion with M(III) as the "wing" and Cu(II) as the "body", as is revealed by X-ray diffraction. The compounds were characterized by various physical methods including variable-temperature (2–290 K) susceptibility and variable-temperature variable-field (VTVH) magnetic measurements. The experimental magnetic data were simulated by using appropriate models Complexes 9 and 10 display antiferromagnetic exchange coupling of the neighboring metal centers, due to the "spin-frustration" or more precisely competing exchange interactions between the spin carriers complex 10 exhibits irregular spin state structure with ST = 2 ground state. While strong wing-body interactions over body-body interaction, stabilizes ST = 4 ground state in complex 9. Two tetranuclear complexes, MnII4 11, MnIII4 12 based on salicylaldoxime ligand are described. One of them, 11 contains metal-centers in "star-shaped" arrangement while the complex 12 in which the four manganese(III) centers are disposed in a tetrahedral fashion, as is revealed by X-ray diffraction. The compounds were characterized by various physical methods including variable-temperature (2–290 K) susceptibility and variable-temperature variable-field (VTVH) magnetic measurements. The experimental magnetic data were simulated by using appropriate models Complexes 11 and 12 display weak ferromagnetic exchange coupling of the neighboring metal centers, and yield high-spin ST = 10 and ST = 8 ground states for the complexes 11 and 12 respectively. Also hexa-and nonanuclear complexes have been synthesized and are described in chapters six and seven. The hexanuclear complexes, composed of two edge-sharing triangular units, are also subjected to magnetostructural studies as described in chapter five, while sixth chapter describes two rare examples of nonanuclear Ni(II) and Cu(II) complexes. Two nonanuclear complexes, NiII9 16, CuII9 17 based on (pyridine-2-aldoximato)nickel(II) unit and N,N'-(2-Hydroxypropane-1,3-diyl)bis(benzoylacetoneimine) respectively are described. Both of them, 16 and 17, contain two irregular tetrahedra connected to a centrally placed M(II) ions, as is revealed by X-ray diffraction. The compounds were characterized by various physical methods including variable-temperature (2–290 K) susceptibility measurements and variable-temperature variable-field (VTVH) magnetic measurements. Complexes 16 and 17 display antiferromagnetic exchange coupling of the neighbouring metal centers. The experimental magnetic data were simulated by using appropriate models.