Synthesis and Characterization of Some Nitrogen Heterocycles and Their Coordination Complexes

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In this thesis, new synthetic methods and the coordination behavior of some multidentate nitrogen heterocyclic ligands have been reported. Firstly, a new efficient route for the synthesis of three 2,4,-bis(2-pyridyl)-6-(pyridyl)pyrimidines, by employing respective acetylpyridine, sodium hydroxide and 2-cyanopyridine as well as coordination behavior of thus synthesized 2,4,6-tris(2-pyridyl)pyrimidine (L1) have been described. Structural characterization of complexes [Ni(L1)(H2O)3](NO3)2·4H2O, [Ni(L1)2](NO3)2·2H2O and [Co(L1)2](NO3)2·1.5H2O, revealed that L1 behaves as a tridentate ligand with Ni(II) and Co(II) ions having a distorted octahedral geometry. Here, the mono- and bis-chelated Ni(II) complexes were isolated by controlling the stoichiometry of the reactants and only the bis-chelated Co(II) complex formed, irrespective of the reactant ratios. Results of DFT calculations performed on all the three 2,4,-bis(2-pyridyl)-6-(pyridyl)pyrimidines were reported. Two new binuclear complexes of composition [Ni2(L4)2(Cl)2(H2O)2]Cl2·12H2O (4), and [Ni2(L4)2(N3)3](N3)·5H2O, (5) were isolated using N-(3-(pyridine-2-yl)imidazo[1,5-a]pyridine-1-yl)picolinimidamide (L4). Complex 4 was obtained by treating L4 with NiCl2·6H2O, in 1:1 ratios while 5 was isolated by using two equivalent of NaN3 additionally. Structural characterization revealed that the nickel center has pseudo-octahedral coordination geometry in both the complexes with L4 acting as a bridging bis(bidentate) ligand. In 5, out of the three coordinated azide ions, two are bound as a monodentate fashion at each metal center and the third one in μ-1,3 (end-to-end) bridging fashion. A method for the construction of imidazo[5,1-a]isoquinoline nucleus from 1-isoquinolinemethylamine, various aldehydes and selenium dioxide have been described. Using this method several 3-substituted imidazo[5,1-a]isoquinolines were synthesized and characterized using spectroscopic and single crystal X-ray diffraction methods. The 1-isoquinolinemethylamine alone afforded 3-(1-isoquinolinyl)imidazo[5,1-a]isoquinoline (IQ-1) as the only product in absence of aldehydes. The ligand IQ-1 is fluorescent in nature in most of the common solvents. This developed synthetic method could be applicable to wide variety of substrates and believe that imidazo[5,1-a]isoquinoline moiety can easily be incorporated in substrates having a aldehyde group provided other functionalities are stable towards selenium dioxide. These compounds with suitably substituted groups could find their utility in various fields. As a continuation, utility of this method for the synthesis of 3-substituted imidazo[1,5-a]pyridines has also been described along with the scope and limitations. In this procedure, by reacting 2-picolylamine and different aldehydes in presence of selenium dioxide as the oxidant, a series of 3-substituted imidazo[1,5- a]pyridines were isolated in good yields.
Supervisor: V. Manivannan