Synthesis, characterization and reactivity of new nickel(II) complexes towards urea and carbon dioxide

Inspired by biological systems, the field of catalytic hydrolysis of small molecules such as urea and carbon dioxide by nickel(II) complexes has been actively pursued for several years. Few complexes with catalytic activity towards Thai-Micheal addition and carbon dioxide fixation have been reported in the literature, and they remain largely an unresolved issues. In an attempt to develop model catalysts that would hydrolyze urea and carbon dioxide, a dinucleating ligand N,N'-Bis[2-carboxybenzomethyl]-N,N'-Bis[carboxymethyl]-1,3-diaminopropan-2-ol, H₅ccdp and its nickel(II) complexes have been synthesized and characterized. Under different reaction conditions and ligand to nickel(II) stoichiometric ratios, four new complexes have been synthesized. The complexes range from mono-nickel (K₂[Ni(Hccdp)].8H₂O (K₂[1].8H₂O) and Ni(H₂O)₆[Ni(Hccdp)(H₂O)₂]₂.12H₂O (Ni(H₂O)₆[2]₂.12H₂O) to the di-nickel (K₂[Ni₂(ccdp)(C₅H₇O₂)(H₂O)₂].7H₂O (K₂[3].7H₂O)), and tetra-nickel Cs₆[Ni₄(ccdp)₂(µ-%eta;2-%eta;2-CO₃)₂].23H₂O (Cs₆[4]*23H₂O) in nuclearity. In these reactions, H₅ccdp ligand has shown considerable flexibility in coordinating with nickel(II) ions to produce the complexes. The complexes have been characterized using Uv-Vis, IR, X-ray crystallography, mass spectroscopy and electrochemical techniques. Of the synthesized nickel complexes, the complex K₂[3].7H₂O has shown activity towards carbon dioxide, urea and acetonitrile in basic aqueous media via a bis-µ-OH tetra-nickel reactive intermediate to produce the Cs₆[4].23H₂O complex. In this thesis, a detailed discussion of the synthesis, characterization and reactivity of these nickel complexes and the in situ synthesized [Ni₄(ccdp)₂(µ-OH)₂]4- intermediate towards urea, carbon dioxide and acetonitrile is described.