Non-innocence of the diiminepyridine ligand in its cobalt complexes
This thesis focuses on the properties of the diiminepyridine (DIP) ligand and its transition metal complexes, especially cobalt complexes. Existing and new X-ray structures of five-coordinate DIP Fe and Co dihalide complexes have been analyzed using the two-angle criterion ω. Substituent effects (less than 6 kcal/mol) and metal effects (mostly less than 6 kcal/mol) on structure distortion have been explored by density functional theory (DFT). The small energy barrier indicated easy distortion of the coordination geometries. The same strategy was also applied to the analysis of iron dialkyl complexes. There seems to be no direct correlation between structural preference and catalytic activity in olefin polymerization. Ligand parameters of DIP-type ligands, which intend to measure the σ-donor and π-acceptor ability, were developed using DFT calculation. The stabilization energy of the metal complexes was decomposed assuming a linear energy relationship. The results showed that the standard DIP ligand is both a strong σ-donor and a strong π-acceptor, and inferior only to the bis(carbene)pyridine ligand. A mild way to make (DIP)CoR using labile-ligand cobalt dialkyl precursors has been explored. A simple and easy way to synthesize (Py)2Co(CH2SiMe3)2 has been developed. This compound is stable at room temperature and can be further converted to (TMEDA)Co(CH2SiMe3)2 in high yield. The X-ray structure of the analogous (Py)2Co(CH2CMe2Ph)2 showed a structure similar to its iron analog. Application to DIP ligands indicates that the π-acceptor ability of the ligand determines whether cobalt(I) or cobalt(II) dialkyl will be obtained. However, steric protection is important in obtaining stable cobalt(I) alkyl complexes. Hydrogenolysis of LCoCH2SiMe3 (L: 2,6-[2,6-Me2C6H3N=C(CH3)]2C5H3N) generated LCo(N2) complex in the presence of dinitrogen. When reacted with organic halides, especially aryl chlorides, LCo(N2) broke the carbon-halogen bond through a binuclear oxidative addition mode to generate two cobalt(I) products. The radical mechanism proposed was supported by DFT studies. The resulting cobalt(I) aryl products can further react with activated alkyl halides to generate cross-coupled products, probably also through a radical mechanism. LCo(N2) can also be used to break the acyl carbon-oxygen bond of esters, although less efficiently.
diiminepyridine ligand, cobalt complexes, non-innocence