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- Early Transition Metal Complexes Stabilized by Bulky Aminopyridinato Ligands (2008)
- A series of early transition metal complexes stabilized by aminopyridinato ligands have been synthesized. Many of these complexes have been studied in terms of their structure and have been evaluated in terms of applications in catalysis. The overall evaluation tells about the importance of electrophilicity of the metal centre, the steric bulk of the applied ligands, and the route of syntheses. Trialkyltantalum complexes were synthesized by salt elimination or toluene elimination by reacting the corresponding lithiated ligand with trialkyltantalum dichloride or the corresponding ligand with pentabenzyltantalum, respectively. These trialkyltantalum complexes are unusually thermally stable towards alpha-H elimination and form rather unstable organocations. Bis(aminopyridinato) complexes of zirconium were prepared using salt elimination route. The steric bulk of the ligands prevented the redistribution to tris- or tetrakis(aminopyridinato) zirconium complexes. These zirconium complexes are thermally robust, highly active and selective ethylene polymerization catalysts. Ethylene is polymerized highly selectively out of a mixture of ethylene and propylene. Slight changes in the steric demand of the bulky ligand periphery can be used to tune the nature of the formed polymers by maintaining the selectivity issue. The Zr alkyl cations of the sterically more demanding version of the ligands are able to polymerize ethylene in a living fashion at 50 °C. We also became interested in toluene elimination chemistry and observed that the bulky aminopyridinates that give selectively bis(aminopyridinato) complexes via salt metathesis chemistry lead selectively to mono(aminopyridinato) tribenzyl Zr/Hf complexes. In the solid state, one of the three benzyls is eta -2-coordinated and rest are eta-1-coordinated to the electron deficient metal centres. One of the three benzyls has been partially abstracted using B(C6F5)3. The phenyl ring of B-bounded benzyl in these complexes shows an eta-6-coordination and essentially blocks the vacant site of the metal centre, consequently, preventing it to polymerize ethylene at room temperature. At elevated temperature a moderate single site polymerization activity with the formation of high molecular weight polyethylene was observed for these zwitterionic complexes. The attempted abstraction of the second benzyl group failed when the zwitterionic complexes were reacted with an additional equivalent of B(C6F5)3. However using one equivalent of [R2(Me)NH][B(C6F5)4] (R = C16H33–C18H37) instead of B(C6F5)3 give catalysts which show moderate activities in ethylene polymerization. Treatment of the aminopyridinato metal tribenzyls with [R2(Me)NH][B(C6F5)4] (R = C16H33–C18H37) give active ethylene polymerization catalysts, which produce low molecular weight polyethylene for the zirconium complexes and high molecular weight for the hafnium ones. Propylene polymerization under the same conditions failed, whereas during copolymerization ethylene-propylene copolymers with separated propene units and alternating sequences were observed. The versatility of these ligands was flourished by synthesizing a titanium alkyne complex stabilized by aminopyridinato ligands which may show a very multifaceted chemistry. The reactivity of this complex was studied by the insertion of acetone into titanium carbon bond. The complex is not only quite stable at room temperature but also in solution at high temperatures under argon atmosphere despite a weakly bonded acetylene ligand. The chemistry of low valent chromium stabilized by sterically demanding aminopyridinato ligands has been explored and first non-bridging η2-coordinated chromiumII complexes were synthesized using such ligands. It was foud that reacting deprotonated ligands of the same steric bulk with the corresponding salts of the low valent chromium II/III can lead to mono(aminopyridinato) dimeric chromiumII or monomeric chromiumIII complexes, respectively. It is worth to note that gradual decrease in the steric bulk leads to bis(aminopyridinato) mononuclear chromium complexes. The ability of aminopyridinato ligands to stabilize transition metals in low oxidation state has been highlighted by the synthesis of a dimeric chromiumI complex. The X-ray crystal structure analysis revealed an exceptionally short chromium-chromium distance of 1.7488(18) Å, the shortest metal-metal bond reported so far for a stable compound. The homobimetallic chromium complex was synthesized by the reduction of aminopyridinato ligand stabilized chromiumII/III chloride precursor with KC8. Anaylsis of its electronic structure indicates quintuple bonding.