N, N, N-Tridentate iron(II) and vanadium(III) complexes

• Published in: Journal of molecular catalysis. A, Chemical Chemical, 2004-11, Vol.222 (1), p.27-45
• Main Authors: Schmidt, Roland, Das, Paritosh K., Welch, M. Bruce, Knudsen, Ronald D.
• Format: Article
• Language: English
• Subjects: Catalyst/cocatalyst interactions; Iron(II)-complexes; MAO-activated complexes; UV–vis studies; Vanadium(III)-complexes
• ISSN: 1381-1169; 1873-314X
• DOI: 10.1016/j.molcata.2004.06.026

UV-Visible spectroscopic studies on methyl aluminoxane activated N,N,N-tridentate Iron(II) and Vanadium(III) complexes. ▪ The ethene oligomerization and polymerization activity of N, N, N-tridentate pyridine-diimine vanadium(III)- and iron(II)-complexes showed a remarkable time and temperature dependence after their activation with methyl aluminoxane (MAO). It was observed that the complex/MAO ratio and the time elapsed after MAO-addition to the catalyst precursor are critical factors for its overall performance. Different results were obtained from the same catalyst depending on the time that had passed from the time the catalyst was prepared until it was tested. Similarly. It was also found that an increase in reaction temperature decreased the catalytic activity. In order to investigate these observations, we employed UV–vis absorption spectroscopy on two representative vanadium(III)- and two iron(II)-based N, N, N-tridentate pyridine-diimine complexes activated with MAO. On time-scales of minutes to days, we have observed growths and decays of spectral band-systems, some of which are presumably related with chemical changes leading to active catalyst forms for ethene polymerization/oligomerization. Tentative explanations of the spectral changes that were observed for one of the vanadium systems under varying conditions of relative MAO concentrations are proposed. While the chemistry of the reactions of the complexes with MAO has still remained ill understood, the kinetic data based on spectral changes would be of empirical value to monitor and possibly correlate catalyst performances. This is the first glance into causes due to changes in the activated catalyst depending on concentration and time.