(Arylimido)niobium(V) Complexes Containing 2-Pyridylmethylanilido Ligand as Catalyst Precursors for Ethylene Dimerization That Proceeds via Cationic Nb(V) Species

Abstract

(Arylimido)niobium(V) complexes containing 2-pyridylmethylanilido ligand Nb(NAr)X2(L) [L = 2-(2,6-Me2C6H3)NCH2(C5H4N); X = NMe2 (2a,b), OCH(CF3)2 (3a–c), Me (4a–c), CH2SiMe3 (5a); Ar = 2,6-Me2C6H3 (a), 2,6-iPr2C6H3 (b), 2-MeC6H4 (c)] have been prepared, and structures of 3a,b, 4b, and 5a were determined by X-ray crystallography. The dimethyl complexes (4a,b) exhibited catalytic activities for ethylene dimerization in the presence of methylaluminoxane (MAO), whereas the activity by 4c was negligible under the same conditions. Complex 4b showed the highest activity, and the activity at 50 °C was higher than those conducted at 25 and 80 °C. The major product was 1-butene, and 1-hexene was formed by subsequent reaction of ethylene with 1-butene accumulated in the reaction mixture. A first-order relationship between the activity [turnover frequency (TOF)] and ethylene pressure was observed, suggesting that the metal-alkyl species would play a role in this catalysis. The activities further increased remarkably upon addition of [Ph3C][B(C6F5)4] at 50 °C; TOF at the initial stage (5 min) of 2 100 000 h–1 (583 s–1) has been attained. Reactions of the dimethyl complexes (4a,b) with 1.0 equiv of [Ph3C][B(C6F5)4] in Et2O afforded [Nb(NAr)Me(L)]+[B(C6F5)4]−(Et2O)2 (6a,b), and the reaction of 6b with ethylene afforded 1-butene and 1-hexene even in the absence of MAO, clearly suggesting that the cationic species play a role in this catalysis. X-ray absorption near edge structure spectra of the catalyst solutions containing 4b (in toluene at 25 °C) and MAO (10 and 50 equiv) showed no significant differences in the pre-edge peak positions and intensities from that in the dimethyl complex (4b), strongly suggesting that both the oxidation states and the basic structures are maintained upon addition of MAO in these catalyst solutions.