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

(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.