Synthesis and Structural Analysis of (Imido)vanadium Dichloride Complexes Containing 2-(2′-Benz-imidazolyl)pyridine Ligands: Effect of Al Cocatalyst for Efficient Ethylene (Co)polymerization

Abstract

(Imido)vanadium(V) dichloride complexes containing 2-(2′-benzimidazolyl)-6-methylpyridine ligand (L) of type V(NR)Cl 2 (L) [R = 1-adamantyl (Ad, 1 ), C 6 H 5 ( 2 ), and 2,6-Me 2 C 6 H 3 ( 3 )] have been prepared, and their structures were determined by X-ray crystallography as distorted trigonal bipyramidal structures around vanadium. Reactions with ethylene using 1–3 in the presence of methylaluminoxane (MAO) afforded a mixture of oligomer and polymers, and the compositions were affected by the imido ligand employed. By contrast, 1–3 exhibited remarkable catalytic activities for ethylene polymerization in the presence of Me 2 AlCl; the phenylimido complex ( 2 ) exhibited the highest activity [80 100 kg-PE/mol-V·h turn over frequency (TOF, 2 850 000 h –1 , 792 s –1 )]. The ethylene copolymerizations with norbornene afforded ultrahigh-molecular-weight copolymers with uniform molecular weight distributions and compositions [e.g., M n = 1.71–2.66 × 10 6 , M w / M n = 2.27–2.53]. On the basis of V nuclear magnetic resonance ( 51 V NMR), electron spin resonance, and V K-edge X-ray absorption near-edge structure (XANES) spectra of the catalyst solution, the observed difference in the catalyst performance in the presence of (between) MAO and Me 2 AlCl cocatalyst should be due to the formation of different catalytically active species with different oxidation states. Apparent changes in the oxidation state were observed in the (especially in the NMR and XANES) spectra upon addition of Me 2 AlCl, whereas no significant changes in the spectra were observed in presence of MAO.

Solution X-Ray Absorption Spectroscopy (XAS) for Analysis of Catalytically Active Species in Reactions with Ethylene by Homogeneous (Imido)vanadium(V) Complexes—Al Cocatalyst Systems

“…The (2-anilidemethyl)pyridine analogue containing arylimido ligand, V(NAr)Cl 2 [(2-ArCH 2 )C 5 H 4 N] (7), also showed the activity for ethylene polymerization, whereas the adamantylimido analogues showed remarkable catalytic activities for ethylene dimerization in the presence of MAO, as described above (Scheme 2, Table 1) [60,81,85]. As shown in Table 2, the activity by 8 increased upon addition of ETA [22], and this should be a unique contrast to that observed in the phenoxide analogue (4) [83].…”

“…It should also be noted that the XANES spectra, the pre-edge intensities as well as the edge absorptions, in the reaction of 4 with Me 2 AlCl or Et 2 AlCl are apparently different from those in the reaction of 9a,b with Al i Bu 3 . Moreover, the pre-edge intensities in the reaction of 4 with Me 2 AlCl are also different from that in the reaction of V(NAd)Cl 2 (L) (8, L = 2-(2'-benzimidazolyl)-6-methylpyridine) with Me 2 AlCl [22]. Further decrease in the pre-edge intensity was observed when ETA (Cl 3 CCO 2 Et) was added into the solution containing 8 and Me 2 AlCl, whereas reaction of 4 with Me 2 AlCl led to slight decrease in the intensity.…”

“…It turned out that the coordination number of the V-N bond decreased upon addition of Me 2 AlCl, suggesting the ligand dissociation (probably L) with addition of Me 2 AlCl. Presence of three (more than two) V-Cl bonds (2.455(7) Å), which became apparently weak compared to those in the original (2.293(3) Å) [22]. These analysis results clearly demonstrate that different (imido)vanadium(III) species, which contain a different number of V-Cl bonds (2.34-2.46 Å) as a neutral lone pair donor bridged through Al, play a role in these catalytic reactions; the species containing one V-Cl bond were formed from the reaction of 9a with Al i Bu 3 , whereas the species containing two and three V-Cl bonds were formed from the reaction of 4, V(NAd)Cl 2 (L) (8, L = 2-(2'-benzimidazolyl)-6-methylpyridine) with Me 2 AlCl, respectively.…”

“…It is thus assumed that addition of ETA would be thus effective for generation (increased percentage) of catalytically active species [22]. Figure 7b shows FT-EXAFS spectra and the analysis data are summarized in Table 5 [22]. Three V-N and two V-Cl bonds including the bond distances observed in the spectrum for 8 clearly suggest that the complex 8 preserves basic trigonal bipyramidal structure (determined by the X-ray crystallography) in solution.…”

“…Remarkable decrease in the activity was observed in the polymerization by 4 upon addition of Cl 3 CCO 2 Et (ETA, run 5), which are widely used as activators in (especially classical Ziegler type) vanadium catalysts [33][34][35][36][37][38]; use of halogenated Al alkyl should be prerequisite for exhibiting the high activity by 4 (runs 2-4, and 6-8). The 2-(2'-benzimidazolyl)pyridine analogue (8) also showed the notable activities in the presence of Me 2 AlCl cocatalyst, whereas the resultant products with ethylene were a mixture of polymer and oligomers if the reaction by 8 was conducted in the presence of MAO [22]. The (2-anilidemethyl)pyridine analogue containing arylimido ligand, V(NAr)Cl 2 [(2-ArCH 2 )C 5 H 4 N] (7), also showed the activity for ethylene polymerization, whereas the adamantylimido analogues showed remarkable catalytic activities for ethylene dimerization in the presence of MAO, as described above (Scheme 2, Table 1) [60,81,85].…”