XAS Analysis of Reactions of (Arylimido)vanadium(V) Dichloride Complexes Containing Anionic NHC That Contains a Weakly Coordinating B(C6F5)3 Moiety (WCA-NHC) or Phenoxide Ligands with Al Alkyls: A Potential Ethylene Polymerization Catalyst with WCA-NHC Ligands

• Published in: ACS omega 2019-11, Vol.4 (20), p.18833-18845
• Main Authors: Nomura, Kotohiro, Nagai, Go, Izawa, Itsuki, Mitsudome, Takato, Tamm, Matthias, Yamazoe, Seiji
• Format: Article
• Language: English
• ISSN: 2470-1343; 2470-1343
• DOI: 10.1021/acsomega.9b02828

(Arylimido)­vanadium­(V) dichloride complexes containing anionic N-heterocyclic carbene (NHC) ligands that contain weakly coordinating B­(C6F5)3 moieties (WCA-NHC) of the type [V­(NAr)­Cl2(WCA-NHC-Ar′)] (5, Ar = 2,6-Me2C6H3, Ar′ = 2,6- i Pr2C6H3) showed significant catalytic activity for ethylene polymerization in the presence of Al cocatalysts (MAO and Al i Bu3); the activity by the 5–MAO catalyst (19 500 kg-PE/mol-V·h; TOF 11 600 min–1) is the highest among those reported using the other (imido)­vanadium­(V) complexes in the presence of MAO, and the 5–Al i Bu3 catalyst showed higher activity (66 000 kg-PE/mol-V·h; TOF 39 200 min–1). The V K-edge X-ray absorption near-edge structure (XANES) analyses (in toluene) strongly suggest the formation of certain vanadium­(III) species by reduction with Al i Bu3 accompanying structural changes; the EXAFS analysis suggests the presence of the arylimido ligand and one V–Cl bond (2.34 ± 0.04 Å), which is longer than those [2.1901(8)–2.2462(8) Å] in the reported (imido)­vanadium­(V) complexes. The XANES analysis of [V­(NAr)­Cl2(OAr)] strongly suggests the formation of the other vanadium­(III) species by reduction with Me2AlCl or Et2AlCl, and the EXAFS analysis suggests the presence of the arylimido ligand and two V–Cl bonds (2.45 ± 0.03 Å). The XANES spectra showed no significant changes in both the pre-edge peak(s) and the edge peak when these complexes were treated with MAO, suggesting that the basic geometry and the high oxidation state were preserved under these conditions.