Activated Niobium and Tantalum Imido Complexes: From Tuneable Polymerization to Selective Ethylene Dimerization Systems

Abstract

The niobium and tantalum imido complexes [CpMCl 2 (NDipp)], [MCl 3 (NR)(dme)] (R = t Bu, Ph, 2,6-i Pr 2 C 6 H 3 (Dipp), and Mes), and [TaCl 3 (NDipp)(tmeda)] were tested in combination with EtAlCl 2 for the dimerization of ethylene. The niobium systems afforded dimers or polymers, depending on the nature of the imido ligand, with overall productivities in the range 720 to 13,720 (mol C 2 H 4 )(mol Nb) À 1 . The nature of the polyethylene produced (LDPE or HDPE) depended on the imido ligand and the niobium concentration at which catalysis was run. In contrast, the tantalum/dme systems all mediated ethylene dimerization with productivities of up to 4,503 (mol C 2 H 4 )(mol Ta) À 1 , with overall selectivities to butenes of between 73-81 wt %; selectivity within the dimer fraction to 1-butene was in the range 72 to 100 %. The productivity of [TaCl 3 (NDipp)(tmeda)] was six times higher than that of its dme-bearing counterpart, but at the cost of selectivity to 1-butene. For the tantalum imido-mediated ethylene dimerization the composition of the product slate formed is indicative of a metallacyclic mechanism being operative.[a] Dr.

Synthesis and Structural Analysis of Four Coordinate (Arylimido)niobium(V) Dimethyl Complexes Containing Phenoxide Ligand: MAO-Free Ethylene Polymerization by the Cationic Nb(V)–Methyl Complex

“…33 Resonances ascribed to the proton (carbon) assigned to the Nb−methyl bond could be observed with disappearance of resonances ascribed to Nb-NMe 2 ligand in the 1 H-( 13 C-) NMR spectra. 32 Note that treatment of the dimethyl complex 5a with [Ph 3 C][B(C 6 F 5 ) 4 ] (1.0 equiv) in THF gave the cationic methyl complex, [Nb(N-2,6-i Pr 2 C 6 H 3 )Me(O-2,6-t Bu 2 C 6 H 3 )-(THF)] + [B(C 6 F 5 ) 4 ] − (6a, Scheme 3); the 1 H-, 13 C-, and 19 F-NMR spectra suggested the exclusive formation of 6a. 32 As described below, 6a could be used as MAO-free ethylene polymerization catalyst.…”

“…1 138.7 (s,Ar),126.5 (s,Ar),126.4 (s,Ar),123.8 (s,Ar),123.3 (s,Ar), 122.9 (s, Ar), 122.7 (q, 1 J CF = 281.8 Hz, CH(CF 3 ) 2 ) 121.9 (s, Ar), 121.5 (s, Ar), 80.4 (br, CH(CF 3 ) 2 ), 38.7 (br, N(CH 3 ) 2 ), 35.5 (s, C(CH 3 ) 3 ), 32.1(s, C(CH 3 ) 3 ), 31.0(s, C(CH 3 ) 3 ), 28.1 (br, CH-(CH 3 ) 2 ), 24.5 (br, CH(CH 3 ) 2 ). 19 (12 mL) containing 3a (155 mg, 0.182 mmol) at room temperature. After the mixture was stirred for 3 h, the resultant mixture was passed through a Celite pad and the filter cake was washed with toluene.…”

“…17 , exhibited high activity for selective dimerization of ethylene (MAO cocatalyst), 18 and NbCl 3 (NPh)(dme) (dme = 1,2-dimethoxyethane)−Et 2 AlCl catalyst afforded 1butene accompanied by byproduction of polyethylene. 19 Imido (M = NR) ligand, employed in this study, has been known as an effective ligand for stabilization of the high oxidation state in early transition metals 20 even in their reactions with Al alkyls, recently exemplified by (arylimido)vanadium complexes by solution V K-Edge EXAFS (extended X-ray absorption fine structure) analysis. 21 As summarized in Chart 2, there are thus many reports for synthesis and reaction chemistry of the niobium complexes containing both tert-butylimido and β-diketiminate ligands (also shown as D in Chart 1), 22,23 and some reports for the (arylimido)niobium(V) complexes 6b,c,18,22−26 especially containing monoanionic chelate guanidinato ligands 25 were known.…”

Alkene oligomerization via metallacycles: Recent advances and mechanistic insights

“…Analogous imido niobium precursors were synthesized and tested under the same conditions (Scheme 20). [56] Two main observations were noted : PE is produced in higher quantity (up to 70%) and about 15% of butenes were isomerized to 2-butene. Secondly, the bulkier the imido moiety, the higher the PE production was.…”

“…Thereafter, in 2019, Dyers and co-workers reported the synthesis of imido tantalum precursors (Scheme 20) able to selectively dimerize ethylene to 1-butene (up to 80,7 wt%) after activation with EtAlCl 2 . [56] The by-product is mainly 1-hexene. The very low PE production as well as the 100% selectivity in 1butene in the butenes fraction is said to be in accordance with a metallacycle pathway although no mechanistic studies have been reported.…”