One-pot synthesis of cobalt complexes with 2,6-bis(arylimino)phenoxyl/phenthioxyl ligands and catalysis on isoprene polymerization

Abstract

Several cobalt complexes supported by 2,6-bis(arylimino)phenoxyl/phenthioxyl ligands κ²_N,X-^Ar[NXN]CoCl(THF) (1a, X = O, Ar = 2,6-Me₂C₆H₃; 1b, X = O, Ar = 2,6-ⁱPr₂C₆H₃; 2a, X = S, Ar = 2,6-Me₂C₆H₃; 2b, X = S, Ar = 2,6-ⁱPr₂C₆H₃) were synthesized by direct oxygen(sulfur) insertion into the C–Co bond of the mixed-valence cobalt complexes {κ²_C,N,η^6-Ar[NCN]Co-κ_N-CoCl(μ-Cl)}₂. Crystallization of 1b in the presence of water gave the hydrolysis product 1b′. Treatment of ^Ar[NCN]Li with dioxygen followed by the addition of CoCl₂ afforded the heteroatomic complexes {κ²_N,O-^Ar[NON]Co(μ-Cl)₂Li}₂ (3a, Ar = 2,6-Me₂C₆H₃; 3b, Ar = 2,6-ⁱPr₂C₆H₃) or κ²_N,O-^Ar[NON]Co₂Cl₂(μ-Cl)₂Li(THF)₂ (4a, Ar = 2,6-Me₂C₆H₃; 4b, Ar = 2,6-ⁱPr₂C₆H₃) depending on the amount of CoCl₂ used. The Co(III)/Li heterometallic complex 3b′ with imino-phenoxyl-amino ligands was formed probably via a redox reaction of 3b. The reactions of ^Ar[NCN]Li with elemental sulfur and CoCl₂ gave κ²_N,S-^Ar[NSN]Co₂Cl₂(μ-Cl)₂Li(THF)₂ (5a, Ar = 2,6-Me₂C₆H₃; 5b, Ar = 2,6-ⁱPr₂C₆H₃) respectively. These complexes were well characterized by FT-IR and elemental analyses, and the molecular structures of 1b′, 3b′, 4a, and 4b were confirmed by X-ray crystallography. Upon activation with Al₂Et₃Cl₃ in toluene, these complexes showed high activities in isoprene polymerization affording cis-1,4 enriched polymers with a moderate molecular weight (0.85–4.72 × 10⁴ Da).