Bis(imino)quaterpyridine-bearing multimetallic late transition metal complexes as ethylene oligomerisation catalysts

Abstract

The bis(imino)quaterpyridine, 6,6‴-{(2,4,6-i-Pr₃C₆H₂)NCMe}₂-2,2′:6′,2″:6″,2‴-C₂₀H₁₂N₄ (L_tripp), can be prepared in good yield by the condensation reaction of 6,6‴-bis(acetyl)quaterpyridine with 2,4,6-triisopropylaniline; the 2,6-diisopropylphenyl derivative 6,6‴-{(2,6-i-Pr₂C₆H₃)NCMe}₂-2,2′:6′,2″:6″,2‴-C₂₀H₁₂N₄ (L_dipp) is also reported. Reactions of L_tripp with 2.5 equivalents of MX₂ (MX₂ = CoCl₂, CoBr₂, (DME)₂NiCl₂ and FeCl₂) in n-butanol at 100 °C overnight results in self-assembly to form the paramagnetic salts [(L_tripp)M₂X₃]₂[MX₄] [M = Co, X = Cl (1a); M = Co, X = Br (1b); M = Ni, X = Cl (2); M = Fe X = Cl (3)] in moderate yield. The molecular structures of the acetonitrile/water adducts 1a(NCMe)₂, 1b(NCMe)(OH₂) and 2(NCMe)₄ reveal that L_tripp acts as a bis(tridentate) ligand that can support a range of structural motifs for the cationic units including [(L_tripp)CoCl(μ-Cl)CoCl(NCMe)]⁺, [(L_tripp)CoBr₂CoBr(OH₂)(NCMe)]⁺, [(L_tripp)CoBr(μ-Br)CoBr)]⁺ and [(L_tripp)NiCl(NCMe)(μ-Cl)NiCl(NCMe)]⁺. On activation with MAO (methylaluminoxane), 1-3 show modest activities for ethylene oligomerisation (1a/MAO ∼1b/MAO > 2/MAO) or are inactive (3/MAO). Notably, the two cobalt systems (1a/MAO, 1b/MAO) afford highly linear α-olefins while the nickel system (2/MAO) yields methyl-branched waxes composed of mostly internal unsaturation along with lower levels of α-olefins. In addition to 1a(NCMe)₂, 1b(NCMe)₂(OH₂) and 2(NCMe)₃, a single crystal X-ray determination has been performed on the structurally related salt [(L_tripp)Ni₂Br₃(OH₂)₂][Br] (4).