No abstract

A new family of Cs‐ and C1‐symmetric 2,6‐bis(imino)pyridyl ligands has been synthesised. These ligands, which differ from their known counterparts in the contemporaneous presence of alkyl and aryl substituent at the ketimine nitrogen atoms, form stable, five‐coordinate complexes with FeCl2 and CoCl2. All ligands and catalysts have been fully characterised both in the solid state and in solution. A single‐crystal X‐ray analysis of [CoCl2{2‐[CMe=N−(2,6‐dimethylphenyl)]‐6‐(CMe=N−C6H11)C5H3N}]·H2O shows the metal centre to adopt a highly distorted square‐pyramidal geometry. In combination with methylaluminoxane (MAO), both iron and cobalt compounds act as effective and selective (> 99%) catalysts for the oligomerisation of ethylene to a Schulz−Flory distribution of α‐olefins, with the α factor ranging from 0.61 to 0.91. The iron catalysts exhibit TOFs as high as 7·105 mol C2H4 (mol of cat × h)−1 and are much more active than their cobalt analogues. Experiments at different ethylene pressures with the iron catalysts showed that both the propagation rate and the chain transfer rate are first order in ethylene pressure. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2003)

Mixtures of Cp*TiR3 (with Cp* = C5(CH3)5 and R = CH3, 13CH3) and equimolar amounts of B(C6F5)3 or C(C6H5)3B(C6F5)4, active in the syndiospecific polymerization of styrene, have been investigated by means of ESR spectroscopy. At room temperature the formation of Cp*TiR+ complexes has been observed. Rapid insertion of styrene occurs in Ti−R bonds of these cationic Ti(III) complexes, strongly suggesting their role in the stereospecific polyinsertion.

The oligomerization of ethylene to α-olefins has been obtained by MAO activation of tetrahedral CoII complexes of iminopyridines substituted in the 6-position of the pyridine ring by thiophen-2-yl, furan-2-yl, or phenyl groups. An intriguing heteroatom effect has been noticed on both activity and selectivity, together with a change in the CoII spin state upon activation of the dichloride precursors with MAO.

The position of the sulfur atom in the thienyl group of 6-(thienyl)-2-(imino)pyridine ligands strongly affects the catalytic activity of the corresponding tetrahedral high-spin dihalide CoII complexes in the oligomerization of ethylene to R-olefins upon activation with methylaluminoxane (MAO). Complexes with the sulfur atom in the 3-position of the thienyl ring catalyze the selective conversion of ethylene to 1-butene, while catalysts containing thien-2-yl groups give C4-C14 R-olefins. In situ EPR experiments showed the occurrence of a spin state changeover with the formation of low-spin CoII species upon activation of the catalyst precursors by MAO. DFT calculations suggest that only thien-2-yl rings allow for the coordination of the sulfur atom to the cobalt center in the MAO-activated systems