The structure-activity relations in the alumina-supported cobalt catalysts are studied at the realistic conditions of Fischer-Tropsch synthesis using in situ time-resolved XRD and catalytic measurements. Cobalt sintering during the first 3-5 h of the reaction and cobalt carbidisation at a longer time on stream (>8 h) coincide with catalyst deactivation.

Summary: The stereochemistry of propylene insertion/propagation reactions with a variety of Cs and C1 symmetric, bridged cyclopentadienyl‐fluorenyl ligand containing metallocene catalysts are investigated. It is shown that active species with a local Cs symmetry and enantiotopic coordination positions, in the general contest of the chain migratory insertion, behave syndioselective. However, the bilateral symmetry rule, “central dogma” of syndioselectivity, is not a catholic prerequisite and may be ignored by the Cs systems whenever symmetric “excess” in steric bulk is present. An asymmetric “excess” of steric forces, on the other hand, will perturb the migratory insertion process completely and induce a full tactic inversion in resulting C1 symmetric catalysts. The findings also reveal that C2 symmetry and helicity of homotopic catalysts, conditions thought to be essential for high isoselectivity, can be lifted in light of recent developments in high performance centrally chiral, C1 symmetric enantiotopic catalysts.Ligand outline, chain orientation, and monomer coordination mode with enantiomorphous active species.imageLigand outline, chain orientation, and monomer coordination mode with enantiomorphous active species.

Titanocene and zirconocene complexes with a phenyl or I-naphthyl substituent in one ,B-position of each Cs-ring ligand and with an interannular tetramethylethano-or dimethylsilano-bridge have been prepared. Racemic complex isomers, which are formed together with varying amounts of their meso isomers, were separated from the latter and characterised by lH NMR spectroscopy and X-ray crystallographic studies. * For Part XXII see ref. 15.

The stereochemistry of propylene insertion/propagation reactions with a variety of Cs symmetric fluorenyl‐ containing single site catalysts is discussed. Our recent results indicate that independent of the chemical composition of the ancillary ligand fragments, or nature of the transition metal, active sites with local Cs symmetry and enantiotopic coordination positions behave syndioselectively in the general context of chain migratory insertion mechanism. Perfect bilateral symmetry neither exists nor is required in these processes. In this context the mechanism of syndiospecific polymerization is revisited by taking into account the structural characteristics and catalytic behavior of the original metallocene based (η5‐C5H4‐CMe2‐η5‐C13H8) MCl2/ MAO; M = Zr (1), Hf (2) catalyst systems and new syndiotactic specific systems including (η5‐C5H4‐CPh2‐η5‐3,6‐di‐tBut‐C13H6)ZrCl2 (3), η1,η5‐(μMe2Si)(3,6‐di‐tBut‐Flu)(t‐ButN)MCl2/ MAO; M =Ti (4), Zr (5) and η1,η5‐(μMe2Si)(2,7‐di‐tBut‐Flu)(t‐ButN)MCl2/ MAO; M = Ti (6), Zr (7).

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