Synthesis and characterization of bridged half-sandwich amides of titanium and zirconium

Cyclopentadienyl-based Mg complexes in the intramolecular hydroamination of aminoalkenes: mechanistic evidence for cationic versus neutral magnesium derivatives

Mechanistic evidence in the catalytic hydroamination of aminoalkenes for a cationic magnesium derivative.

Counterion effects on propylene polymerization using two-state ansa-metallocene complexes

Propylene polymerization using unsymmetrical, ansa-metallocene complexes Me(2)Y(Ind)CpMMe(2) (Y = Si, C, M = Zr, Y = C, M = Hf) and the co-initiators methyl aluminoxane (PMAO), B(C(6)F(5))(3), and [Ph(3)C][B(C(6)F(5))(4)] was studied at a variety of propylene concentrations. Modeling of the polymer microstructure reveals that the catalysts derived from Me(2)Si(Ind)CpZrMe(2) and each of these co-initiators function under conditions where chain inversion is much faster than propagation (Curtin-Hammett conditions). Surprisingly, the microstructure of the PP formed was essentially unaffected by the nature of the counterion, suggesting similar values for the fundamental parameters inherent to two-state catalysts. The tacticity of PP was sensitive to changes in [C(3)H(6)] in the case of catalysts derived from Me(2)C(Ind)CpHfMe(2) and PMAO, or [Ph(3)C][B(C(6)F(5))(4)], but the average tacticity of the polymer produced at a given [C(3)H(6)] decreased in the order [Ph(3)C][B(C(6)F(5))(4)] > PMAO. With B(C(6)F(5))(3), the polymer formed was more stereoregular, and its microstructure was invariant to changes in monomer concentration. The PP pentad distributions in this case could be modeled by assuming that all three catalyst/cocatalyst combinations function with different values for the relative rates of insertion to inversion (Delta) but otherwise feature essentially invariant, intrinsic stereoselectivity for monomer insertion (alpha, beta), while the relative reactivity/stability (g/K) of the isomeric ion-pairs present seems to be only modestly affected, if at all. Similar conclusions can also be made about the published propylene polymerization behavior of the C(s)-symmetric Me(2)C(Flu)CpZrMe(2) complex with different counterions. For every counterion investigated, the principle difference appears to be the operating regime (Delta) rather than intrinsic differences in insertion stereoselectivity (alpha). Surprisingly, the ordering of the various counterions with respect to Delta does not agree with commonly accepted ideas about their coordinating ability. In particular, catalysts when activated with B(C(6)F(5))(3) appear to function at low values of Delta as compared to those featuring B(C(6)F(5))(4) (less coordinating) and FAl[(o-C(6)F(5))C(6)F(4)](3) (more coordinating) or PMAO (more coordinating) counterions where the ordering in Delta is MeB(C(6)F(5))(3) < B(C(6)F(5))(4) < FAl[(o-C(6)F(5))C(6)F(4)](3) approximately PMAO. Possible reasons for this behavior are discussed.

Complete defluorination of 1,2,3,4-tetramethyl-5-(trifluoromethyl)cyclopentadiene by titanium tetrakis(dimethylamide)--selective formation of a cyclic hexanuclear titanium fluoroamide and 6,6-dimethylaminotetramethylfulvene

1,2,3,4-Tetramethyl-5-(trifluoromethyl)cyclopentadiene (Cp*CF3-H, 1) reacts with [Ti(NMe2)4] (2) under mild conditions to give [Ti(mu-NMe2)(NMe2)(mu-F)(F)]6 (3) in nearly quantitative yield. The molecular structure of 3 consists of a ring of six [TiF2(NMe2)2] edge-bridged octahedra. Titanium complexes containing the Cp*CF3 ligand, which was the primary intention of these investigations, were not observed. C5Me4=C(NMe2)2 (4) was isolated as a by-product. The complete defluorination of an aliphatic CF3 group occurs during the reaction. The reaction mechanism involves the primary formation of a difluorofulvene intermediate C5Me4=CF2 (5), which was monitored by NMR studies. Density functional theory calculations predict a highly charged C6 atom (+0.87) in 5, which is discussed as the driving force of the reaction.