Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization

Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.

Social Entrepreneurship and Social Business: Retrospective and Prospective Research

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Social Entrepreneurship and Social Business: Retrospective and Prospective Research

Ye

Agostic or not? Detailed Density Functional Theory studies of the compounds [LRh(CO)Cl], [LRh(COD)Cl] and [LRhCl] (L = cyclic (alkyl)(amino)carbene, COD = cyclooctadiene)

Detailed Density Functional Theory (DFT) studies were conducted on the rhodium compounds [LRh(CO)Cl] (1), [LRhCl] (2) and [LRh(COD)Cl] (3) (L = cyclic (alkyl)(amino)carbene, CAAC; COD = cyclooctadiene). Particular attention was paid to the two cyclohexyl hydrogens of the CAAC which are in close proximity to the metal centre. Bader and NBO analyses confirmed an agostic interaction, and NBO analysis revealed that in the case of 1, the Rh–CO antibonding orbital acts as an acceptor. Removal of the CO ligand (2) did not significantly change the agostic interaction of the two cyclohexyl hydrogens or the geometry of the cyclohexyl ligand. Replacement of the 2e− donor CO with the 4e− donor COD gives a different picture. Although both cyclohexyl hydrogens are still in close proximity to the metal centre, neither are agostically bound to it. In fact, the very weak interaction of one of them is of the same order as that present in the Cl–H bond. We thus suggest a revised description of agostic bonding

Dft investigation of the 'quasi-living' propene polymerisation with Cp*TiMe3/b(C6F5)(3): the 'naked cation' approach

Some time ago we reported the quasi-living polymerization of propene with the catalytic mixture of Cp*TiMe3 and B(C6F5)3 (Cp* = C5Me5). Surprisingly, this mixture is extremely sensitive towards the nature of the anion and the presence of aluminium alkyl. This intriguing observation led us to the attempt to unearth the underlying reaction mechanism using a computational approach. In this communication, we are reporting the first results with the naked cation approach. We obtained evidence, that the 1,2 insertion is the predominant reaction pathway. Whereas initial 1,2 and 2,1 insertion barriers are comparable, consequent second insertion is more discriminating between the two. Although we obtained evidence for the formation of -H agostic bonds, we found that -H elimination is a rare event due to the rather high activation barrier. We can conclude that the quasi-living polymerisation is at least partly an intrinsic property of the cation