New divalent samarocenes for butadiene polymerisation: influence of the steric effect and the electron density on the catalytic activity

Two new divalent samarocenes, Cp^*′₂Sm(THF) (1) and (Cp^Ph3)₂Sm(THF) (2) (Cp^*′=C₅Me₄ⁿPr, Cp^Ph3=H₂C₅Ph₃-1,2,4), were synthesized and characterized by ¹H NMR and elemental analysis. The activity of 1 and 2 as butadiene polymerisation catalysts was studied, in the presence of MAO and MMAO, and compared to this of Cp^*₂Sm(THF)₂ (3) and (Cp⁴ⁱ)₂Sm (4) (Cp^*=C₅Me₅, Cp⁴ⁱ=C₅HⁱPr₄), in the same conditions. The 1/MAO system presents the highest activity. The less active 2/MAO system leads to a high cis-1,4 regular structure up to 97%. The MMAO cocatalyst is found very sensitive to the steric hindrance of the samarocenes: the activity decreases from 1/MAO to 1/MMAO, and no activity is observed in the case of complexes 2 and 4, associated to MMAO. Complexes 1 and 2 can be both oxidized with AlMe₃ to give the corresponding Sm/Al bimetallics and , respectively.     

Photochemistry of tris(pyrazolyl)borate titanium(IV) complexes

The electronic features and photochemistry of TpTiCl₃ (1) (Tp = hydrotris(pyrazol-1-yl)borate) and Tp*TiCl₃ (2) (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) were studied in THF. Reactive decay of the excited states produced either (or ) and metal center Ti(III) radicals via homolytic cleavage of the Tp → Ti (Tp* → Ti) bond. Cleavage of the Tp → Ti and the Tp* → Ti bond as a primary photoprocess is shown to be consistent with LMCT Tp → Ti and Tp* → Ti excitation. TpTiCl₂(THF) (3) and Tp*TiCl₂(THF) (4) were also prepared by stoichiometric reduction of 1 and 2 with Li₃N. The THF ligand in 3 and 4 was replaced by the stable nitroxyl radical TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) to provide the new complexes TpTiCl₂(TEMPO) (5) and Tp*TiCl₂(TEMPO) (6) in which the TEMPO ligand is η¹ coordinated to Ti(IV). Photolysis of 5 and 6 generate Ti(III) and the TEMPO radical in the primary photochemical step.