Synthesis, crystal structures and some properties of dimethylsilylene-bridged ansa-permethyltitanocene [Ti(IV), (III) and (II)] complexes

Article abstract of DOI:10.1016/s0022-328x(96)06917-3

Dimethylsilylene-bridged complexes Me₂Si(C₅Me₄)₂TiCl (2), Me₂Si(C₅Me₄)₂Ti[η²-C₂(SiMe₃)₂] (3) and Me₂Si(C₅H₄)₂Ti[η²-C₂(SiMe₃)₂] (4) have been prepared by the general methods which are known for obtaining of analogous non-bridged titanocene complexes. X-ray crystal structures of Me₂Si(C₅Me₄)₂TiCl₂ (1), 2, and 3 reveal that the dihedral angle ? between the least-squares planes of cyclopentadienyl rings increases in the order 2 < 3 < 1. Comparison with the structures of analogous (C₅HMe₄)₂Ti and (C₅Me₅)₂Ti compounds shows that the value of ? increases in the series (C₅Me₅)₂Ti < (C₅HMe₄)₂Ti < Me₂Si(C₅Me₄)₂Ti, e.g. in the bis(trimethylsilyl)acetylene complexes from 41.1° for (C₅Me₅)₂Ti[η²-C₂(SiMe₃)₂] (9) to 50.0° for (C₅HMe₄)₂Ti[η²-C₂(SiMe₃)₂] (8) and to 53.5° for 3. Compounds 3, 8 and 9 induce the head-to-tail dimerization of 1-hexyne with the selectivity of 72percent, 21percent and ca. 100percent respectively. The discrepancy between the selectivities and the values of ? for 3 and 8 is accounted for by a larger flexibility of the titanocene skeleton in 8, affording a larger space for a non-specific coordination of 1-hexyne. The effects of the μ-Me₂Si group in 2 and 3 on some of their properties are compared with the effects of Me and H substituents in the non-ansa compounds with controversial results. For instance, the affinity of 2 to 2-methyltetrahydrofuran approaches that of (C₅H₂Me₃)₂TiCl whereas the shift of the v(C≡C) vibration in 3 indicates a stronger metal-acetylene bond than in 9.