SYNTHESIS, CRYSTAL-STRUCTURES AND SOME PROPERTIES OF DIMETHYLSILYLENE-BRIDGED ANSA-PERMETHYLTITANOCENE [TI(IV), TI(III) AND TI(II)] COMPLEXES

Dimethylsilylene-bridged complexes Me2Si(C5Me4)(2)TiCl (2), Me2Si(C5Me 4)(2)Ti[eta(2)-C-2(SiMe3)(2)] (3) and Me2Si(C5H4)(2)Ti[eta(2)-C-2(SiMe 3)(2)] (4) have been prepared by the general methods which are known f or obtaining of analogous non-bridged titanocene complexes. X-ray crys tal structures of Me2Si(C5Me4)(2)TiCl2 (1), 2, and 3 reveal that the d ihedral angle phi between the least-squares planes of cyclopentadienyl rings increases in the order 2 < 3 < 1. Comparison with the structure s of analogous (C5HMe4)(2)Ti and (C5Me5)(2)Ti compounds shows that the value of phi increases in the series (C5Me5)(2)Ti < (C5HMe4)(2)Ti < M e2Si(C5Me4)(2)Ti, e.g. in the bis(trimethylsilyl)acetylene complexes f rom 41.1 degrees for (C5Me5)(2)Ti[eta(2)-C-2(SiMe3)(2)] (9) to 50.0 de grees for (C5HMe4)(2)Ti[eta(2)-C-2(SiMe3)(2)] (8) and to 53.5 degrees for 3. Compounds 3, 8 and 9 induce the head-to-tail dimerization of 1- hexyne with the selectivity of 72%, 21% and ca. 100% respectively. The discrepancy between the selectivities and the values of phi 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-h exyne. The effects of the mu-Me2Si group in 2 and 3 on some of their p roperties are compared with the effects of Me and 11 substituents in t he non-ansa compounds with controversial results. For instance, the af finity of 2 to 2-methyltetrahydrofuran approaches that of (C5H2Me3)(2) TiCl whereas the shift of the nu(C=C) vibration in 3 indicates a stron ger metal-acetylene bond than in 9.