CYCLIC ISOMERIC 4-MEMBERED AND 5-MEMBERED HETERODINUCLEAR CARBENE COMPLEXES - FORMED FROM A TITANOCENE VINYLIDENE FRAGMENT AND METAL-CARBONYLS

The four-membered heterodimetallic carbene complexes Cp2* TiC(=CH2)C(=ML(n))O 16 [L(n)M: Cr(CO)5 (a), MO(CO)5 (b), W(CO)5 (c), CpRe(CO)2 (f), Mn2(CO)9(g), Re2(CO)9 (h), Fe(CO)4 (i), Rh(acac)(CO) (i)] are easily prepared by trapping the vinylidenetitanocene [Cp2*Ti=C=CH2] fragment 6 with the corresponding metal carbonyls. The vinylidenetitanocene 6 is generated in a favourable manner from Cp2*Ti(CH3)CH=CH2 (13) or Cp*(Fv)TiCH=CH2 (14) [Fv: C5(CH3)4CH2] by a 1,3-H shift (a elimination) at room temperature. The formation of 16 is characterized by a preferred suprafacial [2+2] cycloaddition. Starting from Cp2*TiC(= CH2)CH2CH2 (5) or 14 and transition metal carbonyls unexpected isomeric five-membered titanacycles Cp2*TiCH= CH-C(=ML(n))O 8 [ML(n) = a-c, MnCp(CO)2 (d), MnCp'(CO)2 (e), f-i] are obtained at higher temperatures (70-100-degrees-C). These oxatitanacyclopentenes 8 are formed also upon heating via the kinetic product 16 with high yield, as a result of a 1,2-hydrogen shift, proved for c, g, h. From the spectroscopic data an eta1-vinylcarbene structure is deduced, exhibiting a cis- (16) or trans-configurated (8) L(n)M=C-C=C sequence. An X-ray structure analysis shows the importance of the acyl resonance form in 8a, typically observed for such heterodimetallic carbene complexes. The reversible liberation of the metal carbonyl is a remarkable property of complexes 16 in solution, whereas the thermodynamic product 8 shows the typical properties of Fischer carbene complexes.