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The titanocene–bis(trimethylsilyl)acetylene (BTMSA) complexes Cp′2Ti[η2-C2(SiMe3)2] (Cp′C5H5−nMen; n=0–5) react with acetone azine Me2CNNCMe2 (AA) in two different ways depending on the number of Me substituents at the Cp′ ligands (n). For n=0–2, BTMSA is replaced by AA, which then undergoes an oxidative addition accompanied by a proton transfer, affording titana-2-isopropyl-4-methyl-2,3-diazacyclopent-3-ene complexes A0–A2. For n=3–5, replacement of the acetylene is followed by the splitting of AA to give either (C5Me5)2Ti(III)(NCMe2) (C5) or the Ti(IV) (Cp′A)2Ti complexes B3 and B4 for n=3 and 4, respectively. The intramolecularly bridging Cp′A ligand 1-(η5-2,3,5-trimethylcyclopentadienide)-2-(η1-amide)-2,2′-dimethylethane arises from a formal insertion of the CN bond of isopropylidene amide into a CH bond of one methyl group of the cyclopentadienyl ligands before or after splitting of AA. Crystal structures of C5 and (η5:η1-C5HMe3CH2CMe2NH)2Ti(IV) (B4) were determined.

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 72%, 21% and ca. 100% 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.

The (C5H5−nMenTi[η5-C2(SiMe3)2] (n = 0–5) (1A–1F) complexes were prepared by the reduction of corresponding titanocene dichlorides with magnesium in THF in the presence of bis(trimethylsily)acetylene (BTMSA). All of them were characterized by spectroscopic methods and (C5HMe2Ti[η5-C2(SiMe3)2] (1E) by the X-ray crystal analysis. The complexes decompose at temperatures in the range 100–200°C. Those with n = 0–3 yield (μ-η5 : η5-fulvalene)(di-μ-hydrido)bis (η5-cyclopentadienyltitanium) (2A) and its methylated analogues (2B–2D) whereas BTMSA is released. The crystal structure of 2D showed that the hexamethylfulvalene ligand contains non-methylated carbon atoms in inner alternate positions. Complex 1E afforded a mixture of products. Among them only volatile isomers (η3:η4-1,2-dimethyl-4,5-dimethylcyclopenteny)(η5-tetramethylcyclopentadienyl)titanium (2Ea) and (η3:η4-1,3-dimethyl-4, 5-dimethylenecyclopentenyl)(η5-tetramethylcyclopentadienyl)titanium (2Eb) have been so far isolated as minor products. The C5Me5 comples 1F yields quantitatively (η3:η4-1,2,3-trimethyl-4,5-dimethylenecyclopentyl)(pentamethylcyclopentadienyl)titanium (2F) and BTMSA is hydrogenated to a mixture of cis- and trans-bis(trimethylsilyl)ethene.

The reaction of (η5-C5H5)TiCl2 with K(C5Bz5)(Bz = benzyl) affords (η5-C5 H5)(η5-C5Bz5TiCl (1 in a high yield. The oxidation of 1 with AGCl in THF yields (η5-C5H5)(η5-C5Bz5)TiCl2 (2), whereas the reduction of 1 with Mg in the presence of bis(trimethylsilyl)acetylene gives (η5-C5H5)(η5-C5Bz5)Ti [η2-(CSiMe3)2] (3). The X-ray analyses of 1, 2, 3 revealed, that in 1 two, whereas in 2 and 3 only one, of the Bz substituents are inclined towards the Ti atom. The ESR spectra of 1 in solution and in the glassy state showed that it is monomeric in toluene from 23 to − 140°C; however, in 2-methyltetrahydrofuran it coordinates the solvent at low temperature.

The cyclotrimerization of 1-phenyl-2-(trimethylsilyl)acetylene (PTMSA) catalysed by the TiCl 4 /Et 2 AlCl (Ti: Al = 10–20) system affords 1,3,5-triphenyl-2,4,6-tris(trimethylsilyl)benzene ( I ) (minor product) and ethyl-triphenyl-bis(trimethylsilyl)benzene ( II ) (major product) in low yields. An X-ray crystal structural study of I revealed that there are two different conformers in the unit cell. Compound I has a planar central ring with alternating values of angles at carbon atoms bearing the trimethylsilyl group (av. 116.8(6)°) and at carbon atoms bearing the Ph group (av. 123.1(9)°). The steric hindrance between the substituents is relieved by the almost perpendicular orientation of the Ph rings with respect to the central ring (81 ± 5°). The replacement of one trimethylsilyl group by an Et group from Et 2 AlCl probably precedes the ring closure yielding II . The presence of trimethylsilyl groups on the carbon atoms involved in this step hinders the ring closure and gives rise to what are probably linear oligomers of PTMSA.

The catalytic system Et 2 AlCl/TiCl 4 induces Diels-Alder addition of phenyl(trimethylsilyl)acetylene (PTMSA) to 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-pentadiene, and [6+21 addition of PTMSA to 1,3,5-cycloheptatriene. The 1-phenyl-2-(trimethylsilyl)-cyclohexa-1,4-dienes obtained undergo thermolysis, yielding the corresponding ortho -(trimethylsilyl)biphenyl derivatives. The cyclohexadienes containing vicinal methyl and trimethylsilyl groups evolve methane at 300°C (the 3-methyl group is released). All other derivatives release hydrogen at only 260°C.

The catalytic system Et 2 AlCl/TiCl 4 induces Diels-Alder cycloaddition of bis(trimethylsilyl)acetylene to 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and ( E )-1,3-pentadiene affording 1,2-bis(trimethylsuyl)cyclohexa-1,4-dienes in high yields. The cyclohexadienes are readily converted to the corresponding 1,2-bis(trimethylsilyl)benzenes upon heating to 240°C. Mass, infrared, 1 H, 13 C and 29 Si NMR spectra of all the products obtained are reported and briefly discussed. The crowded character of aromatic compounds is reflected in their mass, 13 C and 29 Si NMR spectra.

The reaction pathway between the components of the catalytic system Cp 2 TiCl 2 -LiAlH 4 -unsaturated hydrocarbon, depends considerably on the nature of the reaction medium. In dienes which are able to form stable η 3 -allyltitanocene derivatives, these represents the main reaction product; they are catalytically active in the double bond shifts. In dienes not forming stable η 3 -allyltitanocene derivatives and in α-olefins, the catalytically-active η 1 -alkenyl- and alkyl-titanocenes are transiently formed, deactivating rapidly to give the [(C 5 H 5 )(C 5 H 4 )TiHAlR 2 ] 2 complexes and minor amounts of η 3 -allyltitanocene derivatives in a side reaction involving intermolecular hydrogen transfer. In all α-olefinic hydrocarbons, the formation of alkyl-, η 1 -alkenyl- or η 3 -allyl-titanocene derivatives is preceded by the hydroalumination reaction, yielding Cp 2 TiCl 2 AlR 2 complexes. In internal olefins the hydroalumination reaction does not occur and the alkyltitanocenes which are formed catalyze the transformation of (Cp 2 TiCl) 2 into μ-(η 5 : η 5 -fulvalene)-di-μ-chlorobis(η 5 -cyclopentadienyltitanium), thus inducing the self-deactivation of the system.