From a historical point of view “From Silicon to Titanium and Back to Silicon”, the genesis of the titanocene complex of bis(trimethylsilyl)acetylene (1) is explained. Similarities to the known 1-silacyclopropenes led us to describe the bonding in 1 in terms of a 1-titanacyclopropene. Complex 1 is a useful source of the highly reactive bis(cyclopentadienyl)titanium(II), which undergoes both stoichiometric and catalytic reactions that bear a striking similarity to some of the reactions of the isolobal silicon analogues.

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The rather exotic and uncommon five-membered metallacyclocumulenes (1-metallacyclopenta-2,3,4-trienes) Cp′ 2 M(η 4 -RC 4 R) (Cp′ ) substituted η 5 -cyclopentadienyl) are formed by "cis" complexation of titanocene or zirconocene fragments with 1,3-butadiynes or by coupling of two σ-acetylide groups in the coordination sphere of the metals. Examples of these complexes, Cp 2 M(η 4 -RC, and [Cp 2 Zr] 3 [1,3,5-(η 4 -t-BuC 4 ) 3 C 6 H 3 ], are discussed with regard to their genesis, synthesis, dynamics, structures, and bonding. These compounds are compared with the also uncommon and very similar five-membered metallacyclopentynes (1-metallacyclopent-3-ynes) Cp′ 2 M(η 4 -R 2 C 4 R 2 ) (M ) Zr: Cp′ ) Cp, R 2 ) t-Bu/H, Me 3 Si/H, H 2 ; Cp′ ) t-BuC 5 H 4 , R 2 ) H 2 ; M ) Ti: Cp′ ) Cp, R 2 ) H 2 ) formed by analogous "cis" complexation of 1,2,3-butatrienes, in terms of their chemistry, structures, and bonding. Furthermore, reactions in which metallacyclocumulenes were assumed to be intermediates are described, together with the reactivity of metallacyclocumulenes toward different complexes and substrates: e.g. metal complexes, carbon dioxide, diisobutylaluminum hydride, and tris(pentafluorophenyl)borane. Upon complexation with a second transition-metal complex fragment either bridging formal µ-"cis" or µ-"trans" complexes of 1,3-butadiynes and 1,2,3-butatrienes were formed.

The CC coupling of aryl nitriles at Group 4 metallocenes leads to unusual ring-strained 1-metalla-2,5-diaza-cyclopenta-2,4-dienes. The structural, energetic, and chemical properties of these complexes are described. The reactions of these compounds towards CH3 CN, H2 , CO2 , and HCl usually lead to the release of one nitrile and its replacement by different co-substrates.

The synthesis and characterisation of a 1-titanacyclobuta-2,3-diene complex, an organometallic analog of elusive 1,2-cyclobutadiene, is presented.

The reaction of trimethylsilyl-substituted 2-aminopyridines with mixed chloro(dialkylamido)metal complexes (titanium and zirconium) leads via amine elimination to octahedral group 4 metal complexes that contain amine, amido, and aminopyridinato ligands. The X-ray crystal structure analyses of (4-Me-TMS-APy)(NMe(2))(HNMe(2))TiCl(2) (1) (crystallographic data: P2(1)/c (No. 14), monoclinic, a = 16.754(2) Å, b = 14.395(2) Å, c = 17.890(3) Å, beta = 110.28(1) degrees, Z = 8) and (6-Me-TMS-APy)(NEt(2))(HNEt(2))ZrCl(2) (2) (crystallographic data: P2(1)/n (No. 14) monoclinic, a = 10.125(1) Å, b = 16.331(1) Å, c = 15.276(2) Å, beta = 93.90(1), Z = 4) prove the compounds to be mononuclear with a cisoid arrangement of the two chloro ligands embedded in a reactive pocket determined by the steric demand of the three nitrogen containing ligands. Oligo- and polymerization studies with propene and 1-butene reveal the following results. First, 1 is a remarkably active precatalyst in contrast to the very low activity of 2. Second, MAO, a 1:1 mixture of i-Bu(3)Al/B(C(6)F(5))(3) (homogeneous polymerization) and ethylaluminum sesquichloride (if 1 is incorporated in a MgCl(2)-matrix) have shown to be the most active cocatalysts. Third, the polymers and oligomers are atactic.

COMMUNICATIONSIn conclusion. we have demonstrated that the oxide anion accelerated retro-Diels-Alder reaction can be employed for the facile generation of cyclic enediynes from 1,5-diyne precursors. The use of 9-BOM-anthracene (2) to mask the double bond of the enediyne offers the advantages of excellent stability towards a variety of synthetic transformations and convenient removal when desired. Although the enediyne precursor is highly stable under acidic or neutral conditions, treatment with base leads to rapid cycloreversion and development of the enediyne. This methodology offers significant scope for the generation of new cyclic enediynes. Precursors such as 11 have potential as novel prodrugs when conjugated to appropriate delivery systems.

Reactions of Group 4 metallocene alkyne complexes [Cp'2M(η(2)-Me3SiC2SiMe3)] (1: M = Zr, Cp' = Cp* = η(5)-pentamethylcyclopentadienyl; 2 a: M = Ti, Cp' = Cp*, and 2 b: M = Ti, Cp'2 = rac-(ebthi) = rac-1,2-ethylene-1,1'-bis(η(5)-tetrahydroindenyl)) with diphenylacetonitrile (Ph2CHCN) and of the seven-membered zirconacyclocumulene 3 with phenylacetonitrile (PhCH2CN) were investigated. Different compounds were obtained depending on the metal, the cyclopentadienyl ligand and the reaction temperature. In the first step, Ph2CHCN coordinated to 1 to form [Cp*2Zr(η(2)-Me3SiC2SiMe3)(NCCHPh2)] (4). Higher temperatures led to elimination of the alkyne, coordination of a second Ph2CHCN and transformation of the nitriles to a keteniminate and an imine ligand in [Cp*2Zr(NC2Ph2)(NCHCHPh2)] (5). The conversion of 4 to 5 was monitored by using (1)H NMR spectroscopy. The analogue titanocene complex 2 a eliminated the alkyne first, which led directly to [Cp*2Ti(NC2Ph2)2] (6) with two keteniminate ligands. In contrast, the reaction of 2 b with diphenylacetonitrile involved a formal coupling of the nitriles to obtain the unusual four-membered titanacycle 7. An unexpected six-membered fused zirconaheterocycle (8) resulted from the reaction of 3 with PhCH2CN. The molecular structures of complexes 4, 5, 6, 7 and 8 were determined by X-ray crystallography.