Visiting the Limits between a Highly Strained 1-Zirconacyclobuta-2,3-diene and Chemically Robust Dizirconacyclooctatetraene.

The reaction of the allene precursor Li (Me SiC SiMe ) with [Cp ZrCl ] (Cp=cyclopentadienyl) was examined. The selective formation of hitherto unknown linear, allene-bridged dizirconocene complexes [(Cp ZrCl) {-μ-(Me Si)C (SiMe )-}] and [(Cp Zr) {-μ-(Me Si)C (SiMe )-} ] was observed. Upon σ coordination of the allenediyl unit to {Cp Zr}, pyrophoric Li (Me SiC SiMe ) is tamed stepwise to yield a surprisingly robust 1,5-dizirconacyclooctatetra-2,3,6,7-ene with cumulated double bonds. This complex is unexpectedly inert against moisture, air, water and acetone. Surprisingly, it degrades under MS conditions to give the highly strained 1-zirconacyclobuta-2,3-diene. All compounds isolated have been fully characterised and the molecular structures are discussed. The stability and reactivity of these complexes are rationalised by DFT computations.