1-Zirconacyclobuta-2,3-dienes: synthesis of organometallic analogs of elusive 1,2-cyclobutadiene,unprecedented intramolecular C–H activation,and reactivity studies

The structure, bonding, and reactivity of small, highly unsaturated ring systems is of fundamental interest for inorganic and organic chemistry. Four-membered metallacyclobuta-2,3-dienes, also referred to as metallacycloallenes, are among the most exotic examples for ring systems as these represent organometallic analogs of 1,2-cyclobutadiene, the smallest cyclic allene. Herein, the synthesis of the first examples of 1-zirconacyclobuta-2,3-dienes of the type Cp′₂Zr(Me₃SiC₃SiMe₃)] (Cp′₂ = rac-(ebthi), (ebthi = 1,2-ethylene-1,1′-bis(η⁵-tetrahydroindenyl)) (2a); rac-Me₂Si(thi)₂, thi = (η⁵-tetrahydroindenyl), (2b)) is presented. Both complexes undergo selective thermal C–H activation at the 7-position of the ansa-cyclopentadienyl ligand to produce a new type of “tucked-in” zirconocene system, 3a and 3b, that possesses a η³-propargyl/allenyl ligand. Both types of complexes react with carbonyl compounds, producing enynes in the case of 2a and 2b, as well as η¹-allenyl complexes for 3a and 3b. Computational analysis of the structure and bonding of 2a and 3a reveals significant differences to a previously described related Ti complex. All complexes were fully characterised, including X-ray crystallography and experimental results were supported by DFT analysis.

A detailed study of structure, bonding and reactivity of new 1-zirconacyclobuta-2,3-dienes is presented in comparison to a lighter Ti analog. We found a unique C–H activation at the widely used rac-(ebthi) ligand for that only occurs for Zr.