Cyclic Bis‐alkylidene Complexes of Titanium and Zirconium: Synthesis, Characterization, and Reaction.

Transition‐metal alkylidenes have exhibited wide applications in organometallic chemistry and synthetic organic chemistry, however, cyclic Schrock‐carbene‐like bis‐alkylidenes of group 4 metals with a four‐electron donor from an alkylidene have not been reported. Herein, the synthesis and characterization of five‐membered cyclic bis‐alkylidenes of titanium (4 a,b) and zirconium (5 a,b) are reported, as the first well‐defined group 4 metallacyclopentatrienes, by two‐electron reduction of their corresponding titana‐ and zirconacyclopentadienes. DFT analyses of 4 a show a four‐electron donor (σ‐donation and π‐donation) from an alkylidene carbon to the metal center. The reaction of 4 a with N,N′‐diisopropylcarbodiimide (DIC) leads to the [2+2]‐cycloaddition product 6. Compound 4 a reacted with CO, affording the oxycyclopentadienyl titanium complex 7. These reactivities demonstrate the multiple metal–carbon bond character. The reactions of 4 a or 5 a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine‐coordinated zirconacyclopentadiene 9, respectively, which exhibit two‐electron reductive ability. [ABSTRACT FROM AUTHOR]