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.