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-alkylidene of group 4 metals having a four-electron donor from an alkylidene has not been reported. Herein, we report the synthesis and characterization of five-membered cyclic bis-alkylidenes of titanium ( 4a,b ) and zirconium ( 5a,b ), as the first well-dcefined group 4 metallacyclopentatrienes, by two-electron reduction of their corresponding titana- and zirconacyclopentadienes. DFT analyses of 4a show a four-electron donor (σ-donation and π-donation) from an alkylidene carbon to the metal center. The reaction of 4a with N,N'-diisopropylcarbodiimide (DIC) leads to [2+2]-cycloaddition product 6. 4a reacted with CO affording the oxycyclopentadienyl titanium complex 7 . These reactivities demonstrate the multiple metal-carbon bond character. The reactions of 4a or 5a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine-coordinated zirconacyclopentadiene 9 , respectively, which exhibit their two-electron reductive ability.