Synthesis and Characterization of Two “Tied-Back” Lithium Ketimides and Isolation of a Ketimide-Bridged [Cr₂]⁶⁺ Dimer with Strong Antiferromagnetic Coupling

Reaction of 1 equiv of KN­(SiMe₃)₂ with 9-fluorenone results in the formation of (Me₃Si)­NC₁₃H₈ (1) in high yield after work-up. Addition of 1 equiv of phenol to 1 results in rapid desilylation and formation of 9-fluorenone imine, HNC₁₃H₈ (2). Subsequent reaction of 2 with 1 equiv of LiNⁱPr₂ results in deprotonation and formation of [Li­(Et₂O)]₄[NC₁₃H₈]₄ (3) in good yield. Reaction of 1 equiv of KN­(SiMe₃)₂ with 2-adamantanone for 7 days at room temperature results in the formation of (Me₃Si)­NC₁₀H₁₄ (4) in good yield. Dissolution of 4 in neat MeOH results in rapid desilylation concomitant with formation of 2-adamantanone imine, HNC₁₀H₁₄ (5). Subsequent reaction of 5 with 1 equiv of LiNⁱPr₂ results in formation of [Li­(THF)]₄[NC₁₀H₁₄]₄ (6). Both 3 and 6 were characterized by X-ray crystallography. Finally, reaction of CrCl₃ with 3.5 equiv of 6 results in formation of the [Cr₂]⁶⁺ dimer, [Li]­[Cr₂(NC₁₀H₁₄)₇] (7), which can be isolated in modest yield after work-up. Complex 7 features a Cr–Cr bond length of 2.653(2) Å. Additionally, solid-state magnetic susceptibility measurements reveal strong antiferromagnetic coupling between the two Cr centers, with J = −200 cm^–1.