Reactions of Dilithium Dibenzopentalenides with Cr(CO)3 (CH3 CN)3 : Unexpected Formation of a Cubic Tetramer of an Anionic Hydrodibenzopentalenyl Complex.

To explore the coordination chemistry of dibenzopentalene dianion, reactions of two dilithium dibenzopentalenides having different silyl substituents with Cr(CO)3 (CH3 CN)3 were investigated. The products were unexpected anionic complexes, [Li(Et2 O)]+ [Cr(η5 -9-hydrodibenzopentalenyl)(CO)3 ]- . The proton at the 9-position is derived from Cr(CO)3 (CH3 CN)3 , as evidenced by the use of Cr(CO)3 (CD3 CN)3 . The X-ray diffraction analysis revealed that the chromium is coordinated by an anionic five-membered ring of the pentalene skeleton, and the lithium atom is coordinated by oxygen atoms of the carbonyl groups. The complexes form a dimer or a cage-like tetramer via carbonyl-lithium interactions, depending on the bulk of the silyl groups. The cubic tetramer appears to retain its cage structure in nonpolar solvents such as benzene.