Two-Fold Reduction of Dibenzo[ a, e]cyclooctatetraene with Group 1 Metals: From Lithium to Cesium

Chemical reduction of sym-dibenzo[a,e]cyclooctatetraene (DBCOT, 1) has been investigated using group 1 metals ranging from Li to Cs as the reducing agents. With all alkali metals, DBCOT readily undergoes a 2-fold reduction to afford the doubly reduced DBCOT2- (12-) anion. Seven products of 12- with different alkali-metal counterions have been isolated and fully characterized. Their X-ray diffraction structural analyses reveal the ligand core planarization upon addition of two electrons as well as the versatility of 12- in alkali-metal coordination. The negative charge localization at the central eight-membered ring of 12- identified by theoretical calculations is fully consistent with the X-ray crystallographic and NMR spectroscopic results. A broad range of solid-state structural arrangements of the resulting products stems from different metal binding patterns and secondary interactions involving coordinating agents used for crystallization (THF vs 18-crown-6 and [2.2.2]cryptand). This family of alkali-metal salts of 12- provides useful reagents for ligand metathesis reactions that can be used in the preparation of various transition-metal or lanthanide-based organometallic complexes.