Doubly-Reduced Pentacene in Different Coordination Environments: X-ray Crystallographic and Theoretical Insights into Structural and Electronic Changes

Chemical reduction of pentacene (C22H14, 1) with Group 1 metals ranging from Li to Cs revealed that 1 readily undergoes a two-fold reduction to afford a doubly-reduced 1(2-) anion in THF. With the help of 18-crown-6 ether used as a secondary coordinating agent, five pi-complexes of 1(2-) with different alkali metal counterions have been isolated and fully characterized. This series of complexes enables the first evaluation of alkali-metal ion binding patterns and structural changes of the 1(2-) dianion based on the crystallographically confirmed examples. The difference in coordination of the smallest Li+ ion vs. heavier Group 1 congeners has been demonstrated. In addition, the use of benzo-15-crown-5 in the reaction of 1 with Na metal allowed the isolation of the unique solvent-separated ion product with a "naked" dianion, 1(2-). The detailed structural analyses of the series revealed the C-C bond alteration and core deformation of pentacene upon two-fold reduction and complexation. The negative charge localization at the central six-membered ring of 1(2-) identified by theoretical calculations corroborates with the X-ray crystallographic results. Subsequent in-depth theoretical analysis provided a detailed description of changes in the electronic structure and aromaticity of pentacene upon reduction.