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

Abstract

Chemical reduction of pentacene (C 22 H 14 , 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 2À anion in THF. With the help of 18-crown-6 ether used as a secondary coordinating agent, five π-complexes of 1 2À 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 2À 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 2À . The detailed structural analyses of the series revealed the CÀ 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 2À identified by theoretical calculations corroborates with the Xray crystallographic results. Subsequent in-depth theoretical analysis provided a detailed description of changes in the electronic structure and aromaticity of pentacene upon reduction.

Interplay of Charge and Aromaticity Upon Chemical Reduction of p-Quinquephenyl with Alkali Metals

“…Notably, the Na2 ion is η 6 -coordinated to the adjacent phenyl ring with a smaller range of Na–C distances (2.86(4)–2.93(4) Å). This asymmetric metal binding of Na + ions has been observed in a shorter paraphenylene p -terphenyl, with the Na + ions being coordinated to either two different phenyl rings or the same one. , Both Na + ions are capped by three THF molecules, and the Na–O THF distances of 2.19(4)–2.301(8) Å are consistent with those previously reported. − …”

“…Similar to 4 , the Rb + ion is coordinated to the phenyl ring next to the central one but in an asymmetric η 6 -fashion with the Rb–C distances ranging over 3.195(1)–3.574(1) Å (Figure ). The Rb + ion is also entrapped by an 18-crown-6 ether (Rb–O crown : 2.755(10)–2.813(10) Å) with all Rb–C and Rb–O distances being close to those previously reported. ,− …”

“…The K + ion is η 2 -coordinated to the phenyl ring next to the central one, with the K–C distances of 3.168(3) Å and 3.189(3) Å (Figure ). A similar binding mode has been observed in the doubly reduced [6]­CPP and [8]­CPP, with the K–C distances being slightly longer due to the curvature of these macrocycles. , The coordination environment of the K + ion in 4 is completed by an equatorial 18-crown-6 ether (K–O crown : 2.752(2)–2.871(2) Å) and an axial THF molecule (K–O THF : 2.747(3) Å), with the K–C and K–O distances being comparable to those reported in the literature. ,,,− …”

“…Considering the nature of the doubly reduced product, the signals of 1 2– in 2 are shifted slightly downfield than those for complexes 4 and 5 (see the Supporting Information for more details), but shifted slightly more upfield compared to 3 , which may stem from weaker interactions of Na + ions to the doubly reduced quinquephenyl core in solution. Recently, we investigated the reduction of pentacene through NMR spectroscopy and revealed that the signals of the doubly reduced pentacene shift considerably (up to 4.46 ppm) compared to neutral parent. This is a stark contrast to the solution behavior of p -quinquephenyl and its doubly reduced state, where the proton signals are shifted to a far lesser extent.…”

Topology-Dependent Synthesis, Structures, and Bonding Interactions of Uranium Polyarene Complexes

“…Encouraged by the recent report on the synthesis of pentacene dianion 30 with a variety of alkali metals by Petrukhina and co-workers, 100 we also aimed to synthesize a uranium pentacene complex. However, due to the low solubility of pentacene in THF and the instability of uranium precursors 1 and 7 under the reaction conditions for a long time, we were not able to isolate any uranium pentacene complex.…”