Big, Strong, Neutral, Twisted, and Chiral π Acids

Abstract

General synthetic access to expanded π-acidic surfaces of variable size, topology, chirality, and π acidity is reported. The availability of π surfaces with these characteristics is essential to develop the functional relevance of anion-π interactions with regard to molecular recognition, translocation, and transformation. The problem is that, with expanded π surfaces, the impact of electron-withdrawing substituents decreases and the high π acidity needed for strong anion-π interactions can be more difficult to obtain. To overcome this problem, it is herein proposed to build large surfaces from smaller fragments and connect these fragments with bridges that are composed only of single atoms. Two central surfaces for powerful anion-π interactions, namely, perfluoroarenes and naphthalenediimides (NDIs), were selected as fragments and coupled with through sulfide bridges. Their oxidation to sulfoxides and sulfones, as well as fluorine substitution in the peripheral rings, provides access to the full chemical space of relevant π acidities. According to cyclic voltammetry, LUMO levels range from -3.96 to -4.72 eV. With sulfoxide bridges, stereogenic centers are introduced to further enrich the intrinsic planar chirality of the expanded surfaces. The stereoisomers were separated by chiral HPLC and characterized by X-ray crystallography. Their topologies range from chairs to π boats, and the latter are reminiscent of the cation-π boxes in operational neuronal receptors. With pentafluorophenyl acceptors, the π acidity of NDIs with two sulfoxide groups in the core reaches -4.45 eV, whereas two sulfone moieties give a value of -4.72 eV, which is as low as with four ethyl sulfone groups, that is, a π superacid near the limit of existence. Beyond anion-π interactions, these conceptually innovative π-acidic surfaces are also of interest as electron transporters in conductive materials.

Core‐Substituted Naphthalenediimides: LUMO Levels Revisited, in Comparison with Preylenediimides with Sulfur Redox Switches in the Core

“…LUMO levelso fN DIs 9-12 have been previously reported as part of ac omprehensive series on aryl sulfides in the NDI core. [15] The E LUMO = À3.96 meV of phenyls ulfide 9 was found DE LUMO = À40 meV below the E LUMO = À3.92 eV of alkyl sulfide 6.T his change with secondary phenyls on primary sulfides in the core was clearly less pronouncedt han the DE LUMOÀ 100 meV with secondary phenyls on primary imides in NDI 7. Secondary pentafluorophenyl substituents on the primary sulfides in NDI 10 loweredt oL UMO level to E LUMO = À4.13 eV.…”

“…In assessments of the LUMO level of core-substitutedN DIs in functional systems, contributionsf rom the secondary substituents of the intrinsic primary imides remain underappreciated. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] This seemed justifiedb ecause these substituents were expectedt ob el argely decoupled at the nitrogen atom. In functional systems, the main differencea tt he imide periphery concerns aromatic versus aliphatic substituents.…”

“…To complete the picture, the previously reported [15] LUMO levels for NDIs 11 and 12 with only one arylsulfide in the core were added to the graphicals ummary in Figure 1. Viewed from unsubstituted NDI 1,t he contributions were nearly additive.…”

“…LUMO levels wereobtained by differential pulse voltammetry (DPV)inC H 2 Cl 2 and are reported in eV relative to À5.10 eV for Fc + /Fc. Values given in italics are extrapolated from literature [15,26,28] and are shownf or comparison and completion (15)(16)(17)(26)(27)(28)(29)(30). ChemistryOpen 2016, 5,219 -226 www.chemistryopen.org 14 with two sulfoxides in the core.…”

“…[1,14] Most importantly,c ontributions from secondary substituents on the intrinsic imide and primary substituents in the core have been largelyn eglected. [1,15] Considering this situation, we decided to prepare af ocused collection of NDIs and determinet heir LUMO levels from the unambiguous maximum in their DPV under identical, clearly definedc onditions. As ar esult,w eh ere provide the pertinent data set needed to dissect individual contributionsf rom substituentst ot he LUMO levels of NDIs.…”