Anion-π Complexes Research Articles

Boundaries of Anion/Naphthalenediimide Interactions: From Anion−π Interactions to Anion-Induced Charge-Transfer and Electron-Transfer Phenomena

The recent emergence of anion-π interactions has added a new dimension to supramolecular chemistry of anions. Yet, after a decade since its inception, actual mechanisms of anion-π interactions remain highly debated. To elicit a complete and accurate understanding of how different anions interact with π-electron-deficient 1,4,5,8-naphthalenediimides (NDIs) under different conditions, we have extensively studied these interactions using powerful experimental techniques. Herein, we demonstrate that, depending on the electron-donating abilities (Lewis basicity) of anions and electron-accepting abilities (π-acidity) of NDIs, modes of anion-NDI interactions vary from extremely weak non-chromogenic anion-π interactions to chromogenic anion-induced charge-transfer (CT) and electron-transfer (ET) phenomena. In aprotic solvents, electron-donating abilities of anions generally follow their Lewis basicity order, whereas π-acidity of NDIs can be fine-tuned by installing different electron-rich and electron-deficient substituents. While strongly Lewis basic anions (OH(-) and F(-)) undergo thermal ET with most NDIs, generating NDI(•-) radical anions and NDI(2-) dianions in aprotic solvents, weaker Lewis bases (AcO(-), H(2)PO(4)(-), Cl(-), etc.) often require the photoexcitation of moderately π-acidic NDIs to generate the corresponding NDI(•-) radical anions via photoinduced ET (PET). Poorly Lewis basic I(-) does not participate in thermal ET or PET with most NDIs (except with strongly π-acidic core-substituted dicyano-NDI) but forms anion/NDI CT or anion-π complexes. We have looked for experimental evidence that could indicate alternative mechanisms, such as a Meisenheimer complex or CH···anion hydrogen-bond formation, but none was found to support these possibilities.

RI-MP2 and MPWB1K Study of π-Anion-π' Complexes: MPWB1K Performance and Some Additivity Aspects.

Several sandwich complexes of hexafluorobenzene, trifluorobenzene, s-triazine, and trifluoro-s-triazine with halides, nitrate, and carbonate anions have been optimized at the RI-MP2/6-31++G** (full and frozen core), B3LYP/6-31++G**, and MPWB1K/6-31++G** levels of theory. All possible combinations of the π-systems and anions (to generate the sandwich π-anion-π' complexes) have been computed and analyzed using the aforementioned levels of theory. This allows us to evaluate the reliability and the performance of the MPWB1K functional to compute the binding energies of the anion-π complexes and to analyze the additivity of the interaction in π-anion-π' complexes where the aromatic rings are of different nature (π-acidity). We have also explored the Cambridge Structural Database and several interesting X-ray structures that support the theoretical calculations that have been found.

On the directionality of anion–π interactions

The directionality of two important noncovalent interactions involving aromatic rings (namely anion-π and cation-π) is investigated. It has been recently published that the anion-π interactions observed in X-ray structures where the anion is located exactly over the center of the ring are scarce compared to cation-π interactions. To explain this behavior, we have analyzed how the interaction energy (RI-MP2/aug-cc-pVDZ level of theory) is affected by moving the anion from the center of the ring to several directions in anion-π complexes of chloride with either hexafluorobenzene or trifluoro-s-triazine. We have compared the results with the directionality of the cation-π interaction in the sodium-benzene complex. The results are useful to explain the experimental differences between both ion-π interactions. We have also computed the van der Waals radii of several halide anions and we have compared them to the neutral halogen atoms.

On the importance of the inclusion of the basis set superposition error counterpoise correction during optimization of ion-π complexes

Several complexes of hexafluorobenzene and trifluorobenzene with halide anions have been optimized using the HF, B3LYP and MP2 methods and either the 6-31++G ∗∗ or 6-311++G ∗∗ basis sets. The BSSE (basis set superposition error) counterpoise correction (CP) was included during the geometry optimization for all complexes at the six levels of theory. The results were compared with cation-π complexes of benzene and trifluorobenzene. The effect of inclusion of BSSE-CP correction during the optimization is very important in some anion-π complexes.

Induced-Polarization Energy Map: A Helpful Tool for Predicting Geometric Features of Anion-π Complexes.

In this manuscript we propose the use of a new tool that we have found useful to predict the geometries of ion-π complexes. This tool is entitled the Induced-Polarization Energy map (IPE map). The novelty of this representation is that in the map only the contribution of the ion-induced polarization term to the total interaction energy for a given noncovalent interaction is contoured in a 2D region. The IPE map has been found useful to predict and explain geometries of several complexes of a tetrahedral 2 anion (BF4(-)) with perfluoropyrazine, perfluoropyridazine, perfluoropyrimidine, the three isomers of perfluorotriazine, and the three isomers of perfluorotetrazine.