58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Using a library of electron-deficient 1,4,5,8-naphthalenediimide (NDI) derivatives with tunable electron accepting capacity, i.e., variable LUMO levels, we have demonstrated that in aprotic solvents, such as DMSO, DMF, MeCN, THF, and ODCB, (1) strongly Lewis basic anions (OH^–, F^–) trigger thermal electron transfer (ET) to NDIs generating NDI^¥– radical anions and NDI^2– dianions, (2) less Lewis basic anions (AcO^–, H₂PO₄^–) that cannot trigger thermal ET to NDIs, can initiate photoinduced ET to the photo-generated SOMO-1 of excited NDIs (i.e., anion-induced photoreduction of NDI to NDI^¥–); (3) poor Lewis basic anions (Cl^–, Br^–, I^–), which cannot not produce any NDI^¥– radical anions via thermal or photoinduced ET, can form charge transfer (CT) complexes via orbital-mixing; and (4) non-Lewis basic, charge-diffuse anions (TfO^–, ClO₄^–) can bind NDIs through nonchromogenic anion–¹ and CHáááanion interactions.

Thermal and photoinduced ET and CT events involving Lewis basic anions and NDIs have been confirmed by UV/Vis, NMR, and EPR spectroscopies. The UV/Vis spectroscopic changes of DPNDI during the F^– titration are essentially identical to those engendered by its stepwise electrochemical reduction to DPNDI^¥– radical anion (–450 mV vs. Ag/AgCl in DMF) and DPNDI^2– dianion (–900 mV vs. Ag/AgCl in DMF). Furthermore, the characteristic ¹H NMR signals of DPNDI disappear in the presence of 1 equiv. F^– and OH^– ions, while the characteristic EPR signals of paramagnetic DPNDI^¥– radical anion emerge. The DPNDI^¥– radical anion can be oxidized back to neutral form with an oxidizing agent (NOBF₄), as the characteristic signals of neutral DPNDI return to the full glory. These powerful spectroscopic results unequivocally demonstrate that thermal ET from the Lewis basic anions to ¹-acidic DPNDI is indeed responsible for the formation of paramagnetic DPNDI^¥– radical anion and rule out alternative possibilities such as a covalent Meisenheimer complex (C–F s-bond) formation or deprotonation of core-Hs of DPNDI. We have further demonstrated that although less Lewis basic anions, such as AcO^–, Cl^–, Br^–, and I^– do not trigger thermal ET to DPNDI, they can reduce a stronger ¹-acidic dicyano-NDI (DCNDI) receptor that has a much lower LUMO level to DCNDI^¥– radical anion. While both thermal and photoinduced ET from anions to NDIs produce paramagnetic NDI^¥– radical anions, anion/NDI CT complexes remain diamagnetic and their CT absorption spectra are distinctly different from the NDI^¥– radical anion spectra. In protic solvents (H₂O, alcohols), F^– becomes highly solvated and so stabilized that ET is turned OFF. Although the reduced NDI species are stable in dark, inert environment and can be oxidized back to neutral forms with NOBF₄, the oxidized anions generated upon ET to NDIs act as sacrificial agents, as they are consumed irreversibly by radical reactions (e.g., H-abstraction) with solvents and counterions. This work was published in a J. Am. Chem. Soc. (2012) Article, which was also featured on the journal cover.

Using X-ray crystallography, we have shown that in coordination polymers comprised of DPNDI ligands, non-Lewis basic TfO^– and NO₃^– bind with the electron-deficient imide rings of the ¹-acidic ligands via anion–¹ interactions. Furthermore, charge-diffuse ClO₄^– ions form CHáááanion H-bonds with DPNDI ligands and template the formation of a non-catenated square-grid metal–organic framework, cavities of which are occupied by the templating counterions. These anions do not participate in ET or CT interactions and therefore, no spectroscopic changes are observed. These results were published in a Chem. Commun. (2013) Communication.

To demonstrate the generality of tunable electronic interactions between anions and ¹-acidic receptors, namely, ET, CT, and anion–¹ interactions, we have introduced electron-deficient 3,4,9,10-perylenediimide (PDI) to various anions. UV/Vis, NMR, and EPR studies confirmed that analogous to NDI derivatives, PDI is also reduced to its PDI^¥– radical anion and PDI^2– dianion by OH^– anion, while F^– reduces it only to PDI^¥– radical anion. This work was published in Organic and Biomolecular Chemistry (2013).