Anion-Induced Electron Transfer

As counterintuitive as it might seem, in aprotic media, electron transfer (ET) from strong Lewis basic anions, particularly F^-, OH^-, and CN^-, to certain π-acids (πA) is not only spectroscopically evident from the formation of paramagnetic πA^•- radical anions and πA^2- dianions, but also thermodynamically justified because these anions' highest occupied molecular orbitals (HOMOs) lie above the π-acids' lowest unoccupied molecular orbitals (LUMOs) creating negative free energy changes (Δ G°_ET < 0). Depending on the relative HOMO and LUMO energies of participating anions and π-acids, respectively, the anion-induced ET (AIET) events take place either in the ground state or upon photosensitization of the π-acids. The mild basic and charge-diffuse anions with lower HOMO levels fail to trigger ET, but they often form charge transfer (CT) and anion-π complexes. Owing to their high HOMO levels in aprotic environments, strong Lewis basic anions, such as F^- enjoy much greater ET driving force (Δ G°_ET) than mild and non-basic anions, such as iodide. In protic solvents, however, the former become more solvated and stabilized and lose their electron donating ability more than the latter, creating an illusion that F^- is a poor electron donor due to the high electronegativity of fluorine. However, UV-vis, EPR, and NMR studies consistently show that in aprotic environments, F^- reduces essentially any π-acid with LUMO levels of -3.8 eV or less, revealing that contrary to a common perception, the electron donating ability of F^- anion is not dictated by the electronegativity of fluorine atom but is a true reflection of high Lewis basicity of the anion itself. Thus, the neutral fluorine atoms with zero formal charge and F^- anion have little in common when it comes to their electronic properties. The F^- anion can also legitimately act as a Brønsted base when the proton source has a p K_a lower than that of its conjugate acid HF (15), not the other way around, and ET from F^- to a poor electron acceptor is not thermodynamically feasible. While there is no shortage of indisputable evidence and clear-cut thermodynamic justifications for ET from F^- and other Lewis basic anions to various π-acids in aprotic solvents, because of the aforesaid misconception, it had been posited that F^- perhaps formed diamagnetic Meisenheimer complexes via nucleophilic attack, deprotonated an aprotic solvent DMSO against an insurmountably high p K_a (35) leading to a π-acid reduction, or formed [F^-/πA^•+] complexes via a thermodynamically prohibited oxidation of π-acids. Unlike AIET, however, none of these hypotheses was thermodynamically viable nor supported by any experimental evidence. First, by defining the thermodynamic criteria of AIET pathways and all other alternate hypotheses and then evaluating the spectroscopic signals emanating from the interactions between different anions and π-acids and Lewis acids in the light of these criteria, this Account comes to a conclusion that AIET is the only viable mechanism that can rationalize the reduction of π-acids without violating any thermodynamic rules. The paradigm-shifting discovery of AIET not only exposed a common misconception about the electron donating ability of F^- but also enabled naked-eye detection of toxic anions, electrode-free silver plating, luminescent silver nanoparticle synthesis, light-harvesting, and conductivity enhancement of conjugated polymers, with more innovative applications still to come.