Redox Active Ion-Paired Excited States Undergo Dynamic Electron Transfer

Abstract

Ion-pair interactions between a cationic ruthenium complex, [Ru(dtb)(dea)][PF], C1 where dea is 4,4'-diethanolamide-2,2'-bipyridine and dtb is 4,4'-di-tert-butyl-2,2'-bipyridine, and chloride, bromide, and iodide are reported. A remarkable result is that a 1:1 iodide:excited-state ion-pair, [C1, I], underwent diffusional electron-transfer oxidation of iodide that did not occur when ion-pairing was absent. The ion-pair equilibrium constants ranged 10-10 M in CHCN and decreased in the order Cl > Br > I. The ion-pairs had longer-lived excited states, were brighter emitters, and stored more free energy than did the non-ion-paired states. The H NMR spectra revealed that the halides formed tight ion-pairs with the amide and alcohol groups of the dea ligand. Electron-transfer reactivity of the ion-paired excited state was not simply due to it being a stronger photooxidant than the non-ion-paired excited state. Instead, work term, ΔG was the predominant contributor to the driving force for the reaction. Natural bond order calculations provided natural atomic charges that enabled quantification of ΔG for all the atoms in C1 and [C1, I] presented herein as contour diagrams that show the most favorable electrostatic positions for halide interactions. The results were most consistent with a model wherein the non-ion-paired C1 excited state traps the halide and prevents its oxidation, but allows for dynamic oxidation of a second iodide ion.

Excited-state proton-coupled electron transfer within ion pairs

“…Quantum yields of 0.60-0.70 were calculated for all R-SA À derivatives. These cage escape yields are signicantly larger than those found in the diffusional excited-state electrontransfer oxidation of iodide by ruthenium excited states (<0.10) 66,72 and larger than ion-pairs formed between iodide and ruthenium polypyridyl complexes with cape yields of 0.34 73 and 0.25-0.50. 21 For PCET in hydrogen-bonded systems, Ø CE is oen not measured, or irrelevant because the products never dissociate.…”

“…Due to the complexity of charge distribution in the ruthenium complexes and salicylate derivatives, developing a reasonably accurate approximation for the electrostatic work terms is difficult. 46,73,81,82 However, within a single ruthenium complex, such as Ru-Bpz, these work terms are not be expected to vary signicantly, and as such we chose to model solely the Ru-Bpz data as it provided the largest uniform series, Fig. 10A.…”

Kinetics teach that electronic coupling lowers the free-energy change that accompanies electron transfer

“…Such bimolecular chemistry, and others in general, involves diffusion of the A and the D to form an encounter complex prior to electron transfer (11,36). The freeenergy change associated with the encounter complex formation is small in polar solvents, but becomes more significant in low dielectric media (37). If coupling within the encounter complex is strong at the instance of electron transfer, an adiabatic pathway may be operative that is expected to decrease the yield of products from that calculated based on formal reduction potentials.…”

Resolving orbital pathways for intermolecular electron transfer

“…In particular, the roles that intermolecular interactions have in mediating electron transfer are very challenging to define. During the last two decades, a body of literature has shown that electrostatic interactions 9 , 10 , hydrogen bonding 11 – 13 , and other non-bonding interactions 14 – 19 can enhance IET rates, but interactions that are too strong may suppress reactivity by lowering the driving force for reaction 20 . This scenario provides the imperative to understand how weak intermolecular interactions can increase IET rates without compromising chemical reactivity.…”