n-Doping of Organic Electronic Materials Using Air-Stable Organometallics: A Mechanistic Study of Reduction by Dimeric Sandwich Compounds

Several 19‐electron sandwich compounds are known to exist as “2×18‐electron” dimers. Recently it has been shown that, despite their air stability in the solid state, some of these dimers act as powerful reductants when co‐deposited from either the gas phase or from solution and that this behavior can be useful in n‐doping materials for organic electronics, including compounds with moderate electron affinities, such as 6,13‐bis[tri(isopropyl)silylethynyl]pentacene (3). This paper addresses the mechanisms by which the dimers of 1,2,3,4,5‐pentamethylrhodocene (1 b2), (pentamethylcyclopentadienyl)(1,3,5‐trialkylbenzene)ruthenium (alkyl=Me, 2 a2; alkyl=Et, 2 b2), and (pentamethylcyclopentadienyl)(benzene)iron (2 c2) react with 3 in solution. Vis/NIR and NMR spectroscopy, and X‐ray crystallography indicate that the products of these solution reactions are 3.− salts of the monomeric sandwich cations. Vis/NIR kinetic studies for the Group 8 dimers are consistent with a mechanism whereby an endergonic electron transfer from the dimer to 3 is followed by rapid cleavage of the dimer cation. NMR crossover experiments with partially deuterated derivatives suggest that the CC bond in the 1 b2 dimer is much more readily broken than that in 2 a2; consistent with this observation, Vis/NIR kinetic measurements suggest that the solution reduction of 3 by 1 b2 can occur by both the mechanism established for the Group 8 species and by a mechanism in which an endergonic dissociation of the dimer is followed by rapid electron transfer from monomeric 1 b to 3. Doped up: Air‐stable dimers of pentamethylrhodocene and pentamethylcyclopentadienyl arene ruthenium and iron can be used to n‐dope acceptors such as bis[tri(isopropyl)silylethynyl] pentacene. NMR crossover experiments and variable‐temperature Vis/NIR kinetic measurements indicate that, depending on the reaction conditions and the choice of dimer and acceptor, this doping can take place by two different mechanisms (see scheme).