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 b ₂), (pentamethylcyclopentadienyl)(1,3,5-trialkylbenzene)ruthenium (alkyl=Me, 2 a ₂; alkyl=Et, 2 b ₂), and (pentamethylcyclopentadienyl)(benzene)iron (2 c ₂) 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 b ₂ dimer is much more readily broken than that in 2 a ₂; consistent with this observation, Vis/NIR kinetic measurements suggest that the solution reduction of 3 by 1 b ₂ 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).