Multinuclear iron–phenyl species in reactions of simple iron salts with PhMgBr: identification of Fe4(μ-Ph)6(THF)4 as a key reactive species for cross-coupling catalysis

Abstract

The first direct syntheses, structural characterizations, and reactivity studies of iron–phenyl species formed upon reaction of Fe(acac)₃ and PhMgBr in THF are presented. Reaction of Fe(acac)₃ with 4 equiv. PhMgBr in THF leads to the formation of [FePh₂(μ-Ph)]₂²⁻ at −80 °C, which can be stabilized through the addition of N-methylpyrrolidone. Alternatively, at −30 °C this reaction leads to the formation of the tetranuclear iron–phenyl cluster, Fe₄(μ-Ph)₆(THF)₄. Further synthetic studies demonstrate that analogous tetranuclear iron clusters can be formed with both 4-F-PhMgBr and p-tolylMgBr, illustrating the generality of this structural motif for reactions of simple ferric salts and aryl Grignard reagents in THF. Additional studies isolate and define key iron species involved in the synthetic pathway leading to the formation of the tetranuclear iron–aryl species. While reaction studies demonstrate that [FePh₂(μ-Ph)]₂²⁻ is unreactive towards electrophile, Fe₄(μ-Ph)₆(THF)₄ is found to rapidly react with bromocyclohexane to selectively form phenylcyclohexane. Based on this reactivity, a new catalytic reaction protocol has been developed that enables efficient cross-couplings using Fe₄(μ-Ph)₆(THF)₄, circumventing the current need for additives such as TMEDA or supporting ligands to achieve effective cross-coupling of PhMgBr and a secondary alkyl halide.