Electronic and Steric Effects on the Reductive Elimination of Anionic Arylferrate(III) Complexes.

Arylferrate(III) complexes Ph3 FeR- (R=para- and ortho-substituted aryl) are proposed as model systems for the in-depth investigation of reductive eliminations from organoiron(III) species. Electrospray ionization transfers the arylferrate complexes prepared in situ from solution into the gas phase, where mass selection ensures a well-defined population of reactant ions. Upon gas-phase fragmentation, the arylferrate complexes undergo reductive elimination of the cross-coupling product PhR as well as the homo-coupling product Ph2 . The measured branching ratios between the two competing reaction channels are used to construct a Hammett plot, which shows that electron-donating aryl groups R favor the formation of the cross-coupling product. In this way, the complexes avoid the build-up of too much electron density at the iron center during the reductive elimination. ortho Substitution in R increases the fraction of the homo-coupling product, presumably by hindering the approach between the two aryl groups participating in the reductive elimination. The obtained mechanistic insight substantially advances our understanding of one of the central elementary steps of transition-metal-catalyzed cross-coupling reactions.