First Evidence of the Oxidative Addition of Fe0(N,N)2 to Aryl Halides: This Precondition Is Not a Guarantee of Efficient Iron-Catalysed C-N Cross-Coupling Reactions

It has been shown by cyclic voltammetry (CV) that the electrochemical reduction of [Fe(acac)(3)] in the presence of 1,10-phenanthroline (phen; >= 2 equiv.) or FeCl3 in the presence of N,N'-dimethylethylenediamine (dmeda; 2 equiv.) in DMF leads to [Fe-0(phen)(2)] and [Fe-0(dmeda)(2)], respectively. They undergo unprecedented oxidative addition to aryl halides to generate [(ArFeX)-X-II(phen)(2)] (X = I, Br) and [(ArFeX)-X-II(dmeda)(2)] (X = Br), characterized by CV, ESI-MS and DFT. The order of reactivity in the oxidative addition was deduced by CV and supported by DFT: [Fe-0(phen)(2)] < [Fe-0(dmeda)(2)]. [ArFeIIX(phen)(2)] and [(ArFeX)-X-II(dmeda)(2)] are nucleophilic reagents (reaction with H+ and CO2) and do not react with the investigated N-nucleophiles (imidazole or pyrazole) often tested as reagents in catalytic C-N cross-coupling reactions, even in the presence of a base. Moreover, it has been established that {[Fe(acac)(3)] + 2 phen} and {FeCl3 + 2 dmeda} cannot be reduced in situ to Fe-0(N,N)(2) (precondition required to activate ArX) by N-nucleophiles (pyrazole, imidazole) even in the presence of a base. All this explains why {[Fe(acac)(3)] + 2 phen} and {FeCl3 + 2 dmeda} are inefficient precatalysts for C-N cross-couplings reactions, as recently reported in the literature.