C-H Activation from Iron(II)-Nitroxido Complexes.

The reaction of nitroxyl radicals TEMPO (2,2',6,6'-tetramethylpiperidinyloxyl) and AZADO (2-azaadamantane-N-oxyl) with an iron(I) synthon affords iron(II)-nitroxido complexes (Ar L)Fe(κ1 -TEMPO) and (Ar L)Fe(κ2 -N,O-AZADO) (Ar L=1,9-(2,4,6-Ph3 C6 H2 )2 -5-mesityldipyrromethene). Both high-spin iron(II)-nitroxido species are stable in the absence of weak C-H bonds, but decay via N-O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4-cyclohexadiene. Whereas (Ar L)Fe(κ1 -TEMPO) reacts to give a diferrous hydroxide [(Ar L)Fe]2 (μ-OH)2 , the reaction of four-coordinate (Ar L)Fe(κ2 -N,O-AZADO) with hydrogen atom donors yields ferric hydroxide (Ar L)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C-H substrate and are consistent with rate-determining hydrogen atom transfer.