Taking ferrocenium up a notch

Few organometallic compounds are as well known as ferrocene; it was one of the first compounds identified as having metal–carbon bonds and inspired decades of work studying metallocenes. It has a rich history in chemical catalysis and medicinal chemistry in the +2 oxidation state, where its stability and simple derivatization have proven to be beneficial, but although most stable in this state, it is also often encountered as the ferrocenium cation in its +3 oxidation state. Simple complexes of iron in its +4 oxidation state are, however, quite scarce, typically limited to fleetingly stable species (for example FeF₄) or those with metal–ligand multiple bonds. Early work in the area suggested that the permethylated derivative of ferrocene could be oxidized twice, but efforts to isolate the dicationic product were regularly met with defeat, implying that the Fe(IV) analogue of ferrocene would not be stable enough to characterize.

Now, a team led by Moritz Malischewski at the Free University Berlin and Karsten Meyer at Friedrich-Alexander University have successfully prepared — and isolated — various salts of the decamethylferrocene dication through oxidation of decamethylferrocene with AsF₅, SbF₅ and ReF₆ in liquid SO₂ (Science 353, 678–682; 2016). These extraordinarily powerful oxidants proceed to convert the orange, 18-electron Fe(II) complex to the brown, 16-electron Fe(IV) complex in a stepwise fashion. Malischewski, Meyer and colleagues confirmed the oxidation- and spin-states of the products with magnetic susceptibility measurements and Mössbauer spectroscopy; both were consistent with a Fe(IV) complex, with S = 1.

Credit: © 2016 AAAS

In contrast to previous efforts to prepare the decamethylferrocene dication using transition-metal oxidants, the salts prepared in this work are stable at room temperature under an inert atmosphere. Whereas the Fe(II) and Fe(III) complexes have linear structures, the decamethylferrocene dication is bent along the Cp*−Fe−Cp* axis, the extent of this depending on the size of the counteranion. These unusual structures can be explained using the 'polarizable ion model', which accounts for similar effects in decamethyltitanocenium complexes. Although applications of such Fe(IV) complexes are unclear at present, even isolating these rare organometallic complexes was little more than a pipe dream even a few years ago, underscoring the importance of fundamental inorganic synthesis in modern chemistry.