A Family of Decamethylmetallocene Charge-Transfer Salt Magnets Using Methyl Tricyanoethylenecarboxylate (MTCE) as the Electron Acceptor

Methyl tricyanoethylenecarboxylate, MTCE, has been used as a one-electron acceptor building
block for the synthesis of isomorphous decamethylmetallocene charge-transfer salt magnets of the formula
[MCp*2][MTCE], M = Cr, Mn, and Fe. Functionally and electrochemically, MTCE is a hybrid between
tetracyanoethylene (TCNE) and dimethyl dicyanofumarate (DMeDCF), two acceptors that have previously
been found to support ferromagnetism. The X-ray crystal structure of the chromium analogue, [CrCp*2][MTCE], shows it to exist in the expected mixed stack structure in the orthorhombic space group Pnma
with a = 14.739(3) Å, b = 10.7869(19) Å, and c = 15.771(3) Å and Z = 4. As anticipated, all three family
members exhibit dominant ferromagnetic coupling, which is presumed to reflect intrastack interactions.
However, the bulk magnetic properties mostly differ from simple interpolations or extrapolations of the
properties of their TCNE and DMeDCF analogues. Density functional theory calculations have been used
to shed some light on this observation.