Strong π-Backbonding Enables Record Magnetic Exchange Coupling Through Cyanide

摘要

The paramagnetic cyano-bridged complex PhB((t)BuIm)(3)Fe-NC-Mo((NBuAr)-Bu-t)(3) (Ar = 3,5-Me2C6H3) is readily assembled from a new four-coordinate, high-spin (S = 2) iron(II) monocyanide complex and the three-coordinate molybdenum(III) complex Mo((NBuAr)-Bu-t)(3). X-ray diffraction and IR spectroscopy reveal that delocalization of unpaired electron density into the cyanide pi* orbitals leads to a reduction of the C-N bond order. Direct current (dc) magnetic susceptibility measurements, supported by electronic structure calculations, demonstrate the presence of strong antiferromagnetic exchange between spin centers, with a coupling constant of J = -122(2) cm(-1). To our knowledge, this value represents the strongest magnetic exchange coupling ever to be observed through cyanide. These results demonstrate the ability of low-coordinate metal fragments to engender extremely strong magnetic exchange coupling through cyanide by virtue of significant pi-backbonding into the cyanide ligand.