Systematic Investigation of Trigonal-Bipyramidal Cyanide-Bridged Clusters of the First-Row Transition Metals

Reactions between [M‘III(CN)6]3- anions (M‘ = Co, Cr, or Fe) and mononuclear complexes of MII ions (M = Cr, Mn, Co, Ni, or Zn) produce a family of pentanuclear clusters {[M(tmphen)2]3[M‘(CN)6]2]}. The core of the clusters is formed by five metal ions that are bridged through six CN- linkers into a trigonal bipyramid, with M and M‘ ions occupying equatorial and axial positions of the bipyramid, respectively. Three of the CN- ligands from each M‘ center remain terminal and point toward the outside of the cluster, along the trigonal axes. Studies of magnetic coupling in the {[M(tmphen)2]3[Cr(CN)6]2]} family of clusters revealed a similarity between the observed magnetic exchange constants and the values estimated for the molecule-based magnets of the Prussian blue family. The type of the magnetic exchange varies across the series, changing from antiferromagnetic for M = Cr and Mn to ferromagnetic for M = Co and Ni. Complexes {[M(tmphen)2]3[Co(CN)6]2]}, which contain diamagnetic CoIII ions in the axial positions, serve as convenient model compounds for an accurate assessment of the magnetic parameters for the equatorial M ions in the absence of magnetic interactions. The {[Co(tmphen)2]3[Cr(CN)6]2]} cluster exhibits cyanide linkage isomerism, the relative amount of which depends on the synthetic conditions.