Unique Dimerization Topology and Countercation Binding Modes in 12-Metallacrown-4 Compounds

Seven dimeric metallacrowns (MC) based on Ln[12-MCM(III)N(shi)-4], where Ln(III)=Dy, Ho, Yb, or Y, M-III=Mn or Ga, and shi(3-) is salicylhydroximate, have been synthesized and characterized by single-crystal X-ray diffraction, and for the dysprosium-manganese dimers, the magnetic properties have been measured. In each dimer two Ln[12-MCM(III)N(shi)-4] units are linked by four bridging dicarboxylate anions (isophthalate, trimesate, dinicotinate, or 2,2 '-dithiodibenzoate). Three different countercations (sodium, gallium(III), or pyridinium) were used to maintain charge balance of the dimer. While pyridinium does not bind to the dimer, the choice of the dicarboxylate dictates where the countercations Na+ or Ga-III bind. With isophthalate and trimesate, the sodium ion binds to the central MC cavity opposite of the Ln(III), and with dinicotinate the sodium or gallium(III) ions bind to the pyridyl nitrogen of the dinicotinate. All three Dy2Mn8 dimers exhibit an out-of-phase magnetic susceptibility signal consistent with a shallow barrier to magnetization relaxation.