Single-Ion Anisotropy and Exchange Interactions in the Cyano-Bridged Trimers MnIII 2MIII(CN)6 (MIII = Co, Cr, Fe) Species Incorporating [Mn(5-Brsalen)]+ Units: An Inelastic Neutron Scattering and Magnetic Susceptibility Study

The electronic structures of the compounds K[(5-Brsalen)2(H2O)2-Mn2MIII(CN)6]·2H2O (MIII = CoIII, CrIII, FeIII) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn−MIII−Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm−1 established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)2(H2O)2-Mn2Cr(CN)6]·2H2O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)2(H2O)2-Mn2Fe(CN)6]·2H2O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100−800 Hz. INS measurements reveal an anisotropic ferromagnetic manganese(III)−iron(III) exchange interaction, in accordance with theoretical expectations based on the unquenched orbital angular momentum of the [Fe(CN)6]3− anion, giving rise to an M s ∼ ±9/2 ground state, isolated by ∼11.5 cm−1 from the higher-lying levels. The reported INS and magnetic data should now serve as a benchmark against which theoretical models that aim to inter-relate the electronic and molecular structure of molecular magnets should be tested.