Single-Ion Anisotropy and Exchange Interactions in the Cyano-Bridged Trimers (Mn^III₂M^III(CN)₆ (M^III = 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-Mn₂M^III(CN)₆] ·2H₂O (M^III = Co^III, Cr^III, Fe^III) 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-M^III-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 similar to +/-9/2 ground state, isolated by similar to 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.