Analysis of the magneto-structural correlations in the meso-tetraphenylporphyrinatomanganese(iii) tetracyanoethenide family of molecule-based magnets

Abstract

The dependence of the nearest neighbor magnetic interaction, J_AB, as a function of geometry for [Mn(porphyrin)][TCNE] materials has been evaluated by the results of CASSCF, CASPT2 and B3LYP calculations. The B3LYP results were validated for seven representative [Mn(porphyrin)][TCNE] molecule-based magnets and were found to be in good agreement with the experimental data for observed values of J_AB/k_B less than −150 K, and the CASPT2 method for J_AB/k_B greater than −150 K. Then, the J_AB was systematically evaluated as a function of the intermolecular coordinates that define the position and orientation of [TCNE]˙⁻ relative to [Mn(porphyrin)]⁺. The calculated J_AB/k_B (Ĥ = −2ΣJ_ABŜ_AŜ_B) are antiferromagnetic in all regions, with values in the −112 to −457 K range. The most negative J_AB values occur for the smallest plane angles between the [Mn(porphyrin)]⁺ and [TCNE]˙⁻ planes. The B3LYP results were then compared to those obtained using the three spin exchange mechanisms (i.e. orthogonality, spin coupling, and CI mechanisms), and none of these mechanisms give trends in accord with the B3LYP results. Therefore, first principles methods, such as DFT or CASPT2, are needed for more detailed calculations.