Heisenberg exchange in dinuclear manganese complexes:: A density functional theory study

This work presents a systematic investigation of the performance of broken symmetry density functional theory for the evaluation of Heisenberg exchange constants. We study dinuclear Mn-IV-Mn-IV complexes with bis(mu-oxo), bis(mu-oxo)(mu-carboxylato), and tris(mu-oxo) cores for this purpose, as these are of fundamental biological interest as well as being potential precursors for molecular magnets based on manganese complexes, the so-called Mn-12 magnets. The obtained results indicate that quantitative agreement with available experimental data for the Heisenberg exchange constants can be achieved for most of the investigated complexes but also that there are significant failures for some compounds. We evaluate factors influencing the accuracy of obtained results and examine effects of different mappings between broken symmetry and Heisenberg Hamiltonian states in an attempt to formulate a reliable recipe for the evaluation of magnetic coupling in these complexes. An assessment of the bonding situation in the molecular system under investigation is found crucial in choosing the appropriate scheme for evaluation of the Heisenberg exchange constants.