Adducts of manganese diketonates with redox-active ligands: Computational modeling of valence tautomeric systems

Abstract

The potential valence tautomeric (VT) properties of 1:1 adducts of MnII bis-malonates and MnII bis-hexafluoroacetylacetonates with bidentate redox-active ligands – o-benzoquinone, its imine and diimine derivatives (complexes I) and 1,4-diazabuta-1,3-diene (complexes II) were computationally studied. According to the DFT B3LYP∗/6-311++G(d,p) calculations, the ground electronic states of the mixed-ligand complexes I are represented by the high-spin electromeric forms HSMnIII–L− exhibiting strong antiferromagnetic exchange coupling between unpaired electrons of the paramagnetic centers located at the metal ion and radical-anionic ligand. Same type magnetic ordering is characteristic also of the structures containing manganese center in its intermediate spin-state. Stabilization of the electromers HSMnII–L0 containing a redox-active ligand in the non-reduced form is achieved in the complexes formed by MnII hexafluoroacetylacetonate with benzoquinone imine and diimine and adducts on the basis of 1,4-diazabuta-1,3-diene (II) possessing stronger than carbonyl oxygens ligating centers (NH). The adducts I (X=Y=NH; R=CF3), (X=O, Y=NH; R=CF3) and II (R=H) characterized by the narrow energy gaps between the HSMnIII–L− and HSMnII–L0 electromeric forms and low energy barriers (estimated as minimum energy crossing points MECP at seams of intersection of the corresponding potential energy surfaces) for their interconversion may be considered as promising candidates for the observation of thermally driven valence tautomeric rearrangements.