Ferromagnetism in polynuclear systems based on non-linear [Mn II2MnIII] building blocks

Abstract

The design of new polynuclear transition metal complexes showing large total spin values through parallel alignment of the spins is an important challenge due to the scarcity of bridging ligands that provide ferromagnetic coupling. Herein, we report two new complexes, a [Mn^II₄Mn^III₂] system containing two non-linear [Mn^II₂Mn^III] units and a 1D chain system with [Mn^II₂Mn^III] units that are assembled through dicyanamide bridging ligands coordinated to one of the terminal Mn^II centers. In both cases, the main exchange interaction is between Mn^II⋯Mn^III, showing a relatively strong ferromagnetic coupling. Density functional theory calculations corroborate such ferromagnetic interactions and also provide one magnetostructural correlation, showing that larger Mn^II–O–Mn^III angles enhance the strength of the ferromagnetic coupling. Thus, the non-linear [Mn^II₂Mn^III] units present in these two complexes are specially suited because of their larger Mn^II–O–Mn^III angles compared to similar previously reported systems containing a linear [Mn^II₂Mn^III] unit.

The collaboration between Dr Wu's and Professor Ruiz's groups performs research on the synthetic chemistry of transition metal complexes with magnetic properties (Jilin University) and utilizes computational tools (University of Barcelona) to gain further insight into the nature of the physical properties of these systems. In this study, the interaction between our groups focused on the study of polynuclear systems containing an [Mn^II₂Mn^III] motif displaying ferromagnetic couplings. Such exchange pathways are needed to reach a large total spin value, which is one of the requirements to design single-molecule magnets. Thus, the synthesis of polynuclear complexes by Wu's group and subsequent density functional theory (DFT) calculations by the Ruiz group were performed in order to determine all the exchange interaction constants, corroborating the experimental data and provide further insight into the magnetic behavior of these systems. Previous collaborations between our groups included a similar approach for the study of [Mn₁₀] cluster systems also showing ferromagnetic behavior due to Mn^II–Mn^III interactions.