Magnetic Behavior of Heterometallic Wheels Having a [MnIV6M2O9]10+ Core with M = Ca2+ and Sr2+

Two new heterometallic MnIV–M2+ compounds with formula [Mn6M2O9(4-tBuC6H4COO)10(4-tBuC6H4COOH)5] (M = Ca2+ (1), Sr2+ (2)) have been crystallized. The core of both compounds consists of a planar Mn6 ring, where the MnIV ions are alternatively bridged by (μ3-O)2(μ-RCOO) and (μ4-O)­(μ-RCOO)2 ligands, and the two alkaline earth ions are located to both sides of the wheel, linked to the oxo bridges, generating three fused [Mn2M2O4]4+ cuboids. These compounds show a net antiferromagnetic behavior, more important for 2 (Sr2+) than for 1 (Ca2+). The fitting of the experimental data was performed with the support of DFT calculations, considering four different exchange pathways: two between adjacent MnIV ions (J1 and J2) and two between nonadjacent MnIV ions (J3 and J4). The results of the analysis show that J1 and J2 are of the opposite sign, the ferromagnetic contribution corresponding to the [Mn2(μ4-O)­(μ-RCOO)2]4+ unit (J2). The influence of the M2+ ions in the magnetic behavior is analyzed for 1 and 2 and for three hypothetical models with the structural parameters of 1 containing Mg2+, Sr2+ or without the M2+ ions. In spite of the diamagnetic character of the alkaline earth ions, their influence on the magnetic behavior has been evidenced and correlated with their polarizing effect. Moreover, the magnetic interactions between nonadjacent ions are non-negligible.