Impact of 3d Metal Ions on the Magnetic Dynamics in Isostructural Tetranuclear Butterfly {Ni₂IIDy₂III} and {Co₂IIDy₂III} Complexes

This paper describes the synthesis, structures, and magnetic properties of four isostructural butterfly-type heterometallic tetranuclear 3d-4f complexes, [Ni2Dy2(HL1)2(μ3–OH)2(piv)6]·2CH3CN (1), [Ni2Dy2(HL2)2(μ3–OH)2(piv)6]·2CH3CN (2), [Co2Dy2(HL1)2(μ3–OH)2(piv)6]·2H2O (3), and [Co2Dy2(HL2)2(μ3–OH)2(piv)6]·2CH3CN (4), derived from two closely related Schiff base ligands H2L1 and H2L2 in the presence of pivalate (piv) ions. X-ray crystallographic studies of 1–4 revealed that all of the complexes are constructed with a type-II butterfly core in which DyIII ions are located at body positions, while the 3d metal ions (NiII and CoII) occupy the wingtip positions. It is noteworthy that all reported systems containing NiII and CoII have been isolated as type-I butterfly structures, with only one report each on the Ni2IIDy2III and Co2IIDy2III type-II butterfly core systems, which have been both structurally and magnetically characterized. Direct current magnetic susceptibility measurements performed on 1–4 revealed that the susceptibility response is dominated by the depopulation of Stark sublevels of the DyIII ions and the ZFS of the CoII cations in the case of 3 and 4 and the weak intramolecular superexchange interactions have been calculated by means of CASSCF and CASSCF/NEVPT2 calculations. Dynamic magnetic studies revealed that only CoII analogues (3 and 4) display single-molecule magnet behaviors in which the additional anisotropy of the CoII centers is responsible for the occurrence of the out-of-phase response in contrast to 1 and 2 with poorly anisotropic NiII cations.