Modulating the structural topologies from star-shape to cross-shape for Co–Dy heterometallic complexes with slow magnetic relaxation behavior

Abstract

Two novel Co–Dy heterometallic complexes, namely, [Co^III₂Dy^III₂(HL)L{(py)₂CO₂}{(py)₂C(OCH₃)O}(NO₃)₄CH₃OH]·2CH₃CN·2.5H₂O (1, H₃L = 2-((2-hydroxy-benzylidene)amino)propane-1,3-diol, (py)₂C(OH)₂ = the gem-diol form of di-2-pyridyl ketone (dpk) and (py)₂C(OCH₃)OH = the hemiacetal form of dpk) and [Co^III{(py)₂C(OH)O}₂][Co^II₂Dy^III₄Dy^III(HL)₄(L)₄]·3CH₃CN·15H₂O (2), have been obtained by employing H₃L, dpk and Dy(NO₃)₃·5H₂O reacting with Co(NO₃)₂·6H₂O or anhydrous Co(CH₃CO₂)₂ in the solutions of CH₃OH and CH₃CN (V/V = 1/20) containing N(Me)₄OH·5H₂O or triethylamine. Structural analysis indicated that two Dy^III and two Co^III in 1 formed a star-like topology, while the metal ions in [Co^II₂Dy^III₄Dy^III(HL)₄(L)₄]⁻ of 2 formed a cross-shaped topology, in which one Dy^III and two Co^II formed a linear Co^II₂Dy^III paramagnetic unit. Interestingly, the paramagnetic Co^II₂Dy^III units in 2 were isolated not only by four diamagnetic Co^III in the [Co^II₂Dy^III₄Dy^III(HL)₄(L)₄]⁻, but also by the diamagnetic [Co^III{(py)₂C(OH)O}₂]⁺. More interestingly, the distortion of the square antiprism (SAP-8) coordination geometry of Dy^III in 2 can be ascribed to the presence of four long Dy–O bonds and four short Dy–O bonds, which could result from the discrepancies of the polarization of Co^II and Co^III. Magnetic studies indicated that 1 exhibit two types of magnetic relaxation behavior, in which the magnetic relaxation of the fast relaxation (FR) phase is mainly governed by QTM, while that of the slow relaxation (SR) phase is mainly governed by Orbach and Raman processes with an effective energy barrier (U_eff) value of 59.4(9) K. To our knowledge, the U_eff value is relatively high among the reported Co^III–Dy^III heterometallic complexes. For 2, it only shows slow relaxation behavior with a U_eff of 6.43 K. Ab initio calculations revealed that the SR phase and FR phase are governed by the magnetic relaxation of Dy2 and Dy1, respectively, though the axial symmetry of Dy2 (muffin, C_s) is lower than that of Dy1 (triangular dodecahedron, D_2d). However, for 2, the theoretical calculations indicated that the severely compressed SAP-8 geometry of Dy^III leads to a relatively low U_eff value.