Coordination control of a semicarbazide Schiff base ligand for spontaneous aggregation of a Ni2Ln2 cubane family: influence of ligand arms and carboxylate bridges on the organization of the magnetic core

Abstract

A family of tetranuclear Ni-4f coordination aggregates (CAs) from the support of a carbazido ligand is described. The option of altering the 4f ions during synthesis within the family, without altering the core topology, gave the four members of the family. Tetranuclear (2 : 2) heterometallic compounds [Ni^II₂Ln^III₂(μ₃-L)₂(μ₃-OH)₂(μ-OAc)₃(AcO)(H₂O)₃]Cl₂·4H₂O (Ln = Gd, 1; Tb, 2; Dy, 3; Ho, 4) have been assembled through the utilization of all the coordination sites of the anionic form of 1-(2-hydroxy-3-methoxybenzylidene)semicarbazide (HL) and bridging supports from HO⁻ and AcO⁻ ions. The four metal ion centers of two different types, are held together by two μ-L⁻, two μ₃-HO⁻ and three μ_1,3-AcO⁻ bridges to self-assemble into Ni₂Ln₂O₄ hetero-cubane structures. In each cube, Ln^III ions are linked by two hydroxido and one phenoxido bridge and each Ni^II ion is linked by two phenoxido and one hydroxido bridge, so that each hydroxido group in μ₃-mode is linked to two Ln^III and one Ni^II and each phenoxido ion in μ₃-mode is linked to two Ni^II and one Ln^III. Variable temperature magnetic properties have been examined to find the role of magnetic anisotropy for the single molecule magnet (SMM) behavior. All of them show large ground state magnetic moment values and absence of out-of-phase ac susceptibility signals. The absence of SMM behavior is ascribed to the lack of axial magnetic anisotropy.