Filling the Missing Links of M3n Prototype 3d-4f and 4f Cyclic Coordination Cages: Syntheses, Structures, and Magnetic Properties of the Ni10Ln5 and the Er3n Wheels

Abstract

In this paper, we proposed a number rule for 3d-4f and 4f cyclic coordination cages (CCCs); that is, CCCs consisting of vertex-sharing M₄(μ₃-OH)₄ (M = 3d transition metal or 4f lanthanide ions) units should have 3 × n metal centers (abbreviated M_3n), where n represents the number of the M₄(μ₃-OH)₄ subunits. Under this number rule we reasoned that some species of CCCs, for example, the pentadecanuclear 3d-4f wheel and the pure 4f wheels with 9 or 18 centers, should practically have existed. However, there are no such complexes reported in the literature. To realize such CCCs we employed a mixed-ligand approach, that is, to simultaneously use the primary and the ancillary ligands for syntheses. This approach successfully leads to the isolation of two families of CCCs, namely, the Ni₁₀Ln₅ (Ln = Gd and Y) mixed-metal wheels and the Er_3n (n = 4, 5, and 6) pure 4f metal wheels. These two families of CCCs unambiguously fill the missing links of the M_3n prototype CCCs. Moreover, dominated ferromagnetic interaction indicates high ground-spin state for the Gd₅Ni₁₀ wheel. The ferromagnetic interactions between the nickel centers are verified using the diamagnetic Y(III) analogue, which reveals an averaged coupling constant (J = 2.7 cm^-1), while accompanied by a large negative zero-field splitting parameter (D = -6.1 cm^-1) for single Ni(II) ions. Interestingly, the Y(III)-diluted Er₁₂ wheel shows slow magnetic relaxation behavior, presumably indicating the magnetically anisotropic nature of the erbium(III) ions.