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 M3n), 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 Er3n (n = 4, 5, and 6) pure 4f metal wheels. These two families of CCCs unambiguously fill the missing links of the M3n 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.