Synthesis and magneto-structural studies on a new family of carbonato bridged 3d–4f complexes featuring a [Co II3Ln III3(CO3)] (Ln = La, Gd, Tb, Dy and Ho) core: slow magnetic relaxation displayed by the cobalt(ii)–dysprosium(iii) analogue

Abstract

A new family of [3 + 3] hexanuclear 3d–4f complexes [(μ₃-CO₃){Co^IILn^IIIL(μ₃-OH)(OH₂)}₃]-(ClO₄)·mC₂H₅OH·nH₂O (1–5) [Ln = La (1), Gd (2), Tb (3), Dy (4), and Ho (5)] have been prepared in moderate to high yields (62–78%) following a self-assembly reaction between the ligand 6,6′,6′′-(nitrilotris(methylene))tris-(2-methoxy-4-methylphenol) (H₃L), Co(OAc)₂·4H₂O and the lanthanide ion precursors in the mandatory presence of tetrabutylammonium hydroxide. During the reaction, atmospheric carbon dioxide is fixed in the product molecule as a bridging carbonato ligand which connects all the three lanthanide centers of this molecular assembly through a rare η²:η²:η²–μ₃ mode of bridging as revealed from X-ray crystallography. The metal centers in all these compounds, except the Gd^III analogue (2), are coupled in antiferromagnetic manner while the nature of coupling in the Co^II₃Gd^III₃ complex is ferromagnetic. DFT calculations revealed that this ferromagnetic interaction occurs most likely by the Co^II–Gd^III superexchange, mediated via the bridging oxygen atoms. Only the Co^II–Dy^III compound (4) displayed a slow relaxation of the magnetization at a very low temperature as established by AC susceptibility measurements. The data provides an estimation of the activation energy U/k_B = 9.2 K and the relaxation time constant τ₀ = 1.0 × 10⁻⁷ s.