Effect of ligand substitution on the SMM properties of three isostructural families of double-cubane Mn4Ln2 coordination clusters

Abstract

Three isostructural lanthanide series with a core of Mn^III₂Mn^II₂Ln₂ are reported. These three families have the formulae of [Mn^III₂Mn^II₂Ln₂(μ₄-O)₂(H₂edte)₂(piv)₆(NO₃)₂] {no crystallization solvent, Ln = La, Ce, Pr, Nd, Eu (1–4, 6); solv = 3MeCN, Ln = Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y (5, 7–13)}, where H₂edte = N,N,N′,N‘-tetrakis(2-hydroxyethyl)ethylenediamine and piv = pivalate; [Mn^III₂Mn^II₂Ln₂(μ₄-O)₂(H₂edte)₂(benz)₆(NO₃)₂], where benz = benzoate, or [Mn^III₂Mn^II₂Ln₂(μ₄-O)₂(edteH₂)₂(benz)₆(NO₃)₂]·2MeCN {Ln = Gd, Tb, Dy (14–16); and [Mn^III₂Mn^II₂Ln₂(μ₄-O)₂(edteH₂)₂(piv)₈].solv {solv = 4MeCN, Ln = La (17); solv = 2MeCN·tol·H₂O, Ln = Pr, Nd, Sm, Tb (18–20, 22); solv = 2MeCN·H₂O, Ln = Gd (21). These compounds crystallize in two different systems, namely, monoclinic in the space groups P2₁/n for 1–4, 6, and 14–16 and C2/c for 5, 7–13, 18–20, and 22 and triclinic in the space group P for 17 and 21. The crystal structures of these compounds display a face-fused dicubane structure connected by different types of bridged oxygen atoms. Solid-state dc magnetic susceptibility characterization was carried out for 1–22, and fitting showed that Mn^III⋯Mn^III is antiferromagnetically (AF) coupled and Mn^II⋯Mn^III, Mn^II⋯Ln and Mn^III⋯Ln are weakly ferromagnetically coupled. In addition, ac measurements were carried out and showed that only 7, 15, and 22 for Tb, 8 and 16 for Dy, and 20 for Sm exhibited slow magnetization relaxation. In the case of 15, it was possible to determine the energy barrier of the slow-relaxation behavior by fitting peak temperatures to the Arrhenius law, which gave a value of U_eff = 21.2 K and a pre-exponential factor of τ₀ = 4.0 × 10⁻⁹ s.