Family of Double-Cubane Mn₄Ln₂ (Ln = Gd, Tb, Dy, Ho) and Mn₄Y₂ Complexes: A New Mn₄Tb₂ Single-Molecule Magnet

The synthesis and characterization of a family of Mn2IIIMn2IILnIII2 complexes (Ln = Gd (1), Tb (2), Dy (3), and Ho (4)) of formula [Mn4Ln2O2(O2CBut)6(edteH2)2(NO3)2] are reported, where edteH4 is N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine. The analogous Mn4Y2 (5) complex has also been prepared. They were obtained from reaction of Ln(NO3)3 or Y(NO3)3 with Mn(O2CBut)2, edteH4, and NEt3 in a 2:3:1:2 molar ratio. The crystal structures of representative 1 and 2 were obtained, and their core consists of a face-fused double-cubane [Mn4Ln2(μ4-O2–)2(μ3-OR)4] unit. Such double-cubane units are extremely rare in 3d metal chemistry and unprecedented in 3d–4f chemistry. Variable-temperature, solid-state dc and ac magnetic susceptibility studies on 1–5 were carried out. Fitting of dc χMT vs T data for 5 gave Jbb (MnIII···MnIII) = –32.6(9) cm–1, Jwb (MnII···MnIII) = +0.5(2) cm–1, and g = 1.96(1), indicating a |n, 0, n⟩ (n = 0–5) 6-fold-degenerate ground state. The data for 1 indicate an S = 12 ground state, confirmed by fitting of magnetization data, which gave S = 12, D = 0.00(1) cm–1, and g = 1.93(1) (D is the axial zero-field splitting parameter). This ground state identifies the MnII···GdIII interactions to be ferromagnetic. The ac susceptibility data independently confirmed the conclusions about 1 and 5 and revealed that 2 displays slow relaxation of the magnetization vector for the Mn4Tb2 analogue 2. The latter was confirmed as a single-molecule magnet by observation of hysteresis below 0.9 K in magnetization vs dc field scans on a single crystal of 2·MeCN on a micro-SQUID apparatus. The hysteresis loops also displayed well-resolved quantum tunneling of magnetization steps, only the second 3d–4f SMM to do so.