Unusual undecanuclear heterobimetallic Zn4Ln7 (Ln = Gd, Dy) nano-sized clusters encapsulating two peroxide anions through spontaneous intake of dioxygen

Abstract

The synthesis, characterization and properties of two unprecedented undecanuclear heterobimetallic Zn₄Ln₇ complexes of formula [Zn₄Ln₇(L)₈(O₂)₂(OH)₄(Cl)₄(H₂O)₄]·Cl·4H₂O·4CH₃CN (Ln = Gd (1), Dy (2)) encapsulating two peroxide anions are presented, representing a very rare example of a 3d-peroxo-Ln system and expanding the realm of metal-peroxo complexes. These eleven metal ions are arranged in a peculiar structural motif, where Zn₄ is located at the peripheral shell wrapping Ln₇ in the inner core. The Zn ions are penta-coordinate in all cases, linked to the NO₂ donor atoms from the L²⁻ ligand and to a hydroxyl group, and the apical position is occupied by a chloride anion. All Ln^III ions in these systems are octa-coordinate with LnO₈ and LnNO₇ coordination spheres. Magnetocaloric effect (MCE) behavior has been found in the Gd analogue due to multiple low lying excited states arising from antiferromagnetic Gd–Gd exchange interactions. The Dy derivative shows frequency dependent out-of-phase signals indicating the presence of slow relaxation of magnetization below 8 K under zero applied direct current (dc) field, but without reaching a maximum, which is due to a faster quantum tunneling relaxation. The effective barrier extracted from the frequency dependent data is U_eff = 11.2 K and a τ₀ of 4.18 × 10⁻⁶ s.