Introduction of ester and amido functions in tetrairon(iii) single-molecule magnets: synthesis and physical characterization

Abstract

Tetrairon(III) complexes with a propeller-like structure derived from [Fe₄(OMe)₆(dpm)₆] (1) (Hdpm = 2,2,6,6-tetramethylheptane-3,5-dione) are providing a growing class of Single Molecule Magnets (SMMs) displaying unprecedented synthetic flexibility and ease of functionalization. Herein we report the synthesis, crystal structures and magnetic properties of two novel tetrairon(III) SMMs, [Fe₄(esterC5)₂(dpm)₆] (2) and [Fe₄(amideC5)₂(dpm)₆]·Et₂O·4MeOH (3·Et₂O·4MeOH), in which functionalization of the cluster core is achieved using ester and amido linkages, respectively. To this aim, two new tripodal ligands were prepared by acylation of pentaerythritol (2,2-bis(hydroxymethyl)propane-1,3-diol) and TRIS (2-amino-2-(hydroxymethyl)propane-1,3-diol), namely H₃esterC5 = RC(O)OCH₂C(CH₂OH)₃ and H₃amideC5 = RC(O)NHC(CH₂OH)₃ with R = n-butyl. The compounds were structurally investigated by single-crystal XRD, which demonstrated coordination of the tripodal ligands to the cluster core. The products display SMM behavior with anisotropy barriers U_eff/k_B≅ 11 K due to a high-spin (S = 5) ground state and an easy axis anisotropy, described by D = −0.421 cm⁻¹ in 2 and −0.414 cm⁻¹ in 3·Et₂O·4MeOH. The departure of U_eff from the total splitting of the S = 5 ground multiplet, U/k_B≅ 15 K, has to be ascribed to the sizeable rhombic anisotropy that characterizes the two compounds (E = 0.021 cm⁻¹ in 2 and 0.019 cm⁻¹ in 3·Et₂O·4MeOH), as confirmed by master matrix calculations of the temperature-dependent relaxation time.