Introduction of Ester and Amido Functions in Tetrairon(III) Single-Molecule Magnets: Synthesis and Physical Characterization

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 whichfunctionalization 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 Ueff/kB~=11 Kdue to a high-spin (S = 5) ground state and an easy axis anisotropy, described by D = -0.421 cm-1 in 2and -0.414 cm-1 in 3·Et2O·4MeOH. The departure of Ueff from the total splitting of the S = 5 ground multiplet, U/kB~=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 matrixcalculations of the temperature-dependent relaxation time.