Tetrairon(ii) extended metal atom chains as single-molecule magnets

Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon(ii), halide-capped complexes [Fe-4(tpda)(3)Cl-2] (1Cl) and [Fe-4(tpda)(3)Br-2] (1Br), obtained by reacting iron(ii) dihalides with [Fe-2(Mes)(4)] and N-2,N-6-di(pyridin-2-yl)pyridine-2,6-diamine (H(2)tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mossbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with chi T-M at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron(ii) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy (D = -11 to -19 cm(-1), vertical bar E/D vertical bar = 0.05-0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy (D = 8-10 cm(-1), vertical bar E/D vertical bar = 0.06-0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1-Fe2 and Fe3-Fe4 interactions and antiferromagnetic Fe2-Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.

Automated summary

Iron-based extended metal atom chains show potential as high-spin molecules and candidate single-molecule magnets. Tetrairon(ii) halide-capped complexes were compared in terms of their structural, electrochemical, and magnetic characteristics. The electronic structure of iron(ii) ions in each compound displayed distinct magnetic anisotropy, contributing to the observed magnetic behaviors. The competition between super-exchange interactions and noncollinear anisotropies at metal sites results in the weakly magnetic ground state observed in both derivatives, explaining their single-molecule magnet behavior.