Selective arc-discharge synthesis of Dy(2)S-clusterfullerenes and their isomer-dependent single molecule magnetism

A method for the selective synthesis of sulfide clusterfullerenes Dy(2)S@C(2n) is developed. Addition of methane to the reactive atmosphere reduces the formation of empty fullerenes in the arc-discharge synthesis, whereas the use of Dy(2)S(3) as a source of metal and sulfur affords sulfide clusterfullerenes as the main fullerene products along with smaller amounts of carbide clusterfullerenes. Two isomers of Dy(2)S@C(82) with C(s)(6) and C(3v)(8) cage symmetry, Dy(2)S@C(72)-C(s)(10528), and a carbide clusterfullerene Dy(2)C(2)@C(82)-C(s)(6) were isolated. The molecular structure of both Dy(2)S@C(82) isomers was elucidated by single-crystal X-ray diffraction. SQUID magnetometry demonstrates that all of these clusterfullerenes exhibit hysteresis of magnetization, with Dy(2)S@C(82)-C(3v)(8) being the strongest single molecule magnet in the series. DC- and AC-susceptibility measurements were used to determine magnetization relaxation times in the temperature range from 1.6 K to 70 K. Unprecedented magnetization relaxation dynamics with three consequent Orbach processes and energy barriers of 10.5, 48, and 1232 K are determined for Dy(2)S@C(82)-C(3v)(8). Dy(2)S@C(82)-C(s)(6) exhibits faster relaxation of magnetization with two barriers of 15.2 and 523 K. Ab initio calculations were used to interpret experimental data and compare the Dy-sulfide clusterfullerenes to other Dy-clusterfullerenes. The smallest and largest barriers are ascribed to the exchange/dipolar barrier and relaxation via crystal-field states, respectively, whereas an intermediate energy barrier of 48 K in Dy(2)S@C(82)-C(3v)(8) is assigned to the local phonon mode, corresponding to the librational motion of the Dy(2)S cluster inside the carbon cage.