Hetero triply-bridged dinuclear copper(II) compounds with ferromagnetic coupling: a challenge for current density functionals.

Seven new hetero triply-bridged dinuclear Cu(II) compounds have been synthesized and characterized corresponding to a series with general formula [Cu(2)(L)(2)(μ-OH)(μ-OH(2))(μ-O(2)CR)]X(2) (where L = bpy = 2,2'-bipyridine, 4,4'-dmbpy = 4,4'-dimethyl-2,2'-bipyridine and 5,5'-dmbpy = 5,5'-dimethyl-2,2'-bipyridine; R = H for formate, CH(3) for acetate, CH(2)CH(3) for propionate and C(CH(3))(3) for trimethylacetate and X = CF(3)SO(3)(-) and ClO(4)(-)). All compounds exhibit ferromagnetic behavior with the experimental J values derived from magnetic susceptibility measurements being in the 73-104 cm(-1) range. The overall qualitative behavior is reproduced by state of the art density functional theory based methods. However, none of the functionals is able to reproduce the fine details along the series which constitutes an excellent benchmark for future developments.