Unexpected Ferromagnetic Interaction in a New Tetranuclear Copper(II) Complex: Synthesis, Crystal Structure, Magnetic Properties, and Theoretical Studies

The new tetranuclear carbonate complex [Cu2L)2(CO3)]·8H2O (1·8H2O) (H3L = (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) has been obtained by two different synthetic routes and fully
characterized. Recrystallization of 1·8H2O in methanol yields single crystals of {[(Cu2L)2(CO3)]}2·12H2O (1·6H2O),
suitable for X-ray diffraction studies. The crystal structure of 1·6H2O shows two crystallographically different
tetranuclear molecules in the asymmetric unit, 1a and 1b. Both molecules can be understood as self-assembled
from two dinuclear [Cu2L]+ cations, joined by a μ4-η2:η1:η1 carbonate ligand. The copper atoms of each
crystallographically different [(Cu2L)2(CO3)] molecule present miscellaneous coordination polyhedra:  in both 1a
and 1b, two metal centers are in square pyramidal environments, one displays a square planar chromophore and
the other one has a geometry that can be considered as an intermediate between square pyramid and trigonal
bipyramid. Magnetic studies reveal net intramolecular ferromagnetic coupling between the metal atoms. Density
functional calculations allow the assignment of the different magnetic coupling constants and explain the unexpected
ferromagnetic behavior, because of the presence of an unusual NCN bridging moiety and countercomplementarity
of the phenoxo (or carbonate) and NCN bridges.