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

Abstract

The new tetranuclear carbonate complex [Cu₂L)₂(CO₃)]·8H₂O (<b>1</b>·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\ncharacterized. Recrystallization of <b>1</b>·8H₂O in methanol yields single crystals of {[(Cu₂L)₂(CO₃)]}₂·12H₂O (<b>1</b>·6H₂O),\nsuitable for X-ray diffraction studies. The crystal structure of <b>1</b>·6H₂O shows two crystallographically different\ntetranuclear molecules in the asymmetric unit, <b>1a </b>and <b>1b</b>. Both molecules can be understood as self-assembled\nfrom two dinuclear [Cu₂L]⁺ cations, joined by a μ₄-η²:η¹:η¹ carbonate ligand. The copper atoms of each\ncrystallographically different [(Cu₂L)₂(CO₃)] molecule present miscellaneous coordination polyhedra: in both <b>1a</b>\nand <b>1b</b>, two metal centers are in square pyramidal environments, one displays a square planar chromophore and\nthe other one has a geometry that can be considered as an intermediate between square pyramid and trigonal\nbipyramid. Magnetic studies reveal net intramolecular ferromagnetic coupling between the metal atoms. Density\nfunctional calculations allow the assignment of the different magnetic coupling constants and explain the unexpected\nferromagnetic behavior, because of the presence of an unusual NCN bridging moiety and countercomplementarity\nof the phenoxo (or carbonate) and NCN bridges.