Two types of nitrito support for μ4-oxido-bridged [Cu4] complexes: synthesis, crystal structures, magnetic properties and DFT analysis

Novel nitrito supported and mu(4)-oxido bridged Cu-II aggregates have been found in two tetranuclear complexes, [Cu-4(mu(4)-O)L-2(mu(1,3)-ONO)(4)] (1) and [Cu-4(mu(4)-O)L-2(mu(1,3)-OAc)(2)(mu(1,2)-NO2)(2)] (2), of the chiral Schiff base HL (HL = 4-methyl-2,6-bis-(1-phenyl-ethylimino)-methylphenol). The structures contain either in situ generated or externally added peripheral mu-nitrito groups, in kappa O-2/O and kappa N-2/O bridging modes. Four NO2- bridges in 1 and two AcO-co-ligands along with two NO2- bridges in 2 are essential for the stabilization of these tetranuclear aggregates. The complexes have been characterized by X-ray crystal structure determination, spectroscopic and magnetic measurements, and density functional theory (DFT) analysis. They are formed from the assembly of two [Cu2L](3+) fragments around a water-derived oxido ligand under the control of nitrite or mixed nitrite/acetate bridges. Variable temperature magnetic susceptibility measurements reveal strong intramolecular antiferromagnetic exchange interactions within the tetranuclear clusters to yield S-T = 0 ground state. The capacity of the two different nitrite bridging modes to mediate magnetic coupling has been examined through measurements and numerical fitting procedures, and rationalized by means of DFT calculations.