Synthesis, crystal structures and magnetic properties of bis(μ-dialkoxo)-bridged linear trinuclear copper(II) complexes with aminoalcohol ligands: a theoretical/experimental magneto-structural study

The bis(mu-dialkoxo)-bridged trinuclear copper(II) complexes [Cu-3(ap)(4)(ClO4)(2)EtOH] (1), [Cu-3(ap)(4)(NO3)(2)] (2), [Cu-3(ap)(4)Br-2] (3) and [Cu-3(ae)(4)(NO3)(2)] (4) (ae = 2-aminoethanolato and ap = 3-aminopropanolato) have been synthesised via self-assembly from chelating aminoalcohol ligands with the corresponding copper(II) salts. The complexes are characterised by single-crystal X-ray diffraction analyses and variable temperature magnetic measurements. The crystal structures of complexes 1-4 consist of slightly bent linear or linear trinuclear [Cu-3(aa)(4)](2+) (aa = aminoalcoholato) units to which the perchlorate, nitrate or bromide anions are weakly coordinated. The adjacent trinuclear units of 1-4 are connected together by hydrogen bonds and bridging nitrate or bromide anions resulting in the formation of 2D layers. Magnetic studies of 1, 2 and 4 show that J values vary from -379 to + 36.0 cm(-1) as the Cu-O-Cu angle (theta) and the out-of-plane shift of the carbon atom of the bridging alkoxo group (tau) vary from 103.7 to 94.4 degrees and from 0.9 to 35.5 degrees, respectively. Magnetic exchange coupling constants calculated by DFT methods are of the same nature and magnitude as the experimental ones. For complexes 1, 2 and 4, which have complementarity effects between the theta and tau angles (small theta values are associated with large tau values and vice versa), an almost linear relationship between the calculated J values with theta angles could be established, thus supporting that the theta and tau angles are the two key structural factors that determine the magnetic exchange coupling for such a type of compounds. Complex 3 does not obey this linear correlation because of the existence of counter-complementarity effects between these angles (small theta values are associated with small tau values and vice versa). It is of interest that the theoretical calculations for the magnetic exchange interaction between next-nearest neighbours indicate that the usual approximation in experimental studies of neglecting the magnetic coupling between the next-nearest neighbours in linear trinuclear complexes could lead to considerable errors, especially when J(1) and J(2) are of the same order of magnitude as J(3).