Magneto-structural correlations and DFT calculations in two rare tetranuclear copper(II)-clusters with doubly phenoxo and end-on azido bridges: Syntheses, structural variations and EPR studies

By slightly changing the synthetic conditions, we have prepared two closely related linear tetranuclear Cu-II complexes with the symmetrical ONNO donor tetradentate Schiff-base ligand [H2L = (OH)C6H4 (CH3)C=N(CH2)(3)N=C(CH3)C6H4(OH)] and with azide ions. These two distinctly coloured crystalline products were characterized by elemental analysis, IR and UV-Vis spectroscopy, CV, EPR spectra and variable temperature magnetic measurements. Single crystal X-ray diffraction studies of the green [Cu-4(mu-L)(2)(mu(1,1)-N-3)(2)(N-3)(2)] (1) and the red [Cu-4(mu-L)(2)(mu(1,1)-N-3)(2)(N-3)(2)(H2O)(2)] (2) crystals show that the coordination environment of the two mu-phenoxo and mu(1,1)-azido bridged isomorphous tetranuclear Cu-II complexes are slightly different. Thus, both complexes are formed by very similar building units, although with a significant variation in the bridging Cu-O(phenoxo)-Cu and Cu-N(azido)-Cu bond angles. The consequences of these structural variations on the magnetic properties have been investigated from both the experimental and theoretical points of view by variable temperature magnetic measurements and DFT calculations.