Competing Large and Small Angles in a Double End-On Azido Copper(II) Binuclear Complex: A Combined Experimental and Theoretical Study of Magnetic Interactions

The structure and magnetic properties of double azido-bridged Cu-II binuclear complex 1 with the chelating chiral ligand (S,S)-2,2'-isopropylidenebis(4-phenyl-2-oxazoline) were analyzed by combining experimental and theoretical techniques. The Cu-II ions adopt a square pyramidal geometry with different degrees of distortion, whereas the two end-on azido bridges disposed at the equatorial positions exhibit significantly different Cu-N-Cu angles, 110.6 and 97.3 degrees, which are respectively smaller and larger than the critical 108 degrees value distinguishing the ferromagnetic and antiferromagnetic regimes. The asymmetry in 1 arises from the use of the bulky asymmetric ligand, giving rise to two different magnetic pathways between the Cu-II ions. The magnetic pathway along the large Cu-N-Cu angle value dominates over the small one, resulting in a net antiferromagnetic behavior with J = -78.6 cm(-1). On the basis of wavefunction calculations, we investigate the exchange interactions in synthetic compound 1 and fictitious analogs 2 and 3 holding either two large (i.e., 110.6 degrees) or two small (i.e., 97.3 degrees) Cu-N-Cu angles. The calculated exchange interaction in 1 (-104 cm(-1)) is in relatively good agreement with the experimental value and corresponds precisely to the average between the antiferromagnetic value in 2 (-218 cm(-1)) and the ferromagnetic one in 3 (21 cm(-1)). The significant enhancement in the antiferromagnetic contribution accompanying the expansion of one of the Cu-N-Cu bridging angles is undoubtedly the driving force for the observed antiferromagnetic behavior in 1. The control of the local metal environments allowed us to monitor the exchange coupling interactions. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)