The Impact of Anion-Modulated Structural Variations on the Magnetic Coupling in Trinuclear Heterometallic CuII-CoII Complexes Derived from a Salen-Type Schiff Base Ligand

Three new trinuclear heterometallic [((CuL)-L-II)(2)(CoX2)-X-II] complexes [H2L = N,N'-bis(salicylidene)-1,3-propanediamine and X = thiocyanate (1), benzoate (2), or azide (3)] have been synthesized by reacting the metalloligand [CuL] with Co-(ClO4)(2)center dot 6H(2)O and the NH4+ or Na+ salt of the corresponding anion in methanol. Structural characterization reveals that the central Co-II ion is connected to two terminal metalloligands through mu(1,1)-diphenoxido bridges in all three complexes. However, two monodentate thiocyanato ions, which are mutually cis coordinated to the Co atom in 1, generate a bent structure, whereas the trans-coordinated syn-syn bridging benzoato (1 kappa O:2 kappa O') and the end-on bridging azido (mu(1,1)) coligands in 2 and 3, respectively, produce linear structures. The changes in the number and nature of the bridges with a shortening of the distances between the metal centers leads to a concomitant decrease of the average Cu-II-O-Co-II bridging angle from 99.3(2) to 97.1(4) and 91.5(1)degrees for 1, 2 and 3, respectively. Variable-temperature magnetic susceptibility measurements show the presence of a dominant antiferromagnetic coupling between the Cu-Co pairs in all three complexes. However, a steady decrease of the magnitude of the exchange coupling constant (J(Cu-Co)) is observed from -33.4 (for 1) to -11.4 (for 2) and -2.15 cm(-1) (for 3). This trend suggests that larger Cu-O-Co angles are associated with stronger antiferromagnetic coupling.