The impact of anion-modulated structural variations on the magnetic coupling in trinuclear heterometallic Cu^II–Co^II complexes derived from a salen-type Schiff base ligand

Abstract

Three new trinuclear heterometallic [(Cu^IIL)₂Co^IIX₂] 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•6H₂O 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 μ_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 (1κO:2κO′) and the end-on bridging azido (μ_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)° 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.