Control of nuclearity in heterometallic Cu^II-Mn^II complexes derived from asymmetric Schiff bases: structures and...

Two new N₂O₃ donor ligands N-(3-methoxysalicylidene)-N'-(salicylidene)-1,3-propanediamine (H₂L²) and N-(3-methoxysalicylidene)-N'-(α-methylsalicylidene)-1,3-propanediamine (H₂L³) and their Cu(ii) complexes have been synthesized. Using these complexes and another Cu-complex with one asymmetric N₂O₂ donor ligand N-(α-methylsalicylidene)-N'-(salicylidene)-1,3-propanediamine (H₂L¹) as metalloligands, four new heterometallic Cu(ii)-Mn(ii) complexes [(CuL¹)₂Mn₂Cl₄]·3CH₃OH (1), [(CuL¹)₂MnCl₂]·2CH₃CN (2), [CuL²MnCl₂]·H₂O (3) and [CuL³MnCl₂] (4) have been synthesized with MnCl₂·4H₂O. The tetranuclear (1) and trinuclear (2) complexes were prepared by varying the ratio of the metalloligand [CuL¹] and MnCl₂ whereas dinuclear complexes 3 and 4 are the sole products irrespective of the metalloligand : MnCl₂ ratio. All the complexes 1-4 were structurally characterized by single crystal X-ray diffraction. The tetranuclear (CuMn) structure of 1 is formed by the dimerization of phenoxido bridged Cu^IIMn^II units whereas the bent CuMn^II trinuclear species 2 is formed by the coordination of two metalloligands (CuL¹) to Mn(ii) via a diphenoxido bridge. Complexes 3 and 4 possess a similar dinuclear Cu^IIMn^II structure in which two phenoxido groups form a bridge between the two metal centres. The variable temperature dc magnetic susceptibility measurements reveal that the Cu(ii) and Mn(ii) ions are antiferromagnetically coupled in all the complexes, 1-4. The values of exchange coupling constants (J_Mn-Cu) are -45.54, -39.66, -71.54 and -48.40 cm^-1 for complexes 1-4, respectively. A DFT study of all the complexes has also been performed to rationalize the experimental results of magnetic measurements.