Copper(II) compounds with NNO tridentate Schiff base ligands: Effect of subtle variations in ligands on complex formation, structures and magnetic properties

Abstract

The formation and the magnetic properties of the copper(II) compounds [Cu(L1)(py)](ClO4) (1a) and [Cu(L2)(py)](ClO4) (2a), bearing the NNO tridentate Schiff base ligand L1−=(E)-2-((3-aminoethylimino)methyl)phenolate or L2−=(E)-2-((3-aminopropylimino)methyl)phenolate (obtained by monocondensation of salicylaldehyde, salH, and ethylenediamine, en, or 1,3-propylenediamine, tn, respectively) and pyridine (py) are presented. These complexes are converted into new mono-, di- and trinuclear derivatives, whose nature depends on the length of the diamine used and hence on the size of the corresponding metallacycle. Pyridine can be substituted by a molecule of N,N-dimethylformamide (DMF) in compound 2a, leading to the mononuclear [Cu(L2)(dmf)](ClO4) (2b), while 1a undergoes only decomposition under similar conditions. Pyrazine does not act as bridging ligand between two copper centres by substitution of py, but its reaction with 2a yields either the dinuclear compound [Cu2(L2)2(ClO4)2] (2c), with exclusion of pyridine, or the trinuclear [Cu3(L2)3(μ3-OH)](ClO4)2 (2d), where pyrazine acts as base generating OH−, and it does not appear in the product. Reaction of 1a with pyrazine yields only the trinuclear [Cu3(L1)3(μ3-OH)](ClO4)2 (1d). Also with 2,2′-bipyridine (2,2′-bpy) dinuclear complexes are not formed, but bpy acts as bidentate ligand to copper yielding the pentacoordinated mononuclear compounds [Cu(L1)(2,2′-bpy)](ClO4) (1e) and [Cu(L2)(2,2′-bpy)](ClO4) (2e). The crystal structures of compounds 2b, 2c and 1e have been solved and are reported. The magnetic susceptibilities χM(T) of 1a and 2a have been studied, showing the absence of any measurable Cu–Cu interaction for 1a (en, five-membered ring), while a weak but interesting intermolecular Cu–Cu ferromagnetic coupling (J=+0.96(3)cm−1) through the short dimeric Cu···O contacts is detected for 2a (tn, six-membered ring). The X-band EPR spectrum of 2a in a frozen methanol solution at 70K shows the hyperfine coupling of mononuclear copper with the three coordinated 14N atoms, yielding seven narrow lines.