Synthesis of mononuclear nickel(II) and copper(II) complexes using compartmental ligands: X-ray and electrochemical studies

A series of mononuclear complexes ML1 (M=Ni, Cu) where L1=N,N′-bis[2-hydroxy-3-(piperidin-1-ylmethyl)-5-methylbenzyl]alkyldiimine (alkyl=ethylene L1a, propylene L1b, butylene L1c) and ML2 (M=Ni, Cu) where L2=N,N′-bis[2-hydroxy-3-(piperidin-1-ylmethyl)-5-bromobenzyl]alkyldiimine (alkyl=ethylene L2a, propylene L2b, butylene L2c) have been synthesized and characterized by C, H, N analysis, IR, electronic and ESR spectra. The crystal structures of the complexes NiH2L1a (1) and CuH2L1a (6) were determined and the geometry around the metal ion is square planar. Electrochemical studies of the complexes show a quasi-reversible single redox wave at negative potential in the range 0 to −1.3 V. For nickel complexes (1–5) Epc=−1.0 to −0.80 V and for copper complexes (6–9) Epc=−0.90 to −0.72 V. The ESR spectra of nickel complexes are inactive due to square planar nature of the complexes whereas copper complexes show an ESR spectrum characteristic of square planar complex with nuclear hyperfine spin 3/2. Copper complexes show a normal magnetic moment value of μeff=1.70–1.75 B.M. which is close to the spin-only value of 1.73 B.M.