Structural variations in nickel(II) and copper(II) MN4 Schiff-base complexes with deprotonated tetradentate N,N′-bis(5-aminopyrazol-4-ylmethylene)polymethylenediamine ligands

Abstract

Two series of copper(II) and nickel(II) complexes [ML¹]–[ML⁶] containing deprotonated tetradentate N,N′-bis(5-aminopyrazol-4-ylmethylene)polymethylene diamine ligands L¹–L⁶ with varying n(2–4) of the bride –HCN(CH₂)_nNCH– have been prepared and investigated by ¹H NMR, UV/VIS and ESR spectroscopy and cyclic voltammetry measurements. Complexes [CuL⁴]–[CuL⁶] display a gradual alteration of the configuration at the metal centre from planar to pseudo-tetrahedral when going from n= 2 to 4 as indicated by the variations in their ESR spectral parameters. This was also reflected in the electronic spectra which exhibited a red shift of the ligand-field bands in this series. The nickel(II) complexes with n= 2 or 3 possess a planar structure both in solution and in the solid state, whereas in the case of [ML⁶](n= 4), diamagnetic in the solid state, a rapid (on the NMR time-scale) equilibrium between planar low-spin (S= 0) and tetrahedral high-spin (S= 1) forms was found in chloroform solution with 45% of the high-spin species at 331 K. At low temperatures the planar ⇄ tetrahedral interconversion is not accompanied by R⇄S enantiomerization at the metal centre due to the higher barriers to inversion of the seven-membered metallocycle which was followed at elevated temperatures by coalescence of the diasterotopic NCH₂ and CCH₂ protons. The crystal structure of [NiL⁶][H₂L⁶=N,N′-bis(5-amino-1,3-diphenylpyrazol-4-ylmethylene)butane-1,4-diamine] revealed a planar-to-tetrahedral distorted metal configuration with an angle between the N(1)NiN(2) and N(5)NiN(8) planes of 21° and twist-chair conformation of the seven-membered metallocycle. Cyclic voltammetry showed a reversible Ni^II→ Ni^III oxidation at potentials 0.47–0.51 V and irreversible Cu^II→ Cu^I transformations.