Homo-dinuclear hexadentate copper(II) and nickel(II) cryptates: synthesis, spectroscopic and electrochemical characterization of the perchlorate salts

Abstract

Two homo-dinuclear cryptates of Cu^II and Ni^II with a ditopic cryptand incorporating both hard and soft donors are described. The electronic and EPR spectral data for the dicopper cryptate are consistent with a distorted octahedral coordination geometry. The EPR spectra of this complex is axial in nature at 77 K corresponding to the d_x²-y² ground state. A room temperature magnetic moment number (µ_eff/µ_B) of 1.98 per copper indicate that the two Cu^II centres are not magnetically coupled. Cyclic voltametric experiments show a quasi-reversible (ΔE_p = 115 mV) reductive response at E_f = -0.17 V (vs SCE) corresponding to the Cu^II/Cu^I couple. Exhaustive controlled potential coulometry on this cryptate at 295 K gives n = 1.96 e^-, signifying that the cyclic response is a two-electron process involving two independent Cu^II centres. The dinickel cryptate, on the other hand, does not show any cyclic response in the range +1.0 to -1.0 V. However, the electronic spectral and magnetic susceptibility data obtained on this cryptate are consistent with a distorted octahedral geometry around each Ni^II. The Schiff base precursor of the cryptand also readily forms a dicopper cryptate having similar electronic spectral, EPR and magnetic properties as the dicopper cryptate described above. However, the Cu^II/Cu^I couple for this cryptate appears at E_f = +0.14 with ΔE_p = 95 mV. The difference in E_f values for the two cryptates are attributable to the greater σ-donor ability of the amino nitrogens compared to the imino nitrogens.