Shift of the reduction potential of nickel(ii) Schiff base complexes in the presence of redox innocent metal ions

Abstract

With the objective of gaining insight into the modulation of the reduction potential of the Ni(II/I) couple, we have synthesized two mononuclear nickel(II) complexes, NiL^en (H₂L^en = N,N′-bis(3-methoxysalicylidene)-1,2-diamino-2-methylpropane) and NiL^pn (H₂L^pn = N,N′-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane) of two N₂O₄ donor ligands and recorded their cyclic voltammograms. Both the nickel complexes show reversible reduction processes for the Ni(II/I) couple in acetonitrile solution but the reduction potential of NiL^pn (E_1/2 = −1.883 V) is 188 mV more positive than that of NiL^en (E_1/2 = −2.071 V). In the presence of redox inactive metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺ and Ba²⁺), the reduction potentials are shifted by 49–331 mV and 99–435 mV towards positive values compared to NiL^en and NiL^pn, respectively. The shift increases with the decrease of the pK_a of the respective aqua-complexes of the metal ion but is poorly co-linear; however, better linearity is found when the shift of the mono- and bi-positive metal ion aqua complexes is plotted separately. Spectrophotometric titrations of these two nickel complexes with the guest metal ions in acetonitrile showed a well-anchored isosbestic point in all cases, confirming the adduct formation of NiL^en and NiL^pn with the metal ions. Structural analysis of single crystals, [(NiL^en)Li(H₂O)₂]·ClO₄ (1), [(NiL^pn)Li(H₂O)]·ClO₄ (2), [(NiL^pn)₂Na]·BF₄ (3) and [(NiL^pn)₂Ba(H₂O)(ClO₄)]·ClO₄ (4), also corroborates the heterometallic adduct formation. The orbital energies of the optimised heterometallic adducts from which electron transfers originated were calculated in order to explain the observed reduction process. A strong linear connection between the calculated orbital energies and the experimental E_1/2 values was observed. According to MEP and 2D vector field plots, the largest shift for divalent metal ions is most likely caused by the local electric field that they impose in addition to Lewis acidity.