Competitive electronic effect of ligand substitution over the role of metal ions (Ni and Co) on unusual amine–imine interconversion in conjugated amine–ene–imine ligands

Abstract

Two sets of maleonitrile-tethered, N-atom-donor tridentate ligands which are configurationally isomeric, 2-((E)-(aryl-methylene)amino)-3-((pyridin-2-ylmethyl)amino)maleonitrile (HL^1-Ar) and 2-((aryl-methyl)amino)-3-((E)-(pyridin-2-ylmethylene)amino)maleonitrile (HL^2-Ar) have been synthesized and fully characterized (where the aryl groups are 4-methoxyphenyl (HL^1-OMe/HL^2-OMe), 4-(trifluoromethyl)phenyl (HL^1-CF3/HL^2-CF3) and 2,4,6-trimethylphenyl (HL^1-Mes/HL^2-Mes)). The competitive role of the ligand's electronic effect and the type of metal ion were explored on the reactivity of Co(II) and Ni(II) metal ions with differently electronically tuned ligands. An Ni(II) metal ion was found to be effective towards the irreversible transformation of HL^1-Ar to HL^2-Ar (for all substitutions on the aryl group) where the simultaneous oxidation of amine and reduction of imine take place within a single conjugated maleonitrile-tethered ligand. Whereas, Co(II) interacted differently with each member of HL^1-Ar. With the presence of an electron-donating group (EDG), a methyl group (at ortho and para positions) in the phenyl ring of HL^1-Mes, Co(II) could selectively perform the ligand transformation. On the other hand, HL^1-CF3 and HL^1-OMe (with the presence of less EDG in the phenyl ring in comparison to HL^1-Mes) did not respond to the amine–imine interconversion process. The isolated Co(II) and Ni(II) complexes were fully characterized using crystallography, cyclic voltammetry, NMR and UV-Vis spectroscopy and the correlation of spectral data changes with the different electronic environments of the ligands was well explained. The redox instability of Co(II) complexes with HL^2-Ar towards aerobic oxidation was studied using spectro-electrochemical analysis. Finally, a mechanism for the amine–imine interconversion process was proposed based on experimentally identified intermediates as well as DFT calculations.