New binuclear Ni(II)–glycinate homologues: electrochemically distinguishable diastereomers

Highlights: • The influence of the configuration of the stereocenters of an homologous series of binuclear diastereomeric (SR)- and (SS)-Ni(II) α-amino acid /Schiff-base complexes on their electrochemical behavior is reported. • A phenomenon of an alternation of the reduction and oxidation sites in the homologous series of the (SR)-diastereomers was uncovered. • Cyclic voltammetry constitutes an electrochemical probe to determine the absolute configuration of the α-carbon atoms in these complexes. - Abstract: A voltammetric investigation of the electrochemical reduction and oxidation of a homologous series of binuclear diastereomeric (SR)- and (SS)-Ni(II) α-amino acid /Schiff-base complexes in which two Ni-containing moieties are linked via glycine fragments (LNi–Gly–(CH₂)_n–Gly–NiL; n = 0,1,2) and which contain an auxiliary chiral aromatic moiety – (S)-o-[N-(N'-benzylprolyl) amino]benzophenone was performed at a Pt electrode in acetonitrile. These complexes constitute an interesting example of the influence of the absolute configuration of the stereocenters on the electrochemical behavior of the diastereomers and on the redox potential values. To distinguish between the diastereomers, cyclic voltammetry is more sensitive than UV–vis spectroscopy. For n = 0, the difference between the λ_max values for the (SR) and the (SS) diastereomers is large enough to differentiate them. With an increase in the number of the methylene units between two Ni–glycine moieties in the binuclear complex spectral characteristics for (SR) and (SS) diastereomers become almost identical whereas the voltammetric characteristics remain sufficiently different to allow the differentiation even for n = 2. ¹H, ¹³C NMR data and DFT computation revealed that the electronic density in (SR)- LNi–Gly–(CH₂)_n–Gly–NiL (n = 0,1,2) is distributed non-uniformly between the two parts of the diastereomeric molecule both in the neutral complexes and their radical cation. Their dipole moment as well as their frontier orbitals were calculated. The HOMO and LUMO are not located on the same mononuclear units and their location alternates between the two units in the homologous series. Thus, there is an alternation of the reduction and oxidation sites. As a consequence, both the oxidation and the reduction of the (SR)-complexes are easier than those of to their (SS) counterparts, which results in a smaller difference between the oxidation and the reduction potentials for the SR diastereomers. This provides an electrochemical way to probe the absolute configuration of the α-carbon atoms of the diaminodicarboxylic acid moiety in these Ni(II) binuclear complexes.