Synthesis and spectral characterization of nickel(II) and copper(II) complexes with the hexadentate (±)-1,3-pentanediamine-N,N,N′,N′-tetraacetate ligand and its pentadentate derivative: Stereospecific formation and crystal structure of [Mg(H2O)6][Ni(1,3-pndta)]·4H2O

Abstract

Structural variations modulated by ethyl side group have been analyzed for nickel(II) and copper(II) complexes with (±)-1,3-pentanediamine-N,N,N′,N′-tetraacetate ligand (1,3-pndta) and its pentadentate derivative (±)-1,3-pentanediamine-N,N,N′-triacetate (1,3-pnd3a). X-ray results obtained for the octahedral 1,3-pndta-Ni(II) complex indicate the distinct correlation of the central chirality of the metal with the chirality at the diamine α-carbon, which leads to the formation of a racemic mixture of ΔΛΔ(R) and ΛΔΛ(S) enantiomeric anionic complexes, where the R and S labels designate the absolute configuration at the ligand (ethyl-substituted) stereogenic center. These stereoisomers are those required to maintain an equatorial ethyl group on the central six-membered chelate ring, whose conformation is λ twist-boat for the ΔΛΔ isomer and δ twist-boat for the ΛΔΛ isomer. Ethyl group discernibly affects the variation of the Ni–N and Ni–O bond lengths compared to the analogous complex lacking the substitution of one of the diastereotopic diamine CH₂ hydrogens and functions as steric bulkiness which hampers an efficient and of high symmetry packing, observed in the crystals of the parent complex. The infrared and electronic absorption spectra of hexadentate Mg[Ni(1,3-pndta)]·10H₂O (1) and Mg[Cu(1,3-pndta)]·7H₂O (3), and pentadentate Mg[Ni₂(1,3-pnd3a)₂]·8H₂O (2) and Mg[Cu₂(1,3-pnd3a)₂]·7H₂O (4) complexes are presented and discussed in comparison with those of the analogous Mg[Ni(1,3-pdta)]·8H₂O (5), Mg[Cu(1,3-pdta)]·8H₂O (6) and Mg[Cu₂(1,3-pd3a)₂]·7H₂O (7) complexes of known crystal structures.