An X-ray crystallographic and DFT study of bis(pyrrolide-imine) nickel(II) Schiff base chelates

Abstract

Two tetradentate N2,N2′ donor bis(pyrrolide-imine) nickel(II) chelates, 2,2′-{(2,2-dimethylpropane-1,3-diyl)bis[nitrilo(E)methylylidene]}bis(pyrrol-1-ide)nickel(II) and 2,2′-{(2-hydroxypropane-1,3-diyl)bis[nitrilo(E)methylylidene]}bis(pyrrol-1-ide)nickel(II), have been synthesized and studied by X-ray crystallography and DFT methods. Both compounds crystallized in the triclinic space group P-1 with three molecules in the asymmetric unit. The chelates have nominally square planar coordination geometry but exhibit puckering to various degrees. The hydroxyl substituent on the di(azomethine) linkage of 2,2′-{(2,2-dimethylpropane-1,3-diyl)bis[nitrilo(E)methylylidene]}bis(pyrrol-1-ide)nickel(II) leads to complementary hydrogen bonding which yields a trimeric supramolecular structure. DFT simulations at the PBEPBE/6-311G(dp) level of theory suggest that a slight puckering of the structures minimizes non-bonded repulsion between the pyrrole δ hydrogen atoms. The puckered structures are ca. 6kJmol−1 lower in energy than a planar geometry. TD-DFT was used to delineate the experimental UV–Vis spectra. These simulations accurately predicted the relative intensities of the transitions as well as the wavelengths of the higher energy (π→π∗) transitions.