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

Two tetradentate N-2, N-2' 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 delta hydrogen atoms. The puckered structures are ca. 6 kJ mol (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 (pi -> pi*) transitions. (C) 2014 Elsevier B.V. All rights reserved.