An unusual asymmetric pseudomacrocyclic free base ligand and nickel(II) chelate: X-ray crystallographic and DFT studies

Abstract

A novel tetradentate dipyrrolidequinoxaline ligand, 5-(3-{5-[(E)-(propylimino)methyl]-1H-pyrrol-2-yl}quinoxaline-2-yl)-1H-pyrrole-2-carbaldehyde (H2L1) and its Ni(II) complex, 2-[3-(5-formyl-1H-pyrrol-2-yl)quinoxaline-2-yl]-5-(prpylimino)methyl]propyl-1-ide nickel(II), [Ni(L1)], have been synthesised and studied by X-ray crystallography. The asymmetric pseudomacrocyclic ligand, which has an unusual NN′N″O donor atom set, crystallised in the orthorhombic space group P212121. Interestingly, the X-ray data show that the ligand is present as a zwitterion in the solid state with the trans imine and pyrrole N-atoms protonated. The zwitterionic structure leads to two intramolecular hydrogen bonds with hydrogen bond lengths (D⋯A) for Npyrrole–H⋯Npyrrole and Nimine–H⋯Oaldehyde which measure 2.591(2) and 2.836(2)Å, respectively. The Ni(II) chelate crystallises in the monoclinic space group Cc and adopts a square planar coordination geometry. DFT-simulated structures of both H2L1 and [Ni(L1)] in vacuo are in good agreement with those obtained experimentally in the solid state with RMSDs of 0.1710Å and 0.3214Å, respectively. DFT studies showed that the experimentally determined zwitterionic structure of H2L1 is significantly lower in energy than the other possible canonical forms. ESP maps provide insight into the hydrogen bonding patterns of the ligand. TD-DFT calculations were used to delineate the experimental UV–Vis spectra.