Palladium(II) complexes of 5-acetylmethyl-1,2,4-oxadiazole: versatile ditopic acceptors for halogen bonding

Abstract

The cocrystallization of three palladium(II) complexes with 3-R-5-acetylmethyl-1,2,4-oxadiazoles (R = Me (1), Et (2), Ph (3)) as ligands with halogen bond (HaB) donors 1,4-diiodotetrafluorobenzene (1,4-FIB) and 1,3,5-triiodotrifluorobenzene (1,3,5-FIB) gave cocrystals 1•1,3,5-FIB, 2•1,4-FIB, and3•1,4-FIB, whose structures were determined by single-crystal X-ray diffraction (XRD). Geometric analysis of intermolecular contacts revealed I⋅⋅⋅N HaBs involving the nitrogen atoms of the 1,2,4-oxadiazole fragments, with the nature of these interactions confirmed by DFT calculations using the Douglas-Kroll-Hess second-order scalar relativistic (DKH2) approach under periodic boundary conditions, complemented by topological analysis of electron density via Bader's quantum theory of atoms in molecules (QTAIM) and examination of electron localization function (ELF) projections. The HaB directs the assembly of alternating complex and donor molecules into extended one-dimensional chains -linear when involving 1,4-FIB and zigzag in the case of 1,3,5-FIB. The complexes additionally form offset stacks via π-stacking involving either the metal center and 1,2,4-oxadiazole fragment (in 1•1,3,5-FIB and 2•1,4-FIB) or organic ligand fragments (in 3•1,4-FIB). The electrophilic-nucleophilic duality of palladium(II) in intermolecular interactions within 1•1,3,5-FIB and 2•1,4-FIB, as well as the dual electron-donating and electron-accepting roles of coordinated 1,2,4-oxadiazoles, were established through reduced density gradient (RDG) analysis and examination of electron density (ED) and electrostatic potential (ESP) minima along interatomic lines.