Different Stacking Types in a Single Hybrid Cocrystal System: π···π- and π–Hole-Based Organic–Inorganic Planar Assemblies

Abstract

The planar bis-chelated complex [Pd(N^∩O)₂] (1; N^∩O = 4-MeC₅H₃NNC(O)NMe₂) exhibits two distinct stacking modes with electron-deficient aromatics: π···π stacking with hexafluorobenzene (C₆F₆) versus charge-transfer π–hole interactions with 1,2,4,5-tetracyanobenzene (TCB). Cocrystallization of the complex with C₆F₆ or TCB yields cocrystals 1·3(C₆F₆) and 1·2TCB, respectively, which display different colors and stacking patterns despite similar structural motifs. Comprehensive analysis using X-ray diffraction, combined with quantum theory of atoms-in-molecules (QTAIM), an independent gradient model based on Hirshfeld partition (IGMH), extended transition state natural orbital for chemical valence theory with charge displacement function (ETS-NOCV/CDF), many-body interaction analysis, and symmetry-adapted perturbation theory (SAPT), reveals fundamentally different interaction mechanisms. In 1·3(C₆F₆), the stacking is primarily governed by intermolecular polarization without significant charge transfer, with dispersion forces contributing approximately 70% of the attractive energy. In contrast, 1·2TCB exhibits pronounced charge transfer (35 me) and significant inductive components alongside dispersion forces, characteristic of π–hole interactions. This dichotomy in stacking behavior provides new insights into the nature of organic–inorganic planar assemblies and demonstrates that seemingly similar structural patterns can arise from distinctly different combinations of noncovalent forces, which is essential for rational crystal engineering of hybrid materials.