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

The planar bis-chelated complex [Pd(N^∩O)₂] (<b>1</b>; 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 <b>1</b>·3(C₆F₆) and <b>1</b>·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 <b>1·</b>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, <b>1</b>·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.