Dichotomy of π-stacking-directing noncovalent forces in organic–inorganic planar assemblies: the case of halo-substituted benzoquinones π-stacked with a platinum(ii) square-plane

Abstract

The cyclometalated square-planar platinum(II) complex 1, bearing a 2-mercaptopyridine 1-oxide chelate ligand, was cocrystallized with any one of the planar π-hole donating tetrahalo-1,4-benzoquinones QX (X = F, Cl, Br, I) to give a series of cocrystals 1·QX; the latter were studied by X-ray crystallography and ¹⁹⁵Pt MAS-NMR spectroscopy. These studies revealed that all four cocrystals exhibit almost the same π-stacked organic–inorganic structural pattern. The complex plane is linked to QX through overall π–π stacking, which includes metal-involving C⋯Pt tetrel bonding. The internal forces holding the two interacting planes together differ on going from the structure with X = F, Cl (mostly determined by electrostatic and dispersion interactions involving charge transfer d_z²(Pt) → π*(QX)) to that with X = I (no significant charge transfer is observed and the interaction occurs predominantly through electrostatic forces with a concomitant increase of dispersion forces). The dichotomy of π-stacking-directing noncovalent forces (namely, the same organic–inorganic π-stacked motif, but different interplanar holding forces) was studied theoretically using QTAIM and IGMH methods, molecular electrostatic and London dispersion potentials, charge transfer (CDF, ETS-NOCV, and GKS-EDA analyses), interactions and binding energies. The reported findings suggest that π-stacking could be much more complicated than is commonly believed and the available concepts of the π-stacking require rethinking.