When Donors Cannot Choose, the Metal Decides: Acceptor-Site Hierarchy and Polyfurcated, Metal-Involving Halogen Bonds in Platinum(II) Cocrystals

Abstract

The square-planar complex [Pt(ppy){S2CN(CH2)6}] (1; ppyH = 2-phenylpyridine) presents, within a single molecule, a set of chemically distinct halogen-bond (HaB) acceptor sitesthe nucleophilic dz2-PtII center, the dithiocarbamate sulfur atoms, and the metalated C6H4 ring of the phenylpyridyl ligand. To probe how an incoming σ-hole selects among these competing sites, 1 was cocrystallized with a series of polyfluorinated iodoarenes of increasing topicitymonotopic 4-FIBP, ditopic 1,4-FIB and 4,4′-FIBP, and tritopic 1,3,5-FIBwhose σ- and π-holes are deliberately non-codirectional. Four cocrystals were obtained and characterized by single-crystal X-ray diffraction. Across all structures the iodine σ-hole preferentially engages the metal center, and every I···Pt contact is accompanied by an ancillary I···S and/or I···C linkage, giving rise to asymmetric three-center I···(Pt,S)/(Pt,C), four-center I···(Pt,S,C), and, in one case, a five-center I···(Pt,S,C,C) bond, in which the I···Pt component dominates. A complementary set of theoretical bonding descriptors (AIM, ELF, IGMH, IBSIW, NBO, ETS-NOCV, EDD, and CDF) corroborates this site hierarchy and shows that several attractive contacts are not flagged by bond critical points, underscoring the need for a multi-descriptor analysis of heavy-atom interactions. The supramolecular dimensionality scales with donor topicitydiscrete assemblies, one-dimensional chains, and two-dimensional layers for mono-, di-, and tritopic donors, respectivelyyet the observed non-ideal stoichiometries reveal that the donors do not always realize their maximal topicity toward the preferred acceptor sites, an outcome traced to σ-hole anticooperativity and crystal-packing effects. In short, when the donor cannot reconcile its holes to satisfy any single site, the metal decides: the dz2-PtII center wins the σ-hole and organizes the assembly around itself.