Isomorphous Series of Pd^II-Containing Halogen-Bond Donors Exhibiting Cl/Br/I Triple Halogen Isostructural Exchange

Two isomorphous series including the adducts trans-[PdI₂(CNC₆H₄-4-X)₂]·2I₂ [(1–3)·2I₂; X = Cl 1, Br 2, I 3] and the complexes cis-[PdCl₂(CNC₆H₄-4-X)­(PPh₃)] (4–6; X = Cl 4, Br 5, I 6) were characterized by X-ray crystallography. Inspection of the X-ray diffraction data allowed the identification of halogen-bonding C–X···I–I in (1–3)·2I₂ and C–X···Cl–[Pd] in 4–6 thus providing a rare example of the triple Cl/Br/I exchange of σ-hole donating halogens. In accord with the conducted theoretical considerations, the strength of these XBs are in the range of 0.9–2.8 kcal/mol, and these contacts are of a noncovalent nature. The calculated maximum electrostatic potentials on the σ-hole donating X centers of CNC₆H₄-4-X on the 0.001 au molecular surfaces for both (1–3)·2I₂ and 4–6 series are increased along with the decrease of relative XB lengths. These data are consistent with Politzer and co-workers conclusions that are based exclusively on theoretical ground.