Competition between Hydrogen and Halogen Bonding Interactions: Theoretical and Crystallographic Studies

Crystal structures of six iodopyridinium tetrahalocuprate­(II) salts are reported, (<i>n</i>IP)₂CuX₄, where X = Cl or Br, <i>n</i>IP is the <i>n</i>-iodopyridinium cation, and <i>n</i> = 2, 3, or 4. The supramolecular structure of these salts is developed based on N–H···X hydrogen bonding and C–I···X halogen bonding interactions. Comparing these structures with the previously published structures of the general formulas (<i>n</i>CP)₂CuX₄ and (<i>n</i>BP)₂CuX₄, where <i>n</i>CP⁺ and <i>n</i>BP⁺ are the <i>n</i>-chloropyridinium and <i>n</i>-bromopyridinium cations, respectively, allows us to investigate the competition between the halogen and hydrogen bonding interactions. Henceforth, the general formula (<i>n</i>YP)₂CuX₄ will be used to represent the 18 structures where <i>n</i>YP⁺ is the <i>n</i>-halopyridinium cation. Isomorphism has been observed in these structures. Isomorphic structures are divided into four sets. Analysis of the isomorphic structures allows us to apply the separation of variables principle; upon comparison of isomorphic structures, complications arise from geometrical factors due to the isomeric nature of the <i>n</i>YP⁺ cation and effects of intermolecular forces other than N–H···X hydrogen bonding, and C–I···X halogen bonding interactions are minimized and hence can be ignored. Comparing halogen and hydrogen bonding interaction parameters within each isomorphous set allows us to investigate the competition between these interactions. As the organic halogen becomes heavier and the halide ligand is unvaried, the N···X distance is either unvaried or becomes longer. In contrast, the Y···X distance becomes shorter even though heavier halogens have a larger radius. For example, for the isomorphous structures (2BP)₂CuCl₄ and (2IP)₂CuCl₄, the N···Cl distances are 2.926 Å and 3.070 Å, respectively, whereas the corresponding Y···Cl distances are 3.322 Å and 3.316 Å. Theoretical calculations have shown that bifurcated hydrogen bonding interactions are stronger than the corresponding linear ones. Also, calculations have shown that as the organic halogen becomes heavier, the halogen bonding interactions become stronger. This agrees with crystal structure data; as the organic halogen gets heavier and the halide ligand is unvaried, the difference between the two legs of the bifurcated hydrogen bond becomes larger (weaker hydrogen bonding interactions). For example, the three (4YP)₂CuBr₄ structures are isomorphous; the difference between the two legs of the hydrogen bond are 0.117 Å, 0.191 Å, and 0.246 Å for (4CP)₂CuBr₄, (4BP)₂CuBr₄, (4IP)₂CuBr₄, respectively. Surprisingly, the above two trends are valid in all isomorphous sets without exception, which is rare in solid state chemistry. Analysis of the Cu–X bond distances indicates that the Cu–X bond distance of the halogen acceptor is always shorter than that of the corresponding proton acceptor; which agrees with the theoretical calculations; hydrogen bonding interactions are stronger than the corresponding halogen bonding interactions