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

Crystal structures of six iodopyridinium tetrahalocuprate(II) salts are reported, (nIP)(2)CuX4, where X = Cl or Br, nIP is the n-iodopyridinium cation, and n = 2, 3, or 4. The supramolecular structure of these salts is developed based on N-H center dot center dot center dot X hydrogen bonding and C-I center dot center dot center dot X halogen bonding interactions. Comparing these structures with the previously published structures of the general formulas (nCP)(2)CuX4 and (nBP)(2)CuX4, where nCP(+) and nBP(+) are the n-chloropyridinium and n-bromopyridinium cations, respectively, allows us to investigate the competition between the halogen and hydrogen bonding interactions. Henceforth, the general formula (nYP)(2)CuX4 will be used to represent the 18 structures where nYP(+) is the n-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 nYP(+) cation and effects of intermolecular forces other than N-H center dot center dot center dot X hydrogen bonding, and C-I center dot center dot center dot 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 center dot center dot center dot X distance is either unvaried or becomes longer. In contrast, the Y center dot center dot center dot X distance becomes shorter even though heavier halogens have a larger radius. For example, for the isomorphous structures (2BP)(2)CuCl4 and (2IP)(2)CuCl4, the N center dot center dot center dot Cl distances are 2.926 angstrom and 3.070 angstrom, respectively, whereas the corresponding Y center dot center dot center dot Cl distances are 3.322 angstrom and 3.316 angstrom. 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)(2)CuBr4 structures are isomorphous; the difference between the two legs of the hydrogen bond are 0.117 angstrom, 0.191 angstrom, and 0.246 angstrom for (4CP)(2)CuBr4, (4BP)(2)CuBr4, (4IP)(2)CuBr4, 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.