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

Crystal
structures of six iodopyridinium tetrahalocuprate­(II) salts
are reported, (nIP)2CuX4, 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···X
hydrogen bonding and C–I···X halogen bonding
interactions. Comparing these structures with the previously published
structures of the general formulas (nCP)2CuX4 and (nBP)2CuX4, 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)2CuX4 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···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.