The role of Fe-X•••X-Fe contacts in the crystal structures of [(2-iodopyridinium)2FeX4]X (X = Cl, Br)

The analogy of chloride-chloride contacts in compounds containing Fe-Cl1 center dot center dot center dot Cl2-Fe synthons with well-studied organic C-Cl1 center dot center dot center dot Cl2-C interactions has been investigated. The crystal structures of the two tetrahaloferrate(III) salts, [(2-iodopyridinium)(2)FeX4]X (X = Cl, Br) have been determined. Analysis of these two isomorphous structures and related published structures shows that the arrangement of Fe-Cl1 center dot center dot center dot Cl2-Fe synthons is similar to that of C-Cl1 center dot center dot center dot Cl2-C with the Fe-Cl1 center dot center dot center dot Cl2 and Cl1 center dot center dot center dot Cl2-Fe angles being similar to 150A degrees. While inter-chlorine distances are less than the sum of van der Waals radii in C-Cl1 center dot center dot center dot Cl2-C units, they are equal to, or longer, than the sum of van der Waals radii in the corresponding Fe-Cl1 center dot center dot center dot Cl2-Fe contacts. This might indicate that the arrangement of Fe-Cl1 center dot center dot center dot Cl2-Fe synthons occurs predominately to reduce repulsive forces rather than as a result of attractive forces. However, it is observed that the halide-halide distance in [(2-iodopyridinium)(2)FeBr4]Br is shorter than in the isostructural chloride species, which can be explained by the fact that bromine is softer than chlorine. Several intermolecular forces unite the cations and anions within the crystalline lattice of [(2-iodopyridinium)(2)FeX4]X including N-H center dot center dot center dot X-, C-I center dot center dot center dot X-Fe, N(pi)center dot center dot center dot X-Fe, N(pi)center dot center dot center dot I-C, and Fe-X1 center dot center dot center dot X2-Fe contacts. The calculated electron density and electrostatic potential of the [FeX4](-) anions and the organic iodopyridinium cations was used to describe the arrangement of these synthons and the hierarchy of the strengths of the respective contacts.