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

• DOI: 10.1007/s11224-012-0090-3
• Publication Date: 2012-07-05T13:54:49Z
• Authors: Firas F. Awwadi; Deeb Taher; Alaa Maabreh; Fadel Z. Alwedian; Hamdan Al-Ebaisat; Tobias Rüffer; Heinrich Lang
• Keywords: Synthon; Isostructural; Crystal Engineering; Crystal (programming language)

The analogy of chloride–chloride contacts in compounds containing Fe–Cl1···Cl2–Fe synthons with well-studied organic C–Cl1···Cl2–C interactions has been investigated. The crystal structures of the two tetrahaloferrate(III) salts, [(2-iodopyridinium)2FeX4]X (X = Cl, Br) have been determined. Analysis of these two isomorphous structures and related published structures shows that the arrangement of Fe–Cl1···Cl2–Fe synthons is similar to that of C–Cl1···Cl2–C with the Fe–Cl1···Cl2 and Cl1···Cl2–Fe angles being ~150°. While inter-chlorine distances are less than the sum of van der Waals radii in C–Cl1···Cl2–C units, they are equal to, or longer, than the sum of van der Waals radii in the corresponding Fe–Cl1···Cl2–Fe contacts. This might indicate that the arrangement of Fe–Cl1···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)2FeBr4]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)2FeX4]X including N–H···X−, C–I···X–Fe, N(π)···X–Fe, N(π)···I–C, and Fe–X1···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.