Abstract
This paper focuses in the influence of halogen atoms in the design and structural control of the crystal packing of Group VIII halogenated metallocenes. The study is based on the present knowledge on new types of intermolecular contacts such as halogen (X⋯X, C-X⋯H, C-X⋯π), π⋯π, and C-H⋯π interactions. The presence of novel C-H⋯M interactions is also discussed. Crystal packings are analysed after database search on this family of compounds. Results are supported by ab initio calculations on electrostatic charge distributions; Hirshfeld analysis is also used to predict the types of contacts to be expected in the molecules. Special attention is given to the competition among hydrogen and halogen interactions, mainly its influence on the nature and geometric orientations of the different supramolecular motifs. Supramolecular arrangements of halogenated metallocenes and Group IV di-halogenated bent metallocenes are also compared and discussed. Analysis supports halogen bonds as the predominant interactions in defining the crystal packing of bromine and iodine 1,1′-halometallocenes.
Highlights
Several reports of what could be considered weak noncovalent interactions between molecules are found in the literature as early as the end of the 19th century [1]
In the beginning of the 20th century they were identified as van der Waals forces [2,3,4] and hydrogen bonds [5,6]; it was Pauling who used for the first time the term “hydrogen bond” in its 1931 paper on the nature of chemical bonds [7]
He assumed an electrostatic character for hydrogen bonds D-H· · · A, that should result in their formation only when the donor atom (D) and the acceptor (A) were electronegative atoms like N, O, F, Cl, Br and I
Summary
Several reports of what could be considered weak noncovalent interactions between molecules are found in the literature as early as the end of the 19th century [1]. The involvement of other donor atoms was first proposed by Pimentel and McClellan in 1960 [9], further reinforced in 1993 by Steiner and Saenger [10], that defined hydrogen bond as “any cohesive interaction X-H· · · A where H carries a positive charge and A a negative charge and the charge on X is more negative than in H” This definition includes donor atoms like C, P, As, as well as π acceptors, creating the possibility for the existence of strong and weak hydrogen bonds, depending on the properties of the atoms involved [11]
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