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Weak hydrogen bonds as a driving force for polymorphism in Cd(II)-based supramolecular networks

Polymorphism, the occurrence of a compound in two or more crystal forms, is an important challenge in crystal engineering that has arguably received more attention from the organic solid-state community than from inorganic chemists. Despite the fact that we have moved closer towards reliable and reproducible supramolecular synthesis, the appearance of polymorphic structures is still a phenomenon that frequently provides unexpected and very difficult practical and theoretical problems. Here we present two polymorphs of bis-(2-animopyrazine) di-iodocadmium(II) which are different from one another only by the relative arrangements of their bilayers while all other structural features in the two forms remain the same, resulting in barely ascertainable differences in physical properties. The structural studies are combined with spectroscopic, thermal and computational tools which collectively offer a protocol for extracting key information about a system that displays only very subtle structural difference. The relative importance of different metal-bound halide ions for the formation of extended supramolecular networks is also explored, which may have broad implications regarding our understanding of polymorphic transition-metal architectures.

weak hydrogen bonds; polymorphism; cadmium(II); supramolecular network

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