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Competition between Charge Assisted Hydrogen and Halogen Bonding in Pyridinium Trichloroacetates

Over the past decades of ever more intensive work in the field of crystal engineering hydrogen bonds have always been and remained the most typical tool for design of crystal structures. This is due to their strength and directionality which makes them quite reliable, and allows a certain degree of predictability of the supramolecular motifs which will form by hydrogen bonding of given hydrogen donors and acceptors. More recently, halogen interactions have also been recognised as useful interactions in crystal engineering, since they can be comparable in strength to hydrogen bonds. As halogen bonds are by nature electrostatic interactions between a (partial) negative charge on a “halogen acceptor” atom and a partial positive charge on a halogen atom which is present due to the anisotropy of electron density, they are also highly directional. As the partial positive charge on a halogen atom will be better defined as the size of the halogen increases, the most uses of halogen bonds in crystal engineering have involved iodine and bromine derivatives. Here we will present the results of our study of trichloroacetates of different organic bases. Trichloroacetate ion has three chlorine atoms, which are relatively poor halogen bond donors, but at the same time the halogen acceptor (oxygen) carries a net negative charge, rendering it a strong acceptor capable of forming charge assisted halogen bonds. Out of 7 structures studied thus far three indeed show short (< sum of van der Waals radii) O•••Cl contacts. In other structures both carboxylate oxygen atoms participate in strong hydrogen bonds with hydrogen donors, usually from the organic base molecules. It appears therefore that there is a competition between halogen (chlorine) and hydrogen bond donors for the same (carboxylate oxygen) acceptor. As halogen bonding is present only when there is an oxygen atom which participates in no hydrogen bonds, it would seem that hydrogen bond tends to be victorious in this competition.

hydrogen bond; halogen bond; crystal structure; trichloroacetate

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