Investigation of steric influences on hydrogen-bonding motifs in cyclic ureas by using X-ray, neutron, and computational methods

McCormick, Laura J., McDonnell-Worth, Ciaran, Platts, James A., Edwards, Alison J. and Turner, David R. (2013) Investigation of steric influences on hydrogen-bonding motifs in cyclic ureas by using X-ray, neutron, and computational methods. Chemistry - An Asian Journal, 8 (11), 2642-2651. (doi:10.1002/asia.201300530).

Abstract

A series of urea-derived heterocycles, 5N-substituted hexahydro-1,3,5- triazin-2-ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5-position (which is derived from the corresponding primary amine), that is, methyl (1), ethyl (2), isopropyl (3), tert-butyl (4), benzyl (5), N,N-(diethyl)ethylamine (6), and 2-hydroxyethyl (7). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen-bonding tape motif that consists of self-associative R₂ ²(8) dimers. The results from X-ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen-bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the α-carbon atom of the N substituent. With the less-hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the α-carbon atom, as in the isopropyl and tert-butyl derivatives, a different 2D hydrogen-bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N-(diethyl)ethylamine derivatives have methylene units at the α-carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen-bond donor/acceptor in the 2-hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the N-H×××O-C interactions. The geometry around the hydrogen-bearing nitrogen atoms, whether planar or non-planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O-C-N-H torsion angles occurs to maintain almost-linear hydrogen-bonding interactions. The incredible bulk: A series of cyclic ureas has been studied to examine the influence of bulky substituents on the hydrogen-bonding motifs that form and the degree to which the urea group can be distorted to maintain strong intermolecular contacts.

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Contributors

Author: Laura J. McCormick

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