Optimal Spatial Placement of Hydrogen-Bonding Groups Identified for Recognition of Tetrahedral Oxo Anions
Theory has established the basis for a first-time-ever molecular design of an anion receptor from first principles. Anion complexation by synthetic host molecules is an increasingly important theme in supramolecular chemistry. Interest is reinforced by the importance of anions in biology, and the possibility of applying anion receptors in sensors and separations. One of the key challenges is the design of hosts that recognize specific anion shapes. According to the principle of complementarity, recognition is achieved when the binding groups of the host molecule are positioned in space to match the bonding preferences of the guest species.
Benjamin Hay, a PNNL chemist, working with colleagues Timothy Firman and Bruce Moyer at Oak Ridge National Laboratory, hypothesized that maximum recognition may be achieved within an identified set of donor groups by optimizing their positions around the guest, a task that should be readily carried out by computer. To explore the validity of this hypothesis, they used electronic structure calculations to evaluate the optimal arrangement of urea binding sites about three monoanions of different shapes: chloride, nitrate, and perchlorate. Nature employs hydrogen-bond donors to bind anions, and urea emerges as ideal in being able to provide two hydrogen bonds simultaneously, one to each of two oxygen atoms in an oxo anion. Calculations reveal that urea is most complementary to the tetrahedral anion perchlorate among the three model anions examined. The results further establish significant differences in the optimal urea placement about spherical, trigonal planar, and tetrahedral anions, and reveal instances where host architecture might be exploited to achieve recognition on the basis of anion shape. The next step, now in progress, is to perform host design by determining the optimal linking groups that can join multiple urea groups in such a way that they will lie in their optimal positions. Actual candidate host molecules can then be synthesized and tested for their anion-binding ability.
Differences in the placement of three urea groups about chloride (left), nitrate (middle), and perchlorate (right) anions. Two views are provided for each structure: top - down the C3 axis, bottom - rotated 90° about x-axis.
This study has been published in the Journal of the American Chemical Society: Hay BP, TK Firman, and BA Moyer. 2005. "Structural Design Criteria for Anion Hosts: Strategies for Achieving Anion Shape Recognition through the Complementary Placement of Urea Donor Groups." Journal of the American Chemical Society 127(6):1810-1819. DOI:10.1021/ja043995k.