PACKING OF BRUCINE CO-CRYSTALS

Robert O. Gould,¹ Fokke J.J. Dijksma,¹ Paul Taylor² and Malcolm D. Walkinshaw,²

¹Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland
²Department of Structural Biochemistry, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JR, Scotland.

The alkaloid brucine has long been used as a resolving agent, mainly for chiral molecules capable of existing as anions, because of its strong stereoselectivity in crystalline salt formation. We will present a survey of some 30 structures containing brucine in terms of their packing, showing the environements which brucine normally provides. By far the most important packing modes involve sheets separated by layers of cations and water, these sheets being composed of parallel ribbons of brucine molecules arranged "head-to-tail" as in the figure.¹ The unit mesh of these structures has symmetry pg and a mesh of 12.3(3) A (along the ribbons) by 7.7(2) A (normal to the ribbons), these often being recognisable in the cell dimensions of a crystal. The third cell dimension depends on the size of the counterion as well as the layer stacking. The most frequent type has antiparallel layers with a cell repeat every two layers (ideal symmetry P2₁2₁2₁). A less common variant has parallel layers, ideal symmetry P2₁, and a repeat every layer.

Another important mode has the sheets centered with respect to one another, ideal space group C222₁. Less frequently encountered are structures based on "head-to-head" ribbons and columnar structures.

¹Dijksma, Gould, Parsons, Taylor and Walkinshaw, J. Chem. Soc. Chem. Commun., 1998, 745-746.