Pseudopolymorphism in brucine: brucine-water (1/2), the third crystal hydrate of brucine.

The structure of a third pseudopolymorphic hydrate of brucine, brucine-water (1/2) [systematic name: 2,3-dimethoxystrychnidin-10-one-water (1/2)], C(23)H(26)N(2)O(4).2H(2)O, has been determined at 130 K. The asymmetric unit comprises two independent brucine molecules and four water molecules of solvation. The four water molecules form uncommon cyclic hydrogen-bonded homomolecular R(4)(4)(8) tetramer rings, which then form primary hydrogen-bonded chain substructures extending down the 2(1) screw axis in the unit cell. The two brucine molecules are linked peripherally to these substructures by either single O-H...N(brucine) or asymmetric three-centre O-H...O(brucine) hydrogen bonds.