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  • Title: Chiral versus racemic building blocks in supramolecular chemistry: malate salts of organic diamines.
    Author: Farrell DM, Ferguson G, Lough AJ, Glidewell C.
    Journal: Acta Crystallogr B; 2002 Jun; 58(Pt 3 Pt 2):530-44. PubMed ID: 12037343.
    Abstract:
    (S)-Malic acid forms a salt with N,N'-dimethylpiperazine, [MeN(CH(2)CH(2))(2)NMe]H(2)(2+) x 2C(4)H(5)O(5)(-) (1) (triclinic, P1, Z' = 1), in which the cations link pairs of hydrogen-bonded anion chains to form a molecular ladder. With 4,4'-bipyridyl, (S)-malic acid forms a 1:1 adduct which crystallizes from methanol to yield two polymorphs, (2) (triclinic, P1, Z' = 1) and (3) (monoclinic, C2, Z' = 1), while racemic malic acid with 4,4'-bipyridyl also forms a 1:1 adduct, (4) (monoclinic, P2(1)/c, Z' = 1). In each of (2), (3) and (4) the components are linked by O[bond]H...N and N[bond]H...O into chains of alternating bipyridyl and malate units, which are linked into sheets by O[bond]H...O hydrogen bonds. In each of the 1:1 adducts (5) and (6), formed by, respectively, (S)-malic acid and racemic malic acid with 1,2-bis(4'-pyridyl)ethene, the diamine is disordered over two sets of sites, related by a 180 degrees rotation about the N...N vector. In (5), (C(12)H(10)N(2))H(+) x C(4)H(5)O(5)(-) (triclinic, P1, Z' = 1), the components are again linked by a combination of N[bond]H...O and O[bond]H...O hydrogen bonds into sheets, while in (6) (triclinic, P1;, Z' = 0.5) there is only partial transfer of the H atom from O to N and the malate component is disordered across a centre of inversion. With 1,4-diazabicyclo[2.2.2]octane, racemic malic acid forms a 1:2 salt, [(C(6)H(12)N(2))H(2)](2+).2C(4)H(5)O(5)(-) (7) (monoclinic, P2(1)/c, Z' = 2), while (S)-malic acid forms a 1:1 adduct, (8) (monoclinic, P2(1), Z' = 3). There are thus six independent molecular components in each. In (7) the ions are linked by an extensive series of N[bond]H...O and O[bond]H...O hydrogen bonds into a three-dimensional framework, but in (8) there is extensive disorder involving all six components, and no refinement proved to be feasible.
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