Order and Disorder in the Structures of Two Crystal Polymorphs of the Adduct Bis(Quinolinium-2-Carboxylate) DL-Malic Acid

Abstract  

The structures of two polymorphs of the anhydrous cocrystal adduct of bis(quinolinium-2-carboxylate) DL-malic acid, one triclinic the other monoclinic and disordered, have been determined at 200 K. Crystals of the triclinic polymorph 1 have space group P-1, with Z = 1 in a cell with dimensions a = 4.4854(4), b = 9.8914(7), c = 12.4670(8) ?, α = 79.671(5), β = 83.094(6), γ = 88.745(6)°. Crystals of the monoclinic polymorph 2 have space group P2₁/c, with Z = 2 in a cell with dimensions a = 13.3640(4), b = 4.4237(12), c = 18.4182(5) ?, β = 100.782(3)°. Both structures comprise centrosymmetric cyclic hydrogen-bonded quinolinic acid zwitterion dimers graph set R₂²(10)] and 50% disordered malic acid molecules which lie across crystallographic inversion centres. However, the oxygen atoms of the malic acid carboxylic groups in 2 are 50% rotationally disordered whereas in 1 these are ordered. There are similar primary malic acid carboxyl O–H···O_{quinaldic acid} hydrogen-bonding chain interactions in each polymorph, extended into two-dimensional structures but in 1 this involves centrosymmetric cyclic head-to-head malic acid hydroxyl-carboxyl O–H···O interactions graph set R₂²(10)] whereas in 2 the links are through single hydroxy-carboxyl hydrogen bonds.