Crystal Structure of Linagliptin Hemihydrate Hemiethanolate (C25H28N8O2)2(H2O)(C2H5OH) from 3D Electron Diffraction Data, Rietveld Refinement, and Density Functional Theory Optimization

The crystal structure of linagliptin hemihydrate hemiethanolate was solved by direct methods and refined by least-squares on the basis of 3D electron diffraction data. Linagliptin is one of the largest organic structures solved ab initio by electron diffraction. The good quality of the data allows the recognition of subtle symmetry reductions and of fine differences in the configuration of the two independent molecules that make up the structure. The structure was then further refined using synchrotron X-ray powder diffraction data and optimized using density functional techniques. The linagliptin hemihydrate hemiethanolate structure is very similar to the known linagliptin water/methanol/ethanol Form I, and crystallizes in space group P21212 (#18) with a = 24.85091(13), b = 21.56916(9), c = 9.74376(4) Å, V = 5222.79(3) Å3, and Z = 4. Although the compounds are similar, they are not the same, so the linagliptin hemihydrate hemiethanolate structure is a novel one. The powder pattern from a Le Bail fit to synchrotron data is included in the Powder Diffraction File as entry 00-066-1626.