Crystal and Molecular Structures of Two Isostructural 2,5-Diphenyl Pyrimidines

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Abstract
The crystal structures of 5-phenyl-2-(4-n-propoxyphenyl)-pyrimidine (5-PPrPP) and 5-phenyl-2-(4-n-butoxyphenyl)-pyrimidine (5-PBuPP) have been determined as part of a study of the conformational behavior of mesogenic phenyl pyrimidines. The two compounds are isostructural and crystallize in space group P21/c with four molecules per unit cell. The lattice dimensions are (5-PPrPP): a = 12.961(2), b = 15.186(1), c = 7.882(1) Å, β = 98.47(1)° and (5-PBuPP): a = 12.116(1), b = 15.172(1), c = 9.098(1) Å, β = 96.581(9)°. The structures have been solved by direct methods and refined to R values of 0.037 and 0.066, respectively. The molecules of both compounds adopt a stretched form. Their phenyl rings are twisted with respect to the central pyrimidine ring in the following extent: the inter-planar angles amount to 4.8° (5-PPrPP) and 7.6° (5-PBuPP) for the 2-phenyl pyrimidine and to 39.1° and 37.7°, respectively, for the 5-phenyl pyrimidine fragment. The molecules are arranged in the crystalline state nearly parallel to each other forming sheets parallel to the crystallographic ac plane.