The metoprolol free base has been characterized in the solid state by X-ray diffaction (both single-crystal and variable-temperature powder diffraction) and differential scanning calorimetry. These studies are supplemented by molecular modelling and Hirshfeld surface analysis. Structural relationships with the strictly related betaxolol are discussed.
Metoprolol {systematic name: ( RS)-1-isopropylamino-3-[4-(2-methoxyethyl)phenoxy]propan-2-ol}, C 15H 25NO 3, is a cardioselective β 1-adrenergic blocking agent that shares part of its molecular skeleton with a large number of other β-blockers. Results from its solid-state characterization by single-crystal and variable-temperature powder X-ray diffraction and differential scanning calorimetry are presented. Its molecular and crystal arrangements have been further investigated by molecular modelling, by a Cambridge Structural Database (CSD) survey and by Hirshfeld surface analysis. In the crystal, the side arm bearing the isopropyl group, which is common to other β-blockers, adopts an all-trans conformation, which is the most stable arrangement from modelling data. The crystal packing of metoprolol is dominated by an O—H⋯N/N⋯H—O pair of hydrogen bonds (as also confirmed by a Hirshfeld surface analysis), which gives rise to chains containing alternating R and S metoprolol molecules extending along the b axis, supplemented by a weaker O⋯H—N/N—H⋯O pair of interactions. In addition, within the same stack of molecules, a C—H⋯O contact, partially oriented along the b and c axes, links homochiral molecules. Amongst the solid-state structures of molecules structurally related to metoprolol deposited in the CSD, the β-blocker drug betaxolol shows the closest analogy in terms of three-dimensional arrangement and interactions. Notwithstanding their close similarity, the crystal lattices of the two drugs respond differently on increasing temperature: metoprolol expands anisotropically, while for betaxolol, an isotropic thermal expansion is observed.