The solid-state structure of the β-blocker metoprolol: A combined experimental and in silico investigation

Metoprolol {systematic name: (RS)-1-isopropylamino-3-[4-(2-methoxyethyl)phenoxy]propan-2-ol}, C15H25NO3, is a cardioselective beta1-adrenergic blocking agent that shares part of its molecular skeleton with a large number of other beta-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 beta-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— HN/NH—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 OH—N/N—HO pair of interactions. In addition, within the same stack of molecules, a C—HO 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 beta-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