Zirconium(IV) N-Methoxybenzamidate vs. N-Methoxybenzamide: Synthesis, Crystal Structure, and Phase Transitions

First example of a Zr(IV) complex with an aromatic carboxyamidate ligand, Zr(mba)₄ (mba = N-methoxybenzamidate), is prepared. Its structures in the solution (¹H, ¹³C{¹H} NMR) and in the crystal phase (single-crystal XRD) are determined and compared with those of the initial N-methoxybenzamide Hmba. During the complexation, mba^– ligands exhibit a bidentate cyclic function through oxygen atoms. The coordination polyhedron of the metal atom is a distorted square antiprism, the lengths of Zr–O bonds with carbonyl and methoxy groups fall within 2.08-2.09 Å and 2.29-2.33 Å, respectively. Despite the presence of available phenyl rings, the Zr(mba)₄ and Hmba crystals contain no intermolecular π–π-interactions, but exhibit C–H⋯π and N–H⋯O contacts, respectively. According to the DSC data, Hmba undergoes no phase transitions from 130 K up to the melting point (335.0±0.5 K, ΔH = 15.1±0.1 kJ/mol, ΔS = 45.2±0.2 J/(mol·K)), while Zr(mba)₄ exhibits a reversible solid-phase transition (T_onset = 177.4±0.5 K, ΔH = 0.68±0.20 kJ/mol, ΔS = 3.8±0.1 J/(mol·K)). Studying the Zr(mba)₄ crystals at 150 K and 200 K shows that this transition is not accompanied by the crystal deformation and is apparently caused by the rotation of phenyl groups. Due to this conversation, the space group changes from P2₁/c to P2₁/n, and one of the unit cell parameters increases by 3 times.