Reticular Chemistry of Uranyl Phosphonates: Sterically Hindered Phosphonate Ligand Method is Significant for Constructing Zero-Dimensional Secondary Building Units.

Designability is an attractive feature for metal-organic frameworks (MOFs) and essential for reticular chemistry, and many ideas are significantly useful in the carboxylate system. Bi-, tri-, and tetra-topic phosphonate ligands are used to achieve framework structures. However, an efficient method for designing phosphonate MOFs is still on the way, especially for uranyl phosphonates, owing to the complicated coordination modes of the phosphonate group. Uranyl phosphonates prefer layer or pillar-layered structures as the topology extension for uranyl units occurs in the plane perpendicular to the linear uranium-oxo bonds and phosphonate ligands favor the formation of compact structures. Therefore, an approach that can construct three-dimensional (3D) uranyl phosphonate MOFs is desired. In this paper, a sterically hindered phosphonate ligand method (SHPL) is described and is successfully used to achieve 3D framework structures of uranyl phosphonates. Four MOF compounds ([AMIM] (UO )(TppmH )⋅H O (UPF-101), [BMMIM] (UO ) (TppmH ) ⋅H O (UPF-102), [Py14] (UO ) (TppmH ) ⋅3 H O (UPF-103), and BMIM (TppmH )F ⋅2 H O (UPF-104); [AMIM]=1-allyl-3-methylimidazolium, [BMMIM]=1-butyl-2,3-dimethylimidazolium, [Py14]=N-butyl-N-methylpyrrolidinium, and [BMIM]=1-butyl-3-methylimidazolium) are obtained by ionothermal synthesis, with zero-dimensional nodes of uranyl phosphonates linked by steric tetra-topic ligands, namely tetrakis[4-(dihyroxyphosphoryl)phenyl]methane (TppmH ), to give 3D framework structures. Characterization by PXRD, UV/Vis, IR, Raman spectroscopy, and thermogravimetry (TG) were also performed.