Assembly of Robust and Porous Hydrogen-Bonded Coordination Frameworks: Isomorphism, Polymorphism, and Selective Adsorption

By using the tripodal ligand ntb (tris(benzimidazole-2-ylmethyl)amine) and lanthanide nitrate, three isomorphous series of coordination frameworks of the general formula [Ln(ntb)(NO3)3]·solvents (series 1: monoclinic C2/c, Ln = Gd3+ and Yb3+; series 2: hexagonal P31/c, Ln = Nd3+, Eu3+, Gd3+, and Er3+; series 3, cubic Pa3̅, Ln = Gd3+ and Er3+; solvent = H2O or CH3OH) have been assembled and characterized with IR, elemental analyses, and single crystal and powder X-ray diffraction methods. In all isomorphous complexes, analogous [Ln(ntb)(NO3)3] coordination monomers of the same structure act as the building blocks to be assembled via hydrogen bonds into three-dimensional (3D) frameworks. So the complexes of the same lanthanide ion (for example, the Gd3+ ion) from three isomorphous series form polymorphs, for example, monoclinic polymorph 1-Gd, hexagonal polymorph 2-Gd, and cubic polymorph 3-Gd. The single-crystal analyses revealed that the polymorphism was related to different fashions of hydrogen bonding interactions, which was caused by different crystallization conditions, leading to the formation of different 3D hydrogen-bonded frameworks showing distinct porous and topological structures. The monoclinic and hexagonal crystals contain 1D channels, while the cubic crystal is nonporous. The thermogravimetric analyses indicated that all polymorphic crystals have high thermal stability against the removal of guest molecules, and the robust porosity of the hexagonal crystals has been verified by temperature-dependent single-crystal-to-single-crystal measurements upon guest removal/uptake. The solvents adsorption study disclosed that the porous frameworks show high selectivity of benzene against toluene and xylene, while the gas adsorption measurements indicated a moderate H2, CO2, and MeOH storage capacity in contrast to low N2 uptake. The solid-state photoluminescence of the Eu3+ and Nd3+ complexes in the near-infrared and visible region has also been investigated, offering examples with optical properties tunable by means of isomorphous replacement.