Lanthanide(III) Cation Size Selective Formation of Two Different Metal–Organic Frameworks

The reaction between a 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin-FeCl linker (TCPP-Fe) and lanthanide ions (Ln3+) in excess of l-proline coligand provides a synthetic route to two structurally different metal–organic frameworks (TCPP-FeOH0.5/H2O0.5)(l-proline)2Ln2(H2O)(DMF)0.5 (where Ln = La3+, Ce3+, Pr3+, Nd3+, Sm3+, and Eu3+) and (TCPP-FeOH)(l-proline)Ln1.5 (where Ln = Gd3+, Tb3+ Tm3+ and Yb3+). The selection between the two different structures is dependent on the lanthanide ion atomic number. From the different Ln3+ ions used in this study, early-to-mid lanthanides, La3+, Ce3+, Pr3+, Nd3+, Sm3+, and Eu3+, give a structure consisting of discrete Ln8 building units (1-Ln), while mid-to-late lanthanides, Gd3+, Tb3+, Tm3+, and Yb3+, give a framework built upon one-dimensional Ln3+ chains (2-Ln). Therefore, the size of the lanthanide ion seems to play a key role in the structure selection and stability, which contrast with the commonly accepted behavior of lanthanides. Activation and subsequent argon gas sorption analyses done using 1-Nd and 2-Tb showed that 1-Nd is permanently porous with a determined surface area of 1223 ± 4 m2/g, while 2-Tb undergoes a structural change significantly decreasing its surface area (236 m2/g) from its expected value (ca. 900 m2/g). Stability tests on the activated samples revealed that 1-Nd lost its crystallinity after 1 month of exposure to atmospheric moisture, whereas 2-Tb retained its crystallinity, underscoring the higher long-term stability of 2-Tb compared to that of 1-Nd.