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

Abstract

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-FeOH_0.5/H₂O_0.5)(l-proline)₂Ln₂(H₂O)(DMF)_0.5 (where Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺) and (TCPP-FeOH)(l-proline)Ln_1.5 (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.

Two new lanthanide-based MOFs were synthesized from Fe-porphyrin tetracarboxylate linkers and l-proline coligands under a single set of synthetic conditions by varying the lanthanide(III) species. The specific structure is determined by the size of the lanthanide(III) cation with the smaller radii late lanthanides preferring the structure with smaller coordination number, thus demonstrating the lanthanide contraction effect in the MOF assembly.