Lanthanide‐Based T2ex and CEST Complexes Provide Insights into the Design of pH Sensitive MRI Agents

The CEST and T₁ /T₂ relaxation properties of a series of Eu³⁺ and Dy³⁺ DOTA‐tetraamide complexes with four appended primary amine groups are measured as a function of pH. The CEST signals in the Eu³⁺ complexes show a strong CEST signal after the pH was reduced from 8 to 5. The opposite trend was observed for the Dy³⁺ complexes where the r_2ex of bulk water protons increased dramatically from ca. 1.5 mm ⁻¹ s⁻¹ to 13 mm ⁻¹ s⁻¹ between pH 5 and 9 while r₁ remained unchanged. A fit of the CEST data (Eu³⁺ complexes) to Bloch theory and the T_2ex data (Dy³⁺ complexes) to Swift–Connick theory provided the proton‐exchange rates as a function of pH. These data showed that the four amine groups contribute significantly to proton‐catalyzed exchange of the Ln³⁺‐bound water protons even though their pK _a’s are much higher than the observed CEST or T_2ex effects. This demonstrated the utility of using appended acidic/basic groups to catalyze prototropic exchange for imaging tissue pH by MRI.     

Fine Tuning of MOF‐505 Analogues To Reduce Low‐Pressure Methane Uptake and Enhance Methane Working Capacity

We present a crystal engineering strategy to fine tune the pore chemistry and CH₄‐storage performance of a family of isomorphic MOFs based upon PCN‐14. These MOFs exhibit similar pore size, pore surface, and surface area (around 3000 m² g⁻¹) and were prepared with the goal to enhance CH₄ working capacity. [Cu₂(L2)(H₂O)₂]_n (NJU‐Bai 41: NJU‐Bai for Nanjing University Bai's group), [Cu₂(L3)(H₂O)₂]_n (NJU‐Bai 42), and [Cu₂(L4)(DMF)₂]_n (NJU‐Bai 43) were prepared and we observed that the CH₄ volumetric working capacity and volumetric uptake values are influenced by subtle changes in structure and chemistry. In particular, the CH₄ working capacity of NJU‐Bai 43 reaches 198 cm³ (STP: 273.15 K, 1 atm) cm⁻³ at 298 K and 65 bar, which is amongst the highest reported for MOFs under these conditions and is much higher than the corresponding value for PCN‐14 (157 cm³ (STP) cm⁻³).