Understanding Correlation Between CO₂ Insertion Mechanism and Chain Length of Diamine in Metal-Organic Framework Adsorbents

Although CO₂ insertion is a predominant phenomenon in diamine-functionalized Mg₂(dobpdc) (dobpdc⁴⁻=4,4-dioxidobiphenyl-3,3′-dicarboxylate) adsorbents, a high-performance metal-organic framework for capturing CO₂, the fundamental function of the diamine carbon chain length in the mechanism remains unclear. Here, Mg₂(dobpdc) systems with open metal sites grafted by primary diamines NH₂−(CH₂)_n−NH₂ were developed, with en (n=2), pn (n=3), bn (n=4), pen (n=5), hn (n=6), and on (n=8). Based on CO₂ adsorption and IR results, CO₂ insertion is involved in frameworks with n=2 and 3 but not in systems with n≥5. According to NMR data, bn-appended Mg₂(dobpdc) exhibited three different chemical environments of carbamate units, attributed to different relative conformations of carbon chains upon CO₂ insertion, as validated by first-principles density functional theory (DFT) calculations. For 1-hn and 1-on, DFT calculations indicated that diamine inter-coordinated open metal sites in adjacent chains bridged by carboxylates and phenoxides of dobpdc⁴⁻. Computed CO₂ binding enthalpies for CO₂ insertion (−27.8 kJ mol⁻¹ for 1-hn and −20.2 kJ mol⁻¹ for 1-on) were comparable to those for CO₂ physisorption (−19.3 kJ mol⁻¹ for 1-hn and −20.8 kJ mol⁻¹ for 1-on). This suggests that CO₂ insertion is likely to compete with CO₂ physisorption on diamines of the framework when n≥5.