Understanding Correlation Between CO 2 Insertion Mechanism and Chain Length of Diamine in Metal‐Organic Framework Adsorbents

Although CO 2 insertion is a predominant phenomenon in diamine‐functionalized Mg 2 (dobpdc) (dobpdc 4− =4,4‐dioxidobiphenyl‐3,3′‐dicarboxylate) adsorbents, a high‐performance metal‐organic framework for capturing CO 2 , the fundamental function of the diamine carbon chain length in the mechanism remains unclear. Here, Mg 2 (dobpdc) systems with open metal sites grafted by primary diamines NH 2 −(CH 2 ) n −NH 2 were developed, with en ( n =2), pn ( n =3), bn ( n =4), pen ( n =5), hn ( n =6), and on ( n =8). Based on CO 2 adsorption and IR results, CO 2 insertion is involved in frameworks with n =2 and 3 but not in systems with n ≥5. According to NMR data, bn‐appended Mg 2 (dobpdc) exhibited three different chemical environments of carbamate units, attributed to different relative conformations of carbon chains upon CO 2 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 dobpdc 4− . Computed CO 2 binding enthalpies for CO 2 insertion (−27.8 kJ mol −1 for 1 ‐ hn and −20.2 kJ mol −1 for 1 ‐ on ) were comparable to those for CO 2 physisorption (−19.3 kJ mol −1 for 1 ‐ hn and −20.8 kJ mol −1 for 1 ‐ on ). This suggests that CO 2 insertion is likely to compete with CO 2 physisorption on diamines of the framework when n ≥5.