Understanding Correlation Between CO2 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) adsorbents, high-performance metal-organic frameworks for capturing CO 2 , the fundamental function of diamine's carbon chain length in the mechanism remains unclear. Here, we report on Mg 2 (dobpdc) systems with open metal sites grafted by primary diamines NH 2 -(CH 2 ) n -NH 2 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 infrared (IR) results, CO 2 insertion is involved in frameworks with n = 2 and 3 but not in systems with n ≥  5. According to nuclear magnetic resonance (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 , our DFT calculations indicate that diamine inter-coordinated open metal sites in adjacent chains bridged by carboxylates and phenoxides of dobpdc 4- . Computed CO 2 binding enthalpies (less stable than -28 kJ mol -1 ) for CO 2 insertion were within the physisorption range. This suggests that CO 2 molecules are likely to be physisorbed on diamines of the framework without CO 2 insertion in Mg-N bond when n ≥  5.