a, On cooling mmen-Mg2(dobpdc) from 150 °C (red) to 30 °C (blue) under 5% CO2, changes to the aliphatic C–H vibrations of mmen are apparent on adsorption of CO2. Furthermore, ammonium formation from neutral secondary amines is indicated by the appearance of a new, broad feature centred on 3,000 cm−1. b, Room temperature, in situ infrared spectroscopy measurements of mmen-M2(dobpdc) (M = Mg, Mn, Fe, Co, Zn, Ni) under N2 (blue) and CO2 (green) atmospheres. Grey lines mark diagnostic carbamate bands at 1,690 and 1,334 cm−1. c, On cooling under flowing 5% CO2 in N2 from 150 to 30 °C, the normalized mass increase of mmen-Mg2(dobpdc) measured by thermogravimetric analysis (black line) can be compared with the normalized integrated area of the infrared active bands at 1,330 and 660 cm−1. The bands at 1,330 and 660 cm−1 can be assigned to ν(C–N) and [β(OCO) + β(NCO)] modes that are characteristic of the highly ordered ammonium carbamate chains. d, In contrast to carbamate that is coordinated to a metal site through a nitrogen atom (top), changes in electron resonance configurations give rise to a feature at 1,334 cm−1 characteristic of CO2 insertion into the metal–nitrogen bond (bottom). e, Infrared spectroscopy clearly indicates that CO2 adsorption via metal-amine insertion occurs even in the presence of H2O owing to the presence of the aforementioned band at 1,334 cm−1.