2.2 Solid-state NMR High-resolution solid-state NMR spectra were obtained using either a Bruker Avance III HD spectrometer operating at 1 H, C and 2 H NMR frequencies of 499.70 MHz, 125.65 MHz and 76.71 MHz respectively, or a Bruker Avance III HD spectrometer operating at the corresponding frequencies of 400.17 MHz, 100.62 MHz and 61.42 MHz. Samples were packed into 4 mm zirconia rotors. The 13 C shift scale was referenced with respect to neat tetramethylsilane (TMS) by setting the highest frequency peak of adamantane to 38.5 ppm. The 13 C spectra were used to fingerprint the structure and assess stability with respect to solvent loss; see Section 3 of the Supplementary Information for further details. The 2 H shift scale was referenced with respect to neat TMS by setting the peak of D2O to 4.81 ppm. Temperatures are quoted to the nearest 5 K to reflect uncertainties in absolute temperature calibration (details of the temperature calibration can be found in the data archive).