*2.1. General Considerations*

All reactions were carried out under an argon atmosphere using Schlenk line techniques. Chloroform was dried over a P2O5 still, while diethyl ether, THF, petroleum ether and chloroform used in the work-up procedures of the product were dried over 4 Å molecular sieves. FT-IR (Fourier Transform Infrared) spectra were recorded using KBr pellet samples and a Shimadzu IRAffinity-1 FTIR spectrophotometer. 1H NMR and 31P NMR (Nuclear Magnetic Resonance) spectra were collected on a Bruker Ascend NMR spectrometer with a probe having a set frequency of 500.13 MHz for 1H NMR and 202.457 MHz for 31P NMR. Gas chromatography Mass spectroscopy (GC MS) data was collected using a Thermo DSQ II GC/MS spectrometer with samples being prepared by dissolution of the products in chloroform. Single-crystal X-ray diffraction data was collected on a STOE STADIVARI diffractometer.

### *2.2. Synthesis of N,N*-*,N",N*---*-Tetraisopropylpyrophosphoramide (O((iPrNH)2PO)2)*

The synthesis of O((*i*PrNH)2PO)2 **1** was performed through a modification of the synthesis of the analogous O((Me2N)2PO)2 (Schradan) as described by Goehring, M. and Niedenzu, K. in 1956 [22]. Iso-propyl amine (5 mL, 0.058 mol) was dissolved in 10 mL of chloroform as a solvent. The mixture was cooled to −78 ◦C and pyrophosphoryl tetrachloride (1 mL, 0.007 mol) was added dropwise to the reaction solution using a glass syringe. Upon addition of pyrophosphoryl tetrachloride, the formation of white fumes was noted, along with the formation of a solid crystalline mass. The reaction was left at a temperature of -78 ◦C until the white fumes dissipated and was subsequently allowed to reach room temperature. The reaction mixture was left to react at room temperature overnight. The white suspension thus formed was subsequently heated to 60 ◦C for 3 h to complete the reaction. The yellow solution obtained was left overnight to form a clear colourless crystalline mass (*i*PrNH3Cl). The mixture was then filtered to collect a clear colourless crystalline mass (*i*PrNH3Cl) and a yellow solution. The latter was layered with diethyl ether to yield a white solid. Yield, 41% (crude product with respect to pyrophosphoryl tetrachloride), 1H NMR (CDCl3): 8.31 ppm (s, 1H, NH3), 3.64 ppm (td, 1H, CH), 3.40 ppm (m, 1H, CH), 2.26 ppm (s, 1H, NH), 1.39 ppm (d, 2H, CH3) and at 1.14 ppm (t, 6H, CH3), 31P {1H} NMR (CDCl3): 14.33 ppm (s), FT-IR (KBr, cm<sup>−</sup>1): 3400 (w), 3252 (sb), 2965 (s), 2870 (m), 1637 (m), 1527 (m), 1466 (m), 1432 (m), 1396 (m), 1367 (m), 1257 (s), 1229 (s), 1167 (m), 1137 (w), 1055 (s), 1026 (s), 945 (m), 914 (m), 886 (s), 804 (m), 750 (m), GC MS (EI; 70 eV) *m*/*z*: 44.12, 58.08, 79.01, 93.97, 137.07, 179.10 [(*i*PrNH)2PO2]+, 195.16.

Crystals suitable for SXRD studies were obtained through liquid–liquid diffusion crystallisation: A sample of the white solid product was dissolved in a minimum volume of chloroform to yield a saturated solution. This was layered with 30–40 ◦C petroleum ether in a 1:5 volume ratio of chloroform/petroleum ether. This produced a liquid-liquid diffusion crystallisation set up which yielded a white crystalline solid over the course of two days. The solid was collected by cannula filtration and crystals suitable for SXRD studies were collected from this solid.

### *2.3. Purification by Column Chromatography*

Although single crystals of O((*i*PrNH)2PO)2 were obtained from the product, it is evident from NMR and GC MS data that this solid was not pure and therefore a sample of O((*i*PrNH)2PO)2 was also purified by column chromatography to determine whether this was a possible method of purification. The sample was dissolved in dichloromethane and activated Keiselgel 60 was used as the stationary phase, while THF/acetonitrile (1:1 *v*/*v*) was used as a mobile phase. 1H NMR (CDCl3): 3.37 (m, 1H, CH), 2.29 (bt, 1H, NH), 1.16 (t, 6H, CH3), 31P {1H} NMR (CDCl3): 14.33 ppm (s), FT-IR (KBr, cm<sup>−</sup>1): 3414 (w), 3255 (bm), 2965 (s), 2870 (m), 1465 (m), 1428 (m), 1367 (m), 1257 (s), 1205 (s), 1167 (m), 1137 (m), 1051 (s), 1021 (s), 916 (m), 886 (m), 833 (sh), 800 (vw), 771 (m), GC MS (EI; 70 eV) *m*/*z*: 44.12, 58.11, 79.01, 93.93, 137.09, 179.10 [(iPrNH)2PO2] +, 195.16.
