**4. Conclusions**

The crystal structure of the mono-N-substituted pyrophosphoramide, O((*i*PrNH)2PO)2 **1** has been determined from single-crystal X-ray diffraction data while the chemical identity of the species was supported by IR, 1H NMR, 31P NMR, and GC MS data (see SI). A thorough structural comparison of **1** with other pyrophosphoramides for which the crystal structures have been published previously was carried out. **1** forms a novel supramolecular motif previously unattested for mono-N-substituted pyrophosphoramides. This motif was composed of two different synthons with P=O···H–N interactions. The grea<sup>t</sup> impact of this type of hydrogen bonding on the supramolecular motifs in all previously published

mono-N-substituted pyrophosphoramides could be confirmed also for **1**. Trends regarding the effects of the various organic moieties within the different compounds were difficult to describe due to the lack of systematic data. No comparison of **1** with the di-N-substituted pyrophosphoramides was undertaken, given that the trends in packing are significantly different between this group and the mono-N-substituted pyrophosphoramides.

The differences observed between the supramolecular motifs present in **1** and the supramolecular features of other mono-N-substituted pyrophosphoramides indicate that there are possibly other supramolecular motifs that have not ye<sup>t</sup> been discovered yet. The crystal structure of **1** further expands the diversity of possible supramolecular synthons. The different synthon in **1** (R<sup>1</sup> 2(8)ring), not known in previously described mono-N-substituted pyrophosphoramides, adds a new strong structural motif to the viable synthons known experimentally for mono-N-substituted pyrophosphoramides. It can also be considered to be a viable option for the discovery of new co-crystals of these species [20,21]. Different solid-state forms of pyrophosphoramides as well as co-crystals formed with other organic and inorganic species will have different physical properties and, at times different chemical properties compared to the currently marketed compounds without changing the actual molecule [26–30]. This is important for mono-N-substituted pyrophosphoramides given their possible use as a pesticide. The formation of different forms with different thermodynamic and kinetic stabilities can aid in complexation reactions and e.g., diminish decomposition on storage, a property of grea<sup>t</sup> importance for use in agriculture [28,29,31,32].

However, given the disparity between the various experimental techniques used in both the structures described in the literature and the current study further work at different temperatures, including X-ray data from powders, must be undertaken to obtain a more comprehensive understanding of the relationship between the chemical and physical properties and the crystal structures of various pyrophosphoramides.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2624-854 9/3/1/13/s1. Interpretation of IR, NMR and mass spectra. Figure S1: IR spectrum of O((*i*PrNH)2PO)2. Figure S2: IR spectrum of O((*i*PrNH)2PO)2 obtained from column chromatography. Figure S3: 1H NMR spectrum of O((*i*PrNH)2PO)2. Figure S4: 1H NMR spectrum of O((*i*PrNH)2PO)2 from column chromatography. Figure S5: 31P{1H} NMR spectrum of O((*i*PrNH)2PO)2. Figure S6: 31P{1H} NMR spectrum of O((*i*PrNH)2PO)2 from column chromatography. Figure S7: Gas chromatograph of O((*i*PrNH)2PO)2. Figure S8: Gas chromatograph of O((*i*PrNH)2PO)2 from column chromatography. Figure S9: Mass spectrum of O((*i*PrNH)2PO)2 from column chromatography. Table S1: 1H NMR experimental data and assignment for proton peaks of O((*i*PrNH)2PO)2 in CDCl3. Table S2: 1H NMR experimental data and assignment for proton peaks of O((*i*PrNH)2PO)2 from column chromatography in CDCl3. Table S3: Mass spectra peak data for the O((*i*PrNH)2PO)2 Gas chromatography peaks at 6.618 and 7.313 min.

**Author Contributions:** Conceptualization, D.M., U.B. and L.V.-Z.; Methodology and Practical Chemical and Spectroscopic studies, D.M.; XRD data collection and structure solution, U.B.; Writing— original draft preparation, D.M.; Writing—review and editing, U.B. and L.V.-Z.; Project administration, U.B. and L.V.-Z.; funding acquisition, L.V.-Z. and U.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research work disclosed in this publication is partially funded by the Endeavour Scholarship Scheme (Malta). Scholarships are part-financed by the European Union—European Social Fund (ESF)—Operational Programme II—Cohesion Policy 2014–2020 "Investing in human capital to create more opportunities and promote the well-being of society". It was also funded by the project: "Setting up of transdisciplinary research and knowledge exchange (TRAKE) complex at the University of Malta (ERDF.01.124)", which is being co-financed through the European Union through the European Regional Development Fund 2014–2020 (L.V.-Z. and U.B.).

**Data Availability Statement:** The data presented in this study are available in the supplementary materials.

**Acknowledgments:** The authors would like to thank Robert M. Borg for his contribution regarding NMR data collection and analysis and Godwin Sammut for his contribution in GC MS data collection, both of the Department of Chemistry, Faculty of Science, University of Malta. We would also like to thank Jens Meyer from STOE & Cie GmbH for his contribution in SXRD data collection.

**Conflicts of Interest:** The authors declare no conflict of interest.
