*Article* **Exploring the Structural Chemistry of Pyrophosphoramides:** *N***,***N-***,***N--***,***N---***-Tetraisopropylpyrophosphoramide**

**Duncan Micallef, Liana Vella-Zarb and Ulrich Baisch \***

> Department of Chemistry, Faculty of Science, University of Malta, Msida MSD2080, Malta; duncan.micallef.11@um.edu.mt (D.M.); liana.vella-zarb@um.edu.mt (L.V.-Z.) **\***

 Correspondence: ulrich.baisch@um.edu.mt; Tel.: +356-2340-3425

**Abstract:** *N*,*N*-,*N*--,*N*----Tetraisopropylpyrophosphoramide **1** is a pyrophosphoramide with documented butyrylcholinesterase inhibition, a property shared with the more widely studied octamethylphosphoramide (Schradan). Unlike Schradan, **1** is a solid at room temperature making it one of a few known pyrophosphoramide solids. The crystal structure of **1** was determined by single-crystal X-ray diffraction and compared with that of other previously described solid pyrophosphoramides. The pyrophosphoramide discussed in this study was synthesised by reacting iso-propyl amine with pyrophosphoryl tetrachloride under anhydrous conditions. A unique supramolecular motif was observed when compared with previously published pyrophosphoramide structures having two different intermolecular hydrogen bonding synthons. Furthermore, the potential of a wider variety of supramolecular structures in which similar pyrophosphoramides can crystallise was recognised. Proton (1H) and Phosphorus 31 (31P) Nuclear Magnetic Resonance (NMR) spectroscopy, infrared (IR) spectroscopy, mass spectrometry (MS) were carried out to complete the analysis of the compound.

**Keywords:** *N*,*N*-,*N*--,*N*----Tetraisopropylpyrophosphoramide; pyrophosphoramide; synthons; supramolecular motifs; X-ray crystallography
